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Polyurethanes (PUs) are a class of versatile materials with great potential for use in different applications, especially based on their structure-property relationships. Their specific mechanical, physical, biological, and chemical properties are attracting significant research attention to tailoring PUs for use in different applications. Enhancement of the properties and performance of PU-based materials may be achieved through changes to the production process or the raw materials used in their fabrication or via the use of advanced characterization techniques. Clearly, modification of the raw materials and production process through proper methods can produce PUs that are suitable for varied specific applications. The present study aims to shed light on the chemistry, types, and synthesis of different kinds of PUs. Some of the important research studies relating to PUs, including their synthesis method, characterization techniques, and research findings, are comprehensively discussed. Herein, recent advances in new types of PUs and their synthesis for various applications are also presented. Furthermore, information is provided on the environmental friendliness of the PUs, with a specific emphasis on their recyclability and recoverability.
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Polyurethane types, synthesis and applications
a review
John O. Akindoyo,*
a
M. D. H. Beg,*
a
Suriati Ghazali,*
a
M. R. Islam,*
b
Nitthiyah Jeyaratnam
a
and A. R. Yuvaraj
c
Polyurethanes (PUs) are a class of versatile materials with great potential for use in dierent applications,
especially based on their structureproperty relationships. Their specic mechanical, physical, biological,
and chemical properties are attracting signicant research attention to tailoring PUs for use in dierent
applications. Enhancement of the properties and performance of PU-based materials may be achieved
through changes to the production process or the raw materials used in their fabrication or via the use
of advanced characterization techniques. Clearly, modication of the raw materials and production
process through proper methods can produce PUs that are suitable for varied specic applications. The
present study aims to shed light on the chemistry, types, and synthesis of dierent kinds of PUs. Some of
the important research studies relating to PUs, including their synthesis method, characterization
techniques, and research ndings, are comprehensively discussed. Herein, recent advances in new types
of PUs and their synthesis for various applications are also presented. Furthermore, information is
provided on the environmental friendliness of the PUs, with a specic emphasis on their recyclability and
recoverability.
1. Introduction
Polyurethanes (PUs) are a special group of polymeric materials
that are in many ways dierent from most of the other plastic
types. They can be incorporated into many dierent items, such
as paints, liquid coatings, elastomers, insulators, elastic bres,
foams, integral skins, etc. Several forms in which PUs appear
today are mere improvements in the invention of the German
professor (Professor Dr Otto Bayer) and his co-workers.
1
Fig. 1
illustrates the most important types of PUs and some common
examples of their uses. The invention of the diisocyanate poly-
addition technique by these researchers led to the creation of
the PU industry in 1937, with PU produced through the reaction
between diisocyanate and polyester diol.
2,3
PU was rst developed as an alternative for rubber during
World War II. The versatility of this material as well as its
suitability to replace other scarce materials led to its incorpo-
ration in several applications. For instance, PU coatings were
specically used for impregnating paper and producing
garments that were resistant to mustard gas and corrosion.
They were also used as chemically-resistant coatings for wood,
masonry, and metal, and as high gloss nishes for airplanes.
4
The early industrial production of PU coatings began with
dierent formulations for specic purposes. By the mid-1950s,
PU coatings were commonly found in elastomers, coatings,
rigid foams and adhesives.
5
Towards the later part of the 1950s,
comfortable and convenient cushions made from exible foams
were commercially available.
4
Additionally, the development of
exible foams from cheap polyether polyols led to several
automotive and upholstery applications that are still relevant
today. Continuous improvements in processing techniques,
additives types and in the formulations have contributed to
these materials being used in a wide range of applications.
6,7
Currently, PUs are one of the most common, versatile and
researched materials in the world.
8
These materials combine
the durability and toughness of metals with the elasticity of
rubber, making them suitable to replace metals, plastics and
rubber in several engineered products.
7,9
They have been widely
applied in biomedical applications, building and construction
applications, automotive, textiles and in several other indus-
tries due to their superior properties in terms of hardness,
elongation, strength and modulus.
6,8,1014
The urethane group is the major repeating unit in PUs, and
is produced from the reaction between alcohol (OH) and
isocyanate (NCO); albeit PUs also contain other groups, such as
ethers, esters, urea and some aromatic compounds.
8,15
Due to
the wide variety of sources from which PUs can be synthesized
and given their wide range of specic applications, they can be
a
Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang
Lebuhraya Tun Razak, Gambang 26300, Kuantan, Malaysia. E-mail: dhbeg@yahoo.
com; blessedbode@ymail.com
b
Malaysian Institute of Chemical and Bioengineering Technology, University of Kuala
Lumpur, Bandar vendor, Taboh Naning, Alor Gajah 78000, Melaka, Malaysia. E-mail:
muhammad.remanul@unikl.edu.my
c
Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang Lebuhraya
Tun Razak, Gambang 26300, Kuantan, Malaysia
Cite this: RSC Adv.,2016,6, 114453
Received 4th June 2016
Accepted 21st November 2016
DOI: 10.1039/c6ra14525f
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grouped into several dierent classes based on the desired
properties: rigid, exible, thermoplastic, waterborne, binders,
coating, adhesives, sealants and elastomers.
16
Among the major
applications, PU foam is one of the most prominent PU-based
products, and is used globally in signicant amounts. About
50% of all polyurethane foam production is consumed by the
market demand for rigid PU foam.
17
Worldwide there are
dierent types of PU production, with an estimated forecast up
to 2020 given herein in Fig. 2.
18
PU foams can be easily tailored
to obtain specic products by merely changing the types and
quantities of the surfactants, catalysts, blowing agents, isocya-
nate and polyol used in their fabrication, as well as the extent of
intercalation and exfoliation between the llers and matrices to
meet the desired purpose.
19,20
Specically, PUs nd wide appli-
cation in coatings due to their specic properties, such as their
excellent mechanical strength, toughness, good abrasion,
corrosion and chemical resistance and low temperature exi-
bility.
21
One of the most important categories of PUs is that of
PU elastomers, which have been widely incorporated into
dierent engineered products and have been proven to oer
highly impeccable properties.
22
They are malleable polymers
and can be easily processed, both by extrusion and injection
moulding, and also oer a high possibility for recycling.
23
PUs have been prepared from dierent diisocyanates, poly-
ols and chain extenders and their properties investigated.
8
Initially, most of the polyols used to prepare PUs were obtained
from petroleum sources, but the high energy demands of the
production process as well as environmental concerns have
increased the necessity for more suitable and environmentally
friendly substitutes. This has recently drawn enormous
commercial and academic attention to renewable resources,
such as vegetable oils.
2428
Vegetable oils mainly consist of
triglyceride molecules with dierent reactive sites, such as
carboncarbon double bonds, ester and hydroxyl groups. The
conversion of these oils to polyols may be achieved through
dierent techniques, such as epoxidation and ring-opening
polymerization,
29
ozonolysis,
30
transesterication
31
and hydro-
formylation.
32
Polyols obtained from vegetable oils have been
found to be capable of partially replacing petroleum-derived
polyols, especially when they are cross-linked with dierent
isocyanates for PU production. As an advancement in product
development, attention has been drawn to the manufacture of
nanomaterials-based PUs following the novel production of
a nanocomposite from nylon and clay for use in Toyota cars.
33,34
Thus, the incorporation of nanomaterials has been suggested to
oer several advantages related to desirable performance in
a wide range of areas.
35,36
Although a huge amount of research has been carried out on
PUs, to the best of our knowledge, there are no reports in the
literature compiling information on the improvement in the
types and synthetic routes of PUs. Therefore, this review
provides a summary of the advances made in the types and
synthesis of PUs, as well as their recent incorporation into
several engineered products. The importance of the individual
components of PUs is fully highlighted, including how each
component may determine the application for which the
synthesized PU may be used.
1.1 Chemistry of polyurethanes
The chemistry of PUs leads to them being grouped with other
compounds that are oen collectively referred to as reaction
polymers. These compounds include phenolics, unsaturated
Fig. 1 Important types of PUs and common examples of their applications.
Fig. 2 Worldwide PU production and an estimated forecast up to
2020.
18
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polyesters and epoxies.
37,38
Generally, PUs are oen synthesized
from the reaction between an isocyanate and polyol molecule in
the presence of either a catalyst or ultraviolet light activation.
39
These isocyanate and polyol molecules should necessarily
contain two or more isocyanate groups (R(N]C]O)
n$2
) and
hydroxyl groups (R0(OH)
n$2
), respectively.
37
The exhibited
properties of the PUs usually depend on the types of polyols and
isocyanates from which they were made.
40
Generally, soelastic
polymers can be produced from exible long segments of pol-
yols, whereas rigid and tough polymers are obtained via
a higher amount of cross-linking. Stretchy polymers can be
obtained through long chains with low cross-linking, whereas
hard polymers can be obtained from shorter chains with high
cross-linking. On the other hand, a combination of long chains
with average cross-linking would produce polymers that are
suitable for foam making.
10
Due to the cross-linking in PUs, they
oen possess an innite molecular weight with a three-
dimensional (3D) network build-up. This is the reason why
a small fraction of PUs may be referred to as a giant molecule
and this explains why typical PUs oen will not go soor melt
when they are heated. The incorporation of dierent additives
alongside the isocyanates and polyols, as well as modications
to the processing conditions, makes it possible to obtain a wide
range of characteristic features, which makes them suitable for
various applications.
41
Polyols used for PU synthesis oen consist of two or more
OH groups. There are dierent kinds of polyols available that
can be prepared in laboratories by various ways. For example,
polyether polyols are obtained through the copolymerization of
propylene oxide and ethylene oxide with a compatible polyol
precursor,
42
whereas polyester polyols are synthesized in
a similar manner to the way polyesters polymers are prepared. A
distinct type of polyether polyol, poly(tetramethylene ether)
glycols, can be prepared by polymerizing tetrahydrofuran for
usage in highly ecient elastomeric applications.
43
An example
of preparing isocyanate-terminate prepolymers using poly-
tetrahydrofuran and their characterization studies were re-
ported by Rajendran and co-workers.
44
Polyols are oen used as
mixtures of molecules that are similar in nature but with
dierent molecular weights. Their molecules possess dierent
number of OH groups. Therefore, it is oen necessary to state
the average functionality of the polyols.
42
Despite the mixture
complexity of polyols, industrial grade polyols have composi-
tions that have been carefully controlled to obtain consistent
properties, which are necessary for producing PUs with specic
properties. For example, rigid PUs are made from low molecular
weight polyols (a few hundred units), whereas exible PUs are
obtained from high molecular weight polyols (around ten
thousand and above units).
42
Dierent structures of various
polyols are presented in Fig. 3.
45
A comparison of the advan-
tages and disadvantages of dierent polyols from various
sources are listed in Table 1.
45
On the other hand, isocyanates are incorporated into PU
synthesis via a hydroxyl-group-containing compound due to
their high reactivity, although the reaction is slow at room
temperature.
45
This slow speed may be due to the phase
incompatibility of the polar and less dense polyol phase and the
relatively non-polar and denser isocyanate phase. Therefore,
a suitable surfactant and suitable catalysts are required to get
Fig. 3 Comparison of basic polyol structures.
45
Table 1 Advantages and disadvantages of dierent polyols from dierent sources
45
Polyol type Advantages Disadvantages
Polyether polyols based on propylene
oxide and ethylene oxide
Hydrolytic stability, cost, viscosity, exibility Oxidative stability, modulus/strength,
thermal instability, ammability
Aliphatic polyester polyol Oxidative stability, modulus/strength Viscosity, hydrolytic stability
Aromatic polyester polyol Flame retardance, modulus/stiness Viscosity, low exibility
Polyether polyols based on
tetrahydrofuran
Hydrolytic stability, modulus/strength Oxidative stability, viscosity, cost
Polycarbonate polyols Hydrolytic stability, oxidative stability,
modulus/strength
Viscosity, cost
Acrylic polyols Hydrolytic/oxidative stability, hardness Viscosity, cost, low exibility
Polybutadiene polyol Low temperature exibility, solvent resistance Viscosity, thermal oxidizable
(unless hydrogenated), cost
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a faster reaction rate between them. The actions of catalysts
based on polarization of either the isocyanate or hydroxyl
compound through polar interactions are shown in Fig. 4.
45
As
per the model, enhancement of the electrophilic nature of
isocyanate can be performed by the removal of electron density
from the nitrogen or oxygen of the NCO group. Aromatic
isocyanates, such as toluene diisocyanate (TDI) and diphenyl-
methane diisocyanate (MDI), are usually more reactive
compared to their aliphatic counterparts, such as isophorone
diisocyanate (IPDI) and hexamethylene diisocyanate (HDI).
Most isocyanates are difunctional in nature (each molecule
possesses two isocyanate groups), except for a few members,
such as diphenylmethane diisocyanate, which comprises
molecule mixtures containing either two or more isocyanate
groups (in this case, the compound usually possess an average
functionality higher than two, usually 2.7 due to the higher
number of isocyanate groups). Modications and changes to
the types of raw materials and to the synthesis routes of PU
determine the properties shown by the nal product.
2. Types of polyurethanes
2.1 Rigid polyurethane foams
Rigid PU foams represent one of the most commonly known
versatile and energy saving insulation materials. These foams
can signicantly reduce energy costs on the one hand and can
make commercial and residential appliances more comfortable
and ecient on the other. Reports from the U.S. Energy
Department show that heating and cooling is one of the major
consumers of energy in the majority of homes
46
and are
responsible for around 48% of the total energy consumption in
a typical U.S. home.
47
To ensure a stable temperature as well as
a reduced noise level for both home and commercial appli-
ances, builders oen resort to using polyisocyanurate and PU
foams. These foams have been proven to be eective as insu-
lation materials, and hence have been applied in window
insulations and wall and roof insulations as well as in barrier
sealants for air and doors.
The preparation of rigid PU foams can be performed using
petroleum-based polyols as well as with bio-based polyols from
vegetable oils or plant-based lignin. The properties of the
formulated PUs depend on the category of the hydroxyl group
present in the polyols. For example, glycerine, which is
a petroleum-based polyol, contains a primary hydroxyl group.
On the other hand, vegetable oil (for example, castor oil)-based
polyols contain secondary hydroxyl groups. Thus, PUs synthe-
sized from these two categories of polyols exhibit dierent
physical and mechanical properties.
48
In addition, the reaction
between a secondary hydroxyl-group-containing polyol and
isocyanate is slower compared to the reaction between
a primary hydroxyl-group-containing polyol and isocyanate.
Therefore, a mixture of primary and secondary hydroxyl-group-
containing polyols is oen used to reduce the consumption of
petroleum-based polyol.
48
For example, rigid PU foams with
high physical and mechanical properties have been reportedly
produced through a mixture of glycerine and castor oils.
10
Moreover, the presence of dierent polyols may aect the
physical properties of the PUs. For example, the presence of
transesteried palm olein-based polyol, which contains
a secondary hydroxyl group, can decrease the reactivity of the
foaming prole of rigid PU.
49
The reported consequences are an
increased gel time, rise time, cream time and tack-free time of
the formulated PU foam compared to petroleum-based polyol-
prepared PU foams.
To obtain properties comparable to petroleum-based polyols
from vegetable-based ones, a signicant amount of structural
modications or changes is oen required. For instance, in
a research study, the hydroxylation of soybean oil was per-
formed using formic acid and peroxide.
50
Furthermore, trans-
esterication was carried out through the help of the addition of
some polyfunctional alcohol to increase the OH function-
ality.
50
In another study, high ame-retardant rigid PU foams
were synthesized from phosphorylated polyol obtained from
epoxydized soybean oil. The properties of the product were
comparable with commercial polyol-based foams and were
found to have a high ame-retardant capacity.
48
Similarly, in
another research study, the ame-retardant properties of rigid
PU foams were improved through the incorporation of certain
nitrogenphosphorus-based ame retardants, such as dihydro
oxa phosphaphenanthrene oxide-benzylideneaniline (DOPO-
BA). Beyond the ame-retardant properties, the thermal and
physical properties of the resulting foam were reported to be
reasonably improved. The limiting oxygen index (LOI) value of
the synthesized rigid PU was specically found to increase from
20.01% to 28.1% when 20 wt% DOPO-BA was incorporated into
the rigid PU foam formulation.
51
The synthesis route for the
high ame-retardant DOPO-BA material is illustrated in Fig. 5.
Likewise, rigid PU foams have been produced from cardanol-
and melamine-derived polyols. The material was found to
possess high ame-retardant properties, with highly improved
compressive strength and thermal stability compared to other
conventional PU foams.
52
The full synthetic route of the
melamine/cardanol Mannich-based polyol is illustrated in
Fig. 6. Also in another research study, ame retardants obtained
from castor oil were used to synthesize rigid PU foams. The
produced PU foams were found to suit a wide range of appli-
cations based on the many observed property improvements.
53
In another vein, a literature survey revealed that nanoclays,
such as montmorillonite, can also impart properties, such as
high thermal stability, light weight, improved compressive
strength and good ame-retardant properties, to several poly-
meric systems.
5456
It was observed, however, that the
Fig. 4 Examples of bond polarization mechanisms of catalyst action.
45
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hydrophilic nature of nanoclay oen leads to poor interfacial
adhesion between the polymer matrix and the nanoclay
ller.
57,58
This makes it necessary to modify nanoclays before
they are incorporated into polymeric systems. Modication
could help to improve the eciency of load transfer through
better compatibility and improved dispersion of the ller within
the matrix.
59
In a bid to investigate this, a study was carried out
on the production of rigid bio-based PU foams using nanoclay
as the reinforcing agent. Dierent weight contents of nanoclay
were incorporated into oil-palm-based PU to investigate its
inuence on the thermal and mechanical properties of the PU
foam. The addition of up to 4% modied nanoclay (dia-
minopropane montmorillonite) (DAP-MMT) was found to
improve the thermal, morphological and compressive proper-
ties of the rigid PU foam.
41
Overall, modication of the nanoclay
was found to enhance compatibility and dispersion of the ller
within the matrix.
41
2.2 Flexible polyurethane foams
Flexible PU (FPU) foams comprise some block copolymers whose
exibility is based on the phase separations between the soand
hard segments.
60
Thus, PU foams may be modied through
deliberate control of the individual compositional ratios of these
segments. Depending on some physical characteristics they may
be classied as exible PUs; for example, in terms of density,
durability, rmness, tearing resistivity, combustibility, surface
elasticity, etc.,whereacombinationofthesepropertiescan
Fig. 5 Synthetic route for dihydro oxa phosphaphenanthrene oxide-
benzylideneaniline (DOPO-BA).
50
Fig. 6 Synthesis of melamine-modied cardanol-based Mannich polyether polyol (MCMP).
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ensure a good exibility in the PU compound. Flexible PU foams
nd application as cushion materials for a wide range
of consumer and commercial products, including carpet under-
lays, furniture, bedding, automotive interior parts, packaging,
biomedicine and nanocomposites.
6164
The synthesis of exible PU foams oen involves two major
steps: blowing and gelling. From the blowing reaction, carbon
dioxide and urea are produced, which expand and are entrap-
ped by the reaction mixture, while the urethane linkages are
formed by reactions of the isocyanate and hydroxyl group of the
polyol. There are a few parameters that dictate the morphology
and microstructure of the PU foam, including the degree of
cross-linking aer the reaction between the polyol and diiso-
cyanate, the segmental movement of the urea group, the nature
of the interaction between the polyol and urea, etc. In a research
study, special attention was given to the preparation of exible
PU foam from lignin or oxypropylated lignin.
65
Some technical
aspects were reported that could improve the exibility of the
materials; for example, the cross-linking density may be kept
low by reducing the NCO/OH ratio, also, introducing a exible
chain to the main backbone of PU through a chain extender
could help reduce the glass transition temperature to obtain
a highly exible PU. These types of exible PUs are chemically
resistant due to the high degree of cross-linking and adequate
crystallinity, but they are weak in terms of their tensile and tear
properties. To overcome these shortcomings, hybrid laminated
high exible PU foam was prepared and analysed.
66
It was then
suggested that the exible PU foam needs to be reinforced with
textile-based bres, such as aramid, carbon, basalt and glass.
Furthermore, due to the high combustible properties of FPUs,
large volumes and toxic gases, such as CO, NO
x
and HCN, may
be released to the environment during their combustion.
Therefore, anti-ammable properties need to be incorporated
into their formulation during production.
66
2.3 Thermoplastic polyurethanes
Thermoplastic polyurethanes (TPUs) reveal vast combinations
of both physical properties and processing applications.
Usually, they are exible and elastic with good resistance to
impact, abrasion and weather. With TPUs, there is the possi-
bility for colouring as well fabrication using a wide range of
techniques. The incorporation of TPUs could therefore improve
the overall durability of many products.
67,68
TPUs are melt-
processable, like other thermoplastic elastomers. They may be
fabricated using extrusion, blow, compression and injection-
moulding equipment.
69
They may also be solution-coated or
vacuum-formed, which makes them suitable to be made by
a large range of fabrication techniques. The several property
combinations of TPUs makes them suitable for many applica-
tions, such as in automotive, footwear and construction.
67,70
The synthesis of TPUs includes from novel fatty-acids-based
diisocyanates, where the synthesized material was reported to
display considerable thermal stability without any signicant
loss of weight at temperatures below 235 C.
67
The successful
synthesis of rigid spiroacetal-moieties-based renewable ther-
moplastic has also been reported, with the corresponding
materials produced in a very high yield.
71
Analysis of the
product with DMTA and DSC revealed a glass transition
temperature of around 85 C. Also, during hydrolytic stability
analysis, there was no noticeable acid-mediated degradation of
the synthesized PU. In another research study, fully bio-based
thermoplastic PU was synthesized from dimer-fatty-acid-based
diisocyanate and some other renewable diols. A one-step bulk
synthesis method was used to produce the PU material, which
was found to be suitable for coatings, automotive, building,
adhesives and textile applications.
40
Furthermore, due to the
water-insoluble, non-ionic and inert properties of TPUs, they
have been successfully utilized in applications such as polymer
controllers for drug release in vaginal rings
72
and in medical
tubing, because their high mechanical properties could permit
the use of tubes with thin walls without necessarily incorpo-
rating a plasticizer.
69
2.4 Polyurethane ionomers
The presence of ionic groups in the polyurethane backbone
chain has many advantages, such as better dispersion in polar
solvents due to their enhanced hydrophobicity and improved
thermal and mechanical properties.
73
In particular, the shape
memory and biocompatible features provide the materials the
facilities to be used in biomedical devices.
7389
Shape memory
PUs (SMPUs)
7984
possess a thermo-responsive shape memory
eect (SME), and consequently exhibit dierent mechanical
properties than the other PUs. The presence of hard (respon-
sible for the frozen phase) and sosegments (responsible for
the reversible phase) enable the PUs to memorizethe perma-
nent shape.
73,85
The permanent shape can be recovered from the
temporary shape aer heating the materials above a switch
temperature.
73
The sosegment and its glass transition
temperature are related to the switch temperature and tempo-
rary deformation, whereas the hard segment is responsible for
permanent shape memory. The content of hard and so
segments in the PU molecules and their molecular structure has
an eect on the PU's SME. The variation of the glass transition
temperature of the sosegment and crystallization of the hard
segment have important eects on the SME. Those properties
can be changed due to the presence of ionomers in PUs. The
incorporation of ionic groups can be performed by using either
ionic diols or ionic groups containing diisocyanate during the
PU preparation.
73,74
Anionomers can be prepared by post-
functionalization of the PU.
75
An example of a sulfonic ion-
omer prepared by Fragiadakis et al. and its synthesis route are
presented in Fig. 7 (ref. 74) and 8,
90
respectively. PU-Based cat-
ionomers can be prepared by ternization of a sulfur atom or
quaternization of a nitrogen atom. In this case, the diol used for
Fig. 7 Sulfonic ionomer.
74
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PU preparation should contain nitrogen or sulfur.
7678
A shape
memory PU was synthesized from MDI and a,u-poly(butylene
adipate)diol. Chain extension of the diol was carried out by
using MDEA and butane diol, whereas quaternization was
carried out using acetic acid at 40 C.
77,78
Another important feature of polyurethane ionomers (PUIs)
is their biocompatibility. Sulfonate and phosphatidylcholine
groups have been studied to develop PUIs for blood compati-
bility.
8688
An improved haemocompatibility of segmented PU
and poly(urethane-ureas) was observed by Li et al., where
a comparison with medical-grade PU showed a better perfor-
mance of the former.
88
The successful application of these
materials is found in dierent medical applications for articial
hearts, connector tubing for heart pacemakers and haemo-
dialysis tubes.
73
2.5 Coatings, adhesives, sealants and elastomers
There is a growing range of applications and advantageous
markets that may be derived from the use of PUs as coatings,
adhesives, sealants or elastomers (CASE). This is because PUs
oen reveal excellent and versatile mechanical, chemical and
physical properties.
91,92
PU adhesives can oer good bonding
properties, whereas very tight seals may be obtained from PU
sealants. For a PU to be suitable for coating applications, it
needs to possess good adhesive properties, high chemical
resistivity, excellent drying, low temperature exibility and
adequate scratch resistivity.
93,94
Sometimes, to impart anti-
corrosive properties into the material, dierent types of nano-
particles, such as titanium oxide, silicon dioxide, may be used
for high-performance coating applications. The appearance of
this product may be improved and also the lifespan may be
extended. It is noteworthy that despite the suitability of PU
coatings to oer certain desirable properties, their impact
resistance insuciency and susceptibility to UV degradation
when used for outdoor purposes can reduce their use.
9597
Consequently, improvements in these shortcomings using
several synthetic methods for producing PU coatings with
enhanced properties have been reported in the literature.
98
Recently, environmental adhering PU coatings were synthesized
from vegetable oils (cotton seed and karanja oil) using a green
solvent approach. The product was thoroughly characterized for
its thermal and physico-chemical properties. Results obtained
for the material in terms of its adhesion, impact resistance,
exibility and gloss properties showed that it is suitable for
coating applications, even at the industrial scale.
99
Also in
a recent research study, certain isocyanate-free consumer-
applied novel bonding and adhesive materials were obtained
from a hybrid of PU and polyhydroxyurethane. This material
was also found to be a suitable replacement for isocyanate-
based PUs.
100
Most of the adhesive materials commonly found in wood
composites and other sandwich materials are based on phenol
formaldehyde and urea formaldehyde.
101
They are oen used as
binders for interior and exterior solid wood and plywood
components, but are found to cause severe damage to the
integrity of the environment due to the release of organic
solvents.
102
They are found to also contribute to the large
dependence of industries on petroleum consumption.
101
Recently, research has shied towards the production of
adhesives and other binders from renewable materials, espe-
cially plant oils. This is based on their environmental, societal
and economic advantages.
101,103
Jatropha oil, for example, has
been reported to oer various advantages as adhesive materials.
This is due to its oil-containing gums, which may be converted
into renewable-material-based adhesives.
101
In fact, other liter-
ature reports have also revealed that PU wood adhesives have
been successfully synthesized from jatropha oil, and the adhe-
sive was found to have highly desirable properties.
102,104
Also,
the jatropha-oil-based adhesive is renewable, which makes it
conveniently possible to incorporate in adhesive applications.
This also enhances its potential replacement of conventional
petroleum-based materials.
101
Furthermore, the cheap cost of
jatropha oil in comparison with other oils, such as rapeseed,
and soybean,
105
has attracted the attention of researchers,
particularly for its potential in wood adhesives applications.
Apart from jatropha oil, other oils have also been widely
investigated, such as palm oil.
106,107
PU elastomers are another type of important materials,
which may be used for a variety of useful application, such as
shoe soles, household items, suroards, goggles and ski
boots. They may be fashioned into a wide variety of shapes,
colours and design. They are lighter compared to metals, and
can provide highly desirable stressrecoverypropertiesaswell
as can withstand several environmental factors.
15,23
Although
they have an elastic property, they also possess some degree of
plastic nature. Thus, in practice, the highly desired elastic
property cannot be maximally obtained. In a bid to overcome
this challenge, the incorporation of graphene oxide into the
PU formulation was investigated.
108
Also, hybrid llers, such as
titanium oxide or carbon nanotubes, are added to serve
specic purposes. In most cases, PU-elastomer-based nano-
composites are prepared via the common solution cast
method. From the literature, it was found that the elastomeric
properties of PUs were exploited to produce graphene-oxide-
based dielectric materials.
109
The suitability of the material
for this application is based one the ease of their actuation in
an electric eld. Thus this kind of materials are very useful for
their tolerance to high physical strains, such as during
shrinkage and expansion during the application of electric
voltage.
Fig. 8 Synthesis of sulfonic ionomer.
90
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2.6 Binders
PU binders are oen used to bond dierent types of bres and
other materials to each other. Binders made from PU help to
provide a permanent gluing eect between organic materials
and long-strand lumbers, oriented strand boards, laminated
veneer lumber, medium density bre boards, particle boards
and straw boards. As a binding material, the ratio of the hard-/
sosegments of PUs should be high and good thermal stability
is required. Sometime a specic or moderate acid number (not
too high or not too low), weak crystallinity, limited molecular
weight and narrow particle distribution (if PU dispersion) are
preferred for a good quality binder. To impart excellent chem-
ical resistivity in PU binders, hybridization with acrylic polymer
is also preferable. The main areas of application are in elasto-
meric or rubbery ooring surfaces, wood panel manufacturing,
ink-jet printing, foundry industries and sand casting.
110,111
Among these, the major application to which PU binders are put
is in the production of oriented strand board (OSB). The use of
these panels cut across ooring and structural sheathing, shop
panels, joist and beams and other manufactured housing
applications. Also, the fabrication of rebonded foams, which are
used as carpet underlay, mainly take advantage of PU binders
and other chemicals to adhere exible scrap PU foams to the
underlay carpeting. Due to its excellent binding properties, PU
has been proposed as a suitable alternative to binders based on
organic solvents.
112,113
Apart from the use of PUs as adhesives,
sealants, foams and coatings, they may be used also as rocket
propellants and polymer-bonded explosives (PBX).
114116
The
polyfunctional groups present in the material makes it easily to
cure by a diisocyanate.
117
Among the various used ones, pre-
polymers containing hydroxyl-terminated polybutadiene
(HTPB) groups have been widely used as binders for solid
composite propellants as well as PBX.
114,118
This binder was re-
ported to oer structural integrity as well as dimensional
stability to the explosive material. This could be accrued to the
mechanical properties contributed by the urethane reaction
between the HTPB end chain hydroxyl groups and isocyanates,
which produced the PU elastomer. However, due to the pot life
limitations of propellants and PBX, whenever TDI is applied as
a curative agent, IPDI (which is less reactive) could be used as
a suitable alternative if a longer pot life is desired for the PBX
paste during the manufacture of large-sized products.
119
Whichever the case, the mechanical properties of the material
are majorly dependent on the extent of the reactions between
the PU components.
114,120
2.7 Waterborne polyurethane dispersions
Coatings and adhesives that make use of water primarily as the
solvent are oen referred to as waterborne polyurethanes
(WPUs).
121,122
There are several pieces of legislation that place
restrictions on the amount of allowed volatile organic solvents
and other hazardous air pollutants that may be released into the
environment. Most commercial and industrial applications are
therefore dependent on polyurethane dispersions (PUDs), or
waterborne polyurethane dispersions (WPUDs).
123125
PUDs
have the unique advantage that the viscosity of the dispersion is
not dependent on the molecular weight of the polymer. There-
fore, high solid-content WPUs (HSCWPUs) can be prepared by
the drying process only. The dispersion is a two-phase colloidal
system, which includes the polyurethane particles and the water
medium. Several pendent acid or tertiary nitrogen groups in the
PU chain are neutralized to form salts, which basically create
centres for water dispersibility. The types and amount of polyol,
isocyanate, ionomers and chain extender used are responsible
for dierent properties of this dispersion.
Recently, a new method (a two-step emulsication process)
was developed for the synthesis of HSCWPUs,
126
where distri-
bution of the bimodal particle size was strictly controlled. This
was due to the high importance of particle size distribution as
a parameter in the determination of the viscosity and the solid
content interrelationship.
127
This type of high solid-content
materials has also been reported to raise the space and time
yield of reactors, as well as reduce the time needed for lm
forming.
128
One recent research study involved the synthesis of
new WPU novel medium-length uorinated diols. For this
study, the uorinated diol 3-(bis-(N,N-dihydroxyethyl))dodeca-
uoroheptyl acrylate (DEFA) was rst produced from dodeca-
uoroheptyl acrylate and diethanolamine using the Michael
addition method, as shown in Fig. 9. Several uorinated WPU
emulsions were then synthesized, as illustrated in Fig. 10. The
organic solvent/water resistance as well as the mechanical and
thermal properties of the produced material were found to be
greatly improved. For instance, the tensile strength was
observed to increase from 9 MPa to 15 MPa, whereas the
extensibility was found to decrease from 520% to 280%.
129
Other
notable WPUs that have been investigated include a poly-
carbonatediol-based WPU, which was reinforced with silica.
The produced material was reported to have suitable coating
application for exible materials, such as fabrics, paper and
leather, especially in cases where high abrasion resistance is
required.
130
A novel synthesis route had also been provided for
fully bio-based WPUs. The method was cleared to adhere strictly
with requirements for environment safety, and the synthesized
product was shown to possess great hydrophobic and thermal
properties. It was also proposed as a suitable alternative for
conventional petroleum-based materials.
131
Fig. 9 Synthesis of dihydroxyethyl dodecauoroheptyl acrylate
(DEFA).
129
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3. Polyurethane synthesis
PUs may be produced through dierent routes.
38
The most
important and more useful method is through the reaction
between a polyol (an alcohol that has two or more hydroxyl
groups within a molecule) and a diisocyanate.
1,10,132
Fig. 11
illustrates the synthesis of a typical PU. Other suitable addi-
tives and catalysts may also be incorporated for the PU
synthesis.
Additives that may be incorporated into the PU synthesis
include ame retardants, pigments, cross-linkers, llers,
blowing agents and surfactants. PUs may be fabricated into any
fashion with a variety of properties, such as hardness and
density, by merely varying the quantity and types of the polyol,
isocyanate or additives. The most common components that
may be found in typical PUs and the reasons for their inclusion
are presented in Table 2.
3.1 Polyols
Polyols may be largely grouped into either polyether polyols or
polyester polyols. Polyether polyols are obtained from the
reaction between an epoxide and an active hydrogen-containing
compound. They can also be prepared from the ring-opening
polymerization of epoxy monomers.
133,134
Another category is
polyester polyols, which can be obtained from the poly-
condensation of hydroxyl compounds and multifunctional
carboxylic acids. Also, polyols may be classied based on their
end use. The high molecular weight polyols (MW ranging from
2000 to 10 000) are mainly used for the synthesis of exible PUs,
whereas low molecular weight PUs are used for producing rigid
Fig. 10 Synthetic route for isophorone diisocyanate dihydroxyethyl dodecauoroheptyl acrylate polyurethane (IPDI-DEFA-PU) aqueous
emulsion.
129
Fig. 11 Common route for the synthesis of polyurethanes.
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PUs.
10,140,141
Polyols that are applied for exible PUs oen make
use of initiators with low functionality, such as glycerine (f¼3),
dipropylene glycol (f¼2) or a solution of water and sorbitol (f¼
2.75). On the other hand, polyols for rigid PUs require initiators
with higher functionality, such as sorbitol (f¼6), Mannich
bases (f¼4), sucrose (f¼8) and toluenediamine (f¼4). Usually,
ethylene oxide and/or propylene oxide is incorporated into the
initiators until the expected molecular weight is reached. It is
noteworthy, however, that the amount and order in which the
oxide is added can dene many of the polyol properties.
142
This
includes its water solubility, reactivity and compatibility. Poly-
ols produced from propylene oxide only, are oen terminated
with secondary OH groups
12
and they are usually less reactive
compared to polyols containing ethylene oxide (containing
primary OH groups). Grapolyols, otherwise called polymer or
lled polyols, comprise nely distributed acrylonitrile, styrene-
acrylonitrile or polyurea polymer particles, which are chemically
graed onto a polyether ketone with a higher molecular weight.
Most oen, they are incorporated into high-resiliency low
density foams, to improve their load-bearing capacity. They may
also be applied for adding toughness to cast elastomers and
other microcellular foams. Rigid foam polyols with low molec-
ular weight may also be produced by using triethanolamine or
ethylenediamine as initiators. These kinds of polyols possess
inherent catalytic properties due to the nitrogen atoms present
in the backbone. Another class of very important polyether
polyol is called poly(tetramethylene ether) glycol (PTMG), which
is obtained from the polymerization of tetrahydrofuran, and is
mainly applied for high-performance wetting, elastomers and
coating applications.
Polyester polyols are derived from virgin raw materials and
are oen produced through the direct polyesterication of very
pure diacids and glycols. One example is 1,4-butanediol and
adipic acid. Usually, polyester polyols are more viscous,
142
as
well as more expensive, compared to polyether polyols.
However, they are still very important because they produce PUs
with better abrasion, solvent and cut resistance. Another group
of polyester polyols are derived from raw materials that have
been reclaimed. They are produced through transesterication,
otherwise known as the glycolysis of recycled poly(-
ethyleneterephthalate) (PET) or distillation bottoms of dimethyl
terephthalate (DMT) with glycols (for example, diethylene
glycol). These aromatic polyols, which also have a low molecular
weight, are oen used for the production of rigid foams because
they oer reduced cost and good ammability properties to
polyisocyanurate (PIR) boardstock foam as well as to PU insu-
lation foams.
A group of polyols called the speciality polyols are required in
the manufacture of sealants, elastomers and adhesives that
need superior qualities to withstand chemical and environ-
mental factors. Some of these polyols are polysulde polyols,
polycaprolactone polyols, polycarbonate polyols and poly-
butadiene polyols.
Dierent polyols may be obtained from natural and renew-
able sources, such as vegetable oils. These renewable materials
can either be fatty acids or dimer fatty acids.
140,143
The vegetables
oils from which polyols may be obtained include castor,
soybean, Pongamia glabra, neem and cotton seed.
144
Oils derived
from these vegetable oils are mainly used to produce exible
moulded foams, exible bunstocks and elastomers. The pres-
ence of triacylglycerides in vegetables oils makes them suitable
for manufacturing several polymeric materials.
145147
Polyols
from renewable sources can be reacted with isocyanates to
produce PUs with special properties that are suitable for a wide
range of applications.
148
In one study, modications were made
to both soya bean oil and castor oil to make them suitable for
Table 2 Components of polyurethanes and reasons for their inclusion
Additives Reasons for use Ref.
Isocyanate Responsible for the PU reactivity and curing
properties
124
Polyols Contributes exible long segments, which
produces soelastic polymers
135
Catalysts To speed up the reaction between the isocyanate
and polyols and to allow reaction at a lower
reaction temperature
6
Plasticisers To reduce material hardness 69
Pigments To produce coloured PU materials, especially for
aesthetic purposes
116
Cross-linkers/chain extenders For structural modication of the PU molecule
and to oer mechanical support that will
enhance the material properties
136 and 137
Blowing agents/surfactants To aid the production of PU foams, to help
control the formation of bubbles during
synthesis and to control the foam cell structure
138
Fillers To minimize cost and to improve the material
properties, such as stiness and tensile strength
19
Flame retardants To reduce material ammability 139
Smoke retardants To reduce the rate of possible smoke generation
when the material is burnt
51
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manufacturing rigid PU foams. The mechanical properties of
the synthesized foams were found to be reduced compared to
commercial polyol-based foams. However, it was found to
present signicant application opportunities for rigid PU foam
production since it was obtained from sources that are
renewable.
149
In another vein, the copolymerization of tetrauoroethylene
or chlorotriuoroethylene with vinyl ethers containing hydrox-
yalkyl vinyl ethers could lead to the production of uoro-
ethylene vinyl ethers (FEVE) polyols. Fluorinated PUs
synthesized from two components and involving the reaction of
polyisocyanate with FEVE uorinated polyols have been
explored for the synthesis of ambient cure coating/paints. Due
to the high amount of uorinecarbon bonds (the strongest
chemical bond) in uorinated PUs, they have been observed to
possess good resistance to UV, alkalis, chemicals, acids,
solvents, corrosion, weathering, fungi and other microbial
attacks. These qualities make them highly desirable for high
quality paints/coatings.
3.2 Isocyanates and non-isocyanates
Isocyanates are very necessary components for PU preparation.
They can be categorized as difunctional or heterofunctional and
aromatic or aliphatic in nature. Among the several available
options, the most commonly used ones are methylene diphenyl
diisocyanate (MDI), toluene diisocyanate (TDI) and aliphatic
diisocyanates. The structures of some common isocyanates are
illustrated in Table 3. Generally, MDI and TDI are cheaper and
more reactive compared to other isocyanates. Industrial grade
MDI and TDI, which are isomeric mixtures, most oen
comprise polymeric materials. They are usually used for
producing exible foams, such as moulded foams used for car
seats or as slabstock foam for mattress production.
2
They can
also be used for producing rigid foams, such as refrigerator
insulating materials, and for producing elastomers (such as for
shoe soles), etc. Modication to isocyanates may be achieved
through the partial reaction with polyols or by incorporating
certain materials to reduce the volatility and invariably the
toxicity of the isocyanates. This could also reduce the freezing
point, such that they become easier to handle, as well as
enhance the properties of the resulting polymers.
Other groups of less oen used isocyanates are the aliphatic
and cycloaliphatic isocyanates. These nd applications in
coatings and other areas where transparency and colour are
highly desired. This is because aromatic isocyanate-based PUs
usually darken when exposed to light.
116
Common among the
Table 3 Structures of some important isocyanates
10
Code Structures
MDI
HDI
IPDI
TDI
HMDI
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aliphatic and cycloaliphatic isocyanates are 1-isocyanato-3-
isocyanatomethyl-3,5,5-trimethyl-cyclohexane (isophorone dii-
socyanate, IPDI) 4,40-diisocyanato dicyclohexylmethane,
(hydrogenated MDI or H
12
MDI) and 1,6-hexamethylene diiso-
cyanate (HDI).
Despite the importance of diisocyanates, environmental
concerns have led researchers to investigate better means of
reducing or possibly avoiding their use, specically to reduce
the environmental problems and other diisocyanate-related
toxicity issues. For example, a study was carried out to
produce sustainable PU from carbonated soybean oil, 3-ami-
nopropyltriethoxysilane and lignin.
150
The non-isocyanate route
followed the reaction between cyclic carbonate with amines,
151
and the polyol could be prepared from lignin via an oxy-
propylation method.
152
In another study, oligomeric poly-
butadiene diisocyanate was used for the preparation of lignin-
based PU.
153
In a similar study, lignin-aminated polyol and
diphenyl diisocyanates were reacted to prepare PUs. Most of the
reports revealed that the properties of the formulated non-
isocyanate-based PUs depend mostly on the lignin content.
The lignin content determines the cross-linking and modulus of
the materials. The typical synthetic routes are shown in the
scheme in Fig. 12.
3.3 Catalysts
The catalysts that are oen incorporated into PUs may be
grouped into two main categories: metal complexes and amine
compounds. Amine catalysts traditionally consist of tertiary
amines, such as dimethylcyclohexylamine (DMCHA), dimethy-
lethanolamine (DMEA), 1,4-diazabicyclo[2.2.2]octane (DABCO)
and triethylenediamine (TEDA). The selection of tertiary amine
catalysts is based on their ability to drive either the urea,
urethane or isocyanate trimerization reactions. Metal
complexes from compounds of bismuth, lead, zinc, tin and
mercury may also be used as catalysts for urethanes. For the
production of PU sealants, coatings and elastomers and
mercury carboxylates have been found to be specically eec-
tive. This is due to their preferential selection towards the polyol
and isocyanate-related reactions. However, they are reportedly
toxic, consequently leading to the recent use of carboxylates
from zinc and bismuth as replacements. Several types of
application also make use of alkyl tin carboxylates, mercaptides
and oxides. Specically, tin mercaptides are usually incorpo-
rated into water-containing formulations because carboxylates
of tin may be undesirably inuenced by hydrolysis.
Most oen, catalysts are used in the formulation of dierent
kinds of PUs for selective purposes. For example, novel
CuCo
2
O
4
/graphitic carbon nitride nanohybrids have been used
for the reduction of CO generation and re hazards.
153
The
reactivity of these catalysts is dierent in terms of their nature.
A comparison was drawn for the case of the catalytic activity
shown by two catalysts, namely zirconium and tin, for the
preparation of isophorone diisocyanate (IPDI)-based water-
borne polyurethanes.
154
It was found that in the case of the tin
catalyst, the reactivity of the isocyanates was dierent, while in
the case of the zirconium catalyst, it was the same.
3.4 Chain extenders and cross-linkers
Another group of compounds that oen play important roles in
the polymeric morphology of PU are the chain extenders (f¼2)
and cross-linkers (f¼3 or more). These compounds are usually
amine and hydroxyl terminated, with low molecular weights.
They are highly useful for improving the morphology of PU
adhesives, elastomers, bres and some other important
microcellular and skin foams.
155,156
The elastomeric features of
these compounds are obtained from the copolymer interface of
the soand hard segments of the polymer. As such, the
domains of the hard segment urethane serve as cross-linkers for
the domains of the sosegment amorphous polyester (or pol-
yether). This interface separation arises due to the incompati-
bility and immiscibility (while both phases are amorphous) of
the sosegments (low melting and non-polar) with the hard
segments (high melting). Therefore, crystallization does not
have any eect on phase separation.
Generally, the hard segments, which are produced from
isocyanate and chain extenders, are immobile and sti, while
on the other hand, the sosegments, which are produced from
the polyols (high molecular weight), can move freely and oen
appear in foil forms. Covalent coupling between the hard and
sosegments leads to plastic ow inhibition within the poly-
mer chains, thereby producing elastomeric resiliency.
Mechanical deformation of these compounds lead to the
uncoiling of certain portions of the stressed sosegment,
making the hard segments align along the direction of the
stress. The realignment of hard segments coupled with
a subsequent strong hydrogen bond produces high tensile
strength, tear resistance and good elongation properties.
157,158
Proper selection of the chain extender could also inuence the
chemical resistance, heat and exural properties of the PU.
Some of the most commonly used chain extenders include 1,4-
butanediol (BDO), cyclohexane dimethanol, ethylene glycol,
Fig. 12 Synthetic route for PU preparation using castor oil and lignin in
an isocyanate-free mechanism.
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hydroquinone bis(2-hydroxyethyl)ether (HQEE) and 1,6-hex-
anediol. Some examples of biodegradable PUs and their
synthetic routes are presented in Fig. 13 and 14 using water and
ethylene glycol as the chain extenders. These glycols can be used
to manufacture thermoplastic PUs. They also form well-
organised hard segment domains, which separate well and
can be processed in the molten state. The only exception is
ethylene glycol, whose derived bis-phenyl compound is
susceptible to undesirable degradation if the level of hard
segments becomes too high.
37
3.5 Surfactants
Surfactants are oen used to improve the properties of foam as
well as non-foam PU polymers. They resemble block polymers
of polydimethylsiloxanepolyoxyalkylene, nonylphenol ethox-
ylates, silicone oils and some other organic compounds. In
applications that involve foams, they are applied for the
emulsication of liquid components, the regulation of cell
sizes and for stabilization of cell structures to guide against
collapse as well as against voids at the sub surface. For non-
foam applications, they are applied as anti-foaming and air
release agents, and as wetting agents. They may also be used to
remove surface imperfections, such as sink marks, orange
peels and pin holes. There are dierent kinds of surfactants
available for the preparation of PU materials, including non-
ionic
159
and cationic
160
surfactants. The ndings of using non-
ionic surfactants revealed an excellent surface activity without
having a xed critical micelle concentration. On the other
hand, cationic surfactants were found to be better to use for
corrosion resistivity. There are a few drawbacks involved with
the usage of surfactants for the synthesis of PU. For example,
low molecular weight conventional surfactants are responsible
for delamination and corrosion.
161
Also, they can sometimes
migrate easily to the surface of PU materials. Therefore,
a surfactant-free PU was also proposed by a dierent
researcher.
162
4. Advances in polyurethane
synthesis
4.1 Click chemistry
Nowadays, PUs can be prepared following a new reaction
approach called click chemistry. Click chemistry is well known
for producing a single product with a very high yield and high
tolerance of functional groups. There are many benecial
aspects of the click reaction compared to other traditional
processes. For example, it has been observed to be fast, highly
selective, with a high possibility of working with either homo-
geneous or heterogeneous systems, with an insensitivity to the
solvent as well as it can proceed with a moderate reaction
temperature.
Following this technique approach, ame-retardant PUs
were prepared by using a copper(I)-catalyzed azidalkyne
cycloaddition (CuAAC) technique with alkyne-polyol and
azidoalkylmonophosphonate.
163,164
The experimental
process involved four steps: preparation of azidoalkyl
Fig. 13 Production of biodegradable PU using water as a chain
extender.
Fig. 14 Production of biodegradable PU using ethylene glycol as a chain extender.
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monophosphonate compounds through the nucleophilic
substitution of bromoalkylphosphonates with NaN
3
; formation
of terminal alkyne groups attached to polyols from the reaction
of glycidyl propargyl ether and propylene oxide via a mechanism
of anionic ring-opening copolymerization; clickingthe azi-
doalkylphosphonate to the polyol and nally synthesis of the PU
foam with 2.4 wt% of click polyol. The schemes for all the four
steps are illustrated in Fig. 15. Recently, castor-oil-based poly-
functional polyurethane acrylate was prepared by following
photo-click chemistry.
165
The functionalization was performed
via thiolene photo-click chemistry with b-mercaptoethanol,
which was very ecient compared to the traditional method.
The result showed a 100% conversion of the double bonds of
castor oil, which was conrmed by real-time Fourier transform
infrared spectroscopy (FTIR). A similar study was performed
using click chemistry to obtain highly branched PUs by dierent
researchers.
166,167
The improvement in thermal, mechanical,
anti-microbial and anti-corrosion properties are associated with
the use of 1,2,3-triazole-rich polyether polyols and the incor-
poration of carbon nanotubes. In a dierent study, click
chemistry and atom transfer radical polymerization were used
for the deposition of a dopamine-assisted lubricating and
antifouling coating on PU surfaces.
168
A graing process was
involved by using poly(N,N0-dimethyl-(methylmethacryloyl
ethyl)ammonium propanesulfonate) (PDMAPS) and poly(2-
methacryloyloxyethyl phosphorylcholine) (PMPC) in order to
improve the surface hydrophilicity and lubricating properties.
Click chemistry was also used for the functionalization of
waterborne PU by using Cu(I)-mediated azidealkyne through
a cycloaddition reaction.
169
Some other examples of research
work using click chemistry include the preparation of methoxy
polyethylene glycol (MPEG)-functionalized polyurethanes (PUs)
(PU-g-MPEG),
170
the preparation of waterborne siloxanepoly-
urethane nanocomposites reinforced with nanosilica
171
and the
synthesis of a rigid PU foam from novel renewable polyols based
on limonene.
172
4.2 Nitrogen- and phosphorus-containing polyurethanes
Due to the susceptibility of re attack, PU and PU-based mate-
rials need to be re and ame retardant for safe use. To serve
this purpose, usually halogen-based compounds are added.
These compounds are, however, considered to be toxic and not
environmentally friendly. Furthermore, besides the toxicolog-
ical eects of using halogen-related compounds or additive-type
llers for their anti-ammable properties, they can also reduce
the mechanical properties of the PU. Therefore, reactive-type
llers for anti-ammable properties are preferable instead of
the additive types. To justify this assertion, environmentally
friendly materials made from nitrogen and phosphorus are
being exploited. A novel phosphorusnitrogen ame-retardant-
based PU was prepared from benzaldehyde, aniline and 9,10-
dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) by
using the condensation reaction.
173
A schematic illustration of
the preparation of the ame-retardant-based PU is shown in
Fig. 5. The ame-retardant rosin-based rigid polyurethane
foams were prepared from DOPO-BA. Similar research work was
performed to prepare ricinoleic-acid-based phosphorous- and
nitrogen-containing polyol, which was later used to synthesize
a PU for sealing applications. Reports showed that the PU
synthesized from phosphorus- and nitrogen-based materials
could oer the dual benet of environmental protection as well
as improved mechanical properties.
173
In a dierent study,
a heat release study was conducted for PU containing a phos-
phorous ame-retardant.
174
The ame retardants were mixed
with PU via solvent mixing and the copolymerization method.
Per the results, it was found that the mixing condition had an
eect on the heat release rate. The type of reaction and type of
bonding (covalently or non-covalently bonded) between the
ame retardant and PU are other important factors in reducing
the release of heat. An interesting work was performed using
both phosphorous (BHPP)- and nitrogen (MADP)-containing
polyol to improve the ame retardancy of rigid PU foam.
175
The results showed that the optimal weight percentage of BHPP
Fig. 15 Theoretical mechanism for the photoclick chemistry for PU preparation.
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and MADP was 1 : 1 to achieve an improved thermal stability
while limiting the oxygen index. In addition, a small amount (15
wt%) of expandable graphite added extra value by improving the
limiting oxygen index to 33.5% and reduced the rate of heat
release by 52.4%. Expandable graphite and phosphorous were
also used to improve the mechanical and thermal properties of
the PU.
176
Besides ame retardancy, an improvement in the
adhesion property of PU dispersion was also documented by
using phosphorous in a previous study.
177
(Bis)phosphonic acid
moieties were used as adhesion promoting reactive sites to
build covalent bonds through an end-capping reaction to
isocyanate-reactive polyurethane particles under aqueous
conditions. A detailed analysis of the re toxicity of PU foam can
be obtained from the previous literature.
178
4.3 Carbon- and nanomaterial-based polyurethanes
To improve many materials' characteristics, the incorporation
of nanoparticles or nanomaterials is now the new trend among
researchers. There are a variety of nanoparticles that can serve
specic purposes. However, a reasonable amount of intensive
studies over the years have concentrated on nanostructures of
nanocrystalline cellulose (NCC).
179181
Some of the properties
that favour this material include the fact that it is a biopolymer
with great natural abundance. Its industrial production has
been estimated to be around 1012 tonnes annually combined
with its renewability and biodegradation features.
181
Further-
more, the structural geometry of NCC oers highly desirable
mechanical properties, with elastic properties similar or even
higher than Kevlar. Its tensile strength has been reportedly
estimated to be about 10
3
MPa or above.
182
However, it has been suggested that if the full potential of
NCC is to be exploited, there is a need to incorporate it as
a reinforcing material into nanocomposites.
182
Extensive
studies have been carried out on this subject.
183187
As part of
this orchestrated research, studies have also been conducted on
bio-based PUs reinforced with NCC, with a number of publi-
cations produced. One group of researchers worked on the
production of NCC-based PU material using dierent ranges of
concentrations (0.25 wt%) of the reinforcement.
188191
Based on
the results obtained from using FTIR, the authors reported that
there was hydrogen bonding between the NCC and the hard
segments with an increased phased separation of the soand
hard segments as the NCC was incorporated. The diameters of
the cells were reported to decrease as the NCC content was
increased; however, there was a non-monotonous eect on the
density with respect to the NCC content. It was also stated that
despite the NCC being believed to act as a nucleating agent,
there was no signicant change to the existing chemical struc-
ture of the PU with the NCC addition. Overall, the incorporation
of NCC was reportedly found to suitably improve the mechan-
ical properties of the PU material. A group of other researchers
also studied the incorporation of micro/nanocrystals of cellu-
lose into PU based on rapeseed oil.
192
It was reported that
incorporating micro-crystalline cellulose (MCC) did not oer
much observable modication to either the thermal conduc-
tivity, the closed cell or the apparent density of the resulting PU.
However, there was reportedly an increase in water absorption
in line with the concurrent increase in the MCC content.
192
Furthermore, it was observed that there was an increased glass
transition temperature as well as rigidity towards compression.
Other research studies have also been carried out on the
incorporation of cellulose bres into PU. However, the mate-
rials produced were classied as composites instead of
nanocomposites.
193,194
Recently, research on the incorporation of nanomaterials
has grown wider than just the scope of cellulosic materials
alone. Other materials, such as carbon nanobres (CNFs),
carbon nanotubes (CNTs) and clays, are attracting signicant
interest as important additions into polyurethane foams.
195,196
This is based on the suitability of these nanollers to achieve
nano-scale dispersion and thereby improving the properties of
PU materials.
195
These properties include mechanical proper-
ties, such as stiness, toughness, mould shrinkage and hard-
ness,
195197
thermal properties,
198,199
water solubility,
200
barrier
properties
201,202
and other functional properties.
196,201
It should
be noted, however, that the inuence of nanollers on the
properties of PU and other composites depends on several
factors, such as aggregate size, particle size, shape, morpho-
logical characteristics and the degree of dispersion.
196,202,203
Among the most commonly investigated, nanoclays, such as
mica, hectorite, montmorillonite and saponite, have been the
choice of many researchers.
204
This is because they have been
perceived to oer specic properties, such as ame retardancy,
improved thermal stability, improved compressive strength and
light weight, to the composite material.
205
However, there are constraints to these types of materials
due to the hydrophilic nature of the nanoclay, which could lead
to weak interfacial bonding between the ller and the hydro-
phobic polymer matrix.
59
It is therefore necessary to modify the
nanoclay to enhance its compatibility and dispersion within the
matrix, such that the overall load transfer capacity of the
composite material can be improved. Among the very many
possible methods, the use of organic modiers for the ion
exchange process
59
and ultrasonic treatment
206,207
have been
reported to be suitable for eective modications of nanoclays.
Some of the organic modiers that have been used include
diamine-based modiers, trihexyl tetradecylphosphonium
(THTDP) and alkylammonium.
208
Moreover, in a bid to reduce the cost of production, to
minimize the synthetic bre usage and to produce more
property-tailored products, there is presently research ongoing
into hybrid nanocomposites. This is because hybridization can
overcome the shortcomings of one-material reinforcement
through the incorporation of other reinforcing materials.
209,210
5. Recent advances in polyurethane
applications
The general properties (physical and chemical) of any PU are
dependent on the nature of the individual reactants (especially
the R
1
and R
2
groups) from which it is produced. Generally, the
properties of the polyols, such as the number of functional
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groups that are reactive within each molecule, their molecular
structure and their molecular mass, all dene the characteristic
features of the nal PU material as well as how it will be used.
Several research studies have been carried out on the produc-
tion of PU and its potential applications. Some of the recently
reported types of PU and their methods of synthesis are pre-
sented in Table 4.
5.1 Building and construction applications
Present day buildings need to meet certain requirements in
terms of the use of construction materials, including high-
performance strong materials, light weight, easy to install,
durable and versatile. These may be achieved through the
incorporation of PUs into building and construction materials.
In fact, the use of PUs could oer great conservation of natural
resources and help the environment through reduced energy
consumption. The use of PUs for construction and building
applications is on the increase due to their specic properties,
such as excellent heat insulation capacity, highly desirable
strength-to-weight ratio, versatility and durability. An experi-
mental work was performed to determine the reduction of heat
loss through a building envelope for the case of
thermoregulating microcapsules contained in PU foam.
211
Analysis showed that with an incorporation of 40% microcap-
sules it was possible to produce a thermoregulating foam with
two possible advantages: energy accumulation and insulation
during the transient state. Furthermore, the cheap cost of these
high-performance materials coupled with their comfort ability
have made PUs an integral part of many homes. PUs can be
used in almost any part of the home, such as for oors, e.g. in
the form of pads of a exible cushion for carpets, or for roong,
e.g. in form of heat and light reecting materials. In the roong
application, the plastic coverings on the PU surface can help to
keep the house cool on the one hand, and help to reduce energy
usage on the other hand. Generally, PU materials help to add
exibility to new homes, such as the entry door and garage
doors, which contains panels with foam cores. The foam-core
panels also provide a lot of colour variation and proling for
roofs and walls.
5.2 Automotive applications
The areas of PU application in the automotive industry are vast.
Aside from its common use as a foam to make vehicle seats
more comfortable, it may also be used in car bodies, bumpers,
Table 4 Some common methods used to synthesize dierent types of PU
Dierent synthesis methods Dierent types of PU and references
Two-step emulsication process High solid content WPUs
126
Thiolene coupling PUs based on aromatic cardanol-based polyols,
213
vegetable-oil-based
WPUs
131
Step growth polymerization Non-isocyanate PUs from secondary amines
214
Prepolymer Vegetable-oil- and phosphorylated-polyols-based PUs,
48
tannic-acid-
based PUs,
215
phosphinated PUs,
216
biodegradable and electroactive
PUs,
217
WPU based on UV absorption groups,
218
isocyanate-trimers- and
polyester-polyols-based PUs,
219
folate-conjugated PUs,
220
iodo PUs,
221
high solid content WPUs,
222
hyperbranched WPUs,
223
PU/
polyhydroxyurethane hybrids,
100
transparent PUs lms from fatty acid,
92
polycarbonatediols-based WPUs,
224
biodegradable polycaprolactone/
PUs,
225
polyrotaxanes cross-linked PUs,
226
biodegradable low cost
aliphatic PUs,
227
carbohydrate cross-linked PUs,
228
renewable
thermoplastic based on rigid spirocetal moieties,
71
environment-
friendly WPUs,
229
hyperbranched PUs
230
Prepolymer (one step) Fluorine-based PUs,
231
Prepolymer (two step) Sulfadiazine-based PUs,
232
PU based on cellulose nanobres,
233
PUs
based on cardanol- and melamine-derived polyol
52
Solvent/emulsier free Fluorinated WPU acrylate
234
Inverse emulsication PU based on side-chain triethoxysilane and colloidal silica
235,236
Hydroxylation and a subsequent alcoholysis/epoxidation Jatropha-oil-based PUs
101
Microwave-assisted Cyclodextrin PUs
237
Michael addition reaction followed by self-emulsication PUs based on medium length uorinated diols
129
Polycondensation Chitosan-based PUs,
238
polyester-polyols-based PUs,
239
polyfunctional
PU foams
138
Polyaddition Natural-rubber-based PUs
240
Hydrolysis and condensation WPUs based on PU/silica hybrids
130
Green solvent Cotton-seed- and karanja-oil-based PUs
99
Non-isocyanate reaction Soya-bean- and lignin-based PU
241
One shot Liqueed-lignin-based PUs
60
Solgel synthesis method followed by supercritical CO
2
drying PU aerogels
242
Free rise Modied-tung-oil-based PUs foams
243
Cross-linking Terpene-based PUs
244
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doors, windows and ceiling sections. PUs also help to provide
better automobile mileage through reduced weight, increased
fuel economy, good insulation with proper sound absorption,
212
great comfort for passengers
245
and high corrosion resistance
properties. Deng R. and his colleague opined that since a clear
majority of vehicular seats are mainly foams, the dynamic
comfort of users may be controlled by modifying the foam
properties to obtain the desired quasi-static features.
245
PU
foams have been reported to occupy the largest fraction of the
global polymeric foam market.
212
Due to the low density of PU
foams, they are suitable for the manufacture of stiand light
components, which may then be used as interior panels in
aircras, structural shapes, such as bulkhead cores, stringers
and transform cores in reinforced plastic boats, etc.
212
Several
other sandwich materials found in high-end sporting cars,
ships, aircras and racing cars are also based on PU. This is
because the PU material can help to provide heat shielding and
structural stiness as well as crash energy management.
212
PU
adhesives are also used as PUaluminium laminates for auto-
motive applications.
246
These adhesives are prepared from pol-
ycaprolactone polyols and a mixture of aromatic and
cycloaliphatic diisocyanates. The adhesion property has been
found to be inuenced by the structure of the PU used. Coatings
are another prime need for automobiles and can also be
prepared by using PU. The development of modern technology
related to nanollers or nanoparticles can add some important
features in PU-based advanced coating materials for automo-
biles. The relationship between the hard segments of a PU chain
and llers was determined by Verma et al.,
247
who showed that
intercalated and exfoliated clay platelets have a preferential
relationship with the hard segments of the PU chains.
247
Furthermore, the dispersion and morphology of clay can
determine the eective sites for interfacing with PU chains.
5.3 Marine applications
PU materials have contributed a large innovation to the recent
development in boat technology. PU-Based epoxy resins help to
protect boat hulls from weather, corrosion and water as well as
other substances that may increase drag. In addition, PU-based
rigid foam helps to insulate boats from extreme temperatures
and noise. It helps to provide increased tear and abrasion
resistance, and oers good load-bearing properties even at
minimum weight. Based on these, the maritime industries
oen incorporate several thermoplastic PUs into various prod-
ucts for the specic advantages they provide, including elas-
ticity, durability and ease of processing ability with good
suitability for cable and wire coatings, drive belts, hydraulic
seals and hoses and engine tubing as well as ship construction.
Some PUs can also be used to recognize certain active mate-
rials,
248
and for removing certain organic substances from water
bodies. Cyclodextrin PU, for example, has been reported to be
eective towards the removal of certain organic materials, such
as paraben, from water.
249
Recently, a microwave-assisted
technique was used to produce cyclodextrin PU. The material
was fully characterized using
1
H and
13
C NMR spectroscopic
methods and was found to be soluble in organic matters, but
insoluble in water. Therefore, the synthesized PU was reported
to be ecient for removing phenol as well as Nile red dye from
water. It was also projected for further possible application in
the removal of toxic substances from the environment.
237
In
a similar research, iodo PUs were synthesized and were reported
to be ecient for removing dyes, such as crystal violet and
aniline blue, from laundry waste water.
221
For marine applications, accelerated weathering or ageing
analyses of the materials are very important. The possible
outcomes of these analyses may include swelling, debonding of
the llers, hydrolysis, plasticization and loss of mechanical
strength. There are dierent approaches for measuring the
performance activities during the period of usage in the contact
water or seawater. According to the ISO standard test method
11346, accelerated ageing can be performed at elevated
temperature along with applying the Arrhenius expression to
determine the relationship with the material's behaviour at low
temperature or high duration. Sometime a linear extrapolation
of a xed time frame can be used for lifetime prediction. A study
was conducted over a long period (2 and 5 years) observing the
polyether-based PU materials under seawater and adverse
conditions.
250
The ndings indicated that under seawater
conditions, the tensile properties could be retained 100%
during the stated period of immersion in seawater. This sug-
gested that the PU material could help retain the mechanical
integrity of products even under adverse environmental
conditions.
5.4 Coating applications
Over the years, there has been continuous research on suitable
materials for coating applications. PUs have been reported to
possess great potential as paint and surface-coating mate-
rials.
251,252
Research in this area saw the development of certain
non-linear hyperbranched polymers, which have meta-
morphosed into other hyperbranched PUs with gloss, high
solubility and exible coating properties.
253
However, reports in
the literature revealed that most of the synthesized hyper-
branched polymers cannot withstand re outbreak as they are
non-ame retardants. To modify these hyperbranched mate-
rials for certain ame-retardant coating applications,
compounds containing nitrogen, halogen or phosphorus may
be incorporated into them.
252,254
Recently, triol, tris(bisphenol-
A)mono phosphate, which contained phosphorus, was reacted
with polyethylene glycol and castor oil using dierent diiso-
cyanates, such as toluene diisocyanate (TDI), hexamethylene
diisocyanate (HMDI) and isophorone diisocyanate (IPDI). A
highly ame-retardant hyperbranched PU was produced, which
was suitable for application in nanocomposites and nano-
coatings.
230
In another research, a two-step, one-pot pre-
polymerization approach was used to manufacture
hyperbranched castor-oil-based PUs, which were observed to
have highly desirable potential to be used as advanced surface-
coating materials.
255
Another type of coating material suitable
as a marine antifouling material was produced from polyester-
based polyol. Pentaerythritol and trimethylolpropane were
used as initiators and polycondensation was done with 3-
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caprolactone, using the cross-linker hexamethylene diisocya-
nate trimer. The synthesized antifouling coating material was
also found to be highly degradable.
239
Other sources from which
PU have been recently synthesized for coating applications
include fatty acids,
92,143
soybean and lignin
241
isocyanate trim-
mers and polyester polyols.
219
5.5 Medical applications
PUs are used in several medicine-related applications,
including, but not limited to, general purpose tubing, surgical
drapes, catheters, hospital bedding, wound dressing and
several other injection-moulded equipment. They are used for
these applications due to their availability, good mechanical
and physical properties and biocompatibility.
256258
However,
the most frequent use is in short-period implants. The incor-
poration of PUs in medicine-related application helps to oer
cost eectiveness and provides adequate room for toughness
and longevity of materials.
259
This feature has allowed polymeric
materials to replace the conventional materials, such as metals,
ceramics and metal alloys. The global bio-based PU market was
1534 tonnes in 2012.
18
Polyurethane hence obtained has a bio
content ranging from 30% to 70%, depending largely on the
type of bio-based feedstock employed for manufacturing the
polyols. The global polyols market in 2015 reached almost USD
19.5 billion, with an annual growth of 8.5%. The bio-polyol
market is currently worth USD 5.03 billion.
260
The global bio-
based polyurethane market is expected to reach USD 37.5
million by 2020, or less than 0.07% of the total PU market
according to a new study by Grand View Research, Inc.
261
In one study, crystalline prepolymers were used to produce
biodegradable PU, using water as a chain extender. Properties of
the synthesized PU were compared with those obtained via
a polyaddition reaction using ethylene glycol as a chain
extender. It could be seen that there was an improvement in
mechanical and degradation properties of the new material,
which was also found to possess suitable application as an
element for joint endoprostheses.
142
Full synthetic routes for the
production of biodegradable PU using water and ethylene glycol
as chain extenders are shown in Fig. 8 and 9, respectively. Also,
due to the pH changes that oen occur during sexual inter-
course, special drug delivery systems, such as vaginal pessaries
and microbicides, which could help to prevent the spread of
sexually transmitted diseases, including HIV-AIDS, have been
suggested.
262,263
For this purpose, highly sensitive and smart
PUs for vaginal drug delivery were synthesized.
264
Moreover, PUs
have been conveniently used for other purposes, such as drug
delivery systems specically made for the colon
265,266
and as
intra-vaginal rings.
267
Recently, the suitability of carbohydrates as biomedical
devices was investigated. Castor-oil-based biodegradable and
biocompatible PUs were synthesized using polypropylene
glycols (PGs) as the polyol and with dierent carbohydrates
incorporated as cross-linkers.
228
The properties of the produced
PUs were investigated and it was found that the incorporation of
carbohydrates inuenced the thermal, mechanical and degra-
dation properties of the material due to the variety in
carbohydrate structures.
264
The characterizations performed
revealed that the carbohydrates served as suitable components
of biodegradable and biocompatible PUs. This strategy could
therefore be used for developing certain biomedical devices.
228
This report conforms to the observations reported by other
researchers who reported that incorporating modied starch
and cellulose crystals into PUs could enhance their biocom-
patibility and biodegradability as well as their mechanical
properties.
268,269
Other research studies on the applicability of
PU for medical devices can also be found in the literature,
including work reporting on the production of a low-cost
biodegradable aliphatic PU, which had a high saline stability
up to about 37 C without a signicant decrease in mechanical
strength.
227
Furthermore, other medical-application-based
studies were performed, including studies on a chitosan-
based PU for antibacterial properties
238
and biodegradable
electroactive PUs for cardiac tissue engineering.
217
From these
research studies on the medicinal applications of PUs, it was
observed that some of the produced materials oen perform
only at a moderate level, especially in terms of their resistance
towards bacterial adhesion. This is because most of them are
susceptible to bacterial attack, thereby leading to the risk of
infection.
238
New strategies for producing antibacterial PUs have
therefore become necessary. These could be achieved via the
incorporation of certain surfaces that have the capability to
resist or repel the attachment of bacterial to the material
surface.
238
These bacterial-resisting surfaces could be produced
either through the incorporation of some antibacterial coatings
or via some other surface modications that could enhance the
antibacterial or anti-biofouling properties of the materials.
5.6 Appliances, ooring and packaging applications
Most of the appliances that consumers use these days are based
on PUs. Rigid PU foams lead the way in the number of appli-
cations as they can be used as thermal insulators for refrigera-
tors and freezers. These materials have become so essential due
to their cost eectiveness, which make them suitable for use to
meet the required energy ratings in most freezers and refriger-
ators. The advantages that rigid PU foams provide to these
appliances are due to the combination of cell gases and ne
foams with a closed-cell structure, which helps to prevent heat
transfer.
For ooring purposes, PUs have several specic applications,
such as top coatings or as carpet underlay foams. They can help
to make oors more durable, aesthetically pleasing and easy to
maintain. The lifespan of carpets and their appearance can be
increased though the use of PU foam underlays, which can also
help to provide better comfort with reduced ambient noise. PU-
Based protective nishes can also be used as oor coatings,
where they can provide solvent and abrasion resistance on the
one hand and ease of cleaning and maintenance on the other
hand. Except for those properties, the lifetime or service period
is also equally important to consider. In one study, the time
temperaturestress dependent shear creep behaviour of PU
foam was analysed and Findley's power law, extended to include
Arrhenius equations, was used to propose a model for the
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temperature dependency on the viscoelastic parameters.
270
Combining all the properties, PU nishes can oer a better look
to new wood, cement or parquet oors, and can also oer
a regenerated appearance to older oors.
For packaging applications, PU can also be used as a printing
ink or as packaging foams. A PU plasticizer was prepared from
palm olein and castor oil for packaging applications.
271
This PU
plasticizing resin showed high exibility with good mechanical
and freeze resistivity. On the other hand, PU packaging foams
(PPFs) oer a wide range of packaging options, which should
help to overcome most onsite packaging challenges. The
versatility of these foams has also been explored for the cost-
eective packaging of items that demand special protection
during transit, including medical equipment, electronics, large
machine parts and delicate glassware. Custom-t packaging
materials have also been made available to almost all ship-
ments using PUs.
5.7 Apparels applications
Initially when PU was discovered to be a good t for apparels,
where PUs are converted to thin threads and incorporated into
nylon to produce garments that are stretchable and lightweight.
Recently, PUs have been technologically developed into more
improved spandex bres and thermoplastic elastomers. With
the advancement in techniques for producing PUs, it has
opened up the possibilities for producers to manufacture a wide
variety of PU-based leathers, bra cups and man-made skins,
which may be also used for several sport attires and a wide
range of accessories. Among the PU types, aqueous dispersions
of WPUs have been widely incorporated into textile-related
applications.
218
Properties that favour the use of WPUs as n-
ishing agents include permeability, the special structure of their
molecules, abrasive resistance and soness. Also, crock fast-
ness, fastness of washing and the soap fastness of reactive dyes,
acid dyes and direct dyes on dyed fabrics may be greatly
improved by using WPUs as dye nishing agents.
272,273
Recently,
UV absorption groups were incorporated into a WPU to enhance
its washing fastness, UV protection and rubbing fastness of the
material. It was also aimed at ensuring the retention of the
wrinkle recovery angle of the WPU. For this purpose, N,N-
dimethyl allyl p-benzoyl benzyl ammonium bromide was used
as a UV absorber.
218
The product was found to oer great UV
dyeing protection to cotton fabrics, and it also showed great
suitability for several other textile applications.
218
In a dierent
study, a low molecular weight of chitosan was used to extend the
PU prepolymer chain for the preparation of a chitosanPU
dispersion.
274
This dispersion was applied on dierent quality
plain weave poly-cotton dyed and printed fabric pieces to obtain
improved stiness, pilling resistance and better mechanical
properties. It was suggested that the quality of pure cotton and
woollen fabrics can also be improved by applying this
technique.
5.8 Wood composite applications
PUs are very important inclusions in many present day mate-
rials, including wood composites. Recently PU-based at
composites were prepared by using activated carbon for elec-
tromagnetic interference (EMI) shielding.
275
Dierent amounts
of activated carbon were loaded into PUs for microwave
absorption and complex permittivity. The results showed the
suitability of the composites in place of materials based on
polyethylene and polyester lled with metal additives. In
adierent study, PU/wood composites were prepared from
wood wastes and polyols.
276
The polyols were obtained from the
chemical modication of poly(ethylene terephthalate) (PET)
and commercial polyols. The chemical modication of PET was
achieved through glycolysis. Although eective load transfer
was found from the matrix to the dispersed phase, there was,
however, no improvement in the thermal stability. However, the
modication of PET-produced materials was also performed
with dierent molecular weights and various physical charac-
teristics. These changes were associated with several factors,
such as the glycerol content, condition of the reaction and the
stoichiometric ratio of the reactants.
277
The signicance behind
the use of natural bres or wood for PU-based composites is
that they are hydroxyl-enriched substances, which can undergo
chemical bonding easily with diisocyanate.
278
In accordance
with the ndings, cellulose nanocrystals (CNC) were used in
a very low amount (0.5 wt%) with high solid-content PUs to
obtain higher values for the glass transition temperature (76
C), Young's modulus (1.52 GPa), abrasion resistance, etc.
279
A
covalent bond was conrmed between the PU chains and CNC
during the polymerization process.
6. Recycling of polyurethanes
The demand for PU products is increasing day by day. Conse-
quently, recovery and recycling processes have become impor-
tant to attend to the pressing demands for more
environmentally friendly materials.
280282
Indeed, the recycling
process for PU is benecial both in terms of the environmental
as well as from an economical point of view.
13
Usually, the
recycling process is done under four classes:
283
advanced
chemical and thermochemical recycling, mechanical recycling,
energy recovery and product recycling (Fig. 16).
All these four methods contribute unique advantages to PU
fabrication and utilization.
280,281
The material recycling process
needs physical treatment, whereas chemical and thermochem-
ical recycling need chemical treatment to generate feedstock
chemicals for industry.
280282
Also, energy recovery involves the
partial or complete oxidation of waste materials,
285
which result
in the production of electricity and gaseous fuels.
286
The by-
products from the recycling process are non-hazardous and
thus are disposable to the environment.
286
However, chemical,
mechanical and thermochemical recycling and energy recovery
are the main pathways to recycle PUs. Usually, mechanical
recycling can be achieved by regrinding the PU foams into
powder wherein the PU can be used again.
284
The processes
used are compression moulding, adhesive pressing and exible
foam bonding.
284
On the other hand, PU granules can be coated
with a glue (binder) and further cured under pressure and heat.
This process is carried out to fabricate oor mats and tyres.
285,286
Pump and mother housing are, however, made by the
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compression moulding of PU granules under high pressure and
heat. In the energy recovery process, the PU is burned fully to
generate the maximum amount of electricity.
285
In particular,
the thermochemical and chemical recycling processes are based
on several chemical reactions, such as, hydrogenation, pyrol-
ysis,
265,287
hydrolysis
288
and glycolysis.
289,290
The recycling process
of PU is economical
285
and practical
283
because of the PU's rigid
and semi-rigid nature. Hence, recycled PU may be successfully
used in the manufacture of quarter panels, wheel covers,
steering wheels, bumper covers and cores in automotive vehi-
cles as well as for the manufacture of other domestic and
industrial parts.
283
PU is environmentally non-hazardous and
more economical
285
compared to other conventional polymers
due to its recycling and recovery.
7. Conclusion
Polyurethanes (PUs) are some of the most common, versatile
and researched materials in the world. They combine the
durability and toughness of metals with the elasticity of rubber,
making them suitable replacements for metals, plastics and
rubber in several engineered products. They have been incor-
porated into many types of industrial equipment and for
making numerous items, such as paints, liquid coatings, elas-
tomers, rigid insulations, elastic bres, soexible foams and
even as integral skins. PUs may be produced from a wide range
of diisocyanates, a variety of polyols and other chain extenders
and cross-linking agents. This makes it possible to obtain
a large range of tailored materials that can serve many specic
applications. Initially, most of polyols used to prepare PUs were
obtained from petroleum sources, but the high cost and energy
demands as well as environmental concerns have increased the
necessity for a more suitable and environmentally friendly
substitute. This has recently drawn enormous commercial and
academic attention to renewable sources, such as vegetable oils.
The last decade has witnessed a clear majority of studies
appearing in the literature on the use of vegetable oils as
alternatives to petroleum-based materials for PU production.
However, there are certain shortcomings associated with these
kinds of materials, especially in terms of performance. The use
of nanomaterials has been suggested to oer additional
advantages for desirable performance. Hence, the incorpora-
tion of nanoparticles that can suitably replace the hard
segments from isocyanate precursors has therefore been widely
investigated. Thus, materials such as carbon nanobres (CNFs),
carbon nanotubes (CNTs) and clays are attracting signicant
interest as important additions into PU products. With all this
enormously diverse research on PU, recyclability of the product
is very important. Fortunately, the recycling processes of PU
have been reported to be economical and practical. Thus, PU
could be considered to be environmentally non-hazardous and
more economical compared to other conventional polymers,
due to its good recycling and recoverable properties.
Abbreviations
13
C NMR Carbon nuclear magnetic resonance
1
H NMR Hydrogen nuclear magnetic resonance
31
P NMR Phosphorus nuclear magnetic resonance
AFM Atomic force microscopy
ATR-FTIR Attenuated total reection Fourier transform
infrared
AV Acid value
BDO Butanediol
CASE Coatings, adhesives, sealants and elastomers
CNF Cellulose nanobre
CNF Carbon nanobre
CNT Carbon nanotube
CO
2
Carbon dioxide
CPU Cardanol-based polyurethane
DABCO Diazabicyclo octane
DAP-MMT Diaminopropane montmorillonite
DEA Diethanolamine
DEFA Dihydroxyethyl dodecauoroheptyl acrylate
DFHA Dodecauoroheptyl acrylate
DLS Dynamic light scattering
DMA Dynamic mechanical analysis
DMCHA Dimethylcyclohexylamine
DMEA Dimethylethanolamine
DMPA Dimethylolpropanoic acid
DMT Dimethyl terephthalate
DMTA Dynamic mechanical thermal analysis
DOPO-BA Dihydro oxa phosphaphenanthrene oxide-
benzylideneaniline
DSC Dierential scanning calorimetry
EDX Energy dispersive X-ray analysis
ESCA Electron spectroscopy for chemical analysis
ESR Electron spin resonance spectrometry
fFunctionality
FEVE Fluoroethylene vinyl ether
FPU Flexible polyurethane
FTIR Fourier transform infrared spectroscopy
GPC Gel permeation chromatography
HDI Hexamethylene diisocyanate
HQEE Hydroquinone hydroxyethyl ether
Fig. 16 PU recycling using a closed loop.
284
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HR-MS High resolution mass spectrometry
HTPB Hydroxyl-terminated polybutadiene
HSCWPU High solid-content waterborne polyurethane
ICP-OES Inductively coupled plasma optical emission
spectrometry
IPDI Isophorone diisocyanate
IPDA-DEFA-
PU
Isophorone diisocyanate dihydroxyethyl
dodecauoroheptyl acrylate polyurethane
IR Infrared
LOI Limiting oxygen index
MALDI-TOF
MS
Matrix-assisted laser desorption ionization time-
of-ight mass spectrometry
MCC Micro-crystalline cellulose
MCMP Melamine-modied cardanol-based Mannich
polyether polyol
MDI Methylene diphenyl diisocyanate
MDSC Dierential scanning calorimetry in modulated
mode
M
w
Molecular weight
NCC Nanocrystalline cellulose
NCO Isocyanate
NMR Nuclear magnetic resonance
OSB Oriented strand board
PBX Polymer-bonded explosive
PCDL Polycarbonatediol
PCL Polycaprolactone
PDI Particle polydispersity index
PDM Pyridinedimethanol
PEG Polyethylene glycol
PET Poly(ethyleneterephthalate)
PG Polypropylene glycol
PIR Polyisocyanurate
PLLA Polylactic acid
PPF Polyurethane packaging foam
PSD Particle size distribution
PTMC Poly(trimethylene carbonate)
PTMG Poly(tetramethylene oxide glycol)
PU Polyurethane
PUD Polyurethane dispersion
PUF Polyurethane foam
PUI Polyurethane ionomer
RIM Reaction injection moulding
SAXS Small-angle X-ray scattering
SEC Size exclusion chromatography
SEM Scanning electron microscopy spectroscopy
SME Shape memory eect
SMPU Shape memory polyurethane
SPU Segmented polyurethane
TDI Toluene diisocyanate
TEA Triethylamine
TEDA Triethylenediamine
TEM Transmission electron microscopy
TGA Themogravimetric analysis
THTDP Trihexyl tetradecylphosphonium
TPU Thermoplastic polyurethane
UV Ultraviolet
UV-VIS Ultraviolet visible
WPUD Waterborne polyurethane dispersion
WPU Waterborne polyurethane
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