Recent Advances in Functional Polyurethane and Its Application in Leather Manufacture: A Review

Abstract

Over last few years, polyurethane (PU) has been applied in a number of areas because of its remarkable features, such as excellent mechanical strength, good abrasion resistance, toughness, low temperature flexibility, etc. More specifically, PU can be easily “tailor made” to meet specific demands. This structure–property relationship endows great potential for use in wider applications. With the improvement of living standards, ordinary polyurethane products cannot meet people’s growing needs for comfort, quality, and novelty. This has recently drawn enormous commercial and academic attention to the development of functional polyurethane. Among the major applications, PU is one of the prominent retanning agents and coating materials in leather manufacturing. This review gives a summary of academic study in the field of functional PU as well as its recent application in leather manufacture.

Full text

polymers
Review
Recent Advances in Functional Polyurethane and Its
Application in Leather Manufacture: A Review
Saiqi Tian
College of Education, Wenzhou University, Wenzhou 325035, China; tiansaiqi@wzu.edu.cn;
Tel.: +86-0577-8668-9665
Received: 12 August 2020; Accepted: 29 August 2020; Published: 2 September 2020


Abstract:
Over last few years, polyurethane (PU) has been applied in a number of areas because of
its remarkable features, such as excellent mechanical strength, good abrasion resistance, toughness,
low temperature flexibility, etc. More specifically, PU can be easily “tailor made” to meet specific
demands. This structure–property relationship endows great potential for use in wider applications.
With the improvement of living standards, ordinary polyurethane products cannot meet people’s
growing needs for comfort, quality, and novelty. This has recently drawn enormous commercial and
academic attention to the development of functional polyurethane. Among the major applications,
PU is one of the prominent retanning agents and coating materials in leather manufacturing.
This review gives a summary of academic study in the field of functional PU as well as its recent
application in leather manufacture.
Keywords: functional polyurethane; leather; recent advances; review
1. Introduction
Polyurethane (PU) was first introduced by a German professor (Professor Dr. Otto Bayer) and his
co-workers in the 1940s [
1
], and has been applied in a very broad range of commercial and industrial
fields due to its unique combination of unusual features including excellent mechanical strength,
good abrasion resistance, toughness, low temperature flexibility, corrosion resistance, processability,
etc. The basic repetitive unit in PUs is the urethane group (–NHCOO–), which is produced from the
reaction between isocyanate (–NCO), polyols (–OH), and other additives [
2
]. Segmented polyurethanes
are composed by two blocks: the soft segment forms by a macrodiol (polyether or polyester diol),
and the hard segment is composed by a diisocyanate and a low molecular weight chain extender or
crosslinkers [3].
Generally, PU’s structure is determined by the attributes of the raw materials, such as whether they
contain hard or soft segments, their molecular weight, polydispersity, and crosslinking ability. It can be
easily designed by changing the types and quantities of isocyanate, polyol, surfactants, catalysts, fillers,
and matrices during the manufacture process or via advanced characterization techniques, so as to
meet the desired performances [
4
,
5
]. Therefore, polyurethane products show various types, including
elastomers, sheets, adhesives, coatings, and foams.
With the improvement of living standards, ordinary polyurethane products cannot meet people’s
growing need of comfort, quality, and novelty. Consequently, significant interest has been directed
to the development of functional polyurethane [
6
,
7
]. Normally, functional polyurethanes can show
stimuli response to the environment or possess unique characteristics, like thermosensitivity, shape
memory, self-healing, and photochromicity.
Overall, there are two widely used approaches to obtain functional features: One is to adjust the
structure of PU by controlling raw materials, hard and soft segment, molecular weight, polydispersity,
and crosslinking ability. The other one is to incorporate of functional molecules into PU backbones or
Polymers 2020,12, 1996; doi:10.3390/polym12091996 www.mdpi.com/journal/polymers

Polymers 2020,12, 1996 2 of 16
networks. Accordingly, PU can not only maintain its own properties but also acquire the characteristic
of functional groups.
Among the major applications, PU plays an important role during leather manufacturing, which is
mainly used in retanning and finishing process Waterborne PU is one of the best candidates for leather
retanning. Owing to the similarity between the urethane group in PU and the peptide chain in collagen,
the resultant leathers retanned by PU can keep the feeling of genuine leather [8]. PU is also desirable
coating materials in finishing, exhibiting many preeminent properties, such as excellent flexibility,
superior handling, and adhesive strength [
9
]. It is found to be capable to hide crust leather defects or
irregular appearance and confer specific properties. Nowadays, leather products are required to have
higher comfort, quality, and novelty, which can be realized by functional PU.
To the best of our knowledge, no effort has been made in the literature compiling functional PU
and its application in leather manufacturing. The main objective of this review is to give a summary of
the academic study in the field of functional PU as well as its recent application in leather manufacture.
This is illustrated by reviewing preparation methods for the realization of polymeric materials and
mechanism of these unique performances. This review will report a substantial number of examples
that we considered suitable to provide readers with illuminating information about the main topic.
The examples here reported come predominantly from recent relevant literature.
2. Functional Polyurethane
Varieties of functional polyurethane have been made into commercial production. However, most
still need to be explored and improved. According to the application, the five important categories
PUs are (1) anti-fouling, (2) self-healing, (3) anti-bacterial, (4) luminescent and color-tunable, and (5)
shape memory.
2.1. Anti-Fouling PU
Inspired by the super-hydrophobic surface of the natural lotus leaf, the anti-fouling PU can be
realized through two approaches: decreasing surface free energy and increasing surface roughness.
Surface free energy of materials is determined by the chemical composition. As a rule, PU is
modified by organosilicone or organofluorine.
Attributed to its unique structure, organosilicone has the features of both organics and inorganics.
The non-polarity of –Si–O– in its backbone results in fairly low surface tension, ensuring remarkable
hydrophobicity. Usually, polysiloxane is covalently incorporated into PU chains to obtain a block
copolymer. For example, Rahman et al. [
10
] prepared polysiloxane–polyurethane through a prepolymer
process and investigated the potential application of it in marine coatings. Polydimethylsiloxane (PDMS)
and poly (tetramethyleneoxide glycol) were used as the soft segments. Results showed that the coating
was smooth on account of the surface enriched by silicone in PDMS with a content of 15.76 wt.%.
This effectively prevented marine fouling in sea water. Additionally, siloxane–polyurethane was
prepared and the effect of the attachment of silicone oils into the fouling–release coating system was
explored by Galhenage and coworkers [
11
]. A thicker interfacial layer was formed by phenylmethyl
silicone oil, resulting in improved hydrophobic performance.
In recent years, there has been an increasing interest in fluorinated polymers with unique low
surface energy, high hydrophobicity, and non-sticking behavior. Generally, to introduce organofluorine
units into polyurethane, the fluorine unit was embedded into the backbone of polyurethane chain
through a covalent bon [
12
–
14
]. Wen [
15
] prepared a fluoro alcohol-terminated isocyanate trimer
by 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol and hexamethylene diisocyanate trimer, and then
fabricated a series of fluorine-containing water-based polyurethanes (Figure 1). By controlling the
fluorine content, the PU was endowed with low surface free energy and great wetting ability. Next,
a different fluoroalcohol-terminated isocyanate trimer was utilized to synthesize a series of fluorinated
waterborne polyurethanes with various lengths of fluorinated side chains [
16
]. It is indicated that

Polymers 2020,12, 1996 3 of 16
the long fluorocarbon chain plays an important role in the molecular design of low surface energy
polyurethane coating materials.
Polymers 2020, 12, x FOR PEER REVIEW 3 of 16
chains [16]. It is indicated that the long fluorocarbon chain plays an important role in the molecular
design of low surface energy polyurethane coating materials.
Figure 1. Structures of fluorine unit for preparing polyurethane (PU) was endowed with low surface
free energy and great wetting ability [15].
According to the Cassie–Baxter equation [17], the hydrophobicity of materials increases with
increasing surface roughness. In current research, hydrophobicity of PU was achieved by a
hierarchical morphology comprising of packed nanoparticles, which was induced in thermoplastic
polyurethane (TPU)/silica nanocomposite coatings (Figure 2) [18]. After a certain processing time,
the formation of a greatly packed morphology by silica nanoparticles appeared, which lead to a
significant surface roughness. Through the roughness studies, the sample processed at a pressing
time of 5 min was proved to display higher roughness parameters than the one processed at 1 min.
The presented pressing method has promising potentials even in turning hydrophilic polymers like
TPU into superhydrophobic surfaces with self-cleaning behavior.
Particularly, incorporation of functional units into PU matrices can realize self-cleaning
behavior. For instance, nano-TiO2 is a highly efficient photocatalyst to decompose dirt, on account of
its low cost, non-toxicity, and high stability [19–22]. In the application of self-cleaning polymer
coatings, the introduction of photocatalytic activity can endow self-cleaning ability [23]. In a
research, it provided a novel and eco-friendly approach for fabrication of waterborne polyurethane
acrylate with self-cleaning performance in photocatalysis by incorporating surfactant-modified
TiO2/reduced grapheneoxide (TiO2/rGO) nanocomposites [24]. The dispersibility of TiO2/rGO
nanocomposites in the polymer matrix was improved by incorporation of cationic surfactant
hexadecyl trimethyl ammonium bromide (CTAB). Methyl orange (MO; 88.3%) was selected as the
model dirt. After 6 h of illumination by visible light, MO was decomposed in the existence of the
sample of 0.5% C-TiO2/rGO-WPUA. This WPUA composites presented appealing self-cleaning
ability in photocatalysis.
Figure 2. Schematic representation of the pressing method for fabrication of thermoplastic
polyurethane/silica (TPU/silica) coatings (left) and cross-sectional morphology along with the water
drop profile for the superhydrophobic sample (right) [18].
Figure 1.
Structures of fluorine unit for preparing polyurethane (PU) was endowed with low surface
free energy and great wetting ability [15].
According to the Cassie–Baxter equation [
17
], the hydrophobicity of materials increases with
increasing surface roughness. In current research, hydrophobicity of PU was achieved by a hierarchical
morphology comprising of packed nanoparticles, which was induced in thermoplastic polyurethane
(TPU)/silica nanocomposite coatings (Figure 2) [
18
]. After a certain processing time, the formation
of a greatly packed morphology by silica nanoparticles appeared, which lead to a significant surface
roughness. Through the roughness studies, the sample processed at a pressing time of 5 min was
proved to display higher roughness parameters than the one processed at 1 min. The presented pressing
method has promising potentials even in turning hydrophilic polymers like TPU into superhydrophobic
surfaces with self-cleaning behavior.
Polymers 2020, 12, x FOR PEER REVIEW 3 of 16
chains [16]. It is indicated that the long fluorocarbon chain plays an important role in the molecular
design of low surface energy polyurethane coating materials.
Figure 1. Structures of fluorine unit for preparing polyurethane (PU) was endowed with low surface
free energy and great wetting ability [15].
According to the Cassie–Baxter equation [17], the hydrophobicity of materials increases with
increasing surface roughness. In current research, hydrophobicity of PU was achieved by a
hierarchical morphology comprising of packed nanoparticles, which was induced in thermoplastic
polyurethane (TPU)/silica nanocomposite coatings (Figure 2) [18]. After a certain processing time,
the formation of a greatly packed morphology by silica nanoparticles appeared, which lead to a
significant surface roughness. Through the roughness studies, the sample processed at a pressing
time of 5 min was proved to display higher roughness parameters than the one processed at 1 min.
The presented pressing method has promising potentials even in turning hydrophilic polymers like
TPU into superhydrophobic surfaces with self-cleaning behavior.
Particularly, incorporation of functional units into PU matrices can realize self-cleaning
behavior. For instance, nano-TiO2 is a highly efficient photocatalyst to decompose dirt, on account of
its low cost, non-toxicity, and high stability [19–22]. In the application of self-cleaning polymer
coatings, the introduction of photocatalytic activity can endow self-cleaning ability [23]. In a
research, it provided a novel and eco-friendly approach for fabrication of waterborne polyurethane
acrylate with self-cleaning performance in photocatalysis by incorporating surfactant-modified
TiO2/reduced grapheneoxide (TiO2/rGO) nanocomposites [24]. The dispersibility of TiO2/rGO
nanocomposites in the polymer matrix was improved by incorporation of cationic surfactant
hexadecyl trimethyl ammonium bromide (CTAB). Methyl orange (MO; 88.3%) was selected as the
model dirt. After 6 h of illumination by visible light, MO was decomposed in the existence of the
sample of 0.5% C-TiO2/rGO-WPUA. This WPUA composites presented appealing self-cleaning
ability in photocatalysis.
Figure 2. Schematic representation of the pressing method for fabrication of thermoplastic
polyurethane/silica (TPU/silica) coatings (left) and cross-sectional morphology along with the water
drop profile for the superhydrophobic sample (right) [18].
Figure 2.
Schematic representation of the pressing method for fabrication of thermoplastic
polyurethane/silica (TPU/silica) coatings (
left
) and cross-sectional morphology along with the water
drop profile for the superhydrophobic sample (right) [18].
Particularly, incorporation of functional units into PU matrices can realize self-cleaning behavior.
For instance, nano-TiO
2
is a highly efficient photocatalyst to decompose dirt, on account of its low
cost, non-toxicity, and high stability [
19
–
22
]. In the application of self-cleaning polymer coatings,
the introduction of photocatalytic activity can endow self-cleaning ability [
23
]. In a research, it provided
a novel and eco-friendly approach for fabrication of waterborne polyurethane acrylate with self-cleaning
performance in photocatalysis by incorporating surfactant-modified TiO
2
/reduced grapheneoxide
(TiO
2
/rGO) nanocomposites [
24
]. The dispersibility of TiO
2
/rGO nanocomposites in the polymer
matrix was improved by incorporation of cationic surfactant hexadecyl trimethyl ammonium bromide
(CTAB). Methyl orange (MO; 88.3%) was selected as the model dirt. After 6 h of illumination by visible
light, MO was decomposed in the existence of the sample of 0.5% C-TiO
2
/rGO-WPUA. This WPUA
composites presented appealing self-cleaning ability in photocatalysis.

Polymers 2020,12, 1996 4 of 16
2.2. Self-Healing PU
Self-healing materials are able to recover their fundamental properties after damage has
occurred [
25
–
28
]. This ability in materials increases lifetimes, and is especially important to perform
in a designed manner for significant times where repair is not possible. Imparting self-healing
functionalities to PU may be achieved through two different approaches: blended and intrinsic
methods [
29
]. For blended methods, healing agents and catalysts are embedded into the PU matrix
through capsule or vascular, the cracks of which can induce polymerization of healing agents, realizing
the recovery of damage. Conversely, a limited number of healing agents and catalysts cannot repair
PU repeatedly. Furthermore, the quantity of healing agents and catalysts in PU is finite, which
cannot repair PU repeatedly, after they are consumed. Thereby, intrinsic methods become a desirable
approach to prepare self-healing PU. In this case, repairing is achieved through the inherent reversibility
of bonding in PU backbones, which acts as a healing agent, ideally without external input [
25
,
30
].
Intrinsic self-healing behavior is mainly endowed by two types of bonds: One is reversible noncovalent
bonds, such as the hydrogen bond and
π
–
π
bond. The other one is covalent bonds, such as the bond
formed by Diels–Alder (DA) reaction and disulfide bond.
Hydrogen bond is the most common noncovalent bonds with the feature of selectivity, cooperativity
and reversibility. For example, a self-healing covalent PU is successfully prepared, which contains
urea, amide, and urethane groups with plenty of hydrogen-bonding sites, offering numerous lateral
interactions between the polymeric chains [
23
]. A good proportion of the hydrogen-bonding network
between the polymer chains is extremely stable even at high temperatures, which provides appealing
autonomous self-healing ability. A healing efficiency of approximately 85% after heating to 100
◦
C for
24 h is achieved.
The DA reaction is a thermo-induced [4 +2] cycloaddition reaction by furan group and maleimide
group, which are generally utilized as a typical diene and dienophile, respectively (Figure 3).
Cleavage reaction (r-DA reaction) of the DA adducts occurs at high temperature, subsequently
regenerating the corresponding diene and dienophile. Once the temperature reduces sufficiently,
the DA reaction takes place, forming a DA adduct [
31
,
32
]. DA/retro-DA chemistry is regarded to
be simple and effective, occurs under mild reaction conditions, and has minimal side reactions to
build self-healing systems [
33
–
35
]. In a research, special attention was given for the a facile route to
design and prepare a self-healing and recycling PU based on reversible DA/retro-DA reactions [
36
].
A study was carried out on covalently conjugation of a novel diol containing DA bonds in a PU
backbone. After being cut into two pieces, the PU films were thermally treated at 130
◦
C for 30 min.
It was confirmed that the molecular chains of PU were cleaved into small molecular weight fragments
through retro-DA reactions. When exposed to 65
◦
C for 24 h, most of the dissociative maleimide and
furan moieties relinked again via DA reaction. The mechanical performance was restored, which
was attributed to hydrogen bonds surrounding the cracked location. PU presented the self-healing
efficiencies of 92.5% with the 15.6% solid content of DA diol (Figure 4).
Polymers 2020, 12, x FOR PEER REVIEW 4 of 16
2.2. Self-Healing PU
Self-healing materials are able to recover their fundamental properties after damage has
occurred [25–28]. This ability in materials increases lifetimes, and is especially important to perform
in a designed manner for significant times where repair is not possible. Imparting self-healing
functionalities to PU may be achieved through two different approaches: blended and intrinsic
methods [29]. For blended methods, healing agents and catalysts are embedded into the PU matrix
through capsule or vascular, the cracks of which can induce polymerization of healing agents,
realizing the recovery of damage. Conversely, a limited number of healing agents and catalysts
cannot repair PU repeatedly. Furthermore, the quantity of healing agents and catalysts in PU is
finite, which cannot repair PU repeatedly, after they are consumed. Thereby, intrinsic methods
become a desirable approach to prepare self-healing PU. In this case, repairing is achieved through
the inherent reversibility of bonding in PU backbones, which acts as a healing agent, ideally without
external input [25,30]. Intrinsic self-healing behavior is mainly endowed by two types of bonds: One
is reversible noncovalent bonds, such as the hydrogen bond and π–π bond. The other one is covalent
bonds, such as the bond formed by Diels–Alder (DA) reaction and disulfide bond.
Hydrogen bond is the most common noncovalent bonds with the feature of selectivity,
cooperativity and reversibility. For example, a self-healing covalent PU is successfully prepared,
which contains urea, amide, and urethane groups with plenty of hydrogen-bonding sites, offering
numerous lateral interactions between the polymeric chains [23]. A good proportion of the
hydrogen-bonding network between the polymer chains is extremely stable even at high
temperatures, which provides appealing autonomous self-healing ability. A healing efficiency of
approximately 85% after heating to 100 °C for 24 h is achieved.
The DA reaction is a thermo-induced [4 + 2] cycloaddition reaction by furan group and
maleimide group, which are generally utilized as a typical diene and dienophile, respectively
(Figure 3). Cleavage reaction (r-DA reaction) of the DA adducts occurs at high temperature,
subsequently regenerating the corresponding diene and dienophile. Once the temperature reduces
sufficiently, the DA reaction takes place, forming a DA adduct [31,32]. DA/retro-DA chemistry is
regarded to be simple and effective, occurs under mild reaction conditions, and has minimal side
reactions to build self-healing systems [33–35]. In a research, special attention was given for the a
facile route to design and prepare a self-healing and recycling PU based on reversible DA/retro-DA
reactions [36]. A study was carried out on covalently conjugation of a novel diol containing DA
bonds in a PU backbone. After being cut into two pieces, the PU films were thermally treated at 130
°C for 30 min. It was confirmed that the molecular chains of PU were cleaved into small molecular
weight fragments through retro-DA reactions. When exposed to 65 °C for 24 h, most of the
dissociative maleimide and furan moieties relinked again via DA reaction. The mechanical
performance was restored, which was attributed to hydrogen bonds surrounding the cracked
location. PU presented the self-healing efficiencies of 92.5% with the 15.6% solid content of DA diol
(Figure 4).
Figure 3. General equation of the Diels–Alder reaction.
Figure 3. General equation of the Diels–Alder reaction.

Polymers 2020,12, 1996 5 of 16
Polymers 2020, 12, x FOR PEER REVIEW 5 of 16
Figure 4. Schematic illustration of the self-healing mechanism of WPU-DA-x [36].
Disulfide bonds can initiate chain exchange reaction under exposure to heat, UV light, and
redox conditions [37–40] (Figure 5). More specifically, disulfide exchange can be activated at
moderate temperatures (about 60 °C) and without external stimuli, able to endow disulfide-based
PU efficient room temperature self-healing features [41–44]. Kim [38] proposed a transparent and
easily processable polyurethane (IP-SS) using bis (4-hydroxyphenyl) disulfide as the aromatic
disulfide component embedded in the hard segments. This PU possesses the highest reported tensile
strength and toughness (6.8 MPa and 26.9 MJ m
−3
, respectively). After it is cut in half and
reconnected, the mechanical properties are able to recover to more than 75% of those of the original
sample within 2 h under room temperature (Figure 6).
Figure 5. Healing illustration of self-healing polyurethane based on the combination of disulfide
bonds and shape memory effect.
Figure 6. (a) Photograph of the TPU film (25 mm × 25 mm × 0.3 mm) of IP–SS. (b) Optical microscopy
images of the X-shaped scratch on the TPU film of IP–SS before and after healing for 2 h at 25 °C. (c) IP–SS
film cut in half, respliced, and healed for 2 h (+4 h) at 25 °C, followed by a 5 kg dumbbell lifting test [45].
Figure 4. Schematic illustration of the self-healing mechanism of WPU-DA-x [36].
Disulfide bonds can initiate chain exchange reaction under exposure to heat, UV light, and redox
conditions [
37
–
40
] (Figure 5). More specifically, disulfide exchange can be activated at moderate
temperatures (about 60
◦
C) and without external stimuli, able to endow disulfide-based PU efficient
room temperature self-healing features [
41
–
44
]. Kim [
38
] proposed a transparent and easily processable
polyurethane (IP-SS) using bis (4-hydroxyphenyl) disulfide as the aromatic disulfide component
embedded in the hard segments. This PU possesses the highest reported tensile strength and toughness
(6.8 MPa and 26.9 MJ m
−3
, respectively). After it is cut in half and reconnected, the mechanical
properties are able to recover to more than 75% of those of the original sample within 2 h under room
temperature (Figure 6).
Polymers 2020, 12, x FOR PEER REVIEW 5 of 16
Figure 4. Schematic illustration of the self-healing mechanism of WPU-DA-x [36].
Disulfide bonds can initiate chain exchange reaction under exposure to heat, UV light, and
redox conditions [37–40] (Figure 5). More specifically, disulfide exchange can be activated at
moderate temperatures (about 60 °C) and without external stimuli, able to endow disulfide-based
PU efficient room temperature self-healing features [41–44]. Kim [38] proposed a transparent and
easily processable polyurethane (IP-SS) using bis (4-hydroxyphenyl) disulfide as the aromatic
disulfide component embedded in the hard segments. This PU possesses the highest reported tensile
strength and toughness (6.8 MPa and 26.9 MJ m
−3
, respectively). After it is cut in half and
reconnected, the mechanical properties are able to recover to more than 75% of those of the original
sample within 2 h under room temperature (Figure 6).
Figure 5. Healing illustration of self-healing polyurethane based on the combination of disulfide
bonds and shape memory effect.
Figure 6. (a) Photograph of the TPU film (25 mm × 25 mm × 0.3 mm) of IP–SS. (b) Optical microscopy
images of the X-shaped scratch on the TPU film of IP–SS before and after healing for 2 h at 25 °C. (c) IP–SS
film cut in half, respliced, and healed for 2 h (+4 h) at 25 °C, followed by a 5 kg dumbbell lifting test [45].
Figure 5.
Healing illustration of self-healing polyurethane based on the combination of disulfide bonds
and shape memory effect.
Polymers 2020, 12, x FOR PEER REVIEW 5 of 16
Figure 4. Schematic illustration of the self-healing mechanism of WPU-DA-x [36].
Disulfide bonds can initiate chain exchange reaction under exposure to heat, UV light, and
redox conditions [37–40] (Figure 5). More specifically, disulfide exchange can be activated at
moderate temperatures (about 60 °C) and without external stimuli, able to endow disulfide-based
PU efficient room temperature self-healing features [41–44]. Kim [38] proposed a transparent and
easily processable polyurethane (IP-SS) using bis (4-hydroxyphenyl) disulfide as the aromatic
disulfide component embedded in the hard segments. This PU possesses the highest reported tensile
strength and toughness (6.8 MPa and 26.9 MJ m
−3
, respectively). After it is cut in half and
reconnected, the mechanical properties are able to recover to more than 75% of those of the original
sample within 2 h under room temperature (Figure 6).
Figure 5. Healing illustration of self-healing polyurethane based on the combination of disulfide
bonds and shape memory effect.
Figure 6. (a) Photograph of the TPU film (25 mm × 25 mm × 0.3 mm) of IP–SS. (b) Optical microscopy
images of the X-shaped scratch on the TPU film of IP–SS before and after healing for 2 h at 25 °C. (c) IP–SS
film cut in half, respliced, and healed for 2 h (+4 h) at 25 °C, followed by a 5 kg dumbbell lifting test [45].
Figure 6.
(
a
) Photograph of the TPU film (25 mm
×
25 mm
×
0.3 mm) of IP–SS. (
b
) Optical microscopy
images of the X-shaped scratch on the TPU film of IP–SS before and after healing for 2 h at 25
◦
C.
(
c
) IP–SS film cut in half, respliced, and healed for 2 h (+4 h) at 25
◦
C, followed by a 5 kg dumbbell
lifting test [45].

Polymers 2020,12, 1996 6 of 16
Compared to disulfide bonds, diselenide bonds own a lower bond energy (diselenide bonds:
172 kJmol
−1
; disulfide bonds: 240 kJmol
−1
) [
46
]. This indicates that diselenide bonds can be much
more readily induced under milder conditions. Successful synthesis of different PU materials by using
di-(1-hydroxyundecyl) diselenide (DiSe) with different ratio as chain extender has been reported [
47
].
Those materials possessed different healing properties. Under merely visible light, the materials could
heal themselves to various extents. Moreover, the healing process could be improved with shorter
healing time but superior healing results, with irradiation by directional blue laser. The sample was
able to retain its mechanical property and the integrity during the healing process, by using laser
without generating heat.
2.3. Antibacterial PU
PU can cause the growth of bacteria under certain temperature and humidity conditions during
usage and storage [
48
–
50
]. It can be easily decomposed by bacteria, resulting in age and color changes
before breaking. This may essentially threaten human health. Thereby, it is significant to avoid the
damage in consequence of microorganism-induced erosion, and prevent the reproduction as well as
spread of pathogenic bacteria in PU products.
Antibacterial property can be acquired through introduction of a metal acetate, such as Ag
+
[
51
],
Cu
2+
[
52
], and Zn
2+
[
53
]. An example of a Cu–Ag-sputtered PU catheter prepared by Rtimi et al.
and an accompanying illustration are presented in Figure 7[
54
]. PU catheters with different atomic
ratios of sputtered Cu:Ag led to different optical properties and antibacterial activities. The bacterial
inactivation dynamics were also investigated, manifesting that the bacterial inactivation time was
accelerated to 5 min on a 50%/50% Cu–Ag PU catheter compared to Cu or Ag deposited independently
on PU catheters.
Polymers 2020, 12, x FOR PEER REVIEW 6 of 16
Compared to disulfide bonds, diselenide bonds own a lower bond energy (diselenide bonds:
172 kJmol−1; disulfide bonds: 240 kJmol−1) [46]. This indicates that diselenide bonds can be much
more readily induced under milder conditions. Successful synthesis of different PU materials by
using di-(1-hydroxyundecyl) diselenide (DiSe) with different ratio as chain extender has been
reported [47]. Those materials possessed different healing properties. Under merely visible light, the
materials could heal themselves to various extents. Moreover, the healing process could be
improved with shorter healing time but superior healing results, with irradiation by directional blue
laser. The sample was able to retain its mechanical property and the integrity during the healing
process, by using laser without generating heat.
2.3. Antibacterial PU
PU can cause the growth of bacteria under certain temperature and humidity conditions during
usage and storage [48–50]. It can be easily decomposed by bacteria, resulting in age and color
changes before breaking. This may essentially threaten human health. Thereby, it is significant to
avoid the damage in consequence of microorganism-induced erosion, and prevent the reproduction
as well as spread of pathogenic bacteria in PU products.
Antibacterial property can be acquired through introduction of a metal acetate, such as Ag+ [51],
Cu2+ [52], and Zn2+ [53]. An example of a Cu–Ag-sputtered PU catheter prepared by Rtimi et al. and
an accompanying illustration are presented in Figure 7 [54]. PU catheters with different atomic
ratios of sputtered Cu:Ag led to different optical properties and antibacterial activities. The bacterial
inactivation dynamics were also investigated, manifesting that the bacterial inactivation time was
accelerated to 5 min on a 50%/50% Cu–Ag PU catheter compared to Cu or Ag deposited
independently on PU catheters.
In another research, an antimicrobial polyurethane foam was prepared by copper oxide
nanoparticles as antimicrobial agents [55]. The PU foams not only maintained good tensile strength,
but also were endowed with appealing antimicrobial activity against nosocomial infections. The
novel property makes it suitable for applications on antimicrobial hospital mattress to control
hospital infections.
Self-antibacterial PU anchors antibacterial groups to its chains chemically, ensuring desirable
durability. A study was performed on production of a UV-curable waterborne polyurethane with
pendant from 4-NCO pre-polymer and modified by guanidinoacetic acid (GAA). The hydrophilic
groups of coating surface were increased by GAA and pendant amine, leading to an optimized
antibacterial performance [49]. The novel PU had outstanding properties both in Gram-negative
(92.05%) and Gram-positive (94.77%) antibacterial tests. In addition, antibacterial efficiency still
maintained 87.94% after 12 times washing.
Figure 7. Antibacterial illustration of Cu–Ag-sputtered polyurethane (PU) catheters [54].
2.4. Luminescent and Color-Tunable PU
There are two types of luminescent PU: fluorescent PU and long afterglow PU. The research of
fluorescent PU originates from the exploration of colorful PU. Initially, dyes and pigments are
simply mixed with PU matrix. Afterwards, fluorescent PUs were synthesized through chemically
embedding fluorescers in PU backbones. Hu’s group [56–60] reported a series of fluorescent PUs by
Figure 7. Antibacterial illustration of Cu–Ag-sputtered polyurethane (PU) catheters [54].
In another research, an antimicrobial polyurethane foam was prepared by copper oxide nanoparticles
as antimicrobial agents [
55
]. The PU foams not only maintained good tensile strength, but also were
endowed with appealing antimicrobial activity against nosocomial infections. The novel property makes
it suitable for applications on antimicrobial hospital mattress to control hospital infections.
Self-antibacterial PU anchors antibacterial groups to its chains chemically, ensuring desirable
durability. A study was performed on production of a UV-curable waterborne polyurethane with
pendant from 4-NCO pre-polymer and modified by guanidinoacetic acid (GAA). The hydrophilic
groups of coating surface were increased by GAA and pendant amine, leading to an optimized
antibacterial performance [
49
]. The novel PU had outstanding properties both in Gram-negative
(92.05%) and Gram-positive (94.77%) antibacterial tests. In addition, antibacterial efficiency still
maintained 87.94% after 12 times washing.
2.4. Luminescent and Color-Tunable PU
There are two types of luminescent PU: fluorescent PU and long afterglow PU. The research of
fluorescent PU originates from the exploration of colorful PU. Initially, dyes and pigments are simply

Polymers 2020,12, 1996 7 of 16
mixed with PU matrix. Afterwards, fluorescent PUs were synthesized through chemically embedding
fluorescers in PU backbones. Hu’s group [
56
–
60
] reported a series of fluorescent PUs by incorporating
the different molecular structures of fluorescers into the PU chain, and systematically investigated
their fluorescent properties.
Fluorescence is caused by radiation, which ceases almost immediately after the incident radiation
stops [
60
]. Long afterglow is long persistent luminescent phenomenon, whereby the visible luminescent
emission remains visible for an appreciable time—from seconds to many hours—after the excitation
has stopped [
61
–
63
]. This facilitates applications of long afterglow PU as night-vision materials in
many important fields, such as displays, decorations, traffic signage, medical diagnostics, emergency
signs, and military. Nevertheless, long afterglow phosphors are almost always composed of silicate,
phosphate, and aluminate activated by rare earth ions. As a result of the weak interaction between
inorganic phosphors and organic polymers, physically doping phosphors into PU matrices inevitably
lead to incompatibility with the matrix [
64
,
65
]. Our groups selected 3-aminopropyltriethoxysilane
to encapsulate SrAl
2
O
4
:Eu
2+
,Dy
3+
phosphors [
66
]. The surface of the phosphors is decorated
by both organic coatings and –NH
2
groups. Because of the interaction between the –NH
2
of
amino–SrAl
2
O
4
:Eu
2+
,Dy
3+
and the –NCO of the prepolymer, the compatibility of the two components
increases. Thus, the resulting PU obtains better mechanical properties, storage stability, and thermal
properties than the phosphors blending sample (Figure 8).
Polymers 2020, 12, x FOR PEER REVIEW 7 of 16
incorporating the different molecular structures of fluorescers into the PU chain, and systematically
investigated their fluorescent properties.
Fluorescence is caused by radiation, which ceases almost immediately after the incident
radiation stops [60]. Long afterglow is long persistent luminescent phenomenon, whereby the visible
luminescent emission remains visible for an appreciable time—from seconds to many hours—after
the excitation has stopped [61–63]. This facilitates applications of long afterglow PU as night-vision
materials in many important fields, such as displays, decorations, traffic signage, medical
diagnostics, emergency signs, and military. Nevertheless, long afterglow phosphors are almost
always composed of silicate, phosphate, and aluminate activated by rare earth ions. As a result of the
weak interaction between inorganic phosphors and organic polymers, physically doping phosphors
into PU matrices inevitably lead to incompatibility with the matrix [64,65]. Our groups selected
3-aminopropyltriethoxysilane to encapsulate SrAl2O4:Eu2+,Dy3+ phosphors [66]. The surface of the
phosphors is decorated by both organic coatings and –NH2 groups. Because of the interaction
between the –NH2 of amino–SrAl2O4:Eu2+,Dy3+ and the –NCO of the prepolymer, the compatibility of
the two components increases. Thus, the resulting PU obtains better mechanical properties, storage
stability, and thermal properties than the phosphors blending sample (Figure 8).
Figure 8. Synthesis route and molecular structure of Amino-SrAl2O4:Eu2+,Dy3+ [66].
Major long persistent luminescent polymers reported now are blue and green. It makes it
difficult to mimic the gorgeous color changes found in nature, therefore limiting its wider
applications [67,68]. Thus, we further designed and manufactured a novel PU via incorporating
NH2–SrAl2O4:Eu2+,Dy3+ and the red color conversion agent
1-[(2-hydrocyethly)amino]-anthraquinone [69]. The photoluminescence emission spectrum of
phosphors and the excitation spectrum of color conversion agent overlap well. The color conversion
agent is proved to be able to be excited by phosphors in the dark. The final PU displays red emission
in daylight and emits yellow light in the dark.
Stimuli-responsive materials can be utilized to endow luminescent PU with a more vivid color
change. A thermochromic luminescent polyurethane was developed via introducing
SrAl2O4:Eu2+,Dy3+ phosphors and thermochromic pigment (TP) [64]. TP, which is considered as a
smart material, can change its optical properties along with the temperature, based on the the
formation or destruction of a colored complex. Under natural light, PU presents as deep red at room
temperature and the color disappears when the temperature is higher than 35 °C. In dark, TP can
only absorb a small quantity of yellow light, which can be transmitted through polyurethane under
35 °C. The great mass of the green luminescent energy from the phosphors was shielded by the
optical barrier formed by TP. After it is warmed to 35 °C, the red optical barrier by TP is removed. As
a consequence, the luminescence from phosphors can completely transmit through the
polyurethane, and we see the green light with naked eyes (Figure 9).
Figure 8. Synthesis route and molecular structure of Amino-SrAl2O4:Eu2+,Dy3+[66].
Major long persistent luminescent polymers reported now are blue and green. It makes it difficult to
mimic the gorgeous color changes found in nature, therefore limiting its wider applications [
67
,
68
]. Thus,
we further designed and manufactured a novel PU via incorporating NH
2
–SrAl
2
O
4
:Eu
2+
,Dy
3+
and the
red color conversion agent 1-[(2-hydrocyethly)amino]-anthraquinone [
69
]. The photoluminescence
emission spectrum of phosphors and the excitation spectrum of color conversion agent overlap well.
The color conversion agent is proved to be able to be excited by phosphors in the dark. The final PU
displays red emission in daylight and emits yellow light in the dark.
Stimuli-responsive materials can be utilized to endow luminescent PU with a more vivid color
change. A thermochromic luminescent polyurethane was developed via introducing SrAl
2
O
4
:Eu
2+
,Dy
3+
phosphors and thermochromic pigment (TP) [
64
]. TP, which is considered as a smart material, can
change its optical properties along with the temperature, based on the the formation or destruction of
a colored complex. Under natural light, PU presents as deep red at room temperature and the color
disappears when the temperature is higher than 35
◦
C. In dark, TP can only absorb a small quantity of
yellow light, which can be transmitted through polyurethane under 35
◦
C. The great mass of the green
luminescent energy from the phosphors was shielded by the optical barrier formed by TP. After it is
warmed to 35
◦
C, the red optical barrier by TP is removed. As a consequence, the luminescence from
phosphors can completely transmit through the polyurethane, and we see the green light with naked
eyes (Figure 9).

Polymers 2020,12, 1996 8 of 16
Polymers 2020, 12, x FOR PEER REVIEW 8 of 16
Figure 9. The digital image of thermochromic luminous phenomena and thermosensitive process
together with luminescence model in darkness [70].
Mechanochromic materials are able to convert mechanical stimuli into optical signals, and thus
the abilities to sense stress and show internal damages are vital to monitor failures like fractures,
fatigue, and hysteresis [71,72]. This color change allows adjusting the materials before disastrous
failure and further increase in the reliability. An example of thermoplastic polyurethane elastomers
(TPU-BBS) with reversible mechanochromic behavior prepared by blended with
bis(benzoxazolyl)stilbene dyes is reported in Cellini’s article [67]. Upon large deformation of the
polymeric structure, the fluorescence emission shifted from the excimer band to the monomer band,
which is caused by reorganization of dye aggregates. The largely reversible mechanochromic
behavior is able to controlling by the initial dye concentration in the polymer. Compared with
TPU-BBS blends with dye concentrations of 0.1 wt.% and 1.5 wt.%, the one with 0.5 wt.% showed a
higher relative variation of the emission ratio during stretching. These results provide a promising
reference for preparation of mechanochromic sensors (Figure 10).
Figure 10. (a) Photographic image of three TPU-BBS samples with concentrations 0.1, 0.5, and 1.5
wt.%. The image is obtained through a commercial linear plastic polarizer. (b) Fluorescence spectra
for the three concentrations are reported in the graph. Spectra are normalized so that the emission is
equal in correspondence of the monomer peak at 436 nm. The fluorescence emission of dye
aggregates increases with dye concentration and is associated with the progressive shift of material
coloration from blue to green [73].
2.5. Shape Memory PU
Shape memory polymers have the ability to remember their original shape after being
deformed and recover it under appropriate stimulus such as temperature, light, electric field,
magnetic field, pH, specific ions, or enzyme [74,75]. The incompatibility between soft segments and
hard segments in PU leads to microphase separation, which depends on block lengths, hydrogen
Figure 9.
The digital image of thermochromic luminous phenomena and thermosensitive process
together with luminescence model in darkness [70].
Mechanochromic materials are able to convert mechanical stimuli into optical signals, and thus the
abilities to sense stress and show internal damages are vital to monitor failures like fractures, fatigue,
and hysteresis [
71
,
72
]. This color change allows adjusting the materials before disastrous failure and
further increase in the reliability. An example of thermoplastic polyurethane elastomers (TPU-BBS) with
reversible mechanochromic behavior prepared by blended with bis(benzoxazolyl)stilbene dyes is reported
in Cellini’s article [
67
]. Upon large deformation of the polymeric structure, the fluorescence emission
shifted from the excimer band to the monomer band, which is caused by reorganization of dye aggregates.
The largely reversible mechanochromic behavior is able to controlling by the initial dye concentration in
the polymer. Compared with TPU-BBS blends with dye concentrations of 0.1 wt.% and 1.5 wt.%, the one
with 0.5 wt.% showed a higher relative variation of the emission ratio during stretching. These results
provide a promising reference for preparation of mechanochromic sensors (Figure 10).
Polymers 2020, 12, x FOR PEER REVIEW 8 of 16
Figure 9. The digital image of thermochromic luminous phenomena and thermosensitive process
together with luminescence model in darkness [70].
Mechanochromic materials are able to convert mechanical stimuli into optical signals, and thus
the abilities to sense stress and show internal damages are vital to monitor failures like fractures,
fatigue, and hysteresis [71,72]. This color change allows adjusting the materials before disastrous
failure and further increase in the reliability. An example of thermoplastic polyurethane elastomers
(TPU-BBS) with reversible mechanochromic behavior prepared by blended with
bis(benzoxazolyl)stilbene dyes is reported in Cellini’s article [67]. Upon large deformation of the
polymeric structure, the fluorescence emission shifted from the excimer band to the monomer band,
which is caused by reorganization of dye aggregates. The largely reversible mechanochromic
behavior is able to controlling by the initial dye concentration in the polymer. Compared with
TPU-BBS blends with dye concentrations of 0.1 wt.% and 1.5 wt.%, the one with 0.5 wt.% showed a
higher relative variation of the emission ratio during stretching. These results provide a promising
reference for preparation of mechanochromic sensors (Figure 10).
Figure 10. (a) Photographic image of three TPU-BBS samples with concentrations 0.1, 0.5, and 1.5
wt.%. The image is obtained through a commercial linear plastic polarizer. (b) Fluorescence spectra
for the three concentrations are reported in the graph. Spectra are normalized so that the emission is
equal in correspondence of the monomer peak at 436 nm. The fluorescence emission of dye
aggregates increases with dye concentration and is associated with the progressive shift of material
coloration from blue to green [73].
2.5. Shape Memory PU
Shape memory polymers have the ability to remember their original shape after being
deformed and recover it under appropriate stimulus such as temperature, light, electric field,
magnetic field, pH, specific ions, or enzyme [74,75]. The incompatibility between soft segments and
hard segments in PU leads to microphase separation, which depends on block lengths, hydrogen
Figure 10.
(
a
) Photographic image of three TPU-BBS samples with concentrations 0.1, 0.5, and 1.5 wt.%.
The image is obtained through a commercial linear plastic polarizer. (
b
) Fluorescence spectra for the
three concentrations are reported in the graph. Spectra are normalized so that the emission is equal in
correspondence of the monomer peak at 436 nm. The fluorescence emission of dye aggregates increases
with dye concentration and is associated with the progressive shift of material coloration from blue to
green [73].
2.5. Shape Memory PU
Shape memory polymers have the ability to remember their original shape after being deformed
and recover it under appropriate stimulus such as temperature, light, electric field, magnetic field, pH,
specific ions, or enzyme [
74
,
75
]. The incompatibility between soft segments and hard segments in PU
leads to microphase separation, which depends on block lengths, hydrogen bonding, and crystallization

Polymers 2020,12, 1996 9 of 16
extent. The hard segments are related to the fixed points or frozen phases, which remain hard during
temporary shape. The crystalline melting temperature of the soft segment is the shape recovery
temperature. The soft segment is recognized as “molecular switch”, with their crystalline melting
temperature being the shape recovery temperature (T
s
). Reversible phase is controlled upon heating
above Ts, and cooling below Ts, respectively [76–78].
The shape-memory behavior of polymers has been known for over half a century and has been
widespread in numerous applications. Traditionally, shape memory polymers are triggered by heat.
Direct heating is not safe enough and realistic for implementations of many devices, therefore limiting
the use more in complex applications. More recently, there is an increasing trend in the exploration of
multifunctional shape-memory effects [79–81].
The multiple shape effects require at least two segregated domains associated with two distinct
thermal transitions for fixing each temporary shape by the corresponding domain. For example,
Ban [
82
] presented a shape memory PU, which was responsive to both UV light and thermal stimuli.
4,4-Azodibenzoic acid (Azoa) was used to build the PUs because Azoa shows typical trans-cis
photo-isomerization with light [
83
]. This novel PU was shape-deformed under UV light and capable
of shape fixation in visible light. Finally, the shape recovery was facilitated at higher temperature via
weakened hydrogen bond interactions (Figure 11).
Polymers 2020, 12, x FOR PEER REVIEW 9 of 16
bonding, and crystallization extent. The hard segments are related to the fixed points or frozen
phases, which remain hard during temporary shape. The crystalline melting temperature of the soft
segment is the shape recovery temperature. The soft segment is recognized as “molecular switch”,
with their crystalline melting temperature being the shape recovery temperature (Ts). Reversible
phase is controlled upon heating above Ts, and cooling below Ts, respectively [76–78].
The shape-memory behavior of polymers has been known for over half a century and has been
widespread in numerous applications. Traditionally, shape memory polymers are triggered by heat.
Direct heating is not safe enough and realistic for implementations of many devices, therefore
limiting the use more in complex applications. More recently, there is an increasing trend in the
exploration of multifunctional shape-memory effects [79–81].
The multiple shape effects require at least two segregated domains associated with two distinct
thermal transitions for fixing each temporary shape by the corresponding domain. For example, Ban
[82] presented a shape memory PU, which was responsive to both UV light and thermal stimuli.
4,4-Azodibenzoic acid (Azoa) was used to build the PUs because Azoa shows typical trans-cis
photo-isomerization with light [83]. This novel PU was shape-deformed under UV light and capable
of shape fixation in visible light. Finally, the shape recovery was facilitated at higher temperature via
weakened hydrogen bond interactions (Figure 11).
Figure 11. Illustration of the staging-responsive shape memory effect (SME) compared to the
thermally responsive and light-responsive SMEs. (A traditional TSMP is deformed into a temporary
shape and then recovered by “hot programming”, while a traditional LSMP can be deformed under
UV light and promptly recovered using another light stimulus, e.g., Vis light. For the SR-SMPs,
samples were first directionally oriented. They spontaneously deformed into a temporary shape by
“UV programming”, and this deformed shape was unchangeable even under irradiation by Vis light.
Only “hot programming” allowed the initial shape to be recovered [82].)
pH stimulus is also a good candidate for the design of new shape memory PU. The carboxylic
acid in the side chains of waterborne PU is sensitive to pH, which in acid formed dimers, while in
alkaline transform from acid to carboxylate to disrupt the dimers. Moreover, the carboxylic dimers
are demonstrated to be affected by temperature to dissociate and associate as the temperature rise
and down [84]. Based on the above consideration, a polyurethane with thermo-induce triple shape
memory effect and pH-sensitive dual shape memory effect was developed [85]. The glass transition
of soft segments and the association and disassociation of carboxylic dimers are as two switches to
control triple-shape memory property. pH stimulus is realized through the carboxylic dimers to
associating in acid and dissociating in alkaline.
3. Application in Leather Manufacture
3.1. Retanning
Retanning process is honored as the Golden Touch in leather technology, which follows the
primary tanning process to overcome some drawbacks of chrome tannage and could help to
improve the physical-mechanical properties of leathers. Waterborne polyurethane (WPU) retanning
Figure 11.
Illustration of the staging-responsive shape memory effect (SME) compared to the thermally
responsive and light-responsive SMEs. (A traditional TSMP is deformed into a temporary shape and
then recovered by “hot programming”, while a traditional LSMP can be deformed under UV light and
promptly recovered using another light stimulus, e.g., Vis light. For the SR-SMPs, samples were first
directionally oriented. They spontaneously deformed into a temporary shape by “UV programming”,
and this deformed shape was unchangeable even under irradiation by Vis light. Only “hot programming”
allowed the initial shape to be recovered [82].)
pH stimulus is also a good candidate for the design of new shape memory PU. The carboxylic
acid in the side chains of waterborne PU is sensitive to pH, which in acid formed dimers, while in
alkaline transform from acid to carboxylate to disrupt the dimers. Moreover, the carboxylic dimers
are demonstrated to be affected by temperature to dissociate and associate as the temperature rise
and down [
84
]. Based on the above consideration, a polyurethane with thermo-induce triple shape
memory effect and pH-sensitive dual shape memory effect was developed [
85
]. The glass transition of
soft segments and the association and disassociation of carboxylic dimers are as two switches to control
triple-shape memory property. pH stimulus is realized through the carboxylic dimers to associating in
acid and dissociating in alkaline.
3. Application in Leather Manufacture
3.1. Retanning
Retanning process is honored as the Golden Touch in leather technology, which follows the
primary tanning process to overcome some drawbacks of chrome tannage and could help to improve the

Polymers 2020,12, 1996 10 of 16
physical-mechanical properties of leathers. Waterborne polyurethane (WPU) retanning agent, which
disperses in aqueous media, is one of the environmentally friendly leather chemicals in leather industry.
Carboxyl groups ensure excellent solubility; meanwhile, their coordination with Cr
3+
immobilizes
polymer chains in collagens, so as to disperse fibers well (Figure 12).
Polymers 2020, 12, x FOR PEER REVIEW 10 of 16
agent, which disperses in aqueous media, is one of the environmentally friendly leather chemicals in
leather industry. Carboxyl groups ensure excellent solubility; meanwhile, their coordination with Cr3+
immobilizes polymer chains in collagens, so as to disperse fibers well (Figure 12).
Figure 12. Mechanism of polyurethane retanning agent.
Specific properties of final leather can be obtained via retanned by functional WPU. A
fluorescent WPU was reported for leather retanning. This retanning agent shows remarkable
fluorescent stability compared to other leather chemicals, and the resultant leather exhibited an
obvious fluorescence effect [86]. Zhang presented a facile and green approach to prepare
phosphorus-nitrogen-containing waterborne polyurethane/graphene nanocomposite as
flame-retardant retanning agent [87]. Longer flameless combustion time and higher limit oxygen
index (LOI) values of leather retanned by flame-retardant WPU denote the enhanced flame
retardancy and ameliorated smoke suppression.
Another interesting report recently emerged involved a polyurethane retanning agent with the
function of reducing free formaldehyde in leather [88]. Chromotropic Acid (CA) from was used as a
chain extender to synthesize this retanning agent (CAGAPU). Free formaldehyde can react with CA
monomers, forming interaction of two naphthalene rings. As a result, CAGAPU has a positive effect
on reduction of free formaldehyde during the retanning process in leather manufacture. (Figure 13)
The work provides an efficient way to solve the trouble of free formaldehyde in leather.
Figure 13. Reaction mechanism of CAGAPU with formaldehyde [88].
Figure 12. Mechanism of polyurethane retanning agent.
Specific properties of final leather can be obtained via retanned by functional WPU. A fluorescent
WPU was reported for leather retanning. This retanning agent shows remarkable fluorescent stability
compared to other leather chemicals, and the resultant leather exhibited an obvious fluorescence effect [
86
].
Zhang presented a facile and green approach to prepare phosphorus-nitrogen-containing waterborne
polyurethane/graphene nanocomposite as flame-retardant retanning agent [
87
]. Longer flameless
combustion time and higher limit oxygen index (LOI) values of leather retanned by flame-retardant WPU
denote the enhanced flame retardancy and ameliorated smoke suppression.
Another interesting report recently emerged involved a polyurethane retanning agent with the
function of reducing free formaldehyde in leather [
88
]. Chromotropic Acid (CA) from was used as a
chain extender to synthesize this retanning agent (CAGAPU). Free formaldehyde can react with CA
monomers, forming interaction of two naphthalene rings. As a result, CAGAPU has a positive effect
on reduction of free formaldehyde during the retanning process in leather manufacture. (Figure 13)
The work provides an efficient way to solve the trouble of free formaldehyde in leather.
Polymers 2020, 12, x FOR PEER REVIEW 10 of 16
agent, which disperses in aqueous media, is one of the environmentally friendly leather chemicals in
leather industry. Carboxyl groups ensure excellent solubility; meanwhile, their coordination with Cr3+
immobilizes polymer chains in collagens, so as to disperse fibers well (Figure 12).
Figure 12. Mechanism of polyurethane retanning agent.
Specific properties of final leather can be obtained via retanned by functional WPU. A
fluorescent WPU was reported for leather retanning. This retanning agent shows remarkable
fluorescent stability compared to other leather chemicals, and the resultant leather exhibited an
obvious fluorescence effect [86]. Zhang presented a facile and green approach to prepare
phosphorus-nitrogen-containing waterborne polyurethane/graphene nanocomposite as
flame-retardant retanning agent [87]. Longer flameless combustion time and higher limit oxygen
index (LOI) values of leather retanned by flame-retardant WPU denote the enhanced flame
retardancy and ameliorated smoke suppression.
Another interesting report recently emerged involved a polyurethane retanning agent with the
function of reducing free formaldehyde in leather [88]. Chromotropic Acid (CA) from was used as a
chain extender to synthesize this retanning agent (CAGAPU). Free formaldehyde can react with CA
monomers, forming interaction of two naphthalene rings. As a result, CAGAPU has a positive effect
on reduction of free formaldehyde during the retanning process in leather manufacture. (Figure 13)
The work provides an efficient way to solve the trouble of free formaldehyde in leather.
Figure 13. Reaction mechanism of CAGAPU with formaldehyde [88].
Figure 13. Reaction mechanism of CAGAPU with formaldehyde [88].

Polymers 2020,12, 1996 11 of 16
3.2. Finishing
Finishing is able to modify the shade/gloss/handle, hide any defects or irregular appearance,
improve physical properties, and offer functional properties to the final leather. A series of mechanical
operations are carried out during this processes. Normally, polymeric coatings are applied to
the leather surface. Among diverse alternatives, PU has recently become the most widely used
coating-forming material. For instance, Wang reported an anti-biofouling PU with zwitterionic
sulfobetaine side groups in leather coatings [
89
]. The incorporation of zwitterionic groups into the
hard segment of polyurethane created more hydrogen bonding and polar interactions within this
region, and thus made the hard components more thermodynamically and thus incompatible with the
soft segments. The presence of zwitterionic group results in good anti-mold adhesion performance of
polyurethane coatings. The appearance of this coating may extend the lifespan of the leather product.
Although it has antimicrobial adhesion effect, these zwitterionic polyurethanes are not bactericidal.
In this regard, Xu [
90
] covalently conjugated sulfanilamide (SA) into PU backbone as chain extender,
and yielded a PU with enzymatically switchable antimicrobial capability as leather-finishing material.
The contaminant-derived urease was utilized as trigger targets, SA can be released from the covalent
bonded PU coatings. Without urease, the SA-conjugated PU coating displayed excellent hydrolytic
resistance, which stopped any SA release from the conjugation. The antimicrobial ability was able to
rapidly switch on, in case of exposure to urease. This freely delivered SA by urease-catalyzed break of
urea linkages (Figure 14).
Polymers 2020, 12, x FOR PEER REVIEW 11 of 16
3.2. Finishing
Finishing is able to modify the shade/gloss/handle, hide any defects or irregular appearance,
improve physical properties, and offer functional properties to the final leather. A series of
mechanical operations are carried out during this processes. Normally, polymeric coatings are
applied to the leather surface. Among diverse alternatives, PU has recently become the most widely
used coating-forming material. For instance, Wang reported an anti-biofouling PU with zwitterionic
sulfobetaine side groups in leather coatings [89]. The incorporation of zwitterionic groups into the
hard segment of polyurethane created more hydrogen bonding and polar interactions within this
region, and thus made the hard components more thermodynamically and thus incompatible with
the soft segments. The presence of zwitterionic group results in good anti-mold adhesion
performance of polyurethane coatings. The appearance of this coating may extend the lifespan of the
leather product. Although it has antimicrobial adhesion effect, these zwitterionic polyurethanes are
not bactericidal. In this regard, Xu [90] covalently conjugated sulfanilamide (SA) into PU backbone
as chain extender, and yielded a PU with enzymatically switchable antimicrobial capability as
leather-finishing material. The contaminant-derived urease was utilized as trigger targets, SA can be
released from the covalent bonded PU coatings. Without urease, the SA-conjugated PU coating
displayed excellent hydrolytic resistance, which stopped any SA release from the conjugation. The
antimicrobial ability was able to rapidly switch on, in case of exposure to urease. This freely
delivered SA by urease-catalyzed break of urea linkages (Figure 14).
Figure 14. Schematic illustration of possible mechanism by which the SA-conjugated PU leather
coating functions [90].
We prepared a diverse color-tunable luminous polyurethane leather coating (CLPU) which can
reversibly change color as well as fluorescent emission through the UV–Vis or UV–darkness circle,
via covalent incorporation of amino-functionalized phosphors and photochromic
1-(2-hydroxyethyl)-3,3-dimethylindolino-6′-nitrobenzopyrylospiran (SP) [91]. Before UV irradiation,
SP was colorless, which allowed the phosphors green light to pass through the polyurethane matrix.
The colored open-ring merocyanine (MC) induced by UV irradiation formed an optical filter and
shielded most of the energy from the phosphors. As a result, only part of the luminescent can pass
through resulting in an orange coloration. Finally, the finished leather exhibited diverse
color-tunable luminous phenomena. In a different study, a photosensitive silicone-containing
polyurethane acrylate resin (Si-IPDI-HEA) was prepared for leather finishing [86]. The properties of
polysiloxane and polyurethane are maintained. Besides, polymerization of Si-IPDI-HEA can rapidly
realized under UV irradiation. The import of silicon into pre-polymer could improve the
thermostability. The dispersion surface energy of the UV-cured film can be reduced by the change of
microstructure as well.
Figure 14.
Schematic illustration of possible mechanism by which the SA-conjugated PU leather coating
functions [90].
We prepared a diverse color-tunable luminous polyurethane leather coating (CLPU) which can
reversibly change color as well as fluorescent emission through the UV–Vis or UV–darkness circle,
via covalent incorporation of amino-functionalized phosphors and photochromic 1-(2-hydroxyethyl)-
3,3-dimethylindolino-6
0
-nitrobenzopyrylospiran (SP) [
91
]. Before UV irradiation, SP was colorless,
which allowed the phosphors green light to pass through the polyurethane matrix. The colored
open-ring merocyanine (MC) induced by UV irradiation formed an optical filter and shielded most of
the energy from the phosphors. As a result, only part of the luminescent can pass through resulting in
an orange coloration. Finally, the finished leather exhibited diverse color-tunable luminous phenomena.
In a different study, a photosensitive silicone-containing polyurethane acrylate resin (Si-IPDI-HEA) was
prepared for leather finishing [
86
]. The properties of polysiloxane and polyurethane are maintained.
Besides, polymerization of Si-IPDI-HEA can rapidly realized under UV irradiation. The import of
silicon into pre-polymer could improve the thermostability. The dispersion surface energy of the
UV-cured film can be reduced by the change of microstructure as well.

Polymers 2020,12, 1996 12 of 16
4. Summary and Outlook
Since first prepared by Bayer in the 1940s, polyurethane has been one of the most common,
versatile, and researched materials in the world. PUs can be produced from a variety of diisocyanates,
polyols, chain extenders, and crosslinking agents, making it possible to obtain a wide range of tailored
materials. There is no doubt that functional PU, such as anti-fouling PU, self-healing PU, antibacterial
PU, luminescent and color-tunable PU, as well as shape memory PU, will be the main research points
in both academic and commercial fields. Even if a lot of progress has made during the last few decades,
various new and intriguing challenges remains to be resolved. For example, the demand for PU
products is increasing day by day, so recyclability of the product is of great significance. Majority of
studies on the use of vegetable oils as substitutes to petroleum based materials for PU production.
Nevertheless, there are certain drawbacks associated with these kinds of materials especially in
regard to performance. Recent advances have focused on bi-functional PU, even multifunctional PU.
The enormous number of available functional PU open unlimited possibilities for the development of
advanced leather products. We expect that these concepts may steer innovative, smart, intelligent,
environmental, and recycling materials to be used in daily life.
Funding:
This research was funded by Introduce Talent Foundation of Wenzhou University grant
number [135010120719].
Acknowledgments:
Reproduced from the works in [
64
,
76
] with permission from the Royal Society of Chemistry.
Reprinted with permission from the authors of [
29
]. Copyright (2018) American Chemical Society. Reprinted with
permission from the authors of [47]. Copyright (2016) American Chemical Society.
Conflicts of Interest: The author declares no conflict of interest.
Abbreviations
PU polyurethane
TiO2/rGO TiO2/reduced grapheneoxide
CTAB cationic surfactant hexadecyl trimethyl ammonium bromide
MO methyl orange
TPU thermoplastic polyurethane
DA Diels Alder
DiSe di-(1-hydroxyundecyl) diselenide
GAA guanidinoacetic acid
TP thermochromic pigment
TPU-BBS thermoplastic polyurethane elastomers blended with bis(benzoxazolyl)stilbene dyes
Tsshape recovery temperature
Azoa 4,4-azodibenzoic acid
SME shape memory effect
WPU waterborne polyurethane
LOI limit oxygen index
CA chromotropic acid
SA sulfanilamide
CLPU color-tunable luminous polyurethane
SP 1-(2-hydroxyethyl)-3,3-dimethylindolino-60-nitrobenzopyrylospiran
MC merocyanine
Si-IPDI-HEA silicone-containing polyurethane acrylate resin
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