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Introduction to polymer adhesion

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Ever since the invention of polymeric material it is widely accepted in various industries. However, as adhesives polymer has low surface free energy and insufficient polar groups which results in poor adhesion. Therefore, an understanding of the polymer adhesion mechanism is absolutely necessary in order to investigate the adhesion strength promotion. This paper reviews recent research papers to updates the many debating adhesion theories and the adhesion improvement methods for polymer bonding. In part I, seven different theories/models of polymer adhesion mechanisms were proposed with a general introduction to polymer adhesion background. They are mechanical interlocking model, electrostatic theory, diffusion theory, wettability model, acid-based theory, weak boundary layer theory and chemical bonding theory. In part II, technologies and methods for adhesion improvement on contact surface areas between polymer and ceramics, metal, other polymer and et.al. were summarized, followed by a conclusion section.
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Introduction to polymer adhesion
Dr. Amyl Ghanem, Yujie Lang
Department of process engineering and applied science, Dalhousie University, 1360 Barrington Street,
Halifax, Nova Scotia, Canada
Abstract
Ever since the invention of polymeric material it is widely accepted in various industries. However, as
adhesives polymer has low surface free energy and insufficient polar groups which results in poor adhesion.
Therefore, an understanding of the polymer adhesion mechanism is absolutely necessary in order to
investigate the adhesion strength promotion. This paper reviews recent research papers to updates the many
debating adhesion theories and the adhesion improvement methods for polymer bonding. In part I, seven
different theories/models of polymer adhesion mechanisms were proposed with a general introduction to
polymer adhesion background. They are mechanical interlocking model, electrostatic theory, diffusion
theory, wettability model, acid-based theory, weak boundary layer theory and chemical bonding theory. In
part II, technologies and methods for adhesion improvement on contact surface areas between polymer and
ceramics, metal, other polymer and et.al. were summarized, followed by a conclusion section.
Key words: polymer, adhesion, molecular bonding, mechanical interlocking, thermodynamic, diffusion,
ceramics, metal, tissue, surface roughness
Contents
1. Polymer adhesion mechanism .............................................................................................................. 2
1.1 Introduction to polymers as adhesives ........................................................................................ 2
1.2 Background terms of adhesion mechanism................................................................................. 2
1.2.1 Surface free energy ......................................................................................................... 2
1.2.2 Surface tension ................................................................................................................ 2
1.2.3 Intrinsic adhesion ............................................................................................................ 3
1.2.4 Plasma treatment ............................................................................................................. 3
1.3 Theories of adhesion ................................................................................................................... 3
1.3.1 Mechanical interlocking .................................................................................................. 3
1.3.2 Electrostatic theory ......................................................................................................... 4
1.3.3 Diffusion theory .............................................................................................................. 4
1.3.4 Wettability theory ............................................................................................................ 5
1.3.5 Acid-base theory ............................................................................................................. 5
1.3.6 Weak boundary layer....................................................................................................... 6
1.3.7 Chemical or molecular bonding ...................................................................................... 6
2 Surface treatment and adhesion improvement .................................................................................. 7
2.1 Polymer-ceramics adhesion ........................................................................................................ 7
2.1.1 Decreasing and cleaning ................................................................................................. 7
2.1.2 Roughening ..................................................................................................................... 7
2.1.3 Acid-etch ......................................................................................................................... 8
2.1.4 Silane treatment .............................................................................................................. 8
2.1.5 Laser treatment ................................................................................................................ 9
2.1.6 Flame treatment .............................................................................................................. 9
2.2 Polymer-metal adhesion .............................................................................................................. 9
2.2.1 Plasma treatment ............................................................................................................. 9
2.2.2 Laser treatment ................................................................................................................ 9
2.2.3 Silane coupling agents ..................................................................................................10
2.2.4 Grafting .........................................................................................................................10
2.3 Polymer-polymer adhesion .......................................................................................................10
2.3.1 Flame treatment ............................................................................................................10
2.3.2 Plasma treatment ...........................................................................................................10
2.3.3 Corona treatment ...........................................................................................................11
2.3.4 Chemical treatment .......................................................................................................11
2.3.5 UV radiation treatment ..................................................................................................11
2.4 Polymer-fiber adhesion .............................................................................................................11
2.5 Polymer-glass adhesion.............................................................................................................11
2.6 Polymer-wood adhesion ............................................................................................................11
3 Conclusion ...........................................................................................................................................12
References.....................................................................................................................................................13
1. Polymer adhesion mechanism
1.1 Introduction to polymers as adhesives
An adhesive is a material applied to surfaces
to permanently join them by a bonding process. It
is a substance capable of forming bonds to each of
the two or more part interfaces comprising the final
object [1]. The resulted adhesion from applying
adhesive on substrate surfaces is the interatomic
and intermolecular interaction at the interface [2].
Due to a competitive pricing, polymer
adhesive is widely accepted and applied worldwide.
The automobile and aerospace industries have used
polymers in an adhesive system found in the
automotive industry: the attachment of a paint
coating to a polymer bumper bar [2]. In
civil/building industry, polymeric material also
plays a major role as polymer sealant. A good
application of polymer adhesive in biomedical
industry would be polymer glue gun to fix and
repair human bones during surgery [3]. Although
the study of adhesion mechanisms can be traced
back to the 1930s [4], the adhesion mechanism of
polymers until today is still under debate. An
understanding of polymer adhesion mechanism is
therefore of growing importance.
1.2 Background terms of adhesion mechanism
1.2.1 Surface free energy
The excess energy necessary to form a unit
area of new surface or the energy necessary to move
a molecule from the bulk to the surface [4].
1.2.2 Surface tension
Surface tension is defined as the work required
to increase the area of a surface isothermally and
reversibly by unit amount. Surface tension of
polymers can be divided into two components:
polar (γp) and dispersion (γd), to account for the
type of attraction forces at the interfaces. Polar
component is comprised of various polar molecular
interactions, including hydrogen bonding, dipole
energy, and induction energy, whereas the
dispersion component arises from London
dispersion attractions [5].
The attractive forces (van der Waals and
London dispersion) are additive, which results in
the surface tension components being additive:
γ = γp + γd [5].
1.2.3 Intrinsic adhesion
Intrinsic adhesion is the direct molecular
forces of attraction between the adhesive and the
substrates [4]. Osouli-Bostanabad et al. suggested
that to make adhesion strong enough, it is necessary
to excite the intrinsic adhesion forces such as
dipoles across the interface which consequently
increases a bonding strength due to Van der Waals
forces; but secondary forces activation depends on
surface regulation levels.
1.2.4 Plasma treatment
Plasma (glow discharge) sometimes refers to
the fourth state of matter. It is produced by exciting
a gas with electrical energy. It is a collection of
charged particles containing positive and negative
ions [6]. It can be used to treat parts to make their
surfaces harder, rougher, more or less wettable, and
more conducive to adhesion [6].
Figure 1.2.4.1: Introduction of plasma state
Different types of plasma treaments such as
argon, oxygen, nitrogen, fluorine, carbon dioxide,
and water can produce the unique surface properties
required by various applications. For example,
oxygenplasma treatment can increase the surface
energy of polymers, whereas fluorineplasma
treatment can decrease the surface energy and
improve the chemical inertness [7].
Corona treatment and low pressure and
atmospheric pressure plasma treatment methods are
all based on the dielectric barrier discharge
phenomenon [7].
Figure 1.2.4.2: Complexity of operations and cost
of treatment methods based on dielectric barrier
discharge [6].
1.3 Theories of adhesion
Due to the complexity of adhesion mechanism,
its phenomena is hard to be described by a single
theory. Thus far, seven theories can be attributed for
polymeric bonding with valid reasons. They are
mechanical interlocking, electrostatic, diffusion,
wettability, chemical bonding, weak boundary layer
and acid-base. The last four mechanisms are all
based on adsorption/surface reaction. All seven
adhesion theories are summarized in table 1.3.1.
Table 1.3.1: Summary of adhesion theories [7]
No.
Theories of
adhesion
Scale of action
1
Mechanical
interlocking
Macroscopic
2
Electrostatic
Macroscopic
3
Diffusion
Molecular
4
Wettability
Molecular
5
Acid-base
Molecular
6
Weak boundary
layer
Molecular
7
Chemical
bonding
Atomic
1.3.1 Mechanical interlocking
The mechanical interlocking model was first
introduced by MacBain in 1925. The model
suggests that the adhesion is induced by the
mechanical keying of the adhesive into pores,
cavities, and other surface irregularities of the
substrate/adherend surface [4, 7], after the
displacement of the trapped air at the interface. The
determinant factors are the degree of roughness,
porosity and the surface area available for bonding,
with sufficient wettability by the adhesive [4].
Consequently, an abraded, porous surface would
result in higher adhesion than smooth surface.
Figure 1.3.1.1 is a demonstration of mechanical
interlocking model [2].
Figure 1.3.1.1: Illustration of mechanical coupling
between two substrates [2].
Mechanical interlocking theory is one of the
earliest proposals for adhesion mechanism and yet
it is not universally accepted because very strong
adhesion can also occur between smooth surfaces
[7]. Osouli-Bostanabad et al. [8] revealed that the
polished surfaces have shown the highest adhesion
strength among steel surfaces with different
roughness levels; it also can be observed that by
reducing surface roughness, the adhesion strength
between the GFRECPs and steel component
improves.
Moreover, mechanical interlocking model
does not consider factors of molecular level for
adhesive and substrate adhesion into consideration.
Therefore, this theory can only be used as an
introductory level of adhesion mechanisms.
1.3.2 Electrostatic theory
The electrostatic theory applies the electrical
double layer concept for the explanation of
electrostatic charges formation. Essentially, as a
result of unlike electronic band structures, electrons
transfer/attraction happen between the adhesive and
the adherend (as electrostatic effects), resulting in
adhesion between the adhesive and the substrate [7].
Figure 1.3.2.1: Electrical double layer at polymer-
metal interfaces [9]
Specifically, forces of attraction occur
between two surfaces when one surface carries a net
positive charge and the other surface carries a net
negative charge [4].
The electrostatic theory is the widely accepted
model for the description of adhesion mechanism
and provides helpful insights for the bonding
between polymer and metal [7]. One thing should
be noted is that this model is only applicable for
incompatible materials. In this case it means
polymer and metallic adherend [4].
1.3.3 Diffusion theory
The diffusion theory was proposed by
Voyuskiin in 1963 [10]. This theory explains that
interdiffusion of the macromolecules between the
adhesive and adherend would lead to adhesion.
According to the diffusion theory, it would be
applicable when both the adhesive and the adherend
are mutually miscible and compatible polymers
with relatively long-chain molecules capable of
movement [4, 7]. Figure 1.3.3.1 presents the
diffusion adhesion theory in a graphic way by
Fourche et al [11].
Figure 1.3.3.1: Diffusion theory of adhesion (a)
Interdiffusion of adhesive and (b) substrate
molecules [11].
The illustration provides a vivid explanation of
how the adhesion between two polymers can occur:
A transient zone can be formed by the interaction
between the interdiffusion of the macromolecules
of the superficial layers, resulting in adhesion
between the adhesive and the adherend.
In this theory, the macromolecules is vital in
determining interaction process and adhesion
strength and it is influenced by the chain length of
the macromolecule, the concentration, and the
temperature.
Several mathematical equations can be applied
for relative theoretical calculations.
Molecular diffusion coefficient:
(1) [4]
where η is the bulk viscosity, A is Avogadro’s
number, ρ is the density, k is Boltzmann’s constant,
M is the molar mass distribution and T is the
absolute temperature.
Cohesive energy density:
(2) [7]
Solubility parameter:
(3) [7]
Where Ecoh is the amount of energy required to
separate molecules to an infinite distance, V is the
molar volume, and δ is the solubility parameter.
Nonetheless, the diffusion theory can only
explain the bonding of rubbery polymers and
solvent welding thermoplastics [12] and is still
under debate.
1.3.4 Wettability theory
Theoretically, wetting is a process of
establishing continuous contact between the
adhesive and the adherend [7]. The wettability
theory proposed that molecular interactions of two
surfaces and the developed surface forces would
contribute to the adhesion formation [7].
A complete wetting would be having the
adhesives filled in all the crevices on the substrate
surface, maximizing the contact area. An
incomplete wetting, on the other hand, would be
having less contact surface area and generating
interfacial defects [7]. Examples of a good wetting
and a bad wetting are demonstrated in figure 1.3.4.1.
Figure 1.3.4.1: Examples of good and poor wetting
by an adhesive spreading across a surface [13].
As a result, a lower surface tension of the
adhesive than the adherend is a necessity when
conducting good wetting ability [7].
1.3.5 Acid-base theory
Compared with other conventional adhesion
mechanism models, the acid-base theory is a new
approach to explain the adhesion of polymer on
substrates.
The acid-base theory is based on the chemical
concept of Lewis acid and base, which was came up
with by J. N. Bronsted and G. N. Lewis separately
back in the early 20th century [7]. This theory
suggest that the adhesion can be resulted from the
attraction of Lewis acids and bases (cations and
anions) at the interface which comes from the
interactions between compounds capable of
electron donation and acceptance [7].
The relation of measurement of the degree of
acidbase interaction which is based on the
enthalpy of adduct formation is shown in equation
4 as follows:
(4) [14]
where −ΔHab is the enthalpy of adduct formation
per mole.
1.3.6 Weak boundary layer
The weak boundary layer theory was proposed
by Bikerman. The theory approaches the adhesion
mechanism by stating that for performing optimum
adhesion results, there should be no weak boundary
layer or a cohesive break, of which have negative
impact on the failure at the interface between
adhesive and adherend [4, 7]. A weak boundary
layer can be caused by the impurities in the contact
surfaces of either the adhesive or the adherend [4].
Also bonding environment and plasma treatment
can be factors for the formation of a weak boundary
layer. The previous one would not make the
adhesive wet the adhered and cause a weak
boundary layer to form and the latter one would
degrade polymeric substrates leading to a weak
boundary layer [4].
Figure 1.3.5.1: Seven classifications of weak
boundary layers [15].
Figure 1.3.5.1 demonstrates seven different
classifications of weak boundary layers and it can
be observed that air pores, impurities and reactions
between adhesives and adherents are major
contributes.
1.3.7 Chemical or molecular bonding
The chemical or molecular bonding theory is
one of the oldest proposed theories among all and
yet the most widely accepted one for describing the
adhesion mechanism. Hutchinson and Iglauer
[16]studied tack and peel tests of foam and sealants
used in building construction and found that
chemical bonding of the substrates at the interface
is the primary adhesive mechanism, with no sign of
diffusion/interdiffusion or electrostatic effect and a
marginal effect caused by mechanical interlocking.
The chemical bonding theory proposed that
the adhesion mechanism can be attributed to the
interfacial forces and the presence of polar groups
from two similar or dissimilar intimate contact
materials [2, 4, 7]. However, to achieve a good
adhesion, intimate contact alone is not sufficient
enough due to the presence of defects, cracks and
air bubbles [17]. It entails intermolecular forces
between adhesive and substrate such as dipole-
dipole interactions, van der Waals forces and
chemical interactions such as ionic bonding,
covalent bonding and metallic bonding [4]. A
demonstration of molecular bonding mechanism is
shown in figure 1.3.7.1.
Figure 1.3.7.1: Molecular bonding between two
substrates [2]
The exact nature of the interactions for a given
adhesive bond depends on the chemical
composition of the interface [2]. In terms of
adhesion improvement, the adhesion promotor
molecules are called coupling agents and is used to
improve the adhesion strength of the interface area
between adhesives and adherends [18, 19]. The
coupling agents achieve this purpose by reacting
with both the substrate and the adhesive chemically
in order to serve as a chemical bridge at the
interface [4]. Also, one thing worth pointing out is
that silane molecules are the most common tyoe of
adhesion promotors for coupling agents [4].
2 Surface treatment and adhesion
improvement
Most industrially applied polymer resins and
composites have low surface energy and lack polar
functional groups on their surface, resulting in poor
adhesion properties [4]. Therefore, because
adhesive bonding is a surface phenomenon,
preparation prior to bonding is crucial for the
development of strong, durable adhesive joints [20].
As a result, it is crucial to apply surface
treatments prior to bonding as the later formation of
a weak layer on the substrate surface can be
removed and prevented at the beginning. The
interactions between the adhesive and the substrate
surfaces can also be maximized for sufficient joint
strength. [1].
Adhesive thickness and temperature change
can be taken into consideration for adhesion
strength modification. The effect of adhesive
thickness on tensile and shear strength of a
polyimide adhesive has been investigated. Results
shown that the tensile strength of the butt joints
decreased with increasing adhesive thickness and
zero effect on shear strength of single lap joints [21].
Besides, temperature is also important in
determining the surface behaviors. Reis et al. [22]
stated that as temperature increases, both stiffness
and ultimate tensile strength decrease for
PolyAnchor 4100 HTP adhesive.
2.1 Polymer-ceramics adhesion
In modern society, ceramics play a vital role in
various area applications such as teeth repair in
dentistry and avoidance of corrosion in industry.
Theoretically, ceramics can be classified as
crystalline ceramics and non-crystalline ceramics.
Compared with polymeric materials, ceramics have
inherently high surface energy and a low coefficient
of thermal expansion (CTE). They are also usually
wetted at contact angles less than 90°, which is
lower than that of most plastics [23].
The bonding surface of ceramics must be
completely cleaned to remove all oils, grease, and
organic contaminants as they are acting as
inhibitors for forming adhesive bonding [23]. The
contamination would also reduce the surface free
energy of the ceramic substrates [24]. Over years,
many treatments have been used for modifying the
adhesion strength of polymer bonding to ceramics.
Common ceramic treatment methods for adhesive
bonding purpose are listed in table 2.1.1 [23].
Table 2.1.1: Techniques applied for ceramic
bonding treatments [23]
No.
Methods
1
Degreasing and
cleaning
2
Roughening
3
Acid etch
4
Silane treatment
5
Laser treatment
6
Flame treatment
2.1.1 Decreasing and cleaning
Decreasing and cleaning of the surface is an
important step in treatment as unnecessary dirt and
impurities can be cleaned away to increase adhesion
strength when bonding. They can be accomplished
by methyl ethyl ketone, acetone, or iso-propanol
wash with a metal brush if it is required. The
washing step is followed by drying with ambient air
or clean compressed air. The determination of
whether the surface is clean enough is to place a
couple of water drops on the treated surface area. If
the water spreads to cover the area with a
continuous film, the bond area is clean. If the water
beads, that means the surface needs further
treatment [23].
2.1.2 Roughening
Roughing on the surface or abrasion is
important in achieving higher adhesion strength as
it would obtain the strongest and most durable
bonds. Abrading a surface removes surface films,
scale, and oxides and also gives a more suitable area
for the adhesive to contact or grip [23].
For ceramics materials grit blasting is superior
to sand paper (120200 grit), emery cloth, or steel
wool [23]. In an attempt to test the effects of various
treatments on the bond strength of porcelain
materials, Thurmond [25] discovered that grit
blasting tends to lead to a better adhesion strength
as it removes loose substances on the bonding
interface as well as increases the surface contact
area of both adhesives and adherends.
2.1.3 Acid-etch
The treatment of acid-etching is to apply acid
on a ceramic surface in order to create pores for
better bond strength of an adhesive to the surface.
Examples of etching acids include hydrofluoric,
phosphoric, and hydrochloric acids.
Etching of dental porcelain with hydrofluoric
acid, HF, followed by silanization was reported to
give higher bond strength of resin composite to
porcelain as compared to without acid etching [26].
However, as acid-etching is not an effective
surface treatment for acid-resistant ceramics, other
methods to produce micromechanical retention
have been used, including airborne particle
abrasion systems and coarse diamond rotary
instruments [27].
However, some pointed out that the acid-
etching process is not always beneficial to the
enhancement of the surface bond strength of two
substrates. Kato et al [28] after their experiment of
testing the effect of etching and sandblasting on
bond strength to sintered porcelain of unfilled resin,
stated that some acid residues from the etching
process tend to remain in the surface pores of
ceramic adherend and can damage the adhesive
layer and weaken or cause failure of the bond.
Also, some acids in use for treatment such as
hydrofluoric acid (HF) can arise health concern and
destroy nearby bonding surface as it is toxic and
corrosive. To avoid working with the hazardous HF,
acidulated phosphate fluoride is used [23].
2.1.4 Silane treatment
Mechanism
Silane groups usually have two different
reactive groups. One group is reactive to the
substrate and the other to the adhesive [23]. Silane
treatment is proved to greatly improve the adhesion
strength on the interfacial layer of the adhesive and
the adherend.
In terms of how silane agents work, silicon-
hydroxyl (silanol) groups are formed in the bonding
process with silanes, this would then form bonds
with inorganic surface hydroxyl groups from
substrates by covalent or hydrogen bonding, thus
promote the adhesion strength. The X, or organic,
group on the silane is typically a reactive group with
which the adhesive will react or interact. Figure
2.1.4.1 and 2.1.4.2 present the basic structure of a
silane agent, an example of silane agent and the
mechanism of silane agent bonding to the substrates.
[23].
Figure 2.1.4.1: (a) General structure of silanes.
Si=silicon; RO=interacts/reacts with inorganic
materials (ceramic); X=Reactive groups form bond
with organic materials (adhesive). (b) An example
of a silane coupling agent is 3-isocyanatopropyl
triethoxy silane [23].
Figure 2.1.4.2: Reaction mechanism of silane with
iscocyanate end group with a hydroxyl group [23].
Results
The application of silane coupling agents on
ceramics are mostly found in dental area. Research
shows that airborne particle abrasion methods using
alumina particles or silicamodified alumina
particles (silica-coated) produced greater surface
roughness values and that silica-coated surfaces
showed a significant increase (76 %) in the
concentration of silicon, which should enhance
bonding to resin via silane coupling agents [29, 30].
Based on the results performed by Bona et al.
[27], silane bonds to Si-OH on a ceramic surface by
condensation reaction and the methyl methacrylate
double bonds provide bonding to the adhesive.
Good bonding can be achieved when there are
adequate SiOH sites on the ceramic surface.
2.1.5 Laser treatment
Recent studies have shown that laser
treatment can result in great improvement of
bonding strength and the effectiveness of laser
treatment is dependent on the type of ceramic [31].
With the use of laser treatment on ceramic
surfaces, the ultraviolet radiation at the very top
surface layer can be absorbed which could cleans
up surface contaminants such as fluorocarbons and
silicones completely, and also provide an extremely
effective means for structuring ceramic surfaces via
the action of ablation, hence improving the
adhesion bond strength [23].
2.1.6 Flame treatment
Flame treatment is a recent technology applied
on ceramics treatment. Jenda et al. [32] in 2003
started using flame treatment on ceramics surface
treating as a new technology. Specifically, the
PyrosilPen flame treatment technology was
investigated by surface-treating silicate, aluminum
oxide, and zirconium oxide ceramics. The results
showed that although flame treatment can improve
certain adhesion strength sandblasting tends to
result in a stronger bond [23].
2.2 Polymer-metal adhesion
In comparison with polymeric materials,
metals have high surface energy which leads to
stronger adhesion strength. However, most metals
require unique methods of treatment for optimal
bond strength formation [33].
2.2.1 Plasma treatment
Plasma treatment was started to put to use for
the treatment of surface adhesion improvement
after 2000s. Over time, various types of plasma
treatments are developed and tested. Nitrogen
plasma is proved to have a positive effect in
increasing adhesion between metal and polymer
substrates. The current plasma treatments include
nitrogen plasma treatment, atmospheric pressure
plasma treatment, low pressure plasma tretament
and cold plasma treatment.
Soroceceanu et al. [34] discovered that after
nitrogen treatment the cohesion interactions are
lower than the adhesion ones and an increase in
surface polarity was reflected in a higher work of
adhesion between metal and polymer substrates,
particularly with gold.
The test with atmospheric plasma treatment
also shows positive results. With the atmospheric
plasma treatment on the surface of polyimide (PI)
sheet (for joining to titanium with high temperature
adhesive), it was observed that the surface energy
increases with increase in exposure time [35]. In
comparison with other available methods
(Chemical treatment and physical treatment),
atmospheric pressure plasma treatment generated a
higher hydrophilic behavior, which presented a
better outcomes of adhesion strength [36, 37].
It has been shown that with the use of low
pressure microwave plasma treatment, the steel
surfaces have witnesses a great enhancement on
adhesion with an epoxy adhesive. Enhanced
wettability of the steel surface was attributed to
increased surface roughness and oxide deposition
[38, 39].
Cold plasma treatment represents an efficient,
non-polluting and economical alternative to clean
and activate aluminium surfaces. In particular, it
has been demonstrated that an oxygen cold plasma
treatment improves wettability and adhesion of the
Al2024 surface [40].
2.2.2 Laser treatment
The effectiveness of laser treatment on
adhesive bonding is also studied by researchers.
Rotella et al. [41] indicated that laser irradiation
effectively enhances the overall mechanical
behavior of the joint under shear and peel loading.
Titanium also performs great adhesion
improvement after applying laser treatment when
bonding with epoxy materials [42].
2.2.3 Silane coupling agents
Silane is probably the most common adhesion
promotor applied in adhesion improvement. In the
study of adhesion between uncured natural rubber
and carbon steels (CS) by the surface modification
of the CS with silane coupling agents comprising
amino, thiol, glycidoxy, and isocyanate organic
functionality, adhesion was optimized and when 3-
(trimethoxysilyl)propylamine (APS) was used to
modify the CS [43].
2.2.4 Grafting
Chen et al. [44]discovered that when applying
grafting maleic anhydride onto polypropylene, an
improvement of shear strength of aluminium-
polypropylene lap joints can be observed. It is due
to the chemical interactions between OH, Al3+ or
amino group NH2 at the surface of the aluminium
sheets and the polar functional anhydride groups
and carboxylic groups COOH on PP-g-MAH at
the interface.
2.3 Polymer-polymer adhesion
An important and relevant difference between
metals and plastics is their surface energy. Polymers
have inherently lower surface energy than metals
and tend to form intrinsically poor adhesion bonds.
Treatment only impacts the region near the
surface and does not alter the bulk properties of the
plastic parts. Not all methods have wide
commercial application. Some of the techniques are
limited in the scope of their use. For example,
chemical treatment (acid-induced oxidation) is the
most frequently used method to impart adherability
to plastic surfaces. Plasma treatment is limited to
smaller components and parts. Flame and corona
treatments are effective for continuous films (often
called webs) and thin sheets of plastic, usually
operated at high speeds [6].
Figure 2.3.1: classification of polymer-polymer
treatment
2.3.1 Flame treatment
Flame treatment is a well-known method of
surface treatment to impart adherability to a number
of plastics such as polyolefins and polyvinyl
fluoride. Flame treatment oxidizes the surface of
polymeric materials to introduce polar reactive
groups such as hydroxyl and carboxyl which
improves surface free energy and consequently the
wettability and adherability of their surfaces [1].
2.3.2 Plasma treatment
Plasma treatment oxidizes the surface of the
polymer in the presence of oxygen. It can thus
remove organic contaminants from the surface.
Early studies have concluded that the crosslinking
of low molecular weight surface species is the
mechanism for eliminating a weak boundary layer.
More recent research has attributed the
effectiveness of plasma treatment to surface
cleaning, ablation of surface polymer chains,
surface crosslinking of polymer chains, and
introduction of polar functional groups that result in
increased surface energy [45]. After examination,
both flame and plasma treatment are effective
methods for improvement of adhesion strength [46].
With the use of low pressure plasma treatment,
the adhesion strength of LDPE films to polyolefin
foams and polyphenylene sulfide (PPS) were
observed to be greatly improved [47, 48].
2.3.3 Corona treatment
Corona treatment is believed to roughen the
plastics by the degradation of amorphous regions of
the polymer surface. The belief is that corona
treatment does not impact the crystalline region of
the surface, preferentially attacking the relatively
weak amorphous regions. Degradation and
subsequent removal of the amorphous material
leads to the increased roughening of the surface of
plastics such as polyethylene. A rough surface
provides a much larger adhesive contact area than a
smooth surface [45].
2.3.4 Chemical treatment
Chemical treatment or etching oxidizes the
plastic surface similarly to corona treatment. For
instance, chromic acid is used to etch the surface of
polyethylene and polypropylene. An increase in
etching time and temperature intensifies the surface
treatment by increasing the degree and depth of
oxidation [45].
In an attempt to increase the surface adhesion
with silane-based treatment, results showed that the
silane compound with epoxy functional group
significantly increased adhesion strength of acrylic
lacquers with carboxyl functionality to the flame-
treated PP surface [49].
Rosin acid can be used as an addition during
thermoplastic polyurethane synthesis to adhesion to
PVC materials. The increase in the amount of rosin
acid in the prepolymer led to an increase in average
molecular weight and the viscosity of TPU
solutions, improved rheological properties, reduced
crystallinity and slower kinetic of crystallization
[50].
2.3.5 UV radiation treatment
The effect of exposure to different ozone
concentrations, in conjunction with UV radiation,
on the surface modification and adhesion properties
of a block synthetic styrene-butadiene-styrene (S6)
rubber was studied. The paper indicated that
adhesion was highly improved after UV and UV/O3
treatments of S6 rubber, more markedly with
increasing treatment time [51].
Also, Landete-Ruiz and Martin-Martinez
[52]found out that the adhesion between a
polychloroprene adhesive and an ethylene vinyl
acetate copolymer containing 12 wt% vinyl acetate
(EVA12) was improved by treatment with UV
radiation. Treatment of EVA12 with UV radiation
increased its wettability and carbonoxygen polar
moieties were produced and roughness was created.
2.4 Polymer-fiber adhesion
In the investigation of the effects of
atmospheric pressure plasma treatment on the
surface energy of polyetheretherketone (PEEK),
carbon fibers (CF) and glass fiber (GF) reinforced
polyphenylene sulfide (PPS), a substantial
improvement in the surface energy is observed with
the polar component of surface energy responsible
for the increase in total surface energy. Also, in
comparison with low pressure plasma treatment,
the surface modification of polymer by atmospheric
pressure plasma is more effective for surface energy
and bonded joint strength [53].
2.5 Polymer-glass adhesion
Atmoshpheric pressure plasma can be a good
method on treated glass materials with
polyurethane served as an adhesive. Abenojar et al
[54]. concluded that a there was cleaning, etching
and activation effects on the glass surfaces after
plasma treatment, with a change in the locus of
failure from adhesive to cohesive in some cases,
and a reduction in data standard deviation.
The use of an excimer laser treatment, as a way
to control the adhesion performances of
glass/epoxy and carbon/epoxy composites has been
investigated and Benard et al. [55]reported that an
increase in adhesion was observed after the
treatment.
2.6 Polymer-wood adhesion
Bonding phenomena between polymer and
wood is one of the major research areas for polymer
bonding. The adhesion between wood polymer
composites components is not good due to low
surface energy and the hydrophobic nature of the
most widely used polymer matrices, i.e. polyolefins.
In the study of analyzing effective methods that
could be applied to improve the adhesion of WPC
materials made from 60% wood flour, hydrogen
peroxide solutions, hot air and flame demonstrate
good ability in improving strength of adhesion. A
further treatment using halogen heat lamp failed to
show an improvement in adhesion at any of the
speeds studied (10–50 mm s−1) [56].
The influence of the solvent N,N-
dimethylformamide (DMF) on one-component
moisture-curing polyurethane (1C-PUR) bonded
wooden specimens was investigated, and Klausler
et al. [57] presented the results confirming that a
primer can influence both, the adherend and the
adhesive polymer.
In a testing of a new adhesive with the main
ingredients include maleic anhydride (MAH) and
high density polyethylene (HDPE) that is MAH
grafted onto HDPE (PE-cg-MAH) for play wood,
the results showed that the properties of the
resulting plywood using PE-cg-MAH as an
adhesive can meet the standard of Type I plywood
and the optimum hot-press conditions were 160
165 °C and 5 min [58].
Performed by Moghadmazadeh et al. [59] , the
combination surface treatment of mechanical
abrasion and corona discharge treatment was
particularly effective in improving bonded joint
strength of an adhesive bonding of a Wood Polymer
Composite (WPC) material.
The use of flame and corona treatments can
also improve adhesion strength of the wood
substrates. The use of flame ionisation technology
witnessed an improvement on the wettability and
adhesion properties of wood [60]. In a finding
presented by Acda et al. [61],the use of plasma
treatment resulted in significant improvement in
work of adhesion for the three wood species
investigated: Shorea contorta (white lauan),
Gmelina arborea (yemane) and Acacia mangium. In
comparison with sanding treatment, plasma
treatment results in a higher surface adhesion
increase [62]. Custodio et al. [63]observed that the
effects of two surface pre-treatments (corona
discharge and flame ionization) on three timbers
(maritime pine, iroko, and European oak) showed
increased surface free energy for all samples, with
corona treatment comes with a higher surface free
energy, less susceptible to variation, and longer
lasting treatment effects.
3 Conclusion
To understand the complexity of polymer
bonding mechanism seven theories are proposed
and discussed for their validity. Among them,
molecular bonding is the most accepted one
because of its wide applications.
In terms of adhesion promotors, bonding of
polymers with metals, ceramics, other polymeric
materials, glass, wood and fibers are investigated.
The polymer-polymer adhesion and polymer-
ceramics adhesion sections are strongly based on
published book theories whereas the rest references
from the research papers. It can be summarized that
plasma treatment and chemical treatment are the
most applied ones among all methods discussed in
the paper.
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... It consists of the application of electrical discharges capable of uniformly modifying the polymer surface. This discharge leads to the generation of reactive oxidants -such as ozone, oxygen freeradicals, or oxygen atoms -responsible for moistening the surface, thus facilitating adhesion [15]. ...
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