Evidence of Acid-Base Interfacial Adducts in Various Polymer/Metal Systems by IRAS: Improvement of Adhesion

The physical interactions of polymers with inorganic substrates are determined by two major contributions: Van der Waals forces and acid-base interactions, taken in the most general “Lewis” electron acceptor-donor sense. The present work shows that the work of adhesion can be very appreciably increased by the creation of interfacial acid-base interactions. Practically, polymers such as poly(ethylene-co-vinyl acetate) (EVA), terpene-phenol resins (TPR), and their blends, were solution cast on basic and acidic substrates. The nature of the interfacial bonds and the enthalpy of adduct formation through electron exchange are evidenced by Fourier transform infrared reflection-absorption spectroscopy (IRAS). Moreover, it is shown that, on the one hand, modification of the electron donor ability of the polymer functionalities reveals the amphoteric character of the substrate and, on the other hand, modification of the electron donor ability of the substrate changes the nature of the species involved in interfacial adduct formation. Then, practical adhesion tests were carried out in order to correlate the nature and strength of interfacial acid-base bonds with simultaneous increases in adhesive strengths. Thermodynamic considerations allowed us to propose estimated values of the acid-base work of adhesion, W^ab , and of the density of acid-base sites, n^ab .     

Evidence of Acid-Base Interfacial Adducts in Various Polymer/Metal Systems by IRAS: Improvement of Adhesion

The physical interactions of polymers with inorganic substrates are determined by two major contributions: Van der Waals forces and acid-base interactions, taken in the most general “Lewis” electron acceptor-donor sense. The present work shows that the work of adhesion can be very appreciably increased by the creation of interfacial acid-base interactions. Practically, polymers such as poly(ethylene-co-vinyl acetate) (EVA), terpene-phenol resins (TPR), and their blends, were solution cast on basic and acidic substrates. The nature of the interfacial bonds and the enthalpy of adduct formation through electron exchange are evidenced by Fourier transform infrared reflection-absorption spectroscopy (IRAS). Moreover, it is shown that, on the one hand, modification of the electron donor ability of the polymer functionalities reveals the amphoteric character of the substrate and, on the other hand, modification of the electron donor ability of the substrate changes the nature of the species involved in interfacial adduct formation. Then, practical adhesion tests were carried out in order to correlate the nature and strength of interfacial acid-base bonds with simultaneous increases in adhesive strengths. Thermodynamic considerations allowed us to propose estimated values of the acid-base work of adhesion, W^ab, and of the density of acid-base sites, n^ab.     

The Effectiveness of Silane Adhesion Promoters in the Performance of Polyurethane Adhesives

A model polyurethane (PUr) adhesive has been modified by the addition of various silane adhesion promoters and used to bond PVC, ABS, a polyblend and glass. Inverse gas chromatography (IGC) data showed that epoxy silane substantially increased the surface reactivity of the adhesive while maintaining its amphoteric character. An aminosilane shifted the PUr surface to basicity, while vinyl, mercapto and chlorosilanes promoted surface acidity. Lap-shear data identified the amphoteric epoxy silane as the most successful adhesion promoter in all polymer and glass assemblies, increasing their initial bond strengths and also their residual bond strengths following accelerated aging. Elsewhere, the success of silane additives reflected the strength of interfacial acid/base interactions, the aminosilane being favored for bonding PVC, the others being preferred for the basic ABS and polyblend substrates. Correlations were developed between residual bond strength and initial bond properties of the assemblies and also between these system characteristics and an acid/base interaction parameter. The correlations may be useful as guidelines to the formulation of superior adhesives for bonding with substrates of known acid/base interaction potential.     

ADHESION ENHANCEMENT THROUGH CONTROL OF ACID-BASE INTERACTIONS

Adhesive bond strengths have been determined for lap-shear joints of PS/LLDPE and PS/CPE, a chlorinated version of polyethylene. Joints were formed at temperatures in the range of 180-280°C. In PS/LLDPE, bond strength at lower joining temperatures is compromised by the inability of LLDPE to act as electron acceptor to the donor properties of PS. However, at T ≥ 260°C, PS becomes a fluid capable of interacting through dispersion forces only, leading to enhanced diffusion across the PS/LLDPE interface and much stronger adhesive bonds. An acid-base pairing is in effect in joints of PS/CPE, resulting in strong joints made at T ≤ 240°C. The probable loss of acid-base interaction between the polymers at higher T, coupled with a failure of diffusion across the interface, leads to a lowering of the joint bond strength. Control over interfacial interactions is demonstrated to be a vital factor in the development of adhesive bonds.     

Semi-Structural Hot Melt Adhesives Based on Crosslinkable Functionalized Polyolefins

A structural or semi-structural adhesive is usually applied to the substrates as monomers, oligomers, or melts of polymers with reactive groups and is then polymerized or crosslinked in situ in the joint between the substrates. We have been studying a number of crosslinked functionalized polyolefins blended with tackifier used as semi-structural adhesives for bonding to oily galvanized steel surfaces. The functions of takifier, surface properties of adhesive and substrate, geometry effects of lap joints, adhesive T_β, chain end defects, network chain length, and cure kinetics of these systems will be discussed. Our experimental results indicate that lap shear strengths of galvanized steel joints depend on adhesive storage modulus to the power of roughly 1/2. A rough estimate of the fracture energy of the adhesive bond, G_a could be obtained from this relation. Although some estimated G_a values are too low while the others are too high, they seem to be in rough accord with the degree of interfacial bonding and the locus of failure of the lap shear bonds.     

ADHESION ENHANCEMENT THROUGH CONTROL OF ACID-BASE INTERACTIONS

Adhesive bond strengths have been determined for lap-shear joints of PS/LLDPE and PS/CPE, a chlorinated version of polyethylene. Joints were formed at temperatures in the range of 180–280°C. In PS/LLDPE, bond strength at lower joining temperatures is compromised by the inability of LLDPE to act as electron acceptor to the donor properties of PS. However, at T?≥?260°C, PS becomes a fluid capable of interacting through dispersion forces only, leading to enhanced diffusion across the PS/LLDPE interface and much stronger adhesive bonds. An acid–base pairing is in effect in joints of PS/CPE, resulting in strong joints made at T?≤?240°C. The probable loss of acid-base interaction between the polymers at higher T, coupled with a failure of diffusion across the interface, leads to a lowering of the joint bond strength. Control over interfacial interactions is demonstrated to be a vital factor in the development of adhesive bonds.     

Acid–base interactions and covalent bonding at a fiber–matrix interface: contribution to the work of adhesion and measured adhesion strength

The work of adhesion, W _A, and the practical adhesion in terms of the interfacial shear strength, τ, in some polymer-fiber systems were determined to establish a correlation between these quantities. An attempt was made to analyze the contributions of various interfacial interactions (van der Waals forces, acid-base interaction, covalent bonding) to the 'fundamental' and 'practical' adhesion. The surface free energies of the fibers were altered using different coupling agents. To characterize the strength of an adhesion contact, the ultimate adhesion strength, τ_ult, was determined for the onset of contact failure. The adhesion of non-polar polymers occurs through van der Waals interaction only; therefore, fiber sizing does not affect the adhesion strength. For polar polymers, such as poly(acrylonitrile butadiene styrene) and polystyrene, adhesion is sensitive to fiber treatments: suppression of the acid-base interaction by using an electron-donor sizing agent γ-aminopropyltriethoxysilane results in a decrease of both 'fundamental' and 'practical' adhesion. In the case of epoxy resins, the main contribution to the work of adhesion is made by covalent bonds. Since the process of their formation is irreversible, the work of adhesion determined from micromechanical tests seems to be more reliable than indirect estimations, such as from wetting and inverse gas chromatography techniques. Fiber treatment by sizing agents results in considerable changes in the intensity of adhesional interaction with the epoxy matrix. A correlation between the work of adhesion, the ultimate interfacial shear strength, and the strength of macro-composites has been found.     

Peel Adhesion Behaviour of Carboxylic Elastomers

The introduction of carboxylic acid groups into a poly(butyl acrylate) adhesive greatly increases its bond strength to a glass substrate, as may be seen in a changed force-rate relationship for separation by peeling. By selective carboxylation of the bulk only or the surface only of the adhesive, it is possible to discriminate between bond enhancement by an interfacial effect (presumed to involve interfacial hydrogen bonding) and that by a bulk effect (change in visco-elastic response resulting from carboxylation). The interfacial effect provides a somewhat lower contribution towards the improvement of bond strength than does the bulk effect. Energetic considerations show that the presence of 10% by weight of copolymerised acrylic acid increases the thermodynamic work of adhesion by a factor of about 1.5.     

Peel Adhesion Behaviour of Carboxylic Elastomers

The introduction of carboxylic acid groups into a poly(butyl acrylate) adhesive greatly increases its bond strength to a glass substrate, as may be seen in a changed force-rate relationship for separation by peeling. By selective carboxylation of the bulk only or the surface only of the adhesive, it is possible to discriminate between bond enhancement by an interfacial effect (presumed to involve interfacial hydrogen bonding) and that by a bulk effect (change in visco-elastic response resulting from carboxylation). The interfacial effect provides a somewhat lower contribution towards the improvement of bond strength than does the bulk effect. Energetic considerations show that the presence of 10% by weight of copolymerised acrylic acid increases the thermodynamic work of adhesion by a factor of about 1.5.     

Thermoplastic bonding to metals via injection molding for macro-composite manufacture

In this work, we explore a new method of in-situ joining of polymers to metals in injection molding to allow direct bonding between thermoplastic and metal parts. Such a method can integrate several downstream steps in product manufacture, allow optimal design of products and joints, and avoid adhesive application, assembly, and associated difficlties. A variety of process parameters and their effects upon the interface tensile strengths were examined. A full factorial experiment was conducted involving four of the critical process parameters identified. The effects upon tensile strength at break of the following process parameters were studied: (1) adherend surface temperature, (2) screw linear velocity, (3) bondline thickness, and (4) pack and hold pressure. The fracture surfaces and the thermoplastic metal interfaces were analyzed. The bonds fabricated with higher adherend surface temperatures have increased mean tensile strength and less adhesive failure. This increase in mean bond tensile strength and less adhesive failure was due to increased polymer penetration of the adherend surface roughness, at the micrometer level, as shown in the analysis of the polymer-metal interface by a scanning electron microscope (SEM).     

Synthesis of boron‐containing coupling agents and its effect on the interfacial bonding of fluoropolymer/TATB composite

The four kinds of boron‐containing coupling agents were synthesized, which were applied to improve the interfacial bonding of fluoropolymer/1,3,5‐triamino‐2,4,6‐trinitrobenze (TATB) composite. The boron atom of the coupling agent molecule can form coordinate bond with the fluorine atom of fluoropolymer; and hydrogen atom in its amino group can form hydrogen bond with oxygen atom in nitro‐group of TATB. The interfacial performance of the composite was investigated by the measurement of contact angle, surface and interface tension, and adhesive work, and the interfacial bonding mechanism was studied by FTIR and XPS analysis. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Heterogeneity in the strength of interfacial bonds and the resultant synergism in promoting polyurethane/SBR adhesion strength

Polymer adhesion and its evaluation are very important both from academic and industrial points of view. Adhesion phenomenon depends mainly on the strength of interfacial bonds and the deformability of adhering partners, which act as another energy absorbing term. Using a combination of acid-base interactions and a coupling agent at the interface of styrene-butadiene (SBR)-polyurethane system, a synergistic adhesion promotion was observed. The SBR surface was treated with an acidified aqueous solution of calcium hypochlorite to obtain polar groups, which can interact with the isocyanate groups of the polyurethane system. Aminopropyltriethoxysilane, a coupling agent, was also deposited at the SBR surface, after treating it with the aforementioned solution. The polar groups on the SBR apparently interact with the OH sites of the coupling agent at the surface and push the amino groups toward the polyurethane surface. This interesting finding, synergistic adhesion promotion, was attributed to the tortuous path for the crack growth at the interface, which was created by formation of the interfacial bonds with different strengths (heterogeneous bond strength). Furthermore, to obtain the highest synergistic effect, it seems that certain ratio of bonds with different strengths should be formed. The ratio itself depends on the deformability ratio of the adhering materials.     

Adhesion and interfacial healing between supramolecular hybrid networks and glass substrates

Adhesion towards glass and interfacial healing of partially supramolecular hybrid polymer networks featuring a range of H-bonds content were investigated through two dedicated adhesion test methods. In a first series of tests, adhesion strength was measured by separating two substrates containing a cured inner resin layer, and shown to decrease with increasing H-bonds content in the polymer network (from 0 to 50%) as the mechanical strength of the polymer also decreased while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with glass substrates. In a second step, the test was used to evaluate interface restoration through healing of the polymer matrices and results showed an increased from none to a tensile strength recovery up to 70% after 1 h healing time for the 50% H-bond polymer. Then, self-adhesion of freshly cut polymer surfaces to glass substrates was investigated, showing increasing tack with increasing H-bonds content. The influence of glass surface treatments on adhesion and interfacial recovery properties was also explored: while aminosilanes did not influence the interfacial behavior of partially supramolecular self-healing polymers towards glass, trimethoxy (octadecyl)silane (ODS) modification strongly hindered their adhesion abilities, further highlighting the fundamental role of hydrogen bonds interaction with the substrates.     

On The Strength of Adhesive Bonds Between Rigid, Non-Crystalline Polymers

The relatively high strengths exhibited by well-prepared, rigid adhesive bonds partly reflect the crack-growth inhibiting properties of the surrounding bulk. Dispersion forces alone are unable to inhibit crack growth significantly, as is indicated, for example, by the low strengths of low molecular weight glassy polymers. The source of crack-growth inhibition of adhesive bonds was revealed by examining crack fronts with a microscope. Examined were crack fronts along the self-bond between pieces of poly(methyl methacrylate) and along the adhesive bond between pieces of poly(methyl methacrylate) and polystyrene. Associated with each crack front were two sets of interference fringes, indicating the presence of a craze preceding the crack. Crazes form in high molecular weight brittle polymers, and their presence along the adhesive bond ahead of the crack indicates the involvement of the high molecular weight bulk polymer adjacent to the bonding plane. Crazes ahead of cracks are known to inhibit fracture by distributing the load surrounding the cracks and causing any growth to consume large amounts of energy.     

Thermal bonding of nonwovens as simulated by polypropylene films: Effect of time,temperature, and molecular weight

Many nonwoven fabrics are made by melt spinning semicrystalline fibers followed by thermal bonding using heated calendar rolls. In this work, we have studied thermal bonding of polypropylene films to simulate bonding of nonwoven fibers. We have tried to relate the thermal bond strengths with the concepts of chain dynamics via interfacial adhesion development at symmetric polymer interfaces. This requires relating the microscopic dynamics of chains with macroscopic interfacial adhesion measurements. It was found that the interfacial bond strength was proportional to the fraction of the crystals melted. This required heating the interfacial region between the polymer layers into the melting region. Bond strengths were also related to process time as t^1/2. This dependence is consistent with the literature for reptation, but is also due to the required thermal diffusion to bring the interfacial region to the bonding temperature. Finally, the bond strength is also dependent on the polymer molecular weight as 1/M^1/2, which is consistent with forming the bonds via chain reptation, provided that the bonding time is less than the reptation time. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems Part 2. The Effect of Surface Treatment

The effect of surface treatments on the bond strength in glass fibre-polyester resin composites has been investigated using single fibre interfacial shear strength specimens and the short beam shear test for interlaminar shear strength.

A range of bond strengths was obtained by using, either alone or in combination, the various components of the size formulation which is normally applied to the fibres, so that the interaction between the glass surface and the polyester ranged from Van de Waal forces through hydrogen bonding to covalent bonding, the bond strength increasing in that order.

The relative contribution to bond strength of mechanical bonding due to thermo-mechanical mismatch between the two components and of chemical bonding or physical interaction between the three phases, glass-surface treatment-resin, has been evaluated and found to be one third and two thirds respectively.     

A Water Soluble Hydrated Metal Oxide primer for Adhesively Bonded Joints

Weight saving and manufacturing cost benefits have led to the increase in use of adhesively-bonded structures in the automotive, aerospace and marine industries. In order to be a viable alternative to, for example, metal fasteners, these adhesive bonds should maintain the strength typical of conventional fastener systems. In many applications, the bonds are put under a variety of environmental and mechanical stresses. For example, frequently these bonds are exposed over long periods of time to wet environments which can result in a loss of bond strength. The loss of strength can result from the extension of cracks and other deformations that occur in the adhesive or metal oxide which are accelerated by the moist environment. As a result of this deficiency, extensive research and development efforts have been undertaken to define methods and identify materials which improve bonded joint performance in humid conditions. For example, it is known that surface preparation is important in the bonding of aluminum and titanium, and cleanliness in the bonding of ceramic articles. Thus, it is essential that, before bonding, the adherend is cleaned and chemically pretreated to produce a surface which in combination with the adhesive develops the bond strengths which meet application requirements. The normal procedure after surface treatment is to apply a corrosion-inhibiting primer by a spray technique for surface protection prior to bonding and to insure resin penetration into the oxide structure which provides improved environmental resistance. A major drawback of spray application is the large volume of organic solvent (normally MEK) emitted to the atmosphere. A successful alternative is the recently-developed electrodeposited primer by Northrup Corp., which consists of water solubilized primer particles which migrate in an electric field to a conductive work piece where they are deposited in a dense, continuous coating.¹ The primer was developed for use with 121°C (250°F) curing epoxy adhesives. An Air Force sponsored contract is currently under way, the objective of which is to develop an electrodeposited water-based primer for use with 177°C (350°F) curing epoxy systems.² A water-based epoxy primer system for application using the more conventional spray techniques has also been decribed.³     

A Water Soluble Hydrated Metal Oxide primer for Adhesively Bonded Joints

Weight saving and manufacturing cost benefits have led to the increase in use of adhesively-bonded structures in the automotive, aerospace and marine industries. In order to be a viable alternative to, for example, metal fasteners, these adhesive bonds should maintain the strength typical of conventional fastener systems. In many applications, the bonds are put under a variety of environmental and mechanical stresses. For example, frequently these bonds are exposed over long periods of time to wet environments which can result in a loss of bond strength. The loss of strength can result from the extension of cracks and other deformations that occur in the adhesive or metal oxide which are accelerated by the moist environment. As a result of this deficiency, extensive research and development efforts have been undertaken to define methods and identify materials which improve bonded joint performance in humid conditions. For example, it is known that surface preparation is important in the bonding of aluminum and titanium, and cleanliness in the bonding of ceramic articles. Thus, it is essential that, before bonding, the adherend is cleaned and chemically pretreated to produce a surface which in combination with the adhesive develops the bond strengths which meet application requirements. The normal procedure after surface treatment is to apply a corrosion-inhibiting primer by a spray technique for surface protection prior to bonding and to insure resin penetration into the oxide structure which provides improved environmental resistance. A major drawback of spray application is the large volume of organic solvent (normally MEK) emitted to the atmosphere. A successful alternative is the recently-developed electrodeposited primer by Northrup Corp., which consists of water solubilized primer particles which migrate in an electric field to a conductive work piece where they are deposited in a dense, continuous coating.¹ The primer was developed for use with 121°C (250°F) curing epoxy adhesives. An Air Force sponsored contract is currently under way, the objective of which is to develop an electrodeposited water-based primer for use with 177°C (350°F) curing epoxy systems.² A water-based epoxy primer system for application using the more conventional spray techniques has also been decribed.³     

Photochemical surface modification of polymers for improved adhesion

A method using photoactivatable reagents is described to modify organic polymer surfaces without changing the bulk properties of the material. The reagents contain a benzophenone or other photoactivatable group which, when exposed to light of appropriate wavelength, generates highly reactive intermediates that covalently bond with nearly any organic material. Some general surface characteristics that can be achieved by this approach, on a wide range of materials, are good wettability, good lubricity, passivation, and priming for either adhesion or immobilization of other molecules. This technology provides tremendous flexibility for tailoring surface characteristics for a broad range of applications. Some materials that have desirable bulk properties for specific applications, however, have surface characteristics that make bonding them to other materials difficult. By photocoupling water-soluble polymers onto the surfaces of such materials, the surface properties can be modified to achieve greatly increased bond strengths with conventional adhesives. For example, using such techniques, the strength of bonding two pieces of high-density polyethylene to each other using a cyanoacrylate adhesive was increased by about 17-fold. Similarly, in preliminary experiments, the bond strengths of silicone rubber to polyvinyl chloride, using cyanoacrylate adhesive, were increased by more than 18-fold. This technology offers great potential for surface modification for improved adhesion.     

Quantitative characterization of the acid-base properties of solvents,polymers, and inorganic surfaces

The growing realization of the importance of intermolecular acid-base interactions in promoting the solubility, adsorption, and adhesion of polymers to other materials has caused a demand for the quantitative characterization of the acid-base properties of the commonly used solvents, polymers, and inorganic fillers and substrates. There have been several recent advances in the measurement techniques for such determinations, especially in the fields of inverse gas chromatography, microcalorimetry, ellipsometry, FTIR, NMR, and XPS spectroscopy, all leading to the capability of determining the Drago E and C constants or the Gutmann acceptor numbers (AN) or donor numbers (DN) for the acidic or basic sites of solvents, polymers, or inorganic surfaces. In the last year, new studies have also allowed the characterization of the specific acid-base cohesive interactions in solvents and polymers, and the determination, from contact angle measurements on polymers, of the surface concentration and strength of acidic and basic surface sites. All of these techniques are discussed in this paper and it is expected that they will soon become standard laboratory practices.