The role of chemical bonding in the adhesion of elastomers

A review is given of several studies of the effect of interfacial bonding upon the mechanical strength of an adhesive joint. In the first, polybutadiene layers were crosslinked by a free radical process whilst in contact with silane-treated glass. A direct proportionality was found between the minimum peel strength of the joint, at high temperatures and low rates of peeling, and the vinyl content of the silane treatment liquid. Covalent bonding between the diene polymer and vinyl groups on the treated glass was inferred. When radioactively tagged silanes were employed, extensive combination with the glass substrates was demonstrated. Again, the greater the amount of vinyl silane found on the treated glass surface, the greater the mechanical strength of adhesion between the treated glass and a polybutadiene overlayer. In another series of experiments two partially-crosslinked sheets of polybutadiene were pressed together before the crosslinking was taken to completion. The additional crosslinking was determined from measurements of the elastic properties and of the degree of equilibrium swelling by a compatible liquid. Again, the mechanical strength of adhesion between the two sheets under threshold conditions was found to be directly proportional to the inferred degree of interfacial interlinking. Thus, at least at high temperatures and low rates of peel, there is substantial evidence for a direct correlation between the mechanical strength of a joint and the degree of interfacial chemical bonding. Moreover, the relationships established in these studies allow other bonding systems to be diagnosed as chemical or physical in nature. For example, a dramatic increase in the strength of adhesion between two crosslinked polybutadiene layers was observed if they were exposed to air or oxygen for periods of an hour or two before being pressed into contact. It is inferred that interfacial chemical bonds are formed as a consequence of rapid surface oxidation reactions.     

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.     

Adhesion and self-adhesion of immiscible rubber blends

Rubber blends of synthetic polyisoprene rubber (IR) and hydrogenated acrylonitrile butadiene rubber (HNBR) are prepared with different compositions. First, DSC results confirm that IR and HNBR are incompatible rubbers. A tensile testing machine equipped with a tack probe test allows us to measure the level of adhesion at rubber blends/glass as well as rubber blends/pure rubber interfaces, for contact times ranging from 0.1 s to a few hours. The adhesive properties of rubber blends were compared with those of pure rubbers. Adhesion energy G of IR/HNBR blends onto glass increases with the IR content in disagreement with a simple law of mixtures because of the influence of bulk properties of blends (morphology and mechanical behaviour). For a given blend, G increases with contact time certainly due to an interfacial reorganisation. Self-adhesion energies G_S of pure rubbers and IR/HNBR blends increase also with contact time, thanks to mainly an interdiffusion phenomenon of the rubber chains through the interface. Self-adhesion energy of blends in contact with pure IR follows a simple law of mixtures as a function of IR content. On the contrary, the variation of self-adhesion energy of these blends in contact with pure HNBR is more complex.     

Reactive bonding of natural rubber to metal by a nitrile–phenolic adhesive

The mechanism of adhesive bonding of rubber to metal using an interlayer of bonding agent (adhesive) is discussed with respect to various physical and chemical events such as adsorption at the metal surface, chemical crosslinking within the adhesive, interdiffusion, and formation of interpenetrating networks at the rubber–adhesive interface. An investigation on the peel strength of a natural rubber (NR)–adhesive–metal joint, made by vulcanization bonding using nitrile–phenolic adhesive containing various concentrations of toluene diisocyanate–nitrosophenol (TDI–NOP) adduct, is presented. A single‐coat adhesive, consisting of a p‐cresol phenol formaldehyde resin, nitrile rubber (NBR), and vulcanizing agents in methyl ethyl ketone solvent, was selected for the study. Considerable improvement in the peel strength was obtained by the incorporation of TDI–NOP adduct into the nitrile–phenolic adhesive. The peel strength increases as the concentration of TDI–NOP adduct in the adhesive composition increases, then levels off with a transition from interfacial failure to cohesive tearing of rubber. The peel strength improvement is believed to be attributed to the interfacial reactions between the bonding agent and natural rubber, when TDI–NOP adduct is incorporated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2597–2608, 2001     

Independent investigation on the influences of the processing conditions on the reclamation of crosslinked isoprene rubber after the impregnation of a reclaiming reagent

In this study, the reclamation of sulfur (S)‐cured isoprene rubber (IR) was investigated independently after the impregnation process of the reclaiming reagent diphenyl disulfide (DD) into the crosslinked IR matrix with supercritical carbon dioxide (scCO₂) as the transmission medium. According to the mass uptake of DD into IR and scanning electron microscopy–energy‐dispersive X‐ray spectrometry measurements, DD was highly impregnated and homogeneously dispersed in the network under 12 MPa at 80°C for 11 h in scCO₂. During the impregnation process, almost no reclaiming reaction occurred. Then, through three different reclaiming methods, a mechanochemical method, a chemical method with oxygen, and a chemical method without oxygen, the influences of the shear force, reclaiming atmosphere, reaction time, and amounts of reclaiming reagent on the reclamation with crosslinked IR with pre‐impregnated DD were independently investigated and compared with those of the reaction without pre‐impregnated DD. The sol fraction of the reclaimed rubber and molecular weight of the sol were measured. The results show that the reclaiming speed greatly depended on the amount of reclaiming reagent and that the reclaiming reaction was dramatically accelerated when the reclaiming reagents were pre‐impregnated into the crosslinked IR under the same processing conditions. This indicated that the impregnation time of the reclaiming reagent into the crosslinked network constituted a large proportion of the reclaiming time. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40298.     

Bonding of Natural Rubber to Steel: Surface Roughness and Interlayer Structure

This paper is concerned with two aspects of the adhesion produced by the vulcanisation bonding of a simple natural rubber (N.R.) compound to mild steel. Adhesion was measured using a 45° peel test.

When the N.R. was bonded, using a proprietary bonding agent (Chemlok 205/220), to ‘smooth’ steel (acid etched) or to ‘rough’ steel (phosphated) high values of peel energy (≥ 4.5 kJm^?2), and good environmental resistance to water were obtained, with failure cohesive largely within the rubber. The highest values of peel energy (≈ 7.5 kJm^?2) were associated with a phosphated surface which consisted of plate-like crystals which directed the stresses away from the substrate in a way which produced a failure surface within the rubber which showed extensive tearing and cracking.

The nature of the layer formed in the interfacial region by interaction between bonding system and rubber was investigated using a chlorinated rubber as a ‘model compound’ representing the adhesive and uncompounded N.R. to represent the rubber. When a blend of the two was heated in air at 150°C, evidence was found of a solid state chemical reaction in which carbonyl groups were incorporated into the blend which became visually homogeneous. Further evidence points to the relevance of this change to adhesion in rubber-to-metal bonding.     

The separate roles of phase structure and interfacial adhesion in toughening a brittle polymer

The effects of rubber particle size and rubber/matrix adhesion on the impact properties of a brittle polymer have been separated using polystyrene (PS)/acrylonitrile-butadiene rubber (NBR) as a model system in which interfacial chemical reaction could be controlled. It has been proven that the interfacial adhesion between the rubber phase and the PS matrix not only greatly aids in reducing the rubber particle size but also plays a further role in improving the impact properties of the matrix polymer. The impact energies of PS/NBR blends with interfacial chemical bonding are four to ten times as high as those without interfacial bonding for the same average rubber particle size. However, at temperatures below the glass transition temperature of the rubber, there is no difference in impact energies with or without interfacial chemical bonding. It has been found that the optimum rubber particle size for toughening PS is influenced by interfacial adhesion. Smaller optimum rubber particle size is observed for blends with greater amounts of interfacial chemical bonding.     

Influence of Covalent Bonds on the Adhesion Energy at Elastomer-Glass Interfaces

The aim of this study was to analyze the effect of interfacial covalent bonds on the adhesive behavior of an elastomer, a crosslinked polydimethylsiloxane, and a glass substrate. These covalent bonds were created by applying to both materials an appropriate surface treatment by means of plasma polymerization. Adhesion measurements were carried out by analyzing the contact area between a rubber hemisphere and a flat rigid glass plate. The contact was forced under a given compressive loading for different times t_c, then the load was removed and the fracture propagation at the interface was recorded as a function of relaxation time t_r. Finally, adhesion energies were also determined by means of a probe test using a tensile testing machine.     

The prediction of adhesion between polymer matrices and silane-treated glass surfaces in filled composites

Extending the database and the analysis of work reported earlier, the practical adhesion between a glass filler, modified by various silane coupling agents, and different polymeric matrices is measured and compared with predictions based on a generalized thermodynamic criterion. The criterion used is the magnitude of the (negative) molar Gibbs free energy of mixing, (-ΔGmix)_0.5, for a pseudo-solution consisting of equal molar amounts of the repeat units of the polymer matrix and the organo-functional group of the silane coupling agent. It is computed using the group contribution method of UNIFAC, in which molar volumes, molar areas, and molar interaction energies are constructed from contributions of the functional groups which make up the molecules of the solution. Measurements leading to the values of the adhesion strength are carried out using the singleparticle composite method, in which a rectangular polymer specimen containing a single silane-treated glass bead is subjected to increasing uni-axial tensile stress until interfacial failure, as observed using a microscope, occurs at one of the poles of the sphere. Earlier work reported a good correlation between the local stress at the pole computed from such measurements and the value of (-ΔG _mix)_0.5 computed using UNIFAC for ten different organo-silane-modified spheres imbedded in a poly(vinyl butyral) matrix. The present work extends the database to two additional matrices, viz. poly(methyl methacrylate) and poly(ethyl methacrylate). In addition, elastic fracture-mechanics theory is used to deduce specific adhesion energies in all cases. The relative values of the latter are all found to be in good correlation with the predictive thermodynamic criterion.     

Bacterial Adhesion to Low Energy Solid Surfaces: A Surface Thermodynamics Approach

A desired approach to reduce bacterial adhesion to ship hull, heat exchanger and medical device surfaces is to make them less attractive for bacteria by applying anti-fouling or foul-release surface coatings. However, the selection of a useful anti-fouling coating is a difficult problem and surface thermodynamics may guide us in this respect. In this work, we investigated the independent contributions of substrate–water, γ _SW ^Tot, substrate–bacteria, γ _SB ^Tot, and bacteria–water, γ _BW ^Tot, interfacial free energies to the total free energy of adhesion, ΔG _SWB ^Tot, of Pseudomonas fluorescens bacteria on the Si- and SiN-doped DLC coated glass slide surfaces using the Lifshitz–van der Waals and acid–base surface free energy components theory. It was found that mostly acid–base interactions determine the bacterial removal properties. The repulsion between bacteria and the solid surface in water increases if γ _SB ^AB is large and γ _SW ^AB is small, when they are both positive. When Lifshitz–van der Waals and acid–base components of free energy of adhesion are considered, it was found that the effect of ΔG _SWB ^LW was very small and the main effect on bacterial removal was found to depend on the ΔG _SWB ^AB parameter and bacterial % removal increased linearly with the increase of both ΔG _SWB ^AB and ΔG _SWB ^Tot parameters for all the samples.     

Effect of contact time and temperature on adhesion between components of rubber-polyethylene thermoplastic composites

—The effect of contact time and temperature on the adhesion between rubber and polyethylene has been studied. The degree of adhesion between natural rubber (NR) and polyethylene (PE) was varied by using physical (EPDM) and chemical interaction promoters (ENR/PE_m). It was observed that the peel strength increases with an increase in time of contact at a particular temperature. The adhesion strength varies with the square root of the contact time for all the systems with the exception of NR/PE/DCP at 75 and 100°C, EPDM/PE at 100°C, and NR/ENR/PE_m/PE at 100°C. With an increase in temperature, however, only EPDM-containing systems show higher values of adhesion between components. EPDM enhances the strength of the interface of the NR/PE joint, especially at longer contact times and higher temperatures. However, the chemical modifier is active only when the joining temperature is 150°C. On mastication of NR up to 15 min, the adhesion between natural rubber and polyethylene increases. The tack strength of NR-PE composites is increased with the introduction of physical and chemical modifiers.     

Interfacial adhesion in sisal fiber/SBR composites: an investigation by the restricted equilibrium swelling technique

Short sisal fiber-reinforced styrene butadiene rubber (SBR) composites were prepared and characterized by the restricted solvent swelling technique. The solvent swelling characteristics of SBR composites containing untreated and bonding agent-added mixes were investigated in a series of aromatic solvents, such as benzene, toluene, and xylene. The diffusion experiments were conducted by the sorption gravimetric method. The adhesion between the rubber and short sisal fibers was evaluated from the restricted equilibrium swelling measurements. The anisotropy of swelling of the composite was confirmed by this study. The effect of fiber orientation in controlling the anisotropy of restricted swelling was also demonstrated. As the fiber content increased, the solvent uptake decreased, due to the increased hindrance and good fiber-rubber interactions. Bonding agent-added mixes showed enhanced restriction to swelling, due to the strong interfacial adhesion. The bonding system containing hexa-resorcinol in the mix produces an in-situ resin, which binds the fiber and the rubber matrix firmly. In addition, as the penetrant size increases from benzene to xylene, the uptake decreases. The swelling index values of the composites support this observation. Due to the improved adhesion between the short sisal fiber and SBR, the ratio of the volume fraction of rubber in the dry composite sample to the swollen sample (V _T) decreases. The extent of fiber orientation of the composites was also analysed from the restricted swelling method. SEM studies of the composite revealed the orientation of short fibers. The sorption data support the Fickian diffusion trend, which is typical in the case of cross-linked rubbers.     

Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase

The objective of this study was to examine the effects of interfacial chemistry on the interfacial micromechanics of cellulose fiber/polymer composites. Different interfacial chemistries were created by bonding polystyrene (a common amorphous polymer) to fibers whose surfaces contained different functional groups. The chemical compatibility within the interphase was evaluated by matching the solubility parameters (δ) between the polymer and the induced functional groups. The physico-chemical interactions within the interphase were determined using the Lifshitz–van der Waals work of adhesion (W _a ^LW) and the acid–base interaction parameter (I _a?b) based on inverse gas chromatography (IGC). The micromechanical properties of the fiber/polymer interphase were evaluated using a novel micro-Raman tensile test. The results show that the maximum interfacial shear stress, a manifestation of practical adhesion, can be increased by increasing the acid–base interaction (I _a?b) or by reducing the chemical incompatibility (Δδ) between the fibers and polymer. A modified diffusion model was employed to predict, with considerable success, the contribution of interfacial chemistry to the practical adhesion of cellulose-based fibers and amorphous polymers. The increased predictability, coupled with the existing knowledge of the bulk properties of both fibers and matrix polymer, should ultimately lead to a better engineering of composite properties.     

The effect of liquid-induced adhesion changes on the interfacial shear strength between self-assembled monolayers

Friction between chemically-modified tips and surfaces has been studied with chemical force microscopy (CFM) to evaluate the effect of changing solid/liquid free energy on energy dissipation in sliding tip-surface contact. Well-controlled conditions were necessary to attain a single asperity contact in these experiments. We found that in a series of methanol- water mixtures the interfacial shear strength between CH3-terminated surfaces of the siloxane self-assembled monolayers (SAMs) was independent of the adhesion force. The shear strength value of 10.2 ± 1.0 MPa found for this interface under methanol-water media is consistent with the previous studies of similar systems under dry gas conditions. A comparison to available data on interfacial shear strengths demonstrated that siloxane monolayers were much more effective in reducing friction than various carbon coatings.     

Duality of the interfacial thermal conductance in graphene-based nanocomposites

The thermal conductance of graphene–matrix interfaces plays a key role in controlling the thermal properties of graphene-based nanocomposites. Using atomistic simulations, we found that the interfacial thermal conductance depends strongly on the mode of heat transfer at graphene–matrix interfaces: if heat enters graphene from one side of its basal plane and immediately leaves it through the other side, the corresponding interfacial thermal conductance, G_across, is large; if heat enters graphene from both sides of its basal plane and leaves it at a position far away on its basal plane, the corresponding interfacial thermal conductance, G_non-across, is small. For a single-layer graphene immersed in liquid octane, G_across is ∼150 MW/m²K while G_non-across is ∼5 MW/m²K. G_across decreases with increasing multi-layer graphene thickness (i.e., number of layers in graphene) and approaches an asymptotic value of 100 MW/m²K for 7-layer graphenes. G_non-across increases only marginally as the graphene sheet thickness increases. Such a duality of the interface thermal conductance for different probing methods and its dependence on graphene sheet thickness can be traced ultimately to the unique physical and chemical structure of graphene materials. The ramifications of these results in areas such as the optimal design of graphene-based thermal nanocomposites are discussed.     

Cut growth properties of styrene–butadiene copolymers

The cut growth properties of styrene–butadiene block and random copolymers are considered in terms of the tearing energy theory. It is found that the value of T₀ (the minimum value of tearing energy below which no cut growth takes place in the absence of chemical effects) is far higher for a styrene–butadiene resin copolymer system with a high amount of bound styrene resin than for a conventionally vulcanized SBR elastomer. Similarly, it is shown that the value of T₀ for a butadiene–styrene block copolymer (thermoplastic rubber) is considerably reduced when the material is crosslinked. It is proposed that the value of T₀ is influenced by the hystersial properties of the rubber.     

The rheological behavior and dynamic mechanical properties of syndiotactic 1,2‐polybutadiene

The rheological behavior and the dynamic mechanical properties of syndiotactic 1,2‐polybutadiene (sPB) were investigated by a rotational rheometer (MCR‐300) and a dynamic mechanical analyzer (DMA‐242C). Rheological behavior of sPB‐830, a sPB with crystalline degree of 20.1% and syndiotactic content of 65.1%, showed that storage modulus (G′) and loss modulus (G″) decreased, and the zero shear viscosity (η₀) decreased slightly with increasing temperature when measuring temperatures were lower than 160°C. However, G′ and G″ increased at the end region of relaxation curves with increasing temperature and η₀ increased with increasing temperature as the measuring temperatures were higher than 160°C. Furthermore, critical crosslinked reaction temperature was detected at about 160°C for sPB‐830. The crosslinked reaction was not detected when test temperature was lower than 150°C for measuring the dynamic mechanical properties of sample. The relationship between processing temperature and crosslinked reaction was proposed for the sPB‐830 sample. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

The role of the polymer/metal interphase and its residual stresses in the critical strain energy release rate (Gc) determined using a three-point flexure test

The critical strain energy release rate (G _c), the residual stresses (σ), Young's modulus (E), and the practical adhesion, characterized by ultimate parameters (F_max or d_max), of organic layers made of DGEBA epoxy monomer and IPDA diamine hardener were determined. The prepolymer (DGEBA-IPDA) was deposited both as thick coatings and as a mechanical stiffener onto degreased aluminum alloy (5754) or chemically etched titanium alloy (Ti-6Al-4V). During the three-point flexure test used as a practical adhesion test [this test is also called the double cantilever adhesion test (DCAT)], the failure may be regarded as a special case of crack growth. To understand the real gradient properties of the interphase, substrate, and bulk polymer properties, a three-layer model was developed for quantitative determination of the critical strain energy release rate (G_c). The particular characteristic of this model was to consider the residual stresses developed within the entire three-layered system, leading to an intrinsic parameter representing the practical adhesion between a polymer and a metallic substrate. Moreover, to determine the residual stresses generated in such three-layer systems, the gradient of interphase mechanical properties was considered. The maxima of residual stress intensities are found at the interphase/substrate interface, leading to an adhesional (interfacial) failure that is experimentally observed. The determination of the critical strain energy release rate by the three-point flexure test (DCAT) shows that residual stresses cannot be neglected. A comparison between the results obtained from the three-point flexure test (DCAT) and those obtained by the tapered double cantilever beam (TDCB) test is presented.     

Cellulose-Nanofiber-Reinforced Rubber Composites with Resorcinol Resin Prepared by Elastic Kneading

A resorcinol resin/water dispersion and a rubber latex are added to 1% 2,2,6,6,-tetramethylpyperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TEMPO-CNFs) dispersed in water, followed by oven drying at 40 °C for 20 h to prepare a dried TEMPO-CNF/resorcinol resin/latex rubber (DTRL) mixture with a weight ratio of 1/0.5/3. DTRL is then added to a nitrile-butadiene rubber (NBR) or a carboxy group-containing NBR (X-NBR) sheet, and the mixture is kneaded by a two-roll mill at 20–30 °C with high shear forces. The tensile strength and Young's modulus of the crosslinked DTRL/rubber composite sheets remarkably increased from 10 and 12 MPa, respectively, for the reference sheet to 24 and 82 MPa, respectively, for the DTRL/rubber composite sheets containing ≈10 vol% TEMPO-CNFs. Scanning electron microscopy revealed that no TEMPO-CNF agglomerates are present in the DTRL/rubber composite sheets. The tensile properties of the composite sheets are the best when a X-NBR sheet and NBR latex are used as the matrix rubber and latex in DTRL preparation, respectively. When water-extracted DTRL (WDTRL, mass recovery ratio ≈94%) is used in place of DTRL, the WDTRL/rubber composite sheets show sufficient water resistance, while the tensile properties are almost the same as those of the DTRL/rubber composite sheets.