Effect of Wetting Liquids on the Strength of Adhesion of Viscoelastic Material

The effect of a variety of wetting liquids on the resistance to peeling separation for a lightly crosslinked rubbery adhesive in contact with a Mylar substrate has been studied over a wide range of peeling rates and at two temperatures. Although the magnitude of the peel strength is much greater than the thermodynamic work of detachment, it is reduced by alcohols and alcohol/water mixtures in good agreement with calculated reduction factors. It is concluded that the measured strength is a product of two terms: the thermodynamic work, and a numerical factor, generally large, denoting inefficiency. The latter term is strongly dependent on peel rate and temperature for viscoelastic adhesives. Two anomalies are pointed out: particularly low adhesion is observed at low rates of peel for certain liquids, attributed to swelling of the adhesive, and smaller effects are found for some other liquids than predicted.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

An Elasto-Plastic Investigation of the Peel Test

This work outlines an elasto-plastic investigation of two common peel tests which use high and low yield strength aluminium adherends. An elastic, large-displacement, finite element program has been extended to include elasto-plastic material behaviour. This has been used to analyse both peel tests. The adhesive stresses near the crack tip have been shown to be finite while the corresponding strains remain singular. A failure criterion based on a maximum adhesive strain has been used to predict the relative strengths of the peel test. The amount of energy dissipated in the plastic deformation of the peeling adherends has been assessed by a series of tests and has been shown to be a considerable amount of the total energy supplied to the peeling system. Further, although the two aluminium alloys considered have grossly different yield strengths the energies dissipated in plastic deformation are similar. Material data for the finite element analysis and the plastic work calculations have been obtained from uniaxial tensile tests of both the adherends and the adhesive and actual peel strengths have been measured in a series of peel tests.     

An Elasto-Plastic Investigation of the Peel Test

This work outlines an elasto-plastic investigation of two common peel tests which use high and low yield strength aluminium adherends. An elastic, large-displacement, finite element program has been extended to include elasto-plastic material behaviour. This has been used to analyse both peel tests. The adhesive stresses near the crack tip have been shown to be finite while the corresponding strains remain singular. A failure criterion based on a maximum adhesive strain has been used to predict the relative strengths of the peel test. The amount of energy dissipated in the plastic deformation of the peeling adherends has been assessed by a series of tests and has been shown to be a considerable amount of the total energy supplied to the peeling system. Further, although the two aluminium alloys considered have grossly different yield strengths the energies dissipated in plastic deformation are similar. Material data for the finite element analysis and the plastic work calculations have been obtained from uniaxial tensile tests of both the adherends and the adhesive and actual peel strengths have been measured in a series of peel tests.     

Peel Strength of Aluminium-Aluminium Joints Bonded by Adhesive Based on Carboxylated Nitrile Rubber-Chlorobutyl Rubber Blend

The peel strength of aluminium-aluminium joints bonded by an adhesive based on carboxylated nitrile rubber and chlorobutyl rubber was found to depend on surface topography and use of a silane primer. Anodization causes a marginal increase in bond strength while the silane primer improves the adhesive joint strength remarkably.

The peel strength was also found to be dependent on test conditions (test rate and temperature). The threshold peel strength value obtained by measurements at low peel rate and high test temperature was found to depend on the type of failure during peeling (cohesive or interfacial) which, in turn, is controlled by the presence of silica filler in the adhesive. Two different threshold values of peel strength were obtained: 60 N/m for interfacial failure (in silica-filled adhesive), 140 N/m for cohesive failure (in unfilled adhesive).     

Failure mechanisms in peeling of pressure-sensitive adhesive tape

Studies on the peel behavior of pressure-sensitive tape comprising a polyester backing and polyacrylate adhesive have shown that, in peeling from a plane glass surface, three fundamentally different modes of peeling may be distinguished, depending upon the rate of pulling. At low rates, deformation by flow of the adhesive appears to determine the peel behavior and the peel force is strongly rate dependent. At high rates, little or no viscous deformation of the adhesive occurs and the peel force is independent of rate. At intermediate pulling rates, cyclical instability of made of failure involving alternate storage and dissipation of elastic energy in the backing, results in the phenomenon of “slip-stick” peeling, in which failure is jerky and regular. Results have been obtained which show how the pulling rates at which transitions from one mode of peel to another occur, and the peel force values for a given type of failure, depend upon such factors as molecular weight of adhesive, thickness of backing film, and angle of peeling.     

Peel mechanics of adhesive joints

A review is given of the mechanics of peeling rupture of an adhesive joint, consisting of a flexible adhering strip peled away from a layer of adhesive. Attention is drawn to a number of anomalous results that cannot be accounted for solely, in terms of the thermodynamic work of formation of two new surfaces. The work of detachment is found to be generally much larger than the theoretically-predicted amount. Moreover, the value obtained is greater for thicker layers of adhesive, and for detachment at a peel angle of 180° rather than at 90°. Also, it is found to increase with increasing thickness of the adhering strip, passing through a maximum value in some cases and then decreasing as the strip thickness is increased still further. All of these effects are attributed to dissipative processes, for example, plastic yielding, in one or both of the adhering layers as they are peeled apart. Some quantitative relationships are given for the additional peel forces arising from plastic yielding of the adherend or the adhesive.     

Effect of Peel Load on Stringiness Phenomena and Peel Speed of Pressure-Sensitive Adhesive Tape

The factors governing interfacial separation in lightly cross-linked polymer adhesives at low pulling rates as demonstrated by their stringiness phenomenon are investigated.

Cohesive failure and adhesive/substrate interfacial separation of uncross-linked polymer adhesives have been adequately explained. However, in lightly cross-linked polymer adhesives, where cohesive failure cannot occur because there is no viscous flow, there are two regions of interfacial separation at low rate and this phemonenon cannot be readily explained by present viscoelastic theories.

Investigation of the stringiness phenomenon of peeling pressure-sensitive adhesive tapes at constant loads shows that two peeling speeds exist for any peeling load up to the vicinity of 200 g/25 mm. Also it is clear that stringiness structure differs greatly at each peeling speed. The stringiness phenomenon of each of these two regions is analyzed using Miyagi's observation apparatus. These two measurements are then reversed and a comparison shows that the two peeling speeds correspond to each steady peeling region.

This field of investigation, when added to the present viscoelastic property studies, should lead to a new peeling adhesive theory which, in turn, may lead to the development of new high peel force pressure-sensitive adhesives.     

Calorimetric Measurements of The Heat Generated by The Peel Adhesion Test

The peel test is a simple mechanical test commonly used to measure the adhesion of flexible films bonded to rigid substrates. When the film is deformed elastically during peeling, the peel force is a direct measure of the strength of the interface. However, when plastic deformation takes place, the work of detachment is much larger than the thermodynamic work of forming the fracture surfaces. Simultaneous mechanical and calorimetric measurements of the work of detachment and the heat generated during the peeling of polymeric films from metal substrates and metal films from polymeric substrates have been made. An energy balance for peeling has been proposed. Most of the work of peeling was consumed by plastic deformation. The peeled polymer dissipated approximately one half of the work of peeling as heat and most of the remainder was stored in the peeled material. The peeled metal dissipated most of the work of peeling as heat.     

On the work of adhesion and peel strength between pressure sensitive adhesives and the polymeric films used in LCD devices

Peel strength, a convenient measure of bond strength in adhesive/adherend systems, is known to be a function of various factors such as the thermodynamic work of adhesion, rate of measurement, thermal history, and temperature. Generally, it is believed that the work of adhesion is primarily involved in the first stage of adhesion through wetting phenomenon and beyond that its role diminishes in that the portion of thermodynamic contribution to actual bond strength is insignificant. In practice, however, we often observe that a suitable surface treatment increases the surface energy of the substrate, which further enhances the bond strength. One practical example is the surface treatment carried out in LCD industry to obtain sufficient bond strength between pressure sensitive adhesives and polymeric films. To further our understanding of the effect of surface treatment, we attempted to establish a possible correlation, if any, between the thermodynamic work of adhesion and peel strength. For this, we carefully measured the contact angles of water and diiodomethane against various polymeric films, and calculated the surface energy and the thermodynamic work of adhesion using the two widely used approaches: Young-Fowkes-Girifalco-Good, and Wu methods. Before establishing a correlation, some general aspects of the above two methods are discussed. The values of the work of adhesion obtained were compared with the measured peel strength. Indeed, we observed a clear correlation between the two quantities: the increase of the work of adhesion led to the increase of peel strength. As a reason for this correlation, we proposed that the increase of surface energy might be associated with the increase of various surface functional groups, which, in turn, contributed to the formation of chemical bonding with the PSA leading to the increase of peel strength.     

Effect of adhesive thickness on the stringiness of crosslinked polyacrylic pressure‐sensitive adhesives

The effect of adhesive thickness on stringiness behavior during 90° peel testing was investigated for crosslinked poly(n‐butyl acrylate‐acrylic acid) (A) and poly(2‐ethylhexyl acrylate‐acrylic acid) (B) with a constant crosslinker content. The adhesive thickness was varied over the range from 15 to 60 μm. All adhesive thicknesses exhibited sawtooth‐type peeling with a front frame for B, but only the 30‐μm thickness generated a front frame‐type for A. The peel rate decreased from 15 to 45 μm and plateaued above 45 μm under a constant load test. These results indicate that the adhesion strength increases with adhesive thickness, but reaches a constant value at high thicknesses. The stringiness was also analysed for B and the sawtooth interval observed to increase with increasing thickness. This means the sawtooth number decreased. As a result, the concentrated stress per sawtooth induces easier peeling and so this factor tend to increase the peel rate. Conversely, the stringiness width increased with increasing thickness. The stress load over the stringiness region decreased with an increase in thickness, meaning that a decrease in the concentrated stress decreases the peel rate. The actual peel rate is influenced by the contributions of these two factors. The strain rates during constant peel rate tests decreased slightly with increasing thickness, due to a reduction in the apparent modulus. The molecular mobilities near the adherend and the backing surfaces were evidently restrained by these surfaces, and the relative rates of motion of such restrained molecules decrease with increased thickness. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42210.     

Stress Relaxation in Peel Adhesion

The peeling of an adhesive joint consisting of an SBS copolymer and two Mylar film substrates proceeds by cohesive rubber rupture, and the strength increases with test rate. Stress relaxation during peeling is shown to account for this behavior and relaxation data after peeling is used to predict the rate dependence of the peel force.     

Nonstationary peeling apparatus

A piece of apparatus for measuring the peel adhesion behavior of pressure-sensitive adhesive tapes was developed. The apparatus allows peel rates to be varied as any functions of time or peel distance. In a cycle run of the accelerating and decelerating peel processes, nonstationary peeling was investigated over a wide range of peel rates using relatively short-length samples. The resultant behavior in nonstationary peeling indicated that in the interfacial failure regime, a good agreement with the normal stationary peeling can be obtained. This nonstationary peeling apparatus is expected to be useful for evaluation of peel adhesion. © 1994 John Wiley & Sons, Inc.     

An analytical model for interfacial stresses in double-lap bonded joints

Due to their many advantages, adhesively bonded joints are widely used to join components in composite structures. However, premature failure due to debonding and peeling of the joint is the major concern for this technique. Existing analytical models suffer from two major drawbacks: 1) not satisfying zero-shear stress boundary conditions at the adhesive layer’s free edges^[1] and 2) failure to distinguish the peel stress along two adherend/adhesive interfaces^[2]. In this study, we develop a novel three parameter elastic foundation (3PEF) model to analyze a representative adhesively bonded joint, the symmetric double-lap joint, which is believed to have relatively low peel stresses. Explicit closed-form expressions of shear and peel stresses along two adhesive/adherend interfaces are yielded. This new model overcomes the existing model’s major drawbacks by satisfying all boundary conditions and predicting various peeling stresses along two adherend/adhesive interfaces. It not only reaches excellent agreement with existing solutions and numerical results based on finite element analysis but also correctly predicts the failure mode of an experimentally tested double-lap joint. This new model therefore reveals the peel stresses’ significant role in the failure of the double-lap joint, but the classical 2PEF model cannot create it.     

Analytical Peel Load Prediction as a Function of Adhesive Stress Concentration

Peel data for two epoxy adhesives and a recent model of the adhesive stresses in the peel geometry are used to investigate the effectiveness of two constitutive models and several adhesive failure criteria. The failure criteria are based on either the critical strain energy-release rate or the critical von Mises strain at the peel root, both taken as functions of the “loading zone length” (LZL), defined as a measure of the degree of stress concentration at the root of the peeling adherend. The peel model uses LZL as an independent parameter that captures the effects of the peel angle, adherend thickness, and the mechanical properties of the adhesive and adherend. Both the energy- and strain-based failure criteria can be used to predict the steady-state peel load with an average absolute error of less than 10% over the range of conditions that were examined.     

A generalized cohesive zone model of the peel test for pressure-sensitive adhesives

The peel test is a commonly used testing method for adhesive strength evaluation. The test involves peeling a pressure-sensitive tape away from a substrate and measuring the applied peel force. In the present study, a cohesive zone model, in which the adhesive fails cohesively, is proposed to analyze the mechanics of the peel test. The proposed model is capable of predicting the dependence of the peel force, as well as the traction distribution across the peel front, on factors such as the peel rate, the peel angle, the nature of the adhesive, the mechanical properties and geometries of the backing and the substrate.     

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.     

Comparison of Peel Tests for Metal-Polymer Laminates for Aerospace Applications

Standard peel tests for aerospace laminates based on metal-polymer systems, namely floating-roller and climbing-drum peel methods, have been accommodated in a unified theory of peeling. This theory also accommodates more basic peel tests such as T-peel and fixed-arm peel and also newer methods such as mandrel peel. These five methods have been applied to two aerospace laminate systems to critically examine their use in the determination of adhesive strength. The theory has been used to unify the outputs from the tests in terms of adhesive fracture toughness. In this way, the comparative merits of the methods can be commented on.

The validity of the standard methods has been put in doubt because of the absence of a correction for plastic bending energy and also because of the poor conformance of the peel arm to the roller system used in these methods. The unified theory and some measurements of peel-arm curvature help but not completely overcome some of these difficulties.

A further complication that arises in peel is a change in the plane of fracture. This reflects a transition from cohesive fracture in the adhesive to an adhesive fracture at the interfaces among adhesive, primer, and substrate. It is likely that such plane-of-fracture phenomena are intrinsic to evaluation of the laminate and that contemplation of cohesive fracture toughness for the adhesive cannot accommodate such events.     

Sawtooth-shaped stringiness with front frame formation for polyacrylic pressure-sensitive adhesives with two different molecular structures

The formation of sawtooth-shaped stringiness during 90° peeling was investigated using crosslinked poly(n-butyl acrylate–acrylic acid) and poly(2-ethylhexyl acrylate–acrylic acid) random copolymers with an acrylic acid content of 5 wt.% and different crosslinking degrees as pressure-sensitive adhesives (PSAs). The gel fraction was measured by toluene extraction of PSA, and it increased with crosslinker content for both systems. The observed stringiness was sawtooth-shaped, but there were three different types; both the typical sawtooth shape and the frame formed at the front tip with interfacial failure, and the sawtooth shape formed with cohesive failure. The change in the stringiness shape was affected strongly by the gel fraction of PSA. The peel rate under constant peel load was measured and revealed that the peel rate was lowest upon formation of the front frame type. A good relation was found between peel rate and peel strength, with a greater peel strength upon formation of the front frame type. The concentrated stress at the peeling tip is released by progress of peeling and deformation of the adhesive layer (stringiness) for no frame type. On the other hand, the sufficient interfacial adhesion delays the progress of peeling, and the applied larger stress causes cavitation in the PSA layer for front frame type. The formed cavity grows and the front frame type formed as a result. That is, internal deformation occurred preferentially over peeling. In order to improve the peel strength, the front frame type is the most useful stringiness shape.