Peel mechanics for an elastic-plastic adherend

The force required to propagate a 180° bend in an elastic-plastic strip has been calculated from elementary bending theory. Measured forces for Mylar strips of various thicknesses, bent to various degrees, were in good agreement with these calculated values. The corresponding additional stripping force in a peeling experiment will depend upon the thickness of the elastic-plastic adherend, becoming zero both for infinitesimally thin adherends and for those exceeding a critical thickness t_c and passing through a maximum value at intermediate thicknesses. Published data are in good agreement with these conclusions. For a strongly adhering strip, higher peel strengths are found for a peel angle of 180°, compared to 90°, and the effect is greater than can be accounted for solely by plastic yielding of the adherend. It is attributed in part to greater energy dissipation within the adhesive layer.     

Effect of Peel Angle Upon Peel Force

Measurements of peel force P per unit width are reported for samples of three adhesive tapes, adhering to two different substrates. In all cases, the work of detachment per unit area of bonded interface was found to depend upon the angle θ of detachment, increasing as θ increases. This effect is attributed to dissipation of energy in bending the tape away from the substrate at the line of detachment, to a greater degree as θ increases. Extrapolation to θ = 0 is suggested as a simple way of minimizing contributions to the observed work of detachment that arise from bending an imperfectly-elastic adhering layer as it is peeled away from a flat rigid substrate. But at small peel angles the tape tends to stretch appreciably. Peeling at 45° is recommended to minimize both effects.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY–METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

Effect of Peel Angle Upon Peel Force

Measurements of peel force P per unit width are reported for samples of three adhesive tapes, adhering to two different substrates. In all cases, the work of detachment per unit area of bonded interface was found to depend upon the angle θ of detachment, increasing as θ increases. This effect is attributed to dissipation of energy in bending the tape away from the substrate at the line of detachment, to a greater degree as θ increases. Extrapolation to θ = 0 is suggested as a simple way of minimizing contributions to the observed work of detachment that arise from bending an imperfectly-elastic adhering layer as it is peeled away from a flat rigid substrate. But at small peel angles the tape tends to stretch appreciably. Peeling at 45° is recommended to minimize both effects.     

Viscoelastic analysis on peel adhesion of adhesive tape by matrix method

Analysis by means of matrix method is presented on the phenomenon of peel adhesion for 90° peeling of adhesive tape. A model of framed structure was assumed to duplicate the viscoelastic behavior of the tape: The adhesive layer is composed of a network structure made by elastic members for lattice elements and viscous members for diagonal elements. Calculated force distribution near the bond boundary showed good agreement with the experimental results of Kaelble. It was also found that the curve of peel rate versus peel force for the cohesive failure occurred in the adhesive layer was S-shaped; the change of peel force was affected severely by particular range of peel rate. For the interfacial failure at the bound boundary, on the other hand, the peel force possessed a maximum value for medium peel rate. Predicted failure mode for the adhesive tape changed from cohesive failure to interfacial failure with increasing rate of separation. Analytical results for the dependences of thickness of flexible members and adhesive layers on peel forces showed qualitative correlation with the experimental results.     

The use of peel tests to examine the self adhesion of polyimide films

Peel tests were used to examine the adhesion between two layers of the polyimide pyromellitic dianhydride-oxydianiline (PMDA ODA). The main thrust of this work was to examine these tests with particular emphasis on yielding in bending of the peeled strips. Two peel geometries and a range of sample thicknesses were used to study interfaces whose strength could be varied over a wide range by changing the cure schedule. The peel strength varied with strip thickness and often reached a peak at an intermediate thickness. The results were shown to agree qualitatively with a combination of two theoretical models for the effects of yielding on peel tests. It was also found that a second problem of polyimide adhesion, the effect of solvent swelling, could significantly enhance the adhesion between polyimide layers.     

THE DEVELOPMENT OF A MANDREL PEEL TEST FOR THE MEASUREMENT OF ADHESIVE FRACTURE TOUGHNESS OF EPOXY-METAL LAMINATES

A mandrel peel test is established for measuring the adhesive fracture toughness of a metal/rubber-toughened epoxy laminate system. By adopting an energy balance analysis it is possible to determine directly both adhesive fracture toughness and plastic work in bending the peel arm around the mandrel. The suitability of the procedure is examined for various types of metal peel arms, which are classified in terms of their ability to deform plastically during the test. The plastic work is also predicted theoretically, and comparisons are made between the measured and calculated values. The fracture energies determined from the mandrel tests are compared with those obtained from 90° fixed-arm peel tests. For the calculations of plastic work in bending in the fixed arm test, various options are used when modelling the tensile stress-strain behaviour of the peel arm material. In addition, the adhesive layer thickness is considered in terms of its influence on the calculation of adhesive fracture toughness.     

A peel adhesion study of electroless Cu layers on polymer substrates

The peel adhesion between two different electroless-plated Cu layers and polymer substrates was studied. Cu was electroless-plated onto polymer substrates using two different commercial solutions with different compositions. The adhesion strength between the electroless Cu layers and polymer substrates was measured with the 90° peel test. The adhesion was influenced by the coverage, grain size, and the thickness of the electroless Cu layer. Poor coverage of the electroless Cu layer increased the density of the pores at the interface between the Cu layer and the substrates, thereby degrading the adhesion strength because of a decrease in the contact area. In addition, the electroless Cu layers with larger nodules and larger grains were softer and had higher peel adhesion since the soft and ductile Cu layer promoted a greater amount of plastic deformation during the peel test. This led to enhanced peel adhesion. Finally, as the thickness of the electroless Cu layer increased, the peel adhesion decreased. The thicker Cu layers are not easily bent. Poor bending of the Cu layer induced less plastic deformation, causing a decrease in the peel adhesion. In conclusion, soft and thin electroless Cu layers with greater coverage are preferred in order to obtain good adhesion.     

Two-Dimensional Analysis of Peeling Adhesive Tape from Human Skin

Adhesive tapes are attached to human skin for various purposes. When they are removed by peeling, discomfort or trauma may occur. Typically, the removed tape is partially covered by skin cells, and peeling involves failure within the substrate (skin), rather than just interfacial failure between the adhesive and the substrate, or cohesive failure within the adhesive. As an edge of the tape is pulled, first the skin deforms outward, and then peeling occurs after some threshold is attained. The literature is reviewed first, and then a two-dimensional analysis is developed. The tape is modeled as an extensible elastica, while the skin is represented as a nonlinear elastic strip with no bending stiffness. In the numerical results, the peel angle varies from 90° to 170°. Shapes of the tape and skin during pulling are determined, and the corresponding force is computed. For a certain peel criterion, the peel force is obtained.     

Strain energy release rates of a pressure sensitive adhesive measured by the shaft-loaded blister test

The elastic solution to the shaft-loaded blister test (SLBT) was adopted to measure the applied strain energy release rate ( G ) of Kapton® pressure sensitive adhesive (PSA) tape bonded to a rigid substrate. The substrates used were either aluminum or Teflon®, a high-energy surface and low-energy surface, respectively. The values of G were calculated from three different equations: (1) load-based, (2) hybrid, and (3) displacement-based. An experimental compliance calibration was utilized to measure the film's effective tensile rigidity, ( Eh ) eff , the results of which are presented in an appendix. Plastic deformation at the contact area of the shaft tip and adhesive results in an overestimated displacement ( w 0 ) (relative to the elastic model), leading to disagreement among the values of G calculated. Estimation of the effective membrane stress in the film, ( N eff ), as well as the reasonable agreement between the value of ( Eh ) determined from a stress-strain experiment and the compliance calibration, suggest that, in spite of the plastic deformation, the assumption of linear elasticity in the crack growth region and hence the validity of the model, is reasonable. The compliance calibration has been shown to improvethe agreement among the values of G calculated from the three equations. Using the load-based equation, the assumed "correct" value of G may be obtained for a thin adhesive coating independent of the film's stiffness even in the presence of plastic deformation at the shaft tip. Comparing the value of G obtained by a pull-off test and the 90° peel test for a single ply indicates that the value of G obtained by the SLBT is of reasonable magnitude, being less than that obtained by the more firmly established pull-off test, and also that undesirable plastic deformation is reduced relative to the 90° peel test. An experimental configuration for studying the effects of liquids on the fracture energy has been demonstrated for the SLBT. This study indicates that the SLBT is an attractive and convenient test method for measuring the strain energy release rate of adhesive films, because of the insensitivity of the load-based equation to the coating stiffness, plus the independence of the value of G on the plastic deformation at the shaft tip, and the reduced plastic deformation at the crack front relative to the 90° peel.     

PEEL ADHESION PROPERTIES OF AIR-DRIED PHARMACEUTICAL PRESSURE SENSITIVE ADHESIVE

ABSTRACT

The performance of a pharmaceutical pressure sensitive adhesive, whose liquid formulation is based on a multicomponent mixture of solvents, has been examined during two peel adhesion types of tests (90° dynamic adhesive strength peel test and 180° release liner peel test). The experiments were carried out under various drying temperatures, initial coating thickness, and types of backing film and release liner. The results show that the peel force depends mainly on the dry film weight of the tested adhesive. The type of the backing film which is used to form the adhesive also affects its peel adhesion properties.     

Strength of adhesive joints with adherend yielding: II. Peel experiments and failure criteria

Peel tests were conducted with an epoxy adhesive on nine rigid-flexible peel configurations: combinations of 1, 2, and 3 mm aluminum adherend thickness and 30°, 60°, and 90° peel angle. The peel model described in an accompanying paper was used to calculate the stress and strain distributions in the adhesive, the strain energy release rates, and the root curvature of the adherend corresponding to steady-state peel failure. Two failure criteria were examined: the critical von Mises strain and the critical fracture energy, G c . The first criterion was found to be essentially independent of the peel angle but dependent on the thickness of the peel adherend. It produced predictions of the peel force that had an average error of 11%. The fracture energy criterion showed that G c depended on the average phase angle of the loading. This criterion was preferred, having an average prediction error of 6% over the nine experimental cases, and requiring fewer free parameters.     

Plasticity in peeling

Contrary to classical theory, a high proportion of bond failures by peeling involve progressive plastic adherend flexural yield. Such yield occurs with adherend thicknesses below a critical value, T_c, which is shown calculable by combining elastic peel mechanics with plastic bending criteria. The geometry of such “peel with yield,” and thence the moment-controlled peel forces, can be accounted for only if the adhesive is also recognized as behaving essentially plastically. Subsequent plastic adherend unbending is important with highly extensible adhesives. The geometry of “legging” peel in such cases is best described by fully plastic mechanics. These are derived and shown to account for literature data on dependencies of peel force upon peel rate and adhesive thickness. “Stick-slip” peel phenomena are indicated to be controlled by recurring interacting plastic–elastic transitions, in both adhesive and adherend: adhesive strain rate is critical in such phenomena. Four regimes of peel behavior can therefore apply as adherend thickness (T) increases, with peel forces proportional respectively to T⁰, T^2/3, T^3/2 (above T_c) and finally controlled by moment limitations due to joint configurational constraints (“cleavage”).     

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.     

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.     

Influence of Layer Thickness on Cohesive Properties of an Epoxy-Based Adhesive—An Experimental Study

Cohesive laws are determined for different layer thicknesses of an engineering adhesive. The shape of the cohesive law depends on the adhesive layer thickness. Of the two parameters of the cohesive law—the fracture energy and the strength—the fracture energy is more sensitive to thickness variation than the strength. The fracture energy in peel mode (Mode I) increases monotonically as the thickness is increased from 0.1 to about 1.0 mm. At an adhesive thickness of 1.5 mm, the fracture energy is slightly lower than for a 1.0 mm adhesive thickness, indicating a maximum between 1.0 and 1.5 mm. In shear mode (Mode II), the thickness dependence is not as strong, but an increasing trend in fracture energy with increasing adhesive thickness is evident. A slight decrease in strength with increasing adhesive thickness is found in both loading modes.     

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.     

Elasto-Plastic Analysis of Adhesively Bonded Symmetric Single Lap Joints Under In-Plane Tension and Edge Moments

An analysis is presented that predicts adhesive shear and peel stresses and strains in an adhesively bonded single lap joint having symmetric configuration with adhesive behavior. The single lap joint is under tension loading together with moments induced by the interactions of the geometric eccentricity and the boundary conditions of the joint. The von Mises yielding criterion is used to relate the adhesive stress components within the yielded region. The adhesive strains are computed from the relative displacements of the adherends and can be considered as an average of the strain variation through the adhesive thickness direction. Example calculations show that the predicted adhesive shear and peel stress and strain profiles are well matched to detailed finite element analysis results. Generally, the analytical model predictions are found to be more accurate when the adhesive thickness is small.     

Elasto-Plastic Analysis of Adhesively Bonded Symmetric Single Lap Joints Under In-Plane Tension and Edge Moments

An analysis is presented that predicts adhesive shear and peel stresses and strains in an adhesively bonded single lap joint having symmetric configuration with adhesive behavior. The single lap joint is under tension loading together with moments induced by the interactions of the geometric eccentricity and the boundary conditions of the joint. The von Mises yielding criterion is used to relate the adhesive stress components within the yielded region. The adhesive strains are computed from the relative displacements of the adherends and can be considered as an average of the strain variation through the adhesive thickness direction. Example calculations show that the predicted adhesive shear and peel stress and strain profiles are well matched to detailed finite element analysis results. Generally, the analytical model predictions are found to be more accurate when the adhesive thickness is small.     

Theory and Analysis of Peel Adhesion: Adhesive Thickness Effects

The existing assumptions concerning boundary stress concentrations in peel adhesion are extended to treat the effects of adhesive thickness. In adhesive bonds involving the all-angle peeling of a flexible elastic adherend from a rigid substrate the varying of adhesive thickness is shown theoretically to predict a proportional increase of peel force (P) with adhesive interlayer thickness (a) when the product (βCa) of the cleavage stress concentration β, cavitation scale factor C, and adhesive thickness a is less than unity. When the product (βCa) becomes greater than unity the new theory predicts that cleavage stresses concentrate within a fractional layer of the total adhesive thickness f(a) and the peel force P tends to achieve a constant value P_max. This new theory is verified by experimental studies and the experimental analysis suggests new optimizations in the design and measurement of the peel adhesive bond.