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.     

Evaluation of the Constrained Blister Test for Measuring Adhesion

The constrained blister test (CBT) was evaluated as a method for measuring adhesion using a model system, electrical tape bonded to polystyrene. Pressure is applied through a circular inlet hole in the substrate, causing the adhesive to “blister” up and peel radially away from the substrate. A glass constraint, placed some distance above the adhesive, limits deformation of the adhesive in the vertical direction and promotes radial peel. By operating at low spacer height (the distance of the constraint above the adhesive) and very low growth rates, the energy spent for deformation of the adhesive and viscoelastic dissipation is minimized. Blister radial growth was linear with time, and growth rate increased linearly with the second power of the energy input. An intrinsic, rate-independent adhesion energy was obtained by extrapolation to zero crack growth rate. The CBT was compared with two peel tests. The dependence of the growth rate on energy input was different, but the extrapolation to zero growth rate gave the same value of the intrinsic adhesion energy.     

PEEL ADHESION PROPERTIES OF AIR-DRIED PHARMACEUTICAL PRESSURE SENSITIVE ADHESIVE

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.     

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.     

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.     

Quantifying the Relationship Between Peel and Rheology for Pressure Sensitive Adhesives

An attempt is made in this work to model quantitatively the peel force vs. rate behavior of a pressure sensitive adhesive (PSA) tape. The approach follows suggestions of previous authors in modeling the deformation of the PSA as uniaxial extension of individual strands. A debonding failure criterion based on stored elastic energy density is used. In this work, experimental measurements of dynamic mechanical master curves are used to provide the mechanical properties of the PSA in the model. The predictions are compared with experimental peel force vs. rate master curves on tapes made from those same adhesives. The only adjustable parameter for the fitting is the quantity related to the debonding criterion. In this set of natural-rubber-based PSAs, the general shape of the peel master curve and the changes in peel behavior associated with tackifier loading and rubber molecular weight are well explained by the model. The effect of changes in substrate chemistry are not well explained.     

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.     

Adhesion of styrenic triblock copolymers to miscible blends of polystyrene and a phenylene ether copolymer

The adhesion of a triblock copolymer having short styrene end-blocks and a hydrogenated mid-block to a polystyrene containing substrate was studied using both lap shear and peel test methods. The two approaches gave very similar results. Within the limits examined, the adhesive bond strength did not depend significantly on bonding temperature or time. However, the adhesive strength did increase substantially as a phenylene ether copolymer or PEC, essentially poly(phenylene oxide), was added to the substrate. This effect is believed to be the result of the exothermic mixing of PEC with polystyrene that causes an additional driving force, other than combinatorial entropy, for interpenetration of segments of the substrate and the styrenic phase of the block copolymer at the interface. Attempts to use a block copolymer having longer styrenic segments resulted in adhesive bond strengths so large that cohesive failure occurred first.     

Energy and force distributions during mandrel peeling of a flexible tape with a pressure-sensitive adhesive

Peel tests are commonly used to investigate the strength of adhesive joints. In the mandrel peel test the curvature of the flexible tape within the zone of detachment is controlled by the radius of the mandrel and this, coupled with an energy balance approach, allows separate determinations of the energy terms associated with any plastic or inelastic deformation of the tape and the de-adhesion or peel energy. If, in addition, the force on the mandrel is monitored then the position of the force vector within the separation or de-adhesion zone can be established. This, in turn, permits a closer comparison with predictions from a peel model which allows non-linear behaviour of both the tape and the adhesive layer. The model has been validated by peel tests on cellulose, PVC, aluminium and Teflon tapes carried out at sufficiently low speeds for rate effects within the adhesive to be small. Both the measured values of the de-adhesion energy, which varied from 60 to 160 J/m², and the positions of the force vector within the de-adhesion zone correlated well with those predicted from load-extension tests on samples of the tapes and bulk samples of similar polymeric adhesives carried out at similar rates of deformation.     

The effect of substrate roughness on tape peeling

This study explores the effect of substrate roughness on peel force for tape peeling. In order to assess the effect of substrate roughness on peel force for tape peeling, a new approach based on a peel zone model was established, including rough surfaces as substrate. This approach is applicable for rough surfaces and accomplished by using contact and adhesion theories. Experimental values were used to calculate peel forces for surfaces with a different level of roughness. The results indicated that the peel force for a rough substrate for the same peel angle is smaller than the one for a flat substrate and that increasing root mean square roughness decreases the peel force. This study is a different approach to prove conclusions that may be proved with experiments by using real animals such as Gecko, lizard, etc. Also, this theory is useful for explaining how some animals such as Gecko peels-off from rough surfaces more easily and quickly than from smooth ones.     

The effect of fluorination on the surface characteristics and adhesive properties of polyethylene and polypropylene

Polyethylene and polypropylene have been treated with fluorine/nitrogen or fluorine/oxygen/nitrogen mixtures at atmospheric pressure. Changes in surface chemistry and topography as well as depth of fluorination have been examined using Fourier transform infra-red analysis, X-ray photoelectron spectroscopy (X.p.s.), atomic force microscopy (AFM) and ellipsometry. Even very short exposure times caused a large substitution of the hydrogen atoms by fluorine. No change in surface topography was noticed at magnifications of up to 5000 times with the scanning electron microscope (SEM), but AFM showed that fluorination led to an increase of microroughness. The influence of fluorine or fluorine/oxygen concentration, as well as time of treatment and time of storage before adhesive bonding, on adhesion of polypropylene to steel was investigated with a bending peel test. Significant improvement in peel strength was already achieved with minor fluorination intensity. Increase of fluorination intensity did not lead to further improvement in peel strength. Analysis of the fracture surfaces was carried out with the SEM and by energy dispersive X-ray spectroscopy and X.p.s. The findings showed that the samples failed cohesively in the polymer or directly beneath the fluorinated layer. A model to describe the formation of specific interactions between substrate and adhesive is suggested.     

Shear,tack, and peel of polyisobutylene: Effect of molecular weight and molecular weight distribution

The effect of polymeric molecular weight and molecular weight distribution on pressure-sensitive adhesive performance was studied in a model system of commercial polyisobutylenes. Molecular weight was characterized by size exclusion chromatography and membrane osmometry. Pressure sensitive adhesive performance was assessed by shear, peel, and probe tack testing based on standardized test methods. Lower molecular weight polyisobutylenes (Mw < 600,000) are successful in peel and probe tack testing due to their ability to flow quickly and wet the substrate test surface. They do not function as well in shear, however, where the polymer must resist flow under a load. High molecular weight species, by contrast, perform well in shear resistance tests and less successfully in peel and probe tack testing. Where high and low molecular weight polyisobutylenes are blended to broaden the molecular weight distribution while maintaining constant weight average molecular weight, adhesive performance in shear, peel, and probe tack are improved. All of the adhesive properties tested were found to have their foundation in some fundamental rheological properties of the polymers (e.g., shear viscosity and tensile creep compliance). This suggests the use of fundamental rheological characterization for screening of adhesive formulations over more empirical adhesive testing methods.     

Effects of peel angle on peel force of adhesive tape from soft adherend

In the case of the peeling of adhesive tapes from soft adherends, the contributions of the compressive force at the adhered portion as well as the larger deformation of adherend have essential roles in determining the peeling properties. In this paper, the peel force of an adhesive tape from a soft adherend has been measured to understand the peeling mechanism, which is greatly affected by the peel angle. A commercially available pressure-sensitive adhesive was used as the tape, and a cross-linked polydimethylsiloxane (PDMS) was used as the soft adherend. The purpose of this study is to clarify the effects of the peel angle on the peel behavior of this system at room temperature under different material specifications and different experimental conditions. The factors that affect the peel force of the PDMS adherend included the degree of cross-linking in PDMS, the thickness of PDMS, peel angle, and peel velocity. Two characteristic peel patterns were observed, which depended on the material specifications and different experimental conditions. The peel mechanism was discussed in terms of the deformation of the adherend.     

Investigation of adhesive performances by scanning force microscopy

In this work an original investigation of the polymer adhesive properties by scanning force microscopy (SFM) is proposed. Polyethylene terephthalate (PET)/ethylene vinylacetate copolymers (EVA) assemblies have been separated using a peel test. SFM (tapping mode) is used to study the adhesive surface (EVA) after the mechanical separation. The topographic analysis allows to determine the average roughness due to the polymer surface deformation. A tentative correlation between the surface deformation and the adherence force is proposed. The result show that the polymer which possess the higher adhesive strength presents the greater surface deformation, measured on the SFM image. A relatively good correlation between the evolution of the adherence force and the variation of the surface topography analysed by SFM is highlighted in this original work. Received: 16 July 1997/Revised: 2 September 1997/Accepted: 8 September 1997     

Adhesive failure peak in peel spectra

Peel force spectra for pressure‐sensitive adhesive tapes provide a peel peak in the adhesive failure region. The observed peak behavior is coincident with calculation based on a viscoelastic peel model. It turns out that the origin of the peak is significantly associated with viscoelasticity or short relaxation time of the adhesive. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 264–266, 1999     

Mechanical analysis of structural adhesive for marine joints

A structural adhesive from I.C.R., X07 Extreme®, was fully characterised through tensile test, static and dynamic lap-shear strength test, cleavage peel test and DSC. Lap-shear tests were performed by using either aluminium, stainless steel and glass fibre reinforced polymer as substrate and after aging the specimen under extreme environmental conditions. The performance of this novel adhesive was compared to that of a structural adhesive of a competitor. The work summarises the typical procedures intended for the qualification of a structural adhesive for marine joints.     

Evaluation of natural rubber latex-based PSAs containing aliphatic hydrocarbon tackifier dispersions with different softening points—Adhesive properties at different conditions

Natural rubber latex-based water–borne pressure sensitive adhesives (PSAs) have been formulated with three aliphatic hydrocarbon water-based dispersions (varying softening points) at two different resin addition levels (25% and 50%). Time–temperature superposition analysis using WLF approximations for adhesive peel has revealed that the adhesives formulated with 50% resin addition level show good adhesive behavior. It has also been determined from time–temperature superposition analysis that peel force increases systematically with softening point and peel rate. Correlation of viscoelastic behavior with adhesive properties suggests that at least 50% resin addition level is needed to bring the natural rubber-based formulations into PSA criteria as defined by Dahlquist and others. Adhesive property evaluations performed on a high surface energy substrate (stainless steel) and low surface energy substrate (LDPE) suggested that optimum tack, peel and shear properties at room temperature were obtained for a formulation containing a higher softening point dispersion (95 °C) at 50% resin addition level. Adhesive peel and tack tend to follow softening point trends as well. A 25% tackifier dispersion addition level did not provide any significant adhesion. Humid aging (50 °C and 100% relative humidity) evaluations of the water–borne adhesives seem to correlate well with the room temperature adhesive property observations.     

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.     

Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths

Adhesive behavior in blends of high molecular weight poly(N-vinyl pyrrolidone (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) has been studied using a 180° peel test as a function of PVP-PEG composition and water vapor sorption. Hydrophilic pressure-sensitive adhesives are keenly needed in various fields of contemporary industry and medicine, and the PVP-PEG blends, pressure-sensitive adhesion has been established to appear within a narrow composition range, in the vicinity of 36 wt% PEG, and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers (enhancers of adhesion) in the blends with glassy PVP. However, PEP alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a standard substrate (at PEG content less than 36 wt%), but the same amounts of sorbed water are also capable of depressign adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The PVP-PEG blend with 36% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effect on adhesion has been found to be reversible. The micromechanics of adhesive joint failure for PVP-PEG hydrogels involves the fibrillation of adhesive polymer, followed by fibrils stretching and fracturing as their elongation attains 1000-1500%. Peel force to rupture the adhesive bond of PVP-PEG blends increases with increasing size of the tensile deformation zone, increasing cohesive strength of the material, and increasing tensile compliance of the material, obeying the well-known Kaelble equation, derived originally for conventional rubbery pressure-sensitive adhesives. The major deformation mode upon peeling the PVP-PEG adhesive from a standard substrate is extension, and direct correlations have been established between the composition behaviour of peel strength and that of the total work of viscoelastic strain to break the PVP-PEG films under uniaxial drawing. As a result of strong interfacial interaction with the PET backing film, the PVP-PEG adhesive has a heterogeneous two-layer structure, where different layers demonstrate dissimilar adhesive characteristics.     

PRESSURE-SENSITIVE ADHESION IN THE BLENDS OF POLY(N-VINYL PYRROLIDONE) AND POLY(ETHYLENE GLYCOL) OF DISPARATE CHAIN LENGTHS

Adhesive behavior in blends of high molecular weight poly(N-vinyl pyrrolidone (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) has been studied using a 180° peel test as a function of PVP-PEG composition and water vapor sorption. Hydrophilic pressure-sensitive adhesives are keenly needed in various fields of contemporary industry and medicine, and the PVP-PEG blends, pressure-sensitive adhesion has been established to appear within a narrow composition range, in the vicinity of 36 wt% PEG, and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers (enhancers of adhesion) in the blends with glassy PVP. However, PEP alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a standard substrate (at PEG content less than 36 wt%), but the same amounts of sorbed water are also capable of depressign adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The PVP-PEG blend with 36% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effect on adhesion has been found to be reversible. The micromechanics of adhesive joint failure for PVP-PEG hydrogels involves the fibrillation of adhesive polymer, followed by fibrils stretching and fracturing as their elongation attains 1000-1500%. Peel force to rupture the adhesive bond of PVP-PEG blends increases with increasing size of the tensile deformation zone, increasing cohesive strength of the material, and increasing tensile compliance of the material, obeying the well-known Kaelble equation, derived originally for conventional rubbery pressure-sensitive adhesives. The major deformation mode upon peeling the PVP-PEG adhesive from a standard substrate is extension, and direct correlations have been established between the composition behaviour of peel strength and that of the total work of viscoelastic strain to break the PVP-PEG films under uniaxial drawing. As a result of strong interfacial interaction with the PET backing film, the PVP-PEG adhesive has a heterogeneous two-layer structure, where different layers demonstrate dissimilar adhesive characteristics.