PEELING SHEAR AND CLEAVAGE FAILURE DUE TO TAPE PRESTRAIN

In 1960 Kaelble published data for the peeling of several tapes secured by pressure-sensitive adhesives at a range of peeling angles. Several of the cellophane tapes showed a small dropoff or “jog” in the value of the peeling force of approximately 0.1-0.2 lbf when the peel angle was in the range of 20-40 degrees. The jog was associated with a relatively rapid change in decohesion mechanism from one of cleavage to one in which shear played a much larger role, and a similar but much larger effect was noted with metal foil tapes. These observations were in contrast to data presented some months earlier in which no such “jog” had been seen using a similar cellophane tape tested in much the same way. The setup for all of these tests consisted of a stripping wheel to which the tape had been roll-bonded by a wheel of 1/2 inch diameter loaded at an intensity of 6 lbf/inch. Although there have been several attempts to explain the dropoff in peel force, none have been entirely successful so far. An analysis of this effect is presented based on the magnitude of the prestrain in the tape introduced by the roller bonding method of attaching the tape to the stripping wheel. This is consistent both with these observations and some further tests we have ourselves conducted at lower than usual peeling angles.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T, where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T, i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

The Use of Large Diameter Monofilament for Improving Peel Resistance of a Brittle High Temperature Adhesive

Peel force measurements as a function of adherend thickness are reported for adhesively bonded specimens based on a cyanate ester resin and aluminium adherends. It has been demonstrated that by incorporating large diameter (0.28mm) PTFE monofilament within the adhesive bond then the peel force and associated fracture energy can be increased significantly over that for specimens based on adhesive alone. Fracture energy measurements are derived for specimens with peeling adherend thickness of up to about 0.6 mm using the 90° peel test. Fracture energies are also derived for peeling of more practically-representative 1.6mm thickness adherends using a single cantilever beam experiment. In-situ photoelasticity and SEM microextensomctry experiments are reported which show the stress fields and displacements associated with the presence of the monofilament. It is believed that the reported increase in measured fracture energy is partly due to the crack pinning effect of the monofilament, and partly due to the monofilament creating a “load shadowed” region between adherend and monofilament which prevents the interfacial crack from propagating between adherend and adhesive.     

Deformation mechanisms at the interface between grafted polyethylene and ethylene/vinyl alcohol copolymer

The adhesion of grafted polyethylene (PEg) to an ethylene/vinyl alcohol copolymer (EVOH), has been studied using different peel configurations and angles. Overall peel energies have been obtained and found to depend on peel angle. Experimental and theoretical studies of local peel arm curvature and opening angles near the crack front led to good agreement, the latter being based on elastic foundation theory and global elasto-plastic analysis. Having established the validity of the analysis used, the contribution to the peel energy pertaining to bulk bending of the peel arm(s) was estimated, allowing the local adhesion energy to be isolated. This was found to be virtually independent of peel angle. Scanning electron microscopy examination revealed a plastic, fibrillar craze zone in the PEg corresponding to a Dugdale zone. Nevertheless, adhesion energy was higher than expected from the Dugdale model. Energy dissipation in the vicinity of the Dugdale zone associated with shear deformation, and thus without apparent cavitation, may contribute to fracture energy. A rough estimate of the energy expended during the observed change in orientation of fibrils in the relaxation zone after the crack tip shows this contribution to be significant.     

Application of the Island Blister Test for Thin Film Adhesion Measurement

The island blister test has recently been proposed as an adhesion test which allows the peel of thin, well-adhered films without exceeding the tensile strength of the film. The island blister test site is a modification of the standard blister test site, consisting of a suspended membrane of film with an “island” of substrate at the film center. The membrane support and island are secured to a rigid plate and the film is pressurized, peeling the film inward off the island. A model for this inward or “annular” peel indicates that even for systems of good adhesion, peel can be initiated at low enough pressures to prevent film failure by making the center island sufficiently small relative to the size of the film.

We have fabricated island blister test sites using micromachining techniques and have used them to measure the debond energy of polymer films on various substrates. The peel data obtained from these island sites match well to the behavior predicted by a simple fracture mechanics analysis. This paper reports the fabrication of the island test sites, the experimental verification of the test, and the results of application of the test to polyimide films on metallic and polymeric substrates.     

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.     

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.     

The Influence of Surfactants on the Peel Strength of Water-based Pressure Sensitive Adhesives

In order to study the effect of surfactants on the adhesive properties, peel measurements were performed with two series of model polymers of ethylhexylmethacrylate (PEHMA), the first prepared by emulsion polymerization with four anionic surfactants, and the second by post-adding the same surfactants to a surfactant-free latex. Cohesive fracture is observed at low peel rates; the peel strength depends on the bulk mechanical properties and is independent of the emulsifier. A transition to another type of separation occurs at higher peel rates, which seems to be an interfacial failure by visual inspection. Surface analytical studies, however, give evidence that this “interfacial” failure is, in fact, a mixed failure, leaving traces of the polymer on the substrate surface. The peel rate at this transition as well as the peel strength at mixed fracture are influenced by the surfactants. Large differences were observed between the four surfactants as well as between both series of polymers, leading to the conclusion that the surfactants have a different mobility within the film. This is also reflected by a different aging behaviour of the films.     

Stress Distribution during Peeling of Adhesive Tapes

The pattern of stress distribution observed during the peeling of pressure sensitive tapes is not adequately described by existing theoretical analyses of peel, which make over-simplifying assumptions. In particular, the consequence of filamentation or “legging” in the peeling zone is neglected by the theories. In the present work an attempt is made to assess the effect of filamentation by analysis of the peeling profile obtained by photography. The deflection of the backing film from its unrestricted “bent beam” configuration is interpreted in terms of a “filamentation force”. The stress distributions obtained show that filamentation makes an important contribution to the peel force and show good correlation with the results obtained from related systems by a different experimental method.     

The Contact Adhesion of Self-Adhesive Strain Gauges

Thin pieces of flexible polymers may be adhered to rigid substrates merely by pressing the two surfaces into intimate contact. The peel strength of the bond is poor, but the shear strength can be sufficiently high for strain gauge operation. This paper offers an explanation for the adhesion which can account quantitatively for the behaviour of various materials. Frictional phenomena, resulting from a normal force produced by the surface tension of a surface film, can explain why a gauge with a PVC body does not slip. With other polymers, other liquids or electrostatic effects may be of importance. Any surface “fluid” film of sufficiently high viscosity that viscous effects are important will behave as an integral part of the elastomer.     

Interfacial and Bulk Contributions to Peeling Energy

Thin polyurethane films, having low adhesion to dried protein, were developed as candidate materials for non-adhesive surgical dressings. In order to model wound-adhesion, gelatine was cast from solution on to the film and allowed to dry. The film was peeled from the gelatine at 180° peel angle, and the peel force measured as a function of the temperature of test. The dynamic mechanical properties of the films were measured over the range -90°C to 110°C and values of tan δ were determined at the temperatures employed for peeling. Thus, a correlation was obtained between peeling energy and tan δ for each of eight films.

The generalised theory of fracture mechanics states that the adhesive failure energy is given by the product of an interfacial energy term and a “loss function” involving the hysteresis ratio of the material. If the strains are small the hysteresis ratio is proportional to tan δ. The experimental results show excellent agreement with the theory, but the interfacial term turns out to be much greater than the true interfacial energy (or thermo-dynamic work of adhesion). The reason for this result is discussed.     

Predicting Thermal Efficiency in Timber Radio Frequency Vacuum Drying

In this work, the efficiency of transforming dielectric energy into evaporated water is analyzed for the case of timber radio frequency vacuum drying. Based on well-known heat and mass transfer equations, a simplified mathematical model is proposed that estimates the drying efficacy in regards to the thermo-physical properties of wood. Although not exact, the theoretical results are close to the experimental observations and elucidate some phenomena like the tendency of the timber to dry from inside to outside, and the drying rate increase with the rise of the timber gas permeability. The theoretical efficiency model also predicts a range of wood permeability values for which the drying efficiency changes from 100 to 0%, thus providing a quantitative scale for classifying the spectrum of “difficult-to-dry” all the way to “easy-to-dry” wood species when using radio frequency vacuum technology.     

Studies on Adhesion Between Natural Rubber and Polyethylene and the Role of Adhesion Promoters

Studies on adhesion between natural rubber (NR) and polyethylene (PE) with different levels of interaction (physical and chemical) have been carried out. Ethylene propylene diene rubber (EPDM) and chlorinated polyethylene (CPE) were used as physical promoters and epoxidised natural rubber/modified polyethylene (ENR/PEm) and sulfonated ethylene propylene diene rubber/modified polyethylene (S-EPDM/PEm) were used as chemical adhesion promoters. The failure surfaces were examined with the help of scanning electron microscopy (SEM), optical photography and electron spectroscopy for chemical analysis (ESCA) techniques.

The peel strength between natural rubber and polyethylene as measured in this study is 140 J/m². With the incorporation of physical promoters such as EPDM, the peel strength increases twenty fold because of structural similarity of EPDM with PE and the rubbery nature of EPDM. Similarly, the other promoters show significant improvement in peel strength. At high temperature and low rate of peeling, the nature of failure is mainly “stick-slip” for joints with interaction promoters. The average peel strength increases with increase in test rate and decrease in test temperature for most of the joints. All the data could be shifted onto a master curve indicating that the increase in strength is a result of viscoelastic dissipation. NR/EPDM/PE and NR/CPE/PE systems, however, behave in a different way probably because they alter the nature of crack propagation at or near the interface. ESCA results of the peeled PE surface show a chemical shift of C_1S peak. SEM photographs also indicate interaction at the interface when modifiers are used. An increase in crystallinity of PE from 30% to 64% and modulus increase the peel strength of NR/PE joints by a factor of four. The results of peel strength measurement at 90° are lower than those at 180°. Lap shear results are in line with peel strength.     

A Review of Adhesion Mechanisms Using the Peel Test in Air and Liquid Media

This paper deals with the analysis of peel energy of assemblies measured in different environments, i.e. in air and in the presence of liquids, and constitutes a brief review of the work of Professor Schultz' team in this domain. It is shown how such measurements can lead to a better knowledge of the nature as well as of the magnitude of fundamental interactions established at the interface between two solids. Earlier experiments have shown that peel energy can be expressed as a product of three terms corresponding, respectively, to the reversible energy of interfacial adhesion, the hysteretic losses of the bulk materials and the molecular dissipation near the crack front during peeling. This approach is well-verified when only physical interactions (van der Waals) are involved at the interface. However, more complex cases correspond to systems where specific interactions are also established between both materials, in particular acid-base interactions and creation of chemical bonds. In both cases, peel measurements in liquid media can lead to the determination of fundamental parameters, such as the interfacial density of specific interactions at the interface and the acid-base or chemical components of the work of adhesion. Finally, the effect of interdiffusion phenomena on peel energies can also be investigated in the case of elastomer/elastomer assemblies.     

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.     

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.     

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.     

ON THE MODELLING APPROACHES OF BIOMASS BEHAVIOUR IN BIOREACTOR

This paper offers a critical review of various modelling techniques to describe the behaviour of microorganisms in a bioreactor. Several approaches from the simplest unstructured to the structured stochastic type are examined to highlight the basic methodologies, the capacity to describe the phenomena occurring and the diOiculties on the experimental validation. The effects of macromixing are updated using the chemical reactor methodologies. The inside mechanisms of microorganisms and their interaction with reactor fluid dynamics are outlined in term of comparison of their relaxation time; this suggestion provides a useful tool to analyze continuous bioreactors. Lastly the potentialities of qualitative and semi-quantitative modelling approaches are outlined to study biochemical phenomena and bioreactor features, focusing the attention on “fuzzy” and o[M] (order of magnitude) techniques.     

A Kinetic Approach to the Oxidation of Linseed Oil as Influenced by Fruit Peel and Seeds of Pomegranate

The objective of this research is to analyze the kinetic parameters of linseed oil by treating it with pomegranate peel and seeds at 353, 368, and 383 K using Rancimat. There are no significant differences (p < 0.05) between the oxidative stability indices of samples containing pomegranate peel and pomegranate seeds. In addition, the indices pertaining to the oxidative stability of linseed oil increase in value as the concentrations of pomegranate peel and seeds increase. Apart from the pomegranate peel at 0.1% and the quercetin, all other antioxidants are able to reduce the severity of temperature‐related parameters (i.e., temperature coefficient and Q₁₀ values). In addition, these antioxidants are able to form an activated complex with lower levels of thermal energy (by reducing activation energy and enthalpy) but of more structured configuration (by reducing the frequency factor and entropy). A high correlation is found between the Gibbs free‐energy of activation and the oxidative stability index of samples. The most substantial increase in the Gibbs free‐energy of activation occurs by TBHQ, followed by gallic acid, quercetin, and pomegranate peel at 1%. Practical Applications: Linseed oil is characterized by its high amounts of essential polyunsaturated fatty acids (PUFAs). The high PUFA content of linseed oil contributes to its rapid oxidation. Pomegranate peel and seeds are valuable sources of natural antioxidants. Investigating the effect of pomegranate peel and seeds in reducing the dependence of linseed oil oxidation on temperature provides a range of valuable kinetic parameters. Nevertheless, this subject has received little attention so far. There are few published data regarding the effect of natural antioxidants on lipid oxidation by this approach. Accordingly, this study is designed to investigate the oxidation kinetics and mechanisms of linseed oil as manipulated by fruit peel and seeds of pomegranate. The results show that the fruit peel and seeds of pomegranate can improve the oxidative stability of linseed oil and reduce the severity of effects caused by undesirable temperatures that may increase the oxidation rate of linseed oil.     

Fracture Toughness of a Silane Coupled Polymer-Metal Interface: Silane Concentration Effects

Fracture toughness of joints made from a glassy, 343,000 molecular weight polystyrene block bonded to chromic-sulfuric acid etched or phosphoric acid anodized aluminum are investigated. The fracture tests are performed with a 90-degree peel apparatus under “dry” laboratory conditions and “wet” conditions created by submerging the apparatus in a temperature controlled water bath. The bond strengths are controlled using various concentrations of styrl silane coupling agent added directly into the styrene monomer solution that polymerizes against the aluminum. Ellipsometric measurements on smooth silicon surfaces verify that the thickness of bound polymer is controlled by the silane to polystyrene mole ratio. X-ray photoelectron spectroscopy (XPS) analysis of fractured surfaces indicates that the fracture is near the aluminum surface. Both the wet and dry fracture energy as a function of bound polymer thickness on acid etched aluminum joints resemble quite closely the adhesion literature results obtained by fracturing pairs of fused, immiscible glassy polymers. Reasons for this similarity are discussed.