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.     

Experimental determination of the peel adhesion strength for metallic foils

This work addresses the problem of experimental measurement of peel adhesion in cases where a non-recoverable (plastic) deformation energy of the peeled foil, plus frictional losses, constitutes a significant portion of the total peel energy. In standard tests, when the true adhesion strength is desired, the plastic energy has to be calculated and deducted from the total energy. Several studies have been dedicated to the modelling and calculation of the energy dissipated through plastic deformation so that the net adhesion energy could be deduced. These calculations are cumbersome and impractical for general use. A simple experimental technique for the determination of the net adhesion strength is proposed. Experimental results with ～ 0.1 mm thick foils of stainless steel, nickel, and titanium confirm the theoretical predictions regarding the energy balance during peeling. Using the proposed methodology, there is no need to calculate or otherwise determine the deformation energy losses of the peeled foil or the frictional dissipation. The method is not limited to a particular material and can be used successfully for strain hardening plastic as well as metallic foils. Peel tests on adhesively bonded specimens of stainless steel and nickel and of a thermal spray-coated Ti alloy foil were carried out to demonstrate the applicability of the proposed method.     

Changes in the Peeled Surfaces of Copper Thin Films Deposited on Polyimide Substrates After Heat Treatment

Peel strength between a copper (Cu) thin film and a polyimide (pyromellitic dianhydride-oxydianiline, or PMDA-ODA) substrate is reduced by heat treatment at 150°C in air. In this work, we investigated the peel strength, the morphology of the interface between Cu films and polyimide substrates using optical microscopy and electron microscopy, and chemical change of the interface using Auger electron spectroscopy (AES) and micro X-ray photoelectron spectroscopy (XPS). The analysis showed that CuO “lumps” were present on the peeled surface of PMDA-ODA after heat treatment at 150°C in air. The peeled surfaces of other polyimide substrates were also analyzed: biphenyl dianhydride-para phenylene diamine (BPDA-PDA) and biphenyl dianhydride-oxydianiline (BPDA-ODA). CuO lumps were present on the peeled surface of BPDA-ODA after the heat treatment, but not that of BPDA-PDA. Compared with the adhesion strength for the Cu thin film, the adhesion strength was high for the Cu/PMDA-ODA and Cu/BPDA-ODA laminates, but the adhesion strength was very low for the Cu/BPDA-PDA laminate. This low strength is the reason that CuO lumps were not detected on the peeled surface of the BPDA-PDA substrate. These CuO lumps were related to the adhesion degradation of the Cu/polyimide laminates after the heat treatment.     

The Effects of Viscoelasticity in the Peeling of Polymeric Films

A method to compute the interfacial fracture energy of a polymer film bonded to a rigid substrate, using the measured quantities of a peel test, is presented. The formulation is general and will accept any polymer which can be modeled as a linear viscoelastic material. The method is used to analyze the peeling of a polyimide film and a relationship between the fracture energy, the peel force, the speed of peeling and the thickness of the film is derived.     

Temperature and humidity effects on adhesion of polyimide film to a silicon substrate

A 90° peel tester with substrate heating capability was built to evaluate the adhesion strength of polyimide films to a silicon substrate. The effects of polyimide film thickness and peel rate on polyimide adhesion to a silicon substrate under high or low humidity, and at elevated temperatures, have been evaluated. In a high humidity environment, a low peel strength was measured. The influence of moisture on the peel strength increases with decreasing peel rate. Peeling at elevated temperature reduces the moisture effect even under high humidity conditions. Using a low peel rate in a high humidity environment, the measured peel strength showed a maximum as the polyimide film thickness increased. No striations in peeled polyimide films were observed for peeling in a high humidity environment.     

Improvement in the adhesion of polyimide/epoxy joints using various curing agents

An improvement in the adhesion strength of polyimide/epoxy joints was obtained by (1) introducing a functional group on the polyimide surface, (2) improving the mechanical properties of the epoxy adhesive, (3) increasing the curing temperature, and (4) using polyamic acid as an adhesion‐promoting layer. The functional group on polyimide was introduced via treatment with aqueous KOH. An adhesion‐promoting layer was formed by spin coating polyamic acid onto a modified polyimide surface. The maximum adhesion strength of the polyimide/epoxy joint was obtained using polyamic acid as both the adhesion‐promoting layer and as the curing agent. The surface energy of the modified polyimide was examined using contact angle measurements and Fourier transform infrared spectroscopy, and the peel strength was determined by the T‐peel method. The peeled surfaces were analyzed using scanning electron microscopy and X‐ray photoelectron spectroscopy.© 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 812–820, 2002     

Studies on the adhesion of polyimide coatings on copper foil

The peel strength and the color of the copper foil peeled at 90 degrees from five different polyimide films were studied. The interfacial surfaces of copper foil and polyimide were examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersion analysis by X-ray (EDAX). There is a correlation between peel strength, and the color of the interfacial side copper caused by oxygen diffusion. Study of the imidization process carried out in vacuum indicates that the geometric arrangements of the atoms of polyimide also play a very important role in peel strength.     

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.     

Mechanical integrity of ceramic coatings on Kapton made by a dry aerosol deposition of lunar mare simulant

The purpose of this paper is to demonstrate the use of lunar regolith and the dry aerosol deposition (DAD) method to produce ceramic coatings on polyimide polymer, and to test their mechanical integrity. Ceramic films were produced on Kapton substrates using a custom-built DAD system and lunar mare simulant (LMS) feedstock. Ultrafine grains and impact densification were confirmed using atomic force and electron microscopy. Mechanical properties of the DAD–LMS Kapton samples were evaluated with indentation, tensile, and mandrel bend tests. DAD–LMS coatings tripled the hardness and doubled the indentation modulus of the Kapton surface. Coatings 3–16 µm thick did not have a predictable effect on the ultimate tensile strength or elongation to failure; however, the apparent modulus of elasticity did increase. The coatings were able to withstand significant bending before damage, with critical bend radii of 5 and 1.5 mm for 7.5 µm and 120 nm thicknesses, respectively. Modest heat treatment was shown to reduce the bending strain of coated substrates. Advanced ceramic coatings are of interest for the protection of space and satellite polymers from micrometeoroid impacts, radiation, and atomic oxygen. Lunar posts and habitats will require the development of space manufacturing techniques and in situ resource utilization.     

Study of the correlation between flexible food packaging peeling resistance and surface composition for aluminum-metallized BOPP films aged at 60°C

ABSTRACT

This study investigated the correlation between surface composition and peeling resistance in food packaging films by studying the heat aging of fabricated films over varying periods of time. The films consisted of a layer of aluminum (Al) metallized, biaxially oriented polypropylene (BOPP) bonded with a polyurethane (PU) adhesive onto another polymeric layer of low-density polyethylene (LDPE). The Al metallized films were prepared by physical vapor deposition (PVD) and aged at 60°C for either 5 or 15 days. The resulting aluminum surfaces were analyzed using X-ray photoelectron spectroscopy (XPS) and found to contain aluminum oxide (Al₂O₃) and trihydroxide (Al(OH)₃). The XPS characterization also revealed a 29% increase in the Al(OH)₃ layer thickness of the aged sample relative to a non-aged sample. Atomic force microscopy (AFM) was applied on investigations of possible morphology changes. The aluminum and PU adhesive surface energies were also determined using contact angles measurements and the aluminum surface energy was found to increase by as much as 11.7% compared to the non-aged sample, while the PU adhesive surface energy was at least 65% higher than that of the metallic substrate. The peeling resistance of the laminated aluminum was determined by average peel strength measurements and it was found that the variation in peel strength was related to changes in the Al₂O₃ layer thickness. The delaminated samples were analyzed using scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) and showed the cohesive failure of the aluminum film.     

Nano‐alumina modified epoxy based film adhesives

Epoxy based film adhesives were developed that contained nano‐sized alumina particles of different size and shape. These nano‐scale materials were incorporated at 5 and 10 weight percent into an adhesive formulation that was filmed on polyester random mat scrim. Films were applied to composite substrates for fracture testing and aluminum substrates for lap shear and peel testing. Scanning electron microscopy was used to characterize the surfaces of both the aluminum and composite specimens. Results from the bonded aluminum samples showed that in almost all cases the additives increased the peel and shear strength of the adhesives. Composite samples yielded results that were less clear, but still demonstrated that nano‐scale additives can increase the fracture toughness of a thermosetting adhesive.     

A Comparison of Energy Release Rates in Different Membrane Blister and Peel Tests

The relationship between energy release rates and coating stresses in various coating adhesion fracture tests is investigated. In spite of the apparent difference in test geometries and loading conditions, an equivalent peel test can be found for each of the membrane peeling tests examined in this paper. The results suggest that these tests and other membrane peeling tests are special cases of the peel test if examined near the debond front. In addition to clarifying the relationship between peel tests and other membrane peeling tests, the limitations of all possible membrane peeling test geometries with coatings having a tensile prestress are investigated through the study of the fracture efficiency parameter for the most general coating peeling problem. The results suggest that developing high fracture efficiency tests for coating adhesion measurement seems unlikely.     

Peeling of heterogeneous thin films: Effect of bending stiffness,adhesion energy,and level of heterogeneity

The peeling behaviour of a heterogeneous thin film bonded to a rigid substrate was investigated by using both experiments and finite element modeling. The enhancement in peel force was studied specifically for heterogeneous thin films with periodic stiff and compliant portions along the length. Peel tests with homogeneous thin films (uniform film thickness) showed that the maximum peel force can be observed before the onset of steady state peeling process. Moreover, this maximum peel force was observed to be a function of the bending stiffness of the film and adhesion energy at the film-substrate interface. For the heterogeneous thin films, maximum peel force can be observed either before the onset of steady state or when the peel front traverses from compliant to the stiff portion of the film. The three-dimensional finite element model, based on cohesive zone technique was developed, which provided further insight into the enhancement in peel force. The maximum force was shown to be dependent on the level of heterogeneity in addition to adhesion energy and bending stiffness as was observed with homogeneous films. The improvement in peel force was found to be prevalent at relatively low adhesion energy. This study may be helpful for the better design of homogeneous and heterogeneous thin film-substrate systems having improved bonding strength.     

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.     

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.     

Peel Strength and Failure Mechanisms in Oxidized Copper-Polyethylene Lap Joints Bonded with Flexible Epoxy

A study was made of the failure mechanisms in peeling of a composite consisting of copper-copper oxide-flexible epoxy-polyethylene. The morphology of the copper oxide (needle shaped) had a profound effect on the peel strength. The surfaces of the oxidized copper and the peeled specimens were examined with a scanning electron microscope and the results were used as a basis for analyzing the load transfer mechanisms at the interfacial regions. It is pointed out that the higher tensile strength of the oxide needles and the increased adhesion at the polymer-oxide interface are both important for the improvement in the peel strength. The failure process in polyethylene was shown to be equivalent to a continuous sequence of the reversed indentation problem of Prandtl. Using Prandtl's model it was possible to explain the regular ridge patterns observed at the surfaces of the peeled samples and relate the failure mode to the peel strength.     

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.     

Thermodynamic work of adhesion and peel adhesion energy of dimethoxysilyl‐terminated polypropylene oxide/epoxy resin system jointed with polymeric substrates

The characterization of the interfacial surface of a dimethoxysilyl‐terminated polypropylene oxide (DMSi–PPO)/diglycidylether of bisphenol A (DGEBA) system, which has the phase structure of DGEBA particles in a DMSi–PPO matrix, was investigated by using model joints with polymeric substrates. The surface free energy (γ) of the DMSi–PPO/DGEBA system had varied with the γ of each substrate. When the system contacted to low surface free energy materials such as Teflon, polypropylene, and polyethylene, the γ of the system showed about 14.3–31.6 mJ/m²; on the other hand, when the system contacted to high surface free energy substrates such as polyethylene–telephthalate and polyimide, the γ of the system showed 50.4 and 64.6 mJ/m², respectively, because the concentration of the DGEBA as a polar component in the system changed around these interfaces. In the low surface energy substrates used, the actual peel adhesion energy value was in good agreement with the thermodynamic work of adhesion (Wa) determined independently. However, in the high surface energy materials used, the peel adhesion energies were 10³–10⁴ times larger than Wa because the energy was dissipated viscoelastically at the jointed points. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1920–1930, 2001     

Plasma-Sprayed Aluminum and Titanium Adherends: III

The durability of aluminum and titanium adherends, plasma-sprayed with polymeric coatings, and bonded with an epoxy and a polyimide adhesive has been investigated. Organic-polymeric coatings were plasma-sprayed using epoxy, polyester, polyimide, and cyanate ester components. Durability was investigated using a wedge-type specimen by exposing the specimens to an environmental cycle that included low temperature, high relative humidity at elevated temperature, high temperature at atmospheric pressure in air, high temperature in a vacuum, and room temperature. The systems exhibiting durability comparable with that for adherends treated using standard solution methods, included aluminum or titanium coated with a bis-maleimide/cyanate ester (B-CE) or a bis-maleimide-LaRC TPI-1500^® (B-TPI) mixture and bonded with an epoxy or a polyimide adhesive. For these B-CE- and B-TPI-coated specimens, failure during exposure to the environmental cycle occurred in the adhesive, indicating a favorable adherend/plasma-sprayed coating interaction.