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.     

Physico-Chemical Criteria for Maximum Adhesion. Part II: A New Comprehensive Thermodynamic Analysis

In this paper, two parameters defined as the relative work of adhesion [W_A/γ_L] and the relative interfacial energy [γ_SL/γ_L] have been examined for their assumed usefulness in correlating the thermodynamic properties of the components of the system substrate/ adhesive with its practical performance (strength). It is shown that the minimum value of [γ_SL/γ_L] relevant to conditions for the maximum adhesion becomes zero only for those systems (relatively rare) for which interaction factor Φ₀ is equal to 1.0.

Several transition points were identified for boundary conditions acquired at θ = 0° and θ = 90° which can be used to predict the properties and performance of an adhesive joint. These transition points are: a_MIN—energy modulus of the system (E. M. S.), relevant to the minimum interfacial energy; a_S—E. M. S. where self-spreading of adhesive occurs; a_CRIT—E. M. S. relevant to conditions under which the thermodynamic work of adhesion becomes negative and the system exhibits a tendency for self-delaminating or has “zero-strength”; a_CF—E. M. S. beyond which the geometry of the interface at any interfacial void or boundary of the joint may be regarded as a crack tip.

It is shown that only in those systems for which Φ₀ = 1.0 can a minimum contact angle of 0° indicate a condition for the maximum strength. If Φ₀ is known, the optimum contact angle can be estimated and hence the optimum surface energy of the substrate (adjusted by surface treatment, etc.) for the maximum adhesion.     

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.     

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.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

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

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

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

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

Peel Adhesion of Solid Films-The Surface and Bulk Effects

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

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

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

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

Adhesion of Pure and CaCO3 Filled Latex Films on Poly(ethylene terephthalate)

Adhesion to poly(ethylene terephthalate) of carboxylated styrene-butadiene latex films, filled with calcium carbonate particles, was studied in this work. The acid content (2, 4 or 6 wt%) the degree of crosslinking (25, 50 or 75 wt% of insoluble polymer) of the latex particles, and the percentage of filler (0, 20, 40, 60, 80 or 90wt%) were varied. A peel test at 180° was used.

It was shown that, for the lowest filler percentages (up to 60 wt%), the films were non-porous and adhesion decreased when the peel rate, the percentage of filler, the degree of crosslinking and the acid content increased. Failure was always localized at the film-support interface (except at very low peel rate). At low peel rate, stick-slip was observed. The adhesion lowering can be explained by a decrease of the energy dissipation during peeling due to a reduced mobility of the polymeric chains. At high filler percentage (80 or 90 wt%), the films became porous and the level of adhesion very low. Failure occurred in the bulk of the latex film. The peel rate dependence was reversed; adhesion increased at higher speed. Owing to its marked importance in this system, the mechanism of the stick-slip phenomenon is especially discussed.     

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.     

Hydrogen Bonding and the Interfacial Component of Adhesion: Acid/Base Interactions of Corona-Treated Polypropylene

The effect of activation of the surface of polypropylene sheet, by a corona discharge, upon the contact angles of liquids and on the surface free energy parameters γ^LW, γ^⊕ and γ^⊖, was determined. Both advancing and retreating contact angles were measured. The “acid/base” theory of the components of surface free energy was employed.

The contact angles of water and glycerol were initially lower by as much as 30°, after treatment, and that of diiodomethane was lower by about 5°. With time, the advancing angles rose, and the γ^⊕ and γ^⊖ parameters fell, towards the values on the untreated solids, and attained more or less steady values after 5 to 10 days. The basic component, γ^⊖, was the most strongly affected by the corona treatment; it rose, typically, from 2.2 to as high as 25 mJ/m². The acidic component, γ^⊕, rose from zero to as high as 1.9 mJ/m². Its decay with time was only qualitatively the same as that of γ^⊖. The retreating angles, and the corresponding energy components, were changed in the same direction, and somewhat more strongly, than were the “advancing” data.

The well-known improvement in the property of forming strong joints or adherent coatings, after corona treatment, is no doubt due to the formation of sites or areas on the polymers where hydrogen bonds can be formed. The decay of the strength of adhesion with time is, no doubt, due to the decay of these sites or areas.     

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.     

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.     

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 Adhesion: Influence of Surface Energies and Adhesive Rheology

The adhesion properties of polymers are known to be influenced by both intermolecular forces operative at the interface and the rheological history of both bonding and unbonding. Recent adsorption and viscoelastic theories of adhesion and cohesion are implemented in a comprehensive examination of these phenomena. Eight peel force “master curves” extending over 14 decades of reduced rate and representing glassy state to flow region rheology are superimposed to provide a composite response envelope. Each master curve represents rate-temperature reduced adhesion of an alkyl acrylate adhesive (γ_c = 26 dyne/cm) to substrates ranging from low adhesion fluorinated polymers (γ_c = 15 to 17 dyne/cm) to polar poly-amide surfaces (γ_c = 45 dyne/cm) and glass. The rate dependent transition from interfacial to cohesive failure, a subject not treated by adsorption theory, is shown to be coincident with the onset of entanglement slippage within the polymeric adhesive. Thermodynamic criteria of polymer adhesion are shown to be applicable only to the flow region of polymeric response. This study indicates that measured surface tensions or calculated surface energies of polymeric solids do not properly account for the contributions of three dimensional network structure of the polymeric bulk phase to its total work of cohesion. Evidence of true interfacial failure of a polymer-polymer bond is supported by critical surface tension measurements.     

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.     

A New Approach to the Copper/Epoxy Joint Using Atmospheric Pressure Glow Discharge

A new approach for adhering copper to an epoxy resin was studied. In this new approach, the copper surface was first treated with hydrogen plasma generated by the atmospheric pressure glow (APG) discharge. Then a thin film of γ-aminopropyltriethoxysilane (γ-APS) was formed on the treated copper surface. The copper oxide formed by air on the copper surface deteriorated the adhesion by forming a weak boundary layer, part of which could separate from the surface. This oxide layer was reduced when an APG hydrogen plasma was applied for a couple of minutes at a frequency of 13.56 MHz and a power input of 200 W. The resulting peel strength at the copper/epoxy interface increased up to ca. 0.9 Kg/cm. Curing temperature of γ-APS was also an important factor in obtaining good adhesion at the copper/epoxy interface, with the highest value of peel strength occurring at a curing temperature of 120°C.     

Sol-gel processed barium titanate thin films

Ferroelectric barium titanate (BaTiO₃) thin films have been prepared by sol-gel technique from barium acetate [Ba(CH₃COO)₂] and titanium (IV) isopropoxied [Ti(CH₃)₂CHO)₄] precursors. The as-grown films were found to be amorphous which crystallized to tetragonal phase after annealing at 700°C for one hour in air. The room temperautre dielectric constant (ε) and loss tangent (tanδ) of the films were found to be 370 and 0.02 respectively. The values of the spontaneous polarization (P_s), remanent polarization (P_r) and coercive field (E_c) of the films determined from the polarization-field (P-E) hysteresis were found to be 14.0, 3.2μC-cm^-2 and 53KV-cm^-1 respectively. The coercive field of the film determined from the capacitance-voltage (C-V) characteristics is slightly lower than that determined from the P-E hysteresis loop.     

Adhesion and Scratch Resistance of Polycarbonate Films on Ferroplate Substrates: Effect of γ-APS Primers

The influence of γ-aminopropyltriethoxysilane (γ-APS) primers on the adhesion and scratch resistance of polycarbonate (PC) films on ferroplate substrates was determined from the critical normal loads at which debonding of the films from the substrates occurred during scratch testing. The critical load was a strong function of the concentration of the aqueous solutions from which the γ-APS primers were adsorbed and of the thickness of the primer films. Thus, the critical normal load increased from 0.09 ± 0.02 N to 0.31 ± 0.07 N as the concentration of the γ-APS solutions increased from 0.05% to 0.2%, respectively. However, the critical load increased only slightly as the solution concentration increased beyond 0.2%. The increase in critical load as concentration of γ-APS solutions increased was related to the formation of an interphase involving chemical reaction and physical entanglement of PC and γ-APS molecules. The critical load for debonding of PC films from the substrates also depended strongly on the temperature at which the γ-APS films were dried before application of the PC films. Thus, the critical normal loads for debonding were 0.31 ± 0.07, 0.20 ± 0.02, and 0.05 ± 0.01 N for γ-APS films that were dried for 15 min at room temperature, 60°C, or 110°C, respectively. The decrease in critical load with increasing drying temperature was attributed to the greater cross-link density in γ-APS films that were dried at elevated temperatures, which limited interdiffusion and physical entanglement of PC and γ-APS molecules. High reaction temperature of γ-APS and PC induced a fragmentation of amine. However, it also increased the probability of amines to react with carbonate because of increasing mobility of PC chains. Optimization of these two factors was required to obtain the greatest adhesion and scratch resistance. Chemical reactions occurring between PC films and γ-APS primers were investigated by reflection-absorption infrared spectroscopy (RAIR) and X-ray photoelectron spectroscopy (XPS) using diphenyl carbonate (DPC) as a model compound. The carbonyl absorption band of neat DPC was observed at 1780 cm^-1. However, two carbonyl bands were observed at 1738 and 1652 cm^-1 in RAIR spectra of γ-APS films that were reacted with DPC and were assigned to urethane and urea groups, respectively. XPS results revealed that urethane was the main reaction product between DPC and γ-APS. It was concluded that urethane groups formed by the reaction of PC with γ-APS were responsible for adhesion and scratch resistance of PC to ferroplate substrates that were primed with γ-APS.     

Effect of the Molecular Length of n-Alkylsilanes on the Adhesion at a Rubber/Glass Interface

The effect on the peeling energy, G, of glass/styrene-butadiene rubber (SBR) assemblies of the length, N, of the alkyl chain, ranging from 4 to 30 carbon atoms, of silane coupling agents is determined. Experimentally, it is shown that G strongly increases with N. Therefore, considering that the rheological model of adhesion (or model of multiplying factors) is valid, G is assumed equal to the product of three terms: the reversible adhesion energy, W. at the interface, the viscoelastic dissipation factor, φ, of SBR and a “molecular factor” to be determined. Finally, it appears that this latter factor depends linearly on N. Such a result should be consistent with an extraction (“suction”) process of the silane alkyl links from bulk SBR during peeling experiments.     

Wave Phenomena in Low Angle Peeling

The angular dependence of peeling has been investigated over a wide range of peel angles for a rubber strip peeled from glass. At low peel angles the peel front becomes “V”-shaped, wave phenomena are often observed and the peel energy can increase by an order of magnitude or more. A tentative theory, which appears to give the correct magnitude in a worked example, is advanced to account for the energy increase. The influence of factors such as electrostatic charge, deformation and rate on the observed phenomena are discussed.     

Mode I fracturing properties of epoxy bonding paste

The wedge-splitting method by Tschegg was applied for the investigation of mode I fracture behavior of epoxy-based adhesives. Specific fracture energy and notch tensile strength of “traditional” thickened laminating resin were measured between −40 and +45 °C and compared to those of four new trial compounds. The adhesives in bulk were investigated as well as the interfaces between these adhesives and glass fiber-reinforced plastic. The wedge-splitting method has turned out suitable for epoxy resin. The quality of thickened laminating resin was not reached by the new trial compounds. Adherend pre-treatment with laminating resin and peel ply improved interfacial properties.