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.     

Plasticity in peeling

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

Thermoplastic character of polyimide blends

Polyimide blends consisting of pyromellitic dianhydride/4,4′-oxydianiline (PMDA/ODA) and biphenyl-tetracarboxylic dianhydride/p-phenylene diamine (BPDA/PDA) show a distinct glass transition behavior at temperatures lower than each component does. Disruption of molecular packing by blending of polymers having dissimilar interaction sites leads to a significant increase in molecular mobility at much lower temperatures. This is examined by laminating two pieces of film cast from the blend and measuring the adhesive strength at the interface. A strong adhesion, 11.5 N/cm (6.6 lbf/in) by 180° peel test, was achieved indicating interpenetration of polyimide molecules. It was also found that the polyimide blends can be converted into highly ordered states by mechanical deformation of the blends above their glass transition temperatures (T_gs).     

Protocols for the Measurement of Adhesive Fracture Toughness by Peel Tests

This article reports on the work of the European Structural Integrity Society Technical Committee 4 (ESIS TC4) and its activities in the development of test protocols for peel fracture. Thirteen laboratories have been working on peel test methods in ESIS TC4 since 1997 and their activities are ongoing.

The aim of the work is to develop robust and credible test methods for the determination of adhesive fracture toughness by peel tests. Several geometric configurations have been used, namely, multi-angle fixed arm peel, T-peel, and roller assisted peel in the form of a mandrel test.

The starting point of their work is an established analysis of a peel method that is often developed from a global energy approach. The adopted analysis is combined with an experimental approach in order to resolve ambiguities in the determination of adhesive fracture toughness (G _A ). The test methods involve the measurement of peel strength in order to calculate the total input energy for peel (G) and the calculation of the plastic bending energy (G _P ) during peel. The latter is often obtained from a measurement of the tensile behaviour of the peel arm. Adhesive fracture toughness is then G – G _P .

Four ESIS TC4 projects are described. The first relates to fixed arm peel whilst the second and third involve both fixed arm and T-peel. The fourth project combines mandrel peel and fixed arm peel. Each project uses different types of polymeric adhesives in the form of quite different laminate systems. The selection of the laminate system enables all characteristics of laminate property to be embraced, for example, thin and thick adhesive layers, polymeric, and metallic peel arms and a range of flexibility in the laminates.

The development of the enabling science required to establish the test protocols is described and software for conducting all calculations is referenced.     

Adhesion of PMDA-ODA polyimide to silicon oxide surface: sensitivity to fluorine contamination

The adhesion of pyromellitic dianhydride-oxydianiline polyimide (PMDA-ODA) to clean and fluorine-contaminated silicon oxide (SiO₂ and F-SiO₂, respectively) surfaces was studied by the peel test. A 50% drop in the peel force was observed for PMDA-ODA on F-SiO₂ as compared to that on SiO₂ measured at low (11-22%) ambient humidity. It was also found that the PMDA-ODA adhesion strength to F-SiO₂ was a strong function of the humidity. A 21/2-fold increase in the humidity caused over an order of magnitude decrease in the peel force of PMDA-ODA on F-SiO₂, which was only about 5% of the peel force measured under the same ambient conditions for PMDA-ODA on the SiO₂ surface. A careful analysis further demonstrated that the basic mechanical properties of the polyimide film were unaffected to within experimental error. The most probable cause of the adhesion loss is thus attributed to moisture attack in the polyimide-substrate interface region. The locus of failure in the case of PMDA-ODA on SiO₂ is in an apparent weak boundary layer of the polyimide, while for polyimide on F-SiO₂ the failure locus is mixed within the polyimide and the F-SiO₂. An estimate of the work of adhesion (interface fracture energy) was made using the high (40-55%) humidity peel data for PMDA-ODA on the F-SiO₂ surface, giving a value of 6 J/m².     

Polyester hot-melt adhesives. I. Factors affecting adhesion to epoxy resin coatings

The peel strength and tensile shear strength of polyester hot-melt adhesives on metals coated with epoxy resins are affected by four characteristics of the polyester: (1) inherent viscosity, (2) glass transition temperature (T_g), (3) degree of crystallinity, and (4) melting point. The inherent viscosity affects the strength, toughness, and crystallinity of the adhesive. The T_g and degree of crystallinity affect the low-temperature adhesive properties; the peel strength is relatively low when the T_g is appreciably above the use temperature. The T_g, degree of crystallinity, and melting point affect the high-temperature adhesive properties. A hot-melt adhesive with high peel and tensile shear strengths from 0° to 120°C is the polyester of 1,4-butanediol and trans-1,4-cyclohexanedicarboxylic acid.     

Synthesis and characterization of pyridine‐containing poly(imide‐siloxane)s and their adhesion to copper foil

A series of pyridine‐containing poly(imide‐siloxane) (PIS) copolymers with different amounts of PDMS with various segmental lengths were synthesized from 2,6‐diaminopyridine (DAP), α,ω′‐aminopropylpoly(dimethylsiloxane) (PDMS), 1,3‐bis(4‐aminophenoxy)benzene (APB), and 4,4′‐oxydiphthalic dianhydride (ODPA). A modified synthetic approach was applied instead of approaches commonly reported in the literature, to ensure the incorporation of DAP and PDMS. The effects of the content and the segmental length of PDMS on the thermal glass transition temperature (T_g), dielectric constant, and surface electrical resistivity of the copolymer are investigated. The copolymers were attached to copper foil by hot‐pressing, and changes in wettability caused the peel strength of the laminates to increase with the PDMS content, but to decrease as the DAP content increased. Furthermore, X‐ray photoelectron spectroscopy was employed to determine the loci of failures (LOF) of the laminates and to monitor the movement of LOF, which varies with the PDMS content. For those laminates with good peel strengths, the LOF occur in the interior of PIS layer, indicating that the adhesion is cohesive rather than adhesive. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Polybutadiene adhesive compositions

This paper deals with the development of adhesives that are useful for preparing flexible laminates. The adhesive compositions are based on the chemistry of 1,2-polybutadiene polymers and their reactions via chain extension and epoxidation. The resulting materials can be suitably formulated to produce resins that can be used to impregnate fibrous reinforcements which can be B staged; when laminated to copper, these materials give products with good peel strength, solder resistance, tear strength, and flame resistance. These adhesives can also be used to bond polyimide and polyester film to themselves and to copper. Typical properties of selected laminates include peel strength values of 8 lb/in. width, tear strength of 1–2 lb, oxygen index from 29 to 37, and excellent solder resistance.     

Adhesion to Plasma-Modified LaRC-TPI II. Effect of Plasma Treatment on Peel Strength

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. A 180[ddot] peel test with an acrylate-based pressure sensitive adhesive tape as an adherend was utilized to study the interactions of the plasma-treated polyimide surface with another polymeric material. The peel strengths of the pressure sensitive adhesive tape on the plasma-treated LaRC-TPI fell below the level of the non-treated controls, regardless of the plasma treatment used. Failure surface analysis by XPS revealed the presence of polyimide on the pressure sensitive adhesive failure surface, indicating failure in the plane of a weak boundary layer created by plasma treatment. The removal of the weak boundary layer by a solvent rinse restored the peel strength to the level of the control. Comparison with tape adhesion peel strengths of oxygen plasma-treated high density polyethylene showed that the physical condition of a polymer surface following plasma treatment plays an important role in determining the level of adhesion which can be achieved.     

Structure Mechanical Property Changes Occurring in Ethylene Acrylic Acid Copolymer Adhesive During Aging of Bimetallic Laminates

The increase in peel strength after thermal aging of aluminum and copper to tin-plated steel laminates was studied using adhesive samples recovered from copper to tin-plated steel laminates. Morphological changes of the ethylene acrylic acid copolymer adhesive were observed. Calorimetric and dynamic mechanical measurements on adhesive copolymers recovered from Cu-tin plated steel laminates suggest that the crystallinity of EAA increases due to thermal aging as well as oxidative processes. A higher degree of crystallinity and effective crosslink density are consistent with the higher elastic (E′) modulus for aged specimens. Increase in crystallinity of an ethylene-acrylic acid copolymer resulted in a higher tensile strength, which appears related to the increase in peel strength of the laminate.     

Effect of different skin permeation enhancers on peel strength of an acrylic PSA

Different amounts of two skin permeation enhancers, Oleic acid (OA) and Propylene glycol (PG), were mixed thoroughly with solution of a commercial acrylic pressure sensitive adhesive (Duro‐Tak). Films with different adhesive layer thickness (30 and 60 μm) were prepared by casting of formulations with a film applicator on a PET 80‐μm film and drying of solvents. Peel test was done on different formulations according to ASTM D3330. Surface study and thermal analysis were used for explaining the results. It was shown that the effect of interfacial work of adhesion on peel strength was too low to be considered. PG had no significant effect on peel strength, which was related to effect of hydrogen bonds between PG and copolymer chains acting as crosslinks. OA decreased peel strength significantly, which is due to important changes in copolymer structure. These changes can be found by relatively sharp drop in T_g values. Adhesive–cohesive transition occurred in OA formulations as a result of OA crystals formation. OA migration to surface in concentrations of more than 10 (w/w %) was confirmed by results of DSC and surface study. In contrast with PG, doubling of thickness had no effect on peel strength. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2987–2991, 2003     

Nanotailoring of polyurethane adhesive by polyhedral oligomeric silsesquioxane (POSS)

The development and commercialization of nanoparticles such as nanoclays (NCs), carbon nanotubes (CNTs) and polyhedral oligomeric silsesquioxanes (POSS) offers new possibilities to tailor adhesives at the nanoscale. Four types of POSS, with reactive mono-functional groups of isocyanatopropyl, glycidoxypropyl, aminoethyl and non-reactive octaphenyl, were incorporated in concentrations of 1, 3 and 5 wt% into a polyurethane (PU)-based adhesive. Thermo-mechanical bulk properties were studied using dynamic mechanical analysis (DMA). Adhesive properties were characterized in shear and peel modes. Atomic force microscopy (AFM) was used to study the nanoscale morphology. DMA measurements indicated that the neat PU possessed a glass transition temperature (T _g) of ≈ 30°C. The T _g of PU/POSS-glycidoxypropyl nanocomposite adhesive increased gradually with POSS concentration to 50°C for 5 wt%. PU/POSS-octaphenyl nanocomposite adhesive exhibited an increased T _g by 10°C for 5 wt%. The incorporation of POSS-isocyanatopropyl in the PU had no effect on the T _g. With respect to shear properties of POSS-octaphenyl-, POSS-isocyanatopropyl- and POSS-glycidoxypropyl-based PU nanocomposite adhesives, shear strength improved by 230, 178 and 137%, respectively, compared to neat PU. POSS-aminoethyl exhibited lower shear and peel strengths, while POSS-isocyanatopropyl provided the best balance of both higher shear and peel strengths compared to neat PU. It was concluded that the grafted functional group on the POSS and its reactivity with the PU network components were the decisive factors with respect to the thermo-mechanical, morphological and adhesive properties of the resulting nanocomposite adhesives. Consequently, the POSS/polyurethane based nanocomposite adhesives could be tailored for a large range of applications.     

Interlaminar Bond Strength and Failure Mechanisms in Commercial Flexible Polymer-Metal Laminates

An alternative to the 180° “T” peel test (called simply the “T-peel test” in the USA) was developed by Cropper and Young for the measurement of interlaminar bonding in three-ply polypropylene-aluminium-polyester laminates used in food packaging applications. The effect of temperature on the interlaminar bond strength of three laminate systems has since been studied. In particular, the effect of temperature on both the failure mode and on the adhesive's appearance after testing has been determined. It is shown that as the temperature is raised about 23°C, the laminating adhesive begins to soften and the failure mode changes from almost exclusively adhesive failure at the polyurethane adhesive-aluminium interface to cohesive failure of the polyurethane adhesive itself. The change in the failure mode is accompanied by the appearance of a meniscus instability. The temperature at which the meniscus instability patterns become more prominent correspond to the temperature at which the maximum interlaminar bond strength is attained.

It is thought that this new test can be used to characterise the behaviour of laminating adhesives more fully, both in their change in appearance with temperature, and in their effectiveness in bonding layers together as temperatures are increased above ambient conditions.     

Polymer Viscoelasticity: Relation to Peel Strength in Structural Adhesives

The two tests of most importance in evaluating structural adhesives for metals are (1) lap shear strength and (2) peel strength. Epoxies perform well in the first due to high tensile and shear strength. They are poor in the second unless modified to reduce brittleness. We have developed a urethane modified epoxy for this purpose. By taking climbing drum peel data in which both the temperature and the peel rate are varied, the time-temperature superposition principle can be tested. This principle is most generally applicable to thermoplastic materials between T_θ and T_θ + 100 °C (T_θ = glass transition temperature), and serves as a measure of viscoelastic response in the polymer. First, good agreement was found for a thermoplastic adhesive (PE-AA film). This was done to verify that climbing drum peel data can be used in this manner. Next, data were taken for our urethane modified epoxy. Results showed adherence to the superposition principle only above the heat distortion temperature of the cured polymer. These results indicate, among other things, that our point of failure upon peeling is within the body of the adhesive rather than within a urethane-rich layer at the metal-adhesive interface.     

Cleavage behavior of bonds made with adherends capable of plastic yield

The failure behavior of adhesive joints under cleavage stresses depends upon the thickness of the adherend. With thick, rigid adherends failure occurs by rapidly propagated adhesive rupture. Thinner adherends can exhibit plastic flexural yield, the subsequent adhesive failure then being progressive and strain-limited, and occurring only in the region of bond directly adjacent to the yielding adherend. A fairly sharp discontinuity between these two types of behavior occurs over a small range of adherend thickness T. Work to rupture can differ by more than an order of magnitude, for otherwise identical joints having T above or below the transitional range (around T_c). For T > T_c the applied load P causing rupture is proportional to T^1.5 while the moment arm remains constant, as predicted by Yurenka. For T < T_c the turning moment during failure is proportional to T² and is substantially independent of the nature of the adhesive. Empirically, the radius of the yielded adherend after failure is proportional to T. The manner of interaction of various adhesive mechanical properties in defining P in the two ranges and, thereby, T_c, are related to this and other empirical correlations. The initial free moment arm in the joint, L, determines the stability of peel at initiation of adhesive rupture. Reducing L leads ultimately to instability. The change of controlling factors as L → 0 is discussed.     

Thermal residual stresses in an adhesively-bonded functionally graded single-lap joint

This study investigates three-dimensional thermal residual stresses occurring in an adhesively-bonded functionally graded single-lap joint subjected to a uniform cooling. The adherends are composed of a through-the-thickness functionally graded region between Al₂O₃ ceramic and Ni metal layers. Their mechanical properties were calculated using a power law for the volume fraction of the metal phase and a 3D layered finite element was implemented. In a free single-lap joint the normal stress σ_xx was dominant through the overlap region of the upper and lower adherends and along the adhesive free edges, whereas the transverse shear stress σ_xy concentrations appeared only along the free edges. The peel stress σ_yy and the transverse shear stress σ_xy became dominant along the free edges of the adhesive layer. In addition, the von Mises stress decreased uniformly through the adherend thickness from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer. In addition, the layer number had only a minor effect on the through-the-thickness stress profiles after a layer number of 50, except for the peak stress values in the ceramic layer. In a single-lap joint fixed at two edges both adherends underwent considerable normal stress σ_xx concentrations varying from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer along the free edges of both adherend–adhesive interfaces, whereas the peel stress σ_yy and transverse shear stress σ_xy reached peak levels along the left and right free edges of the adhesive layer. The layer number and the compositional gradient exponent had only minor effects on the through-the-thickness von Mises stress profiles but considerably affected the peak stress levels. The free edges of adhesive–adherend interfaces and the corresponding adherend regions are the most critical regions, and the adherend edge conditions play more important role in the critical adherend and adhesive stresses. Therefore, the first initiation of the joint failure can be expected along the left and right free edges of the upper and lower adherend–adhesive interfaces.     

Geometry and nature of the interfacial layer in polyimide coatings

A new approach to the quantitative evaluation of the thickness h_i and the volume fraction V_i of the interfacial layer in polyimide coatings has been developed using the model of a one-sided polymer coating as a three-phase system (a substrate, an interfacial layer and a polymer matrix) and the method of differential scanning calorimetry. The new approach is characterized by the fact that a “pure” polymer matrix is modeled with the help of free films obtained on mercury. The formulas for the calculation of the thickness and volume fraction of the interfacial layer have been proposed. The value h_i has been determined in polymer coatings of different thicknesses on aluminium foil. This parameter has been shown to be independent of the film thickness if the latter is not larger than the length of the interfacial layer. An assumption has been made about the “mechanical” origin of thick (> 1–5 μm) interfacial layers, and their relaxation character has been revealed.     

CRACK GROWTH OF STRUCTURAL ADHESIVE JOINTS IN HUMID ENVIRONMENTS

The adhesive fracture energy, G _c , of aluminiumalloy and steel joints bonded with a rubber-toughened epoxy adhesive has been measured using monotonicallyloaded tests. Such tests have been conducted at different levels of relative humidity, and two surface pretreatments have been employed for the substrates prior to bonding: a simple grit-blast and degrease (GBD) pretreatment or a silane primer (GBS) pretreatment. When G _c was plotted against the crack velocity, three regions of fracture behaviour could be distinguished. At low rates of displacement the crack grew in a stable manner, visually along the interface, and relatively low crack velocities could be readily measured. This was termed “Region I”, and here the value of the adhesive fracture energy was relatively low and decreased steadily as the relative humidity was increased. On the other hand, at relatively high rates of displacement the crack grew in a stick-slip manner mainly cohesively in the adhesive layer at approximately 20 km/min. This was termed “Region III”, and here the value of G _c was relatively high and independent of the relative humidity. In this region the crack was considered to grow faster than the water molecules were able to reach the crack tip, which explains the independence of G _c upon the test environment. In between Region I and Region III a transition region was observed, which was designated “Region II”. The major effect of the GBS pretreatment, compared to which the GBD pretreatment, was to increase the value of G _c both in Regions I and III, although the presence of the silane primer had the greater effect in Region I.     

Polymer/clay aerogel‐based glass fabric laminates

Laminates of polymer/clay aerogels and glass fabric sheets were prepared with varying epoxy adhesion application levels. A poly(amide‐imide) and an epoxy (1,4‐butanediol diglycidyl ether/2,6‐diaminopyridine) were chosen as the two “foam core” polymers; both single‐layered and double‐layered glass fiber laminates were investigated. The adhesion between polymer clay aerogels and glass fibers was quantified using the T‐peel method. The peel strength properties were found to increase as adhesive loading increased up to an optimal value, after which peel strength declines. Flexural and compressive testing of the laminates was also performed as a way of measuring mechanical strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of cure history on dynamic mechanical properties of an epoxy resin

The effect of cure history on the dynamic thermomechanical properties of a high temperature curing epoxy resin has been studied using torsional braid analysis. In isothermal cures “full cure” is not possible except at temperatures above the maximum glass transition temperature (T_g) of the cured resin, hence the necessity of a “post-cure” after lower temperature isothermal cures. The highest T_g and maximum cross-linking in the cured resin was for a linear heating rate of 0.05°C/min from 30 to 200°C; higher heating rates lead to lower glass transition temperatures.