Peel Strength of Composite Layered Joints

1. Master curves of R/A/R (rubber/adhesive/rubber) type adhesion tests were generated by laterally shifting the rate curves of peel response with empirically determined shift factors. The universal form of the WLF Equation (3) did not shift the data into continuous master curves, due to the individual contributions of each deforming layer. 2. For the weakly-bonded A/EPM system, rate curves were shifted into good agreement using the universal form of the WLF relation and the Tg of EPM. However, in the case of EPM/A, shift factors calculated by the WLF Equation (3) did not create a smooth master curve. Lateral shifting produced a master curve of EPM/A peel response and resulted in experimentally determined shift factors which fell between the shift factors of the EPM and the adhesive. Apparently, the EPM substrate of low Tg completely dominated the viscoelastic response of the A/EPM joint and even exerted some influence over the debonding process for the EPM/A joint, where the detaching layer was comprised only of adhesive. 3. In the case of strongly bonded chloroprene systems, CR/A/CR data were shifted laterally, but the CR/A and A/CR data shifted well with log aT values calculated from the WLF equation. Adhesive and CR Tg's were within ten degrees, and the rubber layer therefore appeared to contribute little to the overall viscoelastic response of CR/A/CR and CR/A. 4. Detachment of EPM systems was entirely interfacial, except in regions of rate where the adhesive was forced to fail in an unfavorable mode (the low-rate EPM/A experiment and the high-rate A/EPM experiment). A distinct transition of interfacial failure was observed near the glass-transition temperature of the adhesive. As the adhesive failure site changed from substrate to backing, even at the same rate and temperature, a three-fold drop in adhesive fracture energy occurred. 5. CR systems debonded through cohesive rupture of the adhesive layer at low rates. Clean interfacial failure occurred at intermediate to high peel rates. A failure site transition occurred in the same rate and temperature region as the weakly-bonded transition. This failure site change was again associated with a three-fold drop in adhesive-fracture energy. However, the actual peel force difference resulting from this transition in CR/A/CR was on the order of 1000 N/m, while the weakly bonded EPM/A/EPM system experienced a drop of approximately 100 N/m. This disparity in the two transitions suggests that the driving mechanism for the substrate-to-backing failure change is not caused simply by added adhesive bending forces in the low-rate failure mode. 6. Relative contributions of each deforming material to the total peel response for an R/A/R system cannot be determined by examining the shift factor dependence upon temperature. However, R/A and A/R models of the R/A/R detachment modes illustrate the contributions of each layer: as the adhesive debonds from the substrate, both adhesive and rubber contribute to the viscoelastic response of peel, except in the case where both layers exhibit similar Tg's; during adhesive failure from the backing, only the rubber backing contributes to the peel response.