Peel strength effective factors

While cure time is of substantial importance, other variables have also been found to result in major property changes. A semiquantitative comparison of the importance of three factors - multifunctional acrylate concentration, photoinitiator concentration, and cure time - is given in Table 9 for peel strength, shear, and the adhesive/cohesive peel mode. We have assigned relative values of high, medium, and low to each of the independent variables for their effect on the dependent properties. 

Clearly, the peel strength is not a fundamental property for an adhesive. The value of force per unit width required to initiate or sustain peel is not only a function of the adhesive type, but also depends on the particular test method, rate of loading, thickness and stiffness of the adherend(s) and adhesive as well as other factors. Thus, peel tests generally do not yield results that may be used in quantitative design. This does not imply, however, that the peel test is not a useful test. Peel tests provide quantitative comparisons between different adhesive systems, insight into rate and temperature effects, etc. Additionally, peel tests can be used to provide fracture mechanics information as will be discussed in the next section. In the author s opinion, the latter aspect of peel tests has been perhaps most adroitly exploited by Gent and Hamed [18-20] who used peel tests in conjunction with fracture mechanics to obtain insights into time-temperature effects, the role of plasticity, and many other aspects of adhesive fracture. 

Adhesion is not an intrinsic property of a materials system, but is dependent on many factors. By now it should come as no surprise that the measurement of adhesion is sensitive to both rate and temperature as all of the other mechanical properties have been. In fact, adhesion can often be transformed by the WLF (Williams, Landel, Ferry) equation or Arrhenius transformation in the same maimer as modulus and other properties. Figure 11.7 shows the transformation of isothermal peel data transformed into a master curve along with the polyester adhesive s shear and tensile strength properties. In another study investigating the effect of temperature and surface treatment on the adhesion of carbon fiber/epoxy systan, five epoxy systems were found to fit an overall master curve when corrected for the material T. This result is quite remarkable and is shown in Fignre 11.8. 

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