Peel strength mechanism

Speciali2ed copolymer latices, which are inherently and permanently tacky, are available as pressure-sensitive emulsions. They are mechanically stable and have excellent machinabiUty. They are compatible with many other PVAc latices and, therefore, can be easily blended with other resins for modification of surface tack, peel strength, and creep. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

A standard test report usually documents the resulting measurements, such as tensile shear strength and peel strength. It should also indicate all the pertinent conditions that are required to ensure reproducibility in subsequent testing. It is often very useful to describe the failure mode of the tested specimens. An analysis of the type (or mode) of failure is an extremely valuable tool to determine the cause of adhesive failure. The failed joint should be visually examined to determine where and to what extent failure occurred. The percent of the failure that is in the adhesion mode and that in the cohesion mode should be provided. A description of the failure mode itself (location, percent coverage, uniformity, etc.) is often quite useful. The purpose of this exercise is to establish the weak link in the joint to better understand the mechanism of failure. 

Figure 12 represents a cross section of a plated sample of ULTEM 1000 having a peel strength of 118 g/mm. Little physical surface change of the plastic has occurred as a result of the pretreatment steps. Figure 13 is a cross section of copper plated ULTEM 2312. While a mechanical component to adhesion is present, it is much less than that found in traditional swell and etch treatment (Figure 2). Physical alteration of the plastic is confined to the outermost 25ii. The bulk properties of the plastic (flexural strength, electrical resistivity) are unaffected by this new process (Table I).

It could be shown by the adhesion strength experiments that the best results are obtained if the ratio of free SiOH phCH = 0.18 and bridged SiOHrphCH is less than 0.05. Higher and lower values lead to a decrease of adhesion, however, the overall adhesion is only in the range of about 3-3.5 N cm , determined as peel strength. Investigations of the fracture mechanism show that the seal shows a brittle fracture behavior, as indicated in Fig. 9. 

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