Polyethylene weak boundary layers

It is important to recognize that polypropylene, which is the major constituent of TPO, is a typical degrading-type polymer in the radiation chemistry of polymers, i.e., once a free radical is formed on a polymer chain, the free radical unzips the chain rather than cross-links. CASING effect was first found with polyethylene [24], which is a typical cross-linking-type polymer. The same CASING effect, however, could not be anticipated with the treatment of the degrading-type polymers because the degradation of substrate polymer enhances the extent of weak boundary layer. 

Chemical or physical surface treatments are especially required for structural bonding of low-surface-energy plastics. Low-surface-energy plastics include polyethylene, polypropylene, TPO, and fluorinated polymers. These surface treatments are designed to increase the critical surface tension and improve wetting and adhesion. In addition to increasing the critical surface tension, surface treatments are designed to remove contaminants or weak boundary layers, such as a mold release. 

Two examples of a weak boundary layer effect are polyethylene and metal oxides. Conventional grades of polyethylene have weak, low-molecular-weight constituents evenly distributed throughout the polymer. These weak elements are present at the interface and contribute to low failing stress when polyethylene is used as an adhesive or adherend. Certain metal oxides are weakly attached to their base metals. Failure of adhesive joints made with these adherends will occur cohesively within the weak oxide layer. Weak boundary layers can be removed or strengthened by various surface treatments. 

Early work by Schonhorn [119] showed that orientated monolayers of amphipathic molecules, such as stearic acid, could be employed as extremely effective adhesives in the bonding of polyethylene to aluminium indeed, so effective were these adhesives that joint strengths often exceeded the cohesive strength of the polyethylene Multilayer adsorption lowered the joint strength not because of less intrinsic adhesion of the amphipathic molecule to the substrate, but because the relatively thick layer possessed low cohesive strength and thus behaved as a weak boundary layer. Chemisorption was thought to occur for the stearic acid on the aluminium oxide [120,121], and this... 

Table 1.1 presents information on the effect of additives upon the adhesion strength of an adhesive based on ED-20 resin cured hy polyethylene polyamine (PEPA). It is evident that the lowest adhesion strength is exhibited by the adhesive based on the unpurified resin. The sorption of low-molecular weight fractions of an epoxy resin from the interphase boundary caused by addition of alcohols to this adhesive, i.e. resin purification, results in increase of the adhesion strength. Adding further alcohol to a purified resin results in formation of a weak layer on the interphase boundary, producing some decrease of the adhesion strength. 

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