Abstract
Compounds of poly(ethylene-co-vinyl acetate) (EVA with vinyl acetate content 33%) with three different organic per- oxides, namely, dialkyl peroxide, peroxyester peroxide, and peroxyketal peroxide, were prepared with a twin screws extruder and a two-roll mixing mill. The cure behavior of the EVA compounds was analyzed from rheographs, which were obtained by a moving die rheometer (MDR) at various curing temperatures between 150?C to 170?C. The effects of organic peroxides on cure behavior were examined. The dynamic curing obtained by the torque rheometer provided sufficient experimental data to show that dialkyl peroxide is not suitable because it has a high half-life temperature and its by-products can discolor the final product. Peroxyester peroxide is good for curing at temperatures in the range of 150?C to 160?C, which accomplished an ultimate cure within 5 to 8 minutes. Also, the peroxyketal peroxide has higher performance, which decreased the optimum cure time to 3 minutes. The thermal decomposition mechanism of organic peroxide was applied to explain how the cure behavior is affected by generated free radicals.
Highlights
EVA are most commonly used worldwide in a variety of commercial applications, especially as encapsulating materials for solar cell photovoltaic modules, because of its outstanding properties: flexibility, toughness and excellent adhesion on various substrates, high electrical resistivity which makes it as a good electric insulator, low processing and crosslinking temperature, very low water absorption ratio, good optical transmission and high transparency [1,2]
The cure behavior of the EVA compounds was analyzed from rheographs, which were obtained by a moving die rheometer (MDR) at various curing temperatures between 150 ̊C to 170 ̊C
The thermal decomposition of dicumyl peroxide (DCP) is demonstrated in Scheme 1
Summary
EVA (ethylene vinyl acetate) are most commonly used worldwide in a variety of commercial applications, especially as encapsulating materials for solar cell photovoltaic modules, because of its outstanding properties: flexibility, toughness and excellent adhesion on various substrates, high electrical resistivity which makes it as a good electric insulator, low processing and crosslinking temperature, very low water absorption ratio, good optical transmission and high transparency [1,2]. Crosslinking EVA encapsulant resists degradation from weathering better than conventional polyethylene. The main compositions can be divided into three groups: the EVA resins, curing agents (i.e., organic peroxides) and other additives. The first grade developed was the standard cure grade, which used dialkyl peroxide as the curing agent with a cure time of approximately 25 minutes. The second generation so-called fast cure grade used peroxyester peroxide as the curing agent, with a cure time of 7 minutes. From the above additive list, it is implied that the main purpose is to improve the weathering degradation resistance, to which the degree of crosslinking of EVA encapsulant is a key factor. Crosslinking reactions induced by organic peroxides and silane coupling agent are essential phenomena during the lamination-curing process. Understanding the cure characteristics is important to improving the quality of encapsulating materials
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