Cross-linking EVA

A substantial part of the market for the ethylene-vinyl acetate copolymer is for hot melt adhesives. In injection moulding the material has largely been used in place of plasticised PVC or vulcanised rubber. Amongst applications are turntable mats, base pads for small items of office equipment and power tools, buttons, car door protector strips and for other parts where a soft product of good appearance is required. Cellular cross-linked EVA is used in shoe parts. 

Zinc borate in conjunction with ATH or MDH is used in many halogen-free polymers. Based on DTA and DSC analyses, it was also demonstrated that the partial replacement of ATH with Firebrake ZB in cross-linked EVA can delay and reduce the thermal oxidative peak (Figure 9.17). The mode of action of this zinc borate and metal hydroxide is summarized as follows ... 

Tih-Tay Dual chambers, polyethylene from cables (cross-linked EVA) 1998... 

A substantial use of EVA copolymers is as was additives and additives for hot-melt coatings and adhesives. Cellular cross-linked EVA copolymers are used in shoe parts. 

Table 6.11 Effect of MPBD in a magnesium hydroxide-filled, peroxide cross-linked EVA...

Interest in EVA as a cable-insulating material has arisen because of the good resistance to stress cracking and because the polymer may be more easily cross-linked (see Table 11.12). 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

Obviously, there exists severe interplastics competition, e.g. PP vs. ABS, clarified PP vs. PS, PA, PVC, HDPE and PS (Table 10.7). A wide range of cross-linked and thermoplastic elastomer applications, from footware to automotive parts and toothbrushes, are adopting new metallocene-catalysed polyolefin elastomers (POEs). These low-density copolymers of ethylene and octene were first accepted as impact modifiers for TPOs, but now displace EPDM, (foamed) EVA, flexible PVC, and olefinic thermoplastic vulcanisates (TPVs). Interpolymer competition may also result from... 

Fukamachi, M., Matsui, T., Shimoda, M., and Osajima, Y. (1996). Cross-linking treatment of EVA film to improve flavor preservation. J. Agric. Food Chem. 42,1989-1992. 

The implant consists of a tablet-shaped ganciclovir reservoir. The drag is initially completely coated with poly(vinyl alcohol) (PVA) and then coated with a discontinuous film of hydrophobic, dense poly (ethylene-co-vinyl acetate) (EVA). Both polymers are nonerodible and hydrophobic (the PVA used in the implant is cross-linked and/or high molecular weight, to ensure it does not dissolve when exposed to water). The entire assembly is coated again with PVA to which a suture tab made of PVA is attached (Figure 4.5). 

None 200-220 PP, PS, HDPE, EVA, ABS, SMA, polyolefin copolymers, blends, cross-linked polymers... 

Mishra S., B. Baweja and R.J. Chandra. 1999. Studies on dynamic and static cross-linking of EVA and EP diene ter-polymer blends. J. Appl. Polym. Sci. lA 2756-63. 

Figure 3 shows the performance during static oven aging at 170°C (338°F) of a stabilized EVA polymer. The base stabilization of the EVA polymer by the producer using AO-1 provided an unsatisfactory level of stability. The presence of skinning and a more pronounced level of discoloration in the base AO-1 stabilized EVA requires additional antioxidant to meet the performance needs of a HMA. Upon the addition of AO-2 to the base polymer, it is clear that the stability of the EVA is improved significantly, skin formation is not observed, and color development is reduced substantially. The formation of insoluble gel as a result of cross-linking is also reduced dramatically with the addition of AO-2, as shown in Fig. 4. 

Irradiation doses were commonly used in the presence of cross-linking agent, such as trimethylol propane trimethylacrylate (TMPTMA) or ditrimethylol propane tetraacrylate (DTMPTA), in blends containing isotactic (i)PP/LDPE or LDPE/EVA,55 5 respechvely. In such blends, irradiation in the presence of trimethylol propane acrylates was particularly efficient, regarding tensile strength, when the LDPE content was high. 

ENR/EVA blends. Irradiation produced the initiating free radicals that then reacted with a bismaleimide or a multifunctional acrylate molecule through unsaturation to produce cross-linking. The mechanism is given in Figure 9.7. The effective cross-linking by HVA-2 or TMPTA was confirmed by dynamic mechanical analyzer (DMA). 

The cross-linking of a polymer could be enhanced by the incorporation of a second polymer easily cross-linkable. In PP/EVA blends containing 70% of PP and 30% of EVA, EVA was described to cross-link under low doses of irradiation, whereas PP underwent chain scissions. Minkova et al. found that higher amounts of EVA in PP matrix favored cross-linking and increased the cross-linking density. EVA in PP slows the oxidation of alkyl radicals of PP. °... 

Dalai and Wenxiu draw similar conclusions about PE/EVA blends irradiated by y-irradiation in air. evA could enhance efficiently the cross-linking of LDPE while the amorphous phases of both polymers are miscible. Zhang et al7 showed that the radiation cross-linking behavior of LDPE/ EVA blends fits an equation between radiation dose and gel fraction developed initially for pure polymers. They concluded that LDPE and EVA are miscible in amorphous phase. On the contrary, the amorphous phases of EVA and HDPE are only partially miscible. Therefore, when EVA is incorporated into HDPE, poor enhancement of PE cross-linking is obtained under irradiation. ... 

In the same manner as blends of thermoplastics or thermoplastic/ elastomer, irradiated blends of elastomers can undergo chain scission due to degradation or cross-linking, depending especially on the dose range. For example, Zurina et al. i measured tanS versus temperature for 50/50 epoxidized natural rubber (ENR-50)-EVA blends by DMA. At 60 kGy the irradiation-induced cross-link enhanced the Tg of the blend, whereas at higher dose (100 kGy), the Tg decreased due to the occurrence of oxidative degradation that broke the cross-link structure. 

Their results were explained on the basis of elastic recovery (it means cross-linking density) and crystallinity. EVA increased the amorphous fraction of the blends and enhanced its radiation cross-linking. Heat shrinkability increased with the increase in crystallinity and decreased with the increase in gel fraction. DMPTMA level, EVA content, and radiation dose decreased the heat shrinkability. The amnesia rating also decreased with increasing radiation dose and EVA content. 

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