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Cross-Linking of Polyolefin Film Packaging Applications
his article was originally published on www.uvebtech.com by Im Rangwalla.View the original article here.
Guangdong Shunde KSL Co., Ltd. is a professional manufacturer specialized in Cross-link shrink film. cross-link shrink film has demonstrated comparable or even better performance versus other shrink film. It is proved by versatility across a broad range of shrink equipment, especially high-speed application. KT Series is an all-purpose shrink film with superior strength, strong seals, high abuse resistance and superior optics and clarity. Abstract Radiation cross-linking comprised of gamma rays from a Co60 source and even high-energy electron cross-linking from > 1 MeV accelerators have been commercial since 1960s. These applications were mostly for the wire and cable industry, rubber tires and some high-barrier shrink film applications for meat packaging introduced by Cryovac. Since the development of low-voltage electron beam accelerators in the range of less than 300 kV – in particular in the 125 kV range – EB cross-linking has found new applications and markets, particularly in the packaging industry. This report will discuss these markets, along with the properties achieved by electron beam irradiation of polyolefin films used in packaging. In addition, theory of electron beam cross-linking will be discussed in detail. Introduction When irradiated either by gamma rays from a Co60 source or high-energy electrons generated by electron accelerators, polymers either cross-link or undergo chain scission, depending on the chemical structure. Both processes take place simultaneously and, depending on the chemical structure, either cross-linking or chain scission becomes the rate-limiting step and the end result of the irradiated polymer. Cross-linking usually results in molecular weight increase and is directly proportional to the dose of irradiation. The molecular weight increases with dose until a three-dimensional network is formed, making the polymer thermoset. When chain scission reaction predominates, then the molecular weight decreases with dose, resulting in reduction of the polymer’s mechanical properties. Table 1 indicates which polymers cross-link or undergo chain scission.1 Radiation cross-linking of natural rubber for the tire industry and cross-linking of polyethylene for packaging have been used since the 1960s, using high-voltage 400 kV to 1 MeV type EB accelerators. Even for the wire and cable industry, radiation cross-linking to provide temperature resistance was the favored curing option over thermal cross-linking using peroxides. Significant research and commercial activity in radiation cross-linking of polyolefins has occurred, especially focused on polyethylene films for packaging applications. Properties of ordinary polyethylene film made by the bubble process were significantly enhanced by electron cross-linking, as shown in Table 2.2 As can be seen in the table, EB irradiation provides significant enhanced physical properties required for heavy-duty high-shrink bags for meat/poultry packaging. The process is shown in Figure 1. The extruded thick-walled tube is irradiated using a high-energy 1 MeV type accelerator before being oriented in a bubble form. The irradiation step cross-links the polymer molecules, then it is biaxially stretched and oriented. The irradiation imparts the memory effect to the polymer, resulting in higher shrink ratios. The cross-linking effect increases the molecular weight of the polymer, resulting in higher temperature-resistance, broader heat-seal range and enhanced mechanical properties, as shown in Table 2. Copolymerizing of EVA copolymers and EB cross-linking results in excellent shrink bags. Electron beam cross-linking has two major advantages over other methods: FDA approval and lack of residual chemical contaminants. As a result, greater than 90% of frozen turkey is packaged in high-barrier EB cross-linked PE films. To be continuned…… |