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Static electricity is a common problem in the world of polymers, which themselves act as electrical insulators. One of the more annoying effects of static electricity is the tendency for surfaces to pick-up dust or to stick to each other. A more dangerous consequence is its sudden discharge which, for example, can lead to a dust explosion. One remedy is the addition of an antistatic agent to the base polymer. This lowers surface resistivity to such a level that the static charge can dissipate. Non-migrating additives are permanent Non-permanent antistatic agents are typically based on hygroscopic compounds with a low molecular weight, such as glycols, sulphonates or amides. They are more-or-less incompatible with the base polymer and therefore migrate to the surface once the final product is formed.The result is the formation of a thin surface layer, which, with sufficient air moisture, absorbs water to become conductive. The speed of migration to the surface will depend on the chemical structure of the molecules. The antistatic effect is not permanent, as the content of the antistatic agent in the polymer will decline after time. Permanent antistatic agents are usuallybased on polymers. During processingthey form a co-continuous network within the polymer they need to render antistatic. Consequently it is functional almost immediately and without activation time, while the network is also the reason why no migration takes place. This brings with it two major benefits: • The antistatic effect remains over time,as the agent is not consumed. • The effect limits itself to the intended,specific layer of a part/package. There is no danger that the agent has a negative effect on the packaging content or – if required for decoration – the adhesion properties of the surface. Common permanent antistatic agents are based on polyamide or polyesters and are therefore incompatible with polyolefins, which may have a negative effect on processing. The newly-developed permanent antistatic agent Entira™ AS MK400 from DuPont is based on an ethylene ionomer. Due to its polyolefinic nature, it has excellent compatibility in processing with polyethylene homo- or co-polymers as well as polypropylene. The properties of this novel, food-approved antistatic agent are summarized in Table 1. Laboratory testing In order to further illustrate and evaluate these merits, the newlydeveloped antistatic agent was processed by DuPont in conjunction with a variety of polymers, ranging from polyethylene (PE) and polypropylene(PP) to ethylene-copolymers (EMA and EVA) to engineering polymers (ABS and polyamide). Processing methods employed included blown film,extrusion blow moulding, injection moulding and roll mill mixing. Properties that were evaluated included surface resistance, static decay and dust pick-up after generating static electricity on the finished piece. The latter was measured by rubbing the testing object against a cotton cloth before bringing it in contact with Bonita Flakes (fish flakes). The amount of antistatic agent used for these tests varied between 10 and 30 wt.%. In polyethylenes, the tests demonstrated that the desired low surface resistance and required low static decay time could be achieved with additive levels between 15 and 20 wt.%(measured at 50% RH). This translates into an absence of friction charge on the plastic part and therefore no attraction of dust or flakes. Figure 2 shows the effect of the new antistatic agent on the results of the rubbing test performed on a coextruded multilayer film with an outer layer made from OPET and a tri-layered metallocene-PE inner layer. Its mPE heat-sealable layer is treated with an antiblock (AB) additive, and the outermost mPE layer – only 15μm thick – contains the antistatic agent. The result is significant–even after rubbing with a cotton cloth, the surface does not attract particles. Figure 3 demonstrates the correlation between the effect of Entira™ AS MK400 and humidity when tested on bottles made from PE-HD and PP using coextrusion blow moulding. As in the previous test, the additive is applied to the thin outerlayer, yet it determines the performance of the complete bottle. Migration to the other layers does not take place. The new permanent antistatic agent maintains most of its performance down to humidity levels of approximately 30%RH. A further version is planned for commercial availability in 2007 that will be able to maintain favourable antistatic properties down to a humidity level of 12%RH. A similarly good result can be obtained with EVA blown films with varying VA content.High functionality was also illustrated in tests with DuPont Elvaloy®1224AC, a high mineral-filled ethylene-methyl-acrylate(EMA), which is used, for example, to produce soft-touch packaging (bottles, films),and with the engineering polymer ABS. Applications The most obvious applications for Entira™ AS MK400 are multilayer films for packaging powder-based products and multilayer extrusion blow-moulded bottles. In such cases, the effect of the antistatic agent can be concentrated on the layer where it is needed most, without any fear of migration into other layers or negative influence on decoration. Due to its excellent compatibility with polyolefins, it is possible to create products with a smooth, if not glossy,surface. Another application is the use of Entira™ AS MK400 as an additive in polyethylene for pallet stretch hoods. Here the antistatic agent may help to facilitate the opening of the hood in order to allow faster cycle times. This is possible due to the permanent and immediate action of Entira™ AS MK400 without the necessity to migrate to the interior surface. The latter would be difficult, as the interior of the bubble of the stretch hood does not represent a surface where migrating additives can easily migrateto. Further applications in other industries(for example, tubing, pipes or cables) are currently under development. A soon-tobe-available version, that is effective at lower relative humidities, will provide specific benefits for the packaging of electrical components. |
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