The choice of a correct antistatic is crucial and is determined by a wide range of factors such as polymer type, processing conditions and end application. By careful selection of the correct blend of antistatic ingredients, Wells Plastics has developed a dynamic range of products to cover the whole spectrum of requirements.
Masterbatches can be fast acting to reduce dust attraction in food packaging and display applications and can be formulated to give longer-term effects in demanding applications such as flooring.
Combinations with antiblock and slip products for use in the film industry allow cost effective and high performance to be available for polyethylene and polypropylene film extruders.
Sheet, Film, Fibres, Injection Moulding and others.
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Most plastics have a surface resistivity (when dry) of 1015—1016 ohms.sq and at this level they are good insulators being quite effective at holding on to any static charge.
The level of resistivity at which they will allow the charge to decay to earth (although more slowly than a conductive material) is around 1012 down to perhaps 108 ohms.sq. At this level of surface resistivity the tendency to pick up dust (and retain powdered products) is reduced and the overall tendency to accumulate static charge and perhaps cause dust explosions is also reduced.
At surface/volume resistivities below the arbitrary level of approximately log 8 you start to approach products which actively and rapidly dissipate charge. Lower than around log 3 are conductive matrices and at this level of resistivity you can produce earthing mats, protect computer chips from earthing damage and perhaps totally negate the risk of static charge explosions.
Internal antistatic additives are substances which are added to polymers/plastic articles during thermoforming or compounding in order to minimize the natural tendency of most plastics to accumulate static charge.
Generally these additives work by migrating to the surface of the polymer, usually in a molecular or multi-molecular layer, and attracting water molecules from the atmosphere. The water lowers the surface resistivity from the approximate value of 1014 - 1015 ohms for most polymers (including styrenics and polyolefinics) to 1011- 108 ohms.
The additive molecule normally consists of a hydrophobic part, which confers a level of compatibility with the polymer and a hydrophilic part which attracts the water that forms the surface conductive path. The type of additive used depends on achieving appropriate compatibility with the polymer in question, an appropriate rate of migration, an appropriate persistence of effect and the ability to undergo the manufacturing process, both of the additive concentrate and subsequent conversion into the finished product.
The speed of antistatic effect and its duration can be optimised through careful choice of the additive. The migration rate is determined by a number of factors including the relative compatibility of the additive and the polymer, polymer crystallinity, total additive formulation, concentration of antistat present and temperature.
There are four classes of antistatic additive:
The active part of these molecules is the cation - usually an alkyl radical such as ammonium, phosphonium or sulphonium. The associated anion is generally a chloride, methosulphate, nitrate or something similar. They work best in more polar substrates such as styrenics or PVC, but have the disadvantage of being rather thermally unstable and not suitable for food contact use. Furthermore the addition rate to achieve an equivalent effect to anionic antistatics is 5 or 10 fold.
With these additives the active component is the anion, normally an alkylsulphonate or something similar. The cation is generally sodium or another common alkali metal. These, like the cationic antistatics, are very suitable for use in polar polymers such as styrenics, but they have the advantage of achieving a potent effect at relatively low addition rates and they are also suitable for food contact use. The disadvantage is they do not always work well in polyolefins.
These are uncharged compounds having interfacially active structures with very low polarity. They are typically waxy substances such as polyethylene glycols, organic stearates, organic amides and ethoxylated amines. As they have low or no polarity they are very compatible with polyolefins and their long alkyl chains make them fairly hygroscopic when they migrate to the surface. Disadvantages are their low compatibility with polar polymers and the fact that their presence on the polymer surface can adversely affect some post production processes such as printing etc.
This class is often referred to as permanent as it consists of non-migratory, hygroscopic, polymeric materials that are melt incorporated to form a conductive, percolating network. This effectively dissipates charge both upon and within the polymer carrier. They are typically low molecular weight substituted polyamide copolymers. They work well in in polyolefinic substrates and have also been found to be effective in some styrenics such as ABS.
Advantages are their permanence, efficacy in low relative humidity environments and lack of any tendency to disrupt post-forming operations. Disadvantages include cost, relatively high addition rates, relatively poor surface resistivities achieved (1010 at best) and some potential processing difficulties (the need for high temperatures and low shear production regimes).
The precise choice of additive class is critical to achieve the desired effect and then within that class the correct compound must be used to give the correct balance of effectiveness, speed of effectiveness and persistence. The graph in Figure 1 demonstrates the general effects of three major classes of antistatic additives - ethoxylated amine (EA), glycerol monostearate (GMS) and lauric diethanolamide (LDN).
To achieve good performance the antistatic additive must be homogeneously distributed through out the polymer matrix. This is often best achieved by the use of a masterbatch as this ensures that the antistat is already well dispersed and the similar physical form makes mixing with the polymer feedstock easier.
Wells Plastics can readily offer additive packages to reduce the surface resistivity of polymers using a variety of systems as discussed. Wells has many years of experience in developing the correct formulation for precise requirements and we are able to either offer a product from our large range of standard grades or produce a carefully formulated “special” to meet even the most demanding application.
This information is correct to the best of our knowledge, but we would recommend that users make their own assessment to confirm that the material meets their requirements. We accept no liability for any damage, loss or injury resulting from the use of this information. Freedom from patent rights must not be assumed.
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