Antistatic additives migrating

Obviously, cost and application requirements will determine the choice of an antistatic additive. Migrating antistats, for instance, have cost advantages, but also have usage limitations that industry observers have noted. Along with the pluses and minuses discussed in Section 6.1.1, there are other specific important factors to consider when using migrating antistats ... 

The tendency to form electrostatic charges could, therefore, be decreased by increasing the conductivity of the polymer. To obtain drastic effects in this direction, carbon black is added to the polymer for less stringent requirements, antistatic additives are used, which, blended into the polymer in small quantities, tend to migrate to the surface and form a very thin surface layer, which considerably reduces the surface resistance, and which is continuously renewed. 

In some cases, however, the use of additives is based on their migration to the surface antistatic additives, and also agents to reduce friction of films, perform as desired when they are present on the surface of the article, forming a thin skin. 

Slip and antistat additives, which fnnction at the snrface of the plastic part, are traditionally migratory. The additives are difficult to predict and control because migration occurs over time and depends on part/film thickness and polymer crystallinity. 

The composition of polymer surface layers often differs from the bulk, due to the migration of organic antistatic additives. The surface resistivity of the polymer can dominate the insulation resistance. The concept of sruface resistivity implies the existence of a surface layer on top of an insulating substrate. The surface resistivity ps is related to the layer thickness t and volume resistivity p by... 

Migratory antistats (MAS). Migratory antistats have chemical structures that are composed of hydrophilic and hydrophobic components. These materials have limited compatibility with the host plastic and migrate or bloom to the surface of the molded product. The hydrophobic portion provides compatibility within the polymer and the hydrophilic portion functions to bind water molecules onto the surface of the molded part. If the surface of the part is wiped, the MAS is temporarily removed, reducing the antistat characteristics at the surface. Additional material then migrates to the surface until the additive is depleted. These surface-active antistatic additives can be cationic, anionic, and nonionic compounds. 

Antistatic additives can be classified by application method, as internal and external, and by chemistry, as anionic, cationic, and non-ionic. Internal agents are normally compounded at 0.1-3.0% by weight and have a slight compatibility with the polymer, but the molecule has a hydrophilic head forcing it to migrate to the surface and attract moisture from the environment, which increases the surface conductivity. These are easy to use and have low addition rates, often also providing other benefits such as improved processability and mould release. 

Internal antistatic additives are incorporated into the polymeric matrix. The additives migrate onto the surface of the poljmtier and they function by two mechanisms ... 

The selection of the types of compounds used as antistatic additives is governed by complex considerations, one of which is that it should have the correct degree of comparability with the base polymer which will enable it to migrate to the polymer surface at a controlled rate during service life. 

Examples of the possibility of a degradation of an additive and its effect on the design of the analytical procedure are supported next by means of an example, i.e., the antistatic additive lauric diethanolamide which has been used in foodgrade polyolefin and polystyrene formulations. Lauric diethanolamide upon migration... 

A wide variety of low molecular weight additives, such as plasticizers, antioxidants, UV stabilizers, pigments and antistatic agents are frequently incorporated into polymers to improve a property. Often these additives are simply dissolved in the polymer matrix and, consequently, they are liable to be extracted at a later time. This is especially the case if they are volatile or are soluble in a solvent which may later contact the polymer. One of the most common polymers in which additive migration is encountered is poly(vinyl chloride). 

Certain polymer substrates may also contain sliding (slip) agents, antistatic additives, anti-block additives or other components that may migrate to the surface if stored for long periods of time prior to the screen printing process. These additives and work-in-process delays can impair the adhesion of the screen ink to the substrate or cause poor printability. 

Decrease in Coefficient of Surface Friction Here, the additive acts entirely as an external lubricant friction during processing occurs between lubricant-coated PVC and lubricant-coated process surface, instead of between PVC and metal. The interaction is much weaker, with lower frictional development of charge. For this to occur, the antistat additive must be partly compatible with the polymer. That is, its solubility parameter must be lower than that of PVC (18-19 SI units) or plasticized PVC (usually 17-18 SI units), but not so low as to be incompatible (more than 3-4 SI units difference). Friction-reducing antistats, as with external lubricants, tend to contain both polar and nonpolar sections. For external action, the nonpolar part should have polarity similar to the nonpolar area in PVC (i.e., a hydrocarbon section). The polar part of the additive should be less polar than the carbon-chlorine bond in PVC, so as to permit migration to the surface (in competition with internal attraction to the polymer or to polar fiUer particles). If the polar section of the additive is chosen to be more polar than the polar part of the polymer, it wUl function primarily internally, as a filler treatment, internal lubricant, or both. 

Three properties that contribute the most to migration are (1) incompatibility with the resin, (2) low melt point, and (3) low molecular weight. Examples of noncolorant additives that are prone to migrate are (1) lubricants/dispersants such as metal stearates and bis-stearamides, (2) mold release and slip agents, and (3) antistatic agents. All of these ingredients serve useful purposes in color formulations. If you exceed their solubility limit in the end-use resin, however, you risk blooming and its consequences. 

---