Oleamide and erucamide fatty acid derivatives are the most common slip agents used in polyethylene film. Oleamide is derived from mono-unsaturated C18 oleic acid while erucamide is the amide of C22 mono-unsaturated erucic acid. Each offers distinct benefits and advantages when used in specific processing situations.
The rate of diffusion of slip additive to the surface of the polymer film varies as a function of chemical structure. The lower molecular weight oleamide migrates to the film surface more rapidly and is often referred to as a 'fast blooming' slip (see Figure 1). Although 'slow blooming' erucamide develops its lubricating effect at a slower rate initially, after 4-6 hours it produces lower COF values than equal quantities of oleamide (see Figure 1 and Figure 2).
Antiblocking performance is another key characteristic of amide slips. ' Blocking' is the tendency for polyethylene film sheets to adhere to one another as they are separated vertically; antiblocking is therefore the ability to reduce inter-sheet adhesion characteristics. In most cases, the effectiveness of amorphous and crystalline silicas, diatomaceous earths and talc antiblocks far outweigh the marginal antiblock properties provided by amide slip agents. However, because inorganic antiblocks increase haze levels and can negatively affect other optical and physical film properties, some high clarity film applications rely exclusively on the slip agent to provide the necessary reduction in blocking force. In these cases, erucamide provides superior antiblocking compared to equal levels of oleamide. Stearamide (another fatty acid amide) provides even better antiblocking, but since it does not function effectively in reducing COF, it is usually used in combination with oleamide or erucamide to achieve a desired balance of slip and antiblock characteristics.
Thermal stability is also an important feature to consider when selecting a slip agent. Erucamide has a lower vapour pressure and is less volatile than oleamide, making it more suitable for higher temperature processing. Lower volatility means that more of the slip will stay on the film surface and not end up venting off as smoke or depositing on metal die surfaces. In addition, erucamide is less prone to thermal oxidation during processing and contributes less color (i.e. yellowing) and odor to the final product. The chemical stability of erucamide also translates into lower rates of oxidation, rancidification and color formation during product storage.
Oleamide is often the slip agent of choice for in-line bag converting operations where a low COF is needed in a short period of time. Because of its faster migration, a lower level of oleamide can be used in place of erucamide. Since oleamide is generally less costly, the savings are two fold. The slower blooming rate of erucamide can be an advantage in roll stock applications, where film that is too slippery causes winding difficulties and telescoping of rolls. The slower exudation of erucamide can also result in more effective in- line corona treatment. Because of its low volatility and chemical stability, erucamide is also ideally suited for most high quality food packaging applications.