Antiblocking agents come in various forms. The most common is silica, which is a more scientific term for a finely ground sand. A form typically used and a larger word for the same is “diatomaceous earth”. The layman’s term “crushed sea shells” is sometimes stated as the source of antiblock. Talc is an option which is a very soft crushed rock. Finally, there are antiblocks drawn from feedstocks and the more exotic and less common, of course, will be more expensive. When the surface of a film containing antiblock is examined under an electron microscope, the surface will have a ‘moonscape’ appearance with peaks and valleys. Scuffing of the film surface can sometimes result as an undesireable effect of antiblock.
The scuffing occurs when the film or bag surfaces slide or rub against each other or against another surface. This can happen during winding at extrusion, unwinding during converting, shipping and packing of converted bags, and handling of bags or packaging films. The powdery appearance that results is a visual concern to some, but not ‘contamination’ as sometimes perceived by the lesser informed.
Elements that can contribute to scuffing include:
- The type of antiblock used.
- The particle size and consistency of particle size.
- The dispersion of the particles within the polyethylene masterbatch.
- Roll wind – less tight a wind may allow surface movement wrap to wrap.
- Boxing / packing converted bags- movement within cased boxes of bags may contribute to scuffing or degree of scuffing from one delivery to the next.
- Gauge of the film is a factor as the heavier gauge films contain greater amounts of antiblock.
- Consumer handling of the packaging film or bags may contribute to the scuffing effect.
As indicated, a number of contributing factors may be involved in the cause and control of antiblock scuffing. It is difficult to manage and maintain specific roll hardness regarding roll wind. The wind factors within plastic rollstock are going to change and the film moves somewhat even after rolls are wound. Our film suppliers control the particle size and dispersion of antiblocking agents from the resin producers. Barring the identification of an obvious, measurable and controllable quality issue within the resin, film, or handling perhaps the best option to eliminating the potential for antiblock scuff is through the choice and the type of antiblock used. The type of course can also impact the price.
In the case of ATMI; infrared analysis was run comparing two lots of 6 mil (heavy gauge) polyethylene film from the same supplier. One had the preferred clarity and the other lot examined was complaint film. The complaint film showed little white spots in places across the inner surface of the film. We believe this to be ‘micro-scuff’ marks produced by movement of film surfaces against the peaks of antiblock.
The infrared analysis showed a similar loading of antiblock between lots indicating the “formulation” was not the culprit. While we do not know the manufacturing or post manufacturing step at which time the optical qualities were compromised, there is some thought that the tremendous draw length on this size bag, made for the first time, may explain why the effect was not seen in previous lots. The bag length with the good optics was one half the length. The customer had also stated the effect had not been seen previously and questioned the difference.
At this time, we are working with our film supplier regarding their thoughts for a potential cause and solution and will report the results as they become available.