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CA® – Block

Blocking is a common problem encountered by manufacturers of polyolefin films and coatings. There is an array of antiblock types available. The overview covers the fundamental reasons for considering and using antiblock additives. A review of the most commercially important grades is offered, with general guidelines relative to the user’s needs. Natural silica (diatomaceous earth) and talc prove to be of greatest interest for most commodity applications.

Blocking ist the adhesion of two adjacent layers of film. it is a problem most associated with polyethylene and polypropylene films (either blown or cast), and to a lesser extent in extrusion coated or laminated products.

it is thought that blocking of adjacent film layers occurs due to the presence of Van der Waal’s forces between the amorphous regions of the polymer. There forces increase with reduced distance between the two layers, thereby increasing blocking when two layers are pressed together (e.g. winding onto a takeup roll or stacking of finished, converd films). Another possible reason for blocking is the presence of low molecular weight species (such a oligomers), which tend to migrate to the surface of the film.

The most effective method for combating these handling issues is to add an antiblock additive. An antiblock additive present in the resin microscopically protrudes from the film surface. This creates asperities (i.e. “little pumps”), which help to minimize the film-to-film surface contact, increasing the distance between the two layers, thereby minimizing blocking.

The blocking between adjacent layers results in increased friction (resistance to motion), and the addition of an antiblock generally contributes to a reduction in the film-to-film coefficient of friction (COF). COF is a measure of the relative difficulty with which one surface will slide over an adjoining surface. The greater the resistance to sliding, the higher the COF value (e.g. “low-slip” or “no-slip” film, sometimes referred to as “high COF” film).