Page 355 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.35

5.5 LUBRICANTS AND PROCESSING AIDS

The term lubricants is used by a variety of specialists to cover a range of chemicals that
are added to plastics to improve a variety of performance characteristics in processing or
in final properties. When they are used to improve processing, the field may expand to in-
clude other types of processing aids. When they are used to improve final properties, the
field may expand to include other properties beyond lubrication.
Most of these characteristics are difficult to quantify, difficult to correlate between lab-
oratory and commercial practice, and depend very much on the individual processor, pro-
cessing equipment, process, and type of product. Theoretical understanding is limited,
problems are solved by trial and error, and secrecy is dominant, making this one of the
most obscure and difficult areas in the entire field of plastics additives.

5.5.1 Lubricants in Processing
Lubricants may improve processing in a number of different ways. These are discussed
below.
When pellets or powder fail to flow smoothly down the feed hopper, they tend to bridge
or arc. This may require mechanical tapping to keep them flowing, and coating the pellets
or powder particles with a small amount of lubricant may help them to flow more freely.
Screw feed produces frictional heating, which speeds fusion of pellets or powder into a
fluid mass. When processors want to delay fusion, usually in processing rigid vinyl, lubri-
cants that reduce friction between the resin and the steel screw and channel may be useful
to reduce frictional heating and thus delay fusion.
Lubricants that are liquid and miscible in the molten polymer can act as plasticizers to
decrease melt viscosity, decrease power requirements, and give faster flow.
Shear degradation occurs when polymer molecules are very stiff and/or very entangled,
and friction from the screw forces them to flow anyway. Then, the molecules may simply
break and degrade to lower molecular weight. Lubricants can prevent shear degradation,
either by plasticizing the disentanglement of the polymer molecules or by reducing the
friction between the polymer and the steel screw.
Steel molds, extrusion dies, and rolls have high surface energy; molten polymers have
much lower surface energy. This makes the polymer coat and stick to the steel equipment.
Lubricants that interface between polymer and steel can help to release the plastic product
from the mold, die, or roll. This can speed processing and avoid distortion of the product
when it is pulled away from the steel.
Die swell following extrusion can be reduced by addition of lubricants. They may
function by plasticizing the polymer, permitting molecules to disentangle and accept the
shape of the die, or they may function by reducing adhesion between the polymer extru-
date and the steel die, thus releasing the product without distortion.
Gloss may be hurt by friction between polymer and extrusion die, producing micro-
scopic roughening of the surface. A lubricant that reduces adhesion and friction between
polymer and steel can thus reduce roughening and increase surface gloss.
More serious adhesion and friction between polymer and extrusion die can cause melt
fracture in the form of sharkskin, matte, or orange peel. All of these can be reduced by a
lubricant that reduces adhesion and friction at the interface.
“Slip agents” are added to polymers for film production. They exude to the surface of
the film and lubricate it, keeping the films from sticking to each other and making it easier
for them to slide over each other and to separate from each other. This is helpful in pro-
cessing, both for speeding production and to avoid distortion.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.