Page 356 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.36 CHAPTER 5

5.5.2 Lubricants in Final Products
Lubricants may be added to a polymer to produce lubricity in the final product. The effects
are discussed below.
Antiblocking agents (“flatting agents”) may be added to polymers for film production.
They roughen the surface of the films, reducing surface contact and thus reducing adhe-
sion between them, making it easier to separate them in use. This is particularly important
in packaging film and bags.
Gears and bearings made from metals have high polar attraction, which produces high
friction and requires excessive use of contaminating lubricants such as motor oil and
grease. By comparison, organic plastics have much lower attraction and may require much
less use of contaminating lubricants. Even here, however, they may not give perfect self-
lubrication. Thus, it may be very helpful to add lubricants that come to the surface of the
gears and bearings and make them completely self-lubricating.

5.5.3 Lubricant Chemistry
It is common to distinguish between “internal” and “external” lubricants. The worst defi-
nition is based on who adds the lubricant: if the supplier added it, it comes to the processor
as an “internal” lubricant; if the processor must add it himself, it is an “external” lubricant.
A semimeaningful distinction: an internal lubricant is compounded into the polymer; an
external lubricant is applied to the surface of the mold. The proper distinction is based on
miscibility/immiscibility of the additive in the polymer: an internal lubricant is miscible
with the polymer and acts like a small amount of plasticizer to increase molecular flexibil-
ity and mobility, and to help the disentanglement and flow of the polymer molecules in the
melt. An external lubricant is immiscible in the polymer and tends to exude to the surface
of the plastic and form an abhesive (nonstick) interface between the polymer and the steel
process equipment, or it tends to come to the surface of the finished plastic product and
give it continual lubricity in the use of the product.
Some lubricants are purely internal; some are purely external. Many exhibit a balance
of internal/external activity, which depends both on the lubricant and on the polymer in
which it is used. This depends on the relative polarity of the polymer and the lubricant. If
they have very similar polarity, the lubricant is completely soluble in the polymer and acts
as an internal lubricant. If they have very different polarity, the lubricant is insoluble in the
polymer and exudes to the surface, where it acts as an external lubricant. If the lubricant
molecule contains a nonpolar segment, -----, and a polar segment, *****, the relative
length or strength of these segments
-------***
-----*****
---*******
can be adjusted to balance the miscibility of the lubricant in the polymer and thus adjust its
relative internal versus external performance.

5.5.4 Major Market Classes of Lubricants
The major market classes of lubricants are as follows:
• Fatty esters (mono, di, and triglycerides and straight-chain esters)
• Fatty amide waxes (erucic, oleic, ethylene-bis-stearamide)

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.