Page 116 - Handbook of Plastics Technologies

THERMOPLASTICS

2.56 CHAPTER 2

FIGURE 2.42 Preparation of vinylidene chlo-
ride polymers.

for food wraps. Commercial polymers are all copolymers of vinylidene chloride with vi-
nyl chloride, acrylates, or nitriles. Copolymerization of vinylidene chloride with other
monomers reduces the melting point to allow easier processing. Corrosion-resistant mate-
rials should be considered for use when processing PVDC.

2.3 ADDITIVES

There is a broad range of additives for thermoplastics. Some of the more important addi-
tives include plasticizers, lubricants, anti-aging additives, colorants, flame retardants,
blowing agents, cross-linking agents, and UV protectants. Fillers are also considered addi-
tives but are covered in Chap. 1.
Plasticizers are considered nonvolatile solvents. 400 They act to soften a material by
separating the polymer chains, allowing them to be more flexible. As a result, the plasti-
cized polymer is softer, with greater extensibility. Plasticizers reduce the melt viscosity
and glass transition temperature of the polymer. For the plasticizer to be a “solvent” for the
polymer, it is necessary for the solubility parameter of the plasticizer to be similar to the
polymer. As a result, the plasticizer must be selected carefully so it is compatible with the
polymer. One of the primary applications of plasticizers is for the modification of PVC. In
this case, the plasticizers are divided into three classes, namely, primary and secondary
plasticizers and extenders. 401 Primary plasticizers are compatible, can be used alone, and
will not exude from the polymer. They should have a solubility parameter similar to that of
the polymer. Secondary plasticizers have limited compatibility and are generally used with
a primary plasticizer. Extenders have limited compatibility and will exude from the poly-
mer if used alone. They are usually used along with the primary plasticizer. Plasticizers are
usually in the form of high-viscosity liquids. The plasticizer should be capable of with-
standing the high processing temperatures without degradation and discoloration, which
would adversely affect the end product. The plasticizer should be capable of withstanding
any environmental conditions that the final product will see. This might include UV expo-
sure, fungal attack, or water. In addition, it is important that the plasticizer show low vola-
tility and migration so that the properties of the plasticized polymer will remain relatively
stable over time. There is a wide range of plasticizer types. Some typical classes include
phthalic esters, phosphoric esters, fatty acid esters, fatty acid esters, polyesters, hydrocar-
bons, aromatic oils, and alcohols.
Lubricants are added to thermoplastics to aid in processing. High-molecular-weight
thermoplastics have high viscosity. The addition of lubricants acts to reduce the melt vis-
402
cosity to minimize machine wear and energy consumption. Lubricants may also be
added to prevent friction between molded products. Examples of these types of lubricants
403
include graphite and molybdenum disulphide. Lubricants that function by exuding
from the polymer to the interface between the polymer and machine surface are termed ex-
ternal lubricants. Their presence at the interface between the polymer and metal walls acts
to ease the processing. They have low compatibility with the polymer and may contain po-
lar groups so that they have an attraction to metal. Lubricants must be selected based on
the thermoplastic used. Lubricants may cause problems with clarity, ability to heat seal,
and printing on the material. Examples of these lubricants include stearic acid or other car-

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