PVC Lubricants

Lubricants classically function in two ways: (1) externally to reduce friction between the polymer melt and hot metal surfaces during processing; and (2) internally to reduce frictional forces within the polymer matrix and, in so doing, lower the effective viscosity. External lubricants, by definition, have poor compatibility with PVC and migrate to the surface to create metal release.

Internal lubricants are more compatible, have high polarity, and are therefore more soluble in PVC. Both types are essential for processing of PVC-U formulations, particularly taking account of the differing shear conditions resulting from extrusion, calendering, or injection moulding. In practice, most lubricants have both external and internal characteristics, but vary in the ratio of one to the other in the way they perform. However, the correct balance and type of internal to external and addition level are crucial due to the influence on gelation level, plate-out of incompatible materials in the die and calibration equipment, and physical properties.

The main chemical classes that make up lubricants are all waxes or soaps and are as follows:

• Amide wax – Ethylene Bisstearamide (EBS) has a balance of internal and external behavior.
• Hydrocarbons, which can be split up as:

- Paraffin waxes: linear alkanes with chain lengths varying from C20 to C50 which are very external in nature. The lower molecular weights show some internal character.

- Polyethylenes (PE) with molecular weights ranging from 500 to 1000 (C35–C70) with linear or branched molecular chains, differing from the paraffin waxes in end groups.

- Oxidised polyethylene waxes which result from oxidation of PE waxes causing breaks in the molecular chains and the formation of acid groups. They are very external in nature because of excellent metal release characteristics.

• Esters of many types such as:

- Glycerol esters such as the liquid Glycerol Mono Oleate (GMO) and the solid Glycerol Mono Stearate (GMS) which are mostly internal functioning with GMS having some external properties.

- Montan esters, obtained from brown coal, and consisting of montanic acid esters with long chain aliphatic alcohols. Chain length is in the region C28–C34 with a broad range of lubricant functionality and high compatibility. Particular use is in profile and calendered formulations.

• Fatty acids such as stearic acid (can be a mixture of C16 and C18) or hydroxy stearic acid. External lubricating behavior with some internal lubrication.
• Metallic soaps, in particular calcium stearate, which acts as an internal lubricant by improving flow but increases internal shear and mould release. Also acts as an acid scavenger. It is a very commonly used lubricant.

Assessment of lubrication performance, on a processing machine, is carried out taking account of energy consumption (motor load, screw speed, and output), surface finish, melt flow, and physical properties. In the laboratory, gelation measurement based on time and melt viscosity is carried out using a torque rheometer or mill. This takes account of the internal lubrication effect (lowering of melt viscosity with no significant change on fusion or mill stick times) and external lubrication (little influence on melt viscosity but does significantly influence fusion or mill stick times).

The addition level of internal lubricants can be in the region 0.3–1.0 phr, whereas external lubricants are added at much lower levels, e.g., 0.05–0.3 phr.

For extrusion and injection moulding of PVC-U formulations, it has become common practice to use one-packs, comprising of the stabiliser and a balanced lubrication package, which are formulated to match the particular processing and end-use performance requirements.

In PVC-P applications, the plasticiser acts as an internal lubricant and external lubricants can be added to increase output, and so on. Excessive levels or incorrect choice can cause problems with post-treatment, e.g., printing or blooming on ageing.