In processing high melt viscosity polymers such as polyvinyl chloride (PVC) by extrusion, milling, calendering and injection molding, the shear forces applied cause excessive frictional heat which may lead to serious thermal stability problems. Another problem in processing PVC is to assure that the polymer releases from metal surfaces of the processing equipment. To solve these problems two types of lubricants are used.Lubricants which lower the melt viscosity and control frictional heat build-up are called internal lubricants while substances which promote release are called external lubricants. These materials are used in relatively small amounts since an excess will cause processingand stability problems and structural weakness in the ultimate product. In the processing of polymers such as PVC discrete particles are subjected to stress and heat until there is fusion of the discrete particles and a resulting loss of particle identity. An excess amount of an external lubricant will tend to coat the individual particle and while promoting a slippage between particles will delay fusion.
The role of the internal lubricant is to reduce the internal friction within the polymeric melt, which includes reducing heat build-up when the polymer is subjected to stress.Because of the characteristic high melt viscosity of rigid PVC an internal lubricant is usually viewed as being necessary to improve flow properties. Their use will result in an economic advantage in that less work will be expended at a given set of processing conditions. In addition, improved product appearance usually results, particularyly with respect to improved surface appearance. An internal lubricant will promote fusion.
Other distingguishing characteristics of internal and external lubricants are the effects they have on fusion time. Internal lubricants show no change in fusion time as the concentration of lubricant increases in the polymer system; external lubricants lengthen fusion time with increasing concentration.
Some lubricants exhibit properties of both internal and external lubricants and are identified as internal/external. The degree of each type of lubricity imparted in a specific application is depending on the type and concentration of lubricant employed, the composition of the plastic system, the type of processing equipment, and the operating parameters of the processing system.
In some instances, one encounters undersiable side effects in the use of lubricants, most notably in the reduction of heat stability which can lead to such major production problems as:
Thermal degradation of the thermalplastic material within the extruder requires a halt of process operations for cleaning out.
Recycling of materials is limited.
The use of thermoplastic materials in light colored goods is limited.
High level of expensive heat stabilizers may be required.