Classification of PVC Lubricants According to Their Specific Action

2 Classification of PVC Lubricants According to Their Specific Action 

Lubricants may be divided into two basic groups, external and internal, according to their action [8,24,25]  

External Lubricants. External lubricants are substances with very low solubility in the polymer they exude partially from the bulk, creating a separating layer between the polymer and the metal surface of the processing machine. It is believed that the polar end of the lubricant molecules is strongly adsorbed to the metal surface so as to form a stationary layer of lubricant molecules that then initiates the formation of a multimolecular layer as shown in Fig. 2.6 [19]. The strength of the bond between a lubricant molecule and the metal surface depends on the nature of the metal surface it determines whether the adsorp- 

Internal Lubricants. Internal lubricants are substances that have a good affinity to the polymer and reduce its viscosity in the melt state. Moreover, they reduce the heat produced by the action of the friction forces. They also affect 

It seems that most of the substances used as lubricants fulfill more or less two functions, internal and external lubrication. The limit of affinity between a polymer and a lubricant is the criterion for the differentiation between the two types of lubrication [27]. In fact, a lubricant begins to act as an external lubricant above a given concentration in the polymer. If its affinity to the polymer is limited, the mechanical properties of the polymer containing the lubricant vary with its concentration before the limited affinity is attained. In such cases that substance functions as a plasticizer. 

---