Lubricant is one of the most essential additives after Heat Stabilizer yet most difficult to adjust for the processing of rigid PVC. PVC requires various additives to prevent PVC from degrading when heated and sheared and get the desired and perfect articles. Lubricant reduces various types of frictions within the polymer matrix and between the polymer matrix and processing equipment. It influences the viscosity and flow behavior of PVC compositions under the heat and shear experienced in the processing machinery so that the processing is made easier and smooth.
The rheology of PVC is adjusted with the help of various lubricants depending upon the process and application. Goldstab uses these lubricants in the manufacturing of One Pack Stabilizers and adjusts the lubrication required according to the application.
There are a wide variety of lubricants used for processing PVC, each having its unique motive
A.
External Lubricant – Paraffin wax, Polyethylene wax, Fischer-Tropsch wax
B.
Internal Lubricant – Oxidised Polyethylene wax, Ester wax
A. External Lubricant
External Lubricant is responsible to reduce friction between PVC melt and the metal surface of machinery. It provides operational benefits of easier movement of the stock through the machinery and prevention of its adhesion to the hot metal that would quickly result in the local formation of stagnant layers and their burning. It increases the fusion time and stability of PVC and delays gelation. It also provides an additional feature of gloss to the end product.
1. Paraffin Wax
Paraffin wax is the most prevalent external lubricant used for U-PVC applications such as pipes and fittings. It is derived from petroleum refining and has a dropping point between 55-75°C. These long branched chain wax segments tend to form crystallites at the surface of U-PVC melt and form a layer between metal surface and matrix. This arrangement reduces the friction between the surface of the machine and the resin. It improves the surface gloss for rigid application but produces haze in the transparent application. Due to its low melting point, easy evaporation and low melt viscosity, it has a narrow application range. It is suitable for single-screw and twin-screw extruders. The general dosage for paraffin wax is 0.1-0.8 PHR.
2. Polyethylene Wax
Polyethylene wax is the low molecular weight (ranging from 1000 to about 20000) polymeric chains of ethylene. These low molecular weight polymer waxes need to go under a separation process in which it is separated from water, residual catalysts and other contaminants, are then known as Synthetic Polyethylene Wax and Refined Polyethylene Wax. These are further arranged according to their molecular weight before packaging. A change in i) monomer purity; ii) degree and length of polymer branching; iii) monomer to polymer composition; will alter the physical characteristics of the polyethylene wax, such as viscosity, hardness, melting range, reactivity, etc.
The most used low density, low molecular weight polymeric chains of ethylene wax have dropping points between 85-115°C. They have characteristics of good colour & stability better than paraffin wax. They have excellent electrical properties and can also provide clean release from the conductor during stripping for termination. The resulting surface of the PVC-PE monolayer is more readily printed than the PVC surface with paraffin wax.
