Processing external lubricant

As previously stated, internal lubricants are soluble in the resin system and act by reducing friction between polymer molecules, leading to lower melt viscosity and lower energy requirements needed for processing. External lubricants are generally incompatible with the polymer and act to reduce the friction at the interface between the polymer and the surface of the processing equipment. 

By far the most common lead salt used for PVC stabilization is tribasic lead sulfate. It can be found either alone or combined with another lead salt in almost every lead-stabilized PVC formulation. Many of the combinations are actually coprecipitated hybrid products, ie, basic lead sulfophthalates. Dibasic lead stearate and lead stearate are generally used as costabilizers combined with other primary lead salts, particularly in rigid PVC formulations where they contribute lubrication properties dibasic lead stearate provides internal lubrication and lead stearate is a good external lubricant. Basic lead carbonate is slowly being replaced by tribasic lead sulfate in most appHcations due the relatively low heat stabiHty of the carbonate salt which releases CO2 at about 180°C during PVC processing. 

Poly(ethylene terephthalate). PET is a crystalline material and hence difficult to plasticize. Additionally, since PET is used as a high strength film and textile fiber, plasticization is not usually required although esters showing plasticizing properties with PVC may be used in small amounts as processing aids and external lubricants. Plasticizers have also been used to aid the injection mol ding of PET, but only at low concentrations. 

Substituted Amide Waxes. The product of fatty acid amidation has unique waxlike properties (13). Probably the most widely produced material is N,1S7-distearylethylenediarnine [110-30-5] which has a melting point of ca 140°C, an acid number of ca 7, and a low melt viscosity. Because of its unusuaHy high melting point and unique functionaHty, it is used in additive quantities to raise the apparent melting point of themoplastic resins and asphalts, as an internal—external lubricant in the compounding of a variety of thermoplastic resins, and as a processing aid for elastomers. 

Laboratory tests and experience during use have demonstrated that the nylons have extremely good abrasion resistance. This may be further improved by addition of external lubricants and by processing under conditions which develop a highly crystalline hard surface e.g. by use of hot injection moulds and by annealing in a non-oxidising fluid at an elevated temperature (150-200°C for nylon 66). 

Alkanesulfonates act as an external lubricant in PVC, polystyrene, and engineering thermoplastics. They have a good release effect and assist flow. Addition is in the concentration range between 0.1 and 2.0 parts per 100 parts resin (phr). Because of their low volatility, alkanesulfonates are also used as a processing aid for high-melting engineering thermoplastics. 

Lubricants can be classified as internal or external lubricants. Internal lubricants should be partially miscible with the polymer at processing temperatures (i.e., behave similar to a plasticiser), but phase separate at ordinary temperatures. Whereas plasticisers are completely miscible with the bulk polymer, lubricants have a limited solubility. 

A study was made of the effects of processing conditions and of blowing and nucleating agents and external lubricants on the apparent density, mechanical properties, linear thermal expansion coefficient and surface roughness of extruded cellular MDPE. Based on the results, recommendations were formulated for the properties of cellular MDPE pipes for the protection of light conducting cables. 16 refs. (Translation of Polimery, Tworzywa Wielkoczasteczkowe, No. 10, 1996, p.580). 

Some of these processing aids are incorporated into the bulk of the material rather than being directly applied to the surface and are known as internal release agents. In the process of migration to the surface they have an inevitable internal lubrication effect. Most internal lubricants also have an external lubricant effect, particularly at higher concentrations because usually some material finds its way to the exterior. 

Otsuka, M., Sato, M., and Matsuda, Y. (2001), Comparative evaluation of tableting compression behaviors by methods of internal and external lubricant addition Inhibition of enzymatic activity of trypsin preparation by using external lubricant addition during the tableting compression process, AAPS PharmSci, 3(3), article 20. 

The characteristics of lubricants, their effects during plastics processing and their influence in the calendering process are discussed in depth. Attention is paid to the different intemal/external behaviour of lubricants, viscosity reduction by internal lubricants, fusion delay by external lubricants, shear liquefaction by lubricants and the suitability of various lubricants for the manufacture of calendered PVC films in relation to melt elasticity, release effect, flow and plate-out. 

I) PE blends with a small quantity of "external lubricant" fluoro-polymers, siloxanes, PE-waxes, etc. These blends are primarily formulated for Inqirovement of processability without affecting the PE performance (2 ). 

Syncrolube. [Croda Universal Ltd. Croda Surf. 1 ] Intonalaitd external lubricants and processing aids. 

Stickiness is the ability of a rubber compound to stick to a nonrubber surface such as a metal or textile fabric surface. Excessive stickiness to metal can result in poor release from mills and problems in process equipment. On the other hand, the destruction of compound stickiness can cause slippage of the compound against metal surfaces in extruders or the rotors of an internal mixer. Various compounding additives, such as mill release agent.s or external lubricants, are used to control the level of stickiness. Note, compound stickiness and tackiness are not the same property. 

In plasticized PVC it is common practice to incorporate a lubricant whose main function is to prevent sticking of the compound to processing equipment [14]. The material used should have limited compatibility such that it will sweat out during processing to form a film between the bulk f the compound and the metal surfaces of the processing equipment. The additives used for such a purpose are known as external lubricants. 

An unplasticized PVC formulation usually contains at least one other lubricant, which is mainly intended to improve the flow of the melt, i.e., to reduce the apparent melt viscosity. Such materials are known as internal lubricants. Unlike external lubricants they are reasonably compatible with the polymer and are more like plasticizers in their behavior at processing temperatures, whereas at room temperature this effect is negligible. Among materials usually classified as internal lubricants are montan wax derivatives, glyceryl monostearate, and long-chain esters such as cetyl palmitate. 

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