Lubricants are essential additives in PVC processing. Adding an appropriate amount of lubricant to PVC can reduce the mutual friction between the particles before the PVC melts and the macromolecules in the PVC melt; reduce the mutual friction between the PVC melt and the mechanical contact surface of the plastic. Appropriate amount of lubricant can improve the fluidity of PVC melt, improve production efficiency, prevent PVC degradation caused by frictional heat generation, and improve the appearance quality of products.
According to function, it can be divided into external lubricant and internal lubricant. The external lubricant has poor compatibility with PVC, and it is easy to migrate from the inside of the resin melt to the surface to form a lubricant interface layer, thereby reducing the mutual friction between the particles before the PVC melts and between the PVC melt and the mechanical contact surface of the plastic. The internal lubricant contains polar groups and has good compatibility with PVC, which can reduce the intermolecular force of PVC, reduce melt viscosity and improve melt fluidity.
The distinction between internal and external lubricants is only relative, and there is no strict division standard. In resins with different polarities, the role of internal and external lubricants may change. For example, stearic acid alcohol, stearic acid amine, butyl stearate and stearic acid monoglyceride play an internal lubricating role for polar resins (such as PVC and PA), but for non-polar resins (such as PE , PP), it shows external lubrication. On the contrary, polymer paraffin has poor compatibility with polar resins. For example, it is used as an external lubricant in polar PVC, while it is an internal lubricant in non-polar resins such as PE and PP. At different processing temperatures, the role of internal and external lubricants will change. For example, stearic acid and stearyl alcohol are used in the early stage of PVC calendering. Due to the low processing temperature and poor compatibility with PVC, they mainly play the role of external lubrication. After the temperature rises, the compatibility with PVC increases, and it turns into an internal lubricant.
There are many varieties of lubricants. Usually two or more lubricants are used to form a PVC lubricating system to reduce its ability to migrate to the surface of the product; improve the dispersion of lubricants and balance the effects of internal and external lubricants.
Commonly used lubricant varieties
According to the composition of lubricants, it can be divided into: saturated hydrocarbons, metal soaps, aliphatic amides, fatty acids, fatty acid esters and fatty alcohols.
Saturated hydrocarbons can be divided into non-polar hydrocarbons (such as polyethylene wax and polypropylene wax) and polar hydrocarbons (such as chlorinated paraffin, oxidized polyethylene, etc.) according to polarity. According to the molecular weight, it can be divided into: liquid paraffin (C16~C21), solid paraffin (C26~C32), microcrystalline paraffin (C32~C70) and low molecular weight polyethylene (molecular weight 1000~10000), etc., mainly used for PVC non-toxic External lubricant.
Metal soap is not only an excellent heat stabilizer, but also a kind of lubricant. It has both internal and external lubricating functions. Different varieties have slightly different emphases. Commonly used metal soaps are added in the following amounts when used in lubricants:
Fatty acid amides include mono-fatty acid amides and bis-fatty acid amides. Mono-fatty amides are mainly used for internal lubrication, and the main varieties are stearic acid amide, erucic acid amide, ricinoleic acid amide, etc.; bis-fatty acid amides are mainly used for external lubrication. , the main varieties are N, N’-ethylene bis-stearamide, N, N’-ethylene ricinoleic acid amide and so on.
Fatty acids mainly include (1) stearic acid, which is the most widely used lubricant after metal soaps. When the dosage is small in PVC, it plays an internal lubrication role; when the dosage is large, it plays an external lubrication role. Generally, the added amount of stearic acid is less than 0.5 part. (2) Hydroxystearic acid has good compatibility with PVC and shows internal lubrication, but has poor thermal stability.
Fatty acid esters mainly include (1) butyl stearate, which is colorless or light yellow oily liquid in appearance. It is mainly used for internal lubrication and external lubrication in PVC. The dosage is 0.5~1.5 parts. (2) Glyceryl monostearate, codenamed GMS, is a white waxy solid in appearance. It is an excellent internal lubricant for PVC with little effect on transparency. The addition amount is less than 1.5 parts. It can be used together with stearic acid. (3) Ester wax and saponified wax mainly refer to the subsequent products made from montan wax as the main raw material through bleaching and other processes. Bleached wax has S wax and L wax, esterified wax has E wax and EG wax, and saponified wax has O wax and OP wax. Mainly used for hard PVC, the dosage is 0.1~0.3 parts.
Fatty alcohols mainly include (1) stearyl alcohol, which is white bead-like in appearance, plays an internal lubricating role, and has good transparency. The dosage in PVC is 0.2~0.5 parts. (2) Pentaerythritol, PVC high temperature lubricant, the dosage is 0.2~0.5 parts.
With the expanding application of PVC products, strict requirements are put forward for the hygiene, transparency and processability of plastic additives, thus promoting the development and application of new lubricants. Among them, ester lubricants have developed rapidly, and the proportion of dosage has been increasing. Among the Loxiol G series lubricants of Henkel Company in Germany, there are both stearyl stearate (G30), oleic acid monoglyceride (G10) and castor oil. acid monoglyceride (G11), and stearic acid monoglyceride (GMS). Higher alcohol esters of dicarboxylic acids, such as bis(octadecyl) adipate, have been used as excellent internal lubricants. The oligomeric compound ester of fatty acid and polyol has a large molecular weight, is resistant to extraction, and does not bloom. It has both internal and external lubricity, which is the development direction and development focus. Its representative products include Henkel’s G70, G70S, G71, G72, G74, etc. Since the mid-1980s, my country’s plastic additives research institutes have successfully developed a variety of compound ester lubricants. The main varieties that have been produced and applied are LC-1, LC-2, LC-4, LC-6, LC-7, RH-60, WH-70, RW-100, ZB-16, ZB-74, H-16, H-74, etc. On this basis, multi-functional synergistic compound additives for PVC (such as compound heat stabilizers/lubricants, etc.) have been further developed.
It is worth noting that lubricants have been added to many composite heat stabilizers. In the production of PVC products, the compound heat stabilizer should be used reasonably according to the product specification and reference formula. If there are no special process requirements, no additional lubricant is generally added to avoid adverse situations such as “over-lubrication”.
In rigid PVC plastics, excess lubricant leads to a reduction in strength and also affects process operation. For injection products, peeling will occur, especially near the gate. For injection products, the total dosage of stearic acid and paraffin is generally 0.5 to 1 part; for extruded products, generally no more than 1 part.
In the formulation of soft products, too much lubricant will bloom and affect the strength of the product and high-frequency welding and printing. Too little lubricant will stick to the roller. For blown film, too little lubricant will stick to the die, which is easy to coke the plastic in the mold. At the same time, in order to improve the stickiness of the blown film, it is advisable to add a small amount of internal lubricant monoglyceride. When producing PVC soft products, the amount of lubricant added is generally less than 1 part.
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