Polyvinyl chloride (PVC for short) is the most widely used general-purpose plastic in the world. Different additives can be added according to different uses. According to the amount of plasticizer, PVC products can be divided into PVC hard products and PVC soft products. Different additives cause PVC plastics to exhibit different physical and mechanical properties. PVC products of the same use have large differences in product quality due to material formula, quality of auxiliary materials, production equipment, production technology and other reasons. This article will express my personal thoughts on the application of lubricants in PVC rigid products.
We divide the lubricants of PVC hard products into internal and external lubrication, but with the continuous changes in the quality requirements of products and changes in market demand, the original lubrication theory can no longer meet the technical needs of specific production.
The classic internal and external lubrication mechanism is: using internal lubrication as an effective means to reduce the viscosity of PVC, thereby reducing the generation of frictional heat and improving the fluidity of materials. In particular, it also ensures the melt strength of some products and increases the back pressure; and the external lubricant, as a “lubricating layer”, exists between the material and the equipment, ensuring the brightness of the product and adjusting the plasticizing time.
However, with the development of the domestic industry, the current manufacturing of PVC rigid products requires more exploration and experience.
1. Product formula of high calcium system
From 25phr pressure water pipes, to 50phr pipe fittings, to 100phr micro-foaming decorative panels, to 250phr ceiling gussets, etc., compared with similar foreign products, domestic products are all high-calcium PVC products.
Stearic acid is theoretically a processing aid for internal lubrication, but in the application in the domestic market, we are interested in its contribution to the release of the product, but because of its low price, it has led to excessive use by manufacturers. Disadvantages are highlighted. So we almost no longer pay attention to the internal and external lubricity of stearic acid, but more about how bad stearic acid is, how their volatility in the mold area affects precipitation, and so on.
I have seen some downstream customers using coupling agents, and the coupling agents themselves are frequently used in PE filled masterbatches. Certain coupling agents are also attractively priced. They activate calcium powder to satisfy its oil absorption and are also better than stearic acid. At the same time, in high-calcium products, the good mechanical properties are given to the products, and the effect is far greater than that of resin-based CPE and ACR. Therefore, in the high calcium system, the definition of internal and external lubricity is relatively weak, and the model of the classical theory is relatively weak. Because the proportion of PVC resin in the formula is small, the model of internal and external lubricants has little significance. More need to discuss lubricants and calcium carbonate problems in between. As long as the product can be stably produced, and the processability can be improved accordingly, even if the product quality can be slightly improved, it is good.
2. Low quality of additives
In calcium and zinc stabilizers, fatty acid salt lubricants dominate, or in zinc-based non-toxic stabilizers, zinc stearate is more accurate.
Zinc stearate accounts for nearly 30% of the stabilizer formulation, and it is meaningless to discuss its internal and external lubricity at this time. Because when the lubricant is added in a large amount, even if it is originally an internal lubricant, the added amount far exceeds the absorption of PVC primary particles, so it can be seen on the torque rheometer that zinc stearate is in the stabilizer. It has obvious external lubricity, and it is due to excessive use, which leads to its obvious external lubricity. Similarly, soaps also showed similar volatilization of fatty acids in the mold area. Then metal compounds without fatty acids are welcome, and even metal compounds without lubricity, such as polyol zinc glycerol zinc, and complexes of metals and organic heterocycles, etc. Subtraction from the lubricating system, separation of metal elements and lubricity, and excessive use of cheap materials are also problems faced by hard zinc-based stabilizers for a long time.
B Coal chemical wax
Raw materials with low molecular weight, low viscosity and low melting point, in the production process, the conversion rate and denaturation rate are destined to be large.
Due to the pursuit of price, this material is used in a large amount in stabilizers, with excessive zinc stearate to replace the external lubricity similar to lead stearate.
From the traditional definition of wax, “in order to meet the gloss of the product and adjust the degree of plasticization”, it has become “delayed degree of plasticization”, “even if there are problems such as precipitation in the downstream, regardless of the pursuit of cost and price reduction” .
For any system, the richer the better, the better the mix. Esters can be used to replace part of the fatty acids, or alkane waxes can be used for functional matching before and after. More excellent polar wax as lubricants can also be used to ensure the plasticization degree of the product and the excellent wettability of the equipment surface to obtain better surface gloss. Abundant, efficient, far more viable than cheap, overused.
3. Equipment structure and market orientation
In the production of national standard PVC products, more conical double machines are used in China, rather than flat double machines. Therefore, the selection of raw materials brought about should also be biased towards performance. In addition, in recent years, major domestic enterprises have also improved the process of conical double high-speed extrusion for national standard PVC products, so they are more sensitive to the performance requirements of lubricants.
The direct problem is that, under the double-cone high-speed extrusion process, the good appearance flatness of the product is still guaranteed to maintain qualified physical indicators. High temperature resistance of lubricants, viscosity and back pressure parameters of materials, control of plasticization degree of materials, production cycle of continuous start-up, etc. All need to be tested and concluded.
In high-grade PVC rigid products, the degree of plasticization is a very important indicator. In terms of equipment, the requirements for screws and molds are more stringent, not only for the lubricating system, but also for the scope of processing aids. Also not good for the old-fashioned way: increase the feed, decrease the speed, and thus increase the degree of plasticization. Ignoring shearing and abandoning the device structure is putting the cart before the horse. The difference between the plasticization peak and the bottom valley is a functional index to ensure processability. It should not be adjusted by formula, but by equipment shearing.
In terms of the market, places such as hotels and KTVs are frequently renovated. Fast-packing products are often produced with a large amount of crushed and recycled materials. Here, high-priced esters and high-molecular-weight alkane waxes can also effectively protect continuous start-up, and are a good choice for the production of recycled materials. More and more high-end raw materials are used in low-end PVC products, which can bring a wider processing window, ensure the yield and reduce the overall cost of customers.
Summary: We believe that metal soaps should be weakened, and popular lubricants should be partially replaced and enriched in use. Products return performance and enhance lifespan, and non-standard products should not be given up on quality.
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