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Jiuzhi Plastics News] In the automotive sealing strip industry, most of the sealing strip products are mainly soft PVC (hardness range is HA50-HA85). Since the glass transition temperature of PVC resin is about 75 ° C, it is particularly sensitive to temperature changes, and it is extremely brittle when the temperature is low. Therefore, it is an urgent task for automotive
PVC seal strip manufacturers to improve the cold resistance of PVC seals. So, how to improve the cold resistance of PVC sealing strips? The formula can be adjusted from the following aspects.
1, PVC resin
Resin grade or viscosity number
analysis:
PVC resin is a non-crystalline, polar polymer with a glass transition temperature of 75-105 C. The greater the degree of polymerization, the higher the relative molecular mass and the higher the viscosity. High polymerization degree PVC has larger crystallinity and cross-linking structure than conventional PVC resin, which makes the sliding between macromolecules difficult, the elasticity increases, the molecular weight increases, the intermolecular van der Waals force and the intramolecular chemical bonding force increase to obtain excellent cold resistance. .
Suggest:
In the conventional PVC formula, if it is only necessary to cope with the cold weather in the northern winter, it is possible to select a resin having a slightly higher viscosity and a higher degree of polymerization. In some special requirements, such as can be resistant to -25 C, a high degree of polymerization of polyvinyl chloride resin (average degree of polymerization greater than 2000) can be used.
2, plasticizer
Type and amount of plasticizer
species
analysis:
As an important formula component of PVC soft products, plasticizers have great influence on the performance of soft products. If the products are required to be used at low temperatures, the type of plasticizer must be selected.
Suggest:
At present, mainly used as a cold-resistant plasticizer are a fatty acid dibasic acid ester, a phthalic acid ester of a linear alcohol, a fatty acid ester of a glycol, and the like. It has been reported that naphthenic dicarboxylates, chloromethoxy fatty acid esters, etc., are also cold-resistant plasticizers having excellent low-temperature properties.
Dosage
analysis:
Improving the cold resistance of PVC soft products can generally be obtained by increasing the amount of cold resistant plasticizer. DOA (dioctyl adipate), DIDA (diisodecyl adipate), DOZ (dioctyl sebacate), DOS (dioctyl sebacate) are representative of use as a cold-resistant plasticizer Variety.
Suggest:
Since the compatibility of general cold-resistant plasticizers with PVC is not very good, it can only be used as an auxiliary plasticizer for improving cold resistance, and its dosage is usually 5-20% of the main plasticizer.
3, modifier
An effective way to improve the poor low temperature performance of PVC is to add a polymer having a low glass transition temperature and exhibiting high elasticity at room temperature, collectively referred to as a modifier. The high polymer added should have similar solubility parameters to PVC, and have a certain mutual solubility ability, and can form a blend of two structures, thereby improving the low temperature impact strength of the product. Commonly used modifiers are CPE (chlorinated polyethylene), NBR (powder nitrile rubber), EVA, ACR (polyvinyl chloride), ABS, SBS, and the like.
analysis:
CPE can improve the low temperature performance, impact strength and the like of the product. As the amount of CPE increases, the impact properties of PV C products will gradually increase. When the dosage is increased to a certain extent, the low-temperature impact performance of the PVC product tends to be stable, and the appropriate performance-price ratio is achieved at about 8 or 9 parts.
The increase in the amount of NBR increases the low-temperature impact strength of the hard PVC.
EVA has good flow performance, low glass transition temperature, good low temperature toughening effect, but high cost.
ACR has excellent low-temperature impact strength and weather resistance, and can improve the appearance of the product. Generally, 5 parts can be added to achieve good results.
MBS has a low glass transition temperature and has a good improvement effect on the low temperature brittleness of PVC materials, but has poor weather resistance.
ABS can improve the low temperature impact strength of PVC materials while improving the appearance of the products.
Some substances, such as SBS, which contain a rubber phase and have a low glass transition temperature, also have the effect of improving the impact strength and cold resistance of PVC.
4, filler
analysis:
The effect of filler on the cold resistance of soft PVC is related to the absorption of plasticizer. The general trend is that the filler with low plasticizer absorption has little effect on cold resistance, so the plasticizer such as carbon black and hard clay absorbs a large amount. The filler will make the cold resistance of PVC have a significant reduction.
The addition of fillers such as nano-calcium carbonate and ultra-fine calcium carbonate will have a similar modifier effect due to the small size effect, and can improve the low-temperature performance of the PVC material within a certain range, but since there is no low glass transition temperature, The effect is not obvious, and the low temperature brittleness of the material increases when added to a certain amount.
In short, through the following series of formula adjustment methods, the cold resistance of PVC materials can be improved to meet the requirements for low temperature use:
1. Use a resin of suitable grade and degree of polymerization
2. Select/replace the plasticizer with better cold resistance and the appropriate dosage
3. Introduce some polymer modifiers with cold resistance
4. Add filler with low plasticizer absorption
At the same time, processing temperature, cooling temperature, traction rate, structural design, etc. will also have a certain impact on the cold resistance of PVC products. Therefore, when designing PVC formula, it is necessary to comprehensively consider the application conditions, product structure, processing equipment, process conditions and other factors together with the formula, and adjust accordingly through the test to finally obtain PVC formula with excellent cold resistance. .
