PVC plastic is one of the five general synthetic plastics and is the second largest plastics in the world after polyethylene. PVC plastic has good physical and mechanical properties and can be used in the production of building materials, packaging materials, electronic materials, consumer goods, etc., widely used in industrial, agricultural, construction, transportation, power telecommunications and packaging. Due to its poor cold resistance and low temperature impact resistance, the lower temperature limit of hard PVC is generally -15oC, which limits the application of PVC materials in some aspects. Through the adjustment of PVC resin and additives, it can effectively improve the cold resistance of PVC plastics and meet the needs of low temperature. This article focuses on some methods to improve the cold resistance of PVC from the perspective of formulation, for your reference.

1.PVC resin

PVC resin is a kind of non-crystalline, polar polymer. Its glass transition temperature is 75~105oC. The higher the relative molecular mass, the higher the viscosity. The PVC macromolecular chain van der Waals gravitation or entanglement correspondingly Increase, PVC segment growth, the better the low temperature resistance of the material.

In the conventional PVC formula, if it is only necessary to cope with the cold winter weather in the north, it is possible to select a PVC resin with a slightly higher viscosity, that is, a slightly larger average molecular weight, which may be a PVC with a higher viscosity value in the same grade or a lower grade resin.

In addition, in some special requirements of products, such as -30oC blood bags and other products, the choice of high polymerization degree PVC resin (average degree of polymerization greater than 2000) [1], because the high degree of polymerization PVC has Compared with the conventional PVC resin, the crystallinity and the cross-linking structure make the sliding between the 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.

2. Plasticizer

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. 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 an epoxy fatty acid monoester. It has been reported that N,N-disubstituted fatty acid amides, cycloalkane dicarboxylates, and chloromethoxy fatty acid esters are also cold-resistant plasticizers having excellent low-temperature properties.

Improving the cold resistance of PVC soft products can generally be obtained by increasing the amount of cold resistant plasticizer. DOA (dioctyl adipate), DIDA (diisononyl adipate), DOZ (dioctyl sebacate), DOS (dioctyl sebacate) are representative of use as a cold-resistant plasticizer Variety, because the compatibility of general cold-resistant plasticizer and PVC is not very good, in fact, it can only be used as an auxiliary plasticizer to improve cold resistance, and its dosage is usually 5-20% of the main plasticizer. [2]

In addition, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIB), butyl stearate, LHAT acid diethylene glycol 1-2 ethyl 2-ethyl ester, etc. The product also has the function of a cold resistant plasticizer.

Domestic researchers have pointed out that in order to improve the cold resistance and low temperature elongation of the film, it is preferred to use a cold resistant plasticizer together with hexamethylphosphoric triamide. Because hexamethylphosphoric triamide itself is not a cold-resistant plasticizer, it can effectively reduce the freezing point of various plasticizers and achieve the purpose of strengthening the cold resistance of the film.

3. Modifier

An effective way to improve the low-temperature impact strength 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.

The research results show that CPE can improve the low temperature performance, impact strength and so on. 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 PV C 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 powdered nitrile rubber (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. The high impact type MBS has a low glass transition temperature and has a good improvement effect on the low temperature brittleness of the PVC material, but has poor weather resistance. ABS can improve the low temperature impact strength of PVC materials while improving the appearance of the products. In addition, some substances such as SBS which contain a rubber phase and have a low glass transition temperature also have an effect of improving the impact strength and cold resistance of PVC.

The results of domestic research on the cold resistance of modified soft PVC show that the cold resistance of blended soft PVC is obviously affected by the variety and dosage of polymer modifier. Through various methods, it was found that polymer modifiers such as Elvaloy 711 (ethylene-vinyl acetate carbon monoxide copolymer), NBR-26 (blocky butadiene rubber), and Chemigum P83 (pre-crosslinked powdered butadiene rubber) can be significantly improved. The cold resistance of soft PVC, while some polymer modifiers such as CPE and EVA with low VA content will impair the cold resistance of soft PVC. The butyl rubber modifier can improve the low temperature performance of soft PVC immersed in soft PVC by improving the resistance of the flexible PVC to gasoline (gasoline).

Cold-resistant agents introduced on the market, such as K-175C and N-550C, are actually a styrenic modifier developed to improve the low-temperature flexibility and impact strength of PVC. Due to its low glass transition temperature and good compatibility with PVC, it has certain plasticizing and toughening effects. Therefore, after adding in PVC, it can improve and improve the low temperature performance of the modified PVC.

4. Thermoplastic elastomer

Thermoplastic elastomers (TPE) are a class of synthetic materials that exhibit rubber elasticity at room temperature and can be plasticized at high temperatures. Therefore, these polymers combine the characteristics of rubber and thermoplastics, and they can be used as composite materials. Tougheners can also be used as matrix materials for composite materials. These materials mainly include polyurethanes, styrenes, polyolefins, polyesters, syndiotactic 1,2-polybutadienes and polyamides. . At present, more used as a composite toughening agent are styrenes and polyolefins.

The cold resistance of PVC-TPE products is at least not less than that of soft PVC. When using cold-resistant plasticizers and cold-resistant formulas, PVC-TPE still maintains good elasticity at -45. PVC-TPE is also popular in cold and seawater resistant products such as marine seals, container seals, marine hoses, and the like. For example, TPE of H4040, H3303 and other grades have good compatibility with PVC. After addition, the low temperature flexibility of PVC can be obviously improved, the resistance to winding is significantly improved, and the embrittlement point is reduced.

Japanese plastics technology company has also recently developed a thermoplastic polyurethane-polyvinyl chloride elastomer. This material is blended with TPU and PVC and the third component. It fully utilizes the excellent properties of both TPU and PVC. It has the following advantages: (1) TPU is used as a plasticizer for PVC, eliminating the past The problem of migration and volatilization of plasticizers in soft polyvinyl chloride. (2) The embrittlement temperature of PVC material is also reduced from -30 °C to -68 °C, reaching a special cold resistance level.

5. Filler

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, while the plasticizer such as carbon black and hard clay absorbs a lot. The filler will make the cold resistance of PVC significantly lower.

The addition of fillers to rigid PVC tends to affect the impact properties, especially the low temperature brittleness increases with the amount of filler. This is because when the filler is added to the PVC as an inorganic particle, it is filled between the molecular chains. When the amount is small, it is filled into the gaps of some molecular chains to enhance the effect; or it is filled between the molecular chains to increase the distance between molecules and increase the toughness of the system. However, as the amount of the molecule increases, the inter-molecular force is destroyed as the distance between molecules increases. When the temperature is low, the mobility of the molecular segment decreases, and the ability of the material to resist external impact is drastically reduced. Therefore, it has an adverse effect on the low temperature impact properties of hard PVC.

After the filler is treated, the tensile properties of the material are improved, but the improvement of the low-temperature impact resistance is not obvious. The reason is related to the fact that the filler particles occupy the active space of the PVC molecular chain. Although the binding force of the active filler to the PVC molecular chain is increased, the increase is only increased when the molecule is subjected to tensile force, and the brittleness of the material is only increased by the addition of the filler particles.

Nano-calcium carbonate and ultra-fine calcium carbonate are added to PVC. Due to the small size effect, it has a similar modifier effect, and can improve the low-temperature performance of PVC materials 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.

6. Other additives

In the general formulation, it is possible to increase the amount of the internal lubricant which promotes the plasticization, and to reduce the amount of the processing aid, but this method is not recommended in the cold resistant formula. Because the role of processing aids is not only to improve the processability of PVC, but also its role in the low temperature performance of the product has a greater improvement, this role lubricant can not be replaced.

The most commonly used soft PVC flame retardant plasticizer is tricresyl phosphate, but the low temperature performance of tricresyl phosphate is very poor, so it is more suitable to use alkyl phosphate in the case where cold resistance is required.

Commonly used stabilizers have a negative effect on the cold resistance of PVC products. The stabilizers of different varieties have different effects on the cold resistance of products due to their different shapes and physical properties. Because of the necessity and limited amount of use, this problem is rarely considered in general formulation design.

In short, by selecting/changing the additives with better cold resistance and introducing a series of formula adjustment methods such as polymers with cold resistance, the cold resistance of PVC materials can be improved and the requirements for low temperature use can be achieved. At the same time, we should also pay attention to the processing temperature, cooling temperature, traction rate, structural design and many other aspects, and 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, and finally obtain a PVC formula with excellent cold resistance.

Wuxi Jiahong Plastic Technology Co., Ltd. has nearly 30 years of experience in research and development and granulation of modified PVC pellets. The professional technical service team can provide one-stop system solutions for customers. For more information about the product, please visit our official website: m.bdlingxin.cn, consult online customer service or call the hotline. Fixed line: 0510-68755207 Mobile: 15190220696, we will be happy to help you.

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