How to improve the cold resistance of PVC formulations
Aug 19, 2021
Ways to improve the cold resistance of PVC formulations
PVC plastic has good physical and mechanical properties and can be used in the production of building materials, packaging materials, electronic materials, daily consumer goods, etc., and is widely used in various fields such as industry, agriculture, construction, transportation, power telecommunications and packaging.
Due to its poor cold resistance and low temperature impact resistance, the lower limit of the use temperature of hard PVC is generally -15ºC, which limits the application of PVC materials in certain aspects. Through the adjustment of PVC resin and additives, the cold resistance of PVC plastic can be effectively improved to meet the needs of low temperature.
Xiongxing Group relies on its own more than 26 years of PVC production experience and a professional R&D team to write this article, focusing on some methods to improve the cold resistance of PVC from a formula point of view, for everyone to learn and refer to.
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PVC resin is a kind of non-crystalline, polar polymer. Its glass transition temperature is 75~105℃ according to the molecular weight. The larger the relative molecular mass, the higher the viscosity, and the degree of van der Waals attraction or entanglement between PVC macromolecular chains. Correspondingly, the PVC chain segment increases, and the material's low temperature resistance is better.
In conventional PVC formulations, if you only need to cope with the cold northern winter climate, you can choose a PVC resin with a slightly higher viscosity, that is, a slightly larger average molecular weight. It can be a PVC resin with a higher viscosity in the same grade or a lower grade resin.
In addition, in some products with special requirements, such as blood bags that can withstand -30ºC, high polymerization degree polyvinyl chloride resin (average degree of polymerization greater than 2000) can be used. This is because high degree of polymerization PVC is better than conventional PVC. The large crystallinity and cross-linked structure of the resin make it difficult to slide between macromolecules, increase the elasticity, and increase the molecular weight, and increase the intermolecular van der Waals force and intramolecular chemical bonding force to obtain excellent cold resistance.
As an important formulation component of PVC soft products, plasticizers have a great impact on the performance of soft products. If products are required to be used at low temperatures, the type of plasticizer must be selected. Currently used as cold-resistant plasticizers mainly include fatty acid dibasic acid esters, linear alcohol phthalate esters, dihydric alcohol fatty acid esters, and epoxy fatty acid monoesters. According to reports, N,N-disubstituted fatty acid amides, naphthenic dicarboxylic acid esters, and chloromethoxy fatty acid esters are also cold-resistant plasticizers with excellent low-temperature performance.
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 azelate), DOS (dioctyl sebacate) are representative of cold-resistant plasticizers Variety, because the compatibility of general cold-resistant plasticizers with 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.
In addition, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIB), butyl stearate, LHAT acid diethylene glycol di-2-ethyl ester, etc. The product also functions as a cold-resistant plasticizer.
Researchers from the scientific research team of Xiongxing Group pointed out that it is better to use cold-resistant plasticizer and hexamethylphosphoric triamide together for the purpose of improving the cold-resistant toughness and low-temperature elongation of the film. Although hexamethylphosphoric triamide itself is not a cold-resistant plasticizer, it can effectively reduce the freezing point of various plasticizers and achieve the purpose of enhancing the cold-resistant effect of the film.
An effective way to improve the poor low-temperature impact resistance of PVC is to add polymers with a low glass transition temperature and high elasticity at room temperature, collectively referred to as modifiers. The added polymer should have similar solubility parameters with PVC, have a certain mutual solubility, and can form a blend of two structures, thereby improving the low-temperature impact strength of the product.
CPE can improve the low temperature performance and impact strength of products. As the amount of CPE increases, the impact performance of PV C products will gradually improve. When the dosage is increased to a certain extent, the low-temperature impact performance of PV C products will tend to be stable, reaching a suitable cost-performance ratio at about 8 or 9 parts. As the amount of powdered nitrile rubber (NBR) increases, the low-temperature impact strength of hard PVC will gradually increase.
EVA has good flow properties, 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, a good effect can be achieved by adding 5 parts. The high-impact MBS has a low glass transition temperature, which has a good effect on improving the low-temperature brittleness of PVC materials, but has poor weather resistance.
ABS can increase the low-temperature impact strength of PVC materials, and at the same time improve the appearance of products.
In addition, SBS and other substances that contain a rubber phase and have a lower glass transition temperature can also improve the impact strength and cold resistance of PVC.
Not only that, researchers from Xiongxing Group studied the cold resistance of modified soft PVC and showed that the cold resistance of blended modified soft PVC is obviously affected by the variety and dosage of polymer modifiers. Through various methods of testing, it is found that Elvaloy 711 (ethylene-vinyl acetate carbon monoxide copolymer), NBR-26 (bulk nitrile rubber), Chemigum P83 (pre-crosslinked powder nitrile rubber) and other polymer modifiers can significantly improve The cold resistance of soft PVC, and certain polymer modifiers such as CPE and EVA with low VA content will damage the cold resistance of soft PVC. The nitrile rubber modifier can improve the medium (gasoline) resistance of soft PVC, thereby improving the low temperature performance of soft PVC soaked in gasoline.
The cold-resistant agents on the market, such as K-175C, N-550C and other products, are actually a styrene modifier developed to improve the low-temperature flexibility and impact strength of PVC. Because of its low glass transition temperature and good compatibility with PVC, it has certain plasticizing and toughening effects. Therefore, after adding to PVC, it can improve and improve the low temperature performance of PVC.
Thermoplastic elastomer (TPE) is a kind of synthetic material that shows rubber elasticity at room temperature and can be plasticized and formed at high temperature. Therefore, this type of polymer has the characteristics of rubber and thermoplastics, and it can be used as a composite material. Toughening agent can also be used as the matrix material of composite materials. Such materials mainly include polyurethane, styrene, polyolefin, polyester, syndiotactic 1,2-polybutadiene and polyamide products. . At present, styrenes and polyolefins are more commonly used as toughening agents for composite materials.
The cold resistance of PVC-TPE products is at least not lower than that of soft PVC. When cold-resistant plasticizers and cold-resistant formulas are used, PVC-TPE still maintains good elasticity at -45. In terms of cold-resistant and seawater-resistant products, such as ship seals, container seals, and marine hoses, PVC-TPE is also popular. TPEs such as H4040, H3303 and other brands have good compatibility with PVC. After adding, it can significantly improve the low-temperature flexibility of PVC, significantly increase its resistance to bending, and reduce the embrittlement point.
A Japanese plastic technology company has also developed a thermoplastic polyurethane-polyvinyl chloride elastomer. This material is made by mixing TPU and PVC and the third component after mixing. It gives full play to the excellent characteristics of TPU and PVC and has the following advantages:
(1) TPU is used as a plasticizer for PVC, eliminating the plasticizer migration and volatilization problems that existed in soft PVC in the past.
(2) The embrittlement temperature of PVC material has also been reduced from -30°C to -68°C, reaching a special cold-resistant grade.
The influence of fillers on the cold resistance of soft PVC is related to the absorption of plasticizers. The general trend is that the absorption of plasticizers is small. Fillers have little effect on cold resistance, while the absorption of plasticizers such as carbon black and hard clay is large The filling agent of PVC will significantly reduce the cold resistance of PVC.
The addition of fillers to rigid PVC often affects the impact performance, especially the low-temperature brittleness will increase with the increase in the amount of fillers. This is because when the filler is added to PVC as an inorganic particle, it will fill in between the molecular chains. When the amount is small, it fills into the gaps of some molecular chains to play a reinforcing role; or fills in between the molecular chains to increase the distance between molecules and increase the toughness of the system. But when its dosage increases, as the intermolecular distance increases, the intermolecular force is destroyed. When the temperature is added, the mobility of the molecular chain is reduced, and the material's ability to resist external impact is drastically reduced. Therefore, it has a bad influence on the low temperature impact performance of hard PVC.
The addition of fillers to rigid PVC often affects the impact performance, especially the low-temperature brittleness will increase with the increase in the amount of fillers. This is because when the filler is added to PVC as an inorganic particle, it will fill in between the molecular chains. When the amount is small, it fills into the gaps of some molecular chains to play a reinforcing role; or fills in between the molecular chains to increase the distance between molecules and increase the toughness of the system. But when its dosage increases, as the intermolecular distance increases, the intermolecular force is destroyed. When the temperature is added, the mobility of the molecular chain is reduced, and the material's ability to resist external impact is drastically reduced. Therefore, it has a bad influence on the low temperature impact performance of hard PVC.
After the filler is processed, the tensile properties of the material will be improved, but the improvement of the low-temperature impact resistance is not obvious. The reason is that the filler particles occupy the active space of the PVC molecular chain. Although the binding force between the active filler and the PVC molecular chain increases, this increase will only increase the strength of the molecule when it is stretched, and the brittleness of the material will only increase due to the increase in 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. Within a certain dosage range, it can improve the low temperature performance of PVC materials, but because there is no low glass transition temperature, The effect is not as obvious as the modifier, and after a certain amount is added, the low-temperature brittleness of the material will increase.
In general formulations, it is possible to increase the amount of internal lubricant that promotes plasticization while reducing the amount of processing aids, but this method is not recommended in cold-resistant formulations. Because the function of processing aids is not only to improve the processability of PVC, but also to greatly improve the low-temperature performance of products, which can not be replaced by lubricants.
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 when cold resistance needs to be considered.
Common stabilizers have a negative effect on the cold resistance of PVC products. Different types of stabilizers have different effects on the cold resistance of products because of their different shapes and physical properties. Because it is indispensable and the amount is limited, this aspect is rarely considered in general formulation design.
In short, by selecting/changing additives with better cold resistance, introducing some cold-resistant polymers and a series of formula adjustment methods, the cold resistance of PVC materials can be improved to meet the requirements of low-temperature use. At the same time, we should also pay attention to many aspects such as processing temperature, cooling temperature, traction speed, structural design, etc., which will also have a certain impact on the cold resistance of PVC products.
Part of the experimental scientific research room of Xiongxing Group
In summary, the scientific research personnel of Xiongxing Group suggest that when designing PVC formula, all factors such as application conditions, product structure, processing equipment, process conditions, etc. must be considered together with the formula, and corresponding adjustments should be made through experiments. Obtain a PVC formula with excellent cold resistance.
