PVC plastic has good physical and mechanical properties, can be used for the production of building materials, packaging materials, electronic materials and consumer goods. It is widely used in industry, agriculture, construction, transportation, telecommunications and packaging and other fields.
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 material in some aspects. Through the adjustment of PVC resin and additives, can effectively improve the cold resistance of PVC plastic, to meet the needs of low temperature.
This article focuses on the formula from the point of view to improve the cold resistance of some of the methods for your reference.
First, PVC resin
PVC resin is a non-crystalline, polar polymers, the glass transition temperature molecular weight of 75 ~ 105 º C, the larger the relative molecular mass, the higher the viscosity, the PVC macromolecular chains Van der Waals attraction or degree corresponding Increase, PVC segment growth, the better the low temperature resistance of the material.
In conventional PVC formulations, PVC resins with slightly higher viscosities, ie with slightly higher average molecular weights, may be used as long as they cope with the northern winter's cold weather and may be PVC or lower grade resins of the same brand with a higher viscosity number.
In addition, some special requirements of products, such as blood bags can be resistant to -30 º C and other products, the choice of high degree of polymerization of polyvinyl chloride resin (average degree of polymerization greater than 2000), because the high degree of polymerization PVC has a PVC The large crystallinity and cross-linked structure of the resin make it difficult for the macromolecules to slide and increase the elasticity, meanwhile, the molecular weight is increased, and intermolecular van der Waals forces and intramolecular chemical bonding force are increased to obtain excellent cold resistance.
As an important component of PVC soft plasticizer, plasticizer has a great influence on the properties of soft products. If it is required to use the product at low temperature, the type of plasticizer must be selected. Currently used as cold plasticizers are mainly fatty acid dibasic acid esters, linear alcohol phthalates, dihydric fatty acid esters and epoxy fatty acid monoesters. N, N-disubstituted fatty acid amides, naphthenic dicarboxylates, and chloromethoxy fatty acid esters have also been reported as cold-resistant plasticizers with excellent low-temperature properties.
To improve the cold resistance of PVC soft products, generally by increasing the amount of cold plasticizer to obtain. DOA (dioctyl adipate), DIDA (diisodecyl adipate), DOZ (dioctyl azelauate), DOS (dioctyl sebacate) are representative of the use as cold-tolerant plasticizers Varieties, due to the general cold plasticizer compatibility with PVC are not very good, in fact, can only be used as an auxiliary plasticizer to improve the cold resistance, the amount is usually 5 to 20% of the main plasticizer.
In addition, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIB), butyl stearate, LHAT acid diglycol monobasic 2- The product also has the role of cold plasticizer.
Jiang Peifen study pointed out that in order to improve the film cold toughness and low temperature elongation for the purpose, so that cold plasticizer and hexamethylphosphoric triamide used better. Because hexamethylphosphoric triamide itself is not a cold plasticizer, but it can effectively reduce the freezing point of various plasticizers to achieve the purpose of strengthening the film cold resistance.
An effective way to improve the poor performance of low temperature impact strength of PVC is to add high polymer with low glass transition temperature and high elasticity at room temperature, which is collectively referred to as modifier. The added polymer should have similar solubility parameters with PVC, have certain miscibility, can form a blend of two structures, so as to improve the low temperature impact strength of the product.
Zhang Zhenghong research results show that,
CPE can improve the product's low temperature performance, impact strength and so on. With the increase of CPE dosage, the impact performance of PV C products will gradually increase. When the dosage increases to a certain extent, the low temperature impact performance of PV C products tends to be stable, and the suitable cost performance is reached at around 8 and 9 parts. With the increase of the dosage of NBR, the low temperature impact strength of hard PVC will be gradually increased.
· EVA flow well, low glass transition temperature, 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 add 5 copies can achieve good results. High impact MBS glass transition temperature is low, low temperature brittle PVC material has a good improvement, but poor weather resistance.
· ABS improves the low temperature impact strength of PVC materials while improving the appearance of the product.
· In addition, some materials such as SBS that contain a rubbery phase and have a low glass transition temperature also improve the impact strength and cold resistance of PVC.
Han Huisheng study of modified soft PVC cold resistance results show that the cold-resistant properties of blends modified soft PVC significantly affected by the type and amount of polymer modifier. It has been found through various methods that polymer modifiers such as Elvaloy 711 (Ethylene-Vinyl Acetate Monoacetate Copolymer), NBR-26 (Block NBR), Chemigum P83 (Pre-crosslinked Powder NBR) Soft PVC cold hardiness, and some polymer modifiers such as CPE and low VA content of EVA will damage the soft PVC cold hardiness. NBR rubbers modifiers improve the low-temperature performance of soft PVC soaked in gasoline due to the media-resistant (gasoline) properties of soft PVC.
Market launch of cold agents, such as K-175C, N-550C and other products is actually to improve the low temperature flexibility and impact strength of PVC developed a styrene-based modifier. Because of its lower glass transition temperature, and with PVC has good compatibility, with a certain plastic and toughening effect. Therefore, after adding PVC, PVC can improve and improve the low temperature performance.
Fourth, thermoplastic elastomer
Thermoplastic elastomers (TPE) are a class of synthetic materials that exhibit rubber elasticity at room temperature and plasticize at elevated temperatures. Therefore, these polymers combine the characteristics of rubber and thermoplastics, both as a composite Toughening agent, but also can be used as the matrix material of composite materials, such materials mainly include polyurethane, styrenes, polyolefins, polyesters, syndiotactic 1,2-polybutadiene and polyamides and other products . Currently more used as a composite toughening agents are styrenes and polyolefins.
The cold resistance of PVC-TPE products is at least not less than that of soft PVC. PVC-TPE maintains good elasticity at -45 with cold-resistant plasticizers and cold-resistant formulations. PVC-TPE is also popular in cold-resistant and seawater-resistant products such as marine seals, container seals and marine hoses. Such as H4040, H3303 and other brands of TPE and PVC have good compatibility, adding can significantly improve the low temperature flexibility of PVC, significantly improve its ability to resist song around, reducing the embrittlement point.
Japanese plastic technology company also developed a thermoplastic polyurethane - polyvinyl chloride elastomer. This material to TPU and PVC and the third component after mixing by mixing, give full play to the excellent properties of both TPU and PVC, has the following advantages:
(1) The use of TPU as a plasticizer for PVC eliminates the problem of plasticizer migration and volatilization in the past with soft polyvinyl chloride.
(2) The embrittlement temperature of PVC material is also reduced from -30 ℃ to -68 ℃, reaching a special cold-resistant level.
The effect of filler on the cold resistance of soft PVC is related to its plasticizer absorption. The general tendency is that filler with a small amount of plasticizer absorbs less impact on cold resistance, whereas plasticizer such as carbon black and hard clay have a large absorption Of the filler, it will make PVC cold resistance significantly reduced.
Addition of fillers to hard PVC tends to affect the impact properties, especially low temperature brittleness increases with the amount of fillers. This is because when fillers are added to PVC as inorganic particles, they fill the interchain chains. When the dosage is small, it fills in the gaps of some molecular chains, plays a reinforcing role; or fills in the intermolecular chains and plays the role of increasing the intermolecular distance and increasing the toughness of the system. However, as the dosage increases, the intermolecular forces are destroyed as the intermolecular distance increases, and the activity of the molecular segments decreases at low temperatures, dramatically reducing the ability of the material to resist external impacts. So the low temperature impact of hard PVC has a negative impact.
After the filler has been processed, the tensile properties of the material will be improved, but the improvement of low temperature impact resistance is not obvious. The reason, and filler particles occupy the PVC molecular chain activity space. Although the binding force between active filler and PVC molecular chain increases, this increase only increases the tensile strength of the molecule, while the brittleness of the material increases only due to the increase of filler particles.
Nano calcium carbonate, calcium carbonate added to the PVC, due to the small size effect, it has a similar role as a modifier within a certain amount of range can improve the low temperature properties of PVC materials, but because there is no low glass transition temperature, The effect is not modifier obvious, and add to a certain amount, the material's low temperature brittleness will rise.
Sixth, other additives
In the general formula, you can use to increase the amount of plasticizer with an increase in the amount of internal lubricants and reduce the amount of processing aid, but the cold-hard formulations are not recommended for this method. Because the role of processing aids not only to improve the processing of PVC, and its role for the product of the low temperature performance has greatly improved, this role can not be replaced by lubricants.
Soft PVC flame retardant plasticizer is the most commonly used tricresyl phosphate, but the low temperature performance of tricresyl phosphate is poor, so the need to consider the occasion of cold hardiness alkyl phosphate ester is more suitable.
Commonly used stabilizers on the cold resistance of PVC products have a negative effect, different varieties of stabilizer because of their different morphological and physical properties of the cold-affected products are not the same. Due to the indispensable and limited amount, the problems in this respect are seldom considered in the general formula design.
In short, through the selection / replacement with better cold-resistant additives, the introduction of a number of cold-resistant polymers and a series of formula adjustment methods, can make the cold resistance of PVC materials can be improved to meet the requirements of low temperature. At the same time, we should also note that processing temperature, cooling temperature, traction rate, structural design and many other aspects, but also PVC products have a certain impact on the cold resistance.
Therefore, in the design of PVC formulations, we must consider the application conditions, product structure, processing equipment, process conditions and other factors, together with the formula considered, and adjusted accordingly to the test, and finally obtain PVC formulations with excellent cold resistance.