PVC plastic has good physical and mechanical properties, can be used in the production of building materials, packaging materials, electronic materials, consumer goods, etc., widely used in industry, agriculture, construction, transportation, power telecommunications and packaging and other fields.
Due to its poor cold resistance and low temperature impact resistance, the lower limit of temperature for hard PVC is usually -15 ° C, which limits the application of PVC materials in some aspects.Through the adjustment of PVC resin and auxiliary agent, the cold resistance of PVC plastic can be effectively improved to meet the needs of low temperature.
PVC resin is a kind of noncrystalline, polar high polymer, the glass transition temperature is 75~105 ° C, the relative molecular weight is larger, the viscosity is higher, the van der Waals attraction or winding degree between PVC macromolecular chains increases correspondingly, the PVC chain segment grows, the material temperature resistance is better.
In the conventional PVC formula, if it only needs to cope with the cold climate in the north winter, PVC resin with slightly higher viscosity number, i.e. slightly larger average molecular weight, can be selected, which can be PVC resin with higher viscosity value or even lower in the same brand.
In addition, in some special requirements of products, such as resistant to - 30 DHS C bags and other products, can choose high polymerization degree PVC resin (average degree of polymerization is greater than 2000), this is because compared to the conventional PVC resin with high polymerization degree PVC crystallinity and crosslinking structure, make it difficult to slide between macromolecules, increase elasticity and molecular weight increases at the same time, the van der Waals force between the molecules and molecular chemical bonds within the force increases with the excellent cold resistance.
plasticizer
Plasticizer as an important component of PVC soft products formula, the performance of soft products has a great impact, such as the requirements of products used at low temperature, must choose a good plasticizer type.At present, the main types of plasticizers used as cold-resistant plasticizers are dibasic fatty acid esters, phthalic acid esters of straight-chain alcohols, fatty acid esters of dibasic alcohols and epoxy fatty acid monoesters.It has been reported that N, n-disubstituted fatty acid amide, naphthenic dicarboxylate, and chloromethoxy fatty acid ester are also cold-resistant plasticizers with excellent low-temperature performance.
Improve the cold resistance of PVC soft products, generally by increasing the amount of cold plasticizer to obtain.Adipic acid dioctyl ester (DOA), DIDA (adipate isodecyl ester), DOZ (azelaic acid dioctyl ester), DOS (sebacic acid dioctyl ester) is the representative varieties used as cold-resistant plasticizer, due to the generally cold resistant plasticizer and the compatibility of PVC is not very good, actually can only be used as a plasticizer to improve cold resistance of auxiliary, the dosage is usually 5 ~ 20% of plasticizer.
In addition,2, 2, 4-trimethyl-1, 3-pentaerylene glycol diisobutyrate (TXIB), butyl stearate, LHAT acid 120-2-ethylate and other products also have the effect of cold-resistant plasticizer.
In order to improve the cold-resistant toughness and elongation of thin films, it is better to combine the cold-resistant plasticizer with hexamethylphosphotriamine.Although hexamethylphosphotriamine itself is not a cold-resistant plasticizer, it can effectively reduce the freezing point of various plasticizers to achieve the purpose of strengthening the cold-resistant film.
modifier
The effective way to improve the low temperature impact strength performance 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 high polymer should have similar solubility parameters with PVC, have a certain mutual solubility, can form a blend of two structures, so as to improve the low temperature impact strength of products.
Zhang zhenghong's research showed that
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 increases to a certain extent, the low-temperature impact performance of PV C products tends to be stable, reaching an appropriate performance-to-price ratio around 8 or 9 parts.The low temperature impact strength of hard PVC was gradually increased with the increase of the dosage of NBR.
EVA has good flow performance, low vitrification temperature and good toughening effect at low temperature, but the cost is high.
ACR has excellent low temperature impact strength and weather resistance, and can improve the appearance of products, generally add 5 parts can achieve good results.The glass transition temperature of high impact MBS is low, which can improve the low temperature brittleness of PVC materials, but the weather resistance is poor.
ABS can improve the impact strength of PVC materials at low temperature and improve the appearance of products.
In addition, some SBS containing rubber phase, and a lower glass transition temperature of the material, also improve the impact strength and cold resistance of PVC.
Han huisheng et al. studied the cold resistance of modified soft PVC, and the results showed that the cold resistance of modified soft PVC was significantly affected by the variety and amount of polymer modifier.Through a variety of methods to test found that Elvaloy 711 (ethylene vinyl acetate copolymer) carbon monoxide, NBR - 26 eyes (block butyl rubber), Chemigum P83 eyes (accepting the al powder butyl rubber), and other polymer modifier can significantly improve the performance of cold on the soft PVC, and some polymer modifier such as content of CPE and low VA EVA will damage the soft PVC cold resistant performance.Butenzing rubber modifier can improve the medium resistance of soft PVC (gasoline), so it can improve the low temperature performance of soft PVC soaked in gasoline.
The cold-resistant agent on the market, such as k-175c, n-550c and other products, is actually a styrene 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, the addition of PVC can improve and improve the low temperature performance of PVC.
Thermoplastic elastomer
Thermoplastic elastomer (TPE) is a kind of at room temperature shows that the rubber elasticity, under high temperature and plasticizing molding composite material, therefore, this kind of the characteristics of both rubber and thermoplastic polymer, it can be used as a composite material toughening agent, and can be used as the matrix of the composite material, this kind of material is mainly include polyurethane, styrene, polyolefin, polyester, syndiotactic 1, 2 - polybutadiene and polyamide, etc.At present, styrene and polyolefin are widely used as toughening agents for composites.
The cold resistance of pvc-tpe products is at least not lower than that of soft PVC. When adopting cold-resistant plasticizer and cold-resistant formula, 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, etc.For example, TPE of H4040, H3303 and other brands have good compatibility with PVC. After addition, the low temperature softness of PVC can be significantly improved, its bending resistance can be significantly improved, and the brittle point can be reduced.
Japanese plastics technology company also developed a thermoplastic polyurethane - PVC elastomer.This material is mixed with TPU and PVC and the third component. It gives full play to the excellent characteristics of both TPU and PVC and has the following advantages:
(1)TPU is used as plasticizer for PVC, eliminating the problems of plasticizer migration and volatilization existing in soft PVC in the past.
(2) the embrittlement temperature of PVC material also decreased from -30℃ to -68℃, reaching the special cold resistance level.
Filling agent
The influence of filler on cold tolerance of soft PVC is related to the absorption amount of plasticizer. The general trend is that filler with small absorption amount of plasticizer has little influence on cold tolerance, while filler with large absorption amount of plasticizer, such as carbon black and hard clay, will make PVC cold tolerance have a relatively obvious reduction.
The addition of filler in hard PVC often affects the impact performance, especially the brittleness at low temperature will increase with the increase of the amount of filler.This is because when a filler is added to PVC as an inorganic particle, it fills the intermolecular chain.When the dosage is small, it fills in the gaps of some molecular chains, plays a reinforcing role;Or fill in the molecular chain to increase the distance between molecules and increase the toughness of the system.However, when the dosage increases, with the increase of the distance between molecules, the force between molecules will be destroyed. In addition, when the temperature is low, the activity of molecular chain segments will be reduced, and the ability of materials to resist external impact force will drop sharply.Therefore, the low temperature impact performance of hard PVC has a bad impact.
After the packing is treated, the tensile properties of the material are improved, but the impact resistance at low temperature is not obviously improved.The reason is related to the activity space of PVC molecular chain occupied by packing particles.Although the binding force between active filler and PVC molecular chain increases, this increase only increases the strength of the molecule under tensile force, while the brittleness of the material only increases with the addition of filler particles.
Nanometer calcium carbonate, ultra-fine calcium carbonate added to PVC, due to the small size effect, has the similar effect of modified agents, within the scope of a certain dosage can improve the low temperature performance of PVC material, but with no low glass transition temperature, no modifier effect, and add to a certain amount, after the material brittleness at low temperature will rise.
Other auxiliaries
In the general formula, it can be used to increase the amount of internal lubricant with promoting plasticizing effect and reduce the amount of processing assistant, but this method is not recommended in the cold resistant formula.Because the role of processing AIDS is not only to improve the processing of PVC, and, its role for the products of low temperature performance has a greater improvement, this role of lubricant can not be replaced.
The most commonly used soft PVC flame retardant plasticizer is trimethylphosphate, but trimethylphosphate has poor low temperature performance, so it is more suitable to use alkyl phosphate in the occasions requiring consideration of cold resistance.
Commonly used stabilizers have a negative effect on the cold resistance of PVC products. Different varieties of stabilizers have different effects on the cold resistance of products due to their different forms and physical properties.Due to the necessity and limited dosage, the general formula design rarely consider this aspect.
In short, the cold resistance of PVC materials can be improved to meet the requirements of low temperature by selecting/changing the additives with better cold resistance and introducing a series of formula adjustment methods such as some polymers with cold resistance.At the same time, we should also note that processing temperature, cooling temperature, traction rate, structural design and many other aspects, will also have a certain impact on the cold resistance of PVC products.
When designing the 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 make corresponding adjustments through the test, and finally obtain the PVC formula with excellent cold resistance.
Reproduced on the Internet, for reference only.
