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1. The structure and performance of PVC resin
2. Modification and application of PVC resin
Physical properties
The density of rigid PVC is usually 1.38~1.46g/cm3, the water absorption rate is less than 0.5%, soaked in water for 24 hours, the water absorption rate is less than 0.05%;
Mechanical properties
Rigid PVC mainly includes tensile strength, tensile modulus, elongation, impact strength, hardness, creep properties, etc.;
1. Tensile performance
1.1 Influencing factors of stress-strain curve:
a. It is related to the speed of the test: the tensile speed increases, the yield stress increases, and the fracture stress increases;
b. Related to the test temperature: it is sensitive to temperature. When the temperature decreases, the tensile strength, tensile modulus, elongation at yield, and hardness increase, and the elongation at break and impact strength decrease; vice versa.
1.2 Influence on tensile strength and elongation:
a. The influence of PVC molecular weight: as the molecular weight increases, it increases;
b. The influence of toughening agent: when using toughening agent, it will increase the toughness of the product, the elongation will increase, but other properties such as tensile strength will decrease;
c. The influence of plasticizers: usually can improve the processing flow properties of the compound, but the tensile strength will be significantly reduced;
d. The influence of filler: Generally, the use of filler will reduce the tensile strength and impact strength of the product, except for individual fillers (such as glass fiber, etc.).
2. Impact performance
The main factors affecting impact performance are as follows:
2.1 Test loading speed:
2.2 Sensitivity of the gap:
2.3 Effect of Temperature: strongly dependent on temperature changes
2.4 The influence of ingredients: each component and dosage of the formula will affect the impact strength of the product
a. The influence of molecular weight: the increase in molecular weight, the increase in impact strength
b. The influence of modifier: Under normal circumstances, the increase in the amount of modifier increases the impact strength of the product, but after the increase in the amount reaches a critical value, the increase in the amount of modifier will significantly reduce the increase in impact strength. The effect is not very obvious, and the modifier must be increased to a certain amount before there is obvious modification effect;
c. The influence of plasticizer: there is an anti-plasticizing effect. After the amount of plasticizer has passed the anti-plasticizing effect, the impact strength will increase with the increase of the amount;
d. The influence of filler: Usually the use of filler will reduce the impact performance of the product. However, when using a small amount of ultra-fine filler and reinforcing filler, the impact strength of the product can be increased;
e. The influence of processing conditions: The main requirement is to achieve uniform plasticization, and control the degree of curing and plasticization between 55% and 65%.
3. Hardness
4. Heat resistance
Processing rheological properties
Mainly refers to the change of the melt viscosity of PVC compound during processing and its influencing factors.
The rheological properties of PVC-a typical pseudoplastic fluid-the phenomenon of shear thinning, that is, the shear speed increases and the melt viscosity decreases.
1. Factors affecting shear viscosity and flow curve:
a. PVC molecular weight: high molecular weight, high melt viscosity, not conducive to processing
b. PVC resin particle shape: The shape and structure of the resin surface is loose, which is conducive to the absorption of plasticizers, lubricants and additives, which is conducive to plasticization and has good processing properties.
c. Plasticizer: reduce melt viscosity and improve melt flow processing performance
d. Lubricant: reduce the viscosity of the melt
e. Processing aids: different types of processing aids can play different roles (see the role of processing aids for details)
f. Shear speed: the shear speed increases, the viscosity decreases
2. The elastic effect of the melt : the high-molecular polymer melt is accompanied by reversible high elastic deformation during the flow process, including the normal stress effect (wrapped axis phenomenon), the Ballas effect (outlet expansion) and the melt Fracture phenomenon.
a. Extrusion expansion phenomenon: refers to the phenomenon that the cross-sectional area of the extrudate is larger than the cross-sectional area of the die after the melt is extruded from the die.
Usually the molecular weight is high, the melt viscosity is large, the movement of the chain segment requires a long time, the elastic relaxation becomes slower, the elastic effect is obvious, the outlet expansion degree is relatively small, and the expansion ratio is small: vice versa.
b. Unstable flow-melt fracture phenomenon.
Modification purpose
Some defects of PVC materials: such as poor thermal stability, brittleness of rigid PVC products, poor heat resistance, high melt viscosity, poor fluidity, difficult processing, unstable plasticization and easy precipitation, polar polymers and hydrophilic properties Poor biocompatibility.
Modification method
It can be divided into chemical modification and physical modification through different modification principles
Usually, the most commonly used in the production process is physical modification, including compound modification, blending modification and other physical modification. Among them, compound modification mainly includes filling modification and fiber-reinforced modification, and blending modification is Mainly refers to mixing PVC with other polymers to obtain a blend with good comprehensive performance.
Key points of blending modification
1. If it is used as a polymer to improve toughness and processing properties, it requires partial compatibility with PVC;
2. If it is used as a polymer for permanent plasticization and improving heat distortion temperature, it requires complete compatibility with PVC.
The purpose of blending modification
Improve toughness, heat resistance, processability, flame retardancy, etc.
1. Improve toughness : usually use CPE, MBS, ABS, NBR, EVA and some rigid polymers, etc.
a. CPE modifier;
b.EVA；
c.MBS；
d.ABS；
e.NBR；
f. Other impact modifiers.
2. Heat resistance : use heat-resistant modifier
3. Processing and molding performance : The so-called processing aid refers to a special compounding agent that can significantly improve the processing performance of PVC when added in a small amount. It can be roughly divided into two categories that can promote the plasticization of PVC and give it rubber elasticity and lubricity. .
3.1 The main functions of the rubber elastic polymer processing aids are :
a. Promote plasticization and improve the gloss of products;
b. Improve the melt strength when the melt is broken;
c. Prevent shrinkage during blow molding and vacuum molding;
d. Improve the storage property, roll-wrapping property and melt uniformity during the calendering process;
e. It can make the extruded and foamed cells uniform;
f. Improve the appearance of the product;
g. Prevent swirling during injection molding;
h. It can improve the dispersibility of fillers and pigments.
3.2 The main functions of lubricating polymer processing aids are :
a. Delay plasticization and reduce molding load;
b. Improve the metal peeling performance of the melt;
c. Prevent the surface sticking material and reduce other properties.
3.3 Main principles :
a. Promote and extend plasticization;
b. Endowment of rubber elasticity;
c. Improve the melt strength during molding;
d. Improve foam molding;
e. Improve injection performance;
f. Improve calendering performance;
g. Provide lubricity.
4. Other modifications : such as flame retardant performance, antistatic performance, etc.
