PVC formula design and influence
PVC is a highly amorphous polymer with a glass transition temperature of about 80 ° C. It is very brittle, and has poor thermal stability and light resistance. It can be modified by adding related additives or fillers. Multi-component composite materials are polymer modified materials. Therefore, we can choose different materials to modify the PVC according to its characteristics to achieve the purpose of use. For example: according to the amount of plasticizer used to control the PVC products are rigid, semi-rigid, soft or paste state. Choose to make the product transparent, translucent, opaque or colored according to the resin and additives. According to the choice of additives and modifiers, the properties of the product, such as tensile, bending, impact resistance, etc., can be changed, and the properties of flame retardancy, insulation, density, etc. can also be changed according to different additives. We refer to these material choices and component amounts as PVC formulations. In practice, the use of different raw materials and components to change the physical properties of PVC products is very obvious. We believe that the addition of any raw material component has a modifying function in the PVC formulation system. Therefore, the additives used in PVC formulations can be considered as modifiers. In order to better distinguish the main uses of raw materials, it is convenient to judge that process adjustments are divided into: thermal stabilizers, lubricants, modifiers, grafting agents, fillers, and so on. From the perspective of technology, it is more convenient and understandable for the individual to distinguish into process component formula and modified component formula for analysis.
1. Process formula
The PVC formula has multiple components, and the PVC resin is a loose porous powder. When processing PVC, the components and some of the raw materials in the formula must be reasonably dispersed into the PVC pore system. Only in this way can the PVC processing process be completed more favorably. The process formula is relatively simple, after understanding the required product performance, equipment and operating speed. Firstly design a basic formula reasonably, let the component materials be reasonably dispersed under high-speed mixing, and then adjust the PVC plasticization by the screw to achieve the desired effect of the melt. Therefore, the process formula cannot be adjusted by relying solely on the formula, it is also greatly related to the equipment and process parameters. For the simple melt plasticization process, the main adjustment direction is to coordinate the use of stability and lubrication. At the same time, the impact of the operation process on plasticization also needs to be adjusted. Different equipment temperature screw and mold temperature, speed and current, the requirements for lubrication and stabilizer lubrication are very different. Therefore, it is not reasonable to rely on the formula alone to meet the requirements of the equipment, so it is easy to form excessive component residues. Personally, I believe that the process formula is mainly composed of dispersion, shearing ability and pressure, temperature and speed.
According to the porosity and porosity of PVC resin, in the mixed use of PVC, it is necessary to ensure sufficient dispersion to allow all components to be fully distributed in the material. Then pass the extrusion equipment to make the plasticized melt form perfect uniformity to achieve the best effect. In the process formula, we can not only consider the shear of the mixer and the return section to disperse the raw materials. Reasonable addition of better dispersing lubricant can make the dispersion better, so that the component material can obtain a uniform distribution effect. There are many techniques for dispersion, and many practical methods have been found in experiments over the years. For example, when the high filling reaches 200 parts, the requirement for the calcium powder mesh number is reduced to about 800 mesh. If it is used too fine, serious current fluctuations will occur. On this basis, the addition of fine calcium can allow fine calcium (2500 mesh) to be partially drilled into the resin particle holes to achieve the purpose of dispersion, which can significantly improve the performance of PVC products. Dispersion is targeted, and different methods for different substances are not necessarily all lubricants. Of course, the lubricant will obviously disperse various plastics and rubber bodies during the shearing process. Make the distribution of components in the melt more uniform. Lubricant dispersion can be regarded as powder dispersion and melt dispersion. The reasonable choice of lubricants in processing according to the equipment conditions has a significant effect on the overall formulation system.
2. Cutting ability and pressure
Rigid PVC forms a melt during shearing. As the temperature rises, shearing can make the component distribution more uniform. In particular, the cone melt is sheared back and forth in the reflow section to obtain a preliminary melt. After a certain pressure, the melt fusion is better. This process can be viewed as an evolution from elastomer formation to highly elastomeric. If the feed is forced to increase the pressure, the shear dispersibility will be relatively weakened, and severe localized plasticization points will appear severely. Therefore, the pre-lubrication in the cone twin screw can allow PVC to be fully kneaded and sheared. It is also necessary to retain a part of the high melting point lubricant to have a certain dispersion effect on the melt. With the backward movement, the melt pressure becomes larger, the macromolecules have a certain movement, the viscosity of the melt of PVC increases, and the lubricating part migrates to the surface of the melt in the later stage, which improves the melt fluidity. Therefore, generally, the lubrication with mold release is considered to be an excellent shear dispersant in the formula. Generally, the amount of lubrication that has mold release effect on PVC is small. Too much is easy to reduce the affinity between melts and the quality of the product is deteriorated.
Third, the temperature
There are two main sources of temperature during PVC processing, external heating and shear heat. When PVC does not form a viscoelastic melt, external heating is needed to soften the PVC and gradually develop it to viscoelasticity in order to obtain a kneaded melt. The return section is the main location for the formation of viscoelastic bodies, so sometimes the two phenomena of heating and cooling are compensated. The viscoelastic body enters the lead and develops into a viscous state. At this time, the macromolecule's ability to move increases, and a large amount of heat begins to be generated during shearing. External cooling is required to control heat dissipation, or to increase lubrication to reduce pressure and the thermal energy of motion between molecules. Therefore, the approximate state of the melt can be preliminarily determined according to the temperature performance of the lead region of the screw, which provides a certain reference for the amount and selection of stabilizers and lubricants in the formulation system. Personal habits are based on the performance of the last paragraph of the lead (barrel 4) to judge the amount of lubricant and stabilizer. If the long-term high temperature indicates that the frictional heat is too high, it is necessary to reduce the speed of the host or reduce the heat by compensating the lubricating body to prevent the PVC from decomposing. If the temperature shear heat is not high in each area of the screw, it means that the amount of lubrication is in a controllable range or high. Of course, this has a lot to do with the speed of the host. The faster the speed and the higher the relative heat of shear, the greater the need for stability and lubrication.
Fourth, the extrusion process formula
Reasonably adjust the lubricating content according to the temperature control phenomenon of the front, middle and late stages, and fully judge the relationship between stability and lubrication according to the smoothness of the inside and outside of the product. For example, the melt of the product is too thin, and the viscosity is too good. It is difficult to control the decomposition of PVC. After the mold is released, pits appear to indicate that the melt is decomposed. If the tube is white, it means less lubrication. And if the inside and outside smooth product turns yellow, it means less stability. Many performances can be adjusted for basic process recipes. Stabilizers are also stable in the early and late stages. The stability in the later stage is very important. Under normal circumstances, the temperature of the die is raised very high, and the product does not change color even if it has pitting points.
The extrusion process formula is relatively simple. That is, the process of cooperation between thermal stability and lubrication. relatively simple. The initial grasp is difficult, and as experience grows, it can be adjusted slightly without affecting it. Mastering the use of multi-component lubrication far exceeds independent lubrication. Because the fewer types of lubrication are used, independent lubrication will form an interface and cause harm to the affinity between the melt components.
Five, process formula
Formulation system for process requirements. The process of making reasonable adjustments to the amount of use based on the performance of the equipment. According to personal experience, the techniques for adjusting the process formula: (1) the effect of the host speed on the formula, the faster the host speed, the better the shear dispersion effect, the better the dispersion of the component materials in the melt. (2) The faster the speed of the host, the higher the shear heat and the viscous flow state, and the melt decomposition is more likely to occur. So speed has a very obvious demand for lubrication. (3) When using high speed and high current, the viscosity of the melt increases, and the stability and lubrication requirements are increased again. To sum up, the mechanic can design a basic formula first, and then control the basic formula according to adjusting the host speed and current control. At the same time, the process formula is reasonably optimized according to the phenomenon of the equipment.
The performance formula is much more complicated than the process formula. It is mainly based on the physical properties of the PVC products to modify the PVC based on the process formula. Therefore, the performance formula is a reasonable screening of modified raw materials based on the established process formula, and then a reasonable compounding process. Not only should you be familiar with the properties of various raw materials, but also pay attention to the influence of its process formula and the mutual influence on the performance of other component materials. For example, as the CPE rises, the fluidity will obviously decrease, and the lubrication will increase. After adding ACR to the extrusion system, the MBS lubrication will decrease relatively. When adding ABS system, dispersant should be added appropriately. If the melt is too soft, you can choose to add a small amount of phosphite to improve processability. The performance formula is to add different auxiliaries or synthetic plastic substances according to the body shape. This is equivalent to the category of modified plastics, and PVC is much more difficult to process than other plastics, plus special needs for modification. So it's very difficult, and start from the basic process to learn performance. When the basic melt is understood, there will be more and more material technologies to choose from, and individual skills will become stronger and stronger. When it reaches a certain level, it will return to the original simple process to solve the problem, but it retains a little personal reliance on long-term use of materials to meet the needs of product modification.