PVC is a non-crystalline polymer with high brittleness at the glass transition temperature of around 80℃. It has poor thermal stability and light resistance, so it needs to be modified with relevant additives or fillers before it can be used.The multi-component composite material is a polymer modified material.So we can choose different materials according to the characteristics of PVC to modify it to achieve the purpose of use.For example: according to the amount of plasticizer to control PVC products for hard, semi-hard, soft or paste state.Make the product transparent, translucent, non-transparent or color based on resin and additives.According to the choice of additives and modifiers for the product, the tensile, bending, impact resistance and other performance changes, can also be selected according to different additives for Flame Retardant, insulation, density and other performance changes.We refer to these material choices and the amount of ingredients used as PVC formulations.In actual operation, the physical properties of PVC products are significantly changed by the use of different raw materials and components, and we believe that the addition of any raw material components has a modification function in the PVC formula system.Therefore, the additives used in PVC formulation can be considered as modifiers.In order to better distinguish the main USES of raw materials, the process adjustment can be divided into: Heat Stabilizer, lubricant, modifier, graft agent, filler and so on.From the point of view of process, I think it is more convenient and understandable to distinguish the formula of process components from that of modified components.
I. process formula
PVC formula has multiple components, and PVC resin is a porous powder of loose type, when processing PVC, the components and part of the raw materials in the formula should be reasonably dispersed into the PVC hole system.This can be more favorable to complete PVC processing technology.Process formula is relatively simple, after the required product performance, equipment and operating speed to understand.First, a reasonable design of the basic formula, in high speed mixing, let the components of the material reasonable dispersion, and then through the screw adjustment of PVC plasticizing melt to achieve the ideal effect.So the process formula can not only rely on the formula alone to adjust, it and equipment and process parameters are also greatly involved.In the simple melt plasticization process, the main adjustment direction is to stabilize and lubricate and coordinate the usage amount, and the influence of operation process on plasticization also needs to be adjusted.Different equipment temperature screw and mold temperature, speed and current, on the lubrication of the stabilizer lubrication requirements are very different.Therefore, it is not reasonable to rely on the formula alone to force the requirements of the equipment, which is easy to form excessive residual components.I think the process formula is mainly based on dispersion, shear capacity and pressure, temperature and speed.
Second, the decentralized
According to the porosity and porosity of PVC resin, in the use of PVC mixture, it is necessary to ensure sufficient dispersion, so that all components are fully distributed in the material.The plasticizer melt is then extruded to achieve perfect uniformity to achieve the best effect.In the process formulation, the shear of mixer and return section should not be considered only to disperse the raw materials.Reasonable addition of good dispersion lubricant can make the dispersion more excellent, so that the distribution of component materials uniform effect.There are a lot of techniques for dispersion, and many of them have been found to be very useful in experiments over the years.For example, when the high filling reaches 200 parts, the requirement for the number of calcium powder mesh drops to about 800 parts. If too fine, serious current fluctuations will occur.On this basis, the addition of micro - calcium, can allow the micro - calcium (2500 mesh) part of the hole into the resin particles to achieve the purpose of dispersion of PVC products to significantly improve the performance.Dispersion is targeted, the use of different methods of dispersion of different substances may not be all lubricants to complete.Of course the lubricant in the shear process of various plastics and rubber will appear obvious dispersion.The composition distribution in the melt is more uniform.Lubrication dispersion can be regarded as powder dispersion and melt dispersion.According to the equipment condition in the processing to give a reasonable choice of lubricants, the overall formula system has an obvious role.
2. Cutting ability and pressure
Rigid PVC forms a melt in shear as the temperature rises, shear can make the component distribution more uniform.In particular, the initial melt can be obtained when the cone is cut back and forth in the reflux section.After a certain amount of pressure to make the melt fusion better.This process can be regarded as the evolution from elastomer formation to high elastomer.If forced feeding makes the pressure increase, the shear dispersion will be relatively weak, serious will appear local difficult plasticizing point.Therefore, in the middle and early stage of the conical twin screw lubrication can allow PVC to fully kneel and shear, but also to retain part of the high melting point lubrication on the melt has a certain degree of dispersion.With the backward passage, the melt pressure increases, and the macromolecules have a certain movement. The viscosity of PVC melt increases, and the lubrication part migrates to the melt surface at the later stage to improve the melt fluidity.Therefore, lubrication with demultifying effect is generally considered to be an excellent shear dispersant in the formulation.There is a demoulding effect on PVC lubrication generally add less, too much easy to melt between the ability to decline, the quality of products become poor.
Third, the temperature
PVC processing temperature is mainly from two sources, external heating and shear heat.In PVC does not form a viscoelastic melt, the need for external heating, let the PVC softening gradually to viscoelastic development, in order to get kneading melt.The backflow section is the main location of viscoelastic body formation, so sometimes both heating and cooling phenomena are compensated.The viscoelastic body enters the lead and develops to the viscous flow state. At this point, the movement ability of the macromolecules is improved and a large amount of heat is generated during shear.External cooling is needed to control the release of heat, or to increase lubrication to reduce the heat capacity of pressure and movement between molecules.Therefore, the approximate state of the melt can be preliminarily determined according to the temperature performance of the lead zone of the screw, providing certain reference for the dosage and selection of stabilizer and lubricant in the formula system.Personal habits according to the last lead (cylinder 4) to judge the amount of lubrication and stabilizer.If the long-term high temperature indicates that the friction heat is too high, the need to reduce the main engine speed or by compensating the lubricant to reduce the heat, to prevent PVC decomposition.If the screw temperature is not high shear heat, indicating that the use of lubrication in the controllable range or high.Of course, this also has a lot to do with the speed of the mainframe.The faster the speed, the higher the relative shear heat, the greater the need for stability and lubrication.
Iv. Extrusion process formula
According to the machine temperature control phenomenon in the middle and late period, the lubrication content can be adjusted reasonably, and the relationship between stability and lubrication can be fully judged according to the internal and external smoothness of the product.For example, the melt of the product is too dilute and soft, and the viscosity is too good to control the decomposition of PVC.If the pipe is white, less lubrication.If the inside and outside smooth product color yellow, indicating less stability.Many performances can be adjusted to the basic process formula.Stabilizers are also divided into early stability and late stability.Later stability is very important, under normal circumstances, the mouth mold temperature rise is very high, even if the product does not change color, indicating that the late stability is very good, that is, the mold can be significantly high temperature production.
Extrusion process formulation is relatively simple.That is, the use of thermal stability and lubrication process.It's relatively simple.Initial grasp difficulty is high, with the experience grows, can adjust slightly to its influence is not big.Master multi - component lubrication, far more than independent lubrication.Because of the fewer types of lubrication used, independent lubrication will form an interface, causing damage to the affinity between melt components.
Five, technological formula
Formulation system for process requirements.Then according to the performance of the equipment to do a reasonable adjustment between the use of the process.According to my personal experience, the adjustment skills of the process formula :(1) the influence of the host speed on the formula, the faster the host speed is, the better the shear dispersion effect is, and the better the dispersion of the component materials in the melt is.(2) the faster the host machine is, the greater the increase of shear heat, the higher the viscous flow state, and the easier the melt decomposition is.So the speed of lubrication demand is very obvious.(3) when high speed and high current are used, the viscous flow performance of the melt increases, and the requirements for stability and lubrication are raised again.Based on the above, the mechanic can design a basic formula first, and then adjust the speed and current control of the main engine to control the basic formula.At the same time according to the phenomenon of equipment to optimize the process formula.
Vi. Performance formula
The performance formula is more complex than the process formula, mainly from the PVC products physical performance requirements for the purpose, on the basis of the process formula to modify PVC.Therefore, the performance formula is based on the established process formula, the modified raw materials are screened reasonably, and then the reasonable compounding process is carried out.It is important not only to be familiar with the properties of various materials but also to pay attention to the effects of their technological formulations on the properties of other components.For example, with the rise of CPE, liquidity will decline significantly and lubrication is required to rise.When ACR is added to the extrusion system, MBS lubrication decreases relatively.Dispersant should be added when adding ABS system.For the melt soft too high can choose to add a small amount of phosphite ester to improve the processing.Performance formula is based on the need to add different additives or synthetic plastic material.This is equivalent to the category of modified plastics, and PVC processing is much more difficult than other plastics, plus special needs to modify.So the difficulty is very high, from the basic process to continue to learn performance.As the basic melt is understood, the number of material technologies available will increase and the performance of individual skills will increase.When it reaches a certain level, it will return to the original simple process to solve the problem, only to retain a little personal dependence on the long-term use of materials to meet the modification needs of products.
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