The formula of PVC plastic products is mainly composed of PVC resin and auxiliaries, and the additives are divided into heat stabilizer, lubricant, processing modifier, impact modifier, filling agent, aging agent, colorant and so on. Before designing the PVC formula, we should first understand the properties of PVC resins and various auxiliaries.
The resin for the production of PVC plastics is polyvinyl chloride resin (PVC). PVC is a polymer made from vinyl chloride monomer. The output of polyvinyl chloride is only second to PE, which is the second place.
PVC resin can be divided into two types: loose type (XS) and compact type (polymer) due to different dispersants in polymerization. The loose particle size is 0.1 - 0.2mm, the surface is irregular, porous, cotton ball shaped, easy to absorb plasticizer, compact particle size below 0.1mm, surface rules, solid, table tennis, not easy to absorb plasticizer, at present the use of loose type more.
PVC can also be classified into ordinary (toxic PVC) and sanitary (non-toxic PVC). Hygienic requirements for vinyl chloride (VC) content below lOXl0-6 can be used in food and medicine. The synthesis process is different. PVC can also be divided into suspension PVC and emulsion PVC. According to the national standard GB / T5761-93 "general polyvinyl chloride resin test standard for suspension method", the suspension method is divided into PVC-SGl to PVC-SG8Jk resin, of which the smaller the number, the greater the degree of polymerization, the greater the molecular weight and the higher the strength, but the more difficult the melting flow is, the more difficult the processing is.
When choosing a soft product, PVC-SGl, PVC-SG2 and PVC-SG3 should be used in general, and a large amount of plasticizer should be added. For example, polyvinyl chloride membrane uses SG-2 resin and 50~80 plasticizer is added. When processing hard products, plasticizer is not added or very small, so PVC-SG4, VC-SG5, PVC-SG6, PVC-SG7 and PVC-SG8 are used. For example, PVC hard pipes use SG-4 resin and plastic door and window profiles to use SG-5 resin, and hard transparent tablets use SG-6 resin and rigid foamed profiles to use SG-7 and SG-8 resin. The emulsion PVC paste is mainly used for artificial leather, wallpaper and floor leather and dipped plastic products. Some PVC resin manufacturers produce PVC resin according to the degree of polymerization (the degree of polymerization is the number of unit chain, the degree of polymerization is multiplied by the molecular weight of the chain is equal to the molecular weight of the polymer), such as the PVC resin produced by the Shandong Qilu Petrochemical plant, and the factory products are SK-700; SK-800; SK - 1000; SK - 1100; SK-1200 and so on. The degree of polymerization of the SG-5 resin is 1000 - 1100.
The PVC powder is a white powder with a density between 1.35 and 1.45g / cm3 and the apparent density is 0.4-0.5g / cm3. The size of the plasticizer can be soft and hard products. The general plasticizer content is 0-5 hard parts, 5-25 semi hard products, and more than 25 parts are soft products.
PVC is a non crystalline, polar polymer with high softening temperature and melting temperature. Pure PVC can be plasticized at 160 to 210~C, because the polar bonds between large molecules make PVC display hard and brittle properties. Moreover, PVC molecules contain chlorine groups. When the temperature reaches 120~C, pure PVC will start to react with HCl, which will lead to thermal degradation of PVC. Therefore, various additives should be added in processing to modify and impact modification of PVC, so that it can be processed into useful products.
PVC resin is mainly used in the production of various kinds of film (such as daily printing film, industrial packaging film, agricultural greenhouse film and heat shrinkable film, etc.), various kinds of plates and sheets (its sheet can be used for plastic products), all kinds of pipes (such as non-toxic water pipe, building thread pipe, transparent hose, etc.), various types of profiles (such as doors, windows, decorative plates), hollow blowing bottles ( Used in cosmetics and beverages), cables, injection products, artificial leather, floor leather, plastic toys, etc.
The pure PVC resin is sensitive to the heat pole. When the heating temperature reaches 90Y: above, a slight thermal decomposition reaction will occur. When the temperature rises to 120C, the decomposition reaction intensifies. At 150C, 10 minutes, the PVC resin becomes yellow to red to Brown - black. The decomposition process of PVC resin is caused by a series of chain reactions caused by HCL reaction. The thermal stability mechanism to prevent the thermal decomposition of PVC is achieved through the following aspects.
By capturing the HCl produced by PVC thermal decomposition, the catalytic degradation of HCl is also prevented. Besides, metal soap, organotin, phosphite and epoxide are also used. Replacements of active allyl chloride. Metal soap, phosphite and organotin can act on this mechanism. Reacts with free radicals to terminate the reaction of free radicals. The growth of the conjugated chain is inhibited by conjugated double bond addition. Organotin and epoxies are based on this mechanism. Decomposition of peroxides to reduce the number of free radicals. Passivation has a metal ion that catalyzes the removal of HCl.
The effect of lubricants is to reduce the friction between the material and the surface of the material and processing equipment, thus reducing the flow resistance of the melt, reducing the viscosity of the melt, improving the fluidity of the melt, avoiding the adhesion of the melt to the equipment, and improving the finish of the surface of the product. According to different molding methods, the lubrication effect is different.
Calendering, to prevent the melt adhesive roll;
Injection molding, improve flow and improve demoulding.
Extrusion molding improves flow and improves die separation.
Pressing and laminating are beneficial for separating the pressboard from the product.
Due to poor melt ductility of PVC melt, it is easy to cause melt fracture; PVC melt relaxation is slow, which can lead to rough surface, luster and shark skin. Therefore, PVC processing often requires additional processing aids to improve the melt defects.
The processing aids are the auxiliaries which can improve the processability of the resin. There are three main modes of action: to promote the melting of the resin, to improve the rheological properties of the melt and to give the function of lubrication.
To promote resin melting: when the PVC resin is heated in a state of heating, when the resin is melted under a certain shear force, the processing modifier first melts and adheres to the surface of the PVC resin particles. Its compatibility with the resin and its high molecular weight make the viscosity and friction of PVC increase, and thus effectively transfer the shear stress and heat to the whole PVC resin. To accelerate the melting of PVC.
Improving melt rheological properties: PVC melts have disadvantages such as poor strength, poor ductility and melt fracture, and processing modifiers can improve the rheological properties of melts. The mechanism is to increase the viscoelasticity of PVC melt, thereby improving the mold expansion and enhancing the melt strength.
Lubricity: the compatible part of the processing modifier first fused with the PVC to promote melting, while the incompatible part of the PVC moved outside the molten resin system to improve the demoulding property.