10-20 COMPOUNDING
10-20 COMPOUNDING
Modification of the polymer properties is accomplished by mixing it with various ingredients such as polymeric resins, plasticizers, fillers and reinforcing agents, various stabilizers, lubricants, coloring agents, flame retardants, and crosslinking agents, when needed.
Blending of Polymers. Blending is a process of mixing or reacting two or more polymer resins to obtain a product with improved properties. espcially impact resistance. Blend ing can be accomplished by physical blending, interpolymerization, and graft polymerization. Physical blending is achieved by milling together two incompatible polymers and heating them to above their softening points. On cooling, a two-phase system is formed that consists of a continuous matrix in which the particles of the other constituent are dispersed as spheres, cylinders, rods, or lamellae of varying dimensions. The microstructure and morphology of such a system greatly affect the toughness of the final product. A variety of microstructures are possible, and the dispersed particles may range from 1 to 20 µ m, but better properties result with particles from 1 to 5 µ m. An example of such a system is the so-called toughened polystyrene (TPS), also called high-impact polystyrene. which consists of a polystyrene matrix with a dispersed rubber phase such as potybutadiene or styrene—butadiene rubber.
Interpolymerization involves the polymerization of a styrene solution of rubber using organic peroxides or azo compounds as catalysts. With increasing polymerization a continuous matrix of polystyrene is formed in which rubber droplets form the dispersed phase. Another two-phase polymer mixture, acrylonitrile-butadiene-styrene, is produced by graft polymerization of butadiene—acrylonitrile or polybutadiene as the rubbery component and styrene-acrylonitrile as Interpolymerization involves the polymerization of a styrene solution of rubber using organic peroxides or azo compounds as catalysts. With increasing polymerization a continuous matrix of polystyrene is formed in which rubber droplets form the dispersed phase. Another two-phase polymer mixture, acrylonitrile-butadiene-styrene, is produced by graft polymerization of butadiene—acrylonitrile or polybutadiene as the rubbery component and styrene-acrylonitrile as
Plasticizers. Plasticizers are materials that are added to polymeric materials to increase their plasticity and flexibility. Plasticizers range from solvents with high boiling points to nonvolatile oils and resinous materials. There are three general types of materials used as plasticizers: (1) vegetable oils (nondrying type): (2) monomeric chemicals of high boiling points: and (3) polymeric resinous materials of low molecular weight. The action of plasticizers is considered to
be the weakening of the intermolecular forces between macromolecules. This results in greater freedom of movement of the polymeric macromolecules, increasing the flexibility and plasticity of the material but, at the same time reducing its tensile strength and chemical resistance. The
effect of plasticizers is to lower the T g of amorphous polymers and to decrease the degree of crystallinity of crystalline polymers. The amount of plasticizer added controls the final properties of plastics. For example. PVC is produced as rigid nonplasticized, rigid partially plasticized, and flexible rubbery-type polymer. The most widely used plasticizers are various phthalates.
Fillers and Reinforcing Agents. In order to counteract the weakening effect of the plasticizer, fillers and various reinforcing agents are added. Fillers improve the strength, impact resistance, heat resistance, dimensional stability, and other similar properties of the polymers. The quantitative aspect of the reinforcement effects of various fillers and fibers is discussed in Chapter 14.
Stabilizers and Other Additives. Stabilizers are added to prevent decomposition or deterioration of the polymer under the action of light, atmosphere, and other possible external environ mental factors. For example. the addition of carbon black, which is also a very effective filler, to polyethylene resin prevents photochemical degradation of the material by excluding light. Antioxidants are a special kind of stabilizer that prevent or inhibit the oxidation of the polymeric resin. Other additives used are antistats, blowing agents, colorants, organic peroxides, flame retardants, lubricants, crosslinking agents, and various processing aids.