In order to impart certain specific properties, polymers are generally compounded with additives. Additives modify and improve certain characteristics such as stiffness, strength, color, weatherability, flammability, are resistance for electrical applications, and ease of subsequent processing. These additives are described below.
These materials are generally wood flour, silica flour, various minerals, powdered mica, and short fibers of cellulose, glass, and asbestos. Depending on their type, fillers improve the strength, hardness, toughness, abrasion resistance, and stiffness of plastics. These properties are maximized at various percentages of different types of polymer/tiller combination.
The effectiveness of a filler depends on the nature of the bond between the filler material and the polymer chains. Because of their lower cast, fillers also reduce the overall cost per unit weight of the polymer. Most thermoplastics and some thermosetting plastics contain fillers.
These are added to give flexibility and softness to the polymer by lowering the glass-transition temperature. Plasticizers are low-molecular-weight solvents (with high boiling points, i.e., nonvolatile) that reduce the strength of the secondary bonds between the long-chain molecules of the polymer, thus making it soft and flexible. The most common application is in polyvinyl chloride (pvc), which remains flexible during its many uses.
Most polymers are adversely affected by ultraviolet radiation and oxygen which weaken and break the primary bonds of the long-chain molecules. The polymer then becomes rigid and brittle. This is known as degradation. A typical example of protection against ultraviolet radiation is the compounding of rubber with carbon black (soot). The carbon black absorbs a high percentage of the ultraviolet radiation. Protection against degradation by oxidation, particularly at elevated temperatures, is done by adding antioxidants to the polymer. Various coatings are another means of protection against degradation.
The great variety of colors available in plastics is obtained by the addition of colorants. These are either organic (dyes) or inorganic (pigments). The selection of a colorant depends on service temperature and exposure to light. Pigments, which are dispersed particles, generally have greater resistance to temperature and light than dyes do.
If the temperature is sufficiently high a polymer will ignite. The flammability (the ability to support combustion) of polymers varies considerably, depending on their composition, such as the chlorine and fluorine content. Polymethylmethacrylate, for example, continues to burn when ignited, whereas polycarbonate extinguishes itself. The flammability of polymers can be reduced either by making them from less flammable raw materials for by adding flame retardants to the compound. Common flame retardants are chlorine, bromine, and phosphorus compounds.
To reduce friction during subsequent processing and use and to prevent sticking to the molds, lubricants are added to polymers. Lubrication is also important in preventing thin polymer films from sticking together.
To give them a rubbery behavior, polymers can be blended with small amounts of rubbery polymers. These are finely dispersed throughout the polymer and improve its impact strength. These polymers are known as rubber-modified.
Post time: 02-02-2017