The polymers

Polymers are chemical compounds whose molecules are very large. And they are often resembling long chains made up of a seemingly endless series of interconnected links. The size of these molecules, as is explained in the chemistry of industrial polymers, is extraordinary, ranging in the thousands and even millions of atomic mass units (as opposed to the tens of atomic mass units commonly found in other chemical compounds). The size of the molecules, together with their physical state and the structures that they adopt, are the principal causes of the unique properties associated with plastics — including the ability to be molded and shaped.

Thermoplastic and thermosetting

Polymers that are classified as plastics can be divided into two major categories: thermoplastics and thermosets. Thermoplastics such as polyethylene and polystyrene are capable of being molded and remolded repeatedly. Thus, a foamed-polystyrene cup can be heated and reshaped into a new form—for instance, a dish. The polymer structure associated with thermoplastics is that of individual molecules that are separate from one another and flow past one another. The molecules may have low or extremely high molecular weight, and they may be branched or linear in structure. But the essential feature is that of separability and consequent mobility.

Thermosets, on the other hand, cannot be reprocessed upon reheating. During their initial processing, thermosetting resins undergo a chemical reaction that results in an infusible, insoluble network. Essentially, the entire heated, finished article becomes one large molecule.

Properties

The physical state and morphology of a polymer have a strong influence on its mechanical properties. A simple measure of the differences produced in mechanical behavior is the elongation that occurs when plastic is loaded (stressed) in tension. A glassy polymer such as polystyrene is quite stiff, showing a high ratio of initial stress to initial elongation. On the other hand, polyethylene and polypropylene, two highly crystalline plastics, are usable as films and molded objects because at room temperature their amorphous regions are well above their glass transition temperatures. The leathery toughness of these polymers above Tg results from the crystalline regions that exist in an amorphous, rubbery matrix. Elongations of 100 to 1,000 percent are possible with these plastics.

Almost all plastics exhibit some elongation on being stressed that is not recovered when the stress is removed. This behavior, known as “creep,” may be very small for a plastic that is well below its Tg, but it can be significant for a partly crystalline plastic that is above Tg.

The most commonly specified mechanical properties of polymers include stiffness and breaking stress, quantified as flexural modulus and tensile strength. Another important property is toughness, which is the energy absorbed by a polymer before failure—often as the result of a sudden impact. Repeated applications of stress well below the tensile strength of plastic may result in fatigue failure.

Most plastics are poor conductors of heat; conductivity can be reduced even further by incorporating a gas (usually air) into the material. Plastics also are electrical insulators unless especially designed for conductivity. Besides conductivity, important electrical properties include dielectric strength (resistance to breakdown at high voltages) and dielectric loss (a measure of the energy dissipated as heat when an alternating current is applied).

Additives

In many plastic products, the polymer is only one constituent. In order to arrive at a set of properties appropriate to the product, the polymer is almost always combined with other ingredients, or additives, which are mixed in during processing and fabrication. Among these additives are plasticizers, colorants, reinforcements, and stabilizers. These are described in turn below.

Plasticizers

Plasticizers are used to change the Tg of a polymer. Polyvinyl chloride (PVC), for instance, is often mixed with nonvolatile liquids for this reason. The vinyl siding used on homes requires an unplasticized, rigid PVC with a Tg of 85 to 90 °C (185 to 195 °F). A PVC garden hose, on the other hand, should remain flexible even at 0 °C (32 °F). A mixture of 30 parts di(2-Ethylhexyl) phthalate (also called dioctyl phthalate, or DOP) with 70 parts PVC will have a Tg of about −10 °C (15 °F), making it suitable for use as a garden hose.

Although other polymers can be plasticized, PVC is unique in accepting and retaining plasticizers of widely varying chemical composition and molecular size. The plasticizer may also change the flammability, odor, biodegradability, and cost of the finished product.

Colorants

For most consumer applications, plastics are colored. The ease with which color is incorporated throughout a molded article is an advantage of plastics over metals and ceramics, which depend on coatings for color. Popular pigments for coloring plastics include titanium dioxide and zinc oxide (white), carbon (black), and various other inorganic oxides such as iron and chromium. Organic compounds can be used to add color either as pigments (insoluble) or as dyes (soluble).

Reinforcements

Reinforcements, as the name suggests, are used to enhance the mechanical properties of plastic. Finely divided silica, carbon black, talc, mica, and calcium carbonate, as well as short fibers of a variety of materials, can be incorporated as particulate fillers. (The use of long or even continuous fibers as reinforcement, especially with thermosets, is described below in Fibre reinforcement.) Incorporating large amounts of particulate filler during the making of plastics such as polypropylene and polyethylene can increase their stiffness. The effect is less dramatic when the temperature is below the polymer’s Tg.

Stabilizers

In order for a plastic to have a long and useful life in any application, the properties of that plastic should change as little as possible with time. Stabilizers are added, usually in small quantities, to counter the effects of aging.

Because all carbon-based polymers are subject to oxidation, the most common stabilizers are antioxidants. Hindered phenols and tertiary amines are used in plastics in concentrations as low as a few parts per million. For example, butylated hydroxytoluene (BHT) is used in polyolefin packaging films for foods and pharmaceuticals. PVC requires the addition of heat stabilizers in order to reduce dehydrohalogenation (loss of hydrogen chloride [HCl]) at processing temperatures. Zinc and calcium soaps, organotin mercaptides, and organic phosphites are among the many additives found to be effective. Other stabilizers are designed specifically to reduce degradation by sunlight, ozone, and biological agents.

Fillplas has products made of polymers. Please kindly visit our website to have more information.