How Plastics Are Made(1)-The Basics of Plastic Manufacturing

The Basics of Plastic Manufacturing

The term “plastics” includes materials composed of various elements such as carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur. Plastics typically have high molecular weight, meaning each molecule can have thousands of atoms bound together. Naturally occurring materials, such as wood, horn and rosin, are also composed of molecules of high molecular weight. The manufactured or synthetic plastics are often designed to mimic the properties of natural materials. Plastics, also called polymers, are produced by the conversion of natural products or by the synthesis from primary chemicals generally coming from oil, natural gas, or coal.

Most plastics are based on the carbon atom. Silicones, which are based on the silicon atom, are an exception. The carbon atom can link to other atoms with up to four chemical bonds. When all of the bonds are to other carbon atoms, diamonds or graphite or carbon black soot may result. For plastics the carbon atoms are also connected to the aforementioned hydrogen, oxygen, nitrogen, chlorine, or sulfur. When the connections of atoms result in long chains, like pearls on a string of pearls, the polymer is called a thermoplastic. Thermoplastics are characterized by being meltable. The thermoplastics all have repeat units, the smallest section of the chain that is identical. We call these repeat units unit cells. The vast majority of plastics, about 92%, are thermoplastics1.

The groups of atoms that are used to make unit cells are called monomers. For some plastics, such as polyethylene, the repeat unit can be just one carbon atom and two hydrogen atoms. For other plastics, such as nylons, the repeat unit can involve 38 or more atoms. When we combine monomers, we generate polymers or plastics. Raw materials form monomers that can be or are used to form unit cells. Monomers are used form polymers or plastics

When the connection of the carbon atoms forms two and three-dimensional networks instead of one-dimension chains, the polymer will be a thermoset plastic. Thermoset plastics are characterized by not being meltable. Thermoset plastics, such as epoxy adhesives or unsaturated polyester boat hulls and bathtubs or the phenolic adhesives used to make plywood, are created by the user mixing two chemicals and immediately using the mixture before the plastic “sets up” or cures.

The formation of the repeat units for thermoplastics usually begins with the formation of small carbon-based molecules that can be combined to form monomers. The monomers, in turn, are joined together by chemical polymerization mechanisms to form polymers. The raw material formation may begin by separating the hydrocarbon chemicals from natural gas, petroleum, or coal into pure streams of chemicals. Some are then processed in a “cracking process.” Here, in the presence of a catalyst, raw materials molecules are converted into monomers such as ethylene (ethene) C2H4, propylene (propene) C3H6, and butene C4H8 and others. All of these monomers contain double bonds between carbon atoms such that the carbon atoms can subsequently react to form polymers.

Other raw material chemicals are isolated from petroleum, such as benzene and xylenes. These chemicals are reacted with others to form the monomers for polystyrene, nylons, and polyesters. The raw materials have been changed into monomers and no longer contain the petroleum fractions. Still other raw materials can be obtained from renewable resources, such as cellulose from wood to make cellulose butyrate. For the polymerization step to work efficiently, the monomers must be very pure. All manufacturers purify raw materials and monomers, capturing unused raw materials for reuse and byproducts for proper disposition.

Monomers are then chemically bonded into chains called polymers.There are two basic mechanisms for polymerization: addition reactions and condensation reactions. For addition reactions a special catalyst is added, frequently a peroxide, that causes one monomer to link to the next and that to the next and so on. Catalysts do not cause reactions to occur, but cause the reactions to happen more rapidly. Addition polymerization, used for polyethylene and polystyrene and polyvinyl chloride among others, creates no byproducts. The reactions can be done in the gaseous phase dispersed in liquids. The second polymerization mechanism, condensation polymerization, uses catalysts to have all monomers react with any adjacent monomer. The reaction results in two monomers forming dimers (two unit cells) plus a byproduct. Dimers can combine to form tetramers (four unit cells) and so on.  For condensation polymerization the byproducts must be removed for the chemical reaction to produce useful products. Some byproducts are water, which is treated and disposed. Other byproducts are raw materials and recycled for reuse within the process.  The removal of byproducts is conducted so that valuable recycled raw materials are not lost to the environment or exposed to populations. Condensation reactions are typically done in a mass of molten polymer. Polyesters and nylons are made by condensation polymerization.

Different combinations of monomers can yield plastic resins with different properties and characteristics. When all monomers are the same, the polymer is called a homopolymer. When more than one monomer is used, the polymer is called a copolymer. Plastic milk jugs are an example of homopolymer HDPE. Milk is satisfactorily packaged in the less expensive homopolymer HDPE. Laundry detergent bottles are an example of copolymer HDPE. The aggressive nature of the detergent makes a copolymer the right choice for best service function.  Each monomer yields a plastic resin with specific properties and characteristics. Combinations of monomers produce copolymers with further property variations. So, within each polymer type, such as nylons, polyesters, polyethylenes, etc, manufacturers can custom make plastics that have specific features. Polyethylenes can be made to be rigid or flexible. Polyesters can be made to be low temperature melting adhesives or high temperature resistant automobile parts. The resulting thermoplastic polymers may be melted to form many different kinds of plastic products with application in many major markets.The variability of the plastic either within plastic family types or among family types permits a plastic to be tailored to a specific design and performance requirements. This is why certain plastics are best suited for some applications while others are best suited for entirely different applications. No one plastic is best for all needs.

Some examples of material properties in plastic product applications are:

  • Hot-filled packaging used for products such as ketchup
  • Chemical-resistant packaging used for products such as bleach
  • Impact strength of car bumpers
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Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold

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