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
The Structure of Polymers
As we have discussed, polymers can be homopolymers or copolymers. If the long chains show a continuous link of carbon-to-carbon atoms, the structure is called homogeneous. The long chain is called the backbone. Polypropylene, polybutylene, polystyrene and polymethylpentene are examples of polymers with homogeneous carbon structure in the backbone. If the chains of carbon atoms are intermittently interrupted by oxygen or nitrogen, the structure is called heterogeneous. Polyesters, nylons, and
polycarbonates are examples of polymers with heterogeneous structure. Heterogeneous polymers as a class tend to be less chemically durable than homogeneous polymers although examples to the contrary are numerous.
Different elements can be attached to the carbon-to-carbon backbone.
Polyvinyl chloride (PVC) contains attached chlorine atoms. Teflon contains attached fluorine atoms.
How the links in thermoplastics are arranged can also change the structure and properties of plastics. Some plastics are assembled from monomers such that there is intentional randomness in the occurrence of attached elements and chemical groups. Others have the attached groups occur in very predictable order. Plastics will, if the structure allows, form crystals. Some plastics easily and rapidly form crystals, such as HDPE—high density polyethylene. HDPE can appear hazy from the crystals and exhibits stiffness and strength. Other plastics are constructed such that they cannot fit together to form crystals, such as low density polyethylene, LDPE. An amorphous plastic typically is clear in appearance. By adjusting the spatial arrangement of atoms on the backbone chains, the plastics manufacturer can change the performance properties of the plastic.
The chemical structure of the backbone, the use of copolymers, and the chemical binding of different elements and compounds to a backbone, and the use of crystallizability can change the processing, aesthetic, and performance properties of plastics. The plastics can also be altered by the inclusion of additives.
When plastics emerge from reactors, they may have the desired properties for a commercial product or not. The inclusion of additives may impart to plastics specific properties. Some polymers incorporate additive during manufacture. Other polymers include additives during processing into their finished parts. Additives are incorporated into polymers to alter and improve basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer processing properties such as melt flow; to provide product color; and to provide special characteristics such as improved surface appearance, reduced friction, and flame retardancy.
Types of Additives:
- Antioxidants: for plastic processing and outside application where weathering resistance is needed
- Colorants: for colored plastic parts
- Foaming agents: for expanded polystyrene cups and building board and for polyurethane carpet underlayment
- Plasticizers: used in wire insulation, flooring, gutters, and some films
- Lubricants: used for making fibers
- Anti-stats: to reduce dust collection by static electricity attraction
- Antimicrobials: used for shower curtains and wall coverings
- Flame retardants: to improve the safety of wire and cable coverings and cultured marble
The Two Plastic Types, Based on Processing
A Thermoset is a polymer that solidifies or “sets” irreversibly when heated or cured. Similar to the relationship between a raw and a cooked egg, a cooked egg cannot revert back to its original form once heated, and a thermoset polymer can’t be softened once “set”. Thermosets are valued for their durability and strength and are used extensively in automobiles and construction including applications such as adhesives, inks, and coatings. The most common thermoset is the rubber truck and automobile tire. Some examples of thermoset plastics and their product applications are:
• Boat hulls
• Bath tubs and shower stalls
• Adhesive glues
• Coating for electrical devices
• Helicopter and jet engine blades
• Oriented strand board
• Electrical appliances
• Electrical circuit boards and switches
A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding, or pressing. Ice cubes are common household items which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times. Thermoplastics offer versatility and a wide range of applications. They are commonly used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and carbonated soft drink bottles. Other examples of thermoplastics are:
• Electrical insulation
• Milk and water bottles
• Packaging film
• House wrap
• Agricultural film
• Carpet fibers
• Automotive bumpers
• Microwave containers
• External prostheses
Polyvinyl Chloride (PVC):
• Sheathing for electrical cables
• Floor and wall coverings
• Automobile instrument panels
Thermoplastic and Thermoset Processing Methods
There are a variety of different processing methods used to convert polymers into finished products. Some include:
Extrusion - This continuous process is used to produce films, sheet, profiles, tubes, and pipes. Plastic material as granules, pellets, or powder, is first loaded into a hopper and then fed into a long heated chamber through which it is moved by the action of a continuously revolving screw. The chamber is a cylinder and is referred to as an extruder. Extruders can have one or two revolving screws. The plastic is melted by the mechanical work of the screw and the heat from the extruder wall. At the end of the heated chamber, the molten plastic is forced out through a small opening called a die to form the shape of the finished product. As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling or onto rollers for cooling or by immersion in water for cooling. The operation’s principle is the same as that of a meat mincer but with added heaters in the wall of the extruder and cooling of the product. Examples of extruded products include lawn edging, pipe, film, coated paper, insulation on electrical wires, gutter and down spouting, plastic lumber, and window trim. Thermoplastics are processed by continuous extrusion. Thermoset elastomer can be extruded into weatherstripping by adding catalysts to the rubber material as it is fed into the extruder.
Calendering – This continuous process is an extension of film extrusion. The still warm extrudate is chilled on polished, cold rolls to create sheet from 0.005 inches thick to 0.500 inches thick. The thickness is well maintained and surface made smooth by the polished rollers. Calendering is used for high output and the ability to deal with low melt strength. Heavy polyethylene films used for construction vapor and liquid barriers are calendered. High volume PVC films are typically made using calendars.
Film Blowing – This process continuously extrudes vertically a ring of semi-molten polymer in an upward direction, like a fountain. A bubble of air is maintained that stretches the plastic axially and radially into a tube many times the diameter of the ring. The diameter of the tube depends on the plastic being processed and the processing conditions. The tube is cooled by air and is nipped and wound continuously as a flattened tube. The tube can be processed to form saleable bags or slit to form rolls of film with thicknesses of 0.0003 to 0.005 inches thick. Multiple layers of different resins can be used to make the tube.
Injection Molding - This process can produce intricate three-dimensional parts of high quality and great reproducibility. It is predominately used for thermoplastics but some thermosets and elastomers are also processed by injection molding. In injection molding plastic material is fed into a hopper, which feeds into an extruder. An extruder screw pushes the plastic through the heating chamber in which the material is then melted. At the end of the extruder the molten plastic is forced at high pressure into a closed cold mold. The high pressure is needed to be sure the mold is completely filled. Once the plastic cools to a solid, the mold opens and the finished product is ejected. This process is used to make such items as butter tubs, yogurt containers, bottle caps, toys, fittings, and lawn chairs. Special catalysts can be added to create the thermoset plastic products during the processing, such as cured silicone rubber parts. Injection molding is a discontinuous process as the parts are formed in molds and must be cooled or cured before being removed. The economics are determined by how many parts can be made per cycle and how short the cycles can be.
Blow Molding - Blow molding is a process used in conjunction with extrusion or injection molding. In one form, extrusion blow molding, the die forms a continuous semi-molten tube of thermoplastic material. A chilled mold is clamped around the tube and compressed air is then blown into the tube to conform the tube to the interior of the mold and to solidify the stretched tube. Overall, the goal is to produce a uniform melt, form it into a tube with the desired cross section and blow it into the exact shape of the product. This process is used to manufacture hollow plastic products and its principal advantage is its ability to produce hollow shapes without having to join two or more separately injection molded parts. This method is used to make items such as commercial drums and milk bottles. Another blow molding technique is to injection mold an intermediate shape called a preform and then to heat the preform and blow the heat-softened plastic into the final shape in a chilled mold. This is the process to make carbonated soft drink bottles.
Expanded Bead Blowing – This process begins with a measured volume of beads of plastic being placed into a mold. The beads contain a blowing agent or gas, usually pentane, dissolved in the plastic. The closed mold is heated to soften the plastic and the gas expands or blowing agent generates gas. The result is fused closed cell structure of foamed plastic that conforms to a shape, such as expanded polystyrene cups. Styrofoam™ expanded polystyrene thermal insulation board is made in a continuous extrusion process using expanded bead blowing.
Rotational Molding - Rotational molding consists of a mold mounted on a machine capable of rotating on two axes simultaneously. Solid or liquid resin is placed within the mold and heat is applied. Rotation distributes the plastic into a uniform coating on the inside of the mold then the mold is cooled until the plastic part cools and hardens. This process is used to make hollow configurations. Common rotationally molded products include shipping drums, storage tanks and some consumer furniture and toys.
Compression Molding – This process has a prepared volume of plastic placed into a mold cavity and then a second mold or plug is applied to squeeze the plastic into the desired shape. The plastic can be a semi-cured thermoset, such as an automobile tire, or a thermoplastic or a mat of thermoset resin and long glass fibers, such as for a boat hull. Compression molding can be automated or require considerable hand labor. Transfer molding is a refinement of compression molding. Transfer molding is used to encapsulate parts, such as for semi-conductor manufacturing
The formation of plywood or oriented strand board using thermoset adhesives is a variant of compression molding. The wood veneer or strands are coated with catalyzed thermoset phenol formaldehyde resin and compressed and heated to cause the thermoset plastic to form into a rigid, non-melting adhesive.
Casting – This process is the low pressure, often just pouring, addition of liquid resins to a mold. Catalyzed thermoset plastics can be formed into intricate shapes by casting. Molten polymethyl methacrylate thermoplastic can be cast into slabs to form windows for commercial aquariums. Casting can make thick sheet, 0.500 inches to many inches thick.
Thermoforming – Films of thermoplastic are heated to soften the film, and then the soft film is pulled by vacuum or pushed by pressure to conform to a mold or pressed with a plug into a mold. Parts are thermoformed either from cut pieces for thick sheet, over 0.100 inches, or from rolls of thin sheet. The finished parts are cut from the sheet and the scrap sheet material recycled for manufacture of new sheet. The process can be automated for high volume production of clamshell food containers or can be a simple hand labor process to make individual craft items.
1 American Chemistry Council, Plastics Industry Producer Statistics Group, 2005