This article gives complete industry insights on plastic materials.
Read further to learn more about topics such as:
- What is a plastic material?
- Advantages of plastics
- Production of raw plastics
- Types of plastics
- Plastic fabrication process
- And much more...
Chapter 1: What is a Plastic Material?
Plastic materials are objects artificially made from organic compounds called polymers along with other additive components. They possess excellent formability, making them extremely versatile for many different fabrication and manufacturing processes. The term “plastic” is widely used for materials that are easily shaped or molded.
Previously, metals, glass, and wood were the common raw materials. Plastics easily replaced these materials in several applications because of their attractive inherent properties. Being highly formable, they are suitable for manufacturing processes such as packaging materials and container production. Aside from formability, plastics are generally known to be lightweight, flexible, durable, corrosion-resistant, and cost-effective.
Plastics can also be engineered to have different properties—not only is their physical shape easily manipulated; their intrinsic properties are as well. Different chemical processing techniques yield varying degrees of strength, toughness, resilience, hardness, heat resistance, and so on.
History of Plastics
The invention of plastics came after several breakthroughs in the polymerization process, the main chemical process for creating plastics. Key milestones and precursors in the development of polymerization were achieved through the accidental discovery of polystyrene by Eduard Simon, the invention of Shellac by Critchlow and Peck, and the vulcanization of natural rubber by Charles Goodyear to name a few.
The first man-made plastic can be traced back to 1855 when Alexander Parkes patented Parkesine, a semi-synthetic celluloid plastic. This was considered the birth of the plastics industry. Parkesine was created by reacting a naturally occurring compound called cellulose with nitric acid, then dissolving the product in alcohol. The result was an elastic material that was made formable when heated and hardened when cooled.
Further advancement in technology led to the rise of fully synthetic plastic. The first was Bakelite. It was created in 1907 by Leo Baekeland, who also used the term “plastics”. Bakelite was formed from the reaction of phenol with formaldehyde. It was successfully mass-produced and was used as a raw material for products such as sealants, lacquers, laminations, and moldable materials.
Throughout the 20th century, the emergence of different types of plastics followed. Some of these plastics were polystyrene, polyester, PVC, polyethylene, and nylon. Mass production experienced a boom when World War II began. Plastics were extensively used in the military for producing synthetic silks, vehicle parts, and containers. After the war, the surge in demand settled. In time, production continued with the intent of satisfying the consumer goods market. Since then, the plastics industry has grown exponentially.
Chapter 2: Advantages of Plastics
Plastic materials were once considered a “wonder material.” They surpass steel in almost every aspect of engineering design. Plastics have many desirable inherent characteristics that most metals do not possess. They are also cheaper to produce, making them suitable for mass production. The only drawback in using plastics is their threat to the environment.
Below are some of the advantages of plastics.
Plastics are great materials when it comes to formability. Plastics can be molded, cast, rolled, pressed, stamped, extruded, and so on. They can be formed into complex shapes, including those that are difficult or impossible for other materials to achieve. The dies and tools used to form plastics are also easier to make.
Resistant to degradation from chemicals and water:
Plastics do not corrode or degrade the same way as metals. Metals develop rust, which weakens the structural integrity of the product. Rust also poses a threat of product contamination, especially for food and pharmaceutical products.
Plastics have densities around 0.8 to 1.5 times that of water. Steels have densities of around 7.8 times that of water, while glass and ceramics are around 2 to 3 times the density of water. This shows that plastics are significantly lighter than metals and glass, and we know they can be used for many of the same applications. Moreover, some plastics are engineered to have a high strength-to-mass ratio.
ACan be made extremely flexible or high strength:
Each type of plastic has its inherent mechanical properties. These properties are modified by compounding special additives that can improve their flexibility and strength. Examples of these additives are glass and carbon fibers. Adding fibers into a plastic matrix creates a composite material with better tensile and flexural strength.
High impact and tear resistance:
Plastics are made from long, chained molecules that arrange themselves in crystalline structures or amorphous structures. Their structure gives them their inherent elasticity. Plastics do not fail easily through brittle fracture and cracking. Tearing is an issue that is resolved by including additives or by using a polymer base with high tensile strength.
Good aesthetics and surface characteristics:
Plastic can be made into clear, translucent, or fully opaque products. They can also be made into different colors by adding pigments. When it comes to surface characteristics, plastics possess a variety of finishes and textures, negating the need for expensive secondary operations.
Long service life:
Because of its chemical and wear resistance, plastic does not degrade easily under normal conditions. This gives them a long service life. Some plastic additives further enhance their durability by imparting resistance to oxidation and ultraviolet radiation. However, the downside of their long life is their negative impact on the environment. When not managed properly, they can quickly accumulate and harm ecosystems.
Like glass and metals, some varieties of plastics can be recycled. Traditionally, plastics are recycled through heating and melting. Through heating, plastics are melted and formed into raw materials for manufacturing new plastic products. However, melting is only applicable to thermoplastics. Advanced processes are also being developed for processing other types. In general, these methods chemically convert plastics into monomers that are used as fuels for power generation.
Low production cost
Plastics are easy to form. They require less energy to produce than metal and glass. When heated, plastics are easily shaped; shaping requires only a moderate amount of pressure. Plastics can even be formed by compressed air. The temperature in their melted state is not as high as that of metals and glass. Plastics in this state can be injected and molded without the need for expensive dies and tools.
Chapter 3: Production of Raw Plastics
The manufacture of plastic goods starts with the production of raw plastics. Raw plastics are materials containing the basic properties of the polymer from which they were made. They are produced in petrochemical plants where petrochemical feedstocks are converted into raw plastics through a chemical process known as polymerization. Raw plastics are supplied to manufacturing and fabrication shops typically in liquid, powder, or pellet form. There, they are further processed to create a final product.
Polymerization is the process of forming macromolecules called polymers by joining hundreds to thousands of unit molecules called monomers. Polymerization is made possible by the fact that organic compounds contain double-bonds and active functional groups to form long-chained molecules.
Different polymerization processes exist to produce a specific type of polymer. Examples of such processes are bulk, solution, suspension, and emulsion polymerization. Each process has different chemical mechanisms to carry out the reaction.
Polymerization can also be done with one or more types of monomer feedstocks. Combining two or more types of monomers is a common way to give raw plastics better characteristics. Polymers made from more than one monomer are called copolymers.
After polymerization, plastics are further processed by blending an initial set of additives. Additives such as stabilizers and antioxidants prevent the raw plastic from degrading when exposed to air, light, or heat. This stabilizes the raw plastic so that it can endure further processing and storage.
To produce a commodity plastic with desired properties, they are further blended and compounded. Specific formulations impart a specific set of physical, mechanical, electrical, and chemical properties to the plastic. Aside from the previously mentioned stabilizers and antioxidants, plastic additives include processing aids, performance enhancers, and aesthetic modifiers.
Another set of additives such as pigments, fillers, and reinforcing materials are added to the plastic in manufacturing plants and fabrication shops. These additives give the plastic its final specifications according to the manufacturer’s standards to suit an end-use.
Chapter 4: Types of Plastic Materials
Plastic polymers can be broadly classified as thermoplastic and thermosetting polymers.
Thermoplastic polymers or thermoplastics have polymer molecules that can be repeatedly rearranged by heating and cooling. Heating thermoplastics liquifies or softens them. No chemical change takes place during this process. This is because of the absence of crosslinking that is evident in thermosetting polymers. Subsequent cooling returns the material to its solid-state. This heating and cooling process allows the plastic to be formed into different shapes.
Plastics made from these types of polymers have functional groups that form the crosslinks between the molecules. Thermosetting polymers or thermosets cannot be softened through heating. Once heated, they undergo a chemical reaction that permanently changes their properties. Processing thermosets includes an additional process called curing. Curing is the process of creating crosslinks between polymer chains. This finalizes the properties of the plastic.
In addition to being classified as thermosetting or thermoplastic, plastics are divided according to the type of polymer used in producing the raw resin.
Polyethylene is the most extensively used plastic material. PE has many desirable characteristics such as easy processability, toughness, and flexibility, which are all retained even at low temperatures. PE is odor and toxin-free and has excellent clarity, good water barrier properties, good electrical insulation properties, and a low cost. It has two main types: high-density polyethylene (HDPE) and low-density polyethylene (LDPE).
High-Density Polyethylene (HDPE)
Among the types of polyethylene, HDPE is the more dominant raw material in terms of market share. Its molecular structure is linear with little branching, resulting in higher intermolecular forces. This gives HDPE its high specific strength.
Low-Density Polyethylene (LDPE)
LDPE has a branched polymer chain that has weak intermolecular forces. This results in lower tensile strength and barrier properties. Nevertheless, it has better impact strength and resilience than HDPE.
PP is a polymer that can have a wide range of properties, which depend on its molecular weight, morphology, crystalline structure, additives, and copolymerization. It can be made into polymers with a high degree of crystallinity, resulting in higher tensile strength and hardness, which is comparable to HDPE. Moreover, it can withstand higher temperatures without loss of strength or degradation. The disadvantage of using PP is its susceptibility to UV degradation and oxidation.
PU is produced from polyester or polyether polyols, diisocyanate compounds, curatives, and additives. They are suitable for making high-performance, engineering-grade products. Their mechanical properties can vary from soft and flexible to hard and rigid.
Polyvinyl Chloride (PVC)
PVC is a plastic that can be formulated with different stabilizers, plasticizers, impact modifiers, processing aids, and other additives. It can be made into rigid or flexible plastic by modifying the amount of plasticizers. Moreover, they offer better clarity than other versatile plastics. However, PVCs have the potential to release harmful pollutants, acids, and toxins during processing or degradation. Its compounding ingredients are now being regulated by FDA, EPA, and other organizations.
Polyethylene Terephthalate (PET):
PET, specifically, biaxially oriented PET, is known for its low permeability to moisture, carbon dioxide, and alcohol. It also has an excellent intrinsic viscosity. The downside of using PET, however, is its affinity for water. It tends to absorb water, which makes processing difficult as the resin needs to be dried before extrusion.
PS is another versatile plastic modified by copolymerization and the addition of additives. They can be made into flexible, rigid, or cellular (foam) plastic forms. PS is generally prone to oxidation. Thus, repeated recycling is not recommended. Furthermore, their sensitivity to oxidation causes their color to become yellowish.
PA is considered an engineering plastic characterized by its high toughness, high impact strength, resistance to solvents, abrasion resistance, and ability to be modified to possess heat resistance. PA production mostly goes into the manufacture of fibers. Only about 10% of PA production volume is used in plastic forming processes.
Acrylonitrile Butadiene Styrene (ABS)
ABS is a common plastic material characterized by good hardness and rigidity with some degree of toughness. Protective coatings are usually applied due to the material’s poor resistance to UV and merely adequate resistance to most acids and alkalis.
PC is easily processed by different molding methods, with injection molding and sheet extrusion being the most common. Polycarbonates are known for their high impact strength, heat resistance, good electrical insulation, transparency, good water barrier properties, and inherent flame retarding properties.
PTFE is one of the most common types of fluorocarbon polymers. It has many desirable characteristics such as low coefficient of friction, self-lubrication, chemical resistance, and hydrophobicity. This makes PTFE desirable as a coating material. Its hydrophobic property also prevents the growth of microbes, which further extends its applications to food and drug manufacture.
Polymethyl Methacrylate (PMMA):
This type of plastic is also known as acrylic. It is a type of thermoplastic known to have distinctive properties such as superb transparency, lightness, tensile and flexural strength, and UV resistance. They are commonly used as a substitute for transparent glass. Examples of their applications are windows, lenses, safety barriers, and screens.
Chapter 5: Plastic Fabrication Processes
Because of their excellent formability, different fabrication methods for plastics have been developed. Plastics can easily be molded, cast, extruded, stretched, and spun. They tend to flow according to the profile of the mold or die without the need for extreme heat and pressure. After undergoing the primary fabrication processes, their mechanical properties allow them to undergo secondary processes such as trimming, cutting, grinding, drilling, gluing, and welding similar to that of metals.
Enumerated below are the primary fabrication processes for plastic materials.
Injection molding is one of the most common plastic forming processes. It involves injecting molten plastic into a closed chamber or mold. This process has four main operations:
- Heating and grinding the plastic until it flows under pressure.
- Injecting the plastic inside the mold.
- Cooling the molded plastic.
- Opening the mold to eject the product.
Injection molding is limited to producing plastic parts that are open on one side. By itself, injection molding is not suited for producing closed, hollow products such as plastic bottles. To produce these products, an inert gas is released into the mold partially filled with molten plastic. This pushes the plastic on the surface of the mold creating a hollow part. This process is known as gas-assisted injection molding.
Casting is the basic process of pouring liquid plastic into a mold without the help of pressure. This process is used in processing both thermosets and thermoplastics.
- Liquefying and blending the resin. Some resins are already in liquid form. For solid or viscous plastics, heat is applied.
- Pouring the liquid resin into the mold.
- Removing trapped air bubbles using a vacuum.
- Hardening and cooling the molded plastic. Curing is required to harden thermosets.
- Mold opening and releasing the product.
Similar to injection molding, casting is not suitable for producing hollow parts. This process is limited to producing simple, solid shapes. Moreover, additional machining processes are required to remove flashes and extra material from gates, risers, and runners.
Blow molding forms plastic hollow products by inflating a softened plastic compound inside a mold. The main operations of blow molding are:
- Heating the plastic and forming it into a tube called a parison or preform.
- Enclosing and clamping the preform between two dies.
- Inflating the preform.
- Cooling and ejecting the product.
Blow molding can be categorized into two main types: extrusion blow molding and injection blow molding. Extrusion blow molding extrudes the preform into a hollow tube suspended on one end. Injection blow molding, on the other hand, creates the preform by injecting plastic into a mold with a core for air supply. Both processes use air to shape the preform against the mold.
Rotational molding, commonly referred to as “roto molding,” is a plastic casting technique used to produce hollow and seamless plastic products. This process does not use high pressures for melt extrusion or injection. Instead, it forms the container by spreading the plastic melt on the inner surfaces of the mold through rotation. Its operation is summarized as follows:
- Loading the powdered plastic resin into the mold.
- Heating and melting the plastic while rotating the mold.
- Cooling the molded plastic.
- Demolding and unloading the product.
Since there are no high pressures involved, the molds used for rotational molding are inexpensive. This allows the fabrication of larger products with minimal investment. Rotational molding can also produce double-walled parts without any secondary processing.
Compression molding shapes the plastic resin by pressing it against two molds. This process is preferred when forming large thermosetting plastics products. The process is summarized below.
- Placing a compounded plastic charge with predefined mass onto the lower mold.
- Compressing the plastic by lowering the upper mold.
- Curing of the plastic resin.
- Cooling and removing the product from the mold.
Typically, the compression press is downward closing. Upward closing compression presses are also available. The mold has internal heating elements that soften the plastic charge. This allows the plastic to flow according to the shape of the mold. The heat also cures the plastic. During curing, some plastic may release gases that are vented through an additional phase called degassing.
Plastic extrusion is the process of forcing molten plastic through a die, producing a product with a continuous shape. This is a common method of producing films, sheets, rods, and tubes. Extrusion is also combined with other processes such as blow molding where the plastic is first processed and fed by an extruder, followed by a molding process. The operations involved in plastic extrusion are outlined below.
- Feeding the powdered or granular plastic resin into the extruder.
- Heating, kneading, compounding, conveying, and pressurizing the resin as it passes the extruder.
- Introducing the pressurized molten plastic against the die.
- Curing and cooling the final product.
Plastic extrusion covers different types of related processes based on the product. Examples are sheet and blown-film extrusion. Extrusion is also employed for applying coatings and jacketing to wires and cables.
Calendering is a forming process that involves heating and rolling a plastic mass into films, sheets, or lamination coatings. This process is widely used in processing rubbers but is now being adopted in the fabrication of thermoplastics. It consists of the following stages:
- Heating of the plastic mass.
- Squeezing the mass through an initial set of rolls, forming a continuous sheet.
- Progressive rolling to produce the desired thickness and surface qualities.
- Passing the plastic sheets into cooling rolls and a thickness gauge for final dimension checking.
Calendering is most suitable for producing multilayered products. Textile or paper are examples of materials that are fed into the final rolling stages together with the plastic sheet or film. This process creates a double-ply product that combines the strength of the main material and the surface and barrier properties of plastic.
Thermoforming is the process of heating thin plastic sheets to their forming temperature and stretching them over a mold. It is a secondary plastic forming process. It does not use raw plastic resin for compounding. Rather, it uses a plastic sheet or film produced from preliminary processes such as extrusion or calendering. The steps involved in thermoforming are:
- Heating the plastic sheet.
- Forming the plastic sheet using mechanical or pneumatic action to give its three-dimensional shape.
- Trimming the formed part from the rest of the sheet.
There are four different methods to create the three-dimensional shape of a thermoformed product. These are vacuum, pressure, mechanical, and twin sheet forming. Each method differs in how pressure is applied to create the thermoform. Vacuum, pressure, and twin sheet processes all use compressed air to press the plastic sheet against the mold. Mechanical thermoforming has two dies that press against each other to deform the plastic.
Thermoforming is limited to producing parts with relatively thin walls. Moreover, the process is prone to defects such as inconsistent thickness, webbing, and warping.
Spinning:Spinning, in plastic fabrication, refers to the method of twisting and stretching short strands into fibers with continuous lengths. The product is a synthetic fiber that can be used for making synthetic textiles, ropes, and cables. A typical spinning process involves:
- Liquefying the solid plastic resins.
- Pumping the molten polymer or polymer solution.
- Filtering and spinning the polymer into fibers.
- Solidification and cooling of fibers.
The steps mentioned above are the general operations for plastic fiber spinning. Spinning can be further divided into three main types: melt, dry and wet spinning. These processes differ in how the dimensional stability of the fiber is attained.
- Plastic materials are highly formable materials that are artificially made from organic compounds called polymers along with additive components.
- Aside from formability, plastics are generally known to be lightweight, flexible, durable, corrosion-resistant, and cost-effective.
- Polymerization is the process of converting petrochemical feedstocks into raw plastic resins. Production of raw plastic resins is done in a petrochemical plant.
- Plastic polymers can be broadly classified as thermoplastic and thermosetting polymers. They can further be divided according to their main polymer.
- Several fabrication processes for plastics include injection molding, casting, blow molding, rotational molding, compression molding, extrusion, calendering, thermoforming, and spinning.