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Plastic containers are material storage products made of a variety of plastics. Plastics are an ideal material for creating containers. They are known for their excellent formability, degradation resistance, durability, lightweight, recyclability, and controllable mechanical properties.
Plastic has been around as early as the 1840s when a German named Eduard Simon discovered polystyrene. This finding was followed by the discovery of celluloid. These breakthroughs were the earliest demonstrations of the polymerization process, the main chemical process for creating plastics. The first plastic containers were plastic bottles. The boom in plastic bottle production came in the 1930s which used cellulose acetate as raw material. Further development led to the use of low-density polyethylene (LDPE) and polyvinyl chloride (PVC) which were more suitable for mass production.
Today, plastic containers are available in different shapes and sizes. Advances in manufacturing methods brought large bulk containers, thin and transparent clamshell packs, and impact-resistant plastic cases. The global market size of plastic containers is estimated to be around $86 billion in 2018. This is expected to grow by 4% from 2019 to 2025. Examples of industries that have a high demand for plastic containers are beverage, food, pharmaceutical, cosmetics, household consumer goods, and industrial lubricants, and petroleum products.
The most commonly used material for plastic containers is polyethylene terephthalate (PET). It is used for producing mineral water, carbonated drinks, and juice bottles. Next is HDPE which is used for the packaging of household goods such as shampoo, cleaning materials, detergent, etc.
The basic chemicals for producing plastics typically come from petroleum and other fossil fuel-based compounds. Examples of such chemicals are ethylene, propylene, styrene, and acetylene. These monomers are simple organic molecules with a double bond and some functional groups. The presence of double bonds and functional groups allows monomers to react with one another to form long, chained molecules or polymers. This process is known as polymerization.
One or more types of monomers can be polymerized to form a polymer chain. Combining two or more types of monomers is a common way to give plastics better characteristics. The resulting polymers are then mixed with different ingredients through the process of compounding. Additives, fillers, and reinforcements are compounded to add additional properties intended to suit a particular application. Examples of these imparted properties are thermal stability, aging resistance, flame resistance, clarity, and color. After compounding, plastics are ready to be formed into different products including containers.
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 it. 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, it undergoes a chemical reaction that permanently changes its properties. Processing thermosets include an additional process called curing. Curing is the process of creating the crosslinks between polymer chains. This finalizes the properties of the plastic.
Since its mass production from the 1950s, plastics were regarded as the "wonder material". Though considered as an environmental problem today, it will be difficult for other materials to take their place. Plastics are cheap, light, strong, tough, and do not corrode. That is why plastics are suited not only for producing plastic containers but other everyday products as well. Below are some of the advantages of plastic containers.
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. The dies and tools used to form plastics are also easier to make. Forming plastic containers does not require as much pressure as that of glass or metal containers forming.
Plastic containers do not corrode or degrade the same way as metals. Metals develop rust which weakens the structural integrity of the container. Rust also poses a threat of product contamination, especially for food and pharmaceutical products. Only a few materials can compare to the degradation resistance of plastic. An example is glass which has many limitations when used as containers.
Plastics have densities around 0.8 to 1.5 times that of water. Steels have densities of around 7.8 times while glass and ceramics are around 2 to 3 times. This just shows that plastics are significantly lighter than metals and glass for the same application. Moreover, some plastics are engineered to have a high strength-to-mass ratio. Containers made from these plastics have thinner walls which further reduces their mass.
Plastic containers can be made from different types of chemicals and through various processes. Each type of plastic has its inherent mechanical properties. These properties are modified by compounding special additives that can improve its 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.
Plastics are made from long, chained molecules that arranged 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.
The appearance of plastic containers is fully customizable. Plastic containers can be made into clear, translucent, or fully opaque products. They can also have different colors by adding pigments. When it comes to surface characteristics, plastics have a variety of finishes and textures without using expensive secondary operations.
Because of their chemical and wear resistance, plastic containers do not degrade easily under normal conditions. This gives them their long service life. Some plastic additives further enhance their durability by imparting resistances 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, plastic containers can be recycled. Traditionally, plastics are recycled through heating and melting. By 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. In general, these methods chemically convert plastics into monomers that are used as fuels for power generation.
Plastic containers are easy to form. They require less energy to produce than metal and glass containers. When heated, plastics are easily shaped which requires only a moderate amount of pressure. Plastics can even be formed by compressed air. The temperature in its melted state is not as high as metals and glass. Plastics in this state can be injected and molded without the need for expensive dies and tools.
Most plastic containers have a number embossed at the bottom. These numbers indicate the type of plastic that is used in making the container. This system allows a straightforward identification for proper segregation during recycling. The type of plastic is determined by the type of polymer used in creating the resin. Each polymer type is processed and recycled differently.
A good rule to follow is illustrated below. Numbers three and six are not readily accepted by most recycling systems. The rest can be recycled normally.
Numbers one to six are common materials enumerated in order below. The last number, number seven, is composed of miscellaneous plastics such as polycarbonate and ABS.
PET is the most used plastic material for plastic containers. This plastic is known for its low permeability to carbon dioxide, particularly the biaxially oriented type. This makes the material desirable for producing bottles for carbonated beverages. PET containers used in food and beverage packaging are designed to be for single use only. This plastic is prone to develop bacterial growth.
HDPE is a type of polyethylene (PE). It is characterized as having a high strength-to-density ratio. Its molecular structure is linear with little branching that results in higher intermolecular forces. HDPE is commonly used for making stiffer containers and bottles. Examples are water gallons, milk jugs, detergent bottles, and plastic drums. HDPE can both be reused or recycled. It is considered one of the safest forms of plastics.
Polyvinyl Chloride is a type of 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 containers 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.
Low-density Polyethylene is another type of PE. It has a branched polymer chain linked by weak intermolecular forces. This results in lower tensile strength and barrier properties. Nevertheless, it has better impact strength and resilience than HDPE. They are used for making flexible containers and bottles such as squeezable bottles, dispensing bottles, and sampling containers.
Polypropylene is a versatile polymer that can have a wide range of properties. Its properties depend on its molecular weight, morphology, crystalline structure, additives, and copolymerization. It can be made into polymers with a high degree of crystallinity. This results in higher tensile strength and hardness that is comparable to HDPE. Moreover, they can withstand high temperatures without loss of strength or undergoing degradation. The disadvantages of using PP are its susceptibility to UV degradation and oxidation. PP is commonly used for producing drums and pails.
Similar to Polypropylene, Polystyrene is another versatile plastic modified by copolymerization and the addition of additives. They can be made into flexible, rigid, or cellular (foam) plastic forms. Polystyrene is generally prone to oxidation. Thus, repeated recycling is not recommended. Furthermore, their sensitivity to oxidation causes their color to become yellowish. PS is often used for making disposable containers such as clamshell food packaging and disposable cups.
Polycarbonate is easily processed by different molding methods, with injection molding and sheet extrusion being the most common. Its usual applications are beverage bottles and food storage containers. Polycarbonates are known for their high impact strength, heat resistance, good electrical insulation, transparency, good water barrier properties, and inherent flame retarding properties.
Acrylonitrile Butadiene Styrene is a common plastic material characterized by having good hardness and rigidity while having some degree of toughness. Protective coatings are usually applied due to their poor resistance to UV and only adequate resistance to most acids and alkalis. They are commonly used for making battery containers.
Polyamide is considered an engineering plastic characterized by its high toughness, high impact strength, resistance to solvents, good abrasion resistance, and heat resistance. The common application of PA is automotive fuel tanks.
The type of plastic forming process employed depends on the form of the plastic container. Plastic containers can be in the form of intermediate bulk containers (IBCs), drums, bottles, bags, and enclosures. Each type of plastic container has its distinct shape that suits one type of fabrication process better than the others. Below are the common methods for producing plastic containers.
Injection molding is a plastic forming process that involves injecting molten plastic into a closed chamber or mold. This process has three main operations:
The most popular injection molding machine is a reciprocating screw-type extruder. Plastic is compounded by the repeated kneading and mixing of the screw. When the plastic is ready to be injected, the screw moves to push the plastic out of the extruder and into the mold.
The mold contains the shape of the plastic container. It is usually composed of two halves. One is stationary while the other can move. After molding, one half moves to release the product. The mold has several openings or channels. These are used for introducing plastic into the mold, for venting air, and for permitting some plastic to flow out of the mold.
Injection molding is limited to producing containers that are open on one side. Examples are pails, tubs, cups, bowls, and food containers. By itself, injection molding is not suited for producing closed, hollow products such as plastic bottles. To produce these products, an inert gas is introduced 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.
Blow molding forms plastic containers by inflating a softened plastic compound inside a mold. The main operations of blow molding are:
As the injection molding process, blow molding also uses screw extruders for compounding and heating the plastic. In creating the preform, the process is divided into two 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.
Blow molding is best suited for producing hollow plastic products such as bottles, water jugs, tube packaging, intermediate bulk containers (IBCs), and storage tanks.
Rotational molding, or commonly referred to as "roto molding", is a plastic casting technique used to produce hollow and seamless plastic containers. This process does not use pressure to form its product. 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:
There are different rotational molding machines. They basically differ on the number and configuration of the molds and how the molds are heated and rotated. Examples of rotational molding machines are clamshell, turret, shuttle, and swing machines.
Since there are no high pressures involved, the molds used for rotational molding are inexpensive. Larger products can be formed. Rotational molding can also produce double-walled containers without any secondary processing.
Some products of rotational molding are industrial and agricultural storage tanks, drums, carboys, insulating boxes, and trash bins.
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:
There are four main types of thermoforming. These are vacuum, pressure, mechanical, and twin sheet forming. Each method differs on how pressure is applied to create the thermoform. Vacuum, pressure, and twin sheet thermoforming 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. It is not suited for producing containers requiring rigidity.
Common thermoforming containers are those used for food and pharmaceutical applications. They are designed for single use. Examples are disposable cups, clamshell containers, and trays.
Compression molding shapes the plastic resin by pressing it against two molds. This process is more preferred when forming large thermosetting plastics products. The process is summarized below.
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.
Compression molding is also used for molding containers with fiber components. Glass or carbon fibers can be added while preforming the plastic charge.
Like injection molding which uses two mold halves, compression molding is limited to producing containers that are open on one side. Examples of compression molding containers are large bins, tubs, and trays. Plastic caps for bottles can also be made by compression molding.
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