Maya Ellison
Plastic is everywhere, and it's not all the same. There are hundreds of types of plastic, or polymers, but only a handful that we interact with regularly. In 1988, the Society of the Plastics Industry introduced a system to categorize plastic resins into seven groups, each with unique properties, uses, and environmental impacts. These categories are essential for understanding the complexities of recycling and upcycling, as well as the health factors associated with plastic. While plastic has benefited humanity in some ways, its excessive use has also led to environmental concerns, especially regarding waste management and plastic pollution.
| Characteristics | Values |
|---|---|
| Number of categories | 7 |
| Plastic resins introduced by | Society of the Plastics Industry |
| Year of introduction | 1988 |
| Purpose | To facilitate the recycling of post-consumer plastics |
| Safer options | Polyethylene Terephthalate (PET), High-Density Polyethylene (2-HDPE), Polypropylene (5-PP) |
| Recyclable plastics | Polyethylene Terephthalate (1-PET) and High-Density Polyethylene (2-HDPE) |
| Easily recyclable | 1, 2, and 5 |
| Plastic types | Thermoplastics and thermosets |
| Percentage of plastic manufactured that is classified as thermoplastic | 80% |
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What You'll Learn
Plastic categories: recyclable vs non-recyclable
Plastic is everywhere, but it's not all the same. There are hundreds of types of plastic, but only a handful that we interact with regularly. In 1988, the Society of the Plastics Industry introduced a system to categorize plastic resins, dividing them into seven categories to "provide a consistent national system to facilitate recycling of post-consumer plastics." These categories are:
Polyethylene Terephthalate (PET)
PET is one of the most commonly used plastics. It is lightweight, strong, and typically transparent, making it ideal for beverage bottles, food jars, and even fabrics like polyester. It is also one of the most widely recycled plastics, with a global collection of approximately 7.5 million tons in 2011. However, it is important to note that not all countries have the same recycling rates, and some, like the US and China, are still struggling to scale up their recycling efforts.
High-Density Polyethylene (HDPE)
HDPE is another commonly used plastic, known for its strength and resistance to moisture and chemicals. It is used in various products, including milk cartons, detergent bottles, and toys. HDPE is one of the easiest plastic polymers to recycle and is accepted at most recycling centers worldwide. The US recycling rate for HDPE bottles is around 30%, and it is often downcycled into products like plastic lumber and tables.
Polyvinyl Chloride (PVC)
PVC is a hard and rigid plastic that is resistant to chemicals and weathering, making it desirable for building and construction. It is also widely used in medical applications due to its impermeability to germs and ease of disinfection. However, PVC is challenging to recycle and should be avoided whenever possible.
Low-Density Polyethylene (LDPE)
LDPE is characterized by low-density molecules, giving it a thinner and more flexible design. It is commonly used in plastic bags, six-pack rings, containers, dispensing bottles, and plastic wraps. LDPE is not often recycled through curbside programs and is considered a hard-to-recycle plastic.
Polypropylene (PP)
Polypropylene is the second most widely produced commodity plastic, known for its hardness and sturdiness. It can withstand high temperatures and is used in products like Tupperware, car parts, thermal vests, and even disposable diapers. It is resistant to fatigue, making it suitable for living hinges.
Other Plastics (Group 7)
Any plastic that doesn't fit into the first six categories falls into Group 7. These plastics are often made from multiple types of plastic that cannot be easily recycled. Examples include polycarbonates (PC), which are used in eye protection, electronics, and compact discs (CDs). While some Group 7 plastics may be recyclable, they typically require specialized handling.
While recycling is critical to improving the environment and waste management, not all plastics are widely recyclable. Some plastics, like bioplastics, composite plastic, plastic-coated wrapping paper, and polycarbonate, are considered non-recyclable. Additionally, certain plastic products, such as thin bags, can clog recycling machines and are therefore not accepted by recycling programs. However, advancements in technology and environmental legislation are leading to more plastic types being collected and recycled over time.
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Thermoplastics vs thermosets
Plastic is categorized into seven different types, each with distinct characteristics and applications. These categories are identified by the Resin Identification Code (RIC) system, which was introduced to facilitate the recycling of post-consumer plastics. Despite this categorization, the world of plastic encompasses hundreds of types or polymers, with unique properties that influence their use and recyclability.
Among these diverse polymers, two prominent classes stand out in the realm of plastic injection moulding: Thermoplastics and Thermosets. These two types of plastics exhibit markedly different behaviours, advantages, and limitations. Understanding these distinctions is essential for making informed decisions about the choice of material for specific projects.
Thermoplastics are distinguished by their remarkable ability to be reshaped repeatedly without compromising their chemical structure. This unique trait stems from the nature of their bonds, which are weaker intermolecular bonds that can be broken and reformed without affecting the strong covalent bonds within the polymer chains. Consequently, thermoplastics can be heated, cooled, and remoulded multiple times, making them highly versatile and recyclable. A well-known example of a thermoplastic part is the LEGO brick, which is made from acrylonitrile butadiene styrene (ABS).
On the other hand, thermosets, also known as thermosetting plastics or polymers, undergo an irreversible transformation when heated. During the curing process, the polymers within the material 'cross-link', forming unbreakable and irreversible covalent bonds with each other. This process results in a three-dimensional network of bonded molecules, rendering the thermosets permanently set in their physical and chemical composition. Once cured, thermosets cannot be melted or reshaped, even when exposed to extreme temperatures. Common examples of thermoset plastics include epoxy, silicone, polyurethane, and phenolic.
The key advantage of thermosets over thermoplastics lies in their superior physical properties, including excellent dimensional stability, heat resistance, and chemical resistance. Thermosets are ideal for applications where parts need to maintain their shape and integrity in extreme climates or varying temperature conditions. Additionally, thermosets are easy to work with due to their low viscosity and liquid form at room temperature. The injection moulding process for thermosets also requires less heat and pressure, making it a more cost-effective and time-efficient option compared to thermoplastics.
However, the irreversible nature of thermosets presents a significant limitation: they cannot be remoulded or recycled. This contrasts with thermoplastics, which offer the advantage of recyclability due to their remouldable nature. Thermoplastics are also preferred in certain product applications, despite the emergence of thermosets as attractive alternatives.
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Polyethylene: High-Density, Low-Density, and Linear Low-Density
There are hundreds of types of plastic, but only a handful are commonly used. In 1988, the Society of the Plastics Industry introduced a system to classify plastic resins into seven categories, each with unique properties, uses, and recycling considerations. One of the most common types of plastic within these seven categories is Polyethylene, which is further classified into three types: High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Linear Low-Density Polyethylene (LLDPE).
High-Density Polyethylene (HDPE) is known for its strength and resistance to moisture and chemicals. Its durability makes it ideal for various applications, including cartons, containers, pipes, and building materials. Examples of products made from HDPE include milk cartons and bottles, detergent bottles, cereal box liners, toys, buckets, park benches, and rigid pipes. HDPE is one of the safer plastic options and is commonly picked up by recycling programs.
Low-Density Polyethylene (LDPE), in contrast, is characterized by low-density molecules, resulting in a thinner and more flexible design. It has the simplest structure among plastics, making it easy and inexpensive to produce. LDPE is commonly used in plastic bags, six-pack rings, dispensing bottles, and plastic wraps. However, it is not often recycled through curbside programs.
Linear Low-Density Polyethylene (LLDPE) combines the best characteristics of LDPE and HDPE. It offers improved strength, flexibility, and impact resistance compared to LDPE, while being more cost-effective than HDPE. LLDPE is commonly used in packaging applications, such as plastic bags, stretch wrap, and product packaging. It is also used in the manufacturing of toys, containers, and various consumer products.
Polyethylene, in all its forms, is incredibly versatile and widely used. However, it is essential to understand the specific characteristics and applications of each type to make informed decisions about their use and proper disposal or recycling. Proper waste management and recycling practices are crucial to mitigating the environmental impact of plastic waste.
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Polypropylene: a flexible thermoplastic
Plastic is broadly categorized into two types: thermoplastics and thermosets. However, there are hundreds of types of plastic, or polymers, with seven categories of plastic resins defined by the Society of the Plastics Industry's Resin Identification Code (RIC) system. These seven categories are further divided into numerous types of plastic, including LDPE, PE, Polyester, HDPE, PVC, LDE, PP, PS, and PC.
Polypropylene (PP) is a flexible thermoplastic that is harder than PE but retains flexibility and does not crack under repeated stress. It is also durable, heat-resistant, acid-resistant, and cheap, making it ideal for laboratory equipment, automotive parts, medical devices, and food containers. Polypropylene is the second most widely produced commodity plastic, with a forecasted market growth in the coming years. Its strength and resistance to fatigue make it suitable for living hinges, the thin piece of plastic that allows a product part to move. Polypropylene is considered a safer plastic option and is easily recyclable, although not all recycling programs accept it.
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Plastic additives: plasticizers, flame retardants, fillers, colourants, etc
Plastic is a complex material with hundreds of types (also called polymers), though only a handful are commonly encountered. The seven most common types of plastic are numbered according to their recycling codes, with the seventh category serving as a catch-all for plastics that don't belong in the other six categories. These seven types of plastic are:
- Polyethylene Terephthalate (PET)
- High-Density Polyethylene (HDPE)
- Polyvinyl Chloride (PVC)
- Low-Density Polyethylene (LDPE)
- Polypropylene (PP)
- Polystyrene (PS)
- Polycarbonate (PC)
Each type of plastic has unique properties, such as flexibility, hardness, and resistance to heat and chemicals. These properties are influenced by additives, which are essential compounds that improve a plastic's functionality. Additives are added to plastics depending on their intended application and can account for a significant portion of the plastic's composition.
Plastic additives can be classified into several groups, including functional additives, fillers, reinforcements, and colourants. Functional additives are the most critical group as they enhance the functional properties of plastics. They include stabilizers, flame retardants, plasticizers, curing agents, and antioxidants. Flame retardants and plasticizers are especially prevalent in indoor environments, where they are released from various sources under normal living conditions and accumulate in the air and dust. Phthalic acid esters and phosphororganic compounds (POCs) are commonly used as flame retardants and plasticizers and are known as semivolatile organic compounds (SVOCs).
Filler additives, meanwhile, are used to reinforce and bulk up plastics. They can be made from natural or synthetic materials, with common natural fillers including wood flour, talc, and calcium carbonate, and common synthetic fillers including glass fibres and carbon black. Colourants are another important group of additives, though they can cause issues in the recycling process by resulting in dark-coloured or black recycled products. As a result, there is a growing interest in developing additive extraction methods to remove unwanted colourants and other additives, thereby broadening closed-loop recycling options.
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