Chloe Zhang
Plastic products are often identified by the numbers at the bottom of plastic containers. These are known as Resin Identification Codes (RICs), a standardised numbering system developed by ASTM International. There are seven main types of plastics, each with slight distinctions in how they feel, their use, colour, and chemical composition. For example, PET (polyethylene terephthalate) plastic bottles are often clear, strong, and lightweight, and commonly used for shampoos and lotions. HDPE (high-density polyethylene) plastic bottles are known for their strength and versatility, and are commonly used for liquid products in the food, chemical, and pharmaceutical industries.
| Characteristics | Values |
|---|---|
| Chemical structure | Polymer's backbone and side chains |
| Chemical process used in synthesis | Condensation, polyaddition, cross-linking |
| Physical properties | Hardness, density, tensile strength, thermal resistance, glass transition temperature |
| Resistance and reactions | Exposure to organic solvents, oxidation, ionizing radiation |
| Manufacturing or product design | Thermoplastics, thermosets, conductive polymers, biodegradable plastics, engineering plastics, elastomers |
| Toxicity | PVC is widely considered the most toxic and hazardous |
| Resin Identification Codes (RICs) | A standardized numbering system developed by ASTM International |
| Synthetic plastic | Derived from crude oil, natural gas, or coal |
| Biobased plastic | Derived from renewable products such as carbohydrates, starch, vegetable fats, and oils |
| Identification | Look at the number at the bottom of plastic containers |
| Plastic type | Various types of plastics |
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What You'll Learn
Resin Identification Codes (RICs)
Plastic products are often identified by Resin Identification Codes (RICs), a standardised numbering system developed by ASTM International. The RIC system was introduced in 1988 by the Society of the Plastics Industry, and categorises plastic resins into seven different categories. The purpose of the system is to "provide a consistent national system to facilitate the recycling of post-consumer plastics".
The seven categories are as follows:
- PET (polyethylene terephthalate): PET plastic bottles are often clear, but can be dyed different colours. They are strong, lightweight, and have high impact and chemical resistance levels. PET bottles are often used for shampoos, lotions, and other personal care products.
- HDPE (high-density polyethylene): HDPE plastic bottles are known for their strength and versatility. They have good impact and chemical resistance and are commonly used for liquid products in the food, chemical, and pharmaceutical industries.
- PVC (polyvinyl chloride): PVC is a well-known and commonly used plastic that can be manufactured to possess rigid or flexible properties. It is often used in construction materials, bottles, and non-food packaging.
- LDPE (low-density polyethylene): LDPE has a thinner and more flexible design and is the simplest structure of all the plastics, making it easy and cheap to produce. It is often used in plastic bags, six-pack rings, and plastic wraps.
- PP (polypropylene): Polypropylene is hard and sturdy, withstanding high temperatures. It is found in products such as Tupperware, car parts, thermal vests, and disposable diapers.
- PS (polystyrene): Polystyrene can be solid or foamed and is used in a variety of applications, including laboratory bottles, disposable cutlery, and packing peanuts.
- Other: Any resin that is not categorised in the previous six classifications is combined to make type #7. This group includes polycarbonates (PC), which are used to build strong and tough products, such as lenses for sunglasses and CDs.
It is important to note that not all plastics are easily identifiable by their RICs, such as polyurethanes (PURs), which encompass many chemically diverse formulations.
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Polymer resin blends
Plastic products are identified by manufacturers through Resin Identification Codes (RICs), a standardised numbering system developed by ASTM International. There are seven main types of plastics, each with slight distinctions. The differences can range from how it feels and how it is used to colour and chemical composition.
Polymer blends are also used in the food-packaging industry to improve the processability of a natural polymer to suit industrial applications, improve the physical, chemical, and mechanical properties of the biodegradable material, and adapt to frequent changes in demand for new materials.
Waterborne acrylic resins/polyester polymer blends are used as binders in coatings, paints, and ink systems. These blends exhibit superior pigment grinding and film properties compared to polyester or acrylic polymer alone.
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Chemical structure
Plastics are classified by the chemical structure of the polymer's backbone and side chains. The polymerisation process generates thick, viscous substances as resins, which are used to make plastic products.
Plastics are derived from natural, organic materials such as cellulose, coal, natural gas, salt and crude oil. Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used. The production of plastics begins with the distillation of crude oil in an oil refinery, which separates the heavy crude oil into groups of lighter components, called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds made up of carbon and hydrogen), which differ in terms of the size and structure of their molecules.
One of these fractions, naphtha, is the crucial compound for the production of plastics. Two main processes are used to produce plastics – polymerisation and polycondensation – and they both require specific catalysts. In a polymerisation reactor, monomers such as ethylene and propylene are linked together to form long polymer chains. The monomer used in LDPE and HDPE is ethylene, but there is a difference in the degree of branching.
The largest application for plastics is as packaging materials, but they are used in a wide range of other sectors, including construction, textiles, consumer goods, transportation, electronics and as machine parts.
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Physical properties
Plastic products are identified by manufacturers through their physical properties, which vary depending on the type of plastic.
Plastics are polymers that can be moulded into desired shapes and sizes when soft and then hardened to produce durable items. The word 'plastic' is derived from the Greek word 'plastikos', which means 'to mould'. They are lightweight, chemically stable, and easily moulded, with good impact resistance, transparency, and wear resistance. However, they have poor dimensional stability and can be easily deformed.
Plastics can be broadly categorised into two types: thermoplastics and thermosetting plastics (or thermosets). Thermoplastics, such as polyethylene (PE), polyvinyl chloride (PVC), and polystyrene (PS), can be softened when heated and then cut or moulded again. They tend to be less stiff, softer, and more malleable than thermosetting plastics. Thermoplastics are commonly used in packaging, cable jacketing, pipes, and industrial fabrics due to their good insulation properties.
On the other hand, thermosetting plastics can only be moulded once and cannot be softened with heat. Examples include vulcanized rubber, Bakelite, and polyurethane. These plastics degrade and become damaged when exposed to high temperatures.
The physical properties of plastics can be altered by adding other materials or additives during processing and manufacturing. These additives include plasticizers, colourants, reinforcements, and stabilisers. Additionally, the density and degree of branching in the structure of plastics like polyethylene can result in different mechanical properties and melting points.
Overall, the physical properties of plastic products play a crucial role in their identification and suitability for various industrial applications. Manufacturers carefully consider these properties to ensure the plastic material meets the specific requirements of the intended use.
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Synthetic vs biobased
Plastic is an essential material in modern life, but conventional plastics derived from fossil fuels are a significant contributor to climate change. Bioplastics, which are plastics produced from renewable biomass sources, have emerged as a more sustainable alternative. However, the term "bioplastics" encompasses a wide range of materials, and it is important to distinguish between synthetic and biobased plastics to make informed decisions about their environmental impact.
Synthetic plastics, also known as conventional or fossil-fuel plastics, are derived from petroleum or natural gas. They have been widely used due to their durability and low cost. However, they have come under scrutiny for their environmental impact, particularly their contribution to greenhouse gas emissions and pollution. Synthetic plastics often contain harmful chemicals, such as phthalates and brominated flame retardants, which can leach into the environment during production and end-of-life disposal. While some synthetic plastics are biodegradable, the majority are not, leading to their accumulation in the environment.
On the other hand, biobased plastics are derived from biological resources such as vegetable fats and oils, corn starch, straw, woodchips, and recycled food waste. They are marketed as a more sustainable alternative due to their renewable and biodegradable nature. Biobased plastics have a lower carbon footprint compared to synthetic plastics and can be compatible with existing recycling streams. Additionally, they can be used for foodservice packaging, providing compostability options at the end of their life.
However, it is important to note that not all biobased plastics are biodegradable, and their environmental benefits depend on their full life cycle assessment. The degradation and disposal of biobased plastics must be carefully managed to ensure they decompose in a controlled manner. Unregulated biobased plastics can have negative environmental impacts, similar to synthetic plastics. Additionally, biobased plastics face challenges such as higher costs, competition with food production, and unclear end-of-life management.
To address the confusion around different types of plastics, standardization and certification bodies have developed standards for biobased, biodegradable, and compostable plastics. These standards aim to evaluate the environmental constraints and ensure the overall environmental impact of their production and consumption is positive. However, as of 2025, there is no comprehensive EU law or international sustainability criteria specifically for biobased plastics.
In conclusion, while biobased plastics offer a promising alternative to synthetic plastics, a comprehensive understanding of their environmental impact throughout their life cycle is crucial. Careful assessment, regulation, and responsible management of these materials are necessary to ensure their sustainability and prevent potential trade-offs that may outweigh their benefits.
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Frequently asked questions
Plastic products can be identified by looking at the number at the bottom of the plastic containers. There are seven main types of plastics, each with slight distinctions. The seven plastic resin identification codes were introduced by the Society of the Plastics Industry in 1988.
The seven main types of plastics are: PET (polyethylene terephthalate), HDPE (high-density polyethylene), LDPE (low-density polyethylene), Polypropylene, Polystyrene, Polycarbonate (PC), and Polyethylene.
Resin Identification Codes (RICs) are a standardized numbering system developed by ASTM International to identify different types of plastics. RICs are used to facilitate the recycling of post-consumer plastics.
