- What is plastic?
Although plastic has only been around for just over a hundred years, it is one of the most widely developed and applied materials in human history. This rapid development is primarily due to its excellent performance. Essentially, plastic is a synthetic or semi-synthetic material made from petroleum. It is easy to process and is used in methods such as injection molding, extrusion, and blow molding.
- Properties of plastic
Some of the prominent properties of plastic include:
2.1 Plasticity
The raw material used to produce plastic is petroleum, which is abundant, making plastic production cost-effective. We can significantly reduce production costs due to its thermoplastic properties, even though the cost of oil is not low.
Plasticity refers to the ability to form a new shape after being cooled, having been softened by heat. This allows for the creation of products with complex shapes, which are easier to produce and more efficient in processing compared to metals. Over the past 100 years, numerous inventions and improvements in plastic molding processes, especially injection molding, have made it possible to create complex products with ease.
2.2 Low density
With the development of the plastic industry, materials like wood, steel, fabric, shoes, and many other traditional materials have been largely replaced by plastic. Some plastics, like polyethylene (PE) and polypropylene (PP), have a density of less than 1g/cm³, which means they can float on water. The density of foam plastic is around 0.1g/cm³. Some plastics, such as PVC, have a density of 1.4g/cm³, and PTFE has a density of 2.2g/cm³, but they are still lighter than metals and ceramics.
Although plastics have low density, their strength is high. For example, nylon has one-tenth the density of steel, but its bending strength is half that of steel. While plastics are not as hard as metals, when compared to metals and ceramics, they are lightweight, have good mechanical properties, high mechanical strength, and are wear-resistant.
This allows for the creation of lightweight products that are still durable. Currently, a portion of plastics is used to produce many components of automobiles, with plastics accounting for around 50% of an automobile’s material, reducing the car’s weight to a third. A lighter car results in lower energy consumption.
2.3 Corrosion resistance
Corrosion resistance is an important issue that needs to be addressed. In China alone, the direct economic loss due to corrosion is at least 20 billion yuan annually. Most plastics have strong corrosion resistance and do not react with acids or alkalis, which is why they are commonly used for chemical containers. Plastics do not rust or decay in humid environments, nor are they corroded by microorganisms. Therefore, plastics are often used for windows and doors in buildings and stores.
2.4 Excellent electrical and thermal insulation
The polymer chains in plastics are formed by covalent bonds, not ionic or donor-acceptor bonds. This gives them an electrical resistance of 10^14 ~10^16 Ω. As a result, plastics are excellent electrical insulators and can be used for electrical switches, household electrical devices, insulation for wires and cables. They are widely used in fields such as electronics, radar, television, telecommunications, multimedia, and computers.
One important point to note when utilizing plastics is their weaknesses: poor heat resistance, deformation at high temperatures, and flammability. They are also prone to oxidation under the influence of light, oxygen, heat, water, and humidity. Additionally, due to their low surface hardness, plastic products are more prone to damage. Being insulative, plastics are also susceptible to static electricity and dust accumulation.
The main drawback of plastics is that they do not biodegrade, which is extremely harmful to the environment. They do not decompose even if buried for hundreds of years. As society accelerates its pace of life to meet the demands for convenience and health, single-use food containers, plastic bags, and other products are proliferating in daily life. Although the convenience and low cost of plastics bring many benefits, they pose significant risks to ecosystems and the environment if improperly disposed of. This phenomenon is referred to as “white pollution.”
- Applications of plastic
With its excellent properties, plastic can be processed into a wide range of products, from soft materials like silk to hard ones like steel, and clear materials like glass. Today, plastic products are present in every aspect of life. It is not an exaggeration to say that without plastic, there would be no life as we know it. Key applications include:
- Structural materials
- Insulating materials
- Building materials
- Containers
- Consumer goods
- Electrical wire coatings, electrical panels, electrical components
- Plastic pipes, gas pipes
- Plastic bags, synthetic fabric bags, plastic boxes
- Office supplies, toys
- Transport tools, vehicle parts
- Fabrics, carpets, artificial grass
- Types of plastic
There are over 300 types of plastic produced worldwide, and classification methods vary and can overlap. For convenience, we can classify plastics based on their physical properties, application properties, and processing methods.
4.1 Classification by physical properties
Based on physical properties, plastics can be divided into two types: thermosets and thermoplastics. Thermoplastics have linear or branched molecular structures that soften under high temperatures and harden again when cooled, and this process can be repeated multiple times without changing their physical state. Therefore, waste or scrap from processing can be recycled by grinding. Thermoplastics include PE, PP, PVC, PS, ABS, PA, PO, PC, PMMA, etc.
Thermosetting plastics are flexible with a linear structure before heating. When heated, the polymer chains can form chemical bonds, creating a network of cross-links that form an irreversible three-dimensional structure. They do not melt or dissolve after this process, and their structure cannot change again. Both chemical and physical transformations occur during processing. Waste or scrap from thermosetting plastics cannot be reused.
4.2 Classification by processing methods
Plastics can also be classified according to processing methods such as casting, injection molding, and blow molding.
4.3 Classification by product
Plastics can be divided into products such as plastic films, plastic pipes, cable coatings, building materials, and plastic foams.
- Methods of coloring plastics
Coloring refers to the process of changing the color of an object or adding color to a colorless material. Color is exhibited through the selective absorption and reflection of certain light wavelengths, which can be classified as pigments and dyes.
Dyes are synthetic organic substances that can dissolve in most solvents and dyeing agents. Dyes have good transparency, strong coloring ability, and low density. Unlike dyes, pigments do not dissolve in water, oils, or plastics. They are typically dispersed in plastic and give color to the material.
Furthermore, pigments do not react with the material being colored and can only be evenly dispersed in plastic through physical methods to achieve optimal coloring performance. Despite these differences, it is difficult to define them precisely. Pigment classification is not clear-cut. Some inorganic pigments may dissolve in certain types of polymers. For example, Red 254 (DDP red) is a bright red pigment that does not dissolve in most polymers.
Dyes are organic compounds, while pigments can be either inorganic or organic. They differ in properties. Inorganic pigments are salts or oxides of metals. Inorganic pigments of the same type have many common features. They are insoluble in common solvents and have better heat resistance than organic pigments. On the other hand, organic pigments generally have higher color intensity.
In summary, colors used for plastics include organic pigments, inorganic pigments, and dyes.