Omar Khalid
Plastic is a synthetic material composed of organic polymers that can be moulded into a variety of shapes and sizes. It is a broad term that encompasses various chemicals and synthetic materials. The chemical properties of plastics vary depending on their composition, with some plastics being biodegradable and others non-biodegradable. They are generally classified according to the chemical structure of their polymer base and side chains, with categories including acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. The versatility of plastics is due to their adaptability and unique properties such as low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production.
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What You'll Learn
Plastic's resistance to chemical reagents
Plastic is a synthetic material created from a broad range of organic polymers that have become indispensable in our daily lives. Plastics are generally classified according to the chemical structure of the polymer base and side chains. They are also classified by the various physical properties they possess, such as tensile strength, hardness, heat resistance, density, and glass transition temperature.
Plastics have unique chemical properties, such as their resistance to certain chemical reagents or their chemical reactivity. The chemical composition of the polymer constituting the plastic determines its resistance to chemical reagents. For instance, plastics do not rust and are less susceptible to chemical reactions that affect metals, such as oxidation or rusting. They are also resistant to corrosion.
The resistance of plastics to chemical reagents is evaluated using ASTM D543, a standard developed by ASTM International. This standard is widely used in the polymer industry to assess how different plastic materials withstand exposure to various chemicals. It is crucial for manufacturers, engineers, and quality control professionals to ensure the durability and suitability of plastic products in diverse applications. The standard covers the evaluation of all plastic materials, including cast, hot-molded, cold-molded, laminated resinous products, and sheet materials.
The evaluation procedures include practices such as immersion tests, mechanical stress, and reagent exposure under standardized conditions of applied strain. These practices involve measuring changes in weight, dimensions, appearance, colour, strength, and other mechanical properties. The choice of reagent types and concentrations, duration of exposure, level of stress, and temperature are important factors in these evaluations.
The resistance of plastics to chemical reagents is crucial in various applications, such as in the automotive industry, where plastics must resist exposure to fuels, oils, and coolants, and in the medical field, where plastics must withstand disinfectants and bodily fluids.
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Plastic's chemical reactivity
Plastic is a synthetic or semi-synthetic material created from organic polymers. The defining characteristic of plastics is their plasticity, which allows them to be moulded, extruded, or pressed into various solid forms. This plasticity is often combined with other properties such as low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production.
The chemical properties of plastics vary depending on their composition. For example, halogen-containing polymers like PVC are inherently flame retardant due to the release of halogen gases that interrupt the free radical oxidation chain reaction when heated. On the other hand, the addition of plasticizers to PVC makes it flammable. The combustion products of plastics are generally similar to those of wood, paper, and textiles due to their similar chemical components. However, the combustion products depend on the burning conditions, and insufficient oxygen can lead to the formation of toxic carbon monoxide and smoke.
Plastics can be classified into two main categories: thermoplastics and thermosets. Thermoplastics do not undergo chemical changes when heated and can be remoulded multiple times. Their chemical structures and compositions remain constant, allowing them to be softened and moulded repeatedly. Examples of thermoplastic polymers include polyethylene, polystyrene, and polypropylene. Thermosets, on the other hand, undergo an irreversible chemical change when heated and can only be moulded once. This process is known as curing, and the transformation from a liquid to a solid state is irreversible, resulting in an ultra-strong end product.
The chemical structure of plastics can be altered by using copolymers and different chemical bonding techniques. Additionally, the use of crystallisability can change the processing, aesthetic, and performance properties of plastics. The wide variety of product types and additives available makes understanding the capabilities and limitations of plastics crucial for suppliers, manufacturers, and product developers.
The chemical reactivity of plastics also varies. For instance, acetal plastics are resistant to chemical solvents and moisture absorption, while acrylic plastics are strong, transparent, and resistant to weathering and UV radiation. Vinyl plastics are versatile, durable, and exhibit excellent flame-retardant properties due to the presence of chlorine and ethylene. However, they are sensitive to prolonged exposure to ultraviolet rays. High-performance plastics, such as aramids and ultra-high-molecular-weight polyethylenes (UHMWPE), exhibit superior properties, including high-temperature resistance, chemical corrosion resistance, and excellent mechanical and electric properties.
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Plastic's effect of temperature on chemical composition
Plastics are semi-synthetic or synthetic organic polymers known for their malleability, making them mouldable into various shapes. Each type of plastic has unique physical and chemical properties. While all plastics are mouldable, some can be remoulded, and others cannot.
The two primary forms of plastic are thermoplastic and thermoset plastics, the use of which depends on the application. The main physical difference between the two is that one can be reused, and the other cannot. Thermoplastics do not undergo any chemical changes when subjected to high temperatures and can be remoulded multiple times. When thermoplastics are heated, there is no chemical bonding, and the material's physical properties remain unaffected. They can be moulded, melted, and remoulded repeatedly into various shapes, sizes, and objects. Thermoset plastics, on the other hand, undergo an irreversible chemical change when heated and can only be moulded once. Once formed and solidified, the plastic becomes "set", and the transformation process is irreversible.
The rate of loading, or how the plastic is heated, is a key component of how we perceive its performance. Prolonged exposure to heat while subjected to a load or force can cause plastic to deform or "creep" over time. As the temperature increases, material stiffness will decrease, and the plastic will expand. This can be a consideration when the plastic is mated with another material, such as metal, that may have conflicting thermal expansion rates. If the dimensional change is obstructed, excessive stress can be induced in the plastic part.
The Continuous Use Temperature Rating is based on a thermal aging test that predicts the temperature at which a 50% loss of the original mechanical properties will occur after 100,000 hours of continuous exposure at that temperature. The quantity of heat that passes through a cube of the material in a given period when the temperature difference across the two surfaces is one degree is also a factor. Plastic materials generally have a much lower thermal conductivity than metals.
In summary, the effect of temperature on the chemical composition of plastics varies between types of plastics. Thermoplastics remain chemically unchanged when heated, while thermosets undergo an irreversible chemical change. Prolonged exposure to heat can cause plastic to deform, and higher temperatures generally lead to greater wear and faster degradation.
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Plastic's resistance to corrosion
Plastic is a synthetic material that has become indispensable in our daily lives. It is made from a wide range of organic polymers and is known for its malleability and mouldability. One of its most useful features is its resistance to corrosion.
Plastics are generally classified according to the chemical structure of their polymer base and side chains. The chemical structure of plastics can be altered by using copolymers and the chemical bonding of different elements and compounds. The wide variety of product types and additives available makes it important to understand the capabilities and limitations of different plastics.
The chemical resistance of plastic comes from its specific chemical formula. Each type of plastic has a unique chemical composition, which means they interact with different materials and chemicals in distinct ways. For example, a plastic that is great at resisting the corrosion of acid might still melt when in contact with a solvent. Therefore, it is important to understand the specific corrosive agents that a plastic product will be exposed to when determining its resistance to corrosion.
Some common plastics that exhibit high corrosion resistance include:
- CPVC (chlorinated polyvinyl chloride) - a high-heat, corrosion-resistant plastic used in industrial and plumbing systems.
- Kynar® - a chemically inert plastic product that is highly resistant to a wide range of chemicals.
- High-Density Polyethylene (HDPE) - offers superior corrosion resistance, a higher working temperature range, and higher tensile strength compared to Low-Density Polyethylene (LDPE).
- UHMW (Ultra High Molecular Weight) - a corrosion-resistant plastic that is also static dissipative and UV resistant. It is highly resistant to abrasion, outperforming carbon steel by 15 times.
- PVC (polyvinyl chloride) - a corrosion-resistant plastic polymer used in industrial applications such as valves, processing tanks, and plumbing.
The resistance to corrosion in plastics makes them suitable for a wide range of applications, from plumbing and electrical cable insulation to holding corrosive materials in various industries.
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Plastic's effect of ionising radiation
The effects of ionizing radiation on plastics have been studied for almost 40 years, with a focus on the safety evaluation of plastics for use in food irradiation. Ionizing radiation consists of particles such as X-rays or gamma rays that have sufficient energy to cause ionization in the medium through which it passes. The shorter the wavelength of radiation, the higher the susceptibility of a plastic to damage.
The overall service life of a plastic is dependent on the total amount of radiation absorbed. Materials like PEEK and polyimide show good resistance against gamma radiation and X-rays, while PTFE and POM are very sensitive and are, therefore, less suitable for applications involving radiation exposure. The suitability of plastics for use in radiation environments is evaluated by considering their structural properties and correlating them with changes in their chemical structure. For example, Nylon shows good performance under gamma-ray doses of up to 5.3 MGy, with no change in ultimate tensile strength and an increase in stiffness.
The effects of ionizing radiation on plastics are dependent on the chemical structure of the polymer, the composition (including additives), the processing history of the plastic, and the irradiation conditions. Irradiation causes physical and chemical changes in plastic packaging materials, including changes in infrared and UV/VIS spectra, and the development of brittleness in the polymer. However, the permeability of plastic films is generally not affected, and deterioration of mechanical properties can usually be controlled with adequate stabilizers.
In terms of specific plastic materials, studies have been conducted on the effects of gamma-radiation doses on commercial monolayer flexible packaging films such as ethylene vinyl acetate (EVA), high-density polyethylene (HDPE), polystyrene (PS), and low-density polyethylene (LDPE). These studies have shown that radiation doses of 5, 10, and 30 kGy do not induce any significant changes in the permeability of the films to gases (oxygen and carbon dioxide) and water vapour. Additionally, the mechanical properties (tensile strength, percentage elongation at break, and Young's modulus) of these films remain unaffected after absorbed doses.
Weather influences, particularly UV radiation, can negatively impact the optical and mechanical properties of plastics. Black coloration can protect plastics against weather influences, while fluorinated polymers such as PTFE and PVDF demonstrate good UV stability in their natural state.
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Frequently asked questions
Plastics are synthetic or semi-synthetic materials composed primarily of polymers. Their chemical properties include their resistance to chemical reagents, chemical processes, and their reactions to various substances.
The main categories of plastics are acrylics, polyesters, silicones, polyurethanes, and halogenated plastics.
Plastics are classified by the chemical structure of the polymer base and side chains, the chemical process used in their synthesis, and their various physical properties, such as tensile strength, hardness, and density.
Plastics are lightweight, durable, flexible, and have low toxicity. They are also easy to mould and work with due to their elasticity.
Many additives and derivatives in plastics can persist in the environment and bioaccumulate in organisms, which can have adverse effects on human health and the environment.
