What are the properties of plastics?

Plastic is a polymer that, like synthetic fibres, can be moulded into the desired shape and size when soft and can harden to produce durable articles. The term “plastic” comes from the word “plastikos”, which means “to mould” in Greek, and is used to refer to a wide range of semi-synthetic or synthetic organic polymers. Different types of plastics are known to have different physical and chemical properties. Many items, such as chairs, tables, buckets, toys, balls, etc., are made of plastic.

Most plastics contain organic polymers. A large part of these polymers consists of chains of carbon atoms, either pure or with oxygen, nitrogen or sulphur. The chains have several repeating units, composed of monomers. Each polymer chain has several thousand repeating units. The backbone is the main section of the chain that links a large number of repeating units.

Different molecular groups “hang” from this backbone to customise the properties of the plastic. These pendants are usually “dangling” from the monomers until the monomers are linked together to form the polymer chain. It is the arrangement of these side chains that influences the properties of the polymer. The molecular structure of the repeating units can be fine-tuned to affect the specific properties of the polymer.

Chemical properties.

• They possess plasticity, a typical property of these materials. Plasticity is the ability of a material to deform without breaking.
• They are polymers, organic compounds with a macromolecular structure formed by groupings of monomers, which are obtained by polymerisation processes.
• They are of low density.
• Easy to mould and work with due to their elasticity.
• Resistant to corrosion, as well as to other chemical formulas.
• Good thermal and electrical insulators, although they do not withstand high temperatures.
• They are pollutants, either when burned or because of their difficulty to be recycled or biodegradable.

Classification of plastics.

Plastics are generally classified according to the chemical structure of the polymer base and side chains. The main categories of these classifications include acrylics, polyesters, silicones, polyurethanes and halogenated plastics. Plastics are also classified by the chemical process used in their synthesis, such as cross-linking, condensation and polyaddition.

Plastics can also be classified by their various physical properties, such as tensile strength, hardness, heat resistance, density and glass transition temperature, and their chemical properties, such as the organic chemistry of the polymers and their resistance and reactions to various other materials and chemical processes, such as ionising radiation, oxidation and organic solvents.

Other classifications depend on characteristics relevant to the manufacturing process or product design. Examples of these qualities and classes are conductive polymers, thermoplastics and thermosets, engineering plastics and biodegradable plastics and similar plastics with unique structures, such as elastomers.

Thermoplastics

Thermoplastics are plastics that do not undergo a chemical change in composition when heated, so they can be moulded several times. Examples include polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) and polystyrene (PS).

Thermosets

Thermoset polymers are plastics that can be melted and moulded into any shape once. They undergo an irreversible chemical reaction when heated, so if heated again, they decompose rather than melt.
Conductive polymers

Intrinsically conductive polymers (ICP) are electrically conductive organic polymers. Example: Polyacetylene.

Biodegradable plastics

Biodegradable plastics are plastics that degrade or break down when exposed to sunlight or ultraviolet radiation, bacteria, certain enzymes, moisture or water, or wind abrasion. In certain circumstances, rodents, pests or insect attacks can also act as modes of biodegradation or environmental degradation. Example: Starch-based plastics, Cellulose-based plastics, Soy-based plastics.

Bioplastics

While most plastics are products of petrochemicals, bioplastics are plastics produced substantially from renewable plant materials such as cellulose and starch. Due to the finite limits of petrochemical resources and the risk of global warming, bioplastics remain a growing field.

Plastic recycling

The plastic recycling process is an essential procedure. If not recycled at the right time, plastic is mixed with other chemicals or materials, making it difficult to recycle and a source of contamination.

They are not biodegradable and therefore cannot be broken down by microbial activity. To avoid this, it is essential to use biopolymers or biodegradable polymers.

The following is a classification of the most common plastics:

1. Polyethylene terephthalate (PET or PETE):

PET is one of the most widely produced plastic materials in the world. It is considered safe for food and beverages and has a high capacity to prevent oxygen from entering the packaging and spoiling food. It is a highly recyclable, cheap and strong plastic with a very good strength-to-weight ratio. It is used to make food packaging, plastic bottles and polyester fibre such as that found in clothing. It is also used in a wide variety of industrial applications, such as the manufacture of glass fibre and carbon nanotubes.

2. Polyethylene (PE):

It is the most common plastic on the planet and can be manufactured in different densities. Each density gives the final plastic unique physical properties. As a result, polyethylene is found in a wide range of products.

Low Density Polyethylene (LDPE)

LDPE has high ductility but low tensile strength, making it more flexible than other plastics. It is used to make products such as plastic bags, transparent food packaging, disposable packaging and cable insulation, among others.

Medium Density Polyethylene (MDPE)

Having more polymer chains and therefore higher density, medium density polyethylene is often used in gas pipes, shrink film, carrier bags and screw caps.

High Density Polyethylene (HDPE)

HDPE is considered environmentally friendly, and the manufacture of this type of plastic requires only a small fraction of the energy that would be needed to produce steel from iron ore. It is resistant to degradation, environmental stresses and is quite rigid, so it is used to manufacture a multitude of products such as material containers, buckets, signage, sprockets, water and sewer pipes.

3. Polypropylene (PP)

Polypropylene is a very hard, heat-resistant, semi-transparent plastic that retains its shape after much twisting, bending or folding. Its widespread use and popularity are undoubted, as it is one of the most flexible thermoplastic polymers on the planet. Durable, flexible, heat-resistant, acid-resistant and inexpensive, polypropylene sheet is used to make laboratory equipment, automotive parts, hinges, medical devices, food packaging and more.

4. Polycarbonate (PC)

Strong, stable and transparent, polycarbonate is an excellent engineering plastic that is as clear as glass and two hundred and fifty times stronger. Clear polycarbonate sheets are easy to work with, easy to mould and, while extremely strong and impact resistant, polycarbonate plastic has an inherent design flexibility. It is found in a wide variety of products, such as greenhouses, DVDs, sunglasses, riot control material, etc.

5. Polyvinyl chloride (PVC)

PVC is a polymer that possesses rigid or flexible properties and is well known for its ability to blend with other materials. For example, expanded PVC sheet is a foamed polyvinyl chloride material ideal for products such as kiosks, shop displays and exhibits. The rigid form of PVC is commonly used in building materials, doors, windows, flooring, cladding, etc. With the addition of plasticisers such as phthalates, the softer, more flexible form of PVC is found in plumbing products, electrical cable insulation, clothing, medical tubing and other similar products.

6. Polystyrene (PS)

This is a transparent thermoplastic that can be found in both solid plastic and rigid foam material. Polystyrene is widely used in packaging, medical devices such as test tubes or Petri dishes, Styrofoam peanuts, parts of household appliances, automobiles and computers, among others. In industry, it is used in particular to manufacture sprockets for roller chains, rod frames or pulleys.

According to their heat behaviour.

As organic materials, polymers and plastics have, with few exceptions, a much lower heat resistance than metals, especially in the presence of oxygen. Among the common polymers, one exception is tetrafluoroethylene, which has a very high thermal stability because it has only C-C and C-F bonds, both of which are very stable. When heated to different temperatures, thermoplastic materials slowly change from more or less rigid solids to very viscous liquids. Although thermoset materials do not soften with heat, excessive or prolonged heating can cause over-hardening, shrinkage, carbonisation or disintegration. Plastics have coefficients of thermal expansion (4 to 20 x 10-5/°C; 2 to 11 x 10-5/°F) considerably higher than those of base metals (1.0 to 2.5 x 10-5/°C; 0.6 to 1.4 x 10-5/°F). Polymers and derived plastics are generally good electrical insulators and some, such as polytetrafluoroethylene, are excellent.

Fire resistance

Plastics exhibit a wide range of fire behaviour: some are flammable, some are self-extinguishing, others are slow to fast burning. The flammability depends on the polymer and other components such as fillers, reinforcing materials, plasticisers or flame retardant additives.

Halogen-containing polymers, such as PVC or chlorinated PVC, are inherently flame retardant; when heated, they release halogen gases that interrupt the free radical oxidation chain reaction. However, the addition of most plasticisers to PVC makes it flammable. The flame retardant properties of plastics can be improved by incorporating suitable additives (CBD 154) or by using polymers with built-in flame retardancy.

The combustion products of most plastics are similar to those of wood, paper and textiles because their chemical components are essentially similar. However, the products of combustion depend not only on the chemical nature of the material, but also on the conditions under which it burns. For example, with sufficient air, the main combustion products of most plastics, wood and textiles are carbon dioxide (harmless) and water, but if there is a deficiency of oxygen, large volumes of toxic carbon monoxide and smoke are formed. In addition, flame-retardant plastics quickly produce dense smoke that is not easily removed by ventilation; and if the plastics contain combined chlorine, fluorine, nitrogen and sulphur (or their derivatives), these elements or their derivatives will also be present in the smoke.

Composition of plastics.

Plastics are mainly based on carbon atoms. Silicones are an exception, as they are based on the silicon atom. The carbon atom can be bonded to other atoms with up to four chemical bonds. In plastics, carbon atoms also bond with hydrogen, oxygen, nitrogen, chlorine or sulphur. When the bonding of these atoms results in long chains, like the beads of a pearl necklace, the polymer is called a “thermoplastic”. Thermoplastics are meltable. All thermoplastics have repeating units, i.e. the smallest identical section of the chain. The vast majority of plastics are 92% thermoplastics.

A group of atoms called “monomers” is used to form the unit cells, the combination of which gives rise to the polymers or plastics. All monomers contain double bonds between the carbon atoms, so that these can subsequently react to form polymers.

The plastic behaviour of polymers is influenced by the large-scale arrangement of their molecules. In other words, polymers are amorphous or crystalline. The arrangement of the molecules in the amorphous state is random and intertwined. In the crystalline state, the arrangement of molecules is closely identifiable. Semi-crystalline materials, on the other hand, have crystalline regions, called crystallites, within an amorphous matrix.

The chemical structure of plastics can change, with the use of copolymers, and the chemical bonding of different elements and compounds and, on the other hand, the use of crystallisability can change the processing, aesthetic and performance properties of plastics. Alteration of plastics can also occur through the addition of additives.

In short, the wide variety of product types and additives available makes understanding the capabilities and limitations of a material a key issue for suppliers, manufacturers and product developers at all levels of the industrial chain. Mechanical, thermal, optical, rheological behaviour and climatic tests provide a better understanding of the material, the product and its performance.

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