Advantages and disadvantages of plastics

Plastics have many advantageous qualities as outlined in the diagram; they have low density making them light in most applications, they do not corrode easily making them suitable for use outdoors or in harsh environments, they are easily moulded into complex shapes and a host of others as shown.

Figure 1 Advantages of plastics

Unfortunately though, as with all materials, plastics also have their disadvantages as shown in the second diagram. For example, they exhibit time dependent behaviour, tending to lose strength over time, they can only be used in a limited temperature range, and perhaps of the greatest significance is concern around their environmental impact.

Figure 2 Disadvantages of plastics

Properties

Like the properties of metals, the properties of polymers tend to vary considerably.  Just as aluminium alloys do not all have the same properties, neither do those of, for example, polyethylene.  Only the data supplied by the actual manufacturer should be considered accurate.

Some general properties of plastics and polymers are given in the following table, along with steel for comparison.

Table 1 Comparison of properties of plastics and other materials

The following two graphs give an indication of strength to weight ratio. The first shows a broad comparison between polymers, elastomers and metals, whilst the second shows more detail on polymers.

© Gareth Bradley. UHI

Structure of Thermoplastics

Thermoplastics are made up of repeating chains of monomers such as ethylene (C₂H₄) held together in a long chain to create a polymer – such as polyethylene as shown in the diagram.

Figure 3 Structural and rod-and-ball structure of polyethylene. © Gareth Bradley, UHI.

The structure and some example uses of some of the most common thermoplastics are given in the table.

Table 2 Structure and uses of common plastics

The chains are not cross-linked (there are no primary bonds between the polymer chains), although they may occasionally branch.  Weak, secondary (Van der Waals or hydrogen) bonds bind the molecules together; on heating these melt (losing their strength) allowing the polymer to become a viscous liquid.

Crystallinity

Crystalline plastics have an ordered structure, whereas amorphous ones have a random structure.  Polyethylene and nylon have a high degree of crystallinity, although the structures should only be considered partially crystalline if compared to a metal.

• Plastics that are crystalline typically have:
• higher density,
• rigidity, especially at elevated temperatures,
• low friction and harder wearing,
• increased hardness,
• resistance to environmental stress cracking,
• can be reinforced,
• can be stretched,
• better creep resistance.

But, crystalline plastics are always  opaque and exhibit greater shrinkage during moulding.

Copolymers

Crystalline plastics have an ordered structure, whereas amorphous ones have a random structure.  Polyethylene and nylon have a high degree of crystallinity, although the structures should only be considered partially crystalline if compared to a metal.

• Plastics that are crystalline typically have:
• higher density,
• rigidity, especially at elevated temperatures,
• low friction and harder wearing,
• increased hardness,
• resistance to environmental stress cracking,
• can be reinforced,
• can be stretched,
• better creep resistance.

But, crystalline plastics are always  opaque and exhibit greater shrinkage during moulding.

Figure 4 Pictorial representation of the various types of copolymers.

Acrylonitrile butadiene styrene (ABS) [(C₈H₈)_x· (C₄H₆)_y·(C₃H₃N)_z]_n is a copolymer consisting of polymers made from the following monomer units:

• Acrylonitrile, C₃H₃N (15 - 35%);
• Butadiene, C₄H₆ (5 - 30%);
• Styrene, C₆H₅CH=CH₂ (40 - 60%);

The resulting polymer benefits from the strength and stiffness of the acrylonitrile and styrene polymers and the toughness of the polybutadiene rubber, and it is used for a wide variety of products, from Lego to hard hats.

Structure of Thermosetting Plastics

The chains in thermosetting plastics are cross-linked, and the cross links consist of strong primary (covalent) bonds.  When the plastic is heated these bonds do not melt and hence the plastic burns or chars.

Thermosetting plastics include epoxies, polyesters (some polyesters are thermoplastics), polyurethanes, Bakelite (phenol-formaldehyde).

Figure 5 Rod-and-ball representation of a thermosetting plastic

Due to this cross – linking, the structure of thermosetting plastics is considerably more complex than that of thermoplastics, as is demonstrated in the diagram.

Figure 6 Production of a thermosetting plastic. © Gareth Bradley, UHI.

The table shows some types of thermosetting plastics along with their uses. The structure is not included this time as it is nearly always amorphous as stated.

Table 3 Uses of thermosetting plastics

Additives

A variety of substances can be added to polymers to enhance their properties. These range from pigments to give them a specific colour, to flame retardants to stop them from igniting or burning.

Typical additives used in the production of plastics include:

• Antistatic agents – most polymers are poor conductors, additives that improve the surface conductivity reduce the likelihood of a spark or discharge occurring.
• Coupling agents – these improve the bonding between the plastic and any inorganic fillers such as glass or carbon fibres.
• Fillers – some fillers, such as short fibres or particles of inorganic materials improve the structural properties.  Other fillers, called extenders,  increase the volume of the material whilst reducing the amount of the (expensive) polymer required.  Calcium carbonate, silicates and clay are common extenders.
• Flame retardants – most polymers are flammable, the addition of chemicals that contain bromine, phosphorous or metallic salts reduce the flammability.
• Lubricants – these reduce the viscosity of the molten plastic enhancing the forming characteristics.  Wax and calcium stearate being examples.
• Pigments – these allow the plastics to be coloured.
• Plasticisers – plasticisers are low molecular weight materials that alter the properties and forming characteristics of the plastic.  Flexible PVC is an example of their use.
• Reinforcement – the strength and stiffness of polymers can be increased through the addition of fibres such as glass or carbon.
• Stabilisers – these prevent the deterioration of the polymer due to environmental factors.  Antioxidants are added to acrylonitrile butadiene styrene (ABS), polyethylene and polystyrene.  Heat stabilisers in the processing of PVC .  Stabilisers are also used to prevent plastics deteriorating due to exposure to ultra-violet light.

Variations in properties

As with metals, two nominally identical plastics may have slightly different properties.

For example, the same plastic from different manufacturers may have a different spectrum of molecular lengths due to slightly different processing methods.  The degree of polymerisation may change and the amount of molecular branching and crystallinity.

Figure 7 Plastic pipes. [Pixabay]

Glass Transition Temperature

As the average lengths of the polymer chains increase the tensile strength and glass transition temperature (T_g) also increase.

T_g – the glass transition temperature – is the temperature at which the weaker secondary bonds “melt”, resulting in a softening of the plastic.  Below T_g most polymers have a modulus of ~3 GPa. Above T_g the secondary bonds creep and the modulus falls.  For this reason, the mechanical properties of polymers are typically time dependent.

T_g also has an effect on the fracture toughness.  Near T_g plastics are typically quite tough, but this reduces at temperatures below T_g.  The result of this is that they become brittle.

The modulus of the plastic decreases slightly as the temperature increases up to T_g.

Once T_g is attained the modulus decreases rapidly.  At a temperature of ~1.5T_g the plastic becomes a viscous liquid.

T_g range from -70 °C for rubber to over 100 °C for plastics such as polytetrafluoroethylene (PTFE) and polycarbonate.

Tensile Properties

The tensile properties of plastics can vary considerably depending on the plastic, the rate of loading and the temperature:

where t = time and T = temperature

Polystyrene, acrylic and epoxy are relatively brittle materials at room temperature, with failure strains of <=3%.  In contrast, low density polyethylene and flexible PVC have failure strains >300%.

The rate of applying the force has an effect on the tensile properties.

Reduced rates allow the plastic to creep and therefore the strain increases at a greater rate.  The result of this is that both the modulus and yield strength decrease.

The graph shows this difference experimentally

Graph 3 Fracture Behaviour of plastic. © Gareth Bradley, UHI.

Thermoplastics – Manufacturing

Due the fact they soften on the application of heat thermoplastics lend themselves to a range of processing routes:

• Injection moulding
• Extrusion
• Thermoforming
• Blow moulding
• Production of thin films and coatings

Many of the secondary processes used for metals can be utilised, e.g. milling, turning, drilling, welding etc. although very often the primary process is able to produce the finished product to a satisfactory standard with no further processing needed.

Figure 8 A schematic of an injection moulding machine. © A.Henderson, UHI.

Elastomers

Elastomers (elastic polymers) are polymers that have a low elastic modulus and high failure strain (up to 700%). They have glass transition temperatures that are usually below room temperature, hence they have a rubbery consistency.

They are usually thermoset polymers, but cross-linking is often low.  A process known as vulcanisation is used to increase cross-linking. Elastomers include natural rubber and butyl rubber.

When unstressed, the polymer chains in an elastomer are tangled and chaotic, becoming straight under stress – it is this change that allows for significant increases in strain prior to failure.

Figure 9 Representation of the polymer chains in stressed and unstressed elastomers. © Gareth Bradley, UHI.

Vulcanisation

The polymer chains of unsaturated elastomers are very long, with little, or no, cross-linking, and so they have properties that lie somewhere between those of thermoplastics and thermosets.  For example, natural rubber becomes soft and sticky in summer, but hard and brittle in winter.

Charles Goodyear, after many years of experimenting, found by accident that the addition of sulphur to the rubber converted it to a tough, elastic substance stable to both heat and cold. The main polymers subjected to vulcanization are polyisoprene (natural rubber) and styrene-butadiene rubber (SBR).

The process of adding sulphur is known as vulcanisation and results in cross-linking occurring between the polymer chains of the rubber and hence it becomes a thermosetting plastic, albeit with a glass transition temperature -70 °C.

The diagram shows the difference between vulcanised and unvulcanised rubber, and shows the behaviour of vulcanised rubber in both stressed and unstressed states.

Figure 10 Vulcanised and unvulcanised rubber. © Gareth Bradley, UHI.