Copyright © A.F.Billington 2022. All rights reserved.
Mr BILLINGTON
Class 2 -
Plastics have a huge range or differing properties depending on their chemical makeup and additives. There are, however, some properties common to plastic materials;
1. Plastics are considered to be good insulators of heat as they restrict the movement of thermal energy. When any two objects of differing temperatures touch there will always be thermal conduction, but some materials have lower thermal conductivity than others. Plastics are used where thermal transfer must be restricted, eg. in home cavity insulation, cooking utensils and pans & beverage cups. Polyurethane and polystyrene have some of the lowest thermal conductivity. They are commonly used as foam to increase the insulating properties through trapping pockets of air within the material structure
------------------------------------------------------------------------------------------------------
2. Most plastics are also excellent insulators of electricity due to their high resitivity. The valence electrons in the outer atomic band of plastics have low energy and no 'free' electrons as found in metals. They will therefore not allow the transfer of current through them from an external source. This, coupled with the malleable/ductile properties of plastic, make it ideal for insulating copper electrical wire. PVC is commonly used where electrical conductivity must be restricted for safety or to prevent short circuits within a product. This could be likened to electrical resistance as insulators restrict current flow
Although plastic is non-conductive, no material is considered a perfect insulator due to charge carriers existing in all materials. If a large amount of voltage is applied a 'breakdown voltage' can be reached which forces the material to conduct, breaking the electrons away from their atomic structure. Products are classified according to the insulation applied -
- Class 1; all exposed metal parts & casing on the product must be earthed
- Class 2; 'double insulated' - by fully insulating all electrically energised parts within a casing to prevent 'live' contact
------------------------------------------------------------------------------------------------------
3. Elasticity is a property common to plastics and details a materials ability to resist a deforming force and to return to its original size and shape when that force is removed. As with insulators, no perfectly elastic material exists and all have an 'elastic limit' which determines the extent to which they can be stretched.
The elasticity of a material is measured as 'Youngs Modulus', which is measure of a materials performance under Stress (force applied) & Strain (distance covered). A high youngs modulus would mean the material is considered 'stiff' or ridgid and resists deformation without returning to its original shape. Polymers, in particular rubbers, are considered 'flexible' and have a low youngs modulus making them elastic. Low density Polyethelene (LDPE) is an elastic thermoplastic used commonly used for clingfilm products for this reason
-----------------------------------------------------------------------------------------------------
4. Toughness is a materials ability to absorb energy and plastically deform without fracturing. This is not to be confused with Strength which concerns the force a material can support, although they are often linked together. For a material to have good toughness it must have both strength and ductility, which is a materials ability to deform under tension (copper wire).
4. Toughness is a materials ability to absorb energy and plastically deform without fracturing. This is not to be confused with Strength which concerns the force a material can support, although they are often linked together. For a material to have good toughness it must have both strength and ductility, which is a materials ability to deform under tension (copper wire).
In order to test the toughness of a material, a sample can be taken and a force applied to it by a swinging pendulum mass. The height the pendulum fell, minus the height it rose after hitting the sample, multiplied by the mass of the pendulum would calculate the energy absorbed by the sample. The more energy absorbed without fracture, the higher the material toughness. PVC is a high toughness polymer
------------------------------------------------------------------------------------------------------
5. Chemical resistance is common amoungst plastics and is a materials ability to resist molecular reaction with liquids other than water. This property is rated from A-D as follows;
- A/E = No Attack, possibly slight absorption. Negligible effect on mechanical properties.
- B/G = Slight attack by absorption. Some swelling & a small reduction in mechanical properties likely.
- C/N = Moderate attack of absorption. Material will have limited lifespan
- D/F = Material will decompose or dissolve in a short period of time
This property led them to be widely used in the food and drinks industry for storage to prevent contamination from the material over a long period of time. However, the prescence of bisphenol A (BPA) in some plastics meant it could contaminate the contents leading to the common use of BPA Free plastics.
Material Properties - Plastic Properties
- Class 1