1.3 Enhancement of Materials

Introduction

Material enhancement is a way of improving a materials mechanical properties to better suit the requirements of the final product.

Enhancements are commonly carried out on polymers, woods and metals, less so with the other materials studied as they are often enhanced while being produced.

As an example, some metals are very brittle which means they cannot be bent easily into shape without fracture. The use of heat treatments can alter the crystalline structure of the metal to improve its toughness and making it less likely to fracture.

In this unit we will look at enhancing the materials mechanical properties rather than physical appearance which can often be done with finishes.

See unit 1.5 for more information on finishing materials.

Polymer Enhancement

Polymers are generally inert so they will resist most chemicals and fungal attack, but they can still be enhanced through the use of additives.

Additives to make polymer easier and less expensive to process. 

• There are many ways to process polymers, but most involve melting the granules or powder and reforming. Additives called ‘process aids’ make the polymer flow more easily:

• Lubricants such as wax make the polymer less ‘sticky’ and help it to flow easier when moulding.

• Thermal antioxidants prevent the polymer discolouring due to heat.

• Pigments are tiny particles mixed into the polymer to give colour to the final product.

Additives to improve product function

• Antistatics are used to give the polymer greater resistance to static. Polymers have poor electrical conductivity so static can easily build up. These antistatics help to reduce static discharge/shock.

• Flame retardants such as bromine or metals (Asbestos in the past) help to reduce the likelihood of the spread of fire especially in engine components and plug sockets.

• Plasticisers allow the polymers to become less hard and brittle. Used extensively in the food industry to make cling film and other food packaging.

• Fillers have a dual function – to bulk out the product or to improve the polymer properties. Common bulk fillers include sawdust and wood flour.

• Mineral fillers such as chalk and clay can increase thermal conductivity meaning they heat up and cool down more quickly to enable shorter mould cycle times.

Additives to prolong life (prevent degradation)

Antioxidants help to reduce the environmental deterioration of the polymer from exposure to oxygen in the air. Degradation leads to polymers becoming brittle, discolouring and developing surface cracks.

UV light stabilisers prevent the polymer chain being broken down by the effects of exposure to UV rays which are present in sunlight. Polymers become discoloured and become brittle. White pigment becomes yellow and other colours turn milky and/or faded - Often used in public seating and uPVC windows and doors.

Additives to encourage degradation

• Biodegradable plasticisers are similar to plasticisers used to enhance processing and make the polymer more flexible, softer and easier to break down which means faster degradation time.

• Bio-batch additives make the polymer break down in certain conditions:

• Oxy-degradable polymers degrade over time in the presence of oxygen  - Often used in single use items such as plastic carrier bags and food packaging.

• Photodegradable polymers degrade when exposed to UV light.

• Hydro-degradable polymers degrade in the presence of water.

These polymers can be engineered to last from a few weeks or months to many years and will not contribute long term to landfill issues.

Wood Enhancement

Natural wood can have defects such as splits or knots which can affect the overall stability of the wood. This is why manufactured boards such as MDF and Plywood were developed. Natural wood has several disadvantages:

• strength decreases when the wood gets wet

• wood is highly combustible

• wood is susceptible to rot as well as insect and fungal attack

• wood is anisotropic which means it has different properties in different directions such as the wood grain

There are several ways to enhance wood...

Enhancement using preservatives

• Wood for buildings is highly sought after due it its strength and beautiful grain as well as its insulating properties as well as being a renewable source. However, it is subject to attack from insects and rot as well as weathering. Preservatives are used to enhance the wood and extend its life. See more in section 1.5 on finishes

• Wood can also be treated to make it harder in high wear situations such as flooring where the wood is impregnated with a modified natural polysaccharide similar to cellulose. These effectively cure within the wood cell structure to increase toughness and stability.

Metal Enhancement

There are a wide range of metals, each with their own working properties such as toughness, hardness, malleability etc. This means that not all enhancements are necessary for every metal. For example, Ferrous metals require a finish or coating to prevent them from rusting while Non-ferrous metals don’t.

• Work hardening - When a metal is ‘cold worked’ the crystals within the metal are distorted and changed leading to improved tensile strength and hardness. This is known as work hardening. The affects of work hardening can be removed by annealing.

• Annealing is used to make work hardened metal easier to work by making it less brittle and more ductile. The metal is heated then slowly cooled. In industry, the process is carried out in a specific temperature-controlled furnace.

• Case hardening is a process used for hardening the surface of steels with less than 0.4 % carbon. This makes the surface of the steel more resistant to indentation and abrasion. There are two stages:

1. Carburising which changes the chemical composition of the surface of low carbon steel so it can absorb more carbon. This is often done in a box with carbon powder where it is heated to around 950 degrees C. - The longer it is ‘baked’, the thicker the carbon layer.

2. Quenching where the hot metal is quenched in water. This fast cooling seals the hard surface without affecting the inner core.

Hardening and tempering

• Hardening is the process of heating medium and high carbon steels to alter the crystalline structure, holding them at this temperature for a given time then quenching them in water, oil or salt baths. The hardening and subsequent quenching greatly increase the hardness property but also increase the brittleness.

• Tempering is a heat treatment for medium and high carbon steels that is carried out after hardening to reduce some of the excess hardness and increase the toughness. This reduction in hardness results in greater ductility and reduces the brittleness of the metal. To temper a metal it is heated to below the critical point for a given time then air cooled. The rise in tempering temperature results in a reduction of hardness and increase in toughness

Revision Notes

Enhancements are different to finishes as they often affect the mechanical properties of materials, but occasionally there is some crossover such as preservatives for wood which enhance the material but are often achieved through the use of applied finishes.

It is unusual for a student to need to change the mechanical properties of a material they are using in D&T (more likely on engineering based courses) and you would generally choose a material which has the necessary physical and mechanical properties for the intended function.

Most materials can be enhanced in some way but concentrate on those in this unit which cover wood, metals and polymers.

Tanalising is one way of preserving wood by impregnating it with copper sulphate but this is covered in the units on woods 1.2 and finishes 1.5.