B Thermoplastics and thermosetting plastics

The page goes on to look at types of polymer.

Synthetic polymers can be divided into two main categories:

Thermoplastics can be melted by heat, and formed in shaped containers called moulds. After the liquid plastic has cooled, it sets to form a solid material. A thermoplastic is a type of plastic that can be heated and moulded numerous times. Examples of thermoplastics that are common in engineering include:

-ABS (acrylonitrile butadiene styrene) - stiff and light, used in vehicle bodywork

-polycarbonate - used to make strong, transparent panels and vehicle lights

-PVC (polyvinylchloride) - a cheaper plastic used for window frames and pipes.

Thermosetting plastics, also called thermosets, can be heated and moulded like thermoplastics. They may also be mixed from cold ingredients. Fiowever, during cooling or mixing, a chemical reaction occurs, causing thermosets to cure. This means they set permanently, and cannot be moulded again. If a thermoset is heated after curing, it will burn. Examples of thermosets used in engineering are:

-epoxy resins - used in very strong adhesives

-polyimides - strong and flexible, used as insulators in some electric cables.

Two more categories of polymer are engineering plastics and elastomers. Engineering plastics are mostly thermoplastics that are especially strong, such as ABS and polycarbonate. Elastomers are very elastic polymers which can be stretched by force to at least twice their original length, and can then return to their original length when the force is removed.

 

14.1 Circle the correct words to complete the text. Look at A opposite to help you.

A lot of rubber is made from latex, a (1) natural / synthetic polymer which comes from rubber trees. However, not all rubber comes from trees. Synthetic rubber is a (2) manmade / natural polymer with similar properties to latex. Plastics are also polymers. Like rubber, they consist of long chains of (3) atoms / molecules which form extremely large (4) atoms / molecules.

14.2 Read the extract describing a plastic panel manufacturing process. Then decide whether the sentences below are true or false, and correct the false sentences. Look at B opposite to help you.

 

By this stage of the process, the plastic is solid, and has fully cooled. Selected panels can now undergo quality-control testing, to check they are strong enough to cope with the tough conditions they will be exposed to in use. Tests include tensile testing, where narrow lengths of panel are subjected to high tension loads to check they do not stretch or fracture. More tests are carried out to check the panels' resistance to impacts and scratching. Any products that fail the tests are returned to the beginning of the production process, melted down, and their material is reused.

 

1. The plastic was heated earlier in the process.

2. The plastic has now set.

3. The plastic is now liquid.

4. To pass one of the tests, the plastic must be an elastomer.

5. The description suggests the plastic is a type of engineering plastic.

6. The material is a thermosetting plastic.

7. The material is a thermoplastic.

 

Over to you

Talk about specific types of polymer that are used in your industry, or an industry you're familiar with. How are they used? Which of the categories mentioned in A and B opposite do the polymers belong to?

 

Minerals and ceramics

A Mineral and ceramic engineering materials

A mineral is a natural, inorganic material (one that is not living) which is found in the ground, often within rocks. Minerals are quite pure. Rocks, on the other hand, can be mixtures of several minerals, and may also contain previously organic material. Examples of minerals include different types of ore - from which metal can be extracted - such as iron ore. Non-metallic minerals include:

-diamond, an extremely hard form of carbon (C), which is used as an abrasive (very hard and rough) material in cutting tools - frequently referred to as industrial diamond when used in engineering

-silicon (Si), found in sand as silica (silicon temperatures to make glass.

 

Generally, inorganic, non-metallic materials that have been formed by heating are called ceramics. Glass is therefore a ceramic. When materials are heated to extremely high temperatures to form ceramics that are glass-like - that is, with a structure like that of glass - we say that they are vitrified.

Ceramic materials are used to make construction materials such as bricks. These are made from clay, and are then fired in a kiln - that is, heated to a high temperature in an industrial oven. Clay can also be vitrified - for example, to make waterproof pipes.

 

B Glass

A technical adviser for a glass manufacturer is giving a briefing to a group of engineers at a trade fair.

‘Sheets of glass, which are obviously flat and thin, are called float glass. This refers to the manufacturing technique where molten glass is floated on molten tin, to produce flat sheets. Usually, after float glass has been formed, it’s annealed - it’s left to cool slowly. But if it’s left in this state, and the glass later gets broken, it breaks into dangerous, sharp pieces. So for most engineering and architectural uses, annealed glass is unsuitable. We need to use what we call safety glass.’

‘One type of safety glass is toughened glass, also called tempered glass. As the term suggests, the glass is tempered - it’s heated and kept hot for a certain time, to change its structure. Then if tempered glass is broken, it shatters - it breaks into tiny pieces. These are a lot safer than the long, sharp pieces produced when annealed glass breaks. The disadvantage of toughened glass is that it can’t withstand impacts from small objects, such as flying stones.

So, for instance, that makes it unsuitable for vehicle windscreens. So in cases where impacts are a problem, another type of safety glass - laminated glass - is generally used. This is made by laminating glass with a polymer - in other words, making a glass and polymer ‘sandwich’, with a sheet of polymer in the middle and sheets of glass at either side. The advantage of having a laminated material is not just that it’s very strong. The layers of glass are bonded to a layer of polymer - they’re stuck to the polymer - so if the glass does break, the broken pieces are held together, and don’t fly.’

 

15.1 Decide whether the sentences below are true or false. Then, change one word in each of the false sentences to correct them. Look at A opposite to help you.

1. Minerals are organic.

2. Minerals can be found in rocks.

3. Silica is a compound containing silicon.

4. Minerals can be metallic or non-metallic.

5. Industrial diamond is an abrasive, metallic mineral.

6. In order to become ceramics, materials must be vitrified.

7. Clay can be fired to produce material with a glass-like structure.

 

15.2 Complete the article about bulletproof glass from a science and technology magazine, using words from В opposite. Sometimes, more than one word is possible.

‘Bulletproof is a loosely used word, suggesting something is totally unbreakable. But technically speaking, how accurate is the term ‘bulletproof glass'? Outside of Hollywood movies, can glass really stop bullets? The answer is, not on its own. But if several (1).........of glass are sandwiched with a high-strength polymer to form (2)......... glass, a bullet-resistant, if not completely bulletproof, barrier can be obtained.

 

The technique of sandwiching polymer and glass is nothing unusual. Car windscreens are made by (3)........glass to a polymer, such as polyvinyl butyral (PVB), to form a type of safety glass.

Unlike the other main type of safety glass - (4)......glass - laminated glass remains intact on breaking. If a stone hits a windscreen, even though a small section of the glass on the outside may crack, the polymer behind it will stop the stone, and also ensure the entire piece of

glass doesn't (5).......... Bullet-resistant glass uses the same principle, but must be much tougher. A stronger polymer is therefore used - often polycarbonate - as well as a greater number of (6)....... of glass and polymer.

 

Over to you

Think about the different ceramics and minerals used in your industry, or in an industry you're familiar with. What types of material are used, and why?

 

Concrete

A Gravel - coarse aggregate

Cement is a key material in construction. It consists of a very fine powder. When water is added to cement, a chemical reaction occurs, and the cement begins to set - it starts to become solid. The most widely used cement-based material is concrete, which is made from cement, fine aggregate (sand), coarse aggregate (gravel) and water. After concrete has set, it needs time to reach its structural strength - the strength needed to perform effectively. Generally, engineers consider that this strength is reached after 28 days - a point called 28-day strength.

Concrete mix designs, which are specified by engineers, state the proportions of cement, fine aggregate and coarse aggregate to be used for specific structures. For example, a 1:2:4 (one- two-four) mix consists of one part cement, two parts fine aggregate and four parts coarse aggregate. For mixing precise quantities - known as batching - proportions are measured by weight. Mix designs also specify the water-cement ratio - the amount of water added relative to the amount of cement used. Excess water reduces the strength of concrete, so the quantity of water is kept to a minimum. But as drier concrete is more difficult to work with, an additive (added chemical substance) called a plasticizer is often used. This helps the concrete to flow more easily. Other additives can also be used - for example, a retarder may be added to delay setting, which gives workers more time to pour (place) the concrete.

B Reinforced concrete

Reinforced concrete (RC) structures contain steel bars. Steel reinforcement is needed mainly because concrete is weak in tension - that is, bad at resisting stretching forces. As steel is strong in tension, reinforcing bars overcome thisweakness.

In order to form the different parts of structures, formwork - sometimes also called shuttering - is used. This consists of moulds of the required size and shape, made from steel or timber, which are

used to contain the concrete until it has set. In-situ reinforced concrete being poured

When wet concrete is cast (placed) in its final position, it is called in-situ concrete. Instead of being cast in-situ, reinforced concrete elements can also be precast - cast at a factory - then delivered to the construction site ready for assembly. Sometimes, precast concrete is also prestressed. With prestressing, tension is applied to the reinforcing bars, by machine, usually before the concrete is poured. The bars are then held in tension while wet concrete is poured around them. After the concrete has fully set, the bars become ‘trapped’ in tension. This increases the concrete’s ability to resist bending forces.

16.1 Find words and expressions in A opposite to match the descriptions (1-10).

1.	gravel used in concrete	......
2.	sand used in concrete	.......
3.	powder that enables concrete to set	.....
4.	mixing concrete accurately	......
5.	specification of concrete ingredients	....
6.	effective structural capability of concrete	......
7.	affects the wetness and strength of concrete	.....
8.	different types of chemical put in concrete	.....
9.	allows concrete to stay wet for longer	....
10.	makes drier concrete easier to work with	.....

 

16.2 Complete the textbook extract about a type of prestressed concrete using the words in the box. Look at B opposite to help you.

 

cast	formwork	pouring	prestressing	structural
concrete	in-situ	precast	reinforcement

 

Prestressing techniques

In the production of reinforced concrete components, the process of (1)

usually involves holding the (2).....in tension while...the concrete. This form of prestressing is called pre-tensioning, as tension is applied before the concrete is poured. The technique is often used in the manufacture of floor components, which are small enough to fit on the back of a truck, and can therefore be (4)...... at a factory.

 

A less common prestressing technique is post-tensioning (applying tension offer the concrete has set). This is more suitable for large elements, especially long beams, which cannot be transported, and therefore need to be poured (5)..... Before the concrete is poured, ducts (usually plastic tubes) are placed inside the (6)..... along the length of the beam. These ducts contain steel cables. After the concrete has been (7)..... and has gained sufficient (8)..... strength, the cables are put in tension, using jacks at either end of the beam. This is only possible because the cables are free to move within the ducts - it is not possible with pre-tensioned reinforcing bars, which are held fast by the hard (9).... surrounding them.

The ends of the cables are then permanently anchored at either end of the beam.

 

Over to you

Think about a reinforced concrete structure in tour area - for example. a building or a bridge. In what sequence do you think it was built? Do you think it was poured in-situ, or were its parts precast?

Wood

A Categories of wood

The two main categories of wood are:

- hardwood - usually from deciduous trees, which lose their leaves in autumn, although some hardwood (for example, gram tropical hardwood) comes from other types of tree

- softwood - from coniferous trees, which remain green throughout the year.

In engineering, wood can be categorized as:

- solid wood - softwood or hardwood that has been sawn into specific shapes and sizes, but whose natural structure, consisting of grain and knots, remains intact

- engineered wood - made by bonding (sticking together) layers of solid softwood or hardwood, or by mixing quantities of wood particles and bonding them with resin.

 

Notes:

in industry, wood is often referred to as timber (BrE) or lumber (AmE). In American English, timber generally means wood that is still growing in trees. Knot is pronounced /not/ (the k is silent).

 

B Solid structural timber

The text below is from a technical handbook about structural timber - wood intended to support loads in a structure.

Generally, timber is cut to the required section - the width and depth that determine its cross- section - at a sawmill, where a range of section sizes are produced. Timber from sawmills is generally supplied in rough-sawn sections. This refers to the surface texture produced by sawing timber with a circular saw. If the timber needs to have a smooth finish - for example, because it will be visible in the structure - it can subsequently be planed to smooth its surface.

 

Because the strength of wood varies, structural timber must be stress-graded. This means its strength is tested in order to give it a stress grade - a standard strength value which an engineer can use for design calculations. Timber can be mechanically stress-graded, where its strength 4 is checked by machine. It can also be visually stress-graded, where the wood is examined by an inspector who looks for potential weaknesses - in particular, the position of knots.

 

C Engineered wood

Engineered wood covers a range of softwood and hardwood materials.

It includes:

- cheap, low-strength boards, such as particle board (often called chipboard) and medium- density fibreboard (MDF)

- stronger boards suitable for structural use - primarily orientated strand board (OSB), which is made from strands of wood bonded with resin, and plywood, which consists of several plies (layers) of solid wood, bonded so that the grain of each ply runs at 90 degrees to that of the adjacent plies, to provide increased strength

- glue-laminated sections - sometimes called glulams - which can be used as major structural elements, such as beams, in large buildings.

 

17.1 Match the two parts to make correct sentences about wood. In each case, there is more than one possible answer. Look at A opposite to help you.

1. Engineered wood

2. Softwood

3. Solid wood

 

a. comes only from coniferous trees;

b. comes only from deciduous trees;

c. can come from either coniferous or deciduous trees.

d. specifically describes single pieces of timber, not multiple pieces that have been bonded together;

e. is always made from multiple pieces or particles of wood;

f. may have knots in it.

 

17.2 Complete the sentences below using words and expressions from B opposite.

1. Wood has a smooth finish after it has been..........

2. Wood cut with a circular saw is called...... timber.

3. After timber is tested for strengths and weaknesses, it is given a........

4. When timber is inspected by a person who looks for weaknesses, it....

5. When timber is inspected by a machine which tests its strength, it is.....

 

17.3 Complete the article about the environmental considerations of wood using words from B and C opposite.

From an environmental perspective, wood has many advantages. Firstly, it comes from a sustainable source. Coniferous trees grow relatively fast, providing a rapidly replaceable source of (1)............... Secondly, almost all the timber in a tree can be utilized, leaving little or no waste. The best quality wood can be used for structural applications, where solid, (2)....... sections are required by engineers; for high-strength elements such as (3)..... beams; and in the high-quality plies used to make (4)..... Smaller strands can be made into engineering wood with structural properties, such as (5)......

And small particles and fibres, including those from waste timber, can go into cheaper materials, like (6)...... board and (7)......

 

Over to you

What types of timber are used in your home and/or office, both as building materials and to make fixtures and furniture within the building?

 

Material properties 1