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Synthetic Materials Answers Worksheet 1 A synthetic fibre that replaces silk Time to Think (p. 9) 1. Nylon stockings were much cheaper than silk stockings, and they were smoother, stronger and more elastic. 2. Fabrics made from nylon is soft, light, strong, elastic, and resistant to chemicals. 3. A water molecule is given out. 4. Diacid has two acid groups and diamine has two amino groups. Each acid group of diacid and each amino group of diamine can react to form strong bonds. 5. Both of the monomers of nylon and silk are joined by strong bonds formed between acid groups and amino groups. 6. Swimwear, curtain, string or rope, raincoat, racket string, dental floss, etc. 7. Biological washing powder can digest silk, which is a protein. So it cannot be used for clothes made from silk. However, it can be used for nylon fabrics because nylon is not a protein. 8. Polyester, acrylic, Dacron, Teflon, etc. © Times Publishing (Hong Kong) Limited -1- Synthetic Materials Answers Worksheet 2 Composite materials Time to Think (p. 15) 1. The wall of the houses may be hard and strong, but may be brittle and may not be resistant to weathering and fire. 2. In a mud brick, the mud resists compression and the straw stands tearing. Thus the composite material has good compressive strength and good tensile strength. 3. The matrix material is cement and the reinforcement materials are small stones and sand. 4. Steel rods make the cement stronger so that it is more capable of standing compression and bending. 5. CFRP is lighter and stronger than metals, and can block the spread of a crack that appears on the body. 6. Fibreglass can undergo more elongation before it breaks than carbon fibre composite. 7. (a) Wood is made up of a large number of thick-walled cells. The cell wall of these cells contains an organic matrix embedded with fine fibres of cellulose and lignin. The cellulose fibres make the wood tough and strong. Lignin gives the wood additional strength and rigidity. (b) Bone is made up of an organic matrix embedded with protein fibres and calcium salts. The protein fibres make the bone strong and flexible. The calcium salts make the bone hard and rigid. -2- © Times Publishing (Hong Kong) Limited Synthetic Materials Answers Worksheet 3 Metal with memory Time to Think (p. 21) 1. They wanted to test the flexibility of the alloy strip. They were surprised to observe that the metal strip immediately reverted to its original shape when it was heated with a lighter. 2. Buehler and Wang thought that the property change of Nitinol might be due to a change in the atomic arrangement or crystal structure of the alloy. 3. Pressure has no effect on the shape of the A-phase, but will change the shape of the M-phase. 4. The A-phase has a symmetrical cube-like crystal structure which is very rigid and cannot be deformed by pressure easily. The M-phase has a less symmetrical crystal structure which is less rigid and can be deformed easily by pressure. 5. A dental brace made from SMA applies a constant pressure on the teeth. The wire does not need to be retightened when it contracts as the teeth move, unlike the conventional stainless steel wires. 6. The stent expands to its original shape when its temperature rises to that of body temperature. 7. The SMA wire can exert a constant force to hold the separated parts of a fractured bone tightly together until the fracture of the bone heals. 8. No definite answers. Students may obtain the relevant information on websites such as: http://www.stanford.edu/~richlin1/sma/sma.html http://webdocs.cs.ualberta.ca/~database/MEMS/sma_mems/sma.html http://en.wikipedia.org/wiki/Shape_memory_alloy © Times Publishing (Hong Kong) Limited -3-