1. Tutorial 1 (MATS 602) 6 th Semester 2011 Prepared by: Prof. Dr. Sayed Abdelbary Teaching Assistant: Yasser Sheasha

2. 1-Materials expand when heated because: a) The mean interatomic distance increases as the thermal energy of the atoms increases. b) The heat takes up extra volume forcing the material to expand c) The rubber melts d) There are more chemical bonds being formed 2- When rubber is put under uniaxial tension it: a) Contracts b) Extends c) Explodes d) Rises 3-Rubber differs from all other materials in that: a) The density of cross-links is higher b) It can be extended by more than 100% c) Its stiffness increases with increasing temperature d) It demonstrates visco-elastic behavior Question 1 Choose the correct answer:

3. 4-Approximately how many monomers are there in between cross-links in rubber? a) 5-10 b) 50 – 100 c) 500-1000 d) 5000 - 10000 5-If a load is suspended from a rubber strip at room temperature, and the temperature is then reduced by 20°C, the load will be: a) Pulled upwards b) Lowered c) Released d) Unaffected 1) a 2) b 3) b, c, d 4) c 5) b

4. You have four polymers having the following date: TmTm TgTg -150 o CPolymer A 175 o C0 o CPolymer B Cross-linked--81 o CPolymer C -100 o CPolymer D Which of them have good mechanical properties. Why? Solution Polymer A, because it has glass transition temperature above room temperature. Polymer B, because it has melting temperature (crystalline) Polymer C, due to the presence of cross-links Question 2

5. Mechanical Properties of Polymers In order to have a polymer with reasonable mechanical properties at least one of the following conditions should be fulfilled: 1-Tg of the polymer is above room temperature 2-Have a reasonable degree of crystallinity. 3-Crosslinked

6. The glass transition temperature, Tg of polyethylene, PE is about -80 o C and for NR (natural rubber) is about -70 o C. explain why the mechanical properties of non-vulcanized NR is much less than PE. Solution: Because PE is a crystalline polymer which improve its mechanical properties while NR isn’t crystalline. Question 3

7. During the winter months, the temperature in some parts of Alaska may go as low as -55 o C. Of the elastomers natural polyisoprene, styrene- butadiene, acrylonitrile-butadiene, chloroprene, and polysiloxane which would be suitable for automobile tires under these conditions? Why? Useful temperature range ( o C)elastomer -60 to 120Natural polyisoprene -60 to 120styrene-butadiene -50 to 150acrylonitrile-butadiene -50 to 105chloroprene -115 to 315polysiloxane Question 4

8. From the above table, only natural rubber, poly(styrene-butadiene), and polysiloxane have useful temperature ranges that extend to -55 o C. At temperatures below the lower useful temperature range limit, the other elastomers listed in this table become brittle, and therefore, are not suitable for automobile tires. Solution:

9. Why styrene- butadiene-styrene block copolymer can’t be used in manufacturing of car tires? Although SBS is an elastomeric material, it can’t be used for car tire because during rolling of the tire its temperature reaches to 120 o C which is above the glass transition temperature of styrene blocks leading to flow of styrene and the cross-links damaged. Solution: Question 5

10. Which of the following elastomers can be used as: 1- oil seal. 2- ozone (O 3 ) resistant. b) Polybutadiene c) Ethylene-propylene rubber d) Styrene-Butadiene Rubber, SBR a) Polychloroprene Question 6

11. Solution: 1- Oil seal: Polychloroprene due to the presence of polar bond C-Cl 2- Ozone resistance: Ethylene-propylene rubber because all the bonds are single bonds and there is no double bond which cause the attack of ozone.

12. Explain why EP random copolymer is elastomer (not crystalline) material. Solution: Because introducing propylene units along polyethylene chain randomly disturb the structure and leading to irregularity and the resulting polymer will be amorphous. Question 7

13. OH a) b) For the above two equations which of them leads to more elastic polymer? Why? Solution: The product from equation (a) is more elastic than that of equation (b) because of the presence of extra phenyl (delocalized electron) group in (b) restrict the rotation of the chains (T g increase). 1, 4-dihydroxy benzene Question 8

14. Describe the influences of the acrylonitrile content of NBR on their properties. Question 9 Acrylonitrile contains the polar Nitrogen group which produces intermolecular forces and high chain packing. Thus the presence of the Nitrile group gives this elastomer superior oil resistance properties. Applications requiring good oil resistance are the principal uses of nitrile rubber compounds. The automotive industry has been the primary consumer of nitrile rubber based products with hoses, fuel lines, and numerous O-rings and seals Solution:

15. Compare the properties of CR, NBR and butyl rubber with those of natural rubber on the basis of their structure. Question 10 Butyl Rubber: its primary use is due to its low permeability to small- molecule diffusants as a result of their efficient intermolecular packing (101–103) as evidenced by their relative high density (density of 0.917 g/cm3). This efficient packing in isobutylene polymers leads to their low fractional free volumes and low diffusion coefficients for penetrants. The highly saturated chain gives the rubber higher resistance to oxydation Polychloroprene: a butadiene elastomer with chlorine present in the backbone,the polymer exhibits excellent tensile strength and low hysteresis, much like natural rubber. The polarity imparted by the chlorine atom improves the oil and solvent resistance approaching those of nitrile polymers. The polymer is also a high flame retardant

16. Natural Rubber: The structure of NR is of 98% cis Polyisoprene. The polymer is very flexible and is self stiffening as it gets crystallized under tension. NR has very high mechanical properties (strength, resilience, tear strength,…). NR is weak against oxygen and ozone also against heat aging due to the large degree of unsaturation in the backbone Nitrile Butadiene rubbers: The ratio of acrylonitrile to butadiene has a direct effect on the properties and the nature of the polymers. As the acrylonitrile content increases, the oil resistance of the polymer increases. As the butadiene content increases, the low temperature properties of the polymer are improved. The primary benefit of the polymer is its oil and solvent resistance due to the polar interaction by the Nitrogen.

17. Question 11 The low temperature flexibility of NR is better than NBR Write explanatory notes on each of the following statements: NR has much higher gum tensile strength than SBR Due to the high chain stereo-regularity, stretching of NR vulcanisates leads to crystallization which leads to strengthening of rubber. This self-reinforcement properties result in high tensile strength. This stereo-regularity is missing in SBR random copolymer so it is not self-reinforced and need fillers to enhance its mechanical properties. EPDM and EPR have better ozone resistance than NR Ozone always attack points of high electron density (Unsaturated bonds) causing chain scissoring. EPR and EPDM are totally saturated chains with no double bonds in case of EPR and side double bonds in EPDM (not in the main chain) so both rubbers are highly resistant to ozone attack The structure of NR is very flexible (double bonds and single bonded carbon atoms) and giving it very low value of Tg so even at low temperatures the rubber still possesses higher flexibility.

18. Question 12 What are the requirements needed for the polymer to have elastomeric nature Answer in lecture!!

19. Question 13 What is a copolymer and how are they formed? A copolymer is formed when more monomers are mixed together than are required for normal homopolymer formation. For addition polymerization this means that two different monomer types are required. For condensation polymerization three different monomer types are required. When these monomers are mixed, the resultant polymer has some of each of the monomers within it. These monomers can be arranged in four different patterns. The properties of the copolymer are normally intermediate between the properties of the homopolymers that would have been formed from pure, single-type monomers.

20. Question 14 Discuss how copolymerization would affect crystallinity. The nature of copolymerization would cause more randomness to occur along the molecular chain than would the corresponding homopolymers. Therefore, would be lower

21. Question 15 What is the mechanism and thermodynamic explanation that explains why elastomers recover to their original shape after being elastically stretched? When an elastomer is stretched the molecules become more ordered. This causes a decrease in entropy. Since entropy naturally tends to increase, when the stretching force is removed the natural tendency of entropy to increase supplies the energy for the molecules to return to their more energetically favored, random configuration.

22. Question 16 Explain why polybutadiene is softer than polyisoprene. Polyisoprene, either cis or trans, has a methyl group that is pendent to the four-carbon backbone repeating unit of the polymer. Polybutadiene has no methyl group attached to its four-carbon repeating unit. This methyl group in polyisoprene interacts with other molecules as the molecules are moved past each other and also causes some restrictions to bending and twisting within each molecule. These intermolecular and intramolecular interactions cause the polyisoprene to have higher energy that is manifested as higher hardness and higher tensile strength.

23. Question 17 Describe the change in properties that are likely to occur when the temperature is lowered below the Tg for an elastomer. What does this indicate about the usable lower temperature range for elastomers? As with most plastic materials, when the temperature of an elastomer is lowered below its Tg, the polymer will embrittle. This occurs because the long-range flexibility of the polymer is decreased and the intermolecular forces increase as the polymer motion is reduced with the reduction in temperature. Therefore, below Tg, elastomers are not elastomeric but are, rather, hard and brittle. The lower temperature range for elastomers is, therefore, the Tg.

24. Question 18 Explain why copolymers of ethylene and propylene can be thermoplastic elastomers when the pure polyethylene and pure polypropylene are not elastomeric. Discuss whether you would expect a blend of polyethylene and polypropylene to be elastomeric? The homopolymers of ethylene and propylene are nonelastomeric because they both have high crystallinity. Even low molecular weight polyethylene has considerable crystalline content. However, when the materials are copolymerized, the ability of the polymers to pack together tightly is significantly reduced when compared with the homopolymers. (The copolymer is much like atactic polypropylene which is very soft and flexible.) A blend of polyethylene and polypropylene would likely have regions of crystallinity and would not, therefore, be elastomeric

25. Question 19 Why would fluorocarbon elastomers be so effective in resisting solvents? As with fluorocarbon molding resins, the presence of fluorine in the elastomer reduces all chemical activity, including solvation effects. This occurs because of the strength of the carbon-fluorine bond and the tightness with which the fluorine electrons are held. These tightly held electrons and the strong, short carbon-fluorine bond mean that the fluorocarbon molecules are repellent to attack from other atoms or molecules. Hence, their solvent resistance is extremely high.

26. Question 20 1. Polychloroprene, though containing more unsaturation, has higher resistance to ozone than nitrile rubber. Furthermore, nitrile rubber cures readily with sulfur, while polychloroprene does not. Explain. NBR is a copolymer of butadiene and acrylonitrile. As with BR, unprotected double bonds in the butadiene portion are readily cleaved by ozone and can undergo crosslinking via sulfur. The double bonds in CR are protected by a polar, electron withdrawing chlorine group. This reduces reactivity of the double bonds towards ozone and towards the curing reaction with sulfur.

27. Question 21 Homopolymerization of ethylene or propylene results in a hard plastic, while a 50/50 copolymer is an elastomer. Why? Homopolymers of ethylene and propylene have low glass transition temperatures, but they have regular structures and are crystalline solids at room temperature. Copolymerization creates molecular chains with an irregular micro structure that cannot fit together into a crystal lattice. Hence, the copolymer is an amorphous rubbery material with low Tg.