Text 10

Thermoplastic elastomers

Physical properties and applications

The development of that part of the rubber industry concerned with the fabrication of rubber articles has been primarily concerned with two processes. First, in order that an article of a given shape can be made by some kind of molding process, the viscous properties of raw material must be utilized and possibly enhanced by mastication and degradation. Once the article is shaped, however, viscous flow becomes undesirable, and intermolecular motion must be prevented while retaining sufficient chain segment mobility to provide elastomeric properties.

The thermoplastic industry was the first concerned with rigid polymers, which displaced traditional materials partly because of the economic advantages of high automatic production rates made possible by easy flowing melts and rapid set up on cooling. In recent years, the thermoplastic industry has introduced flexible material such as plasticized PVC and ethylene - vinyl acetate copolymer. Though these materials have some rubbery charater, they do not compare with conventional vulcanizates as regards rapid retraction with low set from high elongation, resilience, and other physical properties, or as regards versatility and aesthetic appeal in many applications.

There are many diverse materials which require chemical reaction (usually at elevated temperature) to make useful articles, and these are loosely classed as thermosetting materials. They range from hard rigid products, through flexible materials to soft high elastic rubber. Another loose classification can be termed thermoplastic. Representatives of theses materials occur only in the first categories, however, the rigid and the flexible.

Thermosetting Thermoplastic

Rigid Epoxies

Phenol- Formaldehyde

Urea- Formaldehydes

Hard rubber Polystyrene

Poly- Vinyl Chloride

Polypropylene

Flexible Highly Vulcanized Rubbers Polyethylene

Ethylene-Vinyl-Acetate Copolymer

Plasticized PVC

Rubbery Vulcanized

Rubbers (NR, SBR, IR, etc,) Thermoplastic Elstomers

Thermosetting and thermoplastic material

Description of new products

The materials are block copolymers of styrene and butadiene. Their low viscous component results from long chain entanglements and van der Waal's forces between polymer chains. No covalent or chemical crosslinking, that is, no vulcanization or cure reaction is involved in their use. At lower temperature the interchain forces are sufficiently strong and the sites sufficiently immobile to permit typical snappy rubbery behavior, tensile strength up to 5.000 psi, high resilience, etc.; yet, at elevated temperatures, they are labile with the result that these materials have melt viscosities very similar to conventional thermoplastics at normal processing temperatures.

The thermoplastic elastomers are somewhat superior to conventional natural rubber or SBR vulcanizates in retaining their elastomeric properties down to low temperatures. The flexible thermoplastic, on the other hand, lose their flexibility at much higher temperatures, and in fact can become unusably stiff under fairly mild climatic conditions.

(to be continued)

Exercises

I. Answer the following questions:

1. What processes are generally used in the fabrication of rubber articles?

2. What is influence of viscous flow on the shaping process?

3. Is viscous flow desirable when the article is shaped?

4. Why do thermoplastic elastomers flow at elevated temperatures?

5. Why did thermoplastic polymers replace traditional materials?

6. What flexible thermoplastics are widely known at present?

7. Why are thermoplastic elastomers superior to conventional natural rubber?

8. What are the main characteristics of thermosetting material?

II. Translate the following sentences into Vietnamese, paying attention to the words in bold type:

1. A great number of experiment concerning the nature and extent of filler- rubber adhesion have been conducted?

2. The thermoplastic industry was first concerned with rigid polymers, which displaced traditional ones

3. Artificial ageing tests concerned are an extremely important part of rubber testing.

4. The development of that part of the rubber industry concerned with the fabrication of rubber articles has been primarily concerned with two processes.

III. Translate the following sentences into Vietnamese, paying attention to the words in bold type:

1. Though thermoplastic elastomers have some rubbery characters, they do not compare with conventional vulcanizates as regards rapid retraction with low set from high elongation, resilience, or as regards versatility and aesthetic appeal in many applications.

2. Petroleum and its distillates can not be regarded as a source of raw elemental sulphur.

3. The problems regarding the use of pigments in latex are similar to those in other types of coatings and finishes.

4. Gerlach developed an interesting table with regard to the boiling points of glycerol solutions.

IV. Form nouns from the following adjectives:

capable

flexible

labile Mobile

Versatile

V. Translate into Vietnamese, paying attention to the Prefixed:

covalent

degradation

displace

interchain

intermolecular

immobile Premastication

Recycle

Unaffected

Undesirable

Unduly

Unusable

VI. Translate into Vietnamese paying attention to the Gerund

1. Reheating rubber is always attention by increasing its volume.

2. Merret identified the component responsible for producing the sol with the graft copolymer.

3. Many surface coatings shrink slowly on having been aged.

4. Natural rubber vulcanized with zinc oxide in the absence of free sulphur is known for being resistant to aeing.

5. The highly coiled rubber chains permit their being extended up to seven times their original length.

VII. Give a short summary of the text.

VIII. Translate the following sentences and state whether the ing- form is expressed by the gerund or by a participle:

1. The thermoplastic elastomers are somewhat superior to conventional natural rubber in retaining their elastomeric properties down to low temperatures.

2. The novel products permit the manufacture of elastomeric articles by the use of high speed thermoplastic processing techniques.

3. The new materials can be modified to impart desirable properties without affecting the strength of the polymer network or the capability of obtaining desirable melt flow characteristics.

4. The thermoplastic elastomers are capable of modification in many directions to give special properties while retaining their basic dual elastomeric nature.

5. Stress- strain properties are essentially unaffected by repeated melting and recovery of solid polymer.

6. The latex rubber, having had no mastication, produces a tough, strong film with superior ageing properties.

7. If the latex or latex compound is mixed with a certain thickening agent the amount of compound which clings to the surface of the forms after being immersed is increased.

IX. Translate the following sentences into English:

1. C¸c ®å vËt nhùa ®­îc ®Þnh h×nh cã thÓ ®iÒu chÕ ®­îc tõ hçn hîp c¸c chÊt cã tÝnh nhít.

2. Ng­êi ta cho r»ng, tÝnh ch¶y dÎo cña hçn hîp cÇn gi¶m ®i khi c¸c ®å vËt võa míi ®­îc h×nh thµnh.

3. Nh­ ®• biÕt, gÇn ®©y nh÷ng vËt liÖu nhiÖt dÎo,r¾n vµ cøng ®• thay thÕ c¸c kim lo¹i trong hµng lo¹t c¸c lÜnh vùc kü thuËt.

4. C¸c chÊt nhùa ®µn håi cã nh÷ng tÝnh chÊt gièng c¸c vËt liÖu nhiÖt dÎo, còng nh­ lµv c¸c tÝnh chÊt cña cao su.

5. Mét mÆt, ë nhiÖt ®é trung b×nh vµ thÊp nhùa ®µn håi gi÷ ®­îc c¸c tÝnh chÊt cña nhùa, nh­ng mÆt kh¸c ë nhiÖt ®é cao chóng cã kh¶ n¨ng ch¶y nhít.

6. Nh­ ®• biÕt, c¸c chÊt ®µn håi lµ copolyme cña styren vµ buta®ien.

7. C¸c nhµ khoa häc cho r»ng, nhùa ®µn håi kh«ng chøa bÊt kú mét liªn kÕt ho¸ häc hoÆc mét liªn kÕt cÇu nµo.

8. Ng­êi ta cho r»ng, ®Ó chÕ biÕn nhùa ®µn håi cã thÓ dïng c¸c thiÕt bÞ b×nh th­êng cña nhµ m¸y s¶n xuÊt nhùa.