Text 9

Storage hardening

The presence of microgel particles in natural rubber latex has a directly practical consequence in the phenomenon of storage hardening. Rubber, either in the form of latex or solid sheet, when stored for considerable periods develops an increased hardness (as measured by Mooney viscosity or Williams plasticity), the change in the solid sheet being generally greater the lower the initial hardness.

The changes are greatly influenced by the relative humidity of storage. In dry air the rate at which hardening occurs increases with decreasing sample thickness or with increasing temperature. The removal of acetone soluble and nitrogenous components does not prevent the hardness increase.

One might expect that the increase in hardness, which occurs when ammoniated latex is stored, is a result of intra- particle crosslingking and microgel formation. With solid rubber the crosslinking process is no longer confined to within the original latex particles, but there is no reason to suppose that the mechanism of the crosslinking reaction differ in the two cases.

Although the nature of the spontaneous hardening reaction remains obscure considerable information about the functional groups involved and on methods of control has recently been forthcoming. Rubber hydrocarbon interacts with oxygen to form hydroperoxidic compounds and the decomposition of such compounds could result either in degradation or crosslinking of the rubber molecules, according to condition. However, crosslinking via hydroperoxide decomposition does not appear to be a major cause of storage hardening. Since hardening can be almost completely suppressed by the addition of the latex of monofunctional carbonyl reagents (especially demendone which is a specific reagent for aldehyde groups) it has been inferred that the crosslinking reaction involves carbonyl groups in the rubber. The number of such carbonyl groups per rubber molecule can be estimated by measuring the concentration of hydroxylamine required to inhibit completely storage hardening; value of 9-29 was found for a number of clonal rubber. Difunctional amnines, such as benzidine, in contract to their monofunctional analogues, promote crosslinking by coupling reaction.

If it is accepted that storage hardening results from the presence of carbonyl groups in very small proportion, it is then desirable to determine how these groups originate. It has been suggested that they are incorporated at intervals along the "hydrocarbon chain", but it has still to be shown that they are an integral feature of natural rubber molecules. Biologically induced oxidation could proceed in the vicinity of the tapping cut or in the latex vessels. The generation of oxygenated groups in secondary reactions is consistent with our present knowledge of polyisoprene biosynthesis.

Exercise

I. Answer the following question:

1. What is storage hardening?

2. In what way can one measure rubber hardness?

3. Does the removal of acetone- soluble components prevent rubber harness?

4. How can the hardening process be suppressed?

5. The nature of spontaneous hardening reactions is obscure to investigators, isn't it?

6. In what way is the number of carbonyl groups per rubber molecule estimated?

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

1. Repeated washing of latex in a centrifuge, followed by drying, results in rubber suited for insulation purposes.

2. Homologs of aniline result from the reduction of their corresponding nitro compounds.

3. In the experiment conducted in the laboratory the resultant product is a vitreous solid that softens at 1000.

4. The fatty acids resulting from the hydrolysis are neutralized by the use of sodium or potassium hydroxide.

5. If chain molecules are dissolved in a low- molecular- weight solvent, the resulting solution is nonbifringent.

6. It is accepted that storage hardening results from the presence of carbonyl groups in very small proportion.

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

1. Unvulcanized rubber is both plastic and elastic.

2. Rubber either in the form of latex or solid sheet, when stored for considerable periods develops increased hardness.

3. The generation of oxygenated groups in secondary reactions is consistent with our present knowledge of polyisoprene biosynthesis.

4. It has been suggested that the carbonyl groups (in storage hardening) are incorporated at intervals along the "hydrocarbon chain".

5. As a result of Goodyear's work, along with improvements by Hancock, rubber goods of wide utility and durability could be made.

6. Modern industry could no longer function properly without reinforced rubber.

7. Polystyrene has been used commercially long before the high styrene resins.

IV. Put question to the words in bold type:

1. The Hardness of rubber is measured either by Mooney viscosity or Williams plasticity.

2. The generation of oxygenated groups in some secondary reaction is consistent with our present knowledge of polyisoprene biosynthesis.

3. The infra-red spectrum is closely similar to that of natural rubber.

4. The stress produced by shearing is very low compared with that of natural rubber.

V. Translate the following sentences paying attention to the function of the verb to do:

1. Monosaccharides do not hydrolize to simpler substances.

2. An organic chemist has much to do with different reaction.

3. It is known that most fibers when suspended in an alkaline aqueous medium have s negative charge as do the particles in alkaline latex.

4. Upon what structural arrangement does the colour of an organic compound depend?

5. Some grades of synthetic material have a lower cis- content than does natural rubber.

6. The results of the experiment show that adhesion does increase for a time after curing, at least when measured at room temperature.

7. On the basis of oxidizability, the zinc salt had the effect, as did mercaptobenzimidazole itself.

VI. Translate the following sentences paying attention to the "Complex Object":

1. We consider the natural rubber to be isomerized.

2. Investigators find the infra-red spectrum to be closely similar to that of natural rubber.

3. One might expect the increase in hardness to be a result of intra- particle crosslinking.

4. Researchers dealing with latex know biologically induced oxidation to proceed in the vicinity of the tapping cut.

VII. Translate into English:

1. CÇn nhÊn m¹nh r»ng ®é cøng cña cao su trong latex chøa NH3 ®­îc t¨ng c­êng trong qu¸ tr×nh b¶o qu¶n.

2. Ng­êi ta cho r»ng sù t¨ng ®é cøng cña cao su , mét mÆt ®­îc g©y ra bëi sù dÝnh kÕt néi h¹t, mÆt kh¸c bëi sù t¹o c¸c gel vi m«.

3. Còng nh­ ®• biÕt qu¸ tr×nh t¨ng ®é cøng cña cao su trong latex cã thÓ bÞ ngõng b»ng c¸ch thªm vµo ®ã c¸c hîp chÊt cacbonyl.

4. Nh­ chóng ta ®• biÕt, c¸c amin l­ìng chøc kh¸c víi c¸c amin ®¬n chøc lµ chóng xóc tiÕn cho qu¸ tr×nh nèi kÕt trong ph¶n øng kÕt hîp.

5. Trong kh«ng khÝ kh«, tèc ®é t¨ng ®é cøng cña cao su t¨ng khi gi¶m ®é dµy cña mÉu còng nh­ khi t¨ng nhiÖt ®é.