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Rubber latex

Latex, before the advent of synthetic rubber, was a term applied solely to the milk of tropical plants and rubber trees which are known to yield a rubber latex, a natural emulsion of rubber particles.

Today, latex is a term also used to refer to any rubber- like polymer in emulsified form.

The rubber molecules in the form of very long hydrocarbon chain.

It may have a molecular weight as high as 250.000. In natural rubber the molecules are intertwined and held together by van der Waals forces to give "drop" of rubber about 3 in diameter. These drops are suspended in water, when first obtained from the plant, the system resembling an emulsion. Proteins, fats, soaps, and other substances are present in the milky fluid, which is called "latex".

The usual process applied to rubber latex is to add acid, usually formic, which renders the emulation unstable and coagulates the rubber. A spongy mass is produced which is passed through rolls to squeeze out excess latex and to form a sheet of rubber. The sheet is usually "smoked" with the fumes from burning green wood, which helps to preserve the rubber. This product is called "crepe rubber" and is the raw material for most rubber manufacture.

It is necessary, for certain purposes, to make use of the latex itself as a raw material. This necessitates two modifications to the natural latex. Firstly a preservative must be added to suppress bacterial action, which would eventually lead to coagulation. This is achieved by the addition of ammonia. This prevents the development of acidity, which is the means of causing coagulation. The natural product contains only 35 percent rubber suspended in about 60 percent of water. Concentration may be effected by using a centrifuge, in a manner similar to the centrifuging of milk to give cream. It has recently been shown that the addition of konnyakn meal, which is a complex carbonhyrate soluble in water, to latex is very effective in producing a rich cream of high rubber content. Rubber is lighter than water and hence form a cream and not sediment.

The creaming is done in a tank. The initial effect of the creaming agent is to cause a clustering of the rubber particles. The resulting large effective size of the units produces their Brownian motion and increases the speed with which they rise through the liquid. Clustering differ from coagulation in that it is reversible, suitable changes being able to redisperse the particles. This is of importance since coagulation would prevent the use of latex for special methods of manufacture. Soon after the addition of the creaming agent to the tank, a very deep cream layer is formed, which is built of clusters linked together, with water filling the spaces. Water then passes out of the cream layer, the lower boundary line gradually ascending. After about four days the liquid may be run off from valve A and the latex cream from valve B. Proteins and swollen meal particles may then centrifuged off, and the cream is stabilized with ammonia for shipment in drums.

The colloidal behavior of latex rubber is largely the result of the protective layer of protein with which the particles are surrounded. Clustering is influenced by the action of the creaming agent on this surface layer. Coagulation occurs when the acidity is such as to render the adsorbed protein molecules electrically neutral. These facts are closely related to the protective action of proteins on gold sols.

Exercises

I. Answer the following question:

1. What is latex?

2. What components of latex do you know?

3. What is meant by "crepe rubber"?

4. What is difference between "a ream" and "a sediment"?

5. Can you explain what is meant by the term "clustering"?

6. What is coagulation?

7. When is latex used as a raw material?

II. Describe the process of rubber latex production.

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

1. The colloidal behavior of latex rubber is largely the result of the protective layer of protein with which the particles are surrounded.

2. It is necessary, for certain purposes, to make use of the latex itself as a raw material.

3. Proteins, fats, soaps, and other substances are present in the milky fluid, which is called "latex".

4. We shall not consider synthetic plastic materials, partly because of their similarity.

5. The utility of rubber is ultimately related to the ease, with which it sustains very large distortions, returning to its original shape when released.

6. To coat positively charge electrodes with the particles, use can be made of the electric charge on the rubber particles,

7. The latex is mixed with sulphur, accelerators, zinc oxide, etc., in the form of powders dispersed in water with a protective colloid such as gelatin present.

8. Concentration may be effected by using a centrifuge, in a manner similar to the centrifuging of milk to give cream.

9. Numerous ingredients other than sulphur are normally included in the rubber mixing, the proportions depending on the grade of rubber required.

10. The rubber molecule may have a molecular weight as high as 250.000.

11. The sulphur, zinc oxide and accelerators are referred to collectively as the vulcanizing system.

12. Apart from its use in such specialized products as adhesive, the full potentialities of rubber latex are only realized on vulcanization.

IV. Give Vietnamese equivalents:

to break down

to build from

to centrifuge off

to correspond to

to rise through to run off

to split up

to squeeze out

to pass through

V. Translate into Vietnamese paying attention to the word with:

1. to coat positively charge electrodes with the particles;

2. to combine with rubber molecules;

3. to extract with acetone;

4. to mix with sulphur;

5. to smoke with fume;

6. to stabilize with ammonia;

7. when tested with iodine;

8. some substances remain with impurities;

9. with avoidance of agitation;

VI. Translate the following sentences into Vietnamese paying attention to the - ing forms:

1 Latex may be used for impregnating paper, leather, or cloth, the rubberized product being water proof.

2. The initial effect of the creaming agent is to cause a clustering of the rubber particles.

3. Clustering is influenced by the action of the creaming agent on this surface layer.

4. Soon after the addition of the creaming agent to the tank, a very deep cream layer is formed, which is built of clusters linked together, with water filling the space.

5. Articles are formed by dipping shapes in the latex, drying and vulcanizing in hot air.

6. The "drops" of rubber are suspended in water, when first obtained from the plant, the system resembling an emulsion.

VII. Translate the following sentences into Vietnamese and state the function of will and would:

a)

1. When stretched rubber flows, it will not return to its original shape.

2. If heating is carried on long enough in the presence of curatives, the latex will not form a continuous film on drying but will crack into a multitude of small pieces.

3. Styrene will polymerize to produce polymers other than rubber- like bodies.

4. Ethylene chloride will react with sodium polysulfide to form high molecular weight rubber- like substances.

b)

5. Coagulation would prevent the use of latex for special methods of manufacture.

6. To suppress bacterial action, which would eventually lead to coagulation, a preservative must be added to latex.

7. Were unstretched rubber cooled, a slow crystallization would take place, giving a harder and less extensible material.

8. The atoms of the molecule share in the general thermal motion at any temperature, so that a free molecule would be continually coiling, twisting and, changing its shape.

9. Density considerations make it impossible to accept a theory, which would require substances to polymerize with the decease in density as, would be the case if the polymers spin.

VIII. Translate the following sentences into English:

1. Ngµy nay cao su tæng hîp ®­îc sö dông réng r•i ®Ó thay thÕ cao su tù nhiªn còng nh­ ®Ó s¶n xuÊt c¸c vËt dông cã tÝnh chÊt ®Æc dông.

2. Chóng ta biÕt r»ng, latex, ®­îc ®iÒu chÕ b»ng ph­¬ng ph¸p Gevei, lµ hÖ phan t¸n cña c¸c h¹t cao su vµo n­íc.

3. T¸c dông cña c¸c chÊt t¸ch kem ë giai ®o¹n ®Çu lµ g©y ra sù ®«ng tô cña c¸c h¹t cao su.

4. Nh­ ®• biÕt, sù ®«ng tô lµ qu¸ tr×nh keo tô mµ cã thÓ lµ thuËn nghÞch tøc lµ trong mét sè ®iÒu kiÖn thÝch hîp th× ®¸m ®«ng tô Êy cã thÓ bÞ ph©n t¸n ra.

5. Ng­êi ta ®• x¸c ®Þnh ®­îc r»ng vÒ b¶n chÊt sù keo tô kh«ng ph¶i lµ qu¸ tr×nh ho¸ häc mµ lµ mét qu¸ tr×nh vËt lý.

6. Sù ®un nãng, sù lµm qu¸ l¹nh, sù khuÊy trén m¹nh,... lµ nh÷ng yÕu tè bªn ngoµi g©y ra sù keo tô.