a
BUILDING A NATURAL RUBBER LATEX COMPOUND' DALE W. BUTTON Globe Manufacturing Company, Fall River, Massachusetts
NaTunAL rubber is a high polymer made up of isoprene u d s as the building blocks. Modern viscosity methods indicate that the natural rubber molerules2 have an average molecular weight of 1,000,000. This means 14,706 isoprene units make up the linear chain. An X-ray diffrartion pattern of stretched rubber shows an ordered structure indicating a cis 1,4 config~ration,~ that is, t,he isopreue units are joined to each ot,her tail t,ohead t,o form a long linear chain. Unil = 9.13 A
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C=CH CHI CH? C=CH CH2 CHr-~--/ \(\ /\/ \/\ / -----CHr C Ht C=CH ?Hz CH5 C=CH
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Katural rubber comprises about 95% rubber hydrocarbon and about 5% of materials consisting of proteins, fats, sterols, organic acids, and salts. The primary source of all natural rubber is the ruhber tree Hwea Rrasiliensis. Natural ruhber latex 4.5.6 is the milk-like liquid which flows from the cut in the inner bark. Approximately 100 ml. of normal latex (.10%40% total solids) is obtained a t each tapping. Since latex is readily subject to pntrifaction as a result of bacterial or enzymatic action, it is necessary to add a preservative. Ammonia is commonly used for this purpose because it can be removed later if the necessity arises. Latex is normally shipped as a concentrate of 62.07,-68.0y0 total solids which is prepared by rentrifnging or rreaming.
' Presented a t the 289th Meeting oE the New England Association of Chemistry Teachers, February 9, 1957, at the Hmdford Durfee Technical Institute, Fall River, Massachusetts. FISC~IER, H. L., Sci. Am., 195, 74 (1956). =FIESER, L. F., AND M. FIEGER, "Organic Chemistry," 3rd od., Reinhold Publishing Corp., New Yorlt, 1956, p. 848. ' BARRON, H., "Modern Rubber Chemistry," D. Van Nostrand Ca., Inc., New York,.1948. ' DAVIB,C. C., J. T. BI~AKE, "Chemistry and Technology of Rubber," A.C.S. Monograph No. 74, Reinhold Publirhing Corp., New York, 1937. ' "The Vanderbilt Later Handbook," R. T. Vmderhilt Co., Inc., New York, 1954. VOLUME 34, NO. 5, MAY, 1957
Natural rubber latex is a complex colloidal system consisting of negatively charged particles of rubber suspended in a watery medium referred to as the serum. Each ruhber particle is stabilized by an envelope of naturally occurring proteins and soaps adsorbed a t the rubber-water interface. Electron microscopy7 has shown that most of the particles are in a size range of 0.05 to 0.15 micron. Particles larger than 0.4 micron, which comprise about 4% of the total number of particles, account for about 85% of the weight of ruhber present. As an article of commerce, natural rubber is imported as a preserved liquid concentrate or in the form of dried acid-coagulated sheets. Raw natural rubber, whether in sheet or latex form, has no particular commercial a ~ ~ l i c a t i oasn such. Raw rubber has lo~v physical strength and is easily torn or rupt,ured. It softens badly with the application of heat. Although it is elastic to a limited extent, it distorts out of shape easily. It has poor solvent resistance. In addit,ion, raw rubber is readily deteriorated by atmospheric conditions. Charles Goodyear is generally given credit for the discovery in 1839 that ruhber and sulfur could he heated together with t,he resultant improvement in these properties. This discovery was termed uulcanization and today is commonly referred to as cure. BUILDING A NATURAL RUBBER COMPOUND & .
The modern rubber products of today were developed over a long period of years. Progressive developments in rubber c ~ m p o u n d i n g ~have , ~ shown that enhanced properties are obtained by vulcanization with heat, by control of curing rate with accelerators and activators, and by the use of antioxidants to retard deterioration. The exact meaning of these terms will be defined further. Vulcanization. I n a broad sense, vulcanization is t,he term applied to the treatment of raw rubber which increases its elasticity and tensile strength, eliminates tackiness, and decreases plasticity, solubility in sol-
'
WEITBY,G. S., "Advances in Knowledge of Hevea Latex," lecture from summer course on "The Chemistry and Physics of Elastomers," University of Akron, 1956. 255
vents, and sensitivity to temperature changes. Commonly, vulcanization is effected with sulfur, accelerators, and heat. Other methods of vulcanization have been discovered, such as heating with peroxides, selenium, tellurium, nitro compounds, or halogenated quinones. This discussion will be confined, however, to sulfur vulcanization alone. Accelerators. Accelerators are organic chemicals which shorten the time required for vulcanization and give the mbber desirable properties. They also make it possible t o obtain optimum physical properties with reduced quantities of sulfur. The 7-10 parts of sulfur required without an accelerator are reduced to 3 parts and less with an accelerator. Modern acceleration systems began with the discovery by George Oenslager in 1906 that the organic base, aniliie, enhanced vulcanized rubber properties. Aniline, being toxic, was soon displaced by materials safer to handle. The accelerators of today consist of materials from the classes of aldehyde amines, guanidines, and carbon disulfide reaction products such as dithiocarbamates, xanthates, thinram disulfides, and mercaptobenzothiazole. Examples of some accelerators to illustrate the various classes are:
Heuamethylenetetramine
I $ -0 Diphenylguanidie
Zinc diethyldithioearbnmate
CH, HsC-A-4-C-S-Na
w
w
Symmetrical dibetsnsphthyl-p-phenylenediamine
CxHs CtHs 2,2-Methylenebis(4ethyl-, Gtertisry-butyl phenol)
Deterioration of a rubber vulcanizate occurs as a result of the chemical reaction with its environment, usually the atmosphere. Ozone and oxygen are the primary factors. Moisture may be another, especially in combination with ozone and oxygen. Cross linking of the vulcanizate may occur, however, as a self reaction in the presence of light or heat in the absence of other chemicals. The following chemical changes are considered to occur during aging: (1) The long molecular chains which form the major structure of a linear polymer may be cut int,o smaller chains. This is known as scission. (Depolymerization.) (2) The linear chains may be tied together by cross links. (3) The chemical nature of the side groups along the molecular chain may be modified. Antioxidants are specific chemicals which are added to rubber to retard these deteriorating effects. LATEX COMPOUNDING
I
C& Sodium isopropyLxanthate
used to retard deterioration of a rubber product. Moreau and Dufraisse are credited with the discovery about 1918 that certain organic compounds greatly retarded oxidation of other organic compounds. They termed these materials "antioxygens." Today they are known as antioxidants. As classes, these materials consist of waxes, amines, amine condensation products, and phenolics. Examples of some antioxidants illnstrating the materials used are: selected waxes from petroleum, symmetrical dibetanaphthyl-p-phenylenediamine and 2,2-methylenebis (4-ethyl-, 6-tertiarybutyl phenol).
Zinc mercaptobenaothiara
CHs
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N-CSH
Tetramethylthiuram disulfide
Accelwator Activator. It was further discovered that zinc oxide enhanced the action of many accelerators. In fact, the advantage of zinc oxide was so great it has resulted in its use with practically all modern accelerators. Because of its beneficial action, zinc oxide was termed an aclivator. Antiozidants. Antioxidants are chemicals which are
Since latex is a liquid rather than a solid, the compounding system must necessarily be a liquid rather than the dry phase. For this reason, water-insolul-
