1
INDUSTRIAL AND ENGINEERING CHEMISTRY
DECEMBER, 1939
This is actually a slight corruption of the integrated form of Arrhenius’ equation for dependence of reaction rate upon temperature, which would have us plot log t against the reciprocal of the absolute temperature to obtain a straight line. However, the above form serves for practical purposes in the comparatively narrow temperature range used for most rubber vulcanization. Given either the proper times of cure a t two different temperatures or the proper time of cure a t one temperature and a value for temperature coefficient of vulcanization, we can obtain values for constants M and K and then calculate proper conditions a t any time or temperature. COEFFICIENTS OF TABLEIv. TEMPERATURE Compound By free sulfur By T-50 By modulus
Av.
A 2.14 2.05 1.88 2.02
B 2.51 2.50 2.42 2.48
C 2.28 2.28 2.22 2.26
VULCANIZATION
D
E
2.57 2.47 2.34 2.46
2.28 2.19 2.11 2.19
Reviewing the values found for temperature coefficients of vulcanization by the three different methods (Table IV), it is to be noted that the values obtained from free-sulfur data are higher for every compound than those determined by modulus. Several sets of data in the literature on various accelerators also bear out this behavior (3). The values determined by T-50 are, in general, between those by the other two methods and very close to the average values for all three methods. This may be considered another advantage for the T-50 method in that i t does not give extreme values either way. All methods agree very well in indicating that compounds containing reclaimed rubber give higher values for temperature coefficient of vulcanization than a compound containing no reclaim. Also, the alkali whole-tire and tread reclaims give higher values than acid-type whole-tire or red tube reclaims. No general theory seems to be available to explain this behavior of compounds containing reclaim except the assump-
1485
tion that reclaimed rubber promotes a different type of acceleration, very much as a different accelerator might. Other compounds that have given high values for temperature coefficient of vulcanization in the literature have been plain rubber-sulfur (100 to 6 . 5 ) ,crotonaldehyde-aniline, and butyraldehyde-aniline. The literature on activated us. unactivated mercaptobenzothiazole type accelerators has been somewhat contradictory.
Conclusions All compounds containing reclaimed rubber gave considerably higher values for temperature coefficient of vulcanization than the compounds containing no reclaim. Of the compounds containing reclaim, B and D gave somewhat higher values than C and E. Temperature coefficient values determined from modulus data were, in general, somewhat lower than those obtained by the free-sulfur method. The values determined by T-50 lay between those by the other two methods and very close to the average values for all three methods. Of the three methods, the T-50 has the advantage of simplicity and speed and is quite adequate for most practical purposes; the free-sulfur method is probably most accurately reproducible and not unduly difficult to perform; the modulus method, although preferred by some investigators because of the greater practical significance of this physical property, involves considerable discretion in selection of conditions.
Literature Cited
c.
(1) Brittain, L., IND. ENQ.C H E M . , 21, 362 (1929). (2) Eliel, K. W., Rubber Chem. Tech., 11, 101 (1938). (3) Gerke. R.H.. IR’D. ENO.CHEM..31. 1478 (1939). (45 Gibbons, W. A., Gerke, R . H., an‘d Tingey, H. C., Ibid., Anal. Ed., 5, 279 (1933). (5) Oldham, E. W., Baker, L. M., and Craytor, M. W., Ibid., 8 , 4 1 (1936). (6) Tuley, W. F., I n d i a Rubber World, 97, 39 (1937).
Vulcanization of Latex C. E. BRADLEY United States Rubber Company, Mishawaka, Ind.
During the last decade latex has established its position as an important raw material of commerce, supplementing dry rubber in many cases rather than displacing it. Principles of compounding and vulcanization adapted to its own peculiar nature have been developed. Research is steadily improving the quality and methods of transportation, and simplifying its control in manufacturing processes. Although fundamentally its vulcanization technique is similar to that of the parent industry, latex permits a wider latitude in choice of operating conditions which bids fair to increase its usefulness and extend its application. This paper covers latex vulcanization in general, including rubber deposited from compounded latex as well as vulcanization of latex per ae.
HE commercial possibilities of rubber latex were first
T
outlined by LaCondamine and Fresneau nearly 200 years ago. They made a memorable report on this product which they obtained from certain trees in South America during explorations in that country. I n connection with his description of methods for preparing various useful articles from latex, Fresneau (9) says: “But all these things can only be executed on the spots where the trees grow, as these juices soon lose their fluidity.” Although his statement was well grounded and the inherent difficulties in transportation of latex did long delay its general use, today, thanks to industrial research, fleets of steamers are conveying millions of gallons of latex annually across the seas, and the tank car is delivering this material a t the factory door wherever it may be situated. Almost 100 years later, in 1824 and 1830, Hancock (3) filed patents on “impregnating felt, wool, cotton, hair and fibrous materials”, and the making of “dress or wearing apparel, fancy ornaments, figures, etc.”, using the milk of the rubber tree, imported from South America. I n his experiments on shipping latex in “good sound barrels”, most of t h e material coagulated, however, and he considered the plan impracticable.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Small importations of ammonia-preserved latex from Heveo braziliensis are reported to have been made to Europe in 1853, but not until rubber plantations had been established in the far East did a real study of latex transportation begin. In 1911 latex preserved with ammonia and packed in 5gallon tins was imported by the United States Rubber Company. This was followed by drum shipments which in succeeding years gradually increased. In 1922 the first bulk shipment in steamer tank was made from Sumatra to New York. Transportation of latex on a commercial scale was now established.
NATIVELATEXSRoE FROM THE AMAZONCoUNTHl' Received in shipment of Para rubbor about 1900, it contains 0.35 per cent sulfur, which indicates that sulfur had been used in ita preparation. Its age is unknown but it is in a good state of preservation. Charles Goodyear apparently did not have access to s u p plies of latex and therefore none of his inventions specify the use of this material. It is vel1 recognized, however, that he laid the foundation for the broad application of latex since the major portion of this material is subjected in its cormnercia1 use to the process of vulcanization. Although accurate statistics are difficult to obtain, i t may be assumed that most of the latex consumed today is eventually vulcanized in some manner.
Principles of Compounding
As in dry rubber compounding, the essential ingredients for proper vulcanization are sulfur, accelerator, and zinc oxide. In order to obtain the best possible homogeneity of the latex mix, these materials must be finely ground and dispersed in water with appropriate dispersing agents. I n addition, provision must he made to keep them in suspension permanently while the mix is being used. This may he achieved by increasing the viscosity of the mix to the point where settling will not occur or by keeping the latex well agitated during the period of use. It is usually necessary to grind sulfur in pebble mille for protracted periods in order to obtain the best results. As a rule, commercial dry sulfur is too coarse for the most exacting work. Accelerators should also be very finely divided. If the particle size is already sufficiently small, dispersion in water by means of a high-speed mixer or a so-called colloid mill may suffice. Many accelerators and other powders, however, have a tendency to form agglomerates which are broken up only with difficulty. By a proper choice of combinations of wetting and dispersing agents, it is frequently possible to ob. tain good dispersions in the colloid mill, but when hard aggregates and large particles are present, pebble milling should he employed. Overnight grinding is usually sufficient.
VOL. 31. NO. 12
If water-soluble accelerators are to he employed, the problem of distribution is simplified. Accelerators, such as sodium dibutyl dithiocarbamate, the potassium salt of mercaptohenzothiazole, and others, are available as aqueous solutions. Piperidonium pentamethylene dithiocarbamate, tricthyltrimethylenetriamine, and other water-soluhle materials sold as powders, crystals, or viscous liquids may be simply dissolved with a suitable quantity of water and added directly to the mix. Water-insoluble liquids such as heptaldehyde-aniline and butyraldehyde-aniline condensation products are readily emulsified. A good colloid mill or homogenizer is essential to secure fine-grained stable emulsions. In choosing the proper accelerator for latex work. several factors must be kept in mind. The final choice will be governed by the following requirements: speed of vulcanisation required, temperature of cure, physical properties desired (high or low modulus, transparency, etc.), effect on stability of mix, rate of precure of mix, and method of coagulation. Ultra-accelerators find ready application in latex work since the danger of mill or calender scorching is absent. Very rapid cures may therefore be obtained at low temperatures. Such mixes, however, have a tendency to precure rapidly at ordinary room temperatures and must he used within a relatively short time after mixing. The life of the mix may be extended by refrigerating to temperatures at which the accelerator becomes less active. A latex compound that tends to cure progressively in the liquid state shows increasingly poorer physical properties. Even at comparatively low states of precure, the coagnlated films may show poor wet strength. If the mix is intended to be used over a period of time, it is generally better to rely on high-temperature aGeelerators unless refrigeration is available. That latex can vulcanize in t.he liquid state may seem somewhat remarkable from the scientific point of view. Approximately 50 per cent of the latex particles are reported to be less than 0.2 micron in diameter with about 10 per cent greater than 0.5 micron (6). From 0.5 to 2.75 per cent of sulfur is ordinarily used, and of this amount only portion enters into chemical combination with the rubber. Since both rubber and sulfur are present as discrete particles and not in perfect solution, and the latter arc greatly outnumbered hy the former, one might expect a rather heterogeneous mixture of latex particles in different states of vulcanization. On the other hand, the odor of hydrogen sulfide frequently present in compounded latex containing sulfur indicates that sulfur has been dissolved by the ammonia or fixed alkali usually present and has thus given rise to sulfides and probably polysulfides. Since the latter are soluble, they would be evenly distributed and therefore cause uniform vulcanization of the latex particles. Zinc oxide also is dissolved by ammonia and its activity presumably increased. Many accelerators, however, are relatively insoluble in aqueous alkalies and yet exert a powerful vulcanizing action at low t.emperatures. Their action may possibly be explained on the basis that a portion of the solid accelerator phase dissolves in the aqueous medium and is adsorbed on the surface of the latex particles. It may then dissolve in the rubber hydrocarbon, and vulcanization ensues when the proper concentration is reached. Water-soluble accelerators are perfectly dispersed and therefore might be expected to be more active. It has been observed that at the beginning of vulcanization a solidification or gelling occurs at the surface of the particles and proceeds toward the interior. Green ( 1 ) followed the reaction between sulfur and a latex particle by means of ultraviolet light and deduced that the internal liquid portion reacted with sulfur but that the superficial layer was unaEected.
DECEMBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
The complete mechanism by which latex particles become vulcanized is still rather vague and worthy of more scientific investigation. Some accelerators have a pronounced thickening or even coagulating effect on latex mixes. I n this class belong accelerators such as mercaptobenzothiazole and the guanidines. The acidity of these materials may cause thickening, or their use may result in polyvalent ion formation with consequent precipitation of protective colloids in the latex (7). This type of thickening should not be confused with that due to slight prevulcanization of the latex, in which case the stability may not be affected. The selection of a water-soluble or -insoluble accelerator depends on the method of vulcanization to be employed, apart from other considerations. If the article is to be dried completely before final vulcanization or a dry heat cure is used, there should be no objection to the use of water-soluble accelerators. A different case is encountered when the article is to be cured in water after coagulating but before final drying. In this procedure a certain amount of the accelerator is likely to be lost along with other latex serum constituents, owing to syneresis and leaching out by the hot water. Since this paper deals primarily with vulcanization, no attempt will be made to discuss other phases of compounding. Many fillers, antioxidants, etc., may exert a definite accelerating or retarding action on the cure and may be employed specifically for either purpose. Such effects must naturally be studied in formulating a compound for a given use.
Zinc Oxide Compounding Although zinc oxide is practically indispensable in the majority of compounds, it has also proved somewhat troublesome to the latex technologist. I n ammonia-preserved latex, zinc oxide instability is due to interaction with ammonia, which leads to the formation of polyvalent zinc-ammonia complexes. Jordan (4)showed that the KOH number of latex is a valuable aid in compounding to, obtain stable zinc oxide mixes. This property is defined as the number of grams of potassium hydroxide necessary to neutralize the acidic constituents in 100 grams of latex solids and is determined by electrometric titration of the latex.
GRAMS KOH PER 100 GRAMS LATEX SOLIDS
FIGURE1. TITRATIONCURVESFOR VARIOUS LATICES
I n practice, ammonia is reduced to a low figure (0.1 to 0.2 per cent based on the rubber content) by aeration or by addition of formaldehyde, which results in the formation of hexamethylene tetramine, and potassium hydroxide is added in an amount equal to the KOH number, plus sufficient potassium hydroxide to neutralize added acidic materials such as casein, glue, mercaptobenzothiazole, gum arabic, etc.
1487
Figure 1 shows titration curves for several types of latex obt,ained by means of the glass electrode. Where alkali stabilization is undesirable, other means of stabilization may be employed. Frequently, it is feasible to omit zinc oxide and depend on zinc-bearing accelerators or high temperatures for cure. Good cures are obtainable with zinc salts of substituted dithiocarbamates a t fairly low temperatures. Heptaldehyde-aniline condensation product and tetramethylthiuram disulfide produce good technical cures without zinc oxide a t elevated temperatures. For certain purposes some authorities (8) recommend that the latex mix “mature” by standing for varying lengths of time before use. This procedure tends to stabilize the viscosity and the curing characteristios of the compound and improve the quality of the optimum vulcanizate. Slower accelerators or combinations should be employed in order to avoid precure in the liquid state.
Methods of Vulcanization The transformation of raw latex into finished products follows definite procedures or steps, such as processing, coagulation, vulcanization, drying, and finishing. The point a t which vulcanization is carried out depends on the process and type of goods to be manufactured. Three possible sequences of latex manufacturing operations follow: I
I1
I11
Vulcanization Processing Coagulating Drying Finishing
Processing Coagulating Vulcanization Drying Finishing
Processing Coagulating Drying Vulcanization Finishing
VULCANIZEDLATEX. The first method, in which vulcanization precedes other operations, is based on the work of Schidrowitz (8) who discovered that latex may be vulcanized in the liquid state. Originally he vulcanized latex with alkali polysulfides as the source of sulfur. I n present-day practice vulcanized latex is obtained by (a) high-temperature vulcanization in pressure vessels, (6) low-temperature vulcanization in open tanks, or (c) ultra-accelerators acting a t room temperature. Before use the latex may be compounded with fillers, wetting agents, etc., similar to ordinary latex. No further vulcanization is required after the article is dried. This is of advantage where vulcanizing equipment or the time element is a large economic factor. On the other hand, it is obviously not possible to vary the degree of cure as desired. VULCANIZATION BEFORE DRYING. A second and widely used method of manufacturing latex articles consists of making the mix, forming the article, and coagulating by any desired means, vulcanizing, and drying. Such processes are applicable where the coagulum is too tender to be handled and must be vulcanized to become strong enough to stand subsequent operations. They are also applicable in the case of dipped goods or other materials where i t is desired to leach out water-soluble materials during the cure, a hot water cure being employed for this purpose. Probably the most important example of this method is the process for making latex foam rubber sponge. The compounded latex mix is frothed to a predetermined density, a sensitizing agent added, and the mix poured into a suitable mold. I n a short time the foam sets to an irreversible gel, or, if the mix is of the heat-sensitive type, heat may be used to accelerate gelling. This gel cannot be stripped from the molds without undue distortion and must be cured in the wet state. The wet coagulum is therefore vulcanized by any of the wellknown methods-. g., boiling water, hot air, or steam. After stripping, excess water is removed by centrifuging and the article dried a t low temperature.
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INDUSTRIAL AND ENGINEERING CHEM ISTHY
VULCAXIZATION AFTER DRYINQ. The majority of articles manufactured from latex fall into this category. Wherever a solid article or sheet of good physical properties is required and porosity is a disadvantage, good drying before vulcauization is required. This method offers more latitude in choice of vulcanizing conditions and compounding. Owing to absence of scorching tendencies, continuous manufacture and curing are possible over a wide range of temperatures from that of the room up to 500" F. (260" C.). Latex-coated and doubled fabrics, rug backing, etc., can he simply dried after latexing, and vulcanization can proceed at room temperature; or if a quicker cure is desired, the fabric may be conveyed continuously through a heated chamber or over hot rolls. Latex thread and latex-insulated wire are cured continuously by leading the strands of thread or wire from the drying into the curing chamber. Here vulcanization is carried out in superheated steam or combustion gases at temperatures of 300Oto 500" F. (150'to 260" C.). In the case of some coatings, adhesive compositions, impregnated articles, and thc like, where p0rosit.y is not objectionable or even desired, vulcanization may proceed simultaneously with drying. Thus, sheet latex foam sponge rubber may be cured and dried in open pans, the temperature being adjustcd so that curing and drying are essentially complete at the sa.me time. Some articles, such as dipped gloves, air bags, shoes, etc., must undergo some finishing or assembling operations after drying, in which the maint,emancc of tack or adhesiveness is required. In such cases the article must bc dried at temperatures which leave tho coniporind essentially nncnred. In the making of spread shcet or latex-coated wire where a uumher of coats are to be applied and dried by heat., no cure should take place during the spreading or dipping operation in order to ohtaiu good adhesion of consecutivc coat,s. Likewise, impregnated tire cord must preserve its adhesiveness during assembling operations and must, cure satisfactorily along with the: balance of the tire Methods of Heat Transfer Virtually all known methods of vulcanization may be used successfully with latex compounds. Low-temperature cures in air, steam, or watcr have already been mentioned and need no further discussion. Suitable modifications in the compound enable one to use almost any desired t.emperature compatible with good physical properties. Other methods of vulcaniaation less frequently used may be worthy of note. Thin dipped goods may be made up without accelerator and cured by immersion in a hot aqueous solution of a wateraoluble accelerator. Obvioiisly this method is limited in scope since with thick-walled articles only a surPace cure is obtained in a reasonable length of time. Furthermore, the concentration of accelerator in the bath must he kept adjusted carefully. If considerable decomposition of the accelerator occurs, the method may bo costly, for the bath will have to be discarded at the end of a run or changed frequently if used for long periods. Salts such as calcimu chloride have been advocated to raise the boiling temperature and decrease loss of water-soluble constituents. Leduc (5) proposed curing by diathermy. In this process the article t.o he cured is placed in tho electrical field of a highfrequency alternating current. The high dielectric loss of the article leads to a rapid heating of the mass, and vulcanization of thick articles may he carried out comparatively quickly. Heating is uniform throughout the entire mass, which is refiected in a rapid aud uniform cure; other nretliods of heat transfer depend upon slow penetration of lieat by conduction from the outside or mold surfaces, and the tcndency is for the latter to be overcured before the center is
VOL. 31, NO. 12
adequately heated. Success of the new process seems to depend on finding suitable materials for mold construction and reduction in cost of the apparatus required.
Hard Rubber Compounds Certain types of hard rubber are advantageously prepared from latex. Compounding is essentially similar to dry rubber practice with modifications to obtain good dispersion and stability. Here again the widest application lies in fields not easily conquered by dry rubber technology because of processing advantages. For example, hard rubber tank linings may be applied by spraying; solvent hazards are thus avoided i n closed vessels. After drying, the lining is vulcanized by presslire steam within the vessel.
Nonsulfur Vulcanization Although nonsulfur vulcanization does not appear to be of great technical importance, it is nevertheless of considerable scientific interest. The classical methods of Ostromislensky using dinitrobenzene, l,:j,$t.rinitrobenzene, and benzoyl peroxide as vulcaniaing agents have been extended to latex (11).
More recently Spencc and Ferry (10) obtained vulcanizates by heating latex free from diffusible serum constituents with potassium ferricyanide in the absence of air. Similar effects were obtained with mercuric and ferric diloride and pentaamminocarbonatocobaltic nitrat.e.
Acknowledgment The writer acknowledges the assistance of E. G . Bargmeyer and I). E. Cable in t . 1 preparation ~ of this paper.
Literature Citations (1) Flint, C . F., "Chemistry and Teehnolmy of Robber Latex". DV. 34&8. New York. 1 ). Van Nostrand Co.. 1938.
(2) Ibid., p. 585. (3) Hamock, Thomas, "I'ersonal Narrative". 1867. (4) Jordan. H.F., Proc. Rubber Tech. Cord., London, 1938,111-25. (5) Ledua, H. A,, Ibid., 1938, 381-7. (6) I,ucas, IND.END.CNEM.,30,146-.53 (1938). (7) Pestalozza, U., Brit. Patent 411.202 (June 7, 1034). (5) Sehidrovitz, P., >/,id.. 193,451 (Fcb. 20, 1928) and 208,235 (Seut. 14. 1922): U. S. Patent 1.082.857 (Seat. 4. 1928). (9) Seelighann, Toriiitron, and Feiconnet. "Indra Rubber and GuttePeiehs", I). 6, London, Soott, Greenwood & Son. 1910. (10) Spenee, D., and Forry, J. D., J Soo. Cham. Ind., 56, 24F-9 (19371: Kzbbrr Chern. Tech.. 10. 762-7 (1937). (11) Stevens. Brit,. Patent 324.287 '(March 19,'l'J29)
Caurleiy, The Coodyeor Tire & Rubber Conrpanv
IMPR~YED METHODSOF BUD GRAFTINGASRIJRE PROGREE SIVELY h C R E A s I K G YIELDB PER RUBBER TREE
