February 1951
w.
INDUSTRIAL AND ENGINEERING CHEMISTRY
accurate measurements of the area torn in such cases, and experience so far indicates that the lower limit for the energy input should be such that tearing is complete within the first hundred blows. This leaves a wide latitude for investigating different energy levels. Under certain conditions some compounds do not tear in such a way as t o propagate the initial cut horizontally, but torn ahear planes are formed in the body of the rubber until the tear has proceeded t o one of the metal surfaces of the end pieces. In Figure 14 such a test piece is shown; the raised surface of the test piece on the right is the part of the rubber which was bonded to the metal. A profile of this’test piece is shown in Figure 15; once again the distance the tear has traveled with each of the three successive blows is clearly indicated. Many of the variables associated with this test method have still to be evaluated more systematically before the full poentialities of the method are known.
38 1
Although the method has been used in testing bonded units, there is no reason why it should not have a much wider application. The lack of correlation between tear tests (crescent, Delft, and angle) and service is due, among other reasons, to the fact that measurements are made on relatively thin test pieces and it is not permissible to extrapolate the results to thicker test pieces and hence to the performance of actual articles in service.
Literature Cited ( 1 ) Am. SOC.Testing Materials, D 4 2 9 4 7 T . (2) Buist, J. M., and Naunton, W. J . S., Trans. Inst. Rubber I n d . , 25,378 (1950). (3) Mullins, I,., J . Rubber Research, 16, 275 (1947). (4) Mullins, L., Proc. 3nd Rubber Tech. Conf. London, 1948, 179. (5) Naunton, W. J. S., and Waring, J. R. S.,PTOC.Rubber Tech. Conf. London, 1938,805. (6) Villars, D. S., J . Applied Phys., 21, 565 (1950). (7) Werkenthin, T. A., Rubber Age ( N . Y . ) ,1946 and 1949. RECEIVED September 30,1950.
Direct Reinforcement of Natural Rubber Latex Mixes Jean Le Bras and Ivan Piccini InetZtut Fran~aZede Caoutchouc, Par& France
R
T h e direct utilization of latex has up to the present time been limited to a relatively small number of applications because of lack of knowledge of how to communicate to articles sufficient hardness, modulus, resistance to tear, and resistance to abrasion. But it is possible to obtain such properties by combining the molding of latex (thermosensitized by the action of trypsin) with the addition of partially condensed resins. Numerous types of resins can be utilized, but the best results have been obtained by use of resorcinol-formaldehyde resins. The mechanical properties of vulcanizates thus prepared are extremely high-for example, the tensile strength may reach 7500 pounds per square inch (with an elongation of 700%), and the resistance to tear, 900 pounds per inch. Operating conditions that must be observed for the formation of the resin are described in detail, as well as the influence on the properties of vulcanizates of different factors: length of condensation of the resin, conditions of vulcanization, proportion of catalyst, molecular ratio of the constituents, conditions of drying, etc. A new way is therefore opened for obtaining vulcanizates of natural rubber of outstanding mechanical properties.
. I
T
HE advantages of the direct use of latex are well known, be-
cause of the simplicity andeconomyof the practical techniques Because the rubber has not been submitted t o mastication, it retains its intrinsic qualities unimpaired. Yet the use of latex has been until now limited to a relatively small number of applications, because it does not permit the fabrication of articles combining medium or high moduli and a hardness greater than 45 Shore with sufficiently great tensile strength and resistance t o abrasion or tear. Loading materials can be added t o latex, but the reinforcing agents, which have such interesting effects on the properties of masticated rubber, do not exercise similar action in the case of latex. The reinforcement of latex is a problem which has often attracted the attention of investigators, in view of its great interest as well as the mechanical properties of the articles obtained.
from both the theoretical and practical points of view. However, little progress was achieved in this field prior t o the authors’ work (14, Z J ) , which has led, apparently for the first time, t o a solution of the reinforcement problem. There is room for a precise definition of the term “direct reinforcement of mixes of latex,” or more briefly “reinforcement of latex.” This term should be interpreted to mean an improvement of the intrinsic properties of rubber obtained by the addition to a latex compound of a product or composition with “reinforcing” character, the final article being obtained without recourse t o mixing on a mill at any time during its manufacture. For historich1 interest only, reference is made t o work dealing with the introduction of loading materials into latex for the purpose of assuring their perfect dispersion before coagulation and mastication. These experiments had t o do with the usual loading materials, such as carbon black, zinc oxide, and calcium carbonate (4, 11, 19, 27, S I ) , or more special products, such as the Marbon resins ( 2 0 ) and Indulin (12, 34) Numerous experiments were carried out, involving addition of resins, either by direct introduction of dispersions or by formation of the resins in the body of the latex. The operation was always followed by coagulation and thereafter mastication-for example, experiments with phenol-formaldehyde resins a t various stages of condensation (9, 10, 22, XI),alkylated resins (@, glyptal resins ( 2 2 ) , resins from condensation of fatty acids or their esters with maleic anhydride (26),and polymers of acrylonitrile, acrylic esters, and styrene (1, 2 ) . The attempts to reinforce latex by the incorporation of mineral loading materials in the form of stabilized dispersions have not been successful. A certain increase of the modulus, hardness, and sometimes tear resistance is observed (3, 23, 26), but the mechanical properties diminish rapidly. The expectations of van Rossem ( 2 9 ) on bentonite have not been confirmed (28). Pliolite Latex 190, a copolymer of styrene and butadiene, possesses good reinforcing qualities in the case of synthetic rubber latices (‘7, 33), but its effect on natural rubber latex is negligible. This also seems t o be the case with colloidal silica (6). Twiss, Neale, and Hale (32)experimented with the preparation of rubber mixes, reinforced by formation of the constituents of
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the mix in the body of the latex. Although their initial purpose was to obt’ain a coagulum designed t o be milled thereafter, they nonetheless made certain useful observations on the compatibility of resins and latex. According t o these authors, “it is possible by effecting the condensat,ion of a polyhydroxyphenol, such as resorcinol, quinol, or pyrogallol with formaldehyde in the presence of an alkaline solution as catalyst, t,o carry the resin formation further without separation of the resin. The solution of soluble resin can be mixed uniformly with latex, causing thickening of the latter; evaporation of a layer of the mixture causes further condensation or polymerisation of the resin constituent and yields a uniform sheet of leatherlike consistency.” This may indicate a certain degree of reinforcement, but Twiss and eo-n-orkers have not pursued their investigations in that, direct.ion. They give no numerical dat,a, and conclude t,hat “none of the esperinierits provides a satisfact,ory solution t o the problem of compounding natural rubber by the formation of t.he compounding ingredients in situ. The potential importance of the problem, however, is evident. and each experiment in the desired direction, even if disappoint,ing in itself, enables subsequent attempts t,o be made with more definite control of favourable factors and with an increased prospect of success.” The authors have studied, in a detailed and systematic manner, the addition of resins a t various stages of condensation, using a thermosensitive latex which permits the easy fabrication of objects by molding. Their aim in this study was t o reconcile the conditions of drying and vulcanization of the gel with those of the condensation of the resin. They thus established that, by judicious regulation of the different factors, it was possible t o obtain rciiiforcement; in particular, certain soluble resorcinol-formaldehyde resins, condensed in alkaline medium, confer except,ional mechanical properties on vulcaiiizatcs of natural latex, surpassing those of the best mixtures of masticated rubber based on reiriforciny blacks.
Uetermlnation of Conditions of Reinforcement Experimental Technique. The latex (BOY0 solids) was prepared for molding by a thermosensitiaation treatment before addition of the vulcanization ingredients or other additives. The procedure employed depended on the action of trypsin (16-17). The latex was treated with 0.5 t o 2.OY0 of powdered pancreas of swine which was diluted a t first with 10 parts by weight of water. In general, this was done a t room temperatures. The thermoseiisitization, in the presence of the same quantity of zinc oxide and for the same temperature of gelation, is then a function of the amount of pancreas powder added, the temperature, and the time of reaction. To obtain samples, the best conditions were used for mixing the thermosensitive latex with the necessary ingredients for ultimate vulcanization, with variable quantities of the loading materials being investigated. The ingredients were added in the form of aqueous dispersions t o avoid coagulation of the latex and t o obtain a homogeneous mixture. The basic mixture, control mixture ?VI, corresponded t o the formula (parts by weight): Dry rubber Sulfur Zinc oxide Zino diethyldithiocarbamate
100 2 3
1
After drying, it was vulcanized for 1 hour in the oven a t 100’ C. The gelation was carried out in a metallic mold in hot water; then, after drying, the sheet was vulcanized at optimum conditions. Finally, the various properties were systematically determined by means of appropriate tests on the vulcanizates. The loadings were appropriately incorporated in the form of dispersions or solutions, depending on the nature of the loading material (solid or liquid). It was usually necessary t o stabilize
Vol. 43. No. 2
the dispersions or solutions against coagulation and makc, tiiciii compatible with the heat-sensitized latex. The nature and properties of the loading materials being investigated are very different and, in each case, i t was necessary to determine the conditions of compatibility with the latex. Incorporation of Dispersed Fillers. On examination it was found that the organic or inorganic loadings function diff ereritly according t o their physical state. The organic or inorganic loadings, which exist as insoluble powders and are added to the latcx in the form of dispersions, act as inert fillers in masticated rubber. The stiffening of the mix, .rT-hichincreases with the quantity of filler incorporated, is accompanied by a decrease of the mechanical properties.
Table I.
Characteristics of Latex
Tensile a t break, kg./sq. om. ilIodulus at 3007,, kg./sq. c m . Elongation a t break, % ’ Abrasion resistance, cc./hp./hour Shore hardness Resistance t o tear, kg./cm.
Latex 330 15 750 600 42
50
“Reinforced” Latex 35s 90 676 3.50 72 8.5
The organic fillers in soluble form or in colloidal suspension behave differently; stiffening of the mixture is always observed, but the mechanical properties are not altered as long as the concentration of the filler in the rubber does not exceed a certain variable, critical limit on the order of 10 to 25%. In addition, there is generally an increase in tear resist,ance. This was observed, for example, with the urea-formaldehyde active resin 24562 (Socikte Kobe1 Franqaise) or the Pliolite Latex Type 190. I n spite of the numerous tests performed and all the attention given the mixing, the expected results were not obtained. Formation in Situ of Synthetic Resins. INTRODUCTION OF ELEXENT.4RY CONSTITUENTS. For SatkfaCtOry Operation Of the test, the constituents should: Kot coagulate nor flocculate t,he latex a t the time of their iticorporation Mix easily with the latex and form a homogeneous medium Not hinder gelation S o t inhibit vulcanization The most encouraging results were obt’ained in the case of the vinyl resins, the aminoplast,~,and, principally, the phenoplasts. Among the phenoplasts, the resorcinol-formaldehyde resins were noteworthy . Nevert.heless, the direct introduction into the thermosensitive latex of the resin, the formaldehyde, and the alkaline catalyst proved impossible. There v a s observed: The instantaneous desbbilization of the ammoniated latcx, followed by it,s coagulation if there is an excess of aldehyde. The impossibilit,y of the formation of the resin if there is an excess of ammonia. The impossibility of gelation if there is an excess of alkaline catalyst or the presence of a stabilizer. Attempts were made thercfore to introduce the aldehyde in a less active form. Trioxymethylene may be used; its addition to the latex mix does not cause abrupt coagulation. It is thus possible t o obtain a certain reinforcement, as is shown in Table I, in which are indicated the principal characteristics of the latex mix and of the same latex in which has been incorporated 13% by weight of a preparation comprising 3 parts of resorcinol, 3 parts of trioxymethylene, 1 part of 15% sodium hydroxide, and 6 parts of water. There will be observed a distinct increase in hardncss anti modulus, and a substantial improvement in resistance t o abrasion and tear, as well as a certain increase in tensile. But the stability of this latex mix is still insufficient t o allow practical application. Nevertheless, these r e s u h encouraged the authors t o
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383
strength, and elongation a t break, there were employed dumbbell test pieces (Type I.F.C. j, 2 mm. thick, with a constricted area 2 INTRODUCTION OF INTERMEDIATE RESINIFICATION PRODUCTS. mm. wide and 25 mm. long (18). The resistance t o tearing was evaluated by the American Society for Testing Materials test Here again the authors have examined a number of types of method with specimens in the form of a crescent having one cut. resins, principally the aminoplasts and the phenoplasts. HowFor abrasion resistance, standard test specimens were prepared ever, the present communication discusses only the resorcinolfor test on the D u Pont-Grasselli machine by molding. formaldehyde resins, which gave the best results. Because the literature contains but little concerning the formation of these resins, the authors conducted a detailed study of the resorcinol-formaldehyde condensation reaction in aqueous medium a t high concentrations. [Soon after this work had been 30-900completed, several analogous studies were published, notably by Little and Pepper (f8),in which are found certain results which agree with the present authors'.] They were able t o determine 800the influence of various important factors such as concentration, 9075. pH, catalysts, temperature and time of condensation, molecular ratio of reactants, and exothermic character of the reaction, as 700well as the limits of operating conditions between which is ob80tained a single homogeneous liquid phase perfectly miscible with 600water. For a resin t o be compatible with the vulcanizable latex, thermo70sensitized by trypsin, i t was necessary that the resin:
undertake a study of the incorporation of intermediate products of resinification, which might lead to further improvement.
'
500-
Be present in the form of a liquid with sufficiently reduced viscosity Be of pro er concentration Be free ofuncombined formaldehyde Have a p H which does not prevent the formation of the sensitizing zinc-ammonium ion Not have a n unfavorable effect on the vulcanizing system of the latex mix The experimental conditions were chosen to comply with these requirements. If soluble resins suitably condensed were incorporated in the thermosensitive latex, the gelation of the resulting mix could be controlled. Under these conditions the formaldehyde is totally combined with the resorcinol a t the moment of introduction of the resin. The ammonia of the latex does not then have a tendency t o react arid continues t o fulfill its dual functions of preserving the latex during the period of manipulation and contributing t o the formation of zinc-ammonium sensitizers.
Inf laenee of Resoroinol Formaldehyde Resins on Reinforoement
I?
Operating Conditions. After preparation of the resin, it was mixed with the dispersion of the vulcanization ingredients and added t o the 60% latex, previously treated with trypsin. This dispersion had the following composition, and was added in the ratio of 10 parts per 100 parts of 60% latex (parts by weight ). Zinc oxide
Sulfur
Zinc diethyldithiooarbaniate Dispergine CB a t 20% Gum tragacanth a t 4% Distilled water
300 200 100 30 140 890
This mix was poured into a metal mold, whereby slabs were obtained. The slabs most frequently used had a thickness after drying of about 2 mm. The mold was placed for 10 minutes in 70" C. water t o effect gelation. The samples were dried for 2 days in ambient air and 2 days in a n oven a t 40" C.; then they were vulcanized by heating for 1 hour a t 100 a C. in hot air. The stability of the latex-resin mixture varies, according t o the nature of the latex and the state of the resin, between 30 minutes and several hours. The delay must be sufficient t o permit casting in molds under satisfactory conditions. On the other hand, the gelation is more rapid in the presence of resin; the gels are firmer and more coherent than in the case of latex alone and can be manipulated easily without deformation. From the vulcanized sheets finally obtained, specimens were cut for mechanical tests. For the determination of modulus, tensile
so.
60- 400-
3005025
aoo40-
t Dt tC
E
A
c
-+%
1
TENSILE STRENGTH AT MODULUS 300% MODULUS 400 y o
+.
cm.
c- HARDNESS D- ABRASION
B - ELONGATION PER CENT
RESiSTANCE
ou.cm. / h p / h r .
Figure 1. Influence of Molecular Ratio, FIR, on Properties of Mixes Reinforced by 10% o f Resin Time of Resin Condensation. After numerous preliminary tests, for a systematic examination a resin of the following mole composition was selected (preparation RI): Resorcinol Formaldehyde (as 30% aqueous solution) NaOH (as 1 N solution) Fe (as 0.5 M FeCls solution)
1 1
0.05 0.0001
T o a flask are added resorcinol, formaldehyde solution, and sodium hydroxide; the flask is stoppered and agitated rapidly, ferric chloride is added, and the Condensation is allowed to proceed in the closed flask. The reaction is exothermic and, in order t o prevent precipitation, or a too abrupt condensation, it is necesmry to keep the temperature below 50" C. and above 25" C. A series of resins was prepared in the above manner, using variable times of condensation. These resins, used in the proportion of 10 grams of resin t o 100 grams of 60y0latex, gave the results indicated in Table 11. The reaction time is seen t o be of great importance and the maximum reinforcement occurs only when the condensation has been carried out for about 1 hour. For practical reasons (viscosity, hygroscopicity, etc. j i t is preferable not to go beyond this time of condensation. Influence of Conditions of Vulcanization. The optimum vulcanization of control mixture M, without resin, corresponds t o a heating of 1 hour at 100" C. in hot air. I t was necessary to deter-
INDUSTRIAL AND ENGINEERING CHEMISTRY
384
Table 11. Time of Resin Condensation Condensation Time, Min.
Tensile a t Break, Kg./Sq. Cni. 300 390 450 490 445 455 450 490 330
Table 111.
Effect of Time and Temperature of Vulcanization
Influence of 'Time of T'ulcanization %t1000 c. Min. Kg./sq. cni. 30 40 50 60 70 80 90 100 110 120 130
426 415 460 475 485 480 475 470 450 440 440 445
160
Influence of Temperature for
1-Hour Heating _~__ _~_ ____
T,
C.
Kg./sq. cin.
70 80 90 100 110 120 130 140 160
360 450 475 475 465 325 175 150 150
mine whether the addition of the resin modified these conditions. By varying the time of vulcanization a t 100' C. or the temperature of heating for 1 hour, there were found the values for the tensile at break shown in Table 111. The addition of the resin does not noticeably influence the conditions for optimum vulcanization, which are around 60 to 70 minutes a t 100" C. Proportion of Catalyst. The catalyst regulates the nat,ure and the molecular iveight of the components of the resin. It, is not surprising to find, as shown in Table IV, that, the concentration of the alkali in t,he reaction mixture directly influences t,he reinforcement.
Table IY.
Effect of Concentration of Catalyst
Conon. of NaOH in Preparation R I
Tensile a t Break, Kg./Sq. Cm.
n
2nn ..
0.01
330 400 425 435 460 475 480 455 390
0.02 0.03 0.04
0.05 0.06 0.08 0.10 0.12
I t is sufficient t o have a small quantity of iron, the optimum corresponding t o the proportion given in preparation R,. Less favorable results obtained in its absence show that iron enters into the catalysis; it can be replaced by other metals, such as t,in or aluminum. Molecular Ratio of Constituents. The proportion of formaldehyde in preparat,ion R I was varied betn-een 0.5 and 3 moles, while remaining within the limits of temperature indicated. The resin was incorporated with the latex 1 hour after the start of the condensation reaction. In Figure 1 are collected the results of the measurements made on the vulcanizates. It was found that the molecular proport.ion of t,he constituents present is of primary importance. The hardness and the modulus of these vulcanizates are increased with t,he formaldehyde-resorcinol ratio, whereas the elongation a t break undergoes very little variation. The tensile strengt,h and
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the resistances t o tear and t o abrasion display a maximum for a value of t,he molecular ratio in the neighborhood of 1. Proportion of Resin. The quantity of resin added to the latex plays an equally important role with reference to the various properties. Figure 2 shows that there is a maximum of reinforcement for a proportion of around 10 grams of preparation R, for 100 grams of 60% latex. The curves of comparison shown in Figure 2, relating t o other loading materials (calcium carbonate, zinc oxide and Pliolite, Latex Type 190) make the reinforcing effect obtained by the new method clearly evident. I t appeared interesting t o compare in Table V the principal characteristics of three vulcanizates corresponding t o the mixture of latex reinforced with 10% of resin R,condensed for 1hour; the control latex n i x ; and a carbon black mix, prepared by mastication (t,ypical tire tread stock), vulcanized in a p r ~ s efor 60 minutw a t 143" C.: Smoked sheet Zinc oxide Stearic acid Sulfur Pine t a r Phenyl-2-naphthylamine Rlercaptobenzothiazole 1 I P C black
IO0 6 3 2.85 2 1
0.75 40
I t is seen that t.he vulcariizate of reinforced latex has a set of remarkable properties, clearly superior t o any present knon-n properties of rubber. Other characteristics, not, set forth in Table V ai'e likewise very good-for instance, permanent set and t,he resistance to flcx cracking are comparable t,o those of the control latex. In coniparison t o a carbon black tread st'ock, the dielectmric properties arc better and the thermal conductivity is greater, while the heat build-up on the Goodrich flexometer and traction hysteresis are slightly lower. Aging is excellent. Figures 3 and 4 show the characteristic appearance, after breaking, of the test pieces used for traction and tear resistance. Instead of the clean-cut seet,ions which are usually observed, the material here shows a ragged rupture. Influence of Vulcanizing System. INGREDIEXTS OF KORMAL MIX. This compound contains 2 parts of sulfur, 3 parts of zinc oxide, and 1 part of zinc diethyldithiocarbamate. It can be seen (Table VI) that small variations taken individually in the proportions of each of these ingredients has no effect, on the tensile strength and that,, for each of t,hese, the optimum corresponds t o the quantity utilized in the normal mixturc:.
-
-
-
7 FILLER CONTENT, G.FOR 100 G. OF 60 % LATEX
Figure 2.
Tensile Strength of Reinforced Latex
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951 Table V.
Characteristics of Vulcanizates Reinforced Latex
Tensile a t break, kg./sq. cm, blodulus a t 300%, kg./sq. cm. Elongation a t break, % Resistance t o abrasion, cc./ hp./hour Density Shore hardness Tear resistance, kg./cm.
Table VI. Influence of - . . Sulfur S Kg./sq. om. 1 1.5 2 2.5
3.5 5
Latex Control
460 60 750 300
Carbon Black Mix
330 15 750 600
310 100 500 350 I .2
0.93 42 50
0.95
62 100
68 90
Influence of Vulcanizing System Influence of Zinc Diethyldithiocarbamate % Accel. Kg./sq. cm.
Influence of Zinc Oxide % en0 Kg./sq. am.
1 2 3 4 5 7.5 10
275 440 450 475 470 460 445
0.5
420 475 475 460 455 460 465
170 275 470 465 450 415 385
0.5 0.75 1 1.25 1.5 2 3
On the other hand, if the three ingredients are varied simultaneously by adding different proportions of the normal dispersion t o the latex--e.g., 5 t o 20% by weight-significant changes are observed. The hardness and the modulus increase very nearly linearly with the percentage of ingredients, s n d the elongation at break diminishes, while the resistance t o tear, t o rupture, and t o abrasion passes through a very sharp maximum for a proportion of 10 parts by weight. The tensile strength values are respectively, 325, 460, 370, and 270 kg. per sq. cm. for 5 , 10, 15, and 20 parts loading of ingredients. OTHER ACCELERATORS. If the effect of different types of accelerators is examined, i t is observed that many of them do not function in the presence of the resorcinol-formaldehyde resins. Vulcanization is very much retarded or does not take place at all. I t is possible t h a t the chemical constitution of the accelerator permits reaction with the partially condensed resin. The dialkyldithiocarbamates give the best results; in particular, cadmium diethyldithiocarbamate permits vulcanizations a t 120" C. in hot air, with an extended plateau. Influence of Drying Conditions. The drying conditions employed in these experiments were determined empirically. By eliminating the period of drying in ambient air, which is difficult to control, by use of a ventilated oven (air circulation of 1600 liters per hour with a capacity of 0.2 cu. meter) and by adjusting air t o normal humidity, the authors have been led t o some interesting observations. First of all, during the period in the oven, vulcanization may
Table VII.
Figure 3.
Tensile Test Pieces after Rupture Reinforced latex on right
be made t o take place at the same time as the drying; furthermore, the mechanical properties are further improved and thus vulcanizates were obtained with tensiles of 530 kg. per sq. cm., and tear resistance values of the order of 130, and, under certain conditions, of 160 kg. per cm. It is equally possible, with good results, t o combine a drying time of shorter duration-for example at 30" or 40" C.-with a complementary vulcanization of 1 hour a t 100" C. in hot air (Table VII). By contrast, if drying is effected under the conditions generally recommended for objects molded from latex-with an initial high humidity decreasing progressively with a n increase in temperature-reinforcement does not take place. The drying conditions are thus of primary importance. Aging of Vulcanizates. The good aging of the vulcanizates is another condition indispensable t o their technical value. Therefore, the behavior in the Geer oven, at 70" C., of a vulcanizate of latex reinforced with 10% resin has been compared with a control mixture of latex without resin. The results are indicated in Table VIII. The vulcanizate made with a reinforced latex, without addi-
Effect of Drying Conditions
Conditions of Drying and Vulcanization Days = C.
4 2.5 1.5
Tensile a t Break, Kg./Sg. Ctn.
40 50 60
2 1 hour 2 1 hour
1%
520 530 495 500
1
480
Effect of Aging
Table VIII. Days a t 70° C. Reinforced latex Tensile a t break, kg./sq. em. Loss, % Control latex Tensile a t break, kg./sq. om. Loss, %
385
0
2
5
10
20
30
40
470
460
450
420
400
370
340
...
2
4
11
15
'21
28
320
335
325
295
235
190
170
...
...
..,
8
26
40
47
Figure 4.
Tear Test Pieces after Testing Reinforced latex on right
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
tional antioxidant, has normal aging, rather better than that of the control latex, which is known t o exhibit good aging. In addition, after 40 days in the Geer oven, the mechanical properties are still superior t o those of a nonaged latex mix without resin.
Conclusions These experiments show t h a t i t is possible, by addition of certain partially condensed resins to the latex under suitable conditions, t o realize a pronounced reinforcement, the vulcanizates presenting a combination of exceptional mechanical properties. T h e way is thus open for the direct use of latex in applications (which could not be thought of until the present) requiring resistance to tear, high modulus, and hardness. There still remain points to clear up and improvements t o make before such a method can be employed technically. Improvements are needed in controlling stability and extending the range of vulcanization conditions. The studies which have been purmed in this direction have given encouraging results; moreover, either with or without the addition of ordinary loading materials, t h e procedure lends itself t o many practical applications. From a theoretical point of view, the authors offer no hypothesis on the mechanism of this reinforcement; the observations involved are recent and the example of the extensive studies carried out on the mechanism of the reinforcement of milled rubber with carbon black suggests caution. It does not appear, however, that the resins intervene directly in the vulcanization; the operating conditions and the proportions are very different from those indicated in the work of van der Meer and the Rubber-Stichting (21, 36). Nor does there seem t o be a simple chemical combination of the resins t o the rubber, inasmuch as the curve of Gee ( 6 ) , relating the swelling in various solvents to their energy of cohesion, does not show displacement of the maximum in comparison to the latex control. Many years may elapse before it will be possible to provide a satisfactory explanation of this phenomenon, thanks t o which vulcanixates of natural rubber have been given a combination of properties never before attained.
Literature Cited Bacon, R. G. R., Farmer, E. H., and Schidrowita, P., Proc. Rubber Tech. Conf. London, 1938, 5 2 5 , Bacon, R. G. R., and Schidrowitz, P., T r a n s . I n s t . Rubber I n d . , 15, 152 (1939). Belmas, R., “Le Latex,” p. 20, Paris, R. C. P., 1950.
Vol. 43, No. 2
(4) Chapman, W.H., Patterson, 1’. D., and Dunlop Rubber Co,, Ltd.. Brit. Patent 318.029 (1929). (5) Du Pont de Nemours &- k o . , Ltd.,lE. I., “Ludox, Colloidal Silica in the Rubber Industry,” 1949. (6) Gee, G., “Advances in Colloid Science,” Vol. 2, p. 145, Xcrr York, Interscience Publishers, 1946. (7) Goodyear Tire & Rubber Co., “Pliolite 190 for Latex Reinfor(:ing,” 1947. (8) Grupe, H. L., Kienle. R . PI., and British Thomson-Houston C’o., Brit. Patent 395,217 (1938); I n d i a Rubber J., 86, 429 (1938). (9) Hopkinson, E., Brit. .Patent 204,487 (1923). (10) I. G. Farbenindustrie, A,&., French Patent 630,983 (1927). (11) K. D. P., Ltd., Austrian Patent 112,957 (1930); ILunststo$’e, 20, 67 (1930). (12) Keilen, J. J., and Pollak, A.: IND.ENG.CHEM.,39, 480 (1947). (13) Le Bras, J., and Jarrijou, A., Rev. gdn. caoutchouc, 21, 133 (1944). (14) Le Bras, J., and Piccini, I., B7111.soc. chim. France, 17, 215 (1950). (15) Lepetit, F., Rev. 0th c o o i ~ l c / ~ a u26, c . 675 (1949). (16) Lepetit, F., T r a n s . Inst. R 7 ~ b h e iInd., 23, 104 (1947); Rev. gBn. caoutchouc, 24, 390 (1947); 26, 167 (1949); Rubber Chem. and T’echnol., 22, 912, 923 (1949). (17) Lepetit, F.,and Hoorcman, M., Rer. ge’n. caoutchouc, 25, 3 (1948). (18) Little, G. E., and Pepper. K. W., B r i t . Plastics, 19, 430 (1947). (19) hIcMahon, \v., and I
