NONSOLVENTS IN
RUBBER CEMENTS T. FOSTER FORD The B. F.Goodrich Company, Akron, Ohio
Data comparing various nonsolvents as ingredients of rubber cements are reported. The results are for benzene, Varnish Makers and Painters’ naphtha, and ethylene dichloride cements, to which are added varying proportions of the more common alcohols, ketones, and esters. The properties studied are the film thickness deposited by dipping and the film-breaking tendency. Isopropanol permits most satisfactory simultaneous control of these properties in a benzene cement of the composition employed. For such a cement, graphical data are given which show the interrelated effects of varying the proportion of isopropanol, the concentration of nonvolatile ingredients, and the time of mastication of the rubber stock. Results of a study of the effects of temperature and humidity in the dipping room upon film thickness and film breaking for a benzene-isopropanol cement also are reported.
satisfactory control of both filni breaking and f ilm thickness was possible. As here reported, the results afford a basis for comparison among the more common nonsolvents in cements in which the solvents used are benzene, Varnish Makers and Painters’ (V. M. & P.) naphtha, or ethylene dichloride; and for a benzene-isopropanol cement they show the nature of the effects upon the properties studied, of simultaneously varying the proportion of nonsolvent, the concentration of nonvolatile ingredients, and the degree of mastication of the rubber stock. Results on temperature and humidity control are included to illustrate the importance of these factors in dipping-room operations. The nonvolatile ingredients of the cement used in these studies were as follows (in grams) : Rubber (pale crepe)
Sulfur Clay
100
60 52
Carbon black Mineral oil Aldehyde-amine accelerator (liquid)
5 50 2
Despite the specific nature of this recipe the results are not limited in their significance, since the properties of a cement generally are influenced far more by the concentration and kind of rubber present than by pigment loading. For the experimental work, cements containing solvent but no nonsolvent, were prepared in laboratory churns, and the measurements were made upon aliquot parts to which varying proportions of the nonsolvents were added. -4ll cements were prepared under comparable conditions, and each set of experiments was performed a t one time, PO that errors due to temperature and humidity variations are negligible. The rubber stocks for the various cements were all mixed on the same laboratory mill by the same operator, and a definite procedure was followed. All of the nonvolatile ingredients except the mineral oil were mixed on the mill, and the oil was added in the churn. The time of mastication was measured from the start of breakdown of the rubber. Film thickness was calculated from the dry weight and the area of a deposited film; and the film-breaking tendency was measured as “film-breaking time,” in seconds required for a standard film to break or as “per cent break”-i. e., the number of films breaking per hundred standard films formed over small holes in a perforated panel, as explained in detail later, The experimental results are presented in the order in which they were obtained, falling into three groups: (a) the comparative effectiveness of various nonsolvents in causing film breaking; (b) the effects upon film thickness and film breaking of varying the proportion of nonsolvent, the concentration of nonvolatile ingredients, and the degree of mastication of the rubber stock for a benzene-isopropanol cement; and (c) the effects of temperature and humidity upon these properties.
E
THANOL has long been used for reducing the apparent viscosity of rubber cements. Other rubber nonsolvents, especially acetone and amyl acetate, are sometimes similarly employed. However, there are few data in the literature from which adequate comparisons among nonsolvents may be deduced, nor has any systematic study of the properties of a cement , containing nonsolvents been reported.’ Whitby ( I )studying a long list of chemicals as solvents, records comparative swelling power for rubber and nitrocellulose; Whitby and Jane (3) find the viscosity of dilute rubber solutions to be reduced by a number of these materials; and Kawamura and Tanaka (1) obtain the following order of decreasing precipitant value for a 0.25 per cent solution of rubber in xylene-methanol, ethanol, n-butanol, isobutanol. With the exception of the swelling experiments, these results, as is the case with practically all published cement studies, are for dilute solutions only and therefore are of little technical value. The present experiments were performed with a commercial cement and are the result of a wish to control particularly two properties-the thickness of rubber film deposited upon an article by dipping and the tendency of the wet 6lm t o break across small holes in perforated objects. The addition of a suitable nonsolvent to a cement improved the film-breaking property, and by changing the proportion of the nonsolvent, with proper regard for other factors,
Comparison of Nonsolvents
this paper wae written, Fabritriev et al. [Korhevenno-Oburnoyo Prom., 14,614-18 (193511have reported experiments on t h e s a m e nonsolvents used here. 1 Since
Since nonsolvents are useful in cements because of their effect upon film breaking, this property was employed as the 915
INDUSTRIAL AND ENGINEERING CHEMISTRY
916
VOL. 2 8 . YO. 8
FIGURE1. EFFECT o s FIGURE 2. EFFECT O S FIGURE 3. EFFECTO S FILM FIGURE4. EFFECT ON FILX FILMBREAKIXG O F SIMPLE FILMBREAKIXG OF XosBREAKISGOF SONSOLVESTS BREAKINGOF SON~OLVENTS IS DILUTION OF CEMESTS MADE SOLVENTS IN A BEXZESE IN Y. M. & P. NAPHTHAAS ETHYLESEDICHLORIDE CEIN REPRE~ESTATIVE SOLCEMENT CEYEST MEST VENTS R u b b e r stock masticated 15 mi n u tea.
R u b b e r stock masticated 90 minutes.
basis for comparison among them. To cenieiits prepared as previously described, the nonsolvents were added stepwise and film-breaking time was measured. In these experiments the standard film was that formed over a small round hole (0.63 em. in diameter) centered 0.63 cni. from one end of a 5 X 12.5 X 0.078 em. iron panel. Panels were dipped to a dept'h of 5 em. in cements free froin air bubbles or scum, and were withdrawn slowly at a standard rate. Time ivas measured froni the instant of withdrawal of the film through the cement surface. Three solvents studied (benzene, V. AI. 8: P. naphtha, and ethylene dichloride) were considered the most representative of the aromatic, the aliphatic, and the commercial chlorinated solvent's, respectively. In cases where combinations of solvents within a homologous series are desirable for control of evaporation rate, it is probably safe to assume that the present results will hold for the coniliination about as well as for the single solvents studied here.
R u b b e r s t o c k masticated 15 minutes.
R u b b e r stock masticated 15 niiriutei
Effectivenessof Nonsolvents in Benzene Cement Figure 2 shows the nature of the changes in film-breaking time as a benzene cement is diluted with various nonsolvents. These curves were obtained by successive additions of nonsolvent to a benzene cement originally containing 250 grams of nonvolatile ingredients per liter of cement. For comparison, a blank curve shows the changes on simple dilution of this cement with benzene. All of the nonsolvent curves terminate at the last value obtained before coagulation of the cement occurred. These curves show, therefore, not only effects upon film breaking, but the relative tolerance of a benzene cement for the different nonsolvents as well. Upon the basis of this diagram and of supplementary data for benzene cements and concentrations of diluent between 10 and 20 volume per cent, the various liquids can be arranged in the following order of decreasing effectiveness in film breaking: isopropanol,
Effect of Simple Dilution on FilmBreaking Time The effect of s i m p l e d i l u t i o n upon filmbreaking time for cements made in benzene, V. hI. R- P. naphtha, and ethylene dichloride is shown by Figure 1. Within the concentration range studied, ethylene dichloride is evidently a much more powerful solvent than either of the others, if by solvent power iq meant ability to dissolve a large amount of rubber to form a cement of low apparent viscosity. There seems to be comparatively little difference b e h e e n benzene and V. 31. & P. naphtha. However, different orders of solvent power may be obtained, depending upon the viscosity (film breaking) desired, because the two curves cross each other. It is not correct to assume that the properties of a cement always vary with dilution in the same way regardless of solvent. Obviously. the choice of solvent may sometimes be greatly influenced by the particular combination of concentration and flowing qualities desired in a cement.
OF -4 BENZESE-ISOPROPANOL %KING CHARICTERISTICS FIGURE 5 . FILM-BRE C E M E N T 4T DIFFERENT COSCEXTRATIONS O F RUBBER AND ISOPROPANOL
R u b b e r s t o c k masticated 90 minutes.
AUGUST, 1936
ISDUSTRIAL AND ENGINEERING CHEMISTRY
91;
ethanol, Pentasol (technical grade), b u t a n o l (technical grade), amyl acetate, butyl acetate, acetone, methyl ethyl ketone, benzene. Although there is actually almost no difference between isopropanol and ethanol in volume-forvolume effect on film breaking, the tolerance for ethanol is much less than for isopropanol. If over 10 per cent of ethanol is used. coagulation occurs, whereas more than 15 per cent of isoi60 50 propanol may be present without coagulation. In deposited films, the simultaneous evaporation B of benzene and ethanol apparently causes wide 5 ’O i ;30 variation> in alcohol content so that films sometimes break and sonietiines do not. With isopropanol this trouble is not encountered. Branching from the isopropanol curve in Figure 2 is a horizontal line marked “butanol.” This means that, after dilution of the cement n ith isopropanol to this point, butanol was added. FIGURE 6. FILM-RREAKINGC H 4 R 4 C T E R I S T I C S O F L BEXZE~E-ISOPROP.~VOL CEMEST AT DIFFEREUT CONCEUTRLTIOSS O F RUBBER LTD I s o P R O P a N O L The curve shows that the further dilution with butanol gave no further change in film breaking Rubbei . t w L masticated 11 75 houlq until a total dilution was reached where a change TI-ould have occurred had the entire dilution carefully drilled with one hundred holes 0.48 cm. in diameter, been with butanol alone. It is also interesting that the evenly spaced on 0.i3-cm. centers, were employed. These curve for dilution with V. 11.B: P. naphtha falls somewhat panels were slowly withdrawn from the cements and the numa b m e the curve for benzene. The practical conclusion from ber of holes over which the films broke were recorded as “per this latter observation is that cements which have lost solcent break.” For the measurement of average film thickness, vent by evaporation should be “made up” only with the 5 x 12.5 x 0.0i8 cm. iron panelswere immersed in the cements solvent originally present. to a depth of 10 cm. and withdrawn slowly at a uniform rate, Effectiveness of Nonsolvents in V. M. & P. the dry weight of the deposited film being found by difference.
Naphtha Cement
Figure 3 is similar to Figure 2 and was obtained by successive additions of nonsolvent to a V. M. B: P.naphtha cement originally containing 250 grams of nonvolatile’ ingredients per liter of cement. Upon the basis of this diagram, for Tr. RI. &- P. naphtha cements the order of decreasing effectiveness in film breaking for concentrations of nonsolvent betmeen 10 and 20 volume per cent is: ethanol, isopropanol, butanol, methyl ethyl ketone, butyl acetate, T’. 11. B: P. naphtha. Approximately 20 per cent of ethanol can be added to a V. AI. B: P. naphtha cement, in contrast to the ~naximuinof 10 per cent which can be added to a benzene cement. From the standpoint of tolerance, therefore, ethanol is evidently as good as isopropanol in gasoline cements.
Effectiveness of Nonsolvents in Ethylene Dichloride Cement Figure 4 is similar to Figures 2 and 3. Upon the basis of this diagram for ethylene dichloride cements for concentrations of nonsolvent between 10 and 20 volume per cent, the order of decreasing effectiveness in film breaking is : butanol, butyl acetate, ethylene dichloride, ethyl acetate. Ethanol, acetic acid, dichlorodiethyl ether, methyl ethyl ketone, and isopropanol merely cause coagulation.
Simultaneous Control Of Thickness and Film Breaking The imPortaIlt factors in the control of the Properties of a rubber cement are: the proportion of nonsolvent in the liquid system, the concentration of nonvolatile ingredients, and the degree of mastication of the rubber stock. F~~the study of the effects upon film thickness and film breaking of varying these three factors simultaneously, the benzeneisopropanol solvent mixture was selected. In the experi”Ork it was found to breaking time” by measurement Of ‘‘Per cent break.” For this purpose small aluminum panels, 0.157 cm. thick and
Optimum Per Cent Isopropanol Figures 5 and 6 show the relations between concentration of n o n olatile ~ ingredients, volume per cent isopropanol, and film breaking for benzene cements made from rubber stocks masticated 1.5 and 11.76 hours, respectively. Diagrams for intermediate mastication times are similar. The valleys represent the areas where maximum film breaking occurs. Evidently for thi. cement, regardless of concentration of nonTolatile ingredients or of time of mastication, the maximum film-breaking effect is obtained when the cement contains about 11 per cent by volume of isopropanol.
Correction of Film Thickness Although the film-breaking properties of a cement are greatly improved by adding isopropanol, film thickness is a t the same time reduced. This effect can he offset by increasing the mastication time of the rubber stock and the concentration of nonvolatile ingredients. Figure i shows the relation of film thickness and per cent break to time of inastication of stock and concentration of cement for the benzene cement described which contains 11.2 per ceut by volume of isopropanol. All data are for 25’ C. and 20 per cent relative humidity. For this cement and these conditions: 1. To obtain 0.0025-em. film thicknesz and 90-100 per cent break, the rubber stock must he masticated one hour and the cement must contain 250 grams of nonlolatile ingredients per liter. (In mastication, time is measured from the start of the breakdown of crude rubber and include- mixing of pigments The stock \\as mixed and masticated on a cold mill. The hatch size was 1000 grams. A 12-inch laboratory mill was used with rolls 6 inches in diameter and a differential roll speed of 1.5 to 1 with the front roll speed 20 r. p. m. The mill opening, as measured by the thickness of the rubber sheet, !vas 0.25 inch. A continuous bank was maintained. and the hatch was cut from left to right and folded back every 30 seconds.) obtain 0.0063-cm. film thickness and 90-100 per cent 2. To break, the stock must he 10 hours and the cement must contain 400 grams of nonvolatile ingredients per liter.
918
ISDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 28, NO. 8
pearance suggests a t least an incipiently discontinuous liquid system.
Effect of Temperature and Humidity
HOURS MASTICATION OF RUBBER STOCK FIGURE7. RELATIONOF FILMTHICKNESS AND P E R C E N T BREAKTO MASTICATION OF RUBBERSTOCK AND CONCENTRATION OF CEMENTFOR A BENZENE CEMENT CONTAINING. 11.2 P E R C E N T BY VOLUME OF ISOPROP.4NOL
For a cement of the type used in the preceding experiments the relation of film thickness obtainable a t 100 per cent break to the temperature and humidity of the dipping room is shown in Figure 8. The concentration of nonvolatile ingredients necessary to obtain each film thickness shown is given by curve C C C (Figure 7 ) . Figure 8 was simplified from extrapolated data for illustrative reasons. It shows the importance of careful control of t e m p e r a t u r e a n d humidity in d i p p i n g o p e r a t i o n s . Evidently for this particular cement a t e m p e r a t u r e of about 200 (7. and moderate h u m i d i t y w o u l d
Physical Chemistry of Film Breaking with Isopropanol When a film deposited from a benzene-isopropanol cement breaks, it breaks across a tiny pool of clear solvent. These pools can be observed in the process of formation. They occur all over the surface of a film (not just where the film covera a hole), and their craters leave the dried film with a dull appearance. If a little isopropanol is stirred into benzene, an emulsion is formed. Not until about 50 parts isopropanol per 100 parts benzene have been added is a clear solution obtained. That is, in concentrations below about 33 per cent by volume, isopropanol does not completely dissolve in benzene. If for any of the benzene-isopropanol cements the FIGURE 8. RELATION OF FILM THICKNESS OBTAINABLE AT 100 PERCENT BREAKTO TEMPERATURE AND HVMIDITY OF THE DIPPING ROOMFOR percentage isopropanol in total solvent is calculated A BENZENE-ISOPROPANOL CEMENT CONTAININQ 11.2 PER CENTBY VOLfor any point where coagulation just occurs (shown UME OF ISOPROPANOL by downward arrows in Figures 5 and 6), this value Rubber stock masticated 5.5 hours. will be very close to 33 per cent. The suggested explanation of film breaking is that in be good working conditions. However, in forming surgeons' the film deposited from the cement, as in the cement and in gloves where a naphtha cement is used, the temperature of a simple benzene-isopropanol mixture, the isopropanol is in a the dipping room is held a t about 30" C. and the humidity state of line dispersion. Since, in the drying film, benzene is very high. Since film thickness and film breaking are evaporates more rapidly than isopropanol, the concentration both closely related to solvent evaporation rate, it is obvious of isopropanol gradually increases until it reaches that point that, for any cement, optimum temperature and humidity where it is capable of dissolving all of the benzene, and then depend upon the nature of the solvents present. precipitation of the rubber occurs. The effect is the same whether a given percentage of isopropanol is reached by Literature Cited addition of isopropanol or by evaporation of benzene. That isopropanol in benzene cements removes a portion of (1) Kawamura, J., and Tanaka, K., J . SOC. Chem. Ind. Japan, the benzene from its role as solvent might be concluded from 35, Suppl. Binding 186-8 (1932); reprinted in Rubber Chem. Tech., 5, No.4 (Oct., 1932). the observation that curing cements containing isopropanol (2) Whitby, G.S., Colloid Symposium Monograph, IV, 203-23, eap. are more likely to gel than those which do not contain it, 204-10 (1926). presumably because of greater effective total solids concentra(3) Whitby, G. S., and Jane, R. S., Ibid., 11, 16 (1924). tion in the benzene remaining free. Furthermore, the fact that such cements which have gelled have a granular apRECEIVED August 9, 1935.
