Weather
esistance of Vulcanizates
EFFECT OF COMPOUNDING INGREDIENTS D. C. THOJIPSON 4 N D N. L. CATTON Rubber. Lnbovalory, E . I . d u Pont de .Veinours & Company, Zitc., JVilmirzgtoi~,Del.
It is generally recognized tlmt black neoprene compounds givc hotter outdoor service than light colored compounds. Becawe, however, in many "consuiner" proclucte eye appeal dictates the use of bright colors, it becanie desirable to develop compounding techniques that would result in optimum weather aging of nonblack stocks. ilpplications such as oolored garden hose covers prompted a study of the effect of compounding ingredients on thc weather resistance of neoprene vulcanizates. Tests have shown that compounding ingredients exert, a more pronounced effect; on the weather resistance of neoprene than was previously recognized. These effects, especially pronounced in nonblack stocks, are also encountered in stocks containing carbon black.
T h e superiority of neoprene 'L ulcanizates in services involving outdoor exposure has been well established. Results presented in this paper show t h a t compounding ingredients exert a more pronounced effect on the weather resistance of neoprene vulcanizates than has been recognized previously. This is especially true of nonblaclc stocks. Exposure tests conducted in Florida and Delaware have shown t h a t the amount of loading, while of secondary importance to the type of loading, influences weather resistance. Data are presented which show that the weather resistance of neoprene vulcanizates is increased b y certain loading materials. Within defined loading limits materials such as carbon blacks and clays improve weather resistance, whereas materials such as whiting are detrimental even when present in small amounts. The effect of substituting varj ing amounts of GR-S or rubber for part of the neoprene in a clay-loaded vulcanizate containing 6070 elastomer by volunie was studied. Increasing the concentration of the less weatherresistant elastomer bas a deleterious effect, although the decrease may not alwaj s be linear.
OUTDOOR T w r
Outdoor csposure of teat speciinens under variou? condit,ions in Florida and Delaivare \vas >elected as a method of studying the effect of compounding ingi~cdioutson the weather resistance of neoprene vulcanizates. l l o s t of the work covered in this paper is based on outdoor esposurcs of 6 mont,hs' duration, confirmed in part by longer tcst, periods up t,o nearly 2 years. Test specimens have been exposed both s t h s s e d and unstressed, inclined 45 degrees facing south. In most cases a inet,hotl similar to that described by Fielding ( 1 ) 'iws used. The assembly is shown in Figure 1. Die-cut spcwimeris, 0.075 inch thick, tapering from a width of 1 inch to 0.5 inch over a length of 5 inches, n-ere st,retchel to 5.76 in( ' 011 a n-oodcn frame giving 15% over-all elongation. This provided a range of strains increasing froiii very slight a t the \vide rwci u p t,o approximately 25% a t the narrow end. Selected coinpouiitis also have been tested uritlcr actiml service conditions i n the foi~mof garden hose covers. Aniong the conipoundiiig inglcdierits first tested were sntioxidmt,s, nhicih are ne11 rwogiiizctd as dcbiroble adtlitiws for
T
HE superior durability of neoprene products exposed t,o suii-
light has been well established during the past 17 years. This property has been responsible for its widespread use in industrial products for which rubber &*as inadequate. Most of these applications involved carbon black-loaded compounds. More recently, however, it has been recognized that these properties are inherently valuable in inany consumer types of products. Greatly extended usage in this field has resulted in increased interest in colored neoprene products.
Figure 1.
nim-wm
Assembly for Outdoor Testing
892
May 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
893
bon black. The finger on the right, which was rubbed over the surface of the channel carbon black sample, is unsoiled. EEFECT O F MINERAL FILLERS
Carbon black-loaded neoprene stocks are inherently superior to gum neoprene vulcanizates in resistance to weathering. This is not true of all nonblack fillers. Figure 3 shows the effect of G months' outdoor exposure on a series of neoprene stocks containing various nonblack fillers, all of which contained 60% neoprene by volume. In this and subsequent experiments the neoprene was compounded according to the following base formula : Parts by Weight Neoprene Type G N Stearic acid Extra light calcined magnesia Zino oxide Filler Light process oil
Figure 2.
Press cure
Effect of Carbon Black on Weather Resistance of Neoprene
MT BLACK
optimum general aging in all neoprene vulcanizates. A study of the effect of a wide variety of antioxidants on the weather aging of nonblack neoprene compounds, however, indicated that except for antioxidants of a type that darken severely in sunlight little if any benefit is derived. The discoloring types obviously are impractical in products requiring reasonable color retention in outdoor service. Waxes have not been found to provide the added protection in neoprene compounds that they give in rubber compounds, subject to weathering under static conditions. The reasons for this are not clear, but it would appear that the inherent resistance to ozone attack of neoprene minimizes the benefits that make waxes valuable in rubber compounds. The conclusions drawn relative to the effects of antioxidants and waxes were based on earlier work using a commercial neoprene garden hose cover compound loaded with clay and whiting.
100 0.5
4 5 As indicated .\pqrox. 10% of weight, of filler 20 minutes a t 307' F. for 0.075 inch thick sheets
GUM
CLAY
% INEOP'RE BY VOL.UI
EFFECT O F CARBON BLACKS GUM A
HARb CLAY
B
C
1)
PF D. C A I >a NAT. WHITING BARYTES
CASiOs ASBESTINE
MsCOs
Figure 3. Effect of Mineral Fillers on Weather Resistance of Neoprene 6 months' exposure
Although both channel carbon black and soft carbon black stocks, if properly compounded, develop no visible cracking or crazing after years of outdoor exposure, soft carbon black stocks after a few weeks' weather exposure develop a surface covered with released carbon black in the form of a fine powder. This difference is demonstrated in Figure 2. Although the channel carbon black conipound had been exposed for a considerably longer period than the soft carbon black compound, it underwent no visible surface c h a n g e . T h e d a r k spot on the finger on the left, which gives the illusion of a fingernail, is actually a smudge on the ball of the finger obtained by rubbing it over the surface of the sample containing MT car-
Figure 4.
Effect of M.T. Carbon Black and Clay
I
In order to demonstrate the degree of surface deterioration, the tapered specimens were removed from the exposure frame and bent over a l/s-inch mandrel before photographing. The bend was made across the area that was under the higher elongation during exposure, because, in general, this area showed the greatest effect. This series of nonblack fillers developed an over-all surface condition which shows the following decreasing order of weather resistance: hard clay, precipitated calcium carbonate, asbestine, magnesium carbonate, barytes, whiting, and calcium silicate. The stocks containing clay developed a hard glazed surface which remained so flexible that when the samples were bent, surface cracking was insignificant. Clay was the only nonblack filler tested which produced a stock superior in weather resistance to a gum compound. It was difficult to differentiate among the various clays tested. Loading with magnesium carbonate produced a hard glazed surface of good appearance, but when the samples were bent cracking occurred, probably as a result of a thick surface film. For this reason it was considered inferior to the gum control, particularly for applications such as garden hose covers where the product is subject to bending after exposure. Natural whiting, barytes, and asbestine are representative of unsatisfactory fillers for products requiring good resistance to
894
INDUSTRIAL AND ENGINEERING CHEMISTRY
weathering, whiting being outstandingly poor. I n addition to an unsightly, eroded surface, the stock containing whiting exhibited rather deep cracks when folded, which indicates that deterioration has extended well beneat,h the surface. On the other hand, a stock containing precipitated calcium carbonate was noticeably superior to t.he v-hiting stock, although it vas inferior to t.he clay-loaded stock. It shoxed an eroded surface but, considerably less depth of cracking. The fa(,t that two fillers,
% NEOPRENE BY VOLUME
97
(GUM)
75
60
50
Vol. 42, No. 5
EFFECT OF CARBOS BLACK A N D CLAY
I s it had been demonstrated that the type of filler has a marked effect upon the weather rezistance of neoprene vulcanizat,rs, it !vas desirable to investigat,e the effect of amount of filler e m p l o y d I n general, the amount' of loading has less influence than The type. Loading materials which improve weather resistance (compared to a gum neoprene control) may he increased in quantity \vit,hin reasonable limits without, an adverse effect. Figure 4 s h o w the effect' of 6 months' outdoor exposure on neoprene vulcanixates loaded with hIT carbon black or hard clay. The amount of loading used was such that the st,ocks contained 75, 60, 50, and 40% neoprene by volume. Wit,h 1IT carbon black, increasing the loading to give 5070 elastomer cont,ent did not, affect the v-eather resistance adversely. A loading such that the stock contained 40% elastomer by volume resulted in the development of an occasional crack perpendicular to the direction of st,rain after exposure. As with the soft carbon black, increasing the amount, of clay did not affect the weather resistance of the vulcanizat,e seriously. At the highest loading t,esteti, hoxever, 40y0 neoprene by volume, the glazed surface became brittle. While flat. its appearance was good, but on folding it tended to crack. This indicates that no lees than 50% neoprc~n(~ h y volume should be used for good weather a,ging. EFFECT OF CLAY 4hW W H l l I S ( :
40
Figure 5 , Effect of Whiting
both basically of the same chemical composit,ion, give different degrees of weather resistance indicates that particle size, shape, and dispersion, rather than chemical nat,ure of the filler, are t'he influencing factors. Bzrytes and ashestine stocks alsc developed eroded surfaces, but ir-ith a depth of cracking less than the natural whiting stock, A4sbestineappeared only slightly inferior to precipitated calcium carbonate, but barytes gave nearly as poor aging stocks as whiting. St,ocks loaded xvit8hcalcium silicate hardened during weather exposure, but showed surface erosion. When bent over a mandrel, severe cracking was observed. The mineral fillers, like the soft carbon black, were released on the surface of the neoprene st,ocks after a few weeks' weather exposure. This was true with clay loading notwithstanding the superior surface appearance of the exposed vulcanizates. I n order to show in the photographs the true surface appearance of the specimens discussed in this paper, t,his poxder \vas wiped off. % NEOPRENE BY VOLUME
In the case of fillers, such as natural whiting, which have an adverse effect o n Jveathcr resistance, the amount used hecomes an important factor. Figure 5 shows the adverse effect of using increasing amounts of whiting. At all loadings it, produred poorer weathering stocks than the gum cont,rol. The stock containing only 40% neoprene broke after about, 2.5 months' exposure. The question naturally arises: What is the effect of adding small amounts of a filler such as whiting to a stock that has been compounded n-ith clay? Investigation has shown that as far as weather resistance is concerned, even small quantities of Tvhiting in a clay stock are undesirable. Figure 6 shows the effect of replasing with whiting part of the clay in the stocks previously studied. I n one series, half, and in the other, a quart,er, of t8he clay was replaced by whiting. Even t8he smallest amount of whiting studied has an adverse effect and incrwsing the ratio of whiting t,o clay as well as increasing the loading for a given ratio inarkedly decreases rwather resistance. It frequently is desirable to obtain the utmost weather resistance in a stock hiLving the properties conferred by loading, with clay. I n such a case, the addition of small amounts of channel black is of considerable benefit. Homver, there exists a minimum level of carbon black belovi which little or no benefit is derived. Figure 7 shows the effect of the addition of EPC
RATIO 1 to 1
1
2 5 10 30 PARTS EPC BLACK PER 100 PARTS NEOPRENE
Figure 7. Effect of Carbon Black on Weather Resistance of Clay-Loaded Neoprene Figure 6.
Effect of Clay and Whiting
60% neoprene by volume
May 1950
NEOPRENE
INDUSTRIAL AND ENGINEERING CHEMISTRY
GR-S
