1
JOSEPH GREEN1 and E. F. SVERDRUP
U. S.
Rubber Reclaimhg
Co.,Inc.,
Buffalo, N.
Y.
Carboxylic Rubbers from Scrap Vulcanized Rubber
SCRAP
vulcanized rubber has been used principally for the manufacture of reclaimed rubber, which exhibits properties inherent in the original polymers of the scrap. Little has been found in the literature on the utilization of scrap vulcanized rubber as a low-cost starting material for controlled polymer synthesis. I n the present investigation scraps containing natural and Type S synthetic rubbers have been modified to produce chemically different polymers possessing properties not usually associated with the initial elastomers. T h e authors believe that reactions with vulcanized rubber are not usually the same as reactions with the raw polymers and in this work the physical means of accomplishing the reaction are different. I n 1938 Bacon and Farmer (2) reported that when masticated raw natural rubber and maleic anhydride were dissolved in a solvent and the solution was heated in the presence of benzoyl peroxide, the ingredients reacted, yielding a variety of tough, fibrous, or resinous products (6, 27). When vulcanized natural and Type S synthetic rubber scraps were reclaimed in a Reclaimator (8, 9) (a specially designed extruder type plasticator, made by the U. S. Rubber Reclaiming Co., Inc.) in the presence of a critical concentration of certain activated unsaturated compounds, a reaction occurred between the unsaturated compound and the scrap vulcanized rubber. With maleic anhydride, the resulting product was a carboxylated and replasticized rubber. This elastomer exhibited vulcanizing versatility via the carboxyl groups--Le., curing with bivalent metallic oxides, diamines, glycols, epoxy resins, and diisocyanates. T h e polarity imparted by the carboxyl groups and the degree of cross linking of the polymer appear responsible for its oil resistance, a property not normally present in a tire reclaim. The blocking of the double bonds, either by reaction a t the double bond or by steric hindrance, added to the good aging properties anticipated with nonsulfur vulcanizates.
been combined with vulcanized natural and Type S synthetic rubber scrap. This work is based on the utilization of such compounds, wherein the electrophilic group is a carboxyl group. The reaction products have a lower benzene extract and higher tensile strength, modulus, cured hardness, and torsional hysteresis when contrasted with the corresponding reclaims. The products also exhibit oil resistance and good aging properties. The maleic anhydride and vulcanized
truck and bus peel scrap form a product very different from the corresponding reclaim (control) containing no maleic anhydride, data on which are shown in Table I. The following test formula was used : I’aTts Hydrocarbon 2-Mercaptobenzothiazole DPG Stearic acid Sulfur Zinc oxide
100.0 0.5 0.2 2.0 3.0 5.0
I
10
I 0
I 2
I
3
CONCN. OF ACTIVE INGREDIENT
0 CONCN.
8
lb
25
OF ACTIVE INGREDIENT
Figure 1. Relution of Mooney to concentration
Figure 2. Relation of benzene extract to concentration
Figure 3. Relation of 300% modulus to concentration
Figure 4. Relution of torsional hysteresis ta concentration
Characterization of Active Ingredient Unsaturated compounds containing a n electrophilic or electron-accepting group alpha to the double bond have I Present address, 91 Kindall Court, Dover, N. J.
2 1 38
INDUSTRIAL AND ENGINEERING CHEMISTRY
Various organic acids, unsaturated (70) and saturated, when used as reclaiming agents in small quantities generally upgrade the final product. Use of larger quantities of certain unsaturated acids, however, radically changes the characteristics of the polymer (77). Comparative data on the effect of unsaturated and saturated acids are shown in Table 11, where the effect of maleic anhydride is contrasted to that of succinic anhydride, which differs from maleic anhydride only in being saturated. The succinic anhydride produces a polymer of slightly higher tensile and elongation than the control, but otherwise similar in properties and characteristics. The polymer containing maleic anhydride, however, shows a n anomalous increase in Mooney viscosity, modulus, and cured hardness. Further increase in concentration of the maleic anhydride shows corresponding changes in physical properties (Figures 1 to 4), in contrast to the changes observed with the succinic anhydride. The concentration of maleic anhydride used to prepare the sample shown in Table I1 is equivalent to that of the sample represented by concentration 4 in the figures. The effect of the unsaturated acid is therefore not attributed to the carboxyl or acid groups alone. Comparative data on the effect of unsaturated compounds having an acid group alpha to the double bond and other unsaturated compounds are shown in Table 111. Fresh styrene N-99 monomer (Dow Chemical Co., Midland, Mich.) was used both with and without inhibitor. The data obtained were essentially the same for the two; Table I11 shows the results obtained with the inhibitor removed. It has been concluded, in part from these results, that the general class of unsaturated compounds does not give the desired result. Table I V shows the results obtained by using a series of unsaturated acids to give carboxylated elastomers exhibiting special characteristics common to the group. Mooney viscosities, moduli, and cured hardnesses are high as contrasted to the control. The initial reading obtained on the Mooney viscometer using the large rotor was above 200 for the maleic anhydride sample; hence the result reported was obtained using the small rotor. These materials have high torsional hysteresis (25). The restilt obtained with the control is unusually high for this particular type of reclaim. Benzene extracts were made on uncured samples which had been extracted with acetone for 16 hours. Table IV shows that citric acid imparts the physical characteristics of a carboxylic rubber t o the elastomer. Citric acid, tribasic and saturated, does not meet the conditions for the active ingredient; however, at the temperature
of the reaction it decomposes to give unsaturated acids. T h e literature (4, 26) describes the decomposition of citric acid, whereby it loses water t o form the unsaturated tribasic aconitic acid which may then lose carbon dioxide to form the unsaturated dibasic itaconic acid. These acids have the electrophilic carboxyl groups alpha to the unsaturation. Decomposition of the citric acid took place, as evidenced by the large evolution of gas during the reaction.
Table 1. Physical Properties of Truck and Bus Peel Scrap Carboxylated with Maleic Anhydride Maleic M i n . Control Anhydride Concn. of active ingredient ML/212/3-1 Cure at 287O F." 300% modulus, Ib./sq. inch 25 35 45
Theoretical Considerations
Tensile, Ib./sq. inch
Bacon and Farmer (2) and Le Bras and Compagnon (22, 23) had postulated that the reaction between maleic anhydride and masticated raw natural rubber was an addition a t the double bonds of the rubber molecule. At a later date, however, Farmer (73, 75) concluded that reactions between olefins or unconjugated polyolefins and maleic anhydride can be promoted a t elevated temperatures, wherein the reaction is a substitution a t the a-methyl-
Table It.
Hardness, Shore A
4.0 54
530 610 640
1510 1640 1675
25 35 45
1100 1140 1140
2250 2300 2300
25 35 45
440 440 425
520 495 490
25 35 45
37 38 38
63 64 65
Test formula.
Relation of Unsaturated and Saturated Acid Active Ingredients as Shown by Physical Properties of Plasticized Rubbers
Concn. of active ingredient ML/212/3-1 Cure at 287O F.O 300'% modulus, lb./sq. inch
Tensile, lb./sq. inch
Elongation,
Yo
Hardness, Shore A
a
Elongation, %
0 44
Min.
Control 0 30
30 40 50 30 40 50 30 40 50 30 40 50
590 640 680 820 880 940 380 380 380 40 41 41
Maleic Anhydride 4.0 93
Succinic Anhydride 4.0 30
1060 1150 1250 1390 1420 1490 430 390 380 60 61 63
690 695 720 1200 1110 1120 440 420 400 44 45 46
Test formula. ~
Table 111.
~~
-~
~~
~
Physical Properties of Vulcanized Rubber Scrap Replasticired in Presence of Unsaturated Compounds vs. Unsaturated Acids Min.
Concn. of active ingredient ML/212/3-1 ~
Cure at 287' F.b 30070 modulus, lb./sq. inch
Control 0 28
25 515 35 570 45 620 Tensile, Ib./sq. inch 25 745 35 745 45 770 Elongation, yo 25 400 35 370 45 350 Hardness, Shore A 25 43 35 44 45 44 Phillips Petroleum Co., Bartlesville, Okla. b Test formula.
Cinnamic Acid 6.2 26 540 650 740 825 910 965 430 400 380 53 54 54
Styrene N-99 4.25 28 490 545 580 790 810 850 430 410 400 43 44 44
VOL. 48, NO. 12
0
Isooctenesa 4.6 26
Fumaric Acid 4.8 42
515 600 640 765 810 860 420 380 380 42 43 44
DECEMBER 1956
715 880 940 1020 1090 1160 410 380 370 53 54 54
2139
ene groups of the olefin. Adler, Pascher, and Schmitz (7) have shown that the substitutive addition of maleic anhydride to simple olefins takes place a t elevated temperatures. This reaction proceeds with particular ease in the system -CHz-CMe=CH-CHzof rubber. In the presence of benzoyl peroxide the rubber molecules are capable of forming free radicals a t the a-methylene atoms. The radicals of rubber then add the maleic anhydride, the double bonds of which thereby disappear; the double bonds of rubber, however, remain intact (75).
‘d
I
HC--CHB I
1
ob ho
‘d
When rubber is masticated in the presence of air (77, 78, 79) a t temperatures normally encountered in the Reclaimator (8, 9, 20), hydroperoxides, peroxides, and free radicals are formed mainly a t the a-methyl carbon and not a t the double bond (72, 74, 29). The initiating reaction may take place a t the double bond. The reaction between rubber and maleic anhydride should therefore proceed with ease in the present system in the absence of a free radical catalyst. This has been found to be the case; no difference was found, with or without the use of benzoyl peroxide.
Table IV.
c
R-CH=CH
This electron-poor double bond of maleic anhydride would be expected to add readily to the electron-rich double bond of natural rubber. The reaction under consideration is a substitution a t the a-methylene carbon of the rubber. This reasoning can, however, be extended. The assumption is made that substituents, which influence the polarity of the double bond, can similarly influence the polarity of an adjacent free radical, and the attraction of the negative radical for the positive double bond appears to be an important factor in facilitating this reaction (7, 27). The steric factor, also influences the reaction. Table IV contains data obtained by using a series of unsaturated acids and esters. Careful examination of the data, especially the 300% modulus and the Shore i\ hardness, show that the reactivity of the active agent decreases from left to right. Thus maleic anhydride is most active, but when the ring is open as in maleic acid or maleic esters, activity is greatly reduced. Steric factors also appear to be responsible for
P -
R-CH-CH
-
-+
COOH
Because the free radical rubber is relatively negative, there is a strong electrical factor facilitating combination a t the beta carbon of the acid. For diethyl maleate not more than one carboxyl group at a time can be coplanar with the double bond due to steric hindrance :
The reaction between the free radical and the double bond of the maleate is preferentially in the plane perpendicular to the atoms attached to the double bond, and, because the rubber radical is a “negative” radical, it should add
Diethyl
D ibuty 1 Maleate
6.1 42
7.0 29
9.3
830 920 985
730 810 860
530 570 595
355 395 420
1240 1390 1395 450 440 410
1410 1460 1430 475 440 410
1200 1200 1175 440 400 370
920 940 950 440 410 410
770 740
53 53 53
47 48 50
46 48 50
54 56
39
58
40
33 33 34
...
0.114
0.100
0.100
0.162
...
...
12.7
12.0
12.8
...
...
*..
...
Maleic Anhydride
Maleic Acid
0 31
4.0 67”
4.8 70
8.0 44
4.75 67
5.3 42
7.1 45
510 540 525
1150 1320 1420
1065 1155 1215
1045 1190 1205
1015 1085 1195
760 870 990
770 905 900 390 415 410
1430 1480 1530 390 340 330
1460 1555 1565 425 410 390
1370 1410 1375 400 360 350
1310 1330 1375 390 370 340
37 37 37
58 59 61
53 55 55
50 51 52
0.099
0.179
...
15.6
12.3
...
M/S 212/3-1. Test formula. On uncured acetone-extracted sample.
2 140
-+ COOH
s+
the decreased activity observed when fumaric, itaconic, and cinnamic acids are used. Chloromaleic anhydride appears to be slightly more active than maleic anhydride, as the former produces a polymer having a lower benzene extract and a slightly higher modulus, Shore A hardness, and torsional hysteresis (Table V). This is explained by the electrical effect of the chlorine atom (7). The carboxyl group of the unsaturated acid polarizes the n electrons in such a way that the beta carbon is positively charged.
Effect of Active Ingredient on Properties of Replasticized Vulcanized Scrap Rubber
Min. Control Concn. of active ingredient ML/2 1 2 / 3 4 Cured at 287’ F.b 300% modulus, Ib./sq. inch 30 40 50 Tensile, Ib./sq. inch 30 40 50 Elongation, yo 30 40 50 Hardness, Shore A 30 40 50 Torsional hysteresis 40 48-hr. benzene extract, %c
Conceivably, an olefin may behave as either an electron donor or electron acceptor. However, olefins are predominantly electron donors, and natural rubber would be expected to be especially so, because of the inductive effect of the side methyl group. An electrophilic substituent-i.e., a substituent which normally directs meta in the benzene ring-attached to the double bond of an unsaturated compound will withdraw electrons from the double bond.
INDUSTRIALAND ENGINEERINGCHEMISTRY
Citric Acid
Fumaric Acid
Itaconic Acid
Aconitic Acid
Cinnamic Acid
Muleate
40
24
780
470 44 0 430
preferably at the positive beta-carbon of the maleate (27). As can be seen from the above formulation, there will be hindrance in this case which is not present in maleic anhydride or diethyl fumarate, which is the trans form (7, 27, 28). Table I V shows the inactivity encountered with diethyl and dibutyl maleates.
Method of Preparation The reaction was accomplished in a Reclaimator (8, 9 ) , in which the ground vulcanized rubber scrap was subjected to heat, chemical action, and mechanical work. The Reclaimator was heated to 325' F. by means of oil jackets and the mechanical working raised the temperature of the material being processed to 400" to 450" F. The screw was designed with special masticating sections to enhance the working and the intimate, mixing of the ingredients. The vulcanized polymer used was fabric-free whole tire or peel scraps ground to 30-mesh size. The total rubbery hydrocarbon content of fabricfree whole tire scrap was about 550j0, which consisted of approximately 50% natural and 50y0 Type S synthetic rubbers. The peel scrap was considerably higher in natural rubber content, but, it is considered that the butadiene units in the Type S synthetic scrap also take part in the reaction. Analyses of typical scraps are shown in Table VI. The ground vulcanized polymer and reclaiming ingredients were mixed in a pug mill-type mixer and the acid was added in crystal, flake, or powder according to the form available. The reaction took place simultaneously with the reclaiming action. The time of contact in the Reclaimator was the same as for the reclamation of the scrap rubber, about 3 minutes. These reaction conditions were also used for the controls. As the product was extruded, it was cooled immediately to prevent excessive oxidation. The samples were then refined on a two-roll laboratory-size refiner and allowed to age a minimum of 24 hours before the testing was continued.
Description of Uncured Polymer Various types of carboxylic rubber polymers have been made, depending on the active ingredient, its concentration, and the scrap used. In addition, the highly reacted types have been extended with large quantities of petroleum and ester oils. Each type has its special characteristics which are developed to the fullest extent through proper compounding and processing. As the concentration of active ingredient is increased upward from the critical point, a gradual transition may be observed from tough but still rubbery products (Figures 1, 3, and 4) to hard
and brittle resins. This progressive modification of physical properties can be a convenient adaptable feature. In their laboratory work the authors have been concerned generally with four grades of carboxylated rubber known as Bisonide 400, P-400, 1600, and 1630 (approximate data are shown in Table VII). The Bisonide 400 is a general all-purpose polymer, possessing high green
Table V. Effect of Maleic Anhydride and Chloromaleic Anhydride on Physical Properties
Min. Concn. of active ingredient ML/212/3-1 Cure 287O F.a 300% modulus, lb./sq.inch 25 35 45 Tensile, lb./sq. inch 25 35 45 Elongation, %
Hardness, Shore A
Torsional hysteresis 48-hr. benzene extract, % b
Maleic Anhydride
Chloromaleic Anhydride
4.0 88
5.4 85
1150 1225 1260
1235 1350 1385
1375 1455 1450
1255 1350 1385
25 35 45
390 370 360
305 300 300
25 35 45
58 59 61
61 63 63
35
0.234
0.298
12.3
8.3
' Test formula. On uncured acetone-extracted sample.
Table VI.
Typical Analysis of Scrap Used
h'o. 1 Passenger Truck Whole and Bus Tire Peels Acetone extract 9.8 7.7 Ash 6.1 3.8 Carbon 25.4 28.4 Total rubber hydrocarbon 55.7 57.1 Direct rubber hydrocarbon (natural) 28.2 44.5 Difference (Type S synthetic) 27.5 12.6
Table VII.
Properties of Carboxytic Rubbers
Grade 400 Plasticity (Mooney), ML/ 212/5-1 40-75 Specific gravity 1.15 Rubber hydrocarbon, % 48-50 Acetone extract, yo 20
1600
1630
75-125 1.20
3045 1.17
45-50 25
37-40' 35
' Oil-extended types are treated a s if rubbery hydrocarbon were 50%.
strength, made from whole-tire vulcanized rubber. The P-400 is made from vulcanized truck and bus peels. I t breaks down more readily on the mill and has a lower plasticity and more tack; the vulcanizate has a higher tensile strength and elongation. Sulfur is normally required for vulcanizing both grades, although sulfur requirements are reduced. Bisonide 1600 is a tough, nervy, nontacky, but still rubbery material made from whole-tire vulcanized rubber. I t is highly resistant to mechanical breakdown, needs bivalent metallic oxides, diamines, or glycols for vulcanization, and its vulcanizates possess oil resistance and good oven-aging properties. Bisonide 1630 has the same concentration of active ingredient as the 1600, but is enriched with oil. The reaction and extension with oils are accomplished in a single operation. This material is soft and tacky and more easily worked on the mill, although it still possesses mill nerve and high green strength. Figure 2 shows that the benzene extract of uncured samples, which have been extracted in acetone for 16 hours, decreases with increased concentration of active ingredient.
Compounding Techniques Processing and compounding techniques vary with the grade of carboxylic rubber polymer used. Although the carboxylated reclaims are generally tough, and the more highly reacted grades lack tack, they respond well to processing. Even after they have been warmed on a mill, they have a rough appearance, which persists in very lightly loaded compounds, but smooth out as the loading is increased. O n a cold mill these elastomers soften slightly during the early stages of milling. Unlike natural rubber, however, the carboxylic rubbers are not progressively softened by continued milling. Exceptions to this rule are the P grades and 1630 grade, which can easily be broken down on the mill. The Bisonides generate considerable heat on a mill or in a Banbury; hence the use of plasticizers is recommended. The self-reinforcing nature of these carboxylic rubbers is seen in their very high moduli and cured hardnesses. I t has been reported (5) that carboxylic nitrile rubbers do not require reinforcing pigments for high tensile strengths. When the carboxylic rubbers from scrap vulcanized rubber under discussion are blended with polychloroprene (neoprene), the characteristic high modulus and cured hardness are retained. I t therefore appears possible to obtain the desired modulus characteristics in polychloroprene compounds by use of these carboxylic rubber hydrocarbons rather than reinforcing fillers. The carboxylic VOL. 48, NO. 12
DECEMBER 1956
2 14 1
Table VIII.
Bisonide 1600 Stearic acid
Zinc Oxide as a Curing Agent 200.0
... ...
Zinc oxide
200.0
200.0
200.0
1.0
1.0
1.0
2.5
5.0
7.5
Table X. Bisonide P-400Compounds of 70 and 80 Shore A Bisonide P-400 200.0 200.0 Antioxidant 2.0 2.0 Diphenylguanidine 0.24 0.24 Benzothiazyl disulfide 0.8 0.8 Sulfur 3.6 3.6 Stearic acid 2.4 2.4 FT carbon black 20.0 75.0 Zinc oxide 6.0 6.0 Total 235.04 290.04
it4in.
Cure at 307' F. Tensile, lb./sq. inch Elongation, % Hardness, Shore A
15 30 45
530 705
15 30 45
390 395
815 935
. . a
1010
1260 1280 1310
1685 1650 1625
300 310 360
240 250 265
210 230 230
...
15 30 45
55
41 43
70 71
56 58
...
Cure
Tensile,
at287' F.,
Lb./Sq.
Mi%.
80 80
71
Inch 1430b 1575 1625
Curing with Amines and Diethylene Glycol CuT'uring with Diethgl-
Curing with Amines
hfin.
Bisonide 1600 MT carbon black Hexamethylenediamine Diethylene glycol
Elongation, %
Hardness, Shore A
200.0
90.0
90.0
... ... ...
Hexamethylenetetramine
Cure at 307O F. Tensile, lb./sq. inch
200.0
1075
15 20 30
200
15 20 30
2 142
...
...
...
...
1450
... ...
... ...
170
...
*..
150
...
... 69
...
75
...
Uncompounded carboxylic rubber polymer can be vulcanized by heat alone, but the physical properties of the vulcanizates are greatly enhanced by the use of curing agents and compounding ingredients. These rubbers require a
10.0
1390
...
Curing Mechanism
... ...
... ... ...
. I .
rubbers utilize the same curing mechanism as the polychloroprene. Polymers of desired levels of modulus may be synthesized by incorporating the carboxyl groups in the molecules of the polymer itself, as above indicated, rather than by reliance on reinforcing fillers (30). Different modulus levels can also be attained by varying the amount of curing agent. These carboxylic rubbers are effective stiffening agents for unvulcanized rubbers and contain a large portion of useful rubber hydrocarbon. They prevent excessive softness and low plasticity in stocks being processed and could be effective in reducing product defects such as blistering and collapsing. They provide excellent mill roll release, and with the accompanying stiffness should be beneficial in calendered stocks.
200.0
1.0
...
15 20 30
ene Glycol
...
Table XII.
78
200.0 . . e
... . . I
200.0
a
...
b
... 5.2
530 665 705
1090 1220 1220
390 395 370
330 300
41 43 44
Elong.,
Shore
330 270 250
75 79
%
A
80
Temp. of testing, 86.5' F. Temp. of Testing, 88' F.
-
Tensile, Lb./Sq. Elong., Shore
270
;Mi%. Inch
Modulus
%
A
200%
220 210
78 78
1325 1375
Cure at
56
57 58
307OF.
10 1350 15 1375
Comparison of Neoprene, Whole Tire Reclaim, and Bisonides in Blends with Neoprene
Neoprene G N W. T. reclaim
Bisonide P-800 Bisonide 800 Bisonide 1600 Bisonide 1630 Magnesia Antioxidant Stearic acid Light Process oil MT carbon black SRF carbon black Zinc oxide Total Cure 20 min. at 307' F. 100% modulus, lb./sq. inch Tensile, Ib./sq. inch Elongation, % Hardness, Shore A
100.0
... ... ... ... ...
8.0 4.0 2.5 15.0 45.0 80.0 5.0
259.5
990 1850 210 79
Heat aged, 70 hr. at 212O F. Shore A gain, pts. Tensile gain, % Elongation loss, %
2 38
A.S.T.M. oil 3 Volume swell, % Tensile loss, % Elongation loss, %
67 29 5.
INDUSTRIAL AND ENGINEERING CHEMISTRY
68 69 69
Table XI. Bisonide 1630 Compound Bisonide 1630 162.0 Stearic acid 2.0 SRF carbon black 40.0 Sulfur 0.5 Hexamethylenetetramine 1.5 Zinc oxide 5.0 Total 211.0
* s *
...
A
440 390 330
1770 1750 Tensile, Lb/Sq.
Table IX.
Shore
%
1705a
15 30 45
80
Elong.
Inch
10
70.0 60.0
... ... ... ...
6.0 4.0 2.5 15.0 45.0
70.0
... 60.0 ... ...
...
6.0 4.0 2.5 15.0 30.0 80.0 70.0 5.0 5.0 _ _ _ 287.5 262.5
70.0
... ... ...
...
...
... ... 60.0 ...
70.0
60.0
70.0
... ... ... ...
5.0 - 267.5
60.0 6.0 4.0 2.5 10.0 30.0 70.0 5.0 __ 257.5
990 1560 190 80
6.0 4.0 2.5 15.0 30.0 70.0 5.0 ~ 262.5
6.0 4.0 2.5 20.0 30.0 70.0
1195 200 74
1110 1580 160 83
1125 1450 150 84
15 22 40
9 17 25
7
7
8
17 33
26 25
10 32
89
60 57
55 53
50 43
0
0
64 46 21
710
75 13
...
1120 1305 120 85
Table XIII.
Comparison of Neoprene and Neoprene-BisonideBlend in Compound, ' 60 Durometer 100.0
Neoprene GN Bisonide 1600 Magnesia Antioxidant MT carbon black Light process oil Stearic acid Petrolatum Zinc oxide 2-Mercaptoimidazoline Total Specific gravity
66.7 66.7 2.7 2.0 16.7 10.0 0.5 1.0 8.3 0.5 175.1 1.29
... 4.0
2.0 75.0 10.0 0.5 1.0 5.0 0.5 198.0 1.42
Tensile, Lb./Sq. Elong., Min. Inch % Cure at 307' F. 10 1460 650 15 1460 590 Heat aged, 70 hr. at 212O F. (10-min. cure) Tensile change, % -1 Elongation loss, % 43 Shore A gain, points 16
To ble XIV.
Shore A 52 52
Tensile, Lb./Sq. Inch 1525 1450
I
c=o
b ill
Cure at 307' F.
Heat aged 70 hr. at 212O F. change
Min. 10 20
10 20 Oil aged, % volume change ASTM oil 1 ASTM oil 3
100.0
Elong.,
% 370 320
Shore A 61 61
+2817 11
66.7 66.7 2.7 2.0 120.0 15.0 1.0 1.0 8.3 0.33 0.33 284.06 1.43
... 4.0
2.0 180.0 15.0 1.0 1.0 5.0 0.5
... 308.5 1.54
Tens., Lb./Sq. Elong., Inch % 1105 280 1210 230
Shore A 76 79
Tens., Lb./Sq. Elong., % Inch 1220 180 1240 140
Shore A 79 80
Per cent +12 -70 3 -52
+ 18 pts. + 16 pts.
Per Cent +I2 -47 $15 -32
$11 pts. +10pts.
+
- 1 +33.5
curing mechanism distinct from that of other polymers and the mechanism varies according to the maleic anhydride that has reacted. Although the 400 grades normally require some sulfur for vulcanization, the requirements are reduced. Zinc oxide as the sole vulcanizing agent cures the 1600 grade to produce very snappy and tight vulcanizates. Table VI11 shows the data obtained by incremental increases of the concentration of zinc oxide. Curing with amines or diethylene glycol is shown in Table IX. Cured hardness is unusually high and elongation is low. It is conceivable that mono- or diamines could react with the carboxyl groups of the chain to form antioxidant elements which are actually built into the vulcanized network (30). The use of accelerators appears to be of
I I
o=c 0-Zn-0
L o I
-0.5 $37.5
little benefit and certain organic compounds normally used as rubber accelerators, notably 2-mercaptobenzothiazole, retard vulcanization. Polychloroprene-carboxylic rubber blends can be cured to form practical compounds. Raw natural or Type S synthetic rubber added to the carboxylic rubber does not enhance the physical properties; these are, in fact, lower than those obtained with either rubber alone. Bivalent metallic oxides, such as zinc oxide, are recommended vulcanizing agents. Consequently, in mixing they should be the last ingredients added, just as the sulfur or accelerator is held until last when a rubber batch is mixed. After the metallic oxide has been added, the batch should be treated with the same care as a rubber batch containing an ultra-accelerator.
c=o I I
0
An
I
AH
0
OH
L o
!
I
ComDarison of Neowene and Neoprene-BisonideBlend in Compound, ' 80 Shore A '
Neoprene GN-A Bisonide 1600 Magnesia Antioxidant MT carbon black Plasticizer Stearic acid Petrolatum Zinc oxide 2-Mercaptoimidazoline Hexamethylenetetramine Total Specific gravity
Brown and Duke ( 5 ) have represented the linkages potentially present in zinc oxide vulcanizates of carboxylic nitrile elastomers as follows :
c=o I
The fact that the carboxylic rubber polymer under consideration can be vulcanized by bivalent metallic oxides, diamines, and glycols suggests that the rubber chains contain carboxyl groups. A procedure has been adapted to the-determination of the carboxyl groups on the rubber in question. vu1canizeb. rubber scraD was made to react with an excess of maleic anhydride and the resulting product was hydrolyzed. The product was thoroughly washed and allowed to stand overnight in a known concentration of sodium hydroxide, forming the sodium salt of the acid. Titration with acid to determine the unused portion of sodium hydroxide indicated that 0.84 mole of maleic acid added to the vulcanized rubber per mole of isoprene units. I t has been concluded that a definite chemical reaction occurs between a mixture of vulcanized natural and Type S synthetic rubbers and unsaturated compounds containing an electrophilic group alpha to the double bond, resulting in a n elastomer with carboxyl groups on the polymer chain.
Properties of Vulcanizates When cured in the appropriate recipe, these carboxylic rubbers are characterized by good tensiles, high moduli, and high cured hardnesses. ?he right-hand portion of the graph of Figure 3 shows how the 30001, modulus increases with increase in concentration of maleic anhydride above the critical concentration. Table I V illustrates the magnitude of the modulus increase due to the various active ingredients. Table X I shows the increase in modulus that can be expected by blending neoprene and the carboxylic rubbers. When cured in the test formulation, the increase in cured hardness obtained by the use of maleic anhydride, as compared to the corresponding reclaim, is about 20 points Shore A. Use of other unsaturated acids gives increases of 10 to 20 Shore A points. Table X illustrates a typical formulation for compounding the P-400 grade and the physical properties that can be expected. Shore A hardness is high, with relatively little carbon black or other VOL. 48, NO. 12
DECEMBER 1956
2143
Table XV.
Comparison of NeoDrene and Neoprene-Bisonide Blend in Conmound, ’80& 90 Shore A
Neoprene GN-A Bisonide 1600 Bisonide 1630 Magnesia Antioxidant Stearic acid Light process oil Wood cellulose flock M T carbon black
SRF Zinc oxide 2-Mercaptoimidazoline Total Specific gravity Cure 20 min. at 310’ F. Tensile, lb./sq. inch Elongation, % Hardness, Shore A Heat aged, 70 hr. at 212’ F. Hardness gain, pts. Tensile gain, % Elongation loss, % ASTM oil 3 Volume swell, % Tensile loss, yo Elongation change, Yo Compression set, method B 22 hr. at 158’ F., % Plug aged 22 hr. at 158’
F.,70
...
100.0
8.0 4.0 2.5 15.0
2.0 4.0 1.0 20.0 30.0 65.0 90.0 5.0 1.0 318.0
100.0
...
...
45.0 80.0 5.0
...
259.5 1.49
1.50
70.0
... 60.0
70.0 60 0
...
6.0 4.0 2.5 10.0
15.0
30.0 70.0 5.0
45.0 80.0 5.0
257.5
289.5
...
~-...
1.41
8.0 4.0 2.5
...
...
1.43
1850 210 79
1330 110 92
1560 190 80
1410 110 91
10 2 38
7 14 73
8 10 32
5 14 36
67 29 -5
33 5 f27
64 46 -21
49 23 -9
30
22
33
33
25
21
26
27
fillers. These test results were obtained at temperatures of 86”to 88” F. Table X I illustrates the formulation and results obtainable with the 1630 grade. Although the 1630 grade is extended with a large quantity of petroleum oil and contains relatively little additional filler, Shore A hardness and modulus are high. Table X I 1 compares an all-neoprene compound, a whole tire reclaim-neoprene blend, and carboxylic rubber-neoprene blends in approximately the same formulations. Higher cured hardnesses and moduli are obtained when the carboxylated rubber is used. Tables X I I I . X I V , and X V show comparative data obtained with neoprene compounds and carboxylic rubber-neoprene blend compounds. The oven aging of the carboxylic rubbers is good and in blends with neoprene perhaps slightly improves the aging characteristics of the neoprene. The Shore A increase on oven aging of the blends is less than that obtained with similar allneoprene compounds. The blends generally show higher increases in tensile strength. Although the initial elongations of the blends may be lower than the corresponding all-neoprene compound, the former retain a greater percentage of their elongation on oven aging (24) (Tables X I I I , XIV, and XV). The volume swell of the blends in ASTM oi!s 1 and 3 is generally of the same order of magnitude as the allneoprene compound (perhaps lower). Consideration of the degree of loading is important. Shore A hardness change of the blends is low and equivalent to the all-neoprene compound. Tensile
2 1 44
... ...
strength change in ASTM oil 1 shows the blends have the advantage and the elongation changes of the two are about the same in magnitude. The all-neoprene compound retains more of its tensile \vhen aged in ASTM oil 3 ; the elongation change varies (6, 27). Compression set data are shoivn in Table XV. Some data indicate that zinc oxide acts as a persistent cross-linking agent in the carboxylic rubbers. Conditioning the plug by accelerated aging before testing improves the compression set. Unpublished data show the improvement of set due to conditioning to be far greater for the blends than for the all-neoprene compounds. As the concentration of maleic anhydride is increased above the critical point. the torsional hysteresis (23) greatly increases, as shown in Figure 4. Data on results obtained when other active ingredients were used are shown in Table IV. The more highly reacted products show high heat build-up on flexing. The similarity of curing mechanism and properties of the Bisonides and neoprene suggested beginning the development program with combinations of the two. Tables X I I I , XIV, and X V show the general properties obtainable by blending the low-cost carboxylic rubbers with neoprene. These compounds should be of sufficient interest to merit further evaluation along practical lines.
Elgin in the preparation of this article, to thank B. R. Wendrow for reviewing it, and to thank the management of the U.S. Rubber Reclaiming Co. for permission to publish this work.
References ( 1 ) Alder, K., Pascher, F., Schmitz, A , , Ber. 76, 27 (1943). (2) Bacon, R . G. R., Farmer, E. H.,
Proc. Rubber Technol. Conf., London, 1938, p. 256. ( 3 ) Barron, H., Rubber and Plastics Age 35, No. 12, 579 (1954); report o n lecture by W. J. S, Naunton, at Southampton, England. (4) Brewster, R . Q., “Organic Chemistry,” vol. I, p. 382, Prentice-Hall, New York, 1948. ( 5 ) Brown, H. P., Duke, N. G., Rubber World 130, No. 6, 784 (1 954). ( 6 ) Compagnon, P., Bonnet, O., Rei. gin. caoutchouc 19, 79 (1942). (7) D’Alelio, G. F., “Fundamental Principles of Polymerization,” pp. 4246, Wiley. New York, 1952. (8) Elgin, J. C., Sverdrup, E. F. (to U. S. Rubber Reclaiming Co.), U. S. Patent 2,653,348 (Sept. 29, 1953). ( 9 ) Ibid.. 2,653,349. (1 0) Ibzd., 2,653,916. (11) Essex, Mi. G., Zbid., 2,154,894 (April 18,1939). (12) Farmer, E. H., Zndia Rubber J . 112, 119 11947). Farmer, E. H., Rubber Chem. and Technol. 15, 765 ( I 942). Zbzd., 20,366 (1947). Farmer, E. H., Sundralingam, A , , J . Chem. Sos. 1942, Part 1, 121. Fisher, H. B., McColm, E. M., INU. ENG.CHEIM. 19, 1328 (1927). Green, 3. (to L.S. Rubber Reclaiming Co.), U. s. Patent pending. Ioannu. J. P. (to Pennsylvania Salt Manufacturing C o . ) , S. Patent 2,069,151 (Jan. 26, 1937). (19) Kirby, W. G., Steinle, L. E. (to United States Rubber Co.), Ibid., 2,279,047 (-4pril 7, 1942). (20) LeBean, D. S., Zndia Rubber World 118, No. 1, 59 (1948). (21) Le Bras, J., Rev. Len. caoutchouc 19, 43 (1942). (22) Le Bras, J., Compagnon, P., Bull. soc. chim. France 11, 553 (1944). (23) Le Bras, J., Compagnon, P., Compt. rend. 212. 616 11941). (24) Le Bras. J.: Merher, J. de, Zbzd., 231, 230 (1950). (25) Moonev, M.. Gerke. R. H., Rubber Chern: and Technol. 14, No. 1, 35 (1941 ). (26) Chas. Pfizer 8i Co., Inc., New York, “Pfizer Products as Industrial Chemicals,” p. 2. (27) Price, C. C., “Mechanisms of Reactions at Carbon-Carbon Double Bonds,” pp. 95-8, Interscience, New York, 1946. (28) Remick, A. E. .‘Electronic Interpretations of Organic Chemistry,” p. 463, Wiley, Sew York, 1943. (29) Shelton, J. R., Natl. Bur. Standards, Circ. 525, 159 (1953). (30) Whitby, G. S., IND.ENG.CHEM.47, 806 (1955). \
>
e.
RECEIVED for review November 4, 1955 ACCEPTEDJuly 11, 1956
Acknowledgment The authors wish to acknowledge the valuable advice and guidance of J. C.
INDUSTRIAL AND ENGINEERING CHEMISTRY
68th Meeting, Division ofRubber Chemistry ACS, Philadelphia, Pa., November 3, 1955.
