Control of Crys
in
L. R. MAY0 Rubber Laboratory, E . 1. d u Pont de Nemours & Company, I n c . , Wilmington, Del.
C
ERTAIN elastomers, T h e ability of general-purpose neoprenes to crystallize may Variations from, or addinotably natural or may not have a favorable influence on the behavior of tions to, these formulas are described in the text and rubber and the g e n e r a l these elastomers or their vulcanizates depending on the all concentrations are expurpose neoprenes, will unspecific application in mind. Means of inhibiting crystalpressed in parts per 100 dergo the reversible process lization include copolymerization of a small amount of of crystallization as a consecond monomer during manufacture of the elastomer, parts of elastomer. Unless o t h e r w i s e indicated, slab sequence of the application addition of specific crystallization retarders, and adequate cure in the presence of sulfur. Data include x-ray diffraccures were 15 ininut,es at of stress or of exposure for a tion measurements, determination of changes in hardness, 153" C. for the gum stock sufficient time in a favorable compressive stress, and tensile stress with time of exposure, temperature range and 20 minutes a t the same - ( 1 , 2, S, 6). Physical manifestations and stress-strain figures. temperature for the carbon of this effect m a y h a v e black compound. considerable influence, Methods by which cry*favorable or otherwise, on the ease with which a compound tallization and its effects were evaluated include the following: processes or on the service rendered by the eventual vulX-RAYDIFFRACTION LIEASUREMENTS. The instrument used canized product. Crystallizable elastomers characteristically was the Geiger counter x-ray spectrometer manufactured by the show high tensile strengths in gum-type vulcanizates; they North American Phillips Company, Inc. All measurements were made using copper radiation, maximuin amplification, and mealso show a delayed stiffening in either the uncured or vuldium slits. Elongated samples were prepared by stretching thin canizate form when exposed a t a temperature favoring crys(0.015-inch) strips over a smooth wooden block just prior to xtallization (6, 0 ) . At this same temperature, their vulray examination. canizates may take a very high set if held in compression for CHANGES IN SHOREA H a R D N E b S WITH TIfifE. The test samples consisted of 3 X 3 X 0.25 inch blocks; the durometer was mainseveral days ( l o ) , or, if exposed when stretched, they may untained a t the specified temperature throughout the test. dergo a stress relaxation and even a spontaneous extension ( 1 , 8, RELAXATION OF TENSILE STRESS. This was judged by the l a ) . Effects such as these, particularly those associated with length of time a t 0" C. required for a stretched sample to show the temperature of exposure, give rise to many more problems in the first signs of sagging. RELAXATIolV OF COMPRESSIVE STRESS. This was evaluated by use of neoprene than in rubber, largely due to the difference in set values following conditioning a t 0" C. under compression. the temperature a t which crystallization proceeds most readily in The method followed was essentially that of Xorris and associthe two elastomers-a relatively high 0 C. for neoprene ( 2 , 6 ) as ates (10). An initial deflection of 30% was used and air \?-asemployed as the cooling medium. hleasurements were made followcompared with -25' C. for natural rubber ( 2 , S, 13). ing a 30-minute recovery a t the test temperature. Information previously published on the control of crystallizaOTHER PHYSICAL PROPERTIES. Other physical properties, tion in neoprene vulcanizates (5, 7 ) emphasizes the benefits stress-strain data for example, were determined following procewhich result from a high state of cure preferably in the presence of dures outlined by the American Society for Testing Materials. added sulfur, Unfortunately, in addition to requiring a cure which may be undesirably long, such a method exerts no control EFFECT OF COPOLYMERIZATION over crystallization in the unvulcanized state and may have a Synthetic elastomers of the copolymer type such as GR-S somewhat adverse effect on certain physical properties. or the specialty Neoprene Type FR Sholiy no evidence of crystalliAdditional methods for the control of crystallization in neozation (6). I t is believed that their failure to crystallize is due t o prene which are effective in uncured as well as in vulcanized the irregularity of structure in the copolymer which prevents compounds will be discussed in this paper, arid include the modiorientation into the ordered pattern of a crystallite. This has fication of the elastomer itself during manufacture by the cosuggested the possibility of copolymerizing a limited amount of a polymerization of a small amount of a second monomer, and the second monomer during the manufacture of a general-purposeaddition of specific high molecular weight compounding intype elastomer in the hope of introducing sufficient irregularity gredients. PROCEDURE into the polymer chain to retard objectionable crystallization without appreciable sacrifice in other properties. A new generalThe t v o basic compounds used in this evaluation, one a gum purpose elastomer, 3-eoprene Type RT, in which the second stock and the other containing a loading of 40 volumes of hIT monomer is stvrene appears to fulfill this objpctive. carbon black, are shown in Table I. X-ray diffraction experiments constitute a recognized means of studying crystalline structure. The reflecting-type instrument permits rapid evaluation of crystallite development since the TABLE I. BASICFORMULAS investigation may be restricted to that portion of the x - ~ a y Gum MT Black spectrum in the vicinity of a critical Bragg angle 20, specific for 100 Neoprene (type as indicated) 100 2 Neosone A" .. the crystallite being studied. Well-developed crystallinc strucStearic acid 0.5 '4 4 Extra light calcined magnesia ture is indicated by a sharp peak in diffraction intensity a t this MT carbon black 58 critical angle which in the case of chloroprene polymers is a t Zinc oxide '5 6 approximately 19 30'. I n Figure 1, x-ray diffraction intensities Phenyl-n-naphthylamine a t 19"30' for uncured blocks of Xeoprene Type R T are compared
696
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1950
i1I
NEOPRENE
NEOPRENE TYPE 00
----neroRe
-AFTER
CONDITIONINQ CONDlTlONlNO
* t
t
E
11
697
I'\ A A NEOPRENE TYPE
NEOPRENE TYPE RT
I 'A
---_
-
UNSTRETOHEP
z
NEOPRENE TYPE
I-
t w
2
STRETCHED
NEOPRENE TYPE RT
t
NEOPRENE TYPE
w
NEOPRENE TYPE FR
19'30'
19.30'
19.30'
19'30'
19'30'
BRAGG ANGLE 2 8
Figure 1. X-Ray Diffraction Intensity before and after 4 Days' Exposure at 15.5' C.
with those of the general-purpose Neoprene Types Gn', CG (a fast crystallizing type used in quick setting cements), and F R (a noncrystallizable copolymer). The lower dotted lines (amorphous halos) represent measurements made immediately after warming at 70 O C. to destroy residual crystallization. The solid line peaks were obtained following 4 days' exposure a t 15.5' C. and indicate a pronounced development of crystalline structure in Neoprene Type CG as a result of this exposure. Less crystallinity is shown in Neoprene Type GN, only a slight amount is indicated in Neoprene Type RT, and none a t all is apparent in Neoprene Type FR. Figure 2 consists of diffraction intensities measured for gum vulcanizates of the same 4 elastomers before and after stretching to an elongation of 400'%. I n this case, amorphous halos, indicated by the lower dotted lines, represent readings made prior to stretching, whereas the solid lines show peaks developed as a consequence of strain. The degree of crystallization indicated follows the same trend as that for the uncured polymers exposed a t 15.5 O C. It will be noted, however, that the crystal development exhibited by the Neoprene Type R T vulcanizate following stretching is much greater relative to the other types than is that shown for the uncured polymer in Figure 1. From this it may be concluded that crystallization due to exposure a t a low temperature is inhibited to a greater extent in Neoprene Type R T than is that due to strain. The development of diffraction peaks at the same Bragg angle as a result of exposure a t a favorable temperature (Figure 1) and straining (Figure 2) supports previous opinion that crystallites resulting from both of these causes are structurally identical (3, 4, 6). A serious objection to the use of easily crystallizable neoprenes has been the delayed stiffening in both uncured and vulcanized form which results from exposure a t moderately low temperatures. This type of behavior may complicate processing cycles or may, temporarily a t least, alter the physical properties of a vulcanizate in service. The value of the partially inhibited crystallization of Neoprene Type R T in overcoming such objections is illustrated in Figure 3. Hardness increases for M T carbon black-loaded stocks of Neoprene Types GN and RT are plotted against time at 15.5' C. for uncured, and a t 0 C. for the vulcanized compounds. Under both circumstances, Neoprene Type R T compounds show a reduction both in rate and degree of hardening. In spite of its complete reversibility, the relaxation of either compressive or tensile stress as a result of exposure a t a moderately low temperature is a serious problem in virtually all mechanical applications. A probable explanation of these effects is that crystallites formed under stress have their axes oriented in the direction of the resulting strain and that further crystallization due t o favorable conditions of temperature tend to proceed in the same direction even after complete relaxation of the initial stress. The relationship of this effect to elastomer crystallization is shown in Table I1 in which the time in hours at 0" C. required for gum-type vulcanizates of Neoprene Types CG, GN,
19'30' BRAGG
19'30'
19'30'
ANGLE 2 8
Figure 2. X-Ray Diffraction Intensity before and after Stretching to 400% Elongation
'"t
I40 $
z m
30
NEOPRENE TYPE GN
f
-
NEOPRENE TYPE RT
Y
/'
'
200 TIME
Figure 3.
-UNOURED-----CURED-
-
400 HOURS
EXPOSED AT 15 5*0.
EXPOSED A7 0%
600
Hardness Change on Exposure €or
MT Carbon Black Stocks
TYPE 0
% ELONGATION
Figure 4. Stress-Strain Curves for Gum Vulcanizates of Neoprene Types GN and RT Cured 15 min. at 153O C.
and R T to lose completely a stress resulting from an initial extension of 20% is recorded. Relaxation is very rapid for the Neoprene Type CG vulcanizate, appreciably delayed in the case of Neoprene Type GN, and is very decidedly delayed for the Neoprene Type R T vulcanizate. The temperature retraction test proposed by Forman and Radcliff ( 7 ) as a rapid method for estimating crystallization tendencies is apparently dependent on this same type of stress relaxation effect. The reduced tendency to crystallize shown by Neoprene Typc RT is not reflected in a significant shift in stress-strain properties. Figure 4 shows that the stress-strain curve for a gum vulcanizate of Neoprene Type RT approaches that of a comparable Neoprene Type GN vulcanizate so closely that the two curves are difficult t o distinguish. This is in contrast t o the fact t h a t compounds of completely noncrystallizable elastomers such as GR-S and Neo-
698
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
J
PROCESS OIL
-
50
yo
p
p ,/
30-
Y)
-- -
/x-
-x
0-
0
-0
40-
- - -- -
I O PARTS PETROLEUM PROCESS OIL
60
P R O C E S OIL
---x
x-x
;so 2
IO
PARTS PHC
v)
4 m
z
0 cn r Y
px
Vol. 42, No. 4
40-
< PHC
__--,
-----,-
3 0 ; ----
400
200 TIME
600
- HOURS
20
Figure 5. Exposure Hardening of Neoprene Type GN Stocks, Influence of Additives
20
60
80 TIME
Figure 6.
MT carbon black stocks
40
100
120
140
160
200
180
AT 24-C.
Uncured Neoprene T ~ p eCG Cement Adhesion Effect of PHC
0
200
400
600
800
TIME - H O U R S
Figure 7.
Effect of PHC Concentration on Hardening at 0' C.
N e o p r e n e T y p e GI\-BIT carbon hlttck stork
t 0
2000
crease in hardness in an XT carbon black-loaded Neoprene Type CG vulcanizate in t,hc presence of 15 volumes of the rna under test. In considering d a h in Table 111,it should be borne in mind that the effects of thermoplmticity as well as those due to cryst,allization may occur simult,aneously during the first, few hours of exposure a t 0 " C. Since this experiment was not designed t o diiferentiate betyeen these two effectjsduring this initial cooling pcriod, small variations in the rate of hardening are not necessarily indications of real differences in susceptibility to crystallizat,ion. It is particularly noteworthy that t,lie ester-type plasticizers which impart outstanding resistance to einbrittlernent owing to amorphous freezing at very low temperatures do not inhibit stiffening due to crystallization. Several materials, however, all of which consist, in part at least, of polymeric hydrocarbons derived from petroleum or coal t,ar show a promising retarding action. One such material, a viscous petroleum base liquid, was selected for further investigat,ion ill Xeoprene Type G S cnnipounds. I n Figure 5, an MT carbon black-loaded Neoprene Type GI$ compound cont,aining 15 parts of this material (hereafter designated PHC, indicative of its polymerized hydrocarbon content) is compared with a similar compound with 16 parts of light process
-
Y)
Y v)
a ! i 1000
T.LBLE
-
200
400
600
090
to00
ELONGATION
Figure 8.
Effect of PHC on Stress-Strain
prene Type FR are generally considered practical orilv in the presence of a reinforcing filler. E F F E C T O F ADDED INGREDIENTS
Theoretical considerations suggest the possibility of retarding crystallization by the addition of compounding ingredients capable of physically interfering with the process. A list of materials representative of a large number investigated in an effort to find such ingredients is found in Table 111. Their relative effectiveness as crystallization inhibitors is indicated in this same table as the time a t 0" C. required for a 20-point in-
11.
T I h l E FOR &PRESS ItELAXATIOS AT
0"
c:.
(Gum stocks, cured 15 niiri. a t 1 5 3 " C.) Elastomer Hours t o Relaxa Neoprene Type CG 6 Neoprene Type G K 150 1000 Neoprene Type R T a Relaxation indicated by first signs of srtgging in strip initially stretched 20%.
TABLE 111.
RETARDATIOK O F HARDEXIXG I N XEOPRENE TYPIC
ccr VULC.4PiIZaTES~
(Effect of added ingredients) Time for 50-Point Rise in Shore A Durometer Hardness, Hr. 2 Factice 4 Control Processed castor oil 1 Petroleum process oil fi Tributoxyethyl phosphate 2 Mineral rubber 6 Dicapryl phthalate 2 Gilsonite A Triglycol octoate 2 Mixed organic nitrile 8 Butyl acetyl ricinoleate 2 Burgundy pitch 8 High molecular weight polyetber 2 Asphalt 12 High molecular weight polyester 2 Coal t a r base polymeric hydroNeoprene Type F R 4 carbon (liquid) 14 Keoprene ?$De S 4 Petroleum base polymeric hy4 drocarbon (liquid) 35 GR-S 4 Coal t a r base polymeric hydroButyl Smoked sheets 4 carbon (solid) 50 a Methods effective i n retarding Neoprene Type CG crystallization have the same relative effect i n other neoprenes. Type CG was used t o accelerate the test.
699
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1950
TABLE IV. EFFECT OF P H C CONCENTR~TION ON PROPERTIES OF
I
NEOPRENECOMPOUND CONTAINING M T CARBONBLACK P H C , parts
3000
Stress e t 300% elongation, Ib./sq. inoh Tensile a t break, lb./sq. inch Elongation a t break, 3' ' Hardness, Shore-type Yerzley resilience, Yo Compression set, method B T-50point C. Solenoid bihttle point, C. Slab cures 20 rnin. a t 153' C. Pellet cure's, 26 min. at 153' C
2
'd*-ZOO0 I lu v)
100
200
SO0
400 100 600 ELONGATION %
700
800
900
0 5 10 20 1220 590 740 400 1750 2700 2000 2625 520 1000 1040 950 63 58 60 52 78.1 76.7 77.0 75.9 39 37 40 43 +7.3 +5.1 4-1.7 -4.6 36 - 40 - 38 -35
-
1000
Figure 9. Effect of MT Black Loading (Volumes) in Presence of 15 Parts PHC on Stress-Strain Properties of Neoprene Type GN Vulcanizate
oil. The increase in hardness of the uncured compounds at 15.5' C. and the vulcanized compounds a t 0" C. plotted against time of exposure clearly demonstrates the retardation of hardening effected by the addition of PHC. T o substantiate further the effect of P H C in retarding crystallization prior to cure, 10 parts were added to 1 portion of a Neoprene Type CG cement while 10 parts of process oil were added t o another portion of the same cement. Adhesion strips bonded with these two cements were tested after various periods of aging at 24" C. Figure 6, a graphical representation of the resulting data, shows a delayed development of adhesion in the presence of P H C attributable to retarded crystallization. The effect of P H C concentration on the retardation of crystallization is illustrated in Figure 7 , which shows a delay in hardening a t 0 C. in a X'eoprene Type GN-MT carbon black stock in proportion to the amount of P H C added over the range from 0 to 20 parts. The effect of adding substantial quantities of any material, particularly one known t o influence crystallization, on the physical properties of resulting vulcanizates is of considerable interest. Figure 8 shows the distinct difference between the stress-strain curves of the 40-volume MT carbon black-Neoprene Type GN stock containing 15 parts of added P H C and one containing 15 parts of light process oil. PHC, although a less effective softener than light process oil, very markedly reduces intermediate modulus values, an effect which appears to be associated with inhibited crystallization in this region. . The effect of varying P H C concentration (over the range of 0 t o 20 parts) on the physical properties of an MT carbon blackloaded Seoprene Type GK vulcanizate is shown in Table IV. As would be expected, the addition of increasing amounts of PIIC produces softer stocks as shown by the progressive decrease in stress a t 300% elongation. Coincidentally, however, the elongation a t break increases with the result that much higher tensile strengths are shown for these stocks containing PHC. The increase in tensile strength appears to be dependent upon the amount of P H C used up to a Concentration of 10 parts. The trend in Yerzley resilience and A.S.T.M. Method B compression set a t 70" is moderately adverse, and as would be expected T-50 values decrease progressively as crystallization is inhibited more completely. Solenoid brittle point temperature, however, increases with concentration of PHC. This latter observation serves to emphasize the complete independence of effects due to amorphous freezing and those attributable to crystallization. Since varying the concentration of P H C had produced such striking changes in the physical properties of the resulting vulcanizates, the effect of varying the concentration of filler with a single concentration of P H C wm studied. For this work a concentr%
ON
GNtS
GNtPHO
GNtPI10
t s
Figure 10.
RT+PHG +S
Compression Set after 7 Days af
oo c.
tion of 15 parts of P H C was used while the concentration of M T carbon black was varied from 10 t o 100 volumes. The stressstrain curves of the resulting vulcanizates are plotted in Figure 9. As the concentration of filler is increased, the ultimate elongation is decreased, and above 40 volumes of loading the loss in tensile strength reaches serious proportions. At these higher loadings, in contrast to the results a t the lower loadings, the tensile strengths of the vulcanizates are considerably below those which would be obtained from similar stocks in which a light process oil replaced the PHC. It is probable that a similar loss in tensile strength would be encountered at high loadings of fillers other than the M T carbon black tested and this phenomenon will undoubtedly limit to some extent the utility of PHC. DISCUSSION
The mechanism by which crystallization is inhibited by PHC and related materials is not clearly understood. It seems likely to consist of a physical interference with crystallite formation dependent on molecular size and configuration. Preliminary separation of P H C along the lines outlined by Rostler and Sternberg (1 1 ) indicates that the ingredients responsible for retardation of crystallization are found in the rather indeterminate group of pentane insolubles designated as the asphaltenes. The failure of a material such as gilsonite (high asphaltene content) to retard effectively crystallization, however, makes it clear that not all asphaltenes are useful for this purpose. The work of Rostler and Sternberg (11) further suggests the possibility of significant variation in the concentration of the crystallization-inhibiting ingredients in commercially available materials. The fact does remain, however, that a physical inhibition of crystallization by the addition of a high molecular weight compounding ingredient has been demonstrated which is sufficient t o justify further investigation of this type of effect. USE OF COMBINED METHODS
The methods already discussed for the control of crystallization may be combined effectively. The results obtainable are
700
INDUSTRIAL AND ENGINEERING CHEMISTRY
tlemoiistrat,ed most readily by measurement of t'he relaxation or compressive st'ress a t 0" C. Such set data following a conditioning Iwriod of 7 days are summarized in the bar graph chart Figure 10. All stocks were loa.ded with 40 volumes of MT carbon black and cured 45 minutes a t 153" C. The Neoprene Type GK vulcaiiizat,e, with 15 parts of ester plasticizer, takes a set of essentially 100%. The addition of sullur to this compound or the replacement' of the ester with an cyual amount of PI-IC effect an appreciable reduct,ion in set. I stock combining the advaiitages of both P H C and sulfur reduces the set value markedly, and the substitution of n'eoprene Type ItT for Seopreiie Type GN in this latter compound brings ahout un even greater improvrmc:nt. SUMMARY
Crystallization and its consequences in neoprene may be conlrolled in both uncured and vulcanized stat,eby: 1. The copolymerization of a small amount of a second niononier, such as styrene, during the manufacture of the elastomer. 2 . The addition of certain types of polymerized hydrocarbons cqiable of physically interfering Trith crystdlization. 111 addition,
vulcanizate crystalliza.tion may also be inhibited by:
3. Vulcanization to a high state of cure.
4. Vulcanization by a long or accelerated cure in the presence of added sulfur.
These methods may be used successfully in combination mit,h
Vol.. 42, No. 4
benefits which are least additive. In all cases consideration should be given to possible effects on other required properties. The seriousness of problems arising from the crystallization oi' neoprene is tempered by the reversibility of the process which eliminates or minimizes crystallization in products subject even intermittently t o fairly high temperatures or mechanical work. LITEHATURE CITED
(1) AIEi,ey, T., a n d Mark, H., K u b h e i Chern. T'eci~nol.,14, 526 (1941). ( 2 ) ,Ani. Soc. T e s t i n g Materials, Designation D 832-46T, p. 203 (1949).
(8) Hekkedahl,
K.,J . Research S a t l . B U T . S'fundards, 13, 411 ( 1 9 3 4 ) ; Rubber L'hem. Technol., 8, 5 (1935). (1) Clark, G . I,., Kothius, E., and S m i t h , W ,H., J . Re Bur. S t a n d a d s , 19, 479 (1937). (5) Forman, D. R.,du Pont Co.. Rubber C h e m . Div.. Rept. BE-196 (1945). (6) Formail, D. B., ISD. >;.VC:. CHERT., 36, 738 (1944). and Rndcliff, R.R., Ibid.,38, 1048 (1946). and Field, 3 . E.. J . A p p l i e d Phys., 10, 564 (1939). D. Exa. CHEM., 36, 40 (1944). (10) h l o r r i s , R. E.. Hollister. J. TY,, aiid Slallard. 1'. -i., I n d i a Rubber F o r l d , 112, 455 (1945). ( I I) Rostler, F. S.,and Sterriberg, IT. W , , 1x1).&:NO. C:EII:II.. 41, 598 (1949). (12) Smith, W . H., a n d S a y l o r , C . P...I. Bbaenrch, AVatl. Bur. ,S'ta7~durds, 21, 257 (1938). (13) IYood, L. A , , Eekkedahl. N . , and Gibson, 1%.E , , Ihid.. 35, 876 (1945). R E C E I V EJune D 2 5 , 1949, l'reeerited befose the spring meeting of the Division of Rubber Chemistry of the A V ~ R I C A SCHENICAL S o c I m y , Boston, Mass., M a y 1949.
J
N. R. BHOBV
AN^
Polytechnic I n s t i t u t e
Styrene is copolymerized by the mass method with commercial fatty acids having conjugated unsaturation, and the rates of reaction are reported for various amounts of excess styrene. Alkyd resins are made by reacting the sty-renated fatty acids of dehydrated castor oil with phthalic anhydride and glycerol. A laboratory process is described for completing both the copolymerization and the alkyd resin reactions in from 4 to 6 hours. Evalua-
s
NEIB'MX- F. PAIPiE
of Brooklyn, Brooklyn, S. Y .
T P R E N E is becoming an increasingly important raw niaterial for use in organic surface coatings. However, a t the present time it is riot used in coatings as monomer nor as polystyrene but rat,her as copolymers with such materials as butadiene, drying oils, and most recently nith alkyd resins. The volatility of the monomer and lack of compatibility of the polymer are the principal deterrents from the use of these niaterials as such. The advantages obtained from the styrene-copolyinerized oils and alkyds are faster drying, harder film, and bett,er water arid chemical resistance t,han can be o b t a i n d rrith the straight oils or alkyd resins. However, the copolymers of styrene and various niaterials retain some of the sensitivity of polystyrene to certain hydrocarbon solvents. The tremendous 1)roduction capacity for styrene, resulting from its extensive use i n the synthetic rubber program during the last war, its wlarively loa cost, and very high degree of purity make it of defiriit'e iuterest for the surface-coating industry. One of the first methods proposed for the reaction of styrene with a drying oil was disclosed in a British patent (6) in 1931.
tion of the experimental stj-renated alkyds in clear films and in white enamels shows that they have superior t l r j i n p time and chemical resistance to conventional phthalic alkyds. In view- of the present and possible future price structure of styrene and the other resin-forming ingredients it appears certain thal styrenated alkyds will be important in the surface-coating industry from both economic and performance considerations.
This describes the polymerization of an aqueous emulsion of styrene and tung oil with hydrogen peroxide as catalyst. The next development was the use of the solvent method for copolymerization of styrene and film-forming materials in inert solvents in 1934 ( 7 ) . This work was investigated further by 14'akoford and Hewitt ( 8 ) , Wakeford, Hewitt, and Armitage ( I O ) , and Wakeford, Hewitt, and Davidson ( 1 1 ) from 1942 on\vards in a number of British patents. The mechanism of copolymerization between styrene and various drying oils i s described by IIewitt and A4rinitagc.( 5 ) anti the effect of various solvents waR studied by Armitage, IIewit't,, and Sleightholme ( 2 ) . They used as a standard formula 50 parts solvent, 25 parts oil, and 25 parts styrene without catalyst. They also applied the same method for styrenation of a prepared alkyd; however, t,he method requires about 30 hours for reaction. Dunlap (4)and Takrford, €iewit,t, and Armitage (8) in 1945 investigated the mass method of copolymerizing styrene and various drying oils. The Inass method is much faster than the solve~itmethod hut only limited amounts of styrene can be co-
