PROPERTIES AND APPLICATIONS OF hWns=l,4=PO LY ISO P R EN E E. G. K E N T A N D C. B. S W I N N E Y Polymer Gorp., Ltd., Sarnia, Ontario, Canada
Synthetic trans-1 ,4-polyisoprene, a stereospecific rubber, is almost identical with naturally occurring trans1,4-polyisoprene (a polymer found in natural balata). It is similar to natural rubber and most synthetic rubbers, in that it is vulcanizable with sulfur, but differs from most rubbers in that it is relatively crystalline at room temperature. The crystals, however, melt at 130' F. (54.5' C,),so that trans-l,4-polyisoprene exhibits thermoplastic characteristics. These characteristics, combined with high tensile strength and cut abrasion and scuff resistance, make frans-l,4-polyisoprene ideal for use as the base polymer in gold ball cover stock. Natural balata is suitable for a variety of other applications, but its use has been limited by uncertainty of supply and unpredictable price fluctuations. With the advent of uniformly high-quality synthetic trans-l,4-polyisoprene at a stable price, trans-polyisoprene can now be considered for many new applications in addition to those traditionally served by balata.
N
occurring trans-l,4-polyisoprene (found in balata or gutta percha) has been used as insulation for submarine cables and electrical wires, for impregnation and coating of porous material, and for belting. The first gutta-insulated telegraph wires were produced in 1847 and the first experimental length of submarine cable was laid in 1849. The favorable electrical insulating properties of trans-I ,4-polyisoprene, its low water absorption and slow waterdiffusion rates, and its excellent mechanical strength were attractive for these applications. A more recent use is in neon sign lead wires, where high resistance to ozone is important. Adhesion and thermoplastic properties are desirable in the impregnation and coating of porous material for use as transmission and conveyor belts, V-belts, elevator belts, and drive ropes. Impregnation with trans-l,4-polyisoprene improves the mechanical strength, abrasion resistance, and coefficient of friction, and provides the belt with form stability, resistance to water and vegetable oils, and freedom from odor and taste. Battery separators, heat and sound insulators, and vibration dampeners are further examples of porous materials which are improved by impregnation with trans-I ,4-polyisoprene. Paper, metal foil, Cellophane, corks and gaskets, fabrics for footwear, repair patches, and adhesive tape are typical examples of materials which may be coated. The scarcity of natural balata, the resulting high cost, and the large demand for golf ball covers discouraged its widespread use in other areas. As a result, new and less costly synthetic polymers have displaced naturally occurring balata from uses for which it was well suited. The introduction of a synthetic balata, readily available, a t a stable price, and essentially identical to natural balata, will result in its use in new as well as traditional applications. ATURALLY
Chemical Structure
Balata is an isomeric form of natural rubber. Natural rubber is the cis form, with chain bonds on the same side of the double bonds, and balata is the trans form, with chain bonds on opposite sides of the double bonds. 134
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
CHI
CHI
\
\
C=CH
\
C=CH
trans (Balata)
CHz
cis (Natural rubber)
/
C=CH
/
CHI
CHa
CH3
\
\ \ C CHz C / \\ / \ / \\
CH?
CH
CHz
/\ CH
The difference in the stereoisomeric structure of the two polymers is responsible for the differences in their properties. Typical physical properties for the raw polymers are listed in Table I. The large difference in properties is partially explained by the tact that natural rubber (cis-1,4-polyisoprene) crystallizes very slowly a t room temperature. Therefore it usually exists as a noncrystalline polymer characterized by low raw polymer hardness and tensile strength. t r a w l ,4-Polyisoprene or balata, on the other hand, crystallizes rapidly at temperatures below 140' F. (60' C . ) and at room temperatures exists as a crystalline polymer characterized by high raw polymer hardness and tensile strength. Synthetic and Natural tranr-l,ePolyisoprenes
Natural trans-1,4-polyisoprene is found in balata or gutta percha, and in latex form in the wild rubber trees of the Sapotaceae family known as Palaquium guttas (7). Jungle balata is obtained by felling the tree and stripping the bark. The gum, found beneath the bark, is removed in a semicoagulated condition, boiled in water, and made into a sheet or block. Balata can be recovered from the latex form by drying thin layers of the balata latex in the sun, to produce a sheet form, or
Table 1.
Raw Polymer Properties of cis- and Imnr-1.4 c Polyiroprenes Tested at 77' F. (25' C.) cis tram
Hardness, Shore A, Tensile strength, kg./s Elongation at break,
2
cm ( p s i ) '
' ' '
30-35 21(300) 1200
95+ 352(5000) 475
by heating the latex until it coagulates and then, while it is stil hat, compressing the balata into blocks. This crude balata contains 20 to 70y0 trans-1,4-polyisoprene 5 to 10yosand, bark, and wood chips; 30 to 75% mixture o natural resins and hydrocarbonse.g., the albane resins-ani up to 50% moisture. Generally, the balata is refined b reprecipitation to 97% tranr-l,4-polyisoprene content. Th remaining impurities are in approximately the same ratio a before reprecipitation. Some of the impurities act as anti oxidants and prevent degradation of the polymer. Synthetic tranr-l,4-polyisoprene is obtained from the poly merization of isoprene in hydrocarbon solvent in the presena recovered by precipitation contains 95 to 98% polyisoprene ii the trans form. The remaining 2 to 5% consists of an anti oxidant, which is added immediately prior to precipitation and a small amount of isoprene in the cis form which is in carporated into the polymer chains during polymerization. Similarity. INFRARED SPECTRA.The spectra in Figure were obtained from a synthetic polymer sample in which tw( crystal forms co-existed, and from a natural polymer sample ii which there was only one crystal form. The natural ani synthetic polymers are not basically different; each can hi obtained in identical crystal forms by appropriate therma treatment. The minor absorption bands shown in the spectrum fo natural rubber (Figure 1) are removable by further purificatioi of the polymer. The similarity of the physical prop PHYSICAL PROPERTIES. erties of svnthetic and natural balata is shown in Table 11. Mooney viscosity, a function of the average molecular weigh t or chain length, can be varied in the synthetic polymer. Relation of Crystal Structure to Physical Properties The crystals of trons-polyisoprene form supercrystalline struc tures known as spherulites. The nucleation and growth atf spherulites of tram-polyisoprene are shown in Figure 2. trnns-1,4-Palyisoprene exists in three polymorphic forms alpha, beta, and gamma. T h e beta and gamma forms exist ii1 the unstressed polymer. The third or alpha farm is presen t ~. ... . .. . only in stressed specimens. 'I'he gamma modification, a highmelting form, is converted into the beta or low-melting form by
50
850
750
iic and natural frans-l,4-
Table
Viscosiry,
?s of Synthetic and
M L - ~ar L I L P. (IUU
35-25 70-76 Hardness, instantaneous, Shore C Tensile strength, kg./sq. cm. (psi.) 352(5000) Elongation a t break, yo 460-5on Tear strength, kg./cm. (Ib./inch) 20.5(1
C.)
-
,-,,
352(5000) A&"-WI"
-
Tabla 111.
Effect of Crvrtal Form on Phvrical Pronerties of trans-1 +Polyisoprene Befa Gmm. lulus at 300% ," elone.. kz./sa. ~
I
~
(psi) iile strength, kg./sq. cm. ( p s i ) gation at bzeak yo .ing point, F. ( ' C.) 1.
188(2675) 348(4930) 480
147(64)
190(2700) 358(5070) 475 165(74)
Figure 2. Progressive stages of spherulitic growth of frans1 ,4-polyisoprene
thermal treatment, which causes the isoprene units to rotate about the single bond which joins them to each other. The beta form can he produced from the gamma form by heating above 150' F. (65' C . ) and quickly chilling the melt. Tensile strength, modulus, and elongation are similar for the two crystal forms (Table 111). Figure 3 shows x-ray diffraction diagrams of natural and synthetic balatas (upper) and of natural and synthetic trans1,bpolyisoprenes stretched to 400% elongation (lower). There is no difference in the crystal structures of the synthetic and natural polymers. As with all polymers, melting does not occur at a sharply defined temperature, because of the size, distribution, and imperfection of the crystals. The effect of temperature on hardness and tensile strength is shown in Figures 4 and 5. The values for both properties decrease with increasing temperature. The decrease in hardness and tensile strength with increasing temperature arises primarily from the decrease in crystallinity caused by the progressive melting out of small and imperfect crystallites, Thus, a t higher temperatures the spherulites contain a larger fraction of amorphous material, resulting in lower hardness and tensile strength. Properties. T h e high tensile strength of tmns-1,4-polyisoprene is attributed to the crystalline nature of the polymer. The polymer melts at about 140' F. (60' C , ) , but regains its crystallinity on cooling to room temperature. It may, therefore, be compression-molded, injection-molded, extruded, and calendered by rubber or plastics techniques. O n cooling it resumes its crystal structure, strength, and hardness. Films
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of trons-1,4-polyisaprene may be formed to thicknesses of as little as 0.012 mm. (0.0005 inch) and oriented to give tensile strength values in excess of 700 kg. per sq. cm. (10,000 p.s.i.). Vulcanization. The physical properties of golf ball cover stocks cured below and above the crystallization temperature of irons-1,4-polyisoprene are shown in Table IV. Compound A contains crystals which did not melt at the low cure temperature, and displays the physical properties of a crystalline polymer. Compound B was cured above the crystallization temperature and exhibits the physical properties of a noncrystalline polymer-e.g., lower hardness, lower modulus, and higher elongation a t break. Compound C, cured at the same temperature as compound B, was rested for 72 h o u n a t 41' F. (5' C.) before testing. During this rest period, compound C recrystallized. Table I V shows it to have higher hardness and modulus than compound B. Sulfur was omitted from compound A to prevent premature cure, because trans-1,4-polyisoprene must be heated above its melting point to incorporate compounding ingredients. Vulcanization can take place if both accelerator and sulfur are present during prolonged mixing a t these high temperatures and particularly if a low temperature accelerator is used. Vulcanization may also occur during prolonged storage. Compounds B and C had both accelerator and sulfur incorporated but under controlled mixing conditions. No delay was required as with compound A between mixing and curing. The physical test results a t zero time indicate no crosslinking. Table V shows the difference in tensile strength of cured and uncured cornpounds tested at 180' F. (82' C.).
Figure 3. Upper.
Effect of Cure of Golf Ball Cover Stocks Bared on Synthetic trtmr-l.4-Polyisoprene 1rom-l,4-Polyisoprene 80.0 parts .~ Pale crepe 20.0 parts Table IV.
Titanium dioxide
Zinc oxide
Piperidinium pentamethylene dithiocarbamate Cure conditions Exposed to
Lover.
Lefl. Righl.
X-ray diffraction patterns
fran~-1,4-Polyifoprene Imnr-l.4-Polyit~preneat 400%
elongation
Synthetic Natural
10.0 parts 5 . 0 Darts 1 .O part
1 .O part sulfur incorporated in
cornpound A B C Days at Min. at 270 F. Min. at 270 F. 107 F. (143" C.) 1143" C.). (430 C.) iested 7Zhr. at 41' F. sulfur
Compound
( 5 0 C.)
elong., kg./sq. cm. ( p s i . )
Elongation at break,
'70
Table V.
Figure 4. Dependence of hardness on temperature trans-1 ,rl-polyisoprene
0 520 2 400 4 400 8 400 16 420
0 500 2 640 4 660 8 620 1 6 620
0 2 620 4 620 8 610
16 560
Hot Strength of Cured and Uncured tmm-l,4Polyisoprene
Tensile strength, kg./sq. cm. (p.s.i.) Elongation at break, 70
Cured
Uncured
4.5(64) 1600
0.64(7) 1000
of
Crystallinity is destroyed in cured and uncured compounds when the test temperature is above the crystallization temperature oftranr-1,4-polyisoprene. Table V shows that the uncured cornpound has no tensile strength and high elongation, while the cured compound has some tensile strength and higher elongation. The values for tensile strength and elongation a t break for the cured compound are evidence of the presence of crosslinks. Compounding. hnnr-1,4-Polyisoprene may be extended by adding fillers or by blending with other polymers. Although blends are used primarily to lower costs, in some instances the modification of properties is important. The addition of
iv TER
(26.6mm. 0 . d )
INNER WINDINGS ( 3 . 0 m m . wide) o u m’ WINDINGS (2.Omm. wide) COVER (l.7mm. thick)
\ 0
25
I
I
I
I
30
35
40
45
TEMPERATURE
Figure 6.
Diameter of outer ball, 42.1 mrn. Windings approximately 0.38 mm. thick
2 50
55
60
(*C)
Figure 5. Strength loss in trans-1 ,rl-polyisoprene with temperature increase
extenders or compounding ingredients restricts the degree to which the polymer crystallizes and thus has an adverse effect on properties which depend on crystallinity, such as tensile strength, hardness, and rate of hardening. Other Physical and Chemical Properties. Since cis-l.4polyisoprene (natural rubber) and truns-l,4-p011 isoprene (synthetic or natural balata) are stereoisomers based on the same monomer (isoprene), it is not surprising to find many similarities in their chemical behavior. Pure trans-l,4-polyisoprene is subject to rapid oxidation, and the rate of oxidation increases markedly after an initial induction period characterized by the formation of peroxides. Consequently, peroxide-destroying substances inhibit deterioration of the polymer. In the natural polymer, these substances are present as hydrocarbon ester gums and resins. In the synthetic polymer, antioxidants are added to destroy peroxides. Raw trans-l,4-polyisoprene is resistant to ozone. Specimens of this polymer bent about a ‘/Z-inch diameter glass mandrel and exposed at 120’ F. (49’ C.) to an atmosphere containing 25 p.p.m. of ozone showed no signs of cracking after 150 hours. cis-l,4-Polyisoprene, on the other hand, exhibits extensive cracking after 5 hours’ exposure. trans-1,4-Polyisoprene is particularly resistant to concentrated hydrofluoric and hydrochloric acids, alkalies, vegetable oils, and fats, but reacts u i t h concentrated sulfuric and nitric acids. Pure trans-I ,4-polyisoprene is soluble a t room temperature in mcst arcmatic hydrocarbons, chlorinated hydrocarbons, ether, and carbon disulfide. I t is essentially insoluble in most straight-chain saturatcd hydrocarbons, esters, and acetone. The rates of water absorption and of diffusion of water for trans-1,4-polyisoprene are very low. Other physical properties are: Dielectric constant Refractive index at 68 F. (20 O C.) Mill shrinkage, 70 Cold brittle point, F. ( ’ C.) Molecular weight Specific heat, cal./g. Coefficient of expansion (per C.)
Cross section of golf ball
2.6 1.55 49 -80( - 6 2 . 2 ) 30,000-50,000 0,67 0,008
Present and Potential Applications
The largest present-day application of truns-l,4-polyisoprene is in cover stocks for golf balls, which evolved from a smooth sewn leather casing stuffed with feathers. About 1850, when
gutta percha was appearing as a commercial product, solid “guttas,” solid balls with smooth surfaces, were introduced. Through the introduction of dimpled surfaces to improve aerodynamic properties, u p to today’s high compression, lvound, and covered balls, trans-l,4-polyisoprene has remained one of the most important materials in the production of golf balls. A typical golf ball is shoum in cross section in Figure 6. The center is molded and wound with a natural or synthetic cispolyisoprene thread under an elongation of approximately 1000%. These Ivindings are susceptible to heat degradation; consequently, the cover compound must be capable of penetrating the Ivindings and taking the desired shape at relatively l o ~ vtemperatures. A golf ball cover must be hard at outdoor summer temperatures, to help provide the satisfying “click” of a \\-ell-hit ball. I t must accept paint, have cut and scuff resistance, and have sufficient strength and pliability to xvithstand the impact and recover deformation caused by the club striking the ball. trans-l,4-Polyisoprene is \vel1 adapted to these requirements. I t can be molded at relatively lo\\ temperatures [as lotv as 140’ F. (60’ C,)], and can be vulcanized to develop resistance to heat and chemicals, yet it retains its crystal structure under service conditions. \Vhen cured, the cover \vi11 not deform or lose strength at high summer temperatures, and it will resist the common solvents used in paints and coatings. Potential Applications. HEAT-SEKSITIVE ADHESIVES. trans1,4-Polyisoprene can be used as a heat-sealable coating on paper, fabiic, and leather, and repair patches, shoes, pocket books, leather bags, and cigarette boxes. Such coatings can be formulated Lvith various resins as a cement. hot dough, or calendered film: depending on the applicaticn. PRESSURE-SENSITIVE ADHESIVES.\l‘hen added to rubber adhesives, trans-1,4-polyisoprene increases the crystallization rate, making them pressure-sensitive. CAULKING COMPOUNDS. trans-I ,4-Polyisoprene may be used with butyl rubber to make caulking compounds lvhich are pliable and sticky \\hen tvarm, but set to a tough semirigid condition Lvhen cooled. The hardening process can be controlled by varying the composition, thereby permitting alignment of components in the assembly before the sealant sets. THERMQPLASTICS. The impact resistance and elongation at break of thermoplastic compounds are improved lvhen trans1,4-polyisoprene is blended into the compound. SAFETY APPLICATIOSS. trans-l,4-Polyisoprene may be suitable for inserts for the toes of safety boots or for impregnating the fabric of safrty clothing. SPECIALTY APPLICATIONS. Because it can be formed at relatively low temperature, trans-l,4-polyisoprene may be useful for gaskets for loxv temperature service and coatings for mining equipment, cable covering, and tank linings. VOL. 5
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Acknowledgment
literature Cited
The authors thank the management of Polymer Corp., Ltd., for permission to publish this paper, and acknowledge the contribution of E. Fisher to the section on the crystal structure of trans- 1,4-polyisoprene.
(1) Davis, C. C., Blake, J. T., eds., "Chemistry and Technology of Rubber,'' pp. 706, 707, Reinhold, New York. RECEIVED for re' iew August 21, 1964 Dec. 3. 1965 RESUBMITTED ACCEPTED April 1; 1966
NEW LOW ODOR ETHYLENE-PROPYLENE COPOLYMER CROSS-LINKING SYSTEMS L
. P . L E N A S , Enjay Polymer Laboratories, Linden, N . J.
Several peroxides containing more than one peroxide group were evaluated as curing agents for ethylenepropylene rubber. These aliphatic and cycloaliphatic ketal peroxides yield vulcanizates possessing much lower odor than those resulting from dicumyl peroxide, and in the presence of a polyfunctional coagent yield pleasantly mild and, in some cases, nearly odorless vulcanizates. On the basis of active oxygen content, they are less efficient than dicumyl peroxide but are competitive with it when compared on a weight basis. The ketal peroxides possess a superior cure cycle, requiring only 9.5 minutes at 300' F. whereas dicumyl peroxide requires 19 minutes at 320' F. for an equivalent cure. Dicumyl peroxide, however, has an advantage in scorch safety.
DOR
and long cure times are the major problems associated
0 with the usual peroxide-coagent vulcanization of ethylene-
propylene rubber (EPR). The proper selection of peroxide and coagent can minimize these problems and make EPR more acceptable for mechanical and electrical goods ( 7 , 4, 7). Undesirable odor results from the decomposition products of peroxides, and sulfurous by-products when sulfur is used as a coagent. Replacing sulfur with polyfunctional coagents significantly improves odor. These coagents also improve the cure cycle and/or modulus but generally at the expense of tensile strength, especially in carbon black-filled vulcanizates (6, 73). Therefore, the odor and cure cycle can best be improved through the proper choice of peroxide. The present paper reports the evaluation of new organic peroxides as low odor and low temperature curatives for EPR. The new peroxides discussed in this paper are of the ketal (aliphatic and cycloaliphatic) and alkyl-aralkyl type, containing more than one peroxide group. They were studied in moderately and highly loaded carbon black compounds and also in mineral-filled systems. The effectiveness of the new peroxides was compared with that of dicumyl peroxide, which is widely used in EPR vulcanization, Efficient peroxides such as dicumyl peroxide are normally used at a concentration level of 0.16 gram of active oxygen per 100 grams of rubber. The new peroxides, however, were evaluated at an active oxygen concentration of 0.32 gram, since earlier work indicated that peroxides containing more than one peroxide group generally are not as efficient as the monoperoxides (7). Five peroxide half lives at 300' F. was the cure cycle employed in the comparison of peroxides. However, 320' F. was also employed for dicumyl peroxide vulcanizates, since it permits the use of shorter cure times and is the cure temperature most frequently used for this peroxide.
Materials Used
Enjay EPR 404 was the ethylene-propylene copolymer used in this study. I t contains 43 weight yo of ethylene, and has a 138
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Mooney viscosity of 40 (8 M L at 212' F.). Information on the new peroxides evaluated is presented in Table I (72). Trigonox X-29/40 is the only peroxide available in semicommercial quantities. The other ne\v peroxides are experimental at the present time. No particular precautions were taken in handling the curing agents. The safety and stability of related materials are described in the literature (2, 77). The coagents studied included sulfur, ethylene dimethacrylate (EDMA), and Buton 150. Ethylene dimethacrylate is a reactive difunctional monomer. Buton 130 is a liquid butadiene polymer with a ratio of 1,2 to 1,4 monomer addition of about 3 to 1. Hydrated silica (Hi-Si1 233) and hard clay (Dixie clay) were the mineral fillers studied. Carbowax 4000, a polyethylene glycol, was used as an additive in mineralfilled compounds. Information on the carbon blacks and other materials tested is given in Table I1 ( 5 ) . Odor
The classification of peroxides in terms of vulcanizate odor is somewhat subjective. Nevertheless, differences between peroxides are detectable and preferences can be established. Of the peroxides evaluated in this work, three yield low odor levels in EPR vulcanizates. The cycloaliphatic ketal peroxides yield the lowest odor, followed closely by the alkyl-aralkyl peroxide. Table I11 shows a qualitative evaluation of these peroxides. EDMA improves odor when it is used in place of sulfur. The odor of peroxides dissipates gradually and heating in air or steam accelerates this process. Postaging can produce nearly odorless vulcanizates Jvith Trigonox X-29/40. Additional reduction in odor can be achieved by incorporating 15 phr of bituminous coal fines (Austin black) into the EPR compound ( 9 ) Peroxide Evaluation in Carbon Black-Filled EPR
The new peroxides studied contain more than one peroxide group. The cycloaliphatic ketal peroxides (Trigonox X-29/40 and Perkadox Y-I 2/40) possess the lowest decomposition temperature and, therefore, the shortest half lives at a given temperature. O n the other hand, the alkyl-aralkyl peroxide (Perkadox Y-14/40) is the most thermally stable peroxide investigated.
