structure and properties of polypentenamer

Structure and Properties of Polypentenamer HAROLD TUCKER, R. J. MINCHAK and

J. H. MACEY

B . F. Goodrich Research and Development Center

Brecksville, Ohio

Trans-1,s-polypentenamer (TPP) has some similarity to natural rubber partly because of properties that relate to crystallinity and to the position of the crystalline melting point. This similarity makes TPP a unique rubber among other synthetic hydrocarbon polymers. Requirements for attaining a good balance of physical properties include adjustment of both micro and macrostructure with processability. Natta, Dall'Asta, Haas and Pampus have described the preparation of polypentenamers based on tungsten or molybdenum catalysts. Since Eleuterio made his disclosure, there have been many important contributions disclosing special conditions for preparing TPP or variations in catalyst preparation including many catalyst activators. Natta and Dall'Asta vulcanized both the TPP and the amorphous cis-1,5-polypentenamer (CPP). They showed that TPP (melting point 23°C) gives good tensile properties even in pure gum vulcanizates characteristic of rubbers that crystallize on stretching. CPP gave better low- temperature characteristics than other hydrocarbon elastomers ( SBR rubber, propylene oxide/allyl glycidyl ether copolymer, cis-1,4-polybutadiene). For example, the CPP vulcanizates were less brittle down to -90°C measured by 100 per cent moduli and, in a comparison of temperatures at which retraction occurred, CPP showed a superiority. With CPP from 25°C to -7O"C, both tensile strengths and moduli increased without appreciable variation of elongation at break. Since the crystalline melting point at rest is near 20°C for TPP, the elastic behavior is governed by this transition rather than the glass transition point ( -9OOC).

The rate of crystallization for TPP is more rapid compared to natural rubber. Although vulcanization is a factor on elastic behavior, we suggest that further compromise may be necessary to balance the desirable properties related to crystallinity while maintaining elasticity at lower temperatures. The summary of the Haas paper noted that TPP rubber is outstanding except that the abrasion, wet skid and heat build-up are inferior to existing tread rubber types. Our efforts suggest that TPP is not inferior.

In our examination of TPP's having varied or lowered melting points, vulcanizates (tread recipes) with good low temperature flexibility were developed from TPP with T , of 5°C. Since tack and green strength are dependent on both the micro and macrostructure, properties lost by decreasing the trans content or the T , were offset by increasing the molecular weight. With higher molecular-weight TPP, other properties such as heat build-up and abrasion were improved or made equivalent to other tire rubbers. Thus, by optimizing molecular weight, oil level and processability with the microstructure, a good balance of properties may be produced for TPP rubber.

INTRODUCTION yclopentene polymerization and polypentenamer

C r u b b e r s have been receiving much attention in the technical literature. The discovery a n d develop-

ment of this new polymer chemistry represents one of the most important a n d exciting advances since the disclosure of the Ziegler-Natta catalysts in the 1950's. The transformation of alicyclic unsaturated

360 POLYMER ENGINEERING AND SCIENCE, MAY, 1975, Vol . 15, No. 5

Structure and Properties of Polypentenamer

molecules such as cyclopentene, cyclooctene and bi- cycloheptene-2, for example, to high molecular weight products with a conservation of unsaturation from monomer to polymer is credited mainly to the efforts of Eleuterio (I), Anderson (2), and Truett (3) from duPont; Natta (4) and Dall'Asta (5) from Montedison; Haas (6) and Pampus (7) from Bayer; and Calderon ( 8 ) arid Scott (9) from Goodyear. The list of all the contributors is much larger. There is the related application of this chemistry to acyclic molecules by Bailey and others (10) from Phillips.

Transpolypentenamer is in a unique position in relation to other synthetic rubbers in that its crystalline melting point is near room temperature (4, 6) . This suggests that it may have some of the properties of natural rubber, a rubber with a crystalline melting point just above room temperature. Like natural rubber, transpolypentenamer is self-reinforc- ing or stress crystallizes. On the other hand, since the unsaturation in the polypentenamer chain is unsubstituted, a comparison with polybutadiene rubbers is a natural one to make.

POLYMERIZATION There have been many important contributions to

preparative methods for polypentenamers. Since the early disclosure by Eleuterio (1) who used molybdenum or tungsten oxide, hydrogen treated, with metal hydride activator, many improvements and variations have been described for catalyst preparation. Tungsten halides are preferred, but a large variety of organo-metallic compounds can function as the second component. The list of necessary promoters for fast reaction or active polymer formation includes a wide assortment of compounds. Vari- ous methods of catalyst addition and orders of mixing are described.

Natta and co-workers (4) used WCl, and aluminum alkyls for preparing the trans polymer. They used MoCl, with aluminum alkyls for the cis product. In the application to larger ring systems, WCl, plus alkylaluminum dichloride ( 11 ) is emphasized including alcohol modification ( 8, 12). Dall'Asta ( 5 ) broadened the catalyst to include WCl,, WWl4, M&l, with Et3A1, Et,AlCl, EbBe plus an oxygen- ated compound such as benzoyl peroxide, t-butyl hydroperoxide, air, water, and ethanol. He made reference to the tra.nsition metal fluorides, acetyl- acetonate, phenolate, pyridinate and to the organometallic compounds of Mg, Zn, Ca. His most suit- able charging order specified the addition of organometallic compound last. Another disclosure (13) used A1C1,/WC12 from A1 metal and WCl, with Et2A1Cl, Et3A12C1, EtAICl,, EbAlBr or EtAlBr, plus benzoyl peroxide. Natta (14) claimed high molecular weight transpolypentenamer in addition to a process using titanium, zirconium, molybdenum and tungsten salts I: halides, acetyl acetonate) plus R,Al, R,AlCI, R2AlH, R,Be, RLi.

Calderon (15) claimed a new composition of matter from WC1, with different alkyl or aryl alumi-

num compounds including halides, dihalides, hy- drides and dihydrides plus one compound with the formula ROH where R is alkyl, aryl, alkaryl, aryl- alkyl, alkenyl, alkoxy, aryloxy, alkaryloxy or at least one hydroxyl substituted compound from the first five examples of R. Another variation by Uraneck and Trepka (16) for cyclopentene polymerization employed tantalum and niobium salts with R,Al, RzAIH or RA1H2. Marshall (17) used a novel metal alkyl-free catalyst combining AlBr, and WCl,. A related system AN, and WCl, added a third component RLi, R4Sn or R2Zn (18). Ofstead (19) used the 1,s-cyclooctadiene-W ( CO ) complex with EtAlCI, and either 02, Br,, AlBr,, I,, C12 or cyanogen halide.

The polymer formation data described by Haas (6) and Pampus (7 ) expanded somewhat on the organometallic compound and accented the development of activators such as 1,3-dinitro-2,5-dichloro- benzene, cyclopentene-2-hydroperoxide ( 20 ) , alkyl hypohalite ( 21 ), 2-chloroethanol ( 2 2 ) , ( 23 ) , 2-halo- cyclohexanol (24), epichlorohydrin (25), sodium peroxide (26) and irradiation (27). Included with the WCl, were wOC14, WBr5, TaCl,, TaBr,; the organometallic compound included iBu,Al, Et,AICI, R,AlOR and Be, Mg, Zn, B compounds. Pampus changed the microstructure from high trans to 83 percent cis-polypentenamer by lowering the Et3AI,C13/WFo ratio ( 6 ) , (28). Natta (13) showed a similar effect getting 91 percent cis at the low ratio of Et2A1C1 to A1C1,/WClZ. Dall'Asta (29) described the preparation of high cis-l,S-polypentenamer with MoCI, and Et3A1 catalyst. He varied the trans content from 0.6 to 15 percent between -80" to 30°C. From differential thermal analysis he compared the transition temperatures of the high cis and transpolypentenamers. See below:

Glass Crysta I I ine temperature Melting point

Tg ("C) Tm ("C) cis - 114 -41 trans - 97 18

At the ACS Polymer Division Meeting at New York in August 1972, Pampus (30) showed WCl,/ Et4Sn for preparing either high trans polymer or 90 percent cis polypentenamer. The latter formed at low temperature. Minchak showed the depen- dence of microstructure on polymerization temperature with a very sharp change between -25" and -40°C changing the structure from high trans to 99 or 100 percent cis (31). Catalyst was charged in the following order: ( a ) triethylaluminum, ( b ) tungsten hexachloride, and ( c ) benzoyl peroxide. Excluding vessel size, other polymerization pro- cedures and techniques were similar to those described by Kormer (32). Recently, Hein (33) reported additional work with WCI6 and various tin compounds. Several reviews of the chemistry in cyclopentene polymerization, catalyst systems are available (34,35).

POLYMER ENGINEERING AND SCIENCE, MAY, 1975, Vol. 15, No. 5 361

Harold Tucker, R. 1. Minchuk and I. H . Macey

PROPERTIES OF POLYPENTENAMER The literature stresses the evidence for self-rein-

forcing or crystallization of transpolypentenamer under elongation. Natta (4) reported this effect in pure gum vulcanizates and in vulcanizates reinforced with black, The stress-strain curves showed a strong upward curvature similar to data from natural rubber. In addition, the pure gum vulcanizates from polypentenamer reached a minimum resilience or rebound only at very low temperature (below -60°C) similar to cis-l,4-polybutadiene.

Haas (6) and Pampus (7) made a most extensive contribution to the testing and development of trans-1,s-polypentenamer and its vulcanizates. They showed self-reinforcement with uncured tire tread compounds containing 50 parts of black per hundred of polypentenamer. The natural rubber and polypentenamer stress-strain curves were almost identical turning upward strongly. Polybutadiene or SBR con- trols turned downward. With transpolypentenamer vulcanizates, self-reinforcement developed even at higher black levels-75 parts phr. Although isomer control during polymerization optimizes crystallization, the authors demonstrated additional isomer control during compounding, processing, or curing.

Haas and Pampus showed that Mooney viscosity was higher for polypentenamer compared to cis- l,.l-polybutadiene at the same intrinsic viscosity. LMore interestingly, their Mooney viscosity-temperature curve for high molecular weight polypentenamer changed little from 60" to 140°C. Thus, shear forces during mixing would bring about optimum or constant dispersion of black and curing agents over a wide temperature range. The steep shear gradient from internal mixer diagrams sug- gested easy incorporation of compounding ingredi- ents for polypentenamer.

Haas and Pampus considered or proposed transpolypentenamer for tire use, specifically to carcass application. The rubber tolerated both heavy loading of black and oil, yet retained green strength and tack. The requirements for curatives were less than for other rubbers. For comparison, equivalent hardness was reached by adding higher levels of plasti- cizer. Other supporting data besides green strength were tensile properties, resilience values, compression set, abrasion resistance, wet skid, and heat build-up.

Haas (36) and Theisen (37) expanded on the above, and by a regression analysis examined the vulcanization behavior of transpolypentenamer. The effect of zinc oxide level, stearic acid, accelerator ( N-cyclohexyl-2-benzothiazol sulphenamide ) , sulfur levels and curing temperature were determined. They presented three dimensional graphic repre- sentations and determined the effects on tensile strength, elongation at break, compression-set, and abrasion resistance. The best tensile properties were obtained at low concentrations of zinc oxide and stearic acid (about 0.5 phr) using approximately 2.0 phr sulfur and 0.2 phr accelerator at 170°C. This

362

was the maximum curing temperature in the evalu- ation. Sulfur, accelerator and temperature decreased the elongation at break somewhat as they were increased. 'Hie optimum compression-set was obtained at 170°C with medium levels of zinc oxide, stearic acid and sulfur, about 2.0 phr, and low accelerator concentration. The effects on abrasion resistance were complex, However, at the higher temperature the best results were found at very low zinc oxide level and at 0.75 phr stearic acid. The best choice of sulfur under these conditions was 2.0 phr and the accelerator 0.2 phr for abrasion resistance. Under these preferred conditions, the polypentenamer vulcanizates were resistant to temperature and to re- version.

Natta (14) showed supporting data on pure gum vulcanizates in his composition of matter patent for crystalline transpolypentenamer. Only stress-strain values (tensile strength, modulus, percent elongation) and Shore A hardness were listed. In claiming a polymerization method, Uraneck and Trepka (16) gave compounding results in a tread stock recipe. In addition to stress-strain data and hardness, they listed percent compression set, heat build-up, and percent resilience.

Dall'Asta (29, 38) described some elastomer properties of an all cispolypentenamer at low-temperature use. By annealing for one week at -75"C, he was able to show a crystalline melting at -41°C. He reported that the cis polymer was a poor elastomer at room temperature, especially as a pure gum vulcanizate. Processing was difficult on an open mill. The room temperature stress-strain data are similar to that for other amorphous rubbers using compara- ble loading of black and oil curatives. Tensile strength and modulus increased greatly without appreciable change in percent elongation as the testing temperature was dropped to 50" and -90°C. Dall'Asta interpreted this as a real enhancement of the low-temperature properties. Tear strength increased with the tensile strength, and the 100 percent modulus was relatively unchanged from 23" to -80°C. Other low-temperature rubbers ( SBR, propylene oxide/allylglycidyl ether copolymer, cis- 1,4-polybutadiene) gave increased 100 percent moduli which was interpreted as a transition to a brittle product and loss of rubber properties. The cis-1,4-polybutadiene performed well in this test, ranking second to the cispolypentenamer. Another test compared temperatures of retraction (10 percent and 70 percent) after quenching elongated (250 percent ) samples of reinforced vulcanizates. The 10 percent retraction was related to brittle temperature and the 70 percent recovery to the compression set. Only part of Dall'Asta's data (29, 38) is shown

__-___ 1 O % O % ( R x a c t i o n ) Temperature, O C

S B R - 44 - 36 cis-1, 4-polybutadiene - 92 - 42 transpolypentenarner - 4a 0 c ispo lypentena rner < - 100 - 85

POLYMER ENGINEFRING AND SCIENCE, MAY, 1975, Vol. 15, No. 5


below. Again, cispolypentenamer was superior to cis-lY4-polybutadiene as a low-temperature rubber.

EXPERIMENTAL The polymerization procedure is described in the

ACS Polymer Division preprints from August 1972 (30). The cyclopentene monomer was mixed with solvent, benzene, or toluene. Next the olefin modifier was charged, followed by catalyst. In some experi- ments, the extender oil was added to the polymer cement before polymer recovery. The compounding recipe is given below.

ComDoundina Recipe for Polwentenamer

Rubber Antioxidant Zinc Oxide Black Oi I Stearic Acid1 Santocure Sulfur Time Temperature

Parts

100 1.0 1.3 Varied (1) Varied (1) 1.0 0.6 3.0 40 min. 320°F

-

RESULTS AND DISCUSSION The crystalline melting point of high transpoly-

pentenamer at rest is near 20°C; therefore, the elastic behavior is governed by this transition rather than the glass transition temperature (-90°C). Compared to natural rubber, the rate of crystallization is more rapid. Although vulcanization is a factor in improving elastic behavior at low temperature, our data suggest that further compromise may be necessary to balance the desirable properties related to crystallinity while maintaining good low-temperature properties.

Lowering the polymerization temperature lowers the crystalline melting point (31) of the polypentenamer. Melting point data from differential scan- ning calorimetry (39) are presented in Fig. 1. From room temperature runs to polymerizations at O'C, the T, varied from greater than 10" to less than -10°C. A similar plot may be made from the microstructure measurements (40). Instead, we compare T, directly to the percent trans content in Fig. 2. The trans content varies from 85 to 75 percent over this range.

The low-temperature behavior of polypentenamers after compounding and vulcanization was measured in the Gehman low-temperature torsion test (41). The plot in Fig. 3 gives the stiffening temperature at twice the room temperature modulus ( T2) and the T, of the raw polymer. The second plot in Fig. 3 uses the T2 value of the raw rubber against its Tm. Using the T2 from the Gehman test in the comparison may be a rather severe restriction; however, we see that by loading and vulcanizing the elastic behavior of transpolypentenamer may be extended downward by 30 centigrade degrees. If we consider T2 at -25°C as tolerable for winter temperatures,

ISL .-

- " 0

i !! I-

-I I I I I I . d 0 5 m I5 m 25

WLYERIZATION TEYPERATURE,*C

Fig. 1 . Polypentenamer crystalline melting point us p o l y m - ization temperature.

POLY YERlZ ATION TEYPERATURE

C - 2 2 O C a..J 5 h.".,.l 0 ......... 1)

75 80 85 -v 10 % TRANSPOLYPENTENAYER ( I R )

Fig. 2. Crystalline melting point vs percent transpolypentenamer.

a compromise of properties may be made selecting a crystalline melting point between -5" and 5°C. This optimization of melting point corresponds to 77 to 80 percent trans structure.

The tack (42) and green strength are dependent on both micro and macrostructure. Therefore, properties lost by decreasing the trans content or Tm can be offset by increasing the polymer molecular weight. The change in tack with micro and macrostructure

mLYMER ENGINEERING AND SCIENCE, MAY, 1975, Vol. IS, No. 5 363

Harold Tucker, R. J .

-IcQ

Minchak and 1. H . Macey

U....D 0 ......V ARIEO VULCANIZATTES

I I I I I I I

WLYNERlZATlON TEMPERATURE

O....tPC RAW ?LINER a..... 15 d ....... 1

6.P INHERENT VISCOSITY fDSV1 POLYYERIZATION TEYPERLTURE DSV ...........

..... ............. 2.8 DSV A

0 ............. 0

22.C 0 I5 0 4.1

1.3 DSV

/o'

I I I I I I -10 -5 0 5 10 15

Tm, *C (DSC)

Fig. 4 . True tack of mixed stock (uncured) 1;s polypentenamer crystalline melting point.

POLYYERlZATlMl TEYPERATURE DSV

............... 22.C 0....2.8 a ............. I5 O....J.I h.. 7 A-6.2 P ............. 0

..........

I I I I I I -IS -10 -5 0 5 10 I S

Tm, 'C (DSC)

F i g . 5. Green strength of mixed stock [uncured) us polypentenamer crystalline melting point.

't \

Y

4

\A * \ a....-.11 h ...... I7 V.." .̂ ... 0

WLYYERIZATION TEYPERATURE

*...-.22'C a....-.11 h ......

V

A HEVEA

((t_LLLY 0 0 I 2 POLYPENTENAYER 3 4 DSV, d 14 I 6 7

Fig. 6 . Vulcanizate flexometer heat rise at 212°F 1;s polypentenamer inherent viscosity (DSV).

Table 1. Polypentenamer Compounding Data

Run 2 3 4 5 6 8 22 9 11 Polymerization temperature, "C 22 22 15 15 15 15 10 7 7

Tm, "C (DSC) 12 8 8 -1 4 3 -2 6 -3

Inherent viscosity 2.9 2.9 3.9 6.0 4.4 4.0 3.5 2.9 3.1 Trans, % 82 83 81 80 82 79 76 82 76

Black, phr 75 75 75 75 75 75 75 75 75 Oil, phr 50 50 50 50 50 50 50 50 50 Stress-strain, 25°C

Tensile, Ib/in.2 Elongation, %

Hardness, shore A

2660 2800 2660 2120 2190 2360 2600 2020 1870 455 460 540 455 470 540 515 455 400

54 54 55 56 56 59 60 54 58

364 POLYMER ENGINEERING AND SCIENCE, MAY, 1975, Vol. 15, No. 5


herent viscosity. The heat build-up improves up to approximately 3.5 inherent viscosity and approaches 40" to45"F.

A summary of polymers cured to an optimum crosslink density is given in Table 1. The polymers include both high cis and high transpolypentenamers, and all permit a high loading of oil and black. The stress-strain properties at room temperature and hardness values are presented in this summary. At maximum vukanizate tensile, we plot the oil requirement of high transpolypentenamer against the polymer inherent viscosity, Fig. 7. With 5 or 6 inherent viscosity, the polymers tolerate 100 phr of oil. The tensile strengths level out at approximately 2,800 psi. Similarly, the hot tensile properties (100°C) increase up to 3 or 4 inherent viscosity and level out about 1,300 to 1,400 psi. See Fig. 8. Again, the loading is increased in these high molecular weight polypentenamers.

We have other vulcanizate properties that are optimum at 3 to 4 inherent viscosity. The hot tear properties ( 4 5 ) ( l 0 O O C ) in Fig. 9 show a decline from about 100 lb/in. as molecular weight increases.

I .c loo -

rn W

CIS- I, 4- WLYWTADINE

I 2 3 4 5 6

s n - I- = B d 0

P

INHERENT VISCOSITY

Fig. 7 . Oil requirement for maximum vulcanizate tensile strength vs polypentenamer molecular weight.

14 15 16 18 19 20 0 0 0 - 15 - 30 - 60 6.5 4.4 4.2 4.2 11.6 6.0

12 78 76 37 16 4

15 75 85 95 115 15 50 50 75 15 100 50

2400 2530 1850 1840 1230 1570 520 400 440 290 395 685 60 59 56 63 62 61

- - - 14 -1 - 11 -

A UEVEA

CIS-I,4-POLIL)UTADIENE

y 12 55 L W I- 'T

01 I I I I I I 2 3 4 5 6

INHERENT VISCOSITY

Fig. 8. Vulcanizate tensile strength at 100°C vs polypen- tenumer molecular weight.

,i" HEVEA

a W

I-

T 1 I I 1 I I I e 3 4 5 6

Fig. 9. Vulcaniulte hot tear strength us polypentenamer molecular weight.

The Pic0 abrasion index (46) gives a maximum value of 120 at 4 inherent viscosity, Fig. 10. The ring crack growth test (47) at 70°C is outstanding with flexing over 160 hours without failure for 3 to 5 inherent viscosity. We use natural rubber (plasticized Hevea 5256 ) and cispolybutadiene for comparison.

CONCLUSIONS The transpolypentenamer has properties in rein-

forced compounds that compare well with those from existing general purpose rubber types. Some

INHERENT VISCOSITY

POLYMER ENGINEERING AND SCIENCE, MAY, 1975, Vol. 15, No. 5 365

Harold Tucker, R . I. Minchuk and I . H . Macey

“t

80

60

TRANSPOLYPEWTCWAYER

/* q\* HEVEA

40t ml I I I I I

2 3 4 s 6 7 INHERENT VISCOSITY

Fig. 10. Vulcanizate Pic0 abrasion index vs polypentenamer molecular weight.

of its good properties give it similarity to cis polybutadiene. In addition, unvulcanized, it shows tack and green strength properties that are important in tire making and are currently maintained by natural rubber use. Objections to poor low-temperature properties in polypentenamer vulcanizates are over- come with microstructure adjustment. Both microstructure and macrostructure are important in preparing a useful polypentenamer rubber.

Other advantages of polypentenamer are high loading or oil extersion and low curative levels. The high tensile properties expected in a crystallizing rubber such as natural rubber have yet to be achieved.

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structure and properties of polypentenamer

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