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A review of isobutylene-based elastomers used in automotive applications.

Isobutylene polymers

Isobutylene-based elastomers include butyl rubber, the copolymer of isobutylene and isoprene, halogenated butyl rubbers, star-branched versions of these polymers and the terpolymer isobutylene-para-methylstyrene-bromo-para-methylstyrene (BIMS). A number of recent reviews on the manufacture, physical and chemical properties and applications of isobutylene-based elastomers is available (refs. 1-5).

Structure

In butyl rubber (IIR) (figure 1), isoprene is enchained by 1,4-addition in the trans configuration (ref. 6). Depending on the grade, the unsaturation level due to isoprene, incorporation is between 0.5-2.5 moles per 100 moles of isobutylene monomer. The low content of isoprene along with a reactivity-ratio product near unity indicates that there is a random distribution of unsaturation throughout the chain. The molecular-weight distribution of butyl rubber also depends on the grade, with many products having a [M.sub.w]/[M.sub.n] of 3-5. The methyl groups adjacent to the unsaturation in butyl rubber prevent halogen addition across the carbon-carbon double bond. Figure 2 shows the predominant structure (ca. 90%) as determined by [sup.13]C NMR spectroscopy (refs. 7 and 8) that results from the introduction of bromine or chlorine at approximately a unit molar ratio of halogen to the unsaturation level in butyl rubber. The butyl polymer backbone appears to be relatively unaffected by the halogenation process.

[Figures 1-2 ILLUSTRATION OMITTED]

Brominated isobutylene-co-para-methylstyrene (BIMS), is the elastomer formed by brominating poly(isobutylene-co-para-methylstyrene). The para-methylstyrene monomer can be present between 2-8 moles per 100 moles of isobutylene monomer, with the bromine level ranging from approximately 20-50% of the para-methylstyrene content.

Physical properties

The physical properties of butyl rubber are shown in table 1 (ref. 2). The physical properties of polyisobutylene, chlorobutyl rubber (CIIR) and bromobutyl rubber (BIIR) are similar to those of butyl rubber.
Table 1 - physical properties of butyl rubber
(ref. 2)

Property Value Composition(a)
Density, g/[cm.sup.3] 0.917 B
 1.130 CBV
Coefficient of volume expansion 560x10" BV
 (1N)(V/T), K 460x10" CBV
Glass-transition temperature,
 [degrees] C -75 to -6 B
 1.95 B
Heat capacity, [C.sub.p]kJ/tkg.K)(b) 1.85 BV
 0.130 BV
Thermal conductivity, W/(m.K) 0.230 CBV
Refractive index, [n.sub.p] 1.5081 B


(a). B - butyl rubber; BV = vulcanized butyl rubber; CBV = vulcanized butyl rubber with 50 phr carbon black

(b.) To convert J to cal, divide by 4,184

The rotational restriction of the polyisobutylene backbone due to the presence of the geminal-dimethyl groups results in a high monomeric friction coefficient and unique William-Landel-Ferry constants (ref. 9) compared to hydrocarbon elastomers of similar glass-transition temperature, for example natural rubber. The low vapor permeability of the isobutylene-based elastomers is apparently due to a low diffusion constant caused by limited chain mobility and to the low solubility of gases in these saturated polymers (ref. 10).

Chemical properties

Polyisobutylene and butyl rubber have the chemical resistance expected of saturated hydrocarbons. Oxidative degradation is slow and the materials may be further protected by antioxidants, for example hindered phenols. Oxidative attack results in a loss of molecular weight, rather than in embrittlement.

In butyl rubber, the hydrogen atoms positioned [Alpha] to the carbon-carbon double bond permit vulcanization into a crosslinked network with sulfur and organic accelerators (ref. 11). The low degree of unsaturation requires the use of ultra-accelerators, such as thiuram or thiocarbamates. Phenolic resins, bisazidoformates (ref. 12), and quinone derivatives can also be employed. Vulcanization introduces a chemical crosslink approximately every 250 carbon atoms along with the polymer chain, producing a molecular network. The number of sulfur atoms per crosslink is between one and four or more (ref. 13). Sulfur crosslinks have limited stability at elevated temperature and can rearrange to form new crosslinks. This results in permanent set and creep for vulcanizates exposed for long periods of time at high temperature. Resin cure systems provide carbon-carbon crosslinks and heat-stable vulcanizates; alkyl phenol-formaldehyde derivatives are usually employed. Typical vulcanization systems are readily available (ref. 2).

The presence of allylic halogens in halobutyl elastomers allows crosslinking by metal oxides and enhances the rate of sulfur vulcanization over that of butyl rubber. Halobutyl elastomers can be crosslinked by the same materials as used for butyl rubber, and by zinc oxide, bismaleimides, diamines, peroxides and dithiols. The allylic halogen allows more crosslinking than is possible in elastomers with only allylic hydrogens. Halogen is a good leaving group in nucleophilic substitution reactions. When zinc oxide is used to crosslink halobutyl rubber, carbon-carbon bonds are formed through dehydrohalogenation to form a zinc halide catalyst (ref. 14). A very stable crosslink system is obtained for retention of properties and low compression set.

Brominated isobutylene-co-para-methylstyrene crosslinking involves the formation of carbon-carbon bonds, generally through alkylation chemistry or the formation of zinc salts, for example zinc stearate (refs. 15 and 16). Sulfur vulcanization is achieved by using thiazoles, thiurams and dithiocarbamates. Diamines, phenolic resins and thiosulfates (ref. 17) are also used to crosslink BIMS elastomers. The stability of these bonds combined with the chemically saturated backbone of brominated isobutylene-co-para-methylstyrene yields excellent resistance to heat and oxidative aging, and to ozone attack.

Tire applications

The tire is a high performance polymeric composite of many rubbery components consisting of a tread area which includes the tread, base and cushion compounds, and the casing which includes the bead, carcass plies and belts, and the sidewall, inner liner, apex and chafer compounds (ref. 18). Each component serves a specific and unique function, yet all function synergistically to produce the desired performance.

Isobutylene-based elastomers are used commercially in a number of the rubbery components of the tire, including the inner liner, non-staining black sidewall, white sidewall and white sidewall coverstrip compounds, and have been evaluated for use in tread compounds.

Inner liner

The inner liner is a thin layer of rubber laminated to the inside of a tubeless tire to ensure retention of compressed air. The liner is most commonly formulated with halobutyl rubber in order to provide good air and moisture impermeability, flex-fatigue resistance and durability (ref. 18). The impermeability is thought to be caused by the close packing of the methyl side groups along the polymer backbone, resulting in slow movement of the chains (ref. 19). The integrity of the tire is improved by using halobutyl rubber in the inner liner since it minimizes the development of inter-carcass pressure which could lead to belt edge separation and adhesion failures, and inhibits the rusting of steel tire cords (ref. 20).

Inner liners for passenger tires can be formulated with a blend of chlorobutyl rubber and natural rubber (refs. 19-26). Hopkins, Jones and Walker (ref. 20) established that a chlorobutyl rubber/natural rubber liner would have to be thicker than that of a 100% chlorobutyl rubber liner in order to obtain the same air impermeability. The permeability increases essentially linearly with increasing natural rubber content (refs. 20 and 23). Von Hellens (ref. 23) showed that a 20 phr replacement of chlorobutyl rubber by natural rubber adversely doubles the permeability and reduces by half the adhesion to a natural rubber carcass compound.

Bromobutyl rubber is used extensively in inner liner formulations (refs. 19, 20, 23, 26-33). Voigtlander (ref. 19) stated a number of factors that favor the use of bromobutyl rubber over chlorobutyl rubber. They are:

* Superior adhesion and balance of properties (ref. 23);

* increasing use of speed rated fires with lower profiles having higher surface area to air volume ratios;

* requirement for fighter tires to reduce rolling resistance;

* use of high-pressure space-saver spare tires requiting a more impermeable liner;

* better flex-cracking resistance after aging; and

* bromobutyl liners, at half the gauge, are cheaper in material costs.

A typical bromobutyl rubber finer formulation is shown in table 2 (ref. 28). Von Hellens (ref. 23) showed that increasing levels of processing oil and of N660 carbon black increases the permeability of a bromobutyl rubber compound. Alternatively, Navakoski, Juengel and Laube (ref. 31) reported that use of higher loadings (up to 145 phr) of a low surface area carbon black continually improves the permeability of a bromobutyl rubber compound, even at higher oil loadings.
Table 2 - bromobutyl rubber inner liner
formulation (re. 28)

Bromobutyl rubber 100 phr
N660 carbon black 60
Flexon 876 oil 15
Stearic acid 1
Zinc oxide 2
MBTS accelerator 2
Sulfur 1


Star-branched bromobutyl rubber (SB-BIIR) was developed for use in tire inner liner compounds to improve the processability of bromobutyl rubber (ref. 32). The polymer contains two distinct fractions, a linear chain and a star fraction containing as many as 30-40 arms similar in length to the linear chains. The very high molecular weight of the minor star fraction (5-20% by weight) contributes to increased green strength of the polymer without adversely affecting the relaxation process. This results in reduced mixing cycles to yield a similar mix quality, significantly lower die swell at the higher shear rates, improved extrudate surface quality and calendered surface appearance, and reduced shrinkage. In inner liner compounds, the modulus values are slightly higher and the elongation at break values are somewhat shorter.

Brominated isobutylene-co-para-methylstyrene has been evaluated for use in inner liners (refs. 28, 33 and 34) and affords improved heat and flex characteristics in off-the-road tires compared to a 100% bromobutyl rubber inner liner (table 3) (ref. 33).
Table 3 - comparison among inner lines (ref. 33)

 CIIR BIIR BIMS
Polymer 1066 2222

Mooney viscosity, MV 1+4 @ 100 [degrees] 46 44 56
Mooney scorch, T5 @ 135 [degrees] C, min. 13 16 22
T90 (@ 160 [degrees] C, min. 15 12 12
Hardness, Shore A 40 42 40
100% modulus, MPa 1.0 1.0 1.0
Tensile strength, MPa 92 10 9
Elongation @ break, % 715 745 950
Strain energy (TS X elongation)
 Initial 6,578 7,450 8,550
 3 days @ 125 [degrees] C 3,791 4,878 7,986
 4 days @ 100 [degrees] C 4,034 4,075 7,769
 7 days @ 180 [degrees] C 0 0 2,682
Monsanto flex, kc
Initial 360 85 660
 3 days @ 125 [degrees] C 53 23 260
 4 weeks @ 100 [degrees] 25 11 200


Black sidewall

The black sidewall is the outer surface of the tire between its bead and tread that protects the casing against weathering. It is formulated for resistance to weathering, ozone, abrasion and tear, radial and circumferential cracking, and for good fatigue life (ref. 18). Traditionally, a blend of natural rubber and butadiene rubber is used along with carbon black, curatives and a high concentration of antidegradants to provide weather resistance (ref. 35). However, an in situ surface-discoloration problem occurs upon exposure to ozone as a result of using N, N'-disubstituted-para-phenylenediamine antiozonants as protectants (ref. 36)

In order to achieve a high-gloss black sidewall over the life of a tire, inherently ozone-resistant, saturated-backbone polymers are used in blends with diene rubbers. Elastomers such as ethylene-propylene-diene terpolymers (EPDM), halogenated butyl rubbers and brominated isobutylene-co-para-methylstyrene elastomers have been used in conjunction with natural rubber and/or butadiene rubber (ref. 36). The ozone-resistant polymer must be used in sufficient concentrations and also be sufficiently dispersed to form domains that effectively block the continuous propagation of an ozone-initiated crack through the diene rubber phase within the compound.

Halobutyl rubbers have been evaluated for use in tire black sidewalls (refs. 37-43). Lodocsi and Young (ref. 37) used a blend of chlorobutyl rubber, EPDM and natural rubber to improve static and dynamic ozone, and flex resistance of the compound. Particularly good results were obtained when 25% or greater chlorobutyl rubber and 20% EPDM rubber are the elastomers used. Flowers, Fusco and coworkers (ref. 43) reported that a black sidewall consisting of a blend of chlorobutyl rubber for heat and flex resistance, and EPDM rubber for ozone resistance, offered a polymeric protection system that eliminated the need for chemical protectants for a natural rubber/butadiene rubber sidewall compound. Superior dynamic and static ozone resistance was obtained, along with equivalent fatigue crack growth when compared to a natural rubber/butadiene rubber black sidewall compound.

Young, Kresge and Wallace (refs. 40 and 41) made cut growth/tearing energy measurements on four factory-mixed black sidewall compounds, two conventional natural rubber/butadiene rubber formulations and two compounds blended with chlorobutyl rubber. Laboratory results showed excellent tearing energies and cut growth rates for the chlorobutyl rubber-containing black sidewalls (figure 3). Tire test data showed lower cut growth rates for the chlorobutyl rubber blends. Young and Doyle (ref. 42) studied the fatigue crack growth of natural rubber, butadiene rubber, chlorobutyl rubber and bromobutyl rubber, noting that the halobutyl rubbers showed a very large reduction in crack propagation (ca. 100X) upon addition of carbon black.

[Figure 3 ILLUSTRATION OMITTED]

Brominated isobutylene-co-para-methylstyrene is used in non-staining passenger tire black sidewall formulations (refs. 43-48). It has been determined that at least 40 phr of BIMS rubber is needed to protect the natural rubber from ozone attack (ref. 43). The bromination and the para-methylstyrene comonomer levels of the brominated isobutylene-co-para-methylstyrene rubber are important factors for ozone resistance.

Flowers, Fusco and Tracey (refs. 44 and 45) reported on a brominated isobutylene-co-para-methylstyrene rubber/natural rubber/butadiene rubber compound with improved cured adhesion and tear properties by using a compatibilizer and oil blend. Tires built with black sidewalls containing BIMS rubber performed well compared to the general-purpose rubber control compound: DOT testing was comparable, high speed testing was improved, but rolling resistance was also increased. Fleet evaluations indicated that the experimental tires with BIMS sidewalls retained their black color throughout the testing period, while the chemically protected general-purpose rubber sidewall surface discolored to a reddish-brown.

McElrath and Tisler (ref. 48) studied the large-scale internal mixing of a brominated isobutylene-co-para-methylstyrene rubber/natural rubber/butadiene rubber sidewall compound. Transmission electron microscopy followed by image analysis was used to relate the polymer phase morphology to physical properties. They determined that the BIMS rubber phase must be highly dispersed to minimize crack growth, and a three-step remill-type mixing sequence was suitable. Use of a lower bromination level in the BIMS rubber improved crack growth resistance, but also resulted in decreased ozone resistance and adhesion to a carcass compound. However, use of a BIMS rubber having a lower bromination level in conjunction with an increased para-methylstyrene comonomer content resulted in generally improved sidewall compound properties (table 4). Tires containing BIMS elastomers having both lower bromine and higher para-methylstyrene levels in the black sidewall were built and tested (ref. 49), with those having a BIMS robber with ten weight-% para-methylstyrene performing equivalent to a NR/butadiene rubber control in the ozone wheel test. Tires with a BIMS rubber having twelve weight-% para-methylstyrene and 0.85 mole-% benzylic bromide outperformed the NR/BR control in this test. All tires having BIMS elastomers in the black sidewall enhanced tire appearance.
Table 4 - BIMS elastomer black sidewall compound(a) properties
(ref. 48)

 MDX EDU
 93-4(b) 043(c)
Mooney Scorch @ 135% (min.
 to 5 pt. rise 9.1 11.2

ODR @ 160 [degrees] C (1 [degrees] arc)
Maximum torque, dNm 23 21
Ts2, min. 4.0 4.1
T90, min. 22.3 18.3
Stress-strain
Modulus (a) 200%, MPa 3.2 2.8
Tensile strength, MPa 12.6 13.3
Elongation, % 556 618
Shore A hardness 45 45
Fatigue-to-failure (101% elongation,
 100 c/min.) 307,314 594,024
DeMattia flex (pierced, 60 degree
 bend, 300 c/min.)
Crack length, mm @ 4.5 kc 4.5 3.6
 @ 90 kc 8.9 6.0
 @ 2,200 kc 10.7 6.7
1" strip adhesion (avg. peel @
 100 [degrees] C, #/in.)
 to self 115 >200
 to carcass 131 146
Ozone test (40 [degrees] C, 25 pphm
 03, 33 days)
 Static No cracks No cracks
 Dynamic No cracks No cracks
Outdoor flex (27 days) 1 minor edge No cracks
 Crack after 14 days


(a.) Formulation: BR, 50 phr; NR 10; BIMS elastomer, 40:N351 black, 40; oil, 12; tackifying resin, 5; Struktol, 4; SP 1068, 2; stearic acid, 0.5; sulfur, 0.4; zinc oxide, 0.75; Rylex 3011,0.6; MBTS, 0.8.

(b.) 7.5 wt % PMS, 1.2 mol % BrPMS.

(c.) 9.5 wt % PMS, 1.0 mol % BrPMS

Mouri (ref. 50) studied non-staining black sidewall compounds and showed that a blend of brominated isobutyleneco-para-methylstyrene/natural rubber/butadiene rubber performed better than EPDM/NR/BR compounds in laboratory and tire testing. Dispersion of the polymer and fatigue to failure (cut growth) of the BIMS sidewall were superior to the EPDM compounds, even when the EPDM compound was phased mixed.

Mason (ref. 51) reported the use of a blend of brominated isobutylene-co-para-methylstyrene and EPDM rubbers [NR/BR/BIMS/EPDM (40/10/30/20)] to form a non-staining tire black sidewall with improved adhesion to a carcass compound and flex resistance compared to a NR/BR/BIMS (10/50/40) control compound. Tires had improved ozone resistance on an aged ozone wheel test, and high speed and rolling resistance.

White sidewall and cover strip

Chlorobutyl rubber/EPDM rubber/natural rubber blends are used in tire white sidewall compounds (refs. 35 and 52-56), and in white sidewall cover strip compounds (refs. 26 and 56). The chlorobutyl rubber imparts flex resistance to the compounds.

Treads

The tread is the wear-resistant component of a tire which comes in contact with the road. It is designed for abrasion resistance, traction, speed, stability and casing protection. The tread rubber is compounded for wear, traction, low rolling resistance and durability (ref. 18). It is normally composed of a blend of SBR and BR elastomers for passenger tires.

Isobutylene-based elastomers have been studied in treads with blends of natural rubber, styrene-butadiene rubber and butadiene rubber (refs. 26, 34 and 57-67). Sabey and Lupton (ref. 58) measured the friction of nine tread compounds prepared with natural rubber, NR/SBR, oil-extended SBR, butadiene rubber, EPDM rubber and butyl rubber. The butyl rubber compound had the highest friction value on a number of surfaces ranging from a very smooth, very highly polished surface with a texture depth of [is less than] 0.001 inches, to rough coarse textured, polished stones with a texture depth of 0.04 inches. The skid resistance of the butyl rubber compound maximized at about 30 [degrees] C, showing a minimum at -10 [degrees] C.

Cohen (refs. 59 and 60) reported that copolymers of isobutylene and cyclopentadiene improved the wet skid resistance of a carbon black-filled SBR/BR tread compound.

Keller (ref. 61) studied carbon black-filled chlorobutyl rubber/SBR blends, and showed that as the percentage of chlorobutyl rubber used was increased, the rebound decreased linearly and the skid resistance increased linearly. Tire testing revealed that 30 phr chlorobutyl was needed for a 5% improvement in skid resistance; however, at this level the relative treadwear rating was 4% lower than the SBR/BR control in a low severity road wear test. Increased chlorobutyl rubber use further decreased relative treadwear. Hirakawa and Ahagon (ref. 62) evaluated carbon black-filled chlorobutyl rubber/NR/BR tire treads, concluding that lower hysteresis and equal or better wet skid resistance could be obtained by using chlorobutyl rubber.

Fusco and Young (ref. 26) blended bromobutyl rubber with oil-extended SBR in carbon black-filled compounds, improving wet traction, but decreasing treadwear in tire tests (table 5). Similar results were obtained when blending bromobutyl rubber with non-oil-extended styrene-butadiene rubber (SBR 1502) (ref. 63). Wilson (ref. 1) increased compound wet traction based on laboratory dynamic properties using bromobutyl robber in blends with SBR and BR; however, abrasion loss also increased. He concluded that poor abrasion is an inherent property of butyl rubber and that tread compounds containing HIIR will, in general, have poorer wear properties than similar compounds in which the HIIR is not present (ref. 1).
Table 5 - BIIR blend tread evaluation in high
performance tires (ref. 26)

Compound 1 2

SBR 1712 137.5 110
BIIR 2255 0 20
N110 black 85 85
Aromatic oil 17.5 25
Stearic acid 2.0 2.0
6PPD(a) 2.0 2.0
Zinc oxide 3.0 3.0
OBTS(b) 2.0 2.0
Sulfur 2.5 2.5
Mooney scorch, T10 @ 135 [degrees] C 11.8 8.4
Mooney viscosity, M L(2+8) @ 100 [degrees] C 63 77
Rheometer, 160 [degrees] C, 1 [degrees] arc
 MH-ML 25.3 23.9
 Ts2 5.4 5.6
 Tc90 11.3 12.6
Press cure 15 min. @ 160 [degrees] C
 Hardness, Shore A 73 72
 300% modulus, MPa 11.3 11.5
 Tensile strength, MPa 18.5 15.0
 Elongation, % 460 380
Wet traction tire test results
 20 mph, peak (index) 100 110
 60 mph, peak (index) 100 113
Tire treadwear results
 Wear index, crown 100 75
 Wear index, shoulder 100 69
Pico abrasion (mean of 4 samples) 114 106
Dynamic properties @ 25 [degrees] C
12 Hz, [+ or -] 5% strain (mean of 2 samples)
 G', N/[mm.sup.2] 3.72 4.02
 G", N/[mm.sup.2] 1.48 1.60
 Tan [Delta] 0.40 0.40
At 0 [Delta]C, 100 Hz [+ or -] 70N load
 (mean of 2 samples)
 G', N/[mm.sup.2] 16.4 22.1
 G", N/[mm.sup.2] 3.43 3.83
 Tan [Delta] 0.209 0.173

Compound 3

SBR 1712 110
BIIR 2255 20
N110 black 85
Aromatic oil 25
Stearic acid 2.0
6PPD(a) 2.0
Zinc oxide 3.0
OBTS(b) 1.80
Sulfur 2.3
Mooney scorch, T10 @ 135 [degrees] C 8.6
Mooney viscosity, M L(2+8) @ 100 [degrees] C 80
Rheometer, 160 [degrees] C, 1 [degrees] arc
 MH-ML 22.1
 Ts2 6.1
 Tc90 15.4
Press cure 15 min. @ 160 [degrees] C
 Hardness, Shore A 72
 300% modulus, MPa 11.4
 Tensile strength, MPa 15.3
 Elongation, % 390
Wet traction tire test results
 20 mph, peak (index) 111
 60 mph, peak (index) 111
Tire treadwear results
 Wear index, crown 80
 Wear index, shoulder 75
Pico abrasion (mean of 4 samples) 108
Dynamic properties @ 25 [degrees] C
12 Hz, [+ or -] 5% strain (mean of 2 samples)
 G', N/[mm.sup.2] 3.72
 G", N/[mm.sup.2] 1.48
 Tan [Delta] 0.397
At 0 [Delta]C, 100 Hz [+ or -] 70N load
 (mean of 2 samples)
 G', N/[mm.sup.2] 19.3
 G", N/[mm.sup.2] 3.90
 Tan [Delta] 0.202


(a.) Santoflex 13F;

(b.) Santocure MOR

Mroczkowski (ref. 64) studied blends of bromobutyl rubber, star-branched bromobutyl rubber and brominated isobutylene-co-para-methylstyrene in carbon black- and silica-filled SBR/BR compounds. Increased tangent delta values at low temperatures (-30 to +10 [degrees] C) and decreased tangent delta values at higher temperatures ([is greater than] 30 [degrees] C) were obtained compared to a carbon black-filled NR/BR/SBR tire tread composition (figure 4). Laboratory abrasion resistance was comparable. Costemalle, Hous and McElrath (ref. 34) reported that use of bromobutyl rubber or brominated isobutyleneco-para-methylstyrene increased the tangent delta values at 0 [degrees] C of carbon black-filled emulsion-(SBR 1502) and solution-SBR (sSBR 1216)/BR compounds.

[Figure 4 ILLUSTRATION OMITTED]

Zanzig and coworkers (ref. 65) evaluated brominated isobutylene-co-para-methylstyrene in blends with isoprenebutadiene rubber, butadiene rubber, natural rubber with and without styrene-butadiene rubber in silane-coupled silica-filled compounds, and found increased tangent delta values at 0 [degrees] C when using BIMS.

Rogers (ref. 66) reported that use of brominated isobutylene-co-para-methylstyrene and silane-coupled silica-filled in butadiene rubber/styrene-butadiene rubber compounds afforded increased tangent delta values at 0 [degrees] C and decreased tangent delta values at 60 [degrees] C in laboratory tests, with only slight reductions in treadwear based on tire tests using sectional retreads (figure 5). Hojo (ref. 67) used a hydrazide compound and brominated isobutylene-co-para-methyl-styrene to lower the heat generation and improve the wet gripping property of a carbon black and silane-coupled silica-filled natural rubber compound.

[Figure 5 ILLUSTRATION OMITTED]

Automotive applications

Hose

Hose for automotive applications requires an elastomer that is resistant to the material it is transporting, low permeability, low compression set and resistance to the increasing underhood temperatures seen in today's automobiles. Also, there is increasing focus on noise, vibration and harshness (NVH), and therefore, a material that can provide damping to minimize vibration transmission is desirable. Isobutylene-based polymers satisfy all of these characteristics and hence are used commercially in this application. An important application of isobutylene-based polymers in automotive hose is in air-conditioning hose. Their use in some fuel line hose, as well as brake line hose, has been reported.

Air conditioning hose requires a material with good barrier properties to minimize refrigerant loss and reduce moisture ingression. It requires good compression set to help ensure coupling integrity. Also, resistance to high temperatures is required. Damping of compressor vibration and noise is also desirable.

Air conditioning hose is typically a composite construction, made of layers of rubber and rein forcing yam. In some constructions, a layer of thermoplastic resin is as a barrier to refrigerant loss; typically, this thermoplastic resin is a polyamide (ref. 68).

Halobutyl rubber is typically used in automotive air conditioning hose cover largely due to its barrier properties and its resistance to moisture ingression. Pilkington, Cole and Schisler (ref. 69) reported a chlorobutyl rubber compound used as a cover for an air conditioning hose. Use of a chlorobutyl rubber cover provided better resistance to moisture ingression than an EPDM cover, and also was compatible with operating temperatures up to 120 [degrees] C.

Various techniques have been described that improve the adhesion of butyl polymer compounds to various substrates, including polyester, nylon and other materials. Keller and Kuhnhein (ref. 70) show how the use of a blend of a functionalized ethylene polymer with butyl or halobutyl rubber can improve the adhesion to polyester yam typically used as a reinforcing material. The ethylene polymer is functionalized with methyl acrylate and acrylic acid. Shiota and Kitani (ref. 71) report a process for the manufacture of a flexible hose that is suited for use as an automotive air conditioning hose where a butyl rubber and halobutyl rubber blend is used as the intermediate layer between the nylon-based innermost layer and the cover. The need for an adhesive is obviated by the use of the butyl/halobutyl recipe.

Fusco and Kruse (ref. 72) investigated the aging properties of brominated isobutylene-para-methylstyrene (BIMS) and they report improved aging properties over halobutyl and comparable to peroxide cured EPDM. The aging conditions used were one week at 170 [degrees] C. This increased heat resistance makes them attractive for use in automotive hose applications. Costemalle, et. al. (ref. 73) disclose the use of a compound containing BIMS for use in a hose composition. An example which shows the improved physical property retention is shown in table 6.
Table 6-heat resistant BIMS compounds (ref. 73)

Polymer BIMS([dagger])

N326 30
N770 30
Siloxane treated clay 20
ASTM type 104B oil 15
Fatty acid mixture 3
Low molecular weight polyethylene 4
Stearic acid 2
Zinc oxide 0.5
MBTS
TMTDS
DPTHS 1

Cured physical properties, cured 20'
 @ 160 [degrees] C
Hardness, Shore A 55
100% modulus, MPa 1.6
Tensile strength, MPa 12.7
Elongation, % 570

Aged physical properties, aged 168
 hours @ 150 [degrees] C
Hardness, Shore A 65
100% modulus, MPa 2.8
Tensile strength, MPa 10.6
Elongation, % 350

Polymer BIMS([double dagger])

N326 30
N770 3O
Siloxane treated clay 20
ASTM type 104B oil 15
Fatty acid mixture 3
Low molecular weight polyethylene 4
Stearic acid 2
Zinc oxide 0.5
MBTS
TMTDS
DPTHS 1

Cured physical properties, cured 20'
 @ 160 [degrees] C
Hardness, Shore A 61
100% modulus, MPa 2.3
Tensile strength, MPa 11.4
Elongation, % 440

Aged physical properties, aged 168
 hours @ 150 [degrees] C
Hardness, Shore A 73
100% modulus, MPa 4.4
Tensile strength, MPa 10.6
Elongation, % 240

Polymer BIIR(#)

N326 30
N770 30
Siloxane treated clay 20
ASTM type 104B oil 15
Fatty acid mixture 3
Low molecular weight polyethylene 4
Stearic acid 2
Zinc oxide 5
MBTS 2
TMTDS 1
DPTHS

Cured physical properties, cured 20'
 @ 160 [degrees] C
Hardness, Shore A 57
100% modulus, MPa 1.0
Tensile strength, MPa 10.3
Elongation, % 780

Aged physical properties, aged 168
 hours @ 150 [degrees] C
Hardness, Shore A 69
100% modulus, MPa 2.1
Tensile strength, MPa 4.5
Elongation, % 330


([dagger]) 5.0 wt% para-methylstyrene, 0.5 mol% benzyllic bromine; from Exxon Chemical Company.

([double dagger]) 10.0 wt% para-methylstyrene, 0.8 mol% benzyllic bromine; from Exxon Chemical Company.

(#) Exxon Bromobutyl grade 2233, from Exxon Chemical Company.

McElrath and Measmer (refs. 74 and 75) studied heat resistant BIMS compounds, for use in automotive hoses. The use of glycols and hydrotalcite in conjunction with a zinc thiocarbamate accelerator was shown to improve the heat aging properties of BIMS compounds. They reported good physical property retention after aging at 175 [degrees] C, as well as good compression set (ref. 74). Results are shown in table 7. The effects of different compounding additives on reversion and heat aging were also evaluated. Stearic acid is shown to improve the scorch/cure rate balance, as well as improve the heat aging properties (ref. 75). Also, BIMS compounds that use accelerators without zinc in the structure have more scorch safety than those with zinc, yet still retain good aging properties (ref. 75).
Table 7-effect of glycol and hydrotalcite on heat aging of
BIMS compounds

Formulation PHR

BIMS([dagger]) 100
N330 55
Paraffinic oil 5
Polyethylene wax 4
Stearic acid 2
Zinc oxide 1
Zinc Diethyldithiocarbamate 1
Triethylene glycol 2
Hydrotalcite 2
 Aging conditions,
 (Hours/temperature, [degrees] C)
Properties Initial 22/150 70/150

Tensile strength, MPa 14.5 13.4 13.8
Elongation, % 229 216 211
100% modulus, MPa 4.2 4.1 4.6
Hardness, Shore A 60 60 64

Properties 170/150 96/175 70/200

Tensile strength, MPa 13.1 12.5 6.9
Elongation, % 161 162 101
100% modulus, MPa 6.0 6.0 6.8
Hardness, Shore A 73 69 67


([dagger]) BIMS - Exxpro MDX 90-10 by Exxon Chemical Company

Isobutylene-based polymers are also used in automotive hose applications other than air conditioning. Stefano and Arvada (ref. 76) detail the use of a halobutyl interlayer in a hose that is especially suited for automotive coolant systems. The halobutyl interlayer is used to bond dissimilar rubbers that are used in the tube and cover. The construction with the halobutyl rubber interlayer had lower coolant loss than those without.

Trexler (ref. 77) reported on bromobutyl compounds for use in brake hose applications. Of several materials tested, bromobutyl rubber provided the most resistance to Delco Supremen II brake fluid, and was also impermeable to the fluid.

A bromobutyl compound is reported by Dunn (ref. 68) that has good resistance to alternate fuels such as methanol and a M85 (85:15 methanol:gasoline blend). Data are shown in table 8 that compare the utility of a bromobutyl compound versus a nitrile compound in hose for this application. The bromobutyl has better retention of physical properties and lower permeabilty than the nitrile control compound.

Table 8-comparison of bromobutyl and nitrile compound in alternate fuels (ref. 68)
Bromobutyl(dagger]) 100
NBR([double dagger]) 100
Stearic acid 1 1
N550 70
N762 75
Atomite 30
Magnesium oxide 0.3
DOP 5
MBTS 1
Zinc oxide 3 5
Sulfur 1.25
TMTD 0.4
TMTM 0.5

Physical properties, cured 10'
 @ 166 [degrees] C
Hardness, Shore A 75 68
100% modulus, MPa 2.9 3.4
300% modulus, MPa 9.0 15.4
Tensile, MPa 9.5 19.5
Elongation, % 320 440

Aged in methanol, 168 hrs.
 @ RT-change
Hardness, )pts.) -2 -8
Tensile strength, (%) +4 -22
Elongation, (%) +5 -31
Volume (%) -2 +11

Aged in M85, 168 hrs.
 @ RT- change
Hardness, (pts.) -26 -16
Tensile strength, (%) -21 -37
Elongation, (%) -22 -44
Volume (%) +29 +24

Aged in Fuel C, 168 hrs.
 @ RT- change
Hardness, (pts.) -43 -26
Tensile strength, (%) -63 -56
Elongation, (%) -67 -59
Volume (%) +220 +51

Permeability (weight loss in
 grams after 14 days)
Methanol 0.2 1.48
M*% 0.42 4.20


([dagger]) Bromobutyl - Polysar Bromobutyl 2030

([double dagger]) NBR - Polysar Krynac 3450

Automotive dynamic parts

Isobutylene-based polymers are used for various types of automotive mounts because of their ability to damp vibrations from the road or engine. Warley has published data that compare the dynamic properties of butyl rubber versus those of natural rubber (ref. 78). Reed and Warley show that for equal dynamic spring rate to static spring rate ratios ([k.sub.d]/[k.sub.s]), the butyl compound will exhibit higher damping or hysteresis (ref. 79).

Butyl and halobutyl rubber is used in automotive mounting applications where a degree of damping is required. Some uses include body mounts and in medium damping engine mounts (ref. 80). Exhaust hanger straps also use halobutyl due to its heat resistance. Dunn has published work on a bromobutyl compound cured with zinc oxide and diamine antioxidants.

Martin discloses a resilient engine mount in which a high damping material is used in conjunction with a resilient material to resist transmission of vibrations from the engine at both high and low amplitude vibrations. The high damping material is preferably an isobutylene-based rubber (ref. 81).

Tabar and Kilgoar (ref. 82) disclose a soft, fatigue resistant compound that uses a blend of natural rubber and bromobutyl as its base. Also, the compound uses uncured polyisobutylene as an additive to improve its durability. Blends of NR/BIIR showed greater fatigue resistance than the natural rubber compound. The addition of polyisobutylene to a natural rubber and bromobutyl rubber blend also gave improved fatigue resistance.

McElrath and Measmer (refs. 83 and 84) report on the use of natural rubber and BIMS blends to achieve improved heat aging over conventional natural rubber compounds. It is shown that the addition of BIMS increases the damping at low temperatures, but at room temperature, the effect is slight, and at elevated temperatures, there is no increase. Also, blends of BIMS and natural rubber provide more stable dynamic properties after heat aging than the natural rubber control, and they also delay the onset of stiffening. Heat aging properties are also improved.

Mouri (ref. 85) has developed an application that makes use of butyl rubber's damping properties. In it, an elastomeric coupling, made of butyl rubber blended with natural rubber, is used to transmit steering wheel input to the tires. It is said to improve the steering response and give improved driving feel.

Summary

Isobutylene-based elastomers have a unique combination of material properties that are exploited for use in automotive applications. Some of these properties include low permeability to gases and liquids, high damping and chemical resistance. Halogenated isobutylene-based-elastomers have these same properties, and also high heat resistance. Brominated isobutylene-co-para-methylstyrene have all of these properties, but with even greater heat resistance and inherent ozone resistance.

Tire inner liners make use of halobutyl rubber's barrier properties and its ability to adhere to diene-rubber based carcasses. BIMS is used in blends with other elastomers in tire black sidewalls to eliminate the need for chemical protectants, which thereby eliminates the bloom of these protectants and the staining of the sidewall. Blends containing chlorobutyl rubber are also used in white sidewall compounds and white sidewall coverstrips due to their good flex properties. The use of blends of BIMS with other rubbers are being evaluated in tire treads to improve traction without an increase in rolling resistance.

Other automotive applications include hose and dynamic parts. Hose applications make use of halobutyl rubber's low gas and liquid permeability, and good heat resistance and compression set. BIMS has shown promise in hose applications due to its further improved heat aging properties. Body mounts use butyl rubber because of its high damping. Blends of BIMS and natural rubber are being evaluated as a means of improving the heat resistance of engine mounts.

References

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John E. Rogers and Walter H. Waddell
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Author:Waddell, Walter H.
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