Advances in NR science and technology.

Advances in NR science and technology

There have been many advances in the field of natural rubber science and technology over the past decade, but it is necessary within the constraints of one article to restrict it to highlighting some of the more significant developments. These include the modification of natural rubber by various means. First, by epoxidation to give a new polymer with properties different to those of the original rubber. Epoxidized natural rubber has oil resistance, gas permeability and damping performance more like some of the specialty rubbers, while for tire applications, wet traction is improved without the loss of the low rolling resistance characteristic of natural rubber. Secondly, controlled oxidative degradation to produce liquid natural rubber of very low molecular weight, which is attractive to the tire manufacturers as a vulcanizable process aid. New materials are now available. Thirdly, thermoplastic natural rubber produced by blends with polypropylene. Hard compounds with small proportions of natural rubber are low temperature impact resistant materials, while the softer grades with a larger proportion of rubber are slowly taking over a part of the vulcanized rubber market owing to their simple processing and recycling characteristics. Blends of natural rubber with synthetic rubbers have also received considerable attention, and a technique has not been developed whereby the crosslink density in each rubber in the blend can now be measured. Although in the majority of its applications, such blending of natural rubber is commonplace, there exists little knowledge of the phase morphology of the blends or of the distribution of curing ingredients or black. A method whereby the crosslink density in each polymer can be assessed has indicated substantial differences when the rubbers have a marked difference in polarity.

No article on natural rubber can omit developments in the field of tires where over 60% of polymer is used. Since the advent of the pre-cure process for truck tire retreading, natural rubber based compounds have been largely replaced by all-synthetic formulations which have been found to give excellent wear characteristics under low severity conditions. Until recently, reversion of natural rubber in service was the prime cause for its inferior wear performance for high mileage treads. However, development of the cure system by additional fatty acid has arrested this problem, and mileage comparable to synthetic rubbers can now be obtained with the added benefit of lower rolling resistance. Leading in from the use of oil-extended natural rubber in winter tires, its value in the fast developing all-season tire has been assessed. Systematic replacement of oil-extended SBR in all-season tires with oil-extended NR gives improved ice-traction, lower rolling resistance, no loss of wet grip and better wear at temperatures below 15 [degrees]C. However, during the summer months, when ambient temperatures are above 15 [degrees]C, wear rates become higher than those of oil-extended SBR.

Finally, there have been significant developments in the engineering uses of rubber in particular for base isolation applications. It is over 30 years ago since bridge bearings regularly became made from natural rubber, both to isolate the traffic deck from the piers and to accommodate the horizontal movement of the bridge caused by changes in ambient temperature. This highly successful use of NR in these laminated bearings led to base isolation per se, where many actual buildings were placed on natural rubber mounts to isolate them, for example, from the London underground. The most recent development has been the extension of this concept to seismic bearings, where buildings are isolated from earthquakes by placing them on suitably designed natural rubber steel-laminated bearing. The Foothill Communities Law and Justice Center in San Bernadino was one of the first buildings to be constructed on such bearings, which are now becoming increasingly used in many other parts of the world.

As this is essentially a review article, no experimental details will be given, although reference to the original papers listed will provide the appropriate information where needed.

Results and discussion

Epoxidized natural rubber

It has been known for many years that chemical modification can alter the physical properties of natural rubber quite substantially. For example, ethyl N-phenylcarbamoylazoformate (ENPCAF) when caused to react with natural rubber in up to 15 mole % has been shown (ref. 1) to reduce resilience, improve linear swelling in petroleum ether and reduce permeability to nitrogen. However, ENPCAF was a relatively expensive reagent and, above all, it was found that it could not be stored on a large scale. One cheap method of modifying NR is epoxidation. This reaction goes back to 1922 when it was first discovered by Pummerer and Burkard (ref. 2). In spite of the process being extensively studied by many workers, it was not until Gelling (ref. 3) controlled the reaction to eliminate ring opening that a useful material was discovered. Although in theory any degree of epoxidation may be achieved, it is thought that initially interest will center on two `grades.' These are 25 and 50 mole % epoxidized natural rubber (ENR-25 and ENR-50, respectively).

Both materials should really be considered in their own right as new polymers because their properties are significantly different to those of natural rubber itself. As with the ENPCAF modification, the epoxidation reaction was found (ref. 4) to alter the glass transition temperature (approximately 1 [degree] per mole %) consequently reducing room temperature resilience, improving oil resistance and reducing gas permeability. Because of this, there has been a tendency to compare ENR with the relevant specialty synthetic rubbers. For example, 50 mole % epoxidized NR is comparable in oil resistance to a medium nitrile rubber (table 1), although the ENR-50 has a higher tensile strength since, like NR, it is a strain crystallizing rubber. Also, it has been shown (ref. 5) that 50 mole % epoxidized NR has similar air permeation properties to butyl rubber (figure 1), although its low temperature properties make it unsatisfactory for articles such as tire inner liners in cold climates. Its high damping characteristics, however, make it suitable for many engineering applications, where it can be blended to the customers' needs, although it is incompatible with many other rubbers owing to its polar nature.

TABLE : Table 1 - oil resistance of ENR compared to NR and NBR (% volume increase after 4 days at 23 [degrees]C)

                   NR    ENR-25    ENR-50    NBR
ASTM no. 1 oil     15      12        0.1     0.2
ASTM no. 2 oil     28       3        0.6     0.2
ASTM no. 3 oil     78      40        1.1     0.7

In the longer term, 25 mole % epoxidized natural rubber may have potential in high performance tire treads. It has been shown by Morton and Krol (ref. 6) that most general purpose rubbers fall on a single line when wet grip is plotted against rolling resistance; i.e., any attempt to improve the wet grip of a tread compound results in that compound exhibiting a higher rolling resistance. With automobile manufacturers seeking high performance tires with improved wet grip and yet requiring lower rolling resistance for improved fuel economy, the synthetic rubber industry is tailoring its new polymers to meet this demand. Owing to its hysteresis/temperature profile (ref. 7), it has been observed that ENR-25, especially when reinforced with black and silica, can provide an almost unique combination of high wet grip and low rolling resistance (figure 2). A this time, the wear performance requires to be improved to match current tread compounds, but this is expected to be achieved by suitable blending with polybutadiene rubber (BR). Epoxidized NR is now in production in Malaysia under license to Kumpulan Guthrie Berhad.

Liquid natural rubber

Liquid natural rubber (LNR) has been available for many years, but there has been a resurgence of interest in it by the tire manufacturers for use as a vulcanizable process aid. Liquid natural rubber is generally made by a depolymerization process employing a peptizer and long mixing times in an internal mixer at 120 [degrees]C. Depending upon conditions, liquid rubbers of various viscosities or molecular weight are obtained. The process is dirty and the liquid rubber colored and of limited use. More recent work by IRCA (ref. 8) uses phenylhydrazine at 70 [degrees]C in an oxidizing medium on latex, which produces LNR with a molecular weight, according to time, of 3,000 to 20,000. Small commercial amounts are available from Cote d'Ivoire and tire companies are now assessing the potential of this new cleaner material as a vulcanizable process aid for hard compounds. A process has also been developed in our laboratories (ref. 9) which produces very clean liquid natural rubber with a molecular weight ([M.sub.n]) of about 8,000 (table 2). Its molecular weight distribution compared to that of the original SMR gum rubber and `overmilled' material is shown in figure 3. This material is almost as clean as the liquid polyisoprene marketed by Kurare, and is currently under scale up at the Rubber Research Institute of Malaysia.

TABLE : Table 2 - molecular weight of liquid NR compared to other NRs

             [M.sub.n]   [M.sub.w]   [M.sub.z]     d     Gel%
SMR L         236,000     836,400    1,337,000    3.54   15.7
Milled NR     154,200     424,700      710,100    2.75    0.4

Overmilled

NR            100,300     195,400      329,200    1.95    0.0
Liquid NR       8,130      26,300       61,300    3.24    0.1

Thermoplastic natural rubber

Thermoplastic elastomers (TPEs) are a rapidly expanding group of new materials (10-15% per annum) and are likely to take over a significant proportion of the current vulcanized rubber market in the coming years. They offer several advantages over thermoset rubbers requiring no compounding or vulcanization time, scrap can be recycled and production rates are very high. Thermoplastic natural rubber blends are a group of thermoplastic elastomers prepared by blending NR with a polyolefin, particularly polypropylene, in varying proportions (ref. 10). Basically, two distinct types are obtainable. First, semi-rigid rubber modified polypropylenes when the rubber content is low, and secondly, softer thermoplastic elastomers per se when the rubber content is high.

The hard grades are rubber toughened forms of polypropylene with flexural moduli in the range 300-1,000 MPa (ref. 11). These materials are often used in automotive applications when low temperature impact strength is required, especially where the vehicles are subject to very cold climates. In this respect, natural rubber based materials offer advantages over EPDM/polypropylene materials on account of natural rubber's inherent low temperature performance. This is indicated in figure 4 where it can be seen that the Izod impact strength of hard TPNR is higher than that of the EPDM based equivalent at temperatures down to -50 [degrees]C. Thus, if low temperature impact strength is of critical importance, NR modified polypropylene will provide the best performance.

The greater interest, however, in thermoplastic natural rubber lies in the softer blends containing higher proportions of NR. These can range in hardness from 50 to 90 Shore A according to the NR content. Some typical properties are shown in table 3. The blends are dynamically vulcanized to retain natural rubber's good strength and recovery properties, and indeed it has been shown (ref. 12) that TPNR differs from other dynamically vulcanized blends in having a continuous NR phase. In spite of this, resistance of TPNR to aging is better than that of most general purpose vulcanizates, as shown by accelerated aging data. For example, a 70 Shore A TPNR retained 96% of its modulus, 93% of its tensile strength and 95% of its elongation at break after 14 days at 100 [degrees]C. Ozone resistance of TPNR is also good, even the softest blends showing no cracking after seven days at 20% strain and 40 [degrees]C in 100pphm ozone. These materials are now commercially available as `DVNR' from Teknor-Apex Co. (U.S.) and Vitacom Ltd. (U.K.), and represent an attractive cost/performance profile.

TABLE : Table 3 - typical properties of TPNR thermoplastic elastomers

Hardness, Shore A 50 60 69 80 90

Modulus at 100% elong.,

  MPa                             3.1    4.1    5.3    6.3    8.2
Tensile strength, MPa             6.5    8.8   11.2   13.2   15.3
Elongation at break, %            285    315    340    370    405
Tear strength, die C, kN/m         22     31     38     43     51
Tenset, 10m at 100%, %              9     10     14     18     23
Comset, 24h at 70 [degrees]C, %    30     35     36     41     55

Rubber-rubber blends

In a very high proportion of its applications, natural rubber is blended with other rubbers and, in the majority of cases, the formulations for the cure system have been derived empirically without any real knowledge of the distribution of crosslinks between the polymers in the final vulcanizate. Although the distribution of blacks has been widely studied, for example by Cotton (ref. 13), there has been no technique established to determine the distribution of crosslinks in a blend. However, this problem has recently been addressed and solved by Loadman and Tinker (ref. 14). They have demonstrated that the peak broadening in the continuous wave [sup.1.H] NMR spectra of swollen vulcanizates ran be related to the degree of crosslinking in gum vulcanizates of natural rubber and acrylonitrile rubber. Figure 5 shows the olefinic signal in the NMR for swollen NR vulcanizates at two different crosslink densities. The peak broadening in the more highly crosslinked vulcanizate has been found to be linearly related to crosslink density. By making up a series of vulcanizates of NR and NBR with different known crosslink densities, the peak broadening can be calibrated and hence the crosslink density in blends of the two determined. By this means it was discovered, perhaps not unexpectedly from the different polarities of the rubbers, that in blends of NR and NBR, there is a strong preferential distribution of crosslinks in the NBR phase. Indeed, it was found that the NR phase was crosslinked to a much lesser degree than was originally thought. This technique is now being extended to blends of natural rubber with other rubbers, and has been shown to be also valid in black-filled vulcanizates, albeit considerably more difficult to carry out. Ultimately this technique can be used to monitor the crosslink density of natural rubber in blends, and hopefully redress imbalances by suitable choice of curing ingredients with appropriate solubilities in each polymer.

Natural rubber in truck tire retreads

Few heavy duty truck tires are essentially made from natural rubber for several major reasons; notably for its high green strength, tack and adhesion properties essential for maintaining green tire uniformity during building; for its superior adhesion to brass-plated steel cord; for its low heat generation in service; for its low rolling resistance; and for its high resistance to cutting, chipping and tearing. Until 20 years ago, such tires were also retreaded with NR based stock, since, again, its tack was required for building and its low heat generation for the thick shoulder region of the tire. However, about that time the pre-cured tread process was developed where the tread is pre-cured as a flat strip and then bonded onto the buffed tire with a cushion gum, so eliminating the need for natural tack in the compound. At the same time, the thickness of the shoulder region was substantially reduced, obviating the need for low heat build-up properties. As a result, SBR/BR formulations were adopted and found, particularly in America, to exhibit extremely good wear performance under low severity conditions (long haul/trailers).

In the early 80s, it was established (ref. 15) that a normal semi-EV 80/20 NR/BR formulation exhibited reversion during the long service life of retreads subjected to low severity conditions. Indeed, its wear-out mileage was only 72,000 km compared to 113,000 km for an SBR/BR reference tread. However, it was discovered that if the level of stearic acid in the 1.2 S/1.2 CBS semi-EV cure system was increased from 2 to 6 parts phr, the laboratory abrasion on overcure was unchanged and on the road in service trials, tread lives were increased by up to 42% to 103,000 km under the low severity conditions.

In the mid-80s a second service trial was mounted (ref. 16) in which the high stearic acid formulation was again examined, but this time in formulations containing 35 parts BR. Two 65/35 NR/BR compounds were studied, one with normal black and oil (55/8) and the second with high black and oil (62/12). This latter compound, under the low severity conditions, outlasted the SBR/BR control with a tread life of 118,000 km against 111,000 km for the all-synthetic formulation. However, the same treads were also examined under high severity conditions, where NR normally reigns supreme, and in this case the 65/35 NR/BR compound was only slightly superior to the SBR/BR control (63,000 km and 59,000 km, respectively).

To improve this wear performance under high severity conditions, a third service trial was run (ref. 17) in which a high zinc stearate formulation was studied in a more conventional cure system. The change to zinc stearate was to eliminate the bloom observed with the high stearic acid formulations, while the more conventional cure system was considered likely to improve wear characteristics under the high severity conditions. Laboratory data indicated that this formulation still exhibited good retention of abrasion resistance on overcure. In this trial (table 4), two NR/BR ratios were examined, 80/20 and 60/40, where the former contained N110 black, and the latter N110 and N234 blacks with the N110 preferentially located in the NR phase. The results of this trial are shown in figure 6. Unfortunately, not all the compounds could be tested on all the test vehicles. However, all were examined under a range of test conditions, essentially high and low severity on tippers and very low severity on trailers. Figure 6 indicates that the 60/40 NR/BR compound outlasted the best European commercial synthetic tread under high severity 4th axle tipper conditions by 53,000 km compared to 46,000 km. It also gave a marginally longer tread life compared to the 80/20 NR/BR compound on the less severe 3rd axles. In the very low severity trailer application, again the 60/40 NR/BR compound gave a higher mileage than the SBR/BR commercial compound (154,000 km to 140,000 km), while on the extremely low severity trailer test 1st axle, it gave an outstanding 275,000 km tread life.

TABLE : Table 4 - tread compounds for heavy duty truck tires (third retread trial)

                      80/20      60/40
                      NR/BR      NR/BR       SBR/BR
SMR 20                   80         60
Medium cis 1,4 BR        20         40           40
SBR 1712                                       82.5
Struktol A82            1.2        1.2
N110 black               50         30
N234 black                          25           55
High aromatic oil         2                       5
Zinc oxide              4.2        4.2            4
Zinc stearate           6.6        6.6    Not known
Antidegradant*            2          2            2
Sulphur                 2.2        2.1    Not known
MBS                     0.8        0.9    Not known

*N-(1,3-dimethylbutyl)-N'-phenyl-p- phenylenediamine

These trials indicate that natural rubber-rich truck tire tread compounds have now been developed which are capable of outperforming the best synthetic treads in wear resistance over a range of European test severities. In addition, these compounds exhibit lower rolling resistance, thereby improving fuel economy for the truckers. It has also been found that for normal unmodified cure systems, wear is adversely affected through the long-term effects of heat generation in service, and not by the effects of surface overcure during manufacture as originally thought.

Oil-extended natural rubber in all season tires

The use of oil-extended natural rubber (OENR) in winter tires was established in the 70s in both Europe and America to provide ice traction without need for studs. Since that time, changing technology has seen the introduction of all-season tires and the advent of low rolling resistance requirements. Oil-extended NR has therefore been examined as a replacement for oil-extended SBR in all-season tires (ref. 17).

Table 5 shows the compound studied as OESBR is substituted systematically in steps of 20 parts by OENR, from 80/20 OESBR/BR to 80/20 OENR/BR. Clearly, ice and snow traction will improve as the proportion of OENR increases, as will other low temperature properties, but other tire criteria require to be studied, notably the effect of OENR on rolling resistance, wear and wet traction.

Rolling resistance and wet traction - New 165 x 13 steel radial tires were retreaded with the five compounds in table 5 and tested accordingly. The rolling resistance measurements were made on a tire test rig, and the indices for the various compounds, using the OESBR/BR control as 100, are shown in figure 7. It was observed that rolling resistance was reduced by some 8-12% on the substitution of OESBR by 20, 40 and 60 parts of OENR, but considerably more (25%) when replaced altogether by OENR. The same tires were then placed on a Schallamach trailer and tested for wet grip on the MIRA proving ground by measurement of skid path lengths from 40 mph on asphalt. It was found that there was no change in wet grip within experimental error as the OESBR was replaced by OENR, i.e., all five compounds in table 5 exhibit equally good wet traction performance. [Tabular Data Omitted]

Wear - The test tires were subjected to two series of trials. One, trailer testing at temperatures below 15 [degrees]C, and the second, on actual passenger cars at temperatures generally above 15 [degrees]C. These results are shown together in figure 8. At below 15 [degrees]C, there was no change in wear rating when up to 40 parts OESBR were substituted by OENR, but at higher substitution levels, the addition of OENR gave 10% improvement in wear. However, as would be expected since NR is known to be inferior in wear performance to SBR above 15 [degrees]C, the trials on passenger cars showed that under normal temperature conditions, the successive substitution of OESBR by OENR gives increasingly reduced wear performance.

On the other hand, 40/20/40 OENR/BR/OESBR compound only showed a 5% loss in wear, and it is unlikely that over a whole calendar year, the average motorist would detect this, bearing in mind that during the winter months there would be no loss of wear performance. These trials, therefore, indicate that up to 40 parts OENR can be incorporated into all-season tires to improve ice and snow traction and other low temperature properties with no change in wet grip, a small improvement in rolling resistance, and probably insignificant loss in wear performance over any 12 month period.

Natural rubber in seismic bearings

The use of natural rubber in engineering applications is an ever expanding story. However, one of the most interesting developments is its initial use in bridge bearings which moved on to base isolation of buildings, and now to seismic isolation.

The first British natural rubber bridge bearings were used in the construction of the Pelham Bridge (ref. 18) in 1957, when the traditional steel bearings were replaced with steel laminated rubber bearings. This departure from conventional practice was the result of collaboration between W.S. Atkins and Partners, consultant engineers for the bridge, NRPRA as it was then, and the Andre Rubber Company who manufactured the bearings. All the initial concern over the use of these rubber bearings proved to be totally unfounded when in 1965, after eight years in service, the bearings were examined and found to be free from any surface cracks combined with negligible creep. Since that time, many bridges and fly-overs have been built on natural rubber bearings the world over, and their durability and longevity proved without doubt.

The second stage in the development of natural rubber bearings was in the mid 60s when a block of flats, `Albany Court' in London, became the first building in the U.K. to be isolated from low-frequency groundborne vibrations (ref. 19). At that time, due to the value of land, there was considerable pressure to develop over railways where noise under redevelopment, and the previous building on the site suffered clearly perceptible vibrations from the London Underground, totally unacceptable to the standards of amenity of the day. To isolate the building, special versions of the rubber bridge bearings were devised with the metal plates completely embedded in the rubber. There were additional problems too of designing the bearings to take into account possible wind-excited vibrations. The total load was about 1,400 tons and the largest bearing supported 216 tons (see figure 9). Creep was found to be no problem with a maximum movement of 1/8 in. over 50 years, less than the normal settlement of a building, and the differential between one bearing and its neighbor would only be a small proportion of this.

It was perhaps not surprising, therefore, that this development of the 60s led to the latest development of the 80s to place buildings on natural rubber bearings to protect then from earthquakes (ref. 20). The success of the bearings to isolate buildings above from traffic noise and vibration was built upon to study the feasibility of designing bearings for seismic isolation, and this ultimately led to shaking table tests using model bearings in a joint research program between MRPRA and the Earthquake Engineering Research Center of the University of California, Berkeley.

Following a delegation from San Bernadino to the U.K. in 1983, to examine several of the buildings in London where natural rubber bearings had been successfully used in civil engineering projects, it was finally decided to use natural rubber seismic base isolation for the prestigious Foothill Communities Law and Justice Center.

This building, completed in 1985, stands on 98 large circular bearings, each 760 mm in diameter and 400 mm high. They are manufactured in a range of stiffnesses to carry loads ranging from 32 to 270 tons. Under normal conditions, the bearings will support the building rigidly, but in an earthquake, they allow the building to move up to 450 mm in any horizontal direction. This freedom of movement results in much smaller forces being transmitted to the building.

Even the maximum credible earthquake should cause no significant damage to the building or its contents. There has already been a minor earthquake in the region since the Law and Justice Center was built, and the data from this is shown in table 6. The benefits are clear. Several other buildings around the world are now being or have been erected on natural rubber bearings of this type and it is hoped that this new application of natural rubber will lead to the saving of many lives in the future. [Tabular Data Omitted]

Conclusion

It is hoped that this article has indicated some of the more important developments in the science and technology of natural rubber over the past decade. Although the oldest rubber, it has been shown how natural rubber can be modified, both chemically and physically, to meet the challenges of the latest synthetic rubbers and the demands of the modern world.

Epoxidized natural rubber, liquid natural rubber and thermoplastic natural rubber are all new materials widening the scope of natural rubber's potential for the future. Meanwhile, modern compounding techniques have been developed to enable natural rubber formulations to match synthetic rubber treads in wear performance for heavy duty truck tire retreads and to provide snow and ice traction in all-season tires. The base isolation capability of natural rubber has been extended to protect buildings against earthquakes - helping mankind to survive against nature itself.

References

[1.] D. Barnard, K. Dawes and P.G. Mente, Proc. Intern, Rubber Conf. Kuala Lumpur, 4,215 (1975). [2.] R. Pummerer and P.A. Burkard, Ber. 55,3458 (1922). [3.] I.R. Gelling, Rubber. Chem. Technol. 58,86 (1985). [4.] C.S.L. Baker, I.R. Gelling and R. Newell. Rubber Chem. Technol. 58,67 (1985). [5.] I.R. Gelling, Elastomerics, 18 (June 1989). [6.] P.F. Morton and L.H. Krol, presented at a meeting of the Rubber Division, American Chemical Society, Chicago 1982; abstract in Rubber Chem. Technol. 56.508 (1983). [7.] C.S.L. Baker, I.R. Gelling and I.R. Wallace, Elastomers, 25 (August 1989). [8.] R. Pautrat and J. Leveque. Proc. of UNIDO-sponsored Symposium on Powdered, Liquid and Thermoplastic Natural Rubber, Phuket, 207 (1981). [9.] 49th Annual Report, MRPRA (1987). [10.] D.J. Elliott and A.J. Tinker, Blends of natural rubber with thermoplastics. Natural Rubber Science and Technology, ed A.D. Roberts, Oxford University Press. ch. 9, p. 327 (1988). [11.] A.J. Tinker, NR Technology, 18, 30 (1987). [12.] A.J. Tinker, R.D. Icenogle and I. Whittle, Rubber World, 25 (March 1989). [13.] G.R. Cotton, Proc. of Seventh Australian Rubber Convention (April 29 - May 1, 1986). [14.] M.J.R. Loadman and A.J. Tinker, Rubber Chem. Technol. 62,234 (1989). [15.] D. Barnard. C.S.L. Baker and I.R. Wallace, Rubber Chem. Technol. 58,740 (1985). [16.] C.S.L. Baker and I.R. Wallace, Proc. of Seventh Australasian Rubber Convention (April 29 - May 1, 1986). [17.] C.S.L. Baker, I.R. Gelling and I.R. Wallace, Elastomerics, 20 (July 1989). [18.] Rubber Developments, 19, 30 (1966). [19.] Rubber Developments, 19, 91 (1966). [20.] Rubber Developments, 36, 101 (1983).

PHOTO : Figure 1 - comparative air permeation properties of various epoxidized natural rubbers

PHOTO : Figure 2 - Morton and Krol type diagram of wet grip against rolling resistance for ENR-25 compared to NR and OESBR

PHOTO : Figure 3 - molecular weight distribution curve for liquid NR compared to other NRs

PHOTO : Figure 4 - low temperature impact strength of TPNR

PHOTO : Figure 5 - broadening of the olefinic signal in the continuous wave `H NMR spectra for swollen NR gum vulcanizates caused by two different crosslink densities

PHOTO : Figure 6 - wear performance of heavy duty truck tire retreads (MRPRA third service trial)

PHOTO : Figure 7 - relative rolling resistance of various all-season compounds

PHOTO : Figure 8 - relative wear ratings of various all-season compounds above and below 15 [degrees]C

PHOTO : Figure 9 - position of rubber bearings isolating Albany Court