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Precured urethane treads are here.

by Peter Astles, Compounding Ingredients, Ltd., and E.N. Gomberg, Synair Corp.

The development of a successful urethane to rubber adhesive makes the commercial application of urethane precured treads a reality. They look the same as precured rubber treads. They feel the same. They are applied exactly the same by the retread shop. They just run twice as far on the road.

The precured urethane tread (TyrKast) system has been under continuous test for the past four years. Tires recapped with urethane treads have passed all tests conducted by the Malaysian Rubber Producers' Research Association for on-the-road truck tires. The test results indicate that the experimental urethane treads will run 140,000 to 150,000 kilometers versus the rubber tread control tires which are estimated to run 70 to 80 thousand kilometers.

In order to evaluate the performance of precured urethane treads in different climates and operating conditions, testing is currently under way in South Africa and is shortly to commence in Malaysia, Indonesia, Thailand and India. Maxipress in South Africa has been running experimental 11.00 x 22.5 urethane tread capped tires for the past five months. They see no evidence of wear.

Synair, in the U.S., had developed and patented special adhesives designed to bond a self-healing urethane liner to the butyl liner inside pneumatic tire casings as an essential part of the TyrLyner system. The complex application procedure of these adhesives was not suitable for use in the average recap shop for applying precured urethane treads to buffed tire casings. The challenge of developing a suitable bonding system for the retreader that would meet the extreme demands of an on-the-road tire was taken up.

The new proprietary adhesive which was finally developed, (TyrBond RC-1), met all the necessary performance parameters. With this new one component adhesive it was possible for the first time to create a truly vulcanized bond between polyurethane and rubber.

The unique characteristics of the new proprietary adhesive coupled with the precured urethane tread elastomer system opens a new field for the manufacture of bead to bead recapped tires as an alternative to recapping with precured treads.

The two viable alternatives, direct casting or a premolded polyurethane tread, each have advantages and disadvantages. Direct casting requires expensive mixing and dispensing equipment for injecting the liquid precured urethane tread polyurethanes into a mold. However, the precured urethane tread elastomer is exothermic. It requires no external heat source to cure and bond to the rubber casing. The initial investment in meter mixing equipment and molds would be offset by the energy savings of a system that requires no other heat input.

The advantage of the premolded tread is that it is a tried and proven system with which most recap shops are familiar. Its disadvantage is that it is more labor intensive and requires substantial energy input.

Retreading or remolding of pneumatic tires, particularly car and trucks, is well accepted nowadays not only as a cost effective alternative to new tires, but also as a means of recycling used tires. The problems of disposing of old tire casings is a worldwide concern, with the limited number of alternative disposal methods available all raising environmental issues. The simple fact is that the recycling of used tire casings through retreading or remolding processes is the most satisfactory method from the environmental standpoint.

Of the two established methods of tire retreading, hot cure and precure, it is the precure method which is now the most widely used, primarily because of the relatively low capital investment necessary to set up a manufacturing unit. In Scandinavian countries, precure retreading now predominates, accounting for 90% of the market, and in most other countries the trend is more and more towards adoption of the precured method.

A standard 11.00 x 22.5 tread matrix was obtained, and with minimal modification a number of polyurethane treads were cast. The polyurethane system used was selected after a series of laboratory tests to determine the optimum mix of properties to offer the most technological and cost effective performance. The properties evaluated included hardness, compression set, tensile and tear strength, cut resistance, abrasion resistance, coefficient of friction and hysteresis. The formulation finally selected was referenced T-K 65.

Experimental test tires were built initially at Colway Tires, Bradford, and later at the Malaysian Rubber Producers' Research Association. The initial work at Bradford convinced all parties that the only commercially acceptable product would be one virtually identical in usage to that of current NBR/SBR treads. The concept that the tire builder need not necessarily know whether he was building a tire using a conventional NBR/SBR tread or a PU precured tread became the main objective once we had developed the adhesive and optimized the polyurethane.

Briefly, the tire building process involves locating the buffed casing on a tire building machine, priming the buffed casing with a typical tack (tie) cement and drying as for any other tread. Once the tack cement is dry, the PU tread is unpackaged and located on the tread using a minimum amount of stretch. Cutting the thread is preferred (using a guillotine) and the tread length must involve s dmsll smount og stretch or zero stretch.

The tire is then placed into the vacuum bag and cured in an autoclave for a period which vulcanizes the cushion gum layer. In other words, it is the cushion gum layer on the PU precured tread composite which determines the cure schedule in the autoclave.

When a 11 R 22.5 truck tire was retreaded with a PU precured urethane tread, the characteristics shown in tables 1-3 were obtained. Tires are still undergoing tests and prediction of a maximum tire life of over 150,000 km are not unrealistic. The potential for PU precured treads is not limited to retreads. There is no reason why new tires could not include a PU wear strip or industrial tires be recapped to provide new performance properties in toughness and non-marking capability.
Table 1 - typical physical properties of the cured elastomer

Hardness Shore [degrees]A 65 ASTM D 2240
Rebound resilience (%) 40 ASTM D 2632
Tensile modulus (Kg/cm2) 100% 25 ASTM D 412

Tensile strength (Kg/cm2) 300% 50 ASTM D 412
Elongation at break (%) 350 ASTM D 412
Tear resistance (Kg/cm) 550 ASTM D 412
Abrasion resistance (mg loss) 50 ASTM D 624
 (1000 cycles, 1000 gm load)
Abrasion resistance (mm3) 10 Taper, H-2
Compression set (%) 55 DIN 53516
Method B 35 ASTM D-395
Goodrich flexometer delta T ([degrees]C) (22 hrs @ 70[degrees]C
D-623 Method A 25 ASTM
Table 2 - data on a retreaded 11R22.5 truck tire

EC tire R16 test (EC regulation 54) Passed type
Approval Testing
Temperature rise delta T[degree] Maximum 50[degrees]C
compared to [greater than or less than]
53[degrees]C for a
conventional rubber tire.

Expected life of a tractor unit 1.5 to 2 x standard
NR tire high quality

Wet skid testing Equal to an NR tire

Rolling resistance (data to be confirmed)
Table 3 - additional features of the PU
retreaded tires

No toxic extender oils which harm the environment when
tires wear.
An insignificant loading of carbon black - will lead to a
cleaner environment when PU tires are used.
The PU is ultimately biodegradable.
Improved fuel consumption due to a lower rolling resistance.
For industrial use these PU treads are non-marking - ideal
for clean environments.
Superior oil, fuel and chemical resistance - ideal for
applications where rubber tires are currently inadequate.

The testing undertaken to date has proved the integrity of the bond between the polyurethane tread and the tire casing, to the extent that we can say that the new proprietary adhesive is well capable of maintaining the absolute bond required to deal with all the operating conditions a road going tire is likely to meet.

With regard to the specific polyurethane system used, it is certain that further modification would produce even higher mileages, but the additional performance would be gained at a cost premium which may cancel out the performance improvement. At the end of the day the issue is not one of technical achievement, however interesting or even revolutionary the PU trend concept is, but of cost effectiveness. A truck fleet operator is unlikely to be interested in the nature of the polymer employed, but he or she will certainly want to know what cost per mile or kilometer is possible in terms of tire and fuel cost. There is a very real connection between reduced rolling resistance and fuel economy, and we would expect the precured urethane retread to produce very significant fuel savings.

Also, there is growing concern about the air pollutants produced directly from tire wear. Tire rubber often contains aromatic extender oil which may well be a potential carcinogen. The quantities of these aromatic oils released into the atmosphere is very considerable given the millions of tires traveling on our roads. The precured urethane tread does not contain any such extender and must therefore be seen as a safer alternative in terms of environmental damage.

It would, however, be naive to pretend that there is only good news. There is at the present time a worldwide shortage of good quality casings for retreading.

The high mileage potential of precured urethane treads will inevitably place greater demands an the casing to which it is attached. The selection criteria required for casings suitable for precured urethane treads will undoubtedly create an additional problem for the retreader. There is little point in attaching a tread capable of twice or three times normal mileage to a casing which may fail before the full mileage potential is realized.
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Title Annotation:tire innovations
Author:Gomberg, E.N.
Publication:Rubber World
Date:Apr 1, 1997
Previous Article:Physical properties and their meaning.
Next Article:Stabilization strategies for weatherable PU.

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