GB2210001A - Tyre provide with a precured tread and method of making same - Google Patents

Abstract

A tyre to be rebuilt with a precured tread 12 is prepared for curing by securing the precured tread against uncured rubber 11 applied to a buffed area of the tyre casing 10. The junction of the precured tread and the sidewall on each side of the tyre is covered by a thin layer of uncured rubber 16 meeting both the tread and the sidewalls. If the tread is formed in sections, the splice 13 or splices between the ends of any sections thereof are covered with a caulk layer 17. The tyre is then placed in a curing chamber and subjected to heat and pressure which may be in conventional ranges and which results in the curing of the layers 16 and 17. Curing time is reduced from the usual period by about 50% if the tyre is cured by steam under pressure and by about 25% if the tyre is cured by air under pressure. In either case, the curing medium is blocked from entry between the tread 12 and the casing 10 and the bond between the casing and the precured retread is characterized by the intermediate layer being free of distortions and by there being a substantial increase in resistance to tread separation in standard pull tests. <IMAGE>

Description

TYRE PROVIDED WITH A PRECURED TREAD AND METHOD OF MAKING SAME A large percentage of retreaded tires is produced using precured treads Assuming that a used tire is found suitable for reuse, the tire is buffed and, if necessary repaired. A cushion layer of uncured rubber is placed on the buffed and cemented area and is then covered by the precured tread. The ends of the precured tread, and sections thereof if the required tread length is so established, are temporarily joined by staples and are filled with uncured rubber to form splices.

Before the thus prepared tire is placed in a curing chamber, there to be subjected to heat and pressure for the required length of time, it is provided with an inner tube and mounted on a curing rim. The precured tread is covered with an envelope which is marginally sealed against the tire, vented to the atmosphere and securely holds the precured tread when the tube is inflated.

One type of envelope seal which is used commercially, see United States Patent No. 3,136,673, employs annular rims which are caught between the flanges of the curing rim and seal the margins of the envelope against the casing of the tire near its beads when the tube is inflated. Such envelopes are vented laterally of the tread. In practice and by way of example, in the case of truck tires of average size, the tubes are inflated to have 115 PSI (gauge pressure). Each tire is placed in an autoclave type of a curing chamber where the steam pressure is less, say 85 PSI (gauge pressure).

Another type of commercially used way of sealing envelopes, see United States Patent No. 3,778,833 for example, employs an annular receiver dimensioned to receive the envelope covered tire with the envelope sealed by the side walls of the tire against the side walls of the receiver when the tube is inflated. Steam is introduced into the receiver adjacent the precured tread and the pressure to which the envelope is exposed is in the order of 70 PSI (gauge pressure). The tube pressure is in the order of 100 PSI (gauge pressure).

The length of time required for curing of a tire within a curing chamber varies with the tire size but with the average truck tire size 11R24.5 the curing time, using the above types of envelope seals is about two hundred twenty minutes and one hundred minutes respectively and in the case of the average passenger car tire about one hundred eighty and seventy-five minutes.

In addition to the length of time and the energy required for curing, the cost of envelopes and the means by which they are sealed, the use of envelopes has the disadvantageous feature that the uncured rubber layer underlying the precured tread becomes more or less distorted which distortions are reflected in conventional pull tests in which samples of a standard width are pulled from the casing with the pounds of pulling force required to do so duly recorded.

The general objectives of the present invention are to simplify the curing of tires provided with precured treads, shorten curing times and at the same time provide retreaded tires of better quality.

In accordance with the invention, these objectives are attained by varying the previously described preparation of radial tires for entrance into a curing chamber by applying as final steps, the application of layers of uncured rubber, preferably, but not necessarily, a high tensile strength, extrudable natural rubber to overlie the side edges of the precured tread and the exposed margins of the buffed area in order to provide at each side, when the tire is cured, an annular blending of substantially uniform width, such rubbers are commercially available.

If the precured tread consists of one or more sections cut from a roll of precured tread material or from other sections assembled about the casing, a layer of uncured rubber is inserted edgewise between the ends of the section or sections and the ends of the sections are then stapled together. With care, the sections can be so fitted together as to make the use of caulking materials unnecessary but, under production conditions, the splices, which should not exceed one sixteenth of an inch, are typically covered with a suitable caulk which will prevent the curing medium from entering the splices.

Each such rebuilt tire, if a radial tire, without a tube, curing rim or tube, is then placed in the curing chamber where the exterior and interior surfaces of the tire are directly exposed to heat and pressure, typically 60 to 75 PSI (gauge pressure) steam. The surfaces of the blending layers become cured before the uncured rubber cushion layer between the casing and the precured tread loses its ability to hold the precured tread and they hold the precured tread in place until the cushion layer becomes cured and in addition prevents the entry of steam under the tread and cushion layer. The caulk applied to the splices prevents the entry of steam into the splices of the tires. The curing time is typically less than 508 of the time required where envelopes are employed.

If curing is effected in chambers where the heat transfer is by latent rather than by sensible heat, the reduction in curing time is about 25% of that required when the tires are within envelopes. In chambers where the curing medium is hot air under pressure, the position of the precured tread is maintained by tensioned, peripheral straps and the caulk covering the splices and the blending layers are effective barriers against the pressurized air.

In the case of bias weave tires, they must, if to be cured in an autoclave chamber be provided with an inner tube and mounted on a rim with the tube so inflated that Nylon shrinkage is prevented. If such tires are to be cured by pressurized air, it is necessary also to employ tensioned straps to anchor the tread.

The cured tires, made in accordance with the invention have an esthetically acceptable appearance and have the valuable characteristic that the cured rubber layer is substantially free of distortions so that pull test recordings are materially higher.

Other objectives of the invention and the manner by which they are attained will be apparent from the following description of a preferred embodiment and the appended claims.

The accompanying drawings illustrate preferred embodiments of the invention and Fig. 1A illustrates one prior art method of and apparatus for use in curing a tire which has been rebuilt with a precured tread; Fig. 1B illustrates another prior art method of an apparatus for use in curing a like rebuilt tire; Fig. 2 is a view, in perspective of a tire of the radial type rebuilt in accordance with the present invention; Fig. 3 is a cross sectional view of the tire. shown in Figure 2; Fig. 4 is a view showing a radial tire rebuilt in accordance with the invention positioned within a chamber wherein curing is effected by steam under pressure; Fig. 5 is a partly sectioned view of a tire of the radial type ready to be placed in a chamber to be cured by heated air under pressure; ; Fig. 6 is a cross sectional view of a tire of the bias weave type ready to be placed in a chamber to be cured by steam; and Fig. 7 is a like view of a tire of the bias type ready to be placed in a chamber to be cured by pressurized hot air.

In accordance with the invention, a tire 10 of the radial type which has been carefully examined and found to be suitable for retreading is buffed in a conventional manner.

The buffed area, see Figures 2 and 3, is then cemented and covered with a cushion layer 11 of uncured rubber against which the precured tread 12 is seated. If the tread 12 is formed by a section or sections, the ends thereof are spliced together with a strip 13 of uncured rubber, typically one-sixteenth of an inch in thickness, positioned edgewise between its ends and the ends of any sections required to establish the required precured tread length. Staples 14 are temporarily employed in connecting such ends.

The rebuilding of tires with precured tread in the manner above summarized is conventional. As previously stated, such rebuilt tires are covered by envelopes, provided with tubes and mounted on rims before being placed in a chamber in which the inflated tires are cured.

In Figure 1A, the tire 10A has its envelope 15A sealed thereto as shown and described in United States Patent No. 3,136,673 while in Figure 1B, the tire 10B has its envelope 15B marginally sealed as detailed in United States Patent No. 3,778,833.

In accordance with the invention, there is the additional rebuilding step before the tire 10 is placed in the curing chamber of applying an annular layer 16 of uncured rubber to each side of the tire 10 which blends the brushed or sanded margins of the tread 12 to the buffed area of the tire 10 at or near the base line of that area. The layers 16 are preferably a high tensile strength natural rubber extrudable compound which cures at a suitably low temperature and has been so prepared that it can be applied as smooth, even layer to provide a smooth transition between the tread and the casing with the thickness of the blending layers varying between their margins. In practice, the layers 16 are applied using an extruder gun which is in practice of a hand held type.

Where the tread 12 has one or more splices, it is usually necessary, before placing the tire in a curing chamber, to cover the splice or splices with a caulk 17 which will become a barrier against the entry of the curing media into and through them. Suitable caulking materials are Butyl rubber (both air drying and steam curing types) and Silicone rubber as these have the advantage that they effectively cover splices up to one eighth of an inch. After the tire has been cured, the caulk layer is or may be removed as are the staples.

A tire is then placed in the curing chamber 18, see Figure 4, which may be that of an autoclave. then steam is admitted into the chamber is, the temperature reaches a high of about 300"F within five minutes and the pressure will have then risen to about 50 PSI (gauge pressure). During this interval, at least the surfaces of the rubber polymer layers 16 are cured and during this interval, the blending layers 16 have blocked steam from penetrating under the tread and the cushion layer and the caulk layers, if such are used, are cured to block steam from entering the splice or splices. The tread 12 thus becomes secured to the casing of the tire by the blending layers 16 before the cushion layer 11 loses its ability to hold the tread 12 in place.After the end of the required time for curing, typically about forty-five minutes for a truck tire of average size and about thirty minutes for a passenger car tire of average size, the tire or tires are removed from the curing chamber 18, the caulk 17 pulled free and the staples 14 removed.

Curing chambers such as the curing chamber 18 are conventionally provided with trap-equipped, condensate drains D, see Figure 4, connected to a return line L to the boiler, not shown. In order to avoid the over curing of the cushion layer 11 in the upper portion of a tire suspended in the chamber 18 or the under curing of the layer 11 in the bottom portion thereof, it is necessary to siphon away condensate as it collects in the bottom of the interior of each casing. Such a siphon is schematically illustrated at S and is shown as discharging into the line L through a trap T.

It has also been found desirable to ensure uniform steam distribution throughout the chamber 18 by as means of a steam distributing manifold M connected to the boiler and extending lengthwise of the interior of the chamber 18 adjacent the bottom thereof.

If the curing medium in the chamber 18 is hot air under pressure, the transfer of the latent heat to the tire results in a slow rise in its temperature with the consequence that the blending layers 16 are cured too slowly to secure the tread. For that reason, see Figure 5, the tread is anchored by fabric straps 19 extending about the periphery of the tire with the ends of each strap interconnected by a ratchet type coupling 20 enabling the straps 19 to be suitably tensioned to hold the tread until curing is initiated.

The temperature of the air is in the approximate range of 210 F to 2400F and the air pressure is in the approximate range of 70 PSI to 90 PSI (gauge pressure).

The time required for curing a truck tire of average size is about one hundred fifty minutes and about seventy-five minutes in the case of a passenger car tire of average size. Both the blending layers and the splice-covering caulk layer or layers block air from adversely affecting the curing process by penetrating the otherwise vulnerable areas.

A valuable feature of the invention is that since the interior of the casing of a radial tire is open during the curing period, repairs with uncured rubber may be made to its interior while being rebuilt.

Tires of the bias weave type require that, see Figure 6, such a tire 21 be provided with a tube 22 and mounted on a rim 23 and the tube inflated to prevent the Nylon from shrinking with the internal pressure of that tire about 30 PSI (gauge pressure) greater than that of the curing medium. If the curing medium is hot, pressurized air, it is also necessary to employ fabric belts 19A which can be tensioned as by the couplings 20A, see Figure 7.

Tires rebuilt and cured in accordance with the invention are esthetically acceptable in appearance, have perfect splices and cushion layers that are not distorted and have significantly higher adhesion pull strength values than retreads conventionally cured within curing envelopes.

Claims (16)

Claims

1. The method of retreading a tire with a precured tread without creating distortions in the cushion layer underlying the tread, said method consisting of the steps, after the tire has been buffed and, if necessary, repaired and rebuilt with a cushion layer of uncured rubber cemented to the buffed area, with the precured tread seated against the cushion layer, and the rebuilt tire, then stitched, of applying a blending layer of uncured rubber to the annular areas at the sides of the tire between the margins of the tread and the buffed area, and then curing the uncured rubber layers by subjecting the tire directly to a curing medium under a wanted pressure and at a selected temperature for a predetermined length of time, the blending layers serving as barriers against the entry of the curing medium.

2. The method of claim 1 in which the precured tread consists of one or more sections seated against the cushion layer with their ends spliced together by staples with an edgewise layer of uncured rubber between spliced together ends and the additional step of covering each splice with a layer of caulk material of a type which will not affect the uncured rubber and which serves as a barrier against the entry of the curing media into the splices.

3. The method of claim 2 in which the caulk material is a selected one of the group consisting of air and heat curing Butyl rubbers and Silicone rubber.

4. The method of claim 1 in which the tire is a radial type, the curing medium is steam the temperature and pressure of which is such that at least the surfaces of the blending layers become cured before the cushion layer loses its ability to hold the tread in place.

5. The method of claim 4 in which the temperature of the steam is in the approximate range of 295"F to 325"F and the pressure is in the approximate range of 45 PSI to 80 PSI, gauge pressure, and the curing time of the cushion layer is approximately forty-five minutes for a truck tire and approximately thirty minutes for a passenger car tire of average size.

6. The method of claim 4 in which the steam temperature and pressure are such that the blending layers become cured on their surface before five minutes of exposure to such steam.

7. The method of claim 1 in which the tire is of the radial type, the curing medium is heated and pressurized air and the additional step of anchoring peripheral tensioned straps lengthwise of the tread and utilizing the blending layers as barriers against the entry of air to either surface of the tread establishing and maintaining the air temperature in the approximate range of 210 F to 240"F and the air pressure is in the approximate range of 70 PSI to 90 PSI, and exposing the tire to the heated and pressurized air for about one hundred fifty minutes in the case of a truck tire of average size and about seventy-five minutes for a passenger car tire of average size.

8. The method of claim 1 in which the tire is of a bias weave type, the curing medium is heated air under pressure, and the additional steps, prior to the exposure of the tire to the curing medium of providing the tire with a tube, mounting the tire on a rim and inflating the tire.

9. The method of claim 8 in which the curing medium is steam under pressure.

10. The method of claim 8 in which the curing medium is heated and pressurized air, utilizing the blending and caulk layers as barriers against the entry of air to the covered areas, establishing and maintaining the air temperature in the approximate range of 70 PSI to 90 PSI, and exposing the tire to the heated and pressurized air for about one hundred fifty minutes in the case of a truck tire of average size and about seventy-five minutes for a passenger car tire of average size.

11. The method of claim 10 anchoring peripheral tensioned straps lengthwise of the tread to anchor the tread.

12. The method of claim 4 in which a repair to an interior area of the tire is necessary and the additional step prior to the application of the air curing caulk of effecting the repair by applying uncured rubber to said area.

13. A retreaded tire of the type having a precured tread overlying a layer of originally uncured rubber on the annular buffed zone of the casing and having the junctions between the margins of the tread and the sidewalls of the casing covered by blending layers of originally uncured rubber with the uncured rubber of said layers cured by heat and pressure applied directly against the surfaces of the rebuilt tire, the layer of now cured rubber under the tread distinguishable, if the tire is sectioned, from the tread and casing and substantially free of distortions throughout the length and width thereof.

14. A retreaded tire of the type having a precured tread overlying a layer of originally uncured rubber on the annular buffed zone of the casing and having the junctions between the margins of the tread and the sidewalls of the casing covered by blending layers of originally uncured rubber with the uncured rubber of said layers cured by heat and pressure applied directly against the surfaces of the rebuilt tire, the layer of now cured rubber under the tread effective to establish an adhesion pull value of the bonding of the cushion layer to the casing and tread material significantly higher than a retread cured within a conventional curing envelope.

15. A-method of retreading a tyre substantially as hereinbefore described with reference to and as illustrated in Figures 2 to 7 of the accompanying drawings.

16. A tyre retreaded by the method of any preceding claim.