PROCESS FOR RETREADING TIRES

Abstract: A process for retreading a tire including preparing a casing of a pre-cured tire, forming a gum into a strip of about .030 inch thickness, placing thermoplastic covers over each side of the gum and irradiating both sides of the gum. The gum is irradiated with about 20 megarads of continuous electron bombardment. The gum is subsequently further cured by heat treatment for about 4 minutes at 280 DEG F. Subsequently, the partially pre-cured gum, a pre-cured tread and casing are assembled and finally cured for about three hours at 280 DEG F.

Description: PROCESS FOR RETREADING TIRES BACKGROUND OF THE INVENTION This invention relates to a method for retread¬ ing tires, and in particular the use of an irradiated pre-cured gum in such process. When the tread on an originally manufactured tire wears out, it is very common to place a new tread on the old casing. The retread process and the suppliers of the components for the process have become a significant industry because of the efficiencies involved. One of the most popular methods of retreading tires is to place pre-cured gum around the circumference of a tire casing, cover it with a pre-cured tread, and cure the comnbina- tion with heat and pressure. While this method ' is gener¬ ally satisfactory, the time, cost and e ectiveness of the pre-cured gum are major factors in the success of the retread process. A sample of such a process is shown in Patent No. 4,434,018, the disclosure of which is incorpo¬ rated herein by reference. Industry practices have also been published by the Rubber Manufacturing Association, Inc. (RMA) in their bulletins, and in particular Bulle¬ tins Nos. 8, 10, 14, 18, 19, 23, and 25, which are also incorporated herein by reference. The process shown in the RMA bulletins typically uses a gum about .060 in. thick. It is pre—cured for about 7.5 minutes at about 280 F. When this is done, the gum meets industry standards of a tensile strength of 3500 psi, a 3007o modulus of 1050 psi, and an elongation of 5507o. Using the heat pre-cured gum, the Einal curing of the casing, gum, and tread typically takes about five hours. The present invention has found a method which reduces by about 40% the time necessary to pre-cure the gum with heat and the time necessary for the final cure of the retread tire. This reduction in pre-cure and cure time nearly doubles the output and capacity of a given amount of equipment and plant. Moreover, this invention reduces to about one-half the necessary thickness of the gum needed in the retread process. These decreases in time and material and increases in productivity are quantum jumps over the prior art. This invention attains the lower cure times by irradia-ting the gum with nuclear radiation prior to its pre-cure. The gum is normally natural rubber with con¬ ventional additives which has been formed into strips, with a thin, strippable, thermoplastic covering on each side. The strip gum is passed in proximity to a nuclear source which changes the molecular bonding to a more stable form. Even though the gum is only about .030 inch thick, it remains tacky following irradiation. The irradiated gum is then cured with heat or about 4 minutes instead of the usual 7-1/2 minutes. The final cure process of the tire is about 3 hours instead of the previous 5 hours. The efficiencies are clear. SUMMARY OF THE INVENTION The process of preparing gum for retreading a tire casing includes the addition of additives to raw rubber to form a gum which can be formed into strips and pre-cured with radiation and heat. The gum is subse¬ quently placed around a casing, a tread is placed over the gum, and the combination is finally cured with a combination of heat and pressure. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a block diagram of the pro¬ cess of this invention; FIG. 2 is a schematic representation of the nuclear radiation used in treating the gum; FIG. 3 is a graphic representation of the degree of dosage versus penetration; and FIG. 4 is a schematic representation of the final curing apparatus for a tire. DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a process of retreading a tire casing according to this invention. The tire is first buffed and repaired according to standard operating procedures which are well known in the art. An example of such procedure is set forth in Rubber Manufacturing Association, Inc. Bulletin No. 8. In general, the tires are irst cleaned and inspected for foreign .objects and for any type of leak or cut. The tire is then cleaned, repaired, and buffed. Buffing should be uniformly done and should avoid any damage to the casing. Subsequently, a cement, such as Hercules Tire & Rubber Company's bonding cement, is placed on the tire in order to hold an irradiated gum in place. RMA Bulletin No. 10 is an example of- such cementing process. Tread is cemented over the gum, and the combination is cured. In this particular invention, the gum is first formed into a strip and then irradiated. The type of gum that may be used is available from Hercules Tire £. Rubber Co., Model Ultra-Cushion 06. It includes natural rubber and conventional rubber addi¬ tives such as carbon black, zinc oxide, stearic acid, antioxidant and antiozonant materials. Other gums would be operable in this process. Such gums can be polymers made from conjugated diene monomers containing from 9 to 12 carbon atoms, preferably from 4 to 6 carbon atoms, or in combination with a vinyl-subs ituted aromatic contain¬ ing from 8 to 12 carbon atoms. Examples of specific diene monomers include butadiene, isoprene, pentadiene, hexadiene, etc., with butadiene and isoprene being pre¬ ferred. An example of a preferred vinyl-substituted aromatic is styrene. A copoly er made from butadiene and styrεne, that is, poly(butadiene-styrene) , is also pre¬ ferred. Previously, the gum was about .060 inch thick. This was necessary in order to reach the desired degree of pre-curing while continuing to have tackiness. That is, the gum with the cement had to be sticky enough to hold the tread in place on the casing during the build-up of the tire and during the final curing process. The .060 inch thick gum took about 7-1/2 minutes at 280°F to cure to 907o. The final characteristics of the 907, cured gum included a tensile strength of about 3500 psi, a 3007c modulus of 1050 psi, and an elongation of 5507 o . With these cured physical properties and with 907o pre- cured treads, the combination of the tread, gum and cas¬ ing used to take about 5 hours for the final curing pro¬ cess. The final curing process, as well as the time and treatment of the pre-cured gum, was a major expense .and time consideration in the preparation of retreaded tires. As shown in FIG. 1, this- invention has found that if the gum layer is first irradiated with nuclear radiation, the further pre-cure treatment of the gum, as well as the final curing process, are greatly reduced. The substantial increase in the efficiency of the process of forming retreaded tires greatly enhances the possible use of existing machinery as well as providing a superior product. It is anticipated that there are several types of irradiation procedures and nuclear radiation that could be used with this invention. These would include without limitation electron and gamma radiation. As illustrated in FIG. 2, a strip of gum 10 preferably hav¬ ing thin sheets of thermoplastic on each side is passed between two sources of radiation 20 and 22. The thermo¬ plastic sheets prevent adhesion of the gum to itself and other items during the pre-cure process. One source of radiation could be used if the strip were passed by it twice, once for each side. In the preferred embodiment, the radiation sources 20 and 22 are continuous electron beams delivering from 10—30 megarads on each side. Typi¬ cally, twenty (20) megarads (10 per side) are delivered to a gum strip .030 inch thick. The amount of radiation actually delivered to the strip of gum will depend upon the intensity of the radiation and the speed and thick¬ ness of the strip of gum. While different combinations exist for irradiating the strip gum to the necessary pre-cured degree, one successful example has been to have the gum travel at 50 feet per minute in an Electrocurtain TM machine built by Energy Sciences, Inc. It is anticipated that the gum may travel from 20-200 ft./min. and that intensities of radiation may be utilized. In one preferred embodiment, a window of 1-1/2 x 40 inches is used with the source of radiation in order to focus the electron beams on either side of the gum strip. Obviously, the size of the window will depend upon, and be greater than, the width of the strip being irradiated. The strip of gum 10 passes from a roll 12 over rollers 14 and 16 through which, cooling fluid op¬ tionally may be utilized, since the radiation causes some heating of the gum. The gum is then, rolled up on an out¬ going roller 18. The intensity of the electron source depends upon the voltage and current utilized by the radiation source (FIG. 3). For example, 20 megarads (10 per side) would be delivered to a strip of gum traveling at 50 feet per minute and utilizing 100 milliamps in the above-noted machine. Whatever the parameters, it is imperative that molecular changes occur which act to pre-cure the gum. Such molecular changes are known (see, for example, Patent No. 4,221,253, which is incorporated herein by reference) . After the irradiation of the strip of gum, a certain degree of pre-cure is attained which manifests itself in the physical characteristics of the gum. The degree of pre-cure is verified by further curing a sample with heat to test the e ectiveness of the radiation. Verification is achieved when the gum has the character¬ istics of a 907. cured gum as explained above. It is anticipated by the applicant that the gum may actually be from 10 to 60 mils thick with appropriate voltages and currents applied to the source of radiation to give the partial cure that is .needed. However, the thicker the gum, the more radiation is required and, accordingly, the gum should be made as thin as possible while still retaining the necessary physical and molecu¬ lar characteristics to successfully work in the final curing process. FIG. 3 illustrates a graph which shows the variation of dose on the Y axis with the depth of pene¬ tration on the X axis. In the graph, the top surface is considered 1007o. The percentage of the dose of radiation at the top surface is plotted at different depths. For example, on the 150— ilovolt curve, there is only 507. of the radiation of the top surface at a depth of approxi¬ mately 4 mils. Accordingly, if greater depths of pene¬ tration are required, the voltage must be raised. Alter¬ nately, the current can be raised or the density of the gum can be changed. The particular example shown in FIG. 3 utilizes the type of gum illustrated in this inven¬ tion. Obviously, the graphs will change with the differ¬ ences in material. After the irradiation treatment, the gum is then cured physically by placing it in an environment of about 250°F to 300°F for approximately 4 minutes. The time may be as low as 2 minutes and as high as 6 minutes if required. In practice, the cure time is significantly lower and almost half of the previous 7—1/2 minutes required for the same type of gum used for the same application. Accordingly, there is a significant reduc¬ tion in time in the curing process by the use of irradi¬ ation. Once the gum has been pre-cured by irradiation and heating, it has physical characteristics similar to gum which was heated for 7—1/2 minutes. That is, it has a tensile strength of from about 2500 to about 3500 psi, with approximately 3000 psi being preferred, a 3007, modu¬ lus of about 800—1200 psi, with 1000 psi being preferred, and an elongation of about 400-600,7, with about 507, being preferred. In general, different times, speeds, doses, etc. may be used, but the gum should be about 907, cured after the irradiation and heat pre—curing. Typically, the gum is then used by either the retread manufacturer or a retread dealer by stripping off the previously applied plastic from the gum and rolling it onto a casing that has been prepared. Pre-cured tread is then placed over the gum. Cement is used as known to the art and to the literature. The final curing process may be done in any one of a number of processes. As par¬ ticularly set forth in the Rubber Manufacturing Associ¬ ated, Inc.'s pre-cured tread retreading Bulletin No. 28, there are at least five different types of chambers where final curing may occur. These chambers are known as a smooth matrix, an outside steam bag, a matrix chamber- . envelope, a matrix chamber-diaphragm and a pressure chambers-envelope. FIG. 4 is one example of the environment in which the final curing occurs. It is the type known as a "Pressure-Chambers Envelope." It includes a pressure chamber 30 applying air or fluid pressure to the outer wall 32 and having a tube 34 inside the tire 36. An envelope 38 surrounds the outside of the tire and is held in place by a rim 40. Air is exhausted through a valve 42 and line 44. Air pressure is supplied via line 46 and valve 48. The tire shown in FIG. 4 was previously cured for approximately 5 hours. By utilizing the irradiation set forth in this application, the final cure time has been reduced to approximately 3 hours. This 407, savings in time is a major step forward over the prior art, and is an unexpected result. While the invention has been shown and described with respect to a particular embodiment thereof, this is for the purpose of illustration rather than limitation, and other variations and modi ications of the specific embodiment herein shown and described will be apparent to those skilled in the art all within the intended spirit and scope of the invention. Accordingly, the patent is not to be limited in scope and effect to the specific embodiment herein shown and described nor in any other way that is inconsistent with the extent to which the progress In the art has been advanced by the invention.

Claims: WHAT IS CLAIMED IS: 1. A process of retreading a tire comprising: (a) preparing the surface of the casing of a tire so that it does not have any significant defects that would interfere with its adhesion to the tread; (b) irradiating a gum with nuclear radia¬ tion to partially cure the gum; (c) heating the gum to further pre-cure it; (d) placing the irradiated gum around the circumference of the casing; (e) applying a pre-cured tread over the gum; and (f) curing the tread and gum while on the casing. 2. The process of claim 1, wherein radiation changes the molecular bonds of the molecules. 3. The process of claim 1, wherein the radia¬ tion is a continuous electron beam. 4. The process of claim 1, wherein the gum has a thickness of from 10-60 mils. 5. The process of claim 4, wherein the gum is from 10-30 mils thick. 6. The process of claim 5, wherein the gum is from 20-30 mils thick. 7. The process of claim 5, wherein the gum is irradiated with a total of approximately 20 megarads. 8. The process of claim 7, wherein the gum is irradiated with about 10 megarads per side. 9. The process of claim 8, wherein the irradi¬ ated and heated, partially cured gum has a tensile strength of about 3000 psi, a tensile strength of about 2500 to 3500 psi, and a 3007,. modulus of about 1050 psi. 10, The process of claim 9, wherein the irradi¬ ated gum is pre-cured with heat from about 2 to 4 minutes at 250-300°F. 11. The process of claim 9, wherein the irradi¬ ated gum is cured with heat at about 280°F. for about 4 minutes. 12. The process of claim 5, wherein the tread, gum, and casing are cured from about 2-4 hours at 200-300°F. 13, The process of claim 12, wherein the tread, gum, and casing are cured for about 3 hours at about 280°F. 14. The process of claim 5, wherein the gum is irradiated on both sides. 15. The process of claim 5, wherein the gum is irradiated by radiation sources on both sides of the gum. 16. The process of pre-curing gum for use in retreading tires, comprising * . putting additives in rubber to form a gum; ' forming the gum into a strip of material; irradiating the gum with nuclear radiation; and heating the gum to further pre—cure it. 17. The process of claim 16, wherein the gum is irradiated from both sides with approximately 20 megarads of radiation and further wherein the gum is about 10 to 30 mils thick. 18. The process of claim 16, wherein the pro¬ cess of pre-curing gum includes irradiating said gum with a continuous electron beam simultaneously from both sides thereof. 19. The process of claim 16, wherein the gum is cured withh hheeaatt aatt aabboouutt 225500°°FF to 300°F for about 2 to 4 minutes after irradiation.

Inventors: SHANK BYRON E, SHANK, BYRON, E.
Applicants: HERCULES TIRE & RUBBER CO
Application year: 1988
Main classification: B29D30
IPC codes: B29D30