Patent Application
20160368231
Many rubber articles are made from extruded, calendered, or molded components whose ends are adjoined to form a continuous surface. Each set of adjoined ends is referred to as a splice.
Splices are a well-known source of uniformity problems in curable rubber articles. Splices tend to induce non-uniformities in rubber articles that can result in inconsistent product performance. Splicing the ends of an uncured rubber strip, such as a tire tread, can be significantly more challenging where the rubber composition of the strip contains a high content of precipitated silica, where the content of such silica in the rubber composition can significantly exceed the content of the rubber itself Such high content of silica may reduce the building tack of the uncured rubber composition.
Methods have been proposed to ensure splice integrity for the useful life of a rubber article. These attempts typically include using a hydrocarbon-based adhesive solvent or cement to secure a splice. For example, skived tread splices are sometimes secured using a hydrocarbon-based rubber solvent or cement. However, the use of hydrocarbon cements is undesirable due to environmental concerns. It can also be difficult to use a solvent or cement to secure a butt splice (i.e, a splice in which there is no rubber overlap at the adjoining location), which is used in many types of rubber articles.
There is a continuing need for a method for securing a splice in a curable rubber article. The method should also be effective for all types of splices.
The invention is directed toward methods of securing a splice in a curable rubber article and toward splices secured using these methods. As used herein, the term “curable rubber article” refers to an article made from a compound containing one or more types of rubber having double bonds therein and being sulfur-curable.
It has now been found that splices in curable rubber articles may be improved by inserting one or more elongate fasteners made of a stiff rubber compound. The fasteners may include a compound comprising syndiotactic 1,2-polybutadiene having a melting point of less than about 175° C. Having these fasteners inserted into the splice tends not to affect the uniformity of the rubber article, because the syndiotactic 1,2-polybutadiene layer melts into the rubber article during vulcanization.
Furthermore, the syndiotactic 1,2-polybutadiene is crosslinked, or cures, with the rubber article during vulcanization. Thus, the fasteners made of syndiotactic 1,2-polybutadiene do not create a significant size bulge in the rubber article above splices secured using methods of the invention. Significant size bulges in a curable rubber article can be a source of uniformity problems. In addition, rubber compounds used to make rubber articles do not have to contain syndiotactic 1,2-polybutadiene for methods of the invention to be useful or for splices resulting from methods of the invention to be secure.
Methods of the invention include inserting one or more elongate fasteners into a splice and two ends of uncured rubber in a curable rubber article with syndiotactic 1,2-polybutadiene, and adhering the syndiotactic 1,2-polybutadiene to the uncured rubber. Typically, the syndiotactic 1,2-polybutadiene is adhered to the uncured rubber using heat and pressure. Preferably, the syndiotactic 1,2-polybutadiene is melted in the uncured rubber at a temperature of at least about 150° C. and not greater than about 175° C. and under enough pressure to adhere the syndiotactic 1,2-polybutadiene to the compound. Generally, the melt process is performed in less than about 10 seconds. These insertion and adhering processes take place prior to vulcanization of the rubber article. The curable rubber article having the syndiotactic 1,2-polybutadiene therein is subsequently cured using methods known in the art.
The preferred syndiotactic 1,2-polybutadiene generally has a relatively low melting point that approximates the temperature that the rubber article reaches near the end of its cure cycle. Syndiotactic 1,2-polybutadiene having this approximate melting point tends not to remelt prior to the latter portion of the cure cycle. If the syndiotactic 1,2-polybutadiene tends to melt prior to the latter part of the cure cycle, then the splice is more likely to come apart before vulcanization. In one embodiment, the melting point ranges from 40 to 170° C. In one embodiment, the melting point ranges from 50 to 150° C. The syndiotactic 1,2-polybutadiene elongate fasteners are typically from about 0.3125 inch to about 0.25 inch thick, and from about 0.25 inch to about 4 inch long. The methods of the invention and splices resulting therefrom are applicable to any type of splice, including butt and skived splices.
The invention is directed toward methods for securing splices in curable rubber articles. Examples of curable rubber articles useful in the invention include, tires, hoses, conveyor belts, tubes, bladders and belts. It has found that, by inserting fasteners made with a stiff curable rubber compound in these types of rubber articles, splices can be secured.
Any type of uncured, but curable, rubber article, or component thereof, made from a first rubber compound containing any unsaturated rubber that is sulfur-curable is useful in methods of the invention. Rubber compounds useful in the invention typically contain natural rubber and/or synthetic rubber, silica, carbon black, oil, curing agents and accelerators in amounts as are known in the art. These types of rubber compounds are well known for making rubber articles, such as tires, belts, hoses, conveyor belts, tubes and bladders. In particular, tread compounds with high silica contents ranging from 50 to 150 phr that are difficult to splice are especially amenable to the methods of the invention.
For example, high silica containing uncured rubber compounds disclosed in U.S. Pat. Nos. 8,312,905 and 8,302,643 are useful in methods of the invention. In general, rubber compounds having a higher level of unsaturation have a greater number of potential cross-link sites and a higher probability of bonding with syndiotactic 1,2-polybutadiene placed in splices in rubber articles. Thus, using rubber compounds having higher degrees of unsaturation are most preferred in methods of the invention.
In general, any type of stiff rubber compound that can be formed into insertable fasteners that can be vulcanized with unsaturated rubber compounds is useful in the present invention.
In one embodiment, the fasteners may be made from a compound comprising syndiotactic 1,2-polybutadiene. For example, the syndiotactic 1,2-polybutadiene disclosed in U.S. Pat. No. 5,278,263 is useful in the invention. The syndiotactic 1,2-polybutadiene can be used in methods of the invention in any form that facilitates formation of fasteners suitable for insertion into a splice.
In one embodiment, the syndiotactic 1,2-polybutadiene polymers has a melting point ranging from 40 to 170° C., alternatively from 50 to 150° C. With this range of melting temperatures, the syndiotactic 1,2-polybutadiene can efficiently be adhered to the rubber compound using heat and pressure.
In one embodiment, the syndiotactic 1,2-polybutadiene has a vinyl content on average of at least about 80 percent by number and of not greater than about 95 percent. By “vinyl content,” it is meant the weight percent of the polymer which has the vinyl structure. Preferably, the syndiotactic 1,2-polybutadiene used in the invention also has a crystallinity of at least about 40 percent and of not greater than about 60 percent. Syndiotactic 1,2-polybutadiene compounds may have these characteristics in order to insure that the syndiotactic 1,2-polybutadiene used in the invention is sufficiently thermoplastic for methods of the invention. An example of syndiotactic 1,2-polybutadiene that is useful in the invention is disclosed in U.S. Pat. No. 5,307,850.
When using syndiotactic 1,2 polybutadiene in the fastener, the syndiotactic 1,2 polybutadiene may be used alone or in a rubber compound including additives as listing above and optionally other elastomers.
In one embodiment, the fasteners may be made from a rubber compound that has been cooled to stiffen the compound such the fasteners are insertable into the rubber compound of rubber article to be spliced. In this embodiment, the fasteners may be made from a rubber compound that is identical to that to the rubber compound of the rubber article to be spliced. In this embodiment, the fasteners are cooled to a temperature low enough to prevent flexure of the fastener during the insertion. Shaped fasteners may be cooled cryogenically, for example, to a temperature below the glass transition temperature of the fastener compound, and then the cooled fasteners may be immediately inserted into the splice. Glass transition temperature of a rubber compound may be conveniently determined, for example, by differential scanning calorimetry as the inflection point of a plot of heat flow versus temperature taken at a scanning rate of 10° C. per minute.
In various embodiment, the stiff rubber compound used in the invention is shaped into fasteners such as pins, screws or staples. The fasteners may have a length sufficient to penetrate at least halfway into the depth of the spliced material. In one embodiment, the fasteners have a length ranging from 0.25 inch to 4 inch, and a thickness ranging from 0.03125 inch to 0.25 inch.
Fasteners made be made from stiff rubber compound using molding techniques as are known in the art, such as injection molding. Alternatively, pins for example may be made by punching from an extruded sheet of material.
Generally, methods of the invention are performed during construction of curable rubber articles. It is during construction that rubber components are configured to form rubber articles, and the configuration process typically involves splicing two ends of rubber components together.
In splicing a rubber article, two ends of a rubber component are adjoined, as a stiffened rubber fastener is applied to the rubber component in such a matter that the fastener penetrates the splice and two ends of the rubber component.
The two types of splices shown in the figures are merely exemplary of the splices in which methods of the invention may be used. For example, staples may be used in both butt splices as shown in
The splice fasteners are distributed axially (with respect to the tire axial and radial directions) across the splice, with a plurality of fasteners spaced appropriately to ensure splice integrity. Such spacing may be determined without undue experimentation. In one embodiment, the axial distance between adjacent fasteners ranges from 0.5 to 5 cm. In one embodiment, the axial distance between adjacent fasteners ranges from 1 to 4 cm.
Heat and pressure are used to melt the syndiotactic 1,2-polybutadiene to adhere it to the rubber compound, such as during a tire curing process in a mold. Enough heat and pressure are used to sufficiently melt the syndiotactic 1,2-polybutadiene. If too little heat and pressure are used, then the syndiotactic 1,2-polybutadiene may not penetrate into the rubber compound and, thus, not have sufficient mechanical adhesion to hold the splice together. Enough heat and pressure are applied to the syndiotactic 1,2-polybutadiene to melt it as observed by the human eye. Typically, it takes about 5 to 10 seconds for the syndiotactic 1,2-polybutadiene to melt.
Once melted, while the rubber article is cured, the syndiotactic 1,2-polybutadiene can be vulcanized with the rubber compound that is included in the rubber article. The rubber article can be vulcanized using methods known in the art.
The above specification and example provide a complete description of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.
