Rubber products are often prepared by assembling a multiplicity of uncured rubber components, although sometimes a combination of uncured and cured rubber components is employed, which desirably requires them to have a somewhat tacky surface to enhance the stability of their assembly, followed by curing the resulting assembly under conditions of elevated temperature and pressure. Such tack is often referred to as building tack. The use of building tack in fabricating rubber products is well known to those skilled in such art. Usually building tack is used in the context of uncured rubber and, in this description, it is also used in the context of partially cured rubber. In this description, cured rubber is normally equated with partially cured rubber for the treatment purposes.
For such purpose, a conventional practice is to treat at least one of the uncured rubber surfaces with a solvent in order to both make the rubber surface tacky and, of a usually equal importance, to remove surface bloom from the rubber surface. Sometimes, the surface is treated with a rubber cement based on such solvent. The surface bloom is often in the form of various oils, fatty acid salts, antidegradants and waxes which have migrated to the surface of the rubber over a period of time and which often tend to inhibit a strong cure between the assembled rubber component surfaces.
Thus, in the description of this invention, for the treatment purposes, uncured rubber means uncured compounded rubber. Compounded rubber is used in a conventional sense, namely, for rubber which has been mixed with compounding ingredients.
While various aromatic solvents have heretofore demonstrated an ability to provide adequate building tack for uncured rubber surfaces, it is desired to provide an essentially aliphatic solvent for such surface treatment which has an adequate drying time, essentially eliminates or removes uncured rubber surface bloom, provides uncured rubber surface building tack, provides or enables an interfacial rubber surface adhesion after curing the rubber assembly and contains less than about one weight percent aromatic components.
Accordingly, it is desired to provide a solvent for such purposes which contains less than one weight percent and, thus is basically free, of aromatic materials such as, for example, benzene, alkyl substituted benzenes, xylene and alkyl substituted xylenes as designated aromatic and substituted aromatic hydrocarbons. It is also desired that it contains less than about one weight percent n-hexane or cyclohexane.
It is desirably required of such solvent that it effectively tackifies rubber surfaces, particularly uncured rubber surfaces, to provide adequate building tack so as to hold the rubber components together prior to curing the assembly and, further, to have a satisfactory short drying time in a practical application.
The solvent ability of a solvent is considered to be important for removing surface bloom from the rubber surface. It is believed that a suitable test for such solvent ability is a Kauri-Butanol (KB) test.
By experience, it is considered herein that a hydrocarbon solvent or solvent mixture having a KB value in a range of about 32 to about 38, preferably about 34 to about 36, is generally satisfactory for the solvency purposes of this invention.
The KB (Kauri-Butanol) values are conventionally determined by first dissolving 100 grams of Kauri Gum in 500 grams of butanol (thus, the designation KB). The desired solvent, or solvent mixture, is titrated into the KB solution to determine its KB value. KB values have been published in the literature for various aromatic and aliphatic hydrocarbon solvents. The KB value is, generally, an indication of the solubility of the Kauri Gum resin (contained in the butanol solution) in the solvent being tested.
For example, a KB value for toluene or benzene would be about 105-110; cyclohexane about 50; and heptane would be about 29.
A reference to the KB test may be found in "Physical And Chemical Examination of Paints, Varnishes, Lacquers And Colors", Eleventh Edition, 1950, pages 449-451, by Gardner and Sward, originally distributed by The Henry Gardner Laboratory, Inc., Bethesda, Md.
Various solvent systems were evaluated for use in treating uncured rubber to aid in the fabrication of rubber products.
Initially, heptane, an aliphatic C.sub.7 hydrocarbon with a KB value of about 29, was evaluated. However, it was found to be inappropriate because its observed drying time was found to be unacceptable because it was too slow and it was observed to not provide adequate building tack for the treated uncured rubber surface.
However, n-heptane was considered to be a good solvent if it could be successfully blended with other hydrocarbons because of its nearly adequate drying time and its KB value.
Accordingly, it was decided to proceed to evaluate hydrocarbon solvent blends which had a heptane base for the rubber tackifying purposes, which would have the properties of an adequate drying time, bloom removal and the providing of an observed satisfactory building tack for a rubber surface, particularly an uncured rubber surface.