Heretofore, rosins and rosin esters being the derivatives thereof have been used variously as tackifiers for hot-melt adhesives and pressure-sensitive adhesives, modifiers for rubbers and various plastics, base materials for chewing gum, and raw materials such as resins for traffic paints, sizing agents for paper making, emulsifiers for synthetic rubbers, resins for ink and resins for coating, and the like. However, appearance of the above-mentioned rosin compounds was colored in yellow or yellowish brown, and the compounds were not satisfactory from the viewpoints of odor, thermal stability and weatherability (hereinafter referred to thermal stability and weatherability as stability), and the like.
Therefore, in order to dissolve the above-mentioned problems in the rosin compounds, a disproportionated rosin having an improved thermal stability which is prepared by disproportionating an unpurified rosin, a hydrogenated rosin having a somewhat improved stability which is prepared by hydrogenating an unpurified rosin, or their ester compounds which are prepared by esterifying the above-mentioned disproportionated rosin or hydrogenated rosin are commercially available. However, each and every one of these rosin compounds does not have satisfactory color tone, stability, and the like.
Japanese Examined Patent Publication No. 33771/1970 and Japanese Examined Patent Publication No. 20599/1974 disclose a method for disproportionating rosins or rosin compounds with a specific organic sulfur compound. However, the disproportionated rosin esters or the disproportionated rosins which are obtained according to these methods are not satisfactory from the viewpoints of color tone, stability and odor.
Japanese Unexamined Patent Publication No. 9605/1980 discloses a process for preparing rosin esters having a higher stability, comprising purifying a disproportionated rosin to remove high molecular weight materials, unsaponifiable materials and the like which are contained in the raw rosin and successively esterifying the purified disproportionated rosin with an alcohol. The rosin ester obtained according to the process is relatively superior in light color and stability to the known rosin esters. However, the obtained rosin ester is colored in the above-mentioned esterifying step and does not have a satisfactory heat coloration resistance. Accordingly, the obtained rosin ester should be improved upon the properties mentioned above.
Also, Japanese Unexamined Patent Publication No. 186783/1988 discloses a process for preparing a colorless rosin ester having a high stability. Japanese Unexamined Patent Publication No. 85265/1989 discloses a process for preparing a colorless rosin having a high stability. However, a hydrogenating step is indispensable to these processes. In the processes, an advanced high-pressure-proof apparatus is necessarily used, or a large quantity of hydrogen is consumed. The cost of preparation is remarkably increased thereby. Therefore, these processes are disadvantageous.
Further, Japanese Unexamined Patent Publication No. 230072/1984 discloses a process for preparing a light-colored rosin ester having a high stability, comprising esterifying a purified rosin by distillation with an alcohol in the presence of a specific organic sulfur compound having abilities of both disproportionating and light-coloring. However, the obtained rosin ester does not have satisfactory color tone and stability and there is a problem that the obtained rosin ester has strong sulfuric odor which is generated from the organic sulfur compound when the rosin ester is heated.
As mentioned above, any conventional rosin esters, rosins, disproportionated rosins and hydrogenated rosins have not yet been satisfactory in odor, color tone and stability concurrently. In addition, the above-mentioned rosin compounds could not have competed at all in the above-mentioned properties with the hydrogenated petroleum resins, which are employed in the same uses as the rosin compounds. Therefore, it has been desired to find a rosin ester and a rosin which are colorless and have a high stability.
In consideration of the above-mentioned circumstances, the present inventors have studied to develop novel processes for preparing a rosin ester and a rosin of which various properties such as color tone, odor and stability have been further improved, in comparison with those of conventional rosin esters and rosins, which enable to cheaply provide such a rosin compound.