This invention relates to a modified vegetable protein adhesive binder and a process for producing the same.
Vegetable protein materials are well-known as adhesive binders for pigment containing coatings that are used for the coating of paper. The pigment containing coatings provide the paper with a desirable finish, gloss, and smoothness. The functions of the pigment in the coating are to fill in the irregularities of the paper surface and to produce an even and uniformly absorbant surface for printing. The adhesive also functions to bind the pigment particles to each other as well as to the surface of the coating. The selection of a suitable adhesive is, therefore, an important factor in the quality of a coating for paper.
Vegetable protein materials have been extensively used as the adhesive binders for paper coatings and among these materials have been the vegetable protein isolates, typically soy isolate. Isolated soy protein is produced by the treatment of oil free soybean flakes with an alkaline solution to dissolve the protein which is removed from the non-soluble materials by filtration or centrifugation. The protein is then recovered from the solution by the addition of acid in order to precipitate the protein at its isoelectric point. The precipitated protein is then in an unhydrolyzed or generally unmodified state and can be dried and subsequently dispersed in an alkaline medium to form an adhesive binder for paper coating pigments.
The unhydrolyzed or unmodified soy protein isolates are less desirable as adhesive binders for paper coatings because of low solubility in weak alkali solutions and sensitivity to heat, tending to form gels at temperatures of about 160.degree. F. Because of these disadvantages, unmodified or non-hydrolyzed soy protein is usually modified in some way to lower the viscosity of coating compositions containing soy protein and increase the adhesive strength of the protein when it is dispersed in weak alkaline solutions. Modification or hydrolysis of the protein also reduces the sensitivity of the protein to heat. Modification of the protein usually consists of hydrolysis or treatment of the extracted curd in an aqueous dispersion with various alkaline reagents under controlled conditions of pH, temperature and time. These conditions dissociate the native or unmodified protein structure into smaller subunits thereby improving the suitability of this material as a binder in paper coatings. A process involving the hydrolysis of soy protein with an alkaline reagent in a jet cooker at temperatures above 270.degree. F. is illustrated in U.S. Pat. No. 3,356,516.
Another means of modifying the protein which is described in U.S. Pat. No. 2,274,983 is to treat the unmodified protein with hydrogen peroxide in a mildly alkaline medium in order to produce a modified or oxidized protein which has a much lower viscosity. Treatment of isolated soy protein with sodium peroxide is also taught in U.S. Pat. No. 2,246,466 in order to improve the color, adhesion, viscosity and gelling tendency of vegetable protein adhesive binders. Either of the processes described in these patents describe the treatment of the protein with peroxide in an alkaline medium followed by precipitation of the modified protein material at the isoelectric point of the protein.
U.S. Pat. No. 3,411,925 describes the treatment of a protein and starch composition at an alkaline pH of 8-10 with an oxidizing agent followed by jet cooking of the composition at a temperature of 220.degree.-350.degree. F. It is indicated that this process results in grafting of the starch and protein molecules to modify the viscosity characteristics of the composition and improve its usefulness as an adhesive binder in paper coating compositions.
Modification of vegetable protein isolates, such as soy isolate as generally described above, alters rheological properties of these protein materials such that they are similar to but not identical with casein, which has also long been used as an adhesive binder for pigment containing coatings for paper. In spite of these attempts at modification of isolated soy protein, particularly by oxidation, a need has still existed for modified proteinaceous adhesives which have rheological properties comparable or better than casein and can be modified by an oxidizing agent in a manner which is efficient and convenient to perform. Typical problems with carrying out the mild oxidation of protein in an alkaline medium include difficulties with pH regulation during the oxidation and uncontrollable foaming of the reaction mixture. For example, it is necessary that pH modification be carried out under alkaline conditions and the pH of the reaction mixture is constantly dropping as reactant is consumed thereby requiring continual adjustment of the pH to keep the reaction mixture on the alkaline side.
These objectives were achieved in the present invention and not only were the noted problems with modification of the protein material overcome, but the reaction resulted in a modified proteinaceous adhesive binder having an unexpected improvement in rheological and paper coating properties.
It is, therefore, an object of the present invention to produce a modified proteinaceous adhesive binder of good rheological properties.
It is also an object of the present invention to produce a modified vegetable protein adhesive binder having rheological characteristics which are similar to casein.
It is a further object to produce a modified vegetable protein adhesive binder with good pigment binding and paper coating characteristics.
It is a further object to modify the vegetable protein adhesive binder by a process that is both reliable and convenient to practice.