US2032615A - Paper making - Google Patents

Description

Patented Mar. 3, 1936 UNITED STATES PATENT OFFICE No Drawing. Application December 28, 1933, Serial No. 704,332

3 Claims.

This invention relates to paper making; and it comprises a method of preparing a paper stock for paper making wherein a major-proportion of wood pulp made by the kraft sulfate process and incomplete bleaching is beaten together with a minor proportion of a fully bleached sulfite pulp; and it further comprises a paper stock preparation of superior whiteness and strength and possessing a paper making degree of freeness, said preparation comprising a beaten mixture of incompletely bleached and unbeaten kraft pulp with a minor proportion of unbeaten fully bleached sulfite pulp; all as more fully hereinafter set forth and as claimed.

Wood pulp of the type known as kraft pulp and made from various woods by the kraft or sulfate pulping process, has come into wide use in paper making because of the economy of this pulping process and the strength of fiber of the pulp made. For making the cheaper grades of paper such as wrapping papers from wood the sulfate process has to a great extent displaced the older soda process and the sulfite process.

Kraft pulp is generally used when strength of paper is particularly desired.

When it is desired to make a smooth white finely finished paper for writing or printing, certain difiiculties arise in using kraft pulp. One of the chief difficulties is in bleaching; kraft pulp being a hard bleaching pulp. The drastic chemical action required for full bleaching results usually in loss of fiber strength and in loss of cellulose; out-Weighing in some cases the advantages of economy and strength of fiber inherent in the sulfate pulping process.

Another difliculty arising in the use of kraft pulp for making printing and writing papers is the drastic beating required for good conformation of the paper produced. To make good paper of maximum strength from kraft pulp it is necessary to beat the pulp with water in the beating engine for unduly long periods of time or to treat the pulp extensively in a Jordan engine. Long beating or extensive J ordaning is required to bring the pulp to a degree of hydration or gelatinization which puts the pulp in a proper condition of freeness or slowness for working in the paper machine. Long beating requires the expenditure of much power and plant equipment and the Jordan treatment shortens the cellulose fibers and weakens the paper. Thus again the economy and strength of kraft pulp are lessened.

It is an object of the present invention to utilize kraft pulp for making the finer grades of paper without the disadvantages above described.

I have found that by admixing with partially bleached kraft pulp prior to its beating a relatively small amount of fully bleached but unbeaten sulfite wood pulp, the kraft pulp can be made available for fine papers with less drastic bleaching and with shorter beating time. The paper made from the beaten mixture of'pulps is whiter and closer as well as stronger and less expensive than fine paper made from kraft pulp alone or from beaten kraft mixed with other cellulose materials after the beating.

Perhaps the most striking result obtained by mixing a minor proportion of sulfite pulp with the kraft pulp before beating is the shortening of the beating time required. Almost as striking however is the effect of a small amount of fully bleached sulfite pulpin whitening a kraft pulp which has been only partially bleached. This permits a less drastic bleaching of the kraft pulp with maintenance of its fiber strength and conservation of the cellulose which would otherwise be lost in bleaching.

I have found that a satisfactory degree of freeness or slowness is produced in kraft pulp with a great reduction of the beating time and without much J ordaning by adding to the kraft pulp prior to beating a proportion of sulfite pulp. A mixture of per cent kraft and 25 per cent sulfite, for example, has been found to reach a normal slowness with a beating time from 25 to 45 per cent less than the beating time required for kraft pulp alone. Expressed in another way, the 33 per cent admixture of sulfite with the kraft produces a paper of equal or greater closeness and. strength with a beating time from 55 to 75 per cent of the normal beating time for kraft alone.

It has been found by actual test that a paper made from a cobeaten mixture of partially bleached kraft and a small proportion of fully bleached sulfite pulp has greater strength than paper made from fully bleached kraft alone in spite of a shorter beating time for the mixed pulps. The superior strength of the kraft fiber is not lessened by the sulfite admixture. The whiteness of the paper is equal to that of fully bleached kraft and the strength is greater. And the paper is cheaper to make.

The character of kraft pulp fibers is such that a sheet of paper made entirely of kraft pulp becomes rough from erasures of writing. A cobeaten mixture of kraft and sulfite forms a close smooth sheet that does not roughen with erasures.

An additional beneficial result of a small admixture of sulfite with kraft pulp in the beating engine is the whitening effect upon the paper formed from the beaten mixture. Kraft pulp of a slight cream tint when admixed prior to beating with about one third its weight of fully bleached sulfite makes a paper having a whiteness almost equal to that of pure white sulfite paper. The improvement in whiteness is greater in proportion than that corresponding to .the amount of sulfite admixture.

I have found that by the present invention it becomes possible to dispense with the usual drastic bleaching of the kraft pulp which weakens the fiber and causes loss of cellulose in the bleaching. The two pulps are advantageously separately bleached by the process described and claimed in my copending application Serial .No. 691,075, filed Sept. 26, 1933 (Patent No. 1,990,942) pretreatment with a solution formed by treating limestone with chlorin and water followed by ordinary bleaching with a bleach liquor. The sulfite is readily bleached by that process to a pure white color. The more difficult bleaching of the kraft pulp requires a relatively less drastic action in that process and the kraft may be incompletelybleached .with 'a minimum loss of cellulose and of fiber strength, the pulp being left with awslight cream tint. The beating of the kraft and sulfite admixture to a paper making condition in the limited time required results in a, paper possessing a whiteness corresponding to that of a fullybleached kraft pulp'paper and superior strength and more uniform conformation.

Kraft pulp can be bleached to a slightly creamy tint (say about 70 per cent white by the T. A.-P. P. I.standard) with no material loss'of .fiber or of fiber strength and with only about halfthe available chlorin required to reach 85 per-cent white. The cost of bleaching-kraft to 70 whiteis about half that of bleaching to 85-white. A paper of strength as measured bythe Mullen test approximating that of unbleached kraft paper but having a whiteness well over 80 per cent can be cheaply produced by cobeating a mixture of about 75 per cent kraft bleached to about '70 whiteness with about 25 percent of fully bleached sulfite.

The minor proportion of sulfite admixed with a major proportion of kraft pulp may be varied within wide limits to meet varying requirements in the paper to .be produced. Generally speaking the greater the proportion of sulfite the less is the beating time required and the better the whiteness of the paper. is usually cheaper in cost of production, it is desirable to keep the proportion of sulfite at a minimum. .In most cases an admixture of one part of *sulfitewith three parts of kraft suffices to give the desired whiteness of paper and shortincompletely bleached raw kraft -;pulp in a 'major' proportion with a minorproportion-of 'a :fully bleached raw sulfite pulp and beating thermixed But as the kraft pulp pulps to a condition of freeness suitable for 7 paper making.

2. In-the preparationof raw kraft-woodzpulp for paper making, .a method of reducing thev time required in beating the pulp to a #desired condition of slowness whichzcomprisesadmixing in proportions of about per cent'kraftto125.

per cent sulfite, beating said mixture to a .de-

sired condition of slownessrandformingg-paper-of said beaten mixture.

.1 CLARK .T. .HENDERSON.