US2916413A - Paper manufacture - Google Patents

Description

Dec. 8, 1959 Filed April 15. 1957 J. A. HARPHAM PAPER MANUFACTURE 2 Sheets-Sheet 1 JOHN A. HAR PHAM INVENTOR.

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AGENT Dec 8, 1959 J. A. HARPHAM PAPER MANUFACTURE 2 Sheets-Sheet 2 Filed April l5, 1957 .md 2.o of@ mnd @no @No @No 2.o 2.o @0.o

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BY vl-Maf www AGENT `United States Patent() PAPER MANUFACTURE John A. Harpham, Wilmington, Del., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware Application April 15, 1957, Serial No. 652,898

12 Claims. (Cl. 162-157) purification and either with or without mechanical puri` fication. The term purified with reference to cotton linters or linters is used herein to designate raw cotton linters which have been chemically purified by conventional techniques Which are well known in the art, and

whenever' mechanical purification alone is meant, that will be so specified. Degree of substitution (D.S.) is the number of hydroxyalkyl groups per anhydroglucose unit of cellulose.

One of the important properties of paper is its strength as measured by various tests such as Mullen burst, Elmendorf tear, fold strength, tensile strength, and the like. Heretofore, difficulty has been experienced in using purified cotton linters yas a feed in commercial manus facture of fine papers, such as writings, bonds, ledgers, blueprints, and the like, one of the reasons being the failure of purified cotton linters to develop sufficient strength under the usual conditions of treatment. The strength properties of the purified cotton linters can be developed by unusual conditions of treatment in beating, jordaning, and other similar operations, but such unusual treatments are uneconomical and impractical in commercial operations.

In general, there is a desire in the paper industry for improved strength levels in paper from all types of furnishes, and the paper industry is endeavoring to improve these levels by such means as internal additives, wet and dry strength resins, and the like.

In accordance with this invention, it has been found that the strength properties 0f paper can be substantially improved by employing as a feed in the preparation of paper raw cotton linters which have been chemically modified and then purified. The chemical modification of the linters adds hydroxyalkyl substituent groups to the anhydroglucose unit in such anamount that the chemically modiried linters retain substantially the fibrous form of unmodified linters and are both Water-insoluble and alkali-insoluble. Purification is by well-known techniques which will be disclosed more fully hereinafter.

A unique, very important and surprising feature of this invention is the discovery that the specific chemical modification of the raw linters which adds hydroxyalkyl substituent groups provides properties in the paper not obtainable with other chemical modifications of the raw linters, such as 'carboxymethylation Thus, introduction of hydroxyalkyl substituents, for example, hydroxyethyl substituent, provides an improvement in paper strength properties such as burst, fold, tensile and tear not obtainable with carboxymethyl substitution.

Furthermore,

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an additional yand significant distinction is also observed in the sizing of the chemically modified cotton linters in the formation of sized paper. The improved strength properties resulting from chemical modification of the linters paper furnish by hydroxyalkylation carry over into the sized paper to a larger extent, making it possible to produce sized paper of still greater strength properties than it is possible to obtain from other chemically modified linters paper furnishes, such as carboxymethylated furnish. This latter distinction is significant in the use of the hydroxyalkyl-substituted linters in the production of sized paper.

In the art of paper making, it is common knowledge that the levels of certain paper properties, eg., tear strength, opacity, and porosity, are lowered during refining. Tear strength actually increases during the initial stages of refining, but in the commercial range of refining the tear 'strength level is decreasing; the levels of other paper properties, e.g., fold strength, burst strength, tensile strength, and density, are increasing. The rates at which these various properties change during refining are substantially fixed for a given cellulose furnish. However, the same furnish, chemically modified in accordance with this invention, shows a disproportionation of the rates at which the various properties develop such that the end result is a paper with an entirely different cornbination of properties.

Improvements in desirable paper properties other than those discussed y'above are realized by practicing this invention. While these additional properties are highly desirable, no standard means has been established by the industry for measuring them. For example, in the fine paper industry the stiffness, surface and feel of the sheet are important. Heretofore, when attempting to use unmodified cotton linters, the industry has been unable to obtain a high stiffness, the type of surface and the feel 0f the sheet which it regards as being indispensable. The present invention makes it possible to overcome these drawbacks.

Advantages to be gained by practicing this invention are demonstrated by the examples below. In examples l to 8, all the samples of either unmodified and purified cotton linters or chemically modified and purified cotton linters were refined at a 2.5% consistency (neutral pH) in a 1% lb. Valley laboratory beater for 2.5 hoursl with a 6-kg. bedplate load. In all the examples the refined samples were handsheeted using Noble and Wood handsheeting equipment to a 40 lb. basis weight (24 x36- 500) and the handsheets were then conditioned and tested by standard TAPPI procedures (Technical Association of the Pulp and Paper Industry). These tests included Mullen burst, Elmendorf tear, tensile strengths and MIT folding endurance. The strength data were converted to a 40 lb. basis weight using the linear relationships between the various strengths and basis weight. Percent as used herein is on a weight basis.

.EXAMPLE 1 A sample of second-cut cotton linters that had been mechanically and chemically purified was prepared into handsheets in the manner described above. The purification was conventional and included digesting in caustic at an elevated temperature `and pressure and bleaching to give a high brightness cotton cellulose pulp. This example represents a control run, the data of which are to be compared with the data of Examples 2-8 listed in Table I hereinafter. Y A

EXAMPLES 2, 3 AND 5 Three samples of raw mechanically purifiedrsecond-cut cotton linters were steeped in aqueous caustic, reacted with gaseous ethylene oxide, and then cooked and bleached' 3 as in Example l above. The resulting linters samples, having hydroxyethyl D.S. of v0.05, 0.1 and 0.2 (shown as HEC), were then handsheeted and tested in the manner described above. The 4data are listed in Table I below.

EXAMPLE 4 A sample of raw, mechanically purified second-,cut cotton linters was steeped in aqueous caustic, reacted with gaseous ethylene oxide, and then cooked and bleached as modified and purified cotton linters, prepared from mechanically purified raw cotton linters as in the preceding examples, were refined for two hours in a cycle beater at 3.6% consistency with 1GO-lb. roll pressure. In all the examples, the samples were removed and treated with size. Then the sized samples were handsheeted, tested, and the paper strength data converted to a 40-lb. basis weight, all as described above for Examples l-8. By percent size as shown in Table II hereinafter is meant the amount of IhEIXIDHe O'B1(TISU1I1S lill-lflel) having a: hYdTOXX- l0 size started with and not the amount retained on the e y o s own as were tiieri l'iannhamisheetg sheeted and tested in the manner described above. The EXAMPLE 9 data are listed in Table -I. This example is the same as Example 7 of my application, Serial No. 503,803, filed A Sample of Second'ut cotton-hnters that had been April 25, 1955, and my copending application, Serial No. mechanically and diemlcauy plmfd as m Example I 587,471, med May 28 1956 except that thestrength data above was treated with gum rosin size, then handsheeted listed in Table I hereinafter have been converted to a 40- and tested' Thls example represents. a control run the lb. basis weight for comparison purposes. data of which 'fue to. be compared with the data of EX' amples 10-13 listed in Table II hereinafter. A l f EXAMPIIIE 6 l zo EXAMPLES 10 AND 11 samp e o raw mechanica y pur' ed second-cut cotton linters was steeped in aqueous caustic, reacted with csamls (if hydsiyetllged cotto? hlftsll propylene oxide, and then cooked and bleached as in ast ant glmg a '.0 t/erellzed ted! el Example 1. The resulting linters, having ahydroxypropyl stemoun S gum msm Slze en an s ee an D.S. of 0.05 (shown as HPC), were then handsheeted and tested in the manner described above. The data are listed EXAMPLES 12 AND 13 in Table I. This example is the same as Example 6 of my These examples were similar to Examples 10 and 11 copending application, Serial No. 534,881 filed September with the exception that an alkyl ketene dimer (Aquapel 16, 1955, and it is the same as Example 9 of my applicaas marketed by Hercules Power Company, Wilmington, tion, Serial No. 503,803, filed April 25, 1955, except that 30 Delaware) was employed as the sizing material.

' Table 11 Example 9 Example 10 Example 11 Example 12 Example 13 Paper-Finnish unmodified Purified HEC HEC HEC.. HEC.

Cotton Linters. D.s 0.05 0.05 0.05 0.05. lzahgi slum Rosin. 2Guin Rosin. luapel.. (.tiqunpel Mefen BmS'iTrJE-.iijj Elmendorf Tear, g./sheet MIT Folds, double the strength data listed in Table I hereinafter have been converted to a 40-lb. basis weight for comparison purposes.

EXAMPLE 7 A sample of raw mechanically purified second-cut cotton linters was steeped in aqueous caustic, reacted with butylene oxide, and then cooked and bleached as in Example l. The resulting linters, having a hydroxybutyl D.S. of 0.01 (shown as HBC), were then handsheeted and tested in the manner described above. The data are shown in Table I.

EXAMPLE 8 A sample of mechanically purified second-cut raw cot- Table l The advantages of this invention are realized by chemically modifying cellulose in such a manner that hydroxyalkyl substituent or group is introduced to the anhydroglucose units and employing the resulting chemically modified cellulose, having a low degree of substitution, in paper manufacture. Among the hydroxyalkyl substituents that are introduced to the cellulose are the hydroxyethyl, hydroxypropyl, hydroxybutyl, and similar radicals or groups, either singly or in combination.

For the practice of this invention, the degree of substitution of the hydroxyalkyl cellulose is such that the cellulose derivative is water-insoluble and alkali-insoluble, and the chemically modified cellulose retains substantially the fibrous form of the unmodified cellulose. The actual degrees of substitution necessary to impart either alkali solubility or water solubility to a chemically modified cellulose are either known or easily determinable and depend to some extent upon the method of preparation of the modified cellulose and upon the uniformity of the substitution.

In the practice of this invention, it has been found Example 1 Exagiple Example 3 Example Example Example Example 5 6 7 8 Unmodled Purled Cotton Linters.

Paper Furnlsh D.S Mullen Burst, lb./sq. in

MIT Folds, double Tensile. lb./in

In the following Examples 9-13 all the ,samples of either unmodified andpurified cottonlnters or Acheiriically HEO BEPC necessary to employ a hydroxyethyl cellulose having a degree of substitution not in excess of about 0.20. There are a number of reasons for this. One reason for this restriction as to D.S. is that the higher the D S.v the greater the hydrophilic nature of the linters and as a consequence, under conditions of high humidity, the increased hydrophilic nature is apt to cause a decrease in the improved stiffness obtained by chemical modification. Another consequence of the increased hydrophilic nature is that it resultsin production problems such as difficulty in drying the modified linters and excessive shrinking of the paper sheet during drying. Another reason for this restriction as to D.S. is that D.S and paper sheet opacity vary inversely. Above a D.S. of about 0.20 the decrease in opacity has become quite undesirable. A further reason for the D.S. restriction is that, based on applicants finding, the over-al1 strength properties of the paper level off at a D.S. of about 0.20.

Thus by operating at a D.S. not exceeding about 0.20, applicant avoids the difficulties discussed above and still obtains a paper having substantially maximum strengths.

In considering the strength of paper it is the over-all strength properties, rather than an individual strength property, which are important. Such a measure of the over-all strength properties of paper, as disclosed above, has been referred to in the art and trade as the strength number and expressed as` 3V Tear burst X log fold see TAPPI 33, 370 (1950).

Figs. l-4 of the accompanying drawing clearly show that the over-all strength properties of paper level off at a D.S. of about 0.20. Data for Figs. 1-3 were obtained by preparing and testing five samples, one sample in exactly the same manner as Example 1 hereinbefore (i.e., an unmodified cotton linters control) and four samples in exactly the same manner as Examples 2, 3 and 5 hereinbefore, except for using beating times of 2 and 2.5 hours and varying amounts of ethylene oxide` to give D.S. levels of 0.11, 0.21, 0.28 and 0.43. Thus, the unmodified and the modified linters were refined and portions thereof were removed from the beater after 2.0 and 2.5 hours beating and handsheeted. The handsheet properties of tear, burst and fold Werevmeasured according to TAPPI methods. These properties are shown graphically in Figs. 1-3, D.S. having been plotted against tear in Fig. l, against burst in Fig. 2, and against fold in Fig. 3. From Figs. l-3 tear, burst, and fold values respectively were read off the graphs at each beating time (2 and 2.5 hours) for the D.S. values of 0, 0.10, 0.20, 0.30 and 0.40. These tear, burst and nfold values were then used to determine the strength number of the paper at all of these latter D.S. values for each beating time according to the formula Va Tear burst X log fold and these strength numbers Were plotted graphically against these latter D.S. values in Fig. 4. It is clear from Fig. 4 that the "strength number levels ofi at a D.S. of about 0.20.

Thus by practicing this invention one obtains an improved paper having substantially increased over-all strength properties (Fig. 4) without encountering production problems and without a sacrifice in other desirable properties of paper such as high stiffness and opacity.

The chemically modified cellulose for use in the practice of this invention is usually formed by first converting mechanically purified raw cotton linters to alkali cellulose by steeping in an aqueous caustic solution. The alkali cellulose is then reacted with a suitable hydroxyalkylating agent, such as ethylene oxide, in order to introduce the desired amount of hydroxyalkyl substituent into the anhydroglucose unit. The reaction can be carried out in a slurry medium, such as an aqueous solution of a lower aliphatic alcohol, for example, propanol, isopropanol, tertiary butanol, and the like, but if desired vthe slurrying medium need not be used.

To form the improved paper product of this invention, the chemically modified cellulose is made into an aqueous suspension, and commercially this suspension usually has a consistency not greater than about 5%. The aqueous suspension can then be passed to a suitable beater, and it will usually be found that the strength properties of the paper product depend to a substantial extent upon the length of the beating cycle. It is preferred that the beating cycle not exceed a period of several hours for economical commercial operation. Actually, if given an adequate beating'period, unmodified purified secondcut cotton linters can be used to form a paper having satisfactory properties for many purposes. However, in order to impart satisfactory strength to such papers, it is necessary to provide the linters With an unusual and uneconomical refining treatment. By practicing this invention the strength development with chemically modified second-cut cotton linters can be achieved in conventional retining cycles. After beating, the aqueous suspension is used to form paper in any of the conventional procedures. For example, the aqueous suspension can be continuously flowed onto a traveling endless tinemeshed screen called a Wire, and water is drained from the suspension through the wire. The chemically modified cellulose forms a matted or felted web of wet fibers on the wire from which additional water is removed by suction. The web is then carried through several pairs of press rolls to squeeze out as much additional water as possible. Most of the remaining Water is subsequently removed by evaporation as the wet web is pressed against successive smooth surfaces of revolving steam-heated cylinders or drums. The dried web usually is next passed between calender rolls for smoothing and then reeled up.

Paper produced in accordance with this invention can be given conventional treatments used in paper manufacture, such as addition of various modifying materials, for example, size, coloring materials and mineral fillers. A satisfactory method of introducing these modifying materials involves their addition to the aqueous suspension of chemically modified cellulose prior to its passage onto the traveling wire, for example, during the beating operation.

In practicing this invention, the chemically modified cellulose, such as hydroxyethyl cellulose, can be used alone or in admixture with other cellulosic materials for the preparation of paper by conventional refining techniques having the desired strength properties.

While the chemically modified cellulose herein disclosed is particularly suitable for the production of fine papers such as bonds, ledgers, blueprints and the like it may also be used in the production of other types of papers. Thus, by changing refining procedures'as, for example, by shortening the refining time, it is possible to obtain paper having the requisite porosity, absorbency and strength to adapt it for use in filter paper, battery separator plates, laminates and the like. Such papers, compared to those produced from cellulose which has not been chemically modified as herein described, are considerably stronger at the same porosity levels and considerably more porous at the same strength levels.

This application is a continuation-in-part of my copending applications, Serial No. 534,881, filed September 16, 1955, and Serial No. 587,471, filed May 25, 1956, which in turn are continuation-impart of my application, Serial No. 503,803, led April 25, 1955, the latter three now abandoned.

What I claim and desire to protect by Letters Patent it:

1. In the method for forming paper wherein an aqueous suspension of a fibrous material is dewatered on a screen to form a matted web and said matted web is dried, the improvement which comprises employing as a fibrous material a water-insoluble and alkali-insoluble hydroxyalkyl cellulose whereby a paper of improved strength is obtained, said celluose resulting from etherifying raw cotton linters to a D.S. not in excess of about 0.20 and then chemically purifying same.

2. In the method for forming paper wherein an aqueous suspension of a fibrous material is dewatered on a screen to form a matted web and said matted web is dried, the improvement which Acomprises employing as a fibrous material a water-insoluble and alkali-insoluble hydroxyethyl cellulose whereby a paper .of improved strength is obtained, said cellulose resulting from etherifying raw cotton linters to a D.S. not in excess of about 0.20 and then chemically purifying same.

3. In the method for forming paper wherein an aqueous suspension of a fibrous material is dewatered on a screen to form a matted web and said matted web is dried, the improvement which comprises employing as a fibrous material a water-insoluble and alkali-insoluble hydroxypropyl cellulose whereby a paper of improved strength is obtained, said cellulose resulting from etherfying raw cotton linters to a D.S. not in excess of about 0.20 and then chemically purifying same.

4. In the method for forming paper wherein an aqueous suspension of a fibrous material is dewatered on a screen to form a matted web and said matted web is dried, the improvement which comprises employing as a fibrous material a water-insoluble and alkali-insoluble hydroxybutyl cellulose whereby a paper of improved strength is obtained, said cellulose resulting from etherifying raw cotton linters to a D.S. not in excess of about 0.20 and then chemically purifying same.

5. In the method for forming paper wherein an aqueous suspension of a fibrous material is dewatered on a screen to form a matted web and said matted web is dried, the improvement which comprises employing as a fibrous material a water-insoluble and alkali-insoluble hydroxyethyl hydroxypropyl cellulose whereby a paper of improved strength is obtained, said cellulose resulting from etherfying raw cotton linters to a D.S. not in excess of about 0.20 and then chemically purifying same.

6. As a new article of manufacture, a sheeted paper product of improved strength comprising fibrous raw cotton linters modified chemically with hydroxyalkyl substituent and then chemically purified such that the modified cellulose is water-insoluble, alkali-insoluble, has a D.S. not in excess of about 0.20 and retains substantially the fibrous structure of the unmodified cellulose.

7. As a new article of manufacture, a sheeted paper product of improved strength comprising fibrous raw cotton linters modified chemically with hydroxyethyl substituent and then `chemically purified such that the modified cellulose is water-insoluble, alkali-insoluble, has a D S. not in excess of about 0.20 and retains substantially the fibrous structure of the unmodified cellulose.

8. As a new article of manufacture, a sheeted paper product of improved strength `comprising fibrous raw cotton linters modifiedl chemically with hydroxypropyl substituent and kthen chemically purified such that the modified cellulose `is Water-insoluble, alkali-insoluble, has la D.S. not in excess ,of `about 0.20 and retains substantially the fibrous structure of the unmodified cellulose.

9. As a new article of manufacture, a sheeted paper product of improved strength comprising fibrous raw cotton linters modified with hydroxybutyl substituent and then chemically purified such that the modified cellulose is water-insoluble, alkali-insoluble, has a D S. not in excess of about 0.20 `and retains ,substantially the fibrous structures of the unmodified cellulose.

10. As a new article o f manufacture, a sheeted paper product of improved strength comprising fibrous raw cotton linters modified with hydroxyethyl and hydroxypropyl substituents and then chemically purified such that the modified cellulose is water-insoluble, alkaliinsoluble, has a D.S. not in excess of about 0.20 and retains substantially the fibrous structure of the unmodified cellulose.

1l. As a new article of manufacture, a sheeted paper product of improved strength comprising fibrous raw cotton linters modified chemically with hydroxyalkyl substituent and then chemically purified such that the modified cellulose s water-insoluble, alkali-insoluble, has a D.S. of not in excess of about ,0.20 and substantially retains the fibrous structure of the unmodified cellulose, said paper product being sized with gum rosin size.

12. As a new article of manufacture, a sheeted paper product of improved strength comprising fibrous raw cotton linters modified chemically with hydroxyalkyl substituent and then chemically purified such that the modified cellulose is water-insoluble, alkali-insoluble, has a D.S. of not in excess of about 0.20 and substantially retains the fibrous structure of the unmodified cellulose, said paper product being sized with an alkyl ketene dimer.

References Cited in the le of this patent UNITED STATES PATENTS 1,941,278 Schorger Dec. 26, 1933 2,038,679 Richter Apr. 28, 1936 2,172,109 Reichel et al. Sept. 5, 1939 2,190,445 Ellsworth Feb. 13, 1940 2,268,112 Dreshfield Dec. 30, 1941 2,533,145 Schorger Dec. 5, 1950 2,535,690 Miller et al. Dec. 26, 1950 2,627,477 Downey Feb. 3, 1953 FOREIGN PATENTS 674,577 Great Britain June 25, 1952

Claims (1)

1. IN THE METHOD FOR FORMING PAPER WHEREIN AN AQUEOUS SUSPENSION OF A FIBROUS MATERIAL IS DEWATERED ON A SCREEN TO FORM A MATTED WEB AND SAID MATTED WEB IS DRIED, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS A FIBROUS MATERIAL A WATER-INSOLUBLE AND ALKALI-INSOLUBLE HYDROXYALKYL CELLULOSE WHEREBY A PAPER OF IMPROVED STRENGTH IS OBTAINED, SAID CELLULOSE RESULTING FROM ETHERIFYING RAW COTTON LINTERS TO A D.S. NOT IN EXCESS OF ABOUT 0.20 AND THEN CHEMICALLY PURIFYING SAME.

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