US 1998758 A
Description (OCR text may contain errors)
Patented Apr. 23, 1935 1,998,758 PATENT OFFICE TREATMENT OF PAPER PULP Harold S. Holt and George L. Schwartz, Wilmington, Del., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application April 3, 1933, Serial No. 664,286
This invention relates to the treatment of paper pulp, and more particularly to an improved starch impregnated paper or felt.
The incorporation of starch in paper has been known in the prior art, such processes generally consisting in passing the finished web of paper through a suitable solution of starch or by mixing starch with the paper pulp in a heater before depositing the web. The former process is known as tub sizing, while the latter process is known as beater or engine sizing. Both methods of starch sizing are usually employed as a means of imparting a leathery feel to the paper and as a means of modifying the properties of the paper in order that the finishing operation will impart a smooth, shiny surface to the paper. Starch sizing also increases the hardness and stiffness of the paper and reduces absorbency toward ink, especially in the case of book and writing papers. In addition to being non-absorbent products, starch impregnated papers, as conventionally produced, have a lower tearing strength than the untreated paper. According to Bureau of Standards Technical News Bulletin #162, the tearing strength of papers which have been surface sized with starch according to the common commercial procedure undergoes a decrease which is roughly proportional to the amount of starch taken up.
This invention has as an object a starch impregnated paper or felt having an exceptionally high tearing strength, good flexibility, and little 'or no tendency to exhibit paper break. A further object is a process of manufacturing astarch treated pulp which may be made into paper exhibiting a tearing strength markedly greater than paper made from untreated pulp. Further objects will appear hereinafter.
In carrying out our invention, we hydrate a superficially mercerized or caustic treated paper pulp such as disclosed in Patent 1,857,100 by beating this artificially crinkled pulp in water and treating the pulp before, during or after the hydration with starch in accordance with the procedure more fully disclosed hereinafter.
We have found that this caustic treated 'pulp after hydration by long beating in water, although undergoing a considerable decrease in freeness from the high value of the superficially mercerized pulp, is capable of absorbing an increased amount of starch with consequent increase in tearing strength. The increased amount of starch absorbed and the nature of the hydrated pulp contribute to the increased strength and toughness of our product. The beating of the pulp in water is conducted in a manner which prevents cutting or abrasion of the fibers so that the decrease in freeness is caused almost entirely by the hydration of the pulp. In the present process the amount of starch absorbed rises as the freeness decreases. This is altogether unexpected because it is directly opposite from the results obtained with other colloidal impregnated media, such as resins and rubber. The amount of these latter materials absorbed increases as the freeness increases.
The objects of our invention in the preferred embodiment are accomplished, by the steps which consist in treating ordinary paper pulp with strong caustic solution under conditions which avoid drastic action between the caustic and the fiber, beating the caustic treated pulp with water in a special manner which causes hydration of the pulp without substantial shortening of the fiber length, and then incorporating starch into the hydrated pulp.
As the initial.-rnaterial, we use pulp such as wood pulp composed of fibers that are long enough (2.0 mm. to 4.0 mm. mean fiber length) for making strong paper. The pulp may be made by any of the usual chemical methods of manufacturing paper pulp from the raw cellulose material. The fibers are not purified further than that which takes place in the ordinary sulfate, sulfite, or soda processes of cooking the wood chips in the manufacture of the pulp which constitutes the raw material in the present process. Rope, flax, hemp and other long fibers may be used, providing the fiber length has been reduced by cooking or beating, or both, to a fiber length of 2.0 to 4.0 mm. average length. A highly purified pulp is undesirable in the practice of our invention and we therefore hydrate the pulps having a pentosan content above and an alpha cellulose content below 93%.
In producing a superficially mercerized pulp which is to be subsequently hydrated and impregnated with starch as described hereinafter, we proceed in accordance with the process set forth in detail in the patent mentioned above. Briefly, this consists in dispersing the pulp with caustic of mercerizing activity and promptly diluting the caustic in contact with the pulp below mercerizing activity as soon as the pulp has been dispersed into the caustic liquor. In the present process, however, it is not desirable to use concentrations of sodium hydroxide above 14% because of the long time required to hydrate the pulp by beating it in water. It is to be understood, however, that with the exception of this disadvantage the higher caustic concentrations up to 35%, where the wetting properties of the solution become very poor,'can be used, but in any event the best results are obtained when'the treatment is conducted so as to prevent extended time action of the strong caustic on the pulp. In the practice of our invention, we may use sodium hydroxide concentrations above 6.5%, which is Just slightly below the concentration required for a. solution of mercerizing activity. A concentration of 8% is preferred. The concentrations mentioned are those which have suitable activity at room temperatures. It will be understood that if other mercerizing agents or temperatures are used the content of the reagent in the solution will be varied in the known manner in order to give solutions of equivalent activity. In order to minimize the drastic action of the caustic on the pulp and to prevent the action of the strong caustic on the fibers from extending beyond a superficial or skin effect, the pulp is dispersed at high consistencies, i. e., consistencies above 10% and preferably about 18%, the ratio of caustic to pulp at an 18% consistency being 4.56 parts by The ratio of the treating solution to the dry fibers is very important for the reasons just mentioned and because of the economy of operation, reduction of weight loss, as well as certain mechanical advantages obtained by dispersing the pulp with the caustic at high consistencies. The speed with which the sodium hydroxide solution is dispersed into the pulp is very important. Slow dispersion results in obtaining fibers treated with different concentrations due to dilution, especially when moist pulp is used. The best conditions are obtained when a sodium hydroxide solution is dispersed into the pulp instantaneously. A longer time of treatment than is required for good dispersion of the pulp with the caustic is undesirable. The freeness of the pulp treated with the caustic alkali solution in the manner just described, as tested by the special freeness tester mentioned in the patent noted above, will as a rule vary from 60 seconds for a 6.5% caustic treatment to 31 seconds for a 14% caustic treatment, these values being also dependent upon the kind of fibers used.
The mechanism for dispersing the pulp with the caustic is preferably the disc refiner in the patent mentioned above. Other mechanisms (such as the ordinary Hollander beater with the beater roll raised slightly above the beater plate) capable of exerting a squeezing rather than a cutting action on the fibers during the mixing, may however be used. The time of treatment, that is, the time the pulp is in contact with the strong caustic is usually not more than 15 seconds when using a swift dispersing mechanism such as the disc refiner. With slower types of mixing apparatus, such as the Hollander beater, the time required for efficient dispersion may be as long as 15 minutes. A time as long as half an hour, even at high pulp consistencies, would be undesirable for most pulps.
In the treatment of the pulp with the caustic the temperature is a very important factor and room temperature, that is a temperature between 20-40 C., is preferred. For temperatures below 20 C. the concentrations of treating liquor must be reduced to conform to the increased mercerizing efiect. However, treatments with low temperature solutions give products that are much more diihcult to hydrate by beating than products obtained at room temperature or above. Temperatures above 40 C. are avoided because of the considerable degradation of cellulose which takes place at higher temperatures.
The caustic in contact with the pulp is diluted below mercerizing activity promptly after the solution of caustic and pulp are dispersed. This dilution is effected with the 4% caustic resulting from a countercurrent washing system in order to reduce the amount of fresh water taken into the system and to minimize the expense of recovering the sodium hydroxide. The pulp, after it has been washed on the counter-current washing system and formed into wet lap (the washing system disclosed in the mentioned patent or an Oliver filter being used for this purpose), is ready for the last or hydration step.
In beating the treated and washed pulp with water in order to hydrate it, the beater roll must be close enough to the bed plate to cause pressure and squeezing action on the pulp but without cutting or abrasion of the fibers. The temperature during the beating of the pulp with water should not be allowed to rise above 50 C. In order to get the proper squeezing action it is important, not only that the beater roll be set at a brushing position to avoid cutting the fibers, but that the pulp be present in the aqueous dispersion at a consistency somewhat higher than is ordinarily used in processes involving beating pulp in water. For this reason we prefer,to operate with consistencies between the limits of about 4% to 5%. While it is possible to operate with consistencies as low as 3% and as high as ti this practice is undesirable because of the excessive care required to prevent bruising or cutting at the lower consistencies and to prevent overheating with consequent retardation of hydration at the higher consistencies. Those skilled in the art will understand what is meant by a light or medium brush setting of the beater roll which does not reduce the freeness appreciably by cutting action. The following is a description of the beating conditions in terms of units of measurement for a 10 pound laboratory beater: Consistency of pulp in water 5%; tem perature during beating 25 to 40 C.; bed plate 12" x 3" with bars and between bars, bars set diagonal to beater bars; beater bars broad with 1" spacings; roll 12" face and 14" diameter; peripheral speed of roll 1570' per minute; pressure on bed plate, which is movable, 27,000 grams (this pressure is obtained by a leverage ratio of 4:1). I
While a more or less completely mercerized or highly purified pulp may be successfully hydrated by conducting the beating in water under the special conditions described above, the beating time required is very long and the product obtained after impregnation with the starch is less desirable for the purposes of the invention.
The following example is illustrative of the preferred method of carrying out our process:
Air dry kraft pulp having a freeness of seconds was treated with a 9% solution of sodium hydroxide at 25 C. by continuously feeding the shredded pulp and sodium hydroxide solution into a disc pulper so that the consistency of the pulp in contact with the caustic solution was 20%. The pulp as it left the disc was intimately mixed with the caustic solution and fell into a stream of a 4% caustic solution at 25 C. in sufficient quantity to give a consistency of 0.5%. The caustic solution was washed from this pulp by means of a continuous segmented suction filter and the last traces of alkali were neutralized with a dilute solution of sulfurous acid. The treated pulp having a freeness of 53 seconds was then charged into a Hollander type beater at a consistency of 5% and at a temperature of 25 C. The beater was run for 30 minutes with the roll in' a light brushing position and during the next two hours the roll was gradually let down to a medium brushing position which was maintained for four hours more without cutting the fibers until the freeness of the pulp was 150 seconds. A 4% solution. of swollen corn starch was then added to this pulp in sufficient amount to give 30 parts dry weight of starch to every 100 parts dry weight of fiber. The temperature of this pulp solution at the end of the beating period was 30 C. This mixture was passed through the regular preparation equipment emerging onto the Fourdrinier wire at a consistency of 0.45%. It was formed into a sheet of paper by the regular wet press and drier can method followed by calendering through a regular paper machine stack of calenders. The final product contained 10.4% of starch and had an Elmendorf tear of 709 grams for a sheet weighing 0.87 pounds per 36" x 40" area.
It is desirable to control rather carefully the degree of hydration of the fibers. With many starches the amount of starch retained by the fibers, as well as the tearing strength of the paper fluencedto a considerable extent by the degree of hydration. This is illustrated by the data in Table I which shows the amounts of starch retained by the alkali treated pulp which has been hydrated to different degrees, as well as the tearing strengths of the paper sheets after starch treatment according to the process disclosed herein. The data in Table I were obtained from air dried sheets prepared in the laboratory and the tearing strengths obtained were higher than the tearing strengths obtained from similar paper made on a Fourdrinier machine; hydration on the tearing strength of starch treated pulp made on a Fourdrinier machine is given in Table II. The first item in Table I, the
'pulp having 32 seconds freeness, received the caustic treatment described in the example, but was not hydrated. The remainingrsamples received the same caustic treatment but were hydrated to different degrees as indicated by the slower freeness values. The degree of hydration is measured by determining the freeness of the pulp according to the method described in the mentioned patent. Hydrating the fibers has the effect of decreasing the freeness of the pulp, i. e., the more highly a pulp is hydrated the slower it becomes on the Fourdrinier wire.
Table I Tearing Freeness in strength h in z as ue Seconds paper gg i gg to has ting 32 8. 6 1210 Standard (not by rated) 43. 7 l0. 2 i335 l0. 3
Table II Tearing Per tent in- Freenass in 22; 2 2 strength crease in seconds a r after starch strength due D De treatment to hydration It is evident from the data in Table II that the effect of hydration is more noticeable when operating'in the plant, i. e. on a paper machine, than with hand sheets prepared in the laboratory.
The above tearing strength figures are one. basis of sheets having a weight of 0.87 pounds The effect of per 36" x 40" area. All the freeness values mentioned herein are obtained with the special freeness tester described in detail in the patent referred to above. Corn starchwas used for making the samples given in the table. Because of the difficulties in accurately determining the starch content, the figures relating thereto should be considered as approximate only and as showing generally the fact that the tearing strength for a given amount of starch increases with the degree of hydration and that a small increase in starch content causes a considerable increase in tearing strength as the degree of hydration (as evidenced by the decrease in freeness) increases.
Although the tearingstrength of the paper after starch treatment increases with increasing hydration of the fiber, there are other factors that limit the degree of hydration which should be attained in actual practice. drated pulp in many cases absorbs starch to such a large extent that the final product after drying is stiff and boardyand the starch treated pulp when freshly deposited as a web is very slimy, making it difficult to handle on a paper machine. Maximum strength and a high degree of final pliability are obtained if the crinkled fibrous base material is beaten (hydrated) to a freeness of 150 to '200 seconds before the starch treatment. Pulps within this range are still considerably freer than pulps used in the manufacture of other types of papers, these latter pulps seldom having a freeness of less than 300 seconds.
Our preferred practice is to hydrate the pulp before adding the starch. If the paper m ll is using a closed system for the water, the starch is added before hydrating the pulp,
A number of precautions should also be observed in the preparation of the starch solution and in the starch treatment of the crinkled pulp after hydration. According to the preferred practice of our invention, we mix the hydrated crinkled pulp with a 4% solution of swollen starch in water. The starch solution is prepared by adding a paste containing 40% starch and 60% cold water to suflicient hot water C) to give a solution whose starch content is 4%. Starch solutions of a lower solids content may be used, but solutions of higher solids content are liable to be too viscous to be conveniently handled. This applies largely to solutions containing starches which have not been chemically treated to increase their solubility in water. Starch solutions having solids contents higher than 4% may be used in case the starch gives a low viscosity solution in water. The temperature of the water to which the starch paste is added must be high enough to cause a swelling of the starch, since unswollen starch does not impart the properties to the paper that are obtained with swoolen starch. The hot solution of swollen starch prepared according to the above process should be cooled to about 45 C. be. ore it comes in contact with the hydrated pulp, since the starch will not be absorbed as readily by the pulp fibers when the temperature of the starch solution is too high.
The ratio of starch to pulp may be varied over wide l mits, depending on such factors as the amount of crinkling the fibers have undergone.
the degree of hydration of the pulp, the type of pulp used, and the type of starch used. The preferred ratio of starch to pulp varies from 0.3 pound of starch. per pound of pulp to 1.25 pounds of starch per pound of pulp. Good results may be obtained, however, with ratios varying from 0.1
A very highly hypound of starch per pound of pulp to 2.0 pounds of starch per pound of pulp. The most desirable ratio of starch to pulp is that ratio which will produce maximum tearing strength without imparting an undesirable stiffness to the final prodnot.
It is important that the mixing of hydrated pulp and starch solution be carried out under carefully controlled conditions. Prolonged or vigourous mixing will result in the formation of lumps throughout the mass of crinkled base material. I'hese lumps are known as birds' eyes, and their formation is accompanied by a serious loss in the tearing strength of the paper after it is formed. In order to avoid the formation of birds eyes, therefore, gentle stirring is employed and in stirring is preferably carried out intermittently and for only short periods of time.
In the conversion of the starch treated pulp into paper as, for example, with the Fourdrinier machine, it is desirable to maintain a certain concentration of swollen starch in the white water or back water which carries the pulp over the wire. A concentration varying from 0.5% to 1.5% has been found suitable for this purpose. The use of back water containing swollen starch results in a paper of higher tearing strength than is obtained when fresh water is used to carry the pulp over the Fourdrinier wire. This precaution also tends to give a slow stock having better formation than is obtained when fresh water is used.
Any fibrous cellulosic material commonly used in paper practice such as kraft, sulfite, soda pulp, cotton linters, alpha-cellulose, rag, and rope may be treated according to the process herein described. Best results, however, are obtained with kraftand sulfite pulps. Soda pulp and cotton linters are the least desirable of the pulps; the former becoming excessively brittle and weak and the latter possessing relatively low tearing strength after starch treatment.
The type of starch used in the present invention influences to a considerable extent the properties of the final product. For maximum tearing strength, we prefer to use a starch manufactured from corn, potato, wheat, tapioca, barley, cassava, or rice, said starch having been purified by washing and settling treatments such as are commonly used in the purification of commercial starches. starches of this type must be swollen by treatment with hot water before they are effective in this process, and this invention therefore deals withthe treatment of crinkled cellulosic materials with swollen starch regardless of whether the starch is swollen immediately before use or whether the starch has been previously swollen to render it dispersible in cold water.
Certain types of starches known as soluble starches prepared by separating the contents of the starch cell known as granulose from the cell wall or starch cellulose may also be used in this process. The solubility of such starches depends on the high granulose content of the starch. The sc-called prepared starches; i. e., starches which are chemically treated with acids, concentrated alkalies, or oxidizing agents to render them more readily soluble in water and to impart low viscosity characteristics to their solutions are much less effective in this process than the starches previously described. In general, those starches which have been degraded or partially hydrolized by chemical treatment and which contain appreciable amounts of dextrin as a results of degradation, or which contain appreciable amounts of sugars as a result of hydrolysis, are to be avoided if optimum results are desired.
It is possible under certain conditions to increase the adhesiveness of starch by treating its solutions in water with alkalies, boric acid, certain inorganic salts, etc. Under these conditions degradation and hydrolysis are largely avoided and the viscosity of the starch solutionis not greatly reduced. Starch solutions treated to in crease adhesiveness without degradation and/or hydrolysis may also be used in this process with good results. Improved tearing strengths are obtained by using a. mixture of starch and flour (preferably containing 75% starch) which has been treated by a small amount of alkali insufficient to degrade the flour. Just a trace of caustic not more than about /2 sodium hydroxide solution is used. The mixture is boiled slightly and carefully so that degradation does not proceed appreciably further than required to get the starch in solution.
Several alternative methods for incorporating starch with the alkali treated pulp are possible, the choice of methods being determined largely by the properties of the starch which is used. Instead of mixing the hydrated pulp and starch solution in a chest, it is possible to add an aqueous starch solution directly to the beater following the hydration of the pulp. In case a cold water dispersible starch is used, it is possible to add the dry material directly to the beater following hydration of the pulp. It is also possible to use two different types of starch in conjunction with one another in the process: for example, a cold water dispersible starch (an example of which is a commercial pre-swollen corn starch) in conjunction with the familiar types of hot water dispersible starches. In this case the cold water dispersible starch may be added in the dry state to the beater containing the hydrated pulp and the hot water dispersible starch may be used to form back water orwhite water which is used to carry the pulp over the Fourdrinier wire. Still other methods of incorporating starch with pulp will be obvious to those skilled in the art, and we do not wish to limit ourselves in this respect.
The tearing strength imparted to the crinkled cellulosic base by starch is not obtained with such hydrophilic colloids as gum arabic, pectin, inulin, alginates, viscose, casein, gelatin or glue. On the other hand, certain mucilaginous products obtained from the endosperms of vegetable seeds and sold under various trade names impart some of the properties of starch but to a lesser degree.
The process described herein may be adapted to any of the well-known types of paper-making machines, as for example Fdurdrinier, cylinder, or Harper-Fourdrinier machines.
It will be understood that the hydration referred to in the present invention is that caused by the water-treatment of the superficially mercerized pulp and is reversible by boiling with water as distinguished from the hydration caused by caustic treatment which is not so reversible. The starch treated pulp and paper produced in accordance with the present invention responds to the conventional tests for the mercerization and hydration of cellulose fibers. The mercerization of the fibers may be identified by viewing them through the microscope and noting the change from the natural ribbon-like form, characteristic of cellulose fibers, to a Wire-like form substantially circular in cross section. When the mercerizing solution used is sodium hydroxide, as is the preferred practice, the product shows a blue stain when treated with zinc chloriodine solution in the manner fully described in the mentioned patent. The light blue stain obtained when the sodium hydroxide concentration used was just at the border line of mercerizing activity, around 7% at room temperature, as well as the darker stain obtained with pulp treated with higher caustic concentrations, is easily distinguished from the slight coloration imparted to pulp which has been manufactured by cooking wood chips with sodium hydroxide and which has not re"- ceived further treatment with mercerizing caustic as described herein. The presence or absence of reversible hydration in the pulp may be determined by boiling the pulp in water for fifteen minutes and noting whether an increase in freeness occurs. If the pulp has been first mercerized and then hydrated by a process such as we have described the increase in freeness due to reversion may vary from about up to 90% depending on the conditions present during mercerization and beating. The presence of the starch may be determined by boiling the product in dilute acetic acid until the starch is hydrolyzed to glucose and the glucose may then be determined by titration with Fehlings solution, which involves the precipitation of copper.
The following table shows the excellent pliability and high tearing strength exhibited by paper made from our starch treated pulp as compared to paper made by adding starch to pulp which has received no treatment or only the caustic treatment disclosedherein. The first item in the table is ordinary kraft pulp which was not given the caustic treatment. The second item represents this pulp after it has received the caustic treatment but not the hydration treatment of the example, and the third item represents this caustic treated pulp after hydration has been effected in accordance with the process set forth in the example.
Table III The figures in the above table are on a basis of sheets having a weight of 0.81 pound per 36 x 40" area. i
It is to be understood that the starch contained in the pulp is combined in a manner which 1 of the starch by washing with prevents rem water. The colloidal particles of the starch are probably adsorbed on the surfaces of the fibers, whereas the particles of other colloidal impregnating materials are contained, for the greater part at least, within the voids between the caustic treated fibers.
The starch impregnated fabrics prepared in the manner described above have been shown to possess better transpiration or breathing properties than uncoated leather. Furthermore, the product has good aging qualities, shows a relatively small paper break when bent or sharply flexed, and has a softer feel than ordinary kraft paper.
The fabrics made by impregnating the mercerized pulp with starch, and particularly the starch impregnated mercerized pulp which has been hydrated, may be improved for certain purposes by impregnating the starch treated sheet with certain other colloidal impregnating materials. Thus, we have produced an improved paper product consisting of a strong pliable sheet made of artificially crinkled fibers, starch, and a softening agent such as glycerin, a hygroscopic salt, or a synthetic resin such as castor oil modified polyhydric alcohol-polybasic acid resin. For waterproofing, we may use waxes, bitumens, natural and synthetic resins, as for instance, phenol-aldehyde and polyhydric alcohol-polybasic acid resins as well as other water repellent materials which are added either to the pulp or to the wet or dry sheet, or we may use water soluble materials such as saponified rosin which forms with alum a water repellent substance. In this instance, the water soluble material is added to the pulp before sheet formation and the aqueous pulp mixture is treated with alum. The starch impregnated paper may be softened with polyhydric alcohols (glycerol),-or with'hygroscopic salts and similar agents. Excellent colored fabrics may be obtained with dyes or by adding a water dispersion of a pigment to the starchtreated pulp. Fire retarding pigment such as gypsum can be used to advantage. The sheet, due to the starch contained therein, has an unusual capacity for retention of the pigment. We have produced a strong sheet of paper of unusually high absorption power so that it can be readily and thoroughly, impregnated with various substances such as melted waxes, petrolatum, solutions of natural or synthetic resins, drying oils, or solutions of cellulose derivatives such as pyroxylin. The advantages of this paper over standard papers, such as strong kraft or rope paper, is that for unit strength greater amounts of impregnating substances can be placed in a sheet due to its porosity, and more complete imbe used in such products as cable wrapping material, insulating paper, paper toweling, paper bag material, etc.
As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that we do not limit ourselves to the specific embodiments thereof except as defined in the following claims.
1. A process for making starch treated hydrated mercerized paper pulp which comprises beating mercerized paper pulp in water without substantial cutting action on the fibers until the freeness of the pulp is reduced to within the range of 150 seconds to 200 seconds, and incorporating starch into the pulp.
2. A process for making paper from starch containing hydrated mercerized paper pulp which comprises beating mercerized paper pulp in water without substantial cutting action on the fibers until the freeness of the pulp is reduced to within the range of 150 seconds to 200 seconds, mixing starch solution with the pulp and'felting the starch treated pulp into paper.
3. A process for making starch treated hydrated mercerized paper pulp which comprises dispersing ordinary chemical paper pulp having a mean fiber length of 2-4 mm. with sodium hydroxide within the range of concentration of at least 7% at a consistency above 10% and at a temperature of from about 20 C. to 40 C., washing out the caustic, beating the pulp with water at a consistency of about 4% to 5% without substantial cutting or abrasive action on the fibers until the freeness is reduced within the range of 150 seconds to 200 seconds, and mixing the pulp with starch solution.
' in freeness when the hydration is reversed by boiling with water.
6. Starch impregnated hydrated mercerized paper pulp having a freeness of 85 to 200 seconds, said pulp being increased at least 15% in freeness when the hydration is reversed by boiling with water.
7. Paper comprising felted starch impregnated hydrated mercerized paper pulp, said pulp being increased at least 15% in freeness when the hydration is reversed by boiling with water.
8. Paper comprising felted starch impregnated hydrated mercerized paper pulp; said paper having an Elmendorf tearing strength of from 1200 grams to 1600 grams based on an air dried sheet having a weight of 0.87 pounds per 36" x 40 area, said pulp being increased at least 15% in freeness when the hydration is reversed by boiling with water.
9. A process which comprises hydrating mercerized paper pulp by subjecting the pulp admixed with water to a continuous mixing without substantial cutting of the fibers, and incorporating starch into the hydrated pulp.
HAROLD S. HOLT. GEORGE L. SCHWARTZ.