PAPER HAVING 60-97 PERCENT HYDRATED CELLULOSIC FIBERS AND 3-40 PERCENT UNHYDRATED CELLULOSIC FIBERS
United States Patent 3839144
Novel paper is prepared by employing a furnish containing from about 60 to about 97 percent of hydrated, gelatinized paper-making fibers and from about 3 to about 40 percent of substantially unhydrated paper-making fibers. This paper is dense and erasable and has a hold out time of at least 4 seconds by the Tappi T-454 -TS- 66 turpentine test.
Application Number:
05/359601
Publication Date:
10/01/1974
Filing Date:
05/11/1973
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Export Citation:
Primary Class:
Other Classes:
162/187, 162/146
International Classes:
D21H11/12; D21H11/18; D21H17/00; D21H11/00; D21H5/14
Field of Search:
162/141,139,187,146
Primary Examiner:
Bashore, Leon S.
Assistant Examiner:
Smith, William F.
Attorney, Agent or Firm:
Howson, And Howson
Parent Case Data:
This application is a continuation of application Ser. No. 74,172 filed Sept. 21, 1970, now abandoned, which was a continuation-in-part of application Ser. No. 871,955, filed Oct. 28, 1969, also now abandoned.
Claims:
1. A dense, erasable paper consisting essentially of a pressed and dried water-laid web of a blend of from about 60 to about 97 percent of hydrated, gelatinized cellulosic paper making fibers having a Schopper-Riegler freeness of less than 340 cc and from about 3 to about 40 percent of substantially unhydrated cellulosic paper making fibers having a Schopper-Riegler freeness above about 680 cc, said percentages being based on the weight of the total content of paper making fibers, said paper having a hold-out time of at least about 4 seconds by the TAPPI
2. The paper of claim 1 wherein said hydrated, gelatinized fibers are hydrated to the extent equivalent to a Schopper Riegler freeness of from
3. The paper of claim 2 wherein said substantially unhydrated fibers are
4. The paper of claim 3 wherein said substantially unhydrated fibers are
5. A dense, erasable paper consisting essentially of a pressed and dried water-laid web of a blend of from about 60 to about 97 percent of hydrated, gelatinized natural cellulosic paper making fibers having a Schopper-Riegler freeness of less than 340 cc and from about 3 to about 40 percent of substantially unhydrated natural cellulosic paper making fibers having a Schopper-Riegler freeness above about 680 cc, said percentages being based on the weight of the total content of paper making fibers, said paper having a hold-out time at least about 4 seconds by the TAPPI
6. The paper of claim 5 wherein said hydrated, gelatinized fibers are essentially from wood pulp containing less than 92 percent, by weight, of alpha cellulose the balance being essentially hemicelluloses, and wherein said substantially unhydrated fibers consist essentially of high alpha cellulosic fibers containing at least 92 percent, by weight, of alpha
7. The paper of claim 6 wherein said wood pulp containing less than 92 percent, by weight, of alpha cellulose is kraft wood pulp, and wherein
8. The paper of claim 5 wherein said hydrated, gelatinized fibers and said substantially unhydrated fibers are essentially the same type of fibers
9. The paper of claim 8 wherein said hydrated, gelatinized fibers and said substantially unhydrated fibers are essentially from wood pulp containing less than 92 percent, by weight, of alpha cellulose the balance being essentially hemicelluloses.
2. The paper of claim 1 wherein said hydrated, gelatinized fibers are hydrated to the extent equivalent to a Schopper Riegler freeness of from
3. The paper of claim 2 wherein said substantially unhydrated fibers are
4. The paper of claim 3 wherein said substantially unhydrated fibers are
5. A dense, erasable paper consisting essentially of a pressed and dried water-laid web of a blend of from about 60 to about 97 percent of hydrated, gelatinized natural cellulosic paper making fibers having a Schopper-Riegler freeness of less than 340 cc and from about 3 to about 40 percent of substantially unhydrated natural cellulosic paper making fibers having a Schopper-Riegler freeness above about 680 cc, said percentages being based on the weight of the total content of paper making fibers, said paper having a hold-out time at least about 4 seconds by the TAPPI
6. The paper of claim 5 wherein said hydrated, gelatinized fibers are essentially from wood pulp containing less than 92 percent, by weight, of alpha cellulose the balance being essentially hemicelluloses, and wherein said substantially unhydrated fibers consist essentially of high alpha cellulosic fibers containing at least 92 percent, by weight, of alpha
7. The paper of claim 6 wherein said wood pulp containing less than 92 percent, by weight, of alpha cellulose is kraft wood pulp, and wherein
8. The paper of claim 5 wherein said hydrated, gelatinized fibers and said substantially unhydrated fibers are essentially the same type of fibers
9. The paper of claim 8 wherein said hydrated, gelatinized fibers and said substantially unhydrated fibers are essentially from wood pulp containing less than 92 percent, by weight, of alpha cellulose the balance being essentially hemicelluloses.
Description:
Papers of the grease proof type made from hydrated and gelatinized (well beaten) fibers have traditionally had certain properties that are desirable and certain properties that are undesirable. Among the advantages of such papers are good transparency, good pick resistance for printing, good ink holdout for brilliant colors, good grease resistance, good ultraviolet light transmission, high tensile strength and high burst (Mullen) strength. Among the disadvantages are high density, poor dimensional stability, brittleness, poor tearing strength, poor folding and aging characteristics, lack of compressibility, slow surface ink drying rate and poor opacity.
Papers made from substantially unhydrated fibers, including especially the relatively expensive rag or cotton bond paper, have good tearing strength, good to excellent typewriting and printing characteristics, high folding qualities and resist embrittlement. However, they posses limitations; for example, it is difficult to erase typewritten images from them. Thus, U.S. Pat. No. 2,676,119 is directed to a paper coating method adapted to make such paper erasable.
There have been suggestions to make paper from blends of hydrated fibers and relatively unhydrated fibers. U.S. Pat. No. 1,765,774 relates to an insulating sheet having high dielectric strength made from a blend of about 60 percent of normally beaten fibers and about 40 percent of high gelatinized fibers. U.S. Pat. No. 1,850,139 discloses a paper-making composition composed of 85-95 percent unhydrated fibers and 5-15 percent hydrated fibers. U.S. Pat. No. 3,332,833 relates to paper made from 70-95 percent of mercerized fibers and 5-30 percent of hydrated, gelatinized fibers.
The present invention involves, in the process for the manufacture of paper wherein a web is formed by draining water from a furnish of hydrated, gelatinized cellulosic paper-making fibers, and the web is pressed and dried, the improvement wherein there is present in the furnish at the time the web is formed substantially unhydrated cellulosic paper-making fibers blended with the hydrated, gelatinized fibers in a proportion of from about 3, preferably from about 5, to about 40 percent, by weight, of the substantially unhydrated fibers to from about 97, preferably from about 95, to about 60 percent of the hydrated, gelatinized fibers. As will appear hereinafter there are preferred ranges of proportion of the two types of fibers depending at least in part on the grade or use to be made of the product.
The resulting product is thus a paper sheet consisting essentially of a pressed and dried water-laid web of cellulosic paper-making fibers wherein the fibers in said web are a blend of substantially unhydrated cellulosic paper-making fibers and of hydrated, gelatinized cellulosic paper-making fibers in the stated proportions.
It has been found that a minor proportion of substantially unhydrated fibers blended with the hydrated, gelatinized fibers (the latter being of the type of which grease-proof paper is made) has a marked effect on the general qualities of the resulting paper. The density is sharply reduced and, with it, a corresponding marked change in other qualities. However, certain of the qualities characteristic of grease-proof paper and other dense papers, especially typewriting erasability, are largely retained. One particular use for this paper, as a result of its characteristics, is as an erasable typewriter bond paper. Thus, the present process can produce a paper having good typewriting erasing qualities while possessing the qualities of a good bond paper. Moreover, the inclusion of the minor proportion of substantially unhydrated fibers produces a surprising marked increase in tearing strength. This increase is accompanied by little or no decrease in burst (Mullen) strength, whereas in normal paper making a substantial increase in tear strength is accompanied by a substantial reduction in burst strength. Thus, the present invention is also applicable to the production of other types of papers, for example, those useful as tracing papers, master sheets for the diazo copy process, printing papers and cover (book or magazine) stock with excellent ink holdout, base papers for release coatings, and so on.
Aside from the nature of the furnish, containing the two types of fibers in the stated proportions at the time of sheet formation, the procedures and equipment employed in forming the sheet and ultimate paper are conventional. Thus, the sheet is formed by draining water from the furnish and this may be done on a cylinder or on a Fourdrinier type of paper machine or on variations of these. The draining of water causes matting (felting) of the fibers and the resulting water-laid web of fibers contains about 80 percent water. This wet web may then be pressed, on the usual press rolls, mechanically to remove further water following which the sheet is finally subjected to evaporative removal of water (drying) by passage over heated drying drums. The web, at some time after the formation and before, during or after final drying, may be treated, as is conventional, with a size, brightener, dye or the like.
Likewise, the equipment and procedural techniques employed in preparing the furnish, except for the provision of the stated blend of fibers, are conventional for the production of dense papers, like grease-proof paper, and include refining of the hydratable fibers for a prolonger period as is customary in making grease-proof papers, and optional addition of fillers, sizes, elastomeric binders and/or coloring dyes.
The provision of the stated blend of fibers may be accomplished in either of two general procedures: (1) subjecting all fibers to a common refining operation, the fibers which are to be hydrated and gelatinized being of the type that are relatively quick and easy to hydrate and the fibers which are to be relatively unhydrated being of the type which are relatively difficult and slow to hydrate; or (2) subjecting only the fibers which are to be hydrated and gelatinized to refining to bring about hydration and gelatinization and then mixing relatively unhydrated fibers therewith just prior to sheet formation.
Referring to the first embodiment referred to above, refining may take place in conventional pulp refining apparatus as used in the manufacture of dense papers, like grease-proof papers, which may be of batch beater type of continuous such as the well known conical or disc type continuous refiners. Usually the refining apparatus is equipped with special fillings so that the hydratable fibers will hydrate more rapidly. In the case of this embodiment, however, one type of fiber subjected to refining will be of a type that is relatively quick and easy to hydrate and gelatinize, namely wood pulp containing less than 92 percent, preferably in the range of from about 85 to about 89 percent, by weight, of alpha cellulose, the balance being essentially hemicelluloses. Examples of such pulps are sulfate (kraft) pulps, Mitscherlich sulfite pulps, semichemical pulps and soda pulps having the stated alpha cellulose and hemicellulose contents. In this embodiment, the other type of fiber which is present during refining is of a type which is slow and difficult to hydrate, namely high alpha cellulose fibers (at least 92 percent alpha cellulose) such as cotton fibers (from linters, new cloth or old rags), flax fibers, hemp fibers, rayon (regenerated cellulose) fibers, wood pulps having an alpha cellulose content of at least 92 percent, and the like. Preferably the relatively unhydrated fibers will be "long" paper-making fibers. The fibers used for both types may be bleached or unbleached. Cotton fibers are the preferred high alpha cellulose fibers.
In the second embodiment referred to above, that is, where the relatively unhydrated fibers are mixed with the hydrated, gelatinized fibers just prior to sheet formation, the hydrated, gelatinized fibers will be of the same type as described above in connection with the first embodiment and will have been refined in equipment as described above. The relatively unhydrated fibers may be any type of cellulosic paper-making fibers, including each of the types mentioned above in connection with the first embodiment, so long as they have not been refined if they are of the type that is easily and quickly hydrated and gelatinized. In this embodiment, the two types of fibers are mixed just prior to sheet formation, as in the stock chest or other mixing apparatus.
The principal difference between the two types of fibers employed in accordance with the present invention is one of hydration, the predominant fiber being hydrated while the other is substantially unhydrated. "Hydration" as used herein refers to the enhancement of the fiber's affinity for water brought about by beating or refining. This procedure also causes swelling (gelatinization), cutting, fraying and fibrillation. One way of distinguishing between the two types of fibers is on the basis of differences in "freeness" caused by variation in the degree of hydration. A substantially unhydrated pulp will allow water to drain therefrom relatively quickly, whereas a well beaten pulp (hydrated) will lose water slowly. The hydrated, gelatinized fibers used according to the present invention will be hydrated to the extent equivalent to a Schopper-Riegler freeness of less than 340 cc., and preferably from about 180 to about 280 cc., that is substantially to the extent of fiber used in the manufacture of grease-proof paper. The substantially unhydrated fibers, on the other hand, will be equivalent to a Schopper-Riegler freeness of at least 680 cc., and preferably from about 800 to about 850 cc.
The present invention will be more readily understood from a consideration of the following specific examples which are given for the purpose of illustration only and are not intended to limit the scope of the invention an any way. "Erasability" characteristics set forth in the examples are determined by typing an image on the paper, erasing the image with an eraser, as commonly found on a lead pencil, noting the image remaining, if any, and noting the damage to the paper, if any, retyping over the same area and noting the residue of the original image, if any. The results are characterized as follows: excellent -- a faint trace of original image and no damage to paper after erasing but original image not discernible after retyping; good -- same as "excellent" but a faint evidence of original image after retyping; fair -- same as "good" but original image fairly obvious after retyping; and poor -- sheet damaged on erasing and original image is pronounced after retyping. Conventional typewriter letter head papers would have an erasability rating under this criterion of "poor."
EXAMPLES 1 - 2
Twenty five parts, by weight, of cotton pulp (98 percent alpha cellulose content) and 75 parts of southern kraft pulp (88-90 percent alpha cellulose, about 9-10 percent hemicelluloses) were mixed, and refined in conventional beaters of the type used to make grease-proof paper. Approximately 4 percent, by weight, of titanium dioxide was added as an opacifying agent. The mixed pulp was beaten for 90 minutes and then formed into paper on a conventional Fourdrinier paper-making machine, and dried. Papers of two different basis weights were made.
Characteristics of the papers are set forth below and compared to those of a typical grease-proof paper made of 100 percent hydrated, gelatinized fibers, and having a nominal basis weight of 16 lbs.:
Table 1 ____________________________________________________________ ______________ Example 1 2 Grease-Proof ____________________________________________________________ ______________ Basis weight 500)" × 22" 9.8 lbs. 16.3 lbs. 16 M.D. Tear (TAPPI T- 414-TS-65) 14 g 34 g about 30 Burst (TAPPI T-403- TS-63) 14 27 about 30 Density (g/cc) 0.72 0.73 do. 0.90 Caliper (inches) 0.0020 0.0031 do. 0.0022 Turpentine test (TAPPI T-454-TS-66) 4 secs. 5 secs. 300-1000 secs. Erasure excellent excellent excellent ____________________________________________________________ ______________
EXAMPLES 3 - 4
Eighty parts, by weight, of northern bleached kraft pulp (88-90 percent alpha cellulose about 9-10 percent hemicelluloses), 10 parts of Mitscherlich sulfite pulp (85-86 percent alpha cellulose, balance essentially hemicelluloses) and 10 parts of kraft alpha pulp (97.4 percent alpha cellulose) were blended together in a hydropulper. Four percent of titanium dioxide was added as an opacifying agent. The blended pulp was passed through continuous, Jordan-type refiners as used in the production of grease-proof paper and formed into paper using a conventional Fourdrinier paper-making machine, and dried. Papers of two different basis weights were made.
Characteristics of the papers are set forth below and compared to those of a typical grease-proof paper made of 100 percent hydrated, gelatinized fibers, and having a nominal basis weight of 16 lbs.:
Table 2 ____________________________________________________________ ______________ Example 3 4 Grease-Proof ____________________________________________________________ ______________ Basis Weight 500)" - 22" 9.1 lbs. 15.8 lbs. 16 lbs. Caliper (inches) 0.0018 0.0029 about 0.0022 Density (g/cc) 0.76 0.78 do. 0.90 Burst (TAPPI T-403-TS-63) 17 31 do. 30 M.D. Tear (TAPPI T-414-TS-65) 15 37 do. 30 Opacity (TAPPI T-425-OS-60) 54 70 do. 66 Turpentine Test (TAPPI T-454-TS-66) 75 secs. 144 secs. 300-1000 secs. Erasure excellent excellent excellent ____________________________________________________________ ______________
EXAMPLES 5 - 8
A bleached northern kraft pulp of about 89 percent alpha cellulose content was hydrated to a Schopper Riegler freeness of about 200 cc. in a continuous Jordan-type refiner as used in the manufacture of grease-proof paper (fiber A). Separately, a bleached kraft pulp of 97.5 percent alpha cellulose content and having a Schopper Riegler freeness of about 800 cc. was dispersed in water (fiber B). Portions of each of these fiber dispersions were blended together, in proportions as set forth in the following Table 3, and hand sheets were made according to TAPPI Standard Procedures T-205-m-58. Test results are set forth in Table 3 below (bearing in mind that hand sheets are normally less dense than machine-made paper using the same paper-making fibers).
Table 3 ____________________________________________________________ ______________ Example 5 6 7 8 ____________________________________________________________ ______________ Percent fiber A 100 90 80 70 Percent fiber B 0 10 20 30 Caliper (inches) 0.0032 0.0034 0.0039 0.0040 Opacity (TAPPI T-425-OS-60) 60 63 74 70 Density (g./cc.) 0.74 0.69 0.62 0.59 Erasability excellent excellent fair* poor* ____________________________________________________________ ______________ *Paper made from these fibers on a paper-making machine would be expected to have better erasability than these hand sheets.
Similar results can be observed by employing, in place of the 97.5 percent alpha bleached northern kraft pulp, cotton or rag fibers. Likewise, unrefined regular kraft or sulfite pulps (85-92 percent alpha cellulose content, and a Schopper Riegler freeness above 680 cc.) and unrefined semichemical or soda pulps (Schopper Riegler freeness above 680 cc.) can also be used in place of the 97.5 percent alpha bleached northern kraft pulp, although the effect on caliper and density is a little less pronounced.
EXAMPLES 9 - 12
Blends of cotton fibers (98 percent alpha cellulose) and bleached southern kraft wood pulp (89.5 percent alpha cellulose content the balance essentially hemicelluloses), in proportions as set forth in the following Table 4, were mixed together and refined for 90 minutes in a laboratory beater. Hand sheets were then made according to TAPPI Standard Specification T-205-m-58. Results were as follows (bearing in mind that hand sheets are normally less dense and would be expected to have a lower erasability rating than machine-made sheets from the same paper-making fibers):
Table 4 ______________________________________ Example 9 10 11 12 ______________________________________ Percent cotton fiber 0 10 25 40 Percent wood fiber 100 90 75 60 Density (g./cc.) 0.77 0.74 0.71 0.63 Burst (TAPPI T-403-TS-63) 138 115 112 104 Erasability good good good fair ______________________________________
Specifications for typical paper grades or uses producible according to the present invention are as follows:
Proportion (%) of un- hydrated fibers base wt. (lbs.) ______________________________________ general preferred (17 × 22", 500) ______________________________________ erasable bond 5 - 40 10 - 25 6 - 30 tracing paper 5 - 40 5 - 15 6 - 24 master sheets for diazo printing 5 - 40 5 - 15 6 - 24 printing paper 5 - 40 10 - 25 6 - 50 cover stock 5 - 40 15 - 35 50 - 100 base paper for release coating 5 - 40 15 - 35 6 - 50 ______________________________________
EXAMPLE 13
One hundred percent sulfite mitscherlich pulp was first beaten in a commercial lava-filled beater under conditions normal for the manufacture of glassine or greaseproof papers until the Schopper-Riegler freeness was approximately 350 c.c. The stock was then fed through commercial jordans with wide, dull bars. Freeness after jordaning was 235 cc.
Laboratory hand sheets (TAPPI) were first made using the 100 percent sulfite fully refined pulp. Basis weight was approximately 74 pounds 24 × 36-500.
In a separate operation, a long-fibered sulfate alpha pulp was dispersed without refining to any extent. The pulp was approximately 98 percent alpha cellulose. Average fiber length was 2.20 milimeters. This alpha pulp was blended with the fully refined mitscherlich sulfite pulp described above in various proportions, ranging from 10 percent to 30 percent, by weight, based on the combined weight of the fibers. Hand sheets were made similarly as sheets made from 100 percent sulfite. Basis weight, tear, and mullen tests were run on samples that had been conditioned at 50 percent relative humidity and 72°F., and the results were as follows:
Percent Burst Basis Weight Alpha Pulp (Mullen)* Tear* ______________________________________ 73.7 lbs. 0% 79 59 gr. 71.6 lbs. 10% 65 90 gr. 77.6 lbs. 20% 65 143 gr. 75.2 lbs. 30% 65 246 gr. ______________________________________ *TAPPI as in preceding examples.
EXAMPLE 14
A blend of 90 percent bleached northern kraft pulp with 10 percent sulfite pulp, both approximately 88 percent alpha content, was dispersed and refined in lava-filled jordans under commercial papermaking conditions common to the manufacture of extremely dense papers. Schopper-Riegler freeness was 230 c.c. after refining.
Separately, rayon pulp, commercially identified as American Viscose SN 8510 (1.5 denier 1/4 inch bright), was dispersed in water sufficiently to form a completely uniform mixture that could be easily blended with the refined pulp described above.
Laboratory hand sheets (TAPPI) were made, using no rayon pulp and then increasing percentages ranging from 10 percent to 35 percent rayon fibers based the total furnish. Results were as follows:
Percent Basis Burst Rayon Pulp Weight Tear* (Mullen)* Thickness ______________________________________ 0% 94 lbs. 82 gr. 97 .007" 10% 87 lbs. 124 gr. 94 -- 15% 80 lbs. 142 gr. 99 .008" 25% 79 lbs. 151 gr. 81 .008" 35% 88 lbs. 173 gr. 81 .009" ______________________________________ *TAPPI as in preceding examples.
EXAMPLE 15
A blend of 90 percent unbleached northern kraft and 10 percent sulfite, refined to 230 c.c Schopper-Riegler freeness as used in Example 14 was used in this example.
However, various percentages of long fibered manila hemp rope pulp dispersion prepared for papermaking use under commercial conditions was blended with the refined pulp in various proportions, and hand sheets (TAPPI) were made. Results were as follows:
Percent Basis Burst Hemp Fiber Weight Tear* (Mullen)* ______________________________________ 0% 73.5 lbs. 64 gr. 110 10% 73.5 lbs. 146 gr. 115 25% 75 lbs. 140 gr. 120 50% 76 lbs. 154 gr. 110 ______________________________________ *TAPPI as in preceding examples.
EXAMPLE 16
A blend of 90 percent unbleached northern kraft and 10 percent sulfite were refined together under commercial papermaking conditions in lava-filled jordans until the Schopper-Riegler freeness was 230 c.c.
Separately, a long fibered, sulfate alpha pulp, as used in Example 13, was dispersed without refining, and blended with the above pulp in the ratio of 20 parts alpha per 80 parts of refined pulp.
To this blend, an elastomeric polyacrylate emulsion (Rohm and Haas E-631) was added in varying proportions, by weight, based on the dry weight of the resulting sheet. Retention aid was added to the pulp before the acrylic emulsion was added in an amount of 1 percent based on the polyacrylate solids.
Laboratory hand sheets (TAPPI) were made and tested and the results were as follows:
Percent Basis Burst Acrylic Weight Tear* (Mullen)* ______________________________________ 0% 75 lbs. 125 gr. 85 10% 75 lbs. 160 gr. 91 ______________________________________ *TAPPI as in preceding examples.
Papers made from substantially unhydrated fibers, including especially the relatively expensive rag or cotton bond paper, have good tearing strength, good to excellent typewriting and printing characteristics, high folding qualities and resist embrittlement. However, they posses limitations; for example, it is difficult to erase typewritten images from them. Thus, U.S. Pat. No. 2,676,119 is directed to a paper coating method adapted to make such paper erasable.
There have been suggestions to make paper from blends of hydrated fibers and relatively unhydrated fibers. U.S. Pat. No. 1,765,774 relates to an insulating sheet having high dielectric strength made from a blend of about 60 percent of normally beaten fibers and about 40 percent of high gelatinized fibers. U.S. Pat. No. 1,850,139 discloses a paper-making composition composed of 85-95 percent unhydrated fibers and 5-15 percent hydrated fibers. U.S. Pat. No. 3,332,833 relates to paper made from 70-95 percent of mercerized fibers and 5-30 percent of hydrated, gelatinized fibers.
The present invention involves, in the process for the manufacture of paper wherein a web is formed by draining water from a furnish of hydrated, gelatinized cellulosic paper-making fibers, and the web is pressed and dried, the improvement wherein there is present in the furnish at the time the web is formed substantially unhydrated cellulosic paper-making fibers blended with the hydrated, gelatinized fibers in a proportion of from about 3, preferably from about 5, to about 40 percent, by weight, of the substantially unhydrated fibers to from about 97, preferably from about 95, to about 60 percent of the hydrated, gelatinized fibers. As will appear hereinafter there are preferred ranges of proportion of the two types of fibers depending at least in part on the grade or use to be made of the product.
The resulting product is thus a paper sheet consisting essentially of a pressed and dried water-laid web of cellulosic paper-making fibers wherein the fibers in said web are a blend of substantially unhydrated cellulosic paper-making fibers and of hydrated, gelatinized cellulosic paper-making fibers in the stated proportions.
It has been found that a minor proportion of substantially unhydrated fibers blended with the hydrated, gelatinized fibers (the latter being of the type of which grease-proof paper is made) has a marked effect on the general qualities of the resulting paper. The density is sharply reduced and, with it, a corresponding marked change in other qualities. However, certain of the qualities characteristic of grease-proof paper and other dense papers, especially typewriting erasability, are largely retained. One particular use for this paper, as a result of its characteristics, is as an erasable typewriter bond paper. Thus, the present process can produce a paper having good typewriting erasing qualities while possessing the qualities of a good bond paper. Moreover, the inclusion of the minor proportion of substantially unhydrated fibers produces a surprising marked increase in tearing strength. This increase is accompanied by little or no decrease in burst (Mullen) strength, whereas in normal paper making a substantial increase in tear strength is accompanied by a substantial reduction in burst strength. Thus, the present invention is also applicable to the production of other types of papers, for example, those useful as tracing papers, master sheets for the diazo copy process, printing papers and cover (book or magazine) stock with excellent ink holdout, base papers for release coatings, and so on.
Aside from the nature of the furnish, containing the two types of fibers in the stated proportions at the time of sheet formation, the procedures and equipment employed in forming the sheet and ultimate paper are conventional. Thus, the sheet is formed by draining water from the furnish and this may be done on a cylinder or on a Fourdrinier type of paper machine or on variations of these. The draining of water causes matting (felting) of the fibers and the resulting water-laid web of fibers contains about 80 percent water. This wet web may then be pressed, on the usual press rolls, mechanically to remove further water following which the sheet is finally subjected to evaporative removal of water (drying) by passage over heated drying drums. The web, at some time after the formation and before, during or after final drying, may be treated, as is conventional, with a size, brightener, dye or the like.
Likewise, the equipment and procedural techniques employed in preparing the furnish, except for the provision of the stated blend of fibers, are conventional for the production of dense papers, like grease-proof paper, and include refining of the hydratable fibers for a prolonger period as is customary in making grease-proof papers, and optional addition of fillers, sizes, elastomeric binders and/or coloring dyes.
The provision of the stated blend of fibers may be accomplished in either of two general procedures: (1) subjecting all fibers to a common refining operation, the fibers which are to be hydrated and gelatinized being of the type that are relatively quick and easy to hydrate and the fibers which are to be relatively unhydrated being of the type which are relatively difficult and slow to hydrate; or (2) subjecting only the fibers which are to be hydrated and gelatinized to refining to bring about hydration and gelatinization and then mixing relatively unhydrated fibers therewith just prior to sheet formation.
Referring to the first embodiment referred to above, refining may take place in conventional pulp refining apparatus as used in the manufacture of dense papers, like grease-proof papers, which may be of batch beater type of continuous such as the well known conical or disc type continuous refiners. Usually the refining apparatus is equipped with special fillings so that the hydratable fibers will hydrate more rapidly. In the case of this embodiment, however, one type of fiber subjected to refining will be of a type that is relatively quick and easy to hydrate and gelatinize, namely wood pulp containing less than 92 percent, preferably in the range of from about 85 to about 89 percent, by weight, of alpha cellulose, the balance being essentially hemicelluloses. Examples of such pulps are sulfate (kraft) pulps, Mitscherlich sulfite pulps, semichemical pulps and soda pulps having the stated alpha cellulose and hemicellulose contents. In this embodiment, the other type of fiber which is present during refining is of a type which is slow and difficult to hydrate, namely high alpha cellulose fibers (at least 92 percent alpha cellulose) such as cotton fibers (from linters, new cloth or old rags), flax fibers, hemp fibers, rayon (regenerated cellulose) fibers, wood pulps having an alpha cellulose content of at least 92 percent, and the like. Preferably the relatively unhydrated fibers will be "long" paper-making fibers. The fibers used for both types may be bleached or unbleached. Cotton fibers are the preferred high alpha cellulose fibers.
In the second embodiment referred to above, that is, where the relatively unhydrated fibers are mixed with the hydrated, gelatinized fibers just prior to sheet formation, the hydrated, gelatinized fibers will be of the same type as described above in connection with the first embodiment and will have been refined in equipment as described above. The relatively unhydrated fibers may be any type of cellulosic paper-making fibers, including each of the types mentioned above in connection with the first embodiment, so long as they have not been refined if they are of the type that is easily and quickly hydrated and gelatinized. In this embodiment, the two types of fibers are mixed just prior to sheet formation, as in the stock chest or other mixing apparatus.
The principal difference between the two types of fibers employed in accordance with the present invention is one of hydration, the predominant fiber being hydrated while the other is substantially unhydrated. "Hydration" as used herein refers to the enhancement of the fiber's affinity for water brought about by beating or refining. This procedure also causes swelling (gelatinization), cutting, fraying and fibrillation. One way of distinguishing between the two types of fibers is on the basis of differences in "freeness" caused by variation in the degree of hydration. A substantially unhydrated pulp will allow water to drain therefrom relatively quickly, whereas a well beaten pulp (hydrated) will lose water slowly. The hydrated, gelatinized fibers used according to the present invention will be hydrated to the extent equivalent to a Schopper-Riegler freeness of less than 340 cc., and preferably from about 180 to about 280 cc., that is substantially to the extent of fiber used in the manufacture of grease-proof paper. The substantially unhydrated fibers, on the other hand, will be equivalent to a Schopper-Riegler freeness of at least 680 cc., and preferably from about 800 to about 850 cc.
The present invention will be more readily understood from a consideration of the following specific examples which are given for the purpose of illustration only and are not intended to limit the scope of the invention an any way. "Erasability" characteristics set forth in the examples are determined by typing an image on the paper, erasing the image with an eraser, as commonly found on a lead pencil, noting the image remaining, if any, and noting the damage to the paper, if any, retyping over the same area and noting the residue of the original image, if any. The results are characterized as follows: excellent -- a faint trace of original image and no damage to paper after erasing but original image not discernible after retyping; good -- same as "excellent" but a faint evidence of original image after retyping; fair -- same as "good" but original image fairly obvious after retyping; and poor -- sheet damaged on erasing and original image is pronounced after retyping. Conventional typewriter letter head papers would have an erasability rating under this criterion of "poor."
EXAMPLES 1 - 2
Twenty five parts, by weight, of cotton pulp (98 percent alpha cellulose content) and 75 parts of southern kraft pulp (88-90 percent alpha cellulose, about 9-10 percent hemicelluloses) were mixed, and refined in conventional beaters of the type used to make grease-proof paper. Approximately 4 percent, by weight, of titanium dioxide was added as an opacifying agent. The mixed pulp was beaten for 90 minutes and then formed into paper on a conventional Fourdrinier paper-making machine, and dried. Papers of two different basis weights were made.
Characteristics of the papers are set forth below and compared to those of a typical grease-proof paper made of 100 percent hydrated, gelatinized fibers, and having a nominal basis weight of 16 lbs.:
Table 1 ____________________________________________________________ ______________ Example 1 2 Grease-Proof ____________________________________________________________ ______________ Basis weight 500)" × 22" 9.8 lbs. 16.3 lbs. 16 M.D. Tear (TAPPI T- 414-TS-65) 14 g 34 g about 30 Burst (TAPPI T-403- TS-63) 14 27 about 30 Density (g/cc) 0.72 0.73 do. 0.90 Caliper (inches) 0.0020 0.0031 do. 0.0022 Turpentine test (TAPPI T-454-TS-66) 4 secs. 5 secs. 300-1000 secs. Erasure excellent excellent excellent ____________________________________________________________ ______________
EXAMPLES 3 - 4
Eighty parts, by weight, of northern bleached kraft pulp (88-90 percent alpha cellulose about 9-10 percent hemicelluloses), 10 parts of Mitscherlich sulfite pulp (85-86 percent alpha cellulose, balance essentially hemicelluloses) and 10 parts of kraft alpha pulp (97.4 percent alpha cellulose) were blended together in a hydropulper. Four percent of titanium dioxide was added as an opacifying agent. The blended pulp was passed through continuous, Jordan-type refiners as used in the production of grease-proof paper and formed into paper using a conventional Fourdrinier paper-making machine, and dried. Papers of two different basis weights were made.
Characteristics of the papers are set forth below and compared to those of a typical grease-proof paper made of 100 percent hydrated, gelatinized fibers, and having a nominal basis weight of 16 lbs.:
Table 2 ____________________________________________________________ ______________ Example 3 4 Grease-Proof ____________________________________________________________ ______________ Basis Weight 500)" - 22" 9.1 lbs. 15.8 lbs. 16 lbs. Caliper (inches) 0.0018 0.0029 about 0.0022 Density (g/cc) 0.76 0.78 do. 0.90 Burst (TAPPI T-403-TS-63) 17 31 do. 30 M.D. Tear (TAPPI T-414-TS-65) 15 37 do. 30 Opacity (TAPPI T-425-OS-60) 54 70 do. 66 Turpentine Test (TAPPI T-454-TS-66) 75 secs. 144 secs. 300-1000 secs. Erasure excellent excellent excellent ____________________________________________________________ ______________
EXAMPLES 5 - 8
A bleached northern kraft pulp of about 89 percent alpha cellulose content was hydrated to a Schopper Riegler freeness of about 200 cc. in a continuous Jordan-type refiner as used in the manufacture of grease-proof paper (fiber A). Separately, a bleached kraft pulp of 97.5 percent alpha cellulose content and having a Schopper Riegler freeness of about 800 cc. was dispersed in water (fiber B). Portions of each of these fiber dispersions were blended together, in proportions as set forth in the following Table 3, and hand sheets were made according to TAPPI Standard Procedures T-205-m-58. Test results are set forth in Table 3 below (bearing in mind that hand sheets are normally less dense than machine-made paper using the same paper-making fibers).
Table 3 ____________________________________________________________ ______________ Example 5 6 7 8 ____________________________________________________________ ______________ Percent fiber A 100 90 80 70 Percent fiber B 0 10 20 30 Caliper (inches) 0.0032 0.0034 0.0039 0.0040 Opacity (TAPPI T-425-OS-60) 60 63 74 70 Density (g./cc.) 0.74 0.69 0.62 0.59 Erasability excellent excellent fair* poor* ____________________________________________________________ ______________ *Paper made from these fibers on a paper-making machine would be expected to have better erasability than these hand sheets.
Similar results can be observed by employing, in place of the 97.5 percent alpha bleached northern kraft pulp, cotton or rag fibers. Likewise, unrefined regular kraft or sulfite pulps (85-92 percent alpha cellulose content, and a Schopper Riegler freeness above 680 cc.) and unrefined semichemical or soda pulps (Schopper Riegler freeness above 680 cc.) can also be used in place of the 97.5 percent alpha bleached northern kraft pulp, although the effect on caliper and density is a little less pronounced.
EXAMPLES 9 - 12
Blends of cotton fibers (98 percent alpha cellulose) and bleached southern kraft wood pulp (89.5 percent alpha cellulose content the balance essentially hemicelluloses), in proportions as set forth in the following Table 4, were mixed together and refined for 90 minutes in a laboratory beater. Hand sheets were then made according to TAPPI Standard Specification T-205-m-58. Results were as follows (bearing in mind that hand sheets are normally less dense and would be expected to have a lower erasability rating than machine-made sheets from the same paper-making fibers):
Table 4 ______________________________________ Example 9 10 11 12 ______________________________________ Percent cotton fiber 0 10 25 40 Percent wood fiber 100 90 75 60 Density (g./cc.) 0.77 0.74 0.71 0.63 Burst (TAPPI T-403-TS-63) 138 115 112 104 Erasability good good good fair ______________________________________
Specifications for typical paper grades or uses producible according to the present invention are as follows:
Proportion (%) of un- hydrated fibers base wt. (lbs.) ______________________________________ general preferred (17 × 22", 500) ______________________________________ erasable bond 5 - 40 10 - 25 6 - 30 tracing paper 5 - 40 5 - 15 6 - 24 master sheets for diazo printing 5 - 40 5 - 15 6 - 24 printing paper 5 - 40 10 - 25 6 - 50 cover stock 5 - 40 15 - 35 50 - 100 base paper for release coating 5 - 40 15 - 35 6 - 50 ______________________________________
EXAMPLE 13
One hundred percent sulfite mitscherlich pulp was first beaten in a commercial lava-filled beater under conditions normal for the manufacture of glassine or greaseproof papers until the Schopper-Riegler freeness was approximately 350 c.c. The stock was then fed through commercial jordans with wide, dull bars. Freeness after jordaning was 235 cc.
Laboratory hand sheets (TAPPI) were first made using the 100 percent sulfite fully refined pulp. Basis weight was approximately 74 pounds 24 × 36-500.
In a separate operation, a long-fibered sulfate alpha pulp was dispersed without refining to any extent. The pulp was approximately 98 percent alpha cellulose. Average fiber length was 2.20 milimeters. This alpha pulp was blended with the fully refined mitscherlich sulfite pulp described above in various proportions, ranging from 10 percent to 30 percent, by weight, based on the combined weight of the fibers. Hand sheets were made similarly as sheets made from 100 percent sulfite. Basis weight, tear, and mullen tests were run on samples that had been conditioned at 50 percent relative humidity and 72°F., and the results were as follows:
Percent Burst Basis Weight Alpha Pulp (Mullen)* Tear* ______________________________________ 73.7 lbs. 0% 79 59 gr. 71.6 lbs. 10% 65 90 gr. 77.6 lbs. 20% 65 143 gr. 75.2 lbs. 30% 65 246 gr. ______________________________________ *TAPPI as in preceding examples.
EXAMPLE 14
A blend of 90 percent bleached northern kraft pulp with 10 percent sulfite pulp, both approximately 88 percent alpha content, was dispersed and refined in lava-filled jordans under commercial papermaking conditions common to the manufacture of extremely dense papers. Schopper-Riegler freeness was 230 c.c. after refining.
Separately, rayon pulp, commercially identified as American Viscose SN 8510 (1.5 denier 1/4 inch bright), was dispersed in water sufficiently to form a completely uniform mixture that could be easily blended with the refined pulp described above.
Laboratory hand sheets (TAPPI) were made, using no rayon pulp and then increasing percentages ranging from 10 percent to 35 percent rayon fibers based the total furnish. Results were as follows:
Percent Basis Burst Rayon Pulp Weight Tear* (Mullen)* Thickness ______________________________________ 0% 94 lbs. 82 gr. 97 .007" 10% 87 lbs. 124 gr. 94 -- 15% 80 lbs. 142 gr. 99 .008" 25% 79 lbs. 151 gr. 81 .008" 35% 88 lbs. 173 gr. 81 .009" ______________________________________ *TAPPI as in preceding examples.
EXAMPLE 15
A blend of 90 percent unbleached northern kraft and 10 percent sulfite, refined to 230 c.c Schopper-Riegler freeness as used in Example 14 was used in this example.
However, various percentages of long fibered manila hemp rope pulp dispersion prepared for papermaking use under commercial conditions was blended with the refined pulp in various proportions, and hand sheets (TAPPI) were made. Results were as follows:
Percent Basis Burst Hemp Fiber Weight Tear* (Mullen)* ______________________________________ 0% 73.5 lbs. 64 gr. 110 10% 73.5 lbs. 146 gr. 115 25% 75 lbs. 140 gr. 120 50% 76 lbs. 154 gr. 110 ______________________________________ *TAPPI as in preceding examples.
EXAMPLE 16
A blend of 90 percent unbleached northern kraft and 10 percent sulfite were refined together under commercial papermaking conditions in lava-filled jordans until the Schopper-Riegler freeness was 230 c.c.
Separately, a long fibered, sulfate alpha pulp, as used in Example 13, was dispersed without refining, and blended with the above pulp in the ratio of 20 parts alpha per 80 parts of refined pulp.
To this blend, an elastomeric polyacrylate emulsion (Rohm and Haas E-631) was added in varying proportions, by weight, based on the dry weight of the resulting sheet. Retention aid was added to the pulp before the acrylic emulsion was added in an amount of 1 percent based on the polyacrylate solids.
Laboratory hand sheets (TAPPI) were made and tested and the results were as follows:
Percent Basis Burst Acrylic Weight Tear* (Mullen)* ______________________________________ 0% 75 lbs. 125 gr. 85 10% 75 lbs. 160 gr. 91 ______________________________________ *TAPPI as in preceding examples.