Method of making glassine paper

Patented Aug. 13, 1940

2,211,348

UNITED STATES PATENT OFFICE

2,211,348

METHOD OF MAKING GLASSINE PAPER

Robert F. Nelson, Ardmore, Pa., assignor to The Glassine Paper Company, West Conshohocken, Pa., a corporation of Delaware

No Drawing. Application June 11, 1936, Serial No. 34,706

Claims. (Cl. 92—68)

This invention relates to an improved glassine and method of making the same, and has particular reference to a glassine of more uniform quality than that heretofore produced and a method 5 whereby it may be produced with considerably less waste of time than the present method involves. , ,.

Glassine paper has heretofore been produced by first forming a sulphite spruce pulp paper 10 commonly known as “greaseproof” paper, this paper being produced in a substantially dry state and containing only about 2% to 10% of moisture removable by heating it to 212° F. This paper is then moistened, for example, with a spray of water to add an amount which will raise the moisture content to around 20 to 30 %. The moistening is usually accomplished by unwinding a roll of the greaseproof paper, subjecting it to a spray of water and then rewinding it. The -:!) water to added is not initially uniformly distributed but remains on the paper in the form of droplets. Accordingly the paper is permitted to stand for a period ranging generally from about three hours to three days, in order that the 25 water sprayed upon it will uniformly spread throughout the roll producing a greaseproof paper containing the desired percentage of water. Following such spread of the moisture, it is supercalendered in the well known fashion by passing 30 it through a stack of heated supercalendering rolls. The supercalender stack usually comprises both hollow steel rolls arranged to receive steam, and paper rolls.

tn accordance with the present invention, the 35 necessity for the interruption of the procedure involved in permitting the sprayed roll of paper to become uniformly moist is entirely avoided.

In the preferable form of the invention, the paoer machine is operated in the usual fashion 40 to produce a greaseproof paper from suitable conventional stock. However, the drying is not carried out to the extent of obtaining a dry greaseproof paper. In general, the paper, after passing the suction boxes and suction, roll without 45 a pressure roll, and finally a suction roll with which is associated a pressure roll, will contain from 50 to 70% of water. 50% of moisture is approximately the upper limit which may be included when supercalendering is to take place. 50 Preferably, after leaving the suction roll associated with the pressure roll, the paper is passed through at least some of the usual paper machine dryers, preferably such as to reduce its moisture content to about 20 to 25%.

The paper is then directly supercalendered, be ing run through the supercalender stack operating in the usual fashion.

For the accomplishment of this last step, several variations may be introduced into the process. Under ideal conditions such as are presented 5 with a narrow paper machine (from which the entire web may be run to a conventional supercalender without being slit), the greaseproof paper containing the proper amount of moisture to produce glassine may run directly from the 10 dryers of the paper machine through the superCallender stack, it is only necessary in such case that the paper machine and supercalender stack should be operating at corresponding speeds. , Paper machines such as are used in producing 1» glassine can be satisfactorily run through a large range of speeds, for example, fifty to four hundred feet per minute. The supercalenders can likewise be run quite satisfactorily through a considerable range of speeds from about thirty-five ₂o feet to five hundred feet per minute. It will be obvious, therefore, that there is no difficulty in synchronizing the paper machine with a supercalender stack either by providing a common mechanical drive or by operating the two machines 25 through the medium of synchronous or induction motors 'which have substantially constant speed characteristics.

The supercalender stack may be operated under conventional conditions which, as is well ₃₀known, may vary quite widely. The steam rollers used in supercalendering are generally heated to a point in the neighborhood of about 300° F.. though a lower temperature may be used or, if desired, this temperature may be exceeded. In gen- ₃₅eral, what is considered primarily is the effect of the high temperature upon the life of the paper ' rolls in the supercalender stack, the temperature being generally maintained as high as possible consistent with a long life for these rolls. ₄₀

The feeding of a supercalender stack directly from the paper machine, even when the supercalender stack may take care of a web of the full width of the paper machine, is generally attended with some difficulties which make it desirable to provide for accidental stoppage of the continuity of the paper. In general, the operation of a paper machine is considerably more reliable than that of a supercalender stack, which may fail, for various reasons, to give satisfactory results. In gQ the majority of cases, however, the failure of the supercalender stack is minor and can be corrected very easily, for example, when the failure is due to a sticking of paper to one of the rolls. A difficulty of this sort may be corrected by stopping eg

2,211,348 the operation of the stack for a very short time, releasing the pressure, cleaning it out, and then restarting the operation, all of which may take only a very short time. Because of minor 5 failures of this nature, it is desirable to provide, between the paper machine and the supercalender stack, a means such as a festooning arrangement, which can take up the amount of paper delivered by the machine in a period of, say, five 10 to ten minutes, so that the operation of the paper machine need not be interrupted. The festooning arrangement may be of conventional type, normally being out of action but becoming effective to provide a reservoir for the paper from the 15 machine for a period such as that indicated during which the operation of the supercalender stack may be corrected. Following such a correction, the supercalender stack may be operated at a somewhat higher speed than the machine 20 to take up the accumulated paper.

Provision is also desirably made, however, to take care of more serious failures of the supercalendering stack by the provision of a reel on the end of the paper machine to reel up the paper ²⁵ having the desired moisture content, which paper can be thereafter run directly through the same or an auxiliary supercalender stack. If desired, in fact, the web need not run directly from the paper machine to the supercalender stack in any ³⁰ event, but the paper may be wound up on a reel and the reel then moved to the supercalender stack so as to provide, in effect, a process having a short discontinuity-. The delay, however, need never approximate that necessary in the ⁰ present method and, furthermore, all spraying operations are avoided.

If the supercalender stacks which are used are narrower than the paper machine and it is desired to run the paper machine at full capacity, 40 the sheet may be split'and wound on separate reels prior to supercalendering, though after splitting, the separated webs may be run directly to individual supercalenders.

Quite frequently another matter must be taken 45 into account, namely, the fact that the paper coming from the paper machine generally has edge portions which are irregular and usually thicker than the center portions of the web. The thickness may be such as to prevent proper super50 calendering. Accordingly, between the paper machine and the supercalenders there are preferably provided rotary knives bearing against a hardened steel roller for the purpose of removing such portions of the edge as might interfere 55 with the supercalendering operation.

The introduction of the end of the web into the supercalender may be very quickly accomplished in the conventional fashion involving locating in the supercalender a web of paper 50 while the pressure is removed from the supercalender rolls, attaching, by the use of adhesive, the end of the paper web to the trailing end of the paper strip in the supercalender, pulling rapidly through the supercalender the leading end of the 05 web, using the strip as a feeder, and then applying the pressure to the rolls and carrying on the supercalendering at the proper speed.

Usually in producing greaseproof paper little attention need be paid to the fact that the cen7° tral portion of the paper web dries more slowly than the edge portions, since the entire web is run to the condition of substantial dryness. However, in the present instance the web should have a substantially uniform moisture content 75 throughout its width, when the moisture content is of the proper amount for supercalendering, namely, between, say, 20 to 30%. The attainment of uniformity of moisture content across the entire width of the web may be effected through the use of a blast of hot air directed 5 . against the central portions of the web as it passes over the drying rollers of the paper machine. The same result may be accomplished by partitioning the suction boxes and suction rolls so that when leakage is taken into account a 10 greater suction is produced in the central portion of the boxes or rolls, thereby providing a lower moisture content thereat than at the edges. Thereafter, in passing through the drying rolls, the normal removal of a greater amount of 15 moisture from the edges will result in the attainment of substantial uniformity throughout the entire width.

While a skilled paper machine attendant can, by feel, quite accurately determine the moisture 20 content of a web of paper, it is desirable, in the present process, to provide automatic control of the moisture content by the use of known devices which regulate the entrance of steam into the dryer rolls and thereby maintain substantially ²⁵constant the desired percentage of moisture in the product. Control should, in general, be such as to maintain the moisture content just prior to the supercalender stack at about 20 to 30%, 22% being about the optimum moisture content. It ³⁰will be understood, however, that the moisture content may vary considerably from this amount. About 15% is the minimum moisture content which is consistent with high transparency, though even a less amount of moisture, say 10%, ³⁵may be used where surface finish is more to be desired than transparency. The maximum amount of water which may be used is such as to insure that the sup ?rcalendered glassine is substantially dry when it leaves the supercalender stack and, 40 of course, also such that there «will be provided a sheet which can stand the supercalendering operation without breaking or sticking to the supercalender rolls. The maximum amount of moisture dictated by these considerations is about 45 50%.

A plasticizer may be incorporated in the greaseproof paper in its passage through the paper machine. The plasticizer, which may be of any well known composition, for example, that given in ⁶⁰Bidwell Patents 1,914,798 and 1,914,799, of course adds considerable moisture to the paper. The addition after the paper is partially dry, has the advantageous effect of promoting uniformity of the moisture content throughout the width of ⁵⁵the web. Drying after the addition of the plasticizer should be carried out to such extent as to reduce the moisture content to the amount indicated above which is desirable for supercalendering to produce a highly transparent glassine. ⁶⁰

A further variant of the process involves the printing of the glassine before supercalendering. In this variation the moisture content may, if the ink requires a low moisture content to print ₆₅properly, be reduced in the machine dryers somewhat below the amount which would be satisfactory for immediate supercalendering for the formation of glassine, say, between 10 to 15%, though preferably the moisture content is left 70 as high as possible. This amount of moisture will not generally prevent printing of the paper by the use of inks such as those of “vapor” type, which are adapted to be dried by the application of heat. A press driven in timed relationship 75

2.211.348 out the sheet cannot be attained by a second moistening of a dried sheet. A high degree of transparency can only be approached when the moisture content is uniform throughout even quite small areas of the web. ⁵