US2414833A - Thermoplastic paper and process of preparing the same - Google Patents

Description

Juli. 28, 1947. OSBORNE 2,414,833

THERMOPLASTIC PAPER AND PROCESS OF PREPARING THE SAIIB Filed Kay 9, 1944 FAY H. OSBORNE Patented Jan. 2a, 1947 THERMOPLASTIC PAPER AND PROCESS OF PREPARING THE SAME Fay H. Osborne, Windsor Locks, Coma, assignor to C. H. Dexter & Sons, Inc., Windsor Loc Conn., a corporation of Connecticut Application May 9, 1944, Serial No. 534,7 81

4 Claims. 1

The present invention relates to the art of Paper making and is more particularly concerned with the production of a thermoplastic specialty paper finding general utility in the packaging and other arts. In its more specific aspects it concerns a process for preparing a novel filtering paper having thermoplastic, heat-sealing characteristics rendering it particularly useful in the manufacture of tea bags.

In my prior United States Patents Nos. 2,045,095 and 2,045,096 I have disclosed a process and apparatus for the preparation of a light-weight, long fibered, non-hydrated paper which is of uniform texture and thickness throughout and which is extremely porous. This paper, because of the foregoing characteristics and further because of its non-toxicity and freedom from taste and odor, has attained widespread use in the industry as a filtering medium in the preparation of tea bags.

In the manufacture of this porous paper, long, non-hydrated fibers are suspended in an extreme- 1y large proportion of water, and the dilute suspension thus formed is fed onto a transverse moving screen, the water falling freely and rapidly therethrough, leaving the fibers deposited on the screen. The dilute suspension preferably contains about one pound of fiber to twenty-five tons of water; that is, it has a consistency expressed as .002%. Higher consistencies such as .012% may be used, but in any case it will be recognized by those skilled in the art that even in this case the suspension is properly characterized as dilute to distinguish it from ordinary paper making suspensions whose consistency is of an entirely different order, being between .2% and .5%. This highly diluted, non-hydrated stock is supplied to a traveling, upwardly inclined portion of a wire screen. The water flows freely and very rapidly with an avoidance of eddy currents through the screen and through the web of fibers deposited on the screen. The fibers are also deposited evenly on the screen in a web without any rolling or rippling efiects whereby a paper web of any desired length and of uniform and homogeneous texture throughout and free of clots may be had. These results, as clearly pointed out in my prior Patent No. 2,045,095, are not possible with Fourdrinier machines as heretofore made.

The paper produced in accordance with the foregoing procedure has, as has been previously pointed out, found utility as a filtering paper in the manufacture of tea bags. It is made up of extremely long fibers, from 3 to 9 mm. in length, loosely formed into a very low density paper having many interstices or openings between the fibers. These openings permit the rapid passage of liquids therethrough but are so small that they retain very small solid particles. When used for tea bags in the brewing of tea, for instance, the paper permits hot water to enter the ba and extract the tea flavor therefrom, without permitting even the smallest particle of tea to pass from the interior of the bag to the surrounding hot water. This infusion is very rapid and complete within a. very few minutes.

In spite of the advantages enjoyed by this specialty filtering paper in the manufacture of tea bags, the industry for the most part has been forced, prior to the advent of the present invention, to make up these bags by stitching, tying or stapling the paper around the tea. The disadvantages of these operations are obvious. They are cumbersome, necessitate expensive packing apparatus, and incur the additional expense and cost of the thread, staples, etc.

One of the most important objects of the present invention is to provide for the production of a filtering paper of the general type described in my prior patents, while at the same time obviating the necessity of stitching, tying and stapling in subsequent tea bag manufacture. This is accomplished in accordance with the invention by rendering paper of this character thermoplastic in nature so that when the edge of one sheet is superposed upon the edge of another sheet, and

both edges submitted to heat and pressure, they,

are effectively sealed one to the other. Such a good bond results that the edges will not separate when immersed in boiling water for a considerable period of time.

Another object isto produce papers of the foregoing character in which the porosity is equal to and in some cases greater than that possessed by the paper described in my U. S. Patent No. 2,045,- 096 but still possessing the advantages of being thermoplastic and heat-sealing.

In the accompanying drawing there is shown, more or less diagrammatically, and for illustrative purposes only, an improved machine on which the step of forming the thermoplastic web may into a save-all I and returned to the system through pipe 8. In Fig. 1 these fibers, which are non-thermoplastic and which form the so-called "base of the novel paper are represented by fine lines 9. The sheet of paper is not completely formed until it reaches the point which is the point of contact, of the level of the fiber suspension in the head box with the moving wire screen. From the point III the paper passes on to be couched and dried in the usual paper making procedure.

So far the description of the paper making process has followed quite closely the process set forth in my prior Patent No. 2,045,095. Now, in accordance with the present invention, there is introduced at a point II, just under the level of the fiber suspension in the head box, another very dilute water suspension of low melting point, thermoplastic, synthetic fibers, such as so-called Vinyon fibers, which are made up of a copolymer of vinyl acetate and vinyl chloride. The synthetic fibers in this suspension are indicated by the reference number II, the suspension being fed into the head box at a point, II by means of an inclined trough It provided with baliles (not shown), if necessary. The outlet of trough 13 extends completely across the head box. The consistency of the thermoplastic fiber suspension is approximately the same as the consistency of the non-thermoplastic fiber suspension.

The point of introduction of the dilute suspension of the thermoplastic synthetic fiber into the head box is a very critical feature of the present invention. It will be noted that the point of introduction I I is so situated that due to speed of flow of both suspensions, the commingled fibers fiow away from and are first deposited on the screen at'a point considerably removed from the point H where the non-thermoplastic fibers first come in contact with the paper making wire screen I. In other words, the thermoplastic fibers, being fed into the head box at the point I l, intermingle with the non-thermoplastic fibers 9 therein and the mixture of both fibers begins to be deposited on the mat of non-thermoplastic fibers on the screen at about the point l5. Be-

tween the points It and IE it will be understood that the non-thermoplastic fibers Shave had a chance to be somewhat matted but they still have a very low consistency. As a matter of fact, the mat being formed between the points I4 and I5, still contains about 99% water. On the other hand. this mat leaving point I! is of sufiicient consistency to allow the fiber mix formed by the confluence of non-thermoplastic and thermoplastic fibers to fiow uniformly onto the upper surface of said mat, between points [5 and I0, and also through the surface to an extent sufiicent to bond them to the non-thermoplastic mat. It is very necessary that not only shall the web of non-thermoplastic fiber be of very low consistency to afford bonding between the two types of fibers but that the thermoplastic fibers be prevented from so completely permeating the nonthermoplastic fiber web that the thermoplastic fibers be allowed to reach the under side of the sheet as it is being formed. There must be some intermingling of the two types of fiber however, because if the thermoplastic fibers as such were on the upper surface of the sheet, i. e., if the latter were deposited for the first time at the point Ill just as the sheet leaves the suspension level, they would fall away from the sheet upon d ying.

At, the point III, which is the point on'the suspension level in the head box as it contacts the upwardly inclined, continuously moving wire screen, the sheet of paper is completely formed. Here, the top of the sheet has some non-thermoplastic fibers and some thermoplastic fibers, with the latter greatly predominating. Also, at this point, the bottom of the sheet is made up entirely of non-thermoplastic fibers.

In the dried and finished sheet the fibers are arrangedin a manner illustrated in Fig. 2 of the drawing. In this figure the thermoplastic fibers are represented as thick fibers by the reference numeral I8 whereas the non-thermoplastic fibers are represented as thin fibers by the reference numeral II. It will be understood that this is for-illustrative purposes only and that actually the two types of fibers may be of the same or different thickness. It will be observed that the top of the sheet contains fibers which are predominantly thermoplastic although some nonthermoplastic fibers may be present. The bottom of the sheet is, and must be, composed entirely of non-thermoplastic fibers for reasons which will -be pointed out more fully hereinafter. As seen in Fig. 2, further, the center of the sheet will be made up of a mixture of the two fibers. The intensity of the thermoplastic fiber is the greatest at the top of the sheet, becoming gradually less and less from top to bottom, and becoming completely exhausted before the bottom is reached. In this way the thermoplastic fibers are very well bound into the sheet but are not permitted in any way to be present in the under surface thereof.

If it is desirable to lock the thermoplastic fibers into the sheet of paper somewhat stronger than can be accomplished by the use of thermoplastic fibers alone then there can be mixed with the thermoplastic fiber suspension some extremely highly beaten hard fiber such as caroa; manila hemp. or the like, which will take a hard beating action and hydrate to a point where it shows no freeness on the Schopper Freeness test. This very highly beaten fiber which I call flock, and which is described in my pending application Serial No. 470.637, is a very strong binding agent and employed in strengths as low as 2% will bond synthetic fibers so they will have as high bursting, tear, and tensile strength as natural fibers.

Since packaging machines using the heat-sealing principle usually have heated jaws operated under pressure, it is important that the thermoplastic material be kept out of contact with the hot jaws, otherwise the material would stick to the jaws and pull the seal apart and also gum up the jaws. This invention overcomes this difilculty by producing a thermoplastic sheet one surface of which is entirely non-thermoplastic, thus permitting the surface used next to the jaws to be free from thermoplastic material when the thermoplastic surfaces are placed face to face prior to heat sealing.

In making thermoplastic filtering paper for tea bag use it has been found that a very strong pound per ream of 480 sheets 24 x 36 inches fin-- ished thermoplastic tea bag paper, approximately 6.8 pounds per ream of non-thermoplastic fiber and approximately 1.7 pounds per ream of thermoplastic fiber should be used. When employing Vinyon fibers in these proportions the resulting paper will give a very strong seal at 280 to 300 F. For many uses, as low as 1.3 pounds per ream of Vinyon fiber will be suflicient to make a strong seal.

Although throughout the foregoing description Vinyon has been indicated as the desired synthetic thermoplastic fiber, because of its nontoxicity and low fusing temperature of 260 to 300 F., it is to be understood that other thermoplastic fibers may be used. Cellulose acetate fibers, for instance, may be employed but because of its relatively high fusing temperature (about 500 F.) it is desirable. when using this fiber, to lower the fusing temperature to about 400-450 F. by employing a plasticized fiber. Any suitable plasticizer, such as dimethyl phthalate, will serve the purpose of lowering the fusing point.

The Vinyon fibers employed in the present invention may be spun to different deniers and cut to different lengths. Deniers of from 1.2 to 15.0 and lengths of from inch to inch have been employed. Preferably the fiber should be from 3 to 15 denier since a fiber under 3 denier disperses rather difiicultly and fiber distribution is not particularly even. Also, a Vinyon fiber length of inch to inch is preferred, since fibers longer than A; inch are difiicult to disperse and fibers shorter than 1 inch do not lock into the sheet as well as longer fibers.

In some cases it has been found that Vinyon fiber has an afiinity for air, and,in the presence of air bubbles, the fibers'will collect the bubbles to such an extent that there may be a tendency for the fibers to rise to the top of the suspension. This is undesirable because, as pointed out, Vinyon fibers entirely on the surface of the sheet have little or no anchorage to the base with the result that they tend to drop off the sheet in subsequent operations.

If this trouble is encountered it may be obviated or avoided in two ways:

1) A wetting agent may be added directly to the suspension of Vinyon fibers and water. Any number of wetting agents may be used but they must either be non-toxic initially or capable of being neutralized or washed out later to produce a finished paper which is non-toxic, odorless and tasteless. Some of the common wetting agents, such as Tergitol (sodium sulphate derivative of 3,9-diethy1tridecanol-6) have been used successfully.

(2) The Vinyon fibers may be soaked in a mild sodium hydroxide solution for several hours before using. The concentration of sodium hydroxide to water should be approximately 1% per cent. After the Vinyon fibers are se into the sheet the alkali may be washed out or neu tralized in subsequent operations.

The non-thermoplastic fibers may be any of the well-known paper-making fibers but it is very important that they be extremely long, unbeaten and non-hydrated, such as the fibers described in my U. S. Patent No. 2,045,096. They may be either natural fibers such as manila hemp, caroa,

6 jute or Indian hemp, or they may be synthetic fibers, such as viscose fibers.

As many changes could be made in the above process and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawmg shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language used in the following claims is intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. A thermoplastic, highly porous, long-fibered,

extremely thin and light-weight specialty waterfiltering medium in the laid paper useful as a manufacture of tea bags, which comprises a sheet composed of a mixture of thermoplastic and long, unbeaten, and non-hydrated non-thermoplastic fibers, the top of the sheet having thermoplastic fibers predominant, the latter being thoroughly anchored to and extending down into the nonthermoplastic fibers but not to the bottom of the sheet, the bottom being composed entirely of,

non-thermoplastic felted fibers.

2. A thermoplastic, highly porous, long-fibered, extremley thin and light-weight specialty waterlaid paper useful as a filtering medium in the manufacture of tea bags, which comprises a sheet composed of a mixture of fibers of a copolymer of vinyl acetate and vinyl chloride and long, unbeaten, and non-hydrated neutral cellulosic fibers, the top of the sheet having fibers of a copolymer of vinyl acetate and vinyl chloride fibers predominant, the latter being thoroughly anchored to and extending down into the cellulosic fibers but not to the bottom of the sheet, the bottom being composed entirely of cellulosic felted fibers.

3. A process-for the prepartion of thermoplastic, highly porous, long-fibered, extremely thin and light-weight specialty paper, useful as a filtering medium in the manufacture of tea bags,

- which comprises continuously advancing an exconsistency suspension of said fibers,

tremely dilute water suspension of long nonthermoplastic fibers towards a continuously trav eling, inclined wire screen to create, in advance of the screen, a phase of an extremely thin and low the speed of the suspension feed and the speed of the screen travel being such that said suspension is in a state of formation and is non-set, continuously and forcibly feeding directly into said phase of suspended long fibers over the screen and at a point immediately following the point where said phase starts depositing its fibers on the screen, an extremely dilute suspension of thermoplastic fibers to form a second phase of a dilute suspension of a mixture of both fibers having a predominance of the thermoplastic fibers in the upper part of this second phase and gradually diminishing to zero at the bottom of this phase, allowing said dilute suspension of the mixed fibers in this second phase to drain continuously on the screen to form a third phase comprising a web having a predominance of the thermoplmic fibers on the upper surface and locked into and extending down through the non-thermoplastic fibers, gradually diminishing to zero at the bottom surface of the web, and then drying said web to form a thermoplastic paper having the foregoing characteristics.

4. A process for the preparation of thermoplastic, highly porous, long-fibered, extremely thin and light-weight specialty paper, useful a a 111- tering medium in the manufacture of tea base. which comprises continuously advancing an extremely dilute water suspension of long, natural, cellulosic fibers towards a continuously traveling, inclined wire screen to create, in advance of the screen a phase of an extremely thin and low consistency suspension of said fibers, the speed of the suspension feed and the speed of the screen travel being such that said suspension is in a state of formation and is non-set. continuously and forcibly feeding directly into said phase of suspended long fibers over the screen and at a point immediately following the point where said polymer of vinyl acetate and vinyl chloride to form a second phase of a dilute suspension of a mixture of both fibers having a predominanc of the fibers of a copolymer of vinyl acetate and vinyl chloride in the upper part of this second phase and, gradually diminishing to zero at the bottom at this phase, allowing said dilute suspension of the mixed fibers in this second phase to drain continuously on the screen to form a third phase comprising a web having a predominance of the Vinyon fibers on the upper surface and locked into and extending down through the cellulosic fibers, gradually diminishing to zero at the bottom surface of the web, and then drying said web to form a thermoplastic paper having the foregoing characteristics.

FAY H. osnomm.

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