US2904893A - Process of humidifying hardboard - Google Patents

Description

Sept. 22, 1959 a. s. WlLLEY v PROCESS OF HUMIDIFYING HARDBOARD Filed July 5, 1956 IN V EN TOR. $2441 1 5. at? a m M BY J flak,

United States Patent PROCESS OF HUIVIIDIFYING HARDBOARD Grant S. Willey, Lake Forest, 111., assignor to United States Gypsum Company, Chicago, 111., a corporation of Illinois Application July 5, 1956, Serial No. 595,840

1 Claim. (Cl. 34-23) The present invention relates to a rapid process for the humidification of hardboard for the purpose of imparting to the board a degree of moisture at which it is in substantial equilibrium with the atmosphere.

One of the objects of the present invention is to produce hardboard which, when properly humidified according to this process, will be substantially free from substantial dimensional change upon subsequent exposure to atmospheres of varying humidity.

Further objects of the invention are to provide a method for the proper conditioning of pressed lignocellulosic hardboard with a low moisture content to a moisture content substantially in equilibrium with that of the circumambient atmosphere and to attain such desired humidification in a minimum of time (eg. 30 minutes or less) and at the lowest possible expense.

In the manufacture of hardboard the process most generally employed at the present time comprises the comminution of suitable woody material into fibers of a suitable type which are formed in the presence of water into thick sheets on a suitable conventional board-making machine. The sheets thus produced are dried and then pressed under conditions of elevated temperature and pressure to consolidate the fibers into a lignocellulosic sheet having substantial strength and rigidity. As a result of the application of the high pressure and high temperature during the consolidation process, the lignocellulosic constituents of the wood become plasticized and are welded together to form a strong dense coherent board. The product, known as hardboard, is discharged from the presses in an essentially bone dry condition. It difiers greatly not only from wood, but also from socalled insulation board and cardboard. Hardboard, though of a woody origin, is usually more dense than wood and is readily produced in large homogeneous sheets, being considerably stronger than the original wood in many cases. Insulation board is a lightweight unconsolidated mat of fibers, in sheet form, admirably adapted for insulation purposes because of its cellular structure, but not as strong as hardboard. In use, hardboard is generally used in place of wood or even plywood, where a grainless uniform product, strong in all directions, is desired.

It is known that bone dry hardboard produced by the conventional methods has a decided tendency to absorb water from the atmosphere under ordinary conditions of storage and use. Thus these products have a tendency to warp and twist as a result of moisture absorption. For example, when a sheet of bone dry hardboard is applied to a vertical surface to which it is secured by nailing or screwing, the two sides are not normally equally exposed to the same conditions of moisture. The exposed side absorbs more moisture, will tend to expand, and hence will cause the board to bulge, sometimes with force sufficient to loosen the fastening means. If, on the other hand, the opposite side becomes more moist, the board will warp in the other direction and the fastening means will be pulled from the periph- 2,904,893 Patented Sept. 22, 1959 ice 2 cry of the board at the points where it is secured to the supports. Both of these conditions are highly undesirable and it has long been the practice to humidify hardboard after it has been formed whereby dimensional changes in the product during storage or use are minimized.

These prior art methods of humidification have generally been very time-consuming and involve the gradual absorption by the hardboard of anywhere from 3% to 10% of moisture upon exposure thereof to a suitably moistened or humidified atmosphere. This usually has been accomplished by passing the boards, supported in such a manner as to afford adequate access of moist air to their fiat surfaces, through a chamber or tunnel to which damp air was supplied. Under the conditions known to the prior art, the operation requires about six or more hours to effect the desired humidification of the hardboard. Therefore large apparatus and much space is required to carry on that part of the operation of commercial hardboard manufacture.

The present improvement is predicated upon the discovery that the desired amount of moisture, say from 2% to 8% by weight, may be very rapidly introduced into a preheated hardboard at temperatures which are usually considered for drying rather than humidifying. It is generally known that water is evaporated when heated to its boiling point and it would be expected that the subjection of hardboard to a moistened atmosphere at a temperature at or above the normal boiling point of water would serve only to dry the board. However, it has been unexpectedly found, in accordance with this invention, that hardboard can be rapidly and easily humidified to any desired moisture content at temperatures in excess of the boiling point of water and under controlled conditions.

The problem of introducing moisture into a hardboard through its surfaces is further aggravated greatly because the smooth, lustrous and highly desirable surface finish imparted thereto by the press platens during the process of consolidation of the lignocellulosic material into a coherent homogeneous body by means of heat and pressure must be retained in its original unblemished condition after humidification. During the humidification of hardboard care must be taken that liquid water in the form of droplets or otherwise does not contact the board surface, since the fibers on the surface will be caused to rise above and protrude from the original surface plane to mar and blemish the original smooth lustrous condition of the hardboard surface. These blemishes are further accentuated and high-lighted when decorative and preservative finishes are applied to the humidified product. Though such surface blemishes can usually be removed by expert refinishing, nevertheless unnecessary eiiort, time and cost are incurred. It is, therefore, highly desirable to avoid such fiber raising during the process of humidifying pressed lignocellulosic bodies. The present invention quite unexpectedly solved this problem of re-moistening or humidifying hardboard without the loss of its initial desirable lustrous smooth outer surface.

In accordance with this invention, it has been found that if the humidification be efifected within a rather narrow temperature range and under careful predetermined conditions of relative humidity, the desired amount of mois ture, say from 2% to 8% by weight, may be safely and easily introduced into the hardboard, provided that the humid atmosphere employed for the process of humidification is maintained at an elevated temperature which is not more than about 40 F. above the boiling point of water, and provided that the dew point of the humid atmosphere is at a temperature point lower than the temperature of the circumambient atmosphere in which the treatment is carried out.

In accordance with one embodiment of the present invention, hot hardboard in an essentially bone dry condition having a temperature above 212 F. is discharged from the consolidating press or from a baking step after treatment with oils and is placed in a suitable chamber to which there is admitted by suitable means a hot mixture of water vapor or steam and air. It is important that the temperature of the steam or vapor admitted be above the temperature of the dew point of the chamber mixture to avoid the formation therein of water droplets, particularly upon the surfaces of the board. An important feature of this invention resides in the discovery that the desired humidification may be efiectively carried out at a temperature of between about 200 F. and about 250 F. under conditions in which the dew point of the gaseous humidifying medium is below the temperature at which the treatment is carried out. Thus, for example, if the temperature existing within the humidifying chamber is 215 F., then the dew point of the mixture should be preferably between 205 F. and 210 F. for practical operations. The temperature differential between the humidifying temperature and the dew point of the hurnidifying gases is usually less than 40 F., and, in practical operations to prevent condensation, preferably greater than F.

It has been found that if large sheets of substantially dry, hot hardboard, say 4 ft. by 12 ft. in size, are treated in a suitable closed chamber at substantially atmospheric pressure with a mixture of steam and air having a dew point of about 200 F., but with the mixture heated to 220 F., that the boards therein in as little as 30 minutes or even less will have a moisture content of from 3% to 5%. Obviously, the exposure can be for any desired length of time to obtain any desired degree of water absorption within the board, but the absorption of 3% to 5% moisture sufiiciently normalizes the hardboard and it can then acquire further amounts of water from the air while it is stored Without fear of development later of waviness, surface blemishes and the like.

In carrying out the present invention, for instance, a chamber may be provided conveniently adjacent to the point of discharge of the pressed boards from the hot press. Inasmuch as methods of making hardboard and the apparatus employed therefor are completely well known at the present time, it is not deemed necessary to describe them in detail nor to illustrate them by drawings. Sufiice it to say that the hot boards usually having a temperature of 400 to 500 F. may be mechanically or otherwise transferred to the humidifying chamber. In this chamber the boards may be conveyed either horizontally, suitably spaced from each other, or they may be conveyed through the chamber while resting upon their edges with the boards extending vertically, care again being taken to leave enough space between them to permit the circulation of the humidifying gaseous mixture of air and Water vapor. While the boards are traversing the hurnidifying chamber there is circulated through it a humidifying mixture of gases maintained at a suitable temperature as above indicated. This mixture may be produced in several manners; steam may be admitted into suitably preheated air under such conditions that the eventual mixture will have the desired temperature, or steam may be mixed with air and the resultant mixture then superheated, that is to say, partially dried by raising its temperature by the passage of the mixture over suitable heating surfaces, for example, steam pipes, electrically heated surfaces or the like. If desired, the humidification can take place in an atmosphere of steam alone, provided the surface temperatures of the hardboard are above the dew point of the water vapor.

If, for example, the initial mixture of steam and air has a temperature of, say, 170 F., this may be suitably heatedto bring it to 220 F. and the mixture then passed 7 into the chamber. However, the moisture content of the humidifying gas is controlled so that the dew point is above F., preferably above 200 F. Thus, in accordance with this invention, recourse is had to the use of suitable hygrometric instrumentalities to establish a condition in the humidifying chamber where the dew point of the mixture is, say, between 10 to 20 F. below the temperature of treatment. This can be most readily accomplished by means of dry bulb and Wet bulb thermometers, the temperature of the wet bulb thermometer being a good indication of the dew point of the mixture. If, for example, the thermometers located in the humidifying chamber show a wet bulb temperature of 200 F., then the dry bulb temperature should be about 220 F. It has been established as a result of numerous investi gations that the temperature conditions for humidifying are within the relatively narrow range of from 200 to 250 F. When the temperature of the treating chamber is increased above 250 F., the rate of humidification becomes slower. Eventually it is possible under some conditions to abstract moisture from the boards if they contain it, rather than to impart it to them.

The boards may be conveyed through the treating chamber by any suitable mechanical means, for example, by having the edges of the boards ride on rollers which are positively driven at a suitable rate so as to convey the board through the chamber in a period of from thirty to sixty minutes. After the boards leave the chamber they may then be trimmed and stacked, or prepared for shipment or use. It will be found when this process is followed that the original lustrous smooth surface remains unimpaired and unblemished. Further they are now substantially free of warping tendencies when commercially applied for their intended uses.

With reference to the attached diagrammatic drawing, there is shown in elevation a humidification chamber constructed and operated in accordance with the process of this invention. The hot pressed boards 10 are pressed into and through the humidification chamber 12 on positively driven rollers 14. A mixture of air and steam is preheated in the preheater 16 and passed into the chamber 12 through a manifold 18, the moisture content and temperature of the mixture being controlled as herein set forth. The heated moisture-laden gas comes into contact with the boards 10 and then escapes from the chamber through the vents 20. The boards emerging from the chamber are. thus humidified in the chamber in accordance With this invention.

As specific examples of the carrying out of the present invention, the following examples are given:

Example 1 Quick humidification of 4; inch hot, bone dry hardboard weighing 750 pounds per 1000 square feet:

Dry bulb temperature at the beginning of the operation F 226 Dew point temperature F 198 Difierence F 28 Exposure time minutes 30 At the end of the operation the dew point temperature had increased to 202 F. but the other temperature had not changed. Under these conditions the board was found to contain from 4.1 to 4.9% of moisture.

Example 2 Example 3 Humidification of /s inch boards weighing 750 pounds per 1000 square feet. With temperatures at the dry bulb varying between 218 at the start and 220 F. at the end of a 30 minute period, and the dew point varying between 203 and 205 F., the absorption was 4.03 In a 45 minute exposure under the same conditions, the boards were found to contain 5.77% of moisture.

Example 4 In this case the same hardboard was treated for a period of 30 minutes, the temperatures being somewhat lower, the dry bulb thermometer varying between 216 and 219 F., with the dew point between 194 and 197 F. Under these conditions the amount of moisture imparted varied between 3.2 and 4.2%.

It will thus be seen from the examples that adequate humidification of hardboard may be obtained within a comparatively short period instead of the time-consuming and expensive conventional methods. The hardboards that are treated in accordance with the process of this invention are usually those that have a density in excess of that which corresponds to 500 pounds weight of inch per 1000 square feet.

A feature of the present invention is that the humidification can be carried out much more rapidly than by prior art procedures resulting in a tremendously increased speed of operation of the hardboard plant. Therefore very valuable commercial results both from the point of view of saving in space and also of a corresponding saving in time are attained. It would hardly have been expected that humidification to this extent could be carried out rapidly without fiber-raising and destruction of the original lustrous surface at temperatures which normally are used to dry articles rather than add moisture to them.

Thus it had always been believed necessary in conditioning of lumber and other lignocellulosic material that long periods of exposure to a damp atmosphere were necessary to attain the proper degree of moisture, and

while numerous suggestions have been made in the prior art that lumber may be subjected to steam and air, no directions were given as to temperatures, times, and particularly the correlation between the temperature of the conditioning medium and its dew point. Furthermore, the problem of fiber raising is not one which is of any real concern in the treatment of lumber or insulation board products. In view of the fact that the depositron of water on hard pressed boards of the character herein contemplated would be destructive of its desired smooth and lustrous surface, it will be seen that a great advance has been made in this art.

While particular embodiments of this invention are shown above, it will be understood, of course, that the invention is not to be limited thereto, since many modifications may be made, and it is contemplated, therefore, 'by the appended claim, to cover any such modifications as fall within the true spirit and scope of this invention.

1 claim:

A rapid process for humidifying hardboard to a moisture content of between about 2 and about 8 percent by weight which comprises passing a bone dry hardboard having a temperature above 212 F. into a humidifying zone through which a moisture-laden gas is passed into contact with the surfaces of said board and controlling the temperature of said gas in said zone to a temperature between 200 and 250 F., the moisture content of said gas being controlled to have a dew point below the temperature of the hardboard and between 5 and 40 F. less than the temperature of the gas in said zone and the time of contact of said gas with said surfaces being less than about minutes.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,071 Nevin Mar. 14, 1939 1,941,536 Boehm Jan. 2, 1934 2,400,051 Pasquier May 7, 1946

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