US2919492A - Process for humidifying hardboard - Google Patents

Description

Jan. 5, 1960 c. F. STORY 2,919,492

PROCESS FOR HUMIDIFYING HARDBOARD Filed March 15, 1957 TIME IN HOURS x Q Q (I I I, q 657 x I: Q '0 n I n N e /V/ K938 3801 9/0 .1 NJJUJd INVENTOR CHARLES E STOP) B d-ww AITORNEY United States Patent PROCESS FOR HUMIDIFYING HARDBOARD Charles Story,-Laurel, Miss., assignor to Masonite Corporatron, Laurel, Miss, a corporation of Delaware Application MarchlS, 1957, Serial No. 646,469

4 Claims. (Cl. 34-26) The present invention relates to the humidification of lignocellulose hardboard products. More particularly, the invention relates to a novel humidification method adapted to effect moisture regain in hardboard products of all types, thicknesses, and densities. The novel method is outstanding in its provision of a rapid humidification treatment which, at the same time, effects moisture regainin hardboardsheets without appreciable damage to the surfaces of such sheets.

In the manufacture of lignocellulose hardboard sheet articles, the finished products are usually characterized by a substantially complete lack of moisture subsequent to the consolidation thereof which is carried out under considerable heat and pressure. In many instances, hardboard sheets are also subjected to additional finishing treatments wherein the boards are impregnated with drying oils and the like materials and subjected to a baking cycle in order to polymerize the compositions with which they have been treated. In any case, before the hardboard sheets are sold and are installed by customers, it is desirable and, indeed, even necessary that they be conditioned to effect a moisture uptake, or regain, equivalent to that normally encountered under atmospheric conditions. The conditioning of the hardboard prevents undue dimensional changes therein during use, thus substantially eliminating unsightly warping and/or shrinkage. Manufacturers have generally subjected their hardboard products to a humidification treatment wherein from about 2% to about 6% of moisture is forced into the hardboard sheets as rapidly as possible. The amount of moisture absorbed by the Sheets usually lies within the above stated range which represents substantially equilibrium moisture content for most atmospheric conditions.

According to prior art practices, hardboard sheets have been charged in spaced apart relationship to a humidification chamber and therein subjected to a warm, humid atmosphere. Normally, temperatures have not appreciably exceeded 110 F. since it has been known that temperatures above about 110 F, when employed together with high relative humidity as, for example, 90%-95% have caused swelling and rupture of the large fibers which are always present to some degree in the surfaces of the sheets. Inasmuch as these swollen fibers never return to their originalcondition, the surfaces of the hardboard sheets have been permanently disfigured and have not been acceptable commercially. The humidification conditions which have been acceptable to the art, however, necessitate the employment of extremely lengthy moisture regain cycles. For example, it has often been necessary to humidify relatively dense hardboard sheets for a period of 20 hours or more in order to effect a moisture regain of about 4% to about 6%.

p In the case of sheets which have been impregnated with drying oils and the like compositions, these extremely lengthy. humidification cycles are always required with the result that the moisture regain treatment is uneconomical and highly undesirable.

lt is a primary objectof the invention to provide a humidification, or moisture regain, method which overcomes the above described shortcomings of the prior art treatments.

Another object of the invention resides in the provision of a novel humidification method which permits rapid moisture regain by hardboard products without deleterious results.

A further object is the provision of a novel method which is applicable to all types of hardboard products regardless of thickness or density.

Further objects of the invention will be apparent from the following detailed description thereof. The description will pertain to hardboard products in general but it will be directed particularly to the accompanying drawing in which:

The single figure is a chart illustrating hardboard moisture regain cycles for various humidifying conditions as applied to different hardboard products.

Briefly, the invention stems from the discovery that under certain conditions, namely, where the humidifying atmosphere is so regulated as to prevent appreciable condensation of moisture on the surfaces of the hardboard sheets, the moisture regain treatment may be carried out under relatively high temperatures as, for example, 140 F.-200 F. Within this temperature range the relative humidity of the humidifying atmosphere may vary within relatively wide limits as for example from about 50% to about 95%. This range of relative humidity represents approximately wet bulb temperatures of from about 115 F.138 F. to about 165 F.--198 F. at the respective lower and upper dry bulb temperature limits. The differential between the dry bulb temperature and the wet bulb temperature within the humidifier may therefore lie within the range of from about 2 Ffto about 25 F. depending on the particular temperature and humidity required to effect a moisture regain in the hardboard sheets of from about 2% to about 6%. Within this broad range of critical dry bulb temperature, it is preferable that the humidification cycle be performed at about F.-190 F. in order to obtain the benefits of extremely short moisture regain cycles.

It has been discovered that the above described conditions will permit rapid moisture regain in hardboard sheet products without damage to the surfaces thereof where the sheets are charged to the humidification chamber at relatively elevated temperatures. Broadly, the hardboard sheets should be charged into the chamber at a temperature preferably not more than about 10 F. below the Wet bulb temperature within the chamber. Generally speaking, hardboard products are characterized by a specific heat value of about 0.4 while the boards themselves may range in weight from about 0.9 to about 1.3 or more pounds per square foot. It may therefore be calculated that a differential of 10 F. between the board temperature and the wet bulb temperature within the chamber will result in approximately 0.4% of moisture by weight being condensed on the surface of the board. This figure is particularly applicable to a wet bulb temperature of about P. where the relative humidity of the chamber is approximately 90%. However, the same relationship holds true throughout the complete humidification range recited hereinbefore. While 0.4% moisture is seemingly slight, it should be pointed out that this amount of moisture is condensed entirely on the surfaces of the hardboard sheet material. Therefore, the initial percentage moisture content at the board surfaces is of the order of magnitude of 10% or greater over the period of time required to raise the board temperature to that of the wet bulb temperature in the chamber. Accordingly, while a 10 F. differential may be tolerated, a greater difierential invariably leads to 3 severe rupture of the surface fibers of the hardboard sheets. When the surface fibers are once swollen, they never regain their original size or shape and surfaces so disfigured are undesirable from a commercial viewpoint.

The novel method of the invention is applicable to all commercially available hardboard products. As will be seen from the chart of the single figure, the table A, which applies to hardboard sheets /8 inch thick and having a specific gravity of approximately 1.0, illustrates that such sheets may be conditioned to about 2% moisture content within a period ranging from about 30 minutes to about 1.5 hours at dry bulb temperatures of about 190 F.140 F., respectively. Table C shows that hardboard sheets inch thick and having specific gravities of about 1.1- or greater require from about 1.5 to about hours over the same dry bulb temperature range. Table B, which is applicable to intermediate thicknesses, shows that the sheets will require humidification time cycles intermediate between the two extremes in hardboard thickness. Where it is desired to effect a 5% moisture regain in hardboard sheets 7 inch thick, the humidification cycle will be correspondingly greater. For example, at a dry bulb temperature of 190 F., 5% moisture will be absorbed in about 5 hours. At a dry bulb temperature of 140 F., however, about 12 hours are required. Within this range, the relative humidity is so regulated within the chamber that the moisture is regained under conditions of equilibrium. That is to say, the desired moisture regain is effected over the stated time cycles and will not be appreciably exceeded if the treated sheets are allowed to remain within the chamber for extended periods of time.

While it is not known with exact certainty, it is believed that the dry bulb temperature within the humidification chamber is the force which drives the moisture into the sheet. The relative humidity at any given temperature determines the amount of moisture which will be forced into the sheet. In other words, it has been found that the rate of moisture regain in hardboard products increases in approximately direct proportion with increase in temperature.

It will be apparent that the chamber used for carrying out the novel method of the invention must be so constructed as to prevent condensation of moisture on the walls thereof. Such condensation has, in the past, resulted in disfigurement of the hardboard surfaces due to condensed moisture dropping on the surfaces. A preferred humidification device is disclosed and claimed in copending patent application Serial No. 646,451 of Howard Max Wassem, filed on even date herewith. Briefly, that invention is directed to a double walled humidification chamber containing gated openings for charging and discharging hardboard sheets. The space between the walls is heated. The walls of the inner chamher, which is the humidification area, are maintained at a temperature of such proximity to the wet bulb temperature within the chamber that condensation is thereby prevented on the inner walls. The differential in temperature between that of the inner walls and the space between the double walls above referred to is dependent on the thermal conductivity of the construction materials employed. Obviously, this may vary within relatively wide limits. The novel humidifier, however, represents a decided improvement over prior art humidification devices which have been so constructed that the walls were insulated but were not heated to insure the proper relationship to the wet bulb temperature within the inner chamber.

In the case of hardboard sheets or inch thick, it is permissible that two such sheets may be charged to each wicket of the above described humidifier. Under these conditions four percent moisture, for example, will be absorbed by the sheets in about 1.5 hours or less at 4 a dry bulb temperature of 180 F., wet bulb temperature of F., and relative humidity of 70%.

From the foregoing description it will be seen that the invention provides a greatly improved method for effecting moisture regain in all types of hardboard products. Whereas the prior art methods of humidification entailed extremely long time cycles, the novel method of this invention permits moisture regain up to about 6% or more over relatively short periods of time. The novel method also provides for the substantial elimination of condensation of moisture on the board surfaces and thereby prevents rupture of surface fibers. The novel method therefore presents to the art a new and hitherto unthought of humidification method which represents a decided economic advantage over prior art practice.

I claim:

1. A method of humidifying dense hardboard sheets having specific gravities within the range of about 1.0 to about 1.3 and thicknesses of about inch to about A inch which comprises charging said hardboard sheets to a closed chamber in spaced-apart position, subjecting said sheets to a humid atmosphere at about 140 F.- 200 F., dry bulb temperature for a period of time sufficient for said hardboard sheets to absorb about 2%6% by weight of moisture, said heated humid atmosphere having a relative humidity of at least about 50% so that the said range of absorbed moisture represents substantially equilibrium moisture uptake within said dry bulb temperature range, said hardboard sheets being at a temperature not substantially lower than about 10 F. below the wet bulb temperature within said chamber when charged thereto, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal relationship, and cooling said sheets to room temperature in stacked condition.

2. A method of humidifying dense hardboard sheets having specific gravities within the range of about 1.0 to about 1.3 and thicknesses of about inch to about i inch which comprises charging said hardboard sheets to a closed chamber in spaced-apart position, subjecting said sheets to a humid temperature at about 140 F.- 200 F. dry bulb temperature for a period of time within the range of about 30 minutes to about 12 hours, said heated humid atmosphere having a relative humidity of at least about 50% so that the said range of absorbed moisture represents substantially equilibrium moisture uptake within said dry bulb temperature range, said hardboard sheets being at a temperature not substantially lower than about 10 F. below the wet bulb temperature within said chamber when charged thereto, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal relationship, and cooling said sheets to room temperature in stacked condition.

3. A method of humidifying dense hardboard sheets having specific gravities within the range of about 1.0 to about 1.3 and thicknesses of about inch to about 4 inch which comprises charging said hardboard sheets to a closed chamber in spaced-apart position, subjecting said sheets to a humid temperature at about F.- F. dry bulb temperature for a period of time sufficient for said hardboard sheets to absorb about 2%6% by weight of moisture, said heated humid atmosphere having a relative humidity of at least about 50% so that the said range of absorbed moisture represents substantially equilibrium moisture uptake within said dry bulb temperature range, said hardboard sheets being at a temperature not substantially lower than about 10 F. below the wet bulb temperature within said chamber when charged thereto, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal relationship, and cooling said sheets to room temperature in stacked condition.

4. A method of humidifying dense hardboard sheets 5 having specific gravities within the range of about 1.0 to about 1.3 and thicknesses of about ,4 inch to about 95 inch which comprises charging said hardboard sheets to a closed chamber in spaced-apart position, subjecting said sheets to a humid atmosphere at about 170 F.- 190 F. dry bulb temperature for a period of time within the range of about 30 minutes to about 12 hours, said heated humid atmosphere having a relative humidity of at least about 50% so that the said range of absorbed moisture represents substantially equilibrium moisture uptake within said dry bulb temperature range, said hardboard sheets being at a temperature not substantially lower than about 10 F. below the wet bulb temperature within said chamber when charged thereto, thereafter removing said hardboard sheets from said chamber, stacking said sheets in horizontal relationship, and cooling said sheets to room temperature in stacked condition.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,071 Nevin Apr. 14, 1942 1,160,724 Landin Nov. 16, 1915 2,284,838 Oholm June 2, 1942 2,728,444 Richardson et al Dec. 27, 1955 UNITED STATES PATENT OFFICE CERTIFICATE @l QQERREQTEUN Patent N00 2,91%492 January 5, 196

Charles F Story It is herebj certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4L lines 41 and 61 for 'temperature", each oecurrence read atmosphere *"9 Signed and sealed this 6th day of September 1960a (SEAL) Attest:

EIgNEST wer aw ess Attesting Ofiicer ROBERT C. WATSON Commissioner of Patents

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