US2874481A - Method of treating fibre boards - Google Patents

Description

United States Patent t a a 2,874,481 METHOD OF TREATING FIBRE BOARDS Bror Liifgren, Stockholm, and Nils Jul (igland, Svanskog,

Amal, Sweden, assignors to Aktiebolaget Svenska Flaktfabriken, Stockholm, Sweden Application June 28, 1954, Serial No. 439,848

Claims priority, application Sweden June 29, 1953 6 Claims. (CI. 34-16) The present invention relates to an improved method of treating fibre boards and like materials manufactured by defibrating fibrous raw materials. .The invention has particular utility in treating materials which have been formed, dehydrated, and driedin a pressing device.

Hard fibrous sheets, 'such as wall-board are manufactured from cellulose materials such as wood, straw, bagasse, etc, The manufacture is commonly carried out in such a way that the raw material, after having been defibrated, is formed into a web which is then cut into sheets. The sheets are then dehydrated and dried in a pressing device. The pressingis usually performedto produce one smooth surface and one puckered or uneven surface by pressing the sheet between a plane plate and a wire mesh. In certain cases, when manufacturing sheets smooth on both sides, the sheets are bone 'dried before pressing and are then subjected only toheat and-pressure in the pressing device. e

In the pressing cycle, the 'wet sheets are charged into the pressing device and are then exposed to pressure and heat to be dehydrated asthe temperature of the sheets 'is increased. At a certain time, the temperature of'the sheets reaches such a value that the sheets are practically bone dry. The "sheets areusually allowed to remain in v the pressing device untilanother time, in which period, the sheets are heatedunder pressure so that they reach a temperature which is almost equal to the temperature of the heated surfaces of the pressing devicel ln many operations, the procedureis interrupted during this latter period and thesheets-are 'then heat-treatedso called hardening+in a separate device. The reasonfor this is that the pressing device is an expensive part of the plant, and thus", heat-treating the sheets while in the pressing device would be 'expensive. The object of the heat treatment is to somewhat increa se the strength of the sheets against bending and also to reduce their power of moisture absorption'and theirswellingdue to moisture absorption. The sheets pressed and dried in this way are these and other disadvantages of the prior methods of treating hard, wooden fibre sheets, boards or the like.

It is also an object of theinvention to improvethe quality of the moistened sheets. 1 e

The invention is mainly characterized in that the material, after having been heat-treated, is charged into the moistening chamber, and the moistening isperformed by substantially air-free steam and is followed by cooling. It is possible to use saturated or superheated steam and the'steam may be at atmospheric pressureor at super pressure. It is also possibleto combine the heat treatment and the moistening of the sheets, and 'we;have found that if this combined heat and moistening treatment is performed under controlled conditions, it is possible to obtain improved results with respect to the decrease in the sheets power of moisture absorption and their swelling, resulting in a worthwhile increase in the quality of the sheets. i j Other objects of the invention will be evident from the claims and the description and with reference to the accompanying drawings wherein: U 1

Fig. l is a curve illustrating a conventional pressing cycle; 1 e

Fig. 2 is a curve of the hygroscopic properties of fibre: board in an atmosphere of steam, showing the equilibrium relation between the moisture ratio of the sheets and the temperature of the steam at atmospheric pressure;

Figs. 3 and 3a are curves showing cycles of stepwise moistening in accordance with the invention;

Fig. 4. is a curve illustrating a cycle of continuous'rnoistening of sheets in accordance with the invention;

Fig. 5 shows an apparatus for heat-treatingor'heating and moistening and cooling the sheets in accordance with the invention; Fig. 6 illustrates apparatus for treating sheets under high pressures in an autoclave in accordance with the invention; and '*Figs. 7 and 8 arecurves illustrating respectively the variations of the temperature and pressure during a cycle of the treatment in apparatus such as shown in Fig. 6.

When'a sheet is moistened in steam according to the invention, the moistening initially has ateridency to proceed very rapidly because of the great difference in vapor pressure which exists between the moistening medium and the sheet. almost immediately will attain a moisture ratio equalto the equilibrium moisture ratio of the sheet in the steam. The moistening is accompanied by a swelling of the sheet. In accordance with the invention, the moistening then moistened to a degreefinor'der to'have their state in equilibrium with the temperature and the relative humidity existing in the place wherethe sheets are to be used. The prior conve'ntional moistening in moist air is often time-consuming. f V i The moistening is usually performed byf air which is practically saturated 'with'water vapor, and the temperature is maintained at al moderate level, preferably about 315degrees to 60 degreeshc. At high temperatures, the time for moisteningjdecreases because the, diffusion of watervapor in the sheet takes place more rapidly at high temperatures thanat low temperatures. However, when using high temperatures and a high relative humidity in the air, in ordertoaccelerate thepenetrating'of the moisture into the sheets, the smoothness of the sheets is spoiled (i; e., the sheetloses part of its lustre). In addition, a so-called rise of the fibres occurs in the surface layer of the sheets. i

is slowed so that moisture from the surface layer has time to penetrate the lower layers to permit these layers toswell to a certain degree. In the present case, it may 'beassurned that the molecules of water simultaneously penetrate the layers of the fibres both in the center of the sheet and at the surface of the sheet. In a pure atmosphere of water vapor, the air molecules in the sheet compete with the water molecules of the steam within capillary tubes of the sheets. We have found that in spite of this, the moistening proceeds more rapidly in steam than in moist air. At high temperatures, the rapidity of the movement of the molecules israther great. The water vapor penetrates rapidly into the cavities, be-

cause of the high diffusion constant existing at'the high- -temperature. 1

. When moistening in superheated steam, there exists a simple relationship between the temperature of the steam and the moisture ratio of the sheet whereby, as the temperature of the steam increases, the moisture ratio of the sheet decreases. This relationship is diagrammath cally illustrated in Fig. 2. Y The curve of Fig. 2 'is somewhat similar to the curves of the hygroscopic properties of selected materials which are available in the Inter-z The surface layer of the sheet therefore national Critical Tables. For example, in the volumes published for the National Research Counsel by Me- Graw-Hill Book Company in 1933, such curves appear in volume 2 at pages "321 et seq. These curves plot the moisture ratio of the material against the relative humidity of the surrounding moist air at equilibrium, and it is a simple matter to derive a curve such as shown in Fig. 2 which plots the moisture ratio of the material against the temperature of superheated steam.

With the curve of Fig. 2, the state of the steam (in i degrees of superheat or in temperature and pressure) which is in equilibrium with fibreboard having a selected moisture ratio, may be determined. The percentage moisture, which is the weight of moisture divided by the weight of bone dry material is plotted as the ordinate, and the temperature expressed in degrees centigrade is plotted as the abscissa. The curve plots the values for steam at one atmosphere of pressure. Reading from the curve, fibreboard having a moisture ratio of 22% would be in equilibrium with atmospheric steam at degrees centigrade. Atmospheric steam at 104 degrees centigrade would bein equilibrium with fibreboard having a moisture ratio of 0.12 (see point vD on the curve). Atmospheric steam at degrees centigrade would be in equilibrium with fibreboard having a moisture ratio of 0.095 (see point E). Atmospheric steam at degrees C. would be in equilibrium with fibreboard having a moisture ratio of 0.07 (see point G). By using the curve of Fig. 2, the state of the steam at atmospheric pressure and any selected temperature may be expressed in terms of the moisture ratio (Uv) of the fibreboard in equilibrium therewith. Thus, steam at one atmosphere and degrees C. may be defined as having a Uv of 0.06 (see point H). At other pressures and temperatures, the Uv of the steam may still be 0.06, in accordance with the values of the humidifying force of the steam at such pressures and temperatures.

The curve of Fig. 1 illustrates a standard pressing cycle showing the increase in temperature as the pressing cycle progresses. The work sheets are charged into the pressing device at the time 0 and are exposed to pressure and heat. At time A, the temperature of the sheet reaches a level where the sheets are practically bone dry. The sheets are usually retained in the device until the time B, during which period they are heated to the temperature of the surfaces of the pressing device. In many operations, the procedure is interrupted at the time C and the sheets are heat-treated in a separate device.

When a sheet is discharged from the pressing device, a certain amount of time will elapse before the sheet is transported to the succeeding treatment. During this time, the sheet will cool. If the sheet is exposed to heat treatment, its temperature will be raised to a temperature of about 170 to'200 degrees C. The sheet then can be charged into the moistening apparatus with a temperature of about -160 degrees C. When moistening in superheated steam at this temperature, the material must have an equilibrium moisture ratio of about,0.0l to 0.02 or a percentage moisture of about 1 to 2 percent. By moistening, a percentage moisture of about 7% is desired in order to obtain equilibrium with air at normal atmospheric conditions. In order to attain this equilibrium condition, it is necessary to lower the temperature of the steam successively down to about 110 degrees C. (see point G on the curve of Fig. 2). It is possible to proceed with the treatment as indicated in Figs. 3, 3a or 4.

Figs. 3 and 3a illustrate the cycle of moistening in a chamber Where the sheets are stationary and the condition of the treating medium is gradually changed stepwise during the treatment period. Fig. '4 shows a cycle in apparatus for continuously moistening sheets. In this case, the sheet is moved eounter'current to a steam current, the relative moisture or superheating of which is varied with respect to the time. In carrying out the invention, the moistening steps should be controlled so that the state of the steam in terms of the moisture ratio of material in equilibrium with the steam, is gradually varied in relation to the actual moisture ratio of the sheet. This variation is continued until the moisture ratio of the material reaches about /3 of the intended final moisture ratio of the sheet, for example, as indicated at the time F in Figs. 3, 3a and 4, and then the state of the steam is adjusted so that the moisture ratio of the material in equilibrium with the steam is equal to the desired final moisture ratio of the sheet. In the curves Uv indicates the state of the steam in terms of the moisture ratio of the material in equilibrium with the steam, and Um indicates the actual moisture ratio of the material. Umf indicates the desired final moisture ratio of the sheet.

Referring to Fig. 3, during the moistening cycle, the Uv of the steam is gradually increased stepwise as the Um of the material increases. If the moistening occurs at atmospheric pressure, the temperature of the steam s gradually reduced. The values for Um may be estimated to obtain the curve shown in Fig. 3, or the moistening cycle may be run and test samples taken intermittently to determine the Um. When the curves Uv and Um have been determined, it is not necessary to take repeated samples or readings of the Um, and the Uv of the steam may be increased according to the pattern initially determined. A

The regulation may also be performed as shown in Fig. 3a in such a manner that the ratio Uv/ Um is equal to 2-5, preferably about 3. As shown, the Uv of the steam is changed in steps from 0.01 to 0.12 approximate- 1y, which in atmospheric pressure, would mean a tem perature variation in steps from about 200 degrees C, down to 104 degrees C., at the time F the Uv is reduced to the value of Umf, for example by raising the temperature to 110 degrees C.

Fig. 4 shows the cycle where the state of the steam 1s changed continuously, for example as when the material travels through a tunnel ranging in temperature from approximately 200 degrees C. down to 110 degrees C.

After the moistening cycle, the sheet has a moisture ratio nearly equal to Umf. If the sheet is not heattreated before moistening, for instance during the hardening, it is necessary to heat the sheet to prevent condensation. I The sheet should be heated to or above the dew point of the steam used in the moistening. The sheet should preferably be heated to a temperature corresponding to the dry bulb temperature of the steam at the beginning of the moistening to insure against water vapour condensation at the surface of the sheet at the beginning of the moistening;

At the end of the moistening the sheet has a temperature corresponding to the desired equilibrium moisture ratio. When moistening in super-heated steam the sheet will thus reach a temperature of about 110 C. corresponding to a normal moisture content of the sheet. If such a warm sheet is directly introduced into a normal room-atmosphere at about 20 C. and 50% relative humidity, the sheet will lose a part of its moisture content while being cooled. In order to prevent this, the sheet should in accordance with the invention be cooled to or heart-o the room temperature by means of air whose condition is maintained near the equilibrium state intended for the sheet, so that the sheet during its cooling neither gives off nor absorbs moisture from the cooling air. It is also possible to proceed in such a manner that the sheets are moistened somewhat above the normal moisture ratio and then discharged while warm. When the sheets cool; they give oif a small part of their Water content to the surrounding cool roomatm p Heat treatment (or only heating) and'moistening and cooling of the sheets may be performed in apparatus ac- I the duct 13.

cording to fig. 5. .fIhe plant principally consists of two q ambcts 1an .pra sdw f eale closing s 3. Bymeans of partition 14 the chambers are divided into a treating space 1b and 2b respectively and a channel 1a and 2a respectively for'a circulating medium. In each of the circulating channels there is a fan 4 "driven by a motor 5. These fans are intended to circulate the treating medium through the goods (the sheets) 6 piled for instance on trucks. The chamber 1 is furthermore provided with one or more *jets 7 for supplying steam into thechambenand. also with. heating elements 8 for the supply of heat to the sheets and for the super-heating of steam; The supply of moisture to the jets 7 is controlled by a va'lve7a,, andthe heating elements are controlled by a valve 8a. The chamber 2 is provided with jets 9 (controlled by a valve 9a) for steam or water and drop traps 10 for eliminating water drops from the jets. In order o m k rsssib tq ha s gases rat d: dur ing the heat treatmentthardening) the chamber 1 is provided with a chimney 11. The chamber 2 is provided with an inlet 12 for air and an outlet 13 for ventilating air, the object of which air is to carry away heat from the sheets during the cooling. The flow of air through the inlet and outlet may be controlled by dampers 16 and 17.

The treatment of the sheets is performed in the following manner. The sheets delivered from the pressing device are piled on a truck charged into the chamber 1. By means of the heating element 8 and the circulating fan 4 thesheets are rapidly heated up to the hardening temperature usually 170-200 C. This temperature is maintained during a certain time often about 3-5 hours. While the sheets still are warm the valve 7a is operated .to supply steam through the jet 7, whereby air and gas are forced out from the chamber and caused to escape through the chimney 11. p The pressure of the chamber is then regulated by means of a damper arranged in the chimney. The temperature of the super-heated steam is gradually reduced by operating the valve 8a to a value causes the Uv of the steam to rise, for example, as indicated in Figs. 3, 3a, or 4. When the sheet is moistened it will have a temperature somewhat above 100 C. andthe sheet must therefore be cooled before it can be chamber 2 where they are cooled by air supplied through 'the duct 12. The used air (having, higher water and heat contents than-the supplied air) will escape through In order to make it possible to regulate the humidity of the air during the cooling so that the desired moisture ratio is maintained in the sheet, the

, stored. The sheets are for'this' reason transported to the valve 911 is operated to supply the necessary quantity of moisture through the'jet 9. Any water drops are eliminated in the drop trap 10. The sheet can also be cooled by means of cool air without any addition of water, but the moisture which has beenadded to the sheet in th moistening apparatus will partly be lost. V

The heat treatment of the sheets can also be performed result of the combinedinfluence of moisture, pressure and temperature will be that the fibres of the sheets are converted (plastified) and that the absorption of moisture as The treatment can be carried out in a closed. chamber (autoclave) substantially o'f the design shown in. Fig. 6. The apparatus consists of a receptacle 21 constructed to withstand high pressures. In this receptacle there is an inner casing 22 designed to contain. a truck 23011 which the sheets to be treated are piled. .Said inner casing is on its bottom provided with two rails 24 for the truck. The receptacle21 is at one ofits ends providedwith a swinging door 25 where a gas distributing means 26 (for instance a perforated plate) fastened so that it canrfollow'the movements of the door. The distributing means is adapted to register with the surface ofthe inner cjasing 22. In the rear part of the inner casing there is a similar distributing means 27 at some distance from the 'rear end wall 28 of the inner casing. 1 In the wall128 there. is a centralioutlet for, gaseous medium to. whieh" a fan 29 is connected. The fan is'driven by meansof a shaft 30 passing through the rear wall. 35 f the receptacle. For the shaft 30, a paclgipg'boxjgis mounted in thewall 35. A channel for circulating medium"isformed between the outer and inner casings of the apparatus and in this channel there are heating elements 32 controlled by a valve 36 and a steam inlet 33 controlled by a valve 37. The purpose of the inlet for steam is partly to supply moisture to the sheets (boards) and partly to maintain the pressure in the receptacle.

The treatment is performed in the following manner. The receptacle is charged with a truck with piled sheets to be treated. The sheets are then heated to a temperature equal to or somewhat above the steam temperature at which the sheets obtain the intended moisture-ratio. The sheetswill reach about 5% of moisture ratio at atmospheric pressure and at a temperature of about 105110 C. At a super-pressure of 2 atmospheres and with saturated steam in the receptacle the sheets will have a moisture ratio of about 15%. For the conversion (plastifying) of the sheets it is then necessary to raise the temperature and the pressure, as the conversion takes place in the sheets more rapidly at a high pressure and at a high temperature. The maximum temperature which can be used depends on the origin of the cellulose fibres. For fibres of different kinds it is possible to use temperatures between 160-220 C. and super-pressures of about 6-24 atmospheres corresponding to the temperature of the saturated steam. When the conversion has continued .to a certain degree the pressure and the temperature is I pressure producesbesides an improved plastic quality,

If the sheets are moistened before bep a ing exposed to the relatively high heat treatment-temperature, to amoisture percentage of about 1-45%, the

aireduced absorption of moisture and a reduced swelling-the advantage of a shortened pressing time resulting in an increased production by the existing equipment. This is of a great economic importance as the pressing device is more expensive than the heat treating apparatus.

'Wh at we claim is:

- 1. An improved method of treating fibre boards and like materials manufactured by defibrating fibrous raw materials, forming, dehydrating, and drying in a pressing device the dehydrated material, comprising the steps of heat-treating the material up to a temperature of 180 0., preferably 160 C.,subsequently moistening the material while. at said temperature by air-free steam, and cooling the material after moistening, said moistening step being performed for at least an initial and major portion of said step by gradually changing the state of the steam in relation to the actual mean-moisture ratio of the material in such a manner that the ratio of Uv to Um is maintained at a substantially constant value The invention is of course not limited to the zsvaasz (where uv stands for the state of the steam in terms of the moisture ratio o'f the material which would be in equilibrium with said steam and Um is the actual moisture ratio of said material).

2. A method according to claim 1 wherein in said heattreating step the material is heated to a temperature corresponding to the dry bulb temperature of the steam employed at the start of the moistening step.

3. YA method according to claim 1 wherein said cooling step cools the moistened material approximately to room temperature by means of air having a relative humidity and temperature providing a humidifying force equal to the humidifying force of atmospheric air.

4. A method according to claim 1 wherein said heattreating step comprises heating the material to at least the dew-point of the air-free steam in the moistening step.

5. A method according to claim 4 wherein said ratio of Uv/ Um is in the range between 2 and 5.

, t 8 v V 7 6 A methea according as than 5 wherein said are of Uv/Um is maintained until themate'rial attains 'a moisture ratio equal to of'the desired'fin'al valueand thereafter the state of the steam is maintained constant so as to make Uv equal to the desired final moisture ratio.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,071 Nevin Apr. 14, 1942 1,421,685 Glessner a July 4, 1922 2,000,663 Darrah May 7, 1935 2,376,612 Muench et ai. May 22, 1945 FOREIGN PATENTS 335,942 Great Britain Oct. 3, 1930 416,552 Great Britain Sept. 17, 1934

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