US2317394A - Process for making hardboard - Google Patents

Description

Patented Apr. 27, 1943 UNITED STATES PATENT OFFICE 2,317,394 I rnocEss roa MAKING mannom William H. Mason, Robert M. Boehm,and George G. Simpson, Laurel, Miss., assignors to Masonite Corporation, Laurel, Miss., a corporation of Delaware No Drawing. Application November 18, 1939, Serial No. 305,102

. ZClaims.

material is reduced to fiber, felted into sheets;

and the sheets are dried, and the final hardboard product obtained by making an application of heat and pressure. to the dried sheet for a sustained period of time by means of pressing plates.

This invention is an improvement on U. S. Patent No. 2,120,137to W. H. Mason. In that patent there is but one activation of the self-bondin properties of the ligno-cellulose fibers which is performed in the final step of making a sustained application of heat and pressure to a dry sheet of ligno-cellulose fiber. With the present invention, however, the self-bonding properties of the ligno-cellulose fiber are activated twice; first, by making a material and controlled application of heat treatment, in the presence of a large percentage of moisture, to the ligno-cellulose fiber or ligno-cellulose material from which the fiber is made, and, second, are again and further activated inthe dry state by the use of heated pressing surfaces for applying consolidating pressure to a dried sheet of this fiber. Further, with the present invention, by reason of these self-bonding properties having been activated in the preparation of the fiber, the final step of making a sustained application of heat and pressure to the 'dry sheet composed of fiber so prepared can be be developed by a heat treatment which leaves the material in a predominantly fibrousstate adapted for giving high strength to the'board made therefrom. Hardwoods or deciduous woods such as the various varieties of gum, and mag- -nolia, poplar, cottonwood and the like are so adapted to the greatest extent, also the woody fiber of cane and similar annular growths can be used and such materials will be. grouped under self-bonding properties" and products made from I it lacking resistance to absorptionof water and wet strength.

.Neither mechanical lulose material without heat treatment, such as .used in making ground wood for example, nor

by controlled heat treatment applied in the presence of a large percentage of moisture, and after the sheet of so-treated material. is formed and dried, the self-bonding properties of the lignocellulose fiber material are again and further ace tivated by controlled application of heat and pressure to the dried sheet of such material during the consolidation thereof into a board or other product.

Another object of the invention is the provision of a process by which a greatly improved fiber hardboard is produced from a dried sheet, of ligno-cellulose fiber in a relatively short interval of time and without resort to the application of excessive temperature and pressure.

A further object is the provision of a very stiff hardboard having good properties of dry and wet strength and water resistance, but without being necessarily made excessively thin and dense.

. To secure the advantages of the invention to the fullest extent, the ligno-cellulose material taken should be of such character that the selfbonding properties of fiber made therefrom can fibration by explosion not involving a heat treatment which will serve to activate the self-bonding properties of the fiber throughout the whole of the material taken for treatment is adapted for preparing fiber material usable for the purposes of the present invention; For example, the explosion with steam carried out with merely such short, exposure to steam as to permit penetration of ligno-cellulose chips followed quickly by pressure release and explosion to obtain a fiber, which is the substantial equivalent of ground wood fiber as pointed out in the U. S. Patents Nos. 1,663,505 and 2,120,137 to W. H. Mason, does not involve a heat treatment of the entire charge taken for such explosion with steam which is adapted for preparing fiber material usable for securing the benefits of the present invention. Fiber so exploded, ground wood fiber and the like, are hereinafter referred to as raw fiber. The table given hereinafter shows the improvement obtained by the present invention over use of such raw fiber.

0n the other hand, while the heat treatment must be fairly considerable inorder to activate the self-bonding properties of the ligno-cellulose fiber in the .presence of considerable proportions of moisture, it should not be carried to such extent that the whole mass of material taken for treatment is converted to predominantly plastic state, as for producing the very dense bodies described in U. S. Patent No. 2,080,078, or bodies which can be consolidated by substantially momentary application of heat and pressure defibration of the ligno-cel scribed in U; S. Patent No. 2,140,189. If such predominantly plastic. material, which has little or no fibrous structure after hot pressing, were given the hereinafter described treatment in the press which gives good results with the present invention, only a brittle product would be produced. I

Depending on the type of ligno-cellulose material, the size of the pieces of wood, such as chips,

moisture content of the raw material and other variables, the time and temperature and the disintegration treatment are adjusted with the present invention so as to subject the ligno-cellulose -material to a heat and moisture treatment and disintegration adapted to secure a material which is predominantly fibrous and in which the selfbonding properties of the ligno-cellulose fiber throughout the entire charge of'material taken for treatment are activated to a substantial extent. The heat treatment, consisting in application of heat in the presence of moisture, is preferably performed in a closed chamber or gun by use of high-pressure, preferably saturated, steam upon the ligno-cellulose material, which material is preferably charged into the gun in somewhat subdivided-state, as in the form of chips about one inch or less in length, the chips ordinarily containing a material proportion, as 25% or more, of water.

The time and the temperature of heat treatment are somewhat interchangeable, longer time being required at relatively low temperature, and

'the time being considerably decreased at higher steam temperature and pressure. The temperature should exceed about 20090., which is sublbs. per sq. in. (temperature 01' about 255 C.) in

about 85 seconds, and maintaining it at this latter pressure for about 20 to 30 seconds additional, after which by opening a. small outlet the entire mass is explosively discharged from the region of high pressure to atmopsheric pressure, which disintegrates the material to fiber and terminates the action.

In order to ensure best penetration of the steam throughout thechips, it is of advantage to use lower temperature at first and then higher temperature, in which case the time of the applica tion of the higher temperature will be less than would be needed if only'the higher temperature is applied, as may be done. For example, with gumwood chips having about the same moisture content, good results may be obtained by admitting saturated steam to raise the pressure from atmospheric to approximately 500 lbs. per sq. in. (temperature of 245 C.) in about 30 seconds, following this by increasing the pressure to about 1000 lbs. per sq. in. (temperature of 285 C.) in about l'second, and maintaining this pressure for about 5 or 6 seconds followed by explosive discharge.

-It is not necessary to explosively discharge the aterial at the termination of the heat treatment although this will stop the heat treatment quickly and secure fibration of the wood or wo dy material. The heat treatment can be terminated without explosion of the wood or woody material, as by bleeding oi! the steam, and this heat-treated material may later be mechanically fibrated by known methods, or the material may be refined while under the pressure applied for the heat treatment. The heat treatment may precede, accompany or follow the disintegration treatment so long as the'material for making the sheets going to the press has undergone heat treatment as herein described or the equivalent thereof. Little refining, or beating, if any, need be given to: the fiber when .flbrated by explosion with steam and, if it is put through a refiner, this serves principally to secure more uniformity in' its formation into sheets.

The fibrous material obtained as described contains a considerable proportion, as about oi material soluble or dispersible in water, and

in order to secure best resistance to water absorption in the product, this material is reduced to a lesser proportion, as for example itmay be "re-.

duced to 5%, and this may be done by expression from the fiber of water containing water-solubles which are usable for manufacture of bill-Products, or by treatment of the fiber with water, which can, be conveniently accomplished in the operation of forming the fiber into sheets from water, or both.

The fiber produced as described is suspended in a large proportion of water, as 1% of fiber by weight to 99% of water, and formed into sheet form from the water. A good grainless, felted formation can be obtained with this fiber, and this is of advantage in getting a. good surface and uniform high strength of the final product.

After forming into sheet form from water the sheet is preferably passed between squeeze rolls which reduce the water content, as to approximately 'or less, after which the continuously formed sheet is preferably cut into board length to be later dried and hot pressed.

While binding agents from extraneous sources are not needed, they could be incorporated, if

desired.

We preferably incorporate waterproofing size for enhancing the resistance to water absorption and the wet strength of the product. In using hydrocarbon size, such as petrolatum, for example, the water bath or stock chest may be maintained at a temperature above the melting point of petrolatum, and molten petrolatum, to the extent of around 1% to 5% on the weight of the dry fiber, may be incorporated with the pulp in the water bath. Other waterproofing sizes, such as rosin size and alum, for example, may be used, and instead of maintaining the water bath at a temperature above the melting point'of the size, which is preferable with the use of ,petrolatum size, the waterproofing agent may be emulsified and introduced cold, if desired, orin yet other ways. The sizingv material, as petrolatum for example, may be put into the gun with the chips 11' desired.

The wet sheet formed from-the water batlr'isdried, preferably completely dried, but drying to complete dryness in a drier is an expensive and protracted operation, and after going through a drying operation which is commercially practicable, the sheet may still contain a small percentage of moisture, as 1 to 2% of moisture.

This small percentage of water can be removed in the press as hereinafter described. The drying is preferably done without application of pressure, but pressure may be applied during the sorb as much as 7 or 8% of moisture.

4 pors are permitted to escape.

drying operation if desired. When dried without pressure, the sheets made as ,described are ordinarily of about 0.4-0.6 sp. gr.

The sheets are preferably charged into the or explosion of the product upon opening the press while fully heated.

The press platen temperatures and the pressure required to again and further activate the self-bonding properties of the fiber and to thoroughly bond the heat-treated fiber together into a stiff, hard, strong, final product depend to a considerable degree on the particular wood or by sufficient roll pressure can be applied to effect some consolidation of the sheet as to about 0.? sp. gr., for example. This makes a firmer sheet,

' better adapted to withstand rough handling and to avoid scufilng of the sheet in charging into the press, which could cause defects in the finished board. Inasmuch as this rolling is not a finishing treatment, and the boards are to be later subjected to heat treatment in the press, the rolls, if used, need not be highly heated. A roll temperature of about 150'-250 C. and a roll pressure of about 40,000 lbs. per linear foot of roll bite are ample to give a firm sheet well adapted for handling into the press. Rolls so used for only a partial compaction of the sheet can be run at practically any desired speed as moisture and vapors can readily escape without injury to the sheet because of the sheet bein compacted only partially.

If the sheets are allowed to stand and pick up atmospheric moisture after drying, they may ob- In such case, before they are charged into the press, they are preferably redried to make them completely dry, or to reduce the moisture content to about 2% or less.

The dried sheets or blanks, whether or not partially consolidated by heated rolls, are finished by pressing in a hot press, preferably a hydraulic press, and preferably amultiple platen press, having for example openings for hot pressing 20 sheets at a time, between heated pressing surfaces which may consist of the press platens or surface plates, and subjected therein to heat and pressure for a sufficient time to effect their final and permanent consolidation. When it is desired to make a board smooth on both sides. both pressing surfaces are smooth, continuous and imperforate. However, vapor escape during hot pressing is not detrimental so long as the pressing surfaces are adapted to supply heat eff ciently to the sheets, and if, for example, only one smooth side is wanted, the other pressing surface may be faced with fine wire mesh or the like, or patterned as desired, and the surface of the finished board will be the counterpart of the pressing surface.

It is desirable, particularly if the sheets contain residual moisture when charged into the press, to-at first merely close the press thereon, making little or no pressure application beyond perhaps the mere weight of the platens. Thesheets can thus be heated to substantially the temperature of the press' platens without material pressure application while moisture or va- After a short time, as about 1-'-4 minutes, the pressure is raised to the extent required for the final bonding and consolidation of the sheet. In this way, even though a sheet may contain some moisture when charged into the press, and even though continuous imperforate pressing surfaces be used, the final hot pressing can be performed with no blisterin woody material used, and the amount of heat treatment which has already been given to the initial wood or woody material, and the product desired. In general hard or deciduous wood is preferred, with which a press platen temperature of at least 200 C. is desirable to secure activation of the self-bonding properties of the fiber containing natural encrustants and obtain good bonding and consolidation of the previously heattreated material in a reasonably short time.

Higher temperatures may be employed such as about 240 C. so long as the tempertaure is not sufficient to cause material loss in strength with the time and pressure employed.

The pressures applied in the press are largely interchangeable with the temperature and time, and may be varied to give a board product of the desired density. Pressures of approximately 500;. per sq. in. to 1'500# per sq. in. or higher may be used, depending not only on the variables mentioned above but also on the amount of refining which may be given to the fiber.

, For any given time in the press, the press temperature and pressure required for pressing the heat-treated fiber of the present invention are both less than those required with fiber which has not been given. a heat treatment adapted to activate the self -bonding properties. If raw fiber is used, such as ground wood or equivalent, and

substantially all the activation of the self-bonding properties of such raw ligno-cellulose accomplished in the press alone, the sheet must be given a large amount of heat treatment and high pressure in the press, or a relatively poor product made, since raw ligno-cellulose material does not have good bonding power whenhot pressed in the absence of moisture unless the press temperatures and pressures are sufliciently increased and the times sufliciently prolonged to secure such effect.

This-comparative situation will be illustrated by the following typical instances, using a pressing time of 3 minutes in each case: With the fiber of wood or woody material in a raw or substantially raw state, as by being exploded with steam at 1000# per sq. in., but with little or no dwell at bonding properties of the ligno-cellulose fiber in the press alone, and secure tolerably good consolidation and strength in 3 minutes pressing time. treatment in accordance with the present invention, however, as by means of steam raised to 500# per sq. in. in 30 seconds followed by increasing the steam pressure to 1000# per sq. in. in

' 1 second, with a dwell of about 6 seconds at such higher pressure, followed by explosion, a product which is much stronger arid more resistant to absorption of water is obtained in 3 minutes pressing time with a pressing temperature of only about 210 C. and pressure of only about 700# per sq. in. The following table shows the above comparison, with additional details, the fiber sheets used in each case having been thoroughlydried before I.

With the material given a preliminary heat I pressing, and each board containing about 2% of petrolatumior sizing:

V Table Heat-treated hardwood Fiber 3 min 240 C 1.1 I Modulus of rupture dry 8000! sq. in W000i sq in 24 hr. water absorption 12% 8%.

6000lsq. in.... 9000! sq m Modulus of rupture wet (after soaking 24 brainwater).

With the present invention, important advantages are obtained; not only is the temperature and pressure and/or the pressing cycle greatly reduced but the product secured is very much superior in substantially all physical properties.

1. The double-activation process of making superior hard fiber board, which comprises the steps of subjecting wood or woody ligno-cellulose material, composed of ligno-cellulose fibers con-' taining the natural fiber encrustat-ion, in small pieces, to the action of steam at a temperature over 200 C. and corresponding pressure maintaining the treatment with steam at such temperature and pressurefor a time that it is too long with relatiom to the steam temperature for production of a raw fiber by means of explosion with steam, but

being much stifi'er, stronger, harder and more water-resistant than if substantially raw ligno cellulose fiber material were used, and this prodnot is welladapted for replacing higher cost products for many uses.

\ Another advantage of the process of the present invention is the fact that pressing equipment may begmuch lighter and less expensive since the hydraulic pressures required are lower, and lower steam pressures can be used for supplying steam for heating the press platens.

. Another advantase is that the heat-treated and shorter fiber may be easily and uniformly felted on the forming machine, leading to a smooth and uniform surface on the board or other product.

A further advantage of the invention is that,

if press pressure is used as high as required with and more water-resistant product in the final hot-pressing operation even when using lower pressures and temperatures and/or times in this hot-pressing operation than where the sheets are made of raw fiber, it is apparent that these good qualities of the final product are secured partly by the preliminary heat treatment and partly by the final hot-pressing operation performed upon the preliminarily heat-treated material. However, it is to beobserved that theconditions for the two heat treatments are not altogether the same, the preliminary heat treatment being performed in the presence of water and followed by very considerable reduction of temperature in forming the sheet but nevertheless contributing very materially to the good properties of the final board, whereas the final hot-pressing operation and the heat treatment in such connection is not long enough to produce a general breakdown of fiber structure or conversion to a state of predominant plasticity; to thereby produce a material initial activation of the self-bonding properties of the ligno-cellulose fibers, reducing the steam treated fibrous material to greater fine ness, removing at least part of the water-soluble constituents, forming a sheet of the fine fiber, drying the sheet, subjecting the sheet to sustained consolidating pressure of at least 500 lbs. per sq. in. in a platen press between pressing surfaces heated to a temperature over 200 C. and maintaining the application of said pressure and temperature to the sheet for a period of time which with pressure and temperature used is sufiicient to activate the self-bonding properties of the ligno-cellulose fibers asecond time and produce the superior hard fiber board, the time of sustained pressure application being approximately three minutes when platen, pressure 01' 700 pounds per square inch-is used with pressing surfaces at a temperature of 210 C.

2. The double-activation process of making a superior hard fiber board, which comprises the steps of subjecting ligno-cellulose material, consisting at least in large part of deciduous wood and. with the component lignocellulos'e fibers containing the natural fiber encrustation, in small pieces, to the action of high pressure steam at temperatures from about ZOO-285 C., and maintaining the steam treatment at such temperatures for periods ranging from about 15m 20 minutes at 200 C. and about 20 to 30 seconds at 255 C. (600 pounds steam pressure) to about ,5 to 6 seconds at 285 C. (1,000 pounds steam performed in the absence or substantially in the absence of water and results immediately in the finished board. Accordingly it is to be understood that our use of the expression activation of self-bonding properties or equivalent expressions in connection with each of these two heat treatments, is not to be construed as meaning that they are alike in all respects.

We claim:

pressure), which said steam treatment is too great for production of raw fiber by means of explosion with steam and is not sufficiently ex tensive to produce'a general breakdown of fiber structure or conversion to a state of predominant plasticity, to. thereby produce a material initial activation of the self-bonding properties of the ligno-cellulose fibers, reducing the steam-treated fibrous material to greater fineness, removing at least part of the water-soluble constituents therefrom, forming the fine fibrous material into a sheet, drying the sheet, and subjecting the sheet to sustained consolidating pressure of at least 500 lbs. per sq. in. in a platen press between pressing surfaces heated to a temperature over 200 C. and maintaining the application of said pressure and temperature for a period of-time which with pressure and temperature used is sufilcient to cause the self-bonding properties of the ligno-cellulose fibers to be activated a second time and to produce a superior and highly strong hard fiber board, the time of sustained pressure application being approximately three minutes when platen pressure of 700 pounds per square inch is used with pressing surfaces at a temperature of 210 C.

. WILLIAMH. MASON ROBERT M. BOEHM. GEORGE G. SIMPSON.