US2150841A

US2150841A - Process of normalizing hot pressed plywood

Info

Publication number: US2150841A
Application number: US199585A
Authority: US
Inventors: James V Nevin
Original assignee: Individual
Current assignee: Individual
Priority date: 1938-04-02
Filing date: 1938-04-02
Publication date: 1939-03-14
Anticipated expiration: 1956-03-14
Also published as: USRE22071E

Description

Patented Mar. 14,1939

UNITED STATES 2.150.841 v rhoonss or NORMALIZING no'r mssnn I ruwoon lamest. Nevin, Aberdeen, Wash.

No mm. Application April z, 1988, serial No. 199,585

PATENT omen 4 assum APR 14.1942;

4 Claims. (01. 34-124) method of treating a press. This heat and pressure treatment converts' the thermo-setting glue into an insoluble and infusible bonding agent. permanently welding the veneer sheets to the cores or cross banding and forming'a plywood panel. This hot pressing operation expands the wooden veneer sheets and cores and since the operation is usually carried out at temperatures above 212 F., themoisture content of the wood is also greatly reduced.

The expansion of the wood veneers and-cores is greatest across the grain of the wood so that when the plywood'panelsare cooled afterthe glue sets, stresses will occur along the glueline of the panel. These stresses will build up or accumulate until a check or crack is formed along the glue line. Such checks or cracks, of course, greatly diminish the shear strength of the panel. If the panels are then subjected to weather conditions or to other than substantially bone dry atmospheres, they will warp and blister, due to an uneven moisture absorption into the wood.

I have found that hot pressed plywood panels can be normalized to prevent the formation of .checks and blisters and to forestall possible future warping. In accordance with this invention the hot pressed plywood panels are heat-treated in a hot, humid atmosphere immediately after the hot pressing operation and before the same have cooled. This treatment plasticizes and softens the wood along the glue line to relieve stresses. At the same time, the wood absorbs moisture from the humid atmosphere so that it will not warp under any weather conditions.

Humidification or treatment of the hot pressed panels in a moist atmosphere is not sufllcient to normalize the panels. I have found that the humid atmosphere must'be heated. A temperature range of from 150 to 210 F. is desirable.

The humidity of this atmosphere should be maintained as high as possible without actually condensing water on the panels. A relative humidity around 98% is desirable.

It is, then, an object of thisinventio'n' to norm'ali'ze hot pressed plywood panels. A further object of this invention is to treat prevent the same from check- 4 hot pressed plywood panels forrelieving stresses developed during the hot pressing operation and for imparting a moisture content to the wooden veneers and cores that will forestall warping of the panels. I

Another object of thisinventlon is to heat treat hot pressed plywood panels in a humid atmosphere immediately after the hot pressing operation. v I

I Other and further objects of the invention will become apparent to those skilled in the artfrom the following detaileddescrlption of the preferred processand from the appended claims.

Natural wood veneer sheets to be used in the I manufacture'of hot pressed plywood panels are prepared in the usual way by peeling wooden logs on a lathe and then drying the resulting peeled sheets. The sheets are dried to a moisture content between 2 and 4%. 7 The sheets selected'for plywood cores or cross banding are coated on both broad facesther'eof with an aqueous solution of a cresylic acid-formaldehyde partial condensation product. The co'n-.

densation product is prepared by reacting metacresylic acid (meta-cresol) and formaldehyde exothermically inthe presence of caustic soda until somewhat constant temperatures around. 208-F. are reached. The reactionis then'arrest- .ed by the incorporation of a weak aqueous alkaline solution of sodium hydroxide. To insure the preparation of ,a condensation product that is soluble in water in all proportions, the ratio of sodium hydroxide to meta-cresol should notbe less than 16 to 100 parts by weight. Equivalent amounts of other alkali metal hydroxides can be used.

The aqueous resin solution is preferably applied to thecore sheet by a seriesof coating" operations. Each coating is dried toa desired moisture content and the final coating may be utilized to impart a desired moisture content at the glue line of the plywood panel duringthe hot press ng I operation.

The core sheets are sandwiched between veneer sheets to form a" stack of the desired numberof plies-u The stack is then placed between. the

platens of'a hot'press. Thepress is heated to temperatures around-330 to 340 for converting 'the meta-cresol-form'aldehyde resin into a 'hard, infusible and insoluble binder permanently uniting the veneers to the core. The stacks or piles in the press are subjectedtopressure varying withthe nature of the wood. Pressures of "120 lbs. per square'inch are used for Redwood sheets are heated to temperatures somewhat lower than the temperatures of the hot platens. This causes a drying of the wood as well as an expansion of the same. The, wood will expand crosswise of its grain to a much greater extent than the expansion along the grain. Since adjoining sheets are arranged with the grains thereof running at right angles to each other, stresses will occur along the glue lines between the sheets.

After the resin has set, due to the hot pressing operation, the panels are removed from the presses and, in accordance with this invention, are

placed in kilns where they are heat-treated in humid atmospheres before they cool.

The panels are preferably stacked between racks and weighted to keep them in a flat condition while in the kilns.

' The kiln is heated to temperatures between 150 to 210 F. and the atmosphere therein is maintained at a high relative humidity.- A relative humidity of from 90 to 98% is desirable.

The panelsremain in the kiln until the moisture content thereof is raised preferably to above 6%. A range of 6 to, 12% moisture content is desirable for the normalized product. The amount of time that the panels must remain in the kiln depends upon the number of plies in the panel and the thickness of the panel. The following table will illustrate a suitable schedule for the normalizing operation on three, five and seven-ply panels:

- Weight of Number Weight Percent 32:5 panel cm of hours of panel moisture in panel panel tering humidileaving content kiln lied kiln of panel Inch: 7

5 a 157- 50 19 I. (X) 8. 03

After treatment at elevated temperatures in the humid atmosphere until the desired moisture content has been reached in the panels, the same are cooled gradually before removal from the kiln. The panel should be cooled to temperatures ,izing not above 150 1". beforethey are removed from the kiln.

From the above description it'should be underof hot plywood panels at elevat d temperatures in humid atmospheres to'impart a. moisturecontent between 6 to 12% to the panels. The humid atmosphere treatment must be carried out at elevated temperatures to plasticize that this inventionrelates to the norma and soften the wood so that stresses set up during the hot pressing operation are relieved.

I am aware that numerous details of the process may be varied through a wide range without departing from the principles of this invention, a'ind I, therefore, do not purpose limiting the patent grantedhereon otherwise the prior art.

than necessitated by I claim as'my invention:

1. The method of normalizing hot pressed ply-. wood panels composed of a plurality/0f bondedtogether wood plies with adjoining plies having the grains thereof running at anangle to each other which comprises placing the panels while still in a heated condition from the hot pressing plywood operation into a humid, heated atmosphere until the panels absorb an appreciable amount of moisture and then slowly cooling the panels before removing the same from said heated atmosphere.

2. The method'of normalizing hot pressed plywood panels composed of a plurality of bondedtogether wood plies with adjoining plies havin the grains thereof running at an angle to each other which comprises treating the panels while still in a heated and expanded condition from the hot pressing operation in an atmosphere heated between 150 to210 F. and havinga high rela'-- tive humidity until said panels have a moisture content of from 6 to- 12%, and thereafter gradually cooling the panels before removal from the heatedatmosphere. I

3. The method of normalizing hot pressed plywood panels composed ofa plurality of bonded-' together wood plies with adjoining plies having the grains thereof running at an angle to each other to relieve stresses set nup in the panels during the hot pressing operation and to humidify the panels for preventing subsequent warping which comprises introducing the hot pressed panels while the same are still in a heated condition into'a humid atmosphere maintained at temperatures between 150 and 210 F. until the panels have a moisture content of at least 6%.

4. The process of normalizing hot pressed plypanels composed of a'plurality of bondedtogether wood plies with adjoining plies having the grains thereof running at' an angle to each other whichcomprises heat treating the panels while still in a heated and expanded condition from the hot 'pressing'operation at, temperatures between 150v to 210 F. in a humid atmosphere having a relative humidity of from- 90 to 98% until 22d panels have a moisture content not below s JAMES V. NEVIN.