CA1136976A - Poplar bark as extender for glue mix used in manufacture of plywood - Google Patents

Abstract

Abstract of the Disclosure The invention relates to the manufacture of plywood. It is known to use in adhesive for plywood manufacture various extenders and a commonly used one is comminuted corn cobs. Comminuted corn cobs have recently been in short supply, which has led to attempts to find other satisfactory extenders. It has now been found that comminuted bark of aspen trees can satisfactorily be used in place of comminuted corn cobs and that there arises an unexpected advantage when using the comminuted bark. The maximum assembly time, i.e. the time which can elapse between the start of laying up of the veneers to form the plywood and the application of heat and pressure to cure the adhesive, is increased.

Description

1~3~97'~

The present invention relates to the manufacture of plywood.
It is known to make pl~Yood by la~ing up sheets, or veneers, of wood with adhesive between sheets and then curing the adhesive with heat and pressure to form the desired plywood sheet. In a typical industrial application a first veneer is laid flat and there is then laid on the upper surface of the veneer cross banding in the form of pieces of wood whose grain runs in a direction perpendicular to the direction of the grain in the venecr. The cross banding is supplied from between rolls and as it passes through the rolls adhesive is applied to both sides of the cross banding. The pieces of wood forming the cross banding are laid side^by-side so that in the finished product they will form a complete core. Then, a top veneer is laid on the cross banding. If the plywood sheet being formed is three ply then this veneer forms one of the surfaces of the finished plywood sheet.
If the sheet being formed is five ply then more cross banding, bearing adhesive, and a top veneer ar~ applied. A plurality of such laid up sheets are fed to a press where they are subjected to heat and pressure to cure the adhesive and ensure a proper bond between the components of the plywood sheet. Typically, the sheets are subjected to a pressure of 200 p.s.i. and a temperature of 300F. The pressure is applied by hydraulic rams and the pressure is observed. A pressure drop occurs as a consequence of compression of the components of the plywood sheet, migration of glue and the like. As soon as the pressure drops below 200 p.s.i. the hydraulic rams supply another pressure pulse so that the pressure is increased again to above 2Q0 p.s.i. Further pressure pulses occur as necessary to maintain the pressure above 200 p.s.i.
The pressure is maintained for a predetermined period of time which depends upon the thickness and the num~er of the veneers forming the 113697~

plywood sheet. The time for which pressure i5 maintained is known as the press time and it is usually in the range of from 4 1/2 to 6 minutes. The time from the commencement of application of glue to the cross banding to the application of pressure is known as the assembly time. It should be noted that the termination of the assembly time and the commencement of the press time occur at the same point in time.
There is a desirable time range for the assembly time. In order that the adhesive has some time to penetrate into the wood it ; is nccessary that there shall be an interval between the application of the adhesive and the commencement of curing in the hot press. This interval should normally be at least about 6 minutes and is preferably at least 7 1/2 minutes. Hence this interval imposes a minimum value on the assembly time. There is also a maximum which is imposed upon the assembly time, created by drying out of the adhesive. If the adhesive is left upon the wood for too long the adhesive will dry in particular areas and when the plywood sheet is pressed and cured there will be no bond in those areas where the adhesive had dried. Hence the product obtained is unsatisfactory as it will break down in the unbonded areas on exposure to the atmosphere. It is desirable that the maximum assembly time shall be as long as possible, as there can always occur interruptions in the manufacturing process which delay the passing of assembled veneers into the press and thereby prolong the assembly time.
The adhesive used in plywood manufacture is usually a phenol-aldehyde resin, frequently phenol itself and formaldehyde. A preferred adhesive isan aqueous solution of phenol ormaldehyde resol also containing caustic soda which is available from Reichhold Chemicals Ltd. under the trade mark BB-002 plyophen . Another adhesive is another aqueous solution of phonol for;aldehyde resol containing a greater atount of ' :

~36~76 caustic soda which is available from Reichhold Chemicals Ltd. under the trade mark Bs-Qo6 Plyophen. The adhesive is mixed with additional water, wheat flour, soda ash and an extender, for about 20 to 25 minutes and then further phenol formaldehyde and water are added and mixing is continued for a further 5 minutes. The adhesive is then ready for use, i.e. application by means of rolls to the veneers of wood which are to form the plywood.
There have been suggested many oxtenders for the adhesive ; to be used in plywood manufacture for instance wood flour, wheat flour, ground bark, for example alder bark, walnut shell flour, and partially hydrolyzed lignocellulosic materials such as the residues from manufacture of furfural (see U.S. Patent No. 3,965,056 issued June 22, 1976 to MacMillan Bloedel Ltd.). One commonly used extender is comminuted corn cobs, available under the trade marks Norprofil and Cobfil. Comminuted corn cobs have recently been in short supply, which has led to attempts to find a satisfactory replacement for comminuted corn cobs. There has now been found a material which not only is a suitable replacement for comminuted corn cobs as extender in the adhesive used for plywood manufacture but which, unexpectedly, yields the advantage that it retards the drying out of the adhesive so that ; the maximum value for the assembly time is increased.
Accordingly the present invention provides in a process for manufacture of plywood which comprises applying phenol aldehyde adhesive to veneers, laying up the veneers and subjecting them to heat and pressure to cure the adhesive and thereby form a sheet of plywood, the improvement of incorporating into the phenol aldehyde adhesive as an extender comminuted bark of an aspen tree.
The bark which is used as an extender is preferably comminuted _ 3 _ 113~976 so that not more than 20 % of the bark is retained on a 200 mesh U.S.
standard screen. Jt is necessary that the extender is digested into the adhesive composition during the mixing process. During the mixing of the components of the adhesive composition there occurs an exothermic reaction which causes an increase in temperature. To obtain a satisfactory adhesive, the temperature is allowed to rise to about 90F and mixing is continued for the amount of time necessary for this temperature to be reached. This time is usually about 20 to 25 minutes and the extender part~cles should h0 digested in this time. It is found that particles which pass a 200 mesh U.S. standard screen are digested in this time and that up to about 20 % of particles which do not pass this screen can also be digested. Hence, up to 20 % of particles which do not pass the screen, based on the total weight of particles, can be added to the adhesive composition. Alternatively, of course, the larger particles can be subjected to further comminution.
Bark to be comminuted is first preferably passed through a hammer hog where it is broken down so that the larger fines will not exceed one half inch in size. Pieces larger than this are returned to the hammer hog for resizing. The pieces of bark of less than half inch size are then dried to a constant moisture level. It is important that the moisture level is constant, although it is not important what the level is. This is because it is necessary to control the amount of water present in the adhesive composition. As long as the moisture content of the bark is known, it is possible to compensate for the amount of water added with the bark when determining how much water should be added to the composition. It has been found convenient to use bark which has been dried to a 20 % moisture content. The dried bark is then comminuted in a pinwheel`grinder until at least 80 % of 1131~976 it passes a 200 mesh U.S. standard screen. Tree bark is composed essentially of three components, cork, sclerenchyma tissue and parenchyma tissue. It should be noted that in accordance with this invention the bark which is used is whole bark which has simply been dried and comminuted, as described above. Before it is incorporated into the adhesive composition the bark is not treated with any chemical reagent.
The invention will be further illustrated with reference to the following examples.
For the purpose of understanding the examples it is necessary to describe the test which is used to determine whether the components of a plywood sheet are satisfactorily bonded.
The test is described with reference to the accompanying drawings in which:
Figure 1 is a plan view of a plywood test piece prior to the test;
Figure 2 is an elevation of the test piece of Figure l; and Figure 3 is an elevation of part of the test piece after testing.
In this test a piece of, say, three ply wood is tested.
A rectangular piece of wood three inches long by one inch wide is taken, with the grain of the outer sheets of wood running in the direction of the longest dimension. On one surface of the wood a cut 1 is made perpendicular to the longest side and one inch from the end. The depth of the cut is such that it penetrates just into the core of the middle ply of the sheet of plywood. On the other surfsce of the piece of wood a similar cut 2 is made, one inch from the other end of the piece of wood. Again the depth of the cut is such that it just penetrates into the middle ply. The piece of wood is then gripped at both ends :
~ ~ - 5 -113~;~76 by gripping means indicated at 3 and 4 and subjerted to a pull parallel to the longest dimension, i.e. in the directions of the arrows, until the two ends of the piece of wood separate when the middle ply ruptures between the two cuts. If there are good bonds between the core and the inside surfaces of the outer veneers then the rupture must occur actually in the wood of the core, so that core wood will adhere to the two inside surfaces of the two outer veneers. From the test there are obtained two pieces of wood, each including a one inch square and each having adhering wood from the ruptured corc. One such square is shown as 5 in Figure 3 and wood from the core adhering to it is indicated at 6. As a measure of how good a bond is obtained, the squares 5 are examined and a determination is made of how much of the area of the squares 5 is covered by wood fibres from the core. If at least 78 % of the total area is covered by wood fibres from the core the bond is considered to be satisfactory, although values of 80 % or higher are preferred. Values below 78 % are considered to be unsatisfactory.
In the following tests 1/10 Douglas fir veneers were used.
A 1/10 veneer is in fact a veneer which is cut to have a thickness of 104 thoùsandths of an inch. The veneer is dried to a moisture content of approximately 3 % and then is laid up, pressed and cured as described above. The compression to which the veneer is subjected reduces the thickness of the veneer to approximately 100 thousandths of an inch in the finished plywood.
In the tests three adhesives were used, for purposes of comparison.
The first was the conventional adhesive made up as follows:

~136976 Phenolic resin (BB-002 Plyophen4700 lb from Reichhold ~hemicals Ltd) Water 800 lb Comminuted corn cob extender 1150 lb Whea~ flour 200 lb Soda Ash 200 lb These components were mixed for approximately 25 minutes, until the temperature of the mixture reached 90F. There were then added a further 2144 lb of phenolic resin and a further 850 lb of water and the mixture was stirred for a further 5 minutes.
The second adhesive was made up in identical manner except that the 1150 lb of comminuted corn cobs was replaced by 1150 lb of comminuted aspen bark in accordance with this invention.
The third adhesive was made up in identical manner except that the 1150 lb of comminuted corn cobs was replaced by 1150 lb of comminuted green pea hulls. The green pea hulls had been previously dried to a moisture content of 20% and ground to powder form with no more than 20% being left on a 200 mesh screen. With this adhesive there was encountered the difficulty that the green pea hulls tended to separate from the adhesive mixture, rather than be digested.
'~ In the first test plywood sheets were laid up and subjected to pressing as quickly as possible, i.e. the assembly time was as short as possible and below the minimum normally required to obtain a satisfactory `product. Three press times were used for each adhesive mixture. For each adhesive mixture and for each press time six tests were carried out as described above and the average of the six tests was taken.
The results, expressed as percentages of area covered with wood fibres, ~' , :

.

.. . . .
., ~13~7S

are given in Table 1.

Press time Green Pea HuIls Comminuted Corn Cobs Comminuted Poplar Bark 4 1/2 mins. ~ 16 ~~~~~~~- 18 0 31 19 9 cut 10 4 wrongly 3 11 5 " 7 0 _ __ __ ____ _.
Average 13 8 7 __~

5 1/4 mins. 45 65 57 52 29 56 __ Average 44 49 45 6 1/4 mins. 55 21cut wrongly 65 81 89 27" " 55 55 33 67 77" 74 90 61 ,.
It will be noted that in none of the tests was the percentage :
of area covered with wood fibres as high as 78 %. It should be noted, however, ~hat the results obtained with the green pes hulls are inferior to those obtained with the comminuted corn cobs and the comminuted poplar bark, whereas the results Gbtained with these latter two are approximately comparable.
In a further test 1/10 Douglas fir veneers were used and three ply sheets were laid up using the same three adhesives. The laying up ~as conducted with five diferent assembly times and after assembly each sheet was subjected to pressing with a press time of 4 1/2 minutes. The sheets were then subjected to tests described above 1~ 3~5'76 and the average of six tests was taken. Results are given in Table

2.

., _ Assembly Green Pea ~ulls Comminuted Corn Cobs Aspen Bark time 5 mins. 32 84 77 94 36 95 ~ _~ ~ __ _ ___ ._ 15 mins. 88 74 78 77 92 100 96 85 72 95 98 lay up wrong 94 83 93 100 97 " _ 20 mins. 64 93 90 91 lay up wrong 86 95 91 85 ll ll 66 84 93 100 .. ..

25 mins. 92 81 100 98 98 100 79 80 100 97 lay up 97 wrong 82 _ _ _ 97 _~
30 mins. 57 65 88 88 91 86 _ _ 81 47 67 73 _ _ _ __ _ __ _ _ _ _ _ _ _ __ __ 76 _ _ _ ~ _ _ _ _ g _ 113~976 From these results it can be seen that the adhesive with green pea hull extender gives satîsfactory results when the assembly time is from 15 to 2S minutes inclusive. The adhesive with comminuted corn cobs also gives satisfactory results when the assembly time is from 15 to 25 minutes inclusive and within this range results are superior to those obtained with green pea hulls. The adhesive with aspen bark in accordance with the invention gives satisfactory results over an assembly time of 15 to 30 minutes inclusive. Over the range where all three adhesives give satisfactory results the adhesive containing aspen bark is superior to the adhesive containing green pea hulls and is comparable with or superior to the sdhesive containing comminuted corn cobs.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the manufacture of plywood which comprises applying phenol-aldehyde adhesive to veneers, laying up the veneers and subjecting them to heat and pressure to cure the adhesive and thereby form a sheet of plywood, the improvement of incorporating into the phenol-aldehyde adhesive as an extender comminuted bark of an aspen tree.

2. A process as claimed in claim 1 wherein the bark used as extender is dried to a moisture content of about 20 % and is then comminuted until at least 80 % of the comminuted bark will pass a 200 mesh U.S.
Standard screen.

3. A process as claimed in claim 1 wherein the phenol-aldehyde adhesive is a phenol formaldehyde adhesive.