US1883617A - Method of making plywood - Google Patents

Description

No Drawing: I

product.- r

- .of description we will 4 of the structure Patented Oct. 18, 1932 ED" \STATES,

PATENT OFFICE THEODORE Winn mus min, or NEW WESTMINSTER, BRITISH COLUMBIA, cANAnA,

ND IRVING r.j n ucxs AND CHARLES N. CONE, or SEATTLE, WASHINGTON, ASSIGN- ons, BY MESNE ASSIGNMENTS, TO LAMINATING PATENTS CORPORATION, or SEAIILE, WdSHINGTON, A CORPORATION OF DELAWARE I METHOD or MAKING rLYwoon Our invention relates aim; art ofgluing and, more particularly, tolpro'cedur'es adapted for the formation of plywood and the like and involving the application of the adhesive in discrete-particle for1n,"such for example, as a-dry powder or suspension of discrete particles in a fluid; and has among its objects the controlling of certain of the factors which efiec't and areirelated to the gluing operation in suchamanner, as .to' yield particularly desirable results and, .to provide an improved A particular featureof the invention is the control of the size of the particles of the adhesive-base material applied. I

For purposes of clearness and definiteness set forth our inventi onas applied to the highly exacting art of manufacturingplywood or veneering, a field characterized by presenting not only a wide range of conditions and important factors but by these being most diflicult and complex in the t me I By plywoodfwe 'means to include all structures embodying. at least oneply or lamination of wood, whose thickness is not greater than can be caused to adhere to the balance under the conditions of our process. That is, of sufiicient thinness 'so that the heat of the hotpress can penetrate to the glue line sufiiciently to cause setting of the adhesive, and weinclude in the term plywood, alsothe term veneering.

In this connection it is to be understood that the term fwood is intended to include .not'only the natural product, but also artificial woods having a porous structure similar feet as of plaster on lath,

. Application n; 'ipm1 m. Seria1 No. 351,822.

the penetrating elements are attached. The surfaces of Wood prepared for joining as seen under'a microscope are very rough and perforated with numerous openings or pores, some of which extend for a relatively great distance into the wood.

Whether the bond between glue and wood is formed bytrue adhesion, i. e., specific adhesion, between the lue substance and the wood substance, or W ether the bond is mechanical, that is, due to the glue penetrating into the numerous irregularities. of the surface of the wood and on being hardened in place, producing a bond due to a keying efit is obvious that other things being equal, the greater the contact the glue has with the surface of the wood, the better will be the bond formed.

However, if the glue penetrates too deeply into the pores and other openings in the wood, the supply of glue between the surfaces to be joined may be so depleted that a strong transversely continuous film joining the two surfaces cannot be formed, and an inferior bond will result. This condition is known as a starved joint. When two pieces of wood are joined with an adhesive, it will be obvious that there must be a continuous film of adhesive between the two pieces, in a transverse direction. It may be that penetration to a certain extent is necessary beyond the boundaries of this continuous film, but the first requisite is the transversely continuous film; this is the foundation of adhesion.

The extent to which the glue penetrates into the wood depends partly. upon the nature of the wood, which varies as to size and number of openings or pores. It depends also upon the eiiective pressure exerted upon the glue and also upon the fluidity or mo flucnce flow, including also plasticity of a discrete particle or collection ofparticles. We recognize that in case of a suspension, if the orifice is smaller than the swollen particle size, then the rate of flow may depend on the plasticity of the swollen particles themselves.

In considering the penetration of an ad-.v hesive into wood under pressure the'structure of wood must be borne in mind, First in importance are the pores of the wood, which, in general in veneer, liein a direction roughly parallel to the faces of the panel. However, many of the-pores are intersected by the faces so that they present openings on the faces into which glue can readily penetrate if it is sufficiently fluid. There are also other openings (rays) in some woods in a di-- rection perpendicular to the faces. The wood cells themselves are much smaller but do permit penetration by the more fluid glues. Besides these openings natural tothe wood, .there are thechecks or cracks that are formed in the cutting of theveneer as wellv as other irregularities of surface- Thus with Wood there are'abundant opportunities for a fluid to escape from the surface.

For purposes of discussion,we classify the commercial methods of plywood manufacture with dispersed glues as follows:

A Process -'-hot press, wet veneer B Process-hot press, dry veneer C Processcold press, dry veneer persible adhesive base in suspension in water.

The invention contemplates the regulation of the coarseness of the powder'inaccordan'ce with the character of theadhesive and the character of the wood and other factors as hereinafter set forth.

The powdered adhesive may be spread on the plies of wood; If the wood-is undried containing a large part of its original sap moisture, or is wet from lying in water, no Water need be added. If the wood be dry, water may be sprayed on the-adhesive layer or the tors influencing penetration.

Penetration may take place either (a) after the press is closed and pressure is applied,

to recognize and properly control those facor. (6) before pressure is applied. The foldered animal glue at rate of 40 lbs. per thousand. Four-panels were made as follows:

Panel 1 AGlue particles through mesh, pressure 25 lbs. per sq. in.

Panel 1 B-Glue particles on 40 mesh, pressure 25 lbs. per sq. in.

Panel 2 A Glue particles through '60 mesh, pressure 100 lbs. per sq. in;

Panel .2 BGlue'particles on 40 mesh, pressure 100 lbs. per sq. in.

All four panels were pressed with hot plates heated at 100 lbs. steam pressure. The results showed the following differences:

1 A bond-"A total failure, joint completely starved and no glue appeared on the surface of the faces of the panel. No shear strength. i

, 1 B .bondPerceptible but extremely weak, joint badly starvedand no glueon faces. 37 lbs..shear strength.

2 A,bond-Fair bond,"joint much less starved, considerable glue substance pressed through the wood and appearing on the surface of the faces. 128 lbs. shear strength.

2 B bond Fair, some glue appeared on the surface of the faces but markedly less than in 2A. 100 lbs. shear strength. i

This experiment was then'repeated with green gum veneer at 80% moisture and green birch at with plates heated with steam at 50 lbs. The results were as follows:

1aFailure, no bond with either wood.

,lb Failure, trace of bond with both woods spontaneously failing as the panels cooled.

2alVeak bond, much glue substance on the face surfaces, shear strengthsgum 54 lbs, birch 65 lbs.

2b-Fair bond, less glue substance on the face surfaces, shear strengths-gum '98 lbs., birch 130 lbs.

Penetration may also be regulated through a control of the plasticization factor of the adhesive. i

The following indicates the effect of the use of a dispersible adhesive base having a low plasticization factor:

A variety .of animal glue was selected and tested for viscosity with a given amount of water. It was found to have the property of absorbing a relatively large amount of water. and produce'a solution of a given viscosity. Another variety of animal glue (N. B, the market recognizes many varieties of animal glue) was also similarly tested and found to be able to absorb a much lesser 1,ses,e 17

amount of water to roduce a solution of the same viscosity. e first glue was found to have .nearl double the water requirement of the secon The low water sample was designated #1 and the high water #2.

Two anels were then glued up with partly dries cottonwood veneer, one panel being spread with a definite amount of #1 glue vreduced to a coarse powder and the other with #2. The panels were then pressed in the usual way, all the conditions being maintained identical in each case except for the difference in'the water requirement of the glue.substance. v

#1. panel-Weak bond Outer surface of panel. badly stained with glue, 61 lbs. shear strength. V

. #2 panel-Much better bond, less stained, 110 lbs. shear strength.

This ex riment was repeated with other woods an conditions and in each case the glue substance, with the higher water re quirement gave markedly better results than a low-water requirement glue, both as to actualstrength of bond and as to visual evidence of glue penetrating complletely'through the plies and appearing on t e outer surface of the. anel.

T e main control, however, is exerted through the size of the particles which influences the penetration and affects the rate at which water is imbibed.

The size of particle has a marked effect when time is taken into consideration. Small particles imbibe Water faster than larger so at any given instant before'maximum imbibition has been reached an adhesive, un-

der the conditions given, will be more fluid if composed of smaller particles.

In all the experiments with dispersible ma terials listed previously and subsequently, two sets of experiments were made under each of the given conditions, one being with small particle sizeand the other with larger size.

- VVi'thout exception the larger size particles gave the best results, whether with low or high hydraulic pressure, low or high water requirement glue, low or high wood moisture content, low or high heat, or with all other factors favorable.

\Ve have therefore discovered that the selection of large particles is an important means of favorably controlling results in our process when using highly dispersible adhesives. l/Vith more slowly dispersible or nondispersible materials particle size will of course be of less vital importance.

A series of experiments were tried with dispersible adhesives and green woods of three species representing a wide range of unfavorable types including cottonwood at 125% moisture, gum at 82% and birch at 69%. Pressure, heat, particle size of glue substance and kind of glue substance as to moisture requirement, were controlled in favorable directions.

Shear strength for cottonwood averaged 107 lbs.

Shear strength for gumwood averaged 122 lbs.

Shear strength for birch averaged 140 lbs.

As these results with control indicate very fair bonds in all cases, while without control no bonds are obtained, it is evident that by use of our discoveries of means of control completely dispersible adhesives maybe employed successfully in our process and with completely green woods which represents the most unfavorable condition as far as the moisture content of the wood is concerned.

It will be noted that it is the property of the adhesive of being dispersible in water that makes it necessary to use care and to closely regulate the other influencin factors, if good results are to be obtained. uch care,

while it is not impossible, is often beyond the limits of practicability in commercial operations.

It must be remembered that the condition met in gluing wet veneer with dry gluea condition the difficulty of which we purpose to solve and a condition usually obtaining in wet veneer or pliesis that there is a great excess of water in the veneer over what is required by the glue. Under these conditions especially with green veneer the greatest advantages are realized with the non-dispersible adhesvies, as with green veneer it is difficult to control the factors influencing the degree of hydration.

Thereis found, moreover, a class of substances, or a state of matter, that is distin guished by (1) becoming plastic in contact with water, (2) drying to a strong continuous film, (3) not being dispersible by hot or cold water to any great extent. These materials are admirably adapted because of these properties to be used as dry adhesives in the gluing of green veneer.

Where dealing with a wood presenting coarse pores, for example cottonwood, it is in general desirable to apply the adhesive base in particles of a size for instance of 2035 mesh for a highly dispersible adhesive base, and. for instance, -80 for a non-dispersible material: and where dealing with a wood of fine pore structure, for example birch, in general it is desirable to apply a dispersible adhesive base in particles of a size for instance of 3040 and a non-dispersible base in particles of a size for instance of 120. Again, with a wood of moisture-content exceeding 25 percent, it is desirable to employ adhesive basc particles of 2580 percent, the coarser particles with a more dispersible adhesive; while with woods of loss moisture content it is desirable to employ particles for instance of 30120 mesh size, the dispersible adhesive being the coarser. In general also, the viscosity desirable may range for example from about 4704150 poises (as measured by an clfiuxtype viscosimeter).

As examples of non-dispersible materials,

' reference is made to the following: soya bean flour, wheat flour (feed grade), casein, pea flour, corn meal, oatmeal, ground rice, potato flour, corn starch (edible) 1., linseed meal, wheat gluten, dry powdered bentonite, wheat gluten, wheat flour (feed grade) soya bean flour, casein, laundry starch 2., laundry starch 3.,crn starch (edible) 1., corn starch (edible) 2., potato flour, rice flour, wheat gluten, wheat flour (feed grade), soya' bean flour, casein, laundry starch, corn starch (edible), potato flour, rice flour, soya bean flour, wheat gluten, casein, soya bean flour.

Among the dispersible materials which may be utilized are animal glue or gelatin, blood albumin, soluble starch/es, and sodium silicate.

Penetration before pressure is applied can also be controlled by decreasing the moisture content of the veneer. For example, with completely dispersible adhesives like animal glue we find no difiiculty in producing a strong bond on cottonwood with any moisture content in the wood below 25% without any other method of control, but with increasing moisture content other means of control have to be used and with the full moisture content of 125% animal glue can be used successfully only by rigid control of all the other available factors. Cottonwood is one of the wettest of woods in its green state and it is therefore to be understood that with other woods of lower moisture content the problem of control of dispersible adhesives requires less extreme methods.

With dispersible materials there is no limit to the amount of water which is taken up by the adhesive, that is, if more water is present it mixes homogeneously with the adhesive layer and so the fluidity keeps on increasing as long as more water is presented. In this case, therefore, it is more difficult to insure against excessive penetration, especially with wet wood. As much insurance as possible must be obtained b increasing the particle size so that imbibition will be slower. Further insurance must then be had by increasing the rate of drying to a point so that the water of the wood will be driven quickly from it, thus decreasing the supply of water avail able to migrate to the adhesive layer; if both of the above regulations are not sufficient,

further control must be exercised by previously decreasing the amount of moisture in the wood itself so that there will not be enough present to dilute. the adhesive layer to the point u'hereexcessive penetration results.

It will he noted that our process is also applicable to the. gluing of dry veneer by the hot press method 8" process.

The moisture of the veneer, of course, is

:7 subject to control if our process is desired to be carried out by providing such control; in other words, the veneer can be dried to any degree desired before the adhesive is applied to it in the dry state. For example, it can be dried down to the point where it will supply just enough moisture to the adhesive, due regard being had to the amount of moisture which will escape when the heat is applied.

Also, if desired, the veneer may be dried to a very low content of moisture, so low in fact that it cannot supply any moisture tothe adhesive substance. Under these conditions we then provide the proper amount of moisture to secure the desired fluidity in the adhesive layer by adding moisture either to thesurface of the wood to be glued or to the dry adhesive which has previously been spread on the surface of the wood, that is, the dry adhesive is first spread on the dry veneer by a suitable apparatus which gives a uniform spread and then the required amount of water is added to this dry adhesive by any means which will give a uniform amount of water.

T he amount of water, of course, which is to be added must be under control and may be pro-determined by trial, that is, it is found what amount of water best suits the wood and the adhesive to secure the proper conditions of fluidity and penetration. The plies are then assembled and pressure and heat is applied to dry the adhesive layer and set it to a permanent bond.

While in this joint application the disclosure, dealing as it does with a complex inven-.

tion, is identical in part to that of a co-pend- 10o ing application, Serial No. 335,998, filed by Theodore lVilliams Dike, as sole inventor, nevertheless in this application the claims only are made for such part of said disclosure as constitutes our joint invention.

WVe claim:

l. The method of making plywood comprising applying to the plies an adhesive base substance in the powder form dry at the time of spreading and controlling the degree of 110 penetration of the adhesive material, an element of such control being by providing the particles of said powder of a predetermined size with respect to the magnitude of the pores of the plies.

2. The method of making plywood comprising applying to the plies an adhesive base substance in the powder form dry at the time of spreading and controlling the degree of penetration of the adhesive base substance, an 12 element of such control being by regulating the size of the particles and the moisturecontent of the veneer, the size of the particles increasing as the moisture content increases.

3. The method of making plywood com- 15 prising applying to the plies an adhesive base substance in the powder form dry at the time of spreading and controlling the degree of penetration of the adhesive base substance, factors of such control being by regulating 1 the pressure, the water requirements of the adhesive base substance, the temperature at the time of pressing, the size of the particles, the moisture content of the veneer, and the migration of water from the veneer to the of penetration of the adhesive base substance" into the plies, factors of such control being by regulating the fluidity of the adhesive substance through the amount of water which is imbibed by the adhesive base, a lesser amount of water causing a.le sser penetration, said amount of water being governed by the size of the particles, the larger particle size imbibing a lesser amount of water.

5. In the method of making plywood, the steps of applying to the plies an adhesive base substance in dry discrete particle form at the time of spreading; and controlling the degree of penetration of the adhesive base substance into the plies, factors of such control being by regulating the fluidity of the adhesive substance through the amount of water which is imbibed by the adhesive base, a lesser amount of water causing a lesser penetration, said amount of water being governed by (a) the size of the particles, the larger particle size imbibing a lesser amount of water, and (b) the moisture content of the veneer, the higher moisture content causingthe greater amount of water to be imbibed.

6. The method of gluing porous materials in the nature of wood, which comprises applying to a surface to be incorporated an adhesive-base substance in discrete-particle form, assembling and subjecting to bonding conditions, and controlling the degree or penetration of the adhesive material into the wood or the like by regulating the coarseness of the powder.

7. The method of gluing porous materials in the nature of wood, which comprises applying to a surface to be incorporated an ad hesive-base substance in-powdered form, assembling and subjecting to bonding conditions, and controlling the degree of penetration of the adhesive material into the wood or the like by regulating the coarseness of the powder.

8. The method of gluing porous materials in the nature of wood, which comprises applying in powdered form to a surface to be incorporated an adhesive-base substance of a character which is dispersible in water, assembling and subjecting to bonding conditions, the degree of penetration of the ad hesive material into the wood or the like being controlled, an element in such control being the inclusion of particles of the adhesive-base substance of a relatively large size.

9. The method of gluing porous materials in the nature of wood, which comprises applying in powdered form to a surface to be incorporated an adhesive-base substance of a character which is non-dispensible in water, assembling and subjecting to bonding conditions, the degree of penetration of the adhesive material into the wood or the like being controlled, an element in such control being limiting the maximum size of the particles of the base substance applied.

10. The method of gluing porous materials in the nature of wood, which comprises applying to a surface to be incorporated an adhesive-base substance in powdered form, assembling and subjecting to bonding conditions, the degree of penetration of the adhesive .material into the Wood or the like being controlled, elements in such control being the regulation of the plasticization factor of the base substance utilized, the use of a base substance having a lower plasticization factor being employed to obtain less penetration, and the regulation of the coarseness of the powder, a greater coarseness being utilized to obtain less penetration.

In witness whereof, we hereunto subscribe our names this 20th day of March, 1929.

THEODORE WILLIAMS DIKE.

IRVING F. LAUCKS.

CHARLES N. CONE.