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Introduction

Wood furniture and wood construction have been used by mankind probably since 10000 years.

In the past, wood was assembled by mechanical assembly such as dovetails, tenon and mortise, tongue and groove, and then the assembly was secured by nails or dowel pins.

Later the Egyptian and others started to use some glues based on animal glues such as gelatin, bone glue, blood glue for furniture, and tar or asphalt as sealants for the boats.

Modern synthetic wood glues started only in the 20 th century with the discovery of phenolic and urea-formaldehyde glues in 1920, of polychloroprene adhesives in the forties, the development of vinyl emulsion glue in the fifties, the polyurethane starting in the fifties, the hot melts in the sixties…

In this chapter, we will study the modern synthetic adhesives, their technical characteristics, the bonding techniques, the machines which are used for bonding and the different application of wood: bonding panels, millwork, laminating, construction, furniture and decoration.

But first, we must spend some time to understand the properties of wood and see how they affect bonding.

Physical properties that affect bonding

Wood is porous and heterogeneous:

It is made of long fibers, its cells have different shapes and composition: the heartwood or core is different from the outer part ( sapwood ) refer to figure 1 and 3b. Specific gravity and porosity are different from one wood species to the other and also between heartwood and sapwood.

Figure 1: The various layers of a tree Heartwood is more dense than Sapwood.

Therefore the penetration of the adhesive into the cells and the absorption of the water contained in the adhesive will depend on the structure of the wood and the wood cells, and of the wood species.

Wood is anisotropic:

All fibers are aligned in one direction, ( refer to figure 3 ) and we call "grain" the direction of the wood fibers in the tree. The grain will be quite different according to the way the wood is cut, as we show on figures 2.

Figure 2a: The 3 different ways of cutting a wood log.

	Type of cut	Characteristics
	Radial cut	straight grain uniform in width shrinks less than tangential cut
	Traversal cut	show-end grain wood cells are cut and may absorb water
	Tangential cut	plain-sown boards high shrinkage during drying

Appearance and characteristics of the wood boards according to the 3 ways of cutting the logs

Traverse cut log will be porous to liquids such as glue or water from the glue, the glue will penetrate more into the wood fibers and fluid glues may be completely absorbed into the fibers.

Figure 3a: Structure of wood in a tree

Figure 3b: Tracheids and rays in a wood

Early wood = wood found early in the season
Late wood = wood found lately

All the wood characteristics vary with the direction of the cut: specific gravity, mechanical resistance, dimensional stability / shrinking , absorption of water and glue…

Wood is hygroscopic:

Wood absorbs or loose moisture according to the temperature and the humidity of ambient air. The cells enlarge or shrink during these variations so that the shape and dimensions of the wood will vary. This is a very important point to consider when bonding wood as we will see.

The lumber cut from a freshly cut tree contains from 50 to 100 % water. The wood must be carefully dried before it can be used to make furniture or millwork because, during drying it will change of shape and dimensions.

When the moisture content lowers to 20- 30 %, the wood starts to shrink, and the shrinkage depends on several factors:

direction: lengthwise shrinking is very small, radial shrinkage is 4 to 8 % according to the wood species and tangential shrinking 5 to 12 %, ( total shrinkage between 30 % moisture and fully dried state ),
final moisture, that depend on the humidity of the ambient air,
wood species, for instance oak displays high shrinkage, 10 % tengentially, resinous woods ( Spruce, Douglas fir ) 9 % tangentially, sipo or iroko shrink less 6 to 7 % tangentially.
When the tangential shrinkage is much higher than the radial shrinkage, there are risks of cracks.

Figure 4 shows what happens when the wood dries: the wood deformations depend on where it has been cut in the log.
Obviously tangential cut may be a problem for bonding , mostly if large variations of ambient humidity are expected.

Figure 4: Shrinking and deformation of wood after drying according to the ways of cutting/sawing the log

Calculation shows that, when the moisture content drops from 30 % to 10 %, a plank 2 meters long and 50 cm wide may shrink of 3 to 6 mm. Therefore we can understand that if this plank was bonded to another wood piece already dried before bonding, the bond will fail because shrinkage develops high forces that the glue line could not withstand of course.

The first precaution, when bonding wood to wood is to make sure that the different pieces of wood are dried to a certain amount of residual moisture so that when the bonded parts will be placed in their normal life conditions, there will not be a differential shrinkage.

Equilibrium moisture content: ( EMC )

Wood exchanges humidity with the ambiant air and its moisture content will stabilize at an equilibrium value, as it is indicated on figure 5.

Figure 5: Wood equilibrium humidity curves as a function of outside air humidity and temperature

Example: If the dry temperature is 40°C and the relative humidity of the air 60%, the wood humidity content will reach equilibrium at 10%.

The most important case is that of furniture inside a house: in summer if the temperature is 20 °C and relative humidity 65 %, the EMC will be of 12 %. In winter, due to the heating, temperature will be 21 °C and RH only 30 %, so that EMC will be only 6 %, and this will cause shrinkage between summer and winter conditions.

On the opposite, if a parquet was very dry at the time of installation, for instance at 4 % humidity, and later there is a water leak in the room or high ambient humidity, the wood will swell, and if the parquet is blocked against the wall it may lift from the floor and the adhesive bond between the parquet and the concrete floor will be broken. ( because the bond line cannot resist to the very high stresses that develop in the wood when it swells or dries: remember the old Egyptians who used to split stones by inserting a wood wedge into the stone and then soak it with water, making it swell strongly )

In South China I have experienced myself these problems with wood furniture which was manufactured in hot and humid weather, 35 °C and 85 % RH for instance. After we placed the furniture in our dry, air conditioned apartment, ( 20 °C and 40 % RH ) the wood table top and the large side panels shrunk and large joints or even cracks 2 mm wide appeared ! The same thing happens when these furniture are shipped to some European countries where the temperatures and Relative humidity are much lower than in South East Asia !.

The remedy for the furniture manufacturer is to dry their wood to the low humidity of their customer's countries and also work in air conditioned workshops !

Therefore the most important rule, when bonding wood, is to dry it ,just before bonding , to a humidity content which should be very close to the humidity content it will have during service life. If we take the example above of the furniture inside a house, with EMC varying from 12 down to 6 %, the wood should be dried to 9 % just before bonding.

Flatness of the wood surface:

Most of the wood adhesives may only bond and dry or cure as thin films, a few hundredths of a mm thick.

Also wood boards are stiff and if the boards are not flat ( warped or uneven ) after the pressure of the press is released the boards "spring back" to their original shape.

Therefore the wood surfaces should be very flat in order to get a good bond, planning or sanding machines must be used to get such a flat and smooth surface, and the 2 parts should mate perfectly before pressing it together.

However, in some cases, laminated veneers may be bended in a press and then bonded with high modulus, stiff and high resistance adhesives. For instance we will discuss later in the Applications section the bended laminates.

Wetting of the wood surface:

We will see later in this chapter that, in order to get a good bond, the glue or adhesive should wet the wood.

The wetting of wood depends on the type of wood as we will discuss hereunder and the time elapsed between cutting / planning and coating of the adhesive.

A simple test may be done by placing a drop of adhesive on the wood surface and measuring the contact angle (figure 6 ): the lower the angle, the easier the wood will be wetted by the adhesive.

Figure 6: Measurement of wetting by measuring the contact angle

a) poor wetting
b) good wetting

Wood species which are difficult to bond, surface preparation of wood before bonding

Some wood species are difficult to bond and we indicate hereunder why and how to prepare it in order to get good adhesion.

Resinous woods and woods containing fatty products:
For example: teck, resinous woods, pine tree, snakewood…
These woods should be prepared by drying first at high temperatures ( 70 °C ), in order to have the resins and fatty products exude from the wood, then cleaning the surface with solvent, then abrading the surface. The bond should be made immediately after these treatments, to prevent resins and fatty products to exude again, and under a high pressure. Preferred adhesives in these cases are phenolic glues.
Porous woods and traverse cut:
Examples: beech, poplar, pine tree, with traverse cut.
The glue may be completely absorbed into the wood cells, there will not be enough glue to fill the joints and the bond strength will be low.
In order to improve the bond it will be necessary to increase the viscosity and dry solids of the glue, to apply 2 coats of glue or one coat on each substrate and thicker coats, and to apply a moderate pressure in order to prevent squeezing the glue out of the joint.
In any case, avoid bonding end cuts because the wood absorb too much the glues.
High density woods:
Examples: iroko, doussie, ebony, rosewood, wenge, azobe…
Here the water of the glue will not be absorbed by the wood, so that the drying of the glue will be slow and the adhesion will be low.
It is recommended to abrade both surfaces to be bonded in order to allow some penetration of the glue into the cells, apply glue on both surfaces so that the liquid glue could penetrate, wait enough time before assembly to let the glue penetrate before pressing.
In some cases it will be necessary to use Polyurethane adhesives, because their adhesion does not depend on penetration but is only a chemical adhesion, as we will discuss later.
Acidic woods:
For example oak ( pH 3.5 ), western red cedar ( 3 to 4 )
High acidity will slow down the curing of some adhesives such as resorcinol-formaldehyde, and on the contrary increase the setting speed of urea-formaldehyde glues.
Therefore the RF glues should be cured at higher temperature ( 60 °C ) and the UF glue should be used with a slower hardener.
Burnt wood:
Some sensitive woods may be burnt during machining, planning… The surface becomes harder and difficult to wet by the glues.
To solve this problem it is recommended to abrade the burnt surface, sharpen frequently the cutting tools and increase pressure during bonding.
Wood treated with insect protection:
Several chemicals used for the protection of woods prevent a good bonding.
For instance woods protected with boric acid cannot be bonded with UF glues.
Copper-chromium-arsenic compounds are used for protection against termits of glulam, before bonding with resorcinol-formaldehyde. They reduce wetting and adhesion. It is necessary to wait several days after treatment, sand the wood immediately before bonding, increase glue coverage to 500 g/m² and increase pressure and press time.
Also the treatment could be done after bonding, and in this case it will not prevent adhesion.

The user should ask advice to the supplier of the protective chemicals and run tests before bonding.

Other woods may be difficult to bond for other reasons:

woods sensitive to staining,
woods protected with fireproofing agents,
cemented woods, ( dried too fast, at too high temperatures )
woods protected with waterproofing products.

Theory of adhesion to wood surfaces

How glues and adhesives bond on wood ?

First the adhesive must spread onto the whole surface in order to get maximum contact: this is what we called wetting of the substrates.
Then the adhesive if it is enough fluid could penetrate into and between the wood fibers where it will be locked after drying or curing. ( mechanical entanglement )
However the adhesives penetrate only 2 or 3 layers of fibers, ie a few hundredths of a mm.

Intermolecular forces appear when the adhesive molecules come very close to the wood cellulosic molecules.

Van der Waals bonds ( dipole to dipole, dipole to induced dipole and dispersion forces ) provide bond energy of 1 to 20 kJ/mole, which is low compared to true chemical reactions, but it may be enough to get a good bond.
Wood is a polar surface and thus polar adhesives will be attracted to its surface, for instance UF resins, is OCyanates from the PU adhesives, epoxydes, vinyl adhesives.

Hydrogen links: The cellulose molecules in the wood contain hydroxyl group -OH and when the adhesive contains also hydroxyl groups (such as phenolics, UF or some PU rubbers ) these will build Hydrogen links as shown in figure 7. Hydrogen links will also form between cellulose and UF, RF, PU or epoxy molecules.

Figure 7a : Hydrogen links between the -OH groups of the cellulosic fibers and -OH group of a phenolic resin.

True chemical links: Stronger links could be obtained when a chemical reaction occur between the cellulose and the adhesive molecules, for instance with polyurethane or is ocyanate adhesives as shown on figure 7.
However some scientists have tried to measure the strength of covalent bonds between adhesives and wood and they concluded that there are very few cases where covalent bonds occur, so that it seems that secondary bonds ( Van der Waals forces and hydrogen links ) are the main causes of adhesion to wood.

Figure 7b : True chemical reaction and links obtained by chemical reaction between isocyanate groups and the hydroxyl groups of the cellulosic fibers.

For more information, our readers may refer to the chapters devoted to Theories of adhesion later in this adhesives program on the Special Chem site.

In the next part, we will study the technical characteristics of the glues and adhesives which are used for woodworking, then in a next part we will study the different chemical types of wood adhesives and glues.

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