- General classification of joinery adhesives
- How to choose the right adhesive
- Preparation of parts for gluing
- Adhesive application and curing
- Positioning, compression of parts
- Processing of glue lines
Carpentry is based on precise fitting and gluing of parts, and it is the last part that is responsible for the strength and durability of the product. There are quite a few varieties of joinery glues, and today we will talk about the difference between them and the rules of application..
General classification of joinery adhesives
The modern assortment of adhesives for joinery includes more than a hundred items. Therefore, to begin with, let’s briefly go over the types of glue, their properties and the most popular brands..
By origin, all types of glue can be divided into compositions of natural origin and synthetic. In turn, natural adhesives can include binders of animal and vegetable origin, while synthetic adhesives are divided according to the method of production into condensation (formaldehyde) and polymerization (PVA, polyamide). The list of natural adhesives includes compounds based on casein, collagen and albumin, chemical compositions include PVA, formaldehyde, PVC, epoxy, as well as most universal types of glue. As you might guess, the most widespread are adhesives of natural origin, primarily due to low cost and well-established production technology..
Wood is a material with high porosity, which means that almost any adhesive can be used for gluing. However, since most of the joinery is used for domestic purposes, it is not customary to use adhesives based on volatile solvents for gluing them. This is the second advantage of natural glue – it provides sufficient strength despite the fact that it does not emit any toxic substances into the environment..
Polyurethane adhesive for wood
However, for a number of products, it is required to provide resistance to various kinds of external influences. Mostly, the struggle is being carried out to increase moisture resistance, because most varieties of natural glue lose their strength when wet. It may also require an increase in adhesion, mainly this need arises when gluing parts made of hard and dense species of small-vascular wood. Here, the best performance is demonstrated by joints on synthetic adhesives.
How to choose the right adhesive
If you are not a professional carpenter, you should not be intimidated by such a variety of compounds for gluing wood. You can go an easier way – use time-tested types of wood glue, environmentally friendly and with a certain set of special properties.
Natural glue is supplied mostly in dry form and requires preparation before use. The main disadvantages of natural adhesives are their short pot life (up to 2–3 days) and low bond strength due to the low degree of chemical purity and mistakes made during preparation. At the same time, natural glue is the best choice for bonding irrelevant parts with a significant area of adjoining surfaces.
Most carpenters use factory-made PVA-based compounds as a universal glue. Well-established manufacturers – Titebond, Kleiberit, Moment, are listed in order of decreasing product cost and, accordingly, quality. These types of glue are classified according to the reliability and quality of the connection by the D index with four serial numbers. The higher the number, the more pronounced the moisture resistance and strength of the seam, and additional properties may appear. So, Titebond III glue, aka D4 in the general classification, allows contact with food after drying, this glue is mainly used to assemble end cutting boards.
If the contact area of the parts to be glued is not high, high gluing accuracy or resistance to stress is required, it is better to use compositions based on synthetic resins. Even ordinary epoxy resin EDP in such cases will give odds to any water-soluble glue. It is especially advantageous to use it for gluing hard wood, including thermally modified wood, as well as when joining wood with foreign materials..
Bonding wood to plastic with epoxy glue
Preparation of parts for gluing
When gluing wood, the general rules for making glued joints apply. The fixation will be the stronger, the smaller the thickness of the glue line, the deeper the impregnation and the lower the content of impurities. For these purposes, the surfaces to be glued must be carefully prepared..
Gluing of wooden parts is carried out before finishing grinding. Pre-contact surfaces must be carefully sanded with abrasive paper of at least 300 grit to remove fine lint that interferes with the absorption of the glue. It is important to remember that the cleaned surface is suitable for gluing only for a certain time: over time, the top layer of wood oxidizes, the vessels become clogged with dust and new portions of pile rise.
Degreasing is not necessary when using a water-borne adhesive. However, when gluing on epoxy resin or polyurethane hot melt glue, it is desirable to remove residues of wood resins and natural emissions from the surface using concentrated technical acetone. It is worth mentioning the moisture content of the parts to be glued separately: a moderately high moisture content (20-22%) when working with water-soluble adhesives slightly slows down the hardening process, but at the same time promotes deeper absorption and ultimately has a positive effect on the bond strength. But when using adhesives on liquid resins, an increased (more than 12-14%) moisture content is categorically unacceptable, therefore, the adjacent surfaces are pre-dried with hot air for 2-3 minutes.
Adhesive application and curing
The glue is applied to the surfaces to be joined as evenly as possible. The layer thickness should be as small as possible, but still sufficient to compensate for curvature. The presence of air pockets in the glue seam affects the reliability of the connection extremely negatively. Particular attention is paid to seams with uneven thickness, reaching a few millimeters at the peak: in such joints, it is recommended to mix the glue with wood flour or cellulose fiber..
In certain cases, the adhesive does not have the main bonding function, but is used as a plastic filler. Such situations include the strengthening of joints on spikes and lamellas with glue; splicing of wood using the groove and micro-spike method also belongs to this category. In such cases, the glue is applied with a clear excess, after which a rather painstaking work is carried out to remove the protruding residues.
It is convenient to apply glue on even and smooth surfaces with a small flexible spatula, which can be used as a regular plastic card. In the grooves and small holes, as well as on the spikes and folds, the glue is applied with a brush with synthetic bristles. When gluing parts with a large contact area, it is necessary to apply a large amount of glue as quickly as possible so that it is absorbed evenly. For example, you can take all the same end boards and furniture boards: in order to glue several dozen bars, the glue is quickly rolled over them with a roller. The same applies to gluing veneer and gluing flat pieces of plywood..
Almost all types of adhesives require curing after application in the open air. Upon contact with oxygen, the glue undergoes primary curing, which speeds up the drying process of the assembled parts. For PVA-based adhesives, the exposure time can be up to 20-30 minutes, for natural adhesives – up to several hours. Manufacturers may give special recommendations, for example, polyurethane glue is cured until the viscosity is completely lost, after which it is activated by the temperature method.
Positioning, compression of parts
Holding the parts helps to increase the viscosity of the adhesive, which is good for holding the parts in the glued position. However, this approach is used primarily in modeling and making irresponsible glue seams. In most joinery glue joints, rigid fixation of parts is required using clamps and stops.
Fastening the parts with an additional clamp not only allows them to maintain their position during the drying of the glue, which is almost always accompanied by shrinkage and warping of the wood from local moisture. As the glue becomes more viscous, the pressure helps to crush the composition that has not yet set deep into the pores, which increases the strength of the glue line..
In joinery, a huge number of clamps of different types and sizes are used. For gluing flat parts, you can also use ordinary oppression, pressing the product to the table with a weight of 7-10 kg. But to connect linear elements, it is necessary not only to maintain the corners, but also to squeeze the parts, which can be very dimensional. Conventional linear clamps do an excellent job with this task; if there is a lack of length, a stop can be attached to the parts, but only if the place of its fixation is covered by another part of the product. It is also widely practiced to temporarily fasten the stops to made with clamps..
For gluing parts at an angle, clamps of a special shape are used, which have two pairs of clamps and fixed jaws, rigidly fixed to the frame in a given position. This tool can be replaced by a number of fixtures, which is often practiced when gluing at angles other than straight. For example, wedges cut in a common pattern can act as a temporary stop. With this positioning method, it is important that the clamping force is directed as close as possible to the perpendicular to the glue line. For these purposes, the occipital parts of the clamps and stops can take on a wide variety of shapes..
Processing of glue lines
In conclusion, we will give a couple of practical tips on how to ensure the invisibility of the glue seams. Most joinery adhesives have a translucent white color for a reason: after drying, slight streaks can be easily hidden under a layer of protective and decorative coating. But there are also a number of exceptional situations..
For example, initially invisible traces of glue getting on wood are clearly visible when impregnated with oil. If such a coating of the product is planned, you need to work with glue as carefully as possible. Excess glue protruding from the seam should not be wiped off immediately, it is better to wait for their preliminary setting, and then cut off with a model knife. The story is the same with drips: attempts to wipe away will only lead to deeper absorption, so it is better to just blot the drop with a foam sponge, and after drying, clean the defective place with a cycle.
Solvents can be used to remove small residues of leaked adhesive. Depending on the type of glue, these can be acetone (for most adhesives based on synthetic resins), isopropyl alcohol (for PVA-based adhesives), toluene (for universal rubber adhesives and epoxy resins), and ordinary soapy water (for natural reversible adhesives). Wipe off dried excess with an absolutely clean rag or cotton napkin strictly in the direction of the adhesive seam.