Wood glue

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Wood glues are adhesives made from natural or synthetic raw materials for joining wood and wood-based materials .

Stress groups

Wood glues are divided into different stress groups, new designation DIN / EN204 (D1, D2, D3 and D4), old designation according to DIN 68602: B1, B2, B3 and B4. The load groups arrange the glues according to their minimum shear strength values and their behavior when exposed to moisture and water:

  • D1 (old name B1 ): Suitable for indoor use, whereby the wood moisture must remain below 15%.
  • D2 (old name B2 ): Suitable for indoor areas with occasional short-term exposure to water or condensation, whereby the wood moisture may not exceed 18%.
  • D3 (old name B3 ): Suitable for indoor areas with frequent short-term exposure to water or higher air humidity. Suitable for outdoor use without exposure to direct weathering.
  • D4 (old name B4 ): Suitable for indoor areas with frequent and long-lasting exposure to draining water or condensation water. Outside and exposed to the weather with adequate surface protection.

Types of glue

There are different types of glue, which differ from one another in their properties, their processing and their composition:

Glutin glue (hot glue)

Glutine glue is obtained from animal waste from higher animals (“glue-giving bodies”) by boiling it out. This creates a jelly that is called "glue" when it is dry. The main component of Glutin glue is Glutin , a protein compound . According to their origin, a distinction is made between different glue glues, e.g. B. bone glue, hide glue (leather glue), rabbit glue, isinglass glue or isinglass glue . These glues are usually heated so that they can be processed.

Casein glue

This glue consists of casein and slaked lime . Again, it is a natural glue, which, unlike the glowing glue, is waterproof and very heat-resistant. Nevertheless, it is used less and less today and, due to its properties, is almost exclusively used in building joinery. It is available in load groups D2 and D3. It is available as a powder mixture that can be mixed with water, or it can be made by thoroughly mixing low-fat quark with slaked lime (ratio approx. 4–5: 1).

  • Production: milk protein and calcium compounds.
  • Commercial form: As a white powder. Since casein paste powder is hygroscopic, it must be kept tightly closed.
  • Shelf life: one to two years. Storage beyond this time is not recommended as the solubility slowly decreases.
  • Preparation: In earthenware, glass or porcelain vessels, not in metal vessels. Stir 1 part by weight with 1.5 to 2 parts by weight of water quickly. The casein absorbs all the water, the paste becomes gritty and firm. Leave. The action of calcium dissolves the casein within a quarter to half an hour into a new, viscous substance, calcium caseinate, which from now on slowly sets. Set glue substance cannot be liquefied again by adding water (so do not prepare for stock).
  • Standing time: With suitable additives, the commercially available casein glue is adjusted in such a way that the applied glue remains spreadable for approx. 8 hours. Cold spots increase the service life.
  • Spreadability: Mixing proportions that give the best cohesion are tough and cause considerable effort when applying glue. We recommend using a spatula or roller for larger areas.
  • Wetting time: This is 5–10 minutes. For the best possible gluing, glue should therefore be applied to both woods.
  • Setting process: This takes place a) by chemical conversion of the liquid glue into a gelled state. b) By dehydration through the wood. Since the woods are glued cold, they are not dried out more strongly by warming them up, as is the case with the glowing glue. The dehydration takes place more slowly with casein gluing than with glutin gluing. The binding force therefore increases more slowly.
  • Setting time: After 3 hours there are usually no more glue breaks. The maximum binding force is reached after 6 hours.
  • Pressing time: The gluing of unclamped wooden parts requires 1 to 1 1/2 hours of pressing time. Tight joints 3 to 6 hours.
  • Compression pressure: Since casein glue joints remain tough and elastic and even thick joints do not become brittle, high compression pressures are good, but not absolutely necessary.
  • Binding strength: This is higher than with glutin glue and higher than the wood strength.
  • Embrittlement: Even thick joints are pulled together when they set and do not become brittle. This makes the casein glue an excellent construction and assembly glue.
  • Water resistance: Set casein glue can no longer be liquefied by water, but swells up to a slushy state. The bond strength is then reduced to about half. During the subsequent drying process, the joint no longer achieves its previous strength because the pressure is missing. Tight joints can fall apart completely. So one can only speak of a much better water resistance than with Glutin glue. Fungi can also damage casein glue when it is moist.
  • Staining: Casein glue can cause dark stains on the wood. For this reason, casein glue is more likely to be used for painted woodwork or for joints that are not visible.
  • Tool wear: It is greater than with ember glue, but less than with many synthetic resin glues.

Urea-formaldehyde resin glue (also polycondensation glue or UF glue)

This type of glue consists of synthetic resins combined with formaldehyde . It is offered in three forms of trade:

  • One-component glue: hardener is already mixed in.
  • Two-component glue: glue and hardener available as powder or liquid. The correct hardener must be used depending on the processing.
  • Glue film: Adding extenders (e.g. rye flour to make the glue joint more joint-filling).

The unpleasant, pungent and conspicuous formaldehyde odor is striking (therefore good ventilation of the room is advisable). It is available in load groups D3 to D4.

Phenol-formaldehyde resin glue (PF glue)

Phenol-formaldehyde resin glues or urea-formaldehyde resin glues are dark, weather-resistant, boil-proof and fill the joints. Used in the joinery industry, manufacture of chipboard and plywood .

Resorcinol formaldehyde resin glue (RF glue)

It consists of liquid glue and powdered hardener. This glue is boil-proof, has very good weather and heat-resistant properties and is mainly used in boat building, in glider construction and in the manufacture of waterproof sheet materials.

Low-formaldehyde polycondensation glue

Formaldehyde is required to harden the glue resins. In most cases, the proportion of formaldehyde is higher than that of the resins, so that good curing takes place. However, with this polycondensation glue, the reduction of free formaldehyde is achieved by adding a formaldehyde scavenger or reducing the proportion of formaldehyde. Thus, the glue has a lower bond strength.

Formaldehyde-free dispersion glue (PVA or white glue)

The well-known PVAC ( P oly V ynyl Ac etatleim or simple white glue) is a formaldehyde-free dispersion glue. This is commercially available as a dispersion. The binding agent in this glue is the polyvinyl acetate, a crystal clear, clean mass. This glue is supplied ready-to-use and is available as cold glue , quick binder, veneer glue, hardener glue, lacquer glue and hot glue. It is available in stress groups D2 – D3.

D4 glueing requires a D3 glue to which isocyanate crosslinking agent (usually an HDI) is added before it is used. Such two-component adhesives have a pot life of several hours. There are also systems that are based on a D2 glue to which a hardener (a Lewis acid such as aluminum chloride ) is added for crosslinking . One advantage of this mixture is the much longer pot life. This type of D4 glue is often preferred, especially for glue application machines. One-component D4 adhesives have been on the market for some time, especially for smaller consumption quantities.

Some data:

  • Basis: dispersion
  • Color: white, hardened: transparent
  • pH value: approx. 3–5
  • Shelf life: at 20 ° C approx. 4–6 months

Polyurethane adhesive (PUR or PU glue)

The most modern glues for the wood sector are one-component adhesives based on polyurethane . They are usually referred to as PU or PUR glue. They are waterproof and not only glue wood, but almost all glueable materials. The achievable strengths correspond to the D4 requirements according to DIN EN 204/205 or the higher requirements according to DIN EN 15425 or 302-1. They are solvent-free reactive adhesives that cure with the help of moisture. In many cases there is a slight foaming, which is caused by the release of carbon dioxide. On the one hand this leads to a gap filling, on the other hand the foam reduces the ultimate strength of the bond. Sufficient pressure is necessary so that the foaming adhesive does not force the parts apart.

In the uncured state, polyurethanes are allergenic when inhaled and when they come into contact with the skin because of the isocyanates they contain . According to the manufacturer's instructions, the adhesives should therefore only be used with protective equipment (protective gloves and, if there is insufficient ventilation, with a respirator ).

Epoxy resin (EP glue) and unsaturated polyester resin (UP glue)

Epoxy and polyester resins are usually used as 2-component adhesives for waterproof bonds. You can join almost any material together. The high strength of these resins is usually not necessary for bonding softwood, so that they are rarely used here. Since 2-component resins do not require a solvent , these adhesives do not lose any mass during curing and can therefore also be used to fill gaps.

Melamine glue (MUPF glue)

Melamine-urea-phenol-formaldehyde glue consists of a mixture of melamine, urea, phenol and formaldehyde; In the variant without the addition of phenol, it is also used as MUF glue.

PMDI glue

PMDI is used as a binder in wood -based panels , especially OSB panels , and is a starting material for the production of polyurethanes.

processing

The processing is different depending on the manufacturer. The stress groups alone say nothing about the glue base, i.e. the glue base material.

White glue is applied on one or both sides, depending on the absorbency of the surfaces to be glued; for highly absorbent surfaces, a double application of glue with a flash-off time of ~ 10 minutes is necessary. White glues can be diluted with water if the tightening time (the time until the glue bonds the workpieces together after they are joined) needs to be increased; this is z. B. necessary for form gluing or difficult pressing situations. Due to their basis, white glues (PVAc) are thermoplastics . This is z. B. exploited in the edge coating in that both contact surfaces are wetted with white glue, dried and then joined together while hot.

Hot glue, e.g. B. Kaurit , is primarily used for gluing on veneers. Kaurit glue is usually in the form of a powder that contains both the actual glue and a chemical hardener component. The addition of water activates the Kaurit glue, which is then applied to one side of the support surface. The surfaces are pressed briefly, about 3–5 minutes, with high pressure (0.5 N / mm², corresponding to 50 t / m²) and temperatures between 70 and 120 ° C. After pressing, the (steaming) workpieces are ventilated upright.

For types of wood with tannic acid , e.g. B. oak , or when using iron-containing applicators, the wood can be discolored. Kaurit glues can lead to undesirable reactions with wood stains. If surfaces cannot be joined with sufficient pressure, gap-filling but non-expanding adhesives such as polyester and epoxy resin are recommended.

The room, material and glue temperature should be 18–20 ° C, the relative humidity 50–70% and the wood humidity 8–10%.

  • White point: +3 ° C to +8 ° C is the limit temperature below which glue does not yet form a "film" when it dries and therefore dries to a white, rubbery layer. Around 15 ° C is optimal.
  • Open time: 8-10 min (at approx. 200 g / m²) bring the joint parts together and press them within the open time.
  • Pot life: shelf life or after adding hardener; this is the time that a glue can remain in the glue container from its usability until it begins to set. At the end of the pot life the glue is unusable.
  • Glue application rate: for surface gluing 80–140 g / m², for assembly 150–180 g / m²
  • Compression pressure: 0.1–0.5 N / mm² (corresponding to 10–50 t / m²)
  • Minimum pressing time: surface gluing of laminates in short-cycle presses (+70 ° C) from 1 min, assembly gluing 8–15 min, board joint and block gluing 20–40 min.

literature

Individual evidence

  1. ↑ From the carpenter's profession; unpublished, ca.1970.
  2. ↑ Safety data sheet Ponal Konstruktions PUR-Kleber, (pdf; 86.57 kB) .
  3. ^ Kaurit from BASF .