Why use polyurethane adhesives for structural wood or used in extreme conditions

Because it is a very moisture resistant adhesive, so resistant that it is quite possible for the wood to rot and the glue line to remain intact. Because the glued wood can be treated in autoclavesat high temperatures and pressures. Because end grain crushers with polyurethane adhesive can be washed in the dishwasher. Because the glued wood can be boiled in water for several hours without splitting into component parts. Because it can be used for obtaining wooden structures used in the construction of houses. I think you already understand that when glued wood needs high resistance, both mechanically and to extreme treatments, a very good choice is polyurethane adhesive. Let's see why it is so resistant and how it should be used to achieve the desired results.

What is a polyurethane adhesive

The adhesive in liquid form contains urethane groups (obtained, as in polyurethane varnishes, from the reaction of a polyol with an isocyanate) which, through polymerisation, form the solid adhesive that bonds the elements to be glued. The increased strength comes from the advanced cross-linking achieved through polymerisation. Polyurethane adhesives can be two-component or one-component. In bicomponent adhesives the reaction is triggered by mixing with a hardener, whereas monocomponent adhesives react with moisture in the atmosphere. They can bond wood, metal, ceramics, cork, plastic, stone and other materials. They are used in areas such as furniture and carpentry, construction, automotive, footwear, flooring and house cladding. Due to the high strength of the bond formed, they are preferred mainly in industry, but are also used for DIY projects with more specific strength requirements. In terms of moisture resistance, polyurethane adhesives are class D4.

There are several types of polyurethane adhesives, each formulated for its field of use. Before curing they are white or brownish in colour and after curing they generally become transparent. Those used for gluing wood are very elastic and are suitable for laying flooring or gluing beams. The volume of the adhesive increases during the curing reaction, so it can also be used on surfaces with small holes or imperfections, which it fills after expansion. The reaction time can be from 20-30 minutes to hours. After the initial bonding time, when the object is removed from the press, there is a reaction completion time after which the bond reaches maximum strength. This time can be of the order of days.

When choosing a polyurethane adhesive

As mentioned at the beginning, they are chosen for the very good strength of the bond formed between the glued elements. Like epoxies, they are structural adhesives, but unlike epoxies, they have much better strength on the outside. They react even in very humid environments (in water) and the adhesive film resists high temperature, humidity and pressure.

Polyurethane adhesives are chosen when the need arises:

• High resistance to weathering and solar radiation,
• High resistance to ageing,
• High structural strength,
• High impact and shear strength,
• Good properties for gluing on large surfaces (parquet, floorboards).

Polyurethane adhesives for wood are easy to work with, with no special equipment requirements or conditions, and can be used for different surfaces/materials, both indoors and outdoors.

Requirements when applying polyurethane adhesives

Single-component adhesives are more commonly used in the wood industry. They are applied at room temperature, the minimum application temperature being, as with PVA adhesives5-10°C. Unlike PVA adhesives, however, they have no special requirements for frost resistance. After applying the adhesive, the elements must be put in contact. The open phase does not have to last long because, when reacting, the adhesive tends to increase its volume (it releases carbon dioxide forming a foam). Depending on the type of adhesive, the open phase can be between 10 and 30 minutes.

The adhesive is applied by brush, roller or gun. If one of the elements has become dirty, it can be wiped off immediately with acetone, alcohol or universal thinner. Dry adhesive is easily removed with a scraper, chisel or sanding. For bonding, the elements must be fixed together using clamps or a press. It is not necessary to apply a lot of pressure but only that the elements are in direct and even contact. Uniform pressure prevents voids from forming which will be filled by expanding adhesive. A uniform appearance will result, but the bond will be less strong in the space where the adhesive is expanded.

The affinity for moisture makes adhesives very sensitive to humidity. The packaging should not be kept open for long and once opened, the shelf life will be greatly reduced. Don't buy adhesive in large packaging if you don't plan to use it in a short time, because unused adhesive will harden and cannot be used. Moisture in the wood is needed to cure the polyurethane adhesive. Very dry wood will need moistening before application. Also the preference for moisture makes it hard to remove from the skin. Wear gloves when working with polyurethane adhesive!

TISZAPUR, polyurethane adhesive used in the production of strength structures for houses

TISZAPUR D4 4401 Special, distributed by the company Szolvegy from Târgu Mureș, is a highly resistant polyurethane adhesive that can be used to obtain beams and posts used in the construction of wooden houses. It has proven its qualities over time, being used for more than 15 years by house builders. Dimmer to obtain laminated wood structures. The high impact and shear strength, high elasticity make the structures obtained very strong and elastic at the same time. It is the kind of adhesive that lasts longer than glued wood.

The adhesive is one-component, brown in colour, becoming transparent after polymerisation. The quantity applied must not exceed 250 g/m². The curing time in the press is between 8 and 12 hours, depending on the quantity applied. After removal from the press, the panels should be left for 24 to 48 hours to complete the reaction. After this time the bonding reaches maximum strength.

Tiszapur D4 Special can be used to obtain outdoor furniture or fittings, beams and the load-bearing structures of houses, gluing wood to be treated in autoclaves, gluing parquet and stair treads, gluing boats or elements immersed in water. In the case of autoclave treatment, a period of min. 7 days is recommended between leaving the press and the start of the temperature and pressure treatment.

Polyurethane adhesives are the safe choice where wood has to withstand high pressure and shear forces, the aggressive action of permanently wet environments or the degrading action of UV radiation. When there are no special requirements, even in damp environments such as bathrooms or kitchens, PVA adhesives remain the most suitable for gluing wood.