This post may contain affiliate links. We may earn a small commission from purchases made through them, at no additional cost to you. You help to support resin-expert.com
There are many different adhesives available on the market which have different properties and are more or less suitable for really firm bonding. One of these adhesives is polyurethane adhesive – an adhesive that can stand out with many favorable properties. But what is polyurethane adhesive? We will explain it to you – in our PU adhesive guide.
Table of Contents
- 1 What Is Polyurethane Glue?
- 2 Different Types of Polyurethane Glues
- 3 Polyurethane Two-Component Adhesives
- 4 Difference between Urethane and Polyurethane?
- 5 Polyurethane 1K Glue
- 6 Hot Melt Glues (Hotmelts)
- 7 Conclusion of the Different PU Adhesives
- 8 Storage and Durability of Polyurethane Adhesives
- 9 Safety Instructions for Processing PU Glue
What Is Polyurethane Glue?
Polyurethane adhesives, also known as PU adhesives, are adhesives used for higher strength constructions and can be used as both inelastic and permanently elastic adhesives. They are based on polymers, but often contain other components such as various additives.
PU adhesives are often used as two-component adhesives. But there are also one-component adhesives available. Both are based on the chemical reaction of different ingredients. This reaction often involves catalysts such as light, heat, or moisture. The glue is paste-like and shrink-free. Once it has cured, it is waterproof and can be painted or sanded. The adhesive also compensates for small unevenness in the workpiece. These and various other properties make it an unbeatable assistant for all kinds of bonding work.
Not to be underestimated, however, is its health hazard. It is, therefore, subject to the Chemicals Prohibition Ordinance and may not be sold to persons under 18 years of age. Furthermore, specific safety measures must be taken when using it, which we will discuss below.
Different Types of Polyurethane Glues
Polyurethane glues are available in different versions. Besides the one-component adhesives, there are two-component adhesives. However, both are based on the same principle, the chemical reaction of different ingredients. Furthermore, hot melt adhesives are available, also called hotmelts. All types have their advantages and disadvantages. Hotmelts are particularly robust, but offer the user only a very limited open time during which the adhesive can be processed.
In the following sections, we list all polyurethane adhesive properties of the different types of adhesive.
Polyurethane Two-Component Adhesives
PU 2K adhesives are those that bond surfaces together by the chemical reaction of two components, usually a resin and a hardener. Both elements must be mixed in a specific ratio. There are different mixing ratios. The most common mixing ratios of adhesive to hardener are either 100:15 or 100:30. A good alternative is epoxy resin glue.
Polyurethane two-component adhesives are divided into different degrees of hardness. A distinction is made:
- Adhesives that are designed for high loads and must have a high shear strength
- Adhesives that must withstand high shock loads and elongation at break
- Adhesives that combine the positive properties of both of the adhesives as mentioned above and are therefore considered viscoplastic
Two-component adhesives are characterized by various positive polyurethane adhesive properties:
Fields of Application
PU adhesives are suitable for various materials and applications. PU adhesives can be used not only for bonding metal and strongest wood glue, but also for plaster, cork, plastic, and other materials. PU glues are particularly suitable for combination bonding, i.e., bonding between different materials.
For example, if you want to renew the floor covering of your house and glue floorboards or parquet flooring, PU glue could be the right choice for you. Of course, there are also some disadvantages: For example, this adhesive is relatively expensive in comparison and loses its good adhesion if too much time is lost. So you have to invest some money and work fast. This makes the PU adhesive more suitable for experienced artisans. Nevertheless, its many advantages outweigh the disadvantages. For example, high-quality PU adhesive is solvent-free, so you do not have to air your floor for days after laying it before you can safely enter the room. Its adhesive effect, when used correctly, is enormous and withstands the highest demands.
A great advantage is the fact that PU adhesives can be used to bond natural stone without causing discoloration or greasing. Even fire-protected elements on the building site – for example, gypsum fibers or gypsum plasterboards – can be bonded without any problems.
Difference between Urethane and Polyurethane?
Urethane and Polyurethane are both polymers. The term Polyurethane is widely used, if the polymer consists of more than one urethane group, therefore it is called Polyurethane.
How to Glue Polyurethane?
The processing of the 2K PU adhesive is very easy. First, the components are weighed and mixed together. Please observe the manufacturer’s instructions. Change the mixing ratio, and it will influence the properties of the adhesive, and it may not cure sufficiently. When mixing, you should also not add too much air, because every smallest air bubble is a weak point in the cured adhesive.
Practical double cartridges are also available to simplify the mixing of the two components.
It is sufficient to apply the adhesive on one side. Brush the surface to be bonded thoroughly with the PU glue, but do not use too much of it, otherwise the glue could leak out at the joints. If adhesive does leak, apply it immediately with a cloth. Dried adhesive can be removed with a very sharp blade, such as a razor blade.
It is essential to pay attention to the pot life during application. Once this has elapsed, the adhesive can no longer be used. For better handling, you can either add an inhibitor to extend the pot life or an accelerator.
The final strength, i.e., the last degree of curing, is reached after about four to seven days, and the minimum strength usually after about twelve hours. Make sure to fix the workpieces during this time, for example with screw clamps.
Polyurethane 1K Glue
Polyurethane 1K adhesives consist, as the name suggests, of only one component. They absorb moisture from the air after application and thus cure. They form a skin after only a few minutes. The curing speed is, therefore, very high.
The paste-like PU 1K adhesives are applied in industrial plants in a bead-like manner. For the small user, the use with a hand pistol is interesting.
The following advantages can be found with PU 1K adhesives:
Fields of Application
Like the 2K PU adhesive, the 1K PU adhesive is also suitable for a wide range of applications. Thus, wood, plastics, gypsum boards, and metal can be bonded without any problems.
Since the PU 1K adhesives lead to a soft-elastic bonding, they are especially popular in the field of flooring work. If you want to lay a large-format plank or parquet floor, this type of adhesive should be your choice.
You should make sure that the parts to be bonded are free of grease and dust. You can clean them with a lint-free cloth and some solvent before using the 1K PU adhesive. To improve the adhesion of the surfaces, you can roughen them slightly with some sandpaper.
Now apply the adhesive on one side and spread it evenly over the surface to be bonded. After this, you glue both surfaces together. For better adhesion, you can fix the surfaces with screw clamps until the glue has dried.
Hot Melt Glues (Hotmelts)
Hot melt adhesives are adhesives based on thermoplastic polymers. This means that they have a stable state of aggregation at room temperature and become liquid when heated. They can only be processed in the liquid state. After drying, they return to their solid state of aggregation.
Hotmelt adhesives often contain various additives, including antioxidants, wax, and plasticizers. However, other additives can be added depending on the desired properties.
Unlike one- or two-component PU adhesives, hot melt adhesives do not only work on porous substrates. In addition, the drying process is not a chemical but a physical process, as it is not caused by the reaction of different components but by cooling.
Two types of hot melt adhesives are known: EVA hotmelts and PUR hotmelts. The former have the advantage of being reactivatable. This means they can be melted again after curing, which makes recycling possible.
In the following you will find the advantages and disadvantages of hotmelts:
Fields of Application
The areas of application of hot-melt adhesives are very diverse. Hotmelts are not only used in the packaging industry and bookbinding, but also in the production of cardboard and labels, in the automotive industry, and in assembly. In principle, Hotmelts can be used to bond almost any material imaginable: on metals, rubber, plastics, glass and ceramics, wood, cardboard, cork, and various fabrics. Felt and leather can also be bonded with them.
This means that hot-melt adhesives are widely used not only in the private but also in the industrial sector. Both the manufacturing and the health and hygiene industry benefit from the positive properties of these adhesives.
For handicraft work with the family, a hot-melt glue gun is, in principle, indispensable. Especially in the hobby area, hotmelts are often used.
Usually, hotmelts are applied by small users using an industrial glue gun, which releases the heated adhesive by finger pressure. The adhesive is applied to the clean surfaces to be bonded, and these are then joined together. You should proceed quickly, as you only have a limited time window for application. If the hotmelt cools down, it becomes solid and can no longer be processed.
However, hotmelt can also be processed with the aid of tools other than the glue gun, such as extruders and spiral syringes. The adhesive is available either in the form of glue sticks, films, blocks, or even granulates.
Conclusion of the Different PU Adhesives
Although PU adhesives are often slightly more expensive, they make up for this minor disadvantage with their favorable properties. They can, therefore, be applied to a wide range of materials. Their adhesive properties are excellent, and they adhere surfaces to each other very well if they are applied on one side. Their handling is also quite simple. Even with the PU 2K adhesives, you as a user, usually get along well, as double cartridges are now also available. This saves you the mixing.
Furthermore, PU adhesives have a high tensile strength as well as excellent weather resistance.
Storage and Durability of Polyurethane Adhesives
There are several things to consider when storing polyurethane adhesive:
- The adhesive should be stored in a cool and dark room; the optimal temperature of the storage room is between +10 and +25° Celsius
- Protect the cartridge from wet, cold and moisture
- Process older adhesive first – first in, first out
- Check the packaging regularly for damage
After working with the PU adhesive, the cartridge should always be well sealed to prevent the glue inside from hardening. Otherwise, the remaining adhesive cannot be used.
The durability of PU adhesives is limited. Depending on the adhesive, it is between a few months and a few years. More detailed information can be found in the enclosed datasheet.
Used PU cans must be disposed of separately. It is, therefore, better to deliver them to a collection point for harmful substances instead of putting them in the regular household waste.
Safety Instructions for Processing PU Glue
When working with PU glue, you should always pay attention to safety. Always use safety glasses, a breathing mask and disposable gloves to protect your skin as well as your eyes and mucous membranes from contact with the glue. Various ingredients are suspected to be carcinogenic. Therefore, you should also air the room in which you work well.
Before you start working, you should read the safety data sheet carefully and, if necessary, contact qualified personnel if you have any questions.
If the PU adhesive does come into contact with your skin, remove the adhesive residue thoroughly with soap and warm water immediately. If the adhesive is still moist, it can also be removed with a cloth. To remove hardened adhesive residues, you must use a razor blade – but of course, you should be very careful with it!
PU adhesives are therefore recommended if you want to achieve high strength connections, but they are quite expensive. You should also always pay attention to your safety.