Until the middle of the last century, rubber or cork were mainly used to seal various joints in engineering and seal joints in construction. These were expensive materials, and they needed to find a cheap alternative, an adhesive substance that could be produced in virtually unlimited quantities using available inexhaustible resources.
The first experiments on the synthesis of polyamides began in the United States, but the German scientists who soon joined the problem were more fortunate than the American researchers: they were able to obtain polyurethane elastomers by combining polyols with some diisocyanates. Later, as a result of various experiments, polyurethanes known today were created.
Why polyurethane sealant has become such a popular building material?
This is due to the fact that the sealant is polyurethane based:
- possesses extremely high elasticity (sometimes reaching 1 000%);
- demonstrates excellent adhesion to many materials, including concrete and bricks, metal, wood and glass;
- has excellent self-adhesion;
- resistant to moisture and ultraviolet radiation;
- provides high-quality sealing and waterproofing of structures for a long time;
- can withstand negative temperatures up to -60 ° C;
- can be used when working in the winter, if the ambient temperature does not fall below -10 ° C;
- does not flow (if the thickness of the applied layer is less than one centimeter) from the vertical planes of the structures;
- gives zero shrinkage after polymerization;
- dries quickly and hardens;
- may be colored or transparent;
- does not emit harmful substances after solidification (and therefore it can be used in bathrooms, in kitchens, and in living rooms);
- polymerized by the action of moisture in the air.
However, the polyurethane sealant offered by manufacturers today has its drawbacks. The main ones are listed below.
- Its adhesion is not sufficient to ensure a reliable strong connection and good sealing of the seams of products, the material of which are some types of plastics.
- Polyurethane sealant should not be applied to surfaces whose moisture content exceeds 10%. In this case, to increase adhesion, you must use special primers.
- It loses its qualities upon prolonged exposure to temperatures in excess of 120 ° C.
- Disposal of polymerized polyurethane sealant is an expensive, complex manufacturing operation.
What else can you say about the technical characteristics and features of the use of polyurethane sealant for sealing seams?
When installing door / window constructions, all joints are sealed with their help.
In the jewelry industry, the use of polyurethane sealants (especially transparent) for fixing natural stones provides barely noticeable neat joints. And since this material is produced in different colors, it is easy to find such a shade of it that will be very close to the color of the stone used in the decoration. The use in this case of silicone-based (even transparent) sealant is unacceptable, because the latter can not only change the color of a precious or semi-precious stone, but also destroy it over time.
In those places of structures, in which there are significant vibrations, it is better to use polyurethane sealants, which are not prone to shrinkage and change in shape. That is why they are often used in the automotive industry.
If it is necessary to perform suture joints exposed to significant temperature changes, the use of a polyurethane sealant is also recommended, as it is also very elastic and resistant to punctures and abrasion.
In the case of waterproofing works for a bathroom, in a fountain, in external ponds or on the roof, a polyurethane sealant suitable for its technical properties can also be successfully used. After curing, the layer of polyurethane has a sufficient density to ensure its resistance to moisture.
Polyurethane based adhesive can be either one component or two component.
One component sealant
It is a pasty substance, the main component of which is a polyurethane prepolymer. Such a single-component polyurethane adhesive has a high adhesion to most building materials. It meshes well with ceramics and glass. After applying this one-component sealant at the joints, the process of its polymerization begins due to exposure to moisture contained in the ambient air.
Single-component compositions are easy to use, as it does not require mixing of any components, which ensures guaranteed quality of joints. Such sealants can be used for both repair and construction work, and in particular for sealing:
- building structures;
- roof joints;
- car bodies;
- installed in the car windows.
In this case, the sealants used in the latter case are often referred to as glass. They are used when performing operations on pasting autoglasses and when installing fiberglass decorative elements in automobiles, as well as when you need to firmly glue glass or plastic parts to a metal base that constantly experiences strong vibrations, temperature changes, water and moisture during operation. .
The disadvantage of one-component compositions is that they cannot be used at an ambient temperature of less than -10 ° C because:
- as the temperature decreases, the humidity of the air decreases, and, as a result, the rate of glue polymerization decreases;
- an increase in the hardening time of the sealant ultimately leads to a decrease in its elasticity, adhesion and hardness;
- due to the increase in viscosity of a single-component polyurethane adhesive under these conditions, the work associated with its use is complicated.
Two component sealant
In the package of such polyurethane glue there are two separately packaged components:
- a paste containing polyols;
- special hardener.
As long as the substances are not mixed, they can be stored for a long time, since they do not interact with the environment.
The main advantage of two-component sealants is that they can be used at negative temperatures, since when they freeze, the moisture contained in the air does not participate in this process. At the same time, these compounds, as well as the one described above, provide strong, elastic and durable seams.