Polyurethane Adhesive Construction Technology

Surface treatment

One of the conditions for forming a good bond is the necessary treatment of the substrate surface.

There are often weak interface layers such as grease and dust on the surface of the adherend. Affected by them, the bonding strength obtained by bonding on the weak treatment is one of the first steps to improving bond strength.

Cleaning and degreasing

The surface of some metal and plastic substrates is often easily contaminated by sweat, oil, dust, etc. In addition, there is a release agent on the surface of the plastic, so such a plastic and the adhesive layer only form a weak bonding interface.

For Polyurethane adhesives, the grease on the metal or plastic surface has poor compatibility with polyurethane, and the existing moisture will react with an NCO group in the adhesive to generate bubbles, which will reduce the contact surface area between the adhesive and the substrate, and make the adhesive layer cohesive. Therefore, surface cleaning and drying must be carried out before bonding. Generally, it is washed with alkaline water containing surfactants and organic solvents, then washed with water and dried, or directly washed with organic solvents (such as acetone, carbon tetrachloride, ethanol, etc. ). For metal with rust, generally use sandpaper and a wire brush to remove the surface rust.

Roughening treatment

The smooth surface generally needs to be roughened to increase the contact area between the glue and the substrate. The adhesive penetrates into the recesses or pores on the surface of the substrate, and after curing, it acts as a "nail, hook, and stick"-like embedding, which can firmly stick the substrates together.

Commonly used methods include sandblasting wood rasping, and sandpaper polishing. However, if it is too rough, the infiltration of the adhesive on the surface will be affected, and it is easy to remain in the recess or generate air bubbles, which will reduce the bonding strength. If it is easy to damage the substrate by sanding and other methods, it is advisable to use primer coating, etching, corona treatment, and other methods to change its surface properties, so that it can be easily bonded by polyurethane adhesives.

Chemical treatment of metal surface

Derusting, degreasing, and slight corrosion treatment can be performed on the metal surface at the same time. Most polar plastics and rubbers only require surface roughening and solvent degreasing. However, the surface energy of polyolefin is very low, and chemical methods can be used to increase its surface polarities, such as the solution oxidation method, corona method, and oxidation flame method.

Primer

In order to improve the bonding performance, a thin layer of primer(primer)can be applied on the surface of the treated substrate. The primer can also protect the surface of the newly treated adherend from corrosion and pollution, and prolong the storage time. The commonly used primers for polyurethane adhesives and sealants are polyurethane varnish (such as a dilute solution of polyurethane adhesive or coating); polyisocyanate adhesive (such as a dilute solution of PAPD); dilute solution of silicone coupling agent; dilute solution of epoxy resin, etc.

Adhesive preparation

One-component polyurethane adhesives generally do not need to be formulated and can be used directly according to operational requirements, which is also the convenience of single-component adhesives.

For two-component or multi-component polyurethane adhesive, it should be prepared according to the instructions. If the hydroxyl content and isocyanate group content of the components is known, the distribution ratio of each component can be determined by chemical calculation. The isocyanate index R=NCO/OH is generally in the 0.5-1.4 range.

Generally speaking, when two-component solvent-based polyurethane adhesives are formulated, the latitude of the two-component distribution ratio is larger than that of non-solvent-based adhesives, but if there are too many NCO groups in the adhesive, the curing will be incomplete, and the cured The adhesive layer is hard and even brittle; if there is too much hydroxyl component, the adhesive layer will be soft and sticky, with low cohesion and poor bonding strength. The latitude of the solvent-free two-component adhesive is smaller than that of the solvent-based adhesive because the initial molecular weight of each component is small. If one of the components is excessive, the curing will be slow and not easy to complete, and the surface of the adhesive layer will be sticky. Low strength.

The prepared glue should be used up every day because the prepared glue has a limited shelf life. The pot life is the time for which the formulated adhesive can maintain its operable construction. The viscosity increases with the standing time, so it is difficult to operate until the glue loses its fluidity and gels and fails. Different varieties and grades of polyurethane adhesives have different pot life, ranging from a few minutes to a few days. When it is widely used in industrial production, the pot life test should be done in advance.

If the adhesive component contains a catalyst or a catalyst is added during compounding in order to speed up the curing speed, the pot life will be shorter. In addition, the ambient temperature has a great influence on the pot life, and the pot life is short in summer and long in winter. Two-component polyurethane adhesive diluted with urethane-grade organic solvent for extended pot life. General solvent-based two-component adhesives, such as two-component polyurethane adhesives for soft plastic composite films, should have a pot life of more than 8h (ie, one working day).

Bonding construction

a. Glue

Coating (gluing)methods include spraying, brushing, dipping, roller coating, etc., which are generally determined according to the type of glue, viscosity, and production requirements. The key is to ensure that the glue layer is uniform, with no bubbles, and no lack of glue.

The amount of glue (actually related to the thickness of the glue layer)is also an important factor affecting the shear strength, and the shear strength is usually higher within a certain range. If the adhesive layer is too thin, the adhesive cannot fill the uneven gap on the surface of the substrate, leaving a gap, and the bond strength is low. When the thickness of the adhesive layer increases, the bond strength decreases. It is generally believed that when the lap shear specimen bears the load, the adherend, and the adhesive layer are deformed by themselves, and the adhesive layer is destroyed into a peeling state, and the effect of the peeling force reduces the apparent shear strength value.

b. Dry and place

For solvent-based polyurethane adhesives, it needs to be left in the air for a few minutes to tens of minutes after applying the glue, so that most of the solvent in the adhesive is volatilized, which is conducive to improving the initial adhesion. If necessary, it should be properly heated for air drying (such as the composite film lamination process). Otherwise, due to a large amount of solvent remaining in the glue, the curing process is likely to form bubbles in the glue layer, affecting the bonding quality. For solvent-free polyurethane adhesives, the adherents can be attached after gluing.

c. Bonding

This step is to stick the adhesive surfaces of the glued adherents together. You can also use a clamp to fix the adhesive parts to ensure that the adhesive surfaces are completely fitted and positioned. If necessary, apply a certain pressure to make the adhesive better. Produces plastic flow to wet the adherend surface and maximize the contact between the adhesive and the substrate surface.

Adhesive curing

Most polyurethane adhesives do not have high bond strength immediately upon bonding and require curing. The so-called curing refers to the process in which the liquid adhesive becomes solid, and the curing process also includes post-curing, that is, the reactive groups in the initially cured adhesive further react or crystallize to obtain the final curing strength. For polyurethane adhesives, the curing process is the process of making the NCO groups in the adhesive react completely, or the solvent is completely volatilized, and the polyurethane molecular chain is crystallized so that the adhesive and the substrate have a high enough adhesion process.

Polyurethane adhesives can be cured at room temperature. For reactive polyurethane adhesives, if it takes a long time to cure at room temperature, a catalyst can be added to promote curing. In order to shorten the curing time, heating can be used. Heating is not only conducive to the curing of the adhesive itself but also to accelerating the reaction of the surface of the substrate and facilitating molecular movement, finding a "partner" that generates a molecular force on the bonding interface.

Heating is beneficial to improve adhesion. The curing heating methods include an oven or drying tunnel, drying room heating, fixture heating, etc. For metal substrates with fast heat transfer, fixture heating can be used, and the adhesive layer is heated faster than the oven.

The heating process should be gradually heated up. A solvent-based polyurethane adhesive should pay attention to the volatilization speed of the solvent. During the drying process, most of the solvent has been volatilized, and the remaining solvent will slowly diffuse outward through the adhesive layer. If the heating is too fast, the solvent will vaporize and bubble in the softened adhesive layer, forming bubbles in the joint. In severe cases, most of the uncured, viscous adhesive can be squeezed out of the joint, and the formation of vacancies will affect the bonding strength. For two-component solvent-free adhesives and one-component moisture-curing adhesives, the heating should not be too fast, otherwise, the NCO group will accelerate the reaction with the moisture in the adhesive or the surface of the substrate and the air, and the generated CO₂ gas will not have time to diffuse, and the adhesive layer Viscosity builds up quickly and air bubbles remain in the bond line.

The one-component moisture-curing polyurethane adhesive is mainly cured by moisture in the air, so a certain air humidity should be maintained, and it is advisable to cure slowly at room temperature. If the air is dry, a small amount of water can be added to the coated surface to promote curing. If the glue is sandwiched between dry and hard small amount can be injected into the glue moisture.