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Introduction of construction technology of polyurethane potting glue

1. 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 this, the bonding strength obtained by bonding on the weak interface layer is not easy to improve. For those substrate surfaces that do not match the surface tension of the adhesive, it must also be Chemical treatment. Surface treatment is one of the first steps to improve bond strength.

2. 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 plastic and the adhesive layer only form a weak bonding interface. For polyurethane adhesives , The grease on the surface of metal or plastic 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 reduce the cohesion of the adhesive layer, thus bonding The surface must be cleaned and dried before cleaning. Generally, it is cleaned with alkaline water containing surfactants and organic solvents, then washed and dried, or directly cleaned with organic solvents (such as acetone, carbon tetrachloride, ethanol, etc.). The rusted metal is generally first removed with sandpaper and wire brush to remove the surface rust.

3. Roughening treatment

The smooth surface generally needs to be roughened to increase the contact area between the adhesive and the substrate. The adhesive penetrates into the cavities or pores on the surface of the substrate, and after curing, it acts as a "nail, hook, rod"-like embedding, which can be firmly Glue the substrates together. Common methods include sandblasting, wood rasping, sandpaper grinding, etc., but too rough will affect the infiltration of the adhesive on the surface, 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.

Fourth, metal surface chemical treatment

Derusting, degreasing, and slight corrosion treatment can be performed on the metal surface at the same time. There are many treatment agents available. Generally, it is an acid treatment liquid. For aluminum and aluminum alloys, potassium dichromate/concentrated sulfuric acid/water (mass ratio of about 10/ 100/300) mixture, soaked at 70-12°C for 5-10min, washed with water, neutralized, washed with water again, and dried.

For iron, it can be mixed with concentrated sulfuric acid (hydrochloric acid) and water 1:1, soaked at room temperature for 5-10 minutes, washed with water and dried. Or treated with potassium dichromate/concentrated sulfuric acid/water mixture.

5. Surface chemical treatment of plastics and rubber

Most polar plastics and rubbers only need roughening treatment and solvent degreasing treatment on the surface. However, the surface energy of polyolefin is very low, and chemical methods can be used to increase its surface polarity, such as solution oxidation method, corona method, and oxidizing flame method. Wait.

(1) The chemical treatment solution can be potassium dichromate/concentrated sulfuric acid/water (mass ratio 75/1500/12, or 5g/55ml/8ml, etc.), PP or PE at 70℃ for 1-10min or at room temperature for 1.5 minutes After h, wash with water, neutralize, wash with water, and dry.

(2) Corona treatment High-frequency high-voltage discharge is used to partially oxidize the plastic surface by oxygen in the air to generate polar groups such as carbonyl groups. It is often a combination of several surface treatment methods, such as sanding → corrosion → cleaning → drying.

6. Primer

In order to improve the adhesion performance, a thin layer of primer (primer) can be applied to 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 varnishes (such as dilute solutions of polyurethane adhesives or coatings); polyisocyanate adhesives (such as dilute solutions of PAPI); dilute solutions of silicone coupling agents; dilute solutions of epoxy resins, etc. .

Seven, bonding construction

1. Glue

The methods of coating (gluing) 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, no bubbles, and no lack of glue.

The amount of glue applied (actually related to the thickness of the adhesive 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 surface of the substrate. When the thickness of the adhesive layer increases, the adhesive 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 in which the effect of peeling force reduces the apparent shear strength value.

2. Let it dry

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 needs to be properly heated for air drying (such as composite film lamination process). Otherwise, since a large amount of solvent remains in the glue, bubbles are easily formed in the glue layer during the curing process, which affects the bonding quality. For solvent-free polyurethane adhesives, the glue can be glued after coating Matter fit.

3. Bonding

This step is to stick the adhesive surfaces of the glued adherends together, or 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. Plastic flow is generated to wet the surface of the adherend and maximize the contact between the adhesive and the surface of the substrate.

Eight, adhesive curing

Most polyurethane adhesives do not have high bonding strength immediately when they are bonded, and need to be cured. The so-called curing refers to the process of the liquid adhesive becoming solid, and the curing process also includes post-curing, that is, the initial curing adhesive in the adhesive. The reactive groups can further react or crystallize to obtain the final cured strength.

Heating methods for curing include oven or drying tunnel, oven heating, fixture heating, etc. For metal substrates with fast heat transfer, fixture heating can be used, and the adhesive layer is heated faster than oven heating.