Polyurethane bonding process

1. One of the conditions for the surface treatment to form a good bond is to perform the necessary treatment on the surface of the substrate.
On the surface of the adherend, there are often weak interfacial layers such as grease, dust and the like, and it is difficult to increase the bonding strength obtained by the bonding on the weak interface layer. For those surfaces that do not match the surface tension of the adhesive, they must also be chemically treated. Surface treatment is one of the first steps in improving the bonding strength.
2. Cleaning and degreasing The surface of some metal and plastic substrates is often easily contaminated by perspiration, oil, dust, etc. In addition, there is a release agent on the plastic surface, so the plastic and adhesive layer only form a weak bonding interface. For polyurethane adhesives, the grease on the surface of metal or plastic is poorly compatible with the polyurethane, and the presence of moisture reacts with an NCO group in the adhesive to generate bubbles, which reduces the surface area of ​​the adhesive in contact with the substrate and makes the adhesive layer cohesive. Lower, so the surface must be cleaned and dried prior to bonding. It is generally washed with alkaline water containing a surfactant and an organic solvent, washed with water, or washed with an organic solvent (such as acetone, carbon tetrachloride, ethanol, etc.). For rusted metals, first use sandpaper, wire brush to remove surface rust.
3. Roughening Generally smooth surfaces are 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 acts like a "nail, hook, or stick" to hold it firmly after curing, which firmly holds the substrates together. The commonly used methods are sand blasting, raft roughening, sandpaper sanding and the like. However, if the surface is too rough, the adhesive will be affected by the infiltration of the surface, and the recesses will easily remain or generate air bubbles, which in turn will reduce the adhesive strength. If it is easy to damage the substrate by sanding and other methods, it is better to change the surface properties by applying primer, etching, corona treatment, etc. to make it easy to be bonded by polyurethane adhesive.
4. Metal surface chemical treatment can simultaneously perform rust removal, degreasing, and slight corrosion treatment on the metal surface. There are many available treatment agents. Generally acidic treatment liquid. For aluminum and aluminum alloys, a mixture of potassium dichromate/concentrated sulfuric acid/water (about 10/100/300 mass ratio) can be used, immersed at 70-12°C for 5-10 minutes, washed with water, neutralized, washed with water, and dried.
Concentrated sulfuric acid (hydrochloric acid) can be mixed with water in 1:1 for iron, immersed at room temperature for 5-10 minutes, washed and dried. Or with potassium dichromate / concentrated sulfuric acid / water mixture treatment.
5. Surface chemical treatment of plastics and rubbers Most polar plastics and rubbers require only roughening of the surface and solvent degreasing. However, the surface energy of polyolefins is very low, and the surface polarity can be increased by chemical methods, such as solution oxidation method, corona method, oxidation flame method, and the like.
(1) Potassium dichromate/concentrated sulfuric acid/water (mass ratio 75/1500/12, or 5g/55ml/8ml, etc.) may be used as the chemical treatment solution, and PP or PE may be soaked at 70°C for 1-10 minutes or at room temperature. After h, wash, neutralize, wash, and dry.
(2) The corona treatment uses high-frequency and high-voltage discharge to partially oxidize the surface of the plastic by oxygen in the air to produce polar groups such as carbonyl groups. It is often a combination of several surface treatments such as sanding → corrosion → cleaning → drying.
6. Primer In order to improve the adhesion performance, a very thin primer (bottom adhesive) 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, extended storage time.
Primers commonly used for polyurethane adhesives and sealants are: polyurethane varnish (such as a dilute solution of polyurethane adhesive or paint); polyisocyanate adhesive (such as PAPI dilute solution); dilute solution of organic silicon coupling agent; dilute epoxy resin solution, etc. .
7. Preparation of Adhesives One-component polyurethane adhesives generally do not need to be formulated. They can be used directly according to the operating requirements. This is also the ease of use of single-component adhesives.
For two-component or multi-component polyurethane adhesives, they should be formulated according to the instructions. If you know the hydroxyl content of the components and the content of isocyanate groups, the distribution ratio of each group can be determined by chemical calculation. The isocyanate index R=NCO/OH is generally 0.5-1.4 range. In general, when two-component solvent-based polyurethane adhesives are used with rubber, the two sets of distribution ratio latitude are larger than those of non-solvent types, but if too much NCO groups are used in the compound, the curing is not complete and the cured adhesive is cured. The adhesive layer is hard and even brittle; if the hydroxyl component is excessively large, the adhesive layer is soft, the cohesive force is low, and the adhesive strength is poor. Solvent-free two-component adhesives have a lower latitude than solvent-based ones, because the initial molecular weight of each component is small. If one of the components is excessive, curing is slow and not easy, and the surface of the adhesive layer is sticky. Low intensity.
It is advisable that the prepared rubber be used up in days because the formulated rubber has a limited pot life. The pot life of the formulated adhesive can maintain its operational construction time. The viscosity increases with the standing time, and thus the operation is difficult until the glue loses its fluidity and gels and fails. Different types and grades of polyurethane adhesives have different pot life, ranging from a few minutes to several days. When used in large quantities in industrial production, a pot life test should be done in advance.
If the adhesive component contains a catalyst, or if the catalyst is added in order to speed up the cure, the pot life is shorter. In addition, the ambient temperature has a greater impact on the application period, with a short period of application in summer and a long period in winter. Two-component polyurethane glue diluted with urethane grade organic solvents can be extended for a longer period of time. General solvent type two-component adhesives For example, two-component polyurethane adhesives for soft plastic composite films, the pot life should be greater than 8h (ie one working day).
If there is no use for the same day, it can be diluted properly, covered with a lid, stored in a cool place, and checked for turbidity or gelation on the second day of work. If there is no obvious change in the appearance of the glue, the liquidity is good. It can still be used, and it can be used in batches in batches. If it has deteriorated, it should be discarded.
In order to reduce the viscosity, it is easy to operate, so that the glue is evenly coated, and it is advantageous to control the thickness of the glue. Dilution can be done by adding organic solvents. The diluents available for polyurethane adhesives include acetone, butanone, toluene, ethyl acetate, and the like.
Adding a catalyst can speed up the curing of the glue. Curing catalysts are generally organotin compounds.
8. Bonding construction
1. Coating method (coating) includes spraying, brushing, dipping, roller coating, etc. It is generally determined according to the type, viscosity, and production requirements of the adhesive. The key is to ensure that the adhesive layer is uniform, bubble free, and adhesive free. .
The amount of glue applied (in fact, related to the thickness of the adhesive layer) is also an important factor that affects the shear strength, and usually has a higher shear strength in a certain range. If the adhesive layer is too thin, the adhesive cannot fill the uneven surface of the substrate surface, leaving a gap and a low adhesive strength. When the thickness of the adhesive layer increases, the bonding strength decreases. It is generally believed that when the lapped shear specimens are loaded, the adherend and the adhesive layer are deformed by themselves, and the adhesive layer is broken into a kind of peeling state, and the effect of the peeling force reduces the apparent shear strength value.
2. Airing For solvent-based polyurethane adhesives, it takes several minutes to tens of minutes for the adhesive to dry after the adhesive is applied, so that most of the solvent in the adhesive is volatilized, which is conducive to improving the initial adhesion. If necessary, appropriate heating, blast drying (such as composite film lamination process). Otherwise, due to a large amount of solvent remaining in the glue, the curing process easily forms bubbles in the adhesive layer, which affects the bonding quality. For non-solvent polyurethane adhesives, the adherends can be adhered after gluing.
3. This step of bonding is to glue the adhered surface of the adherend that has been coated with glue, and it is also possible to use a jig to fix the adhesive to ensure that the surface of the adhesive is completely conformed, and if necessary, a certain pressure is applied to make the adhesive more stable. Good plastic flow is generated to wet the surface of the adherend to maximize contact between the adhesive and the substrate surface.
9. Curing of the adhesive

Most polyurethane adhesives do not immediately have high bond strength when they are bonded and must be cured. The so-called solidification refers to a process in which the liquid adhesive becomes a solid, and the curing process also includes post-curing, that is, the reactable group in the adhesive after preliminary curing further reacts or crystallizes to obtain the final curing strength. For polyurethane adhesives, the curing process is to complete the reaction of the NCO groups in the gel, or to completely volatilize the solvent, and to crystallize the molecular chains of the polyurethane, so that the adhesive and the substrate produce a sufficiently high adhesive force. Polyurethane adhesives can be cured at room temperature. For reactive polyurethane adhesives, curing at room temperature takes a long time and catalysts can be added to promote curing. In order to shorten the curing time, heating method can be used. Heating not only facilitates the curing of the adhesive itself, but also facilitates the reaction between the NCO groups in the adhesive and the active hydrogen groups on the surface of the substrate. Heating can also soften the adhesive layer to increase the wetting of the substrate surface and facilitate molecular motion, finding a “partner” that produces molecular forces at the bonding interface. Heating is good for improving adhesion. A two-component polyurethane adhesive bonded steel plate, the bonding strength at different curing temperature and time.

Cured heating methods include oven or drying tunnel, oven heating, and fixture heating. For a metal substrate with fast heat transfer, the fixture can be used for heating, and the adhesive layer heats faster than the oven.

The heating process should be gradually warmed up. Solvent-based polyurethane adhesives should pay attention to the volatilization rate of solvents. During the open process, most of the solvent has volatilized, and the remaining solvent slowly diffuses through the adhesive layer. If the heating is too fast, the solvent vaporizes in the softened adhesive layer and forms bubbles in the joint. Seriously, most of the uncured, streamlined adhesives can be squeezed out of the joints to form gaps that can affect the bond strength. For two-component non-solvent adhesives and single-component moisture-curing adhesives, the heating should not be too fast. Otherwise, the NCO groups react with water in the adhesive or on the surface of the substrate and in the air, and the generated CO2 gas is too late to diffuse. The viscosity increases rapidly and bubbles remain in the adhesive layer.

One-component moisture curing polyurethane adhesives are mainly cured by the moisture in the air, so it should maintain a certain degree of air humidity, it is appropriate to slowly solidify at room temperature. If the air is dry, a small amount of water can be applied to the coating surface to promote solidification. If the glue is sandwiched between dry and hard adherends and the adhesive layer is thick, moisture at the interface and outside is not easily penetrated into the glue, so the curing is incomplete. In this case, a very small amount of glue can be injected. Moisture.

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