High viscosity, water-resistant, cold-resistant, and heat-resistant! However, there are nine important considerations for polyurethane adhesives.

Polyurethane adhesives, with their high viscosity, water resistance, cold resistance, and heat resistance, are widely used across various industries. Below are the nine key considerations for polyurethane adhesives.

1. Surface Preparation

One of the key conditions for achieving excellent adhesion is proper surface preparation of the substrate.

The surfaces of the substrates often have weak interface layers such as oils and dust, which can affect the bond strength. For substrates with surface tensions that do not match those of the adhesive, chemical treatment is also necessary. Surface treatment is one of the primary steps to enhance bond strength.

2. Cleaning and degreasing

The surfaces of some metal and plastic substrates are often contaminated by sweat, oil, dust, etc. Additionally, plastic surfaces may have release agents, so such plastics only form weak bonding interfaces with the adhesive layer.

For polyurethane adhesives, the oils on metal or plastic surfaces are incompatible with polyurethane, and any moisture present reacts with the NCO groups in the adhesive to produce bubbles, reducing the contact area between the adhesive and the substrate and lowering the cohesive strength of the adhesive layer. Therefore, surface cleaning and drying must be performed before bonding. Typically, cleaning is performed using an alkaline solution containing surfactants and organic solvents, followed by rinsing and drying, or direct cleaning with organic solvents such as acetone, carbon tetrachloride, or ethanol. For metal surfaces with rust, rust should first be removed using sandpaper or a wire brush.

3. Roughening treatment

Smooth surfaces generally require roughening treatment to increase the contact area between the adhesive and the substrate. The adhesive penetrates into the depressions or pores on the substrate surface, and after curing, it acts like “nails, hooks, and rods” to securely bond the substrates together.

Common methods include sandblasting, roughening with a wood file, and sanding with sandpaper. However, excessive roughening can impair the adhesive's wetting ability on the surface, leading to residue or gas bubbles in recesses, which may reduce bonding strength. Methods like sanding can also damage the substrate, so it is advisable to use methods like applying a primer, etching, or corona treatment to alter the surface properties, making it more suitable for bonding with polyurethane adhesives.

4. Chemical Treatment of Metal Surfaces

Metal surfaces can be simultaneously treated for rust removal, degreasing, and mild corrosion. There are many available treatment agents, typically acidic solutions. For aluminum or aluminum alloys, a mixture of potassium dichromate/concentrated sulfuric acid/water (mass ratio approximately 10/100/300) can be used. Immerse for 5–10 minutes at 70–12°C, rinse with water, neutralize, rinse again, and dry. For iron, a mixture of concentrated sulfuric acid (or hydrochloric acid) and water in a 1:1 ratio can be used, with immersion at room temperature for 5-10 minutes, followed by water rinsing and drying. Alternatively, a mixture of potassium dichromate, concentrated sulfuric acid, and water can be used for treatment.

5. Chemical treatment of the surface of plastics and rubber

Most polar plastics and rubber only require surface roughening and solvent degreasing. However, the surface polarity of polyolefins is very low, so chemical methods can be used to increase their surface polarity, such as solution oxidation, corona treatment, or oxidation flame treatment.

(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 is immersed at 70°C for 1–10 minutes or soaked at room temperature for 5 hours, then rinsed with water, neutralized, rinsed again, and dried.

(2) Corona treatment uses high-frequency high-voltage discharge to partially oxidize the plastic surface with oxygen in the air, producing polar groups such as carbonyl groups. Often, several surface treatment methods are combined, such as sanding → etching → washing → drying.

6. Applying a primer

To improve adhesion performance, a very thin layer of primer (base adhesive) can be applied to the surface of the pre-treated substrate. The primer also protects the freshly treated substrate surface from corrosion and contamination, extending storage time.

Common primers for polyurethane adhesives and sealants include: polyurethane varnish (such as a dilute solution of polyurethane adhesive or coating); polyisocyanate adhesives (such as a dilute solution of PAPI); dilute solutions of organosilicon coupling agents; and dilute solutions of epoxy resins, etc.

7. Preparation of Adhesives

Single-component polyurethane adhesives generally do not require preparation and can be used directly according to operational requirements, which is one of the advantages of single-component adhesives.

For two-component or multi-component polyurethane adhesives, preparation should be carried out according to the instructions. If the hydroxyl content and isocyanate content of the components are known, the mixing ratios can be determined through chemical calculations. The isocyanate index R = NCO/OH is typically within the range of 0.5 to 1.4.

Generally, when mixing two-component solvent-based polyurethane adhesives, the tolerance for the ratio of the two components is greater than that of non-solvent-based adhesives. However, if the NCO groups are excessively abundant during mixing, the adhesive will not cure completely, and the cured adhesive layer will be hard or even brittle. If the hydroxyl components are excessively abundant, the adhesive layer will be soft and sticky, with low cohesive strength and poor bonding strength. The tolerance for mixing ratios of solvent-free two-component adhesives is smaller than that of solvent-based adhesives, as the initial molecular weights of the components are smaller. If one component is excessive, it will result in slow curing and incomplete curing, with the adhesive layer appearing sticky and having low strength.

Once mixed, the adhesive should be used on the same day, as the adhesive has a limited pot life. The pot life refers to the time during which the adhesive can maintain its workability after mixing. Viscosity increases with time, making application difficult, until the adhesive loses its flowability, gels, and becomes ineffective. The pot life varies among different types and grades of polyurethane adhesives, ranging from a few minutes to several days. When used in large quantities in industrial production, a service life test should be conducted in advance.

If the adhesive components contain a catalyst, or if a catalyst is added during mixing to accelerate curing, the service life will be shorter. Additionally, environmental temperature significantly affects the service life, with shorter service life in summer and longer in winter. Two-component polyurethane adhesives diluted with amide-grade organic solvents can extend the pot life. For general solvent-based two-component adhesives, such as two-component polyurethane adhesives for soft plastic composite films, the pot life should exceed 8 hours (i.e., one working day). If the mixed adhesive is not used up on the same day, it can be appropriately diluted, covered, and stored in a cool place. The next day, check for any cloudiness or gelation. If the adhesive surface shows no obvious changes and maintains good flowability, it can still be used, typically by mixing small batches into newly prepared adhesive. If it has deteriorated, it should be discarded.

To reduce viscosity, facilitate application, ensure even adhesive distribution, and control adhesive thickness, organic solvents can be added for dilution. Suitable diluents for polyurethane adhesives include acetone, butanone, toluene, and ethyl acetate.

Adding a catalyst accelerates curing speed. Curing catalysts are typically organic tin compounds.

8. Adhesive Application

(1) Adhesive Application

Adhesive application methods include spraying, brushing, dipping, and rolling, typically determined based on the adhesive type, viscosity, and production requirements. The key is to ensure an even adhesive layer without bubbles and intact adhesive.

The adhesive application amount (actually related to adhesive layer thickness) is also a critical factor affecting shear strength, typically reaching higher shear strength within a certain range. If the adhesive layer is too thin, the adhesive cannot fill the uneven gaps on the substrate surface, leaving voids, resulting in low bonding strength. As the adhesive layer thickness increases, bonding strength decreases. Generally, when the lap shear specimen bears a load, the bonded materials and the adhesive layer itself deform, causing the adhesive layer to be damaged into a peeling state, and the peeling force reduces the apparent shear strength value.

(2) Curing

For solvent-based polyurethane adhesives, after applying the adhesive, it must be allowed to cure for several minutes to several tens of minutes to allow most of the solvents in the adhesive to evaporate, which helps improve initial tack. When necessary, appropriate heating and forced-air drying (e.g., in composite film lamination processes) may be required. Otherwise, a large amount of solvent remaining in the adhesive may cause bubbles to form during the curing process, affecting the bonding quality. For solvent-free polyurethane adhesives, the bonded objects can be joined immediately after adhesive application.

(3) Bonding

This step involves bonding the adhesive-coated surfaces of the substrates together. Clamps may be used to secure the bonded components, ensuring that the bonding surfaces are fully aligned and positioned. When necessary, apply sufficient pressure to facilitate plastic flow of the adhesive, allowing it to penetrate the surface of the substrate and achieve maximum contact between the adhesive and the substrate surface.

9. Adhesive curing

Most polyurethane adhesives do not immediately achieve high bonding strength upon application and require curing. Curing refers to the process by which liquid adhesive transforms into a solid. This process also includes post-curing, where reactive groups in the adhesive undergo further reactions or crystallization after initial curing to achieve final curing strength. For polyurethane adhesives, the curing process involves the complete reaction of NCO groups in the adhesive, or the complete evaporation of solvents, followed by crystallization of the polyurethane molecular chains, resulting in sufficient bonding strength between the adhesive and the substrate.

Polyurethane adhesives can cure at room temperature. For reactive polyurethane adhesives, if room-temperature curing requires a longer time, a catalyst can be added to promote curing. To shorten the curing time, heating can be employed. Heating not only facilitates the adhesive's own curing but also accelerates the reaction between the NCO groups in the adhesive and the active hydrogen groups on the substrate's surface. Heating also softens the adhesive layer, enhancing wetting of the substrate surface and facilitating molecular movement to find “partners” for molecular interactions at the bonding interface.

Heating is beneficial for enhancing bonding strength. Curing heating methods include oven or tunnel heating, chamber heating, and fixture heating. For metal substrates with rapid heat transfer, fixture heating can be used, as the adhesive layer heats up faster than in an oven.

The heating process should involve gradual temperature increase. For solvent-based polyurethane adhesives, attention must be paid to the evaporation rate of the solvent. During the curing process, most of the solvents have already evaporated, and the remaining solvents slowly diffuse outward through the adhesive layer. If heating is too rapid, the solvents may vaporize and form bubbles in the softened adhesive layer, creating bubbles at the joint. In severe cases, this can force most of the uncured, flowable adhesive out of the joint, creating voids that impair bonding strength. For two-component solvent-free adhesives and single-component moisture-curing adhesives, heating should also not be too rapid. Otherwise, the NCO groups in the adhesive or on the substrate surface may react too quickly with moisture in the air, producing CO₂ gas that cannot diffuse in time, causing the adhesive layer's viscosity to increase rapidly and leaving bubbles trapped within the adhesive layer.

Single-component moisture-curing polyurethane adhesives rely primarily on moisture in the air for curing, so a certain level of air humidity should be maintained, and curing at room temperature is preferable. If the air is dry, a small amount of moisture can be applied to the adhesive surface to promote curing. If the adhesive is sandwiched between dry, hard substrates and the adhesive layer is thick, moisture from the interface and external environment may not penetrate the adhesive, leading to incomplete curing. In such cases, a very small amount of moisture can be introduced into the adhesive.