Polyurethane adhesives classified by components

1.polyisocyanate adhesive

Polyisocyanate adhesive is an early product of polyurethane adhesive composed of polyisocyanate monomer or its low molecular derivatives. As a reactive adhesive, polyisocyanate adhesive has high bonding strength and is especially suitable for bonding metals with rubber and fibers.

Polyisocyanate adhesive mainly has the following characteristics:

(1) It has high reactivity, is easy to infiltrate and penetrate, promotes the formation of adhesion, and can generate covalent bond force with many adhered materials with active hydrogen atoms on the surface, such as metal, rubber, fiber, wood, leather, plastic, etc., and contains carbamate, urea bond and polar bonds and groups after curing, and is easy to generate secondary bond force with the substrate. These chemical and physical bonding forces work together to produce high bonding strength between the bonded substrates.

(2) The polyisocyanate adhesive is good at resisting heat and solvents, can be cured at room temperature or by heating, and has an easy-to-produce cross-linked structure.

(3) The common polyisocyanate adhesive has a small relative molecular weight, and it can self-polymerize on the alkaline adherend, resulting in chemical bonds and cross-linked structures on the interface, which can improve the bonding performance. It can be dissolved in most organic solvents, so it is easy to diffuse to the surface of the substrate, and it is also easy to penetrate some porous bonded substrates, thus further improving the bonding performance.

(4) Because it contains more free isocyanate groups, it is sensitive to moisture and toxic, and usually contains organic solvents. Attention should be paid to waterproofing and moistureproofing during storage and ventilation during operation.

(5)The relative molecular weight of polyisocyanate adhesive is small, the content of the NCO group is high, and the cured adhesive has high hardness and brittleness. Therefore, it is often modified by rubber solution and polyether.

Commonly used polyisocyanate adhesives include JQ-1 adhesive and JQ-4J adhesive. The former is 4,4',4"- triphenylmethane triisocyanate dissolved in dichloroethane to prepare a 20% glue solution. The latter is a 20% colloidal solution prepared by dissolving p-triphenyl-thiophosphate in dichloroethane, which can be cured at 140℃ for 20~30min under the pressure of 3MPa.

2. prepolymer polyurethane adhesive

The one-component type of prepolymer polyurethane adhesive can react with water vapor in the air or on the adherend at room temperature to cure-moisture curing polyurethane adhesive. This adhesive is easy to use and has a certain toughness, but the humidity in the air has a certain influence on the bonding speed and bonding performance. When ammonium chloride and urea are added as catalysts, they can be cured at room temperature or heated.

Due to the release of carbon dioxide, the wet curing adhesive often has bubbles in the adhesive layer, which leads to defects. Carbon dioxide absorbent or adsorbent is often mixed with adhesive to overcome the above shortcomings, which is a good adhesive for wood, civil construction and structure; It is also often used as sealant, which plays an important role in automobile, construction and machinery industries. Two-component series is composed of polyester resin and polyester-modified diisocyanate.

In order to improve the strength of adhesives, cross-linking agents can generally be added. All multifunctional active hydrogen compounds can be used as cross-linking agents, commonly used are polyols and polyamines. The obtained cross-linked polyurethane adhesives can be used as structural adhesives for metal-to-metal, metal-ceramic bonding, as well as non-structural materials such as rubber, plastic and wood.

3.End-sealed polyurethane adhesive

End-capped polyurethane is a product with a urethane structure produced by the reaction of phenol or other hydroxyl groups (such as alcohols and 3-diketones) with terminal isocyanate groups, which temporarily seals the active isocyanate groups to make them stable in water, and solves the shortcoming of solidification due to absorption of moisture in the air during storage. When it is used, it can dissociate the blocking agent at a certain temperature, release active isocyanate groups, and play its inherent functions. Although this kind of adhesive is liquid and easy to use, it usually needs to be heated above 120℃ to play its role. The dissociated sealant bubbles when it volatilizes, or remains in the adhesive layer, which is not good for bonding.