what is polyurethane Adhesive?
Adhesives containing urethane groups (-NHCOO-) and/or isocyanate groups (-NCO) in the molecular chain.
The isocyanate group has high chemical activity and can react with water molecules, hydroxyl groups, and hydrogen in amines. Polyurethane molecules contain highly polar urethane groups. Polyurethane glue has a wide bonding surface and high bonding strength. An appropriate amount of water molecules can cross-link polyurethane prepolymers with terminal isocyanate groups. One-component polyurethane is moisture-cured, which is why polyurethane hot-melt adhesives post-cure after hot-melt gluing and are irreversible.
The molecular chain structure of polyurethane has a hard segment phase and a soft segment phase. The hard segment phase is composed of isocyanate structural unit segments, and the soft segment phase is composed of polyether or polyester unit segments. The hard segments are gathered together by intermolecular forces and hydrogen bonding forces to form dots in the network structure of the cured material, and the soft segments are dispersed among the dots. Compared with the dots in cured epoxy, which are composed of curing agent molecules, the dots in cured polyurethane have lower strength and the distance between dots is longer than that of epoxy. As a result, the mechanical strength of polyurethane is lower than that of epoxy, and its flexibility is better than that of epoxy. The high temperature resistance is worse than that of epoxy.
2. Polyurethane properties
advantage:
•High initial bonding strength
• No by-products when cured
• Isocyanate molecules are small in size, easy to diffuse/penetrate strongly, and improve adhesion.
• Small internal stress and excellent fatigue resistance
• Good abrasion/oil/chemical/ozone and bacteria resistance
shortcoming:
Excellent low and ultra-low temperature performance
Do not stick if the relative humidity is higher than 70%
Poor high temperature resistance
3. Classification
• Polyisocyanate adhesives
Polyisocyanate monomer added to 20% methylene chloride solution
• Prepolymer adhesives
One-component product of the reaction of isocyanate and polyol (polyester or polyether).
Two-component glue is composed of polyester resin and polyester modified diisocyanate
• End-blocked polyurethane adhesive
The terminal isocyanate reacts with phenol or other hydroxyl groups to form a product with a urethane structure, which temporarily blocks the active isocyanate group and can be formulated into an aqueous solution or emulsion adhesive when used.
4. Single and double component performance
two-component
• Different components have different curing speed, elasticity and fluidity
• Good adhesion to plastics
• Good bonding properties to composite materials
• Extremely sensitive to moisture during curing
single component
• Easy to operate and use
• Requires moisture curing
• Different elasticity depending on formula
• Can be made into hot melt adhesive
• Good bonding performance to composite materials