1. High temperature all polymers exposed to high temperature will undergo some degree of degradation and mechanical properties will be reduced.For high temperature resistant adhesives, melting point or softening point must be high, and should be antioxidant.Most thermoplastic waterborne polyurethane adhesives can achieve good bonding effect at room temperature. However, once the temperature reaches the glass transition temperature of adhesives, the rubber layer will be deformed and the internal polymerization strength will be reduced.Thermosetting adhesives have no melting point and are composed of highly crosslinked networks of macromolecules, most of which are suitable for use at high temperatures.The key problem of thermal curing is the rate of strength reduction caused by thermal oxidation and pyrolysis.High temperature resistant adhesives usually have rigid polymer structure, high softening temperature and stable chemical groups, which bring difficulties to the bonding process.Therefore, only a few thermosetting adhesives can be used at 177℃ for a long time.
When the temperature drops from room temperature to -253℃, the adhesive whose shear strength exceeds 6.89MPa is defined as ultra-low temperature adhesive.Ultra-low temperature adhesives are mainly used for bonding metals and non-metals with their external insulators, but can also be used as sealants. Most airfoil-type fuel tanks and pressure-resistant bulkheads are sealed with adhesives.Many problems existing in joints under ultra-low temperature condition are the result of stress concentration and stress gradient in joints.The main causes of stress concentration are as follows: a.B. Volume shrinkage of adhesive during curing;C. Envelop or release gas when gluing;D. The difference of elastic modulus and shear strength between the adhesive and the adhesive;E. Residual stress retained by the adhesive after removal by pressure applied in bonding;F. Inelasticity of adhesives or objects to be glued;G. Plasticity of adhesives or objects to be glued.
Polyurethane is one of the best ultra-low temperature adhesives. At -253℃, the current room-temperature curing polyurethane adhesives have better shear and tensile strength, peel strength and impact performance than the earlier polyurethane adhesives, which is contrary to most structural adhesives.The strength of polyurethane adhesive increased at -253℃, but decreased at room temperature and high temperature.
2. Moisture and immersion: for polyurethane containing ester bond, it will hydrolyze and lose its strength and hardness in hot and humid environment, and even liquefy in serious cases.Water can also penetrate the adhesive layer, replacing the adhesive on the interface, which is the most common reason for the decrease in bonding strength in a humid environment.The rate of hydrolysis depends on the chemical structure of the base material, the type of catalyst, the amount and the flexibility of the binder.Certain chemical bonds, such as esters, ammonia esters, amides, and ureas, can be hydrolyzed.

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