Nondestructive testing of defects in adhesive joints

Experimental Procedure
Liquid Castable Polyurethane Elastomer
Cast elastomers are usually slightly cross linked thermoset polymers. In the production of
urethane elastomer the heated prepolymer is mixed with a selected curative in specific proportion.
Prepolymers contain terminal isocyanate (NCO) groups, which react chemically with hydroxyl (OH) or
amine (NH2) groups in the curative. The mixture is poured into a heated mould where the components
rapidly react to form a solid elastomeric article.
Reaction
HO – A – OH + OCN – R1 - NCO
Polyol Diisocyanate
OCN – R1 – HNCOO – A – OCONH – R1 –NCO + (n-1)HO – R2 – OH
Prepolymer Alcohol
- [-O – A – OCONH – R1NHCO –(O – R2 – OCONH – R1 – NHCO-)-n-1]-
Polyurethane Elastomer
The flexibility is imparted by the long chain glycols, whereas the rigidity is obtained by short
chain glycols. There are two types of polyurethane elastomers, depending upon the polyol used. These are
called polyether based polyurethane elastomers and polyester based polyurethane elastomer. Polyether
based polyurethane elastomers give lower densities and better hydrolytic stability, fungus resistance and
electrical properties. Polyester based polyurethane elastomers result in higher densities – better tear and
abrasion resistance and less oil and solvent absorption. Polyesters are also less easily oxidized and resist
higher temperatures than polyethers.
PROCESSING
Process Conditions
Procedure
Reaction Temperature 70 – 80°C
Stoichiometry 95%
Mixing Time 3 min
Oven Temperature 120°C
Cure Condition 16 hours at 120°C
Post-cure condition Two weeks at room temperature
The desired amount of prepolymer is weighed into a suitable container and the prepolymer is
heated to 70 – 80°C. The over heating should be avoided. The required amount of plasticizer and filler are
added in the case of filled products. The reaction is carried out in vacuum and with suitable amount of
curatives. The mixed reactants are degassed and poured into a preheated (120°C) mould and is allowed
for 16 hours to complete the reaction (curing). Further, the product is kept for two weeks time at room
temperature for post curing before being tested.
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