Nondestructive testing of defects in adhesive joints

Results and Discussion
Properties of Unplasticized and Unfilled Polyrethane Elastomer
The curing efficiency of four types (X1, X2, X3, and X4) of hydroxyl terminated curatives have
been tested on three types of polyether based isocyanate terminated prepolymer (A,B and C) with
different functional additives. Among the curatives, the curative X1 imparts highest pot life (working
time) towards the polyether prepolymers (A, B and C) tested in this investigation, whereas the curative
X4 imparts shortest pot life and other two curatives X2 and X3 lie between these two extremes. This
behavior may be attributed to the reactive nature and the percentage of isocyanate groups and hydroxyl
groups present in the polyether prepolymers and curatives respectively. The pot life for prepolymer A
obtained, when mixed with curatives X1, X2, X3, and X4 are 75, 62, 47 and 29 minutes at 100°C
respectively.
Hardness of the cured polyurethane elastomers is determined using the durometer and the results
are obtained for different combinations of prepolymer and curatives. Hardness, shore A obtained for the
polyurethane elastomer resulted from prepolymer A with curatives X1, X2, X3 and X4 are 57, 54, 53, and
45 respectively. Among the polyurethane elastomer obtained, the product from prepolymers A with
curative X1 results in highest hardness (shore A57) which may be explained due to the influence of
higher cross link density resulted from the molecular structure. Similarly, among the prepolymers A, B
and C, the product polyurethane elastomer obtained from prepolymer C with corresponding curative
imparts higher hardness, shore A69, than prepolymers A and B, due to higher percentage of isocyanate
and hydroxyl groups, which in turn enhances the crosslink density.
Tensile strength determined using tensile tester for the different polyurethane elastomers are
presented. Tensile strength results are merely reflections of hardness values, the product from prepolymer
C with curative X1 possess higher tensile strength (42.7 kg/cm 2 ) than from other combinations of
prepolymers A and B curatives X2, X3 and X4. Percentage elongation results obtained for different
polyurethane elastomers are studied. The product resulted from prepolymer B with curative X4 exhibits
highest elongation (500%), while for the product from prepolymer C with curative X1 gives lowest value
(120%). The percentage elongation values are inversely proportional to the hardness and link density of
the products.
Modulus, tear strength, and density for different combinations of elastomers are studied. All these
values increase with increasing crosslink density and hardness due to higher percentage of reactive groups
in the prepolymers. Compression set values for the polyurethane elastomers tested after 22 hours at 70°C
are given. The elastomer from the prepolymer A with curative X1 exhibits lowest value of compression
set 1.6%, while 3.19% is obtained for the product from prepolymer B with X1, these results reveals that
the former AX1 imparts better dimensional stability than the latter BX1.
Solvent resistance characteristics of different combinations of elastomers are carried out by
immersing them in different organic solvents for the period of 7 days. The solvent resistant
characteristics are expressed in the form of percentage weight gain of the elastomer. In this respect, the
product from CX1 gives the lowest value (135%) and the product from BX4 gives the highest value
(505%), this is due to nature of molecular structure and types of secondary forces involved. Among the
solvent system used, ethyl acetate influences more swelling of elastomers than others due to dipole-dipole
attraction. The moisture absorption characteristics of different elastomer combinations are also studied.
The product from CX4 absorbs lowest percentage (1.5%) of moisture; whereas the product from AX1
absorbs highest percentage (3.1%) of moisture. This behavior is due to the number of hydrogen bonds
involved in the elastomer structure.
3