Nondestructive testing of defects in adhesive joints

PCNSL-1 to PCNSL-3. This is expected to be due to the increase in cohesive strength of the PSA
based on it due to the increase in molecular weight / viscosity up on changing from PCNSL-1 to
PCNSL-3. Also, at higher solids contents the nature of failure changed from ‘adhesive’ to ‘cohesive’
NR / CR / PCNSL (phr) 100 / 0 / 5 0 / 100 / 5
Storage time (days) 1 7 1 7
TSC (%) Mean T-peel strength (N/25mm)
PCNSL - 1 15 5.4 9.2 13.7 16.5
PCNSL - 2
PCNSL -3
20 8.8 10.4 21.5 33.7
25 9.1 11.5 34.8 41
15 5.3 7.3 14.6 18
20 8.7 11 18.8 30
25 9.3 12.1 33.3 45.3
15 6.3 7.4 15 16.9
20 9.7 14.1 26 33.1
25 13.4 17.1 32.6 43.9
Table 1. Variation in T-peel strength of rexin / rexin bonded with PCNSL based PSAs
type similar to that of the commercial sample. The adhesion strength of rexin specimens bonded with
PCNSL based PSAs increase with storage time from 1 to 7 days. This is more prominent in the
adhesive containing higher solids content, higher proportion of CR and PCNSL having a higher
extent of oligomerization / phosphorylation. Time dependent diffusion of the PSA to the substrate
matrix aiding ‘wetting’ may be one of the reasons for the increase in adhesion strength. A similar
time dependent increase in self adhesion strength (tack) of PCNSL modified NR has been reported
earlier 5 which was ascribed to higher molecular diffusion of NR assisted by the plasticizing effect of
PCNSL. The T-peel strength of rexin bonded with Fevicol SR 998 showed a peak value (19N/25 mm)
after storing for five days. This may probably be facilitated by its very low value of viscosity.
However, it can be noted that the increase in adhesion strength is comparatively greater for the
PCNSL modified CR based PSAs, particularly at the higher solids contents, as shown in Table 1. The
progressively decreasing viscosity at higher shear rates of PCNSL – 1, PCNSL-2 and PCNSL-3
based PSAs may aid the flow of the adhesive and wetting of the substrates, facilitating development
of a higher bond strength over a longer period of time. This increasing adhesion strength may also be
due to possible physico-chemical interaction between CR and PCNSL as mentioned in an earlier
work 7 where-in the polar phosphate group of PCNSL can interact with the chloro-group of CR. Polar
– polar interaction between an additive and PSA has been reported as a factor responsible for a high
value of T- peel strength. 8 Table 2 shows the mean T-peel strength of rexin bonded with PSAs based
on unmodified blends of NR and CR, tested after 24 h. The lower values of adhesion strength here is
in sharp contrast to the considerably higher values for the PCNSL based PSAs shown in Table 1.
3