Nondestructive testing of defects in adhesive joints

of CR and solids content, all of the unmodified adhesives show low values of adhesion strength. The
lower adhesion strength of the unmodified adhesives may be due to the absence of a plasticizer and /
or tackifier which are known to be essential constituents of an elastomeric PSA.
Figure 3 shows that the PCNSL based PSAs show a maximum in the T-peel strength at a CR
content of 90 phr. Also, at all the different proportions of NR and CR there is a gradual increase in T-
peel strength (in the order PCNSL-1 < PCNSL-2 < PCNSL-3) with a change in the type of PCNSL.
Mean T-peel strength (N/25mm)
55
50
45
40
35
30
25
20
15
10
5
B
C
D
TSC = 20%
PCNSL - 5 phr
PCNSL - 1
PCNSL - 2
PCNSL - 3
0
0 20 40 60 80 100
Dosage of CR (phr)
Fig.3. Variation in T-peel adhesion strength of rexin / rexin bonded specimens with dosage of CR
in the NR/CR/PCNSL blend and at 20% TSC of adhesives.
This can be attributed to the increase in molecular weight of the resin in that order, contributing to the
cohesive strength of the adhesive. Zosel has reported that above a minimum molecular weight of the
modifier, tendency for fibrillation increases leading to an increase in cohesive strength of PSAs. 9 A
similar mode of cohesive failure was observed for rexin specimens bonded with the PCNSL based
PSA containing PCNSL-3. This indicates the possibility for entanglement of the pre-polymer chains
of PCNSL with macromolecular chains of CR and NR with consequent increase in cohesive strength
and T-peel adhesion strength. Also, the small proportion of NR (10 phr) may facilitate the visco-
elastic deformation / flow characteristics of the adhesive facilitating quick and uniform wetting of the
substrates and development of a high value of adhesion strength. The comparatively lower values of
viscosity of Fevicol SR 998 at various shear rates may be one of the factors for its high value of
adhesion strength. Thus, the low viscosity of this adhesive leads to almost complete wetting of the
substrates as shown in the stereo optical micrograph of the failure surface, given as Fig.4 (a). The
optical micrograph of the failure surface of rexin bonded with PCNSL based PSA having 25% solids
content is given as Figures 4(b).This shows almost complete wetting of the substrate, accounting for
5