Nondestructive testing of defects in adhesive joints

Results and discussion
Mechanical properties
In order to understand the effectiveness of various coagents in the PP/EOC TPVs, it is
necessary to understand the performance of different coagents only in the EOC compound
(without PP). The reactivity and efficiency of different coagents were characterized by cure study
on gum EOC vulcanizates. Figure 1 shows the rheographs of peroxide cured EOC vulcanizates
containing various coagents at 20 meq concentration and compared with the control sample
(without addition of any coagent). Irrespective of different coagents taken for the investigation, a
considerable improvement in the maximum torque (Max S) and delta torque (Max S - Min S)
values were inferred upon addition of coagent. This is mainly due to the improved crosslinking
efficiency of DCP in presence of coagent. It is clear from the Figure-1 that, TAC shows the
higher torque values followed by HVA-2 and TMPTA. Mechanical properties of the TPVs
prepared by three different coagents with varying concentration were summarized in the Table 1.
Mechanical properties are considerably improved by the dynamic vulcanization process. It is
clear from the result that blend properties are sensitive to the type of coagents. Among the
coagents used, HVA-2 shows the best overall balance of mechanical properties. TPVs prepared
using HVA-2 as coagent, exhibit gradual increase and provide superior tensile strength, modulus,
and tear strength values relative to the other coagents used. It is expected that HVA-2 can act as a
reactive compatibiliser as well as crosslinking agent in this particular blend system. It has been
previously reported that HVA-2 can act as a reactive compatibiliser in the NR-PP blend system. It
generates a low degree of crosslinking in the NR phase and forms a block or graft copolymer in
the NR-PP interface. It is clearly seen that, TMPTA shows lower delta torque value and there by
exhibiting higher elongation at break. TAC shows the lowest value and HVA-2 shows the
intermediate value of elongation in TPVs. It was generally accepted that a low crosslink density
compound is indeed accompanied with the higher elongation at break. Since HVA-2 can act as a
crosslinking agent as well as a compatibilising agent, where crosslinking decreases the elongation
and compatibilisation increases the same. In this case both the effects are very sensitive in
determining the final elongation at break.
Morphology
A SEM photomicrograph of the PP/EOC TPVs, in which PP phase was preferentially
extracted by etching with hot xylene, is given in Figure 2. Qualitatively it shows crosslinked EOC
particles are dispersed throughout the PP matrix (droplet and matrix morphology). Since EOC
content is more than PP, the particle-particle association is strong to form aggregates and these
aggregates can agglomerate. The crosslinked rubber aggregates are embedded in the PP
macromolecules via joint shell mechanism and/or segmental interdiffusion mechanism.
Rheological properties
In the solid state, the properties of the TPVs are determined by the matrix molecular
weight (which has a direct consequence on the percent crystallinity and entanglements density),
extent of crosslinking, degree of dispersion, size and deformability of dispersed phase as well as
morphology persist. In the melt state, changes in the morphology originating from matrix
molecular weight (crystallinity) can be excluded and the influence of other factors can be studied.
The rheological properties of 50/100 PP/EOC TPVs prepared by three different coagents are
represented in Figures 3 to 6. Dynamic vulcanization blends show improved dynamic modulus
and viscosity values (dynamic functions) than uncured blends. As expected, addition of coagent
further improved the dynamic functions than the control one. Among the coagents, HVA-2 shows
better dynamic functions and more nonlinear behavior followed by TAC and TMPTA. At equal
formulation volume fraction, smaller particles can impart greater viscosity and more nonlinear
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