Nondestructive testing of defects in adhesive joints

ehavior. Here two possible effects can account: particle size reduction by improvement in
interfacial tension cause rise in viscosity and these small particles have higher tendency to
agglomerate or aggregate, which results in more nonlinear behavior. In other words, presence of
secondary clusters increases the viscosity and enhances the viscoelastic nonlinearity.
Payne effect: For a constant frequency, with increasing the strain amplitude dynamic storage
modulus decreases. In general, the decrease in dynamic functions (nonlinearity) is related to the
disintegration of secondary structures. Indeed it may also relate to the bonding and debonding of
dispersed phase from the matrix phase. TPVs show more progressive nonlinear behavior i.e,
decrease of dynamic functions with increasing strain amplitude is observed (Figure 3). In TPVs
the secondary structure corresponds to the agglomeration or aggregates of crosslinked EOC
particles dispersed in PP matrix. In simple blends the nonlinearity at high strain is due to both the
disintegration of entanglement network and debonding of molecules of PP anchored in EOC
matrix phase.
Modulus recovery: Another important aspect in the mechanism of nonlinearity in TPVs is
restoration of moduli following the large strain amplitude effects. Figure 4 shows the effect of
complex modulus on the subsequent strain sweep experiment results. It is clear that only minor
effects occurred in modulus values. Furthermore, the critical strain amplitude (where the
nonlinearity effect occurs) is slightly moved towards lower amplitude region in the subsequent
strain sweep test. It is anticipated that interfacial slippage between crosslinked EOC domains and
PP matrix, cause to reduce the linear viscoelastic region.
Frequency dependence of viscoelastic behavior: Dynamic frequency sweep tests were conducted
in linear viscoelastic region to further study on network formation and microstructural changes in
detail. Figure 5 shows that the frequency (ω) depends of storage modulus (G’) in unvulcanized
and dynamically vulcanized blends. It is well established that dynamical vulcanization increase
storage modulus values especially at low frequency region. As the frequency increases the curves
become close to each other. HVA-2 based TPVs show higher values in the entire range of
frequency studied. Low frequency improvement in G’ indicates strong interaction between
crosslinked EOC phase and PP matrix. Polymers are non Newtonian liquids and their viscosity
decrease with increasing shear rate. Figure 6 shows the log complex viscosity (ή * ) vs log ω. It can
be seen that the viscosity is highly sensitive with significant drop at higher shear rate, so the
related structure are of pseudoplastic nature. Formation of agglomeration and aggregated
structure of dispersed phase may be responsible for the high initial value of complex viscosity.
The frequency dependence after the strain amplitude sweep was reproducible confirming that the
strain sweep does not significantly deform microstructure and the deformation induced is
reversible.
Conclusion
Comparative studies of the mechanical, microstructure and rheological properties were
carried out in PP/EOC TPVs prepared by three different coagents. Among the various coagents
taken for the investigation, most interesting properties were observed for HVA-2 containing
TPVs. It is found to give better solid and melt state properties, which may be attributed to the
strengthening of interfacial adhesion between blend components. HVA-2 is shown to effectively
behave as a crosslinking agent and compatibilizer for the PP/EOC blend system and thereby
improvement is significant.
References
1. Coran, A.Y. In: Thermoplastic Elastomers - A Comprehensive Review; Legge, N. R.;
Holden, G. Eds.; Hanser Publisher: Munich, 1987.
2. Naskar, K. Rubber Chem. Technol 2007, 80, 504.
3. Henning, S. K.; Costin, R. Fundamentals of curing elastomer with peroxide and coagents.
Paper presented in Spring 167 th ACS technical meeting.
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