Nondestructive testing of defects in adhesive joints

Effect of pristine nano silica filler on oxidative degradation characteristics of
EVA/LDPE blends
The control blend (EL6/4) exhibited greater thermal stability than the pure EVA and LDPE
[Figure 1(a)]. Also, EVA was also found to be thermally more stable than LDPE. Thus in
presence of EVA, LDPE was stabilized. In all silica filled blends there were two distinct and well
separated steps in the thermogravimetric curves (TG) (corresponding weight loss peaks in
derivative thermgravimetric, DTG) curves as represented in Figure 1 (b) and (c). The first step
(309-315 0 C) for all samples (unfilled and filled) was possibly due to de-acetylation of vinyl
acetate group of EVA with the elimination of acetic acid. As a result, double bonds were formed
[10] in the main chain. The second step (413-418 0 C) might be assigned to the further degradation
of polyacetylene-ethylene chains formed in the first step accompanied with the degradation of
LDPE [10].
As compared to inert atmosphere, it was observed that the initial stages of degradation for
all samples were accelerated in the presence of oxygen [Fig. 2(a)]. Thus the rate of reaction might
be controlled by O2 diffusion in the polymeric matrix. There was also an initial gain in weight
prior to deacetylation in the presence of oxygen. This suggested a rapid initial oxidation of the
blends. Due to the presence of SiO2 particles, the onsets of degradation of filled blends were
shifted towards higher temperatures as compared to the unfilled one. Therefore, the filled systems
were thermally more stable than the pure blend. But interestingly, at very high temperature (~
above 425 0 C) the presence of nano silica accelerated degradation of filled blends. This might be
due to the fact that at these temperatures silica acted as an acid catalyst. The outcomes from TGA
are represented in Table 2. As compared to samples prepared following sequence-2,
ELS6/4/3-1 exhibited higher thermal stability at lower temperature ranges. This indicated
the occurrence of intermixing of LDPE and EVA in ELS6/4/3-1. In TEM observations
also silica particles were found to be dispersed in both phases as well as in the interface
for this system. Si69 increased the compatibility between two phases (in ELS6/4/3-2-
Si69) and made silica well dispersed in the polymeric matrix. Hence, ELS6/4/3-1 and
ELS6/4/3-2-Si69 exhibited greater thermal stability than the rest of the samples. The
degradation behavior was well correlated with the morphology analyzed by TEM and
FESEM. The details of the analysis are not given in this brief article. It will be mentioned
during the presentation.
The iso-conversional integral method named Flynn-Wall-Ozawa method suggested independently
by Ozawa [11] and Flynn and Wall [12] uses Doyle’s approximation [13] of the temperature
integral. From Eq. (1) and using Doyle’s approximation, the result of the integration after taking
logarithms is:
log β = log (AE/g (α)R) - 2.315 – 0.457 E/RT.......................(1)
where β is the heating rate, A is the pre-exponential factor, E is the activation energy, α is the
degree of conversion and T is the temperature. Thus, for α = constant, the plot of log β versus
1000/T obtained at several heating rates should yield a straight line whose slope can be used to
evaluate the activation energy.
All the filled samples showed higher activation energy of degradation than the control sample.
This implied that the thermal stability of the silica filled nanocomposites was higher than the neat
control blend. From plots (not shown in text) it can be observed that the best fitted straight lines
(with at least 90% correlation) were nearly parallel to each other and thus confirmed the
applicability of this method with in the conversion range studied. Interestingly, mean activation
energy of ELS6/4/3-2 was higher than that of ELS6/4/3-1. The kinetic analysis was performed by
both dynamic and static methods and the results were well correlated. However, the details of
kinetic analysis will be discussed during presentation.
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