Nondestructive testing of defects in adhesive joints

Once the free-radical species (A • ) are formed, they produce cellulose macroradicals via
direct abstraction of hydrogen atom cellulose molecules.
•
•
Cell - OH + A → Cell - O + HA …..……………..(2)
where Cell – OH represents cellulose molecule. Cellulose macroradicals may also be formed by
direct attack of Ce 4+ ions on cellulose molecule via H abstraction.
Cell - OH
4+
•
3+
+ Ce → Cell - O + Ce + H
+
………………..(3)
These cellulose macroradicals, so produced, combine with monomers AAm to induce
graft-copolymerization.
3.1. Effect of reaction temperature
The temperature of the reaction system does influence the rate of graft polymerization and the
grafting field. We varied the temperature of the monomer/crosslinker solution, in the range 10
to 15°C and determined the percent grafting. The results, as shown in the Fig.1 reveal that 30°C
is the optimum reaction temperature for maximum grafting. Below 30°C, the percent grafting is
poor due to insufficient formation of free radicals at lower temperatures which is commonly
observed phenomenon [16-17]. However, when temperature is increased beyond 30°C, the
percent grafting again shows decreasing trend. This may possibly be explained on the basis of
the fact that at higher temperature, the acid catalyst may induce degradation of cellulose chains,
thus lowering the number of grafting sites in substrate. It was also observed that when reaction
temperature was 50°C, the filter paper became brittle and was torn to small pieces indicating
enhanced breakdown of cellulose chains by HNO3. In addition, the possibility of recombination
of free of free radicals at higher temperature should also be not ruled out.
3.2. Water uptake analysis
Fig.2 depicts the water uptake behavior of grafted – filter paper (GFP) in distilled water
at 30ºC. It is clear that when GFP is put in water, it begins to take up water as indicated by
increasing percent mass swelling. This may simply be attributed to the fact that water enters into
grafted polymer network of the filter paper, thus allowing it to absorb water which is retained
within the network. After some time the water uptake attains maximum value.
3.3. Loading of Silver nanoparticles into GFP
As mentioned in the section introduction, the nano silver-loaded filter paper has been
prepared by loading silver nanoparticles into the grafted filter paper, utilizing our newly
developed approach [8]. The overall process of entrapment of nano silver may be explained as
follows: when grafted filter paper is equilibrated in distilled water, the network swells due to
hydrophilic nature of monomer and plasticization of macromolecular chains. On dipping swollen
filter paper into aqueous solution of AgNO3, Ag + ions enter into the swollen grafted network.
Later on when this Ag + containing filter paper is put in the sodium citrate solution, a uniformly
distributed array of Ag nanoparticles is obtained due to reduction of Ag + ions. The crosslinked
three dimensional network serves as stabilizer for silver nanoparticles and prevents them for
aggregation.
Fig.3 clearly describes the color change observed due to reduction of Ag + ions into silver
nanoparticles. It is very clear that filter paper turns brown due to presence of silver nanoparticles.
3.4.Characterization
Fig.4 (A) shows the TEM image of the silver nanoparticles. The image indicates nearly
uniform distribution of silver nanoparticles. In addition, a typical selected area electron
diffraction (SAED) pattern of a collection of silver nanoparticles is also shown (see inset). The
3