Nondestructive testing of defects in adhesive joints

due to adsorption on rubber particles and convert into fatty acid during coagulation. This
uniformly dispersed fatty acids help in activating cure characteristics of rubber.
3. Mechanical properties .
Soap coagulated rubber recorded a higher modulus, tensile strength, and hardness. Heat -
build up and compression set were comparable to that of control (Table 2) .The NR/BR 80/20
and 60/40 blends also showed a higher modulus tear strength and hardness in comparison with
blends prepared using conventionally obtained NR.
Improvement in mechanical properties is attributed to the formation of higher level of crosslinks
and other interactions involving filler and rubber. Oxygen containing groups present in rubber
have a considerable effect on vulcanization kinetics and have a role in interaction with rubbers. It
is expected that there is better polymer filler interaction involving uniformly dispersed fatty acid
soaps though the actual mechanism is not clear from this study. There are reports that use of
higher dosage of stearic acid in tread formulation enhanced in abrasion resistance. In this study it
is inferred that fatty acids soaps are formed during vulcanization from added surfactant and it acts
as lubricant, reducing the abrasion loss [7]
SEM photographs of tensile fracture and abraded surfaces of NR vulcanizates are shown in
Figures 2 and 3. The tensile fracture surface of NR prepared from soap sensitized coagulation has
a more uniform surface and less matrix removal, as compared to conventional NR fillerwhich
shows enhanced filler dispersion in presence of in situ formed fatty acids. The abraded surface of
the sample from soap coagulation shows fine ridges while the conventional mix has bigger ridges
and foldings. It is known that wear occurs as a result of two processes; local mechanical rupture
(tearing) and general decomposition of the molecular network to a low molecular weight material
(smearing). [8, 9] It is known that molecular rupture under frictional forces followed by
stabilization of the newly formed polymeric radicles by reaction with oxygen or with other
polymer molecules or with other macroradicals take place during the abrasion process. In rubber
obtained by soap sensitized coagulation mainly due to higher levels of crosslinking there is lower
crack growth and removal of the matrix The ageing characteristics are shown in Table 3. The
blends prepared from in situ formed fatty acid has a higher retention of tensile strength and less
variation in modulus and elongation at break as compared to blends prepared from conventional
sheet rubber. The enhancement is attributed to higher level of vulcanization and better filer
dispersion.
CONCLUSION
The carboxylic acid soaps added to latex get adsorbed on rubber particles and are retained in
rubber after coagulation. The fatty acids retained on rubber activate the vulcanization and
improve the mechanical and ageing characteristics of recovered rubber. NR/BR blends prepared
from fatty acid incorporated rubber have higher level of vulcanization, and better mechanical
and ageing properties as compared to blends prepared from conventional rubber.
REFERENCES
1. Corish PJ, Powell B.D.W. Rubber Chem. Technol. 1974:47:481-509
2. Gallen G.E, Hess W.M., Scott, C.E, Rubber Chem. Technol 1971:44: 814-837
3. Blackley, D.C. (1997). Polymer Latices, Science and Technology, Vol. 2. Types of lattices ,
Chapter 9.
4. Alex, R., Premalatha, C.K., Nair, R.B. and Kuriakose, B. (2003). Journal of Rubber Research,
6(4): 221-230.
5. Cockbain, E.G. (1952). Transactions, IRI, 28: 297-302.
6. Franta, I. (1989). Elastomers and Rubber compounding materials manufacture properties and
applications. Elsiever Publications, Chapter6.