Nondestructive testing of defects in adhesive joints

11 . Skim rubber and DPSR differed most obviously from ISNR 20 in the high levels of fatty acid and
protein content and hence the acetone extractable of skim rubber and DPSR were higher than that of
ISNR 20. Mooney viscosity of the three samples was within the processable limit.
Cure characteristics and mechanical properties
Cure characteristics and mechanical properties of mixes 1, 2 and 3 are given in Table 3. The
results showed that there is attainment of good level of cure and cure rate even after addition of 20 phr
skim rubber or DPSR to ISNR 20 with no extra curatives, compared to the conventional ISNR 20 mix.
This shows that both in skim rubber and DPSR there is sufficient quantity of amines and proteins that
have an accelerating effect on vulcanization. ISNR 20 vulcanizate and the vulcanizate containing
additional 20 parts of skim rubber showed comparable values of moduli, tensile strength, tear strength
and abrasion resistance and these values were higher than that observed for ISNR 20 vulcanizate
containing 20 parts DPSR. Skim rubber incorporated mix showed slightly higher resistance to crack
initiation and crack failure when compared to ISNR 20 mix. However, comparable flex resistance was
obtained for ISNR 20 mix and DPSR (20 parts) incorporated mix. It is reported that the proteins
present in skim rubber can improve the modulus and resistance to flex cracking of rubber vulcanizate
owing to its reinforcing characteristics 12 . Comparatively lower values of resilience, higher heat buildup
and compression set were recorded for the mix containing 20 parts of skim rubber. The proteins
present in skim rubber are known to adversely affect dynamic properties like compression set,
resilience and heat build-up. Blend which contains 20 parts of DPSR showed slightly better dynamic
properties compared to normal skim rubber. The improvement may be attributed to the reduction of
protein content and partial removal of unsaturated fatty acids. The above results showed that
incorporation of 20 parts of skim rubber or deproteinized skim rubber to ISNR 20 imparts cure
characteristics and mechanical properties similar to that attained by ISNR 20 without the
incorporation of additional curatives.
Cure characteristics and mechanical properties of mixes 4, 5 and 6 are also given in Table 3.
Cure characteristics were comparable but the values were lower than that which contains the usual
dosage, i.e. for mixes 1, 2 and 3. Same trend was observed for the mechanical properties.
The ageing resistance in modulus and tensile strength of the vulcanizates after ageing at 70°C
for 7 days are given in Figs. 2 and 3. For mixes 1, 2 and 3, retention of modulus and strength for the
blend which contain DPSR were better than ISNR 20 mix and the blend containing skim rubber. The
improvement in ageing characteristics may be attributed to the partial removal of unsaturated fatty
acids and metal ions that are pro-oxidants during deproteinization and creaming process. For mixes 4,
5 and 6 retention of modulus and strength were comparable but the values were lower than that
obtained for the mixes 1, 2 and 3.
Conclusions
Incorporation of 20 parts of skim rubber or deproteinized skim rubber to ISNR 20 imparts
cure characteristics and mechanical properties similar to that attained by ISNR 20 without the
incorporation of additional curatives. Skim rubber incorporated mix showed a slightly higher
resistance to crack initiation and crack failure when compared to ISNR 20 mix. Blend which contains
20 parts DPSR showed slightly better dynamic properties as compared to normal skim rubber. For the
system which contains the normal dosage of curatives, retention of modulus and strength for the blend
which contain DPSR were better than ISNR 20 mix and the blend containing skim rubber.
References
1. Subramanian, A.RRIM Technol Bulletin, 1980, 4, 1.
2. Yeang, H. Y, Eshah, Y, Samsidah, H. J Rubb Res, 1995, 10, 108.
3. Smith, M.G. J Rubb Res Malaya, 1969, 23, 1, 70.
4. John, C.K, Sin, S.W. J Rubb Res Inst Malaya, 1973, 23, 4, 257.
5. Morris, J. E. Proc 3 rd Rubb Tech Conf, London, June 1954, 13.
6. Sakdapipanich, J.T, Nawamawati, K, Tanaka, Y. J Rubb Res, 2002, 5, 1, 1.