Nondestructive testing of defects in adhesive joints

Mechanical properties of natural rubber / poly butadiene rubber blends
prepared using fatty acid incorporated natural rubber
ABSTRACT
R. Alex*, T.Cherian, S.Joseph, and K.T Thomas
Rubber Research Institute of India, Kottayam -9, Kerala
Email: rosammaalex2000@yahoo.com
Natural rubber(NR) can be sensitized for quick coagulation by addition of suitable fatty
acid salts as stabilizers. A portion of the fatty acids formed remains on rubber after coagulation
Fatty acids play a major role on cure characteristics and mechanical properties of recovered
rubber. Use of this rubber in blends with polybutadiene rubber (BR) can alleviate some of the
problems of rubber blends like cure mismatch and unequal filler distribution. In this paper an
investigation on the preparation, cure characteristics and mechanical properties of NR/BR blends
prepared using fatty acid incorporated NR is carried out.
Natural rubber latex is treated with a required quantity of fatty acid soap and then
coagulated by addition of suitable acids. The coagulum is washed well to remove acids and then
dried at 70 0 C in an air oven to get dry rubber. Rubber compounds based on pure NR, 80/20 and
60/40 NR/BR blends are prepared using conventional mixing methods. The compounds are
vulcanized and tested as per standard test methods. The fatty acid soaps added to latex get
adsorbed or rubber particles and get converted to the corresponding fatty acids by reaction with
acids during the process of coagulation. Due to presence of fatty acids, the pure and NR/BR
blends show better cure characteristics as revealed from a higher level of vulcanization. Due to
the higher level of crosslinking and better filler dispersion, pure and blend vulcanizates show a
higher modulus, tensile strength, and hardness along with comparable dynamic properties like
heat build-up and compression set in relation to conventional rubber vulcanizates. The presence
of fatty acids also helps in a better dispersion of filler in pure rubbers as revealed from scanning
electron microscopy (SEM) studies. A noticeably higher ageing resistance is also observed for
NR/BR blends prepared by the new process due to the presence of better interaction of filler with
rubber.
INTRODUCTION
Blends based on natural rubber and polybutadiene rubber are extensively used in tire
sector due to the enhanced mechanical properties like low heat build-up, and abrasion resistance
realized in the vulcanizates. The basic problems with such rubber blends are the inherent
incompatibility, unequal filler distribution and uneven crossslinking. In general elaostomer blends
are microheterogenious, the continuous phase being either the polymer in highest concentration
or polymer of lowest viscosity. Filler distribution is influenced by the point of addition of filler,
mixing method, surface polarity of filler and other factors like unsaturatiion , viscosity and
polarity of blend components. The incorporation of 50/50 elastomer preblends indicated that
black affinity decreased in the order, SBR, CR, NBR, NR, EPDM, IIR.[1,2] Curative distribution
is influenced by the level of unsaturation , viscosity and polarity of blend constituents. The type
and nature of colloidal stabilizers retained on rubber during the coagulation process also affect the
cure characteristics and filler dispersion.
Both natural and synthetic rubber latices, (depending on the polymerization technique)
have fatty acid salts as stabilizers. In the case of fresh NR latex the colloidal stability is afforded
by protein anions and to a small extent by fatty acid anions. In NR latex , the sensitivity to
coagulation by acids is controlled by the type of colloidal stabilizers and the fatty acid anions
have more sensitivity to coagulation by acids. [3] By using suitable fatty acid soaps the
coagulation characteristics and non rubber constituents retained in rubber can be adjusted. By this
process the cure characteristics, along with filler dispersion can be controlled so as to have