Nondestructive testing of defects in adhesive joints

enhancement in mechanical properties. There are no systematic reports on use of in situ
formation of fatty acids that can affect cure characteristics, filler dispersion and hence
mechanical properties of rubber blends.
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.
EXPERIMENTAL
Fresh Natural rubber (NR) latex used in this study was obtained from Rubber Research
Institute of India. Poly butadiene rubber (CISAMER ) was obtained from Indian Petrochemicals
Corporation Ltd, Vadodara , Other ingredients used were rubber grade chemicals .
Preparation of rubber by fatty acid sensitized coagulation of NR latex
NR latex was mixed with the quantity of fatty acid soap required for sensitization to
quick coagulation as reported earlier [4]. The latex was then diluted to a dry rubber content of
20% and coagulated by addition of 10% acetic acid. The coagulum was washed free of acid and
dried at 70ºC in a laboratory oven. The control NR was prepared as per the conventional method
of sheet preparation. .The NR prepared by the two methods were blended with BR in 80/20 and
60/40 proportions by conventional mixing method as per formulation in Table 1.
Cure behaviour, mechanical properties and SEM evaluation
The vulcanization characteristics were determined using moving die rheometer
(RheoTech MD ) Test samples for determination of mechanical properties were vulcanized to
optimum cure time in a hydraulic press at 150 0 C. Filler dispersion in rubber was assessed using
a JOEL model scanning electron microscope. Tensile fracture and abraded surfaces of
vulcanizates were coated with gold to conduct SEM study
SEM was carried out using a scanning electron microscope model using sputter coated samples.
Analysis was conducted on fractured surfaces and abraded surfaces.
The mechanical properties were determined from relevant ASTM standards.The
ageing tests were carried out as per ASTM method after ageing the samples at 100 0 C /3days.
RESULTS AND DISCUSSION
1. Effect of fatty acid soaps on coagulation of latex
Fatty acid sensitized fresh NR latex coagulated immediately on addition of acids. On
addition of fatty acid soaps to latex they cause displacement of proteins and gets strongly
absorbed on rubber particles. In this way the protein stabilized latex gets transformed into a soap
stabilized system. On addition of acids to soap treated latex the adsorbed soap anions react with
acid to form un-dissociated fatty acid, and deprive the latex particles of stabilizers. As a
consequence, latex coagulates immediately. [3,5]
2. Vulcanization characteristics
The cure characteristics of the blend mixes along with pure NR mixes are given in Figure
1.The rubber recovered from soap-sensitized coagulation showed better overall cure
characteristics as compared to conventional mix. It had a higher level of cross slinking and a
faster onset of cure which is attributed to the fatty acids retained on rubber. There was
enhancement of level of vulcanization in 80/20 and 60 /40 NR/BR blends.
As an activator of vulcanization ZnO requires sufficient amount of fatty acids which
converte it into rubber soluble form . Though NR contains a certain amount of these acids it is
usually insufficient. Therefore their contents are adjusted to the required level by addition of
commercial stearic acid which can be replaced by lauric acid that is more soluble in rubber[6]
Hence it is inferred that when fatty acid is added as soap to latex it disperses uniformly in latex