OSCILLATING DISC RHEOMETER
PRINCIPLE :
The oscillating Disc type Rheometer is an efficient, simple and reliable testing equipment. It is quite easy to operate. The Rheometer describes precisely and quickly curing & processing characteristics of vulcanizable rubber compounds. It works on a very simple principle.
A test piece of rubber compound is contained in a sealed test cavity under positive pressure and maintained at a specified elevated temperature. A Rotor (biconical disc) is embedded in the test piece and is oscillated through a small specified rotary amplitude. This action exerts a shear strain on the test piece and the torque (force) required to oscillate the disc depends upon stiffness (shear modulus) of the rubber compound. The stiffness of the specimen compound increases when crosslinks are formed during cure. The direct proportionality posited between the shear modulus and the cross linking density is based on the statistical theory of rubber elasticity (cf. Nitzsche/wolf: struktur und physikalisches verhalten von kunststoffen, Berlin, Heidelberg, Vienna: Springer - Verlag, 1962, pp.234ff).
It gives the equation:
G = v.R.T.
where
G is shear modulus, in n/MM²
v is cross linking density, in Mol/MM3.
R is the universal gas constant, 8.313 J/(Mol.k)
T is the thermodynamic temperature, in K
If strain is directly proportional to the force applied then:
t = G.y = v.R.T.y
where
y is shear strain, equal to the tangent of the shear angle.
t is the shear stress in n/MM².
if T and y are constant, then t is proportional to v.
At a constant temperature of the test, a cross linking isotherm is the function of time of that property which serves to measure the course of the cross linking reaction. In the context of Rheometer, the cross linking isotherm is thus the function of time, of the oscillating shear force F, or of the Rheometer indication proportional to it, occurring at a given temperature as a result of vulcanization and expressed as F = f(t)
where t is vulcanization time.
A complete “Cure Curve” is obtained when the recorded torque value either increases to an equilibrium value or a maximum value. The time required to obtain a “Cure Curve” is a function of the test temperature and the vulcanization characteristics of the Rubber compound specimen.