Density
An important parameter for polyethylene, affecting most end product physical
properties like stiffness, impact strength and optical properties. The density
is normally given in g/cm3, although the correct SI unit would be kg/m3. Two
basic measuring methods are used: (a) Density gradient column is the basic
method for density measurement. In it a density gradient is prepared by mixing
two liquids of different densities so that density increases uniformly from top
to bottom. Small test pieces are immersed into this density gradient, and the
density of the polymer is determined based on the equilibrium position of the
pieces and calibration floats. (b) Pyknometer measurements rely on calibrated
volumes and the density difference to a reference fluid; compression-moulded
samples are normally used here.
Dynamic rheometry
Dynamic rheometry gives information both about flow and elastic properties of
polymer melts. In a dynamic measurement the sample is put between two round
plates or between a cone and a plate in the oven. The system is heated to a
desired temperature and a sinusoidal deformation at different frequencies is
applied.
As a result we get storage and loss modulus as a function of frequency. Storage
modulus is connected with elastic energy, while loss modulus corresponds to
viscose energy. From the above parameters we can calculate complex viscosity as
a function of frequency.
With most of polyolefin products this is the same as the conventional viscosity
function, which is viscosity as a function of shear rate. The dynamic
measurements are limited to relatively low shear rates. On the other hand, a
benefit compared to capillary rheometry is, that we get reliable information
also about elasticity.
In addition, the obtained properties are much more sensitive for small
differences in polymer structure which play an important role in processing and
end-use properties.
Differential Scanning Calorimetry
In Differential Scanning Calorimetry (DSC) the sample and a reference material
are heated and cooled in a defined manner. DSC provides a rapid method for the
determination of the temperature profile of a polymeric material. This includes
e.g. measurement of heat capacity, specific heat, melting, crystallisation and
glass transition temperatures, heat (enthalpy) of fusion, crystallisation, study
of thermal stability or reaction kinetics and in certain cases identification of
the polymer or polymer mixture.
Dynamic Mechanical Thermal Analysis
Dynamic Mechanical Thermal Analysis (DMTA): Many polymeric materials exhibit
time-dependent, reversible viscoelastic properties in deformation. Dynamic
mechanical tests are characterized by an application of a small stress in
sinusoidal fashion and a continuous scan of resonant frequency of vibration and
damping vs. temperature is produced. This is transformed into dynamic storage
modulus, dynamic loss modulus and damping factor by the instrument software.
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