(657d) Rheological Properties of Hdpe/CO2 and Ldpe/CO2 Solution Under High Pressure

Rheological
properties of HDPE/CO₂ and LDPE/CO₂ solution under high
pressure

A
high-pressure rheometer contained a rotor was used to determine the effect of
temperature, static pressure and CO₂ concentration on the viscosity
of linear high-density polyethylene (HDPE) and branched-chain low-density polyethylene
(LDPE). First, the molecular parameters of the two polymers were characterized
by high-temperature Size-Exclusion-Chromatography (SEC). The linear HDPE possessed
narrow molecular distribution of the branched chain while branched-chain LDPE
present broad distribution. The CO₂ solubility and diffusivity in
the two polymers was investigated by using magnetic suspension balance (MSB). The
gas diffusion coefficient was used to estimate the time required to saturate
the sample in the gap according to the one-dimensional diffusion equation of
Fick¡¯s second law.

Considered
the potential existence of signal delay in the magnetic driven process and the
frictional resistance, the test reliability was inspected by comparing the
results of the plate geometry with the rotor using dynamic frequency sweep
test. The results indicated that at low frequency range where is critical to
the zero-shear viscosity, the rotor generate a little higher viscosity than the
plate, however, the mechanical delay may overlaps the signal from the sample
results to large error at high frequency range. Another problem that hampered
the test was the stability of the sample after about 38 hours saturation at
high temperature. The molecular parameters were selected to compare with the
virgin pellets, indicated that the degradation was minor and acceptable.

The
effect of temperature on the viscosity was studied using the rotor in the high
pressure chamber with diluent-free polymer though the test may inaccurate
compared with the plate geometry. The activation energy of LDPE is slightly
higher than HDPE due to the relatively high free volume fraction. An inert gas,
helium, was used to characterize the effect of static pressure on the
rheological properties. The pressure shift factor was fitted with Barus
equation, shows that there¡¯s turning-point for HDPE which is also put forward
firstly to the authors¡¯ knowledge. At low pressure (<10 MPa), the pressure
coefficients is slightly higher than LDPE, while at high pressure the
rheological properties is insensitive to the pressure increase. This phenomenon
can be attributed to the fractional free volume. Moreover, the pressure coefficients
of HDPE decrease as temperature increase which is the exact opposite of LDPE.
In addition, The combined effect of CO₂ pressure and concentration
on the rheology was investigated after more than 10 hours¡¯ saturation. Compared
the thickening effect of static pressure, the plasticization of dissolved gas
is predominant of the polymer rheology. Lastly, the effect of gas concentration
and pressure was assumed to be separated, and the F-K model was used to
describe the effect of gas concentration on the viscosity alone.

Figure 1. Schematic of high pressure attachment for the
rheometer, which consist of a magnetic clutch, a pressure cell, a rotor and
stator (dimensions in mm)

Figure 2. The temperature, CO₂
concentration and pressure shift factors, the reference temperature T_r = 150 ^oC,
pressure P_r=1.0 MPa.
where shift factor a=h_0-exp/h_0-ref