Isotactic polypropylene solidification under pressure and high cooling rates. A master curve approach

Measurement of cloud point temperature in polymer solutions

A temperature-controlled turbidity measurement apparatus for the characterization of polymer solutions has been instrumented and set up. The main features are the coupled temperature-light transmittance measurement and the accurate temperature control, achieved by means of peltier cells. The apparatus allows to measure cloud point temperatures by adopting different cooling protocols: low rate for quasi-equilibrium measurements and high rate for detect kinetic effects. A ternary polymeric solution was adopted as case study system showing that cooling rate affects the measured cloud point temperature.

Evolution of Morphology of iPP in Processing Conditions

A model for crystallization kinetics that accounts for the formation of different crystalline phases and is able to describe the morphological characteristics of samples solidified under quiescent conditions, has been enriched to account for the effect of solidification pressure. The effect of pressure was considered by assuming a linear increase of melting and glass transition temperatures (which are involved in the description of the growth rate and nucleation density of the alpha phase). Moreover, pressure was incorporated in the kinetic constant adopted to describe the evolution of the mesomorphic phase. The parameters of the model were identified on the basis of literature data on the distribution of crystalline phases in samples solidified under different pressures. The modified model also satisfactorily described PVT curves up to 100 MPa, and is now able to describe the evolution of morphology during solidification at cooling rates as fast as several hundreds of Kelvin degrees per second and under pressures of as high as 100 MPa.

A Dilatometer to Measure the Influence of Cooling Rate and Melt Shearing on Specific Volume

We developed a dilatometer to investigate the specific volume of polymers as a function of pressure (to 100 MPa), temperature (to 260 oC), cooling rate (to 100 oC/s), and shear rate (to 80 1/s). The dilatometer is based on the principle of confined compression and comprises of a pressure cell used in combination with a tensile testing machine with rotation capability. The design of the pressure cell is a mixture of a traditional ‘pistondie type’ dilatometer and a Couette rheometer, i. e. piston and die make up an annular shaped sample spacing. Typical dimensions of annular samples are: inner radius ri = 10.5 mm, outer radius ro = 11.0 mm, height h = 2.5 mm, and a typical mass of about 60 to 70 mg. Silicon grease is used to reduce loss of hydrostatic pressure in the sample due to friction occurring between the solidifying sample and the dilatometer wall. Specific volume measurements at low cooling rate using an isotactic polypropylene (i-PP) are compared with measurements performed using a commercial bellows type dilatometer, showing relative differences in the range of 0.1 to 0.4%. Finally, results for an isotactic polypropylene are presented showing a profound influence of cooling rate and melt shearing on the evolution of specific volume.

A Novel Dilatometer for PVT Measurements of Polymers at High Cooling – and Shear Rates

A novel dilatometer to investigate the specific volume of polymers as a function of the combined effect of pressure (100 MPa), temperature (300°C), cooling rate (100°C/s) and shear rate (200 l/s) was developed. The dilatometer consists of a pressure cell, which in design is a combination of a traditional “piston-die type” dilatometer and a Couette rheometer, embedded in a custom made frame, which allowed for scaling down to a “table-sized” machine that requires only standard laboratory supplies, like pressurized air and tap water, for operation and cooling. We implemented software for fully automated control of procedures to operate non-isothermal experiments with shear steps applied at predefined temperatures. The sample rings (m ≈ 65 mg) used in the dilatometer are made with a micro injection moulding machine. Experiments with two commercial isotactic Polypropylene (iPP) grades at low cooling rates, performed by two independent groups, were compared with measurements from a commercial confined fluids dilatometer showing small relative differences in the range of 0.03 to 0.3%. As an example, additional results of an isotactic polypropylene were chosen to show the profound influence of cooling rate and melt shearing on the evolution of specific volume.