Since solidification during thermoplastic processing very often involves high velocity gradients, high thermal gradients and high pressures, the development of a model able to describe polymer behavior turns out to be really complex. Due to the experimental difficulties, the study of polymer structure developed under processing conditions has been mainly performed using conventional techniques such as dilatometry and DSC. The investigations possible using these techniques normally involve experiments under isothermal conditions or non-isothermal conditions but at cooling rates several orders of magnitude lower than those experienced in industrial processes, often leading to quite different structures and properties /3/.
4.1 Rapid cooling methods with defined cooling rates
Spherulitic growth rates were studied by Ding and Spruiell /52,53/ based on light depolarizing microscopy at different cooling rates up to 100 K/s. The Polymer between two thin glass plates is cooled down on the microscope sample stage by a gaseous liquid supplied at constant temperature. The temperature of the polymer is measured by a thermocouple embedded directly in the sample. The sample geometry was chosen by heat transfer analysis in order to neglect a temperature distribution across the sample. The light scattering effect occurring when crystallization takes place was observed and the corrected depolarized light intensity was measured in order to study crystallization kinetics.
Malkin et al. /127/ observed the crystallization kinetics of 2-3 cm thick Polyamid 6 (PA6) plates, which were cooled from 205 to 120 °C in 80 min, reaching only a cooling rate of 0.05 K/s. They determined the temperature along the thickness of the sample by means of three thermocouples.
Piccarolo et al. /78/ developed a new experimental set up in order to study nonisothermal crystallization at high cooling rates. A schematic drawing of the apparatus and the samples in the samples holder is shown in figure 4.1. The sample is wrapped in aluminium film to prevent leakage of the hot melt and placed between two metal slabs and heated to the desired temperature by means of an electric heater. After the necessary holding time the sample is placed in the lower part of the box and water is sprayed onto the sample holder in order to cool the sample to below room temperature. The temperature is measured by a fast response thermocouple embedded in one of the metal slabs made of a copper-beryllium-alloy. The slabs are slightly pressed onto the sample by means of metal springs in order to compensate the shrinkage during solidification and to guarantee thermal conductivity. Cooling rates can be varied by changing the cooling medium, its temperature and its flow rate (pressure and spray nozzle geometry) as well as by choosing thicker metals slabs i.e. by acting on the removed heat flux or on the thermal capacity of the sample assembly. The exact cooling rates are determined after the cooling from the temperature-time curves by taking the first derivation at a temperature, which shows the maximum of crystallization rate.
N2outlet Thermocouple
Heaters
H2O inlet H2O inlet
N2inlet
H2O outlet Spray nozzles
N2outlet Thermocouple
Heaters
H2O inlet H2O inlet
N2inlet
H2O outlet Spray nozzles
N2outlet Thermocouple
Heaters
H2O inlet H2O inlet
N2inlet
H2O outlet Spray nozzles
Polymer Cu-Be
T0
b
d 2l
Al Polymer
Cu-Be
T0
b
d 2l
Al
a) b)
Fig. 4.1 (a) Scheme of the experimental set-up for quenching experiments; (b) sample assembly and temperature profiles. b = 1–2 mm; l = 50–100 µm; d = 10 µm /3/.
Moneke /128/ developed an apparatus for rapid cooling of disc-shaped polymer samples of different thickness in a cylinder made by a special isolating material. The sample is heated and cooled by two different pistons subsequently reaching cooling rates of 200 K/s at a pressure of 30 bar. The temperature-time curves are obtained by a thermocouple embedded between two polymer discs at varying distance from 0.01 to 1 mm from the cooling surface.
Volume changes have been determined separately by means of standard dilatometers and temperature conduction was measured. Materials studied were PP, PET homo-polymers and with varying glass fiber content. However the pressure in the cooling experiments was not varied. Also an influence of the thermocouple itself on the crystallization behavior is notable caused by nucleation processes on the surface boundary.
Brucato, La Carruba, et al. /129,130/ designed a new equipment in order to approach typical pressure as used in injection molding and measuring the temperature in the cavity during polymer solidification at the same time. They used a modified injection molding machine in order to supply a pre-determinable and maintainable pressure. The mould consists of two parts; the fixed part with a preheated cavity and a pressure sensor in order to measure the pressure during filling and the movable part containing the cooling liquid system. The cavity is closed towards the spray side with a copper-beryllium diaphragm containing a thermocouple allowing the temperature to be tracked during the cooling process. It is worth noticing that in order to avoid the influence of the temperature of the mould on the temperature measurement during cooling, an insulating ring of a special ceramic material is placed between diaphragm and mould. Finally the hot melt is filled into the cavity. The cooling liquid is sprayed on the diaphragm and the polymer solidifies only in one direction from diaphragm to the sprue. The cooling rate measured at the diaphragm and can be changed either by increasing the thermal mass of the diaphragm (higher thickness) or increasing the heat flux of the spray liquid. Two cooling rates taken at 70 °C, 100 and 20 K/s, at different
pressures (0, 1, 8, 24, 40 MPa) have been reported. The experimental scheme of the mould is shown in figure 4.2.
Coolant Spray
„nozzle“
Beryllium Copper diaphragm
Pressure Sensor
Injection Sprue Thermocouple Heated „Mold“ Cavity
Thermocouple Coolant
Entry
Coolant Spray
„nozzle“
Beryllium Copper diaphragm
Pressure Sensor
Injection Sprue Thermocouple Heated „Mold“ Cavity
Thermocouple Coolant
Entry
Fig. 4.2 Modified injection mould for defined cooling under pressure used by La Carruba /130/