Table of Contents ECNDT '98
Session: Materials characterization
Monitoring of Curing Reaction of Polycondensating Thermosets at Press and Injection MouldingW. Stark1, J. Döring
Federal Institute for Materials Research and Testing,
12205 Berlin, Unter den Eichen 87, Germany
Kiev Polytechnic Institute, KPI-2240, Peremogy Ave. 37, 252056 Kiev-56, Ukraine
Engineering College Iserlohn, Frauenstuhlweg 31, 58644 Iserlohn, Germany
Corresponding Author Contact:
|TABLE OF CONTENTS|
'- dielectric permittivity, ''- dielectric loss, h - ' at high frequency limit, l - ' at low frequency limit, - ohmic conductivity.
Therefore softening and crosslinking should influence the characteristic dielectric values via a change of D and . The first term in c'' indicates that the contribution of ohmic conductivity at low frequencies is high.
Also mechanical behaviour is determined by the mobility of molecular segments which are characterised by a relaxation time. The complex modulus (e.g. shear modulus G*(w)) is therefore also dependent on the measuring frequency:
|Fig. 1: Dielectric equipment for frequencies up to 1 MHz: a) for basic investigations, b for on-line monitoring: 1 - sample holder with temperature regulation, 2 - sample with electrodes, 3 - analyser, 4 - PC, 5 - temperature controller, 6 - press mould form, 7 - movable stamp, 8 - dielectric sensor, 9 - thermocouple, 10 - PC plug in card|
|Fig. 2: Block-diagram of the rectangular waveguide impedance method: 1 - plate-shape sample; 2 - shorted waveguide; 3 - heating or cooling system; 4 - microwave generator; 5 - probe measuring line; 6 - indicator||Fig. 3: Block- diagram of the open-ended coaxial line resonator method: 1 - sample; granulated material, powder, bulk, etc.; 2 - pressform; 3 - coaxial sensor; 4 - heating system; 5 - microwave sweeping generator; 6 - microwave reflectometer; 7 - display.|
|Fig. 4: DSC measurements of 1-original PF51, 2-cured PF51, 3-novolac||Fig. 5: Torsion pendulum measurement of original and cured PF51|
Pendulum measurements show a decrease in modulus which is related to softening begin at about 30 °C. At 120 °C an increase in modulus similar to that at the beginning sets in, which is caused by crosslinking. The loss modulus shows two maxima, typical for high energy loss by passing two times the resonance between measuring frequency and relaxation time.
The same behaviour was observed again in dielectric parameters. Softening of novolac and PF51 causes an increase, crosslinking causes a decrease. The very high ´ and ´´ values at low frequencies hint on high ohmic conduction on softening. High ´ is caused by interface polarisation (Maxwell-Wagner-effect). ´´ is directly influenced by ohmic conductivity.
|Fig. 6: Dieletric spectroscopy of novolac and PF51 as function of temperature|
Ohmic conduction depends on the number of charge carriers and their mobility. Assuming that at softening the number of charge carriers does not increase it is plausible that the existing carriers now become mobile.
Vice versa their mobility decreases by crosslinking. At temperatures over 180 °C there may be an increase in the number of charge carriers, as the modulus does not indicate distinct additional softening.
In the microwave region the fundamental properties of the "pure" substance, mobility of the molecules play the main role and are not masked by the conductivity. To prove the efficiency of the microwave methods we investigated 4 sorts of thermosets, belonging to 2 different types: PF, UF. Increasing of '(T) and "(T) at T > 50 °C corresponds to softening, decreasing at T > 110 °C - to curing. If the microwave and low frequency values of '(T) and "(T) are compared, we can see a big difference in the absolute values: at low frequencies ' 102, " 102 (tan 1); at microwaves ' 3 to 8, " 0.1 to 0.8 (tan 10-2).
Low values of the microwave dielectric permittivity and losses prove that microwave dielectric parameters are defined by the fundamental polarization mechanism, caused by the dynamics and kinetics of the polymer structure parts [5,6]. Two maximaof ´(T) and ´´(T), corresponding to the curing process, are observed in UF and PF. The second maximum of microwave dielectric parameters is caused by a second kind of the crosslinking reaction as seen in Fig. 4 by DSC.
Fig. 8: UF and PF ' variation during press moulding at 160 °C for frequencies, 1 - 1 kHz, 2 - 10 kHz, 3 - 100 kHz
There is an important difference between both kinds of thermosets. UF shows the expected picture: after closing the mould and pressing softening is seen by increase of ´. After a short time the increase of temperature leads to crosslinking, and a decrease of ´ follows, as known from the basic experiments. This is the opposite for PF. Here, softening is also observed, but where a decrease caused by crosslinking is expected, an increase occurs which remains constant over a longer time period. UF and PF crosslinking reaction is a polycondensation reaction where by-products, such as ammonia (PF) or water (UF) are produced. In the mould these by-products have no chance to disappear. In the mould these by-products have no chance to disappear. Obviously in PF the conductivity is considerably increased by this effect so that hardening is masked. Therefore the dielectric method in the low frequency range does not work with all thermosets. Many tests gave the results, that crosslinking monitoring by the medium frequency dielectric method works well with EP, UF (known from literature) and also with UF (unknown before).
|Fig. 7: Temperature dependence of the microwave dielectric parameters of two UF sorts - a, b and two PF sorts - c, d at 10 GHz during curing process (measurements in a rectangular waveguide, samples were prepared from the preformed pressed but not cured polymers).|
Therefore monitoring of UF manufacturing in injection moulding process could be tested. UF is widely used for electrical installation articles. The influence of different production parameters was successfully simulated. Because of the limited space only one characteristic result - the influence of mould temperature - is given in Fig. 9. The sensor is situated at a distance of about 100 mm from the nozzle. That means when the resin comes in contact with the sensor the softening process is over and the material has already begun to cure. The signal reduces towards the end of reaction. There are valuable results for producers because the cycle time may be optimized and reduced.
|Fig. 9: UF131.5 " variation during injection moulding, influence of mould temperature|
One option is to increase the mould temperature, so that the heat transfer rate to the resin is increased in order to reach the starting temperature earlier and also to increase reaction speed. But without insight into the process this can not be done effectively because of the danger of under- or overcuring. Now we have the possibility to watch directly when curing is finished. At 170 °C an important reduction of cycle time seems to be possible.
To find also possibilities for cure monitoring of the thermosets which are unsuitable for low frequency dielectric method we selected methods which should not be influenced by conductive reaction products. These methods should use ultrasound or microwaves. With ultrasound and microwaves we were successful. The ultrasonic results are presented in the same proceedings .
For microwave measurements under realistic conditions a sensor was developed for use in the mould. The measuring arrangement for the open-ended coaxial resonator is given in Fig. 3. Results for PF in rectangular waveguide and at press moulding using the open-ended coaxial resonator are shown in Fig. 10.
Fig. 10: Temperature dependence of the microwave dielectric parameters of PF at 10 GHz during curing process. Curves 1 and 2 correspond to: 1 - measurements in a rectangular waveguide under no pressure, 2 - measurements by the coaxial sensor under conditions, close to the industrial press and injection moulding process, with granualated polymers inserted into the pressform
Both results correlate with each other. Products of the curing reactions do not influence the microwave dielectric parameters.