Technology


The DC-based real-time Tg and degree of cure prediction is a unique accomplishment which can revolutionize the whole range of composites manufacturing.

Process Monitoring using DC-based Sensing Tools

The use of DC-based conductivity in cure monitoring has beenexplored by several researchers [1, 2] while an intersting and extended report on using the DC-based cure monitoring for vinylester resins can be found here. The DC-based cure monitoring was always tempting for industrial cure monitoring as more recent research results on using DC-based electrical measurements for cure monitoring in aerospace composites can be found in [3, 4]. However besides the numerous advantages of using the DC-based cure monitoring in composites the main obstacle was the lack of reliable equipment to measure such a low conductivity materials and it was this reason that drove researchers to use AC-based conventional dielectric equipment where a range of sinusoidal electrical excitations are applied to the electrodes of a sensor which are in contact with the material under investigation. Although significant effort has been devoted in this technology for more than 30 years only laboratory scale equipment and limited industrial applications exist.

It was only in 2009 that the first DC-based cure monitoring system was introduced by Synthesites with a clear focus on composites manufacturing. The system that has been developed [5] measures directly the resin's resistivity and the temperature using proprietary sensors, advanced electronics and proprietary software and is capable of the in-situ monitoring of the full transformation of thermoset or thermoplastic resins i.e. from very low viscosities at high temperatures to fully cured resins at room temperature with the resistance ranging from 10^5 Ohm (100 KOhm) up to 10^14 Ohm (100 TOhm). Through the years, this technology has been adopted by major R&D and University centres across Europe but what is the most promissing is the adoption of Synthesites technology by major industrial players in aerospace, automotive and wind energy sectors.

Comparison between the DC sensing and commercial dielectric systems both using durable sensors showed the superiority of the DC sensing particularly after gelation where conductivity is measured by the DC-based system in a more reliable and faster way. Furthermore, the DC sensing is relatively cheaper and requires simpler sensors which can be more flexible in geometry and robust and can be installed in several locations in the mould, in the die, in the feeding or in the evacuation lines so a global process monitoring is possible. Last but not least, in contrast to the through-thickness measuring nature of the dielectric systems, the DC sensing is less vulnerable to carbon fibres in the cavity due to its inherited “surface” measuring nature so it may be used in industrial production of carbon fibre parts even without protection. All our cure sensors have an integrated temperature sensor providing the temperature of the resin which is absolutely necessary in conjunction with its conductivity for the calculation of the resin state.

As can be seen in figure 1, viscosity is directly related to electrical resistance (reversely to ion viscosity). In a special equipment which measures simultaneously the viscosity and the resistivity of a resin at controllable temperature 4 repetitive trials were performed with neat epoxy resin that was not reacting. As can be seen in figure 1 both viscosity and resistance are degreasing together as temperature was rising.

Figure 1. Simultaneous measurements of electrical resistance and viscosity of 4 repetitive trials with changing the temperature of neat epoxy resin (unreacted).

Correlation between Electrical Resistance and Processing Parameters

Viscosity is the most important property for the first step of composite moulding which is the fibre impregnation. During this step it is important to maintain the viscosity below a certain limit in order to ensure good product quality. Using this DC-based monitoring system it is possible to monitor this viscosity in real-time and in the mould in order to check that the fibre impregnation is progressing as planned. After that it is important to identify the gelation and the end of cure.

Figure 1 shows the simultaneous comparison between the measured viscosity, the degree of cure and Tg evolutions versus the measured electrical resistance for low-temp epoxy resin. Tests were performed at lab scale conditions for a popular epoxy resin by Tecnalia showed that the evolution of viscosity at the early stages of the process can be directly

Figure 2. Electrical resistance, degree of cure, Tg and viscosity correlation at an isothermal curing of an epoxy resin (Isabel Harismendy, Tecnalia).

Figure 3. Electrical resistance of an epoxy system at three isothermal cure cycles and the corresponding Tg evolution (resin datasheet).

correlated to resistance. Then, there is a point where a sharp increase of viscosity occurs due to curing and the two curves start to diverge. This divergence becomes more obvious at the calculated gelation point at a=0.69 where the resistance curve starts to follow the calculated degree of cure and Tg. If we compare the 3 models with the signal, in this particular case, it can be concluded that from a=0 to a=0.3 the DC monitoring system provides information about the viscosity evolution and from a=0.3 to the end of the reaction the signal can be correlated with the evolution of the degree of cure or even better, with Tg. When vitrification occurs and reaction stops (da/dt=0), resistance curve flatens and depends only on temperature.

The direct correlation between the electrical resistance and the Tg build-up in the curing of thermoset resins has been extensively studied (see for example [4]) and is clearly demonstrated in figure 3 for a typical epoxy system in isothermal cures at different temperatures.

On the other hand research based on dielectrics has failed to extract in real-time accurate information such as Tg or degree of cure in a reliable way especially if carbon fibres are involved in the process or temperature is not constant.

Synthesites has developed and applied successfully an unique intelligent algorithm for the real-time calculation of the Tg and degree of cure that can be used in all industrial conditions no matter if carbon fibre is used (figure 4).

In a series of demo trials of the production of a large cfrp part with sharp exotherms  it was proven that the Tg calculated in real-time in all of the six sensors used in each trial was within the statisitcal error (±5%) from the Tg calculated by DSC after demoulding as can be seen in figure 5.

Figure 4. Isothermal curing of Epoxy resin: Correlation between resistance, Tg real-time prediction and Tg as provided in the TDS.


Figure 5.  Comparison between Tg real-time predicted by Optimold and DSC-measured after demoulding for all six sensors.

 

 REFERENCES

[1] Tajima Y. Monitoring Cure Viscosity of Epoxy Composite. Polymer Composites, 3, pp.162-169 (1982).

[2] Schwab S., Levy R., Glover G. Sensor System for Monitoring Impregnation and Cure During Resin Transfer Molding, Polymer Composites 17, pp. 312-316 (1996).

[3] G.Boiteux, P.Dublineau, M.Feve, C.Mathieu, G.Seytre and J.Ulanski, Dielectric and viscoelastic studies of curing epoxy-amine model systems, Polymer Bulletin 30, 441-447 (1993).
[4] C. Garschke, C. Weimer, P.P. Parlevliet, B.L. Fox, Out-of-autoclave cure cycle study of a resin film infusion process
using in situ process monitoring, Composites: Part A 43 (2012) 935–944.

[5] Pantelelis N., Bistekos E.Process monitoring and control for the production of CFRP componentsin “Proceedings of Conference SAMPE’10”, Seattle, USA (2010)

[6] Stephan F., Fit A. and Duteurtre X. In-Process Control of Epoxy Composite by Microdielectrometric Analysis, Polymer Engineering and Science 37, pp.436-449 (1997)