The storage (chemical) aging of a thermoset polymer directly affects its viscosity and consequently its electrical resistance as has been experimentally shown. For liquid resins, the OptiMold system in combination with permanent sensors can be installed in a pressure pot to monitor the aging of the resin within a safety margin. Tests that were contacted by NTUA (figure 1) the storage age of a resin batch is directly related to its resistivity, so depending on the electrical resistance of a batch it is possible to decide if the batch is fresh enough, if the addition of fresh resin is necessary or the batch is too aged for use.
Figure 1. Correlation of electrical resistance with the storage age of a monocomponent resin: resin batches of same storage age correspond to the same zone of electrical resistance.
Accordingly, the storage (chemical) aging of prepregs can be also identified if a small piece of prepreg is placed on a special press for measuring its electrical resistance.
Using an in-mould sensor as can be seen in the figure below, viscosity and resistivity goes together independently of the chemical age of the resin. In the following figure batches with various chemical age, fresh and thermally-aged monocomponent epoxy resins were mixed in various mixing ratios, were tested in a representative heating cycle (80oC/120oC/180oC) measuring simultaneously the resin’s viscosity and resistance. The resin supplier specifications allow the resin for 24 hours at 80oC. In these trials a fresh resin batch (case 1 in black), an ‘in-specs’ aged resin (case 2 in red), an ‘out-of-specs’ aged resin (case 3 in blue) and a 50-50 mixture of case 1 and case 3 resins (case 4 in green) were tested. As can be seen in the same figure, during the first stage at 80oC the resins’ viscosity is directly related to the measured resistance and are in accordance to their ‘degree of aging’. As expected the ‘degree of aging’ affects directly the viscosity of the resins at 120oC (injection phase) as well as the time where resins’ viscosity is below a threshold. The behaviour of the mixture (case 4) coincides well both in viscosity and resistance with the equally aged resin batch of case 2. Furthermore, the direct correlation of the viscosity and the resistance of all the resin batches should be highlighted allowing for the secure prediction of the resin's viscosity by measuring itsresistivity and the temperature of the resin.