JEC COMPOSITES MAGAZINE - Issue #63 - March 2011 - 89

recommended amount of cardanol-based novolac was found to be 18 phr. All the thermograms reported in Figures 1 and Figure 2 bear the evidence of the polymerization mechanisms described above. The profiles include a low exothermic peak occurring at lower temperatures and a sharper peak at higher temperatures. The first peak results from the reaction between the epoxy groups and the phenolic hydroxyl groups; the following overlapping peak denotes that the secondary hydroxyl groups, formed during the former reaction, have reacted with the remaining epoxy groups as well. The comparison of the data reported in Table 1 and Table 2 shows that, when using the same epoxy molar amount of catalysts, the Ep60-Nov18 system has a higher transition temperature than the Ep50-Nov18 system. This is easily explained by a higher cross-linking density of the former system, in virtue of its lower overall epoxy equivalent weight and, thus, its higher number of reactive groups per unit mass. Moreover, although the polymerization enthalpy per unit mass rises with the catalyst amount for both systems, the transition temperatures follow a different trend: maximum values are reached at 5% of the epoxy molar amount of imidazole for both resins. When a catalyst amount higher than 5% is used, the number of activated reactive groups is too high with respect to the non-activated groups, resulting in a lower cross-linking density, as discussed in previous studies (5-8). Due to its high Tg and acceptable tackiness at room temperature, the Ep60-Nov18 system with a catalyst molar amount of 5%m was chosen to produce, first, the unidirectional carbon fibre prepreg tape and, then, the composite laminate, whose features are discussed in the following paragraphs.

Rheological measurements A preliminary measurement of the uncatalyzed Ep60-Nov18 system’s viscosity at different temperatures was carried out in order to set Fig. 3: Viscosity of the uncatalyzed Ep60-Nov18 up the correct system at different temperatures parameters for manufacturing the prepreg tape. Since viscosity values ranging from 40,000 to 70,000 cPs are recommended for the hot melt process, resin temperatures dropping within the range of 5055°C were found to be ideal for the prepreg tape manufacture, as shown in Figure 3.

Izod impact test The Izod impact strength of specimens made from the Ep60Nov18 system, cured at 120°C for 1 hour, was evaluated in order to demonstrate the toughening effect of the cardanolbased novolac on the epoxy resin. For comparison purposes, a traditional amine-cured , epoxy system, named Ep50+Am, was also , prepared and tested. This , system is based on a 50/50 weight ratio , Ep1/Ep2 blend cured with aromatic amines. It , is commonly used by Technologycom s.r.l. for , , the manufacture of certain prepregs. The , results are shown in Figure 4. The impact resistance improved by 163% when using the cardanol-based novolac as Fig. 4: Izod impact test results hardener for the epoxy blend. This behaviour can be explained by the fact that the alkenyl chain of cardanol hinders the reciprocal movements of the broken macromolecules while the crack spreads throughout the specimen. Prepreg manufacture A unidirectional carbon fibre prepreg, using the Ep60-Nov18 system as the matrix, was manufactured by the hot melt process that usually consists of two different steps: filming and impregnation. During the filming step, the liquid resin is cast on a siliconed paper tape and spread in a uniform thickness when it goes through a set of compaction rolls. In the subsequent impregnation step, two resincoated paper tapes, coming from two different delivery spools, are pulled while the reinforcement is pressed between them. The fibres are progressively wetted with the resin as the tape goes through the compaction rolls. Afterwards, one of the paper tapes is peeled off and the other, that brings on the prepreg, is wound around a take-up roll. The prepreg is then stored at low temperature. The prepregs discussed in this work were manufactured by Technonologycom s.r.l. technicians, using a CD 6015-300 tape
Fig. 5: Resin filming and prepreg plant

No63 March 2011 / jec composites magazine

Research & development

JEC COMPOSITES MAGAZINE - Issue #63 - March 2011

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #63 - March 2011