Hardening systems

Epoxidized phenol novolak and cresol novolak are the most common curing agents. The composition of the resin and hardener system is optimized for each specific appHcation eg, incorporating phenol novolaks in the matrix resin can iacrease cure speed. 

Comparison of hardening systems Miscellaneous Epoxide Resins... 

Fig.51 Effect of the hardening system in an epoxide resin powder coating on the coloristic properties of the paint. Pigment Dibromoanthanthrone (Pigment Red 168). Cross-linking conditions 15 minutes at 180°C, respectively.

P.Y.139 is sometimes used in conjunction with inorganic pigments for paints, especially to replace Chrome Yellow pigments. The systems are fast to overpainting (up to 160°C for 30 minutes), but they are not entirely fast to acids. The pigment performs very poorly in contact with alkali, therefore it is not suitable for use in amine hardening systems or in emulsion paints which are to be applied on alkaline substrates. 

Materials Description. Three CIBA-GEIGY epoxy/hardener systems were studied Araldite 6010/906, Araldite 6010/HY 917 and Araldite 6010/972 with stoichiometries 100/80, 100/80 and 100/27, respectively. Araldite 6010 was a DGEBA epoxy resin. The hardeners 906, HY 917 and 972 were, respectively, methyl nadic anhydride (MNA), methyltetrahydro phthalic anhydride (MTPHA) and methylene dianiline (MDA). These systems were investigated previously for the matrix controlled fracture in composites (6-8). The curing cycles used can be found in (6). The ideal chemical structures of the systems are shown in Table I. Neat resins were thoroughly degassed and cast into 1.27 cm thick plates for preparation of test specimens. 

The hardener systems are complex. The most employed hardener system contains a three-part system the actual radical-producing molecule, here a cumene hydroperoxide. 

The carboxyl terminated polybutadiene (C-3000) is about equally effective to CTBN in heat distortion temperature and impact but considerably less effective in strength. From the haze data (the percent haze of ERL-4221 modified with 10 phr of CTBN and C-3000 were 17 and 85% respectively) it is quite clear that this elastomer (C-3000) is highly incompatible with the epoxy-hardener system in the cured state. A 2000 molecular weight polybutadiene elastomer, containing no carboxyl groups, was completely incompatible with the epoxy system and segregated in the cured state. 

Zannoni, R., et al. (1991). Limestone hardening system for brindisi s power station. First limestone recarbonation unit installed in Italy for industrial water production. Institution ofChem. Engineers Symp. Series, Proc. Twelfth Int. Symp. Desalination and Water Re-Use, Apr. 15-18, Malta, 2, 125, 351-357. Inst, of Chemical Engineers, Rugby, England. 

This is a simplification of the process occurring in a curing resin-hardener system and a detailed discussion may be found in Pascault et al (2002), Williams et al (1997) and Inoue (1995). The main parameter that it is important to control in the reactive phase separation is the diameter of the elastomer particle. This is because the toughness of the resulting network is controlled by the energy-absorbing mechanisms such as particle cavitation and rubber bridging of cracks. Also of importance is the limitation of the effect of the rubber dispersed phase on the critical properties of the cured epoxy resin such as the stiffness and Tg. This will be affected by the extent to which the rubber dissolves in the matrix-rich phase. 

Figure 3.35. A correlation plot of rheological gel time (from crossover of G and G") and the maximum in chemiluminescence (CL) intensity over the temperature range 140-200 °C for the following epoxy-resin/hardener systems o, Tactix 742/27% DDS A, Shell 1153/22% DDS and Shell 1071/27% DDS. Adapted from Kozielski et al. (1995).

Figure 3.50. The relationship between PCI score and percentage cure as determined by DSC for the epoxy-resin/hardener system DER 332/Jeffamine T-403 in the following mixing ratios o, 100/45 and X, 100/55. Aust et al. (1997). Copyright 1997 by Society for Applied Spectroscopy reproduced with permission of Society for Applied Spectroscopy.

Water Absorption. The water absorption was tested according to ISO 62-1980, method 1 (immersion in water of 23°C for 24 1 hours), using a test specimen, of 50 mm in diameter and a thickness of 3 mm. The water uptake was 0,05 %. Comparable novolak hardened systems have slightly smaller water absorptions in the range of 0,03 to 0,04 %. 

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