Thermoplastic-modified epoxy

Figure 8.9 TEM photographs of thermoplastic-modified epoxy networks. The system is DGEBA - 4,4 -methylene bis [3-chloro-2,6 diethylaniline], MCDEA, with 10 wt% polyphenylene ether, PPE (a) is cured at 135°C (T < Tg,ppE) and (b) at 240°C (T > Tg,PPE). (a) Epoxy-rich substructures are visible inside PPE-rich particles (b) the particle sizes decrease with an increase in cure temperature and a binodal distribution is observed. (From LMM Library.)... 

Several basic morphologies are observed in thermoplastic-modified epoxies and, indeed, other thermosets. Homogeneous [Fig. 6(A), in which no phase separation is observed] and particulate [Fig. 6(B), in which the modifier phase separates to produce small domains] morphologies occur at low concentrations of modifier. In these cases, the thermoplastic modifier is encapsulated within a thermoset matrix, whereas in the phase-inverted morphology, [Fig. 6(C)] the minor thermoplastic component is the continuous phase surrounding large, discontinuous domains of the major... 

There has been much work on characterizing the phase separation and network development for thermoplastic-modified epoxy-resin systems. Initial work (Yamanaka and Inoue,... 

Rheological Monitoring of Phase Separation Induced by Chemical Reaction in Thermoplastic-Modified Epoxy... 

Chen et al. (12) showed that phase separation in elastomer-modified epoxies can also be detected with much more sensitivity using small-angle X-ray scattering (SAXS). Then, the choice of the cloud point as a criterion for detecting the beginning of phase separation can be discussed. We have not performed SAXS studies on our systems because phase separation is much faster in thermoplastic-modified epoxy and the scanning time is too long. 

Figure 12.11 Moisture enhancement of matrix resins Unmodified epoxy Fibredux 924E (o) thermoplastic modified epoxy Fibredux 924T ( ) Fibredux 924T within a composite (estimated) ( ) carbon fibre Fibredux 924C composite ( ) [7].

Thermoplastics are sometimes added to epoxy resins. Thermoplastic-modified epoxy resins [43,44] based on tri- and difunctional epoxy resins cured with DDS and blended with polyethersulfone form the basis for the matrix material in a composite used for the Boeing 777 aircraft. The incorporation of the thermoplastic helps the processing characteristics and also improves the mechanical properties, notably the toughness. The thermoplastic is able to phase separate from the epoxy phase and acts as a reinforcement for the epoxy matrix, enhancing its high temperature properties. The maximum use temperatures of all these resins will typically be 30 to 50 degrees lower than the cited Eg, assuming the same cure schedule. 

A J MacKinnon, S D Jenkins, P T McGrail and R A Pethrick, A dielectric, mechanical, rheological and electron microscopic study of cure and properties of a thermoplastic modified epoxy resin . Macromolecules 1992 25 3492. 

Mur Murakami, A., Saunders, D., Ooishi, K., Yoshiki, T., Saitoo, M., Watanabe, O., Takezawa M. Fracture behaviour of thermoplastic modified epoxy resins. J. Adhesion 39 (1992) 227-242. 

Pea Pearson, R. A., Yee, A. F. Toughening mechanisms in thermoplastic-modified epoxies 1. Modification using poly(phenylene oxide). Polymer 34 (1993) 3658-3670. 

Mac MacKinnon, A. J., Jenkins, S. D., McGrail, P. T., Pethrick, R. A. Cure and physical properties of thermoplastic modified epoxy resins based on polyethersulfone. J. Appl. Polym. Sci. 58... 

Thermoplastic-modified epoxy resins cured with different functionalities amine mixtures. Kinetics and miscibility study. Journal of Thermal Analysis and Calorimetry, Vol.97, No.3, (June 2009), pp. 969-978, ISSN 1388-6150. 

Particulate thermoplastics toughen by crack bridging, crack pinning and by stress concentration which produces shear banding. Amine curing agents are most often used to cure thermoplastic modified epoxies. 

Girard-Reydet E., Pascault J.P., Sautereau H., In-situ studies of phase separation process in thermoplastic modified epoxy networks TEM, SAXS and LS technics, in ref 59 pags 328, 528... 

In the case of thermoplastic-modified epoxies, the proposed role of elongated rigid plastic particles is to span the two fracture surfaces and apply tractions that effectively reduce stress intensity factor applied at the crack tip (42). Additional contribution to the energy dissipation is assumed from the plastic deformation of the inclusions (43). Pearson and Yee (44) suggested that the contribution to... 

Figure 8.10 TEM photographs of thermoplastic-modified epoxy networks. The epoxy system is DGEBA-MCDEA (same as in Fig. 8.9), and the thermoplastic is a polyetherimide, PEI. The figure illustrates both the influence of the cure schedule and the PEI concentration, (a) 10 wt% PEI precured at 80°C and postcured at 190°C (b) 20 wt% PEI, same cure schedule (c) 10 wt% PEI precured at 160°C and postcured at 190°C (d) 20 wt% PEI same cure schedule as (c). Phase inversion is around 20 wt%. (From LMM Library.)...

Figure 11 Mechanical properties of a thermoplastic-modified epoxy (DGEBA/ piperidine). Note that the toughness increases with thermoplastic content while the yield strength remains unchanged.

Compared to rubber, thermoplastic-modified systems show a much more complicated phase separation and structure evolution, and consequently very few studies were initially undertaken related to phase separation and rheological behavior. Examples of these studies included Pethrick et cd. [47,48], who applied a curome-ter to monitor the pot life and gel time of thermoplastic-modified epoxy systems. In 1999, Bormet and Pascault et al. [49] were the first to conduct a study of the relationship between rheological behavior and thermoplastic concentration in pol-yetherimide-modified epoxy-amine systems. By using an isothermal steady shear test with parallel plates at low deformation (1%), these authors showed that the rheological behavior at phase separation was greatly dependent on the initial concentration of thermoplastics. 

While examining other thermoplastic-modified epoxy systems (ABS-modified DBEBA/DDS system), Jyotishkumar and Thomas et al. [67] recently found that this type of exponential growth of viscosity has connections with the thermoplastic content By comparing the ABS content from 3.6% to 12.9%, the reference relaxation time ts values obtained from the stimulation of WLF equation were found to increases with the ABS content It is believed that a lower viscosity of the epoxy/ DDS in a small content of the ABS-modified systems would favor the polymer chains flowing. 

Recently, numerous research groups have shown that nanofillers can have dramatic effects on the morphology and properties of either rubber- or thermoplastic-modified epoxy systems [92-95]. Although such rheology studies are still in progress, it is expected that any differences in viscoelastic behavior would further influence the phase separation process, and so have serious effects on the rheological behavior. 

Mackinnon, A.( Jenkins, S.D., McGrail, P.T., and Pethrick, R.A. (1993) Dielectric, mechanical and rheological studies of phase-separation and cure of a thermoplastic modified epoxy-resin -incorporation of reactively terminated polysulfones. Polymer, 34 (15),... 

Liu, Y, Zhong, X,H and Yu, Y,F, (2010) Gelation behavior of thermoplastic-modified epoxy systems during polymerization-induced phase separation. Colloid Polym. Sci., 288 (16-17), 1561-1570,... 

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