DGEBA/CTBN

From this, the time scale for diffusion of the rubber particles to coalesce to form a particle may be estimated from the SEM data showing the distance, L, between phase-sepamted particles. This was found to be of the order of 10 pm in the DGEBA-CTBN system (Manzione and Gillham, 1981). Since... 

The simple thermodynamic model derived in Sect. 2.1 has been useful to get a qualitative insight into the phase separation process. When one intends to apply it to an actual system, the significant influence of polydispersity is clearly evidenced. For example, Fig. 13 shows the experimental cloud-point curve for a DGEBA-CTBN binary mixture, together with binodal and spinodal curves calculated by assuming monodisperse components [66] (curves are arbitrarily fitted to the critical point). The shape of the CPC and precipitation threshold temperature (maximum of the CPC) appearing at low modifier concentrations are a clear manifestation of the rubber polydispersity [77]. 

Spinodal decomposition creates a unique morphology in that the thickness of the interface between coexisting phases increases as the temperatirre approaches the spinodal temperature, i.e., the temperature at the spinodal curve. Spinodal decomposition leads to periodic structines with some d ree of cormectivity. This theory makes predictions about particle size which have been validated for mixtures of di(2,3-epoxypropoxy)- 2,2 -bis(4-hydroxyphenyl)propanefcarboxyterminated-butadiene acrylonitrile/polyamide (DGEBA/CTBN/polyamide) (11). 

DGEBA/CTBN resin studies were made with DGEBA cured with 5 parts per hundred piperidine reinforced with 5, 10 and 15 parts per hundred CTBN rubber cured at 120 C for 16 h. It was found from this study that there is epoxy present in the separated rubbery particles, and that rubber is dissolved in the epoxy matrix. 

With some elastomer systems, such as the DGEBA/CTBN/polyor propyleneamine system, there is an intermixed boundary between the elastomer and the epoxy phases which provides a graduated modulus. This type of intermixed boundary has been proven to increase interfacial bonding and reduce stress concentration between the phases (10). 

In another study (10) of the DGEBA/CTBN system, choice of curing agent was found to influence of the rubbery phase. When curing is done with a polyamide, no... 

Morphology of TETA-DGEBA-CTBN systems (a) low-resolution image showing the distribution of CTBN phases in the matrix (b) enlargement of one of the CTBN phases showing the included epoxy nodules within the CTBN phase. 

Figure 8.1 Cloud-point temperatures versus volume fraction of modifier, for mixtures of diglycidyl ether of bisphenol A, DGEBA (n = 0.15) with two CTBN copolymers with different acrylonitrile content 18 and 10 wt%. (Reprinted from Verchere et ai, 1989, Copyright 2001, with permission from Elsevier Science)...

Figure 8.7 SEM photograph of a fully cured rubber-modified epoxy network. The rubber CTBN (26 wt% AN) is first pre-reacted with a large excess of diglycidyl ether of bisphenol A (DGEBA) to obtain an epoxy-terminated rubber. Then an equivalent of 15 wt% initial CTBN is introduced in DGEBA-4,4 -diamino diphenyl sulfone, DDS, system precured at 135°C (time > tgei) and then postcured at 230°C. Rubber-rich particles are spherical, D 2.8 0.5 gm, and well dispersed. (From LMM Library.)...

Figure 13.8 Fracture energy of epoxy networks cured with DDS (diamino dipheny sulfone) versus the initial DGEBA (diglycidyl ether of bisphenol A) ( ) neat systems ( ) with 10% CTBN (27% AN). Rubber = CTBN carboxy-terminated butadiene acrylonitrile random copolymer. (Pearson and Yee, 1989 with kind permission from Kluwer Academic Publisher.)... 

Another approach for improving adhesion is to introduce a small amount of a third component that may simultaneously be dissolved in the TP and form chemical bonds with the thermoset. One example is the addition of CTBN in a PSF DGEBA DDS system, where CTBN is preferentially dissolved in the PSF phase but can react with the epoxy groups, promoting adhesion between both phases (Pascault and Williams, 2000). 

Woo et al. (1994) studied a DGEBA/DDS system with both polysul-fone and CTBN. The thermoplastic/rubber-modified epoxy showed a complex phase-in-phase morphology, with a continuous epoxy phase surrounding a discrete thermoplastic/epoxy phase domain. These discrete domains exhibited a phase-inverted morphology, consisting of a continuous thermoplastic and dispersed epoxy particles. The reactive rubber seemed to enhance the interfacial adhesive bonding between the thermoplastic and thermosetting domains. With 5 phr CTBN in addition to 20 phr polysul-fone, Glc of the ternary system showed a 300% improvement (700 Jm-2 compared with 230 J m 2 for the neat matrix). 

The epoxy-nitrile adhesives were introduced commercially in the late 1960s. They consisted primarily of DGEBA epoxy resin modified with carboxyl-terminated butadiene nitrile (CTBN) rubber. These first nitrile copolymers were available from B.F. Goodrich under the trade name of Hycar. The most convenient form of epoxy nitrile adhesive, especially when one is bonding large parts (aircraft structures), is a supported film. However, solvent solutions of epoxy-nitrile adhesives have also been commercially available. 

Hycar CTBN-Bisphenol A-Epon 828 Cast Samples. A standard recipe follows 100 parts DGEBA resins (Epon 828), 24 parts bisphenol A, five parts liquid rubber (CTBN), and five parts piperidene. It is often desirable to have a stock mixture of bisphenol A and Epon 828. Since the shelf stability of the mixture is poor at room temperature, it should be refrigerated. Melt the bisphenol A in Epon 828 with stirring... 

Fig. 4 Plot of fracture toughness vs. resin molecular weight for neat and CTBN-modified DGEBA-DDS. (From Reft f)...

The separate phases will be rich in one component but may have the other present as a minor component. In order to control compatibility the elastomer may have reactive end groups to enhance interfacial adhesion. A common example in epoxy-resin technology is the carboxy-terminated butadiene-acrylonitrile copolymer (CTBN). The structure is shown in Scheme 1.47. In this resin the solubility in the epoxy resin is conferred by the acrylonitrile group, and an increase in the fraction present decreases the upper critical solution temperature, with 26% acrylonitrile conferring total miscibility of CTBN with a DGEBA-based epoxy resin (Pascault et al, 2002). 

In a fundamental study of the factors affecting the growth of the rubber domains in a CTBN-toughened DGEBA epoxy resin cured by piperidine (Manzione and GilUiam, 1981) the kinetics of phase separation were linked to the diffusivity, Uab, of the rabber (A) dissolved in the epoxy resin (B). The relevant dependence on the molar volume of the rubber (proportional to the radius of the rubber molecules when dissolved in the epoxy resin) at the viscosity for the temperature T of reaction is given by the Stokes-Einstein equation ... 

Pearson and Lee (1991) examined the effects of particle-size and particle-distribution effects on rubber-toughened epoxy resins. They examined a variety of CTBN liquid rubbers and a methacrylated butadiene styrene core-shell particle in a DGEBA-piperidine system. They found that the toughening mechanism for small particles was internal cavitation of the... 

The material used was a diglycidyl ether of bisphenol A (DGEBA) based epoxy resin (Ciba-Geigy, GY250) cured using stoichiometric amounts of 4,4 -diamin-odiphenyl sulfone (DDS). The rubber used for the modifications was Hycar car-boxy-teminated butadiene-acrylonitrile (CTBN) rubber (1300 x 13). The curing schedule for all the rubber-modified epoxy-DDS systems was as follows first the rubber and then DDS were mixed with the epoxy resin and stirred at 135 °C until the DDS was dissolved the systems were cured for 24 h at 120 °C and then postured for 4 h at 180 °C. The control epoxies were cured according to the same schedule. 

Chen et al. (20) found that the miscibility of CTBN with DGEBA decreased with the decrease in the AN content of the rubber. 

In this paper, we report the results of a morphological characterization of a DGEBA-based epoxy resin, a CTBN-modlfled DGEBA resin and their corresponding graphite flber-relnforced composites. 

Prepare CTBN (1300X13)/DGEBA Epoxy (EEW, 185) at 40/60 (See Table III)... 

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