Rubber-modified epoxy

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

A further increase in the amount of solvent leads to the development of a bi-modal pore size distribution, as observed with SEM on samples prepared with concentrations of 10-15 wt % hexane (Fig. 18c,d). Similar bimodal distributions have also been reported with the octane and decane based systems [88,89] as well as in in rubber-modified epoxies prepared via phase separation [67,95-98]. 

In comparison to the results obtained for the samples prepared with hexane, it can be concluded that the mean pore size and volume fraction do not depend on the initial concentration of the solvent, ( )o, but mainly on the difference between ( )o and (Fig. 25). Similar qualitative results are also reported for rubber-modified epoxies prepared via reaction induced phase separation [103]. 

While the surface modification is not effective to suppress cavitation, Yee and coworkers performed an experiment to suppress the cavitation mechanically in a rubber-modified epoxy network. They applied hydrostatic pressure during mechanical testing of rubber toughened epoxies [160]. At pressures above BOSS MPa the rubber particles are unable to cavitate and consequently no massive shear yielding is observed, resulting in poor mechanical properties just like with the unmodified matrix. These experiments proved that cavitation is a necessary condition for effective toughening. 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

Tough matrices, such as thermoplastics and rubber-modified epoxies, are particularly useful for high fracture toughness and damage tolerance against... 

Fig. 8.3. In-situ scanning electron microphotographs of mode I interlaminar fracture of Hexed T6T145 carbon liber composites containing (a) 1155 unmodifted epoxy matrix, and (b) FIS5 rubber-modified epoxy matrix. Reprinted from Bradley (1989b), with kind permission from Hlsevier Seience-NL, Sarti burger hart straat 25, 1055 KV Amsterdam, Fhc Netherlands,...

F155NR. FI85NR. unmodified epoxies FI55, F185. rubber-modified epoxies Lexan, polycarbonate. 

Fig. 8.6. Mode I potential energy release rate, GJ, plotted as a function of crack extension, Aa, for carbon fiber composites containing different matrices E (pure epoxy) ER (rubber-modified epoxy) ERF (short fiber-modified epoxy) ERP (rubber-and particle-modified epoxy). After Kim et al. (1992).

Kim, J.K., Mackay, D.B. and Mai, Y.W., (1993). Drop weight impact tolerance of CFRP with rubber modified epoxy matrix. Composites 24, 458-494. 

In the same manner, with decreasing of diffusion coefficient and interaction parameter, the spinodal is reached during the evolution of the system in the pregel stage. The very low values of interfacial tension in rubber modified epoxies (interfacial tension of polymer-polymer-solvent system were reported in range of 10-4-10-1 mN/m) therefore lead to an NG mechanism for phase separation. 

J. A. Manson, R. W. Hertzberg, G. Attalla, D. Shah, J. Hwang and J. Turkanis (Lehigh University, Bethlehem, PA, USA) Fatigue in Neat and Rubber-Modified Epoxies... 

Fig, 3. Transmission electron micrograph of osmium-tetroxide stained section of a typical rubber-modified epoxy thermosetting polymer... 

Fig. 6. Stress-intensity factor, KIc, at the onset of crack growth as a function of temperature for unmodified and rubber-modified epoxy polymers 81...

Fig. 7. Value of the shift factor, aT, needed for superpositioning of bulk mechanical data as a function of temperature, 5>. The various compositions of the rubber-modified epoxies are indicated...

Fig. 11. Transmission optical micrograph, taken using cross polarizers, of fracture region of a rubber-modified epoxy showing shear-yield bands 37)...

Fig. 14. Crack opening displacement, 8te, as a function of test temperature for a rubber-modified epoxy polymer 44)...

Fig. 17a and b. Variation of Klc/Klcs ratio with j/e44 a Unmodified, simple epoxy polymer b Rubber-modified epoxy polymer AO values of q deduced from Eq. (6) A.0 measured values ofp Full curve Theoretical relation from Eq. (12)... 

High resolution l3C NMR is also used in the determination of the composition of the dispersed phase in cured rubber modified epoxies in order to analyze the chemical structure of the mobile segments 152). In this case quantitative analysis is possible because the areas under each peak are approximately equal to the number of carbons contributing to the peak, and the intensities of the broad lines from the rigid phase are very low, almost indistinguishable from the baseline noise. The structure of crosslinked networks based on poly(3,4-pyrrolidinediethylene), synthesized by different methods, was determined from gels swollen in water and chloroform 153). 

Studies of the particle—epoxy interface and particle composition have been helpful in understanding the rubber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in rubber-modified epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and tf-butyl acrylate—acrylic acid copolymers (309), have been used for toughening epoxy resins. 

Mechanical properties of rubber-modified epoxy resins depend on the extent of mbber-phase separation and on the morphological features of the mbber phase. Dissolved mbber causes plastic deformation and necking at low strains, but does not result in impact toughening. The presence of mbber particles is a necessary but not sufficient condition for achieving impact resistance. Optimum properties are obtained with materials comprising both dissolved and phase-separated mbber (305). 

A proposed mechanism for toughening of rubber-modified epoxies based on the microstructure and fracture characteristics (310—312) involves mbber cavitation and matrix shear-yielding. A quantitative expression describes the fracture toughness values over a wide range of temperatures and rates. 

Although CTBN and derivatives still constitute the most important group of modifiers used in rubber-modified epoxies, several other types of modifiers, such as vegetal oils (castor oil), have been proposed as well. 

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.)...

The effects of strain rate and temperature are correlated, and can be modeled (Kinloch and Young, 1983, Kinloch, 1985). For different temperatures and strain rates, GIc and the time to failure, tf, were measured. Using the time-temperature superposition principle, shift factors (aT) applicable to the time to failure tf, were determine. Shift factors plotted against (T — Tg) are independent of the type of test used (Fig. 12.14). The construction of a typical master curve GIc versus tf/aT is shown in Fig. 12.15 (Hunston et al., 1984). The value of GIc may be predicted for any strain rate/temperature combination. This model can also be applied to rubber-modified epoxies (See chapter 13). 

Figure 12.14 Values of the shift factor aT needed for superposing of bulk mechanical data as a function of temperature for neat and rubber-modified epoxy networks. (Hunston et al., 1984. Reprinted with permission from Kluwer Academic/Plenum Press.)...

Fracture Modeling of Rubber-Modified Epoxy Networks... 

The model was successfully applied to rubber-modified epoxy networks, taking into account both the test temperature and the rate effect (Fig. 13.5). 

Figure 13.7 Variation of yield stress (

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). 

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