Phase-Separation and Transition Phenomena in Toughened Epoxies

11 Phase-Separation and Transition Phenomena in Toughened Epoxies Wai H . Lee , Kenneth A. Hodd , and William W. Wright

Downloaded by COLUMBIA UNIV on June 20, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0222.ch011

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Ciba-Geigy Plastics, Bonded Structures, Duxford CB2 4QD, England Department of Materials Technology, Brunel University, Uxbridge UB8 3PH, England Department of Materials and Structures, The Royal Aircraft Establishment, Farnborough GU14 6TD, England

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Three epoxy resins with different cross-link densities were modi fied with low-molecular-weight carboxyl-terminated butadiene -acrylonitrile (CTBN) rubbers. The phase-separation process was investigated by using light microscopy and transmission electron mi croscopy. It was found that the phase-separation process is favored by increasing the cross-link density of the epoxy matrix, but that the rubber particles in the more highly cross-linked epoxies undergo cavitation upon cooling from theirfinalcuring temperatures. Possible explanations for these observations were investigated. The measured volume fractions of the phase-separated rubber particles were com pared with the theoretical volume fractions. Dynamic mechanical thermal analysis was used to characterize the transition phenomena of the cured toughened epoxies. The heights, positions, and areas of their tan δ peaks were related to the microstructures of the rubber -toughenedresins.

RESISTANCE AND FRACTURE TOUGHNESS

IMPACT of thermoset epoxy resins were improved by the incorporation of liquid rubber, as demonstrated in the late 1960s by Sultan and McGarry (1). Since then, these modified resins have been subjected to intensive investigations (2-7). Because of their en hanced fracture properties, they have found wide application as adhesives and matrix resins in the aerospace and other industries. The properties of rubber-modified epoxy are strongly dependent on the 0065-2393/89/0222-0263$07.25/0 © 1989 American Chemical Society

In Rubber-Toughened Plastics; Riew, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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morphology generated during its cure (5-7). The morphology in turn is determined by factors such as the before-cure compatibility of the rub ber-resin system, the choice of curing agent, and curing time and temper ature (5, 7). The effect of acrylonitrile content on the phase-separated particles was demonstrated by Rowe et al. (7). Generally, the higher the acrylonitrile content of the carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber, the smaller the particle size. Siebert and Riew (8) also described the chem istry of the particle formation in an epoxy system involving a C T B N rubber, a diglycidyl ester of bisphenol A epoxy resin, and piperidine catalyst. A c cording to them, three reactions can occur between C T B N rubber and epoxy resins. The three reactions are (1) epoxy-acid (rubber) reaction (esterification); (2) acid-aliphatic hydroxyl (from reaction 1) reaction (esterification); and (3) epoxy-aliphatic hydroxyl reaction (etherification). Reactions 1 and 2 are the important reactions, and reaction 1 is termed the "chain extension reaction" (i.e., building up of molecular weights by reaction of carboxyl and epoxy groups). Riew et al. (9) later showed the superiority of carboxyl terminal groups over others such as phenol, epoxy, hydroxyl, and mercaptan. We now report an investigation of the influence of both the compatibility of the resins with the rubber and the cross-link density of the cured resin on the morphology of the transitions of rubber-epoxy system networks.

Experimental Details Materials. D i g l y c i d y l ether of bisphenol A ( D G E B A ) resin (Ciba-Geigy MY750) and tetraglycidyldiaminodiphenylmethane ( T G D D M ) resin (Ciba-Geigy MY720) were modified with C T B N rubbers from B F Goodrich. The types and prop erties of the C T B N rubbers are given in Table I, and the structures of the resins and hardeners are shown in Chart 1. The epoxy and epoxy-rubber systems were cured with either piperidine or diaminodiphenylsulfone (DDS) and boron trifluoromonoethylamine ( B F M E A ) . For D D S - c u r e d systems, a rubber-epoxy prereaction was prepared at 80 °C for MY750 resin and at 100 °C for MY720 resin. Triphenylphosphine catalyst was added in the case of rubber-MY750 resin. T h e reaction was carried out under nitrogen atmosphere and followed by determination of the carboxyl content at regular intervals 3

(10). Table I. Properties of Hycar C T B N Elastomers Property Molecular weight Functionality Acrylonitrile content, % Solubility parameter, C a l cm Specific gravity at 25 ° C / 2 5 °C

CTBN 1300X13

CTBN 1300x8

3500 1.85 27 9.14 0.96

3500 1.85 17 8.77 0.948



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Chart 1. Structures of epoxy resins, curing agents, and CTBN rubbers.

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R U B B E R - T O U G H E N E D PLASTICS

The reaction was stopped when the acid value had been reduced to 0.05 m m o l / g or below (usually 3-4 h). T h e prereaction was cooled and stored at 5 ° C . The prereaction was used in combination with additional resin to obtain the desired rubber content. T h e mixing was carried out by heating the mixture (pre reaction, resin, and D D S as curing agent) to 100 °C and stirring for 15 min. T h e temperature was then raised to 135-140 °C, and the mixture was degassed until all the D D S had dissolved. The mixture was cooled to 110 ° C , and B F M E A catalyst was added. After 3 min more of stirring, the mixture was poured into preheated glass molds and cured in a heated oven for the required time. For the rubber-modified piperidine cure system, the desired rubber content was added to 100 parts of resin. T h e n the mixture was heated and degassed to 80 °C for 30 min before being cooled to 30-40 ° C . Piperidine was added, and the mixture was stirred for 5 min before it was poured into a preheated glass mold and cured. For cast resin without rubber, the premixing stage with the rubber was omitted. A l l the formulations and curing schedules used are given in Table II. Also included are the notations used to describe the different formulations.


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Characterization Techniques. Optical microscopes ( O M , Zeiss or Microstar) equipped with polarizers were used to examine phase-separated particles. Specimens were prepared by placing a speck of uncured sample on precleaned glass slides or cover slips and curing in situ. Submicrometer specimens (

Phase-Separation and Transition Phenomena in Toughened Epoxies

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