Hard segment와 soft segment를 가지고 있는 강인화제를 이용한 다성분계 에폭시수지 시스템

최철림,김준경

고분자하이브리드연구센터

한국과학기술연구원

MULTIPHASE  EPOXY SYSTEMS TOUGHENED BY

MOLECULES WITH HARD AND SOFT SEGMENTS

Chul Rim Choe and Junkyung Kim

Polymer Hybrids Research Center

Korea Institute of Science & Technology

Introduction

The incorporation of reactive rubber has long been the most effective technique to get  toughened  epoxies, although strength and stiffness could not be maintained due to the soft elastomeric materials added to the systems. However, Bucknall's report[1] that brittle epoxies could be successfully toughened by the inclusion of thermoplastics without sacrificing their inherent properties has raised the importance of thermoplastics in toughening epoxies to such an extent that engineering thermoplastic polymers are now widely used as epoxy tougheners, especially in the area of high performance composite matrices. Our intensive studies[2-4] have provided us with an insight into several new systems, in which the toughener's molecules with hard and soft segment together are designed by physical mixing(separated two component system) or by chemical bonding(copolymer system).

In this paper PES/CTBN mixture and PBT-PTMG copolymer were considered as a toughener with hard and soft segments in one molecule.

Experiment

Bisphenol A type epoxy resin, Epon 828  with epoxy equivalent weight of 184-190 g/mol, was used as a base resin, and 4,4'-diamino diphenyl methane(DDM) and piperidine as curing agents respectively for PES/CTBN and PBT-PTMG systems. In the system of epoxy/PES/CTBN specimen was prepared by two steps with the aid of methylene chloride, giving a homogeneous mixture of PES/epoxy and for epoxy/PBT-PTMG copolymer system dissolution/precipitation(by crystallization) method was used for the specimen preparation. Double-torsion test was used to measure fracture energy. Fracture surfaces were observed by SEM and process zone at a crack tip was examined by polarized optical microscopy.

` Results and Discussion

Two phase morphologies are fixed by phase separation during process,

quite differently for PES/CTBN/epoxy and PBT-PTMG/epoxy. While two stage phase separation can be observed in PES/CTBN/epoxy, say, macro-phase separation during mixing and micro-phase separation during curing, phase separation in PBT-PTMG/epoxy  seems to be initiated by the crystallization of PBT segment during dissolution/precipitation process.

During mixing of homogeneous binary mixtures of PES/epoxy and CTBN/epoxy, phase separation occurs immediately. The morphology of maco-phase separation  is found to be sensitive to the composition PES and CTBN : PES-rich spherical domains at lower composition of PES(the total modifier content is fixed at 10 wt. %), co-continuous structures at the PES content of ca. 7 wt. % and phase inversion at higher PES content. Phase separation occurs again microscopically during curing within the unaffectedly remained macro-phase separated morphology.

For PBT-PTMG/epoxy the phase separation and morphology of dispersed phases are found to be sensitive to the copolymer composition and the total amount of copolymer in the system. With the increase of PBT composition in the copolymer the size of precipitated crystalline phase increases and its perfectness, which seems to be the dorminant factor for the fracture toughness enhancement, decreases.

While the strength and stiffness remain unaffected in PBT-PTMG/epoxy and change monotonously in PES/CTBN/epoxy with the composition of hard/soft segments, the fracture toughness of these two multi-phase toughened epoxies exhibits a synergy effect(Fig.1 and 2) irrespective of the toughener systems, implying that the presence of hard and soft segments together is more important than the way they are bonded(physically or chemically).  This synergism is inferred from the observation of the fracture surfaces by SEM and the examination of the process zone at the crack tip by polarized transmission optical microscopy, and hence believed to be due to the positive interaction between various toughening mechanisms resulted from the coexistence of hard and soft segments in the toughener's molecules. The main cause for the synergy effect seems to be the positive interaction of crack bridging and rubber cavitation process in PES/CTBN/epoxy blends : the crack bridging by dispersed hard molecules(PES phase) tends to decrease the yield stress of nearby matrix, which results in easier rubber cavitation process. In case of PBT-PTMG/epoxy systems the synergism in fracture toughness seems to come from so-called phase transformation mechanism directly related to the crystal perfectness of PBT(Fig.3) affected by the PTMG composition: the higher degree of crystal perfectness is given under the higher composition of PTMG and needs more energy for the phase transformation under load of PBT crystals.

Although different systems get involved in different synergy mechanisms, it seems that synergy effect in toughness enhancement is a general feature for the modified epoxies, if molecules with hard and soft segments together are applied as tougheners.

References

1. Bucknall,C.B. and Patridge,K., Polymer, 24, 639 (1983)

2. Kim,S.T., Kim,J. and Choe,C.R., J. Mater. Sci., 27, 3523 (1996)

3. Kim,S.T., Dissertation, Seoul National University, Seoul, Korea (1997)

4. Kim,S.T., Kim,J., Lim,S.H., Choe,C.R., and Hong,S.I., J. of Mater. Sci., 33,     2421 (1998)

Fig. 1 Effects of composition on the toughness      Fig. 2. The effect of copolymer composition

of PES/CTBN/epoxy ternary blends                 on the fracture toughness of PBT-

(Total wt% of PES & CTBN: 10 wt%)             PTMG/epoxy blends

Fig. 3   Polarized transmission optical micrographs of thin section of modified epoxy with PBT

contents in 10 wt% PBT-PTMG copolymer: (a) 46, (b) 66, and (c) 87