Epoxy/CTBN 계의 기계적 성질

이재영, 이홍기*, 김상욱

서울시립대학교 화학공학과, *우석대학교 화학공학과

Mechanical Properties of Epoxy/CTBN System

Jae-Young Lee, Hong-Ki Lee* and Sang-Wook Kim

Dept. of Chem. Eng., The Univ. of Seoul, Seoul 130-743

*Dept. of Chem. Eng., Woosuk Univ., Wanju 565-800

Introduction

Epoxy resins have many advantages such as high modulus and strength, low creep, chemical resistance and reasonable high temperature performance and they are nowadays used in adhesives and matrix resins for fiber reinforced composites. These characteristics of epoxy resins are obtained when the cross-link density is moderate because too high cross-link density results in brittle failure and too low density results in unsuitableness for construction materials. Therefore, many researchers have studied and proposed to enhance the toughness of the cured epoxy resin[1∼10] and the most successful methods are 1. addition of soft segment to the backbone of base epoxy resin or curing agent, 2. introduction of reactive additives during the cure reaction, which make the cross-link density decreased and 3. incorporation of reactive liquid elastomers such as CTBN (carboxyl-terminated butadiene acrylonitrile copolymer), ATBN(amine-terminated butadiene acrylonitrile copolymer), CR(chloroprene ruber) etc. In this study, the 3rd method was used to modify the brittle behavior of the epoxy system, and the impact and tensile properties were investigated.

Experiment

Diglycidyl ether of bisphenol A(DGEBA, Epon 828 grade) supplied by Shell Co. was used as a base resin and 4,4'-methylene dianiline(MDA) was used as a curing agent. Synthesized phenyl glycidyl ether(PGE)-acetamide(AcAm) was used as a reactive additive and carboxyl-terminated butadiene acrylonitrile copolymer(CTBN) manufac- tured by B. F. Goodrich Co. under the trade name of Hycar 1300×8 was used as a toughener.

DGEBA/MDA(30 phr)/PGE-AcAm(10 phr)/CTBN (0, 5, 10, 15, 20 and 30 phr) were well-mixed and cured at 150℃ for 1.0 hr after curing at 80℃ for 1.5 hr. Tensile properties was measured by Universal Testing Machine(Shimadze AGS 1000D) at the cross-head speed of 10 mm/min by the recommendation of ASTM D638 and impact strength was carried out by Izod impact tester. To estimate the glass transition temperature(Tg), cured samples were analyzed by Chan DSC at the heating rate of 10 ℃/min.

Results and Discussion

It is very important to improve impact strength, for polymer materials are frequently subjected to high impact loading during the service life and it results

Fig.1 Impact strength of DGEBA/

MDA/PGE-AcAm/CTBN system

Fig.2 Tg of DGEBA/MDA/PGE-

AcAm/CTBN system

in severe loss when impact energy could not be absorbed. Bumper of an automobile was the typical example. Therefore, the modification of impact resistance for polymer materials should be investigated, even though it made other mechanical properties be in loss. In this study, CTBN, a reactive liquid rubber was introduced as a toughening modifier.

Fig.1 shows the change of impact strength for DGEBA/MDA/PGE-AcAm system with various CTBN contents. The impact strength increased with the increment of CTBN content and at 40 phr, it was improved about 22%. The modification of impact strength was explained by the effect of CTBN domains dispersed in the epoxy matrix, which was generated during the cure reaction[11∼13]. While the well-mixed epoxy/CTBN system was cured, the CTBN molecules aggregated due to the lack of compatability between epoxy and CTBN. If the cured epoxy resin was loaded by impact, the rubber domains effectively absorbed the impact energy.

Glass transition temperature(Tg) of polymer is thermal property and it is also closely related to mechanical properties. Tg is affected by the polymer structure, such as the softness or rigidity of backbone chain, the number, size and species of side chains and the crosslink density. Therefore, Tg should be considered to study the impact and tensile properties. Tg of the epoxy system with various contents of CTBN was shown in Fig.2. With the increment of CTBN content, Tg value decreased and this result implied that the crosslink density of epoxy system was decreased due to the disturbance of CTBN domains. The generated domains were located at the space between the functional groups in epoxy system, so the chemical reaction was disturbed and the crosslink density decreased.

Fig.3 and 4 show the tensile strength and Young's modulus for DGEBA/MDA/PGE-AcAm system with various CTBN contents, respectively. The decrement of tensile strength was explained by the rubber domains. When tensile

Fig.3 Tensile strength of DGEBA/

MDA/PGE-AcAm/CTBN system

Fig.4 Young's modulus of DGEBA/

MDA/PGE-AcAm/CTBN system

load was added to the specimen, the interface between CTBN domain and epoxy matrix was acted as a defect, so crack easily initiated from this region. The weak crosslink structure was also the reason of the decrement of tensile strength. The decrement of Young's modulus was due to the plasticizing effect of the CTBN domain.

Fig.5 shows the decrement of elongation for the epoxy system with the increasing CTBN content.

Conclusions

From these results, the following conclusions were obtained.

1. Impact strength increased with the increment of CTBN contents.

2. Tg, tensile strength, Young's modulus and elongation were decreased with the increment of CTBN contents.

3. These were explained by the generation of CTBN domain and the decrement of crosslink density. The CTBN domain absorbed the impact energy, however the interface of CTBN/epoxy was acted as crack initiation region.

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PGE-AcAm/CTBN system

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