등온 경화 조건에서 에폭시-제올라이트계의 유리화

여만경, 김상욱

서울시립대학교 화학공학과

The Glass transition of Epoxy-Zeolite System in The

Isothermal Curing Conditions

Mangeng Lu, and Sangwook Kim

Department of Chemical Engineering, The University of Seoul

．Department of Life Science, The University of Seoul

Introduction

Although a considerable amount of research has been undertaken with regard to the cure kinetics of epoxy resins and their composites with different techniques, relatively little information has been reported on epoxy systems containing fillers.

Natural zeolite is one of the important inorganic fillers. It is very significant to exploit and utilize zeolite in industry. Many works performed in our laboratory have shown that zeolite is a useful filler for epoxy resins. However, as known, the composite system often exhibits the unusual cure behavior and results in a deviation from the model. There are often conflicts in the kinetic parameters due to the complex in cure process for thermosets because many reactive processes occur simultaneously, and there are other parameters besides that increase the complexity of the curing process, such  as the phenomena of gelation and vitrification, the change from chemical kinetic control to control by diffusion in the advance of the curing. Moreover, data can been interpreted by different methods to produce different results. in this present paper the change in glass transition and vitrification have been observed for the zeolite filled epoxy system.

Experimental

Materials

The epoxy resin systems used in this study consist of a commercial diglycidylether of bisphenol A (DGEBA) resin (Shell Chemicals, Epon 828) and 4, 4′-methylene dianiline (MDA) hardener (Fluka Chemie Co.). These materials were used as supplied without further purification. The epoxide equivalent weight of the resin was taken as 184. Natural zeolite is used as an inorganic filler. It was washed, dried and pulverized. Before being used to fill the epoxy system, it was dehydrated at 120℃ for about 5 hours under a vacuum to a constant weight and stored in a desiccator.

The resin was cured employing stoichiometric quantities of diamine (epoxy/amine ratio=1). The epoxy resin was mixed with zeolite and melted MDA under good stirring about 80℃, then cooled rapidly to room temperature.

DSC Characterization

Samples consisting of 3-6 mg mixture are placed in aluminum DSC pans and run on a SEIKO Ⅰ- 5000 series. In the isothermal scans, the samples were heated to the test temperature at 100℃/min and measured at four temperatures between 70-130℃. The carrier gas is nitrogen at a flow of 50 ml/min. The samples for measurement of glass transition temperature were placed in thermostatic oven at different curing temperature for various periods of curing time, or cured inside the DSC module if the curing time was less than 3 hours.

Results and Discussion

As mentioned previously, the reactive system can vitrify for the low curing temperatures, which dramatically affects the progress of the curing reaction since the reaction process becomes practically diffusion-controlled.

Vitrification involved a physical transformation from a liquid or rubbery state to a glass state as a result of an increase in crosslinking density of the sample. It is well known that this phenomenon occurs when the glass transition temperature, Tg, becomes equal to the curing temperature, Tc. For a given curing temperature, as would be expected, the Tg increases and the residual heat of curings decreases with the curing time as a consequence of the increase in the degree of crosslink. The higher the curing temperature, the shorter the time required to reach the maximum glass transition temperature Tg∞ of the resin will be.

Fig. 1 is a plot of Tg to curing time at curing temperatures of 70℃ and 130℃. The reaction time and degree of conversion required to reach Tg=Tc is defined as vitrification time tv and □v. The values of tv at different Tc can be obtained from Tg versus time as shown in Fig. 1, and □v was calculated from the residual heat involved at tv. It is clear from Table 1 that the vitrification was involved for the curing temperature of 70℃ since the degree of cure is only about 0.76 when vitrification occurs. For the high curing temperature, the vitrification is negligible because the curing reaction at tv is approximately completed although the curing temperature is still lower than the maximum glass transition temperature Tg∞.

Table 1   The effect of curing conditions on vitrification

Filler content (%)	70 (℃)		90 (℃)		110 (℃)		130 (℃)
	Tg∞ (℃)	□v	Tg∞ (℃)	□v	Tg∞ (℃)	□v	Tg∞ (℃)	□v
0	106.3	0.72	124.2	0.94	142.8	0.96	156.4	0.97
10	104.8	0.73	123.6	0.93	142.1	0.95	154.7	0.97
40	103.9	0.76	121.8	0.94	141.3	0.97	152.8	0.96

Fig. 1 Tg as a function of             reaction time.

Fig. 2 Plot of lnt vs. Tc for a         given glass transition.

Different from the curing rate, the glass transition temperature only increases with increasing filler content at the initial curing stage for the given curing temperature and time because of the autocatalytical reaction by zeolite that results in the increase of the initial crosslinking degree of resin. However, a slight reduction in the Tg of the fully cured epoxy resin was observed as the zeolite content increased, which can be attributed to the reduction in crosslink density due to the presence of zeolite in the network. It is also possible that the filler present in system increases the free volume in glass state of cured resin, such decreasing the Tg. Similar result in the filled epoxy system has also been obtained by some researchers.

A correlation betweeen the curing time and the corresponding curing temperature for a given glass transition was obtained by DSC scans of partially cured samples. A log plot  curing time t as a function of curing temperature is given in Fig. 2. The observed dependence of t and Tc can be described by the following exponential equation. Same relationship between Tg and curing temperature Tc was also observed.

t =7.76×10-11exp(3.67×104/RTc)                       (1)

Tg=1.32×1010exp(2.86×104/RTc)                        (2)

Conclusions

The DSC experimental measurements indicate that the cure reaction can be retarded at low temperatures because vitrification may involve in the curing process. The glass transition temperatures of cured resin were slightly decreased with the addition of zeolite filler. The temperature dependence of glass transition and curing time at fixed Tg can be approximated by the exponential equations.

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