균일상 반응 모델에 의한 Epoxy/Zeolite 복합재료의 경화 특성

여만경, 심미자．, 김상욱

서울시립대학교 화학공학과, ．서울시립대학교 생명과학과

The Cure Characterization of Epoxy/Zeolite Composite

by Homogeneous Reaction Model

M. G. Lu, M. J. Shim． and S. W. Kim

Dept. of Chem. Eng., The Univ. of Seoul

．Dept. of Life Sci., The Univ. of Seoul

Introduction

Epoxides constitute a most important class of thermosetting polymers used in adhesives, matrices for fiber-reinforced composites, and coatings. Fillers such as short fiber, metal or inorganic particle, and various minerals have been widely used in the practical formulations. Therefore, the kinetic characterization of filled epoxy systems is necessary for a better understanding of structure-property relationships and improved product quality.

In general, kinetic expressions may be phenomenological or mechanistic. All kinetic studies can start with the basic equation:

[eqn: g( alpha )= INT { d alpha } over {f( alpha ) } = INT k dt]                                   (1)

where □ is the fractional conversion at any time t; k, the Arrhenius rate constant, and f(□), a function form of □ that depend on the reaction mechanism.

Isothermal differential scanning calorimetry (DSC) has been used for the estimation of kinetic parameters in which the fractional conversions are determined from the ratio of the partial to total peak areas by assuming that the exothermic heat evolved during cure is proportional to the extent of reaction. In this paper, the isothermal experiments have been conducted using differential scanning calorimetry. The widely accepted homogeneous reaction models including n-order and autocatalytical reaction models have been used to fit the experimental data.

Experimental

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. Natural zeolite is used as an inorganic filler; it is known that the zeolite is composed mainly of clinoptilolite which coexists with some quartz and feldspar.

The resin was cured employing stoichiometric quantities of diamine. The epoxy resin was mixed with zeolite and melted MDA under good stirring about 80℃. 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.

Results and Discussion

For the isothermal reaction with the temperature being constant, eq. (1) can be changed to

g(□) =  kt                                                 (2)

The values of activation energy can be obtained by assuming a Arrhenius form of rate constant k.

[eqn: k=Aexp(- { { E}_{a } } over {RT } )]                                         (3)

where Ea is the apparent activation energy; A is the pre-exponential factor.

Various kinetic parameters in the different models are listed in Table 2.

Table 2   The kinetic parameters by first-order reaction model

Filler cont. (%)	E1 (KJ/mol) 0-40%	E2 (KJ/mol) 40-95%	K1 102min-1	K2 102min-1
10	43.2	33.7	3.54	0.108
20	48.0	31.6	4.47	0.081
30	50.9	33.3	5.12	0.144
40	56.7	38.5	8.18	0.469

Table 2  The kinetic parameters by several different models

Filler cont. (%)	First-order reaction model			Second order reaction model			Autocatalytic reaction model
	Ea (KJ/mol)	k 102(min-1)		Ea (KJ/mol)	k 102(min-1)		Ea (KJ/mol)	k 102(min-1)
0 10 20 30 40	42.2 38.2 35.9 38.8 44.6	6.61 6.94 7.24 8.70 12.2		62.9 40.6 64.5 65.4 63.8	8.96 12.5 10.0 13.4 25.1		38.3 37.4 35.0 34.7 30.5	11.6 12.4 7.09 14.3 25.3

The homogeneous reaction model is the simplest in which assumes that the reaction is occurring simultaneously at the molecular level throughout the sample. Fig. 1 is a plot using the homogeneous first order kinetic model. It shows the rate constant k is not a constant as expected in the whole cure time range and there is a marked change in slope at around 45% conversion.  Similar results were obtained for the samples with various filler contents in the different curing temperatures.

The rate constant k is a measure of reaction rate in which the reaction activation energy is contained, so the change of rate constant also indicated a marked change in the reaction energies beyond 40% conversion. As shown in table 1, the value of the activation energy obviously decreases at the higher conversion range. Such a decrease in apparent activation energy have been frequently observed in heterogeneous reactions. Therefore, it is clear that the diffusional factors become significant beyond 40% conversion in the curing process of the filled epoxy systems. Otherwise, we can find that the values of the activation energy were not interrelated to the extents of filler beyond 40% conversion. From the results reported here, it can be inferred that at the early stage of cure process, reaction is homogeneous no matter whether it has or not filler. With the proceeding of cure, heterogeneous reaction occurred but, even in it, there is a difference in mechanisms. As a reflection, the much faster reaction rate was observed over 40% filler contents.  It is consistent with the results obtained from half-time of cure.

Fig. 1  Analysis of isothermal data using first-order model for 10% filled sample at 90℃.

Fig. 2 Analysis of isothermal    data using autocatalytical      model for 10% filled sample.

Homogeneous second order reaction model obtained the similar results, but didn't fit the data as well as first order model.

Fig. 2 shows the analytical results by autocatalytic reaction model. As expected, it fits the data well and shows the hydroxyl groups existed in zeolite accelerate the curing reaction of epoxy resin as catalysts as reported in most of literature. It is possible for filled zeolite system, but a number of works have shown that zeolite catalyzes the ring-opening reaction of epoxide as a supporting-reagent due to its unique properties. Zeolite not only provides a surface area large enough to ensure the dispersion of amine reactant, but also possesses strong acid sites to activate electrophiles effectively at a appropriate temperature.

Summarizing all the facts observed previously, a feature on the cure process of systems can be proposed in rough, that is, at the early stage of cure, reactions are homogeneous; and then heterogeneous reactions occur. The filler can catalyze the cure reactions mainly by dispersing the curing agent and enhancing the cleavage of carbon-oxygen bond, but this catalysis very depends on the reaction temperature and would be retarded at low temperatures.. Highly filled samples can provide the extra reaction sites, thus further accelerate the reaction rate at the high temperature.

Conclusions

The kinetic effects caused by the presence of a filler zeolite during the cure of an epoxy-amine system have been studied. The experimental cure measurements indicate that the cure reaction can be accelerated effectively by filler over 70℃. The kinetic analysis have been conducted using homogeneous reaction models. The autocatalytical reaction model fit the experimental data well in the whole curing range. However, the cure reaction can only be represented up to 40% conversion by the first-order reaction mechanism.

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