에폭시 몰딩 컴파운드의 물리적 성질에 미치는 수분의 영향

여만경, 이철호, 김상욱*

화인폴리머(주), *서울시립대학교 화학공학과

Effects of Moisture on Physical Properties of Epoxy

Molding Compounds

M.G. Lu, C.H. Lee and S.W. Kim*

Fine Polymer Co. Ltd., ．Dept. of Chem. Eng., The Univ. of Seoul

Introduction

Polymeric materials differ from other structural materials in that, at ambient temperatures, low molecular weight substances can easily migrate in them freely. Early studies of the sorption mechanisms and diffusion of small molecules in plastic materials arose from the desire to prepare barrier materials, mainly against gases and moisture. The environmental degradation of the mechanical properties of the epoxy polymers is associated to the plasticization and micromechanical damage induced by the sorbed moisture. Water molecules are reported to act as plasticizers or crazing agents for the epoxides, strongly influencing the properties of the materials subjected to temperature, humidity and stress fatigue tests.

In this paper, we reported the effects of water molecules on the mechanical properties glass transition temperature and degradation behavior of epoxy molding compounds. The fact that properties differences between desiccant and soaked samples exist, supports the concept that physical modifications are introduced into the polymer network by the moisture-temperature aging, although the exact nature of this change has not been completely identified yet.

Experimental

The resin used in this study is a commercial mineral filled DGEBA-based epoxy compound (MG6-0330, The Dexter Corporation, USA), which contains acid anhydride hardeners. The samples for mechanical property and water-resistance tests were processed by RTM operations.

The tensile strength measurements were conduced with an Instron mechanical testing machine (Shimadzu, Japan) according to the procedures described by ASTM D 543 and 1259. A Instrumental Specialists Incorporated TGA-1000 Thermogravi- metric Analyzer is used to obtain weight loss data for the degradation of various types of samples. Water equilibrated samples were dried in vacuum before TGA measurements. For the dynamic thermal degradative runs, the samples are purged with nitrogen gas at a flow rate of 50 ml/min. Constant heating rates of 5, 10, 20 and 50 ℃/min are used. The sample weights are in the range of 20-50 mg in all cases. The glass transition temperature was measured by DSC at a heating rate of 10 K/min under nitrogen atmosphere.

Results and Discussion

The mechanical properties, water uptake and glass transition temperatures of epoxy molding compounds before and after soaked in water were listed in Table 1. As can be seen, the glass transition temperature was depressed due to the plasticization of sorbed water.  It is clear that the apparent water uptakes in materials were saturated after three days, then slightly decreased. It has been reported that at lower temperatures the translational freedom of the water molecules is strongly hindered by the stiffness of the polymer chain segments, and the formation of clusters is kinetically unfavored and is usually not observed in short-time experiments. The slight drop in water uptakes with time may be associated with a progressively greater damage developed in this inhomogeneous materials, or with the solution of some additives in compositions. It is also possible that an over-saturation was reached during the first short-time period, then the over-sorbed water was released with time.  The details were not clear now.

Table 1. Physical properties of dry and water equilibrated epoxy molding compounds

at 30℃ and different times.

time in water, h	□, MPa	□, %	water uptake, %	Tg, ℃
0	7.62	1.08	0	101.3
24	7.66	1.16	0.014	95.8
72	7.06	1.05	0.047	92.5
120	6.66	1.27	0.044	92.7
168	5.74	0.87	0.035	91.2

It was also found that the tensile strength decreased with increasing the time of samples in water. However, as shown in Table 1, the elongations at break did not seem to be changed markedly. The glass transition temperature did not decrease further with time in water, and  practically equilibrated after 24 h.  So the decrease in mechanical strength can be attributed to the crazing effect by water, in other words, water acted predominantly as a crazing agent for these observed systems, which resulted in a continued decrease in the mechanical strength with time (also see Fig. 1).

The low moisture adsorption in epoxy molding compounds was related to the low resin component, which also affected the other properties mentioned above, and the application of acid anhydride curing agent that got rid of the formation of hydrogen-bond  as in amine-cured epoxy systems, where moisture uptakes of 2-5% can easily be reached under environmental conditions, leading to a significant drop of Tg, even well below the application limits of most of epoxy materials.

It was observed that the degradation behaviour of epoxy molding compounds was not drastically affected by water. Both systems had almost same decomposition activity energy calculated from Kissinger's expression. The values were 121.4 and

Figure 1. The tensile strength and glass transition

temperature varied with time in water.

115.6 kJ/mol, respectively. The onset and maximum decomposition temperatures were given in Table 2. It is clear that these systems showed very excellent temperature-resistance although their glass transition temperature were lower than that of aromatic amine cured systems.

Table 2. The characteristic temperatures of materials by TGA analysis.

heating rate, K/min	Tonset		Tmax		E, kJ/mol
	dried	120h	dried	120h	dried	120h
5	312.3	292.7	402.5	396.1	121.4	115.6
10	339.3	318.9	412.7	417
20	325.3	355.1	431.6	427.4
50	412.6	425.8	471.7	460.1

Conclusions

The analysis of the changes in tensile strength, glass transition temperature and thermal decomposition behaviour revealed that the moisture in epoxy molding compounds can affect their properties markedly. Although the sorbed water plasticizes the materials, it acts predominantly as a crazing agent, such continuously decreasing the mechanical strength with time in water. The fact that properties differences between desiccant and soaked samples exist, supports the concept that physical modifications are introduced into the polymer network by the moisture-temperature aging.

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