Materials Science Research International, Vol. 9, No. 3 pp. 230-234 (2003)

General paper

Corrosion Behavior of Epoxy Resin Cured with Different Amount of

Hardener in Corrosive Solutions

Hideki SEMBOKUYA*, Yoshiyuki NEGISHI*, Masatoshi KUBOUCHI* and Ken TSUDA* *Chemical Engineering Department, Tokyo Institute of Technology,

Tokyo, 152-8552, Japan

Abstract: Epoxy resin systems with different amounts of hardener were prepared. Bisphenol A type epoxy resin prepolymer was used and the hardener was m-xylylenediamine. Specimens were immersed in water, 10wt%sulfuric acid solution and 10wt% sodium hydroxide solution at 80℃. The epoxy resins showed high corrosion

resistance to water and sodium hydroxide solution regardless of the amount of hardener. On the other hand, color

changed layers were observed on the both surfaces of specimen uniformly in sulfuric acid solution. Sulfuric acid did not penetrate into color unchanged layer. From the results of weight change in sulfuric acid solution, it was suggested that the larger the amount of hardener, the faster was the penetration of sulfuric acid solution. Especially for specimen

cured with the largest amount hardener, cracks and defects were observed. The flexural strength, which was measured immediately after taking from sulfuric acid solution, decreased as immersion time increased. The flexural strength

recovered by drying. However, the specimen with the largest amount of hardener indicated irreversible degradation due to formation of cracks. The results suggested that excess hardener promote the penetration of sulfuric acid

solution. The amount of hardener must be determined carefully in order to avoid corrosion.

Key words: Epoxy resin, Corrosion, Hardener, Sulfuric acid, Degradation, Polymer, Curing agent, Strength

1. INTRODUCTION

Since epoxy resins have good chemical, mechanical and adhesive properties, they have been widely applied to corrosion protection fields. They are used as lining materials or coatings to metallic materials or concrete structures. In chemical industries, fiber reinforced plastic (FRP) tanks are used to store corrosive chemicals. Epoxy resins are sometimes selected as the matrix resin of FRP. Epoxy resin system consists of epoxy resin prepolymer, hardener and other additives such as inorganic fillers, modifiers, and so on. The chemical structure of epoxy resin prepolymer and hardener affects diffusion of water into cured epoxy resin [1,2]. There are 2 kinds of hardener of epoxy resin; one is an amine type and the other is acid anhydride type. The chemical resistance of cured epoxy resin to a solution is governed by the types of hardener [3,4]. Epoxy resin cured with amine has good resistance to alkaline solutions and that cured with acid anhydride has good resistance to acid solutions.

An epoxy resin prepolymer and a hardener must be mixed with hardener before curing. In that case, nonuniform distribution of hardener concentration is sometimes caused by insufficient mixture. This ununiformity of hardener affects mechanical properties [5] and corrosion resistance [6] for unsaturated polyester.

Recently, degradation of epoxy resin coated on concrete structures of sewerage systems due to generation of sulfuric acid synthesized by bacteria has been reported [7]. For the case of concrete lining at the site, resins are cured under atmospheric temperature. If the lining is undertaken in winter (or at a cold district), excess hardener may be often added in order to accelerate the curing reaction.

Although the effect of amount of hardener on static

mechanical properties has been studied, there were few research works on the corrosion resistant with different hardener contents. The authors investigated the effect of amount of hardener on corrosion behavior of thermosetting resins. The larger the amount of hardener (methylethylketone peroxide, MEKPO) of unsaturated polyester resin, the lower was the corrosion resistance in nitric acid solution due to local corrosion [6,8]. The degradation was due to the diluent of MEKPO added for the purpose of safety storage of MEKPO. However, the effect of amount of hardener on the corrosion behavior of epoxy resins is not clarified yet [9]. In this study, corrosion behavior of epoxy resins cured with different amount of amine hardener was investigated under acid and alkaline condition in order to make clear the effect of amount of hardener.

2. EXPERIMENTAL

2.1. Materials and Specimens In this study, bisphenol A type epoxy resin, EPIKOTE

828 was cured with amine type hardener, m-xylylenediamine. The chemical structures of epoxy resin and m-xylylenediamine are illustrated in Fig. 1.

Five kinds of epoxy resin systems (consisting of a common quantity of epoxy resin in varying hardener contents) were prepared. The weight contents of hardener were 0.9, 13.4, 17.9, 22.4 and 26.9wt%. These contents correspond to epoxy equivalent ratios (ER) of 0.50, 0.75, 1.00, 1.25 and 1.50, respectively. For simplicity in notation, these 5 kinds of materials with different hardener contents are expressed as ER0.5, ER0.75, ER1.0, ER1.25, ER1.5, respectively. ER1.0 means that the number of epoxide and that of amine before curing are theoretically identical. Specimens of ER0.5 and ER0.75

Received September 25, 2002

Accepted July 4, 2003230

Corrosion of Enoxv with Different Amount of Hardener

are deficient hardener and ER1.25 and ER1.5 are excess hardener under the curing reaction.

Fig. 1. Chemical structures of bisphenol A type epoxy resin prepolymer and amine type hardener.

Epoxy resin and hardener were cast into a mold andwere pre-cured at 50℃ for 24 hours and then post-cured

at 100℃ for 10 hours. Specimens were cut from the

molded 2mm thick plate. The dimensions of specimens were 60mm in length, 25mm in width and 2mm in thickness. Flexural modulus and flexural strength of the specimens are shown in Fig. 2. The effect of amount of hardener on the flexural modulus is small. The flexural strength increases slightly as the amount of hardener increases. Thus, static mechanical properties are not seriously affected by a little excess or deficiency of hardener.

Fig. 2. Mechanical properties of specimens with different hardener content.

2.2. Corrosion Test Specimens were immersed in deionized water, 10

wt% sulfuric acid solution and 10wt% sodium hydroxidesolution. The temperature was kept at 80℃ during

immersion tests. After a certain immersion time, specimens were taken from the environmental test vessel. Specimens were rinsed by pure water and remaining water was absorbed carefully by filter papers. Then, specimens were left in laboratory air for 1 hour and the weight of specimens was measured. Immediately after the weighing, 3 point bending tests were carried out followed by ASTM D790. The measured strength was defined as the "wet flexural strength". After immersion test, somespecimens were dried in an oven at 50℃ for a week and

the weight was measured as the "dry weight". Dry flexural strength was also obtained by conducting flexural tests. Degradation was evaluated by analyses based on visual observations, scanning electron microscope (SEM) observations, elementary analysis by using energy dispersion spectroscopy (EDS), and infrared spectroscope (FT IR) analysis.

3. RESULTS AND DISCUSSION

3.1. Effect of Hardener Content on Weight Change First of all, the effect of solutions on the corrosion

behavior was studied. Figure 3 shows the relation between the change of weight and immersion time of ER1.0 specimen in water, 10wt% sulfuric acid andsodium hydroxide solution at 80℃. The weight gain of

ER1.0 in sulfuric acid solution is very large. On the other hand, the weight gains in water and sodium hydroxide solution are small. The difference of the weight change between water and sodium hydroxide solution is small. Furthermore, there was no significant visual change for the specimens immersed in water and sodium hydroxide solution. The authors reported that the other epoxy resin cured with amine has good corrosion resistance to alkaline solution [3]. The same result is obtained for the materials tested in this study. The reason why the weight change in water is larger than that of sodium hydroxide solution is the difference of the osmotic pressure of water. Specimens with the other content of hardener showed a similar tendency. Thus, we focused on water and sulfuric acid solution in this paper.

Fig. 3. Weight change of ER1.0 specimen immersed in water, sulfuric acid solution and sodium hydroxide solution.

Figure 4 is the relation between the change of weight and square root of immersion time of all specimens in water. The results of ER0.75, ER1.0 and ER1.25 are almost the same. For ER1.5 specimen, the penetration rate which is defined as the slope of the tangent to initial curve in Fig. 4 is larger and the saturated water absorption weight is also larger than ER0.75, ER1.0 and ER1.25. These 4 kinds of specimens indicate "Fickian curves". On the other hand, the behavior of ER0.5 is quite different from the others. There seems to be no

proportional region in the curve of Fig. 4. This behavior may be attributed to low network density or surplus

231

Hideki SEMBOKUYA, Yoshiyuki NEGISHI, Masatoshi KUBOUCHI and Ken TSUDA

Fig. 4. Relation between weight change and square root of

immersion time in water for specimens with different

amount of hardener.

Fig. 5. Relation between weight change and square root of

immersion time in sulfuric acid solution for specimens

with different amount of hardener.

epoxide group due to the lack of amine hardener. Figure 5 shows the relation between weight gain and

root of immersion time in sulfuric acid solution. The weight gain in sulfuric acid solution is roughly 10 times larger than that in water. The effect of hardener content is different from the result in water. The larger the amount of hardener was, the faster the penetration of sulfuric acid solution was. ERO.5, ERO.75 and ER1.0 specimens did not level off before about 1000 hours. ER1.25 specimen shows a clear Fickian curve. Although ER1.5 specimen indicates basically Fickian curve, there seems to be one more step of increment of weight gain after saturation. For the ER1.5 specimen, cracks can be observed at 25 hours immersion as shown in Fig.6. Cracks were

generated by internal stresses caused by swelling. So, the weight gain of ER1.5 after saturation may be due to the

penetration of the solution into the cracks. In Fig.6, small defects also appeared inside the

specimen. The shape of the defects is a disk, and they look like micro penny cracks. However, we can not confirm if the defects have free surfaces or not. The mechanism of defect formation is under investigation. They may be crazes caused by the internal stresses due to the penetration of sulfuric acid solution. The number of the defects increased during immersion. The defects can be observed in ER1.25 specimen. Many defects also can be observed in the ER1.5 specimen immersed in water as

Fig. 6. Observation of ER1.5 specimen after 25 hours immersion in sulfuric acid solution. Cracks and defects (see Fig. 7) appeared.

Fig.7. Observation of ER1.5 specimen after 1200 hours

immersion in water. Many defects were observed.

shown in Fig. 7.

3.2. Effect of Hardener Content on Flexural Strength Figure 8 illustrates the relation between retention of

flexural strength (normalized by the strength before immersion shown in Fig. 2) and square root of immersion time in sulfuric acid solution. The flexural strength in this figure was measured under wet condition. The flexural strength of ER0.5 did not decrease within 2300 hours immersion. The higher the hardener content was, the larger was the decrease of retention of flexural strength. For the case of ER1.0, the flexural strength decreases to 75% of initial value after 2300 hours immersion. ER1.5 indicates the largest degradation and the flexural strength decreases to 35% of initial value.

Figure 9 shows the change of retention of "dry" flexural strength for the specimens immersed in sulfuric acid solution. Although scatters were large, values of dry flexural strength for all specimens were recovered to the strength before immersion except for ER1.5. The decrease of flexural strength due to the penetration of sulfuric acid solution shown in Fig. 8 is basically reversible. Thus, the true flexural strength did not decrease by immersing in sulfuric acid solution for

232

Corrosion of Epoxy with Different Amount of Hardener

Fig. 8. Retention of "wet" flexural strength for specimens

with different amount of hardener immersed in sulfuric

acid solution.

Fig. 9. Retention of "dry" flexural strength for specimens with different amount of hardener immersed in sulfuric acid solution-

ER0.5-1.25 specimens. For ER1.5 specimen, the flexural

strength decreased essentially by means of crack

development. The effect of the formation of defects on

flexural strength seems to be small because the retention

of flexural strength of ER1.25 specimen in which defects

were observed, recovered well.

3.3. Corrosion Mechanisms Figure 10 shows the cross section of ER1.0 specimen

immersed in sulfuric acid solution for 300 hours imaged by SEM. In this photograph, the result of sulfur (S) element line analysis by EDS is superimposed on the SEM photograph. Color changed layers were observed at the both surfaces of specimen uniformly. Sulfur element did not exist in the color unchanged layer. The border between two layers is very clear and sulfuric acid solution did not penetrate into color unchanged layer thoroughly. This feature can be seen in all types of specimens.

Figure 11 indicates the relation between penetration depth and square root of immersion time, where penetration depth is defined as the thickness of color changed layer. This figure suggests that the penetration depth increase linearly with respect to the root of immersion time. The effect of amount of hardener on the penetration rate is very large. The higher content of hardener was, the faster the penetration of sulfuric acid solution into the specimen was.

Fig. 10. Result of sulfur element line analysis of ER1.0 specimen immersed in sulfuric acid solution for 300 hours.

Fig. 11. Relation between penetration depth and square

root of immersion time for specimens with different

amount of hardener immersed in sulfuric acid solution.

Figure 12 shows the IR spectra of ER1.0 specimen. The upper chart is the IR spectrum before immersion. The other two charts are the spectra of the specimen immersed in sulfuric acid solution for 600 hours. The middle chart corresponds to the color unchanged layer, and the lowest one corresponds to the surface of the specimen. The spectrum before immersion and that of the color unchanged layer are almost the same. It is very interested that the spectrum of the surface (color changed layer) is also similar to the others. In detail, a peak at 1130cm-1 appeared in the spectrum at the surface. This peak relates to sulfone (>SO2). It may be due to sulfuric acid hydrates or sulfone amide. If a polymer is decomposed chemically such as hydrolysis of ester, a clear change in IR spectrum will be observed [10]. It is concluded that the color unchanged layer is identical with the state before immersion. On the other hand, the color

233

Hideki SEMBOKUYA, Yoshiyuki NEGISHI, Masatoshi KUBOUCHI and Ken TSUDA

Fig. 12. FT-IR spectra of ER1.0 specimen. The upper one is a spectrum before immersing in sulfuric acid solution. The middle one is a spectrum of color unchanged layer. The lower one is a spectrum of color changed layer.

changed layer is the sulfuric acid solution penetrated region. In the color changed layer, the chemical structure of epoxy resin did not change significantly.

The authors have investigated corrosion behaviors of polymeric materials systematically and discussed about the relation between penetration rate of environmental solution into material and reaction rate of environmental solution and material [11,12]. Corrosion behaviors are classified into 3 types, i. e, surface reaction type, corroded layer forming type and penetration type. If the penetration rate is much larger than the reaction rate, the corrosion behavior will be penetration type. In the case of this study, the behavior has something in common with "corroded layer forming type." For example, color changed layers appeared and the intensity of sulfur element was almost constant in color changed layer as shown in Fig. 10. For corroded layer forming type, corroded layer is degraded irreversibly by means of chemical decomposition. However, the "dry" strength did not decrease by immersing in sulfuric acid as shown in Fig. 9 and no reaction of breakage of chemical bond was observed. So, the behavior is classified into the penetration type essentially. From these points of view, the corrosion behavior observed in this study can be termed "quasi-penetration type."

The corrosion behavior of all specimens with different hardener content was quasi-penetration type. The penetration rate is strongly affected by hardener content. Epoxy resins cured with amine are widely used as lining materials. Excess hardener may be added if the atmospheric temperature is quite low. The results of this study suggest that excess hardener may promote the penetration of sulfuric acid solution. The amount of hardener must be determined carefully in order to avoid solvent penetration.

4. CONCLUSIONS

Five kinds of epoxy resin systems with different hardener contents of which epoxy equivalent ratios (ER) are 0.50, 0.75, 1.00, 1.25 and 1.50 were prepared. The effect of amount of hardener of epoxy resin on corrosion behavior was investigated. Following results have been obtained. (1) In sulfuric acid solution, the weight change was large

for the epoxy resin tested in this study. Color changed layers were observed at the both surfaces of specimen

uniformly. On the other hand, the weight changes in water and in sodium hydroxide solution were small

and significant degradation did not appear. (2) The results of weight change in sulfuric acid solution

suggested that the larger the amount of hardener, the faster was the penetration of sulfuric acid solution.

Especially for the largest amount hardener (epoxy equivalent ratio was 1.50), cracks and defects were

observed. (3) Chemical decomposition at color changed layer in

specimens immersed in sulfuric acid solution was not observed. Although the flexural strength decreased

under wet condition, the strength recovered by drying. However the strength of the specimen with the largest amount of hardener did not recover due to the

generation of cracks.

REFERENCES 1. J. Zhou and J. P. Lucas, Polym., 40 (1999) 5505. 2. C. Damian, E. Espuche and M. Escoube, Polym.

Degradation and Stability, 72 (2001) 447. 3. H. Hojo, K. Tsuda, K. Ogasawara and T. Takizawa, Polym.

Mater. for Corrosion Control, ACS Sympos. Ser., ACS, 322, (1986) 314.

4. H. Sembokuya, T. Nishiyama, M. Kubouchi and K. Tsuda, Mater. Sci. and Techn., 39 (2002) 121.

5. T. Sakai, H. Hojo, S. Ono, K. Arai and K. Tsuda, Proc,12th Int. Conf. Compos. Mater., Paris (1999) (CD-ROM).

6. K. Arai, H. Hojo, K. Adachi and Y. Murakami, Reinforced Plastics, 44 (1998) 475 (in Japanese).

7. K. Enda, Mater. Sci. and Techn., 39 (2002) 2 (in Japanese). 8. T. Sakai, H. Sembokuya, M. Kubouchi, K. Arai and K.

Tsuda, Network Polymer, 22 (2001) 2 (in Japanese). 9. Y. Negishi, T. Sakai, H. Sembokuya, M. Kubouchi and K.

Tsuda, Proc. 5th Int. Conf. Durability Analysis of Composite Systems, Tokyo (2001) 333.

10. T. Tomiyama, M. Kubouchi, H. Sembokuya and K. Tsuda, Mater. Sci, and Techn., 39 (2002) 33 (in Japanese).

11. K. Tsuda and M. Kubouchi, Bulletin of the Society of Sea Water Science, 52 (1998) 14 (in Japanese).

12. H. Sembokuya, M. Kubouchi, K. Tsuda, K. Okamura and H. Hojo, Kagaku Kogaku Ronbunshu, 27 (2001) 373 (in Japanese).

234