A study of the corrosion resistance of glass fibre reinforced polymers

Composite Structures 2 (1984) 191-200

A Study of the Corrosion Resistance of Glass Fibre Reinforced Polymers

G . W . E h r e n s t e i n a n d R. S p a u d e Institute for Material Science and Technology, Universityof Kassel, West Germany

A BS TRA C T Specific interactions between chemical environments (hydrochloric acid, sulphuric acid and distilled water) and glass fibre cause stress corrosion cracking in the glass fibre surface. The etching of the glass fibre gives rise to an extraction process. Axial or spiral cracks can then be observed. These effects depend on the fibre diameter, the etching time and the chemical environment and cause a drop in tensile stresses. The glass fibre crumbles with increasing etching time. Strict etching procedures lead to definite extraction processes and crack structures in the glass fibres and will be discussed in connection with strength tests. In addition to investigations of individual elements, e.g. glass fibres, it is also possible that whole glass fibre reinforced composites are damaged during service under the influence of aggressive surrounding media. In such cases, circular or spiral-shaped cracks can also be observed preferentially in the glass fibre. The fibres can then no longer contribute to an increase in strength and the result is the untimely failure of the composite material.

1 INTRODUCTION The high demands made on the quality of materials under very different conditions require more and more exact explanations about the chemical and physical behaviour of the individual components. Special attention must be given to the structure and strength of the individual components 191 Composite Structures 0263-8223/84l$03.00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

192

G. W. Ehrenstein, R. Spaude

of glass fibre reinforced resins. This is especially important in aggressive media surroundings. The glass fibre inserted in the resin will probably be more sensitive to the aggressive surrounding media than the resin itself. During investigations on individual elements, e.g. glass fibres, crack initiation was observed in the glass fibre surface. Cracks can result from the interaction of the residual stresses in glass fibres and corrosion. 1-5 Strict etching procedures on glass fibres and glass fibre reinforced resins give information about the kind of crack initiation and strength of these materials.

2 EXPERIMENTAL METHODS Studies of corrosion in individual glass fibres were conducted with the E-glass, R-glass, C-glass and modified E-glass fibre types. The composite material studied consisted of polyester resin with E-glass fibres forming the reinforcing material. Table 1 gives details of the composition and microstructural elements of the fibres. Most of the fibres studied had diameters between 10 and 14/zm. TABLE 1 C h e m i c a l C o m p o s i t i o n o f the G l a s s F i b r e s

Constituents (wt% ) Si02 E-glass D-glass R-glass C-glass

54.5 74.0 60-0 65-0

fe203 A1203 B203 0.5 0-2 0-3

14.5 -25.0 4-0

7.5 22.5 -5.0

CaO 17.0 0.5 9-0 14.0

MgO K20 Na20 Ti02

BeO

4-5 0-2 6-0 3.0

---1.0

0.8 1.5 -8.0

1"3 -0.5

0.1 -0-1 --

The glass fibres with different diameters were exposed to various corrosive media (see Table 2). The glass fibres were kept in the corrosive media for up to 600 h at various temperatures. The expected decrease in strength was determined by tensile strength tests. The material properties, i.e. tensile strength and modulus of elasticity, of various glass fibre types are shown in Fig. 1. Mechanical measurements were used to determine the residual strength of the composites when

193

Corrosion resistanceof glassfibre reinforcedpolymers TABLE 2

Research Programme

Corrosive media E-glass D-glass R-glass C-glass Modified E-glass GF-UP (~ = 22 wt%)

HCI HCI HCI HCI HCI --

Distilled H20 Distilled H20 Distilled H20 Distilled H20 Distilled H20 --

H2SO4 ----H2SO4

t h e r e are s i m u l t a n e o u s influences o f static p o w e r and aggressive surr o u n d i n g m e d i a ( T a b l e 2). P h o t o g r a p h s w e r e t a k e n using a scanning e l e c t r o n m i c r o s c o p e in o r d e r to o b s e r v e local f r a c t u r e m e c h a n i s m s o f the composites. T h e investig a t i o n s o f the f r a c t u r e surface should allow a conclusion to be d r a w n f r o m the glass fibre b e h a v i o u r in the c o m p o s i t e w h e n t h e r e are s i m u l t a n e o u s m e c h a n i c a l and chemical influences.

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194

G. W. Ehrenstein, R. Spaude

3 RESULTS AND DISCUSSION

3.1 Free single glass fibres The aggressive influence of weak concentrated chemical environments on glass fibres could be observed using a microscope. The glass fibres are attacked rapidly under the influence of weak acids and high temperature (80°C). The release of some chemical components in the glass fibre surface leads to a change in the refractive index, so that the extraction process can be observed with the microscope. A nuclear layer and a surface layer are then seen. The boundary between the nuclear and the surface layer moves continuously until the extraction process is finished. The glass fibre is then observed as a new continuum. U n d e r the conditions given the process described lasts for 2 h and depends on the concentration and etching temperature of the acid. Figures 2(a) and (b) show the nuclear process in an E-glass fibre at two times, t = 15 rain and t = 22

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Corrosion resistance ofglassfibrereinforcedpolymers

195

min. Figure 2(a) shows the beginning of the nuclear process and Fig. 2(b) the continued process. Figure 3 shows the relationship between nuclear diameter dK divided by fibre diameter dF from the same E-glass fibre and etching time and concentration of sulphuric acid. Although there is a divergence in the measurements with increasing etching time, a 2"5 wt% sulphuric acid can be seen to have a greater influence on the extraction process than a 1 wt% sulphuric acid. However, in either case complete extraction of the glass 1"0~---

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fibre takes place. Simultaneous with the extraction process is a decrease of the tensile strength of the glass fibre. Figure 4 shows the relationship between tensile strength, etching time, aggressive medium and temperature of an E-glass fibre. A 0"5N hydrochloric acid and a 1N hydrochloric acid cause a strong decrease of the tensile strength of an E-glass fibre. It has been found that A!203 leads to a small corrosion resistance of E-glass fibres. With increasing etching time the extraction process moves on, i.e. network changing cations leave the structural unit by diffusion. 6 The influence of the surrounding medium, e.g. distilled water, can also lead to a decrease in strength. Therefore it can be concluded that absorbed distilled water influences fibre strength. Figure 5 shows the influence of tempering on fibre strength; a decrease in the strength level is observed. The reason for this is increasing surface failure of the glass fibre with rising temperature. This leads to a decrease

196

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in strength. The modulus of elasticity also decreases with rising temperature. 7 It has been found that commercial glass fibre does not have a simple strength distribution but has different strength maxima. Carefully produced glass fibres have a high tensile strength of about 2000 N/mm 2. Tempered glass fibres only have a low tensile strength as a consequence of surface failure. Figure 6 shows the decrease of strength of different glass fibre types

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under the influence of IN hydrochloric acid and distilled water. C-glass fibres and D-glass fibres show good corrosion resistance. The network producing element B203improves corrosion resistance and decreases the surface stress. There is also a good relationship between B203and A1203. This leads to a small decrease in tensile strength with increasing etching time. R-glass fibres and modified E-glass fibres show insufficient corrosion resistance although there is a high original tensile strength. The reason for this is the existence of AI203 (Table 1).

3.2 Composites There are hardly any details of corrosion resistance of glass fibres inserted in a resin matrix. In many papers it is suggested that the boundary between the glass fibre and the resin shows no good corrosion resistance and that this is the reason for a decrease in strength. However. our investigations show that there is a dominant attack on the single glass fibre and only a small attack on the boundary in the composite. This is always observed when there is a static or dynamic load so that microcracks in the resin lead to an acceleration of the diffusion of media on the glass fibre surface. Figure 7 shows the decrease of tensile strength of a glass fibre reinforced resin (0 = 22wt%) as a function of the etching time (H~SO4). After I000 h etching time a decrease in strength of about 50% of original strength was observed. Figure 8 shows the fracture surface of this

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Corrosion resistanceof glassfibre reinforcedpolymers

199

Fig. 8. Fracture surface of a glassfibre reinforced resin, axial and radial cracks. laminate. The aggressive attack gives rise to crack initiations in the glass fibre surface in the resin. The result of the corrosion attacks is an untimely failure of the composite. In this case we can preferentially observe axial or spiral cracks. The reasons for these cracks are residual stresses in glass fibres, s The cracks then give rise to multiple fibre fractures. As a consequence the fibres can no longer contribute to an increase in strength. The mechanical behaviour of the composite decreases with the increasing influence of the aggressive media.

4 CONCLUSIONS The mechanical behaviour of glass fibres and glass fibre reinforced plastics is d e p e n d e n t on the influence of aggressive surrounding media.

200

G. W. Ehrenstein, R. Spaude

Specific interactions between chemical environment and the glass fibre generate extraction and nuclear growth in the glass fibre. The consequence is a decrease in tensile strength, Scanning electron micrographs of the fracture surface of glass fibre reinforced resin give information about the failure mechanisms of this material if there is simultaneous mechanical and chemical stressing. The problem of boundary adhesion between the glass fibre and the polymer matrix is superposed on crack initiation in the glass fibre surface. Crack initiation in glass fibres leads to a decrease in strength and untimely failure of the composite.

ACKNOWLEDGEMENT The authors would like to thank Dr.-Ing. A. Bledzki, Institute for Chemical Technology, University of Stettin, Poland, for supporting this work and the German Research Community for financial backing.

REFERENCES 1. Torp, S. and Arvesen, R., Influence of glass fiber quality on mechanical and strain corrosion properties of FRP. Proposed method for quality control, Proc. 34th Annual Tech. Conf. on Reinforced Plastics~Composites, SPI, New Orleans, 1979. 2. Schmidt, K., Textilglasfiir Kunststoffverstiirkung, Speyer, Zechner & Hfithig Verlag GmbH, 1972. 3. Donnet, J., Battistella, R. and Chatenet, B., Comportement particulier d'un filament de verre en milieu chlorhydrique, J. Microskopie, 9 (1970) 273-6. 4. Metcalfe, A., Gulden, M. and Schmitz, G., Spontaneous cracking of glass filaments, Glass Technology, 12(I) (1971) 15-23. 5. Haarsma, J., Proc. 34th Annual Tech. Conf. on Reinforced Plastics/ Composites, SPI, New Orleans, 1979. 6. Wiedemann, G., Untersuchungen zur Morphologie der Glasseide, Faserforschung und Textiltechnik, 22 (1971) 192-202. 7. Barteniev, G. M., The structure and strength of glass fibers, J. Non Crystalline Solids, 1 (1968) 69-90. 8. Ehrenstein, G. W. and Spaude, R., Crack initiation in glass fibres under the influence of chemical environment and high temperature, J. Materials Technology and Testing, 14(3) (1983) 73-81.