Textile Research Journal

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Chemical Composition and Mechanical Properties of Basalt and Glass Fibers: A Comparison Abstract The geometrical and mechanical properties and chemical composition of different basalt and glass fibers have been investigated. Tensile tests were performed on short basalt fiber made by melt blowing, glass fiber and three different types of continuous basalt fibers made by spinneret method. The chemical composition was evaluated by plasma atomic emission spectroscopy. The geometrical and mechanical properties of continuous basalt and glass fibers were similar to each other in terms of diameter, tensile strength and modulus. Short basalt fibers had considerably lower average diameter and mechanical performance with relatively high standard deviation. The SiO2 and Al2O3 content of basalt fibers showed correlation with tensile properties of fibers. Results revealed that continuous basalt fibers were competitive with glass fibers and short basalt fibers were weaker in terms of quality and mechanical properties. It was observed that the joint SiO2 and Al2O3 content of basalt and glass fibers showed correlation with tensile properties of fibers.

Tamás Deák and Tibor Czigány1 Department of Polymer Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Muegyetem rkp. 3, Hungary

Key words

Basalt fibers, glass fibers, mechanical properties, chemical composition

In the last two decades, basalt fibers have come into consideration as potential reinforcement of composite materials. Basalt is a common volcanic rock that can be found in most countries around the globe and is directly suitable for fiber manufacturing. Its chemical structure is nearly related to glass. The most important components of basalt are SiO2, Al2O3, CaO, MgO, Fe2O3 and FeO [1–3]. The different oxides compose a large crosslinked molecule with primary bonds, therefore basalt and glass can be regarded as a special kind of polymer. Basalt rocks are molten approximately between 1350 and 1700 °C [4–6]. When cooled rapidly, basalt solidifies in a glassy amorphous phase. Slower cooling results in a partially crystalline structure, an assembly of minerals. Basalt fibers are good electric insulators, biologically inactive and ± environmentally friendly. The average density of basalt is 2.6–2.7 g/cm3, while glass has a density of 2.5–2.6 g/cm3 [7, 8].1

Basalt materials are classified according to their SiO2 content as alkaline basalts (up to 42 % SiO2), mildly acidic basalts (43 to 46 % SiO2) and acidic basalts (over 46 % SiO2). The color of basalt ranges from brown and gray to dull green depending on the chemical composition. Basalt fibers are more resistant to strong alkalis than glass fibers, but glass can better withstand strong acids. Basalt fibers can be used over a wide range of temperature, from –200 °C to +600 °C [9–11]. At higher temperatures structural changes occur. For continuous fiber manufacturing, basalt rocks must meet the following requirements: SiO2 content above 46 % (acidic basalt) with constant composition, ability to melt without solid res-

Textile Research Journal Vol 79(7): 645–651 DOI: 10.1177/0040517508095597

www.trj.sagepub.com © 2009 SAGE Publications Los Angeles, London, New Delhi and Singapore

1 Corresponding author: Department of Polymer Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Muegyetem rkp. 3, Hungary. e-mail: [email protected]

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Figure 1 Optical micrographs of fiber heads on short basalt fiber made by Junkers method.

idue, appropriate melt viscosity for fiber formation and ability to solidify in a glassy phase without noticeable crystallization [12, 13]. Basalt fibers are produced in one step, directly from crushed basalt stone. Some melt blowing technologies (e.g. Junkers method) are suitable for producing cheap, short basalt fibers, but such fibers have relatively poor and uneven mechanical properties. In melt blowing technologies, the molten basalt rock is poured onto an ensemble of rotating steel cylinders. As the melt is blown off from the cylinders by air jets, fibers are formed in the air blast and solidify quickly in a glassy amorphous phase. The characteristics of Junkers technology cause the formation of the so-called fiber heads. The fiber heads are spherical objects with the diameter of 10 to 100 times the fiber diameter. While some of them break from the fibers, the others – mostly the smaller ones – remain on the fiber ends (Figure 1) [14]. Continuous basalt fibers are made by spinneret method, similarly to glass fibers (Figure 2). The basalt broken stone is molten in a rho-

dium-platinum pot, lead to a spinneret made from the same material and spun gravitationally through holes in the spinneret bottom at 1350–1420 °C. The filament bundle is taken up downwards at about 2000–5000 m/min, prepared at 1.0 to 1.2 meters below the spinneret, then spooled. In glass fiber manufacturing, predominantly overhead gas burners are used for heating the melt. In the case of basalt, it raises difficulties because due to its dark color it absorbs infrared energy near to the surface, thus homogeneous heating is rather challenging. This can be overcome by holding the melt in the reservoir for a longer time or by electric heating using electrodes immersed in the bath. Basalt stone is molten in two steps: in the initial furnace it is fused, then conveyed to the secondary heating zone feeding the extrusion bushings, equipped with a precise temperature control system [11, 15]. The idea of using basalt fibers as reinforcement of composite materials first emerged in the former Soviet Union in an aerospace research program. Today most of the continuous basalt fibers is manufactured in Russia and Ukraine [11]. The aim of this study was to evaluate the mechanical properties and chemical composition of different basalt and glass fibers.

Materials and Methods Three types of fibers were tested: E-glass, short basalt fibers made by melt blowing (Junkers method) and continuous basalt fibers from three different manufacturers. Table 1 shows the type and manufacturers of investigated fibers. Continuous basalt and glass fibers are produced in rovings. Table 1 Basic data of investigated fibers. Figure 2 A simplified scheme of a basalt fiberization processing line: 1) crushed stone silo; 2) loading station; 3) transport system; 4) batch charging station; 5) initial melt zone; 6) secondary heat zone with precise temperature control; 7) filament forming bushings; 8) sizing applicator; 9) strand formation station; 10) fiber tensioning station; 11) automated winding station.

Marking Type of fiber

Manufacturer

SB

Short basalt fiber

Toplan Ltd., Hungary

CB1

Continuous basalt fiber

Kamenny Vek Co., Russia

CB2

Continuous basalt fiber

D.S.E. Group, Israel

CB3

Continuous basalt fiber

Technobasalt Co., Ukraine

GF

E-glass fiber

Skoplast Ltd., Slovakia

Chemical Composition and Mechanical Properties of Basalt and Glass Fibers T. Deák and T. Czigány The mechanical properties of the fibers were investigated by tensile tests. The elementary fibers were stuck to paper windows and their diameter was measured on a Projectina 4014/BK-2 projection microscope fitted to an image processing system and equipped with a CCD camera, with 400x magnification. The diameter of fibers was measured at three different points to determine the variations in diameter. Subsequently the specimens were clamped to the testing machine, the paper window was cut and the fiber was tensioned. The tests were executed according to the EN ISO 5079 : 1999 standard, with 25 mm gauge length on a Zwick Z002 testing machine, at ambient temperature. The test speed was v = 2 mm/min. 100 specimens of each material were tested and the mean values and standard deviations were calculated. The tensile tests of short basalt fibers have been presented in our previous work [16]. The tensile strength was defined as the tensile stress at break, while the elastic modulus was the gradient of the stress-strain curves between 0.05 and 0.25 % nominal strain. The chemical composition of the fibers was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) method, using a Labtest Plasmalab ICP spectrometer. The amount of oxides was calculated from the elementary composition. The entire Fe content was considered as Fe2O3.

Results and Discussion The chemical composition of investigated fibers can be seen in Table 2. SiO2 was a basic component of both basalt and E-glass. Its proportion was relatively uniform, between 50 and 56 mass percentage (m%) in continuous basalt fibers. Short basalt fibers had a lower SiO2 content, while in glass fibers it was over 58 m%. E-glass had less diverse chemical composition than basalt fibers. It was mainly built

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up from four essential compounds: SiO2, Al2O3, CaO and B2O3. Although boron was not evaluated in our experiments, it is known from literature that B2O3 can be found in E-glass, but it is absent from basalt rock [4, 16]. All other oxides were below 1 m% in E-glass. Some compounds which scarcely occurred in E-glass fibers, could be found in basalt in a large quantity, e.g. Fe2O3, K2O, MgO, Na2O and TiO2. These compounds determined the differences between basalt and glass fibers. The higher heat resistance and dark color of basalt fiber were mostly due to its Fe content. The results of optical, density measurements and tensile tests can be seen in Table 3. The tensile strength of short basalt fibers was considerably lower compared to other invesigated fibers. The elastic modulus of short basalt fibers was also relatively small and had a large deviation. The moduli of other fibers concurred, although CB3 continuous basalt fibers and glass fibers had slightly smaller elastic modulus. The diameter of short basalt fibers was small and varied in a broad range. The tested E-glass fiber had relatively large diameter, while the continuous basalt fibers had a diameter between 12 and 15 µm uniformly. The relation between the average breaking strain values (εfs) was similar to that of the tensile strength. This was evident, because the modulus of glass and continuous basalt fibers was in the same order of magnitude. Figure 3 shows the tensile test diagrams of fibers. All tested fibers had a rigid behavior, without plastic deformation. The modulus and failure method of continuous basalt fibers and glass fiber were quite similar, while short basalt fibers were considerably less stiff. The relationship between geometrical and mechanical properties of all three tested continuous basalt fibers and glass fiber had a similar nature, thus only CB1 is presented on diagrams in Figures 4–7. On the other hand, short basalt fibers had notably different properties. Tensile strength of short basalt fibers showed a large hyperbolic dependence on diameter below 9 µm. Over this value, the

Table 2 Chemical composition of basalt and glass fibers. Element

Oxide

SB

CB1

CB2

CB3

GF

Element

Oxide

Element

Oxide

Element

Oxide

Element

Oxide

Element

Oxide

m%

m%

m%

m%

m%

m%

m%

m%

m%

m%

m%

m%

Al

Al2O3

9.17

17.35

8.20

15.44

6.49

14.21

9.51

17.97

6.30

11.86

Si

SiO2

19.76

42.43

26.04

55.69

24.95

53.36

23.66

50.62

27.24

58.25

Ca

CaO

6.35

8.88

5.31

7.43

5.54

7.74

6.32

8.85

15.05

21.09

Fe

Fe2O3

8.17

11.68

7.55

10.80

7.68

10.98

7.77

11.11

0.21

0.30

K

K2 O

1.94

2.33

1.25

1.51

0.88

1.06

1.43

1.73

0.36

0.43

Mg

MgO

5.70

9.45

2.45

4.06

3.22

5.35

3.13

5.19

0.32

0.54

Na

Na2O

2.81

3.67

1.78

2.40

2.81

3.79

1.76

2.38

0.22

0.30

Ti

TiO2

1.53

2.55

0.74

1.23

1.04

1.73

0.66

1.10

0.25

0.41

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Table 3 Results of fiber optical, density measurements and tensile tests.

Marking

Diameter

Cross-section

Maximum force

Extension at failure

Tensile strength

Breaking strain

Elastic modulus

Density

Dav [µm]

Af [µm2]

Ffs [N]

∆lfs [mm]

σfs [MPa]

εfs [%]

Ef [GPa]

ρ [g/cm3]

SB

10.3 ± 3.1

90.2 ± 56.7

0.05 ± 0.04

0.32 ± 0.12

602 ± 295

1.29 ± 0.48

48.2 ± 20.6

2.66

CB1

14.2 ± 1.4

160.2 ± 30.3

0.32 ± 0.09

0.89 ± 0.22

2016 ± 434

3.56 ± 0.89

61.9 ± 3.5

2.56

CB2

12.7 ± 1.5

128.1 ± 31.5

0.21 ± 0.07

0.68 ± 0.17

1608 ± 350

2.72 ± 0.67

62.0 ± 3.6

2.64

CB3

14.1 ± 2.9

163.5 ± 63.3

0.30 ± 0.13

0.87 ± 0.18

1811 ± 331

3.47 ± 0.70

53.2 ± 7.4

2.63

GF

16.8 ± 1.6

223.4 ± 42.0

0.32 ± 0.08

0.68 ± 0.22

1472 ± 395

2.71 ± 0.86

57.0 ± 3.0

2.61

Figure 3 Typical tensile diagrams of basalt and glass fiber tensile tests (see markings in Table 1).

average tensile strength was nearly constant (Figure 4(a)). In the case of CB1 continuous basalt fiber, the same connection showed a slightly decreasing trend, but due to the poor correlation coefficient (R = 0.14), it could rather be regarded as stochastical arrangement, the tensile strength having irrelevant coherency with fiber diameter (Figure 4(b)). The elastic modulus of short basalt fibers also had a tendency to grow below 9 µm (Figure 5(a)). The elastic modulus of continuous basalt fibers had little standard deviation and it was independent from the diameter (Figure 5(b)).

The relationship between the breaking strain and tensile strength of short basalt fibers had a linear characteristic with a relatively low correlation coefficient (R = 0.42). It could be ascribed to the inequality of elastic modulus of SB fibers (Figure 6(a)). CB1 had a similar characteristic with better correlation coefficient (R = 0.90), due to the smaller standard deviation of elastic modulus (Figure 6(b)). The constancy of elastic modulus (i.e. the linear connection between deformation and stress) could be characterized by the relationship of breaking strain and elastic modulus (Figure 7). The elastic modulus of short basalt fibers was decreasing with increasing breaking strain. Generally, SB fibers with a lower elastic modulus had a tendency to reach a higher breaking strain (Figure 7(a)). The elastic modulus of CB1 had a significantly smaller standard deviation and it was independent from breaking strain (Figure 7(b)). The mechanical properties of investigated continuous basalt and glass fibers were quite independent from the diameter. In contrast with this, the tensile strength and elastic modulus of short basalt fibers were higher at diameters under 9 µm. Supposedly continuous basalt fibers also have a tendency to become stronger at smaller diameters, but in our case, they all had a diameter above 10 µm, so this phenomenon could not occur. Continuous basalt fibers and short basalt fibers had two main differences: first, SB had greater deviances in diameter and elastic modulus, and secondly, continuous fibers showed higher modulus and strength values. Both phenomena arose mainly from technological reasons. The production technology of continuous basalt fibers is characterized by long stages of melting and vitrification, degassing, homogenization and melt chilling, which – together with the fiberization by spinneret method – ensures the uniformity of diameter and physical-chemical properties. On the other hand, short basalt fibers are produced by a technology where the time of material going through the melting and fiberization process is not sufficient for equalizing its chemical composition through thermal diffusion and for stabilizing the process of vitrification and homogenization. Supposedly the inhomogenity of melt is the main reason of irregular fiber and fiber head diameters [15]. Generally,

Chemical Composition and Mechanical Properties of Basalt and Glass Fibers T. Deák and T. Czigány

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Figure 4 Diameter-tensile strength relationship: (a) SB; (b) CB1.

Figure 5 Diameter-elastic modulus relationship: (a) SB; (b) CB1.

Figure 6 Breaking strain-tensile strength relationship: (a) SB; (b) CB1.

tensile strength has a larger statistical deviation than elastic modulus. The presence of volume defects, such as cracks and cavities and surface defects (microcracks, indents, swellings), has an irrelevant effect on elastic mod-

ulus, but produces a decrease in tensile strength while the elastic modulus remains substantially constant. According to literature data, the mechanical behavior of basalt, glass and ceramic fibers is strongly dependent

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Figure 7 Breaking strain-elastic modulus relationship: (a) SB; (b) CB1.

Figure 8 Correlation of ceramics (SiO2 and Al2O3) content and tensile strength (a) and elastic modulus (b) of investigated fibers.

on their Al2O3 content. This recognition has resulted in attempts to enhance the strength of these fibers by adding additional Al2O3. SiO2 is the fundamental component of glass and basalt fibers [17, 18]. This led to the hypothesis that a relationship can be found between the amount of abovementioned components and mechanical properties of investigated fibers. Our measurements did not reveal a correlation between the average tensile strength or elastic modulus and Al2O3 or SiO2 content, but the effect of these two components together (Al2O3 + SiO2) was demonstrable. As these compounds are classified as ceramics, we denominated them ceramic-like materials. Figure 8 shows the relationship between the ceramic-like material content and the tensile strength of investigated fibers. The correlation coefficient here was R = 0.93. The tensile strength of basalt fibers was growing in the function of ceramic content, at the same time glass fibers had weaker tensile strength than continuous basalt fibers, although the ceramic content of glass was not lower. However, it must be taken into consideration that the glass fibers had quite different chemical composition and had the largest average diameter among investigated fibers, which may have caused smaller tensile

strength due to the diameter dependence of strength. If glass fiber was excepted from the comparison, unequivocal relationship could be observed. The correlation coefficient was R = 0.99 in this case. Elastic modulus showed a similar characteristic to tensile strength (Figure 8(b)). A difference between this and the character of tensile strength shown on Figure 8(a) was that measured points of elastic modulus showed no monotonic growth in the function of ceramic-like material content, on the other hand short basalt fibers clearly had a smaller average modulus combined with lower ceramic content. The correlation coefficient was R = 0.77 with glass fibers and R = 0.80 excluding glass fibers.

Conclusions The chemical, geometrical and mechanical properties of basalt and glass fibers have been investigated by microscopy, tensile tests and plasma atomic emission spectroscopy analysis. The basalt fibers represented two different

Chemical Composition and Mechanical Properties of Basalt and Glass Fibers T. Deák and T. Czigány production technologies: short basalt fibers made by melt blowing (Junkers method) and continuous basalt fibers made by spinneret method. The main components of glass were Al2O3, SiO2, CaO and B2O3, while basalts lacked boron and contained considerable amounts of TiO2, K2O, MgO, Na2O and Fe2O3. The geometrical and mechanical properties of continuous basalt and glass fibers were similar to each other in terms of diameter, tensile strength and modulus. Short basalt fibers had considerably lower average diameter and mechanical performance with relatively high standard deviation. Short basalt fibers showed a steep growth in modulus and tensile strength below 9 µm diameter. Other investigated fibers did not feature such dependence of properties on diameter, due to their higher diameters, which well exceeded 9 µm. The joint SiO2 and Al2O3 content (denominated as ceramic-like materials) of basalt fibers showed correlation with tensile properties of fibers, especially if basalt fibers were considered without glass fibers. It was concluded that continuous basalt fibers were competitive with glass fibers and short basalt fibers were weaker in terms of quality and mechanical properties.

4.

5.

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7.

8.

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11. 12.

Acknowledgement Kamenny Vek (Russia), Technobasalt (Ukraine), D.S.E. Group (Israel) and Toplan Ltd. (Hungary) are kindly acknowledged for the provision of basalt fibers. This work was supported by the Hungarian Scientific Research Fund (OTKA K61424).

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