4 Fundamentals
6.5 Comparison of different Borosilicate Glasses
It can be summarized that:
- Decreasing wall thickness at constant inner diameter accompanied with decreasing resistance against inner pressure. In that case Griffith theory is not valid on hollow fibers and the pressure resistance follows the Barlow’s formula. Decreasing wall thicknesses led to decreasing pressure resistances.
- Keeping the free space constant Griffith’s theory was valid also for hollow glass fibers. The reduction of wall thickness accompanied with a reduction of the outer and inner diameter and resulted in higher pressure values. Under these circumstances small and thin walled hollow glass fibers showed an increased pressure resistance with smaller wall thicknesses.
- Consequently, the reduction of the wall thickness led to an increased pressure resistance but a simultaneous decrease in size had to be proceeded as well.
Table 37: Chemical main components of tested borosilicate glass fibers, as determined in chemical analysis
Components [ma-%]
Borosilicate 3.3 (DURAN) [148], [154]
Borosilicate C5 [151]
Borosilicate C1S [177]
SiO2 80.5 72.0 65.0
Na2O 3.5 6.7 2.5
Al2O3 2.4 6.8 7.3
K2O 0.5 2.4 3.0
B2O3 12.8 11.4 18.0
BaO 2.6
The changes of chemical composition entail variations in physical properties which are given in Table 38. The characteristic temperatures like annealing, softening and working point are different for each type. Consequently the thermal history after drawing the thin hollow fibers out of the glass tube differs for each type. Also the mechanical properties changed with varying the chemical composition, as can be seen in the different Young’s modulus of borosilicate 3.3 and C5.
The investigation of the effect of the different borosilicate glasses on the pressure resistance contained four test series with a minimum sample size of 30 samples per series. A direct comparison of all three glasses to each other was not possible due to the non-existence of suitable raw-tubes. During the drawing process the dimension is reduced many times. The open area and the ratio of diameter to wall-thickness were not substantially changed. Consequently borosilicate C5 was taken as a reference value and compared once to borosilicate C1S and twice to DURAN.
Table 38: Physical properties of tested borosilicate glasses
Physical property
Borosilicate 3.3 (DURAN) [148], [154]
Borosilicate C5 [151]
Borosilicate C1S [177]
Density [g cm-3] 2.23 2.33 2.27
Transformation
temperature Tg [°C] 525 465
Annealing point [°C at 1013 dPa s]
560 570 480
Softening point [°C at 107.6 dPa s]
825 785 715
Working point [°C at 104 dPa s]
1260 1140 1130
Young’s modulus E [(kN mm-2)]
63 102
CTE α25-300°C
[(1 K-1) 10-6]
3.3 5.5 4.7
Hollow fibers made of borosilicate C5 and C1S featured dimensions of do = 488 µm, di = 441 µm and a wall thickness of s = 23 µm. The characteristic test results are given in Table 39. It is predictable that the results are quite similar. The characteristic pressures P and the maximum burst pressures of both test series are close together.
The lower form parameter of borosilicate C1S can be explained by the lower minimum burst pressure which created a wider spread of the measured data.
Fibers made of DURAN as well as the corresponding borosilicate C5 fibers had the dimensions of do = 478 µm, di = 400 µm and s = 39 µm. The outer diameter is nearly the same of C1S fibers but the wall thickness is almost doubled. Therefore higher burst pressure was expected which can be seen by comparison of both C5 test series. The doubling of wall thickness at the same outer diameter led to a doubling of minimum and maximum burst pressure as well as characteristic pressure value. A comparison of borosilicate C5 to DURAN fibers shows a significant difference. All pressure values of DURAN fibers are 30% lower than those of borosilicate C5. The form parameter exhibits
a higher value of b = 10.6 which can be attributed to the lower range between minimum and maximum burst pressure.
Table 39: Characteristic test results of hollow glass fibers made of three different borosilicate glasses and different dimensions
Material
Dimension [µm]
Min. burst pressure pmin [MPa]
Max. burst pressure pmax [MPa]
Form parameter
b
Characteristic pressure P
[MPa]
C5 do = 488 di = 441 s = 23
13.0 26.9 7.1 22.5
C1S 10.4 26.5 5.6 21.8
C5 do = 478 di = 400 s = 39
29.0 50.2 8.5 43.1
DURAN 21.6 36.8 10.6 31.4
A visualization of calculated failure probabilities plotted against the burst pressure is given in Figure 48. Thereby the distinctions between the different borosilicate glasses can be seen.
Comparing the S-curves of borosilicate C5 and borosilicate C1S with a wall thickness of s = 23 µm a likewise development of both graphs is observable. The similar test results of minimum and maximum burst pressure, as well as characteristic pressure and form factor, which are illustrated in Table 39, lead to comparable progression and slope of both curves; especially, in the upper range both curves overlay each other.
What is clear to see is the difference of the S-curve of borosilicate C5 glass with higher wall thickness to the curve of borosilicate 3.3. Comparing both graphs of borosilicate C5 to each other, the doubling of pressure resistance by doubling the wall thickness is detectable. The curve of borosilicate C5 with higher wall thickness slid to the right of the diagram significantly. Borosilicate 3.3 shows a complete different behavior. The corresponding graph slid only slightly to the right due to the higher wall thickness. A sharper increase of the curve can be recognized because of the smaller range between the minimum and the maximum value of burst pressures. Nevertheless, the pressure resistance of borosilicate 3.3 is much smaller than of borosilicate C5.
Figure 48: Failure probability of hollow fibers made of different types of borosilicate glass with similar outer diameter but different wall thickness (do = 488 µm, di = 441 µm, s = 23 µm respectively do = 478 µm, di = 400 µm, s = 39 µm) A conclusion of the varying pressure resistance of the different borosilicate glasses could be given by the differences in chemical composition. The addition of substances to the glass mixture leads to changes in a multitude of chemical as well as mechanical properties of the glass itself. Alkaline and earth alkaline components cause a decrease of melting and working temperature in comparison to pure quartz glass [3], [4].
Simultaneously the network structure of the glass is weakened due to the formation of disconnecting points by breaking the bridge structure oxygen [3]. The addition of Al2O3 and B2O3 leads to the reduction of disconnecting points and a stabilization of the network. The stabilizing effect depends on the concentration in the chemical mixture and the percentage of alkaline and earth alkaline components [3], [4]. Attention should be paid especially to the concentration of B2O3. Due to its open structure, high concentrations lead to the destabilization of the network structure which appears, for example, in the decrease of the Young’s modulus [3].
A comparison of the chemical compositions given in Table 37 reveals main distinctions in the percentage of SiO2, Al2O3 and B2O3. Borosilicate C5 and C1S reached nearly the
same test results with fibers of similar dimensions. Thereby C5 glass features a significantly higher percentage of SiO2 than C1S glass. Nevertheless, the burst pressures and both failure probability curves showed similar developments. Alkaline like Na2O and K2O lead to the degradation of solidity of the network structure of glass.
Borosilicate C5 contains about 10% of these two alkaline substances. However, the addition of 6.8% of Al2O3 and 11.4 % of B2O3 acts against that effect and stabilizes the glass structure.
Borosilicate C1S contains less SiO2 as the main network builder. The concentration of Al2O3 increases slightly from 6.8% to 7.3%. But the B2O3 concentration in comparison to borosilicate C5 significantly increases from 11.4% to 18.0%. Due to that high percentage B2O3 loses the stabilizing effect and the resulting network exhibits a wide and open structure. In that case, however, the addition of alkaline substances reinforces the open network of boron and leads to a stiffening. Therefore the slight increase of Al2O3 and the presence of alkaline substances act on the network-weakening effect of boron and stabilize the glass network and thus a similar pressure resistance can be achieved.
The comparison of borosilicate C5 and C1S to borosilicate 3.3 shows an obvious higher percentage of SiO2 but lower amounts of alkaline substances. However, borosilicate 3.3 exhibits the lowest amount of alumina oxide of the three tested glasses. At the same time, the concentration of boron oxide is slightly higher than borosilicate C5. It can be assumed that the existing concentration of boron oxide leads to a strength-decreasing influence by opening the network [3]. The missing stabilizing effect of Al2O3 and the small amounts of potassium and sodium oxide cannot act against that influence.
Therefore the structure of the network is weaker than the other borosilicate glasses and in the comparison of Young’s modulus of borosilicate C5 and 3.3. The higher Young’s modulus of a material the higher is the stiffness of its structure. The elastic modulus of borosilicate C5 is almost doubled in comparison to borosilicate 3.3. A value of elastic modulus of borosilicate C1S was not available in the literature. Hence, it can be assumed that the amount of alumina oxide and boron oxide influence the mechanical resistance of the glass and both interact. Consequently a weakening effect of boron oxide on the solidity of the network can be countered by the addition of alumina oxide.
The chemical resistance of glass can be influenced massively by the addition of B2O3 [4]. Its positive effect on the firmness of the network seems to be mainly linked to the concentration and the presence of Al2O3. Similar concentrations of B2O3 but obvious distinctions in the amount of Al2O3 of borosilicate 3.3 in comparison to C5 lead to significant decreases of pressure resistance. Borosilicate C5 and C1S exhibit comparable or even higher percentages of B2O3 than borosilicate 3.3. The increase of
alumina and alkaline oxides in the mixture of the glass acts against the negative effect of boron and lead to higher resistance against inner pressure load.
All samples were investigated under the light microscope before preparing and testing.
No relevant or meaningful defects on the surface or in the volume could be detected.
Therefore the different behaviors can be attributed to the effect of chemical composition in all probability.
In following the test results of glass fibers made of different borosilicate glasses can be summarized as:
- In comparison to borosilicate C5 fibers made of borosilicate 3.3 and borosilicate C1S were produced and tested. Differences of their chemical composition led to deviations of physical properties of the borosilicate glasses. Both types borosilicate C5 and C1S exhibited a lower amount of silica as main component.
- The amount of Al2O3 was increased which acts as network stabilizer. The effect on the glass structure thereby depends on the concentration. The percentage of boron oxide, as well classified as stabilizer, was decreased for borosilicate C5 and increased for borosilicate C1S.
- Borosilicate C5 reached significant higher pressure resistances as borosilicate 3.3.
The slightly higher amount of boron oxide of borosilicate C5 with concurrent increase of aluminum oxide led to a stiffening of the network.
- The effect of the even higher percentage of Al2O3 in borosilicate C1S was equalized by the high amount of B2O3. Nevertheless, C1S featured similar results of pressure resistance as borosilicate C5. Consequently increasing the amount of Al2O3 in borosilicate glasses leads to increased pressure resistance.
Table 40: Development of pressure resistance of different borosilicate fibers compared to a borosilicate C5 fiber
C1S DURAN
Pressure resistance Slightly decreased Decreased