INFLUENCE OF MECHANICAL AND THERMAL PREHISTORY ON THE STRUCTURE OF GLASS FIBERS

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INFLUENCE OF MECHANICAL AND THERMAL PREHISTORY ON THE STRUCTURE OF GLASS

FIBERS

R. Brückner, H. Stockhorst

To cite this version:

R. Brückner, H. Stockhorst. INFLUENCE OF MECHANICAL AND THERMAL PREHISTORY ON THE STRUCTURE OF GLASS FIBERS. Journal de Physique Colloques, 1985, 46 (C8), pp.C8-527- C8-531. �10.1051/jphyscol:1985883�. �jpa-00225236�

INFLUENCE OF MECHANICAL AND THERMAL PREHISTORY ON THE STRUCTURE OF GLASS FIBERS

R. Bruckner and H. S t o c k h o r s t

Institut für N i c h t m e t a l l i s c h e Werkstoffe, Anorganische Werkstoffe, Technische U n i v e r s i t ä t Berlin, Englische Strasse 20, 1000 Berlin 12, F.R.G.

Résumé - Cet article explique comment les propriétés des fibres de verres, qui sont sensibles à la structure, dépendent des para- mètres de tirage des fibres. Une comparaison est faite avec les mêmes propriétés du verre massif et également entre les propriétés de fibres ayant des histoires différentes. Il apparaît des effets évidents d'anisotropie et d'orientation dans la structure des fibres.

Abstract - This paper demonstrates how structure-sensitive pro- perties of glass fibers (three types: a linear chain structure, with a cross-linked and with a three dimensional network struc- ture) depent on the variety of drawing parameters during fiber preparation and how the structure of fibers depent on these pa- rameters as compared with the bulk glasses and with fibers of different prehistories. The properties indicate significant an- isotropics and orientation effects in the fiber structures.

I - INTRODUCTION AND GOAL

The rheology of the glass fiber drawing process is well understood to- day [1-3]. Various parameters determine this process and in connection with that the properties and structure of glass fibers are determined by these drawing parameters. The independent parameters are: nozzle temperature, pressure on the nozzle and drawing speed; the dependent parameters are: mass flow, drawing force, drawing stress and strain, fiber radius, and cooling rate.

Mainly three of these parameters have the most dominating influence on properties and structure of glass fibers: cooling rate and nozzle tem- perature (thermal influence) and the drawing stress during fiber pre- paration (mechanical influence). By systematic variation of the above mentioned parameters it is possible to separate the dominating influ- ences in order to find out how the structure is determinded by a

(nearly) pure thermal and by a (nearly) pure mechanical prehistory of the glass fibers [4, 5 ] .

In the present paper the results of three types of glass fibers, prepa- red under the mentioned aspects, have been investigated by structure- sensitive properties as optical birefringence, density, thermal shrink- age, and X-ray diffraction. The selected glass fiber types are: E-glass as an example for a three-dimensional connected borosilicate network;

an alkali metaphosphate glass ("NaPoLi": 25 Li2O, 25 Na2O, 50 P2O5) and an alkaline earth metaphosphate glass ("CaBa": 25 CaO, 25 BaO, 50 P205) as examples for a one- and two-dimensional connected network, respectively.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985883

C8-528 JOURNAL DE PHYSIQUE

I1 - STRUCTURE-SENSITIVE PROPERTIES OF GLASS FIBERS

I, In K

-0- 1623 -=- 1573

-A- 1523

E-Gloss

A

20 LO 60 80 100

- Draw~ng Stress o, In MPa

-

Figs. 1 a-d: Birefringence as a function of drawing stress during the fiber spinning process

a. E-glass fibers, parameter: nozzle temperature b. CaBa-glass fibers, parameter: nozzle temperature

c. NaPoLi-glass fibers at constant nozzle temperature ( 4 5 0 ~ ~ ) and two mass flows m = 0.85 and 6.5 mg/s

d. NaPoLi-glass fibers at constant nozzle temperature ( 3 7 5 O ~ ) and two mass flows m = 1.0 and 2.3 mg/s

As was shown in [4, 61 and in a not yet published work of the authors of this paper [ 7 ] the birefringence An can be determined with the help of the fiber drawing apparatus described in [3] as functions of the following parameters:

An = f(vZ, T ) at p = constant D

An = f(RE, T ) at p = constant D

An = f (FZ, T ~ , R ) at fi = constant E

The best description of the fiber birefringence may be done, however, with the drawing stress a and nozzle temperature T at constant mass flow fn. Figs. la to d shoz this behaviour for the tRree investigated types of glass fibers as a comparison. It is remarkable that the bire- fringence increases with increasing nozzle temperature at constant stress levels. Obviously, the higher the temperature, the larger are the frozen-in strains and orientations. This effect is very significant for the E-glass and NaPoLi-glass fibers.

The largest birefringence values obtained at high stresses show the sequence E-glass + CaBa + NaPoLi glass fibers, indicating an increasing anisotropy and orientation capability. The maximum birefringence values up to 10 000 nm/rnm are comparable with values obtained from linearly connected organic polymers [ 8 , 91 and point to oriented chain structure in the NaPoLi fibers. These fibers show also three effects which don't occur in the CaBa nor in the E-glass fibers: a mass flow dependent birefringence and a non-linear dependence (more than and less than linear) of birefringence on stress at high levels (figs. lc and d).

These effects can be interpreted, that the chain formation process and the orientation process are intensified with increasing mass flow, which means increasing drawing force and delay time within the gob during the fiber spinning process. Therefore the birefringence will be increased with mass flow (figs. lc and d) and additionally with stress (fig. lc). The less than linear increase with drawing stress is due to the fact, that disorientation acts already at room temperature (time between drawing process and measurement), if very high stress levels are applied as is the case for the samples of fig. Id.

11.2 - DENSITY AND SHRINKAGE

The density doesn't show only an open network structure, produced by rapid quenching, but also an anomalous temperature behaviour, result- ing from drawing stress. Thus, fibers with constant diameters, drawn at higher nozzle temperatures, are more dense than those which have been drawn at lower temperatures. From the density values of the pri- stine fibers and their shrinkaae at Tg for 15 hours and from the den- sity values of the fully relaxed fibers after total shrinkage it is possible to calculate the degree of anisotropy, at least partly, as was shown in [ 4 1 for the case of E-glass fibers only, to which the reader will be refered. The main results for all three glass fiber types of this paper are gathered in table 1 in which the smallest and largest obtained values are listed together with the maximum values of birefringence and some other characteristics. These resulkindicate an isotropic, open, hiah-temperature structure, frozen-in by rapid quen- ching (thermal prehistory), superimposed by an anisotropic open network structure with anisotropic properties and orientations in the direction of fiber axis, produced by the drawing stress (mechanical prehistory).

11.3 - X-RAY DIFFRACTION PATTERN

The large angle X-ray diffraction patterns have shown, that this direct structural method is much less structure-sensitive than the above ones.

While the X-ray diffraction shows a significant orientation effect for those NaPoLi fibers with extreme drawing conditions (high degree of

stretching, fiber diameters 8 pm) for nozzle t m eratures 375 and 5 0 0 ~ ~ with corresponding halos found at 5.5 and 3.5

8-7

(27 pm fiber diameter and more), for which birefringence was found.

C8-530 JOURNAL DE PHYSIQUE

Also, no indications were found for ansymmetric diffraction halos for the CaBa and E-glass fibers, even not for extreme drawing conditions.

Table 1 RANGES FOR FIBER PROPERTIES

Fig. 2: Large angle X-ray diffraction at tern of NaPoLi fibers with the

1

.

375OC nozzle temperature, 2.0 mg/s mass flow,

. 8 pm fiber diameter,

: drawing stress 80 MPa

!

_ I

.

...!

;.;!kt.:

t.---

,

"4

',i +' .,: . ' a ^,*.!.+.ib -': '

+ densities of the bulk alasses or totally relaxed fibers, respectively Anmax

in nm/mm at 80 MPa

4 5 2 60 8500 fiber diam.

in ~.lm

5 + 50 7 + 40 6 ⁺ 40 fiber type

E-glass CaBa NaPoLi

I11 - CONCLUSIONS

density-3 in g cm

2.518 + 2.535 (2.5585)+

3.105 + 3 . p 7 (3.1336) 2.411 + 2.419

(2.4238) nozzle temp.

in OC

1150 + 1300 700 ⁺ 800 400 ⁺ 500

From the results the following conclusions may be drawn about the structure of the three types of fibers with respect to that of their bulk glasses.

fiber type

E-glass CaBa NaPoLi

The NaPoLi glass fibers are built up by phosphate anion chains growing in length with decreasing temperature. With respect to the applied nozzle temperatures, both, the chain forming and the orientation pro- cess are acting in an opposite sense: the increasing chain lengths lead to entanglements and thus, to increasing difficulties for orientation with decreasing temperature as the birefringence measurements show.

This is confirmed by the very low shrinkage values of maximal 4 % only, compared with those of the CaBa and E-glass fibers of maximal 1.5 %

apparent Poisson ratio Ar/r/Al/l 0.43 + 0.78 0.01 + 0.31 0.28 ⁺ -0.42

anisotropic 3 portion (10 ) All1 - Ar/r 0.8 ⁺ 4.8 2.5 + 9.6 1.1 + 42.5 total contraction (10 3 )

axial Al/l 3.5 + 8.4 3.6 + 9.8 1.5 ⁺ 30

radial Ar/r 2.81 + 3.7 0.11 ^-+ 2.72 0.42 -+ -12.5

a P-0-P-bond switching mechanism within chains and between different chains which produces an increasing entanglement. The shrinkage pro- cess itself will be much smaller in this way as if a shrinkage of un- changed chains without bond switching would take place. This interpre- tation is confirmed by the calculated radial dilatation during annea- ling (table l), which can be understood by the described entanglement process, too.

The structure of the CaBa glass fibers may be suspected to consist of double chains, tighted together by the bivalent alkaline earth ions.

However, the low birefringence values indicate, that the phosphate chains in the CaBa fibers are intensively cross-linked. In this way a partly three-dimensional network is formed, however, in not such a strict manner as in typical silicate glasses or silicate glass fibers.

Therefore the orientation effects in the CaBa fibers are much less than in the NaPoLi fib'ers, but somewhat more pronounced than in the E-glass fibers as indicated by the experimental results.

The structure of the E-glass fibers was interpreted in [4] as an uni- axial deformation of a three-dimensional network with frozen-in struc- tural strains and very strong disordering forces. Statistically, a prefered orientation of the bridging oxygen atoms in axial fiber direc- tion and of non-bridging oxygen atoms perpendicular to that can be suggested from the results here and from [5].

Furthermore, as was concluded in [ 5 1 , all three types of glass fibers contain flaws which obviously are sharply stretched and oriented along the direction of fiber axis and which determine with their very small cross section the fracture behaviour and the well-known high tensile strength.

IV - REFERENCES

[l] Manfre', G.: Glass Technol. 10 (1969) 99.

[2] Glicksman, L.R. and ~rishnan,~.: Amer.Soc.mechan.Engrs. 93D

(1971) 355.

[3] ~ t e h l e , M. and Briickner, R.: Glastechn.Ber. 52 (1979) 82 and 105.

[4] Stockhorst, H. and Briickner, R.: J . ~ o n - ~ r ~ s t x o l i d s 49 (1982) 471.

[5] Pahler, G. and Briickner, R.: Glastechn.Ber. 2 (1985) 33 and 45.

[61 Stockhorst, H.: PhD-thesis, Berlin 1984.

[7] Stockhorst, H. and Briickner, R.: J.Non-Cryst.Solids,in progress.

[81 Liska, E.: Kolloid-Z. 251 (1973) 1028.

[9] Falkai, B.: Angew.Makrom.Chemie

108

Acknowledgement: The authors are greatful to the Deutsche Forschungs- gemeinschaft, Bonn-Bad Godesberg, FRG, for financial support and to Prof. Dr. Dr. h.c. R. Hosemann and Dr. M . Hentschel f0r.X-ray diffrac- tion patterns of fiber bundles.