Glass composition for spinning fibre

Glass composition for spinning fibre

122 Journal of Non-Crystalline Solids 80 (1986) 122-134 North-Holland, Amsterdam GLASS COMPOSITION FOR SPINNING FIBRE S. K U M A R Central Glass and...

700KB Sizes 0 Downloads 14 Views

122

Journal of Non-Crystalline Solids 80 (1986) 122-134 North-Holland, Amsterdam

GLASS COMPOSITION FOR SPINNING FIBRE S. K U M A R Central Glass and Ceramic Research lnsatute, Calcutta, India

The performance of polymer composites reinforced with neutral glass fibres was compared with that of "E" glass reinforced composites. The strength and fatigue characteristics of composites, and the polymerisation kinetics at the glass-polymer interface, were studied. The results show that "N" glass fibre composites possess superior wet strength and are suitable for general application.

Broadly, the use of reinforcement fibres of glass can be classified in two groups: (1) polymer matrix based composites, (2) brittle matrix, i.e. cement and gypsum based composites. T h e development of polymer matrix composites took place in order to utilise the high intrinsic strength of glass in fibre form where the polymer matrix serves to hold the fibres, to space them, to transfer the load to individual fibres and to protect them from mechanical and chemical damage. T o fulfil the above functions the interface between the fibre and the polymer has to play a crucial role. In fact, successful development of polymer composites has been closely linked with the development of interfacial keying agents [1]. In the case of the second group of composites, i.e. brittle matrix composites, glass fibres are incorporated to change the brittle nature of the matrix to a quasi-plastic one, thereby preventing a sudden failure under load, particularly under impact. Incorporation of glass fibres not only modifies the fracture mode but also increases the impact strength, by at least an order of magnitude; the tensile and the flexural strengths also increase significantly. T h e bond between the fibre and the matrix in these composites appears to be of the frictional type and therefore is different from that in polymer matrix composites [2]. A glass composition suitable for the manufacture of commercial reinforcement fibres should satisfy certain prerequisites. It should be cheap, melt at temperatures not too high for c o m m o n glass melting practice and show favourable working characteristics, so that fibres can be drawn without devitrification. It should also possess high mechanical strength and elastic modulus and adequate resistance to fatigue and chemical attack. A 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

S. Kumar / Glass composition for spinning fibre

123

v a s t n u m b e r of c o m p o s i t i o n s h a v e b e e n r e p o r t e d in the l i t e r a t u r e for the p r o d u c t i o n of r e i n f o r c e m e n t fibres. H o w e v e r , d e p e n d i n g o n a p p l i c a t i o n n e e d s , five p r e d o m i n a n t glass c o m p o s i t i o n s a r e u s e d in the p r o d u c t i o n of c o n t i n u o u s f i l a m e n t fibre glass p r o d u c t s - t h e " A " type, a s o d a l i m e glass, the " E " o r e l e c t r i c a l type, the " C " t y p e - a glass with i m p r o v e d c h e m i c a l d u r a b i l i t y , the " S " t y p e for high p e r f o r m a n c e a p p l i c a t i o n s m o s t l y in a e r o s p a c e a p p l i c a t i o n a n d , lastly, the " A R " o r a l k a l i - r e s i s t a n t fibres for r e i n f o r c e m e n t of c e m e n t a n d c o n c r e t e (table 1).

E-glass: " E " glass d o e s n o t h a v e a d e f i n e d c o m p o s i t i o n . T h e c o m p o s i t i o n r a n g e for this f a m i l y of glasses is s h o w n in t a b l e 2. T h e b u l k of the c o n t i n u o u s glass fibres p r o d u c e d is of the " E " type. A l t h o u g h o r i g i n a l l y d e v e l o p e d for e l e c t r i c a l uses, the " E " glass t o d a y finds a l m o s t u n i v e r s a l application. N o t w i t h s t a n d i n g this, t h e " E " glass suffers f r o m s e v e r a l l i m i t a t i o n s . B e i n g v e r y low in a l k a l i e s , t h e m e l t i n g a n d refining t e m p e r a t u r e s of these glasses a r e high, a r o u n d 1550°C. A t t h e h i g h f o u n d i n g t e m p e r a t u r e s e m p l o y e d , " E " glass is h i g h l y c o r r o s i v e o n r e f r a c t o r y m a t e r i a l s . C h r o m i u m o x i d e p r o v i d e s the b e s t glass c o n t a c t r e f r a c t o r y , f o l l o w e d b y d e n s e zircon. D u e to low alkali c o n t e n t , the v a p o u r p r e s s u r e of b o r i c o x i d e at the m e l t i n g t e m p e r a t u r e is h i g h a n d a c l o s e c o n t r o l of c o m p o s i t i o n is difficult. A l l these f a c t o r s a d d e d to t h e b a s i c c o s t of the glass itself, call for the a d o p t i o n of s o p h i s t i c a t e d p l a n t s r e q u i r i n g h i g h c a p i t a l input. T h e s e a r e s o m e of the m a j o r r e a s o n s w h y t h e t e c h n o l o g y for the p r o d u c t i o n of fibre glass has b e e n c o n f i n e d to a few l a r g e i n t e r n a t i o n a l c o m p a n i e s [3,4].

Table 1 Glass composition used in major applications of continuous filament fibre glass Constituents

A

E

C

AR

S

SiO2 A12Os + Fe203 CaO MgO Na20 K20 B203 BaO TiO2 ZrO2 SO3 AS205

72.0 0.6 10.0

54.3 15.2 17.3 4.7 0.6

64.6 4.1 13.4 3.3 7.9 1.7 4.7 0.9 tr

60.9 0.27 4.8 0.1 14.3 2.7 6.5 10.2 0.2 -

65.0 25.0 10.0 -

F 2

-

14.0 0.7 tr -

8.0

0.1

S. Kumar / Glass composition for spinning fibre

124

Table 2 Composition range of "E" glass fibre Constituents

Typical "E" glass

Free barium

Boron and fluorine free

With boron and fluorine replaced by barium

SiO2 AlzO3

55.2 14.8 7.3 3.3 18.7 0.3 0.2 0.3 0.3 -

54-64 3.5-5 4.5-8 0-4.5 7-12 14-16 0-2.5 0-0.5 -

55-63 11-18 0-10 9-25 0-2.5 0-1 2-5 0-4 0-1.5 0-2.5 0.3-2.5 0-1.5 0-2.0

58-64 3-6 4-9 3-7 12-19 -

n203 MgO CaO Na20 K20 Fe203 F2 TiO2 ZnO SrO BaO Li20 MnO

-

ZrO2

-

1.5-4

T h e s t r e n g t h and m o d u l u s values of " E " glass are marginally higher than those of " C " glass and resistance to acid attack is v e r y poor. For these reasons the c o m p o s i t e s w h i c h will be in c o n t a c t with acidic materials are always p r o v i d e d with a layer r e i n f o r c e d with chemically resistant " C " glass for p r o t e c t i o n of the p a r e n t " E " glass laminate [5]. It is w o r t h noting that " A " glass c a n also be used for such applications [6,7]. In view of these facts, it w o u l d be worthwhile to reexamine w h e t h e r fibres m a d e out of o t h e r glass c o m p o s i t i o n s could be used with g r e a t e r a d v a n t a g e with respect to cost of p r o d u c t i o n and p e r f o r m a n c e . Here, we p r o p o s e to present s o m e of o u r w o r k on the p r e p a r a t i o n and evaluation of r e i n f o r c e m e n t fibres f r o m c o m m e r c i a l soda glass, ( " A " glass) and neutral glass ( " N " glass). T h e r e a f t e r , we will briefly consider the future o u t l o o k of high m o d u l u s and high strength fibres, w h i c h m a y effectively c o m p e t e with o t h e r high p e r f o r m a n c e fibres, particularly c a r b o n fibre.

A-glass: leaving apart the specialised applications, the " A " type, one of the earliest glass c o m p o s i t i o n s used in spinning fibre, has b e e n retained only for a few non-critical applications. H o w e v e r , a vast and c h e a p supply of " A " glass c a n be d e r i v e d f r o m bottle and plate glass cullets. T w o m a j o r applications of " A " glass are for r e i n f o r c e m e n t of plaster of paris and b i t u m e n felt. Since these are i m p o r t a n t building materials p r o d u c e d in large volumes, i n c o r p o r a t i o n of a low cost fibre is particularly a d v a n t a g e o u s . " A " glass can also be used for resin r e i n f o r c e m e n t for

S. Kumar / Glass composition for spinning fibre

125

certain non-critical applications. Several authors [3] have indicated that the mechanical properties of " A " glass composites are well within the minimum requirement of performance laid down for " E " glass composites. However, the strength of the fibres has been reported to decrease during drying of the cakes, and the data on long term performance of " A " glass composites are meagre. A simple method of drawing " A " glass fibre from waste strips of glass sheets has been worked out. In this process, glass is drawn from a supply point, i.e. the heated end of a strip of sheet glass. The fibre cools and solidifies as it is drawn. The drawing force is applied through the solidified fibre by a mechanical means, i.e. winding of the fibre into a rotating drum. The drawing force necessary to produce a fibre is largely determined by the viscosity of the glass, the optimum viscosity being in the region of 104 P. The fibre drawing assembly used for the purpose is shown in fig. 1. 50-100 sheet glass strips are suspended from a perforated MS plate which is hung from two ends of a grooved horizontal shaft. The DC drive of this shaft feeds the glass strips uniformly at a predetermined rate into an electrically heated rectangular muffle furnace which has two slit openings one at the top, for the entry of the strips and the other at the bottom for the exit of fibres. To ensure uniform distribution of temperature along the horizontal direction, silicon carbide plates are placed between the furnace elements and the glass strips. As the fibre comes out, it passes over an applicator and a gathering shoe and is then wound on a collect drum. The

Fig. 1. An assembly for drawing fibres from waste glass strips.

126

S. Kumar / Glass composition [or spinning fibre

description of this equipment is the same as that of a conventional fibre drawing procedure. Although the fibres are drawn from rectangular strips, the section is nearly circular as shown in fig. 2. The " K " type fibres having a diameter of about 12-14/zM can be drawn from strips of 4 m m x 10 mm size, by an appropriate setting of the rate of feeding the strips into the furnace, the drawing speed and the furnace temperature. The output is about 3 kg per hour and energy consumption is about 2.5 kW per kg of fibre. The salient features of the process are: effective utilisation of waste glass, low energy consumption and freedom from pollution hazards. The technology is simple, yet competitive. N-glass: fibres produced from a commercial neutral glass composition might be a better candidate for resin reinforcement. Keeping this in view we attempted to produce fibre glass from a conventional neutral glass compostion ("N" glass) which is in commercial production. The composition and some of the properties of the glass are shown in table 3. The acid and alkali resistance of this glass is shown in figs. 3 and 4, respectively, along with that of " E " glass. It should be possible to draw "N" glass fibres from conventional bushings using commercial cullets as raw material. However, for the present studies glass fibres were drawn from rods of 3.5-6 mm diameter by the process mentioned earlier.

Fig. 2. Sectionalview of "A" glass fibres drawn from strips.

S. Kumar I Glass composition for spinning fibre

127

Table 3 Properties of "N" glass Compositions (wt%) SiO2 AI203 Fe203 68.41 5.05 0.17 Melting temperature

TiO3 Tr

CaO 7,12

MgO O. 19

B2O 3 6.25

ZnO 1.00

K20 3.46

Na20 7.84

1500°C

Devitrification range

825-1075°C

Liquidus

1075°C

6 5

~4

f

~' 't/

Oo~.. ..........

_

E

....

N FIBRE GLASS

: .....

F,BR

GLASS

,~

,,~

TIME (HOURS)

Fig. 3. Acid resistance of E and N types of fibre glass.

E

"O

E uO u~

"~

0. J ~J ~ 0 _J

--'E' G L A S S --'N' G L A S S

2

3

O

io

2'o

TIME IN HOURS --~

3'o

4~o

ALKALI RESISTANCE

Fig. 4. Alkali resistance of E and N types of fibre glass.

i

5O

128

S. Kumar / Glass composition for spinning fibre

A detailed study was made on the fibre size formulation based on a silane coupling agent ~/-methacryloxy-propyl-tri-methoxy silane (A174 - Union Carbide) compatible with polyester resin. The optimum formulation, which showed the best result, was used for coating the fibres. The properties of "N" glass fibre have been compared with that of " E " glass fibre - commercially available in India. The findings are shown in figs. 5to9. Fig. 5 shows the diameter distribution of experimental "N" glass and commercial " E " glass fibres for 100 monofilaments segregated from 1200 tex roving. The diameter was measured by using a Vicker's image shearing microscope. The mean of two readings varying by not more than ±0.03/~m was taken as the average diameter. The histogram of the diameter distribution shows a mean value of 14.8/~m for "N" glass fibre and 14.25/~m for " E " glass fibres. The range of distribution of diameter is more or less the same for both. However, in the case of "N" glass there is a small percentage of fibre having a diameter below 11 ~m. It may be mentioned that diameters were measured on coated fibre and, therefore, the values may be slightly on the high side. No attempts were made to determine the strength of virgin monofilament or monofilaments collected from roving, since it was felt that an investigation on the composite strength would be more realistic as far as application is concerned. Unidirectional flexural strength measurements were carried out to evaluate the composites, since the 0 ° flexural strength is considered to be a filament-dominated property. For this purpose unidirectional composite rods of dia(6.5 ± 0.3) mm diameter were prepared by drawing a bunch of roving impregnated with resin through a glass tube. The resin used was of isopthalic polyester type (HSR8131 - Bakelite Hylam Ltd, India). The composite rods were post-cured at a temperature of 80°C for 4 h as per the recommendation of the resin manufacturer. The glass content of all the rods, as determined by burning off the resin, was (73 + 1)% by weight. The flexural strength of the composites was determined in 3-point bending following the procedure of BS 3691. The composite rods were boiled in distilled water for varying periods of time and the flexural strength of the composites was again determined following the procedure mentioned above. For each data point at least 5 samples were used. the results of this investigation are shown in figs. 6 to 8. The mean dry strength of "N" glass composites is 1123 MPa which is about 10% less than that of " E " glass composites. However, the strength of both the composites becomes equal after subjecting them to boiling water for about 10 h, as shown in fig. 7. Thereafter, the strength of "N" glass composites shows a higher retention trend than that of " E " glass on further boiling. After about 240 h of boiling, the strength of both the composites levels off, at 60% for "N" glass and 45% for " E " glass as shown in fig. 8.

44

4

4

8

I0

IO

14

16

14

16

"N'GLASS

18

T8

MEAN

---"

2O

i

C.V.- I0%

DIAMETER-14"25#m

E GLASS

20

MEAN DIAMETER-14"B#m C.V. - 130 Z

M I C R O N S ---*

12

M ICRONS

12

--

Fig. 5. Diameter distribution of "E" and "N" glass fibre.

z

28 h 0 t~ 2 O nl 1:13 Zl2

<

~0 4 4 uJ J ~-36

Z

~r

1:1312

Ix;

u12) 2 8

~36

U3 L~J

9OO

20

24 I

4

8

HO0

30 0

!,

I 5~00

MEAN FLEXURALVALUE 1,1235(M. Pa) C , V - - 8 o ./.

N GLASS

FLEXURAL STRENGTH(MPa)

II00

1500

STRENGTH (M P~)

1300

MEAN FLEXURAL STRENGTH

FLEXURAL

[

strength)

'E' GLASS

(Dry

Fig. 6. Flexural strength of unidirectional and "E" and "N" glass composites.

900

bD ,2

<) 1 6

J

Z

r~ UJ ,'n

u_ 12 0

~" < 16 {.,9

u3 ,,,20 J n

24

~C

E

S. Kumar / Glass composition for spinning fibre

130

--'E" FIBRE GLASS

I

---'N' FIBRE GLASS 1400

r-m 1200 x c'~O IIOO r

800

-I

m

600

Z 400 "1-

,-, 2 0 0

3:

I

4

0

8I0

TIME

,

i

I

120

(HOURS)

I 160

I 2 O0

"---"

Fig. 7. Retention of flexural strength of unidirectional "E" and "N" glass composites in boiling water.

! I00t~

9oJ~ IV,o _ 80'~ 70 ~

"'0.

o~

60

X c

o - - - - - o . . _ _ . _ _ _ -o-

. . . . . .

.-o-

. . . . . . .

JO-'N'

5o ~

'

E

'

40 --4

ao --I -'- 2 0 To

i

0

20

i

i

60

,

r

IO0

i

i

140

i

180

~

i

220

i,

,

T I M E (HOURS) Fig. 8. Percentage retention of flexural strength of unidirectional "E" and "N" glass in

composites in boiling water. As mentioned earlier, it is expected that the degradation of "N" glass fibre in water will be less than that of "E" glass. However, the observed reduction of strength in a composite is an interrelated phenomenon involving the polymer matrix, the reinforcement and the interface. Since the polymer matrix is the same in both cases, it may be concluded that the

S. Kumar / Glass composition lot spinning fibre ----....... --

UNTREATED 'NEUTRAI~ GLASS (19t'c) TREATED 'NEUTRAl' GLASS (tB4"e) POLYESTER RESINCISOPTHALIC)(JgB'O TREATED "E' GLASS (18¢'0 UNTREATED 'E'GLASS (l't6"c)

]91 184"(2 I t776 *C

200

C

~

iB~ C ~ , . . . . . ~ ~wq-)i 6 0

Y w 20

131

Catalyst: M,k "P{roxid¢ : I °/~

Accelerator: B© - obolt 0opthonot,

98°C

l/ ;~/ ~ t// ~ .:/(~

__1%

40

O

0

I

I

I

I0

20

30

I

40

TIME (Mins) Fig. 9. Influence of glass composition on polymerisation of resin.

o b s e r v e d effect may be due to the interface between the resin and the fibre, and the fibre itself. T h e influence of glass composition on the polymerisation characteristic of the resin was studied. 300 mesh (BS sieve) glass powders were prepared from heat-cleaned glass fibres. T h e powder was washed with acetone and a 20 g portion was mixed with 100 g of polyester resin which was activated with 1% M E K P and 1% cobalt napthanate. T h e resin container was kept in a Brooke-Field constant t e m p e r a t u r e bath whose t e m p e r a t u r e was maintained at (40 ± 0.1)°C. T h e rise in temperature with time was recorded with a c h r o m e l - a l u m e l thermocouple. In another set of experiments 2 0 g of powder of both " E " and " N " glass were treated with a hydrolysed solution of 0.3% A174 silane coupling agent in water. T h e treated powder was dried at r o o m t e m p e r a t u r e for 24 h and at 80°C for 4 h. T h e exothermic peak was again determined following the procedure mentioned above. T h e rise in t e m p e r a t u r e with time is shown in fig. 9. T h e results indicate that the exothermic peak appears earlier in the presence of " N " glass, whereas when " E " glass is present the peak is developed after a longer period. This suggests that in the presence of " N " glass polymerisation takes place at a faster rate leading to the formation of a more dense matrix around the

132

S. Kumar / Glass composition[or spinning fibre

fibres. Higher wet strength retention in the case of " N " glass composites may be ascribed to the formation of the denser layer of polymer at the interface. At the time of this report, the sudy of the fatigue characteristics is still in progress. Initial results show that both the composites have similar fatigue characteristics in flexure. No failure took place in both the composites at 10% of the ultimate strength even after 30000 cycles. Further investigations of the fatigue and dynamic characteristics for both the composites are being pursued.

High performance fibres: in recent years there has been an increasing demand for fibres having high strength and moduli, especially for aerospace and other critical applications. However, in spite of considerable R & D efforts, modulus values higher than 100 GPa could not be achieved in a glass. In table 4 the properties of high modulus glass fibres are compared with those of carbon and alumina fibres. The modulus value of "S" glass is about 20% higher than that of " E " glass and is about the highest that could be achieved in a commercial glass. By incorporation of beryllium, lanthanum, etc, the value can be pushed up further but such glasses are of limited commercial utility for reasons of health hazards and the high cost of production. In any case, the properties are decidedly inferior to those of the other competing materials, such as carbon fibres. During the last ten years or so, substantial progress has been made towards the production of high modulus glass-ceramic fibres through controlled nucleation and crystallisation of glass fibres [8-11]. One of the essential prerequisites for the production of glass-ceramic fibres is that while fibres are drawn through a bushing, crystal formation should be minimum. The glass composition has to be suitably tailored so that the crystallisation rate at the drawing temperature is not more than a few microns per minute [12]. This has been achieved through the incorporation Table 4 High strength and high modulus fibres Fibre type A. S-glass B. Glass ceramic crystal phase (i) cordierite (ii) cordierite/beryl (iii) /3-eucryptite//3-spodumene C. Carbon (i) high strength (ii) high modulus D. Alumina

Modulus (GPa)

Strength (MPa)

85.5

2414.0

144.0 128.0 147.0

5324.0

226.0 392.0 289.7

2740.0 2060.0 2448.0

S. Kumar / Glass composition [or spinning fibre

133

of certain rare earths which retard crystal growth. However, contrary to previous belief, in the presence of a small volume of crystals, an increase in the modulus has been observed without lowering of the strength. T h r o u g h controlled nucleation and crystallisation of glass fibres substantial imp r o v e m e n t s in strength could be achieved. A wide variety of glass-ceramic compositions have been d e v e l o p e d for the purpose of yielding cordierite, beryl, spodumene, encryptite, diopside, spinel, etc, as the m a j o r crystal phases. In table 4 a few of the results are shown. T h e strength and modulus values upto 5 3 0 0 M P a and 128 GPa, respectively, could be obtained in glass-ceramic fibres. Also, by stretching the fibres to 10% of the length during nucleation and crystallisation, the modulus has been reported to increase from 86 to 147 GPa. It may be mentioned here that m a n y of the glass-ceramic fibres also possess superior resistance to alkali attack as c o m p a r e d to commercial " A R " glass, and there is a distinct possibility of using this type of fibre in cement composites as well.

Summary (1) T h e E-glass c o m m o n l y used for resin reinforcement suffers from several limitations. It has a high melting temperature, a strong corrosive action on alumino-silicate refractories and a p o o r resistance to acids. (2) A glass can be used for reinforcement of gypsum and bitumen and in resin composites for non-critical applications. It can be produced cheaply from strips of waste sheet glass. T h e technology is simple and competitive. (3) Resin composites reinforced with commercial neutral glass possess superior wet strength retention properties and resistance to acid attack as c o m p a r e d to E-glass reinforced composites. (4) Glass-ceramic fibres possessing high strength and modulus appear to hold considerable promise for reinforcement of high p e r f o r m a n c e c o m posites and also c e m e n t composites.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

E.P. Plueddemann, ed., Composite materials, Vol. 6 (Academic Press, New York, 1974). J.F. Ryder, BRE Notes, Reference No. 211/69 (1969). F.F. Javay, 28th SPI Annual Technical Conf.; Section 3a (1973). K.L. Loewenstein, The Manufacturing Technology of Continuous Glass Fibres (Elsevier, Amsterdam, 1973) p. 39. National Bureau of Standards (USA) Product Standards, PS 15-69 (1969) p. 2. J.A. Schlarb, 19th SPI Annual Technical Conf., Section 4-A (1964). E.P. Plueddemann and G.L. Stark, 28th SPI Annual Technical Conf., Section 21 -E (1973). J. Economy, ed., New and Specialty Fibres (Wiley, New York, 1976) p. 117. A. Maries, Nature 256 (5516) (1975) 401. A. Maries and P.S. Rogers, J. Mater. Sci. 13 (1978) 2119. H.L. Ritter, USP 4199336 dt. (April 22, 1980).

134

[12] (a) (b) (c) (d) le) (~) (g) (h) (i) (j) (k) (1)

S. Kumar / Glass composition [or spinning fibre Bayer Aktien Gesellschaft, BP 1415628 dt. (November 26, 1975). Bayer Aktien Gesellschaft, Neth P 7604473 dt. (May 24, 1977). W.H. Brueggemann, USP 3900306 dt. (August 19, 1975). W. Schartan and F. Schwochow, Ger P 2532842 dt. (February 10, 1977). H. Taki et al., Jap P 7546721 dr. (April 25, 1975). Bayer Aktien Gesellschaft, Neth P 7604473 dt. (May 24, 1977). M. Sopicka-Lizer et al., Bull. Soc. Fr. Ceram, 128 (1980) 13 (in French). R.W. Jones and P.W. MacMillan, J. Non-Cryst. Solids 38-39, Pt. II (1980) 705. T. Kokubo et al., J. Ceram. Soc., Japan 89(7) (1981) 375 (in Japanese). S. Meriani and G. Soravu, Riv. Stn. Sper. Vetro (Muxano, Italy), 12(3) (1982) 118. S. Meriani et al., Riv. Stn. Sper. Vetro (Muxano, Italy) 12(4) (1982) 147. S. Meriani et al., Thermal Analysis, Vol. 1, ed., B. Miller, (Wiley, New York, 1982) p. 660.