Biosoluble pot and marble-derived fiberglass

Glass compositions suitable for pot and marble fiberization display excellent chemical resistance to both acids and moisture while being highly biosoluble at the same time. The glass compositions are characterized by ratios of components which are reflective of acid resistance, biosolubility, and moisture resistance. Preferred glasses have a difference between HTV (10.sup.3 poise) and liquidus greater than 500.degree. F., and a biodissolution greater than about 350 ng/cm.sup.2 /hr.

TECHNICAL FIELD 
The present invention pertains to fiberglass products prepared from glass 
compositions suitable for fiberization by the pot and marble process. The 
glass fibers exhibit enhanced biosolubility while maintaining other 
desirable properties. 
DESCRIPTION OF THE RELATED ART 
Fiberglass has a myriad of uses, including the reinforcement of polymer 
matrix composites; preparation of thermoformable intermediate products for 
use as headliners and hoodliners in vehicles; air and water filtration 
media; and sound and thermal insulation products. The preparation and/or 
subsequent processing of such materials often involves handling steps 
which result in cut or broken fibers which may be inhaled. As it is 
impractical or impossible to remove such fibers from the body, it has 
become important to create glass compositions which exhibit high degrees 
of biosolubility, i.e. which are rapidly solubilized in biological fluids. 
If high biosolubility were the only factor which need be considered, a 
solution to the biosolubility problem would be rapidly attained. However, 
in addition to being biosoluble, glass fibers must also possess a number 
of other physical and chemical characteristics. For example, in many 
applications such as in battery separators, high chemical (e.g. acid) 
resistance is required. As can be readily imagined, high chemical 
resistance and high biosolubility are largely conflicting characteristics. 
Glass fibers must also be strong and moisture-resistant. If moisture 
weakens glass fibers appreciably, their applicability to many uses 
suffers. Weakened glass fibers not only possess less than desired tensile 
strength and modulus, but also break and fracture more easily, thus 
increasing the risk of inhalation, etc. By the same token, moisture 
resistant glass fibers which have low strength to begin with also do not 
fulfill many requirements. For example, building insulation is shipped in 
compressed form. If the glass fibers of the insulation product are weak or 
brittle, many fibers will be broken during compression, not only 
increasing the number of small fibers which are bioavailable, but also 
producing an inferior product which may not recover a sufficient amount of 
its pre-compressed thickness. Strong fibers which are not moisture 
resistant also exhibit a great deal of breakage, especially under humid 
storage, as illustrated hereinafter. Finally, glass fibers must be 
prepared from glass compositions which can be economically processed. 
The two principle methods of glass wool fiber production are the pot and 
marble process and the centrifugal or "rotary" process. In the latter, 
molten glass enters a centrifugal spinner from the forehearth of a glass 
melting furnace. As the centrifugal spinner rotates, relatively large 
diameter glass strands stream from orifices located in the spinner's 
periphery. These large diameter strands immediately contact an intense hot 
gas jet produced by burners located around the spinner. The hot gas 
attenuates the large diameter strands into fine, elongated fibers, which 
may be collected on a moving belt. 
As glass is an amorphous rather than crystalline "solid", crystallization 
in the melt or during fiberization will disrupt the fiber glass forming 
process with disastrous results. In the rotary process, the glass 
ingredients are first melted in a glass melter prior to their entry into 
the forehearth. Thus, the feed to the forehearth is high temperature, 
molten glass. From the forehearth, the molten glass fed to the spinner is 
cooled to the HTV (high temperature viscosity) or "fiberization" 
temperature. Because the forehearth is fed with hot, molten glass, and the 
temperature of the glass in the forehearth is above the HTV, the 
difference in temperature between the HTV and liquidus (".DELTA.T"), the 
temperature which defines the boundary of crystallization, may be quite 
small in the rotary process. 
In the pot and marble process, relatively large diameter "primary" strands 
of glass (primaries) exude from holes located in the bottom of the pot. 
Because room temperature marbles are continuously or incrementally added 
to the pot, numerous locations will exist within the pot where the 
temperature might fall below the liquidus temperature, thermodynamically 
favoring crystallization and disrupting the process. To ensure that the 
process is not disrupted, glass compositions must be used which exhibit a 
significant difference, minimally 300.degree. F., between the HTV and 
liquidus temperatures. Thus, glass compositions formulated for the rotary 
process, having low .DELTA.T, are not suitable for use in pot and marble 
process. 
The primaries exiting the pot from the pot and marble process are flame 
attenuated rather than hot gas attenuated, thus exposing the glass fibers 
to higher temperatures than in the rotary process. These higher 
temperatures cause a loss of the more volatile compounds of the glass 
composition from the outside of the fibers, resulting in a "shell" which 
has a different composition than the fiber interior. As a result, the 
biosolubility of glass fibers prepared from pot and marble fiberglass is 
not the same as that derived from the rotary process. As glass fibers must 
necessarily dissolve from the fiber ends or the cylindrical exterior, more 
highly resistant shell will drastically impede the biodissolution rate. 
SUMMARY OF THE INVENTION 
It has now been surprisingly discovered that glass fibers of enhanced 
biosolubility may be prepared from glass compositions suitable for pot and 
marble processing, which exhibit minimally about a 350.degree. F. 
difference in HTV and liquidus, and which have well defined formulations 
meeting both narrow mol percentage composition as well as meeting each of 
three specific "C-ratios" which govern chemical resistance, moisture 
resistance, and biosolubility. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The subject invention glasses have HTV and liquidus which are suitable for 
production of glass fibers in the pot and marble process. Such glass 
generally must have an HTV (10.sup.3 poise) of 1800.degree. F. to 
2100.degree. F., preferably 1900.degree. F. to 2000.degree. F., and 
exhibit a liquidus which is minimally about 350.degree. F., preferably 
425.degree. F., and more preferably 500.degree. F. or more lower than the 
HTV. These characteristics are necessary to prepare glass fibers 
economically on a continuous basis. 
The glass composition must fall within the following range of composition, 
in mol percent: 
______________________________________ 
SiO.sub.2 
66-69.7 
Al.sub.2 O.sub.3 
0-2.2 
RO 7-18 
R.sub.2 O 
9-20 
B.sub.2 O.sub.3 
0-7.1 
______________________________________ 
where R.sub.2 O is an alkali metal oxide and RO is an alkaline earth metal 
oxide. R.sub.2 O is preferably Na.sub.2 O in most substantial part, while 
RO may be MgO and/or CaO, preferably both, in a molar ratio of MgO/CaO of 
1:3 to 3:1, more preferably 2:3 to 3:2. The chemical behavior of the glass 
is dictated by three ratios which the glass composition must meet, 
C(acid), C(bio), and C(moist). These ratios are defined compositionally as 
follows, all amounts being in mol percent: 
EQU C(acid)=SiO.sub.2 !/(Al.sub.2 O.sub.3 !+B.sub.2 O.sub.3 !+R.sub.2 
O!+RO!) 
EQU C(bio)=(SiO.sub.2 !+Al.sub.2 O.sub.3 !)/(B.sub.2 O.sub.3 !+R.sub.2 
O!+RO!) 
EQU C(moist)=(SiO.sub.2 !+Al.sub.2 O.sub.3 !+B.sub.2 O.sub.3 !)/(R.sub.2 
O!+RO!). 
In these ratios, C(acid) is the ratio which pertains to chemical resistance 
in acid environments, C(bio) is the ratio which is most closely linked to 
biosolubility, and C(moist) is the ratio which relates to retention of 
properties in moist environments. It is desired that C(acid) and C(moist) 
be as large as possible, while C(bio) should be as low as possible. At the 
same time, the HTV and liquidus of the overall composition must be 
suitable for glass fiber processing. It has been found that pot and marble 
glass of high biosolubility, while yet maintaining other necessary 
physical properties such as chemical resistance and moisture resistance, 
is obtained when C(acid).gtoreq.1.95, C(bio).ltoreq.2.30, and 
C(moist).gtoreq.2.40. 
Preferably, the biosoluble fiberglass of the subject invention has a 
composition which falls within the following ranges (in mol percent): 
______________________________________ 
SiO.sub.2 
66-69.0 
Al.sub.2 O.sub.3 
0-2.2 
RO 7-16 
R.sub.2 O 
9-19 
B.sub.2 O.sub.3 
0-7.1 
______________________________________ 
Most preferably, the biosoluble glass fibers of the subject invention have 
a composition which falls within the following most preferred range: 
______________________________________ 
SiO.sub.2 
66-68.25 
Al.sub.2 O.sub.3 
0-2.2 
RO 7-13 
R.sub.2 O 
11-18 
B.sub.2 O.sub.3 
0-7.1 
______________________________________ 
With respect to the performance characteristics of the glass fibers of the 
subject invention, it is preferred that C(acid) be greater than or equal 
to 2.00; C(bio) be less than or equal to 2.23, more preferably less than 
or equal to 2.20; and that C(moist) be greater than or equal to 2.50, 
preferably greater than or equal to 2.60. As discussed previously, it is 
most desirable that C(acid) and C(moist) be as high as possible. For 
example, C(moist) values of 3.00 or greater are particularly preferred. It 
should be noted also, that the various C-ratios are independent in the 
sense that a more preferred glass need not have all "more preferred" 
C-ratios. 
Acid resistance may be measured by battery industry standard tests. For 
example, a typical test involves addition of 5 grams of nominally 3 .mu.m 
diameter fiber in 50 mL of sulfuric acid having a specific gravity of 
1.26. Following refluxing for 3 hours, the acid phase may be separated by 
filtration and analyzed for dissolved metals or other elements. 
The procedure used to evaluate biodissolution rate is similar to that 
described in Law et al. (1990). The procedure consists essentially of 
leaching a 0.5 gram aliquant of the candidate fibers in a synthetic 
physiological fluid, known as Gamble's fluid, or synthetic extracellular 
fluid (SEF) at a temperature of 37.degree. C. and a rate adjusted to 
achieve a ratio of flow rate to fiber surface area of 0.02 cm/hr to 0.04 
cm/hr for a period of up to 1,000 hours duration. Fibers are held in a 
thin layer between 0.2 .mu.m polycarbonate filter media backed by plastic 
support mesh and the entire assembly placed within a polycarbonate sample 
cell through which the fluid may be percolated. Fluid pH is regulated to 
7.4+0.1 through the use of positive pressure of 5% CO.sub.2 /95% N.sub.2 
throughout the flow system. 
Elemental analysis using inductively coupled plasma spectroscopy (ICP) of 
fluid samples taken at specific time intervals are used to calculate the 
total mass of glass dissolved. From this data, an overall rate constant 
could be calculated for each fiber type from the relation: 
EQU k=d.sub.o .rho.(1-(M/M.sub.o).sup.0.5 !)/2t 
where k is the dissolution rate constant in SEF, d.sub.o the initial fiber 
diameter, .rho. the initial density of the glass comprising the fiber, 
M.sub.o the initial mass of the fibers, M the final mass of the fibers 
(M/M.sub.o =the mass fraction remaining), and t the time over which the 
data was taken. Details of the derivation of this relation is given in 
Leineweber (1982) and Potter and Mattson (1991). Values for k may be 
reported in ng/cm.sup.2 /hr and preferably exceed a value of 150. 
Replicate runs on several fibers in a given sample set show that k values 
are consistent to within 3 percent for a given composition. 
Data obtained from this evaluation can be effectively correlated within the 
sample set chosen--dissolution data used to derive k's were obtained only 
from experimental samples of uniform (3.0 .mu.m) diameter and under 
identical conditions of initial sample surface area per volume of fluid 
per unit time, and sample permeability. Data was obtained from runs of up 
to 30 days to obtain an accurate representation of the long term 
dissolution of the fibers. Preferred biodissolution rate constants in 
ng/cm.sup.2 /hr are greater than 150 ng/cm.sup.2 /hr, preferably greater 
than 200 ng/cm.sup.2 /hr, more preferably greater than 300 ng/cm.sup.2 
/hr, and most preferably greater than 400 ng/cm.sup.2 /hr.

Having generally described this invention, a further understanding can be 
obtained by reference to certain specific examples which are provided 
herein for purposes of illustration only and are not intended to be 
limiting unless otherwise specified. 
EXAMPLES 
Comparative C1 and C2 
C-ratios are calculated for a conventional C-glass (chemically resistant 
glass) and a "soluble" glass as disclosed in Examples 1a and 2b in Table 1 
of U.S. Pat. No. 5,055,428. The glass composition is in weight percent. 
HTV (10.sup.3 poise) and liquidus are as reported in the '428 patent. 
TABLE 1 
______________________________________ 
Comparative Example 1 
Comparative Example 2 
(Wt %) (Wt %) 
______________________________________ 
SiO.sub.2 
66.4 66.7 
Al.sub.2 O.sub.3 
1.2 1.0 
B.sub.2 O.sub.3 
11.0 10.0 
Na.sub.2 O 
12.9 13.9 
K.sub.2 O 
0.2 0.2 
CaO 4.8 5.5 
MgO 3.2 2.5 
C(acid) 2.03 2.07 
C(bio) 2.09 2.13 
C(moist) 3.41 3.29 
HTV 1965.degree. F. 1949.degree. F. 
(10.sup.3 poise) 
Liquidus 1738.degree. F. 1702.degree. F. 
______________________________________ 
As can be seen from Table 1, the C-ratios of these rotary process glasses 
indicate that they should both have good performance with respect to acid 
resistance, moisture resistance, and biosolubility. The Comparative 
Example 2 glass is reported by the patentee to have a dissolution rate in 
model physiological saline (composition not disclosed) of 211 ng/cm.sup.2 
/hr. However, examination of the HTV and liquidus temperatures reveals 
that these differ only by 227.degree. F. and 247.degree. F., respectively. 
Thus, these glass compositions cannot be used in pot and marble 
fiberization. These Comparative Examples serve to illustrate the ease with 
which higher biosolubility can be obtained in rotary processable glass. 
These glasses cannot be used to manufacture fiberglass by the pot and 
marble process. However, even were this possible, the flame attenuation 
and consequent loss of volatile oxides from the fiber surface would be 
expected to lower the measured biodissolution rate by a factor of from 
about 2 to 4. 
Examples 1 and 2 
Two glass formulations were processed into marbles for use in pot and 
marble fiberization, and glass fibers prepared in the conventional manner. 
The formulations, C-ratios, HTV (10.sup.3 poise), liquidus, and measured 
biosolubility are presented in Table 2. The ingredients are in mol 
percent. 
TABLE 2 
______________________________________ 
Example 1 (mol %) 
Example 2 (mol %) 
______________________________________ 
SiO.sub.2 67.24 67.18 
Al.sub.2 O.sub.3 
1.04 1.02 
B.sub.2 O.sub.3 
6.08 5.99 
CaO 4.99 4.87 
MgO 5.24 5.26 
Na.sub.2 O 15.22 15.45 
K.sub.2 O 0.26 0.23 
C(acid) 2.05 2.05 
C(bio) 2.15 2.14 
C(moist) 2.89 2.87 
Biosol K(dis) 
350 426 
HTV 1972 1981 
Liquidus 1435 &lt;1325 
______________________________________ 
The C-ratios indicate that the glasses of Table 2 should exhibit desirable 
chemical resistance (both acid and moisture) as well as high 
biodissolution. The high biodissolution is confirmed by actual tests, 
being in both cases, considerably greater than 300 ng/cm.sup.2 /hr. 
Example 3, Comparative Examples C3 and C4 
A subject invention glass is compared with two commercial glasses for acid 
resistance and moisture resistance, respectively. The formulations (mol 
percent) are as follows. 
TABLE 3 
______________________________________ 
Example 3 Example C3 
Example C4 
______________________________________ 
SiO.sub.2 
67.28 65.36 57.53 
Al.sub.2 O.sub.3 
1.04 1.83 3.11 
B.sub.2 O.sub.3 
6.00 4.59 7.23 
CaO 4.00 6.27 8.82 
Na.sub.2 O 
15.20 15.56 16.24 
K.sub.2 O 
0.26 0.45 0.71 
F.sub.2 -- 1.43 -- 
C(acid) 2.06 1.96 1.35 
C(bio) 2.16 2.14 1.54 
C(moist) 2.89 2.67 2.11 
MgO 5.23 4.52 6.36 
______________________________________ 
The acid resistance of the Example 3 glass was compared with that of 
Comparative Example C3. It is noted that the Comparative Example C3 glass 
meets the C-ratio requirements but not the compositional limitations. The 
results of the acid resistance test are presented below in Table 3a. 
TABLE 3a 
______________________________________ 
Example 3 Example C3 
Glass Quantity Dissolved 
Quantity Dissolved 
Element (ppm) (ppm) 
______________________________________ 
Al 187 453 
Ca 2831 4110 
Mg 854 938 
______________________________________ 
To determine moisture resistance, a stress corrosion test is used in which 
the fibers are stressed by bending in a controlled humidity and 
temperature test chamber. Fibers which exhibit moisture resistance under 
these conditions take longer to break. The Example 3 glass was compared to 
Comparative Example C4 glass, a glass used commercially for building 
insulation where compression of insulation and storage generates the 
potential for fiber breakage as a result. After 50 hours, only 12% of the 
Example 3 glass had broken, while all of the Comparative Example C4 fibers 
had failed. 
Comparative Examples C5 and C6 
C-ratios are calculated for the rotary process glasses of Example 3 of U.S. 
Pat. No. 4,510,252, and Example 2 of U.S. Pat. No. 4,628,038. Composition, 
calculated C-ratios, liquidus, and estimated HTV (10.sup.3 poise) are 
given below in Table 4, in mol percent. 
TABLE 4 
______________________________________ 
Example C5 
Example C6 
______________________________________ 
SiO.sub.2 68.2 66.67 
Al.sub.2 O.sub.3 
2.2 2.25 
B.sub.2 O.sub.3 
5.0 4.78 
Na.sub.2 O 9.2 8.68 
K.sub.2 O -- 0.26 
CaO 11.9 14.77 
MgO 3.5 3.60 
C(acid) 1.85 1.94 
C(bio) 2.03 2.15 
C(moist) 2.54 2.70 
HTV 2280.degree. F. 
2210.degree. F. 
(10.sup.3 poise), est. 
Liquidus 1983.degree. F. 
2035.degree. F. 
______________________________________ 
As can be seen from the table, the acid resistance of Comparative Example 
C5 is expected to be low, and the biodissolution is expected also to be 
low, although the glass should display good moisture resistance. However, 
the difference between HTV (10.sup.3 poise) and liquidus is only about 
297.degree. F., and thus this glass is not suitable for use in a pot and 
marble process. The glass of Comparative Example C6 exhibits C(acid) close 
to an acceptable value, although C(bio) is too high. The glass should have 
good moisture resistance. However, the glass cannot be used in a pot and 
marble process as the difference between liquidus and HTV (10.sup.3 poise) 
is only 175.degree. F. 
Comparative Example 7 
C-ratios and composition data (mol percent) are presented for Example 6 of 
U.S. Pat. No. 5,108,957. 
TABLE 5 
______________________________________ 
Example C7 
______________________________________ 
SiO.sub.2 
69.55 
Al.sub.2 O.sub.3 
0.08 
CaO 7.46 
MgO 4.30 
Na.sub.2 O 
15.05 
K.sub.2 O 
0.04 
B.sub.2 O.sub.3 
3.52 
C(acid) 
2.28 
C(bio) 2.29 
C(moist) 
2.72 
HTV 2003.degree. F. 
(10.sup.3 poise) 
Liquidus 
1706.degree. F. 
______________________________________ 
The biodissolution for this glass should be marginal, however the moisture 
and acid resistance should be acceptable. However, the difference in HTV 
and liquidus (.DELTA.T) indicates that this glass is unsuitable for pot 
and marble fiberization. 
Examples 4-12 
Additional glass compositions which fall within the subject invention 
parameters are presented in the following table. 
__________________________________________________________________________ 
EXAMPLES 4-12 
Example 4 
Example 5 
Example 6 
Example 7 
Example 8 
Example 9 
Example 10 
Example 11 
Example 12 
Mol % Mol % 
Mol % 
Mol % 
Mol % 
Mol % 
Mol % Mol % Mol % 
__________________________________________________________________________ 
SiO.sub.2 
67.28 
67.4 66.38 
66.9 65.96 
68.03 
69.08 67.96 68.61 
Al.sub.2 O.sub.3 
1.04 1.03 2.35 2.37 2.33 0 0 0 0 
B.sub.2 O.sub.3 
6.00 6.06 3.49 7.03 6.93 0.75 3.42 3.37 6.8 
Na.sub.2 O 
15.20 
15.25 
17.13 
16.08 
9.25 16.08 
17.58 11.63 9.83 
K.sub.2 O 
0.26 0.19 0.52 0 0.51 0 0 0 0 
CaO 4.99 4.83 4.87 3.76 7.42 7.43 4.89 8.46 7.28 
MgO 5.23 5.23 5.27 3.87 7.63 7.71 5.02 8.58 7.48 
__________________________________________________________________________ 
By the term "consisting essentially of" is meant that additional 
ingredients may be added provided they do not substantially alter the 
nature of the composition. Substances which cause the biodissolution rate 
to drop below 150 ng/cm.sup.2 /hr or which lower the .DELTA.T to a value 
below 350.degree. F. are substances which do substantially alter the 
composition. Preferably, the glass compositions are free of iron oxides, 
lead oxides, fluorine, phosphates (P.sub.2 O.sub.5), zirconia, and other 
expensive oxides, except as unavoidable impurities. It should be noted 
that while rotary process glass compositions are in general unsuitable for 
pot and marble fiberization, the reverse is not true, and the subject 
invention glass compositions should yield fibers prepared by the rotary 
process which have yet higher rates of biodissolution. 
Having now fully described the invention, it will be apparent to one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.