SYNTHETIC VITREOUS FIBERS 163
4. CHEMICAL AND PHYSICAL INFORMATION
4.1 CHEMICAL IDENTITY
Synthetic vitreous fibers are inorganic substances, largely composed of aluminum and calcium silicates
that are derived from rock, clay, slag, or glass (IARC 1988, 2002). While naturally occurring mineral
fibers such as asbestos are crystalline in structure, synthetic vitreous fibers are amorphous materials.
There are several methods of categorizing synthetic vitreous fibers based either on origin, chemical
structure, morphology, application, or method of manufacturing. The most recent classification scheme
proposed by the International Agency for Research on Cancer (IARC) has divided these compounds into
two broad classes: filaments and wools. The filaments contain continuous glass filaments, while the
wools contain glass wool, rock (stone) wool, slag wool, refractory ceramic fibers, and other newly
engineered biosoluble fibers (IARC 2002). Glass wools are further subdivided into insulation wools and
special purpose wools (see Figure 2-1). Continuous filament products are produced by drawing or
spinning the molten mix from nozzles, while the wools are manufactured with a rotary or centrifugal
process without using a nozzle (see Chapter 5 for details). Generally, the wool fibers tend to be shorter
and finer than the continuous filament fibers, and their diameters may be more variable (IARC 1988).
The typical chemical composition of these types of synthetic vitreous fibers is represented in Table 4-1.
Special purpose glass fibers are sometimes used in high technology industries and have very specific
properties that are tailored to their specific use. Although the procedures used to make these fibers are
similar to those of glass wool, the operating parameters are usually adjusted to create products with
extremely small diameters. One example of a special purpose glass fiber is included in Table 4-1.
Fibrous glass products are derived from powdered sand and largely consist of silicon and aluminum
oxides. The final properties of the glass are dictated by the percent composition of other oxides including
alkali metal oxides, alkaline earth oxides, and metal oxides like ZrO
2
and Fe
2
O
3
. Glass, like other
insulating materials, provides a high resistance to the passage of electricity. Electrical glass (E-glass) is a
continuous filament type of fibrous glass developed for electrical applications that has excellent heat and
water resistance (IARC 1988, 2002). The high resistivity of E-glass is related to its low alkali oxide
content. The majority of continuous filament fibrous glass produced is E-glass (IARC 1988, 2002).
Other types of glass are used for certain types of specialized purposes, and relatively small changes in the
SYNTHETIC VITREOUS FIBERS 164
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-1. Chemical Identity of Some Types of Synthetic Vitreous Fibers
a,b
Percent
com-
position
E-
Glass
S-
Glass
AR-
Glass
Glass
wool
Rock wool
from basalt
melted in a
furnace
Rock wool from
basalt and other
material melted
in a cupola
Slag wool
melted in
a cupola RCF kaolin
RCF
zirconia
Special
purpose glass
fiber 475
formulation
c
SiO
2
52–56 65 60.7 55–70 45–48 41–53 38–52 49.5–53.5 47.5–50 57–58
Al
2
O
3
12–16 25 0–7 12–13.5 6–14 5–15 43.5–47 35–36 5–6
B
2
O
3
5–10 3–12 10–11
K
2
O 0–2 2 0–2.5 0.8–2 0.5–2 0.3–2 <0.01 <0.01 2–3
Na
2
O 0–2 13–18 2.5–3.3 1.1–3.5 0–1 0.5 <0.3 10–11
MgO 0–5 10 0–5 8–10 6–16 4–14 <0.1 0.01 0–0.5
CaO 16–25 5–13 10–12 10–25 20–43 <0.1 <0.05 2–3
TiO
2
0–1.5 0–0.5 2.5–3 0.9–3.5 0.3–1 2 0.04 0–0.1
Fe
2
O
3
0–0.8 0.1–0.5 1 <0.05 0–0.1
FeO 11–12 3–8 0–2
Li
2
O 1.3 0–0.5
SO
3
0–0.5
S 0–0.2 0–0.2 0–2
F
2
0–1 0–1.5
BaO 0–3 5
ZrO
2
21.5 0.1 15–17
P
2
O
5
0–0.5
Cr
2
O
3
<0.03 <0.01
ZnO 4
a
Navy Environmental Health Center 1997; TIMA 1993
b
As is standard practice, the chemical composition of the elements are reported as oxides, even though no such individual crystals
exist in the fibers.
c
There are several formulations applicable to this category and formulation 475 is generally representative.
AR-glass = alkali resistant glass; E-glass = electrical glass (so called because the low alkali oxide content makes it useful for
electrical applications); RCF = refractory ceramic fiber; S-glass = high tensile strength glass (stronger than E-glass)
SYNTHETIC VITREOUS FIBERS 165
4. CHEMICAL AND PHYSICAL INFORMATION
chemical composition of the glass can result in significant changes to its optical, electrical, chemical, and
mechanical properties. Chemical glass (C-glass) is highly resistant to attack by chemicals such as
hydrofluoric acid, concentrated phosphoric acid (when hot), and superheated water. The chemical
resistance is determined by the relative amounts of the acidic oxides (SiO
2
, B
2
O
3
), basic oxides (CaO,
MgO, Na
2
O, K
2
O), and amphoteric oxides (Al
2
O
3
). High-strength glass (S-glass) is almost completely
composed of aluminum, silicon, and magnesium oxides and finds use in sophisticated high technology
applications where high tensile strength is required; its tensile strength is typically 30–40% greater than
E-glass. Alkali resistant glass (AR-glass) contains a high percentage of zirconium oxide, which makes
this type of glass highly resistant to acidic and alkaline compounds.
The term mineral wool is often used to collectively refer to rock wool and slag wool, although
occasionally, glass wool was included in this category. Similar to other glass fibers, the chemical
composition of rock wool and slag wool are primarily aluminum and silicon oxides. However, these
wools possess a higher alkaline earth oxide content (MgO and CaO) and lower alkali metal oxide content
(Na
2
O and K
2
O) than glass wool. Rock wool is derived from igneous rocks such as diabase, basalt, or
olivine, while slag wool is derived from blast furnace slag from the steel industry (Navy Environmental
Health Center 1997).
Refractory ceramic fibers are a specialized type of synthetic vitreous fiber that are highly heat resistant
and thus find use in high-temperature applications. Refractory ceramic fibers contain a much higher
concentration of alumina than the other fibers listed in Table 4-1 and are sometimes referred to as
aluminosilicate glasses. Although refractory ceramic fibers are amorphous at low temperatures, they
undergo partial crystallization (devitrification) to quartz, cristobalite, or tridymite at elevated temperatures
(Maxim et al. 1999b).
4.2 PHYSICAL AND CHEMICAL PROPERTIES
The important physical properties that are pertinent for organic compounds are generally not applicable to
inorganic materials like fibrous glass. Properties such as vapor pressure, Henry’s law constant, and
octanol/water partition coefficient are exceedingly low and not measurable. Even properties like melting
point are difficult to define since fibrous glass products are amorphous and do not experience a distinct
melting point as crystalline materials do, but soften over a fairly broad temperature range. The term
softening point is used for materials that do not possess a definite melting point and is often employed
SYNTHETIC VITREOUS FIBERS 166
4. CHEMICAL AND PHYSICAL INFORMATION
when discussing synthetic vitreous fibers. It represents the temperature at which plastic flow becomes
viscous flow, and is specifically defined as the temperature at which the viscosity of the partial molten
glass is 10
7.6
poise (TIMA 1993). Since synthetic vitreous fibers are often used in textile products as a
reinforcing material, the softening point is an important physical property. Some physical properties of
the synthetic vitreous fibers listed in Table 4-1 are shown in Table 4-2. Since the final products within
each class of fibers can be varied according to manufacturing specifications, the values listed in Table 4-2
should only be considered representative of the properties for each class in a very general sense.
Synthetic vitreous fibers are not actually soluble in water, but the term dissolution is often used to
describe the durability of synthetic vitreous fibers, especially as it pertains to biological fluid. This
should not be confused with solubility, which is the amount of material that dissolves in solution before it
reaches chemical equilibrium. The dissolution rate is the rate at which a fiber reacts with a solution and is
degraded in it. Under alkaline and acidic conditions, the silicate network of synthetic vitreous fibers can
be attacked, resulting in the leaching of individual ions and the eventual disruption of the entire fiber
network. Due to the larger surface area, fine fibers have greater dissolution rates than course fibers (see
Section 3.4 for details).
Because the toxicity of fibers is related to their physical dimensions, it is important to characterize the
size of synthetic vitreous fibers. In a typical glass fiber product, the average length is usually on the order
of several centimeters, but the average diameter is usually on the order of a few microns. The nominal
diameter is defined as the average fiber diameter in the finished product and varies according to fiber
type, use, and manufacturing process involved (ACGIH 2001). The diameters of airborne fibers are an
important physical property from a biological standpoint because thin fibers are considered respirable and
may be deposited in the peripheral lung airways. Airborne fibers with diameters <3 µm are generally
considered respirable in humans. There is also a strong correlation between the fiber diameter and the
airborne fiber levels found in workplaces (Esmen and Erdal 1990; Esmen et al. 1979a, 1979b). Generally,
the greater the fiber diameter, the lower the airborne fiber concentration. The nominal fiber diameter of
continuous filament fibrous glass is usually in the range of 3–25 µm, depending upon the application,
with typical diameters in the range of 6–15 µm (Navy Environmental Health Center 1997). The method
of producing continuous filament fibers allows for excellent control of the preset fiber diameter and as a
result, there is little variation in range of diameters for the resulting product. The production of rock
wool, slag wool, and glass wool includes a rotary or centrifugal process resulting in nominal fiber
SYNTHETIC VITREOUS FIBERS 167
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Physical Properties of Some Types of Synthetic Vitreous Fibers
a
Property E-Glass S-Glass
AR-
Glass
Glass
wool
Rock
wool
Slag
wool
Refractory
ceramic
fibers
Special purpose
glass fiber 475
formulation
b
Molecular
weight
N/A N/A N/A N/A N/A N/A N/A N/A
Density (g/cm
3
) 2.60–
2.65
2.5 2.52 2.40–
2.55
2.7–2.9 2.7–2.9 2.6–2.7 2.4
Softening point
°C
835–
860
970 680 650–
700
No data No data 1,740–1,800 650
Dielectric
constant at
1 MHz
5.8–6.4
c
4.9–5.3
c
No
data
No data No data No data No data No data
Modulus of
elasticity (GPa)
70–75 85 70–75 55–62 55–62 48–76 76–100
d
No data
Refractive
index
1.55–
1.57
1.52 1.525 1.51–
1.54
1.6–1.8 1.6–1.8 1.55–1.57 1.53
Tensile
strength (MPa)
3,400
c
4,590
c
3,700
c
No data 482–689
d
482–
689
d
1,000–
1,300
c,e
No data
a
All data derived from TIMA 1993 unless otherwise noted.
b
There are several formulations applicable to this category and formulation 475 is generally representative.
c
Fitzer et al. 1988
d
Navy Environmental Health Center 1997
e
There are various commercial products of boron or silicon carbide filaments or yarns with high tensile strength, but
these are crystalline fibers and technically not synthetic vitreous fibers.
N/A = not applicable
SYNTHETIC VITREOUS FIBERS 168
4. CHEMICAL AND PHYSICAL INFORMATION
diameters in the range of about 3–7 µm for rock wool and slag wool and 3–15 µm for ordinary glass wool
(Navy Environmental Health Center 1997). The smaller diameters of these fibers in comparison to
continuous filament fibers, allows for the possibility that a small fraction of these fibers may be respirable
when they become airborne. Special purpose glass fibers are produced by a flame attenuation process
whereby the hot, molten glass is poured in front of a high temperature gas flame, resulting in fibers with a
mean diameter of <3 µm and very often <1 µm. Refractory ceramic fibers (RCFs) are produced by
melting and spinning or blowing of calcinated kaolin, aluminum silicates and metallic oxide blends, and
high purity aluminum silicate. The typical fiber diameter of RCFs is 1–5 µm.
Christensen et al. (1993) employed light microscopy (LM) and scanning electron microscopy (SEM) to
measure the length-weighted diameters of 22 synthetic vitreous fiber products obtained from 11 different
manufacturers. In this study, nine different glass wool products, nine rock wool or slag wool products,
three refractory ceramics, and a single special purpose glass fiber were analyzed. The results of this study
are summarized in Table 4-3.
The results of a recent comprehensive workplace monitoring study using transmission electron
microscopy (TEM) was reported by Mast et al. (2000), which characterized the airborne fiber dimensions
of refractory ceramic fibers. Measurements of 3,357 fibers obtained at 98 workplaces yielded an airborne
diameter range of 0.067–4.0 µm. The arithmetic mean and standard deviation were reported as 1.05 µm
and 0.64 µm, respectively, while the geometric mean and standard deviation were reported as 0.84 µm
and 2.05 (the geometric standard deviation is unitless), respectively (Mast et al. 2000). Fiber lengths
ranged from 0.6 to 138 µm, with an arithmetic mean length and standard deviation of 20.6 µm and
19.3 µm, respectively. The geometric mean length and geometric standard deviation were reported as
14.1 µm and 2.48, respectively. The size distributions of airborne synthetic vitreous fibers at different
locations under a variety of occupational settings were summarized in the most recent IARC monograph
(IARC 2002) and these data are condensed in Table 4-4.
SYNTHETIC VITREOUS FIBERS 169
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-3. Measured Diameters of Glass Wool, Rock Wool, Slag Wool, Refractory
Ceramic Fibers, and a Special Purpose Glass Fiber
a
Number of
products
studied
Arithmetic mean
diameter range
(µm) LM
Geometric mean
diameter range
(µm) LM
Arithmetic mean
diameter range
(µm) SEM
Geometric mean
diameter range (µm)
SEM
Glass wool
9 2.4–8.1 1.7–6.6 1.2–7.7 0.8–6.3
Special purpose glass fiber
b
1 Not applicable
b
Not applicable
b
0.6 0.4
Mineral wool
9 2.5–4.7 1.7–3.3 2.4–5.3 1.7–4.0
Refractory ceramic fibers
3 2.3–3.9 1.5–2.8 2.4–3.8 1.7–2.8
a
Data obtained from Christensen et al. (1993); for all samples, between 400 and 490 individual fibers were
measured in order to derive the statistical quantities presented in the table.
b
A single special purpose glass wool fiber was studied with a diameter too small to be accurately measured by LM.
LM = light microscopy; SEM = scanning electron microscopy
SYNTHETIC VITREOUS FIBERS 170
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-4. Statistical Analysis of Airborne Fibers Under Different Occupational
Settings
a
SVF product or setting
GM diameter
(µm)
GSD
diameter
GM length
(µm)
GSD
length
Length-diameter
correlation
Rock wool production 0.3–0.5 1.9–2.7 7.0–9.0 2.2–3.0 0.4–0.6
Rock wool use 1.2 2.7 22 4.0 0.7
Glass wool use 0.75 2.8 16 3.5 0.7
Glass wool use 0.8–1.9 1.4–1.9 9.5–30 1.4–2.5 0.2–0.7
Rock wool use 1.6–1.9 1.6–1.9 19 1.7–2.7 0.4–0.6
Glass wool house
prefabrication
0.91–1.2 1.7–1.8 9.2–9.3 2.3–2.5 No data
Rock wool house prefabrication 1.3–1.7 1.9 12–17 2.5–2.8 No data
Installation of SVF batts 0.9–1.3 2.2 22–37 2.8–2.9 0.5–0.6
Installation of loose SVF with
binder
1.0–2.0 1.8–2.2 30–50 2.3–2.6 0.4–0.6
Installation of loose SVF
without binder
0.60 1.9 14–15 2.4–2.6 0.5–0.6
RCF production and use 0.84 2.05 14.1 2.5 0.4
RCF factory 0.96–1.2 1.7–1.9 12–19 2.4–2.6 No data
RCF factory 0.86 1.9–2.0 11–13 2.4–2.6 No data
a
IARC 2002
GM = geometric mean; GSD = geometric standard deviation; RCF = refractory ceramic fiber; SVF = synthetic
vitreous fiber
