REINFORCED PLASTICS SYMPOSIUM
GLASS FIBER REINFORCEMENT FOR PLASTICS ROBERT M . McMARLIN New glass technology portends potential for higher strength reinforced plastics and increasing use ofglassjbers as reinforcing agents f o r resinbased composites o understand many of the basic properties of glass
Tand glass fibers and their beneficial effects in rein-
forcing plastics, it is best first to examine the structure of the material. The major constituent of most commercial inorganic glasses is silica. The silicon dioxide molecule is composed of a central silicon ion surrounded by four oxygen ions in a tetrahedral configuration (Figure 1). In quartz, the room temperature modification of silicon
dioxide, the tetrahedral molecules form an ordered threedimensional structure. When melting takes place, the ordered silica network breaks down and becomes random in nature, as shown in Figure 2 in two dimensions. As long as the silica is in the molten state, the bonds in this more or less random network are constantly breaking and reforming. Upon cooling, however, the molten glass becomes too viscous to allow the silica molecules to revert readily to an ordered crystalline arrangement. The final structure can be visualized as a three-dimensional amorphous polymer. This general theory of glass structure is called the random network hypothesis. Molten silica can incorporate almost every chemical element. For example, sodium ions or .calcium ions would fit into "holes," or interstices, within the random network represented in two dimensions in Figure 3.
Sliced end of reinforced rod stock magniJicd 73,090times shows 0.00038- to 0.09042-inchdiameter glassfberfilanunts in resin M Iw
VOL 5 8
NO. 3
OIE"ISORNIN0
i#eEs(il** C o w .
MARCH 1966
21
Note that the secondary ions have broken some of the oxygen-silicon bonds, thus making the glass more fluid when molten. These incorporated ions are known as network modifiers. A second theory of glass structure, the crystallite hypothesis, has also been developed (4).Various investigators have found evidence in certain glasses of crystalline domains which are limited to a few structural units of 7-15 A. These crystallites are apparently surrounded by more deformed intermediate zones. There is a third theory of glass structure, the micelle hypothesis, which accounts for a reported structure composed of individual elements (micelles) of globular, laminar, or chainlike shape. These micelles are reported to have a definite chemical composition and to extend over a range of from 20-200 A. Even though there is a wide difference of opinion regarding the structure of glass, each of these theories contributes something to our understanding. For simplicity, we prefer the structural theory which assumes a more or less random network composed of silica tetrahedra. According to this theory, the degree of randomness appears to depend on the silica content. With less silica in the glass, more randomness is possible; conversely, with more silica there is a greater possibility of finding small crystalline areas. Alkali and alkaline earth ions are capable of modifying this structure by entering interstices in the network. Under certain conditions, various added ions (Al+a, Mg+2, and Z ~ I + ~ , for example) may even enter actively into the silica network. Property Regulation by Composition Control
How this general structural theory provides an explanation of several of the more common properties of glass and glass fibers becomes apparent by examination of two properties, viscosity and density. Fused silica has a viscosity of 1OI2 poises at 2550" F. The addition of only 20% S a z O lowers the viscosity at this teinperature to l o 2 poises. I t is assumed that the Na+ ion enters holes within the silica network. To maintain electrical neutrality, the silica network must be broken at several points. These points correspond to oxygen ions bonded to only one silica. I t is this breaking of bonds which lowers the viscosity. Density variations caused by the addition of different oxides can be explained in a similar manner. If we assume that the addition of certain ions to a silicate glass extends the network, we find that an actual density measurement is higher than expected. The only conclusion is that the ions have not materially increased the glass volume, but have entered existing interstices in the silica network. This is true of ions such as Li+, K+, Ba+*, Pb+2, Sr+2,and Ca+2. We cannot always assume, however, that ions smaller than some of these will also enter holes in the network. Density measurements indicate that the addition of ions such as Mg+2, Zn+2, Al+', Ti+4, and Zr+4 produce a lower density than expected. This means that the ions have entered the network struc22
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
ture itself and have caused an increase in the glass volume. These and other properties of glass depend to a large extent on the chemical makeup of this material. Yet certain properties also depend on other factors. These often become all-important when a bulk glass is fiberized. A common window glass composition, NazO-CaOSiO2, is durable in bulk form. When it is fiberized, however, it has a low resistance to water attack. The large surface area of the fibrous Na20-CaO-SiOZ glass accelerates the attack which proceeds by a leaching of the alkali component. Other compositions containing a high alkali content behave in a similar manner.
TABLE I.
C H E M I C A L COMPOSITIONS OF T H R E E TYPES O F GLASS
Constztuent Si02 A1203 CaO
1
1
E-Glass
I
54.0
15.0 17.0 5.0
~
MFO
1
C-Glass 65.0 4.0
I
14.0
I
3.0
S-Glass
id
TABLE I I . M O D U L U S O F ELASTICITY AND C H E M I C A L COMPOS IT I ON Constituent 1
E-Glass
Si02
54.0
A1203
15.0 17 .O
CaO MgO B,>Oa
5 .O 8.0
1 1 j
K20
I
Fez03 Be0 ZrO2
... ... ...
... ... ...
2
P.S.I.,
...
...
IilO
Modulus,
... ...
il.0
TiOz
1
M-Glass
65 25
