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present can polymerize, various solutes and colloids present can react with the leached layer, and stress buildup can cause cracking and spalling. The characteristics of the leaching solution are very important, especially in long-term test, where the solution may become saturated and various crystalline phases may precipitate altering the concentration of leached species and the pH of the solution. The evaluation of glasses for hazardous waste disposal, where dissolution is over a very long time, requires careful examination of the solution characteristics.

Fiber Glass

A discussion of glass would not be complete if some mention of glass fibers were not made. The corrosion of fibers is inherently greater than bulk glass simply because of the larger surface-to- volume ratio. Since one of the major applications of fibers is as a reinforcement to some other material, the main property of interest is that of strength. Thus, any corrosion reactions that would lower the strength are of interest. This effect is important both when the fiber is being manufactured and after it has been embedded in another material. For example, the strength of E- glass (borosilicate) fibers in dry and humid environments was studied by Thomas [2.81], with the observation that humid environments lower strength. The mechanisms of environmentally enhanced stress corrosion of glass fiber are discussed in more detail in Chap. 8, page 360, Glassy Materials.

Wojnarovits [2.82] reported that multicomponent glass fibers exhibited a variation in dissolution in acid and alkaline environments due to the existence of a layered structure, each having a different dissolution rate, with the core generally having the highest rate. Single component fibers (i.e., silica) did not show this layering effect and thus no variation in dissolution rate.

Bioactive Glass

Bioactive glasses were first discovered by Hench in 1969. The special chemistry of these glasses allowed them to bond to living

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Chapter 2

bone. These Na2O–CaO–P2O5–SiO2 glasses have been trademarked as Bioglass® and marketed under several other names depending upon the application. The beneficial effect of these glasses is their controlled release of soluble silicon and calcium ions. In this way, the glass acts as a substrate for the growth of new cells. Newer forms of these glasses have been prepared via sol-gel routes that contain numerous very fine interconnected pores. Dissolution kinetics are a function of the following variables [2.83]:

1.Composition

2.Particle size

3.Pore size distribution, average size, and volume percentage

4.Surface area

5.Thermal stabilization temperature

6.Chemical stabilization temperature

The alumina content of bioactive glasses is very important in controlling the durability of the glass surface. The bioactivity, although dependent upon the bulk composition of the glass, decreases beyond acceptable levels once the alumina content rises above 1.0–1.5 wt.% [2.49]. This same phenomenon is present for glass compositions containing cations such as Ta2O5 except higher levels are tolerable (1.5–3.0 wt.%).

Rare earth aluminosilicate (REAS) glasses have been developed for applications as delivery agents for radiation in the treatment of various cancerous tumors [2.84]. In these cases, the glass must be sufficiently durable to allow the release of beta-radiation over a specified period of time (about 2 weeks) while being lodged within the malignant tumor. Once the radiation treatment has been completed, then the REAS can be resorbed into the body. It is important that these glasses not dissolve while being radioactive, which would release radioactive species into the other parts of the body damaging healthy tissue. These glasses are generally incorporated into the body as microspheres about 30 µm in diameter. A 90Y- containing radiotherapeutic REAS is sold under the trade name

Copyright © 2004 by Marcel Dekker, Inc.

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TheraSphere™ .* White and Day [2.84] reported no detectable weight loss of a 1×1×0.2 cm glass sample before 6 weeks in 100 mL of distilled water (pH=7) or saline (pH=7.4) at 37°C, 50°C, or 70°C. Dissolution rates of =3×10-9 g/cm2.min were determined after 6 weeks. In a comparison study of fused silica, a Corning glass (CGW-1723™), and yttria aluminosilicate (YAS), Oda and Yoshio [2.85] showed that YAS was significantly more durable than fused silica in saturated steam at 300°C and 8.6 MPa. The dissolution mechanism is very important for applications in the human body; however, it is very difficult to determine whether these glasses exhibit congruent or incongruent dissolution. Surface analyses of microspheres and bulk glasses indicated that the mechanism was congruent [2.84]. Using inductively coupled plasma and atomic adsorption spectroscopy, it has been determined that the yttrium release from YAS microspheres in distilled water or saline at 37°C or 50°C was below detectable limits [2.86].

More recently, Conzone et al. [2.87] have reported the development of borate glasses for use in treatment of rheumatoid arthritis since these glasses are potentially more reactive with physiological liquids. Borate glasses containing only alkali ions dissolved uniformly (i.e., congruently) in simulated physiological liquids at temperatures ranging from 22°C to 75°C. When the borate glasses contained other cations (such as Ca, Mg, Fe, Dy, Ho, Sm, and Y) in amounts ranging from 2 to 30 wt.%, dissolution was nonuniform (i.e., incongruent) with the formation of new compounds. Day [2.88] gave an example of Dy2O3-containing borate solid glass microspheres that reacted to form hollow spheres, shells of concentric layers, or microspheres filled with homogeneous gel-like material depending upon the Dy2O3 content. The dissolution mechanism involved the selective leaching of lithium and boron allowing the rare earth (i.e., Dy) to react and form an insoluble phosphate.* When calcium-containing borate

* TheraSphere™ is manufactured by MDS Nordion located in Ottawa, Ontario, Canada.

Copyright © 2004 by Marcel Dekker, Inc.