Compositions of water soluble glass

Details Compositions of water soluble glass Compositions of water soluble glass

2001-01-18

2003-04-29

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S045000

Reexamination Certificate

active

06555491

ABSTRACT:

This application is the U.S. national phase application of PCT International Application No. PCT/GB99/01677 filed May 27, 1999.
The present invention relates to a composition for a water-soluble glass, especially for a glass adapted for fibre production.
It is known that certain glasses in which the usual glass former, silicon dioxide, is replaced with phosphorous pentoxide are soluble in water and body fluids. The rate of dissolution is controlled largely by the addition of glass modifiers such as calcium oxide. In simple terms, the greater the concentration of the modifier the slower the rate of dissolution. The rate of dissolution may range from minutes through to several years.
Soluble phosphate based glasses which have demonstrated good biocompatability can incorporate inorganic metals such that a sustained release of the metals can be provided at the wound site. Such materials can also find use in mechanical applications where, for example, slow release of an anti-corrosion agent may be beneficial.
Certain applications require that the glass is in the form of wool or fibres for mechanical applications such as insulation wool and packaging. Thus, for example, Mohr et al in “Fibre Glass” (Van Norstrand, Reinhold Company, New York 1978) and Jaray in “A New Method of Spinning Glass Fibres” (28th Annual SPI RP/C Institute Proceedings 1973, Section 3-A) describe the production of wool and fibres, respectively, from molten glass. The glass fibres can be used for insulation, construction or even communication purposes. Glass wool also finds uses in packaging and insulation applications.
Normally, glass fibres are produced from molten glass using traditional fibre pulling techniques; whereby is filaments of high temperature molten glass (850°-1300° C.) are formed into strands and stretched over pull rolls before being collected onto a reel.
Glass wool formation is similar in that the glass is initially melted in a crucible. The crucible has suitable apertures to allow filaments of glass to flow downwards, which are then “blown” into wool using jets of either steam or compressed air. Alternatively, glass wool can be formed using a flame attenuation process, developed by Owens-Corning Fibreglass Corporation circa 1940. In this process molten glass passes through a bushing stage where primary filaments approximately 1 mm wide are formed. The fibres are then aligned into an exact uniformly juxtaposed array, using a fibre guide, into a jet flame issuing from an internal combustion burner. The jet flame causes thinning and lengthening of the fibres before they are collected on a steel mesh belt.
In both cases, the glass is either supplied in molten form direct from a crucible or from a temperature-gradient furnace.
Generally, water soluble glasses do not lend themselves to these traditional fibre and wool forming techniques. As an example, U.S. Pat. No. 4,604,097 of Graves et al. discloses a water soluble drawn fibre, composed primarily of calcium oxide and phosphorous pentoxide. The fibre produced has a very low tensile strength, compared to fibres spun from non-soluble glass compositions.
Further, water soluble glasses can also be chemically aggressive when molten, unlike traditional glasses where silicon dioxide is used as the glass former. Additionally, the fibres produced are prone to thermal shock and can suffer from devitrification or crystallisation.
To combat problems of devitrification and crystallisation, water soluble glass fibres have been previously produced in exacting conditions. Thus, for example, Zimmer et al. in WO-A-92/07801 discloses drawing fibres from a water soluble glass composed primarily of phosphorus pentoxide, calcium oxide and iron oxide. In order to keep the viscosity of the glass suitable for drawing, the fibres were drawn at 1200° C. Also as a result of the chemically aggressive nature of the glass at that temperature the glass was pulled in an oxygen rich atmosphere (as high as 80% oxygen by volume). Obviously the commercial production of glass fibres under these high temperature controlled atmospheric conditions is expensive.
The problems of working with water soluble glass are compounded by the very nature of the glass. Metal oxides of elements such as lead and tellurium have previously been used in glass as additives to affect qualities of the glass; crystallisation temperature, viscosity and density, for example. As a result of environmental concerns and particularly when the glasses are to be used in a biological application these additives must be avoided and replaced by more acceptable alternatives.
Our copending application, Ser. No WO-A-98/54104, describes a method of producing water soluble glass fibres. We have now found a particular type of composition which is especially suited to this method. In the prior art compositions described therein a mixture of Group I and Group II compounds are present, and a typical phosphorus pentoxide based glass composition comprises approximately 30 mole % Na
2
O and approximately 15 mole % CaO.
The composition of the present invention comprises a glass former (which is typically phosphorus pentoxide) and an alkali earth metal compound. Preferably no alkali metal (Group I) elements or compounds containing such elements are present, although very low quantities (eg 1-2 mole % up to as much as 5 mole %) may be useful to adjust the rate of dissolution of the glass. This low level or absence of alkali metal compounds forms a distinction over the water soluble glass compositions currently available. The reference to alkali metal compounds herein refers to compounds of Group Ia (Li, Na, K, Rb, Cs and Fr) and the reference to alkali earth metal compounds herein refers to compounds of Group IIa (Be, Mg, Ca, Sr, Ba, Ra).
The present invention thus provides a water soluble glass composition formed from a glass former and at least one alkali earth metal compound, characterised in that said glass composition contains a maximum amount of alkali metal compounds of 5 mole %.
Phosphorous pentoxide (P
2
O
5
) is preferably used as a glass former and desirably the glass former will preferably be mainly P
2
O
5
. Solution rate control and stability can be affected by the addition of other glass formers such as boron, silica, alumina, sulphur, germanium, arsenic etc. Generally the mole percentage of phosphorous pentoxide in the glass composition is less than 85 mole %, preferably less than 60 mole % and especially between 30-60 mole %.
Alkali earth metals and lanthanide oxides or carbonates are preferably used as glass modifiers. Generally, the mole percentage of alkali earth metals and lanthanide oxides or carbonates is less than 60 mole %, preferably between 40-60 mole %.
Boron containing compounds (eg B
2
O
3
) are preferably used as glass additives Generally, the mole percentage of boron containing compounds is 15 mole % or less, preferably 10 mole % or less, especially 5 mole % or less.
As indicated above, other compounds may also be added to the glass to modify its properties, for example SiO
2
, Al
2
O
3
, SO
3
, sulphate ions (SO
4
2−
) or transition metal compounds (eg. first row transition metal compounds), but these will be present only in very low quantities, for example up to a total amount of 5 mole % or less of the glass composition.
Typically the soluble glasses used in this invention comprise phosphorus pentoxide (P
2
O
5
) as the principal glass-former, together with any one or more glass-modifying nontoxic materials such as magnesium oxide (MgO), zinc oxide (ZnO) and calcium oxide (CaO). The rate at which the glass dissolves in fluids is determined by the glass composition, generally by the ratio of glass-modifier to glass-former and by the relative proportions of the glass-modifiers in the glass. By suitable adjustment of the glass composition, the dissolution rates in water at 38° C. ranging from substantially zero to 25 mg/cm
2
/hour or more can be designed. However, the most desirable dissolution rate R of the glass is between 0.01 and 2.0 mg/cm
2
/hour.
The water-soluble glass is thus preferably a

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