Amorphous Phosphate Fibers

Much work has been expended on unsuccessful attempts to prepare useful spun fibers of phosphate glasses similar to silicate fiber glasses. No problems exist in obtaining spun fibers. It is relatively easily accomplished. The primary problem 

Some spun fibers are reported to be very useful in bone implants and similar applications, where it is desirable that a fiber degrade and be absorbed in a relatively short time. Here, these fibers would lend green strength to a composite that is ultimately replaced by normal bone. Some more recent systems are also showing some promise as high-temperature spun amorphous phosphate fibers. This utility may eventually be possible. 

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Numerous long-term projects have been initiated in an attempt to spin amorphous phosphate fibers. Until now they have been unsuccessful for long-term applications, but can be safe for the same reasons that crystalline phosphate fibers are safe. Amorphous fibers have been used successfully in some bioimplants. Later we will discuss some amorphous fibers that should be safe and benign, but in fact seem to have a potential to be deadly. Phase chemistry and thermodynamics of polyphosphates will be discussed in more detail in Chapter 3 and their uses discussed in Chapter 8. Some amorphous phosphate-silicate fibers have been found to be very toxic when injected into rats. 

A spun amorphous phosphate fiber could be of use in areas where it is desired that the fiber degrades rapidly, but adds green strength to a reinforced item. In certain bone prostheses this could be highly desirable. In other areas, where a soluble strong fiber that is biocompatable is needed, as in sutures, spun phosphate... 

The chemistries of phosphates and silicates are similar, but the morphology of the crystals of the sparingly soluble phosphates are unsuited for fiber applications. Amorphous phosphate glasses can be easily spun into fibers in a process similar to the manufacture of fiberglass. Unfortunately, amorphous phosphates lack both strength and hydrolytic stability. 

Alkali metal and alkaline earth polyphosphates crystallize as short chains, two to six phosphate groups per chain or very long chains with hundreds to thousands of PO3 per chain. All polyphosphates in the alkali metal and alkaline earth systems are amorphous in the intermediate chain lengths. Control of the short chain length polyphosphates, both crystalline or amorphous, is a function of R, the M2O-P2O5 ratio. The control of the chain lengths of very long crystalline polyphosphates as Maddrell s salt, Kurrol s salt, and calcium phosphate fibers is not well understood. 

Three possibilities exist when a salt with a polyphosphate x-ray pattern crystallizes from a melt containing an excess of phosphorus pentoxide. 1. The phosphorus pentoxide is incorporated into the polyphosphate chains converting the chains to crystalline ultraphosphates. 2. The excess phosphorus pentoxide does not enter the polyphosphate crystal structure, but forms an amorphous phase between the crystals of polyphosphate. The amorphous phase is not detected by x-ray. 3. The excess phosphorus pentoxide does not enter the crystal structure of the polyphosphate, but forms as an ultraphosphate between the crystalline polyphosphate crystals as a eutectic phase. (This latter case is precisely what happens in the calcium sodium ultraphosphate system from which calcium phosphate fibers are grown (21) and the phase diagram of Hill et. al. is obeyed as it should be.)... 

In the potassium Kurrol s salt phase system the crystalline analogues to Ca2P60i7 or CaP O i were not found by a literature search. (22) The amorphous potassium ultraphosphate systems have been studied. (23) Amorphous condensed phosphates are seldom, if ever, single compounds, but are a random mixtures of compounds and can be large and very complex. If the ultraphosphates were embedded between crystals of pure potassium phosphate fibers they would be very difficult to detect. 

Amorphous spun phosphate fibers can create effectiveness problems when used for reinforcement if they are inclined to devitrily. If crystallization is catastrophic, an entire fiber can be converted to a string of fine powdered crystals as a phase transition occurs. If decomposed fibers are exposed to an environment, then a dusting problem may also result. Amorphous fibers are also much more inclined to be hygroscopic than are their crystalline counterparts. Usually, a crystalline fiber is superior to an amorphous fiber, when short reinforcing fibers are compared for utility and a reliable long life. 

When preparing synthetic fibers it is usually preferable to avoid crystals that can undergo phase transitions in the temperature ranges of their intended application. This is one reason why amorphous inorganic phosphate fibers may not function well for an extended period. Attempts to prepare phosphate glass fibers have always met with failure in past years when refractory requirements were desired. Additionally, molecular weights in glasses are relatively small when compared to crystals of a similar composition. Two problems have usually been encountered. 

In both instances fibers lose their strength and usually disintegrate to powders. Therefore, if a condensed phosphate is to be chosen for its lasting physical properties, the chances of satisfactory results are better if crystalline fibers can be grown, rather than attempting to spin amorphous fibers. 

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