Zinc Phosphate Ceramics

As mentioned in Chapter 2, zinc phosphate dental cements were discovered over a century ago, and their development has continued since then [1-9]. A brief history of this development is given in that chapter. For a detailed history of these cements and properties of contemporary formulations, the reader is referred to the book by Wilson and Nicholson [10]. Because, the kinetics of formation of these cements has not been discussed in these earlier publications, we will emphasize it in this chapter and present the earlier work in light of the solubility characteristics of zinc oxide and its products in an acid phosphate solution. 

The role of aluminum in the zinc phosphate cements was considered very important. Aluminum oxide greatly moderated the reaction of zinc oxide and phosphoric acid, and this effect was attributed to the formation of an aluminum phosphate gelatinous coating on zinc oxide particles. In fact, Wilson and Nicholson [10] believe that the gelatinous 

The final cement is an opaque solid that consists of excess zinc oxide coated and bonded by possibly aluminum phosphate and zinc phosphate gels. The cement is porous and permeable to dyes [10]. 

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As discussed earlier, ceramic is formed by the reaction of calcined magnesium oxide (MgO) with a solution of phosphoric acid or an acid phosphate in these products. The quick-setting reaction results in products similar to those found in zinc phosphate ceramics. The major products can be represented by the formula, Mg(X2P04)2 H2O or MgXP04 H20, where X is hydrogen (H), ammonium (NH4), sodium (Na), or potassium (K). The reaction products are listed in Table 2.2. 

The literature review summarized in Chapter 2 and discussed in Ref. [10] indicates that zinc phosphate ceramics have been synthesized only in small sizes as dental cements, and no attempt has been made to cast large forms of these cements. In spite of this. 

Zinc phosphate, Zn2(P0 2> forms the basis of a group of dental cements. Chromium and zinc phosphates are utilized in some metal-treating appHcations to provide corrosion protection and improved paint adhesion. Cobalt(II) phosphate octahydrate [10294-50-5] Co2(P0 2 8H20, is a lavender-colored substance used as a pigment in certain paints and ceramics. Copper phosphates exhibit bioactivity and are used as insecticides and fungicides. Zinc, lead, and silver phosphates are utilized in the production of specialty glasses. The phosphate salts of heavy metals such as Pb, Cr, and Cu, are extremely water insoluble. 

One great advantage with phosphate bonded ceramics in biomaterial or dental applications is the phosphate ions in their structure. Bones contain calcium phosphate, and hence phosphate bonded ceramics are generally biocompatible with bones. While chemically bonded calcium phosphate ceramics have been difficult to produce, magnesium and zinc based phosphate bonded ceramics have been more easily synthesized and used as structural and dental cements. 

The literature review in Chapter 2 reveals that divalent metal oxides such as oxides of calcium, magnesium, and zinc (CaO, MgO, and ZnO) are the major candidates for forming phosphate ceramics. These oxides are sparsely soluble in acidic solution, and as we shall see in Chapter 4, they are the most suitable ones to form ceramics. In addition, following the methods discussed in subsequent chapters in this book, aluminum oxide (alumina, AI2O3) and iron oxide (Fe203), which are abundant in earth s crust have excellent potential to form low cost CBPCs. For this reason, we have provided relevant information on these oxides. Table 3.2 gives some details. 

The process of fabrication of zinc phosphate cements is very similar to that of magnesium phosphate ceramics. Direct reaction with phosphoric acid is fierce and needs to be slowed down. This is done by the following methods. 

Considerable development has occurred on sintered ceramics as bone substitutes. Sintered ceramics, such as alumina-based ones, are uru eactive materials as compared to CBPCs. CBPCs, because they are chemically synthesized, should perform much better as biomaterials. Sintered ceramics are fabricated by heat treatment, which makes it difficult to manipulate their microstructure, size, and shape as compared to CBPCs. Sintered ceramics may be implanted in place but cannot be used as an adhesive that will set in situ and form a joint, or as a material to fill cavities of complicated shapes. CBPCs, on the other hand, are formed out of a paste by chemical reaction and thus have distinct advantages, such as easy delivery of the CBPC paste that fills cavities. Because CBPCs expand during hardening, albeit slightly, they take the shape of those cavities. Furthermore, some CBPCs may be resorbed by the body, due to their high solubility in the biological environment, which can be useful in some applications. CBPCs are more easily manufactured and have a relatively low cost compared to sintered ceramics such as alumina and zirconia. Of the dental cements reviewed in Chapter 2 and Ref. [1], plaster of paris and zinc phosphate... 

Arar, H. H. and Bajpai, R K., insulin delivery by zinc calcium phosphate ceramics, Biomed. Sci. Instrum., 28, 173, 1992. 

Calcium phosphate A family of calcium phosphate ceramics including aluminum calcium phosphate, ferric calcium phosphate, hydroxyapatite and tricalcium phosphate (TCP), and zinc calcium phosphate which are used to substitute or augment bony structures and deliver drugs. Glass-ceramics A glass crystallized by heat treatment. Some of those have the ability to form chemical bonds with hard and soft tissues. Bioglass and Ceravital are well known examples. 

Di-t-butyl phosphate complexes of zinc were synthesized as precursors for ceramic material formation. A tetrameric zinc complex was characterized from the treatment of zinc acetate with the phosphate resulting in a compound with a bridging oxo at the center, [Zn4(/i4-0)(di-t-butyl phosphate)6]. In the presence of auxiliary donor ligands such as imidazole or ethylenediamine, monomeric complexes are formed, [Zn(di-t-butyl phosphate)2(imidazole)4]. It is also possible to convert the tetramer into the monomer by treating with a large excess of imidazole.41... 

Chapter 8 provides a review of the phosphate mineralogy that facilitates discussion on the applications discussed in the latter part of the book. The next five chapters discuss individual phosphate (magnesium, zinc, aluminum, iron, and calcium) ceramics that have potential for use in current and future applications. The approach provided in this book should guide researchers to many more formulations for specific needs in the future. 

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