Zinc polycarboxylate cement

The nature of the poly(alkenoic add) can affect the hydrolytical stability of metal oxide cements (Hodd Reader, 1976). For example the B203-poly(ethylene maleic add) cement, unlike its poly(acrylic add) counterpart, is not hydrolytically stable. 

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Polyelectrolytes are polymers having a multiplicity of ionizable groups. In solution, they dissociate into polyions (or macroions) and small ions of the opposite charge, known as counterions. The polyelectrolytes of interest in this book are those where the polyion is an anion and the counterions are cations. Some typical anionic polyelectrolytes are depicted in Figure 4.1. Of principal interest are the homopolymers of acrylic acid and its copolymers with e.g. itaconic and maleic adds. These are used in the zinc polycarboxylate cement of Smith (1968) and the glass-ionomer cement of Wilson Kent (1971). More recently, Wilson Ellis (1989) and Ellis Wilson (1990) have described cements based on polyphosphonic adds. 

Nevertheless adhesive materials were developed, for in 1968 Deimis Smith announced the zinc polycarboxylate cement (Smith, 1968,1969) and... 

Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn and Ca +) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. 

Table 5.2. Composition of zinc polycarboxylate cements (Bertenshaw Combe, 1972a,b, 1976)...

Typical compositions of zinc polycarboxylate cements are given in Table 5.2. 

The zinc polycarboxylate cement sets within a few minutes of mixing and hardens rapidly. Strength is substantially developed within an hour. However, even when fully hardened the cement exhibits marked plastic behaviour. Its most important property is its ability to bond permanently to untreated dentine and enamel. 

The early zinc polycarboxylate cement did not possess the ease of mixing characteristic of the zinc phosphate and zinc eugenolate cements. It suffered because it was expected to mix exactly as a traditional zinc... 

Table 5.3. Properties of zinc polycarboxylate cements (Jendresen Trowbridge, 1972 Plant, Jones Wilson, 1972 Paddon Wilson, 1976 Powers, Johnson Craig, 1974 Powers, Farah Craig, 1976 Chamberlain Powers, 1976 Levine, Beech Carton, 1977 0ilo Espevik, 1978 Bertenshaw, Combe Grant, 1979 Peddy, 1981 Hinoura, Moore Phillips, 1986)...

An unfortunate characteristic of early zinc polycarboxylate cements was the early development of elastomeric characteristics- cobwebbing -in the cement pastes as they aged, thus shortening working time (McLean, 1972). Improvements in cement formulation, the addition of stannous fluoride to the oxide powder (Foster Dovey, 1974, 1976) and modifications in the polyacid have eliminated this defect. However, the cements have to be mixed at quite a low powder/liquid ratio, 1 -5 1 0 by mass, when used for luting. 

In vivo studies show that zinc polycarboxylate cements are much less... 

The poly(alkenoic acid)s used in glass polyalkenoate cement are generally similar to those used in zinc polycarboxylate cements. They are homopolymers of acrylic acid and its copolymers with itaconic add, maleic add and other monomers e.g. 3-butene 1,2,3-tricarboxylic add. They have already been described in Section 5.3. The poly(acrylic add) is not always contained in the liquid. Sometimes the dry add is blended with glass powder and the cement is activated by mixing with water or an aqueous solution of tartaric add (McLean, Wilson Prosser, 1984 Prosser et al., 1984). 

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean Wilson, 1977a,b,c Wilson McLean, 1988 Mount, 1990). 

Compressive strengths of these cements were found by Bertenshaw et al. (1979) to range from 20 to 50 MPa and tensile strengths from 5 to 9 MPa. These values are inferior to those of the conventional glass polyalkenoate cements but similar to those of the zinc polycarboxylate cements. They are reported to have a good translucency and have a low solubility in water. These materials do not appear to be manufactured commercially. 

Barnes, D. S. Turner, E. P. (1971). Initial response of the human pulp to zinc polycarboxylate cement. Journal of the Canadian Dental Association, 37,265-6. 

Chamberlain, B. B. Powers, J. N. (1976). Physical and mechanical properties of three zinc polycarboxylate cements. Journal of the Michigan Dental Association, 58, 494-500. 

Crisp, S., Lewis, B. G. Wilson, A. D. (1976a). Zinc polycarboxylate cements. A chemical study of erosion and its relationship to molecular structure. Journal of Dental Research, 55, 299-308. 

Main, J. H. P., Mock, D., Beagrie, G. S. Smith, D. C. (1975). Investigations of possible oncogenic action of zinc polycarboxylate cement. Journal of Biomedical Materials Research, 9, 69-78. 

Wilson, A. D. (1982). The nature of the zinc polycarboxylate cement matrix. Journal of Biomedical Materials Research, 16, 549-57. 

In vivo studies have indicated that zinc phosphate cements erode under oral conditions. Also, cements based on zinc oxide, including the zinc phosphate cement, are less durable in the mouth than those based on aluminosilicate glasses, the dental silicate and glass-ionomer (Norman et al., 1969 Ritcher Ueno, 1975 Mitchem Gronas, 1978,1981 Osborne et al., 1978 Pluim Arends, 1981, 1987 Sidler Strub, 1983 Mesu Reedijk, 1983 Theuniers, 1984 Pluim et al., 1984, Arends Havinga, 1985). However, there is some disagreement on whether the zinc phosphate cement is more durable than the zinc polycarboxylate cement. 

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