Polycarboxylate cements

Polycarboxylate Cements. Polycarboxylate cements (30,31) are made by mixing a 2inc oxide-based powder and an aqueous solution of poly(acryHc acid) [9003-01 ] or similar polyacid (see Acrylic acid). The biological effects of these cements on soft and minerali2ed tissues are mild (32). 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

The compressive strength of polycarboxylate cements at cementing consistency is 55—85 MPa (8,000—12,000 psi). Typical diametral tensile strength ranges from 8—12 MPa (1160 1740 psi). The solubiHty and disintegration in distilled water after 7 days at 37°C is 0.04—0.08 wt %, and is not reflected in clinical performance. 

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... 

The most common poly(alkenoic acid) used in polyalkenoate, ionomer or polycarboxylate cements is poly(acrylic acid), PAA. In addition, copolymers of acrylic acid with other alkenoic acids - maleic and itaconic and 3-butene 1,2,3-tricarboxylic acid - may be employed (Crisp Wilson, 1974c, 1977 Crisp et al, 1980). These polyacids are prepared by free-radical polymerization in aqueous solution using ammonium persulphate as the initiator and propan-2-ol (isopropyl alcohol) as the chain transfer agent (Smith, 1969). The concentration of poly(alkenoic add) is kept below 25 % to avoid the danger of explosion. After polymerization the solution is concentrated to 40-50 % for use. 

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. 

Beech, D. R. (1972). A spectroscopic study of the interaction between human tooth enamel and polyacrylic acid (polycarboxylate cement). Archives of Oral Biology, 17, 907-11. 

Beech, D. R. (1973). Improvement in the adhesion of polycarboxylate cements to human dentine. British Dental Journal, 135, 442-5. 

Brown, D. Combe, E. C. (1973). Effects of stainless steel filler on the properties of polycarboxylate cement. Journal of Dental Research, 52, 388. 

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. 

Pulp reaction to a polycarboxylate cement in monkeys. Journal of Dental Research, 53, 15-19. 

Lawrence, L. G. Smith, D. C. (1973). Strength modification of polycarboxylate cements with fillers. Journal of the Canadian Dental... 

McLean, J. W. (1972). Polycarboxylate cements. Five years experience in general practice. British Dental Journal, 132, 9-15. 

---