Cement-forming acids poly

Denis Smith, and the development showed that satisfactory cements could be made by reacting heat-treated zinc oxide of the type used in zinc phosphate dental cements with concentrated solutions of poly (acrylic acid). This demonstrated that an alternative cement-forming acid was available, and one, which offered the prospect that cements formed from it, would adhere to the tooth [22,23],... 

It was not a straightforward matter to take the next step of making an acceptable cement from dental silicate glass and aqueous poly(acrylic acid) [18], When it was first tried, the result was a disappointing material that set very slowly and was extremely weak. It was so poor that the result was not reported at the time in a pioneering study of novel cement-forming acids [24], It was only some years later that Wilson mentioned this experiment and its unfortunate outcome [18],... 

Poly(acrylic acid) and its salts have been known to have useful binding properties for some thirty years they have been used for soil consolidation (Lambe Michaels, 1954 Hopkins, 1955 Wilson Crisp, 1977) and as a flocculant (Woodberry, 1961). The most interesting of these applications is the in situ polymerization of calcium acrylate added to soil (de Mello, Hauser Lambe, 1953). But here we are concerned with cements formed from these polyacids. 

The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. 

Using this information. Crisp et al. (1977, 1979) and Hornsby et al. (1982) selected candidate minerals for cement formation with poly(acrylic acid) and found a number of minerals that formed cements (Table 5.4). 

Cement-forming liquid 45 % poly(acrylic acid)... 

Vashkevitch Sychev (1982) have identified the main reaction product of the cement-forming reaction between copper(II) oxide and phosphoric acid as Cu3(P04)2. SHjO. The addition of polymers - poly(vinyl acetate) and latex - was found to inhibit the reaction and to reduce the compressive strength of these cements. However, impact strength and water resistance were improved. 

Ellis and Wilson studied cements formed from concentrated solutions of poly(vinylphosphonic acid) (PVPA) and oxides and silicate glasses, which they termed metal oxide and glass polyphosphonate cements (Wilson ... 

In 1968 Smith (4) replaced phosphoric acid by poly(acrylic acid) to give the zinc polycarboxylate cements, and in 1971 Wilson and Kent (5) reported a cement formed by the reaction between an ion-leachable glass and an aqueous solution of poly(acrylic acid). They termed this material the glass-ionomer or ASPA cement. (ASPA is the acronym of Alumino Silicate PolyAcrylic Acid). The cement-forming mechanism may be represented as a reaction between two pol3nners to form a third which acts as a cementing matrix ... 

AB cements are not only formulated from relatively small ions with well defined hydration numbers. They may also be prepared from macromolecules which dissolve in water to give multiply charged species known as polyelectrolytes. Cements which fall into this category are the zinc polycarboxylates and the glass-ionomers, the polyelectrolytes being poly(acrylic acid) or acrylic add copolymers. The interaction of such polymers is a complicated topic, and one which is of wide importance to a number of scientific disciplines. Molyneux (1975) has highlighted the fact that these substances form the focal point of three complex and contentious territories of sdence , namely aqueous systems, ionic systems and polymeric systems. 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

In their original form these cements came as a zinc oxide powder and a concentrated solution of poly(acrylic acid) (Wilson, 1975b). Since then they have been subject to a number of chemical modifications. 

The cement sets as the result of an acid-base reaction between a zinc oxide dental powder and a poly(alkenoic acid). The pH increases and an insoluble amorphous salt is formed which acts as the cement matrix. A general account of the gelation processes is given in Section 5.4. 

Most orthosilicates reacted completely with poly(acrylic acid) solution an exception was andradite, CagFOg [SiOJg. Even so, the cements of gehlenite and hardystonite were very weak and affected by water. Only gadolinite and willemite formed cements of some strength which were unaffected by water, probably because one contained beryllium and iron and the other zinc. 

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

The two matrices in these cements are of a different nature an ionomer salt hydrogel and polyHEMA. For thermodynamic reasons, they do not interpenetrate but phase-separate as they are formed. In order to prevent phase separation, another version of resin glass polyalkenoate cement has been formulated by Mitra (1989). This is marketed as VitraBond, which we term a class II material. In these materials poly(acrylic acid), PAA, is replaced by modified PAAs. In these modified PAAs a small fraction of the pendant -COOH groups are converted to unsaturated groups by condensation reaction with a methacrylate containing a reactive terminal group. These methacrylates can be represented by the formula ... 

Zinc polycarboxylate, the first polyelectrolyte dental material, was developed and used as early as 1968 [124]. These materials are formed by the reaction of a zinc oxide powder with an aqueous solution of poly(acrylic acid). The zinc ions cross-link the polyacid chains and form a cement. A few years after the development of zinc polycarboxylate cements, Wilson and Kent introduced the first glass-ionomer cement (GIC) [125]. Glass-ionomer cements are formed... 

Composite resins consist of blends of large monomer molecules, filled with unre-active reinforcing filler. As such, they are hydrophobic, which means that they are unable to bond to the hydrophilic prepared tooth surface [1]. Glass-ionomer cements, by contrast, consist of aqueous solutions of polymeric acid, typically poly(acrylic add) and powdered reactive glass. These two components react together in an acid-base reaction, and thus cause the cement to set. These materials are hydrophilic, and therefore capable of wetting the prepared tooth surface and forming tme adhesive bonds. 

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