Лако-красочные материалы — производство

Although originally used as direct filling materials, the glass ionomer (GI) cements have since proven their worth in a number of different dental applications, including the pre­viously discussed luting of restorations. Their use as cavity base and lining materials has increased rapidly in recent years, and in this area the glass ionomers have established themselves as a major materials class for reasons of compatibility with resin restorations, biological acceptability, good thermal insulating properties, fluoride release,[37] and good strength and bonding characteristics, the latter accentuated by a low coefficient of thermal expansion (typically, 15 x 10“6 °C_1), matching that of dentin (Table 2), thus minimizing microleakage caused by expansion differentials under thermocycling conditions. Similar in composition and hardening properties to the luting variety, the lining materials contain an ion-leachable (generally, calcium fluoride-modified) sodium fluoroaluminosilicate glass and an acrylic acid homo — or copolymer as the principal reactants. The powder-liquid products comprise the finely ground glass filler in the powder component, and an aqueous solution of poly(acrylic acid) or copolymer, sometimes in combination with tartaric acid. A high aluminum content in the glass serves to increase the reactivity with the polyacid. The tartaric acid additive undergoes early complex formation with Al3+ ions liberated from the glass surface, thus facilitating calcium ion accessibility in the glass for acid attack; it remains an important participant in subsequent reaction steps leading to ultimate cross­linking.

Powder-water products differ from the powder-liquid products insofar as the solid component contains both the glass and the anhydrous polyacid, whereas the aqueous phase here is either plain water or a diluted aqueous tartaric acid solution. Both applica­tion forms produce the same type of end product, a cement comprising surface-gelled glass particle filler and polyacid matrix cross-linked through three-dimensional calcium and aluminum salt formation. Residual free carboxyl groups in the cement are left available for calcium salt formation involving adjacent dentin, and this represents an important, although weak chemical dentin-bonding mode utilized to advantage in GI applications. A further increase in the dentin-GI bond strength reportedly results from preconditioning the exposed dentin surface with aqueous poly(acrylic acid) solution; the conditioner etches the surface and serves to dissolve (and, perhaps, reprecipitate) the so-called smear layer, a thin (ca. 1 pm), mineral-rich zone of dentinal debris collecting on the freshly prepared dentin surface, which, if left untreated, is widely considered detrimental to the bonding process. Deep dentin surfaces, which possess a lower apatite content and show stronger resistance than upper dentin surfaces to bonding, can be activated for bonding by a mineralization treatment, which induces calcium phosphate crystallization and thus increases the Ca2+ ion concentration on the dentinal adherend.

GI adhesion to the enamel of the tooth structure is more efficacious than to dentin because, in addition to the calcium carboxylate bond formation with Ca2+ present in a higher concentration in the enamel adherend (Table 1), the free polyacid in the cement exerts an etching effect on the enamel surface, resulting in increased surface roughness and concomitantly improved micromechanical retention. Typical GI-enamel and GI-dentin bond strength data are provided in Section II. B.6. While GI liners are rarely used under amalgam, their beneficial application under composite fillings, where resin compatibility is important, has been widely accepted. Adhesion between liner and composite filling mate­rial can be improved further by etching the liner surface with phosphoric acid prior to packing of the restorative. Shear bond strength values of joints so prepared typically average 10 MPa (6.5 MPa without liner etching), and fracture occurs predominantly in the cement [19]. A weakened bond between liner and dentin, on the other hand, is often a consequence of this treatment. As the packed composite undergoes polymerization shrink­age, the firmly bonded GI liner, being subjected to tensile and/or shear stresses, tends to retreat from the dentin surface and, in the process, cause detrimental enhancement of leakage in the liner-dentin interface. With increasing success in research toward composite materials devoid of polymerization shrinkage (see Section V. B.2), one can expect this liner-dentin debonding problem to become less relevant.

The advent of light-activation methods for resin composites has prompted research into light-activated GI cements. Representative products now on the market are powder/ liquid combinations. The powder, again, constitutes an ion-leachable fluoroaluminosili — cate glass containing a light-activated initiator. The liquid is an aqueous solution of a polycarboxylic acid modified with methacryloyloxy side groups, for example, a poly(acrylic acid-co-methacryloyloxyethyl acrylate), hydroxyethyl methacrylate, and the light-sensitizer part of the light-activating system. Combinations of this type provide adequately long working times, as the purely chemical hardening process, utilizing the calcium cation-carboxylate interaction, proceeds at a conveniently slow rate. After completed placement, the material may be light activated, which initiates polymer­ization of the methacrylate side groups and entails rapid hardening. The presence of residual free carboxyl groups ensures chemical cement bonding to the enamel-dentin adherend as in the conventional products. Slow continuing reaction of polyacid and glass filler, following the light-curing step, leads to further maturation of the cement. Typical shear bond strength values for light-cured GI cements bonded to dentin range from about 3 to 5 MPa (occasionally even higher [20]), and similar values are obtained for bonds to amalgam.

Numerous other so-called ‘‘light-curing’’ GI cements have recently been commer­cialized that are related to the glass ionomers only insofar as they contain a powdery filler made up of GI powder and calcium phosphate as the principal ingredients. The matrix component of these materials is a light-curing mixture of mono — and diacrylate monomers. As a consequence, their setting shrinkage is considerably larger than that of the conventional GI cements [21]. Furthermore, containing no polyacids, these materi­als are unable to undergo the chemical bonding reaction to enamel-dentin character­istic of the glass ionomers proper, although other bonding mechanisms associated with the acrylate monomers may be quite efficacious. Procedural details for GI liner appli­cation have been described [22], and a good review of developments in this field is available [23].