Salicylate cements

The manipulation of calcium hydroxide paste is not easy, however, and Dougherty (1962) introduced the calcium hydroxide salicylate cements. These are based on the reaction between calcium hydroxide and salicylate esters and come in two-paste packs which are easy to mix in the dental surgery. 

Calcium Chelates (Salicylates). Several successhil dental cements which use the formation of a calcium chelate system (96) were developed based on the reaction of calcium hydroxide [1305-62-0] and various phenohc esters of sahcyhc acid [69-72-7]. The calcium sahcylate [824-35-1] system offers certain advantages over the more widely used zinc oxide—eugenol system. These products are completely bland, antibacterial (97), facihtate the formation of reparative dentin, and do not retard the free-radical polymerization reaction of acryhc monomer systems. The principal deficiencies of this type of cement are its relatively high solubihty, relatively low strength, and low modulus. Less soluble and higher strength calcium-based cements based on dimer and trimer acid have been reported (82). 

Prosser, Stuart Wilson (1979) and Prosser, Groffman Wilson (1982) examined the setting of a number of these cements using infrared spectroscopy. The infrared spectrum of the alkyl salicylates showed an O-H stretch band at 3190 cm" and a C-O stretch band at 1675-95 cm" . 

These materials are hydrolytically unstable and weaken when stored in water for a week (Bryant Wing, 1976b). Prosser, Groffman Wilson (1982) found that calcium and hydroxide ions and salicylates were released and that the rate of release was controlled by the plasticizer used in the cement formulation. Hydrophilic sulphonamide plasticizers allowed ready ingress of water and promoted decomposition, whereas the hydrophobic hydrocarbon plasticizer repelled water and retarded hydrolytic decomposition. 

In the absence of knowledge of the surface area of cement hydrates available for adsorption at the time of addition, it is difficult to estimate how many layers of water-reducing admixture molecules are adsorbed, but attempts have been made [40] indicating that over 100 layers may be formed with calcium lignosulfonate and salicylic acid at normal levels of addition. However, these calculations were based on specific surface areas of 0.3-1.0 m g-l, whereas other studies [27, 38, 39] have indicated... 

Quantitative data on the rate of consumption of pfa are few and somewhat variable. Those based on differences between the CH contents of pure Portland and pfa cements are suspect, because the calculation involves the effects of pfa substitution both on the rate of consumption of the clinker phases and on the compositions of the products, which are not fully understood. Unreacted pfa has been directly determined by dissolution of the other phases with HCl (C43) or with salicylic acid in methanol followed by HCl (T44), chemical separation of the residual pfa followed by QXDA determination of its content of crystalline phases (D12) and a trimethylsily-lation method (U19). A method based on EDTA extraction was found unsatisfactory (L46). 

At concentrations above 2% salicylic acid has a keratolytic effect, causing the keratin layer of the skin to shed. Keratolysis is achieved by increasing the hydration of the stratum corneum, softening the cells and facilitating dissolution of the intracellular cement that bonds the cells together so that they separate and detach (desquamate). Moisture is essential to this process and is provided by either the water in the formulation or the occlusive effect produced by its application to the skin. 

XRD (Cu Ka) data for (A) raw Portland cement clinker, (B) Residue 1 from which most of the C3S has been removed and (C) Residue 2 from which C3S and C2S have been removed. Silicate phase extraction was conducted using a salicylic acid methanol (SAM) mixture using the method described by Taylor." In Residue 2 the presence of the alkali sulfate phases [denoted Arc for arcanite K2SO4 and Ap for aphthitalite K3Na(S04)2] is clearly evident. Notably, the material used in these plots was derived from a different cement plant to the one illustrated in Figure 10. 

Salicylic acid is a P-hydroxy acid that is thought to function through solubilization of intercellular cement, again reducing comeocyte adhesion. It appears to eliminate the stratum comeum layer by layer from the outermost level downward. This contrasts with the a-hydroxy acids, which preferentially diminish cellular cohesion between the cor-neocytes at the lowest levels of the stratum comeum. 

Numerous other applications of ETA have been described (182,188.189). Compounds or materials studied by this technique include barium salts of phthalic. isophthalic, terephthalic, benzoic, salicylic, 1.4-aminobenzoic. 1,2-dichlorobenzoic and 1.2-diiodobenzoic acids (197) uranyl gels (198. 204) ferric oxide (199) cement (200, 201) powder metallurgy 1192) Ge0, 5Te0.6o Se0 i5 1202) anatase (TiOi), gibbsite, and kaolinite (193) thoria iThCK) powder (204) and many others. 

Setting calcium hydroxide cements are typically based on liquid alkyl salicylates, and they are supplied to the clinician as a two-paste pack [36], Alkyl salicylates that have been used include methyl salicylate, isobutyl salicylate and 1-methyl trimethylene disalicylate [37], These cements set because the alkyl salicylate contains a phenolic -OH group which has acid character, and this means it can react with the alkaline calcium hydroxide [38],... 

Takashima (1972) describes progressive dissolution of alite and beta belite with an accompanying ease of detection of alpha by XRD using methyl-ethyl ketone (MEK) and salicylic acid. Alpha belite was most easily detected by XRD when a 1.0-g cement sample was combined with 3.0 g of salicylic acid and 100 mL MEK. Details of the test are given in his article. 

The extraordinary photocatalytic performance of AEROXIDE TiOj P 25 in comparison to other nanoscaled titania particles has been published in several papers It is, for example, useful in the degradation of humic acid [71], of phenol and salicylic acid [72], of l,4dichlorobenzene [73], and in the photocatalytic reduction of Hg(II) [74]. It is also used in the oxidation of primary alcohols to aldehydes [75] or in the photopolymerization of methyl methacrylate [76]. Its use in cement can help reduce environmental pollution [77, 78]. A detailed study is reported by Bolte [79]. The results show that crystal size and filling ratio in mass are more important than the modification of the titania. Pyrogenic titania is not only useful in photocatalysis but also in other catalytic applications. 

In general, retarders are effective in the very early hydration of cement. For a given percentage, Na gluconate, citric acid, or sucrose are better retarders of initial set of a normal low alkali cement than lignosul-fonate, salicylic acid, Na heptonate, or boroheptonate.1 1... 

A complex results when a cement is hydrated in the presence of salicylic acid. Such a product, when added to cement, slows down the hydration. TG and other methods have been used to identify this complex. 

Figure 4.2 XRD scan of a portland cement (CEM I 52.5 N) (top) and its pattern decomposition calculated by Rietveld QPA (downwards from XRD scan). At right the QPA results are presented for indicative purposes. Arcanite was identified by means of a salicylic acid/methanol selective dissolution treatment.

As an aid in minor phase identification, a SAM extraction was performed on the as-received cement. To this purpose, 15 g of cement was mixed with a solution of 900 mL methanol and 75 g of salicylic acid. The suspension was stirred and left to react for 30 min then it was filtered and rinsed twice with methanol. The filter residue was dried at 60°C and measured by XRD. 

---