Cement-forming liquids

Cement-forming liquids are strongly hydrogen-bonded and viscous. According to Wilson (1968), they must (1) have sufficient addity to decompose the basic powder and liberate cement-forming cations, (2) contain an acid anion which forms stable complexes with these cations and (3) act as a medium for the reaction and (4) solvate the reaction products. 

Generally, cement-forming liquids are aqueous solutions of inorganic or organic adds. These adds include phosphoric add, multifunctional carboxylic adds, phenolic bodies and certain metal halides and sulphates (Table 2.1). There are also non-aqueous cement-forming liqtiids which are multidentate acids with the ability to form complexes. 

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

A deficiency of water in the cement liquid has the same effect and this occurs when the H3PO4 content exceeds 60%. Wilson Mesley (1968) noted that in a cement formed from a solution of 65 % H3PO4 there was evidence of incomplete reaction even after 6 hours. We have noted in Section 6.5.3 that there is a sharp decline in the rate of reaction when the orthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14). The avidity of cements to absorb water from humid surroundings also increases sharply when the phosphoric acid in the cement-forming liquid exceeds 60%. It is difficult to avoid the conclusion that these two phenomena are related and that a deficiency of water retards the cementforming reaction. 

Park, C.K., Silsbee, M.R., and Roy, D.M. (1998) Setting reaction and resultant structure of zinc phosphate cement in various orthophosphoric acid cement-forming liquids. Cement and Concrete Research 28,141-150. 

The liquid is an aqueous solution of phosphoric acid, always containing 1 to 3 % of aluminium, which is essential to the cement-forming reaction (Table 6.2). Zinc is often found in amounts that range from 0 to 10% to moderate the reaction. Whereas zinc is present as simple ions, aluminium forms a series of complexes with phosphoric acid (Section 6.1.1). This has important consequences, as we shall see, in the cement-forming reaction. 

Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. 

The addition of aluminium to the liquid slowed down the reaction. An amorphous cement was formed and there was no crystallization in the bulk of the cement. However, after some time crystallites were formed at the surface. Thus, the presence of aluminium exerts a dedsive influence on the course of the cement-forming reaction. This effect is to be attributed to the formation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1). These complexes may delay the predpitation of zinc from solution and also introduce an element of disorder into the structure, thus inhibiting crystallization. It is significant that zinc, which does not form complexes, has little effect on the nature or speed of the reaction. 

Skinner, Molnar Suarez (1964) studied the cement-forming potential of 28 liquid aromatic carboxylic acids with zinc oxide. Twelve yielded cohesive products of some merit. Of particular interest were cements formed with hydrocinnamic, cyclohexane carboxylic, p-tertiary butyl-benzoic, thiobenzoic and cyclohexane butyric acids. One of these cements is on the market as a non-eugenol cement. It is very weak with a compressive strength of 4 0 MPa, a tensile strength of 11 MPa and a modulus of 177 MPa, and is only suitable as a temporary material (Powers, Farah Craig, 1976). 

Wilson, A. D. Mesley, R. J. (1974). Chemical nature of cementing matrices of cements formed from zinc oxide and 2-ethoxybenzoic add-eugenol liquids. Journal of Dental Research, 53, 146. 

Dental cements are a diverse class of material of widely different chemistries and applications (2,3). However, all may be classified as acid-base reaction cements formed by mixing a powder(base) with an acidic liquid. A typical example, the traditional zinc phosphate, is the product of the reaction between a zinc oxide powder and a concentrated solution of phosphoric acid. The cement sets, within minutes, as an amorphous zinc orthophosphate gel is formed. 

Material that holds parts together by surface attachment. Examples include glue, mucilage, paste and cement. Various forms of adhesives include liquid or tape adhesives (physical type) and silicate or resin adhesives (chemical type). 

Industrial finishing systems are applied to a wide variety of substrates, the majority of which are metallic, but they are also applied to paper, wood, wood composites, cement products and plastics. Often a high quality of decoration is required, as well as protection from a number of hazards, such as knocks, abrasions, bending or forming and contact with non-corrosive liquids. Resistance to the weather may be required. Outdoor finishing systems, and many others, are also required to protect metal against corrosion. 

The PMMA bone cement is formed from a mixture of prepolymer PMMA powder, which contains a free-radical initiator, and liquid MMA monomer. In the operating theatre the powder and liquid are mixed, causing the initiator to dissolve and bring about polymerisation in the monomer component. The powder pre-polymer does not dissolve in the monomer but remains in the newly polymerised materials as a kind of reinforcing filler. 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

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