Acrylic cement

Figure 5,8 The comparative durability of polymer-modified cements. - - -. - = unmodified cement -------= acrylic-modified cement ------= styrene-butadiene-rubber-modified...

MAJOR USES Used in the manufacture of acetyl cellulose, varnish and finish removers, organic compounds, pharmaceutical products, resins, paints, soaps and scouring compounds, rubber cement, acrylic adhesives, degreasing agents used in leather cleaning and photography. 

Orthopedic Adhesives/Bone Cements. Acrylic bone cements [198] are the only group of materials currently used to anchor long-term implantable devices to the neighboring bone. Though the bone cements have drawbacks, including trauma created by a highly exothermic hardening process, over 90 % of hip and knee repairs function well for 15 years. The bone cements consist of separate powder and liquid components which are mixed carefully prior to application. 

Uses. The largest use for sodium thiocyanate is as the 50—60 wt % aqueous solution, as a component of the spinning solvent for acryUc fibers (see Fibers, acrylic Acrylonitrile polymers). Other textile appHcations are as a fiber swelling agent and as a dyeing and printing assist. A newer commercial use for sodium thiocyanate is as an additive to cement in order to impart early strength to concrete (376). 

In paints, zinc oxide serves as a mildewstat and acid buffer as well as a pigment. The oxide also is a starting material for many zinc chemicals. The oxide supphes zinc in animal feeds and is a fertilizer supplement used in zinc-deficient soils. Its chemical action in cosmetics (qv) and dmgs is varied and complex but, based upon its fungicidal activity, it promotes wound healing. It is also essential in nutrition. Zinc oxide is used to prepare dental cements in combination with eugenol and phosphoric and poly(acrylic acid)s (48) (see Dental materials). 

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). 

Solution Polymerization. This method is not commercially important, although it is convenient and practical, because it provides viscous cements that are difficult to handle. Also, the choice of the solvent is a key parameter due to the high solvent chain-transfer constants for acrylates. 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

Acetylene works Acrylates works Aldehyde works Aluminum works Amines works Ammonia works Anhydride works Arsenic works Asbestos works Benzene works Beryllium works Bisulfate works Bromine works Cadmium works Carbon disulfide works Carbonyl works Caustic soda works Cement works Ceramic works Chemical fertilizer works Chlorine works Chromium works Copper works Di-isocyanate works Electricity works Fiber works Fluorine works Gas liquor works Gas and coke works Hydrochloric acid works Hydrofluoric acid works Hydrogen cyanide works Incineration works Iron works and steel works... 

Another development has been the advent of the dual-cure resin cements. These are hybrids of glass polyalkenoate cements and methacrylates that set both by an add-base cementation reaction and by vinyl polymerization (which may be initiated by light-curing). In these materials, the solvent is not water but a mixture of water and hydroxyethylmethacrylate which is capable of taking dimethacrylates and poly(acrylic add)-containing vinyl groups into solution. In the absence of light these materials set slowly and... 

This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated... 

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. 

Water occurs in glass-ionomer and related cements in at least two different states (Wilson McLean, 1988 Prosser Wilson, 1979). These states have been classified as evaporable and non-evaporable, depending on whether the water can be removed by vacuum desiccation over silica gel or whether it remains firmly bound in the cement when subjected to such treatment (Wilson Crisp, 1975). The alternative descriptions loosely bound and tightly bound have also been applied to these different states of water combination. In the glass-poly(acrylic acid) system the evaporable water is up to 5 % by weight of the total cement, while the bound water is 18-28 % (Prosser Wilson, 1979). This amount of tightly bound water is equivalent to five or six molecules of water for each acid group and associated metal cation. Hence at least ten molecules of water are involved in the hydration of each coordinated metal ion at a carboxylate site. 

The effectiveness with which divalent ions cause gelation of poly(acrylic add) has been found to follow the order Ba > Sr > Ca (Wall Drenan, 1951) and this has been attributed to the formation of salt-like crosslinks. Gelation has been assumed to arise in part from dehydration of the ion-pairs (Ikegami Imai, 1962), and certainly correlates with predpitation in fairly dilute systems. Indeed, the term precipitation has sometimes been applied to the setting of AB cements derived from poly(acrylic add) as they undergo the transition from soft manipulable paste to hard brittle solid. 

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. 

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. 

This is a reasonable inference, because site binding is significant only with multivalent cations and strong electrostatic interactions. Under these conditions ion polarization occurs and bonds have some covalent character (Cotton Wilkinson, 1966). This is illustrated by the data of Gregor, Luttinger Loebl (1955a,b). They measured the complexation constants of poly(acrylic acid), 0 06 n in aqueous solution, with various divalent metals, which, as it so happens, are of interest to AB cements (Table 4.1). The order of stability was found to be... 

Crisp, S., Prosser, H. J. Wilson, A. D. (1976). An infra-red spectroscopic study of cement formation between metal oxides and aqueous solutions of poly(acrylic acid). Journal of Materials Science, 11, 36-48. 

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. 

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. 

Table 5.1. Compressive strength of metal oxide-poly acrylic acid) cements Elliott, Holliday Hornsby, 1975 Hornsby, 1977)...

The nature of the poly(alkenoic add) can affect the hydrolytical stability of metal oxide cements (Hodd Reader, 1976). For example the B203-poly(ethylene maleic add) cement, unlike its poly(acrylic add) counterpart, is not hydrolytically stable. 

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. 

There are several reasons why these cements are bland. Acid irritation is probably minimal. Poly(acrylic acid) is a weak add and, in addition, because of its high molecular weight will not readily diffuse along dentinal tubules and is also immobilized by phosphatic material in these tubules. Moreover, once set these cements rapidly become neutral. 

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). 

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. 

Chain silicates, consisting of connected metasilicate units, (SiOg) " (Q ), and of an open structure. Wollastonite Ca(SiOg) reacted completely with poly(acrylic add), but the cement was much affected by water. 

Sheet silicates (Q ) with significant isomorphic replacement of Si by AF+ or Fe +. These were decomposed by poly(acrylic add) to silica gel. The chlorite, thuringite, formed a strong cement but was much affected by water. 

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

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). 

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