Hardness, enamel

Enamel Hard ceramic that covers and protects the exposed part of the tooth. 

Eimar, H. et al. (2012) Regulation of enamel hardness by its crystallographic dimensions. Acta Biomater., 8 (9), 3400-3410. 

Abrasion Resistance. Porcelain enamel is the most scratch resistant and hardest of commercial coatings (see Hardness). This property is used to distinguish between porcelain enamel and organic enamel or painted coatings. The rate of abrasive wear in surface abrasion increases with time, and the subsurface abrasion which follows exhibits a higher, but constant rate of wear. Abrasion resistance can be evaluated by loss of gloss or weight (35). 

Epoxy ester Epoxy esters are a type of alkyd where a high molecular weight resin is reacted with alkyd resin. The curing mechanism remains primarily through the oil-oxidation reaction and their properties are in no way similar to the chemically reacted epoxies. They have similar properties to alkyds although with improved chemical resistance but inferior appearance. They form a reasonably hard, oil-resistant coating, which can sometimes be suitable for machinery enamels, but are primarily for interior use, since they tend to chalk rapidly on exteriors. Their best use is for chemical or water resistance where circumstances dictate that finishes that are more superior cannot be used. 

A vitreous enamel coating is, as the name implies, a coating of a glassy substance which has been fused onto the basis metal to give a tightly adherent hard finish resistant to many abrasive and corrosive materials. The purpose of modern vitreous enamels is twofold, i.e. to confer corrosion protection to the metal substrate and at the same time to provide permanent colour, gloss and other aesthetic values. 

The hardness of an enamel surface is an important property for such items as enamelled sink units, domestic appliances, washing machine tubs which have to withstand the abrasive action of buttons, etc. On Moh s scale most enamels have a hardness of up to 6 (orthoclase). There are two types of hardness of importance to users of enamel, viz. surface and subsurface. The former is more important for domestic uses when one considers the scratching action of cutlery, pans, etc. whereas subsurface hardness is the prime factor in prolonging the life of enamelled scoops, buckets, etc. in such applications as elevators or conveyors of coal and other minerals. 

The interior surface finish, to comply with EEC and other health standards, must be rustproof, cleanable, and free from any crevices which can hold dirt. Bare timber in any form is not permitted. Most liners are now aluminium or galvanized steel sheet, finished white with a synthetic enamel or plastic coating. GRP liners are also in use. Floors are of hard concrete or tiles. Very heavy working floors may have metal grids let into the concrete surface. Floor concrete is coved up at the base of the walls to form a protective curb. 

In the laboratory, alkyd paints are preferable due to their durability and cleanability. Although latex paints have come a long way since their development, they do not measure up to the alkyd variety in laboratory applications. The popular latex enamels, in particular, are inferior to a good alkyd enamel. They are hard to apply smoothly and they pick up dirt far too fast. 

The mechanism of adhesion to various substrates has not been fully explained. Brauer Stansbury (1984b) consider that bonding to composite resins occurs by the diffusion of methacrylate polymer chains into the resin. Bonding to base metals is, perhaps, by salt or chelate bridges. Here it is significant that ZOE cements do not bond, so perhaps bonding is due to the action of free EBA on the substrate. The adhesion to porcelain is surprising. Porcelain is inert so that the attachment can hardly be chemical. Also, it would be expected that if a cement adheres to porcelain then it should adhere to untreated enamel and dentine, but this is not so. 

Teeth, the hard conical structures embedded in their jaws that vertebrate animals use to chew food, consist of two layers of compact matter surrounding a core of soft, living tissue. The inner layer is composed of dentine, also known as ivory, whose composition is similar, but not identical, to that of bone it contains less collagen and is harder than bone. The thin outer layer of the teeth, the teeth s enamel, includes even less collagen and other organic matter than dentine and is the hardest substance produced by animals (Hilson 1986a Kurten 1986b 1982). 

Our researchers have worked very hard to accomplish our goals by doing things we felt would enhance our synthetic bone materials and their performance to enable them to equal and often exceed the performance of autograft as implants as well as in other types of bone augmentation and replacement. The nonporous tooth enamel solid calcium phosphate materials have flexural strengths of over 20,000 lb in.2 However, without pores it would take an extremely long time to resorb this nonporous bioceramic. 

The connection between the use of ultrasound for the machining and drilling of hard materials (see below) and dentistry is clear since tooth enamel is a very hard material. In the dentist s surgery a new instrument has been introduced which is essentially a small ultrasonic probe operating at 25 kHz with a variety of attachments. Although... 

Cement, dentin, and enamel are bone-like substances. The high proportion of inorganic matter they contain (about 97% in the dental enamel) gives them their characteristic hardness. The organic components of cement, dentin, and enamel mainly consist of collagens and proteoglycans their most important mineral component is apatite, as in bone (see above). 

All enamels had excellent adhesion, solvent resistance, and flexibility. Enamels made from LC diols Ib-lg were far superior to those made from control diols 2a-2g and 3a-3g in both hardness (5H-6H vs. H-2H 5H corresponded to about 20 KHN on a Tukon tester) and impact resistance (50 to >80 in-lb vs. 8 to 15 in-lb). Odd spacers lb and Id afforded the best properties in these formulations. Spacer variations did not measurably affect enamel properties in the control oligoesters. 

Crosslinked Enamels. Properties of enamels made from LC diols optimized with slight variations in HMMM concentration gave 5H hardness (20 KHN Tukon tester) and >80 in-lb. impact resistance. The best properties obtainable for enamels made from non-LC diols were 2H hardness and 45 in-lb. impact resistance. 

The greatly enhanced hardness and impact resistance of enamels made from LC diols is not simply explainable by the monomer raising the Tg of the coating. In fact, Tg of the crosslinked enamels of Ib-lg are abnormally low for hard coatings and are similar to the much softer control enamels (Table II). 

The preceding chapter (7 ) described synthesis of model LC oligomeric diols and their incorporation into coatings binders of the baking enamel type. The enamels had far better hardness and impact resistance than control enamels made from amorphous or crystalline oligomeric diols. The model LC oligomers were synthesized by a Schotten-Bauman method that would be costly for large-scale production. 

This chapter is a preliminary report of our results. We found a procedure by which PHBA can be reacted with pre-formed amorphous oligoester diols at 230°C with minimal formation of phenol, an undesirable by-product. The products of such reactions are oligomers containing amorphous and ordered phases. They appear liquid crystalline. While the oligomer structures formed in the procedure described here differ from those of the previous paper, they form baked enamel films of exceptional hardness, toughness, and adhesion. 

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