Enamel mechanical properties

Zimmerman, B. et al (2010) Aiteration of dentin-enamel mechanical properties due to dental whitening treatments. 

The evaluation of the components of the tinplate container showed that the preferred enamel for irradiation processing was the epoxy phenolic the preferred end-sealing compound was the blend of cured and uncured isobutylene—isoprene copolymer. Component testing of tinplate and solder for possible changes in mechanical properties, microstructure, and corrosion resistance indicated that the radiation caused... 

Cements based on phytic add set more quickly than their glass polyalkenoate or dental silicate cement cormterparts, but have similar mechanical properties (Table 8.2). They are unique among add-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone... 

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Ca5(P04)30H]2, usually associated with around 5% carbonate (and referred to as carbonated apatite). The bones of the endoskeleton and the dentin and enamel of teeth have a high mineral content of carbonated apatite, and represent an extraordinary variety of structures with physical and mechanical properties exquisitely adapted to their particular function in the tissue where they are produced. We begin by discussing the formation of bone and then examine the biomineralization process leading to the hardest mineralized tissue known, the enamel of mammalian teeth. 

Table 5.14 Selected Mechanical Properties of Human Dentin and Enamel...

Mechanical Properties of Candidate Materials. The mechanical properties of enamel and dentin were presented earlier in Table 5.14. We will use these values as the basis for our material selection process. Of these properties, compressive strength is the most important. The candidate material should have a compressive strength at least that of enamel, which is about 384 MPa. 

Whilst the use of enamel and dentine as test substrates is widespread, they are complex materials to work with due to the natural variability both within and between specimens. A number of authors have examined alternative materials, which have similar mechanical properties to enamel and dentine, to use as test substrates. Acrylic [19, 20] and synthetic hydroxyapatite [21] have been proposed as suitable materials for abrasion testing, where mechanical effects dominate. These materials have several advantages since they are available as relatively large, smooth samples and exhibit better intra- and inter-sample reproducibility than their natural counterparts. This may, therefore, give better discrimination between test products for formulation development. However, the use of natural enamel and dentine is preferred, particularly for studies that aim to understand interactions between toothpaste products and tooth hard tissues. Other methods for assessing toothpaste abrasivity to hard tissues include gravimetry [22], scanning electron microscopy [23] and laser reflection [24]. 

This chapter describes the results of an ongoing study we are conducting into the nanoscale mechanical properties, chemical composition and structure of healthy enamel, carious lesions and the acquired salivary pellicle layer. A variety of material characterization techniques are being used, including nanoindentation, scanning electron microscopy (SEM), electron microprobe analysis (EMPA), scanning acoustic microscopy, atomic force microscopy (AFM) and time-of-flight secondary ion mass spectroscopy (TOF SIMS). 

Mechanical and Chemical Characterization Enamel has often been viewed as a homogeneous solid [2, 3], but Knoop microhardness tests [4, 5] and compression tests [6] have shown that the Young s modulus (E) and hardness (H) are higher for cusp (or surface) enamel than for side (or subsurface) enamel. Depth-sensing Vickers indentation [7] has shown that the H and E obtained from an occlusal section of enamel are higher than those for an axial section. The variations in mechanical properties with location have been explained in terms of the degree of tissue mineralization. Notably,... 

Fig. 1. Young s modulus, E, (a) and hardness, H, (b) for the enamel of the mesial half of a maxillary 2nd molar as determined by nanoindentation. The standard deviations for these averages range from 0.2 to 0.3 GPa for hardness and from 2 to 5 GPa for modulus. Note the wide variation in mechanical properties between the enamel surface and the EDJ. Average values of H and E that have been reported earlier by other researchers are included for comparison. 

For all mineralized tissues, the environment in which they are tested can significantly affect their mechanical properties. For bone, tests in aqueous and in simulated physiological solutions can change the hardness and elastic modulus by 20% [16, 17]. For enamel and dentin, the difference between the dry and wet mechanical properties can be 10% [18, 19]. Earlier studies [9] found... 

From a materials perspective there are two possible reasons why dental enamel shows the large variations in mechanical properties shown in figure 1 firstly, chemical variations in apatite composition and, secondly, changes in enamel structure with position from the occlusal surface to the EDJ. The chemical composition of enamel can be examined with a lateral resolution of 1-10 pm with electron microprobe analysis. Enamel structure can be obtained with SEM. To perform an accurate microprobe analysis, natural and synthetic minerals are used as standards to calibrate the instrument. This is fairly routine for geologists and earth scientists who are able to obtain chemical compositions with an accuracy of <0.1% for a wide range of elements simultaneously (including Na, Mg, Al, Si, P, K, Ca, Ti, Cr, Mn, Fe, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Hf, Ta, Pb, Th, U, F and Cl). In enamel only a few of these (Na, Mg, Al, P, K, Ca, Ti, Cl and F) are above the detection limit. The Ti is likely to be an impurity or contaminant rather than a constituent of enamel. This technique does not work for lighter elements such as C, S, O and N which may be present in enamel. 

Figure 1 also includes previously reported values for H and E for comparison with the mechanical property maps of Cuy et al. [12]. It is clear that nanoindentation is able to sense a much wider variation in these properties than previously observed. This is at least partially because the past investigations of mechanical properties yielded mainly average values of H and E that correspond to the values for the interior enamel. Many of these previous studies did not show the extreme local variations that can be measured with nanoindentation. Only the earlier nanoindentation studies have shown any evidence for the highest E and El found by Cuy et al. [12] using nanoindentation. Willems et al. [3] reported E = 90.59 16.13 GPa and Mahoney et al. [21] reported H = 4.88 0.35 GPa. 

Preliminary nanoindentation results on other teeth (premolars, incisors and canines) indicate variations in mechanical properties as large as those discussed for molars [unpubl. data]. In each case the exact distribution of mechanical properties within the enamel appears to correlate with the extent of mechanical loading experienced by the tooth during mastication. However, there appears to be an increase in the viscoelasticity (loss modulus) for the enamel of anterior teeth when compared to posterior teeth, again this may be related to their function. 

Xu HHK, Smith DT, Jahanmir S, Romberg E, Kelly JR, Thompson VP, Rekow ED Indentation damage and mechanical properties of human enamel and dentin. J Dent Res 1998 77 472-480. Kodaka T, Debari K, Yamada M, Kuroiwa M Correlation between microhardness and mineral content in sound human enamel. Caries Res 1992 26 139-141. 

Cuy JL, Mann AB, Livi KJ, Teaford MF, Weihs TP Nanoindentation mapping of the mechanical properties of human molar tooth enamel. Arch Oral Biol 2002 47 281-291. 

Chapter 5 concerns the mechanical properties of tooth mineral, with particular emphasis on the use of nanoscale hardness measurements to elucidate the variations across the tooth surface and how they may be associated with tooth function. The influence of environmental factors, such as those described in Chapter 4, are also discussed. In addition, the authors present very recent studies, employing a variety of state-of-the-art techniques, on pellicle-coated enamel and on the early carious lesion, which complements the work described in Chapters 2 and 4, respectively. 

The diacid mainly used in the production process for poly(ester-imide) based wire enamel is terephthalic acid (or its dimethylester) (6). Resins are known where isophthalic [46] and phthalic acid [47] are used. The large tonnage products all contain the terephthalic unit. The cured films have better thermal and mechanical properties (e.g., a higher hardness), important when the coated wires are processed in high speed winding machines. 

Additives used in formulations have the purpose of improving the flow of the enamel, to improve the thermo-mechanical properties like heat shock, or to give better adhesion of the cured film to the copper surface. Some of the additives have multiple effects. Because these effects are very specific to a given poly(ester-imide) resin and varnish formulation, the product classes are here only enumerated. Phenolics [112,113], epoxies [114,115], and silicones [116] are well known and some of them were claimed. Better understood is the effect of the phenol... 

Water based poly(ester-imide) wire enamels were developed in the 1960s and 1970s. The resins were made water soluble in different ways. Resins with a defined acid number were neutralized with alkanolamines [144-148] or ammonia [149]. In another process the poly(ester-imide) resins were submitted to an ami-nolysis with alkanolamines [150-153] or ammonia [154-156], when the resin network is more or less degraded. Solvents for this poly(ester-imide) is water and usually a small amount of high boiling solvents like N-methylpyrrolidon or diethylene glycol monomethylether. Titanium catalysts stable to hydrolysis, like titanium-ammonium lactate and titanium lactate, were used [157]. To improve thermal and mechanical properties, phenol blocked isocyanates can be added to the water based poly(ester-imide)s. The blocked isocyanates are dispersed by means of an ethoxylated nonylphenol and are added to the water based wire enamel. Improvement of the property level is claimed [158]. 

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