Depth of cure

The selection of the cure system in these applications is directed by constraints such as location of the adhesive in terms of confined space, speed and depth of cure, etc. The volumes of silicones typically applied are relatively small. In general, the uncured adhesive needs to be dispensed in a well-defined and limited area, and needs to stay in place without flowing during cure. No by-products of the cure reaction are acceptable as they may contaminate other sensitive areas of the devices. These constraints often direct the choice to the platinum-catalyzed hydrosilylation cure system that is relatively expensive. 

With the light curing mechanism, there is a limitation to the penetration of the light. The dentist may have to place a restoration that is 6 + mm thick, whereas the light may penetrate only 2 mm [182]. Factors that affect this penetration are the translucence of the material, the color or shade used to match the tooth, the ability to place the light source close to the material being polymerized, and the intensity of the source. Under relatively ideal conditions, the mean depth of cure is approximately 4-5 mm. Thus, the dental application requires that the material be placed in layers. Due to the oxygen inhibition of the outside surface of the resin layers, additional layers can be laminated and cured with the appearance of uniformity of the final restoration. 

The developmenf of self-curing resins, i.e., systems curing without photoinitiators or, in some cases, with just small amounts of photoinitiators, has been reported recently. Such resins are synthesized by Michael reaction of acrylic functional materials with Michael donor compounds such as acetoacetates. The resulting product has an increased molecular weight compared to the parent acrylate(s). This provides resins with reduced volatility and propensity for skin absorpfion. This new technology is versatile and flexible and opens a possibility of synfhesis of a large number of different acrylate resins. The novel resins reportedly exhibit unique depth of cure capability. In the absence of a photoinitiator (PI), film of approximately 10 mils (0.25 mm) thick can be cured at a line speed of 100 fpm (30.5 m/min). When only 1% of PI is added, the thickness of film that can be cured increases to over 100 mils (2.5 mm). 

Although the processing and final physical properties of epoxy-curing agent systems depend primarily on their chemical composition and degree of cure, the corresponding relations are often empirical or semiempirical and are not well understood. The tie between the cure chemistry and structure and properties of the cured resins consists in the theoretical and experimental study of network formation as a function of the depth of cure. 

Figure 6.23. The effect of quartz filler on the depth of cure at three irradiation times. [Adapted, by permission, from Murata N, Nishi S, Hosono S, J. Adhesion, 59, Nos. 1-4, 1996, 39-50.]...

MRM to determine the time-dependent depth of cure had been able to observe only the disappearance of the monomer. ""... 

Type of pol5mer Temperature Type of irradiation source Dose Chemistry, crystalhnity, density, branching structure Mobility of chains and reaction rates increase with temperature Type of grafting/crosslinking reaction possible, depth of cure penetration Number of reaction sites increases with dose... 

The disadvantages of UV adhesives are that one transparent substrate is normally required they suffer from oxygen inhibition and only a limited depth of cure can be achieved. The latter problem has been tackled by the development of dual-cure adhesives. In these systems, two independent curing mechanisms are incorporated into a single system. Thus the adhesive is cured first to a chemically stable form by UV irradiation and subsequently led to fiiU cure by a second means, for example, thermal cure. 

Depth of cure is tested in the laboratory by curing a sample of composite typically in a metal mould, removing it and scraping away the bottom with a plastic spatula [17], This technique is widely used, and it is certainly able to establish whether or not there is uncured composite paste at the lower end of the specimen. However, it is somewhat crude test. It also suffers from questionable clinical relevance, as the optical properties of a metal or even a plastic mould differ substantially from those of the natural tooth. Consequently, light transmission is likely to be different, resulting in a different pattern of curing. 

The alternative approach used in published studies has been to determine the micro-hardness at varying depths [18,19], an approach that assumes the existence of a correlation between micro-hardness and degree of conversion (DC) of monomer to polymer. Studies using this approach have shown that depth of cure does not correlate well with measurements made using a proper hardness measurement technique, in this case Vicker s hardness [20], In particular, for the so-called bulk fiU materials the ISO 4049 method, which involves scarping away unset material from the base of a cylinder of composite that has been exposed to light from the top, was found to overestimate the depth of cure compared to that determined from Vickers hardness number. 

B. K. Moore, J.A. Platt, G. Borges, T.M. Chu, I. KatsUieri, Depth of cure of dental resin composites ISO 4049 depth and microhardness of types of materials and shades, Oper. Dent. 33 (2008) 408 12. 

H. Matsumoto, J.E. Gres, V.A. Marker, T. Okabe, J.L. Ferracane, G.A. Harvey, Depth of cure of visible light-cure resin clinical simulation, J. Prosthet. Dent. 55 (1986) 574—578. 

S. Flury, S. Hayoz, A. Peutzfeldt, J. HUsler, A. Lussi, Depth of cure of resin composites is the ISO 4049 method suitable for bulk fill materials Dent. Mater. 28 (2012) 521-528. 

J.G. Leprince, P. Leveque, B. Nysten, B. GaUez, J. Devaux, G. Leboup, New insight into the depth of cure of dimethacrylate-based dental composites. Dent. Mater. 28 (2012) 512-520. 

A.C. ShorthaU, H.J. Wilson, E. Harrington, Depth of cure of radiation-activated composite restoratives - influence of shade and opacity, J. Oral Rehabil. 22 (1995) 337-342. 

R.W. Mills, K.D. Jandt, S.H. Ashworth, Dental composite depth of cure with halogen and blue hght emitting diode technology, Br. Dent. J. 186 (1999) 388-391. 

S.M. Mousavinasab, I. Meters, Comparison of depth of cure, hardness and heat generation of LED and high intensity QTH light sources, Eur. J. Dent. 5 (2011) 299-304. 

M. Nishimaki, Depth of cure and hardness of indirect composite resin materials polymerized with two metal halide laboratory curing units, J. Oral Sci. 54 (2012) 121-125. 

Depth of cure in polyacid-modified composites has been studied and compared with that in conventional composites [12]. Two techniques were used to inspect the behaviour of the specimens that had been prepared in split metal mould and cured from one end for 40 s. Depth of cure was then measured either using a penetrometer or by scraping away the inadequately cured material from the end furthest away from the curing lamp using a plastic spatula. These two techniques gave very similar results for all materials. Overall the study showed that clinical materials are able to cure to a variety of depths, depending on the brand and the shade [12],... 

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