Flowable composites

M. Helvatjoglu-Antoniades, Y. Papadogiannis, R.S. Lakes, P. Dionysopoulos, D. Papadogiannis, Dynamic and static elastic moduh of packahle and flowable composite resins and their development after initial photo curing. Dent. Mater. 22 (2006) 450-459. 

N. Nakamura, A. Yamada, T. Iwamoto, M. Arakai, K. Tanaka, S. Aizawa, K. Nonaka, S. Fukumoto, Two-year chnical evaluation of flowable composite resin containing pre-reacted glass-ionomer, Pediatr. Dent. J. 19 (2009) 89-97. 

An alternative approach was taken by Hasel (1999, 2001), who developed materials capable of being placed through a hypodermic syringe, from 25 to 16 gauge needles (0.260 to 1.194 mm internal diameter) (Fig. 9.4). Retaining the monomer system used with conventional materials, these new systems initially contained less filler content. This filler also included colloidal silica of particle size 0.04 yum. These are termed flowable composites , and are now widely available (Clinical Research Associates, 2002). Recent generations of flowable composites have been introduced that are at or above conventional paste filler rates (e.g. up to 80wt%). This advance may open up the use and applications of flowables to any situation where conventional paste composites have been used. 

Illustration of the thixotropic behavior of a flowable composite (Aria ). 

It has been found that, for a fixed mineral filler content, the viscosity of PMF-based composites increases when the coat is made of polyethylene [164, 209, 293], poly(vinyl chloride) [316] and polypropylene [326, 327], The picture was different, however, for composites based on the ethylene/vinyl acetate copolymer to which kaolin with grafted poly (vinyl acetate) was added [336]. Addition of PMF with a minimum quantity of grafted polymer results in a sharp drop of flowability (rise of viscosity), in comparison to addition of unmodified filler but with a further increase of the quantity of grafted polymer the flow gradually increases and, depending on the kaolin content and quantity of grafted polymer, may even become higher than in specimens with unmodified filler, for equal concentrations. 

This difference in spatial characteristics has a profound effect upon the polymer s physical and chemical properties. In thermoplastic polymers, application of heat causes a change from a solid or glassy (amorphous) state to a flowable liquid. In thermosetting polymers, the change of state occurs from a rigid solid to a soft, rubbery composition. The glass transition temperature, Tg, ... 

Electroconductive resin compositions, which are useful for packaging electronic devices, have been described. In general, electroconductive resin compositions are made up from a thermoplastic resin and an electroconductive filler, mostly carbon black. Polyphenylene ether) resins are known to impart heat resistance. For general purposes, a poly(styrene) (PS) resin and an ABS resin are superior to other resins in that even if carbon black is incorporated in a large amount, there will be no substantial decrease in the flowability or... 

The use of poly(alkylene oxide)s in ASA molding compositions results in improved properties. They reduce the susceptibility to electrostatic charging, improve the flowability and reduce Shore hardness (9). 

This paper reviews the results of investigations into low-frequency mechanical and high-frequency (ultrasonic) vibration effects upon flowable polymeric systems, primarily, on molten commercial thermoplastics. We tried to systematize possible techniques to realize vibration in molding of polymers. Theoretical and experimental corroboration is provided for major effects obtained at cyclic (shear and bulk) strains of molten polymers and compositions based thereon. It is demonstrated that combined stress of polymeric media is attained under overlapping vibrations and this results in a decreased effective viscosity of the melts, a drop i the pressure required to extrude them through molding tools, increased critical velocities of unstable flow occurrence and a reduced load on the thrust elements of extruder screws. 

Bilgrien CJ et al. (1992) Storage-stable flowable organosiloxane composition powders and their preparation. US Patent 5 153238... 

UV-curable dental cement composites consisting of 20% asymmetrical 1,3,5-oxadiazine-2,4-dione trimethacrylate derivatives have been prepared that have lower shear viscosities than their symmetric triazole counterpart. This property is particularly needed for preparing flowable pre-cured cement paste. 

Note The remainder of the composit consisted of 80 wt% filler consisting of barium- aluminum-boron silicate glass powder, silicon dioxide-zirconium dioxide, and ytterbium fluoride. For dental composites lower viscosities are preferred because of their flowability. 

The properties of a powder can be subdivided into those related to the particle itself and those of the ensemble of particles (bulk properties). The major particle properties include particle size and size distribution, shape, density and porosity, surface properties (van der Waals attractions, electrostatic charge), moisture content and composition. Particle properties influence the bulk properties of powders/particulates. There are a vast number of bulk properties including moisture content, bulk density, bed porosity, compressibility, flowability, permeability, sinkability, wettability and dispersibility, among others. 

The shape of particles is normally that of more or less regular spheres, dense or hollow, with smooth surfaces and sometimes cracks. This is related to the composition and the rate of solvent evaporation, with possible existence of internal pressure inside the drops when a rigid surface layer is being formed (Walton and Mumford 1999). All these characteristics will have some effect on handling properties of powders such as bulk and tapped densities, particle density, (mixing with other powders, storage) wettability and solubility, porosity, specific area (rehydration, instantisation) flowability (size, surface asperities), friability and creation/existence of dust, stability in specific atmosphere and medium (oxidation, humidification, active component release) (Huntington 2004). 

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