Kotok
R.U, Stepanova S.V.
State University “Dnipropetrovsk
Medical Academy of the Ministry of Healthcare of Ukraine”
Department of Dentistry, Faculty of
Postgraduate Studies
Effect
of temperature on the conversion of dental composite. (Literature overview)
Summary. In this
overview, the authors have studied the physicochemical properties of a dental
restorative composite. The following topics are the main focus: the discussion
of the monomer conversion concept, and the effect of temperature on the degree
of conversion of the polymer methacrylate matrix and the mechanical properties
of composite restorative materials.
Key words: composite
conversion, polymerization, double bonds.
Photocurable composite
filling materials are firmly established in the everyday practice of the
dentist. They allow performing high quality aesthetic and reliable restorations
of all teeth groups. Despite all significant improvements, dental composite
materials have two drawbacks that make their use difficult: incomplete
polymerization (conversion) and polymerization shrinkage, which leads to
various complications. One of the main drawbacks of composite materials is the
presence of a residual unreacted monomer in the fully cured material after the
polymerization reaction is completed [1,2].
The choice and ratio of
monomers affects the reactivity, viscosity, polymerization shrinkage,
mechanical characteristics, water absorption of cured composites. Binding of
the filler to the matrix significantly increases the mechanical performance
(flexural strength, pressure, Vickers hardness). However, the bond of the
filler to the matrix is still a certain weak point of the composite materials.
During the
polymerization, not all double bonds of composite materials are converted. Only
85% of the monomer reacts despite the full adherence to the polymerization
regime. This indicates the presence of a certain percentage of the residual
monomer. In addition, during the polymerization, new reaction products are
formed that are not present in the starting material. Partially unreacted
initiators and stabilizers also stay in the resinous matrix. These substances
may have toxic potential. In particular, the residual monomer may have an
irritant effect on the pulp. The allergic and general toxicity effects of
individual components are currently poorly understood, so the level of
toxicological risk can not be determined. Unreacted monomers can also act as
plasticizers, reducing the mechanical strength of the restoration and enhancing
its swelling. Oxidation of unsaturated methacrylate groups may cause a change
in the color of the composite and formation of formaldehyde [4,5].
The monomer is
continuously released from the polymerized composite material to a greater or
lesser degree. At present, a number of methods are used to determine the
residual monomer in polymeric materials, such as polarographic, bromination,
gas chromatography, IR spectroscopy, and thermal analysis methods [3].
The volume of
polymerization of the composite is expressed in the degree of conversion of
monomeric bonds to polymeric [7,8]. The degree of conversion affects the
physical and mechanical properties of the polymer [9,10]. Strengthening the
conversion of bonds makes the polymer surface harder, increases flexural
strength and elastic modulus, increases resistance to fracture, increases the
ultimate tensile strength and wear resistance [6].
When polymerizing, the
conversion of monomers and, accordingly, the properties of the polymer are
affected by temperature. As the temperature increases, the mobility of the
radicals increases and, as a result of reducing the viscosity of the system,
additional polymerization takes place [12].
A number of scientific
papers have noted the relationship between the conversion of monomers and
temperature, when the temperature of the composite is increased, the free
volume is additionally increased, giving the captured radicals greater
mobility, and as a result, the conversion of monomers increases. Some studies
have focused on heating composites after light-polymerization to improve the
conversion of monomers and the mechanical properties of the material.
The study revealed that
with an increase in temperature from 25 ° C to 40 ° C, the conversion of
unsaturated radicals increases by 14.5%. With an increase in temperature from
40 ° C to 70 ° C, an increase in the conversion of monomers to 72% is noted.
That is, with a rise in temperature, a statistically significant increase in
conversion was established [3]. . With an increase in the polymerization
temperature, an increase in the level of structurization and a decrease in the
defectiveness of the material is observed, which has a favorable effect on
mechanical properties [11].
Conclusions. The change
in the polymerization temperature leads to a change in the conversion of the
polymer methacrylate matrix and the change in the mechanical properties of the
composite restorative materials. At present, most of the work is devoted to the
effect of temperature on the conversion of a composite upon heating of the
material to polymerization. Few works describing the structural changes in
photopolymer composites under the influence of temperature (heating), even
after polymerization, in the manufacture of indirect composite restorations.
Literature
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Клемин В.А.
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Чистякова Г.Г.
Композиционные материалы светового отверждения: учеб.-метод. Пособие/Г.Г.
Чистякова, О.Н. Манюк.- Минск: БГМУ, 2012.-42с.
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Чистякова Г.Г.
Метод термографического анализа определения конверсии мономеров в
композиционных материалах/Чистякова Г.Г., Сахар Г.Г. //Стоматологический
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Lovell L.G., Newman
S.M., Bowman C.N. The effects of light intensity, temperature, and comonomer
composition on the polymerization behavior of dimethacrylate dental resins// J
Dent res.-1999- 18.- P1469-1476.
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S.A. Remaining methacrylate groups in composite restorative materials //
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Мандра Ю.В.
Изучение образцов композитного материала повышенной конверсии после разрушения
при сжатии и изгибе по данным сканирующей электронной микроскопии / Мандра
Ю.В., Ивашов А.С., Зайцев Д.В. // Проблемы стоматологии -2014.-№5.-С.18-19.
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