Composite resins
I Generalities
I.1 Definitions
I.2 Composition
I.2.1 Organic phase
I.2.2 Charges
I.2.3 Organo-mineral coupling agent
II Classification of composite resins
II.1 Depending on viscosity, polymerization mode, clinical indications
II.2 Depending on the size of the loads
II.3 Depending on the size of the charges and the viscosity
III Polymerization
III.1 Polymerization of dental composite resins
III.1.1 Chemo-polymerization
III.1.2 Photopolymerization
III.2 Main modes of polymerization of dental composites – Summary
IV Properties of composites
IV.1 Importance of the inorganic phase: percentage of charges
IV.2 Mechanical properties
IV.2.1 Bending strength
IV.2.2 Tensile strength
IV.2.3 Young’s modulus
IV.2.4 Hardness
IV.2.5 Aging and wear
IV.3 Physicochemical properties
IV.3.1 Polymerization contraction
IV.3.2 Thermal properties
IV.3.3 Water absorption and solubility
IV.3.4 Optical and radiographic properties
IV.3.5 Adhesion
Conclusion
INTRODUCTION
Aesthetic and electrochemical problems of amalgams, fragility, solubility and poor biocompatibility of silicates and PMMA (Poly Methyl MethacrylAtes) resins,
led to the development of a new type of material in the 1960s: composite resins. These were therefore developed to overcome in particular the aesthetic inadequacies of previous fillings: silicates and acrylic resins.
1953: Bowen adds quartz fillers to epoxy resins
1955: Buonocore introduces the concept of “mordanting”
1956: Bowen creates Bis-GMA (bisphenol A glycidyl dimethacrylate),
1962: Bowen files the patent for Bis-GMA.
I GENERALITIES:
I.1 DEFINITIONS:
In dentistry, a COMPOSITE RESIN is a material made up of an ORGANIC RESIN MATRIX and a reinforcement made up of FILLERS. The cohesion between these two materials is ensured by a coupling agent, a SILANE.
Schematic representation of a composite resin
Composite resins
I.2 COMPOSITION
I.2.1 Organic phase
The organic phase (= continuous or dispersing phase) constitutes on average 24 to 50% of the volume of the composite. It includes the matrix resin, viscosity reducers, the polymerization system and various additives.
1 – Matrix resin:
Matrix resins are the chemically active components of the composite. These are all
monomers “R – di methacrylates”, thus making all composite resins compatible with each other and with adhesives. They are derived from Bis-GMA and polyurethanes .
● Bis-GMA
Bis-GMA and its derivatives form the basis of most matrix resins. Its synthesis is carried out in two stages:
1/Esterification reaction:
Esterification reaction of methacrylic acid and glycidyl alcohol
2/ Reaction by addition:
Water hydrolyzes the ester bond of glycidyl methacrylate and Bisphenol A giving
birth at Bis-GMA.
Addition reaction of glycidyl methacrylate and Bisphenol A giving rise to
at Bis-GMA
Characteristics of the Bis-GMA molecule:
– aromatic rings stiffen the molecule
– the presence of a phenol cycle makes it possible to reduce the setting shrinkage but leads to a
high viscosity.
– hydroxyls cause significant viscosity of the unpolymerized matrix.
● Urethanes:
UDMA formation reaction
UDMA Features:
– high molecular weight with low toxicity to the pulp.
– lower viscosity than Bis-GMA but high setting shrinkage
– no ester bond to reduce the risk of matrix hydrolysis.
2 – Viscosity controllers
Bis-GMA and diurethane dimethacrylate monomers are very viscous liquids. The addition of a large amount of fillers causes the formation of a material with a consistency too thick for the clinic. Therefore, to counteract this problem, low viscosity monomers (diluents) are added, the most commonly used of which is:
– TEGDMA : Tri Ethylene Glycol Di Methacrylate
TEGDMA
Effect of the diluent on physical properties:
– increased grip retraction
– makes the resin more flexible and less brittle
– reduces its resistance to abrasion.
3- Polymerization agents
The polymerization of composites is based on the decomposition of a molecule (INITIATOR) by an ACTIVATOR into FREE RADICALS (R*). Free radicals initiate the opening of the vinyl bond of the monomer and the elongation of the polymer.
Schematic representation of the chain polymerization mechanism of radical polymerization
R* = free radical.
● Chemopolymerization agents
– The main ACTIVATORS:
– amines (DMPT, para amino methyl acetate and its derivatives): inhibited by humidity,
turn brown as they age.
– para-toluene-sulfinic acid : unstable, does not turn brown, is inactivated by atmospheric oxygen
– substituted thioureas
– ascorbic acid .
– The main PRIMERS: These are peroxides:
– benzoyl peroxide
– cumene peroxide
– tributylhydroperoxide.
● Photopolymerization agents
– ACTIVATOR = photons (light) at a certain wavelength
– INITIATOR = tertiary amine (DMAEMA : DiMethylAminoEthylMethAcrylate) + PHOTOSENSITIZER.
The most widely used photosensitizer is CAMPHOROQUINONE (CQ) (peak absorption
in the blue at 466.5 nm) but two other molecules are also used: Lucirin
TPO and Phenyl Propanedione (absorption peaks closer to UV).
After irradiation, there is formation of a PHOTOSENSITIZER–AMINE complex (ex CQ-DMAEMA) which generates a free radical.
4- Inhibitors of taking
In order to prevent spontaneous polymerization during storage of composite materials, phenol derivatives are added as polymerization inhibitors:
– hydroquinone (may cause discoloration)
– hydroquinone monomethyl ether
– BHT: (2, 4, 6-tritertiary-butyl phenol)
I.2.2 Charges:
The inorganic phase consists of the fillers that reinforce the material. These fillers
are linked to the matrix via a silane.
1- Nature of charges:
The fillers, mostly mineral, vary from one composite to another but are composed of SILICA (SiO2) in different forms and other types of particles:
● Mineral fillers
Mineral fillers are formed from:
– SILICA (SiO2) in different forms:
o in crystalline forms (crystobalite, tridymite, quartz): these forms are hard and resistant.
o in non-crystalline form (glass: borosilicate glass): interesting mechanical and aesthetic qualities.
– HEAVY METAL GLASSES which give the material its radio opacity:
o barium or strontium glass silicate
or zirconium dioxide glass
o trifluorinated yttrium or ytterbium (YbF3).
● Organic charges:
These are fillers based on organically modified ceramics, called OrMoCers . They are macromonomers composed of an inorganic silica core grafted with multifunctional methacrylate groups.
We also find organo-organic charges: (+/- 20 μm of TriMethylolPropane
Trimethacrylate).
● Organo-mineral fillers:
Organo-mineral fillers have a mineral core (vitreous silica or aerosil) and a
polymerized resin matrix that coats the core.
Microcharges are used exclusively in this form.
2 – Size of loads:
| Kind | Charge | Examples |
| Macrocharged | 1 to 40µm | Concise® |
| Microcharged | 0.04 µm | Durafill VS® Filtek A 110® |
| Hybrid | 0.5 to 30 µm | Ouixfil ® |
| Microhybrid | 0.1 to 10 µm | Z100®, Z250®, Tetric Ceram®, Miris®, Tetric Flow® |
| Nanocharged | 2 to 70 nm | Filtek Supreme® Tetric EvoCeram® |
Average particle size of fillers in composite resins
● Properties of charges
– high hardness
– chemical inertia
– refractive index close to that of resin matrices
– opacity controlled by addition of titanium dioxide pigments (TiO2).
3- Morphology and granulometry
The shape of the charges varies depending on the method of preparation:
• angular: obtained by grinding and attrition
• rounded: result from sintering
• spherical: sol-gel process (emulsion) or atomization.
3- Load rate
The proportion of fillers can be expressed as a mass fraction (% by weight) or a volume fraction (% by volume).
THERE IS EVERY INTEREST IN INCREASING THE LOADS AND REDUCING THEIR DIMENSION BUT THESE TWO POINTS INCREASE THE VISCOSITY OF THE COMPOSITE.
| Family | Viscosity | % loads (weight) | % charges (volume) |
| Macrocharged | Average | 78 | 67 |
| Microcharged | Average | 57.1 | 42.7 |
| Fluid | 52 | 38.1 | |
| Reinforced microcharged | Average | 72.8 | 58.5 |
| Compactable | 77 | 46 | |
Hybrids | Average | 78 | 60 |
| Fluid | 64.5 | 45.9 | |
| Compactable | 79.1 | 66.1 | |
| Ormocers | Average | 76.7 | 59.4 |
| Fluid | 63 | Not available |
Loading rate of composite resins
I.2.3 Organo-mineral coupling agent
An organo-mineral coupling agent is a bifunctional molecule that achieves cohesion between the fillers and the organic phase. This molecule is generally a silane.
This bifunctional molecule (ex γ- (methacryloxyl)propyltrimethoxysilane) has:
• at one end a Si atom binds to three OH groups which interact with the free OH functions on the surface of the charge (sizing).
• at the other end a methacrylate group which reacts with the matrix resin during polymerization.
y-(methacryloxyl)propyltrimethoxysilane (silane)
The hydrolysis of the bonds established between the fillers and the matrix leads to the decohesion of the organic and mineral phases, causing premature and rapid aging of the composite resin.
II CLASSIFICATION OF COMPOSITE RESINS
II.1 CLASSIFICATION OF COMPOSITE RESINS ACCORDING TO THEIR VISCOSITY:
VISCOSITY: Fluids < Average < Compactable.
II.2 CLASSIFICATION OF COMPOSITE RESINS ACCORDING TO THEIR POLYMERIZATION METHOD:
POLYMERIZATION MODE: Chemopolymerizable, Photopolymerizable, Dual (chemo and photo polymerizable).
II.3 CLASSIFICATION OF COMPOSITE RESINS ACCORDING TO THEIR CLINICAL INDICATIONS:
CLINICAL INDICATIONS REQUIRED: Anterior, Posterior, ‘Universal’.
III POLYMERIZATION
III.1 POLYMERIZATION OF DENTAL COMPOSITE RESINS:
The polymerization of a dental composite is a chain polymerization reaction. The
starting point is the decomposition of an initiator into free radicals (= activated state). Activation can be of thermal origin (thermopolymerization), chemical (chemopolymerization) or photochemical (photopolymerization).
Schematic representation of initiator activation in polymerization
In direct technique restorations, only chemo- and photopolymerization are
used.
Mechanism of radical polymerization
= addition of type “homogeneous radical” in 3 PHASES:
– INITIATION phase = activation of monomers by free radicals (slow, energy)
– PROPAGATION phase = addition, growth of the polymer (fast)
– TERMINATION phase = stop (disappearance of free radicals) – (recombination).
End of reaction = TERMINATION PHASE:
– combination of 2 growing chains by their radical end (addition)
– attachment of a fragment of initiator on the radical end
– saturation of the radical end by H+ (disproportionation)
III.1.1 Chemo-polymerization
III.1.1.1 Principle of chemo-polymerization
An electron from nitrogen (N) attaches to peroxide (benzoyl) creating a free radical (R*).
This is an oxidation-reduction reaction.
Chemopolymerization reaction
R*: free radical, M: Monomer, M*: Activated monomer
III.1.2 Photopolymerization:
Schematic representation of photopolymerization
R*: Free radical, M: Monomer, M*: Activated monomer
PhotoP / ChemoP advantages :
– Less porosity
– Homogeneous distribution of activators
– Flexible working hours
Quality of photopolymerization depends on :
– Useful power of the lamp
– Light source / shutter distance
– The color, rate and nature of the charges.
Degree of conversion :
The degree of conversion is the percentage of [C=C] double bonds that convert to [CC]
during the polymerization reaction.
It is never 100% and does not exceed 50 to 60% for photopolymerizable composite resins. The degree of conversion depends on the polymerization method:
chemo- < photo- < thermo polymerization.
It depends on:
. of the nature of the matrix and the charges
. of the size of the loads
. of the composite tint
. of the power of the light source
. of the irradiation time.
Main modes of polymerization of dental composites
IV.2 MECHANICAL PROPERTIES:
IV.2.1 Modulus of elasticity:
| Family | Viscosity | Modulus of elasticity (GPa) |
| Macrocharged | Average | 12.3 |
| Microcharged | Average | 5.2 |
| Fluid | 4.4 | |
| Reinforced microcharged | Average | 6.8 |
| Compactable | 6.5 | |
Hybrids | Average | 9.3 |
| Fluid | 4.5 | |
| Compactable | 9.5 | |
| Ormocers | Average | 6.9 |
| Fluid | 4.4 |
Modulus of elasticity of composite resins
IV.2.2 Bending resistance:
| Family | Viscosity | Flexural strength (MPa) |
| Macrocharged | Average | 109.7 |
| Microcharged | Average | 66.3 |
| Fluid | 96 | |
| Reinforced microcharged | Average | 90.4 |
| Compactable | 125 | |
Hybrids | Average | 109 |
| Fluid | 88 | |
| Compactable | 112 | |
| Ormocers | Average | 100.4 |
| Fluid | 86.7 |
Flexural strength of composite resins
IV.2.3 Hardness:
| Family | Viscosity | Vickers hardness |
| Macrocharged | Average | 62.3 |
| Microcharged | Average | 30.8 |
| Fluid | 30 | |
| Reinforced microcharged | Average | 35 |
| Compactable | 41.5 | |
Hybrids | Average | 57 |
| Fluid | 20.6 | |
| Compactable | 50.2 | |
| Ormocers | Average | 49.1 |
| Fluid | 20.6 |
Hardness of composite resins
IV.2.4 Wear:
Being the weak point of these materials, several theories propose an interpretation of the wear and degradation of composites:
– Microfracture theory: Based on the difference in matrix/load elastic moduli.
– Theory of silane hydrolysis: Hydrolysis of the matrix/charge bond in the presence of water and saliva.
– Chemical absorption theory: Interaction between salivary and food components
– Protection theory: Wear in areas not exposed to occlusal contacts.
IV.2.5 Aging mechanisms:
Aging can be due to the breakdown between the silane and the fillers or to the breakdown of the
polymer chain:
1. Breakage between organosilanes and fillers :
The primary causes of failure between silane and fillers are:
– surface defects (cracks linked to polymerization shrinkage, porosities created by removal of fillers during polishing).
– air bubbles in the composite.
– incomplete polymerization of the material.
2. Breaking of links within the chain :
Under the combined actions of temperature, light, and oxygen in the air.
The most resistant composites are generally hybrids, but the material is not the only factor responsible; the size and location of the restoration as well as the implementation of the material must be added.
In summary, wear is greater in molars than in premolars and other teeth, in large restorations than in small ones, in areas undergoing occlusal contacts, and in the first years after placement of the material.
IV.3 PHYSICO-CHEMICAL PROPERTIES
IV.3.1 Polymerization contraction:
| Family | Viscosity | Modulus of elasticity (GPa) | Flexural strength (MPa) | Vickers hardness | Socket retraction |
| Macrocharged | Average | 12.3 | 109.7 | 62.3 | 1.8-2.4 |
| Microcharged | Average | 5.2 | 66.3 | 30.8 | 3.08 |
| Fluid | 4.4 | 96 | 30 | Not available | |
| Reinforced microcharged | Average | 6.8 | 90.4 | 35 | 3.15 |
| Compactable | 6.5 | 125 | 41.5 | 2.7 | |
Hybrids | Average | 9.3 | 109 | 57 | 3.04 |
| Fluid | 4.5 | 88 | 20.6 | 4.68 | |
| Compactable | 9.5 | 112 | 50.2 | 2.58 | |
| Ormocers | Average | 6.9 | 100.4 | 49.1 | 2.05 |
| Fluid | 4.4 | 86.7 | 20.6 | 2.92 |
Setting shrinkage of composite resins
The polymerization shrinkage of acrylic matrix composite resins is inherent to the polymerization reaction itself and depends on their chemical composition, the volume fraction of the fillers and the degree of conversion (measure of the degree of polymerization) during polymerization which is never total and uniform.
These constraints can have bad clinical consequences:
• Tensions in dental tissues which can lead to bending of the cusps, weakening or breakage of the enamel.
• +/- extensive and deep tears at the joint level with creation of a peripheral hiatus promoting marginal percolation, discolorations, inflammatory pulp reactions, recurrence of caries.
• Internal constraints in the material promoting partial or complete rupture of the resin-particle bond, appearance of cohesive fractures in the material.
• Decrease in mechanical resistance.
To limit the clinical consequences, different procedures adopted for a long time remain current: obturation technique (stratification, quantity of material/layer), polymerization (eg: “soft start” polymerization), configuration factor (factor C), etc.
Composite resin lamination technique
IV.3.2 Thermal properties
They also intervene in the integrity of the peripheral seal:
IV.3.2.1 Coefficient of thermal expansion:
The coefficient of thermal expansion of composite resins is 2 to 4 times greater than
that of dental tissues:
• 25.10-6/°C < macrocharged composites < 35.10-6/°C
• 22.10-6/°C < hybrid composites < 35.10-6/°C
• 45.10-6/°C < microcharged composites < 70.10-6/°C
Reminder: Email: 11.4.10-6/°C & Dentine: 8.3.10-6/°C
Stresses may appear at the material/tooth interface during changes of
temperature.
IV.3.2.2 Thermal conductivity:
Composite resins have low thermal conductivity (1.09 Wm-1.K-1), close to
that of enamel (0.93 Wm-1.K-1) and dentin (0.64 Wm-1.K-1), unlike amalgam (83 Wm-1.K-1).
IV.3.3 Water absorption:
The water behavior of composite resins is directly related to the quality of polymerization.
• Water absorption between 0.2 and 2.2 mg/cm2.
• Solubility in water after 2 weeks varies between 0.01 and 2.2 mg/cm2,
• The resulting volumetric expansion (0.3 to 4%) compensates for the polymerization shrinkage.
IV.3.4 Optical and radiographic properties:
The reduction in the number of shades available in a system and the appearance of new terminology characterizes the colorimetric evolution of new composite resins.
The differences in opacity are achieved by the differences in refractive index between the
mineral charges and the matrix; the different saturation levels are obtained thanks to variable concentrations of metal oxides.
The heavy elements contained in the charges (high atomic number) allow radiographic visualization.
IV.3.5 Adhesion
A composite resin does not adhere spontaneously to dental tissues. For there to be adhesion to dental tissues, an adhesive system must be used:
– Etching of enamel (micro-retentions), dentin (opening of tubules), conditioning or elimination of dentin smear.
– Enamel-dentin coupling agent: the best results are obtained with loaded adhesives and having a solvent not containing acetone (volatile, very operator dependent).
CONCLUSION
The structural evolution of composites has mainly been marked by the increase in the percentage of fillers, the decrease in their size, the modification of their shape, the improvement of the coupling agent (silane) and the use of less hard filler particles. The consistency of these materials has also been diversified since initially proposed in a single “classical” viscosity, medium, they now also exist in the form of fluid composites (“flowables”) and compactable composites (“packables”).
From macrocharged composites to “nano” technology, including microhybrids
and ormocers, composite resins have developed strongly.
Currently, three families of composite resins can be distinguished: MACROFILLERS, HYBRIDS and MICROFILLERS, with hybrids containing the largest number of materials . Among these, we can distinguish microhybrids (average particle size of fillers < 1 μm) and nanofilled microhybrids (containing nanometric particles).
Composite resins
Early cavities in children need to be treated promptly.
Dental veneers cover imperfections such as stains or cracks.
Misaligned teeth can cause difficulty chewing.
Dental implants provide a stable solution to replace missing teeth.
Antiseptic mouthwashes reduce bacteria that cause bad breath.
Decayed baby teeth can affect the health of permanent teeth.
A soft-bristled toothbrush preserves enamel and gums.