Resin-Composite Flashcards
Benefits of Acrylics over Silicates x5
- Less prone to erosion
- Less soluble over a wide pH range
- Less acidic
- Less brittle
- Better thermal insulators –> Lower value of thermal diffusivity
Define resin composite
A combination of 2 chemically different materials with a distinct interface separating the components and having properties that cannot be achieved by any of the components acting alone
3 major components of the resin-composite
- Organic resin matrix
- Inorganic filler
- Coupling agent
Which is the most chemically active component of the resin composite?
Organic resin matrix
What reaction does the fluid monomer undergo to become a rigid polymer?
Radical addition reaction
What is the benefit conferred from the fluid monomer –> rigid polymer reaction?
The ability to convert from a plastic mass into a rigid solid allows the material to be used for the restoration of dentition
Name 2 examples of monomers of the resin matrix
- BisGMA
2. Urethane dimethacrylate
Are the monomers of the resin matrix of high or low viscosity?
High (because of high molecular weight) ==> Addition of even a small amount of filler would produce a composite with a stiffness that is excessive for clinical use
Give an example of a monomer added to help control viscosity. Is this monomer of high or low viscosity?
TEGMA; low viscosity
Why is TEGMA added to the resin matrix?
To help control viscosity, facilitating manufacturers filler addition and clinical handling
Which compound is added to ensure a long shelf life for the composite?
An inhibitor is included, usually hydroquinone in amounts of 0.1% or less
What type of compound is hydroquinone?
Inhibitor - it prevents premature polymerisation so as to ensure a long shelf life for the composite
Name the 4 factors of the filler that can control properties
- Type of filler
- Concentration of filler
- Particle size
- Particle size distribution
Examples of fillers x5
- Quartz
- Silica
- Glasses (Aluminsilicates)
- Borosilicates
- Barium oxide
Benefits of inclusion of fillers x5
- By incorporating a large amount of glass filler, the shrinkage is reduced as the amount of resin used is reduced and the filler does not take part in the polymerisation process
- Addition of ceramic fillers reduce the coefficient of thermal expansion, bringing it closer to that of tooth tissues
- Fillers can improve mechanical properties such as hardness and compressive strength
- The use of heavy metals e.g. barium and strontium incorporated in the glass provides radio-opacity
- Fillers provide the ideal means of controlling various aesthetic features e.g. colour, translucency and fluorescence
Effect of addition of filler on composite shrinkage
Addition of filler reduces the amount of shrinkage as the amount of resin used is reduced and the filler does not take part in the polymerisation process
Effect of addition of filler on coefficient of thermal diffusivity
Addition of filler lowers the coefficient of thermal diffusivity, as methacrylate monomers have a high coefficient of thermal expansion (~80ppm/deg).
Effect of addition of fillers on mechanical properties e.g. surface hardness
Higher the filler content, higher the value of surface hardness (VHN)
Function of the coupling agent
The coupling agent transfers the stresses generated under loading from the rigid and brittle filler to the more flexible and ductile polymer matrix.
Most commonly used coupling agent
y-methacryloxypropyltriethoxysilane (y-MTPS)
What will a lack of bonding between the resin and glass filler particle create?
Crack initiation sites
What is a fundamental problem of resins? What is the solution to this problem?
Resins are hydrophobic, whereas silica-based glasses are hydrophilc due to a surface layer of hydroxyl groups bound to the silica. Hence, the resin does not have a natural affinity to bond to the glass surface.
The solution lies in the use of a suitable coupling agent - e.g. the silane coupling agent
Types of classification of resin-composite (x4)
- Method of activation - light vs chemical
- Filler particle size and distribution
- Handling characteristics - packable vs flowable
- Intended clinical application - type I vs type 2
What is Type I resin-composite used for under ISO4049?
Restoration of cavities involving the occlusal surfaces
What is Type 2 resin-composite used for under ISO4049?
All other polymer based filling and restorative materials
Name the 3 chemical wear mechanisms of resin composites
- Abrasive
- Erosive
- Fatigue
Describe the abrasive wear mechanism of resin-composite
Hard asperities penetrate the polymer matrix removing materials by shearing and cutting
Describe the erosive wear mechanism of resin-composite
Impact of asperities produces damage of the polymer matrix
Describe the fatigue wear mechanism of resin-composite
Localised deformations of the polymer are subjected to repeated stresses, resulting in its failure
Name the 3 stages of wear as visualised by Ag staining
- Little wear + No subsurface damage
- Higher wear + considerable damage
- Catastrophic wear as a result of extensive subsurface damage
