L05: Types Of Dental Materials: Polymers, Polymerisation Flashcards
What is a polymer?
A long chain molecule with chains being held together by covalent bonds and bonds between chains being secondary bonds
Why are resin-based systems easy to use chairside?
Because it’s a light cured setting reaction. Very versatile and can make minor adjustments to change properties.
In what way are polymers versatile?
Less dense than metals/ceramics
Some compatibility with human tissue
Electrical insulation
Resists corrosion
What is PMMA?
Polymethyl methacrylate
What are the types of polymers?
Plastics
Elastomers
Long chain molecules
Hydrocarbons
Examples of commodity thermoplastics
Polyvinyl chloride (PVC)
Polypropylene
Polystyrene
Polyethylene
Examples of engineering thermoset polymers
Polyurethanes
Epoxies
Polyesters
More complex polymer chains (held together by cross-links or branching), cannot be softened with heat, will just breakdown
What polymers do we mainly deal with?
Silicones, polyesters, some epoxies
What is Bowen’s resin made of?
25% weight polymerisable resin
75% weight quarts or aluminosilicate glass filler
BisGMA or TEGDMA
Describe the properties of Bowen’s resin
Difunctional (methacrylate groups on either side).
High molecular weight monomer
Very viscous and bulky providing structural integrity to the resin matrix that’s used for resin-based composites
High weight of inorganic component (filler) needs to be added to strengthen it
Mix with lower viscosity resin to allow this
Define polymer, monomer, polymerisation and free radical.
Polymer: long chain molecules of repeating units
Monomer: units from which the chain is built
Polymerisation: the process of converting monomers to polymers
Free radical: chemical species with an unpaired electron
What is an addition reaction?
Reaction of 2 molecules to make a larger molecule
Same empirical formula as the monomer
Used more in dentistry
What is a condensation reaction?
Reaction of 2 molecules to make a larger molecule with the elimination of a smaller molecule
Different empirical formula
Briefly describe free radical addition polymerisation
Activation: provides a reactive species reacts with the monomers. Activation, usually by temperature.
Initiation: free radical reacts with monomer and polymerisation process is initiated.
Propagation: free radical reacts with more monomer, repeats and polymer chain grows. Self-limiting.
Termination: terminated either by lack of mobility, or mutual annihilation (2 radicals joining together).
How is propagation self limiting?
Because as the polymer chain grows, the mobility of the polymer chain reduces, so probability of radical going on to react with other chains reduces until the material freezes/vitrifies/forms a glass. Once there is no mobility left, reaction will terminate.
Why is incremental addition needed?
During termination you may be left with residual monomer and/or oligomers (uncured parts left within the material). Limited depth of cure so incremental addition is needed.
What is degree of polymerisation?
Number of repeating monomer units.
Depth of cure
What is molecular weight?
Number of repeating units multiplied by molecular weight of repeating unit. Can be changed readily to change the properties of the polymer.
How do properties of restorative materials and dentures relate to their thermoset or thermoplastic nature?
Resins are more branched and thermoset whereas denture acrylics are more thermoplastic and unbranched. Restorative purposes need to withstand higher amounts of stress, so thermoset cross-links resins are needed. Dentures need to have more flexibility etc.
What are thermosetting polymers?
They form a 3D network on polymerisation and if heated will decompose.
Can you remould/reshape thermosetting polymers?
No, as you increase temperature, the cross-links will break and material will decompose.
What are cross-linked polymers?
Cross-linked polymers form a 3D network by joining the individual chains
What are the main properties of cross-linked polymers?
Mechanical tensile strength, flexibility
What are tensile strength, extension and flexibility of a polymer dependent on?
Chain length, degree of cross linking and bond strength of the polymer chains
They also influence glass transition temperature
Structure of thermoplastics
Linear structure: no cross-linking. Held together by weak secondary bonds
What is glass transition temperature (Tg)?
The temperature at which the polymer chains begin to flow past each other. Below Tg the polymer is solid, above Tg the polymer can flow.
What can change how a polymer flows, affecting its glass transition temperature?
Molecular weight and cross-linking can change how a polymer flows
How is silicone polymer a unique type of plastic?
It has a low Tg. We make use of it above its Tg meaning it becomes very flexible. Still a solid due to its cross-links but becomes more useful as an impression material, where it can be solid but also flexible without it distorting or tearing.
What is the application of PMMA as a denture base?
Make use of the plastic the lower it’s glass transition temperature. It is solid and rigid and less flexible at room temperature. It is used below its glass transition temperature.
How is the peroxide imitator activated for different uses?
Heat (>65C) eg acrylic denture base
Chemical eg cold cure acrylic/chemical cure RBC
How is light activation for RBCs done?
Activating benzoyl peroxide with use of camphoroquinone dye and co-initiator
A yellow soluble powder at 470nm
