Glass Polyalkenoate Flashcards
What compounds react to form polycarboxylate cements?
List some features of the set cement:
How can we improve the features?
ZnO reacts with polycarboxylic acid to form polycarboxylate cements
- adhesive to tooth substance
- poor appearance
- high solubility
- poor mechanical properties
Replace ZnO with ion leachable glass for better appearance, less solubility and better mechanical properties.
What are the three presentations of glass polyalkenoates and which form is used today?
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- powder and liquid
- powder mixed with water
- encapsulated form (used today)
Name the chemicals in the powder and liquid form of glass polyalkenoate:
What is the setting reaction:
Powder: sodium aluminosilicate glass with approx 20% CaF
Liquid: aqueous solution of acrylic/itaconic acid copolymer, and tartaric acid to control setting characteristics
- essentially acid base reaction
- unreacted glass cores, swimming in a silaceous hydrogel plus water
This is another example of a composite structure!
What materials are involved in the powder/water form of glass polyalkenoate?
Powder: sodium aluminosilicate glass with approx 20% CaF, plus vacuum dried polyacid
It is reconstituted by mixing either water from dropper botle, or with dilute aqueous solution of tartaric acid
Describe the porosity of hand mixed formulations:
Compare this with mechanical mixing:
Cements are viscous so difficult for air to get in during hand mixing, therefore low porosity
More likely for air to get in, increasing the porosity due to the frothing effect when the material is mixed mechanically.
Explain why manipulation is governed by chemistry:
Briefly describe the setting chemistry of glass polyalkenoates:
Fluoride containing aluminosilicate glass will react and cross link with a suitable polyacid: - polyacrylic or - polymaleic
- requires modifying and cross-linking agents in the mix
Acid base reaction –> salt and water
- results in the set material consisting of glass particles and a silaceous hydrogel
What purpose does the unreacted glass cores serve?
What are the two phases of the reaction?
Is there anything we need to be careful of?
Unreacted glass cores confer translucency
Setting reaction has two phases:
- initial set ( first few minutes after mixing)
- final set (~24 hours after)
Hydrogel is moisture sensitive, so need to consider this when finishing the restoration to protect against moisture.
How does the setting reaction occur?
Which ions predominate at which phase of the setting reaction?
- Setting involves the cross linking of polyacid chains by ions liberated from the glass particles
At initial set, Ca2+ predominates
At final set, Al3+ predominates
List some factors which affect which ions predominate at which phase:
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- Ca2+ is more accessible being on the outer surface of the glass particle
- Ca2+ is more mobile than Al3+ due to:
- ionic radii (Al 5.0nm, Ca 9.9nm)
- as a result, upon hydration the aluminium ion is more heavily solvated that the calcium ion, and is less mobile
- Al3+ wants to cross-link with 3 chains, as opposed to Ca2+ only needing 2 chains
What are the implications for clinicians?
- material is not fully set until 24 hours after finishing, so must delay final finishing for at least 24 hours
- material is sensitive to moisture until final set is complete, so must protect from moisture by applying varnish or an unfilled resin
- if polishing is necessary, vaseline should be applied to void dessication (dehydration) of the material
List some properties of glass polyalkenoate:
- adhesion to tooth substance: may enhance with application of a tooth cleanser e.g. citric acid
- tooth coloured and translucent: unreacted glass cores reflect colour of surrounding tooth and transmits light
- fluoride release from glass itself: sponge mechanism (takes up fluoride from oral cavity and releases it locally at a slow rate)
- generally considered to be biocompatible
- brittle and poor abrasion resistance
- can be used for restorations, luting and lining/base
How does glass polyalkenoate bond to tooth substance?
How may bonding be enhanced?
What should you consider when enhancing bond strength?
- bonding to tooth substance due to cross-linking with Ca ions
- also bonds to collagen by H bonding and metallic ion bridging
May be enhanced with tooth cleanser
- citric acid removes smear layer, no increased bond strength
- polyacrylic acid increases bond stength (micro-etching)
Can only increase bond strength to the limit of tensile strength of glass polyalkenoate cement, if greater, this will result in cohesive failure (bond remains intact)
What is the difference between modified composite to resin composite and what is its aim?
How do modified composites set?
What is the composition of giomers?
What are the two types of giomers?
- filler is replaced with aluminosilicate glass, to encourage fluoride release
- Setting by light cured free radical polymerisation
Giomers composition is as for modified composite but aluminosilicate glass is pre-reacted with polyacid to form a pre-reacted glass polyalkenoate complex
- surface reaction type: better physical properties
- full reaction type: fluoride release occurs
List the components in resin modified glass ionomer cements:
Powder: ion leachable glass
Liquid:
- methacrylate resin
- polyacid
- HEMA
- Water
- polymerisation activators and stabilisers
List advantages of glass polyalkanoates:
Disadvantages:
Advantages:
- inherent adhesion
- little shrinkage
- fluoride release
- biocompatible
Disadvantages:
- brittle
- soluble
- not radiopaque
- wear prone
- water sensitive
- slow set
