Cements 2 Flashcards
How is Zinc oxide/eugenol cement supplied?
- as a powder and a liquid
- or as 2 pastes
Properties of eugenol
- reactive groups at adjacent positions
- phenolic - OH
- methoxy - O-CH3
- vinyl group in unreactive
Zinc oxide properties
- acts as basic oxide
- ionic form of zinc - accelerator
Setting reaction of ZnO/eugenol cement
- acid + base -> salt and water
- also coordinate/dative bond
- forms chelate salt
Ionic reaction in ZnO/eugenol cement setting
- zinc acetate added to powder (1-5%) typically as an accelerator
- water is an ionic liquid so can speed up reaction
- increasing temp can too
- slow chairside (dry/cool) but rapid in the mouth (wet and warm)
Chemical properties of ZnO/eugenol cement
- biocompatible
- thermal insulator
- electrical insulator when dry
- protects pulp from chemical effects
- high solubility
ZnO/eugenol cement is biocompatible - it’s used for … cavities
deep
ZnO/eugenol cement has high solubility. This means what?
- unsuitable for luting
- used as temp cement
Mechanical properties of ZnO/eugenol cement
- develops strength quickly
- resists flow
- relatively weak and brittle
How can the weakness of ZnO/eugenol cement be improved?
- added resins
- compressive strength increases from 20 to 40 MPa
There’s some concern about methacrylate polymers. Why?
- inhibits methacrylate polymerisation of chemically activated polymerisation
- some evidence shows it effects light activated polymerisation but less than chemically activated
- many dentists use other materials as a base for composite therefore
What is EBA cement?
Composition
- closely related to ZnO/eugenol cement
- powder and liquid (powder is ZO reinforced with resin/filler, liquid is EBA - o-ethoxybenzoic acid)
- eugenol may be present mixed with EBA
- acid and ethoxy groups at adjacent positions
Compare stregnths of ZnO/eugenol cement and EBA
- EBA stronger
- 85MPa
Compare solubility of EBA to ZnO/eugenol cement
- less than ZnO/eugenol cement
- but still a problem
Uses of EBA cements
- cavity lining
- luting/cementation
How does zinc phosphate cement come?
- as a powder or liquid
- or encapsulated
Composition of zinc phosphate cement
- powder is zinc oxide and magnesium oxide
- liquid is aqueous phosphoric acid, buffered with zinc oxide and aluminium oxide
Setting reaction of zinc phosphate cement
- 3ZnO + 2H3PO4 + H2O -> Zn3(PO4)24H2O
With these bases, setting reaction is quick/slow so WT is…
- quickly
- short
Factors that affect setting time are…
- particle size
- powder to liquid ratio
- temperature
How does particle size affect setting time?
- smaller the particle, the faster the setting
How does powder to liquid ratio affect setting time?
- common to add increments of powder and mix
- rather than in 1
How does temperature affect setting time?
- lower temp increase working time
- cool glass slab often used
- beware dew point - condensed water can dilute the acid
How to take care of these liquids?
- important to only take lid off when needed
- to stop water evaporating, increased acid concentration
- acid may crystallise in hot countries
- small changes in liquid may largely affect performance
Difference properties of zinc phosphate cements can be obtained by …
altering powder:liquid ratio
Proportioning of zinc phosphate cements
- thick (3.5:1) - stornger and less soluble
- thin (3:1) - weaker, more soluble, thin film
Zinc phosphate cements are stronger than …, … and …
Give 2 types
- ZOE, EBA, Ca(OH)2
- lining cements (thick) 140MPa
- luting cement (thin mix) 80MPa
Zinc phosphate cements are alkaline/acidic
Explain consequences
- acidic (1.5)
- irritant - if not much residual dentine preesent
- may need a sub-lining
Properties of zinc phosphate cements
- stronger than others
- acidic
- lower solubility than others
- forms thin films
- white, opaque appearance
Zinc cements have lower solubility and form thin films of …micrometres
Consequence?
- 25
- good for luting
Why do zinc phosphate cements have a white opaque appearance?
- unreacted zinc oxide
- white line effect
Uses of zinc phosphate cements
- cavity base
- luting agent
Requirements of luting cements
- resist fracture in function
- form thin film
- low solubility
- adhesion to enamel/dentine, alloys/ceramics
Why must luting cements resist fracture in function?
- a fracture would lead to loss of retention in device
Why must luting cements form a thin film?
- cement is weakest component
- thicker the film, the more stress the cement experiences
- thinnest film possible is ideal
- too thick can alter fit of the device
Why must luting cements have low solubility?
- because cements exposed at margins
Why must luting cements be adhesive?
- to enamel/dentine
- to alloys/ceramics
- many cements aren’t adhesive but rely on mechanical strength on rough surfaces
- retentive designs may be needed to prepare underlying tooth
What are polycarboxylate cements?
- forerunner of glass ionomers
- use the zinc oxide of zinc phosphates but a weaker acid
- reduce risk of patient pain
Composition of polycarboxylate cements
- powder - zinc oxide and other oxides
- liquid is aqueous solution of polyacrylic acid
What is a problem with polycarboxylate cements?
Solution
- shelf life issues - polyacrylic acid was crystallising in bottle
- developed powder/water materials
- powder contains solidified acid and fluoride, liquid is pure water
Setting reaction of polycarboxylate cements
- acid and base -> salt and water
- polyacid chains cross-linked with zinc ions
- 3D polymeric structure formed
- residual cores of zinc oxide remains
Properties of polycarboxylate cements
- adhesion
- strength
- irritancy
- solubility
- thin film producing
- opaque appearance
How are polycarboxylate cements adhesive?
- carbon dioxide group can react with calcium ions in enamel and dentine
- react with metallic ions in stainless steel
- can be difficult to remove from instruments too
polycarboxylate cements strength and solubility is about the same as …
zinc phosphate
polycarboxylate cements is less/more irritable than zinc phosphate
Why?
- less
- weaker acid
Why are polycarboxylate cements opaque?
- due to unreacted zinc oxide particles core
Uses for polycarboxylate cements
- luting
- orthodontic (bands)
What are resin cements?
- composites used with a bonding agent
- low viscosity composite, sometimes dual-cured
Resin cements have functional groups that react with …
- enamel and dentine
- metals and alloys
- ceramics but not optimally
Adhesion in resin cements
- acid-etch bonding to enamel
- bonding to dentine with bonding agent
- adhesive may be incorporated or separate
Some resin cements require air what?
What prevents this?
- air block
- gel prevents oxygen inhibition
Resin cements are … and … than acid-base cements
stronger, tougher
Film thickness of resin cements
Zinc phosphate thickness
- over 100 micrometres
- less than 25 micrometres
Uses of resin cements
- non-retentive crowns
- resin-bonded bridges
- bonded ceramics (e.g veneers)
- bonded amalgams
- orthodontics
Are resin cements easy to remove?
- no
- difficult to remove
- not ideal if revisions are needed
