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Dental Materials > Cements 2 > Flashcards

Cements 2 Flashcards

1
Q

How is Zinc oxide/eugenol cement supplied?

A
  • as a powder and a liquid
  • or as 2 pastes
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2
Q

Properties of eugenol

A
  • reactive groups at adjacent positions
  • phenolic - OH
  • methoxy - O-CH3
  • vinyl group in unreactive
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3
Q

Zinc oxide properties

A
  • acts as basic oxide
  • ionic form of zinc - accelerator
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4
Q

Setting reaction of ZnO/eugenol cement

A
  • acid + base -> salt and water
  • also coordinate/dative bond
  • forms chelate salt
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5
Q

Ionic reaction in ZnO/eugenol cement setting

A
  • 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)
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6
Q

Chemical properties of ZnO/eugenol cement

A
  • biocompatible
  • thermal insulator
  • electrical insulator when dry
  • protects pulp from chemical effects
  • high solubility
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7
Q

ZnO/eugenol cement is biocompatible - it’s used for … cavities

A

deep

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8
Q

ZnO/eugenol cement has high solubility. This means what?

A
  • unsuitable for luting
  • used as temp cement
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9
Q

Mechanical properties of ZnO/eugenol cement

A
  • develops strength quickly
  • resists flow
  • relatively weak and brittle
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10
Q

How can the weakness of ZnO/eugenol cement be improved?

A
  • added resins
  • compressive strength increases from 20 to 40 MPa
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11
Q

There’s some concern about methacrylate polymers. Why?

A
  • 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
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12
Q

What is EBA cement?
Composition

A
  • 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
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13
Q

Compare stregnths of ZnO/eugenol cement and EBA

A
  • EBA stronger
  • 85MPa
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14
Q

Compare solubility of EBA to ZnO/eugenol cement

A
  • less than ZnO/eugenol cement
  • but still a problem
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15
Q

Uses of EBA cements

A
  • cavity lining
  • luting/cementation
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16
Q

How does zinc phosphate cement come?

A
  • as a powder or liquid
  • or encapsulated
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17
Q

Composition of zinc phosphate cement

A
  • powder is zinc oxide and magnesium oxide
  • liquid is aqueous phosphoric acid, buffered with zinc oxide and aluminium oxide
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18
Q

Setting reaction of zinc phosphate cement

A
  • 3ZnO + 2H3PO4 + H2O -> Zn3(PO4)24H2O
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19
Q

With these bases, setting reaction is quick/slow so WT is…

A
  • quickly
  • short
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20
Q

Factors that affect setting time are…

A
  • particle size
  • powder to liquid ratio
  • temperature
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21
Q

How does particle size affect setting time?

A
  • smaller the particle, the faster the setting
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22
Q

How does powder to liquid ratio affect setting time?

A
  • common to add increments of powder and mix
  • rather than in 1
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23
Q

How does temperature affect setting time?

A
  • lower temp increase working time
  • cool glass slab often used
  • beware dew point - condensed water can dilute the acid
24
Q

How to take care of these liquids?

A
  • 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
25
Q

Difference properties of zinc phosphate cements can be obtained by …

A

altering powder:liquid ratio

26
Q

Proportioning of zinc phosphate cements

A
  • thick (3.5:1) - stornger and less soluble
  • thin (3:1) - weaker, more soluble, thin film
27
Q

Zinc phosphate cements are stronger than …, … and …
Give 2 types

A
  • ZOE, EBA, Ca(OH)2
  • lining cements (thick) 140MPa
  • luting cement (thin mix) 80MPa
28
Q

Zinc phosphate cements are alkaline/acidic
Explain consequences

A
  • acidic (1.5)
  • irritant - if not much residual dentine preesent
  • may need a sub-lining
29
Q

Properties of zinc phosphate cements

A
  • stronger than others
  • acidic
  • lower solubility than others
  • forms thin films
  • white, opaque appearance
30
Q

Zinc cements have lower solubility and form thin films of …micrometres
Consequence?

A
  • 25
  • good for luting
31
Q

Why do zinc phosphate cements have a white opaque appearance?

A
  • unreacted zinc oxide
  • white line effect
32
Q

Uses of zinc phosphate cements

A
  • cavity base
  • luting agent
33
Q

Requirements of luting cements

A
  • resist fracture in function
  • form thin film
  • low solubility
  • adhesion to enamel/dentine, alloys/ceramics
34
Q

Why must luting cements resist fracture in function?

A
  • a fracture would lead to loss of retention in device
35
Q

Why must luting cements form a thin film?

A
  • 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
36
Q

Why must luting cements have low solubility?

A
  • because cements exposed at margins
37
Q

Why must luting cements be adhesive?

A
  • 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
38
Q

What are polycarboxylate cements?

A
  • forerunner of glass ionomers
  • use the zinc oxide of zinc phosphates but a weaker acid
  • reduce risk of patient pain
39
Q

Composition of polycarboxylate cements

A
  • powder - zinc oxide and other oxides
  • liquid is aqueous solution of polyacrylic acid
40
Q

What is a problem with polycarboxylate cements?
Solution

A
  • shelf life issues - polyacrylic acid was crystallising in bottle
  • developed powder/water materials
  • powder contains solidified acid and fluoride, liquid is pure water
41
Q

Setting reaction of polycarboxylate cements

A
  • acid and base -> salt and water
  • polyacid chains cross-linked with zinc ions
  • 3D polymeric structure formed
  • residual cores of zinc oxide remains
42
Q

Properties of polycarboxylate cements

A
  • adhesion
  • strength
  • irritancy
  • solubility
  • thin film producing
  • opaque appearance
43
Q

How are polycarboxylate cements adhesive?

A
  • 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
44
Q

polycarboxylate cements strength and solubility is about the same as …

A

zinc phosphate

45
Q

polycarboxylate cements is less/more irritable than zinc phosphate
Why?

A
  • less
  • weaker acid
46
Q

Why are polycarboxylate cements opaque?

A
  • due to unreacted zinc oxide particles core
47
Q

Uses for polycarboxylate cements

A
  • luting
  • orthodontic (bands)
48
Q

What are resin cements?

A
  • composites used with a bonding agent
  • low viscosity composite, sometimes dual-cured
49
Q

Resin cements have functional groups that react with …

A
  • enamel and dentine
  • metals and alloys
  • ceramics but not optimally
50
Q

Adhesion in resin cements

A
  • acid-etch bonding to enamel
  • bonding to dentine with bonding agent
  • adhesive may be incorporated or separate
51
Q

Some resin cements require air what?
What prevents this?

A
  • air block
  • gel prevents oxygen inhibition
52
Q

Resin cements are … and … than acid-base cements

A

stronger, tougher

53
Q

Film thickness of resin cements
Zinc phosphate thickness

A
  • over 100 micrometres
  • less than 25 micrometres
54
Q

Uses of resin cements

A
  • non-retentive crowns
  • resin-bonded bridges
  • bonded ceramics (e.g veneers)
  • bonded amalgams
  • orthodontics
55
Q

Are resin cements easy to remove?

A
  • no
  • difficult to remove
  • not ideal if revisions are needed

Dental Materials (17 decks)

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