1 Pavel Bradna Dental cements and composites Chemistry, composition, properties Institute of Dental Research Materials used for: Luting, fixation, cementation - i.e. luting inlays crowns, bridges, veneers on the prepared tooth To protect pulp from heat (”thermal insulation“) and from chemical irritation (liners and bases) - to stimulate secondary dentin formation Temporary filling material What are cements? A material which ”glues“ various things together e.g. embedded particles in minerals are glued in a compact body (sand, particles of rocks are ”glued“ using Portland cement in a concrete) In the Oxford dictionary: In dentistry:
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Pavel Bradna
Dental cements and composites
Chemistry, composition, properties
Institute of Dental Research
Materials used for:
Luting, fixation, cementation - i.e. luting inlays crowns, bridges, veneers on the prepared tooth To protect pulp from heat (”thermal insulation“) and from chemical irritation (liners and bases)
- to stimulate secondary dentin formation Temporary filling material
What are cements?
A material which ”glues“ various things together e.g. embedded particles in minerals are glued in a compact body (sand, particles of rocks are ”glued“ using Portland cement in a concrete)
In the Oxford dictionary:
In dentistry:
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Cements are formed from:Materials formed by:
1. Reaction of acid and alkali (base) components – setting via acid-base reaction (neutralization in water based cements)
2. Free radical polymerization (the sameas that of acrylics and also composites) or
3. Via combination of the free-radical polymerization and the acid-base reaction
Setting reaction is free radical polymerizationHybrid glass ionomerResin cement
Water-based
Water-based
Terms and definitions:
Working time (WT) – time period measured from the beginning of mixing to the maximum time atwhich viscosity (consistency) is low enough to flow easily under pressure to a thin film
* ČSN EN ISO 9917-1 Water-based cements – Part 1: Cements powder/liquid setting through acid-base reaction
Setting time (ST) – time interval measured from the beginning/end of mixing* till cement reaches such resistance that external force will not cause itpermanent deformation
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Area of elastic behavior - according
to the Hook law
Area of plastic/irreversible
behaviour
Deformation
Ultimate strength compressive or tensile strength
Strength and creep
Long-term stress
Plastic/irreversible
deformation
Permanent deformation even though in area of elastic
behaviour
Short-term stress – fully
elastic/reversible
deformation
forc
e/ar
ea
Setting reaction - neutralizationwater
base + acid salt + water
Why water is needed?
- dissolves acids
- enables dissociation of acidic groups
- hydrates particles of cements and releases alkaline ions from their surface
Water-based cements
powder liquid matrix
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Zinc phosphate cementsIn dentistry used since 19th century
Main components:Powder: ZnO (90%) + MgO (10%)
Deactivated – sintering at 1100 – 1200oC (reducesspecific surface area and densifies ZnO) Grinding to 10 – 20 µm
Dying with pigments
Liquid: 33 – 40 % aqueous phosphoric acid H3PO4: Partly neutralized (buffered) by Al(OH)3 (app. 3 %) and ZnO (0-10 %) to slow setting reaction during mixing
Zn3(PO4)2.4 H2OStrongly exothermic3ZnO+2H3PO4+H2O
crystalline material resembles mineral
Hopeit
a surface Zinc aluminophosphate (hydrated amorphous gel) Zn3(PO4) 2 . 4H2O + AlPO4.nH2O surface hopeit amorphous gel
Precipitation of amorphous Zn, Al phosphates on the surface of ZnO particles, which inhibits penetration of phosphoric acid to particle core and fast crystallization of hopeit
Setting reaction:
Fast setting reaction, fast crystalization unsuitable properties
1. Reaction of pure ZnO:
much slower formation and crystallization of Zn3(PO4)2.4 H2Oslower setting reaction
H3PO4 partially neutralized with Al ions, different setting mechanism:
2. Presence of Al:
Slower reaction, suitable manipulationproperties – prolonged working time
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Structure of set cements:
2. High porosity (diameter app. 0.5 µm) arising from excess unbound waterwhich is then lost by diffusion – porosity decreases strength of cement and makes it permeable to dyes
1. Particles of incompletely dissolved ZnO covered with Al phosphate in matrix of brittle amorphous Zn phosphate
Acid-base Cements Their Biomedical and Industrial Applications, A.D. Wilson, J.W.NicholsonCambridge university press 1993
higher amount of water in the acid decreases cement strength (effect of higher porosity)
1. Higher powder/liquid ratio – (lower fraction of Al to ZnO surface) – faster setting
2. Higher temperature (37 oC) – a decrease in setting time app. 5 times (cooled mixing glass slab) – cement is dispensed in the crown not on a prepared tooth
3. Fast mixing – prolongation of setting breaking the formed cement structure into fragments
1. Finer particles – increase in particle surface accesible to an acid attack (faster setting)
2. Dilution of liquid – lower Al concentration (faster setting)
Water lost or liquid dilution – faster setting
Hygroscopic /alkaline powder absorbs CO2 – cement deterioration
How can be setting affected:
In the cabinet:
Improper manipulation:
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The very first translucent ”aesthetic“ anterior restorative material (1900-1950)
Silicate cements (silico phosphate cements)
Too acidic, irritating pulp, brittle and highly soluble, but releasing F- ions.
Even-though not used anymore it suggested how to prepare aesthetic cements with anticariegenic potential
Powder: particles of acid-soluble calcium fluoroalumino-silicate glass (oxide composition SiO2-Al2O3-CaO/CaF2)
Liquid: solution of app. 50 % phosphoric acid H3PO4, partially neutralized with Al a Zn
Zinc polycarboxylate (polycarboxylate)cementsInvented by Smith in 1968
Main components:
Powder – similar to that used for zinc phosphate cement,Al2O3, SnF2 are also added to improve its strength, to release F- and improve its manipulation (dried polyacid)
Liquid: 40 – 50 % aqueous solution of polyacrylic acid or copolymers of acrylic acid with ithaconic or maleic acids (molecular weight app. 20 000-50 000 - viscous)
CH CH CH2
OHC
O
CH2
COHO
polyacrylic acid
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C = CH H
H COOH
C = CH H
HOOC COOH
C = CH COOH
H CH2COOH
Acrylic acid
Maleic acid
Ithaconic acid
Higher pH than for Zn phosphate cementLower disintegration in the mouth environment Very good biological properties Adhesion to the tooth tissues
Shorter working time, worse manipulation properties – too high liquid viscosityLower resistance to the mechanical loadHigh creep
Main components:Powder: particles app. (10 – 20 µm) of acid-soluble calcium fluoroaluminosilicate glass, with high content of Ca (Sr, La-RTG), Al, P, F- and:
- freeze-dried polyacid to improve manipulation properties and pigment
Reactivity of glass particles must be decreased by special (thermal or acidic treatment – depletion of surface ions)
if Ag particles are added – metal-reinforced glass ionomer cement (cermets)Liquid: solution (viscous) 25 – 40 % of poly(ithaconic, acrylic, maleic) acid or their copolymers
- tartaric acid app. 5 % (to prolonge WT)
Chemically curing glass ionomer cements
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1. Particle surface dissolution by polyacid attack – cationsrelease
2. Reaction of Ca and Al cations with COOH groups and formation of amorphous, cross-linked polyacrylates