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:

2

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

Types of dental cements:

Zinc phosphate

Silicate (silico phosphate, not used anymore)

Polyalkenoate: – Zinc oxide polycarboxylate (polycarboxylate)

– Glass ionomer

Calcium hydroxide

Zinc oxide-eugenol

Resin cements

3

Types according to setting reaction:Setting reaction is acid-base:

Zinc phosphateSilicatePolyalkenoate: – Polycarboxylate– Glass ionomer

Calcium hydroxideZinc oxide-eugenol

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

4

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

5

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

6

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

8

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

Advantages:

Disadvantages:

CH CH CH2CH2

O O

C

OO

C

Ca Tooth surface

Comparison with zinc phosphate cements

9

• Chemically curing, classical, autocuring

- setting via neutralization reaction

• Hybrid, fortified, reinforced, resin-modified glass ionomer cement, dual-cured

- setting reaction is free-radical polymerization and neutralization

Glass ionomer cements(GIC – Glass-Ionomer Cements)

First prepared by Wilson, Kent and McLean, 1971

Types:

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

Setting reaction:

CH CH CH2CH2

O HOC

OHOC

CH CH CH2

HO O

CH2

COHO

C

Ca

waterbase + acid polysalts + water

( Ca+2 + - COO- - COOCa + H2O)

Glass core

Layer of hydrated glass (gel SiO2)

Matrix of Ca, Al polyacrylates(hydrogel)

Scheme of set GIC

Advances in Glass-ionomer Cements, Davidson CL, Mjor IA, Quintessence Publishing Co, 1999

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Why tartaric acid is needed ?

To prolong manipulation time (WT),

and to accelerate setting („snap“) set

• Powder: similar to that of classical GIC Ca (Sr, La), Al, P, Si, F glass (5–20 µm)

- decreased reactivity by thermal, acid treatment (depletion of surface cations)

- surface modified by silanes (coupling agents, see polymeric materials for details)

- freeze-dried polyacid with pendant (methacrylate /acrylic) groups (double bonds)

fortified, reinforced, resin-modified glass ionomer cement, dual-cured, LC light-cured

Hybrid glass ionomer cements

12

• Liquid: an aqueous solution of poly(acrylic, ithaconic, maleic) acids or their copolymers with pendant methacrylic /acrylic groups

• HEMA (2-hydroxyethylmethacrylate)

• components of an initiating system (camphorquinone)

1. Free-radical polymerization of monomeric HEMA and pendant double (methacrylate/acrylate) bonds on a polyacid backbone

CQ, peroxide

- C - C. + M - C - C - M.

Setting reaction:

initiators: camphorquinone/amine, dibenzoyl peroxide/amine

HEMA

C=C-

C=C

-

C=C-

C=C

-

C=C

--C

OO

H

-COOH

-CO

OH

-COOH

-COOH

-COO

H

-CO

OH

-CO

OH

C=C

-

(compression strength: 50 – 70 MPa/10 min.)

2. Neutralization (acid-base) reaction (compression strength150 – 180 MPa/24 h)

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• Thermal and mechanical properties similar to human dentin

• F- ions release

• Natural adhesion to the tooth tissues (Ca apatite)

• Good biological properties (mainly chemically curing types)

• Fast treatment

• Acceptable aesthetic properties

Sensitive to the loss and water sorptionLong maturationLower resistance to the mechanical load, creep and abrasion (plastification with water)

Advantages:

Disadvantages:

Maturation – strength increase in the first month after cement set

100

140

180

220

260

0 5 10 15 20 25 30

Days

Co

mp

ress

ion

str

en

gth

[M

Pa

]

Some typical properties of GIC

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2

2.5

3

3.5

4

4.5

5

5.5

0 10 20 30 40 50 60 70time [min]

pH

Fast pH increase after cement set

polycarboxylate

GIC

zinc phosphate

0

1

2

3

4

5

6

0 5 10 15 20 25Time [hours]

Defo

rm

ati

on

[%

]

Creep

zinc phosphate

GIC

polycarboxylate

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Delivered as powder/liquid – liners, bases, temporary fillings, root canal sealers, paste/paste – temporary cementation

Zinc oxide-eugenol (ZOE, ”phenolate“) cements

Powder: ZnO, covered with Zn stearate, Zn acetate –accelerators, Al2O3 – to improve strength

Liquid: eugenol, oil, rosin, acetic acid - accelerator, poly(methylmethacrylate) to improve strength

base + acid chelate + water

Setting reaction:water (as an accelerator)

Donor of electrons Electron acceptor

zinc eugenolate

Advantages:• Good biological properties (pH 7 after setting)

• Antibacterial properties

• Sedative effect

• Fast setting in the mouth environment (presence of water)

• Ease to dislodge

Disadvantages:

Low strength, not adhesiveHigh solubility in water

!! Inhibits free radical polymerization!!

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Non- eugenol cements for temporary cementation are frequently based only on EBA type structures – to avoid eugenol

inhibiting effect

Incorporation of ethoxybenzoic acid (EBA) increases strength of ZOE cements

Modified zinc oxide-eugenol /ethoxybenzoic acid (EBA) cements

COOH

OC2H5

+ ZnO

COO

O

C2H5

Zn2+

OOC

O

C2H5

+ H2O

Mixtures 62.5 % EBA and 37.5 % eugenol, powder contains up to 30 % of Al2O3 – to increase strength

Paste B: salicylates, disalicylates, fillers - BaSO4 (RTG), TiO2, CaSO4

paste/paste, used as liners, bases, indirect or direct pulp capping

Calcium hydroxide (salicylate) cements

Paste A: Ca(OH)2, plasticizer (N-ethyl toluenesulphonamide)

Setting reaction:

Ca-chelatemethylsalicylate

OH

COOCH3

O

O

H3C

Ca2+

O

O

O

C

C

O CH3

Ca(OH)2+

Low strength but alkaline – they can neutralize acids, antibacterial effect, induce formation of secondary

dentin

Setting types:

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Varnishes – suspensions of Ca(OH)2 and film-forming agents in organic solvents

Indirect/direct pulp capping

Varnishes and Ca(OH)2 suspensions

Water suspensions of Ca(OH)2 – water, Ca(OH)2, thickening agent

Heterogeneous materials composed of a matrix (e.g. polymeric) and particulate or fibrous fillers

Main components:• matrix

• filler

• coupling agents (bonds, sizings)

Resin Composites

What are composites?

Main use:Filling, luting, build-ups material

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TypTypes es of compositesof composites

Particulate composites - concrete, various parts for car making, sport appliances etc.

Fibrous composites „Fibre Reinforced Composites“ (FRC) – wood, bones, glass/carbon/aramide laminates

matrix - transfers mechanical load on reinforcing filler particles

- protects filler particles against environmental degradation

filler - acts as a ”load carrier“

coupling agents – enable stress to be optimally transferred on the filler

- facilitate particle dispersion in a monomers

Role of individual components:

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Matrix - monomers, initiators, polymer inhibitors

- (2,2-bis[4-(2hydroxy-3-methacryloyloxypropoxy)phenyl]propan)

1. BIS-GMA

methacrylicacid methacrylic

acid

Intermolecular H-bridges – high

viskosity

H CH3H CH

3

H

H

H

H

H

H

H

H

Smaller polymerization

contraction (shrinkage)

app. 5-6 vol. %

Sterically hindered rotation -rigid part of a molecule

General structure

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- Triethylenglycoldimethacrylate (low viscosity diluent)

- 1,6 hexandioldimethacrylate

- Urethandimethacrylate (2,2,4-trimethylhexamethyl-bis-(2-carbamoyl-oxyethyl)dimethacrylate)

3. TEGDMA

4. HDMA

2. UDMA

Two-paste systems, base paste (amine)/catalyst(peroxide) paste

Also powder/liquid systems (old fashioned)

Setting reaction: free-radical polymerization(similar to polymerization of acrylic monomers)

Chemically cured: build-ups, fixation – where sufficient light penetration cannot be guaranteed

Light cured: fillings, one syringe systems

Dual cured: fixation cements, build-ups materials

Two-component systems

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ethyl-4-(N,N’-dimethylamino)benzoate (4EDMAB)

N,N’-dimethylaminoethylmethacrylate (DMAEMA)

Amines

1-phenyl-1,2 propandion (λ = app. 420 nm)

camphorquinone

Initiators:Light cured composites:

(setting accelerators)

(CQ)

Chemically cured composites:

(dibenzoyl peroxide + terc. amine)

Reaction products colour the material after short period

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Fillers:50 – 90 wt. % of silanized quartz particles, finely groundBa-Sr glass, synthetic Zr glass (Zr-silica), pyrogenic silicaSiO2 (silica), pigments

Coupling agents:

γ-methacryl-oxypropyltrimethoxysilane (A 174)

Treated surface Untreated surface

HO-Si-

HO-Si-

HO-Si-CH2=C(CH3)-R

OCH3

OCH3

OCH3

Gla

ss p

artic

le s

urfa

ce

H2 O

-O-Si-

O-Si-

O-Si-

Gla

ss p

artic

le s

urfa

ce

O

CH2=C(CH3)-R

O-

O-

O-

CH2=C(CH3)-R O-

1. Hydrolysis of methoxy groups to silanol groups and their reaction withsurface silanol groups on glassparticle surface (methanol isreleased)

CH3 OH

2. Copolymerization of silane double bonds withmonomers – covalent bond between filler particles and amatrix

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Classification of composites according to particle size

~ 0.10 – 0.04

1 – 3

1. nanoparticles

2. nanoclusters – nanoparticleagglomerates

Nanocomposites

<1

0.02-0.04

1. Ba-Sr glass, synthetic Zr-Si (zirconium-silica)

2. amorphous Si02

Microhybrid

1-50.02-0.04

1–50

0,04

5 – 50

Particle size[µm]

1. Ba-Sr glass2. amorphous Si02

Hybrid

1. “Splitter“ – prepolymerized ground with amorphous Si02(d=0.04 µm) “organic filler“2. amorphous Si02

Microfilled

Ba-Sr glass, quartzTraditional/Conventional(macrofilled)

composite Filler

Filler load app. 55-85 wt %, amorphous Si02 app. 3-10 wt. % to control thixotropic properties

COMPOMERS COMPOMERS A hybrid A hybrid materialsmaterials COMPOsite and glassCOMPOsite and glass--ionoMERionoMER

((Polyacid modified composite resinsPolyacid modified composite resins))

a) From compositesMechanical resistanceExcellent aesthetic propertiesOne-component

b) From glass ionomers

F- releaseAdhesion to the tooth tissues

Synergic combination of composites and glass ionomer cements

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Ca, Sr reactive glass similar to that of GIC, Ba glass (RTG), YbF3

Main components:

1. Matrix:

Monomers with COOH groups (app. 5 %), BIS-GMA, TEGDMA, UDMA etc.

2. Fillers:

Properties similar to composite materials but lower resistance to mechanical forces, abrasion etc.

e.g. alkyltetracarboxylic acid and HEMA aduct

Properties similar to composites, but lower mechanical resistance

Setting reaction

- Prevails free-radical polymerization

- Neutralization between COOH groups of monomers and basic cations only in a small extent after water sorption (long-term effect)

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Some properties of dental cements and composites

moderate

mild

mild

mild-moderate

mild

moderate

Pulp irritation

10-20**

0

0

7-10*

1-3

0

Adhesion[MPa]

Elution F-

[µg/cm2]Solubility

%CS

[MPa]FT

[ µm ]ST

[min]Cement

0-50-0,01200-40010-30

2-4

CompositeChem. cured

05-20-3-4Calcium hydroxide

00,0420-50(100 EBA)

254-10Zinc oxide-eugenol(luting type)

150-6001,3170-20020-254-6Glass ionomer

00,0640-6020-305-6Polycarboxylate

00,0690-1202015-6Zinc phosphate

Phillips´s Science of Dental Materials, KJ Anusavice, Sounders 2003

ST- setting time, FT- film thickness , CS - compression strength after 24 hours, *usingprimers and **adhesive systems, 1max 25 µm (ČSN EN ISO 9917-1)