glass ionomers 7 Flashcards
Polyelectrolytic cements class:
- Zinc polycarboxylate cements
2. Glass ionomer cements(GIC)
GLASSIONOMERS -Terminology-
“glass” refers to the glassy ceramic particles and the glassy matrix (non-crystalline) of the set
material, while
= “ionomer” refers to ion-crosslinked polymer
Polyalkenoic or Polyalkenoate terminology
refers to polymer chain which incorporates alkenoic monomers such as acrylic acid, tartaric acid, maleic acid, etc.
Conditioning terminology
use of an acid with a low concentration, without hydroxyapatite crystals removal
!!! CONDITIONING ≠ ETCHING(DEMINERALIZATION)
PACKING
Powder and liquid = dosage + mixing (many disadvantages)
• Predosed capsules and automatic mixing with the gun
COMPOSITION
The acid component (polyacid, -COOH, H+ donor) = concentrated aqueous solution of polymers and copolymers of some unsaturated carboxylic acids – LIQUID 1. The base component = silicate of Al, Ca and F, proton receivers (H+), donors of metallic ions – POWDER 3. +/- resin (the liquid has monomers) = resin modified glass ionomers(RMGIC)
powder(%) • Si O 29,0 • Al O3 16,6 • Ca F2 34,3 • Na3AlF6 5,0 • Al F3 5,3 • Al PO4 9,9 particles’ sizes 47,5 % solution of: =>till 50μm for fillings =>till 20μm for lutting
Liquid – different copolymers of: 47,5 % solution of: =>till 50μm for fillings =>till 20μm for lutting water tartaric acid= influences the COMPOSITION polyacrilic acid itaconic acid = reduces the viscosity of the polyacrylic acid and thus preventing its gelation working time and the setting time
SETTING REACTION
acid – base reaction
OS•SiO2 + H2A → MA + SiO2+H2O
Glass oxides + acid → salt + sillica gel + water
There are 3 stages of the setting reaction, due to the gradual release of the glass ions – first of them being the Ca ions:
- dissolution (Ion-leaching phase)
- gelation (Hydrogel phase)
- hardening (Polysalt gel phase)
SETTING REACTION acid - base
1. Dissolution:
After mixing the liquid with the powder,
• The acid attacks the outer surface of the glass , which it will release cations –Al+++, Ca++, F+
• the released H ions of the carboxyl groups from the polyacid chains, diffuse into the glass and thus the cations will be released
- Gelation
begins 5-10 min after mixing=> initial setting
This initial setting is owed to the quick action of the Ca ions which firstly react with the carboxyl groups of the acid (Ca bivalent, Al trivalent) → thus it results a gel phase of the GIC
!!! it’s a critical phase – it needs perfect isolation
• Thus:
• the Al ions may dissolve into saliva = the gel isn’t stable => reduced properties
• water loss from GIC = unfinished setting reaction
•water / blood sorption => aesthethic changes of the restoration, reduced mechanical properties
- Hardening:
This process may last up to 7 days or several months
• generally it needs around 30 min for the Al ions to link within the network, and thus transforming the gel phase into a solid one (through cross-linking – because they are trivalents)
• GIC releases slowly after setting, Ca and F ions – with beneficial effects
• RMGIC has the possibility to simultaneously make a chemically /light induced polymerization
Manipulation
The glass composition has very important effects over the setting process and over the material’s handling:
- TheAl:Siratioishighercomparingtothatofsilicatecements= more reduced working time
- The tartaric acid reacts with the Ca ions prolonging the working time, but in the same time it stimulates the 3D-network formation with Al ions and thus it reduces the setting time
- Mixing time= 20sec
- Working time= 75sec
- Setting time = 2min
- Finishing only after = 7-10 min
advantages of Adhesion to the hard dental tissues
Major advantages:
2. a. Adhesion to the hard dental tissues
• Direct adhesion to the hard dental tissues
• Bulk application (not in successive layers as for the light-cured RBCs)
The –COOH groups of the polyacid chemically react :
- with–OHgroupsofthehydroxyapatitefromtheenamelordentin (replacing the Ca) OR with Ca from apatite
The formed bonds are => H bonds (reduced 2-7 MPa, but long-lasting if GIC is used for class V cav)
- -
Thefracturesarecohesive(withintheGICmasse) weakH+link=>thereducedtensionstrengthoftheGIC(7MPa)=> brittle material
-
The adhesion is more stronger to the enamel than to dentin!! => the
dentin needs to be conditioned
how to increase the efficiency of adhesion for gi
More efficient adhesion if:
• an acid is used to condition the dental tissues
Enamel
• PAA (polyacrylic acid 10%) short time applied= clean the dental surface, removes the smear layer and exposes the collagen fibers (0,5-1μm) conditioner
It results a similar adhesion to that of the self-etch adhesive systems
DIFFERENCE – high molecular weight of the polycarboxylic polymer which limits its capacity to infiltrate into the dentin:
› thin hybrid layer,
› !!! they cannot infiltrate into decalcified dentin depth made with orthophosphoric acid (in this case it must NOT be used for GIC)
Adhesion to the other substrates
GICs adhere to the active polar surfaces:
- non-noble metal used for the PFM restorations
- They don’t adhere to the ceramic and noble metals
- GICs used for putting:their adhesion decreases with the increase of the viscosity surface won’t be glassy (dehydration=dessication)
Aesthethics
Porous surface,more reduced for the RMGICs
- Roughness through:
- mechanical working (low speed; it needs continuous cooling)
- in time due to the oral cavity conditions
- micro fissures due to dehydration=dessication,soafteritssetting => protective varnish layer (NOT for RMGIC)
- Reduced translucency compared to RBC
- Colourchangeintime,fasterthanforRBCs(watersorption), especially on their surface, not on the margins
- Marginal pigmentation is more reduced => less stress during setting at the interface (the acid-base reaction generates a more reduced polymerization shrinkage than for linear polymerization)
