Dental Materials Flashcards
1
Q
What are the 3 phases within Composite, which are `hydrophilic/phobic?
A
1) Organic Matrix (Hydrophobic)
2) Inorganic Filler (Hydrophilic)
3) Coupling Agent (Silane)
2
Q
Why isn’t MMA used as monomer in Composite organic matrix?
A
Its molecular weight and volume are too low –> resulting in polymer shrinkage
3
Q
What monomer(s) are usually found in organic matrix of Composite? What polymerisation reaction do they undergo?
A
ALL MONOMERS HAVE DIMETHACRYLATE GROUPS + UNDERGO FREE RADICAL ADDITION POLYMERISATION (no by-products, reduced shrinkage)
BisGMA
or UDMA/ Ethoxylated BisPhenol A derivative (less viscous)
Diluent monomer(s) added to reduce viscosity = Glycol dimethacrylates (e.g. TEGDMA, EDGMA and THFMA)
4
Q
In composites, what are “diluent monomers” added to organic matrix?
A
Added as viscosity controllers to reduce viscosity (which increases with filler)
They’re Glycol Dimethacrylates (e.g. TEGDMA, EDGMA and THFMA)
5
Q
What is Camphorquinone?
A
Activator/Initiator system for light cure composites, Camphorquinone absorbs light within range 460-480nm to produce free radicals and initiate polymerisation setting reaction
6
Q
What are the main components (monomers) within (new) Bulk Fill Composite?
A
Aromatic Dimethacrylate monomers: DDDMA + UDMA
Silorane Resin: Made from Siloxane and Oxirane monomer ring, which opens upon polymerisation to reduce polymerisation shrinkage
7
Q
What are 5 advantages of inorganic filler in composites?
A
1) Reduced polymerisation shrinkage
2) Reduced thermal expansion coefficient
3) Enhanced mechanical properties
4) Radiopacity
5) Improved aesthetic control
8
Q
What are the 3 main types of Inorganic Filler and how do they differ (Composites)?
A
1) Macro-filler (QUARTZ)
Large particle size, good strength but dull appearance - POSTERIOR
2) Micro-filler (COLLOIDAL SILICA)
Small particle size, less strength - ANTERIOR
3) Hybrid filler
Either Hybrid/Blended or Small Particle Hybrid
9
Q
Other than primary monomer (e.g. BisGMA), what are 5 other components within the organic matrix?
A
1) Hydroquinone (retarding agent?)
2) Activator/Initiator system (Room Temp Cure = DHPT or Benzoyl Peroxide & Light Cure = Benzoyl Peroxide or Camporquinone)
3) Pigments (e.g. Iron Oxide)
4) UV Stabilisers
5) Optical Brighteners (fluorescence)
10
Q
What is the filler loading + maximal theoretical packing density?
A
Filler loading = 30-70 vol% (50-85 wt%)
Theoretical packing density = 74%
This is least in MICROfillers… (20% wt)
11
Q
What are 2 advantages of Coupling Agents in Composites?
A
1) Bridge between hydrophilic filler and hydrophobic matrix
2) Improved mechanical properties
12
Q
What does the Coupling Agent bond to (Composites) and what reactions take place?
A
1) Hydrophilic filler via Condensation reaction (filler -OH and coupling agent -OCH3/-OH)
2) Hydrophobic matrix via chemical bond (matrix methacrylate group and coupling agent methacrylate group)
13
Q
What is meant by Composite “Degree/Depth of Conversion” and which type of cure has the best?
How do we overcome restrictive curing issues?
A
The proportion of reacting C=C double bonds, want it as high as possible.
Light Cure = 65-80%
Room Temp Cure = 60-75%
Heat Cure = 90% (but cannot be used at chair side)
Limited depth of cure overcome by Incremental curing (2mm)
14
Q
What are the 4 main methods of light cure for Composites? What is the wavelength of the first 2 and how does this relate to Camphorquinone?
A
1) Quartz- Tungsten Halogen (broad spec: 400-500nm)
2) LED (narrow spec: 460-480nm)
3) PAC (Plasma Arc)
4) Argon Laser
Camphorquinone = activator which absorbs light in 460-480nm range, so 2 can only use Camphorquinone, whereas 1 can use multitude of photoinitiators
15
Q
What is the Oxygen Inhibition Layer of Composites?
How can it become an issue and what are 3 main ways a clinician could overcome these?
A
Air and resin sticky interface layer that helps the next layer of composite to stick in incremental placement. Issue when its top layer 1) Clear matrix strip use 2) Overfill, cure and polish down 3) Apply bonding agent (clinician fav)
16
Q
What is the difference between “Flowable” and “Packable” Composites?
A
Flowable = Less resin, reduced mechanical properties
Difficult to control and used on Anterior teeth
Packable = More resin, enhanced mechanical properties but compromised aesthetics (Posterior teeth)
17
Q
Which Impression materials are used on:
1) Dentate Px?
2) Edentulous Px?
A
1) Elastic
Hydrocolloids (Agar or Alginate)
Elastomers (Addition/Condensation Silicone, Polysulphides or Polyethers)
2) Non-Elastic (Impression Plaster, Compound or ZOE)
18
Q
What is the difference between Agar and Alginate?
A
Agar = Reversible Hydrocolloid (undergoes PHYSICAL reaction) Alginate = Irreversible Hydrocolloid (undergoes CHEMICAL Cross-linking reaction)
19
Q
Alginates can be fast, regular or slow set, what does this depend on?
A
Depends on composition, how much CaSO4 (cross-linking agent, speeds up setting) vs. Na3PO4/Na2CO3 (retarder agent, slows down set)
20
Q
What is the bulk component of Alginate (70%)?
A
Diatomaceous earth (filler - for strength)
21
Q
What are the 6 main components of Alginate and their functions?
A
1) Sodium/Potassium Alginate (thickener)
2) Diatomaceous earth (filler - for strength)
3) CaSO4 (cross-linking agent)
4) Na3PO4/ Na2CO3 (retarding agent - slows reaction)
5) Sodium Silicofluoride/Fluorotitinate (pH controller)
6) Magnesium Oxide (pH controller)
22
Q
What are the 3 main pH controllers present in Alginate? Why are they necessary?
A
1) Sodium Silicofluoride
2) Sodium Fluorotitinate
3) Magnesium Oxide
pH changes occur throughout the setting reaction - initially decreases to below 3.5 then increases to 9 upon set due to build up of sodium and potassium ions
23
Q
Outline the main steps in the Alginate setting reaction…
A
Water added to powder and forms “Colloidal suspension” (large powder particles suspended in water)
1) CaSO4 (cross-linking agent) ionises to Ca and SO4 ions
2) Na3PO4 (retarder) ionise to Na and PO4 ions
3) Ca and PO4 ions react to form insoluble calcium phosphate
Cross-linking reaction suppressed whilst Phosphate ions used up (N.B CO3 may be present instead of phosphate)
24
Q
What is an example of a “Dust-free” Alginate?
A
Triethanolamine alginate
25
How is Alginate disinfected?
| In what material, how long for and why?
Disinfected in: Sodium Hypochlorite
For: 10 mins
Why: Too little = Swelling (imbibes water), Too much = Shrinkage (water soluble ions leach out)
After: Rinse under tap water, wrap in damp gauze and place in polyethene bag (cast within 24 hours)
26
How does Agar set?
PHYSICAL (instead of chemical) reaction so "Reversible" Hydrocolloid
Solid (gel) at room temp
Heat to 60 C = Viscous liquid
(Cool to 40 C before placing in mouth)
27
What are the 5 main components of Agar and their functions?
1) Agar (colloid)
2) Borates (strengthen gel - also found in Gypsum as retarder)
3) Potassium sulphates (accelerates stone set)
4) Thixotropic material/ Wax (filler - strength)
5) Water (dispersion medium)
28
What is meant by the following (N.B. Both occur in Agar and Alginate)
1) Synerisis?
2) Imbibition?
1) Synerisis = Expulsion of liquid from solid due to shrinkage from water loss
2) Imbibition = Swelling due to absorption of water (e.g. from disinfectant)
29
What is the viscosity of elastomers affected by?
| What are the 5 viscosity levels and which is used on clinics?
```
Affected by filler content
More filler = More viscous
1) Putty (most filler)
2) Heavy-bodied
3) Medium-bodied - Used on clinic!
4) Light-bodied
5) Wash (least filler)
N.B. Mixture can be used (e.g. putty for strength with light-bodied/wash for fine details)
```
30
Out of the Elastic Impression Materials, which are Hydrophobic and which are Hydrophilic?
```
Hydrophobic = Addition & Condensation Silicones
Hydrophilic = Hydrocolloids (Agar and Alginate), Polysulphides and Polyethers
```
31
In Addition and Condensation Silicones:
1) What type of reaction takes place (duh)?
2) What monomer is present (therefore what polymer is made)?
1) Addition = Addition
Condensation = Condensation (Ethanol by-product so shrinkage)
2) Addition = PVDMS Poly(Vinyldimethylsiloxane)
Condensation = PDMS Poly(Dimethylsiloxane)
32
What are 8 desirable properties of an Impression material?
1) Easy to use
2) Good odour, taste, texture and colour
3) Cheap
4) Good dimensional stability (and readily disinfected without losing accuracy)
5) Good (long) storage life
6) Good tear strength
7) Good elastic recovery and flexibility
8) Non toxic/Irritant
33
What are the 3 main Non-elastic Impression Materials?
| Which ones are Mucostatic and which are Mucocompressive - what does this mean?!
Mucostatic = Doesn't displace soft tissues
Mucocompressive = DOES displace soft tissues
1) Impression Plaster (Mucostatic)
2) Impression Compound (Mucocompressive)
3) ZOE (Mucostatic)
34
Which 2 Elastomeric Impression materials set via Addition reaction?
Addition Silicone
| Polyethers
35
Which 2 Elastomeric Impression materials set via Condensation reaction?
Condensation Silicone
| Polysulphides
36
What are the 2 main problems with bonding/adhesion to enamel and how do we overcome these problems?
1) Enamel surface area/tension lower than that of adhesive resin (34-38 mJ/m2) and made even smaller by enamel pellicle (28mj/m2) which reduces adhesive wettability
2) Adhesive resin non-polar, requires dry environment!
Enamel surface acid-etched with 37% (30-50%) Phosphoric Acid to remove enamel pellicle and disrupt enamel prism structure. Allows increased wettability (enamel surface tension increased to 48mj/m2) and formation of Micromechanical bonds as unfilled resin readily flows down etched prism areas to form "Resin tags"
37
What are the 2 main problems with bonding/adhesion to dentine and how do we overcome these problems?
1) Formation of smear layer when dentine cut into (collagen/HAP/debris/bacteria) - must be removed as contamination issue & reduces adhesion
2) Exposed dentine tubules - must be filled to prevent dentine hypersensitivity (and any adverse effects on pulp)
38
What is the "Smear Layer"?
0.5-5 nanometer layer formed from fluid expulsion when dentine tubules first cut into.
Contains:
- Denatured collagen (type 1)
- Inorganic HAP crystals
- Debris (may contain bacteria)
Can retract back into exposed dentine tubules to form "Smear Plugs" - bacteria contaminant.
39
What is the "Hybrid Zone/Layer"?
```
Interpenetrating layer of:
- Sealer
- Coupling Agent
- Demineralised dentine
Forms strong micromechanical bonds (resin tag formation) and composite restoration sits on TOP of this Hybrid zone, via Chemical bonds
```
40
What are the 3 steps in Dentine adhesion?
1) ETCH (Acid/Conditioner)
2) PRIME (Coupling/Bonding Agent)
3) BOND (Sealer)
41
In 3 stage dentine bonding, what are the 3 bonds present and between which chemical groups?
1) Dentine collagen (hydroxy/amine group) bonds to hydroxy group on Primer ("Molecular entanglement")
2) Methacrylate group on Primer bonds to Methacrylate group on Sealer (chemical bond)
3) Methacrylate group on Sealer bonds to Methacrylate group on Composite Resin (BisGMA/UDMA) - (chemical bond)
42
The setting reaction of an Acid-Base cement (e.g. ZOE/CH/GIC) is NEVER complete, what happens to unreacted particles and what 3 factors affect the Acid-Base cement setting?
Unreacted particles form "cored" structure and act as fillers.
1) Particle size (smaller = increased SA = faster set)
2) Powder-to-Liquid ratio (more powder = faster set)
3) Temperature (Exothermic reaction: higher temp = faster set)
43
What are the 6 Acid-Base Cements you must know?
How can they be classified based on:
1) Bonding?
2) Liquid composition (water vs oil based)?
3) Powder (base) composition?
Zinc Phosphate, Zinc Polycarboxylate, Zinc Oxide Eugenol (ZOE), Calcium Hydroxide, Ethoxybenzoic Acid Cements (EBA) and Glass Ionomer Cements (GICs)
1) PHOSPHATE = Zinc Phosphate
POLYCARBOXYLATE = Zinc Polycarboxylate and GICs
PHENOLATE = ZOE, Calcium Hydroxide and EBAs
2) WATER-BASED = Zinc Phosphate, Zinc Polycarboxylate and GICs (phosphate and polycarboxylate bonded)
OIL-BASED = ZOE, Calcium Hydroxide and EBAs (phenolate bonded)
3) ZINC OXIDE = ALL except GIC (= Ion Leachable Glass)
44
What are the 3 catagories of Acid-Base Cements based on Application?
(What is the particle size associated with the first 2?)
TYPE 1 = Luting (Particle size = 25 microm)
TYPE 2 = Restorative or Lining (40 microm)
TYPE 3 = Lining or Base
N.B. Acid-Base cements can also be used as root canal sealers and in orthodontic attachment.
45
What occurs in an Acid-Base cement reaction?
| What type of reaction is this (3)?
Acid (liquid) + Base (powder) -> Salt + Water
- Acid-Base reaction
- Neutralisation reaction
- Exothermic reaction (heat given off, which then speeds up reaction further)
46
What is the liquid and powder composition of Zinc Phosphate Cements?
1) LIQUID
45.3-63.2% Phosphoric Acid
"Hygroscopic" so easily absorbs water, bottle should be tightly closed and water used to accelerate set
May be partially neutralised by Zinc/Aluminium cations to slow set
2) POWDER
(90%) Zinc Oxide
Additions (10%) of MgO, Al2O3 or SiO2 for strength
Zinc/Magnesium Oxides "heat-treated" to reduce reactivity
SnF2 (short term fluoride release - BUT weakens cement
47
What is the Compressive strength (range) for Zinc Phosphate and Zinc Polycarboxylate Acid-Base Cements? Why are they different?
Their tensile strengths are similar (brittle) what is this?
```
Phosphate = 40-140 MPa
Polycarboxylate = 55-85 MPa
```
Phosphate generally stronger but wider range as they're more affected by powder-to-liquid ratio used
(Similar) Tensile strength = 5-7 MPa
48
What is the liquid and powder composition of Zinc Polycarboxylate Cements?
1) LIQUID
30-45% Polyacrylic Acid (PAA)
Other unsaturated carboxylic acids added (e.g. Iatonic or Maleic)
Liquid may be freeze dried to powder, water = activator
2) POWDER
(90%) Zinc Oxide
Additions (10%) of MgO or SnO, Al2O3 or SiO2 for strength - Zinc/Magnesium/Tin Oxides "heat-treated" to reduce reactivity
"Bismuth salts" to modify set
SnF2 or Tannin Fluoride (short term fluoride release)
49
What occurs in the setting reaction of Zinc Phosphate?
| What is the name of the final product? Is water bound or unbound at this resulting stage?
1) ZnO (base) reacts w/ Phosphoric Acid to form salt and unbound water
2) Salt reacts with water and excess ZnO to form "Hopeite" (new salt w/ bound water - no by products)
50
Not all Acid-Base cements have natural adhesion to the tooth, which 2 do? How is this achieved?
All Polycarboxylate bonded (Zinc Polycarboxylate and GICs)
1) ZINC POLYCARBOXYLATE
Acid-Base cross-linking reaction between Zinc ions (ZnO) and Carboxyl group of PAA
Chemical bonds form between Phosphate group of HAP (on tooth) and Carboxyl group of PAA via Calcium ions on tooth surface/saliva
2) GICs
[Further details on initial Acid-Base reaction due to Ion Leachable Glass base]
(AS ABOVE) Chemical bonds form between Phosphate group of HAP (on tooth) and Carboxyl group of PAA via calcium ions on tooth surface/saliva
51
What are the 4 material choices to line a deep cavity that extends near the pulp?
What are 3 features we want a cavity liner to have?
1) Cavity Base
2) Cavity Liner
3) Cavity Varnish
4) Nothing!
1) Protective (mechanical/chemical/electrical/thermal)
2) Palliative (calming effect on pulp)
3) Therapeutic (barrier to prevent dentine sensitivity)
52
What are the 3 possible cavity lining materials and where are they placed in proximity to each other?
How does their thickness vary?
1) Cavity Base
At "base" below liner and varnish
Thickest layer - over 0.75mm
Reduces bulk of restorative and blocks out undercuts
2) Cavity Liner
Between base and varnish
Thinner layer - below 0.5mm
3) Cavity Varnish
Closer to cavity surface (final layer)
Thinnest layer - 0.002-0.005mm (2-5μm)
53
What are the Pulpal protection properties of a Cavity:
1) Base?
2) Liner?
3) Varnish?
1) BASE
Mechanical protection (thickest layer - 0.75mm)
Thermal/Electrical protection (insulator)
2) LINER
Thermal/Electrical protection (insulator)
Pulpal medication and Anti-bacterial
```
3) VARNISH
Chemical protection (seals dentine to reduce micro-leakage)
```
54
What is an example of a "Suspension Liner"?
| What can be added to improve its strength?
A non-setting cavity liner placed thicker
E.g.
Non-setting Calcium Hydroxide placed 20-25μm
Methyl/Ethyl Cellulose added for strength
55
What is the main composition of a Cavity Varnish (4) ?
| name generic material then give example
1) Natural Resin (E.g. Copal)
2) Synthetic Resin (E.g. Polystyrene)
3) Solvent (E.g. Ethanol, Acetone or Ether)
4) Calcium Hydroxide or Zinc Oxide may be added...
56
What are 6 comparisons between Calcium Hydroxide and Zinc Oxide Eugenol in use as Cavity Liners?
1) Both sets accelerated by moisture (n.b. excess water weakens resulting CH and can reverse ZOE set)
2) ZOE stronger (15-25MPa) and less soluble than CH (20MPa) - Reinforced ZOE = 40MPa!
3) ZOE has calming effect on pulp (obtundent)
4) CH more alkaline pH (12, reduces on set), ZOE = 5.5-6 (increases to 6-8 on set)
5) Eugenol component of ZOE inhibits vinyl polymerisation (cannot be used with Composite/Compomer)
6) CH stimulates secondary dentine formation
57
What is Zinc Oxide Non-Eugenol?
| What is its use and why might this be favourable?
ZOE alternative that completely removes Eugenol acid/liquid component and replaces it with other oil acid (e.g. Nonanoic Acid)
Allows it to be used with Composites/Compomers as Eugenol previously inhibited vinyl polymerisation
58
Both Calcium Hydroxide and ZOE are Cavity Liners that set through Acid-Base reactions, what is the (Acid/Base) composition of each?
CALCIUM HYDROXIDE
1) "Catalyst" (Acid acting)
- 50% Calcium Hydroxide
- 40% Plasticiser
- 10% ZnO
2) Base
- 40% (Di)Salicylate Ester
- Filler
ZINC OXIDE EUGENOL
1) Acid
- Eugenol/"Oil of Cloves"
-
59
Both Calcium Hydroxide and ZOE are Cavity Liners that set through Acid-Base reactions, what is the (Acid/Base) composition of each?
CALCIUM HYDROXIDE
1) "Catalyst" (Acid acting)
- 50% Calcium Hydroxide
- 40% Plasticiser
- 10% ZnO
2) Base
- 40% (Di)Salicylate Ester
- Filler
ZINC OXIDE EUGENOL
1) Acid
- Eugenol/"Oil of Cloves"
- Other oils (e.g. Olive oil) - modify viscosity
- 1% Acetic Acid - accelerate set
- Water (small amount)
2) Base
- ZnO (MgO - viscosity)
- Filler
- Dicalcium Phosphate, Mica or Rosin - improve mixing
- 1% Zinc Salt -accelerate set
60
For Calcium Hydroxide cavity liner, what is the:
1) Mixing Time?
2) Working Time?
3) Setting Time?
1) 5-30 secs (10 secs)
2) 30-60 secs (2 mins 20 secs)
3) 1-2 mins (2.5-3.5 mins)
61
What happens to the setting time of acid-base cements as:
1) powder/liquid ratio decreases?
2) rate of powder incorporation increases?
3) mixing temperature increases?
4) water contamination occurs?
1) Increased
2) Decreased
3) Decreased
4) Decreased
62
What happens to the compressive strength of acid-base cements as:
1) powder/liquid ratio decreases?
2) rate of powder incorporation increases?
3) mixing temperature increases?
4) water contamination occurs?
1) Decreases
2) Decreases
3) Decreases!!
4) Decreases
63
What happens to the solubility of acid-base cements as:
1) powder/liquid ratio decreases?
2) rate of powder incorporation increases?
3) mixing temperature increases?
4) water contamination occurs?
1) INCREASES
2) INCREASES
3) INCREASES
4) INCREASES!
(Bad as we dont want to be that soluble)
64
What happens to the initial acidity of acid-base cements as:
1) powder/liquid ratio decreases?
2) rate of powder incorporation increases?
3) mixing temperature increases?
4) water contamination occurs?
1) Increases (increased liquid acid component)
2) Increases!!
3) Increases!!
4) Increases
65
What is the only restorative material to have NO shrinkage?
| What is the polymerisation shrinkage of Composites
GIC
N.B. Amalgams only have small amount (0.02-0.2%)
Composite polymer shrinkage = 2-3%
66
```
Out of the following, which causes the most vs. least pulpal irritation?
Calcium Hydroxide
Zinc Polycarboxylate
Zinc Oxide Eugenol
Zinc Phosphate
GIC
```
Calcium Hydroxide and ZOE cause least pulpal irritation (used as Cavity Liners!)
1) CH and ZOE (least)
2) Zinc Polycarboxylate
3) GIC
4) Zinc Phosphate (most)
67
```
Place the following in order of most and least acidic:
Calcium Hydroxide
Zinc Phosphate
Zinc Polycarboxylate
Zinc Oxide Eugenol
GIC
PAA
Phosphoric Acid
```
1) Phosphoric Acid (most - pH 1.5)
2) PAA (pH 2.5)
3) Zinc Phosphate (pH 3)
4) GIC (pH 3.5)
5) Zinc Polycarboxylate (pH 4)
6) ZOE (pH 5.5-6)
7) CH (least - pH 9-12)
68
What is the Primary polymer present in:
1) Condensation silicones?
2) Addition silicones?
1) PMDS
Poly(Dimethyl siloxane)
2) PVDMS
Poly(Vinyl-dimethy siloxane)
69
What is the main resin in RMGIC?
HEMA or PAA
70
What are 4 advantages of Light-Cure composites?
1) Command Set
2) Less discolouration
3) No mixing - less porosity
4) Thinner Oxygen Inhibition layer produced
71
What are 4 disadvantages of Light-Cure composites?
1) Limited depth of cure (2mm)
2) Time consuming to cure in increments
3) Clinician hazard (retinal)
4) Light sensitive during application (may react to fast from UV sunlight)
72
What are 4 things that may affect depth of cure?
1) Distance between light and restoration
2) Type of light (and bulb life - e.g. Quartz Tungsten Halogen depth of cure reduces with bulb life)
3) Type of Composite (darker shades take longer)
4) Presence of contaminants on light tip end
73
What 3 bond types are present in Dentine-Resin Adhesion (3 step process)?
1) Micro-mechanical/ Molecular Entanglement
(between dentine + prime/coupling agent)
2) Chemical
(between prime/coupling agent + bond/sealer and bond/sealer with filled restorative)
3) Secondary atomic bonds: Polar hydroxyl groups on HAP and primer/coupling agent with Ca ions in saliva
74
In Enamel-Resin adhesion, what is the bond strength produced from Acid-Etch process?
20 MPa
75
In amalgam, what is meant by "Condensing"?
What type of condenser tip and pressure is used if the final mercury composition is:
1) 45% Lathe-Cut?
2) 40% Spherical?
Condensing = Incremental packing of amalgam into cavity AFTER Trituration to give good marginal seal
1) Small condenser tip + High pressure
2) Large condenser tip + Low pressure
76
In amalgam, what is meant by "Trituration"?
What is the outcome if amalgam is:
1) Under triturated?
2) Over triturated?
3) Properly triturated (what are these conditions)?
Trituration = Mechanical mixing of (40-45%) Mercury with Silver-Tin alloy
Mixed at 3000rpm for 5-20 seconds
1) Dry, dull, crumbly mix
2) Shiny, hot, wet mix - hard to separate from plastic capsule
3) Shiny mix - easily separates from plastic capsule in single mass
77
How do the following factors affect Gypsum product setting time:
1) Powder/Liquid ratio?
2) Spatulation Time?
3) Rate of Spatulation?
4) Temperature (minimal effect...)?
1) More powder = Reduced setting time BUT more viscous and increased porosity
2) Increased spatulation time = Reduced setting time (as continual breaking of crystals forms new sites for growth) BUT → increased expansion and porosity risk
3) Increased rate of Spatulation = Reduced setting time (Same as above - with increased expansion and porosity)
4) Increased temperature = Reduced setting time
78
Gypsum products contain 4% Accelerators and/or Retarders, what are 3 of each and how do they function?
ACCELERATORS = Reduce setting time and expansion
1) LESS than 20% NaCl (produce additional nucleation sites)
2) CaSO4.H2O (as above)
3) K2SO4 (Reacts with water to form "Syngenite" which increases crystal growth. 2% solution reduces setting time 10 → 4 mins)
RETARDERS = Increase setting time and expansion
1) MORE than 20% NaCl (affect crystal growth)
2) Borax/Borates (interfere w/crystal formation)
3) Potassium Citrate (as above)
79
What manufacturer reaction produces (3) Gypsum products from Gypsum?
From this, what reaction do Clinicians induce to set Gypsum products?
Gypsum → Gypsum Products
"Calcined" = Heated to remove one water molecule (Dihydrate → Hemihydrate + Water)
Gypsum Products → Set
REVERSE REACTION
Water added: Hemihydrate + Water → Dihydrate
Exothermic reaction, so heat by-product
80
What is the general composition of Gypsum Products? (5)
1) 𝝰 or β-Hemihydrate (75-85%)
2) Unreacted Dihydrate Gypsum (5-8%)
3) Fast-set soluble and slow-set insoluble anhydrites (5-8%) - formed by over heating of Gypsum above 130ºC
4) Accelerators and/or Retarders (4%)
5) Impurities (4%)
81
What is the general composition of Gypsum Products? (5)
1) 𝝰 or β-Hemihydrate (75-85%)
2) Unreacted Dihydrate Gypsum (5-8%)
3) Fast-set soluble and slow-set insoluble anhydrites (5-8%) - formed by over heating of Gypsum above 130ºC
4) Accelerators and/or Retarders (4%)
5) Impurities (4%)
82
What is Gypsum? What are the 3 main dental products made from it and how are they formed?
(How does each differ, hydrate type and porosity)
Gypsum = Calcium Sulphate Dihydrate (CaSO4.2H2O)
"Calcined" (Heated to remove one water):
Dihydrate -> Hemi-hydrate
1) PLASTER (β-Hemihydrate)
Heated (kettle) to 120ºC
WEAK: Small, Irregular and Porous particles
2) STONE (𝝰-Hemihydrate)
Heated (autoclave, under steam and pressure) to 120-130ºC
STRONG: Small, Regular and Non-porous particles
3) IMPROVED STONE (𝝰-Hemihydrate)
Heated (boiling) with "Deflocculants" MgCl2/CaCl2 (separate particles, avoiding agglomeration which would reduce SA and reduce set)
STRONG: Small, Regular and Non-porous particles
83
What is the difference between a Negative and Positive Replica of the Oral Cavity?
Which Gypsum product can produce both?
Negative Replica = Impression
Positive Replica = Cast model (made from impression)
Plaster (β-Hemihydrate) can produce both (Non-elastic impression only for edentulous Px)
84
How would you compare the 3 Gypsum products for:
1) Compressive and Tensile strength?
2) Surface Hardness?
3) Scratch Resistance?
4) Dimensional Stability?
1) Lowest in plaster (porous) --> Highest in Improved Stone
Compressive strength continues to increase upon set, and is affected by too much or too little water
2) As above
3) As above
4) All dimensionally stable - little/no change with water
85
Why is the concentration of NaCl added to Gypsum products so important?
LESS than 20% NaCl = ACCELERATOR
Speeds up setting reaction and reduces expansion by:
Providing additional crystallisation sites
MORE than 20% NaCl = RETARDER
Slows down setting reaction and increases expansion by:
Preventing crystal growth
86
What is the ONLY Dental material to have NO SHRINKAGE?
| Which one is second best and has very little shrinkage only (0.02-0.2%)?
```
GIC = NO SHRINKAGE
2nd best (only 0.02-0.2%) = Amalgam
```
87
Place the 6 Acid-Base Cements (except EBAs) in order from most to least acidic - why is this?
```
MOST ACIDIC
1) Zinc Phosphate - (Phosphoric acid most acidic)
2) GIC - 50% PAA
3) Zinc Polycarboxylate - 30-45% PAA
4) ZOE
5) CH
MOST ALKALINE
```
88
What happens to the setting time of Zinc Phosphate when water is added?
Reduces, as water accelerates set
| BUT Water weakens final material compressive strength...
89
Place the 6 Acid-Base cements in order of COMPRESSIVE strength?
```
STRONGEST
1) GIC
2) Zinc Phosphate
3) Zinc Polycarboxylate
4) ZOE
5) EBA
6) CH
WEAKEST
```
90
Place the 6 Acid-Base cements in order of TENSILE strength?
```
STRONGEST
1) Zinc Polycarboxylate
2) Zinc Phosphate
3) GIC
4) ZOE
5) EBA
6) CH
WEAKEST
```
(Essentially, learn compressive, then learn that GIC and Zinc Polycarboxylate switch for tensile)
91
What is the difference between Thermal "Conductivity" and "Diffusivity"?
What restorative has the highest and lowest for both?
```
Conductivity = Constant (measured once)
Diffusivity = Transient (changes over time)
```
```
CONDUCTIVITY = Highest (Amalgam) + Lowest (ZOE)
DIFFUSIVITY = Highest (Amalgam) + Lowest (GIC)
```
92
List the 6 Acid-Base cements in order of Most → Least pulpal response (hint: last 3 = all same)
THINK:
Acid content and strength
Which are used near pulp!
```
MOST
1) Zinc Phosphate - Phosphoric acid highest pH
2) GIC - 50% PAA
3) Zinc Polycarboxylate - 30-45% PAA
4) EBA, ZOE + CH
LEAST
```
