Materials Exam Flashcards
What are the three main classes of materials?
Metals
Polymers
Ceramics
Mechanical Properties - Metals
Hard
Ductile – Tough
Strong
Mech Properties- Polymers
Soft
Ductile - Tough
Weak
Mech Properties - Ceramics
Hard
Brittle
Strong
Bonding - Metals
¢ metallic elements have 1,2,3 electrons in their outer shell ¢ electrons are key to metallic bonds ¢ electrons are loosely bound to nucleus ¢ electrons have free mobility thermal and electrical conductivity ductility-bend without breaking
Bonding - Ceramics
¢ ionic and covalent bonds associated with ceramics ¢ both are stronger than metallic bonds
¢ covalent >ionic
¢ ionic bonds - electron donor and electron acceptor ¢ covalent bonds-equally shared electrons
¢ non mobile ions
Bonding - Polymers
Covalent bonds
¢ High molecular weight
¢ Long molecules composed principally of nonmetallic elements (organic chemistry C,O,N,H)
Polymers are Entangled Long Chains “Cooked Spaghetti”
Derive Strength and Properties From the Entanglement
POLYMERIZATION PROCESS
¢ Light Activation of Initiator ¢ Initiation of Monomer
¢ Propagation of Free-Radical ¢ Termination of Free-Radical
Thermal Properties
(1-15 ppm/°C)
Ceramic processing
Processed by Sintering or Melting at High Temperatures (porcelein)
- What factors contribute to each materials mechanical properties?
Metals - electrons and microstructure
Polymers - Monomethacrylates vs dimethacrylates (
Ceramics - Crystalline vs noncrystalline - Most dental ceramics are semicrystalline or polycrystalline
polymerization can be initiated by
light, heat and chemical mixing
RULE OF MIXTURES
By knowing the phases present in the structure of any material and interfacial interactions, it is possible to predict the overall properties fairly well
Fillers Affects Properties
Increase filler vol: increase strength, modulus, viscosity, decrease shrinkage.
Increase filler size: increase surface roughness
Understand thermal expansion coefficient
most things expand when heated and contract when cooled
Measure:LCTE-linear coefficient of thermal expansion- cm/cm/°C (ppm/°C)
Understand the chemical implication of heat flow
Pulps can can withstand small temperature changes for short times (42°C for 60 sec) restricted circulation of pulp cannot dissipate heat and carry it away.
Metals have high thermal conductivity so they need thermal insulator like base
Composites have low thermal conductivity so they do not need base
Color
Know that color is defined in a 3D coordinate system
Hue
Wavelength
Color (Roy G Biv)
Value
intensity
brightness
Chroma
Purity
Density or concentration
Mercury issues with Amalgams
(a) Disposal
(b) Patient issues
Some patients may exhibit an allergic skin reaction to dental amalgams
(c) Operator issues
Because of mercury toxicity, US government has set threshold limit value (TLV) for sustained (40 hr/wk) exposure at 0.05 mg Hg/m3
Creep
Creep is only mechanical property correlated with clinical marginal fracture of low-copper amalgam restorations (no correlation for high-copper products – all have low creep)
Creep mechanism is grain boundary sliding of 1 phase (blocked by η in high-copper amalgams)
The pro- gressive deformation of a material at constant stress is called creep
Amalgam corrosion
Galvanic corrosion at interproximal contacts with gold alloys
Electrochemical corrosion because multiple phases
Crevice corrosion at margins
At unpolished scratches or secondary anatomy — lower pH and oxygen concentration of saliva
Corrosion under retained plaque because of lower oxygen concentration
Chemical corrosion from reaction with sulfide ions at occlusal surface
Gamma 1
Strongest phase – incompletely consumed starting alloy particles
Gamma 2
Weakest phase – 2 in low-copper amalgams (most corrosion prone)
Effect of Alloying on Amalgam
Setting expansion changes, other compositional changes.
Elements that are useful in Alloying
Copper- High-copper products should be selected — benefit: greater clinical longevity of restorations — much lower creep values measured in laboratory
Zn: Zinc is considered to facilitate machining lathe-cut particles (makes ingot more brittle) and improves corrosion resistance of amalgam, but results in less plastic amalgam mix
Expansion vs. Contraction amalgam
Initial contraction from absorption of Hg (diffusion) by amalgam alloy particles
Can be subsequent expansion from formation and growth of 1 and 2 or Cu-Sn (η) phases (matrix)
Final absorption of mercury by remaining amalgam alloy particles causes contraction
No free mercury in final set dental amalgam
Lathe cut vs. Spherical
Lathe: Filing or lathe-cut (machined from cast ingot)
Spherical (molten alloy blown through nozzle)
Spherical: Spherical particles are wetted with a lower mercury:alloy ratio than lathe-cut particles
Spherical particles resist forces of condensation less than lathe-cut particles
Phases formed
Strongest phase – incompletely consumed starting alloy particles ()
Weakest phase – 2 in low-copper amalgams (most corrosion prone)
Completely interconnected nature of 2 can result in bulk corrosion of low-copper dental amalgam
High-copper amalgams – Cu6Sn5 (η) is corroding phase that provides margin-sealing – because η is not interconnected, corrosion limited to marginal regions without bulk corrosion
Weaker interface between alloy particles and reaction phases
Heat treatment - amalgam
Eliminates compositional nonuniformity that exists in ingot before lathe-cutting (machining) or in spherical alloy particles (from freezing process in both cases)
Relieves stresses in alloy particles (both lathe-cut and spherical)
Provide manufacturer control of setting time ― great clinical importance
Hydrophobic vs. Hydrophilic - surface properties
Hydrophilic: ¢ Allow penetration into areas with water
¢ Usually mixed with a solvent (alcohol, acetone)
¢ Very low viscosity
¢ Allows for attachment to composite
Hydrophobic: Wetting can be anticipated on the basis of the Hydrophobicity (water-hating) and hydrophilicity (water loving) of materials
Contact Angle
contact angle used to measure how liquid interacts with solid
Good wetting –low contact angle (approach 0)
Poor wetting- high contact angle (approach 180)
Clinical consequence: Bonding and adhesives Protein and cell attachment Hyrophilic/phobic Surfacearea/rougness
Molecules can have both
Hydrophobic and hydrophilic parts
Types of corrosion
Galvanic: Metal 1 beside metal 2.
Structure selective corrosion: Phase 1 and 2 interspersed - difference in solidification patterns.
Crevice Corrosion: Crack tip is anode, surrounding is cathode.
Stress corrosion: stress point is anode, unstressed is cathjode.
METAL CORROSION
Active - lead to destruction (Gamma 2)
¢ Passive - produce corrosion film that prevents further
corrosion (titanium implants) ¢ Immune - noble metals (gold)
Galvanic
metal 1 beside metal 2
Structure selective corrosion
Phase 1 and 2 interspersed - difference in solidification patterns.
Crevice Corrosion
Crack tip is anode, surrounding is cathode.
Stress corrosion
stress point is anode, unstressed is cathjode.
Ceramics - Hydrolysis
¢ Chemical Dissolution –normally occurs through dissolution of oxides created by hydrogen bonding effects of water in local areas of high acidity.
¢ Examples
Acids dissolve HA (caries, acid etch for enamel bonding)
Acidulated fluoride treatments dissolve ceramic crowns may roughen surface or remove surface stain
Polymers- Hydrolysis
Hydrolytic degradation and release of components (primary bonds)
Water, enzymes (esterases), bacterial byproducts Contributes to wear problem with composites
Degradation products are related to
toxicity
Stress-Strain Curve
Review this
Toughness
before failure occurs
Modulus
Stiffness
creep
(strain relaxation)
Deformation over time in response to low constant
stress
Fracture toughness
Measures a Materials Resistance to Crack Propagation
¢ Has High Clinical Correlation to Clinical Wear Data (Composites)
Teeth are not completely Rigid can lead to
abfrations- cyclic stresses
Buccal-Lingual Stability
Cusp flexure, stress transfer lessened due to filling rigidity.
Know the components of composite resins
A composite filling is a tooth-colored plastic and glass mixture used to restore decayed teeth. Composites are also used for cosmetic improvements of the smile by changing the color of the teeth or reshaping disfigured teeth.
Resin helps
processing and handling
Filler helps
modulus and wear resistance
Know what a coupling agent does
as a compound which provides a chemical bond between two dissimilar materials, usually an inorganic and an organic.
Filler size
Viscosity and Strength Controlled by Filler Loading - Increased Filler Loading = Increased Viscosity
and Modulus
Glass ionomer cement components
Glass other phase, mixed with liquid.
Glass ionomer mech
l GICs contain a basic glass and an acidic polymer liquid, which set by an acid-base reaction. The polymer is an ionomer, containing a small proportion - some 5 to 10% - of substituted ionic groups. These allow it to be acid decomposable and clinically set readily.
The glass filler is generally a calcium alumino fluorosilicate powder, which upon reaction with a polyalkenoic acid gives a glass polyalkenoate-glass residue set in an ionised, polycarboxylate matrix.
The acid base setting reaction begins with the mixing of the components. The first phase of the reaction involves dissolution. The acid begins to attach the surface of the glass particles, as well as the adjacent tooth substrate, thus precipitating their outer layers but also neutralising itself. As the pH of the aqueous solution rises, the polyacrylic acid begins to ionise, and becoming negatively charged it sets up a diffusion gradient and helps draw cations out of the glass and dentine. The alkalinity also induces the polymers to dissociate, increasing the viscosity of the aqueous solution.
The second phase is gelation, where as the pH continues to rise and the concentration of the ions in solution to increase, a critical point is reached and insoluble polyacrylates begin to precipitate. These polyanions have carboxylate groups whereby cations bind them, especially Ca2+ in this early phase, as it is the most readily available ion, crosslinking into calcium polyacrylate chains that begin to form a gel matrix, resulting in the initial hard set, within five minutes. Crosslinking, H bonds and physical entanglement of the chains are responsible for gelation. During this phase, the GIC is still vulnerable and must be protected from moisture. If contamination occurs, the chains will degrade and the GIC lose its strength and optical properties. Conversely, dehydration early on will crack the cement and make the surface porous.
Glass ionomer cements release
releases fluoride, can be water based
Understand similarities and differences between adhesives and composites
ADHESIVES
¢ Based on Composite Resin Chemistry
¢ Contains Acidic Groups to Promote Bonding ¢ Dimethacrylates that Are Visibly Light Cured ¢ Unfilled or Lightly Filled
Composites tend to be hydrophobic, so you need to be able to bridge that gap.
Hydrophilic modifiers
promote bonding to tooth structure.
How sealents differ from composities:
Also hydrophilic, but made with a BisGMA-TEGDMA system. Polymeric coating for protection.
oxygen inhibition layer
Oxygen Interferes with Radical Polymerizations
¢ Surface of VLC Materials are often Uncured
¢ For thin Layers such a Sealants more of a Problem
hydrocolloid vs. elastomers
Hydrocolloid: Also called “alginate impression material” Used to make dental impressions for:
- removable partial dentures
- preliminary impressions for complete dentures - orthodontic models and study models
Elastomers: rubber-like polymers that show an elastic behavior at the time of loading.
Alginates are
Water based, Calcium Crosslinked
Alginate problems
Dimensional stability:
Because it is a gel, it undergoes shrinkage or expansion upon loss or gain of water.
Syneresis
Loss of water to the surroundings
Imbibition
Pick up of water from the surroundings
For least dimensional change/avoid distortion: Store impressions in 100% relative humidity Pour quickly after removal from mouth
Limited detail reproduction
Low tear resistance L Single pour only
Quick pouring required
Low dimensional
Alginate benefits
¢ Generally easy to use
¢ Powder (supplied in can) shaken up before
use for aeration
¢ Water to Powder (W/P) ratio to be carefully
followed as specified by the manufacturer ¢ Lower W/P ratio increases strength, tear
resistance, and consistency; decreases
working and setting times and flexibility
¢ Lower water temperature increases working and setting times
¢ Insufficient mixing produces a grainy mixture and poor recording of detail
¢ Adequate spatulation produces smooth, creamy mix with minimum of voids
Polysiloxanes
Hydrophobic - Consequences for taking impressions: Hydrophobic but Chemically and Dimensionally Stable n Addition type can be made hydrophilic by:
simple addition of surfactant, or
chemical incorporation of hydrophilic moieties into the silicone backbone.
Can Cause Voids Upon Addition of Stone Slurry
¢ Slurry in Water Based
Have Good dimensional stability
Polysiloxanes Crosslinked by hydrosilation reaction
Additiona silicone
Part A: Siloxane with terminal silane groups
¢ Part B: Siloxane with vinyl terminal groups
¢ Many will Contain surfactants to make this material
hydrophilic.
¢ Cross linking Initiated by chloroplatinic acid.
¢ For extensive impressions, working time of addition
silicones may be lengthened by the addition of a retarder without adversely affecting the other properties.
Polyethers
Hydrophobic - but better surface characteristics
Many of the Same Characteristics of
Polysiloxanes ¢ Differences
Hydrophillic Bad Taste
Good Dimensional stability
Gypsum
Water cured- calcination occurs to be rid of H20. When mixed with water, reverse reaction takes place, forming stone and releasing heat.
(c) Properties dependent on density and crystal structure
(d) Dental stone used for models not plaster
Denture Base and Provisionals composition differences
Different fillers provide Hardness and Rigidity to the
Denture base is normally
VLC
Provisionals are normally
chemically cured
Mouthguards
Thermoplastic not Thermoset
Has a Low Softening Point
¢ Thermally Processed with Hot Water ¢ Vacuum Formed – Requires a Die
Copolymer controls properties
Role and function of cements
Used to Bind Restorations of Appliances to Tooth
Structure. Sometimes Light Cure Not Possible ¢ Cold-Cure for Methacrylate
¢ Water Cure for Cements
Requirements for cements
Low Viscosity but Good Mechanical Properties
¢ 25 μm Thickness
How viscosity and handling properties are modified
¢ For Glass-Ionomers it is the Powder to Liquid Ratio
¢ Powder = Filler
¢ More Filler = More Viscosity More Filler = More Modulus
Resin Cements
Typically One Component Systems
¢ Viscosity and Strength Controlled by Filler Loading
¢ Also by Low Molecular Wt. Monomers
¢ Increased Filler Loading = Increased Viscosity
and Modulus
¢ Orthodontic Bonding Needs to be Reversible for Example
¢ Different Balance of Properties
Glass-ionomers
Just know this can be used as a cement
Understand the function of bases and liners
Liner: relatively thin layer of material used to protect the dentin from residual reactants that diffuse out of the restoration or oral fluids that may penetrate leaky restoration interface
¢ Thin Layer so Materials are Lower Viscosity
Bases: Are used to provide thermal protection for the pulp and to supplement mechanical support for the restoration by distributing local stresses from the restoration across the underlying dentin surface during amalgam condensation (or cementation of indirect restorations)
¢ Require Greater Mechanical Properties
Calcium Hydroxide
Pulp capping
ZOE
ZOE - Pain reduction
Requirements of waxes
Temporary Materials
¢ Should Melt at Lower Temperatures ¢ Be Carveable
¢ Burn off
¢ Dimensionally stable
Lower molecular weight materials inbetween polymers and organic liquids
Polysiloxanes
Hydrophobic covering materials. Really good tear resistance, very stable. Won’t have water issues.
Polyethers
Similar to siloxanes. A little more hydrophilic. Kind of a half way house.
Zinc
improves corrosion, makes it more brittle.
Alloy
improve corrosion resistance, mechanical properties.
Linear part of stress-strain
modulus (stiffness). This is how much stress it takes to start the bend. Deformation in this region is considered to be reversible. Past this is irreversible. Area under curve is amount of energy material can absorb before failure.
Fillers are primarily composed of
resin. Composites are 70-80% filled, adhesives are around 20%.
Mouthguard
two monomers, these two work together to control properties.
Waxes
in between, not low MW, but doesn’t have entanglement issues as much as polymer.
Stress
(weight) how much it deforms.
Strain
– a pull – how much the stress relaxes.
Glass ionomers
Liquid w/acidic polymer (acrylic acid ex) w/basic glass. Mixing forms a salt, with slight exothermic reaction. This forms aqueous cement. Due to acidity, it bonds to tooth structure. Glass has fluoride, slowly releases over time.
More crosslinking
higher modulus
Polymerization
oxygen quenches free radicals, hence why there is an exposed top layer.
TRIAD
Used for custom tray materials. Comes as a sheet composed of polymethyl… uncured methaculoid monomer in there. This allows the sheet to be flexible, and once it goes into curing machine, it solidifies.
TRIM/JET
Cold cured – provisional materials. In general these do not have dimethacylates as TRIAD. As it polymerizes it does essentially the same thing as TRIAD.
Crosslinking
higher modulus, also free monomer can leech out in lower crosslinked materials.
Bond
You need an acidic portion of a molecule to bind to HA. You also need methacylate portion for polymerization (for adhesive), and it needs hydrophobic portion for binding to composite.
