Dental Material Science Flashcards
what does success of dental materials depend on
Selection of material- which material is most suitable
Use of material (instructions)- has it been mixed/ set correctly
Quality of material- has it been manufactured well. Must follow instructions. To must look for the ‘CE’
how can dental materials be tested
Clinical trials- may not answer everything, expensive
Laboratory evaluation- may be better, cheaper, no patients, compare a new product with a product already used that works well.
ISO and BSI standards
what are the various stages that the properties of dental materials is important at and why
After manufacture, during storage (shelf life)
- Shelf life – long shelf life allows large quantities to be bought
- Storage conditions – temperature and light exposure
- Dispensing mixing/manipulation
- During setting
- after setting – the patient e.g. young/old, diet, work
- after ageing – once patient has left the clinic. E.g. Durability
what forms can the dental materials be in
powder/liquid, pastes
what is shelf life
- How long the materials can be stored and still be used as the manufacturer intends
- Long shelf life means large orders can be placed – reduced costs
- Careful stock control
what is shelf life affected by
Temperature:
- Products that contain volatile components may require refrigeration. This may affect performance.
- liquids may be unstable at low temperature, e.g. components may crystallise- changes the chemical structure
Light:
- Some products may be unstable in visible light – need storage in cupboard or require special packaging e.g. foil packaging or amber glass bottle. Must inspect the packaging to see if it has been damaged as it may not behave as you expect. DO NOT use product if this is the case.
If the shelf life is exceeded, you must not use the product.
what are the methods of mixing
- Hand mixing
- Mechanical mixing
- No mixing
what are the features of hand mixing
Powder/liquid, paste/paste, paste/liquid
Mixed using a spatula on a pad or a mixing bowl
Cheap equipment e.g. spatulas and bowls
Technique sensitive:
- Must learn how to mix them correctly within a given time
- Quality of materials and procedure depends on skill level
- unpredictable results, quality depends on skill
- reduction in technique sensitivity (below)
what are the features of mechanical mixing
Capsules and cartridges
Consistent mixing in capsule
Mixed using special equipment
More expensive – due to need of equipment e.g. rotating mixture
Less technique sensitive
what are the features of No mixing
Single paste
Mixed by manufacture
Need careful storage to prevent premature setting
Needs special equipment to start setting – more expensive
Lowest technique sensitivity
what are the Properties important during setting
Working time (WT)-How long have you got to mix the components
Setting time (ST)- How long until you can proceed to the next procedure
what is the working time (WT)
- Measured from start of mix
- Till material can no longer be effectively used
e. g. filling must be in cavity by this stage
e. g. impression material should be seated in mouth at this stage
Measured at room temperature
what is the setting time (ST)
- From start of mix
- Till material achieves minimum properties for function
- e.g. filling can be polished. Impression can be removed
- Does not imply 100% completion of reaction
- Sufficient elasticity for impression to be removed from patients mouth
Measured at appropriate temperature
- e.g. at mouth temperature for materials which set in mouth
- Set in the mouth - 32 and 37 degrees- difference between an open and closed mouth
- Set chair site- setting time at room temp
what are the methods to measure WT and ST
- Rheology
- Thermal effects
- Dimensional changes
what is rheology
The study of deformation and flow of materials
Measure how viscosity changes
How fluid a material is
- Fluid does not mean liquid – fluid means IT FLOWS
what are thermal effects
- Exothermic reactions
* Temperature rise
what are Dimensional changes
Expansion or contraction
outline rheology (viscosity) in terms of extruding from a syringe
Low viscosity – plunger moves easily – e.g. water- FLOWS EASILY
High viscosity – plunger harder to move – e.g. treacle
-FLOWS SLOWER
what is viscosity related to (2 things)
pressure and speed (rate)
in terms of K what is the Newtonian (linear) behaviour and for what compound would this be true for
K=1
It doesn’t matter how fast we push plunger the viscosity will always be the same
water
in terms of K what is the pseudoplasticity behaviour and for what compound would this be true for
K < 1
viscosity reduces as shear rate increases (push the plunger faster rate increases as viscosity decreases)
e. g. ketchup
- Shake the ketchup it will start to flow
- As we increase the shear rate the viscosity goes down
in terms of K what is the dilatant behaviour and for what compound would this be true for
K>1
viscosity increases as shear rate increases (as you mix something it gets harder to mix
e. g bullet proof vests
- Some Endontontic materials are dilatant- as you place them down the canal they get more and more viscous
what is thioxtropy
no flow until sufficient pressure is applied
E.g. Nail varnish- until you brush sufficiently hard it wont move
What are the implications of viscosity for practice?
- Ease of manipulating
* Ability to flow and adapt
how is ease of manipulating implicated in practice
Ease of mixing
- Low viscosity is better- easier to get the components to mix
- May affect how you mix materials (e.g. may not be able to mechanically mix)
Ease of transfer (e.g. to impression tray)
- High viscosity is better (stops spills) e.g. impressions- move material into patients mouth it won’t spill.
- E.g cup of tea- if its full its likely to spill.
what is the importance of the ability of dental materials to flow and adapt
e. g. detail in impression
e. g. adaptation of fillings to cavity- low enough viscosity to to take the detail but high enough to transfer it.
(to lower the viscosity the easier it is to flow)
why are many dental materials difficult to mix
A - a high initial viscosity
B – a rapid increase in viscosity during setting
How are manufacturers helping dental professionals with mixing dental materials ?
solvents – reduce viscosity.
Retarders – delay setting- delays the increase of visocity over a given period of time so there is time to mix things
how can Temperature change during setting affect the materials
- Many materials set through an exothermic reaction
- Once it gets to a maximum temperature from mixing, that is the setting time
- Useful for determining rate of set
- The setting reaction slows down after the max. temperature
Can influence structure / properties
- High temperature rise can cause porosity – leads to weak strength
- Trapped air bubbles make something weak e.g. filling/denture
what clinical problems can temperature change cause
Pulp is sensitive to temperature change (rise of 5°C can damage it)- dentine protects the pulp but if there is a deep cavity, we wont have as much dentine left- can damage the pulp by putting the restorative (filling) material in.
what are dimensional changes that can take place during setting
- Expansion
* Contraction
what is expansion and what clinical issues can this cause
Reactions involving crystal growth, e.g. amalgam, gypsum
- If things expand when made in the lab it may not fit
- Meet each other- expansion
- Potential damage to tooth (restorations)
- Inaccuracies in devices fitting (crowns, orthodontics)
what is Contraction and what clinical issues can this cause
Reactions involving polymerisation
- Greyish brown marks- tooth coloured restoration has shrunk during setting-
- May lead to marginal staining, secondary caries
- Inaccurate impressions- shrink
Casting of alloys
- Large temperature decrease (heating then cool- hot things expand, cold things contract)
- Inaccuracies in devices fitting (crowns)
what are the features of the oral environment that can affect the properties of dental materials
- Temperature
- pH variations- restoring function
- Mechanical stress
- Abrasive factors
- Bacteria
how does temperature affect materials
Cold drinks snd Hot food/drinks can cause Thermal cycling (5°C to 60°C) -natural material in the patients mouth cope well. If we are restoring teeth we should consider how those materials react to temperature changes.
how do pH variations affect restoring function
Plaque (~pH4) oCaused by fermentation oBelow critical pH-l loss of enamel and dentine oEnamel and dentine don’t cope well oDental materials struggle at low pHs
Acidic drinks (pH 1-3) oCarbonated drinks
Alkaline medication (~pH 12)
Toothpaste with chalk (~pH 12)
we require our materials to be stable in these pH ranges
how does mechanical stress affect teeth and dental materials (what are the types of mechanical stress)
High stress leads to fracture
e.g. on Incisal edges
Low stress over repeated over time (low repeated cycling)
- Fatigue- not necessarily high stress in one bite
Sudden, rapidly applied stress
- Impact failures
- Enamel and dentine good at absorbing energy but they do fail
- Dentures susceptible to breaking. Shatter- breaking into lots of different pieces.
how do abrasive factors affect dental materials
Abrasive food- e.g. seeds
Abrasive toothpaste- removes plaque and stains by being abrasive. Restorative/ denture materials not as hard - scratches which bacteria can enter
Solvents causing softening- alcohols are solvents. Softening means they can be scratched easier. Mouthwashes are also solvents
how do bacteria affect dental materials
- Breakdown of resins
- Oral bacteria can break down the fillings
what are properties of the set enamel
- Biocompatibility- toxicity, irritancy, allergies
- Appearance - aesthetics
- Thermal properties - expansion/contraction & heat transfer
- Chemical properties - solubility, corrosion, leaching
- Mechanical properties - strength, toughness, stiffness, hardness etc
- Adhesion - bonding of filling to tooth
why is safety of materials important
Important for all materials
Patient should not be harmed by treatment- allergies e.g. nickel. Consider medical history
Don’t forget dental staff
why is accuracy of materials important
- Important for impressions and models
- Very good reproduction of the oral anatomy
- Cant get accurate adhesive
why is durability of materials important
- Important for restorations, prosthodontic devices
- Things places in patients mouth
- Things that are a permanent solution (lasts around 5 years)
why is conservation of materials important
- Very important for restorations
- Conservative dentistry -move away from ‘drill and fill” and conserve as much of natural hard tissue as possible – minimal intervention
why is prevention of materials important
- Longevity of ‘restored tooth’ more important than the material
- Make sure that every treatment doesn’t cause damage to the tooth or other teeth
why is aesthetics of materials important
- Important for ‘visible’ restorations
- Invisible restorations
- Less aesthetic may last longer
how are dental materials developed towards to satisfying appearance and aesthetics
Colour, shade, translucency
•Not available for all materials (e.g. amalgam, gold)
•Tooth-coloured materials come in many shades
•Use of shade guide to match with natural tooth
•Build up different shades to match the tooth
• However Can (and will) change over time- diet/abrasion etc. should advise patients.
Surface roughness, gloss
•Altered by scratching, wear, erosion, stains
•Affected by polishing
what are the chemical properties of materials
- Solubility-
- Leaching
- Corrosion
what is the effect of solubility on materials
Dissolution in a solvent
(CaOH2) – pulp capping- water soluble so cover needs to be put on so it maintains it function
Durability requires low solubility
Dietary factors by the patient can affect this
what are the positive effects of leaching
Stable in an aqueous environment
fluoride leaching– GIC naturally contains fluoride so when you put in an aqueous solution such as saliva the fluoride leaches come out
oFluoride is antibacterial
oit might allow fluoride appetite to forms- less susceptible to erosion and caries. Better fluoride releasing dental materials = better materials
what are the negative effects of leaching
Aligners contain plasticisers. (plasticisers make things softer). When plasticiser leaches out the denture gets harder and harder and the denture will have to be removed
how does corrosion effect dental materials
if we have 2 metals and they come into contact then we can get a galvanic cell formed- flow of electrons
they must be in an electrolyte
-Saliva is a very good electrolyte
2 different metal touching above the electrolyte
-Part of amalgam filling covered in saliva- this is still sufficient for corrosion to occur
how is corrosion graded
in terms of electronegativity – which is likely to Form anode and cathode
-More difference in electronegativity the more likely they are to form a galvanic cell.
what are the consequences of erosion of dental materials (amalgam)
Metallic taste tells us we have a change in chemistry
We start Weakening the material – restoration fails
Amalgam composed o different alloys- if there’s saliva its possible to get an electrochemical cell to develop and get corrosion
Mercury produced from amalgam- not toxic to patients or practitioners
what are the thermal properties of materials
•Materials expand and contract as temperature changes (Hot it expands, Cold it contracts)
•For restorations this can lead to marginal gaps forming
- Possibly leading to staining and secondary caries
what is the amount of expansion and contractions is related to
the coefficient of thermal expansion (Units °C -1 )- governs expansion and contraction
how are the Selected values for Coefficient of Thermal Expansion clincally relavent
Dental materials Expansion & Contraction different to tooth
When things cool down- amalgam shrinks twice as fast as tooth and composite 4x as much so may have gap forming. (staining)
As we heat up and cool down it starts to pump liquid around. May allow bacteria in - secondary caries
what can changes in oral temp cause
Cause pain – hypersensitivity (Cold) , burns (hot)
Damage the pulp- shine a bright curing light/hot drink may get temp rise above 5 degrees
what is thermal conductivity and what are enamel, dentine, amalgam and composite
How well materials transfer heat is termed
- Enamel and dentine are insulators (low conductivity)
- amalgam -conductor
- composite- conductor
what are the clinical implications of amalgam being a conductor
- If deep cavity is produced an insulating liner may be needed
- Amalgam restoration may feel pain when they drink a hot or cold drink.
what are the clinical implications of Composites (made from acrylic resin) being insulators
Are liners (between restoration and pulp) still needed? – lack of clinical evidence on which is the best
Gold- 200x better
what is thermal diffusivity
How quickly material reacts to a sudden temperature change
-i.e. quickly raises to normal after cold drink-
(Conductivity only really covers static conditions- (mouth stays at a constant temp) – not what happens therefore diffusivity is needed.
- Low diffusivity doesn’t react
- High diffusivity does react
what is the equation related to thermal diffusivity (D)
. D (m2 s-1) = l / r Cp
where r is density
Cp is heat capacity
- heat capacity - heat required to raise 1g of material by 1°C
- Units J g-1 °C-1
- Measured using a thermocouple in a defined volume of material
- Low diffusivity doesn’t react
- High diffusivity does react
in most cases what type of diffusivity is required and why
low diffusivity is desired
-i.e. a restoration does not transfer the heat from a hot drink to the pulp
why would dentures ideally have high diffusivity
to prevent scalding
If the denture has a low diffusivity they don’t recognise that the liquid is too hot as they lose some sensation and burn mouth
what are the mechanical properties of dental materials related to
Force
Stress
how does force affect materials and what is its eq
Results from an outside agency acting upon a body to change its momentum
Force (N) = load (kg) x acceleration (ms-2)
(Weight x acceleration due to gravity)
•Static load acts under gravity (e.g. 1 kg acting under gravity gives force of 9.8N)
how does stress affect materials and what is its eq
•internal forces are set up inside a body to oppose an externally applied force
- e.g. internal forces need to develop to oppose biting on restoration
Magnitude of stress is function of applied force and dimensions of the object to which force is applied
what are the types of stress
what are most stresses in the mouth
simple – tensile, compressive, shear
complex- flexural (tension and compression), torsional (twisting) & diametral
In the mouth most stresses are complex
i.e. they combine simple stresses
what stress to dentures undergo
e. g. dentures undergo flexural stress (mixture tension and compression)
- top is compression
- under is tension
what is stress
Internal forces are set up inside a body to oppose an externally applied force
Magnitude of stress is function of applied force and dimensions of the object to which force is applied
what are the types of stress
- simple – tensile (pulling) , compressive (squashing) , shear (pushing out of line) - stress applied in one direction
- complex- flexural (dentures) , torsional (twisting) , diametral - stress applied in different directions
outline the calculation of simple stress with units
- Stress = Force/Area
- Units of stress Nm-2 = Pa
- (MPa = 1000000 Pa)
- (GPa = 1000000000 Pa)
what is strength
- The maximum stress which can be withstood before breaking
- E.g. if you bite and apply so much stress that you break the filling, the stress would be greater than the strength of the filling.
what is strain
When stress is applied the material will change dimensions
The amount of change that occurs due to an applied stress
how can strain be calculated
Ratio of new length / original length
no units
May be expressed as a %.
what are the different types of deformation
elastic deformation
plastic deformation
what is elastic deforamation
Material returns to original dimension
when is elastic deformation clinically applicable
If we have made a denture or placed a filling
Because we don’t want the filing/denture to change dimensions everytime the patient chews as it will fail very quickly
what is plastic deformation
Material is permanently changed
when is plastic deformation clinically applicable
When placing filling material into the cavity and pushing it into the cavity you want the deformation to be plastic- filling material deformed permanently so it fits into the cavity.
what is viscoelastic deformation
Material slowly return to original dimensions OR material only partially returns to original dimensions
explain elastic deformation in terms of a spring
- When load applied deforms quickly
- If load held it stays deformed
- When load taken off, quickly returns to the original dimensions
- Pulling spring down- very quickly itll reach the length being pulled too. Stays deformed while we apply the load and returns to normal length when removed.
explain plastic deformation in terms of a dashpot
- When load applied deforms quickly
- If load held it stays deformed
- When load taken off, stays deformed
what are the models of viscoelasticity
•Viscoelasticity is a combination of elastic and plastic deformation
Models involve both springs and dashpots
- Maxwell model – describes when only some deformation returns (partial)
- Voigt model – all deformation returns but slowly
explain viscoelasticity in terns of a spring connected to a dashpot
maxwell model -only some defromation returns
Spring connected to dashpot- starts deform, slow increase. When we let go the spring will close, however dashpot not closed and wont return to original dimensions. the deformation that happened after we wont be able to cover it. Left with permanent deformation, it will never return to original dimensions.
how do Liners on dentures to make more comfortable behave
they are viscoelastic (maxwell model)
called ‘Temporary’ liners as they need to be replaced- wont fit as well over time
how do impression materials behave
viscoelastic
if you place impression in patients mouth, let it set and try and remove it. removing it might do some deformation and
elastic deformation recovered but we also recover the plastic deformation until we’re back in the original dimension.
when does more deformation occur
More deformation and the longer time period we do it over
either the more permanent deformation we get or the longer it takes us to return to the original dimension
Technique- not much deformation in impression tray then it wont take much time to return to original.
what is stiffness
the measure of resistance to deformation
- It does not matter whether this is elastic, plastic or viscoelastic deformation
- The higher the stiffness the harder it is to deform something
- How easy is it to change its dimensions
- Stifness and strength work together in dental materials
what factors contribute to strength and stiffness of a material. relate this to dentures
strength- how easily it BREAKS
stiffness- how easily it DEFORMS
Denture in patients mouth and the patient bites with such force that the denture will break the denture will fail. If they bite and it deforms then it may fail too- the flexing may cause discomfort and patient may stop wearing denture
what is ultimate tensile
point where apply sufficient strength it breaks
how is stiffness shown on the graph
• Stiffness is the gradient of the curve where it starts to bend
- Lower stiffness - shallow
- Higher stiffness - steeper
how can stiffness me measured
Modulus of elasticity
Young’s modulus
what is Modulus of elasticity
and Young’s modulus
Modulus of elasticity - the rate of change of unit stress with respect to unit strain
Young’s modulus- measure of the ability of a material to withstand changes in length when under lengthwise tension or compression.
what is yield stress
Stress required to permanently deform material- precise definition.
what is Proof stress
Easier to measure than yield stress as its more expensive to do yield. Draw a line parallel to that line to 0.1% across on the x axis – stress required to do 0.1% permanent deformation
the offset point of yield stress which is not easily defined on a graph
what is - Proportional limit
Easier to measure than yield stress- must be similar to yield stress
what is - Ductility (elongation)
How much can something be pulled until it breaks- measure the value. More you can pull the more ductile
what is - Malleability
How much can something be compressed till it breaks
what is - Resilience
– How much energy can something take before it deforms- measure area under straight line on graph
what is Toughness
How much energy can something take before it breaks- all the area under the curve.
how do ductile materials behave
Ductile materials can be deformed large amounts
• The are often deformed elastically and, then, plastically
• They may show “necking” – thin areas prior to breaking. Happens too fast in dentistry to be used in mouth
• Pull- weaknesses, stress drops and fails
how do brittle materials behave
- The can only be deformed elastically
- Less than 1% plastic deformation will break then
- The smallest amount of deformation breaks them
- Depends on temperature and how fast we’re deforming them.
most materials can be ductile and brittle, what does the behaviour type depend on
- Temperature
* Strain rate (how fast it is being deformed).
what is Toughness
How much energy something takes before it breaks
• Useful to understand what happens when things are deformed very quickly:
• E.g. dropped or patient trips
why is toughness used over stress
• Energy is more useful than stress, so toughness used.
how can toughness be measured
- To measure toughness an impact test is used.
- Compare energy required to break pre-cracked (notched) and un-notched (no cracks) specimens.
- If energy to break un-notched specimens is»_space; than notched material is notch sensitive
what is meant when a material is notch sensitive
any cracks and scratches can make material break easier. If energy to break an unnotched specimen than that to break a notched
what is Fatigue
- Materials fail due to:
- Repeated cycles of stress
- Often fail at stresses much lower than strength
- may fail at the biting stress due to fatigue
when is fatigue common in the mouth
- E.g. Dentures
- They flex repeatedly during chewing and talking
- Fracture down the midline
what is fatigue life and what are the 2 ways materials can act
cycles (or time/age) survived at a value of stress
A-Initially a higher strength, the more cycles we apply the lower the strength becomes till it breaks at 1/5 of its ultimate tensile strength. Traditionally chosen for a denture but now we thing B might be better
B- starts off with a lower strength but doesn’t decrease over time. Going to survive longer and better. ‘Fatigue life”. Material B will never get to 60
what is fatigue limit
stress below which material survives indefinitely
what is Hardness and how is it measured
• How likely a material is to be scratched?
pressing something hard into the surface of something (usually a diamond)
Polishing discs contain hard particles to polish- things in mouth allow scratches
why are dentures likely to be scratched
Acrylic resin has a VHN of 20 which is v low so can be scratched by harder material
what is Resistance to indentation under load related to
- A factor in determining durability
* Related to scratching, wear
what are some uses of polymers
- Impressions
- Dental composites
- Denture bases
- Artificial teeth
What are the important characteristics of polymers?
Made up of many regular repeating units – termed mers • Based on C, O, N, H • Covalently bonded • Monomer – one mer • Polymer – many mers
The mers join to form long chains
• Formed by covalent bonds mainly
• As the mers join the chain the molecular weight increases- want a high molecular weight (lots of the monomers to join onto chains)
what is the difference between a linear polymer and crosslinked(network) polymer
No links – called a linear polymer
With links (cross links) – termed a network or cross-linked polymer o Changes of properties
what is the difference between a homopolymer and copolymer
Only one monomer makes up polymer – homopolymer
Two or more monomers – copolymers
what are the 3 ways Monomers may join, explain them
Random (Homopolymer)- Most likely- join together randomly, no distinct structure
Regular- monomers alternate in every chain (condensation polymers form in this way)
Block- blocks of M1, blocks of M2 etc.
what are the two methods of polymerisation common in dentistry
Addition polymerisation
- two molecules join to form a bigger molecule
Condensation polymerisation
- two molecules join to form a bigger molecule and a bi-product (often water but H202, O2, CO, CO2, ethanol etc can also be produced)
(toxic bi-product should be considered if the materials is to set in the patients mouth)
what are the distinct reaction stages that take place in addition polymerisation
- Activation
- Initiation
- Propagation
- Termination
what must free radicals have and give an example of an initiator used in dentures
- A molecule with a weak bond (initiator)
- A means to break (activate) this bond
- Energy used to break bond – heat, light, etc.
An example initiator – benzyl peroxide (BPO)
•Commonly used initiator for dentures
•The O-O bond is a weak bond
•As temperature increases this breaks up more (atoms vibrate more and move further away)
what are the Types of monomers
•Vinyl monomers – have a carbon-carbon double bond C=C
can change the R1 and R2 groups to form -Very similar structures -the minor changes lead very different polymers
what are the different polymers that can be made from vinyl monomers
PMMA – Plexiglass, denture bases
- Hard
- Add pigments
- Transparent
PE – drinking bottles, hip replacements
-Difficult to break by hand
PS – heat-proof cups, packaging
- Not transparent
PVC – clothing, food packaging
-Fake leather
