Theme 1 Flashcards
Why different materials need different properties. What do the properties affect
- used for different scenarios and in different patients
- Cavity sizes are different and oral anatomy differs in patients with different age, sex, ethnicity etc.
- Affects storage conditions, shelf life, whether it needs mixing, setting time, working time, how long it lasts
what is shelf life and its importance
- How long materials can be stored and still be used as the manufacturer intends.
- Long shelf life means large orders can be placed which saves money
- If you exceed the shelf life, it may be unreactive, set too quickly or slow, release toxic substances etc.
What are the 3 mixing methods and their pros and cons (hand, mechanical, no mixing)
- Hand mixing: cheap equipment however technique sensitive so unpredictable results that depends on skill
- Mechanical mixing: special equipment used so more expensive, but less technique sensitive
- No mixing: pre-mixed by manufacturer so lowest technique sensitive method. But more expensive and needs careful storage
What is working time and setting time
- Working time: how long you have got to mix the components before properties change/ needs to be put in patient’s mouth. Measured from start of mix to when it can no longer be effectively used. Want it long enough so you are not rushing but not too long
- Setting time: measured from start of mix to when the material achieves minimum properties for function (it doesn’t have to be fully finished its reaction, only so it has sufficient strength etc. before moving onto next patient)
What is viscosity. Pros and cons of high and low viscosity.
What do the terms dilating, newtonian and pseudo plastic mean
- How easily a material flows
- Low viscosity (water) means it is easy to mix and to extrude from a syringe. It can flow easier so can mould round shapes. BUT spills
- High viscosity means its is harder to exclude as more pressure is needed. But stops spills
- Viscosity usually increases during setting
- Dilatant= viscosity increases as shear rate of force increases (bullet proof vest)
- Pseudoplasticity= viscosity reduces when rate of force increases (ketchup)
- Newtonian behaviours means there is a linear relationship between viscosity and pressure & speed. No matter how fast you apply the pressure, viscosity is the same (water)
Why are solvents and retarders used when mixing dental materials
- Many materials are difficult to mix due to high initial viscosity and rapid increase in viscosity when mixing. So substances are added to make mixing easier.
- Solvents reduce initial viscosity so easy to mix
- Retarders delay setting so enough time for mixing. Then makes the setting time quick so can move onto next patient
Temperature change during setting of materials: how it is used to determine rate of set, how it influences structures, and its clinical problems
- Exothermic
- The setting reaction slows down after the temperature has peaked, so setting time is often measured as the time to reach maximum temperature
- High temperature may increase porosity, as material evaporates quicker than it sets, leading to weakness and failure of material
- High temperatures bad as pulp is sensitive to heat and so causes damage
How the oral environment affects dental materials
- Temperature: from food and drink
- pH variations: plaque, acidic drinks, alkaline meds, toothpaste
- Mechanical stress from mastication
- Abrasion from food etc.
- Bacteria can breakdown resins
What factors are considered for the efficacy of materials
-Safety, accuracy, durability, aesthetics, solubility, corrosion, leaching.
What is leaching and how it can have a positive or negative effect
- Loss of a component into saliva
- Positive effect: fluoride leaching out can be antibacterial.
- Negative effect: plasticiser from polymers can make them uncomfortable to wear
What is corrosion. Consequences of corrosion of amalgam.
-anatural phenomenon. Requires at least 2 metals in contact and an electrolyte solution (saliva)
-The metals have different electrochemical potential so form an anode and cathode. An anodic reaction resulting in a loss of electrons and a cathodic reaction that gains.
=oxidation and reduction reactions
-Corrosion of amalgum causes loss of metal ions, causing metallic taste, changes its appearance and weakens the material, so restorations can fail
Consequences of expansion and contraction of materials, due to different temperature changes
- Each material has a thermal expansion coefficient. Materials have higher value than teeth so can shrink more than teeth
- Contraction is caused by temperature decrease. Gaps can form so liquid and bacteria can enter, leading to staining or secondary caries. Also can cause inaccuracies in crowns or impressions
- Expansion of fillings causes potential damage to tooth like cracking. Inaccuracies in crowns and orthodontics as well.
Thermal conductivity meaning of materials and their clinical considerations
-ability for materials to transfer heat
-Transfer heat effectively = conductor (amalgam)
Transfer hear ineffectively = insulator (enamel and dentine)
-As enamel and dentine are low conductors, if these are removed due to caries, then filling with amalgam will increase conductance. Heat conducted to pulp more so more sensitive to heat. Insulating liner used to protect the pulp.
What is thermal diffusity. In what scenarios is low and high more favourable
- thermal diffusivity = thermal conductivity, divided by density and specific heat capacity
- the rate at which a material transfers heat from hot end to cold end.
- Low is more desirable for a restoration as it doesn’t want to transfer heat to the pulp quickly as this damages the pulp and causes pain
- For dentures it is more ideal if they have high diffusivity so that temperature changes can be recognised to prevent scalding. [However they usually have quite low diffusity so temperature only really felt with very sudden temperature change so risk of scalding.]
Difference between force and stress. Different types of complex and simple stress.
What is strength
- Force = static load acting under gravity
- Stress =The force applied to a particular area. Takes into consideration the size of the object.
- Simple types of stress in one direction = tensile (pulled), compressive (squashed), shear (like scissors)
- Complex types which are a combination of simple stresses= flexural, torsional, diametral
-strength= the maximum amount of stress that can be withstood before breaking. So a filling needs to have a strength greater than the biting stress
What is strain. What is elastic, plastic and viscoelastic deformation. Give examples of when each are needed
- When a material changes dimensions due to stress being applied. Higher stress means more strain
- How much the length has changed over the original length
- Elastic= when load taken away, material returns to original dimension. Reversible.
- Plastic= permanently changed so doesn’t return
- Viscoelastic= combination of reversible and permanent deformation. it returns very slowly or it only partially returns
- Impressions or fillings need to shape round something when being fitted and need to be permanent so are plastic.
- Once shaped, impressions change to elastic to remove the impression tray
What is stiffness
- Stiffness= how difficult is it to deform the material
- High stiffness means it is very resistant to deformation (either elastic/plastic/viscoeleastic)
Yield stress. Clinical considerations (if biting stress lower or higher than yield stress)
- Yield stress – Stress required to permanently deform material. So when it reaches plastic deformation.
- High yield stress means more stress is required to get deformation
- If biting force is below the yield stress, then it won’t change dimensions permanently. But biting above yield stress causes permanent deformation. (plastic)
- Pressing into impression tray requires a force above the yield strength so we get plastic deformation
Ductility and maleability. And brittle behaviour. What 2 factors affects these behaviours
- Ductile materials can be deformed easily.
- Ductility (elongation): How much can something be pulled
- Malleability: How much can something be compressed
- Both can be deformed large amounts before breaking
- Brittle: can only be deformed small amounts before breaking. Less than 1% plastic deformation will break them
- Temperature and strain rate (how fast it is being deformed) affect if something is brittle or ductile
Resilience and toughness. What it means when materials are notch sensitive
- Resilience – How much energy can something take before it deforms permanently.
- Toughness – How much energy can something take before it breaks. So beyond plastic deformation.
- If Notched/ cracked materials require less energy to break than un-notched material then they are easier to break and are termed notch-sensitive
Fatigue. Fatigue life and limit.
- Materials have increased fatigue if they fail due to repeated cycles of stress. It depends on the stress applied. So higher forces of mastication will mean less cycles are needed to cause it to fail.
- Fatigue life= number of cycles it can survive, at a value of stress
- Fatigue limit= stress value where the material will survive indefinitely.
Hardness. Clinical implications. Is amalgam or teeth harder
- Ability to resist being scratched and worn
- Harder material (porcelain) will scratch softer materials (enamel)
- Amalgam is softer than enamel so opposing teeth can damage the filling
What are polymers and polymerisation, and what dental materials do they make
-Impressions, denture teeth, composites. Materials undergo polymerisation when they set.
- Polymers: made of repeating units of mer molecules. {Usually made of N, O, C , O atoms}.
- Polymerisation: monomers join together (usually with covalent bonds) to form long chains - polymers
- Chains may either be not linked (linear polymer) or linked (network/ cross linked polymer)
What are homopolymers and co-polymers. What are random, regular and block polymers
- Homopolymers -made of one type of monomer
- Copolymers- made up of 2 or more types of monomers
- Monomor join in different ways when forming a polymer. The different arrangements affects its properties.
1. Random= monomers randomly placed
2. Regular copolymer= monomers alternate regularly
3. Block polymers= same type of monomer will appear in a block, followed by the next block of another type of monomer
Addition and condensation polymerisation. What needs to be considered with condensation reactions
-Addition polymersiation= a series of addition reactions where 2 molecules join to form a bigger molecule, so chain grows.
- Condensation= 2 molecules join to form a bigger molecule and also form a bi-product
- If either water or oxygen is the bi-product this is tolerable in the mouth, however carbon monoxide for example can be produced when it sets which can be toxic in the mouth, so not safe to use
What are the steps involved in addition polymerisation. What is required
- Requires production of ion/ radical with spare electrons so molecules can be added
- A molecule with a weak bond (eg. O-O)
- Energy to break the bonds
1-Activation: 65-75 degrees vibrates the initiator molecules and splits bonds to activate it. It forms free radicals
2-Initiation: monomers added to free radicals. Because the C=C bond of monomers breaks and reacts with radicals [initiator = benzyl peroxide]
3.Propgation: monomers continue to react and radicals shift. Chain grows
4. Termination: chain cannot grow anymore.
4 ways how termination occurs. What early termination leads to
- Viscosity increases so monomers no longer react.
- Or if no more monomers are left to react.
- Or if free radicals cancel each other out.
- Or if impurities have been added, such as oxygen.
-Terminating reaction too early leads to a weak polymer as not enough monomers have been added.
What are vinyl monomers. Give examples of these vinyl monomers/ polymers
- Monomers with a C=C double bond.
- Different types with slightly different structures depending on what molecules are attached to the Carbons. But they lead to very different polymers.
- Methylmethacrylate= used for denture bases
- Ethylene = dunking bottles, hip replacements
- Styrene =packaging, heat-proof cups
- Vinyl chloride= clothing, food packaging
Requirements for condensation polymerisation. Give examples of polymers
- Monomers must have 2+ reactive groups capable of condensation reaction
- 3+ functional groups to allow cross-linking
- Polyester= used for fabrics
- Polyamide= nylon for clothing or denture bases
- Polydimethylsiloxane= impressions, bath sealant
How cross-linking of polymers affects its properties. Clinical consideration
- Cross linking is when polymer chains link to form a network
- Makes it harder to permanently deform when compressed, because when the chains move they can return to original shape ( unlike in linear polymers) - so increases stiffness
- Improves stability in aqueous environment. Polymers less likely to float off so therefore it can retain its shape. So polymers used in the mouth are cross linked so they don’t deform when forces act on them
How can benzyl peroxide initiator be activated in polymerisation
- 65-75 degrees Celsius
- However this is impractical in a patient’s mouth so:
- Adding a chemical activator (tertiary amines) allows the bonds to break at a lower temperature
Cons of heat and chemical curing. Pros of light curing.
- Heat/ chemical = requires mixing of components so it technique sensitive. Setting time may be long
- Light curing= no mixing needed and quick setting time when light shone on it. practical in the mouth.
