Tamir Ordeman Dental Materials
Tamir Ordeman Dental Materials 2011
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Comparison of Metals, Ceramics, and Polymers
Stress, Stain, Elasticity, Plasticity Mechanical Properties
Viscosity, Hardness Mechanical Properties
Thermal properties, Electrical-, Electrochemical properties
Chemistry, manufacturing, Apparent density of Gypsum Products
The Setting process (reaction, water requirement, stages in setting) of
Gypsum Products
The rate, and effect of additives on setting reaction of Gypsum Products
Manipulation and disinfection of Gypsum Product
Surface Energy, Wetting, Capillary Penetration in Adhesion
Adhesion to tooth structures (Enamel and Dentin)
Basic about polymers (Composition, Molecular weight, Properties)
Polymerization initiation, Propagation, Termination
Denture base polymers: Classification, Important Contents of powder
and liquid of heat cured acrylic
Description of heat cured acrylic, Autopolymerizing denture base,
Injection molded, and Light activated materials.
The chemistry of denture base polymers after mixing the powder and
liquid (Heat cured acrylic)
Denture base soft liners
, Non elastic Type of Impression Materials
Elastic impression Material
:Classification, Composition of Polymeric Restorative Materials
:Properties Polymeric Restorative Materials
Phosphate-Based Cements
Phenolate-Based, and Glasionomer Cements
Polycarboxylate-Based, and Polymer-Based Cements
Structure and Properties of Metals and Alloys, Basic about metals
Precious/ noble alloys
Ignoble Alloys, Base Metal Casting Alloys
Porcelain-fused-to-Metal
All-ceramic Crowns
Waxes, Guttapercha
Impression Compound
Definition, and Types of Biocompatibility Stages of injury, DEJ
Types of testing the biocompatibility of materials
Corrosion and Biocompatibility
Dental Amalgam
Color in Dentistry
Topic 1 – comparison of metals, ceramics and polymers.
Topic 2 – stress. Stain elasticity, plasticity mechanical properties
Stress (F/CM2) – which is in simple words is the force which is applied to the material
doesn’t depend only on the force but also on the shape (cross sections) that the load acts
upon. We can say that the stress is inversely proportional to the cross sectional area and
directly proportional to the load. There are 33 basic types of stresses in dental structure:
1. Tensile.
2. Compressive.
3. Shear.
all three of them are present in each case of stress on a structure and all three operate
under the same premise that if the stress value is bigger than the strength of the material,
the structure will fail.
Tensile forces are always present in critical area – where breakage occurs.
Strain – when a block of material is subjected to a tensile stress and it temporarily become
longer – this temporary elongation is called strain.
By dividing the increase in length by the original length you get the value of strain.
Elasticity the property of having a constant ratio of stress to strain is called elasticity and
this constatnt is called the modulus of elasticity. In simple words this property states that if
we doubled the stress (weight for example) the strain (length) will also double and so on.
This constant ratio which for each material is different because of the different characteristic
of the materials is called the modulus of elasticity.
There are a few important terms we need to know:
Proportional limit/elastic limit – the greatest stress at which stress is proportional to strain
Yield strength/ yield point – strength measure at the stress at which a small amount of
plastic strain occurs.
Ultimate tensile strength this is the stress at the point of breakage, after the material was
stretched to the max.
Work hardening – when most metals are stressed further then their porprtional limits, they
go through a process called work hardening which makes them stronger and harder but
more brittle.
Elongation – the total strain (elastic and plastic) at fracture.
Modulus of elasticity is an inherent property of a material and cannot be altered by
any conditioning
Plasticity when the elastic or porortional limit is exceeded in a material, it exhibits plastic
behavior. There are 2 types of materials from this point:
1. Ductile materials – materials that experience a large amount of plastic behavior or
permanent deformation.
2. Brittle materials – undergo a little or no plastic behavior.
Toughness – the entire area under the stress strain curve – how much energy was required
to fracture the material.
Resilience The area under only the elastic region of the stress strain curve, measure of how
much elastic energy can be stored by the material.
Notes
Fatigue – cycles of loading and unloading like mastication cause fractures despite
not reaching total ultimate tensile strength.
Topic 3 – viscosity, hardness mechanical properties
Viscosity – duo to the fact that many dental materials are in fluid state when they are
formed it is important to know the viscous flow characteristics.
Impression materials and amalgam involve a viscoelastic phenomenon: when shear stress
strain rate (flow rate) plots are obtained, they enable us to classify viscous materials in
several ways.
A Newtonian fluid shows constant viscosity, , which is independent of strain rate.
Where is the viscosity in poise,
strain or flow rate.
is the shear stress acting on the fluid and
is the
Fluids differ in their flow responses to the level of stress applied:
a. Plastic fluids – flow only from minimum stress.
b. Pseudoplastic fluids – become more free flowing with increasing shear.
c. Dilatant fluids – increase in rigidity as more pressure is applied.
Hardness – the resistance of a material to indentation or penetration.
There are several tests for testing the hardness of a material:
1. Brinell hardness number (BHN) – uses a small, hardened steel ball that is
forced into the surface of a material under a specified load. And by measuring
the diameter of the round dent created.
2. Vickers hardness number/diamond pyramid hardness test (VHN) – uses
a pyramid shaped diamond indenter, hardness is determined by measuring
the diagonals of the square shaped indentation and taking the average of the
two dimensions
In case you want to compare the 2 values you use the following equation:
VHN 1.05 + BHN.
Topic 4 – thermal properties, electrical, electrochemical properties
Thermal properties
As we know metals are good conductors of heat most of the time, and this must be taken
into consideration when doing metallic restoration. Dentin is a thermal insulator, thus if we
have enough dentin the patient will not be sensitive to heat or cold of the metallic
restoration, in case we don’t have enough dentin we would have to create a base.
Thermal conductivity/thermal diffusivity is defined as the rate at which heat flows through
a material.
Thermal conductivity (K) is measured by the speed heat (calories) travels through a given
thickness of material (1 cm) when one side of the material is maintained at a constant
temperature that is 1C higher than the other side.
Thermal diffusivity – this talks about the interior surface/ the structure of the material
conducting. It gives us an idea how fast will the interior surface of a crown will heat up when
the exterior surface is heated.
Thermal diffusivity (h) of a material (mm2/sec) is dependent on its thermal
conductivity, heat capacity (Cp) and density (p).
Thermal expansion – some restorative materials have coefficients of thermal expansion that
are markedly different from tooth structure. In such cases damage can occur to the tooth
and the restoration itself, that is why it is important to use a porcelain and a metal in a
porcelain fused to metal restoration with the same contraction rate on cooling from the
firing temperature.
Electrical and electrochemical properties
Electrochemical series is a listing of elements according to their tendency to gain or lose
electrons in a solution. The reference standard for this series is the potential of a standard
hydrogen electrode (0.000 V).
Metals, such as platinum and gold, who have large positive electrode potential are more
resistant to oxidation and corrosion in the oral cavity.
If there is a large difference between the electrode potentials of two metals in
contact with the same solution, such as between gold and aluminum, an electrolytic
cell may develop which will cause discomfort to the patient.
Electric resistivity – the resistance of a material to the flow of an electric current.
Resistance can be calculated with the following formula:
)
R – Resistance (ohms)
P Resistivity
l – Length
A – Cross sectional area.
Topics 5 8 – chemistry, manufacturing, apparent density of gypsum products.
Apparent density
The apparent density of a powder is the reciprocal of it bulkiness and so gives a measure of
it packing ability. Hemihydrate (by dry calcination) have low apparent density duo in part to
the rough irregular shapes of the individual particles. But the most important factors is the
adhesion of particles to their neighbors. Caused by high surface free energy resulting from
crystal imperfections and adsorption of gases during calcination. This adhesiveness makes it
more likely that particles will stick together so we can say that.
Dry calcination (high surface free energy – crystal imperfections):
Particles more likely to sticky together > more bridges between particles in powder > more
voids – so low apparent density.
Wet calcination (low surface free energy – crystal with significantly less imperfections):
Powder particles are smooth and dense > less crystallographic strain => lower surface
energy – so better packing ability and higher apparent density.
Topic 9 10 – surface energy, wetting, capillary penetration in adhesion …
Topic 11 – basic about polymers
A polymer is a molecule made up of many units. An oligomer is a short polymer usually has
fewer than 10 mer units. A mer is the simplest repeating chemical unit of polymer. For
example polystyrene is a polymer composed of styrene units.
Monomers are the molecules that unite to form a polymer, and the process by which this
occurs is termed polymerization. If two different monomers are joined a copolymer is
formed. The atoms/molecules are joined by covalent bonds.
Molecular weight – or molar mass of a polymer is the mass of one polymer molecules and is
often calculated approximately from the sum of the molecular weight of the mers of which it
is made.
Degree of polymerization – defined as the total number of mers in a polymer
molecule.
Spatial structure – there are 3 basic spatial structures to polymers:
1. Linear structure – comprised of a single unbranched polymer which is connected to
other molecules through weak physical bonds.
2. Branched polymer – comprised of a single branched polymer (the intersections are
the weak points in terms of covalent bond) which is connected to other molecules
through weak physical bond.
These 2 structures change upon heating and cooling, by heating the ability of the
chains to slide one on top of the other which results in softened material and by
cooling the material hardens – these materials are termed thermoplastic
(polystyrene, polyvinyl acrylics and poly methyl methacrylate (PMMA)).
3. Cross linked polymer creates strong covalent bonds between adjacent chains of
polymers, and it becomes strong bonded unit. This inhibits the breaking of bonds
upon heating and with that the sliding of the chains – these materials are called
thermosets. (cross linked PMMA, CIS polyisoprene, bisphenol, silicones etc)
Properties
These are effected by the degree of polymerization, the number of cross linked chains and
by the chemical composition of the polymer. But in general we can say that longer chains
and higher molecular weight result in the polymer's increased strength, hardness, stiffness
and resistance to creep along with increased brittleness.
The amount of Crystallinity present in a polymer affects its properties. Materials that
are highly crystalline have atoms with a very regular arrangement in space and are
stronger, stiffer and absorb less water than do noncrystalline materials.
Small plasticizer molecules when added to a stiff uncrossed linked polymer, reduced
its rigidity. The small molecules surround the big ones, allowing them to move more
easily. A plasticizer therefore lowers the glass transition temperature (Tg) of the
polymer (the temperature at which a polymer ceases to be glassy and brittle and
becomes rubberlike), so a material that is normally rigid at a particular temperature
may become more flexible.
During polymerization the following occurs:
1. Volumetric decrease occurs, shrinkage.
2. Varying ability to absorb water depending on the polymer (small expansion may
occur).
3. Change in shape upon reheating – called warpage.
Topic 12 – polymerization initiation, propagation, termination.
We usually have two type of polymerization reactions:
1. Addition polymerization – no by products is formed, materials like:
PMMA = used in dentures.
Bisphenol A glycidyl methacrylate (bis GMA) – common component of the
matrix of resin composites.
2. Condensation polymerization – a low molecular by product, like water or alcohol, is
formed, materials like:
Polysulfide rubber and some silicone rubber impression materials.
In the free radical addition polymerization reaction there are 3 stages which may be
accelerated by heat, light or small amount of peroxides.
Initiation – this step produces free radicals, which initiate the growth of polymer chains. As
we know free radical have unshared electrons. The most common initiators are thermal
initiators. Which have one weak bond that breaks at a significant rate at a moderate
temperature to yield free radicals. (Usually peroxide O O or azo N N).
Initiators can also be created from photo dissociation – UV light breaks the bond
and creates a radical. Or from redox reaction which involves the transfer of an
electron and the creation of a Radical.
Propagation – activated monomers (Radicals) attack the double bond of additional available
monomers, resulting in rapid addition of monomer molecules to the free radical.
Termination – we have several mechanisms for the termination of the growing free radical,
and can result in the formation of branches and cross links.
Small amounts of inhibitors, like hydroquinone, are added to the monomer to increase
storage life. They react with free radical, reduction the rate of initiation.
Topic 13 16 – denture base polymers
Classification
Denture base is the material in which the teeth of a denture are set and which rests on the
supporting tissues when the denture is in place in the mouth
The polymeric denture base can consist of:
1. Simple stiff base on which the teeth are arranged.
2. Sandwich of stiff base and a resilient liner to provide greater retention and comfort.
If irritation occurs on the soft tissue we can apply Tissue conditioner to relieve the
symptoms.
Topic 17 18 – non elastic and elastic impression materials
Topic 19 20 – polymeric restorative materials
Topic 21 23 – cements
Topic 24 – structure and properties of metals and alloys, basic about metals.
A wide variety of metals are used in dentistry. When elements are alloyed together to
change their properties, the single meting temperature is changed to a range of
temperature in which there is an equilibrium between the solid crystals nucleated in the
liquid metal.
The upper temperature for the liquid solid alloy range is called liquidus temperature
– it is the temperature at which solid crystals start to nucleate during cooling and it
is the temperature where solid crystals start to dissolve during heating.
The lower temperature limit is called solidus temperature – it is the point at which
the last liquid solidifies on cooling or vice versa.
Unit cells of crystal lattices
Liquid metals nucleate crystals on cooling. The atoms joining the crystals from a packing
arrangement in the space that is characteristic of the metal or alloy at equilibrium.
Unit cell is the smallest division of the crystalline metal, when it is repeated in space it forms
the crystal lattice structure of a crystalline solid.
The atom at each corner is shared among the adjacent eight unit cells
Nucleation and polycrystalline grain structure
As the melted metal is cooled, clusters of atoms from a solid crystal nuclei. These nuclei will
be stable and grow into crystallites or grains if the energy of the system is favorable, or in
other words more energy is lost by bonding than gained by increasing the interfacial surface
area.
This nucleation can occur by 2 processes:
1. Homogeneous nucleation – it is enhanced by rapid cooling or supercoiling of the
nuclei. More energy is lost, and more nuclei are formed per unit volume –
polycrystalline grains is formed by these nuclei.
The more nuclei formed, the smaller the grain size.
2. Heterogeneous nucleation – adding to the melted metal a foreign solid particle or
surface ti which the atoms are attracted, this will decrease grain size.
Grain size and properties
By decreasing the grain size we see a number of advantages for the cast alloy structure of a
crown or removable partial denture.
1. Can raise yield stress.
2. Increase the ductility.
3. Raise the ultimate strength.
Alloy system
An alloy is formed when two metals form a solution the in the liquid state so their atoms mix
randomly.
Different grains may be practically pure if their elements are insoluble in each other's
lattices on the solid state. They are more likely to combine if:
1.
2.
3.
4.
Have the same atomic lattice type.
Have similar atomic radii.
Have the same valence number.
Form bond to other atoms with strengths similar to those they form among
themselves.
If they "like" each an intermetallic compound may be formed, at a specific ratio.
Topic 25 26
Topic 27 – porcelain fused to metal
Dental porcelains are used to bond with metals for a natural appearing outer layer.
These porcelains developed during the 1950s.
Topic 28 – all ceramic crowns
Topic 29 – Waxes. Guttapercha
Waxes
These are organic polymers consisting of hydrocarbons, and their derivatives. Dental waxes
are blends of ingredients including natural waxes, synthetic waxes, natural resin, oils, fats,
gums and coloring agents.
Dental waxes are classified according to their applications into 3 groups:
1. Pattern waxes – there are several types of pattern waxes:
a. Inlay waxes (kerr dental) – are used to make inlay, crown and pontic replicas
(used in lost wax casting technique)
There are 2 type of inlay waxes: type 1 (soft, used for the indirect inlay
technique or miscellaneous attachment) and type 2 (hard, used for preparing
direct patterns in the mouth).
Inlay waxes are provided in geometric and anatomic forms as well as in bulk.
b. Pattern resins/resin waxes – characterized by higher strength and resistance to
flow than waxes, good dimensional stability and burnout without residue.
A pattern is fabricated by applying 3 5 mm layers of resin and curing in a
light chamber or with a hand held light curing unit. Resin is removed from
mold before heat casting (690 degrees 45 minutes)
Full crowns patterns of pattern resin and inlay waxes have similar marginal
discrepancies.
c. Casting wax – used for thin sections of certain removable and fixed partial
denture patterns.
Convenient in preparation of copins or clasp – thin regions, and they are
supplied in sheets, rods and in bulk.
d. Base plate wax – supplied in sheets, and used in the construction of full denture
patterns and for occlusal rims. There are 3 types of base plate wax: type 1 – soft
base plate wax for veneers and contours, type 2 is a medium hardness base
plate wax for use in temperate climates and type 3 which is the hardest base
plate and is used for tropical climates (hardness based on amount of flow at 45
degrees).
2. Processing waxes – there are several type of processing waxes –
a. Boxing wax – used to form containers for poring casts and to fabricate
replacement pontics for provisional fixed partial dentures.
b. Sticky wax – used to join materials temporarily.
c. Carding wax – used for attaching parts and in some soldering techniques.
d. Blackout wax – used to fill voids and undercuts for removable partial denture
fabrication.
e. White wax used for making patterns to simulate a veneer facing.
f. Utility wax – used for various laboratory procedures.
3. Impression waxes – these include bite wax and mizzy wax, exhibit high flow and
distort on withdrawal from undercuts. The waxes which are specifically for denture
impressions are used only in edentulous regions (without teeth) of the mouth.
Corrective waxes are used as wax washes to record detail and displace
selected regions of soft tissue in edentulous impressions.
Bite waxes – used in certain prosthetic techniques, like bite registration
(occlusal representation of how the maxillary and mandibular teeth
intercuspate).
Properties
May consist of both crystalline and amorphous components, each with a distribution of
molecular weights, and because of that waxes melt over a wide range of temperatures. They
have the highest coefficient of thermal expansion from all the dental materials, which can
lead to poor fitting casting if compensating factors are not present. Also on solidification
(cooling) shrinkage occurs, up to 0.4%.
Flow is a measure of a wax's ability to deform under light forces and is analogous to creep.
Flow is proportional to temperature and force on it.
Wax distortion
Waxes are partly elastic in behavior and tens to return to their original shape after
deformation – this effect is called memory effect, residual stresses also contribute later
distortion. There are 4 ways to minimize pattern distortions:
a. Direct technique waxes should be heated uniformly at 50 degrees for 15 min.
b. Pattern should be invested quickly.
c. Storage in a refrigerator is preferred if there will be a delay in investing (applying),
elastic recovery is slower at low temperatures.
d. Essential that no wax residues are left in the mold after burnout in the lost wax
process.
Gutta percha
It is a non elastic impression material,
Topic 30 – impression compound
Dental compound can be used for the following:
1. Full crown impressions (type 1).
2. Impressions of partially or completely edentulous jaws (type 1).
3. Impression trays in which a final impression is taken with another material (type 2)
It can't be used to record undercuts because it is not elastic.
Composition
Natural resins give the compound its thermoplastic character (40%), waxes about 7%
and stearic acid, about 3%, acts as a lubricant and plasticizer. Fillers and inorganic
pigments account for the remaining 50%.
Thermal and mechanical properties
Can be used at about 45 degrees and then cooled down to oral temperature of about 37
degrees at which it is fairly rigid.
It is a physical process and each type reacts differently:
Type 1 is a flow of about 85% at 45 degrees and less than 6% at 37 degrees.
Type 2 has a flow of about 70% at 45 degrees but less than 2% at 37 degrees.
Dental impressions compounds have low thermal conductivity.
Manipulation
Softened by heating over a flame or water bath and cooled by a water spray, and cooling
must continue until the entire mass is rigid to reduce plastic flow.
Advantages
It is compatible with die and cast materials and is easily electroplated to form accurate and
abrasion resistant dies.
Disadvantages
Handling is very sensitive, can undertake water and by that change composition and lost low
molecular ingredient during heating.
Disinfection
By immersion in sodium hypocholorite, iodophors or phenolic glutaraldehydes.
Topic 31 33 – biocompatibility and corrosion
Topic 34 – dental amalgam
Topic 35 – color in dentistry
Tamir Ordeman Dental Materials
Personal note:
This took me a very long time to make and I want to share it for free with everyone in hope
that others will do the same with other things in the future.
Good luck in your exam I hope to hear from you if it helped you.
Please look at the lecture's pictures they also ask you to recognize pictures and sometimes
even draw if you know my summary by heart and can identify all the pictures (or at least
most of them) you would get a 5 on your final exam!
Let us all help each other to become the best doctors we can be!
Tamir Ordeman
Dentistry Group 13
Tamir Ordeman Dental Materials 2011