a lecture for under grads

1. Dr Mumtaz ul Islam
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Chemical stability
Chemical composition of material remains unchanged
Solubility how much a material is soluble
Erosion dissolution of material with a mild mechanical
action
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When assessing the solubility or erosion rate of
materials it is important to consider the vast range of
conditions which may exist in the mouth
The pH of oral fluids may vary from pH 4 to pH 8.5
representing a range from mildly acidic to mildly
alkaline
Highly acidic soft drinks and the use of chalkcontaining toothpastes extend this range from a lower
end of pH 2 up to pH 12
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It is possible for a material to be stable at near neutral
pH values but to erode rapidly at extremes of either
acidity or alkalinity
This partially explains why certain materials perform
adequately with some patients but not with others
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Often involve the storage of disc specimens of
materials in water for a period of time the result being
quoted as the percentage weight loss of the disc
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Many materials, when placed in an aqueous
environment absorb water by a diffusion process.
Constituents of the material may be lost into the oral
fluids by a diffusion process commonly referred to as
leaching
This may have serious consequences if it results in a
change of material properties or if the leached material
is toxic or irritant
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Occasionally leaching is used to the benefit of the
patient. For example, in some cements containing
calcium hydroxide slow leaching causes an alkaline
environment in the base of deep cavities.
This has the dual benefit of being antibacterial and of
encouraging secondary dentine formation
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Soft acrylic polymers used for cushioning the fitting
surfaces of dentures due to large quantities of
plasticizer
Slow leaching of these plasticizer causes the resin to
become hard and ineffective as a cushion
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Glass ionomer cements release fluorides which produce
strength as well as antibacterial environment
If this release is confined only to ion exchange there
will be no harm at all
If all the material leached out this should be considered
as washout
Disintegration of material is obvious
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Property of metals and alloys
These are good conductors
First stage of corrosion involves formation of an
electrolytic cell
A device which converts electrical energy in to
chemical energy and vice versa
Electrode potential affinity of a material to be reactive
against charges
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measure of the extent to which the reaction
M → M+ (ion) + electron will occur
In an electrolytic cell involving two metals material is
lost from the metal with the most negative electrode
potential
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when zinc and copper come into contact in the presence
of a suitable electrolyte material loss occurs from the
zinc by the reaction:
Zn → Zn2+(ion) + 2 electrons
In this simple cell zinc gives a pair of electrons hence
called as anode
Corrosion occurs here
While copper receives electrons so it is cathode
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chromium is electrochemically active it reacts readily
forming a layer of chromic oxide which protects the
metal or alloy from further decomposition
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Excessive or continued bending can accelerate the rate
of corrosion and may lead to failure by stress corrosion
cracking
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Pits in rough surfaces can lead to the setting up of small
corrosion cells in which the material at the bottom of
the pit acts as the anode and that at the surface acts as
the cathode
It accomplished with oxygen concentration
Deeper area becomes anodic
Corrosion occours
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pain due to the flow of galvanic current
Metallic taste due to the release of ions
Deterioration in appearance and mechanical properties
Increased body burden of metallic ions
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Surface discolouration due to interaction between a
metal and its environment
Usually a sulfide layer is present on the material
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A material should be harmless to the manufacturers and
users
Placed into a patient’s mouth
Non-toxic
Non-irritant
Non-carcinogenic or non-allergic potential
filling material should be harmless to the pulp
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