04/13/2023

Chemical Properties of Dental Materials

Dr Mumtaz ul Islam

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Chemical properties

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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Role of pH

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 chalk-containing toothpastes extend this range from a lower end of pH 2 up to pH 12

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Difference in performance

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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Standard tests of solubility

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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Leaching of constituents

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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Benefits of leaching

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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Disadvantage of leaching

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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Leaching and integrity of material

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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Corrosion

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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Corrosion

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Corrosion

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Corrosion

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Electrode potential

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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Electrolytic cell

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Anode and cathode

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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Nobility

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Passivation

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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Stress

Excessive or continued bending can accelerate the rate of corrosion and may lead to failure by stress corrosion cracking

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Concentration cell corrosion

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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Results of corrosion

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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Tarnish

Surface discolouration due to interaction between a metal and its environment

Usually a sulfide layer is present on the material

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Tarnish

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Tarnish

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Biological properties

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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Thank you

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