5.dental waxes 14
TRANSCRIPT
Introduction
The waxes used in dentistry normally consist of two or more components
which may be natural or synthetic waxes, resins, oils, fats and pigments. Blending
is carried out to produce a material with the required properties for a specific
application. Waxes are thermoplastic materials which are normally solids at room
temperature but melt, without decomposition, to form mobile liquids. They are,
essentially, soft substances with poor mechanical properties.The use of wax in
dentistry is an established phenomenon. Whether it is formation of inlay patterns,
boxing of impressions, taking bite registrations or making bite rims all require the
waxes with specific properties.
The specific use of the dental wax determines the physical properties that
are most desirable for a successful application.
Waxes, gums, fat and resins
Dental waxes may be composed of natural and synthetic waxes, gums,
fats, fatty acids, oils, natural and synthetic resins, and pigments of various types.
The desired waxes working characteristics are achieved by a blend of appropriate
natural and synthetic waxes, resins and other additives.
Historically, waxes have been classified according to their origin: (1)
mineral, (2) plant, (3) insect, and (4) animal; however, a better classification is
based on their chemical composition. The two principal groups of organic
compounds contained in waxes are hydrocarbons and esters, although some waxes
contain free alcohols and acids as well
Natural Waxes Synthetic waxes Additives
Mineral- Paraffin
Microcrystalline
Barnsdahl
Ozokerite
Ceresin
Montan
Plant – Carnauba
Acrawax C
Aerosol OT
Castor wax
Flexo wax
Epolene N 10
Albacer
Aldo 33
Stearic acid
Glyceryl tristearate
Oils-turpentine, colour
Natural resins
Rosin – copal, dammer
Sandarac, mastic
Shellac, kauri
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Ouricury
Candelilla
Japan wax
Cocoa butter
Insect- Beeswax
Animal- Spermaceti
Durawax 1032 Synthetic resins
Polyethylene,polystyrene
NATURAL WAXES
These are classified according to their origin into
Mineral
Plant
Insect
Animal
They may also be classified based on the chemical composition
- Hydrocarbons
- Esters
- Some do contain free alcohol and acids as well.
The natural waxes are complex combinations of organic compounds of
reasonably high molecular weight. The composition varies depending on the
source and the time of collection; therefore the dental manufacturers must blend
the particular batches of wax to obtain the properties desired for a particular
application.
MINERAL WAXES
Paraffin waxes;-
- Obtained principally from high boiling point fractions of petroleum.
- Mixtures of chiefly straight chain saturated hydrocarbons containing 26 to
30 carbon atoms.
- Melting range 40 ºC to 71 °C .melting temperature increases with the
increase in molecular weight, but decreases if oil is present in the wax.
- Paraffin wax used in dentistry is refined and has less than 0.5 percentage
oil.
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- During solidification and cooling, a volumetric contraction occurs that
varies from 11% to 15%.
- Crystallization is in the form of plates predominantly.
Microcrystalline waxes;-
- Obtained from heavier oil fractions in the petroleum industry.
- Higher melting points ranging from 60 to 91 ºC
- They crystallize in small plates and are more flexible and tougher.
- They are branched chain hydrocarbons and have higher molecular weight
41 to 50 carbon atoms.
- Less Volumetric contraction compared to paraffin waxes.
Barnsdahl ;-
- Microcrystalline wax with boiling point 70°C to 74°C used for increasing
melting range and hardness and reducing flow of paraffin waxes.
Ozokerite;-
- An earth wax found near petroleum deposits.
- Melting temperature about 65°C consisting of microcrystalline structure of
needles or short plates.
- Usually added to paraffin wax to improve its physical properties.
Ceresin;-
- Obtained from natural mineral petroleum refining or lignite refining.
- Straight and branched chain hydrocarbons.
- High molecular weight and hardness.
- Used to increase melting range of paraffin wax.
Montan wax;-
- Obtained by extracting various lignites.
- Melting temperature ranges from 72°C to 92°C.
- Mixture of long chain esters of 40 to 58 C- atoms accompanied by free high
molecular weight alcohols and acids and varying amounts of resins.
- They are hard, brittle, lustrous, and blend well with other waxes.
PLANT WAXES
Carnauba and Ouricury waxes;-
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- Composed of straight chain esters, alcohols, acids and hydrocarbons.
- They have high hardness, brittleness, and high melting temperatures.
Carnauba - 84° C to 91° C
Ouricury - 79 ° C to 84° C.
- They have outstanding quality of increasing melting range and hardness of
paraffin wax.
Candelilla wax;-
- Consists of 40% to 60% hydrocarbons containing 29 to 33 carbon atoms,
accompanied by free alcohols, acids, esters and lactones.
- Melting temperature 68°C to 75.
- Hardens paraffin wax.
Japan wax and cocoa butter;-
These are not true waxes but are fats. Japan wax is mixed with paraffin
wax to improve tackiness and emulsifying ability. Cocoa butter is used in
dentistry as a protector against dehydration of soft tissues and to protect silicate
products temporarily from moisture during setting or to protect them from
dehydrating after they set.
INSECT WAX
Beeswax:
- Complex mix of esters, saturated and unsaturated hydrocarbons and high
molecular weight organic acids.
- Melting temperature 63°C to 70°C
- It is brittle at room temperature but plastic at body temperature.
- Mainly used in sticky wax.
ANIMAL WAX
Spermaceti wax:
- Used as a coating on dental floss.
SYNTHETIC WAX:
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Synthetic waxes are sparingly used in dentistry. They are highly refined
with very little contamination and have a limited compatibility with other waxes.
They function as plasticizers and tend to toughen films of wax.
FATS
They are similar to wax in that they are colourless, odourless, tasteless, in
pure form, however waxes are harder and have higher melting points. Fats are
basically esters of fatty acids. Hydrocarbon oils are used to soften wax. Silicone
oils aid in ease of polishing of the waxes.
RESINS
These are complex, amorphous mixtures of organic substances. They
exhibit specific behaviour rather than definite chemical composition. Most are
obtained from plants. Shellac however is an insect product. They are blended with
waxes for dental use. Resins Dammar and Kauri are added to harden the wax.
CHARACTERISTIC PROPERTIES OF WAXES
MELTING RANGE
THERMAL EXPANSION
MECHANICAL PROPERTIES
FLOW
RESIDUAL STRESS
DUCTILITY
Melting range
Waxes have a melting range and not a definite melting point. Different
components in the dental wax affect the melting range. The melting of a mix of
paraffin with high melting waxes results in paraffin melting near its usual
temperature, but the entire wax does not appear melted because the matrix of the
higher melting wax does not melt until a higher temperature is reached.
2.5% carnauba wax has little effect on the melting range.
> 10 % carnauba wax increases the melting range rapidly.
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Thermal Expansion
Waxes expand when temperature rises and contract on decrease of
temperature. Blending of waxes can to some extent control this property. Waxes
have the highest coefficient of thermal expansion compared to other materials
used in dentistry. Mineral waxes have greater thermal expansion than plant waxes
due to weaker secondary valence forces. Many waxes exhibit two ranges of
expansion between 22° and 55°C. These changes occur at transition points and are
attributed to internal structural parts being freer to move. Certain inlay waxes
have more than 2 changes in rate of expansion because the transition points of the
ingredients do not coincide.
Clinical significance:-
Possible distortion to patterns due to temperature changes leading to
inaccurate castings.
Mechanical properties
The moduli of elasticity, proportional limit, compressive strength, are low
for waxes and depend on temperature strongly. Modulus of inlay waxes are
important in hygroscopic casting procedures in which the wax pattern is subjected
to stresses of expansion of the investment during setting. This is controlled by
using waxes of different modulus in different areas. E.g. the lateral walls of the
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crown are prepared using inlay wax and occlusal surface using soft green casting
wax. The ratio of modulus being 7:1 Inlay: Casting wax which is approximately
the ratio needed for most patterns to obtain uniform expansion in the occlusal
compared to the margin areas.
Flow
Flow is generally associated with fluids or plastic substances and is
visualized as slippage of molecules over each other. In the case of waxes in liquid
state flow is synonymous with viscosity or internal friction. Below the melting
point it is a measure of the degree of plastic deformation at the given temperature.
Flow is dependent on temperature, force and duration of force. It greatly increases
as the melting point is approached. Depending on the application the flow may be
useful and in other cases harmful. E.g. direct inlay wax should have good flow
few degrees above room temperature but essentially no flow at room temperature.
Mineral waxes flow upto 50% at 20°C below their melting point due to weak
secondary valence forces. Plant waxes flow 50% only few degrees below their
melting point. The exception to this is Montan mineral wax which has ester
groups like plant waxes.
Residual stresses
Residual stresses are incorporated invariably in the wax pattern regardless
of the techniques. When we consider the thermal expansion curves of annealed
wax with wax that has been cooled under compression or tension, the annealed
wax follows the same curve on cooling or heating. In the case of specimens
prepared by holding under compression during cooling, the residual stresses cause
the molecules to move closer and subsequent heating results in greater expansion
than routine due to the relaxation of the stress.
Similarly when the specimen is prepared by subjecting it to tensile stress, the
curve will be lower than that of annealed specimen and may even lead to
contraction when the residual tensile stresses are released.
Ductility
Ductility increases with temperature. Waxes with lower melting point
have greater ductility than waxes with higher melting point. A blended wax with
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components having wide melting range will show greater ductility then those with
narrow melting range.
DENTAL WAXES
Classification based on use and application.
Pattern Processing Impression
Inlay Boxing Corrective
Casting
- sheet
- ready shapes
- wax-up
Utility Bite
Base plate Sticky
Pattern waxes are used to replicate the restoration. Two important concerns are
thermal change in dimension and tendency for distortion.
Processing waxes act as auxillaries in various procedures.
Impression waxes are seldom used now.
INLAY DENTAL WAX
Requirements of wax-pattern materials The major requirements of waxes used to
construct wax patterns by either the direct or indirect technique are as follows.
(1) The wax pattern must conform to the exact size, shape and contour of the
appliance which is to be constructed.
(2) No dimensional change should take place in the wax pattern once it has been
formed.
(3) After formation of the casting mould, it should be possible to remove the wax
by boiling out or burning without leaving a residue.
The ability to record detail depends on the flow of the material at the
moulding temperature, which is just above mouth temperature for direct
techniques and just above room temperature for indirect techniques. Accuracy and
dimensional stability depend on dimensional changes which occur during solidifi
cation and cooling of the wax. Distortions may also occur if thermal stresses are
introduced.
Composition:
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Paraffin 60%
Carnauba 25%
Ceresin 10%
Beeswax 5%
Inlay waxes come under ADA specification no: 4 and 3 types are recognized.
Types A, B and C. The non volatile residue should be a maximum of 0.1 % in all
the types.
Type A - Used in indirect techniques. It is hard wax and has little flow.
Type B - Medium wax used for making direct patterns in the mouth.
Type C - Soft wax used in indirect techniques and has maximum flow.
Maximum linear thermal expansion permitted between 25°& 30°C is
0.20% and between 30°& 37°C is 0.60%. They should soften without flaking and
during carving should not chip or flake. The flow of inlay wax can be decreased
by adding carnauba wax or selection of high melting paraffin wax.
Flow characteristics of inlay wax.
FLOW % WAX TEMPERATURE ° C
Minimum Maximum A B C
- 1 43 37 34
1 15 46 40 37
50 85 49 43 40
70 90 52 46 43
Thermal coefficient of expansion
The rate of thermal expansion of type B wax is greatest from just below
mouth temperature to just above 45° C. The knowledge of amount of expansion
and contraction is imperative to obtain accurate castings.
Warpage
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Inlay wax patterns tend to warp on standing and this tendency to warp
increases with temperature and prolonged storage time.
Warpage can be minimized by
Softening wax uniformly by heating at 50°C for at least 15 min before use.
Using warm instruments.
Using a warm die.
Adding wax in small increments.
Storing patterns in refrigerator if not being invested within 30 minutes.
Investing the pattern at the earliest.
Using solid wax sprue or hollow metal sprue filled with sticky wax during
spruing will reduce distortion.
Inlay waxes are generally produced in deep blue, green or purple rods or sticks
about 7.5 cm long and 0.64 cm in diameter.
CASTING WAX
Used for the designing of patterns for RPD framework. They are available
as
Sheets of 28 to 30 gauge(.4 & .32 mm)
Readymade shapes – round, half round, half pear shaped rods & wires
Bulk material
Difference between inlay and casting wax is minimal and related to the
physical properties. The composition is similar but the proportions are different.
Casting wax sheets are used to establish minimum thickness in certain areas of the
partial denture framework, such as the palatal and lingual bar. They are also used
for post damming of complete maxillary denture impressions, for checking high
points and for producing wax bites of cusp tips for articulation of stone casts.
Physical characteristics
Casting wax comes under federal specification No: u-w-140. It possesses a
slight degree of tackiness which maintains their position on the cast. According to
the federal specification they are put into 3 classes, however their properties like
flow, breaking point, and working properties are similar for all the classes. The
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wax should be capable of bending double on itself without fracture at a
temperature of 23°C i.e. it should have good ductility.
Most of the pattern waxes are being replaced by preformed plastic patterns.
Type of
casting waxFlow
Breaking
pointWorking properties
Class A
28 gauge
pink
35°C-max
10%
38°C-min 60%
No
fracture at
23°±1°C
- Pliable & readily
adaptable at 40-45°C
- Copy accurately the
surface on which it is
adapted
- Not be brittle on cooling
- Vapourize at 500°C
leaving no film other
than carbon.
Class B
30 gauge
green
Class C
Ready
shapes
blue
BASEPLATE WAX
Base plate waxes are used for
Establishing vertical dimension in complete denture construction
Establishing the plane of occlusion
Forming the initial arch form
Forming the tray
Serves to produce the desired contour after teeth setting
Designing removable orthodontic appliances.
Composition:
Ceresin 80%
Beeswax 12%
Carnauba 2.5%
Resins 3%
Microcrystalline or synthetic waxes 2.5%
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Physical properties: - This wax comes under ADA specification number 24.
Three types of waxes are recognized. These are normally supplied in sheets of 7.6
X 15 X .13 cm. normally marketed in pink or red colours.
Type I - Soft wax for building veneers.
Type II - Medium wax used for patterns to be tried in mouth in temperate
climates.
Type III - Hard wax used in tropical climates.
Type Temperature °C Flow % Practical requirements
Type I
soft
23 - 1 -Linear thermal expansion from
25-40°C <0.8%
-softened sheets should cohere
readily without flaking or
adhering to fingers.
- no tissue irritation
-trim easily with sharp
instrument at 23° C
-smooth surface after gentle
flaming
-no residue on porcelain or
plastic teeth.
-colouring should not separate
or impregnate plaster during
processing.
-no adhering to other sheets or
paper during storage.
37 4
5
85
45 - -
Type II
medium
23 - 0.6
37 - 2.5
45 5
0
90
Type III
hard
23 - 0.2
37 - 1.2
45 5 50
BOXING WAX
It comes under federal specification u-w-138. These are used to make box
around the impression into which freshly mixed plaster or stone is poured and
vibrated. In general it should be tacky and have strength and toughness sufficient
for convenient manipulation.
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UTILITY WAX
It comes under federal specification u-w-156. Utility wax is used when an
easily workable, adhesive wax is desired (e.g. contouring perforated tray prior to
alginate impressions). It is supplied in sticks and sheets. Colour ranges from dark
red to orange. Ductility and flow are the highest amongst dental waxes. It is
comprised of beeswax, petrolatum, and soft waxes.
CORRECTIVE IMPRESSION WAX
Used as a wax veneer over an original impression to contact and register
the detail of the soft tissue. It records tissue in functional state. It is not covered
under any specifications but flow is generally 100% at 37ºC.
BASE PLATE WAX
It is used for articulating accurately certain models of opposing quadrants.
It is generally made from 28 gauge casting wax sheet or from hard Base plate
wax. Certain bite waxes contain aluminium or copper particles. Flow ranges from
2.5% to 22% at 37 ºC and is susceptible to distortion on removal from the mouth.
BITE REGISTRATION WAX
It is covered by federal specification u-w- 00149a. It is formed by
mixing waxes and resins. It is sticky when melted and adheres to the surface
closely. It is firm and free from tackiness at room temperature and is brittle.
Type of wax Flow Colour Working properties
Boxing wax
UW- 138
Green
Black
Smooth glossy surface on
flaming,
Pliable at 21 °C,
Retains shape at 35º C,
Seals easily to plaster with
hot spatula.
Utility wax
UW- 156
At 37.5ºC
65-80%
Orange
Dark red
Pliable at 21-24 ºC
Tacky at 21-24 ºC
Sufficient adhesion to
build up.
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Sticky wax
UW- 00149a
30ºC – 5%
max
43ºC – 90%
min
Dark or
vivid
Sticky when melted
Adheres closely
Not more than 2% residue
on burnout.
Not more than 5%
shrinkage from 43ºC to
28ºC
References
Kenneth Anusavice: Phillips Science of Dental Materials 10th Ed. W B
Saunders & Co 1998.
Robert Craig: Restorative Dental Materials 10th Ed. Mosby Year Book Inc
1997.
Reese J A, Valega T M: Restorative Dental Materials: An Overview
Volume one. Quintessence Publishing Co 1985.
Craig R G, O’Brien W J: Dental Materials, Properties and Manipulation 6th
Ed. Mosby Inc 1996.
John F. McCabe, Applied Dental Materials, Ninth Edition, Blackwell
Publishing
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