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GYPSUM
PRODUCTS

BY SWATI SAGGAR
INTRODUCTION
 Gypsum is a mineral that is mined in various parts of
  the world.
 The word gypsum derived from Greek word meaning
  ¶to cook· referred to burnt or calcined mineral.
 Chemically it is CaSo4;2H2O i.e. calcium sulphate
  dihydrate.
 Three types of gypsum products are available in
  dentistry ²
            1.   Model plaster
            2.   Dental stone
            3.   High strength dental stone or die stone
All these types differ in their physical form ,
  but have the same chemical structure.
 The term Plaster of Paris was given to this
  product as this was obtained by burning
  the gypsum from deposits near Paris ,
  France .
      OTHER FORMS OF GYPSUM
             Gypsum has variety of names
  that are widely used in the mineral trade
1} Selenite                 2} Satin spar
3} Alabaster                4}Plaster of Paris
CLASSIFICATION
ACCORDING TO ADA SPECIFICATION NO 25-

TYPE I -IMPRESSION PLASTER
TYPE II -DENTAL PLASTER
TYPE III - DENTAL STONE
            (Hydrocal, Alpha hemihydrate)
TYPE IV - DENTAL STONE (HIGH STRENGTH)
             (Die stone, Densite,Modified alpha hemihydrate)
TYPE V - DENTAL STONE (HIGH STRENGTH, HIGH EXPANSION)
Types of gypsum products
Gypsum             Manufacture                 Use
Products
Impression         Earlier plaster was         Preliminary impression
Plaster            modified for use as         of edentulous ridge in
(Type ² I)         impression material by      fabrication of dentures.
Now rarely used.   addition of chemicals and   [MUCOSTAIC
                   flavoring agents            IMPRESSION]


Dental Plaster     Heating the gypsum          Weakest and least
(Type ² II)        powder in open container.   expensive of all gypsum
Plaster of paris   This direct and rapid       products.
                   heating results in powder   Making preliminary cast
F-hemihydrate      that consist of porous      for CD
Model plaster      and irregular particles.    To secure cast to
                                               articulator ,
                                               to fill a flask in denture
                                               construction
Dental Stone      Made for gypsum by
(Type ² III)      carefully controlled
Hydrocal          calcinations under steam       Stone is stronger
                  pressure in a closed
Class I stone     container. The method          and more expensive
E-hemihydrate     releases the water of          than plaster.
                  crystallization slowly and     Use for making cast
                  resultant powder particles     for diagnostic purpose.
                  are more uniform in shape      Cast for CD and RPD
                  and less porous.               construction

Dental Stone,     Made by calcining in calcium   It is strongest and most
high strength     chloride solution. This        expensive. Densite is
(Type IV)         calcination result in a        the densest of all the
Class II Stone,   powder particles that is       material.
Densite,          very dense and cuboidal in     Used mainly for cast and
Improved          shape, with a reduced          dies for inlay and crown
stones, Die       surface area.                  fabrication.
Stone
Dental Stone,    Same as Type IV,      For making very
high strength,   Added with            strong dies (has
high expansion   accelerators and      highest
(Type V)         retarders and         compressive
                 surface tension       strength) this is
                 reducing agent like   achieved by
                 LIGNON SULFATE        lowering the w/p
                                       ratio and
                                       increasing the
                                       setting expansion
                                       to compensate
                                       for alloy
                                       solidification
                                       shrinkage.
Based on crystal structure:
1.    Beta hemihydrate- Type I , II
     fibrous aggregate of fine crystals with capillary
     pores.
     Fluffy porous and least dense

2. Alpha hemihydrate- Type III, IV, V
  cleavage fragments and crystals in form of
  rods or prisms
   Higher density and more crystalline
MANUFACTURE


 CaSO4.2H2O           CaSO4.1/2 H2O       CaSO4
 Gypsum     110-130oC       Plaster or Stone     130-200oC
                                                           CaSO4
                                                           Hexagonal
                                        CaSO4                        anhydrite
                                        Orthorhombic
                                       anhydrite
                                                     200-1000oC


 Commercially, the gypsum is ground and
 subjected to temperature 110-120 0C to drive
 off the water of crystallization and produce
 calcium sulphate hemihydrate .
CALCINATION

                  CaSO4;2H2O
                                     boil,
                                      CaCl2

    open kettle    high

                  pressure       Type IV      Type V
                   steam



     Type II



                      Type III
CaSO4.1/2H2O




        -hemihydrate           -hemihydrate

  The difference between            and
hemihydrate are the result of differences
in crystal size, surface area and degree of
lattice imperfection
-hemihydrate              -hemihydrate


y When dihydrate is    y When dihydrate is
  heated under steam     heated in an open
  pressure               kettle or kiln.
y Dense  prismatic    y Spongy  irregularly
  crystals               shaped crystals.
y Stronger  harder    y Less strong products
  products on mixing     on mixing with water
  with water           (Type I,II)
 (type III,IV,V)
y Smaller w/p ratio    y   Larger w/ p ratio
F-hemihydrate       -hemihydrate




          PLASTER                  STONE
PLASTER   STONE   DIE STONE
SETTING OF GYPSUM
PRODUCTS

The following reaction takes place in the hardening
process
CaSO4.2 H2O+ 3H2O 2(CaSO4 .2H2O) + heat
                                   3900Cal/g mol)
The product of the reaction is gypsum.
The heat evolved in the exothermic reaction is
equivalent to the heat used originally in calcination.
This chemical reaction takes place regardless of
whether the gypsum material is used as impression
material , Die material, or a binder in casting
investment.
SETTING PROCESS

 There is distinct difference in the solubility of
 hemihydrate and dihydrate.
 Hemihydrate is 4 times more soluble in water
 than dihydrate near room temp.(200c)
T      s tti g r cti c               rst     s
    f ll s:
          i y r t is ix      it     t r,
    s s      si is f r     t t is fl i
        r    l .
    W t r c         ss t r t     it      i y r t .
    T is s t r t s l ti is s         rs t r t
      it i y r t ,s t l tt r r ci it t s t.

       l ti is     l g rs t r t      it
         i y r t .
        it c ti    s t iss lv , till    f rt    r
     i y r t r ci it t s t f s l ti .
The process continues until most of the
hemihydrate is converted to dihydrate.
The crystals of dihydrate are spherulitic in
nature and grow from specific growth sites
called nuclei of crystallization.
THEORIES OF SETTING

Theories of Setting
of Gypsum products

 Colloidal Theory
 Hydration Theory
 Dissolution precipitation Theory or Crystalline
 Theory
Colloidal Theory:
    When mixed with water ,
  plaster enters into the colloidal state through sol-
  gel mechanism .
   In the sol state ,hemihydrate particles are
  hydrated to form the dihydrate, thereby entering
  into an active state .
   As the measured amount of water is
consumed, the mass converts into a solid gel.
Hydration Theory
  This theory suggests that rehydrated plaster
  particles join together through hydrogen bonding
  to the sulfate groups
 to form the set material.
Dissolution precipitation Theory: by Louis Chatelier in
  1885
  This theory is based on dissolution of plaster and
  instant recrystilization of gypsum, followed by
  interlocking of the crystals to form the set product
NUCLEI
      OF
CRYSTALLIZATION


  SPHERULITIC
    GROWTH




OUTWARD THRUST AS
 SPHERULITES MAKE
     CONTACT
PROPERTIES

                   Water/Powder ratio-
The proportion of water to powder used to make a
workable mix of a particular gypsum product is called
water/powder ratio
It is the quotient obtained when the weight or volume of
the water is divided by the weight of powder.
It is usually abbreviated as W:P
The W:P ratio is an important factor in determining the
physical and chemical properties of the final gypsum
product
Chief factors that determine the amount of
  gauging water required are:-
1. Particle size

2. Total surface area

3. Particle size distribution
´For      t l s ,       xc ss     ount of   sur
    ter is l ys necess ry ove the
 theoretic lly correct    ount required for
 hydr tion hich is . 6%
´The excess is needed to e or le ix
 that can e oured and shaped
´The excess ater is distri uted as free ater in
 the set ass ithout taking part in chemical
 reaction and it contri utes to the subsequent
 porosities or voids in the set product
´The proper W:P ratio for each product depends on
 physical characteristics of powder particles
´Therefore plaster requires more measured water
 (gauging water) to float the irregular porous
 particles than the dense particles of stone
´A mixture of Calcium oxide (0.1 )  Gum arabic
 (1 ) reduces the amount of water necessary to mix
 gypsum products: resulting in Improved properties.
The excess water is distributed as free
 water in the set mass without taking
 part in the chemical reaction .When
 the set mass is dried the excess water
 evaporates  leaves porosity in the
 structure weakening it.
Recommended ranges of w/p ratio:
´Type I ³ 0.50-0.75
´Type II ³ 0.45-0.50
´Type III³ 0.28-0.30
´Type IV ³0.22-0.24
´Type V ³0.18-0.22
gypsum
SETTING TIME
Mixing time-
 Time from addition of powder to water
 until mixing is completed.
 Mechanical mixing:20-30 secs
 Hand spatulation:1 min.
Working Time: The time available to use a workable
 mix, one that maintains an even consistency that
 may be manipulated to perform one or more tasks.
 General 3 min working time is adequate
Setting time
    When the powder is mixed with water, the
time that elapses from the beginning of mixing
until the materials hardens is known as setting
 time.

This is usually measured by some type of
penetration test, using the following instruments-
          Vicat penetrometer
          Gilmore needle
1. Loss of gloss test for initial set.
   occurs hen excess ater in the mix is
   taken up in forming the dihydrate, so the
   mix loses its gloss.
     This occurs approx. at mins.and the
   mass has no measurable compressive
   strength.
 . Initial ilmore test for initial set
      This occurs approx at 13mins this time
   is marked by definite increase in strength.
Initial gillmore test for initial set:-
    the mixture is spread out , the needle of ¼ lb
 weight and dia of 1/12µ is lowered onto the surface
 and the time at which it no longer leaves an
 impression is called the initial set. This time is
 marked by a definite increase in strength.

  Gillmore test for final setting time:-
    this uses a heavier gillmore needle(1lb, 1/24µ). The
  time elapsed when it leaves only a barely perceptible
  mark on the surface is called the final setting time. It
  is 45- 0 min.
It is rarely used as an indication for the ready-for-use
  stage
GILMORE NEEDLES
a) MATERIAL IS UNSET

b) MATERIAL IS SET
3. icat test for setting
  time:it has an inbuilt gauge
  for judging the setting
  characteristics. The setting
  material is indented by a
  needle of 1mm dia. nder a
  load of 300gm.setting time is
  achieved hen needle can no
  longer penetrate to a depth of
    mm into the material.


     Vicat
     penetrometer
´   It consists of a rod eighing 300 g ith a
    needle of 1-mm diameter. A ring container is
    filled ith the mix, the setting time of hich
    is to be measured. The rod is lowered until it
    contacts the surface of the material, then
    the needle is released and allowed to
    penetrate the mix. When the needle fails to
    penetrate to the bottom of the container, the
    material has reached the icat or the initial
    setting time
COMPRESSIVE STRENGTH CORRELATED
WITH      TIME
Ready for use criterion

´ It is a subjective measure of the time at
  which material may be safely handled
´ Technically it is the time when compressive
  strength is atleast 80 of that attained at
  1 hour.
´ Most modern products reach the ready to
  use state in 30mins.
CONTROL OF SETTING TIME(S.T.)


  The .T. of gypsum products can be controlled
 by manufacturers particular formulation
 accordingly either a fast setting or a slow setting
 product can be purchased.
    Theoretically there are 3 methods
1. The solubility of hemihydrate can be increased or
   decreased.
 . o. of nuclei of crystallization can be increased
  or decreased.
3. ate of crystal growth is increased or decreased.
The operator can vary the S.T.within reason
By altering W:P ratio  mixing time.
Decrease in setting time (fast setting) -
  o mixing
  q W:P ratio
  Addition of certain chemicals called accelerators.
    Increase in setting time (slow setting) ²
  q mixing
  o W:P ratio
   Addition of certain chemicals called retarders.
The operator can accelerate the setting time
by adding gypsum (.20 ),Potassium
sulfate or sodium chloride(.28 )
Retarder act by forming an adsorbed layer
on the hemihydrate to reduce its solubility
and on the gypsum crystals to present to
inhibit growth. Organic materials like glue,
gelatin and some gums behave in this
manner.
    Nacl when added in small
   quantities will act as accelerators ,
   BUT in large quantities will act as
   retarder
Other factors-
´Impurities
´Fineness
´Temperature
IMPURITIES


´ Presence of set gypsum particles, shortens
  the setting time because of the increase in
  the potential nuclei of crystallization.
´ Orthorhombic anhydrite: Induction period is
  increased. Hence setting time increased.
´ Hexagonal anhydrite: Induction period is
  decreased.
  hence setting time is decreased.
Fineness:
    Finer the particle size of the hemihydrate«
   «faster is the set.
Temperature:
´ Little change occurs between 0*C to 50*C.
´    If temperature of the plaster exceeds 50*C
  then gradual retardation occurs.
´   As the temperature approaches 100*C, no
  reaction takes place.
´   At higher temperature there is a tendency
  for the gypsum crystals formed to be
  converted back to hemihydrate form
Setting Expansion

´This thrust  results in an external
 expansion with resulting internal porosity
 in set mass.
CONTROL OF SETTING EXPANSION
(S.E)
´Less the W:P ratio o S.E.
       o mixing time o S.E.
´However the most effective method for control of S.E.
 is the addition of chemicals. (accelerators or
 retarders)
´Accelerators or retarders have an effect of reducing
 setting expansion and are referred as Anti expansion
 agents.
ACCELERATORS AND
RETARDERS
Accelerators : It increases the solubility of
 hemihydrates without increasing the solubility
 of dihydrate. Thus accelerates the process.
Eg. NaCl upto about 2
      NaSO4 upto 3.4
      K2SO4          2
    Terra Alba/Set calcium sulfate dihydrate.
         Liquids of High pH accelerate the
        setting reaction while the liquids of low pH
         like saliva retard the setting reaction
Retarders : Certain chemicals form a
  coating on the hemihydrates particles and
  thus prevent the hemihydrates from going
  into solution in the normal manner.
Eg. Citrates, Acetates, Borates
Colloidal system such as agar  alginate
  retard the setting reaction by being
  adsorbed on the hemihydrate or the
  dehydrate nucleation sites thus
  interfering in hydration reaction
HYGROSCOPIC S.E.

´So far we have assumed that the plaster or
 stone is allowed to set in air.
´If the setting process is allowed to occur under
 water, the setting expansion is more than
 double in magnitude.
Stage I : Initial mix is represented
by three round particles of hemihydrate
surrounded by water




           As the crystals of dihydrate
           grow, they contact each other,
           and the S.E. begins
Stage II : Reaction started and crystal
 of dihydrate start forming
´In left, water around particles is reduced by
 hydration and particles are drawn more closely
 together by surface tension action of water.
´In right, because the setting is taking place
 under water, the water of hydration is replaced
 and distance between the particles remains the
 same.
Stage III :
´The water around particles is again decreased in
 the eg. on left.
´The particles with their attached crystals tend to be
 drawn together as before but the contraction is
 opposed by the outward thrust of growing crystals.
´On the other hand the crystals in right diagram are
 not so inhibited, as water is again replenished.
Stage IV  Stage V:
  The effect become more marked.
 The crystals being inhibited on the left
  become intermeshed and entangled much
 sooner than those on right, which grow much
 more freely during early stages before the
 intermeshing finally prevents the further
 expansion.
´The basic mechanism of crystal growth
 is same in both instances, and both
 phenomenon       are    true    setting
 expansion.
´To distinguish between them, S.E.
 without water immersion is often
 termed as Normal setting expansion
 (NSE) where as the expansion that
 occurs under water is known as
 Hygroscopic setting expansion (HSE) .
FACTORS INFLUENCING HSE

 ´q W:P ratio o HSE
 ´o mixing time o HSE
STRENGTH

´The strength of gypsum products is
 usually measured in terms of compressive
 strength.
´As seen in setting reaction the strength
 develops rapidly during the first 30 to 45
 min as the hydration is completed.
 The wet strength of dental plaster is 9MPa and
 Improved stone is 35MPa
 1 hour tensile strength of model plaster is 2.3
 MPa
gypsum
´The strength depends on porosity of set
 material, which is related to W:P ratio
 necessary to make a workable mix.
´Eg ² Plaster (which require the most
 gauging water to make a fluid mix) is the
 weakest, while improved stone is
 strongest.
´The presence or absence of excess of
 free water affects strength and two types
 of strength are recognized.

    Wet strength
    Dry strength
Wet trength : It is the strength
 measured when the sample contains
some or all of the water is excess of the
 theoretical amount required for
 hydration.
  ry trength : It is the strength
measured when the excess of water is not
 present.
    ry strength is two or more times more
          than the wet strength.
FACTORS INFLUENCING THE
STRENGTH
´o W:P ratio q strength.
´o mixing time o strength.
´o accelerator or retarder q strength.
SURFACE HARDNESS AND ABRASIVE
RESISTANCE
´   DEFINITION - IN MINEROLOGY SURFACE HARDNESS OF
    SUBSTANCE IS ITS ABILITY TO RESISTS SCRATCHING

    SURFACE HARDNESS ~ COMPRESSIVE STRENGTH
Detail Reproduction:
ANSI/ADA Specification No.25 requires

´      Type I  Type II to reproduce a
    groove of 75 m
´      Type III , IV  V to reproduce a
    groove of 50 m.
PROPORTIONING, MIXING AND CARING
FOR PRODUCTS
The technical use of gypsum products is relatively
  simple requiring only
´Mixing bowl
´Mixing spatula
´Room-temperature water
´Appropriate gypsum product
PROPOTIONING

 Because the strength of a stone is indirectly
 proportional to the W:P ratio, it is most
 important to keep the amount of water as low
 as possible. However it should not be so low
 that the mix will not flow in every detail of the
 impression.
 Thus, water and powder must be proportioned
 accurately for optimum properties.
a. Measuring the Water
    The water is usually dispensed by volume in
    a graduated cylinder, as 1 gm of water has a
   volume of very close to 1 ml.
b. Measuring the Powder
    The power can be weighed in grams with a
   simple balance or scale.
    Volume dispensers may be used, but volume
   dispensing of the power is not accurate because
   of varying packing effect on the powder.
    Weighing with a scale is a simple and convenient
    method to ensure accurate proportions.
gypsum
WATER FOR REACTION

MIXING WATER:-TOTAL AMOUNT OF WATER
 REQUIRED FOR CHEMICAL REACTION TO OCCUR


REQUIRED WATER:- THAT AMOUNT OF WATER
 THAT REACTS WITH HEMIHYDRATE IS KNOWN AS
 REQUIRED WATER.


STANDARD VALUE 18.6 ML/100GM OF
 POWDER
MANUAL MIXING
´ The preferred method of mixing is to add the
 measured water first, followed by the gradual
 sifting of the pre weighed powder into it.
´ Guess work of repeatedly adding water and
 powder to achieve proper consistency is to be
 avoided, as it may cause an uneven set in the
 cast, resulting in low strength and distortion.
´Hand mixing involves using spatula at a rate of
 2 revolutions /sec.
MIXING
´Done in a flexible plastic or rubber bowl
 with a stiff bladed spatula to combine the
 powder and water.
´The mix should be smooth, homogenous,
 workable and free of air bubbles.
´A minimum of air inclusion in the mixed
 product is desirable to prevent surface
 bubbles and internal defects.
gypsum
VACCUM MIXING
´Often, mixing is done mechanically with a
 vaccum mixing and investing machine.
´This provides a gypsum mix that is free of air
 bubbles and homogenous in consistency.
´Many other devices are available which will
 mix gypsum products mechanically and they
 are used where the application of product is
 critical
VACCUM MIXER
CARING FOR CAST
 Once the setting reaction in the cast have been
 completed, its dimension will be relatively constant
 thereafter under ordinary conditions of room
 temperature and humidity.
 However it is sometimes necessary to soak
 gypsum cast in water. If the stone cast is
 immersed in running water, its linear dimension
 may decrease approximately 0.1 for every 20
 min of such immersion.
 The safest method is to soak the cast in saturated
 solution of calcium sulfate.
Storage of set plaster or stone at room
temperature produces no significant dimensional
change.

However if the storage temperature is raised to
between 90o and 100oc, a shrinkage occurs as
the water of crystallization is removed and the
dihydrate reverts to hemihydrate.

  Therefore, it is not safe to store or heat a stone
cast in air at temperature higher than 55oC
SYNTHETIC GYPSUM

´This is done with waste products or by-
 products of phosphoric acid production
´The synthetic product is usually much more
 expensive than that made from Natural
 gypsum, but when properly made its properties
 are equal to or exceed those of the latter.
SPECIAL GYPSUM PRODUCTS
 In addition to the standardized gypsum
materials there are some that have been
characterized for special purposes
For Eg.
   Orthodontist prefer a white stone or plaster for
   study models and may even treat the surface with
   soap solution for added sheen.
          These products have a longer
          working time for ease of trimming.
´The mounting stones or plasters are used to
 mount the cast on articulator, are fast setting
 and have low S.E.
   The mounting plaster has low strength to
 permit easy trimming and to separate the cast
 readily from articulator
´Since 1991, a plethora of new dental stones
 have appeared mostly as time savers.
´One type is ready to use in 5 min. but it has
 little working time.
´Another product changes color to help denote
 when it is ready for use.
´Most recently, the trend is to add a small
 amount of plastic or resin that reduces
 brittleness and improves the resistance to
 scratching during the carving of wax pattern.
CARING OF GYPSUM PRODUCTS
INFECTION CONTROL
´There is always a risk of cross contamination
 to a dental office personnel by micro-organism,
 including
 hepatitis B and HIV , via dental impressions.
´Thus the impressions should be disinfected
 using spray and immersion disinfecting
 techniques
If the impression is not disinfected it is prudent to
disinfect stone cast
Usual disinfectants for stone cast include spray
disinfectants, hypochlorites and iodophores
Disinfecting solutions can be used that do not
adversely affect the quality of gypsum cast.
Alternatively, dental stone containing disinfectant
may be employed.
When patients with known cases of infection are
being treated, overnight ethylene oxide gas
sterilization is an option.
REFERENCES


´ Phillips Science of
  DENTAL MATERIALS- 10th 11th Edition
´ CRAIG·S Dental Materials- Properties and
  manipulation. 8th edition
´ Clinical aspects of dental materials .Theory,
  practice and cases. Marcia Gladwin.
´ Applied dental Materials. McCabe
THANK YOU

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gypsum

  • 2. INTRODUCTION Gypsum is a mineral that is mined in various parts of the world. The word gypsum derived from Greek word meaning ¶to cook· referred to burnt or calcined mineral. Chemically it is CaSo4;2H2O i.e. calcium sulphate dihydrate. Three types of gypsum products are available in dentistry ² 1. Model plaster 2. Dental stone 3. High strength dental stone or die stone
  • 3. All these types differ in their physical form , but have the same chemical structure. The term Plaster of Paris was given to this product as this was obtained by burning the gypsum from deposits near Paris , France . OTHER FORMS OF GYPSUM Gypsum has variety of names that are widely used in the mineral trade 1} Selenite 2} Satin spar 3} Alabaster 4}Plaster of Paris
  • 4. CLASSIFICATION ACCORDING TO ADA SPECIFICATION NO 25- TYPE I -IMPRESSION PLASTER TYPE II -DENTAL PLASTER TYPE III - DENTAL STONE (Hydrocal, Alpha hemihydrate) TYPE IV - DENTAL STONE (HIGH STRENGTH) (Die stone, Densite,Modified alpha hemihydrate) TYPE V - DENTAL STONE (HIGH STRENGTH, HIGH EXPANSION)
  • 5. Types of gypsum products Gypsum Manufacture Use Products Impression Earlier plaster was Preliminary impression Plaster modified for use as of edentulous ridge in (Type ² I) impression material by fabrication of dentures. Now rarely used. addition of chemicals and [MUCOSTAIC flavoring agents IMPRESSION] Dental Plaster Heating the gypsum Weakest and least (Type ² II) powder in open container. expensive of all gypsum Plaster of paris This direct and rapid products. heating results in powder Making preliminary cast F-hemihydrate that consist of porous for CD Model plaster and irregular particles. To secure cast to articulator , to fill a flask in denture construction
  • 6. Dental Stone Made for gypsum by (Type ² III) carefully controlled Hydrocal calcinations under steam Stone is stronger pressure in a closed Class I stone container. The method and more expensive E-hemihydrate releases the water of than plaster. crystallization slowly and Use for making cast resultant powder particles for diagnostic purpose. are more uniform in shape Cast for CD and RPD and less porous. construction Dental Stone, Made by calcining in calcium It is strongest and most high strength chloride solution. This expensive. Densite is (Type IV) calcination result in a the densest of all the Class II Stone, powder particles that is material. Densite, very dense and cuboidal in Used mainly for cast and Improved shape, with a reduced dies for inlay and crown stones, Die surface area. fabrication. Stone
  • 7. Dental Stone, Same as Type IV, For making very high strength, Added with strong dies (has high expansion accelerators and highest (Type V) retarders and compressive surface tension strength) this is reducing agent like achieved by LIGNON SULFATE lowering the w/p ratio and increasing the setting expansion to compensate for alloy solidification shrinkage.
  • 8. Based on crystal structure: 1. Beta hemihydrate- Type I , II fibrous aggregate of fine crystals with capillary pores. Fluffy porous and least dense 2. Alpha hemihydrate- Type III, IV, V cleavage fragments and crystals in form of rods or prisms Higher density and more crystalline
  • 9. MANUFACTURE CaSO4.2H2O CaSO4.1/2 H2O CaSO4 Gypsum 110-130oC Plaster or Stone 130-200oC CaSO4 Hexagonal CaSO4 anhydrite Orthorhombic anhydrite 200-1000oC Commercially, the gypsum is ground and subjected to temperature 110-120 0C to drive off the water of crystallization and produce calcium sulphate hemihydrate .
  • 10. CALCINATION CaSO4;2H2O boil, CaCl2 open kettle high pressure Type IV Type V steam Type II Type III
  • 11. CaSO4.1/2H2O -hemihydrate -hemihydrate The difference between and hemihydrate are the result of differences in crystal size, surface area and degree of lattice imperfection
  • 12. -hemihydrate -hemihydrate y When dihydrate is y When dihydrate is heated under steam heated in an open pressure kettle or kiln. y Dense prismatic y Spongy irregularly crystals shaped crystals. y Stronger harder y Less strong products products on mixing on mixing with water with water (Type I,II) (type III,IV,V) y Smaller w/p ratio y Larger w/ p ratio
  • 13. F-hemihydrate -hemihydrate PLASTER STONE
  • 14. PLASTER STONE DIE STONE
  • 15. SETTING OF GYPSUM PRODUCTS The following reaction takes place in the hardening process CaSO4.2 H2O+ 3H2O 2(CaSO4 .2H2O) + heat 3900Cal/g mol) The product of the reaction is gypsum. The heat evolved in the exothermic reaction is equivalent to the heat used originally in calcination. This chemical reaction takes place regardless of whether the gypsum material is used as impression material , Die material, or a binder in casting investment.
  • 16. SETTING PROCESS There is distinct difference in the solubility of hemihydrate and dihydrate. Hemihydrate is 4 times more soluble in water than dihydrate near room temp.(200c)
  • 17. T s tti g r cti c rst s f ll s: i y r t is ix it t r, s s si is f r t t is fl i r l . W t r c ss t r t it i y r t . T is s t r t s l ti is s rs t r t it i y r t ,s t l tt r r ci it t s t. l ti is l g rs t r t it i y r t . it c ti s t iss lv , till f rt r i y r t r ci it t s t f s l ti .
  • 18. The process continues until most of the hemihydrate is converted to dihydrate. The crystals of dihydrate are spherulitic in nature and grow from specific growth sites called nuclei of crystallization.
  • 19. THEORIES OF SETTING Theories of Setting of Gypsum products Colloidal Theory Hydration Theory Dissolution precipitation Theory or Crystalline Theory
  • 20. Colloidal Theory: When mixed with water , plaster enters into the colloidal state through sol- gel mechanism . In the sol state ,hemihydrate particles are hydrated to form the dihydrate, thereby entering into an active state . As the measured amount of water is consumed, the mass converts into a solid gel.
  • 21. Hydration Theory This theory suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material. Dissolution precipitation Theory: by Louis Chatelier in 1885 This theory is based on dissolution of plaster and instant recrystilization of gypsum, followed by interlocking of the crystals to form the set product
  • 22. NUCLEI OF CRYSTALLIZATION SPHERULITIC GROWTH OUTWARD THRUST AS SPHERULITES MAKE CONTACT
  • 23. PROPERTIES Water/Powder ratio- The proportion of water to powder used to make a workable mix of a particular gypsum product is called water/powder ratio It is the quotient obtained when the weight or volume of the water is divided by the weight of powder. It is usually abbreviated as W:P The W:P ratio is an important factor in determining the physical and chemical properties of the final gypsum product
  • 24. Chief factors that determine the amount of gauging water required are:- 1. Particle size 2. Total surface area 3. Particle size distribution
  • 25. ´For t l s , xc ss ount of sur ter is l ys necess ry ove the theoretic lly correct ount required for hydr tion hich is . 6% ´The excess is needed to e or le ix that can e oured and shaped ´The excess ater is distri uted as free ater in the set ass ithout taking part in chemical reaction and it contri utes to the subsequent porosities or voids in the set product
  • 26. ´The proper W:P ratio for each product depends on physical characteristics of powder particles ´Therefore plaster requires more measured water (gauging water) to float the irregular porous particles than the dense particles of stone ´A mixture of Calcium oxide (0.1 ) Gum arabic (1 ) reduces the amount of water necessary to mix gypsum products: resulting in Improved properties.
  • 27. The excess water is distributed as free water in the set mass without taking part in the chemical reaction .When the set mass is dried the excess water evaporates leaves porosity in the structure weakening it.
  • 28. Recommended ranges of w/p ratio: ´Type I ³ 0.50-0.75 ´Type II ³ 0.45-0.50 ´Type III³ 0.28-0.30 ´Type IV ³0.22-0.24 ´Type V ³0.18-0.22
  • 30. SETTING TIME Mixing time- Time from addition of powder to water until mixing is completed. Mechanical mixing:20-30 secs Hand spatulation:1 min. Working Time: The time available to use a workable mix, one that maintains an even consistency that may be manipulated to perform one or more tasks. General 3 min working time is adequate
  • 31. Setting time When the powder is mixed with water, the time that elapses from the beginning of mixing until the materials hardens is known as setting time. This is usually measured by some type of penetration test, using the following instruments- Vicat penetrometer Gilmore needle
  • 32. 1. Loss of gloss test for initial set. occurs hen excess ater in the mix is taken up in forming the dihydrate, so the mix loses its gloss. This occurs approx. at mins.and the mass has no measurable compressive strength. . Initial ilmore test for initial set This occurs approx at 13mins this time is marked by definite increase in strength.
  • 33. Initial gillmore test for initial set:- the mixture is spread out , the needle of ¼ lb weight and dia of 1/12µ is lowered onto the surface and the time at which it no longer leaves an impression is called the initial set. This time is marked by a definite increase in strength. Gillmore test for final setting time:- this uses a heavier gillmore needle(1lb, 1/24µ). The time elapsed when it leaves only a barely perceptible mark on the surface is called the final setting time. It is 45- 0 min. It is rarely used as an indication for the ready-for-use stage
  • 35. a) MATERIAL IS UNSET b) MATERIAL IS SET
  • 36. 3. icat test for setting time:it has an inbuilt gauge for judging the setting characteristics. The setting material is indented by a needle of 1mm dia. nder a load of 300gm.setting time is achieved hen needle can no longer penetrate to a depth of mm into the material. Vicat penetrometer
  • 37. ´ It consists of a rod eighing 300 g ith a needle of 1-mm diameter. A ring container is filled ith the mix, the setting time of hich is to be measured. The rod is lowered until it contacts the surface of the material, then the needle is released and allowed to penetrate the mix. When the needle fails to penetrate to the bottom of the container, the material has reached the icat or the initial setting time
  • 39. Ready for use criterion ´ It is a subjective measure of the time at which material may be safely handled ´ Technically it is the time when compressive strength is atleast 80 of that attained at 1 hour. ´ Most modern products reach the ready to use state in 30mins.
  • 40. CONTROL OF SETTING TIME(S.T.) The .T. of gypsum products can be controlled by manufacturers particular formulation accordingly either a fast setting or a slow setting product can be purchased. Theoretically there are 3 methods 1. The solubility of hemihydrate can be increased or decreased. . o. of nuclei of crystallization can be increased or decreased. 3. ate of crystal growth is increased or decreased.
  • 41. The operator can vary the S.T.within reason By altering W:P ratio mixing time. Decrease in setting time (fast setting) - o mixing q W:P ratio Addition of certain chemicals called accelerators. Increase in setting time (slow setting) ² q mixing o W:P ratio Addition of certain chemicals called retarders.
  • 42. The operator can accelerate the setting time by adding gypsum (.20 ),Potassium sulfate or sodium chloride(.28 ) Retarder act by forming an adsorbed layer on the hemihydrate to reduce its solubility and on the gypsum crystals to present to inhibit growth. Organic materials like glue, gelatin and some gums behave in this manner. Nacl when added in small quantities will act as accelerators , BUT in large quantities will act as retarder
  • 44. IMPURITIES ´ Presence of set gypsum particles, shortens the setting time because of the increase in the potential nuclei of crystallization. ´ Orthorhombic anhydrite: Induction period is increased. Hence setting time increased. ´ Hexagonal anhydrite: Induction period is decreased. hence setting time is decreased.
  • 45. Fineness: Finer the particle size of the hemihydrate« «faster is the set. Temperature: ´ Little change occurs between 0*C to 50*C. ´ If temperature of the plaster exceeds 50*C then gradual retardation occurs. ´ As the temperature approaches 100*C, no reaction takes place. ´ At higher temperature there is a tendency for the gypsum crystals formed to be converted back to hemihydrate form
  • 46. Setting Expansion ´This thrust results in an external expansion with resulting internal porosity in set mass.
  • 47. CONTROL OF SETTING EXPANSION (S.E) ´Less the W:P ratio o S.E. o mixing time o S.E. ´However the most effective method for control of S.E. is the addition of chemicals. (accelerators or retarders) ´Accelerators or retarders have an effect of reducing setting expansion and are referred as Anti expansion agents.
  • 48. ACCELERATORS AND RETARDERS Accelerators : It increases the solubility of hemihydrates without increasing the solubility of dihydrate. Thus accelerates the process. Eg. NaCl upto about 2 NaSO4 upto 3.4 K2SO4 2 Terra Alba/Set calcium sulfate dihydrate. Liquids of High pH accelerate the setting reaction while the liquids of low pH like saliva retard the setting reaction
  • 49. Retarders : Certain chemicals form a coating on the hemihydrates particles and thus prevent the hemihydrates from going into solution in the normal manner. Eg. Citrates, Acetates, Borates Colloidal system such as agar alginate retard the setting reaction by being adsorbed on the hemihydrate or the dehydrate nucleation sites thus interfering in hydration reaction
  • 50. HYGROSCOPIC S.E. ´So far we have assumed that the plaster or stone is allowed to set in air. ´If the setting process is allowed to occur under water, the setting expansion is more than double in magnitude.
  • 51. Stage I : Initial mix is represented by three round particles of hemihydrate surrounded by water As the crystals of dihydrate grow, they contact each other, and the S.E. begins
  • 52. Stage II : Reaction started and crystal of dihydrate start forming ´In left, water around particles is reduced by hydration and particles are drawn more closely together by surface tension action of water. ´In right, because the setting is taking place under water, the water of hydration is replaced and distance between the particles remains the same.
  • 53. Stage III : ´The water around particles is again decreased in the eg. on left. ´The particles with their attached crystals tend to be drawn together as before but the contraction is opposed by the outward thrust of growing crystals. ´On the other hand the crystals in right diagram are not so inhibited, as water is again replenished.
  • 54. Stage IV Stage V: The effect become more marked. The crystals being inhibited on the left become intermeshed and entangled much sooner than those on right, which grow much more freely during early stages before the intermeshing finally prevents the further expansion.
  • 55. ´The basic mechanism of crystal growth is same in both instances, and both phenomenon are true setting expansion. ´To distinguish between them, S.E. without water immersion is often termed as Normal setting expansion (NSE) where as the expansion that occurs under water is known as Hygroscopic setting expansion (HSE) .
  • 56. FACTORS INFLUENCING HSE ´q W:P ratio o HSE ´o mixing time o HSE
  • 57. STRENGTH ´The strength of gypsum products is usually measured in terms of compressive strength. ´As seen in setting reaction the strength develops rapidly during the first 30 to 45 min as the hydration is completed. The wet strength of dental plaster is 9MPa and Improved stone is 35MPa 1 hour tensile strength of model plaster is 2.3 MPa
  • 59. ´The strength depends on porosity of set material, which is related to W:P ratio necessary to make a workable mix. ´Eg ² Plaster (which require the most gauging water to make a fluid mix) is the weakest, while improved stone is strongest.
  • 60. ´The presence or absence of excess of free water affects strength and two types of strength are recognized. Wet strength Dry strength
  • 61. Wet trength : It is the strength measured when the sample contains some or all of the water is excess of the theoretical amount required for hydration. ry trength : It is the strength measured when the excess of water is not present. ry strength is two or more times more than the wet strength.
  • 62. FACTORS INFLUENCING THE STRENGTH ´o W:P ratio q strength. ´o mixing time o strength. ´o accelerator or retarder q strength.
  • 63. SURFACE HARDNESS AND ABRASIVE RESISTANCE ´ DEFINITION - IN MINEROLOGY SURFACE HARDNESS OF SUBSTANCE IS ITS ABILITY TO RESISTS SCRATCHING SURFACE HARDNESS ~ COMPRESSIVE STRENGTH
  • 64. Detail Reproduction: ANSI/ADA Specification No.25 requires ´ Type I Type II to reproduce a groove of 75 m ´ Type III , IV V to reproduce a groove of 50 m.
  • 65. PROPORTIONING, MIXING AND CARING FOR PRODUCTS The technical use of gypsum products is relatively simple requiring only ´Mixing bowl ´Mixing spatula ´Room-temperature water ´Appropriate gypsum product
  • 66. PROPOTIONING Because the strength of a stone is indirectly proportional to the W:P ratio, it is most important to keep the amount of water as low as possible. However it should not be so low that the mix will not flow in every detail of the impression. Thus, water and powder must be proportioned accurately for optimum properties.
  • 67. a. Measuring the Water The water is usually dispensed by volume in a graduated cylinder, as 1 gm of water has a volume of very close to 1 ml. b. Measuring the Powder The power can be weighed in grams with a simple balance or scale. Volume dispensers may be used, but volume dispensing of the power is not accurate because of varying packing effect on the powder. Weighing with a scale is a simple and convenient method to ensure accurate proportions.
  • 69. WATER FOR REACTION MIXING WATER:-TOTAL AMOUNT OF WATER REQUIRED FOR CHEMICAL REACTION TO OCCUR REQUIRED WATER:- THAT AMOUNT OF WATER THAT REACTS WITH HEMIHYDRATE IS KNOWN AS REQUIRED WATER. STANDARD VALUE 18.6 ML/100GM OF POWDER
  • 70. MANUAL MIXING ´ The preferred method of mixing is to add the measured water first, followed by the gradual sifting of the pre weighed powder into it. ´ Guess work of repeatedly adding water and powder to achieve proper consistency is to be avoided, as it may cause an uneven set in the cast, resulting in low strength and distortion. ´Hand mixing involves using spatula at a rate of 2 revolutions /sec.
  • 71. MIXING ´Done in a flexible plastic or rubber bowl with a stiff bladed spatula to combine the powder and water. ´The mix should be smooth, homogenous, workable and free of air bubbles. ´A minimum of air inclusion in the mixed product is desirable to prevent surface bubbles and internal defects.
  • 73. VACCUM MIXING ´Often, mixing is done mechanically with a vaccum mixing and investing machine. ´This provides a gypsum mix that is free of air bubbles and homogenous in consistency. ´Many other devices are available which will mix gypsum products mechanically and they are used where the application of product is critical
  • 75. CARING FOR CAST Once the setting reaction in the cast have been completed, its dimension will be relatively constant thereafter under ordinary conditions of room temperature and humidity. However it is sometimes necessary to soak gypsum cast in water. If the stone cast is immersed in running water, its linear dimension may decrease approximately 0.1 for every 20 min of such immersion. The safest method is to soak the cast in saturated solution of calcium sulfate.
  • 76. Storage of set plaster or stone at room temperature produces no significant dimensional change. However if the storage temperature is raised to between 90o and 100oc, a shrinkage occurs as the water of crystallization is removed and the dihydrate reverts to hemihydrate. Therefore, it is not safe to store or heat a stone cast in air at temperature higher than 55oC
  • 77. SYNTHETIC GYPSUM ´This is done with waste products or by- products of phosphoric acid production ´The synthetic product is usually much more expensive than that made from Natural gypsum, but when properly made its properties are equal to or exceed those of the latter.
  • 78. SPECIAL GYPSUM PRODUCTS In addition to the standardized gypsum materials there are some that have been characterized for special purposes For Eg. Orthodontist prefer a white stone or plaster for study models and may even treat the surface with soap solution for added sheen. These products have a longer working time for ease of trimming.
  • 79. ´The mounting stones or plasters are used to mount the cast on articulator, are fast setting and have low S.E. The mounting plaster has low strength to permit easy trimming and to separate the cast readily from articulator ´Since 1991, a plethora of new dental stones have appeared mostly as time savers. ´One type is ready to use in 5 min. but it has little working time.
  • 80. ´Another product changes color to help denote when it is ready for use. ´Most recently, the trend is to add a small amount of plastic or resin that reduces brittleness and improves the resistance to scratching during the carving of wax pattern.
  • 81. CARING OF GYPSUM PRODUCTS
  • 82. INFECTION CONTROL ´There is always a risk of cross contamination to a dental office personnel by micro-organism, including hepatitis B and HIV , via dental impressions. ´Thus the impressions should be disinfected using spray and immersion disinfecting techniques
  • 83. If the impression is not disinfected it is prudent to disinfect stone cast Usual disinfectants for stone cast include spray disinfectants, hypochlorites and iodophores Disinfecting solutions can be used that do not adversely affect the quality of gypsum cast. Alternatively, dental stone containing disinfectant may be employed. When patients with known cases of infection are being treated, overnight ethylene oxide gas sterilization is an option.
  • 84. REFERENCES ´ Phillips Science of DENTAL MATERIALS- 10th 11th Edition ´ CRAIG·S Dental Materials- Properties and manipulation. 8th edition ´ Clinical aspects of dental materials .Theory, practice and cases. Marcia Gladwin. ´ Applied dental Materials. McCabe