gypsum

1
GYPSUM PRODUCTS

• Presented by
  Guided By
• Ashutosh Pai. Dr.
  Amit Jagtap.

2

• INTRODUCTION
• PRODUCTION OF CALCIUM SULFATE HEMIHYDRATE.
• SETTING OF GYPSUM PRODUCTS.
• TESTS FOR WORKING, SETTING, FINAL SETTING
  TIMES.
• CONTROL OF SETTING TIME.

3

• NORMAL SETTING EXPANSION.
• HYGROSCOPIC SETTING EXPANSION.
• TYPES OF GYPSUM PRODUCTS.
• PROPORTIONING, MIXING CARING FOR GYPSUM
  PRODUCTS.
• SPECIAL GYPSUM PRODUCTS.
• INFECTION CONTROL.

4
INTRODUCTION

• OBTAINED BY -
• As a by product of some chemical operations.
• Mined.
• USES -
• Molds casts.
• Making impressions -gt type I.
• Study models for oral and maxillofacial
  structures.

5
INTRODUCTION CONTD . .

• Important auxiliary materials for dental
  laboratory procedures.
• Plaster fillers (forms of silica) gypsum
  dental investments.
• OTHERS USES -
• Artifacts.
• Widely used in industries.
• Making walls of plaster (false ceiling).

6
PRODUCTION OF -- CaSO4 1/2 H2O

• Calcining process
• Gypsum is ground subjected to
  temperatures of 110C to 130C to drive off part
  of the water of crystallization that is the
  amount of water needed to convert gypsum to its
  hemihydrate form.
• It is done in a vat or a kiln open to air.
• Dry calcination wet calcination.

7
PROCESS CONTD..

• As the temperature is raised the remaining water
  is removed , and products are formed as indicated
  .
• 1100 1300
  1300 2000 2000 10000
  CaSO42H2O CaSO41/2 H2O
  CaSO4 CaSO4

CALCIUM SULFATE HEMIHYDRATE
HEXAGONAL ANHYDRATE
ORTHOROMBIC ANHYDRATE
GYPSUM
8

• Depending on the method of calcination, different
  forms of hemihydrate can be obtained like -
• a- hemihydrate (dental stone).
• ß- hemihydrate (dental plaster).
• a- modified hemihydrate.(made by boiling gypsum
  in 30 aqueous solution of calcium chloride and
  magnesium chloride).It is used primarily for
  dies.

9
WHY a ß DESIGNATIONS ?

• For tradition convenience.
• Difference between a ß -
• Difference in crystal size.
• Surface area.
• Degree of lattice perfection.

10
Difference between a- hemimhydrate and ß-
hemihydrate.

• a hemimhydrate
• Type III , IV V
• Produced by wet calcination.
• Requires less water for mixing.
• Better packing ability.
• Low surface free energy.
• Crystal habit of hexagonal calcium sulfate .
• High apparent density.

• ß hemihydrate
• Type I II
• Produced by dry calcination.
• Requires more water form mixing.
• Less packing ability.
• High surface free energy.
• Crystal habit that of hemihydrate.
• Low apparent density.

11

• DENTAL PLASTER
• (ß HEMIHYDRATE)

• DENTAL STONE
• (a - HEMIHYDRATE)

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SETTING OF GYPSUM PRODUCTS

• The reverse of reaction (1) describes the
  reaction of calcium sulfate hemihydrate powder
  with water to produce gypsum -
• (CaSO4 ) 2.H2O 3H2O 2CaSO4 . 2H2O
  
• 
  unreacted (CaSO4 ) 2 1/2 H2O heat
• The heat evolved in the exothermic reaction is
  equivalent to the heat used originally in
  calcination.
• The hexagonal anhydrite reacts very rapidly,
  whereas orthorombic anhydrite requires more time.

13
SETTING REACTION

• PROPOSED THEORIES
• Colloidal theory
• hemihydrate water --gt colloidal state
  through sol gel mechanism.
• In sol state hemihydrate particles are
  converted to dihydrate and as the measured amount
  of water is consumed the mass converts to a solid
  gel.

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PROPOSED THEORIES CONTD . .

• Hydration theory
• suggests that rehydrated plaster particles
  join together through hydrogen bonding to the
  sulfate groups to form the set material.
• Dissolution precipitation theory
• (most widely accepted theory)
• based on dissolution of plaster and instant
  recrystallization of gypsum, followed by
  interlocking of the crystals to form the set
  solid.

15
PROPOSED THEORIES CONTD . .

• Hemihydrate is 4 times more soluble in water than
  is the dihydrate near room temp (20C). Thus the
  setting reaction can be understood as follows--
• hemihydrate water
• Suspension is formed that is fluid and workable.
• Hemihydrate dissolves until it forms a saturated
  solution.
• This saturated solution supersaturated in
  dihydrate, precipitates out dihydrate.

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PROPOSED THEORIES CONTD . .

• Thus solution is no longer saturated with
  hemihydrate, so it continues to dissolve.
• Dissolution of the hemihydrate and precipitation
  of dihydrate proceeds as new crystals form or
  further growth occurs on the present crystals.
• Reaction continues until no dihydrate
  precipitates.

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W/P RATIO

• The ratio of the water to the hemihydrate powder
  is usually expressed as the W/P ratio, or the
  quotient obtained when the weight (or volume) of
  the water is divided by the weight of the powder.
• W/P ratio ? setting time , strength ,
  setting expansion .
• example if 100g is mixed with 60 ml of
  water, the W/P ration is 0.6.

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W/P RATIO CONTD..

• Some typical recommended ranges
• Type II plaster -- 0.45 to 0.50
• Type III stone -- 0.28 to 0.30
• Type IV stone -- 0.22 to 0.24
• In preweighed bags powder mass variation may be
  2

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Compressive strength as a function of w/p ratio
for five types of gypsum products.

• American National Standard Institute/ American
  Dental Association (ANSI/ADA) Specification No 25
  for dental gypsum products and the strength
  values represent the wet strength at 1 hr.

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STAGES IN SETTING

• Upon mixing there is a continuous aqueous(FLUID)
  phase present exhibiting pseudoplasticity
• As the reaction proceeds the clump of growing
  gypsum crystals interact, the mix becomes
  PLASTIC.
• The glossy surface disappears growing crystals
  thrust apart converting the plastic mass into a
  rigid solid, weak and FRIABLE.
• The relative amount of solid phase increases and
  then this friable mass becomes CARVABLE.

21
TESTS FOR WORKING, SETTING AND FINAL SETTING
TIMES.

• MIXING TIME (MT)
• Time from addition of the powder to the water
  until the mixing is completed.
• mechanical mixing 20 to 30 secs
• hand mixing atleast 1 minute.
• WORKING TIME (WT)
• measured from the start of mixing to the
  point where the consistency is no longer
  acceptable for the products intended purpose.
• Generally, a 3 min working time is adequate.

22
TESTS FOR WT, ST AND FINAL ST CONTD..

• SETTING TIME
• The time that elapses from the beginning of
  the mixing until the material hardens is known as
  the setting time.
• Reaction (2) requires a definite time for
  completion .

23
TESTS FOR SETTING TIME CONTD..

• LOSS OF GLOSS TEST FOR INITIAL SET
• The excess water is taken up in forming the
  dihydrate so that the mix looses its gloss.
• This occurs at approximately 9 mins .
• The mass still has no measurable compressive
  strength.

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TESTS FOR SETTING TIME CONTD..

• INITIAL GILLMORE TEST FOR INITIAL SET
• The smaller needle is most frequently used
  for cements but it is sometimes used for gypsum
  products .
• The mixture is spread out, and the needle is
  lowered onto the surface. The time at which it
  no longer leaves an impression is called the
  initial set , noted as Initial Gillmore.
• It takes place at approx 13 mins.

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TESTS FOR SETTING TIME CONTD..

• VICAT TEST FOR SETTING TIME
• Needle with a weighted plunger rod is
  supported and held just in contact with the mix.
• soon after gloss is lost the plunger is
  released.
• The time elapsed until the needle no longer
  penetrates to the bottom of the mix is known as
  the setting time.

26
A
B AVICAT NEEDLE BEING
USED TO MEASURE THE SETTING TIME IN GYPSUM.B
SET OF GILLMORE NEEDLES.
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TESTS FOR SETTING TIME CONTD..

• GILLMORE TEST FOR FINAL SETTING TIME
• Measured by using the heavier gillmore
  needle.
• The elapsed time at with this needle leaves a
  barely perceptible mark on the surface is called
  the final setting time .

28
COMPRESSIVE STRENGTH OF A TYPE II MODEL PLASTER
DURING SETTING. THE W/P RATIO WAS 0.50 .
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READY-FOR-USE CRITERION

• Subjective measure .
• Ability to judge readiness improves with
  experience.
• The previously mentioned tests help us to
  determine whether the set material is ready to
  use but the test conclusions may vary with
  variations in the W/P ratio , the time of mixing
  and the particular product used.

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CONTROL OF SETTING EXPANSION

• The solubility can be increased or decreased.
• The number of nuclei of crystallization can be
  increased or decreased.
• The setting time can be accelerated or retarded
  by increasing or decreasing the rate of growth of
  crystals.

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CONTROL OF SETTING EXPANSION CONTD . .

• HOW DO WE ACHIEVE THIS ??
• IMPURITIES
• Add gypsum setting time shortened.
• (because of increase
  in potential number of nuclei)
• Orthorombic anyhdrite ? induction period
• Hexagonal anhydrite ? induction period

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CONTROL OF SETTING EXPANSION CONTD . .

• FINENESS
• Finer the particle size of the hemihydrate
  faster the mix sets.
• Particles are ground ? rate of
  dissolution.
• 
  number of nuclei.
• W/P RATIO
• Higher w/p ratio fewer nuclei per unit
  volume longer setting time.
• 
  

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CONTROL OF SETTING EXPANSION CONTD . .

• MIXING
• Faster the mix more rapid is set within
  limit.
• TEMPERATURE
• Little changes occurs between 0 to 50 0 c,
  above 50 0 c ,retardation occurs and
  above 100 0 c
• no reaction takes place. As the reaction is
  reversed hemihydrate are formed .

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CONTROL OF SETTING EXPANSION CONTD . .

• ACCELERATORS
• Sodium Sulphate - 3-4
• Potassium Sulphate - 2-3
• Sodium Chloride - 2
• Gypsum - lt 20
• Potassium sulfate
  ROCHELLE
• potassium sodium tartrate
  SALT
• The acceleration caused by an additive
  depends on the amount and rate of solubility of
  the hemihydrate versus the same effect on the
  dihydrate.

35
CONTROL OF SETTING EXPANSION CONTD . .

• RETARDERS
• Act by forming an adsorbed layer on the
  hemihydrate and on gypsum crystals to reduce its
  solubility.(organic materials)
• forms a layer of calcium salt that is less
  soluble than is the sulfate salt.(salts)
• ORGANIC MATERIALS
• Glue, gelatin and some gums.
• SALTS
• Borax
  1-2
• (sodium tetraborate decahydrate)
  
• sodium chloride 20

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10.7 Compressive strength of a model plaster
plotted against time when accelerators and
retarders are added to the plaster. The gain in
strength is a measure of the rate of hardening or
setting.
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SETTING EXPANSION

• Gypsum product shows linear expansion during the
  setting due to outward thrust of crystals that is
  change from hemihydrate to dihydrate.
• Low 0.06 High 0.5
• CaSO42 H2O 3H2O
  2 CaSO4 2H2O
• Molecular mass 290.284 54.048
  344.322
• Density(g/cm³) 2.75 0.997
  2.32
• Equivalent volume 105.556 54.211
  148.405
• Total volume 159.767
  148.405

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SETTING EXPANSION CONTD . .

• Net change in volume is
• (148.405 159.767) 100 -7.11
• 159.767
• CRYSTALLIZATION MECHANISM
• crystals grow ? outward thrust or stress
  develops ? expansion of the entire mass .
• Practically the product is greater in external
  volume but less in crystalline volume.

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HYGROSCOPIC SETTING EXPANSION
40

• TYPES OF GYPSUM
• IT IS OF THE FOLLOWING TYPES
• Impression plaster
  TYPE I
• Model plaster.
  TYPE II
• Dental stone.
  TYPE III
• Dental Stone , high strength . TYPE
  IV
• Dental stone, high strength TYPE
  V
• High expansion.

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IMPRESSION PLASTER TYPE I

• Composed of Plaster Of Paris.(ß- hemihydrate)
• Rarely used now a days for making impressions
  (replaced by hydrocolloids elastomers).
• USES
• Making final impression in constructing complete
  dentures.
• Bite registration material.
• For making impression impressions maxillofacial
  prosthesis.

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MODEL PLASTERTYPE II

• It is called model plaster.(ß- hemihydrate)
• It is less strength and requires more water of
  mixing
• Its compressive strength is as low as 9 MPa and
  tensile strength of 0.6 MPa.
• Uses
• For making study casts models.
• For mounting .
• For flasking.

43
DENTAL STONE TYPE III

• In 1930, a researcher at U.S Gypsum Corporation
  learned that the plaster mold used for forming
  rubber denture bases in a vulcanizer under steam
  pressure became unusually hard over night.
• gypsum had calcined under pressure formed a
  much better quality of crystallized calcium
  sulfate hemihydrate.

44
DENTAL STONE TYPE III CONTD . .

• The product was soon patented as a gypsum.
• Since this discovery the process has been
  performed in an auto clave.
• It has a minimum 1 hr compressive strength of
  20.7 MPa .
• The setting time varies from 30 to 60 mins.
• USES
• For making master casts.

45
DENTAL STONE, HIGH STRENGTH TYPE IV

• It is also called a DENSITE, IMPROVED STONE,
  DIE STONE OR CRYSTACAL .
• The surface dries more rapidly , the surface
  hardness increases more rapidly than does the
  compressive strength .
• This is a real advantage in that the surface
  resists abrasion , whereas the core of the die is
  tough and less subject to accidental breakage.
• It is mainly used as a die stone material.

46
DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION (TYPE
V)

• Exhibits higher compressive strength than type IV
  .
• The setting expansion has been increased from
  0.10 to 0.30
• Rationale
• Certain newer base metal alloys exhibit
  greater casting shrinkage .
• Thus higher expansion is required in the
  stone used for the die in order to compensate for
  the alloy solidification shrinkage .

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TYPICAL PROPERTIES AS RECOMMENDED BY ADA/ANSI
SPECIFICATION NO. 25
48
OTHER TYPES OF GYPSUM

• SYNTHETIC GYPSUM
• Formed as a by product of chemical reaction
  involving phosphoric acid.
• they have better packing ability than the
  hemihydrate but are expensive .
• Very few have succeeded.

49
OTHER TYPES OF GYPSUM CONTD . .

• ORTHODONTIC PLASTER (WHITE STONE)
• Used for making orthodontic models.
• Its wet strength is 25 Mpa at 1 hr dry
  strength is 45 Mpa.
• Its workig time is 7 9 mins sets in about
  14 mins.
• W/P ratio recommended is 37ml/100g.
• setting expansion is 0.20

50
PROPERTIONING MIXING CARINGFOR GYPSUM

• MIXING
• W/P ratio should be optimum.
• Preffered method for mixing is to add the
  measured water first followed by gradual addition
  of the preweighed water. This should be followed
  by approximately 15 sec of hand mixing followed
  by 20 to 30 sec of mechanical mixing under
  vacuum. Guess work should NOT be done!

51
SECTION THROUGH A CAST OF SET STONE THAT WAS
IMPROPERLY PROPORTIONED MIXED
SECTION THROUGH A CAST OF SET STONE THAT WAS
PROPERLY PROPORTIONED MIXED.
52

• DIFFERENT INSTRUMENTS USED FOR MIXING.

53
PROPERTIONING MIXING CARINGFOR GYPSUM CONTD . .

• CARING OF THE CAST
• The gypsum casts are slightly soluble in water .
  If the cast is immersed in running water its
  linear dimension may decrease approximately 0.1
  for every 20 mins. The safest method is to
  immerse it in a solution containing saturated
  solution of calcium sulfate.
• If the storage temperature is raised to around
  100 C shrinkage might occur.

54
PROPERTIONING MIXING CARINGFOR GYPSUM CONTD . .

• CARING OF GYPSUM PRODUCTS
• Hemihydrate takes up water from the air readily.
• If the relative humidity exceeds 70 the plaster
  takes up sufficient water vapour to start the
  reaction .
• This produces a few particles of gypsum on the
  hemihydrate .
• These crystals act as nuclei for crystallization
  and thus there is a decrease in setting time .
• Use of sealed containers is abvisable.
• This can be compensated by increasing the mixing
  time.

55
MICROSTRUCTURE OF CAST GYPSUM

• Set material contains acicular shaped gypsum
  crystals with length ranging from 5 to 20 µm.
• TWO distinct types of porosities are seen
• Due to residual unreacted water. These are
  roughly spherical and occur between clumps of
  crystals. These porosities will be seen in high
  W/P ratio.
• Due to growth of gypsum crystals. These are
  angular spaces and occur between individual
  crystals during their growth. These will be seen
  in low W/P ratio.

56

• A SHOWS POROSITIES DUE TO HIGH W/P RATIO.
• B SHOWS PORPSITIES DUE TO OPTIMUN W/P RATIO.

57
DISINFECTION OF GYPSUM CASTS

• To prevent cross infection.
• The addition of disinfectants to the water used
  for mixing 5 phenol and 2 glutaraldehyde,
  have been proven to be effective and did not
  change the properties of the set material.
• Alternatively, casts and dies may be treated by
  imersion in a disinfecting solution after each
  clinical stage.
• Autoclave sterilization is also possible but it
  should be done under carefully controlled
  conditions.

58
ACCURACY AND POURABILITY OF GYPSUM PREWEIGHED
PACKAGES Journal of Prosthodontics, Volume
10, Number 2, June 2001 , pp. 86-90(5)

• PURPOSE
• The purpose of this study was to determine the
  amount of gypsum provided by the manufacturers in
  their preweighed packages and to determine the
  amount of gypsum dispensed from the packages as a
  function of the method used.
• CONCLUSION
• Within the limits of this study, results
  indicate that the use of a thorough method to
  remove gypsum from preweighed packages was
  important. Ideally, gypsum supplied in preweighed
  packages should be weighed initially to ensure an
  optimal water/powder ratio.

59
SURFACE DETAIL, COMPRESSIVE STRENGTH, AND
DIMENSIONAL ACCURACY OF GYPSUM CASTS AFTER
REPEATED IMMERSION IN HYPOCHLORITE
SOLUTIONDepartment of Prosthetic Dental
Sciences, College of Dentistry, King Saud
University, Kingdom of Saudi Arabia. 8 June 2006

• PURPOSE
• The purpose of this study was to evaluate the
  changes in surface detail quality, dimensional
  accuracy, and wet compressive strength of dental
  casts as a result of repeated disinfection in
  slurry containing 0.525 sodium hypochlorite
  solution.

60
SURFACE DETAIL, COMPRESSIVE STRENGTH, AND
DIMENSIONAL ACCURACY OF GYPSUM CASTS AFTER
REPEATED IMMERSION IN HYPOCHLORITE SOLUTION CONTD
. .

• MATERIAL AND METHODS
• Two different test specimen configurations
  were used, one for evaluating surface detail
  quality and dimensional accuracy and the other
  for compressive strength. Sixty specimens each
  were made of type III and type IV dental stone
  (Excalibur). Thirty specimens were immersed in
  slurry, a supernatant solution of calcium sulfate
  in distilled water (control casts), and 30 in
  slurry with sodium hypochlorite (test casts)
  solutions for 30 minutes and air dried for 24
  hours. This process was repeated 7 times before
  testing. Linear dimensional change, surface
  detail quality, and wet compressive strength were
  determined according to American National
  Standards Institute/American Dental Association
  (ANSI/ADA) standards. The data were analyzed
  using 2-way analysis of variance and 2-sample
  independent t tests (a.05).

61
SURFACE DETAIL, COMPRESSIVE STRENGTH, AND
DIMENSIONAL ACCURACY OF GYPSUM CASTS AFTER
REPEATED IMMERSION IN HYPOCHLORITE SOLUTION CONTD
. .

• RESULTS
• For specimens prepared with type III and
  type IV stone, treatment by immersion in slurry
  (control casts) and in slurry with sodium
  hypochlorite (test casts) resulted in an increase
  (Plt.0001) in linear dimensional change. The type
  IV casts treated with slurry with sodium
  hypochlorite showed significantly less (Plt.0001)
  mean linear dimensional change (0.025) compared
  with type III stone casts (0.063), and the
  values remained within the ANSI/ADA specified
  standards. Both test solutions caused some degree
  of erosion or damage to the surface quality of
  casts made from type III and type IV stones.
  However, the difference between the control and
  test casts was not significant. Slurry with
  0.525 sodium hypochlorite solution decreased the
  compressive strength of both types of stones
  significantly (Plt.001) compared with distilled
  water slurry. However, the values remained close
  to ANSI/ADA standards.

62
SURFACE DETAIL, COMPRESSIVE STRENGTH, AND
DIMENSIONAL ACCURACY OF GYPSUM CASTS AFTER
REPEATED IMMERSION IN HYPOCHLORITE SOLUTION CONTD
. .

• CONCLUSION
• The results demonstrated that repeated
  immersion of type III and type IV stone specimens
  in slurry with distilled water and slurry with
  0.525 sodium hypochlorite, along with drying in
  air, caused a significant increase in linear
  dimension and a significant decrease in wet
  compressive strength. Although both the solutions
  caused some degree of damage to surface details
  for type III and type IV stones, the difference
  was not significant.

63
INFLUENCE OF SURFACE HARDENER ON GYPSUM ABRASION
RESISTANCE AND WATER SORPTIONJPD, VOLUME 90,
ISSUE 5, (NOVEMBER 2003)

• PURPOSE
• This study evaluated abrasion resistance and
  water sorption with 4 commonly used gypsum die
  materials with and without the application of
  surface die hardeners.

64
INFLUENCE OF SURFACE HARDENER ON GYPSUM ABRASION
RESISTANCE AND WATER SORPTION CONTD . .

• MATERIAL AND METHODS
• Three ADA Type IV (Vel-Mix, ResinRock, and
  Silky-Rock) and 1 Type V die material (Die-Keen)
  were evaluated for abrasion resistance after
  application of 2 surface hardeners (Permabond 910
  cyanoacrylate and Clear Coat). Thirty specimens
  of each material were fabricated using an
  impression of a standard brass die machined with
  1-mm high ridges, sloped 45 degrees. Gypsum
  materials were mixed according to manufacturers'
  recommendations and allowed to set 1 hour before
  separating. All replicated dies were allowed to
  bench set for 14 days before testing. One hour
  before testing, specimens were arbitrarily
  assigned to 1 of 3 treatment subgroups
  (n10/group) no treatment (control), coated with
  Permabond 910, or coated with Clear Coat. In the
  coated groups, die hardener was painted over the
  grooves and air dried. Abrasion resistance
  (measured by weight loss) was evaluated using a
  reciprocal abrasion device in which a stylus
  applied a 50-g mass perpendicular to the ridges.
  Mass loss was determined using an analytical
  balance before and after each test cycle. Five
  sets of 20 unidirectional passes were made on
  each specimen. A scanning electron microscope was
  used to evaluate the surface of specimens in each
  treatment subgroup. Water sorption was also
  evaluated using 2 Type IV (Silky-Rock, ResinRock)
  and 1 Type III (Microstone) gypsum materials.
  Specimen dies were separated 1 hour after pouring
  the impression and allowed to bench set 1 week
  before testing. Five specimens from each material
  group received a coating of a surface hardener 1
  hour before testing. Specimens were placed in
  distilled water for 15 minutes and differences in
  mass were determined using an analytical balance
  before and after each test. A 2-way analysis of
  variance was completed followed by a Tukey post
  hoc test (a.05)

65
INFLUENCE OF SURFACE HARDENER ON GYPSUM ABRASION
RESISTANCE AND WATER SORPTION CONTD . .

• RESULTS
• The 2-way analysis of variance revealed an
  interaction between product and surface coating
  (P.0459). Given this interaction, the 12
  combinations determined by surface treatment and
  material type were considered individually using
  the Tukey method. Vel-Mix, control (2.62 2.64
  mg) had the most material loss and Vel-Mix, Clear
  Coat (0.48 0.29 mg) had the least material
  loss. Water sorption results indicated an
  interaction between the gypsum material and the
  surface treatment (Plt.0001). The control groups
  of Microstone (299.2 49.6 mg) and Silky-Rock
  (159.0 8.5 mg) showed the most water sorption
  compared with the other treatment groups.
• CONCLUSIONS
• This study demonstrated that a significant
  improvement in abrasion resistance occurred only
  with specific gypsum/surface hardener material
  combinations. Also, water sorption decreased
  significantly for Microstone and Silky-Rock
  gypsum materials when a surface hardener was
  used.

66
REFRENCES

• PHILLIPS SCIENCE OF DENTAL MATERIALS
• 11th EDITION -- ANUSAVICE.
• DENTAL MATERIALS AND THEIR SELECTION 3RD EDITION
  -- WILLIAM J. OBRIEN.
• RESTORATIVE DENTAL MATERIALS 12TH EDITION --
  GRAIGS.
• JOURNAL REFRENCES.

67
DISCUSSION ..

• X-ray diffraction data suggests that hemihydrate
  particles remain in set product and estimates of
  this data indicates 50 gypsum present in type IV
  and V stones, about 60 in type II die materials
  and more than 90 in plaster . This results
  demonstrate higher concentrations of gypsum in
  the weaker set material.
• ?

68
DISCUSSION CONTD ..

• The acceleration caused by an additive depends on
  the amount and rate of solubility of the
  hemihydrate versus the same effect on the
  dihydrate.
• 
  
• ?

69
THANK YOU ?