- Email: [email protected]
Freeze-drying and scanning electron microscopy of setting dental gypsum
Dent Mater
11:226-230,
July, 1995
Freeze-drying and scanning electron microscopy of setting dental gypsum Mark M. Winklerl, Peter Monaghan”, Jeremy L. GilberF, Eugene P. Lautenschlager” ‘Indiana
Universdy School of Dentistry, Department of Dental Mnkrruk lr~tl~ur~c~puoi~s. i,ldrtrttrfI Sii 2Northwestern University, Dir?ision of’Biological Matclrinl,s. (‘hiwg,ro. Illirms, 1 ‘,V,-\
ABSTRACT Objectives. The initial and final forms of reactive gypsum products have been photomicrographed previously. However, the purpose of this project was to document the microscopic morphology of setting dental stone at various stages during the reaction. Methods. Two dental products, a conventional (Type IV) die stone and a fast-setting (Type Ill) stone, were investigated. At selected times ranging from 1 min to 24 h after mechanically mixing the stone under vacuum, the conversion of calcium sulfate hemihydrate to a dihydrate was suspended by immersion into liquid nitrogen. Water was immediately removed by freeze-drying the specimen to prevent any further reaction so that the specimen could be returned to room temperature for examination in a scanning electron microscope (SEM). Results. Crystal formation appeared to be nearly complete at the 20 min interval for the die stone and at the 10 min interval for the fast-setting dental stone. Transitions noted during these times include the nucleation and growth of small needle-like crystals on and near the larger prismatic-shaped hemihydrate crystals, the concurrent decrease in size and number of the hemihydrate crystals. and the progressive entanglement of the growing dihydrate crystals. Significance. The two-step process of suspending the reaction. then freeze-drying the specimen made it possible to observe and document the intermediate stages during crystal growth of dental stone. These observationsshould be helpful in understanding the structural dynamics of crystal growth during the setting of gypsum dental products. This procedure should be applicable to the study of other water-based dental materials.
INTRODUCTION Gypsum products have been used in dentistry to make impressions of edentulous arches, rigid models and dies, and molds for forming and processing resin bases fol removable prostheses. Also, gypsum products have been used to join dental casts to articulators, and as binders fol various refractory dental investment materials. The exothermic reaction between calcium sulfate hemihydrate 226 Winkler et a/./Electron
mlcroscopy
of dentalgypsum
and water is shown below\ 111the equation I C:tmg:. 193x9; Phillips, 1991: Phillips and Moore, 1994) (‘&\(I, . !.1 t/ ,o 1~1 ‘I II >o--c ‘il\O, . !Il,O calcium sulfattt hemihydratcI dental ~tonca powder
1
<‘)C )I1_Ii:
,!l, ,‘l ‘,<
~~iil~~lUll1 +c\~atr‘l'Lsulf'atc
f
l~~iil
tJihvdrat.c i g”! pun1
’
In 1887, Le Chatelier ascribed the thtaork fill’ the selt~ng of dental stones as a continuing process of dissolution oft hck more soluble hemihydrate particles and concurrent patcipitation ofthe less soluble dihydrate particles which form the growing crystals (Werner, 1942; Docking. 1965 1. Mahler and Asgarzadeh I 1953 I attributed the discrep ancy between the true volumetric contraction during setting and the subsequent volumetric ctxpansion to tht, formation of pores within the stone. I,autenschlagcr n~lc! Corbin i 1969 I found an exponential relationship hetwrclr: micropores (pores not caused by loss of cxces~ \vatcri ;nlrl the amount of expansion. They dcmonstratc~d that thc~ volume increased from 0.0357 cm~‘!cm : of st.ontsto 0.0965 cm:‘/cm” of stone when thfi watcr/powdcr ratio decreased from 0.60 to 0.25. They hypothesized that thts collision 01 the growing crystals which occurs morr ti-equentl! ‘11 lower water/power ratios resulted in greater init,ial setting expansion. The conversion of’calclum sulfate hermhydraL!l t tierltai &one powder) plus water to calcium sulfate dihvdratc t set dental gypsum) has been characterized in v;&ous ~vays: heats of reaction (Craig, 1989 ), change:: m crystal strut ture (Lautenschlager and Corbin, 1969). and reaction rates (Harcourt and Lautenschlager. 1970 I Hot,h t hca initial hemihydrate and final dihydrate forms of’ thib reaction have been photomicrographed prevIousI> 1Phillips
G 0
300
L?! 2 5 200
31
II
x
E :
-73
ICE
I
‘1
100
,-I73
2 ,, ,,,
,, 0.01
0.10
1.00
,, ,f
,, ,ut
10.00
100.00
,,,, j-273 1000.00
Pressure (kPa) Arrowsandcorrespondingnumbers Fig. 7. Phasediagramforwater. indicate path and sequence for freeze-drying process(Atkins, 1986).
and Ito, 1952; Craig, 1989; Phillips, 1991; Phillips and Moore, 1994 1. The purpose of this project was to attempt to visually examine the intermediate stages in the formation of calcium sulfate dihydrate from the hemihydrate form. A freeze-drying technique was used to halt the reaction and remove the water so that the partially set dental stone could be observed and documented.
MATERIALS & METHODS Two dental gypsum products were examined. A common regular-setting die stone (Type IV), Vel-Mix (Kerr Manufacturing Co., Romulus, MI, USA, Batch No. 03148930721, and a new fast-setting dental stone (Type III), Snap-Stone Whip Mix Corp., Louisville, KY, USA), were selected for comparison. Samples of each of these products were fabricated in the following manner. A small quantity ( 100 grams) of stone was incorporated into 24 mL of tap water as recommended by the manufacturer. After ensuring that the calcium sulfate hemihydrate was completely wetted, the dental stone and water slurry were mechanically mixed under vacuum (Vat-U-Spat, Whip Mix Corp.) for 10 s. Portions of this mixed mass were vibrated into 10 identical sample holders, one for each of the selected observation times. Two mixes, one for the first five observation times ( 1, 2: 3, 4, and 5 min) and one for the second five observation times ( 10,20,40, and 60 min, and 24 h), had to be used. Each sample holder was a threaded metal nut (4.2 mm inside diameter and 3.2 mm thick) placed on a piece of aluminum foil. This small holder size was chosen to enhance instantaneous cooling. At selected times ranging from 1 min to 1 d after the incorporation of the powder into water, the conversion of hemihydrate to dihydrate was stopped by immersing the specimen into liquid nitrogen. Specimens were retained in the liquid nitrogen bath for at least 10 min. When removed from the liquid nitrogen bath (-196”C), the samples were immediately transferred to a vacuum chamber (-133 Pa) equipped with a liquid nitrogen-cooled metal heat sink to keep the samples cold during the 2 h evacuation to remove all the water. The freeze-dried samples were then returned to room temperature (-22°C) and pressure
c-1OlkPa) (see steps in Fig. 1) and stored in a desiccator until examined with a scanning electron microscope (SEMI ( S120, Cambridge Instruments Ltd., Cambridge, England, UK). To serve as controls, one of the samples was allowed to set for 24 h and not freeze-dried, and another sample was prepared with the unreacted calcium sulfate hemihydrate. Only the air-exposed surfaces, or the top surfaces, of the samples were examined. To compare these airexposed surfaces with surfaces exposed to an impression material, one portion of the mixed stone was vibrated into a mold (6.35 mm in diameter by 12.7 mrn depth) made of vinyl polysiloxane impression (Mirror 3 Extrude, Kerr Manufacturing Co. 1. In preparation for the SEM observation, samples of dental die stone were sputter-coated with gold and then examined using the backscatter mode. The operating conditions for the SEM were 30 kV and a working distance of 16 mm to 17 mm. Micrographs were taken at a magnification of about 3000x. The SEM photomicrographs of all the specimens were visually compared. At each observation time, the microstructure of the setting stone, including the size, shape, location, and number of particles, was noted. Any differences between adjacent time periods were recorded. For each type of stone, the earliest photomicrograph which was not distinguishable from the 24-h photomicrograph was selected to represent completion of the crystal formation.
RESULTS Conuentional Die Stone (Type IV). Photomicrographs of the intermediate and final stages of setting stone (Type IV) are presented in Figs. 2 and 3. Before water was added, the dry calcium sulfate hemihydrate powder substantially consisted of angular and irregularly-sh.aped fragmented particles that ranged in size from about 1 to 25 pm; the observable surfaces of these particles were relatively smooth. (Fig. 2A). At 1 min, the reaction of water with the hemihydrate crystals results in the nucleation of numerous particles less than 1 pm in size (Fig. 2B). These newly forming particles appear to nucleate on or very near the original powder particles. From approximately 1 min to 5 min (as seen in Figs. 2B and 2C), these newly formed particles appear to grow in size. By 10 min (Fig. 2D), these particles have grown sufficiently large to vaguely resemble t.he familiar crystal pattern of set gypsum. Very little difference can be discerned between the microstructures at 20 min (Fig. 2E), at 60 min, and at 24 h (Fig. 2F) for the freeze-dried, airexposed gypsum surfaces. The large original particles are still visible at 10 min (Fig. 2~. They have almost entirely disappeared by 60 min, while the growing dihydrate crystals have interdigitated with one another in a mesh. Besides the usual elongated prismatic crystals, platty crystals can also be seen. The appearance of a 24-h sample which was not freezedried but was exposed to air is similar to the freeze-dried specimen in Fig. 2F. In contrast, a 24-h sample (Fig. 3) which was allowed to set against a vinyl polysiloxane
Denfd Materials/July
1995
227
Fig. 2. Scanning electron micrograph at 3000x of setting Type IV dental stone (Vel-Mix) at various times after mixing (air-exposed surface). (A) 0 min (no water was added to powder), (B) 1 min, (C) 3 min, (D) 10 min, (E) 20 min, (F) 24 h. Note: Arrows labeled PL indicate platty crystals while those designated PR indicate prismatic crystals (Fig. 2F).
impression material without freeze-drying displays the usual elongated prismatic crystals with little evidence of platty crystals. Fast-Setting Dental Stone (Type III). Photomicrographs of the fast-setting stone powder reveal a different 228 Wfnkler et a/./Electron mlcroscopy of dental gypsum
microstructure. At 0 min {Fig. 4A) before any water 1;s added, the largest particles range in size only from 1 to 4 pm. In addition, there are many smaller particles of less than 1 pm intermixed among the larger ones. The reaction of water with the hemihydrate cryst,als results in the
nucleation of needle-shaped crystals which are approximately 5 urn in length and less than 0.5 urn in width at 1 min after mixing (Fig. 4B). These newly-forming particles also appear to nucleate on or very near the original powder particles. By 3 min (Fig. 40, the original particles are no longer visible, indicating that the dissolution of the hemihydrate was complete. From approximately 1 min to 5 min, the newly formed particles appear to grow in size. At 5 min, the needle-shaped crystals have increased in size, mainly in width, and the original hemihydrate particles can no longer be observed. By 10 min (Fig. 4D!, the width of the needle-shaped crystals ranges from 1 to 3 urn. The branching among the particles appears to have increased from 1 to 10 min. Since no obvious change in structure is apparent in the aging process between 10 min (Fig. 4D) and 1 d, this fast-setting stone appears to be fully set by 10 min.
DISCUSSION Fig.3. Scanning electron micrograph at 3000x of setting Type IV dental stone (Vel-Mix) at 24 h after mixing (impression-exposed surface).
Freeze-drying (i.e., lypholyzation) essentially stopped the reaction between calcium sulfate hemihydrate and water
Fig. 4. Scanning electron micrographat 3000xof setting Type Ill dental stone (Snap-Stone)at (A) 0 min (no water was added to the powder), (B) 1 min, (C) 3 min, (D) 10 min.
various times after mixing (air-exposed
surface).
Denkrl Marferials/July I995
229
as evidenced by the transformation in crystal structure which is delineated in Figs. 2-4. Additional efficacy of the freeze-drying process was demonstrated by the difference in crystal structure among the samples even though all of the samples were photographed in the SEM at least 24 h after mixing. Basically, this technique could be described as thermally stopping the reaction and then removing the water from the setting mass which prevents any further reaction when the partially set stone is returned to room temperature. This process allowed scanning electron microscopic evaluations of the setting dental die stone at various stages. From the photomicrographs, it was seen that small crystals nucleate near, if not on, the original particles of calcium sulfate hemihydrate. Thus, this nucleation appeared to be predominantly heterogeneous. Prism-shaped crystals were observed by 10 min after the start of mixing for the regular-setting die stone but by only 1 min for the faster-setting stone. The quicker setting of the fast-setting dental stone is due at least in part to the smaller initial particle size which contributes to a higher surface areal volume ratio that results in faster dissolution (and hence. quicker supersaturation of the aqueous solution) of the hemihydrate phase. The growth of these crystals appeared to be proportional to the gradual disappearance of the larger hemihydrate particles. The final set (24 hi appearance of the regular-setting set die stone was similar to that found at 20 min, not at 60 min as would be expected for the final set found from x-ray measurements of crystal formation (Harcourt and Lautenschlager, 1970). The only visual difference between a mix at 20 min and one at 60 min is a slightly higher density of crystals at 60 min. In contrast, the crystal structure of the fast-setting crystal stone at 24 h was indistinguishable from that at 10 min. In conclusion, a freeze-drying technique was successfully employed to halt the setting reaction for two dental stones so that scanning electron microscopy could be used to visually evaluate the various stages during the transformation of calcium sulfate hemihydrate into calcium sulfate dihydrate.
230 Winkler et a/./Electron microscopy of dental gypsum
ACKNOWLEDGMENTS This research was supported in part by grant NIDR T32 DE07042 from the National Institute of Dental Research, Bethesda, MD 20892.
REFERENCES Atkins PW 11986). Physical (:hemlstrh l’hlt-d txd. NC~M York: W. H. Freeman and Company. I:%-146 Craig RG i 1989 ). Restorative Dental Materials. 8th c1c-l. ;ir Louis: The C. V. Moshy Company. ::4i-35~ Docking AR 11965 1.Gypsum research III .1uit 1’;+lia I’tit. setting process. Int Dent ,I 15:372. Harcourt JK, Lautenschlager EP (197!) I. Accelerated and retarded dental plaster setting investigat,cd by x-rn;~ diffraction. ,I Dent Res 49(3):502. Lautenschlager EP, Corbin F ( 1969). lnvc~stlgatlon oi t htexpansion of dental stone. J Dent Res 4812 1:206-2 10 Mahler DB, Asgarzadeh K ( 1953). Th e volumetric contraction of dental gypsum materials on sett,ing. *JlZQt?tRr~,c 32(3):354-361. Phillips RW ( 1991). Skinner-s Science ot’Dcnt,ai Maten& 9th ed. Philadelphia: W. B. Saunders C’ompany, 69-92 Phillips RW, Ito BY c1952). Factors afFecting the surface of stone dies poured in hydrocolloidimpressions..Ji+o.sth(~! Dent 2(3 ):390. Phillips RW$ Moore BK ( 1994 j. Elementh $,t‘ i lental Ma& rials for Dental Hygienists and Assistants. 5th ed Philadelphia: W. B. Saunders Company-. 40-56. Worner HK C1942 1.Dental plasters, A~sl .I I)o~// 46( 2 ~:i5.
11:226-230,
July, 1995
Freeze-drying and scanning electron microscopy of setting dental gypsum Mark M. Winklerl, Peter Monaghan”, Jeremy L. GilberF, Eugene P. Lautenschlager” ‘Indiana
Universdy School of Dentistry, Department of Dental Mnkrruk lr~tl~ur~c~puoi~s. i,ldrtrttrfI Sii 2Northwestern University, Dir?ision of’Biological Matclrinl,s. (‘hiwg,ro. Illirms, 1 ‘,V,-\
ABSTRACT Objectives. The initial and final forms of reactive gypsum products have been photomicrographed previously. However, the purpose of this project was to document the microscopic morphology of setting dental stone at various stages during the reaction. Methods. Two dental products, a conventional (Type IV) die stone and a fast-setting (Type Ill) stone, were investigated. At selected times ranging from 1 min to 24 h after mechanically mixing the stone under vacuum, the conversion of calcium sulfate hemihydrate to a dihydrate was suspended by immersion into liquid nitrogen. Water was immediately removed by freeze-drying the specimen to prevent any further reaction so that the specimen could be returned to room temperature for examination in a scanning electron microscope (SEM). Results. Crystal formation appeared to be nearly complete at the 20 min interval for the die stone and at the 10 min interval for the fast-setting dental stone. Transitions noted during these times include the nucleation and growth of small needle-like crystals on and near the larger prismatic-shaped hemihydrate crystals, the concurrent decrease in size and number of the hemihydrate crystals. and the progressive entanglement of the growing dihydrate crystals. Significance. The two-step process of suspending the reaction. then freeze-drying the specimen made it possible to observe and document the intermediate stages during crystal growth of dental stone. These observationsshould be helpful in understanding the structural dynamics of crystal growth during the setting of gypsum dental products. This procedure should be applicable to the study of other water-based dental materials.
INTRODUCTION Gypsum products have been used in dentistry to make impressions of edentulous arches, rigid models and dies, and molds for forming and processing resin bases fol removable prostheses. Also, gypsum products have been used to join dental casts to articulators, and as binders fol various refractory dental investment materials. The exothermic reaction between calcium sulfate hemihydrate 226 Winkler et a/./Electron
mlcroscopy
of dentalgypsum
and water is shown below\ 111the equation I C:tmg:. 193x9; Phillips, 1991: Phillips and Moore, 1994) (‘&\(I, . !.1 t/ ,o 1~1 ‘I II >o--c ‘il\O, . !Il,O calcium sulfattt hemihydratcI dental ~tonca powder
1
<‘)C )I1_Ii:
,!l, ,‘l ‘,<
~~iil~~lUll1 +c\~atr‘l'Lsulf'atc
f
l~~iil
tJihvdrat.c i g”! pun1
’
In 1887, Le Chatelier ascribed the thtaork fill’ the selt~ng of dental stones as a continuing process of dissolution oft hck more soluble hemihydrate particles and concurrent patcipitation ofthe less soluble dihydrate particles which form the growing crystals (Werner, 1942; Docking. 1965 1. Mahler and Asgarzadeh I 1953 I attributed the discrep ancy between the true volumetric contraction during setting and the subsequent volumetric ctxpansion to tht, formation of pores within the stone. I,autenschlagcr n~lc! Corbin i 1969 I found an exponential relationship hetwrclr: micropores (pores not caused by loss of cxces~ \vatcri ;nlrl the amount of expansion. They dcmonstratc~d that thc~ volume increased from 0.0357 cm~‘!cm : of st.ontsto 0.0965 cm:‘/cm” of stone when thfi watcr/powdcr ratio decreased from 0.60 to 0.25. They hypothesized that thts collision 01 the growing crystals which occurs morr ti-equentl! ‘11 lower water/power ratios resulted in greater init,ial setting expansion. The conversion of’calclum sulfate hermhydraL!l t tierltai &one powder) plus water to calcium sulfate dihvdratc t set dental gypsum) has been characterized in v;&ous ~vays: heats of reaction (Craig, 1989 ), change:: m crystal strut ture (Lautenschlager and Corbin, 1969). and reaction rates (Harcourt and Lautenschlager. 1970 I Hot,h t hca initial hemihydrate and final dihydrate forms of’ thib reaction have been photomicrographed prevIousI> 1Phillips
G 0
300
L?! 2 5 200
31
II
x
E :
-73
ICE
I
‘1
100
,-I73
2 ,, ,,,
,, 0.01
0.10
1.00
,, ,f
,, ,ut
10.00
100.00
,,,, j-273 1000.00
Pressure (kPa) Arrowsandcorrespondingnumbers Fig. 7. Phasediagramforwater. indicate path and sequence for freeze-drying process(Atkins, 1986).
and Ito, 1952; Craig, 1989; Phillips, 1991; Phillips and Moore, 1994 1. The purpose of this project was to attempt to visually examine the intermediate stages in the formation of calcium sulfate dihydrate from the hemihydrate form. A freeze-drying technique was used to halt the reaction and remove the water so that the partially set dental stone could be observed and documented.
MATERIALS & METHODS Two dental gypsum products were examined. A common regular-setting die stone (Type IV), Vel-Mix (Kerr Manufacturing Co., Romulus, MI, USA, Batch No. 03148930721, and a new fast-setting dental stone (Type III), Snap-Stone Whip Mix Corp., Louisville, KY, USA), were selected for comparison. Samples of each of these products were fabricated in the following manner. A small quantity ( 100 grams) of stone was incorporated into 24 mL of tap water as recommended by the manufacturer. After ensuring that the calcium sulfate hemihydrate was completely wetted, the dental stone and water slurry were mechanically mixed under vacuum (Vat-U-Spat, Whip Mix Corp.) for 10 s. Portions of this mixed mass were vibrated into 10 identical sample holders, one for each of the selected observation times. Two mixes, one for the first five observation times ( 1, 2: 3, 4, and 5 min) and one for the second five observation times ( 10,20,40, and 60 min, and 24 h), had to be used. Each sample holder was a threaded metal nut (4.2 mm inside diameter and 3.2 mm thick) placed on a piece of aluminum foil. This small holder size was chosen to enhance instantaneous cooling. At selected times ranging from 1 min to 1 d after the incorporation of the powder into water, the conversion of hemihydrate to dihydrate was stopped by immersing the specimen into liquid nitrogen. Specimens were retained in the liquid nitrogen bath for at least 10 min. When removed from the liquid nitrogen bath (-196”C), the samples were immediately transferred to a vacuum chamber (-133 Pa) equipped with a liquid nitrogen-cooled metal heat sink to keep the samples cold during the 2 h evacuation to remove all the water. The freeze-dried samples were then returned to room temperature (-22°C) and pressure
c-1OlkPa) (see steps in Fig. 1) and stored in a desiccator until examined with a scanning electron microscope (SEMI ( S120, Cambridge Instruments Ltd., Cambridge, England, UK). To serve as controls, one of the samples was allowed to set for 24 h and not freeze-dried, and another sample was prepared with the unreacted calcium sulfate hemihydrate. Only the air-exposed surfaces, or the top surfaces, of the samples were examined. To compare these airexposed surfaces with surfaces exposed to an impression material, one portion of the mixed stone was vibrated into a mold (6.35 mm in diameter by 12.7 mrn depth) made of vinyl polysiloxane impression (Mirror 3 Extrude, Kerr Manufacturing Co. 1. In preparation for the SEM observation, samples of dental die stone were sputter-coated with gold and then examined using the backscatter mode. The operating conditions for the SEM were 30 kV and a working distance of 16 mm to 17 mm. Micrographs were taken at a magnification of about 3000x. The SEM photomicrographs of all the specimens were visually compared. At each observation time, the microstructure of the setting stone, including the size, shape, location, and number of particles, was noted. Any differences between adjacent time periods were recorded. For each type of stone, the earliest photomicrograph which was not distinguishable from the 24-h photomicrograph was selected to represent completion of the crystal formation.
RESULTS Conuentional Die Stone (Type IV). Photomicrographs of the intermediate and final stages of setting stone (Type IV) are presented in Figs. 2 and 3. Before water was added, the dry calcium sulfate hemihydrate powder substantially consisted of angular and irregularly-sh.aped fragmented particles that ranged in size from about 1 to 25 pm; the observable surfaces of these particles were relatively smooth. (Fig. 2A). At 1 min, the reaction of water with the hemihydrate crystals results in the nucleation of numerous particles less than 1 pm in size (Fig. 2B). These newly forming particles appear to nucleate on or very near the original powder particles. From approximately 1 min to 5 min (as seen in Figs. 2B and 2C), these newly formed particles appear to grow in size. By 10 min (Fig. 2D), these particles have grown sufficiently large to vaguely resemble t.he familiar crystal pattern of set gypsum. Very little difference can be discerned between the microstructures at 20 min (Fig. 2E), at 60 min, and at 24 h (Fig. 2F) for the freeze-dried, airexposed gypsum surfaces. The large original particles are still visible at 10 min (Fig. 2~. They have almost entirely disappeared by 60 min, while the growing dihydrate crystals have interdigitated with one another in a mesh. Besides the usual elongated prismatic crystals, platty crystals can also be seen. The appearance of a 24-h sample which was not freezedried but was exposed to air is similar to the freeze-dried specimen in Fig. 2F. In contrast, a 24-h sample (Fig. 3) which was allowed to set against a vinyl polysiloxane
Denfd Materials/July
1995
227
Fig. 2. Scanning electron micrograph at 3000x of setting Type IV dental stone (Vel-Mix) at various times after mixing (air-exposed surface). (A) 0 min (no water was added to powder), (B) 1 min, (C) 3 min, (D) 10 min, (E) 20 min, (F) 24 h. Note: Arrows labeled PL indicate platty crystals while those designated PR indicate prismatic crystals (Fig. 2F).
impression material without freeze-drying displays the usual elongated prismatic crystals with little evidence of platty crystals. Fast-Setting Dental Stone (Type III). Photomicrographs of the fast-setting stone powder reveal a different 228 Wfnkler et a/./Electron mlcroscopy of dental gypsum
microstructure. At 0 min {Fig. 4A) before any water 1;s added, the largest particles range in size only from 1 to 4 pm. In addition, there are many smaller particles of less than 1 pm intermixed among the larger ones. The reaction of water with the hemihydrate cryst,als results in the
nucleation of needle-shaped crystals which are approximately 5 urn in length and less than 0.5 urn in width at 1 min after mixing (Fig. 4B). These newly-forming particles also appear to nucleate on or very near the original powder particles. By 3 min (Fig. 40, the original particles are no longer visible, indicating that the dissolution of the hemihydrate was complete. From approximately 1 min to 5 min, the newly formed particles appear to grow in size. At 5 min, the needle-shaped crystals have increased in size, mainly in width, and the original hemihydrate particles can no longer be observed. By 10 min (Fig. 4D!, the width of the needle-shaped crystals ranges from 1 to 3 urn. The branching among the particles appears to have increased from 1 to 10 min. Since no obvious change in structure is apparent in the aging process between 10 min (Fig. 4D) and 1 d, this fast-setting stone appears to be fully set by 10 min.
DISCUSSION Fig.3. Scanning electron micrograph at 3000x of setting Type IV dental stone (Vel-Mix) at 24 h after mixing (impression-exposed surface).
Freeze-drying (i.e., lypholyzation) essentially stopped the reaction between calcium sulfate hemihydrate and water
Fig. 4. Scanning electron micrographat 3000xof setting Type Ill dental stone (Snap-Stone)at (A) 0 min (no water was added to the powder), (B) 1 min, (C) 3 min, (D) 10 min.
various times after mixing (air-exposed
surface).
Denkrl Marferials/July I995
229
as evidenced by the transformation in crystal structure which is delineated in Figs. 2-4. Additional efficacy of the freeze-drying process was demonstrated by the difference in crystal structure among the samples even though all of the samples were photographed in the SEM at least 24 h after mixing. Basically, this technique could be described as thermally stopping the reaction and then removing the water from the setting mass which prevents any further reaction when the partially set stone is returned to room temperature. This process allowed scanning electron microscopic evaluations of the setting dental die stone at various stages. From the photomicrographs, it was seen that small crystals nucleate near, if not on, the original particles of calcium sulfate hemihydrate. Thus, this nucleation appeared to be predominantly heterogeneous. Prism-shaped crystals were observed by 10 min after the start of mixing for the regular-setting die stone but by only 1 min for the faster-setting stone. The quicker setting of the fast-setting dental stone is due at least in part to the smaller initial particle size which contributes to a higher surface areal volume ratio that results in faster dissolution (and hence. quicker supersaturation of the aqueous solution) of the hemihydrate phase. The growth of these crystals appeared to be proportional to the gradual disappearance of the larger hemihydrate particles. The final set (24 hi appearance of the regular-setting set die stone was similar to that found at 20 min, not at 60 min as would be expected for the final set found from x-ray measurements of crystal formation (Harcourt and Lautenschlager, 1970). The only visual difference between a mix at 20 min and one at 60 min is a slightly higher density of crystals at 60 min. In contrast, the crystal structure of the fast-setting crystal stone at 24 h was indistinguishable from that at 10 min. In conclusion, a freeze-drying technique was successfully employed to halt the setting reaction for two dental stones so that scanning electron microscopy could be used to visually evaluate the various stages during the transformation of calcium sulfate hemihydrate into calcium sulfate dihydrate.
230 Winkler et a/./Electron microscopy of dental gypsum
ACKNOWLEDGMENTS This research was supported in part by grant NIDR T32 DE07042 from the National Institute of Dental Research, Bethesda, MD 20892.
REFERENCES Atkins PW 11986). Physical (:hemlstrh l’hlt-d txd. NC~M York: W. H. Freeman and Company. I:%-146 Craig RG i 1989 ). Restorative Dental Materials. 8th c1c-l. ;ir Louis: The C. V. Moshy Company. ::4i-35~ Docking AR 11965 1.Gypsum research III .1uit 1’;+lia I’tit. setting process. Int Dent ,I 15:372. Harcourt JK, Lautenschlager EP (197!) I. Accelerated and retarded dental plaster setting investigat,cd by x-rn;~ diffraction. ,I Dent Res 49(3):502. Lautenschlager EP, Corbin F ( 1969). lnvc~stlgatlon oi t htexpansion of dental stone. J Dent Res 4812 1:206-2 10 Mahler DB, Asgarzadeh K ( 1953). Th e volumetric contraction of dental gypsum materials on sett,ing. *JlZQt?tRr~,c 32(3):354-361. Phillips RW ( 1991). Skinner-s Science ot’Dcnt,ai Maten& 9th ed. Philadelphia: W. B. Saunders C’ompany, 69-92 Phillips RW, Ito BY c1952). Factors afFecting the surface of stone dies poured in hydrocolloidimpressions..Ji+o.sth(~! Dent 2(3 ):390. Phillips RW$ Moore BK ( 1994 j. Elementh $,t‘ i lental Ma& rials for Dental Hygienists and Assistants. 5th ed Philadelphia: W. B. Saunders Company-. 40-56. Worner HK C1942 1.Dental plasters, A~sl .I I)o~// 46( 2 ~:i5.