Viscosity of monophase shear rate Kyoung-Nam Kim, Andrew Koran III,
DDS,a Robert DDS, MSc
Yonsei University College of Dentistry, Dentistry, Ann Arbor, Mich.
addition silicones as a function of G. Craig,
Seoul,
Korea,
PhD,b and
University
and of Michigan,
School
of
The viscosity of monophase addition silicone impression materials was measured as a function of shear rate. The setting of mixed catalyst and base was prevented by addition of a small amount of phenyl propiolic acid. All products showed a B- to lo-fold decrease in viscosity with an increasing shear rate (shear thinning). The addition of phenyl propiolic acid had little or no effect on the viscosity of three materials. However, when added to the catalyst or base only of two products, it increased their viscosity and exaggerated the shear thinning effect. (J PROSTHET DENT 1992;67:794-8.)
A
ddition silicone impression materials have high accuracy, little dimensional change after setting, moderately short working and. setting time, and excellent recovery from deformation on removal. Although they are expensive, some are more rigid than condensation silicones and thus are more difficult to remove from undercuts.1-5 Addition silicones have been traditionally available in four consistencies (viscosities), low, medium, heavy, and putty, which are used in various impression techniques. The low and medium types have been used as syringe materials and the medium, heavy, and putty types as tray materials. Recent advances in addition silicones have involved the elimination of hydrogen evolution after setting and the development of automatic mixers and hydrophilic types. The automatic systems simplified mixing and reduced the number of voids in mixes.6 Incorporation of surfactants increased the wettability of the impression by mixes of gypsum and made them hydrophilic.7 Purification of the reactants or addition of finely divided palladium or platinum eliminated the evolution of hydrogen and allowed the impression to be poured immediately. The introduction of single viscosity or monophase addition silicones, which can be used both in the syringe and the tray, allows the number of consistencies to be reduced. This change is possible because the materials possess the quality of shear thinning, where the viscosity decreases as the shear rate is increased. Thus, a portion of the mix can be used in the syringe, where the viscosity is low because of
aAssistant Professor, Department of Dental Materials, Yonsei University. bProfessor, Department of Biologic and Materials Sciences, School of Dentistry, University of Michigan. cProfessor, Department of Prosthodontics, School of Dentistry, University of Michigan.
10/l/36684
794
Fig. 1. Sketch of water chamber and material container with T-bar: A, viscometer; B, T-bar; C, container; D, water chamber; E, supporting rod and plate.
high shear, and another portion used in the tray, where the viscosity is high because the shear rate is low. Koran et a1.8using a Brookfield viscometer (Brookfield Corp., Brookfield, Mass.), found that some rubber base impression materials were newtonian materials, but several others were nonnewtonian. McCabe and Bowman9 measured the viscosity of rubber base impression materials using an extrusion rheometer and observed that all materials were pseudoplastic, with the viscosity decreasing with increasing shear rates. CooklO studied the polymerization of elastomeric impression materials using viscosity as a measure of the reaction. One difficulty with these studies was that the impression materials were setting when tte viscosity was being measured and the effect of shear rate was being determined. Thus, one was forced to measure the viscosity of the catalyst and base separately or measure the viscosity of mixes
JUNE1992
VOLUME67
NUMBER6
VISCOSITY
OF MONOPHASE
SILICONES
Table I. Addition silicone products and their code, type, viscosity, mixing method, batch number, and manufacturer Products
Code
Baysilex Green-Mousse Hydrosil Imprint Omnisil *No marks
Type
Ba Gr HY Im Om
Viscosity
Type 1 *
High *
‘be 1 Type 1 ‘Ihe 1
Medium Medium Medium
tion of the setting of addition silicones by measuring the of mixes
containing
various
concentrations
of
phenyl propiolic acid (PPA). It was found that the setting could be essentially stopped with small amounts of added PPA. This investigation examined the shear thinning of monophase addition silicones of the two paste type and the automatic mixing type by measuring the viscosity as a function of shear rate with and without this retarder to stop the setting reaction. MATERIALS
AND
METHODS
Five addition silicones developed as monophase systems were evaluated for their viscosity. Codes, types, mixing methods, batch numbers, and manufacturers of the products are listed in Table I. Only monophase addition silicones were used in this study. A rotational viscometer (Rheolog model ‘/ RVT-RL-199, Brookfield Corp.) fixed with special T bar-type spindle (spindle cross-bar length of 5 mm) was used to evaluate the viscosity of the impression materials. Values of viscosity recorded by the pneumatic recorder (Model 5400, Foxboro Corp., Foxboro, Mass.) were calibrated in centipoise (cp) by using a conversion equation obtained from calibrating solutions (N3500, S-8000, and N190000 Certified Viscosity Standard, Cannon Instrument Co., State College, pa .) .s,ll, 12 Cups, 3 cm in diameter and 2 cm deep, were completely filled with the experimental materials and were placed in a water chamber connected to a constant temperature bath (Model NB-Reservoir and Circulating, Brookfield Engineering Lab., Inc., Stoughton, Mass.) that circulated water of 37” C (Fig. 1). The viscosities of the base and the catalyst paste of each impression material were measured at 2.5 rpm spindle speed and the reading was recorded 10 minutes after the viscometer was started. The viscosity of the mixed materials without retarder was measured as a function of time at 0.5 and 2.5 rpm of rotational speed at 1 minute and 1.5 minutes after the e.nd of mixing. All materials were mixed at the same weight ratio of the base and catalyst
paste for hand-mixed types and extruded from the automatic mixer type according to the manufacturer’s instruction for the automix types. The mixes filled the cup with a volume that weighed :!O to 21 gm.
THE
Batch no.
Hand Auto Hand Auto Hand
77171 934935 1018892 P890926 091389A
Manufacturer
Cutter Dental Parke11 L.D. Caulk 3M Dental Coe Lab. Inc.
on products.
at a fixed time. Stannard and Craigll studied the retardaviscosity
Mixing method
JOURNAL
OF PROSTHETIC
DENTISTRY
The viscosity of mixed materials with retarder as a function of rotational speed was measured at five rotational speeds (0.5, 1, 2.5, 5, and 10 rpm) and readings were recorded 10 minutes after the viscometer was started. PPA (Aldrich Chemical Co., Milwaukee, Wis.) was used to maintain the mixed materials in the unpolymerized state. PPA has a retarding (poisoning) effect on the chloroplatinic acid catalyst present in the addition silicones. Since very small quantities of PPA were needed to inhibit the reaction, 1 gm of PPA was dissolved in 10 ml of dibutyl phthalate (DBP) (Fisher Scientific Co., Fair Lawn, N.J.). This solution was used in the following ratios to the impression materials: Ba, 20 gm/0.4 ml solution; Gr, 21 gm/0.5 ml; Hy, 20 gm/0.5 ml; Im, 21 gm/0.5 ml; and Om, 20 gm/0.5 ml. At these ratios the setting reaction was retarded for more than 6 hours. In the hand-mixing type, the base paste and the catalyst paste were mixed together for 30 seconds after the solution was mixed with the catalyst paste for 30 seconds. In automatic mixing types the solution was mixed with the extruded
materials
for 30 seconds.
To compare the effects of the PPA on the tested materials, the same volume of the base and the catalyst paste were mixed$ibh the same amount of PPA as the mixed materials for 30 seconds and the viscosity of each paste was measured at a rotational speed 2.5 rpm 10 minutes after the viscometer was started. All impression materials were mixed at room temperature (23 + lo C) and the viscosity measured at 37 + 0.5’ C. All experiments were replicated five times and the means and standard deviations were calculated. RESULTS The viscosities of the base and catalyst pastes at a rotational speed of 2.5 rpm, 10 minutes after the start of the viscometer, are shown in Table II. The viscosities of the base and the catalyst pastes of individual materials were similar, except for Om. The viscosity of the catalyst paste of Om was approximately 65 % higher than the base paste. The lowest viscosity for Ba.
was observed
for Hy and the highest
The viscosities of the mixed materials with added PPA to retard polymerization are shown in Fig. 2. The viscosity of all materials decreased as the rotational speed increased.
When the rotational speed was increased from 0.5 to 10 rpm the viscosities of Ba and Hy were decreased approximately 795
KIM,
.
CRAIG,
AND
KORAN
e 200
E * t a 0 ii T
100
0 0.0
Fig.
2. Viscosity
of mixed
2.0
materials
4.0
6.0
ROTATIONAL
SPEED,
retarded
Table II. Viscosity of base and catalyst pastes 10 minutes after the viscometer was started at 2.5 rpm and 37O c (x104 cp) Material
SD, Standard
code
Base (SD)
Catalyst (SD)
Ba Gr
65.5
(0.6)
70.6
(1.0)
29.8
(1.3)
25.0
(1.5)
HY
16.1
(0.9)
19.1 (0.8)
Im Om
42.3
(1.1)
50.0
(0.6)
35.2
(0.9)
58.5
(2.0)
deviation.
10 times, and those of Gr, Im, and Om decreased approximately sixfold. All tested materials demonstrated pseudoplastic behavior (or shear thinning) and therefore a decrease in viscosity with an increase in rotational speed (shear rate). In a comparison of the average viscosities of the base and catalyst for individual products from Table II with the values of the same products mixed with PPA (Fig. 2) at 2.5 rpm, the viscosities were comparable for Gr, Im, and Om, but the viscosities of Ba and Hy mixes with PPA were sig796
8.0
10.0
rpm
with PPA as function
of rotational
speed.
nificantly higher (Fig. 2) than the averages of the viscosities of the base and catalyst (Table II). Table III presents the viscosities of mixed materials without retarder at 1.0 and 1.5 minutes and at 0.5 and 2.5 rpm. At 0.5 rpm the viscosities of Ba and Gr increased approximately 80% between 1 and 1.5 minute; however, the viscosities of Hy, Im, and Om increased more than 100%. At 2.5 rpm the viscosity of Hy increased only 18% from 1 to 1.5 minutes, whereas Gr and Om increased 37 % and 50 % and Ba and Im increased 83 % and 116 % , respectively. At comparable times the viscosities of all materials were lower at 2.5 than at 0.5 rpm. Decreases at 1 minute ranged from 25 % to 57 % , whereas decreases at 1.5 minute ranged from 29 % t.o 69 % . Gr had the largest decrease at both times. In a comparison of the values in Tables II and III at 2.5 rpm, the viscosity of Ba was unchanged between 0 and 1 minute, whereas those of Gr, Im, and Om increased by 2 times and Hy by 7 times.
DISCUSSION The recent trend in making clinical impresgions is to make more accurate impressions with simpler techniques.13 To make an accurate impression, it is necessary to recogJUNE
1992
VOLUME
67
NUMBER
6
VISCOSITY
OF MONOPHASE
SILICONES
Eaysilex
Green-Mousse,
HBase
q
Ba+PPA
n
Catalyst
q
Ca+PPA
0
Ba+Ca+PPA
Hydrosil
Imprint
Omnisil
I
Fig. 3. Viscosity of base and catalyst with or without F’PA and mixed material retarded with PPA at 2.5 rpm and 37” C.
nize that the viscosity of impression materials is related to its ability to record detail, because the lower the viscosity the greater the flow into spaces of fine detail. The viscosity may vary with the material, temperature, time after mixing, and mixing method or speed. The addition silicones tested were pseudoplastic (shear thinning) materials, where the viscosity was higher under low shear stress but lower under high shear stress. This characteristic is important in the s,ingle-mix technique. When subjected to low shear stress, as when mixed .material is in a tray, these impression materials have viscosities and do not flow out of the tray. If these materials are used in a syringe, higher shear rates are encountered during extrusion from the syringe tip, the viscosity decreases, and they flow readily into the detailed areas.14 This quality of shear thinning varies among materials. The viscosity of the base and the catalyst paste should be similar to improve the ease of mixing of the hand-spatulated type as well as improve the mixing of the two pastes in the static mixing tip of the automatic mixing system. The base and the catalyst of the individual products tested, except Om, had similar viscosities. Although all materials had pseudoplastic characteristics, the decrease in viscosity with increasing shear rate was not the same for all materials. When the rotational speed increased from 0.5 to 10 rpm the viscosities of Ba and Hy decreased 10 times, whereas those THE
JOURNAL
OF PROSTHETIC
DENTISTRY
XII. Viscosity of mixed materials as function of time after mixing at 0.5 and 2.5 rpm and 37’ C (~10~ cp)
Table
Material code
Ba GI HY Im
OItl
rPm
0.5 2.5 0.5 2.5 0.5 2.5 0.5 2.5 0.5 2.5
Viscosity
Viscosity
at 1 min
at
122.1 68.9 133.7 56.7 194.2 129.4 106.5 79.7 156.6 102.5
(2.8) (2.5) (8.9) (2.9) (8.5) (4.1) (12.2) ( 2.2) (11.8) ( 1.9)
1.5 min
211.2 148.8 247.9 78.0 398.07 153.5*
(14.7) ( 1.2) (14.9) ( 2.8) (7.8)
245.1 (8.9) 146.2 (5.9) 347.1 (5.2) 153.5?
SD, Standard deviation. *Value at. 75 seconds after mixing. TValue at. 77 seconds after mixing.
of Gr, Im, and Om decreased six times. At high shear rates all materials had similar viscosities within 200,000 cp, whereas large differences in viscosities occurred at low shear rates (as high as 2,500,OOOcp). This shear thinning charact,eristic was also shown in Table III when the rotational speed was changed from 0.5 to 2.5 rpm. Between 1
KIM,
and 1.5 minutes the viscosity of Ba and Im increased approximately two times at both 0.5 and 2.5 rpm, but the viscosity of Gr, Hy, and Cm increased approximately two times at 0.5 rpm and approximately one-half at 2.5 rpm. It was presumed that the viscosity of unpolymerized mixed materials would ‘be similar to the average viscosity of the base and the catalyst paste at the same shear rate. In comparing the viscosities in Table II and Fig. 2, Gr, Im, and Om mixed with PPA had the same viscosities as the average viscosities of the base and the catalyst pastes. However, this was not true for Ba and Hy. Thus, the true shear thinning effect as a function of shear rate for Ba and Hy (shown in Fig. 2) is somewhat exaggerated by the presence of PPA. This effect is further illustrated in Fig. 3, in which the viscosities of (1) the base only, (2) the base plus PPA, (3) the catalyst only, (4) the catalyst plus PPA, and (5) the base plus catalyst plus PPA are plotted for the various products. The presence of PPA in Gr and Im had no significant effect on the viscosity, whereas PPA increased the viscosities of Ba and Hy and reduced the viscosities for Om. Thus, although all products showed shear thinning with increasing shear stress (Fig. 2), the magnitude of the effect observed with PPA present is magnified for Ba and Hy and reduced for Om. It appears that PPA affected the viscosity of these two materials by reacting with the filler or polymer to influence the wettability, which could affect the viscosity. It is apparent that the following factors related to viscosity should be considered when single-mix addition silicones are used in the dental office. During spatulation, syringing, and insertion of the tray in the mouth, the imnression materials are under relatively high shear stress, similar to high rotational speed, and will decrease in viscosity. When these materials are stored in the tray under their own weight they are under low shear stress and the viscosity will be higher.i5 With information about the effect of shear rate on viscosity a more successful impression should be made. As previously reported’l PPA functioned as a retarder by interacting with the chloroplatinic acid in the catalyst naste. PPA in appropriate concentrations retarded the __ . setting reaction of the materials for more than 6 hours. Because such small amounts of PPA are needed to completely retard the reaction, it is probably not appropriate material to be used to clinically retard addition silicones. Commercial dental addition silicones are applied with retarders that increase the working and setting times.16 These materials are low molecular weight silicone polymers that have the same general composition as the impression materials and react during setting.14
CRAIG,
AND
KORAN
uated. The viscosity of the base paste and catalyst pastes were determined separately. The changes in viscosity after base and catalyst were mixed as a function of time at 0.5 and 2.5 rpm were also determined. Finally, changes in viscosity of base and catalyst mix with a retarder were measured as a function of rotational speed at 37” C. The base and the catalyst paste of each material except Om had similar viscosities. The viscosity of mixes of base and catalyst decreased as a function of shear rate and increased as a function of time. When the mixes of base and catalyst were retarded with PPA, the viscosity of Ba and Hy were decreased 10 times, and those of Gr, Im, and Om were decreased six times when the rotational speed increased from 0.5 to 10 rpm. REFERENCES 1. Council on Dental Materials, Instruments, and Equipment. Revised American Dental Association Specification No. 19 for non-aqueous elastomeric dental impression materials. J Am Dent Assoc 1977;94:73341. 2. Yeh CL, Powers JM, Craig RG. Properties of addition-type silicone impression materials. J Am Dent Assoc 1980;101:482-4. 3. Johnson GH, Craig RG. Accuracy of addition silicones as a function of technique. J PROSTHET DENT 1986;55:197-203. 4. O’Brien WJ. Dental materials: properties and selection. Chicago: Quintessence, 1989187-94. 5. Farah JW, Powers JM, eds. The dental advisor. Crown and bridge impression materials. Vol 6, No. 2. Ann Arbor: Dental Consultant Inc., 1989. 6. Craig RG. Evaluation of an automatic mixing system for an addition silicone impression materials. J Am Dent Assoc 1985;110:213-5. 7. Pratten DH, Craig RG. Wettability of a hydrophilic addition silicone impression materials. J PROSTHET DENT 1989;61:19’7-202, 8. Koran A, Powers JM, Craig RG. Apparent viscosity of materials used for making edentulous impressions. J Am Dent Assoc 1977;95:75-9. 9. McCabe JF, Bowman AJ. The rheological properties of dental impression materials. Br Dent J 1981;151:179-83. 10. Cook WD. Rheological studies of the polymerization of elastic impression materials. II. Viscosity measurements. J Biomed Mater Res 1982; 16:331-44. 11. Stannard JG, Craig RG. Modifying the setting rate of an addition-type silicone impression materials. j Dent Res 19?9;58:1377-82. 12. Rheological properties of non-Newtonian materials by rotational (Brookfield) viscometer. D2196-86. In: American Society for Testing and Materials, Information Handling Service Inc., V&F 1990:2104~ 2874. 13. Craig RG. Review of dental impression materials. Adv Dent Res 1988;2:51-64. 14. Craig RG. Restorative dental materials. 8th ed. St Louis: CV Mosby, 1989:309-25. 15. Hertfort TW, Gerberich WW, Macosko CW, Goodkind RJ. Viscosity of elastomeric impression materials. J PROSTHET DENT 1977:38:396-404. 16. Stannard JG, Sadighi-Nouri. Retarders for polyvinylsiloxane impression materials: evaluation and recommendations. J PROSTHET DENT 1986;55:7-10.
Reprintrequeststo: DR. ROBERT G. CRAIG DEPARTMENT OF BIOLOGIC AND MATERIALS SCHOOL OF DENTISTRY UNIVEILSITY OF MICHIGAN ANN ARBOR, MI 48109-1078
SCIENCE
SUMMARY The viscosities of five commercial monophase (singleviscosity) addition silicone impression materials were eval-
798
JUNE
1992
VOLUME
67
NUMBER
6
DDS,a Robert DDS, MSc
Yonsei University College of Dentistry, Dentistry, Ann Arbor, Mich.
addition silicones as a function of G. Craig,
Seoul,
Korea,
PhD,b and
University
and of Michigan,
School
of
The viscosity of monophase addition silicone impression materials was measured as a function of shear rate. The setting of mixed catalyst and base was prevented by addition of a small amount of phenyl propiolic acid. All products showed a B- to lo-fold decrease in viscosity with an increasing shear rate (shear thinning). The addition of phenyl propiolic acid had little or no effect on the viscosity of three materials. However, when added to the catalyst or base only of two products, it increased their viscosity and exaggerated the shear thinning effect. (J PROSTHET DENT 1992;67:794-8.)
A
ddition silicone impression materials have high accuracy, little dimensional change after setting, moderately short working and. setting time, and excellent recovery from deformation on removal. Although they are expensive, some are more rigid than condensation silicones and thus are more difficult to remove from undercuts.1-5 Addition silicones have been traditionally available in four consistencies (viscosities), low, medium, heavy, and putty, which are used in various impression techniques. The low and medium types have been used as syringe materials and the medium, heavy, and putty types as tray materials. Recent advances in addition silicones have involved the elimination of hydrogen evolution after setting and the development of automatic mixers and hydrophilic types. The automatic systems simplified mixing and reduced the number of voids in mixes.6 Incorporation of surfactants increased the wettability of the impression by mixes of gypsum and made them hydrophilic.7 Purification of the reactants or addition of finely divided palladium or platinum eliminated the evolution of hydrogen and allowed the impression to be poured immediately. The introduction of single viscosity or monophase addition silicones, which can be used both in the syringe and the tray, allows the number of consistencies to be reduced. This change is possible because the materials possess the quality of shear thinning, where the viscosity decreases as the shear rate is increased. Thus, a portion of the mix can be used in the syringe, where the viscosity is low because of
aAssistant Professor, Department of Dental Materials, Yonsei University. bProfessor, Department of Biologic and Materials Sciences, School of Dentistry, University of Michigan. cProfessor, Department of Prosthodontics, School of Dentistry, University of Michigan.
10/l/36684
794
Fig. 1. Sketch of water chamber and material container with T-bar: A, viscometer; B, T-bar; C, container; D, water chamber; E, supporting rod and plate.
high shear, and another portion used in the tray, where the viscosity is high because the shear rate is low. Koran et a1.8using a Brookfield viscometer (Brookfield Corp., Brookfield, Mass.), found that some rubber base impression materials were newtonian materials, but several others were nonnewtonian. McCabe and Bowman9 measured the viscosity of rubber base impression materials using an extrusion rheometer and observed that all materials were pseudoplastic, with the viscosity decreasing with increasing shear rates. CooklO studied the polymerization of elastomeric impression materials using viscosity as a measure of the reaction. One difficulty with these studies was that the impression materials were setting when tte viscosity was being measured and the effect of shear rate was being determined. Thus, one was forced to measure the viscosity of the catalyst and base separately or measure the viscosity of mixes
JUNE1992
VOLUME67
NUMBER6
VISCOSITY
OF MONOPHASE
SILICONES
Table I. Addition silicone products and their code, type, viscosity, mixing method, batch number, and manufacturer Products
Code
Baysilex Green-Mousse Hydrosil Imprint Omnisil *No marks
Type
Ba Gr HY Im Om
Viscosity
Type 1 *
High *
‘be 1 Type 1 ‘Ihe 1
Medium Medium Medium
tion of the setting of addition silicones by measuring the of mixes
containing
various
concentrations
of
phenyl propiolic acid (PPA). It was found that the setting could be essentially stopped with small amounts of added PPA. This investigation examined the shear thinning of monophase addition silicones of the two paste type and the automatic mixing type by measuring the viscosity as a function of shear rate with and without this retarder to stop the setting reaction. MATERIALS
AND
METHODS
Five addition silicones developed as monophase systems were evaluated for their viscosity. Codes, types, mixing methods, batch numbers, and manufacturers of the products are listed in Table I. Only monophase addition silicones were used in this study. A rotational viscometer (Rheolog model ‘/ RVT-RL-199, Brookfield Corp.) fixed with special T bar-type spindle (spindle cross-bar length of 5 mm) was used to evaluate the viscosity of the impression materials. Values of viscosity recorded by the pneumatic recorder (Model 5400, Foxboro Corp., Foxboro, Mass.) were calibrated in centipoise (cp) by using a conversion equation obtained from calibrating solutions (N3500, S-8000, and N190000 Certified Viscosity Standard, Cannon Instrument Co., State College, pa .) .s,ll, 12 Cups, 3 cm in diameter and 2 cm deep, were completely filled with the experimental materials and were placed in a water chamber connected to a constant temperature bath (Model NB-Reservoir and Circulating, Brookfield Engineering Lab., Inc., Stoughton, Mass.) that circulated water of 37” C (Fig. 1). The viscosities of the base and the catalyst paste of each impression material were measured at 2.5 rpm spindle speed and the reading was recorded 10 minutes after the viscometer was started. The viscosity of the mixed materials without retarder was measured as a function of time at 0.5 and 2.5 rpm of rotational speed at 1 minute and 1.5 minutes after the e.nd of mixing. All materials were mixed at the same weight ratio of the base and catalyst
paste for hand-mixed types and extruded from the automatic mixer type according to the manufacturer’s instruction for the automix types. The mixes filled the cup with a volume that weighed :!O to 21 gm.
THE
Batch no.
Hand Auto Hand Auto Hand
77171 934935 1018892 P890926 091389A
Manufacturer
Cutter Dental Parke11 L.D. Caulk 3M Dental Coe Lab. Inc.
on products.
at a fixed time. Stannard and Craigll studied the retardaviscosity
Mixing method
JOURNAL
OF PROSTHETIC
DENTISTRY
The viscosity of mixed materials with retarder as a function of rotational speed was measured at five rotational speeds (0.5, 1, 2.5, 5, and 10 rpm) and readings were recorded 10 minutes after the viscometer was started. PPA (Aldrich Chemical Co., Milwaukee, Wis.) was used to maintain the mixed materials in the unpolymerized state. PPA has a retarding (poisoning) effect on the chloroplatinic acid catalyst present in the addition silicones. Since very small quantities of PPA were needed to inhibit the reaction, 1 gm of PPA was dissolved in 10 ml of dibutyl phthalate (DBP) (Fisher Scientific Co., Fair Lawn, N.J.). This solution was used in the following ratios to the impression materials: Ba, 20 gm/0.4 ml solution; Gr, 21 gm/0.5 ml; Hy, 20 gm/0.5 ml; Im, 21 gm/0.5 ml; and Om, 20 gm/0.5 ml. At these ratios the setting reaction was retarded for more than 6 hours. In the hand-mixing type, the base paste and the catalyst paste were mixed together for 30 seconds after the solution was mixed with the catalyst paste for 30 seconds. In automatic mixing types the solution was mixed with the extruded
materials
for 30 seconds.
To compare the effects of the PPA on the tested materials, the same volume of the base and the catalyst paste were mixed$ibh the same amount of PPA as the mixed materials for 30 seconds and the viscosity of each paste was measured at a rotational speed 2.5 rpm 10 minutes after the viscometer was started. All impression materials were mixed at room temperature (23 + lo C) and the viscosity measured at 37 + 0.5’ C. All experiments were replicated five times and the means and standard deviations were calculated. RESULTS The viscosities of the base and catalyst pastes at a rotational speed of 2.5 rpm, 10 minutes after the start of the viscometer, are shown in Table II. The viscosities of the base and the catalyst pastes of individual materials were similar, except for Om. The viscosity of the catalyst paste of Om was approximately 65 % higher than the base paste. The lowest viscosity for Ba.
was observed
for Hy and the highest
The viscosities of the mixed materials with added PPA to retard polymerization are shown in Fig. 2. The viscosity of all materials decreased as the rotational speed increased.
When the rotational speed was increased from 0.5 to 10 rpm the viscosities of Ba and Hy were decreased approximately 795
KIM,
.
CRAIG,
AND
KORAN
e 200
E * t a 0 ii T
100
0 0.0
Fig.
2. Viscosity
of mixed
2.0
materials
4.0
6.0
ROTATIONAL
SPEED,
retarded
Table II. Viscosity of base and catalyst pastes 10 minutes after the viscometer was started at 2.5 rpm and 37O c (x104 cp) Material
SD, Standard
code
Base (SD)
Catalyst (SD)
Ba Gr
65.5
(0.6)
70.6
(1.0)
29.8
(1.3)
25.0
(1.5)
HY
16.1
(0.9)
19.1 (0.8)
Im Om
42.3
(1.1)
50.0
(0.6)
35.2
(0.9)
58.5
(2.0)
deviation.
10 times, and those of Gr, Im, and Om decreased approximately sixfold. All tested materials demonstrated pseudoplastic behavior (or shear thinning) and therefore a decrease in viscosity with an increase in rotational speed (shear rate). In a comparison of the average viscosities of the base and catalyst for individual products from Table II with the values of the same products mixed with PPA (Fig. 2) at 2.5 rpm, the viscosities were comparable for Gr, Im, and Om, but the viscosities of Ba and Hy mixes with PPA were sig796
8.0
10.0
rpm
with PPA as function
of rotational
speed.
nificantly higher (Fig. 2) than the averages of the viscosities of the base and catalyst (Table II). Table III presents the viscosities of mixed materials without retarder at 1.0 and 1.5 minutes and at 0.5 and 2.5 rpm. At 0.5 rpm the viscosities of Ba and Gr increased approximately 80% between 1 and 1.5 minute; however, the viscosities of Hy, Im, and Om increased more than 100%. At 2.5 rpm the viscosity of Hy increased only 18% from 1 to 1.5 minutes, whereas Gr and Om increased 37 % and 50 % and Ba and Im increased 83 % and 116 % , respectively. At comparable times the viscosities of all materials were lower at 2.5 than at 0.5 rpm. Decreases at 1 minute ranged from 25 % to 57 % , whereas decreases at 1.5 minute ranged from 29 % t.o 69 % . Gr had the largest decrease at both times. In a comparison of the values in Tables II and III at 2.5 rpm, the viscosity of Ba was unchanged between 0 and 1 minute, whereas those of Gr, Im, and Om increased by 2 times and Hy by 7 times.
DISCUSSION The recent trend in making clinical impresgions is to make more accurate impressions with simpler techniques.13 To make an accurate impression, it is necessary to recogJUNE
1992
VOLUME
67
NUMBER
6
VISCOSITY
OF MONOPHASE
SILICONES
Eaysilex
Green-Mousse,
HBase
q
Ba+PPA
n
Catalyst
q
Ca+PPA
0
Ba+Ca+PPA
Hydrosil
Imprint
Omnisil
I
Fig. 3. Viscosity of base and catalyst with or without F’PA and mixed material retarded with PPA at 2.5 rpm and 37” C.
nize that the viscosity of impression materials is related to its ability to record detail, because the lower the viscosity the greater the flow into spaces of fine detail. The viscosity may vary with the material, temperature, time after mixing, and mixing method or speed. The addition silicones tested were pseudoplastic (shear thinning) materials, where the viscosity was higher under low shear stress but lower under high shear stress. This characteristic is important in the s,ingle-mix technique. When subjected to low shear stress, as when mixed .material is in a tray, these impression materials have viscosities and do not flow out of the tray. If these materials are used in a syringe, higher shear rates are encountered during extrusion from the syringe tip, the viscosity decreases, and they flow readily into the detailed areas.14 This quality of shear thinning varies among materials. The viscosity of the base and the catalyst paste should be similar to improve the ease of mixing of the hand-spatulated type as well as improve the mixing of the two pastes in the static mixing tip of the automatic mixing system. The base and the catalyst of the individual products tested, except Om, had similar viscosities. Although all materials had pseudoplastic characteristics, the decrease in viscosity with increasing shear rate was not the same for all materials. When the rotational speed increased from 0.5 to 10 rpm the viscosities of Ba and Hy decreased 10 times, whereas those THE
JOURNAL
OF PROSTHETIC
DENTISTRY
XII. Viscosity of mixed materials as function of time after mixing at 0.5 and 2.5 rpm and 37’ C (~10~ cp)
Table
Material code
Ba GI HY Im
OItl
rPm
0.5 2.5 0.5 2.5 0.5 2.5 0.5 2.5 0.5 2.5
Viscosity
Viscosity
at 1 min
at
122.1 68.9 133.7 56.7 194.2 129.4 106.5 79.7 156.6 102.5
(2.8) (2.5) (8.9) (2.9) (8.5) (4.1) (12.2) ( 2.2) (11.8) ( 1.9)
1.5 min
211.2 148.8 247.9 78.0 398.07 153.5*
(14.7) ( 1.2) (14.9) ( 2.8) (7.8)
245.1 (8.9) 146.2 (5.9) 347.1 (5.2) 153.5?
SD, Standard deviation. *Value at. 75 seconds after mixing. TValue at. 77 seconds after mixing.
of Gr, Im, and Om decreased six times. At high shear rates all materials had similar viscosities within 200,000 cp, whereas large differences in viscosities occurred at low shear rates (as high as 2,500,OOOcp). This shear thinning charact,eristic was also shown in Table III when the rotational speed was changed from 0.5 to 2.5 rpm. Between 1
KIM,
and 1.5 minutes the viscosity of Ba and Im increased approximately two times at both 0.5 and 2.5 rpm, but the viscosity of Gr, Hy, and Cm increased approximately two times at 0.5 rpm and approximately one-half at 2.5 rpm. It was presumed that the viscosity of unpolymerized mixed materials would ‘be similar to the average viscosity of the base and the catalyst paste at the same shear rate. In comparing the viscosities in Table II and Fig. 2, Gr, Im, and Om mixed with PPA had the same viscosities as the average viscosities of the base and the catalyst pastes. However, this was not true for Ba and Hy. Thus, the true shear thinning effect as a function of shear rate for Ba and Hy (shown in Fig. 2) is somewhat exaggerated by the presence of PPA. This effect is further illustrated in Fig. 3, in which the viscosities of (1) the base only, (2) the base plus PPA, (3) the catalyst only, (4) the catalyst plus PPA, and (5) the base plus catalyst plus PPA are plotted for the various products. The presence of PPA in Gr and Im had no significant effect on the viscosity, whereas PPA increased the viscosities of Ba and Hy and reduced the viscosities for Om. Thus, although all products showed shear thinning with increasing shear stress (Fig. 2), the magnitude of the effect observed with PPA present is magnified for Ba and Hy and reduced for Om. It appears that PPA affected the viscosity of these two materials by reacting with the filler or polymer to influence the wettability, which could affect the viscosity. It is apparent that the following factors related to viscosity should be considered when single-mix addition silicones are used in the dental office. During spatulation, syringing, and insertion of the tray in the mouth, the imnression materials are under relatively high shear stress, similar to high rotational speed, and will decrease in viscosity. When these materials are stored in the tray under their own weight they are under low shear stress and the viscosity will be higher.i5 With information about the effect of shear rate on viscosity a more successful impression should be made. As previously reported’l PPA functioned as a retarder by interacting with the chloroplatinic acid in the catalyst naste. PPA in appropriate concentrations retarded the __ . setting reaction of the materials for more than 6 hours. Because such small amounts of PPA are needed to completely retard the reaction, it is probably not appropriate material to be used to clinically retard addition silicones. Commercial dental addition silicones are applied with retarders that increase the working and setting times.16 These materials are low molecular weight silicone polymers that have the same general composition as the impression materials and react during setting.14
CRAIG,
AND
KORAN
uated. The viscosity of the base paste and catalyst pastes were determined separately. The changes in viscosity after base and catalyst were mixed as a function of time at 0.5 and 2.5 rpm were also determined. Finally, changes in viscosity of base and catalyst mix with a retarder were measured as a function of rotational speed at 37” C. The base and the catalyst paste of each material except Om had similar viscosities. The viscosity of mixes of base and catalyst decreased as a function of shear rate and increased as a function of time. When the mixes of base and catalyst were retarded with PPA, the viscosity of Ba and Hy were decreased 10 times, and those of Gr, Im, and Om were decreased six times when the rotational speed increased from 0.5 to 10 rpm. REFERENCES 1. Council on Dental Materials, Instruments, and Equipment. Revised American Dental Association Specification No. 19 for non-aqueous elastomeric dental impression materials. J Am Dent Assoc 1977;94:73341. 2. Yeh CL, Powers JM, Craig RG. Properties of addition-type silicone impression materials. J Am Dent Assoc 1980;101:482-4. 3. Johnson GH, Craig RG. Accuracy of addition silicones as a function of technique. J PROSTHET DENT 1986;55:197-203. 4. O’Brien WJ. Dental materials: properties and selection. Chicago: Quintessence, 1989187-94. 5. Farah JW, Powers JM, eds. The dental advisor. Crown and bridge impression materials. Vol 6, No. 2. Ann Arbor: Dental Consultant Inc., 1989. 6. Craig RG. Evaluation of an automatic mixing system for an addition silicone impression materials. J Am Dent Assoc 1985;110:213-5. 7. Pratten DH, Craig RG. Wettability of a hydrophilic addition silicone impression materials. J PROSTHET DENT 1989;61:19’7-202, 8. Koran A, Powers JM, Craig RG. Apparent viscosity of materials used for making edentulous impressions. J Am Dent Assoc 1977;95:75-9. 9. McCabe JF, Bowman AJ. The rheological properties of dental impression materials. Br Dent J 1981;151:179-83. 10. Cook WD. Rheological studies of the polymerization of elastic impression materials. II. Viscosity measurements. J Biomed Mater Res 1982; 16:331-44. 11. Stannard JG, Craig RG. Modifying the setting rate of an addition-type silicone impression materials. j Dent Res 19?9;58:1377-82. 12. Rheological properties of non-Newtonian materials by rotational (Brookfield) viscometer. D2196-86. In: American Society for Testing and Materials, Information Handling Service Inc., V&F 1990:2104~ 2874. 13. Craig RG. Review of dental impression materials. Adv Dent Res 1988;2:51-64. 14. Craig RG. Restorative dental materials. 8th ed. St Louis: CV Mosby, 1989:309-25. 15. Hertfort TW, Gerberich WW, Macosko CW, Goodkind RJ. Viscosity of elastomeric impression materials. J PROSTHET DENT 1977:38:396-404. 16. Stannard JG, Sadighi-Nouri. Retarders for polyvinylsiloxane impression materials: evaluation and recommendations. J PROSTHET DENT 1986;55:7-10.
Reprintrequeststo: DR. ROBERT G. CRAIG DEPARTMENT OF BIOLOGIC AND MATERIALS SCHOOL OF DENTISTRY UNIVEILSITY OF MICHIGAN ANN ARBOR, MI 48109-1078
SCIENCE
SUMMARY The viscosities of five commercial monophase (singleviscosity) addition silicone impression materials were eval-
798
JUNE
1992
VOLUME
67
NUMBER
6
