Rheology of Fluoride Gels M. BRADEN and RATNA PERERA Dental School of The London Hospital Medical College, London, England
Six commercial fluoride gels have been studied, using a cone and plate viscometer. Also, the thickening agents have been analyzed using infrared spectroscopy. All gels showed stress thinning, which is the decrease of viscosity with shear rate. Such shear rate dependence is clinically convenient in that the gel will flow readily at the high shear stresses present zwhen the gel is applied but will not flow readily under its own weight when on the tooth. Five materials containing hydroxyalkyl celluloses showed similar degrees of shear thinning. One material with a noncellulosic thickener showed much more extreme stress thinning together with elastic behavior at low shear rates; such behavior may be clinically advantageous. All of the gels showed only slight temperature dependence of rheological properties.
Materials containing fluoride ions occupy an important place in preventive dentistry.' These ions have both systemic and topical effects on teeth. The latter effect can be obtained either by the addition of sodium fluoride to central water supplies or from solutions, gels, pastes, and so forth, applied topically on an individual basis.2 The gels are preferred to aqueous solutions since they can be more easily kept in contact with many of the teeth for the required time.3 However, little information is available regarding the physical characteristics such as the rheology of these gels. The present investigation therefore describes the measurement of the rheological properties and a preliminary study of their content. Dr. Perera was supported by a British Council Scholarship. A grant from the Central Research Fund Committee of the University of London was used to purchase the Weissenberg Rheogoniometer. Received for publication June 10, 1975. Accepted for publication October 15, 1975.
Materials and Methods Six commercial gels were studied. The gels were Pacemaker,a Hifluor Gel,b FluorO-Dent,c Rafluor Gel,d En-De-Kay,e and De Treys, Alpha-Gelf; and are designated A, B, C, D, E, and F, respectively. The rheology of the gel was studied with a cone and plate viscometer.g This consists of a flat plate that is stationary, and a conical plate that can be rotated at different speeds as described elsewhere.4 Measurements of shear rate and shear stress were made, and time-dependent effects, if any, were carefully investigated. Materials were examined over the shear rate range 0.014 to 1,390 sec-1 at both 25 C and 35 C. The six different gels being studied were placed on six different watch glasses. Watch glasses were weighed before and after placement of the gels. These were then kept in a vacuum desiccator with a calcium sulfate desiccant. The watch glasses containing the gels were taken from the desiccator and weighed at convenient time intervals until each reached a constant value. Inorganic ionic material was removed by shaking the gel with an ion-exchange resinh; films of the polymeric thickening agents were then cast, dried in a vacuum desiccator, and the infrared spectrai determined. Films of sodium carboxymethylcellulose, methylcellulose, and hydroxypropyl methylcellulose Pacemaker Corp., Portland, Ore. Allied Laboratories Ltd., London, Eng. Den-Tal-Ez Mfg. Co., Columbus, Ohio. d Pascal Co., Inc., Bellevue, Wash. e Westone Products Ltd., London, Eng. f Amalgamated Dental Co., London, Eng. a
b
9 Weissenberg Rheogoniometer. Sangamo Controls Ltd., Bogner Regis, Eng. h Biodeminrolit BDH Chemicals Ltd., Poole, Eng. i Pye-Unicam Infrared Spectrophotometer SP 1000, Cambridge, Eng.
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353
354
J Dent Res May-June 1976
BRADEN AND PERERA
TABLE 1 Viscosrry VALUES AT Low AND HIGH SHEAR RATES Viscosity (poise)
Material
Gel A Gel B Gel C Gel D GelE Gel F
0.0140 sec-' 35 C 25 C
700
340 280 450 290 650 6,000
600 560
450 780
7,500
1,390 sec-
25 C
35 C
2.1 2.0 1.9 1.6 2.2 0.28
1.8 1.5 1.5 1.0
1.7 0.25
0
10
Shfta, Rats ("c-l
FIG I.-Plot of shear stress against shear rate for material C (crcles), and material F (triangles), slope of Newtonian fluid (broken line) .
were cast, and their spectra determined for comparison purposes.
Results RHEOLOGY OF THE GELS.-Materials A through E behaved very similarly. Therefore, Figure 1 showing shear stress plotted against shear rate, shows data for material C only, together with material F that behaved very differently. (The broken line shows the unit slope that would be obtained for a Newtonian liquid) . In Figure 2 the data are plotted as apparent viscosity (Shear StressShear Rate) as a function of shear rate for
25 and 35 C; again, material C only is seen as typical of materials A through E, together with material F. As in all instances, apparent viscosity decreased monotonically with shear rate. In Table 1 the data for all materials and conditions are given in terms of the apparent viscosity at 0.014 sec-1 and 1,390 sec-1, the limits of shear rates used. Figure 3 shows the torque decay when the rheogoniometer was stopped for material F, the only material that noticeably showed this effect. THICKENING AGENT CONTENT.-About 90 to
95% of the water content of each of the gels was lost during the first two days. Subsequently, the loss per day was very small. This decrease continued for about a month and then the weight fluctuated slightly, depending on the humidity of the room at the time of measurement. Table 2 gives this data.
10 F
FIG 2.-Plot of apparent viscosity against shear rate. Material C: open circles, 25 C; closed circles, 35 C. Material F: open triangles, 25 C; closed triangles, 35 C.
'v
t;"mns
'
FIG 3.-Plot of residual torque as function of time from switching off viscometer. (Normalized
torque - torque at time t/torque at t =
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RHEOLOGY OF FLUORIDE GELS
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FIG 4.-Infrared spectra of material A and E compared with hydroxypropyl cellulose (HPC).
INFRARED SPECTROSCOPY.-Figure 4 shows the comparison of the spectra of materials A and E with hydroxypropyl methylcellulose. Materials B, C, and D were almost identical to A and are not seen here. Figure 5 shows the spectrum of material F. Discussion RHEOLOGY OF THE GELS.-Figure 1 shows that all of the gels are non-Newtonian; material F is peculiar in that the linear plot shows it to be a power law fluid. In fact, it obeys the law r = ka-n (n = 0.342), where r is shear stress and a shear rate. As with most non-Newtonian substances the apparent viscosity (,q) decreased with increasing shear rate (Fig 2) for all gels studied (shear thinning or pseudoplastic flow behavior). All gels except gel F produced a similar pattern (Fig 2). The gel F, however, produced a straight line with a negative slope showing very high viscosity at low shear rate and very low viscosity at high shear rate (Table
2). TABLE 2 WEIGHT FRACrION OF THE DRY RESIDUE OF GEL AFTER WATER REMOVAL Dry Residue
Material
(%)
A B C
10.36
Gel Gel Gel Gel Gel Gel
D E F
4.71 5.91 4.83 5.06 4.29
The shear thinning experienced by all materials suggests that the flow of the material will be facilitated by the higher shear stresses consequent on the pressure during the application of these gels to teeth. However, once the material is applied to the teeth, the retention of the gel by the teeth will be made easier by the highly viscous nature of these gels at the low shear stresses obviously involved in flow under its own weight. The gel F seems to be superior to the other gels in this respect. Gels A through E did not show any noticeable elastic effects. However, gel F showed considerable elasticity at low shear stresses as reflected in the slow decay of the torque at low shear rates (Fig 3). This elasticity may well be significant clinically in that the gel may be more difficult to displace. Temperature has only a small effect when compared with shear rate. This small effect of temperature is probably due to the balancing effect between the result of increase
0 2
z -C
ux
Six commercial fluoride gels have been studied, using a cone and plate viscometer. Also, the thickening agents have been analyzed using infrared spectroscopy. All gels showed stress thinning, which is the decrease of viscosity with shear rate. Such shear rate dependence is clinically convenient in that the gel will flow readily at the high shear stresses present zwhen the gel is applied but will not flow readily under its own weight when on the tooth. Five materials containing hydroxyalkyl celluloses showed similar degrees of shear thinning. One material with a noncellulosic thickener showed much more extreme stress thinning together with elastic behavior at low shear rates; such behavior may be clinically advantageous. All of the gels showed only slight temperature dependence of rheological properties.
Materials containing fluoride ions occupy an important place in preventive dentistry.' These ions have both systemic and topical effects on teeth. The latter effect can be obtained either by the addition of sodium fluoride to central water supplies or from solutions, gels, pastes, and so forth, applied topically on an individual basis.2 The gels are preferred to aqueous solutions since they can be more easily kept in contact with many of the teeth for the required time.3 However, little information is available regarding the physical characteristics such as the rheology of these gels. The present investigation therefore describes the measurement of the rheological properties and a preliminary study of their content. Dr. Perera was supported by a British Council Scholarship. A grant from the Central Research Fund Committee of the University of London was used to purchase the Weissenberg Rheogoniometer. Received for publication June 10, 1975. Accepted for publication October 15, 1975.
Materials and Methods Six commercial gels were studied. The gels were Pacemaker,a Hifluor Gel,b FluorO-Dent,c Rafluor Gel,d En-De-Kay,e and De Treys, Alpha-Gelf; and are designated A, B, C, D, E, and F, respectively. The rheology of the gel was studied with a cone and plate viscometer.g This consists of a flat plate that is stationary, and a conical plate that can be rotated at different speeds as described elsewhere.4 Measurements of shear rate and shear stress were made, and time-dependent effects, if any, were carefully investigated. Materials were examined over the shear rate range 0.014 to 1,390 sec-1 at both 25 C and 35 C. The six different gels being studied were placed on six different watch glasses. Watch glasses were weighed before and after placement of the gels. These were then kept in a vacuum desiccator with a calcium sulfate desiccant. The watch glasses containing the gels were taken from the desiccator and weighed at convenient time intervals until each reached a constant value. Inorganic ionic material was removed by shaking the gel with an ion-exchange resinh; films of the polymeric thickening agents were then cast, dried in a vacuum desiccator, and the infrared spectrai determined. Films of sodium carboxymethylcellulose, methylcellulose, and hydroxypropyl methylcellulose Pacemaker Corp., Portland, Ore. Allied Laboratories Ltd., London, Eng. Den-Tal-Ez Mfg. Co., Columbus, Ohio. d Pascal Co., Inc., Bellevue, Wash. e Westone Products Ltd., London, Eng. f Amalgamated Dental Co., London, Eng. a
b
9 Weissenberg Rheogoniometer. Sangamo Controls Ltd., Bogner Regis, Eng. h Biodeminrolit BDH Chemicals Ltd., Poole, Eng. i Pye-Unicam Infrared Spectrophotometer SP 1000, Cambridge, Eng.
Downloaded from jdr.sagepub.com at UCSF LIBRARY & CKM on June 14, 2015 For personal use only. No other uses without permission.
353
354
J Dent Res May-June 1976
BRADEN AND PERERA
TABLE 1 Viscosrry VALUES AT Low AND HIGH SHEAR RATES Viscosity (poise)
Material
Gel A Gel B Gel C Gel D GelE Gel F
0.0140 sec-' 35 C 25 C
700
340 280 450 290 650 6,000
600 560
450 780
7,500
1,390 sec-
25 C
35 C
2.1 2.0 1.9 1.6 2.2 0.28
1.8 1.5 1.5 1.0
1.7 0.25
0
10
Shfta, Rats ("c-l
FIG I.-Plot of shear stress against shear rate for material C (crcles), and material F (triangles), slope of Newtonian fluid (broken line) .
were cast, and their spectra determined for comparison purposes.
Results RHEOLOGY OF THE GELS.-Materials A through E behaved very similarly. Therefore, Figure 1 showing shear stress plotted against shear rate, shows data for material C only, together with material F that behaved very differently. (The broken line shows the unit slope that would be obtained for a Newtonian liquid) . In Figure 2 the data are plotted as apparent viscosity (Shear StressShear Rate) as a function of shear rate for
25 and 35 C; again, material C only is seen as typical of materials A through E, together with material F. As in all instances, apparent viscosity decreased monotonically with shear rate. In Table 1 the data for all materials and conditions are given in terms of the apparent viscosity at 0.014 sec-1 and 1,390 sec-1, the limits of shear rates used. Figure 3 shows the torque decay when the rheogoniometer was stopped for material F, the only material that noticeably showed this effect. THICKENING AGENT CONTENT.-About 90 to
95% of the water content of each of the gels was lost during the first two days. Subsequently, the loss per day was very small. This decrease continued for about a month and then the weight fluctuated slightly, depending on the humidity of the room at the time of measurement. Table 2 gives this data.
10 F
FIG 2.-Plot of apparent viscosity against shear rate. Material C: open circles, 25 C; closed circles, 35 C. Material F: open triangles, 25 C; closed triangles, 35 C.
'v
t;"mns
'
FIG 3.-Plot of residual torque as function of time from switching off viscometer. (Normalized
torque - torque at time t/torque at t =
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0.)
eAl._
RHEOLOGY OF FLUORIDE GELS
Vol 35 No. 3
355
FIG 4.-Infrared spectra of material A and E compared with hydroxypropyl cellulose (HPC).
INFRARED SPECTROSCOPY.-Figure 4 shows the comparison of the spectra of materials A and E with hydroxypropyl methylcellulose. Materials B, C, and D were almost identical to A and are not seen here. Figure 5 shows the spectrum of material F. Discussion RHEOLOGY OF THE GELS.-Figure 1 shows that all of the gels are non-Newtonian; material F is peculiar in that the linear plot shows it to be a power law fluid. In fact, it obeys the law r = ka-n (n = 0.342), where r is shear stress and a shear rate. As with most non-Newtonian substances the apparent viscosity (,q) decreased with increasing shear rate (Fig 2) for all gels studied (shear thinning or pseudoplastic flow behavior). All gels except gel F produced a similar pattern (Fig 2). The gel F, however, produced a straight line with a negative slope showing very high viscosity at low shear rate and very low viscosity at high shear rate (Table
2). TABLE 2 WEIGHT FRACrION OF THE DRY RESIDUE OF GEL AFTER WATER REMOVAL Dry Residue
Material
(%)
A B C
10.36
Gel Gel Gel Gel Gel Gel
D E F
4.71 5.91 4.83 5.06 4.29
The shear thinning experienced by all materials suggests that the flow of the material will be facilitated by the higher shear stresses consequent on the pressure during the application of these gels to teeth. However, once the material is applied to the teeth, the retention of the gel by the teeth will be made easier by the highly viscous nature of these gels at the low shear stresses obviously involved in flow under its own weight. The gel F seems to be superior to the other gels in this respect. Gels A through E did not show any noticeable elastic effects. However, gel F showed considerable elasticity at low shear stresses as reflected in the slow decay of the torque at low shear rates (Fig 3). This elasticity may well be significant clinically in that the gel may be more difficult to displace. Temperature has only a small effect when compared with shear rate. This small effect of temperature is probably due to the balancing effect between the result of increase
0 2
z -C
ux
