Dent Mater 8:354-358, November,1992
Surface properties and castability of elastomeric impression materials after plasma cleaning C. P. Fernandes, N. Vassilakos, K. Nilner
Department of Prosthetic Dentistry, Center for Oral Health Sciences, Lund University, Sweden
Abstract. The wettability of impression materials is an important requirement for accurate reproduction of intraoral structures. The aims of this study were to evaluate the effect of plasma cleaning on critical surface tension and die stone castability of five silicone impression materials. Critical surface tension was calculated using a contact angle analysis according to Zisman (1964), before and after plasma cleaning. Die stone casts were produced from impressions of a master die and the area extension of the reproduction was measured planimetrically. The results showed a general increase in the critical surface tension and in the polar force component of the composite free energy for materials treated with plasma cleaning. An improvement in castability of all materials was also noted.
INTRODUCTION Voids and incomplete areas in die stone casts are often located in margins, grooves and pinholes of abutment teeth preparations, resulting in unacceptable models for use in restorative procedures. These defects have previously been related to the wetting properties ofimpression materials (Pratten and Craig, 1989; Chong et al., 1990). The wettability of impression materials has been investigated by measuring the contact angles of drops from aqueous solutions ofCaSO 4(Norling and Reisbick, 1979; Pratten and Craig, 1989; Chai and Yeung, 1991; Vassilakos et al., In Press) or of die stone (Lorren et al., 1976; Lacy et al., 1977; Chong et al., 1990) on flat impression surfaces. It has recently been reported that plasma treatment of elastomeric impression materials resulted in lower contact angles of CaSO 4 aqueous solution on the tested materials (Bochiechio et al., 1991; Vassilakos et al., In Press) and in die stone casts with fewer voids (Vassilakos et al., In Press). Plasma is a gas in an excited state consisting of atoms, molecules, ions, free radicals and electrons. Glow discharge is the process when plasma is generated under reduced pressure. The technique of plasma treatment has been previously used to modify the surface properties of polymer materials without affecting the bulk properties (Gombotz and Hoffman, 1987). The use of various CaSO 4 solutions for contact angle analysis (Norling and Reisbick, 1979; Pratten and Craig, 1989; Chai and Yeung, 1991; Vassilakos et al., In Press) only quantifies the interaction of the impression material surfaces with the test products. When more specific information about the surface properties of these materials is desired, more sensitive techniques which can detect changes in the outer354 Femandes et al./Plasma cleaning of impression materials
most layers of the set materials should be used. Contact angle analysis, using a series of pure liquids with known surface tensions, has been extensively used for defining surface properties of biomedical materials (Baier and Mayer, 1983). Some of the advantages of this technique are that it is relatively simple to perform, and it gives information about the outermost layers of the studied substrates (5-10 A). The results of such analysis are usually expressed as critical surface tension of wetting which was described by Zisman (1964) as an empirical parameter of the surface energy state of a given material. The critical surface tension value reflects the surface tension of a hypothetical liquid which spreads on the studied substrate, giving zero contact angle. The contact angle data of this type of analysis can also be used for calculations of surface energy according to the concept of additivity of the composite free energy (Kaelble, 1970), which proposes that the surface free energy could be represented by the sum of the contributions arising from different types of force components (polar and dispersion). The calculation ofthe percentage polarity (or the percentage of dispersion forces) offers the possibility of detecting which type of forces change, if a shift in the surface free energy of a material takes place. In previous investigations, voids in die stone casts were counted, irrespectively of their size, and scores were given as total numbers of voids. This method relies on the observer to identify the voids and on varying criteria for void sizes. Though counting voids in die stone casts might contribute to an overall appreciation of the castability, it cannot accurately depict small changes in surface wetting properties. A more precise alternative would be to use a computerized system to digitally measure the area of die-stone casts reproduced from an original master die. The aim of this study, therefore, was to analyze the surface properties ofset elastomericimpressionmaterials after plasma cleaning by calculating critical surface tension and apparent polarity ratios of the composite free energy. Further, since the wettability of impression materials has previously been related to the quality of die stone casts, a complementary aim was to study the effect of plasma cleaning on the ability of the materials tested to produce complete casts. MATERIALS AND METHODS One condensation and four addition silicone impression materials were used in this study. The type of products tested, their viscosity, commercial names, codes used in the study,
SIDE VIEW
TABLE 1: IMPRESSIONMATERIALSUSEDIN THE STUDY
Type of material Condensation silicone Addition silicone Addition silicone Addition silicone Addition silicone
Viscosity Product
Low
Code Manufacturer BatchNo.
Xantopren CSX
Bayer, Base 8157A Leverkusen, Cat31230 Germany Bayer
ASP
Provil
Low
Coltene AG, Base080191-47 President ASPR Zurich, Cat200491-47 Switerland
Low
Imprint Hydrophylic ASI
3M, St. Paul, 2AN8TA1R MN, USA
Express Hydrophylic ASE
3M
Low
1AL2X70
TABLE 2: TEST LIQUIDS USEDIN THE CONTACTANGLEANALYSISAND THEIR RESPECTIVESURFACETENSIONS
Water Glycerol Thiodiglycol Methylene iodide S-Tetrabromomethane Propylene carbonate
30rmn I .I 4
Base 8009T Cat 8028T
Low
Liquid
1n m
Surface tension mN/m 72.4 63.7 53.5 51.7 49.8 41.8
manufacturer's information and respective batch numbers are given in Table 1. The impression materials were handled according to the manufacturer's instructions. Contact angle analysis. Specimens from the test materials were obtained using a plexiglass mold with dimensions of 20 x 10 x 2 mm. Clean glass slabs were placed on the top of the freshly mixed materials to form flat surface. The materials were allowed to set for 10 min. Then the specimens were carefully inspected, and those with defects were discarded. Twenty specimens of each material were produced with this method. Since an adsorbed salivary film always coats the surface of impression materials in normal clinical conditions, the specimens were dipped in saliva containing glass dishes containing saliva for 10 min. The saliva samples were collected from three healthy individuals, pooled, clarified by means of centrifugation at 15000 x g for 20 rain and finally stored at -80°C until used. After saliva coating, the specimens were thoroughly rinsed with double distilled water and blown dry with filtered compressed air. Six liquids were used for contact angle analysis. The names of these liquids and their respective surface tensions are given in Table 2. Three droplets of 0.05 mL from each liquid were dispensed on the surface of the specimens. After equilibrium, the drops were registered with a video camera (Jai type No 711.16.00 CV, 40-10 Image Analyzer, AMS Ltd, Tokyo, Japan) and documented directly on a video printer (Mitsubishi Video Copy Processor, Tokyo, Japan). Contact angles were then determined with the use of a graduated protractor. The values of all registered contact angles were entered in a computer program (made by Mazurowski and Baier, Cornell Aeronautical Laboratory, Buffalo, NY, USA) that was used to
17.5rmn 175foul1
I=,
I
80
9"5ramI
075 mm 10 mm
1,0n~
2.5 m m
TOP VIEW
Fig. 1. Schematic illustration of the machined brass critical die, used to produce die stone casts for the castability comparisons.
calculate the critical surface tension of wetting 7c of the specimens according to Zisman (1964). Using the concept of additivity of force components, the apparent polar 7p and dispersion force components 7d of the composite surface free energy 7s (Kaelble, 1970) were determined according to the mathematical calculations modified by Nyilas et al. (1977). The polarity of the surface free energy was expressed as percentage polarity % 7p = [ 7p/ (Tp + 7d)] x 100. Contact angles were performed on two groups of ten specimens from each impression material tested. The specimens of the first group were used after saliva coating without any further treatment. The specimens of the second group were plasma cleaned for 5 rain in residual air at low pressure (0.10.05 torr). Plasma treatments were performed with a radiofrequency glow discharge unit type Harrick, PDC-3XG (Harrick Scientific Corp, Ossing, NY, USA). Castability. Impressions were made of a master die, with a custom acrylic tray, using the same impression materials as for the contact angles analysis. A schematic drawing with the dimensions of the machined brass master die is shown in Fig 1. Twenty impressions were made of the master die for each impression material. The impressions were then coated with saliva as described above and divided in two groups of ten impressions each. Those of the first group were poured with a type IV die stone ("Indic-Die-stone", Modern Materials, Miles Inc, St Louis MO, USA) using a water-powder ratio of 22 cc:100 g. The stone was mechanically mixed in vacuum (Degussa Typ R3, Frankfurt, Germany). The impressions were poured 1 rain after the start of mixing, using a frequency vibration of 60 Hz and with the help of a paint brush. The impressions of the second group were first plasma-treated according to the above described procedure and then poured with the same die stone. The produced die stone casts were subsequently placed on the stage of a microscope (Vickers, Photoplan, Frankfurt, Germany) illuminated with low-angle light. Black and white high-contrast photographs were taken of each one of the six squares of the casts at 25x magnification with a camera adjusted to the eye piece. Six photographs were taken in all for each die stone cast. This procedure was also performed for the master die. The area of each square was then measured digitally from the photographs, and a score was given for each cast by summing up the area extension of the six squares. The area calculations were performed using a graphic digitizer (Calcomp 2500, Anaheim, CA, USA) and a mouse pen to follow Dental Materials~November 1992 355
mN,m 80 ~
lDBofo
oa eo,'P, . . . . ,ean. J
% polarity
D Before treatment
•
ASP
ASPR
Plasma cleaned
1
~o T
70
60
60
50
50 40 40 30
30 20
20
10
10
0 CSX
ASP
ASPR Impression
ASI
ASE
0 CSX
Materials
Fig. 2. Mean values and standard deviations of critical surface tension (7,-mN/m) calculated for all impression materials before and after plasma cleaning (p
Surface properties and castability of elastomeric impression materials after plasma cleaning C. P. Fernandes, N. Vassilakos, K. Nilner
Department of Prosthetic Dentistry, Center for Oral Health Sciences, Lund University, Sweden
Abstract. The wettability of impression materials is an important requirement for accurate reproduction of intraoral structures. The aims of this study were to evaluate the effect of plasma cleaning on critical surface tension and die stone castability of five silicone impression materials. Critical surface tension was calculated using a contact angle analysis according to Zisman (1964), before and after plasma cleaning. Die stone casts were produced from impressions of a master die and the area extension of the reproduction was measured planimetrically. The results showed a general increase in the critical surface tension and in the polar force component of the composite free energy for materials treated with plasma cleaning. An improvement in castability of all materials was also noted.
INTRODUCTION Voids and incomplete areas in die stone casts are often located in margins, grooves and pinholes of abutment teeth preparations, resulting in unacceptable models for use in restorative procedures. These defects have previously been related to the wetting properties ofimpression materials (Pratten and Craig, 1989; Chong et al., 1990). The wettability of impression materials has been investigated by measuring the contact angles of drops from aqueous solutions ofCaSO 4(Norling and Reisbick, 1979; Pratten and Craig, 1989; Chai and Yeung, 1991; Vassilakos et al., In Press) or of die stone (Lorren et al., 1976; Lacy et al., 1977; Chong et al., 1990) on flat impression surfaces. It has recently been reported that plasma treatment of elastomeric impression materials resulted in lower contact angles of CaSO 4 aqueous solution on the tested materials (Bochiechio et al., 1991; Vassilakos et al., In Press) and in die stone casts with fewer voids (Vassilakos et al., In Press). Plasma is a gas in an excited state consisting of atoms, molecules, ions, free radicals and electrons. Glow discharge is the process when plasma is generated under reduced pressure. The technique of plasma treatment has been previously used to modify the surface properties of polymer materials without affecting the bulk properties (Gombotz and Hoffman, 1987). The use of various CaSO 4 solutions for contact angle analysis (Norling and Reisbick, 1979; Pratten and Craig, 1989; Chai and Yeung, 1991; Vassilakos et al., In Press) only quantifies the interaction of the impression material surfaces with the test products. When more specific information about the surface properties of these materials is desired, more sensitive techniques which can detect changes in the outer354 Femandes et al./Plasma cleaning of impression materials
most layers of the set materials should be used. Contact angle analysis, using a series of pure liquids with known surface tensions, has been extensively used for defining surface properties of biomedical materials (Baier and Mayer, 1983). Some of the advantages of this technique are that it is relatively simple to perform, and it gives information about the outermost layers of the studied substrates (5-10 A). The results of such analysis are usually expressed as critical surface tension of wetting which was described by Zisman (1964) as an empirical parameter of the surface energy state of a given material. The critical surface tension value reflects the surface tension of a hypothetical liquid which spreads on the studied substrate, giving zero contact angle. The contact angle data of this type of analysis can also be used for calculations of surface energy according to the concept of additivity of the composite free energy (Kaelble, 1970), which proposes that the surface free energy could be represented by the sum of the contributions arising from different types of force components (polar and dispersion). The calculation ofthe percentage polarity (or the percentage of dispersion forces) offers the possibility of detecting which type of forces change, if a shift in the surface free energy of a material takes place. In previous investigations, voids in die stone casts were counted, irrespectively of their size, and scores were given as total numbers of voids. This method relies on the observer to identify the voids and on varying criteria for void sizes. Though counting voids in die stone casts might contribute to an overall appreciation of the castability, it cannot accurately depict small changes in surface wetting properties. A more precise alternative would be to use a computerized system to digitally measure the area of die-stone casts reproduced from an original master die. The aim of this study, therefore, was to analyze the surface properties ofset elastomericimpressionmaterials after plasma cleaning by calculating critical surface tension and apparent polarity ratios of the composite free energy. Further, since the wettability of impression materials has previously been related to the quality of die stone casts, a complementary aim was to study the effect of plasma cleaning on the ability of the materials tested to produce complete casts. MATERIALS AND METHODS One condensation and four addition silicone impression materials were used in this study. The type of products tested, their viscosity, commercial names, codes used in the study,
SIDE VIEW
TABLE 1: IMPRESSIONMATERIALSUSEDIN THE STUDY
Type of material Condensation silicone Addition silicone Addition silicone Addition silicone Addition silicone
Viscosity Product
Low
Code Manufacturer BatchNo.
Xantopren CSX
Bayer, Base 8157A Leverkusen, Cat31230 Germany Bayer
ASP
Provil
Low
Coltene AG, Base080191-47 President ASPR Zurich, Cat200491-47 Switerland
Low
Imprint Hydrophylic ASI
3M, St. Paul, 2AN8TA1R MN, USA
Express Hydrophylic ASE
3M
Low
1AL2X70
TABLE 2: TEST LIQUIDS USEDIN THE CONTACTANGLEANALYSISAND THEIR RESPECTIVESURFACETENSIONS
Water Glycerol Thiodiglycol Methylene iodide S-Tetrabromomethane Propylene carbonate
30rmn I .I 4
Base 8009T Cat 8028T
Low
Liquid
1n m
Surface tension mN/m 72.4 63.7 53.5 51.7 49.8 41.8
manufacturer's information and respective batch numbers are given in Table 1. The impression materials were handled according to the manufacturer's instructions. Contact angle analysis. Specimens from the test materials were obtained using a plexiglass mold with dimensions of 20 x 10 x 2 mm. Clean glass slabs were placed on the top of the freshly mixed materials to form flat surface. The materials were allowed to set for 10 min. Then the specimens were carefully inspected, and those with defects were discarded. Twenty specimens of each material were produced with this method. Since an adsorbed salivary film always coats the surface of impression materials in normal clinical conditions, the specimens were dipped in saliva containing glass dishes containing saliva for 10 min. The saliva samples were collected from three healthy individuals, pooled, clarified by means of centrifugation at 15000 x g for 20 rain and finally stored at -80°C until used. After saliva coating, the specimens were thoroughly rinsed with double distilled water and blown dry with filtered compressed air. Six liquids were used for contact angle analysis. The names of these liquids and their respective surface tensions are given in Table 2. Three droplets of 0.05 mL from each liquid were dispensed on the surface of the specimens. After equilibrium, the drops were registered with a video camera (Jai type No 711.16.00 CV, 40-10 Image Analyzer, AMS Ltd, Tokyo, Japan) and documented directly on a video printer (Mitsubishi Video Copy Processor, Tokyo, Japan). Contact angles were then determined with the use of a graduated protractor. The values of all registered contact angles were entered in a computer program (made by Mazurowski and Baier, Cornell Aeronautical Laboratory, Buffalo, NY, USA) that was used to
17.5rmn 175foul1
I=,
I
80
9"5ramI
075 mm 10 mm
1,0n~
2.5 m m
TOP VIEW
Fig. 1. Schematic illustration of the machined brass critical die, used to produce die stone casts for the castability comparisons.
calculate the critical surface tension of wetting 7c of the specimens according to Zisman (1964). Using the concept of additivity of force components, the apparent polar 7p and dispersion force components 7d of the composite surface free energy 7s (Kaelble, 1970) were determined according to the mathematical calculations modified by Nyilas et al. (1977). The polarity of the surface free energy was expressed as percentage polarity % 7p = [ 7p/ (Tp + 7d)] x 100. Contact angles were performed on two groups of ten specimens from each impression material tested. The specimens of the first group were used after saliva coating without any further treatment. The specimens of the second group were plasma cleaned for 5 rain in residual air at low pressure (0.10.05 torr). Plasma treatments were performed with a radiofrequency glow discharge unit type Harrick, PDC-3XG (Harrick Scientific Corp, Ossing, NY, USA). Castability. Impressions were made of a master die, with a custom acrylic tray, using the same impression materials as for the contact angles analysis. A schematic drawing with the dimensions of the machined brass master die is shown in Fig 1. Twenty impressions were made of the master die for each impression material. The impressions were then coated with saliva as described above and divided in two groups of ten impressions each. Those of the first group were poured with a type IV die stone ("Indic-Die-stone", Modern Materials, Miles Inc, St Louis MO, USA) using a water-powder ratio of 22 cc:100 g. The stone was mechanically mixed in vacuum (Degussa Typ R3, Frankfurt, Germany). The impressions were poured 1 rain after the start of mixing, using a frequency vibration of 60 Hz and with the help of a paint brush. The impressions of the second group were first plasma-treated according to the above described procedure and then poured with the same die stone. The produced die stone casts were subsequently placed on the stage of a microscope (Vickers, Photoplan, Frankfurt, Germany) illuminated with low-angle light. Black and white high-contrast photographs were taken of each one of the six squares of the casts at 25x magnification with a camera adjusted to the eye piece. Six photographs were taken in all for each die stone cast. This procedure was also performed for the master die. The area of each square was then measured digitally from the photographs, and a score was given for each cast by summing up the area extension of the six squares. The area calculations were performed using a graphic digitizer (Calcomp 2500, Anaheim, CA, USA) and a mouse pen to follow Dental Materials~November 1992 355
mN,m 80 ~
lDBofo
oa eo,'P, . . . . ,ean. J
% polarity
D Before treatment
•
ASP
ASPR
Plasma cleaned
1
~o T
70
60
60
50
50 40 40 30
30 20
20
10
10
0 CSX
ASP
ASPR Impression
ASI
ASE
0 CSX
Materials
Fig. 2. Mean values and standard deviations of critical surface tension (7,-mN/m) calculated for all impression materials before and after plasma cleaning (p
