Effectiveness of hardening films on die stone

Surface Coating of Gypsum-Based Molds for Maxillofacial Prosthetic Silicone Elastomeric Material: Evaluating Different Microbial Adhesion

PURPOSE

To compare the adhesion of three microorganisms on modified and unmodified silicone elastomer surfaces with different surface roughnesses and porosities.

MATERIALS AND METHODS

Candida albicans, Streptococcus mutans, and Staphylococcus aureus were incubated with modified and unmodified silicone groups (N = 35) for 30 days at 37°C. The counts of viable microorganisms in the accumulating biofilm layer were determined and converted to cfu/cm² unit surface area. A scanning electron microscope (SEM) was used to evaluate the microbial adhesion. Statistical analysis was performed using t-test, one-way ANOVA, and post hoc tests as indicated.

RESULTS

Significant differences in microbial adhesion were observed between modified and unmodified silicone elastomers after the cells were incubated for 30 days (p < 0.001). SEM showed evident differences in microbial adhesion on modified silicone elastomer compared with unmodified silicone elastomer.

CONCLUSIONS

Surface modification of silicone elastomer yielding a smoother and less porous surface showed lower adhesion of different microorganisms than observed on unmodified surfaces.

Comparison of the Effect of Addition of Cyanoacrylate, Epoxy Resin, and Gum Arabic on Surface Hardness of Die Stone

PURPOSE

To observe the effects of incorporating cyanoacrylate, epoxy resins, and gum arabic on the abrasion resistance of type IV gypsum die materials.

MATERIALS AND METHODS

Forty specimens were prepared and divided into four groups (10 specimens in each group), namely group A (control), group B (die stone mixed with cyanoacrylate), group C (die stone mixed with epoxy resin), group D (die stone mixed with gum arabic). All the specimens were subjected to abrasion testing, wear volume analysis, Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM) analysis.

RESULTS

Abrasion testing showed maximum wear in the control group and minimum wear in the gum arabic group. Intergroup differences were statistically significant (p < 0.001). The largest mean difference was between control and gum arabic. The lowest was between cyanoacrylate and the control group. The mean wear volume was lowest in the gum arabic group (4.23 mm(3) ) and highest in the control group (6.78 mm(3) ). The FT-IR graphs of the gum arabic models showed the presence of CH2 , which is responsible for its binding activity. SEM revealed that the irregular particles of gum arabic display an interlocking arrangement. This jigsaw puzzle pattern results in stronger physical bond formation.

CONCLUSION

Observations from this study showed that the addition of gum arabic increases resistance to abrasion in type IV gypsum. Cyanoacrylates are good adhesives as well, but a major drawback is that they have very low resistance to chemical action with water and physical actions such as sunlight. Epoxy resins are powerful adhesives, but they attain their full efficiency when cured with heat. Cyanoacrylate and epoxy resin displayed poor physical bonding, primarily because of inhomogeneity.

Surface Coating of Gypsum-Based Molds for Maxillofacial Prosthetic Silicone Elastomeric Material: The Surface Topography

PURPOSE

This study aimed to compare the surface roughness of maxillofacial silicone elastomers fabricated in noncoated and coated gypsum materials. This study was also conducted to characterize the silicone elastomer specimens after surfaces were modified.

MATERIALS AND METHODS

A gypsum mold was coated with clear acrylic spray. The coated mold was then used to produce modified silicone experimental specimens (n = 35). The surface roughness of the modified silicone elastomers was compared with that of the control specimens, which were prepared by conventional flasking methods (n = 35). An atomic force microscope (AFM) was used for surface roughness measurement of silicone elastomer (unmodified and modified), and a scanning electron microscope (SEM) was used to evaluate the topographic conditions of coated and noncoated gypsum and silicone elastomer specimens (unmodified and modified) groups. After the gypsum molds were characterized, the fabricated silicone elastomers molded on noncoated and coated gypsum materials were evaluated further. Energy-dispersive X-ray spectroscopy (EDX) analysis of gypsum materials (noncoated and coated) and silicone elastomer specimens (unmodified and modified) was performed to evaluate the elemental changes after coating was conducted. Independent t test was used to analyze the differences in the surface roughness of unmodified and modified silicone at a significance level of p < 0.05.

RESULTS

Roughness was significantly reduced in the silicone elastomers processed against coated gypsum materials (p < 0.001). The AFM and SEM analysis results showed evident differences in surface smoothness. EDX data further revealed the presence of the desired chemical components on the surface layer of unmodified and modified silicone elastomers.

CONCLUSIONS

Silicone elastomers with lower surface roughness of maxillofacial prostheses can be obtained simply by coating a gypsum mold.

Dimensional changes in gypsum fragments bonded with cyanoacrylate

PURPOSE

Accidental fractures may occur during manipulation and transportation of plaster casts. In clinical practice, plaster fragments may be bonded without harming the accuracy of the final denture, provided that the bonding agent does not cause dimensional alterations. Cyanoacrylate could be a good material because of its ease of use, quick set, wide availability, and low cost. The aim of this study was to assess the dimensional alteration of Type IV plaster fragments bonded with a cyanoacrylate-based adhesive.

MATERIALS AND METHODS

Ten hexagonal regular prisms were made of Type IV plaster, with two reference marks on one of the faces. The distance between the marks was measured under a comparison microscope. After this, the prisms were fractured so that the fracture line would be between the two reference marks, bonded with a cyanoacrylate-based universal adhesive and measured again.

RESULTS

The mean difference between the measurements performed before and after fracture and bonding of the fragments was 0.0194 mm. At a level of significance of 0.05, there was no statistically significant difference between the measurements before and after fracture and bonding of the dies (p = 0.1582).

CONCLUSION

It may be concluded that bonding of Type IV plaster fragments with a cyanoacrylate-based adhesive did not cause significant dimensional alterations.

Alterations of surface hardness with gypsum die hardeners

STATEMENT OF PROBLEM

Die stones require abrasion resistance, dimensional stability with time, and high surface wettability material properties.

PURPOSE

The purpose of this study was to compare the surface microhardness (Knoop) of 4 contemporary gypsum materials with and without surface die hardener.

MATERIAL AND METHODS

Materials used were a Type III stone (Microstone) and 3 die stones (Die-Keen, Silky-Rock, and ResinRock). Die hardener was cyanoacrylate (Permabond 910) or Clear Coat. Specimens of stone were hand mixed with distilled room temperature water and vacuum spatulated according to manufacturer's directions. Five cylinders (15 x 15 mm) per group were poured, using vibration, into phenolic ring molds positioned on top of a glass slide. The face of each specimen was polished with 2400-grit Al2O sandpaper. One face of each of 5 specimens/material was coated with cyanoacrylate; 5 specimens/material were coated with Clear Coat, air thinned and dried; and 5 specimens/material had no treatment (control). Knoop hardness (kg/mm2) readings were made on each face (5 readings/time point) 3, 12, and 24 hours after pouring. An ANOVA procedure with post hoc Tukey tests were performed (alpha=.05).

RESULTS

Microhardness did not vary between 3 and 24 hours for any material (P>.05). Microstone had significantly lower surface hardness (P<.0001) than the die stones. Specimens coated with die hardener had lower hardness values (P<.001 in all cases)

CONCLUSIONS

The 3 types of die stones evaluated in this study did not differ significantly in surface microhardness. Under these conditions, die hardener coatings reduced the surface hardness of the gypsum material.

Influence of surface hardener on gypsum abrasion resistance and water sorption

STATEMENT OF PROBLEM

Abrasion is a major concern when gypsum products are used for dies, leading to the frequent recommendation that surface hardeners should be used before waxing or scanning.

PURPOSE

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

MATERIAL AND METHODS

Three ADA Type IV (Vel-Mix, ResinRock, and Silky-Rock) and 1 Type V die material (Die-Keen) were evaluated for abrasion resistance after application of 2 surface hardeners (Permabond 910 cyanoacrylate and Clear Coat). Thirty specimens of each material were fabricated using an impression of a standard brass die machined with 1-mm high ridges, sloped 45 degrees. Gypsum materials were mixed according to manufacturers' recommendations and allowed to set 1 hour before separating. All replicated dies were allowed to bench set for 14 days before testing. One hour before testing, specimens were arbitrarily assigned to 1 of 3 treatment subgroups (n=10/group): no treatment (control), coated with Permabond 910, or coated with Clear Coat. In the coated groups, die hardener was painted over the grooves and air dried. Abrasion resistance (measured by weight loss) was evaluated using a reciprocal abrasion device in which a stylus applied a 50-g mass perpendicular to the ridges. Mass loss was determined using an analytical balance before and after each test cycle. Five sets of 20 unidirectional passes were made on each specimen. A scanning electron microscope was used to evaluate the surface of specimens in each treatment subgroup. Water sorption was also evaluated using 2 Type IV (Silky-Rock, ResinRock) and 1 Type III (Microstone) gypsum materials. Specimen dies were separated 1 hour after pouring the impression and allowed to bench set 1 week before testing. Five specimens from each material group received a coating of a surface hardener 1 hour before testing. Specimens were placed in distilled water for 15 minutes and differences in mass were determined using an analytical balance before and after each test. A 2-way analysis of variance was completed followed by a Tukey post hoc test (alpha=.05).

RESULTS

The 2-way analysis of variance revealed an interaction between product and surface coating (P=.0459). Given this interaction, the 12 combinations determined by surface treatment and material type were considered individually using the Tukey method. Vel-Mix, control (2.62 +/- 2.64 mg) had the most material loss and Vel-Mix, Clear Coat (0.48 +/- 0.29 mg) had the least material loss. Water sorption results indicated an interaction between the gypsum material and the surface treatment (P<.0001). The control groups of Microstone (299.2 +/- 49.6 mg) and Silky-Rock (159.0 +/- 8.5 mg) showed the most water sorption compared with the other treatment groups.

CONCLUSIONS

This study demonstrated that a significant improvement in abrasion resistance occurred only with specific gypsum/surface hardener material combinations. Also, water sorption decreased significantly for Microstone and Silky-Rock gypsum materials when a surface hardener was used.

Film thickness of die coating agents

Die coating agents are often used during the fabrication of porcelain margins for crowns and fixed partial dentures. These agents seal microscopic irregularities within the stone die and resist abrasion. This investigation measured the film thickness of several die coating agents. A specific reference point was used during the measurement process. The results indicate that there is no significant difference between the die coating agents examined under the conditions of the study. The film thickness measurements ranged between 0.367 microns for Duro Super Glue Material and 2.3 microns for Zap CA material.

Mold inversion and abrasion resistance of a type IV stone

The objective of this study was to determine whether placing the impression and diestone so the impression is on the top during setting affects the resulting die's abrasion resistance. Impressions of a brass bar with a milled 90 degrees edge were obtained by use of a polysulfide impression material in a rigid brass tray. A Type IV diestone was vacuum-mixed and vibrated into the impression. Eight procedures were compared: (1) no inversion; (2) inversion at initial set; (3,4) inversion one or two min before initial set; and (5,6,7,8) inversion 1.5, 3, 5, or 12 min after initial set. Each specimen was abraded 24 hours after mixing. Specimens inverted at 1.5 and 3 min after initial set were significantly more abrasion-resistant than were all other treatments.

The effects of a liquid dispersing agent and a microcrystalline additive on the physical properties of type IV gypsum

This study evaluated the effects of a liquid dispersing agent (LDA) and a microcrystalline additive (MCA) on selected physical properties of type IV gypsum. Working consistency, setting time, setting expansion, and compressive strength (1 hour and 7 days) were determined, following ADA Specification No. 25, on a standard, LDA (0.5, 1, 1.5, and 2 mL), MCA (21.1, 24.1, and 27.1 gm), and combination (LDA 0.75 mL + MCA 12.05 gm) mixes per 300 gm of gypsum. Results indicate that the additives affect the consistency of the mix, but consistency can be kept close to that of the standard by lowering the water/powder ratio. Statistical analysis of the data indicated that the additives significantly affected the setting time, setting expansion, and both the 1-hour and the 7-day compressive strengths. SEM examination of fractured surfaces of test mixes indicated improved crystal packing. The properties of type IV gypsum can be improved by optimizing the amount of LDA and MCA additives.

Effect on the fit of removable partial denture frameworks when master casts are treated with cyanoacrylate resin

Master casts of improved dental stone were made from irreversible hydrocolloid impressions of a metal model. Half of the master casts were treated with cyanoacrylate resin and half were untreated controls. They were sent to a dental laboratory for framework fabrication. The returned frameworks were examined for acceptability and their completeness of seating was determined. The frameworks fabricated on the treated master casts fit significantly better than the controls. The improved seating may be attributed to an increased surface hardness, increased abrasion resistance, and improved surface morphology, but further study is necessary.

The esthetic metal-ceramic margin: a comparison of techniques

Metal-ceramic crowns with all-porcelain labial margins may have esthetic and biologic advantages. Several techniques for making these restorations have been described in the prosthodontic literature. The authors used all the techniques and compared the results in terms of quality and degree of difficulty in construction. Clinically acceptable results can be achieved with all described techniques. The advantages and disadvantages of each method are summarized in Table I.

Effects of cyanoacrylates on die stone

An evaluation was made of the effects of the application of an increasing number of coats of cyanoacrylate on die stone. The application of one coat of cyanoacrylate adhesive on the surface of trimmed and marked dies increases the surface hardness, increases the scratch resistance, will not change the dimensions of the die appreciably, and renders the margin marking more permanent. Excess resin should be removed immediately by compressed air. The use of hardener instead of water is recommended for mixing Type IV dental stone. Use of the cyanoacrylate method will aid the dentist and the technician in achieving an accurate crown fit.

Comparison of bond strengths of polymethyl methacrylate and alpha cyanoacrylate

A method of fabricating similar methyl methacrylate cylinders was used to produce specimens to test the bond strength of alpha cyanoacrylate and autopolymerizing methyl methacrylate. Each group was tested immediately and after storage in 37 degrees C water bath for 90 days. Statistical analysis supported the finding that the autopolymerizing methyl methacrylate produced a better bond even without undercuts or mechanical retention.