Accuracy of alginate and elastomeric impression materials

The accuracy of three alginates and seven elastomeric impression materials was determined. Impressions were taken of a truncated cone and poured with stone die. A steel ring fitting the steel cone was placed on the dies, and the discrepancy between the top surface of the ring and the die was measured. From these measurements the deviation between the base diameter of the die and that of the cone was calculated and used to express the accuracy of an impression material. All impressions showed a net shrinkage, resulting in too large a die stone and incomplete seating of the steel ring. The inaccuracy of the alginates varied between 44 and 188 microns. Blueprint regular was found to be significantly more accurate than the two other alginates investigated, and had a degree of accuracy comparable with that of the more accurate elastomeric impression materials. Within this group of materials the inaccuracy varied between 39 and 130 microns. Generally, the addition-curing silicones were more accurate than the polyethers. Baysilex and President regular body had the highest degree of accuracy of the elastomeric impression materials.     

Accuracy of alginate and elastomeric impression materials

Abstract – The accuracy of three alginates and seven elastomeric impression materials was determined. Impressions were taken of a truncated cone and poured with stone die. A steel ring fitting the steel cone was placed on the dies, and the discrepancy between the top surface of the ring and the die was measured. From these measurements the deviation between the base diameter of the die and that of the cone was calculated and used to express the accuracy of an impression material. All impressions showed a net shrinkage, resulting in too large a die stone and incomplete seating of the steel ring. The inaccuracy of the alginates varied between 44 and 188 μm. Blueprint regular was found to be significantly more accurate than the two other alginates investigated, and had a degree of accuracy comparable with that of the more accurate elastomeric impression materials. Within this group of materials the inaccuracy varied between 39 and 130 μm. Generally, the addition-curing silicones were more accurate than the polyethers. Baysilex and President regular body had the highest degree of accuracy of the elastomeric impression materials.     

Accuracy of newly formulated fast-setting elastomeric impression materials

STATEMENT OF PROBLEM: Elastomeric impression materials have been reformulated to achieve a faster set. The accuracy of fast-setting elastomeric impression materials should be confirmed, particularly with respect to disinfection. PURPOSE: The purpose of this study was to assess the accuracy of 2 types of fast-setting impression materials when disinfected with acid glutaraldehyde. MATERIAL AND METHODS: Impressions of the mandibular arch of a modified dentoform master model were made, from which gypsum working casts and dies were formed. Measurements of the master model and working casts included anteroposterior (AP) and cross-arch (CA) dimensions. A stainless steel circular crown preparation incorporated within the master model was measured in buccolingual (BL), mesiodistal (MD), and occlusogingival (OG) dimensions and compared to measurements from recovered gypsum dies. The impression materials examined were a fast-set vinyl polysiloxane (VPS-FS, Aquasil Ultra Fast Set), a fast-set polyether (PE-FS, Impregum Penta Soft Quick Step), and a regular-setting polyether as a control (PE, Impregum Penta). Disinfection involved immersion in 3.5% acid glutaraldehyde (Banicide Advanced) for 20 minutes, and nondisinfected impressions served as a control. Linear measurements were made with a measuring microscope. Statistical analysis utilized a 2-way and single-factor analysis of variance with pair-wise comparison of mean values when appropriate. Hypothesis testing was conducted at alpha = .05 RESULTS: No differences were shown between the disinfected and nondisinfected conditions for all locations. However, there were statistical differences among the 3 materials for AP, CA, MD, and OG dimensions. AP and CA dimensions of all working casts were larger than the master model. Impressions produced oval-shaped working dies for all impression materials. PE and PE-FS working dies were larger in all dimensions compared to the stainless steel preparation, whereas VPS-FS-generated working dies were reduced in OG and MD dimensions. Differences detected were small and may not be of clinical significance. CONCLUSIONS: Impression material accuracy was unaffected by immersion disinfection. The working casts and dies were similar for PE and PE-FS. VPS-FS generated gypsum dies that were smaller in 2 of the 3 dimensions measured and may require additional die relief. Overall accuracy was acceptable for all 3 impression materials.     

Rigidity of elastomeric impression materials

The rigidity of an impression material should be less than a certain value in order that excessive forces are not required when taking impressions of undercut areas. Conversely, the impression should be sufficiently rigid to withstand the weight of stone whilst pouring the cast without distortion. This paper describes a method of determining elastic modulus values at times as early as the setting time of the material. Results indicate that the elastic modulus values increase from the setting time to 30 min after the start of mix. Values are presented for a representative range of elastomeric impression materials.     

Radiopacity of elastomeric impression materials

The purpose of this study was to measure and classify the radiopacity of various elastomeric impression materials available on the open market and to compare their appearance in radiographs of the oral tissues. In order to measure and classify their radiopacity, twenty-eight specimens of various materials were placed and exposed on film together with an aluminium stepwedge. A dummy head was used to investigate the radiographic density of the above specimens in comparison with that of simulated oral tissues.     

Viscoelastic properties of elastomeric impression materials

The variation of torsional shear modulus with time for several commercial elastomeric impression materials has been determined. The effects of mixing proportions and temperature on rheological properties are presented. The relevance of these properties to the actual use of these materials are discussed.     

Mechanical properties of elastomeric impression materials

Dental elastomeric impression materials of different consistencies, representing addition and condensation silicones, polysulphides and a polyether, were studied to determine their mechanical properties in terms of tearing energy, tensile properties and modulus determined at high strain rates. Whilst the polysulphides had the greatest resistance to tearing, they were more extensible, had lower moduli of elasticity and lower tensile strengths than the silicones. The polyether resembled the silicones in many respects.     

Viscoelastic properties of elastomeric impression materials

A method of measuring torsional shear modulus and rigidity of elastomeric impression materials has been developed and is described. In addition, the changes in viscoelastic properties of these materials during setting are discussed.     

Viscosity of elastomeric impression materials.

(1) Some elastomeric impression materials (Omniflex and Impregum) exhibited extreme shear thinning properties. This property was considered useful and these materials can be considered all-purpose. (2) The use of impression materials in syringing applications should not be precluded simply because the substances are thick or heavy during spatulation. A tenfold reduction in viscosity with increasing shear rate was found for three of the materials tested. (3) Viscosity characteristics of the impression rubbers were linked to filler content. (4) The viscosity rise during cross-linking was monitored for various impression products. It was found that each manufacturer's specifications of the length of time for syringing were relatively accurate.     

Comparison of commercial elastomeric impression materials

The properties of some recently marketed rubber impression materials were compared with some earlier products. In general, the qualities of addition silicones and polyethers were superior to polysulfides and condensation silicones. However, there were significant differences in the properties of products of the same type and class and thus the selection of a product for a particular application should be based on actual property data rather than on the type and class of rubber impression material.     

Viscoelastic properties of elastomeric impression materials

A method of measuring the elastic recovery of impression materials after the application of a strain applied at the setting time is described. The results obtained using this method for a representative selection of impression materials are presented and discussed.     

Tear strength of elastomeric impression materials.

1. Under simulated clinical conditions the maximum tear strength of those materials tested was reached in 10 to 15 minutes. 2. The polysulfide materials showed three to six times the tear strength of the silicones. 3. The polyether material displayed a tear resistance slightly higher than that of the silicones but one third to one fifth as high as that of the polysulfides. 4. The clinical significance of the differences in tear strength was undetermined since other factors (such as adhesion) were not considered.     

Hydrophilicity of unset and set elastomeric impression materials

The aim of this study was to compare the initial hydrophilicity of unset and set elastomeric impression materials. Initial water contact angles were studied on thin unset and set films of one polyether and six polyvinyl siloxane (PVS) impression materials using high-resolution drop shape analysis at drop ages of 1 and 3 seconds. All unset PVS materials were very hydrophobic initially but showed different kinetics of hydrophilization. In contrast, the unset polyether was more hydrophilic initially but lacked distinct hydrophilization. All impression materials showed statistically significant contact angle differences between unset and set surfaces (P < .05). Dependent on the drop age, two PVS materials reached or exceeded the hydrophilicity of the polyether (P < .05). It can be concluded that studies on the wetting behavior of elastomeric impression materials should consider both the experimental drop age and set and unset material surfaces.     

Adhesion of elastomeric impression materials to trays

summary The tensile and shear adhesive bond strengths of two addition cured silicones (Provil and Express) and a polyether (Impregum) impression material to brass, poly(methylmethacrylate) and visible light-cured (VLC) tray resin were determined. Adhesive application significantly increased the bond strength; Provil and Express adhered most strongly to brass; whilst the Impregum-VLC combination produced the strongest bond. Indeed, VLC resin generated greater adhesion than acrylic resin. Exchanging the adhesives specified for each silicone material generally resulted in higher bond strengths. No correlation was established between speed of separation of the test surfaces and bond strength. For optimum clinical performance, the impression material (adhesive) tray material giving the highest bond strength should be utilized.     

Wettability of nonaqueous elastomeric impression materials.

The wettability of eight nonaqueous elastomeric impression materials was studied by comparing their contact angles. The materials included three polyethers (one of which was light activated), three hydrophilic poly(vinyl siloxanes), one conventional poly(vinyl siloxane), and one poly(vinyl siloxane) putty. Extracted teeth were prepared to approximate the roughness of a tooth preparation. Contact angles were measured at different time intervals after the start of mixing but were not shown to be significant. The nonhydrophilic poly(vinyl siloxane) materials and the poly(vinyl siloxane) putty were found to be significantly less wettable.