Pressure and temperature changes in heat-cured acrylic resin during processing

Assessment of Adaptability and Linear Dimensional Changes of Two Heat Cure Denture Base Resin with Different Cooling Techniques: An In Vitro Study

AIM

The current study was designed to assess the linear dimensional changes and adaptability of two heat-cured denture base resins using various cooling methods.

MATERIALS AND METHODS

To prepare a total of 90 acrylic resin samples (45 acrylic resin samples for each material), four rectangular stainless-steel plates measuring 25 × 25 × 10 mm were fabricated. For both groups, the material was put into the mold at the dough stage. Group I - SR Triplex Hot Heat Cure acrylic; group II - DPI Heat Cure acrylic. Both groups used the same curing procedure. One of the following three techniques was used to cool the material (15 samples from each material) once the curing cycle was finished: (A) water bath, (b) quenching, and (C) air. A traveling microscope was used to measure the distance between the markings on the acrylic samples. The data was recorded and statistically analyzed.

RESULTS

In SR Triplex Hot heat cure acrylic material, the maximum linear dimensional changes were found in the quenching technique (0.242 ± 0.05), followed by the air technique (0.168 ± 0.11) and the least was found in the water bath technique (0.146 ± 0.01). In DPI Heat Cure acrylic material, the maximum linear dimensional changes were found in the quenching technique (0.284 ± 0.09), followed by the air technique (0.172 ± 0.18) and the least was found in the water bath technique (0.158 ± 0.10). There was a statistically significant difference found between these three cooling techniques. On comparison of adaptability, the water bath technique, the marginal gap SR Triplex Hot was 0.012 ± 0.02 and DPI Heat Cure was 0.013 ± 0.02. In the quenching technique, the marginal gap SR Triplex Hot was 0.019 ± 0.04 and DPI Heat Cure was 0.016 ± 0.04. In the air technique, the marginal gap SR Triplex Hot was 0.017 ± 0.01 and DPI Heat Cure was 0.019 ± 0.01.

CONCLUSION

The present study concluded that among the different cooling methods, the water bath technique had the least linear dimensional change, followed by the air and quenching techniques. When comparing the materials, DPI Heat Cure acrylic resin showed a greater linear dimensional change than SR Triplex Hot heat cure acrylic resin.

CLINICAL SIGNIFICANCE

During polymerization, heat-cured acrylic resins experience dimensional changes. Shrinkage and expansion are dimensional changes that occur in heat-cured acrylic resins and have an impact on the occlusal relationship and denture fit. However, the denture base's material qualities and the different temperature variations it experiences during production may have an impact on this. How to cite this article: Kannaiyan K, Rathod A, Bhushan P, et al. Assessment of Adaptability and Linear Dimensional Changes of Two Heat Cure Denture Base Resin with Different Cooling Techniques: An In Vitro Study. J Contemp Dent Pract 2024;25(3):241-244.

Embedded 3D printing and pressurized thermo-curing of PMMA for medical implants

Poly (methyl methacrylate) (PMMA) is a synthetic polymer commonly used for medical implants in cranioplasty and orthopedic surgery owing to its excellent mechanical properties, optical transparency, and minimal inflammatory responses. Recently, the development of 3D printing opens new avenues in the fabrication of patient-specific PMMA implants for personalized medicine. However, challenges are confronted when adapting medical-grade PMMA to the 3D printing process due to its dynamic viscosity and nonself-supporting characteristics before cured. In addition, the intrinsically exothermic polymerization of MMA brings about bubble generation issues that reduce its mechanical performance harshly. Therefore, in this study, an embedded 3D printing methodology followed by pressurized thermo-curing is proposed and developed: a granular alginate microgel is designed for serving as a supporting matrix when jamming formed between the granules to structurally support the extruded precursor filaments of PMMA-MMA ink during both 3D printing and post-curing; moreover, the autoclave reactor enclosing the alginate matrix and as-sculpted PMMA structures is utilized to generate temperature-dependent pressure, which serves for suppressing the bubbles and solidifying the polymerized MMA during the post-curing process. The 3D printed PMMA is comparably matchable to traditional PMMA castings in terms of their microstructures, density, thermal properties, mechanical performance and biocompatibility. In the future, the proposed embedded 3D printing platform combined with the special post-curing method has great potential for a customized and cost-effective fabrication of patient-specific, complex and functional PMMA implants.

Comparison of Mechanical Properties, Physical Properties, and Biocompatibility of Four Different Denture Base Resins: An In vitro Study

Four new dental replacement base tars were evaluated in vitro to determine their mechanical qualities, authentic properties, and biocompatibility.

Materials and Methods

In this experiment, we employed SR Triplet HOT (a fiber-developed heat fix tar), Sunflex (a multipurpose force fix sap), Trevalon-Hello (a high-impact heat fix tar), DPI (a digital pigment imaging system), and a variety of other pigments and inks (normal power fix tar). For these models, the ISO specification 1567 for dental substitute base gums called for testing of flexural strength, hardness, impact strength, water sorption and dissolvability, and cytotoxicity.

Results

All the strength and mechanical properties tested had a statistically significant difference when intergroup analysis was performed.

Conclusion

The exceptional physical and mechanical capabilities of the Sunflex denture base resin, together with its biocompatibility with oral tissues, make it a good candidate for use as a denture base material in routine clinical practice.

CAD-CAM removable complete dentures: A systematic review and meta-analysis of trueness of fit, biocompatibility, mechanical properties, surface characteristics, color stability, time-cost analysis, clinical and patient-reported outcomes

OBJECTIVES

This review compared Computer-aided designand Computer-aided manufactured (CAD-CAM) and conventionally constructed removable complete dentures (CDs).

DATA

Seventy-three studies reporting on CAD-CAM (milled/3D-printed) CDs were included in this review. The most recent literature search was performed on 15/03/2021.

SOURCES

Two investigators searched electronic databases [PubMed (MEDLINE), Embase, CENTRAL], online search engines (Google) and research portals. Hand searches were performed to identify literature not available online.

STUDY SELECTION

Studies on CAD-CAM CDs were included if they reported on trueness of fit, biocompatibility, mechanical, surface, chemical, color , microbiological properties, time-cost analysis, and clinical outcomes. Inter-investigator reliability was assessed using kappa scores. Meta-analyses were performed on the extracted data .

RESULTS

The kappa score ranged between 0.897-1.000. Meta-analyses revealed that 3D-printed CDs were more true than conventional CDs (p = 0.039). Milled CDs had a higher flexural-strength than conventional and 3D-printed CDs (p < 0.0001). Milled CDs had a higher flexural-modulus than 3D-printed CDs (p < 0.0001). Milled CDs had a higher yield-strength than injection-molded (p = 0.004), and 3D-printed CDs (p = 0.001). Milled CDs had superior toughness (p < 0.0001) and surface roughness characteristics (p < 0.0001) than other CDs . Rapidly-prototyped CDs displayed poor color-stability compared to other CDs (p = 0.029). CAD-CAM CDs d displayed better retention than conventional CDs (p = 0.015). Conventional CDs had a higher strain at yield point than milled CDs (p < 0.0001), and had superior esthetics than 3D-printed (p < 0.0001). Fabrication of CAD-CAM CDs required less chairside time (p = 0.037) and lower overall costs (p < 0.0001) than conventional CDs.

CONCLUSIONS

This systematic review concludes that CAD-CAM CDs offer a number of improved mechanical/surface properties and are not inferior when compared to conventional CDs.

CLINICAL SIGNIFICANCE

CAD-CAM CDs should be considered for completely edentulous patients whenever possible, since this technique offers numerous advantages including better retention, mechanical and surface properties but most importantly preserves a digital record. This can be a great advantage for older adults with limited access to dental care.

Effect of comonomer of methacrylic acid on flexural strength and adhesion of Staphylococcus aureus to heat polymerized poly (methyl methacrylate) resin: An in vitro study

Aims and Objective:

The purpose of the present study was to evaluate and compare flexural strength and Staphylococcus aureus adhesion of heat-activated poly (methyl methacrylate [MMA]) resin modified with a comonomer of methacrylic acid (MAA) and MMA monomer.

Materials and Methods:

Comonomer preparation was done with the addition of varying concentration of MAA (0, 15, 20, and 25 wt %) to the MMA of conventional heat-activated denture base resin to prepare the specimens. Prepared specimens were stored in distilled water at 37°C for 1 day and 1 week before the evaluation of flexural strength and microbial adhesion. Flexural strength was measured using a universal testing machine at a crosshead speed for 2 mm/min (n = 10). Microbial adhesion (colony-forming unit [CFU]) was evaluated against S. aureus using a quadrant streaking method (n = 5). Data were subjected to one-way ANOVA, and the significant differences among the results were subjected to Tukey's HSD test. P < 0.05 was considered statistically significant.

Results:

Addition of MAA to the MMA monomer was found to significantly reduce the adhesion of S. aureus for all the groups. Reduction of CFU of S. aureus was found be more significant for Group 3 as compared to control, both at 1-day (P < 0.001) and 1-week (P < 0.002) storage in distilled water. However, no statistically significant changes in the flexural strength were observed with the addition of MAA at 1-day (P = 0.52) and 1-week (P = 0.88) time interval.

Conclusion:

Addition of MAA to conventional denture base resin reduced the microbial adhesion without significantly affecting the flexural strength.

A Comparative Evaluation of the Linear Dimensional Changes of Two Different Commercially Available Heat Cure Acrylic Resins during Three Different Cooling Regimens

INTRODUCTION

Close mucosal adaptation of denture base to the underlying mucosa is of prime importance for denture stability. This however can be affected by various temperature changes which the denture base undergoes during processing and also to its material properties.

AIM

The aim of the present study was to compare linear dimensional change of heat cure acrylic resin with three different cooling regimens on two different commercially available acrylic denture base resins.

MATERIALS AND METHODS

Six groups of acrylic specimens with 10 samples each were prepared using either PYRAX or DPI acrylic resin, with a standard processing technique. Three different cooling methods were used for both the commercially available heat cure acrylic denture base resins. Linear dimensional changes were measured between three pre-determined points on the specimens of all the groups using a travelling microscope after removing the sample from the flask. One way ANOVA and unpaired t-test was used for statistical analysis.

RESULTS

Linear dimensional change was more in quenching followed by air and water bath method of cooling respectively. Amongst the materials, linear dimensional changes were more in PYRAX than in DPI acrylic.

CONCLUSION

Slow cooling by methods described should be advocated for better mucosal adaptation of the denture base.

Comparative evaluation of different mechanical modifications of denture teeth on bond strength between high-impact acrylic resin and denture teeth: An in vitro study

Aim and Objective:

Acrylic teeth separates from the denture base and remains a major worry in day-to-day routine dental procedure. The present study was conducted to comparatively evaluate different mechanical modifications of acrylic teeth on bond strength between Lucitone 199 heat cure resin and cross-linked teeth.

Materials and Methods:

The test specimens, central incisors (21) were demarcated into four groups. Group 1 was the control group, whereas Group 2, Group 3, and Group 4 were experimental groups modified with round groove, vertical groove, and T-shaped groove, respectively. The preparation of masterpiece was done by aligning the long axis of the central incisor teeth at 45° to the base of a wax block (8 mm × 10 mm × 30 mm), with ridge lap surface contacting the base. These test specimen (21) was prepared by Lucitone 199 heat cure resin. Evaluation of bond strength of all the specimens was done using universal tester (materials testing machine). Shapiro–Wilk Test, one-way analysis of variance (ANOVA), and Bonferroni test were done to do statistical investigation.

Results:

Group 1 specimens prepared by Lucitone 199 heat cure resin showed the lowest bond strength and Group 4 specimens prepared with T-shaped groove packed with Lucitone 199 exhibited the highest bond strength.

Conclusion:

The bond strength between Lucitone 199 heat cure resin and cross-linked teeth was increased when mechanical modifications was done on denture teeth. The specimens prepared with T-shaped groove packed with Lucitone 199 heat cure resin showed the highest bond strength followed by Group 3, Group 2, and lastly Group 1 prepared by Lucitone 199 heat cure resin.

Effect of three investing materials on tooth movement during flasking procedure for complete denture construction

PROBLEM STATEMENT

Tooth movement has been shown to occur during and after the processing of complete dentures. An understanding of this phenomenon may permit one to construct functional complete dentures that require less occlusal adjustment in the articulator and in the patient's mouth.

PURPOSE

The purpose of this study was to examine the effects of three different investing methods on tooth movement occurring during the processing of simulated maxillary complete dentures.

MATERIAL AND METHODS

Forty-five similar maxillary dentures were made using heat-polymerized acrylic resin, and assigned randomly to three experimental groups (n = 15 each) according to investing method: plaster-plaster-plaster (P-P-P), plaster-stone-stone (P-S-S), and plaster-mix (P-M). Specimens in all experimental groups were compression molded with denture base resin. Transverse interincisor (I-I) and intermolar (M-M) distances, and anteroposterior incisor-molar (LI-LM and RI-RM) distances, were measured with digital calipers at the wax denture stage (pre-polymerization) and after denture decasting (post-polymerization). Analysis of variance and Tukey's test were used to compare the results.

RESULTS

M-M, LI-LM, and RI-RM movement was significantly greater in the P-P-P group than in the P-S-S and P-M groups; no significant difference in I-I movement was observed among groups. Transverse movement along M-M and I-I was significantly greater than anteroposterior movement in the P-P-P group; no significant difference among measurements was observed in the other two groups.

CONCLUSION

The study results indicate that the use of dental stone or a 50:50 mixture of plaster and stone for investing of dentures is an important factor in efforts to control the magnitude of tooth movement.

Effect of micro-additions of carbon nanotubes to polymethylmethacrylate on reduction in polymerization shrinkage

PURPOSE

Carbon nanotubes are used in dentistry, although there are no adequate scientific data to support their use in acrylic resins. The polymerization shrinkage that occurs with polymethylmethacrylate (PMMA) resins is well known. This study compared the polymerization shrinkage of denture base acrylic resin with and without micro-additions of carbon nanotubes.

MATERIALS AND METHODS

Two materials were used, PMMA resin and multiwalled carbon nanotubes. Four groups were established of 10 specimens each according to the weight percent of carbon nanotubes dispersed and disintegrated in the monomer: group I (0.5% of carbon nanotubes in monomer), II (0.25%), III (0.125%), and IV (control group, 0%). The polymerization shrinkage of acrylic resin for each group was evaluated based on the distance between the reference points in wax (before polymerization) and in acrylic (after polymerization), measured using a traveling microscope. The data were submitted to Kruskal-Wallis and one-way ANOVA for comparison among the groups, and the results were statistically analyzed.

RESULTS

The Kruskal-Wallis test detected that the different percentages of carbon nanotubes incorporated in the monomer showed significant differences, and the mean ranks of polymerization shrinkage (%) showed differences among all the groups (group IV = 0.126, III = 0.037, II = 0.017, I = 0.006). Hence, the order of severity of polymerization shrinkage was 0% > 0.125% > 0.25% > 0.5% for the amount of carbon nanotubes incorporated in methylmethacrylate.

CONCLUSION

The present study was done to prove polymerization shrinkage in PMMA resins with micro-additions of carbon nanotubes. The results clearly show reduction in polymerization shrinkage when carbon nanotubes are incorporated into the PMMA resin.

The role of polymerization cycle and post-pressing time on tooth movement in complete dentures

This study analyzed the influence of polymerization cycle and post-pressing time on tooth movement in complete dentures. Forty maxillary complete dentures were fabricated and randomly assigned to 8 groups (n = 5); the polymerization cycle (conventional long cycle in water bath, fast cycle in boiling water, and by microwave energy) and the post-pressing time (immediate and 6 hours) were varied. Metal reference pins were placed on the incisal border of the central incisors (RI and LI), on the buccal cusp of the first premolars (RP and LP), and on the mesiobuccal cusp of the second molars (RM and LM). Two transverse and 2 anteroposterior distances were measured with a linear optical microscope (Olympus Optical Co., Tokyo, Japan) with an accuracy of .0005 mm, before and after processing the complete dentures. The data collected were submitted to analysis of variance (ANOVA) and the Tukey Test at a significance level of 5%. When Clássico, a conventional heat-polymerizable acrylic resin, was polymerized by microwave energy, tooth movement was statistically significant for 2 distances, with the highest value being observed for the 6-hour post-pressing time. Onda-Cryl microwave acrylic resin processed by microwave energy presented the lowest tooth movement for 2 distances, considering the 6-hour post-pressing time. QC-20 acrylic resin presented no statistically significant change considering both post-pressing times in each distance evaluated. Tooth movement showed an asymmetric behavior, so the mastering of the processing method is essential to the precise execution of all laboratory steps, irrespective of the polymerization cycle and post-pressing time used.

Minimization of the inevitable residual monomer in denture base acrylic

OBJECTIVES

Residual monomer ([MMA](R)) in denture base acrylic continues to be of concern. The response surface of concentration vs. time and temperature for the equilibration of methyl methacrylate (MMA) and its polymer (PMMA) allows a prediction of the time to the minimum at any temperature for a closed system. It was the purpose here to determine whether this prediction applies to normal denture base processing, and whether optimum conditions could be identified.

METHODS

Denture bases were processed following normal laboratory procedures, including pre-cure for 3 h at 70 degrees C for all tests. Commercial powder and liquid were used at either 95 or 100 degrees C, or a plain PMMA powder and the same liquid at 95 degrees C, for times ranging from 5 to 192 h. Residual MMA was determined by gas chromatography.

RESULTS

[MMA](R) decreased steadily from approximately 0.25% to as low as approximately 0.07% with increasing time at temperature, but did not approach equilibrium. The rate of diffusive loss of MMA appears to exceed the rate of depolymerization.

SIGNIFICANCE

Residual monomer is inevitable for all PMMA-based products no matter what the curing conditions are. However, extended time at high temperature can allow low values to be attained, and the time allowed can compensate for processing temperatures somewhat lower than the ordinarily recommended 100 degrees C. It is suggested that overnight processing at 95 degrees C should be adopted to minimize [MMA](R) and save energy. This result is of importance for work at high altitude.