Physical and Mechanical Properties of Heat activated Acrylic Denture Base Resin Materials

 

Rama Krishna Alla^1*, Raghavendra Swamy KN², Ritu Vyas³, N. B. Prakash Tiruveedula⁴, Alluri Murali Krishnam Raju⁵

 

ABSTRACT:

Heat activated acrylic resin materials are most widely used for the fabrication of denture prosthesis. Flexural strength, impact strength and surface hardness are some of the desirable mechanical properties of denture prosthesis materials. Polymers’ tendency to absorb water or any other fluids changes the dimensions of prosthesis and also influences its mechanical properties. The purpose of this in-vitro study is to determine the physical and mechanical properties of heat activated denture base materials. A total of 40 specimens from each of the selected heat activated acrylic materials were fabricated. Flexural strength was measured using three point bending method with universal testing machine at a crosshead speed of 2mm per minute. Impact strength was determined using Izod impact tester. Surface hardness was measured using Vickers hardness testing machine. Water sorption and solubility were measured by weight gain and loss by denture base acrylic specimens on immersing them in distilled water for about a week and after dessicated for 24 hours respectively. One way ANOVA and TukeyHSD tests were used to analyse the results. Each denture base material used in the study was superior or inferior in one or the other properties tested. Trevlon denture base materials exhibited more flexural strength, Lucitone199 materials exhibited more impact strength and TriplexHot materials exhibited more Vickers hardness. All the denture materials exhibited least water sorption and solubility qualities.

 

 

 

 

 

INTRODUCTION:

Poly methyl methacrylate (PMMA) resin is most commonly used material for the fabrication of denture prosthesis since it was introduced to dentistry by “Walter Wright”^1,2. Various materials have been incorporated to modify the properties of PMMA based denture polymers^3,4. However, these materials suffer with some inherent shortcomings like frequent fracture of dentures because of fatigue and, low thermal conductivity, and ease of microbial adherence to the intaglio surface^3,5-8. The factors which persuade the fracture of acrylic resin dentures include stress intensification and increased ridge resorption leading to unsupported dentures, deep incisal notching at the labial frenum, sharp changes at the contours of the denture inclusions, previous repair and residual methyl methacrylate (MMA)^3,7. Fracture of dentures by impact forces, on the other hand, is due to the accidental dropping of dentures^3,7,910.

 

Various heat activated denture base materials are available in the dental market. Each material is superior or inferior to the other material in some aspects. So, it is necessary to know about the physical and mechanical properties of those materials before they are used for a specific purpose. Hence this study aims to evaluate and compare the flexural strength, impact strength, surface hardness, water sorption and solubility of conventional heat-activated denture base materials.

 

MATERIALS AND METHODS:

Details of the materials used in the present study are presented in Table 1.

 

Table 1: Denture base materials used in the study

S.No.	Name of the Materials	Manufacturer
1	Lucitone199	Dentsply International Inc, USA
2	Trevlon	Dentsply India Pvt. Ltd, India
3	TriplexHot	IvoclarVivadent, USA

 

Preparation of acrylic specimen:

A total of 120 specimens were fabricated using conventional compression molding technique. Acrylic specimens were fabricated in accordance with ADA specification number 12 by investing metal strips of 65x 10 x3 mm, 50x 6x 4 mm and 20x 2 mm for the evaluation of flexural strength, impact strength, surface hardness respectively and metal discs of 50x 2.5 mm for measuring water sorption and solubility. Metal strips and/or discs were carefully removed after the investment material was set. A thin layer of separating medium was applied in the mould space and allowed to dry. Denture base acrylic powder and monomer liquid was mixed as per the manufacturer recommendations and packed into the mould when the mix reached the dough consistency. Then the flask was closed and kept under pressure in a hydraulic press apparatus followed by bench curing for 30 minutes. Then the flask was tightly secured in a clamp and transferred into a thermostatically controlled water bath (Acrylizer, Confident A-73, India) and cured as per the manufacturer’s recommendations. Temperature of the water bath was increased to 73±1°C within 30 minutes and maintained at same temperature for 90 minutes. Then the temperature of the water bath was increased to 100°C and maintained for 60 minutes. After completion of polymerization cycle, the flask was allowed to cool in water bath to room temperature, and the acrylic resin specimens were retrieved after deflasking. The specimen obtained were finished and polished in the conventional manner.

 

Evaluation of flexural strength:

The flexural strength was measured using a three point bending test on a computerized universal testing machine (Instron 8801, United Kingdom) at a crosshead speed of 2mm/minute. The load was applied until the specimen fractured from which the flexural strength value was computed automatically by the machine.

 

Evaluation of impact strength:

The samples were subjected to IZOD type of impact tester (KRYSTAL Industries, India). Digital calliper was used to locate the midpoint of the sample and was marked using a marking pen. A notch of 1.2 mm was made at the mid-point of the specimens using carborundum disc to avoid brittle fracture. The sample was placed in a metal fixture so that the middle of the sample coincided with the striking pendulum. The pendulum struck the sample until fracture of the material was obtained. The energy required to break the sample was measured in Joules/mm².

 

Evaluation of surface hardness:

Hardness was measured using Vickers hardness test apparatus which has a diamond pyramid as indenter (Daksh Quality Systems Pvt Ltd., India). The value of the load (25 gm) and the time duration (25 seconds) that is to be applied was set. The test specimen was held firmly in position and lens were arranged to get the image clearly at its focal length, then the indentation made using set parameters. Indentations focused and the measuring lines were made to interact at two diagonally opposite corner. A total of five indentations were made at different points for each specimen, and the means of individual specimens were averaged. The readouts of the lengths of the diagonals were immediately taken after each indentation, allowing a minimal (as short as 10 s) time interval to elapse between making and reading the indentations. It was assumed that due to the short time elapsed between making and reading the indentation, the viscoelastic recovery of the diagonals after indentation was minimal.

 

Evaluation of water sorption and solubility:

The specimens were placed in open glass bottles, which were placed in desiccators containing freshly dried white silica and kept at 37±1ºC inside a vacuum oven for 24 h (Asian Test Equipments, India). After that, the desiccators containing the specimens were removed from the oven and left at room temperature for 2 hours, which completed a 24-h cycle. The discs were weighed daily using an analytical balance (Dhona Single Pan Balance, Inida) to record 24-h weighing cycles. The complete cycle was repeated until a constant mass (M1) was obtained, i.e., until the mass loss for each specimen was not more than 0.1 mg per 24-h cycle. Thereafter, the specimens were carefully placed back in their labelled bottles, and 15 mL of deionized water (W) were added using manual pipettes. The bottles were sealed, brought back into the oven and kept at a 37º ± 1º C for 7 days.

 

After these periods, all the bottles were removed from the oven and kept at room temperature for 2 hours. The specimens were removed from the bottles, dried with absorbent paper for 15 s and left in a sterile bucket for 1 min. They were then weighed again to obtain M2. After weighting, the specimens were reconditioned in the desiccators until they reached a constant weight (M3) using the cycle describe for M1. The values for water sorption (Wsp) and solubility (Wsl) in micrograms per cubic millimeter were calculated using the following equations: Wsp=(M2-M1)/V; Ws1=(M1-M3)/V.

 

 

Results were subjected to One way ANOVA and TukeyHSD tests for statistical analyses using SPSS for windows, Version 12.0.,SPSS Inc.

 

RESULTS:

The means and standard deviations for mechanical properties of denture base materials are displayed in Table 2. Trevlon denture base material showed more flexural strength than the other denture base materials. Lucitone 199 exhibited more impact strength and TriplexHot showed more Vickers hardness than the other denture base materials. One-way ANOVA (Table 2) showed a significant difference (p=0.000) in the mechanical properties tested among the three groups. The means and standard deviations for physical properties of denture base materials are displayed in Table 3. Trevlon denture base specimens showed more water sorption and solubility than the other materials. One-way ANOVA (Table 3) showed no significant differences in both water sorption (p = 0.708) and water solubility (p = 0.567) of the spcimens tested among the three denture base materials.

 

Table 2: Comparison of Mechanical Properties of Denture base materials using One-way ANOVA.

Groups	N	Flexural Strength (MPa)		Impact Strength (J/mm2)		Vickers Hardness
		Mean ± SD*	Significance	Mean ± SD*	Significance	Mean ± SD*	Significance
Lucitone-199	30$	73.308 ± 2.699	0.000	5.5600±0.25473	0.000	10.9020 ±0.39052	0.000
Trevlon	30$	108.623± 6.282		4.6300±0.38312		14.2870±.49142
TriplexHot	30$	72.142± 2.619		4.4000±0.39158		17.7280±.47356

*SD – Standard Deviation

$ 10 specimens for each test

 

Table 3: Comparison of water soption and solubility of Denture base materials using One-way ANOVA.

Groups	N	Water Sorption (µg/mm2)		Water Solubility (µg/mm2)
		Mean ± SD*	Significance	Mean ± SD*	Significance
Lucitone-199	10	8.1200±2.81141	0.708	0.6100±0.18529	0.567
Trevlon	10	9.0000±2.68038		0.6400±0.11738
TriplexHot	10	8.2700±1.97262		0.5700±0.12517

*SD – Standard Deviation

 

In PostHoc (Tukey’s HSD) test (Table 4) TriplexHot material did not show significant differences in flexural and impact strengths with lucitone199 and Trevlon denture base materials respectively. However, significant differences were observed in Vickers hardness among the groups.

 

Table 4: PostHoc analysis (Tukey HSD) of mechanical properties of Denture base materials

Groups		Flexural Strength (MPa)		Impact Strength (J/mm2)		Vicker’s Hardness
		Mean Difference ± SE	Significance	Mean Difference ± SE	Significance	Mean Difference ± SE	Significance
Lucitone199	Trevlon	35.315 ± 1.89055	0.000	0.93000± 0.15599	0.000	3.38500 ± 0.20302	0.000
	TriplexHot	1.166 ± 1.89055	0.812	1.16000± 0.15599	0.000	6.82600± 0.20302	0.000
Trevlon	TriplexHot	36.481 ± 1.89055	0.000	0.23000± 0.15599	1.000	3.44100± 0.20302	0.000

 

DISCUSSION:

PMMA based resin is currently the most commonly used material for the construction of dentures. However, this material is not ideal because of its relatively low mechanical strength, which can cause the fracture of denture. This study was essentially designed to evaluate the physical and mechanical properties of some of the commonly used heat cure acrylic resins.

 

Clinical failure of denture prosthesis made from PMMA are most likely in the form of fracture either due to fatigue forces of mastication^10,11. Midline fracture of denture prosthesis is the most commonly observed problem and it is due to the stress concentration around the micro cracks formed in the material due to continuous applications of small forces. Repeated application masticatory loads may result in propagation of these cracks which weakens the denture base and finally results in fracture^10-12. In addition, processing errors also may contribute to the fracture of the denture bases¹³. Denture prosthesis is largely subjected to the combination of compressive, tensile and bending stresses under mastication. Hence, it is ideal to characterize them using 3-point or four-point bending stresses. In this study, the specimens were subjected to 3-point bending stress to evaluate the flexural strength¹⁴. From this study it was observed that the Trevlon denture base material exhibited more flexural strength than the other denture base materials. However, the flexural strength of all groups used in this study was more than the ISO 20795-1:2008 (ISO, 2008) requirement. According to ISO 20795-1:2008 (ISO, 2008), the flexural strength of polymeric materials must be at least 50 MPa¹⁵. PostHoc analysis showed no statistical significant differences in flexural strength between TriplexHot and Lucitone199 denture base materials. The flexural strength of denture base specimens are mostly influenced by the specimen size, span length and thickness ratio, fabrication methods, shape and surface finish of the specimen, duration and rate of  loading¹⁴.

 

The other most commonly observed problem in denture base prosthesis is its low resistance to sudden application of forces. Such types of fractures are more likely due to the accidental dropping of dentures on surfaces during cleaning of dentures by patients⁶. From this study it was observed that the Lucitone199 specimens exhibited more impact strength than the other denture base material’s specimens. However, PostHoc analysis showed statistically no significant differences in the impact strength between trevlon and TrplexHot speciemens. The impact strength results observed for Lucitone199 in this study are in compliance with the results observed by Meng TR etal., (2005)⁹. It was reported in the literature that the impact strength of polymers can be significantly improved by graft co-polymerization of polymer with certain rubbers¹⁶. The reason for high impact strength of lucintone199 denture base material could be attributed to the use of rubber modified polymer which has the capability to resist the fractures under sudden blow of forces⁹.

 

An ideal denture base prosthesis must have maximum resistance to abrasion under masticatory forces and also during cleansing. Poor abrasion resistance leads to the formation of surface irregularities on the prosthesis that will encourage the food to stick and leads to unhygienic denture and may also cause denture stomatitis¹⁷. The Vickers hardness test is considerd to be a valid method to evaluate rigid polymers by means of the ability of the material to resist the penetration of a specific load. In this study, TriplexHot specimens exhibited more vicker’s hardness than the other groups and significant differences were observed in the vicker’s hardness among all the specimens. The hardness of acrylic resins is mainly depends on the amount plasticizers present in the material¹⁸.

 

Denture base polymers are capable of absorbing water over a period of time, due to the polar properties of the resin molecules. The absorbed water may act as plasticizer and reduces the strength of the prosthesis¹⁹. Initially water soluble compounds such as plasticizers and soluble compounds may be leached into the water and later, these sites will be ocupied by the water. The balance between these processes alters the dimensional stability which results in misfit of the prosthesis in the oral cavity²⁰. The rate of water sorption is mainly depends on the resin polarity, concentration of polar sites available to form hydrogen bonds with water and network topology²¹. Hence, an ideal denture base material should exhibit least or no water sorption. However denture polymers are least soluble in the water. According to ISO standards specification 1567:1999²² the maximum allowable water sorption by both heat and self-cure denture base materials must be 32 µg/mm and the water solubility must not exceed 1.6 µg/mm³ for heat-cured, and 8.0 µg/mm³ for self-cured matrials.  The materials used in this exhibited least water sorption and solubility and no statistical significant difference was found among all the materials. The results obtained in this study were in complaince with the results indicated by Saini R et al (2016) ²⁰.

 

The soluble materials present in acrylic resins are initiators, plasticizers, and free monomer. It has been suggested that there might be a correlation between residual monomer and the weight loss determined by the solubility test²⁰. The solubility of the denture base materials used in this study had shown least values. The materials used in this study were only heat-curable materials and it can be stated that these materials had very less residual monomer content.

 

CONCLUSION:

Within the limitations of this study, it can be concluded that the each heat-cure denture base material is superior or inferior to the other in the various mechanical properties tested.

 

·         Trevlon denture base material exhibited more flexural strength, followed by Lucitone199 and TripexHot materials.

·         More impact strength was showed by Lucitone199 material followed by Trevlon and TriplexHot.

·         Vickers hardness was more for TriplexHot material followed by Trevlon and Lucitone199.

·         Water sorption and solubility of the three materials tested were very less and were in compliance with ISO specifications.

 

REFERENCES:

1.        Anusavice KJ. Philips, Science of Dental Materials, 11^th edition, Elsevier, New Delhi, India, 721-758.

2.        Alla RK. Dental Materials Science, 1^st edition, Jaypee Brothers Medical Publishers (Pvt) Ltd., New Delhi, India, 248-284.

3.        Alla RK, Sajjan S, Alluri VR, Ginjupalli K, Upadhya N. Influence of fibre reinforcement on the properties of denture base resins. J Biomater. Nanobiotech. 4(1); 2013: 91-7.

4.        Mohamad Qulam Zaki Bin Mohamad Rasidi. Review on History of Complete Denture. Research J. Pharm. and Tech 2016; 9(8):1069-1062.

5.        Arikan A, Ozkan YK, Arda T, Akalın B. Effect of 180 days of water storage on the transverse strength of acetal resin denture base material. J of Prosthodont. 19(1); 2010:47–51.

6.        Alla RK, Swamy KNR, Vyas R, Konakanchi A, Conventional and contemporary polymers for the fabrication of denture prosthesi: Part I – Overview,  Composition and Properties, Int J Applied Dent Sci., 1(4);2015 : 82-89.

7.        Pathmashri. V.P, Abirami. A Review on Denture Stomatitis. Research J. Pharm. and Tech 2016; 9(10):1809-1811.

8.        Swamy KNR, Alla RK, Mohammed S, Konakanchi A, The role of antifungal agents in treating denture stomatitis, Research J. Pharm. and Tech., Accepted for publication and to be published  in April 2018 issue.

9.        Meng TR, Latta MA, Physical Properties of Four Acrylic Denture Base Resins, Journal of Contemporary Dental Practice, 6(4); 2005: 93-100.

10.     Vallittu PK. Fracture surface characteristics of damaged acrylic-resin-based dentures as analysed by SEM-replica technique. J Oral Rehabil. 23(8); 1996:524-9.

11.     El-Sheikh AM, Al-Zahrani SB. Causes of denture fracture: A survey. Saudi Dent J. 2006;18:149-54.

12.     Hirajima Y, Takahashi H, Minakuchi S. Influence of a denture strengthener on the deformation of complete denture. Dent Mater J. 28(4); 2009:507-12.

13.     Nanda Kumar E, Suresh V. Assessing the errors during the processing on complete denture. Research J. Pharm. and Tech 2016; 9(12):2287-2294.

14.     Ginjupalli K, Upadhya N, Alla RK, Nandish BT, Evaluation of cohesive and adhesive strength of Dental Materials, Manipal Odontoscope, 4; 2012: 47-52.

15.     Alla RK, Swamy KNR, Vyas R, Konakanchi A, Guduri V, Gadde P, Influence of Silver nanoparticles incorporation on flexural strength of Heatcure denture base acrylic denture base resin materials, ARRB; 17(4);2017: 1-8.

16.     B. Sehgal, G.B. Kunde. Structure Property Relationship of Polymer Blends: Effect of Grafting on Mechanical Properties. Asian J. Research Chem. 3(3): July- Sept. 2010; Page 637-639.

17.     Farina AP, Doglas Cecchin, Soares RG, Botelho AL, Takahashi JMFK, Mazzetto MO, Mesquita MF, Evaluation of Vickers hardness of different types of acrylic denture base resins with and without glass fibre reinforcement, Gerodontology 29; 2012:  e155–e160

18.     Gracia RCMR, de Souza Junior JA, Rached RN, Del Bel Cury A, Effect of Denture cleansers on the surface roughness and Hardness of a microwave cured acrylic resin and dental alloys, J Prosthodont, 13;2014; 173-178.

19.     Barsby MJ. A denture base resin with low water absorption. J Dent 20; 1992:240‑4.

20.     Saini R, Kotian R, Madhyastha P, Srikant N. Comparative study of sorption and solubility of heat-cure and self-cure acrylic resins in different solutions. Indian J Dent Res 27; 2016:288-94.

21.     Malacarne J, Carvalho RM, de Goes MF, Svizero N, Pashley DH, Tay FR, et al. Water sorption/solubility of dental adhesive resins. Dent Mater 22; 2006 :973‑80.

22.     Council of Dental Materials and Devices. Revised American Dental Association specification no. 12 for denture base polymers. JADA. 90; 1975:451‑8.

 

Accepted on 21.04.2018           © RJPT All right reserved

Research J. Pharm. and Tech 2018; 11(6): 2258-2262.