Osada et al.

Int Chin J Dent 2010; 10: 1-5.

Bending strengths and hardness of autopolymerized acrylic resin
Tomoko Osada, DMD, PhD, Takako Ishimoto, DMD, PhD, Takayuki Aoki, DMD, PhD,Yasunori Suzuki, DMD, PhD, Norio Takishin, DDS, PhD, Chikahiro Ohkubo, DMD,PhD, and Toshio Hosoi, DDS, PhD
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine,
Yokohama, Japan
Purpose: Resins for denture repair must have higher bending strengths and similar hardness compared todenture base resins. This study evaluated the bending strengths and hardness of autopolymerized acrylic resin
applied using the brush-on technique.
Materials and Methods: Square resin rods (2x2x25 mm) and blocks (15x15x5 mm) were fabricated formeasuring the bending strengths and hardness, respectively, of five autopolymerized resins (Unifast II, Unifast
Trad, Provinice, Metafast, and Miky). Two procedures for applying the autopolymerized resins were tested inthis study: the brush-on technique and the conventional mixing (monomer/polymer: 0.5 mL/g) technique. Thebending strengths of the resins (crosshead speed: 5 mm/minute) were measured with a three-point bending testusing autography. The Vickers hardness (1.96 N, 10 s) was also measured using a hardness tester according toJIS 6518. The data (n=5) were analyzed by ANOVA/Tukey’s test (α=0.05).
Results: There were no significant differences (p>0.05) for both bending strengths and hardness between thebrush-on technique and the conventional mixing technique. The bending strengths of Unifast Trad weresignificantly greater than those of Miky (p>0.05). Unifast II had the greatest hardness among all the resinstested.
Conclusion: Although the monomer/polymer ratio for the brush-on technique was lower than for theconventional mixing technique, similar bending strengths and hardness values were found for both techniques.
(Int Chin J Dent 2010; 10: 1-5.)
Key Words: autopolymerized acrylic resin, bending strength, brush-on technique, hardness

Introduction
In prosthetic practice, the breakage of denture bases has frequently been observed. Broken dentures areusually repaired chairside with autopolymerized resin using the brush-on technique.1-3 These resins must havesufficient mechanical properties and strong adhesion to denture base resins in order to prevent the breakage thatis commonly found.

The mechanical properties of polymerized resin are affected by factors such as theoperator’s skill, the differences between the original denture base resins and the repaired resins, and the repair
conditions.4-6
Kamada et al.7 investigated the repair procedures by evaluating the fluidity and hardening time ofautopolymerized acrylic resins applied using the brush-on technique. They found great differences in thesecharacteristics among the autopolymerized acrylic resins tested. The hardening time for the brush-on technique
was shorter than for the conventional mixing technique. Hanatani et al.8 reported that the dimensional accuracyof the autopolymerized resins applied using the brush-on technique was better than that of the conventional
mixing technique. This finding was attributed to the fact that the polymer and monomer ratio was lower and also
the polymerization shrinkage was minimized using the brush-on technique. Suzuki et al.9 evaluated the handlingefficiency of autopolymerized resin applied using this technique and concluded that the handling efficiency of
the autopolymerized resin using the brush-on technique depended on the skill of the operators rather than theresins and brushes.
Although the brush-on technique has been used mainly for chairside denture repair, the mechanical propertiesof the repair resins applied with this technique are not well known. The aim of this study was to investigate thebending strengths and hardness of commercial autopolymerized acrylic resins applied using the brush-on

1

Osada et al.

Int Chin J Dent 2010; 10: 1-5.

technique.

Materials and MethodsFive autopolymerized resins, namely, Unifast II (GC Dental Industrial Corp., Tokyo, Japan), Unifast Trad
(GC Dental Industrial Corp.), Provinice (Shofu Inc., Kyoto, Japan), Metafast (Sun Medical Co., Ltd., Moriyama,Japan), and Miky (Nissin, Kyoto, Japan) were tested in this study (Table 1).
Table 1.

Autopolymerized resins used in this study.

8

Trade name

Manufacturer

Color

L (mL) / P (g)Lot number

Unifast IIGCTokyo, Japan#3 Pink

0.5

Powder 0503172
Liquid 0504261

Unifast TradGC#3 Pink

0.5

Powder 0504261
Liquid 0503221

ProviniceShofuKyoto, Japan

U30.5

Powder 020507
Liquid 030555

Metafast

Sun MedicalMoriyama, Japan#2 Pink

0.7

Powder 41101
Liquid 41103

Miky

NissinKyoto, Japan#20.5

Powder PEIL
Liquid ELG

Fig. 1.

Illustration of the rod and block specimens

Specimen fabrication
Silicone molds were prepared for two types of specimens, rod (2x2x25 mm) and block (15x15x5 mm), tomeasure the bending strengths and hardness, respectively (Fig. 1). Two procedures, the brush-on technique andthe conventional mixing technique, were evaluated.

In the brush-on technique, the brush was soaked inmonomer and then dipped into the polymer according to the manufacturer’s recommendations. The resin slurry
was packed into the rod and block molds in 3 and 10 increments, respectively. In the mixing technique, themonomer and polymer were mixed at the manufacturer’s recommended ratio (P/L: 0.5 mL/g) for 10 s and thenpoured into each type of mold within 10 s. After removal from the molds, the resin rods and blocks werefinished and polished using #2,000 abrasive paper. Five specimens were fabricated for each of the resins (total2Osada et al.

Int Chin J Dent 2010; 10: 1-5.

of 50 rod and 50 block specimens).
Measurement of bending strengths and hardness
After the resin rods were soaked in distilled water (37°C) for 48 hours, they were mounted in a screw-driven
mechanical testing apparatus (Instron 5565, Instron, Canton, MA, USA).

The bending strengths weredetermined by the breaking loads (N) and elastic moduli (GPa) using a three-point bending test (25 mm between
the supporting points) at a crosshead speed of 5 mm/minute.
The resin blocks were soaked in distilled water (37°C) for 24 hours. According to JIS 6518, the Vickers
hardness of the five resins was measured using a hardness tester (HMV, Shimadzu, Kyoto, Japan) with a load of
1.96 N held for 10 s. The hardness numbers were obtained from five specimens of each kind of resin at threearbitrarily chosen sites per specimen. The data (n=5) on both the bending strengths and hardness were analyzed
by ANOVA and Tukey’s multiple comparisons test at a significance level of α=0.05.

Results
Bending strengths
The breaking loads and elastic moduli of the five resins packed using both techniques are presented in Figs. 2and 3, respectively. There were no significant differences for both the breaking load and elastic modulus(p>0.05) between the brush-on technique and the conventional mixing technique.
The bending strengths of all the resins ranged from approximately 13 N to 22 N. For the brush-on technique,Unifast Trad and Metafast had significantly greater strengths than those of Miky (p<0.05). For the mixing
technique, there were significant differences between Miky and the other resins, and between Unifast andUnifast Trad (p<0.05). The elastic moduli of all the resins ranged from approximately 2.7 GPa to 3.5 GPa.
There were no significant differences for both techniques among all the resins tested (p>0.05).

Hardness
The hardness of each resin is shown in Fig. 4. Similar to the bending strengths, there were no significant
differences between the two techniques (p>0.05). Although Unifast II (mixing technique) had the greatest
hardness and Unifast Trad (brush-on technique) demonstrated the lowest hardness, no remarkable differenceswere found.
For the brush-on technique, there were significant differences between Unifast II and Metafast, and between
Unifast II and Unifast Trad (p<0.05). For the mixing technique, significant differences were found between
Unifast II and Unifast Trad, Unifast II and Provinice, and Unifast Trad and Miky (p<0.05).

3

Osada et al.

Int Chin J Dent 2010; 10: 1-5.

Discussion
Compared to the mixing technique, the polymer and monomer ratio (P/L) of the brush-on technique is lowerand inconsistent. In addition, the transpiration of monomer and non-swelling polymers occurs during handling.
However, no clear differences in hardness and bending strengths were found between the two techniques tested.
As for the resin after polymerization, it was shown that the strengths became approximately constant regardlessof the powder/liquid ratio. Differences were found between some resins, but all of them exceeded the minimum
permitted level for clinical use.
Unifast Trad indicated the highest bending strengths, and Miky demonstrated the lowest among all the resinstested for both techniques. In contrast, the hardness was the greatest for Unifast II and the worst for Unifast Trad.
The main component of the monomers of all the resins is methyl methacrylate (MMA) and of the polymers ismethacrylic acid ester.

Unifast II, Unifast Trad, Provinice, and Metafast contain methacrylic acid ester

copolymers, and Miky is composed of poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate)(PEMA).10,11 The polymers of Miky consist of two polymer chains (PMMA and PEMA) intertwined with eachother. In contrast, the other four products contain both MMA and EMA. It is suggested that the difference in thetype of polymerization might influence the bending strength, so the polymer component in Miky may decrease
its bending strengths. However, the relationship of the hardness among the products tested is obvious. The
presence of porosity and non-swelling polymers might also contribute to the differences in the strengths.
All the specimens were fabricated with resins that included color pigment (pink) for denture base repair.
Differences between the resins might be apparent if clear resin was used. In addition, repaired dentures require
not only the mechanical strengths of the autopolymerized resins but also bonding strength to the denture baseresin. Further testing using repaired specimens, in which autopolymerized resins are applied to the original
denture base resin, is necessary to evaluate the total properties for chairside denture repair.
Although the monomer/polymer ratio of the brush-on technique was lower than for the conventional mixing
technique, similar bending strengths and hardness values were found for both techniques.
Acknowledgment
This work was supported in part by research grant to improve the quality of dental products from the Japan DentalAssociation.

4

Osada et al.

Int Chin J Dent 2010; 10: 1-5.

References
1. Beyli MS, von Fraunhofer JA. Repair of fractured acrylic resin. J Prosthet Dent 1980; 44: 497-503.
2. Hirabayashi S, Nasu I, Harashima I, Hirasawa T. Studies on dental methacrylic resins (part 9) on composition of heat-shock,pour-type and self-curing denture base resins. J Jpn Dent Mater 1984; 3: 338-49.
3. Naka S. Transverse strength of repaired denture base acrylic resins. J Fukuoka Dent Coll 1992; 19: 293-317.
4. Harada Y. Study on the self-curing resins for denture base materials. J Stomatol Soc Jpn 1965; 32: 291-312.
®
5. Hoshiai K, Hiranuma K, Kawazoe T, et al. The clinical evaluation of self-curing resin “UNIFAST II ”. J Jpn Prosthodont Soc
1994; 38: 986-96.
®
6. Hoshiai K, Kanazawa T, Hiranuma K, et al. Examination of physical properties of self-curing resin “UNIFAST II ”. J JpnProsthodont Soc 1995; 39: 494-500.
7. Kamada N, Ohkubo C, Shimpo H, et al. Fluidity and hardening time of autopolymerized denture repair acrylic resin: Acomparison of brush-on technique and mixing technique. Prosthodont Res Pract 2008; 7: 19-23.
8. Hanatani S, Shibuya N, Muraishi E, et al. Dimensional accuracy of autopolymerized resin applied using the brush-ontechnique. Int Chin J Dent 2009; 9: 9-13.
9. Suzuki Y, Chiba H, Kamada N, et al. Handling efficiency of autopolymerized resin applied using the brush-on technique. IntChin J Dent 2009; 9: 33-8.
10. Hasegawa A, Katsu M, Hoshiai K, Hiranuma K. Development of new initiator of polymerization for self-curing resin withbarbituric acid derivatives. J Jpn Prosthodont Soc 1997; 41: 613-9.
11. Shimizu H, Yoshinaga M, Habu T, Takahashi Y. Bond strength of a self-curing resin with barbituric acid derivatives to a few
kinds of denture base resin in denture repair. J Jpn Prosthodont Soc 1998; 42: 815-22.

Correspondence to:Dr. Tomoko Osada
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine
2-1-3, Tsurumi, Tsurumi-ku, Yokohama 230-8501, Japan
Fax: +81-45-573-9599 E-mail: osada-tomoko@tsurumi-u.ac.jp
Accepted January 16, 2010.

Copyright ©2010 by the International Chinese Journal of Dentistry.

5