journal of prosthodontic research 58 (2014) 55–61

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Original article

Effects of low-energy electron beam irradiation on flexural properties of self-curing acrylic resin Kyosuke Ito DDS, PhDa,*, Akiko Nomura DDS, PhDb, Shuichi Nomura DDS, PhDa, Kouichi Watanabe DDS, PhDc a Division of Comprehensive Prosthodontics, Niigata University Graduate School of Medical and Dental Sciences, 2-5274 Gakkocho-dori, Niigata city, Niigata 951-8514, Japan b Department of Dental Technology, Meirin College, 3-16-10 Masago, Niigata city, Niigata 950-2086, Japan c Division of Biomaterial Science, Niigata University Graduate School of Medical and Dental Sciences, 2-5274 Gakkocho-dori, Niigata city, Niigata 951-8514, Japan

article info

abstract

Article history:

Purpose: The purpose of this study was to confirm the effectiveness of LEB irradiation onto

Received 1 June 2013

the polymer powder for improving the mechanical properties of self-curing acrylic resin.

Received in revised form

Methods: The polymer powder of self-curing acrylic resin was irradiated with total LEB doses

17 September 2013

of 25, 50, 75 or 100 kGy. Non-irradiated powder was used as a control. After LEB irradiation,

Accepted 29 October 2013

ESR measurement, weight-average molecular weight measurement and three-point bend-

Available online 3 January 2014

ing test were performed. Results: ESR spectrum of control had no peaks. After LEB irradiation, nine peaks were observed

Keywords:

in each ESR spectrum, which indicates the presence of free radicals from main polymer chain.

Self-curing acrylic resin

The quantity of free radicals increased linearly up to 100 kGy. Calibrated weight-average

Low-energy electron beam

molecular weights were as follows: control, 960,000; 25 kGy, 500,000; 50 kGy, 440,000; 75 kGy,

Polymer powder

410,000; and 100 kGy, 390,000. Molecular weight decreased with increasing LEB irradiation

Cross-linking

dose. The mean values of flexural strength (MPa) were as follows: control, 61.5  3.0; 25 kGy,

Flexural strength

68.1  4.0; 50 kGy, 73.0  1.9; 75 kGy, 70.4  3.6; and 100 kGy, 67.7  2.3. The flexural strength of the specimens cured with the LEB-irradiated powder was significantly higher than that of control ( p < 0.01). These results indicate that flexural strength of polymer materials cured with the LEB-irradiated powder increases because of increase in cross-linking structure. Conclusion: It is confirmed that LEB irradiation onto the polymer powder of self-curing acrylic resin improves the flexural strength. Crown Copyright # 2013 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society. All rights reserved.

1.

Introduction

Dental acrylic resin is made from a mixture of methylmethacrylate (MMA) and polymethylmethacrylate (PMMA), and has been widely used in dentistry since it was developed in

1937 [1]. At present, there are heat-curing, self-curing and light-curing resins available, and the appropriate type is selected according to the purpose. Self-curing acrylic resins, which are polymerized by mixing polymer powder and monomer liquid at room temperature, are used in a variety of clinical applications, such as

* Corresponding author. Tel.: +81 25 227 2892; fax: +81 25 229 3454. E-mail address: [email protected] (K. Ito). 1883-1958/$ – see front matter . Crown Copyright # 2013 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2013.10.004

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journal of prosthodontic research 58 (2014) 55–61

fabricating provisional restorations, repairing dentures and adjusting occlusion. However, the chemical and mechanical properties of this resin are inferior to those of heat-curing resin [1–3]. In addition, self-curing acrylic resin often produces fracture of provisional restorations, re-fracture of the repaired denture and early wear of the reconstructed occlusal surface, which causes clinical problems [1,4–7]. Therefore, various techniques have been studied to improve these shortcomings [5–10]. We focused on low-energy electron beam (LEB) irradiation treatment to improve resin properties [11]. In general, electron beam irradiation treatment is used as a post-curing technique for hardening and improving wear resistance of polymer materials [12], and there have been some reports about the mechanical properties of dental acrylic resin being improved by this technique [13–16]. However, the technique has not yet been applied to the dental field because installing electron beam irradiation equipment at individual dental offices is difficult. Therefore, we hypothesized that installation of such irradiation equipment is unnecessary at each dental office, if the mechanical properties of the polymer material are improved by LEB irradiation onto the polymer powder of self-curing acrylic resin prior to polymerization with monomer. The purpose of this study was to evaluate the effectiveness of LEB irradiation onto the polymer powder for improving the properties of the polymer material by examining whether the characteristics of the powder and the flexural properties of the polymer material changed under various LEB irradiation doses.

2.

Materials and methods

2.1.

LEB irradiation

LEB irradiation was carried out at 110 kV and 16 mA in a nitrogen atmosphere at room temperature using a low energy electron beam radiation system (EES-S-MJC01; Hamamatsu Photonics K.K. & Meirin College, Hamamatsu, Japan). The test material used was self-curing acrylic resin supplied in polymer powder and monomer liquid form (Unifast III; GC, Tokyo, Japan). The powder was placed into an aluminum dish, which was set on an irradiation stage in the LEB radiation system. The distance between the irradiation window and irradiation stage was 15 mm, and the stage was moved under the window at a speed of 50 mm/s (Fig. 1). Under these conditions, the LEB energy dose was 25 kGy per movement, and the powder was irradiated with total LEB doses of 25, 50, 75 or 100 kGy. Non-irradiated powder was used as a control.

2.2.

Electron spin resonance (ESR) measurement

In order to directly detect unpaired electrons and investigate the characteristics of free radicals under different LEB irradiation doses, ESR spectroscopy (JES-RE3X; JEOL, Tokyo, Japan) was used for measurement of powder specimens (0.10  0.01 g) placed in glass tubes 1 h after LEB irradiation at room temperature (Fig. 2). Measurement conditions for acquisition of ESR spectral data were as follows: microwave power, 1 mW; frequency modulation, 100 kHz; amplitude modulation, 9 G; time constant, 0.3 s; and magnetic field range, 326–346 mT.

Fig. 1 – Schematic diagram of LEB irradiation. The distance between the irradiation window and the irradiation stage is 15 mm, and the stage is moved under the window at a speed of 50 mm/s.

Fig. 2 – Schematic diagram of ESR measurement.

2.3.

Measurement of weight-average molecular weight

As the molecular weight of the powder affects polymer characteristics, the weight-average molecular weight of the powder specimens was measured by high performance liquid chromatography (HPLC) (LC-10A; Shimadzu, Tokyo, Japan). Solvent in which the powder was dissolved in tetrahydrofuran at a concentration 0.6% (w/v) was measured using four columns connected to the HPLC system at a flow rate of 0.8 mL/min and a temperature of 40 8C. The obtained HPLC peak for each powder specimen was calibrated with an analytical curve prepared using PMMA of known molecular weight. The weight-average molecular weight was estimated using the software installed in this apparatus.

2.4.

Three-point bending test

Self-curing acrylic resin was handled as follows: the powder/liquid ratio (P/L) was 1.3, in accordance with the

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journal of prosthodontic research 58 (2014) 55–61

(a) control

**

** **

Flexural strength (MPa)

**

(b) 25 kGy

(c) 50 kGy

: p 0.05).

2.4

2.2

2.0

4.

Discussion

4.1.

Background of electron beam irradiation

1.8 control

25 kGy

50 kGy

75 kGy

100 kGy

LEB irradiation condition Fig. 6 – Elastic modulus at each condition. Bars represent the mean elastic modulus W standard deviations of ten independent experiments. There are no significant differences in the elastic modulus between each condition ( p > 0.05).

with increasing LEB irradiation dose. These molecular weights were obtained from single sample because our preliminary measurements were very consistent.

3.3.

Three-point bending test

The mean values of flexural strength and standard deviations (MPa) were as follows: control, 61.5  3.0; 25 kGy, 67.9  4.8; 50 kGy, 73.0  1.9; 75 kGy, 70.4  3.6; and 100 kGy, 67.7  2.3. The flexural strength of the specimens cured with the LEBirradiated powder was significantly higher than that of control ( p < 0.01), while the 100 kGy specimen showed lower flexural strength values than the 50 kGy specimen ( p < 0.01) (Fig. 4). A representative strain–stress curve of the specimen irradiated with total LEB dose of 50 kGy as well as a control specimen was shown in Fig. 5. Although the flexural strength of the LEB irradiated specimen increased, compared with a control, there were not different in the initial slope of the stress–strain curve and proportional limit between those two specimens.

Electron beam irradiation treatment of polymer materials was first performed in the 1950s [18,19], and for decades, this method has been used successfully in industry. In particular, post-curing techniques using electron beam irradiation have been widely applied to improve the mechanical properties of polymer materials, such as polyethylene, polystyrene and polycarbonate [20,21]. However, few studies have been published in the dental field [14,15,22,23]. In past studies, electron beam irradiation was mainly used for post-curing technique [12,14–16,23]. Behr et al. reported that the changes in mechanical properties were so small that the expenditure of energy and cost did not justify the use of electron beam post-curing for dental PMMA [14]. Therefore, instead of irradiation with LEB for post-curing, we focused on LEB irradiation onto the powder before mixing with monomer [24,25]. After LEB irradiation onto the powder, we examined whether the characteristics of the powder and the flexural properties of the polymer material were altered under different LEB irradiation doses, and we then assessed the effectiveness of LEB irradiation onto the powder for improving resin properties.

4.2.

Various reactions caused by electron beam irradiation

Generally, there are two competing reactions that occur during electron beam irradiation (cross-linking and chain scission), and the dominant reaction during irradiation depends on different parameters, such as irradiation dose,

journal of prosthodontic research 58 (2014) 55–61

polymeric structure, temperature and functional groups of the polymer [26]. Predicting the results of electron beam irradiation on different polymers is therefore difficult because of diverse irradiation methods and polymeric systems. PMMA is often described in the literature [19,27,28] as a thermoplastic polymer that tends to undergo more chain scission during irradiation. In this study, LEB irradiation was performed with acceleration at 110 kV. It has been reported in the literature that acceleration influences only the depth of electron penetration and not reaction type [29]. Thus, in the powder specimens after LEB irradiation, scission of the polymer chain was predicted to mainly occur under the conditions used in this study. To confirm this phenomenon, all of the present powder specimens were subjected to measurement of ESR spectra and molecular weight.

4.3.

ESR measurement

ESR is a spectroscopic technique based on magnetic resonance, and is able to detect and determine the structures of molecules with an unpaired electron. ESR utilizes the following phenomena; energy transfer occurs when molecules with an unpaired electron, which are orientated in magnetic field, are irradiated by microwaves. It has been established that ESR is a useful experimental technique for studying phenomena associated with radicals originating from various mechanisms, such as fracture, deformation and radiation [30]. ESR selectively detects free radicals, and this is a major advantage of ESR over other methods. Free radicals play an important role in the polymerization reaction of acrylic resin. When considering how LEB irradiation affects polymerization, it is useful to investigate the relationship between LEB irradiation and free radicals. Therefore, the amount and characteristics of free radicals under different LEB irradiation doses were measured by ESR spectroscopy. In Fig. 3, each ESR spectrum showed nine peaks. It is known that the characteristic ESR spectrum of the strong quintet and weak quartet is observed from radicals produced in PMMA [31–35]. Radicals from polymer chains, smaller molecules and initiators were observed at lower temperature [36,37]. Taking into account reactivity of the radicals, the lifetimes of radicals from the smaller molecules and initiators were very short, and therefore it would be difficult to detect these radicals at room temperature in air [37]. As shown in Fig. 3, these phenomena were confirmed; only free radicals originating from the main polymer chain [31–34] were detected. This indicates that main chain scission of the polymer occurred after LEB irradiation and the radicals survived under these conditions. Moreover, strong peaks of ESR spectra were observed with increasing LEB irradiation dose.

4.4.

Measurement of weight-average molecular weight

The molecular weight of the powder affects the characteristics of the polymer material. Generally, when powders of large molecular weight are mixed with monomer, the mechanical properties of the polymer material are improved. On the other hand, solution rate of the powder in the monomer decreases and viscosity of the mixture increases [38]. It has been reported that the molecular weight of powders is decreased

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by irradiation [39], and that there are few differences in the mechanical properties of acrylic resin when powders with molecular weight above 300,000 are used [40]. Molecular weight decreased with increasing LEB irradiation dose. This suggests that main chain scission of the polymer occurred. The rate of change in molecular weight decreased with increasing LEB irradiation dose, although molecular weight at 25 kGy decreased markedly when compared with the control. This suggests that both scission and re-combination reaction by LEB irradiation occurred at room temperature.

4.5.

Three-point bending test

Self-curing acrylic resins are used in dentistry for various purposes although fracture and abrasion are sometimes observed [4]. Improving such mechanical properties is an important subject of dental material research. Therefore, three-point bending test was performed in order to examine the effects of LEB irradiation onto the powder on the mechanical properties of the resin. The flexural strength of specimens cured with the LEBirradiated powder increased significantly in comparison with that of the control. The greatest values for flexural strength were observed after irradiation at 50 kGy (Fig. 4). As stated above, after LEB irradiation onto the powder, free radicals were produced by main chain scission of the polymer and the molecular weight of the powder decreased. It has been noted that chain scission of the polymer contributes to dissolution of chain entanglements [41]. As a result, when the powder specimen and monomer were mixed, both swelling and solubility of the powder occurred quickly [42,43]. Thus, cross-linking and degree of polymerization of the polymer materials are expected to increase. However, after irradiation with 100 kGy, because of excessive scission of the main polymer chain, free radicals were produced in large quantities. Moreover, as some initiators were also resolved during irradiation, the free radicals caused by the resolved initiator decreased after mixing, and the resin was insufficiently polymerized when the powder was mixed with monomer during conventional handling. Therefore, it was thought that the mechanical properties worsened due to a lower average polymerization degree in the polymer material. According to the present results, the optimal LEB irradiation dose for improving flexural properties of self-curing acrylic resin is 50 kGy. For microscopic aspects of polymer fracture, fracture of covalent bonding and slipping of the polymer chain are assumed to play an important role. However, considering the differences in bonding energy between the covalent bond and the intermolecular force (van der Waals force), the slipping mechanism is an important phenomenon for determining flexural strength. The slipping reaction is hindered when crosslinking increases. Arima et al. reported that highly cross-linked acrylic resins had higher transverse bend strength than conventional acrylic resins [44]. Therefore, the amount of cross-linking plays a decisive role in flexural strength. In contrast to flexural strength, the elastic modulus is defined as the ratio of stress to strain [45]. Thus, cross-linking structure affects little to the elastic modulus. When the amount of the cross-linking increases fully, even small deformations, which

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are primarily influenced by van der Waals force, are slightly restricted by the cross-linking structure. The present findings demonstrated that LEB irradiation promoted the solubility and swelling of the powder into the monomer and increase of free radicals. These reactions would produce increase of the number of cross-linking point in the polymer, enabling to develop a three-dimensional network, which contributed to the enhancement of flexural strength. Consequently, the most important finding obtained by LEB irradiation onto the powder is the clear increase in cross-linking after conventional dental polymerization treatment. After LEB irradiation, the flexural strength of the polymer materials was increased. Thus, LEB irradiation onto the powder is effective for improving resin properties. However, based on this experiment, the duration of effects after LEB irradiation is not fully understood. Moreover, it is expected that the amount of unreacted monomer decreases in the polymer materials cured with the LEB-irradiated powder. As the duration of effects and the amount of unreacted monomer are important factors in application to dental practice, further investigations are required in order to identify several factors, such as storage conditions for the powder after LEB irradiation and elution of residual monomer in the polymer materials.

5.

Conclusions

The flexural strength of the polymer materials cured with the LEB-irradiated powder increased because of increase in crosslinking structure. This finding confirms that LEB irradiation onto the polymer powder of self-curing acrylic resin is capable of improving the mechanical strength.

Acknowledgements The authors cordially thank Assoc. Prof. Hiroshi Kobayashi from Division of Comprehensive Prosthodontics, Niigata University Graduate School of Medical and Dental Sciences for his insightful comments and suggestions. The authors are grateful to Dr. Seigo Okawa and Dr. Mitsugu Kanatani from Division of Biomaterial Science, Niigata University Graduate School of Medical and Dental Sciences for their technical assistance and encouragements. The authors are also grateful to Mr. Dai Haraguchi from Hamamatsu Photonics K.K. for the use of LEB irradiation equipment. This work was partially supported by JSPS KAKENHI Grant Number 20390495.

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Effects of low-energy electron beam irradiation on flexural properties of self-curing acrylic resin.

The purpose of this study was to confirm the effectiveness of LEB irradiation onto the polymer powder for improving the mechanical properties of self-...
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