Impact Toughness of Polymer Dental Composites Based on Dimethacrylates Resins

Modern Polymeric Materiale for Environmental Applications, Vol. 3, 2008

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Impact Toughness of Polymer Dental Composites Based on

Dimethacrylates Resins

Joanna Kleczewska a, Dariusz M. Bieliński a, b, Jerzy Morawiec c

a Institute of Polymer & Dye Technology, Technical University of Lodz, Stefanowskiego 12/16, 90-924 Lodz

b Institute for Engineering of Polymer Materials & Dyes, Department of Elastomers & Rubber Technology, Harcerska 30, 05-820 Piastow

c Centre of Molecular & Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz

Abstract Polymer restoratives based on dimethacrylate resins have practically removed toxic amalgams from dental practice [1, 2]. In spite of all aesthetic attributes, low mechanical strength, especially under dynamic conditions, still remains the problem to be solved [3, 4]. More attention should be devoted to mechanical properties of the materials, whereas PN-EN ISO 4049 standard limits the requirements only to static three-point bending test. The paper discusses experimental data of impact toughness for some commercially availaible dental resin composites, varied according to morphology and filler loading. Experiments were performed with an instrumented 1 N Charpy hammer. Characteristics of energy dissipation and force development in the polymer composites were collected during impact. Higher filler content and broader particle size distribution leads to the increase of absorbed energy value. Force dissipated by material grows, but time to achieve the maximum value decreases. Keywords: dimethacrylate resins, dental materials, impact toughness, crack

propagation Introduction Expectations from nowadays dental restorative materials focus on aesthetic and precise „reconstruction” of destroyed tooth tissues [1, 2]. Because of the former, amalgam fillings, despite higher strength [3, 4], are gradually losing their position to polymer composites based on dimethacrylate resins. The specification for restorative resin composites collected in PN-EN ISO 4049 contains requirements on: biocompatibility, hardness, transparency, light sensibility and colour stability, water absorption and solubility of materials. The standard limits


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itself only to three-point bending test, to be carried out on relatively big, inconvenient and far from the reality, samples. Lack of broader requirements on mechanical properties of polymer dental composites is rather surprising. Dental fillings, working usually in occlusal contact, are liable to great loads and stresses, frequently applied under dynamic conditions. It seems legitimate, that complete assessment of polymer composites, requires measurements of their impact toughness, which can be performed using an instrumented Charpy hammer with computerized data acquisition system and analysis software. From a practical point of view also limiting the size of samples is beneficial. Taking into consideration the specific mechanisms of chewing, dynamic examination of restorative materials seems to be more appropriate to simulate real exploitation conditions [5]. Materials and Methods Commercially available resin composite formulations: QuixFil Universal (DENTSPLY DeTrey, Germany), Filtek P60 (3M ESPE, USA) and Enamel GE2 (HFO Micerium, Italy), were objects of examinations. The materials were selected for their different morphology and hard filler content – Table 1. Tab. 1. Characteristics of polymer-based dental composites (manufacturer’s data).

Material Manufacturer Filler Filler content,

vol. % SEM morphology of sample

cross-section [6]

QuixFil Universal

DENTSPLY DeTrey

(Niemcy) www.dentsply.de

szkło Sr-Al-Na-F-P-Si

66

Filtek P60

3M ESPE (USA)

www.3m.com /espe

SiO2, ZrO2 61


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Enamel GE2

HFO Micerium (Włochy)

micerium.it

krzemionka pirolityczna

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Composite samples, having the form of 4 mm height and 2 mm of diameter rods, were polymerized through the thin microscopic slide using an Astralis 5 (Ivoclar Vivadent, Liechtenstein) halogen lamp, emitting light of approx. intensity 650 mW/cm2, in the wave range 400-500 nm. Time of irradiation was applied according to recommendations from manufacturers of dental materials studied. Impact toughness of composites was studied using an instrumented Resil 5.5 Charpy Hammer (CEAST, Italy), operating with mass of 0.238 kg and initial angle of 146 °, what gives impact energy of 0.98 J and working range of 1.7 kN [7]. In order to enable analysis of small specimens, sample holder of the instrument had to be redesigned. Instead of conventional breaking a crushing impact, better reflecting exploitation conditions was applied – Figure 1.

1 – horizontal frame 2 – sample 3 – vertical frame Fig.1. Scheme of a sample holder. Each time 10 samples were measured, from which 6 the closest results were taken for further analysis. Results and Discussion Tab. 2 presents parameters of impact toughness for the dental composites studied. Magnitude of the median for the following experimental values was calculated: maximum force being dissipated (Fmax), time to its achievement (t), and energy absorption (E) during impact test.

HAMMER

1 2 3


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Tab. 2. Impact toughness parameters for the dental composites studied.

Material t [ms] Fmax [N] E [mJ]

QuixFil Universal 0.227 413.7 62.3 Filtek P60 A3 0.248 389.7 56.7 Enamel GE2 0.257 368.9 55.5

Significant scatter of experimental data set for dissipated force and absorbed energy, for series of samples made of one material, confirm general heterogeneous nature of polymer dental composites. Increase in filler loading together with non-uniform size distribution of its particles, make energy absorbed by composite material higher, simultaneously with shorter time for dissipated force maximum to appear (see data for QuixFil Universal). The difference in energy absorbed by sample made of Filtek P60 and Enamel GE2 materials is insignificant. It can be the result of the same spherical shape and only slight difference according to size distribution of their hard phase particles. Various filler content seems not to play any role in ability to energy absorption by the materials. Figures 2–4 present changes of force and the energy versus impact time, registered for the composites studied.

Fig. 2. Force and energy versus impact time curves for QuixFil Universal.


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Fig. 3. Force and energy versus impact time curves for Filtek P60.

Fig. 4. Force and energy versustime curves for Enamel GE2.

Conclusions Crack propagation showed to be less effective in a composite of bimodal filler size distribution (QuixFil Universal) in comparison to other materials studied. It seems not to be associated with filler content, based on similar data obtained for Filtek P60 and Enamel GE2 materials. Despite they vary significantly, according to filler loading, slightly broader particle size distribution [7] is responsible for slightly higher ability to energy absorbance for the former. Additionally, broad, preferentially bimodal particle size distribution for QuixFil Universal, probably also together with their more developed geometry (multiwall seems to have an advantage over circular one) make external energy to be absorbed inside the material faster. The data presented point on


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desired morphology of polymer dental composites from the point of view of material durability [8]. References [1] F.J.M. Roeters, N.J.M. Opdam, B.A.C. Loomans, J. Dentistry, 32 (2004) 337. [2] J. W. Nicholson, Int. J. Adhes. Adhesives, 20 (2000)16. [3] J.L. Ferracane, Dental Mater., 22 (2006) 689. [4] P. Lambrechts, K. Goovaerts, D. Bhardwaj, J. De Munck, L. Bergmans, M. Peumans, B. Van Meerbeek, Wear, 261 (2006) 980. [5] R. Koczorowski, Biotrybologiczna ocena wybranych materiałów protetycznych stosowanych do rekonstrukcji powierzchni zwarciowych stałych protez zębowych –DSc dissertation, Poznan 1996. [6] J. Kleczewska, J. Sokołowski, D.M. Bieliński, L. Klimek, Inż. Mater., 160 (2007) 930. [7] http://www.ceast.com/ceastrel2/pdf/1/Resil-Family-960-06.pdf [8] J. Kleczewska, D.M. Bieliński, A. Pacyk, K. Wichrowska, Polish J. Env. Studies – submitted.

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Impact Toughness of Polymer Dental Composites Based on Dimethacrylates Resins