d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema

Characterization of urethane-dimethacrylate derivatives as alternative monomers for the restorative composite matrix Izabela M. Barszczewska-Rybarek ∗ Department of Physical Chemistry and Technology of Polymers, Silesian University of Technology, M. Strzody 9, 44-100 Gliwice, Poland

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. The aim was accomplished by a comparative analysis of the physicochemical prop-

Received 31 October 2013

erties of urethane-dimethacrylate (UDMA) monomers and their homopolymers with regard

Received in revised form

to the properties of basic dimethacrylates used presently in dentistry. The homologous series

14 August 2014

of UDMA were obtained from four oligoethylene glycols monomethacrylates (HEMA, DEG-

Accepted 23 September 2014

MMA, TEGMMA and TTEGMMA) and six diisocyanates (HMDI, TMDI, IPDI, CHMDI, TDI and MDI). Methods. Photopolymerization was light-initiated with the camphorquinone/tertiary amine

Keywords:

system. Monomers were tested for viscosity and density. Flexural strength, flexural modulus,

Composite resin

hardness, water sorption and polymerization shrinkage of the polymers were studied. The

Urethane-dimethacrylate

glass transition temperature and the degree of conversion were also discussed.

Viscosity

Results. HEMA/IPDI appeared to be the most promising alternative monomer. The monomer

Glass temperature

exhibited a lower viscosity and achieved higher degree of conversion, the polymer had

Polymerization shrinkage

lower water sorption as well as higher modulus, glass temperature and hardness than Bis-

Degree of conversion

GMA. The polymer of DEGMMA/CHMDI exhibited lower polymerization shrinkage, lower

Water sorption

water sorption and higher hardness, however it exhibited lower modulus when compared to

Mechanical properties

HEMA/TMDI. The remaining monomers obtained from HEMA were solids. Monomers with

Alternative monomers

longer TEGMMA and TTEGMMA units polymerized to rubbery networks with high water sorption. The viscosity of all studied UDMA monomers was too high to be used as reactive diluents. Significance. The systematic, comparative analysis of the homologous UDMA monomers and corresponding homopolymers along with their physico-mechanical properties are essential for optimizing the design process of new components desirable in dental formulations. Some of the studied UDMA monomers may be simple and effective alternative dimethacrylate comonomers. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗

Tel.: +48 32 237 17 93; fax: +48 32 237 15 09. E-mail address: [email protected]

http://dx.doi.org/10.1016/j.dental.2014.09.008 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

1.

1337

Introduction

Restorative dental composites undergo hardening due to the polymerization of multifunctional monomers that produce a rigid and heavily cross-linked polymer matrix surrounding the inert filler particles. The most commonly used monomer is the highly viscous 2,2-bis-[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]-propane) (Bis-GMA). Bis-GMA is usually accompanied by triethylene glycol dimethacrylate (TEGDMA), acting as a low viscosity reactive diluent to achieve high filler loading. The stiff molecular structure and hydroxyl groups of Bis-GMA ensure low cure shrinkage, high polymer modulus and desirable adhesion to tooth enamel [1–5]. They can increase the resin’s viscosity, residual unsaturation in the polymer and its water uptake. On the other hand, TEGDMA has been shown to increase matrix water sorption and its polymerization shrinkage [1–3,6,7]. Alternative dental formulations urethane-dimethacrylate monomer–1,6contain bis-(methacryloyloxy-2-ethoxycarbonylamino)2,4,4-trimethylhexane, commonly abbreviated to UDMA, but for the purpose of this work it has been abbreviated to HEMA/TMDI. The advantage of HEMA/TMDI is its lower viscosity, when compared to Bis-GMA. Moreover, urethane linkage can form strong hydrogen bonds and thus improve both durability of the composite’s matrix as well as bonding to the tooth structure [1–3]. Current aims in dental research include identifying new dimethacrylates of moderately low viscosities [8,9], producing polymers with low polymerization contraction [10,11], a high degree of conversion [11], good mechanical properties [8,9,11] and low water sorption [12,13]. Though innovative ormocer [14] and silorane [15] composite systems have been developed, materials based on Bis-GMA, HEMA/TMDI and TEGDMA are still mostly applied [2] in the dental practice. The adequate properties of HEMA/TMDI, in combination with the low price of its production, may determine the work undertaken leading towards the preparation of new monomers of this kind [10,12,16]. In fact, urethane-dimethacrylates (UDMA) are identified by a wide family of monomers. Their chemical structures can easily be tailored through an appropriate choice of the core and wing segments, resulting in diversity of monomers and corresponding polymers with a wide range of chemical and physico-mechanical properties. In the present paper, several UDMA monomers, being the structural analogues of HEMA/TMDI, and their homopolymers were characterized. The monomer cores derived from six commercially available diisocyanates (DI): aliphatic – HMDI and TMDI, cycloaliphatic – IPDI and CHMDI, aromatic – TDI and MDI. The wing structures originated from oligoethylene glycols monomethacrylates (OEGMMA), which have from one to four oxyethylene units in the oligooxyethylene chains (Scheme 1) [17–19]. The monomers were tested for their viscosity and density. Corresponding homopolymers were characterized by the degree of conversion, glass temperature, polymerization shrinkage, water sorption and selected mechanical properties (flexural modulus, flexural strength and hardness).

Scheme 1 – Structures of monomers used.

The aim of this work was to show, how various homologous series of the UDMA structures may influence the monomer/polymer properties in terms of their application in dentistry as Bis-GMA, HEMA/TMDI or TEGDMA substitutes. Understanding the basic individual properties of UDMA monomers and their polymer networks can inspire new monomer designs for improving or maintaining desirable properties. New and more efficient copolymer formulations can be developed, accordingly.

2.

Materials and methods

2.1.

Materials

Urethane-dimethacrylate monomers (UDMA) were synthesized from oligoethylene glycols monomethacrylates (OEGMMA) and diisocyanates (DI) according to the procedure previously reported [17,18]. OEGMMA: DEGMMA, TEGMMA and TTEGMMA were obtained through a trans-esterification reaction of methyl methacrylate (MMA, Acros, Geel, Belgium) with the corresponding glycols: diethylene (DEG, Acros, Geel, Belgium), triethylene (TEG, Acros, Geel, Belgium) and tetraethylene (TTEG, Acros, Geel, Belgium) ones, according to the procedure described previously [17,18]. The Bis-GMA monomer was synthesized from 2,2-Bis[4-(2,3epoxypropoxy)phenyl]propane (BADGE, DER 330, The Dow Chemical Company, Midland, MI, USA, EV = 0.57 mol/100 g epoxy groups), methacrylic acid (MAc, Sigma–Aldrich, St. Louis, MO, USA) and ␣-picoline (catalyst, Fluka, Taufkirchen, Germany) according to the procedure reported in [20]. 2-Hydroxyethyl methacrylate (HEMA, Sigma–Aldrich, St. Louis, MO, USA), 1,6-hexamethylene diisocyanate (HMDI,

1338

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

Fluka, Taufkirchen, Germany), 2,2,4(2,4,4)-trimethylhexyl-1,6diisocyanate (TMDI, Sigma–Aldrich, St. Louis, MO, USA), isophorone diisocyanate (IPDI, Sigma–Aldrich, St. Louis, MO, USA), 4,4 -methylenebis(cyclohexyl isocyanate) (CHMDI, Sigma–Aldrich, St. Louis, MO, USA), 2,4-toluene diisocyanate (TDI, Sigma–Aldrich, St. Louis, MO, USA), 4,4 methylenebis(phenyl isocyanate) (MDI, Sigma–Aldrich, St. Louis, MO, USA) and triethylene glycol dimethacrylate (TEGDMA, Sigma–Aldrich, St. Louis, MO, USA) were used as received. The structure of all the monomers was confirmed in 1H NMR experiments (300 MHz spectrometer, Varian UNITY/INOVA, Palo Alto, CA, USA), performed in CDCl3 solution, using tetramethylsilane (TMS) as a reference (Sigma–Aldrich, St. Louis, MO, USA).

2.5.

2.2.

where Se is experimentally determined polymerization shrinkage, St – polymerization shrinkage extrapolated to the full conversion, MW – a monomer molecular weight. The degree of conversion (DC) was calculated according to the following formula:

Curing procedure

The monomers were mixed with: 0.4 wt.% of camphorquinone (CQ, Sigma–Aldrich, St. Louis, MO, USA)–the photosensitizer, and 1 wt.% of N,N-dimethylaminoethyl methacrylate (DMAEMA, Sigma–Aldrich, St. Louis, MO, USA) – the reducing agent, and poured into moulds. Petri dishes (120 mm in diameter and 4 mm thick) as well as PTFE O-rings placed on a glass surface (15 mm in diameter and 1 mm thick) were used for this purpose. The samples were covered with PET film in order to reduce the effects of oxygen inhibition and then irradiated for thirty minutes. Photopolymerization was initiated with a high pressure mercury vapor lamp (FAMED-1, model L-6/58, Lodz, Poland, power 375 W [21,22]), emitting UV/VIS light, where CQ absorbs in the 420-500 nm range [4]. Liquid monomers were photopolymerized with the above mentioned lamp at room temperature. Solid monomers (HEMA/HMDI, HEMA/CHMDI, HEMA/TDI and HEMA/MDI) were mixed with the initiation system, introduced into moulds and photopolymerized in the molten state as previously stated. Before irradiation, each monomer was fused at a temperature lower than the temperature of its thermal polymerization (Table 1), as exhibited in earlier DSC experiments [23].

2.3.

Viscosity

The monomer viscosity (, Pa s) was measured by means of a rotating spindle viscometer (Brookfield Fungilab Viscometer, Visco Star Plus L, Barcelona, Spain) at 25 ◦ C. Viscosity was measured using the appropriate spindle, at various spindle speeds, which allowed for recording viscosity values between 10 and 90% torque.

2.4.

Glass transition temperature

The glass transition temperature (Tg ) values were taken from our previous studies [17–19]. The rectangular samples of polymers (length × width × thickness: 50 mm × 5 mm × 2 mm) were examined by using Dynamic Mechanical Analysis (Polymer Laboratories MK II DMA apparatus, Shropshire, UK). Experiments were performed in bending mode and a frequency of 1 Hz. The Tg was taken as the temperature at the tan delta peak maximum.

Density and polymerization shrinkage

The densities of monomers (dm ) were measured utilizing a liquid pycnometer at 25 ◦ C according to ISO 1675 [24]. The polymer densities (dp ) were determined according to the Archimedes’ principle, on the Mettler Toledo XP Balance with 0.01 mg accuracy (Greifensee, Switzerland) with the density determination kit at 25 ◦ C. Water was used as the immersing liquid. The volumetric shrinkage of photopolymerized samples was determined by the following equations: Se (%) =

dp − dm × 100 dp

(1)

St (%) =

dm × 2 × 22.5 × 100 MW

(2)

Se × 100 St

DC(%) =

2.6.

(3)

Water sorption

Water sorption was measured according to ISO 4049 [25]. Disc-like specimens (diameter × thickness: 15 mm × 1 mm) of each UDMA polymer network were dried in a pre-conditioning oven at 37 ◦ C until their weight was constant. This result was recorded as m0 (Mettler Toledo XP Balance with 0.01 mg accuracy, Greifensee, Switzerland). The specimens were then immersed in distilled water and maintained at 37 ◦ C for a week. After this time, the samples were removed, blotted dry and weighed (m1 ). Water sorption (WS) was calculated using the following formula: WS (␮g/mm3 ) =

m1 − m0 V

(4)

where V is the initial volume of the sample.

2.7.

Mechanical properties

The flexural modulus (E) and the flexural strength () were determined in accordance with ISO 178 in threepoint bending tests, using a universal testing machine (INSTRON, model TT-CM, Norwood, MA, USA) [26]. Rectangular samples of UDMA polymers (length × width × thickness: 80 mm × 10 mm × 4 mm) were cut from moulds, prepared as previously mentioned. The E and the  were calculated following the relationships, respectively: E(MPa) =

P1 l3 4bd3 ı

(5)

3Pl 2bd2

(6)

and (MPa) =

1339

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

Table 1 – The properties of studied monomers: molecular weight (MW), melting temperature (Tm ), viscosity () and density (dm ). Brackets show standard deviations of five tests. The results of the Student’s t-tests are presented in the Supplementary material. Monomer HEMA/HMDI DEGMMA/HMDI TEGMMA/HMDI TTEGMMA/HMDI HEMA/TMDI DEGMMA/TMDI TEGMMA/TMDI TTEGMMA/TMDI HEMA/IPDI DEGMMA/IPDI TEGMMA/IPDI TTEGMMA/IPDI HEMA/CHMDI DEGMMA/CHMDI TEGMMA/CHMDI TTEGMMA/CHMDI HEMA/TDI DEGMMA/TDI TEGMMA/TDI TTEGMMA/TDI HEMA/MDI DEGMMA/MDI TEGMMA/MDI TTEGMMA/MDI Bis-GMA TEGDMA a b

Tm (◦ C)

MW (g/mol)

77a – – – – – – – – – – – 108a – – – 98a – – – 89a – – – – –

428.5 516.6 604.7 692.8 470.6 558.7 646.8 735.0 482.5 570.7 658.8 746.9 522.7 610.8 698.9 787.0 434.4 522.6 610.7 698.8 510.6 598.7 686.8 774.9 512.6 286.3

 (Pa s)

dm (g/cm3 )

– 14.37 (0.86) 8.71 (0.45) 6.64 (0.26) 6.22(0.42) 2.80 (0.21) 1.44 (0.09) 1.18 (0.07) 12.33 (0.83) 8.80 (0.51) 6.69 (0.27) 4.10 (0.11) – 16.66 (1.29) 12.85 (0.93) 9.68 (0.58) – 13.75 (0.84) 10.27 (0.61) 6.96 (0.39) – 38.97 (2.43) 23.59 (1.34) 8.91 (0.37) 1200b 0.011b

1.131 (0.006) 1.130 (0.007) 1.129 (0.006) 1.126 (0.005) 1.111 (0.005) 1.109 (0.005) 1.104 (0.003) 1.104 (0.004) 1.139 (0.005) 1.137 (0.006) 1.129 (0.006) 1.127 (0.004) 1.137 (0.007) 1.138 (0.007) 1.128 (0.006) 1.126 (0.007) 1.189 (0.007) 1.181 (0.006) 1.177 (0.006) 1.177 (0.005) 1.201 (0.006) 1.203 (0.005) 1.191 (0.007) 1.189 (0.006) 1.152 (0.007) 1.089 (0.001)

As cited in Ref. [23]. As cited in Ref. [4].

where P is the maximum load, P1 – the load at a selected point of the elastic region of the stress–strain plot, l – the distance between supports, b – the width of the specimen, d – the thickness of the specimen, ı – the deflection of the specimen at P1 . The ball indentation hardness (HB) was determined according to ISO 2039-1, on disc-like test specimens (diameter × thickness: 120 mm × 4 mm), using VEB Werkstoffprüfmaschinen apparatus (Leipzig, Germany) [27]. HB was calculated according to: HB(MPa) =

Fm (0.21/(h − hr + 0.21)) dhr

(7)

where Fr is the reduced test load, hr – the reduced depth of impression (hr = 0.25 mm), d – the diameter of the ball indenter (d = 5 mm), Fm – the test load on the indenter (Fm = 490 N), h – the depth of impression.

2.8.

Statistical analysis

The experimental results were analyzed using one-way analysis of variance (ANOVA). The pair-wise comparisons were conducted by means of the Student’s t-test with a significance level (P) of 0.05. For each physical property a set of five samples was tested. The results of measurements were expressed as mean values with associated standard deviations and are presented in Tables 1 and 2. The results of the Student’s t-tests are presented in Supplementary material (Tables S1–S8).

3.

Results and discussion

In this work, a family of urethane-dimethacrylate (UDMA) monomers were tested to verify their potential use as comonomers in dental restorative formulations. Their synthesis was based on an easy and reliable procedure, which has already been described in the literature [17,18]. The variability in the monomer structure and associated wide range of properties were realized through the addition reaction of four monofunctional oligoethylene glycols methacrylates (OEGMMA), having from one to four oxyethylene units in the oligooxyethylene chain, and six diisocyanates (DI): HMDI and TMDI (aliphatic), CHMDI and IPDI (cycloaliphatic) as well as TDI and MDI (aromatic) (Scheme 1). These monomers were photopolymerized to obtain 24 homopolymer networks. The camphorquinone/tertiary amine photo-initiating system was used, as the most common one used in current photoactivated dental materials [2]. Properties of monomers and corresponding polymers changed gradually along each of the homologous series. They were explained by the differences in the monomer chemical structure, the molecule size, elasticity and hydrophilicity. Monomers were tested for their viscosity and density. Polymers were examined for density, polymerization shrinkage, the degree of conversion, water sorption, glass temperature, modulus, mechanical strength and hardness. The UDMA monomer/polymer properties were compared with common dental dimethacrylates: Bis-GMA, HEMA/TMDI, TEGDMA and

1340

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

Table 2 – The properties of studied polymers: glass temperature (Tg ), degree of conversion determined from FTIR measurements (DCIR ), experimental shrinkage (Se ), theoretical shrinkage (St ), degree of conversion calculated from the shrinkage (DCS ), water sorption (WS), flexural modulus (E), flexural strength () and hardness (HB). Brackets show standard deviations of five tests. The results of the Student’s t-tests are presented in Supplementary material. Polymer HEMA/HMDI DEGMMA/HMDI TEGMMA/HMDI TTEGMMA/HMDI HEMA/TMDI DEGMMA/TMDI TEGMMA/TMDI TTEGMMA/TMDI HEMA/IPDI DEGMMA/IPDI TEGMMA/IPDI TTEGMMA/IPDI HEMA/CHMDI DEGMMA/CHMDI TEGMMA/CHMDI TTEGMMA/CHMDI HEMA/TDI DEGMMA/TDI TEGMMA/TDI TTEGMMA/TDI HEMA/MDI DEGMMA/MDI TEGMMA/MDI TTEGMMA/MDI Bis-GMA TEGDMA a b c d e f g

Tg (◦ C)

DCIR (%)

a

139 67a 33a -10a 148a 62a 40a 20a 194a 105a 83a 42a 186b 129b 80b 71b 192 c 111c 65c 33c 165c 109c 75c 46c 171d 65e

f

67 83f 93f 92f 48f 63f 90f 88f 27f 66 f 79f 81f 23f 41f 86 f 85f 23f 47f 84f 89f 31f 67f 64f 69f 39e 75.5e

Se (%)

St (%)

DCS (%)

WS (␮g/mm3 )

E (MPa)

 (MPa)

HB (MPa)

8.35 (0.49) 7.76 (0.57) 7.00 (0.49) 5.38 (0.42) 6.95 (0.42) 6.65 (0.41) 5.96 (0.26) 5.72 (0.34) 5.48 (0.41) 5.09 (0.50) 4.73 (0.51) 4.65 (0.34) 4.45 (0.59) 4.29 (0.59) 4.24 (0.51) 4.09 (0.60) 6.45 (0.55) 5.37 (0.48) 5.08 (0.49) 4.77 (0.41) 4.83 (0.48) 4.22 (0.39) 4.18 (0.56) 3.80 (0.48) 4.71 (0.58) 11.46 (0.08)

11.88 (0.07) 9.84 (0.06) 8.40 (0.05) 7.31 (0.04) 10.62 (0.05) 8.93 (0.04) 7.68 (0.02) 6.76 (0.02) 10.62 (0.05) 8.97 (0.05) 7.71 (0.04) 6.79 (0.02) 9.79 (0.06) 8.38 (0.06) 7.26 (0.04) 6.44 (0.04) 12.32 (0.08) 10.17 (0.05) 8.67 (0.05) 7.58 (0.03) 10.59 (0.06) 9.04 (0.04) 7.80 (0.05) 6.90 (0.04) 10.11 (0.06) 17.12 (0.02)

70.3 (4.5) 78.9 (6.3) 83.3 (6.3) 73.6 (6.1) 65.4 (4.2) 74.5 (5.0) 77.6 (3.5) 84.6 (5.3) 51.6 (4.1) 56.7 (5.9) 61.3 (6.9) 68.5 (5.2) 45.5 (6.3) 51.2 (7.4) 58.4 (7.3) 63.5 (9.7) 52.4 (4.8) 52.8 (4.9) 58.6 (5.9) 62.9 (5.6) 45.6 (4.7) 46.7 (4.6) 53.6 (7.5) 55.1 (7.3) 46.6 (6.0) 66.9 (0.5)

40.87 (2,68) 49.99 (3.11) 62.77 (3.97) 114.72 (6.82) 23.85 (1.55) 26.63 (1.86) 49.57 (2.43) 87.47 (4.64) 28.62 (1.69) 29.86 (2.06) 44.41 (2.89) 114.59 (4.61) 15.90 (1.06) 18.46 (1.22) 21.83 (1.26) 37.74 (2.08) 23.52 (1.73) 34.22 (1.99) 38.08 (2.01) 89.07 (4.28) 21.50 (1.62) 24.53 (1.67) 39.20 (2.68) 47.89 (3.21) 32.18 (1.94) 66.93 (3.99)

3098 (264) 2588 (126) 305 (17) 66 (3) 3488 (243) 2582 (171) 638 (33) 81 (4) 4406 (291) 3431 (189) 2047 (103) 1146 (48) 3347 (309) 2806 (179) 2169 (152) 1154 (58) 5210 (484) 3431 (267) 2252 (112) 950 (56) 4222 (325) 2122 (109) 922 (45) 148 (6) 3872 (215) 3910 (199)

138 (10) 151 (12) –g –g 135 (9) 158 (11) –g –g 85 (6) 139 (10) –g –g 104 (7) 141 (9) –g –g 104 (8) 126 (9) 135 (9) –g 58 (4) 113 (8) 127 (9) –g 110 (7) 89 (6)

133.07 (8.03) 68.71 (3.14) 57.24 (2.53) 29.32 (1.56) 162.04 (7.23) 111.15 (5.16) 73.11 (3.09) 50.00 (2.11) 217.2 (13.38) 178.18 (9.92) 122.83 (6.02) 59.58 (3.02) 219.47 (14.01) 192.16 (12.05) 148.99 (8.93) 111.16 (5.04) 164.17 (10.15) 158.71 (8.75) 133.12 (7.39) 62.36 (3.07) 151.62 (9.94) 138.32 (7.21) 106.08 (5.04) 82.73 (4.07) 73.12 (4.07) 129.32 (7.92)

As cited in Ref. [17]. As cited in Ref. [19]. As cited in Ref. [18]. As cited in Ref. [34]. As cited in Ref. [4]. As cited in Ref. [23]. Did not break in bending tests.

their homopolymers. The monomer physical properties are given in Table 1, while physical properties of corresponding polymers are given in Table 2. The monomer viscosity, as an important parameter in the polymerization kinetics of dimethacrylates and the composite applicability, was first determined. High-viscosity monomers have limited flexibility that improves mechanical properties, but negatively affects the degree of conversion in final polymer networks [1–3,28,29]. On the other hand, monomers of lower viscosity allow for higher filler loading hence improving mechanical properties of the composite [1–3,30]. Most of the studied UDMA monomers were colorless, viscous liquids. HEMA/HMDI, HEMA/CHMDI, HEMA/MDI and HEMA/TDI were crystalline solids, with melting points between 77 ◦ C and 108 ◦ C (Table 1). If they were to be utilized in dentistry as comonomers, they would have to be dissolved in liquid monomers, such as TEGDMA. For this reason their potential application is firmly limited. The viscosity of the remaining UDMA resins ranged from 1.18 to 38.97 Pa s. As shown in Table 1, the longer the OEGMMA unit, the lower viscosity recorded. From the DI perspective, viscosity was found to increase accordingly: TMDI < IPDI < HMDI < TDI < CHMDI < MDI, which is related to the decrease in molecular mobility as well as free volume.

All of the resinous UDMAs revealed viscosities statistically significantly lower than Bis-GMA (P < 0.05, Table S1 in Supplementary material). Strong hydrogen bonds between two pendant hydroxyl groups in the Bis-GMA molecule yield its extremely high viscosity of 1200 Pa s [2,4]. On the other hand, any of the characterized UDMAs could replace TEGDMA for viscosity. All differences in viscosity between particular UDMAs and TEGDMA were statistically significant. Since TEGDMA does not have groups involved in hydrogen bonding, its viscosity is very low −0.011 Pa s [3,4]. When comparing the viscosity of HEMA/TMDI (6.22 Pa s) to viscosities of studied monomers, the latter were usually more viscous. Only viscosities of HEMA/TMDI homologous (from 1.18 to 2.80 Pa s) as well as TTEGMMA/IPDI (4.10 Pa s) were lower than HEMA/TMDI. These differences in viscosity were statistically significant (P < 0.05). The TTEGMMA/HMDI and TEGMMA/IPDI monomers delivered statistically insignificant differences in viscosity to the HEMA/TMDI one, respectively P = 0.12 and 0.65. For polymers with application in dentistry, an especially important parameter to be taken into account is glass temperature (Tg ). The Tg of a dental composite matrix is only of relevance if it is higher than the intraoral temperature. If this temperature exceeds the Tg , the material may be in a rubbery state, unable to withstand the stresses exerted

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

by orthodontic mechanics and variations in the oral environment [31]. Moreover, inadequately low Tg can indicate an insufficient extent of curing [32]. The UDMA polymers tested here have been characterized for the Tg in previous works [17–19]. Their values ranged from −10 to 194 ◦ C and decreased with the lengthening of OEGMMA, as shown in Table 2. The Tg was also influenced by the DI and its values increased in the following general order: HMDI < TMDI< IPDI ≈ TDI ≈ MDI < CHMDI. When comparing only those polymers which are based on HEMA, their Tg followed a more precise trend: HMDI < TMDI < MDI < CHMDI < TDI < IPDI and ranged between 139 and 194 ◦ C. The fully aliphatic networks of HEMA/HMDI and HEMA/TMDI, as the most elastic, had the lowest Tg . The Tg of polymers, which were based on HEMA and aromatic or cycloaliphatic diisocyanates were strongly influenced by the asymmetry in the ring substitution. Polymers with asymmetrically substituted rings (IPDI and TDI) had the highest Tg . Additionally, the introduction of cycloaliphatic structures (IPDI and CHMDI) benefited Tg , by increasing it, if compared to their aromatic analogues (TDI and MDI). Generally, symmetrical aromatic diisocyanates, such as MDI, restrict molecular mobility and improve polymer stiffness [33]. The Tg of HEMA/MDI polymer lower than Tg of HEMA/IPDI polymer may result from higher packing ability of the latter one. MDI and CHMDI are more bulky than TDI and IPDI [33]. Their connection to short HEMA moieties significantly restricts the monomer flexibility. In that case, monomers with asymmetric IPDI and TDI moieties, being still rigid, however possessing higher mobility are able to pack tighter through polymerization, giving rise to the polymer Tg [33]. When comparing polymers obtained only from liquid monomers, poly(HEMA/IPDI) had the highest Tg (194 ◦ C). It was higher than Tg of poly(HEMA/TMDI) (reported form 68 ◦ C [4] to 148 ◦ C [17]), poly(Bis-GMA) (reported from 67 ◦ C [4] to 171 ◦ C [34]) and poly(TEGDMA) (65 ◦ C [4]). Poly(DEGMMA/CHMDI) had the second highest Tg (129 ◦ C). The remaining DEGMMA polymers, TEGMMA polymers (excluding poly(TEGMMA/HMDI) and poly(TEGMMA/TMDI)) as well as TTEGMMA/CHMDI had Tg within the range of 62 to 111 ◦ C. The excluded TEGMMA polymers mentioned above and the remaining TTEGMMA polymers had Tg lower than 50 ◦ C. This indicates that they would be too elastic in the oral environment and therefore, not suitable for dental composites. Another critical limitation of dental resins is the polymerization shrinkage. It occurs as a result of exchanging the van der Waals spaces within the covalent bonds, when monomers are converted into polymer networks. As a result, internal and interfacial contraction stresses are created, which leads to a poor marginal seal, marginal staining and recurrent caries [35]. In Table 2 the experimental and theoretical values of the polymerization shrinkage are presented (respectively, Se and St ). The Se was calculated from the differences between experimentally determined densities of monomers and corresponding polymers. St was calculated from the monomer density, under the assumption that the monomer polymerized up to a 100% degree of conversion. The volume change per mole of methacrylate group was taken as 22.5 cm3 /mol when monomer was polymerized [36]. Predictably, monomers with higher molecular weights had lower Se . It was the most evident along with the extension of

1341

the oligooxyethylene chains. As the DI changed, the Se values followed the order: HMDI > TMDI > TDI > IPDI > CHMDI > MDI. Monomers synthesized from cycloaliphatic or aromatic DIs shrank less (from 3.80% to 6.45%) than fully aliphatic monomers (from 5.38% to 8.35%). Additionally, dimethacrylates with symmetrical CHMDI and MDI cores exhibited the lowest determined contractions, from 3.80 to 4.83%. These values were not statistically significant, when compared to Bis-GMA, which Se = 4.71% (P = 0.78-0.06), but significantly lower (P < 0.05), when compared to HEMA/TMDI (Se = 6.95%) and to TEGDMA (Se = 11.46%) (Table S2 in Supplementary material). HEMA/IPDI and DEGMMA/CHMDI, highlighted earlier for high Tg , had the polymerization shrinkage values of, respectively 5.48% and 4.29%. The Se for the remaining resinous monomers having IPDI, CHMDI, TDI and MDI segments, which were not previously excluded due to the low polymer Tg , was determined between 4.09 and 5.37%. Though a low polymerization shrinkage has a desirable outcome for dental material performance, one must remember that it is a consequence of the polymerization reaction. The higher discrepancies between Se and St results in a lower degree of conversion in the polymer. The extent of polymerization described by the degree of conversion (DC) is very important in that it dictates many of the network physical and mechanical properties, such as glass temperature, mechanical strength, modulus and hardness [4,5,37]. In dental composites, the DC ranges between 55% and 80% [1,2]. However, HEMA/TMDI, Bis-GMA and TEGDMA reach conversions of, respectively: 69.6%, 39.0% and 75.7% when homopolymerized [4]. In Table 2, values of the DC in studied UDMA homopolymers are shown. They were determined by the ratio of Se to St . The DC mainly increased with the increasing length of oligooxyethylene chain. The influence of the DI’s chemical character on the DC was also observed. In fully aliphatic polymers, the DC varied between 65% and 85%. The DC in polymers with cycloaliphatic and aromatic structures ranged from 45% to 68%, which is slightly higher or similar to the DC in Bis-GMA but still lower than those values in HEMA/TMDI and TEGDMA polymers. The DC in TEGMMA- and TTEGMMAbased polymers were found to be the highest. However, it is worth noting, that the increasing molecular mobility with the increasing length of the UDMA molecules gave rise to improvement of the final conversion in corresponding polymers. On the other hand, the increasing length of the monomer creates the higher probability of forming cycles, which negatively affect mechanical properties of the network [2]. The DC values determined in this study follow the previously presented results for the DC obtained for the same UDMA system in FT IR experiments (Table 2) [23]. Dental polymer matrix in the oral environment may absorb water, resulting in swelling of the composite, its expansion and an increase in weight [6,7]. These effects were found to decrease modulus [7], mechanical strength [4], abrasion resistance [38] or even color stability [39]. Water sorption (WS) of a dimethacrylate polymer depends on the resin chemical structure and the network heterogeneity [6]. It is known, that the hydrophilic character of TEGDMA and Bis-GMA resins [6] as well as the modulus of elasticity of their homopolymer networks are similar [21]. It is also known, that hydroxyl groups,

1342

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

form the strongest hydrogen bonds with water (the cohesive energy density of –OH group is 2980 J/cm3 ), urethane groups form weaker hydrogen bonds (the cohesive energy density of –NHCOO– group is 1425 J/cm3 ) and ether groups form the weakest ones (the cohesive energy density of –O– group is 881 J/cm3 ) [6]. In contrast, the TEGDMA polymer has the highest water sorption (67 ␮g/mm3 ), followed by Bis-GMA polymer (32 ␮g/mm3 ) and HEMA/TMDI polymer (24 ␮g/mm3 ) (Table 2). The much higher tendency of poly(TEGDMA) to absorb water was assigned to the lack of hydrogen bonds between TEGDMA units, which creates greater space between the polymer clusters and allows for the accommodation of a larger water quantity [6]. The hydrophilicity of the studied UDMA polymers is due to the presence of the urethane and ether groups. These networks reached WS within 15.90 and 114.72 ␮g/mm3 (Table 2). With the extension of the oligooxyethylene chains, the elasticity of UDMA polymers and their hydrophilic character increased, revealing higher WS. Since some TEGMMA-based polymers and almost all TTEGMMA-based polymers were in a rubbery state, they showed disproportionately high WS. When all monomers forming networks with low Tg were ignored, the WS upper limit decreased to 49.99 ␮g/mm3 . This means that, each remaining monomer can be recommended for dental restorative application, as having WS lower than 50 ␮g/mm3 [13]. WS was also influenced by the DI and increased in the following order: CHMDI < MDI < TDI < TMDI < IPDI < HMDI. Symmetrical cycloaliphatic and aromatic moieties limit the space between macrochains and increase polymer stiffness. Consequently, CHMDI- and MDI-based networks had the lowest WS. Exemplary, poly(DEGMMA/CHMDI) had the lowest WS amongst polymers obtained from resinous monomers (18.46 ␮g/mm3 ). This WS value was also lower than the WS values of dental dimethacrylates and these differences were statistically significant (Table S5 in Supplementary Material). Asymmetry in the monomer core substitution (TDI, IPDI and TMDI) creates larger free volume, thus allowing the network to accommodate more water molecules. For example, poly(HEMA/IPDI) showed WS of 28.62 ␮g/mm3 , which was higher than WS of poly(HEMA/TMDI) but lower than WS of poly(Bis-GMA). Both differences were statistically significant (P < 0.05). HMDI-based networks as the most elastic, having easily rearrangeable chains, were characterized by the highest determined WS values. In this study, elastic modulus (E) and flexural strength () were also determined in three-point bending tests. Their values are presented in Table 2. Modulus was found to be between 66 MPa and 5210 MPa. When polymers being in the rubbery state were omitted, this range started with 922 MPa. E values decreased as the length of oligooxyethylene chains increased. From the DI perspective, they followed the general order: HMDI < TMDI < MDI < CHMDI < IPDI < TDI. This arrangement did not coincide with the monomer viscosity, which can be treated as an indicator of the strength of intermolecular interactions. HMDI and TDI monomers, forming polymers of respectively, the lowest and the highest modulus, had moderate viscosities. IPDI monomers, characterized by viscosities from the lower scale, had high modulus. This is an evident discrepancy and suggests that modulus of the investigated

polymers is strongly influenced by the monomer elasticity and resulting in the degree of conversion of double bonds. The most elastic, fully aliphatic UDMAs lead to polymers with the lowest modulus. Monomers synthesized from IPDI and TDI, which are more elastic than those from CHMDI and MDI, and thus are able to polymerize to higher DC, had the highest modulus. This result was in agreement with the DC values of investigated dimethacrylates presented in Table 2. Polymers of Bis-GMA, HEMA/TMDI and TEGDMA are characterized by similar modulus, between 3500 and 4000 MPa (Table 2). The differences between poly(HEMA/TMDI) and poly(TEGDMA) (P = 0.08) as well as between poly(Bis-GMA) and poly(TEGDMA) (P = 0.71) were not statistically significant (Table S6 in Supplementary Material). Studied here, HEMA-based resins produce polymers with E in the range of 3098–5210 MPa. Amongst them only HEMA/IPDI appeared to be promising. It is a resinous monomer and its network had a modulus of 4406 MPa. The differences in modulus between this value and the values delivered by dental dimethacrylates were statistically significant. The next polymers with the descending modulus can be ordered as such: poly(DEGMMA/IPDI) as well as poly(DEGMMA/TDI) (3431 MPa) and poly(DEGMMA/CHMDI) (2806 MPa). Most differences in E between them and dental dimethacrylates were statistically significant. Only, the comparison of poly(HEMA/TMDI) to poly(DEGMMA/IPDI) and poly(DEGMMA/TDI) resulted in statistically insignificant differences, respectively P equaled to 0.77 and 0.80. Flexural strength increased with increasing OEGMMA length (Table 2). Polymers with TEGMMA and TTEGMMA structures usually did not break under load, except TEGMMA/TDI and TEGMMA/MDI. The  values of polymers that broke in the bending mode varied from 58 to 158 MPa. From the DI perspective, the  values followed the order: MDI < TDI < IPDI < CHMDI < TMDI ≈ HMDI. Aliphatic DIs gave rise to a higher mechanical strength of the polymer than cycloaliphatic and aromatic DIs. Ignoring the polymers produced from solid UDMAs, all the remaining ones had very good mechanical strength. Its values were usually statistically significantly high compared to  of poly(Bis-GMA) (110 MPa) and poly(TEGDMA) (89 MPa). Poly(HEMA/IPDI) was characterized by  of 85 MPa, which was statistically significantly lower than  of poly(Bis-GMA) and poly(HEMA/TMDI) (P < 0.05), but similar to poly(TEGDMA) (P = 0.30). The differences in mechanical strength between poly(HEMA/TMDI) (135 MPa) and the polymers of: DEGMMA/HMDI, DEGMMA/IPDI, DEGMMA/CHMDI as well as TEGMMA/TDI were not statistically significant (P = 0.13–0.97) (Table S7 in Supplementary material). In Table 2 the relationship between Brinell hardness (HB) and monomer chemical structure is shown. Without taking into account polymers in a rubbery state, its values ranged from 68.71 to 219.97 MPa. The HB decreased as the number of oxyethylene units in the chain increased. From the DI perspective the HB followed this order: HMDI < TMDI < MDI < TDI < IPDI < CHMDI. The highest HB was recorded for polymers with both cycloaliphatic structures. Since such DIs are more elastic than aromatic DIs, monomers based on IPDI and CHMDI are capable of polymerizing to higher DC than monomers based on MDI and TDI, confirming hardness as the DC dependant property [37]. The resinous HEMA/IPDI, DEGMMA/IPDI and DEGMMA/CHMDI monomers

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

produced the hardest polymers, with HB higher than 178 MPa. Their HB values were also statistically significantly higher than those of poly(Bis-GMA) (73.12 MPa), poly(HEMA/TMDI) (162.04 MPa) and poly(TEGDMA) (129.32 MPa) (Table S8 in Supplementary material). Most of studied UDMA monomers and corresponding polymers had statistically significant differences in mean values of these particular properties: viscosity, theoretical polymerization shrinkage, water sorption, modulus and hardness. The degree of conversion, flexural strength and experimental shrinkage of polymers delivered a large number of statistically insignificant results. In general, statistical analysis showed that the increasing length of the oligooxyethylene chains in each homologous UDMA series resulted in statistically significant differences in mean values of all studied properties. It means, that the DI structure determines the significance of the property value. However, each particular property was changing with the UDMA chemical structure according to a different pattern.

4.

1343

HEMA/IPDI shrinks slightly more than Bis-GMA, but less than HEMA/TMDI. DEGMMA/CHMDI, if compared to HEMA/TMDI, had lower polymerization shrinkage, lower water sorption, higher flexural strength, higher hardness, however lower modulus. DEGMMA/IPDI and DEGMMA/TDI had lower polymerization shrinkage, higher hardness and similar flexural properties to HEMA/TMDI. Their water sorption was close to that of Bis-GMA. These selected and studied monomers can be used for further testing of their potential application in dentistry, as comonomers. The results can serve as a guide for studying the correlation between the monomer chemical constitution as well as the microstructure and properties of resulting polymers.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.dental.2014.09.008.

Conclusions references

The monomers: HEMA/HMDI, HEMA/CHMDI, HEMA/TDI and HEMA/MDI are solids and thus almost eliminated from potential application. Asymmetry in DI structures (especially in the case of TMDI and IPDI) as well as an increase in the oligooxyethylene chain length result in the reduced UDMA viscosity. However, these decreases are only within one order of magnitude, which is not high enough to produce effective reactive diluents. Polymer properties mainly change with the length of the OEGMMA. The more oxyethylene units in the chain, the lower glass temperature, modulus, hardness as well as polymerization shrinkage is measured. This also results in a higher water sorption, degree of conversion (with increasing cyclization probability) and flexural strength. The chemical character of the DI also influences poly(UDMA) properties. The highest degree of conversion and the highest flexural strength were determined for polymers produced from aliphatic DIs. The lowest polymerization shrinkage as well as the lowest water sorption were achieved when symmetrically substituted cycloaliphatic and aromatic DIs (CHMDI and MDI) were applied. More elastic, asymmetrically substituted cycloaliphatic and aromatic DIs (IPDI and TDI) lead to polymers with higher DC and consequently higher modulus, if compared to symmetrical ones (CHMDI and MDI). The presence of cycloaliphatic structures (IPDI and CHMDI) gave raise to the polymer hardness. Networks with these structures were characterized by the highest hardness. Polymers with TEGMMA and TTEGMMA appeared to be very elastic and would likely occur in a rubbery state at intraoral temperatures. They are also characterized by enormous water sorption. Based on the results of the present study it can be concluded that HEMA/IPDI, DEGMMA/IPDI, DEGMMA/CHMDI and DEGMMA/TDI seem to have the best potential. HEMA/IPDI, having a significantly lower viscosity than Bis-GMA, when polymerized exhibits a higher glass temperature, degree of conversion, flexural modulus as well as hardness and a lower water sorption, if compared to the polymer of Bis-GMA.

[1] Watts DC. Dental restorative materials. In: Cahn RW, Haasen P, Kramer EJ, editors. Materials science and technology: a comprehensive treatment. Vol. 14. Dental and medical materials. New York: VCH; 1992. p. 209–58. [2] Stansbury JW. Dimethacrylate network formation and polymer property evolution as determined by the selection of monomers and curing conditions. Dent Mater 2012;28:13–22. [3] Moszner N, Salz U. New developments of polymeric dental composites. Prog Polym Sci 2001;26:535–76. [4] Sideridou I, Tserki V, Papanastasiou G. Effect of chemical structure on degree of conversion in light-cured dimethacrylate-based dental resins. Biomaterials 2002;23:1819–29. [5] Asmussen E, Peutzfeldt A. Influence of UEDMA, BisGMA, and TEGDMA on selected mechanical properties of experimental resin composites. Dent Mater 1998;14:51–6. [6] Sideridou ID, Karabela MM, Vouvoudi ECh. Volumetric dimensional changes of dental light-cured dimethacrylate resins after sorption of water or ethanol. Dent Mater 2008;24:1131–6. [7] Sideridou ID, Karabela MM. Sorption of water, ethanol or ethanol/water solutions by light-cured dental dimethacrylate resins. Dent Mater 2011;27:1003–10. [8] Pereira SG, Osorio R, Toledano M, Nunes TG. Evaluation of two Bis-GMA analogues as potential monomer diluents to improve the mechanical properties of light-cured composite resins. Dent Mater 2005;21:823–30. [9] Podgórski M. Synthesis and characterization of acetyloxypropylene dimethacrylate as a new dental monomer. Dent Mater 2011;27:748–54. [10] Atai M, Ahmadi M, Babanzadeh S, Watts DC. Synthesis, characterization, shrinkage and curing kinetics of a new low-shrinkage urethane dimethacrylate monomer for dental applications. Dent Mater 2007;23:1030–41. [11] Podgorski M. Synthesis and characterization of novel dimethacrylates of different chain lengths as possible dental resins. Dent Mater 2010;26:e188–94. [12] Kerby RE, Knobloch LA, Schricker S, Gregg B. Synthesis and evaluation of modified urethane dimethacrylate resins with

1344

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1336–1344

reduced water sorption and solubility. Dent Mater 2009;25:302–13. Sideridou I, Achilias DS, Spyroudi C, Karabela M. Water sorption characteristics of light-cured dental resins and composites based on Bis-EMA/PCDMA. Biomaterials 2004;25:367–76. Tagtekin DA, Yanikoglu FC, Bozkurt FO, Kologlu B, Sur H. Selected characteristics of an Ormocer and a conventional hybrid resin composite. Dent Mater 2004;20:487–97. Zakir M, Al Kheraif AAA, Asif M, Wong FSL, Rehmand IU. A comparison of the mechanical properties of a modified silorane based dental composite with those of commercially available composite material. Dent Mater 2013;29:e53–9. Moszner N, Fischer U, Angermann J, Rheinberger V. A partially aromatic urethane dimethacrylate as a new substitute for Bis-GMA in restorative composites. Dent Mater 2008;24:694–9. Barszczewska-Rybarek I, Gibas M, Kurcok M. Evaluation of the network parameter in aliphatic poly(urethane dimethacrylate)s by dynamic thermal analysis. Polymer 2000;41:3129–35. Barszczewska-Rybarek I, Korytkowska A, Gibas M. Investigations on the structure of poly(dimethacrylate)s. Des Monomers Polym 2001;4(4):301–14. Cyra M, Barszczewska-Rybarek I, Gibas M, Kurcok M. Synthesis and polymerisation of bis(methacryloyloxyoligoethylenoxy)-N,N -[4,4 -methylenebis ´ 1999;140:59–62. (cyclohexyl carbamate)]s. Zesz Nauk Pol Sl Gibas M, Szapska B. Preparation of dental composite material with low water sorption. Polish J Appl Chem 1993;3–4:277–89. Barszczewska-Rybarek I. Structure-property relationships in dimethacrylate networks based on Bis-GMA, UDMA and TEGDMA. Dent Mater 2009;25:1082–9. Barszczewska-Rybarek I, Krasowska M. Fractal analysis of heterogeneous polymer networks formed by photopolymerization of dental dimethacrylates. Dent Mater 2012;28:695–702. Barszczewska-Rybarek I. Quantitative determination of degree of conversion in photocured poly(urethane-dimethacrylate)s by FTIR spectroscopy. J Appl Polym Sci 2012;123(3):1604–11. ISO 1675: 1985 Plastics – liquid resins – determination of density by the pyknometer method.

[25] ISO 4049: 2000 Dentistry – polymer-based restorative materials. [26] ISO 178: 2003 Plastics – determination of flexural properties. [27] ISO 2039-1: 2004 Plastics – determination of hardness – part 1: ball indentation method. [28] Lovell LG, Stansbury JW, Syrpes DC, Bowman CN. Effects of composition and reactivity on the reaction kinetics of dimethacrylate/dimethacrylate copolymerizations. Macromolecules 1999;32:3913–21. [29] Leprince JG, Palin WM, Hadis MA, Devaux J, Leloup G. Progress in dimethacrylate-based dental composite technology and curing efficiency. Dent Mater 2013;29:139–56. [30] Kim KH, Ong JL, Okuno O. The effect of filler loading and morphology on the mechanical properties of contemporary composites. J Prosthet Dent 2002;87(6):642–9. [31] Moore RJ, Watts JTF, Hood JAA, Burritt DJ. Intra-oral temperature variation over 24 hours. Eur J Orthod 1999;21:249–61. [32] Sostena MM, Nogueira RA, Grandini CR, Moraes JC. Glass transition and degree of conversion of a light-cured orthodontic composite. J Appl Oral Sci 2009;17(6):570–3. [33] Chattopadhyay DK, Raju KVSN. Structural engineering of polyurethane coatings for high performance applications. Prog Polym Sci 2007;32:352–418. [34] Łukaszczyk J, Janicki B, Frick A. Investigation on synthesis and properties of isosorbide based bis-GMA analogue. J Mater Sci: Mater Med 2012;23:1149–55. [35] Braga RR, Boaro LCC, Kuroe T, Azevedo CLN, Singer JM. Influence of cavity dimensions and their derivatives (volume and ‘C’ factor) on shrinkage stress development and microleakage of composite restorations. Dent Mater 2006;22:818–23. [36] Patel MP, Braden M, Davy KWM. Polymerization shrinkage of methacrylate esters. Biomaterials 1987;8:53–6. [37] Ferracane J. Correlation between hardness and degree of conversion during the setting reaction of unfilled dental restorative resins. Dent Mater 1985;1(1):11–4. [38] Gohring TN, Besek MJ, Schmidlin PR. Attritional wear and abrasive surface alterations of composite resin materials in vitro. J Dent 2002;30:119–27. [39] Shintani H, Satou N, Yukihiro A, Satou J, Yamane I, Kouzai T, et al. Water sorption, solubility and staining properties of microfilled resins polished by various methods. Dent Mater J 1985;4:54–62.