Evaluation of Surface hardness of Denture Base Acrylic Resin Modified with Different Techniques

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INTRODUCTION
Several studies were conducted on using metal oxides to enhance denture base resin.Some have found a significant improvement in the properties of acrylic (1- 15) , other didn't find the desired result (16) .
The important factor that plays a major role in the successful enhancement of the properties of the particulate filled polymer compounds is the strong adhesion of the filler to the interface of the polymer matrix (17, 18)   .Inorganic reinforcing fillers typically display high surface energy due to the ionic hydrophilic nature.However, the waterproof polymer does not wet out or react with the filler due to the difference in surface energies.Therefore, different ways had been demonstrated to recover surface wetting, compatibility and holding between inorganic filler particles and organic matrix materials.Among these methods, silanation is most commonly used for surface modification to get particulate filled acrylic composite having improved properties over pure acrylic (11,(19)(20)(21)(22)(23)(24)(25) .Other researchers achieved a good adhesion of fillers with acrylic resin matrix by modifying the fillers' surfaces with PMMA.They proved that such modification can greatly improve resin's transverse mechanical properties (26)   .Treatment of fillers with a variety of bonding agents or primers preceding to admixing with the polymer in order to create a strong interphase between structural medium and the resin matrix reported an improvement of the acrylic resin properties for provisional fixed restorations (9,19,27) .
ZnO can be considered as a promising metal oxide which can be used to alter the biomaterial for denture bases (28)(29)(30)(31)(32)(33)(34)(35) .It is used as a radio-opaque material to make acrylic resin visibility in x-rays better (36) .It is also utilized as a pigment to develop aesthetics of denture acrylic. (37,38) eover, ZnO was found to increase acrylic hardness (39) and participate in increasing acrylic thermal conductivity and strength and decreasing its roughness (15) .
Hardness is defined as the material's resistance to a permanent indentation (40) .
Therefore, in the present study, the efficiency of the above-mentioned ways of acrylic resin modification was evaluated in term of preservation or improvement of surface hardness..A detailed description of the used materials, treatment of the additives and preperation of the polymerized samples were explained in a previous article (53) .

MATERIALS AND METHODS
After polymerization, The specimens were stored in deionized distilled water at (37

RESULTS
The descriptive statistics of durometer Shore D hardness were represented in  In the preliminary study in our research series (53) , five different ways of to the liquid and mixed, the monomer dissolves and diffuses to the PMMA (59) .
In the current study, the liquid monomer also dissolves the added acrylic acid particles (AA), so it is probable that addition of AA in different percentages may impact the acrylic properties differently.
The result showed the hardness of acrylic modified by 5% AA was greater than that of 10% AA group.This result displayed that a specific amount of AA (5%) could change the polymer matrix saturation for acrylic resin and when the amount of added AA was increased to 10%, a noticeable decrease in hardness occurred.
This decrease may be clarified by the maximum saturation of the polymer matrix, which occurred by adding the 5% AA, and the excess monomer disturbed the crosslinked polymer structure.
Although ZnO is a relatively soft material with hardness of 4.5 on the Mohs scale These values increased depending on the raising of the filler concentration in the cured material from 5% to 10% by weight to become significant in 10% concentration.In inorganic-organic hybrid materials, the organic constituent usually accounts for flexibility of the composites whereas the inorganic constituent is responsible for hardness and mechanical impact resistance (20)(21)(22) and the hardness of heat cured acrylic resin increases with increasing the concentration of incorporated inorganic particles ( 6 1 , 6 2 ) .
The size of filler particles in the polymer matrix has a great influence on the mechanical properties of particulate filled polymer composites (18) .The size of metal oxide particles should be small for proper handling (63) lesser prosity and reduced water sorption (36) .In this study the pretreated ZnO powder has micro/ nano sized particles ranging from (0.1090 -1.0550μm) as determined by LASER Diffraction (53) .This metal oxide particle size was greatly smaller than that of powder resin particles (121.2μm) as mentioned by the manufacturer; therefore, they will fill the interstitials between polymer particles to give a heterogeneous mixture and will not force or displace the segments of polymer chain.In addition, since these particles are of micro/ nano sized, they have large surface area per unit volume leading to better interactions between the particles and polymeric matrix thus exhibiting remarkable mechanical properties (63) .The tiny size, large surface area and quantum effect as well as strong interfacial interaction between the organic polymer and inorganic particles all these factors contributed to improve the mechanical properties of the polymers (64) .Asar et al. (9) stated that using metal oxide with different particle sizes get the advantage of providing additional advance of some mechanical and physical properties of denture base acrylic by inhibiting the spaces among oxide particles and leading to have further particles per unit volume of polymer.This high polymer density leads to increased hardness even at low filler concentration (65) .
Another important consideration playing a major role on the mechanical properties of particulate filled polymer composites is excellent adhesion between reinforcements and polymer matrix (17,18) .Kamonkhantikul., et al (31)  Polymeric materials contain crosslinking agents result in a more rigid polymer structure (66,67) .Exchanging the van der Waal's forces between polymer chains by stronger carbon-carbon (C-C) primary bonds reduces the mobility of polymer segments by holding the chains more tightly together (68) thus increasing polymer hardness.Crosslinking restricts the ability of individual polymer chains to slide pass each other and provides insolubility, rigidity, and stiffness to the polymer (65) .
Therefore, the null hypothesis assumed that these modifying procedures do not diminish the surface hardness of the heat cured acrylic resin was accepted.Further research is required to investigate the possibility of clinical use of these promising acrylic resins as biomaterials.

CONCLUSIONS
This study is a continuance and extension

Declaration of interest
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.Effect of reinforcement of high-impact acrylic resin with zirconia on some physical and mechanical properties.

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°C for (48 ± 2) hours f o r h y d r a t i o n a n d r e s i d u a l m o n o m e r r e l e a s e before measuring (54) .Surface hardness was assessed using Durometer hardness tester (Shore D, Shaw, Model: LD -YJ, China) for hard materials.It was fabricated according to American National Standard / American Dental Association (ANSI /ADA, 2002) (40) .Five measurements were recorded on different places of each s i n g l e specimen and the mean value was calculated.Shore durometer is a dimensionless quantity because it measures the relative movement of the indenter.IBM © SPSS © Version 19 was utelized to analyze the c o l l e c t e d d a t a .
modification were tried; (a) The copolymerization of acrylic resin with acrylic acid, (b) The addition of thermally activated ZnO, (c) The chemical bonding or engagement of Zn ions into the polymer chain by an organic link (ZDA) to get a copolymer, (d) The addition of ordinary inactivated ZnO and (e) The addition of ordinary ZnO with AA.Fourier Transform Infrared Spectroscopy (FT-IR) was used to estimate degree of conversion, to characterize the materials under investigation and to compare between them.The study concluded that the addition of an ordinary ZnO or a thermally activated ZnO to acrylic resin appeared to be just as filling inserted within the inter-chain spaces without chemical bonding with the resin matrix.It also proved that synthesis and copolymerization of ZDA with MMA was the successful way to get a chemical engagement of Zn ions with the resin matrix.The former study was followed by another one (55) customized to estimate the amounts of residual methyl methacrylate (MMA) in heat cured acrylic resin after its modification in the same mentioned ways.The study revealed that incorporation of 5% and 10% weight fraction from each of ZnO and ZDA into the acrylic resin amounts of residual MMA (p ≤ 0.05), while modifying it by addition of AA had no effect on residual MMA amounts.Nevertheless, the residual MMA of all samples were lesser than ADA standardization Specification No. 12 (40) .To the best of the author's knowledge, no study has been carried out to assess hardness property of heat cured acrylic resins modified by the same techniques mentioned in this study.Therefore no comparison with other studies' results was performed.Moreover, since there is no interrnational specification for the hardness of denture base acrylic resin, the hardness values of the modified groups were compared with that of control one in the present study.The results of the current study showed a general increase of the s u r f a c e h a r d n e s s of the experimental groups in comparison with the control group with a significant increase in both acrylic groups modified by 10% ZnO and 10% ZDA in comparison with the unmodified group and the remaining modified samples.The hardness mean values of both acrylic resin groups modified by AA were greater than that of control.This is consistent with Al-Fahdawi (2009cured acrylic resin with poly vinyl chloride (PVC) and stated that the produced copolymer was slightly harder than the unmodified material.Ayaz and Durkan (58) found enhancement of the mechanical characteristics of PMMA after its successful copolymerization with acrylamide monomer.The mechanical properties of acrylic resin are known to depend on ratios of powder/ liquid, the interface between the powder and the matrix and the crosslinking density of the polymer matrix.When the powder added e a d d i t i o n o f t h e p r e p a r e d Z n O i n 5 % a n d 1 0 % t o P M M A i n c r e a s e d hardness values in comarison with an unmodified group.
investigated the effect of adding the similar weight of silanized and nonsilanized ZnO nanoparticles on the antifungal, mechanical and optical characteristics of PMMA.They proved that silanized groups had improved properties.In the present study, the surface of the inorganic filler was modified with organic compound in order to achieve chemical blending or unification of the inorganic filler with the organic matrix of the acrylic resin to enhance denture base properties.This strategy successfully improved surface hardness of the tested heat cured acrylic material.It is clear that acrylic resin samples modified with ZDA have greater hardness than unmodified one.In addition the material became significantly harder as the ZDA percent increased f r o m 5 % to 10% in the modified acrylic.Another reason of the increase of hardness property of the modified acrylic samples with ZDA was thought to be the crosslinking behavior of ZDA.Azevedo et al. (46) found that both Lucitone 550 and Duraliner II materials result in more rigid materials than other tested denture polymers due to their high constituent of cross-linking agents.
of a previously published work where the efficiency of new different modifying ways of denture base acrylic resin was demonstrated.The present study emphasized the improvement of surface hardness of heat cured acrylic resin modified by addition of ZnO in the stated techniques.While taking into consideration all other influences and variables, the good adhesion of fillers with acrylic matrix may have a significant impact in the ability to reach the desired enhancement of acrylic properties.The results of the current study showed a general increase of the s u r f a c e h a r d n e s s of the experimental groups in comparison with the control group.Hardness was significantly enhanced by either adding 10% by weight of thermally activated ZnO or by copolymerizing it with 10% by weight of ZDA to get poly (methyl methacrylate -co-zinc acrylate) copolymer.

Table (
2).This includes means, standard deviations, standard error of deviations, maximum and minimum values of hardness.The highest two Table (2): Descriptive statistics of surface hardness test.