Bond Strength of Reinforced Autopolymerized Acrylic Resin to Denture Base Resin

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Introduction
In the fabrication of denture bases, poly(methyl) methacrylate, known as PMMA, remains the preferred material of choice and has been used in the dental industry ever since its introduction in the 1930s, up until today. PMMA resin has gained immense popularity in the past centuries due to its ease of manipulation and processing, afordability and pleasing aesthetics, adequate strength, and dimensional stability, as well as its biocompatibility [1]. Despite its popularity, conventional PMMA is far from ideal in meeting the mechanical requirements of a dental prosthesis. Te conventional PMMA denture bases are shown to be brittle and weak, and therefore it has insufcient strength to withstand high stress of masticatory forces [2][3][4]. Tis is refected in the unresolved problem of denture fracture and the accompanying costs to repair [5].
Te majority of fractured dentures is repaired with autopolymerizing acrylic resin. Tis is a simple, quick, and low-cost method that can be executed chairside at the clinic [6,7]. Despite the fact that autopolymerized acrylic resin has lower transverse strength than standard heat-cured acrylic resin, knowing that it is frequently fractured again at the mended site, it is widely used and applied as a denture repair material [8]. Many factors infuence the success of PMMA denture repair, including the repair surface design, the surface treatment, the combination of denture base resin, and the repair material applied, as well as the use of adhesives and reinforcements [9]. Te ideal denture repair materials should have adequate strength, good dimensional stability with color match, ease of manipulation, and cost efectiveness. Researchers have made numerous attempts to modify PMMA resins in order to improve mechanical performance as well as bond strength between conventional heat-cured denture base resin and the repair material [7,10]. Adhesion between materials can be improved through mechanical and chemical surface modifcation. Chemical treatment by proper wetting and changes in surface morphology of the repair surface with monomer, acetone, methylene chloride, or chloroform makes a crucial contribution to the strength of repaired acrylic resin [11,12]. Mechanical surface treatment with abrasive air blasting showed signifcant improvement in the denture repairs [13,14].
Particulate fllers are added into the polymer matrix to modify the physical and mechanical properties of the polymers. Adhesion between fllers and matrix is critical for achieving optimal properties because it allows the load to be transferred from the weak matrix to the fllers, providing the necessary reinforcement [1,6,15]. Te bonding is typically based on the silanization of glass fbers, particle sizes and shapes, and particles' surface treatments. Te short fberreinforced resin was investigated and presented as a viable option for the use as a denture material since it is capable of enduring high stress-bearing forces [16]. A previous study showed that adding 1% short E-glass fber signifcantly improved the fexural strength of the autopolymerized acrylic resin [17]. Stipho concluded that the highest transverse strength was found when the denture acrylic resin was enhanced by adding 1% glass fber before and after repair, and that the inclusion of higher than 5% glass fber content yielded no signifcant mechanical benefts [18]. Another study by Deb et al. 2020 revealed that using glass fberreinforced autopolymerized acrylic resin as a repair material showed to have the highest fexural strength compared to unreinforced autopolymerized and light-cured acrylic resin [19]. Te E-glass fber has undergone extensive research, leading it to be widely recognized as one type of PMMA reinforcement. Nonetheless, the optimal concentration for reinforcements related to the dental feld as well as fber sizes remains undetermined. Moreover, UWPE is one of the most durable and versatile materials that have been recently introduced in dentistry as it also possesses signifcant potential applications in many areas especially as fllers in resin polymers [20]. Tis is due to UWPE's high wear-resistance, toughness, ductility, and biocompatibility [21]. UWPE is white in color, allowing it to be implemented in aesthetic applications in the dental feld [22]. However, one documented disadvantage of this material is its inertness. Gutteridge indicated that the addition of 1% UWPE fber yielded promising results for acrylic resin reinforcement, but there was no signifcant efect between surfaces treated with UWPE fber [23]. According to Alla et al., concentrations as low as 1% UWPE can signifcantly improve denture base resin impact strength [24]. Te study by Ranade et al. also stated that inclusion of UWPE improved both the composites' toughness and modulus while decreasing the fexural strength [25].
Although reinforcing fllers and particles help improves the properties of denture repair, there have been insufcient studies, and more research studies are required [26]. To date, the efect of SEG and UWPE as hybrid reinforcement in terms of shear bond strength has not been investigated. As a result, the goal of this research was to determine the infuence of SEG and UWPE fller particle additions on the shear bond strength and mode of failure of repaired autopolymerized PMMA denture base. It was hypothesized that there are no statistically signifcant diferences with respect to the shear bond strength between the repaired surface of autopolymerized acrylic resin with diferent percentages of SEG and UWPE fller particles and rapid heat-cured acrylic resin.

Reinforced Autopolymerized Acrylic Resin Preparation.
Te autopolymerized acrylic resin used in this experiment was Unifast Trad (GC Corporation). A commercial discontinuous short E-glass fber with diameters of 16 μm and 220 μm in length, known as microglass milled fbers (asreceived silanized), was obtained and used as received from Fibertec (Bridgewater). Te UWPE fller particles (Ø150 μm, PSD X50; lot no. 200410018) were obtained from IRPC Public Company Limited, performing further surface treatment with Chromic acid solution (K 2 Cr 2 0 7 : H 2 SO 4 : H 2 O) of (7 : 150 : 12 wt%) following Li et al.'s method [27,28] before being used as a reinforcing fller in this study. Te reinforced autopolymerized acrylic resin was obtained with a combination of resin polymer powder, along with preweighed SEG and UWPE fller particles in the proportions specifed by weight (Table 1). A magnetic stirrer machine set to 450 rpm for a duration of 30 minutes was used to reach an even and consistent dissemination of reinforcement particles that are spread throughout the resin polymer powder mixture.

Specimen Preparation.
Tis experiment was conducted following the Standard Test Method for Shear Strength of Adhesive Bonds Between Rigid Substrates by the Block-Shear Method (ASTM D4501-01(2014)).
A circular stainless-steel mold of diameter 15 ± 0.2 mm and thickness of 3.3 ± 0.2 mm was used to fabricate the specimens with rapid heat-cured resin (Vertex-Dental B.V) according to manufacturer's recommendations (curing for 20 minutes at 100°C). Te specimens were standardized with digital caliper and fxed into PVC mold with autopolymerized acrylic resin (Kerr Corporation). All specimens were polished with an automatic polishing machine (Future-Tech Corp.) with abrasive silicon carbide paper 600 grit (Waterproof abrasive paper DCC; TOA Paint Co., Tailand). Specimens were immersed in an ultrasonic machine (Crest Ultrasonics Corp.) for 1 minute to remove contaminated particles and then were randomly divided into seven groups (n � 8). After that, they were adhered to autopolymerized acrylic resin integrated with various percentages by weight of SEG and UWPE fller particles, according to each test group.

Bonding Procedures.
Te boundaries of the area of repaired surface were set by using a masking tape with a center hole of diameter 5 mm. Te adhered surface was treated by application of MMA monomer for 180 seconds. Ten, silicone mold with a center diameter hole of 5 ± 0.2 mm and 2 ± 0.2 mm height was placed over the PVC mold, ensuring that the center hole of the silicone mold and the masking tape is aligned together (Figure 1). Te reinforced autopolymerized acrylic resin powder was simply mixed with liquid monomer and poured into the silicone mold, achieving minimal excess to account for polymerized shrinkage. Te polyethylene flm and weight pendulum of 1 kg were then placed over the silicon mold. All specimens were immersed in water at 50°C for 5 minutes to achieve a full polymerization reaction. Specimens were then stored in distilled water in an incubator at 37 ± 1°C for 50 ± 2 hours. Tis was then followed by placing all specimens in thermocycling machine (Medical and Environmental Equipment Research Laboratory) at 5°C and 55°C for 5,000 cycles prior to the test.

Shear Bond Strength Test. Te shear bond strength tests
were conducted on all ffty-six prepared specimens under UTM (Shimadzu). Te axis of the specimen was placed in a position where the knife-edge shearing blade was in contact with the junction of rapid heat-cured acrylic resin and reinforced autopolymerized acrylic resin interface, securing a parallel location. Shear force was applied at a crosshead speed of 1.26 mm/min with a 50 N load cell. Te needed forces (N) for the separation of the resin interface were noted.
Te shear bond test ( Figure 2) was used to determine the adhesives' bond strength in a given direction or under a type of stress. Te aim of the shear bond test between the conventional heat-cured acrylic resin and the repair self-cured acrylic resin was to determine the bonding ability between two types of acrylic resins together while under stress, simulating denture repair procedures performed at the clinic. Te resulting force was calculated using the following formula: where S is the shear bond strength (MPa), T is the tension applied (N), and A is the bonded area (mm 2 ).

Mode of Failure.
All of the test specimens were visually analyzed using a stereo microscope at 20× magnifcation for examining mode of failure. Failure was defned according to three types: (1) cohesive failure, (2) adhesive failure, or (3) mixed failure. Te percentage of retained reinforced autopolymerized acrylic resin or dislodged rapid heat-cured acrylic resin on the repaired surface determined the type of failure. Te failure modes were defned as adhesive when appeared up to 25% on the repaired surface, as cohesive where fracture of reinforced autopolymerized acrylic resin or rapid heat-cured acrylic resin exceeding 75%, and lastly as mixed when cohesive and adhesive failures were between 25 and 75%.

Scanning Electron Microscope (SEM) Analysis.
Te repaired surface topography was visualized using a scanning electron microscopy (Oxford X-Max 50). SEM at ×15 and ×1000 examines the mode of failure (adhesive, cohesive, or mixed).  Table 2 summarizes the means, standard deviations, and statistical signifcances of the shear bond strength (MPa) of the tested groups. Te ANOVA results indicated statistically  As shown in Figures 3 and 4, all of the shear bond strength tested specimens were visually analyzed using a stereo microscope at 20× magnifcation and SEM at ×15 and ×1000 to examine the mode of failure. Te images revealed that the majority of mode of failure was a mixture of cohesive and adhesive failures. Te control group, group 6 (1% UMPE), and group 7 (2%UM PE) were noted to have 25% of adhesive failure between interface of reinforced autopolymerized and rapid heat-cured acrylic resin. Another 75% of tested specimens were noted to be of cohesive failure of PMMA within the acrylic resin itself. Tese cohesive failures were seen to be of reinforced autopolymerized acrylic resin. On the other hand, 100% mixed failures occurred for group 2 (2% SEG), group 3 (1% SEG), group 4 (0.5% SEG/UMPE), and group 5 (1% SEG/UMPE). In addition, groups 2 and 3 cohesive failures were noted to be of rapid heat-cured crylic resin while groups 4 and 5 cohesive failures were noted to be of both autopolymerized and rapid heat-cured acrylic resin, as summarized in Figure 5.

Discussion
Tis research was performed to assess the impact of adding SEG and UWPE fllers on the shear bond strength of the repaired autopolymerized acrylic denture base in improving the strength of the repaired denture. Two commonly employed techniques for evaluating the bond strength of dental materials are the shear bond strength (SBS) and the fexural bond strength (FBS) [29,30]. Shear bond strength is not infuenced by the strength of the adhesive itself and provides a straightforward assessment of the adhesive's bond strength, owing to its simple preparation of the specimen and relatively easy testing protocol, as well as the low occurrence of the pretest failure [30]. Considering the previously mentioned advantages of this type of test along with the fact that reinforced acrylic resin has been tested with regards to fexural strength in earlier studies [17], evaluating the strength and durability of the acrylic denture base when shear force is applied would be of high value, in order to further explore and validate the previous outcomes. Out of these points, this study was conducted, revealing that the addition of SEG and UWPE signifcantly afects the shear bond strength between reinforced autopolymerized and rapid heat-cured acrylic resin. Tus, the null hypothesis was rejected.
Te results of the current investigation showed that the SEG and UWPE fllers signifcantly improved the shear bond strength between reinforced repaired acrylic resin and rapidsimplifed heat-cured acrylic resin in comparison to the control group. Moreover, the fller concentration had a direct infuence on increased shear bond strength for UWPE but inversely proportion for SEG. What is noted from the present experiment is also similar to Stipho's study, where the strength and defection of repaired acrylic resin joints, enhanced with various several fber concentrations, were evaluated. Te authors revealed that after repair, 1% glass fberreinforced autopolymerized acrylic resin could recover 65% of the strength of intact fracture load [18]. Te fexural and adhesive properties of conventional resin along with the joint of repair resin directly infuence the defection and strength of the repair units. It takes less energy to break the repair units as   International Journal of Biomaterials they become more rigid. Specimens with 1% glass fber reinforcement had a larger mean defection at failure than those that were not reinforced. Te increased fller loads may result in a more surface area between the fller and the resin matrix. Te decreased in strength could be from additional sites where failure may occur. However, because of the morphology of the UWPE particles being quite irregular, the mechanical interlocking between the resin and the particles could play an additional role in enhancing the mechanical characteristics, which resulted in signifcant superior shear bond strength over the control group. Furthermore, groups with hybrid reinforcement of SEG and UWPE fllers to  PMMA did not add up the properties but rather signifcantly lowered the strength of the PMMA resin compared to the group reinforced with SEG alone.
Te current research fndings also demonstrated signifcant increase of the shear bond strength for the inclusion of 1% SEG compared to all tested groups while the incorporation of 2% SEG lowered the shear bond strength. Krause et al. suggested that due to the rod shape of glass fbers, greater energy levels are required to dislodge the particles from the matrix [31]. Te outcomes of this experiment could be explained by the fact that the low concentration of fllers is due to the homogeneous dissemination of the particles and their capability to occupy the interpolymeric chain spaces, whereas high concentration can result in agglomeration, which creates spaces [32]. Te spaces could provide an explanation for the material's decreased strength and nonhomogeneous mixing within the resin. Tese hollow spaces impair the stress distribution, resulting in structural weak points that eventually weaken the material [33]. As reported by Gad et al., glass fbers' reinforcement could potentially improve the mechanical properties when utilized in low percentages. Alhotan et al. also stated the importance of homogeneous fller distribution and good adhesion between fber and matrix within the resin matrix, as this has a major efect in stress transfer between the matrix and the fbers. Te transverse strength of the material will certainly be infuenced by strong adhesion. He concluded that to provide a desirable reinforcement for PMMA denture base resins, a fller of E-glass fber with a concentration of 3-7% by weight is recommended [34]. Matinlinna et al. reported that the use of silane coupling agents helps promote the adhesion between dental restorative materials [35]. Te UWPE fller used in this study has been surface treated with potassium dichromate and short E-glass fber used was presilanized by the manufacturer. Te use of a silane coupling agent improved the chemical bond between the fller and the PMMA matrix, thus requiring more energy to disintegrate the bonds that formed. Te presilanized E-glass has the potential to form chemical bonds between the fber and matrix. Superior repaired strength can be seen in this study with the addition of 1% SEG, considering that an optimum level of fller to the matrix is reached. Abushowmi et al. also claimed that incorporation of nanofllers (nano-ZrO 2 and nano-SiO 2 ) is advantageous over microfller (glass fber), owing to its even distribution, strong bonding with resin matrix, and ability to fll gaps between polymeric chains [36]. Terefore, the incorporation of nanofllers proved to be an efective method for increasing repair bond strength and avoiding repeated denture fractures.
Kumar et al. tested diferent acrylic resins for denture repair. Te authors demonstrated the heat-cured denture base repaired with autopolymerizing repair resin obtained higher mean shear bond strength, compared to visible lightcured resin [13]. Te interface between the two resins is typically the weakest point of repaired dentures. Several attempts were made to overcome this problem of increasing the bond strength by performing surface modifcation using chemical and mechanical treatments. Mechanical surface treatments prior to denture repair were recommended by many studies [37,38], as these treatments could promote higher debonding force at the interface of two PMMA materials. In this study, abrasive paper 600 grit was implemented for all specimens' bonding interface with the same speed and time, under automatic polishing machine. Artifcial aging via thermocycling was also simulated in this denture repair study. Tis helped to determine the longevity of acrylic resin mimic oral cavity environment. Overtime, penetration of water molecules can cause the softening of the denture base and signifcantly infuence the mechanical properties of the repaired acrylic resin [37]. From the current study, the addition of UWPE and SEG as hybrid reinforcement did not show to have synergy benefts in improving shear bond strength of acrylic resin. It is noteworthy that previous investigations also demonstrated the benefts of chemical treatment of the denture base prior to repairing with autopolymerizing acrylic resin, indicating higher shear strength when this was done, and diferent chemicals suitable for diferent acrylic resins [11]. Terefore, future studies using various chemical treatments, accompanied by the addition of 1% SEG to autopolymerized PMMA as a repair material, which is concluded from this study, would be of great interest in order to reveal whether this combination would give optimal outcomes in denture base repair.
Bonding performance has been evaluated using the mode of failure. Adhesive failures have always been viewed as the least acceptable, followed by mixed failures and cohesive failures [39]. Prpić et al. evaluated the shear bond strength of 10 groups, consisting of combinations of several types of denture teeth with cold-/heat-polymerized, as well as CAD/CAM denture base resin. Te authors also concluded that higher cohesive and mixed failure rates were present with higher shear bond strength values, which is in accordance with this study [40]. Te mode of failure for the majority of specimens was a mixed failure. Group 2 (1% SEG) and group 3 (2% SEG) 100% failure were mixed failure with cohesive breakdown of rapid heat-cured acrylic resin. Tis indicated that the reinforced autopolymerized acrylic resin has superior mechanical properties over that of rapid heat-cured acrylic resin. Te addition of E−glass fber to autopolymerized acrylic resin enhances its strength by serving as a reinforcement. Tese fbers efectively distribute stress across the material, preventing the occurrence of cracks or fractures [17]. Previous reports demonstrated remarkable benefts from adding glass for the reinforcement of PMMA [1]. One essential advantage is that glass fbers ofer excellent mechanical enhancement and aesthetics, compared to other types of fbers [41]. It has been shown that adding glass elevates the strength and the toughness, Vickers hardness, as well as the fexural strength of the denture base [42][43][44][45]. Moreno-Maldonado et al. also concluded that the deformation was also reduced signifcantly (<1%) in fber-reinforced PMMA [42]. Consequently, the autopolymerized PMMA resin will continue to be among the frst options for repairing denture prostheses. Clinically, the simple mixing procedure of E-glass fber to autopolymerized PMMA resin to repair fractured dentures International Journal of Biomaterials may be an efective means and efcient solution, considering the absence of any additional lab work, especially for patients who are waiting for new dentures to be made and with longer denture service life.
Some limitations of this study include its in vitro nature, which did not fully replicate clinical conditions. Furthermore, the more percentages of reinforced materials, the more color change of acrylic denture base. Although different proportions of reinforced materials were tested, surface treatment was standardized and the same method was implemented for all groups as mentioned earlier, i.e., other surface treatments were not examined. Tis could have some infuence on the results, based on the chemical treatment used. Consequently, future research is necessary to evaluate the repair bond strength of reinforced acrylic resin under more realistic clinical conditions and to determine the most suitable surface treatment.

Conclusion
Within the limitations of this research, it can be concluded that (1) In comparison to the control group, the addition of SEG and UWPE fller to autopolymerized PMMA denture base signifcantly improved shear bond strength (2) Te addition of 1% SEG to autopolymerized PMMA denture base signifcantly improved the shear bond strength with rapid heat-cured acrylic resin, and this ratio is recommended as the reinforcement for chairside repair denture base material

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that there are no conficts of interest.

Authors' Contributions
NA conceptualized the study; NA, CA, and PN proposed the methodology; NA and NW validated the data; NA and CA performed formal analysis; NA, BM, and PN performed data curation; NA, CA, and PN prepared the original draft; NA, CA, and BM reviewed and edited the manuscript; and NA, SW, and NW supervised the study.