The Effect Of CAD/CAM Waste on Mechanical Properties of Heat Cure Acrylic Resin

2


Introduction
Biomedical waste management has become a concern in the dental world as different materials start to increase in use.
In dentistry, few studies have suggested to recycle and reuse some of the dental materials (1, 2).The majority of such study population was completely clueless regarding knowledge pertaining to the recycling process.They were unaware of the proper disposal of some nondegradable dental materials (3,4,5,6).Acrylic resin polymethyl methacrylate (PMMA) has been the most popular material for the construction of dentures for many decades as it has many advantages such as stability, good aesthetics, accurate fit, in the oral environment, inexpensive equipment's, easy clinical and laboratory manipulation (7,8,9).Although it is the most commonly used in dentistry for the manufacture of the denture bases, this material is still insufficient to meet the ideal mechanical requirements for dental applications.This problem is mainly due to low plaque accumulation and fracture resistance (10,11).The zirconium oxide Nano-particles powder was determinant to improve the properties of polymethyl methacrylate, as a bio-compatible material possessing high fracture resistance, and to improve fracture toughness of ceramics by developing a new generation of ceramic-matrix composites (12,13,14).Zirconium oxide (ZrO2) is a metal oxide that possesses several advantages such as good mechanical strength, fracture toughness, hardness, wear and chemical resistance, and good thermal stability, thus making it beneficial for use in dental materials such as denture base reinforcement (15,16).ZrO2 is a polymorphic material, is available in three stable phases: monoclinic, tetragonal, and cubic.The properties of ZrO2 differ according to the phase types.The tetragonal phase is a catalytic phase, and it is a mechanically interesting phase compared with the other phases.Tetragonal phase ZrO2 can be stabilized at room temperature with divalent and trivalent cationic species such as magnesium, calcium, and yttrium (17,18).

Zirconia-Waste Fiber Collection
In the current study, a decomposed Zirconia blank was used.The Zirconia fiber was collected after using the CAD-CAM production method (Vita/zahnfabrik, Germany).The result fiber was brushedoff from the CAM machine after dry milling procedure (Roland-DWX-50, USA).Then a magnet is used during vibrating sieving procedure to remove any trace of metal particles may incorporated from the cutting bur.The standard stainless steel sieves of ISO standardization (Italy) no.40 (420μm), 60 (250μm), and 100 (150μm) respectively was used to achieve fine particle grit sizes (19,20).

Preparing the PMMA/Zirconia Composite
The composite materials for the proposed PMMA/Zirconia study were prepared according to them by following the measurements using the sensitive balance as shown in Table (1).To reach an even Zirconia distribution of the fibers inside PMMA powder, each prepared amount was distribute using a dispensing unit at 40 rpm/min for 5 h (12,000 rpm).

Study Sample Grouping
A specimens of 40 disc samples was prepared for this study, and it was divided into four major groups.The subgroup was tested for surface roughness and mechanical hardness tests, (n = 10).
Group I: 10 specimens control group without aiding Zirconia.Group II: 10 specimens with 1% wt Zirconia.Group III: 10 specimens with 3% wt Zirconia.Group IV: 10 specimens with 5% wt Zirconia.The X-ray diffraction (XRD) was used to analyses the effect of the Zirconia filler on the PMMA crystallinity behavior.The XRD technique indicates the normal order of crystalline structure in the polymeric chains.XRD pattern was obtained in the 2θ range between 0 and 90 degree (LabX6000-Shimadzu, Japan

Sample Preparation
The silicone mold for wax disc samples was prepared for surface roughness and hardness tests.The dimensions were 12 (± 0.1) mm in diameter and 2 (± 0.1) mm thickness as shown in Figure ( 1).The treated PMMA and PMMA/Zirconia compounds followed the traditional formulation method using a water bath curing system.The mixing ratio of PMMA powder/liquid is according to the manufacturer's instructions by volume (1/3) (Veracril, Spain).The short-cycle thermoplastic polymerization method was timed for 3house.After curing, the flasks were kept on abench to cool overnight, deflasked, sample flashes removed, and cleaned of the gypsum product using an ultrasonic unit for 15 min.Figure (2) shows the surface hardness that measured using Shore D durometer hardness tester unit (China) and the surface roughness which measured using the profilometer surface roughness tester device (China), the contact pin was using during tracing.The analyzing tip of the durometer traveled 5 mm at the surface center for each specimen.

Statistical Methods
Study data were analyzed by one-way ANOVA (Post-hoc test, Tukey) with a confidence level of 95% and a significant P-value of (p≤.05).

Result
The descriptive statistics of the values including the means and standard deviation (SD) values for each subgroup of surface roughness showed that control group had the higher mean value in surface roughness (1.4620) and the lowest mean value was found in group containing 3%ZrO2 (0.0466) as shown in Tables (2)  and Figure (3

Discussion
The initial mechanical properties of any dental material may predict the primary mode of clinical failure and provide a determination for a specific application.Recently, the development of new materials for the load-bearing areas were suggested to improve the mechanical properties.PMMA was one of the most commonly used denture bases with inferior mechanical properties.In the present study, the surface hardness and roughness of PMMA base resin are evaluated after the addition of 1%, 3% and 5% of Zirconia fiber.

The effect of Zirconia waste fiber on surface roughness
Surface irregularities on denture base materials may act as a reservoir of infection and increase the potential for hosting microorganisms even after dentures have been cleaned, and rough surfaces facilitate penetration of fungal and bacterial cells onto denture base resins (10).It is important to determine the materials surface roughness used in dental prostheses before they are used in the mouth.Rough surfaces can cause discoloration of the prosthesis, be a nuisance to patients and may also contribute to biofilm formation and microbial colonization.Fungal and bacterial species tend to adhere to the rougher denture base materials.Previous studies indicate a surface roughness threshold level for dental materials used in the oral cavity of Ra = 0.2 µm where no further decrease in plaque accumulation would be expected below this level (12).In the present study, result showed that control group with non -additive had the higher mean value in surface roughness (1.4620) and the lowest mean value was found in group containing 3%ZrO2 (0.0466).This may be attributed to, that the Zirconia possesses many advantages like good mechanical strength, hardness toughness, chemical and wear resistance, and good thermal stability, which makes it -  (13,15).Another reason to reduce the surface roughness when a cutting tool is used to cut a workpiece, there will be a residual cutting face on the surface, feed rate, main/auxiliary deflection angle and the radius of the tooltip arc, which will affect the size of the residual face.Adjust the feed rate and angle in the machining process can reduce the surface roughness and cutting area of the part.In addition, the lubricant and cutters selected should conform to the characteristics of the material, to reduce roughness and inhibit the formation of tool burr and scale (10).This coincide with (Kareem and Moudhaffer, 2015;Michael et al., 2019) they found that the addition of Zirconia fiber causes decreased surface roughness (14, 16).

The effect of zirconia waste on surface hardness
Hardness is an important property that acrylic materials to be used as base resins for dentures because of their resistance to occlusal forces.This property also provides resistance to abrasion and scratching, and is directly related to the arrangement of materials, chemistry, and mode of polymerization.Hardness is defined as the resistance of an indentation and is determined by measuring the permanent depth of the indentation using various test methods.Commonly used hardness test methods are Vickers, Brinell, Shore, Knoop, and Rockwell (7).
In the current study it was found that the PMMA+5%ZrO2 yielded the highest average mean values in surface hardness in all the tested group, this the increase in surface hardness of 5%ZrO2 could be related to the particle sizes, their distribution of the Zirconia fillers and the good mechanical properties of Zirconia within the matrix resin denture material (20), followed by the PMMA control group and PMMA+1%ZrO2 while the lowest average mean values were obtained for PMMA +3%ZrO2.The result in this study revealed that the additions of Zirconia waste fiber to the PMMA base resin increased the surface hardness of PMMA than that of non-additives.The annealing process for the material provided a high crystallinity behavior for CAM purposes, the increase in the Zirconia fibers filler percentages could affect positively the surface hardness ( 5).This result came in agreement with (Zidan, 2020;Chathuranga et al., 2021) they state that the material hardness may increase by the presence of polymer of high crystallinity polymer and the addition of Zirconia fiber to heat cure acrylic effects the surface hardness of the material (9, 17).

Conclusion
The 1%, 3% and 5% wt. of Zirconiafibers as a filler would be an effective percentage to decrease the surface roughness of the PMMA.5% wt. of Zirconia -fibers as a filler would be an effective percentage to increase the surface hardness of the PMMA.

Table ( 1
): The composition of PMMA-Zirconia composite for the experimental groups

Table ( 5
): One-way (ANOVA) distributions of micro-hardness of the tested groups.