1. Introduction
Acrylic resins are the preferred materials for several prosthodontic applications such as denture bases due to their ease of fabrication, low cost, low density, acceptable aesthetics, reasonable strength and suitability for the oral environment [
1]. However, the mechanical properties of the acrylic resins still need to be improved where the removable denture bases are subjected to recurrent chewing forces in the oral cavity [
2]. A surface roughness higher than the acceptable limit and degradation of the acrylic resins may also contribute to the adhesion and accumulation of microorganisms, which can increase the occurrence of denture-associated stomatitis [
3]. Different trials have been reported to improve the strength properties of acrylic resins by integrating them with fibres, nanoparticles and nanotubes [
1,
4]. Nevertheless, researchers are still seeking biocompatible additive materials to improve the mechanical and biological properties of the acrylic resins.
The deposition of biofilms on the surface of the acrylic denture bases is enhanced by the properties of the material, especially its porosity, irregularity and absorbency.
Candida albicans yeast is the most common contributor for fungal infection affecting the oral mucosal tissues, especially in elderly patients fitted with complete dentures [
2]. Although antifungal treatments either systemic or topical have been suggested to reduce the fungal infection, many barriers may limit their use such as microbial resistance, poor oral hygiene, cost and the strict routine of local treatment application with proper denture cleaning [
5].
Incorporating different biomaterials with the acrylic resin has been reviewed in the literature, such as titanium dioxide [
6] and zeolite [
7]. Using zeolite as an antimicrobial agent with acrylic resins has shown the potential to elute agents from resins, eliminating bacterial, fungal and yeast infections [
2,
7]. Zeolite has also been shown to reduce the bacterial growth when combined with nonacrylic materials such as dental root fillings [
8] and soft liners [
9]. It is also used as a coating on titanium alloy implants and has been shown to diminish the incidence of peri-implant infections [
10].
Zeolite is a naturally occurring mineral with unique properties, including low toxicity and lack of odour or flavour; therefore, it is used safely as a dietary supplement and in various medical treatments [
2,
5]. Zeolites are hydrated microporous aluminosilicates with an open framework structure and a negatively charged surface consisting of tetrahedra made of silicon and aluminium, which are connected by sharing oxygen [
11,
12]. The negative charge in the zeolites is neutralised by metal cations, either alkali or alkaline earth such as sodium, potassium and calcium, accommodated in the pores and cavities of the zeolite. These cations are easy to exchange with others and strongly affect the structure of the zeolite and thus their properties [
13,
14]. Silver and zinc cations can also be deposited within the pores and cavities of the zeolite and, over time, the free cations can be replaced by other cations from its environment [
2,
7]. Zeolites can be Linde type A (LTA) or NaA or 4A types. A well-known synthesised zeolite having a pore size of 0.4 nm, Si/Al molar ratio of 1 and high exchange capacity to other metal cations, 4A zeolite has sodium ions in its structure, which is easily exchangeable with other cations such as potassium and calcium and can thus be used for various industrial and chemical applications [
15]. Silver ion exchange zeolites have shown excellent antibacterial activity with polymers. The mechanism of the antimicrobial effect of zeolite is explained by the ionic exchange reaction of the antimicrobial cations within which the pores of the zeolite are located. The antimicrobial behaviour of Ag-Zn zeolites has attracted researchers to mix it with the acrylic resins [
5].
Acrylic resins currently do not contain any antimicrobial properties. By adding antimicrobial agents, bacterial and fungal contamination can be reduced without damaging the strength properties of dental materials [
5]. The influence of adding different types of zeolite fillers on some mechanical properties of polymers has also been studied. Zeolite modified by 4-(dimethylamino) benzene diazonium cations was found to improve the compressive and flexural strength of the resin material [
16]. The addition of 13X zeolite into heat cure acrylic resin showed no impact on the impact strength, transverse strength, surface hardness, surface roughness or colour [
5]. In another study, silver-zinc zeolite decreased the flexural strength [
17], and tensile and bending strength of heat-polymerized acrylic resin [
18]. The application of zeolite in the medical fields has been reviewed in recent articles [
19,
20]. Moreover, the addition of silver nanoparticles to acrylic resin decreased the flexural strength of cured material, and this reduction was influenced by the concentration of nanoparticles [
21]. Nanostructured silver vanadate (β-AgV03) also improved the surface hardness and increased the compressive strength of acrylic resin [
22].
Although adding zeolite to the acrylic resin materials showed effective microbial inhibition, there is no clear evidence of its impact on the mechanical or surface properties of the acrylic resins, and it demands further investigation for dental applications. Furthermore, no studies on the surface characteristics of 4A zeolite combined with heat cure (HC) and cold cure (CC) acrylic resins were found.
It is important to investigate the effect of the addition of zeolite as an antimicrobial agent in different types of PMMA for dental prostheses which are placed under recurrent pressure inside the patient’s mouth. Other than providing the antimicrobial characteristics by zeolite, it is also important to ensure that zeolite addition is not going to affect the mechanical properties such as hardness or surface properties such as surface roughness of the PMMA resin. Unfortunately, there is limited research concerning the addition of zeolite material to the denture base resins [
2]. Thus, this study aimed to evaluate the effects of adding two different concentrations (0.50 wt.% and 0.75 wt.%) of the 4A zeolite on the surface and microstructural characteristics of the HC and CC acrylic resins. The null hypothesis was that adding 4A zeolite to the HC and CC resins will not change their surface roughness and hardness.
4. Discussion
Materials that exhibit antimicrobial features have taken an increasing interest in the field of medicine and dentistry to prevent or reduce microorganism accumulation and subsequent infection. Antimicrobial zeolites have been incorporated with dental materials such as tissue conditioners, temporary fillings, and polymethyl methacrylate [
2]. Incorporating zeolite with acrylic resin has effectively reduced bacterial and fungal infections in different studies [
2,
7].
Surface roughness plays a vital role in determining how a real object will interact with its environment. It was known to be a factor in the entrapment of microorganisms on acrylic surfaces. Higher numbers of microorganisms were observed on a rough surface than on a smooth surface. Irregularities and porosities present on the denture surface played a significant role in reducing the activity of denture-cleaning agents and hence an increased stain and plaque retention [
27]. One of the main objectives for the resin restoration is that it produces a highly smooth surface without fine scratches to prevent microorganism collection on the external surface of the final restoration [
28].
Although the incorporation of zeolite with acrylic resin materials showed effective antimicrobial behaviour, no other studies were found in the literature that investigated the effect of the zeolite on the surface roughness and hardness of both the HC and CC resin materials. Thus, to bring novelty, the present study aimed to evaluate the effect of incorporating two different percentages (0.50% and 0.75%) of 4A zeolite on the surface roughness and hardness of the acrylic resins. The zeolite was selected as a vehicle for the antimicrobial cations due to its properties, including prolonged antimicrobial activity, nontoxicity and lack of odour or flavour [
29].
The results confirmed that the surface roughness values significantly decreased when 0.50 wt.% and 0.75 wt.% 4A zeolite were incorporated in the HC specimens compared to the corresponding control groups. However, this reduction was not significant in the case of CC acrylic resin. Therefore, the hypothesis is partially accepted.
The results of the current study showed a reduction in values of the surface roughness when the 4A zeolite was incorporated in both HC and CC acrylic specimens compared to the control group. A significant difference between the control group and HC0.50 and HC0.75 groups was observed, which might be attributed to the HC acrylic resin having fewer porosities and voids which were usually generated between polymer chains during the curing process and uniform dispersion of the zeolite particles within the HC resin matrix. These porosities might be filled by even a small percentage of the Ag-Zn zeolite (0.50 wt.%) with small particle size (1888–1908 nm). Therefore, only a few particles might have appeared on the surface of the specimens resulting in a smooth surface. On the other hand, a nonsignificant difference was found between HC0.50 and HC0.75 groups. The HC0.75 group was smoother than the control but slightly rougher than the HC0.50 group. These results revealed that a slight increase in the Ag-Zn zeolite concentration to the HC acrylic resin might fill the porosities present between polymer chains until saturated, and then any excess zeolite material might gather on the external surface of the acrylic resin.
This result is comparable with that found by Alnamel and Mudhaffer when a small percentage of silicon dioxide nano-filler was added to the acrylic resin [
30], and by Abdulhamed and Mohammed when alumina was added to the acrylic resin [
31]. Both concluded that when a small percentage of filler materials was added with the acrylic resin, a certain amount of filler would be incorporated within the acrylic resin matrix; with the increase in the percentage of the filler materials, more particles would appear on the external surface of the acrylic resin, leading to an increase in the surface roughness. However, the results of this study disagree with Azeeza and Fatah, who found that incorporating 0.50% of Ag-Zn zeolite into HC resin had a nonsignificant effect on the surface roughness [
5].
The incorporation of cold cure acrylic resin with Ag-Zn zeolite at both tested concentrations was found to decrease the value of surface roughness compared to the control group, though the difference was nonsignificant between the CC control group and all the CC experimental groups. This might be related to the fact that the cold cure acrylic resin might produce many porosities and voids due to the evaporation of the monomer during the curing process, which filled with the Ag-Zn zeolite material that led to the production of surfaces smoother than those of the control group. The results also showed a nonsignificant difference between the control group and the CC0.50 group and a nonsignificant difference between the CC0.50 and CC0.75 groups. These results might be related to the fact that the addition of 0.75 wt.% Ag-Zn zeolite filled most of the porosities and voids present within the CC resin matrix, producing a smoother surface than the control group and the CC0.50 group.
In general, the values of surface roughness in the HC resin were lower than those in the CC resin with and without the Ag-Zn zeolite incorporation. This result agrees with Ogle et al.’s study, which showed that heat-polymerised resin had the smoothest surface compared to the self-polymerised and light-polymerised acrylic resins [
32].
There were not enough studies in relation to the addition of Ag-Zn zeolite to acrylic denture base materials to further compare and contrast with this study. However, the beneficial effect of adding Ag-Zn zeolite to other materials was evidenced in the literature. The result of this study is comparable with Ari et al. who found decreasing surface roughness and dimensional change of the casting after the addition of zeolite to phosphate-bonded investment due to its molecular sieve property [
33]. Furthermore, Sadeq and Hummudi found a significant improvement in shear bond strength of the HC silicon soft liner after incorporation of 0.50 wt.% and 0.75 wt.% Ag-Zn zeolite [
24]. However, in the case of CC resin, the same authors found that only 0.75 wt.% Ag-Zn zeolite can generate a surface roughness significantly lower than that of the pure CC resin [
24]. Ning et al. found that the surface roughness of zeolite-polyamide thin-film nanocomposite membranes (TFN) increased by increasing the zeolite loading as a result of the zeolite incorporation, which was characterised by a greater nodular appearance on the surface of TFN membranes [
34]. Al-Azawi and Al-Naqash found that the incorporation of 0.50% of Ag-Zn zeolite into silicon impression material did not affect the setting time [
35].
Although the results obtained in the present study demonstrated the valuable effect of Ag-Zn zeolite in reducing the surface roughness of the HC and CC acrylic resins compared to the pure resins, one limitation of this study was that the surface roughness after the regular polishing procedure was not assessed here. Further investigations could be carried out to study the influence of Ag-Zn zeolite on the surface roughness of the acrylic resins before and after the polishing procedure. Hence, the incorporation of acrylic resins with such biocompatible and inexpensive materials could be promising for improving the surface properties of acrylic resin, particularly the HC one. A further in-depth understanding about the surfaces could be gained with an SEM or an optical surface profilometer.
The incorporation of fillers in plastics may alter the resistance to abrasion, but the hardness of the plastic matrix remains unchanged [
36]. In the present study, the addition of 0.50 wt.% and 0.75 wt.% Ag-Zn zeolite significantly increased the surface hardness of heat and cold cure acrylic resin compared to the control group. The improvement in the surface hardness was dose-dependent. Zeolite consists of silicon oxide with fine particle sizes characterized by a greater surface area, providing the interstitial adhesion with the polymer chains. The random distribution of zeolite particles within polymer chains also leads to an improvement in the surface hardness. The results of this study are in agreement with Azeez and Fatah [
5] who found that the addition of 0.5% of antimicrobial sliver-zinc zeolite into heat cure acrylic increased the surface hardness. Alnamel and Mudhaffar found an increase in surface hardness of acrylic resin after the addition of silicon dioxide nano-fillers [
30].
There was a statistically high significant difference between the CCControl group and CC0.5 group, a high significant difference between the CCControl and CC0.75 groups, and a nonsignificant difference between the CC0.50 and CC0.75 groups. The possible explanation of these results could be attributed to the fine particle size of the zeolite material acting as a filler, which is uniformly distributed within the polymer matrix and fills the porosities which are usually generated due to the evaporation of monomers of cold cure acrylic resin during the polymerization process.
The results of this study are supported by the findings of Yasser and Fatah who found that the addition of zirconium nanoparticles to the acrylic-based soft liner caused an increase in shear bond strength [
37]. Sadeq and Hummudi found that the incorporation of 0.5% and 0.75% of Ag-Zn Zeolite into cold soft liner had a significant effect on the shear bond strength [
38]. On the contrary, it has been stated that currently available acrylic resin materials do not have any antimicrobial properties. With the addition of antimicrobial agents, bacterial and fungal contamination can be reduced without impairing the mechanical and surface properties of the dental materials [
39]. However, the difference between these studies with the current study was the type of resin material used, types of filler and the mechanical tests used [
25].
Overall, the HC and CC resins with the Ag-Zn zeolite infusion can provide a smoother, harder and abrasion-resistant surface along with the bacteria-killing capability, which can improve clinical performance and patient’s satisfaction.