Effect of simulated gastric acid on the mechanical properties of conventional and polypropylene mesh reinforced poly methyl methacrylate denture base resin

Abstract

Aim:

Sustainability of dental polymethyl methacrylate (PMMA) in acidic environments is crucial as oral pH fluctuations can degrade the mechanical properties of denture bases. PMMA strengthening with polypropylene (PP) mesh has improved mechanical properties, but its effectiveness in acidic conditions remains underexplored. This investigation aimed to evaluate the mechanical properties of PP mesh reinforced PMMA (PP-PMMA) on exposure to an acidic environment as against unreinforced denture base resin (UN-PMMA).

Settings and Design:

Experimental In-vitro study.

Materials and Methods:

A total of 120 PMMA samples were prepared as per standardized guidelines and divided into two groups as unreinforced and PP – PMMA. Prepared specimens were immersed in artificial saliva and simulated gastric acid (0.06M HCl, pH 1.2) at 37°C for 96 hours to simulate long-term clinical exposure to an acidic environment. Flexural strength, impact strength, surface roughness and microscopic characteristics were assessed with aid of universal testing machine, Izod’s impact tester, profilometer and high-resolution scanning electron microscope (HR-SEM) in order.

Statistical Analysis Used:

The data were statistically analyzed using ANOVA and independent t-test set at a significance level of P-values less than 0.05.

Results:

PP-PMMA exhibited significantly higher flexural strength in simulated gastric acid (70.03±1.4MPa) compared to UN-PMMA (68.77±2.2MPa) (P < 0.05). In simulated gastric acid (HCl), PP-PMMA showed no significant difference in surface roughness compared to UN-PMMA (P = 0.29). However, in artificial saliva, PP-PMMA exhibited lower surface roughness (0.47±0.14μm) than UN-PMMA (0.56±0.08μm) (P < 0.05), indicating improved smoothness. Impact strength remained comparable between groups in both immersion conditions (P > 0.05). The presence of mesh between the layers restricted the ingress of saliva and HCl as was perceived with rougher surface in the external area.

Conclusion:

PP reinforcement improved the flexural strength of PMMA in acidic environments and maintained surface roughness in acidic environments but did not significantly enhance impact strength. Polypropylene mesh reinforcement effectively reduced porosities and maintained structural integrity within the PMMA matrix. These findings suggested that polypropylene mesh reinforcement enhances the durability of denture base materials and benefits patients with gastroesophageal reflux condition.

In prosthetic dentistry, the superior characteristics of PMMA have made it a widely preferred material for denture base fabrication. Despite its many benefits, PMMA has certain limitations. Its mechanical and surface properties are prone to changes when exposed to the oral environment, which presents various chemical and physical challenges.[1] The oral cavity is a dynamic environment influenced by factors such as saliva, gingival crevicular fluid, temperature variations from hot and cold beverages, food consumption, and exposure to acidic substances. In addition, intrinsic acidic conditions can predispose dental materials to degradation. A significant challenge to PMMA denture bases is their exposure to gastric acid, particularly in individuals suffering from gastroesophageal reflux disease (GERD).[2] GERD is a prevalent chronic gastrointestinal disorder characterized by the regurgitation of acidic stomach contents into the esophagus and oral cavity, leading to symptoms such as heartburn, regurgitation, chronic cough, laryngitis, and dental erosions. The acidic pH of gastric contents, ranging between 1.0 and 1.5, can significantly impact dental materials, causing surface roughness, reduced mechanical strength, and increased microbial adhesion.[3]

The prevalence of GERD is increasing globally, including in India, where an epidemiological study reported a 15.6% prevalence, comparable to that in western countries.[4] The condition affects oral health by altering salivary pH, influencing dental material degradation, and contributing to biofilm formation. Several factors, including dietary habits, smoking, chemotherapy, and the consumption of acidic beverages, further exacerbate these effects. The primary etiology of GERD is transient lower esophageal sphincter relaxations, which allow acidic contents to reflux into the oral cavity. Other contributing factors include hiatal hernias, reduced lower esophageal sphincter pressure, impaired esophageal clearance, and delayed gastric emptying.[3,4]

Given the impact of acidic environments on denture materials, researchers have sought to enhance the properties of PMMA through reinforcements such as nanoparticles, fibers, and mesh structures.[5] Although several modifications have been attempted, each has presented challenges. For instance, nanoparticles improve mechanical properties but may compromise esthetics. Carbon and aramid fibers enhance strength but alter the polymer’s original color.[6] Metal wires and mesh reinforcements contribute to strength but result in poor esthetics and do not prevent fracture failure. Glass fiber reinforcements such as orthopedic casting tape have been explored but suffer from thickness mismatches with the PMMA matrix.[6]

To address these limitations, PP mesh emerged as a promising reinforcement material.[7] PP is a biocompatible, thermoplastic polymer widely used in medical applications, including surgical procedures for hernia repair, due to its inertness, elasticity, tensile strength, and resistance to acidic and alkaline environments.[8] With a low density of 0.91 g/cm³, PP mesh offers a lightweight reinforcement option while maintaining durability.[9]

This study aimed to evaluate the effects of PP mesh reinforcement in heat-activated PMMA denture base resin, particularly its mechanical properties after exposure to simulated gastric acid. By incorporating a sheet of PP mesh between layers of PMMA, the study sought to determine whether this reinforcement enhanced the material’s resistance to bending and impact forces in an acidic environment. Understanding these modifications is crucial for improving the longevity and durability of dentures in patients affected by GERD or other conditions that expose dental prostheses to acidic environments. The null hypothesis proposed in this study stated that no significant difference in mechanical properties would exist between PP-reinforced and conventional PMMA denture resins after immersion in simulated gastric acid.

MATERIALS AND METHODS

This study was commenced after consent granted by the Institutional Review Board (SRMDC/IRB/2022/MDS/No. 204). The total sample size was estimated to be 120, with 10 samples per group with an effect size of 0.6 (α = 0.05) and 80% power of the study. Sixty samples were assigned to each group, one with unreinforced PMMA and the other with PP mesh-reinforced PMMA, with 40 specimens designated for each mechanical test. The specimens were immersed in both simulated gastric acid and artificial saliva (Nanochemazone, Aritech Chemazone Pvt. Ltd) to mimic the dynamic intraoral conditions experienced in clinical scenarios. Artificial saliva was used to simulate the neutral oral environment. The details of the study materials are furnished in Table 1. Simulated gastric acid was prepared by dissolving hydrochloric acid (HCl) to create a 0.06 M (0.113%) solution with a pH of 1.2. PMMA specimens were processed using a compression molding technique with a polymer-to-monomer ratio of 25 g powder to 10 mL liquid. The material underwent heat polymerization at 8000 kPa and 80°C for 20 min. For the PP mesh-reinforced samples, PP mesh was cut and inserted between layers of PMMA following the dewaxing process. All samples were polished and trimmed, and dimensions were confirmed using a digital caliper (Digital Caliper 01407A; Neiko).

Table 1.

Details of materials used

Material	Chemical composition	Processing method
Conventional heat-cure PMMA (DPI Heat cure; DPI)	PMMA	Compression molding technique
PP mesh (Trulene laparo mesh; Healthium)	PP, thermoplastic polymer with high tensile strength, biocompatible	Manual cutting and insertion
Artificial saliva (Nanochemazone; Aritech chemazone Pvt Ltd.)	Composed of inorganic salts to replicate human saliva	-
Simulated gastric acid	HCL 0.06 M (0.113%) solution with a pH of 1.2	Prepared in laboratory
Distilled water	Purified water without any contaminants	-

PMMA: Polymethyl methacrylate, DPI: Dental products of India, HCL: Hydrochloric acid, PP: Polypropylene

Samples were immersed in 5 mL of simulated gastric acid and artificial saliva at 37°C for 96 h to simulate 10 years of clinical exposure. After immersion, samples were rinsed with distilled water and air-dried.[4] This immersion protocol was adapted from the methodology used by Tinastepe et al., where a similar duration and temperature were employed to represent accelerated aging equivalent to several years of clinical use. After immersion, all samples were thoroughly rinsed with distilled water and air-dried prior to further testing.

The flexural strength was evaluated using a three-point bending test performed on a universal testing machine (Autograph, AG-IS 50 kN; Shimadzu Co) [Figure 1a]. Samples were tested according to ISO 20795–1:2013 using a stainless-steel die (65 mm × 10 mm × 3.3 mm ± 1 mm) [Figure 2b]. Specimens were submerged in purified water at 37°C for 50 h before testing. Flexural strength was calculated using the formula FS = 3 × W × I/2 × b × d², where FS = Flexural strength (MPa), W = Fracture load (N), I = Test span (center to center) distance between support points (mm), b = Width of specimen (mm), and d = Thickness of the specimen (mm).[10]

Figure 1.

Testing machines used in the evaluation of mechanical properties. (a) Universal testing machine. (b) Presto Izod impact strength analyzer. (c) Contact profilometer

Figure 2.

Illustration of study specimens prepared for mechanical testing. (a) Surface Roughness specimens (b) Flexural strength specimens (c) Impact strength specimens

The Presto Izod impact strength analyzer (Presto Stantest Pvt Ltd) [Figure 1b] was used to evaluate it. Every specimen was made in accordance with ASTM D-256 specifications, which call for dimensions of 65 mm × 10 mm × 4 mm [Figure 2c]. Every specimen was positioned horizontally and fastened between two fixed supports at a distance of 40 ± 0.2 mm. The pendulum (0.5 J) was raised to a specified height and then released, striking the specimen precisely. The energy absorbed in breaking the specimen was recorded from the digital display. Impact strength was assessed for each specimen in this manner.

The contact profilometer (Surftest SJ 310; Mitutoyo) [Figure 1c] was employed at three distinct sites, with a diamond tip radius of 2 µm, a length of 3 mm, and a speed of 0.5 mm/s. Following 96 h (T96) of immersion in both solutions, the mean Ra values were calculated. At T96, the change in surface roughness was computed. Surface characteristics of the samples were examined in a high-resolution scanning microscope (Apreo S Model Quanta 250 FEG; Thermo Fisher Scientific). The samples were ultrasonically cleaned in distilled water for 15 min and allowed to air dry for 20 s before being analyzed. Prior to examination under the scanning electron microscope (SEM), the samples were sputter coated with gold (Au) using a vacuum coating unit to enhance surface conductivity. This step was essential because the specimens were nonmetallic and required a conductive coating to prevent charging and ensure high-quality imaging. During SEM analysis, the coated specimens were securely mounted onto metal stubs using conductive adhesive tabs for stable positioning [Figure 2a-c].

To assess the thermal stability of PP mesh-reinforced PMMA after immersion in different media, thermogravimetric analysis (TGA) was performed using a PerkinElmer Thermal Analyzer. Specimens were previously immersed in 5 ml of simulated gastric acid (HCl) and artificial saliva at 37°C ± 1°C for 96 h. The TGA analysis involved heating the specimens from 30°C to 600°C at a rate of 15°C/min under a nitrogen atmosphere, and weight loss was recorded as a function of temperature.

RESULTS

Descriptive statistics [Table 2], including mean and standard deviation, were computed for all tested parameters using statistical software (IBM SPSS Version 20, IBM Corp). Data normality was assessed, confirming a normal distribution. One-way analysis of variance was performed to compare differences within and between groups. Independent t-tests were used for multiple comparisons between unreinforced PMMA (UN-PMMA) and PP-reinforced PMMA (PP-PMMA) under different immersion conditions. P <0.05 was considered statistically significant.

Table 2.

Descriptive statistics (mean±standard deviation) for all groups

Parameter	Sub-group	Group	Mean±SD	SE
Flexural strength	AS	UN-PMMA	68.8550±2.01663	0.45093
		PP-PMMA	69.3700±1.63775	0.36621
	HCL	UN-PMMA	68.7750±2.22684	0.49794
		PP-PMMA	70.0300±1.49388	0.33404
Impact strength	AS	UN-PMMA	13.9800±1.22027	0.27286
		PP-PMMA	13.1350±0.94550	0.21142
	HCL	UN-PMMA	12.7950±0.99497	0.22248
		PP-PMMA	12.8150±1.03021	0.23036
Surface roughness	AS	UN-PMMA	0.5650±0.08127	0.01817
		PP-PMMA	0.4750±0.14464	0.03234
	HCL	UN-PMMA	0.5050±0.10501	0.02348
		PP-PMMA	0.4700±0.10311	0.02306

SD: Standard deviation, HCL: Hydrochloric acid, AS: Artificial Saliva, SE: Standard error, PMMA: Polymethyl methacrylate, PP: Polypropylene

The mean flexural strength values of PP-PMMA and UN-PMMA were compared under artificial saliva and hydrochloric acid immersion. In artificial saliva, PP-PMMA exhibited a slightly higher mean flexural strength (69.37 ± 1.63 MPa) compared to UN-PMMA (68.85 ± 2.01 MPa). In hydrochloric acid, PP-PMMA (70.03 ± 1.4 MPa) retained higher strength than UN-PMMA (68.77 ± 2.2 MPa), indicating better resistance to acidic degradation.

Surface roughness values were evaluated for both materials in artificial saliva and hydrochloric acid. In artificial saliva, PP-PMMA exhibited lower roughness (0.47 ± 0.14 μm) compared to UN-PMMA (0.56 ± 0.08 μm), indicating a smoother surface. In hydrochloric acid, PP-PMMA maintained a roughness value of 0.47 ± 0.10 μm, while UN-PMMA had a slightly higher roughness of 0.50 ± 0.10 μm. These findings suggested that PP reinforcement improved surface smoothness, particularly in artificial saliva, potentially enhancing the material’s resistance to biofilm accumulation and wear.

The impact strength of PP-PMMA and UN-PMMA was analyzed under both immersion conditions. In artificial saliva, UN-PMMA demonstrated a higher impact strength (13.98 ± 1.22 MPa) compared to PP-PMMA (13.13 ± 0.94 MPa). In hydrochloric acid, PP-PMMA (12.81 ± 1.03 MPa) and UN-PMMA (12.79 ± 0.99 MPa) exhibited similar impact strength values, with no significant reduction observed. This suggested that reinforcement with PP mesh did not significantly enhance impact strength but maintained comparable performance across both immersion conditions.

Independent samples t-test [Table 3] revealed a statistically significant difference was observed between groups in hydrochloric acid (P < 0.05), indicating that PP-PMMA showed better resistance to acid-induced flexural degradation. No significant difference was noted in artificial saliva (P = 0.38), suggesting similar flexural performance in a neutral environment. A significant difference (P < 0.05) was found in artificial saliva, with PP-PMMA exhibiting lower roughness than UN-PMMA. No significant difference was noted in hydrochloric acid (P = 0.29), implying similar surface roughness performance in acidic conditions. A statistically significant difference (P < 0.05) was found in artificial saliva, indicating that UN-PMMA had slightly higher impact strength than PP-PMMA. No significant difference was observed in hydrochloric acid (P = 0.95), showing that both materials responded similarly to acid exposure.

Table 3.

Independent t-test between and within the groups

Parameter	Sub group	t	P	Mean difference	CI of the difference
					Lower	Upper
Flexural strength	AS	−0.887	0.381	−0.51500	−1.69098	0.66098
	HCL	−2.093	0.043*	−1.25500	−2.46883	−0.04117
Impact strength	AS	2.448	0.019*	0.84500	0.14621	1.54379
	HCL	−0.062	0.951	−0.02000	−0.66833	0.62833
Surface roughness	AS	2.426	0.020*	0.09000	0.01490	0.16510
	HCL	1.064	0.294	0.03500	−0.03162	0.10162

*Significance level was set at P<0.05. AS: Artificial saliva, HCL: Hydrochloric acid, CI: Confidence interval

Microscopic images of PP-reinforced PMMA after exposure to artificial saliva and hydrochloric acid showed PP mesh within the polymeric matrix are shown in Figure 3. The precise location of the mesh between the layers of resin was appreciated from the SEM images. Breakage of the mesh was observed indicating fracture of the specimen during flexural strength testing by application of the load in the central area. Although PP-reinforced PMMA specimens exhibited mesh breakage during the flexural strength test, this does not negate the effectiveness of reinforcement. The failure occurred under standardized loading conditions designed to induce material fracture, and the measured flexural strength values remained above the clinically acceptable threshold of 65 MPa, as recommended by ISO 20795–1 for denture base materials. This indicates that PP mesh reinforcement effectively enhanced the mechanical properties of PMMA, providing greater resistance to bending forces compared to nonreinforced specimens. With PP reinforcement, there was a significant decline in the porosities of polymer. A difference in the surface characteristics of the PP-PMMA after immersion in both the solutions was discerned between the layer above and below the mesh. The presence of mesh between the layers restricted the ingress of saliva and HCl as was perceived with rougher surface in the external area. Like unreinforced group, PP-PMMA observed increased surface roughness on immersion in HCL than in artificial saliva.

Figure 3.

Scanning electron micrographs. (a) Polymethyl methacrylate (PMMA) immersed in artificial saliva. (b) PMMA immersed in simulated gastric acid. (c) Polypropylene (PP)-PMMA immersed in artificial saliva. (d) PP-PMMA immersed in simulated gastric acid

TGA results showed [Figure 4] that both HCl-immersed and artificial saliva-immersed PP-reinforced PMMA specimens exhibited thermal stability up to approximately 300°C, with minimal degradation (<5% weight loss) observed up to this point. The major weight loss phase commenced beyond 400°C, attributed to the decomposition of the polymer matrix. These results confirm the thermal resilience of the reinforced PMMA within physiological and elevated laboratory conditions.

Figure 4.

Thermogravimetric analysis curves. (a) Polypropylene-reinforced polymethyl methacrylate (PMMA) specimens after immersion in simulated gastric acid (HCl). (b) Polypropylene-reinforced PMMA specimens after immersion in artificial saliva (AS)

DISCUSSION

The results of the present study partially rejected the null hypothesis, as both significant and insignificant differences were observed between unmodified and PP mesh-reinforced PMMA after immersion in artificial saliva and simulated gastric acid. The results indicated that in artificial saliva, the difference between PP-PMMA (69.37 MPa) and UN-PMMA (68.85 MPa) was not statistically significant (P = 0.38), suggesting that PP reinforcement did not markedly influence flexural strength in neutral conditions. However, in the hydrochloric acid environment, PP-PMMA exhibited significantly higher flexural strength (70.03 MPa) than UN-PMMA (68.77 MPa) (P < 0.05), indicating enhanced resistance to acidic degradation.

This finding aligns with Vallittu et al., who demonstrated that fiber reinforcement improves the flexural strength of denture base materials.[11] Vojdani and Khaledi et al. discussed various fiber reinforcement methods, including the use of glass, carbon, and polyethylene fibers, and how they affect the denture base resins’ mechanical characteristics. The study emphasized that fiber reinforcement can significantly enhance the strength and durability of denture bases but bonding to PMMA is less reliable without special surface treatments.[12] It may decrease stiffness while improving toughness, potentially affecting denture fit Mathew et al. further emphasized that surface treatments on PP fibers enhance adhesion to PMMA, thereby improving flexural properties.[13] These findings underscore the potential of PP mesh reinforcement in extending denture longevity, particularly in patients exposed to acidic conditions. Surface roughness is a critical factor influencing plaque accumulation and esthetic appearance. In this study, PP-PMMA exhibited significantly lower surface roughness than UN-PMMA in artificial saliva (0.475 μm and 0.565 μm, respectively, P < 0.05), suggesting that PP reinforcement contributes to a smoother surface, potentially improving oral hygiene and patient satisfaction.

However, in hydrochloric acid, the difference in surface roughness was not statistically significant (0.470 μm and 0.505 μm, respectively, P = 0.29), indicating that while acidic exposure affects surface degradation, PP reinforcement does not provide a distinct advantage under these conditions. These findings correlate with Bollen et al., who reported that smoother denture surfaces reduce microbial adhesion.[14] Furthermore, Gad et al. highlighted that fiber reinforcement enhances PMMA’s resistance to environmental wear, supporting the clinical relevance of PP mesh incorporation.[15] While fiber reinforcement has been associated with increased surface roughness in some studies, such as Lee et al., this study observed a reduction in roughness, reinforcing the potential of PP as an effective reinforcement material.[16]

The impact strength results revealed no substantial advantage of PP reinforcement over conventional PMMA. In artificial saliva, UN-PMMA exhibited a slightly higher mean impact strength (13.98 MPa) compared to PP-PMMA (13.13 MPa), with a statistically significant difference (P < 0.05). However, the clinical significance of this variation is minimal. In hydrochloric acid, both groups exhibited comparable impact strength values (12.79 MPa for UN-PMMA and 12.81 MPa for PP-PMMA, P = 0.95), indicating that acidic exposure does not differentially impact the impact resistance of reinforced and unreinforced PMMA. These findings are consistent with Zafar et al., who concluded that specific additives improve PMMA impact strength under simulated oral conditions.[17] In addition, Yerliyurt et al. supported the role of mesh reinforcement in enhancing PMMA’s mechanical characteristics without compromising clinical performance.[9] The absence of a significant impact strength improvement with PP reinforcement may be attributed to its stress distribution properties, as previously suggested by Mowade et al.[18] While alternative reinforcements, such as E-glass fibers, have demonstrated greater impact strength improvements, PP remains advantageous due to its biocompatibility and resistance to moisture related degradation. Alhotan et al. study showed that E-glass fibers significantly improved the impact strength of PMMA. ZrO2 and TiO2 nanoparticles had no discernible effect on impact strength. Mathew et al. found that adding 10 wt% of PP fibers significantly improved the impact strength of the material compared to unreinforced PMMA.[13] The increase in impact strength was attributed to the reinforcement provided by the fibers, which helped distribute applied stresses more effectively.[19]

The thermal behavior observed in TGA analysis indicates that PP mesh reinforcement does not undergo significant degradation at intraoral temperatures (typically below 60°C), even after chemical exposure to acidic or artificial saliva environments. The stability up to ~300°C confirms the material’s suitability for clinical use from a thermal safety standpoint. These results are consistent with the reported high melting point and low thermal expansion characteristics of PP, making it a viable long-term reinforcement agent in prosthodontic applications.

Despite the valuable insights gained, this study has several limitations. The use of artificial saliva and hydrochloric acid to simulate oral conditions may not fully replicate the complex oral environment, which includes microbial interactions, temperature fluctuations, and dietary variations. In addition, the effect of other chemical solvents commonly encountered in the oral cavity such as alcohol-based mouthwashes and acidic food components was not evaluated. Including such factors would provide a more comprehensive understanding of the material’s performance in dynamic conditions. The study focused solely on flexural strength, impact strength, and surface roughness, excluding other critical properties such as tensile strength, fatigue resistance, and esthetics. Furthermore, laboratory-based testing on standardized specimens may not fully represent the behavior of full-scale denture bases in clinical applications. Future investigations could benefit from comparing PP mesh with other reinforcement materials, such as glass fibers, aramid fibers, or metal meshes, to assess their relative advantages. Future research should explore long-term clinical performance and additional reinforcement strategies to optimize denture base materials for enhanced durability and patient satisfaction.

CONCLUSION

Based on the investigation findings,

1. PP-reinforced PMMA exhibited a significant increase in flexural strength after immersion in hydrochloric acid but showed no significant difference in artificial saliva

2. The impact strength remained unaffected in hydrochloric acid but showed a significant difference in artificial saliva

3. Surface roughness did not change significantly after hydrochloric acid immersion but was significantly altered in artificial saliva.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.