Effect of glycerin as a plasticizer on flexural strength in the fabrication of gypsum-based chip

Introduction

The main objective of periodontal treatment is to eliminate the cause of infection and inflammation. To suppress the progression of periodontal infection, a homeostatic relationship between periodontal tissues and their polymicrobial environment must be achieved.¹ Elimination of infectious pathogens can control the infection, halt the progression of tooth decay and promote the healing process.² Several antibacterial agents have been used as adjunctive therapy to promote bacterial elimination. Antibacterial agents can be applied by systemic and local routes.³ The development of drug delivery systems holds a significant interest in medicine due to its characteristics. The latest development in local delivery therapy is the chlorhexidine chip which delivers antibacterial agents right into the periodontal pocket. The chip that is available commercially is made of gelatine or collagen crosslinked with glutaraldehyde to increase the mechanical properties and durability.^4,5

In this study, gypsum will be used as a base material for drug-carrying chips. Ceramic-based drug delivery systems have received a lot of attention with the advancement of medicine, pharmaceuticals, and materials science. The advantage of ceramic-based drug delivery is its adjustable size and structure, making it advantageous for nano-sized drugs and low toxicity because ceramics are biocompatible, biodegradable, and have good biological stability.^6,7 The drug can diffuse through the pores of ceramic materials depending on its solubility and concentration, as well as the porosity of the ceramic which can affect the diffusion of the drug. The chosen ceramic material is gypsum because it can be easily modified by physical and chemical modifications, making gypsum a widely used material in dentistry.⁸ Gypsum is osteoconductive, so it will help in the regeneration process of alveolar bone affected by periodontitis. Gypsum also has biodegradable properties and is biocompatible so it can be an ideal candidate in the treatment for periodontitis.⁸ The biodegradable nature of gypsum allows for the release of the drugs such as antibiotics.⁹ However, gypsum is a brittle material. So, in this study, glycerin was added with various compositions as a plasticizing agent which is expected to reduce the brittleness of gypsum chip. Glycerin is used because it is the most common plasticizing agent to be added to increase the flexibility of the film. In addition, glycerin has a very high water solubility compared to other polyol compounds.¹⁰

Materials and Methods

The sample consisted of 36 specimens. Six samples were evaluated for each group; Group 1 was the glycerin 0% sample (control), Group 2 was the glycerin 5% sample, Group 3 was the glycerin 10% sample, Group 4 was the glycerin 15% sample, Group 5 was the glycerin 20% sample and Group 6 was the glycerin 25% sample.

Calcium Sulfate Hemihydrate powder (Sigma-Aldrich Pte.Ltd., USA) was used in this study. The liquid was prepared by mixing glycerin (Loba Chemie PVT Ltd., India) with distilled water (Puma, Indonesia). The concentrations used for glycerin was 0%, 5%, 10%, 15%, 20%, and 25%. The 5% glycerin liquid means that every 5 ml of the liquid is formed of 0.5 ml glycerin and 4.5 ml distilled water. The formulated liquid was mechanically mixed with a magnetic stirrer (Thermo Scientific, USA) for 10 minutes to ensure proper mixing.

The powder weighed 1 gram with an analytical balance (Shimadzu AX 200, Japan) and mixed with 0.4 ml of the prepared liquid. The liquid was measured with a micropipette (Eppendorf, Germany) to ensure the measurement. Then the powder was mixed with the liquid with a w/p ratio of 0.4 in a bowl with a cement spatula. The powder and liquid were mixed until the paste was homogenous. The mixed paste was then poured into mould with 25 x 3 x 1.5 mm dimension and the excess paste was removed and levelled with a spatula. The samples then dried at room temperature for 24 hours, until the sample was completely dried.

After the samples were completely dried, the samples were taken out of the mould and each sample was measured with a digital calliper (Mitutoyo, Japan). The sample was then stored in a 10cc pot before testing. Three-point bending tests were performed according to ISO 4049 using a universal testing machine (Shimadzu AGS-X, Japan) at a crosshead speed of 0.5 mm/min and preload 5000N. Force, Stress, and Strain were determined and calculated for means. Numerical data were analysed using the Saphiro-Wilk normality test, Levene’s Test for Homogeneity of Variances, and one-way ANOVA and Kruskall-Wallis. Differences were considered statistically significant when p<0.05. All data were tabulated, and statistical tests were performed with IBM SPSS Statistic 26. Alternatively, JASP open software can be used.

Results

Six groups of gypsum chip flexural test specimens were successfully made in the study with a W/P ratio of 0.4 with a glycerin composition of 0%, 5%, 10%, 15%, 20% and 25%. Pictures of gypsum chip specimen groups can be seen in Figure 1 (a-f). In Table 1, the mean stress value of each group was shown a statistically significant difference between the groups (p<0.000). All glycerin groups showed a statistically significant reduction in maximum stress. To analyze the difference between each group, the Tamhane post-hoc test was performed. Tamhane post-hoc test results indicated no significant differences between glycerin 5%, 10%, 15%, and 20% groups (p>0.05). The maximum stress of the Glycerin 25% group was 1.9254 ± 0.5088 MPa. It was shown that the glycerin 25% group had significantly lower mean values compared to other group (p<0.05).

Figure 1. Gypsum Chip Specimen Group with Glycerin 0% (a), 5% (b), 10% (c), 15% (d), 20% (e), 25% (f).¹⁸

The percentage of the maximum strain of each group from the three-point bending test was then tabulated and analyzed. The mean values of the mean strain value of each group showed a statistically significant difference between the groups. Table 2 shows the significant improvement between the control and glycerin groups (p<0.05) except for the glycerin 25% group. This result indicated that the addition of glycerin affected the strain of each group until the 20% concentration. To analyze the difference between each group, the Mann-Whitney test was performed. The test indicated that there was no significant difference between glycerin 5%, 10%, and 15% groups. Figure 2 showed the stress-strain graphic curve of all 6 groups. The graphic showed that the elasticity of the chip increased with the increasing strain percentage even though the mechanical (flexural) stress is declining.

Figure 2. Stress-Strain curve of each group.¹⁹

Discussions

Gypsum is a biocompatible material that has been widely used clinically for periodontitis, endodontic lesions, alveolar bone loss, and maxillary sinus augmentation.^7,8 Gypsum has a unique crystal structure and high calcium content so it has an osteoconductive characteristic.¹¹ However, due to its brittleness, glycerin was added in varying concentrations in this study with the aim of reducing the brittle characteristic of gypsum chips.

Glycerin is commonly used as a plasticizing agent, humectant, emollient, and solvent for food flavoring and coloring. Glycerin is a very hygroscopic molecule that can be added to film-forming solutions to reduce the brittleness of film.¹² In addition, glycerin is hydrophilic, which makes it easily soluble in water so that glycerin has infinite water solubility.¹³

In this study, the percentage of strain was increased by the addition of glycerin (Table 2). However, it was found that the mechanical strength of the gypsum chip decreased in proportion to the increase in glycerin concentration (Table 1). This shows that the gypsum chip has increased elasticity even though its mechanical strength was compromised. Previously, no study has examined the effect of adding glycerin on the strength of gypsum and ceramics in the form of chips or films.

However, this phenomenon has been found in the addition of glycerin to composite. Glycerin with various concentrations is used as a plasticizer in the manufacture of chitosan-based composite films. From the results of the study, it was found that the addition of 10% glycerin increased the tensile strength of nanocomposite films but the tensile strength decreased with the addition of 20% and 30% glycerin. It was also stated that the glycerin content did not have a significant effect on the film because the stiff characteristic of the film base material played a more dominant role compared to the plasticizing effect of glycerin.¹⁴

Research conducted by Zhang et al., regarding the addition of additives to the strength of gypsum stated that the addition of compounds such as retarders like citric acid, tartaric acid, salicylic acid and sucrose can slow down the setting time of gypsum due to the adsorption along the axial axis of crystal growth thereby inhibiting crystal growth and reducing the strength of the gypsum.¹⁵ The strength of the gypsum is due to the mechanical interlocking between crystals.¹⁶ It is possible that the more compound added, the lower the mechanical strength of the gypsum because it can affect the interlocking mechanism between crystals. During crystal growth, additives such as glycerin may be adsorbed and modify the crystal shape. The addition or organic additives is said to produce crystals that are flatter and with a hexagonal shape.¹⁷

Therefore, it is estimated that the addition of materials such as glycerin can increase the gaps between crystalline structures and weaken the bonds between dehydrated crystals. This phenomenon can reduce the mechanical strength of the drug-carrying gypsum chip.

Conclusions

With the limitation of the study, gypsum chips were successfully made. It can be seen that the addition of glycerin can increase the elasticity and decrease the brittleness of the gypsum chip even though the strength of the chip is lowered. It is hoped that this study increases interest in using gypsum as a base material for local drug delivery systems, especially in periodontal pockets. Other plasticizer alternatives can be subjected to experimental testing in order to see how they affect gypsum.

Author contributions

Conceptualization and methodology, S.; validation, S.; investigation, A.M.; resources, S., and B.I.; writing—original draft preparation, A.M., and S.; writing—review and editing, S.; visualization, A.M.; supervision, S., and B.I.; funding acquisition, S.