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 Table of Contents  
Year : 2022  |  Volume : 22  |  Issue : 4  |  Page : 328-337

Comparative evaluation of tensile strength, tear strength, color stability and hardness of conventional and 1% trisnorbornenylisobutyl polyhedralsilsesquioxane modified room temperature vulcanizing maxillofacial silicone after a six month artificial aging period

Department of Prosthodontics, K. M. Shah Dental College and Hospital, Sumandeep Vidyapeeth Deemed to be University, Vadodara, Gujarat, India

Date of Submission17-May-2021
Date of Decision15-Jul-2022
Date of Acceptance24-Aug-2022
Date of Web Publication03-Oct-2022

Correspondence Address:
Drashti Sunil Gandhi
No. 9, Department of Prosthodontics, K. M. Shah Dental College and Hospital, Sumandeep Vidyapeeth Deemed to be University, Piparia, Vadodara - 391 760, Gujarat
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jips.jips_226_21

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Aims: Silicone elastomers, chemically known as polydimethylsiloxane used in maxillofacial rehabilitation, over a period of time, undergo degradation and discoloration once aged, thereby reducing clinical longevity. Many previous studies reinforced the maxillofacial silicone material with stronger materials to increase its mechanical properties. However, no studies have been conducted to evaluate all the primary properties using single reinforcing agent. This study was conducted to evaluate and compare the tensile strength, tear strength, color stability, and Shore A hardness of conventional and 1% trisnorbornenylisobutyl polyhedralsilsesquioxanes (POSS) modified room temperature vulcanizing (RTV) maxillofacial silicone after a 6 - month artificial aging period.
Setting and Design: In vitro comparative study.
Materials and Methods: Eighty-eight silicone samples were fabricated. Therefore for each parameter of tensile strength, tear strength, color stability and hardness, twenty two samples comprising of 11 samples of conventional RTV silicone (Group 1) and 11 for POSS modified RTV silicone (Group 2) were fabricated in stainless steel molds using ASTM D 412–06, ASTM D 624, and ASTM D 2240–15 Standards. Baseline measurements for Shore A hardness and color values were recorded. Samples were then exposed to 6 months of natural weathering process and evaluated for tensile and tear strengths, color stability (ΔE), and hardness.
Statistical Analysis Used: Paired and unpaired t-test.
Results: Intragroup and intergroup comparison was done using unpaired and paired t-test. At the end of 6-month aging period, the tensile strength and tear strength of POSS-modified RTV silicone were significantly higher than conventional RTV silicone (P < 0.0001 and P = 0.00014, respectively). Intragroup comparison of conventional group showed highly statistically notable changes in L, a, and b values (P = 0.01631, > 0.0001, and = 0.0.0067, respectively), whereas the POSS-modified RTV silicone showed statistically nonsignificant results in L, a, and b values' (P = 0.91722, 0.15174, and 0.10847, respectively) comparisons after aging. Intergroup ΔE value comparisons showed an extremely statistically difference (P < 0.0001) within the groups. Intergroup comparisons postaging hardness showed a high statistical difference between both the groups, indicating a significant increase in hardness in the conventional group (P < 0.0001). However, intragroup comparison for hardness values showed a statistically highly significant difference for Group 1 (P < 0.0001) and a nonsignificant difference (P = 0.4831) for Group 2.
Conclusion: After the simulated 6-month aging procedure, 1% NB 1070 trisnorbornenylisobutyl POSS-incorporated RTV maxillofacial silicone showed better tensile strength, tear strength, Shore A hardness and color stability as compared to conventional RTV silicone. Hence, trisnorbornenylisobutyl POSS is a potent cross-linking agent which enhances the primary mechanical properties of RTV silicone can result in in significant increase in the mean life expectancy of RTV silicone even after 6 months of weathering.

Keywords: A-2000 room temperature vulcanizing maxillofacial silicone, color stability, hardness, natural weathering, NB 1070 trisnorbornenylisobutyl polyhedralsilsesquioxanes, tear strength, tensile strength

How to cite this article:
Gandhi DS, Sethuraman R. Comparative evaluation of tensile strength, tear strength, color stability and hardness of conventional and 1% trisnorbornenylisobutyl polyhedralsilsesquioxane modified room temperature vulcanizing maxillofacial silicone after a six month artificial aging period. J Indian Prosthodont Soc 2022;22:328-37

How to cite this URL:
Gandhi DS, Sethuraman R. Comparative evaluation of tensile strength, tear strength, color stability and hardness of conventional and 1% trisnorbornenylisobutyl polyhedralsilsesquioxane modified room temperature vulcanizing maxillofacial silicone after a six month artificial aging period. J Indian Prosthodont Soc [serial online] 2022 [cited 2023 Feb 9];22:328-37. Available from: https://www.j-ips.org/text.asp?2022/22/4/328/357793

  Introduction Top

Maxillofacial prosthodontics is associated with rehabilitation of missing or lost stomatognathic and associated structures by fixed or removable auxiliary alternates.[1],[2] Since decades, many different materials have been used for fabrication of maxillofacial prosthesis such as porcelain, natural rubber, gelatin, latex acrylics, and silicones in anaplastology. Among these material, methylmethacrylate resin is used extensively due to its durable nature.[3] However, acrylics have disadvantages of rigidity and polymerization shrinkage. This often leads to staining and unesthetic appearance and hence have been replaced by silicones as the choice for extraoral maxillofacial prosthesis. Silicones are flexible and soft as they mimic the soft tissues.[4] Pigmentation is done with intrinsic and extrinsic stains to mimic soft tissues. Competency of the silicone makes it clear at the corners, which merges smoothly with the adjoining soft tissues, giving an esthetically pleasing appearance. In spite of these advantages, degradation and discoloration are two major limitations associated with silicone maxillofacial prostheses. Once the prosthesis is delivered, it is exposed to light containing ultraviolet (UV) radiation, air that is polluted with dust and changes in atmospheric pressure, all resulting in wearing of silicone prosthesis.[5],[6] While resting on human skin for extended period, perspiration and sebum get absorbed into the extraoral silicone prosthesis. However, UV radiation increases cross-linking, but breaks down bones of polymer matrix, lowers down the rate polymerization and degrades the silicone, all of which contribute to colour changes and material deterioration.[7],[8],[9],[10] Hence to improve the longevity, many studies have reported use of stronger reinforcing materials. However this has seen limited success.

Silicone elastomers chemically known as polydimethylsiloxane, are widely used due to its unique properties of good consistency, high tear and elongation strength, longevity, good handling properties, improved intrinsic stainability and patient compliance. The essential properties of silicone elastomer depend on the degree of cross linking network, the type and density of filers in the silicone network. Cross-linking of elastomers further depends on type, nature and density of thermal initiator, type of fillers, the reinforcing material, curing temperature and polymerization method.[11],[12],[13]

Incorporation of polyhedralsilsesquioxanes (POSSs) as a reinforcing stabilizer and fortifying agent in elastomers has been reported.[14] POSS as a reinforcing agent contains nano-scale organic-inorganic components that form a 1.5 nm silica cage with eight pendant organic groups. This structure helps in more cross - linking and enhances mechanical properties.[15],[16],[17] Hybrid molecule will contain a 1.5 nm silica cage with eight pendant organic groups. POSS are hybrid nanoscale agent having to class of discrete organic inorganic hybrids particles.

Studies have concluded reinforving silicone with POSS cross-linker have enhanced mechanical properties.[16],[17] Recently, a new interventional POSS named NB 1070 – TrisnorbornenylIsobutyl POSS (Hybrid Plastics, Hattiesburg, MS) has been introduced. It is a colorless liquid and is said to be the most biocompatible with silicone elastomers. Its resin solubility is maximum with silicone elastomers. POSS moieties have been studied for cytocompatibility, antithrombogenicity, and biostability and hence is biocompatible and medical graded.[18] In addition to its biocompatibility and resin solubility, the liquid nature of NB 1070 may enable a complete homogenous dispersion into maxillofacial silicone, which can probably enhance the properties of interest in maxillofacial silicones. This formed the base for our study's research hypothesis.

A comprehensive search of literature databases yielded only one study conducted by Mohammad et al. on the effect of POSS on elongation strength, tear strength of room temperature vulcanizing (RTV) maxillofacial silicone. However, POSS used in this study was the powder form of tris - diimethylvinylisobutyl poss.[19] This study concluded that there was insignificant difference in the tensile strength of silicone at 0% and 5% concentrations of POSS. However, there was notable increase in tear strength of silicone at 1% concentration of POSS. Hence, 1% POSS was used to conduct the study.[19]

No studies have been conducted to compare the effects of POSS on color balance and hardness of maxillofacial RTV maxillofacial silicone. Furthermore, no study exists that has evaluated the properties of maxillofacial silicone after incorporating 1% trisnorbornenylisobutyl POSS. Neither there are studies, that have evaluated the effect of a single cross-linking agent or nanofiller particles on all the mechanical properties such as tensile strength, tear strength, Shore A hardness and color stability of maxillofacial silicone. Hence, this study was planned to measure and compare the effect of trisnorbornenylisobutyl POSS in 1% concentration on tensile strength, tear strength, color stability, and hardness of RTV silicone.

The null hypothesis states that addition of trisnorbornenylisobutyl POSS in 1% concentration in silicone elastomers will not significantly change the tensile, tear, color, and hardness properties of RTV maxillofacial silicone when compared to conventional unmodified RTV maxillofacial silicone.

  Materials and Methods Top

The study was conducted after obtaining necessary permission from Institutional Ethics Committee vide approval no: SVIEC/DN/DENT/BNPG/17018002. On the basis of a previous study by Mohammad et al.,[19] a minimum sample size derived for the study was 9 to achieve a mean reduction difference between 0% concentration and 1% concentration of POSS with standard deviation of 5.5 at 5% risk and 95% power. However, a sample size of 11 was considered for each parameter of tensile strength, tear strength, color stability, and hardness. Thus, the total sample size of 88 was achieved.

Stainless steel metal dumbbell shaped molds with dimensions of the tensile test bar and trouser shaped molds with dimensions of the tear test bar were made as per ASTM D 412-06 standards,[20] and ASTM D 624 standards,[21] respectively. Samples for color stability and hardness were made from stainless steel metal molds with wells of 30 mm × 06 mm, according to ASTM D2240-15 Standards,[22] were fabricated. All the metal mold assemblies consisted of three parts of 6 mm in dimensions of height, breadth, and thickness that were secured tightly in place by wrench screws.

Silicone samples were fabricated using RTV Maxillofacial silicone elastomer. Factor II A -2000 Part A and Part B (Technovent Ltd, UK) were weighed on an electronic balance. Six drops of Factor II Thixo (Technovent Ltd, UK) was added and the mixture was hand spatulated first and then mechanically mixed at 1000 rpm in a high speed mixture (Phillips, India). Intrinsic Pigments (P499, Technovent, UK) were weighed and added drop by drop in sequence of Intrinsic skin shade Biscuit (P415): 7 drops, Ochre (P416): 19 drops, Ivory (P417): 9 drops, Mushroom (P419): 4 drops, Tan (P412):9 drops, Light Buff (P413): 3 drops, Pink (P410): 13 drops and Dark Brown (P418): 4 drops to obtain appropriate skin color. The final mixed silicone was mixed in a vacuum mixing machine to avoid porosities in final mix. The mixture was dispensed into disposable plastic syringes to avoid air incorporation in the material. The silicone material was then dispensed in the stainless steel molds and was sealed completely with tightening of screws. For Group 2 samples, the same procedure was followed except that Factor II Part A was mixed with 1% concentration of NB 1070 trisnorbornenylisobutyl POSS (Hybrid plastics, Hattiesburg) to form reinforced Part A. To promote miscibility, the mixture was heated at 55°C for 15 min. Then, this mixture again was placed in a speed mixer for 2 min at 3000 rpm. The mixture was then placed in a refrigerator to cool for 1 h to prevent spontaneous curing. This modified Part A (15 g) was then mixed with A-2000 Part B (15 g) along with six drops of Factor II Thixo by spatula and then for 1 min at 1000 rpm in the speed mixture and pigmented conventionally. All stainless steel molds were coated with the talcum powder for easy retrieval of the samples.[23] Once the samples were cured, the excess flash of silicone was removed with sharp scissor or BP blade, respectively. Thus, 88 samples were made, which were free of porosities or any kind of defects.

Baseline measurements of color were made using a spectrophotometer (Premier Model: 5100, color scan, Rayscan Equipments and Services Pvt. Ltd.).[22] The samples were kept near densitometer using white background which measures the degree of light passing through or reflected by a subject. The wavelength reflected back was calibrated and the computer gave the L, a, and b readings.

Baseline measurements for hardness of both the groups were made using Shore A hardness durometer[24] (Tool Center, Digital). The measured hardness was determined by the depth of the impaling indenter under the load. The hardness values were expressed in Shore units (range, 0–100).

Eighty-eight samples were now subjected to artificial aging procedures.[22],[25],[26] Artificial sebum and acidic perspiration were prepared using reagents and methods as per routine methods.[8],[16],[27],[28] For first 3 months, daily, all the samples were exposed to daylight exposure for 8 hours [Figure 1] followed by exposure to simulated sebum for 16 hours [Figure 2] followed by cleansing for 5 min using neutral soap solution. Further,daily for next 3 months, they were exposed to daylight exposure for 8 hours, followed by exposure to simulated acidic perspiration for 16 hours [Figure 3] and cleansing of samples for 5 min using neutral soap solution. Lastly all the specimens were exposed to dark storage for 24 hours at 50% ± 5% relative humidity by placing wet cotton saturated with distilled water between the samples [Figure 4]. The artificial aging protocol is summarized in [Figure 5].
Figure 1: Exposure of conventional and POSS-modified samples to UV radiations (day light). POSS: Polyhedralsilsesquioxanes, UV: Ultraviolet

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Figure 2: Exposure of conventional and POSS-modified samples to sebum. POSS: Polyhedralsilsesquioxanes

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Figure 3: Exposure of conventional and POSS-modified samples to acidic perspiration. POSS: Polyhedralsilsesquioxanes

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Figure 4: Exposure of conventional and POSS-modified samples to dark room condition. POSS: Polyhedralsilsesquioxanes

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Figure 5: Form of exposure of all the samples which is equivalent to 6 months of aging of RTV maxillofacial silicone is given in tabular form below. RTV: Room temperature vulcanizing

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The four properties were now evaluated after the artificial aging procedure. Color values and hardness values were evaluated as per the methodology followed for baseline measurements. From the two values (baseline and after 6 months) ΔL, Δa and Δ b values were calculated. Color change ΔE was measured according to CIELab system using the formula ΔE = ([ΔL]2 + [Δa]2 + [Δb]2)½ formula. Further the mean ΔE was classified as per the 3 clinically relevant intervals as follows: ΔE <1 (undetectable color alteration); 1< ΔE <3.3 (clinically acceptable color alteration); and ΔE >3.3 (clinically unacceptable color alteration).[29] The color stability was also quantified and evaluated as National Bureau of Standards (NBS)[30] units by using the formula = ΔE × 0.92. The interpretation of the NBS values was: 0.0–0.5 as trace, 0.5–1.5 as slight, 1.5–3.0 as noticeable, 3.0–6.0 as appreciable, 6.0–12.0 as much and >12 as very much.

For evaluating tensile and tear strengths, the samples were fit in Universal Testing Machine (Tinius Olsen 10ST) with 1 KN load cell and measured at cross head speed of 51 mm/min. The tensile strength and percentage elongation were measured automatically by the software using formula Load = Stress (Nm−2)/Initial cross- -sectional area (mm−2). The tear strength was calculated using the following formula: Tear strength (n/mm) = Load failure in n/thickness of specimen in mm.[31],[32]

  Results Top

Tensile strength comparisons using unpaired t-test [Table 1] showed that Mean Tensile Strength of Group 2 (POSS-modified RTV silicone) is extremely statistically significant when compared to the mean tensile strength of Group 1 (Conventional RTV Silicone) P < 0.0001. Results of unpaired t-test for tear strength comparisons [Table 2] also showed that mean tear strength of Group 2 is extremely statistically significant when compared to the mean tear strength of Group 1 (P < 0.0001).
Table 1: Summary statistics of the unpaired t-test comparison between conventional (Group 1) and polyhedralsilsesquioxanes modified (Group 2) room temperature vulcanizing silicone for tensile strength

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Table 2: Summary statistics of the unpaired t-test comparison between conventional (Group 1) and polyhedralsilsesquioxanes modified (Group 2) room temperature vulcanizing silicone for tear strength

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Color stability comparisons [Table 3] using unpaired t-test showed that the mean ΔE values of Group 2 (POSS-modified RTV silicone) were extremely statistically significant when compared to the mean ΔE values of Group 1 (conventional RTV silicone) (P < 0.0001). The ΔE values obtained were expressed and interpreted according to the NBS.[33] Accordingly, to the NBS units interpretation, the ΔE value for Group 1 (4.9174) is interpreted as appreciable change and ΔE value for Group 2 (1.189) is interpreted as slight change. As per clinical relevance classification,[29] ΔE for the Group 1 was 5.345, which was clinically unacceptable, and ΔE for the Group 2 was 1.293064, which was clinically acceptable.
Table 3: Summary statistics of the unpaired t-test comparison between conventional (Group 1) and polyhedralsilsesquioxanes modified (Group 2) room temperature vulcanizing silicone of (ΔE) values after 6 months of aging

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The results of the hardness values for intergroup and intragroup comparisons are shown in [Table 4]. Baseline comparisons for mean Shore A hardness values of Group 1 and Group 2 were statistically nonsignifiant P = 0.2034. However, hardness values between the groups after 6 months of artificial aging were extremely statistically significant P < 0.0001. Intragroup comparisons (baseline vs. Post aging) was statistically significant P < 0.0001 for Group 1 and non significant for Group 2 P = 0.4831.
Table 4: Summary statistics of the inter- and intragroup comparison between conventional (Group 1) and polyhedralsilsesquioxanes modified room temperature vulcanizing silicone (Group 2) for shore A hardness at baseline and after 6 months of aging

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  Discussion Top

The importance of good looks for social expectance and success has always been overemphasized.[34] Patients having acquired or congenital craniofacial defects have non pleasing facial features that affect an individual's day to day, personal and social interactions. With maxillofacial rehabilitation, the patient's negative approach can be reviewed into a positive approach.[35]

In relation to extra oral defects, when it comes for the fabrication of prosthesis for maxillofacial defects, choice of material is utmost important to replicate the hard and soft tissues. The most favorable qualities include biocompatibility, translucence, color stability, texture, resistance to tear due to physical and chemical insults, and a tactile sensation of softness. Silicone elastomers are flexible and soft and are thermal insulators. They have good oxidative stability. Moreover, they can be pigmented to mimic soft tissues by intrinsic and/or extrinsic stains, which gives prosthesis life like natural appearance.[36]

The choice of the RTV silicone has been inundating. Studies have demonstrated high-temperature vulcanizing (HTV) silicones to be superior in terms of strength, hardness and stiffness than RTV silicone. The prime limitation of HTV silicone is its processing. HTV silicones are less translucent, have more stiffness, have low edge strength, and are very sensitive in processing[2],[37],[38],[39] when compared to RTV silicone. Addition RTV silicones exhibit better mechanical properties apart from ease in fabrication of moulds, manipulation, intrinsic and extrinsic coloring. They are color stable and biological inert when compared to other silicones.[40]

Nevertheless, with any type of silicone, even with improved properties, there is no ideal silicone having all favorable properties which can increase the longevity of maxillofacial prosthesis.[41] Discoloration of prosthesis and deterioration over natural aging are the most serious problems associated with currently available maxillofacial silicones. Physical degradation of material properties results in difficulty in prosthesis repair and shortens life span of prosthesis up to 6 months. However, it also depends on patients' personal habit, climate, and environment, which leads to frequent change of prosthesis over short period of time.[42]

Since past few years, there were various attempts to reinforce the maxillofacial silicone due to its very short mean life expectancy. Various studies have evaluated reinforcements with medical fluids, titanium opacifiers, UV mineral based light protecting agent, nano oxides of titanium, zinc and cerium, UV stabilizers, titanium dioxide nanoparticles and nano ceramic fillers.[16],[26],[28],[29],[31],[35] Reinforcing fibers of tulle, polyester, polyurethane, propylene, nano - reinforcements of zinc oxide, titanium, barium sulfate, silica and carbon nanotubes have also been reported to improve properties of maxillofacial silicone.[43],[44] However, in accordance with all the previous studies, there is no single cross-linking or nanoparticles or oxides reinforcing agents used in a research which has evaluated all the primary properties of maxillofacial prosthesis. Taking all the above consideration, this study was conducted to evaluate the primary properties (tensile strength, tear strength, color stability, and hardness after 6 months of aging using a single reinforcing cross-linking material known as POSS).[19]

The POSS belongs to a class of compact three-dimensional architecture consisting of organic and inorganic compounds with cage dimensional structure with unlike degrees of symmetry, topologically equivalent to a sphere. POSS molecules are easily miscible with polymeric resins. POSS and resins form a soluble compound which forms single-phase material. Among the family of silsesquioxanes, the oligomeric composites can be bifurcated into two main groups: the fully dense POSS and the partially dense POSS. Fully dense POSS represents a fully compact closed architect with the silicone atoms placed apically. Partially dense POSS is an open cage with dangling Si-OH groups. Hence, the large variety of completely concentrated and partially concentrated POSS can refer to the general formula RnSinO1.5n.[45] POSS is a medically graded cross-linking agent. It has also been tested as a biomaterial and was first introduced in breast surgery. Its biocompatibility can be due to the foci of areas that are silicone rich with enhanced surface free energy. POSS molecules are not toxic in nature and cytocompatible.[46]

Hamza et al. in 2014 studied the flexural strength and color change of 4 commercially available interim materials modified with 1 wt% POSS and concluded that 1% POSS-incorporated samples did not show any significant differences in color stability with coffee.[47] A study conducted by Shi et al. in 2014 concluded that the thermal stability improved by cross - linking of POSS into new networks of polydimethylsiloxane (PDMS).[48] Chen et al. in 2010 studied the synthesis, intrinsic and extrinsic characterization of novel RTV silicone rubbers by addition of Vinyl - POSS as reinforcing cross - linking agents.[49] They concluded that the mixture was thermally stable thereby increasing mechanical properties. Stiffness of these RTV silicones improved because of the closed and open concentrated caged networks in PDMS matrix.[50]

Only one study by Mohammad et al. in 2010 evaluated the primary properties by incorporating POSS in different concentration (0%, 0.5%, 1%, 2%, and 5%) in RTV maxillofacial silicone and concluded that tensile strength did not differ between 0% and 5% concentrations of POSS, respectively. However, there was marked increase in tear strength of silicone at 1% concentration of POSS.[19]

State of the matter plays a crucial role in miscibility and homogeneous mixture of solutions. When solutions are made using compound of same state of matter, the final solution prepared would be more homogeneous than using compounds of different state of matter.[50] This formed major part for the research hypothesis and hence hypothesized that incorporation of liquid POSS agent may improve the primary properties of RTV silicone. In order to have complete homogeneous mixture and good miscibility and to ensure uniform dispersion of POSS particles into polymer matrix of RTV silicone, liquid NB 1070 trisnorbornenylisobutyl POSS was chosen.

The tensile strength of POSS-modified RTV silicone significantly increased than conventional RTV silicone after a natural aging for 6 months. The trends in the results for tensile strength and tear strength in the present study were similar with the results of the study conducted by Mohammad et al. in 2010, where the tensile strength was maximum at 1% dimethylvinyl isobutyl and tris-dimethylsilane isobutyl POSS concentration. The strength values were in accordance with the clinically accepted range (2.5–6.5 N/mm2 or Mpa).[19] After incorporation of NB 1070 POSS in A - 2000 RTV silicone, the modulus of elasticity and yield stress decreased significantly both in tension and compression. In tension, the silicone with NB 1070 POSS yield at 14% strain and the modulus reduces with increase in POSS concentration.[50] The yield stress, however, decreases with increase in POSS concentration. In terms of cross-networking density, ideally for NB 1070 POSS-incorporated silicone, the yield stress would increase due to the greater cross - link density. However, this was not observed; the yield stress decreased with POSS loading, thus resulting in increase in resistance to tensile load.[51]

The tear strength of POSS-modified RTV silicone was significantly higher than conventional RTV after a natural aging for 6 months and was in accordance with clinically acceptable values (26–60 n/mm).[29] Tear strength increased on the interaction between the NB 1070 POSS cross-linker and the polymer chains, respectively. NB 1070 POSS being readily miscible has good surface area to maximize the polymer/POSS interactions. Thus, the POSS modified A-2000 RTV silicone resulted in a polymer matrix that is able to withstand significant weathering conditions without deterioration and degradation. The silicone modification by the reinforcing material readily and effectively slides the polymer chains over POSS, thereby making the mixture, a more flexible network which enhances mechanical strength.[52]

Color change calculated as ΔE values for inter-group comparison showed highly statistically significant difference between the groups. POSS modified silicone showing less color change. In addition, ΔE comparisons were further evaluated on color interpretation indices viz. NBS standards and clinical acceptability index. This was done to identify if the samples that showed color change to statistically significant level were clinically acceptable or not. Both the indices interpreted color change in conventional group RTV silicone to be clinically discernible and evident. On the other hand, according to both indices, POSS-modified RTV silicone samples did not show clinically perceivable change in color, thus making it acceptable for clinical use even after 6 months of aging. A direct and indirect comparison of the color stability results obtained in the present study cannot be made with any of the studies available in literature. The present study is first of its kind to evaluate color stability in POSS incorporated maxillofacial RTV silicones.

The reason for high color stability of POSS-modified RTV silicone can be explained on the basis of its chemistry. Alkyl groups break the POSS cage at eight corners of silicone-oxygen bond. The structures of POSS have relative stronger Si-O bonds; however, C-H, C-C, and C-Si are significantly weaker. On exposure to a severe natural aging environment, including tropical heat, high-energy ion beams and oxygen plasma, only Si-O bonds can survive, while others undergo degradation and form volatile organic compounds. More importantly, the bonded Si-O bonds can further form a SiO2-like surface layer on the POSS etching and consumption. Due to exceptional oxidation resistance, POSS structured nano-composite is a promising material that can be utilized for manufacturing photo oxidative resistant materials. Hence, the POSS cage has shown the photo-oxidative stability to polymers through a passivation mechanism.[53],[54]

Baseline comparison of hardness in both the groups showed statistically non-significant difference, indicating that the samples were equally soft at the beginning of the study. After 6 months of aging, there was high marked difference between both the groups, indicating a significant increase in hardness in the conventional group (P < 0.0001). On the other hand, intragroup comparison showed statistical non - significance for the POSS modified RTV silicone and statistically highly significant difference in the conventional silicone group. This means that POSS incorporation helps RTV silicone to retains its softness over 6 months period. Intergroup comparison at 6 months also showed marked significant change between the two groups with POSS-modified group showing lesser loss of softness as compared to conventional RTV silicone. The reason why hardness values did not alter significantly in the POSS modified group can be explained by the nature of POSS bonds in silicone. NB 1070 POSS moieties consist of stronger Si-O bond, which is not degraded even after exposure to environmental factors. The NB 1070 POSS is hydrophobic in nature; hence, the penetration of sebum, perspiration, and water in highly humid climate is significantly reduced. Due to strong Si-O bond, the leaching of the components of RTV silicone is reduced resulting in less rigid material and maintaining its flexibility.

Due to similarity of structure of NB 1070 POSS and maxillofacial RTV silicone, the bond strength remains stronger even after exposure to the natural environmental factors; hence, all the mechanical properties are maintained with little degradation.[47] Hence, NB 1070 is a potent reinforcing agent which has improved all the primary mechanical and physical properties of RTV silicone. Therefore, they null hypothesis is rejected for all the four parameters.

Literature exists on incorporation of various reinforcing agents but is scarce with respect to evaluation of effect of a single agent on all the primary properties viz., tensile strength, tear strength, color stability, and hardness. Further it may be noted that values for all the properties tested in the present study for conventional and POSS-modified RTV silicone were well within the clinically acceptable standards prescribed for clinical use of any maxillofacial silicone.

Results of these study though may not be immediately extrapolated to clinical situation; however, it has given pathways and avenues for further clinical research. The study has a few limitations. The effect of adhesives on the properties of the two groups was not evaluated. In future, the effect of adhesives and an aging time of more than 6 months can be evaluated as clinical studies making the study more externally valid.

  Conclusion Top

Incorporation of 1% NB 1070 trisnorbornenylisobutyl POSS in RTV maxillofacial silicone shows a significant improvement in tensile strength, tear strength, and hardness and improved color stability as compared to conventional RTV silicone after the simulated 6-month accelerated aging procedure. Incorporation of 1% NB 1070 trisnorbornenylisobutyl POSS into RTV silicone improves properties and can prolong the life of maxillofacial RTV silicone.

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Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2], [Table 3], [Table 4]


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