Denture soft lining materials are used in prosthodontics to provide a cushioning layer on the fitting surface of a complete denture1 in the management of traumatized oral mucosa, bruxism, chronic soreness, residual ridge atrophy or resorption, relatively thin and nonresilient mucosa,2 bony undercuts, and for congenital or acquired oral defects requiring obturation.3,4 Additional uses of soft denture liners have emerged in the last few years for transitional prostheses after implant surgery5 When these materials come into contact with fibromucous membranes, they act as shock absorbers, allowing improved comfort and a more homogeneous distribution of occlusal loading forces.3

There are 2 types of resilient lining materials: plasticized acrylic resins and silicone elastomers.2,6 Acrylic resin-based liners contain plasticizers which are responsible for material softness and the leaching of them results in hardening of the liner with time. Silicone-based soft denture liners do not require an external plasticizer7 and inherently soft over a long period.4,8 Furthermore, both autopolymerizing and heat-polymerizing forms of liners are available.2

Silicone-based soft denture liners are similar in composition to silicone impression materials. Both are dimethylsiloxane polymers.5 Polymethyl methacrylate (PMMA) denture base resins and silicone-based lining materials have different molecular structures and they cannot be chemically bonded. A reliable bond between denture bases and soft liner is required for the denture to function properly. Therefore, researchers have attempted to identify other methods to improve the PMMA/resilient liner bond.4,9,10,11,12,13,14,15,16 In the dental literature, the difficulty of obtaining a reliable bond between soft relining materials and PMMA denture base polymers has often been discussed.17 Several studies have been conducted to increase bond strength between liners and acrylics by improving roughness at the acrylic interface using lasers,1,4,9,14 alumina abrading,4,9,10,15,18 chemical etching or primers, acrylic burs, and net woven glass fiber.19

Both 2-hydroxyethyl methacrylate (HEMA)20,21 and isobutyl methacrylate (iBMA)22,23 were used to improve the mechanical properties of PMMA. According to Johnson and Jones,18 increasing the concentration of ethyl and butyl methacrylates resulted in a linear decrease in the mechanical properties measured except molecular weight. Moreover, Vargün et al.24 researched on PMMA copolymerized with HEMA and reported that glass transition temperatures of copolymers were decreased from 119℃ to 100℃ with an increasing amount of HEMA. The Tg of PMMA is lowered by copolymerization with HEMA, which may ease the processing conditions.

Many of the studies have measured the bond strength between the resilient liners and the denture base materials with peel, shear, tear, and tensile bond tests.11,25,26,27 The measured bond strength of resilient liners to PMMA is dependent on the nature of the test method used.25,28

The purpose of this study was to investigate the bonding properties of denture bases to silicone-based soft denture liners immersed in iBMA and HEMA for various lengths of time. The hypothesis tested was that immersion of resilient lining materials into HEMA and iBMA is not an effective method to improve the strength of the bond between resilient liners and PMMA.

A silicone-based soft liner (Lot number: 08034, Permaflex; Kohler, Neuhausen, Germany) and a heat-cured acrylic resin (Lot number: 01.06.30 275, Paladent; Heraeus Kulzer, Hanau, Germany) were used in this study. For tensile bond strength testing of specimens, gypsum (Moldabaster S; Heraeus Kulzer GmBH, Hanau, Germany) molds were prepared with dumbbell-shaped brass patterns, 75 mm in length, 12 mm in diameter at the thickest section, and 7 mm at the thinnest section. The heat-cured specimens were prepared in the molds in denture flasks and were cured in a manner similar to that used in conventional denture construction according to the manufacturer's instructions. Then, 3 mm of the material were cut off from the thin midsection using a water-cooled diamond edge saw (Model No. 11-1280-250; Buhler Ltd., Lake Bluff, IL, USA). Finally, 120 test specimens were prepared. In addition, the PMMA blocks were placed back into the molds and specimens were randomly assigned to five groups (n=15), according to resilient liners immersed into chemical materials.

For each group, 10 g resilient liner was put into a glass and a chemical solution (iBMA or HEMA) was poured in to the glass until the resilient liner was completely covered with the chemical solution. After immersion procedures, relining materials were packed into the space in the molds. The specimens and relining materials were then polymerized according to manufacturer's instructions (2 H in boiling water). The processed molds were left to cool at room temperature for 20 minutes, and then placed under running tap water for 10 minutes. Then, specimens were stored in distilled water at 37℃ for one week. All specimens were placed under tension until failure in a universal testing machine (Lloyd LF Plus; Ametek Inc, Lloyd Instruments, Leicester, UK) at a crosshead speed of 5 mm/min. The maximum tensile stress before failure was recorded for each specimen. Failure strength was recorded in Newtons. Modes of failure was visually determined for every specimen after testing and categorized into one of the following types: Adhesive failure; refers to total separation at the interface between the resilient liner material and acrylic resin, cohesive failure; refers to tears within the resilient liner material, and mixed failure; refers to both.

The mean value and standard deviation of the specimens were statistically evaluated by 1-way ANOVA and post hoc Tukey-Kramer multiple comparison tests (α=0.05) by using a SPSS statistical software program (version 13.0, SPSS Inc., Chicago, IL, USA).

Statistical results for tensile bond strength measurements of the groups are summarized in Table 1. The results of the 1-way ANOVA was F=582.184, P=.05. The highest mean value of (tensile) force was observed in Group 3 specimens, followed by Group 2 specimens. Tukey's HSD test showed that the differences between Group 1 and Groups 2, 3, and 5 were found statistically significant (P<.001), whereas there was no significant difference in bond strength between Groups 1 and 4 (P=.063). Analysis of the data also revealed significant differences between Groups 2 and 3 (P<.001) and Groups 4 and 5 (P<.001).

Modes of failure are presented in Table 2. All specimens of the groups dominated in adhesive failures. 80% of Group 1 specimens, 100% of Group 2 and 3 specimens, and 66% of Group 4 and 5 specimens presented adhesive failures. Moreover, it can be seen that elongation of the resilient liner was the highest degree in Groups 2 and 3 (Fig. 1, Fig. 2, Fig. 3, Fig. 4, and Fig. 5). Both cohesive and mixed failures were prevented by iBMA immersion.

The results of the present study support rejection of the hypothesis because immersion of resilient lining materials into HEMA and iBMA is an effective method to improve strength of the bond between resilient liners and PMMA. However, immersion of resilient liner into HEMA for 1 min was found to be ineffective for increasing the strength of the bond. In the literature, there are no studies about resilient liner treatments with chemical materials. However, only a few studies have been conducted on treatment of PMMA with chemical materials including HEMA and iBMA.20,21,22,23 In addition, Keyf et al.21 advocated that surface treatment process of glass fiber with HEMA monomer and air atmosphere increased the transverse strength and the maximal deflection of a provisional fixed partial denture resin. Furthermore, a conventional heat-polymerized denture base polymer powder was mixed with ethyl, isobutyl and tert-butyl methacrylate monomers respectively by Doğan et al.22 They found that there was no difference in tensile strengths between the polyacrylates and the control group. In addition, Çökeliler et al.20 evaluates the effect of plasma treated E-glass fiber to improve the mechanical properties of PMMA and used three different types of monomer HEMA, triethyleneglycoldimethylether (TEGDME) and ethylenediamine (EDA). They found that there was no statistical difference in the flexural strength between HEMA treated groups and untreated groups. Moreover, Leles et al.23 treated bonding surfaces of hard chairside reline resin and denture base material with methyl methacrylate (MMA) monomer, iBMA monomer, chloroform, acetone, and experimental adhesive. They reported that MMA monomer and chloroform surface treatments produced the highest mean transverse bond strengths, and this was followed by iBMA monomer. The results of the present study were consistent with that of Keyf et al.21 however, contradicted with those of Çökeliler et al.,20 Doğan et al.,22 and Leles et al.23 Furthermore, Leles et al.23 performed surface treatment with iBMA for 180 seconds. Thus, in a preliminary study, 1, 2, 3, and 5 minutes were carried out for immersion time of soft liner. Based on the results of this preliminary study, 1 minute and 3 minutes immersion time were used in the present study.

Effective bond strength between liners and denture bases can be characterized by either high bond values or cohesive bond failures within the lining material.17 Usumez et al.4 reported that lased specimens predominated in mixed failures, but control and alumina-abraded specimens showed adhesive failures. Moreover, according to Kulak-Ozkan et al.7 34 showed adhesive failures, 15 showed cohesive failures, and 23 showed mixed failures out of 72 specimens. In addition, Hatamleh et al.19 reported that all specimens exhibited cohesive failure only because of net fibers. In the present study, in accordance with Usumez et al.4 iBMA groups showed the highest bond strength and adhesive failure only. This result was not in accordance with those of Kulak-Ozkan et al.7 and Hatamleh et al.19 These findings are understandable and can be explained in that the molecular binding of the resilient liner was improved. According to the results of the present study, the iBMA immersion of soft liners significantly increased the tensile strength of the specimens, while all the specimens revealed adhesive failure. It is seldom that specimens with the highest bond strength demonstrate adhesive failure. The explanation of these results is that the physical properties of liners are changed after iBMA immersion. Moreover, it can be seen that elongation of the resilient liner after being immersed into iBMA was higher than both the control and HEMA groups. This indicates that iBMA enables resilient liners to be more resistant against tears or ruptures which is greater than tensile bond strength between resilient liners and PMMA. Therefore, 100% adhesive failures were seen in the iBMA specimens. In addition, another explanation takes into account the increase in tensile bond strength between liners and PMMA. The molecules in the PMMA and chemical materials interacted with each other. Methacrylate substances with high numbers of alkyl groups could interact with C-H groups and form hydrogen bonds. Moreover, application of chemical agents possessing solvent effects on the acrylic resin can cause the formation of roughened surfaces, and this in turn, positively affects the strength of the bond. FTIR is important for determination of the secondary interactions and study of molecular structure.12 However, FTIR analysis could not be performed on the specimens, because the soft structure of the liner prevents getting samples from it for analysis.

On the other hand, researchers2,4,14,17 studied on the bond strength of relining materials to denture base without aging procedures. Furthermore, Kulak-Ozkan et al.7 investigated the effect of thermocycling on tensile bond strength of six silicone-based soft denture liners and no significant difference in tensile bond strength of Permaflex was found after thermocycling. Thus, in the present study, aging of the specimens was not performed.

Within the limitations of this study, chemical treatment of soft denture liner before packing denture base resin could be performed as a part of denture fabrication.