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Year : 2020  |  Volume : 23  |  Issue : 9  |  Page : 1266-1273

Does the plasma application time affect the tensile bond strength between PMMA and a silicone-based denture liner?

1 Department of Prosthodontics, Cukurova University, Faculty of Dentistry, Adana, Turkıye
2 Department of Prosthodontics, Gazi University, Faculty of Dentistry, Ankara, Turkıye
3 Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania, USA

Date of Submission23-Dec-2019
Date of Acceptance08-Apr-2020
Date of Web Publication10-Sep-2020

Correspondence Address:
Dr. E Tamam
Department of Prosthodontics, Gazi University, Faculty of Dentistry, 06510 Emek, Ankara
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/njcp.njcp_692_19

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Aims: This study evaluated the effect of oxygen plasma and argon plasma treatments with different application times on tensile bonding of a silicone-based denture liner to polymethylmetacrylate (PMMA). Methods: Seven groups (n = 5) were prepared and six of them treated by argon plasma or oxygen plasma with 30s, 60s, and 120s, respectively; one group was left untreated served as control. After processing of denture liner, the specimens were deflasked and stored dry for 24 h, and they were then subjected to tensile bond strength testing. Differences in tensile bond strength values were determined using one-way ANOVA (α = 0.05). Results: Highest tensile bond strengths were observed in the oxygen plasma groups, followed by untreated group and argon plasma groups in turn in order. Tensile bond strenght were increased with time for both type of plasma applications tested. Conclusion: This study suggests that the adhesion between PMMA and denture liner is improved under conditions of oxygen plasma treatment with extended exposure time rather than argon plasma treatment.

Keywords: Argon plasma, oxygen plasma, PMMA, silicone denture liner, tensile bonding

How to cite this article:
Soygun K, Tamam E, Dogan A, Keskin S. Does the plasma application time affect the tensile bond strength between PMMA and a silicone-based denture liner?. Niger J Clin Pract 2020;23:1266-73

How to cite this URL:
Soygun K, Tamam E, Dogan A, Keskin S. Does the plasma application time affect the tensile bond strength between PMMA and a silicone-based denture liner?. Niger J Clin Pract [serial online] 2020 [cited 2022 Dec 10];23:1266-73. Available from:

   Introduction Top

The main purpose of the denture liners applied to the tissue surface of denture bases is to increase patient comfort by providing a balanced force distribution and to contribute to retention of the prostheses.[1],[2]

The main drawback related to the usage of denture liners is inadequate adhesion to denture base, especially for silicone-based liners.[3] This needs to be improved by the treatment of polymer surfaces without damaging the structure of polymer. In order to solve the adhesion failure between denture base and denture liner materials, many researches have been carried out by modifying the tissue surface of denture base materials prior to applying the denture liner. The most preferable techniques for surface modification are as follows: chemical modification, chemical modification by radiation, actinochemistry modification, dynamochemical treatment, coupling agent, heat treatment, and surface modification by modifying agent.[2],[4],[5],[6],[7]

Plasma treatment of polymer surfaces is well-introduced technique due to its peerless capability to modify polymer surfaces without affecting their bulk features. It has been shown that substantial changes in the chemical functionality, surface state, wettability, and bonding ability of polymer surfaces could be carried out by plasma treatment.[8],[9] The effect of plasma surface modification mainly depends on the combination of type of the gas and chemical structure of polymer.[8],[10] Up to now, few studies have addressed the effects of plasma treatment on bonding properties of dental polymers.[1],[11],[12],[13] Yavirach et al.[11] investigated the effects of either oxygen, argon, nitrogen, or helium mixed with nitrogen plasmas on adhesion between fiber-reinforced posts and composite core material and reported significant differences between control and other plasma groups. Some of the recent studies have shown that plasma treatment have strengthening effect on the bonding between different type of dental resins.[11],[12],[13],[14],[15],[16] Nishigawa et al.[13] have reported that bond strength between heat-polymerized and auto-polymerized acrylic resins has been increased to a degree that can be achieved with an adhesive primer. As for plasma treatment on silicone rubbers/PMMA, Zhang et al.[1]

reported that 2 min oxygen plasma treatment of an acrylic denture base resin was effective in improving tensile bond strength with an acrylic-based and auto-polymerized denture liner, especially, when the specimens were exposed to air for 1 day after plasma treatment. In another study, Zhang et al.[2] investigated the surface properties and wettability of PMMA following the application oxygen plasma for different durations and reported some changes in chemical composition. Bicer et al.[14] reported that argon plasma treatment with different exposure time have altered the bond strength values between a silicone denture liner and heat-polymerized or auto-polymerized acrylic resins.

The hypothesis tested in this study was oxygen plasma and argon plasma treatments that carried out on PMMA surfaces with different exposure time could increase adhesion (bonding ability) between PMMA and a silicone-based denture liner.

   Experimental Section Top

Materials and experimental groups

In thisin vitro study, the denture liner used was a silicone-based material (Molloplast-B, Detax, Ettlingen, Germany) and the denture base material was a heat-polymerized poly methyl methacrylate (PMMA) acrylic resin (Meliodent, Bayer Dental, Newbury, UK). Seven sample groups were formed and each of the groups consisted of five specimens. Group 1 was control (C) and no treatment was employed, Groups 2, 3, and 4 (AP1, AP2 and AP3) were treated by argon plasma and Groups 5, 6, and 7 (OP1, OP2 and OP3) were treated by oxygen plasma, respectively. To put a finer point on it AP1, AP2, and AP3 refer to argon plasma applied groups for 30s, 60s, and 120s, respectively; OP groups are the same as those of oxygen plasma [Table 1]. Plasma treatments on PMMA surface were carried out with oxygen and argon gases using a plasma system (Diener Pico Type P100, Diener Electronic, Germany).
Table 1: Summary of the test groups

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For tensile bond strength testing, 35 acrylic 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. Denture base resin was polymerized in the sealed molds keeping them in water at 70°C for 1 h followed by boiling in a water bath for 30 min. Then the specimens were removed from the molds and 3 mm sections were cut out from the narrow midsection of specimens using a water-cooled diamond edge saw (Model No. 11-1280- 250, Buhler Ltd., Lake Bluff, IL, USA) in order to create spaces for denture liner placement. PMMA surfaces to be bonded with denture liner were smoothed using 240-grit silicone carbide paper, cleaned and dried. Seventy similar acrylic resin blocks corresponding to 35 acrylic resin specimens were thus obtained. [Figure 1] shows the schematic view of the test specimen.
Figure 1: Schematic view of the test specimen. From a to c, how the specimen was prepared is represented

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In the first group, the PMMA surfaces to be bonded with denture liner were smoothed using 240-grit silicone carbide paper, cleaned and dried. After that, primer agent (Primo, Detax, Ettlingen, Germany) was applied onto the bonding surfaces. After waiting 1 h, Molloplast B was placed and processed for 2 h in boiling water as recommended by the manufacturer. Specimens in Groups 2–7 were produced following the procedures described above except that the surfaces of the acrylic resin to be bonded with the liner were treated differently: The surfaces to be bonded with denture liner in experimental groups 2, 3, and 4 were treated by argon; and the experimental groups 5, 6, and 7 were treated by oxygen in the plasma chamber. Immediately after the plasma treatment, primer agent was also applied to the testing groups. Then all the specimens were stored dry for 24 h before testing, and experiments were carried out at room temperature.

Plasma application

The plasma treatment system used in this study is Diener Pico plasma generator (13.56 MHz/200 W) with a cylindrical vacuum chamber made of stainless steel (30 cm height, 15 cm diameter). In the typical experiment as in a previous study,[15] the samples were introduced by a load-lock system and placed on a grounded aluminum holder; the distance between plasma and the specimen was about 6 cm. The chamber was then closed, and pressure was created in the system. The argon or oxygen was introduced into the generator under the pressure and flow-rate conditions after plasma generator was decontaminated, respectively. When the plasma source was on, the discharge was sustained for the desired frequency and reaction time values at near the room temperature. The gas pressure was fixed at 1 mbar, flow rate of gas was adjusted at 2.50 cm3 per min and the discharge power was set to 60 KHz. The following experimental conditions were employed during the plasma treatments: treating time was set to 30, 60, and 120 s, respectively. At the end of reaction, the power was disconnected and the base pressure was restored in the system. The generator was then repressurized by introducing air, and the specimens were removed.

Tensile test

Tensile bond strength test was performed on a universal testing machine (Instron, Model no 3367, Norwood, Massachusetts, USA) using a cross-head speed of 1 mm/min. Tensile bond strength was calculated from the formula:

S = F/D

where S is tensile bond strength (MPa); F is the force (N); and D is the adhesion surface area (mm2).

Fractured samples were examined with a stereomicroscope to determine the mode of failure. Failure modes were classified as adhesive, cohesive or mixed.

Statistical analysis

All data was statistically compared using one-way analysis of variance and Tukey's test due to the homogeneity of variances was used for post-hoc comparisons between experimental groups. All data were analyzed at a significance level of 0.05 using a software package (SPSS 15.0 for Windows Evaluation Version, SPSS Inc., US).

   Results Top

Means and standard errors (SE) of tensile bond strength regarding different plasma treatments and application periods are presented in [Table 2]. Amongst control and experimental groups, tensile bond strength has ordered in decreasing way as follows; OP Groups, Control, AP Groups, respectively. The results showed that the highest mean tensile bond strength value was obtained for the OP3 group (2.570 MPa) followed by OP2 and OP1 groups (1.849 MPa, 1.703 MPa), respectively.
Table 2: Statistical significance between the groups

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For oxygen plasma treatment groups, OP3 was found statistically different from the others and control group (P < 0.05). In between OP1 and OP2, an increase in tensile bond strength was found, but not statistically significant [Table 2].

As to argon plasma treatment groups, mean tensile bond strength in descending order was AP3, AP2, AP1, respectively, as well of OP groups. All the AP groups showed insignificantly lower tensile strength values with respect to control except for AP 1 (P < 0.05, [Table 2] and [Figure 2]).
Figure 2: Summary of tensile bond strength

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The fracture mode was cohesive for all OP groups. For control group, failure mode was predominantly cohesive; adhesive failure was keep out of sight. As concerns to AP groups, failure modes differed from application phase to the other. Namely, almost all specimens of AP1 group were exhibited adhesive failure; whilst AP2 group had predominantly mixed failure mode, AP3 group had approximately equal failure modes between cohesive and mixed [Table 3]. For all specimens, cohesive failure occurred in the liner material.
Table 3: The Failure Modes of the Specimens†

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   Discussion Top

The fourth state of the matter is known as the plasma state which is composed of highly induced atoms, molecules, ions, and radicals. This unusual and reactive chemical state arises from radio frequency, microwave, or electrons from a hot filament discharge that excite gases into energetic states and several reactions occur between the plasma and the surface. The surface characteristics of inert materials can be altered by the ionized and excited molecules that existed in the high-density plasma. Plasma-based techniques not only unite the benefits of conventional and ion beam techniques but also advance the adhesion strength and surface characteristics of the materials by modifying their surface energy.[16]

So far, few independent data are available regarding the effect of implementation time of plasma on bonding. In this study, effect of the plasma treatment on tensile bond strength of Molloplast B to PMMA denture base material was investigated due to different application periods.

In this study, we used a silicone-based denture liner and a heat-polymerized PMMA denture base material because of superior bond strength values between them and because of their clinical preference by comparison with others.

There have been several recommendations as to the appropriate thickness of long-term denture linings.[17],[18],[19],[20] Although the optimum thickness varies between the brands and/or ingredients, proposed thickness is 2–3 mm. Kawano et al.[17] highlighted that a 2.4 mm layer of soft denture lining material demonstrated good shock absorption. Schmidt and Smith[20] stated that the optimum thickness must be 3 mm and this is necessary for the durability of the resilience of the soft lining material. In this study, thickness of the soft lining material was 3 mm. While preparing the samples, 240-grit silicon carbide papers were used to smooth the surfaces; decrease in grit sizes can affect the surface roughness values and change the results. In our study, there were seven groups, each containing five samples, and two different plasma systems applied in three different periods. In order to perform the binding tests between PMMA samples and silicone-based denture liner, 70 PMMA samples were prepared as shown in [Figure 1], so that a total of 420 times (70 PMMA, 2 plasma systems, 3 periods) plasma was applied.

Plasma conditions are very important for the plasma surface activation and contamination removal. The important factors of plasma process include gases, input power, operating pressure, plasma exposure time, location of the sample in the chamber, and electrode configuration. All of the parameters should be determined carefully for the different applications. Principally, a lower operating pressure needs to be applied in argon plasma process as it is a physical plasma process. However, a higher operating pressure is necessary in oxygen or other reactive gas plasma as chemical reaction is dominant on the surface.[8],[9],[10] Nevertheless, in the current study, except plasma exposure time, all the other plasma parameters were set similar to investigate the time effect only.

Plasma treatment is probably the most versatile surface treatment technique which can be used in many different areas like medical devices and materials by improving biocompatibility and adhesion characteristics, as well as dentistry.[21],[22],[23],[24] Argon, oxygen, nitrogen, fluorine, carbon dioxide, and water can be used in plasma chambers to produce the unique surface properties required by various applications. For example, oxygen-plasma treatment can increase the surface energy of polymers, whereas fluorine-plasma treatment can decrease the surface energy and improve the chemical inertness. Cross-linking of a polymer surface can be enhanced by inert-gas plasma. Modification by plasma treatment is usually confined to the top several hundred angstroms and does not affect the bulk properties. The main disadvantage of this technique is the requirement a high vacuum system, which increases the cost of operation.[24]

Inert gasses such as helium, neon and argon (especially argon depending its relatively low cost), oxygen and oxygen-containing plasmas are most commonly employed to modify polymer surfaces[25] so in the current study, argon and oxygen plasma treatments were chosen. The results of the current study indicated that argon plasma led to a decrease in bond strength with respect to control, conversely to oxygen plasma. Zhang et al.[1] have reported 2.803 MPa tensile bond strength value for untreated samples, in our study we found 1.654 MPa for the same group. This difference might depend on the different brands and/or type denture liner material tested, which was resin based in the mentioned study,[1] whereas we used silicone based one. Although tensile bond strength has increased with time; AP groups, all, exhibited lower bonding values vary from 0.976 MPa to 1.516 MPa in comparison with control. Especially, 30s argon plasma application reduced the bond strength approximately two times (P < 0.05). The mechanism of PMMA surface modification and the effect on the adhesion properties depends on a variety of plasma parameters. Hook et al.[26] investigated the incorporation of hydroxyl groups on PMMA surfaces by means of Ar/H2O RF plasma treatment. They recommended a two-step mechanism for the structural change on the top surface and within a subsurface layer up to 20 nm thick. Surface modification by using plasma allowed to provide special functional groups with uniformity and is applicable for almost all polymer substrates regardless their structure and chemical reactivity.[27],[28]

Although modification of the surfaces has been claimed to be efficient on adhesion, for that matter, there is also disagreement as to weaken the bond strength caused by such modifications. Jacobsen et al.[4] and Minami et al.[5] have reported the probable reason of the reduced bond strength was an insufficient wettability of irregular pores produced by lasing or sandblasting and resultant stresses at the liner–denture base interface caused by mechanical roughening of the denture base material, respectively. Chemical modification has also been found likely to partial dissolution of denture base and its further fracture in service while clinically in service.[29] As explained in the literature, it is very difficult to achieve wetted PMMA surface and to provide an improved bonding between denture liner and acrylic resin without modifying its surface. Additionally, surface modification is desired to succeed in expected features without that affecting the bulk characteristics of denture base.[30] Liebermann et al.[31] showed that Argon plasma application has been shown to reduce bond strength and have no impact on surface roughness. Stawarczyk et al.[32] also stated that plasma treatment had no impact on bond to resin cements. In addition to these, plasma application could not enter the routine due to factors such as the impracticality of processes and their high costs.

The molecules nearby the surface of the polymer gain functionality and form cross-linked polymer chains when they are exposed to appropriate plasma density which is applied in a sufficient amount of treatment time. The typical plasma irradiation process starts with creation of radicals in the middle of the polymer chains by hydrogen abstraction. Plasma gas also creates simple radicals, and recombination occurs between these radicals and simple radicals to create oxygen or nitrogen functionalities. Instead of ions, radicals occurred during plasma process have a crucial role on the adhesion. In general, polymeric materials are in hydrophobic form. Changing the form of polymeric materials from hydrophobic to hydrophilic form mostly enhances the adhesion strength and relevant surface properties. Creating oxygen functionality by plasma application is one of the most beneficial and efficient method to modify the polymeric surfaces. Although oxygen plasma is generally preferred, other kinds of plasma applications such as carbon dioxide, carbon monoxide, nitrogen dioxide, and nitric oxide may alter the characterization of the surface from hydrophobic to hydrophilic form.[33],[34],[35] Results of the current study with the highest bond strength levels of OP groups has also confirmed that the mentioned oxygen functionalities have much more formed, whereupon the surface became more hydrophilic by oxygen plasma rather than argon one.

Since it is very difficult to produce a material that meets all needs alone, the general attitude is to apply some improver treatments to the material to enhance the surface properties. The surface properties can also be selectively modified. Plasma treatment can result in changes of a variety of surface characteristics, for example, chemical, electrical, optical, biological, and mechanical. Polymers exposed to the plasma application exhibit the same chemical and physical properties as their original state. Their composition, chemical structure, and degree of polymerization of the treated polymers are hardly altered and are similar to those of the original polymers.[33] In this study, alterations presumably sourced from surface activation, micro-etching, chain scission, cross-linking, cleaning of surface contamination and/or enhance in wettability and critical surface tension.

Polymers have been exposed to low-power plasma of noble gas ions for one second to several minutes. This exposure is sufficient to remove the hydrogen and to form free radicals at or near the surface, which then interact with unsaturated groups to cross-link with chain scission. The application of plasma also removes low-molecular-weight polymers or transforms them into high-molecular-weight polymers by cross-linking reactions. As a result, the weak boundary layer formed by the low-molecular-weight polymers is removed and much greater adhesive strengths are observed. This treatment has been named as CASING (cross-linking by activated species of inert gases).[35] In this study, argon plasma with prolonged periods might be resulted in the adhesion of weak boundary layer to the surface, whereas more free radicals may have produced by oxygen plasma with time.

Competitive modification and degradation reactions occur when plasma gas interacts with polymer surface. Ion beam interaction, plasma-graft co-polymerization, and plasma polymerization change the characteristics of the polymer when the modification reaction becomes dominant. When degradation dominates the reaction process, the layers of the polymer surface that exposed to plasma for a long time will be etched. Plasma etching of polymers with argon plasma application comes into prominence in the biomaterial researches. Both the nature of the polymeric material and the energy of the plasma affect the rate of weight loss of the material. The high and low plasma susceptibilities of the polymer change due to including oxygen functionality and polyolefin with no substituent, respectively. The outer layers that are exposed to plasma irradiation have higher rate of weight loss than the inner layers.[36],[37] Our study showed that modification effect might be dominated when the plasma has been chosen as oxygen. Conversely, degradation could be clear for the AP groups with the lowest bond strength values even obtained in the control.

Because of their different molecular structures, silicone-based denture liner does not bond readily to the acrylic resin base material, so an adhesive needs to be employed. The fundamental problem is that resins are hydrophobic, whereas silicon-based molecules are hydrophilic due to a surface layer of hydroxyl groups. This adhesion can be achieved using silicon polymer dissolved in a solvent, or by the use of an alkyl-silane coupling agent. Coupling agent is an intermediary substance can be used that is able to bond to both of the materials in question.[38] According to the instruction form of the manufacturer, chemical structure of the Molloplast B primer is mixture of methoxy and ethoxy silane derivates. More precisely, in a previous study, spectroscopic analysis showed that the main component of the primer agent was 3-methacryloxypropyltrimetoksi silane.[39] Methacryl end of this chemical compound is compatible with PMMA and the reactive end with the denture liner. Methacryl groups swells the outer surface of PMMA and allows penetration of the adhesive.[29],[40] The reactive groups are hydroxyl groups which are attracted to the hydroxyl groups of the liner.[41] The initial criterion for adhesion is met, min that intimate contact at the molecular level between the adhesive and the substrate is achieved. If the surface of the substrate is rough enough, the adhesive can penetrate readily into the pits before begins to set.[36],[39] Plasma application, in this study, probably alter the surface roughness of PMMA specimens and thus wettability. Cohesive failure modes seen in the all OP groups may be attributed to the homogenous and decreased pore sizes resulting in increased surface area, increased surface energy, and wettability. Although the cohesive failure observed at the fracture surface of all specimens in OP groups, only the best bond strength results in OP3 can be associated with increased surface roughness, wettability and thus enhanced surface properties with increased application time. AP1 group exhibited predominantly adhesive failure mode indicated that 30 s Argon plasma application is not effective on the bond strength between PMMA and the liner. Even if the extended periods of Argon plasma enhanced the bond strength, failure modes revealed its ineffectiveness at the PMMA- primer interface. Since cohesive failure modes only obtained in the denture liner material for all groups, it can be claimed that hydroxyl end of primer is well matched to the surface of denture liner. These result suggest that oxygen plasma treatment may be clinically more effective.

In short, pursuant to the results of this study the hypothesis could be accepted for only Oxygen plasma application whereas rejected for Argon plasma application. PMMA surfaces subjected to Oxygen plasma ever-increasingly showed significant enhance in the bond strength between PMMA–silicone interfaces.

   Conclusion Top

Plasma treatment of PMMA for improving bond strength still needs to be investigated in detail. Chemical and advanced surface analyses may be useful to define the exact reasons that how the bonding ability is vary one to the other. This study suggests that the adhesion between PMMA and denture liner is improved under conditions of oxygen plasma treatment with extended exposure time rather than argon plasma treatment. Further studies with larger sample sizes are needed for better understanding of this matter.

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Conflicts of interest

There are no conflicts of interest.

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  [Table 1], [Table 2], [Table 3]

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