Bonding metals to poly(methyl methacrylate) using aryldiazonium salts

Highlights

  • Diazonium can be used to improve the mechanical properties of dental devices.

  • Diazonium treatment improved the bonding strength between alloys and PMMA.

  • Diazonium treatment can be applied on dental alloys in a two steps.

  • Benzoyl peroxide was added in the treatment to increase PMMA polymerization rate.

Abstract

Objectives

Many dental devices, such as partial dentures, combine acrylic and metallic parts that are bonded together. These devices often present catastrophic mechanical failures due to weak bonding between their acrylic and metallic components. The bonding between alloys and polymers (e.g. poly(methyl methacrylate), PMMA) usually is just a mechanical interlock, since they do not chemically bond spontaneously. The aim of this study was to develop a new method to make a strong chemical bond between alloys and polymers for dental prostheses based on diazonium chemistry.

Methods

The method was based on two steps. In the first step (primer), aryldiazonium salts were grafted onto the metallic surfaces. The second step (adhesive) was optimized to achieve covalent binding between the grafted layer and PMMA. The chemical composition of the treated surfaces was analyzed with X-ray photoelectron spectroscopy (XPS), and the tensile or shear bonding strength between metals and poly(methyl methacrylate) was measured.

Results

XPS and contact angle measurements confirmed the presence of a polymer coating on the treated metallic surfaces. Mechanical tests showed a significant increase in bond strength between PMMA and treated titanium or stainless steel wire by 5.2 and 2.5 folds, respectively, compared to the untreated control group ( p < 0.05).

Significance

Diazonium chemistry is an effective technique for achieving a strong chemical bond between alloys and PMMA, which can help improve the mechanical properties of dental devices.

Introduction

Chemical bonding between alloys and polymers does not occur spontaneously; indeed, composite materials that combine polymers with alloys often suffer from mechanical failure at the interface between them. One example of this challenge is dental devices, which often present catastrophic mechanical failures due to weak bonding between their metallic and polymeric components . These devices include dental prostheses, combining metallic frameworks and wrought wires with acrylic resin; and orthodontic appliances, combining acrylic resin with stainless steel wrought wires.

Poly(methyl methacrylate) (PMMA) is extensively used in denture materials for dental prostheses and orthodontic devices because of its biocompatibility, excellent esthetic, and mechanical properties . Titanium (Ti) is increasingly used in dental implants, implant abutments, and milled prostheses because of its excellent mechanical properties (i.e. strength to weight ratio) and biocompatibility . PMMA and Ti in dental prostheses are usually bonded by mechanical interlocking the PMMA into the irregularities of the Ti surface . Further improvement in the bonding strength between Ti and PMMA is still needed to prevent debonding, which are otherwise common in clinical practice, and reduce microleaks at the Ti/PMMA interface that causes accumulation of oral debris and discoloration of denture base materials . Ti–PMMA bond can be strengthened by adding a chemical link between PMMA and Ti, since Ti and PMMA do not chemically bind together spontaneously .

Several methods have been tested to increase the bond strength between polymers and alloys in dental prostheses . Table 1 summarizes the strengths obtained by binding PMMA and Ti with different methods, measured with shear bond, four-point bending, and tensile strength tests (it should be noted that the reported values are not comparable with each other as the specimen size, test methods and practice vary between different studies). In general, higher bond strengths are reported for the four-point bending and the shear bond tests (values ranging between 25.5–42.5 MPa and 7.0–46.6 MPa, respectively), while the lowest values are obtained with the tensile strength test (0–23.5 MPa). Indeed, the latter test is the most challenging one; however, it is also the most accurate technique to measure bond strength because it applies a direct and uniform force to the surface . On the contrary, the shear bond and four-point bend tests do not distribute stress uniformly on the surfaces being tested .

Table 1
The bond strengths between titanium and PMMA (MPa) using different bonding methods.
Bonding agent (Commercial name) Surface Topography Type of PMMA used (Commercial name) Testing Technique Bond Strength (MPa) a Ref.
None Sandblasted Self-cured with EGDMA and TBB (Super-Bond C&B) Shear bond 38.1 ± 2.3
Heat-cured Tensile strength 20.0
Shear bond 16.1 ± 1.6
3.2 ± 0.4
Self-cured with BP (Multi-Bond) 13.6 ± 1.6
Self-cured 9.9
MHPA (AZ Primer) 46.6
MDP and VTD (Alloy Primer) 45.7
Self-cured with EGDMA and TBB (Super-Bond C&B) 39.8 ± 2.0
Self-cured with BP (Multi-Bond) 22.0 ± 6.6
Heat-cured 27.5 ± 4.0
16.0 ± 3.6
MDDT and MHPA (Metal Link Primer) Self-cured 45.4
Self-cured with EGDMA and TBB (Super-Bond C&B) 39.6 ± 2.5
Self-cured with BP (Multi-Bond) 16.5 ± 2.3
MATP (Espe-Sil) Polished Heat-cured 0.0
Sandblasted 5.9 ± 2.1
MATP Self-cured Tensile strength 14.3
MATP (Silicoater M D) Shear bond 21.9± 1.7
MATP then Silica-coating (Espe-Sil; Rocatec System) Heat-cured 16.2 ± 2.3
Silica-coating (Rocatec System) Self-cured 38.7
Heat-cured 23.8 ± 1.7
META Heat-cured (Trevalon) Four-point bend 31.9 ± 1.5
Heat-cured (Metadent) 42.5 ± 2.2
Heat-cured Tensile strength 21.0
META (Super bond) Shear bond 19.1 ± 8.9
META (New Metacolor) Self-cured 21.5 ± 2.2
MDP (Estenia Opaque Primer) 42.7
Heat-cured 7.0 ± 3.0
MDP Tensile strength 23.5
Self-cured with EGDMA and TBB (Super-Bond C&B) 21.2 ± 4.7
16.2 ± 5.9
MDP (Cesead) Self-cured Shear bond 19.0 ± 2.2
MEPS (Thermoresin) 14.0 ± 0.6
MPS and n-propylamine Polished Heat-cured Four-point bend 25.5 ± 6.4
MAC (MR Bond) Sandblasted Self-cured Shear bond 7.4 ± 2.1
DOPA 1.8

Abbreviations : BP: benzoyl peroxide; DOPA: 3,4-dihydroxyL-phenylalanine; MAC:11-metacryloyloxyundecan 1,1-dicarboxylic; MATP: Methacryloxypropyl-trimethoxysilane; MDDA:10-methacyloyloxydecyl 6,8-dithioctanoate; MDP:10-Methacryloyloxydecyl dihydrogen phosphate; MDDT:10-methacryloxydecly 6,8-dithiooctanoate; MEPS: methacryloxydecly thiophosphate derivative; META: Methacryloxy ethyl trimellitate anhydride; MHPA:6-Methacryloxyethexy phosphonacetate; MPS:3-Methacryloxypropyl trimethoxysilane; TBB:tribuylborane; VTD:10-Methacryloyloxydecyl dihydrogen phosphate.

a It should be noted that the bond strength values are not comparable with each other as the specimen size, test methods and practice vary between various studies.

Most of the methods reported in Table 1 require sandblasting the metallic surface, and all of them use either silane or phosphonate groups to create a chemical bond between the two surfaces . Silanes and phosphonates covalently bind to Ti, while sandblasting increases the surface area of the exposed Ti, thus increasing the overall bonding strength . The highest bond strengths reported were achieved using phosphonate-based adhesives (MHPA, MDP, and VDT; see Table 1 ) in combination with sandblasting. Specifically, the highest tensile strength reported without sandblasting was 7.4 MPa , while using a combination of sandblasting and bonding agents the tensile strength went up to 23.5 MPa . These values are still too low for dental applications. Overdentures, for example, have to resist biting forces of up to 662.2 N, and pressures of up to 51.1 MPa . This implies that masticatory forces can exceed the strength of the Ti–PMMA bond and lead to prosthesis failure. An ideal goal would be to have a metal/PMMA interface that is at least as strong as PMMA alone, which has a tensile strength of 65 MPa .

Another example of metal–acrylic interface found in dental applications is that between wrought wires and acrylic-based dental devices such as dental prostheses and orthodontic appliances . Wrought wires are usually made of stainless steel or cobalt–chromium alloys, which both lack the ability to bind chemically to acrylic resins . Surprisingly, improving the adhesion between wrought wires and acrylic has hardly been investigated. Dental devices combining wrought wires with acrylic such as acrylic removable partial dentures usually cannot be made when not enough volume of PMMA is available to support the wire . By increasing bond strength between stainless steel wrought wire and PMMA through this treatment, more leverage is possible for fabricating acrylic-based dental devices when not enough volume of acrylic is available to support the wire. In this paper we will show a technique to improve the binding between PMMA and alloys used in dental applications based on diazonium chemistry.

Aryldiazonium salts have been used to modify material surfaces for many applications . Diazonium ions can be produced from aromatic amines and grafted onto almost any surface, including metals, glass, and carbon . Initially, diazonium grafting was performed using electrochemical reduction, but recently this has been achieved using chemical reducing agents in acidic solutions . The reducing agents transform the aryldiazonium salts into aryl radicals, which can covalently bind to the surface of interest . If an extra amino group is present on the aryldiazonium precursor, a polyaminophenylene (PAP) layer is formed on the metallic surface. The amino groups sticking out from the PAP layer can be further activated in a second step, and used to bind a second layer onto the original surface . In this work, we optimize such second step to bind PMMA and metals for dental applications. The aim of this study was to develop a new method of creating a strong chemical bond between alloys and polymers for dental prostheses based on diazonium chemistry.

Materials and methods

Materials

Poly-methyl methacrylate (PMMA) and methyl methacrylate (MMA) were obtained from Great Lakes Orthodontics (Tonawanda, NY), and were used without any further purification. The rest of the reagents were obtained from Sigma Aldrich (St. Louis, MO). P-phenylenediamine (PPD), sodium nitrite (NaNO 2 ), sodium dodecyl sulfate (SDS), benzoyl peroxide (BP), and iron powder (Fe) were used as received. Concentrated hydrochloric acid (HCl) was diluted in distilled water (DW) to a concentration of 0.5 M.

The metallic samples used in the experiments were either orthodontic wrought wires (stainless steel) or polished rectangular bars (Ti). The wrought wires (Tur-Chrome S.S, Rocky Mountain Orthodontic, Denver, CO) had a diameter of 0.6 mm and were cut into 200.0 mm long sections. The Ti samples (Ti alloy grade 2, McMaster-Carr, Cleveland, OH) were obtained as rectangular bars (6.4, 12.7 and 305.0 mm) and cut into smaller sections (12.7, 6.4, and 6.4 mm) using an abrasive cutter (Delta AbrasiMet, Buchler, Whitby, ON).

Preparation of the metallic samples

The Ti samples were polished using a six step polishing method to obtain a flat surface. First, they were polished by means of a water-cooled trimmer and 240–600 grit silicon carbide papers (Paper-c wt, AA Abrasives, Philadelphia, PA). Then, they were further polished on a polishing wheel (LapoPol-5, Struers, Rodovre, Denmark) using two types of polishing cloths; rough-to-intermediate polishing cloth (15–0.02 μm; TexMet C) and final polishing cloth (1–0.02 μm; ChemoMet), with Colloidal Silica Suspension (≤0.06 μm; MasterMet; Buchler, Whitby, ON). The orthodontic wrought wires did not undergo any specific preparation prior to surface treatment besides being cleaned. All metallic samples were cleaned in an ultrasonic bath (FS20D Ultrasonic, Fisher Scientific, Montreal, Canada) with DW, ethanol, and acetone for 5 min in each solution at 37 °C.

Surface treatment of the metallic samples

The surface treatment was performed in a two steps protocol based on p-phenylenediamine diazotization (primer and adhesive). Both steps were carried out in acidic DW solution at pH ≤ 2, since diazonium cations are stable at pH ≤ 2.5, at room temperature in a simple glass beaker . The first step (primer) was conducted as follows: PPD (0.054 g; 0.05 M) and NaNO 2 (0.034 g; 0.05 M) were dissolved in a glass beaker containing 10 ml of 0.5 M HCl. After ultrasonicating the solution for 5 min, all metallic samples except control group were immersed in the solution and Fe powder (0.250 g) was added as a reducing agent. The samples were left to react for 15 min before ultrasonicating them in DW and acetone for 5 min. This first step leads to spontaneous grafting of a polyaminophenylene (PAP) layer on the metallic samples (i.e. titanium and stainless steel wrought wire). These samples are referred to as metal–PAP from here onwards.

Different approaches were investigated in the second (adhesive) step in order to optimize the adhesion of MMA to metal–PAP samples. These approaches can be summarized in four groups ( Table 2 ). All groups share the following process: NaNO 2 (0.034 g; 0.05 M) was dissolved in 10 ml of 0.5 M HCl. Then, the metal–PAP samples were introduced in the solution before adding Fe powder (0.250 g). In the first group, only the monomer (MMA) was added to the solution. In groups 4, 5, and 6, a surfactant (SDS, 0.026 g) was added along with MMA to help emulsify the hydrophobic monomer . The reaction was allowed to continue for 15 min in the ultrasonic bath and for another 30 min on the bench top; during this period the monomer polymerized and formed a layer of PMMA on the metallic surface. In groups 5 and 6, an initiator (benzoyl peroxide, BP) was added after the fifteen minute sonication stage to accelerate the polymerization reaction on the metallic surface. Finally, the samples were thoroughly rinsed with acetone, and then ultrasonicated in DW and acetone for 5 min in order to discard any ungrafted matter.

Table 2
Conditions tested in the second step. The overall solution volume was 12 ml, and was water-based.
Groups Metal Solution Abbreviation HCl 0.5 M; NaNO 2 0.05 M; Fe 0.25 g; MMA 2.0 ml (ml) SDS (M) [BP] (mg/ml)
1 Ti None Control 0 0 0
2 Ti None D 0 0 0
3 Ti MMA emulsion without SDS D + M 12 0 0
4 Ti MMA emulsion with SDS D + M + E 12 9 × 10 −3 0
5 Ti MMA emulsion with SDS and initiator D + M + E + I 12 9 × 10 −3 8–48
6 SS MMA emulsion with SDS and initiator D + M + E + I 12 9 × 10 −3 40
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Bonding metals to poly(methyl methacrylate) using aryldiazonium salts
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