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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 .

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.

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Materials and methods