Initiators dental polymers

Exposure to benzoyl peroxide may occur in its manufacture and use as an initiator in polymer production, food bleaching and rubber curing. Consumer exposure occurs from acne medications and dental products containing benzoyl peroxide. 

Initiator-accelerator systems for acrylic resins and composites in dental use have been previously reviewed ( ). This report updates the information on these systems in dental polymers. 

The most common dental polymers, used for prosthetic purposes and restorative dentistry (filling material), are polymethacrylates.1514 The polymerization process performed directly in the dental cavity has to meet strict demands the reaction must be fast at a temperature below 50 °C and it must avoid the formation of a toxic product. These requirements can be fulfilled by UV curing. For example, a mixture of camphorquinone (586), a chromophore (photoinitiator) with an absorption maximum at 468 nm and an amine 587 as a co-initiator (see also Scheme 6.100), initiates a radical polymerization reaction of the acrylate monomer 588 upon photolysis using a conventional blue lamp or laser (Scheme 6.285). 

Uses Crosslinking agent for actylic/vinyl resins PVC plastisol comonomer (reduces initial vise, and oil extractability. and improves ultimate hardness, heat distort., hot tear strength, and stain resistance) adhesion promoter hardener used in cast acrylic sheet and rod, contact lens, elastomers, ion exchange resins, dental polymers, adhesives, coatings (paper, plastic), cosmetics, paints, sealants, photopolymers, electronics (photoresists, solder masks)... 

The first ever injectable crude biomaterial, that is a dental implant, appeared early in ad 6oo (Fig. 12.1). During those times, Mayan people trimmed seashells into artificial teeth to replace missing teeth (Michael, 2006 Ratner et al., 2004). Early biomaterials also led to problems, including sterilization, toxicity, inflammation, and immunological issues. Since the Mayan s initial use of artificial teeth, biomaterials have evolved to be used in modem artificial hearts, hip and knee pros-theses, artificial kidneys, and breast implants. Materials used in these applications include titanium, silicons, polyurethanes, teflon, polybiodegradable polymers, and most recently bio-nanomaterials (Pearce et al., 2007)... 

We chose to modify the anhydride monomers with photopolymerizable methacrylate functionalities. Methacrylate-based polymers have a long history in biomedical applications, ranging from photocured dental composites [20] to thermally cured bone cements [21]. Furthermore, photopolymerizations provide many advantages for material handling and processing, including spatial and temporal control of the polymerization and rapid rates at ambient temperatures. Liquid or putty-like monomer/initiator... 

Benzoyl peroxide is used as an initiator for polymerization of acrylates (including dental cements and restoratives) and other polymers as a bleaching agent for flour, fats, oils, waxes and milk used in the preparation of certain cheeses in pharmaceuticals for the topical treatment of acne in rubber curing and as a finishing agent for some acetate yams (Anon., 1984 Lewis, 1993 Medical Economics Co., 1996 United States Food and Dmg Administration, 1997). 

A polymer used for making dental impressions must be soft enough initially to be molded around the teeth, but must become hard enough later to maintain a fixed shape. The polymer commonly used for dental impressions contains three-membered aziridine rings that react to cross-link... 

The majority of the polymerization of a dental composite resin occurs very quickly, typically during the 20-40 s or so of light irradiation from the dental cure lamp. However, free radicals within the material do not terminate immediately the lamp switches off. Hence they are able to continue their propagation steps for some time after this initial cure, as growing polymer molecules containing free radical centres continue to incorporate extra monomer molecules [24]. Shrinkage, which is associated with polymerization, has been shown to continue for up to 24h after initial setting [25] in a process known as post-polymerization [26]. 

For light-cured materials, the initiator system can be based on camphorquinone, so that cements can be cured with a conventional dental cure lamp emitting at a maximum wavelength around 470nm. Unlike formulations of composite resin, these materials cannot deploy amines as activators, because they would react with the carboxylic acid groups on the polymer, forming salts. Instead, a substance such as sodium p-toluene sulfinide is used as the activator. In addition, a photo-accelerator such as ethyl 4-NJ -dimethylamino benzoate is included [10]. 

In the preparation of dental base material, poly(methyl methacrylate) (PMMA) is the most commonly used acrylic resin. It is well-mixed with the second component, liquid methyl methacrylate (MMA), prior to use [48]. The prepared in such a way polymer dough is placed in a mould and polymerized in a water bath. The initiator (e.g., benzoyl peroxide) that is usually added to the PMMA powder by a manufacturer decomposed at an elevated temperature and started polymerization and crosslinking. 

After surface preparation, the restorative material is bonded to the tooth structure with auto-polymerizing adhesive resin. One prerequisite for dental adhesives is an ability to cure in the mouth that is why methaciylate-based polymer materials are used in dentistry. Initiation systems are BPO-f amine or TBB (201). Monomers used are MMA or bifunctional methacrylate monomers. The needed optical opacity (215,216) is achieved by various inorganic and metal compoimds incorporated into the formulations. 

We learned in Section 12.2 that polymers can be characterized by their physical properties. Elastomers are used as biomaterials in flexible tubing over leads for implanted heart pacemakers and as catheters (tubes implanted into the body to administer a drug or to drain fluids). Thermoplastics, such as polyethylene or polyesters, are employed as membranes in blood dialysis machines and as replacements for blood vessels. Thermoset plastics find limited but important uses. Because they are hard, inflexible, and somewhat brittle, they are most often used in dental devices or in orthopedic applications, such as in joint replacements. To fill a cavity, for example, the dentist may pack some material into the cavity, then shine an ultraviolet lamp on it. The light initiates a photochemical reaction that forms a hard, highly cross-linked thermoset polymer. 

Initially the implants were intended mainly to replace affected (i.e., broken bones, heart, vein, liver, pancrease) or malfunctioning body p>arts which people cannot live without. Subsequently, the industry of implants developed fast and now there are implants which are used for reconstructing and aesthetic purposes such as breast implants, bone facial implants and skin implants. According to the type of replaced tissue, they are divided into hard and soft implants. Hard tissue implants are usually made of metals or ceramics (bone implants, dental sealant, and joints) while soft tissue implants are made of polymers (blood vessels, skin, plastic surgery). 

Beech [154] points out that, because of the anionic character of CNA polymerization, the product obtained adheres to dental surfaces. CNA polymerization can be initiated by the NH, or OH groups in proteins. Since the initiator remains bound to the monomer, the polymer remains bound to proteinic support. Thus, because of the outstanding importance of oi anic phase in dentine, this hard tissue represents an exquisite support for CNAs. NH2 and OH groups in collagen (the main component in dentine organic phase) amino acids represent active positions... 

GC alone can be a valuable monomer for the synthesis of hyperbranched poly(hydroxyether)s (Scheme 25). In case of polymerization, GC, containing a l,3-dioxolan-2-one ring and hydroxyl group in a single molecule, is considered a latent cyclic AB2-type monomer. The anionic ROP of the GC, which proceeds with CO2 liberation, leads to a branched polyether. l,l,l-Tris(hydroxymethyl)propane or other multihydroxyl molecules are usually used as a initiator-starter and central core of the polyether. The hyperbranched polyglycerol structure is obtained by slow addition of the cyclic carbonate monomer at above 150 °C. Such polymers are characterized by a flexible polyether core and a multihydroxyl outer sphere. They are suitable for preparation of acrylic resins for dental applications or additives for polyurethane foams. Hyperbranched poly (hydroxyether)s from biscyclic carbonate with phenol group (2, Scheme 24) were also reported. 

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