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متعددات التماثر في التعويضاتPolymers in Prosthodontics

Lecture 11

Reference املرجع املعتمد

Dental Materials (Properties and Manipulation)(John M. Powers & John C. Wataha) 2008 Pages 285-301

Titles:

� Polymerization � Acrylics and Free-Radical Polymerization � Complete Dentures � Composition and Manipulation of Polymers in Complete Dentures

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11. 1. Polymers in ProsthodonticsPolymers are widely used in prosthodontics, primarily for removable

prostheses such as complete or partial dentures (Figure 7-1). However, polymers also are used for denture teeth, impression trays, temporary crowns, and maxillofacial prostheses. Prosthodontic polymers have specific compositions and physical and chemical properties to ensure appropriate clinical performance, but the natures of these polymers overlap to some extent with those used for direct esthetic restorative materials, mouth protectors, impression materials, and cements. In this chapter, the focus is on the properties and manipulation techniques that are most critical to prosthodontic polymers.

Prosthodontic polymers are not commonly formed or manipulated by the dental office team. For example, nearly all of the fabrication steps of a removable complete or partial denture occur in a dental laboratory. Yet, all members of the dental team need a basic knowledge of the materials to facilitate the best dental care for patients who need dental prostheses made from these polymers.

Polymers also play important roles in areas of dentistry other than prosthodontics.

Figure 7-1 Polymers are used in a diverse array of applications in prosthodontics (solid arrows). Polymers are used in other areas of dentistry (dashed arrows).

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11. 2. PolymerizationThe polymerization reaction is central to prosthodontic polymers.

Polymerizatic is a chemical reaction that links small organic compounds called monomers into long chains of repeating monomer (“mer”) subunits (Figure 7-2). It is common for a single polymer chain to contain 10,000 to 100,000 linked monomers. Because polymerized chains have molecular weights thousands of times greater than their monomers, the physical and chemical properties of polymers are always distinct from their parent monomers. For example, methyl methacrylate, a common monomer used to make complete dentures, is a liquid that boils at about 100°C. Yet poly (methyl methacrylate) (PMMA) polymers (composed of methyl methacrylate monomer) are fairly rigid solids that decompose before they melt! The simplest polymers consist of many long chains that behave much like long strands of spaghetti. Each polymer strand is not chemically linked to the other, but the strands entangle to create a polymer network that gives the polymer its particular physical properties.

Polymerization is the process of covalently linking many low-molecular-weight compounds called monomers together to form high-molecular-weight polymers.

Figure 7-2 Polymerization is a chemical reaction that links monomers into long chains of repeating monomer units.

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The nature of the chemical reaction that forms a polymer greatly influences the polymer. Each monomer has at least one chemical group that participates in the polymerization reaction. However, for a number of complex reasons, not all monomers may be able to react completely, and any unreacted residual monomer in the polymer may have significant clinical consequences. The extent to which all monomer is polymerized is the degree of conversion. Polymers with high degrees of conversion have low levels of residual monomer.

Polymerization chemical reactions may produce byproducts such as water, hydrogen gas, or alcohols. Because these byproducts form within and are at least temporarily trapped in the polymer network, they are important to the physical nature of the polymer. For example, polymers that form volatile alcohol byproducts commonly shrink as the alcohol evaporates from the polymer network over a period of hours to days. Shrinkage may or may not be important to the performance of the dental prosthesis. However, regardless of the polymerization chemistry, most prosthodontic polymers shrink significantly as polymerization proceeds.

All chains in a polymer network are not identical. As polymerization proceeds, monomer is consumed by thousands of growing chains, each adding monomer units independently of the other. Each polymer chain will grow until monomer is no longer accessible, but the number of monomers (or mer-units) will be somewhat different for each chain. Thus, in a polymer network, the chains will have a distribution of sizes (Figure 7-3). The extent to which monomer is converted into polymer is called the degree of polymerization (to be distinguished from the degree of conversion). Polymers with a high

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degree of polymerization have fewer polymer chains that are longer (on average); those with a lower degree of polymerization have more polymer chains, but each is shorter. The degree of polymerization influences the physical properties of a polymer because shorter strands interact differently than longer strands in the polymer network. As a rule of thumb, high degrees of polymerization lead to more rigid, less soluble polymer networks.

Several types of chemical reactions are used to form polymers. The most common of these in dentistry is the free-radical addition reaction, which is used for nearly all prosthodontic polymer applications. Other chemical mechanisms include ring opening, used to form epoxy polymers, and condensation, used to form some silicones. In this chapter, we focus on the free-radical mechanism. Polymerization of direct restorative materials also proceeds by the free-radical addition mechanism.

Figure 7-3 The monomer may form more chains that are distributed around a lower average molecular weight (left curve) or may form fewer, heavier chains (right curve). The nature of this distribution of chain lengths is called the degree of polymerization. A high degree of polymerization (right curve) implies a higher average molecular weight of individual chains.

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11. 3. Acrylics and Free-Radical PolymerizationIn prosthodontics, free-radical addition forms polymers that are commonly referred to as acrylics. Acrylics are a major class of polymers in prosthodontics used to make complete dentures, portions of removable partial dentures, temporary crowns, custom impression trays, and denture teeth. Acrylics also are used in orthodontic appliances and in several laboratory procedures in casting and soldering. The most common monomer is methyl methacrylate, but hydroxyethyl methacrylate and butyl methacrylate (and several others) also are used. Methyl methacrylate is a sweet smelling liquid that boils at 100°C. The odor of methyl methacrylate characterizes many dental laboratories, but excessive inhalation should be avoided because of potential toxicity to the liver. Methyl methacrylate also is highly flammable and allergenic.

There are several sources of free radicals to initiate acrylic polymerization. Benzoyl peroxide, a solid organic peroxide, is one common source. If heated to about 74°C, benzoyl peroxide decomposes into active free radicals that can initiate the polymerization reaction, and this reaction is the basis for “heat-cured” denture processing. The benzoyl peroxide is therefore the initiator. Alternatively, a chemical reaction between benzoyl peroxide and an aromatic amine (such as N,N dihydroxyethyl-para-toluidine) will cause benzoyl peroxide to decompose into free radicals at room temperature. This chemical reaction is the basis for “chemical-cured” denture processing, and in this reaction, the amine is called an accelerator. Finally, a light sensitive initiator called camphorquinone will form free radicals if exposed to blue light (λ= 462 run) in the presence of an aliphatic amine accelerator. Although the light-mediated mechanism is rarely used for denture processing, it is common for denture repair or fabrication of temporary crowns and custom impression

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trays. Light curing is central to the placement of preventative and esthetic restoration.

11. 4. Complete DenturesComplete dentures use filled PMMA (acrylic) homopolymers for the denture base, chemically bonded to acrylic denture teeth made from filled, cross-linked homopolymers. Occasionally, a denture base is made of a copolymer (e.g., with butyl methacrylate) or filled with special fillers that increase the fracture resistance of the base.

11. 5. Composition and Manipulation of Polymers in Complete Dentures

Most dentures are made using a PMMA homopolymer that is filled but not cross-linked. The materials are purchased as powder-liquid systems, and both the powder and liquid contain multiple components (Figures 7-4 and 7-5). The polymerization of the material may be initiated either by chemicals or more commonly by heat. In heat-cured systems, the powder contains small spheres of prepolymerized PMMA, benzoyl peroxide initiator (1%), ceramic oxide to add translucency, inorganic pigments for color, and small colored fibers to mimic blood vessels. The liquid contains methyl methacrylate monomer and traces (0.1%) of an inhibitor (hydroquinone) to prevent inappropriate polymerization by incident ultraviolet light (the monomer also is stored in dark-colored bottles to prevent light exposure). When the powder and liquid are mixed, the monomer dissolves into the prepolymerized spheres, and the mixture progresses through a series of consistencies. Initially, the mixture is grainy or sandy, but then it rapidly becomes sticky or stringy. Within a few minutes, the mixture becomes doughlike, and it is in this stage that the material is used to “pack” the denture flask. If the mixture sets too long, then the consistency becomes rubbery, which is inappropriate for packing. Packing is accomplished by placing the dough into the flask then

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compressing the flask to remove excess. Several compression sequences are necessary to completely remove the excess material. It is important to note that in heat-cured systems these changes in consistency do Not result from polymerization; rather, they occur as the monomer dissolves and swells the prepolymerized particles. Polymerization of this type of material recurs when the mixture is heated in a water bath to about 74°C for a minimum of 8 hours.

For chemical-cured materials, the process of packing the denture flask is similar, but timing is more critical. Chemical-cured materials contain all of the components of the heat-cured materials but also contain an aromatic amine accelerator in the liquid. When the amine is combined with the benzoyl peroxide in the powder, free radicals are produced, and polymerization begins. The polymerization reaction is therefore superimposed on the dissolution and swelling of the prepolymerized particles that occurs in heat-cured materials.

Figure 7-4 Polymerization of polymers

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Thus, a similar evolution of consistencies (sandy, stringy, doughy, rubbery) occurs, but these stages have shorter lifetimes because the polymerization is occurring at the same time. Denture packing must be done in a shorter time for the same reason.

In other denture base materials, small particles of butadiene-styrene rubber are added to the powder to increase the impact strength of the denture. The rubber is grafted with a methacrylate group so that the particles are covalently bonded into the polymer network. Because denture base materials are radiolucent, pieces of prostheses that might be aspirated during an accident will not be visible radiographically. Bismuth or uranyl salts (10%-15%) can be added to provide radio-opacity, but these chemicals decrease the modulus, increase water sorption, and make processing more difficult.

Figure 7-5 A common heat-cured denture base resin

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