Сraig. Dental Materials

Solution

A will be stronger by at least one order of magnitude. Without flaws, no dislocationmediated sliding can take place and the total of the metallic bonds must be overcome at once. With flaws present, the metal can deform one row of atoms at a time, and thus the deformation occurs at a lower stress (see Fig. 6-5).

Problem 3

If you are given a single crystal of a pure metal with a hexagonal close-packed lattice in the shape of a cube 3 mm on a side, and you test the compressive strength in the horizontal and vertical directions, will the strengths be the same? If you take a sample, the same size, which is a collection of microscopic crystals, what will the result be?

Solution

For the single crystal, the strengths will not be the same because the distribution of the atoms in a crystal lattice are not anisotropic (independent of direction). Because the metal sample is a pure single crystal, the strength in the horizontal and vertical directions will depend on the conformation of atoms in those directions. Because the conformation of atoms in the hexagonal close-packed lattice is different in the horizontal and vertical directions, the horizontal and vertical strength will vary. For the sample with many smaller crystals, the strength will be the same (at least theoretically). This occurs because the different crystals are oriented at random with respect to one another, and any directional difference in properties is averaged out over the sample.

Problem 4

You are given a binary alloy of known composition and the phase diagram for that alloy. Using the phase diagram and knowing the composition of the alloy, can you predict the phases present in your alloy at room temperature?

Solution

No, certain prediction is probably not possible because you do not know if the alloy is at equilibrium or not. Phase diagrams give the phase structure of an alloy at equilibrium. If your alloy was cooled from the molten state such that equilibrium did not have time to occur, then you cannot know where along the temperature axis the alloy was frozen. Furthermore, formation of dendrites and other nonequilibrium anomalies may be present.

Problem 5

You are given two wires, the same diameter and length. One is wrought and the other is cast. Which will have the greatest percentage elongation?

Solution

Chances are that the cast wire will have the greatest elongation. By mechanically working a wire to its wrought form, the dislocations that allow elongation are "used up" and therefore further stress will result in fracture, not elongation. On the other hand, the tensile strength of the wrought wire is probably greater because it resists the deformation leading to fracture.

Problem 6

If a second element is added to pure gold to form an alloy, one finds that the ability of the second element to form a solid solution with the gold depends on the diameter of the second element versus that of the gold atoms. Why might this be true?

Solution

For the second element to be able to substitute at random into the gold crystal lattice, its diameter must be within a certain range of the gold atoms. If the diameter is too large or too small, it cannot substitute without disrupting the lattice. If its diameter is too small, the second element cannot interact properly with the gold atoms. The ability to form solid so-

lutions sometimes also depends on the relative numbers of the two elements. A host crystal lattice can sometimes accommodate a second element only up to a certain concentration, above which the host lattice is overly disrupted.

Problem 7

You are constructing an orthodontic appliance that requires that a wrought wire be soldered to an orthodontic band. What variables in the soldering operation must be controlled to ensure that the fibrous microstructure of the wire is maintained during the soldering?

Solution

The two critical variables are the time it takes to do the soldering and the temperature of the soldering operation. If either variable is inappropriate (too much time or too high a temperature), then the fibrous wire structure will revert to a grain-based structure. The limit on each variables is dependent on the composition of the wire. Each alloy has different tolerances before recrystallization and grain growth occur.

Problem 8

Of the three types of alloys (solid solutions, eutectics, and intermetallic compounds), which is most likely appropriate for long-term dental applications and why?

Solution

Although all three types of alloys occur in dentistry, solid solutions are generally the most useful because they possess high strength, relatively high ductility, and low corrosion relative to eutectics and intermetallic compounds. Solid-solution alloys also offer a flexibility in composition that is not possible with eutectics and intermetallic compounds.

Problem 9

What methods can be used to improve the mechanical properties of a dental casting alloy?

Solution

Perhaps the best approach is to add grainrefining elements to the alloy. Fine grain size generally improves all mechanical properties, including ductility. Another method that can be used is solid-solution strengthening. Strength and hardness can be dramatically improved by small additions of alloying metals, which go into solution. However, with solid-solution strengthening the ductility may be reduced. Other techniques, such as alloying to form second phases to produce eutectic structure or precipitation hardening, are less desirable for use in dentistry because of increased susceptibility to corrosion. Finally, cold working a casting is generally impractical because the shape would be affected.

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186 Chapter 7 POLYMERSAND POLYMERIZATION

B efore the introduction of acrylic polymers to dentistry in 1937, the principal polymer used was vulcanized rubber for denture bases. Polymers introduced since then have included vinyl acrylics, polystyrene, epoxies, polycarbonates, polyvinylacetate-polyethylene, cisand trans-polyisoprene, polysulfides, silicones, polyethers, and polyacrylic acids. In addition, oligomers from bisphenol A and glycidyl methacrylate (such as dimethacrylates) and urethane di-

methacrylates have been applied.

In terms of quantity, the primary use of polymers has been in the construction of prosthetic appliances such as denture bases. However, they have also been used for highly important applications such as artificial teeth, tooth restoratives, cements, orthodontic space maintainers and elastics, crown and bridge facings,obturators for cleft palates, inlay patterns, implants, impressions, dies, temporary crowns, endodontic fillings, and athletic mouth protectors.

CHEMICAL COMPOSITION

The term polymer denotes a molecule that is made up of many (poly) parts (mers). The rner ending represents the simplest repeating chemical structural unit from which the polymer is composed. Thus poly(methy1 methacrylate) is a polymer having chemical structural units derived from methyl methacrylate, as indicated by the simplified reaction and structural formula I.

The molecules from which the polymer is constructed are called monomers (one part). Polymer molecules may be prepared from a mixture of different types of monomers. They are called copolymers if they contain two or more different chemical units and telpolymers if they contain three different units, as indicated by the structural formulas I1 and 111.

As a convenience in expressing the structural formulas of polymers, the rner units are enclosed in brackets, and subscripts such as n, m, and p represent the average number of the various rner units that make up the polymer molecules. Notice that in normal polymers the rner units are spaced in a random orientation along the polymer chain. It is possible, however, to produce copolymers with rner units arranged so that a large number of one rner type are connected to a large number of another rner type. This special type of polymer is called a blockpolymer. It also is possible to produce polymers having rner units with a special spatial arrangement with respect to the adjacent units; these are called stereospeczfic polymers.

MOLECULAR WEIGHT

The molecular weight of the polymer molecule, which equals the molecular weight of the various mers multiplied by the number of the mers, may range from thousands to millions of molecular weight units, depending on the preparation conditions. The higher the molecular weight of the polymer made from a single monomer, the higher

the degree of polymerization. The term polymerization is often used in a qualitative sense,but the degree of polymerization is defined as the total number of mers in a polymer molecule. In general, the molecular weight of a polymer is reported as the average molecular weight because the number of repeating units may vary greatly from one molecule to another. As would be expected, the fraction of low-, medium-, and high- molecular-weight molecules in a material or, in other words, the molecular weight distribution, has as pronounced an effect on the physical properties as the average molecular weight does. Therefore two poly(methy1 methacrylate) samples can have the same chemical composition but greatly different physical properties because one of the samples has a high percentage of low- molecular-weight molecules, whereas the other has a high percentage of high-molecular weight molecules. Variation in the molecular weight distribution may be obtained by altering the polymerization procedure. These materials therefore do not possess any precise physical constants, such as melting point, as ordinary small molecules do. For example, the higher the molecular

weight, the higher the softening and melting points and the stiffer the plastic.

SPATIAL STRUCTURE

In addition to chemical composition and molecular weight, the physical or spatial structure of the polymer molecules is also important in determining the properties of the polymer. There are three basic types of structures: linear, branched, and cross-linked. They are illustrated in Fig. 7-1 as segments of linear, branched, and cross-linked polymers. The linear homopolymer has mer units of the same type, and the random copolymer of the linear type has the two mer units randomly distributed along the chain. The linear block copolymer has segments, or blocks, along the chain where the mer units are the same. The branched homopolymer again consists of the same mer units, whereas the graft-branched copolymer consists of one type of mer unit on the main chain and another mer for the branches. The cross-linked polymer shown is made up of a homopolymer cross-linked with a single crosslinking agent.

The linear and branched molecules are separate and discrete, whereas the cross-linked molecules are a network structure that may result in the polymer's becoming one giant molecule. The spatial structure of polymers affects their flow properties, but generalizations are difficult to make because either the interaction between linear polymer molecules or the length of the branches on the branched molecules may be more important in a particular example. In general, however, the cross-linked polymers flow at higher temperatures than linear or branched polymers. Another distinguishing feature of some cross-linked polymers is that they do not absorb liquids as readily as either the linear or branched materials.

An additional method of classifying polymers other than by their spatial structure is according to whether they are thermoplastic or thermosetting. The term thermoplastic refers to polymers that may be softened by heating and solidify on cooling, the process being repeatable. Typical examples of polymers of this type are poly(methy1 methacrylate), polyethylenepolyvinylacetate, and polystyrene. The term thermosetting refers to plastics that solidify during fabrication but cannot be softened by reheating. These polymers generally become nonfusible because of a crosslinking reaction and the formation of a spacial structure. Typical dental examples are cross-linked poly(methy1 methacrylate), silicones, cis-polyisoprene, and bisphenol A-diacrylates.

Polymers as a class have unique properties, and by varying the chemical composition, molecular weight, molecular-weight distribution, or spatial arrangement of the mer units, the physical and mechanical properties of polymers may be altered.

high-molecular-weight molecules. The polymerization process may take place by several different mechanisms, but most polymerization reactions fall into two basic types: addition polymerization and condensation polymerization. Important addition polymerization reactions are free-radical, ring-opening, and ionic reactions.

ADDITION POLYMERIZATION

Free-RadicalPolymerization Free-radi- cal polymerization reactions usually occur with unsaturated molecules containing double bonds, as indicated by the following equation, where R represents any organic group, chlorine, or hydrogen.

In this type of reaction, no byproduct is obtained. The reaction takes place in three stages, called the initiation, propagation, and termination stages. The reaction may be accelerated by heat, light, and traces of peroxides, as well as trialkyl borane and other chemicals. In any case, the reaction is initiated by a free radical, which may be produced by any of the methods mentioned, as shown in the equation on p. 190.

Sufficient free radicals for polymerization may be produced at room temperatures by the reaction of a chemical accelerator such as a tertiary amine or a sulfinic acid with the organic peroxide. flN-dihydroxyethyl-para-toluidine,

PREPARATION OF PO

Polymers are prepared by a process called polymerization, which consists of the monomer units becoming chemically linked together to form

has commonly been used as an accelerator in dental products.

The initiation stage is followed by the rapid addition of other monomer molecules to the free

radical and the shifting of the free electron to the end of the growing chain (see reactions below), which describes the propagation stage.

This propagation reaction continues until the growing free radical is terminated. The termination stage may take place in several ways, as indicated, where Mrepresents the mer unit and n and m represent the number of mer units.

A study of these termination reactions reveals how branched and cross-linked polymer molecules may be obtained.

Free-radical polymerization reactions can be inhibited by the presence of any material that will react with a free radical, thus decreasing the rate of initiation or increasing the rate of termination. Decreasing the rate of initiation retards the polymerization reaction, and increasing the rate

of termination decreases the degree of polymerization or the molecular weight of the final polymer. Such materials as hydroquinone, eugenol, or large amounts of oxygen will inhibit or retard the polymerization. Small amounts of hydroquinone are used to protect the methyl methacrylate monomer from premature polymerization, which prolongs the shelf life of the monomer.

Another important free-radical polymerization reaction is responsible for the setting of resin restorative composites. The manufacturer prepares a compound from one molecule of bisphenol A and two molecules of glycidyl methacrylate, called 2,2-bis[4(2-hydroxy-3 methacryloyloxy- propy1oxy)-phenyllpropane. The acronym BisGMA has been used to identify this compound. Because it is not, strictly speaking, a monomer, it

is called an oligomer. A simplified structural formula is shown in formula IV.

Lower-molecular-weight difunctional monomers such as triethyleneglycol dimethacrylate are added to reduce the viscosity, and polymerization is accomplished using free radicals. Because the Bis-GMA has reactive double bonds at each end of the molecule, just as the added lower- molecular-weight monomers do, a highly crosslinked polymer is obtained.

Free radicals needed to initiate the reaction are produced in composites by one of the two methods shown below.

Some composites contain oligomers that are urethane dimethacrylates, such as that shown in formula V below.

Polymerization is accomplished by freeradical initiation with a peroxide-amine system or a diketone-amine system and exposure to blue visible light.

The free-radical polymerization of monomers or oligomers with unsaturated double bonds does not result in all the double bonds reacting. The term degree of conversion describes the percentage of double bonds that react; depending on the conditions, the value may vary from 35% at the air-inhibited layer to 80% in the bulk.

Photo-initiation polymerization has become highly popular in dentistry; it has been shown that the degree of conversion with photoinitiation ranges from about 65% to 80%,whereas chemical initiation results in values from 60% to