ANNOTATION TO THE LESSON №15 Polymers are the substances, which molecules consist of many elements of same chemical composition and structure, and these molecules are called macromolecules. Macromolecules can be built of monomers of same chemical structure (such polymers can be also called homopolymers) or different ones (copolymers). During synthesis main molecular chain which consists of same monomers, can graft parts of other monomers, and thus grafted polymers are received. By chemical composition polymers are classified into organic, elementoorganic and inorganic. Organic polymers composition includes carbon atoms, and besides - hydrogen, oxygen, nitrogen, sulfur, halogens atoms as well; and oxygen, nitrogen or sulfur belong to the main chain composition. Elementoorganic polymers – are the substances, in which main chain consists of carbon, and side groups contain geteroatoms (except nitrogen, sulfar, oxygen and halogens), which are connected directly to carbon atoms of main chain. The main chain may contain diffrerent atoms of silica, aluminum, titanium, nickel and other elements. The most spread examples of this group are silicone ones. Inorganic polymers do not contain carbon atoms (silicate glasses, asbestos, isinglass stone etc). The widest group is represented by organic polymers which are divided on base of main chain composition into homochain and heterochain ones. Homochain polymers’ main chains consist of same atoms, for example carbon, sulfur, phosphor etc. If the main molecular chain is made only of carbon atoms, such polymers are called carbochain: their carbon atoms are connected with hydrogen atoms or organic radicals, or functional groups such as hydroxyls, amino groups etc. Heterochain polymers have main chain made of various atoms, which come into composition of organic substances. Atoms of various elements, except carbon, which are present in main chain, change significantly the properties of polymers. Therefore, oxygen atoms induce plasticity of the chain, playing a join role, which provides growth of polymer elasticity; atoms of phosphor and chlorine increase fire resistance; fluoride atoms add to the polymer high chemical stability etc. According to the shape of macromolecules polymers are classified into linear, branched, striped, or ladder, spatial or reticulate.

Linear macromolecules of the polymer are represented by long zigzag or spiral chains. Flexible macromolecules with high toughness along the chain and weak intermolecular connections provide elasticity of the material, its property to get soft at heating and get solid at cooling down. Typical linear polymers are

Fig. 1. Shapes of polymer macromolecules:

а – linear; б – branched;

в – ladder; г – reticulate

(Mashkov Y. K., 2002)

polyethylene, polytetrafluoroethylene, polyvinylchloride.

Branched macromolecules of the polymer have side branches which block approaching of macromolecules and their tight packing. Such polymers (for example polyisobutylene) have decreased intermolecular interaction, decreased toughness, increased density and solubility. Graft copolymers which have different composition of main chain and its branches, also belong to the branched group.

Ladder macromolecules consist of two chains, connected between each other by transversal chemical links or chains. Plymers with ladder macromolecules have stiff main chain, and increased fire resistance. Ladder structure is present in several silicone polymers. Spatial or reticulate polymers are created during connection (stapling) of macromolecules in transversal direction with strong chemical links. In result a reticulate structure appears with different density of the net. Spatial reticulate polymers of high density make a base of constructive polymer materials. On base of physic-mechanical properties (20°С temperature) plastic masses are divided into rigid, semirigid and plastic polymer materials. Rigid polymer materials are hard elastic materials (polymethylmethacrylates, aminoplasts, polystyrene etc.) with mostly amorphous structure, high elasticity modul and low lengthening on extension. Under outer loads, which are below destroying border, they maintain their shape for long time at normal and higher (until definite border) temperatures. Semirigid polymer materials – are hard elastic materials (polyethylene, polypropylene, polyamides), which mostly have crystalline structure, middle values of elasticity modul and residual lengthening on extension. Residual lengthening of these polymers is reversible and usually disappears on heating. Plastic polymer materials — are soft and elastic materials (polyvinylchloride plasticate, polyurethane foam etc.) with mostly amorphous structure and low elasticity modul. They are characterized with relatively high general extension, but low residual extension. Development and disappearing of reversible part of their deformation at normal temperature occurs with slower speed. This is their main difference from elastic plastics, also soft and elastic materials, but with reversible deformation which develops and disappears with big speed (in a moment). According to thermal properties plastic masses are divided into thermoplastic and thermoset. It is a very important and convenient classification, referring mostly

synthetic polymers, underlines principal difference in properties of polymers and their reaction in technological processes of manufacturing and running of the samples. Thermoplastic polymers (thermoplastics) is a name of such polymers, which at heating get plastified and easily formed into samples, and at cooling (after molding) – get solid. Their properties change reversibly. Only during long-term thermal influences irreversible changes are possible. Thermoplastics include: polymethylmethacrylates, polyethylene, polyvinylchloride, polystyrene, polyamides (nylon, caprone), and some other polymer materials. Thermoset polymers (thermosets) are the polymer materials, which are softened only at the moment of molding the sample (during heating and pressure) and lose this property in result of chemical reactions which occur with thermal influence. Stapling of polymer chains occur, and material transits irreversibly into unsoluble and unmalting phase. Thermosets include plastic on base of phenol aldehyde, aminoaldehyde, polyester and some other resins. Classification of dental polymers according to their aim of use:

ü Main (constructive) polymer materials for producing of dentures, splints and apparates;

ü Auxiliary polymer materials (impression, model, modelling, for individual impression trays etc.);

ü Clinical polymer materials, used in dental clinic chairside (impression, modelling, specific types of filling materials, adhesives, sealants etc.).

Classification of dental polymers according to the initiation mechanism of polymerization reaction:

ü Heat activated; ü Self cured (chemical initiation, cold polymerization); ü Light cured (photo polymerization); ü Dual curing (chemical and light polymerization).

BASE MATERIALS Base – is one of main elements of removable dentures, laying on the tissues of prosthetic bed and accurately replicating relief of the mucosa.

Fig. 2. Base of removable partial dentures

In the denture base artificial teeth and retention elements are fixed (fig. 2). Requirements for the base materials (R. Noort, 2002):

ü Biocompatibility; ü High toughness, hardness, impact viscosity; ü Low density; ü Processability; ü Linear stability; ü Natural look (correspondence to the color of oral mucosa); ü Detailed replication of surface; ü Good heat conductivity; ü Good hygiene properties (no odors, stability towards absorption of oral

liquid, stability towards bacterial growth, cleaning should be easily performed);

ü Ability to show strong connection with polymers, ceramic and metals; ü durability; ü low cost; ü ability to be easily repaired.

Classification of polymer materials used for fabrication of bases of removable dentures according to GOST RF 51889-2002, is shown in the table №1.

Тable 1. Classification of polymer materials for bases of removable dentures

Acrylic polymer-monomer materials Thermoplastics (type

3)

Initiating of polymerization with outer energy

Initiating of polymerization with

chemical reaction

For injection molding

For molding of sheet patterns

Heat cured

(type 1)

Microwave cured

(type 5)

Light cured

(type 4)

Self cured (type 2)

For molding For casting Base acrylic plastics of heat polymerization. Classic technology of receiving polymethylmethacrylate supposes the synthesis of polymers (in solution or emulsion). After synthesis and picking out of the environment a polymerizate is received in powder or granules. In heat cured plastics size of granules (balls) makes 50-150 mkm. Afterwards functional and technological additives are introduced into polymer. Composition of acrylic heat-cured

polymers is shown on fig. 3: Initiator – benzoyl peroxide easily dissolves into free radicals under temperature action due to breakage of weak chemical links. Pigments/colorants (oxides of metals etc.) add to polymer materials colours and shades, which mimic dental tissues or oral mucosa. Opacifiers eliminate transparency of polymer materials. Plastificators decrease stiffness and brittleness of polymers.

Fig. 3. Composition of heat cured acryls Powder (polymer)

granules of polymethylmethacrylate

initiator – benzoyl peroxide

pigments/colorants

opacifiers – titanium/zinc oxides

plastificator – dibutyl phthalate

synthetic fibres – nylon/acryl

Liquid (monomer) methyl ether of methacrylic acid

inhibitor – hydroquinone

stapling agent – dimethacrylic ether of ethylenglycole

Fillers change properties of plastics, increase mechanical strength and hardness, decrease shrinkage value during forming of the sample. Mechanical properties, specifically impact resistance increase very well on introducing into plastic of fibrous fillers which play a role of reinforcing elements and reducing brittleness of unfilled plastics. Methyl ether of methacrylic acid is a flying, transparent liquid without colour (density 0,95, boiling temperature 100,3ОС). Inhibitors block irreversible change of plastic properties under action of heat, oxygen of the air, light, moist and other factors. Hydroquinone prolongs shelf life of monomer, by instant entering reaction with free radicals which can spontaneously appear in the liquid and form stable free radical combinations which are not able to initiate polymerization process.

Stapling agent, such as dimethacrylate of ethylenglycole, is included into material composition for improvement of mechanical properties. It is connected in several sites with polymer chain of polymethylmethacrylate and forms transversal staple between itself and neighbouring chain of the polymer with the help of two ending double links. The process of addition (free radical) polymerization of methylmethacrylate (PMMA) occurs as 4 steps:

• activation; • initiation; • propagation; • termination.

On the fig. 4 a scheme of addition (free radical) polymerization of methylmethacrylate is shown.

Activation is achieved by dissolving of initiator (benzoyl peroxide) under heat action. At heating more than 65°С dissolving of benzoyl peroxide occurs with formation of free radicals (•). Free radicals enter reaction with monomer (methylmethacrylate) after which a monomer becomes free radical and in its turn enters reaction with another monomer. Repeating of this process again and again leads to growth of polymer chain (propagation). This process will go on until growing chains will collide into each other or until all free radicals join chemical reaction.

Fig. 4. Scheme of methylmethacrylate polymerization reaction (N.Ray, 1998)

On the fig. 5 some types of heat cured acrylic polymers which are used for fabrication of dentures, are shown.

Acrylic plastic

(Ethacryl-02” is characterized with high technological

properties, increased strength. STOMA

Plastic on base of fluoride-containing acrylic copolymers “Ftoraks”. Bases are highly strong

and elastic. STOMA

Colorless plastic, used when colorants

of base are contraindicated.

Highly strong and transparent. STOMA

Acrylic plastic «ProBASE Hot».

Ivoclar (Lichtenstein)

(Ukraine) (Ukraine) (Ukraine) Fig.5. Heat cured acrylic plastic, used for manufacturing of denture bases

Self-cured acrylic base plastic (chemical initiation, cold polymerization). In self-cured plastics powder component, as well as in heat cured plastics, contains benzoyl peroxide, and liquid contains tertiary amine (n.n-dimethyl-p-toluidine or derivatives of sulfuric acid). Activation of benzoyl peroxide (dissolving into free radicals) occurs under influence of tertiary amine without temperature influence (at room temperature). This cure method is less effective than heat cure and creates a polymer of lower molecular mass. Produced polymer chains are shorter than at heat polymerization. Such state makes a negative influence onto strength properties of the material and also increases rate of residual monomer in it. At the end of polymerization self cure plastic contains till 5% of monomer, which didn’t join polymerization reaction, which is x10 more than in heat cured plastics. Self cure plastics have worse colour stability than heat cured plastics. Besides, during polymerization of self cure plastics big amount of heat is exhaled, which promotes big number of pores and voids. Polymerization of self cure plastics under pressure of 1,5-2 atm in moist environment reduces number of pores and increases their elasticity. Meanwhile, self cure acrylic plastics are widely used during repair of removable denture bases (fixing of cracks and fractures), and during detailing of denture base relief, contacting prosthethic bed tissues (rebasing of the denture). On the fig. 6 there are shown several types of self cure acrylic plastics which are used in dentistry for fabrication of denture bases by casting («CASTDON»), cold polymerization in autoclave under pressure of 2 atm. («ProBASE Cold»), for repairing and rebasing of dentures (“Protacryl-M”, “Redont-03”).

«ProBASECold» - for fabrication of

denture bases. Ivoclar

(Лихтенштейн)

«Protacryl-M» - for repairing of denture bases.

STOMA (Ukraine)

«CASTDON» - for fabrication of denture

bases by casting. Dreve (Germany)

«Redont-03» - for repairing of denture

bases. STOMA (Ukraine)

Fig.6. Acrylic plastics of cold polymerization (self cured) Light cure base plastics (photo polymerization). There are base materials which are polymerized under influence of light with wave length of 400-500 nm (read composite polymer materials). Chemical composition consists of urethane dimethacrylate matrix, which contains a small quantity of colloidal silica oxide to give necessary consistency to the material, and filler – acrylic spheres, which become a part of interpenetrative structure of the polymer net during setting. Activator of polymerization reaction is light with wave length of 400-500 nm. Polymer material “TRIAD”(Dentsply, USA) is one of materials of this group and is used for rebasing of dentures and manufacturing of orthodontic apparates (fig. 7).

Fig. 7. Photo polymerized material

«TRIAD» (Dentsply, USA)

Microwave cure base plastics. Microwave radiation is used for contactless, inner heating of acrylic material in a short period of time, increasing speed of polymerization process. For microwave induced polymerization of plastic a source of microwave radiation is needed as well as special plastic flasks, reinforced with fiberglass and capable of letting microwave radiation through itself. Such plastics as «Microbase» (Dentsply, USA), «Acron MC» (GC, Japan), «Basis MC» (Yamahachi Dental, Japan), «АКР-МВ» (Russia) etc. On the fig. 8 equipment and base plastic for microwave polymerization are shown.

Fig. 8. Microwave cure plastic

«Microbase» (Dentsply, USA)

Thermoplastic base materials, used for injection molding. Thermoplastics form a separate group from all plastics used in dentistry for fabrication of removable dentures (fig.9). This is comparatively new direction, which receives more and more followers, because comparing to acrylic plastics, thermoplastics are more homogenous and elastic. Thermoplastics which are used in dentistry for fabrication of “flexible” denture bases, are represented with such materials as polyamides (nylon), polyoxymethylen, polycarbonates, polypropylene, thermo molding acrylates (monomerless acrylic plastics), ethylenvinylacetate etc.

Fig. 9. «Flexible» base of removable denture, made of thermoplastic with injection molding

Thermoplastics used in dentistry are compound of substances (copolymers) with thermoplastic properties, and fillers which provide color stability of materials. For these materials absence of residual monomer is typical, they don’t contain toxic or allergenic additives, are highly biocompatible and have shape memory. High degree of plasticity, accuracy during manufacturing process, wide shade scale allow to extend possibilities of removable dentures and improve their esthetic properties. The widest use in dentistry belongs to trademarks of thermoplastics, which are shown in the table 2.

Table 2. Thermoplastics, used in dentistry

Base Trademarks

Polyoxymethylen «Biocetal» (Poland), «Dental D» (Italy), «T.S.M. AcetalDental» (San Marino), «Aceplast» (Israel) etc.

Polyamide (nylon)

«Arkon», «Dentiflex» (Poland), «Valplast», «Flexite» (USA), «Flexy-Nylon» (Israel), «Flexiplast», «Bre.Flex» (Germany), «Flexi-J» (San Marino), «Dental D Nylon» (Italy), «Deflex» (Argentine), «Vertex Termosens» (Netherlands) etc.

Polymethylmethacrylate «Polyan» (Germany), «Fusicryl», «Fusicril» (Italy), «Acry-free» (Israel), «Thermo Free» (San-Marino), «Flexite M.P.» (USA) etc.

Polypropylene «LIPOL» (Ukraine) etc.

Ethylenvinylacetate «Flexidy 80», «Flexidy 65», «Flexidy 50» (Italy), «Corflex Orthodontic» (San-Marino) etc.

Polycarbonate «Evihard» (Russia) etc.

Polyetheretherketone «Bio XS®», «Bio HPP» (Germany)

Polyoxymethylen (polyformaldehyde, polyacetale) – is a product of formaldehyde polymerization. Polyoxymethylen doesn’t possess high thermal and chemical stability, but due to its hardness, high melting temperature and stability towards organic dissolvants is widely used in injection molding. Ready polyoxymethylen samples outstand with high rigidity, fatigue strength, low shrinkage during processing, low creep, stability towards wear and moist, alkaline dissolvants. Polyamides (nylon, caprone) – are heteropolymers, containing in the main chain macromolecules – amide groups. In 1935 a group of scientists in Du Pont company, leaded by Wallace Carothers, created a material consisting of hydrogen, nitrogen, oxygen and carbon. Scientists were trying to discover material, similar to silk by its qualitative characteristic. The discovery was made accidentally.

After heating mixtureof coal resin, water and ethanol, scientists found out that something similar to silk was received, transparent and very strong. Its commercial use started in October 1938, and the name of this polymer appeared in 1939 on World fair in New York — «NYlon» - first letters of New-York city. In medicine, and particularly in dentistry only untoxic polyamides are used. Macromolecules of polyamides in solid phase usually have configuration of flat zigzag. Due to amide groups macromolecules of polyamide are connected to each other with hydrogen links, which provide relatively high melting temperatures of crystalline polyamide. Polyamides are processed with molding, injection molding, extrusion and compression. Polyamide samples can be welded (heat welding or high frequency currents) or glued with solutions of same polymer in polyhydric phenols or with formic acid. Polyamide – is the first synthetic polymer, which physical properties overpass properties of some metals. It has inevitable combination of properties – high strength, middle toughness and stability towards high temperature, inflammables and lubricants and most of chemicals. Acrylates — are ethers or salts of acrylic acid. In dentistry polymethylmethacrylate – amorphous transparent thermoplastic is used for injection molding. At heating over 120°С PMMA plastifies, transits into highly elastic phase and is molded easily. Polypropylene — is a thermoplastic polymer of propylene (propene). Polypropylene is received with the help of polymerization of propylene in presence of metallocomplex catalysts. Its main characteristics are similar to nylon but some of its physic-chemical parameters are poorer. Nowadays polypropylene is used for fabrication of prosthodontics constructions as a cheap alternative to nylon. Ethylenvinylacetate – is a substance, referring to polyolefins. It’s received in result of copolymerization of ethylene and monomer of vinyl acetate. Polycarbonates — is a group of thermoplastics, complex polyesters of carbon acid and dialcohols. Polyether etherketone (PEEK) — is a semycrystalline polymer, a material with high mechanical and thermal properties (higher than 240°C). Its characteristics are stable against load and wear. Has low friction coefficient. Physical properties of thermoplastics are shown in the table 3.

Table 3. Physical properties of thermoplastic materials (by I. D. Tregubov, 2007)

Properties

Den

sity

at

200 С

(g

/cm

3 )

Mel

ting

te

mp

erat

ure

0 С

Strength (MPA) on Elasticity modul Im

pa ct

visc

osi

ty

(kJ/

m2 )

Rel

ativ

e ex

ten

sion

(%

)

Imbi

bitio n

with

in

24

hour

s (%

)

Materials

(MPa) on

extru

sion

com

pres

sion

bend

extru

sion

bend

Polyamides 1,1 185-280

50-82,7

46-110

90-100

1,2-2,9

2,1-3,5

50-80

15-90

0,2-2,6

Polyoxymethylene

1,43

173-180

68-71

110-130

100-120 2,9 3,5 90-

120 15-45 0,2

Polypropylene 0,9 172-260

25-40

35-70 80-90 1,1-

2,7 1,5-2,9

40-60

20-100 0,5

Polymethylmethacrylate

1,19

180-240 55 105-

112 110-140

2,5-3,8

2,8-3,1 15 2 0,45-

0,6

Ethylenvinylacetate

1,38

90-160

10,5-23 1-2 300-

400 0,4-0,5

0,7-0,8 170 500-

600 0,4-0,6

Polyetheretherketone 1,3 343-

355 91-112 160-

170 3600 4100 12,4-

25 0,1-0,5

Industrial manufacturing of thermoplastics for injection molding of dentures bases is performed in granules, which are packaged into aluminum containers. Melting of thermoplastic (depending on the composition base, melting temperature varies from 165 till 4000С) and casting under pressure of 4,5 – 5,5 bar is provided with the help of special thermoinjection presses. Polyurethane base materials Polyurethanes – is a class of polymers, in which general feature is the presence of more or less regular urethane groups in the main chain. In 2004 scientists of Research institute of resin and latex fabrics together with МSМSU developed a material for bases of removable dentures – “Dentalur”.

Composition is made of two components – isocyanate and polyol. Polyol component contains in small quantities additives – colorant, catalyst. Componenets of the material are prepackaged into cartridges and mixed in dispenser (fig. 10).

Fig. 10. Material on base of polyurethane for bases of removable dentures

Flowable material is used in methodic of silicone forms molding (free molding) with following temperature influence onto the material within 1200С. Parameters of specific impact strength, relative extension on tear, tests for static bend of polyurethane dentures exceed parameters of classic acrylic polymer-monomer compositions (Y. M. Alter, M. Y. Ogorodnikov, 2007).