MINISTY F EALT F EPUBLI F BELAUS EDUATINAL ESTABLISMENT VITEBSK STATE MEDIAL UNIVESITY ESTATIVE DENTAL PLYME MATEIALS A.S. uryava For Foreig studets of the 1-st year o a specialty 1-79 01 07 Stomatology Vitebsk 2016 1
УДК 547:616.31]=111(07) ББК 56.61я73 93 93 uryava A.S. ESTATIVE DENTAL PLYME MATEIALS: Maual./ A.S. uryava. Vitebsk: VSMU, 2016. 76 p. The maual estorative detal polymer materials for foreig studets of the 1-st year o a specialty 1-79 01 07 Stomatology is composed accordig to the typical academic program o bioorgaic chemistry for specialty of 1-79 01 07 «Stomatology» which is approved o August 20, 2014 (registry ТД-L. 364/тип., Misk) ad cotais program questios rgaic compouds used i stomatology. The issue is iteded for study of bioorgaic chemistry theoretical course for foreig studets of the 1-st year o a specialty 1-79 01 07 Stomatology. Утверждено и рекомендовано к изданию Центральным учебно-методическим Советом непрерывного медицинского и фармацевтического образования Витебского государственного медицинского университета 16 марта 2016 г., протокол 3. УДК 547:616.31]=111(07) ББК 56.61я73 A.S. uryava, 2016 УО «Витебский государственный медицинский университет», 2016 2
NTENTS Page I. ighmolecular compouds. Polymers. 5 1.1. The geeral characteristic of high molecular compouds. Moomer, 5 polymer, oligomer. 1.2. Polymer omeclature. 6 1.3. lassificatio of polymers. 7 1.3.1. lassificatio of polymers accordig to compositio. 7 1.3.2. lassificatio of polymers accordig to the ature of elemets 8 of the mai chai. 1.3.3. lassificatio of polymers accordig to structural characteristics. 9 1.3.4. lassificatio of polymers accordig to costitutioal isomerism 10 ad stereoisomerism. 1.3.5. lassificatio of polymers accordig to meltig behavior. 12 II. The geeral priciples of polymerizatio reactios. 13 2.1. Types of polymerizatio reactios. 13 2.2. hai-reactio (additio) polymerizatio. 15 2.2.1. Free radical chai-growth polymerizatio. 16 2.2.1.1. The mechaism of free radical polymerizatio. 16 2.2.1.2. Free radicals geeratig. Iitiators, activators, fotosesibilizators. 19 2.2.1.3. Ihibitors of free radical reactios. 22 2.2.2. atioic polymerizatio. 23 2.2.3. Aioic polymerizatio. 25 2.2.4. oordiative chai reactio polymerizatio: Ziegler-Natta 28 polymerizatio. III. Polymer detal materials. 30 3.1. lassificatio of detal materials. 30 3.2. Impressio materials. 30 3.2.1. Aqueous elastomeric impressio materials. 31 3.2.1.1. Algiate irreversible hydrocolloid impressio material. 32 3.2.1.2. Agar reversible hydrocolloid impressio material. 36 3.2.2. Noaqueous elastomeric impressio materials. 37 3.2.2.1. Polysulfides. 37 3.2.2.2. Silicoes. 38 3.2.2.3. Polyethers. 40 3.2.3. Ielastic impressio materials. 42 3.2.3.1. Waxes. ompositio of waxes. 42 3
3.2.3.2. Examples of waxes to compose ielastic impressio 43 materials. 3.3. estorative detal materials. 44 3.3.1. lassificatio of restorative detal materials by locatio ad 44 fabricatio. 3.3.2. Performace criteria of direct restorative materials. 45 3.3.3. Polymers o base acrylic ad methacrylic acids ad their esters. 46 3.3.3.1. Acrylic ad methacrylic acids ad their esters. 46 3.3.3.2. Mechaism of acrylates ad methacrylates free radical 48 polymerizatio. Polymers of heat cured ad cold (chem- ical) cured. Depolymerizatio. 3.3.3.3. Polycarboxylate cemets. Glass ioomers. 54 3.3.4. Polymers o base epoxy resis. 58 IV. Moder restorative materials. hemical-cured ad light cured 60 composites. 4.1. verview ad compositio of composites. 60 4.2. igh molecular moomers of the matrix of the moder restorative 61 materials. 4.3. Fillers ad classificatio of composites. 64 4. 4. Pigmets ad other additives. 66 V. Adhesio of restorative materials to tooth tissues eamel ad 67 deti. VI. Polyisopres: atural rubber, gutta-percha. 69 eferece materials 71 Literature 75 4
I. IGMLEULA MPUNDS. PLYMES. 1.1. GENEAL AATEISTI F IG MLEULA MPUNDS. MNME, PLYME, LIGME. igh molecular compouds are large group of atural ad sythetic compouds with high molecular weight raged from several thousad to several millio. Their molecules are called macromolecules. A polymer is a large molecule (macromolecule) cosisted of a umber of smaller repeatig uits that are made from molecules called moomers. Typical simple polymer is polyethylee that is a product of ethylee polymerizatio: 2 = 2 [- 2-2 -] is a umber of moomers ad repeatig uits. A simple repeatig uit is a least structural uit repeated may times alog macromolecular chai. A moomer uit is a repeatig uit formed from a startig moomer molecule. The simple repeatig uit of polyethylee is [- 2 - ], but the moomer uit is 2-2 - made from moomer ethylee. Polymers affect our life i may ways. e of the most sigificat ways is i the atural chemistry of life processes. The DNA that is carrier of the geetic iformatio i our cells is a macromolecule formed from four simple molecules kow as ucleotides; the proteis which costitute much of the structural framework of muscle ad boe, ad ezymes which catalyze biochemical reactios are macromolecules formed from some twety or so simple α-amio acids; cotto (cellulose) ad starch are macromolecules formed from simple carbohydrate molecules. e atural polymers is rubber that is a atural polymer of isopree cotaied all Z (cis-) double bods. Aother way is sythetic polymers. For example, polyacrylates made up from esters of acrylic ad methacrylic acids: 2 2 3 Poly (alkyl acrylate) Poly (alkyl methacrylate) A sample of a polymer cotais the macromolecules with a rage of molecular weights rather tha a sigle molecular weight. There are two defiitios of polymer molecular weight: 1) Number Average Molecular Weight (M ) is the average mass of Avogadro s umber of polymer molecules i a sample; 5
2) Weight Average Molecular Weight (M w ) is the average mass of a polymer molecule i a sample. Both of these average molecular weights ca be measured by appropriate physical methods, ad both give a geeral idea of the size of the polymer molecules i the sample of polymer. The umber average molecular weight M is more widely used. Whe values M ad M w of a polymer are close to each other, the polymer cotais the molecules with very similar molecular weight. This polymer is characterized with arrow molecular weight distributio. If the values M ad M w are the same, polymer is termed moodisperse. Whe the values M ad M w are very differet, the polymer cosists of a mixture of molecules with a wide molecular weight distributio. This polymer is termed polydisperse. The ratio of M ad M w is measured as the polydispersity of the polymer. The molecular weight of polymer macromolecules is characterized by degree of polymerizatio (). It is a measure of the average umber of moomer uits i each polymer molecule. The molecular weight of polymer is obtaied as: M (polymer) = M (moomer uit) eactios occurrig with a high degree of polymerizatio lead to high molecular weight polymers (high polymers). Polymerizatio reactios resultig from macromolecules with relatively low molecular weights are desigated as occurrig with a low degree of polymerizatio. The extet to which the moomer has bee cosumed is called degree of coversio. The course of polymerizatio is characterized i terms of the amout of high polymer preset at low coversio. Macromolecules are broadly categorized by size based o degree of polymerizatio: 1. dimers cotai two moomer uits; 2. trimers cotai three moomer uits; 3. tetramer cotai four moomer uits; 4. oligomers are small macromolecules with 10-50(100) moomer repeat uits; 5. polymers are macromolecules with more tha 100 moomer repeat uits. Polymers may cotai may thousad of moomer uits of the polymer molecule; atural macromolecules cotai millios of repeatig uits. ligomers, cotaied differet umber of moomer uits, have differet physical ad chemical properties. 1.2. PLYME NMENLATUE. There are some logical IUPA rules for amig polymers. hemists ame the moomer ad the add the poly- prefix. Usually chemists use commo ames of moomers more ofte tha their IUPA ames. If the moomer s ame icludes more tha oe word, or if a letter or umber precedes the ame, the moomer s ame is eclosed i paretheses (Table 1). 6
TABLE 1. The ame of moomers ad polymers. Moomer Moomer ame Polymer Polymer ame 2 2 Ethylee 2 2 Polyethylee 2 3 Propylee 2 3 Polypropylee 2 N Acryloitrile 2 N Polyacryloitrile 2 3 Methyl methacrylate 3 2 Poly(methyl methacrylate) 3 3 2 Viyl acetate 3 2 3 Poly(viyl acetate) 2 l Viyl chloride 2 l Poly(viyl chloride) 2 Styree 2 Polystyree 2 2 ad Ethylee glycol Terephthalic acid 2 2 Poly(ethylee terephthalate) 1.3. LASSIFIATIN F PLYMES. 1.3.1. lassificatio of polymers accordig to the compositio. The compositio of polymer is a sequece of moomer repeatig uits that are covaletly boded together. 7
epeatig uits of polymers have a variety of possible structures. Whe all repeatig uits i a polymer have the same structure, this polymer is called a homopolymer A A A A A A A A A A example of a homopolymer is polystyree: 2 2 2 2 or 2 A Styree Moomer Polystyree omopolymer Polystyree epeatig uit Whe the polymer chai cosists of differet repeatig uits, the polymer is called a copolymer. There are three types of copolymers: 1. Alteratig copolymer A B A B A B A B A B 2. Block copolymer ma B A A A A A B B B 3. adom copolymer ma B A A A B A B B B 1.3.2. lassificatio of polymers accordig to the ature of elemets of mai chai. omochai polymers (carbochai) have oly carbo atom i a mai chai of polymers. Examples are polyviyl polymers like polyethylee, polypropylee, polyacryloitrile, polyvyilechloride, polyacrylates, polystyree (see poit 1.2.) ad polybuthadiees like atural rubber. 3 2 2 2 3 2 3 2 2 Natural rubber: cis-1,4-polyisopree eterochai polymers have differet atoms i a mai chai of polymer. There are atural, iorgaic ad elemet-orgaic polymers. eterochai polymers are also classified based o ature of heteroatom or fuctioal group. Examples of itroge cotaiig polymers are both atural protei ad sythetic polyamides (both are polyamides). 8
N protei N 2 m polyamides N N is ( 2 ) x or 6 4 Examples of oxyge cotaiig polymers are: -ethers like polyethyleoxide 2 -polyacetals like atural polysaccharides, for example, amylopecti of starch ( = 20-25) ad glycoge ( = 10-12) 2 2 2 2 2 -ucleic acids. -esters like polyethyleetherephtalate 2 2 Elemet-orgaic polymers have oly heteroatom i a mai chai. For example polysiloxaes, polyalumoxaes Si PLYSILXANES Al PLYALUMXANES 1.3.3. lassificatio of polymers accordig to structural characteristics. 1. A liear polymer is a molecule with a series of coected repeatig uits. A A A A A A A A 9
2. A brached polymer has bods brachig from the backboe of a liear polymer. A A A A A A A A A A A A Examples of brached homopolymers are aturally occurrig homopolysaccharides like amylopecti of starch ad glycoge (see poit 1.3.2.). e polymer may be joied to the other as a brach from the mai chai of the other polymer. The formed copolymer is called also a graft copolymer. A A A A A A A A B B B B 3. ross-liked polymers are liear polymer molecules joied by a brachig coectio. Example is vulcaized rubber. A A A A A A A A S S A A A A A A A A S S A A A A A A A A S S S - coected uit 1.3.4. lassificatio of polymers accordig to the costitutioal isomerism ad stereoisomerism. The three-dimesioal structure ad shape of macromolecule deped o its chemical structure ad arragemet of moomer uits ad side chais to each other i a space. There are regular ad irregular polymer structures. The regular polymer has the same three-dimesioal structure of alterate moomer uits alog macromolecular chai. hage of this alteratio lead to irregular polymer structure. egularity ad irregularity deped o type of additio ad cofiguratio of moomer uits. Polymerizatio of alkees 2 =- lead to differet way of additio: 1. ead to tail (α,β-additio) β 2 α β 2 α β 2 α β 2 α 2. ead to head (α,α-additio) ad tail to tail (β,β-additio) 10
β 2 α β 2 α α β 2 β 2 α α β 2 α,β-additio, α,α-additio ad β,β-additio form regular polymer. Polymerizatio of 1,3-butadiees by 1,4- or 1,2-aditio forms regular polybutadiees. 2 2 2 2 2 2 2 2 1,4-additio 2 2 2 1,2-additio 2 2 2 3. adom additio lead irregular alteratio of α,β-, α,α- ad β,β-additio of poly ethylees ad 1.4-, 1,2-additio of poly butadiees. The polymerizatio of moosubstituted alkees ivolves the formatio of a ew chiral ceter with every molecule of alkee added to the growig polymer chai. 2 2 2 * * 2 * The stereochemical relatioship amog the side groups o the backboe of the polymer chai is called tacticity. There are three geeral ways i which every ew chiral ceter ca be itroduced: 1. I the structure of a isotactic polymer the ew chiral ceter is itroduced with a cofiguratio the same as that of the previous chiral ceter. A isotactic polymer arrages all its substituets o the same side of the polymer chai i a zig-zag coformatio. 3 3 3 3 3 3 3 3 3 isotactic: methyl groups are all 1,3-sy 2. I the structure of a sydiotactic polymer the ew chiral ceter is itroduced with a cofiguratio opposite to that of the previous chiral ceter. The substituets of a sydiotactic polymer are o alteratig sides of the zig-zag structure. 11
3 3 3 3 3 3 3 3 3 sydiotactic: methyl groups are all 1,3-ati 3. The atactic polymer has its substituets arraged radomly o the chai. 3 3 3 3 3 3 3 3 3 atactic: methyl stereochemistry is radom Polymeryzatio of cojugated alkadiees ulike simple alkees leads to polymers (polybutadiees) cotaied double bod of every repeatig moomer uit. There are cis-, tras-isomerism of moomer uits. 2 2 2 2 2 is-isomer 2 2 2 2 2 2 2 Tras-isomer Both cis- ad tras-isomers are stereo regular. adom alteratio of cisad tras-cofiguratio of moomer repeatig uits characterizes stereo irregular polymer. 1.3.5. lassificatio of polymers accordig to the meltig behavior. Accordig to meltig behavior polymers are classified ito thermoplastic ad thermosettig polymers. The liear polymers become plastic at high temperatures ad ca be cast ito a desired shape. Polymer materials are called thermoplastic if the cycle of heatig, meltig ad castig ito shape ca be repeated may times. ross-liked polymers rather tha a liear structure ted ot to melt. ather, they decompose i heatig. These materials are called thermosettig polymers. They caot be heated ad molded, so their use requires them to be i the fial shape whe the polymerizatio reactio occurs. ce a thermosettig polymer has cooled i a particular shape, its shape caot be altered by 12
heatig. They ted to be stroger ad tougher tha thermoplastic materials. Most detal polymers are cross-liked ad are thermoset. II. GENEAL PINIPLES F PLYMEIZATIN EATINS. 2.1. TYPES F PLYMEIZATIN EATINS. The chemical reactio that liks the moomer together to produce a polymer (macromolecule) is called polymerizatio. There are two types of polymerizatio reactio mechaisms: 1. hai-growth polymerizatio (or additio polymerizatio). 2. Step-growth polymerizatio (or codesatio polymerizatio). I a chai-growth polymerizatio oe moomer reacts with aother moomer that cotais the same fuctioal groups. The growig polymer chai is exteded by additio reactio: the additio of reactive species a free radical, a carbocatio, or a carbaio to the π-bod of a usaturated moomer. I a result every moomer uit loses oe π-bod. hai-reactio polymerizatio of alkee moomers lead to formatio of saturated macromolecules. 2 * 2 * i i 2 2 2 2 i * 2 2 i * 2 2 2 i * * =, or = -, -Alk, -N, -Ar, -al,, -N 2, -, - 3, - 3... The mai chai of the additioal polymer does t have ay fuctioal group. Fuctioal group ca be oly i a side chai, ad they determie the chemical ad physical properties of resultig polymer. There are polymerizatios of ethylee, acetylee ad butadiee derivatives. ' ' 2 polyethylees 2 13 poly viylees 2 2 poly butadiees
A step-growth polymerizatio takes place betwee moomers cotaiig differet fuctioal groups. I the reactio mixture each fuctioal group type ca react with the other type but ot with itself. The chai begis to grow whe oe moomer liks to aother moomer cotaiig the other fuctioal group type. The first moomer the reacts with the ed of this two uit chai. The polymer chai grows by the substitutio reactio betwee the differet fuctioal groups with the elimiatio of oe small molecule per moomer durig each reactio. The polymer chai is exteded ot oly durig the codesatio of moomer molecules with the growig macromolecule. Two growig polymer chais also frequetly codese to form a larger macromolecule provided that the required two fuctioal groups are preset ad accessible for reactio. X X Nu ' Nu - X X Nu ' X Nu X - X X Nu ' Nu X X Nu ' Nu Nu ' Nu X Nu ' Nu m Nu ' Nu - X X Nu ' Nu Nu ' Nu Nu ' Nu m Nu ' Nu For example, the codesatio polymerizatio of diamies ad dicarboxylic asids (or their acid chlorides) lead to form poly amides. 2 N ' N 2 N ' N 2-1 2 2 N ' N 2 N ' l l N l 2-1 l The codesatio polymerizatio of diols (ethaediol) ad dicarboxylic acids (terephthalic acid) lead to form poly esters like poly (ethylee therephthalate). 14
2 2 2-1 2 2 2 Differeces betwee two mechaisms are summarized i Table 2. I chai-reactio polymerizatio, the rate of polymerizatio depeds o the umber of active sites available for polymerizatio. The rate of polymerizatio is zero util the polymerizatio is iitiated, ad it rapidly rises to a maximum value limited by the umber of active sites available. While the reactio is proceedig, its rate remais more or less costat util all the iitiator is cosumed. At this time it slowly decreases as the available moomer ad the umber of growig chais decreases. I cotrast to this, the rate of polymerizatio i a step-reactio polymerizatio depeds o the umber of fuctioal group available to participate i the polymerizatio reactio. This umber is at its maximum value at the begiig of reactio, the rate of polymerizatio decreases cotiuously durig reactio as the available fuctioal groups are cosumed. Table 2. Experimetal osequeces of Mechaism of Polymerizatio. hai-eactio Polymerizatio A. hai Growth Additio of moomer to a small umber of active ceters. B. ate of Polymerizatio Iitial rate is zero; it icreases rapidly to a maximum value which remais costat util the iitiator is cosumed, after which it slowly decreases. Moomer is still preset at high coversio.. Degree of Polymerizatio igh polymer preset eve at low coversios. Degree of polymerizatio may be extremely high. Step-eactio Polymerizatio ouplig of ay two species with the appropriate fuctioal groups. Iitial rate is maximum; rate decreases cotiuously durig the reactio. Moomer is rapidly coverted to small oligomers which the slowly codese. Moomer is abset eve at low coversio. igh polymer ot preset util high coversio. Degree of polymerizatio seldom high. 2.2. AIN-EATIN (ADDITIN) PLYMEIZATIN hai-reactio polymerizatio, or additio polymerizatio, is maily used for productio of several ecoomically importat polymers: polyethylee, polypropylee, poly(viyl chloride), polystyree ad may acrylic polymers. 15
The mechaism for a chai-growth polymerizatio ivolves iitiatio, propagatio ad termiatio steps. hai-reactio polymerizatio occurs through a reactive itermediate which may be a free radical, a catio or a aio. There are ioic or radical chai-growth polymerizatios. 2.2.1. Free radical chai-growth polymerizatio. 2.2.1.1. Mechaism of free radical polymerizatio. I free radical polymerizatio, formatio of macromolecule is iitiated by addig free radical to a alkee or alkye. Several polymers, such as polyviyl chloride, polystyree, acrylate polymers, ad polyethylee, are made by free radical additio reactios. The most useful moomers for free radical chaireactio polymerizatio are moosubstituted alkees or 1,1-disubstituted alkees. 1,2-Disubstituted ethylees or more highly substituted ethylee derivatives are usually resistat to free radical polymerizatio. Free radical chai-reactio polymerizatio is a typical free radical chai reactio: it requires iitiatio to geerate the chai carriers, ad it will cotiue util the reactio chai is termiated by a reactio or reactios which remove the chai carriers from the reactio system. So, free radical polymerizatio reactio ivolves three steps: iitiatio, propagatio ad termiatio steps. 1. Iitiatio step. The iitiatio ca be thought of as two reactios. The first ivolves the homolitic cleavage of iitiator molecule to form a free radical: + I the secod reactio the free radical adds to the π-bod of the moomer molecule to make sigle - bod ad aother free radical o the ed of the growig chai. So, the first lik i the growig chai has bee added. 2 ' 2 ' 22.5 kkal/mol 2. Propagatio step. Propagatio step is the growth or legtheig of the chai. The free carbo radical of the iitiated chai is added to the moomer ad the chai is oe moomer loger. The additio follows Markovikov s rule: the more stable of two isomeric radicals are formed as major product. It is secodary or tertiary radical. The product is a free radical, so it ca be added to the ext moomer molecule to give a ew radical. 16
2 ' 2 ' 2 ' 2 ' 22.5 kkal/mol 2 2 2 2 22.5 kkal/mol ' -1 ' ' ' 2 ' The additio reactio is repeated util all moomers are cosumed or util the growth of the free radical polymer chai is termiated whe the chai carrier is lost. 3. Termiatio step. There are several potetial chai termiatio reactios, but most commo are two types. I the first type of termiatio reactio, the free radical chai carriers are removed from reactig system by free radical couplig (combiatio) or atom trasfer (disproportioatio) to form o-radical product. Free radical couplig (combiatio): 2 2 2 2 2 2 2 2 ' ' ' ' ' ' ' ' I a combiatio reactio two free radicals, either of the same or differet legths are combied to form a stable molecule. Atom trasfer (disproportioatio): 2 ' ' ' 2 ' 2 2 2 2 2 ' I a disproportioatio reactio, oe radical is oxidized to a alkee, while aother is reduced to a alkae. I the secod type of termiatio reactio, the reactive free radicals react with molecules to give a ew free radical, which is ot reactive eough to add to the moomer, ad the chai growth is halted. Such a powerful termiator of free radical chai reactios is molecular oxyge, because it is a stable biradical. The 17 ' ' '
molecular oxyge from the air termiates free radical polymerizatio ad provides residual moomer presece o the surface of polymer product. 2 ' 2 ' 2 2 ' ' e importat side-reactio that occurs durig free radical chai-reactio polymerizatio of alkees is pheomeo of chai trasfer. I this reactio oe growig macromolecule chai is termiated by trasfer of a hydroge atom from aother molecule or from a atom withi the same macromolecular chai. The result is creatio of a ew free radical, which the cotiues the chaireactio polymerizatio. hai trasfer reactios give macromolecules with shorter chais ad brachig with irregular degree. ' 2 ' 2 2 2 ' 2 ' 2 ' ' 2 2 ' ertai solvets or impurities may also fuctio as chai trasfer agets, for example tetrachlormethae. 2 2 l l 3 2 2 l l 3 ' ' ' ' These ew radicals the react with a moomer startig a ew growig chai. l 3 2 ' Termiatio reactios are irregular, so free radical polymerizatio leads to form polymers that are polydisperse i their molecular weight. l 3 2 ' 18
2.2.1.2. Free radical geeratig. Iitiators, activators, fotosesibilizators. Excitemet of moomer molecule leadig to high reactive free radical i the iitiatio step of free radical polymerizatio requires much eergy. There is several methods providig trasmissio of eergy to activatio of the moomer: 1) high temperature (heat iitiatio); 2) hard light (light iitiatio); 3) radiatio; 4) iitiators. The iitiatio of free radical chai reactios ivolves the productio of a free radical from a o-radical precursor that is a iitiator. There are several methods for the formatio of free radicals: homolysis of o-radical precursors, fragmetatio, radical additio, atom abstractio ad electro trasfer. Most iitiators geerate free radicals by homolysis of a relatively weak bod or by formatio of a exceptioally stable molecule durig the homolysis. The bods most susceptible to homolysis are σ-bods betwee heteroatoms havig relatively low bod eergy ( 25 kkal/mol or 105 168 kj/mol): Br-Br, l- l, N-, -, -l, N-l ad so o. The most commo compouds geeratig free radicals by homolysis are orgaic peroxides ad hydroperoxides (the weak - bod which easy homolyzes) ad orgaic azo compouds (homolisis produces N 2 that is a extremely stable molecule despite -N bod eergy is eough high - 70 kkal/mol or 293 kj/mol). The most popular radical iitiators are bezoyl peroxide ad azo-bis-isobutyroitrile (AIBN). eatig or irradiatio with ultraviolet light of bezoyl peroxide or of AIBN gives two free radicals. bezoyl peroxide or hν 80-95 0 3 N 3 N N AIBN 3 3 or hν N N N N 3 3 3 3 N The alkyl peroxides ad hydroperoxides udergo homolysis by the same way. Most typical are dicumee peroxide, tert-butyl peroxide ad cumee hydroperoxide: 3 3 3 3 3 T ert-butyl peroxide 3 120-140 0 3 2 3 3 3 3 3 3 120-140 0 2 3 3 Dicumee peroxide 19
3 3 130-140 0 3 3 umee hydroperoxide xyge-cetered radicals alkoxy radicals ad carboxy radicals are typically very high eergy ad extremely reactive. The high eergy of these radicals provides their propesity to fragmet to carbo-cetered radicals ad carboyl compouds. Thus carboxy free radicals rapidly decompose by decarboxylatio to give alkyl radicals ad carbo dioxide. For example, bezoyloxy radical formed by homolysis of bezoyl peroxide is decarboxylated to give pheyl radical ad carbo dioxide. bezoyloxy radical omolysis of iitiators ca be carried out by heat, light or chemical reactios. As a result of various activatio methods, polymerized (cured) materials are classified as heat-activated (also kow as heat-cure), light activated (also kow as photo-cure or light-cure) or chemically activated (also kow as coldcure or chemical-cure). hemically activated polymerizatio is accelerated by the presets of special compouds redactors, which are called activators. Most commo chemical activators are ferrous (Fe 3+ ) ad some other metals (r 3+, V 2+, Ti 3+, u + ) salts, sulfites (S 3 2- ), thiosulfates (S 2 3 2- ), tertiary amies, both aromatic ad aliphatic, for example N,N-dimethyl-p-toluidie or 4-viyl-2-methylailie. 3 3 N 3 N,N-dimethyl-para-toluidie 2-2 - 3 N 2-2 - N,N-dihydroxyethyl-para-toluidie xidatio reductio reactios betwee iitiator ad activator form free radicals leadig iitiatio step of polymerizatio reactios. Examples are the ext: eactio betwee hydroge peroxide ad ferrous salts 2 2 + Fe 2+ + - + Fe 3+ eactio betwee orgaic peroxide ad ferrous salts 20
' Fe 2+ ' Fe 3+ + Fe 2+ - Fe 3+ eactio betwee persulfates ad ferrous salts, sulfites ad thiosulfites 3 S S Fe 2+ 2-3 S 4 S 4 Fe 3+ 3 S S S 2 2-3 3 S 2-4 S 4 S 2 3 - S 4 2 S 4 - eactio betwee peroxide, for example bezoyl peroxide, ad aromatic amie: Ar Ar N 3 3 3 N 3 3 N Ar 3 xidatio reductio reactios (chemical activatio) require less activatio eergy (50-84 kj/mol) tha heat activatio (125-170 kj/mol), so chemical activatio of polymerizatio is accomplished at room temperature. Light-activated polymerizatios use several activator ad iitiator compouds. The activator absorbs light ad the reacts with the iitiator. The polymerizatio process does ot begi util the material is exposed to a very bright light source. Light activatio is accomplished with blue light at a peak wavelegth of about 465 m, which is absorbed usually by a photo-sesitizer, such as camphorquioe. 3 3 3 amphorquioe 21
Several detal materials have both chemical ad light-activated capabilities. These materials are called dual-cure materials. Polymerizatio is started with a curig light, but material that caot be reached by the itese light sets by the chemical activatio mechaism. 2.2.1.3. Ihibitors of free-radical reactios. Polymerizatio reactios ca occur very rapidly. For a polymerized (ad detal also) material to be useful, the reactio must occur whe desired or be delayed after mixig ad occur somewhat slowly ad also whe moomers are stored. The reactio of chemically activated materials is iitially delayed for several miutes by the presece of compouds called a ihibitor. Ihibitors are compouds that prevet the free radical chai mechaism from occurrig ad stop polymerizatio reactio. Ihibitors react with free radicals formed i the iitiatio or propagatio step to form compouds or radicals that are ot iitiators of ew chais due to high stability. This reactio competes with the polymerizatio reactio ad wis. The competig reactio of ihibitor with free radicals delays polymerizatio ad provides workig time for placemet, moldig ad shapig of the material. Without this delay, materials would set too fast for use. After a period of time the ihibitor is used up ad polymerizatio begis to occur. Typical ihibitors are polyhydric pheols (hydroquioe, catechol, pyrogallol), aromatic amies (N-pheyl-2-aphthylamie, 4-amio-1-aphthol), aromatic itro compouds (1,3,5-tryitrobezee, picric acid), sulfur, iodie ad also copper, ferric, zic salts of salicylic, acrylic ad methacrylic acids. The delayed effect of ihibitor depeds o stability of its product with free radicals ad structure of moomer. So, usig of polyhydric pheols as ihibitors of polymerizatio reactios ad atioxidats depeds o their high reactivity i oxidatio reactios. For example, hydroquioe is easy oxidized by free-radical mechaism to form itermediate pheoxy radicals ad aio-radical semiquioe with high stability. [] - + -e - hydroquioe -e - - + stable pheoxy radical semiquioe p-bezoquioe The pheoxy radical ad semiquioe aio radical are stabilized by cojugatio, which ca be represeted as resoace hybrid of four caoical forms 22
Semiquio is relatively stable radical; it does t have ability to abstract atom with upaired electro ad iitiate chai free-radical reactio. But semiquioe ca accept chai carbo radicals ad termiates free-radical reactios. That s why semiquioes are ihibitors of free-radical polymerizatio reactios. 2.2.2. atioic polymerizatio. The chai reactio polymerizatio of electro-rich alkees such as eol ethers, styree ad isobutylee ca be carried out uder coditios where the chai carrier is a carbocatio rather tha a free radical, because these alkees ca react with e electrophile to give a relatively stable catioic itermediate. The example is catioic polymerizatio of isobutylee polymerizatio: 2 Moomers: isobutylee 3 3 BF 3, 2, 80 0 2 3 3 Polyisobutylee The most commo iitiators of catioic polymerizatio are protoic acids ( 2 S 4 ) or Lewis acids (BF 3 ad Til 4 ). Lewis acids are used together with lowmolecular compouds (co-catalysts) like the water or gaseous hydroge chloride, resulted proto releasig: 2 + Til 4 + + [Til 4 ] - l + Til 4 + + Til 5 The iitiatio step i a catioic polymerizatio of isobutylee is to add a proto to the double bod of alkee to form a carbocatio. Electrophilic additio to each alkee moomer occurs with Markovikov rule. The ease of polymerizatio parallels stability of the itermediate carbocatio formed. I our example additio of proto to the double bod of isobutylee leads to formatio of tertiary carbocatio, stabilized by free electro releasig alkyl (methyl) groups. 23
3 3 2 + A - 3 A 3 3 The propagatio step ivolves reactio of a carbocatio with a molecule of alkee to form a ew most stabilized carbocatio. This step repeats itself util the reactio either rus out of reactats or the polymers react i a termiatio step. 3 3 3 3 2 A 2 + A - 3 3 3 3 3 A 3 3 3 2 3 3 A 2 3 3 2 3 2 3 A 3 3 3 3 3 3 2 3 2 3 A 2 3 3 2 3 3 3 3-1 2 3 A 3 The termiatio step. The growig polymer chai may be termiated by loss of a proto from the carbo atom adjacet to the carbocatio ceter as result of chai-trasfer reactios: 1) hai-trasfer to the moomer as base. 2 3 3 3 3 δ- 2 A 2 3 3 3 A 3 3 3 3 3 3 2) hai-trasfer to the aio formed as result of reactio betwee Lewis acid ad co-catalist. 2 3 3 A [Til 4 ] 2 3 3 [Til 4 ] 3 3 3 3 [Til 4 ] 2 Til 4 24
Aother way of chai trasfer reactios ca be carbocatio removig hydride aio from moomer or polymer molecule. 2 3 2 3 3 A 2 2 3 2 3 2 3 3 3 2 3 3 2 A The chai-trasfer reactios do t lead kietic chai termiatio, because oe of products is catio carbocatio or proto that ca start to grow the ew polymer chai. The kietic chai termiatio may be resulted i reactio betwee the growig polymer carbocatio ad aio from Lewis acid: 2 3 2 3 A 3 3 2 2 [Til 4 ] Til 4 3 3 3 3 These termiatio reactios occur fairly readily, so catioic polymerizatio usually leads to lower molecular weight polymers tha those obtaied by free radical polymerizatio. But usig Lewis acids ad co-catalysts for catioic polymerizatio leads to the higher molecular weight polymers. The solvatio ad stability of carbocatio reactive site ad aio is icreased, whe the polarity of solvet is icreased. That s why, probability of termiatio reactio decreases ad higher molecular weight polymer formatio occurs. 2.2.3. Aioic polymerizatio. Alkees are ot ormally susceptible to additio of ucleophiles because the π-bod electro desity of simple alkees is too high. But the presece of electro-acceptig (withdrawig) groups such as a carboyl or alkoxycarboyl group (-=, -), cyao group (-N) ad itro group (-N 2 ) i cojugatio with the alkee π-bod make it more accessible to ucleophiles because cojugatio of π-bod to these groups lowers its electro desity as result of egative iductive ad egative resoace (mesomeric) substituet effects, for example i the molecule of propeeitrile: 2 δ+ δ- N 25
Electro acceptig substituets ca also stabilize a adjacet egative charge of itermediate carbaioe by iductio ad cojugatio, for example: 2 N 2 N ommo iitiators of aioic polymerizatio are strog ucleophiles that ate strog bases such as alkyllithiums ad alkylsodiums or potassium amide. There are Ziegler-Natta catalysts prepared from trasitio metal halide o base elemets of IV-VII groups of periodic table ad metal alkyls based o elemets of I-III groups of periodic table. Aioic chai reactio polymerizatio occurs i much the same maer as free radical or catioic polymerizatio. e example of aioic polymerizatio is polymerizatio of ethylee derivatives with electro acceptig groups i presece of potassium amide i liquid ammoia. 2 KN 2 /N 3 2 Y Y The mechaism of aioic polymerizatio ivolves iitiatio, propagatio ad termiatio steps. Iitiatio step KN 2 K N 2 N 2 2 2 N 2 Propagatio step Y Y 2 N 2 2 2 N 2 2 Y Y Y Y 2 N 2 2 2 2 N 2 2 Y Y Y Y 2 Y 2 N 2 2 2 2 N 2 2 Y -1 Y Y Y Y 26
Ulike free radical or catioic polymerizatio there are o effective reactios for termiatio of aioic polymerizatio uless the additio of a electrophile to reactio mixture. The aioic polymers are called also as livig polymers. The aioic polymerizatio reactio cotiues util all the moomer is cosumed, at this time the polymerizatio reactio ceases. I the absece of termiatio reactio, the eds of polymer chais still carry the reactive aioic species, ad the additio of more moomer to the reactio vessel results i the polymerizatio reactio begiig agai the polymer is alive. This characteristic of aioic polymerizatio is used for producig block copolymers: after the first moomer is cosumed, the secod moomer is added. This process ca be repeated several times to give a block copolymer with desired properties. 2 N 2 2 2 2 N 2 2 Y Y A livig polymer Z Y Z 2 Z 2 N 2 Y 2 Z 2 m A block copolymer Z Termiatio step Sice the chai carriers i aioic polymerizatio are strog ucleophiles, a wide variety of electrophiles may be used to termiate the reactios such as acids, alkyl halides, esters ad epoxides. Electrophiles are deliberately added to the reactio mixture to carry out termiatio reactios. 2 N 2 2 E 2 N 2 2 E Y Y Y Y 2 N 2 2 2 E 2 N 2 2 2 E Y Z m Z Y Z m Z Sodium ad potassium amides are commo iitiators for polymerizatio of derivatives acrylic ad methacrylic acids such as methyl acrylat, acryloitrile ad methacryloitrile. 27
2.2.4. oordiative chai reactio polymerizatio: Ziegler-Natta polymerizatio. Free radical polymerizatio usually produces atactic polymers. Sice may of physical properties of atactic polymers make them less desirable for commercial use tha stereoregular (isotactic or sydiotactic) polymers, coordiative chai-reactio polymerizatio is more used. It is a catalytic, lowtemperature ad low-pressure polymerizatio of alkees, which is kow as Ziegler-Natta polymerizatio after its two discoverers, Karl Ziegler ad Giulio Natta. Ziegler-Natta polymerizatio is catalyzed by a trasitio metal catalyst prepared from a trasitio metal halide ad metal alkyl, more usually from titaium tetrachloride ad triethylalumium. Because the reactio occurs at a metal ceter, this method allows such high levels of cotrol over the stereochemistry ad molecular weight of the polymers produced simply by adjustig the experimetal parameters. For example, oe may produce either sydiotactic or isotactic polypropylee simply by varyig the catalyst ad reactio coditios. The tacticity of polymer strogly affects polymer s properties. Sydiotactic ad isotactic polypropylee are more crystallie tha atactic polypropylee. Atactic polypropylee is a soft, low meltig amorphous solid, but isotactic polypropylee is highly crystallie ad melts at 170. The mechaism with the Ziegler-Natta catalysis begis with formatio of a complex betwee titaium ad alumium. 3 2 3 Al Al( 2 3 ) 2 3 Til 4 l l Ti 2 3 2 3 l l I this complex titaium has a empty orbit available for iteractio with the π electros i the moomer, thus, allowig formatio of π complex. The π complex oriets the methyl group away from the titaium ad towards the alumium. The π-complex the rapidly rearrages: 3 2 3 Al 2 l Ti l l l 2 2 3 3 3 2 3 l Ti l l Al l 2 2 3 3 π omplex of Ti ad alkee 28
The alkyl group of alumium rapidly trasfers to carbocatio so that it does ot have time to rearrage. Thus, the titaium stereo-specifically iserts the propylee group betwee itself ad the ethyl group. 3 2 3 l Ti l l Al l 2 2 3 3 3 3 2 l l 3 2 Al 2 l Ti l 2 3 The titaium complex cotiues to stereo-specifically isert additioal propylee moomers betwee itself ad the last ethyl group added to the chai. This repeatig reactio forms the isotactic polymer. 3 3 2 l l 3 2 Al 2 l Ti l 2 3 2 3 3 2 3 2 3 l l 3 2 Al 2 3 2 l Ti l To ed the reactio, the complex is destroyed by treatig it with methaol: 3 2 3 2 3 l 3 2 3 Al 2 3 2 l Ti Til 4 3 3 3 Al( 3 ) 3 3 l l 3 2 2 2 29
III. PLYME DENTAL MATEIALS. 3.1. LASSIFIATIN F DENTAL MATEIALS. Polymer detal materials are classified by followig categories: 1. Detal materials for the orthopedic stomatology: - Impressio materials for casts ad models; - Deture base resis, materials for crows, a false tooth ad bridges. 2. Direct restorative detal materials for the therapeutic stomatology: - Materials for permaet restoratios to repair aatomy shape ad fuctios of tooth; - Materials for temporary fillig of cavity preparatio durig treatmet of high caries activity; - Materials for liers ad bases before placemet of permaet restoratios; - oot caal fillig materials; - Prevetive materials: pit ad fissure sealats to prevet decay ad caries. 3. Prosthesis called complete detures replaced missig teeth; boe ad gigiva after the teeth have bee lost or extracted. 3.2. IMPESSIN MATEIALS. Impressio materials are used to make a accurate replicas (models or casts) or mold hard ad soft oral tissues. Detists take impressios of teeth ad their supportig structures icludig gigiva, alveolar boe or residual ridge, hard ad soft palate ad freums that are muscle attachmets. The impressio is a egative reproductio of tissues. Fillig the impressio with detal stoe or other model material make a positive replica (cast) that is removed after the model material has set. The positive replicas (casts) are used to fabricate restoratios ad prostheses. Impressio materials set either by a chemical reactio or by a physical chage. The chemical reactio ivolves chai legtheig, cross-likig or both to give thermosettig impressio materials. ther impressio materials set by a physical chage whe they cool, either by solidificatio or by gelatio; they are thermoplastic. I geeral, thermoplastic materials are ot as stable as thermoset materials. Impressio materials set to form elastic or solid ielastic replica. There are several types of impressio materials: 1) Aqueous elastomeric impressio materials: a. Algiate (irreversible hydrocolloid) - thermoset. b. Agar (reversible hydrocolloid) - thermoplastic. 2) Noaqueous elastomeric impressio materials either are thermoset: a. Polysulfides. b. odesatio silicoes. c. Polyethers. 30
d. Additio silicoe. 3) Ielastic impressio materials: a. Wax ad impressio compoud - thermoplastic. b. Plaster ad zic oxide-eugeole - thermoset. 3.2.1. Aqueous elastomeric impressio materials. Aqueous elastomeric impressio materials are based o two polysaccharides - salts of algiic acid ad agar-agar. Both are cosidered to be hydrocolloid materials. Settig of this materials forms colloid of gel type. olloid is a substace cotaiig two or more phases with the uits of at least oe of phases havig a dimesio slightly greater tha simple molecular size (1 to 500 m). Thus colloidal systems are fie dispersios - sols, gels, films, emulsios or foams. Algiate ad agar impressio materials chage from the sol colloid state to the gel colloid state. A sol resembles a solutio, but it is made up of colloidal particles dispersed i liquid; sol is a viscous liquid. Whe sol is cooled or caused to react by addig ay compouds, it is trasformed ito a semisolid, rubbery state, called a gel. A gel has a etagled framework of solid colloidal particles with liquid that is trapped i iterstices ad held by capillarity. Because the liquid phase of sols ad gels is the water, algiate ad agar are called hydrocolloid impressio materials. There are reversible ad irreversible hydrocolloids. Algiate impressio material set via chemical reactio, it is called irreversible hydrocolloid. eatig of settig algiate results i warm algiate, it does ot reverse back to the sol state. Agar gels by physical chage i coolig, it is called reversible hydrocolloid. This impressio material reverse back to the sol state whe heated, the chage agai to the gel state whe cooled. Gels with water liquid phase are hydrophilic ad ted to imbibe large quatities of water whe allowed to stad submerged. The imbibitio leads to swell ad chage physical dimesios. I dry air the gel loses water with a accompayig shrikage. This property of gel makes some advatages ad disadvatages for hydrocolloid impressio materials. e of advatages is that reversible ad irreversible hydrocolloids will wet a tooth surface that is cotamiated by oral fluids. ydrocolloid material absorbs a limited amout of oral fluid. Pourig the impressio with detal plaster to make gypsum casts or models is easier tha with elastomeric impressio material. The gypsum product mixed with water also easily wets the surface of the impressio material. Disadvatage of this material is that water evaporates from the surface of impressio i air. As the result the impressio shriks ad is o loger accurate. 31
3.2.1.1. Algiate - irreversible hydrocolloid impressio material. Potassium ad sodium salts of algiic acid have properties that make them suitable for compoudig a detal impressio material. Algiic acid is prepared from marie plat algae. Algiic acid is a high-molecular-weight block copolymer of β-d-mauroic acid ad α-l-guluroic acid joied together by 1,4- glycosidic likages. β-1,4-glycosidic likage D-mauroic acid L-guluroic acid Properties of algiate materials deped o degree of polymerizatio ad ratio of guluroa ad mauroa block i macromolecules. The mauroa residues are stretched ad flat, the guluroa residues cotribute less flexibility. Potassium ad sodium algiats are lie exteded molecules. Na Na Na D-mauroic acid algiic acid L-guluroic acid sodium algiat Whe lie algiate salt molecules react with polyvalet metal catios the cross-liked etwork polymer is obtaied. ross-likages are formed betwee carboxylic groups of two macromolecules: 2 Na Na Na Na as 4 32
a Na Na a Na Na a Macromolequles ca be cross-liked by chelate complex formatio betwee two vicial hydroxyl groups of oe macromolecule ad catio formed salt with two carboxyl group of other macromolecule. 2 Na Na Na Na as 4 a Na Na a Na a Na Upo mixig with water, the algiate impressio material first forms viscous sol. I followig cross-likig reactio with calcium sulfate the water 33
molecules disperse ito the small pockets betwee liked macromolecules to form a isoluble elastic gel called calcium algiate. The ability of algiates to form gel is maily related to proportio of L-guluroa blocks that maily bid with a 2+. Thus algiates cotaied predomiatly guluroa residues form strog ad brittle gel, algiates rich i mauroa form weaker ad more elastic gels. The quality of gel structure depeds o ature of cross-likig catio ad the rate of its additio. Slow additio of catio leads to formatio of polymolecular ad uder-molecular structures (gel particles) ad followig likig betwee this gel particles to give tree-dimesioal gel structure. This method is preferred for impressio preparatio. That s why maufacturers use partially water soluble salts; it allows slow movemets of catios to the solutio ad slow reactio. The quality ad stregth of cross-likages depeds o complexformig ability of polyvalet catio. Usually maufacturers use partially soluble salts of a 2+, Ba 2+, Pb 2+, S 2+ catios: as 4, (as 4 ) 2 2, BaS 4, Ba 3, SrS 4, PbSi 3. Settig times of algiate impressio materials rage from 1 to 5 miutes. But cross-likages formatio begis immediately after mixig of powder ad water. There are two systems that delay cross-likig reactios: 1. Alkalie compoet acid compoet system; 2. etarder cross-likig aget system. I the first system the alkalie compoet is usually sodium hydroxide or salts of alkalie elemets ad weak acids (Na 2 3, Na 2 Si 3 ) that have alkalie solutio i water (p = 12.0 13.0). For example Na 2 Si 3 + 2 2 2Na + 2 Si 3 I alkalie solutio the algiate macromolecules do t react with calcium or barium catios ad cross-likig do t occur. Workig time is provided by additio of acid compoet i excess towards the alkalie compoet that reacts each other slowly durig tasked time util solutio becomes almost eutral with p = 7.0 8.0. Acid compoets are usually sodium or potassium silicafluorides or titaofluorides. For example, sodium silicafluoride as acid compoet eutralizes alkalie sodium hydroxide: 4Na + Na 2 SiF 6 6NaF + 2 Si 4 2 Si 4 2 Si 3 + 2 Formed silicic acid is water isoluble; it is dispersed i water ad positively affects to set of material. Excess of sodium silicafluoride (Na 2 SiF 6 ) react with water as followig: Na 2 SiF 6 + 2 NaF + 4 Si 4 + F Formed sodium fluoride promotes stickiess missig. 34
The workig time of material with delayig system alkalie acid compoets is the time i which p decreases from 12.0 13.0 to 8.0 ad crosslikig reactio has occurred. Algiate impressio powder icludes special idicators to cotrol this p chage ad process of impressio settig. For example, algiate impressio material Kromora of Italia firm Lascod cotais two idicators: pheolphthalei ad thymolphthalei. ussia materials Стомальгин ad Альгэласт also cotai pheolphthalei as idicator. Mixig the white material powder with water gives paste that is alkalie ad blue-violet colored. While p is decreased due to reactio of alkalie compoet with acid compoet, past color chages to light-pik. I the momet whe color is disappeared at p = 7.0 8.0 settig reactio is started. I the same momet material o the tray is carried i the mouth ad after 40 50 secod set impressio must be removed from the mouth. I the secod system workig time is provided by a competig reactio, that iitially delays cross-likig. I a powder of a algiate impressio compoud soluble algiate, the cross-likig aget, for example calcium sulfate dehydrate ad retarder sodium phosphate are icluded. After mixig powder with water all compoets are disassociate. alcium ios from the calcium sulfate dehydrate reacts preferetially with phosphate ios from the sodium phosphate or pyrophosphate to form isoluble calcium phosphate: Na 3 P 4 + 3aS 4 a 3 (P 4 ) 2 + 3Na 2 S 4. Na 4 P 2 7 + 2aS 4 a 2 P 2 7 + 2Na 2 S 4. alcium phosphate is formed rather tha calcium algiate because it has a lower solubility. The calcium ios do ot react with algiate util all phosphate ios have reacted. Sodium phosphate ca react with carbo dioxide dissolved i water. 2Na 3 P 4 + 2 + 2 Na 2 3 + 2Na 2 P 4 Sodium hydrophosphat i resultig impressio delays settig of plaster material ad decreases its surface stregth. Sodium carboate is also used as retarder: Na 2 3 + as 4 a 3 + Na 2 S 4. Water isoluble calcium phosphate ad calcium carboate serves also as filler i the resulted impressio. Maufacturers adjust cocetratio of sodium phosphate to produce regular- ad fast-set algiates. After retarders - phosphate or carboate ios are depleted the calcium ios react with the soluble algiate to form the isoluble calcium algiate, which together with water forms the irreversible calcium algiate gel. This reactio is irreversible; it is ot possible to covert the calcium algiate to a sol after it has set. 35
ertai particles of calcium sulfate or other cross-likig compoud are covered by calcium algiate formed i reactio. As result the calcium sulfate becomes isolated from the water eviromet ad it does ot react with other molecules of sodium algiate. Therefore, cross-likig aget (as 4 ) is used i fie particles state. The use of suitable fillers i correct quatities produces a cosistecy that is suitable for various cliical uses. Maufacturers adjust the cocetratio of filler to cotrol the flexibility of the set impressio materials from soft-set to hard set. Additio of fillers reduces shrikage ad stickiess. The fillers i most algiate materials are diatomaceous earth or fie siliceous particles. Algiate impressio material is powder cotaied all required igrediets. Preparatio for use requires oly the mixig of measured quatities of powder ad water. Algiate impressio materials are used for a variety of purposes. It is iexpesive ad easy to use but lucks the accuracy for precisely fittig restoratios. Proper mixig ad hadlig will result i acceptable study models ad casts o which to fabricate provisioal restoratios ad removable detal prostheses. 2 D-galactose 2 36 2 β-1,4- α-1,3-3.2.1.2. Agar - reversible hydrocolloid impressio material. eversible hydrocolloid impressio material is premixed by a maufacturer ad supplied as a semisolid material i tubes ad sticks. These sticks feel wet because of their high water cotet. eversible hydrocolloid impressio material is predomiatly water with added agar. Agar or agar-agar is polysaccharide material prepared from red seaweed. Agar cotais 50-80% agarose ad agaropectie. Agarose is liear polysaccharide made up of D-galactose ad 3,6- ahydro-l-galactose uits joied together alteratively by β-1,4- ad α-1,3- glycosidic likages: β-1,4-2 3,6-ahydro-L-galactose Agaropectie is agarose cotaied acid groups; they are prelimiary sulfate groups formed esters with ay alcohol hydroxyl groups of moosaccharide uits.