Plastics are organic materials based on synthetic or natural macromolecular compounds (polymers). The properties of plastics are determined by the properties of the polymers that make up their basis.
Plastics consist of several components: binder, filler, plasticizer, stabilizers, hardeners, pigments and organic and inorganic components can be added.
Fillers are solid materials of organic and inorganic origin. They give plastics strength, hardness, heat resistance, as well as some special properties, such as anti-friction or, conversely, friction. In addition, fillers relieve compression shrinkage.
Plasticizers are non-volatile liquids with a low freezing point. Dissolving in the polymer, plasticizers increase its ability to plastic deformation. They are introduced to expand the temperature range of the highly elastic state, reduce the rigidity of plastics and the brittleness temperature. As a plasticizer, esters, low molecular weight polymers, etc. are used. Plasticizers must remain stable under operating conditions. Their presence improves the frost resistance and fire resistance of plastics.
Stabilizers are added to plastics to improve durability. Light stabilizers prevent photooxidation, and antioxidants prevent thermal-oxidative reactions.
Hardeners change the structure of polymers, affecting the properties of plastics. Hardeners are often used to speed up polymerization. These include oxides of certain metals, urotropine, etc.
Lubricants (stearin, oleic acid) are used to prevent plastic from sticking to equipment during the manufacture and operation of products.
Dyes and pigments give the desired color to plastics.
Thus, plastics in most cases are multicomponent mixtures. An obligatory component is a binder.
Synthesized polymers by polymerization or polycondensation of monomers in the presence of catalysts under strictly defined temperature conditions and pressures.
According to the type of binder, plastics are divided into 4 classes:
1. Based on polymerization products (acriplasts, vinyl plastics, styrene plastics);
2. Based on polycondensation products (phenoplasts, aminoplasts, silyplasts);
3. Based on chemically modified natural polymers (protenoplasts, celloplasts);
4. Based on natural and petroleum asphalts and resins (bntuminoplastics).
Plastics are high-molecular compounds that can be molded under heat and pressure and then stably retain their shape. Depending on the reaction of the binder when heated, plastics are divided into thermoplastic (reversible) and thermoset (irreversible).
Thermoplastic plastics go into a plastic state when heated without chemical change and can be molded under pressure repeatedly (polystyrene, polymethyl methacrylate, etc.).
Thermosetting plastics , when heated, go into a plastic state only at the first stage, and upon further heating, the binder undergoes a chemical change, which leads to irreversibility (phenolic plastics, aminoplasts).
Plastics have the following properties:
Low density (usually 1.0 – 1.8 g / cm 3 , in some cases up to 0.02 – 0.04 g / cm 3 );
· High corrosion resistance. Plastics are not subject to electrochemical corrosion, they are not affected by weak acids and alkalis. There are plastics that are resistant to concentrated acids and alkalis. Most plastics are sanitary harmless;
· High dielectric properties;
Good paintability in any color. Some plastics can be made transparent, which are not inferior in their optical properties to glasses;
· Mechanical properties of a wide range. Depending on the nature of the polymers and fillers chosen, plastics can be hard and strong or flexible and resilient;
· High adhesive properties;
· Good processing properties when casting, pressing, molding using low pressures or in a vacuum.
Plastics used in dentistry are high-polymer organic compounds and represent a group of materials that, under certain conditions, have many positive qualities necessary for prosthetics.
In connection with the conditions of appointment, application and processing, plastics must have the following medical and technical properties:
1. Do not irritate the oral mucosa and be harmless to the body;
2. Have sufficient strength when creating chewing pressure on the prosthesis;
3. Strongly connect with artificial teeth, metal and porcelain (the best connection is a chemical bond, not a mechanical one);
4. Do not deform and do not change volume during the use of the prosthesis, when the temperature in the oral cavity changes;
5. Possess high fatigue resistance to bending due to the pliable mobility of the mucous membrane and variable masticatory pressure on the basis;
6. Have sufficient hardness and low abrasion;
7. Good for grinding and polishing, keep a smooth surface when using;
8. Do not change color when exposed to food, light and other factors;
9. Amenable to repair in case of breakage;
10. Have low thermal conductivity to maintain a constant temperature of the mucous membrane under the prosthesis;
11. Tasteless and odorless, easy to disinfect;
12. Correspond to the color of the oral mucosa or (for bridges) the color of the tooth enamel;
13. Do not adsorb nutrients and microflora;
14. Have a small specific gravity, be cheap in production and not labor-intensive material in processing.
Currently used in dentistry, polymeric materials – plastics have a sufficiently high chemical resistance, do not have a harmful effect on the tissues of the oral cavity and the body as a whole, and can be disinfected by boiling or cold method.
Plastics have high mechanical strength and are easily processed into the desired product. Cosmetically, they mimic soft and hard tissues of the oral cavity well.
Dental plastic materials for various prosthetic purposes are classified into four main groups:
1. Acrylic plastics;
2. Vinyl acrylic plastics;
3. Plastics based on modified polystyrene;
4. Copolymers or mixtures of the listed polymers.
According to technological properties, these materials can be
divided into two groups:
1. Plastics processed into dental products by heat treatment (during polymerization);
2. Self-hardening plastics.
The group of plastics requiring the use of heat treatment by the polymerization method includes: synma, ethacryl, elastoplast, eladent-100. The second group – self-hardening plastics include: protacryl, redont, norakryl, norakryl-100, stadont, carboplast.
The introduction of self-hardening (quick-hardening) plastics into practice has made it possible to simplify individual steps in the manufacture of prostheses, relocate a plastic base that has become unusable without thermal polymerization, and repair a fixed combined prosthesis (metal and plastic) directly in the oral cavity.
Self-hardening plastics have the ability to polymerize at normal air or human body temperature, are well combined with other types of plastics without thermal polymerization, and do not have a harmful effect on the body and organs of the oral cavity.
PROTACRIL. The composition of protacryl includes powder and liquid. The powder is a mixture of finely dispersed polymethyl methacrylate 96.5%, benzoyl peroxide 1.5% and disulfanamine 2% (activator). The powder is pink. The liquid is methyl methacrylate. Dimethylparatoluidine is added to it as an activator in the amount of 0.1 – 0.2%. The factory packaging of protacryl plastic (Fig. 1) contains: powder in a plastic bag – 160 g, liquid in a bottle – 100 g, release varnish in a bottle – 50 g and dichloroethane glue in a bottle – 40 g.
Protacryl has the ability to monolithically chemically combine with the base materials of the acrylic group, sufficient strength, does not change its color in the oral cavity, and corresponds to ethacryl plastic in physical and mechanical properties.
|Rice. 1. Protacryl|
Protacryl is used for the repair of removable dentures, relining and the manufacture of orthodontic appliances.
REDONT. Redont (Fig. 2.) is used for the manufacture of orthodontic appliances of various designs. Consists of powder and liquid. The powder is a finely dispersed copolymer of methyl and ethyl esters of methacrylic acid, dyed pink with fatty dyes. The ratio of methyl and ethyl esters is 90:10. For orthodontic purposes, a transparent polymer is produced. The catalyst is benzoyl peroxide. The liquid is methyl ester of methacrylic acid, dimethyl paratoluidine is introduced into the composition of the liquid as a catalyst and hydroquinone is a stabilizer. There are two bottles in the original packaging: powder – 150 g; liquid – 100 g.
NORACRIL-100. used as a filling material and for the repair of reinforced bridges. Norakryl-100 differs from the previous brand – norakryl – in that it contains an active filler, which significantly reduces water absorption and volumetric change during solidification (self-polymerization). The setting time of noracryl-100 is significantly accelerated, which is important when using it. The hardness of noracryl-100 is slightly higher than that of noracryl. The noracryl kit includes powders of three colors: No. 0, 6 and 10. Powder No. 0 is translucent, can be used alone or mixed with powders No. 6 and No. 10. There are three types of liquid in the package: No. 1 and 2 are catalysts, and liquid No. 3 is pure methyl methacrylate and is used to wash (dissolve) plastic residues from glass plates and tools.
Preparing plastics. The glass plate for the preparation of seals is covered with cellophane, which is included in the kit. The required amount of powder (0.4-0.5 g per filling) is applied to a cellophane sheet with a measuring device. A recess is made at the top of the mound of powder, into which liquids No. 1 and 2 are added with a pipette or from plastic droppers in equal amounts (5-7 drops each). Powder and liquids are mixed with a spatula to obtain a homogeneous mass. The duration of the preparation of plastic should not exceed one minute. The prepared plastic noracryl-100 is introduced into the prepared tooth cavity. Hardening time 5-7 minutes. The surfaces of the filling are polished after hardening. Noracryl-100 plastic can be used to repair composite bridges with a plastic facet. Before repair, it is necessary to carefully free the bed in the prosthesis from the remnants of the plastic facet with a bur, wipe the metal surface with alcohol, monomer, isolate the teeth from saliva, and then apply the prepared plastic.
STADONT. Stadont is a self-hardening plastic of the acrylic group, used for the manufacture of therapeutic fixing splints for periodontal disease. Plastic is made up of powder and liquid. The powder is a finely dispersed copolymer, it contains an opacifier – titanium oxide and pigments that give the plastic the desired shades to match the color of the crowns of the teeth. Liquid – methyl ester of methacrylic acid with the addition of a catalyst – dimethyl paratoluidine 1.25% and a stabilizer – hydroquinone or diphenylpropane. The package contains 50 g powder bottles in colors No. 0, 16, 19. Liquids 120 g.
CARBOPLAST. Plastic is used for the manufacture of temporary plastic crowns, individual impression trays. The composition includes powder 200 g, liquid 100 g. Before making an individual spoon, the plaster model is coated with Isokol-69 insulating varnish or other analogues. The polymerization process takes place in the environment at room temperature or in a polymerizer at a temperature of 40-45°C for 25-30 minutes. The advantages include: fast polymerization without technical means; high plasticity; good marginal fit to the plaster model; precise transfer of details.