Powder-liquid type plastics. Hardeners for cold curing epoxy resins. Basic principles and features of injection molding.

LECTURE 14 POLYMER MATERIALS IN ORTHOPEDIC DENTISTRY. MATERIALS FOR ARTIFICIAL TEETH

Acrylic cold curing materials. Classification of elastic base materials. Comparative evaluation polymer materials for artificial teeth with materials of a different chemical nature.

Acrylic cold-curing plastics are compositions that spontaneously, i.e. without additional external heating energy or light, curing at room temperature. The polymerizate, depending on the composition of the material, can be hard or elastic. Cold-curing plastics are used in dentistry for correcting (relining) dentures, repairing dentures, making temporary dentures, periodontal splints, models, etc. The advantage of these materials over hot-curing acrylic materials is more simple technology. However, they have disadvantages: they are inferior in strength to hot-cured materials, they contain more unpolymerized or residual monomers. According to the requirements of modern standards, taking into account the real possibilities of cold curing materials, their bending strength should be at least 60 MPa, the flexural modulus should be at least 1500 MPa, and the amount of residual monomer, which is recognized as acceptable, should be no more than 4.5% wt. (compare with the norms of the standards for acrylic materials hot curing, lecture 13).

The composition of cold-curing plastics differs from hot-curing plastics in that a larger amount of initiator is introduced into the polymer powder during synthesis (about 1.5% instead of 0.5% for hot-curing materials), and an activator is added to the liquid.

The need to increase the adhesion of the prosthesis to the oral mucosa has led to the emergence of soft elastic lining materials for the bases of removable dentures. Increased elasticity is also needed because some patients cannot use removable dentures with solid bases due to pain. The following medical and technical requirements are imposed on materials for elastic linings:

1) biocompatibility;

2) strong connection with a rigid base material;

3) constancy of elasticity;

4) good wettability with saliva;

5) low water absorption and low degree of solubility (disintegration) in oral fluids;

6) high wear resistance;

7) hygiene, i.e. the ability to be easily cleaned with available means;

8) color fastness;

9) manufacturability.

Materials for elastic linings for denture bases are classified depending on the nature of the material and on the conditions of polymerization or curing (Scheme 14.1).

Scheme 14.1. Types of elastic base materials

Previously, plasticized polyvinyl chloride and vinyl chloride copolymers have been used as elastic base materials.

Temporary elastic pads, or fabric conditioners, are used in the mouth for a short period of about a few weeks, although some successful formulations are known that remain elastic and adhere to the surface of the base for many months. These materials are characterized by specific properties that are fundamentally important for their purpose. One of them is the ability to viscoelastic flow under the action of chewing and other functional loads, for example, during a conversation. Thus, the edematous mucosa, traumatized by the painful fixation of the old prosthesis, gets the opportunity to recover, while the conditioning lining adapts to any terrain. Modern materials for this purpose are mainly acrylic gels.

The purpose of artificial teeth is mainly to ensure the function of the chewing apparatus and improve speech. Another important aspect is the restoration of the dentition in an aesthetic sense. The main criterion for the quality of artificial teeth is their similarity with natural ones. appearance, and chewing efficiency.

Currently, polymeric materials occupy a leading position among materials of a different chemical nature for the manufacture of artificial teeth. In addition to polymers or plastics, porcelain and, to a limited extent, metal alloys are used. Basic requirements for artificial teeth:

• strength and sufficient wear resistance (abrasion resistance);

• moisture resistance and resistance to the action of oral fluids, lack of porosity;

• strong connection with the base material of removable dentures;

• proximity of thermophysical properties (coefficient of thermal expansion) to the properties of the basis;

• conformity in shape and color to natural teeth, preservation of the original color in the conditions of the functioning of the prosthesis for a long time (color fastness);

• ability to be easily processed and polished.

Although there have been attempts to make artificial teeth from various polymers, polycarbonates, polyesters and other materials that have a higher strength than acrylates, top scores in terms of color reproduction and strength of the connection with the base, acrylic materials were nevertheless given. Acrylic artificial teeth were made from copolymers of methyl methacrylate and other monomers of the acrylic series, having a spatial crosslinked structure. Ethylene glycol dimethacrylic ether (DMEG), triethylene glycol dimethacrylic ether (THM-3), oligocarbonate dimethacrylate, etc. were used as bifunctional monomers or crosslinking agents. in relation to the monomers used to prepare the polymer-monomer acrylic composition from which artificial teeth were pressed. This structure of the polymer material gave the artificial teeth increased hardness and heat resistance, as well as increased wear resistance. Increasing the content of the cross-linking agent in the composition over 10% of the mass. led to a decrease in the strength of the bond between artificial teeth and acrylic base material.

Porcelain artificial teeth are obtained by firing a molding compound made from feldspar, quartz, kaolin and additives. All components are pre-ground, the mixture is slightly moistened (up to 1%) and tightly packed in fireclay capsules, which are heated in an oven at a temperature of 1350 ° C for 20 hours. The resulting frit is ground and mixed with pigments. A molding mass is prepared from the ground frit with the addition of plasticizing additives. aqueous solutions starch, oils and cellulose. Such a mass is molded in special metal molds, including special structural elements for mechanical fastening with an acrylic base of a removable prosthesis (crampons - metal pins or cavities and channels).

Metal artificial teeth of stainless steel continue to produce in our country, although in limited quantities. Gradually, they are being replaced by artificial teeth made of plastic and porcelain, since metal teeth do not meet aesthetic requirements and, in terms of their thermophysical characteristics, are very different from the tissues of natural teeth and the polymer material of the prosthesis base.

When comparing artificial teeth made of plastic and porcelain, one can highlight the advantages and disadvantages associated with the chemical nature of these materials. Porcelain teeth are distinguished by higher biocompatibility, color stability and wear resistance, however, their manufacturing technology is more complex, they are not able to adhere to the acrylic base adhesively, they have a higher specific gravity, and dentures with porcelain teeth make an unnatural sound when chewed.

Artificial teeth are produced in sets, sets, different styles and sizes. Each manufacturer presents a map or album of styles and sizes of produced teeth. In most cases, it includes the styles of the anterior (frontal) and lateral (chewing) teeth, divided into several groups. In each group, the anterior tooth sets are of the same width and vary in height and type. Height (h) is determined by the height of the crown of the upper central incisor, width (a) - by the width of the set of the upper front teeth. The anterior teeth vary in shape. They are made of three types: rectangular, oval and wedge-shaped. Moreover, this difference is observed only for the upper front teeth, and the lower teeth are made of one average type.

Based on technological features polyester and epoxy resins, which are also erroneously called compounds. It is more correct to call compounds a mixture of resin with a hardener and a filler, because "compounding" is mixing.

Such mixtures - compounds are cold curing and hot, which is determined by the type of hardener. The essence of the curing process is the transformation of a resin molecule with reactive epoxy groups (= C - C =) into a macromolecule by reaction with amines, organic acid anhydrides, phenol-formaldehyde resins contained in hardeners. With the introduction of hardeners, epoxy resins pass from the liquid-viscous state of oligomers to the solid, infusible and insoluble state of polyepoxide polymers. That is, the epoxy resin molecules crosslink and acquire a network structure.

Heat resistance of cured compounds is 150…250 °C.

Cold curing compounds are the most widely used, as they are the easiest to use. But they have a number of disadvantages, which further lead to cracking, delamination, yellowing of car parts made from them.

This is due to their low resistance to ultraviolet radiation, high temperatures. This can be avoided by coating the surface with varnish, paint and adding filler. The product will be heavier, but also more durable. For the manufacture of carbon, they are used using all types of compounds, which is determined by the size of the part, skills and equipment.

Advantages of epoxy resins and compounds

Epoxy resins for carbon fiber and compounds based on them are a popular and optimal binder for fiber reinforcement materials. And for this they have a wide range of consumer and technological advantages:

• Excellent adhesion to most reinforcing materials, fillers and substrates;
• Large selection of epoxy resin grades and curing agents with a variety of technical parameters, which makes it possible to obtain materials with a wide range of properties after curing;
• The chemical reaction between epoxy resins and hardeners proceeds without the release of water and volatile substances - the process is controlled and safe (it is necessary to take into account the amount of heat in some formulations).
• Curing shrinkage is lower than with phenol-formaldehyde or polyester resins, and its value can be easily adjusted using various fillers;
• Modern modifications of epoxy resins make it possible to choose a brand with a certain temperature, speed and curing time, which is very important in mass production;
• The cured compounds are excellent dielectrics with high volume resistivity.
• They are resistant to water, high temperatures, acids and alkalis.

But initially epoxy resins were used only as universal adhesives, filling the windings of transformers and motors, sealing joints electrical cables, in the manufacture of models and forms.

With the advent of the carbon web and with the development composite materials Epoxy resins are widely used in the manufacture of carbon fiber. Therefore, along with the use of epoxy compounds as adhesives, they are used in the production of laminated plastics and fiber-winding composites in the electronics, chemical, automotive industries and in the manufacture of sports equipment.

Cold curing compounds

"Cold" technologies require lengthy tooling preparation and additional equipment for vacuum processes for removing air from the mixture. This method is laborious and suitable for small-scale production of parts of a certain section. All components must be thoroughly mixed and strictly dosed.

Curing occurs at room temperature or when heated to 70-80 C. All of the above refers to hot curing compounds.

Hot curing compounds

Hot-curing epoxies are stronger, but cure very slowly at room temperature. This property is used in the manufacture of prepregs - blanks for molding. They represent sheets of carbon, in which a resin with a hardener in liquid form is applied to a carbon cloth, and the curing reaction practically does not proceed at room temperature and starts when heated.

Such blanks can be stored from several hours to several weeks, depending on the brand of resin and purpose.

Directly during the manufacture of the part, the heated compound becomes liquid and spreads, filling the entire volume of the working mold and the polymerization process is accelerated.

Each brand of resin has its own temperature and time curing regimes. Therefore, when choosing, you need to pay attention to these parameters and the capabilities of your equipment.

Resin Selection and Compound Preparation Guidelines

When choosing a resin, it is advisable to focus on the dimensions, operating conditions of the finished product and your experience.

1. "Slow" resins are used for large-sized products and if work is carried out at high temperatures, as well as with little experience, in order to have time to straighten all the folds of the fabric and carefully lay out the carbon fabric impregnated with resin before polymerization begins. It must be remembered that an increase in temperature accelerates the polymerization of any compound. The higher the room temperature, the slower the resin needs to be. In a cold room, "fast" resins can be used.

2. If ready product will be used outdoors, it is necessary to choose resins with UV filters and resistant to high temperatures. That is, be sure to pay attention to the workers specifications. It is possible to use varnishes with protective properties with universal resin.

3. The quality of the hardened compound is determined not only by its technical and consumer characteristics, but also by the accuracy of dosage and thorough mixing of all components.

Consumer characteristics of the finished product can be changed by selecting hardeners of various brands.

Attempts to speed up or slow down the polymerization time by independently changing the proportions are fraught with deterioration in the characteristics of the finished compound. An increased dosage of hardener will accelerate hardening, but will brittle the finished product, and the strength will be lower. If the hardener is less than the standard, the mixture may not harden at all.

It is necessary to mix all the components of the mixture for at least a minute.

4. Since the resin is a thermoset, exothermic heat is released during polymerization. passes chemical reaction. And the faster the reaction proceeds, the more heat is released. Therefore, when working, it is necessary to observe safety measures: do not touch with hands, do not inhale fumes, do not use flammable materials.

The time the mixture remains in a liquid state is called the pot life. Time span from liquid state to solid - gelation time. The time interval from complete mixing to complete solidification is the polymerization time.

At the stage of initial hardening (the material is elastic, and when pressed with a fingernail, a trace remains), the next layer of fabric and resin can be applied, because at this moment the new layer is combined with the previous one into one whole, a chemical reaction is still going on. If you miss this moment, in the future, the application of layers is possible only with careful grinding and polishing of the surface. The connection will go already due to the capillary effect. Otherwise, delamination may occur.

It is possible to remove the finished product only after complete hardening. But the final set of strength will go on for another month.

Thus, the gel time needs to be known in order to calculate the compounding and layering times. And the polymerization time determines the holding time and the moment the product is removed from the matrix. Manufacturers offer hardeners of various brands, which can be used to adjust the polymerization time.

In order not to make a mistake when choosing all the components of the mixture on your own, it is better to purchase the resin and hardener in sets, paying attention to the polymerization time of the finished mixture.

Often, when describing resins of general and special purpose indicate the recommended method of application - manual molding, winding, spraying and the field of application - for carbon fiber reinforced plastics, for fiberglass, auto-tuning, decorative panels etc.

Fillers

To increase the density and thickening of the resin, various fillers are used, after thorough mixing of the resin and hardener.

These can be cotton fibers, chopped fiberglass and glass spheres, chopped and ground carbon fiber, metal powder, talc. An exact dosage is usually not required and there is a field for your experiments. Thick mixtures are used to fill gaps and model shapes.

Special additives

To impart special properties such as UV, high and low temperatures, staining, special additives are used.

Excess resin can be easily removed with acetone. This will come in handy. If you get your hands dirty. But it is better to work with gloves.

Taiwan is the main manufacturer of carbon fiber today. Fiberglass (glass mat) is cheaper and it is used for the manufacture of fiberglass or layers of carbon fiber are sandwiched with it. If they try to convince you that carbon fiber can be made from fiberglass, do not believe it. It will just be a different material for the price of carbon.

So, when choosing a compound for carbon important parameters are the following:

• Ratio resin:hardener,
• The viscosity of the mixture according to Brookfield at 22 ° C,
• Pot life at 22 °C,
• gel time,
• Full curing time
• Tensile strength,
• Static bending strength,
• Heat resistance.
• The optimum curing time is 24 hours at 22-24°C.

If there is little experience, a trial mixture test can be done to determine the start time of gelation for the actual temperature and humidity in the room.