Titanium dioxide was discovered in 1971 by a priest and mineralogist. William Gregor produced a white metal oxide by calcination black magnetic sands from Menachan, Cornwall, England. TiO₂ occurs naturally mainly as anatase, rutile, and rarely as brookite. Usually, natural rutile crystals are impure, so the first investigation was limited to ceramic samples. But finally in 1950 a colorless single crystal, a large synthesized rutile crystal, was obtained using the Boule technique.

 TiO₂ is a non-toxic, biocompatible and inexpensive material with high dielectric constant and chemical stability. It is a semiconductor with a band gap of 3 to 2.3 eV associated with the light absorption edge of 387 nm. Depending on its chemical composition, it can provide varying amounts of electrical conductivity due to the presence of oxygen defects. As a UV light absorber, TiO₂ is a white pigment that has been widely used in paints since 1920. TiO₂ replaced the most important white pigment known as “whiting”. In addition to its high corrosion stability, low production cost and non-toxicity make it a good candidate as a food additive. Due to its electrical properties, TiO₂ has been widely studied and used as a photocatalyst for environmental purposes and green synthesis. In this competition, it can be used as an additive in building surfaces due to air detoxification and self-cleaning. TiO₂ in the form of nanoparticles has unique electrical properties and is also a good candidate for use in dye-sensitized solar cells, a photovoltaic technology that converts sunlight into electricity. In addition, taking into account all the mentioned characteristics, as well as outstanding mechanical and rheological behavior, today TiO₂ is one of the most important components of cosmetic products such as sunscreen. Many efforts have been made to synthesize TiO₂ by different methods to adjust its physical and chemical properties and expand its use. In the following, the most important properties of TiO₂ and a wide range of applications of this material are briefly mentioned.

TiO₂ exists in at least 11 crystalline forms, as shown in the figure below. The structural features of this material are summarized in the corresponding table. The 11 crystalline forms are rutile, anatase, brookite, TiO₂(B), pseudohollandite TiO₂(H), pseudoramsdellite TiO₂(R), pseudocolumbite TiO₂(II), pseudobaddelite, TiO₂(OI), pseudocottonite TiO₂(OII) and pseudofluorite cubic phases. In addition, one type of non-crystalline TiO₂ with low density and two types with high density have been reported. The first six species (shown in orange in the figure below) are stable at atmospheric or low pressure and have densities between 3.5 for TiO₂(H) and 4.2 g/cm³ for rutile. The last five species are high-pressure phases (shown in green in the figure below) and have a higher density, such that it is 4.3 for TiO₂ (II) and 5.8 g/cm³ for the cubic phase.

The six stable species of TiO2 at atmospheric pressure include octahedra Ti-O, which share corners, edges, and faces differently. On the other hand, the high pressure modes are octahedral prisms, strengthened or cubic prisms, which are the result of changes due to increased pressure. Four states namely rutile, brookite, anatase and TiO₂(B) are found in nature. High pressure modes are usually obtained in the laboratory under controlled pressure. Anatase and TiO₂(B) contain chains of octagons with shared edges in a single orientation, while this occurs in two directions for rutile, brookite, and TiO₂(H). In addition, the number of shared edges is approximately possible to predict the energy sequence of the structure. The Ti-Ti distance (and energy structure) decreases with increasing number of shared edges. For example, the stability of three widely used types of TiO₂ is rutile, brookite, and anatase, where the number of common edges in each octahedron is 2, 3, and 4, respectively. Similar results have been obtained by calculating the lattice energy for all 6 species, which are stable at atmospheric pressure and their stability is as follows:

Rutile>Brookite>Anatase>TiO₂(B)>TiO₂(R)>TiO₂(H)

In more detail, four of the eight neighbors of each octahedron in anatasehave shared edges, while the rest possess shared corners. In rutile, the octahedral structure has two ridges and eight edges in common. Corner sharing is along the [1 1 0] direction and they are arranged at 90 degrees with their longitudinal axis alternation. The length of Ti-O bond is 1.937 and 1.966 angstroms for anatase, and is 1.946 and 1.983 angstroms for rutile in equatorial and axial directions, respectively. In brockite, both edges and corners are connected.

Structure and defects

Defects in TiO₂ play an important role in almost all applications, as they often determine or influence its physicochemical properties. For example, defects are responsible for the resistive switching properties of TiO₂. It greatly affects the mechanical properties and induces changes that indicate surface interactions in catalytic applications. It also induces electronic changes and provides tuning of optical properties. Based on the dimension (D) of the defective structure, point defects (0D), line defects (1D), surface defects (2D) and mass defects (3D) can be distinguished. Figure 2 shows the size scale of the four classes of defects.

Morphology of TiO₂

The purpose of this section is to present the main morphologies of synthesized TiO₂. Different morphologies offer different surface-to-volume ratios and different orientations, which result in a wide variety of physical and chemical properties for different applications. The morphology of TiO₂ nanoparticles with a size between 1 and 100 nm can be classified into three groups: spherical, elongated and flat. Between spherical nanoparticles, nanospheres, nanogranules, nanoplatelets and nanopores have been reported. Elongated nanoparticles include nanorods, nanotubes, nanowires, nanofibers, nanoneedles, nanopetals, nanowhiskers, nanotrees, and nanostrings. Flat nanoparticles are nanoplates, nanosheets, nanofilms and nanocoatings. Most of the time, morphology deals with the field of application of TiO₂. For example, spherical nanomaterials are preferred in electronics, photodynamic therapy, sensors, probes, catalysts, and antimicrobial applications. Elongated nanomaterials are often used for display technologies, microelectromechanical systems, optical sensors, biological sensing, imaging, and drug release. Finally, flat TiO₂ nanomaterials are used for packaging purposes, coatings, surgical procedures for wound dressings, tissue engineering, and as cell scaffolds. The following images display the main morphology of amorphous, anatase, brookite and rutile species, respectively.

Mechanical properties

The elastic properties of materials are expressed by their bulk modulus. Diamond anvil cell and high-pressure synchrotron XRD techniques were widely used to estimate the bulk modulus of TiO₂. The table below summarizes the bulk modulus and several properties of TiO₂ nanostructured materials. Morphology, crystal structure, particle size and pressure environment and other characteristics of TiO₂ greatly affect its bulk modulus. The results in the figure below show that the bulk modulus of TiO₂ shows a non-uniform trend with particle size. Maximum bulk modulus values (about 250 GPa) are evident for a particle with a size of 15 nm.

Rheological properties

The term nanofluid refers to suspensions of colloidal nanoparticles in various solvents. Nanofluids are used in fields such as advanced heat transfer, solar collectors, microfluidics and drug release. Addition of TiO₂ nanoparticles to liquid paraffin reduces wear for tribological applications. The size of nanoparticles significantly affects the lubrication properties of TiO₂. Viscosity is probably the most important property of nanofluids, affecting not only the mechanical properties but also the convective heat transfer coefficient, implying significant changes in terms of pumping power costs. To investigate the viscosity of nanofluids, both theoretical and experimental approaches have been performed. The results show that nanofluids have higher viscosity values ​​than predicted by the Einstein model, which is due to the aggregation of nanoparticles. In particular, it was found that not only the increase in viscosity of nanofluids compared to base fluids does not follow the functional heat transfer, but it is also worse than the performance of base fluids. Adjusting the pH values ​​can reduce the increase in the viscosity values. Other parameters affecting viscosity are temperature, degree of aggregation, volume fraction and aggregation of nanoparticles. For example, the absolute viscosity of aqueous TiO₂ nanofluids decreases with increasing temperature, while the ratio between the viscosity of nanofluids and water does not depend on temperature. Khadkar et al showed that the viscosity of TiO₂ suspensions in ethylene glycol nanofluids increases linearly with the concentration of nanoparticles. In summary, it is generally believed that low viscosity is preferable for nanoparticle suspensions in practical applications considering pumping power and pressure drop. The properties of TiO₂ allow the use of radiation or an external electric field to adjust the viscosity of nanofluids. An external electric field creates a mass of nanoparticles parallel to the direction of the field, as a result, the viscosity of the nanofluid increases. The effect of UV radiation on viscosity in water (conductor) and silicone oil (insulator) recently studied by Smith et al. The results showed that the viscosity increased about 2.5% and 12.3% in water and silicone oil by UV irradiation (during 30 seconds), respectively. Particle accumulation and electron accumulation have been identified as possible key parameters controlling the phenomenon. The observed increase in viscosity is irreversible up to a certain critical time (unless the nanofluid is modified by ultrasound). After that further increase in viscosity is reversible. The observed photorheological phenomenon is explained with reference to aggregation effects and changes in hydrophilicity caused by UV radiation.

Structure and chemical properties

Polycarbonates are amorphous and linear thermoplastic polyesters obtained from the reaction of carbonic acid with aromatic diols. There is little information on aliphatic, cycloaliphatic and aliphatic aromatic polycarbonates, while aromatic polycarbonates especially those made from bisphenol A have been extensively studied.

Bisphenol A in polycarbonate is obtained from the combination of carbonic acid with phosgene (Phosgene=Carbonyl Chloride=COCCl₂).

Advantages of polycarbonate

• High impact strength
• Low flammability
• Electrical protection capability
• Ability to be sterilized by gamma radiation
• High abrasion resistance        
• High heat distortion temperatures
• Good dimensional stability
• Good electrical properties
• Can be processed through all the special methods of thermoplastic polymers

Polycarbonate is non-corrosive and it can catch fire, but after removing the flame it extinguishes itself. This plastic dissolves in chlorinated hydrocarbon solvents, is attacked by cyclic hydrocarbons and strong alkalis, is insoluble in linear alcohols and is stable against mineral acids. Polycarbonate moisture absorption rate is low. It is resistant to ozone gas, air, ultraviolet rays and environmental conditions.

The table below shows some important chemical properties of polycarbonate.

For many years, it was thought that the parts made of polycarbonate are non-toxic and their contact with food does not cause a problem. However, the research of the last few years indicates that the plastics that contain bisphenol A in their molecular structure are toxic and dangerous because the entry of these substances into the food cycle causes this compound to be absorbed by the human brain, one of the effects of which is Alzheimer’s disease.

Thermal properties

Polycarbonate maintains its rigidity up to 140°C and its toughness up to -20°C.

Polycarbonates made of bisphenol A become brittle only at temperatures lower than -100°C and maintain their resistance up to 120°C.

This plastic has very high dimensional stability. The heat distortion temperature (HDT) of parts made of bisphenol A polycarbonates is in the thermal range of 134-143 ^oC. This temperature increases up to about 15°C using glass fiber reinforcements.

These plastics are flammable in contact with the fire, but after removing the flame, it has the ability to self-extinguish. Polycarbonate can be sterilized by steam. It can be shaped easily with different types of common industrial methods.

The table below shows some of the important thermal properties of polycarbonate.

Mechanical properties

Aromatic polycarbonates (such as bisphenol A polycarbonate) are durable and tough, and retain these properties over a relatively wide range of temperatures.

The mechanical properties of aromatic polycarbonates depend on their molecular weight.

The high impact resistance of this plastic is one of its most outstanding mechanical properties compared to all industrial and engineering plastics. It also has high wear resistance.

Products whose their average molecular weight is less than 10,000 do not have the ability to form a film, but with an increase in molecular weight between 10,000 and 18,000, their mechanical properties become weak. The molecular weights less than 20,000 generally do not have good mechanical behavior, but with the increase of molecular weight up to 25,000, the tensile and impact behavior improves. It is difficult to prepare the molecular weights above 50,000 of this plastic. It has very good dimensional stability, the coefficient of linear thermal expansion of polycarbonates prepared from bisphenol A is much lower compared to other plastics. This value is reduced about 2.3% by using glass fiber reinforcement.

Glass fibers reinforced polycarbonate can be used as a safe substitute for some composites such as polyphenol-formaldehyde. Its processing method is simpler and more economical, and in many cases it shows better properties compared to phenolic resins.

Plastic polycarbonate is very transparent and passing up to 90% of light, but over time, its transparency decreases a little.

The table below shows the change in the transparency of bisphenol A polycarbonate at the beginning of its production and three years after its production.

Bisphenol A polycarbonate has a very low dielectric coefficient, so it has high insulation properties.

Different types and varieties of PC

The variety of commercial types of polycarbonate has increased greatly in recent years. The application of the product and the processing method are two important options for choosing any special types of this plastic.

Types of polycarbonate available in the market include homopolymer, reinforced alloy and improved with special additives. Its homopolymers are used in making films, sheets and parts.

Other types of this plastic can be mentioned as its PC/ABS and PC/PBT blends, which are widely used in the automotive industry.

Improved types of this polymer are flame retardant or stabilized against cracks caused by environmental stresses.

Reinforced types with glass fibers with weight percentages of 20% and 30% have many applications and can produce strong parts.

Branched polycarbonate type is suitable for applications where the melt must have high strength.

The most important differences between species are:

1. Difference in molecular weight

2. Presence of a polyhydric compound

3. Difference in additives

Among the industrial and commercial types of bisphenol A polycarbonate, the following can be mentioned:

1. Suitable for injection molding

2. Suitable for the extrusion process

3. Suitable for blow molding

4. Suitable for structural foams molding

5. Suitable for thermal molding under vacuum

Polycarbonate is processed with excellent quality by all processing methods such as injection molding, extrusion, thermoforming, compression, etc.

Applications

• Electronic and office equipment: office machine bodies, computer peripheral parts, connectors, terminal blocks (terminal modules) and telecommunication components.

• Household appliances: food processors, kitchen electrical components, power tool bodies, refrigerator-freezer drawers and vacuum cleaner components.

• Transportation: Front and rear lights, warning light lenses and bodies, runway lights, air-molded spoilers (A plate or set of plates, comb, tube, rod, or other device exposed to the air current that the whole body is worn by increased friction, or the smoothness of the surface is greatly lost. Such a device is worn from the upper surface by an air layer, increases the friction and reduces the lifting force. It is likely that the bumper and aerodynamic parts of the car are meant here, which is named under the title of the spoiler), and the seat support.

• Safety and sports: sports helmets, hoods of cars, sports and recreational vehicles, car windshields, headlights, boat propellers and sunglass lenses.

• Food related services: special dishes for cooking in the microwave, special trays for eating or serving food, glasses, pitchers, water bottles, baby bottles

• Medical devices: tubing connectors, dialysis components and devices, blood oxygenators, filter bodies, lenses, sterilization or germicidal equipment by gamma rays, and surgical sutures.

• Laminate products: institutional signs, aircraft interior linings, greenhouse windows

• Industrial: mailboxes, material handling tanks, highway designs and images

• This plastic can be used in the construction of computer housings that have higher mechanical, electrical and fire resistance properties, as well as the canopy of supersonic airplane pilots.

• It is also used in the manufacture of molded products, extruded films, pipes, shatterproof glass, windows, automotive and street light bulbs, household uses, astronaut helmet bubbles, bullet-proof glass for banks, riot police shields, and armored cars.

Polyolefin elastomers have the properties of thermoset rubbers, but they can be processed using standard equipment for processing thermoplastic polymers. Products made based on POE provide a specific range of physical-mechanical properties in such a way that they adequately fill the gap between rubber and plastic. The possibility of using thermoplastic processing equipment leads to greater production capability and greater economic efficiency. Recycling of production waste, semi-finished materials left over from the process, reground materials such as the compound in the form of pleats taken out of the main and secondary channels (Runners) between the sprues and the injection throat for filling the injection mold, as well as rejected parts have significant economic advantages.

General physical-mechanical properties of POE with versatile and general applications are given below:

According to the definition, POE materials are defined as materials consisting of 1) compounds of various polyolefins, 2) crystalline plastics and 3) amorphous (non-crystalline) elastomers. The most common types of POEs consist of polypropylene (PP) and ethylene propylene rubber (EPR). EPR can be composed of only ethylene and propylene copolymers or a third monomer (diene monomer) which provides a small amount of unsaturation in the polymer chain so that it can form the crosslink network using the vulcanization process.

Thus, the polymer made of ethylene-propylene-diene monomer is called EPDM rubber. Other olefinic polymers commonly used in POE compounds are: 1) low density polyethylene (LDPE), 2) high density polyethylene (HDPE), 3) linear low density polyethylene (LLDPE), 4) copolymers of ethylene with vinyl acetate (EVA). , 5) ethyl acrylate (EEA), 6) methyl acrylate (EMA), 7) semi-crystalline copolymers of propylene, ethylene and polybutene.

POE products are compounds or mixtures of polymers based on polyolefin. Like most TPEs, they consist of hard and soft zones. The exact size and shape of these zones determine the properties of the mixtures. The properties of products can be determined to a large extent by 1) the manufacturing process of the compound and 2) the chemical composition.

An unlimited number of formulations based on POE mixtures and final application can be designed. This possibility of diverse production is due to a wide range of polyolefin polymers that can be used as needed. Each of these compounds can create a set of specific properties in the final product, which may be useful in some special applications. In most POE compounds, the hard region is composed of isotactic propylene homopolymer or isotactic propylene copolymer with a small amount of ethylene as an auxiliary monomer or comonomer. Ethylene monomer can be distributed either randomly or in the form of block in the copolymer. Some segments or parts of the polymer chain are composed of ethylene and propylene copolymers, while other segments of the polymer chain consist almost entirely of propylene homopolymer. Homopolymer and block copolymers have a crystalline melting point range of 311-329 ^oF, while random copolymers have a melting point range of 293-311 ^oF. The relatively high melting point of the polypropylene portion (PP portion) in the POE microstructure results in products that retain many of their mechanical properties at temperatures close to the melting point (Tm) of the PP resin.

High impact strength at low temperature is achieved through the addition of some ethylene monomer that enters into the microstructure of the copolymer. Also, ethylene reduces the rigidity of the copolymer and increases its flexibility. When the amount of ethylene in the random copolymer increases, the melting point decreases rapidly compared to block copolymers with a steep slope, which means that the effect of the amount of ethylene monomer in reducing the melting point of random copolymers is much greater than that of block copolymers.

The soft region of the POE polymer chain consists of EPR or EPDM rubber. Rubber materials with approximately equal amounts of ethylene and propylene are completely. The softest types of EPDM rubbers are the most effective impact modifiers used as additives in POE formulations.

Some additives used in POE compounds can modify the rubber phase. Hydrocarbon oils increase the softness and flexibility of the POE compound by creating inflation in the rubber phase. Also, polyethylene increases the volume of the rubber phase, but does not have the effect of softening oils. HDPE improves the impact strength of the mixture at low temperatures while keeping stiffness unchanged. In addition, a wide range of other additives are also used in POEs compounding. These additives include: 1) fillers or reinforcing agents 2) lubricants 3) plasticizers 4) heat stabilizers 5) antioxidants 6) UV stabilizers 7) colorants 8) flame retardants 9) foaming agents 10) flow modifiers 11 ) processing aids.

POEs can be formulated to achieve a combination of mechanical properties such as strength and toughness. TPO products are available in a range of 40-70 Shore D hardness with flexural modulus varying from 2,700-300,000 MPa.

Several factors determine the upper limit of the temperature used during operation and at the time of consumption for a product made of POE. The melting temperature of the hard zone in polymer being the most important factor for short-term contacts at that temperature. For most POEs, the melting point of polypropylene (PP) is the limiting factor. PP homopolymers melt in the temperature range of 320-350 ^oF. Most POE (hard/rigid) compounds retain their useful properties at temperatures as high as 280°F. For long-term exposure at these temperatures, the resistance of POE against the wear and aging over time is a very important factor. Its importance at very high working temperatures during use is just as the melting point of polymers with hard zone.

The oxidative stability of POEs is a function of the action of antioxidant and stabilizer additives. The most effectively stabilized hard compounds of POE are formulated with heat stabilizers, antioxidants and reinforcements to easily withstand continuous working temperatures up to 250°F or even higher. Hard compounds of POE are general purpose and multi-purpose, maintaining their physical properties when used for long periods in the temperature range of 180-212°F. One of the prominent properties of POEs is their efficiency at low temperatures. POE compounds retain their flexibility at very low temperatures. Softer commercial types of POE have a break point lower than -112 ^oF when exposed to low temperatures. They are brittle at this temperature which is called brittle point at low temperature.

Almost all POE products retain their physical properties when exposed to sunlight or adverse weather conditions. POE compounds are made without the use of unsaturated polymers in their polymer skeleton and therefore are not susceptible to decomposition, degradation and disintegration through ozone in the air. EPDM is a weather resistant rubber. TPOs are not sensitive to attack by micro-organisms and fungal growth. Many types of POEs have outstanding color stability. These stabilized types are especially designed for automotive unpainted parts. They are exposed to sunlight aging, wear and tear tests in fluoride for more than 2 hours without any significant discoloration.

All POE products are resistant to water and also show moderate chemical resistance to acids and bases. Hot hydrocarbon solvents eventually soften them and cause swelling in POE products. This softening and swelling is typically less for harder formulations, but more severe in softer products. The inactive surface of POE makes it difficult to chemically bond with other materials.

Most POE compounds are good electrical insulating materials. They have good dielectric strength properties and do not absorb moisture.

Many POE components manufactured for automotive applications must be painted with an automotive finishing material.

Advantages of POE

• Excellent electrical insulation properties
• In order to increase the final working temperatures, it can be mixed with nylon.
• Excellent impact resistance
• Excellent fatigue resistance
• Excellent low temperature breaking point for softer types
• Excellent resistance to moisture absorption
• Non-sensitive to ozone destruction
• UV resistant types are available

Applications of POE

• Automotive exterior applications

Interior trim, body or interior molding, bumper covers, bumper end caps, bumper side parts, mudguard liners or covers, windshield sunshades, hood rock and bug guards, and curtain liners.

• Automotive under the hood applications

Bobbin nut of wiring tool, air molded tube or duct, sound absorbing fire wall lining, car speaker box wraps, rocker panel covers, rub strips and car footrests.

• Wire and cable

POEs are used for a number of low voltage wire and cable applications. Also, POEs are used for insulation and jacketing of battery booster cables, portable wires for power storage and submersible pump cable.

• Mechanical goods

Sealants, electrical plugs, extruded sheet and weather tapes.

• Impact modifiers

Polyolefin and PP are used to improve impact strength at low temperatures. POE is added to HDPE to improve stress crack resistance.

Polypropylene (PP) was introduced to the market in the late 1950s and is the most widely used economic-commercial thermoplastic that has grown and developed the fastest in the world. PP is a polymer with multiple and diverse applications that has been used in producing fibers, films, household appliances, and car bumpers. In many applications, PP has replaced other materials such as glass fibers and thermoplastics reinforced with minerals and metals. PP is synthesized by polymerizing polypropylene monomer with a titanium-based catalyst. To start the polymerization reaction, a secondary auxiliary catalyst (triethylaluminum) is added to the reaction medium and hydrogen is used to control the molecular weight of the polymer inside the reactor. This reaction is done using a slurry process or a gas type process. There are three structures of PP: isotactic, syndiotactic and atactic. The main structure of PP is isotactic semi-crystalline material. This structure has good mechanical properties, such as stiffness and tensile strength. These properties can be enhanced by using nucleating agents or fillers such as talc, calcium carbonate or glass fibers. Syndiotactic PP is produced through polypropylene monomer units that are alternately connected to each other in a head-to-tail manner. This structure is more flexible than the isotactic, but it has better impact resistance and more clarity.

Atactic PP (hard wax-amorphous monomer) is a by-product of the PP production process. This polymer is used in tarring the surface of roofs and making adhesives in the shoe industry.

All three PP structures are very sensitive to oxidation caused by the presence of tertiary hydrogen. PP is stabilized and resistant to degradation and thermal decomposition by adding type 1 and type 2 antioxidants. Special neutralizing agents are also used to stabilize small amounts of chloride ash produced during the process. Other special additives are also used, such as antistatic agents, slip agents, and UV stabilizers. PP is sold commercially as homopolymers, random copolymers, or impact copolymers. Its physical properties are changing from a polymer with high strength and stiffness to a flexible polymer with lower strength but greater toughness. PP homopolymer has the highest melting point and hardness along with a wide range of melt flow properties.

Copolymers with small amounts of ethylene in their structure have lower crystallinity are flexible, have a lower melting point, and show improved and better impact resistance properties.

Impact-resistant copolymers are copolymerized and produced through the addition of ethylene in the polymerization reactor. Copolymer (ethylene) acts as a plasticizer and process aid and is uniformly dispersed throughout the homopolymer matrix to obtain a heterophase polymer, i.e. two heterogeneous phases. This copolymer has very high impact resistance even at low temperatures. Copolymers with high impact resistance are prepared through pre-blending of copolymer, additives and EPDM rubber.

The chemical structure and general properties of PP homopolymer are as follows:

PP is a light material with a specific mass lower than water obtained from the polymerization of propylene gas. PP is resistant to moisture, oils and common solvents and melts at about 170°C.

Today, PP has many types with different molecular weights and additives to be used in appropriate applications. PP is solid at room temperature. According to the order of its molecules, PP shows different properties.

PP is classified in the family of semi-crystalline thermoplastics. Isotactic and syndiotactic PP have the ability to crystallize while atactic PP has an amorphous structure.

The main parameters that determine the properties of this polymer are molecular weight and its distribution, polymer melt flow index, isotacticity percentage, as well as polymerization process and type and amount of additives.

For the production of most of the common medium weight fibers, the molecular weight is in the range of 200,000 to 350,000.

PP has a high degree of crystallization, which reaches up to 70%. PP is sensitive to ultraviolet radiation and is destroyed. To prevent this unwanted process, UV light stabilizers are added.

PP is one of the polymers with diverse performance that is used in the production of various plastic parts as well as in the fiber industry. Due to its high crystallinity and non-polar aliphatic structure that does not contain any active agent, this polymer cannot be dyed by conventional methods. Therefore, the polymer mass dyeing method is used to produce colored fibers, which itself has limitations in terms of diversity, color transparency, the possibility of dyeing in each stage of the production of textile products. Therefore, many efforts have been made in the field of producing modified PP fibers that can be dyed with conventional methods and many patents have been registered. Most of these modifications are based on the addition of factors such as PP (in the form of linking on the molecular chain) to improve the absorption of dyes. However, modified PP fibers have not yet been supplied to the market.

Advantages of PP

• Lighter stability with low density

• High melting point

• End use temperatures around 212°F

• Good chemical resistance to hydrocarbons, alcohols and non-oxidizing reagents

• Good fatigue resistance (in hinged doors or caps)

PP can be processed and shaped through all the special thermoplastic resin process methods such as injection molding, compression molding, blow molding, extrusion, film casting and thermoforming.

Disadvantages and limitations of PP

PP is destroyed by UV. It is Flammable, but commercial FR (flame retardant) grades are available. PP is attacked by chlorinated and aromatic solvents.

Applications of PP

PP is processed by various methods such as injection molding, blowing, rotation and extrusion, which depending on the type of use can contain additives, anti-oxidation, UV stabilizers, anti-static materials, nucleating agents, pigments, fire-retardants, fillers, etc.

PP is mostly used in the injection molding process and ranks second in the fiber industry. The consumption of PP fibers has increased in the last decade and it is the second most used synthetic fiber after polyester. PP has been marketed in the form of single fibers and threads for the textile industry. Very fine fibers of PP are used for the production of thermobonding and spinning. Medium fineness fibers are used for spinning and production of knitting and tricot yarn, and its thick fibers are used for the production of floor coverings. The favorable properties and cheapness of these fibers have led to their widespread use in the field of non-woven textiles in structures. PP filament yarn is mainly produced as BCF (Bulk Continuous Filament) yarn, which is used in the production of floor coverings. Its fine yarns are also produced for various applications, especially as industrial yarns, but their use in textiles is limited due to the impossibility of dyeing and their weak texuring ability with the common methods of texuring fine yarns (virtual warping method).

This polymer is used in the automotive industry, interior decoration, propellers, car flooring, packaging and fiber industries. Carpets, artificial grass covers, anti-rot rope and fishing nets are other applications of polypropylene.

• packaging

Flexible packaging films and biaxially oriented packaging films.

• textile

Single strand oriented and stretched and thin strips for textiles, carpet weaving, insulated medical fabrics and backings of woven carpet.

• Automotive applications

Interior components, bumpers, spoilers, exhaust systems, under-hood components, wheel head protective hose

• Medical and personal care

Sanitary products, household goods, medical trays, strainers , and hollow containers.

• Consumer goods

Caps, upper caps, sprays, rigid and semi-rigid packaging, videocassette frames, toys, electrical hardware, household appliances and their components, furniture for field trips and outside the city in the open air, suitcase, etc.

• bottles

Molded bottles by injection blow molding method with excellent rigidity, impact resistance and transparency.

Chemical structure and general properties of acrylic are as follow:

PMMA Properties

Acrylics are composed of polymers and copolymers whose main monomers belong to the two families of ester-acrylates and methacrylates. Transparent and hard acrylic sheets are made from methyl methacrylate, while extrusion and molding resins are made in a continuous solution of methacrylate copolymerized with a small percentage of acrylates or methacrylates.

Methyl methacrylate is produced through a two-step process in which acetone and hydrogen cyanide are reacted to give acetone cyanohydrin. Then, this compound is heated with methanol in the presence of concentrated sulfuric acid to obtain methyl methacrylate monomer. Acrylic monomers are polymerized through free radical polymerization processes initiated by peroxide initiators and form poly (methyl methacrylate). A monomeric initiator which become active at higher temperatures causes a reaction to be progressed that is very intense and exothermic, so that the released heat must be somehow removed from the system.

Different formulations of plastics differ both in molecular weight and in basic physico-mechanical properties such as melt flow index (MFI), thermal resistance, and toughness. There are special formulations that create opaque surfaces or absorb or transmit UV light. They are also available in a full range of colored resins in transparent, semi-transparent (translucent) and opaque forms.

Commercial types of acrylic with high impact resistance are available for injection molding and extrusion. These types of compounds consist of a hard acrylic phase and an acrylic modifier as a soft phase. Acrylic polymers have excellent optical properties and resistance to weathering and environmental conditions. Colorless acrylic resin can transmit white light up to 92%. Acrylics show a very high resistance to sunlight and long-term contact with various elements.

The low strain optical coefficient of acrylics, combined with their ability to be molded under very low stress, make them an ideal material for making video discs. Extruded sheets of a commercially available impact-modified acrylic exhibit excellent thermoforming properties and can be stiffened with glass-reinforced polyester so that they can be used on the inside of bath tubs. The high flow type has the best transparency because it does not contain acrylonitrile (AN), which makes this polymer the most suitable for medical applications where clarity is most important.

Acrylic plastics can be cleaned with solutions of mineral acids, alkalis, and aliphatic hydrocarbons, but chlorinated and aromatic hydrocarbons and ketones will degrade acrylic plastics.

Advantages

Acrylic polymers, such as poly (methyl methacrylate),  show excellent optical and weather-resistant properties, that is, when they are exposed to atmospheric conditions, they are highly resistant to weathering and environmental conditions (temperature, pressure, humidity or steam water). They are also available in a wide variety of colors of transparent, translucent, and opaque.

• Excellent optical clarity

• Excellent surface hardness

• Excellent tolerance to various weather conditions, excellent resistance to weathering and atmospheric conditions, high resistance to sunlight.

• Rigid and inflexible with good impact strength

• Excellent dimensional stability and low in-mold shrinkage

Disadvantages and limitations

• Low solvent resistance especially with ketones, esters, chlorocarbons, and aromatic hydrocarbons.
• flammable: continuous service temperature or continuous working temperature is limited to 160°F.
• Flexible commercial grades are not available.
• Moisture causes dimensional instability in molded parts.

Applications

• Car: Headlights or taillights, parking light lenses, decorative signs and license plates.
• Home Appliances: Holders, lampshades, picture frames, decorative goods.
• Transparent goods: Available in shimmering colors, the ideal material for jewelry packaging and signage.
• Electronic devices: It is used to cover the printed circuit board.

Introduction

Polyamides are high molecular weight polymers that have an amide group in their main chain which is considered as the main component of the polymer chain. The amide groups strongly adhere to each other and ensure high strength. These groups, which have become almost linear due to stretching, create strong hydrogen bonds with each other which are very strong.

Polyamides are classified into two groups according to their constituent monomers. 1) Polyamides that are prepared from the reaction of amino acids (AB) by the addition polymerization method. This method is used to prepare some AB type polyamides whose monomers are cyclic lactams such as ε-caprolactam or pyrrolidinone. 2) Polyamides that are formed from the condensation of bifunctional diamine and diacids that are called AABB type polyamides in which A indicates the amino group and B indicates the acidic group.

Aliphatic polyamides are named from the number that indicates the number of carbons in their constituent monomers. For AABB type polyamides two numbers are used. The first one refers to the number of diamine carbon atoms and the second one shows the number of diacid carbon atoms. Polyamide derived from ε-aminocaproic acid (6-aminohexanoic acid) or lactam is known as polyamide 6. Components with benzene rings are marked with a letter. For example, terephthalic and isophthalic acid are marked with T and I. Some authors also use TA and IA.

Linear polyamides are condensation products of bifunctional monomers. The hydrocarbon part between the amide groups may include branched or linear condensed hydrocarbons, aromatic rings or aliphatic rings, which can also include oxygen, sulfur and nitrogen. The hydrocarbon part used in the chain affects the flexibility of the chain and its structural arrangement, which is an important factor for the formation of the crystalline phase.

Aromatic polyamides are polymers in which an amide bond is placed between two aromatic rings. These polymers are prepared from the reaction of aromatic diamines with aromatic diacids in an amide solvent. Fibers with good heat resistance and high tensile strength and modulus are prepared from these polymers. Due to the unusual physical properties of aromatic polyamides, the general name aramid was chosen for them in 1974.

Aramid is a synthetic fiber made from long-chain polyamide in which at least 85% of the amide bonds (CONH) are directly attached to two aromatic rings.

In commercial aramids, 100% of amide bonds are connected to aromatic rings. With the creation of the new term aramid, synthetic fibers made from long-chain polyamide in which less than 85% of the amide bonds are directly attached to the aromatic ring were called nylon.

Properties

Polyamides comprise properties such as excellent toughness and impact resistance, excellent abrasion resistance, low friction coefficient, high tensile strength properties and optimal creep resistance, maintaining mechanical and electrical properties in a wide range of temperatures, excellent resistance to oils, greases, solvents and bases. These polymers can be processed through all special methods of thermoplastics.

Polyamide 66

Polyamide 66 is produced through condensation polymerization of adipic acid and hexamethylenediamine. The repeating unit of this plastic is as follow:

The molecular mass of polyamide 66 fibers is about (1500 to 13000) g/mol, while for the production of parts, the molecular mass of polyamide is usually higher than 24000.

Advantages of polyamide

• High impact resistance

• High hardness

• Abrasion resistance

• Flexibility

• Stability against many solvents and chemicals

Polyamides show very good resistance against petroleum substances and aliphatic solvents, but they are strongly affected by concentrated mineral acids at room temperature and alkalis at high temperature.

Polyamides are hard to catch fire and if they get out of the flame, they are self-extinguishing. Polyamide 66 fibers are resistant to the attack of insects such as willows.

If they are exposed to UV light, environmental factors and temperature of 130 ^oC during long-term operation, their color will fade and the mechanical properties will become weak and fragile. At a higher temperature, they are destroyed due to hydrolysis. The addition of light and thermal stabilizers slows down the effect of these factors on polyamides.

Research shows that polyamide parts that are used outside the room temperature should be stable and protected from sunlight, so an ultraviolet light absorber suitable for polyamides should be used in the polymeric compound. This can also be done by adding carbon black, so no significant change in their properties and performance in the long term at normal temperature.

Thermal properties of polyamide 66

In DSC thermogram, the melting phenomenon of polyamide 66 occurs around 264 ^oC, and at a temperature of 275 ^oCand above, this polymer is on the beginning of thermal degradation.

Another prominent feature of this plastic is the flexibility and good toughness of its fibers, which has made it widely used in the preparation of textiles, especially for women’s use. The permanent set of these fibers is occurred at 100 ^oC in which if they cooled slowly, the permanent crease will be created. Polyamide 66 maintains its mechanical properties up to 150°C, although conservatively, the temperature applied to this plastic (such as ironing) should not exceed 125°C.

Polyamides benefit from very good electrical insulation capabilities at low temperatures and low humidity, but these properties quickly disappear when the temperature rises or the humidity increases.

Increasing the methylene (-CH₂) functional group in the acidic part of the polyamide chain makes this plastic have a lower melting range and less moisture absorption ability, at the same time it reduces its stiffness and weakens its mechanical properties.

Mechanical properties of polyamide 66

Polyamide 66 contains a set of excellent properties such as mechanical properties (high strengths, high toughness and excellent resistance to wear) and very good thermal and chemical properties.

Two widely used industrial polyamides 6 and 66, despite the differences in molecular structure, are relatively similar to each other in terms of most mechanical and chemical properties.

The most important mechanical properties of polyamides are expressed through the information obtained from measuring of tensile, bending, compressive, shear and hardness tests. The set of results obtained from these 5 tests provides a major part of the technical information needed for the designer.

Polyamide 66 has high tensile strength, high impact resistance, good dimensional stability at relatively high temperatures and good resistance to wear. It also can be lubricated, therefore it is used in the manufacture of bearings.

As a result of stretching, the degree of crystallinity of polyamide fibers increases and many of its mechanical properties are improved. The tensile strength of polyamide 66 fibers is high and reaches more than 8 gr/denier.

At first impression, samples made of polyamide 6 are softer than polyamide 66.

It is worth mentioning that after absorbing moisture, due to the increase in flexibility of polyamide, many of its mechanical properties decrease. Only the impact resistance increases.

The moisture absorption of polyamides, especially polyamide 6 and 66, causes a sharp decrease in their modulus, because the presence of water between the chains (especially in polyamides that have the ability to absorb high moisture) acts as a softening agent for them and causes a drop in the mechanical properties of polyamides. The moisture absorption has a very favorable effect on the electrical properties of these types of plastics.

Types of polyamide 66

Polyamide 66 contains a set of excellent properties and has the ability to be used in many applications and various processes. In the first and most important classification, the types of polyamides 66 are divided into six groups: homopolymer, copolymer, blend, improved, filled, and reinforced. Several varieties of glass fiber reinforced polyamide 66 are made in weight ratios of 10%, 15%, 20%, 30%, 33%, 40%, 50% and 60%.

With the aid of additives and with the aim of improving the properties and increasing the performance of polyamide 66, various types of this plastic have been offered to the consumer market such as the fire-resistant type, which is placed in V-0 site in accordance with UL 94 guidelines.

There is a type of polyamide 66, in which due to mixing with fluoroplastics the decrease in friction is appeared.

Another type of this plastic is offered in black color, which is reinforced against harmful environmental factors such as ultraviolet light. It is worth mentioning that in this type of polyamide the presence of carbon black acts as the ultraviolet light absorber.

There are varieties of polyamide 66 homopolymer that are made for the injection process with low viscosity and high melt flow rate.

Another type of polyamide 66 homopolymer is available in the market, which its high viscosity enables the preparation of thick parts in the injection process.

Another type of polyamide 66 made with molybdenum disulfide additive can show excellent resistance to wear.

Typical applications of polyamide

Transportation: Transportation represents the largest market for polyamides. Applications of unreinforced materials include electrical connectors, wire coatings and lightweight gears, windshield wipers and speedometers. Stone guards and trim clips have been used to protect the windshield of the car. Glass-reinforced polyamides have been used in engine fan shrouds, radiator heads, steering and brake of fluid reservoirs and valves, sensors, and fuel injectors. Mineral reinforced resins have been used in mirror tools and tire hub covers. A combination of glass and mineral materials is used in exterior parts such as fender extensions.

Electrical and electronic applications: Flame retardant polyamides, including those that are performed with UL-94V0 requirements, play a major role in the electrical goods markets (plugs, fasteners or connectors, coils, wiring harnesses, terminal blocks, and antenna mounting tools).

Home Appliances: Polyamides are used not only for components in electrical goods, but also for mechanical parts and tools, power tools, washing machines, and various small household appliances.

Special applications in telecommunications: Power amplifiers or radio/telegraph amplifiers, amplifier stations, and connectors.

Industrial applications: Including hammer or sledgehammer handles, lawnmower parts, non-greased gears, bearings, anti-friction parts and a wide range of applications requiring spring clips or mounting the load on the spring.

Food and textile processing equipment: Including pumps, valves, measuring devices, agricultural and printing devices, office and sales machines.

Consumer Products: Applications of tough and toughened polyamide include ski boots, roller skates and ice skates bases, racket sports equipment, bicycle wheels, kitchenware, toys, and photography equipment.

Polyamide films: These films are used in a wide range for packing all kinds of meats and cheeses, as well as in non-stick bags for cooking and frying food. They are also used as an enclosing coating for making small airplane wings made of thermosetting polymers.

Wire and cable coating: Polyamides are often used as a protective layer on the primary insulation layer.

Piping and piping materials: Polyamides are used to transfer special brake fluids, special fluids for refrigerators, or as an inner lining for flexible cables.

Extrusion: sheets, bars, and handle-like shapes in machining.

• Heat-resistant materials:

This application includes filter bags for hot gases coming out of the chimney, under-press fabrics in industrial presses such as use in the permanent press of the final stage of linen fabric and linen polyester clothes, ironing board cover and sewing thread for very fast sewing, insulating paper for electric motors, tubes made for wire insulation, and dryer belts for papermaking. Another application is as pressure sensors in fuel tanks.

• Flame-resistant materials:

This application includes industrial protective clothing such as welders’ clothing and other protective clothing, firefighter clothing, flight clothing for military pilots and mail bags, carpets, curtains, sofas, fabric and cargo covers, boat covers and tents.

• Materials with dimensional stability

Fire hoses, V-belts, and power transfer belts made with high modulus aramid fibers such as Namax are examples of this application.

• Cases with very high strength and high modulus

These materials are used in V-belts of cables, parachutes, bulletproof vests, rigid reinforced plastics, antenna components, electrical circuit boards, sports equipment, ship ropes, telephone and power line cables, and fiber optic cables. Other usage is as a substitute for fireproof cotton.

• Cases with special properties

This application includes the fabrication of hollow fiber permeable separation membranes used for seawater and saltwater purification.

• Application in automobile industry

Polyamide 6 and polyamide 66 are used in the automotive industry. This industry accounts for 25% of the global consumption of polyamides.

Polyamide 66, in addition to manufacturing engineering parts, is used to make textile fibers in a very large volume. Due to its linear structure and good physical and chemical properties, suitable fibers are obtained from this polymer. First, this fiber replaced silk and was used in textiles, then it was noticed in carpet weaving. In military purposes, it is used to prepare parachutes and life jackets.

The speedometer gear of odometers, and especially the timing gear (chain) of some cars, have been proven to be able to work for years if they are made of polyamide 66.

Woodworking hammer handles are made of polyamide 66 reinforced with glass fibers, which is a good substitute for wood.

Polyamide 6 and 66 are compatible with wool and cotton, so if they are added to natural fibers at a ratio of 30%, they increase their resistance to wear and tear, while also improving their ironability.

Polyamide 66 granules and powder must be dehumidified before the processing, otherwise the fabricated parts will be damaged in terms of mechanical properties and appearance.

The moisture absorbed by the granules of polyamides turns into steam during the molding process. The steam causes the hydrolysis of the polyamide, the reduction of the molecular mass, the loss of mechanical properties. It makes bubbles in the parts and causes some parts of the molded piece to be incomplete and creates adverse effects on the surface.

Moisture absorbed by polyamides changes and loses their good electrical properties. The electrical properties of polyamides are limited to their use in low frequencies because this plastic has polar groups.

The most important uses of polyamide 66, which is an engineering plastic, can be seen in these applications:

Replacement for metals in bearings, gears, rollers (cylindrical rollers) and cams

Power wires are coated with polyamide 66 because it provides a toughness, wear resistance, good insulation properties, and heat stability.

If multi-functional reagents such as triamines, tetraamines and trifunctional acids are used, it leads to the production of network polymers with major differences in their properties.

Product Introduction

Fiber reinforcements

Composites usually consist of a matrix to hold the reinforcing materials. Reinforcing materials are the most important factors to grant strength to a composite system and have different shapes. These materials are able to conduct heat and resist against chemicals. The applications of these materials are also very diverse and range from the manufacture of computer parts to tennis rackets and chemical storage tanks.

Fibers are the main component of fiber-reinforced composite materials. They occupy a significant volume of composite and bear a large portion of the load on the composite structure. The amount, type and alignment of fibers are important factors that can affect the following properties: 1) Specific gravity 2) Tensile strength and modulus 3) Compressive strength and modulus 4) Fatigue strength and failure mechanism 5) Heat and electrical transfer coefficients 6) Price

The reinforcing phase in the composite can be in the form of fibers and particles that can also act as fillers. The fibers can be mineral (glass fibers, carbon fibers, etc.), organic (Kevlar, polyamide, etc.), metallic (boron, aluminum, etc.), or natural.

Due to the wide range of applications of glass fibers, these fibers are discussed below. Glass fibers prepare in a variety of shapes and types, such as continuous or parallel strands, short chopped fibers, or woven coils. These fibers are produced by passing molten glass through a platinum crucible with very small holes. Immediately after the glass filaments leave the crucible, they are quickly cooled by blowing moist air whose temperature and humidity is controlled, as well as by spraying water, and then wrapped in different size and thickness on cylinders for later use.

Glass fiber is the most common and widely used fiber in the composite industry. They are divided into different types according to the type and composition of materials used in their preparation. The main constituent of glass fibers, like ordinary glass (Soda-lime glass), is silica (SiO₂). Other oxides such as B₂O₃ and Al₂O₃ are added to modify the structure of the SiO₂ network as well as to optimize the manufacturing process such as lowering the melting temperature.

The structure of glass fiber is a three-dimensional network of silicon, oxygen, and other atoms that are irregularly gathered together. Therefore, glass fibers have an amorphous structure, which is non-crystalline and isotropic. In isotropic materials the mechanical properties is the same in all directions.

Main advantages of glass fibers

• Low price
• High tensile strength
• High chemical resistance
• Excellent insulation properties

It should be noted that these fibers show low tensile modulus, relatively high specific gravity, sensitivity to abrasion during transport (which often reduces tensile strength), low fatigue strength and high hardness which causes wear of molds and cutting tools.

Different types of glass fibers and their advantages

Glass fibers are classified as A, C, D, E, M and S.

The letter A refers to soda-lime glass and is derived from the word Alkali and is the ordinary glass that was common in the past which has poor thermal and chemical properties and is not suitable for the production of reinforcing fibers. The most common usage of this type of material is for making bottles, glass plates and soda glasses.

The letter C is derived from the word Chemical and is a fiber that has a very high chemical resistance.

The letter D is derived from the word Dielectric and is the fiber with the lowest dielectric constant. The lower the dielectric constant, the more transparent the substance is against electromagnetic waves. Therefore, these fibers are used in the construction of radar shields to protect the radar from atmospheric factors and pass the waves without dropping.

The letter E is derived from the word Electrical. These fibers show good electrical insulation properties. More than 90% of the glass fibers used in the composite industry are of this type. One of the most important capabilities of these fibers is its very high resistance to water, which is especially effective in the manufacture of composites used in humid environments.

The letter M is derived from the word Modulus and are fibers that have a high modulus.

The letter S refers to High Strength. The strength and price of this type of fiber is 20% and 4 times higher than type E, respectively. In addition, these fibers possess good tensile strength at high temperatures with significant resistance to wear. One of the most important applications of these fibers is in the aircraft parts as the engine shell of reactors and wherever there is a need for high efficiency.

The letter Z refers to fiber containing Zirconia and is a glass with excellent resistance to alkalis.

A noteworthy point about strands consisting of low-diameter fibers is that in the brittle fracture due to the growth of cracks, which can be due to the presence of fine cracks or surface flaws on the surface, only individual fibers are broken and the complete failure of the fibers is prevented. Therefore, a strand of fiber has a higher fracture strength than a fiber with a similar overall diameter. Because in a thick fiber the growth of a crack due to surface defects leads to its complete failure.

Affecting Factors on the strength of glass fibers

Studies show that the properties of glass fibers, in addition to the materials used in their structure, also depend on other factors such as environmental conditions. Other factors include the following.

A) Speed ​​of loading: The strength of glass fibers increases with the speed of tension applied during the tensile test.

B) Temperature: The strength of glass fibers decreases with increasing temperature. For example, an increase in temperature from 20 to 100 °C is resulted in a 30% decrease in strength.

C) Moisture: The strength of glass fibers decreases with increasing humidity.

D) The ratio of the reinforcing phase to the matrix phase can have many effects on the properties of the composite. For example, tensile strength increases with increasing percentage of glass fibers, while elongation decreases. Therefore, an optimal limit is selected to obtain the desired characteristics.

Application of glass fibers

Electronics: GRP has been widely used to produce circuit boards (PCBs), televisions, radios, computers, cell phones, electric motor covers, etc.

Home & Furniture: Roofing sheets, bathtub equipment, windows, awnings, shelves, tea tables, hot tubs, etc.

Aviation and Aerospace: GRP has been widely used in aerospace, although it is not widely used to build the original aircraft framework, as alternative materials exist that are more appropriate. Typical applications of GRP are engine covers, luggage racks, toolboxes, caps, channels, storage buckets and antenna shields. It is also widely used in land shipment equipment.

Boat construction and marine applications: This material is ideally suited for boat construction. Although there have been problems with water absorption in the past, modern resins are more durable and are used to make simple boats. In fact, GRP weighs less than other materials such as wood and metals.

Medicine: GRP is widely suitable for medical applications due to its low porosity, non-staining and high wear resistance. From containers to X-ray boards (where X-ray transparency is important) it consists of GRP.

Car: GRP has been widely used in the production of auto parts such as body panels, seat covers, door panels, bumpers and engine covers. It can be said that GRP is generally used to replace metal and non-metal parts in various applications and its processing costs are relatively low compared to metals.

Introduction

Polyamides are high molecular weight polymers that have an amide group in their main chain which is considered as the main component of the polymer chain. The amide groups strongly adhere to each other and ensure high strength. These groups, which have become almost linear due to stretching, create strong hydrogen bonds with each other which are very strong.

Polyamides are classified into two groups according to their constituent monomers. 1) Polyamides that are prepared from the reaction of amino acids (AB) by the addition polymerization method. This method is used to prepare some AB type polyamides whose monomers are cyclic lactams such as ε-caprolactam or pyrrolidinone. 2) Polyamides that are formed from the condensation of bifunctional diamine and diacids that are called AABB type polyamides in which A indicates the amino group and B indicates the acidic group.

Aliphatic polyamides are named from the number that indicates the number of carbons in their constituent monomers. For AABB type polyamides two numbers are used. The first one refers to the number of diamine carbon atoms and the second one shows the number of diacid carbon atoms. Polyamide derived from ε-aminocaproic acid (6-aminohexanoic acid) or lactam is known as polyamide 6. Components with benzene rings are marked with a letter. For example, terephthalic and isophthalic acid are marked with T and I. Some authors also use TA and IA.

Linear polyamides are condensation products of bifunctional monomers. The hydrocarbon part between the amide groups may include branched or linear condensed hydrocarbons, aromatic rings or aliphatic rings, which can also include oxygen, sulfur and nitrogen. The hydrocarbon part used in the chain affects the flexibility of the chain and its structural arrangement, which is an important factor for the formation of the crystalline phase.

Aromatic polyamides are polymers in which an amide bond is placed between two aromatic rings. These polymers are prepared from the reaction of aromatic diamines with aromatic diacids in an amide solvent. Fibers with good heat resistance and high tensile strength and modulus are prepared from these polymers. Due to the unusual physical properties of aromatic polyamides, the general name aramid was chosen for them in 1974.

Aramid is a synthetic fiber made from long-chain polyamide in which at least 85% of the amide bonds (CONH) are directly attached to two aromatic rings.

In commercial aramids, 100% of amide bonds are connected to aromatic rings. With the creation of the new term aramid, synthetic fibers made from long-chain polyamide in which less than 85% of the amide bonds are directly attached to the aromatic ring were called nylon.

Properties

Polyamides comprise properties such as excellent toughness and impact resistance, excellent abrasion resistance, low friction coefficient, high tensile strength properties and optimal creep resistance, maintaining mechanical and electrical properties in a wide range of temperatures, excellent resistance to oils, greases, solvents and bases. These polymers can be processed through all special methods of thermoplastics.

Polyamide 6

It is obtained from polymerization of w-aminocaproic lactam, which is more famous under the name of caprolactam. For this reason, this plastic is also called polycaprolactam. Another chemical name of this compound is 2-oxohexamethyleneimine. By opening the caprolactam ring and the formation of a molecule with two very active ends and joining of these structural units by intercondensation and addition method, the polymerization is carried out and finally the aliphatic polyamide is obtained.

Thermal properties of polyamide 6

The properties of polyamide 6 are very similar to polyamide 66, but there are also differences between them in terms of thermal and mechanical properties. Tensile properties, hardness, thermal properties and density of polyamide 66 is superior to polyamide 6, so the toughness and impact resistance of polyamide 66 is higher than polyamide 6. Polyamide 6 shows high shrinkage after molding due to its semi-crystalline nature. The melting range of polyamide 6 is in the thermal range of 222°C, its glass transition temperature is 53°C and its long-term operating temperature is 90°C.

Polyamide 6 with the high molecular mass and high degree of crystallinity which is used in the casting method, shows improved mechanical and thermal properties compared to other molding methods. Its moisture absorption is also reduced and retains its positive characteristics against changes in humidity.

The table below shows the most important thermal characteristics of polyamide 6.

Mechanical properties of polyamide 6

Polyamide 6 possesses high strength and stiffness, high hardness, good stability against creep, good resistance to wear and thermal aging. Reducing the number of methylene functional groups in the structural unit of the polyamide 6 chain, makes this plastic benefit from a higher melting range of 220, more moisture absorption and higher mechanical properties compared to polyamide 11 and 12. Polyamide 6 fibers have high tensile strength and toughness, high elasticity and luster. These fibers are resistant to wrinkling and wear. Moisture absorption of polyamides, especially polyamide 6 and 66, causes a sharp decrease in their modulus because the presence of water between polyamide chains, which have high moisture absorption capabilities, makes them soft. Polyamide 6 fibers absorb moisture in 50% to 2.7% relative humidity condition, which this amount of water has a favorable reducing effect on its mechanical properties. The amount of moisture absorption of polyamide 6 fibers in saturated relative humidity (100%) is equal to 9.5-11%. The increase in moisture absorption causes a sharp decrease in the mechanical properties of the polyamide 6 products. As the temperature increases, the elastic modulus of the parts made of polyamide 6 decreases sharply.

In the table below, some of the most important mechanical and electrical properties of polyamide 6 are presented.

Different types of polyamide 6

Polyamide 6, like polyamide 66, contains a set of excellent properties, so it can be used in wide range of applications and diverse processes.

In the first and most important classification, polyamide 6 is divided into six groups: homopolymer, copolymer, alloy, improved, filled, and reinforced. In some of the polyamide 6 homopolymer due to the mixing with fluoroplastics at a weight ratio of 15%, 20%, 30%, a fluid-like state and reduction of friction for the final product is occurred.

Homopolymers: There are varieties of polyamide 6 homopolymer that have been developed for the injection process with low viscosity and high melt flow rate. Other types of polyamide 6 homopolymer are suitable for the extrusion process due to their high viscosity.

Reinforced: Reinforced polyamide 6 with glass fibers (long and short) are made in weight ratios of 10%, 15%, 20%, 30%, 40% and 50% to produce composite parts.

Improved: Using additives and with the aim of improving the properties and increasing the efficiency of polyamide 6, various types of this plastic have been supplied to the market, such as the types which are resistant to UV, containing softener or heat stabilizer, or with the higher degree of crystallinity and having a nucleating agent (clarifier). It is worth mentioning that the nucleating agent, in addition to making the product opaque and transparent, increases the speed of the molding and consequently improves the production efficiency. Several types of polyamide 6 have been made with molybdenum disulfide additive which can show excellent resistance to wear.

Filled: There are other types of polyamide 6 that have powdery mineral fillers such as gypsum or lime at 20% and 30% weight ratios.

Polyamide 6 copolymers: Several copolymers and terpolymers have been made from polyamide 6 that some of which have been commercially available for years. The most important polyamide 6 copolymers are 6/610 and 6/66. Copolymer 6/66 was provided by BASF, Germany, which its two comonomers has the ratio of 85:15.

Polyamide terpolymers 6/610/66: These are copolymers with high flexibility and ability to dissolve in water and alcohol (in a mixture), which have high impact absorption ability. Ultramid 1c terpolymer is made of polyamide 6. Terpolyamide include equal amounts of polyamide 6 and polyamide 66 and another amide compound called diaminodicyclohexylmethane. This terpolymer is used as a coating and finishing operation.

Typical applications of polyamide

Transportation: Transportation represents the largest market for polyamides. Applications of unreinforced materials include electrical connectors, wire coatings and lightweight gears, windshield wipers and speedometers. Stone guards and trim clips have been used to protect the windshield of the car. Glass-reinforced polyamides have been used in engine fan shrouds, radiator heads, steering and brake of fluid reservoirs and valves, sensors, and fuel injectors. Mineral reinforced resins have been used in mirror tools and tire hub covers. A combination of glass and mineral materials is used in exterior parts such as fender extensions.

Electrical and electronic applications: Flame retardant polyamides, including those that are performed with UL-94V0 requirements, play a major role in the electrical goods markets (plugs, fasteners or connectors, coils, wiring harnesses, terminal blocks, and antenna mounting tools).

Home Appliances: Polyamides are used not only for components in electrical goods, but also for mechanical parts and tools, power tools, washing machines, and various small household appliances.

Special applications in telecommunications: Power amplifiers or radio/telegraph amplifiers, amplifier stations, and connectors.

Industrial applications: Including hammer or sledgehammer handles, lawnmower parts, non-greased gears, bearings, anti-friction parts and a wide range of applications requiring spring clips or mounting the load on the spring.

Food and textile processing equipment: Including pumps, valves, measuring devices, agricultural and printing devices, office and sales machines.

Consumer Products: Applications of tough and toughened polyamide include ski boots, roller skates and ice skates bases, racket sports equipment, bicycle wheels, kitchenware, toys, and photography equipment.

Polyamide films: These films are used in a wide range for packing all kinds of meats and cheeses, as well as in non-stick bags for cooking and frying food. They are also used as an enclosing coating for making small airplane wings made of thermosetting polymers.

Wire and cable coating: Polyamides are often used as a protective layer on the primary insulation layer.

Piping and piping materials: Polyamides are used to transfer special brake fluids, special fluids for refrigerators, or as an inner lining for flexible cables.

Extrusion: sheets, bars, and handle-like shapes in machining.

• Heat-resistant materials:

This application includes filter bags for hot gases coming out of the chimney, under-press fabrics in industrial presses such as use in the permanent press of the final stage of linen fabric and linen polyester clothes, ironing board cover and sewing thread for very fast sewing, insulating paper for electric motors, tubes made for wire insulation, and dryer belts for papermaking. Another application is as pressure sensors in fuel tanks.

• Flame-resistant materials:

This application includes industrial protective clothing such as welders’ clothing and other protective clothing, firefighter clothing, flight clothing for military pilots and mail bags, carpets, curtains, sofas, fabric and cargo covers, boat covers and tents.

• Materials with dimensional stability

Fire hoses, V-belts, and power transfer belts made with high modulus aramid fibers such as Namax are examples of this application.

• Cases with very high strength and high modulus

These materials are used in V-belts of cables, parachutes, bulletproof vests, rigid reinforced plastics, antenna components, electrical circuit boards, sports equipment, ship ropes, telephone and power line cables, and fiber optic cables. Other usage is as a substitute for fireproof cotton.

• Cases with special properties

This application includes the fabrication of hollow fiber permeable separation membranes used for seawater and saltwater purification.

• Automobile industry

Polyamide 6 and polyamide 66 are used in the automotive industry. This industry accounts for 25% of the global consumption of polyamides.

In addition to making parts, polyamide 6 is also used in the production of a very large volume of textile and non-textile fibers. Due to its linear structure, fibers with particular properties are obtained from this resin.

Polyamide 6 has a wide range of applications in the manufacture of products that comprise high strength, wear resistance, high hardness and resistance to thermal wear, or the part has the ability to be machined.

This plastic is used in making gears, bearings, connections, fibers, car parts, all kinds of films, and films used for food packaging. Polyamide 6 is also used in making the collar of wheelchairs, propellers, strings of musical instruments such as violin, guitar, viola, cello, sitar, and etc.