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Titanium Dioxide (TiO2)

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.

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Polycarbonate (PC)

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).

Bisphenol A repeating unit in polycarbonate

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.

PC Properties

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.

PC Properties

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.

PC Properties

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.

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Polyolefin Elastomer (POE)

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:

POE Properties

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.

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