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85-433: Despite its designation with the recycling symbol, it cannot be as commonly recycled as No. 1 (polyethylene terephthalate) or 2 plastics (high-density polyethylene) . LDPE is defined by a density range of 917–930 kg/m. At room temperature it is not reactive, except to strong oxidizers; some solvents cause it to swell. It can withstand temperatures of 65 °C (149 °F) continuously and 90 °C (194 °F) for

170-400: A "sniffer test" on returned bottles to avoid cross-contamination of flavors. Different applications of PET require different degrees of polymerization, which can be obtained by modifying the process conditions. The molecular weight of PET is measured by solution viscosity. The preferred method to measure this viscosity is the intrinsic viscosity (IV) of the polymer. Intrinsic viscosity

255-425: A byproduct. Polymerization is through a polycondensation reaction of the monomers (done immediately after esterification/transesterification) with water as the byproduct. Polyester fibres are widely used in the textile industry. The invention of the polyester fibre is attributed to J. R. Whinfield. It was first commercialized in the 1940s by ICI , under the brand 'Terylene'. Subsequently E. I. DuPont launched

340-487: A different form, without destroying the polymer; or recycled in a process that includes transesterification and the addition of other glycols, polyols, or glycerol to form a new polyol. The polyol from the third method can be used in polyurethane (PU foam) production, or epoxy-based products, including paints. In 2023 a process was announced for using PET as the basis for supercapacitor production. PET, being stoichiometrically carbon and H 2 O , can be turned into

425-446: A few micrometers ) depending on its crystal structure and particle size. One process for making PET uses bis(2-hydroxyethyl) terephthalate , which can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct (this is also known as a condensation reaction), or by transesterification reaction between ethylene glycol and dimethyl terephthalate (DMT) with methanol as

510-483: A form of carbon containing sheets and nanospheres, with a very high surface area. The process involves holding a mixture of PET, water, nitric acid , and ethanol at a high temperature and pressure for eight hours, followed by centrifugation and drying. Significant investments were announced in 2021 and 2022 for chemical recycling of PET by glycolysis, methanolysis, and enzymatic recycling to recover monomers. Initially these will also use bottles as feedstock but it

595-441: A high modulus observed in the draw direction. Drawn semi-crystalline polymers are the strongest polymeric materials due to the stress-induced ordering of the molecular chains. Other defects, such as voids, occur in the semi-crystalline polymer under tensile stress and can drive the formation of the neck. The voids can be observed via small angle x-ray scattering. Unlike crazes these voids do not transfer stresses. Notably, cavitation

680-734: A machine learning algorithm to be able to tolerate pH and temperature changes by the University of Texas at Austin . The PET-ase was found to able to degrade various products and could break them down as fast as 24 hours. Semi-crystalline polymer Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae , which compose larger spheroidal structures named spherulites . Polymers can crystallize upon cooling from melting, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of

765-426: A more unsustainable rate than other plastics. In a test at the end of 212 days' incubation, emissions recorded were 5.8 nmol g d of methane, 14.5 nmol g d of ethylene, 3.9 nmol g d of ethane, and 9.7 nmol g d of propylene. When incubated in air, LDPE emits methane and ethylene at rates about 2 times and about 76 times, respectively, more than in water. Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic. LDPE

850-453: A multilayer structure is used. PET sandwiches an additional polyvinyl alcohol (PVOH) or polyamide (PA) layer to further reduce its oxygen permeability. Non-oriented PET sheet can be thermoformed to make packaging trays and blister packs . Crystallizable PET withstands freezing and oven baking temperatures. Both amorphous PET and BoPET are transparent to the naked eye. Color-conferring dyes can easily be formulated into PET sheet. PET

935-417: A neck is formed in the amorphous region and propagates down the sample length. During necking, the disordered chains align along the tensile direction, forming an ordered structure that demonstrates strengthening due to the molecular reorientation. The flow stress now increases significantly following neck propagation. Mechanical anisotropy increases and the elastic modulus varies along different directions, with

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1020-437: A short time. Made in translucent and opaque variations, it is quite flexible and tough. LDPE has more branching (on about 2% of the carbon atoms) than HDPE , so its intermolecular forces ( instantaneous-dipole induced-dipole attraction ) are weaker, its tensile strength is lower, and its resilience is higher. The side branches mean that its molecules are less tightly packed and less crystalline, and therefore its density

1105-481: A significant amount of PET microparticles may be precipitated in sewage treatment plants. PET microfibers generated by apparel wear, washing or machine drying can become airborne, and be dispersed into fields, where they are ingested by livestock or plants and end up in the human food supply. SAPEA have declared that such particles 'do not pose a widespread risk'. PET is known to degrade when exposed to sunlight and oxygen. As of 2016, scarce information exists regarding

1190-416: A space with dimensions of a few tens of nanometers, comparable to or smaller than the lamellar crystal thickness or the radius of gyration, nucleation and growth can be dramatically affected. As an example, when a polymer crystallizes in a confined ultrathin layer, the isotropic spherulitic organization of lamellar crystals is hampered and confinement can produce unique lamellar crystal orientations. Sometimes

1275-569: A stabilizer and a bluing agent such as cobalt salt is added to mask any yellowing. In the dimethyl terephthalate (DMT) process, DMT and excess ethylene glycol (MEG) are transesterified in the melt at 150–200 °C with a basic catalyst . Methanol (CH 3 OH) is removed by distillation to drive the reaction forward. Excess MEG is distilled off at higher temperature with the aid of vacuum. The second transesterification step proceeds at 270–280 °C, with continuous distillation of MEG as well. The reactions can be summarized as follows: In

1360-465: A substantial degree, for example about 75% in Switzerland. The term rPET is commonly used to describe the recycled material, though it is also referred to as R-PET or post-consumer PET (POSTC-PET). The prime uses for recycled PET are polyester fiber, strapping, and non-food containers. Because of the recyclability of PET and the relative abundance of post-consumer waste in the form of bottles, PET

1445-415: A tensile stress is applied the semi-crystalline polymer first deforms elastically. While the crystalline regions remain unaffected by the applied stress, the molecular chains of the amorphous phase stretch. Then yielding, which signifies the onset of plastic deformation of the crystalline regions, occurs. The molecular mechanism for semi-crystalline yielding involves the deformation of crystalline regions of

1530-464: A thickness on the order 10–20 nm when crystallized from a dilute solution. The crystal shape can be more complex for other polymers, including hollow pyramids, spirals and multilayer dendritic structures. A very different process is precipitation; it uses a solvent which dissolves individual monomers but not the resulting polymer. When a certain degree of polymerization is reached, the polymerized and partially crystallized product precipitates out of

1615-400: A toughening effect in polymers the interparticle matrix ligament thickness must be smaller than a certain threshold. Crystalline polymers polypropylene and polyethylene display particle strengthening. Plastics are viscoelastic materials meaning that under applied stress, their deformation increases with time (creep). The elastic properties of plastics are therefore distinguished according to

1700-453: A variety of different regimes and unlike simple molecules, the polymer crystal lamellae have two very different surfaces. The two most prominent theories in polymer crystallization kinetics are the Avrami equation and Lauritzen-Hoffman Growth Theory. Below their glass transition temperature, amorphous polymers are usually hard and brittle because of the low mobility of their molecules. Increasing

1785-427: Is isophthalic acid , replacing some of the 1,4-( para- ) linked terephthalate units. The 1,2-( ortho- ) or 1,3-( meta -) linkage produces an angle in the chain, which also disturbs crystallinity. Such copolymers are advantageous for certain molding applications, such as thermoforming , which is used for example to make tray or blister packaging from co-PET film, or amorphous PET sheet (A-PET/PETA) or PETG sheet. On

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1870-478: Is a dimensionless measurement found by extrapolating the relative viscosity (measured in (dℓ/g)) to zero concentration. Shown below are the IV ranges for common applications: PET is often copolymerized with other diols or diacids to optimize the properties for particular applications. For example, cyclohexanedimethanol (CHDM) can be added to the polymer backbone in place of ethylene glycol . Since this building block

1955-440: Is also rapidly gaining market share as a carpet fiber. PET, like many plastics, is also an excellent candidate for thermal disposal ( incineration ), as it is composed of carbon, hydrogen, and oxygen, with only trace amounts of catalyst elements (but no sulfur). In general, PET can either be chemically recycled into its original raw materials (PTA, DMT, and EG), destroying the polymer structure completely; mechanically recycled into

2040-442: Is also technically possible to make PTA from readily available bio-based furfural . There are two basic molding methods for PET bottles, one-step and two-step. In two-step molding, two separate machines are used. The first machine injection molds the preform, which resembles a test tube, with the bottle-cap threads already molded into place. The body of the tube is significantly thicker, as it will be inflated into its final shape in

2125-686: Is expected that fibres will also be recycled this way in future. PET is also a desirable fuel for waste-to-energy plants , as it has a high calorific value which helps to reduce the use of primary resources for energy generation. At least one species of bacterium in the genus Nocardia can degrade PET with an esterase enzyme. Esterases are enzymes able to cleave the ester bond between two oxygens that links subunits of PET. The initial degradation of PET can also be achieved esterases expressed by Bacillus , as well as Nocardia . Japanese scientists have isolated another bacterium, Ideonella sakaiensis , that possesses two enzymes which can break down

2210-430: Is isotropic and static then lamellae grow radially and form larger quasi-spherical aggregates called spherulites. Spherulites have a size between about 1 and 100 micrometers and form a large variety of colored patterns (see, e.g. front images) when observed between crossed polarizers in an optical microscope, which often include the "Maltese cross" pattern and other polarization phenomena caused by molecular alignment within

2295-417: Is lower. When exposed to consistent sunlight, the plastic produces significant amounts of two greenhouse gases : methane and ethylene . Because of its lower density (high branching), it breaks down more easily than other plastics; as this happens, the surface area increases. Production of these trace gases from virgin plastics increases with surface area and with time, so that LDPE emits greenhouse gases at

2380-543: Is much larger (six additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. In general, such PET is known as PETG or PET-G (polyethylene terephthalate glycol-modified). It is a clear amorphous thermoplastic that can be injection-molded, sheet-extruded or extruded as filament for 3D printing . PETG can be colored during processing. Another common modifier

2465-457: Is not observed under compressive stress or shearing. Evidence suggests that cavitation also impacts the onset of yielding. The voids are associated with the breaking of the amorphous phase. The strength of the crystalline phase determines the importance of cavitation in yielding. If the crystalline structures are weak, they deform easily resulting in yielding. Semi-crystalline polymers with strong crystalline regions resist deformation and cavitation,

2550-642: Is permeable to oxygen and carbon dioxide and this imposes shelf life limitations of contents packaged in PET. In the early 2000s, the global PET packaging market grew at a compound annual growth rate of 9% to €17 billion in 2006. Biaxially oriented PET (BOPET) film (including brands like "Mylar") can be aluminized by evaporating a thin film of metal onto it to reduce its permeability, and to make it reflective and opaque ( MPET ). These properties are useful in many applications, including flexible food packaging and thermal insulation (such as space blankets ). BOPET

2635-405: Is produced largely from purified terephthalic acid (PTA), as well as to a lesser extent from (mono-)ethylene glycol (MEG) and dimethyl terephthalate (DMT). As of 2022, ethylene glycol is made from ethene found in natural gas , while terephthalic acid comes from p-xylene made from crude oil . Typically an antimony or titanium compound is used as a catalyst, a phosphite is added as

Low-density polyethylene - Misplaced Pages Continue

2720-1013: Is quite important, because if nothing masks the aroma, even extremely low concentrations (10–20 parts per billion in the water) of acetaldehyde can produce an off-taste. Commentary published in Environmental Health Perspectives in April 2010 suggested that PET might yield endocrine disruptors under conditions of common use and recommended research on this topic. Proposed mechanisms include leaching of phthalates as well as leaching of antimony . An article published in Journal of Environmental Monitoring in April 2012 concludes that antimony concentration in deionized water stored in PET bottles stays within EU's acceptable limit even if stored briefly at temperatures up to 60 °C (140 °F), while bottled contents (water or soft drinks) may occasionally exceed

2805-412: Is still of concern. The Swiss Federal Office of Public Health investigated the amount of antimony migration, comparing waters bottled in PET and glass: The antimony concentrations of the water in PET bottles were higher, but still well below the allowed maximum concentration. The Swiss Federal Office of Public Health concluded that small amounts of antimony migrate from the PET into bottled water, but that

2890-491: Is strong and impact-resistant. PET is hygroscopic and absorbs water. About 60% crystallization is the upper limit for commercial products, with the exception of polyester fibers. Transparent products can be produced by rapidly cooling molten polymer below the glass transition temperature (T g ) to form a non-crystalline amorphous solid . Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an orderly, crystalline fashion as

2975-431: Is strong anisotropy of their mechanical properties along the direction of molecular alignment and perpendicular to it. Above the glass transition temperature amorphous chains in a semi-crystalline polymer are ductile and are able to deform plastically. Crystalline regions of the polymer are linked by the amorphous regions. Tie molecules prevent the amorphous and crystalline phases from separating under an applied load. When

3060-437: Is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods, and thermoforming for manufacturing, and in combination with glass fibre for engineering resins . In 2016, annual production of PET was 56 million tons. The biggest application is in fibres (in excess of 60%), with bottle production accounting for about 30% of global demand. In

3145-538: Is the reduction in space, product handling and energy, and far higher visual quality than can be achieved by the two-step system. PET is subject to degradation during processing. If the moisture level is too high, hydrolysis will reduce the molecular weight by chain scission , resulting in brittleness. If the residence time and/or melt temperature (temperature at melting) are too high, then thermal degradation or thermooxidative degradation will occur resulting in discoloration and reduced molecular weight, as well as

3230-445: Is used in the backsheet of photovoltaic modules . Most backsheets consist of a layer of BOPET laminated to a fluoropolymer or a layer of UV stabilized BOPET. PET is also used as a substrate in thin film solar cells. PET can be compounded with glass fibre and crystallization accelerators, to make thermoplastic resins. These can be injection moulded into parts such as housings, covers, electrical appliance components and elements of

3315-423: Is used in the production of plastic tanks and PET bottles. Some polymers which do not crystallize from the melt, can be partially aligned by stretching. Some elastomers which are amorphous in the unstrained state undergo rapid crystallization upon stretching. Polymers can also be crystallized from a solution or upon evaporation of a solvent. This process depends on the degree of dilution: in dilute solutions,

3400-437: Is why BOPET film and bottles are both crystalline, to a degree, and transparent. PET has an affinity for hydrophobic flavors, and drinks sometimes need to be formulated with a higher flavor dosage, compared to those going into glass, to offset the flavor taken up by the container. While heavy gauge PET bottles returned for re-use, as in some EU countries, the propensity of PET to absorb flavors makes it necessary to conduct

3485-413: Is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic parts for computer components, and various molded laboratory equipment. Its most common use is in plastic bags . Other products made from it include: Polyethylene terephthalate Polyethylene terephthalate (or poly(ethylene terephthalate) , PET , PETE , or the obsolete PETP or PET-P ),

Low-density polyethylene - Misplaced Pages Continue

3570-560: The terephthalic acid process, MEG and PTA are esterified directly at moderate pressure (2.7–5.5 bar) and high temperature (220–260 °C). Water is eliminated in the reaction, and it is also continuously removed by distillation : Bio-PET is the bio-based counterpart of PET. Essentially in Bio-PET, the MEG is manufactured from ethylene derived from sugar cane ethanol . A better process based on oxidation of ethanol has been proposed, and it

3655-422: The EU limit after less than a year of storage at room temperature. Antimony (Sb) is a metalloid element that is used as a catalyst in the form of compounds such as antimony trioxide (Sb 2 O 3 ) or antimony triacetate in the production of PET. After manufacturing, a detectable amount of antimony can be found on the surface of the product. This residue can be removed with washing. Antimony also remains in

3740-401: The PET into smaller pieces digestible by the bacteria. A colony of I. sakaiensis can disintegrate a plastic film in about six weeks. French researchers report developing an improved PET hydrolase that can depolymerize (break apart) at least 90 percent of PET in 10 hours, breaking it down into individual monomers . Also, an enzyme based on a natural PET-ase was designed with the help of

3825-459: The UK were found to contain up to 44.7 μg/L of antimony, well above the EU limits for tap water of 5 μg/L. Clothing sheds microfibres in use, during washing and machine drying. Plastic litter slowly forms small particles. Microplastics which are present on the bottom of the river or seabed can be ingested by small marine life, thus entering the food chain. As PET has a higher density than water,

3910-400: The atactic polypropylene form. Atactic polymers crystallize when the side groups are very small, as in polyvinyl and don't crystallize in case of large substituents like in rubber or silicones . Nucleation starts with small, nanometer-sized areas, where a result of heat motions in some chains or their segments occur parallel. Those seeds can either dissociate, if thermal motion destroys

3995-405: The brand 'Dacron'. As of 2022, there are many brands around the world, mostly Asian. Polyester fibres are used in fashion apparel often blended with cotton, as heat insulation layers in thermal wear, sportswear and workwear and automotive upholstery. Plastic bottles made from PET are widely used for soft drinks , both still and sparkling . For beverages that are degraded by oxygen, such as beer,

4080-523: The chain alignment is parallel to the layer plane and the crystals are organized as ‘‘on-edge’’ lamellae. In other cases, "in-plane" lamellae with chain orientation perpendicular to the layers are observed. The unique crystal orientation of confined polymers imparts anisotropic properties. In one example the large, in-plane polymer crystals reduce the gas permeability of nanolayered films by almost 2 orders of magnitude. Polymers formed via topochemical polymerisation are generally crystalline. In many cases,

4165-430: The context of textile applications, PET is referred to by its common name, polyester , whereas the acronym PET is generally used in relation to packaging. Polyester makes up about 18% of world polymer production and is the fourth-most-produced polymer after polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC). PET consists of repeating (C 10 H 8 O 4 ) units. PET is commonly recycled , and has

4250-427: The degree of crystallinity assume a mixture of perfect crystalline and totally disordered areas; the transition areas are expected to amount to several percent. These methods include density measurement, differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared spectroscopy and nuclear magnetic resonance (NMR). The measured value depends on the method used, which is therefore quoted together with

4335-612: The degree of crystallinity. In addition to the above integral methods, the distribution of crystalline and amorphous regions can be visualized with microscopic techniques, such as polarized light microscopy and transmission electron microscopy . The methods used to determine the degree of crystallinity can be incorporated over time to measure the kinetics of crystallization. The most basic model for polymer crystallization kinetics comes from Hoffman nucleation theory . The crystallization process of polymers does not always obey simple chemical rate equations . Polymers can crystallize through

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4420-408: The digit 1 (♳) as its resin identification code (RIC). The National Association for PET Container Resources (NAPCOR) defines PET as: "Polyethylene terephthalate items referenced are derived from terephthalic acid (or dimethyl terephthalate ) and mono ethylene glycol , wherein the sum of terephthalic acid (or dimethyl terephthalate) and mono ethylene glycol reacted constitutes at least 90 percent of

4505-466: The distance between the parallel chain segments, and it determines the mechanical and thermal properties of the polymer. The growth of the crystalline regions preferably occurs in the direction of the largest temperature gradient and is suppressed at the top and bottom of the lamellae by the amorphous folded parts at those surfaces. In the case of a strong gradient, the growth has a unidirectional, dendritic character. However, if temperature distribution

4590-430: The formation of acetaldehyde , and the formation "gel" or "fish-eye" formations through cross-linking . Mitigation measures include copolymerisation with other monomers like CHDM or isophthalic acid , which lower the melting point and thus the melt temperature of the resin, as well as the addition of polymer stabilisers such as phosphites . Acetaldehyde , which can form by degradation of PET after mishandling of

4675-435: The formation of voids in the amorphous phase, drives yielding. As done in crystalline materials, particles can be added to semi-crystalline polymers to change the mechanical properties. In crystalline materials the addition of particles works to impede dislocation motion and strengthen the material. However, for many semi-crystalline polymers particle fillers weaken the material. It has been suggested that for particles to have

4760-444: The glass transition temperature, the movement of molecular chains is frozen. Nevertheless, secondary crystallization can proceed even below T g , in the time scale of months and years. This process affects mechanical properties of the polymers and decreases their volume because of a more compact packing of aligned polymer chains. The chains interact via various types of the van der Waals forces . The interaction strength depends on

4845-403: The glass transition temperature. Their elastic modulus changes significantly only at high (melting) temperature. It also depends on the degree of crystallinity: higher crystallinity results in a harder and more thermally stable, but also more brittle material, whereas the amorphous regions provide certain elasticity and impact resistance. Another characteristic feature of semicrystalline polymers

4930-434: The good nucleating agents are metal salts of organic acids, which themselves are crystalline at the solidification temperature of the polymer solidification. Crystal growth is achieved by the further addition of folded polymer chain segments and only occurs for temperatures below the melting temperature T m and above the glass transition temperature T g . Higher temperatures destroy the molecular arrangement and below

5015-504: The gradual formation acetaldehyde over the object's lifespan. This proceeds via a Type II Norrish reaction . When acetaldehyde is produced, some of it remains dissolved in the walls of a container and then diffuses into the product stored inside, altering the taste and aroma. This is not such a problem for non-consumables (such as shampoo), for fruit juices (which already contain acetaldehyde), or for strong-tasting drinks like soft drinks. For bottled water, however, low acetaldehyde content

5100-475: The health risk of the resulting low concentrations is negligible (1% of the " tolerable daily intake " determined by the WHO ). A later (2006) but more widely publicized study found similar amounts of antimony in water in PET bottles. The WHO has published a risk assessment for antimony in drinking water. Fruit juice concentrates (for which no guidelines are established), however, that were produced and bottled in PET in

5185-662: The ignition system. PET is stoichiometrically a mixture of carbon and H 2 O , and therefore has been used in an experiment involving laser-driven shock compression which created nanodiamonds and superionic water . This could be a possible way of producing nanodiamonds commercially. PET was patented in 1941 by John Rex Whinfield , James Tennant Dickson and their employer the Calico Printers' Association of Manchester , England. E. I. DuPont de Nemours in Delaware, United States, first produced Dacron (PET fiber) in 1950 and used

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5270-463: The individual lamellae of a spherulite. The above mechanism considered crystallization from the melt, which is important for injection molding of plastic components. Another type of crystallization occurs upon extrusion used in making fibers and films. In this process, the polymer is forced through, e.g., a nozzle that creates tensile stress which partially aligns its molecules. Such alignment can be considered as crystallization, and it affects

5355-439: The life-time of the synthetic polymers in the environment. While most thermoplastics can, in principle, be recycled, PET bottle recycling is more practical than many other plastic applications because of the high value of the resin and the almost exclusive use of PET for widely used water and carbonated soft drink bottling. PET bottles lend themselves well to recycling (see below). In many countries PET bottles are recycled to

5440-598: The mass of monomer reacted to form the polymer, and must exhibit a melting peak temperature between 225 °C and 255 °C, as identified during the second thermal scan in procedure 10.1 in ASTM D3418, when heating the sample at a rate of 10 °C/minute." Depending on its processing and thermal history, polyethylene terephthalate may exist both as an amorphous (transparent) and as a semi-crystalline polymer . The semicrystalline material might appear transparent (particle size less than 500  nm ) or opaque and white (particle size up to

5525-499: The material itself and can, thus, migrate out into food and drinks. Exposing PET to boiling or microwaving can increase the levels of antimony significantly, possibly above US EPA maximum contamination levels. The drinking water limit assessed by WHO is 20 parts per billion (WHO, 2003), and the drinking water limit in the United States is 6 parts per billion. Although antimony trioxide is of low toxicity when taken orally, its presence

5610-411: The material properties. For example, the strength of the fiber is greatly increased in the longitudinal direction, and optical properties show large anisotropy along and perpendicular to the fiber axis. Such anisotropy is more enhanced in presence of rod-like fillers such as carbon nanotubes, compared to spherical fillers. Polymer strength is increased not only by extrusion, but also by blow molding, which

5695-413: The material via dislocation motion. Dislocations result in coarse or fine slips in the polymer and lead to crystalline fragmentation and yielding. Fine slip is defined as a small amount of slip occurring on a large number of planes. Conversely, coarse slip is a large amount of slip on few planes. The yield stress is determined by the creation of dislocations and their resistance to motion. After yielding,

5780-441: The material, is a colorless, volatile substance with a fruity smell. Although it forms naturally in some fruit, it can cause an off-taste in bottled water. As well as high temperatures (PET decomposes above 300 °C or 570 °F) and long barrel residence times, high pressures and high extruder speeds (which cause shear raising the temperature), can also contribute to the production of acetaldehyde. Photo-oxidation can also cause

5865-473: The melt is cooled. While at room temperature the molecules are frozen in place, if enough heat energy is put back into them afterward by heating the material above T g , they can begin to move again, allowing crystals to nucleate and grow. This procedure is known as solid-state crystallization. Amorphous PET also crystallizes and becomes opaque when exposed to solvents , such as chloroform or toluene . A more crystalline product can be produced by allowing

5950-402: The melt. Some polymers retain such a disordered structure upon freezing and readily convert into amorphous solids. In other polymers, the chains rearrange upon freezing and form partly ordered regions with a typical size of the order 1 micrometer. Although it would be energetically favorable for the polymer chains to align parallel, such alignment is hindered by the entanglement. Therefore, within

6035-467: The molecular chains have no connection with each other and exist as a separate polymer coil in the solution. Increase in concentration which can occur via solvent evaporation, induces interaction between molecular chains and a possible crystallization as in the crystallization from the melt. Crystallization from solution may result in the highest degree of polymer crystallinity. For example, highly linear polyethylene can form platelet-like single crystals with

6120-433: The molecular order, or grow further, if the grain size exceeds a certain critical value. Apart from the thermal mechanism, nucleation is strongly affected by impurities, dyes, plasticizers, fillers and other additives in the polymer. This is also referred to as heterogeneous nucleation. This effect is poorly understood and irregular, so that the same additive can promote nucleation in one polymer, but not in another. Many of

6205-416: The molten polymer to cool slowly. Rather than forming one large single crystal, this material has a number of spherulites (crystallized areas) each containing many small crystallites (grains). Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them, causing the resulting solid to be translucent. Orientation also renders polymers more transparent. This

6290-614: The monomer to polymer transition occurs with the retention of crystallinity. Often one can determine the crystal structure of such polymers and the mechanism of polymerisation via single crystal X-ray diffraction. Since the polymerization happens in the crystalline lattice without the aid of solvents or reagents, it comes under the domain of green chemistry. Also, the topochemical polymerizations are mostly atom economical reactions. The product can be obtained without any further purifications. It can achieve unique products which cannot be synthesized through conventional methods. The fraction of

6375-401: The ordered molecules in polymer is characterized by the degree of crystallinity, which typically ranges between 10% and 80%. Higher values are only achieved in materials having small molecules, which are usually brittle, or in samples stored for long time at temperatures just under the melting point. The latter procedure is costly and is applied only in special cases. Most methods of evaluating

6460-589: The ordered regions, the polymer chains are both aligned and folded. Those regions are therefore neither crystalline nor amorphous and are classified as semicrystalline. Examples of semi-crystalline polymers are linear polyethylene (PE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) or isotactic polypropylene (PP). Whether or not polymers can crystallize depends on their molecular structure – presence of straight chains with regularly spaced side groups facilitates crystallization. For example, crystallization occurs much easier in isotactic than in

6545-619: The other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of isophthalic acid, CHDM, diethylene glycol (DEG) or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding ("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages. Polyethylene terephthalate

6630-443: The polymer. The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10 and 80%, with crystallized polymers often called "semi-crystalline". The properties of semi-crystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains. Polymers are composed of long molecular chains which form irregular, entangled coils in

6715-568: The second step using stretch blow molding . In the second step, the preforms are heated rapidly and then inflated against a two-part mold to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as robust and unique containers themselves; besides novelty candy, some Red Cross chapters distribute them as part of the Vial of Life program to homeowners to store medical history for emergency responders. The two-step process lends itself to third party production remote from

6800-425: The solution. The rate of crystallization can be monitored by a technique which selectively probes the dissolved fraction, such as nuclear magnetic resonance . When polymers crystallize from an isotropic, bulk of melt or concentrated solution, the crystalline lamellae (10 to 20 nm in thickness) are typically organized into a spherulitic morphology as illustrated above. However, when polymer chains are confined in

6885-438: The temperature induces molecular motion resulting in the typical rubber-elastic properties. A constant force applied to a polymer at temperatures above T g results in a viscoelastic deformation, i.e., the polymer begins to creep . Heat resistance is usually given for amorphous polymers just below the glass transition temperature. Relatively strong intermolecular forces in semicrystalline polymers prevent softening even above

6970-508: The time scale of the testing to short-time behavior (such as tensile test which lasts minutes), shock loading, the behavior under long-term and static loading, as well as the vibration-induced stress. Crystalline polymers are usually opaque because of light scattering on the numerous boundaries between the crystalline and amorphous regions. The density of such boundaries is lower in polymers with very low crystallinity (amorphous polymer) or very high degree of crystalline polymers, consequentially,

7055-767: The trademark Mylar (boPET film) in June 1951 and received registration of it in 1952. It is still the best-known name used for polyester film. The current owner of the trademark is DuPont Teijin Films. In the Soviet Union, PET was first manufactured in the laboratories of the Institute of High-Molecular Compounds of the USSR Academy of Sciences in 1949, and its name "Lavsan" is an acronym thereof ( ла боратории Института в ысокомолекулярных с оединений А кадемии н аук СССР). The PET bottle

7140-472: The user site. The preforms can be transported and stored by the thousand in a much smaller space than would finished containers, for the second stage to be carried out on the user site on a 'just in time' basis. In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for molding non-standard shapes (custom molding), including jars, flat oval, flask shapes, etc. Its greatest merit

7225-551: Was invented in 1973 by Nathaniel Wyeth and patented by DuPont. PET in its most stable state is a colorless, semi-crystalline resin . However it is intrinsically slow to crystallize compared to other semicrystalline polymers . Depending on processing conditions it can be formed into either non-crystalline ( amorphous ) or crystalline articles. Its amenability to drawing in manufacturing makes PET useful in fibre and film applications. Like most aromatic polymers , it has better barrier properties than aliphatic polymers . It

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