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#6289.

2434) Polyethylene terephthalate

Gist

Polyethylene terephthalate (PET or PETE) is a strong, lightweight, and clear thermoplastic polymer used for everything from food and beverage bottles to clothing fibers. It is made by combining ethylene glycol and terephthalic acid and is valued for its strength, barrier properties, and recyclability. 

Polyethylene terephthalate which is also abbreviated as PET / PETE is mainly used to manufacture the packaging material for food products such as fruit and drinks containers. It is lightweight, transparent and also available in some colour.

Summary

Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), 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 2013, 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 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. PET used in non-fiber applications (i.e. for packaging) makes up about 6% of world polymer production by mass. Accounting for the >60% fraction of polyethylene terephthalate produced for use as polyester fibers, PET is the fourth-most-produced polymer after polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

PET consists of repeating (C10H8O4) units. PET is commonly recycled, and has 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 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 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 a byproduct. It can also be obtained by recycling of PET itself.[10] Polymerization is through a polycondensation reaction of the monomers (done immediately after esterification/transesterification) with water as the byproduct.

Details

Polyethylene terephthalate (PET or PETE) is a strong, stiff synthetic fibre and resin and a member of the polyester family of polymers. PET is spun into fibres for permanent-press fabrics and blow-molded into disposable beverage bottles.

PET is produced by the polymerization of ethylene glycol and terephthalic acid. Ethylene glycol is a colourless liquid obtained from ethylene, and terephthalic acid is a crystalline solid obtained from xylene. When heated together under the influence of chemical catalysts, ethylene glycol and terephthalic acid produce PET in the form of a molten, viscous mass that can be spun directly to fibres or solidified for later processing as a plastic. In chemical terms, ethylene glycol is a diol, an alcohol with a molecular structure that contains two hydroxyl (OH) groups, and terephthalic acid is a dicarboxylic aromatic acid, an acid with a molecular structure that contains a large six-sided carbon (or aromatic) ring and two carboxyl (CO2H) groups. Under the influence of heat and catalysts, the hydroxyl and carboxyl groups react to form ester (CO-O) groups, which serve as the chemical links joining multiple PET units together into long-chain polymers. Water is also produced as a by-product.

The presence of a large aromatic ring in the PET repeating units gives the polymer notable stiffness and strength, especially when the polymer chains are aligned with one another in an orderly arrangement by drawing (stretching). In this semicrystalline form, PET is made into a high-strength textile fibre marketed under the trademarked name Dacron by the American company Invista. The stiffness of PET fibres makes them highly resistant to deformation, so they impart excellent resistance to wrinkling in fabrics. They are often used in durable-press blends with other fibres such as rayon, wool, and cotton, reinforcing the inherent properties of those fibres while contributing to the ability of the fabric to recover from wrinkling.

PET is also made into fibre filling for insulated clothing and for furniture and pillows. When made in very fine filaments, it is used in artificial silk, and in large-diameter filaments it is used in carpets. Among the industrial applications of PET are automobile tire yarns, conveyor belts and drive belts, reinforcement for fire hoses and garden hoses, seat belts (an application in which it has largely replaced nylon), nonwoven fabrics for stabilizing drainage ditches, culverts, and railroad beds, and nonwovens for use as diaper topsheets and disposable medical garments. PET is the most important of the synthetic fibres in weight produced and in value.

At a slightly higher molecular weight, PET is made into a high-strength plastic that can be shaped by all the common methods employed with other thermoplastics. PET films (often sold under the trademarks Mylar and Melinex) are produced by extrusion. Molten PET can be blow-molded into transparent containers of high strength and rigidity that are also virtually impermeable to gas and liquid. In this form, PET has become widely used in carbonated-beverage bottles and in jars for food processed at low temperatures. The low softening temperature of PET—approximately 70 °C (160 °F)—prevents it from being used as a container for hot foods.

PET is the most widely recycled plastic. In the United States, however, only about 20 percent of PET material is recycled. PET bottles and containers are commonly melted down and spun into fibres for fibrefill or carpets. When collected in a suitably pure state, PET can be recycled into its original uses, and methods have been devised for breaking the polymer down into its chemical precursors for resynthesizing into PET. The recycling code number for PET is 1.

PET was first prepared in England by J. Rex Whinfield and James T. Dickinson of the Calico Printers Association during a study of phthalic acid begun in 1940. Because of wartime restrictions, patent specifications for the new material were not immediately published. Production by Imperial Chemical of its Terylene-brand PET fibre did not begin until 1954. Meanwhile, by 1945 DuPont had independently developed a practical preparation process from terephthalic acid, and in 1953 the company began to produce Dacron fibre. PET soon became the most widely produced synthetic fibre in the world. In the 1970s, improved stretch-molding procedures were devised that allowed PET to be made into durable crystal-clear beverage bottles—an application that soon became second in importance only to fibre production.

Additional Information

Polyethylene terephthalate is one of the most common plastics. It’s used in a variety of items from water bottles and product packaging to baby wipes, clothing, bedding and mattresses. You’ll find polyethylene terephthalate written as PET or PETE, or the recycling code #1. On clothing and textile labels, you’ll find it listed as polyester.

The Problem

Making PET is an energy-intensive process. When used in the form of polyester for textiles, it uses far more energy than the manufacturing of other textiles like conventional or organic hemp and cotton, but it’s sold less expensively. In the production process, emissions can severely contaminate water sources with a number of pollutants.

PET does not readily break down in the environment. So, all of those wipes, water bottles, or product packs headed to the landfill will stick around – essentially – forever.

In addition to its issues with biodegradability, PET may pose some toxicity risks. Antimony trioxide is commonly used as a catalyst in the production process. Antimony trioxide is classified as possibly carcinogenic, and some forms are potentially endocrine disrupting.

Researchers have found antimony at detectable levels in polyester textiles. Even at low temperatures, antimony can migrate from polyester to saliva and sweat. One study concluded that exposure to antimony through polyester could result in potential health impacts for groups who wear polyester often and for prolonged period of times. Keep in mind that polyester is often used in active apparel and worn at times when the wearer is sweating.

This research also raises some unanswered questions about our exposure to antimony through polyester bedding while we sleep. Given that we spend about a third of our lives in bed, is it possible that this is prolonged and frequent enough exposure to experience associated health impacts from antimony? No research is available on this subject, so more study is needed.

A number of researchers have also confirmed that other estrogenic compounds are capable of migrating from PET water bottles into its contents.

PET and Plastic Pollution

Because PET doesn’t readily break down, it contributes to plastic pollution. Plastics like PET can break down into tiny pieces called microplastics, which are pervasive in our oceans – as well as our bays, lakes, and even drinking water. Plastics break down into tiny pieces, but they essentially never go away, as petroleum-derived plastic is typically not biodegradable.

Microplastics are often consumed by aquatic life, both large and small. And as the web of life goes, small aquatic animals that have eaten plastic are then consumed by predators. Those predators are consumed by even larger predators. The circle of life – predator becoming prey – allows microplastics to progressively build up with each successive level of the food chain. The largest predator of all? Humans.

Plastic particles have been found in seafood. What this means is that when we consume ocean animals, we may be unknowingly swallowing microplastics.

Unknown Impact

Researchers don’t yet understand how ingesting microplastics – whether from seafood or our drinking water – will impact humans. More study is needed to understand the risks. However, as mentioned above, researchers do know that plastics are capable of leaching toxic substances. Researchers also know that plastic can break down in animals’ stomachs. So it’s very possible that plastic can break down in our stomachs too and that it could be leaching harmful substances in the process.

The problem with plastic pollution isn’t just about humans. Building evidence suggests that marine animals may be threatened by consuming microplastics. And the plastic issue extends beyond consumption; animals can be entangled or smothered by debris, which can injure, debilitate, or even kill them. Some estimates say that if our use of plastic continues at this rate, there will be more plastic than fish in the ocean by 2050!

Finally, the overwhelming majority of plastic is not recycled. About 50 percent of plastic is used for single-use products – those designed to be used once (like a wipe, a to-go container, a water bottle, or a straw) and then thrown away. Less than 10 percent of plastic is actually recycled! That means that while recycling is indeed important, it’s even more important for each of us to refuse the use of single-use plastics.

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2433) Ammonium Nitrate

Gist

Ammonium nitrate is a white, crystalline, water-soluble salt primarily used as a high-nitrogen fertilizer and as a key component in commercial explosives. It is a strong oxidizer that contains both a fuel and an oxygen-producing agent, making it potentially explosive, especially when mixed with a fuel or subjected to heat. Due to its hazardous nature, there are strict regulations surrounding its production, storage, and use.

Ammonium nitrate is primarily used as a component in fertilizers for agriculture and in the manufacture of explosives for mining, quarrying, and construction. It also has niche applications, such as in instant cold packs and as a source for medical-grade nitrous oxide (laughing gas). 

Summary

Ammonium nitrate is a chemical compound with the formula NH4NO3. It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly used in agriculture as a high-nitrogen fertilizer.

Its other major use is as a component of explosive mixtures used in mining, quarrying, and civil construction. It is the major constituent of ANFO, an industrial explosive which accounts for 80% of explosives used in North America; similar formulations have been used in improvised explosive devices.

Many countries are phasing out its use in consumer applications due to concerns over its potential for misuse. Accidental ammonium nitrate explosions have killed thousands of people since the early 20th century. Global production was estimated at 21.6 million tonnes in 2017. By 2021, global production of ammonium nitrate was down to 16.7 million tonnes.

(ANFO or AN/FO, for ammonium nitrate/fuel oil).

Details

Ammonium nitrate, (NH4NO3), is a salt of ammonia and nitric acid, used widely in fertilizers and explosives. The commercial grade contains about 33.5 percent nitrogen, all of which is in forms utilizable by plants; it is the most common nitrogenous component of artificial fertilizers. Ammonium nitrate also is employed to modify the detonation rate of other explosives, such as nitroglycerin in the so-called ammonia dynamites, or as an oxidizing agent in the ammonals, which are mixtures of ammonium nitrate and powdered aluminum.

Ammonium nitrate is a colourless crystalline substance (melting point 169.6 °C [337.3 °F]). It is highly soluble in water; heating of the water solution decomposes the salt to nitrous oxide (laughing gas). Because solid ammonium nitrate can undergo explosive decomposition when heated in a confined space, government regulations have been imposed on its shipment and storage.

Additional Information

Ammonium nitrate, in terms of quantity at least, is the most commonly used oxidizer in improvised explosive mixtures. The salt has been used in many terrorist attacks, particularly those involving large charge weights of over half a tonne. Ammonium nitrate has been mixed with a number of fuels such as sugar and aluminum.

Ammonium nitrate is the main component of slurry explosives used for mining. However, the source of ammonium nitrate used for improvised explosive mixtures is often fertilizer. Fertilizer-grade ammonium nitrate can be powdered or in the form of prills. Fertilizer-grade prills are usually coated to reduce hygroscopicity. However, as prills or powder, fertilizer-grade ammonium nitrate can be mixed with suitable fuels, such as sugar or fuel oil, to produce an effective high explosive. The detonability and explosive power of ammonium nitrate-based improvised explosives is dependent on the particle size, the fuel, stoichiometry, degree of mixing, the packing density, and the degree of confinement. In prill form, the explosive is not usually detonable and requires a booster charge for efficient detonation and complete reaction. It acts as a tertiary explosive. In powdered form, it is often detonable.

Explosive mixtures incorporating ammonium nitrate have large critical diameters and are therefore nonideal explosives, whose energy release during detonation occurs in a time scale insufficient for the majority of it to keep up with the shock front. The loss of energy from the system exceeds the rate of generation and a self-sustaining reaction cannot be maintained. The explosive fails to react completely and can fail to detonate; therefore, large charge weights are often necessary for reliable detonation. Furthermore, in contrast to ideal explosives, confinement has a significant effect on detonation, increased confinement resulting in an increased velocity of detonation (VoD).

Improvised explosives incorporating ammonium nitrate have detonation velocities in the range of 1400–6000 m/s. The TNT equivalence of ammonium nitrate-based improvised explosives ranges from 25% to 100% depending on the factors referred to earlier, for example, packing density and degree of confinement.

Fuels mixed with ammonium nitrate to produce effective improvised high explosives have included sugar, fuel oil, aluminum, nitromethane, and nitrobenzene.

Research has shown that most ammonium nitrate-based improvised explosives are chemically and thermally stable and insensitive to stimuli, friction, impact, and ESD in normal conditions. However, mixtures with aluminum have been found to be sensitive to ESD.

The potential for ammonium nitrate to decompose at temperatures above 200 °C and, as a result of the heat generated by decomposition, to then undergo a runaway reaction is well known. While many fuels have been shown to lower the decomposition temperature, mixtures have, nevertheless, been shown to be relatively thermally stable during storage and handling.

(ESD: Electrostatic discharge).