About Plastics

Polyethylene terephthalate (PET or PETE) is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber. The majority of the world’s PET production is for synthetic fibers (in excess of 60%), with bottle production accounting for around 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. Polyester makes up about 18% of world polymer production and is the third-most-produced polymer; polyethylene (PE) and polypropylene (PP) are first and second, respectively.

Uses:

Because PET is an excellent water and moisture barrier material, plastic bottles made from PET are widely used for soft drinks (see carbonation). For certain specialty bottles, such as those designated for beer containment, PET sandwiches an additional polyvinyl alcohol (PVOH) layer to further reduce its oxygen permeability.

Biaxially oriented PET film (often known by one of its trade names, “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”. Because of its high mechanical strength, PET film is often used in tape applications, such as the carrier for magnetic tape or backing for pressure-sensitive adhesive tapes.

Non-oriented PET sheet can be thermoformed to make packaging trays and blisters. If crystallizable PET is used, the trays can be used for frozen dinners, since they withstand both freezing and oven baking temperatures. As opposed to amorphous PET, which is transparent, crystallizable PET or CPET tends to be black in colour.
When filled with glass particles or fibres, it becomes significantly stiffer and more durable.

PET is also used as substrate in thin film and solar cell. These bottles if placed in direct sunlight they can withstand there for a very long time (five years approximately). As the time passes these bottles become opaque.

Properties:

PET in its natural state is a colorless, semi-crystalline resin. Based on how it is processed, PET can be semi-rigid to rigid, and it is very lightweight. It makes a good gas and fair moisture barrier, as well as a good barrier to alcohol (requires additional “barrier” treatment) and solvents. It is strong and impact-resistant. PET becomes white when exposed to chloroform and also certain other chemicals such as toluene.

About 60% crystallization is the upper limit for commercial products, with the exception of polyester fibers. Clear products can be produced by rapidly cooling molten polymer below Tg glass transition temperature to form an amorphous solid. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an orderly, crystalline fashion as the melt is cooled. At room temperature the molecules are frozen in place, but, if enough heat energy is put back into them by heating above Tg, they begin to move again, allowing crystals to nucleate and grow. This procedure is known as solid-state crystallization.

When allowed to cool slowly, the molten polymer forms a more crystalline material. This material has spherulites containing many small crystallites when crystallized from an amorphous solid, rather than forming one large single crystal. Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them. This scattering means that crystalline PET is opaque and white in most cases. Fiber drawing is among the few industrial processes that produce a nearly single-crystal product.

Polyethylene terephthalate [wikipedia.org]

High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a polyethylene thermoplastic made from petroleum. Known for its large strength to density ratio, HDPE is commonly used in the production of plastic bottles, corrosion-resistant piping, geomembranes, and plastic lumber. HDPE is commonly recycled, and has the number “2” as its recycling symbol.
In 2007, the global HDPE market reached a volume of more than 30 million tons.

Market Study: Polyethylene HDPE [Ceresana Research]

Properties:

HDPE is known for its large strength to density ratio. The mass density of high-density polyethylene can range from 0.93 to 0.97 g/cm3. Although the density of HDPE is only marginally higher than that of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength than LDPE. The difference in strength exceeds the difference in density, giving HDPE a higher specific strength. It is also harder and more opaque and can withstand somewhat higher temperatures (120 °C/ 248 °F for short periods, 110 °C /230 °F continuously). High-density polyethylene, unlike polypropylene, cannot withstand normally required autoclaving conditions. The lack of branching is ensured by an appropriate choice of catalyst (e.g., Ziegler-Natta catalysts) and reaction conditions.

Typical Properties of Polyethylene (PE) [Ides.com]

Compare Materials: HDPE and LDPE [Makeitfrom.com]

Applications:

HDPE is resistant to many different solvents and has a wide variety of applications, including:

• 3-D printer filament
• Arena Board (puck board)
• Backpacking frames
• Ballistic plates
• Bottle caps
• Chemical resistant piping systems
• Coax cable inner insulator
• Food storage containers
• Fuel tanks for vehicles
• Corrosion protection for steel pipelines
• Electrical and plumbing boxes
• Far-IR lenses
• Folding chairs and tables
• Geomembrane for hydraulic applications (such as canals and bank reinforcements) and chemical containment
• Geothermal heat transfer piping systems
• Heat-resistant fireworks mortars
• Hard hats
• Hula hoops
• Natural gas distribution pipe systems
• Fireworks
• Plastic bags
• Plastic bottles suitable both for recycling (such as milk jugs) or re-use
• Plastic lumber
• Plastic surgery (skeletal and facial reconstruction)
• Root barrier
• Snowboard rails and boxes
• Stone paper
• Storage sheds
• Telecom ducts
• Tyvek
• Water pipes for domestic water supply and agricultural processes
• Wood plastic composites (utilizing recycled polymers)

HDPE is also used for cell liners in subtitle D sanitary landfills, wherein large sheets of HDPE are either extrusion or wedge welded to form a homogeneous chemical-resistant barrier, with the intention of preventing the pollution of soil and groundwater by the liquid constituents of solid waste.

HDPE is preferred by the pyrotechnics trade for mortars over steel or PVC tubes, being more durable and safer. HDPE tends to rip or tear in a malfunction instead of shattering and becoming shrapnel like the other materials.

Milk jugs and other hollow goods manufactured through blow molding are the most important application area for HDPE – More than 8 million tons, or nearly one third of worldwide production, was applied here. In addition to being recycled using conventional processes, HDPE can also be processed by recyclebots into filament for 3-D printers via distributed recycling.

Above all, China, where beverage bottles made from HDPE were first imported in 2005, is a growing market for rigid HDPE packaging, as a result of its improving standard of living. In India and other highly populated, emerging nations, infrastructure expansion includes the deployment of pipes and cable insulation made from HDPE. The material has benefited from discussions about possible health and environmental problems caused by PVC and Polycarbonate associated Bisphenol A, as well as its advantages over glass, metal and cardboard.

High Density Polyethylene [wikipedia.org]

Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE (recycling number 4) is recycled. Despite competition from more modern polymers, LDPE continues to be an important plastic grade. In 2009 the worldwide LDPE market reached a volume of US$22.2 billion (15.9 billion Euro)

Introduction to Industrial Polyethylene: Properties, Catalysts, and Processes [Dennis Malpass (28 June 2010)]

“Municipal Solid Waste Generation, Recycling, and Disposal in the United States” [epa.gov]

Properties:

LDPE is defined by a density range of 0.910–0.940 g/cm3. It is not reactive at room temperatures, except by strong oxidizing agents, and some solvents cause swelling. It can withstand temperatures of 80 °C continuously and 95 °C for a short time. Made in translucent or opaque variations, it is quite flexible, and tough but breakable. 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. Also, since its molecules are less tightly packed and less crystalline because of the side branches, its density is lower. LDPE contains the chemical elements carbon and hydrogen.

Chemical Resistance:

• Excellent resistance (no attack) to dilute and concentrated acids, alcohols, bases and esters
• Good resistance (minor attack) to aldehydes, ketones and vegetable oils
• Limited resistance (moderate attack suitable for short-term use only) to aliphatic and aromatic hydrocarbons, mineral oils, and oxidizing agents
• Poor resistance, and not recommended for use with halogenated hydrocarbons.

Plastic Properties of Low Density Polyethylene (LDPE) [Dynalab Corp.]

Applications:

LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic bags for computer components, and various molded laboratory equipment. Its most common use is in plastic bags. Other products made from it include:

• Trays and general purpose containers
• Corrosion-resistant work surfaces
• Parts that need to be weldable and machinable
• Parts that require flexibility, for which it serves very well
• Very soft and pliable parts such as snap-on lids
• Six pack rings
• Juice and milk cartons are made of liquid packaging board, a laminate of paperboard and LDPE (as the water-proof inner and outer layer), and often with of a layer of aluminum foil (thus becoming aseptic packaging).
• Parts of computer hardware, such as hard disk drives, screen cards, and optical disc drives
• Playground slides
• Plastic wraps

LDPE products and applications. [Exxon Mobil Corporation]

DOW LDPE 5004I. [IDES – The Plastics Web]