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Waste composition and recycling by polymer type

Plastic waste consists of various polymer types, its exact composition will vary, but the estimated global average is shown below.^[1]^[2] Polyolefins make up nearly 50% of all plastic waste and more than 90% of waste is make of thermoset polymers. The amount of each polymer which is recovered for recycling also varies, U.S. data is shown as exemplar.^[3]

Global plastic waste production by polymer type (2018)^[1]^[2]
Polymer	Waste production (Mt)	Percentage of all plastic waste	Polymer type	Thermal character
High-density polyethylene (HDPE)	64	19.8%	Polyolefin	Thermoplastic
Low-density polyethylene (LDPE)	45	13.9%	Polyolefin	Thermoplastic
polypropylene (PP)	62	19.1%	Polyolefin	Thermoplastic
Polystyrene (PS)	19	5.9%	Aromatic polyolefin	Thermoplastic
Polyvinyl chloride (PVC)	17	5.3%	Halogenated	Thermoplastic
Polyethylene terephthalate (PET)	35	10.8%	Condensation	Thermoplastic
Polyurethane (PUR)	18	5.6%	Condensation	Thermoset^[4]
PP&A fibers^[5]	51	15.7%	Condensation	Thermoplastic
All Others	12	3.7%	Various	Varies
Total (excludes additives)	324	100%	-	-

Plastic composition of U.S. Municipal solid waste and recycling rates (2018)^[3]
Polymer	Quantitiy (Thousand tonnes)	Percentage of plastic waste	U.S. recycling rate
High-density polyethylene (HDPE)	6,300	17.7%	8.9%
Low-density polyethylene (LDPE)	8,590	24.1%	4.3%
polypropylene (PP)	8,150	22.8%	0.6%
Polystyrene (PS)	2,260	6.3%	0.9%
Polyvinyl chloride (PVC)	840	2.4%	Negligible
Polyethylene terephthalate (PET)	5,290	14.8%	18.5%
All Others	4,160	12%	26.7%
Total	35,680	100%	8.7%

Global plastic waste production by polymer type and U.S. recycling rate (2018)^[1]^[2]
Polymer	Waste production (Mt)	Percentage of all plastic waste	Polymer type	Thermal character	U.S. recycling rate
High-density polyethylene (HDPE)	64	19.8%	Polyolefin	Thermoplastic	8.9%
Low-density polyethylene (LDPE)	45	13.9%	Polyolefin	Thermoplastic	4.3%
polypropylene (PP)	62	19.1%	Polyolefin	Thermoplastic	0.6%
Polystyrene (PS)	19	5.9%	Aromatic polyolefin	Thermoplastic	0.9%
Polyvinyl chloride (PVC)	17	5.3%	Halogenated	Thermoplastic	Negligible
Polyethylene terephthalate (PET)	35	10.8%	Condensation	Thermoplastic	18.5%
Polyurethane (PUR)	18	5.6%	Condensation	Thermoset^[6]	No data
PP&A fibers^[5]	51	15.7%	Condensation	Thermoplastic	Negligible
All Others	12	3.7%	Various	Varies	Varies
Total (excludes additives)	324	100%	-	-

Recycling of polymer types

Plastic composition of U.S. Municipal solid waste and recycling rates (2018)^[3]
Polymer	Quantitiy (Thousand tonnes)	Percentage of plastic waste	U.S. recycling rate
High-density polyethylene (HDPE)	6,300	17.7%	8.9%
Low-density polyethylene (LDPE)	8,590	24.1%	4.3%
polypropylene (PP)	8,150	22.8%	0.6%
Polystyrene (PS)	2,260	6.3%	0.9%
Polyvinyl chloride (PVC)	840	2.4%	Negligible
Polyethylene terephthalate (PET)	5,290	14.8%	18.5%
All Others	4,160	12%	26.7%
Total	35,680	100%	8.7%

^[7]

All common plastics can be recycled, but actual rates of recycling vary significantly between polymers. The physical properties of the plastics and the amount of additives they contain largely control this. Foamed plastics such as polystyrene or polyurethane is usually uneconomical to collect (a truck-full of foam contains little actual plastic) and what little recycling does take place is mostly recovered industrial scrap. Soft plastics, particularly in the form of films and foils are difficult to flake or gravity sort and tend to clog equipment

This is driven by a mixture of economics and technical limitations

PET 725 cheapest HDPE 815 LDPE 100 PP 945

Widely recycled

Sometimes recycled

Usually not recycled

Production

Polyurethanes are produced by mixing two or more liquid streams. The polyol stream contains catalysts, surfactants, blowing agents (when making polyurethane foam insulation) and so on. The two components are referred to as a polyurethane system, or simply a system. The isocyanate is commonly referred to in North America as the 'A-side' or just the 'iso'. The blend of polyols and other additives is commonly referred to as the 'B-side' or as the 'poly'.^{[citation needed]} This mixture might also be called a 'resin' or 'resin blend'. In Europe the meanings for 'A-side' and 'B-side' are reversed.^{[citation needed]} Resin blend additives may include chain extenders, cross linkers, surfactants, flame retardants, blowing agents, pigments, and fillers. Polyurethane can be made in a variety of densities and hardnesses by varying the isocyanate, polyol or additives.

Manufacturing

The methods of manufacturing polyurethane finished goods range from small, hand pour piece-part operations to large, high-volume bunstock and boardstock production lines. Regardless of the end-product, the manufacturing principle is the same: to meter the liquid isocyanate and resin blend at a specified stoichiometric ratio, mix them together until a homogeneous blend is obtained, dispense the reacting liquid into a mold or on to a surface, wait until it cures, then demold the finished part.

Dispensing equipment

Although the capital outlay can be high, it is desirable to use a meter-mix or dispense unit for even low-volume production operations that require a steady output of finished parts. Dispense equipment consists of material holding (day) tanks, metering pumps, a mix head, and a control unit. Often, a conditioning or heater–chiller unit is added to control material temperature in order to improve mix efficiency, cure rate, and to reduce process variability. Choice of dispense equipment components depends on shot size, throughput, material characteristics such as viscosity and filler content, and process control. Material day tanks may be single to hundreds of gallons in size and may be supplied directly from drums, IBCs (intermediate bulk containers, such as totes), or bulk storage tanks. They may incorporate level sensors, conditioning jackets, and mixers. Pumps can be sized to meter in single grams per second up to hundreds of pounds per minute. They can be rotary, gear, or piston pumps, or can be specially hardened lance pumps to meter liquids containing highly abrasive fillers such as chopped or hammer-milled glass fiber and wollastonite. ^{[citation needed]}

A high-pressure polyurethane dispense unit, showing control panel, high-pressure pump, integral day tanks, and hydraulic drive unit
A high-pressure mix head, showing simple controls (front view)
A high-pressure mix head, showing material supply and hydraulic actuator lines (rear view)

The pumps can drive low-pressure (10 to 30 bar, 1 to 3 MPa) or high-pressure (125 to 250 bar, 12.5 to 25.0 MPa) dispense systems. Mix heads can be simple static mix tubes, rotary-element mixers, low-pressure dynamic mixers, or high-pressure hydraulically actuated direct impingement mixers. Control units may have basic on/off and dispense/stop switches, and analogue pressure and temperature gauges, or may be computer-controlled with flow meters to electronically calibrate mix ratio, digital temperature and level sensors, and a full suite of statistical process control software. Add-ons to dispense equipment include nucleation or gas injection units, and third or fourth stream capability for adding pigments or metering in supplemental additive packages.

A low-pressure mix head with calibration chamber installed, showing material supply and air actuator lines
Low-pressure mix head components, including mix chambers, conical mixers, and mounting plates
5-gallon (20-liter) material day tanks for supplying a low-pressure dispense unit

Tooling

Distinct from pour-in-place, bun and boardstock, and coating applications, the production of piece parts requires tooling to contain and form the reacting liquid. The choice of mold-making material is dependent on the expected number of uses to end-of-life (EOL), molding pressure, flexibility, and heat transfer characteristics.

RTV silicone is used for tooling that has an EOL in the thousands of parts. It is typically used for molding rigid foam parts, where the ability to stretch and peel the mold around undercuts is needed. The heat transfer characteristic of RTV silicone tooling is poor. High-performance, flexible polyurethane elastomers are also used in this way.

Epoxy, metal-filled epoxy, and metal-coated epoxy is used for tooling that has an EOL in the tens of thousands of parts. It is typically used for molding flexible foam cushions and seating, integral skin and microcellular foam padding, and shallow-draft RIM bezels and fascia. The heat transfer characteristic of epoxy tooling is fair; the heat transfer characteristic of metal-filled and metal-coated epoxy is good. Copper tubing can be incorporated into the body of the tool, allowing hot water to circulate and heat the mold surface.

Aluminum is used for tooling that has an EOL in the hundreds of thousands of parts. It is typically used for molding microcellular foam gasketing and cast elastomer parts, and is milled or extruded into shape.

Mirror-finish stainless steel is used for tooling that imparts a glossy appearance to the finished part. The heat transfer characteristic of metal tooling is excellent.

Finally, molded or milled polypropylene is used to create low-volume tooling for molded gasket applications. Instead of many expensive metal molds, low-cost plastic tooling can be formed from a single metal master, which also allows greater design flexibility. The heat transfer characteristic of polypropylene tooling is poor, which must be taken into consideration during the formulation process.

bits

Moisture cure polyurethane Polyurethane dispersion Polyurethane urea elastomer

^ ^a ^b ^c Cite error: The named reference Geyer2017 was invoked but never defined (see the help page).
^ ^a ^b ^c Geyer, Roland (2020). Plastic waste and recycling : environmental impact, societal issues, prevention, and solutions. Amsterdam: Academic Press. p. 22. ISBN 978-0-12-817880-5.
^ ^a ^b ^c ""Advancing Sustainable Materials Management: 2018 Tables and Figures" (PDF). U.S. EPA.
^ The majority of polyurethanes are thermosets, however some thermoplastics are also produced, for instance spandex
^ ^a ^b PP&A stand for polyester, polyamide and acrylate polymers; all of which are used to make synthetic fibres. Care should be taken not to confuse it with polyphthalamide (PPA)
^ The majority of polyurethanes are thermosets, however some thermoplastics are also produced, for instance spandex
^ "Plastic Recycling Factsheet" (PDF). EuRIC - European Recycling Industries’ Confederation. Retrieved 9 November 2021.

[Geyer2017-1] Cite error: The named reference Geyer2017 was invoked but never defined (see the help page).

[Geyer2020-2] Geyer, Roland (2020). Plastic waste and recycling : environmental impact, societal issues, prevention, and solutions. Amsterdam: Academic Press. p. 22. ISBN 978-0-12-817880-5.

[US-data-3] ""Advancing Sustainable Materials Management: 2018 Tables and Figures" (PDF). U.S. EPA.

[4] The majority of polyurethanes are thermosets, however some thermoplastics are also produced, for instance spandex

[PP&A-5] PP&A stand for polyester, polyamide and acrylate polymers; all of which are used to make synthetic fibres. Care should be taken not to confuse it with polyphthalamide (PPA)

[6] The majority of polyurethanes are thermosets, however some thermoplastics are also produced, for instance spandex

[7] "Plastic Recycling Factsheet" (PDF). EuRIC - European Recycling Industries’ Confederation. Retrieved 9 November 2021.

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