Materials Reference Guide: Plastic Compounder’s PET Project: Recycling

Nov. 22, 2008
Plastic water bottles can be “upcycled” into durable materials with properties equivalent to virgin polymer.

Rodney W. Fonseca
Valox Technology Manager

Dhaval Shah
Platform Technology Manager, Crystalline Polymers Div.
Sabic Innovative Plastics

Talking about going green has become a national pastime in recent years. Implementing energy-saving processes, using recycled materials, and reducing waste are often easier said than done. However, the production of polybutylene terephthalate (PBT) from recycled polyethylene terephthalate (PET) is one conservation idea that is entering the mainstream.

In the United States alone, consumers generate 4 billion lb of PET bottle waste every year. Although these products are usually marked with a numeral 1 inside the triangle-of-arrows recycling symbol and the abbreviation PETE, only 20% of PET waste is recycled. The rest is dumped into landfills across the country. Nevertheless, postconsumer PET is a very attractive feedstock for which there are established collection processes.

The construction of the PET bottle makes it amenable for recycling. It is a relatively clean polyester feedstock once the glue, labels, and the caps have been removed.

PET bottles are typically formed into bales at recycling-collection centers. To make clean PET flakes, the bales go through cleaning and other processes, including manual separation to remove contaminants, grinding, washing, and drying. Additional extrusion and pelletization may also be involved.

Recycled PET finds its way into carpet fibers, geo-textiles, and fiber fill. The feedstock can be converted into PBT resin, which is widely used in injection or blowmolding applications.

PBT production
A better way to reuse PET might be to convert it into more-valuable “virgin” PBT base stock. To make PBT, cleaned PET flakes are fed into a high-temperature reactor along with a diol such as ethylene glycol or butanediol (BDO) and a catalyst.

Chemical depolymerization breaks the large molecular chain of PET into small repeating sections called oligomers. The process strips the chain’s backbone of its functional groups, spur molecules that give each plastic its unique properties. The next step is to swap out the PET functional groups for PBT functional groups through transesterification, another catalyst-assisted process. Finally, the altered building block oligomers are rejoined, or repolymerized, into a modified PBT copolymer that is chemically identical to one made from scratch.

The process releases ethylene glycol (EG) that’s collected for other uses. The EG represents 28% of the PET’s original mass. The repolymerized PBT contains 60% recycled mass and reduces solid waste by up to 1,918 lb for each ton of PBT produced.

But the big benefit in making PBT from recycled PET is energy savings. Producing PBT from oil takes approximately 50 GJ/ton; the PET-to-PBT process uses about 20 GJ/ton of PBT. The 30 GJ/ton energy savings is the equivalent of keeping 2 tons of carbon dioxide out of the atmosphere.

Environmental benefits and energy savings aside, a big knock on recycling polymers is that the mechanical properties suffer. Each heat cycle lowers molecular weight as chains break apart. Consequently, many postconsumer feedstocks are “downcycled” into less-demanding applications.

The PET to PBT process described above, however, decomposes the PET to its oligomers, erasing the molecules’ heat histories. The monomers are then used to build a PBT molecule that resembles a virgin PBT molecule.

This chemical “upcycling” process turns a polymer used in a nondurable product like water bottles into a durable polymer for automotive uses and other long-lasting applications.

Design decisions
Tests have shown that resins developed by Sabic Innovative Plastics using PBT derived from postconsumer PET have equivalent performance to virgin PBT resins. The company’s Valox iQ* PBT resins and Xenoy iQ* PBT-polycarbonate (PC) alloy have the same mechanical, physical, and chemical properties as their virgin-derived counterparts.

Several formulations showed much higher flow in the postconsumer version than in the virgin PBT version. The Valox iQ 8280SF system, a 40% glass-filled PBT-PET blend formulated for high flow, had a MVR over four times higher than the virgin version. The 30% glass-filled, flame-retardant Valox iQ 4860HR had more than three times the MVR that its virgin counterpart did.

Designers who want to use a resin with lower carbon footprint can use the postconsumer iQ grades as a drop-in replacement for virgin Valox and Xenoy resins. The postconsumer resins are suitable for applications with a longer life cycle than is possible with many recycled materials. The wider list of applications includes structural and impact-resistant parts in automotive, furniture, and appliances.

The Valox iQ* and Xenoy iQ* resins are cost competitive with standard PBT and PC-and-PBT grades, respectively. For some markets where environmental and social responsibility is a selling point, or where raw material provenance is closely monitored by regulators, iQ resins could be a smart choice that does not compromise properties or performance.

Terephthalate testing
To compare the postconsumer-derived resins to virgin resins across the spectrum of PBT-based industrial products, Sabic engineers compounded six different resins with modified PBT copolymer as a building block. The resin formulations are shown in the table.

Physical tests included heat-deflection temperature at 1.8 MPa, specific gravity, and melt-volume rate (MVR). Flexural strength and modulus tests as well as tensile strength and notched Izod impact tests measured mechanical properties.

Tensile elongation at break was measured after hydroaging for 10, 25, and 50 hr at 120°C, 100% relative humidity, and 2 atm of pressure. Notched Izod impact strength was measured after dry heat aging at 140°C for 250, 500, 750 and 1,000 hr.

The test results are shown here graphically. The postconsumer-derived PBT polymers performed equivalently to the same formulations made with virgin PBT in all the mechanical and environmental durability tests. Some postconsumer formulations had much higher flow than their virgin counterparts.

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