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Polybutyrate

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Polybutyrate (short for polybutyrate adipate terephthalate, or PBAT) is a biodegradable random copolymer, specifically a copolyester of adipic acid, 1,4-butanediol and dimethyl terephthalate. PBAT is produced by many different manufacturers and may be known by the brand names ecoflex®, Wango, Ecoworld, Eastar Bio, and Origo-Bi. It is generally marketed as a fully biodegradable alternative to low-density polyethylene, having many similar properties including flexibility and resilience, allowing it to be used for many similar uses such as plastic bags and wraps. The structure of the PBAT polymer is shown to the right. It is depicted as a block co-polymer here due to the common synthetic method of first synthesizing two copolymer blocks and then combining them. However, it is important to note that the actual structure of the polymer is a random co-polymer of the blocks shown.

Contents

History

Production of plastics for use in the industrial sector around the world makes up a very large market. PET (polyethylene terephthalate) is one of the dominant plastics within this market. It is most commonly used for bottle production because of being a rigid container that is very lightweight. However, because of the stability of this polymer, it is also highly resistant to biodegradation, posing a significant environmental threat when it is taken into account the amount of PET produced, sold, used and thrown away on a daily basis. An estimated 30% of the worlds PET goes into making these plastic bottles and anywhere from 15 to 35% end up being recycled; the rest usually end up in a landfill. This has inevitably stimulated research into polymers that can function comparably to PET, but be biodegradable.

Like all developments in the materials world, there are always several requirements for an 'ideal' material. For biodegradable plastics, they would be: cheap, renewable, easy to produce and eco-friendly. In addition to these, the polymer would have to be resistant enough to be functional, such as handling the strain from being pressurized or flexible so that it is easy to mold. There are no polymers that can perfectly contain every one of these features. Therefore, researchers have turned their attention to copolymers: combinations of polymers that have chemical and mechanical properties that compliment each other. This led to identifying Poly(butylene adipate-co-terephthalate) (PBAT) as a potential copolymer for blending.

PBAT, is a random copolymer that is known for being flexible and tough. This makes it ideal for combination with other biodegradable polymers that have high modulus and strength, but are very brittle. This allows for the production of blended copolymers that could potentially substitute industry standard plastics with environmentally safe and biodegradable plastics that will harmlessly disappear over a short amount of time.

The most significant reason for using PBAT as the flexible compliment to other polymers is it will preserve biodegradability; as long as both copolymers can degrade, the blended copolymer will also degrade.

Properties

PBAT is classified as a random copolymer due to its random structure. This also means that it cannot crystallize to any significant degree due to the wide absence of any kind of structural order. This leads to several physical properties: wide melting point, low modulus and stiffness, but high flexibility and toughness. The flexibility and toughness of this polymer makes it ideal for blending with another biodegradable polymer that is strong and rigid for bottle production.

The draw back to this polymer is the same as with any other. When it has high flexibility and toughness, then it will not be strong and rigid. This makes it non-ideal for any situation in which a strong, rigid container is desired. An example of this would be transparent barriers, such as plexiglass (Poly(methyl methacrylate)) used as a transparent glass substitute.

PBAT has been proven to be fully biodegradable when composted, due to the containment of butylene adiapte groups. The high stability and mechanical properties are yielded from the terephthalate portions.

The CAS Registry Number of PBAT is 60961-73-1.

Preparation

PBAT is synthesized through the polymers butylene adipate (polymer of 1,4-butanediol and adipic acid) and dimethyl terephthalate (DMT) with 1,4-butanediol.

Adipic acid and 1,4-butanediol are added together to create their polyester (Step 1). Then, DMT and 1,4-butanediol are added to the reaction to create theirs (Step 2). This polyester is then added to the adipic acid polyester by using TBOT as a transesterification catalyst (Step 3); the over abundance of 1,4-butanediol is to help influence chain lengths. The result is a copolymer of the two previously prepared polymers.

This is a random copolymer, because there is no control on the dispersity of the polymer chain lengths or block structuring in the copolymerization reactions; repeat positions are not being controlled. If A = polyester of adipic acid and B = polyester of DMT, each with 1,4-butanediol, then the chain structuring could look like any of these: AABABBABA or ABABAAAABB or ABABABBBBA; there is no selectivity for A and B reacting with themselves or each other.

Current and Future Uses

PBAT is marketed commercially as a fully biodegradable plastic, with BASF's ecoflex® showing 90% degradation after 80 days in testing. Particular applications that are highlighted by the manufacturers include cling wrap for food packaging, compostable plastic bags for gardening and agricultural use, and as water resistant coatings for other materials, as in paper cups. Due to its high flexibility and biodegradable nature, PBAT is also marketed as an additive for more rigid biodegradable plastics to impart flexibility while still maintain full biodegradability of the final blend.

PBAT is already widely marketed and used for all of the above applications, but is also being investigated as a component in antimicrobial films. In such films, PBAT serves as the bulk of the film with the antimicrobial agent being incorporated during processing. The antimicrobial films would be used in food packaging to inhibit bacterial growth, helping to preserve food products safely.

References

Polybutyrate Wikipedia


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