PBT

A polyester used in toothbrush bristles and moulded keyboard keycaps.

Also known as
polybutylene terephthalate, poly butylene terephthalate

In everyday products

Structure and behaviour

[–O–(CH₂)₄–O–C(=O)–C₆H₄–C(=O)–]ₙ

Terephthalate units are connected by ester links to four-carbon diol segments. PET uses a two-carbon diol segment instead: sharing the same diacid does not make the two polyesters interchangeable.

  • Fine drawn filaments can bend as toothbrush bristles; their diameter, length and tip finish determine the contact feel.
  • The same polymer family can be injection-moulded into rigid keycaps when a suitable grade and geometry are chosen.
  • PBT can crystallise during cooling, so cooling and shrinkage control matter when a small moulded stem must fit accurately.
  • A filament’s orientation is introduced after polymer manufacture; the monomer pair alone does not specify brush performance.
  • PBT keycaps and ABS keycaps reach a similar object through different polymer chemistry and processing choices.

Preparation routes

Terephthalic acid and butanediol route

Esterification and polycondensation. The diacid and diol make ester links, releasing water in the esterification stage; further reaction builds PBT chains. Using a four-carbon diol instead of ethylene glycol changes the repeat unit from PET to PBT, even though the terephthalic acid is shared.

Catalysts and removal of small molecules support chain growth. Filament extrusion and drawing follow polymer manufacture. Later fibre drawing or moulding sets geometry and orientation rather than changing the repeat formula.

  1. Ultradur PBT: chemistry and extrusionBASF
  2. Ultradur PBT: polymer chemistry, crystallisation and moulding shrinkageBASF

Dimethyl terephthalate and butanediol route

Transesterification and polycondensation. Butanediol replaces the methyl ester groups of DMT, releasing methanol during transesterification; further reaction builds PBT. The DMT route and acid route lead to the same PBT backbone but have different initial by-products. The complete pair must still include butanediol.

Industrial catalysts and by-product removal support polymerisation. The product-forming steps occur afterwards. The methoxy groups leave through ester exchange; they are not extra methyl groups in the final polyester repeat.

  1. Ultradur PBT: chemistry and extrusionBASF
  2. Ultradur PBT: polymer chemistry, crystallisation and moulding shrinkageBASF

From p-xylene to terephthalic acid

  1. p-Xylene + Oxygen from air → Terephthalic acid

    Catalytic air oxidation changes both methyl groups into carboxylic acid groups while retaining their opposite positions on the benzene ring. Acetic acid is the process solvent, not an extra carbon fragment in terephthalic acid. Recovery, purification and crystallisation produce the purified diacid used in polyester manufacture.

  1. Purified terephthalic acid: products and manufacturing, annual report 2023, page 94INEOS Quattro

Material limits

PBT bristles, handles, grips and keycap legends use their own materials and processing conditions.

Sources

  1. Ultradur PBT: chemistry and extrusionBASF
  2. Supreme toothbrush fact sheet: PBT filaments, PP handle and TPE gripTePe
  3. Cherry profile double-shot PBT keycapsKeychron
  4. Sustainable plastics: the role of chemistryRoyal Society of Chemistry
  5. Ultradur PBT: polymer chemistry, crystallisation and moulding shrinkageBASF