PBT keyboard keycaps
PBT offers a polyester alternative to ABS for a moulded keyboard keycap.
Material
PBT keycap body: PBT
Material selection rationale
- Rigid moulded detail
- The shell carries finger loads while a precisely shaped stem fits the switch.
- Material and surface wear
- The cited PBT set is designed for repeated contact; texture and moulding also influence how the surface changes with use.
- Different chemistry, same object
- PBT uses a diacid or diester plus a diol; ABS instead comes from three vinyl monomers and a rubber phase.
How it becomes a product
- Mould the body: A suitable PBT grade is moulded with the chosen wall thickness, stem and profile; cooling shrinkage must suit the required fit.
- Form the legend: In a double-shot design, a second moulding operation provides the contrasting legend; its resin need not be assumed identical to the outer body.
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.
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.
From p-xylene to terephthalic acid
- 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.