Improvements in or relating to synthetic fibres

Filaments, films and the like of aromatic linear polyesters are treated with the vapour of sulphuric acid, sulphur trioxide or chlorsulphonic acid. The treatment renders the material more hydrophilic, whereby its dyeing properties are improved and it becomes less liable to accumulate electrostatic charges. The treated material is particularly susceptible to basic dyes in aqueous solution, such as Malachite Green, Methylene Blue and Bismark Brown, and can also be dyed more readily with mordant dyes such as aluminium, iron and chromium mordant dyes. In an example a woven fabric of material sold under the Registered Trade Mark " Terylene " is exposed to the vapour of fuming sulphuric acid at a concentration equivalent to 0.11 gms. sulphur trioxide per litre for 30 secs. at 20-21 DEG C. and washed. After immersion in a 0.5 per cent aqueous solution of Methylene Blue for 10 mins. at 20 DEG C., the material is dyed deep blue, fast to boiling water. Further examples describe different concentrations of sulphur trioxide and different times and temperatures, but conditions preferred are 0.01 to 0.5 gms. sulphur trioxide per litre for 1-60 secs. at 15-25 DEG C.ALSO:Filaments and the like of aromatic linear polyesters are treated with the vapour of sulphuric acid, sulphur trioxide or chlorsulphonic acid. The treatment renders the material more hydrophilic, whereby its dyeing properties are improved and it becomes less liable to accumulate electrostatic charges. The treated material is particularly susceptible to basic dyes in aqueous solution, such as Malachite Green, Methylene Blue and Bismark Brown, and can also be dyed more readily with mordant dyes such as aluminium, iron and chromium mordant dyes. In an example a woven fabric of material sold under the Registered Trade Mark "Terylene" is exposed to the vapour of fuming sulphuric acid at a concentration equivalent to 0.11 gm.s sulphur trioxide per litre for 30 secs. at 20-21 DEG C. and washed. After immersion in a 0.5 per cent aqueous solution of Methylene Blue for 10 minutes at 20 DEG C. the material is dyed deep blue, fast to boiling water. Further examples describe different concentrations of sulphur trioxide and different times and temperatures, but conditions preferred are 0.01 to 0.5 gms. sulphur trioxide per litre for 1-60 secs. at 15-25 DEG C.