Standard Salt Dosing Schedules for Reactive Dyeing Heavy Cotton Canvas
Progressive exponential salt dosing prevents surface strike spikes and forces reactive dye penetration into heavy cotton duck yarn cores.
Alkali Fixation
Chemical Mechanism
Cellulose dyeing processes rely on nucleophilic addition or substitution reactions that link reactive chromophores covalently to ionized hydroxyl groups on cotton fibres. During alkali fixation, alkaline agents like sodium carbonate or sodium hydroxide raise dyebath pH to generate cellulosate ions while concurrently initiating covalent bond formation. The reaction proceeds across specific temperature thresholds depending on dye reactivity, converting physical adsorption into permanent chemical linkage.
Hydrolysis occurs simultaneously as hydroxide ions in water react with dye molecules, creating unfixable hydrolysed dye that wash-off cycles must remove.
Process Control
Precise dosing curves govern liquor alkalinity across modern jet and pad dyeing machines. Automated dosing systems inject soda ash progressively across forty minutes to prevent rapid localized fixation that causes unlevelness. Bath pH readings taken at five minute intervals confirm stability within the targeted 10.8 to 11.5 window for vinyl sulfone chemistries.
Quality Verification
Laboratory evaluations quantify fixation yield through spectrophotometric measurement of dye concentrations before and after alkaline exposure. Wash fastness testing under ISO 105-C06 confirms whether unfixed colourant was cleared after the alkaline phase completed.
Commercial Boundary
Fabric processors manage alkali fixation solely within reactive dye applications, whereas disperse, acid or vat systems follow separate thermal or reduction pathways. Variations in water hardness destabilize alkaline liquor buffers, producing unlevel dyeings across large production lots.