The rubbing fastness of cashmere fibers dyed with madder vegetable dyes
As the most commonly used red natural dye,madder is widely used in the dyeing of silk and cellulose fibers.Nevertheless,it is found that the rubbing fastness of cashmere fibers dyed with madder vegetable dyes with alum as the mordant is very poor,with the dry rubbing fastness reaching grade 1,and the wet rubbing fastness reaching grade 2-3,respectively.In contrast,the dry and wet rubbing fastness of cashmere fibers dyed with madder vegetable dyes with ferrous sulfate as mordant can reach grade 3 and grade 4.To explore the reason why the rubbing fastness of the two is so different,scanning electron microscopy tests were carried out on the cashmere dyed by alum and ferrous sulfate,respectively.The results showed that there was a large amount of solid sediment on the surface of cashmere fibers dyed by alum,but no solid sediment was found on the surface of cashmere dyed by ferrous sulfate.This is because the pH value of the solution during alum dyeing is higher than the precipitation point of the aluminum ion,and a large amount of aluminum hydroxide is deposited on the surface of cashmere during the dyeing process.As a result,the madder dye will react with aluminum ions on the surface of cashmere during dyeing,and the madder dye attached to the surface of cashmere cannot be diffused into the interior of the cashmere fiber,resulting in poor rubbing fastness.Ferrous sulfate dyeing is carried out under weak acidic conditions,ferrous ions are slowly and uniformly absorbed and diffused into cashmere fibers to have coordination reaction with cashmere,and the madder dye can diffuse into cashmere fibers and combine with fibers and ferrous ions during dyeing,so both the dry rubbing fastness and wet rubbing fastness are good.Previous studies have shown that the rubbing fastness of madder to silk and cellulose fibers is excellent,but that of cashmere is poor.In this regard,it is proposed to use hydrogen peroxide to moderate de-flake treatment of cashmere fibers.The results show that the hydrophobic scales on the surface of cashmere affect the rubbing fastness of alum dyeing and madder dyeing of cashmere.Moderate scale removal can effectively reduce the deposition on the surface of cashmere during alum dyeing,which has a certain effect on the improvement of the dry and wet rubbing fastness.The dry and wet rubbing fastness of unscaled cashmere can be improved to grade 2 and grade 3.The alum-organic acid system was used to dye the cashmere,and then madder dyeing was carried out.Consequently,the dry and wet rubbing fastness of the madder dyed cashmere was improved to grade 2 and grade 3～4.This is because acetic acid reduces the pH value of the dye solution and inhibits the hydrolysis of alum.However,due to the volatility of acetic acid,some acetic acid volatilized at 80℃,resulting in the rise of pH value of the dyeing solution,and the generation of a small amount of aluminum hydroxide.Further,the alum-acetic acid system is used to dye cashmere,and the dry and wet rubbing fastness of dyed cashmere fibers is increased to grade 4 and grade 5.This is because tartric acid effectively reduces the pH value of alum dye solution,and Al3+ can be evenly diffused into the interior of cashmere fibers and have coordination reaction with cashmere.During dyeing,madder dye can fully combine with Al3+ and fiber.Moreover,infrared spectrum shows that tartric acid can form hydrogen bond with cashmere,which makes tartric acid,cashmere fiber and madder binding.The dry and wet rubbing fastness of dyed cashmere has been improved to grade 4 and 5,respectively,and the soap fastness and sun fastness are all above grade 4,which lays the foundation for industrial application.
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