For the improvement of the surface whiteness of the color fabric and the maintenance and promotion of the vividness of the surface of the color fabric, it is necessary to go through a good and reasonable main washing process, and bleaching is only an auxiliary means.
Oxidation - The chemical reaction between oxidation or new oxygen and other substances is called oxidation.
Reduction - The chemical reaction between hydrogen or new hydrogen and other substances is called reduction.
Oxidizer - every substance that can supply oxygen is called oxidant.
Reducing agent - all substances that can supply hydrogen are called reducing agents.
The principle of oxidative bleaching: pigment is a very complex organic matter When subjected to oxidation, the pigment molecules immediately split and become other substances, so the pigment is eliminated and becomes a soluble substance that is washed away by water. (Therefore, oxidant bleaching will make the whiteness of the white fabric remain relatively long and not easy to yellow)
The principle of reductive bleaching: the pigment is reduced by the reduction of hydrogen to make the pigment reduce or recessive chromosome compounds, to achieve the purpose of color to plain discoloration. (Therefore, the fruit of reducing agent bleaching is unstable, and when the cryptochromatin compound encounters oxygen, it will be re-oxidized and discolored))
Oxidative bleach: (1) oxidizing chlorine-containing bleach; Calcium hypochlorite, sodium hypochlorite.
(2) Oxidizing chlorine-free bleach; Hydrogen peroxide, sodium perborate
Reducing bleach: low sodium sulfite.
Sodium hypochlorite (Nac130) is a white powder that is easily soluble in water; A colorless or pale yellow liquid with a chlorine odor, its aqueous solution is bleach, and its aqueous solution is unstable when neutral or acidic. The solution should be kept with sufficient alkalinity pH value around 12.
When the pH is below 4.6, the main ingredient is Nac130 and Hock's cl2
When the pH is 4.6-8.4, the main components are Nac130, Nac1 and Hock
When the pH is above 8.4, the main ingredients are Noah, Nalco and Nalco
Sodium hypochlorite is generated by strong bases and weak acids, which can be hydrolyzed in aqueous solutions to generate caustic soda and hypochlorous acid, and the solution is alkaline;
Nac130+H2O=Noah+Chloe
(Sodium hypochlorite) (Water) (caustic soda) (Hypochlorous acid)
Further decomposition of hypochlorous acid; Chloe→Hl+〔O〕
(Hydrogen chloride) (Neooxygen)
The bleaching effect of sodium hypochlorite: it is dependent on this new oxygen with strong oxidation capacity to destroy the color genes in the pigment and eliminate the color to achieve the purpose of bleaching. Factors affecting the bleaching process are pH, temperature, concentration and time.
pH of the bleaching bath: Sodium hypochlorite solution is an unstable complex compound. When the pH of the bleaching bath is 7, the polymerization degree of the cotton fabric fibers will drop below 70% after bleaching, and the damage is very serious. When the bleaching bath is very fast under acidic conditions, the damage to the cotton fiber is minimal, but a large amount of chlorine gas spills out, when the pH of the bleaching bath is 9-11, the cotton weaving is bleached, the damage to the fiber is small, and the bleaching speed is relatively slow, which helps to control the operation. Applied to sodium hypochlorite has two formations of linen white; First, rinsing is carried out with the bath, the pH of the bath is higher, the bleaching speed is slower and the efficiency is not high, and it is generally used in some washing and dyeing processes that do not have high bleaching requirements; The second is independent bleaching, which is generally arranged after the main washing, and the pH of the bleaching bath is about 10, and the bleaching effect is relatively good.
Temperature and time: increasing the bleaching temperature can accelerate the bleaching rate, shorten the bleaching time Oxidation program increases, increases the temperature by 7.5% at a certain temperature, and doubles the bleaching speed, and the temperature is 60 °C time 6 to 9 minutes.
The concentration of the bleaching bath: increase the content of sodium hypochlorite in the bleaching bath, the whiteness of the cotton fabric will increase accordingly, but it is not necessarily proportional, when the whiteness reaches the timing, the concentration increases again, the whiteness does not increase significantly, on the contrary, it will also affect the strength of the bleached fabric, and also waste the drug.
The use of sodium hypochlorite should pay attention to the following points:
(1) The temperature of the bleaching bath does not exceed 60 °C;
(2) Do not use bleach of unknown concentrations;
(3) Do not directly add non-dilution bleaching to the bleaching bath;
(4) Use the appliance to accurately measure the amount of bleach used;
(5) When adding bleach to the washing machine, the direction of the bleaching water should be consistent with the direction of rotation of the washing machine drum.
Calcium hypochlorite commonly known as bleach powder (Caocl2) white powder, the appearance is similar to slaked lime, there is a special odor of near chlorine, when exposed to the air, easy to absorb moisture, carbon dioxide, decomposition quickly and release hypochlorous acid and chlorine gas, therefore, should not be stored for a long time, should pay attention to the hypochlorite 1 calcium formed when the salt is calcium salt is easy to deposit on the surface of the fabric and affect its whiteness, and sodium hypochlorite is formed when bleaching is a sodium salt, easily soluble in water and washed away, does not affect the whiteness of the fabric.
Hydrogen peroxide is also called hydrogen dioxide, commonly known as hydrogen dioxide, molecular formula is H2O2, is an excellent oxidizing bleach, the whiteness and whiteness stability of the bleached product are better than the hypochlorite bleaching products, the damage to the fiber is smaller, the effect is better, there is no risk of macula, no harmful gas is produced in the bleaching process, which is conducive to labor protection, is a multi-purpose bleaching agent, which can be used to bleach fold fabrics, cotton wool, artificial fibers, cellulose, fibers, etc.
Properties of hydrogen peroxide and principle of bleaching
Pure high concentration of hydrogen peroxide (100%); Is an oily liquid- liquid, commonly used concentration of 30% or 35%, such as solution concentration of more than 60%, the temperature is slightly higher, easy to explode in contact with organic matter, hydrogen peroxide is a strong oxidant, although it does not burn itself, but when in contact with flammables, it will cause violent combustion.
Bleaching bath pH: In the range of pH3 to 13.5, all have bleaching effect. pH value between 3 ~ 9, fabric whiteness has a tendency to increase with the increase of pH, pH value in 9 ~ 10, fabric whiteness reached the best level, if further increase the bleaching pH value, fabric whiteness instead declined, from the fabric strength point of view, when the bleach bath pH value is between 3 ~ 10, hydrogen oxide in the pH between 3 ~ 13.5 Although there are bleaching effects, but from the fabric whiteness and fiber damage point of view, bleaching pH value in 10 ~ 11 is more ideal.
Temperature and time: the two are mutually restrictive, generally bleaching bath temperature, the action time is relatively short; The bleaching bath temperature is relatively low, and the action time is relatively long. At room temperature, the speed of hydrogen peroxide drift is relatively slow, in order to increase the speed, generally at 90 ~ 100 ° C, the decomposition rate of hydrogen peroxide is greater than 90%; When the temperature is 60 °C, the decomposition rate is only about 50 °C. Therefore, when the bleaching bath temperature is 90 ~ 100 ° C, the pH value is 10 ~ 11, the bleaching time is 10 ~ 15 minutes.
The concentration of the bleaching bath: Generally, it should be based on the basic principles of not only making the bleached fabric achieve satisfactory whiteness and vividness and removing oxidizable dirt, but also making the cellulose less damaged, and the use of the bath ratio is 6 to 16 grams / liter (30% hydrogen peroxide concentration).
When using hydrogen peroxide bleaching, pay attention to the following problems:
(1) In order to adapt to the conditions of use of hydrogen peroxide, bleaching can be carried out simultaneously in the main washing stage of the fabric, and when the bath temperature is above 90 ° C, the appropriate amount of hydrogen peroxide is slowly filled into the wash solution, and the action time is 10 to 15 minutes.
(2) When the thick hydrogen peroxide solution comes into contact with the skin, it will burn the skin and foam, and it should be safe to use.
(3) The hydrogen peroxide solution should not be placed next to the equipment or in a high-temperature environment, but should be placed in a ventilated and cool place.
Sodium perborate - molecular formula NaBo3 • 4H2O is a white odorless substance or powder crystal, not easily soluble in water, but easily soluble in hot water, its aqueous solution is alkaline, in the dry air without CO2, can not afford any change, in the 40 ° C hot humid air can decompose on its own, and release oxygen. Reactive;
2NaBo3→2NaBo2+O2
(Sodium peroxylate) (Sodium metaborate) (Oxygen)
Sodium perborate: After being dissolved in water, it will not decompose immediately, so the release of hydrogen peroxide is relatively slow, which is conducive to control and management during use. When the solution temperature rises to 40 ° C, the release of oxygen is faster. However, in the application of bleaching, sodium perborate for hot water above 60 ° C, will have a significant effect, and must be used in the same bath with the main washing stage, when the bath temperature reaches 60 ° C, sodium perborate is added, continue to heat up, the action time is 12 to 15 minutes.
Sodium hyposulfite: also known as sodium bisulfate, commonly known as insurance powder, the molecular formula is Na2S2O4, divided into two forms; One is a low sodium sulfite (Na2S2O4) that does not contain crystalline water is a yellowish powder; The other is that sodium monosulfite (Na2S2O4•2H2O) containing two crystals is a slightly grayish white crystalline powder, both of which are easily soluble in water, with a solubility of 20% at room temperature and slightly acidic. At pH 10, it is the most stable, and in case of inorganic acids, it will decompose violently. Therefore, do not touch acids, which are limited to an acid tolerance of up to pH 5.
Insurance powder in the alkaline solution has a stripping and bleaching effect, the use of a concentration of 1 part of insurance powder mixed with 20 parts of water, and add a small amount of alkaline laundry detergent, the solution temperature of 60 ~ 70 ° C, the time of action is 5 ~ 20 minutes, the insurance powder solution should be added to the water, do not add water to the insurance powder, otherwise, the insurance powder will be rapidly decomposed.