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Printing technology promotion : Cotton fabric bleaching methods

After scouring, the fabric most natural and man-made contaminants have been removed , significantly improved the capillary effect , has been able to meet the processing requirements of a number of varieties . But for bleaching colored fabrics and brightly-colored cloth , color cloth, also need to improve the whiteness , and therefore need to further remove pigments on the fabric , the fabric is more white . Then the fabric was going through scouring, especially atmospheric steam cooking practice , there are still some impurities such as whole cottonseed hulls failed removed by the action of bleach , these impurities can be removed completely .
Cotton dyeing widely used hypochlorite , hydrogen peroxide, an oxidizing bleaching agent. Cotton and cotton fabrics bleached , over- acid compounds such as sodium perborate , peracetic acid , sodium carbonate , also had occasional applications of sodium chlorite used for synthetic and blended fabric bleach . Hypochlorite bleach is usually referred to as chlorine bleach , hydrogen peroxide bleaching referred to as oxygen bleach , sodium chlorite bleaching referred to as sub- drift .
(1) a hypochlorite bleach
① Common hypochlorite and sodium hypochlorite bleach two kinds .
Chlorine bleach is made ​​with the effect of lime , such as chlorine gas in the lime milk , containing the active ingredient can be prepared by twice the calcium hypochlorite in chlorine- caustic soda solution , hypochlorite can be obtained . Bleaching powder and the active ingredient is calcium hypochlorite bleaching powder , the overall effect is not as good as sodium hypochlorite. Currently the larger cotton dyeing often create their own sodium hypochlorite or sodium hypochlorite to nearby chemical plant for bleaching purchase , bleaching powder for small cities or rural areas is not easy to get a small dyeing using sodium hypochlorite . Calcium hypochlorite expensive, less use.
Sodium hypochlorite bleaching process and equipment is relatively simple, multi- dimensional for cotton fabrics and cotton blended fabric bleach , sometimes used cotton blended fabric bleach . However, bleach can not be used silk , wool and other protein fibers , because sodium hypochlorite has damaging effects on protein fibers , and to fiber yellowing.
Because of the molecular structure of the fiber 's natural pigment enough research so far is not very clear . From the point of view known to the basics of pigment , natural pigment structure destroyed in the bleaching process, the purpose of eliminating pigment.
In the bleaching process, in addition to the destruction of the natural pigment , the cotton fibers themselves may be affected by injury, therefore, must control the bleaching process conditions , so that the appearance of quality and internal quality are qualified.
② The main methods of bleaching cream bleaching and continuous rolling two kinds.
Bleaching cream is piled on the fabric evenly pour bleach tank , circulation pump will continue to spray bleach solution on the fabric loop at room temperature 1 ~ 1.5h, then washed, acid leaching , washing . Pour bleach non- continuous production , has been rarely used. Continuous rolling bleaching bleaching solution is padding continuous training in rope bleaching machine on , after stacking boxes in pile fabric after washing , rolling acid stacked , piled on the heap wash cloth in the pool , waiting for slitting , rolling water, bake dry .
Cotton fabric rope sodium hypochlorite bleach continuous rolling process is as follows :
Rolling bleaching solution → stacking (→ → piled rolling bleaching solution ) → washing → → stacking → ​​rolling acid washing .
The amount of sodium hypochlorite bleach solution to chlorine computing , cooking pot cloth fabric is generally practiced padding containing chlorine 1.5 ~ 2g / l bleaching solution , stacked about 1h at room temperature and steam cooking and practicing flat pieces of fabric containing chlorine bleach 2-3 3g / l, after rolling bleaching solution piled around 1h. Lower cotton fabric high impurity , chlorine should improve 0.5g / l when the padding . Pickling with sulfuric acid , a sulfuric acid concentration of the fabric rope 1 ~ 3g / l, flat pieces of fabric 2 ~ 3g / l, after rolling acid 30 ~ 40 ℃ stacking 10 ~ 15min. Small plants can be piled cloth in bleach solution after artificial rolling , piled on clean cobbled ground , we must pay attention to labor protection pile cloth.
③ Affect sodium hypochlorite bleach factors:
(a) impact of ph value : When ph value of 7:00 bleach solution , the worst bleaching effect , fiber damage serious ph value of 2 to 4 and 9 to 11:00 , good whiteness , fiber damage is also smaller. However, in acidic solution, sodium hypochlorite decomposition release chlorine, polluted air workshop , affecting the health of workers , but also corrosion of equipment . In the actual production ph value between 9.5 and 10.5 .
(b) the effect of temperature : high temperature , bleaching speed, but the temperature exceeds a certain limit , but also to accelerate the oxidation of cellulose brittle damage , it is generally controlled at 20 ~ 30 ℃. When summer temperatures exceed 35 ℃ , should take measures to cool or adjust other parameters , such as concentration , time, etc. , in order to protect the fibers.
(c) Effect of the concentration : The fabric structure and scouring conditions. To calculate the concentration of available chlorine bleaching , because hypochlorite preparation , the resulting product mixture was mixed with sodium chloride such as sodium hypochlorite , and sodium chloride in no bleaching effect. Hypochlorite chlorine content , and decreased with prolonged storage , and therefore in the preparation of the bleach solution , hypochlorite response analysis to determine the chlorine content , chlorine content in bleach solution accurately to control the production process . After bleaching liquid chlorine content up to a certain value , no further increase in the whiteness of the fabric . High levels of liquid chlorine bleach , but affect the fabric strength . Dyeing factories generally taken to reduce the concentration of chlorine bleach solution , extending bleaching time ways to avoid excessive loss of fiber strength .
(d) dechlorination problem : acid chlorine bleaching the fabric , so that the decomposition is not completely washed out of the chlorine gas , there is still a small amount of chlorine adsorbed to the fabric. Adsorption of residual chlorine will cause the fabric during storage fabric strength decline , yellowing , will also affect the chlorine -sensitive dye. Necessary, the use of chemicals react with chlorine to chlorine thoroughly . Dechlorinating agent is preferably hydrogen peroxide , hydrogen peroxide and chlorine addition reaction itself is a bleach , the bleaching effect can be increased . But more generally using a reducing agent such as sodium hydrogen sulfite , sodium thiosulfate and other processing.
( 2 ) hydrogen peroxide bleaching
Hydrogen peroxide is known as peroxide . Bleached with hydrogen peroxide fabric whiteness better, pure shade , not yellowed during storage , widely used in the bleaching of cotton fabrics . Than oxygen bleaching chlorine bleach have greater flexibility , but hydrogen peroxide, sodium hypochlorite is higher than the price , and the need for oxygen bleaching stainless steel equipment, energy consumption greater costs than chlorine bleach .
① hydrogen peroxide bleaching process
Hydrogen peroxide bleaching more flexible manner , both continuous production , but also on an intermittent equipment available steaming bleach , bleach can also be cold rope available , can also be used flat pieces . Currently dyeing using more open width bleaching steaming method, this method is a continuous degree of automation and production efficiency are high, the process is simple , and does not produce environmental pollution.
Hydrogen peroxide bleaching process is as follows :
Hydrogen peroxide bleaching ran over the steaming liquid → → washing .
The bleaching solution containing hydrogen peroxide (100%) 2 ~ 5g / l, with caustic soda to adjust the ph value of 10.5 to 10.8 , adding a stabilizing amount of a humectant agent , at room temperature, the padding solution bleaching , 95 ~ 100 ℃ steam 45 ~ 60min, then washed out of cloth .
② influence of hydrogen peroxide bleaching factors:
Effect of (a) concentration : similar chlorine bleach , when the hydrogen peroxide in the bleaching solution the concentration of 5g / l , the bleaching has been able to meet the requirements , the concentration of a further increase in whiteness not increase as cotton fibers but lead to brittle damage . Thin fabric can also be appropriate to reduce the bleaching solution, the hydrogen peroxide concentration .
(b) the effect of temperature : hydrogen peroxide decomposition rate increases with increasing temperature , thus increasing the temperature of the available ways to shorten bleaching time , usually 90 ~ 100 ℃ , the hydrogen peroxide decomposition rate of 90 %. whiteness is also the best . Law with cold bleaching hydrogen peroxide concentration should be increased and prolonged bleaching time .
(c) Effect of ph value : Hydrogen peroxide is relatively stable in the acid bath , the industrial use of hydrogen peroxide content of a concentrated solution of about 30% to 35% , which is often added a small amount of sulfuric acid to maintain stability. The decomposition rate of hydrogen peroxide in an alkaline bath and a solution ph value increases with increasing , ph value has a bleaching effect between 3 and 13.5 , but the ph value of 9 to 10, up to the top level of whiteness of the fabric , most of the actual production of the bleaching solution also adjust the ph value to about 10.
(d) Effect of metal ions and the stabilizing agent when the bleach : water, iron, copper , and iron, copper shavings , and dust , etc. , on the catalytic decomposition of hydrogen peroxide has a role in decomposition of hydrogen peroxide into water and oxygen , thus losing the bleaching effect. Oxygen penetrate into the fabric , at a high temperature alkaline conditions of the bleaching of cellulose fibers will seriously degrade , often produce holes in the fabric . In order to prevent these flaws can be added to the amount of stabilizer in bleaching solution to reduce the rate of decomposition of hydrogen peroxide . Silicate stabilizer to use an earlier , stable mechanism of action is not yet very clear water glass , presumably from the calcium or magnesium colloid catalytic decomposition of hydrogen peroxide metal ions adsorption. Silicate inexpensive and easy to get a good stabilizing effect , but long-term use , easy to form silica scale difficult to remove the guide rollers , etc., affect the fabric quality. At home and abroad are studying the use of non- silicate stabilizers, mostly belonging to the organic phosphonates , the effect is good , no fouling , but the price is higher than the silicate , water glass still in production and therefore continue to use. Sometimes blending phosphorus-containing compounds with sodium silicate , can also achieve better results.
③ over other hydrogen peroxide bleaching
In addition to steaming bleaching method widely used , the hydrogen peroxide bleaching process had the following categories:
(a) chlorine - oxygen double bleaching method : first chlorine bleach after oxygen bleaching , oxygen bleaching and bleaching effect both dechlorination , this law can reduce the bleaching solution, the hydrogen peroxide concentration , the process is:
Rolling hypochlorite bleach washing → → → piled ran over steaming hydrogen peroxide bleaching solution → → washing .
Sodium hypochlorite bleach solution containing chlorine 1 ~ 2g / l, hydrogen peroxide bleaching solution containing hydrogen peroxide 1 ~ 3g / l, with the respective bleaching process conditions the same .
(b) cold pad method : To meet the more variety, low-volume, changing requirements, especially small dyeing , bleaching equipment in the absence of oxygen can be used cold bleaching method. This method of hydrogen peroxide in the bleaching solution concentration is high, and added persulfate bleach solution after the fabric rolling , coiling immediately coated with plastic film to prevent evaporation and dried at room temperature and then stacked . Although this method is a long time, production efficiency is low , but more flexible. Bleaching solution containing hydrogen peroxide (100%) 10 ~ 12g / l, sodium silicate 25 ~ 3og / l, persulfate 7 ~ 1og / l, with caustic soda to adjust the ph value of the bleaching solution 10.5 to 10.8 at room temperature for padding, stacked 6 ~ 16h, fully washed.
Concentrated hydrogen peroxide solution with serious burns to the skin effect , should pay attention to labor protection .
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