Liu Yun, Sun Baoguo
(School of Chemical and Environmental Engineering; Beijing Technology and Business University, Beijing, 100037)
Supportted by the National Natural Science Foundation of China(20276002)
AbstractSodium percarbonate, a green ingredient, an effective oxygen-based bleaching agent, has poor storage stability. Small amounts of moisture will cause its decomposition. Low level of heavy metal ions can accelerate its decomposition. Sodium percabonate coated simply with slurry of a group of inorganic compounds got an excellent stability in this paper. It was found that metaboric acid had special stabilization to the product. It was supposed to give the coating colloid property, acidity, and water-absorbing ability. The paper supposed a mechanism of the process of auto catalyst of the product, giving a 4-step reaction, which leads to the decomposition continually.
Sodium percarbonate is well-known as a basic component which is incorporated into an oxygenic bleaching agent or a sterilizer composition in detergents. Generally, oxygenic bleaching agents do not damage fabric anddo notcause yellowing, so they have been allowed enjoy widespread use as oxygenated bleaching agents for household or business use.Sodium perborate is also utilized as a basic component in bleaching or oxidizing agent, and can be incorporated into detergent compositions or the like. Because it must be used at high temperature, and dissolution velocity in water is rather low, sodium perborate is seldom used as a basic component of an oxygenic bleaching agent composition in the countries where washing is set at low-temperature.
Different from sodium perborate, sodium percarbonate exists as an adding of sodium carbonate/hydrogen peroxide (2NaCO.3H), which implies that it has a higher solubility at low temperature and easier release hydrogen peroxide. On the other hand, this formula of sodium percarbonate makes it have a serious drawback of rather poor storage stability as compared with sodium perborate, and fairly rapidly loses its available oxygen during affinity towards water. Even small amount of moisture can cause its decomposition. Very small amount of iron, copper, manganese, cobalt, and other heavy metal ion can accelerate its decomposition. Synthetic zeolite which is recently in wide use in house detergent acts as a strong catalyst to accelerate the reduction of its activity, since synthetic zeolite has too many pores and surface areas to absorb moisture. Therefore, sodium percarbonate is so unstable in detergent especially in phosphorous-free detergent, as to nearly have not any commercial value if its stability is not improved
Up to now, many different approaches have been made with respect to the stabilization of sodium percarbonate. For instance, some hydrophobic substance, such as a blend of hard or soft waxes was used to capsulate bleaching particle . The encapsulated particle is made by spraying molten wax with a melting point of about 40-50onto the particles. Some research disclosed a group of hydrophobic substance and the surfactants being substantially inert in presence of the peroxy compounds. They were: a) Fats, waxes or phosphotides; b) surfactants dissolved in the hydrophobic substance; c) Water swellable grains. Disilicate and some chelating agent, such as glutaric acid, pimelic acid, citric acid, dicarboxylic acids were used to coat the particle of percarbonate[5,6]. Inorganic compounds selected from alkali or alkaline earth metal carbonate, sulphates, chlorides and nitrates have been added to composition of coating agents . Subjecting a dried sodium percarbonate particle and an aqueous solution of borates to solid-liquid mixing to prepare a stable sodium percarbonate, in which temerature is very important for the coating agents to penetrate and coat the product, but the process is not easily controlled.
In this paper a raw sodium percaborate containing stabilizing reagents was prepared by oxydizing sodium carbonate with hydrogen pyroxide and then it was coated simply with slurry of a group of inorganic compounds. The factors influencing the stability of the products is discussed, and a set pf orthogonility of experiments was performed to optimize the coating composition. Also, some mechanism of increasing stability and catalyst of percarbonate are presumed.
All the materials were got from the local chemical markets. Sodium percarbonate was prepared in our laboratory.
Oven with controllable temperature and humidity.
1.3 Preparation of coating and penetrating slurry
(a) A suitable amount of dehydrated carbonate was added to a certain amount of water until dissolved. (b) Other coating and penetrating ingredients were fed into the aqueous carbonate, keeping stirring for 5min.
(a) A suitable amount of starting sodium percarbonate was fed into a vessel. (b) The viscous slurry prepared as above was added dropwise into the vessel during 30 sec with stirring at 250rpm. (c) The wet product was taken out, put on a plate, and was dried with hot air with continually shaking.
1.5 Determination of stability
The test was carried out at the presence of zeolite(4 times of percarbonate, by weight). Each sample was put in a plastic container which was left to stand at 50and 70%RH for 48 hours. Then the residual available oxygen content was determined. The available oxygen content was determined by 0.1N potassium permagnate titration method.
Residual available oxygen content(Wt%)= available oxygen content after storage/ available oxygen content before storage:
2 RESULTS AND DISCUSSION
During the preparation of percarbonate, if the temperature was higher than 5, the product could not crystallize. Table 1 is the effect of the coating ingredients and coating condition.
Table 1. The coating materials and available oxygen lost*
|No.||Coating agent(Wt% ,based on sodium percarbonate)||Water||T(C)||State of coating||Lost of Oav%|
|metaboric acid||Sodium silicate||Magnesium sulfate||Carbo-nate||EDTA.2Na|
* The samples were put into an oven of 50C,70RH% for 48h in the presence of 4 times of 4A zeolite(Wt,on the base of percarbonate).
Carbonate was fed first because of its good solubility. When sodium hydrogen carbonate, or carbonate was used as a single ingredient, the lost of the active oxygen was as high as over 15-32%.
From microcosmic points of view coating layer forms a hetero nucleus outside of percarbonate core. Coating materials can form a unique film to protect the core from decomposition. From macropoints according to thermodynamics balance principle percarbonate delivers oxygen in decomposition:
Along with increasing oxygen pressure the dissociation rate of percarbonate decreases. When percarbonate is coated well the oxygen produced can not transgresses, and make the oxygen pressure go up. As the result, the equation balance moves to the left; the decomposition of percarbonate decreases. If the coating is not uniform, there must be a large number of micro holes on the surface of the core, which make oxygen go out, and lead to the stabilization of percarbonate falls.
In coating process, water was a very important factor to form a uniform layer so as to stabilize percarbonate. If water was too much in making the slurry, it will dissolve the coating agents, and dissolution of surface of the product also took place. If water was not enough to make a slurry coating process would be not compete and become not symmetrical, and the coating agents could not penetrate inside the pores, so the result was not satisfactory.
When the coating agents fed into percarbonate first and then all water was added, the stability of product was not well. In this case it was not possible for percarbonate to be coated completely.
The decomposition of percarbonate can be resumed to be a process of auto catalyst, and the catalyst is just the water.
First percarbonate adds water to form a complex (1). The complex is not stable; it will soon decompose to active intermediate (2). The intermediate soon again decomposes to oxygen and water.
Another explanation may be better that supposes that sodium carbonate in percarbonate absorbs water stronger than the part of hydroxy peroxide.
First under the existence of water, carbonate in percarbonate molecule absorbs water, delivering hydroxyl ions and hydrogen peroxide. It is well known that hydrogen peroxide has higher activity to decompose than in acid or neutral condition. The OOH formed soon becomes oxygen when there are not any bleachable materials and organism. The water produced in 2nd process comes back to 1st step to catalyze the decomposition of percarbonate again. Also the negative hydroxyl group produced in 3rd step comes back to the 2nd step to catalyst hydrogen peroxide again. That leads to the decomposition continually.
Because zeolite absorbs water greatly with its large amount of surface and is the most popular ingredient in phosphors-free detergent, 4A zeolite is selected as the carrier to test the stabilization of percarbonate.
Boric acid is a special momoatomic inorganic acid. Its dissociation constant is 5.8X10-10. its acidity is from adding hydroxyl ions instead of delivering hydrogen ion. Metaboric acid is got from dehydration of boric acid at 169C.
Metaboric acid is a half boric anhydride. It has the ability to absorb moisture that is a valuable property for stabilization of percarbonate. When metaboric acid is added into the solution, sodium silicate delivers silicon gel(SiO), which is a form of colloid with a certain viscscosity. The colloid gives the critical effect to making uniform coating layer.
We tried several organic and inorganic acids to take place of metaboric acid, such as hydrochloric acid, phosphoric acid, citric acid, lauric acid, acetic acid, and sodium borate, and so on. All of them did not have the effect to stabilize percarbonate. Therefore, the special effect of metaboric acid may belong to its special electrical property, and can not only be explained with its acidity.
According to our research on kinetics and property of peroxybleaching[9,10], percarbonate is a critical green ingredient for overcoming “black area” in laundry. Heavy metals, such as Fe, Cu, Mn, Co ions can induce perbonate decomposition. For Fe ions the catalyst mechanism is as the following:
Oxygen molecule do not have direct ability of bleaching and sterilizing. So above radical-type reaction have absolute negative effect for percarbonate.
Sodium silicate is a traditional stabilizer for peroxy agent. Its net-construction is like porous sponge absorbing heavy metal ions which catalyze peroxy compounds and active oxygen groups, such as anionic peroxy ions(OOH), peroxy radical(OOH), and super anionic radical(O-. ). When there is some humidity, magnesium ion and sodium silicate form gluey magnesium silicate which surrounds peroxy ions, and makes peroxy agent release active oxygen slowly. May be magnesium forms a complex with peroxy oxide [Mg(OH)(OOH)], which controls the decomposition of peroxy compounds. typical chelating agents such as EDTA, a typical chelating agent, is also helpful to the stability of percarbonate. The results mentioned above were proved again in this paper.
In order to summarize the effect of coating ingredients, an orthogonality test was performed. The main factors and levels were designed as Table 2, and the test of orthogonality is as Table 3.
Table 2. The factors and the levels in orthogonality
|Factors Levels||O (Wt %,based on coating composition)
|Ingredients (Wt%, based on percarbonate)|
|Metaboric acid (B)||Silicate (C)||Carbonate (D)|
Table 3. Test of orthogonality
|Factors No.||O, Wt%,based on coating.comsition (A)||Metaboric acid, Wt%, Based on percarbonate (B)||Silicate,Wt%, based on percarbonate (C)||Carbonate,Wt%, Based on percarbonate (D)||Residual Oav
|Difference between levels||12.1||15.9||19.2||8.7|
*1 The samples were put into an oven of 50,70 RH% for 48h in the presence of 4times of 4Azeolite to the coating composition.
*2 In all the examples there were 0.8% of magnesium sulfate, and 0.5% of EDTA based on percarbonate
In the range of the levels of the factors the difference of levels teaches that the main factors to influence the residual oxygen content is the amount of silicate, and then metaboric acid, and water. The content of carbonate anhydrous is minor factor. From the trend of change, the maximum composition of the factors was A. From variance analysis the differences is remarkable.
It was also noticed that carbonate could improve the solubility of product because it can absorb water molecule into its molecule as its crystal water. If carbonate was absent in the composition, the lost of oxygen did not increase significantly.
There was not an apparent difference of active oxygen content in coating at 60or at 30. Perhaps at different temperature there was a difference in grade of coating and penetrating, both were beneficial to the stability when the coating agents were in slurry. Slurry could penetrate into inside, and could coating the surface of grain and pores.
The experiments at maximum condition gave 91.8 % of average residual oxygen content. In that case, the loss of available oxygen of untreated starting percarbonate was 56.3% .
In all the cases, if coating was repeated, say to 2 layers or 3 layers, or other methods to be used also, the loss of available oxygen became even smaller.