Formulation screening and optimization of surfactant in electroplating degreasing agent

In the oil removal additives, the most important, the biggest change, and the environmental protection requirements are getting higher and higher. Many laws and regulations on biodegradability of surfactants, foam, COD(chemical oxygen demand) and wastewater treatment have been issued in various countries around the world, which are worthy of further study.

In 1961, the first regulation on detergents was introduced in Germany, stipulating that the biodegradability of surfactants must be greater than 80%. In 1965, the foam problem in surface water and wastewater in Europe was solved. In 1966, LAS(straight chain alkyl benzene sulfonate) was put into commercial production, and DDBS(branched chain alkyl benzene sulfonate) began to withdraw from the historical stage. In October 1990, Germany and the Netherlands had asked the surfactant industry to replace D1821(dioctacoalkyl dimethyl ammonium chloride), and its consumption was reduced by 70% in the following year, and was completely replaced by ester quaternary ammonium salts three years later. In early 2011, nonylphenol (NP) and nonylphenol polyoxyethylene ether (NPE) were listed in the "Catalogue of Toxic Chemicals Strictly Restricted for import and Export in China" issued by the former Ministry of Environmental Protection and General Administration of Customs of China for the first time. Alkyl phenol polyoxyethylene ether compounds (APEO), including NP, OP(octylphenol), DP(dodecyl phenol), DNP(dinonyl phenol), etc., are highly toxic, have the effect of estrogen, are not easy to degrade, will bring serious environmental problems, are gradually being restricted or banned by countries.

At present, the environmental protection situation is grim, energy saving and emission reduction, green environmental protection, resource recovery and recycling have become the guiding spirit of each production enterprise and the wind vane of development.

The mechanism of oil removal is that the oil removal agent removes mineral oil, polishing paste, lubricating oil, powder scale, metal dust, fingerprints, etc. on the surface of the workpiece through the saponification, emulsification, wetting, solubilization, dispersion, dissolution, penetration, etc., to ensure the smooth progress of the following process.

The main functional components of oil remover are:

(1) Emulsifiers: mainly all kinds of surfactants that can emulsify, wetting, solubilizing, dispersing and dissolving, such as octylphenol polyoxyethylene ether, perigat (advanced alcohol polyoxyethylene ether), lauryl alcohol-ethylene oxide condensation, 6501(dodecyl diethanolamide) and so on.

(2) Wetting agent: It is mainly a surfactant that can reduce the surface tension of the oil removal solution and improve the wettability of the workpiece, such AS primary alkyl sulfate salt (AS), quaternary ammonium salt, etc.

(3) Dispersant: It is mainly a coordination agent or inorganic salt that can disperse the solid particles attached to the surface of the workpiece into the solution, such as gluconate, citrate, sodium pyrophosphate, sodium tripolyphosphate, sodium phosphate, etc.

(4) Corrosion inhibitors: mainly agents that can reduce the corrosion of metal by oil removal, such as thiourea, aniline, sodium silicate and so on.

Emulsification mainly involves the surfactant using its own micelles to wrap the oil in micelles and disperse it into the solution. The critical micelle concentration (CMC) of surfactant is an important index to measure its solubilization effect. When the concentration of surfactant is higher than CMC, the solubilization effect is obvious. Therefore, the lower the CMC of the surfactant, the better the oil removal effect, the less the dosage, and the lower the cost. On the other hand, the higher the ratio of hydrophilic and hydrophobic groups (i.e. the balance value HLB) of the surfactant, the weaker the holding power of the hydrophilic group on the oil particles. Therefore, the CMC and HLB of surfactants can be used to select suitable surfactants.

The main disadvantages of most oil removal powders currently on the market are:

(1) Low oil removal efficiency and long processing time;

(2) The processing temperature is high, not enough to save energy;

(3) More foam is generated, which causes difficulties in the production process and wastewater treatment;

(4) surfactants are difficult to biodegrade, causing serious environmental pollution;

(5) Compounds containing phosphorus and nitrogen are easy to cause eutrophication, which has a great ecological impact. Therefore, the search for efficient, low temperature, low bubble, easy biochemical degradation and no phosphorus and no nitrogen degreasing agent has become the main direction of the reform of the plating industry in the 21st century.

The metal surface must be degreased before electroplating. In the oil removal process, if stirring or blowing produces a large amount of foam, it will interfere with the cleaning, resulting in the overflow of the working fluid. This not only causes material waste, increases cleaning costs, but also may produce stains on the surface of the workpiece, which brings inconvenience to the production. In addition, too much foam will hinder the washing of oil, as well as slow the precipitation and separation of dirt. If too much foam is discharged when the oil remover is discharged, it will also increase environmental pollution. Therefore, under the premise of ensuring the degreasing effect of the degreasing agent, the foaming ability of the degreasing agent should be as low as possible. At present, the main way to achieve low foam of degreasing agent is to add defoamer to degreasing agent, but this often has the problem that the defoamer ability is reduced with the degreasing process, and even some silicon-containing defoamer will produce silicone or cause silicon spots in the degreasing process. Another way to obtain the low foaming effect is to improve the oil removal efficiency of the surfactant and reduce the amount of surfactant by screening and mixing. Therefore, to learn more about the relationship between the structure, physical and chemical properties of surfactants and the oil removal effect, to screen several low-foam surfactants suitable for metal surface oil removal, to use the synergistic and synergistic effect between surfactants, to mix several low-foam surfactants, and to obtain the best low-foam oil removal effect, is the current direction of metal oil removal agent research and development.

Commonly used low foam surfactant

The non-ionic surfactants containing EO/PO(ethylene oxide/propylene oxide) block alcohol ethers are easy to form micelles in aqueous solution. Due to the intermixing arrangement of hydrophilic and oleophilic groups, the space obstructs each other and forms a large number of liquid film voids, which weakens the strength of the liquid film. The foam film formed is weak and easy to break, so it has low foaming property. Typical products for sale are:

(1) EO/PO block alcohol ether L-61, L-64, Jiangsu Sixin Surfactant Technology Company;

(2) Iso-alcohol ether E-1307, E-1310, Zhejiang Real Madrid Chemical Co., LTD.;

(3) C12−14 fatty alcohol ethers MOA-3, MOA-5, MOA-7, MOA-9, Anhui COFCO Biochemical Group;

(4) octylphenol polyoxyethylene ether OP-7, OP-10, Jilin Chemical Corporation;

(5) Nonylphenol polyoxyethylene ether TX-10, Jilin Chemical Corporation;

(6) Sorbitan fatty acid ester S-60(Span-60), S-80(Span-80), Hebei Xingtai Blue Sky Auxiliary Factory;

(7) Sorbitan fatty acid ester polyoxyethylene ether T-60(Tween-60), T-80(Tween-80), Hebei Xingtai Blue Sky Auxiliary Factory;

(8) Fatty acid methyl ester ethoxide sulfonate FMES, Shanghai Xihe Fine Chemical Co., LTD.;

(9) Dodecyl benzene sulfonic Acid LAB, Nanjing Jintong Petrochemical Company;

(10) Ethoxylated fatty alcohol polyoxyethylene ether sulfate sodium AES, Light Products Chemical Co., LTD.;

(11) Sodium alkenyl sulfonate AOS, Zhonglight Chemical Co., LTD.;

(12) Isoctyl phosphate RP-98, Haian Sunda Chemical.

Comparison and screening of oil removal properties of single surfactants

In order to analyze the effects of surfactant permeability, emulsification, dispersion, foam and other properties on oil removal, the structure and types of surfactants suitable for oil removal before electroplating were screened in literature [1]. First, a single surfactant was used as the degreasing agent, the degreasing test was carried out under the same dosage and experimental conditions, and the indexes of degreasing rate, foam and COD of wastewater were compared. The results show that 6501 and Tween series of non-ionic surfactants not only have high foam, but also poor oil removal properties such as emulsification and dispersion. Although the foams of block alcohol ether, isooctanol penetrant JFC and Span series are low, the emulsification, dispersion and oil removal properties are very poor;

The oil removal performance of anionic surfactants is worse than that of non-ionic surfactants. For example, the oil removal rate of FMES with the best oil removal performance is only 26%, which is significantly lower than that of OP-10.

6501 and Tween series non-ionic surfactants are not suitable as the main components of oil removal agents because of their poor performance in both foam and oil removal. The oil removal performance of the block alcohol ethers L-61 and L-64, isoctyl alcohol JFC and Pan series is general, although it has the advantage of low foam when used alone, but the foam is not significantly reduced when combined with other non-low foam surfactants, so this product is not suitable for the production of oil removal agents. Considering the foam properties, permeability properties and dispersion properties, the surfactants suitable for oil removal process are determined to be OP-10, TX-10, E-1310 and MOA-5, among which OP-10 and TX-10 have moderate cost and good oil removal effect, but there are problems such as high COD, not easy biochemical degradation and greater harm to the environment. E-1310 has excellent overall performance, but it is the most expensive among non-ionic surfactants.

Although anionic surfactants have poor oil removal performance, they have no turbidity point limitation, good alkali resistance, and low price, so the use of appropriate anionic type products can reduce the cost of oil removal without weakening the performance of other applications of oil remover. After comprehensive evaluation, it is concluded that FMES and LAB are more suitable for metal degreasing.

The surfactants suitable for metal degreasing were TX-10, MOA-5, LAB and FMES. The optimal formula of metal degreasing agent was TX-10 2.0g/L, MOA-5 0.5g/L, FMES 1.5g/L, LAB 0.5g/L. As long as the dosage of 4.50g/L, the oil removal agent obtained with this ratio can obtain a higher oil removal rate than that of other surfactants using 5g/L alone.

The oil remover used in the past is mostly high temperature, up to 90 °C, great energy consumption, does not meet the requirements of energy conservation and emission reduction, so low temperature oil remover has become one of the hot spots of research in recent years. If the oil remover can remove the oil on the surface of the steel under normal temperature conditions, it can not only reduce the production cost, produce considerable economic benefits, but also save a lot of energy and produce significant environmental benefits. At present, the so-called normal temperature metal degreaser products sold in China generally must be above 60 °C to effectively clean heavy oil dirt, and below 50 °C, the degreasing effect is greatly inferior. Surfactants that can have good oil removal function at low temperatures are mainly selected from non-ionic surfactants with low turbidity point and strong wetting and permeability APEO and other compounds containing nitrogen and benzene rings, such as alkyl diethanolamide, isomeric alcohol polyoxyethylene ether, long chain carboxylate polyoxyethylene fatty alcohol polyoxyethylene ether, polyethylpyrrolidone, etc., but for environmental protection considerations, Should not continue to use.

According to the properties of some low-temperature surfactants (see Table 1), An Lei et al. selected LAS, A9, AEO-9 and 168 surfactants as the main components, and added sodium carbonate, homemade organic solvent, Ulotopine and Na2EDTA as additives to prepare a low-temperature and low-foam oil remover.

The mass ratio of the four surfactants was determined to be m(AEO-9):m(A9):m(168):m(LAS)= 6.85:1:3:2. The basic test rules were obtained through a large number of pre-tests and then orthogonal tests were carried out.

According to the results of orthogonal analysis, the mass ratio of additives in the oil removal agent was m(sodium carbonate):m(organic solvent):m(ulotropine):m(Na2EDTA)=1:1.49:0.054:0.082.

By changing the concentration of various chemicals in the oil remover into mass fraction, the final formula of the oil remover was obtained as follows: AO-9 4.79%, A9 0.7%, 168 2.1%, LAS 1.4%, organic solvent 7%, sodium carbonate 4.7%, Ulotopine 0.25%, Na2EDTA 0.38%, water allowance. The degreasing agent has a pH of 10.7 and an oil removal capacity of 95.26% at 35 °C, which has a series of advantages such as energy saving, no harm to operator health, reducing pollution, protecting the environment, non-combustion and low cleaning cost.

Surfactants are more and more widely used in industry, agriculture, medicine, daily chemical and other fields, and the global annual use has exceeded 10 million tons. The extensive use of surfactants has caused serious pollution of soil and water. In order to solve the increasingly serious environmental problems, the green chemistry of surfactants has become the hot spot and frontier of the current chemistry research, and gradually use non-toxic (or low toxic) and easily degradable surfactants. Biodegradability is an important index to evaluate the greenness of surfactants. The biodegradation of surfactants, which is caused by microorganisms, refers to the process in which surfactant molecules are decomposed by microorganisms (mainly bacteria) and converted into microbial metabolites or cellular materials, and carbon dioxide and water are produced

There are many factors that affect the degradation of surfactants, in addition to their own structure, microbial species, light source, concentration, temperature, oxidant, pH and so on.

Biodegradable surfactants

1. Linear alkyl benzene sulfonates (LAS)

LAS is a kind of surfactants that has been studied more, and there are many explanations for its biodegradation mechanism.

2. Alkyl Sulfate (AS)

AS is biodegraded to CO2 and H2O by alkylsulfatase desulphurization and then by dehydrogenase and β-oxidation processes:

3. Alkyl ether sulfonates

The biodegradation of alkyl ether sulfonates is thought to be mainly through the breaking of ether bonds by etherase, followed by progressive degradation by alkyl sulfatase and dehydrogenase

4. Amines and amides

The biodegradation of amines and amides begins with the breaking of the C-N bond, then goes through ω-oxidation and β-oxidation, and finally produces CO2, H2O, NH3 and metabolites.

Relationship between the structure of surfactants and biodegradation

On the basis of summarizing his own and previous research achievements, Swisher summarized the following three general rules on the relationship between biodegradation and structure of surfactants:

(1) The biodegradability of surfactants is mainly determined by the hydrophobic groups, and increases with the increase of the linear degree of hydrophobic groups, and the end-of-season carbon atoms will significantly reduce the degradability.

(2) The properties of hydrophilic groups have only a minor effect on biodegradation, such as the primary biodegradation rate of straight-chain primary alkyl sulfonates (LPAS) is much higher than that of other anions, and the short EO chain polyoxyethylene non-ionic surfactants are easy to degrade.

(3) By increasing the distance between the sulfonate and the hydrophobic end, the primary biodegradability of alkyl benzene sulfonates is increased (distance principle).

Biochemical degradation of anionic surfactants

Among anionic surfactants, LAS, AS, AES and AOS are the most used, so their biodegradability has been studied more. Among them, AS is the most easily biodegradable and can be oxidized to CO2 and H2O by ordinary sulfatase, and the degradation rate is accelerated with the increase of the distance between the sulfonic acid group and the end of the alkyl chain, and the alkyl chain length is 6-12 carbon. When the alkyl chain of anionic surfactant has branch chain, and the length of branch chain is closer to the main chain, the more difficult it is to degrade.

Biochemical degradation of zwitterions and cationic surfactants

Among all surfactants, the environment is the most receptive to amphoteric surfactants, so it is generally opposite