Cutting fluid plays a role in lubrication, rust prevention, cleaning, and cooling during metal cutting. The usage environment of cutting fluid is generally an open system, and during the use process, it is constantly facing attacks from microorganisms (mainly bacteria and molds) in the environment. Microorganisms constantly reproduce in the cutting fluid, ultimately leading to the decay and deterioration of the cutting fluid, producing a foul odor, and having to be replaced. There are generally mechanical, physical, and chemical methods for killing microorganisms in cutting fluids. Compared to mechanical and/or physical methods, chemical methods that add fungicides to cutting fluid to inhibit microbial growth are simpler and more efficient. Cutting fluid fungicides should have broad-spectrum properties, stability, and low toxicity. 01 The Harm of Microorganisms to Cutting Fluids The main reason for changing the cutting fluid is due to the decay and deterioration of the cutting fluid, which is mostly caused by the proliferation of microorganisms. The microorganisms in cutting fluid are mainly bacteria and molds, and their types and composition are closely related to their living environment. The experiment shows that the main microorganisms causing the deterioration of cutting fluid are gas producing bacteria, sulfate reducing bacteria, ordinary Proteus, Escherichia coli, Aspergillus flavus and Aspergillus fumigatus.
Regardless of the type of microorganisms, their proliferation in large quantities can cause harm to cutting fluids, and their degradation effects on cutting fluids include: (1) Anaerobic sulfate reducing bacteria proliferate in large numbers, producing hydrogen sulfide gas and odor, which affects human health; (2) The mineral oil and various functional additives in the cutting fluid are degraded and consumed by microorganisms, which damages the composition of the cutting fluid and reduces the stability of the cutting fluid (emulsion), resulting in phenomena such as oil separation or stratification; (3) Due to the decomposition effect of microorganisms on oily agents, the lubrication performance of the cutting fluid decreases, leading to increased friction during machining, shortening the tool service life, and affecting the quality of the machined parts; (4) The acidic substances produced by metabolic products during microbial reproduction can cause the cutting fluid to become rancid, causing a rapid decrease in the pH value of the cutting fluid, resulting in a decrease or loss of the rust and corrosion resistance of the cutting fluid, and corroding the workpiece and machine tool equipment; (5) When the total number of microbial proliferation exceeds the critical indicator, viscous substances will be generated in the cutting fluid, which will block the filter and circulation pipeline, affecting the cleaning and heat dissipation functions of the cutting fluid. 02 Microbial killing methods The microorganisms in the cutting fluid grow and reproduce rapidly, and under suitable environments and temperatures, they can reproduce one generation approximately every 15-20 minutes, with 72 generations within one day. There are generally three methods (sterilization methods) for killing microorganisms in cutting fluids: mechanical methods, physical methods, and chemical methods. Mechanical methods mainly include filtration and centrifugation, which can effectively remove large particle pollutants such as chips and impurities from the cutting fluid. This can also remove microorganisms adsorbed on the particles. However, the number of microorganisms removed by mechanical methods ranges from 50% to 90%, and the effect is not satisfactory. Physical methods include pasteurization, microwave, ultrasonic, and radiation methods. Pasteurization is a sterilization method that involves heating raw materials to a certain temperature (below the boiling point) and rapidly cooling them after a period of time. This method can effectively control the number of microorganisms, but when heating is stopped for a period of time, surviving microorganisms will rapidly reproduce and cannot be completely eliminated; Microwave method and ultrasonic method use different methods to generate heat to achieve the goal of killing microorganisms; Radiation methods have certain safety hazards, and high doses of ionizing radiation pose health risks.
Whether it is mechanical or physical methods, corresponding equipment needs to be installed at a certain position in the system to process cutting fluid. However, in certain fixed positions or places where the cutting fluid circulation system cannot reach, microorganisms will be difficult to be treated. At present, the most effective method to control the deterioration of cutting fluid is to use a chemical method of adding fungicides, which affect the metabolism of microorganisms and destroy their structure to inactivate them, achieving the goal of sterilization. Using fungicides to extend the service life of cutting fluids is a convenient solution with low investment and high efficiency. 03 Introduction to cutting fluid fungicides 3.1 Types and Performance There are thousands of commercial bactericides at home and abroad. Common bactericides in cutting fluids include triazine, morpholine, 1,2-benzisothiazolin-3-one (BIT), 3-iodo-2-propargyl-butyl carbamate (IPBC), 2-butyl-1,2-benzisothiazolin-3-one (BBIT) and sodium pyrithione. Triazine and morpholine are both formaldehyde releasing fungicides. Formaldehyde can coagulate proteins in microorganisms, reduce amino acids, alkylate protein molecules, and ultimately achieve the goal of sterilization. Formaldehyde releasing fungicides slowly release a very small amount of free formaldehyde for sterilization. A. A ', A "- trimethyl-1, 3, 5-triazine-1, 3, 5 (2H, 4H, 6H) - triethanol (HPT) is a common triazine fungicide. It is mainly targeted at bacteria and has good water solubility. It can be used in both concentrated solution and diluent. Its molecular structure is shown in Figure.
HPT has rapid bactericidal performance, suitable for alkaline environments, and has good bactericidal effects. N. N-methylenedimorpholine (MBM) is a kind of low toxicity, broad-spectrum and efficient bactericide, also a formaldehyde releasing bactericide. Its heat resistance can reach 100 ℃, and it can be soluble in oil or water in any ratio. Its molecular structure is shown in Figure.
MBM has a wide sterilization range and is effective against both bacteria and fungi, effectively preventing the generation of spoilage and foul odor in cutting fluid. 1,2-Benzoisothiazolin-3-one (BIT) fungicide has low toxicity and does not contain heavy metals or halogens. Its thiol sulfonated protein functional group can reduce the production and activity of proteases, and can persistently and stably inhibit microbial growth. BIT has good thermal stability and is suitable for the environment below 150 ℃. It is also a broad-spectrum long-acting bactericide, which can effectively kill a variety of bacteria at low content. Its molecular structure is shown in Figure.
BIT fungicide can effectively sterilize between pH 4 and 12, and can be added to the mother liquor of cutting fluid or directly to the diluent in use. The 3-iodo-2-propargyl-butyl carbamate (IPBC) bactericide mainly oxidizes microbial cells through the iodine on the molecular chain to iodize the sulfhydryl (- SH) and complexine of microbial cells and lose the protein activity, thus achieving the purpose of inhibiting microbial growth. IPBC is a broad-spectrum rapid fungicide, which has high efficiency and lasting fungicidal and antifungal ability against mold and yeast, and is widely used. Its molecular structure is shown in Figure.
IPBC begins to decompose at 180 ℃ and is easily hydrolyzed in alkaline media with poor water solubility. It is not suitable for pool side (on-site) addition and is recommended for use in emulsified oil and semi synthetic cutting fluids. The bactericidal mechanism of 2-butyl-1,2-benzoisothiazolin-3-one (BBIT) fungicide is that the active groups on the heterocycle of isothiazolinone form hydrogen bonds with the bases on DNA molecules in bacterial proteins, and adsorb them on bacterial cells to attack the cell nucleus, thereby damaging the structure of bacterial cell DNA, causing it to lose replication ability, related physiological and biochemical activities, and metabolic activities, leading to bacterial death, Its molecular structure is shown in Figure.
BBIT has a heat resistance of up to 300 ℃ and is stable between pH 2 and 12. It is a broad-spectrum, long-lasting and stable fungicide that is effective against both bacteria and fungi. Sodium pyrithione has a strong metal chelating ability, and its bactericidal mechanism is to interfere with the transportation of microbial cell membranes by affecting the salt balance inside and outside the cell membrane, leading to metabolic disorders of microorganisms and thus inhibiting and killing them. Sodium pyrithione is a kind of mildew and preservative with excellent effect, which has broad spectrum and no formaldehyde. Its molecular structure is shown in Figure.
Sodium pyrithione is stable between pH 4.5 and 9.5. When the pH is below 4.5, it is converted into pyrithione, and when it is above 9.5, it is converted into pyridyl sulfonate. It can withstand temperature for no less than 120 hours at 100 ℃ and has good water solubility. It can be added to the cutting fluid formula and at the pool edge (on-site). However, the combination of sodium pyrithione with non-ferrous metal ions such as iron and copper ions can cause discoloration, and strong oxidants and reducing agents can reduce their bactericidal efficiency. Different bactericides generally only target certain special strains, for example, triazine based bactericides are not very effective against Sulfate-reducing bacteria and mycobacteria; BIT type bactericides have the advantage of not relying on formaldehyde, but they are not very effective against Pseudomonas and Mycobacterium, and will fail when facing Sulfate-reducing bacteria; MBM has broad-spectrum antibacterial properties, but MBM type fungicides can cause eye irritation and skin sensitization. Therefore, it is necessary to choose a suitable fungicide based on the type of microorganisms in the liquid tank. 3.2 Usage in cutting fluid At present, there are two main ways to use fungicides in cutting fluids: one is to directly add them to the formula, and the other is to add them to the liquid tank on site. Most of Japan uses adding fungicides to formulas, while Europe and America mainly use the latter, which directly adds fungicides to diluents. In China, both forms are used. Cutting fluid is not subjected to sterilization treatment during the mixing, filling, and transportation processes, so there are a large number of microorganisms in the cutting fluid stock. Adding an appropriate amount of fungicide to the formula can control the number of microorganisms in the product, while reducing the hassle of additional additives and on-site operations for end users.
Compared to adding fungicides to formulas, direct use of fungicides can control the content of fungicides in diluents based on microbial detection information, carry out targeted sterilization work, and reduce microbial resistance. The use of additional fungicides can maintain the excellent state of cutting fluid for a long time, reduce the frequency of cutting fluid replacement, and reduce the cost of customer fluid use, which is the development trend of cutting fluid technology. No matter which method is used, adding effective fungicide components can to some extent extend the service life of cutting fluid, and there are also two ways to use them simultaneously to obtain comprehensive protection. The bactericidal agent added to the formula may lose its antibacterial effect after a long period of time. Once it fails, adding the bactericidal agent outside the cutting fluid tank can timely control the growth of microorganisms. Therefore, it is necessary to conduct regular on-site monitoring and timely supplement with fungicides. 3.3 Hazards and Protection of Fungicides With the increasing emphasis on health and environmental protection, the toxicity of fungicides is an important consideration in the selection of cutting fluids. Fungicides have a certain degree of toxicity and can inevitably cause harm to human health and the environment. For example, formaldehyde releasing fungicides inhibit microbial growth by releasing formaldehyde, and their original solution has strong irritation to the eyes and nose. When in contact, it can easily cause skin allergies to the operator, and in severe cases, it can cause tissue necrosis. The dosage of formaldehyde releasing fungicides in the formula is generally less than 3.0% by mass, and they need to be diluted about 20 times when used, so the actual content is very low and the harm to the human body is minimal. However, with the increasing awareness of people's health and environmental protection, formaldehyde releasing fungicides are gradually being replaced, and low toxicity and good bactericidal effects are the preferred formulations or on-site supplements. At present, in the actual metal cutting process, the possibility of completely avoiding the use of fungicides is not high. A better way is to minimize contact with fungicides to ensure maximum physical health; At the same time, the use of fungicides in low concentrations in formulas or diluents can further reduce their risk of use. Firstly, adjust the flow rate of the cutting fluid to prevent splashing and install sufficient splash plates; Secondly, operators should wear protective gear, especially protective gloves and goggles; Thirdly, do a good job in personal hygiene and do not use solvents with strong cleaning abilities, as they can easily remove the protective barrier of the skin and exacerbate skin irritation; The fourth is to use skincare products to accelerate skin recovery, and cheap Vaseline is a better choice. 04 Conclusion The proliferation of microorganisms is the main reason for the replacement/deterioration of cutting fluids. Compared to mechanical and/or physical methods, adding fungicides to cutting fluids is a simpler and more efficient chemical method to inhibit microbial growth. The fungicides in cutting fluids should have broad-spectrum properties, stability, and low toxicity, and can be effectively used at low concentrations. Only by comprehensively understanding the performance of fungicides and cutting fluids can fungicides be used more economically and effectively.
