Eco-friendly fungicides

11 March 2010

Dr Lakshmi Muthusubramanian1* CLRI, Chennai, and Rajat B Mitra2, National Chemical Laboratory, Pune, discuss an eco-friendly approach to the synthesis of methylene bisthiocyanate as a fungicide in leather processing

Preservatives are essential in the leather industry, in particular to protect tanned and crust leather against mould damage. Preservative selection is made on the basis of ecological aspects, statutory requirement, official regulation and their effectiveness and cost. Until legislation restricted the use of pentachlorophenol (PCP), phenolics were the predominant wet-blue fungicides but afterwards, methylene bisthiocyanate (MBT) was widely used in the industry. From the workers’ health and safety point of view non-hazardous and non-toxic raw materials such as sodium thiocyanate, water, phase transfer catalyst (benzyl triethylammonium chloride) and halogenating agent (methylene bromide) are used to make the MBT, which is characterised by conventional methods, such as IR, 1HNMR and mass spectral data. This paper reports on a simple and cleaner technology of the synthesis MBT by using non-toxic and non-hazardous raw materials and belongs to the modern concept of environmentally friendly low-waste or non-waste technology (LWNWT).

In the leather industry microbiocides are usually divided into two classes: bactericides for prevention of bacterial damage on hides/skins and fungicides to impart mould and mildew resistance to hides1,2. The impact of Environment and Health and Safety regulations on the leather is becoming wider in scope and more sophisticated. During the past thirty years, attention has been directed primarily to such factors as total oxygen demand, sulphide and chromium levels in tannery wastewater. We are now moving into an era where environmental controls will be much more closely targeted to specific chemicals. Almost by definition, materials that are used to restrict biological activity in an agricultural or industrial sense are likely to have an adverse environment impact if they are not decomposed prior to discharge3-5.

Materials that come within the scope of this are biocides used in the following ways:
(a)    to protect raw hides and skins during storage and intercontinental transport
(b)    to protect pickled stock during storage and transport
(c)    in leather processing, eg in soak liquors and vegetable tan liquors
(d)    to protect simply tanned leather such as wet-blue
(e)    to protect finished leather

Microorganisms shorten the life of industrial and natural products. One of the best ways to prevent damage to the skin is to treat them with a biocide added to the processing liquors. Methylene bisthiocyanate is active against many different microorganism in a concentration of only a few parts per million which makes it an ideal active compound for leather biocidal composites6,7. Earlier it was prepared by reacting dihaloalkane with an alkali metal thiocyanate using an organic solvent such as dimethylformamide8,9. In this procedure the main drawback is that the solution of methylene bisthiocyanate in dimethylformamide either alone or in an a mixture with another solvent slowly decomposes as a result of the concentration of methylene bisthiocyanate, and the solution spontaneously evolves hydrogen cyanide which is highly toxic. Hydrogen cyanide formation can be suppressed or it can be absorbed by the addition of certain compounds such as hydroquinone or hydroquinone monomethyl ether to the solution. This, however, is not completely successful and does not prevent decomposition of methylene bisthiocyanate in dimethylformamide which are particularly toxic in addition to the high toxicity of methylene bisthiocyanate itself. The solvent is also toxic and aids penetration of the skin by the methylene bisthiocyanate, should the solution be accidentally splashed on the body.
The alkaline rare earth metals are also impractical or too expensive and ammonium thiocyanate could be used in place of sodium thiocyanate. However, it is well known that ammonium thiocyanate presents some problems as a reactant due to polymer formation leading to poor yields and/or a red polymer product.
This paper reports a simple and cleaner technology of synthesis of MBT by using environmentally friendly, non-toxic and non-hazardous raw material using methylene bromide containing aqueous sodium thiocyanate with a phase transfer catalyst.

All chemicals unless specified otherwise were obtained from Sigma Chemical and were of analytical grade. Silica gel precoated plates with fluorescent indicator were used for thin layer-chromatography. Developed plates were viewed by UV light. IR spectra were obtained on a Nicolet DXB F.T. spectrometer. 1HNMR spectrum was recorded with a Brucker CXP-90 MHZ pulse spectrometer using TMS as internal standard. Mass spectra were obtained on HP 5890 series II gas chromatograph with HP 5971 as a mass selective detector.
To a stirred mixture of sodium thiocyanate (35.68g, 0.44 mol) 40ml water and methylene bromide (30.00g, 0.17 mol), benzyl triethylammonium chloride (TEBA) (3.00 g, 0.01 mol) were added and heated for 5 hours at 70-72°C. The completion of the reaction was checked by thin layer chromatography. To the reaction mixture 50ml of cold water was added to dissolve the precipitated sodium bromide and the reaction mixture was cooled with agitation. The crystals of MBT were filtered, washed with water and air dried. M.P. 103-104°C, yield: 72% (41.00 g, 0.32 mol).
Methylene bromide was added to a mixture of metal thiocyanate with a small quantity of water containing the phase transfer catalyst (TEBA) to give methylene bisthiocyanate.

Results and discussion
The data used to represent
the reaction and the analysis
of the subsequent product
from the process are shown in Table 1.
The IR spectrum of MBT showed characteristic absorption bands at 2920 cm-1 for aliphatic carbon hydrogen stretching, 2130 cm-1 for an SCN bond and 1600 cm-1 for C º N stretching. The 1HNMR spectrum in (CDCl3)) showed the signal at d 4.42 (2H, s), the proton adjacent to thiocyano group. A M+ peak appeared at 130.

In the leather industry methylene bisthiocyanate is used as a fungicide to prevent mould growth in wet pickled, chrome tanned and vegetable tanned stock. The technology we have developed is economical as well as environmentally friendly from a health and safety point of view. Toxic solvent such as dimethylformamide is not used, while water, sodium thiocyanate, and methylene bromide are economically available, non-toxic and non-hazardous. Therefore, this process belongs to a modern concept of eco-friendly low waste and non-waste technology for producing safer fungicides for use in leather processing.

1    Central Leather Research Institute, Adyar, Chennai
2    National Chemical Laboratory, Pune - 411 008, India


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