In the production and distribution of leather, two types of biodeterioration are encountered: bacterial (which generally applies to hides and skins in the early stages of the process) and fungal (which usually applies after tannage has taken place). Fungal biodeterioration can affect leather both in part-processed condition within the tannery and after it has been made into and distributed as a fully finished article.
Fungal deterioration is apparent from the growth on the leather of moulds. Moulds are part of the group known as fungi – a group of unique organisms that sit uneasily between microbiology and botany. They are neither filamentous bacterium nor are they similar to green plants.
There are many strains of mould that are capable of growing on leather, but often the presence of one strain suppresses the appearance of others.  Aspergillus niger, for example, usually results in the inhibition of other strains, although the others are quite capable of widespread growth in the absence of A. niger.
Like all living organisms they need certain basics to grow and multiply: a source of moisture and a source of nutrient. In wet-blue leather the source of moisture is clear.
In finished leather the moisture content is very much lower and the growth of mould is less, but when transporting leather between different climatic zones, or if storing leather in an inappropriate manner, moisture build up in certain areas, such as just inside any packaging, can readily lead to the growth of mould. Many of the process chemicals used in the manufacture of leather act as nutrients for mould growth, examples include ammonium salts, phosphates, surfactants, fatliquoring agents and other organic agents.
The appearance of the mould growth is related to the type of mould present. In the minds of many people mould is usually associated with green/grey growths. Tanners, however, may also be familiar with the red spots or red discolouration caused by Paecilomyces ehrlichii, P. aculeatum, P. purpurogenum and P. roseopurpureum.
In poorly controlled drying operations, where the humidity remains high or air circulation is poor, other types of growths may also be found, giving rise to other coloured appearance such as green, yellow-brown, dark-brown and grey. Strains associated with damage arising from poor drying control, include A. ochraceus, A. wentii, P. rugulosum, P. funiculosum, P. variotii and V. glaucum.
Fungicides are the chemicals used to control or, better still, prevent mould growth and those used in the leather industry fall into two broad chemical families; phenolics, (which include CMC, OPP and TCP) and heterocyclics (which include TCMTB, OITZ, BMC and DIMTS). In general, fungicides interfere with the fungal germination and penetration process or inhibit fungal growth and reproduction within plant tissues. More specifically, fungicides may interfere with cell division, may inhibit the activity of certain enzymes, or may alter the function of cell membranes.
As a measure of effectiveness fungicides should be demonstrated to have the following properties:

  • high activity
  • a broad antimicrobial spectrum
  • compatibility with leather and with process liquors
  • stability on leather
  • non-discolouring
  • environmentally acceptable
  • low toxicity to humans and other warm-blooded animals
  • cost effectiveness

The greater the amount of applied fungicide the greater the level of protection; however a balance needs to be drawn between adequate protection and excessive cost. The type of fungicide determines the level that should be applied – typical levels of fungicide that have been suggested as being required to be present in the wet-blue to give protection from fungal growth1 are:
TCMTB      250 ppm
CMC        580 ppm
OPP        280 ppm
OITZ       80   ppm
These, however, are the minimum levels of protection. Greater protection requires increased offer of fungicide and the additional concentration will depend on the length of preservation required and the storage temperature of the wet blues. In the application of TCMTB, for example, experience has shown that typical addition levels of a standard 30% TCMTB solution fall in the range 0.05% (short term preservation, 1 month) to 0.15% (6 months preservation). Tropical storage conditions will generally require higher concentrations (0.1 to 0.2%).
Many tanners now use TCMTB and MBT-based fungicides. It is important that the fungicide is obtained from a reputable supplier because although they are highly active against moulds, they can possess some handicaps: for example the thiocyanates could be contaminated with halogen methyl-thiocyanates and isothiocyanates, which are lacromators, causing skin and mucous membrane irritations in tanneries.
Also traces of thiocyanate ions, impurities from the production process, can give rise to red discolouration with iron ions that may be present on wet-blues. OITZ and TCMTB are chemically different, but share certain characteristics: they are both highly effective but can cause skin irritation amongst some workers; they both exhaust rapidly from the process liquor; both are lipophilic and can be prone to inconsistencies arising from process variables.
Some moulds are resistant to certain types of fungicide. However, fungicide failures are often related to application and processing errors.
Selection of a suitable fungicide requires consideration of the material to which it is to be applied and the conditions under which the material is to be stored. Tanners should liaise closely with their fungicide suppliers to ensure both optimum selection and application conditions, as well as ensuring that the fungicides are handled with due regard to health and safety, ie using appropriate personal protective equipment.
BLC can offer wide-ranging testing and problem solving support relating to all aspects of fungicides.
Fungicides by chemical type
Phenolics
CMC               para-chloro-meta-cresol
OPP               ortho-phenylphenol
TCP               2, 4, 6-trichlorophenol
Heterocyclic compounds
TCMTB          2-(thiocyanomethylthio) benzothiazole
OITZ              2-n-octylisothiazolin-3-one
BMC              2-benzimidazolyl-methylcarbamate
MBT              2-mercaptobenzothiazol
P                   sodium pyrithione
Other
DIMTS           diiodomethyltolyl-sulphone
Principal moulds encountered in the leather industryAspergillus sp.
Mucor sp.
Paecilomyces variotii
Penicillium sp.
Rhizopus nigricans
Trichoderma viride
Reference

  • 1. C Hauber, H P Germann, 1997. The addition of fungicides in chrome tannage and their penetration, absorption and distribution in the wet-blue. World Leather, May 1997, pp75-82
  •   For further information contact BLC Leather Technology Centre Ltd on info@blcleathertech.com