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desorption. If the affinity is very high, desorption with water is not always possible.
If after the cleaning process, this surface comes into contact with aproduct with
higher desorption capacity, the residues are released from the surface and replaced
by the new product.
Surveying adsorptive layers is extremely difficult because acleaner is never a
single substance, but amixture ofseveral components. These components have
differentaffinities to the surface and compete in the formation of an adsorbed layer.
For example, when treating a surface with nitric acid, a nitrate layer is
formed on asteel surface. If amixture ofphosphoric and nitric acid is applied,
the phosphate layer is preferentially formed, since the phosphate ion has ahigher
affinity to stainless steel. These monomolecular layers arc not accumulative, which
means they will not build up to form layers with increasing thickness (1).
Surfactants areadifferentcategory.These molecules always have hydrophilic
and hydrophobic groups. The hydrophobic group has ahigher affinity to asteel
surface than to water. Therefore, surfactants are of major concern when examining
residues of cleaners. Asurvey has shown that in case of surfactant residues of
2mg/m 2 may occur on stainless steel (2).
For example, asmall tank of 1-m diameter and 1-m height has avolume of
about 785 Land asurface of about 4m 2 .This means about 8mgofsurfactant
may remain on the surface. When filling the tank with aproduct that completely
desorbs the residues, there will be 8mgofsurfactant in 785 L, which is equivalent
to aconcentration of 0.00255 ppm. When increasing the tank size, the ratio of
volume: surface will increase, decreasing the potential contamination. The most
critical residual substances are surfactants. Nonionic surfactants have a better
solubility in cold than in hot water. When using cleaners containing such
surfactants, the rinse water should always be cold. In addition, the practice of
using demineralized water for rinsing is questionable. Alkaline residues are much
easier to rinse with normal potable water containing some water hardness. In case
of demineralized water, the rinse water consumption may be three times higher
than with hard water.
The adherence of residues to the equipment surface is aphysical process that
cannot be avoided. Therefore, itisimportant to evaluate the impact of the residues
on production. Cleaning agents for the pharmaceutical industry contain only
substances with noncritical LD 50 values. This fact and the extremely low contamination
level permit classification of the potential residues as noncritical.
Besides the adhesion of cleaning agent components to the equipment surface,
other factors can also play arole in the minimizing of residues. Those include the
hygienic design of plant equipment, appropriate surface material finish, internal
geometry, cleanability of equipment, etc.
One has to bear in mind that, whatever theoretical assumptions are made
beforehand, extensive cleaning performance testing and documenting will always
have to be carried out in practice, and successful cleaning processes rely heavily on
previous experiences and chemical supplier expertise (3,4).
ENVIRONMENTAL EFFECTS OF CLEANING AGENT CONSTITUENTS
Rosner
Several factors must be considered when developing cleaning agents. Not only are
effectiveness and performance important, but also wider issues such as toxicity of
the product and its environmental effects. Additionally, the chemical industry has
voluntarily agreed to renounce the use of certain chemical products.