Since the EU Reach chemicals regulation (EC 1907/2006) came
into force, a whole series of environmentally harmful
biocides has disappeared from cooling lubricants. This is
good news, but it is now gradually becom-ing apparent that,
while containing fewer biocides, the new emulsions are
seemingly less compatible with hard plastics and elastomers.
Seals and other components made of these materials are less
resistant to the new coolants than to previous formulations.
"Our companies are reporting increasing problems here. And
so we have launched various initiatives," reports Torsten
Bell, a member of the Technology and Research Depart-ment at
VDW (Verband Deutscher Werkzeugmaschinenfabriken e. V. –
German Machine Tool Builders' Association).
Buckling seals
Since the Reach Regulation came into force, biocides which
had com-monly been used in the past have now disappeared
from the market. "Only a dozen or so are now allowed," says
Bell. "The industry is therefore looking for alternative
ways to keep the microorganisms in check, especially in
aqueous lubricants. Emulsions with alkaline pH values are
frequently used, for example. However, this can have an
effect on the polymer materials commonly used in machine
tools."
According to Bell, seals and O-rings, hoses, wipers, toothed
or other drive belts and even polycarbonate protective
screens (such as those used in cutting machines) are
particularly affected.
Buckling is often observed, for example, in linear seals on
roller blinds and other kinematic coverings. The seals in
glass scales, which are used to measure the travel distances,
can also fail. As a result, the scales become dirty and can
no longer measure accurately. Another problem, according to
Bell, is improper handling of the cooling lubricant. "If,
for example, a machine which is being used with an emulsion
heats up during operation, some of the water will then
evaporate. This and other ingredients in the emulsion must
be regularly readjusted. In one case we heard of, this was
presumably not being done and the proportion of oil in the
lubricant became more concentrated – rising from seven to 20
per cent. There were significant adhesions and deposits in
the machine and the seals and toothed belts failed after
very short periods of time."
Threat of long downtimes
Considerable damage can result. After all, it is not always
a single seal which needs to be replaced: multiple seals and
other machine components such as cables may also be affected.
The machine itself and, in the worst case, the entire
production can be brought to a standstill until they have
all been replaced. In addition, there is a risk of defective
seals damaging other machine parts, such as the machine
spindle.
The failures can usually be prevented by switching to other
materials: to FKM instead of NBR, for example. "With
standard components such as O-rings, good results can also
be achieved by using alternative materials," says Bell.
However, the mechanical and other technical properties also
need to meet the requirements. Furthermore, FKM, for example,
is much more expensive than polyurethane (PU) which is
otherwise commonly used. This is why some components made of
certain materials are not even sold on the mass market.
A standard for cooling lubricant compatibility
But how can plastic resistance to cooling lubricants be
ensured? "Many lubricant suppliers are not prepared to
guarantee the universal stability of their coolants. Using
different materials in the machines for each individual
project is, however, too costly for machine tool
manufacturers. Guidelines would therefore be helpful.
Cooling lubricant manufacturers, for example, could
authorise the use of their media with certain plastics. "It
would be even better if manufactur-ers of components such as
seals were to authorise their products for use with cooling
lubricants in general or for certain CL classes,"
suggests Bell
For this reason, the VDW supports the use of immersion tests
for certain coolant classes initiated by the Verband der
Schmierstoffindustrie (VSI - German Lubricant Industry
Association). The VSI has defined a series of sample
coolants which can be used for testing the CL stability of
plastics. These can be obtained from the VSI members. "These
sample coolant lubricants will serve as the basis of an
eventual standard, which is now to be tackled as the next
step," explains Bell. Machine tool manufacturers as well as
component and coolant manufacturers would then have a set of
regulations at their disposal that confirms the stability of
plastics in use with the more environmentally friendly
lubricants.
Focus on elastomers and hard plastics
However, this is only one of three steps being taken by the
VDW. With regard to elastomers, more is needed than purely
static exposure in the form of immersion tests. The Deutsche
Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.
V. (DGMK – German Society for Petroleum and Coal Science and
Technology) and VDW-Forschungsinstitut e. V. have initiated
a research project that is being carried out by the
Laboratory for Machine Tools and Production Engineering (WZL)
of RWTH Aachen University and the Institute of Machine
Components (IMA) of the University of Stuttgart. The project
is being supervised by VDW's Working Group 5 "Machine Tool
Design". The aims are to obtain a better understanding of
the interactions between the elastomers used in machine
tools and cooling lubricants, and to develop a rapid test
for the elastomer compatibility of coolants. The researchers
have selected three exemplary elastomer components for this
purpose: radial shaft seals, O-rings and coolant hoses.
Finally, a project financed from VDW funds has been running
for a year at the Institute for Machine Tools and Factory
Operation (IWF) at TU Berlin. The focus of this project is
on the stability of polycarbonate guards in contact with the
new lubricants. Tests are currently underway to determine
the extent to which polycarbonate panels can withstand
contact with coolant lubricants. Of practical interest here
are flat viewing panels, but also cold or hot-formed curved
panels.
"Due to their curvature, these cannot be protected
from CL contact by a safety glass panel in the working area,"
explains Bell.
"Protection is therefore more complex, for
example in the form of a durable coating. The trials
determine the retention capacity of artificially aged
polycarbonate in impact tests, for example." The VDW expects
the results of these trials to be available by 2021.
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The German machine tool industry ranks among the five
largest specialist groupings in the mechanical engineering
sector. It provides production technology for metalworking
applications in all branches of industry, and makes a
crucial contribution towards innovation and enhanced
productivity in the industrial sector as a whole. Due to its
absolutely key role for industrial production, its
development is an important indicator for the economic
dynamism of the industrial sector as such. In 2018, with
around 73,500 employees (annual average in 2018, companies
with more than 50 employees), the sector produced machines
and services worth 17.1 billion euros.
