Cooling Lubricant Compatibility of Elastomers and Plastics

In the meantime, reports of incompatibilities have increased again. They have now reached greater numbers than were ever reported in the past. Why is that? In many cases the EU Chemicals Regulation prescribes the use of substitutes or prohibits the use of chemicals with a negative impact either on human health or the environment. In the area of cooling lubricants, this often concerns the biocides previously used to counter the growth of microorganisms, mainly in water-mixed lubricants. The number of biocides still permitted is now down to nearly single figures, which is why the industry is now taking alternative paths. Emulsions with pH values above 9 are often used today, which in turn can have an effect on common polymer materials.

Practical examples: The first case concerns a polyurethane (PU) toothed belt on which the tool stores of the chain magazines of machining centres are mounted. The problem here is that drops of cooling lubricant from the machine's work space enter the tool magazine and the toothed belt via the replaced tools. If the water evaporates there, the concentration increases, which can attack the belt and cause it to decompose. In this specific case, changing to a different belt manufacturer (different PU material) remedied the situation.

Basically, the machine tool manufacturer is convinced that the oil content in semi-synthetic coolant lubricants is constantly being reduced, thereby increasing the risk of corrosion and reducing the lubricity of the coolant. At the same time, the number of unknown additives in cooling lubricants is increasing, which in turn has a negative effect on seals in rotary unions, telescopic covers or enclosures. The only remedy is often to substitute a different material.

A second example concerns seals on kinematic coverings and glass scales which buckle as a result of the CL contact and therefore no longer fulfil their sealing function. The machine manufacturer does not select the seals himself, either for the kinematic coverings or the glass scale coverings. These are actually supplier components that are also installed in large numbers on other machines where they usually cause no problems. The machine concerned was operated in Asia with a CL from a large manufacturer headquartered in Germany.

As far as the machine manufacturer is aware, the CL manufacturer also offers other products in Asia. The CL in question is not available in Germany. The machine manufacturer has not been informed of the eventual solution to the problem but, on the basis of the information available to him, it is highly likely that it was an additive in the CL that attacked the seals. No data sheet was available for the CL. However, it cannot be ruled out that the operator may have added something to the coolant lubricant in this case.

Such cases are by no means limited to distant markets, as a further example involving buckled seals reveals. The machine in question is located in Germany and is operated with a 7-per cent emulsion. As a result of the powerful suction in the machine, 30 to 40 litres of cooling lubricant are consumed and refilled per shift. Before the CL was replaced there was a 20 per cent CL concentration in the coolant tank. There were significant adhesions and deposits in the machine and the seals and toothed belts were failing after very short periods of time.

The following components are most commonly affected by CL compatibility:

• Seals and O-rings,
• (CL) hoses,
• Toothed and other drive belts,
• Wipers, polycarbonate viewing panels, etc. in the working area of the machine.

The failures can usually be eliminated by changing the material. Good results can be achieved with alternative materials for standard components such as O-rings (e.g. FKM instead of NBR). It is more difficult to ensure CL stability when new suppliers or new components are used. In many cases, suppliers are not prepared to guarantee general CL stability. The response to requests for information is references to immersion tests in a defined medium. However, these are of little relevance in practice. On the one hand, in many cases it is not known which medium the end users are actually using. On the other hand, the cost of equipping the machines with different materials for specific projects would be far too high. Appropriate guidelines would therefore be helpful for machine tool manufacturers. A conceivable solution would be for CL manufacturers to authorise certain material classes for their media. It would be even better if component manufacturers were to authorise their products for CLs in general or for certain CL classes.

High speed film of an impact test on aged polycarbonate with a defined projectile. Film courtesy of Institute for Machine Tools and Factory Management (IWF) of the TU Berlin

For some time now, three approaches have been adopted in an attempt to solve the problems described above. The VDW and the Verband Schmierstoff-Industrie e. V. (VSI – Association of the Lubricant Industry), Hamburg, have been cooperating on these. The VDW is involved in Working Group 5 "Machine Tool Design".

1.) Since immersion tests with individual media are only helpful in exceptional cases, the VSI is concentrating on specific cooling lubricant classes in the tests it proposes. The aim is to identify groups of cooling lubricants that are compatible with the materials under consideration. To this end, the VSI and a number of companies defined a series of sample state-of-the-art CLs which contain effective concentrations of the additives attacking the plastic (worst case approach). Following the end of the validation phase, the aim is now to anchor the procedure in a standard. This should enable any laboratory to perform the tests in a valid and reproducible manner. For this purpose, the relevant model CLs should be supplied against payment. Producers of plastic and elastomer materials are also involved alongside the manufacturers of cooling lubricants and machines.

2.) With regard to elastomers, more is needed than purely static exposure in the form of immersion tests. A research project is currently being set up by VDW-Forschungsinstitut e. V. and Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e. V. (DGMK – German Scientific Society for Petroleum, Natural Gas and Coal). The university institutes involved are the Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University and the Institute of Machine Components (IMA) at the University of Stuttgart. The latter has many years of experience with the use of technical elastomers as construction materials in machines and systems. The aim of the project is to make a practical contribution to understanding the interactions between elastomers and the cooling lubricants used in machine tools and to establish a test methodology for the rapid testing of elastomer compatibility with coolant lubricants. In addition to the storage tests, the mechanical stress of the samples is therefore also being considered in analogy tests, and the results validated by examining actual components such as O-rings, media guides and radial shaft seals (functional tests, pressure swing tests, etc.)

3.) The VDW's internally-funded project "KSS-PC", which has been running at the Institut für Werkzeugmaschinen und Fabrikbetrieb (IWF – Institute for Machine Tools and Factory Management) of the TU Berlin for roughly a year now, is investigating the effect of new types of CLs with modified ingredients on the long-term stability of polycarbonate as a separating protector in machining. The focus here is not only on the cooling lubricants which have been changed as a result of the REACH regulation, but also on new technologies, for example on cold formed (bent) PC viewing panels, in the use of polycarbonate. Due to their curvature, these cannot be protected from CL contact by a safety glass pane in the working area. The protection must therefore be provided differently, e.g. in the form of a durable lacquer coat. The retention capacity of artificially aged polycarbonate is determined in the impact test and the elongation at break is determined in the tensile test. In addition, tests are carried out with regard to environmental stress cracking (ESC) with reference fluids. The aim is to gain further insights into the ageing behaviour of PC under the changed conditions. Once the preliminary VDW-funded study is complete, public funding will be sought for the further investigation of the long-term effects.

Header image: Walter Maschinenbau, Tübingen