Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

-1921-

PAPER REF: 5381

CHEMICALLY MODIFIED POLY(TETRAFLUOROETHYLENE) AS

ADDITIVE IN LUBRICANTS FOR CONVEYOR SYSTEMS

Thorsten Hoffmann1(*)

, Dieter Lehmann1, Martin Anders

2, Thorsten Schmidt

2, Markus Michael

3

1 Leibniz-Institut fr Polymerforschung Dresden e.V., Dresden, Germany 2 Fakultt

Maschinenwesen, Institut fr Technische Logistik und Arbeitssysteme, Technische Universitt

Dresden, Dresden, Germany 3 Fakultt Maschinenbau, Institut fr Frdertechnik und Kunststoffe, Technische Universitt Chemnitz,

Chemnitz, Germany

(*)Email: hoffmann-thorsten@ipfdd.de

ABSTRACT

During the irradiation of high molecular poly(tetrafluoroethylene) (PTFE) in presence of

oxygen perfluoroalkyl(peroxy) radicals and functional groups are formed which allow

chemical coupling reactions with oils and plastics. The micropowder resulting from the

irradiation of PTFE are used in base oils to improve the tribological properties significantly if

oil molecules are covalently linked to PTFE particles. Surface charge and the size and

distribution of PTFE particles in the oil affect the resulting properties, e.g., stability of PTFE

particles in the oil phase and, therefore, their tribological efficiency. Tribological properties of

oil-PTFE-cc dispersions (cc = chemical compatibilized) are discussed and specific

performance parameters from practically oriented tests are presented.

Keywords: PTFE, high performance lubricant, oil-PTFE-cc dispersion

INTRODUCTION

Contrary to the well-known inert properties of perfluorinated organic compounds high-

effective additives for various applications can be produced by radiation modification and

subsequent chemical modification of such substances (e.g. for conveyor systems). The

radiation modification of poly(tetrafluoroethylene) (PTFE) is an established method and has

been extensively studied at the Leibniz-Institut fr Polymerforschung Dresden e.V. [1].

The perfluoroalkyl(peroxy) radicals formed during irradiation in the PTFE micropowder can

be react in a radical reaction mechanism with olefinically unsaturated groups of

oils/lubricants. This will lead to chemically compatibilized PTFE particles with oil groups

that prevent the coagulation of the PTFE particles in the oil phase [2]. The reaction between

oil molecules and PTFE particles provokes an enhanced steric stability. In addition, the shear

strain during the preparation process causes a charge separation between oil and PTFE

particles. An electrostatic stabilization of the modified PTFE particles was achieved.

Fig. 1 - Model of the chemical compatibilization reaction (-cc = chemical compatibilized) [2]

that prevent the coagulation of the PTFE particles in the oil phase [2].

oil molecule PTFE particle

temperature, dispersion period

shear strain

functional groups which may be exist at the oil molecule

F2C

CF

F2C

oil-PTFE-cc

Symposium_21

Tribology Trends for Higher Efficiency and Reliability

-1922-

The oil-PTFE-cc dispersions possess mainly anti-wear (AW) properties, but also extreme

pressure (EP) properties can be generated by the use of additional functional groups in the oil

(Figure 1). This contribution shows the suitability of the PTFE micropowder and the chosen

base oils (trimethylolpropane trioleate (TMP), poly--olefin (PAO)) for the use as additive in

lubricants, for example, in the conveying technology sector.

The stability of oil-PTFE-cc dispersions was followed by rheological investigations and by

zeta potential measurement (electroacoustics). For general tribological tests both the VKA test

(point contact) and the Almen-Wieland test (line contact) was used. An oil-wear analysis test

on bearing systems (FE-8 test conditions: axial load = 80 kN, = 80 C, duration = 80 hours)

was performed additionally. To assess the service life of ropes bending fatigue tests and other

wear tests were carried out in different practical environments.

RESULTS AND CONCLUSIONS

To characterize modified wire ropes both visible wire breaks and broken wires within the rope

were counted until to discard (= end of service time), to evaluate the maximum operating

period (DIN 15020-2). Compared to the original lubricated wire rope less wire breaks were

found in the wire rope containing the modified lubricant. This result demonstrates the anti-

wear effect of the modified lubricant. Another application-oriented test (scuffing load test in a

sliding contact) showed an increase of sliding time until the fretting occurs for the modified

lubricant and thus ensures an improved lubricating effect.

Preliminary tests have shown that liquid lubricants based on a chemically modified oil-PTFE-

cc dispersion can increase the service life of HM (high modulus)-HT (high strength) fibre

ropes significantly (higher bending cycles to break compared to the coated HM-HT-fiber rope

with standard sizing). The successful and reproducible production of oil-PTFE-cc dispersions

is realized both in the laboratory and pilot plant scale. Correlations between the stability and

the rheological behavior of such dispersions are shown and discussed [3]. The effect of oil-

PTFE-cc dispersions as AW additive in conveyor systems has also been demonstrated.

Functional groups in the base oil can affect tribological results (especially the EP behavior) of

test products positively. For example in an oil-wear analysis an excellent wear protection

could be achieved with an oil-PTFE-cc dispersion owing additional functional groups on a FE

8 test unit. Wear dimensions of rolling elements were only 1 mg. The studies on the

optimization of AW and EP properties by chemical modification will be continued.

ACKNOWLEDGMENTS

The authors gratefully acknowledged the funding by the Bundesministerium fr Bildung und

Forschung (Nr. 03FO2172), the BMWi for funding the EXIST research transfer project (grant

No. 03EFT8SN34) and the AiF for funding under grant No. KF2954202SL3. Furthermore,

the authors thank the employees of the IME, RWTH Aachen.

REFERENCES

[1]-K. Lunkwitz, U. Lappan, U. Scheler, Jnl. of Fluorine Chemistry, 125 (2004), 863-873.

[2]-EP 2 227 528 B1 (2010).

[3]-T. Hoffmann, C. Bellmann, A. Caspari, U. Geissler, D. Lehmann, Colloids and Surfaces

A, 457 (2014), 297-306.