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SUSTAINABLE TECHNOLOGIES INITIATIVE
Confidential Report
Feasibility of a New Approach to Tin-Based
Material Blends for Brake Pads
STI Reference No: STI3/027
Work Package 1 – Literature Review
For:
Mr David Wilkinson
Business & Environmental Directorate
Department of Trade and Industry
Room 404
151 Buckingham Palace Road
London, SW1W 9SS
By:
ITRI Ltd
Date:
Tuesday, 15 April 2003
Proposal No:
CTD/DEC01/FC11/A
th
Author:
Project Acronym: TIBRAKE
Date:
Dr Paul Cusack, Research Manager – Chemicals Technology Division
Released:
Date:
Dr Ian McGill, Research Director
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Contents
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1. INTRODUCTION.................................................................................................. 1
2. TIN IN FRICTION MATERIAL FORMULATIONS .............................................. 1
2.1. Tin and tin alloys............................................................................................................... 1
2.2. Tin oxides........................................................................................................................... 2
2.3. Tin sulphides ..................................................................................................................... 2
3. FUNDAMENTAL ASPECTS OF TIN – SULPHUR CHEMISTRY ..................... 3
3.1. Tin sulphides ..................................................................................................................... 3
3.1.1. Tin(II) sulphide................................................................................................................ 3
3.1.2. Tin(IV) sulphide .............................................................................................................. 4
3.1.3. Tin(II) tin(IV) sulphide .................................................................................................... 4
3.2. Mixed metal sulphides ..................................................................................................... 4
4. SUMMARY & RECOMMENDATIONS ............................................................... 5
5. REFERENCES..................................................................................................... 6
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1. INTRODUCTION
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Environmental concerns .have been raised regarding the use of lead and antimony
sulphides as friction stabilisers
in brake pad formulations. Tin sulphides have been shown
.
to be technically superior alternatives and are regarded as being more environmentally
acceptable than the lead and antimony compounds. Although tin sulphides already find
some usage in this application, their relatively high cost and somewhat limited supply
have restricted market growth.
The one-year TIBRAKE project explores the possibility of utilising an innovative approach
in which the heat and pressure generated within the brake pad during normal driving and
braking processes are used to form the active tin sulphide species in situ within the pad.
Work Package 1 of the TIBRAKE programme involves the compilation and review of
technical papers and patents that are of relevance to the project objectives. The first part
of this report reviews the use of tin and its compounds in friction material formulations,
with particular emphasis on tin sulphide systems. The second part of the report gives an
overview of fundamental inorganic tin – sulphur chemistry, highlighting specific
information that will be useful with regard to formulation and characterisation of the tin –
sulphur prototype brake pad products.
2. TIN IN FRICTION MATERIAL FORMULATIONS
Although precise details of brake pad formulations are rarely discussed in the open
literature because of proprietary concerns, tin is mentioned widely in the patent literature
on friction materials. Conventional friction materials are principally composed of the
following ingredients – metals (as fibres or powders), fillers (usually inorganic powders or
fibres), solid lubricants, and organic components (e.g. resins, rubbers, organic fibres). Tin
may be present as a metal (either elemental tin or a tin alloy), as an inorganic filler (e.g. a
tin oxide, SnO2 or SnO) or, more pertinently to this project, as a metal sulphide lubricant
(SnS2 or SnS).
2.1. Tin and tin alloys
Numerous patents have been filed by Japanese companies, relating to the inclusion of
elemental tin or tin alloys in sintered friction materials. Nippon Funmatsu Gokin filed a
series of Japanese Patents throughout the 1980s, each claiming a friction material
exhibiting high friction coefficient and superior wear resistance, and utilising either tin
powder,1-4 or a tin – zinc mixture,2-4 at levels of 1 – 20% in a formulation that also
contained copper, graphite, solid sulphide lubricants, metal oxides and hard nitrides.
More recently, Sumitomo Electric Industries have produced sintered brake pad materials
comprising a copper alloy base, optionally containing tin powder, along with numerous
other additives including intermetallic iron alloys, graphite, potassium titanate and a
thermosetting resin.5-7 The sintered products are claimed to have good abrasion
resistance, high friction coefficient, excellent material strength and good squeal
resistance.
1
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Aisin Kako have claimed.a dry friction material showing excellent friction characteristics,
and which comprises a fibre
base material, a bonding agent, a friction conditioner and
.
8
filler. In this case, a tin alloy
. having a fusing point higher than 5000C and a hardness of
less than 4 Mohs is included
. in the formulation at a level of up to 20 % by weight. Typical
tin alloys used include Cu-Sn,
. Cu-Sn-Ni, Cu-Sn-Sb, and Cu-Sn-Zn, each being in the
form of a powder or a fibre.
2.2. Tin oxides
Tin(IV) oxide, SnO2, is a relatively hard and abrasive material, having a hardness of 6.8
Mohs. As long ago as 1980, SnO2 was patented as part of a formulation used for rubberbased brakes for bicycles.9 The tin oxide is used at a level of between 50% and 75% with
respect to the total weight of the friction material and its inclusion is claimed to eliminate
the deterioration of the braking effect in rainy weather.
Very recently, a friction material matrix has been described, containing a tin oxide (e.g.
SnO or SnO2) and a binder system.10 The tin oxide, preferably present in an amount of
0.3 – 20 weight %, is claimed to improve the performance behaviour of a friction couple at
the interface between the friction material matrix and an automotive braking element.
2.3. Tin sulphides
Interest in the use of tin sulphides in friction materials has grown since the early 1990s. A
series of patents by the Austrian company, Chemetall (formerly Chemson PolymerAdditives), relate to solid lubricants for brake and clutch liners, based on tin(IV) sulphide,
SnS2, used in conjunction with other metal sulphides11,12 including Cu2S, TiS2, V2S3, MnS,
FeS, ZnS, MoS2, WS2, Sb2S3 and Bi2S3, or with carbon-based solid lubricants.13
It is claimed that a number of bimetal and trimetal sulphides, prepared either by dry
thermal reaction of metal powders with sulphur or polysulphides, or by aqueous reaction
of metal hydroxides or salts with ammonium sulphide or hydrogen sulphide, give
generally superior properties to single metal sulphide mixtures of analogous
composition.12 Typical compounds described include Mn2SnS4, Fe2SnS4, Cu2SnS3,
Cu3SnS4, Cu5SnS4, Cu5Sn2S7, Cu2FeSnS4 and Cu2FeSn3S8.
The most recent patent from Chemetall describes a method for producing a novel
carbon-containing tin sulphide product, involving dry reaction of tin powder, sulphur (in
excess above the stoichiometric ratio for producing SnS2) and graphite, under an inert
atmosphere at temperatures in the range 200 – 8000C for 6 hours.13 Interestingly, the
product is found to contain both tin(IV) and tin(II) sites in a mixed valence sulphide matrix.
The product is suggested as an environmentally preferred tribological replacement for
antimony trioxide.
Chemetall have also published a comprehensive technical paper in which
thermoanalytical and tribological studies of a wide range of metal sulphides have been
correlated.14 The report concludes that tin sulphides give a high and constant friction
coefficient even at high speeds, they are soft and are not easily oxidised to form abrasive
tin oxides, and are therefore acceptable substitutes for antimony sulphide.
2
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. (formerly TMD Friction, and BBA Friction) has also filed
The German company, Textar
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patents relating to the use. of tin sulphides (either SnS or SnS2) as solid lubricants.15 It is
claimed that, in addition .to their reduced toxicological hazard compared with antimony
trisulphide or lead sulphide,
. the tin sulphides exhibit superior wear resistance and a
substantial reduction of the
. susceptibility to fissures of the frictional counterpart of the
friction lining. More recently, tin sulphides have been claimed as part of a superior
performance friction lining, which also contains an aluminium – zinc alloy.16 In each case,
the tin sulphides are used at a level of 2 – 8% by weight.
Nisshinbo Industries (Japan) have very recently patented a non-asbestos friction material
which exhibits excellent wear resistance and less metal pick-up at high temperatures.17
The material is produced by moulding and curing a composition comprising a fibrous
base, a filler, a rubber-modified phenolic resin binder, and between 0.1% and 15% by
weight of tin and / or a tin sulphide (SnS or SnS2).
3. FUNDAMENTAL ASPECTS OF TIN – SULPHUR CHEMISTRY
3.1. Tin sulphides
Three distinct simple tin sulphides are known – tin(II) sulphide (SnS), tin(IV) sulphide
(SnS2) and a mixed valence tin(II) tin(IV) sulphide (Sn2S3). Some key physical properties
of these compounds are given in Table 1.
Table 1 – Physical Properties of Tin Sulphides
Chemical
Formula
(CAS Reg.
Number)
SnS
(1314-95-0)
Mol.
Appearance
Weight
SnS2
(1315-01-1)
182.8
Sn2S3
333.6
150.8
Crystal
Structure
Grey-black Orthorhombic
‘metallic’
crystals
GoldenTrigonal
yellow
layered
crystals
structure
Grey-brown Orthorhombic
‘metallic’
crystals
Specific
Mohs
Gravity Hardness
Melting
Point
(0C)
Boiling
Point
(0C)
5.22
2
882
1230
4.50
1-2
600
(decomp)
--
4.87
?
745
(decomp)
--
3.1.1. Tin(II) sulphide
Tin(II) sulphide can be prepared using dry or wet chemical methods. Hence, SnS is
formed by the direct reaction of molten tin metal with sulphur at a temperature of 4000C,
in the presence of graphite to prevent oxidation.18
Sn + S (+ C)  SnS
3
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. reacts with an excess of anhydrous
Alternatively, tin(IV) oxide
. of 4500C.19
thiocyanate at a temperature
.
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SnO
. 2 + 2KSCN  SnS + K2S + 2CO + N2
.
molten potassium
In aqueous solution, reaction of tin(II) salts with either hydrogen sulphide or, preferably,
sodium sulphide, leads to precipitation of SnS.20
SnCl2 + Na2S  SnS + 2NaCl
3.1.2. Tin(IV) sulphide
Tin(IV) sulphide can also be prepared using either dry or wet chemical methods. Thermal
reaction of tin (usually in the form of a tin - mercury amalgam) with excess sulphur and
ammonium chloride can be used.21
Sn + 2S (+ NH4Cl)  SnS2
A recently patented method for synthesising SnS2 involves the reaction of tin(II) sulphate,
tin(II) formate or tin(II) oxalate, with elemental sulphur at a temperature in the range of
450 – 7500C, in air.22
SnSO4 + 3S  SnS2 + 2SO2
Aqueous reaction of tin(IV) chloride solution with either hydrogen sulphide or sodium
sulphide, results in precipitation of SnS2.23
SnCl4 + 2Na2S  SnS2 + 4NaCl
3.1.3. Tin(II) tin(IV) sulphide
The most convenient route for preparing Sn2S3 involves the heating of a powdered
mixture of the stoichiometric proportions of tin and sulphur to 7200C in a sealed tube.24
2Sn + 3S  Sn2S3
3.2. Mixed metal sulphides
Apart from the simple tin sulphides described above, a large number of mixed metal
sulphides have been synthesised and characterised (see Table 2).
Table 2 – Complex Tin Sulphides
Tin(II)
BaSnS2
BaSn2S3
Tl2Sn2S3
Sb2Sn2S5
Ga2Sn2S5
P2Sn2S6
Tin(IV)
SnS32SnS44SnS56Sn2S64Sn2S76Sn3S84-
Mixed Valency
Sn(II)4Sn(IV)Sb2S9
* M = Na+, K+, Ca2+, Sr2+, Ba2+, Cu2+, Mn2+, Fe2+, La3+, Eu3+, etc.
4
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. include not only binary metal systems, but also tertiary
These complex sulphides
. , Cu FeSn S and BaCdSnS .
sulphides, such as Cu2FeSnS
4
. 4 2 38
.
Interestingly, iron sulphides
effective than
. (FeS and FeS2) have been claimed to be more
25
elemental sulphur as sulphurising
agents for SnO, SnO2 and metallic tin. Furthermore, it
.
has been shown that tin sulphide species are formed at temperatures that would be
encountered at the braking interface (700 – 8000C), and that this formation is accelerated
in a reducing environment.25 The presence of reducing agents such as graphite, for
example, may affect the chemistry by controlling the extent and reversibility of oxidation
reactions. Hence, tin oxides will reduce to tin metal in the presence of carbon, and tin
metal will combine with sulphur or metal sulphides under appropriate conditions of
temperature and pressure, to form tin sulphide species.
The precise nature of the tin sulphide species formed during reaction of tin, tin alloys or
tin compounds with metal sulphides is uncertain, and will depend on many factors
including temperature, pressure, ratio of the components, presence of other additives,
etc. A comprehensive study of the Fe-Sn-S and Cu-Sn-S ternary systems at 6000C
concluded that, whereas no ternary phases were identified in the iron system, the copper
system is more complex and at least two stable ternary Cu-Sn-S phases exist.26
However, as previously stated, claims have been made that ternary or quaternary
sulphide systems, including Fe2SnS4, Cu5SnS4 and Cu2FeSnS4, may be more effective
than simple tin sulphides when used as friction stabilisers in brake pad formulations.12
4. SUMMARY & RECOMMENDATIONS
Tin, its alloys and compounds, have all found use in friction materials. Interest in tin
sulphides as friction stabilising agents in brake pad formulations has grown recently,
primarily because of environmental concerns regarding conventional lead and antimony
sulphides. Furthermore, there are claims that tin sulphides are superior products
technically, particularly with regard to high and constant friction coefficient, superior wear
resistance and reduced susceptibility to fissures. SnS and SnS2 have both found
commercial application, typical incorporation levels being in the range 2 – 8% by weight.
Inorganic tin – sulphur chemistry is fairly complex. Apart from the simple sulphides, SnS,
SnS2 and the mixed valence, Sn2S3, a number of mixed metal sulphides have been
synthesised and characterised, and these may be relevant to the potential formation of tin
– sulphur species in situ within the brake pad during normal use.
Thermoanalytical techniques, specifically thermogravimetry (TG) combined with
differential scanning calorimetry (DSC), provide a useful means of studying thermal
reactions, and have previously been utilised in studies on metal sulphides.14 A similar
approach is recommended for initial investigation of multi-component systems,
comprising powdered mixtures of tin (or its alloys / compounds), sulphide sources
(elemental sulphur or metal sulphides), and carbonaceous additives (e.g. graphite or
polymeric materials). These experiments, which will be undertaken within WP 3 of the
TIBRAKE programme (‘Thermal Analysis’), should be carried out using various ratios of
the reactants, at a heating rate of 100C per minute (from ambient to 10000C), both under
air and nitrogen.
5
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.
.
. it is evident that there should be an excess of the sulphide
Regarding component ratios,
.
source above the stoichiometric
level that is theoretically required to convert all of the
.
starting tin species to tin. sulphide. Depending on the homogeneity of the powdered
mixture and the subsequent
. reactivity of the components, it is estimated that an excess of
sulphur / metal sulphide over
. tin (compound) of at least twice the stoichiometric ratio, and
possibly as much as five times this ratio, may be required to fully convert the tin to tin
sulphide.
Chemical analysis of the reaction products may be carried out using X-ray diffraction
(solid phase) and evolved gas analysis by FT-infrared spectroscopy (vapour phase).
Thermal analysis should also be used to investigate the chemical interactions that occur
in prototype brake pad formulations, both before and after being subjected to dynometer
tests. Electron microscopy (SEM / EDX) and chemical analysis should be used to
examine test pads in order to determine any changes that occur at the brake surface,
particularly with regard to tin and sulphur contents.
5. REFERENCES
1. Y. Tomiyama, K. Saito & H. Oyabu, Japan Patent Appl. 58,213,740 (1983).
2. Y. Tomiyama, K. Saito & H. Oyabu, Japan Patent Appl. 60,047,062 (1985).
3. Y. Tomiyama, K. Saito & H. Oyabu, Japan Patent Appl. 61,102,390 (1986).
4. Y. Tomiyama, K. Saito & H. Oyabu, Japan Patent Appl. 61,253,985 (1986).
5. K. Kondoh & Y. Takano, Eur. Patent 709,476 (1995).
6. K. Kondoh & Y. Takano, Eur. Patent 731,287 (1996).
7. T. Takemoto & Y. Yamashita, US Patent 6,004,370 (1999).
8. K. Morita, T. Matsukawa & M. Harada, US Patent 5,308,392 (1994).
9. M. Takamatsu, US Patent 4,189,424 (1980).
10. S.K. Kesavan, US Patent Appl. 2003/0,059,645 (2003).
11. I. Buxbaum, L. Buxbaum & M. Geringer, Intern. Patent PCT WO 95/02657 (1995).
12. M. Geringer, US Patent 5,958,846 (1999).
13. R. Huner, B. Melcher, R. Milczarek & H. Kienleitner, US Patent 6,303,545 (2001).
14. B. Melcher & P. Faullant, SAE Technical Paper Series No. 2000-01-2757, SAE
International, Warrendale, PA, USA (2000).
15. M. Hell, Eur. Patent 654,616 (1995).
16. M. Hell, W. Jaworek, W. Huppatz & D. Weiser, US Patent 6,481,555 (2002).
17. K. Takeuchi, T. Nagata & K. Tsugawa, Eur. Patent Appl. 1,227,262 (2002).
18. D.N. Klushin, O.V. Nadinskaya & K.G. Bogatina, J. Appl. Chem. USSR, 1959, 32,
284.
19. G. Brauer, Handbuch der Praparativen Anorganischen Chemie, Stuttgart, Germany,
1960, p.656.
20. G. Tocco & N. Jacob, Gazz. Chim. Ital., 1924, 54, 32.
21. H.E. Swanson, H.F. McMurdie, M.C. Morris, E.H. Evans & B. Paretzkin, Natl. Bur.
Stds. (U.S.) Monogr. No. 25, 1971, p.57.
22. D. Guhl & V. von Drach, US Patent 6,187,281 (2001).
23. K.P. Dubey & S. Ghosh, J. Indian Chem. Soc., 1962, 39, 169.
24. P.P. Seregin & M. Vasilev, Inorg. Materials (USSR), 1973, 9, 119.
25. D.N. Klushin, O.V. Nadinskaya & K.G. Bogatina, J. Appl. Chem. USSR, 1965, 38,
962.
26. G.H. Moh, Carnegie Inst. Washington Yearbook, USA, 1962, p.197.
6