Bearing material

Abstract

A plastic bearing material of a bearing may include a block copolymer. The block copolymer may include structural blocks of two or more different polymers. A first polymer may include at least one of polyamide-imide and polybenzimidazole. A second polymer may include at least one of polydimethylsiloxane, methyl vinyl ether and polyisobutene. A method for manufacturing a plastic bearing material may include synthesizing a structural block of a first polymer and a structural block of a second polymer to define a block copolymer via a condensation reaction.

Claims

1. A plastics bearing material, comprising: a block copolymer including structural blocks of two or more different polymers; wherein the structural block of a first polymer includes at least one of polyamide-imide and polybenzimidazole.

2. A plastics bearing material according to claim 1, wherein the first polymer is polyamide-imide.

3. A plastics bearing material according to claim 1, wherein the first polymer is polybenzimidazole.

4. A plastics bearing material according to claim 1, wherein the structural block of a second polymer includes a polymer having a lower hardness than the first polymer.

5. A plastics bearing material according to claim 1, wherein the structural block of a second polymer includes at least one of polydimethylsiloxane, methyl vinyl ether and polyisobutene.

6. A plastics bearing material according to claim 4, wherein the block copolymer contains more than one of 5 wt %, 10 wt % and 15 wt % of the second polymer.

7. A plastics bearing material according to claim 4, wherein the block copolymer contains less than one of 50 wt %, 40 wt % and 30 wt % of the second polymer.

8. A plastics bearing material according to claim 4, wherein the structural block of the second polymer has a molecular weight greater than one of 500Da, 1000Da, 1500Da, 2000Da, and 2500Da.

9. A plastics bearing material according to claim 4, wherein the structural block of the second polymer has a molecular weight less than one of 1500Da, 2000Da, 2500Da, 3000Da, 3500Da and 4000Da.

10. A plastics bearing material according to claim 4, wherein the structural block of the second polymer has an average molecular weight greater than one of 500Da, 1000Da, 1500Da, 2000Da and 2500Da.

11. A plastics bearing material according to claim 4, wherein the structural block of the second polymer has an average molecular weight less than one of 1500Da, 2000Da, 2500Da, 3000Da, 3500Da and 4000Da.

12. A plastics bearing material according to claim 1, wherein the block copolymer includes structural blocks wherein the block copolymer includes structural of two or more different copolymers that each form a domain within the bearing material.

13. A plastics bearing material according to claim 1, wherein the block copolymer includes a matrix and at least one material is distributed throughout the matrix selected from a metal powder, a fluoropolymer, a silane and a solid lubricant.

14. A plastics bearing material according to claim 13, wherein the metal powder includes aluminium in the form of flakes.

15. A bearing comprising: a plastics bearing material including a block copolymer, the block copolymer including a first structural block of a first polymer and a second structural block of a second polymer; wherein the first polymer includes at least one of polyamide-imide and polybenzimidazole, and the second polymer includes a polymer having a lower hardness than the first polymer.

16. A method for manufacturing a plastics bearing material, comprising: providing a first structural block of a first polymer, the first polymer including at least one of polyamide-imide and polybenzimidazole; providing a second structural block of a second polymer, the second polymer including a polymer having a lower hardness than the first polymer; and synthesising the first structural block with the second structural block to define a block copolymer via a condensation reaction.

17. A method according to claim 16, wherein the second polymer includes at least one of polydimethylsiloxane, methyl vinyl ether and polyisobutene.

18. A bearing according to claim 15, wherein the second polymer includes at least one of polydimethylsiloxane, methyl vinyl ether and polyisobutene.

19. A bearing according to claim 15, wherein block copolymer includes between 10 wt % to 50 wt % of the second copolymer.

20. A plastics bearing material comprising: a block copolymer including structural blocks of two or more different polymers; wherein the structural block of one of the two or more different polymers includes at least one of polydimethylsiloxane, methyl vinyl ether and polyisobutene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which;

(2) FIG. 1 is a cross-section of a plain bearing;

(3) FIG. 2 illustrates a block copolymer structure; and

(4) FIG. 3 shows tapping-mode atomic-force microscopy (AFM) images of hard and soft domains in a plastics bearing material embodying the invention.

DETAILED DESCRIPTION

(5) FIG. 1 shows a cross section through a half shell of a cylindrical sliding bearing comprising a strong backing 1 of steel, a bearing lining layer 2 of a copper-based alloy or an aluminium-based alloy bonded to the backing, and a plastics, polymer-based overlay layer 3 embodying the present invention bonded to the lining layer. In other embodiments of the invention the overlay may be bonded directly to the backing, and the lining layer omitted, depending on the compatibility of the materials used and the intended use of the bearing.

(6) A conventional plastics bearing overlay may utilise, for example, a polyamide-imide (PAI) resin matrix containing aluminium flakes and PTFE particles, and provide good fatigue and wear performance. To produce an overlay with enhanced embedability, embodiments of the invention may replace the resin matrix of the conventional overlay with a modified resin comprising a block copolymer of PAI with a softer polymer such as polydimethylsiloxane (PDMS). PDMS is soft for good embedment whilst also having stability at temperatures suitably in excess of those seen in internal-combustion-engine applications.

(7) A block co-polymer (or co-network polymer) is one where the polymer chains may consist of alternating segments of two or more different polymers. In the case of PAI and PDMS, a condensation reaction may be used to form the copolymer. FIG. 2 schematically illustrates the repeat unit of this copolymer. Each repeat unit comprises a segment or block (A) of PAI and a segment or block (B) of PDMS. The lengths of the A and B segments can be controlled by varying the molecular weights (Mw) of A and B. The molecular weights and the pattern of incorporation of each polymer into the copolymer may determine the weight percentage of each polymer in the copolymer.

(8) in the inventors' experiments, PDMS has been copolymerised with PAI in the range 10-30 wt % and the effect of block size investigated, using PDMS of 1000 and 3000 Da. Initial results indicate that the copolymer comprising PDMS of 3000 Da shows better embedability than the copolymer comprising PDMS of 1000 Da.

(9) Due to the two polymers being chemically bound to each other in the chain structure of the copolymer molecule, there is a reduced risk of failure by fatigue (i.e. there is no separate phase to act as a stress raiser, as there would be if a softer polymer were present as a separate phase or a separate molecule in a PAI matrix).

(10) As the copolymer solidifies, for example on evaporation of a solvent carrying the copolymer, the A and B phases tend to repel and move away from each other. The polymer chains contort and line up so that A segments align with other A segments and B segments align with other B segments. This leads to the creation of physically-localised domains within the polymer matrix. These may be very small nanometer-scale domains. Due to surface energy differences between the polymers, one species (A or B domains) may be slightly raised at the surface. In the case of PAI and PDMS, the PDMS creates nano-scale soft ‘islands’ on the surface of the coating. This may advantageously help with friction reduction and initial running-in.

(11) Other soft segments may be used instead of, or in combination with, PDMS such as Methyl Vinyl Ether (MVE) and Polyisobutene.

EXAMPLES

(12) In specific Examples of the invention, two samples of block copolymers comprising polyamide-imide and PDMS segments were synthesised. The samples were characterised by DMF GPC (Dimethylformamide Gel Permeation Chromatography), Solid State NMR (Nuclear Magnetic Resonance) and TGA (Thermal Gravimetric Analysis), as described further below.

(13) Synthesis Method and Formulations

(14) TABLE-US-00001 TABLE 1 Summary of materials employed in the synthesis of polyamide- imide block copolymer containing PDMS soft segments. All materials were used as received without further purification. Reagent CAS* N-methyl pyrrolidone (NMP) 872-50-4 Tetrahydrofuran (THF) 109-99-9 4,4′-Methylenedianiline 97% (DMA) 101-77-9 Triphenyl Phosphite 97% (TPP) 101-02-0 Pyridine (Py) 110-86-1 LiCl 7447-41-8 Calcium Chloride (CaCl.sub.2) 10043-52-4 Trimellitic Anhydride Chloride (TMA) 1204-28-0 Aminopropyl terminated polysiloxane (3000 g .Math. mol.sup.−1) 106214-84-0 PDMS Aminopropyl terminated polysiloxane (1000 g .Math. mol.sup.−1) 106214-84-0 PDMS *Chemistry Abstracts Service (CAS) Registry number.

(15) TABLE-US-00002 TABLE 2 Table summarises the formulations used to prepare batches of polyamide- imide block copolymer containing PDMS soft segments. Batch Reagent 8/4/14 23/4/14 TMA Chloride (g) 16.90 17.14 NMP (ml) 40 50 PDMS diamine (g/mmol) 8.0 8.0 PDMS molecular weight (g .Math. mol.sup.−1) 1000 3000 THF (ml) 40 40 DMA (g) 15.86 15.86 NMP (ml) 40 40 Pyridine (ml) 20 20 TTP (g) 26 26 LiCl (g) 2 2 CaCl.sub.2 (g) 4 4

(16) Trimellitic anhydride chloride (TMA Cl) was dissolved in N-methylpyrrolidone (NMP) using overhead mechanical stirring at ambient temperature under a constant dry nitrogen stream, typically requiring up to 20 minutes.

(17) A solution of Aminopropyl terminated polysiloxane (PDMS) of the required molecular weight in tetrahydrofuran (THF) was then added drop-wise to the stirred TMA Cl solution at room temperature under N.sub.2 and stirred for 30 minutes. THF was required to solubilise the PDMS.

(18) While still at ambient temperature a solution of 4,4′-methylene dianilene (DMA) in NMP was added with the reaction still under a constant N.sub.2 stream, addition time typically being 20-30 minutes.

(19) After the addition of DMA the reaction was heated to around 100-120° C. and evacuated for 2-3 hours to remove THF added during the PDMS diamine addition. During this heating stage an increase in viscosity was noted.

(20) After the heating stage the TPP and Pyridine catalysts were added along with the LiCl and CaCl; these were typically added one after the other in the order given with no time gaps.

(21) After the last addition the reactor was again heated and maintained at 120° C. for approximately 2-3 hours.

(22) Polymer product was isolated by initially precipitating the contents of the reactor into a large quantity (10-15 reactor volumes) of deionised water with good stirring drop-wise at room temperature. The resultant solid was collected by filtration and then either dried under vacuum or reslurried in acetone and refiltered in order to remove most of any trapped water prior to continuing the purification stage.

(23) Resultant solid polymer at this stage was further purified by redissolution in NMP and reprecipitation into a larger volume (typically 1-2 litres) of either methanol or acetone before filtering and drying to constant weight.

(24) Sample solutions were then prepared by dissolving a known mass of dry polymer in a known volume of NMP at 120° C. with stirring until a homogenous solution was formed (other solvents such as NEP may be used).

(25) Results

(26) Table 3 sets out data characterising the two copolymers, compared with a known material Rhodeftal 210ES which is a PAI resin.

(27) TABLE-US-00003 TABLE 3 PDMS PDMS Onset of M.sub.n M.sub.w block Inclusion decom- (g .Math. mol.sup.−1) (g .Math. mol.sup.−1) PD M.sub.w (wt %) position Concentration Batch a a d (g .Math. mol.sup.−1) b c (% w/w NMP)  8/4/14 33,000 64,000 1.94 1,000 ~15 431° C. 40 23/4/14 40,700 145,600 3.58 3,000 ~19 461° C. 25 Rhodeftal 13,300 30,700 2.31 0 0 437° C. — 210ES a These molecular weights were determined by DMF GPC and are quoted as polymethylmethacrylate-equivalent molecular weights. b The quantity of inclusion of PDMS as a weight percentage in the PAI-PDMS copolymer was determined by solid state NMR employing PDMS calibrants dispersed on polyethylene powder. Error approximately ± 20%. c Onset temperature of decomposition was determined by TGA analysis d Polydispersity (PD), which is M.sub.w/M.sub.n.
DMF GPC Analysis

(28) Samples were made up at approximately 5 mg/ml in DMF and analysed using a Varian 290LC employing DMF as the eluent and two Agilent PL Gel 5 μm Mixed C columns operating in series. The flow rate was lml/min and the temperature of the columns was 60° C.

(29) The system was calibrated with near-monodisperse polymethyl methacrylate standards.

(30) This analysis generated the molecular weight M.sub.n and M.sub.w and PD data in Table 3 above.

(31) Solid State NMR Analysis

(32) To correlate the signal intensity to the absolute amount of siloxane in the polymers, three spectra from a “standard” sample of Aminopropyl terminated polysiloxane (3000 gmol-1) were generated. These were made up from a known mass of the pure Aminopropyl terminated polysiloxane dispersed into a rotor (sample container) containing powdered polyethylene. This gave a calibration plot of signal response per mass of siloxane. The results are shown in Table 4 below.

(33) TABLE-US-00004 TABLE 4 Total mass of Mass of PDMS Batch material (mg) PDMS (mg) content wt %  8/4/14 101 15 ~15 23/4/14 99 19 ~19 Rhodeftal 210ES 143 0 0
TGA Analysis

(34) Samples were analysed using a Perkin Elmer Pyris 1 TGA instrument to assess whether copolymerisation with PDMS had adversely affected the high thermal stability of the PAI, which is desirable in a bearing overlay. The temperature of onset of thermal degradation for the Rhodeftal 210ES as measured was 437° C. The corresponding temperatures for batches 8/4/14 and 23/4/14 were 413° C. and 461° C. In addition, the imidisation temperatures for the Rhodeftal 210ES and the two batches were all the same, at 200-250° C. These results indicate that the copolymers advantageously retained the thermal stability of the PAI.

(35) AFM Analysis

(36) AFM (Atomic Force Microscopy) analysis of spray-cast and cured films of sample 8/4/14 indicated that phase separation of the PDMS blocks in the PA matrix, to form physically-localised domains, occurs as expected.

(37) Tapping mode AFM was used to obtain information on the height and phase of the surface of sample 8/4/14. FIG. 3 shows tapping mode AFM images of height and phase differences, indicating spatial separation of hard and soft copolymer domains on the bearing material surface. The light contrast in the images indicates raised areas, or domains, of the sample surface, which correspond to softer domains of the copolymer.