Mixing and processing of rubber compositions containing polar fillers

Abstract

Rubber compounds comprising one or more rubber with polar functionality and one or more polar filler are prepared by a method comprising the steps: (i) adding a polar volatile liquid to the polar filler; (ii) mixing the one or more rubber with polar functionality with the polar volatile liquid and polar filler to produce a dispersion of the polar filler in the one or more rubber; and (iii) removing the polar volatile liquid from the dispersion. The invention Improves the consistency and uniformity of mixing of rubber compounds based on elastomers having polar functionality, such as epoxidized natural rubber, containing polar reinforcing fillers, such as precipitated silica, The compounds, when vulcanized, have improved mechanical properties. The method is particularly useful in the manufacture of vehicle tires.

Claims

1. A method for preparing rubber compounds comprising one or more rubber containing polar functionality and one or more polar filler which method comprises the steps: (i) adding a polar volatile liquid to the one or more polar filler, wherein the polar volatile liquid is added to the one or more polar filler in an amount of from 0.5% to 10% by weight based on the weight of the one or more polar filler; (ii) mixing the one or more rubber containing polar functionality with the polar volatile liquid and the one or more polar filler to produce a dispersion of the one or more polar filler in the one or more rubber, wherein the one or more rubber containing polar functionality is selected from the group consisting of epoxidized natural rubber, hydroxylated natural rubber, carboxylated natural rubber, epoxidized synthetic rubbers, hydroxylated synthetic rubbers, carboxylated synthetic rubbers, carboxylated nitrile rubber, carboxylated styrene butadiene rubber, hydroxylated styrene butadiene rubber, hydroxylated butadiene rubber and mixtures thereof; (iii) removing the polar volatile liquid from the dispersion; wherein the amount of the one or more rubber containing polar functionality blended with the one or more polar filler is about 100 parts by weight of rubber per 20 to 120 parts by weight of the one or more polar filler.

2. The method according to claim 1, wherein the one or more rubber containing polar functionality is epoxidized natural rubber.

3. The method according to claim 2, wherein the epoxide content of the epoxidized natural rubber is from 5 mol % to 75 mol %.

4. The method according to claim 2, wherein the epoxide content of the epoxidized natural rubber is from 5 mol % to 65 mol %.

5. The method according to claim 2, wherein the epoxide content of the epoxidized natural rubber is from 20 mol % to 55 mol %.

6. The method according to claim 1, wherein the one or more polar filler is selected from the group consisting of precipitated silica and carbon black modified to contain polar functionality.

7. The method according to claim 1, wherein the polar volatile liquid is water, a lower alkanol or a mixture thereof.

8. The method according to claim 7, wherein the lower alkanol is methanol, ethanol or propanol.

9. The method according to claim 8, wherein the lower alkanol is ethanol.

10. The method according to claim 1, wherein the polar volatile liquid is added to the one or more polar filler in an amount of from 2 to 6% by weight, based on the weight of the one or more polar filler.

11. The method according to claim 1, wherein the rubber compound obtained after removal of the polar volatile liquid is subsequently vulcanised.

12. A product obtained by the method of claim 11 which is a vehicle tyre.

Description

(1) The present invention is based on the surprising discovery that the addition of low levels of polar volatile liquids to the compound ingredients prior to mixing, or to the rubber compound during the initial stages of mixing, allows these objectives to be met.

(2) Accordingly, the present invention provides a method for preparing rubber compounds comprising one or more rubber with polar functionality and one or more polar filler which method comprises the steps: (i) adding a polar volatile liquid to the polar filler; (ii) mixing the one or more rubber with polar functionality with the polar volatile liquid and polar filler to produce a dispersion of the polar filler in the one or more rubber; (iii) removing the polar volatile liquid from the dispersion.

(3) The use of the invention results in a more rapid incorporation of polar fillers into the rubber compound where the rubber compound is based on a rubber having polar functionality, thus allowing shorter mixing times. The use of the invention also provides an improvement in the coherence of the compound during mixing and upon dumping of the mix, thus enabling more rapid downstream processing of the compound, for instance when either a train of two roll mills or a dump extruder is used. Furthermore, the use of the invention results in a reduction of the peak Mooney viscosity and also results in improved compound stability which manifests in a more stable Mooney viscosity upon compound storage of compounds based on one or more rubbers with polar functionality containing a polar filler. Furthermore the use of the invention results in a reduction of the amount of extrudate swell of such rubber compounds, an improvement in the surface finish and definition of the edges of the extrudate and an improvement in the degree of surface tack of such rubber compounds comprising one or more rubber having polar functionality and a polar filler. In addition, the use of the invention is effective in improving the mechanical properties of the resulting vulcanised rubber compound. In particular, the Goodrich heat build-up is reduced and Akron abrasion resistance is improved.

(4) The rubber with polar functionality used in the present invention may be, for instance, selected from epoxidized natural rubber, hydroxylated natural rubber, carboxylated natural rubber, epoxidized synthetic rubbers, hydroxylated synthetic rubbers, carboxylated synthetic rubbers, carboxylated nitrile rubber, carboxylated styrene butadiene rubber, hydroxylated styrene butadiene rubber and hydroxylated butadiene rubber. According to a preferred embodiment, the rubber with polar functionality is epoxidized natural rubber. The epoxide content will typically be within the range of from 5 mol % to 75 mol %, preferably from 15 mol % to 65 mol % and more preferably between 20 mol % and 55 mol %.

(5) The polar filler for use in the present invention can be any polar filler having use in rubber compositions. Preferably, the polar filler will be selected from precipitated silica and carbon black that has been modified to contain polar functionality. When the polar filler is a precipitated silica, it will typically have a surface area between 50-250 m.sup.2/g silica.

(6) Typically, the polar volatile liquid used in the present invention will be selected from water and lower alkanols. Examples of suitable lower alkanols include methanol, ethanol and propanol. Preferably, the polar volatile liquid used in the present invention will be water or ethanol.

(7) According to the method of the invention, the polar volatile liquid is added to and mixed with the polar filler before mixing with one or more rubbers having polar functionality. Typically, the polar volatile liquid will be added to the polar filler in an amount of from 0.5% to 10% by weight, preferably from 2 to 6 wt %, based on the weight of the polar filler.

(8) The mixture of the polar volatile liquid and polar filler is then mixed with the one or more rubbers having polar functionality so that the polar filler becomes dispersed in the rubber. Mixing can be effected using any known mixing, blending or extrusion apparatus provided that this ensures that the polar filler will become dispersed, preferably homogeneously, throughout the rubber having polar functionality. Typically, the apparatus used to effect mixing will be an internal mixer such as a Banbury or Intermix mixer or a continuous mixer, such as a twin screw extruder.

(9) Typically, the amount of rubber containing polar functionality used in this mixing/dispersion step will be about 100 parts by weight of rubber per 20 to 120 parts by weight of the polar filler.

(10) In the method of the invention, polar volatile liquid present in the dispersion of the polar filler in the rubber is removed or allowed to evaporate. Preferably, any residual polar volatile liquid should be eliminated before the final stage of mixing when curatives (typically a combination of sulphur and various accelerators) are normally added to the rubber compound. The removal of the volatile polar liquid from the rubber compound is normally achieved during the course of mixing prior to the addition of curatives through normal evaporative processes. The presence of residual polar volatile liquid has a deleterious effect on the cure behaviour of the rubber compound, causing a reduction in available processing time (reduction of scorch safety) and also results in an unpredictable level of cure being achieved, as indicated by the rheometer torque rise.

(11) Indication of the effective removal of residual volatile polar liquid can be provided gravimetrically, comparing the weight of the added volatile polar liquid with the batch weight after mixing, or by weight loss measurement upon efficient drying of the rubber compound. These methods are, however, impractical in large scale mixing in industry where a rapid measurement is often required. An efficient method for determining whether residual polar volatile liquid has been effectively removed has been devised whereby the rheometry characteristics of the compound are assessed prior to curative addition. This is a rapid test using equipment commonly available in a rubber compound mixing facility.

(12) The FIGURE shows a typical rheometer traces for a silica filled ENR tyre compound containing 75 phr silica but no curatives where water has either been added at the first masterbatch mixing stage, or added at a second (final or productive) mixing stage. Water added at the first mixing stage can be subsequently driven off but water added later in the mixing process has little opportunity to be driven off and so excessive residual water still remains in the compound when it is subjected to the high temperatures experienced during curing of a tyre tread compound. The effect of residual water is clear; residual water remaining in the compound results in a significant rapid rheometer torque rise which would result in deleterious compound properties.

(13) The invention is also effective for polar rubbers containing polar fillers, such as silica filled ENR, when used in conjunction with silane coupling agents such as TESPT, or non-coupling silane agents such as octyltriethoxysilane (OTES). The invention is also effective for polar rubbers containing polar fillers, such as silica filled ENR where the silica has a moisture content outside of the normally specified range which for precipitated is between 5-7 wt % moisture. The invention extends the range of climatic regions that are able to make effective use of rubbers containing polar functionality, such as ENR filled with polar fillers such as precipitated silica, to include arid regions where absorbed moisture contents of precipitated silica may be below 5 wt %, and also to include tropical regions where absorbed moisture contents of precipitated silica may be above 7 wt %.

(14) Although we do not wish to be bound by theory, we believe the results achieved by the present invention can be explained as follows.

(15) The polar liquid (water) may be involved in disrupting the otherwise strong fillerfiller particle interactions and thus enabling the filler to be dispersed more easily in the polar polymer. The polar polymer (ENR) is still able to interact sufficiently strongly with the dispersed filler particles so that, during further mixing, dispersion is improved still further to a very high level leading to excellent micro and macro dispersion.

(16) The water (or polar liquid) appears able to modify, on a temporary basis, the nature of the interaction between the rubber and filler, inducing predominantly H-bonding rather than a stronger, more permanent covalent or physical interaction. This change in the nature of the interaction we believe allows the rubber to move around on the surface of the filler rather than being essentially fixed so that during processing (mixing, milling and extrusion) the rubber compound can flow more easily and we achieve the improvements in processing behaviour observed. The improved tack, we believe also comes from the temporary reduction in the strength of interaction between the rubber and the filler so that there are free polymer chains that are free to stick to other rubbers or other surfaces rather than be associated tightly with the silica surface only.

(17) Polar rubbers that interact with polar filler to give high bound rubber and high volume swelling ratio (Vr) undergo a significant reduction in bound rubber and Vr as a result of the addition of the water (or alcohol) but if the excess water is removed by the time the compound is ready to cure, the interaction between the rubber and filler is fully restored after curing, Vr levels are similar to those found where no water was used during initial mixing.

(18) However, it is critical for the excess water to be driven off before curing the rubber compound, as when excess water is present, elevated temperatures appear to promote excessively high degrees of interaction between the rubber and the filler which brings about a very high level of torque as observed in rheometer cure traces and very high modulus in the cured rubber compound.

(19) In the final vulcanized product, there is no evidence of anything unusual having been done to achieve the benefits we see, the water has long been evaporated off.

(20) Experimental Section

(21) The compound formulations shown below in Table 1 were prepared by simple mixing of the ingredients. The compound formulations 2 and 4 were prepared according to the method of the invention whereby a polar liquid (water) was added to one or more polar filler and then a rubber with polar functionality was blended into the polar liquid/polar filler mixture to produce a dispersion of the filler in the rubber. The excess polar liquid was removed prior to curing.

(22) TABLE-US-00001 TABLE 1 Formulation Parts Per Hundred Rubber Compound 1 2 3 4 5 First stage sSBR 96 cis-BR 30 ENR-25 100 100 100 100 Zeosil 1165MP 75 75 75 75 80 N234 Black 5 5 5 5 6.4 Nytex 4700 20 20 20 20 10 Zinc oxide 3 3 3 3 Stearic acid 3 3 3 1 2 6PPD (Santoflex 13) 1 1 1 1 TMQ 1 1 1 1 Calcium stearate 2 2 2 2 Water 3 3 TESPT 6.4 6.4 6.4 Final stage Zinc oxide 3 6PPD (Santoflex 13) 1 TMQ 1 DPG 2 CBS 1.5 Sulfur 1.8 1.8 1.6 1.6 1.5 TBBS 2.4 2.4 2.4 2.4 Abbreviations in Table 1 sSBR - solution polymerised styrene-butadiene rubber (sSBR Buna VSL 5025-2 HM) cis-BR - cis polybutadiene rubber (Europrene NEO CIS BR40) ENR-25 - epoxidised natural rubber (25 mol % epoxide units) Zeosil 1165 MP - amorphous precipitated silica filler (Solvay) N234 Black - carbon black filler Nytex 4700 (Nynas) - heavy black naphthenic tyre oil 6PPD (Santoflex 13) - N-(1,3-dimethylbutyl)-N-phenyl-p-phenylene diamine (antioxidant) TMQ - polymerised 1,2-dihydro-2,2,4-trimethylquinoline (antioxidant) TESPT - Bis[3-(triethoxysilyl)propyl] tetrasulphide (coupling agent) DPG - diphenyl guanidine (accelerator) CBS - N-cyclohexyl-2-benzothiazole sulphenamide (accelerator) TBBS - N-tert-butyl-2-benzothiazyl sulphenamide (accelerator)
Coherence/Appearance

(23) A visual assessment characterising the appearance for masterbatch compounds with water addition and/or with use of silane coupling agent, TESPT.

(24) TABLE-US-00002 TABLE 2 Appearance rating from 1-10 where 10 is good for dumped masterbatches Compound 1 2 3 4 5 Note Control 4% water 6.4 phr 6.4 phr sSBR/ at MB TESPT TESPT 4% BR water at MB Coherence 5 8 7 8 8 Ease of milling 3.5 6 5.5 6.5 7.5 Surface 3.5 7.5 6 7.5 8 appearance
Mooney Viscosity at 100 C.

(25) The Mooney viscosities were measured on the day after masterbatching, remilling and finalising.

(26) TABLE-US-00003 TABLE 3 Mooney viscosity ML (1 + 4 minute) Peak Mooney at Compound Note Stage Mooney 4 minutes 1 Control MB 215.5 105.0 R 116 70 F 90.0 49.0 2 4% water MB 109.0 67.0 at MB R 72.5 51 F 59.5 39.0 3 6.4 phr MB 117.5 59.0 TESPT R 99 53 F 77.0 39.5 4 6.4 phr MB 106.0 51.5 TESPT R 69.5 44.5 4% water F 59.0 34.5 at MB 5 sSBR/BR MB 200.5 156.5 R 143.5 103.5 R2 134.0 99.0 F 105.0 70.0 *Masterbatch stage Mooney viscosities are small rotor values converted to large rotor values using the calculation ML = 2.1031MS 14.911.
Storage Stability

(27) TABLE-US-00004 TABLE 4 Mooney viscosity ML (1 + 4 minute) Maturation, Peak Mooney at Compound Days Mooney 4 minutes 1 1 90.0 49.0 7 158.5 60.5 2 1 59.5 39.0 7 92.0 43.0 3 1 77.0 39.5 7 121.0 47.0 4 1 59.0 34.5 7 98.0 40.5 5 1 105.0 70.0 7 110.0 74.0
ExtrusionFinish, Definition and Swell

(28) Extrusion properties were assessed using an extruder with a 30 mm barrel diameter and a simple helical screw.

(29) Compounds were rated to give a numerical score using ASTM D2230 System A.

(30) TABLE-US-00005 TABLE 5 Appearance ratings using Rating System A, ASTM D2230 extruded with Garvey Die 70 C., Barrel 60 C. and 30 rpm. Swelling Com- and pound Note porosity Edge Surface Corners Total 1 Control 3 3 3 4 13 2 4% water at MB 4 3 4 4 15 3 6.4 phr TESPT 3.5 3 3.5 4 14 4 6.4 phr TESPT 4 3 4 4 15 4% water at MB 5 sSBR/BR 3 1 2.5 3 9.5

(31) Swell properties were assessed using an extruder with a 30 mm barrel diameter and a simple helical screw fitted with a 10 mm diameter, 20 mm long rod die with conditions set as Die temperature 70 C. and Barrel temperature 60 C.
Extrudate swell=((strand diametercapillary diameter)/capillary diameter)100.

(32) TABLE-US-00006 TABLE 6 Swell data for cooled extrudates using a Rod Die Extrudate diameter Compound Note l/d shrinkage (swell) % 1 Control 2 (12.6) 2 4% water at MB 2 0.9 3 6.4 phr TESPT 2 (14.6) 4 6.4 phr TESPT 2 (13.4) 4% water at MB 5 sSBR/BR 2 (19.2)
Tack

(33) A strip of rubber (ca. 20 g) heated to about 90 C. was laid onto a room temperature strip of the same rubber. A weight was applied to the strip, for approximately 5 seconds, and then the strips were upturned and the time that it could hold its own weight was recorded.

(34) Compound 1 with water had the best tack and did not drop.

(35) TABLE-US-00007 TABLE 7 Rating from 1-10 where 10 is good Compound 1 2 3 4 5 Note Control 4% water 6.4 phr 6.4 phr sSBR/ at MB TESPT TESPT 4% BR water at MB Tack rating 2 10 5 7 6 Mooney 79 58 71 58 87 viscosity ML (1 + 4)
Physical Property Data

(36) Test pieces were cured at a temperature of 172 C. for 12 minutes.

(37) TABLE-US-00008 TABLE 8 Akron abrasion and Heat Build Up properties Compound 1 2 3 4 5 Note Control 4% 6.4 phr 6.4 phr sSBR/ water TESPT TESPT BR at MB 4% water at MB Akron Index (%) 161 190 232 235 204 Abrasion Volume 11.8 10.0 8.2 8.1 9.3 loss (mm.sup.3) HBU Set (%) 35.1 30.6 6.5 5.4 5.2 Temp. 101 96 57 52 62 Rise ( C.) The value given is the median