CA1078803A

CA1078803A - Recyclable rubber

Info

Publication number: CA1078803A
Application number: CA236,369A
Authority: CA
Inventors: Timothy C.P. Lee; Stanley H. Morrell; Raymond A. Robinson; Bryan Whittaker; William Millns
Original assignee: Gould Inc
Current assignee: Gould Inc
Priority date: 1974-09-26
Filing date: 1975-09-25
Publication date: 1980-06-03
Also published as: BE833856A; SE7510740L; BR7506209A; DE2541771A1; FR2286164B1; ES441302A1; IN144134B; AT353478B; IT1047570B; ATA737175A; FR2286164A1; DE2541771C2; AU8485675A; JPS5160275A; JPS6111970B2; GB1516090A; NL7511225A

Abstract

ABSTRACT OF THE DISCLOSURE

Finely divided vulcanized rubber is provided in the form of aggregates of particles, the aggregates being of a size gen-erally from 10 to 200 microns and being separable into particles of size generally from 2 to 20 microns by dispersion in a rubber composition; the finely divided rubber is useful in rubber com-positions particularly in association with unvulcanized rubber.

Description

````~ 10'7B803 This invention relates to rubbers and is particularly concerned with the re-use of vulcanised natural and synthetic rubbers including blends of two or more such rubbers.
The re-use of vulcanised rubber in a finely-divided form is well known in the rubber industry. Such material (known as "crumb") is usually prepared by passing through a two-roll mill and normally has a particle size such that it will pass through a "40's mesh" screen (i.e. less than 388 microns in diameter). It is also well known that when this crumb is added to an unvulcanised rubber composition the tensile strength and other important properties of the rubber obtained on vulcanising the composition deteriorate by a substantial amount.
According to the invention there is provided a method of converting vulcanised rubber into finely divided vulcanised rubber which comprises grinding the vulcanised rubber in a colloid mill of the abrasive disc type with an excess of water to obtain a finely divided vulcanised rubber.

, . -" 1 -.

~7l3~ 3 The method of the invention provides a finely divided vulcanised natural or synthetic rubber for re-use in rubber compositions which is in the form of aggregates of particles, the aggregates being of a size generally from 10 to 200 microns and being separable into particles of size generally from 2 -to 20 microns by dispersion in a rubber composition.
Preferably at least 9~/0 by weight of the aggregates or particles have a size within the respective ranges stated above. Preferably most of the aggregates have a size of from 50 to 100 microns.

8~3 The finely divided vulcanised rubber may be natural rubber, styrene-butadiene rubber, acrylonitrile butadiene rubber, polychloroprene, silicone rubber, fluorinated silicone rubber, fluorinated hydrocarbon rubber, chlorosulphonated poly-ethylene rubber, isobutene isoprene rubber, ethylene-propylene copolymer or terpolymer or a polyure-thane. The invention i9 particularly advantageous for the re-use of expensive rubbers as well as large tonnage vulcanised rubber such as waste tyres, The invention includes rubber compositions containing finely divided vulcanised rubber having in general a particle size of 2 to 20 microns. The composition may contain the finely divided vulcanised rubber as the sole rubber component or in admixture with unvulcanised rubber, Rubber vulcanisates made from the composition containing the finely divided vulcanised rubber as the sole rubber component may have tensile strengths above 9 MN/m and tear strengths above 55 N/Std. Rubber vulcanisates containing from 10 to 7~/0 by weight of the finely divided vulcanised rubber may have tensile strengths ranging from 23 to 15 MN/m2, Vulcanised rubber may be converted into the finely divided vulcanised rubber according to the invention by a method which comprises grinding the vulcanised rubber in a colloid mill of the abrasive disc type with an excess of water.
This method gives finely divided vulcanised rubber with an effective particle size of 2 to 20 microns for the purpose of dispersion in a rubber composition although the finely divided vulcanised rubber product itself contains these - particles as aggregates of the sizes indicated above.
The vulcanised rubber used in the method of the invention is in general insoluble in the usual rubber solvents e.g. benzene, chloroform and n-hexane, and remains so when finely comminuted.

3~7~8~13 Prefera~ly, the ratio of water to vulcanised rubber in the mill is in a range from 2:1 to 30:1 by weight an~ more particularly is at least 3:1.
The vulcanised rubber e.g., rubber crumb may be reduced to the required particle size in a colloid mill having rotor and stator stones of e.g. carborundum discs. The rotor is preferably housed in bearings which allow the stones to run under compression e.g., rubber bearings or me-tal spherical - bearings. A particularly suitable form of colloid mill is one feeaing the rubber under pressure to the grinding stones by means of a Mono-type pump.
The grinding process in the colloid mill is carried out with an excess of water to facilitate the reduction of the vulcanised rubber to the required particle size. It is common practice when using such a mill for grinding solid materials to use just enough water to lubricate the stones, but we have found that when grinding rubber crumb, an important feature of the method is preferably to use an excess of water. It has further been found possible to remove this water by means of a centrifugal dryer and recirculate it, there~y conserving the total a unt of water necessary and reducing the e~fluent problem. The use of this excess amount of water also renders unnecessary the use of any of the "dispersion aids" which are advised, these are inorganic minerals such as various types of clays, which cannot easily be separated from the rubber crumb and have ~ deleterious effect on the physical properties of the vulcanisates later prepared from the crumb.
The use of an excess amount o~ water relative to the amount of vulcanised rubber is particularly desirable during the early stage of the grinding process. Thus the excess of water may be varied during the grinding process, the largest excess being used in the initial stage of the grinding process.

,' Alternatively, the excess o~ wa-ter may be kept constant during the grinding process.
Another highly desirable feature of the invention to achieve optimum grinding is to decrease the grit size of the rotor and/or stator progressively during the grinding process.
Thus a slurry of vulcanised rubber and water may be passed through the mill in a series of passes, the ratio of water to rubber and/or the grit size of the rotor and/or stator being decreased after each successive pass.
Conveniently the grit size of the rotor and/or stator may be varied by using more than one mill. -Thus a slurry of vulcanised rubber and water may be passed through a plurality of abrasive disc mills in series, the grit size of the rotor and/or stator being decreased from mill to mill in the series.
The grinding of vulcanised rubber in accordance with the invention gives a micronised rubber crumb with a highly convoluted surface and therefore a high ratio of surface area to volume. The higher this ratio, the better the reinforcing properties of the micronised rubber crumb in rubber compositions or the less fine the crumb needed for a given amount of rein-forcement.
It is an important part of the rubber grinding process that the temperature of the rubber is kept as Iow as possible.
At temperatures above 100C. molecules of natural rubber are known to degrade, giving an inferior material with lower molecular weight. In the case of synthetic rubbers the - reactions are more complicated, in some cases material of low molecular weight is formed and in others additional cross-linking occurs after the initial rupture and a material of such higher molecular weight is produced. In either case the product is inferior to the original material.

' ' - ' ' - ' . , ,, ' ~'788~3 The invention includes a method of making a rubber article which compri~es including in the rubber composition, prior to vulcanising, inely divided vulcanised rubber accord-ing to the invention and thereafter vulcanising the composition.
The ~inely divided vulcanised rubber may be the sole rubber component of the composition but is preferably in admixture with unvulcanised rubber prior to vulcanislng. Thus the rubber composition may contain at least 10 parts by weigh-t, preferably 40 to 90 parts by weight of the finely divided vulcanised rubber per 100 parts by weight of the unvulcanised rubber e.g. styrene butadiene rubber.
Reclaimed rubber or "reclaim" is sometimes used to replace some of the virgin rubber in a compound: this is producçd as a result of the effect of high temperatures (in excess of 150C.) on vulcanised rubber, and gives properties which are markedly inferior to the original vulcanisate.
Commercial reclaims, e.g. whole tyre reclaim are produced in bulk form and are less easily produced as a powder:with the present tendency to use powdered rubbers and powderèd ingredients for automatic weighing and processing, this is a further disadvantage of this existing form of reclaim.
The invention can best be described in greater detail by reference to the following examples. Where not otherwise indicated, the rubber crumb referred to in the Examples was obtained from tread buffings. However it will be appreciated that defibred whole tyre crumb containing only a small percent-age of fibres may be used as the starting material in the method of the invention. In the Examples, the finely divided vulcanised rubber of the invention is referred to as micronised crumb. The "Premier" colloid mills referred to in the Examples are made by Premier Colloid Mills Limited, Hersham Trading Estate, Walton-on-Thames, Surrey, England.

- ~ .

7~ 3 Example I
The hopper of a "Premier 84" colloid mill of size such that the stones are 3~" in diameter (46 grit size) is filled with approximately 15 litres of water. The mill is adjusted to its tightest setting (-3) and the motor which drives the rotor star-ted. The stones are therefore run under the maximum compression available with the mill. Directly a vortex appears in the hopper, 40's mesh vulcanised rubber crumb is scattered on the surface of the water at such a rate that 1 kg is fed into the hopper in a space of 20-30 minutes, whilst maintaining the hopper full of water. When all the crumb has been fed into the hopper, the water level is gradually reduced, until after 1 hour from the start of the experiment all the water has been run through. During the process, care must be taken not to feed the crumb too quickly as this results in uneven running of the stones, resulting in a grating noise, generation of heat in the mill and the appearance of coarse particles in the effluent. About 3~/O of the total crumb is left in the hopper after the process and this has to be dis-carded as it is unsuitable for further processing by this method, The effluent from the mill is fed directly into a centrifugal drier containing a filter bag. It has been found that the best material for this bag is a polyester fabric of 750 denier warp and weft composed of 66 ends per inch warp and 42 picks per inch weft, weighing 11~ ozs per sq. yard and having a scoured "M" finish. A suitable material is marketed by Richard Haywood & Co. Ltd., Crewekerne, Somerset, under the grade CS.5058. This separates the crumb from the water and the latter may then be recycled through the colloid mill.

- ~' ' ' . ~. ' ' ' , . ' 1~3'7~38(~3 The micronised rubber crumb which is collected in the filter bag is passed through the colloid mill again, under the same conditions as above, except that it is now possible to pass the ~ntire batch through this mill in 20 to 25 minutes.
For a comparison of this micronised crumb with 40's mesh crumb and whole tyre reclaimed rubber, 82 parts by weight of each of the three materials were compounded with 100 parts of styrene butadiene rubber (Grade 1500), 3 parts of zinc oxide, 3 parts of stearic acid, 1 part of Flectol* H, 43 parts of HAF black, 10 parts of Dutrex* R softener, 0.9 parts of cyclohexylbenzthiazylsulphenamide, 0.8 parts of diphenyl-guanidine and 1.75 parts of sulphur. Slabs 150 x 150 x 2 mm were press-moulded and vulcanised at a temperature of 150C
for 15 minutes. A similar slab of the base compound (i.e without any form of recycled rubber) was also prepared. On testing by standard methods the following results were obtained:-Table I

(1)(2) (3) (4) Type of rubber 40's mesh whole tyre micronised Additive Nil crumbreclaim crumb Tensile strength (MN/m2) 24.8 16.2 16.5 21.5 Elongation at break (%) 750 470 630 500 Modulus, lOC%
exten~ion (MN/m2) 1.0 1.6 1.5 1~6 Modulus, 3 OG%
extension (MN/m2) 5.4 8.9 5.8 10.0 Hardness (IRHD) 57 63 62 60 Taber Abrasion (loss in weight : g/1000 cycles) 0.14 0.18 0.23 0,15 * Trademark ~ - 8 -~37~
The rubber compound in column (1) above contained none of the thr~e rubber additives. The rubber comyounds in columns ~2) and (3) contained 40's mesh crumb and reclaim respectively whilst the rubber compound in column (4) contained micronised crumb made in accordance with the invention as described in Example I. It will be seen that the tensile strength and abrasion resistance of vulcanisa-te (~) are close to those of vulcanisate (1) and superior to those for vulcanisates (2) and (3). The modulus figures indicate that vulcanisate (4) is also more rigid than vulcanisate (1), Example II
The hopper of a "Premier 200"* colloid mill with rotor and stator stones of ~6 grit size, 200 mm in diameter, is filled with 30 litres of water. The mill is adjusted to its tightest setting and the motor started. Directly a vortex appears in the hopper, 40's mesh vulcanised rubber crumb is scattered on the surface of the water at such a rate that 8 kg is fed into the hopper in the space of 20-30 minutes, whilst ` maintaining the hopper full of water. ~-5 kg of crumb are added per hour for approximately 4-5 hours, by which time the output of micronised crumb is greatly reduced. During the process, care must be taken not to feed the crumb too quickly as this results in uneven running of the stones, resulting in a grating noise, generation of heat in the mill and the appearance of coarse particles in the effluent. About 3~/
of the total crum~ i9 left in the hopper after the process and this has to he discarded as it is unsuitable for further pro-cessing by this method.
The effluent from the mill is fed directly into~a centrifugal drier containing a filter bag as described in Example I in order to separate the rubber from the water. The water may then be recycled through the colloid mill.

* Trademark _ g _ ~ -~.~37~3~3()3 For a comparison of the product with 40's mesh whole tyre crumb and reclaim, compounds were prepared ~s in Example I
and similar results were obtained.
Example III
In this method no hopper is required. A premixed slurry of 40's mesh vulcanised rubber crumb in water is pumped directly onto the grinding stones, set -to run at maximum com-pression. In the initial pass the stator stone consists pre-ferably of 46 grit carborandum, whilst the rotor stone is pre-ferably of larger grit size, i.e. 24 grit to allow passage of the relatively larger particles of crumb. In subsequent passes the grit size of the rotor may be gradually reduced until the total product is of the required fineness ~i.e. when all of the material passes through 46 grit stones). As the gri-t size of the rotor decreases, the flow rate of the slurry decreases accordingly. The concentration of rubber to water in the feed slurry may be increased after each subse~uent pass through finer rotor stones as indicated, by way of example, in the following table:
Table II

Slurry feed Slurry concentration rate(litres/
Rotor grit size (water : crumb) hr) 1st 24 25 : 1 470 final pass 46 12 : 1 120 For a comparison of the micronised crumb product with 40's mesh crumb and reclaim, compounds were prepared as in Example I and simil~r results were obtained.

iL~713E~6~3 E~CAMP~E IV
-The potential use-fulness of the microrl~sed crumb prepared as in Example III can best be described with reference to two typical commercial applications Hitherto only small amounts of conventional (40 mesh) crumb rubber have been used in tyre tread compounds, and certainly very little, if any, conventional reclaimed rubber has been used.
The two compounds (designated Z60 and ~90) given in Table III, contain 25% and 33% micronised crumb respectively and it is clear that in all respects the propertie~s are well within the laboratory specifications for passenger car tyre treads. ~he flex crack initiation and growth data are particularly good, The compounds have been used to retread tyres and these tyres compared with ones commercially available in a short duration, but rigorous, track test. After 1500 miles the following percentage loss of tread rubber was recorded, test tyre retreaded with the compound containing 60 parts of micronised crumb-~/O, test tyre retreaded with the compound containing 90 parts of micronised crumb-~/O, commercially ~vailable retreaded tyre-13%, commercially available first line tyre-8/0. All - these tyres were of a cross-ply construction but the potential usefulness is in no way limited to this type of tyre, satis-factory results would be obtained whatever the tyre construction.

..

~ 788~3 Table III ~est Compounds SBR 1712 137.5 137.5 Vuncan* 3H 75.0 15.0 Dutrex* R 10.0 10.0 Paraffin Wax 1.0 1.0 Santo~lex* 13 1.5 1.5 Flectol* H 1.0 1.0 Zinc Oxide 4.0 4.4 Stearic Acid 2.6 2.9 MICRONISED CRUMB 60.0 90.0 CBS 1.3 1.45 DPG 0.325 0.363 Sulphur 2.28 2.54 Tensile Strength MN/m 19.1 18.6 Elongation at Break % 520 500 100% Modulus MN/m 1.4 1.3 .
30~/O Modulus MN/m 8.4 8.5 Tear Strength ~Std 118 107 Hardness IRHD 57 59 Compression Set % 17.5 20.1 Resilience (Lupke) 36 36 De Mattia. Flex** 105 30 KSC to initiation 115 50 ~150 De MattiaO Flex** ~150 100 KCS to B type crack ~150 80 >150 >150 -`
** Results for three test pieces: tests according to British Standard 903 part A10, * trademark Example V
The excellent dynamuc properties of rubber compounds containing micronised crumb prepared as in Example III can be exploited in their application to rubber mountings. The dynamic moduli and loss factors for a series of compounds (Table IV) containing different levels of micronised crumb up to 60 parts are given in Table V, A ran~e oE frequencies was examined. These properties are in keeping with those of a medium damping rubber compound.
Table IV
GR GRl GR2 GR3 GR4 GR5 Philblack* I 40 40 40 40 40 40 ~onox* ZA1.5 1.5 1.5 1.5 1.5 1.5 Stearic Acid 1.0 1.0 1.0 1.0 1.0 1.0 Zinc Oxide 5.0 5.0 5.0 5.0 5.0 5.0 Dutrex* R4.0 5.1 5.64 6.26.74 7.3 Coumarone Resin 2gog 5.0 5.68 6.036.376.71 7.06 Micronised crumb 0 20 30 40 50 60 CBS 1.5 1.71 1.81 1.912.022.12 DPG 0.5 0.5 0.5 0.5 0.5 0.5 Sulphur 1.5 1.71 1.81 1.912.022.12 * trademark 1(~'7~ 3 Table V
Sample Frequency Tan ~ Complex 2 Hz Modulus MNm 0.03 0.16 5.44 0.3 0.18 6.0 GR 3.0 0.21 6.81 30 0.22 7.45 0.03 0.16 7.26 0.3 0.16 8~05 GRl 3~0 0.16 8.73 0.16 8.9 0.03 0.19 7.59 GR2 0.3 0.1~ 8,46 3.0 0.20 9.35 -,:
0.22 10.07 0.03 0.15 8.04 GR3 0.3 0.18 8.40 3.0 0.19 9.36 0.20 9.99 0.03 0.16 7.55 ¢R4 0.3 0.. 17 8.49 3.0 0.17 9.37 0.21 10.68 0.03 0.15 8.30 GR5 0.3 0.17 8~82 3.0 0.18 9.72 0.18 10.38 Example VI
This example relates to the inve~tion as applied to an expensive synthetic rubber such as Viton* which is manu-factured by ~u Pont and is a copolymer of Vinylidene Fluoride and Hexafluoropropylene.
A standard Viton compound VI was prepared to the specification shown in Table VI. This was then cured into sheets at 163C for 30 minutes and post cured for 24 hours at 250C. The cured sheets were cut into small pieces about half inch in size, then immersed in li~uid nitrogen to embrittle ' the material prior to grinding in a knife mill.
The material which was sufficiently ground passed through a 0.05 cm screen into a container, the material was then dried and sieved through a 40's mesh screen, the ~raction * trademark ~C~781~3 larger than 40's mesh size was kept aside -for further grinding, while the 40's mesh and less ma-terial was passed twice through a colloid mill set at a maximum compression setting, with water as the carrier medium; the product was filtered and dried in the usual manner. Four mixes were prepared using the micronised crumb produced, mix VI, being the control mix containing no recycled Viton* product. Compounds V2, V3 and V4 were prepared by blending, on a two roll mill, 20, 40 and 60 phr respectively of the micronised crumb ln the original compound VI. Compound V5 contained 20 phr of the micronised crumb together with extra curatives equivalen-t to an additional 10 phr of rubber hydrocarbon in compound VI.
All mixes were cured for 30 minutes at 163C and post-cured for 24 hours at 250C. The physical properties of the vulcanisates are shown in Table VI.

* trademark .
.. . . :. . - :

` ~ ~.(3'7B8V3 . VI V2 V3 V4 V5 _ VITON* B 100 100 100 100 100 MAG. ~YIDE 15 15 15 15 16 MT sLAcK 20 20 20 20 20 DIAC* No. 3 3 3 3 3 3.22 RECYCLED ) : VITON*) ~ 20 40 60 20 Tensile Strength (MN/M2)16.614.2 13.0 1~.7 15.7 Elongation at . .
Break (%) 290 280 220 240 240 Modulus 10~/o 2 (MN/M ) 1.7.4.0 4.4 3.5 4.1 Micro-Hardness (IRHD) 72 71 74 74 71 Compression Set (%) 23.024.8 27.0 28.3 23.6 ., .' The presence of the micronised Viton* crumb in the compound gave an increase in the modulus and a fall in the tensile strength of their vulcanisates, while the compression set of samples V2, V3 and V4 show a gradual increase with the increased loading of the recycled Viton* in the compound.
By increasing the level of curatives, sample V5, an improvement in the tensile strength and compression set is observed, which suggests that when using micronised crumb in Viton*
mixes it would be advantageous to adjus-t the amount of curatives relative to the loading of the micronised Viton*
crumb.
.~

* trademark ~3'7~8(,3~

Certain rubber compoundin~ ingredients referred to above are identified more fully below~
Vulcan* 3H Carbon Black (HAF) Philblack* I Carbon Black (ISAF) Dutrex* R Aromatic process oil Santoflex* 13 (Antiozonant) N-phenyl-N'-(1,3-dimethylbutyl)-~p-phenylenediamine.
Flectol* H (Antioxidant) Poly-1,2-dihydro-2,2,4-trimethyl quinoline Nonox* ZA (Antiozonant) ~-isopropyl-diphenylamine C.B.S. (Accelerator) N-cyclohexylbenzthiazylsulphenamide D P.G. (Accelerator) Diphenylguanidine S.B.R. Styrene butadiene rubber The invention will also be further described by wey of example with reference to the accompanying drawings, in which:-Fig. l is a sectional elevation of a colloid mill ; for use in the method of the invention, Fig. 2 is a flow diagram illustrating the method of the invention, Fig. 3 is a Coulter Counter diagram for the micronised crumb of the invention, Fig. 4 is a photomicrograph of 40's mesh rubber crumb, Fig. 5 is a photomicrograph of a 40's mesh rubber crumb dispersed in a rubber composition, and Fig. 6 is a photomicrograph of a micronised cru~b of the invention dispersed in a rubber composition.

* trademark -, '' - ::.: ..
.

` -` 1C1~7l~3 The colloid mill shown in Fig. 1 consists essentiaLly of two horizontal grinding wheels or stones, an upper stationary wheel A and a ~lower rotor B. The wheel A is secured to a top plate C and adjustment of the top plate C may be made by means of screws D to diminish the size of the nip E, 40's mesh ru~ber crumb - either tyre buffings or whole tyre crumb, that is, defibred product from whole -tyres - is fed with water into a hopper F and falls through a safety grid G into the throat of the mill where, by the centrifugal force produced by the rotor and by a spinner H, the rubber is thrown between the two stones into the grinding area and is then discharged through the chute H. The rotor B is provided with V-belt -drive means J.
The method shown in the flow diagram of Fig. 2 is suitable for commercial operation of the invention and may be carried out as follows:
; A premixed slurry of 40's mesh vulcanised rubber crumb in water of concentration 1:7.5 prepared in a small (75 mm) colloid mill is pumped directly into a larger ~200 mm) colloid mill with grinding stones set to run at maximum compression. The stator stone is of 46 grit carborundum whilst the rotor stone is of 24 grit to allow passage of the larger particles of crumb. The product is pumped to a similar mill equipped with a rotor of 36 and a stator of 46 grit stones.
This product is pumped to a third mill equipped with 46 grit stones and is then passed over a screen carrying a fil-ter cloth; the surface water is removed by a centrifugal drier equipped with a filter bag. Final drying is by means of an oven or a fluidised bed drier at 40C. The water separated by means of the filter cloth and filter bed may be mixed with more 40's mesh crumb and recycled.

- 18 _ ~1~7~

~ he product is a fine, free-flowing powder which can be compounded alone or wi-th fresh rubber. Typical formulations and properties are given in Table VII. For comparison, data for compounds containing conventional whole tyre reclaim (WTR3 or the 40's mesh crumb ~eedstock (WTC) are also given, - ~, _ ~ _ ___ R c~ CQ ~ I~ ~ a:~
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In commercial opera-tion of the inven-tion a number of factors have -to be carefully balanced since excessive heat generation can cause softening of the rubber and the production of small rolls of elastic material rather than true grinding.
These factors include (a) the size, shape and grit size of the stones as well as the design of the feed (b) the distance apart of the stones (c) the speed of the xotor (d) the dilution of the 40's mesh crum'Q (e) the rate at which the slurry of crumb and water i5 fed -to the mill (f) the pressure which is exerted on top of the water to force the slurry through the mill (g) the presence or absence of a wetting agent to assist the formation of the slurry and its passage through the mill.
In the foregoing account it has been implied through-out that all the material passes through each of the colloid mills in turn. This is an efficient way of wor~ing, but there are other ways. By a suitable arrangement of pipes it is possible to arrange that only the finer particles from the first mill pass through the second and third mills, the remainder being recycled through the preceding mill in the series. Sometimes it may be desirable to stop production periodically and flush the mill with clean water for a brief period. The mills may also be operated in parallel instead of in series.
The particle size of the micronised crumb of the ; invention may be established by sieving or by means of a Coulter Counter. Fig. 3 shows the graph produced by the Coulter Counter giving a percentage distribution of the various particle sizes of the micronised crumb. It will ~e seen that the total range of sizes is from 10 to 200 microns but that most of the particles lie in the range 50 to 100 microns.

:

However, when such particles are incorporated in pale crepe rubber and the vulcanisate examined by means of an optical microscope, the particle size is evidently very much smaller than this, generally in the 2 to 20 micron range. This is shown in Fig. 6 wherein the magnification is x 200. In contrast, if 40's mesh crumb is subjected to the same procedure of milling with crepe rubber, vulcanising and subjecting to microscopic examination, the size of the particles present in the rubber, as shown in Fig. S, is not substantially different from the original 40's mesh crumb shown in Fig. 4. Figs. 4 and 5 have the same magnification as Fig. 6. It is therefore clear that the particles represented in Fig. 3 are aggregates of much finer material and it may be concluded that the grinding method of the invention does result in primary particles of size generally less than 20 microns but that in the ensuing filtering and drying stages, aggregation of these particles occurs, giving the impression that the product is coarser.
The fineness of the particles of the micronised crumb produced by the present method gives an insight into their capability of incorporation in large amounts into rubber compounds without serious diminution of properties.
They do in fact exert a reinforcing action on the rubber.
This may be shown by the effect of introducing up to 120 parts of micronised crumb into a pure gum SBR compound (Table VIII).
The SsR vulcanisate without micronised crumb is extremely soft and has low tensile and elongation values. With only 20 ~arts of micronised crumb there is a 50/0 increase in tensile strength and tear strength and also an increase in the elongation value.

8i~3~3 As the level of micronised crumb is increased so the tensile strength and tear streng-th are further enhanced modulus and hardness values similarly improve with the loading of micronised crumb.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of converting vulcanised rubber into finely divided vulcanised rubber which comprises grinding the vulcanised rubber in a colloid mill of the abrasive disc type with an excess of water to obtain a finely divided vulcanised rubber.

2. A method according to claim 1, wherein said finely divided vulcanised rubber is in the form of aggregates of particles, the aggregates being of a size generally from 10 to 200 microns and being separable into particles of size generally from 2 to 20 microns by dispersion in a rubber composition.

3. A method as claimed in claim 2, in which the aggregates have a size generally from 50 to 100 microns.

4. A method as claimed in claim 2, in which at least 90%
by weight of the aggregates or particles have a size within the stated respective ranges.

5. A method as claimed in claim 1, 2 or 3, wherein the rubber is natural rubber, styrene-butadiene rubber, acrylo-nitrile butadiene rubber, polychloroprene, silicone rubber, fluorinated silicone rubber, fluorinated hydrocarbon rubber, chlorosulphonated polyethylene rubber, isobutene isoprene rubber, ethylene propylene copolymer or terpolymer or a poly-urethane.

6. A method according to claim 1, 2 or 3, including a step of formulating the finely divided vulcanised rubber having in general a particle size of 2 to 20 microns, as the sole rubber component, in a rubber composition.

7. A method according to claim 1, 2 or 3, including a step of forming a rubber composition comprising in admixture finely divided vulcanised rubber having in general a particle size of 2 to 20 microns and unvulcanised rubber.

8. A method as claimed in claim 2, in which the ratio of water to vulcanised rubber is in a range from 2 : 1 to 30 : 1 by weight.

9. A method as claimed in claim 8, in which said ratio is at least 3 : 1.

10. A method as claimed in claim 1, 2 or 8, in which the colloid mill includes a rotor and stator comprising abrasive discs.

11. A method as claimed in claim 1, 2 or 8, in which the colloid mill has a rotor which is run under compression.

12. A method as claimed in claim 1, 2 or 8, in which the water and rubber are removed from the colloid mill, and the water is separated from the rubber by means of a centrifuge and recirculated.

13. A method as claimed in claim 1, 2 or 8, in which the temperature of the rubber does not exceed 100°C.

14. A method as claimed in claim 1 or 2, in which the excess of water is varied during the grinding process, the largest excess being used in the initial stage of the grinding process.

15. A method as claimed in claim 2, in which said mill has a rotor and stator comprising abrasive discs the grit size of at least one of which is progressively decreased during the grinding process.

16. A method as claimed in claim 15, in which a slurry of vulcanised rubber and water is passed through the colloid mill in a series of passes, the ratio of water to rubber being decreased after each successive pass.

17. A method as claimed in claim 15, in which a slurry of vulcanised rubber and water is passed through the colloid mill in a series of passes, the mill having a rotor disc and a stator disc, the grit size of at least one of said rotor and stator discs being decreased after each successive pass.

18. A method as claimed in claim 1, 2 or 8, in which a slurry of vulcanised rubber and water is passed through a plurality of abrasive disc mills in series, the grit size of at least one of the rotor and stator in said mills being decreased from mill to mill in the series.

19. A method as claimed in claim 2, including a step of incorporating the finely divided vulcanised rubber in a rubber composition prior to vulcanising and thereafter vulcanising the composition to produce a rubber article.

20. A method as claimed in claim 19, in which the finely divided vulcanised rubber is the sole rubber component of the composition.

21. A method as claimed in claim 19, in which the finely divided vulcanised rubber is in admixture with unvulcanised rubber prior to vulcanising.

22. A method as claimed in claim 21, in which the rubber composition contains at least 10 parts by weight of the finely divided vulcanised rubber per 100 parts by weight of the unvulcanised rubber

23. A method as claimed in claim 21, in which the rubber composition contains from 40 to 90 parts by weight of the finely divided vulcanised rubber per 100 parts by weight of the unvulcanised rubber.

24. A method as claimed in claim 21, 22 or 23, in which the unvulcanised rubber is styrene butadiene rubber or natural rubber or a blend thereof.