CA1052063A

CA1052063A - Reclaimed vulcanised rubber

Info

Publication number: CA1052063A
Application number: CA223,360A
Authority: CA
Inventors: Derek W. Sear
Original assignee: Setech International Ltd
Current assignee: Setech International Ltd
Priority date: 1974-03-29
Filing date: 1975-03-27
Publication date: 1979-04-10
Also published as: IT1030491B; FR2265530A1; JPS599341B2; GB1501865A; FR2265530B1; AU7955475A; JPS50134085A; DE2513146A1; DE2513146C2

Abstract

A B S T R A C T

Vulcanised rubber is reclaimed by treating pieces of the rubber to render a surface layer of each piece plastic while the inner core of the piece remains non-plastic. A chemical reagent can be used in the treatment, but it is preferred to render the surface layers of the rubber pieces plastic by a heat treatment which involves direct exposure of the pieces of rubber to either a flame or a heated gas. In a preferred embodiment of the invention vulcanised rubber pieces are dropped under gravity into a flame so that any attached textile residues are burnt away simultaneously with the plasticising of the surface layers of the pieces and the heated pieces then enter a stream of cold air which quenches the heat treatment.
Vulcanised rubber articles made from the rubber reclaimed by the method of the invention have improved properties as compared with articles made by the conventional methods of reusing vulcanised rubber.

Description

l~SJ~D~ 3 This invention relates to methods oi reclaiming vulcanised rubber.

It i8 known to recover vulcanised rubber for re-use.
In most of the known methods for re-use of vulcanised rubber, vulcanised rubber scrap material i8 prepared in one of two forms. Either the treatment may be essentially mechanical, the vulcanised rubber being separated irom contaminants such as pieces of metal or textile residues and subsequently ground and graded to provide a free-flowing rubber crumb in which the rubber is still vulcanised and non-plastic for use as the rubber crumb, or the rubber crumb may be further treated by a combination of mechanical work, ; action of chemical additives and application of heat to form a material which is commonly referred to as "reclaim rubber". "Reclaim rubber" is no longer vulcanised and is once more plastic and capable of shaping and subsequent re-vulcanisation. "Reclaim rubber" takes the form of a homogeneous mass which may be handled in the same way as ordinary unvulcanised rubber.

It will be appreciated that it is con~iderably more expensive to prepare "reclaim rubber" than it is to prepare a vulcanised rubber crumb from scrap rubber materials.

Vulcanised rubber crumb has been included in ; unvulcanised rubber compound, usually in minor propor-tions, to improve ease of processing during shaping, such as extrusion or calendering prior to vulcanisation.
Of increasing importance has been the inclusion Or 105;~0~;3 vulcanised rubber crumb to cheapen the final rubber product since scrap rubber is generally available at lower C09t than unvulcanised rubber compound. However, there i8 a disadvantage that the inclusion of vulcanised rubber crumb in unvulcanised rubber compounds results in weaker products being obtained when the rubber compounds are vulcanised. Weakening becomes particularly apparent when vulcanised rubber crumb is added to the unvulcanised rubber compound such that there is an amount of the order of 25% by weight or more of the vulcanised rubber crumb in the final product.
These mechanical weaknesses are believed to be due to the occurrence on deformation of concentrations of stress at the interfaces between the particles of the vulcanised rubber crumb and the rubber compounds in which they are incorporated. These interfaces are present because the surfaces of the vulcanised rubber crumb are ... ~ ; , ~
non-plastic and are, therefore, not miscible with the ; .
surrounding rubber compound before that rubber compound ' -is vulcanised to form the final product. In consequence a clearly defined multiplicity of adhesive interfaces i9 present and these give rise to the stres~ concentr-ations which lead to mechanical weakness. Accordingly the re-use of vulcanised rubber crumb with rubber com-pounds which are subsequently vulcanised seldom yields a product which is comparable to the same article made from a newly vulcanised rubber compound.
~ On the other hand the fully plasticised "reclaim rubber" which is obtained by breaking down the vulcanis-~0.5~063 ation of the rubber crumb suffers from the disadvantage thatit is so wea~ened by the reclaiming process that products obtained on sub~equent revulcanisation exhibit only relatively moderate strength which is significantly less than the strength of the vulcanised rubber from which the "reclaim rubber" is prepared.

It is an object of the present invention to provide an alternative method of reclaiming vulcani~ed rubber 80 .
, ~ that it may be re-used. ~-~
, .
According to the present invention there is provided a method of reclaiming vulcanised rubber which comprises the step of causing the surface of vulcanised rubber to be Reared by exposing the vulcanised rubOer to a temperature at which the rubber can be destroyed by combustion in air, the exposure being for a time insufficient for the rubber to -~
ignite, whereby a surface layer of the rubber is rendered plastic by devulcanisation while an inner core of the rubber remains vulcanised and non-plastic.
. ~.
f'~ The searing of the surface of the vulcanised rubber by expo~ure to heat may be effected either by causing a flame to play directly upon the surface of the vulcanised rubber or by applying heated gas to the surface of the t vulcanised rubber. The gas may be heated by anyconvenient means. For example, electrical heating may be employed.
~he heated gas may, for example, be air, or a non-oxidizing gas such as gaseous combu~tion products or nitrogen.

For ease of handling, it i8 preferred for the i:

I~A

S~0~;3 vulcsnised rubber to be reclaimed in particulate form.

The particles may be passed through a single zone of heat or through a plurality of zones of heat. The particles may be passed through the zone or zones of ~ .~
heat in a batch process in which the particles sre placed on a substantially horizontal surface and the zone or zones of heat sre causedto play upon the particles, and to be moved across the surface in order to effect pas~age of the particles through the zone or zone8 of heat, or the particles may be treated in a continuous process, for example, by spreading the particles across the surface of a conveyor belt and moving the particles on the conveyor belt through the zone or zone9 of heat. However, . , .
`~ in a preferred method in accordance with the present invention the particles are permitted to fall under -~
gravity through the zone or zones of heat.

~ The or each zone of heat may be constituted by one - or more flames or by a heated gas which may be heated air. -~
ln one of the preferred embodiments of the in~7ention which will be described particles of vulcanised rubber are passed through a flame from a butane gas burner, and in ~^ a second preferred embodiment of the present invention -~
which will be described particles of vl~lcanised rubber are passed through air which has been heated by means of electrical resistance elements.

In one preferred apparatus for reclaiming rubber crumb in accordance with the present invention, the zone of heat is provided in a reaction chamber by a gas burner i ~

105~

mounted at one side of the chamber snd arranged 80 that the zone of heat comprises flame, hot air and gaseous combustion products which stream across the chamber from the said one side of the chamber towards the opposite side of the chamber. There is preferably provided means at the top of the chamber for prouiding a downwardly directed flow of cooling air on the said opposite side of the chamber.
.
Further in accordance with the present invention there is provided rubber material having an inner core of vulcanised non-plastic rubber and a surface layer which is devulcanised and plastic and capable of coalescing with the surface layer of similar rubber material or with :
unvulcanised rubber.

, There will be described later in this Specification the use of this reclaimed rubber material in the formation ` of a shaped article of vulcanised rubber. -~
i The reclaimed rubber material may be shaped together with unvulcanised rubber by moulding the reclaimed rubber material and the unvulcanised r~bber under pressure and heat ; in the presence of sufficient vulcanising ingredients only to vulcanise the unvulcanised rubber and the plasticised rubber in the surface layers of the reclaimed rubber material.

Rubber material reclaimed from vulcanised rubber in 105;~0~3 accordance with the present invention may alternatively be formed into a shaped article without the addition of unvulcanised rubber and the reclaimed rubber material may be moulded under pressure and heat in the presence of sufficient vulcani~ing ingredients only to vulcanise the plasticised rubber in the surface layers of the reclaimed .
rubber material. ~urthermore, it has been found that it i8 not always necessary to introduce additional vulcanising - ingredients to the reclaimed rubber material in order to obtain a vulcanised shaped article by moulding the reclaimed . .
~ rubber material under pres~ure and heat.
.
~ he reclaiming of vulcanised rubber in accordance with the present invention is preferably ef~ected by passing partlcles of vulcanised rubber crumb through a zone or zones of heat of sufficient intensity to cause devulcanisation of the surface layers of the particles during the passage of the particles through the zone or zones.
~ .
~ Examples of vulcanised rubbers which are suitable for - treatment in accordance with the present invention in order to render the ~urface layers of piece3 of the vulcanised rubber plastic include natural rubber, butadiene-styrene copolymers, vulcanised copolymers of isobutylene and vulcanised copolymers of ethylene and propylene.
.
The present invention will be further understood from the *ollowing detailed description which is made by `~ way of example with reference to the accompanying diagrammatic drawings in which:

lOS~0~;3 Figure 1 shows a cross-section through apparatu~ for effecting a particularly preferred method of reclaiming vulcanised rubber crumb, in accordance with the pre~ent invention, Figure 2 shows a cro~s-section through an alternative apparatus for treatment of rubber crumb, in accordance with the present invention, Figure 3 is a diagrammatic view of a third embodiment of apparatus for treatment of rubber crumb, in accordance with the present invention, and Figure 4 shows an enlarged segment of a piece of :~
rubber crumb which has been treated by a method in accordance /

lOS~0~;3 with the present invention.
In the drawings, the same or similar parts are designated b`y like reference numerals.
Referring to Figure 1 of the drawings, the apparatus ; `
T- includes a reaction chamber 1 which i9 advantageouslg made of metal. A hopper 2 i8 provided at the upper end 3 ~
of the reaction chamber 1. The hopper 2 is provided with - -feeder means which includes a feed control mechanism 4 and a chute 5. A gas burner 6 is mounted in the side of the reaction chamber 1 beneath the chute 5.
... . .
~; ~he gas burner 6 is connected to a gas supply ~ . .
through a connecting tube 7, and a plurality of air holes 8 enable air for the combustion of gas from the gas L~`i supply to enter the gas burnèr 6.
When the gas burner 6 is in operation it provides ,~ a region of hot gas comprising flame, hot air and gaseous combustion products which stream across the chamber 1 rom the side of the chamber 1 in which the gas burner 6 ~ ,.1 ' . ~
is mounted towards the opposite side of the chamber 1.
An air inlet 9 i8 mounted in the top 3 of the reaction - chamber 1 towards the oppo~ite side of the chamber 1 from .
r``; the ga9 burner 6. The air inlet 9 i8 connected to an ., ~`, air supply for providing a downwardly directed flow o~
.^ ~, .
'; cooling air on the side of the reaction chamber 1 remote ~' from the gas burner 6. The bottom end 10 of the reaction chamber 1 is open.
In operation of the apparatus Or Figure 1, rubber crumb is loaded into the hopper 2 and is fed at a controlled rate by the feed control mechanism 4 through :

. .

~ OS~0 ~ 3 the chute 5 into the reaction chamber 1, The rubbér crumb fall~ under the action of gravity into the zone of heat provided by the gas burner 6. The stream of flame, heated air and gaseous combustion products from the gas burner 6 cause the surfaces of the rubber crumb particles to be seared and the rubber crumb particles are carried by the stresm across the reaction chamber 1 and into the downwardly directed flow of cooling air from the air inlet 9. The flow of cooling air from the air inlet 9 quenches the heat treatment of the surface~ of the rubber crumb particles and assists in the transfer of the heat treated '`
crumb downwardly to the bottom end 10 of the reaction - chamber 1 from which it may be collected. In addition to quenching the heat treatment of the rubber crumb and assisting in the downward transfer of the treated rubber crumb the downwardly directed flow of cooling air from the air inlet 9 aids the combustion of gas from the gas burner 6 by maintaining a flow of air through the reaction chamber 1.
Treated crumb passing out from the bottom end 10 of the reaction chamber 1 may be collected in a hopper or it may be allowed to fall onto a continuously moving conveyor belt.
The u~e of the apparatus of Figure 1 is particularly preferred because it facilitates the treatment of rubber crumb obtained from tyres. Rubber crumb obtained from tyres usually includes textile residues. The removal of these textile residues has proved extremely difficult and this difficulty has inhibited the re-use of rubber .

~;

105;~0~3 obtained from tyres. When rubber crumb obtained irom tyres is thermally treated in the apparatus Or Figure 1 as described, it is found that textile fibres in the crumb are either carbonised or completely burnt away and there is no evidence remaining of any contaminant iibre.
In test apparatus having the configuration of the apparatus of Figure 1, the reaction chamber 1 was 82cm in height and had an internal diameter of 15.5cm. The chute 5 was 2cm in diameter and the air inlet 9 wa9 also 2cm in diameter. The gas burner 6 was rectangular in cross-section at the point where it entered the reaction chamber 1, having a width of 2cm and a height of 1.5cm.
The gas burner 6 was extended through an orifice,3cm in , - .
, diameter in the' wall of the reaction chamber 1.
' In operation, the feed control mechanism was ,' ',~ adjusted to feed rubber crumb through the chute 5 at a '' ''~,, rate of the order of 200 grams per minute. The gas ; burner 6 was fed with butane gas and the flame was ii adjusted 90 that the "blue zone" of the flame was of the - , order of 3.5cm in length. The chute 5 was situated 90 that the rubber crumb particles were dropped vertically into the "blue zone" of the flame. The air supply was adjueted to provide a downwardly directed flow of cooling air at a ~' rate of 250 litres per minute through the air inlet 9.
`' The ~ollowing ~xamples are illustrative of methods in accordance with the present invention utilising the test apparatus described with reference to ~igure 1.
~ Example 1 ,' Ground whole tyre vulcanised rubber crumb capable :.

lOS~O~i~
of passing though a sieve of 30 meshes/inch was treated in the test apparatus.
The treated whole tyre rubber crumb was tested relative to untreated whole tyre rubber crumb. 100 part~
by weight of the treated whole tyre rubber crumb were blended with 100 parts by weight of an unvulcanised t~
rubber compound which contained styrene-butadiene rubber compound and high abrasion furnace black in a ratio of 100 parts to 72 parts together with 2 to 3 parts of vulcanising agents and antioxidants. The blend of treated crumb and unvulcanised rubber was formed into t inche~ -thick slabs and vulcanised in a moulding press for 25 minutes at 155C. Similar slabs were formed from a blend of 100 parts by weight of untreated whole tyre rubber crumb with 100 parts by weight of the unvulcanised rubber compound. On testing according to British Standard Specification No. 903, the slabs containing the treated crumb were iound to have a tensile strength of 1,523 p. 9 . i .
and an elongation at break of 410%, which were consider-ably superior to the tensile strength of 1,168 p. 8 . i .
and elongation at break of 360% exhibited by the slabs containing the untreated crumb.
Some of the treated crumb was tumble mixed with .: .
powdered vulcanising ingredients according to the form-` ulation:-~arts b~ wei~ht Treated whole tyre rubber crumb100.00 Zinc oxide 2.00 Stearic acid 2.00 -)5'~ ~ 3 Parts by wei~ht Sulphur 1.00 N-cyclohexyl - 2 - benzthiazole sulphenamide 0.32 The formulation was fed into the nip of a two roll rubber mixing mill with the rolls at approximately 40C
to form a rough but coherent sheet on cutting irom the rolls. The formulation was then moulded in a compression mould with a hemispherical cavity of 6cm diameter to produce a hollow hemisphere of 0.5cm wall thickness.
The mould had been preheated in an open daylight press with plattens heated to 170C before charging. The mould was returned to the press and the formulation was vulcan-ised for 3 minutes. On removal from the press, the product was immediately removed from the mould, while it was still hot, and was found to be no longer plastic, but vulcanised and with rubbery properties.
Example 2 Vulcanised natural rubber moulding scrap was ground on a two roll rubber mill to form particles which were ungraded, but which had a maximum size of the order of 0.5m~, and these rubber particle8 were subjected to treatment in the test apparatus. The product of the treatment was 8 loosely coherent fluffy crumb.
The treated rubber particles were mixed on a rubber mill with vulcanising ingredients according to the formulation:-~art~ by wei~ht Treated moulding rubber crumb 100.00 lOSi~0~3 arts b~ weight Zinc oxide 1.00 Stearic acid 1.00 Sulphur 5 s N-cyclohexyl - 2 benzthiazole . sulphenamide 0.16 Some of this formulation was used to form vulcan-ised test pieces which were tested in accordance with British Standard Specification No. 903. The vulcanised test pieces were found to have a tensile strength of 1,815 p.s.i. and an elongation at break of 470~, comparing most favourably with vulcanised products obtained from firæt grade whole tyre reclaim rubber which typically . exhibit tensile strength in the range 850 p. 8 .i. to 900 p.s.i. and elongation at break of 300% to 350%.
A single cavity toroidal O-ring mould wa~ charged with some of the formulation prepared above, and the : formulation was vulcanised in an open daylight moulding press for 10 minutes at 150C. The product was stripped from the mould while still hot, and was found to be non-: plastic but vulcanised and with very good elastic properties.
.~ Example .~
~- Vulcanised natural rubber moulding scrap was ground and treated in the apparatus of Figure 1, as in Example ~: 2. The treated rubber crumb was not mixed with vulcan-ising ingredients, but it was charged directly into a flat sheet plunger mould, inserted into a vulcanising press and heated at 150C for 30 minutes. On opening the --1~+--105;~0~;3 mould it was found that the ground scrap had consolidated to a substantially homogeneous sheet which was removed hot from the mould and was no longer plaætic. It was presumed thatvulcanisation was conseouent upon migration of residual vulcanising ingredients from within the core portions o~
the treated rubber particles into the plastic outer layer~
of the treated rubber particles.
Referring to Figure 2 of the drawings the apparatus includes a substantially cylindrical reaction chamber 1 which is open at both its upper end 3 and its bottom end 1~. At the upper end 3 of the reaction chamber 1, there is provided a hopper 2 having feeder means which comprises a feed control mechaniæm 4 and a chute 5 situated within the open upper end 3 of the reaction chamber 1 to leave a ~ substantially annular opening 11 about the chute 5.
`~ An inlet tube 12 which opens into the side of the reaction chamber 1, is connected to an air supply (not shown). One or more electrical resistance elements 13 are mounted within the inlet tube 12 and are controlled 80 that, in operation of the apparatus of Figure 2j air passing out irom the inlet tube l? into the reaction chamber 1 i8 sufficiently hot that, if a piece of paper were placed at the junction between the inlet tube 12 and the reaction chamber 1, it would ignite substantially instantaneously.
Hot air from the inlet tube 12 rises within the reaction : chambèr 1 and passes out through the annular opening 11.
Provision may be made for closing off parts of the annular - opening 11, for varying the size of the opening from the - inlet tube 12 into the reaction chamber 1, and for varying :

105;~0f~3 the rate of supply of air to the electrical resistance element or elements 13 in order to effect variations in the temperature and flow conditions within the reaction chamber.
A conveyor belt 14 i8 prov~ded beneath the bottom end 10 of the reaction chamber 1 for the removal and cooling of heat treated rubber crumb.
In operation of the apparatus of Figure 2, vulcan-ised rubber crumb is placed in the hopper 2, from which 0 it i9 fed into the reaction chamber 1 through the chute 5 by the feed control mechanism 4. The rubber crumb falls under the action of gravity and encounters hot air rising from the inlet tube 12. The conditions encountered by the rubber crumb become progressively hotter until the rubber crumb passes the inlet tube 12. As the rubber crumb passes down the reaction chamber 1 the surfaces of the pieces of rubber crumb become plasticised. The treated rubber crumb falls through the open bottom end 10 of the reaction chamber 1 onto the conveyor belt 14, and is removed by the conveyor belt 14 for cooling.
In experimental apparatus conætructed in accordance with Figure 2, the reaction chamber 1 had a height of 118cm and an internal diameter of lOcm. The chute 5 had an internal diameter of 2cm and the inlet tube 12 had an internal diameter of 3.5cm. In operation of the apparatus, the feed control mechanism ~ waæ adjuæted to feed rubber crumb through the chute 5 at a rate of 200 grams per minute and the flow of air from the air supply to the inlet tube 12 was adjusted 80 that heated air passed ' lOS;~0~;3 from the inlet tube 12 into the reaction chamber 1 at a rate of 87 li~tres per minute. At the junction of the inlet tube 12 and the reaction chamber 1 the air was at a temperature of the order of 900C.
If desired~ heated gases other than air may be passed through the inlet tube 12 for the treatment o~ the rubber crumb, 90 long as the gases chosen do not react with the rubber in such a way that the ability of the outer layers of the treated rubber crumb to be vulcanised or otherwise 10 cross-linked i9 impairea.
! Referring to Figure 3 of the drawings~ there is shown an apparatus in which rubber crumb is passed through a plurality of zones of heat. Rubber crumb is placed in ¦~ a hopper 2 from which it passes on to a conveyor belt 19, ~ the speed of which is adjustable in order to vary the ~ ,:
rate of supply of rubber crumb from the hopper 2. The conveyor belt 19 feeds the rubber crumb to a reaction chamber 1 through the upper end 3 of the reaction chamber 1, which is open. The conveyor belt 19 and the hopper 2 20 are shielded from heat by a heat shield 20.
i Three opposed pairs of gas burners 6 are disposed at spaced intervals down the length of the reaction ~- chamber 1. The gas burners 6 are similar in construction - to conventional burner elements in domestic gas ovens.
The gas burners 6 are pivotally mounted within the reaction chamber 1 such that they may pivot through an angle of 90 in order to provide adjustment o~ the angles :

105;~0~3 at which flame~ issue from the gas burner~ 6. The gas burners 6 are~connected by connecting tubes 7, each of which include~ a tap 21 to a gas supply.
A pair of air inlet tubes 22 are mounted in the reaction chamber 1, beneath the lowermost pair of ga9 burners 6. Each of the air inlet tubes 22 i9 provided either with a longitudinal slit 23 or a plurality of apertures spaced along its length to allow air to escape ~ into the reaction chamber. The air inlet tubes 22 are ; 10 pivotally mounted within the reaction chamber 1 in a manner similar to the gas burners 6, and they are so arranged that they provide a downwardly moving curtain o*
air within the reaction chamber 1. ~he air inlet tube9 22 are connected to an air supply by a connecting tube 24.
Rubber crumb particles fed to the open upper end of the reaction chamber 1 by the conveyor belt 19 fall under the action of gravity through the reaction chamber 1 between the successive pairs of gas burners 6. The gas burners ~ provide flame, heated-air and gaseous combustion products which cause the surfaces of the rubber crumb particles to be seared. When the rubber crumb particle9 reach the downwardly moving curtain of air provided by the air inlet tubes 22, the heat treatment of the surface9 of the rubber crumb particles is quenched. The downwards movement of the air from the air inlet tubes 22 assist9 the action of gravity in the transfer of heat treated rubber crumb downwardly to the bottom end 10 of the reaction chamber 1. The bottom end 10 is open and allow9 ~,. ..
.-- . .

lOS'~0~i3 the heated rubber crumb to fall, for example, into a hopper or on to a conveyor belt for collection.
Provision may be made for the insertion of thermo-couples at various points in the reaction chamber 1 of the apparatus of Figure 3 in order to assist in achieving optimum temperature distributions within the reactio~
chamber 1.
Rubber particles may be treated by methods in accordance with the present invention by sujecting the rubber particles to the action of heat without causing the rubber particles to fall under gravity, through a zone or zones of heat, as will be understood from the following Example.
Exam~le 4 Approximately 500gm of ground whole tyre vulcanised rubber crumb, capable of passing through a sieve of 30 meshes/inch, was spread evenly and thinly on a metal tray. The flame fro~ a pressure fed butane/air burner was played on the exposed surface of the crumb while the burner was kept in motion such that the crumb fumed but did not reach the point of ignition. After several passes of the flame across the entire exposed surface, and allowing to cool between passes, the crumb was raked to expose fresh surfaces snd the flame treatment repeated.
Two fhrther cycles were repeated, by which time the treated rubber crumb thereby obtained tended to aggregate into fluffy clumps.
The treated rubber crumb prepared above was mixed with ~ulcanising ingredients on a two roll mill, with the 105;~ i3 ~, rolls at 40C and according to the formulation:-parts b~ weight Treated whole-tyre rubber crumb 100.00 Stearic acid 0.25 ~' Zinc oxide 1.25 Sulphur '5 Mercaptobenzthiazole 0.12 ~- Tetramethylthiuram disulphide 0.12 102.25 . . ~ .
A rough band was formed on the mill during mixing.
Some of the formulation was transferred from the mill to ~ ~;
8 flat sheet mould and was vulcanised between the platten~
~` o~ a press heated to 150C for 15 minutes. On stripping ,~ hot from the mould, it was found that the product was no t~`` longer plastic but had been converted into a substantially homogeneous vulcanised product which had good rubbery ,~ ~
9l properties. ~ -`~ Part of the formulation was used to form vulcanised ~ .. : . .
20 test pieces which were tested according to British Standard Specification No. 903. The test pieces exhibited --a tensile strength of 1,~20 p.s.i. and an elongation at break of 250%, which compare ~avourably with the tensile -;..
strengths of from 850 p.s.i. to 900 p.s.i. and elongatio~s at break o~ from 300% to 350% of vulcanised products obtained from first grade whole tyre reclaim rubber.
Accordingly treatment of rubber particles in accord-, ance with the present invention can be effected by the use of movement of either the particles or the zone oi , ...................................................................... .

:.

.

1 ~5'~ ~ 3 heat in a horizontal plane.
The appearance and texture of rubber crumb following thermal treatment by a method in accordance with the present invention varie9 depending upon the extentof the treatment and also the nature and composition of the vulcanised rubber crumb before treatment. For example, the treated rubber crumb may take the form oi a loosely aggregated material having a tacky surface or it may be a free nowing material having a dusty surface appearance.
Figure 4 of the drawings illustrates diagrammatically a`segment which has been cut away from a rubber particle which has undergone thermal treatment in accordance with the present invention. The particle has a core portion 15 which remains non-plastic. ~he core portion 15 is surrounded by an intermediate zone 16 which is partially plasticised and this intermediate zone is in turn surrounded by an outer layer 17 which is substantially completely plasticised and is miscible either with the outer layers of other particles of treated rubber crumb or with unvulcani~ed rubber compound. ~he outer surface 18 of the outer layer 17 may in some instances be coated at least in part with free carbon residue~ from the combustion of either textile residues in the crumb prior to treatment or from limited surface combustion exper-ienced during the heat treatment.
The boundaries between the core portion 15 and the intermediate zone 16 and the intermediate zone 16 and the outer layer 17 are not clearly defined and there is a gradual gradation in the degree of plasticity fr~m the core portion 15 which i~ non-plastic to the outer surface 18 at which the rubber is fully plasticised. When the treated rubber particles are consolidated or when they are blended with unvulcanised rubber compounds no clearly defined interfaces between particles are observable. On cross-linking of the consolidated particles or the blend of particles with unvulcanised rubber compound a substantially homogeneous product is formed which is free from the obvious presence of individual rubber particles.

Rubber material having an inner core of non-plastic ~ubber and a surface layer which is plastic in accordance with the present invention may be used in substantially all the processes which have hitherto employed either - ;
vulcanised rubber crumb or the 80 called "reclaim rubber"
which is a completely plasticised product prepared from vulcanised rubber. However, rubber material reclaimed ~ : .
by methods in accordance with the present invention have ~ ;
the advantage over untreated rubber crumb that they may be used in the manufacture of croæs-linked rubber products ~-without any need for blending with fresh unvulcanised rubber. Oneadvantage of pieces of rubber material treated by methods in accordance with the present invention over "reclaim rubber" is that it may be formed and vulcanised in the presence of sufficient additional ~` vulcanising or other cross-linking agents for cro~s-linking of the plasticised and partially plasticised parts only of the rubber pieces. In cases where the vulcanising or other cross-linking agents in the untreated rubber material were incompletely consumed and remain in the inner cores .. . . .

' 105;~ 3 I

! of the treated rubber pieces they may be sufficient upon migration into the plasticised and partially plasticised parts of the treated rubber either to wholly or partially overcome the need for including additional vulcanising or other cross-linking agents.
The methods which have been specifically described f of treating rubber materials in order to obtain pieces oi rubber material having inner cores of non-plastic rubber and surface layers which are plastic have the advantage 10 that they require a lower energy input in the preparation of a useful material from given rubber materials than f:' the known methods of preparing 'Ireclaim rubber" from those materials. This is in part due to the *act that only the ~ outer layers of the rubber pieces are treated and the core `~ portions of the pieces of treated rubber are substantially unchanged, whereas the known methods of preparing "reclaim rubber" involve plasticising the entire mass of the rubber by a combination of added heat, action of chemical adaitives and mechanical work. The methods which have been specif-20 ically described and which involve a heat treatment alone to obtain pieces of rubber material having inner core9 of non-plastic rubber and surface layers which are plastic have the further advantage over the known methods of preparing "reclaim rubber" that chemical additives are not used. It will be appreciated that savings in energy ; expenditure and/or the use of chemical additives will result in appreciable cost advantages. ~urthermore vulcanised rubber crumb which usually includes textile residue9 may readily be treated as hereinbefore described .. .
,, :. . . ..
.. ..
. : ', " ' lOS~0~3 - by methods in accordance with the present invention, whereas rubber crumb which includes textileresidue~ has generally been unacceptable for the preparation of "reclaim rubber".
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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 reclaiming vulcanised rubber which comprises the step of causing the surface of vulcanised rubber to be seared by exposing the vulcanised rubber to a temperature at which the rubber can be destroyed by combustion in air, the exposure being for a time insufficient for the rubber to ignite, whereby a surface layer of the rubber is rendered plastic by devulcanisation while an inner core of the rubber remains vulcanised and non-plastic.

2. A method according to Claim 1 in which the surface of the vulcanised rubber is seared by direct application thereto of a flame.

3. A method according to Claim 2 which comprises the step of causing vulcanised rubber crumb to fall under gravity into the blue zone of a flame derived from a pressure fed gas/air burner.

4. A method according to Claim 1 which comprises the step of causing vulcanised rubber crumb to fall under gravity through a zone of heated gas.

5. A method according to Claim 1 in which the surface of the vulcanised rubber is seared by passing the rubber through a plurality of zones of heat.

6. A method of reclaiming vulcanised rubber which comprises the step of searing the surfaces of particles of vulcanised rubber crumb by the application thereto of at least one flame from a pressure fed gas/air burner for a time insufficient for the vulcanised rubber to ignite whereby the surface layers of the particles of rubber crumb are rendered plastic by devulcanisation whilst the inner cores of the particles remain vulcanised and non-plastic.

7. A method according to Claim 6 wherein the surfaces of the particles of rubber crumb are quenched by the application of cooling air after the surfaces have been seared.

8. Rubber material having an inner core of vulcanised non-plastic rubber and a surface layer which is devulcanised and plastic, and capable of coalescing with the surface layer of similar rubber material or with unvulcanised rubber.