Note: Descriptions are shown in the official language in which they were submitted.
- 2152641
Improvements in and rel~tin~ to
the reCI~imin~ of elastomeric materials
5 Field of Invention
This invention concerns the recl~iming of elastomeric products such as tyres, mouldings,
gloves and beltings made from natural rubber or synthetic rubber or blends thereof which
were originally vulcanised by the conventional sulphur-accelerated vulcanising systems so that
10 the reclaimed elastomeric material may be recycled.
Prior Art
Recycling of reclaimed rubber from used rubber products is well-known in the industry
where some 200,000 tonnes of recycled rubber is involved per annum. The conventional
rubber recycling processes use high te-npe~alule and catalysts to digest the elastomeric
material being recycled resulting in a high consumption of energy and appreciable
degradation of the elastomeric material being reclaimed. Thus, such reclaimed rubber has
uses which are restricted by its poor physical prope-lies. A typical reclaimed rubber has
20 tensile strength not more than 5 to 6 MPa while raw natural rubber with the same compound
can provide strengths of over 20 MPa. The conventional process is also labour intensive and
is difficult and complicated with respect to quality management and standardisation.
The conventional rubber processes in essence consists of taking vulcanised rubber crumbs,
25 a<1mixing them with catalysts and subjecting the admixture to le-llpelalllres of more than
170C for periods of more than 4 to 6 hours in a digestor. The resulting material is then
subjected to m~tiç~tion until it is rendered into sheet form. Such rubber products are used
in small proportions as processing aids or diluents with fresh rubber compounds. The
presence of recycled rubber in the mixture will adversely affect the physical and dynamic
30 properties of the final vulc~ni~te.
Used tyres and other rubber articles are becoming an environmental hazard globally. There
is a distinct demand for a satisfactory recycling process to address this ever increasing
environmental problem. The used tyre mountains now in existence globally are a fire
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h~7~rd. Many attempts and approaches have been made to assuage this environmental issue.
Among these could be mentioned the use of pelletized tyre crumbs for road surfacing, the
burning of such crumbs to generate energy and so on.
s It is known that hexamethylene tetramine and resorcinol, in the presence of accelerators, do
tend to cut crosslinks in vulcanised rubber and this is reported in liLelaLule em~n~ting from
Czechoslovakia (hereinafter "the Czechoslovakian process"). But this process is
uncontrollable and produces variation and degradation in the resultant recycled rubber.
While the meçh~ni~m of this reaction is not described in detail, it is believed that some form
10 of proton transfer reaction is involved.
Thus, none of the conventional methods or approaches have s~çcee~ed in making any real
progress in solving this vexing global problem.
15 There is, therefore, required a method of effectively recycling used rubber products entailing
a cost effective process which will open up or delink the crosslinks of the vulcanized network
structure in used rubber crumbs without unduly degrading the backbone polymer. The more
successful reclaimed rubber m~int~in.c the original physical and dynamic characteristics of
original natural and synthetic rubbers, the wider the applicability of such reclaimed rubbers
20 in further rubber manufacturing process.
Description of the invention
We have now developed a process which essentially converts used rubber product, preferably
25 in the form of crumbs into a elastomeric material having properties which approach that of
fresh elastomers in similar compounds. This process is elegant, simple and is neither energy
nor labour-intensive.
We have reinvestigated the Czechoslovakian process and have now developed an entirely new
30 system to effect the proton transfer reaction which is believed to be essential. Our process,
however, does not use hexamine which is a hazardous chemical to handle. Instead, we have
relied on chemicals already used in the conventional rubber manufacturing process.
`~ 3 2152641
Our discovery essentially co~ ises of the use of a novel chemical mixture which is capable
of initiating proton exchange in a controlled manner at tempera~ures below 70C, preferably
below 50C, i.e. by opening up or delinking the vulcanised network of elastomer material.
Accordingly, the present invention provides a process for reclaiming elastomeric material
from elemental sulphur-cured elastomeric material, which process comprises treating the said
sulphur-cured elastomeric materials with one or more compounds which are capable of
performing the rubber accelerator functiorl and one or more activators capable of initi~ting
proton exchange at temperalules below 70C and thereby open up or delink the vulcanised
network of the sulphur-cured elastomeric material to provide sulphur-curable reclaimed
elastomeric material.
The invention also includes the delinking compositions which are described herein.
Our novel chemical composition (hereinafter "Delink") comprises one or more chemical
compounds which are capable of performing the rubber accelerator function and one or more
activators. The compounds, which are used conventionally as rubber accelerators, when used
cojointly with one or more activators capable of initiating proton exchange at temperatures
below 70C has to be capable of opening up or delinking the vulcanised network of the
sulphur-cured elastomeric material to provide sulphur-curable reclaimed elastomeric material.
Preferably, the compounds which may perform the function of rubber accelerator comprise
zinc salt of thiocarbamates such as zinc dimethyldithiocarbamate (hereinafter "ZDMC") and
2-mercaplobenzothiazole (hereinfater "MBT"), or derivatives thereof, in the molar ratio in
the range of 1:1 to 1:12. The accelerators are activated by one or more activators,
preferably stearic acid and zinc oxid. Sulphur may be additionally added to the mixture.
ZDMC and MBT being mentioned above as prefelled accelerators may be replaced with
other accelerators some of which may be less active. The following, which are no means
exhaustive, are examples of known accelerators which may replace ZDMC and MBT.
ZDMC may be replaced on a molecular basis by other zinc salts of dithiocarbamates such
as zinc dimethyldithiocarbamate zinc diethyldithiocarbamate (ZDEC), zinc
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dipropyldithiocarbamate, zinc dibutylthiocarbamate (ZBDC) or zinc dibenzyldithiocarbamate
(ZBEC), or by zinc dialkyl dithiophosphates such as zinc dibutyldithiophosphate, and other
chemicals which may pel~ol,l, the function of rubber accelerator.
s Similarly, MBT may be replaced on a molecular basis by other thiazole accelerators such as
benzothiazyl disulphide (MBTS), or zinc 2-mercaptobenzothiazole (ZMBT), or by
suphenamide accelerators such as N-cyclhexyl-2-benzothiazole sulphenamide (CBS) or N-tert-
butyl 2-benzothiazole sulphenamide (TBBS), or by by thiuram accelerators such astetraethylthiuram disulphide (TETD), tetramethylthiuram disulphide (TMTD) or
o tetrabenzylthiuram disulphide (TBETD), or by nitrogen-based accelerators such as
gll~nidinPs, N,N'-diphenylguanidine, d-o~ho-tolylguanidine, and 4,4'-dithiomorpholine, or
any other chemicals which may perform the function of rubber accelerator.
The combination of MBT or derivatives of MBT or other accelerators and ZDMC or
5 derivatives of ZDMC in the molecular proportion initi~tes the proton exchange reaction
which is assisted by the presence of stearic acid and zinc oxide. The presence of a small
amount of sulphur was found to aid ultimate vulc~ni~tion but is not npcess~ry. Similarly,
the presence of diol may help in the dispersion of the powders and perhaps activates the
mixture but this is not essential.
This novel chemical mixture, or Delink (the delinking composition hereof), when blended
with tyre crumbs or any other vulcanised elastomeric crumbs in concentrations of preferably
6 parts Delink per 100 parts of rubber crumbs on a mill, effectively delinks the vulcanised
network and renders the resulting sulphur-curable reclaimed elastomer ready for moulding
25 and vulc~ni7~tion. The total milling period, which occurs preferably at temperatures below
70C, takes only 7 to 10 minutes. Alternatively, the Delink and tyre crumbs could be first
mixed in an intermix and subsequently milled in an open mill.
We have also developed a more convenient method of handling the delinking composition
30 (Delink), i.e. by the "~aslell,alcl method. The Delink is first mixed with fresh or vulcanised
rubber, the ratios of Delink to rubber varying between 90:10 to 40:60. This ma~lelbatcll
mixture can be mixed with vulcanised crumbs in proportions which will ensure that the
ultimate ratio of Delink:rubber is 6:100. Parts are parts by weight.
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Some embodiments of the invention will now be described, by way of illustration, with
reference to the following Examples and Tables, and the properties of the ensuing compounds
illustrate the manner in which this invention is effective in practice.
Example 1
The following materials are mixed in the given proportions:-
1. 2-merc~tobenzothiazol (MBT) 20.0
2. Zinc dimethyldithiocarbamate (ZDMC) 6.0
3. Stearic acid 2.0
4. Zinc oxide 2.0
5. Sulphur 1.5
6. Diethylene glycol 12.0
The powders are first intim~tely mixed with vigorous stirring and then the diethylene glycol
added to render the whole Delink composition into a smooth paste.
500 g of tyre crumbs are milled in a 2 roll mill for 3 mimltes and then 15 g of the Delink
added. After further milling for 2 minlltes another 15 g of the Delink is introduced. The
final 2 minutes of milling are carried out at narrow nip. By this time, it is possible to sheet
2s out the compound which is ready for vulc~ni7~tion at 150C for 15 to 30 mimltes.
Properties of the vulc~ni~tes using tyre crumbs originating from Malaysia and Europe are
shown in Table 1 below.
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Table 1
CHARACTERISTICS OF VULCANISATES
sprepared from tyre crumbs of 100% natural rubber
Country of origin Malaysia Europe
Ch~u~lc-islics 1 2 3 4
1. Size (mm) 0.1 - 0.5 0.1 - 0.5 0.1 - 0.5 > 1.0
o 2. Delink content 6 6 6 6
(parts per hundred of crumb)
3. V~ tion (C) 143 155 143 143
4. Mooney viscosity, Mtl 68 68 75 120
5. Stress at 100% elongation (MPa) 3 3 3.5 5.5
6. Tensile strength (MPa) 12.1 12.7 14.5 16
7. Elongation at break (%) 350 350 375 400
8. Elongation set (%) 15 15 15 12
9. Relative concenll~lion of chains, 1.1 1.2 1.2 1.3
Vte,x104 (mo1e/cm3)*
10. High elastic component of creep, A x103 0.5 0.5 0.4 0.4
(MPa) (aO= 0.8MPa)**
20 * V~l was calculated from the Mooney-Rivlin equation:
vtcl = E
RT ()~ 2)
where E is strain;
~ is the degree of elongation;
R is the gas constant; and
T is temperature
** Creep was c~l~ul~ed as follows:
D(a, T) = Do (a, T) + A (a, T) logT + ~l71 (a~ T)
where D is strain (%), Do is the initial strain at ~ = 1 min (%);
A is the rate constant of creep of high elastic strain (1/MPa);
~1 is viscosity (Pa-s)
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Example 2
The Delink described in Example 1 in this case is incorporated into tyre crumbs and
s pelletised used gloves, again in the proportion of Delink:vulcanised crumbs at 6:100. The
masti~ti~tion and Delink incorporation process are as described in Example 1. The mill
temperature is not allowed to exceed 70C. The total milling time is kept below 10 minutes.
Table 2 below shows the properties obtained from tyre and glove crumbs. For comparison
purposes the general properties obtained from fresh rubber both with (tyre compounds) and
o without fillers (pure gum compound) are shown in Table 2 below.
Table 2
Properties obtained from scrap tyre treads and scrap gloves
in colllparison with compounds using fresh rubber
Material type tyre scrapfresh rubber fresh rubber
scrap glove(tyre compounds)(gum compounds)
Tensile strength (MPa) 13 14.5 18 - 21 21 - 23
Elongation (%) 300 900 350 - 500 700 - 800
Elongation set (%) 10 15
Relative concentration of 1.4 0.6
chains, V,~" x103 (mole/cm3)
Example 3
In this Example, the Delink (as described in Example 1) is admixed with fresh rubber, tyre
30 crumbs and glove crumbs to obtain master batches cont~inin~ 90% Delink and 50% Delink.
The Illas~lbalches so produced are then incorporated sep~ely into tyre crumbs and glove
crumbs in the manner described under Example 1, except that, in the case of the 90% Delink
Illasle~ ch, 6.6 parts of the master batch is used for 100 parts of vulcanised crumbs and for
the 50% master batch, 12 parts per 100 of crumbs are employed. The resultant propel~ies
3s are shown in Table 3.
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Table 3
Properties obtained using Delink masterbatches
Type of fresh NR: NR:DelinkGloves:Delink Gloves:Delink
~I. ,~ Delink 10:90 50:50 10:90 50:50
V~scrap: tyre glovestyregloves tyregloves tyre gloves
tread tread tread tread
Tensile strength (MPa) 11.5 17 11 10 10 13.5 12 10.5
Fl~ n (%) 250 860 250 610 250 780 320 600
Flon~a~ion set (%) 5 10 6 15 10 10 6 15
Relative ~ nn of 1.3 0.7 1.3 0.6 1.2 0.6 1.3 0.6
chains, Vr,l x104 (mole/cm3)
The above examples are illustrative of the versatility and effectiveness of the process of the
present invention to provide recycled rubber compounds from used natural rubbert synthetic
15 rubber vulc~ni~tes and combinations of natural and synthetic rubber vulc~ni~tes.
The resultant reclaimed rubber compounds display satisfactory level of physical and dynamic
characteristics. Such compounds can be directly used in moulded goods or in admixture with
fresh compounds. Examples of such goods which have been made using reclaimed elastomer
20 in conventional methods of moulding and vulcanisation are tyres, mats, carpet underlays,
electrical insulation layers, industrial tyres, tubings and retreads.
The ma~ell,alching process can be further refined with use of styrene butadiene rubber in
place of natural rubber. With careful control of the Delink incorporation process, the milling
25 process and le-l-pelalllre during mastication, the resultant physical and dynamic characteristics
of the final reclaimed compound from tyre and glove wastes can be further enhanced.
The following examples illustrate some cases where MBT is replaced with other accelerators.
Example 4
The following materials are mixed in the given proportions:-
1. Benzothiazyl disulphide (MBTS) 27. 8
2. Zinc diethyldithiocarbamate (ZDEC) 7.6
3. Stearic acid 5.1
4. Zinc oxide 2.5
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Then 10 parts of this Delink mix are added to 100 parts of tyre crumbs and 12.5 parts of rawnatural rubber as described in Example 1. The relevant physical properties of the mix are
shown in Table 4 below.
Table 4
Properties of MBTS mix using raw natural rubber
Tensile strength (MPa) 10.0
Elongation at break (5) 336
o Modulus at 300% elongation (MPa) 5.6
Example 5
15 The following materials are mixed in the given proportions:-
1. N-cyclohexyl-2-benzothiazole sulphenamide (CBS) 43.9
2. Zinc diethyldithiocarbamate (ZDEC) 7.6
3. Stearic acid 5.1
4. Zinc oxide 2.5
Then 10 parts of this Delink mix are added to 100 parts of tyre crumbs and 12.5 parts of raw
natural rubber as described in Example 1. Some relevant physical properties of the mix are
shown in Table 5.
Table 5
Properties of CBS mix using raw natural rubber
Tensile strength (MPa) 11.1
Elongation at break (%) 369
Modulus at 300% elongation (MPa) 7.9
Example 6
The following materials are mixed in the given proportions:-
1. N-tert-butyl-2-benzothiazole sulphenamide (TBBS) 39.8
2. Zinc diethyldithiocarbamate (ZDEC) 7.6
3. Stearic acid 5.1
4. Zinc oxide 2.5
Then 10 parts of this Delink mix are added to 10 parts of tyre crumbs and 12.5 parts of raw
natural rubber as described in Example 1. Some relevant~physical properties of the mix are
45 shown in Table 6.
` -lo- 2152~41
Table 6
Properties of TBBS mix using raw natural rubber and tyre crumbs
Tensile strength (MPa) 12.0
s Elongation at break (%) 353
Modulus at 300% elongation (MPa) 9.3
The invention being thus described, it will be obvious that the specific procedures described
herein may be varied in many ways. Such variations are not to be regarded as a departure
from the scope of the invention.
There are described above novel features which the skilled person in the art will appreciate
give rise to advantages. It is to be noted that in addition to or as an allelllali~e to the use
of stearic acid there may be used methacrylic acid. Also in place of diethylene glycol, there
may be used propylene glycol, dipropylene glycol or triethylene glycol as well as other
suitable diols which can be identified by simple experimentation.
Apart from the compounds which are capable of performing the rubber accelerator function
20 as described above, it should be understood by the person skilled in the art that the said
compounds which fall within the scope of this invention include compounds conventionally
used and compounds that may be used as rubber accelerators.
These are each independent aspects of the present invention irrespective of whether or not
25 they are included within the scope of the following claims. For example, the ingredients of
the "paste" of Example 1 may be varied by plus or minus 20 %, preferably by plus or minus
10%. Accordingly, the delinking composition may co-llptise the (a) ingredient such as
ZDMC in the range of 4.8 to 7.2 parts by weight and the (b) ingredient such as MBT in the
range of 16 to 24 parts by weight. Preferably, there is also present in the delinking
composition stearic acid an zinc oxide both in the range 1.6 to 2.4 parts by weight. Further,
sulphur may be present in the range 1.2 to 1.8 by weight and a diol such as diethylene glycol
may be present in the range 9.6 to 14.4 parts by weight.
It is to be noted that the invention also includes the composition as described above and these
3s may being the form of pastes or ma~l~ll,alches.
