Field of the Invention

This invention relates to the field of recycling rubber crumb, and in particular to a rubber-crumb-based moldable composition in the form of a free-flowing powder, and a method of making the same. The rubber crumb can be derived from synthetic or natural rubber.

Background of the Invention

Rubber crumb is a granular substance typically made from recycled automotive and truck scrap tires, but it more generally includes any vulcanized and granulated particle of post-consumer or post-industrial rubber waste. It can be molded into a number of useful products, such as floor tiles and mats, pipe insulation, garbage cans, and shoe soles. Rubber crumb is classified by mesh size. For example, 10 mesh crumb rubber has passed through a screen with 10 holes per inch resulting in rubber granulate that is slightly less than 1/10 of an inch.

Typically compositions produced from rubber crumb employ a polyurethane binder in liquid form derived from methylene diphenyl diisocyanate (mdi) based poly- isocyanate. Heat, pressure and the presence of water molecules in the slurry result in the polymerization of the polyurethane pre-polymers, resulting in the encapsulation of the rubber particles into a polyurethane matrix.

The handling, dispensing and blending of the polyurethane liquid creates problems of spillage and contamination, both of which contribute greatly to processing cost. Cleaning up the mixing and dispensing equipment is problematic given that the polyurethane cures onto the processing equipment. Cured polyurethane requires operators using jackhammers, reciprocating powered saws and knives to remove most of the mixture (although it is almost impossible to remove it all). Once the polyurethane is mixed with the activator in the blender, it has to be used up within a short time frame (measured in minutes), as a result of which the wastage of raw material (blended crumb and polyurethane) and the variability from batch to batch can lead to process issues including premature curing of the final molded product rendering it as scrap. Also, the polyurethane binder essentially encapsulates the rubber crumb as opposed to creating a bond at the molecular level, thus limiting the physical properties of the final molded product.

Since the polyurethane is a derived material, it is not compatible with the curing of disparate rubber compounds in combination with the polyurethane bound crumb.

The short shelf life of the mdi pre-polymer and fouling of the mixing equipment requiring extensive cleaning and waste due to pre-cure of the crumb binder slurry make the use of polyurethane binder undesirable.

Polyurethane and polyurethane bound crumb disintegrates over a period of time if exposed to rain, snow or moisture more generally. As such products molded with polyurethane bound crumb have a limited life in outdoors and in other applications in which exposure to moisture is prevalent. Additionally, polyurethane disintegrates over a period of time given exposure to uric acid and ammonia. As such, given the prevalence of all of uric acid, ammonia and water in urine, animal mats manufactured using polyurethane bound crumb, as an example, are particularly susceptible to degradation. Summary of the Invention

The present invention activates the surface of a synthetic and/or natural rubber crumb by shear mixing in the presence of virgin diene polymers and vulcanizing agents to promote cross-linking under the application of heat and pressure. The applicants have found that by surface-activating the rubber crumb, it is possible to make comparable products of the prior art without the need for a polyurethane binder. The rubber particles can directly cross link with each other and with cross linking sites in the diene polymer binder.

According to the present invention there is provided a method of making a rubber- crumb-based moldable composition, comprising creating a mixture of rubber crumb and at least 1% by weight of virgin diene polymer in the presence of vulcanizing agents in an intensive or shear mixer to produce a surface-activated free-flowing curable powder.

Virgin in this context refers to fresh polymer that is added as distinct from diene elastomers that may inherently be present in the rubber crumb.

A suitable polymer is styrene butadiene polymer, although the virgin dienes suitable for use in the invention could be any sulfur curable diene elastomer that has a cross- linking site for the sulfur. In addition to styrene butadiene rubber, examples include natural rubber, styrene butadiene rubber, polybutadiene rubber, epdm rubber and nitrile rubber. " .

The rubber crumb may generally be derived from a cured rubber article comprising a cross-linked elastomer, including but not limited to NR (natural rubber), SBR (styrene-butadiene rubber). BR (butadiene rubber), CR (chloroprene rubber), EPDM (ethylene propylene diene monomer (M-class) rubber), NBR (acrylonitrile butadiene rubber), HNBR (hydrogenated NRB rubber), ACM (acrylic rubber), FKM (a fluorelastomer rubber), and IIR (isobutylene-isoprene rubber)

It is believed that the rubber crumb can have a wide range of mesh sizes such as 4 - 60, although 8 - 30 has been found to work best, with 8 - 14 being the most preferred range.

Intensive and high shear mixers are known in the art. A suitable example of a high shear mixer is known as a Banbury™ mixer. The Banbury mixer consists of two rotating spiral-shaped blades encased in segments of cylindrical housings. These intersect so as to leave a ridge between the blades. The blades may be cored for circulation of heating or cooling.

Preferably in a first mixing treatment the rubber crumb is mixed with the virgin diene polymer, and in a second mixing treatment the vulcanizing agents including the accelerators are mixed in to the powder.

The virgin diene polymer should typically be present at less than 30 % by weight, preferably less than 10% by weight. A suitable polymer is styrene butadiene polymer.

The crumb rubber is generally obtained by grinding used tires. It is typically in the form of granules having a size of several hundred microns, which include all components that go into a formulation of a composition for a tire, such as diene elastomers, reinforcing fillers, non-reinforcing fillers, plasticizers, vulcanization additives and protective agents. The rubber crumb also include products formed by reactions undergone by these components during the various stages of manufacture of the composition of the tire, in particular during the vulcanization step, and during the life of the tire. The rubber crumb used in accordance with the invention is not devulcanized. Preferred embodiments of the invention relate to compositions prepared in a Banbury mixer where;

1 -10% of the composition consists of virgin styrene butadiene polymer 90-99% of the composition consists of 8-14 mesh crumb 0.5-5% sulphur

0.5-5% rubber accelerators and vulcanization activators.

The vulcanizing agents include sulphur and accelerators, such as thiazoles, dithiocarbamates and thiurams. Zinc oxide may also be added as a vulcanization activator.

Embodiments in accordance with the invention have several advantages over the prior art. Unlike the prior art, the problem of liquid spillage is avoided as the powder is a liquid solid or semi-solid. The production of free flowing rubber crumb can be produced using existing traditional rubber compounding and molding equipment. The mixture can be stored for months before being used in a molding process, and as such has a very good shelf life.

The composition does not require cleaning up the mixer often and the nature of the cleaning process is very much less labour intensive and invasive if the mixer is required for other uses.

The mixture can be produced with less variability from batch to batch given that the cure is not activated at the mixing stage, but only occurs subsequently when the powder is placed in a compression mold under heat and pressure. The surface-activated free-flowing powder produced by the method of the present invention may then be molded into useful articles under the application of heat and pressure, which causes cross-linking between the surface-granules.

According to another aspect of the invention there is provided a moldable free- flowing curable powder comprising a surface-activated rubber- crumb-based, at least 1 % by weight of virgin diene polymer, and at least 0.5 % vulcanizing agents.

The free flowing powder made in accordance with the invention may be pressure formed in to useful articles, such as rubber slabs, in a mold at an elevated temperature.

Brief Description of the Drawings

This invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which the single figure shows the sequence of steps for making a material in accordance with embodiments of the invention.

Detailed Description of Preferred Embodiments

Referring now to the single figure, in a first step 10 a charge of rubber crumb and virgin diene polymer binder is placed in a mixer, such a Banbury mixer. A diene is a hydrocarbon containing two double bonds, such as styrene butadiene.

The first charge is premixed at step 11 until the temperature reaches a predetermined value, in one example 90°C. Next, at step 12 a second charge including curatives and accelerators is added. The mixture is then mixed at step 13 until a second temperature is reached, in the example, 100°C. The mixture is discharged and allowed to reach room temperature at step 14. It can then be cured and compression molded, for example, in a slab mold at step 15. Example 1 (laboratory demonstration)

A first charge consisting of 900 grams of 8-14 mesh tire crumb and 100 grams of styrene butadiene (SBR 1605 MB) as binder rubber (approx. 9.5 % by weight) was premixed in a Banbury mixer. The Banbury mixer was run at 60Hz to premix the rubber crumb and binder rubber until the temperature reached 90°C.

A second charge consisting of 21 grams (2%) of zinc oxide as a vulcanizing promoter, 10.5 grams (1%) of stearic acid as a surfactant, 10.5 grams (1%) of sulfur as a vulcanizing agent, 5.025 grams (0.5%) of ZDBC (zinc dibutyl dithiocarbamate) accelerator, and 5.025 grams (0.5%) of TBBS (N-tert-butyl-2- benzothiazolesulfenamide) accelerator was added to the Banbury mixer at 90°C.

The Banbury mixer was then run until the temperature reached 100°C. The resulting mix was then discharged and allowed to cool to room temperature (25/30 °C).

The resultant mix was a free flowing crumb mix that was press cured at 149°C (300°F) for 12 minutes in a half inch thick slab mold cavity (16' x 16") under 1800psi (12,410.6 kilopascal) pressure.

Example 2 (plant demonstration)

A first charge consisting of 360 lbs (163 kg) of 8-14 mesh tire crumb and 40 lbs (18kg) of styrene butadiene (SBR 1605 MB) as binder rubber (approx. 9.5% by weight) was premixed in a Banbury mixer. The Banbury mixer was run at 60Hz to premix the rubber crumb and binder rubber until the temperature reached 90°C.

A second charge consisting of 8.4 lbs (3.8kg) of zinc oxide as a vulcanizing promoter, 4.2 lbs (1.9kg) of stearic acid as a surfactant, 4.2 lbs (1.9kg) of sulfur as a vulcanizing agent, 2.1 lbs (0.95 kg) of ZDBC (zinc dibutyl dithiocarbamate) accelerator, and 2.1 lbs (0.95 kg) of TBBS (N-tert-butyl-2-benzothiazolesulfenamide) accelerator was added to the Banbury mixer at 90°C.

The Banbury mixer was then run until the temperature reached 100°C. The resulting mix was then discharged and allowed to cool to room temperature (25/30 °C).

The resultant mix was a free flowing crumb mix that was press cured at 149°C (300°F) for 12 minutes in a half inch thick slab mold cavity (16' x 16") under 1800psi (12,410.6 kilopascal) pressure.

The material obtained in accordance with embodiments of the invention is designated JGMR.

Results

The following table shows the results of tests performed on JGMR material obtained in accordance with embodiments of the invention for different percentages of rubber binder compared with prior art material using a polyurethane binder.

JGMR vs Polvurethane Bound Rubber Crumb

350 400 700 750

347 (2393)

TENSILE PSI (kPa) (2413) (2,757) (4826) (5, 171)

Durometer 70-75 60-65 60-65 60-65 60-65

This table shows that the JGMR material made in accordance with embodiments of the invention compares favorably with the prior art material using polyurethane without the attendant disadvantages.

A notable feature of material made in accordance with embodiments of the invention is that it offers a larger range of achievable properties. The lower and upper limits of hardness are greater as well as the possible upper limit on tensile strength. Generally more tensile and elongation are associated with more durability in service life of elastomer compounds. For example, at 30% virgin diene polymer, the tensile strength is 750 psi

It has been found in the case of the prior art that the addition of more polyurethane above 5% will causes additional equipment problems relating to fouling and pre-cure beyond what is currently experienced.