Thermoplastic Natural Rubber and Process for its Manufacture

Abstract

A process for preparing thermoplastic rubbers from blends of vulcanized rubber scrap materials and virgin Natural Rubbers is provided comprising blending the vulcanized rubber scrap materials with virgin Natural Rubbers (NR) on a two roll mill or an internal mixer to form a polymeric mixture, followed by passing the polymeric mixture with an olefinic plastic through a twin screw extruder along with devulcanizing additives at elevated temperatures. The plastic resin may be a virgin resin, a recycled resin or a mixture thereof. The process is termed “dynamic devulcanization” to denote that the rubber scrap material is being devulcanized and intimately homogenized with virgin NR while it is being blended with the plastic resin to form the said thermoplastic natural rubber. A compatibilizing agent such as phthalic anhydride is introduced at the tail end of the twin-screw extruder to enhance the interface adhesion between the devulcanized rubber, virgin NR and the plastic resin. The resultant thermoplastic natural rubbers exhibit superior physical properties such as tensile strength, elongation, hardness and flexural modulus compared to thermoplastic rubber compositions comprising mixtures of plastics, NR and recycled rubbers heretofore produced. In one embodiment, the thermoplastic rubbers are prepared from blends of partially vulcanized NR known as Superior Processing rubber in lieu of blends of NR and vulcanized rubber scrap materials for better uniformity and consistency of finished thermoplastic rubber products, and virgin plastic resins.

Claims

1. A process for the manufacture of thermoplastic rubber from blend of powdered vulcanized rubber materials and virgin natural rubbers comprising pelletizing the powdered vulcanized rubber materials and virgin natural rubber with epoxidized natural rubber and stearic acid, a zinc salt thereof to form a base mixture and blending said base mixture with an olefinic plastic resin in the presence of devulcanizing additives in a heated twin screw extruder together with a compatibilizer to form said thermoplastic rubber.

2. The process in accordance with claim 1, wherein said devulcanizing additives are selected from the group consisting of zinc oxide, ethylene vinyl acetate copolymer, rubber peptizing agents, at least one rubber accelerator, and mixtures thereof;

3. The process in accordance with claim 2, wherein said rubber accelerator is selected from the group consisting of mercpto benzthiazole, dibenzothiozole disulfide and diphenyl guanidine and or mixtures thereof;

4. The process in accordance with claim 1, wherein a pelletized virgin natural rubber is added during the extrusion of said powdered vulcanized rubber material with devulcanizing additives;

5. A process for manufacture of thermoplastic rubber comprising devulcanizing a vulcanized rubber material while blending with an olefinic plastic resin comprising the steps of: (a) extruding together from about 50 to about 100 parts of a vulcanized rubber material with from about 2.0 to about 7.5 parts of an epoxidized natural rubber and from about 1.0 to about 6.0 parts of stearic acid or salt thereof to form a base mixture; and (b) blending in a twin screw extruder from about 50 to about 85 parts of said base mixture with from about 50 to about 15 parts of an olefinic plastic resin, from about 1.0 to about 5.0 parts of zinc oxide, from about 0.2 to about 2.0 parts of at least one rubber accelerator, and from about 1.0 to about 8.0 parts of an ethylene vinyl acetate copolymer to form a thermoplastic rubber;

6. The process in accordance with claim 5, wherein said rubber accelerator is selected from the group consisting of mercapto benzthiazole, dibenzothiozole disulfide and diphenyl guanidine;

7. The process in accordance with claim 5, wherein said accelerator is from about 0.1 to about 0.5 parts of mercptobenzthiazole, from about 0.2 to about 1.0 parts dibenzothiozole disulfide and from about 0.2 to about 1.0 parts diphenyl guanidine;

8. The process in accordance with claim 5, wherein said blending is conducted in a twin screw extruder at elevated temperature with mixer having a starting temperature of about 180 to about 220.degree. C.;

9. The process in accordance with claim 5, wherein said vulcanized rubber material is derived from rubbers selected from the group consisting of natural rubber rich whole tire recycle rubber;

10. The process in accordance with claim 5, wherein said vulcanized rubber material is selected from the group consisting natural rubber truck tire retread scrap, passenger tire crumb and natural rubber latex glove scraps and mixtures thereof.

11. The process in accordance with claim 5, wherein said olefinic plastic resin is selected from the group consisting of polypropylene homopolymer, polypropylene copolymer, ethylene homopolymer and copolymers, high density polyethylene, low density polyethylene and mixtures thereof:

12. The process in accordance with claim 5, wherein said olefinic plastic resin is selected from the group consisting of virgin polypropylene, recycled polypropylene and mixtures thereof;

13. The process in accordance with claim 5, wherein the olefinic plastic is Styrene Butadiene Styrene block copolymer with 20 to 30% styrene content.

14. The process in accordance with claim 5, wherein the Natural Rubber is selected from virgin Natural Rubber and Superior Processing (SP Rubber) Natural Rubber, or mixtures thereof

Description

DETAILED DESCRIPTION

[0025] The present invention relates to a process for the production of thermoplastic rubber compositions derived from vulcanized scrap rubbers blended with an olefinic plastic resin, and more particularly to a “dynamic de-vulcanization” process. The term “dynamic de-vulcanization” refers to a process in which the devulcanization of vulcanized scrap rubber is accomplished during the mixing with one or more plastic resins at elevated temperatures in the presence of particular devulcanizing additives. Included among the types of vulcanized scrap rubber materials contemplated for use in the present invention are those derived from Natural Rubber, ethylene propylene terpolymer (EPDM), EPM copolymer, Styrene Butadiene Rubber, Polybutadiene Rubber and Nitrile Rubber. Suitable types of scrap rubber sources include, for example, (1) rubber scraps and rejected rubber parts generated from tire manufacturing processes, such scraps constituting mostly styrene butadiene rubber, polybutadiene rubber and natural rubber, (2) rubber scraps generated from automobile parts molding processes, such as molding flashes and rejected rubber parts, such scraps constituting EPDM Rubber, (3) rubber scraps generated from extrusion processes in the manufacture of automobile windscreens and window seals, such as start up rejects, blemished portions and rejected lengths, such scraps constituting EPDM rubber, (4) rubber scraps generated from calendaring processes in the manufacture of EPDM Roofing Membranes, such as torn sections, blemished and short run rejects, and (5) rubber scraps generated in the manufacturing processes of natural rubber latex gloves. Moreover, rubber scrap materials recovered from used articles that are discarded after use, conventionally termed “post consumer rejects”, including, for example, (1) used passenger tires which are ground into fine powders and composed mostly of Styrene Butadiene Rubber, (2) used truck tires ground into powders, most of which is composed of natural rubber and to a lesser smaller degree, polybutadiene rubber, (3) rubber automobile parts obtained from Vehicle Recovery Programs (such as Ford Motor Company's Vehicle Recycling Program) which are segregated into EPDM based parts of windscreens and window seals, and (4) natural rubber latex gloves used in electronic assembly lines and gathered after use for disposal. Suitable types of plastic resin components contemplated for use in the present invention are polypropylene copolymers (PP), polyethylene, low density polyethylene (LDPE) and high density polyethylene (HDPE), and recycled plastic resins of these materials, such as polypropylene derived from recycling processes in the recovery of used battery cases. However, it is to be understood that the types of rubbers and plastic resins disclosed herein are meant to be illustrative only and that scrap rubbers and recycled plastics derived from other sources may be used in the instant invention.

[0026] The process of the present invention comprises a preliminary treatment of the vulcanized scrap rubber material followed by a blending of the treated vulcanized scrap rubber material with an olefinic plastic resin. In both the preliminary treatment and blending operations, at least one additive to promote the devulcanization of the vulcanized scrap rubber material. The preliminary treatment is conducted on a two-roll miller followed by a grinder to produce free flowing particulate while the blending is conducted in an internal mixture or a twin screw extruder. Both operations are continued until a homogenous mixture is obtained. The ratio of the vulcanized rubber scrap material and at least one devulcanizing additive varies according to the desired melt flow properties of the final thermoplastic rubber product. In some instances, the addition of a virgin rubber-based compound of similar composition to that of the vulcanized scrap rubber may be added to the mix when it is desired to modify specific properties of the resulting thermoplastic rubber.

[0027] In the preliminary treatment step, a base mixture is formed by milling together the vulcanized scrap rubber material with epoxidized natural rubber and stearic acid or a zinc salt thereof on a rubber two-roll refiner mill with a friction ratio of the rolls running at about 1:1.8 to about 1:2.5. In order to minimize the strain on the mill and reduce the mixing time, the vulcanized scrap rubbers, if initially obtained in long strips and/or odd shapes, are first granulated or shredded into chips of about ½ inch diameter or less. The chips first are milled alone for a time sufficient for them to breakdown and form a loose band on the rolls. Then, epoxidized natural rubber, stearic acid or zinc stearate are added to the mill. The amount of epoxidized natural rubber varies from about 2.0 to about 7.5 parts/weight (based on the 100 parts of the scrap rubber component) depending upon the type of vulcanized scrap rubber used as the starting material. From about 1.0 to about 6.0 parts/weight (based on the 100 parts of the scrap rubber component) of stearic acid is added to the mixture at the end of the treatment step in order to soften the mixture prior to transferring the base mixture from the mill. The amount of stearic acid is dependent upon the hardness of vulcanized scrap rubber and the type of vulcanized scrap rubber used as a starting material. Zinc stearate may be substituted for the stearic acid.

[0028] The resulting mixture (hereinafter sometimes referred to as the “base mixture”) is first ground into a free flowing powder of between 10 and smaller, then transferred to twin screw extruder which is fitted with pelletizing facilities common in conventional plastic processing lines. An olefinic plastic resin is added to the mixer in a ratio ranging from about 50 parts to about 85 parts of the base mixture to about 50 parts to 15 parts of the olefinic plastic resin to form a rubber/resin mixture. Additionally, the twin screw extruder should have heating/cooling system to maintain the temperature within a certain range. The starting temperature of the mixer should be between about 200 and 250.degree. C.

[0029] The devulcanizing additives are fed together together with the treated rubber base mix powder. Dosage of additives range from about 1.0 to about 5.0 parts of zinc oxide, from about 1.0 to about 8.0 parts of ethylene vinyl acetate copolymer, and from about 0.2 to about 2.0 parts of a rubber accelerator compound (e.g. peptizer), all parts being based on 100 parts of the total scrap rubber component, are added to facilitate the “dynamic de-vulcanization”. Preferred rubber accelerator compounds include dibenzothiozole disulphide, diphenyl guanidine and mixtures thereof. Preferably, from 0.2 to about 1.0 parts of dibenzothiozole disulphide and from about 0.2 to about 1.0 parts of diphenyl guanadine are fed with treated rubber mix along with the other devulcanizing additives. If about 3.0 to about 5.0 parts of zinc stearate was used in the preliminary step, the zinc oxide may be omitted from the mixer. Of course, it is to be understood that the exact amount of each of these devulcanizing additives is dependent upon the type of vulcanized rubber scrap material used, as well as the type of any olefinic plastic scrap material.

[0030] During the extrusion process, the temperature of the mixture rises due to friction. The barrel temperature of the twin screw extruder should maintained such that it does not exceed 500 Deg C. The resulting Thermoplastic Rubber pellets are cooled and stored in bags or in silos. The following examples are provided merely to illustrate the present invention, and it is to be understood that the invention is not limited thereto. All amounts of the various ingredients in the examples and elsewhere in the specification are by weight unless otherwise specified. In each of the examples, one or more rubber scrap material is used as the rubber component. Relevant information regarding each rubber scrap material is provided hereinbelow.

[0031] SC-1: EPMD Roofing Membrane is scrap rubber arising from the manufacturing process of EPDM (ethylene-propylene terpolymer rubber) roof membrane having the following composition:

[0032] Component Parts EPDM Rubber (Grade: Du Pont's Nordel 1070) 80, Pelletized Natural Rubber (NR) 20, Zinc Oxide 5.00 Parafinic Process Oil 80.00 N 650 Carbon Black 120.00 Sulfur 1.50 Captax (rubber accelerator from Vanderbilt Chemicals) 0.50 Unads (Rubber Accelerator from Vanderbilt Chemicals) 1.50

[0033] SC-2: EPDM Automobile Scraps is a scrap rubber derived from automobile under-the-hood body plugs scraps having the following composition:

[0034] Component Parts EPDM Rubber (Grade: Du Pont's Nordel 2744)80.00, Pelletized Natural Rubber (NR) 20, Zinc Oxide 3.00 Stearic Acid 1.00 Parafinic Process Oil 150.00 N 650 Carbon Black 150.00 Sulfur 1.50 Captax (Rubber Accelerator from Vanderbilt Chemicals) 0.50 Unads (Rubber Accelerator from Vanderbilt Chemicals) 1.60

[0035] SC-3: EPMD Automobile Window and Windscreen Seals Scrap is a scrap rubber obtained from the manufacturing operations of Standard Products in Michigan having the following composition:

[0036] Component Parts EPDM Rubber (Grade: Uniroyal EPDM 585) 80.00, Pelletized Natural Rubber (NR) 20, Zinc Oxide 5.00 Stearic Acid 1.00 Parafinic Process Oil 70.00 N 650 Carbon Black 65.00 N 762 Carbon Black 30.00 Whiting 30.00 Azodicarbonamide 6.00 Butyl Zimate (Vanderbilt Chemical—Accelerator) 1.50 Sulfur 1.50 Captax (Rubber Accelerator from Vanderbilt Chemicals) 0.50 Tellurac (Rubber Accelerator from Vanderbilt Chemicals) 0.50 Thiate U 1.00 Elasto Cal 80 1.50

[0037] SC-4: Natural Rubber Truck Tire Retread Scrap is a scrap rubber having the following composition and properties:

[0038] Component Parts Natural Rubber (Grade: SMR 20) 80.00, Pelletized Natural Rubber (NR) 20, Renacit 7 (Peptiser from Bayer Chemicals) 0.40 Zinc Oxide 5.00 Stearic Acid 2.00 Aktiplast T (Bayer Chemicals: Process Aid) 1.50 Process Oil 15.00 N 220 Carbon Black 52.00 TQ (Antioxidant) 1.50 Santoflex 13 (Anti-ozonant from Monsanto Chemicals) 0.60 TBBS (Accelerator) 1.50 Sulfur 1.50 PVI (Prevulcansisation inhibitor) 0.10

[0039] SC-5: Passenger Tire Crumb is a rubber scrap from post consumer scraps (used tires) from NRI Industries having the following composition:

[0040] Components Parts Styrene Butadiene Rubber (Plioflex 1502 SBR) 50.00 TSR 20 Natural Rubber 50.00 Zinc Oxide 3.00 Stearic Acid 1.00 VANAWAX H Special 1.00 Aromatic Process Oil 9.00 N 229 Carbon Black 45.00 Antiozite 67P (Antioxidant/Antiozonant) 1.00 VANAX DPG 0.40 AMAX (Accelerator) 0.80 Sulfur 1.60

[0041] SC-6: Natural Rubber Latex Glove Scraps is a rubber scrap from the manufacture of gloves by Ansell, Inc. having the following composition:

[0042] Component Parts Natural Rubber Latex (Dry Rubber Content 60%) 167.00 Zinc Oxide 40% Dispersion 10.00 Oleic Acid 20% Emulsion 10.00 Aktiplast T (Bayer Chemicals: Process Aid) 1.50 Non staining Antioxidant 40% Dispersion 5.00 Titanium Dioxide 40% Dispersion 10.00 ZDC 40% Dispersion (Accelerator) 2.50 Sulfur 40% Dispersion 2.50

Example 1

[0043] Thermoplastic Rubber from EPMD Roofing Scrap Membrane and Polypropylene

[0044] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0045] EPDM Roofing Membrane Scrap (SC-1) 60.00 parts Epoxidised Natural rubber.sup.1 1.80 parts Stearic Acid 0.70 parts.sup.1 Epoxiprene, a cis 1,4 polyisoprene with epoxide groups randomly dispersed along the polymer backbone, manufactured by Kumpulan Guthrie Berhad of Malaysia as Epoxyprene 25 and Epoxyprene 50, indicating 25% and 50% epoxidation.

[0046] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0047] Base mix composition from preliminary step 60.00 parts Virgin Polypropylene.40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer.sup.3 3.00 parts .sup.2 Polypropylene manufactured by Exxon Chemical Company as Escorene PD 7414. .sup.3 Ethylene Vinyl Acetate copolymer manufactured by AT Plastics, Inc., having 40% vinyl content. The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 2

[0048] The Thermoplastic Rubber from EPDM Roofing Scrap Membrane and Polyethylene

[0049] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0050] EPDM Roofing Membrane Scrap (SC-1) 60.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0051] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0052] Base mix composition from preliminary step 62.50 parts Virgin Polyethylene.sup.4 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts .sup.4 Polyethylene manufactured by Dow Chemical U.S.A. as LDPE-4005M

[0053] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 3

[0054] Thermoplastic Rubber from EPDM Auto and Polypropylene

[0055] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0056] EPDM Auto under hood body plug Scrap (SC-2) 60.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0057] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0058] Base mix composition from preliminary step 62.50 parts Virgin Polypropylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0059] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 4

[0060] Thermoplastic Rubber from Vulcanized EPDM Auto Scraps and Polyethylene

[0061] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0062] EPDM Auto under hood body plug Scrap (SC-2) 60.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0063] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0064] Base mix composition from preliminary step 62.50 parts Virgin Polyethylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0065] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 5

[0066] Thermoplastic Rubber from Vulcanized EPDM Auto Scrap and Polypropylene

[0067] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0068] EPDM Auto under hood body plug (SC-2) 50.00 parts Plasticizer (Parafinic Oil Based) 40.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0069] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0070] Base mix composition from preliminary step 92.50 parts Virgin Polypropylene 10.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0071] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 6

[0072] Thermoplastic Rubber from EPDM Auto Scraps, EPDM Roofing Membrane Scraps and Polupropylene

[0073] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0074] EPDM Auto under hood body plug Scrap (SC-2) 35.00 parts EPDM Roofing Membrane Scrap—Ref. (SC-1) 15.00 parts Plasticizer (Parafinic Oil Based) 20.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0075] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0076] Base mix composition from preliminary step 72.5 parts Virgin Polypropylene 30.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0077] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 7

[0078] Thermoplastic Rubber from EPDM Auto Scraps and Polypropylene

[0079] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0080] EPDM auto window and windscreen seals scrap (SC-3) 60.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0081] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0082] Base mix composition from preliminary step 62.50 parts Virgin Polypropylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0083] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 8

[0084] Thermoplastic Rubber from EPDM Auto Scraps, Virgin EPDM Compound and Recycled Polypropylene

[0085] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0086] EPDM auto window and windscreen seal scrap (SC-3) 50.00 parts Virgin EPDM Compound.sup.5 20.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts .sup.5 EPDM 7 parts, Oil 6 parts, N650 black 6 parts In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0087] Base mix composition from preliminary step 72.50 parts Recycled Polypropylene.sup.6 30.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts.sup.6 Recycled-Polypropylene manufactured by KW Plastics of Bloomfield Hills, Mich. from used battery cases

[0088] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 9

[0089] Thermoplastic Rubber from Tire Retread Scraps, Natural Rubber Compound and Polypropylene

[0090] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0091] Truck Tire Retread Scrap (SC-4) 45.00 parts Natural Rubber Compound.sup.7 15.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts.sup.7 Natural Rubber 10, N 220 Black 5

[0092] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0093] Base mix composition from preliminary step 62.50 parts Virgin Polypropylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0094] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 10

[0095] Thermoplastic Rubber from Passenger Tire Crumb, Natural Latex Glove Rubber Rejuvenate and Polypropylene

[0096] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0097] Passenger Tire Crumb (SC-5) 45.00 parts Natural Latex Glove Rubber Rejuvenate 15.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0098] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0099] Base mix composition from preliminary step 62.50 parts Virgin Polypropylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0100] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 11

[0101] Thermoplastic Rubber from Natural Rubber Latex Glove Scrap and Polypropylene

[0102] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0103] Natural Rubber Latex Glove Scrap (SC-6) 60.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts

[0104] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0105] Base mix composition from preliminary step 62.5 parts Virgin Polypropylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0106] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 12

[0107] Thermoplastic Rubber from Used Passenger Tire Rubber Scrap, Virgin Natural Rubber and Recycled Polypropylene

[0108] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0109] Truck Tire Retread Scrap (SC-5) 48.00 parts Virgin Natural Rubber Compound.sup.8 12.00 parts Epoxidised Natural rubber 1.80 parts Stearic Acid 0.70 parts.sup.8 Natural Rubber 10, N 220 Black 5

[0110] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0111] Base mix composition from preliminary step 62.50 parts Recycled polypropylene 40.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts

[0112] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

Example 13

[0113] Thermoplastic Rubber from EPDM Rubber Scraps, Virgin EPDM Rubber Mix and Polypropylene

[0114] In the preliminary treatment step, a base mix composition was prepared on a refiner mill by mixing the following components for about ten (10) minutes until they were homogenous.

[0115] EPDM Rubber Scrap (SC-3) 50.00 parts Epoxidised Natural rubber 3.00 parts Stearic Acid 1.00 parts

[0116] In the final step, the thermoplastic rubber composition was formed from the following components in a variable speed internal mixer having an initial temperature of 250.degree. F.

[0117] Base mix composition from preliminary step 54.00 parts Virgin EPDM Rubber Mix.sup.9 20.00 parts Virgin Polypropylene 30.00 parts Zinc Oxide 3.00 parts Diphenyl Guanadine 0.25 parts Dibenzothiozole disulphide 0.50 parts Ethylene Vinyl Acetate copolymer 3.00 parts.sup.9 EPDM, Nordel 2744 100 parts, N 650 carbon black 150 parts, and paraffinic oil 150 parts

[0118] The components were in fed to a twin screw extruder fitted with pelletizing die and the extrudate either strand cut or under water pelletized. The resulting mixture was transferred to a dump mill, cooled and granulated.

[0119] Test specimens were prepared from the resulting thermoplastic rubbers of each sample by injection molding and tested in accordance with ASTM standards for physical properties and compared against several commercial grades of thermoplastic rubbers, specifically Santoprene™, manufactured by Advanced Elastomers. The results of these tests are provided in the following table.

[0120] Thermoplastic Tensile Elongation at Modulus at Shore A Flexural Specific Compression Rubber Strength (psi) Break % 50% Strain Hardness Index Modulus Mpa Gravity Set at 23.degree. C. Example 1 1660 170 1630 93 265 1.088 78 Example 2 990 80 910 87 61 1.096 38 Example 3 2600 410 2080 93 237 1.065 89 Example 4 1040 270 680 85 48 1.360 45 Example 5 590 320 440 67 38 1.042 59 Example 6 750 70 670 77 36 1.054 40 Example 7 1650 110 1580 92 286 1.090 58 Example 8 1380 120 1290 92 189 1.110 49 Example 9 1600 120 1280 72 190 1.059 40 Example 10 1490 90 1440 86 172 1.006 59 Example 11 1450 80 1440 80 85 0.957 61 Example 12 1500 90 1470 90 264 1.050 51 Example 13 1400 120 1290 92 189 1.110 48 Santoprene™ Grades Santoprene 101-80A 1120 200 960 84 115 0.960 28 Santoprene 101-73 820 120 710 77 76 0.971 40 Santoprene 101-75 830 210 630 80 67 0.968 29 Santoprene 101-64 680 90 600 61 55 0.914 56

[0121] The results provided in the table show that using the “dynamic de-vulcanization” process of the present invention, it is now possible to produce thermoplastic rubbers from rubber scrap materials having a wide range of flexural modulus levels with a similar Shore A hardness. This is accomplished by varying the composition of the scrap materials and virgin compounds along with suitable plasticizers. Further, the tensile strength of the thermoplastic rubbers produced by the “dynamic de-vulcanization” process of the present invention generally is greater than the Santoprene™ rubbers at comparable flexural modulus levels. Equally significant, the cost of producing thermoplastic rubbers using recycled scrap materials is lower than the typical cost of producing conventional thermoplastic rubbers.

[0122] While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto, and that many obvious modifications and variations can be made, and that such modifications and variations are intended to fall within the scope of the appended claims.