Patent Application
20180066080
The invention relates to the field of tire and plastic recycling.
The proper management of scrap tire rubber and scrap plastic is a worldwide concern. Although numerous attempts have been made to recycle tire rubber by incorporation into recycled thermoplastics, previous attempts have not met with substantial commercial success. Without intending to be bound by theory, known prior attempts involved the use of additives to improve mechanical properties of the blend and, despite extensive experimentation regarding the amount and type of additive to be used, prior marketplace entrants failed to produce a product that could be sold at profit, having regard to the cost of alternatives with similar physical properties and consumer appeal.
A method for producing thermoplastic elastomer forms one aspect of the invention and comprises the step of: blending a mixture including particles of vulcanized rubber material and a molten thermoplastic material such that the rubber material is subjected to mechanical shearing forces and the surfaces of the rubber particles undergo homolytic bond scission to form chains of free radicals which cross-link with the thermoplastic material.
According to another aspect of the invention, the ratio, by weight, of rubber material to thermoplastic material in the mixture can range between about 1:9 and about 4:1.
According to another aspect of the invention, the rubber material can be cryogenically ground scrap tire rubber.
According to another aspect of the invention, the thermoplastic material can be one of PP, HDPE, LDPE, ABS, PET and PVC.
According to another aspect of the invention, the thermoplastic material can be one of PP, LDPE and HDPE.
According to another aspect of the invention, the thermoplastic material can be recycled post-industrial thermoplastic material.
According to another aspect of the invention, the rubber material can have a size between 10 mesh and 100 mesh.
According to another aspect of the invention, the rubber material can have a size between 10 mesh and 60 mesh.
According to another aspect of the invention, the rubber material can have a size between 10 mesh and 40 mesh.
According to another aspect of the invention, the mixture can consist of the rubber material and the thermoplastic material.
According to another aspect of the invention, the mixture can consist essentially of the rubber material and the thermoplastic material.
According to another aspect of the invention, blending can be carried out in an extruder.
According to another aspect of the invention, the extruder can operate at a compounding temperature between about 375 F and about 450 F.
According to another aspect of the invention, the extruder can be a twin-screw extruder operating at a screw speed between about 400 rpm and about 650 rpm.
According to another aspect of the invention, the L/D ratio of the extruder can be about 36:1.
According to another aspect of the invention, the blending step can comprise the following substeps: feeding a particulate thermoplastic material to the extruder to produce, interiorly of the extruder, the molten thermoplastic material; and feeding the particles of rubber material into the extruder and to the molten thermoplastic material to produce the mixture.
According to another aspect of the invention, the mixture can be subjected to said mechanical shearing forces by passage through kneading blocks.
Forming other aspects of the invention are elastomers produced by the method and articles molded from the elastomers.
Forming yet another aspect of the invention is a thermoplastic elastomer comprising: a continuous phase of thermoplastic; and rubber particles dispersed in the thermoplastic phase, the rubber particles having a vulcanized core and a non-vulcanized surface layer cross-linked with the thermoplastic phase.
According to another aspect of the invention, the thermoplastic is one of PP, HDPE and LDPE and the elastomer consists essentially of the rubber particles and the thermoplastic phase.
Apparatus for use with particles of vulcanized rubber material and a thermoplastic material, the apparatus comprising: an extruder adapted to, in use, blend a mixture including said particles of vulcanized rubber material and said thermoplastic material in molten form, such that said rubber material is subjected to mechanical shearing forces and the surfaces of the rubber particles undergo homolytic bond scission to form chains of free radicals which cross-link with said thermoplastic material.
Other advantages, features and characteristics of the present invention, as well as methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description and the appended claims with reference to the accompanying drawings, the latter being briefly described hereinafter.
As an initial matter, and with reference to paragraphs [0024]-[0034], it will be understood that the numeric in the form “XXX/XXX” indicates screw pitch/length.
An exemplary embodiment of the method for producing thermoplastic elastomer of the present invention involves the use of: (i) a multi-barrel, twin-screw, vented, co-rotating, closely-intermeshing, high speed, high torque and horsepower extruder having an L/D ratio about 36:1, operating at a screw speed between about 400 rpm and about 650 rpm and a temperature between about 375 F and about 450 F; and (ii) a screw constructed according to the teachings of the present invention.
A screw for a 58 mm diameter extruder according to the teachings of the present invention can be constructed using combinations of the screw segments shown in
Turning now to the method, same will be described with reference to the zones indicated above:
In the feed zone, which is characterized by a wide pitch screw, thermoplastic material is introduced to the twin screw extruder in any conventional manner. The thermoplastic material can be one of comminuted, recycled, post-industrial PP, HDPE and LDPE. The feed zone can be defined, for example, by the first barrel of a nine barrel extruder.
In this zone, characterized by a gradually decreasing screw pitch, thermoplastic material is moved away from the feed throat and compressed to begin a melting process. This zone can be defined, for example, by the second barrel of a nine barrel extruder.
In this zone, which is characterized by kneading blocks and a reverse pitch screw, shear is introduced, to complete the melting process and produce a molten thermoplastic material. This zone can be defined, for example, by the third barrel of a nine barrel extruder.
In this zone, which is characterized by a medium-wide pitch, material is conveyed out of the Melting zone.
In this zone, which is characterized by a wide pitch screw, crumb rubber is incorporated into molten thermoplastic via a side feeder. The rubber must be steel and fibre free and is introduced in any controlled, conventional manner, in a ratio, by weight, of rubber material to thermoplastic material ranging between about 1:9 and about 4:1. The rubber material can be 10 mesh or finer. Colorant, fire retardant and other conventional additives may also be added here. There is no need for additives for promoting cross-linking; accordingly, the mixture can be fairly described as consisting essentially of rubber and thermoplastic. This zone can be defined, for example, by the 4th barrel of a nine-barrel extruder.
In this zone, which is characterized by a gradually decreasing screw pitch, the partially mixed materials are moved away from the side feeder.
In this zone, which is characterized by kneading blocks, the rubber is distributed throughout the thermoplastic. This zone can be defined, for example, by the fifth barrel of a nine barrel extruder.
In this zone, which is characterized by a gradually decreasing screw pitch, the mixture is conveyed towards zone Mixing (2).
In this zone, which is characterized by kneading blocks, the mixture is subjected to high shear such that the rubber material is subjected to mechanical shearing forces and the surfaces of the rubber particles undergo homolytic bond scission to form chains of free radicals which cross-link with the thermoplastic material. Again, it is emphasized that no catalysts or other active agents are provided to effect this cross-linking. This zone can be defined, for example, by the sixth barrel of a nine barrel extruder.
In this zone, which is characterized by a wide screw pitch, volatiles and moisture are permitted to vent. This zone can be defined, for example, by the seventh barrel of a nine barrel extruder.
In this zone, which is characterized by a gradually decreasing pitch, the blend is moved away from the vent and pressure is built. This zone can be defined, for example, by the eight barrel of a nine barrel extruder.
In this zone, which is characterized by a narrow screw pitch, pressure is built for discharge of the molten thermoplastic elastomer from the extruder. This zone can be defined, for example, by the ninth barrel of a nine barrel extruder.
Once discharged from the extruder, the mixture can be handled in any conventional manner but will typically be pelletized in a conventional manner for subsequent molding use. Useful pellet sizes, for example, can range between 0.125 and 0.1875 inches.
The elastomer end product is characterized by a continuous phase of thermoplastic and rubber particles dispersed in the thermoplastic phase. The rubber particles have a vulcanized core and a non-vulcanized surface layer cross-linked with the thermoplastic phase.
These elastomer pellets can be used like other commodity thermoplastic elastomer pellets.
In terms of utility for molding, the pellets have been found to be quite advantageous, as molded articles made using these pellets can solidify faster and more evenly than articles made using virgin resins. Without intending to be bound by theory, it is believed that, when heated during molding, the temperature of the thermoplastic phase rises faster than the temperature of the vulcanized rubber such that, in the mold, a temperature gradient exists between the vulcanized particles and the thermoplastic phase. When the molten matrix enters the cooling cycle, the different temperatures of the thermoplastic phase and the rubber equilibrate, with the result that the thermoplastic solidifies faster than does virgin resin. This has advantages in terms of cycle time.
In terms of the physical properties obtainable through the process, the method of the present invention was used to produce six thermoplastic elastomers which were tested for melt flow index, density, tensile properties, flexural modulus, hardness, notched Izod and heat deflect temperature, using ASTM methods D1238, D792, D638, D790, D2240, D256 and D648. The results, as set forth in Table 2 below, show that these elastomer products all have physical properties that render them suitable as low-cost commodity thermoplastic elastomers.
Finally, it is to be understood that while but only a few embodiments of the present invention have been hereinbefore shown and described, it will be understood that various changes may be made.
For example, whereas a 58 mm extruder is contemplated above, it will be understood that the process is scaleable and, for use, could be used with similar utility in extruders having diameters ranging from 40 mm to at least 92 mm.
Further, whereas the rubber material is indicated as generally falling in the 10 mesh or finer range, it should be understood that size smaller than 40 mesh provides better surface appearance and smaller crumb rubber generally results in better mechanical properties of the resultant compound. Relatively contaminant free crumb rubber in the 60-100 mesh range is routinely available from cryogenic tire recycling and is advantageously used for many purposes of the present invention.
As well, whereas recycled thermoplastic is specifically mentioned, virgin thermoplastic could be used. Moreover, whereas PP, HDPE and LDPE are specifically mentioned in the detailed description and have been tested, it is contemplated that ABS, PET and PVC can also be used with the method. Similarly, whereas cryogenically-ground tire rubber is specifically mentioned and has been tested, it is contemplated rubber ground to similar dimensions by other conventional methodologies could be used.
Further, whereas a compounding temperature of between about 375 F and about 450 F is specified, it will be understood that the compounding temperature depends on the type of thermoplastic; the same applies to residence time in the extruder.
Additionally, whereas a single screw is described above, modifications to the screw are possible. For example,
Yet further, whereas nine and twelve barrel extruders are specifically mentioned, greater or lesser numbers of barrels can be routinely used.
Accordingly, the present invention should be understood as limited only by the accompanying claims, purposively construed.
This application is a continuation of U.S. patent application Ser. No. 14/134,593 filed on Dec. 19, 2013, which is a continuation of Ser. No. 13/861,725 filed on Apr. 12, 2013, now abandoned, which is a continuation of U.S. patent application Ser. No. 12/892,286 filed on Sep. 28, 2010, now abandoned.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14134593 | Dec 2013 | US |
| Child | 15810772 | US | |
| Parent | 13861725 | Apr 2013 | US |
| Child | 14134593 | US | |
| Parent | 12892286 | Sep 2010 | US |
| Child | 13861725 | US |
