WO2004031277A1

WO2004031277A1 - Method of reclaiming rubber waste

Info

Publication number: WO2004031277A1
Application number: PCT/CZ2003/000053
Authority: WO
Inventors: Miroslav Mojzis
Original assignee: Miroslav Mojzis
Priority date: 2002-10-05
Filing date: 2003-10-03
Publication date: 2004-04-15
Also published as: CZ20023324A3; CZ295220B6

Abstract

In a method of reclaiming rubber waste by which the rubber is first crushed and then devulcanized the rubber crumbs are mixed with a polymeric softener elected from the group of liquid polybutadienes with addition 1,2 - 1,4 having a molecular weight ranging from 250 to 10000 and such a mixture comprising 4 to 20% by weight of a softener and 95 to 80% by weight of rubber crumbs is exposed to a compressive, shear and heat stress at a temperature ranging from 190 to 280 °C. Before the exposing the mixture to the compressive and shear stress the mixture is allowed to swallow for 24 to 80 hours at a temperature 15 to 200 °C.

Description

Method of reclaiming waste rubber

Field of invention

The invention relates to a method of reclaiming waste rubber by which the rubber is crushed and partly or as a whole devulcanized by exposing the rubber crumbs to a compressive, shear and heat stress optionally by simultaneous using of reclaiming agents.

Description of prior art

Waste rabber, the major part of which constitute scrap tires has created a serious technical and social problem, which has not been solved to full satisfaction as yet. The simplest disposal of such a waste i.e. the burning thereof adversely affects the environment and is subject to continuous restrictive measures and standards. Certain environmental problems arise also when using the waste rubber as additives to road asphalt and for various reinforced surfaces. The optimal method of rubber waste disposal is the use of reclaimed rubber for production of new products or as replacement of at least a portion of virgin rubber in curing rubber mixtures. The scrap rubber shall be first devulcanized and prepared in a manner so that in the following admixture thereof in a fresh curing mixture and vulcanization the physical properties of the resulting product, are not improperly adversary affected. Accordingly, the reclaiming process is subject to two criteria such as costs of the process and quality of the final products. The costs of reclaim produced according to the current technologies are mostly too high so that the use of reclaimed rubber as an additive in the amount that would meet the requirements for enormous volume of waste rubber to be recycled with a required quality, i.e. first of all in the tire production, is still significantly restricted.

In the US patent 6 590 042 a chemical composition is disclosed, which is designed to be used as an accelerating vehicle in the process of devulcanization of rubber scrap in combination with a special twin screw extruder. The advantages are low processing temperature and short operation time. Nevertheless, no example of reusing of the reclaimed rubber as a replacement of virgin rubbers in the compounds formulations for tire production is disclosed therein. Less expensive reclaimed rubber exhibit significantly deteriorated quality compared with the virgin elastomers and may be reused only in very small amounts as a part of the curing compound for tire production although it may be fully applicable to the production of final products not requiring high mechanical properties. Such reclaimed rabber is described for example in EP 393 535 where 2 up 6 % of an aliphatic-aromatic soft resin, alternatively in combination with natural rabber are added to the rabber scrap crumbs in the devulcanization process in a calender. The reclaimed rubber may be used for production of plates, mats etc.

According to another of currently used methods the rabber crumb prepared from the scrap truck vehicle tires of butadiene-styrene rabber, car tire treads and various factory scrap rabber or oil resistant scrap rabber with a particle size of up to 5 mm is heated in an autoclave, a press or extrader at a temperature of 140 to 230 °C in the presence of compounds elected from the group including lactams, lactones, organic acids, crude rosin or, as the case may be, polyolefins, polyesters, polyamides and polyurethanes. The resulting raw material is then vulcanized as a mixture of reclaiming agents, virgin rabber, polyolefins, polyesters, polyamides or polyurethanes to obtain a final product. Nevertheless, the physical properties of such final products do not meet the requirements of tire production.

The primary object of the invention is to produce a reclaimed rabber exhibiting such physical properties that make it suitable for use as a component of curing rubber mixtures for production of tires in a minimal amount of up to 5 to 10 % without increasing the costs of production including material, machinery and energy.

Summary of the invention

Based on said prior art and in view of the object of the invention the essential features of the method of reclaiming waste rabber by which the rubber is first crushed and then devulcanized consist in that the rubber crumbs are mixed with a polymeric softening agent elected from the group of liquid polybutadienes with addition 1,2 - 1,4 having a molecular weight ranging from 250 to 10 000 and this mixture is further exposed to compressive, shear and heat stress.

Further objects, advantages and novel features of the invention appear from the following description.

Before exposing the mixture to a compressive shear and heat stress the mixture is allowed to swallow for 24 to 80 hours at a temperature of 15 to 200 °C. Although the higher temperature significantly expedite the swelling process the mixture may be swelled at a room temperature, what takes about 60 to 80 hours of time and reduces the energy consumption to minimum. The swelling process may be supported by a moderate intermittent or continuous stirring of the mixture. The process works without or at presence of reclaiming agent when the mixture is exposed to a compressive and shear stress in a blender or extruder at a temperature of 190 - 280 °C. At temperatures below 100 °C the process is conditional on the presence of higher amount of proper reclaim accelerating agents. Although higher temperature ranges significantly accelerate the degradation of the cross-linking bonds of the scrap rubber the optimal temperature range for the most of the rabber materials according to this invention is 220 to 230 °C.

The liquid polybutadiene may include terminated hydroxyl or carboxyl groups or may be epoxidized by adding epoxy group to double bonds or maleated by ene-addition of maleic anhydride to the tertiary carbon position. The presence of said groups may positively influence the incorporation of the reclaimed rabber into the polymeric network of the resulting cured product.

According to another aspect of the invention the liquid polybutadiene comprises 30 to 98 % of the component with 1,2 addition and 2 to 70 % of the component with 1,4 addition. This composition represents an optimal microstructure of the liquid polybutadiene as a softening and swelling agent.

In accordance with the properties desired in a final product the mixture may comprise 4 to 20 % of the softening agent - liquid polybutadiene and 96 to 80 % by weight of the vulcanized scrap rubber crumb. In view of further processing of the mixture in a blender or extruder and of other aspects such as a suitable viscosity and other physical properties of the cured rubber and the economy of the whole method it appears that the amount of 5 - 8 % by weight of the liquid polybutadiene may be optimal. Generally, the use of said softener - liquid polybutadiene does not exclude its use in combination with certain reclaiming agents such as aromatic oils, aniline and its derivatives etc.

The particle size of the rabber crumbs made specifically of scrap tires is 0.5 to 1.5 mm, wherein the best size enabling optimal swelling process and ensuring homogenous blend appears to be about 1 mm.

During the swelling of rubber crumbs by a polymeric softener such as a low molecular weight liquid polybutadiene the vulcanized rubber polymeric network created by multiple sulfur bonds /cross-links is diluted and consequently the mechanical properties of the rubber subjected to reclaiming are deteriorated. The swelling process significantly facilitates subsequent thermo-mechanical processing of such swollen rubber in a blender or extruder and works with the same effect at a room temperature without any energy consumption as at higher temperatures. The higher temperature and continuous or intermittent stirring expedite the processing time. At higher temperatures of 190 to 200 % even without the presence of reclaiming agents the vinyl groups (addition 1,2) of the liquid polybutadiene begin to react with the sulfur bonds of the reclaimed rubber thus causing the destruction of the polymeric network of the vulcanized rubber scrap. The final destruction occurs in a very short period of time of about 5 minutes in a blender and 4 minutes in an extruder. The processing time depends naturally on the temperature and the intensity of the compression and shear stress in the material when processed in a blender or extrader. In the next curing process the low molecular weight polybutadiene is incorporated into the polymeric network by reaction of its outstanding double bonds and/or terminal functional groups with the virgin rabber and curing agents. The physical properties of the final product, for example a tire, are not deteriorated since the reclaimed rubber becomes a compact part of the polymeric network. In view of a low energy costs and relatively low costs of the softener, which is a commercially produced low molecular liquid polybutadiene and the fact that due to its above-described favorable effects no other or minimum reclaiming agents are necessary the method according to the invention may be used in such a mass and high-volume production as the manufacture of tires. The invention is of course not limited to the reclaiming of scrap tires but is suitable for reclaiming also other cured rabber such as industrial scrap rabber and post-consumer scrap.

Description of preferred embodiment

Example 1

500 gr of car tire scrap rubber crumbs comprising ca 60 % of synthetic rubber and 40 % of natural rubber with a particle size of 0.5 to 1.5 mm were mixed with 5 % by weight of liquid polybutadiene with terminal functional hydroxyl groups having a molecular weight of 2000 and comprising 60 % of the addition 1,2 component and 40 % of addition 1.4 component. The swelling occurred at an ambient room temperature (ca 20 °C ) for 72 hours. The swollen mixture was than blended in labor blending machine (Brabender ) for 5 minutes at a temperature 220 - 230 °C and than refined at a two-roll mill. The reclaimed rubber was used as a component of a virgin rabber mixture for manufacturing car tire sidewalls in an amount of 5 % by weight.

Example 2

The same amount of car tire scrap rubber crumbs exhibiting the same properties was mixed first with 10 % and then with 20 % by weight of liquid polybutadiene having a molecular weight of 1000 and comprising 70 % of the addition 1,2 component and 30 % of the addition 1.4 component and swelled at a temperature of 50 to 80 °C for 50 hours. Then, the mixture was blended in a labor blender at a temperature of 190 to 210 °C for 7 minutes at a temperature of 240 to 280 °C for 4 minutes. The reclaimed rabber was used in an amount of 8 % and 10 % by weight as a component of the rabber mixture for the car tire sidewalls.

Example 3

600 gr of truck tire scrap rabber crumbs comprising ca 40 % of synthetic rubber and 60 % of natural rubber with a particle size of 0.5 to 1.5 mm were mixed with 8 % by weight of liquid polybutadiene with terminal functional hydrocarbon groups having a molecular weight of 5000 and comprising ca 60 % of the addition 1,2 component and 40 % of the addition 1.4 component. The mixture was allowed to swell at about 23 °C for 70 hours. Three samples of the swelled mixture were then blended in a labor blender at temperatures 190 - 200 °C, 220 - 235 °C and 260 - 280 °C. The best physical properties of reclaimed rabber were obtained at temperatures 220 - 235 °C although at higher temperatures the blending time could be reduced as in example 2. Nevertheless at temperatures nearing 280 °C an undesired commencing destruction of the original polymeric chains was monitored. In the low temperature ranges the time necessary for the degradation of sulfur bonds was extended. After refining on a two-roll mill the reclaimed rabber was blended in the amount of 10 % by weight with the virgin rabber for manufacturing tractor tires.

Industrial applicability

The method of reclaiming rubber waste according to the invention is preferably suitable for preparation of reclaimed rubber, which may replace a significant portion of virgin rubber in compounds for manufacturing tires. Further, such reclaimed rubber may be used as a major component of cured rabber mixtures for manufacturing products other than tires.

Claims

1. A methpd of reclaiming rabber waste by which the rabber is first crushed and then devulcanized characterized in that the rabber crumbs are mixed with a polymeric softener elected from the group of liquid polybutadienes with addition 1,2 - 1,4 having a molecular weight ranging from 250 to 10 000 and this mixture is further exposed to compressive, shear and heat stress.

2. A method of claim 1 characterized in that before exposing the mixture to the compressive shear and heat stress the mixture is allowed to swallow for 24 to 80 hours at a temperature of 15 to 200 °C.

3. A method of claim 1 or 2 characterized in that the before exposing the mixture to the compressive shear and heat stress the mixture is allowed to swallow at a room temperature for 60 to 80 hours.

4. A method of claim 2 characterized in that the swelling process is accompanied by continuous or intermittent agitating.

5. A method of claim 1 characterized in that the mixture is exposed to the compressive and shear stress at a temperature ranging from 190 to 280 °C.

6. A method of claim 1 characterized in that a liquid polybutadiene comprises terminated hydroxyl or carboxyl groups or is epoxidized by adding the epoxy group to double bonds or maleated by ene-addition of maleic anhydride to the tertiary carbon position.

7. A method of claim 1 or 6 characterized in that a liquid polybutadiene comprises 30 to 98 % of the component with 1,2 addition and 2 to 70 % of the component with 1,4 addition.

8. A method of any of the claims 1 to 6 characterized in that the mixture comprises 4 to 20% by weight of a softener and 95 to 80 % of rubber crumbs.

9. A method of any of the claims 1 to 8 characterized in that rabber crumbs comprise scrap tire rubber crumbs with a size of 0.5 to 1.5 mm.