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Rubber reclaiming technology

Reclaiming
Reclaiming is the most wide-spread method allowing partial recycling and usage of waste rubber. Common principle of the most existing methods of reclaiming is thermal-oxidative or destruction of  turgid vulcanized rubber.
Regeneration process consists of  production steps as follows: sorting and grinding of rubber, removal of textile fibers and metals, de-vulcanizing and mechanical treatement of the vulcanized rubber . Various reclaiming methods differ mainly in technological execution of devulcanizing process.   Alcaline, acid-based, thermal, steam and solving  reclaiming methods are considered to be outdated . Nowadays there three basic reclaiming methods, namely,  water-neutral method, thermal-mechanic method and dispersion method. The default of water-neutral method is periodicity of its process and low quality of  reclaim owing to high doses of softener,  so the continuous thermomechanical method is now the most popular one. In this case devulcanization is performed in continuos screw devulcanizer in the presence of softener and destruction activator. Dispersion method allows the reclaim of the highest quality,  however there are some diffulties connercted with spray drying of rubber's water dispersion, this is why this method is not widely used now..
Rubber substance of the reclaim consists of  gel-fraction, that preserves dispersed net structure of the vulcanized rubber and sol-fraction, containing short cuts of  branched chains with molecular weight of about 10000. As the reclaim preserves net structure of the vulcanized rubber, injection of reclaim to rubber mixture results in microinhomogeneity, that produces negative effect on structural behaviour of the rubber. Presence of low molecular fractions in the reclaim causes  the decrease in wear resistance of the rubber.  This is why reclaimed rubber is very seldom used in tread rubbers. Nowadayes reclaim in rubber industry is mainly used as processing additive, improving  processibilty of rubber mixtures, it also can be used as raw materials for products of minor importance .
There are several ways of rubber reclaiming, namely/ waterneutral method, thermal and mechancial and dispersions method.

Water neutral reclaiming method  
It includes basic operations as follows: preparation of rubber, preparation of softeners and activators; devulcanization; water separation and drying ; mechanical treatment. The detailed info concerning preparation of rubber for reclaiming- stage of rubber crumbling  may be found here :  LU-YH 7  Rubber recycling production line 7
Flow chart of rubber crumbling line


 

1- Feeding groove; 2 – Muller; 3 – Belt conveyor ; 4 – Elevator; 5 – Vibration sieve ; 6 –Selection conveyor.  


Waste crumbling. Warn tyres, riding air and curing bags are sorted into groups according to the type of caoutchouc contained in them. Formulation and devulanization mode is to be chosen depending of type and caoutchouc content in the ruibber. After that the tires are sent to the jet washer and bead cutters. The cut bead rings containing thick metal cord and metal wire are removed, separated into two parts by  crown and then cut into pieces using power shears. The resultant sections are fed to tire-cutter and cut into pieces of 30-70 mm.  Further crumbling of the rubebr and separation of the cord fibers are performed using  mullers with corrugated surface and  reduction rools aggegated with vibration seeders. The production string can contain one or several tandem rolls.
Vibration sieve is mi\ounted on special mounting area over the rolls or at the second floor.  Initial pieces are fed via the feeding grooves. The rubber passed via mullers 2 is conveyed by belt conveyor 3 to elevator 4 and then vibration sieve 5 to be screened into  fine fraction, selected by conveyor 6 and large fraction, sent to additional crumbling, and textile wastes, taken from the upper net to be sent to the Customer of  for further recycling.

The reclaim is mainly obtained from common caoutchouc rubbers (including natural, isoprene and  butadiene-styrene rubbers) and their mixtures. Small amounts of  butyl rubber, polychlorprene and nitrile rubber based reclaim are also produced . Butyl-based rubber can be reclaimed by irradiationm is this case preliminary crumbling of rubber is not compulsory.

Reckaim grade estimation
For quality estimation the reclaim is subjected to chemical test.  Besides, technologial and mechanical properties of  reclaim-based vulcanized rubber are tested as well. Chemical analysis allows to determine volatile  substances (at 1100) ash textile and softeners contents in the reclaim, its devulcanization degree, acidity and alkalinity. The quality of the reclaim by appearance and processing characteristics is estimated visually by comparison of  layer of the reclaim taken from  refiners with standard. While comparing it is necessary to take into account the density,  degree of roughness and glossiness of the layer's surface as well as  inpuritires of  hard particles of devulcanized rubber (grains). Homogeneity of the reclaim is detemined by microphotoes of the sections. Processing characteristics of the reclaim  are determined by plastoelastic properties. Plasticity, softness and elastiity are mainly determined using compression plastometers, whereas Mooney viscosity is measured by rotation viscosimiters. Determination of Mooney viscosity allows more reproducible results. Mooney viscosity of standard grades of  the reclaim, obtained using waterneutral method at 100oC is 17—60  conventional units.  To estimate mechanical properties if reclaim-based vulcanized rubber, the following standard mixture is used (mass number on 100 mass of pure reclaim): 
Reclaim ...... 100,0
Zinc oxide  . . ..2,5
Dibenztiazolyldisulfide  0.9
Sulphur 1,5
The standard mixture is being vulcanized during 15 min  at 143±1 C.  The resultant vulcanized rubber should have tensile strength of  no less than  5,0—7,0 MPa and specific elongation no less than  350%, depending on the reclaim's grade. Please, mind that the knowledge of mechanical properties of standard mixtures is not sufficient to estimate the quality of  reclaim, as no relation between mechanical indices of vulcanized rubber's reclaim and reclaim-containing rubber mixtures has even been established.  

Using reclaim in rubber mixtures.
Introduction of reclaim to rubber mixtures accelerates the mixing process . This allows to shorten the time of mixture preparation on mullers/internal mixer by several minutes, in some cases it shortens the time of preparation twice. Besides, it diminishes energy consumption during processing of reclaim, as the latter contains dispersed ingredients and has sufficient plasticity. The reclaim in rubber mixtures can be regarded as cross-linked polymer, this causes permanent properties of reclaimed mixtures during their recycling . Reclaim mixtures has lesser shrinkage and has good carcass qualities. When vulcanizing the reclaim-containing product products without molds,  the workpiece has just a slight deformation.  During the second rolling reclaim mixtures are plasticized  less then those prepared using rubber, as they are less sensitive to replastication.  Rather insignificant  thermognesis of reclaim-containing mixtures, reduces the risk of their precuring during their process at rollers, calanders, mixers and extrusion machines. When introducing  reclaim in rubber mixtures it is possible to use accelerated extrusion and calandering speeds with good preservation of the shape of  the workpiece.  Application of reclaim is rather efficient for rubber mixtures used for greasing the materials on calanders. It is preferanle to use reclaim mixtures in production of  the molded articles, especiall big ones, as they spread about slowly and are better in displacement of air from the molds thus preventing bubble formation and undermolding. Using regenerate can reduce the consumption of accelerators and zinc oxide. Reclaim also block the reversion of vulcanization. Among shortcoming of the reclaim that restrict their application in rubber mixtures are reduction of rubber elasticity, module, rupture strength, tear resistance, attrition and fatigue strength. When making reclaim-containing rubber mixtures, rubber and reclaim are first plasticized separately, then mixed. Sulphur and accelerator are introduced into mixture on the basis of total polymer content (rubber and rubber substance of the reclaim). If the reclaimed rubber should have increased aging resistance, sulphuric content is reduced by  20—30%  as compared with its usual content for rubber-based mixtures. Antioxidants and fillers are introduced proceeding from rubber contents only.  This can be explained by their distribution in the rubber; reclaim's particles contain  only the filler that were in initial reclaimed rubber. Reclaim-containing rubber properties may be considerably improved by introduction of active strengthening fillers (high-dispersed carbon, high molecular resins) into the mixtures. Reclaim rubber is used in production of tires, general mechanical rubber goods (conveyor belt, sleeves, gaskets, storage tanks) rubber shoes.  Using of  reclaim in sponge rubbers reduces elasticity of mixture and diminish the difference in pores' size during vulanization. Reclaim added by solvent (usually in presence of the resins) allows valuable glues with high content of rubber substance. It is rather easily dispersed in water and being mixed  with natural or synthetic latex  secures high quality adhesives. Reclaim by itself it mainly used for making non-important goods such as carpets,  house tracks, semi-solid isolating tubes, garden hoses, etc.
Physical methods of recycling of rubber wastes.
Nowadays usage of waste as disperse materials becomes more and more important. Mechanical crumbling  allows better then other method to preserve original structure of rubber and other polymers contained in wastes.
Crumbling (breaking) is the process of  fragmentation of solids into  smaller and smaller particles under external forces. Crumbling (crushing) is fragementation of solid into smaller and smaller forces under extrernal factors. Effectiveness of fragmentation depends on its extend, i.e particles' sizes ratio before and after crumbling.
To find proper crumbling machines (shredders) and to determine their optimal operation conditions it is necessary to establish interrelation between  size of material's particles, their physicochemical and mechanic data and energy consumption required for  crumbling and specifications of crumbling equipment.

Chemicals used for rubber reclaiming

1) Zinc chloride is applied in production of articles as follows: electrolytes for dry elements and electroplating baths in elecroplating ; fluxes for hot galvanization, tinning and leading;fluxes for manual and machine soldering;fibers production;pesticides, pigment (yellow zinc, lithopone);carbons; antiseptic treatment of wood ;additive for rubber vulcanization; cellulose cord solvent during rubber reclaim ; dewatering agent and catalyst in chemical synthesis.

2) Di-xylene Di-sulphide is used as catalyst in production of reclaim rubber. Application of this chemical allows regeneration (devulcanization) both natural and synthetic rubber. It accelerates devulcanization at law temperatures, thus reducing energy consumption and saves time for  recycling, allows regeneration or devulcanization at lower temperature, improves the operating efficiency, saves fuel, improves final reclaim quality and lowers the risk of solidification

3) Disulfide oil
Purity: 99.99% CAS No.: 3812
«Disulfide oil» - is a chemical name of   diaryl disulphide. It is used as raw materials in obtaing reclaim rubber from waste rubber.
Package - barrels (200 L)


4) Specifications:  Pine tar oil

Specific weight (a204): 1.01-1.06
Water content  (V/W) : max 0.50%
Ash content  : max 0.50 %
Viscosity: 180-250/251-350/351-450 
Mechanical a : max 0.03 %
It is made by distillation  from high quality Chinese dry red pine.
Sticky and black phenol mixture withot admixtures is used as softener and plastizer in oil industry for desulphated reclaim rubber in production ot tyres.
It is a raw material or supplemens of timber antiseptics, asphalt felt and pitch.
The Product is packed in barrels or hard carton

An iron barrels bears a net weight of 200kg, cardbaord bears 12-24 kg, the cardboard contains two layers of  PE packages, weight - 1 -4 kg 

Pine tar oil quality index : LY/T1393-1999

Index 1# 2# 3#
Viscosity (Engler Degree) , s (100ml,85 °C ) 180-250 251-350 351-450
Specific weigth (a204) 1.01-1.06 1.01-1.06 1.01-1.06
Volatility , % ≤ 6.50 6.00 5.50
Ash content  , % ≤ 0.50 0.50 0.50
Water content (V/W) , % ≤ 0.50 0.50 0.50
Acidity (acetic acid) , % ≤ 0.30 0.30 0.30
Mechanical admixtures, % ≤ 0.00 0.03 0.03

5) Dipenten-

DESCRIPTIONS: Clear, colorless to pale yellow liquid. As a by-product of the manufacturing process in which high-quality gum turpentine oil or alpha pinene is synthesized into terpineol, it is largely composed of alpha terpinene, dl-limonene, cineol, gamma terpinene, terpinolene and other terpene hydrocarbon.

USES: Mainly used in cost effective fragrances for household products because of its pleasant pine and lime aroma, it can also be used as intermediates in the production of fragrances such as p-cymene and p-menthane. Can be used as a solvent in the reclaim of rubber and as dispersant, wetting and frothing agent for industrial applications. PACKING: IN 145 /175KG GALVANIZED IRON DRUMS.
 

General info
Liquid pyrolysis products can be used as film-fromation solvents, plastisizers, softeners for rubber reclaiming .
Pitch of  pyrolysis resin is a good softener, that can be used by itself or in mixture with other constituents.
Hard pyrolysate fraction as additive to bitumen, used in road construction, can improve its elasticity and its frost and wet resistance.
Gaseos pyrolysis fraction can be used to extract aromatic oils, applicable in production of  rubber mixtures .
Low molecular hydrocarbons can be used as raw materials for organic synthesis and fuel,

Analysis of main producer-gase resins on standards for reclaim rubber softener

Experience in gasification

Type of raw materials

Resins, washed with hot water

Resin moisture  %

Specific weight at 20°

Mechanical admixtures , %

Soluble acids (in terms of acetic acid ), %

Petrol-insoluble substances , %

Resin acids, %

1

Wood chips of resinous woods and pine firewood

From tar extactor

10,5

1,10

1,00

2,45

41,3

21,5

2

Wood chips of resinous woods

From the settling tank

26,7

1,084

0,30

0,93

26,1

30,1

3

Wood chips of resinous woods

Floatable from the settling tank

18,1

1,047

0,24

21,6

27,6

4

Wood chips of pine firewood

Settled from the settling tank

10,5

1,126

1,80

1,41

44,2

25,6

Floatable from the settling tank

7,0

1,123

0,40

1,50

50,6

28,5

5

Wood chips of pine firewood

Settled from the settling tank

14,0

1,110

1,05

1,66

48,8

23,3


Among many method used for reclaiming of rubber, thermomechanical method seems to be the most promising.
This method has advantages as follows: continuous devulcanization process, complete mechanization and automation of production process,  sharp shrinkage of production cycle.
Thermomechanical method is a universal method applicable for recycling of rubber made of both natural and synthetic caoutchouc.
In using this method the reclaim can be obtained by passing the prepared mixture via  worm machine during 3-6 min.
The required temperature mode and worm's rotation speed are set depending of  type of recycled rubber and properties of the reclaim.
Thermomechanical production of reclaim is performed in the following way:
After sorting the used tyres are fed to cutting, washing and  muller. Subsequent crumbling and sieving result in separation of petty crumbled rubber particles and cord fibers. The pneumatic conveyor feeds the crumbled rubber to mixing with aci\tivator and softener solution.
The continuous mixing process is accompanied with  damping of the mixture.
In devulcanizer the rubber is processed at temp withing the range of  150—260° С.
Rotation speed of the devulcanizer's worm is set tentatively.
After devulcanizer the product is cooled in closed reception chamber connected with ventilating device, it  draws off gases and vapors.
In order  to simplify cooling of granulated generator and to prevent  granule-to granule adhesion, the cooling using sprayed emulsion is used.
As differs from common rubber mixture processing worm machines, L/D ratio of devulcanizer's worm is about  10 : 1.
The worms are performed at varied pitch with gradual decrease of   volume betweeb turns, and according to the turns' configurtion are divided into feeding zone, plastisizing zone and  pushing zone.
There's a cone nozzle at the end of the worm, it rotates together with the worm in reverse cone of the machine's head.
Both worm and cone nozzle are made of special  hard alloys to guarantee their long operation life in condtion of intensive attrition.
The gaps between worm's turns as well as special blades and  profile of the worm's turns provide intensive attrition, mixing and plastication of  materials in the machine .

There may be cone nozzles both with corrugated and smooth surfaces
It is recommended to make the gap between cones' surfaces (nozzle and head) adjustable within the range of  0,05 - 5 mm depending on size of the machine.
The speed of worm's rotation should be continuosly adusted within the range of 10-120 rpm
Inner cavity of the worm has a cooling zone .
The machine frame has three zones, all of them are cooled because of high operation temperature .
The machine should have automatic device to keep up and adjust the temperature .
Worm devulcanizers are made with worm diameter of  90, 120 and 320 mm and productivity of 50, 200 and 500 kg/hr 
 

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