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How To Recycle Waste Rubber

2008/8/12 10:44:00 41924

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The disposal of waste rubber is one of the serious problems that people are facing today.

In order to meet the continuous improvement of material performance requirements, rubber is developing towards high strength, wear resistance, stability and aging resistance, but at the same time, it causes the problem that rubber can not be degraded naturally for a long time. A lot of waste rubber causes black pollution which is more difficult to deal with than plastic pollution.

On the other hand, valuable rubber resources are wasted.

Millions of tons of waste rubber are produced every year in the world. The number of waste rubber is so huge that how to deal with them effectively has become a common concern of the whole society.

To this end, in addition to the burning of waste rubber products, since 1910, scientists in various countries have studied more effective waste rubber recycling technology.

Reclaimed rubber refers to the physical and chemical processes such as crushing, heating and mechanical treatment of waste rubber vulcanizates, which change them from elastic state to plastic and viscous, re vulcanized rubber.

The essence of the regeneration process is that under the combined action of heat, oxygen, mechanical action and the chemical and physical effects of reagents, the vulcanizate network is destroyed and degraded. There are both cross links and macromolecular bonds between the cross links.

Rubber regeneration methods can be broadly classified into two types: physical regeneration and chemical regeneration.

1 Physical regeneration

Physical regeneration is the use of additional energy, such as force, thermal force, cold force, microwave, ultrasonic, etc., so that the three-dimensional network of cross-linked rubber is broken into low molecular fragments.

Except microwave and ultrasonic can cause real rubber regeneration, the rest of physical methods can only be a kind of comminution technology, that is, making rubber powder.

When the rubber powder is used in the rubber industry, it can only be used as a non reinforcing filler.

Microwave, ultrasonic and other physical energy can achieve satisfactory rubber regeneration effect, but the equipment requirements are high and energy consumption is large.

1.1 comminution at room temperature

The comminution method at room temperature usually refers to a comminution method of making rubber powder by mechanical grinding of rubber at a temperature of 50 or 5 degrees or slightly higher.

The crushing principle is to cut and crush the rubber through the action of mechanical shearing force.

Therefore, the surface of rubber powder produced by comminution at room temperature is uneven and burr shaped.

Compared with the cryogenic crumb rubber powder, the rubber powder has a larger surface area, which is conducive to the activation modification, and at the same time, it cooperates with the matrix rubber in the new compound.

The earliest normal temperature comminution method is roller crushing method, mainly coarse crushing and fine crushing 2 processes. The coarse crushing equipment is two roll coarse crushing machine with groove on the surface, while the finely crushed two roll finer is used.

After crushing and finely crushing the waste rubber, magnetic separation and separation of iron and fibers are carried out, and then the rubber powder with different particle sizes is sieved.

The rubber powder produced by this method is generally in the range of 0.3 to 1.4mm, which is mainly used for producing intermediate materials for reclaimed rubber or forming elastic floor paving materials by adhesive.

If the roller is used to produce rubber powder and the roller speed exceeds 50m/s, it is called the high-speed grinding method at normal temperature, which can crush rubber and fiber materials at the same time.

Experimental process: scrap tire cleaning, slicing, grinding, sieving, rubber powder [1].

1.2 low temperature comminution

Low temperature comminution is an effective way to crush the rubber below the glass pition temperature by means of liquid nitrogen to cool down at low temperature.

Cryogenic comminution at abroad is mainly done by refrigerating medium liquid nitrogen. Cryogenic comminution can be divided into 3 ways according to the different processes before comminution.

(1) the waste rubber is directly frozen after pretreatment and crushed at low temperature.

(2) 2 processes of crushing and crushing are carried out at low temperatures.

(3) at room temperature, it is first crushed into coarse rubber powder and then crushed at low temperature.

The 3 methods are suitable for grinding at room temperature and low temperature. The production line can flexibly produce normal temperature rubber powder or low temperature rubber powder according to market demand.

Liquid nitrogen cryogenic comminution has developed industrial production lines in developed countries. For example, the United States tire company's low-temperature process for producing rubber powder process is that the waste tires are pre treated and cut into rubber blocks [2], then the crusher is used to break the rubber blocks into 6.5mm rubber particles, and then the liquid nitrogen is sprayed onto the colloidal particles directly by the cryogenic conveyer during the delivery of the colloidal particles, so that they are frozen to above the glass pition temperature, and then sent to the crusher for crushing at low temperature. Finally, the rubber powder with a particle diameter of about 0.42mm is produced.

The pulverizer used for low temperature crushing is generally made of impact crusher.

Ukraine national cryogenic Physics Engineering Institute has also developed the liquid nitrogen cryogenic comminution process. The main process is two parts: comminution and grinding. The two processes are carried out at low temperatures. According to market requirements, the series of rubber powder with particle size of 5mm, 1.25mm, 0.4mm, 0.2mm, 0.1mm and 0.05mm can be produced.

1.3 wet or solution pulverization

Wet or liquid crushing is a method of crushing rubber powder in solvent or solution medium, [3].

The surface state of the rubber powder produced by this method is the same as that of the normal temperature method, but the particle size is small, generally over 200 mesh.

The rubber powder is better than the normal temperature crushing method and the low temperature comminution rubber powder.

Wet or solution grinding was first developed by the British Rubber Plastics Association. The United States used the technology to build an industrial production line. The main grinding equipment used in this method is the disc type colloid grinder.

The crushing process is divided into 3 steps: [3]:

The first step is the coarse crushing of waste rubber, which is carried out by comminution at normal temperature.

The second step is to pretreat the rubber powder with chemicals or water. There are 3 ways of pretreatment: the pretreatment with fatty acids (such as oleic acid) and alkali (such as sodium hydroxide); pretreatment with liquid medium (such as tetrahydrofuran, ethyl acetate and trichloromethane); pretreatment with rubber powder;

The third step is to pre - treat the rubber powder in the millstone type colloid grinder, and to remove the alkali, remove the solvent or dehydrate it.

The appropriate amount of water pretreatment for rubber powder is more suitable for industrial production, but the production cost is high, and it is only used in high-grade products and some special materials.

1.4 microwave regeneration method

Microwave regeneration is a non chemical, non mechanical one-step desulfurization regeneration method.

It uses microwave energy to break S-S and S-C bonds in rubber powder.

When the rubber is placed in the microwave field of f=2 450 or 915MHz, all polar groups will oscillate with the change of the electromagnetic wave due to the change of the direction of the high frequency alternating electromagnetic field. Because of the thermal dynamics of the molecules themselves and the interaction of the neighboring molecules and the inertia of the molecules, the polar groups will be subjected to resistance and interference with the change of the electric field, thus generating huge energy between the polar groups and molecules.

The advantage of the microwave method is that the thermal efficiency is high. In order for the desulfurization to reach the required high heat, the rubber powder used for desulfurization is preferably polar.

Therefore, the thermal effect of microwave Desulphurization on polar rubber is very obvious and strong, but as long as vulcanizate, generally has a certain polarity [4].

Zhao Shugao and Zhang Ping have studied the microwave desulfurization of non-polar vulcanized rubber [5].

The microwave desulfurization method was first studied by the US Novety et al. It has been put into industrial application in the United States. The Japanese patent also has the introduction of microwave desulphurization technology.

In China, Luo Peng, Lian Yongxiang and Dong Chengchun have engaged in research on microwave regeneration of waste rubber.

The microwave desulphurization method has the following characteristics: [6]: good energy saving, good desulfurization effect, high production efficiency, less time consuming, special effect on the rubber with strong polarity, less pollution and good economy.

The disadvantage of microwave desulphurization is that it has certain pollution.

1.5 ultrasonic regeneration method

Akron University invented the ultrasonic regeneration method in l993, which used high density energy field to destroy the cross link and retain the molecular backbone, so as to achieve the purpose of regeneration. The ultrasonic field can produce high frequency expansion stress in a variety of media, and high amplitude oscillatory waves can cause solid crushing and liquid cavitation.

The theoretical explanation is that the mechanism of acoustic cavitation may cause the ultrasonic energy to focus on the local location of the molecular bonds, so that the ultrasonic field with low energy density can be changed to high energy density at the hole destroyed.

After IsayevA.Iet.al. was regenerated with GRT (waste tire rubber powder) by ultrasonic wave, the physical and mechanical properties of the vulcanizate were measured as follows: tensile strength is about 9MPa, elongation at break is 270%, this property is higher than that of ordinary reclaimed rubber.

IsayevA.I also described the process of ultrasonic desulphurization, and established a [7,8].

Ultrasonic desulfurization is a real regeneration of waste rubber, but this method has not yet eliminated the cost and technical obstacles of commercial production. The commercialization of ultrasonic desulphurization will take some time to [8].

The regenerative effect of ultrasonic regeneration is good. Its regenerated vulcanizate is similar to the original gum. Its only drawback is that in the regeneration process, apart from destroying the 3 dimensional network structure, it also causes some macromolecular chains to break [9].

Compared with the microwave method, the ultrasonic method is not as efficient as the microwave method, but its regeneration effect is better than that of the microwave method.

1.6 electron beam regeneration

The electron beam regeneration method mainly uses IIR's unique radiosensitivity and produces a chemical depolymerization effect by means of high-energy electron beams of electron accelerators.

Most rubber elastomers undergo cross-linking reaction under the action of radiation. Only a few of the rubber species containing 4 valence carbon group, such as butyl rubber and butyl vulcanizate, present degradation reaction under high energy radiation field.

Radiation technology is based on the unique radiation chemical properties of butyl rubber, which is generated by electron beam.