Subcategory 1 Tire and Inner Tube Manufacturing

The production of tires and inner tubes involves three general steps: mixing and preliminary forming of the raw materials, formation of individual parts of the product, and constructing and curing the final product. In total, 73 plants use these general steps to produce tires in the United States.

The initial step in tire construction is the preparation or compounding of the raw materials. The basic raw materials for the tire industry include synthetic and natural rubber, reinforcing agents, fillers, extenders, antitack agents, curing and accelerator agents, antioxidants, and pigments. The fillers, extenders, reinforcing agents, pigments, and antioxidant agents are added and mixed into the raw rubber stock. This stock is nonreactive and can be stored for later use. When curing and accelerator agents are added, the mixer becomes reactive, which means it has a short shelf-life and must be used immediately.

Table 2 BPT Limitations for Subcategories of Rubber Processing Industry (kg/kkg of raw material)

Tire and inner

Emulsion

Solution

Latex rubber

Small

Medium

tube plantsb

crumb

crumb

GMEFc

GMEFc

rubber

rubber

Pollutant

Daily 30-max day

Daily 30-max day

Daily 30-max day

Daily max

30-day

Daily 30-max day

Daily 30-max day

avg.a

avg.a

avg.a

avg.a

avg.a

avg.a

COD

12.0 8.0

5.9 3.9

10.0

6.8

bod5

0.60 0.40

0.60 0.40

0.51

0.34

TSS

0.096 0.064

0.98 0.65

0.98 0.65

0.82

0.55

1.3 0.64

0.80 0.40

Oil and

0.024 0.016

0.24 0.16

0.24 0.16

0.21

0.14

0.70 0.25

0.42 0.15

grease

Lead

0.0017 0.0007 0.0017 0.0007

Zinc pHd

Large GMEFc

Wet

Pan, dry

LDEMe

Latex foam

digestion reclaimed

digestion, mechanical reclaimed

Daily 30-max day

Daily 30-max day

Daily 30-max day

Daily max

30-day

Daily 30-max day

avg.a

avg.a

avg.a

avg.a

avg.a

COD

15 6.1

6.2f 2.8

BOD5

3.7

2.2

2.4 1.4

TSS

0.50 0.25

1.0 0.52

0.38 0.19

7.0

2.9

2.3 0.94

Oil and

0.26 0.093

0.40 0.14

0.40 0.14

2.0

0.73

grease

Lead

0.00017 0.0007

Zinc

0.058 0.024

Chromium

0.0086g

0.0036

aComputed from average daily value taken over 30 consecutive days.

bOil and grease limitations for nonprocess wastewater from plants placed in operation before 1959:

aComputed from average daily value taken over 30 consecutive days.

bOil and grease limitations for nonprocess wastewater from plants placed in operation before 1959:

daily max=10 mg/L; 30-day avg.=5 mg/L. cGeneral molded, extruded, and fabricated rubber, limitation is 6-9 pH units for all subcategories. eLatex-dipped, latex-extruded, and latex-molded goods.

'Allowable when the pan, dry digestion, mechanical reclaimed processes are integrated with a wet digestion reclaimed rubber process.

gAllowable when plants employ chromic acid for cleaning operations. Source: USEPA.

After compounding, the stock is sheeted out in a roller mill and extruded into sheets or pelletized. This new rubber stock is tacky and must be coated with an antitack solution, usually a soapstone solution or clay slurry, to prevent the sheets or pellets from sticking together during storage.

The rubber stock, once compounded and mixed, must be molded or transformed into the form of one of the final parts of the tire. This consists of several parallel processes by which the sheeted rubber and other raw materials, such as cord and fabric, are made into the following basic tire components: tire beads, tire treads, tire cords, and the tire belts (fabric). Tire beads are coated wires inserted in the pneumatic tire at the point where the tire meets the wheel rim (on which it is mounted); they ensure a seal between the rim and the tire. The tire treads are the part of the tire that meets the road surface; their design and composition depend on the use of the tire. Tire cords are woven synthetic fabrics (rayon, nylon, polyester) impregnated with rubber; they are the body of the tire and supply it with most of its strength. Tire belts stabilize the tires and prevent the lateral scrubbing or wiping action that causes tread wear.

The processes used to produce the individual tire components usually involve similar steps. First, the raw stock is heated and subjected to a final mixing stage before going to a roller mill. The material is then peeled off rollers and continuously extruded into the final component shape. Tire beads are directly extruded onto the reinforcing wire used for the seal, and tire belt is produced by calendering rubber sheet onto the belt fabric.

The various components of the tire are fitted together in a mold to build green, or uncured, tires which are then cured in an automatic press. Curing times range from less than one hour for passenger car tires to 24 hours for large, off-the-road tires. After curing, the excess rubber on the tire is ground off (deflashed) to produce the final product.

This subcategory is often subdivided into two groups of plants: (a) those starting operations prior to 1959, (applies to 39 plants) and (b) those starting operations after 1959. This subdivision must be recognized in applying limitations on plant effluents of oil and grease because BPT limitations are different for the two groups of plants. For plants placed in operation after 1959, the 30-day average oil and grease limitation is 0.016 kg/kkg of product. For plants placed in operation prior to 1959, the limitation is the same (0.016 kg/kkg) but only for process wastewater. Process wastewater for these pre-1959 plants comes from soapstone solution applications, steam cleaning operations, air pollution control equipment, unroofed process oil unloading areas, mold cleaning operations, latex applications, and air compressor receivers. Water used only for tread cooling and discharges from other areas of such plants is classified as nonprocess wastewater, in which oil and grease levels are limited to 5 mg/L as a 30-day average and 10 mg/L as a daily maximum.

Emulsion polymerization, the traditional process for synthetic rubber production, is the bulk polymerization of droplets of monomers suspended in water. Emulsion polymerization is operated with sufficient emulsifier to maintain a stable emulsion and is usually initiated by agents that produce free radicals. This process is used because of the high conversion and the high molecular weights that are possible. Other advantages include a high rate of heat transfer through the aqueous phase, easy removal of unreacted monomers, and high fluidity at high concentrations of product polymer. Over 90% of styrene butadiene rubber (SBR) is produced by this method. Approximately 17 plants use the emulsion crumb rubber process.

Raw materials for this process include styrene, butadiene, catalyst, activator, modifier, and soap solution.

Polymerization proceeds stepwise through a train of reactors. This reactor system contributes significantly to the high degree of flexibility of the overall plant in producing different grades of rubber. The reactor train is capable of producing either "cold" (277280 K, 103-206 kPa) or "hot" (323 K, 380-517 kPa) rubber. The cold SBR polymers, produced at the lower temperature and stopped at 60% conversion, have improved properties when compared to hot SBRs. The hot process is the older of the two. For cold polymerization, the monomeradditive emulsion is cooled prior to entering the reactors. Each reactor has its own set of cooling coils and is agitated by a mixer. The residence time in each reactor is approximately one hour. Any reactor in the train can be bypassed. The overall polymerization reaction is ordinarily carried to no greater than 60% conversion of monomer to rubber since the rate of reaction falls off beyond this point and product quality begins to deteriorate. The product rubber is formed in the milky white emulsion phase of the reaction mixture called latex. Short stop solution is added to the latex exiting the reactors to quench the polymerization at the desired conversion. The quench latex is held in blowdown tanks prior to the stripping operation.

The stripping operation removes the excess butadiene by vacuum stripping, and then removes the excess styrene and water in a perforated plate stripping column. The water and styrene from the styrene stripper are separated by decanting and the water is discharged to the treatment facility. The recovered monomers are recycled to the monomer feed stage. The latex is now stabilized and is precipitated by an electrolyte and a dilute acid. This coagulation imparts different physical characteristics to the rubber depending on the type of coagulants used. Carbon black and oil can be added during this coagulation/precipitation step to improve the properties of the rubber. This coagulated crumb is separated from the liquor, resuspended and washed with water, then dewatered, dried, and pressed into bales for shipment. The underflow from the washing is sent to the wastewater treatment facility.

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