Presentation of a modern triplex extrusion system for black and silica mixtures.

Tires of passenger cars produced according to the silica concept have been on the European market for some time now. The treads of these tires are made of a mixture that was reinforced partly or completely with silicic acid instead of carbon black. Silicic acid or silica mixtures are usually produced on closed mixers. However, production and processing of silica mixtures are much more difficult than production and processing of carbon black mixtures. Details of the rheological behavior must be known to develop such processes. This article looks into the following subjects:

* How do silica mixtures behave during the mixing process and subsequent treatment compared to carbon black mixtures?

* What are the additional requirements silica mixtures impose on a tread extrusion line?

* What does a modern tread and sidewall line for black and silica mixtures look like?

Silica mixtures

A common silica mixture could be composed of the following components: Polymers (100 phr); silica (70 phr); silane (6 phr); softener (10 phr); zinc oxide (3 phr); crosslinking system (5 phr); and age resister (6 phr).

In this case, the carbon black has been completely replaced by the silica as filler. The silane acts as a coupling component between the silica and the polymers. The strengthening action of silica in such mixtures originates from a chemical compound between the silica and the polymers.

The crosslinking reaction takes place during the vulcanization process.

Carbon black mixtures are different. Parts of the polymer molecules at the carbon black particle are adsorbed during incorporation and dispersion of the carbon black in the polymer. This is a physical bond which is weaker than a chemical bond.

With the right composition of a silica mixture, mixture characteristics such as wet skid resistance, abrasion and other dynamic properties can be optimized and improved compared to the carbon black mixture.

Why silica mixtures?

Using silica instead of carbon black in a tread mixture has the following advantages: A considerably decreased rolling resistance of the tire; a considerably improved snow and ice behavior; much better wet skid properties, with abrasion remaining the same.

Production of silica mixtures compared to carbon black mixtures

The tire industry produces carbon black mixtures in two, or in some cases three steps. In the last mixing step, the vulcanization system is mixed into the compound. During masterbatch production (first mixing step), polymers, fillers, processing oils and other raw materials are mixed to a compound which is as homogeneous as possible. The mixing order normally used for masterbatch mixing is as follows: Adding polymers, masticating, adding fillers and chemicals, incorporating and dispersing, injecting processing oils, incorporating, cleaning step, homogenizing and discharging.

Fillers are delivered as pellets or granulate. During dispersion, the fillers must be broken off the polymer matrix up to an acceptable degree of dispersion. The dispersion process of carbon black mixtures can be explained as follows. Parts of the polymer molecules adsorb at the carbon black surface. The carbon black agglomerates are pulled apart by shearing forces and a relatively high viscosity of the polymer matrix. The high energy dissipation and the resulting mixing temperature rise are side effects of such high shearing forces (shear stresses). Therefore, compounding is limited by the temperature.

An additional mixing step is used before adding the acceleration system if the filler portion is very high or the compound's homogeneity is very critical.

In addition, a chemical reaction takes place during the production of silica mixtures. This reaction is referred to as silanization. Standard silica qualities are highly active. This means that silica can easily adsorb polymer chains, as well as other silica particles, while a chemical bond of polymer chains is not possible. To achieve this, the silica must be silanized. Silane is required for this process. After the silanization process, the silica particles act like sulfur so that the polymers can chemically react on the silica. The surface activity of the silica decreases seriously during the silanization reaction. At the beginning of the silanization reaction, the compound viscosity was extremely high (up to more than 200 Mooney units) and after completion of the reaction the viscosity is very low (down to under 50 Mooney units). The shrinkage behavior of the compound is very distinct and much higher than the behavior of normal carbon black mixtures. A completely silanized compound does not show a yield value.

The silanization reaction is dependent on the temperature, like all chemical reactions. There is a recognizable reaction below 80 [degrees] C, but it takes place very slowly. Above 160 [degrees] C, both silanization and crosslinking start. The compound is burned and cannot be used anymore. The temperature rises during compounding. Therefore, a silica mixture must be discharged from the mixing chamber while the temperature is relatively low. To complete the silanization reaction, additional mixing steps must be used as reaction steps.

The rheological behavior of a silica mixture

Compared to a carbon black mixture, a completely silanized mixture has the following characteristics: Very low viscosity, distinct shrinkage behavior and hardly any yield value. These characteristics show the immense dependence on the degree of silanization.

Effects on the extrusion process

The important differences between carbon black and silica mixtures also have effects on the extrusion process:

1. The temperature of the extrudate must be between 90 [degrees] C (because of the flowability of the compound) and 115 [degrees] C. If the temperature of the extrudate rises too much, ethanol residues (residual products of the silanization reaction) might be gasified in the compound. Inadmissible blisters will then occur in the extruded profile.

2. Compared to carbon black mixtures, silica mixtures have distinct swelling values. This must be taken into consideration when designing extrusion tools. To produce the same profile, final die outlets must be considerably smaller for silica mixtures than for carbon black mixtures.

3. Shrinkage during the process occurs. Especially within the first 15 seconds after leaving the final die, a silica mixture shows distinct shrinkage.

4. After-shrinkage occurs. Silica mixtures have nearly no yield value. Deformation stresses, which occurred during the extrusion process, are not frozen by a filler network, but cause an after-shrinkage effect.

5. Varying the process due to charge variations oct. The silanization reaction has an important impact on the processing characteristics. Each small charge variation results in a varying extrusion process.

Extrusion process requirements

Silica mixtures are much more expensive than carbon black mixtures because of higher raw material prices and production costs. Therefore, not all tire treads consist of a silica mixture. An extrusion line must therefore be suitable for black and silica mixtures. To avoid the above mentioned results and effects, a modern extrusion line must fulfill the following requirements:

1. The extruder and screw geometry must be suitable for carbon black and silica mixtures.

2. A very short take-away conveyor and a longer, adjustable shrinkage conveyor (up to 25%) are necessary to monitor the distinct shrinkage behavior.

3. To avoid free and uncontrolled shrinkage, as few dancer rolls as possible should be used in the downstream equipment.

4. Treads must be wound up instead of being cut and booked. Possible residual stresses in the extrudate cannot or hardly result in a shrinkage on the reel.

5. The extrusion control system must compensate for a huge variety of mixing and processing properties.

6. The extrusion control system must compensate for charge variations.

Extruder characteristics with a silica mixture

Extruder characteristics when using a silica mixture under normal extrusion conditions were examined. A 150 mm pin extruder with length 16D was used. The extruder requirements were fulfilled. The energy balance data indicated that the operating point of the extrusion process was chosen correctly.

Presentation of a triplex extrusion line

In the following, an extrusion line is presented which fulfills the above mentioned requirements. The extrusion line consists of the following components: Feeding device, triplex extruder; triplex extrusion head; shrinkage system; continuous weighing scales and width measuring device; profile scanner; cooling and drying system; cutter; and wind-up system.

An extruder feeding device must guarantee permanent and stress-free feeding of the extruder during the whole extrusion process. Each variation during feeding causes output variations.

A triplex extrusion aggregate consists of three extruders with a common triplex extrusion head. Tools are mounted in the head to produce a co-extrudate from three different compounds. One of the three compounds can be a silica mixture. Today's treads for modern passenger car tires consist of three or even four components, each of them of a separate compound. Each extruder prepares one compound. The compounds are combined in the extrusion tool (figure 1).

[Figure 1 ILLUSTRATION OMITTED]

After leaving the triplex extrusion head, the extruded strip must be cooled down to storage temperature as fast as possible. This takes place on a spray cooling line. Cooling is determined by the line speed and the length of the cooling line.

The temperature of the cooling water is an additional and important parameter.

Between the extrusion tool and the beginning of the cooling line there are also the take-away conveyor for pulling the profile out of the tool; the shrinkage conveyor for relaxation of deformation stresses in the profile; the continuous weighing scales for continuous check of the profile weight; the width measuring device for continuous check of the total profile width; and the profile measuring device for monitoring the cross-section of the profile.

Figure 2 shows the take-away conveyor, the shrinkage conveyor, the continuous weighing scales, the width measuring device and the profile measuring device, as well as the head end of the cooling line.

[Figure 2 ILLUSTRATION OMITTED]

Figure 3 shows the wind-up system. This wind-up system allows stress-free wind-up of treads and sidewalls with a wind-up precision of [+ or -] 1 mm.

[Figure 3 ILLUSTRATION OMITTED]

There are two wind-up positions with two independently driven parallel reels each. They allow independent winding of co-extruded profiles, such as sidewalls, on two different reels. Short wind-up times facilitate a continuous extrusion process.

The extruder control system

Triplex extrusion lines which fulfill the above mentioned requirements should be equipped with the latest control systems. The main tasks of such control systems are monitoring and control of the production process, tracking and monitoring of the material flow, as well as acquisition and evaluation of measuring data.

Characteristic features of such extrusion lines are the great amount of measuring and control parameters, such as extrusion pressure, temperature, meter weight, line speed, extruder speed, profile characteristics, etc. These data must be continuously monitored and re-adjusted.

For some years, Krupp has successfully implemented the so-called distributed control for this purpose. The basic idea is that control functions are decentralized and integrated in an open automation system. The advantages are compatibility of different systems, reliable data communication in industrial environments, and simple mounting on site, hence low mounting and commissioning costs.

Main parts of the automation system

Figure 4 shows the block diagram of an extruder control system. It shows the PLC (user-programmable controller, operator environment, sensor/actuators (I/O-level), intelligent devices, drives (DC, AC and servomotors) and industrial PC.

[Figure 4 ILLUSTRATION OMITTED]

The main parts are connected via bus systems. Summarized, the control system is a combination of three technologies, including one or more PLCs, one microprocessor and industrial communication (field and cell bus).

Variables influencing the automation system

The automation system must fulfill the following tasks: Control of extruder feeding device; control of extruder speed; control of the circuit water temperature in each temperature control zone; monitoring of extrusion head pressure and compound temperature; control of all line speeds; control of hydraulic locking system of the triplex extrusion head; tracking and monitoring of the shrinkage behavior of single groups and the total shrinkage; monitoring of continuous weighing scales and width measuring device; and control of wind-up system, etc.

The automation system must take the following influencing variables into account: Viscosity of the compounds; material temperature; pressure and temperature of the material in the extruder (various zones); output capacity of the extruder; meter weight and/or width of the extrudate; profile characteristics; quality of the extruder (porosity, blisters, etc.); and tension in the extrudate, shrinkage.

Further tasks of the automation system

Further tasks of the automation system include: Control and monitoring of the drives; control of bus systems; control and monitoring of the hot water circuit; control of synchronous start/stop processes; processing of user inputs; communication with intelligent parts; operating data acquisition and processing; acquisition of status data, events, etc.; output of process and alarm signals; state monitoring and diagnostic systems; acquisition and control of the quality; and communication to the host computer.

Conclusion

In an unvulcanized state, the behavior of silica mixtures compared to carbon black mixtures is completely different. Therefore, continuous measures are necessary in a tread extrusion line. The implementation of a modern automation system enables extrusion of carbon black and silica mixtures on the same line.