Review of processing equipment.

In 1839, Charles Goodyear discovered the vulcanization of rubber and since that time, the modern era of rubber was born. In the early years. the mixing of rubber was performed on two-roll mills, until 1916 when the first internal mixer was introduced. There have been many changes through the years to improve the mix, with improved dispersion of carbon and sulfur, faster mixing with reduced temperature gradient and minimized energy cost.

Processing equipment

There is a variety of processing equipment that is required for mixing, sheeting and curing of rubber. The main focus will center on internal mixers, although the following will give you a brief description of the other essential machinery used in rubber processing.

Mixing mills

The open two-roll mill consists of two parallel, horizontal rolls rotating in opposite directions. The rotation of the rolls pulls in the ingredients through the nip, which is the clearance between the rolls. Normally the back roll rotates faster than the front roll, which creates the friction ratio. The front roll which is slower works the material as it is incorporated into the mill.

Mills can be supplied with cored or drilled rolls. Drilled rolls are more advantageous for temperature sensitive materials. The mill is the most common piece of processing machinery in the rubber industry and although the mixer may be more efficient. the mill continues to be the workhorse in most plants.

Extruders

The extruder was first introduced in the 1800s and the two basic designs are a screw type extruder, which is common in the rubber industry and the other design is a ram type extruder, similar to a Barwell preformer.

The screw type extruder is best suited for continuously converting feed to finished form. The most common use is for rod, tube or profile. Usually the feed is in the shape of strips or granules. The screw inside the barrel is where the mixing is performed. The barrel has several zones where cooling or heating can be incorporated. The material. once in the extruder, is commonly forced into the screw. As the screw rotates, the frictional heat created by the shearing action softens the material. Once the material reaches the end of the screw, it is in a viscous state that can be forced through an orifice or die and formed into the desired shape.

There are cold feed extruders and hot feed extruders. The cold feed extruder mixes material that is generally at room temperature. The cold feed extruder has a longer screw length and the flight depth is smaller than a hot feed extruder.

The hot feed extruder is fed material at a temperature above ambient. A hot feed extruder generally accepts material from internal mixers and mills. Its main task is to minimize temperature rise of the material,

Calenders

Since 1825, the calender has been used in the rubber industry. The calender has a variety of roll configurations. horizontal and vertical. Two-roll calenders are commonly used to produce strips and profiles, usually in combination with extruder feeding. This is sometimes called a roller die.

The three-roll calenders are normally used for processing of mechanical rubber goods. The four-roll calenders are found generally in tire plants. The four-roll calender allows simultaneous application of rubber compound on both sides of tire cord fabrics.

The calender has five separate operations in the manufacturing of rubber. They include: sheeting, coating, profiling, frictioning and embossing.

Calenders are diversified machines. They offer manufacturers different sizes from laboratory to production size. They also offer a wide range of diameters, face lengths and configurations. They can also be equipped with cross axis, roll bending and gauging sensors.

Internal mixers

Internal Mixers are best suited to exert a high localized shear stress to the material being mixed and a lower shear rate stirring (homogenizing action). The goal is to have an effective dispersive mixing through high shear stress and large shear deformation. The majority of internal mixer users also want to achieve a high quality mix from an energy efficient process in their compounding operation. Dispersion and energy efficiency are the two limiting factors to mixer system developments. From the viewpoint of both mix quality and energy efficiency, it is widely believed that the rotor design has the greatest effect on mixing efficiency, while rotor speed is the single most important factor influencing the productivity of internal mixers.

Tangential rotors

Tangential rotors perform mixing in four distinct ways.

* Milling: Action of rotors wiping material through the periphery of the bores sides. similar to a mill.

* Kneading: Is caused by the rotor tips when material which has been deformed by mills is carried to the center of the chamber and is relaxed.

* Longitudinal cut-back: When the longitudinal spiral or helix angle to the wing tip forces material to the center of the chamber. Usually a small amount of material is shredded off the center bank at the top by the rotor and, after a trip around the rotor, re-enters the bank at a different point.

* Lateral over-lap: Is when a portion of the material in front of one rotor is pushed over into the opposite chamber or side and is then worked in with material in front of that rotor. As the mix continues, the material is pushed back to the other rotor.

Intermeshing rotors 

Intermeshing rotors provide better heat transfer and mix heat sensitive compounds with lengthy mixing cycles.

Intermeshing rotors advantages include:

* Good dispersion;

* high Mooney reduction;

* low mix temperature;

* wide range and high filler factor; and

* high productivity. Rotor designs may be unimportant as a determinant of internal mixing efficiency. The results of elongational flow in the mixing process, elongational flow being the result of converging flow lines irrespective of rotor design.

Rotor sectional development and design

Four-wing rotors were originally introduced in the 1960s to improve the productivity of internal mixers. Unfortunately, they resulted in poorer dispersion, especially that of sulfur, than with two-wing rotors. Later, attempts were made to overcome this problem but the results were relatively unsuccessful. As a result, an empirical research and design program was carried out in collaboration with the tire industry to determine the best rotor design for intensive mixers and optimize the mixing process.

Phase 1- two dimensional study

This rotor sectional profile study was carried out using a two-dimensional model mixer (straight bladed rotors) with actual tire compounds. Time, temperature, carbon black dispersion and Mooney viscosity breakdown were investigated, enabling the shear work to be studied for various rotor profiles. Two, four and six-wing rotors were tested having a variety of tip widths, tip clearances, operating angles and sections. It was discovered that rotors which had slimmer sections than the traditional rotor profile produced a superior mixing effect.

Four-wing rotors were found to have much improved productivity with respect to two-wing rotors of the same profile. Reduction in usable chamber volume associated with six-wing rotors was so large that productivity was greatly reduced.

Phase 2- three dimensional study

The rotors of this laboratory mixer are fabricated from thin oval section plates, the configuration of which may be easily changed. The mixer chamber is made from clear plexiglass, making it possible to visually study the mix flow, both between the rotors and along the rotor axis. Effects of blade orientation, helix angle and long to short wing ratio on mixing performance can therefore be easily studied. Good mix flow is crucial to dispersion of vulcanizing agents for successful final mixing. It was found in the case of four-wing rotors that by increasing the long to short blade ratio from 2:1 to 4:1 and decreasing the helix angle of the short blade, the back pressure effect of the short blade was decreased. Accordingly, the longitudinal pressure difference between the ends of the rotors has the effect of increasing material flow axially along the length of the chamber and also from rotor to rotor, since the pressure will tend to equalize between the two rotors by material transfer.

Phase 3- production size mixer studies

After rigorous analysis of the results of phase 1 and phase 2, several types of trial rotors were produced and tested in 3.4 and 240 liter internal mixers. From these trials. also carried out using actual tire compounds, it was possible to study production parameters, such as Mooney reduction, carbon black and sulfur dispersion, energy consumption, fill factor and temperature rise. It was also possible to make a comparative study of the productivity of different types of four- and two-wing rotors and further, the mixing performance of the various types of four-wing rotors.

Increasing the number of wings from 2 to 4 gives a four-wing rotor a 30% higher mixing efficiency than a two-wing rotor. Although the net volume of the mixing chamber is reduced by 10% in the case of the four-wing rotor, the fill factor can be 6-7% higher than with the two-wing rotor. The net effect of these differences is that the productivity and mix efficiency is increased by approximately 25% in the case of a four-wing rotor.

In the case of the traditional four-wing rotors, these improvements are at the expense of a reduction in sulfur dispersion. However, in the case of KSBI's four-wing H swirl rotor, the improved sectional profile design and rotor disposition provides a marked improvement in sulfur dispersion, eliminating the problem of poor sulfur dispersion inherent to the earlier models of four-wing rotors.

Four-wing H swirl rotors are fully interchangeable with current four-wing traditional rotors. Although they are also dimensionally interchangeable with two-wing rotors, four-wing rotors generally require 25% more power than two-wing rotors at equal rotor speeds. In the case where it is not possible to increase motor power, four-wing H rotors can be run 25% slower with equal productivity and still realize the benefits of improved ingredient dispersion and mix quality.

Conclusion

* Larger helix angle of long blade gives better mixing performance.

* Better uniformity.

* Interchangeable with all existing 4-wing rotors.

* Higher productivity.

Summary

The goal of this article is to give the newcomer to the rubber industry a brief explanation of the importance of mixing in the rubber industry.

Acknowledgements

"Review of processing equipment" is based on a paper given at the October, 1995 Rubber Division meeting. "Setting up a modern mixing facility" is based on a paper given at the October, 1995 Rubber Division meeting. "Quaternary ammonium salts: new curatives for vulcanization of natural rubber" is based on a paper given at the November, 1994 National Rubber Conference, Cochin, India