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Continuous processing high quality compounds on a co-rotating twin screw extruder.

Traditionally, rubber materials have been compounded in batch equipment that imparts shear stress for mixing through a variety of mechanisms. These include the familiar two roll mills, sigma blade and internal type mixers. This equipment has been successfully used for many years and will continue to be sufficient for most applications. These are all in the class of random order mixers, where all the components will receive the same shear input after some (unknown) time interval. Therefore, the mix cycles, or residence time in the masticated state, are relatively long. This can be several minutes to a half hour or more. Quite often the mixing is finished, as in the case of an internal mixer, on a two roll mill or other secondary equipment, and is operator dependent. Each of these mixing devices has a fixed mechanical configuration, and only the process variables can be manipulated to improve quality or increase throughput. Process optimization focuses mainly on improved batch weigh systems and process variable manipulation.

There is a growing number of applications which demand more consistency, better quality of dispersion and lower heat history than can be attained by these conventional means. These higher quality requirements can be met using ZSK fully intermeshing, co-rotating twin screw extruders. This is a continuous compounding system, as opposed to a batch compounding system. A typical process flow diagram is shown in figure 1.

Description of equipment The key elements that results in improved quality from compound mixed on a ZSK fully intermeshing, co-rotating twin screw extruder are:

* The ZSK produces efficient shear stress transfer into the materials being processed due to the higher surface area to volume ratio than available with batch mixers and the design of the mixing elements.

* The construction of this equipment is modular. This allows the mechanical configuration to be designed for the particular process task at hand.

* The process tasks can be broken down to unique unit operations, and each unit operation can be optimized independent of, or with minimal impact on, the other unit operations.

The design of the fully intermeshing, co-rotating screws results in a shear stress distribution that is essentially uniform across the screw channel. Each particLe of material is subject to the same shear stress in the extruder, resulting in maximally uniform mixing and dispersion. The shear stress input can be manipulated with screw element geometry and operating conditions such as screw speed and throughput rate, providing a high degree of flexibility to balance product quality and process productivity. The material in one screw is wiped by the crest of the oncoming screw for a self-wiping or self-cleaning effect. This results in a short residence time and a narrow residence time distribution. Residence times for rubber compounding are in the order of 45 seconds to one minute. Residence time distributions show no lagging tails, and a narrow bell shaped curve.

The construction of the ZSK extruders is modular, and the barrel sections and screw elements are assembled for a particular process task.

The barrel sections come in various forms, which are totally interchangeable, including open top, side feed and closed. These designs allow for multiple feeding of ingredients, liquid injection, atmospheric venting and devolitilization under vacuum at any point along the process section. All barrels are internally cored for cooling, and are typically supplied with electrical heaters.

There are two basic styles of screw elements, conveying elements and kneading blocks. These are depicted in figure 2. There are different configurations of each, which allows a screw design to be optimized for the particular process task. Each element provides a distinctive conveying, shear and pressurization action which allows for a screw configuration that can accomplish several individual process tasks independently along the process length, in concert with the barrel configuration.

Principles of operation

The above factors provide a great deal of flexibility in designing a new process configuration. With this flexibility, it is possible to break down the overall process task into individual unit operations. Once these are defined, it is relatively easy to design a new process as the summation of the defined unit operations.

The unit operations we are normally concerned with in rubber compounding include:

* Meter feeding of ingredients;

* mastication of base polymer;

* distributive/dispersive mixing;

* plasticizing/extending;

* discharge/shaping.

The meter feeding of ingredients is necessary to maintain the formulation recipe and is one of the process control variables used to control the shear stress as mentioned earlier. Meter feeding of both particulate and liquid feeds uses commercially available equipment. These are usually loss in weight metering devices, where the individual feeders are controlled by a supervisory system, and are not unlike, in concept, the automated batch weigh systems. Traditionally, feeding rubber polymers has been problematic with the rubber supplied in bales. This is solved by incorporating size reduction equipment in line with the feeder.

The mastication of the base polymer is the first of the unit operations that actually is done in the twin screw extruder. This is normally done in the second or third barrel section using the particular arrangement of kneading blocks which will fully masticate the rubber without over working. The peak temperature in the process should occur at this point due to the shear heating response of the rubber. This peak temperature is isolated from the remainder of the process, especially any temperature sensitive ingredients. With the ability to control the shear stress input in the mastication zone, it is not necessary to run "upside down" which is often done in batch mixers to maximize the viscosity and the viscosity related mastication and mixing. Quite often, process aids, lubricants and partial amounts of plasticizing oils are added in the mastication zone. This is done to keep the peak temperature under control and to start to plasticize the rubber polymer.

The distributive and dispersive mixing is done downstream of the mastication zone. The fillers, extenders, carbon blacks, et al are added into the process section with a side feeder. These are conveyed a short distance downstream to a mixing zone. This consists of an arrangement of kneading blocks to first distributively mix these ingredients into the polymer, then disperse them to their finite particle size, fully wetting out the surfaces for maximum effect. The temperature sensitive ingredients such as the accelerator package, or foaming agents can be added with the bulk of the fillers or added further downstream to minimize temperature exposure. The residence time of these temperature sensitive ingredients in the masticated rubber is kept to a minimum, usually only five to ten seconds, which results in a minimal reduction in activity of these ingredients due to thermal degradation.

The plasticizing and extending oils are added both upstream and downstream of the fillers as necessary to control viscosity, temperature and to wet out the fillers. Distributive mixing elements are typically used to quickly incorporate these feedstreams into the compound.

Discharge of the finished compound from the ZSK is usually from the open end of the last barrel into a second stage for pressurization through a forming device, i.e. to make a flat strip, tube or pellets. This is done to prevent temperature buildup at the end of the twin screw due to the pressure restriction a die creates. This also provides an opportunity to create a shape that is most suited to optimize downstream handling of the compound.

Other unit operations that may be important for particular formulations include degassing, devolitilization, dilution, etc. These can all be incorporated into the twin screw process with the appropriate process design.

Expected improvements

The improvements in quality in compound mixed on a ZSK can, in part, be measured directly by traditional testing. For example, improved consistency can be measured with a narrower deviation of Mooney viscosity or ODR test data. Other improvements are not seen as quickly. Improved dispersion of certain ingredients may not affect viscosity. Improvements in dispersion can usually be seen microscopically, but this is a qualitative not quantitative measurement. Measurements of improved mixing, in these instances, require physical and end use testing to understand the improved mixing.

There are some additional advantages which result from using the ZSK for rubber compounding rather than a batch mixer.

Continuous compounding uses constant power to drive the twin screws. Batch mixing is notoriously cyclical, with peak power demands significantly higher than the average power requirement, often with electric power rates based on the peak demand. The lower peak demand of the ZSK results in a significantly lower electric power rate, or demand charge, although the total power consumption per unit output may be equivalent to a batch system.

The reduced temperature exposure in the ZSK may enable a reduction in the amount of accelerators required in the formulation to attain the same degree of cure in the final product, and even may eliminate the need for the retarders so often used in rubber compounds to prevent scorching during the compounding process. Continuous processing on the ZSK opens the door to true SPC and JIT. The variables which control product quality are set points in the process and can be continuously monitored and controlled. Once an SPC program is developed, it is not required to wait for release by QC to use the compound. This certainly improves the probability of meeting JIT.

Summary

In summary, the ZSK fully intermeshing, co-rotating twin screw extruders can improve the consistency, the quality of dispersion, and reduce the heat history in the compounding of rubber formulations. This is a discreet mixer, where every particle of material sees the same shear stress. The equipment is modular, enabling a process configuration optimized for that process task. The process tasks can be broken down to unit operations, and each unit operation can be optimized independent of the other unit operations.
COPYRIGHT 1993 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Ploski, Walter A.
Publication:Rubber World
Date:Jul 1, 1993
Words:1624
Previous Article:Advances in mixing line equipment.
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