Continuing with Part 1 of this article, by Dr.S.N.Chakravarty, President – Elastomer Technology Development Society and Ex-Chairman, Indian Rubber Institute (IRI).
Now, let us look more deeply into PRINCIPLES OF MIXING.
PRINCIPLES OF MIXING
Vulcanizable polymers cannot be used without compounding. Various additives like curative system, protective system, reinforcing agents, cheapeners and other process aids have to be mixed to the polymer or polymer blend “to make a coherent homogenous mass of all these ingredients, which will process satisfactory and on Vulcanisation will give the product capable of giving the desired performance, all with the minimum expenditure of machine time and energy.”
Due to the partly elastic nature and very high viscosity of rubber, power intensive sturdy machinery like mixing mills or internal mixers is necessary to achieve the mixing of additives into the polymer. The ingredients are in form of liquids, solid powders or solid agglomerates.
The mixing of solid ingredients into the solid polymer occurs in phases. During subdivision large lumps or agglomerates are broken down into smaller aggregates suitable for incorporation into the rubber.
For instance carbon black pellets which have dimension of the order of 250-2000 µm get broken down into aggregates with dimensions of the order of 100 µm. Then these aggregates are absorbed or incorporated into the rubber to form a coherent mass.
During mixing, shearing of the rubber generates shearing stress in rubber mass which imposes in turn shear stress on these aggregates and breaks these into their ultimate fine size which in case of carbon blacks is of the order of about 1µm. in size. This phase is also known as intensive mixing or homogenization in micromolecular level.
Distribution or homogenization in micromolecular level or extensive mixing is “the moving of the agglomerates / particles from one point to another, without changing the shape of the particle to increase the randomness of the mixture”. The ingredients incorporation is a very slow process.
Another method of reducing incorporation time is to use powdered rubbers. In a simple ribbon blender the powdered rubbers can be mixed with the other compounding ingredients.
The powdery mass is compacted in another machine and then fed to the internal mixer. Because of the large surface area of the powdered rubbers, the incorporation into polymer is very fast and only a very short mixing cycle in the internal mixer is adequate to achieve the mixing.
Even after all ingredient is incorporated, dispersion/distribution of the ingredient is not complete. Good distribution is comparatively easy to achieve by paying proper attention to cutting and folding operations on a mixing mill or by just prolonging the mixing cycle in an internal mixer.
Dispersion however is dependent on the shear stresses generated within the polymer and hence good dispersion may not be achieved by prolonged mixing . Careful consideration is necessary not only as regards the time of the mixing cycle but also for the order of addition of ingredients to the rubber.
Viscosity break down occurs during mixing and is essential for smooth processing of the stock.
Degree of dispersion of carbon black has profound influence on the physical properties of the vulcanisate. Undispersed carbon black (normally taken as carbon black agglomerates bigger in size than 9µm) act as gritty particles. Under tension, cracks develop at these spots.
Failure properties like tensile strength, tear strength and consequently abrasion resistance come down as the degree of dispersion comes down.
CONDITIONS FOR GOOD DISPERSION OF CARBON BLACK
To achieve dispersion of the carbon black, the polymer mass itself has to exert considerable shear stress on the carbon black agglomerate incorporated inside the polymer. This is achieved by passing the polymer carbon black batch through a narrow nip either between two rolls of a mixing mill moving at frictional speed or that in between rotor tip and chamber wall of an internal mixer.
In internal mixer two additional conditions have to be fulfilled. Chamber loading must correct & Ram pressures must be adequate to hold the stock within the chamber.
CONDITIONS FOR GOOD DISPERSION IN INTERNAL MIXER
- Narrow Clearance between Rotor
- Tip and Chamber wall (High Rate of Shear)
- Correct Volume Loading
- Adequate Ram Pressure
- High Viscosity of Polymer
- Low Polymer Temperature
- (High Viscosity and More Prominent Elastic Characteristics of Raw polymer)
For higher shear stress generation inside the polymer mass, polymer should have high viscosity. The temperature should be low so that thermoplasticity does not lead to lowering of polymer viscosity.
Any sweeping of carbon black at the end of mixing cycle is to be avoided in regular production.
The Master Batch (MB) is aged. Cooled MB goes to the cracker. Mechanical working of the cooled MB improves the degree of dispersion further. Then the MB is worked on Cracker mill, warming mills, feed mill and then to the extruder.
It is possible to follow the mixing process in the internal mixer with the help of power / time curve (or amperage of drive motor / time curve). When carbon black is added the torque does not rise immediately. The carbon black added as palletised black is about 30% higher than the total chamber volume. As the carbon black is slowly absorbed into the rubber the torque increases. As more and more carbon black gets absorbed, stock volume becomes lower and the power curve comes down.
Based on the power curve data on experimental batches, criteria like constant time or constant temperature are selected as dumping criteria. With constant time or constant temperature as the dump criteria, there will be variation in quality of the compound produced.
The better criterion is the constant energy criterion. This is very versatile, and will automatically take care of any minor variation in operating conditions as well as of even major ones to give a consistent quality output. It can also be kept constant even when rotor rpm is changed or ram pressure is increased, while the time or temperature criteria will have to be re-established after a series of experiments.
BLENDING OF POLYMERS
In compounds, sometimes polymer blends are used in order to cover deficiencies of one polymer by partial use of the other. However a homogenous dispersion of two different polymers on molecular scale is not possible.
Most important condition for achieving good blending of polymers is that both polymers should have as near viscosities as possible during blending.
In order to mix uniform, high quality, low-cost rubber in an environmentally clean area, the mixing systems in future must provide the following:-
- Accurate, automatic, clean and flexible weighing of all materials used in the mixed compound.
- Mixers that use :
- Either tangential or intermeshing 4-wing variable speed rotors depending on the product.
- Variable ram pressure and position during the mix cycle.
- Mix time based on feedback from instrumentation sensors that monitor and control in “Real Time “ temperature, viscosity, dispersion and energy.
- Greatly improved dust stops, rotor and chamber metal surfacing as well as mechanical and electrical components that will increase up-time and reduce overall maintenance cost.
- The down-stream equipment will be similar to what is used today but automation will either eliminate or minimise a need for the operator at the mill, former or batch-off unit.
- Online automatic sampling and testing of each individual batch will be performed after the mill or forming machine and this data will be used to make minor adjustments to the formula of the remaining batches as well as further processing down-stream.
- Controls will be more sophisticated with feedback loops to make sure each batch and formula will be compounded properly. They will automatically record and control the conditions of the mixer to provide a more consistent uniform mix.
ZONE ANALYSIS OF UPSIDE DOWN POWER PROFILES :
ZONE – I
Loading + wetting stage – Formation of a Single Mass of filler and rubber – penetration of Polymer in to filler voids – As the C-black is slowly absorbed into the rubber the torque increases. When the volume of rubber + C-black becomes equal to the chamber Vol., the raw comes to the lowest position, the raw hydraulic pressure on the stock disappears. The power shows first peak. More and more C-black gets absorbed, Stock volume becomes lower & the power curve comes down.
ZONE – II
Most of the real dispersion work takes place. The filler agglomerates are gradually distributed through the polymer and then broken down tto their ultimate size. The power curve also starts rising till the whole stock with oil & C-black has consolidated. At this juncture the second power peak occur .
ZONE – III
Plasticization takes place.
The power curve decrease beyond the second power peak has been found to obey first-order kinetic law,
Log [(Po – Pt)/(Pt – Px)] = Kt
The mixing should continue till dispersion half time. (i.e. (Po-Pt)/(Pt-Px) = 0.5) after 2nd power Peak.
TOTAL MIXING TIME = Black incorporation time + Dispersion half-time.
To handle variety of rubber compounds on the same mill required that mill to have.
- Independent speed control on both rolls
- Widely variable speed on both rolls
- Independent temperature control on speed, friction ratio and temp. to be adapted to each individual .
- Hydraulically operated nip adjustment
RECENT DEVELOPMENT FOR IMPROVING MIXING EFFICIENCY :
- Increased rotor speed
- Higher Ram Pressure
- Improved Rotor Design
- Improved Cooling System
- Continuous Mixing Process
MAJOR CHANGES IN RUBBER & PLASTICS MIXING
|i) 2 Speed Rotor (20 – 40 RPM)||Variable (0 – 90 RPM)|
|ii) Low Power (e.g. 11- max. 800 HP)||High Power (e.g 11 Banbury Max 1500 (HP)|
|iii) Low Pressure Ram (40 Bar)||High Pressure Ram (80 Bar)|
|iv) Tap water cooling||Refrigerated water cooling||Tempered water cooling|
|v) Spray side cooling||Cored (channel) Sides cooling||Drilled sides cooling|
|vi) Spring Drop Door||Drop Door|
|vii) Spring Tension Seals||Hydraulic Seals|
|viii) Chrome internal Surface||Alloy internal surface|
|ix) 2- Wing Rotor||4 – Wing Rotor|
|x) Mix unit till it sounds Right||Mix by Power Consumption||Computerized control of all variables|
INCREASED ROTOR SPEED (Size 11 Banbury)
|Rotor Speed(r.p.m)||Mix Time( % )||Out Put rating( % )|
At high pressures the average HP required was found to be inversely proportional to the rotor speed to the 0.97 power.
P1 = P2*(V1/V2) where, P = Horse Power , V = Rotor Velocity
INCREASED RAM PRESSURE (Size No. 11 Banbury, 40 rpm)
|Type Pressure||Ram Pressure (Psi)||Effective Pressure (Psi)||Mix Time (%)||Output Rating|
IMPROVED COOLING SYSTEM
Tempered water / controlled water temperatures are selected relative to the coefficient of friction of the Sp. Polymer being mixed. Lowest possible Temp. at which the polymer gripping the metal surface enabling shear and turbulent flow of the polymer to take place rather than slippage.
|Polymer||Tempered Water Temp. (max.) (°C)|
|Highly Cryst. EPDM||60 – 70|
|Natural Rubber||40 – 60|
|SBR||50 – 60|
|Low Cryst. EPDM||30 – 35|
|Hypalon (CSPE)||30 – 35|
|NBR (Nitrile)||20 – 25|
|IIR (Butyl )||20|
- Total Power consumption is reduced because More time is spent mixing rather than flopping around.
- Greater fill factor is obtained because the mix is hugging the metal during the whole time it is in the mixer.
- Batch to Batch consistency is improved because the temperature of the metal fluctuates in a very narrow range and each batch is exposed to essentially the same metal conditions at each step of the loading and mixing cycle.
- Improved dispersion due to absence of unbroken down polymer lumps.
Dr. Chakravarty can be reached on email@example.com
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