Compounding rotor speed

Initial chamber temperature, 60°C Initial rotor speed, 60 rpm Load the compound, 0-30 s... 

The compounds were mixed in three steps The first two steps were done in an internal mixer with a mixing chamber volume of 390 mL. The mixing procedures employed in the first two steps are indicated in Table 29.2. The starting temperamre was 50°C and the cooling water was kept at a constant temperature of 50°C. The rotor speed was 100 rpm and the fill factor 66%. After every mixing step the compound was sheeted out on a 100-mL two-roll mill. The third mixing step was done on the same two-roll mill. The accelerators and sulfur were added during this step. 

The dump temperature of the compound was varied by changing the mixer s rotor speed and fill factor while keeping the other mixing conditions and the mixing time constant. Under the assumption that the final dump temperature is the main parameter influencing the degree of the sUanization reaction, the effect of the presence of ZnO on the dynamic and mechanical properties of the compound was investigated. ZnO was either added on the two-roll mill or in the mixer. 

One measure to improve the devolatilization of the compound is to work in an open mixer during the silanization step. In the experiments described in the following paragraphs, silanization was done pressure-less in an open mixer for 150 s at a constant temperature (145°C). The temperamre was held constant during the silanization period by adjustment of the rotor speed. After 150 s of silanization the compound was discharged and further analyzed. 

Another measure to improve the removal of ethanol is air injection into the mixer during the silanization step. Air can be injected from the bottom part of the mixer using existing valves without any special outlet for the injected air. In these experiments air injection is switched on once the compound reached the silanization temperature (145°C) and the rotor speed is adjusted in order to maintain the silanization temperature. Figure 29.13 shows the properties of this compound compared to a compound that was silanized under the same conditions except with air injection switched off. Air injection lowers the Payne effect, Mooney viscosity, and water content in the compound, and ethanol removal is more effective. All other properties are comparable to the properties of a standard silica compound. 

Rotor speed control is an integral part of the process of rate of temperature rise. The modem mixer is generally supplied with variable speed, constant torque electric motors driving via gear reducers or by a hydraulic drive system which has a motor shaft mounted tachometer to control rotor speed. Systems that can monitor, adjust and record the mixer rotor speed provide improvement in resultant compound uniformity. Changes in rotor speed within a cycle can be programmed. 

Three types of mixer are recognized, plus a miniature mixer which would provide just enough compound for a curemeter test and one sheet. Mixers types A] and A2 and the miniature device have non-interlocking (tangential) rotors whilst type B has interlocking rotors. Dimensions of the larger mixers are specified fairly precisely, including new and worn rotor clearances, and they are required to have temperature control, a system to record power or torque and a timer. The miniature mixer is only specified in terms of capacity, rotor speed and friction ratio but is required to control of temperature, indication of power and a timer. 

The melt compounding is performed in a thermo-extruder (e.g. Haake MiniLab System) at 380°C, with a rotor speed of 150 rpm. Mixing times of 20 min are applied to attain an adequate mixture. 

Internal mixers must be ran in a full or nearly full condition, so a batch recipe is calculated to provide an appropriate volume. If not filled, the ingredients will not be properly sheared and heat transfer will be compromised. Typical commercial mixers have a batch size of at least 100 pounds of compound. A mixer of this size will have a drive motor of no less than 75 horsepower. Proper dispersive mixing is a balance between proper shear, sequence of addition of ingredients, and thermal stability. Mixers have extensive monitoring instrumentation that provides continuous feedback about thermal conditions, rotor torque, and rotor speed. Once a mixing process has been developed, a standard protocol is followed for preparation of the compound. 

The blends with sulfur and HVA-2 crosslinker were prepared by melt blending of HDPE/NR/TPS in a Haake Rheomix 600 mixer equipped with roller rotors. The mixing was carried out at a temperature of 150°°C and the rotor speed was fixed at 55 rpm. Tables 10.1 and 10.2 show the weight proportions of HDPE/ NR/TPS in the blends and the compounding recipes for the vulcanized blends. The ratio between HDPE and NR was fixed at 70/30 and TPS contents were varied from 5 wt% to 30 wt% relative to the overall weight of the blends. 

The polymer compounds were prepared by homogenisation of the components in the mixing chamber (350) of a Brabender Plasti-Corder PL2000 with a rotor speed of 50 rpm, at 200 °C. Sheets (100 X 100 X 3 mm) were formed using laboratory compression moulding equipment. 

Therefore, the discharge temperature should always be checked against the actual temperature for a specific rotor speed/compound formulation/machine temperature and then the temperature indicator on the mixer should be used as an indication of temperature change and not of the actual temperature of the stock. The same comment applies to infra-red temperature measurement as the infra-red emis-sivity of the stock depends on the colour, texture, etc., of the stock. 

Compounding of TPO with additives is performed in internal mixers whose mixing chambers have a free volume of up to 240 L, rotor speeds between 35-70 rpm and a ram pressure of 0.4-0.6MPa. The compounding temperature ranges between 110-125°C and the mixing time is between 12-15 min. 

Polyepichlorohydrin compounds are typically mixed in a two-stage process. Upside-down and regular mix procedures have been used to make quality compounds, with good dispersion of the other ingredients. One-pass mixing is also feasible with proper control of the chamber and rotor temperatures and rotor speed. A one-pass mix procedure is given in Table 7.14. 

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