Application Engineer, Americas
Mixers have an extensive history, deriving from single rotor wooden mills developed in the 1820s. This technology advanced throughout the 19th and 20th century, from iron mills to two roll mills, eventually leading to the creation of a modern mixer. Mixers are faster, have a greater throughput, and are safer than mills for rubber mixing. They are used today to produce rubber in a fast-paced production environment for many different industries. However, rubber products change significantly based on mixer components and the type of mixer used. This blog highlights the 7 most important mixer-related factors that can change the quality of your mix, and how an RPA can detect these changes.
1. Rotor Type
From two wing rotors to six wing rotors, rotor type influences the quality of mix. Two wing rotors are useful for general purpose mixing and final mix, as they have a slow temperature rise and require low energy to run. This rotor type is ideal for most silica recipes rather than rubber mixes. Four wing rotors require more power than two wing rotors, however they improve mixing. They are superior to two wing rotors, as modifications can be done to better disperse agglomerates. These modifications are known as “H Rotors” or “N Rotors”. The six-wing rotor, which uses various tip clearances along the long wings and short wings of the rotor, improves mixing and dispersion further. Advantages to six wing rotors include a faster cycle time, the highest throughput, reduced energy consumption, excellent dispersion, good temperature uniformity, and reduced Mooney viscosity. More recently in 2016, the 5THR Rotor was invented which is also known as a tangential hybrid rotor. These rotors give better mixing properties and greater mixing productivity for high silica compounds, belt/hose compounds, and pigment masterbatches.
2. Rotor Speed
While the rotor itself is a major variable for mixing quality, the rotor speed also plays a role in mix quality. Whether the rotors are running at an even speed or friction speed. Rotors ran at even speed would display rotors moving at the same rate, while friction speed would display one rotor moving faster than the other. All rotors can run at even speed, but only tangential rotors can run under friction speed. Friction speed was traditionally used to improve the quality of tangential rotors by creating friction within the rubber inside the chamber.
3. Rotor Path
The path the rotor takes within the mixing chamber also plays a crucial role in mix quality. Tangential rotors are run at even or friction speeds, and are used for master-batches, 2nd passes, re-mill and final mixes. Intermeshing rotors, originally called interlock or intermix rotors, heavily rely on geometry, therefore can only be run at even speed. They use a lower fill and smaller chamber with increased dispersion, however, they have longer cycle times than tangential rotors and greater energy use.
4. Mixing Scheme
Mixing scheme, or the order of mixing, can significantly affect the rubber exiting the mixer. Orders of mixing include right side up, up-side down, Y mixing, slush mixing, and additive addition. Right side up mixing consists of putting rubber in the mixer first and then adding additives, where up-side down mixing is the opposite. Y mixing combines two separate mixes to form a third mix, and slush mixing is just combining oil and filler.
Additive additions impact rubber dramatically depending on quantity and at what point it was included in the mix. Oils are added to rubber formulations because it softens the mix and reduces shear, which allows for easier processing. However, if oil is added too early it hinders the dispersion of the material, and if it is added too late the oil is not absorbed.
5. Ram Pressure
The ram pressure has an influence on mixing time and operations. A higher pressure leads to faster mix times, while a lower pressure leads to slower mix times. By having a higher pressure, the rubber is experiencing a greater compression and the mix becomes more dispersed, quicker.
6. Fill Factor
A higher fill leads to a faster mix, to a point. Too much material in the chamber can lead to dead zones and worsen the quality of mix. Lower fills lead to longer cycle times, however if it is filled too low there is a loss of effect on the ram in the mixer. Lower fills give better mixing for stiffer materials.
Because mixers are extensively used, wear and damage over time play significant roles in mixing capabilities. These are most concerning for the rotors, where clearances increase, rotor tips round, and shear decreases. Cycle times will increase, and the properties of the mix change because of that. When working with cold or frozen rubber, it weakens the finish and cracks through recessed areas can lead to water breakthroughs and contaminate future batches.
How Does an RPA Detect Changes in Mix?
Although variability between mixes is common, with the right instrumentation these changes can be detected before they’re processed. Particularly with an RPA (Rubber Process Analyzer), differences in pre-cure properties, processing abilities, and post-cure performance between batches of polymers are detectible by stressing the material in torsion. Specifically for quality of mix, ASTM standards D8059 and D6204 are useful RPA methods that highlight batch variation.
ASTM D5089, known as the Payne Effect, is a useful quality tool, where a strain sweep at a low temperature is performed on the sample to determine dispersion and quality of mix. ASTM D6204 measures processing properties by monitoring the rubber properties at a variety of frequencies. Within these RPA standards, subtests measure the viscosity of the material as well as the mixing capability of samples, which can pinpoint where improvements can be made in the mixing process.