Enhancing the performance of polymer granulates – Gentle touch technology for temperature treatment and soaking
The gentle touch mixing and processing technology from Lindor Products uses a rotating vessel to enable gravity to gently mix powders and granulates and to expose these to other applications like heat treatment and soaking in liquid agents. Amongst others, an Italian producer of thermoplastic polyurethanes and polyesters uses the gentle touch technology to homogenise the pellets and to add a liquid crosslinking agent. Product integrity and pure processing without product degradation lead to important performance improvements of the polymers.
The principle of the gentle touch technology
A gentle touch mixer consists of an inlet, horizontal rotating vessel or drum and a fixed outlet. Material is fed through the inlet in the slowly rotating drum. During the mixing process, the product flows in axial and radial direction over special oriented and finished scoops. A particle is lifted up and slides down a scoop, ending in the ever rolling bed of particles, flowing back to the inlet area and then waiting for another turn. Rotation and gravity are mixing the product randomly, whereby all particles are always in movement. This technology enables a homogenisation within minutes. After opening the outlet, the product slides over the scoops out of the mixing drum into a receiving hopper or to the next station. As the product is always in movement due to the ever-rotating vessel, risk of segregation is minimal. The emptying rate is typically better than 99.9 %.
Advantages of the gentle touch technology
The gentle touch technology enables a gentle mixing process, which is suitable for most products and has exceptional advantages for fragile and heat sensitive powders or granules, as nearly no shear forces occur. The mixers have a space-saving horizontal design, low maintenance costs and are easy to clean. Short mixing times with high homogenisation are achieved with a minimal energy (typically less than 1.5 W/kg). There are no inner moving mixing parts and devices and consequently no shear forces, affecting the product or creating thermic stress. The inner mixing drum is free of dead spaces and allows consequently easy cleaning.
The gentle touch technology allows a flexible filling rate between 10 – 100 % of the maximum batch size, which varies depending on the mixer size between 10 and 25 000 litres. For making the cleaning easier a mixer can be equipped with retractable inlets and outlets, CIP/WIP system and extra-large hatches for cleaning. The mixers can be operated in batch mode of operation as well as in continuous mode.
Liquid dosing on powder and granulate
When using liquid injection for impregnating, soaking, moistening or coating products, a spray or flow of liquid is aimed at the always rolling bed of particles. Every second new powder or granulate particles get exposed to the liquid flow or spray. The dosing rate of the liquids may vary depending on spraying system used and type of nozzle. The liquid dosing can be anywhere between 0,01 % and 60 %.
Single phase nozzles apply a range from 5 – 60 % by weight, dual phase (atomising) nozzles a range of typically 0,1 – 5 % and ultrasonic nozzles for 0.01 – 1.0 % liquid content. Therefore, without loss of product, exactly the required quantity of liquid can be distributed homogeneously over the mixture. When set up correctly, optimised and fine-tuned, a very effective and efficient way of integrating liquids with a powder or granulate can be achieved.
A gentle touch mixing system can also be used as rotating reactor vessel for heating, drying and cooling of products. The mixer can be equipped with double walls at the drum and at the scoops for heating with warm water, steam or thermal oil. Also drying of granulate by applying a continuous air flow through the product during the mixing process is possible.
Gentle touch processing in the plastics and rubber industry
Usually, polymer granulates have good to very good flow properties. Using gentle touch technology this enables even shorter mixing times, achieving high throughputs, especially in case of continuous processing. Capacities of 5 – 15 (and even 20) tonnes per hour are possible. To further simplify the production process, the drum of a batch mixer can be integrated as receiving hopper in a pneumatic conveying system. The granules will be sucked directly out of the silo into the mixing drum on which a vacuum is applied.
Gentle touch processing is suitable for applications such as “powdering” of sticky pellets. The mixture is mixed and treated without negative influence on quality. Breaking of particles, occurring of dust and thermic stress due to friction are nearly excluded.
Homogenising compounds after extrusion
An important application is the homogenisation of compound granules, to equalise colour- and quality variations, significant or however small, caused by a prior extrusion process. A producer of a metal powder/polymer compound uses a large and heavy drum mixer with a capacity of 30 tonnes to homogenise the production of an 8 h extruder shift. Homogenisation secures predictable and equal quality of the compounds which is important for the customer-processor of the compounder.
Soaking compounds with crosslinking agents
For isolating electrical cables, special crosslinked polymers are used. The raw materials used by the cable and wire companies are special compounds which have crosslinking agents adsorbed in the material. These agents become active when the compounds are heated and extruded again.
To adsorb additional crosslinking agent (peroxide) in the pellets after extrusion, a leading manufacturer of extrusion lines (Buss AG from Switzerland) has introduced a post mixing or rather post ‘soaking’ step using gentle touch technology. Temperature control, careful handling of the crosslinking agents and pellet integrity is key. Using the gentle touch technology secures these important pre-requisites. Typical for the configuration of this post blending process are a mixing drum with water jacket for temperature control, temperature controlled liquid dosing and inert gas blanket. For certain grades the purity requirements of the product are so high (less than 1 ppm foreign material) that special seals are applied to avoid even the risk of the smallest seal wear particles to enter the blending and soaking vessel.
Homogenisation and thermal treatment of TPU for ski boots
An Italian producer of thermoplastic polyurethane (TPU) and polyesters for, amongst other markets, manufacturing of ski boots uses the gentle touch process technology for treating its TPU and polyester products. Various large (14 000 litre max. batch size) rotating drum mixers are used to homogenise the pellets and to add a liquid crosslinking agent. One of the machines has a water heated and cooled jacket, to apply a temperature treatment, to further improve the characteristics of some of their high performing polymers.
Dusting of freshly produced TPO in Saudi Arabia
Some thermoplastics are so soft, that they are sticky after production. To process these materials handling problems, a small quantity of anti-sticking-agent (such as talc powder) is “dusted” on the sticky pellets. Using gentle touch technology will do this without smearing, lumping or damaging the pellets. One of the large oil companies has built a petrochemical site in Saudi Arabia. One of the products produced there is thermoplastic polyolefin (TPO). They have installed two continuous drum blenders using gentle touch technology for dusting their freshly produced polymers to avoid them sticking together during packaging and storage.
Gentle touch technology enables gentle mixing and processing. It is used for many applications in polymer processing, especially there where products are fragile or otherwise sensitive, where an even liquid dosing or soaking is required and temperature control is important. Furthermore, the gentle touch technology can be characterised by rapid homogenisation and discharge-free from segregation with a very low level of energy used and dissipated. Gentle touch mixers and processing machines are available in batch sizes ranging from 10 – 25 000 litres and can be used in batch as well as continuous operation.
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