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Home > News > Recycling

The secret of bottle to bottle recycling without pre drying

Source: Date :2012-04-13 Editor :Dr. Axel Hannemann
One special characteristic of PET which can make its processing difficult is its hygroscopic behavior. During storage and during the washing of post-consumer bottles, water molecules from airborne moisture or from surface contact diffuse into the PET. During the extrusion process, these water molecules break down the molecule chains (hydrolysis), resulting in a drastic reduction of the viscosity, unless the water is removed.

In the processing of PET with an extruder at normal processing temperatures, a chemical process takes place in that any water molecules which may be present reduce the PET chain lengths which results in a reduction in the viscosity which in turn reduces the mechanical properties of the final product. This chemical reaction is however reversible and the point of equilibrium can be driven to one or the other side by reducing or increasing the water content.

This phenomenon can be used to positive effect during extrusion. By removing the water molecules during processing, the damaged PET molecule chains can rebuild themselves. The physical prerequisites for the removal of water molecules are described by the diffusion process, which can be formulated with the help of the Fick's first law:



This law shows that the quantity of water removed (Δn) will increase in relation to the residence time (Δt), the larger the melt surface area (F), the driving concentration incline (dc) and the thinner the polymer layer (dx). In other words, the better the polymer is mixed and the better the surface is exchanged, the more the chemical equilibrium will move toward long chain molecules with better mechanical properties.

With the Multi Rotation System (MRS) extruder, PET pellets are processed direct, without pre-drying the material. During extrusion (in the melt phase) the polymer surface is greatly enlarged and exchanged at an extremely high rate, thereby enabling volatiles to be extracted.

The MRS extruder can be described in general as a single screw extruder with a very special degassing section. The polymer melt is delivered into a large single screw drum. The drum contains eight or 10 (depending on the model size) small extruder barrels, parallel to the main screw axis.

Installed in these small extruder barrels are the "satellite" screws, which are driven by a ring gear in the main barrel. The satellite screws rotate in the opposite direction to the main screw while they rotate around the screw axis. This disproportionately increases the surface exchange of the polymer melt.

The extruder barrels which are cut into the drum of the MRS are approximately 30% open to ensure the optimum melt transfer into the barrels and so that the evacuation can take place without restrictions. Further, precise control of the melt temperature is possible as the temperatures of all the surfaces in contact with the melt can be controlled accurately.

Technology based on single screw extruder concept

Because of Gneuss' patented multiple screw section, the MRS makes a very large area available and permits unmatched degassing performance, even with a vacuum of only 20 to 40 mbar. This makes the processing of undried flakes or pellets with up to 1% residual moisture (10.000 ppm) possible.

This evacuation technology is based on the robust and proven single screw extruder concept. In the Multiple Rotation Section, there are eight satellite screws oriented in a ring pattern in the main MRS drum. In this way, the MRS avoids the problems of alternative multiple shaft or screw designs (intermeshing) which are considerably more sensitive to mechanical damage due to their tight clearances. This last point can be decisive in the reprocessing of PET bottle flake which frequently includes coarse contamination.

Further positive effects of the MRS technology are the 100% dehumidification of PET as well as the possibility of increasing the intrinsic viscosity of this material. Due to the multi rotation elements, a melt surface is made available which is far greater than that of conventional extruders. For The melt treatment is extremely gentle, resulting in a particularly high quality final product for example with regard to yellowness.

Compared with other multi screw systems, the MRS is characterized by its compact and rugged design. The rotating satellite screws run in individual bearings and are therefore comparable with a drum containing a number of single screws. The evacuation or degassing system is modular and can be tailored to the individual requirements because of its remarkable performance. The position, length and design of the modules can be varied.

The MRS Extrusion system offers a high level of flexibility. This extruder can be fed with both crystalline and amorphous pellets/flakes with varying viscosities. The viscosity of the material leaving the extruder can be adjusted individually via the vacuum degassing system to suit the requirements of the application. Amorphous pellets or chips made from PET processed on the MRS with a defined viscosity can be processed immediately. Alternatively, the pellets or chips can be crystallized and can even be processed in an SSP where higher viscosities are required.

It is obvious that in the very first section of the extruder the viscosity of the polymer is reduced, because the water content is integrated into the chains of the PET. The more moisture is in the input, the more the decrease. In the degassing section these water molecules are extracted by vacuum (driven by diffusion), here the reversed chemical reaction takes place. Therefore the MRS is able to increase the chain length, the molecular weight, the viscosity and the mechanical properties of the polymer. The viscosity built up can be controlled by the vacuum level itself; consequently it is possible to use an online viscosity measurement to control the vacuum in order to stabilize the viscosity level by a closed control loop.

The viscosity increase in the devolatilization section is influenced and controlled by the vacuum level. The influence of the vacuum on the viscosity is considerable. The increase in viscosity in response to the increasing vacuum level can be seen clearly.

The MRS extrusion process has a letter of non objection (LNO) from the US Food and Drug Administration (FDA) for processing 100% PET bottle flake to food contact products both with and without a subsequent downstream solid state polymerization (SSP) process.

Source: Gneuss

The writer has been the Technical Sales Manager of Gneuss Kunststofftechnik GmbH since 1999
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Source: Date :2012-04-13 Editor :Dr. Axel Hannemann
One special characteristic of PET which can make its processing difficult is its hygroscopic behavior. During storage and during the washing of post-consumer bottles, water molecules from airborne moisture or from surface contact diffuse into the PET. During the extrusion process, these water molecules break down the molecule chains (hydrolysis), resulting in a drastic reduction of the viscosity, unless the water is removed.

In the processing of PET with an extruder at normal processing temperatures, a chemical process takes place in that any water molecules which may be present reduce the PET chain lengths which results in a reduction in the viscosity which in turn reduces the mechanical properties of the final product. This chemical reaction is however reversible and the point of equilibrium can be driven to one or the other side by reducing or increasing the water content.

This phenomenon can be used to positive effect during extrusion. By removing the water molecules during processing, the damaged PET molecule chains can rebuild themselves. The physical prerequisites for the removal of water molecules are described by the diffusion process, which can be formulated with the help of the Fick's first law:



This law shows that the quantity of water removed (Δn) will increase in relation to the residence time (Δt), the larger the melt surface area (F), the driving concentration incline (dc) and the thinner the polymer layer (dx). In other words, the better the polymer is mixed and the better the surface is exchanged, the more the chemical equilibrium will move toward long chain molecules with better mechanical properties.

With the Multi Rotation System (MRS) extruder, PET pellets are processed direct, without pre-drying the material. During extrusion (in the melt phase) the polymer surface is greatly enlarged and exchanged at an extremely high rate, thereby enabling volatiles to be extracted.

The MRS extruder can be described in general as a single screw extruder with a very special degassing section. The polymer melt is delivered into a large single screw drum. The drum contains eight or 10 (depending on the model size) small extruder barrels, parallel to the main screw axis.

Installed in these small extruder barrels are the "satellite" screws, which are driven by a ring gear in the main barrel. The satellite screws rotate in the opposite direction to the main screw while they rotate around the screw axis. This disproportionately increases the surface exchange of the polymer melt.

The extruder barrels which are cut into the drum of the MRS are approximately 30% open to ensure the optimum melt transfer into the barrels and so that the evacuation can take place without restrictions. Further, precise control of the melt temperature is possible as the temperatures of all the surfaces in contact with the melt can be controlled accurately.

Technology based on single screw extruder concept

Because of Gneuss' patented multiple screw section, the MRS makes a very large area available and permits unmatched degassing performance, even with a vacuum of only 20 to 40 mbar. This makes the processing of undried flakes or pellets with up to 1% residual moisture (10.000 ppm) possible.

This evacuation technology is based on the robust and proven single screw extruder concept. In the Multiple Rotation Section, there are eight satellite screws oriented in a ring pattern in the main MRS drum. In this way, the MRS avoids the problems of alternative multiple shaft or screw designs (intermeshing) which are considerably more sensitive to mechanical damage due to their tight clearances. This last point can be decisive in the reprocessing of PET bottle flake which frequently includes coarse contamination.

Further positive effects of the MRS technology are the 100% dehumidification of PET as well as the possibility of increasing the intrinsic viscosity of this material. Due to the multi rotation elements, a melt surface is made available which is far greater than that of conventional extruders. For The melt treatment is extremely gentle, resulting in a particularly high quality final product for example with regard to yellowness.

Compared with other multi screw systems, the MRS is characterized by its compact and rugged design. The rotating satellite screws run in individual bearings and are therefore comparable with a drum containing a number of single screws. The evacuation or degassing system is modular and can be tailored to the individual requirements because of its remarkable performance. The position, length and design of the modules can be varied.

The MRS Extrusion system offers a high level of flexibility. This extruder can be fed with both crystalline and amorphous pellets/flakes with varying viscosities. The viscosity of the material leaving the extruder can be adjusted individually via the vacuum degassing system to suit the requirements of the application. Amorphous pellets or chips made from PET processed on the MRS with a defined viscosity can be processed immediately. Alternatively, the pellets or chips can be crystallized and can even be processed in an SSP where higher viscosities are required.

It is obvious that in the very first section of the extruder the viscosity of the polymer is reduced, because the water content is integrated into the chains of the PET. The more moisture is in the input, the more the decrease. In the degassing section these water molecules are extracted by vacuum (driven by diffusion), here the reversed chemical reaction takes place. Therefore the MRS is able to increase the chain length, the molecular weight, the viscosity and the mechanical properties of the polymer. The viscosity built up can be controlled by the vacuum level itself; consequently it is possible to use an online viscosity measurement to control the vacuum in order to stabilize the viscosity level by a closed control loop.

The viscosity increase in the devolatilization section is influenced and controlled by the vacuum level. The influence of the vacuum on the viscosity is considerable. The increase in viscosity in response to the increasing vacuum level can be seen clearly.

The MRS extrusion process has a letter of non objection (LNO) from the US Food and Drug Administration (FDA) for processing 100% PET bottle flake to food contact products both with and without a subsequent downstream solid state polymerization (SSP) process.

Source: Gneuss

The writer has been the Technical Sales Manager of Gneuss Kunststofftechnik GmbH since 1999
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The secret of bottle to bottle recycling without pre drying

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