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Continuous, highly precise, metal-free polymerisation of PLA using alternative energies for reactive extrusion

Final Report Summary - INNOREX (Continuous, highly precise, metal-free polymerisation of PLA using alternative energies for reactive extrusion)

Executive Summary:
Demand for bio-based polymers is growing fast, but current production technology may use catalysts containing metal which can be an environmental and health hazard.
Mainly focused on the development of an innovative reactor using alternative energies that allow for a continuous and precise polymerization process, the InnoREX project also developed ecofriendly organic catalysts suitable for this new process.
The project demonstrates methods that enhance the production of polymers allowing for a large scale production at a reasonable price.
The particular polymer used by InnoREX is polylactide (PLA) which is mainly used in food packaging and single use cutlery, among other things. PLA is a polymer built up from long chains based on lactic acid molecules. The lactic acid itself is produced by bacteria which are fed by corn, for example, so the feedstock of the polymer is renewable. Another big advantage of PLA is not only that it is biobased but that it is also biodegradable in industrial composting conditions. This means that when it is disposed at composting plants, the polymer is digested by bacteria ultimately resulting in water and CO2.
InnoREX works demonstrated ways to enhance reaction kinetics, speeding up the process of polymerization using twin screw extruders.
Currently twin screw extruders are not used for polymerization on a large scale, because they are not efficient and precise enough as well as not offering sufficient residence times. But InnoREX worked on overcoming this by using alternative energies microwaves and ultrasound. These techniques can achieve an enhanced, controlled and efficient polymerization of PLA in a twin screw extruder.
The consortium managed to introduce microwaves and ultrasound into the extruder which provide additional highly targeted energy and enhance the reaction. The group also adapted an online viscometer which can continuously analyze the material and tell us how complete the polymerization process is. Additionally a second type of extruder was used to purify the product, improving its quality.
Finally the produced materials were optimized and parts were produced from it. A simulative description of the process and an environmental Life Cycle Assessment of the PLA produced according to this process completed this project.

Project Context and Objectives:
As oil scarcity results in instability and rising of the price of oil based raw materials, and since recycled materials are rarely suitable for many high-value applications, the demand for bio-based polymers has been growing. However, due to production processes not yet fully optimized and consisting of many successive batch processes, which also require the application of metal catalysts that may be hazardous and prevent its use in some applications, polylactic acid (PLA) has not yet been fully commercially exploited.
InnoREX has overcome these challenges by developing an efficient, economic and ecologically-feasible production process for PLA. The project developed a novel reaction concept for the continuous, highly precise, tin-free production of PLA via reactive extrusion using the alternative energies microwaves and ultrasound. Conventional co-rotating twin screw extruders have been modified to act as reaction vessels, equipped with the additional input of alternative energies to enhance the polymerization kinetics on the basis of organic catalysts. The alternative energy sources allow a dynamic control of the reaction and the resulting molecular structure of the polymer. An in-line degassing extruder purifies the polymer to remove traces of the remaining catalyst and monomer residues. The efficiency of the complete process has been further increased through online analytics in multiple stages of the reactor and the development of a simulation tool. This combined approach therefore covers development from polymerization to part. By performing small-scale batch reaction trials, catalyst screenings, simulations and continuous high resolution analytics, deep understanding and up-scaling strategies have been developed.

Use of commercially available twin screw extruders as a novel reactor concept
Reactive processes in extruders are attracting increasing interest, as continuous processing offers several advantages over batch processing (including cost-efficiency and energy savings). A further advantage of using extruders for polymer production is their capacity to handle highly-viscous material systems, eliminating the need for any solvent in polymerizations. To perform polymer production in commercially available twin screw compounding systems only minor adaptions to already established hardware are necessary. InnoREX developed a new reactor concept by equipping commercially-available extruders with alternative energy input, online characterization tools and a highly-efficient purification device.

Use of alternative energies for precise, dynamic reaction control
The low intensity but highly-targeted input of alternative energies in the reaction volume ensures a high molecular weight polymerization within the limited residence time of an extruder. For a dynamic control of the reaction, rapidly adjustable energy input is needed. This cannot be achieved by the static energy input of an extruder, but in comparison to an extruder microwaves and ultrasound show nearly no response time. This allows dynamic control of the reaction in the extruder.
Alternative energies also have a significant and highly-differentiated impact on reaction mixtures and polymers. Utilising the different influences of the alternative energies InnoREX showed the possibility to gain a precise dynamic control of the polymerization and the molecular structure (branching, crystallinity, molecular weight, etc.) of the resulting polymer by varying the alternative energies and their modes of operation. For each individual type of alternative energy InnoREX revealed its profile of effects in polymerization reactions and reactive extrusion in general. This allowed the interdisciplinary consortium to utilise the generated knowledge widely beyond the InnoREX project and gain a major technical impact of the InnoREX results and their transferability to other polymerizations, reactive extrusion processes or innovative continuous processes.

Use of metal-free organic catalysts
According to the current state of the art, metal-containing catalysts (e.g.: tin (II) 2-ethylhexanoate) are needed to improve the polymerization rate of lactones to a commercially acceptable level. Health- and environmental regulations dictate that polymer products entering the market must not contain residues of metal catalysts equal or more than 100 ppm. To avoid metal-containing catalysts, organic catalysts have been investigated in the InnoREX project. These catalysts have shown the ability to efficiently control the polymerization of lactide, but yet their performance is insufficient to meet industrial scale standards.

Purification of the InnoREX PLA grade
Purification of the PLA products by removing unreacted monomer strongly influences the stability of the final polymer. This stability is primarily linked to monomer content and catalyst deactivation. InnoREX utilised a specially designed, high-performance MRS extruder type purification device in-line to the polymerization extruder to ensure superior material properties due to high purity. The purification device did not need to incorporate additional high amount of energy in the material (as for heating / melting), and has been able to remove efficiently traces of unreacted monomers and by-products.

Scientific and engineering understanding of the mechanisms behind the alternative energy based processes and of the relations between various parameters influencing those processes
For a deepened understanding of the underlying reaction mechanism the reaction kinetics, the influence of different catalysts and alternative energy sources has been studied thoroughly in small scale batch reactions before realising high throughput continuous polymerizations. In continuous polymerization experiments online analytics have been applied at different stages of the reactor for a deep understanding of all stages of the polymerization. Finally, the resulting polymer has been studied utilising offline chemical analytics and mechanical investigations. Additionally, a Life Cycle Assessment (LCA) study about the new polymer and its production process has been performed, revealing the potential of InnoREX final resulting process for an improvement of the ecologic and economic impact of PLA. Simultaneously the commercially available Ludovic® simulation tool for compounding processes associated to reactive processes has been applied to the simulation of polymerisation reactions identified in academic tasks. The incorporation of additional alternative energy (micro waves) input in twin screw compounding systems has been introduced into Ludovic® thanks to InnoREX project. Additionally, the mechanical interaction between the alternative energy sources, the catalyst and the reaction mixture were studied using molecular dynamics simulation.

Demonstration
A working polymerization line including the incorporation of alternative energy, online characterization technology and a purification device to remove the catalyst has been built up. PLA parts have been produced to demonstrate the different properties of the material which can be achieved by the new process. End-users within the consortium successfully demonstrated these different properties within the case studies: compounding (BHI), extrusion cast-sheet and thermoforming (AIMPLAS), and injection moulding (TaPo). To ensure the projects strategic development to the demands of large scale end-users, additional case studies have been defined strictly by the members of the Industrial Exploitation Board. Furthermore the enhanced Ludovic® software has been described and displayed in dissemination activities showing the potential of the simulation outcome of the InnoREX project.

Project Results:
Université de Mons: Catalyst development for the bulk polymerization of L-lactide
Introduction
There is a perceiving demand of metal-free catalyst development for the bulk polymerization of L-lactide in a controlled manner. For the last few decades metal-free catalysts for ring-opening polymerization (ROP) of cyclic esters such as L-lactide has been the momentum to develop them in solution and in bulk. While many organic catalysts have efficiently promoted the ROP of lactide in solution, there have been relatively few examples that are capable of doing so with a high degree of control under solvent free conditions (bulk) and at high polymerization temperatures (higher than 150 °C). The major issues about these organic catalysts under these conditions are their propensity to promoting undesirable side-reactions such as inter- and intramolecular transesterification reactions and epimerization. In the latter it yields PLA-based materials of low crystallinity features and poor mechanical properties.

Background
Within the InnoREX project consortium UMons developed different catalysts. Among them some of them are active towards bulk polymerization. 4 Dimethylaminopyridine (DMAP) was one of the first organic catalysts, which demonstrated to be capable of catalyzing the ROP of lactide in bulk at 130 °C, although it showed only moderate control under bulk conditions, requiring polymerization times longer. In this respect the combination of DMAP with its conjugated salt(s) of various natures has been considered, resulting in considerable polymerization rate enhancement over DMAP alone for the polymerization of lactide, with a dual activation mechanism proposed (see Figure 1 in attachment).
In this effort among these various conjugate salt pairs with DMAP it resulted with the higher conversion of 54% obtained after 30 minutes. Despite higher monomer conversion, it yielded lower molecular weights and epimerization, i.e. loss of crystalline features of resulting PLA matrices, limiting the use of these catalysts in bulk polymerization. Further work has been undertaken to develop an alternative organic catalyst. Several attempts were carried out and different catalysts were experimented. Among those were 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU), and betaine-type catalyst (see Figure 2 in attachment). Strategies were employed as they were shown to be highly active organic catalysts for lactide polymerizations in solution (Figure 2). Under bulk conditions the catalyst DBU and its conjugated salt combinations however resulted in a significant transesterification extent, leading to a lack of control over the bulk synthesis of polylactide.
For instance, bulk polymerizations were undertaken at a temperature of 130 °C for 30 minutes with increasing amounts of both types of catalyst. Despite high conversion in 30 minutes it again resulted in transesterified and low molecular weight PLA materials. In addition to that high degree of epimerization was observed by 1H NMR.
In order to find efficient catalyst UMons developed new types of catalysts, which are based on carbene-type catalysts. To reduce the extent of these side-reactions, they developed different types of carbene based catalysts that are protected with a series of ligand as shown below (see Figure 4 in attachment). Initial trials were done on five-membered carbenes being commercially available.
This rational choice resulted from the fact that the five-membered carbenes (Figure 3) were not thermally stable enough at higher temperatures. Thereby, UMons developed thermally stable 6-membered carbenes for bulk polymerization (see Figure 4 in attachment). The protection of these 6-membered carbenes was again considered to further control the activity of these catalysts. These catalysts were further investigated as efficient catalyst during the latest period of the project.
N-Heterocyclic carbenes (NHCs) are as a part of catalyst designed in the InnoREX framework. Combined with the aforementioned properties, the flexibility with regard to ring-size, N-substituents, and backbone has won NHCs a prominent position in ROP chemistry. With this new type of protected catalysts UMons was able to decrease the extent of epimerization compared to that of earlier organic based catalysts.

Synthesis
The synthesis of the three NHC’s catalyst has been synthesized in the identical way and protected with the choice of the ligands such as CO2, CS2, MgCl2. Among the three of them catalyst 1,3-dimethyl-3,4,5,6-tetrahydro pyrimidin-1-ium-2-MgCl2 (Mg-NHC) proved to be an ideal choice for bulk polymerization of L-Lactide due to its competence.
N,N’-Dimethyl-1,3-propanediamine (5.39 g, 1 eq), ammonium tetrafluoroborate (5.53 g, 1 eq) and trimethylorthoformate (7.00 g, 1.25 eq) were added to a round bottom Flask and heated at 120 °C for two hours under stirring. The mixture was then filtered to yield clear oil. This clear oil was further treated with potassium tertiarybutoxide under nitrogen atmosphere. The resultant mixture was filtered by using cannula and added to magnesium chloride under anhydrous conditions. Upon prolonged stirring under inert atmosphere a precipitate was obtained which upon filtration dried over vacuum as off-white powder.

Towards Lactide ROP via carbene catalyst(s)
To obtain the optimal results the aforementioned three different carbene catalysts (see Figure 4 in attachment) were tested at high temperature (i.e. 170, 190, 210 °C). Among these three catalysts Mg-based N-Heterocyclic carbene Mg-NHC showed the finest possibilities for bulk polymerization. In addition to its low toxicity for biomedical purposes, it has been proposed that the MgCl2 is released upon deprotection and then acts as a co-catalyst in the polymerization, increasing the polymerization rate. UMons synthesised the magnesium protected 6-membered carbene (see Figure 5 in attachment) to test its performance as a catalyst for the ROP of lactide at high temperatures.
As the activity of the carbene-MgCl2 had been verified, a series of polymerizations were then performed to evaluate its kinetic features towards L-lactide (LLA) ROP at various temperatures and different time intervals without any purposely-added initiator (see Table 1 and Figure 6 in attachment).
These results show that the magnesium-protected carbene performs well for the ROP of LA. Additionally, the 1H NMR results showed no sign of epimerization. The highest monomer conversion could be even obtained at 170 °C after 15 min of polymerization time with 1:400 molar ratio of monomer to catalyst. Five-cycle DSC analyses were also carried out to highlight the low extent of epimerization on high molecular weight PLA obtained at 170 °C (see Figure 7 in attachment). Such approach is to determine whether these metal-free catalysts can further promote any side-reactions after different heating treatment. Interestingly, the crystalline features of high molecular weight PLA are maintained even after 5 cycles, indicating the absence of side-reactions.
A melting temperature of 166.6 °C is noticed, with a melting endotherm of 32.3 J/g. These promising results led us to investigate the synthesis of PLA using a small-scale DSC extrusion compounder at 170 °C under a rotation speed of 75 RPM under a nitrogen flow (Initial molar [L LA]/[carbene-MgCl2] ratio = 400). The L-LA monomer got fed into the DSM machine at 130 °C before increasing the extrusion temperature at 170 °C. The evolution of force recorded with time is reported in Table 2 in the attachment.
UMons found out that the force increased with extrusion time, indicating that this carbene-type catalyst was active enough to efficiently promote the continuous synthesis of PLA. This was further supported by molecular characterizations (GPC and 1H NMR techniques) where a monomer conversion of 80% was recorded, together with Mn = 47,200 g/mol and ÐM = 2.00. No significant epimerization is observed by 1H NMR spectroscopy. Again and to support the absence of epimerization, five-cycle DSC analyses were carried out to on the resulting PLA before and after monomer purification (as obtained by solubilization in chloroform and precipitation from MeOH (see Figure 8 in attachment).
These promising results highlight that the carbene-MgCl2 catalyst is still active and efficient towards the synthesis of high molecular weight PLA before any purification step. After monomer purification the purified PLA chains is of semi-crystalline character with a melting temperature of 166.2 °C after five DSC cycle analyses. This purification procedure enables to remove unreacted monomer as well as the traces of catalyst left in the polymerization medium.
Moreover, the inquisitive reactivity of Mg-based catalyst incites us chemo type-variations for better understanding. Therefore UMons investigated the structure activity relation by altering size of ring, substituents over nitrogen in ring and carbene stabilizing ligands types. Our first choices of modifications were on substituents over nitrogen in the ring and ring size. The proposed structures are in figure 9.
The synthesis and yields of three analogues of catalyst 2 are more similar as scheme 1. The polymerization of Lactide with all above mentioned variations of Mg-NHC catalysts (see Figure 9 in attachment), resulted epimerization as it is confirmed by 1H NMR. The monomer conversion was reached to moderate rate even after 30 minutes in the absence of the initiator. To further attest these features three-cycle of DSC analyses were carried out for PLA samples obtained at 170 °C after 30 and 60 min, which showed the epimerization observed. These results indicate that the Mg-NHC seems to afford the best efficiency towards the continuous synthesis of PLA.

Exploitable results and Conclusion
UMons developed a new reactive organo magnesium-based six-membered catalyst, which gratified in terms of effective and efficient polymerization pathway for the synthesis of PLA. This catalyst complemented with controlled molecular weights of PLA with relatively low dispersities and high optical purity as obtained in bulk at relatively high polymerization temperatures. Another advantage of our new organo catalyst is efficiency in the continuous polymerization of L-LA as carried out in a microcompounder. Such features enable us to consider this catalyst towards the continuous synthesis of polylactide using reactive extrusion technology developed in the frame of InnoREX project.

Hielscher: Ultrasonic Polymerization
Hielscher’s research work in the InnoREX project was focused on the investigation of the ultrasonic effects on the PLA polymerization and the development of an ultrasonic process equipment that allows for the precise control of the most important process parameters – amplitude, pressure, temperature, and retention time – as well as the integration of the ultrasound – before, during and after the extrusion.
Organic catalysts have been shown to efficiently control the polymerization of lactide, but their activity must still be improved to meet industrial standards. Since sonication is well-known for its beneficial effects regarding the initiation and improvement of catalytic reactions, the ultrasonic treatment of various catalyst used for PLA polymerization is a promising field of research. In general, sonication can increase catalyst activity and enable precise control of the reaction by exciting only small parts of the reaction mixture without response time.
In the project, Hielscher tested several ultrasonic flow cells and developed novel reactor systems, where the ultrasonic energy source is introduced into the medium at different process stages.
Three alternative energy sources – ultrasound, microwave or laser irradiation – were investigated for their effect to induce the ring-opening polymerization to ensure the high molecular weight polymerization. During the limited residence time in the reactor chamber, the alternative energy sources introduce the required reaction driving impact into an inline flow cell at a highly-targeted level. It is the goal to avoid thereby the use of metal-containing catalysts such as tin (II) 2-ethylhexanoate, which are used in conventional production processes to raise the polymerization rate of the lactones to an acceptable efficient level.
For the InnoREX pilot plant system, the high power ultrasonic processor UIP2000hdT (see Figure 14 in attachment), which is capable to provide 2 kW of ultrasound power, has been integrated. High power ultrasound is well known for its positive effects on chemical reactions, which is the phenomenon of sonochemistry. When high power ultrasonic waves are introduced into a liquid medium, the waves create high-pressure (compression) and low-pressure (rarefaction) cycles resulting in ultrasound cavitation. Cavitation describes “the formation, growth and implosive collapse of bubbles in a liquid. Cavitational collapse produces intense local heating (~5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (>109 K/sec)” such as liquid streaming with liquid jets of ~400 km/h. (K.S. Suslick 1998)
The ultrasonically generated cavitational forces provide kinetic energy, disperse the particles and create radicals supporting the chemical polymerization reaction.
General positive effects of sonication during a polymerization reaction are:
• initiation of polymerization due to sonochemically created radicals (polymerization kinetics)
• acceleration of the polymerization rate narrower poly-dispersities, but higher molecular weight of the polymers
• more homogeneous reaction and hence a lower distribution of chain lengths

Ultrasonically Enhanced Extrusion of PLA
During the InnoREX project, Hielscher developed and investigated the integration of power ultrasonics at several stages during extrusion. The effects of ultrasonics on the polymerization of lactide to PLA in the extrusion process were investigated in regards to ultrasonic process parameters such as ultrasound intensity, amplitude, temperature, pressure, retention time as well as the point of sonication including pre- and post-extrusion sonication.
To investigate the general effects of sonication, sonication trials with a wide spectrum of varying process conditions were performed at lab scale. Thereby, hundreds of samples were produced. This research allowed depicting the influence of power ultrasound on the lactide / catalyst mixture as well as on PLA (see Figure 10 in attachment). Using the automated data recording of the ultrasonic lab device UP200ST, all relevant sonication data were stored in a CSV file so that an exact analysis was possible.
In the next step, Hielscher set up an ultrasonic recirculation system where the ultrasonic process parameters could be changed in a wider range. This bench-top setup could be operated under elevated pressures up to 10 barg. The effects of sonication to the mixing of lactide and catalyst as well as to the polymerization reaction were investigated. For the mixing of the reactant, a special flow cell insert was used: With the MultiPhaseCavitator insert MPC48, the catalyst was injected via 48 cannulas directly in the ultrasound cavitation zone into the monomer stream (see Figure 13 in attachment).
The final step was the scale-up of the ultrasonic industrial system to the extruder. Power ultrasound can be applied before, during and after extrusion. For each point of the integration of ultrasound, a special setup has been tested.
Pre-Extruder-Sonication: For the sonication before the extruder, the catalyst and monomer are pre-mixed under ultrasonication using the flow cell insert MultiPhaseCavitator MPC48 (see Figure 13 in attachment). The design of the MPC48 allows to inject the catalyst via 48 cannulas into the monomer stream directly before the monomer/catalyst mixture is fed into the extruder. Using the MultiPhaseCavitator MPC48, a very fine-sized, homogeneous monomer/catalyst mixture is obtained, which is important for an optimal polymerization during extrusion.
Ultrasonic Extrusion: For the sonication during extrusion, the ultrasonic transducer and sonotrode are mounted to the extruder block so that the extrusion stream can be sonicated (see Figure 14&15 in attachment). The sonotrode is adapted to high temperatures and pressures in order to ensure a constant high power sonication. By coupling ultrasound waves into the monomer/catalyst stream during reactive extrusion, highly intense shear forces are applied.
Post-Extruder-Sonication: For the sonication behind the extruder, a heatable ultrasonic reactor was built (see Figure 16 in attachment). This reactor can be attached directly to the extruder outlet. The reactor can be heated up to 300 °C to maintain the temperature of the extruded polymer. The pressure in the reactor can be controlled by a back-pressure valve. The post-sonication is applied to improve the final polymerization grade.
Sonication only: Hielscher’s ultrasonic high pressure flow cell reactor can be impinged with pressures up to 140 barg and enables to intensify the cavitational treatment. The ultrasonic setup allows to display the complete polymerization process firstly, by the application of ultrasonically coupled mechanical shear and secondly, by the input of thermal energy.
From the technical standpoint, Hielscher can offer a complete ultrasonic system that can be integrated into any extruder blocks. Both, sonotrodes / ultrasonic horns and ultrasonic reactors are available in different sizes and shapes and capable to perform under high pressure and high temperature conditions.

Modelling
With the goal to display power ultrasound treatments into a model, Hielscher performed a broad range of trials in order to investigate the correlation of amplitude, pressure and temperature. These parameters are required to implement sonication in a numerical software simulation.
The results obtained were used for a regressive analysis. Based on the results of the regression analysis, Hielscher has developed a formula that allows to calculate the ultrasonic surface power output as a function of ultrasonic amplitude, pressure and temperature.

Efficiency and Costs
Efficiency rating: In order to estimate the cost of the sonication procedure, energy input and sonotrode wear have been exactly measured. Since the sonotrode is the only part that is subject to wear and tear and must be replaced after certain time periods depending on the sonication intensity and medium, the average wear has been measured in order to give a profound cost estimation for the ultrasonic extrusion process.

Dissemination and Exploitation
Hielscher offers the ultrasonic equipment for the implementation in extrusion lines. Industrial ultrasonic equipment for the sonication during extrusion as well as for the pre- and post-treatment are available systems for clients and customers worldwide. Hielscher supplies the ultrasonic systems in the field of (reactive) extrusion, polymerization and polymer compounding as well as for ultrasonically intensified processes, to material modifications, and sonochemical reactions.
In order to raise interest from the industry, Hielscher was promoting the ultrasonically assisted reactive extrusion on exhibitions and conferences.

Conclusion
During the InnoREX project, profound tests on the ultrasonic effects on the polymerization of monomer to PLA using various catalysts have been undertaken. Ultrasound has both physical and chemical effects on the polymer melt. It has been shown that sonication influences the polymerization reaction due to mechanical, sonochemical and thermal effects. The benefits of sonication include the accelerated polymerization of lactide as well as the polymerization under lower extrusion temperatures and/or lower rotation speed. It has been shown that ultrasonics influences the polymerization of PLA positively due to its non-thermal shear and sonochemical effects.
The equipment for the ultrasonically assisted extrusion polymerization is commercially available and can be adapted to various processes. From the perspective of commercial integration, Hielscher gained profound knowledge of ultrasonic effects on the reactive extrusion polymerization, which allows for recommendation of the suitable ultrasonic equipment, process parameters and implementation. Industrial ultrasonicators, reactors and extruder block adapters are tested and readily available for commercial extrusion processes.

MUEGGE: Development, design and setup of an extruder block for injection of microwave energy
The major task of MUEGGE in InnoREX project was the development, design and setup of an extruder block to be integrated into the lab-scale twin screw extruder at the premises of project coordinator Fraunhofer Institute for Chemical Technology (ICT) for injection of microwave energy. It was expected that addition of microwave energy during reactive extrusion of polylactic acid (PLA) will accelerate the extrusion process and improve the quality of the resulting PLA.
During the development and design phase of the extruder block, MUEGGE designed a CAD model (see Figure 17 in attachment) for high frequency evaluation of microwave injection into the lab-scale twin screw extruder for heating the lactic acid.
The outer diameter of each extrusion line of the CAD model in figure 17 is Da = 18 mm, and the distance between the centers of the two screws is A = 15 mm. At the length of L = 60 mm, the screw thread is removed from the twin screws (not visible in Figure 17 in attachment) for improved coupling of the 5.8 GHz microwave. Both the R 58 rectangular wave guides (light blue color) and the twin screws are made of metal. The enclosure surrounding both the coupling devices and the material to be heated – omitted for the sake of clarity – is made of metal, too. The sealing elements in purple color inhibit the lactic acid in the extruder from penetrating into the small gap between the coupling devices and their metallic housing and for keeping the coupling devices in their position inside the metallic enclosures in order to maintain a uniform distribution of the mechanical pressure applied by the lactic acid inside the extruder on the coupling devices.
The vertical cross-section in figure 18 in attachment visualizes the transmission path of the microwave, passing the lab-scale twin screw extruder orthogonally to the transporting direction of the lactic acid to be heated. The opposite end of the waveguide is terminated by a shorting plunger for reflection of the fraction of the microwave which has not been absorbed by the lactic acid when passing the twin screw extruder for the first time.
According to the results of simulation of the electrical field strength, there is no direct coupling of the microwave to the twin screws of the extruder made of metal. Furthermore, a homogeneous distribution of the electrical field in the material to be heated is obtained according to the simulation of the distribution of the electrical field strength in the horizontal plane in figure 19 in attachment as well as of the electrical field strength in the vertical plane (see figure 20 in attachment).
According to figure 19, the screw thread of the twin screws is removed at the length of L = 60 mm, resulting in the reduction of the twin screws to their inner diameter of Di = 12 mm. This measure facilitates the coupling of the microwave to the lactic acid around the twin screws.
The corresponding simulation of the distribution of the power density in figure 21 in attachment being equivalent to the microwave energy distribution reveals that the injected 5.8 GHz microwave is almost completely (i.e. by 99%) absorbed by the lactide in the lab-scale twin screw extruder. The microwave energy can be controllably concentrated in the lactic acid to be heated in the 60 mm long area around the twin screws without screw thread.
The spectrum of the microwave power density depicted in figure 21 corresponds to 1 W of injected microwave power. According to simulation, the maximum power density to be achieved in the lactic acid to be heated is 91500 W/m3. When increasing the microwave power injected into the lab-scale twin screw extruder, the rise in the resulting power density is commensurate to the increase in microwave power. Consequently, the maximum achievable power density in the lactic acid is approximately 70 MW/m3 when focusing 750 W of incident microwave power into the lab scale twin screw extruder.
Based on these results for the power density distribution, time-related simulations were performed for calculating the absorption of microwave energy by monomer lactic acid in the lab-scale twin screw extruder and the corresponding temperature profiles. The simulations were focusing on the time frame from powering on the 5.8 GHz microwave until the instant of time when the temperature of the lactic acid passing the extruder block for microwave injection has reached its steady state. These simulations are based on the results of the measurements of the dielectric properties of monomer lactic acid performed by project coordinator Fraunhofer ICT, as the temperature-dependent dielectric loss ε`` of lactic acid directly corresponds to the coupling of the microwave and the equivalent absorption of microwave energy by the lactic acid. Correspondingly, the microwave energy will preferably be absorbed in the sections of the extruder block for microwave injection where the temperature of the lactic acid is maximum at steady state conditions, implying a respectively high dielectric loss ε``.
According to the time-related temperature profiles of the lactic acid in figure 22 in attachment, the maximum temperature increase ΔT in the lactic acid can be found in the center of the extruder block for microwave injection just after powering on the 5.8 GHz microwave. The microwave power applied is 100 W. After 10 s of microwave injection, the peak of maximum power increase is slowly shifting in flow direction of the lactic acid towards the end of the extruder block. Approximately 40 s after powering on the 5.8 GHz microwave, the maximum value of the temperature increase is obtained. Just afterwards, the value of the maximum temperature increase is significantly declining and shows small oscillations. After approximately 70 s, the value of the maximum temperature increase is finally almost constant.
Taking into account the values of the power density in the lactic acid and of the temperature distribution in the individual sections of the extruder block each 4 mm long, the distribution of the injected microwave power at steady state conditions in figure 22 is obtained.
The results in figure 23 in attachment represent the particular space resolved distribution of the microwave power in lactic acid inside the extruder block integrated in the lab-scale twin screw extruder at Fraunhofer ICT at steady state conditions. For monomer materials with different temperature-dependent dielectric loss ε`` (i.e. with different coupling of the microwave and equivalent absorption of the microwave energy) as well as for twin screw extruders with different parameters concerning outer diameter Da of the extrusion lines, inner diameter Di of the twin screws and length L of the twin screws where they are reduced to their inner diameter Di (i.e. where the screw thread is removed), different space resolved distributions of the microwave power at steady state conditions will be obtained.
The results of the space resolved steady-state microwave power distribution inside the extruder block for microwave injection were provided to project partners Sciences Computers Consultants (SCC) and Cranfield University (CrU) for implementing the microwave heating process in their software codes for simulation of the reactive extrusion process of PLA in the lab-scale twin screw extruder at Fraunhofer ICT with additional injection of microwave energy.
Based on the simulations showing optimum results for absorption of microwave energy by the lactic acid passing the extruder block for microwave injection, the final dimensions of the extruder block were determined and all parts of the extruder block constructed and assembled. Figure 24 shows the extruder block and figure 25 the entire microwave injection line including a short piece of R 58 rectangular waveguide, an E/H tuner and a shorting plunger.
In figure 24 in attachment, the short piece of R 58 rectangular waveguide on the left forms the interface between the microwave generator (including magnetron, circulator and water load) and the E/H tuner providing for impedance matching, i.e. for optimized coupling of the 5.8 GHz microwave into the lactic acid inside the extruder block. The shorting plunger at the opposite side of the extruder block (i.e. the microwave component on the right in figure 25 in attachment) is for impedance matching of the part of the incident 5.8 GHz microwave not having been absorbed by the lactic acid after having passed the extruder block for the first time, i.e. reflecting the 5.8 GHz microwave optimally back into the extruder block for generation of a homogeneous electrical field distribution inside the lactic acid. As a consequence, the maximum amount of microwave energy is homogeneously absorbed by the lactic acid inside the extruder block integrated into the lab-scale twin screw extruder at Fraunhofer ICT (see figure 26 in attachment).
Injected microwave energy and throughput of lactic acid were the major parameters varied in the experiments performed by Fraunhofer ICT with the 5.8 GHz microwave injection line integrated into the lab-scale twin screw extruder. When applying high energy settings (high temperature or low throughput of the lactic acid entering the extruder block), additional microwave energy turned out not to be beneficial due to the decreasing molecular weight of the resulting PLA (cf. settings 1-3 in figure 27 in attachment). Additional microwave energy showed to be beneficial for low energy process parameters (low temperature or high throughput of the lactic acid passing through the extruder block) by increasing the molecular weight of the resulting PLA (see settings 4 and 5 in figure 27 in attachment). Setting 6 in figure 27 in attachment corresponds to the InnoREX standard settings: the molecular weight of the resulting PLA could be increased by approximately 20% due to additional microwave energy injection.
Easy scalability is expected when scaling up from laboratory dimensions in figure 26 in attachment to industrial level by decreasing in parallel the microwave frequency from 5.8 GHz down to the industrially relevant frequencies of 2.45 GHz and even 915 MHz. When respecting the reciprocal relation between the microwave frequency and the dimensions of the twin screw extruder, i.e. scaling up the extruder dimensions according to the wavelength of the microwave, the results of the simulation on laboratory level by application of 5.8 GHz microwave are almost directly transferrable to the industrial dimensions corresponding to the microwave frequencies of 2.45 GHz and 915 MHz, respectively.

Gneuss: Degassing extruder and online viscometer development
Removing volatiles from low viscosity liquids is a straightforward task. By reducing the pressure in a vessel volatiles simply boil out and can be easily separated from the liquid. In polymer processing a more sophisticated approach is necessary. Due to the high viscosity of polymers gases do not migrate fast enough to the surface and can be separated from the melt. Since polymers often undergo a thermal degradation the processing time cannot be extended to a high level in order to compensate the slow migration rate of volatiles. Moreover, often effects like forming foam makes it difficult to separate the volatiles from the polymer matrix. The transport mechanism can be described in a good approach by diffusion. According to the well-known Fick’s law of diffusion the diffusion rate in a direction x can be approximated by Δn/Δt = -D A dc/dx. Here n is the particle number of the volatile to be removed, A the surface area of the exchange, c the concentration and D a phenomenological constant, depending from e.g. material and temperature conditions. The Gneuss MRS technology boosts the migration of volatiles out of the polymer by two parallel effects. First the polymer is spread over a relatively large area and second a very thin film is formed. This is done by a dynamic process inside the MRS screw system. Therefore the surface of the polymer is renewed with a high rate without adding a too high shear stress to the polymer system, which might lead to degradation and uncontrolled process conditions.
The system works as follows: The MRS processing section is formed like a single screw degassing screw with a length of 2 to 10 screw diameters. In order to achieve high mixing properties, in the main degassing screw up to eight cylindrical shape cavities are incorporated in the main degassing screws. The bore diameter is approximately a quarter of the main screw diameter and these cavities are opened to the degassing area by a quarter of the contour and in the full length of the degassing section. These cavities literally form a barrel of a small degassing extruder and are equipped with degassing screws rotating in counter-direction of the main screw and a speed of approximately 4 times of the main degassing screw. The counter-rotation of the main screw and the incorporated satellite screws allow a folding of the polymer surface with a high rate whereas the shear stress is comparable to standard single screw degassing systems contrary to state of the art mixing technology like co rotation twin screw extruders.
In work package 4 of the InnoREX project these excellent degassing properties of MRS extrusion systems have been made available for the In-line purification of PLA produced in twin screw extruders. Therefore a melt fed MRS system had to be developed (see figure 28 in attachment).
The MRS system is normally configured with a feeding screw at the inlet and a pumping screw at the outlet. In such configuration plastic material can be processed from pellets to melt (for further production) with intermediate de-volatilization. In the InnoREX project a flexible prototype system was developed. This system can be equipped either with a standard feed screw and hopper or in a second configuration as a melt fed system. Contrary to systems in combination with feed screw the filling grade, the rotational speed of the de volatilization unit and therefore residence time and shear stress in the processing unit can be controlled in a wide range. The melt fed system consists of a small (<4D) side fed feed screw. The whole system is driven by the small melt fed screw, which had to be optimized for a sufficient sealing to the drive system of the MRS to avoid leakage. Based on this system the MRS can be fed with pressures (depending on design) of several hundred bars.
The gear system of the MRS was improved during InnoREX with respect to optimized mixing properties and for shear stress adjustment in the single screw satellite units. This allows the future use of such systems in thermal and shear stress sensitive polymers.
During InnoREX a lab scale extrusion system was developed and built, which also allows processing of small throughputs (less than 20 kg/h) under realistic conditions for later scale up. Therefore also for the development of new processes under research conditions MRS systems will be available in the future due to the InnoREX developments. The advantages of the devolatilization technology could be examined and evaluated during the research phase with the lab size technology. Therefore as an important outcome of InnoREX, the MRS technology became available for research projects in process development and as a result the MRS technology can much easier be evaluated for new production technologies.
This MRS-lab system which is described above was successfully integrated to the Fraunhofer ICT lab system with twin screw reactive extrusion process (see figure 29 in attachment) and a combined and complex lab size process including MRS technology could be demonstrated during the InnoREX project.

Development of online characterization system for extruder integrated viscosity measurement
One of the most important parameters to describe the flow characteristics of a plastic melt is the viscosity function. In an extrusion line the value of the viscosity varies mainly with shear rate (due to the common shear thinning behavior of plastic melts) and the temperature and with minor effect at the typical extrusion conditions with pressure. If theses parameters are constant, the viscosity will be determined by material properties (e.g. chain length of polymer molecules, content of fillers) in a pure plastic melt and will be in direct relationship to the physical properties of the material such as tensile strength and impact resistance. In the context of the InnoREX project the viscosity therefore allows an evaluation of polymerization rate and the amount of low molecular weight content (e.g. monomers). Since the reaction is controlled in a twin screw extrusion system the best results with respect to process control are expected, when measuring the viscosity directly inside the extruder barrel.
Gneuss GmbH developed a rheology sensing unit for integration into a twin screw extruder for InnoREX (see figure 30 in attachment). With respect to an easy, straightforward and robust measurement system the following approach for realizing a viscosity measurement was chosen. By means of a high precision metering gear pump, a small part of the polymer melt is separated from the extruder barrel. A pressure of a few bars in the range of 3-10 bars needs to be built up from the extruder in the section, where the measurement takes place.
The body of the measurement unit is similar to a standard section of the extruder barrel and equipped with the necessary melt channels for feeding the viscosity measurement. The polymer is pumped through a precisely manufactured slot capillary. Both the melt temperature and the melt pressure (measurement in 2 positions) are monitored. Figure 30 in attachment shows the design and measurement principle of the developed rheology sensing unit. The rectangular shape of the capillary allows a rheologically optimized design of the capillary in the region of pressure measurements when state of the art pressure sensors for extrusion are used, because they have a flat surface with perfectly fits the membrane of the sensors. If the two pressure measurements for melt pressures P1 and P2 are separated by a distance L for a given volume flow V ̇ the representative viscosity η_rep and shear rate γ ̇_rep are calculated according to standard formula from the mentioned parameters by the control system attached to the viscosity measurement unit. The depth of the capillary slot is specified according to the material properties within a range of 0.5 to 2.0 mm.
For the capillary a rectangular shape with height H and width B was chosen. This rectangular shape allows a rheologically optimized design of the capillary in the region of pressure measurements when state of the art pressure sensors for extrusion are used, because they have a flat surface with perfectly fits the membrane of the sensors. If the two pressure measurements for melt pressures P1 and P2 are separated by a distance L for a given Volume flow V ̇ the representative viscosity η_rep and shear rate γ ̇_rep are calculated according to standard formula from the mentioned parameters by the control system attached to the viscosity measurement unit. The depth of the capillary slot is specified according to the material properties within a range of 0.5 to 2.0 mm. The body of the unit is designed in cylindrical shape. A gear motor drive is mounted directly to the body of the rheology sensing system.
During the demonstration phase of the project the whole mechanical setup was successfully tested with various processing conditions in order to proof the design concept with the PLA line.

Fraunhofer ICT: Project coordinator combining the InnoREX production line
The role of the Fraunhofer ICT within the InnoREX project was, beneath its coordination, to build up the production line and to combine all parts of technologies which have been developed during the project.
Such the combination of the project, the continuous, highly precise, polymerization of lactide using alternative energies for reactive extrusion has been realized and performed in the labs of the Fraunhofer ICT.
To achieve this first an understanding of the polymerization within the twin screw extruder was build up. Especially the greatly changing viscosities, from the liquid monomer to a high molecular weight polymer, had to be precisely understood, and respected in terms of extrusion process, screw configuration and extrusion parameters. During the course of this optimization non-literature known extrusion phenomena have been revealed by the researchers in course of the InnoREX project. Especially the resulting residence time proofed to be greatly influenced by different evolution of the material viscosity. Consequently lots of effort has been drawn on the screw configuration development and processing temperature profiles to allow handling the material and the polymerization.
Over the lifetime of the InnoREX project, of course the here generated knowledge is of great basis and influence for any coming extrusion process including demanding material behavior. This covers polymerizations within twin screw extruders of different material systems, not only covering ring opening polymerizations. Of course the here generated knowledge may be further utilized for processes showing the opposite viscosity behavior, such as degradation reactions or viscosity and molecular weight optimizations for example, as often performed for Polypropylene type systems. In the future Fraunhofer ICT will be able to offer partners from industry high sophisticated consultation, including handling of material systems in twin screw extruders, which do highly differ from classical compounding tasks.
With hardware supplied by partner Hielscher, Fraunhofer ICT realized the process intensification in twin screw extrusion by means of ultrasound. Using the optimized extruder barrel and sonotrode it could be made sure that large parts of the melt are treated with ultrasound energy. Using the knowledge generated before it could be realized to count for optimal conditions for Ultrasound incorporation under the sonotrode, especially in terms of material viscosity and color (see figure 31 in attachment).
Finally by the researchers of the Fraunhofer ICT it could be realized to intensify the polymerization performance utilizing ultrasound energy. Beneath a beneficial effect on the resulting molecular weight of the produced samples, especially the amount of residual monomer could be considerably lowered for all process settings.
With hardware supplied by partner MUEGGE, Fraunhofer ICT realized the process intensification in twin screw extrusion by means of microwave. Using the newly developed extruder barrel the interaction between the microwave and the metal housing could be minimized, resulting in a homogenous microwave distribution within the extruder without any sparking or other unwanted side effects. Such microwave energy could be included into standard, metal build extruders without the need of special ceramic components for example. Using this setup microwave energy could be successfully incorporated into the reactive mixtures. Unfortunately no that clear positive conclusions can be drawn from the effect of the microwaves on the here evaluated reaction.
Summing up the incorporation of alternative energies, the researchers could not only show the possibility of their incorporation, but also which material requirements do have to be fulfilled for effective material treatment. During the course of the InnoREX project the influence of both alternative energies has been shown on the polymerization of lactide. In the future of course the energy incorporation into other material systems can and should be investigated. Within InnoREX it was only possible to evaluate the energy incorporation onto the polymerization of PLA, but of course all the broadness of applications performed in extrusion systems may be thought of to be intensified by ultrasound or microwave energy. Efficient heating in food production, efficient molecular mixing in reactive extrusions, shearless energy incorporation for sensitive systems or of course additional energy input for reactive extrusions, curing, reaction initiation or else can be thought of as possible further utilization for the here shown proof of principle, of the straight forward microwave and ultrasound incorporation into twin screw extruders.
With hardware supplied by partner Gneuss, the online recording of the viscosity within the processing length of a twin screw extruder was realized. Using the side stream capillary viscosity technology a direct monitoring of the melt viscosity, and extent of the reaction, became accessible for the researchers. As the