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The Development of a Single-Screw Extrusion Process for Production of Low Density Biodegradable PLA Foam for Thermoformed Food Packaging Applications

Final Report Summary - PLA-FOAM (The Development of a Single-Screw Extrusion Process for Production of Low Density Biodegradable PLA Foam for Thermoformed Food Packaging Applications)

Executive Summary:

The overall technological aim is to produce a pre-production foamed PLA meat tray that is comparable in terms of performance to current commercially available EPS offerings. To enable this development we aim to develop:

• a pre-production foam extrusion process for the production of foamed PLA sheets of thickness 5 mm and density 50 kg/m3 using supercritical fluid technology that will be able to achieve an output rate of 10-20 kg/hr.
• a pre-production thermoforming process that can thermoform the PLA shapes to achieve the standard meat tray.

To enable this innovative technology, new knowledge must be acquired concerning the modelling of the effects of supercritical CO2 on the processing behaviour of a range of PLAs during extrusion. Once established, this understanding will be applied to the development of the integrated extrusion system. The in-service performance of the PLA meat trays will be evaluated to determine the fitness for purpose and Life Cycle Analysis against current synthetic polymer solutions.

Through these innovative steps we will produce a system that is able to:
• Develop a theoretical software model to allow modelling of PLA-Foam extrusion process with an accuracy of ±10 % in terms the pressure and temperature.
• Design an extruder screw and die for the production of foamed PLA to a density of 50 kg/m3 at a rate of up to 20 kg/hr.
• Build a CO2 pumping and handling system that can pump up to 0. 4 kg/hr of CO2 at a pressure of up to 10 MPa and a temperature of up to 120 °C.
• Build a pressurised hopper feed system that can feed PLA pellets in a heated and pressurised CO2 atmosphere at a rate of 20 kg of PLA per hour up to 10 MPa and a temperature of up to 120 °C.
• Develop a brush seal system for the single screw extruder that will be able to achieve a pressure differential > 10 MPa.
• Design and build a pressurised venting section that can achieve a pressure of 10-30 MPa.
• Manufacture a modular 30 mm diameter screw for the pre-production prototype extrusion system.
• Design and build a pre-production prototype fully instrumented 30 mm single-screw extruder with pressurised feed section (that can achieve a pressure of up to 10 MPa) and controlled pressurised vent to extrude foamed PLA of density 50 kg/m3 at an output rate of up to 20 kg/hr.
• Develop a lamination process for application of a biodegradable barrier layer.
• Develop a model that will allow the modelling of the thermoforming process with an accuracy of ±10 % using T-Sim.
• Design and manufacture a mould for the thermoforming of the foamed PLA sheets to produce a standard meat tray.

Project background and objectives:

1 To determine effects of scCO2 on the melt temperature (Tm), glass transition temperature (Tg) and viscosity of 3 to 5 PLAs over a range of temperatures and pressures.

2 To determine the effects of decompression on the foaming and crystallisation of scCO2/polymer solutions for 5 PLA materials over a range of temperatures, pressure and pressure drop rates.

3 To develop a theoretical software model to allow modelling of PLA-Foam extrusion process with an accuracy of ±10 % in terms the pressure and temperature.

4 To design an extruder screw and die for the production of foamed PLA to a density of 50 kg/m3 at a rate of up to 20 kg/hr.

5 To build a CO2 pumping and handling system that can pump up to 0. 4 kg/hr of CO2 at a pressure of up to 10 MPa and a temperature of up to 120oC.

6 To build a pressurised hopper feed system that can feed PLA pellets in a heated and pressurised CO2 atmosphere at a rate of 20 kg of PLA per hour up to 10 MPa and a temperature of up to 120oC.

7 To develop a brush seal system for the single screw extruder that will be able to achieve a pressure differential > 10 MPa.

8 To design and build a pressurised venting section that can achieve a pressure of 10-30 MPa.

9 To manufacture a modular 30 mm diameter screw for the pre-production prototype extrusion system.

10 To design and build a pre-production prototype fully instrumented 30 mm single-screw extruder with pressurised feed section (that can achieve a pressure of up to 10 MPa) and controlled pressurised vent to extrude foamed PLA of density 50 kg/m3 at an output rate of up to 20 kg/hr.

11 To develop a lamination process for application of a biodegradable barrier layer.

12 To achieve barrier properties for water and moisture permeability at least equivalent to Polystyrene.

13 To design and manufacture a mould for the thermoforming of the foamed PLA sheets to produce a standard meat tray.

14 To manufacture 5 mm thick PLA foam sheet of density 50 kg/m3 using the PLA-Foam pre-production prototype system.

15 To thermoform 5 thick PLA sheet to produce a standard meat tray.

16 To validate the performance of the PLA foam sheet and thermoformed meat trays against relevant standards (including EN13432: biodegradability and compostability) and competitive materials.

17 To develop scale-up parameters for the design of the full industrial scale PLA foaming process that has the potential of extruding foamed PLA extrudates at a density of 50 kg/m3 and an output rate of up to 100 kg/hr.

18 A complete Life Cycle Analysis (ISO 14040) for the final foamed PLA-Foam product and the production process in comparison with current materials and processes which establishes the materials and energy reductions achievable using the PLA-Foam technology.

Project results:

The main aims of Work Package 1 were to carry out basic studies required on the PLA material itself in order to be able to assess the physical properties and characteristics of the material. This was to identify the optimum conditions that will result in the best foaming conditions for extrusion and in particular to produce a foam of low density of 50kg/m3.

A literature review was carried out on PLA foam and details information currently available on PLA. There are several commercially available biodegradable polymers obtained from renewable sources that are typically either polyesters or polysaccharides. PLA being a polyester, is the only material produced in this group in significant quantities to be widely used for food packaging applications. However, manufacturing foamed products from PLA has proven to be very challenging due to the inherent thermal degradation that occurs during extrusion which hinders foaming of the material. This effectively provides the technical challenge that the project has to overcome.

PLA is a linear aliphatic polyester made up of lactic acid building blocks that can exist in optically active D-or L-enantiomers. The fraction of the enantiomers in the polymer determines the physical properties of the material which can provide different levels of crystallinity, melting temperatures and glass transition temperatures.commercially available PLA tends to be copolymer and the crystallinity is a result of the level of L-enantiomer present; high levels will make it more crystalline, and lower contents result in a more amorphous form. The glass transition temperature of PLA ranges from 50 to 80 ?C, while the melting temperature ranges from 130 to 180 ?C. PLA is very susceptible to thermal degradation (above ?180 ?C) during processing, and results in a significant reduction in the melt viscosity, melt strength and mechanical properties of the material.

The literature review details a number of additives, blends and composites, the effects on the resulting physical properties and some of the applications in which these are used and what is covered by IP and therefore must be avoided in the course of this project. The physical properties and barrier properties of PLA are compared to HDPE, polystyrene, polyethylene terephthalate and polypropylene which are the main competitors to PLA.

The known effects on the physical properties of PLA in supercritical CO2 are summarised in relation to depression of melting point and glass transition temperatures. The effect of nucleation on the resulting physical properties of PLA using DSC techniques and crystallinity information is also summarised. Foaming of PLA is also described in relation to skin effects, soak time and depressurisation rates. The techniques required to produce the best foams and the effects of nanoparticles are also discussed.

A review of lamination materials and techniques currently used in food manufacturing has also been undertaken since it is accepted that following successful production of PLA foam, barrier properties will need to be enhanced by lamination.

The thermal properties of various commercially available grades of PLA from Natureworks were investigated:
1) 3051D, amorphous, Tg 60 °C,
2) 3051D, crystalline, Tg 60 °C and mp 160 °C,
3) 2002D, crystalline, Tg 60 °C and mp 160 °C,
4) 2002D amorphous, Tg 60 °C,
5) 4060D, amorphous, Tg 52 °C and no mp.

In addition the properties of PCL CAPA 6800 properties were investigated.

A range of analyses were performed on these materials to determine the effect of CO2 on their transition temperatures and viscosity. This knowledge was then applied to foam samples, using CO2, in a static environment and subsequently by injection of CO2 into the polymer melt in the extrusion process using PLA.

PLA samples exposed to high pressure CO2 tend to foam during the gas release process allowing the production of microcellular polymers. The addition of CO2 can result in dramatic changes in the physical properties of PLA, such as viscosity, interfacial tension and glass transition temperature and these effects were successfully demonstrated.IR and DSC techniques were used to investigate the effect of CO2 on melting point and Tg of PLA. The effect of the pressure of CO2, temperature and shear effects on viscosity were studied using a high pressure rheometer.

Extrusion studies were carried out on different samples of PLA at different temperatures and flow rates with CO2 being injected into the melt of the polymer with the resulting samples physically characterised.

Work Package 2 concentrated on the modelling of the extruder screw to aid the formation of PLA foams. Temperature and pressure data was collected from the extruder trials carried out at Pera and Ansys produced an accurate model to within ±10 % of the results obtained. The Ansys model provided valuable information on the physical parameters exerted on the PLA during the extrusion process.

For development of a model that included a pressurised hopper, it was thought that the boundary conditions would need to be changed so that pressure was also a factor from the inlet. Following examination of data from work package 4, it was found that this did not impact significantly on the results.

Work package 3 details the design of the pressurised hopper system and the modifications required to the screw and barrel of the extruder. From discussions with Maillefer, who inject nitrogen into extruders to foam polymer materials, the requirements for modification of the extruder were established and the extruder system was produced.

The hopper design from the Annex 1 was modified in order to create a more innovative setup which was both more compact and would ultimately be cheaper to manufacture. This system effectively created a small pressurised section between two valves where CO2 could be injected.

The design supported the ability to inject additional CO2 into the barrel alongside the use of the pressurised feed system if required. The barrel was pressure certified and a full Hazop study was carried out prior to commencing any investigations.

The modified feed system was sequenced so as to pressurise the chamber before opening of the lower valve so as to ensure the PLA was fed effectively into the extruder without causing blocking.

Work package 4 focused on the construction of the extrusion system and included integration of the complete system including the auxiliary system developed in work package 3.

An automated control system was produced based around a modular PLC system to monitor temperatures and pressures and to operate the feed, vent and CO2 valves.

A number of challenges were experienced and overcome during this critical project phase but, foamed PLA was produced from the system for the first time

Work Package 5 covered the development of a thermoforming approach which was able to handle the foam samples produced from the extrusion system. This required production of a die to form a small meat tray and development of a temperature and time profile for the extrusion machine.

In addition, detailed work was carried out on looking at the available films which could be laminated onto the trays and the best methods to achieve this.

Work Package 6 brought together the output of both work package 4 and 5 to validate and optimise the processes. A design of experiments approach was followed so as to optimise the PLA density by varying the key conditions of screw speed, temperature and pressure. This resulted in PLA with the lowest density possible from the system.

Further analysis was carried out on the life cycle of the production system and testing of the finished foamed items.

Potential Impact:

The overall technological aim is to produce a pre-production foamed PLA meat tray that is comparable in terms of performance to current commercially available EPS offerings. To enable this development we aim to develop:

• a pre-production foam extrusion process for the production of foamed PLA sheets of thickness 5 mm and density 50 kg/m3 using supercritical fluid technology that will be able to achieve an output rate of 10-20 kg/hr.
• a pre-production thermoforming process that can thermoform the PLA shapes to achieve the standard meat tray.

To enable this innovative technology, new knowledge must be acquired concerning the modelling of the effects of supercritical CO2 on the processing behaviour of a range of PLAs during extrusion. Once established, this understanding will be applied to the development of the integrated extrusion system. The in-service performance of the PLA meat trays will be evaluated to determine the fitness for purpose and Life Cycle Analysis against current synthetic polymer solutions.

Through these innovative steps we will produce a system that is able to:
• Develop a theoretical software model to allow modelling of PLA-Foam extrusion process with an accuracy of ±10 % in terms the pressure and temperature.
• Design an extruder screw and die for the production of foamed PLA to a density of 50 kg/m3 at a rate of up to 20 kg/hr.
• Build a CO2 pumping and handling system that can pump up to 0. 4 kg/hr of CO2 at a pressure of up to 10 MPa and a temperature of up to 120 °C.
• Build a pressurised hopper feed system that can feed PLA pellets in a heated and pressurised CO2 atmosphere at a rate of 20 kg of PLA per hour up to 10 MPa and a temperature of up to 120 °C.
• Develop a brush seal system for the single screw extruder that will be able to achieve a pressure differential > 10 MPa.
• Design and build a pressurised venting section that can achieve a pressure of 10-30 MPa.
• Manufacture a modular 30 mm diameter screw for the pre-production prototype extrusion system.
• Design and build a pre-production prototype fully instrumented 30 mm single-screw extruder with pressurised feed section (that can achieve a pressure of up to 10 MPa) and controlled pressurised vent to extrude foamed PLA of density 50 kg/m3 at an output rate of up to 20 kg/hr.
• Develop a lamination process for application of a biodegradable barrier layer.
• Develop a model that will allow the modelling of the thermoforming process with an accuracy of ±10 % using T-Sim.
• Design and manufacture a mould for the thermoforming of the foamed PLA sheets to produce a standard meat tray.

Project website:

http://pla-foam. uk-matri. org