Bio-based polymers from poultry feathers -

Bio-based polymers from poultry feathers

Work has begun to replace plastics (polymers derived from crude oil) with sustainable biopolymers. Poultry feathers represent one possible feedstock material for biopolymer development, however the understanding of their thermal processing and melt formation remains in its infancy. Moreover, transitioning from lab scale to industrial scale remains a significant challenge. This paper aims to carry out extrusion of feather-based polymers and further understand the method of polymer formation. A design of experiments (DOE) investigation was conducted to optimize composition and extruder set-up. Results demonstrate shearing forces, pressure and temperature were all key factors for stable feather polymer formation to occur. The investigation concluded that a feather mix with propylene glycol, in a ratio of 70:30 respectively, processed at 125 ◦C represents an optimal set-up in terms of mechanical and processing properties. These outcomes develop a base-line technique for implementing feather-based polymers into the polymer industry. Characterisation showed minimal changes to the thermal properties or structures of produced samples outlining potential reprocessing capability. An estimation of polymer cost generation was conducted, showing a saving of 56% compared to pure polypropylene.

Materials and methods

This investigation consists of 2 parts:

1. Extruding a standard feather polymer mix to review how formation into a polymer occurs. This would include analysis of reprocessing and effect of screw speed.

2. Conducting a design of experiment (DOE) approach with four input variables to determine the optimal extrusion set-up. Analysis on processing and mechanical properties would be conducted.

Feather processing

Poultry feathers were obtained from the production line of a local poultry producer and rinsed with water until filtered clear to remove any dirt or grease on the surface. A Retsch SM 400XL cutting mill with a 4 mm sieve was used to breakdown feathers into a powder. The material was milled again using a 2 mm sieve to further reduce particle size. An Endecotts graduated sieve, with a 500 and 212 µm mesh was used to obtain two feather powder batches of 0–212 and 212–500 µm. Particle size analysis was carried out using a Sympatec HELOS/BF Particle Sizer (Sympatec, Germany). D50 particle sizes of 92.8 and 302.2 µm were obtained for 0–212 and 212–500 µm batches, respectively.

Analysis of formation process

To determine the formation process of feather-based polymers, a standard polymer mix was generated consisting of 625 g of feather powder, 268 g of propylene glycol, 27 g of sodium sulfite and 80 g of deionized water. Samples in batches of 150 g were prepared using a Thermo-Scientific Prism Pilot 3 High Speed Mixer. The samples were removed and stored in plastic containers (airtight) and left overnight to allow integration of plasticiser into feathers. Extrusion was carried out on a Thermo-Scientific Rheomex PTW16 co-rotating twin screw extruder (L/D = 24:1, Thermo-Scientific, US) fitted with a 4 mm circular die. An extruder profile with 4 feeder and 3 mixing zones (2 reverse and 1 forward) was used. The use of reverse mixing zones, providing a reverse flow effect enabled a high mechanical energy input into the material by providing shear to break powder up. The design of the feeder zone enabled a fast material flow that avoided prolonging the mix in the extruder. This reduced the risk of plasticiser loss which could result in a non-cohesive polymer. The set-up enabled a uniform blend to be gained and avoided the mix drying up. A flat temperature profile of 125 ◦C across all heating zones was used, with a screw speed adjusted between 20 and 270 rpm. Material was fed into a Movacolour MC Balance feeder with a motor speed of 35 rpm, allowing for a consistent material feed into the hopper of the extruder. The produced filament was passed along the line and fed into a pelletizer, used to produce uniform polymer pellets. After 10 min of mix feeding the extrusion process was stopped mid-cycle. This allowed a review of how the feather-based polymer mix behaved at each mixing zone and how extrusion occurred at the die. Samples at each zone were collected for further investigation. This process was repeated by feeding extruded samples back into the extruder to review reprocessing ability and secondary formation method. Torque readings were taken from the extruder readout. Mass flow rate values were gained from measuring the weight of material extruded every minute. These values were used to calculate specific mechanical energy (SME – Eq. (1)). SME is a measurement of mechanical energy the extruded material is subject to and is calculated using the formula:

Specific mechanical energy (MJ/kg) = Angular screw speed × Torque Mass flowrate (1)

Characterisation was carried out on collected samples using Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). FTIR was conducted using a Perkin-Elmer Fourier Transform Infrared Spectrometer (Perkin-Elmer, US) at wavelengths 550–4000 cm− 1, a spectral resolution of 4 cm− 1. DSC was conducted on a Perkin Elmer Model DSC 6 with samples weighing 6–9 mg, under a profile of 30–250 ◦C, at a temperature change of 15 ◦C per minute.

Extrusion DOE

Four important factors were determined for use in the design of experiments investigation. These factors were:

A. Plasticiser choice

B. Ratio of feather to plasticiser (composition)

C. Extrusion temperature

D. Particle size range

Variables such as screw speed, screw configuration and feeding method were fixed at values/methods known to work successfully from the formation investigation. Each of these factors consisted of two variables, a low and high value. For the design of experiments investigation, a 4 factor, 2 variable half factorial was used, consisting of 8 individual samples. This would provide detailed results of single and dual factor interactions. A full factorial was not used as it would require high time demands. DSC was carried out on feather mix samples containing propylene glycol and glycerol to determine softening temperatures (TS). The softening temperature was found as 105 ◦C for propylene glycol samples and 125 ◦C for glycerol. Samples were mixed and stored in the same manner to the formation study. Samples consisted of the required amount of feather and plasticiser for ratio, sodium sulfite (4.3% of feather wt.) and de-ionized water (12.9% of feather wt.).

Design of experiment samples were extruded using the same extruder and set-up conditions as the formation study, with temperature adjusted to 105, 125 or 145 ◦C depending on DOE temperature requirement. Screw speed was set at 200 rpm. A continuous production cycle was carried out with no stoppages for material change-over. Samples were collected and labelled. Samples for mechanical testing were developed by compression moulding pellets on a Collin P200P platen press to produce films for mechanical testing. Samples of 2 g in weight were subjected to 110 ◦C at 50 bar for 4 min, followed by 2 min cooling. Mechanical testing was carried out on a Lloyd’s LRX 2.5 kN tensile test machine (Lloyd Instruments, UK) using dog-bone samples (n = 5) of standard ISO–527–2–1BA. Uniaxial tensile testing was carried out at a speed of 0.5 mm/min. DSC was carried out under a profile of 30–250 ◦C, at a temperature change of 10 ◦C per minute.

Results

Analysis of formation process

After stopping the extrusion process, inspection of the extruder profile was undertaken and showed an interesting evolution of feather based polymer materials. When heated to above its softening point, feather mixes became ‘clay-like’ i.e., can be pressed easily in the required shape with strong cohesion, allowing for polymer formation. This formation occurred exclusively at the die, with the material remaining close to a powder in all stages beforehand. The feather and propylene glycol mixture changed into a consistent solid polymer during compression, leading to resemblance to a traditional plastic. Within the first mixing zone the material acted as a compacted feather powder, but via shear forces transformed into a fine powder thereafter. After the 2^nd mixing zone, another reverse zone, a polymer natured film was created. However, the shear in the final mixing zone, a forward zone, broke the film back into a fine powder. This wet powder was compacted at the die with a high compression ratio (reduction of 97%).

Temperature is still required to enable the material to become soft  and to initiate disulphide bond breakage. Processing pellets a second time resulted in similar output. Under heating in the initial mixing zones feather pellets fused into nonuniform soft viscous chunks, which were gradually broken into smaller pieces as they passed through the extruder barrel. The high compression ratio at the die transformed these chunks into a solid cohesive polymer filament under the influence of shear, temperature and pressure. Subjecting the feather-based polymer mixes to elevated temperatures caused a change in colour from light brown of raw feathers to near black at die, with clear gradient change seen during 1^st run.

Analysis of components

To understand the changes occurring for raw powder to secondary processed pellets, components from each stage of the process were taken. DSC was carried out to determine differences in thermal nature and structural properties. Softening temperature for ‘partially fused 1’ sample was 39.2 ◦C lower than that for ‘pellet 1’ samples, despite being derived from the same starting material. These variations are due to a number of factors including changes to surface area, thermal history of material or the level of plasticiser content. The key results from DSC and FTIR data is that feather-based biopolymers experience little change during processing, and secondary reprocessing.  However, it was noted that the powder produced via second cycle was slightly drier due to loss of small amounts of plasticiser, seen in FTIR results. Plasticiser evaporation is more likely given the temperature being used and could be reduced by adding a small amount of plasticizer during each reprocessing cycle, possibly extending the reprocessing cycle of this material.

Effect of screw speed

Screw speed was a critical variable in the formation of feather-based polymers as shear and flow rate of the material, effecting pressure, were influenced. SME was calculated for each screw speed using torque and mass flow rate values gained. Use of screw speeds below 200 rpm was unsuccessful in producing a polymer material. Low screw speeds reduced shearing effect leading to larger powder sizes, causing torque values and resulting in higher apparent mechanical energy values. The longer hold time in the extruder barrel, caused by low flow rate, led to the material drying out. This meant a non-cohesive polymer was extruded through the die. Increasing the screw speed to above 200 rpm led to increased torque and mass flow rate values. This increased the pressure at die, leading to higher SME values with stronger and more consistent filaments produced. In comparison, feather polymers additionally require shear and pressure along with high amounts of energy (12.5 MJ/kg compared to 6.2 for plastics). Feather polymers may also appear to have structurally changed, but this can be almost completely reversed.

Extrusion DOE

For each sample developed in the DOE investigation a miscible polymer was produced with no feather fibres visible, showing a strong reaction between plasticiser and feathers occurred. In order to determine the optimal set-up procedure a normalised points (scale transformation) method was used. Each individual property was adjusted into a range of 0–1, with 16 outputs were obtained during the investigation.  Measured extruder torque, filament length and heat flow were outputs selected for analysing processing of material. Ultimate tensile strength, elongation and Young’s modulus were used for analysing mechanical properties. In this context optimal set up means:

• Low torque value corresponding to lower processing demands.

• Ability to be extrude into a continuous filament.

• Have a high heat flow corresponding to a more thermoplastic nature.

• Have maximised mechanical properties.

Processing properties

Processing properties were recorded for each sample. Material outputs were recorded at 0.1 kg/hr. Increasing the production rate led to a non-cohesive filament or generation of voids in samples. Use of a larger extruder system in future may address these issues by stabilising the process. From these filaments, pellets were produced. Again, the quality and appearance of these pellets varied depending on variables used.

Mechanical properties

Films were compression moulded at 110 ◦C, before DSC was conducted on samples to determine the level of plastic nature. Mechanical data was carried out to determine the properties of ultimate tensile strength, percentage elongation and Young’s modulus.

Set-up selection

To enable optimisation of feather-based biopolymers from all outputs gained, a single output was developed from the normalised point’s method. A single output for both processing parameters and for mechanical was combined to develop a single overall output. These values were used to generate an effect plot helping to determine to optimal set-up and assign percentage contribution to each individual and dual factor interactions. The resulted showed that all four inputs had an impact on the processing and mechanical properties of feather biopolymers. Of the four individual variables, plasticiser choice provided the biggest percentage contribution (21.5%) to material properties. Plasticiser choice effects the viscosity of the polymer melt during extrusion. Glycerol being less viscous at high temperature resulted in lower torque values compared to propylene glycol. Its elastic nature also meant films produced were more flexible with higher elongation values but lower strength versus propylene glycol.

The overall results show that the optimal feather bio-based polymer set-up is:

• Propylene glycol as a plasticiser

• Feather to plasticiser ratio of 70:30

• Processing temperature 20 ◦C above denaturation temperature (125 ◦C for propylene glycol)

• Particle size range of 212–500 µm

While the development of feather based polymers is in its infancy, its warrents continued invetigation. At present with mechanical properties not yet comparable to existing synthetic materials, applications within the agricultural/gardening sectors (silage wrap, flower/tree bags) or secondary packaging where mechanical properties are not prioritized are also worth investigation. The key results from this investigation address previous gaps in knowledge with respect to the use of waste material for polymer conversion.  These results will hopefully continue to draw attention to the potential use of waste based polymers as suitable alternatives to petroleum based polymers, allieviating pressures on both polymer and agricultural industries. To further verify the benefits of feather–based polymers an estimation of producing such a material within a large-scale industrial process was calculated.

Conclusions

The results from this investigation showed that in order for feather based polymer to form the following inputs are required:

• Shear via the mixing zones is needed to break the material up into a fine consistent powder.

• Heating slight above the softening point (~100 ◦C) is required to assist formation and to improve thermoplastic tendencies. Higher temperatures (~140 ◦C) lead to higher plasticiser evaporation and possibility of degradation.

• Pressure via compression is needed to fuse the polymer powder mix into a solid polymer extrudate. Once an understanding of the formation process was gained, the design of experiments investigation was conducted to determine the optimal set-up. Plasticiser selection had the biggest effect, followed by particle size, composition and temperature. Dual interactions of plasticiser with composition and temperature with particle size caused the biggest overall effect on material properties. The optimal set-up in term of processing and mechanical properties was determined as a feather mix with propylene glycol, in a ratio of 70:30, processed at 125 ◦C with a particle size of 212–500 µm. Cost calculations showed such a material could be produced for 52 p/kg, a saving of 56% compared to plastics such as polypropylene. With a new understanding of the formation method for feather biopolymers along with developing a base line extrusion technique, future work can now review transitioning into large scale production. This would eliminate the issues seen with lowmaterial output (0.1 kg/hr), avoid plasticiser loss and reduce mechanical input energy to value closer to traditional plastics. Issues such as the colour of the material, could also be investigated through use of specialised additives, pigments, etc. to change appearance.

Citation:

McGauran, T., Harris, M., Dunne, N., Smyth, B.M. and Cunningham, E., 2021. Development and optimisation of extruded bio-based polymers from poultry feathers. European Polymer Journal, 158, p.110678.