Films from an Aqueous Suspension of Alkaline Pretreated and Fine Milled Chicken Feathers

The purpose was to test the feasibility of preparing cast films directly from an aqueous suspension of alkaline pretreated and fine milled chicken feathers, and to evaluate the impact of different additives on film formation and the tensile properties of the resulting films. The feather suspension consisted of stiff and sharp-pointed fibers together with more round-shaped fines. Films cast from this suspension were opaque and porous. While films without additives were fragile with drying-induced defects, film formation was improved with additives, especially with ethanolamine and maleic acid at 20% and 30% concentrations. A synergistic plasticizing effect was observed with ethanolamine and formamide, and strength of the films was improved with sodium alginate. However, the overall impact of additives on the tensile properties in general and strain at break in specific was limited. This was likely due to the dominating role of the porous film structure and the stiff fibers with a limited reactivity towards the additives.


Introduction
The target of sustainable economy is to utilize biomass-derived materials for high-volume applications [1]. There is interest, for example, in packaging industry towards more sustainable materials, and biobased plastics and biopolymers have been widely studied. Interesting sources of biomass are by-products from animal sources (legs, heads, bones and feather) currently used for fertilizers, animal feed and pet food.
The global production of broiler meat was estimated to reach 98.4 million tons in 2019 [2]. Feathers constitute up to 10% out of chicken body weight [3]. Materials Sciences and Applications This equals 23.4 million tons of feather waste annually. Although chicken feathers and keratin, the main component of feathers, have been tested for various applications, there is still a limited use of feathers as a raw material in industrial applications mainly due to the lack of technologies for processing keratin and established routes for utilizing keratin-based products. Most of feather waste is disposed of in landfills, incinerated, or converted into low-value products. Disposal by landfilling or incinerating is, however, becoming increasingly challenging [4].
Chicken feathers vary in form. In general, they are half fibrous material and half central core with a hollow tube structure [5]. Feather components are hydrophobic by nature resulting in poor wettability compared to cotton and wood pulp fibers [6]. Chemically feathers consist of approximately 90% protein, 8% water, and 1% fat [7]. The amino acids are tightly-packed and crosslinked making extraction of keratin challenging [5] [8]. However, keratin can be extracted by chemical and enzymatic means, superheated water, and with ionic liquids [9] [10]. This adds to the complexity of the feather processing.
The purpose of this study was to test the feasibility of preparing cast films from an aqueous suspension of alkaline pretreated and milled chicken feathers, and to evaluate the impact of additives on the film formation and the tensile properties of the resulting films.

Materials and Methods
This study was carried out at VTT between January and August 2019.

Materials
Sanitized chicken feathers were supplied by Grupo Sada (Madrid, Spain).

Preparing Feather Suspension
Feathers were refined using a combination of mechanical and chemical treatments. Images of original and refined feathers are presented in Figure 1. First, a mild alkali pre-treatment was performed for the feather material. Alkaline treatment was applied with NaOH (2 mol/kg dry feather) and liquid/feather ratio of 5:1 (W/W) at 50˚C for 40 min in order to avoid dissolving. The feather pulp was then washed with deionised water and dried in oven at 105˚C. After the alkaline pre-treatment the feathers were processed using a compactor in which the feathers were pressed through a die using pan grinder rollers and crushed to an approximately 1 -3 cm length to enable better feeding into a grinder.
Pre-treated feathers were soaked in deionised water at 10% consistency for approximately one hour and dispersed using a ladle. The suspension was then The feather suspension was characterized for fiber type and length with an optical microscope (Nikon ECLIPSE Ci) and FiberLab TM analyzer (Metso Automation), and for consistency, pH and conductivity.

Formulations
The feather suspension was diluted with tap water to 4% consistency to lower viscosity.
The diluted suspension was mixed carefully for 20 min at room temperature with a mechanical overhead laboratory stirrer (IKA RW 20 digital). The suspension was placed in a hot water bath at 50˚C and the plasticizer was added to the suspension. Mixing was continued for at least 10 min prior adding other possible additives, and the final formulation was mixed for a further 20 min at 50˚C.
Tested formulations are presented in Tables 1-3.

Film Characterization
The films were evaluated visually, and one film of each formulation was photographed.

Feather Suspension
The feather suspension was a viscous fluid with a brownish color. Consistency of the suspension was 5.68%, pH 8.9, and conductivity 453 µS/cm. Based on the microscope images most of the fibrous material was equal or shorter than 100 µm, but there were also significantly longer fibers, as presented in Figure 2. The fibers were stiff and sharp-pointed, but there were also round-shaped fines. The FiberLab TM analyzer detected fibers with length around 100 µm (number average 80 µm and weight average 110 µm). However, long fibers were excluded from this analysis, although they were obviously present.

Film Formation
Films were prepared successfully from all the formulations, except for formulation 17. Adding 0.25% calcium chloride to a mixture of feathers, ethanolamine, and sodium alginate had a detrimental impact on colloidal stability, and deposits were formed during mixing. Therefore, this formulation was omitted.
Based on visual evaluation of the films prepared from formulations containing feathers and a single plasticizer (Table 1), ethanolamine and maleic acid produced the most uniform films. Other films had drying-induced patterning, but even these were intact and more uniform than the films prepared without any additives. Figure 3 presents films without additives and films from formulations 1 -6. The films without additives were fragile and difficult to handle.   formulation 18 with glyoxal were the most striking with a strong orange color due to an exothermic reaction between ethanolamine and glyoxal.

Film Structure
All the films were opaque caused by the light scattering in a porous film structure. SEM images prepared from films without additives and from formulations

Mechanical Properties
Films without any additives were too fragile for mechanical testing. This was in  line with previous studies indicating that keratin films even with a small amount of plasticizer (0.05 and 0.1 g of glycerol/g of keratin) were too brittle for testing [38]. In this study, all the films with plasticizers could be measured for the tensile properties. However, bonding between the fibers may dominate the mechanical properties of such porous films. Figure 6 shows the tensile properties for the films with a single plasticizer. These films were stronger and more brittle than those prepared previously from thermoplastic feathers and keratin (tensile strength 5.2 -5.9 MPa [18], and strength 0.3 MPa and Young's modulus 25 MPa [9]). The mechanical properties, except of strain at break, were similar to the carboxymethylated keratin films by Schrooyen et al. [38], but weaker than the grafted feather films without and with 30% glycerol (206.3 and 55.7 MPa, respectively) [19]. With 20% plasticizer the strongest films were obtained with sorbitol, 1,4-butanediol and ethanolamine, while the films containing maleic acid, urea, and glycerol were among the weakest. Studies with keratin films have shown that increased glycerol concentration decreases the tensile strength and the elastic modulus, while the strain at break increases [9] [38]. Increasing concentration of 1,4-butanediol decreased strength in this study. However, little effect was observed with ethanolamine, and strength increased with an increasing concentration of maleic acid. Maleic acid must have promoted bonding at the fiber-fiber contacts. Maleic acid is known to improve adhesion in polymer-fiber composites [39].
Increasing the amount of maleic acid increased also the strain at break.  In general, the level of maximum strain was low. 20% urea, glycerol and maleic acid, and 30% 1,4-butanediol resulted in a low elastic modulus. The highest stiffness was obtained with ethanolamine and 30% maleic acid.
Results from formulations with a blend of ethanolamine and maleic acid together with formamide are presented in Figure 7. At 30% concentration of ethanolamine and maleic acid the impact of formamide was logical.  formamide can act as a solvent and prevent phase separation [32]. Such a synergistic effect was observed when the properties of films with 20% ethanolamine and 10% formamide were compared to those with only 30% ethanolamine. Figure 8 shows the impact of other additives on the tensile properties. Together  and Mg 2+ ) were tested. As presented in Figure 9,

Conclusions
An aqueous suspension was prepared from chicken feathers using an alkaline pretreatment followed by a three-stage mechanical milling process consisting of crushing, grinding, and microfluidization. The purpose was to avoid costs and complexity related to extraction of keratin or chemical modification of feathers.
The resulting suspension consisted of stiff and sharp-pointed fibers together with a smaller number of more round-shaped fines.
Films were cast from the feather suspension. Films without additives were of poor quality and fragile. Film formation was improved with additives, and visually the best films were obtained with 20% -30% of ethanolamine and maleic acid as the single additive or together with formamide, sodium alginate, and citric acid. The films were opaque and porous, which would limit their feasibility for applications where transparency and good barrier properties are essential.
The strongest films were obtained with maleic acid, sorbitol, and ethanolamine as the single additive. Adding formamide together with ethanolamine and maleic acid resulted in a plasticizing effect. Sodium alginate as a secondary additive with ethanolamine had a positive impact on tensile strength with and without cationic metal ions used to crosslink sodium alginate. However, the modest impact of additives on the tensile properties, especially strain at break, can be