Thermoplastic Collagen - a new application for untanned byproducts Michael Meyer *), Oliver Kotlarski Forschungsinstitut für Leder und Kunststoffbahnen Meissner Ring 1-5, D-09599 Freiberg *) ++49 3731 366-165; email: michael.meyer@filkfreiberg.de Abstract Dry collagen powder was processed in an internal mixer to degrade the material and on a twin screw extruder to manufacture bands and to calender films. The films were crosslinked by glutaraldehyde to reduce solubility in water at 60 C, the swelling and to increase stability against enzymatic attack. The combination of rheological characterisation and DSC investigation showed, that the material behaved as an interpenetrating network of covalently crosslinked gel and physical gel, stabilized by hydrogen bonds. Introduction Thermoplastic processing is an elegant and widely used technology to form polymer melts into films, tubes, profiles, to blend polymers with additives and fillers and to foam polymer melts. Not only synthetic polymers but also a couple of biopolymers have already been treated by thermoplastic processing. In case of biopolymers the process is often called extrusion cooking, because it has to be performed with water contents from 15 up to 80% and at temperatures around 80 up to 150 C. Thermoplastic processing has several advantages over other polymer treatment technologies. Extrusion as one of the most popular processes is a continuous technology performed mostly free from organic solvents as one step technology. Extrusion cooking is used in food industry inter alia to manufacture ready-to-eat cereals, pasta and to texturize and form doughs [1]. Untanned collagen containing by-products are fleshings, trimmings and splits. Fleshings are separated in their fat and collagen component to manufacture gelatine and hydrolysates from the collageneous part. Because of the low gelatine quality, they are also disposed by making biogas. Trimmings are used to make gelatine and splits can be processed to gelatine, casings or higher quality products as sponges for the cosmetic industry. The known industrial processes of collagen treatment comprise the use as solution or paste. Both are dissolved in neutral or acidic aqueous solvents and then cast or formed and dried.
Casings for sausages are extruded, but this extrusion is performed at about 10 C from a raw stock, that contains about 85% of water. Therefore, the collagen is processed under nondenaturating conditions, so that fibres and triple helices remain stable [2]. In contrast, we have shown, that dry collagen powder, which was produced from dried splits, defatted fleshings and trimmings, could be treated by thermoplastic processing to form films, tubes and coatings. The aim of our present work is to show the potential of thermoplastic processing of collagen and the properties of the resulting extrudates [3][4][5]. Technological aspects Preparation of raw material Limed (unhaired) cattle hides were split, extensively washed and air-dried. The water content was adjusted in the range of 10% to 15%; ph laid between 8 and 9. This parchment like material (dried pelt) was ground into a fine powder. Mixing In earlier investigations it was found that widely insoluble collagen of pelts could be transferred into water-soluble partial hydrolysates by thermo-mechanical treatment in an internal mixer. During this thermo-mechanical treatment the fibrous structure of collagen disappeared. Besides other parameters like ph and temperature the degree of solubility depended strongly on the input of mechanical energy E spec. This specific energy input was calculated to be up to E spec =14.4 MJ/kg, when treated over 30 min in the mixer [3]. The hydrolysates were able to form stiff gels, when a warm aqueous solution was cooled down. Obviously, the increase of solubility showed that the molecular weight was being decreased and covalent bonds were cleaved. In contrast to gelatine manufacture the thermomechanical treatment did not lead to topochemical hydrolysis as it is observed during gelatine production [6]. Extrusion and film calendering To extrude the material, the humidity of the collagen powder was adjusted up to 50% water content. Furthermore glycerol was added up to 40% per dry matter as plasticising agent. This powder was molten on an extruder, which was equipped with a band die. The strong band of 2 mm thickness was calendered into a thin film of ~150µm on a Collin roller. The mechanical energy, which was brought into the material by extrusion, was calculated according to
E spec where 2 n/n = M 0,01 m& max n... rpm of the screws n max... maximum rpm M... torque m&... specific throughput (kg/h) Under stable extrusion conditions the throughput reached 3,6 kg/h. This led to the specific mechanical energy input E spec =0,5 MJ/kg into the material. Therefore, only seven percent of the energy was brought into the material by the used extruder, compared to the specific energy of the soluble material treated in the internal mixer. The extruded material was not soluble in water (60 C) after extrusion, but it melted during the process and seemed to behave thermoplastic. Characterisation Solubility and Swelling The solubility in water at 60 C (2h) and the swelling were measured from calendered films with and without cross-linking by glutaraldehyde (GA). Fig. 2a shows, that the solubility of the non cross-linked films laid around 25%, whereas it decreased strongly down to less than 1% when soaked in solutions of 0,1% 1% GA. The swelling of film samples was also reduced depending on the crosslinking time and the concentration of GA from 500% down to 250% (Fig 2b). The swollen samples (bands, films) looked transparent and felt rubber to gellike. Solubility [%] 30 25 20 15 10 5 0 0 200 400 600 800 1000 crosslinking time [min] Swelling [%] 600 550 500 450 400 350 300 250 200 Increasing glutaraldehyde concentration 0 200 400 600 800 1000 crosslinking time [min] Figure 2a; b: Solubility (left) and swelling (right) of films from thermoplastic collagen (extruded, calendered, 150µm) crosslinked with glutaraldehyde (GA; 0,1, 0,5, 1%). With increasing concentration of GA and increasing soaking time the crosslinking degree increases.
FTIR Measurements The degree of denaturation was measured by FTIR as described by Kaiser and Meyer (2004) [7]. With decreasing degree of denaturation the amide A band is observed to be down shifted and the ratio A 1240 /A 1455 is varied. For a film of acid soluble collagen no denaturation was assumed, whereas full denaturation was postulated for a gelatin solution dried at 50 C to prevent gelling. A film sample was soaked in water, drawn to a thickness of <50µm and dried at room temperature. Using the downshift of the amide A band a helix degree of 78% was found. When using A 1240 /A 1455 a helix degree of 51% was found. Both calculations assumed a linear dependency between 0 and 100%. These values laid between that of renaturated collagen and that of gelled gelatine. When the films of thermoplastic collagen were not dried at room temperature but at 50 C no helix degree could be observed. Rheology and DSC Rheological measurements were performed from the extruded bands to get hints about structure and melting behaviour. The samples were soaked in water and measured between two plates (d 1mm; 20 C). The linear domain extended only up to 0,01% deformation for the cross-linked sample and up to 0,05% for the sample, which was not crosslinked (Fig 3a). These values are very low compared to those of other proteins or synthetic polymers [8]. In contrast, the moduli rested stable over a broad range of frequency from 0,5Hz to 100Hz (Fig 3b), which indicates a rubber-like network with no physical junctions [9]. G' ; G'' Linear Domain G' ; G'' 0,001 0,01 0,1 1 Deformation γ [%] 1e+1 1 10 100 Pulsation ω [rad/s] Figure 3a, b: Dependence of storage modulus G and loss modulus G from bands soaked in water at 20 C on deformation and pulsation frequency. (.. G,..G crosslinked;..g,..g not crosslinked)
Temperature scans were performed at ω=0,1hz; γ=0,01% while heating at a rate of 2 C/min from 5 C to 90 C (1 st run). Then the samples were cooled down at a rate of 2 C/min and heated again at 2 C/min (2 nd run). G' (Pa) 1e+1 20 40 60 80 Temperature [ C] 0,10 0,08 0,06 0,04 0,02 0,00-0,02 Normalized Heat Flow (J/g) Figure 4: DSC (..1 st run;..2 nd run) and rheological measurements (..1 st run;.. 2 nd run) of an extruded band, soaked in water over night. Melting of parts of the material is reflected by the sigmoidal decrease of G and the DSC maxima. The first run showed a sigmoidal decrease of the storage modulus around 40 C. The DSC curve shows an endothermic heat flow maximum at the same temperature. This is interpreted as melting of parts of the material. During the whole temperature scan, the curve of G went in parallel to G and no crossing was observed (data not shown). Therefore, the samples behaved rubber-like (gel-like) in the applied temperature range. When the measurements were repeated with the same sample, both, storage modulus and DSC maximum decreased and laid 10 C lower at around 30 C. Bands, which were treated in 0,1% solution of glutaraldehyde showed only a slight decrease of G over the whole temperature range (Fig. 5). The second run did not differ very much from the first run and G and G went in parallel for both runs (data not shown). Nevertheless, the DSC curves showed maxima for the 1 st and the 2 nd run at the same temperatures as observed for the samples, which had not been crosslinked.
G' (Pa) 1e+1 20 40 60 80 Temperature [ C] 0,10 0,08 0,06 0,04 0,02 0,00-0,02 Normalized Heat Flow (J/g) Figure 5: DSC (..1 st run;..2 nd run) and rheological measurements (..1 st run;.. 2 nd run) of a calendered film, soaked in water over night. Only slight decrease of G, though DSC shows a melting peak. The observed behaviour of our samples is understood as two interpenetrating gels, one stabilized by physical junctions as a gelatine gel and the other stabilized by chemical bonds, namely the natural crosslinks of collagen as outlined in Fig. 6. By rheometry the moduli of both gels are measured as sum. T > T S Fig.6: Model of two interpentrating networks. By increasing temperature the physical (gelatine) gel melts (red) into a gelatine solution (yellow) whereas the chemical gel (black) rests stable. At low crosslinking degree the sigmoidal decrease of G, which correlates with the DSC peaks, is interpreted as melting of the physical gel, though the macroscopic sample continues to behave rubber-like. At high crosslinking degree, caused by the treatment with glutaraldehyde, the contribution of the physical gel to the modules decreases, so that only a low decrease of G is observed. Nevertheless, the thermal effect of melting of the physical gel can be measured by DSC.
References [1] Mercier et al. Extrusion cooking, 1989 [2] Hood, in Advances in Meat research, 1987 [3] Meyer, M., Mühlbach, R., Harzer, D., Polymer Degradation and Stability, accepted for publication [4] Meyer, M., Talk at the VGCT-Tagung, Würzburg 2003 [5] Meyer, M., Talk at the 3 rd Freiberg Collagensymposium 2004 [6] Meyer, M. Dissertation TU Dresden, 2002 [7] Kasier, I., Meyer, M., Poster at the 3 rd Freiberg Collagensymposium 2004 [8] Redl, A., Morel, MH., Bonicel, J., Guilbert, S., Vergnes, B., Rheol Acta 38 (1999) 311 [9] Ross-Murphy, SB., J Rheol. 39 (1995) 1451