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Biocomposites based on flax short fibres and linseed oil
Industrial Crops and Products 33 (2011) 317–324
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Biocomposites based on flax short fibres and linseed oil J. Lazko ∗ , B. Dupré, R.M. Dheilly, M. Quéneudec Laboratoire des Technologies Innovantes (EA 3899), Université de Picardie Jules Verne, CODEM Picardie, 41, rue Paul Claudel, 80480 Dury, France
a r t i c l e
i n f o
Article history: Received 2 September 2010 Received in revised form 18 November 2010 Accepted 20 November 2010 Available online 10 December 2010 Keywords: Biocomposites Flax short fibres Linseed oil Hydrophobicity Polymerization
a b s t r a c t Insulating materials based on flax short fibres were prepared and their functional properties enhanced using linseed oil. In order to improve biocomposites hydrophobicity, the linseed oil was added to the initial formulation, mixed with the fibres and finally dried after the moulding process. Thus, the average water absorption of 10–40/140 g/g oil/fibres samples was reduced up to 10 times during the first hour of immersion, compared to the reference oil-free materials. Moreover, the linseed oil polymerization inside the lignocellulosic matrix, which occurred after 20 days of drying at 50 ◦ C, also improved mechanical and thermal behaviour of biocomposites. Spontaneous combustion phenomena related to the exothermic oxidation–polymerization of linseed oil were described as well. Therefore, the process parameters such as oil/fibres ratio, drying time and temperature, were optimized to ensure the safety of the process and to avoid self ignition of the lignocellulosic fraction at temperatures below 200 ◦ C. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In flax (Linum usitatissimum) long high-quality textile fibres are considered as the noble fraction, and the rest, not <80% of the plant mass, are low-cost by-products. These short fibres, also called “tow”, containing a large proportion of shives, are nevertheless potential lignocellulosic raw materials, perfectly corresponding to actual demand on renewable, recyclable and biodegradable resources. The increasing interest for flax fibres and shives in thermal insulation is particularly well described in literature, and a large number of flax based composites and nonwovens already have their share in transportation and building markets (Satyanarayana et al., 2009; Bos et al., 2006; Flambard et al., 2005; Klamer et al., 2004; Kymäläinen and Sjöberg, 2008; Schartel et al., 2003; AamrDaya et al., 2008). However, these new biocomposites generally contain an important synthetic polymer fraction (10–40%), such as polypropylene for example. Other polyesters and polyamides from petrochemical resources are used to enhance the mechanical properties of the materials, but they also have a quite negative impact on the global biodegradability and recycling aspects. Natural insulation materials from flax short fibres only, without any mineral or synthetic additive, were developed and patented in our laboratory (Queneudec t’Kint et al., 2005). The scale of their thermal and mechanical performances should be suitable for numerous industrial applications. The main disadvantage of these
∗ Corresponding author. Tel.: +32 0 68 27 47 80. E-mail address: [email protected] (J. Lazko). 0926-6690/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2010.11.015
materials is nevertheless, their high hydrophilic character, as it is the case for almost all composites based on vegetal lignocellulosic raw materials. Surface treatments or homogeneous chemical modifications (esterification, acetilation, acylation) are commonly applied to various cellulosic substrates in order to enhance their hydrophobicity (Heinze and Liebert, 2001; Pasquini et al., 2006; Vaca-Garcia and Borredon, 1999). Grafting of aliphatic chains was performed onto flax fibres and shives as well (Hill et al., 1998; Joffe et al., 2003; Sain and Fortier, 2002; Tserki et al., 2005). Results indicate a significant improvement of moisture resistance, mechanical properties and microbial degradation resistance. However, in most chemical modification procedures concerning lignocellulosic substrate, the use of harmful organic reagents, solvents and catalysts, obviously contrasts with the very intrinsic biodegradable and renewable properties of these natural raw materials. The hydrophobisation of biocomposites described in this article was realized with full respect of green chemistry approach. In fact, the hydrophobic treatment was not based on chemical modifications but on formulation modifications: a hydrophobic agent, linseed oil, was introduced into the reaction medium during the process. Thus, the whole biocomposite preparation was realized in aqueous medium, without any use of potentially toxic organic solvent (acetone, toluene, methanol) or acyl chloride and anhydride. The particularity of linseed oil – the latter being commonly used for industrial applications from paints to linoleum – is its high drying ability. In fact, this oil consists of mixed glycerides composed of about 50% linolenic acid, 20% oleic acid, 15% linoleic acid and also containing palmitic and stearic acid (Güner et al., 2006; Sharma and Kundu, 2006). The drying of the linseed oil is
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Table 1 Basic properties and composition of flax short fibres – linseed oil composites. Initial reaction medium
Final composite
Linseed oil/short fibres [g/g]
Mass [g]
0/140 5/140 10/140 20/140 40/140
126.3 132.1 136.6 145.6 160.3
± ± ± ± ±
Composition 3
Density [g/cm ] 0.6 1.4 1.4 0.6 3.0
a polymerization–oxidation process (Abraham, 1996; Meneghetti et al., 1998; Oyman et al., 2005; Stenberg et al., 2005). Both linoleate and linolenate rapidly oxidize, since their unsaturated bonds are close the one to the other, thus facilitating the abstraction of the allylic hydrogen atoms. The following cross-linking step results in formation of three possible polymeric structures: C1 –O–O–C2 , C1 –O–C2 and C1 –C2 . Moreover, the heat liberated during the oxidation–polymerization steps improves even more the reaction kinetics, eventually leading to spontaneous combustion of inflammable materials in contact or in formulation with linseed oil. The present study focuses on the flax short fibres biocomposite process, characterization and on the improvement of their functional properties. The main subject concerns, above all, the enhancement of the composites hydrophobicity using linseed oil and the optimization of process parameters such as oil/fibre ratio, polymerization time and drying temperature. Water absorption kinetics corresponding to different formulations and operational conditions has been discussed. A particular attention was given to spontaneous combustion phenomena, induced by the presence of the drying linseed oil in the core of an insulating and inflammable lignocellulosic matrix. Thus, this study also has a security approach and deals with biocomposites thermal behaviour during the process and their stability at high temperatures. 2. Experimental 2.1. Materials Raw flax short fibres, containing in average 65% of fibres and 35% of shives, were provided by agricultural cooperative of CALIRA, Martainneville, France. Commercial linseed oil was purchased from ONYX-Veolia Environment, Paris, France. 2.2. Biocomposites preparation The flax biocomposites process, also used in this article as the reference process, has been protected and already described in the corresponding patent (Queneudec t’Kint et al., 2005). On the whole, it consists of 5 main steps. Raw short fibres were firstly milled through RETSCH SM 100 (Haan, Germany) cutting mill. 140 g of refined short fibres were then mixed with distilled water in a 5 l CONTROLAB Perrier mixer (Saint Ouen, France). The reaction medium was next treated in Panasonic microwave oven (Plaine Saint-Denis, France). The moulding step allowed preparation of three 13 cm × 4 cm × 4 cm biocomposite test samples, which were finally placed in BINDER drying oven (Tuttlingen, Germany) at 50 ◦ C and dried during 5–6 days to a constant mass.
0.220 0.230 0.238 0.253 0.275
± ± ± ± ±
0.008 0.006 0.002 0.004 0.007
Short fibres [%]
Oil [%]
100 95.8 ± 0.8 92.5 ± 0.6 86.4 ± 0.3 76.8 ± 2.2
0 4.2 ± 0.8 7.5 ± 0.6 13.6 ± 0.3 23.2 ± 2.2
The second reference process modification concerns the drying step. Oil treated samples were placed at 50 ◦ C and the water evaporation occurred in 5–6 days, as it did with non-treated samples. Nevertheless, the composites were kept at 50 ◦ C for at least 20 days to also allow the polymerization of the linseed oil. Therefore, in this particular case of hydrophobic treatment, the prolonged drying step eventually corresponds to composite “maturation”, a term we are now introducing and which will be later related to two main aims: water evaporation and oil polymerization. 2.4. Water absorption Water liquid absorption was performed according to ISO 62 and ASTM D570 methods. 4 cm × 4 cm × 4 cm samples were immersed in distilled water at room temperature, then removed from the tank after 3, 6, 12, 21, 30, 60, 150 and 300 min, dried on blotting paper and weighted. The samples were placed back into water after each measurement. Finally, the water uptake was calculated as the mass difference, and expressed as ratio water/initial fibres (g/g). 2.5. Thermal behaviour Biocomposites 4 cm × 4 cm × 4 cm samples, containing K type thermocouple in their core, were placed in the Nabertherm L3/11 oven (Lilienthal, Germany) and heated at 100 ◦ C, 180 ◦ C, 195 ◦ C and 210 ◦ C for respectively 240, 120, 60 and 60 min. The thermocouple of the same characteristics was placed in the oven near the sample and used to indicate the reference temperature. 2.6. Mechanical properties Maximal compression stress was measured on the PROVITEQ hydraulic testing system (Nozay, France), using 4 cm × 4 cm × 4 cm samples and 200 N/s charge speed. Samples were stored at 50 ◦ C during 48 h before the test, and the measurements were realized in triplicate. 2.7. Microscopy Scanning electron micrographs allowed the visualization of the biocomposites morphology. The micrographs have been generated with FEG XL 30 Phillips scanning electron microscope. In order to facilitate observation, portions of samples were at first dried, and then covered with a thin layer of vacuum sputtered platinum, which acted as a conductor. 3. Results and discussion
2.3. Hydrophobic treatment
3.1. Biocomposites fundamental characteristics and composition
The hydrophobic treatment of flax biocomposites was based on linseed oil which was added in the reaction medium during the mixing step. Thus, 5, 10, 20 and 40 g of linseed oil were introduced in the previous fibres/water mixture.
At first, the high reproducibility of composite’s dimensional characteristics was noticed. Each formulation, containing 0, 5, 10, 20 and 40 g of linseed oil per 140 g of short fibres, was realized in triplicate. Thus, for the whole set of 45 13 cm × 4 cm × 4 cm test
0.29
20
0.28
18
0.27
16
Mass increase / initial oil [%]
Density [g/cm3]
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0.26 0.25 0.24
R 2 = 0.9938 0.23 0.22 0.21 0.00
0.05
0.10
0.15
0.20
0.25
0.30
319
14 12
10 / 140
10
20 / 140 40 / 140
8 6 4 2
0.35
Oil / Short fibres ratio in final composite [g/g]
0 0
Fig. 1. Density as function of biocomposites final composition (oil/fibres ratio).
5
10
15
20
25
Time [days] Fig. 3. Mass evolution of composites containing linseed oil during maturation (oil polymerization) step at 50 ◦ C. Initial oil/fibres g/g ratio: () 10/140, () 20/140 and () 40/140.
900 800
Water uptake [g/cm 3 ]
samples, the mean volume was 192.1 cm3 and the standard deviation was limited to 4.1 cm3 . As the volume remains the same, the mass and consequently the density of biocomposites proportionally increase with the quantity of introduced oil (Table 1). The mean density of the reference composites, based on short fibres only, is around 0.220 g/cm3 , while a density of 0.275 g/cm3 was obtained for 40/140 g/g oil/fibres formulation. According to the results obtained with the reference composite 0/140 g/g, the total mass loss of hydro-soluble fraction during the process represents 10 ± 1% of initially introduced short fibres. The estimation of the final composition of composites containing both fibres and oil is based on two hypotheses. Firstly, the hydrosolubles loss of 10% may remain constant for all formulations. In fact, the inert oil fraction should not modify the solubility of some highly hydroscopic compounds of flax. Secondly, the loss of linseed oil was considered insignificant during the process. These two hypotheses were precisely confirmed by the high linearity between estimated oil/short fibres composition and measured densities (Fig. 1). According to the previous results, the concentration of lignocellulosic fraction (g of flax short fibres per cm3 of biocomposite) remains the same for each sample, independently of the quantity of introduced oil. This remark is particularly important for the following discussion, concerning the influence of composite formulation on water absorption kinetics.
700 0/140 Ref
600
5/140 5d 10/140 5d
500
20/140 5d
400
5/140 20d 10/140 20d
300
20/140 20d
200 100 0 0
60
120
180
240
300
360
Time [min] Fig. 4. Water absorption kinetics of linseed oil/flax short fibres biocomposites as function of oil/fibres initial ratio (0–5–10–20/140 g/g) and maturation time (5 and 20 days). Water uptake expressed as mass increase per unit of sample volume [g/cm3 ].
900 800
80 / 140 g/g Oil / Flax ratio
Water uptake [g/cm 3 ]
Mass increase / initial oil [%]
14 12 10 8 6
700 600
0/140 Ref 40/140 10d
500
40/140 15d
400
40/140 20d 40/140 30d
300 200
4
100
2
0 0
0 0
10
20
30
40
60
120
180
240
300
360
Time [min]
Time [days] Fig. 2. Mass increase of linseed oil/flax short fibres mixture, 80/140 g/g, during the prolonged drying at 50 ◦ C.
Fig. 5. Water absorption kinetics of 40/140 g/g linseed oil/flax short fibres biocomposites measured after 10, 15, 20 and 30 days of maturation at 50 ◦ C.
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Fig. 6. Scanning electron micrographs of shives and fibres in the reference (a and b) and in the treated (c and d) 20/140 g/g linseed oil/flax short fibres biocomposites.
3.2. Linseed oil drying and polymerization The linseed oil drying/polymerization are mainly characterized by oxygen fixation on unsaturated double bounds. Thus, in this study, the drying reaction advancement corresponds to the mass evolution of the oil phase during time. The drying of linseed oil has extremely slow kinetics when the substrate is a continuous compact liquid phase. For example, 5 g oil sample was placed in a 25 ml beaker, and the mass gain measured after 30 days of heating at 50 ◦ C remained inferior to 1.5%. Moreover, the solidification of this sample was only partial and situated on the liquid/air interface. On the other hand, polymerization kinetics was significantly improved when the sample was a dispersed oil phase. Flax short fibres and linseed oil 140/80 g/g were just mixed together, without any moulding, and heated at 50 ◦ C during 30 days. The measures of the mixture mass evolution are represented in Fig. 2. Although the mixture both contains linseed oil and short flax fibres, the mass gain should only be attributed to the oil fraction. A systematic mass increase was observed, especially between the 10th and the 14th day of drying. Finally, the sample mass was sta-
ble after 20 days of drying and the maxima obtained were around 10% of the initial oil mass. In fact, the absorption of linseed oil by flax short fibres would significantly increase the exchange oil/air interface and facilitate oxygen fixation by the unsaturated bonds of linoleate and linolenate. Consequently, the mass gains were 6–10 times superior to values obtained during the drying of a continuous liquid oil phase, under the same operational conditions. Identical observations were made during the prolonged drying at 50 ◦ C of linseed oil/short fibres materials (Fig. 3). All samples presented similar mass evolution during time, independently of oil/fibre ratio. The initial mass loss was due to the water evaporation during the first days of drying. Then, a gradual mass gain was observed and maxima between 10% and 15% were obtained after 15 days of heating. Finally, the masses slightly decreased and stabilized at around 10% – on average – after 20 days. In conclusion, according to the results concerning mass evolution of oil/fibre materials during the drying at 50 ◦ C, two characteristic states could be defined. The first initial state of material maturation, obtained after 5 days, is only relative to water evaporation. The lignocellulosic fraction of these materials is dry but oil polymerization/oxidation phenomena are still not initiated.
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3.3. Water absorption kinetics Water absorption kinetics of the reference composites only based on short fibres, and of composites containing 5, 10 and 20 g of linseed oil per 140 g of short fibres, are shown in Fig. 4. These materials were characterized at the initial and at the advanced maturation state, obtained after 5 and 20 days of drying at 50 ◦ C, respectively. After 5 days of drying, water absorption kinetics of all oil based samples are of the same rate as those of the untreated reference materials. In fact, as soon as the samples are immersed, water penetration and the water/air exchange in the lignocellulosic matrix are extremely fast. For example, the 5/140 and 20/140 g/g oil/fibres samples absorbed 700 mg of water per cm3 in only 3 min, which represents more than 250% of their initial mass. Despite minor differences at the beginning of the analysis, almost the same values were noticed for all ratios after 30 min of immersion. Thus, mass gains between 630 and 770 mg/cm3 , and between 710 and 790 mg/cm3 were measured after 60 and 300 min, respectively. A significant decrease of water absorption kinetics was achieved for mature oil based materials dried during 20 days at 50 ◦ C. The water uptake gradually decreases for the 0/140, 5/140 and 10/140 g/g oil/fibres samples. For example, an improvement of composite hydrophobicity is only partial for 5/140 sample, since the quantity of the oil based polymer might be insufficient to successfully waterproof the lignocellulosic matrix. However, the absorption rate remains stable for ratios above 10/140 g/g and average mass gains between 70 and 80 mg/cm3 and between 100 and 130 mg/cm3 were measured for all samples after 60 and 300 min, respectively. Linseed oil polymerization impact on composite hydrophobic properties was also observed for 40/140 g/g oil/fibres samples (Fig. 5). Water absorption kinetics measures were performed after 10, 15, 20 and 30 days of drying at 50 ◦ C. The most important absorption changes occurred just between the 10th and 15th day. For example, water absorption rate decreased up to 10 times during the first hour of immersion. After 15 days, the composite water behaviour did not evolve any more. Average water absorption of 65, 85 and 145 mg/cm3 was measured after 30, 60 and 300 min respectively, for all 15, 20 and 30 days dried samples. These observations are in agreement with mass evolution data, described previously. During the first 5–10 days of drying, the sample mass remains constant, the linseed oil polymerization is incomplete, so the oil based materials have almost the same water behaviour as the reference composite. The next 10–15 days period is characterized by a significant mass increase, an oil polymerization and consequently a water resistance enhancement. Moreover, after 20 days, the composite mass remains stable as well as the water absorption kinetics. Finally, the hydrophobicity improvement would be closely related to the morphology of the linseed oil treated composites. According to the scanning electron micrographs, the porosity of the lignocellulosic matrix decreases with the quantity of the introduced oil (Fig. 6). Linseed oil largely covers the surface of shives and fibres, and the polymer aggregates are also present in the free space between elemental particles. Compared with the reference materials, the flax hydrophilic fractions coated with the linseed oil would be less accessible, consequently reducing the capillar-
0.7
Maximal Compression Stress [MPa]
The second advanced state of maturation would be characteristic for oil/flax composites after 20 days of drying at 50 ◦ C and to a high degree of linseed oil polymerization. In the following discussion, water absorption kinetics, mechanical and thermal behaviour of oil/flax short fibres materials will be compared at initial and at advanced degree of maturation.
321
0.6
0.5
0.4
0.3 0/140 (Ref)
5/140
10/140
20/140
40/140
Initial ratio Oil / Fibres [g/g]
Fig. 7. Maximal compression stress [MPa] of 0, 5, 10, 20 and 40/140 g/g linseed oil/flax short fibres biocomposites. Measures were realized on 4 cm × 4 cm × 4 cm samples and after 20 days of maturation at 50 ◦ C.
ity phenomena and water diffusion kinetics in the lignocellulosic matrix. 3.4. Mechanical properties The presence of polymerized linseed oil in the flax short fibres matrix not only had an impact on water absorption kinetics, but also on mechanical behaviour of biocomposites. Thus, the compression resistance performances of materials containing 0, 5, 10, 20 and 40 g of linseed oil per 140 g of short fibres were compared (Fig. 7). The maximal compression stress of the reference short fibres material is around 0.35 MPa. An important increase of the mechanical resistance, up to 60%, was observed for all oil based composites, even for relatively low oil/fibres ratios. For example, an average maximal compression stress of 0.58 MPa characterizes the set of 10/140, 20/140 and 40/140 g/g samples. Obviously, the optimization of mechanical behaviour of flax short fibres biocomposites should not be considered as the priority of our study. Nevertheless, these results clearly justify our hydrophobic linseed oil based treatment which not only preserves, but also improves the mechanical properties of materials. Furthermore, the whole process elaboration and conception should be reconsidered in the presence of the oil phase. In fact, the improvement of the mechanical behaviour was only noticed for those samples in which the linseed oil polymerization occurred after the final material moulding. But also, the inversion of the moulding/polymerization steps had a quite negative impact on the mechanical resistance. For example, some composite elaboration tests were made with already treated short fibres, previously described (Fig. 2). The 80/140 g/g oil/fibres mixture was dried at 50 ◦ C during 30 days in order to achieve an advanced polymerization rate. The hydrophobically treated fibres were then mixed with 1000 ml of water, as for the reference process, and finally moulded. Such materials were particularly friable, their maximal compression stress was below 0.15 MPa, and they were even unsuitable for any following manipulation or for water absorption characterization. 3.5. Thermal stability The first results concern thermal behaviour of raw materials and reference composite. All raw materials and biocomposites based on short fibres perfectly resist to the range of temperatures below 195 ◦ C (Fig. 8). The first sights of degradation and an important
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0/140 (Reference composites) T° oven [C°]
40/140, 5 days T° oven [C°] T° sample [C°]
T° sample [C°]
400
Temperature [°C]
Temperature [°C]
400
300
200
300
200
100
100 0
0 0
60
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300
60
120
Time [min]
T° sample [C°]
Temperature [°C]
400
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100
0 120
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480
300
plateau. For high oil/fibres ratios (20/140 and 40/140 g/g) the degradation characteristics are quite different and they could be directly related to the polymerization rate of the oil phase. Concerning 40/140 g/g oil/fibres composites at initial state of oil polymerization (5 days of drying at 50 ◦ C), thermal degradation occurred already at 100 ◦ C (Fig. 10). During the first 3 h of heating at 100 ◦ C, temperature in the core of the sample progressively increased, then exceeded the oven temperature and finally continued to increase exponentially, thus indicating the beginning of the sample consummation. The thermal degradation profiles for 20/140 g/g samples at 100 ◦ C were quite the same, except that the combustion occurred after 4–5 h. In fact, the equilibrium between sample and oven could not be reached under these particular operational conditions. The external heating at 100 ◦ C significantly increases the linseed oil oxidation/polymerization kinetics and finally initiates the spontaneous combustion of the whole material. More specifically, the spontaneous combustion phenomena were favoured by both the insulating and the combustible nature of the lignocellulosic flax materials. Thus, at the first stages, the heat released by oil polymerization was accumulated in the core of insulating composites. Then followed the combustion of the short fibres fraction, as soon as the temperature in the core of the samples reached 210 ◦ C. Similar autoignition phenomena, which occurred even at room temperature, were already reported by Abraham (1996). According to the author, maximum reaction rates were measured when the amount of cellulosic substrate by weight equalled that of the oil. Measures of 20 days dried samples, characterized by an advanced linseed oil polymerization rate, indicated a significant improvement of thermal behaviour. Thus the 40/140 g/g formulations resisted at 100 ◦ C (Fig. 11). However, even after 20 days of drying at 50 ◦ C, spontaneous combustion phenomena occurred at 180 ◦ C for high oil concentration samples (Fig. 12). In fact, a fraction of the linseed oil phase might still remain suitable for exothermic
Linseed Oil
0
240
Fig. 10. Thermal behaviour and degradation of 40/140 g/g linseed oil/flax short fibres composites obtained after only 5 days of maturation. Oven temperature settings: 4 h at 100 ◦ C.
Fig. 8. Thermal behaviour and degradation of the reference flax short fibres based composites. Oven temperature settings: 2 h at 180 ◦ C, 1 h at 195 ◦ C and 1 h at 210 ◦ C.
T° oven [C°]
180
Time [min]
600
Time [min] Fig. 9. Thermal behaviour of the raw linseed oil. Oven temperature settings: 1 h at 180 ◦ C, 195 ◦ C, 210 ◦ C, 225 ◦ C, 240 ◦ C, 255 ◦ C and 270 ◦ C.
smoke release were noticed at 210 ◦ C plateau. During the combustion of short fibre samples, the temperature measured in the core of materials rapidly exceeded the oven temperature and reached 400 ◦ C in <40 min. On the other hand, the linseed oil well resists to all applied temperatures up to 270 ◦ C (Fig. 9). At each heating step, the sample temperature progressively converged to the external temperature. Nevertheless, some other phenomena, such as colour change and solidification of the initially liquid oil, indicated an advanced polymerization degree. The following results concern oil/flax formulations and the influence of the hydrophobic treatment on biocomposites thermal behaviour (Table 2). Biocomposites with low oil/fibres ratios (5/140 and 10/140 g/g), dried during 5 or 20 days, showed the same thermal stability as the reference materials and their degradation occurred at 210 ◦ C
Table 2 Thermal stability of biocomposites at 50 ◦ C, 100 ◦ C, 180 ◦ C, 195 ◦ C and 210 ◦ C, as function of linseed oil/flax short fibres initial mass ratio (0–40/140 g/g) and maturation/polymerization time (5 and 20 days). Initial ratio Linseed oil/short fibres [g/g] 0/140 5/140 10/140 20/140 40/140
Maturation 5 days ◦
Stability [ C] 195 195 195 50 50
Maturation 20 days ◦
Degradation [ C]
Stability [◦ C]
Degradation [◦ C]
210 210 210 100 100
195 195 195 195 100
210 210 210 210 180
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40/140, 20 days T° sample [C°]
T° oven [C°]
Temperature [°C]
400
300
200
100
0 0
120
240
360
480
Time [min] Fig. 11. Thermal behaviour of 40/140 g/g linseed oil/flax short fibres composites obtained after 20 days of maturation. Oven temperature settings: 7 h at 100 ◦ C.
40/140, 20 days T° oven [C°] T° sample [C°] 400
323
cated a partial internal heat release, probably due to the oxidation of the oil phase, during the first hour of heating at 180 ◦ C. The maximal sample temperature was about 20 ◦ C higher than the oven temperature. However, compared to 40/140 g/g samples, the quantity of remaining unsaturated linseed oil for 20/140 g/g formulation was insufficient to induce spontaneous combustion and the internal heat progressively diffused from the core to the external medium. Finally, previous conclusions about oil/fibres ratio, polymerization rate and thermal degradation are closely related to our choice of the process parameters. In fact, the composites maturation step, relative to linseed oil polymerization, was achieved during 20 days and precisely at 50 ◦ C in order to avoid any spontaneous combustion phenomena. Furthermore, in our study, the maximal oil/fibres ratio in materials was limited to 40/140 g/g, for the same security reasons. The oil polymerization kinetics could be increased by addition of inorganic catalysts or, as shown previously, by heating at high temperatures. New formulations, containing some higher oil/fibres ratios, could be also considered. Nevertheless, appropriate technical solutions should be found, in order to ensure a sufficiently fast diffusion of the heat generated in the core of those materials during the fabrication process.
Temperature [°C]
4. Conclusion 300
200
100
0 0
60
120
180
240
300
Time [min] Fig. 12. Thermal behaviour and degradation of 40/140 g/g linseed oil/flax short fibres composites obtained after 20 days of maturation. Oven temperature settings: 2 h at 180 ◦ C.
oxidation/polymerization reactions leading to the combustion of the lignocellulosic fibres. The impact of advanced oil polymerization on thermal behaviour was particularly significant for 20/140 g/g oil/fibres formulations (Fig. 13). These samples resisted henceforth at 100 ◦ C, 180 ◦ C and even 195 ◦ C. Furthermore, the temperature profile indi-
20/140, 20 days T° sample [C°]
T° oven [C°]
Flax short fibres represent a particularly interesting renewable and biodegradable raw material for insulation. Such approach may be economically viable, as the valorization of these low-cost by-products directly concerns the whole flax processing industry. However, several technical improvements should be achieved in order to propose flax short fibres composites as an alternative to some mineral or synthetic insulating materials. Thus, in this study, the high water affinity of the reference short fibres composites was significantly reduced by a linseed oil based hydrophobic treatment. Particularly, the relationships between oil/fibres ratio, polymerization rate and water absorption kinetics were established. In fact, a hydrophobic enhancement was only observed for the advanced polymerization state of the oil phase. Furthermore, polymerization of linseed oil inside the lignocellulosic matrix improved the mechanical properties of biocomposites. But also, an appropriate choice of process parameters, such as linseed oil drying temperature and duration, ensured process security and any spontaneous combustion phenomena were avoided. Further research should be achieved in order to entirely adapt oil/fibres composites to various industrial applications. Biodegradation characteristics and fire-retardant treatments require a particular attention. Finally, this subject also opens a variety of more fundamental research themes, concerning for example oil – fibres or fibres – shives interactions in the lignocellulosic matrix.
400
Temperature [°C]
Acknowledgements 300
The authors wish to acknowledge the Regional Council and the Prefecture of Picardy, AGROTRANSFERT, ITL (Flax Technological Institute) and CALIRA (Abbeville Agricultural Cooperative) for their financial support and technological contribution.
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References
0 0
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Time [min] Fig. 13. Thermal behaviour and degradation of 20/140 g/g linseed oil/flax short fibres composites obtained after 20 days of maturation. Oven temperature settings: 2 h at 180 ◦ C, 1 h at 195 ◦ C and 1 h at 210 ◦ C.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Biocomposites based on flax short fibres and linseed oil J. Lazko ∗ , B. Dupré, R.M. Dheilly, M. Quéneudec Laboratoire des Technologies Innovantes (EA 3899), Université de Picardie Jules Verne, CODEM Picardie, 41, rue Paul Claudel, 80480 Dury, France
a r t i c l e
i n f o
Article history: Received 2 September 2010 Received in revised form 18 November 2010 Accepted 20 November 2010 Available online 10 December 2010 Keywords: Biocomposites Flax short fibres Linseed oil Hydrophobicity Polymerization
a b s t r a c t Insulating materials based on flax short fibres were prepared and their functional properties enhanced using linseed oil. In order to improve biocomposites hydrophobicity, the linseed oil was added to the initial formulation, mixed with the fibres and finally dried after the moulding process. Thus, the average water absorption of 10–40/140 g/g oil/fibres samples was reduced up to 10 times during the first hour of immersion, compared to the reference oil-free materials. Moreover, the linseed oil polymerization inside the lignocellulosic matrix, which occurred after 20 days of drying at 50 ◦ C, also improved mechanical and thermal behaviour of biocomposites. Spontaneous combustion phenomena related to the exothermic oxidation–polymerization of linseed oil were described as well. Therefore, the process parameters such as oil/fibres ratio, drying time and temperature, were optimized to ensure the safety of the process and to avoid self ignition of the lignocellulosic fraction at temperatures below 200 ◦ C. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In flax (Linum usitatissimum) long high-quality textile fibres are considered as the noble fraction, and the rest, not <80% of the plant mass, are low-cost by-products. These short fibres, also called “tow”, containing a large proportion of shives, are nevertheless potential lignocellulosic raw materials, perfectly corresponding to actual demand on renewable, recyclable and biodegradable resources. The increasing interest for flax fibres and shives in thermal insulation is particularly well described in literature, and a large number of flax based composites and nonwovens already have their share in transportation and building markets (Satyanarayana et al., 2009; Bos et al., 2006; Flambard et al., 2005; Klamer et al., 2004; Kymäläinen and Sjöberg, 2008; Schartel et al., 2003; AamrDaya et al., 2008). However, these new biocomposites generally contain an important synthetic polymer fraction (10–40%), such as polypropylene for example. Other polyesters and polyamides from petrochemical resources are used to enhance the mechanical properties of the materials, but they also have a quite negative impact on the global biodegradability and recycling aspects. Natural insulation materials from flax short fibres only, without any mineral or synthetic additive, were developed and patented in our laboratory (Queneudec t’Kint et al., 2005). The scale of their thermal and mechanical performances should be suitable for numerous industrial applications. The main disadvantage of these
∗ Corresponding author. Tel.: +32 0 68 27 47 80. E-mail address: [email protected] (J. Lazko). 0926-6690/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2010.11.015
materials is nevertheless, their high hydrophilic character, as it is the case for almost all composites based on vegetal lignocellulosic raw materials. Surface treatments or homogeneous chemical modifications (esterification, acetilation, acylation) are commonly applied to various cellulosic substrates in order to enhance their hydrophobicity (Heinze and Liebert, 2001; Pasquini et al., 2006; Vaca-Garcia and Borredon, 1999). Grafting of aliphatic chains was performed onto flax fibres and shives as well (Hill et al., 1998; Joffe et al., 2003; Sain and Fortier, 2002; Tserki et al., 2005). Results indicate a significant improvement of moisture resistance, mechanical properties and microbial degradation resistance. However, in most chemical modification procedures concerning lignocellulosic substrate, the use of harmful organic reagents, solvents and catalysts, obviously contrasts with the very intrinsic biodegradable and renewable properties of these natural raw materials. The hydrophobisation of biocomposites described in this article was realized with full respect of green chemistry approach. In fact, the hydrophobic treatment was not based on chemical modifications but on formulation modifications: a hydrophobic agent, linseed oil, was introduced into the reaction medium during the process. Thus, the whole biocomposite preparation was realized in aqueous medium, without any use of potentially toxic organic solvent (acetone, toluene, methanol) or acyl chloride and anhydride. The particularity of linseed oil – the latter being commonly used for industrial applications from paints to linoleum – is its high drying ability. In fact, this oil consists of mixed glycerides composed of about 50% linolenic acid, 20% oleic acid, 15% linoleic acid and also containing palmitic and stearic acid (Güner et al., 2006; Sharma and Kundu, 2006). The drying of the linseed oil is
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Table 1 Basic properties and composition of flax short fibres – linseed oil composites. Initial reaction medium
Final composite
Linseed oil/short fibres [g/g]
Mass [g]
0/140 5/140 10/140 20/140 40/140
126.3 132.1 136.6 145.6 160.3
± ± ± ± ±
Composition 3
Density [g/cm ] 0.6 1.4 1.4 0.6 3.0
a polymerization–oxidation process (Abraham, 1996; Meneghetti et al., 1998; Oyman et al., 2005; Stenberg et al., 2005). Both linoleate and linolenate rapidly oxidize, since their unsaturated bonds are close the one to the other, thus facilitating the abstraction of the allylic hydrogen atoms. The following cross-linking step results in formation of three possible polymeric structures: C1 –O–O–C2 , C1 –O–C2 and C1 –C2 . Moreover, the heat liberated during the oxidation–polymerization steps improves even more the reaction kinetics, eventually leading to spontaneous combustion of inflammable materials in contact or in formulation with linseed oil. The present study focuses on the flax short fibres biocomposite process, characterization and on the improvement of their functional properties. The main subject concerns, above all, the enhancement of the composites hydrophobicity using linseed oil and the optimization of process parameters such as oil/fibre ratio, polymerization time and drying temperature. Water absorption kinetics corresponding to different formulations and operational conditions has been discussed. A particular attention was given to spontaneous combustion phenomena, induced by the presence of the drying linseed oil in the core of an insulating and inflammable lignocellulosic matrix. Thus, this study also has a security approach and deals with biocomposites thermal behaviour during the process and their stability at high temperatures. 2. Experimental 2.1. Materials Raw flax short fibres, containing in average 65% of fibres and 35% of shives, were provided by agricultural cooperative of CALIRA, Martainneville, France. Commercial linseed oil was purchased from ONYX-Veolia Environment, Paris, France. 2.2. Biocomposites preparation The flax biocomposites process, also used in this article as the reference process, has been protected and already described in the corresponding patent (Queneudec t’Kint et al., 2005). On the whole, it consists of 5 main steps. Raw short fibres were firstly milled through RETSCH SM 100 (Haan, Germany) cutting mill. 140 g of refined short fibres were then mixed with distilled water in a 5 l CONTROLAB Perrier mixer (Saint Ouen, France). The reaction medium was next treated in Panasonic microwave oven (Plaine Saint-Denis, France). The moulding step allowed preparation of three 13 cm × 4 cm × 4 cm biocomposite test samples, which were finally placed in BINDER drying oven (Tuttlingen, Germany) at 50 ◦ C and dried during 5–6 days to a constant mass.
0.220 0.230 0.238 0.253 0.275
± ± ± ± ±
0.008 0.006 0.002 0.004 0.007
Short fibres [%]
Oil [%]
100 95.8 ± 0.8 92.5 ± 0.6 86.4 ± 0.3 76.8 ± 2.2
0 4.2 ± 0.8 7.5 ± 0.6 13.6 ± 0.3 23.2 ± 2.2
The second reference process modification concerns the drying step. Oil treated samples were placed at 50 ◦ C and the water evaporation occurred in 5–6 days, as it did with non-treated samples. Nevertheless, the composites were kept at 50 ◦ C for at least 20 days to also allow the polymerization of the linseed oil. Therefore, in this particular case of hydrophobic treatment, the prolonged drying step eventually corresponds to composite “maturation”, a term we are now introducing and which will be later related to two main aims: water evaporation and oil polymerization. 2.4. Water absorption Water liquid absorption was performed according to ISO 62 and ASTM D570 methods. 4 cm × 4 cm × 4 cm samples were immersed in distilled water at room temperature, then removed from the tank after 3, 6, 12, 21, 30, 60, 150 and 300 min, dried on blotting paper and weighted. The samples were placed back into water after each measurement. Finally, the water uptake was calculated as the mass difference, and expressed as ratio water/initial fibres (g/g). 2.5. Thermal behaviour Biocomposites 4 cm × 4 cm × 4 cm samples, containing K type thermocouple in their core, were placed in the Nabertherm L3/11 oven (Lilienthal, Germany) and heated at 100 ◦ C, 180 ◦ C, 195 ◦ C and 210 ◦ C for respectively 240, 120, 60 and 60 min. The thermocouple of the same characteristics was placed in the oven near the sample and used to indicate the reference temperature. 2.6. Mechanical properties Maximal compression stress was measured on the PROVITEQ hydraulic testing system (Nozay, France), using 4 cm × 4 cm × 4 cm samples and 200 N/s charge speed. Samples were stored at 50 ◦ C during 48 h before the test, and the measurements were realized in triplicate. 2.7. Microscopy Scanning electron micrographs allowed the visualization of the biocomposites morphology. The micrographs have been generated with FEG XL 30 Phillips scanning electron microscope. In order to facilitate observation, portions of samples were at first dried, and then covered with a thin layer of vacuum sputtered platinum, which acted as a conductor. 3. Results and discussion
2.3. Hydrophobic treatment
3.1. Biocomposites fundamental characteristics and composition
The hydrophobic treatment of flax biocomposites was based on linseed oil which was added in the reaction medium during the mixing step. Thus, 5, 10, 20 and 40 g of linseed oil were introduced in the previous fibres/water mixture.
At first, the high reproducibility of composite’s dimensional characteristics was noticed. Each formulation, containing 0, 5, 10, 20 and 40 g of linseed oil per 140 g of short fibres, was realized in triplicate. Thus, for the whole set of 45 13 cm × 4 cm × 4 cm test
0.29
20
0.28
18
0.27
16
Mass increase / initial oil [%]
Density [g/cm3]
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0.26 0.25 0.24
R 2 = 0.9938 0.23 0.22 0.21 0.00
0.05
0.10
0.15
0.20
0.25
0.30
319
14 12
10 / 140
10
20 / 140 40 / 140
8 6 4 2
0.35
Oil / Short fibres ratio in final composite [g/g]
0 0
Fig. 1. Density as function of biocomposites final composition (oil/fibres ratio).
5
10
15
20
25
Time [days] Fig. 3. Mass evolution of composites containing linseed oil during maturation (oil polymerization) step at 50 ◦ C. Initial oil/fibres g/g ratio: () 10/140, () 20/140 and () 40/140.
900 800
Water uptake [g/cm 3 ]
samples, the mean volume was 192.1 cm3 and the standard deviation was limited to 4.1 cm3 . As the volume remains the same, the mass and consequently the density of biocomposites proportionally increase with the quantity of introduced oil (Table 1). The mean density of the reference composites, based on short fibres only, is around 0.220 g/cm3 , while a density of 0.275 g/cm3 was obtained for 40/140 g/g oil/fibres formulation. According to the results obtained with the reference composite 0/140 g/g, the total mass loss of hydro-soluble fraction during the process represents 10 ± 1% of initially introduced short fibres. The estimation of the final composition of composites containing both fibres and oil is based on two hypotheses. Firstly, the hydrosolubles loss of 10% may remain constant for all formulations. In fact, the inert oil fraction should not modify the solubility of some highly hydroscopic compounds of flax. Secondly, the loss of linseed oil was considered insignificant during the process. These two hypotheses were precisely confirmed by the high linearity between estimated oil/short fibres composition and measured densities (Fig. 1). According to the previous results, the concentration of lignocellulosic fraction (g of flax short fibres per cm3 of biocomposite) remains the same for each sample, independently of the quantity of introduced oil. This remark is particularly important for the following discussion, concerning the influence of composite formulation on water absorption kinetics.
700 0/140 Ref
600
5/140 5d 10/140 5d
500
20/140 5d
400
5/140 20d 10/140 20d
300
20/140 20d
200 100 0 0
60
120
180
240
300
360
Time [min] Fig. 4. Water absorption kinetics of linseed oil/flax short fibres biocomposites as function of oil/fibres initial ratio (0–5–10–20/140 g/g) and maturation time (5 and 20 days). Water uptake expressed as mass increase per unit of sample volume [g/cm3 ].
900 800
80 / 140 g/g Oil / Flax ratio
Water uptake [g/cm 3 ]
Mass increase / initial oil [%]
14 12 10 8 6
700 600
0/140 Ref 40/140 10d
500
40/140 15d
400
40/140 20d 40/140 30d
300 200
4
100
2
0 0
0 0
10
20
30
40
60
120
180
240
300
360
Time [min]
Time [days] Fig. 2. Mass increase of linseed oil/flax short fibres mixture, 80/140 g/g, during the prolonged drying at 50 ◦ C.
Fig. 5. Water absorption kinetics of 40/140 g/g linseed oil/flax short fibres biocomposites measured after 10, 15, 20 and 30 days of maturation at 50 ◦ C.
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Fig. 6. Scanning electron micrographs of shives and fibres in the reference (a and b) and in the treated (c and d) 20/140 g/g linseed oil/flax short fibres biocomposites.
3.2. Linseed oil drying and polymerization The linseed oil drying/polymerization are mainly characterized by oxygen fixation on unsaturated double bounds. Thus, in this study, the drying reaction advancement corresponds to the mass evolution of the oil phase during time. The drying of linseed oil has extremely slow kinetics when the substrate is a continuous compact liquid phase. For example, 5 g oil sample was placed in a 25 ml beaker, and the mass gain measured after 30 days of heating at 50 ◦ C remained inferior to 1.5%. Moreover, the solidification of this sample was only partial and situated on the liquid/air interface. On the other hand, polymerization kinetics was significantly improved when the sample was a dispersed oil phase. Flax short fibres and linseed oil 140/80 g/g were just mixed together, without any moulding, and heated at 50 ◦ C during 30 days. The measures of the mixture mass evolution are represented in Fig. 2. Although the mixture both contains linseed oil and short flax fibres, the mass gain should only be attributed to the oil fraction. A systematic mass increase was observed, especially between the 10th and the 14th day of drying. Finally, the sample mass was sta-
ble after 20 days of drying and the maxima obtained were around 10% of the initial oil mass. In fact, the absorption of linseed oil by flax short fibres would significantly increase the exchange oil/air interface and facilitate oxygen fixation by the unsaturated bonds of linoleate and linolenate. Consequently, the mass gains were 6–10 times superior to values obtained during the drying of a continuous liquid oil phase, under the same operational conditions. Identical observations were made during the prolonged drying at 50 ◦ C of linseed oil/short fibres materials (Fig. 3). All samples presented similar mass evolution during time, independently of oil/fibre ratio. The initial mass loss was due to the water evaporation during the first days of drying. Then, a gradual mass gain was observed and maxima between 10% and 15% were obtained after 15 days of heating. Finally, the masses slightly decreased and stabilized at around 10% – on average – after 20 days. In conclusion, according to the results concerning mass evolution of oil/fibre materials during the drying at 50 ◦ C, two characteristic states could be defined. The first initial state of material maturation, obtained after 5 days, is only relative to water evaporation. The lignocellulosic fraction of these materials is dry but oil polymerization/oxidation phenomena are still not initiated.
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3.3. Water absorption kinetics Water absorption kinetics of the reference composites only based on short fibres, and of composites containing 5, 10 and 20 g of linseed oil per 140 g of short fibres, are shown in Fig. 4. These materials were characterized at the initial and at the advanced maturation state, obtained after 5 and 20 days of drying at 50 ◦ C, respectively. After 5 days of drying, water absorption kinetics of all oil based samples are of the same rate as those of the untreated reference materials. In fact, as soon as the samples are immersed, water penetration and the water/air exchange in the lignocellulosic matrix are extremely fast. For example, the 5/140 and 20/140 g/g oil/fibres samples absorbed 700 mg of water per cm3 in only 3 min, which represents more than 250% of their initial mass. Despite minor differences at the beginning of the analysis, almost the same values were noticed for all ratios after 30 min of immersion. Thus, mass gains between 630 and 770 mg/cm3 , and between 710 and 790 mg/cm3 were measured after 60 and 300 min, respectively. A significant decrease of water absorption kinetics was achieved for mature oil based materials dried during 20 days at 50 ◦ C. The water uptake gradually decreases for the 0/140, 5/140 and 10/140 g/g oil/fibres samples. For example, an improvement of composite hydrophobicity is only partial for 5/140 sample, since the quantity of the oil based polymer might be insufficient to successfully waterproof the lignocellulosic matrix. However, the absorption rate remains stable for ratios above 10/140 g/g and average mass gains between 70 and 80 mg/cm3 and between 100 and 130 mg/cm3 were measured for all samples after 60 and 300 min, respectively. Linseed oil polymerization impact on composite hydrophobic properties was also observed for 40/140 g/g oil/fibres samples (Fig. 5). Water absorption kinetics measures were performed after 10, 15, 20 and 30 days of drying at 50 ◦ C. The most important absorption changes occurred just between the 10th and 15th day. For example, water absorption rate decreased up to 10 times during the first hour of immersion. After 15 days, the composite water behaviour did not evolve any more. Average water absorption of 65, 85 and 145 mg/cm3 was measured after 30, 60 and 300 min respectively, for all 15, 20 and 30 days dried samples. These observations are in agreement with mass evolution data, described previously. During the first 5–10 days of drying, the sample mass remains constant, the linseed oil polymerization is incomplete, so the oil based materials have almost the same water behaviour as the reference composite. The next 10–15 days period is characterized by a significant mass increase, an oil polymerization and consequently a water resistance enhancement. Moreover, after 20 days, the composite mass remains stable as well as the water absorption kinetics. Finally, the hydrophobicity improvement would be closely related to the morphology of the linseed oil treated composites. According to the scanning electron micrographs, the porosity of the lignocellulosic matrix decreases with the quantity of the introduced oil (Fig. 6). Linseed oil largely covers the surface of shives and fibres, and the polymer aggregates are also present in the free space between elemental particles. Compared with the reference materials, the flax hydrophilic fractions coated with the linseed oil would be less accessible, consequently reducing the capillar-
0.7
Maximal Compression Stress [MPa]
The second advanced state of maturation would be characteristic for oil/flax composites after 20 days of drying at 50 ◦ C and to a high degree of linseed oil polymerization. In the following discussion, water absorption kinetics, mechanical and thermal behaviour of oil/flax short fibres materials will be compared at initial and at advanced degree of maturation.
321
0.6
0.5
0.4
0.3 0/140 (Ref)
5/140
10/140
20/140
40/140
Initial ratio Oil / Fibres [g/g]
Fig. 7. Maximal compression stress [MPa] of 0, 5, 10, 20 and 40/140 g/g linseed oil/flax short fibres biocomposites. Measures were realized on 4 cm × 4 cm × 4 cm samples and after 20 days of maturation at 50 ◦ C.
ity phenomena and water diffusion kinetics in the lignocellulosic matrix. 3.4. Mechanical properties The presence of polymerized linseed oil in the flax short fibres matrix not only had an impact on water absorption kinetics, but also on mechanical behaviour of biocomposites. Thus, the compression resistance performances of materials containing 0, 5, 10, 20 and 40 g of linseed oil per 140 g of short fibres were compared (Fig. 7). The maximal compression stress of the reference short fibres material is around 0.35 MPa. An important increase of the mechanical resistance, up to 60%, was observed for all oil based composites, even for relatively low oil/fibres ratios. For example, an average maximal compression stress of 0.58 MPa characterizes the set of 10/140, 20/140 and 40/140 g/g samples. Obviously, the optimization of mechanical behaviour of flax short fibres biocomposites should not be considered as the priority of our study. Nevertheless, these results clearly justify our hydrophobic linseed oil based treatment which not only preserves, but also improves the mechanical properties of materials. Furthermore, the whole process elaboration and conception should be reconsidered in the presence of the oil phase. In fact, the improvement of the mechanical behaviour was only noticed for those samples in which the linseed oil polymerization occurred after the final material moulding. But also, the inversion of the moulding/polymerization steps had a quite negative impact on the mechanical resistance. For example, some composite elaboration tests were made with already treated short fibres, previously described (Fig. 2). The 80/140 g/g oil/fibres mixture was dried at 50 ◦ C during 30 days in order to achieve an advanced polymerization rate. The hydrophobically treated fibres were then mixed with 1000 ml of water, as for the reference process, and finally moulded. Such materials were particularly friable, their maximal compression stress was below 0.15 MPa, and they were even unsuitable for any following manipulation or for water absorption characterization. 3.5. Thermal stability The first results concern thermal behaviour of raw materials and reference composite. All raw materials and biocomposites based on short fibres perfectly resist to the range of temperatures below 195 ◦ C (Fig. 8). The first sights of degradation and an important
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0/140 (Reference composites) T° oven [C°]
40/140, 5 days T° oven [C°] T° sample [C°]
T° sample [C°]
400
Temperature [°C]
Temperature [°C]
400
300
200
300
200
100
100 0
0 0
60
120
180
240
0
300
60
120
Time [min]
T° sample [C°]
Temperature [°C]
400
300
200
100
0 120
240
360
480
300
plateau. For high oil/fibres ratios (20/140 and 40/140 g/g) the degradation characteristics are quite different and they could be directly related to the polymerization rate of the oil phase. Concerning 40/140 g/g oil/fibres composites at initial state of oil polymerization (5 days of drying at 50 ◦ C), thermal degradation occurred already at 100 ◦ C (Fig. 10). During the first 3 h of heating at 100 ◦ C, temperature in the core of the sample progressively increased, then exceeded the oven temperature and finally continued to increase exponentially, thus indicating the beginning of the sample consummation. The thermal degradation profiles for 20/140 g/g samples at 100 ◦ C were quite the same, except that the combustion occurred after 4–5 h. In fact, the equilibrium between sample and oven could not be reached under these particular operational conditions. The external heating at 100 ◦ C significantly increases the linseed oil oxidation/polymerization kinetics and finally initiates the spontaneous combustion of the whole material. More specifically, the spontaneous combustion phenomena were favoured by both the insulating and the combustible nature of the lignocellulosic flax materials. Thus, at the first stages, the heat released by oil polymerization was accumulated in the core of insulating composites. Then followed the combustion of the short fibres fraction, as soon as the temperature in the core of the samples reached 210 ◦ C. Similar autoignition phenomena, which occurred even at room temperature, were already reported by Abraham (1996). According to the author, maximum reaction rates were measured when the amount of cellulosic substrate by weight equalled that of the oil. Measures of 20 days dried samples, characterized by an advanced linseed oil polymerization rate, indicated a significant improvement of thermal behaviour. Thus the 40/140 g/g formulations resisted at 100 ◦ C (Fig. 11). However, even after 20 days of drying at 50 ◦ C, spontaneous combustion phenomena occurred at 180 ◦ C for high oil concentration samples (Fig. 12). In fact, a fraction of the linseed oil phase might still remain suitable for exothermic
Linseed Oil
0
240
Fig. 10. Thermal behaviour and degradation of 40/140 g/g linseed oil/flax short fibres composites obtained after only 5 days of maturation. Oven temperature settings: 4 h at 100 ◦ C.
Fig. 8. Thermal behaviour and degradation of the reference flax short fibres based composites. Oven temperature settings: 2 h at 180 ◦ C, 1 h at 195 ◦ C and 1 h at 210 ◦ C.
T° oven [C°]
180
Time [min]
600
Time [min] Fig. 9. Thermal behaviour of the raw linseed oil. Oven temperature settings: 1 h at 180 ◦ C, 195 ◦ C, 210 ◦ C, 225 ◦ C, 240 ◦ C, 255 ◦ C and 270 ◦ C.
smoke release were noticed at 210 ◦ C plateau. During the combustion of short fibre samples, the temperature measured in the core of materials rapidly exceeded the oven temperature and reached 400 ◦ C in <40 min. On the other hand, the linseed oil well resists to all applied temperatures up to 270 ◦ C (Fig. 9). At each heating step, the sample temperature progressively converged to the external temperature. Nevertheless, some other phenomena, such as colour change and solidification of the initially liquid oil, indicated an advanced polymerization degree. The following results concern oil/flax formulations and the influence of the hydrophobic treatment on biocomposites thermal behaviour (Table 2). Biocomposites with low oil/fibres ratios (5/140 and 10/140 g/g), dried during 5 or 20 days, showed the same thermal stability as the reference materials and their degradation occurred at 210 ◦ C
Table 2 Thermal stability of biocomposites at 50 ◦ C, 100 ◦ C, 180 ◦ C, 195 ◦ C and 210 ◦ C, as function of linseed oil/flax short fibres initial mass ratio (0–40/140 g/g) and maturation/polymerization time (5 and 20 days). Initial ratio Linseed oil/short fibres [g/g] 0/140 5/140 10/140 20/140 40/140
Maturation 5 days ◦
Stability [ C] 195 195 195 50 50
Maturation 20 days ◦
Degradation [ C]
Stability [◦ C]
Degradation [◦ C]
210 210 210 100 100
195 195 195 195 100
210 210 210 210 180
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40/140, 20 days T° sample [C°]
T° oven [C°]
Temperature [°C]
400
300
200
100
0 0
120
240
360
480
Time [min] Fig. 11. Thermal behaviour of 40/140 g/g linseed oil/flax short fibres composites obtained after 20 days of maturation. Oven temperature settings: 7 h at 100 ◦ C.
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cated a partial internal heat release, probably due to the oxidation of the oil phase, during the first hour of heating at 180 ◦ C. The maximal sample temperature was about 20 ◦ C higher than the oven temperature. However, compared to 40/140 g/g samples, the quantity of remaining unsaturated linseed oil for 20/140 g/g formulation was insufficient to induce spontaneous combustion and the internal heat progressively diffused from the core to the external medium. Finally, previous conclusions about oil/fibres ratio, polymerization rate and thermal degradation are closely related to our choice of the process parameters. In fact, the composites maturation step, relative to linseed oil polymerization, was achieved during 20 days and precisely at 50 ◦ C in order to avoid any spontaneous combustion phenomena. Furthermore, in our study, the maximal oil/fibres ratio in materials was limited to 40/140 g/g, for the same security reasons. The oil polymerization kinetics could be increased by addition of inorganic catalysts or, as shown previously, by heating at high temperatures. New formulations, containing some higher oil/fibres ratios, could be also considered. Nevertheless, appropriate technical solutions should be found, in order to ensure a sufficiently fast diffusion of the heat generated in the core of those materials during the fabrication process.
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4. Conclusion 300
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Time [min] Fig. 12. Thermal behaviour and degradation of 40/140 g/g linseed oil/flax short fibres composites obtained after 20 days of maturation. Oven temperature settings: 2 h at 180 ◦ C.
oxidation/polymerization reactions leading to the combustion of the lignocellulosic fibres. The impact of advanced oil polymerization on thermal behaviour was particularly significant for 20/140 g/g oil/fibres formulations (Fig. 13). These samples resisted henceforth at 100 ◦ C, 180 ◦ C and even 195 ◦ C. Furthermore, the temperature profile indi-
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Flax short fibres represent a particularly interesting renewable and biodegradable raw material for insulation. Such approach may be economically viable, as the valorization of these low-cost by-products directly concerns the whole flax processing industry. However, several technical improvements should be achieved in order to propose flax short fibres composites as an alternative to some mineral or synthetic insulating materials. Thus, in this study, the high water affinity of the reference short fibres composites was significantly reduced by a linseed oil based hydrophobic treatment. Particularly, the relationships between oil/fibres ratio, polymerization rate and water absorption kinetics were established. In fact, a hydrophobic enhancement was only observed for the advanced polymerization state of the oil phase. Furthermore, polymerization of linseed oil inside the lignocellulosic matrix improved the mechanical properties of biocomposites. But also, an appropriate choice of process parameters, such as linseed oil drying temperature and duration, ensured process security and any spontaneous combustion phenomena were avoided. Further research should be achieved in order to entirely adapt oil/fibres composites to various industrial applications. Biodegradation characteristics and fire-retardant treatments require a particular attention. Finally, this subject also opens a variety of more fundamental research themes, concerning for example oil – fibres or fibres – shives interactions in the lignocellulosic matrix.
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The authors wish to acknowledge the Regional Council and the Prefecture of Picardy, AGROTRANSFERT, ITL (Flax Technological Institute) and CALIRA (Abbeville Agricultural Cooperative) for their financial support and technological contribution.
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References
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Time [min] Fig. 13. Thermal behaviour and degradation of 20/140 g/g linseed oil/flax short fibres composites obtained after 20 days of maturation. Oven temperature settings: 2 h at 180 ◦ C, 1 h at 195 ◦ C and 1 h at 210 ◦ C.
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