Polyester-based biocomposites containing wool fibres

Composites: Part A 43 (2012) 1113–1119

Contents lists available at SciVerse ScienceDirect

Composites: Part A journal homepage: www.elsevier.com/locate/compositesa

Polyester-based biocomposites containing wool fibres Lucia Conzatti a,⇑, Francesco Giunco a, Paola Stagnaro a, Massimo Capobianco a,b, Maila Castellano b, Enrico Marsano b,⇑ a b

Istituto per lo Studio delle Macromolecole (ISMAC) – UOS Genova, Consiglio Nazionale delle Ricerche, Via de Marini 6, 16149 Genova, Italy Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31, 16146 Genova, Italy

a r t i c l e

i n f o

Article history: Received 9 November 2011 Received in revised form 1 February 2012 Accepted 13 February 2012 Available online 5 March 2012 Keywords: A. Polymer–matrix composites (PMCs) A. Fibres B. Mechanical properties Biocomposites

a b s t r a c t Biocomposites based on a biodegradable polyester containing different amounts of wool fibres (up to 40 wt.%) were prepared by melt blending in an internal batch mixer. Wool fibres were used as received or pre-treated in order to preserve a high aspect ratio and increase adhesion with polymer matrix. Morphological, thermal, mechanical and dynamic-mechanical properties of the ensuing composites were investigated focusing the attention on fibre length and their distribution as well as on fibre/matrix interaction in order to correlate these aspects with polymer reinforcement. Data from mechanical and dynamic-mechanical analysis were also compared with theoretical models. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Fibre-reinforced polymers have until now been largely applied as materials in many technical applications, where high strength and stiffness are required together with low weight. The good specific (i.e. weight-related) properties are due to the low density of the matrix and to the embedded fibres that provide high values of strength and stiffness. Unfortunately, classic fibre-reinforced polymers often pose considerable problems with respect to their re-use or recycling at the end of their usable lifetime. On the other hand, simple landfill disposal of these materials is becoming increasingly impossible due to problems of environmental sensitivity. Consequently, one of the current challenges is to design materials with structural and functional stability during storage and use, as well as susceptibility to microbial degradation upon disposal with no adverse environmental impact. Biodegradable polymer systems of low mechanical strength are widely used as packaging materials [1]; in order to achieve best performances, maintaining biodegradability, an interesting possibility is to blend these bio-polymers with organic fillers. This class of composites shows some interesting features; one certainly concerns the cost reduction of biodegradable plastics, that is quite reduced since natural organic fillers are extracted from abundant plants or from wastes. The major disadvantage of using organic fibres is that they begin to degrade at temperatures ⇑ Corresponding authors. Tel.: +39 0106475866; fax: +39 0106475880 (L. Conzatti), tel.: +39 0103538727 (E. Marsano). E-mail addresses: [email protected] (L. Conzatti), marsano@chimica. unige.it (E. Marsano). 1359-835X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesa.2012.02.019

concurrent with processing temperatures typically experienced by commodity thermoplastics; indeed, most of the work reported so far deals with non-biodegradable polymers [2], such as polyethylene (PE) [3,4], polypropylene (PP) [5], polystyrene [6], and poly (vinyl chloride) [7,8]. This is mainly because the processing temperature is restricted to temperatures below 200 °C to avoid thermal degradation of the natural fibres. A number of investigations have been made which prove the worth of natural fibres against their synthetic counterparts, such as glass and/or carbon fibres, in reinforcing polymer composites [9,10]. With regard to industrial applications, the most used systems are filled with wood flour or fibres, especially for PP and PE. Some examples of application are door and window framings, furnishings, interior car panels, packaging, scaffolds, light panels, gardening items and, in general, all those applications that do not require particularly high mechanical resistance [11,12]. This also makes it possible to conveniently use, in many cases, recycled polymers in place of virgin resins. Many researchers have investigated the mechanical properties, especially the interfacial performances due to the poor interfacial bonding between the hydrophilic natural fibres, such as sisal, jute, and palm fibres with the hydrophobic polymer matrices [3,4]. The key to successfully enhance mechanical properties of polymers by using fibres of higher modulus or strength is to achieve good polymer/fibre interaction. During melt processing, the molten polymer should adhere to the fibre creating a strong adhesive bond; this means that there must be chemical compatibility between fibre and matrix. In literature very few studies detailing composites made from protein fibres, obtained from agricultural resources, are reported

1114

L. Conzatti et al. / Composites: Part A 43 (2012) 1113–1119

[13–21]. Most of them [13–17] deal with the use of keratin feather fibres as short-fibre reinforcement in composites obtained from various polymer matrices. The keratin feather fibre is obtained from the over four billion pounds of chicken feather waste generated by the USA poultry industry each year [22]. Keratins, which form the main bulk of the horny layer of epidermis and of epidermal appendages, such as hair, nails, claws, scales and feathers, are typically durable, insoluble, and non-reactive toward the natural environment. In particular, wool, which is the most popular natural material among the family of keratins, is a multicomponent fibre and consists of about 170 different protein molecules with a molecular weight distribution from a few thousand up to 100,000 Da [23]. In the textile industries, lots of waste wool fibres and their products induce actions, which lead to the regeneration of wool materials. Despite this, to the best of our knowledge, only a few works deal with polymer composites containing wool in powder [18,19] or pure keratin extracted from wool [20,21]. One of the most important classes of synthetic biodegradable polymers, useful as matrices in composites and available in a variety of types are aliphatic and aromatic polyesters. For instance: polycaprolactone, polyhydroxybutyrate, poly(lactic acid), poly(butylene succinate), poly(butylene succinate-co-adipate), poly(ester amide) and copolyesters based on 1,4-butanediol, adipic and terephthalic acids [24–27]. The main advantage of these polymers is the wide range of mechanical and physical properties that are comparable to synthetic polymers, such as LDPE, HDPE and PP. In this work biocomposites based on a biodegradable polyester containing different amounts of wool fibres (up to 40 wt.%) were prepared by melt blending. Wool fibres were used as received or pre-treated with poly(vinyl alcohol) in order to preserve a high aspect ratio and increase adhesion with polymer matrix. Morphological, thermal, mechanical and dynamic-mechanical properties of the ensuing composites were investigated focusing the attention on fibre length and their distribution, fibre/matrix interaction, and consequent polymer reinforcement. Data obtained from mechanical and dynamic-mechanical investigations were also compared with theoretical models in order to obtain more information on arrangement and dimension of fibres as well as degree of adhesion between fibre and polyester matrix. 2. Experimental 2.1. Materials Fibres of wool (WF) with 20 lm diameter were mixed with a non-commercial biodegradable polyester (BPE), used as the matrix. WF were kindly supplied by CNR-ISMAC UOS Biella. The BPE matrix (IP1669), kindly supplied by Novamont, is a biodegradable thermoplastic material based on a biodegradable copolyester (Novamont’s proprietary technology) made of diacids and a glycol obtained from renewable (from agriculture) and non-renewable resources. Before mixing, WF were cut by scissors to a length of about 2 cm without powder production, and, in order to remove fatty acids and waxes naturally present on their surface (<0.4 wt.%), they were extracted with acetone for 2 h and then dried under vacuum at 105 °C for 4 h. WF were also pre-treated with poly(vinyl alcohol) (PVA) with M n ¼ 22; 000; in order to obtain WF wet with 3 wt.% PVA (WFPVA), the fibres were soaked with an aqueous solution of PVA (obtained by mild warming for 2 h) and dried under vacuum at 60 °C for 24 h. 2.2. Composite preparation BPE-based composites containing 20, 30 and 40 wt.% of WF or 20 and 30 wt.% of WFPVA were prepared into a W50 EHT Plasti

CorderÒ (Brabender) internal batch mixer by melt blending at 160 °C, varying mixing time and rotor speed. After mixing, all the samples were moulded with a P 200E semi-automatic laboratory press (Collin) to form sheets of different thickness necessary for the successive characterizations. 2.3. Characterizations Wool fibre dimensions were measured by using a Polyval Pol (Reichert) optical microscope equipped with a CCD camera. Fibre length in the composites was evaluated by diluting the BPE matrix in CH2Cl2 and measuring the length of at least 150 fibres for each sample. Scanning electron microscopy (SEM) observations were conducted with a LEO 1450VP instrument. TGA analyses were carried out with a thermogravimetric analyser TGA7 (Perkin Elmer) at a heating rate of 20 °C/min between 35 and 700 °C under N2 (flow rate 40 mL/min). Tensile tests were performed by an Instron 5565 machine according to the standard methods ASTM D3379-75 for single WF and ASTM D882-09 for BPE and composites. After determining the range of viscoelastic linearity, dynamic-mechanical measurements were carried out with a strain controlled rotational rheometer MCR301 (Anton Paar). The tests were carried out in a torsion mode with a frequency of 6.28 rad/s and an amplitude of deformation of 0.01% by increasing the temperature from 80 to 100 °C at 2 °C/min. 3. Results and discussion 3.1. Reinforcing effect of fibres In order to get the average stress of the fibres in a composite as close as possible to the maximum bearable, the fibres should be longer than the critical length (Lc), defined in the following, because at this value the average fibre stress is only half of the value achieved with continuous fibres. Several techniques, such as breaking strength of pullout [28,29], microbond [30,31] and fragmentation [32–34] test methods, have been developed to try to measure Lc. Among them, the most popular one is the single fragmentation test (SFCL) [32–36], in which samples constituted by single fibres encapsulated in a matrix and shaped as dog bone, are axially loaded in tension. Upon loading the fibre begins to break and the breaking process continues until all fragment lengths fall below the length necessary to transfer, from matrix to fibre, sufficient stress to cause further break of the fibre. This length is the critical length, Lc, and it is correlated to the average length value of the fragments, Lm , by [32]:

Lc ¼

4 Lm 3

ð1Þ

For the composites containing untreated WF and BPE matrix, Lm was evaluated according the SFCL method obtaining Lc = 0.82 mm. No experimental values of Lc were available for treated WF; however, because of the better affinity of these fibres with BPE, it could be expected a shorter Lc than that obtained for untreated WF. 3.2. Fibre breakage analysis One of the important factors for the strengthening of a composite is the average fibre length that remains after processing. Melt blending process can cause high attrition of the fibres depending on fibre content, viscosity of the polymer melt, melt-flow rate, etc. For constant WF amount (30 wt.%) and temperature (160 °C) the role of rotor speed (30, 60 or 90 rpm) and mixing time (8 or 10 min) is reported in Table 1, where number ðLn Þ and weight ðLw Þ average length of fibres and the relative polydispersity index

1115

L. Conzatti et al. / Composites: Part A 43 (2012) 1113–1119

ðLw =Ln Þ, obtained by measuring the length of 150 fibres in samples which underwent the mixing process, are collected. Increasing the rotor speed from 30 up to 90 rpm leads to a decrease of Ln , that varies from 0.26 to 0.08 mm. On the other hand, maintaining constant the rotor speed (60 rpm) a shorter mixing time results into an increase of the average fibre length up to 0.37 mm. However, it is important to note that the composites obtained by mixing at 30 rpm for 10 min or at 60 rpm for 8 min are not completely homogeneous in terms of macroscopic distribution and dispersion of fibres; for this reason, 160 °C, 60 rpm and 10 min were chosen as proper mixing conditions to have a good compromise between fibre length and effective fibre distribution and dispersion. To decrease the breakage of the wool fibres due to the mixing process and, at the same time, to favour their compatibility with BPE matrix, WF were previously treated with a poly(vinyl alcohol) (PVA) solution. PVA can act as coupling agent [37] toward the polyester matrix. Moreover, being more hydrophilic than the matrix, it could remain preferentially adsorbed on the WF, thus possibly positively affecting fragmentation and dispersion of the fibres into the matrix. For each composite prepared in the same chosen mixing conditions, the length of 150 fibres was measured by optical microscopy; the average fibre lengths and the relative polydispersity index are collected in Table 2. It can be observed that the length of both WF and WFPVA decreases with increasing their weight percentage in the composite, even though WFPVA are significantly longer than WF. The breakage of the fibres correlated to their amount in the blends is due to the increase of the shear stresses in the mixing chamber by increasing the amount of fibres. The expected positive effect exerted by the PVA coating on the reduction of fibre fragmentation was thus confirmed. Cumulative curves, as those reported as an example in Fig. 1 for the two samples containing 30 wt.% of WF or WFPVA, were obtained from dimensional analysis of fibres in the composites. From this analysis, the percentage of fibres longer than Lc was calculated and the resultant values are reported in Table 2. The fractured untreated wool fibres are constituted by very short fragments and, with respect to the Lc value of 0.82 mm, practically 100% of the fragments have a length Lc, for samples containing 20 or 30 wt.% of WFPVA, respectively. It is nevertheless clear that, in any case, the majority of the fibres in these composites are shorter than the critical fibre length, this suggests that they probably are not loaded up to their failure stress within the composite. 3.3. Thermal stability Thermal stability of BPE, WF, BPE-WF and BPE-WFPVA composites was investigated by TGA analysis carried out in dynamic mode under nitrogen; the onset decomposition temperature corresponding to 1–2% weight loss (TD), the temperature of maximum rate of weight loss (Tmr), and the char yield at 700 °C were determined and reported in Table 3. Two weight loss stages were observed in the TG curve of WF: the first, between 35 and 150 °C, corresponds to

Table 2 Number average fibre length, Ln , weight average fibre length, Lw , and polydispersity index, Lw =Ln , of fibres in composites after the chosen mixing process. Sample

Ln (mm)

Lw (mm)

Lw =Ln

WF with L > Lc (%)

BPE-WF-20 BPE-WF-30 BPE-WF-40 BPE-WFPVA-20 BPE-WFPVA-30

0.19 0.18 0.13 0.53 0.40

0.29 0.26 0.29 0.68 0.67

1.55 1.44 2.17 1.30 1.68

1.2 0.7 0.3 14.0 12.5

Fig. 1. Cumulative curves of fibre lengths measured by optical microscopy for BPEWF-30 and BPE-WFPVA-30 sample.

the evaporation of adsorbed moisture, and the second, in the range 210–450 °C, is ascribable to the thermal degradation of the wool fibres. On the other hand, BPE starts to degrade above 400 °C with a char residue at 700 °C of about 4 wt.%. All the composites exhibited TG curves with three weight loss steps, which can be attributed to moisture evaporation and to thermal degradation of WF and BPE, respectively. As a consequence, TD of composites reported in Table 3 corresponds to the degradation of wool fibres, which was shifted toward higher temperatures (from 272 to 300 °C) by incorporation into the BPE matrix. By increasing the amount of WF or WFPVA, both TD and Tmr decrease, still maintaining values significantly higher with respect to that of wool. Besides, composites containing WFPVA exhibit a slightly higher thermal stability (TD). Synergistic effects between wool fibres and BPE, that would be expected on the base of an optimal fibre/matrix adhesion, were not observed. As expected, the amount of char yield at 700 °C increases with the fibre amount in the composite. This behaviour could suggest flame resistant properties.

3.4. Morphology – fibre/matrix adhesion The morphology of fracture surfaces of all BPE-composites was investigated by SEM, and micrographs of samples with different

Table 3 Thermal parameters obtained by TGA analysis under N2 of the BPE-composites. Table 1 Number average fibre length, Ln , weight average fibre length, Lw and polydispersity index, Lw =Ln , of fibres in composites after different mixing conditions. Experimental conditions (°C/rpm/min)

Ln (mm)

Lw (mm)

Lw =Ln

160/30/10 160/60/10 160/90/10 160/60/8

0.26 0.18 0.08 0.37

0.52 0.26 0.14 0.67

1.98 1.44 1.79 1.80

Sample

TD (°C)

Tmr (°C)

Char yield at 700 °C (wt.%)

WF BPE BPE-WF-20 BPE-WF-30 BPE-WF-40 BPE-WFPVA-20 BPE-WFPVA-30

272 413 297 293 290 302 297

376 438 434 430 422 431 428

20 4 10 15 17 10 12

1116

L. Conzatti et al. / Composites: Part A 43 (2012) 1113–1119

amounts of WF are collected in Fig. 2. Independently of their content, the WF appeared well separated from each other and homogeneously distributed within the BPE matrix. A certain degree of fibre/BPE adhesion was found, as the number of broken fibres seems to be higher than that of pulled out fibres. The surface treatment of WF with PVA improves the WF/BPE interaction; as shown in the images of Fig. 3, a higher degree of adhesion between WFPVA and BPE was found. Indeed, at high magnifications (Fig. 3b) a higher number of fibres not pulled out but broken are observed inside the holes.

3.5. Mechanical properties The tensile properties of the composites were evaluated to investigate the reinforcement effect of untreated and treated wool

Fig. 3. SEM micrographs of BPE-WFPVA-30 at different magnifications.

Fig. 2. SEM micrographs of BPE-composites containing 20 (a), 30 (b) and 40 wt.% (c) of WF.

fibres. The obtained values of elastic modulus (E), yield (ry) and break (rb) stresses, elongation at yield (ey) and at break (eb), as well as the increase of E of the composites with respect to that of BPE matrix (DE) are listed in Table 4. Fig. 4 shows E, ry and rb of BPE and BPE/WF composites as a function of wool fibre content (Fig. 4a) and fibre treatment (Fig. 4b). Both ry and E of BPE-composites containing untreated and PVA-treated fibres are higher than those of BPE matrix and E increases with the wool content; however, rb of the composites is lower than that of BPE. Indeed, the tensile strength at break generally depends on the weakest part of the composites, that is the interfacial region between fibre and matrix. As expected, the pre-treatment of fibres with PVA is beneficial on the mechanical properties of the composites (Fig. 4b). In particular, it is interesting to note that the modulus of BPE-WFPVA-30 is significantly higher than that of BPE-WF-30 (437 respect to 338 MPa), and both strengths slightly increase (6.0–6.9 MPa for ry and 6.8–7.9 MPa for rb). This can be correlated to the better wetting of WFPVA by BPE, that leads to improved adhesion between the wool fibres and the matrix, as also testified by SEM analysis. Indeed, some interaction, such as Van der Waals forces between the PVA chains on the wool surface and the BPE matrix, can justify this finding. Moreover, the PVA treatment of fibres leads to a significant improvement in the percentage of fibres longer than Lc. Both these aspects make it possible the stress transfer from the weaker plastic matrix to the stronger wool fibre during loading, thereby improving the strength at break of the composites. However, it is worth noting that, as already reported in the Paragraph 3.2, the length of the wool fibres in the composites was mostly shorter than the critical length (Table 2), in this way limiting the effective reinforcing action of the fibres on the matrix. Lavengood and Goettler [38] related the value of the modulus of composites containing discontinuous fibres to the corresponding oriented moduli according to Eqs. (2) and (3), which arises as the

1117

L. Conzatti et al. / Composites: Part A 43 (2012) 1113–1119 Table 4 Mechanical properties of BPE composites containing untreated or treated WF.

a

Sample

E (MPa)

ey (%)

ry (MPa)

eb (%)

rb (MPa)

DEa (%)

BPE BPE-WF-20 BPE-WF-30 BPE-WF-40 BPE-WFPVA-20 BPE-WFPVA-30

88.4 ± 1.0 290.9 ± 7.0 338.8 ± 6.4 576.4 ± 10.1 333.0 ± 12.4 437.6 ± 5.7

7.5 ± 1.3 3.3 ± 0.4 2.7 ± 0.5 2.2 ± 0.4 2.8 ± 0.2 2.4 ± 0.3

5.2 ± 0.4 6.2 ± 0.4 6.0 ± 0.3 7.3 ± 0.5 6.8 ± 0.3 6.9 ± 0.4

654 ± 56 32.8 ± 3.6 22.4 ± 2.9 3.0 ± 1.9 21 ± 3.1 9.0 ± 1.7

14.3 ± 1.8 8.3 ± 0.5 6.8 ± 0.5 7.4 ± 0.8 9.3 ± 0.5 7.9 ± 0.4

– 329 383 652 377 495

Increase of the Young modulus of the composites with respect that of BPE matrix.

gL ¼

Ef 1 Em Ef þ 2 dlf Em f

Ef

gT ¼ EEmf Em

Fig. 4. Young modulus, ry and rb for BPE-WF composites; effect of amount (a) and treatment (b) of WF.

1 þ2

ð6Þ

ð7Þ

where Ef is the elastic modulus of the fibre. /f was calculated by using 1.18 and 1.32 g/mL as densities of BPE and WF, respectively. In Fig. 5 the experimental moduli obtained for all the samples, that differ for wool content, fibre length and surface treatment of the fibres are reported. The experimental results are compared with the theoretical predictions obtained by applying Eq. (3) (grey plane in Fig. 5), that is valid for a randomly 2-D orientation inplane of the fibres. Supposing a randomly 3-D orientation greater discrepancies between experimental data and theoretical prevision would be found. It can be noted that the values of theoretical modulus increase with: (i) the WF content, maintaining constant the fibre length; and (ii) the fibre length, at constant fibre content. The experimental moduli are, for all the samples, higher than those predicted. These differences can be likely explained in terms of the assumptions introduced in the theory. Moreover, a deeper estimation of the effective fibre length should improve the accuracy of the approach. It should be also taken into account the possibility of a certain degree of fibre orientation in the plane upon the preparation of the film by compression moulding. In particular, for the sample with higher WF content the observed discrepancy could be related to a quite broad distribution of the fibre length (Table 2); as a consequence, the average fibre length used in the theoretical calculus could be not completely representative of the effective

result of an averaging process for composites having, respectively, 3- and 2-D randomly oriented fibres.

Ec ¼

1 4 E11 þ E22 5 5

ð2Þ

Ec ¼

3 5 E11 þ E22 8 8

ð3Þ

where E11 and E22 are the longitudinal and transverse moduli of a unidirectional continuous fibre composite having the same volume fraction of fibres. The values of E11 and E22 can be derived by using the Halpin–Tsai model [39] as follows:

E11 ¼ Em

E22 ¼ Em

1 þ 2 dlf gL /f f

1  gL /f 1 þ 2gT /f 1  gT /f

ð4Þ

ð5Þ

where Em is the elastic modulus of the matrix, lf and df are length and diameter of the fibres, /f the fibre volume fraction, and the parameters gL and gT are given by:

Fig. 5. Comparison between theoretical and experimental Young moduli for BPEbased composites containing WF (s) or WFPVA ().

1118

L. Conzatti et al. / Composites: Part A 43 (2012) 1113–1119

fibre length, that strongly affects the mechanical properties of the composites. 3.6. Dynamic-mechanical measurements Dynamic-mechanical thermal analysis (DMTA) over a wide range of temperatures is useful in understanding the viscoelastic behaviour of composite materials and provide valuable insights into the relationship between their structure and final properties. The parameters obtained from DMTA are the storage (G0 ) and loss (G00 ) moduli, and the loss tangent (tan d), defined as ratio between G00 and G0 . Fig. 6 shows G0 as a function of temperature for BPE matrix and BPE-WF composites at various WF contents. Over the whole range of testing temperatures, the storage modulus of all the composites results significantly higher than that of the BPE matrix. This indicates an increase in rigidity and, hence, in the strength of the composites. The damping property (tan d) is sensitive to all molecular motions and phase transitions occurring in a polymeric material. For composites, the molecular movements at the interface contribute to both intensity and position of tan d peak; this allows to estimate the extent of interaction at the interface between fibre and matrix. The behaviour of tan d as a function of temperature is shown in Fig. 7. The observed lowering in the intensity of tan d peak for the composites with respect to the BPE matrix, suggests that the fibres restrain the matrix mobility. This restriction is enhanced by increasing the fibre content. Moreover, fibre concentration affect also the position of tan d peak (Fig. 7a), that is the glass transition temperature (Tg) of the polymer matrix. Although the differences are not so remarkable, the Tg of the composites increases with the WF content; this may be due to polymer–fibre interactions confirming the existence of a certain degree of adhesion between WF and BPE. The increase of adhesion is further enhanced when PVA-treated fibres are considered (Fig. 7b). In particular, the BPEWFPVA-30 sample exhibits a peak temperature of 25.1 °C, while for BPE-WF-30 and BPE matrix the Tg values are 28.8 and 30.0 °C, respectively. The variation of tan d with WF content can be derived from the Nielsen theory [40] through the following equation:

tan dc ¼ tan dm ð1  /f Þ

ð8Þ

where the indices c and m refer to composite and matrix, respectively. This equation holds for composites based on rigid solid particles excluding any kind of adhesion at the polymer–particle interface. For temperatures around the Tg of the composites, the damping

Fig. 6. Storage modulus as a function of temperature for BPE and BPE-WF composites.

Fig. 7. Loss tangent as a function of temperature for BPE and BPE-WF composites at different WF content (a) and WF treatment (b).

is primarily due to the polymer phase and can be simply related to the polymer volume fraction, as indicated in Eq. (8). If some interaction between polymer and fibre exists, a layer of polymer tend to be immobilized around the fibres, as if the effective matrix volume fraction was reduced. The dotted line plotted in Fig. 8 shows the tan dc/tan dm ratio as a function of /f expected on the base of Eq. (8) in absence of fibre/matrix interaction. The comparison with the experimental values obtained for BPE-composites containing WF evidences a negative deviation from the linear trend, suggesting the existence of interactions between matrix

Fig. 8. Comparison between theoretical (dotted line) and experimental tan dctan dm values for BPE composites containing WF (s) or WFPVA ().

L. Conzatti et al. / Composites: Part A 43 (2012) 1113–1119

and fibre and the formation of appreciable layers of immobilized polymer in the surroundings of the dispersed fibres. A further slight increase of interactions can be also observed in the case of the PVA-coated fibres. 4. Conclusions Homogeneous biocomposites based on a biodegradable polyester containing high amounts of wool fibres (up to 40 wt.%) were successfully prepared by melt blending. A certain degree of fibre/matrix adhesion was obtained in particular for PVA-treated fibres. This resulted in an increment of Young’s modulus up to 500% with respect to the neat polyester matrix. On the other hand, since the length of the wool fibres in the composites was mostly shorter than the critical length, the effective reinforcing action of fibres on matrix was limited, as indicated by the low values of strength at break. A certain degree of consistency was found between the data obtained from mechanical and dynamic-mechanical investigations and those calculated with appropriate theoretical models. In particular, a certain degree of fibre orientation in the plane upon the preparation of the film was observed and the improved fibre/matrix interaction was confirmed for PVA-treated fibres. Acknowledgements Research supported by CARIPLO Bank Foundation Project KEBAB 2009–2011. Thanks are due to Dr. L. Capuzzi (Novamont SpA, I) for supplying the biodegradable polyester IP1669. References [1] Gross RA, Kalra B. Biodegradable polymers for the environment. Science 2002;297:803–7. [2] Dufresne A. Cellulose-based composites and nanocomposites. In: Belgacem MN, Gandini A, editors. Monomers, polymers and composites from renewable resources. Oxford: Elsevier; 2008. p. 401–18. [3] Li Y, Mai YW. Interfacial characteristics of sisal fibre and polymeric matrices. J Adhes 2006;82:527–54. [4] Torres FG, Cubillas ML. Study of the interfacial properties of natural fibre reinforced polyethylene. Polym Test 2005;24:694–8. [5] Ramakrishna M, Kumar V, Negi SY. Recent development in natural fiber reinforced polypropylene. J Reinf Plast Comp 2009;28(10):1169–89. [6] Singha AS, Rana RK, Rana A. Natural fiber reinforced polystyrene matrix based composites. Adv Mater Res 2010;123–125:1175–8. [7] Sombatsompop N, Chaochanchaikul Phromchirasuk C, Thongsang S. Effect of wood sawdust content on rheological and structural changes and thermomechanical properties of PVC/sawdust composites. Polym Int 2003;52:1847–55. [8] Djidjelli H, Martinez-Vega JJ, Farenc J, Benachour D. Effect of wood flour content on the thermal, mechanical and dielectric properties of poly(vinyl chloride). Macromol Mater Eng 2002;287(9):611–8. [9] Joseph S, Seakale MS, Oommen Koshy P, Thomas S. A comparison of the mechanical properties of phenol formaldehyde composites reinforced with banana fibres and glass fibres. Compos Sci Technol 2002;62(14):1857–68. [10] Alves C, Ferrão PMC, Silva AJ, Reis LG, Freitas M, Rodrigues LB, et al. Ecodesign of automotive components making use of natural jute fiber composites. J Clean Prod 2010;18(4):313–27. [11] Netravali AN, Chabba S. Composites get greener. Mater Today 2003;6(4):22–9.

1119

[12] Carroll DR, Stone RB, Sirignano AM, Saindon RM, Gose SC, Friedman MA. Structural properties of recycled plastic/sawdust lumber decking planks. Resour Conserv Recycl 2001;31:241–51. [13] Poole AJ, Church JS, Huson MG. Environmentally sustainable fibers from regenerated protein. Biomacromolecules 2009;10(1):1–8. [14] Barone JR, Schmidt WF. Polyethylene reinforced with keratin fibers obtained from chicken feathers. Compos Sci Technol 2005;65:173181. [15] Barone JR, Schmidt WF, Liebner CFE. Compounding and molding of polyethyelene composites reinforced with keratin feather fiber. Compos Sci Technol 2005;65:683–92. [16] Huda S, Yang Y. Composites from ground chicken quill and polypropylene. Compos Sci Technol 2008;68:790–8. [17] Bullions PA, Gillespie RA, Price-O’Brien J, Loos AC. The effect of maleic anhydride modified polypropylene on the mechanical properties of feathers fiber, kraft pulp, polypropylene composites. J Appl Polym Sci 2004;92:3771–83. [18] Liu X, Xu W, Peng X. Effects of stearic acid on the interface and performance of polypropylene/superfine down powder composites. Polym Compos 2009;30(12):1854–63. [19] Xu W, Wang X, Li W, Peng X, Liu X, Wang XG. Characterisation of superfine wool powder/poly(propylene) blend film. Macromol Mater Eng 2007;292:674–80. [20] Aluigi A, Innocenti R, Tonin C, Vineis C, Freddi G. Studies on keratin/cellulose films from cuprammonium solution. Res J 2004;4(4):175–81. [21] Tonin C, Aluigi A, Vineis C, Varesano A, Montarsolo A, Ferrero F. Thermal and structural characterization of poly(ethylene-oxide)/keratin blend films. J Therm Anal Calorim 2006;89(2):601–8. [22] Parkinson G. Chementator: a higher use for lowly chicken feathers? Chem Eng 1998;105(3):21. [23] Zahn H, Föhles J, Nienhaus M, Schwan A, Spel M. Wool as a biological composite structure. Ind Eng Chem Prod Res Dev 1980;19:496–501. [24] Averous L, Fringant C. Association between plasticized starch and polyesters: processing and performances of injected biodegradable systems. Polym Eng Sci 2001;41:727–34. [25] Lai SM, Huang CK, Shen HF. Preparation and properties of biodegradable poly(butylene succinate)/starch blends. J Appl Polym Sci 2005;97:257–64. [26] Willet JL, Felker FC. Tensile yield properties of starch-filled poly(ester amide) materials. Polymer 2005;46(9):3035–42. [27] Mani R, Bhattacharya M. Properties of injection moulded blends of starch and modified biodegradable polyesters. Eur Polym J 2001;37(3):515–26. [28] Favre J, Merienne MC. Characterization of fibre/resin bonding in composites using a pull-out test. Int J Adhes Adhes 1981;1(6):311–6. [29] Penn LS, Lee SM. Interpretation of the force trace for Kevlar/epoxy single filament pull-out tests. Fibre Sci Technol 1982;17(2):91–7. [30] Gaur U, Miller B. Microbond method for determination of the shear strength of a fiber/resin interface. Evaluation of experimental parameters. Compos Sci Technol 1989;34(1):35–51. [31] Moon CK, Cho HH, Lee JO, Park TW. A solution microbond method for determination of the shear strength of a fiber/thermoplastic resin interface. J Appl Polym Sci 1992;44(3):561–3. [32] Tripathi D, Lopattananon N, Jones FR. A technological solution to the testing and data reduction of single fibre fragmentation tests. Compos Part A-Applied Sci 1998;29(9–10):1099–109. [33] Bascom WD, Jensen RM. Stress transfer in single fiber/resin tensile tests. J Adhes 1986;19(3–4):219–39. [34] Curtin WA. Exact theory of fiber fragmentation in a single fiber composite. J Mater Sci 1991;26(19):5239–53. [35] Waterbury MC, Drzal LT. On the determination of fiber strengths by in-situ fiber strength testing. J Compos Technol Res 1991;13(1):22–8. [36] Baxevanakis C, Jeulin D, Valentin D. Fracture statistics of single-fibre composite specimens. Compos Sci Technol 1993;48:47–56. [37] Kleber R, Stuhler H. Sizing agent for synthetic fibers. US Patent 4399245; 1983. [38] Lavengood RE, Goettler LA. US Government RGD Reports, AD886372, National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22151. [39] Halpin JC, Kardos JL. The Halpin–Tsai equations: a review. Polym Eng Sci 1976;16:344–52. [40] Nielsen LE. Dynamic mechanical properties of polymers filled with agglomerated particles. J Polym Sci Polym Phys 1979;17(11):1897–901.