Influence of retting process on the thermal and mechanical properties of flax/polypropylene composites made of canadian oilseed flax fibers

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Influence of retting process on the thermal and mechanical properties

of flax/polypropylene composites made of canadian oilseed flax fibers

Hu, Wei; Ton-That, Minh Tan; Denault, Johanne; Rho, Denis; Yang,

Jianzhong; Lau, Peter C. K.

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INFLUENCE OF RETTING PROCESS ON THE THERMAL AND

MECHANICAL PROPERTIES OF FLAX/POLYPROPYLENE COMPOSITES

MADE OF CANADIAN OILSEED FLAX FIBERS

Wei Hu1*, Minh-Tan Ton-That1, Johanne Denault1, Denis Rho 2, Jianzhong Yang 2, Peter C.K. Lau 2

1_{Industrial Materials Instiute, National Research Council, Quebec, Canada – Wei.hu@imi.cnrc-nrc.gc.ca} 2 _{Biotechnology Research Institute, National Research Council Canada}

Abstract - Flax fibers are more and more used as reinforcing materials for composites in the field of material research because of their good mechanical properties, low abrasion, environmental friendliness and low cost. In this study, two kinds of Canadian oilseed flax fibers, dew-retted (F1) and enzyme-retted flax fibers (F2) were used as reinforcing material for flax/PP composites to investigate the influence of retting process on the composite properties. Crystallization behavior, thermal stability and mechanical property of flax fiber reinforced PP composites were characterized in detail. It was shown that enzyme-retted flax with higher retting degree could lead to composites with better thermal and mechanical properties than dew-retted flax. PP composite reinforced with F2 presented 73% and 43% of tensile modulus and impact strenght higher than that of pure PP, respectively. In addtion, its tensile and flexural strengh were at least 35% higher than those of F1 reinforced PP composites.

Introduction

Natural fibers as reinforcing material for polymer composites to obtain good mechanical properties and environmental friendly performance have become one of most attractive research subjects in material science [1]. It is known that the properties of composite material are mainly controlled by the property of reinforcing material and the adhesion between the reinforcement and the polymer matrix. Among the different kinds of natural fibers, flax fibers are remarkable for their high specific strength and modulus [2,3]. However, they also present shortcomings as natural fibers, such as hydrophilic nature, high moisture absorption and low thermal resistance. Retting degree is one of the important factors that control the properties of flax fibers[4]. It is characterized by pectin and lignin amount left on the fiber surface and the elementary fiber separation extent after the retting process [3,5,6]. It is known that, one flax technical fiber is composed of several elementary fibrils, which are bundled together by pectin and lignin substances [7]. The retting process remove the binding non-fiber stem tissues, such as pectin, lignin from the fiber bundle to release the individual fiber and increase the fiber strength. So far, there are mainly three kinds of retting process: water-retted treatment, dew-retted treatment [8], and enzyme retting [9]. The water retting process has almost been abandoned because of its bad ecological effect and long time consuming. Dew retting is always limited by climate conditions such as temperature and humidity, which must be suitable for fungal activities. Enzyme retting has been evaluated as a promising retting method for its time-saving, ecology friendlyness and convenient characteristics.

While fibres recovered from linen flax varieties have been well characterized and also used in composite applications mostly in Europe, the oilseed flax fiber varieties typically grown in Canada have not been so well studied [3,10-13]. In this work, dew-retted and enzyme-retted Canadian oilseed flax were used to examine the effect of retting process on the flax/polypropylene (PP) composite properties.

Experimental

Materials

Dew retted fiber (F1) was afforded by the Schweitzer-Mauduit Company. Enzyme retted fibers (F2) were prepared in our laboratories following procedure described in reference [12]. According to the former study on these two kinds of flax fibers [3], the retting degree of F2 seems better than F1. Most of the elementary fibers of F1 were bundled together by thick layer of pectin and lignin, while the elementary fibers of F2 were much separated with much less amount of pectin and lignin left on the surface. Homopolymer Pro-fax PDC 1274 (PP) (Mw) = 250 Kg / mol)

obtained from Basell was used as polypropylene matrix.

Flax/ PP melt compounding

F1 and F2 were first cleaned, and then dried in a vacuum oven at 100ºC for 6h. After that, fibers were stranded into rope for better and easier chopping into 5 mm length. The obtained short fibers were compounded with PP according to the formulation shown in Table 1 in a Brabender. All the components were mixed together for 10 min at 60 rpm at 190 ºC. The obtained composites were kept in a vacuum oven

at 100 ºC for 24h before moulding. Compression moulding of the composites was carried out in a compression moulding press.

Table 1 - Formulation applied in compounding.

F1 (g) F2 (g) PP (g)

PP 50

PPF1 15 35

PPF2 15 35

Differential scanning calorimetry (DSC)

A DSC-7 Perkin-Elmer calorimeter was used to study the melting and crystallization behaviour of PP and flax / PP composites at a heating and cooling rate of 10

º

C / min. The samples were first heated to 200oC and kept for 5 min. And then the samples were cooled down to room temperature and a second heat scan is performed thereafter. The crystallinity of the PP matrix in the composites was determined using the following equation:

Xc = ΔHm / (fP  ΔHf0)  100 %

where ΔHm (J/g) is the enthalpy of melting of PP in the

composite, fP is the PP weight fraction in composites,

and ΔHf0 is the enthalpy of melting of pure crystalline

PP (207.1 J/g).

X-ray diffraction

Bruker Discover 8 diffractometer operating at 40 KV, 40 mA with Cu Ka radiation in reflection mode using a horizontal Bragg-brentano focusing geometry was used for the wide angle X-ray diffraction characterization of the PP crystalline phase.

Scanning electron microscopic (SEM) Observation

JEOL JSM-6100 SEM at a voltage of 10KV was utilized to analyze the dispersion of the flax fibers in the PP matrix using polished surfaces, and the interface between flax and PP matrix using the fractured specimens. Before SEM observation, the samples were coated with Au / Pd to ensure good conductivity.

Thermal Gravimetric Analysis (TGA)

Thermogravimetric analysis were carried out from 25-700 ºC using a TG 96 SETRAM TGA, at a heating rate of 10 ºC / min under an inert atmosphere.

Mechanical Characterization

Tensile and flexural properties were measured according to ASTM test methods D638 and D790, respectively, and an Instron 5500R machine was used. For each reported value at least five specimens were tested.

Results and Discussion

DSC analysis

The melting and crystallization behaviour of the composites were studied by DSC. According to the crystallization curves of PP and flax fiber / PP composites shown in Fig.1, the composites presented melting and crystallization behaviours similar to the PP matrix. The melting temperature of the second scan (Tm2), the crystallization temperature (Tc) and

crystallinity (Xc) for the tested samples are summarized

in Table 2. It is shown that crystallization temperature (Tc) increased from 106 oC to 113 ºC after the addition

of fibers into pure PP. Flax fibers seem to act as nucleating agent for PP and caused the PP crystallization at a higher temperature with 30% of flax added into flax / PP composites, though, Xc of

composites increased not very much compared with pure PP. It is known that nucleating ability of fibers accelerated the crystallization process of PP [14,15]. According to Table 2, there is also indication that, the Tc of PPF2 composite was slightly higher than those of

PPF1 composites suggesting better retting degree can favor the crystallization process of PP.

Figure 1 - The cooling DSC curves of pure PP and flax fiber / PP composites.

Table 2 - Thermal properties (DSC and TGA) of PP and flax fiber / PP composites.

Tc (o_C) Tm2 (o_C) Xc (%) Tmax1 (o_C) Tmax2 (o_C) T5 (o_C) T10 (o_C) PP 106 164 49.0 458 423 435 PPF1 111 165 50.8 349 468 316 341 PPF2 113 166 51.7 359 468 342 356 X-ray Characterization

WAXS patterns of PP and flax / PP composites are shown in Fig. 2. The observed diffraction curves were all characterized by the – monoclinic crystal structure of polypropylene with main reflections at about 14º, 16.8º, 18.5º and 21º of 2corresponding to the (110), (040), (130) and (111) planes, respectively. In addition, there was significant difference in the intensity of the reflections of1 (110) and 2 (040).

90 100 110 120 130 PPF2 PPF1

Temperature

_C

E

n

d

o

(a)

This difference on intensity indicated some orientation of PP crystallite in the composites [16], which was probably produced by the orientation of flax fiber during the compression molding process. Similar orientation phenomenon was also found in PP / clay nanocomposites [17].

Figure 2 - WAXS diffraction curves of PP and flax /PP composites.

SEM Observations

Fig. 3 and 4 are the SEM images of polished and fractured flax/PP composite samples. It is shown from both of them that there were more shive in PPF1 composites than in PPF2 composites. The shive could detrimentally affect the mechanical property of composites. On the other hand, it seems that the amount of elementary fiber and elementary fiber bundle on the sample surface for PPF1 and PPF2 composites was very similar. This could mean that the shear forces during the compounding process of flax and PP matrix can be enough to separate the technical fiber into elementary fiber for both F1 and F2. Therefore, it seems that pectin and lignin content could play a more important role than separation status in the mechanical property of composites. On the other hand, in Fig. 4, it is shown very clearly that the interfacial adhesion between flax fiber, for both of F1 and F2, and PP matrix of composites was poor. This is illustrated by the presence of many pulled-out fibers, holes left on matrix, gap at the interface between fiber and PP matrix, and the clean fiber surface without any adhering polymer.

Thermal Analysis

Fig. 5 shows the TGA thermograms of PP and flax/PP composites. Temperature at 5% (T5) and 10% (T10)

weight loss of all the samples are shown in Table 2. There was a sudden decrease that began at about 250

o

C for the two types of composites in the TGA curves (Fig. 5 (a)). This decrease corresponds to the thermal depolymerization of hemicellulose and the cleavage of glycosidic linkage of cellulose. In addition, T5 and T10

of PPF1 according to Table 2 were 26ºC and 15ºC lower than those of PPF2, respectively, which proved

that more non-cellulose portion, such as

hemicelluloses, lignin, etc. existed in F1 fiber than in F2, confirming the results of Ref. 3.

(a)

(b)

Figure 3 - SEM images of polished composites, (a) F1 / PP; (b) F2 / PP.

(a)

(b)

Figure 4 - SEM images of fractured PPF1 (a);PPF2 (b). 10 15 20 25 4 3 2 1

R

e

la

ti

v

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a

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In the DTG curves of these composites(Fig. 5(b)), two peaks were obtained. The minor peak at about 350 oC was related to the degradation of cellulose, and the higher peak at about 468 oC was corresponded to the degradation of dehydro cellulose [15]. It is shown in Fig. 5 (b) that the F2 composites degraded at higher temperature (359ºC) for the minor peak, which was 10ºC higher than that of PPF1, however, no much difference was observed for the major peaks (468 oC). This may be related to the retting degree difference between F1 and F2. In princilpe, the better retting degree of F2 would lead to the higher degradation temperature. However, it could not affect the degradation temperature of dehydro cellulose, which is related to the major peak. Furthermore, all the composites degrade at higher temperature than PP (458ºC), which meant the thermal stability of composites was better than that of matrix.

Figure 5 - (a) TGA and (b) DTGA curves of PP and composites.

Mechanical Properties

The mechanical results are shown in Table 3. Flax fibers F1 and F2 decreased the tensile strength of the composite in comparison to pure PP. However, the tensile strength of PPF2 was about 35% higher than that of PPF1. All composites showed remarkably higher tensile modulus than pure PP. PPF1 presented

about 46% of increase, and PPF2 composite presented about 73% of increase in terms of modulus compared to pure PP. This is due to the higher modulus of F1 and F2 compared to PP, which is about 48.2 and 57.1 GPa, respectively [13]. The scattering of yield strength shown in Table 3 was similar to that of tensile strength for PP and composites, and the yield strenght of PPF2 was about 36% higher than that of PPF1.

It is known that impact strength is the ability of a material to resist the fracture under stress applied at high speed. Usually, when the flexural and tensile properties of the composite increase, the impact strength will decrease [18,19]. In this work, it is shown in Table 3 that the impact strength of flax / PP composites was higher than that of pure PP, an increase up to 50% is observed.

Table. 3. Mechanical properties of PP and flax/PP composites. Tensile Strength (MPa) Tensile Modulus (MPa) Yield Strength (MPa) Notched Izod Impact Strength (J/M) PP 25.5 ±0.5 2250 ±63 18.2 ±0.3 21.1±3.2 PPF1 17±2 3280 ±230 12.9 ±0.8 31.2±4.5 PPF2 23±1.5 3900 ±210 17.6 ±0.3 30.0±3.3

It is necessary to emphasise that, as shown in Table 3, the strength and modulus of PPF2 composites were higher than those of PPF1 composites, in addition to impact strength. These results are obtained in spite of the fact that the apparent interfacial shear strength of F1 and F2 are similar, IFSS of F1 was 6.3 ± 0.6 Mpa

and that of F2 was 5.7 ± 0.5 MPa [3] considering the error bar, and generally speaking, better interfacial adhesion should result in better mechanical property of composites. However, according to our previous study, the tensile strength of F2 was 456.5 ± 33.9 MPa, which is slightly higher than that of F1 (400.6 ± 40.8MPa) [13]. In addition, the retting degree of F2 was also better than F1. There were more pectin and lignin remained on F1 surface that would affect the mechanical property of composites. Moreover, there were also comparably more kinks on F1 surface than those on F2 surface [3]. The fiber kink bands is believed to result in stress concentration, which could act as sites for the initiation of fiber/matrix debonding as well as for the formation of micro-cracks in the matrix [20,21]. Therefore, according to all the above factors, the mechanical properties of PPF2 composites were higher than that of PPF1 composites.

Conclusions

The crystallization behavior, thermal stability and mechanical properties of Canadian oilseed flax obtained from two different retting processes: dew-retted and enzyme-dew-retted flax reinforced PP composites were studied. Flax fibers acting as nucleating agent was found to increase crystallization temperature of

250 300 350 400 450 500 0 5 10 15 20 25 PPF2 PP PPF1

D

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W

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%

/

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)

Temperature (

C)

(b)

W

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Temperature (

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(a)

flax / PP composites. Better retting degree of F2 seemed to have higher nucleating ability and favor the crystallization process. Furthermore, TGA analysis indicates that less amount of non-cellulose portions in F2 afforded better thermal stability of composites than that of F1. In addition, the tensile and flextural strength of PPF2 were at least 35% higher than that of PPF1. Specially, tensile modulus of PPF2 was 73% higher than that of pure PP, while PPF1 presented 46% increase. Therefore, enzyme retted flax was proved to be a kind of promising reinforcing material for composites compared with dew retted flax. The enzyme retting process afforded higher retting degree, and could contribute to better composite thermal stability and mechanical properties.

Acknowledgements

The authors acknowledge Agriculture and Agri-food Canada and National Research Council Canada for funding via the CBIN and the NBP-2 programs. Wei Hu thanks to the National Science and Engineering Research Council Canada for granting her a Postdoctoral Fellowship.

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