Triticale straw and its thermoplastic biocomposites

1 Introduction

Over the last decade, ecological concerns have initiated a considerable interest in renewable materials to produce “greener” products. Wood fibers have been used as reinforcements for polymers for so many years. Annual crop fibers have also begun to get attention for biocomposites in recent years. Cellulosic fibers become more and more favorable materials for a number of composite products because they are cheap, biologically degradable, and nonabrasive and have low density [1–5]. Beside wood fibers, the annual plant fibers obtained from different sources, such as wheat, sisal, jute, coconut, and flax, have good potential as a source of low-cost reinforcements for polymers [3–9].

Cereal crops have been a pillar of Canada’s agricultural economy for well over a century. Triticale, a hybrid of wheat and rye, is currently grown on an average of 200,000 acres each year in Canada. Canada is edging to be the world leader in the development of triticale into a crop with high yield for industrial use. Beside the seed, triticale straw could also be used to produce composites for industrial application, such as wheat straw. The Canadian Triticale Biorefinery Initiative (CTBI) is a long-term program to develop triticale as a dedicated industrial biorefining crop for Canada [10, 11]. Compared to the country’s major crops such as wheat, corn, canola, and soybeans, triticale currently makes a relatively small impact on the industry. At this stage, there is still very limited information from the literature, which are related to triticale straw and its industrial applications. This paper presents, for the first time, the evaluation of the potential of triticale straw for the production of thermoplastic polypropylene (PP) biocomposites to be used in industrial applications.

2 Materials and methods

3 Results and discussion

In general, chopped triticale straw has light yellow color and consists of a mixture of chopped fibers and shives. The shives were mostly broken up during the process and a part of them attaches onto the fibers. No significant fibers with full separation could be observed. A significant amount of large agglomerates was observed in the received triticale particles as shown in Figure 1A. The triticale agglomerates are approximately 7 mm in diameter, which contain many individual triticale particles. With these agglomerates, it is certainly difficult to mix and disperse them well in the polymer matrix for composite fabrication. Therefore, mechanical treatment is necessary to reduce their size down to a certain level to facilitate its mixing with the PP matrix. Different devices have been used, including granulator and hammer mill, to break down the triticale agglomerates to the size that is suitable for the composite preparation. In this paper, granulated triticale particles are presented (Figure 1B). After granulation, the triticale agglomerates have been broken down significantly into smaller particles. OM was also taken on the triticale before and after granulation as shown in Figure 2. To prepare the slide for the OM, the triticale before and after granulation was gently dispersed in water at room temperature then dropped on a glass slide. One can see in Figure 2 that there is not much different on the individual triticale particle size in terms of length and width before and after granulation. This indicates that granulation did not damage the individual triticale particles but rather break down the agglomerates and separate the particles. Figure 2 also illustrates the nonuniform distribution of the shape of the triticale particle as well as its size that varies from few to few hundred microns in diameter and from a few hundred microns to few millimeters in length.

Figure 1

Photos for triticale (A) obtained from refining wet process and (B) after granulation.

Figure 2

OM images for triticale (A) obtained from refining wet process and (B) after granulation.

Figure 3 shows the SEM photos of the triticale particles obtained after granulation. The photos illustrate that the triticale surface is rough and inhomogeneous. As can also be seen in Figure 3A, the fibers were also not fully separated from each other to form monofilament as bast flax or hemp fibers but mostly remain in bundles and attach with the shives. Figure 3 again confirms the nonuniform distribution of triticale particle shape and size.

Figure 3

Triticale straw surface at different magnifications: (A) 100 times and (B) 700 times.

Very limited information on triticale straw fibers is available in the literature, while information on their composites does exist. Thus, wheat straw was used for comparison. Figure 4 shows the SEM photos of the cross-section of wheat and triticale straws. In general, the triticale straw has a similar structural characteristic as the wheat straw. From the literature, triticale straw has 34% cellulose, 31.7% hemicellulose, and about 17% lignin, while wheat straw contains 39.7% cellulose, 36.5% hemicellulose, and 17.3% lignin [12]. The cellulose concentration in both straws, triticale and wheat, is much lower than in flax and hemp straw, where the cellulose content can go up to 70% or even 80% [13]. In addition, the distribution of fibers in triticale and wheat straw is very different from that in flax and hemp straw. In flax and hemp, the fibers are regrouped in large bundles that are concentrated closely to the straw skin, while the shives in large volume locate near to the core [14, 15]; thus, it is possible to separate the fibers from the shives during the decortication process. However, in wheat and triticale, the fibers are regrouped in much smaller bundles dispersed at various locations from the skin to the core and those small fiber bundles should have very poor integration. Hence, it is very difficult to separate the fiber bundles from the shives without breaking them.

Figure 4

Cross-section SEM photos of wheat and triticale straws.

Crystallization often has strong influence on the polymer physical and mechanical properties, while the presence of cellulosic fibers can alter their crystallization process and crystallinity has been reported. Thus, it is interesting to study the effect of triticale particles on the crystallization of PP (Figure 5). OM coupled with a hot stage was used to observe the influence of a single triticale fiber on the crystallization of PP. The sample was heated from room temperature to the molten stage of PP then cooled down slowly and the growth of spherulites was monitored during the cooling process. One can see that, at about 120°C, the spherulites begin to grow along the triticale fiber surface first then into the bulk PP when the cooling continues. This demonstrates that the triticale fiber plays the role of a nucleating agent that facilitates the crystallization of PP. This phenomenon is similar to the literature where the flax fiber embedded in PP matrix also acts as a nucleating agent for the creation of spherulites [7, 8]. It is also noticed that the crystal growth of PP in the presence of triticale fiber is very rapid and has nearly completed at 108°C (∼35 s from with the cooling rate of 20°C/min).

Figure 5

Nucleating effect of triticale fiber on the crystallization of PP.

The melting temperature (T_m) of the PP and PP/triticale composites determined from the second heating scan in the differential scanning calorimetry (DSC) and the onset crystalline temperature (T_conset) obtained from the cooling scan in the DSC are shown in Figures 6 and 7, respectively. In general, the T_m of all the PP/triticale composites is quite similar and comparable as that of the neat PP (Figure 6). On the contrary, the T_conset behaves differently with the presence of triticale, its concentration and the composite formulation. Figure 7 demonstrates that the addition of triticale directly to formulation without any coupling agent (PPTr30) increased the T_conset, indicating that triticales initiate the crystallization process at a much higher temperature. This is in good agreement with the OM discussion above; the triticale plays the role of a nucleating agent, thus facilitating the initiation of crystal at higher temperature. Figure 7 shows that the presence of coupling agent reduces the T_conset of the composite (comparison between PPTr30 and PPETr30). This may be interpreted that the coupling agent migrates to the triticale particle surface, thus inhibiting the nucleating agent effect of the triticale fiber and hence reduces the T_conset of the PP composite. With the same amount of coupling agent, as the fraction of the triticale increases in the formulation, T_conset is also found to increase (comparison among PPETr20, PPETr30, PPETr40, and PPETr50). It is speculated that, with an unchanged amount of the coupling agent, there is less coupling agent for covering all the triticale particle surfaces when adding more triticale; thus, there is more triticale free surface to nucleate the formation of spherulites in the PP composite. The formulation also influences the T_conset of the triticale composites. The presence of reactive additive CaO increases the T_conset of the composite (comparison among PPTr30, PPETr30, and PPETr30Ca). Basically, the CaO can play the role of nucleating agents. The obtained results illustrate that each component in the formulation has different effect on the T_conset of the PP triticale composites.

Figure 6

Melting temperature of PP triticale composites.

Figure 7

Crystallization temperature of PP/triticale composites.

Figures 8 and 9 present the flexural properties of the PP/triticale composites with different triticale concentrations and formulations. The results demonstrate that the addition of triticale directly to formulation without any coupling agent increased the modulus but reduced the strength of the virgin PP (Figure 8, PPTr30). This means that triticale particles play the role of a filler rather than a reinforcement. The use of coupling agent shows an advantage in the performance. In principle, coupling agent improves the quality of dispersion of triticale in the composites and increases the interface interaction between triticale surface and PP matrix (comparison between PPTr30 and PPETr30), which will be confirmed by microscopic observation. Better dispersion and interface result in an increase in the modulus and the strength of the triticale composite. This effect can also be seen in the literature for the wheat straw polyolefin composites in which the improvement of interfacial adhesion between wheat and polyolefin is observed by using coupling agents [3, 16, 17]. Figure 9 also shows the effect of triticale concentrations on the flexural strength and modulus of the composites (comparison among PPETr20, PPETr30, PPETr40, and PPETr50). The modulus increases almost linearly with the triticale loading, which is reasonable. However, the strength increases slightly at 20% triticale but reduces at higher triticale concentration. In general, the greater the triticale content, the higher the modulus but the lower the strength of composite. Again, at the level of more than 30% of the triticale in the formulation, even with the aid of the coupling agent, the triticale still appears as a filler that increases the modulus while reduces the strength. However, it is interesting to observe that the developed formulation with the combination of coupling agent and reactive additive CaO provides superior strength and modulus compared to the other composites (Figure 8, PPETr30Ca). Thus, this can upgrade the triticale from regular filler to reinforcement category. The additive CaO is a basic reactive filler that has the role to absorb moisture in fibers, neutralize acidity in fiber impurities, and minimize the degradation of fibers [18]; therefore, it improves further the interface of triticale fiber and PP matrix and thus the flexural properties of the composite.

Figure 8

Flexural properties of PP triticale composites with different formulations.

Figure 9

Flexural properties of PP triticale composites with different triticale concentrations.

Figure 10 shows the impact energy of the PP-triticale composite with the presence of triticale in different concentrations and formulations. The results indicate that the presence of triticale, the triticale content, and the formulations have no negative effect on the composite impact energy. The impact energy of all composites remains about the same values as neat PP.

Figure 10

Notched Izod impact strength of PP triticale composites for different triticale concentrations and formulations.

To study the dispersion of the triticale in the PP composite, the SEM observation on the polished surface of the PP triticale composites was taken and is shown in Figures 11 and 12. The triticale particles in general are more hydrophilic, while the PP is more hydrophobic. The incorporation of those triticale particles into PP can be associated with poor triticale dispersion due to the wide differences in polarity between the triticale and PP. Such poor dispersion can be easily seen for the case of 30% triticale in the absence of the coupling agent (Figure 11A). This poor dispersion can result in clumping and agglomeration of triticale, which will act as stress concentration points to initiate cracks during loading and negative effect to mechanical properties of the composites as can be seen earlier in the flexural properties.

Figure 11

SEM photos of PP triticale composites: (A) 30% triticale without coupling agent, (B) 30% triticale with coupling agent, and (C) 30% triticale with coupling agent and CaO.

Figure 12

SEM photos of PP triticale composites: (A) 20% and (B) 50% triticale with coupling agent.

It is evident from Figure 11 that formulation has affected the dispersion but in different extents depending on the components used. At the same triticale concentration of 30%, the presence of coupling agent provides better dispersion of the triticale (Figure 11B vs. Figure 11A). The coupling agent is a chemical substance that is capable to interact with both the fiber and the matrix and promote the dispersion of the fiber in the matrix. The combination of coupling agent and the reactive additive CaO also provides a good dispersion of the triticale (Figure 11C) compared to the formulation without coupling agent (Figure 11A). The increase of the density of the triticale can be observed with its concentration as can be seen in Figure 12, which corresponds to the 20% and 50% of the triticale in the composites.

Fracture surfaces of the PP triticale composites after tensile test were observed by SEM and are shown in Figures 13 and 14. Several triticale particles with quite clean surface can be observed clearly for the formulation of 30% triticale without coupling agent (Figure 13A). This presents bad interface between the triticale and the PP matrix. Figure 13 shows that, at the same 30 wt% of triticale, the presence of coupling agent shows less evidence of clean triticale pullout from the matrix, indicating an improvement of the interface between PP and triticale (Figure 13B) compared to the one without coupling agent (Figure 13A). Similar phenomenon of good interface between PP and triticale also can be seen for the composite that contains both coupling agent and reactive additive CaO (Figure 13C). The matrix attached or even covered on triticale surface demonstrates a good interface between the matrix and the fiber. This good interface observation corroborates well with the increase in the flexural strength of the composite with the presence of coupling agent and both coupling agent and the reactive additive CaO. Fracture surface of the composite also appears differently with the triticale contents (Figure 14). For the 20% triticale (Figure 14A), most of the triticale particles were well enveloped by the PP matrix and fractures that occurred through the PP matrix. With an increasing amount of triticale, for example, 50% (Figure 14B), triticale particles can be observed clearly and the fracture occurred through the interface at some locations probably due to the high concentration of triticale. This corresponds to the loss in the composite strength with high triticale concentration. The results provide a general idea about the role of each component in the formulation and their contribution in the interface between the PP matrix and the triticale particles.

Figure 13

Fracture surface of PP triticale composites: (A) 30% triticale without coupling agent, (B) 30% triticale with coupling agent, and (C) 30% triticale with coupling agent and CaO.

Figure 14

Fracture surface of PP triticale composites: (A) 20% and (B) 50% triticale with coupling agent.

4 Conclusion

The potential use of triticale straw for the fabrication of PP biocomposites with PP was evaluated for the first time. The presence of triticale and CaO promotes the crystallization growth of PP. Triticale plays the role of a filler that increases the modulus while reduces the strength when adding into the PP in the absence of coupling agent. The presence of coupling agent improves the adhesion between the triticale and the PP matrix, thus bringing back a certain level of the strength. The developed formulation with the combination of coupling agent and reactive additive provides superior strength and modulus and thus can upgrade the triticale from filler to reinforcement category.