Wheat straw (WS) has been shown to be effective as a reinforcing filler in common thermoplastics.¹Now there is interest in incoporating WS and other natural fibers into polyamide composites, such as polyamide 6, mainly to decrease density and reduce cost. The challenge is that any natural fiber or filler to be added must be capable of withstanding the high temperatures encountered during compounding, forming, and use. Therefore, a complete and clear understanding of the thermal properties of WS and its composites is essential.

Polyamide 6 is an engineering thermoplastic suited to applications requiring high resistance to heat and stress. Its elevated melting point (T_m) exceeds the temperature at which the onset of thermal degradation in WS occurs. To combat WS property loss during compounding, polyamide 6 can be modified with lithium chloride (LiCl) salt and N-butyl benzenesulfonamide (N-BBSA) plasticizer. LiCl decreases the T_m of the matrix, resulting in a lower compounding temperature. However, it creates a pseudo-crosslinking formation, which limits polymer chain movement and imposes restrictions on processing temperature.^{2, 3} N-BBSA plasticizer can be used to ease the processing procedure and reduce the residence time of the material during the extrusion and injection molding processes.³

We have studied the mechanical properties and degradation kinetics of ground WS, polyamide 6, and extruded polyamide 6-WS composites.^{4, 5} Polyamide 6 and 15wt% WS were compounded with LiCl and N-BBSA in a twin-screw counter-rotating minilab extruder.⁴Two commonly used non-isothermal kinetic models were used to investigate the degradation kinetics of WS and of the composites: the activation energy of WS was identified by the iso-conversional Friedman kinetic method, and composite kinetic studies were based on the Coats and Redfern procedure.⁵Thermal degradation parameters were determined via thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTG), and second time derivative thermogravimetric analysis (D^2TG).

The onset of degradation is defined as the temperature corresponding to a mass loss of 1wt%. As shown in Table 1, the onset of degradation of WS alone, which is dictated by the thermal stability of its lignin content, was found to be around 188^°C, with drastic decomposition commencing at 245^°C. These temperatures define a range that straddles the processing temperatures for polyamide 6 and the required matrix/WS treatments. The maximum rate of decomposition occurred at a peak temperature of 326^°C. The activation energy of WS was found to be around 141.4kJ/mol.

Table 2 shows the measured temperatures corresponding to onset of degradation (T_onset) and maximum rate of weight loss (T_max) for modified polyamide 6 composites and pure polyamide 6. Most samples degraded within a common temperature range. One exception is run #6, whose T_onset and T_max were higher than those of almost all other runs. Another is run #5, whose higher T_max is due to its lower residence time in the extruder. Evidently the addition of plasticizer leads to higher T_max, as compared with the lower values of runs #1–4, reflecting the decrease in flowability caused by the addition of salt.

The activation energy (barrier energy of degradation), and the residual weight percentage at the peak of degradation, were found to be functions of composite formulation. Figure 1 shows the positive correlation that exists between the two values, with higher plasticizer content associated with a lower barrier energy and lower residual weight. This corresponds to the expectation that lowering the energy barrier facilitates degradation. Table 2 shows that run #3 had the highest degradation energy, and thus the highest thermal stability, while runs #4 and 5 had the lowest, perhaps due to evaporation of the plasticizer (which has a low boiling point) simultaneously with degradation of the WS. To determine the extent of degradation due to plasticizer evaporation, one could analyze polyamide 6 with 4wt% of plasticizer and no WS.

Our group recommends the use of extruder and injection molding machines featuring multiple heating zones, both for more efficient heat adjustment during processing and to increase the thermal stability of the resulting composites. We have recently produced chemically modified WS fibers with enhanced heat resistance, and we are now researching the use of these fibers for polyamide reinforcement.