3.1. LOI and UL-94 Vertical Burning Test
The LOI and UL-94 tests are widely accepted to assess the flame retardancy of FR composites and the corresponding results (including dripping behavior) for the PLA/APP and PLA/APP/ST composites are summarized in
Table 1.
Pure PLA did not pass UL-94 test because it was highly flammable with prolific dripping, and the LOI was 19.5%. The addition of 10 wt % APP (PLA/APP10) increased the LOI to 24.4% and the composite achieved a V-2 rating in UL-94 test. The presence of 15 and 20 wt % APP (PLA/APP15 and PLA/APP/20) increased the LOI to 28.5% and 31.9%, respectively, and both composites obtained a V-1 and V-0 rating in UL-94 test respectively. Even so, both of these composites showed some dripping behavior during second flame application.
With the addition of 3 wt % ST (PLA/APP20/ST3), the LOI increased from 31.9% to 34.5%, and the composite retained its V-0 rating in the UL-94 test. There was also no evidence of dripping during first and subsequent test. Higher concentrations of ST (PLA/APP20/ST5 and PLA/APP20/ST7) increased the LOI to 36.2% and 37.3%, respectively, and both composites obtained V-0 classification in the UL-94 test. In this case, however, there was no dripping during either the first or the second test.
The char structure sheltered the underlying material from the external heat source, pyrolysis gases as well as from thermal degradation therefore the ignition process was either delayed or prevented hence enhancing the flame retardancy of starch-based composites. The formation of char is ascribed by the decarboxylation and dehydration reactions caused by the catalytic effect of starch in PLA/APP/ST composites. These results confirmed that the introduction of ST as a natural carbonization agent increased the LOI values of the FR composites significantly while simultaneously inhibiting the melt dripping phenomenon. All composites containing ST managed to obtain a V-0 rating in the UL-94 test. Compared to a similar study done by Marosi et al. [
25], where they achieved LOI value of 34% by adding up to 11 wt % of starch, we managed to achieve LOI value of 37.3% by adding only 7 wt % of starch.
The addition of APP in the concentration range of 10% (
w/
w) to 20% (
w/
w) enhanced the LOI from 24.4% (PLA/APP10) to 31.9% (PLA/APP20) (
Table 1). By increasing the amount of APP, a higher concentration of oxygen is needed to achieve the ignition of the sample due to the dilution of the fuel in the gas phase by the discharge of water vapor as a result of the dehydration of APP. The addition of ST to the formulations not only increased the LOI of the samples but also increased the mass residue, providing enhanced shielding against heat and a barrier against the emission of pyrolysis gases that act as fuel. Therefore, the emission of fuel in the gas phase is minimized by the addition of ST.
The UL-94 test classifies materials based on their ability to either promote or inhibit the spread of fire once it has been ignited. Pure PLA ignited during the first flame application (10 s), and the sample continued to burn until it was fully consumed. Although PLA/APP10 and PLA/APP15 performed better as flame retardants (flame extinguished less than 30 s after each flame application; V-2 and V-1 ratings, respectively), the dripping of the burning sample ignited the cotton placed beneath. Similarly, PLA/APP20 achieved a V-0 rating because the flame was extinguished in less than 10 s, but these samples still showed dripping behavior during second flame application. In contrast, none of the composites containing starch were dripping even after the second application of flame and all achieved a V-0 rating due to the generation of char layer on the surface which isolated the remaining sample and prevented the propagation of the flame. In previous studies [
9,
27], even the addition of 30–40% (
w/
w) PER as a carbonization agent was sufficient to achieve only a V-2 rating, whereas here we found that as little as 3% starch in the presence of 20% APP accomplished the target rating of V-0. Compared to another similar study done by Casetta et al. [
24] where they achieved V-0 rating in UL-94 vertical burning test by adding up to 10 wt % of starch together with 30 wt % of APP, whereas in our results we managed to achieve V-0 rating in UL-94 vertical burning test by adding only 3 wt % of starch together with 20 wt % of APP. Hence, these results are in good relation with the main hypothesis of this study. The photographs of the test samples after UL-94 test are shown in
Figure 2, which confirms the formation of char layer after burning on samples surface containing starch as carbonization agent.
3.2. Cone Calorimetry
Cone calorimetry provides broad information about the combustion behavior of polymers by measuring parameters such as time to ignition (TTI), peak heat release rate (PHRR), and total heat release rate (THR), which can predict their behavior in real-life fires.
The heat release rate (HRR) curves of pure PLA, PLA/APP10, PLA/APP15, PLA/APP20, PLA/APP20/ST3, PLA/APP20/ST5, and PLA/APP20/ST7 are presented in
Figure 3a,b. Following ignition, pure PLA burnt much faster than the other samples and produced a very sharp HRR curve with a PHRR of 570 kW m
−2. For composite PLA/APP20, the PHRR declined to 337 kW m
−2, and with the further addition of 3 wt % ST (PLA/APP20/ST3), the PHRR was even lower, at 212 kW m
−2. At the maximum 7 wt % ST content we tested (PLA/APP20/ST7), the PHRR was only 192 kW m
−2, which is 66.30% of the pure PLA value. These findings indicated that the combined effect of APP and ST allowed the formation of a much thicker char layer on the surface of the composites after ignition, which prevented the degradation of the composite by restricting the fire passage into the polymer matrix.
In IFR systems, flame retardancy is achieved by the swelling of the substrate in the condensed phase, which generates a sponge-like multicellular structure called char that shelters the principal material from heat transfer. The char structure also acts as a physical barrier against fuel and mass transfer from the condensed phase to the site of burning.
Figure 3a,b demonstrates that the heat release rate of the composites containing APP alone (PLA/APP10, PLA/APP15, and PLA/APP20) or together with starch (PLA/APP20/ST3, PLA/APP20/ST5, and PLA/APP20/ST7) changed dramatically in comparison to pure PLA (570 kW m
−2). In samples containing APP alone, the intumescent char layer was thinner and more porous than in samples containing APP and ST. This is because the absence of ST lowered the viscosity of the char layer, in turn allowing vapor and gas bubbles to escape and reducing the degree of swelling because little pressure was allowed to build up. The resulting porous structure allowed further fuel gases and water vapor to pass through the unclosed cells, increasing the PHRR. In contrast, the higher viscosity of the char layer containing ST made the char more compact and prevented the escape of gases and vapor, resulting in a pressure build up that increased the melt viscosity of the condensed phase and resulted in more swelling of the char. The combined effect of APP and ST therefore reduced the PHRR to 192 kW m
−2, which is 66.30% less than pure PLA. The HRR in this study is much lower than reported in other studies of PLA composites containing different carbonization agents [
28,
29,
30].
Table 2 shows that the TTI of pure PLA was 41 s, increasing to 58 s when APP was incorporated into the PLA matrix (PLA/APP20) and to 77 s when the maximum content of starch was included (PLA/APP20/ST7). The ignition of a material is normally dependent on the concentration of pyrolysis gases, which are released when a material is degraded. The concentration of the gases increases during material degradation and ignition starts when they reach a certain threshold. Longer ignition times reflect the slower decomposition of the material mainly due to the presence of starch together with APP. A uniform and compact char structure can hinder the diffusion of pyrolysis gases from the melting substrate to the site of burning. The lower TTI of the samples containing APP alone is mainly due to the emission of more pyrolysis gases, reflecting the weaker swelling of the substrate and the generation of a porous structure as discussed above. The TTI is therefore increased by the more compact char structure in the PLA/APP/ST composites.
Figure 4a,b shows the THR curves of pure PLA and the PLA composites.
Figure 4a indicates that the THR of pure PLA was 58 MJ m
−2 whereas the PLA/APP20 and PLA/APP20/ST7 composites emitted only 37 and 24 MJ m
−2, respectively. The PLA/APP20 and PLA/APP20/ST7 composites therefore limited the total amount of fuel accessible for burning, which confirms the superior FR properties of these composites. The combination of starch and APP makes the composites more flame resistant. The formation of intumescent char on matrix surface improves the thermal insulation between the flame and material’s surface. This extinguishes the flame by preventing access to combustible gases and oxygen at the site of the fire.
The combination of ST and APP makes the composites more flame resistant, and the formation of intumescent char on the matrix surface introduced a layer of thermal insulation between the flame and the surface of the material, which extinguished the flame by preventing contact with combustible gases as well as oxygen. The high concentrations of APP and ST diluted the polymer matrix, providing less material for continued burning. Thermal decomposition therefore led to the dehydration of APP, and the resulting water vapors cooled the gas phase and diluted the fuel, thus reducing the total heat release (THR) in proportion with the increasing APP content. Due to the endothermic decomposition of APP, the heating of the condensed phase was also limited. The presence of ST exacerbated this effect because the emission of pyrolysis gases was inhibited by the formation of the char layer, which provided a physical barrier and enhanced the heat shielding effect. In previous studies involving PLA composites with other carbonization agents, the THR was much higher than the values reported here [
11,
31,
32].
Figure 5a,b shows the residual mass% after burning for pure PLA, PLA/APP, and PLA/APP/ST composites. No residual mass was left following the burning of pure PLA, but both PLA/APP20 and PLA/APP20/ST7 left mass residues corresponding to 22.74% and 43.00% of the starting mass, respectively, as shown in
Table 2. The relatively large proportion of residual mass (char residue) for PLA/APP20/ST7 probably reflects the development of a nanostructure that hindered the passage of fuel and heat during combustion. The higher residual mass correlated with the production of more char, which in turn reflects the lower THR values. The greater residual mass also reflects an increase in char formation due to the combined effect of the acid and carbonization agent. The percentage residual mass achieved in this study is also higher than that reported in previous studies of PLA composites containing alternative carbonization agents [
17,
33].
Figure 6 shows images of the residual samples after cone calorimetry test. As stated above, there was almost no residue of pure PLA, but the samples containing APP and starch presented intumescence with char on the surface, which was thicker and more stable in the case of PLA/APP20/ST7. The char residues of PLA/APP10, PLA/APP15 and PLA/APP20 were loosely bound due to the non-cohesion of the agglomerates, and the structure in each case was porous and discontinuous due to insufficient char formation as indicated by the SEM analysis of char residues in
Figure 7. Heat and mass transfer therefore could not be inhibited effectively in these composites. In contrast, the samples containing ST (particularly PLA/APP20/ST7) produced a more compact char (
Figure 7) with a dense and uniform structure, reducing the heat and mass transfer to inhibit combustion and prevent further burning of the underlying polymeric substrate. These char structures were stable, more uniform, and compact due to the cohesion of the agglomerates. ST particles were supposed to fill the empty spaces between the APP particles, with a resulting increase in density. The thickness of the samples containing ST also increased dramatically due to char formation after burning, from an initial thickness of 3 mm to approximately 1.5–2.0 cm.
3.3. Thermogravimetric Analysis
The thermal decomposition and thermal stability of polymers is most effectively assessed by thermogravimetric analysis (TGA). The thermal degradation and mass residue of the samples were compared to determine the influence of flame retardants and starch on PLA-based composites. TGA curves and data for all the composites heated in a nitrogen atmosphere are presented in
Figure 8a,b and in
Table 3, respectively.
In
Table 3, the temperatures corresponding to 5% and 50% weight loss for each composite are represented by the T5 and T50 values, respectively, whereas the temperature corresponding to the maximum rate of weight losses is represented by T max. The degradation of pure PLA started at 325 °C and 50% loss occurred at 372 °C, with no residue left at 700 °C. A similar trend was observed for PLA/APP10 for the T5 and T50 temperatures, but the residue left at 700 °C was 5.90% of the initial mass. For PLA/APP15 and PLA/APP20, the initial decomposition temperatures and thermal stabilities were greater than the corresponding values for PLA/APP10, with 8.16% and 9.21% residual mass left at 700 °C. The introduction of starch further improved the thermal stability of the composites. The initial decomposition temperatures and thermal stabilities of all composites containing starch are higher compared to composites without starch. For example, composite PLA/APP20/ST3 increased the residual mass at 700 °C from 9.21% to 13.34%, but this increased even further to 19.30% in the case of composite PLA/APP20/ST7.
Figure 8a represents the TGA curves for PLA/APP composites in comparison to pure PLA whereas
Figure 8b shows TGA curves for PLA/APP/ST composites. These composites differ in terms of their initial decomposition temperatures and thermal stabilities. The initial decomposition temperature of PLA/APP20/ST7 was 373 °C, compared to 365 °C for PLA/APP20/ST3, and the residues left at 700 °C were 19.30% and 13.34% of the initial mass, respectively. PLA/APP20/ST7 is therefore more thermally stable, reflecting the denser and more compact char layer as discussed above. These TGA data are in strong agreement with the LOI, UL-94 and cone calorimetry experiments, indicating that composites containing starch are superior in performance to composites containing APP alone.
3.5. Mechanical Testing
The mechanical properties of composites are dependent on the actual stress sharing between matrix and the additives incorporated. Therefore, in order to get better mechanical properties of a composite a uniform interfacial bonding between additives and matrix is needed. Moreover, the size of particles, wt % (w/w) of additives incorporated as well as the adhesion between additives and matrix influence the mechanical properties of polymer composites. As indicated in SEM images in the previous section a weak interfacial bonding between additives and polymer matrix was observed, due to which clustered and agglomerated particles were formed which affected the mechanical strength of the composites.
It can be seen in
Table 4 that the tensile strength and elongation at break of pure PLA was 69.19 (MPa) and 2.49% respectively. However, with the addition of APP alone the tensile strength and elongation at break started to decrease and reached to 45.62 (MPa) and 1.98%, respectively, when 20 wt % of APP (PLA/APP20) was incorporated in PLA matrix. When starch was incorporated together with APP in polymer matrix (PLA/APP/ST), tensile strength and elongation at break was further reduced. The reduction in mechanical properties of PLA/APP and PLA/APP/ST composites is mainly due to weak interfacial bonding initiated by the difference in polarity among PLA matrix, APP, and starch additives.
Another reason of weaker mechanical properties could be due to the degradation of PLA as well as of starch during preparation of PLA composites due to higher extrusion temperature which might have reduced the adsorbed chains mobility on the surface of the particles. Therefore, in order to improve the mechanical properties of PLA composites a uniform dispersion of additives in polymer matrix may be required which sometimes can be obtained by the use of a compatibilizer. Although the addition of starch in polymer matrix decreased the mechanical properties of the composites, however extraordinary improvements in the flame-retardant properties of these composites were seen.