3.3.1. High PLA Content
A reference composition with a high PLA content was formulated with a filler loading of 25 percent chalk and 75 percent of a nine to one mixture of PLA and PBS (overall composition: PLA–PBS–chalk = 67.5:7.5:25). We replaced the mineral filler by SBP in three steps. In addition, we varied the amount of a maleic anhydride-modified PLA coupling agent. From a rule of thumb in natural fiber-based composites, we chose a starting level of one percent coupling agent per 10 percent natural fiber.
Replacing the mineral filler with fine SBP leads, as expected, to a reduced tensile modulus of elasticity (
Figure 4a) due to the weaker inherent stiffness of the SBP in contrast to chalk. Surprisingly, however, for fine SBP and no added compatibilizer (
Figure 5a), blue data points), this decrease is small and almost negligible considering the standard deviation. We suspect that another process is responsible for compensating the expected reduction in modulus caused by changing from chalk to SBP.
One possible reason leading to a higher modulus in a PLA matrix is an increase in crystallinity. According to Aliotta et al. [
38], the amorphous PLA phase contributes 3200 MPa and the crystalline phase contributes 8500 MPa to the modulus. Increasing the crystallinity of the PLA phase in our composites by one percent will therefore raise the tensile modulus by ~35 MPa. Regardless of the amount of SBP and the amount of compatibilizer, the moduli of all the samples containing SBP and a compatibilizer are in the range of ~3650–3850 MPa. Considering the standard deviations, the use of SBP with a compatibilizer leads to an average decrease in the modulus of elasticity of ~8.5 percent (∅~3740/4080) (
Figure 5a) compared to the samples with SBP without a compatibilizer. The expected higher stiffness of the composite by using a compatibilizer for improving the SBP–polymer matrix interface must have been counteracted by morphological changes, e.g., less crystallinity. Nevertheless, the good effectiveness of the coupling agent is shown by the dependence of the tensile strength on the amount of used compatibilizer (
Figure 5b): the strength of uncompatibilized composites is reduced by up to 27 percent at 25% SBP content. Due to the strong difference in their polarities, there is only weak transmission of forces between the components without a compatibilizer. As expected, the strength increases when the compatibilizer is added. Upon adding larger quantities, the SBP composite almost reaches the initial strength of the chalk-filled PLA–PBS blend. However, because of the highly polar nature and the high surface area of the fine type of the SBP in comparison with commonly used natural fibers, amounts of up to 6 percent coupling agent per 10 percent SBP are required, which is not economically justifiable.
Figure 6 shows some cryogen-fractured surfaces of the PLA–PBS–SBP composite with 25 percent fine SBP.
Figure 6a shows an overview of the fracture surface, which does not show any specific fracture boundaries and appears homogeneous in itself. In the magnifications in
Figure 6b,c, holes of pulled-out SBP as well as well-bonded SBP particles can be seen. Additionally, a surface that may be caused by a delaminated particle of SBP is shown in
Figure 6c. In
Figure 6d, a very well-integrated SBP particle is shown.
The use of compatibilized coarse SBP results in a similar picture for the modulus of elasticity and tensile strength (
Figure 7a,b). The modulus is lowered by adding SBP for chalk with no effect of the compatibilizer, considering the standard deviations. In addition, the strength is lowered and almost no dependence on the amount of compatibilizer can be seen. Higher amounts of coupling agent (e.g., 3 or 4% per 10% of SBP,
Figure 7b) even lead to lower values.
However, regarding the strength, a small effect of the compatibilizer can be seen by comparing the values when using no (or 0.5%) compatibilizer per 10% SBP with the ones for higher compatibilizer addition (1.0 or 1.5).
Nevertheless, a plateau value in the range of 46 MPa seems to be achievable, but there is no strong dependency of the strength on the amount of compatibilizer, as in the case of the fine SBP type. This behavior of the strength of the composite may be due to the internal strength of the coarse SBP particle, since SEM images of fracture patterns of cryogenically broken test specimens show cracks in the big SBP particle (
Figure 8a). Therefore, the inherent weakness of compressed cell-scale fragments of SBP is responsible for the reduced strength compared to the fine SBP type with subcellular particles.
The usage of a coupling agent type compatibilizer, anchoring one phase to the other by chemical bonds and chain entanglement, is one way to improve the stiffness and strength of a composite. Since the phases in a composite must be close to each other at the molecular level, the wettability of one phase by the other is crucial. Due to the high content of beta-glycosides and glucuronic acids in SBP (cellulose + hemicellulose + pectin), the surface may be covered with hydroxyl, ester and carboxyl groups, resulting in a highly polar surface with solubility parameters greater than the 29 or 36 J0.5cm1.5 as in methylated, hydroxypropylated or pure cellulose [
39,
40]. Therefore, a modification of the surface by a reactive monomer changing the surface energy and promoting adhesion is another way of compatibilization [
41]. PLA and PBS are mid-polar polyesters with solubility parameters of about 21 to 22 J0.5cm1.5 [
37,
42] using the Hoftyzer–van Krevelen or Hansen three-dimensional model. Using the one-dimensional Hildebrandt model with summarized data from several authors from the book of Robeson [
41], the parameters are about 18.8 to 20.4 J0.5cm1.5 for the glassy state of PLA and 18.3 to 19.9 J0.5cm1.5 for the rubbery state of PBS. Covering the surface of SBP with mid-polar molecules to reduce its polarity closer to that of the polymer components should therefore improve wettability. A commercially available reagent is 1,2,3-propanetriol-glycidyl-ether. This reactive monomer of epoxide-type can react with the acid groups of the pectin or the hydroxyl groups of the cellulose at an elevated temperature in short times.
Figure 9b shows that the surface modification of the fine SBP type by using 1,2,3-propanetriol-glycidyl-ether has a great effect on the strength. Only 0.5 percent per 10 percent of SBP improves the strength by 18 percent, even without the addition of the PLA-based compatibilizer. The further increase in tensile strength with the addition of more epoxide is less pronounced and not completely uniform (1% compatibilizer, 1.5% epoxide). The trend of the data points suggests a maximum tensile strength value in the range of 50–52 MPa. No effect of the addition of the glycidyl ether on the modulus can be seen, taking standard deviations into account (
Figure 9a). In the case of the coarse SBP type, the influence of the adhesion promoter is significantly lower. The strengths of the compounds with 25% SBP and no chalk do not exceed 45 MPa by adding 1% coupling agent and 0.5, 1 or 1.5% adhesion promoter, and the data can also be explained in part by standard deviation (see
Table 3).
Chen et al. [
31] reported a similar effect when treating a PLA–SBP composite with pMDI (polymeric methylene diphenyl diisocyanate). The strength of a 7:3 composite of PLA and SBP increased from 37 to 50–60 MPa when adding 0.5 to 3% pMDI, with the highest value occurring with the application of 2% pMDI. As they mixed the components before extrusion, we believe that the absorbed highly reactive pMDI reacted preferably with the carboxyl and hydroxyl groups at the surfaces of the SBP (a 30 µm-sized type). From our findings, we conclude that in their experimental set-up pMDI acted as an adhesion promoter or surface modification agent and did not behave as a coupling agent, anchoring the phases together, as claimed in their paper.
Taking into account the respective costs of the compatibilizers used and the application-specific requirements for the strength of PLA–PBS–SBP composites, cost optimal composition can develop.
All of the PLA–PBS (9:1)–SBP composites are characterized by brittle fracture failure (
Table 3). The impact strength tends to increase with a lower SBP content. It decreases with an increasing content of fine SBP and is generally poor with coarse SBP, irrespective of the compatibilizer content. All materials possess low notched impact strengths. The elongation at break remains on the level of the reference, with slight improvements upon adding specific impact modifiers (
Table 4).
The addition of a polyolefinic impact modifier (Acti-Tech) improves the impact behavior and the elongation at break only insignificantly when higher amounts are used. However, this is at the expense of the other mechanical characteristics: the modulus decreases by approx. 1 GPa, and the tensile strength by pprox. 20 Mpa. The core-shell-type impact modifier (Biostrength
®) slightly increases the unnotched impact resistance, but does not affect the elongation at break. From acoustic emission analysis, Finkenstadt et al. [
30] deduced that the de-bonding of phases takes place before rupture. Therefore, the fracture mechanics may be determined by the weakest structures of the SBP and its interfaces.
3.3.2. Medium and High PBS Content (PBS Rich Compound)
Choosing a higher PBS content will soften the composite and will make the composite more easily degradable at ambient and slightly elevated temperatures as in composts. In addition, in many commercialized PBS compounds, chalk is replaced by talc. Therefore, we formulated compositions with talc and less filler content due to the higher price and the higher modulus of talc. A filler content of 16% talc and 84% of a four to three or three to four mixture of PLA and PBS (PLA:PBS:talc = 48:36:16 or 36:48:16) was chosen. We replaced the mineral filler by SBP in two steps.
Figure 10 and
Figure 11 show the tensile properties of the PLA–PBS (4:3) and (3:4) composites, respectively, with 0, 8, and 16 percent SBP. In contrast to the PLA–PBS (9:1) composites, the modulus and strength decrease almost linearly with higher amounts of SBP using the fine or coarse type of SBP and small amounts of the compatibilizer. For the composites with the fine SBP type, we tested the influence of higher amounts of the compatibilizer exemplarily. Even though the compatibilizer is based on PLA (PLA-g-MAH; to our knowledge no commercialized PBS-g-MAH is available yet), the compatibilizer also works in PBS-rich composites.
These surprising findings may be due to the following facts: the PLA-g-MAH compatibilizer agent should mainly be solved in the PLA phase due to the poor miscibility of PLA and PBS [
37]. As an example, the relative amounts (mass-based) of PLA-g-MAH in the PLA phase in the composites with higher PBS amounts (PLA–PBS = (4:3) or (3:4), respectively) with 8% SBP and 0.8% PLA-g-MAH or 16% SBP and 1.6% PLA-g-MAH, respectively (
Figure 9a and
Figure 10a), are 1.7 and 4.4%. In the PLA–PBS (9:1)–SBP composites with 8.33 or 16.66% SBP and 0.83 or 1.66% PLA-g-MAH, the relative amounts are 1.2 and 2.5%, respectively. Assuming a PLA coating of the surfaces of SBP in the PLA–PBS = (4:3) or (3:4) composites, the improvement in the modulus and strength at higher amounts of compatibilizer can be explained. Furthermore, the transesterification of PBS with PLA-g-PLA may take place at the higher amount of interfaces in the PLA–PBS = (4:3) or (3:4) composites during compounding, leading to a better compatibility of the polymer phases to some extent.
In contrast to the PLA-rich PLA–PBS (9:1)–SBP composites, an influence of the SBP content on the crystallization of the PLA–PBS (4:3)–SBP composites can be seen. While the SBP-free composites and the composites with 8% talc and 8% SBP crystallize almost completely from the melt upon cooling with 20 Kmin−1, samples without talc and with 16% SBP show significant post-crystallization enthalpies in the range of 20–25% of the sum of the melt enthalpies of PBS and PLA. In PLA–PBS (3:4)–SBP composites, this denucleating effect is only seen to a much more limited extent in the sample with 16% fine SBP and without talc.
The nucleating effect of talc on PLA is well known, and Pivsa-Art Y et al. report that in PLA–PBS (8:2/6:4)–talc composites, the crystallization of PLA is promoted and the crystallization of PBS is inhibited [
43,
44,
45]. Thus, in talc-filled PLA–PBS (4:3)–SBP composites, the replacement of talc by SBP has a denucleating effect. In the talc-free PLA–PBS (3:4)–SBP composites, the denucleating effect of talc on the PBS main phase is outweighed in particular by finely ground SBP. It can be deduced that the chalk and SBP-filled PLA-rich PLA–PBS (9:1)–SBP composites can be adjusted in their thermal mechanical properties to some extent by adding talc, if necessary.
Mechanical data of the PLA–PBS (4:3) and (3:4) composites are summarized in
Table 5. The tensile modulus and strength are reduced in comparison to the PLA-rich composites, as expected since PBS is a softer material. As in the case of the PLA–PBS (9:1)–SBP composites, the notched impact strength is poor for these materials. Due to the higher amount of soft PBS, the elongation at break is somewhat enhanced on average. However, the absolute values still are very low. Requirements from real applications must show if the strength of the composite is high enough for accepting elongation at break in the single-digit range.
In
Figure 12a,b, images of cryogenically fractured samples of PLA–PBS (4:3) composites with 16% SBP are shown.
Figure 12a shows many particle pull-outs and a rough fracture surface. In
Figure 12b, cracks through SBP particles and at the interface are clearly visible.
Figure 12c,d show images of cryogenically fractured samples of compounds with the highest PBS content (PLA–PBS (3:4)). A crack in the matrix phase near the SBP–matrix interface can clearly be seen (
Figure 12c). The fracture pattern in
Figure 12d with well-bonded coarse SBP particles, but also some delamination inside the particles, resembles a delaminated surface (conchoidal fracture).