## 3 Results and Discussion
### Mechanical Properties  
The effects of album sludge addition on the mechanical properties of the composites are presented in Figure 4. The tensile strength results (Figure 4a) indicate that the 2 wt.%  
Figure 3: (**a**) Stress vs. strain curves of degraded samples of neat PBAT, 63 \(\upmu\)m composite, and 250 \(\upmu\)m composite and (**b**,c**) their visual inspection (samples from bottom to top: neat PBAT, 63 \(\upmu\)m composite, and 250 \(\upmu\)m composite).  
composite sample exhibits a value comparable to neat PBAT, albeit with a relatively large error bar. A marginal increase in tensile strength can be observed for the 5 wt.% sample, while a significant reduction occurs at higher filler concentrations (10 wt.% and 20 wt.%). This trend suggests a balance between the reinforcing effect of the filler and the adverse effects caused by processing challenges.  
It is well recognised that particles incorporated into composite materials may act as barriers to stress and crack propagation when they are well-dispersed, enhancing a material's resistance to mechanical deformation and improving its overall strength [28]. In this study, such reinforcement effects were more pronounced in samples with lower filler concentrations. The processing method employed appeared to facilitate better particle dispersion, consistent interfacial bonding, and minimal disruption to the PBAT matrix at 2 wt.% and 5 wt.% filler levels, resulting in an effective reinforcement mechanism. This was particularly evident in the 5 wt.% sample. In contrast, at higher filler concentrations, poor particle dispersion and agglomeration became dominant, leading to detrimental effects. Numerous studies, such as those by [PERSON] et al. [29], [PERSON] et al. [30], and [PERSON] et al. [31], have analysed how agglomeration creates localised stress points that reduce the mechanical strength of composite materials. Similarly, in this study, the agglomerated regions of alum sludge particles in the polymer matrix serve as stress concentration sites, hindering effective load transfer and disrupting matrix continuity. This reduces the composite's overall strength and increases its susceptibility to brittle fracture.  
The % elongation at break of the composite samples was generally lower than that of neat PBAT (Figure 4b), as the addition of particles restricts the matrix's plastic deformation, resulting in a stiffer and less ductile material. Similar trends have been widely reported in the literature [32]. Interestingly, the 5 wt.% sample showed an anomalous increase in % elongation at break (Figure 4e), which could be attributed to an optimised filler concentration that promotes improved dispersion and interfacial bonding of the composite  
Figure 4: Mechanical properties of neat PBAT and its composites with alum sludge at various concentrations: (**a**) tensile strength, (**b**) % elongation at break, (**c**) Young’s modulus, (**d**) representative stress vs. strain curves, and (**e**) an image showing elongation of the 5 wt.% composite sample during tensile testing.  
phases. However, further investigation is needed to confirm this hypothesis. Additionally, the significant error bars observed for many samples emphasise the need to test a larger number of specimens to ensure statistical reliability.  
Figure 4c shows the Young's moduli for the PBAT polymer and the composites. A gradual increase in modulus with a higher filler concentration can be observed, indicating enhanced rigidity due to the reinforcement particles restricting plastic deformation in the matrix. However, this reinforcement effect is relatively modest, which is possibly attributed to the amorphous nature of alum sludge. Previous studies [33] have shown that non-heat-treated, dried alum sludge exhibits low crystallinity, which limits its contribution to modulus enhancement compared to crystalline fillers. Figure 4d depicts the tensile behaviour of the studied samples: a linear elastic region before the yield point, followed by plastic deformation for all samples.