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A response surface methodology was used to analyze the flow rate, power, and time factors of plasma surface treatment. Surface free energy (SFE) of treated glass fiber-reinforced composites showed a strong quadratic dependence on flow rate, power, and time, with significant interaction between time and power. Optimized factors predicted a maximum SFE of 78.63 mN/m, which matched well with the measured value of 77.42 mN/m, accounting for 2.46 times increase in SFE against untreated case. Moreover, with plasma treatment, the SFE’s polar component became dominant (99%) as also confirmed with FTIR spectroscopy. Fracture toughness testing of fresh and aged adhesive joints proved a more stable interface for plasma-treated specimens due to the covalent bonds facilitated by the functional groups formed during the treatment. Consequently, the fracture toughness of the plasma-treated specimens did not drop after seawater immersion, while that for the untreated and sand-treated specimens showed about a 15% drop. 

ABSTRACT This study explores how a sieving step of waste cellulosic fiber and fine (WCFF) mixture affects the performance of WCFF‐loaded polypropylene (PP) composites and whether the separation of fines from fibers offers an added benefit. The WCFF samples were downsized, and four different filler size ranges were sieved using a series of mesh sizes from 4 to 0.85 mm. The WCFF/PP composites were then compounded at 20 wt.% loading of WCFF using a twin‐screw extruder. Incorporating WCFF increased the tensile strength to 41.28 MPa and the modulus to 3207 MPa, accounting for 28% and 38% enhancements, respectively. Interestingly, the greatest improvements were associated with the nonsieved WCFF case, and the sieved WCFF fibers provided only marginal enhancements over virgin PP. The outperformance of nonsieved WCFF was attributed to the synergistic reinforcement of hybrid fibers and fines as well as the maintenance of longer fibers in the system. However, the strain at break and impact strength of PP decreased after introducing WCFF. Moreover, the complex viscosity and storage modulus increased with an increase in the filler size, due to the formation of a more effective percolative network. The PP's crystallinity exhibited a relatively strong dependency on the sieving, where WCFF samples with short‐aspect‐ratio fillers promoted the crystallinity significantly. It was also found that the WCFF degradation onset temperature increased once it was incorporated into PP. This study suggests that waste cellulosic feedstocks can be utilized as a reinforcement without additional sieving to manufacture high‐performance and cost‐effective composites.