Search for: All records

ABSTRACT The processing and performance of multilayer films containing post‐consumer recycled polypropylene (rPP1 and rPP2) materials are investigated to understand the effect of layer multiplying elements (LMEs), die temperature, virgin polypropylene (vPP%) content, and polyethylene (PE) contamination in flexible packaging applications. Three‐layer coextruded films were created with virgin polypropylene (vPP) consistently applied as the outer layers while the core layer comprised recyclates with varying concentrations of polyethylene as an unintended contaminant to polypropylene. To enhance layer uniformity and interfacial interaction, a layer multiplying element (LME) was employed to increase the number of coextruded film layers from 3 to 9. Tensile properties (elongation at break, yield stress, and modulus) were characterized in both machine direction (MD) and transverse direction (TD); after which, multiple linear regression analyses were conducted on 45 observations to model the effect of each factor. The results indicated that the LME significantly enhanced elongation at break in TD by 1280% strain, while temperature and vPP fraction also contributed positively to ductility in TD (+341% and +2373%, respectively). However, PE contamination had a substantial negative impact on elongation in MD (−2449%) underscoring its embrittling role due to lack of compatibility with the PP matrix. Critically, LME partially mitigated the negative PE effect via an interaction term (PE*LME), improving elongation in MD by +3101%. Scanning Electron Microscopy (SEM) revealed a distinct, regular pattern of alternating polyethylene (PE) and polypropylene (PP) domains forming ribbon‐like fibrillar structures. This unique morphological arrangement suggests a self‐organizing behavior driven by immiscibility and flow‐induced alignment under extrusion conditions. The presence of regular alternating domains at near equal concentrations implies a balance among shear‐driven orientation, phase separation kinetics, and crystallization phenomena, resulting in an ordered micro‐fibrillar structure. Importantly, both monolayer and multilayer films containing rPP2 or rPP1/rPP2 blends exhibited these aligned, ribbon‐like fibrils oriented in the machine direction (MD). SEM analysis of fractured specimens further indicated that brittle failure was often associated with interfacial delamination, particularly in recyclate‐rich regions, whereas ductile failure exhibited entangled reinforcing fibrils, suggesting improved energy absorption and interlayer cohesion. Understanding and controlling this self‐organized microstructure could significantly enhance processing stability, mechanical properties, and potential applications of recycled polyolefin blends, offering novel strategies for tailoring recyclate morphology and performance. 

Abstract Additive manufacturing offers reduced lead time between design and manufacturing. Fused filament fabrication, the most common form of material extrusion additive manufacturing, enables the production of custom‐made parts with complex geometry. Despite the numerous advantages of additive manufacturing, reliability, reproducibility, and achievement of isotropic bulk properties in part remains challenging. We investigated the tensile behavior of a model polycarbonate system to explore what leads to different tensile properties, including sources of ductile versus brittle fracture. We utilized a one factor at a time (OFAT) design of experiments (DOE), printed single road‐width boxes, and performed tensile tests on specimens from these boxes. Additionally, we characterized the cross‐sections of parts printed under different conditions and their subsequent fracture behavior. The results demonstrate that isotropic bulk properties are achievable by printing at high speeds, and provide mechanisms to explain why. HighlightsPrinting at high speeds leads to improved mechanical properties.Printed samples undergo a mix of ductile and brittle failure.Jagged fracture path is associated with superior adhesion.High layer times lead to worse interfacial bonding. 

Abstract A ram extruder is described for high‐pressure extrusion of fully compounded thermoset rubber to achieve additive manufacturing. The extruder uses a piston driven by a geared stepper motor to provide volumetric displacement of the rubber charge residing in a temperature‐controlled barrel. Along with activators, accelerators, and a vulcanizing agent, the rubber compound is a formulation of 30 parts carbon black per hundred parts nitrile rubber. Sets of serpentine patterns are printed in parallel and transverse orientations relative to the load direction. From printing to post‐cure, the printed specimens exhibited linear shrinkage of 65% in the print direction. Although printed samples had relatively low void content compared with typical additively manufactured parts by material extrusion, significant decreases in the tensile properties were observed relative to compression‐molded specimens of the same rubber compound. The mean strain to failure was observed as 462% for compression‐molded samples, compared with 347% and 183% for printed specimens with parallel and transverse orientations. To reduce the shrinkage and increase the interfacial area between extruded roads, backstitch and sinewave diddling patterns were implemented to superimpose oscillatory motions along the print path with a periodicity of 1 mm. The specimens printed with the diddling patterns were observed to provide less shrinkage and improved properties compared to the regular serpentine patterns. The mean strain to failure in the transverse orientation, respectively, increased to 218% and 265% for the backstitch and sinewave patterns. Suggested future research is discussed, and the diddling program is provided in the appendix.