Abstract
Due to its relatively simple structure, low-density polyethylene (LDPE) can be considered as a model polymer for the study of its properties. Herein, the effect of processing variables on the microstructure and crystallinity of injection-molded LDPE specimens was quantitatively determined. The polymer was injected at different temperature conditions in the barrel and the mold. The specimens were characterized by scanning electron microscopy and X-ray diffraction. With the data obtained, an analysis of variance (ANOVA) was carried out, and response surface graphs (SRP) were constructed to quantify and to observe the behavior of the processing variables, respectively. Different models were obtained to predict the effect of the experimental factors on the response variables. The results showed that the interaction of the two temperatures has the greatest effect on the size of the spherulite, while the temperature of the mold affects the crystallinity. The SRP showed different behaviors: for the spherulite, the size increases with the mold temperature, while for the crystallinity, higher values were observed at an intermediate mold temperature and a low melt temperature. The results presented herein are valuable for setting empirical relations between the microstructure, crystallinity, and the molding conditions of LDPE.
Highlights
Polyethylene (PE) is a commodity polymer widely used in the plastics market, occupying about one-third of the total production in the world [1]
Because of its semicrystalline nature, the microstructure presents ordered regions of polymer chains known as spherulites, which are conformed of fold back chains or lamellae growing out from nucleation points, interconnected by disordered polymer chains or amorphous regions [6]
At low magnification (100×), spherulites were observed as a discontinuous grain structure, surrounded by continuous regions where the amorphous polymer remained after the etching
Summary
Polyethylene (PE) is a commodity polymer widely used in the plastics market, occupying about one-third of the total production in the world [1]. Leyva-Porras et al [8] developed an empirical approach derived from the ANOVA and surface response methodology (SRM) to predict the Young’s modulus of LDPE processed at different mold and barrel temperature conditions of injection-molded specimens Among their findings, it was reported that mold temperature was the main variable affecting the Young’s modulus, and qualitatively, they discussed the microstructure changes in terms of the degree of crystallinity. It was reported that mold temperature was the main variable affecting the Young’s modulus, and qualitatively, they discussed the microstructure changes in terms of the degree of crystallinity It is Polymers 2021, 13, 3597 still necessary to establish empirical relationships based on phenomenological observations that can quantitatively associate the microstructure with the macroscopic properties. The work contributes to understanding the way the microstructure of LDPE varies with the injection-molding processing conditions and its relation to the macroscopic properties, such as crystallinity
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