Abstract
Low-temperature properties of high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and their blends were studied. The analyzed low-temperature mechanical properties involve the deformation resistance and impact strength characteristics. HDPE is a bimodal ethylene/1-hexene copolymer; LDPE is a branched ethylene homopolymer containing short-chain branches of different length; LLDPE is a binary ethylene/1-butene copolymer and an ethylene/1-butene/1-hexene terpolymer. The samples of copolymers and their blends were studied by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), 13C NMR spectroscopy, and dynamic mechanical analysis (DMA) using testing machines equipped with a cryochamber. It is proposed that such parameters as “relative elongation at break at −45 °C” and “Izod impact strength at −40 °C” are used instead of the ductile-to-brittle transition temperature to assess frost resistance properties because these parameters are more sensitive to deformation and impact at subzero temperatures for HDPE. LLDPE is shown to exhibit higher relative elongation at break at −45 °C and Izod impact strength at −20 ÷ 60 °C compared to those of LDPE. LLDPE terpolymer added to HDPE (at a content ≥ 25 wt.%) simultaneously increases flow properties and improves tensile properties of the blend at −45 °C. Changes in low-temperature properties as a function of molecular weight, MWD, crystallinity, and branch content were determined for HDPE, LLDPE, and their blends. The DMA data prove the resulting dependences. The reported findings allow one to understand and predict mechanical properties in the HDPE–LLDPE systems at subzero temperatures.
Highlights
At the first stage of this study, we investigated a series of bimodal high-density polyethylene (HDPE) samples; low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) were studied at the second stage; and the HDPE/LLDPE blends, at the third stage
It has been demonstrated that while the frost resistance problem is not relevant for HDPE used to manufacture plastic pipes because of the high Eb −45 C and AIzod −40 C values, modification is needed for HDPE used for manufacturing steel pipe coatings with a much smaller layer thickness because low-temperature mechanical properties are significantly deteriorated with decreasing molecular weight of the polymer
When choosing low-density modifying agents for HDPE, it was revealed that the LLDPE terpolymer (C2 /C4 /C6 ) exhibits the best low-temperature properties compared to those of homo-LDPE and binary LLDPE copolymer (C2 /C4 ) due to a combination of short-chain branching involving the 1-butene/1-hexene blend and slight modification of the molecular weight distribution (MWD) according to the two-reactor polymerization scheme
Summary
High-density polyethylene (HDPE) is one of the most common engineering plastics produced on a large scale. Due to its good mechanical properties, rigidity, wear resistance, and chemical inertness, HDPE is widely applied in many areas of human activity. HDPE can be produced as both a unimodal and a multimodal polymer [1]. These products are used for manufacturing plastic pipes, containers, bottles and films (i.e., for domestic consumption). In some specific industries (external coating of gas transportation pipelines), there is a demand for HDPE, but stricter requirements are posed on its mechanical and impact resistance characteristics in this case [2,3]. There currently is interest in wider use of gas because the carbon footprint generated by its conversion is lower compared to that generated by consuming other renewable energy sources, which is an important aspect in terms of the environmental impact
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