Time‐dependent properties of graphene nanoplatelets reinforced high‐density polyethylene

Abstract

The deformation of polymers at constant applied stress is one of their major drawbacks, limiting their use in advanced applications. The study of this property using classical techniques requires extensive testing over long periods of time. It is well known that reinforced polymers show improved behavior over time compared to their neat counterparts. In this study, the effect of adding different amounts of graphene nanoplatelets (GNPs) on the time‐dependent properties of high‐density polyethylene (HDPE) is investigated using short‐term creep tests and load/unload recovery tests. The results are discussed in terms of the test profile and the influence of loading history. Viscoplasticity/viscoelasticity analysis is performed using Zapas model and by comparing creep, creep compliance and pure viscoelasticity curves. The results show that the reinforcement of 15 wt% GNP have the most significant effect on the time‐dependent behavior, reducing the strain by more than 50%. The creep compliance curves show that nano‐reinforced HDPE behaves nonlinearly viscoelastically even at very low stresses. In addition to demonstrating the effect of nano‐reinforcement, the discussion of the results concludes that the influence of loading history can be quite significant and should not be neglected in the design and evaluation of material behavior.

Time‐dependent properties of high‐density polyethylene with wood/graphene nanoplatelets reinforcement

“…For comparison, properties of the neat polymer and other nanocomposites analyzed in previous work are also presented when needed (full set of results are available in Ref. [24]). Specimens with dimensions 200 mm in length, 15 mm in width, and 4 mm in nominal thickness were cut by waterjet followed by thorough drying for 8 h at 80 C before testing.…”

“…Short-term creep tests were performed on a single specimen of each material formulation. One creep test consists of multiple loading ramps at a rate of 3.5 mm/min (based on extensometer) to specific stress for increasing time intervals interrupted by strain recovery steps following the procedure described in [21,24] as shown in Figure 1A. Specimens were loaded to the predefined stress, held at that stress for a time t 1 where creep strain is observed, then unloaded to almost zero load level (4 N) to recover the reversible strains and held at that load for a time 8t 1 .…”

“…In the following, when only VP strains are discussed independently from damage, they are referred to as such (VP strains), otherwise, they are referred to as IR-strains. 24 2. Cyclic loading-unloading-recovery ramps (further referred to as cyclic loading test) were applied to investigate the development of damage and its effect on the material response.…”

“…The efficiency and effects of the single type of nano-or micro-scale reinforcement as well as the synergistic effects of the combination of these two are investigated facilitated by the study and comparison of different concentrations of these reinforcements. An analysis approach presented in [29,32] and used in [12,24,33] has been used to separate the VE and VP responses and to allow studying the effect of the reinforcement on these behaviors individually. To the best of the authors' knowledge, no studies have been performed to characterize the extent to which these additives can affect the time-dependent performance of WPCs, or to separate their effect on the VP and VE behavior.…”

Modelling of Environmental Ageing of Polymers and Polymer Composites—Durability Prediction Methods

“…Creep lifetime is strongly related to the accumulation of irreversible strains; thus, models for predicting their evolution are of great interest. The viscoplastic strain is expressed as nonlinear functions of stress, time, temperature, etc., e.g., Zapas–Crisman model discussed elsewhere [ 65 , 112 , 116 , 117 ]. Identification of multiple model parameters requires an extensive testing campaign that is often not justified in terms of costs.…”