Mechanical characteristics of low-density polyethylene and materials, perspective for applying in impervious structures

Structure protection against groundwater and surface water (spring waters, precipitation and flood) is a topical branch in hydrotechnical, road, industrial and civil engineering. In the process of sumps, waste collectors, heaps and pool converters operation there is a danger of a leakage of ecologically unsafe substances, which differ from each other in composition and the degree of aggressiveness. Natural and artificial ponds rise the groundwater on adjoining territories, which prevents the economic activity of people. Impervious structures are used for ground and soil structures protection [1, 2]. Different ways of installation of impervious structure elements in soil, foundations, slopes, weirs and dams are known. Laying of the polymer panels on the slopes of the soil structures under the layer of the bulk material, trench and trenchless (vibratory) screen drives are applied. Polymer impervious screens, curtains and membranes are widely spread. Most of the film members were made and are made of low-density polyethylene (LDPE), but other polymer and composite polymer materials are becoming more and more popular [3–5].

Almost all materials have properties changing in time (ageing), especially characteristic it is for polymer and composite polymer materials [6–9], as well as steels and alloys [10]. The speed of ageing depends on sensitivity of a material to acting factors and their intensity. The properties changes can be reversible and irreversible. Reversible changes disappear almost entirely after external factors eliminated. The opposite situation takes place in irreversible changes. As we aim to predict the durability of ready-made materials, so we can define ageing as appearance of transformation during storage and exploitation.

The objectives of the present paper are:

1. To estimate the impact of the long-term ageing under load on the mechanical characteristics of the LDPE samples.
2. To compare the results of the experiment with LDPE samples stabilized with 2 % soot content under long-term ageing conditions and under influence of CHP ash.

1. The estimation of the influence of the long-term ageing on mechanical characteristics of low-density polyethylene during storage and under load
   1. The samples and equipment

LDPE samples were made in laboratory complex in All-Union Scientific Research Institute of Hydraulic Engineering named after B. E. Vedeneev. The samples had a shape of a shoulder blade with 25 mm working part length and 3,5 mm width. The thickness was ranged from 0,048…,0,064 mm to 0,16…0,23 mm.

Samples of stabilized LDPE had a shape of a shoulder blade with 30 mm working part length, 3,5 mm width and thickness mm.

LDPE and stabilized LDPE samples were cut in the direction and cross-direction of the film extrusion.

Uniaxial sample tension tests were carried out using FPZ-100/1 and RMI-5 (РМИ-5) installations with different capture displacement speeds (v, mm/min.). Depending on the thickness, the samples were grouped in three, which then were tested and results of the experiment were presented as the mean values. The experiments with long-term loaded samples were held on one specimen. The dimensions of these samples were determined before tension test.

1.2. The results of tests

Mechanical characteristics (σ_ρr– proportional limit, и — limit stress and deformation, — elastic modulus) of low-density polyethylene (LDPE) under tension (v = 50 mm/min., ) are shown in Table 1.

Table 1

LDPE samples (in the direction of the film extrusion) have more stable mechanical properties and higher density than samples, made crosswise of the film extrusion.

Limit stress and the value of the elastic modulus are higher in samples cut in the direction of the film extrusion, than in samples cut in the cross-direction of the film extrusion. The influence of the sample thickness in specified range on LDPE mechanical characteristics (Table 1) is insignificant. The results of tension tests (V = 50 mm/min.) of LDPE samples (along of the extrusion) with thicknesses δ = 0,048…0,064 mm confirm this conclusion.

Table 2

Mechanical characteristics (in the direction of the extrusion) of LDPE samples

LDPE samples (δ = 0,16…0,23 mm) were loaded by constant tensile load during 189–194 months (~16 years). In the first series of experiments (189 months), initial stresses of ageing under load σ_Н were recorded. In the second series of experiments (194 months), beside σ_Н, the relative deformation of LDPE samples was recorded in the beginning (ε_Н) and in the end (ε_к) of ageing. Tables 3–5 show the results of experiments (σ_ut — the durability limit under tension, ε_nt — deformations corresponding to the durability limit). Table 6 shows the data of the comparison of obtained results.

Table 3

Mechanical characteristics of LDPE (in the direction of the extrusion) after long-term (189 months) uniaxial compressive loading

Table 4

Mechanical characteristics of LDPE (in the direction of the extrusion) after long-term (194 months) uniaxial tensile loading

Table 5

Table 6

Mean values of LDPE mechanical characteristics in original state and after long-term ageing under the load

Let us consider the results of the LDPE stabilized by 2 % content of soot tests. Samples of the first series were kept in conditions of heated (without sunlight access) warehouse space during 17 years, and the samples of the second series — in the ash of Magadan CHP during the same time. The samples are oriented in the cross-direction of the extrusion. In each series of the tests, 3 samples were used. Table 7 shows results of the uniaxial tension tests and Table 8 shows their comparisons.

Table 7

Mechanical characteristics of stabilized LDPE samples

Table 8

Comparison (in%) between the values of the LDPE mechanical characteristics of the second series and the first one

Table 9 shows the results of uniaxial tension tests of LDPE samples (stabilized with 2 % soot content) in original state (I) and after ageing (II) during 18 years. The deformation speed was 50 mm/min.

Table 9

Mechanical characteristics of stabilized LDPE in original state and after 18 years of ageing in natural storage conditions

Conclusions

The film of the examined polyethylene of low density (LDPE) in original state has stable mechanical characteristics with some anisotropy related to the direction of the extrusion.

The LDPE long-term ageing (16–17 years) under load in heated warehouse space conditions decreases the proportional limit on 45 % (average), besides the values of the elastic modulus increases on 22 % in the direction of the film extrusion and decreases on 12 % in the cross-direction of the extrusion. The LDPE ageing under load with tensile stresses less than 3 mPa decreases limit stresses under short-term uniaxial tension up to 17 %. The LDPE ageing under load with tensile stresses higher than 3 mPa increases limit stresses under short-term uniaxial tension (in the direction of the extrusion) on 8 % with the deformation decrease more than triple. For samples cut in the cross-direction of the film extrusion the changes are insignificant.

The mechanical characteristics of unstabilized and stabilized by 2 % soot content LDPE are insignificantly different. Natural ageing during 18 years in heated warehouse space conditions of stabilized LDPE does not lead to significant changes of limit stresses and deformations under uniaxial tension.

The influence of the Magadan CHP ash during 17 years does not change significantly the stabilized LDPE mechanical characteristics. The change of the elastic modulus (the average decrease on 29 %) has to be considered in calculations of film impervious structure elements.

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