INVESTIGATION OF THE DURABILITY OF SECONDARY POLYOLEFINS IN AGGRESSIVE MEDIA UDC 539.219
R. A. Andrianov, M. N. Popova, and O. V. Pakhneva
Results of tests of secondary polyethylene and polypropylene used for manufacturing collectors of purification works for industrial enterprises are considered for various aggressive media.
It is very difficult to model any sewage because of its uncertain structure and a large number of its compounds. Therefore, to investigate the properties of secondary materials, we formulated the following problem: to investigate the influence of ferrous metal etching by a sulfuric acid (medium 1) and of liquid aggressive media of coking by-product manufacture (media 2 and 3) on the main sewage properties. After sewage analysis, it was established that the greatest influence on the durability of the examined materials in the above-indicated media have such components as sulfuric acid, phenol, and alkali. To investigate the chemical corrosion resistance of the examined polymers, the concentrations of components were considerably increased, namely, the sulfuric acid concentration was 10, 30, and 50%, the phenol concentration was 5, 10, and 20%, and the alkali (NаОН) concentration was 10, 30, and 50%. The effect of the aggressive media on polyethylene and polypropylene was judged from changes in the specimen weight, tensile strength, and relative elongation at rupture. Changes in the strength properties are characterized by the so-called continuity of the product: ψ=
where σ p is the current value of the tensile strength and σ•p is its initial value. The specimen weights were changed on disks 50 mm in diameter for preset thicknesses of the material (according to GOST 12020-72). The strength and strain parameters were investigated for the specimens cut as blades according to GOST 11262-80 and GOST 14359-69. Aggressive media with sewage characteristics were prepared. The aggressive liquid was poured in a 1-liter chemical glass covered by a tight cap into which a reflux condenser was inserted. The glass was put into a T-10 thermostat filled with water. In experimental design, the temperature is called the optimization parameter. Changes of the strength characteristics were the most important factor of the influence of aggressive media on secondary polyethylene and polypropylene. As indicated above, it is characterized by the continuity factor ψ taken to be the optimization parameter. The problem under study involves investigation of the simultaneous effects of the concentration of the medium, test time, and temperature on polymers. The dependence of the strength characteristics of polymers subject to the simultaneous influence of the above-indicated factors must be described by a first-degree polynomial: ψ = b0 + b1 x1 + b2 x2 + b3 x3 ,
where ψ is the continuity factor, x1 is the concentration of the medium, x2 is the test time, x3 is temperature, and b0 , b1 , b2 , and b3 are coefficients.
To substantiate statistically the reliability of the model, the results were statistically analyzed. To this end, 8–10 parallel tests were carried out, and the variance was calculated for the experiment: n
∑ (ψn − ψ )
Then we calculated the variance characterizing the error in determining the regression coefficient and the confidence interval. If the model coefficients exceeded the confidence interval, they were considered to be statistically significant. Based on the results of tests and investigations of the rupture and aging mechanisms for polyethylene and polypropylene in the aggressive media at increased temperatures, after subsequent analytical data processing we can conclude the following: 1. An experimentally obtained analytical dependences of aging of polyethylene and polypropylene on the external factors including test duration, type and concentration of the aggressive medium, and temperature have the form ψ = ψ 0 ⋅ e− AT ,
where the coefficient A characterizes the aging rate of the material in the aggressive medium that depends on the concentration of the medium, ψ 0 is the parameter depending on the temperature and concentration of the medium, and T is the test period. 2. Temperature dependences of aging rates for polyethylene and polypropylene are exponential. They are described by the Arrhenius equation: A = A0 ⋅ e
where А0 is the frequency factor that also describes the aging process depending on the type and concentration of the aggressive medium, E0 is the activation energy of the oxidation process, R is the universal gas constant, and t is the test temperature. 3. The method of calculating the durability of polyethylene and polypropylene under conditions of aging with allowance for the temperature and concentration of the medium was suggested based on the formula E0 RT е
Tdur = −
λ ⋅ ck
where λ is a constant and ck is the concentration of the medium. 4. Investigations of the durability of polyethylene and polypropylene in an alkaline medium demonstrate that in this case, it can be described by the same equation as in acid media. 5. Investigations of the materials having different thicknesses demonstrate the influence of the scaling factor, that is, the thickness on the durability: Tdur = −