Effect of properties of high-density polyethylene on durability of welded joints

The molecular characteristics of pipe grades of high-density and medium-density polyethylene modified by 1-butene do not affect the durability of welded joints. Their long-term strength depends mainly on the density of the base grade of polyethylene and increases by an order of magnitude when the density is reduced in the investigated range. We have obtained mathematical models approximating the dependence of the durability of welded joints on the most important factors: the degree of crystallinity of the polyethylene and the initial stress in the brittle and mixed brittle-plastic fracture region. The models obtained may be used to assess the quality of the stabilizing system and the effectiveness of the comonomers of ethylene.Kazan State Technological University, Kazan, Tatarstan, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 6, pp. 842–847, November–December 1996.     

Properties of Bitumens Modified by Thermoplastic Elastomers

The possibilities of modification of bitumens by blend thermoplastic elastomers (TPEs) with predominantly maximum long main chains are studied. The evaluation of their properties shows that the blend TPEs display a stronger effect in low-viscosity bitumens, which is associated with improved compatibility of TPEs in bitumens, as evidenced by convergence of the solubility parameters of TPEs and of the aliphatic part of the malthene fraction of bitumens. The relaxation properties of the modified bitumens allow us to judge their stability under operating conditions, to evaluate this effect quantitatively, and to determine the activation energy of the relaxation process.     

Estimation of the molecular characteristics of polymers by the SPRT method and study of their influence on the properties of compositions

Pressure relaxation was examined in the cylinder of an MPT Monsanto processability tester after stopping the piston. The experimental function of the pressure drop F(t) was smoothed over and approximated by cubic splines. The spectra of pressure relaxation times (SPRT) were obtained according to the method of Schwarzl-Staverman. The SPRT method served well for estimating the spectra of the molecular-mass distribution (MMD) of polymers close in their physical sense to the SPRT. The correlation of the characteristic relaxation times and average molecular mass of ethylene-propylene rubbers and polyethylenes obtained by gel permeation chromatography was approximated by optimum models used for calculating the the molecular mass of rubbers according to the measurement results of the relaxation pressure of melts. The SPRT and characteristic relaxation times were used to analyze the significant technical properties of compositions based on polyethylene and rubber. The SPRT method was used to examine the failure of the cure network of butyl rubber and the dependence of the mechanical properties of thermoplastic elastomers on the molecular features of the decomposite.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Kazan State Technological University, Tatarstan, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 5, pp. 691–698, September–October, 1998.     

Spectra of pressure relaxation times of polymer and copolymer melts and their practical use

The possibility of using pressure relaxation of melted polymers for the investigation of the structure of three types of polymers and their compositions and the analysis of the change of their molecular characteristics during aging is shown.Kazan State Technological University, Tatarstan, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 4, pp. 531–538, July–August, 1998.     

Predicting the performance of extruded low-pressure polyethylene from rheological characterstics

Characteristic features of the high-elastic stalling of the melts of modern types of low-pressure polyethylene is investigated. The possibility of predicting the performance of polyethylene pipes from measurements of the following rheological characteristics is demonstrated: melt viscosity, shear-rate gradient at the outset of high-elastic stalling, swelling of the extrudate, and the amplitude of self-sustained fluctuations in shear stress.Kazan' State Technical University, Tatarstan, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 4, pp. 558–563, July–August, 1996.     

Stress Relaxation in Polymer-Modified Bitumens

The efficiency of BNK-40/180 bitumen modification with a blended thermoelastoplast and its components is studied by measuring the kinetics of the stress relaxation in melts. Asphaltene agglomerates are disclosed to be the main kinetic units of the stress relaxation in bitumens and bitumen–polymer compositions. The efficiency of bitumen modification with polymers is mainly affected by the solubility of the latter in the malthene fraction and their adsorption on asphaltenes. The experiments performed made it possible to estimate the behavior of modified bitumens at low shear rates corresponding to the extreme conditions of their use.     

Effect of the Work Expended on Mixing and Extrusion on the Properties of Thermoplastic Elastomer Compounds

The effect of the processing parameters of thermoplastic elastomer compounds on their properties is studied. The compounds consist of ethylene-propylene rubber and high- or low-density polyethylene (HDPE or LDPE) mixed with extenders in equal amounts. The variations in the degree of crystallinity under different mixing modes are examined by means of differential scanning calorimetry. The variations in the molecular weight distribution (MWD) of polymeric constituents of the compounds are estimated based on the results of rheological tests. An increase in the mixing and extrusion rates affects, up to certain limits, the mechanical properties positively. The optimum preparation modes for HDPE- and LDPE-based compounds are different. A noticeable mechanical destruction of HDPE-based compounds, estimated from the MWD parameters, occurs when the work of mixing exceeds 3.4 kJ/g and is accompanied by cross-linking. Up to 7.1 kJ/g, the work of mixing improves the properties of LDPE-based compounds. The extrusion effects do not smooth out the mixing effects — both these effects add up.