• Tomislav Horvat
  • Amanda Salazar del Campo
4. semester, Materials Engineering, Master (Master Programme)
Polymer waste is an issue that has proven to be one of the greatest challenges of our times. Finding new ways of reintroducing waste polymer materials in manufacturing processes has therefore become increasingly important. Presented study is trying to provide a small magnitude contribution to the big problem. In this case, High Density Polyethylene that is used for piping systems is characterized, and so is the recycled High Density Polyethylene(rHDPE). The goal is to analyze and assess weather or not is it possible to reintroduce the rHDPE in the manufacturing process as a part of a blend with vHDPE without compromising the final properties of the end material.

Differential scanning calorimetry (DSC) tests was done to analyze the differences in crystallinity of vHDPE and rHDPE. Following that, it was of interest to measure the stability of rHDPE and vHDPE, particularly due to the concern of possible degradation in rHDPE. This was done by the Oxidation Induction Time (OIT) test, where information about the time of starting degradation of both vHDPE and rHDPE, at certain temperatures were obtained. Furthermore, rheometry (oscillatory and time) tests, using plate-and-plate geometry, was to identify differences in molecular weight and molecular weight distribution, and the flow of rHDPE and vHDPE. Also, Indications on the occurring degradation mechanisms in materials have been traced. Tensile tests were done to see the differences in mechanical properties. Moreover long term tests: creep, relaxation and cyclical tests provided information about vHDPE and rHDPE materials behavior under long term loading conditions.After the characterization of both materials independently, blends were made in following ratios: 30% - 70 %, 50% - 50 %, 70% - 30 %, and assessed in the tensile test. The results are presented in property-composition graphs.

DSC results showed minimum difference between vHDPE and rHDPE crystallinity (50.61 %, and 51.37 % respectively). On the other hand, OIT tests proved that rHDPE is less stable than vHDPE, as it required less time for the start of the degradation. Furthermore, rheology tests displayed that rHDPE starts degrading sooner then vHDPE at 240 °C. It was also proven by means of rheology that rHDPE exhibits lower viscosity, lower molecular weight, and higher molecular weight distribution. Although, the differences in all those parameter between vHDPE and rHDPE are minor. Tensile test results showed lower yield strength and higher strain at yield for rHDPE compared to vHDPE at different testing cross-head speeds. However, the differences between the tested properties lie within the range of 7%-14% of variation, and rHDPE does not perform considerably worse then the vHDPE. Creep tests at 10MPa, and relaxations tests at 6% strain provided results were no differences between rHDPE and vHDPE were seen. However, in creep test at 15MPa, rHDPE exhibited less deformation in 20 minutes than vHDPE, suggested to be due to the presence of cross-links. Indication of cross-links presence in rHDPE was also found in cyclical loading test, were rHDPE fractured at lower number of cycles then vHDPE ( 10 for rHDPE, compared to 300 for vHDPE), at maximum applied stresses close to the yield stress.

For all the blends done in all compositions, these performed lower then vHDPE and higher than rHDPE, with no significant differences in between them. All being said, it is concluded that rHDPE and vHDPE behave alike in low stress and strain conditions, however in long term application, at higher possible loading conditions caution needs to be raised and materials further tested.
Publication date3 Jun 2019
Number of pages72
ID: 304947675