Effect of alkaline solutions on the tensile properties of glass-polyester pipes

Abstract

Construction materials, traditionally used in process equipment, are today successfully replaced by composite materials. Hence, many pipes are made of these materials. The subject of this study was the influence of liquids on the state of stresses and tensile strengths in the longitudinal and circumferential direction of glass-polyester pipes of a definite structure and known fabrication process. These analyses are of great importance for the use of glass-polyester pipes in the chemical industry. The tensile properties (the ultimate tensile strength and the modulus of elasticity) were tested and determined for specimens cut out of the pipes; flat specimens for the tensile properties in the longitudinal direction and ring specimens for the tensile properties in the circumferential direction. First, the tension test was performed on virgin samples (without the influence of any liquid), to obtain knowledge about the original tensile properties of the material composite studied. Subsequently, the specimens were soaked in alkaline solutions: sodium hydroxide (strong alkali) and ammonium hydroxide (weak alkali). These solutions were selected because of their considerable difference in pH values. The specimens and rings were left for 3, 10, 30 and 60 days in each liquid at room temperature. Then, the samples were tested on tension by the standard testing procedure. A comparison of the obtained results was made based on the pH values of the aggressive media in which the examined material had been soaked, as well as based on the original tensile properties and the number of days of treatment. Micromechanical analyses of sample breakage helped in the elucidation of the influence of the liquids on the structure of the composite pipe and enabled models and mechanisms that produced the change of strength to be proposed.

Konstrukcioni materijali tradicionalno korišćeni u procesnoj opremi su danas uspešno zamenjeni kompozitnim materijalima, tako da su i mnoge cevi izrađene od ovih materijala. Uticaj baznih tečnosti na stanje napona i zatezne čvrstoće u uzdužnom i obimnom pravcu staklo-poliester kompozitnih cevi je tema ovog rada. Cevi su definisane strukture i poznatog procesa proizvodnje. Ove analize su od velikog značaja za korišćenje staklo-poliester cevi u hemijskoj industriji. Zatezne osobine (zatezna čvrstoća i modul elastičnosti) su eksperimentalno ispitivane na isečenim uzorcima; ravni uzorci u udužnom pravcu, a uzorci u obliku prstena u obimnom pravcu. Ispitivanje je prvo izvođeno na uzorcima koji nisu bili izloženi uticaju rastvora baza da bi se došlo do saznanja o prvobitnim zateznim svojstvima ispitivanih kompozitnih materijala. Nakon toga, uzorci su stavljeni u rastvore baza natrijum-hidroksida (jaka baza, pH=14) i amonijum hidroksida (slaba baza, pH=12) 3, 10, 30 i 60 dana na sobnoj temperaturi. Nakon toga su ispitivani na zatezanje prema standardnoj proceduri. Poređenje dobijenih rezultata je izvedeno na osnovu rN vrednosti rastvora i broja dana izlaganja rastvorima baza, a na osnovu originalnih zateznih osobina. Mikromehanička analiza se izvodila u odnosu na fotografije sa skening eloktronskog mikroskopa sa prelomnih površina čime se došlo do podataka o uticaju rastvora baza na strukturu kompozitnih cevi i modele i mehanizme koji su dovodili do promene zateznih svojstava.