INFLUENCE OF THE CHEMICAL NATURE OF THE E NVIRONMENT ON THE AGING OF POLYAMIDE 11 USED FOR OFFSHORE FLEXIBLE PIPES

Polyamide 11 is used as a leakproof sheath inside flexible flowlines f or petroleum products. Under some operating conditions, this polymer u ndergoes a degradation of its original physicochemical and mechanical properties, which may be assimilated with a phenomenon of aging. Mater ial exchanges occur between polyamide 11 and the fluid transported. Th e components present in the fluid (water, hydrocarbons) may be absorbe d, and the principal additive of the material (the plasticizer) is ext racted. This study was carried out to determine the influence of the c omposition of the chemical environment of aging on the properties of p olyamide 11. In the first phase, a new analysis method was developed f or quantifying diffusing materials in polyamide 11. Effectively, sever al techniques can be used for determining such materials. however, int erference problems may be encountered when the polymer is in contact w ith oil containing sulfur-bearing products. Likewise, none of these te chniques is capable of simultaneously making a complete analysis of al l the materials. The principle of the method developed consists in per forming a thermodesorption of the different materials present in the p olymer and in analyzing them on line by medium-resolution mass spectro metry (resolution = 3000). This resolution is also capable of determin ing the distribution, by chemical families, of the hydrocarbons absorb ed. The method was checked with aged polyamide 11 samples containing e ither a single type of material or several materials. These materials were analyzed at the same time by thermodesorption coupled with mass s pectrometry and by other techniques. The results obtained show that th is new method is capable of quantitatively and simultaneous determinin g: (1) the residual plasticizer content, (2) the water and absorbed-hy drocarbon content, and (3) the distribution, by chemical families, of the absorbed hydrocarbons (alkanes, cycloalkanes, aromatics). In the s econd phase, since polyamide 11 has varying affinity for the different chemical families present in a petroleum environment, we tried to det ermine the influence of each type of compound on the mechanical proper ties (ultimate elongation) and physicochemical properties (rate of cry stallinity, molecular weight, contents of diffusing materials). Aging was performed in the laboratory in model environments made up in varyi ng proportions, i.e. water, a gas-oil cut composed mainly of aromatic hydrocarbons, a gas-oil cut composed mainly of aliphatic hydrocarbons, The composition of these different aging environments was chosen by m eans of an experimental plan applied to the mixtures, and a polynomial mathematical model has been postulated. The conditions of temperature , pressure and aging time were set at a constant level for all the tes ts, i.e. 140-degrees-C, 6 bar of nitrogen, 15 days. The model obtained was used to plot isoresponse curves and to predict the properties ana lyzed as a function of the composition of the environment. The influen ce of water on the degradation of the mechanical properties (decrease in ultimate elongation) and the physicochemical properties (reduction of the molecular weight and increase in the rate of crystallinity) was revealed. Likewise, it was shown that, no matter what the aging envir onment was at 140-degrees-C, the plasticizer is extracted and the arom atic environment influences the plastication of the material. By means of mass spectrometry, the chemical nature of the aromatic hydrocarbon s absorbed preferentially, and having a pasticating role, was determin ed. These are two-cycle aromatics (alkylnaphthalenes, acenaphthenes, d iphenyls, acenaphthylenes, fluorenes) and sulfur-containing aromatics (benzothiophenes and dibenzothiophenes). The different results led to the conclusion that the principal phenomenon involved in aging is a hy drolysis caused by the presence of absorbed water in the material, whi ch leads to the cutting of the macromolecular chains and embrittlement of the polymer. Lastly, the model created with gas-oil cuts was appli ed to a case of aging of a crude oil, and this led to the satisfactory prediction of how the mechanical and physicochemical properties of th e polymer evolve in such an environment.