Abstract
Nanocomposites based on aliphatic polyurethanes have recently attracted a lot of attention regarding economical and ecological aspects, due to their improved thermal and mechanical properties. The aim of this paper was to investigate the influence of silica nanoparticles, differing in size and specific surface, on thermal stability and degradation, lifetime and mechanical characteristics of the obtained nanocomposites. Two series of nanocomposites based on aliphatic polyurethanes were obtained by using a single-step procedure and by addition of silica nanoparticles of types A380 or N999 at different loadings (0,0 0.15, 0.5, 1,0 and 3.5 wt.%). It was found that the increase in heating rate caused shifting of the onset temperature to higher values (from 283 to 312 ?C). According to the shape of DTG curves, it was observed that the degradation mechanism of prepared nanocomposites consists of two overlapping processes, related to the scission of hard and soft building blocks. Based on DTG results, the addition of larger N999 silica nanoparticles induced lower thermal degradation, shifting the maximum rate temperatures of the first and second degradation stages to lower values, and caused the change in the degradation mechanism. Addition of smaller silica nanoparticles (A380) did not significantly affect the mechanism of the degradation reaction, indicating homogeneity of the obtained nanocomposites. The presence of A380 nanoparticles improved thermal stability of nanocomposites, by increasing the onset temperature from 286 ?C for the pristine elastomer to 303 ?C for the sample containing 3.5 wt.% of silica. Existence of interactions of A380 silica nanoparticles with hard and soft phases was observed, based on the increase in the maximum rates of the first and second degradation steps. The activation energy of thermal degradation of polyurethanes modified with A380 silica nanoparticles was obtained by using the Flyn-Wall and Toop models. Dependence of the activation energy and the lifetime of nanocomposites based on aliphatic polyurethanes on the silica content were estimated. The highest Ea values (determined for 1 and 5 % weight loss) were found for nanocomposites containing 0.5 and 0.15 wt. % of A380 silica nanoparticles (121 and 161.2 kJ/mol). A negative effect of the silica addition on mechanical properties of nanocomposites was observed. The polyurethanes containing smaller SiO2 particles (A380) had a higher tensile strength, elongation at break and hardness as compared to the elastomers filled with larger silica nanoparticles (N999).
Highlights
Uticaj udela nanočestica SiO2 tipa A380 na vrednosti maksimalne temperature kojima nanokompoziti mogu biti izloženi pri termičkoj degradaciji u vremenskom intervalu od 60 min, a da pri tom ne dođe do gubitka mase većeg od 1 i 5 mas. %
Entalpija topljenja tvrdih segmenata za sve uzorke se kretala u opsegu od 18,2-21,5 J/g, na osnovu čega se može pretpostaviti da nije došlo do značajne promene stepena kristaličnosti ispitivanih nanokompozita u odnosu na stepen kristaličnosti čistog poliuretana sa oko 30 mas.% tvrdih segmenata koji iznosi 11 %
Dalje povećanje udela nanočestica dovodi do značajnijeg opadanja vrednosti zatezne čvrstoće i Jungovog modula elastičnosti sintetisanih nanokompozita, i najmanja vrednost parametara σ i E je utvrđena za uzorak PU – 3,50 N999 (18,7 MPa i 75 MPa, redom)
Summary
Alifatični segmentirani poliuretanski elastomeri predstavljaju veoma značajne inženjerske materijale jer su proizvodi degradacije ove vrste poliuretana značajno manje toksični u poređenju sa produktima degradacije poliuretana za čije dobijanje su korišćeni aromatski diizocijanati [1,2,3]. U istoj tabeli su prikazane i vrednosti maksimalne temperature (Tmax) kojima dobijeni nanokompoziti na bazi alifatičnih poliuretana mogu biti izloženi u vremenskom intervalu od 60 min, a da pri tom ne dođe do gubitka mase većeg od 1 i 5 %, usled termičke degradacije. Uticaj udela nanočestica SiO2 tipa A380 na vrednosti maksimalne temperature kojima nanokompoziti mogu biti izloženi pri termičkoj degradaciji u vremenskom intervalu od 60 min, a da pri tom ne dođe do gubitka mase većeg od 1 i 5 mas. Entalpija topljenja tvrdih segmenata za sve uzorke se kretala u opsegu od 18,2-21,5 J/g, na osnovu čega se može pretpostaviti da nije došlo do značajne promene stepena kristaličnosti ispitivanih nanokompozita u odnosu na stepen kristaličnosti čistog poliuretana sa oko 30 mas.% tvrdih segmenata koji iznosi 11 % (određen u prethodnom istraživanju [7]). Temperatura prelaska u staklasto stanje, temperature relaksacije mekih segmenata i temperature topljenja tvrdih segmenata nanokompozita na bazi alifatičnih poliuretana
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