Rheology
Abolfazl Deylami; Abbas Kebritchi
Abstract
The time available for proper casting of the polyurethane (PU) system after isocyanate addition (called pot life) affects the macrodiol microstructure. In this research, the effect of molecular weight ( Mn ) of hydroxyl terminated polybutadiene (HTPB) as an important macrodiol on the pot life of polyurethane ...
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The time available for proper casting of the polyurethane (PU) system after isocyanate addition (called pot life) affects the macrodiol microstructure. In this research, the effect of molecular weight ( Mn ) of hydroxyl terminated polybutadiene (HTPB) as an important macrodiol on the pot life of polyurethane system was rheologically investigated. The chemorheological behavior of polyurethane systems based on five different HTPBs with Mn ranging from 2810 to 4450 g/mol and two common diisocyantes (TDI and IPDI) were studied in situ. The results showed that by increasing Mn , the rotational viscosity as well as the constant reaction rate of urethane formation increases; whereas the pot life of the polyurethane system decreases. The pseudoplastic behavior of the samples showed the minimum rate constant/longest pot life at a critical molecular weight (Mc) of about 3100 g/mol, which may be due to physical chain entanglements. Rheomechanical spectrometry interestingly depicted that tan δ originated-gel point occurs before the intersection of stored modulus (G') and loss modulus (G") (tgel< tG'=G"). The kinetic model developed by Hsich showed excellent agreement with the cross-linking data of three polyurethane systems.
Polyolefin degradation
Abbas Kebritchi; Mehdi Nekoomanesh; Fereidoun Mohammadi; Hossein Khonakdar; Udo Wagenknecht
Abstract
In this work, the effect of hexyl branch content on thermal behavior of a fractionated ethylene/1-octene copolymer with emphasis on high temperatures was investigated. The ethylene/1-octene copolymer was carefully fractionated to different fractions with homogenous hexyl branch (HB) content by preparative ...
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In this work, the effect of hexyl branch content on thermal behavior of a fractionated ethylene/1-octene copolymer with emphasis on high temperatures was investigated. The ethylene/1-octene copolymer was carefully fractionated to different fractions with homogenous hexyl branch (HB) content by preparative temperature rising elution fractionation (P-TREF) method. The P-TREF fractions were thermally analyzed via differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and evolved gas analysis (EGA). The P-TREF profile showed a short chain branch distribution (SCBD) of around 1.24. A linear relationship between P-TREF elution temperature (ET) and methylene sequence length (MSL) was presented. The DSC curves exhibited a monolithically increase in melting temperature (Tm) as well as crystallization temperature (Tc) by decreasing short chain branch (SCB) content. The calculated values of lamellae thickness suggested a linear function of SCB content and Tm. The TGA studies of P-TREF fractions depicted a two-stage thermal degradation behavior: pre-degradation and main degradation stages. Tmax for both pre-degradation and main degradation stages was increased for fractions with less hexyl branch content. As an interesting point the pre-degradation stage was found more intensified for more linear fractions. The concentration of main products was found to be affected by the content of hexyl branches using Py-GC-MS.
Polyolefin degradation
Abbas Kebritchi; Mehdi Nekoomanesh; Fereidoon Mohammadi; Hossein Ali Khonakdar
Abstract
In this work, the role of comonomer content of 1-hexene-medium density polyethylene (MDPE) copolymer, synthesized using Phillips catalyst, on thermal behavior parameters such as: crystallization, melting temperature and thermal degradation was investigated in detail. The copolymer was fractionated to ...
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In this work, the role of comonomer content of 1-hexene-medium density polyethylene (MDPE) copolymer, synthesized using Phillips catalyst, on thermal behavior parameters such as: crystallization, melting temperature and thermal degradation was investigated in detail. The copolymer was fractionated to homogenous short-chain branching (SCB) fractions by "preparative temperature rising elution fractionation" (P-TREF) method and then it was subjected to thermal analyses. A broad chemical composition distribution (CCD) in terms of SCB content and molecular weight (Mw) was observed by P-TREF and gel permeation chromatography (GPC), respectively. Based on P-TREF results, a parabolic relationship between methylene sequence length (MSL) and elution temperature (ET) was presented. Differential scanning calorimetry (DSC) showed distinct, well-defined melting peaks over a 22 °C temperature range for SCB contents of about 3-12 (br/1000 C). The variations in physical characteristics such as melting temperature (Tm), crystallinity (Xc), crystallization temperature (Tc) and lamellae thickness (Lc) against SCB content were correlated. Thermogravimetric analysis (TGA) suggested linear relationships between the temperature at maximum degradation rate (Tmax) as well as the degradation initiation temperature (T5%) versus SCB content. Moreover, the TGA curves exhibited distinct differences at both initiation and propagation stages of thermal degradation at dissimilar comonomer contents.
