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Blown Film Technology
Published in Nicholas P. Cheremisinoff, Elastomer Technology Handbook, 2020
HDPE is made in a low pressure process (300 psi) using a stereospecific catalyst. The catalyst causes the ethylene molecules to be joined in a particular manner, so that there is nearly no branching unless some comonomer is introduced to cause it. A HDPE with a density of 0.955 or greater has only 2 to 3 branches per 1000 carbon atoms. Some larger amount of branching is introduced for certain applications to improve the processing of the polymer. The lack of side chains on the polymer molecule allows HDPE to crystallize to a greater degree, which increases its density, creates a hazy film, reduces the permeation of water vapor through the film, and makes it suffer and more difficult to heat seal. All of these properties are reasons that HDPE is selected for its various applications. To improve processing characteristics, 1-hexene is often added in small quantities as a comonomer. This reduces the density by making some side chains, but some applications require the change in properties.
NMR and EPR Spectroscopy in the Study of the Mechanisms of Metallocene and Post-Metallocene Polymerization and Oligomerization of α-Olefins
Published in Evgenii Talsi, Konstantin Bryliakov, Applications of EPR and NMR Spectroscopy in Homogeneous Catalysis, 2017
Evgenii Talsi, Konstantin Bryliakov
1-Hexene is industrially used as comonomer in the production of linear low-density polyethylene (LLDPE). In industry, 1-hexene is usually obtained by nonselective (statistical) oligomerization of ethylene [177]. The only commercial process capable of selectively producing 1-hexene utilizes chromium-based catalyst [178]. Catalyst systems capable of selective 1-hexene production would be of great industrial and academic interest (see the review by McGuinness [179]). Apart from chromium-based trimerization catalysts, systems relying on titanium complexes have attracted significant interest. Hessen and coworkers discovered a highly active and selective titanium-based catalyst system for this transformation [180]. One of the major recent developments has been the emergence of complex (FI)TiCl3 (9Ti) (FI = phenoxy imine ligand with additional O-donor, Figure 4.56). When activated with MAO, 9Ti produced 1-hexene with exceptionally high activity (up to 132 kg of 1-hexene [g of Ti]−1 h−1 bar−1) [181].
Ti alkoxide-based catalyst system in selective ethylene dimerization: High performance through modifying by alkylsilanes
Published in Chemical Engineering Communications, 2018
Peyman Bigdeli, Majid Abdouss, Sadegh Abedi
In 1950s, the Ziegler growth reaction, which is called Aufbau reaction, significantly changed through the introduction of transition metal compounds in its catalytic system (Hutley and Ouederni, 2016; Steinborn, 2012; Ziegler et al., 1954, 1955). This finding served as a crucial precursor discovery resulted to an important class of the mixtures of the chemical compounds, which were remarkable for their ability to affect the linkage of olefins. The importance of this discovery culminated with the development of commercially viable products such as dimers, oligomers, and polymers. Since then, an increasing number of studies has been conducted by scientists to improve the efficiency of the industrial reactions (Al-Sadoun, 1993; Flisak and Sun, 2015; Keim, 2013; Lancer, 1970; Ma et al., 2014; Metzger et al., 2016; Shamiri et al., 2014; Uglea, 1998; Wang et al., 2014; Zhang et al., 2012; Zhu et al., 2011). In this regard, the discovery of the impressive rheological and mechanical properties of linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE), which was resulted due to the incorporation of low-carbon number alpha olefins including butene-1, hexene-1, and octene-1 in the ethylene polymerization, made their production as a topic of considerable interest in both academia and industry (Ali and Al-Humaizi, 2000; Bollmann et al., 2004; Breuil et al., 2015; Forestiere et al., 2009; Kaivalchatchawal et al., 2011; Kaylon and Yu, 1988; Navaro et al., 1976; Spacer et al., 2005; Suttil and McGuinness, 2012).
Probing PAH Formation from Heptane Pyrolysis in a Single-Pulse Shock Tube
Published in Combustion Science and Technology, 2023
Alaa Hamadi, Leticia Piton Carneiro, Fabian-Esneider Cano Ardila, Said Abid, Nabiha Chaumeix, Andrea Comandini
This research highlights PAH products from n-heptane pyrolysis as a significant new contribution to the literature on the chemistry of this important surrogate component. The experiments were carried out in a single-pulse shock tube coupled to gas chromatographic techniques at a nominal pressure of 20 bar throughout a temperature range of 900–1700 K. Updates to our ongoing PAH kinetic model demonstrate satisfactory predictive performances for the speciation measurements obtained in this work as well as those reported in literature studies on n-heptane pyrolysis. Heptane mainly decomposes through C-C fission and H-abstraction reactions leading to the formation of heptyl, n-butyl and n-propyl radicals. These Alkyl radicals further undergo beta-fission reactions leading to the formation of alkenes, namely, ethylene, propylene, 1-butene, 1-pentene and 1-hexene. The consumption of alkenes results in key intermediates such as methyl, acetylene, ethyl, propargyl, and 1,3-butadiene, which play a significant role in the formation and growth of aromatics. In particular, the formation of benzene relies on C2H3+C4H6 reaction through the intermediate cyclohexene, the H-addition and isomerization of fulvene, and C3+C3 reactions, more specifically, propargyl self-recombination and propargyl+allene reaction. Toluene is the product of the addition reaction of C3H3 to 1,3-butadiene. Indene formation mainly depends on C4H2+C5H5 reaction at low temperatures and on the consumption of C6H5C3H3P_1 and C6H5C3H3A, and the C7H7+C2H2 channel at high temperatures (>1400 K). Indenyl’s subsequent interactions with methyl, propargyl and vinylacetylene lead to the production of naphthalene, acenaphthylene and fluorene, respectively. Naphthyl radicals further participate in the formation of other bigger PAHs including 1-methyl naphthalene and 1-ethynyl naphthalene. Benzyl self-recombination and anthracene isomerization were analyzed to be the greatest contributors to phenanthrene formation. A small amount of phenanthrene is produced through C6H5C2H+C6H5 and C9H7+C5H5 reaction channels. Finally, the findings will help researchers better understand how PAHs form during the combustion of surrogate fuels.