The Effect of Polymers on Ceramic Suspension Rheology and Green Component Properties

1992 ◽  
Vol 289 ◽  
Author(s):  
Jennifer A. Lewis ◽  
Andrea L. Ogden ◽  
David Schroeder ◽  
Kirk J. Duchow
Keyword(s):  
Molecular Weight ◽  
Polymer System ◽  
Polyvinyl Butyral ◽  
Shear Thinning ◽  
Suspension Rheology ◽  
Low Molecular Weight ◽  
Polymer Molecular Weight ◽  
Green Component ◽  
Ceramic Suspensions ◽  
Packing Efficiency

AbstractCeramic suspensions were formulated based on an alumina/plasticized-polymer system. The total polymer volume in suspension was held constant, while the relative amount of high-to-low molecular weight polyvinyl butyral (PVB) in suspension was varied. Experiments were performed to elucidate the effects of polymer molecular weight and distribution on the rheological properties of these casting suspensions as well as on the green microstructure of tape-cast components. The polymer properties affected not only the suspension viscosity at a given shear rate as expected, but also the shear thinning behavior of each suspension. Tapes (thickness ≈ 250 μm) were cast from these suspensions and their properties were evaluated. Pore volume, a measure of the packing efficiency, was found to depend strongly on the polymer molecular weight and distribution. In addition, preliminary lamination studies revealed that dimensional stability and anisotropy were also affected by the relative amount of high-to-low molecular weight PVB. A direct correlation was shown to exist between the shear thinning behavior of these suspensions and the resulting dimensional anisotropy exhibited by the tapes cast from each of them. These results demonstrate that polymeric aids influence not only the suspension rheology, but the green component microstructure as well, and, hence, are an integral aspect of ceramic processing.

The Chemistry of Phenol-Formaldehyde Resin Vulcanization of EPDM: Part I. Evidence for Methylene Crosslinks

1995 ◽  
Vol 68 (5) ◽  
pp. 717-727 ◽  
Author(s):  
Martin van Duin ◽  
Aniko Souphanthong
Keyword(s):  
Molecular Weight ◽  
Phenol Formaldehyde ◽  
Polymer System ◽  
Phenol Formaldehyde Resin ◽  
Formaldehyde Resin ◽  
Low Molecular Weight ◽  
Hydroxyl Groups ◽  
Reaction Products ◽  
Cure Reaction ◽  
Weight Model

Abstract The application of phenol-formaldehyde resins as crosslinking agents is increasing in importance due to the good high temperature properties of the corresponding vulcanizate and the use in thermoplastic vulcanizates. With respect to the chemistry of phenol-formaldehyde cure (reaction mechanism and structure of crosslink) there are still problems that have to be resolved. The reaction products of the phenol-formaldehyde resin curing of EPDM, contain 2-ethylidene norbornene (ENB) as the third monomer, have been studied. Since such an investigation is rather difficult to perform for the polymer system, a low molecular weight model for EPDM was used: 2-ethylidene norbornane (ENBH). Reaction of ENBH and a resole results in scission of the dimethylene ether bridges, i.e. in degradation of the resole into mono-, bis- and terisooctylphenol units. These are consequently converted into products, consisting of two ENBH molecules linked by mono-, bis- and terisooctylphenol units. The solid resole seems to be a technological solution for storing phenol in combination with formaldehyde. These results support the use of 2-hydroxymethylphenol (HMP) as a low molecular weight model for the resole. At low temperatures and/or short reaction times HMP oligomers (= resoles) and HMP oligomers linked to one ENBH molecule are formed, which are converted into ENBH/HMP (1:1) condensation products. The reaction products of ENBH with both the resole and HMP are shown to contain methylene linked structures, as demonstrated by the formation of monisooctylphenol crosslinks and the presence of residual unsaturation and hydroxyl groups, besides chroman linked structures. This is the first experimental evidence that during phenol-formaldehyde resin cure of rubber, formation of methylene bridges occurs.

A General Model for the Ideal Chain Length Distributions of Polymers Made with Reversible Deactivation

2022 ◽  
Author(s):  
Madison Kearns ◽  
Colleen Morleey ◽  
Kostas Parkatzidis ◽  
Richard Whitfield ◽  
Alvaro Sponza ◽  
...  
Keyword(s):  
Molecular Weight ◽  
Chain Length ◽  
Polymer Material ◽  
General Model ◽  
Polymer System ◽  
Polymer Molecular Weight ◽  
Ideal Chain ◽  
Reversible Deactivation ◽  
And Performance ◽  
The Ideal

Polymer molecular weight, or chain length distributions, are a core characteristic of a polymer system, with the distribution being intimately tied to the properties and performance of the polymer material....

A Proposed Method for Estimating Polymer Molecular Weight Distribution without Fractionation

1961 ◽  
Vol 34 (2) ◽  
pp. 453-460
Author(s):  
John Rehner
Keyword(s):  
Molecular Weight ◽  
Distribution Function ◽  
Present Method ◽  
Crosslink Density ◽  
Low Molecular Weight ◽  
Polymer Molecular Weight ◽  
Critical Test ◽  
Molecular Weights ◽  
Experimental Values ◽  
Better Than

Abstract The close agreement which Green has demonstrated between results from the Schulz binomial and the Tung distribution and the applicability of the latter to a variety of fractionation data for different polymers both seem to outweigh possible objection that the present method assumes a particular distribution function. Green's results suggest to us that the apparent exceptions found by Tung for some polyethylenes containing large amounts of low molecular species may possibly be attributed to fractionation inefficiency, rather than to the inadequacy of his function. The internal consistency or smoothness of data does not, of course, constitute proof of precise fractionation. In fact, one can safely say it is no easy matter to find complete fractionation data, of established precision, that can provide a critical test of a distribution function over the entire molecular weight domain. In his study of polyethylenes, Tung suggested that his function tends to exaggerate the low molecular weight end of the distribution, in the sense that the Mn values calculated from his parameters were only about half as large as those obtained by summation of the experimental data for the fractions. His calculated and experimental values of Mw, on the other hand, were in satisfactory agreement. One might be tempted to use this to reconcile the Mn values of the SBR system in Table II with the higher values reported by others. However, this procedure would be indefensible because simply doubling the Mn values would give polydispersity values of less than unity. It would therefore seem incorrect to generalize that the Tung function gives abnormally small Mn values for all polymers, especially when there is some reason to believe that it is better than it seems, even for polyethylenes. Much more likely, the differences between the average molecular weights and the polydispersity of the peroxide-SBR system of Table II, and the corresponding quantities reported by Bueche and Harding for sulfur-SBR and by Booth and Beason for uncompounded SBR, are real and are due to chemical and other factors already mentioned. Although the meager data at hand leave some question as to the accuracy with which the present method can predict absolute values of the various average molecular weights, the key to the matter seems to be the ratio of the true to the physically measured crosslink density for polymers in general, rather than the particular distribution function employed here. Less uncertainty is attached to the polydispersity, since this is a function of only one parameter, b, which is quite insensitive even to large errors in ρ. The method may therefore be useful, even in its present state, for comparative studies in a given system. These might include, for example, the effect of synthesis or processing variables on distribution characteristics and product properties or the effects of stabilizers in aging or other degradative processes.

A high-performance oil-free backing pump for electron microscopes

1978 ◽  
Vol 36 (1) ◽  
pp. 110-111
Author(s):  
G.K.W. Balkau ◽  
E. Bez ◽  
J.L. Farrant
Keyword(s):  
Molecular Weight ◽  
Electron Microscope ◽  
Hot Spots ◽  
High Performance ◽  
Carbonaceous Material ◽  
Contamination Rate ◽  
Low Molecular Weight ◽  
Principal Source ◽  
Rotary Pumps ◽  
Electron Microscopes

The earliest account of the contamination of electron microscope specimens by the deposition of carbonaceous material during electron irradiation was published in 1947 by Watson who was then working in Canada. It was soon established that this carbonaceous material is formed from organic vapours, and it is now recognized that the principal source is the oil-sealed rotary pumps which provide the backing vacuum. It has been shown that the organic vapours consist of low molecular weight fragments of oil molecules which have been degraded at hot spots produced by friction between the vanes and the surfaces on which they slide. As satisfactory oil-free pumps are unavailable, it is standard electron microscope practice to reduce the partial pressure of organic vapours in the microscope in the vicinity of the specimen by using liquid-nitrogen cooled anti-contamination devices. Traps of this type are sufficient to reduce the contamination rate to about 0.1 Å per min, which is tolerable for many investigations.

Low-molecular-weight heparin in the prevention and treatment of thromboembolic disorders

1998 ◽  
Vol 1 (5) ◽  
pp. 166-174 ◽  
Author(s):  
Evelyn R Hermes De Santis ◽  
Betsy S Laumeister ◽  
Vidhu Bansal ◽  
Vandana Kataria ◽  
Preeti Loomba ◽  
...  
Keyword(s):  
Molecular Weight ◽  
Low Molecular Weight Heparin ◽  
Low Molecular Weight ◽  
Prevention And Treatment
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