Initial contact

Ultrasonic Welding. Ultrasonic welding has been appHed to Tefzel with weld strength up to 80% of the strength of the base resin. Typical conditions include a contact pressure of 172 kPa (25 psi) and 1—2 s cycle time. The two basic designs, the shear and butt joints, employ a small initial contact area to concentrate and direct the high frequency vibrational energy. 

Considerable effort has been directed to determining the causes of connection failutes and to learning how to minimize the likelihood of occurrence. Acceptable failute rates range from <1 in 10 operating hours for contacts in air-frame (31) electrical systems and in some telecommunications equipment, to 100—1000 in 10 operating hours in instmments, to even larger rates for contacts in many consumer products. A failute is defined as exceedance of contact resistance, which can be as Httle as twice the initial contact resistance, that causes circuit malfunction. The required lifetimes of connectors may be >20 yr, although most required appHcation times ate shorter (see Materials reliability). 

Air-poUutant effects on neural and sensory functions in humans vary widely. Odorous pollutants cause only minor annoyance yet, if persistent, they can lead to irritation, emotional upset, anorexia, and mental depression. Carbon monoxide can cause death secondary to the depression of the respiratory centers of the central nervous system. Short of death, repeated and prolonged exposure to carbon monoxide can alter sensory protection, temporal perception, and higher mental functions. Lipid-soluble aerosols can enter the body and be absorbed in the lipids of the central nervous system. Once there, their effects may persist long after the initial contact has been removed. Examples of agents of long-term chronic effects are organic phosphate pesticides and aerosols carrying the metals lead, mercury, and cadmium. 

Additionally, the initial contact or suction surface of the blade is of concern. The Elliott program indicates that erosion of the suction surface is almost nonexistent tlierefore, that erosion can be neglected in this instance. This conclusion is substantiated by actual field experience. 

Breakthrough time In the context of chemical protective clothing, the rime between initial contact of the chemical on the barrier material surface and the analytical detection of the chemical on the other side of the material. 

Villermaux, J. and Falk, L., 1994. A generalised mixing model for initial contacting of reactive fluids. Chemical Engineering Science, 49, 5127-5140. 

Many companies are quite wiiiing to share information on their PSM programs with other firms. CCPS and industry associations provide good places for networking to make the initial contacts that begin this. Attending process safety conferences is a good way to meet people who share your interest in PSM. 

With such a wide and important subject it is not possible to provide a full list but the above will indicate some initial contact points. The author hopes that no offence is caused by any omission. 

Temperature effects may also be used in test methods and notably for assessing the effects of inhibitors in acid solutions. The technique is based on that first proposed by Mylius which records the temperature-time behaviour associated with the exothermic reaction resulting from the initial contact of a metal with a corrosive acid solution. The effectiveness of inhibitors may then be determined from their effects on the temperaturetime behaviour. ... 

The surrounding redness caused by the vasodilatation of local blood vessels in the skin (hyperaemia). Histamine released at the site of contact acts on sensory nerve endings in the skin. Impluses travel along the axon to other peripheral branches of the same neuron to cause release of vasodilataory peptide neurotransmitters from nerve endings serving a wider area of skin than the initial contact point. Impluses reaching the CNS are interpreted as itch and pain. 

While it is inherently probable that product formation will be most readily initiated at sites of effective contact between reactants (A IB), it is improbable that this process alone is capable of permitting continued product formation at low temperature for two related reasons. Firstly (as discussed in detail in Sect. 2.1.1) the area available for chemical contact in a mixture of particles is a very small fraction of the total surface (and, indeed, this total surface constitutes only a small proportion of the reactant present). Secondly, bulk diffusion across a barrier layer is usually an activated process, so that interposition of product between the points of initial contact reduces the ease, and therefore the rate, of interaction. On completion of the first step in the reaction, the restricted zones of direct contact have undergone chemical modification and the continuation of reaction necessitates a transport process to maintain the migration of material from one solid to a reactive surface of the other. On increasing the temperature, surface migration usually becomes appreciable at temperatures significantly below those required for the onset of bulk diffusion within a product phase. It is to be expected that components of the less refractory constituent will migrate onto the surfaces of the other solid present. These ions are chemisorbed as the first step in product formation and, in a subsequent process, penetrate the outer layers of the... 

Measuring the radius of a foam lamella plateau border where it initially contacts the oil or of an emulsified drop... 

Dispersion of polymeric viscosifiers is often difficult because the initial contact of the untreated polymer with water results in very rapid hydration of the outer layer of particles which creates a sticky, rubbery exterior layer that prevents the interior particles from contacting water. The net effect is formation of what are referred to as nfish eyes" or "gel balls." These hamper efficiency by lowering the viscosity achieved per pound of gelling agent and by creating insoluble particles that can restrict flow both into the formation and back out of it. The normal remedy for this behavior... 

Equimolar amounts explode while warming together to ambient temperature after initial contact at — 183°C. Dilution of the dichloromethane with trichlorofluo-romethane prevented explosion at — 20°C. 

Breakthrough Time The elapsed time between initial contact of a hazardous chemical with the outside surface of a protective clothing material and the time at which the chemical can be detected at the inside surface of the material. 

Newsletters and support group leaders proved to be fruitful resources for initial contacts, made by telephone prior to my trips. Each person I contacted expressed a desire to share with others the opportunity to give voice to the story of MCS, and volunteered referrals. The few who declined to be interviewed for personal reasons always offered some helpful bit of information or assistance of... 

Figure 2. PMMA-Phe Fluorescence Decay in MEK. Initial contact of MEK and PMMA-Phe produces a sharp drop in intensity. This is followed by the plasticization period, the steady state and the termination.

The process of a solute dissolved in one solvent being pulled out, or extracted into a new solvent actually involves an equilibrium process. At the time of initial contact, the solute will move from the original solvent to the extracting solvent at a particular rate, but, after a time, it will begin to move back to the original solvent at a particular rate. When the two rates are equal, we have equilibrium. We can thus... 

Always the opportunist, Mark used his frequent business travels to initiate contact with other scientists and present lectures. 

In an extensive investigation of the stereochemical memory effect, a series of six diastereomeric pairs of substrates was prepared to probe the effect of single, then multiple substituents on the 5-exo cyclization of amines onto alkene radical cations [144,145]. Overall, these cyclizations were highly dia-stereoselective and were accounted for by a transition-state model employing a chairlike transition state with attack of the nucleophilic amine on the opposite face of the alkene radical to the one shielded by the phosphate anion in the initial contact ion pair (Scheme 34), as exemplified in Schemes 35 and 36. 

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