Dale A. Brandreth

Evaluation of Long-Term Thermal Resistance of Gas-filled Foams: State of the Art

Some cellular plastics (foams) incorporate a chlorofluorocarbon (CFC) to improve their thermal insulating properties. During the service life of this type of insulation air diffuses into the foam cells and the CFC gas diffuses out, each at a rate that depends on type of polymer and temperature, eventually reducing the effectiveness of insulation. This effect is known as aging. Although literature on the aging process is extensive, there are no generally accepted procedures for evaluating long-term thermal resistance of new gas-filled foams. The mechanisms of the aging process are reviewed, and integrated approach to the evaluation of aging of gas-filled foams is proposed.

Insulation Foam Aging- A Review of the Relevant Physical Phenomena

With the advent of alternative fluorocarbon blowing agents which have inherently higher thermal conductivities than FC-11 (trichlorofluoromethane), the aging characteristics of insulation foams blown with these new blowing agents become even more important than before, since some of the advantage of the FC-11-blown foam is lost. The purpose of this paper is to clarify some of the basic technical facts involved in foam aging, because it is clear from reading current literature and hearing discussions of this topic at technical meetings that there is considerable misunderstanding about the basic physical phenomena involved. This misunderstanding is also evident from the literature on the subject, because virtually no attention is given to the areas of greatest potential improvement: attaining a lower fraction of open cells and use of a more impervious facing material. In addition, the validity of the simple relationship between the permeability and the product of the diffusion coefficient times the gas solubility-a keystone in some current models of the aging process-is questionable in this application. If progress is to be made quickly enough to have practical value, it will be necessary to enhance our understanding of the basic physical phenomena and to use improved assumptions in our models. In further-

Clays and Colloidal Silicas

Clays can lay claim to being the starting point for fine particle technology in the history of mankind. Their use in constructing shelters and in pottery manufacture far predates recorded history. Their particular crystalline structure which confers the plastic-like rheological behavior necessary for forming bricks, plugging holes, and making pottery, together with their permanence and ready availability all led inevitably to their extreme importance in mankind’s struggle to survive and prosper.

Small Particles in Paper

The word paper is derived from papyrus, a sheet made in ancient times by pressing together very thin strips of an Egyptian reed, cyperus papyrus. The modern material, paper, consists of sheet materials that are comprised of bonded, flexible, cellulose fibers which, while very short, 0.5–4 mm, are about 100 times as long as they are wide. Small particle fillers or pigments, in the form of clays or other inorganic materials are used to give paper improved properties, e.g. opacity, brightness and printability, or to improve the economics of the papermaking process. In this chapter we will focus on the use of small particles as process aids to improve retention and dewatering on paper machines.

Introduction to Small Particles

One of several ways to classify materials is to divide them into the following groups: 1. Metals 2. Ceramics 3. Polymers-Plastics and Elastomers (Rubber) 4. Cellulose-Wood-Paper

Surfaces of Small Particles

Modern surface physics, using techniques such as x-ray photoelectron spectroscopy, XPS, low-energy ion scattering, LEIS, Auger Electron spectroscopy, AES, secondary ion mass spectroscopy, SIMS, electron microscopy of different types, thermal desorption spectroscopy, and several other techniques, makes possible a detailed characterization of the surface of small particles in particular. Common to all these techniques, however, is that the material must be studied as a dry solid and often under very low pressure. Many of the small particles discussed in this book, on the other hand, are born in water and in most applications they are used, at least initially, in the form of aqueous dispersions. The surface of the small particles will, of course, be affected by the water and indeed other components present in the dispersion. Moreover, the surface properties of small particles in aqueous dispersions can often be drastically changed by deliberate modification of the particle surface. The most important methods for studying the properties of small particles in aqueous dispersions are acid-base titration, electrophoresis, and adsorption of cations, usually metal cations and anions, onto the particles.

Catalyst Supports and Small Particle Catalysts

Many of the useful properties of catalysts arise from reactions between the solid surfaces of the catalysts and various gaseous reactants. Catalysts of large specific surface areas are often desired and they can be obtained by the use of porous bodies. The large surface areas are associated with the internal surfaces of microporous systems which consist of networks of a great many very small pores.

Various Applications of Small Particles

In Chapter 1 it was pointed out that many important technical materials are made of or contained small particles. Some of the appli7)ich the authors have more extensive experience were described in Chapters 7–9. Obviously, there are many other important applications of small particles and a selection of these, to some extent based on the interest and experience of the authors, are discussed in this chapter.

Small Particles of Silica

Except for water, silica is the most abundant substance on the face of the earth. It is the major ingredient of our rocks, sand, and soil.

Small Particles of Hydrous Metal Oxides

As noted in the main introduction, this book focuses generally on a smaller range of particle sizes than older literature on this subject. In the past there have been limitations on producing very fine particles by the conventional method of attrition due to the large amounts of time and mechanical energy required and the often important concomitant problem of contamination by the grinding medium.