Foam
From Wikipedia, the free encyclopedia
 |
This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2009) |
The most general definition of foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. It can also refer to anything that is analogous to such a phenomenon, such as quantum foam. Often the term is used in reference to polyurethane foam (foam rubber), XPS foam, Polystyrene, or many other manufactured foams. It can be considered a type of colloid.
From the early 20th century, various types of specially manufactured solid foams came into use. The low density of these foams made them excellent as thermal insulators and flotation devices, and their lightness and compressibility made them ideal as packing materials and stuffings. Some liquid foams, called fire retardant foams, found use in extinguishing fires, especially oil fires.
Foam, in this case meaning "bubbly liquid", is also produced as an often unwanted by-product in the manufacture of various substances. For example, foam is a serious problem in the chemical industry, especially for biochemical processes. Many biological substances, for example proteins, easily create foam on agitation and/or aeration. Foam is a problem because it alters the liquid flow and blocks oxygen transfer from air (therefore preventing microbial respiration in aerobic fermentation processes). For this reason, anti-foaming agents, like silicone oils, are added to prevent these problems. Chemical methods of foam control are not always desired with respect to the problems (i.e. contamination, reduction of mass transfer) they may cause especially in food and pharmaceutical industries where the product quality is of great importance. In order to prevent foam formation in such cases mechanical methods are mostly dominant over chemical ones.
If foaming is desired, a foaming agent may help.
The term sea foam is used to describe the foam that forms on top of seawater from the action of waves. In some ways, leavened bread is a foam, as the yeast causes the bread to rise by producing tiny bubbles of gas in the dough.
The unique property of gas-liquid foams having very high specific surface area are exploited in the chemical processes of froth flotation and foam fractionation.
Contents |
[edit] Structure of foams
Real-life foams are typically disordered and have a variety of bubble sizes. The study of idealized foams is closely linked to the mathematical problems of space-filling and minimal surfaces. The Weaire-Phelan structure is believed to be the best possible (optimal) unit cell of a perfectly ordered foam[citation needed], while Plateau's laws describe how the soap-films form structures in foams.
Solid foams form an important class of lightweight cellular engineering materials. These foams can be classified into two types based on their pore structure. The first type of foams are called open cell structured foams. These foams contain pores that are connected to each other and form an interconnected network which is relatively soft. The second type of foams do not have interconnected pores and are called closed cell foams. Normally the closed cell foams have higher compressive strength due to their structures. However, closed cell foams are also generally denser, require more material, and consequentially are more expensive to produce. The closed cells can be filled with a specialized gas to provide improved insulation. This is in contradistinction to the open cell foam which will fill with whatever it is surrounded with. If filled with air this could be a relatively good insulator, but if the open cells fill with water, insulation properties would be reduced.
A special class of closed-cell foams is known as syntactic foam, which contains hollow particles embedded in a matrix material. The spheres can be made from several materials, including glass, ceramic, and polymers. The advantage of syntactic foams is that they have a very high strength-to-weight ratio, making them ideal materials for many applications, including deep sea and space applications[1]. One particular syntactic foam employs shape memory polymer as its matrix, enabling the foam to take on the characteristics of shape memory resins and composite materials, i.e., it has the ability to be reshaped repeatedly when heated above a certain temperature and cooled. Shape memory foams have many possible applications, such as dynamic structural support, flexible foam core, and expandable foam fill[2].
The closed cell structure foams have higher dimensional stability, low moisture absorption coefficients and higher strength compared to open cell structured foams. All types of foam are widely used as core material in sandwich structured composite materials.
[edit] Technique monitoring foam stability
Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, hence identifying and quantifying destabilisation phenomena[3][4][5][6]. It works on any concentrated dispersions without dilution, including foams. When light is send through the sample, it is backscattered by the bubbles. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (drainage, syneresis) and global changes in size (ripening, coalescence) are detected and monitored.
[edit] Types
[edit] Integral skin foam
Integral skin foam, also known as self-skin foam, is a type of foam with a high-density skin and a low-density core. They can be formed in an open mold process or a closed mold process. In the open mold process two reactive components are mixed and poured into an open mold. The mold is then closed and the mixture is allowed to expand and cure. Examples of items produced using this process include arm rests, baby seats, shoe soles, and mattresses. The closed mold process, more commonly known as reaction injection molding (RIM), injects the mixed components into a closed mold under high pressures.[7]
[edit] Gallery
 Close-up of sea foam on a tide pool |
 Foamed aluminium |
 Soap foam bubbles |
[edit] See also
[edit] References
- ^ "What is Syntactic Foam?" (HTML). Cornerstone Research Group. http://www.crgrp.net/syntactics.shtml. Retrieved 2009-09-30.
- ^ "Shape Memory Foams" (HTML). Cornerstone Research Group. http://www.crgrp.net/overviews/foams.shtml. Retrieved 2009-09-30.
- ^ I. Roland, G. Piel, L. Delattre, B. Evrard International Journal of Pharmaceutics 263 (2003) 85-94
- ^ C. Lemarchand, P. Couvreur, M. Besnard, D. Costantini, R. Gref, Pharmaceutical Research, 20-8 (2003) 1284-1292
- ^ O. Mengual, G. Meunier, I. Cayre, K. Puech, P. Snabre, Colloids and Surfaces A: Physicochemical and Engineering Aspects 152 (1999) 111–123
- ^ P. Bru, L. Brunel, H. Buron, I. Cayré, X. Ducarre, A. Fraux, O. Mengual, G. Meunier, A. de Sainte Marie and P. Snabre Particle sizing and characterisation Ed T. Provder and J. Texter (2004)
- ^ Ashida, Kaneyoshi (2006), Polyurethane and related foams: chemistry and technology, CRC Press, pp. 79–81, ISBN 9781587161599, http://books.google.com/books?id=IQUd-3aKSD4C.
[edit] External links
 |
Look up foam in Wiktionary, the free dictionary. |
 |
Wikimedia Commons has media related to: Foam |
