Closed cell cellular plastics

The acoustic properties of polymers are altered in a cellular structure. Sound transmission changes only slightly, because it depends predominantly upon the barrier density, in this case the polymer phase. Therefore, closed-cell cellular polymers by themselves are poor materials for reducing sound transmission. They are, however, effective in absorbing sound waves of certain frequencies [64]. Materials with open cells on the surface are particularly effective in this respect. The combination of other advantageous physical properties with fair acoustic properties has led to the use of plastic foams in soundproofing [65,66]. The sound absorption of a number of cellular polymers has been reported [7,64,65,67]. 

Closed-cell foamed plastic (unicellular foam) n. A cellular plastic in, which interconnecting cells are too few to permit the bulk flow of fluids through the mass. 

Closed-cell foam Cellular plastic that is composed predominantly of noninterconnecting cells. 

A low (<0.4 W / (m-K)) thermal conductivity polymer, fabricated into alow density foam consisting of a multitude of tiny closed cells, provides good thermal performance. Cellular plastic thermal insulation can be used in the 4—350 K temperature range. Cellular plastic materials have been developed in... 

Foamed plastics can be classified in different ways, for instance by their nature (flexible vs. rigid), chemical composition of the matrix, density, cell size, cell structure (open-celled vs. closed-celled), processing method, and dimensions. It is the aimed combination of these properties that determines the final application of the cellular polymer. As an example, open-celled ultra-low density foams are highly desirable for acoustical insulation, while rigid foams with closed-cells and elevated densities are preferred as load-carrying core materials in composite materials. 

Foam structures consist of at least two phases, a plastic matrix and gaseous voids or bubbles. A closed-cell or open-cell structure is formed, with cellular walls enclosing the gaseous voids. In closed cell foams, the gas cells are completely enclosed by cell walls, while in open-cell foams, the dispersed gas cells are unconfined and are connected by open passages. Plastic can be stabilized against cell rupture by crosslinking (Chapters 1 and 2). 

Rodents chew through cellular polymers but do not ingest the foam as a foodstuff. The resistance to rot, mildew, and fungi is related to moisture absorption [64]. Therefore, open-cell foams support such growth better than closed-cell foams. High humidity and temperature are necessary for the growth of microbes on any plastic foam. 

ASTM D 2856 is the method used to determine the open-cell content of rigid cellular plastics by use of the air-pycnometer. This method is used where porosity of the cellular plastic has a direct bearing on the end use involved. For example, in thermal insulation, a high percentage of closed cells is essential to prevent the escape of gases and thereby promote low thermal conductivity. Also, in flotation applications high closed-cell contents generally prevent water absorption. 

ISO 4590-81 Cellular Plastics —Determination of Volume Percentage of Open and Closed Cells of Rigid Materials, First Edition, 14 pp... 

A cellular plastic is defined as a two-phase gas-solid plastic system whose apparent density is decreased substantially by the presence of numerous cells or voids disposed throughout its mass. These cells or voids are chiefly formed by using CBAs, PBAs, microballoons, or fillers containing gases or liquids. If these cells are discrete and the gas phase of each is independent of that of the other cells, the material is termed closed cell. 

These test methods are based on the determination of the volume of closed cells in a cellular plastic by the application of Boyle s law to two enclosures in the pycnometer, one of which contains the test specimen (Fig. 7). By equalizing the pressure in the two chambers it is possible to determine the apparent volume of the specimen. 

ISO 4590, Cellular plastics-Determination of volume percentage of open and closed cells of rigid materials, 1981. 

The glass transition temperature lies considerably above the temperature of use with rigid foams and considerably below the temperature of use with flexible foams. The cellular structure may be open, closed, or mixed. All cells are open to each other with open cellular structures. In contrast, each cell is sealed off or encapsulated from the other cells by a plastic wall in closed-cell structures. Structural foams are foams with a dense outer skin and an interior of lower density they are also called integral foams or self-skinning foams. In contrast to normal foams syntactic foams do not directly enclose the gas but contain small hollow bodies of glass, ceramics, or plastics which are under vacuum or are filled with a gas. 

The act of foaming a plastic material results in products with a wide range of densities. These materials are often termed cellular plastics. Cellular plastics can exist in two basic structures closed-cell or open-ceU. Closed-cell materials have individual voids or cells that are completely enclosed by plastics, and gas transport takes place by diffusion through the cell walls. In contrast, open-ceU foams have cells that are interconnected, and fluids may pass easily between the cells. The two structures may exist together in a material so that it may be a combination of open and closed cells. 

Plastic foams can be defined as materials with cellular structure. These cells are normally filled with a gas generated by a foaming agent. The foam properties are largely affected by density and cell geometry. Generally speaking, cellular plastics are classified as open or closed cells. In closed cells structure, thin walls permit the gas to be retained into small bubbles. 

Cell n. (1) A small etched depression in a gravure cylinder that carries the ink. (2) In the cellular-plastics industry, a single void produced by a blowing agent, by mechanically entrained gas, or by the evaporation of a volatile constituent. When the void is completely surrounded by polymer, the cell is said to e closed. A completely open cell has no wall membranes but is part of a three-dimensional network of connected fibers or rods. 

A closed cell is one that is totally closed by its walls and hence not interconnecting with other cells. An open cell is a cell not totally enclosed by its walls and hence interconnecting with other cells. Cell striation refers to a layer of cells within a cellular plastic that differs greatly from the characteristic cell structure of the materials. 

There are several types of foamed plastics including syntactic foam, closed-cell foam, and open-cell foam. Syntactic foam is a special type of plastic foam in which the cellular structure is formed by the use of smaU, hollow spheres made of resin, glass, or other hollow particles. The spheres are dispersed throughout a matrix of resin, usually thermosetting, which when cured resanbles a conventional foam plastic. Once mixed, syntactic foam may be cast or formed in place by a pouring or trowling action. 

A cell may be open because its walls are incomplete or because they are entirely absent. In the latter case the cells take on a filamentary, skeletal outline. Both open and closed cells may coexist to some extent in the same cellular material. Methods of determination of the percentages of open and closed cells in a cellular plastics are given in ASMT D2856. 

Polymer foams, also known as cellular polymers or cellular plastics, are multi-phase material systems (composites) that consist of a polymer matrix and a fluid phase, one usually being a gas. Foam is a general term that refers to a material with any degree of communication between its voids. The word cellular is used as a general term whereby the cells may have any degree of interconnection - expanded polymers are composites with closed cells. 

Foam properties are greatly affected by density and the cell geometry, that is, open versus closed cells, size, and shape. Generally, cellular plastics are classified as open cell... 

The fundamentals of foam formation have been reviewed in detail by a number of authors [17-19]. Therefore, only the main concepts will be reviewed in this chapter. Cellular plastics are produced through several methods, irrespective of cell structure (open or closed). However, the most commonly used methods for cellular PLA are the following ... 

---