Flexible cellular polymer

In contrast to the similar specimen shapes and dimensions used in Europe and the USA (sec Section 3.2 below), when testing the tensile strength of flexible cellular polymers the two continents differ quite widely in the methods used to determine this property for rigid materials. The ISO 1926 [21] and BS 4370. Part 2. Method 9 standards use dumbbellshaped specimens, the size of which can be increased if the thickness of the material is greater than 12.5 mm. 

The tear resistance of flexible cellular polymers can be determined using EN ISO 8067. 1995 [52], which is for materials having a thickness of greater than 24 mm. The specimen (Fig. 14) is placed in the jaws of a machine capable of measuring the force at which rupture of the foam takes place when subjected to a jaw separation speed of 50 500 mm. minute. 

A rigid foam is defined as one in which the polymer matrix exists in the crystalline state or, if amorphous, is below its Tg. Following from this, a flexible cellular polymer is a system in which the matrix polymer is above its Tg. According to this classification, most polyolefins, polystyrene, phenolic, polyycarbonate, polyphenylene oxide, and some polyurethane foams are rigid, whereas rubber foams, elastomeric polyurethanes, certain polyolefins, and plasticized PVC are flexible. Intermediate between these two extremes is a class of polymer foams known as semirigid. Their stress-strain behavior is, however, closer to that of flexible systems than to that exhibited by rigid cellular polymers. 

Flexible Cellular Polymers. The application of flexible foams has been predominantly in comfort cushioning, packaging, and wearing apparel (142,170, 171), resulting in emphasis on a different set of mechanical properties than for rigid foams. The compressive nature of flexible foams (both static and dynamic) is their most significant mechanical property for most uses (Table 3). Other... 

Hilyard, N. C. (ed.), Mechanics of Cellular Plastics , MacMillan, New York, 1982. The mechanical behavior of rigid and flexible cellular polymers is discussed, and the relationship between mechanical properties, structure and composition is shown. 

The thermal, electrical, acoustical, and chemical properties of all cellular polymers are of such a similar nature that the discussions of these properties are not separated into rigid and flexible groups. 

Comfort cushioning is the largest single application of cellular polymers flexible foams are the principal contributors to this field. However, the rapid growth rate of structural, packaging, and insulation applications has brought their volume over that of flexible loams during the past Tew years. Table 5 shows U.S, consumption of foamed plastics by resin and market,... 

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. 

Creyf, H. and Fishbein, J., "Advance of Flexible Polyurethane Foam Technology," in Fire and Cellular Polymers, Eds., Buist, J.M., Grayson, S.J. and Wooley, W.D., Elsevier Applied Science Publishers, New York (1986). 

ASTM D 1565-81 A Standard Specification for Flexible Cellular Materials Vinyl—Chloride Polymers and Copolymers (Open-Cell Foam), 9 pp (DOD Adopted) (FSC 9320) (MR) (Comm D-11)... 

Polyurethanes are produced by the chemical action of di-isocyanate and polyol. The properties can be varied by the type of isocyanate used and the proportion of the two monomers. There are four main groups of classification for the thermoplastic groups of polyurethane, i.e. rigid foam, flexible foam, non-cellular and cellular polymers. Two main isocyanates used are toluene di-isocyanate (TDI) and diphenylmethane diisocyanate (MDI). Polyurethanes have limited application in the pharmaceutical or medical industries. Polyurethane is used as an adhesive for laminations (thermosetting material). Like thermosetting polyurethane, thermoplastic polyurethanes can be found as esters and ethers. 

ASTM D 1565, Flexible cellular materials Vinyl chloride polymers and copolymers (open cell foam). 1981. 

Additives such as antioxidants can cause staining of any surface that comes in contact with or near to the polymer material. The problem is mostly found with rubber or flexible cellular seals and is an important consideration in such consumer products as cars and kitchen appliances. The compounder has to achieve adequate environmental resistance without an unacceptable degree of staining. No staining would be ideal, but in practice some staining may have to be tolerated. To ensure that levels arc acceptable, tests to produce and measure staining arc often included in specifications. 

Some polymers can be used to produce foam possessing a wide range of properties. For example, polyurethane can be made hard or soft, flexible or rigid, at high or low densities. Polyurethane has an exceptional range of physical property variation and provides an unprecedented example to which many of the other cellular polymer test methods can be compared. Indeed, many of the methods used for the polyurethane family of polymers are eommon to the other polymeric foams. This subject is extensive, and a chapter such as this cannot hope to be exhaustive. However, it is hoped that most major physical property measurements are covered or at least guidance given to the reader as to where details can be obtained. 

Most of the test methods outlined in this. section for flexible foams are described in ISO tests or the related BS4443 [44]. These methods usually can be applied to flexible foams in general (The BS standard encompasses flexible cellular materials of polymeric origin ), although in some instances the standards include only certain polymer types, for example ISO 2439, 1980 [45] includes only latex, urethane, and open-cell PVC. Care must be exercised when using the test methods for foams not covered in the scope of the test method. ASTM D 3574-95 [46] relates specifically to urethane foams. 

By using polymers, proper sound insulation can be achieved easily and effectively. Cellular plastic materials (with open celled flexible structures) are shown to be very effective in sound insulation (either for impact noise from footsteps and movement on the top floors of a building, which can usually be eliminated by floor insulation, or airborne noise, which is from noises in the neighbourhood or the street, and which needs wall/party wall insulation for its elimination). Open cell flexible cellular plastic materials also provide acoustic insulation at high frequencies. 

Packaging. Because of the extremely broad demands on the mechanical properties of packaging materials, the entire range of cellular polymers from rigid to flexible is used in this application. The most important considerations are mechanical properties, cost, ease of application or fabrication, moisture susceptibility, thermal conductivity, and aesthetic appeal. 

D3575 Flexible Cellular Materials Made from Olefin Polymers 7214... 

Sims G L A and Sombatsompop N (1996) Pulverised flexible polyurethane foam particles as a filler in natural rubber vulkanisates. Cellular Polymers 15 90-104. 

A form of cellular rubber in which the cells are non-intercommunicating, self-contained units. It has low thermal conductivity. Expanded rubber is buoyant and does not absorb water and was therefore initially used in both the soft rubber and ebonite forms in the construction of lifebuoys and other marine buoyancy equipment. The most commonly used polymer is now polyurethane for both flexible and rigid systems. 

To get polyurethane foams the polymer is formed along with gas evolution. When these two processes take place simultaneously the gas bubbles are trapped in polymer matrix yielding a cellular product. Slightly cross-linked products are flexible while highly cross-linked products are rigid. Both flexible and rigid foams are of commercial importance. 

Foamed polymers are low-density, cellular materials that contain bubbles of gas and are made in a variety of ways out of thermoplastics and thermosets. Their properties vary from rigid to flexible. The rigid foams are best known for their insulation properties (like in ice chests). The flexible foams are used extensively in cushioning (seats, mattresses). 

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