Plastic foam foaming

Creep - [PLASTIC TESTING] (Vol 19) -of plastic foams [FOAMED PLASTICS] (Vol 11) -in tire cord [TIRE CORD] (Vol 24)... 

Cups, foeiined plastics Cushions, carpet and rug plastics foam Foamed plastics products Ice chests or eoolers, portable foamed plastics... 

Schaumgummi foam rubber, foamed rubber, plastic foam, foam... 

Foam controllers Foamed lonomers Foamed materials Foamed plastic Foamed plastics... 

Economic Aspects. In 1994 there were 8 operational insulation board producers in the United States. These mills produced about 1.15 X 10 m (2). The number of mills and total production volume have also decreased in this industry, primarily as a result of changes in building codes and avadabihty of other competitive sheathing products. Both wood composite panels and plastic foam sheathings have captured a segment of these markets. 

A cellular plastic has been defined as a plastic the apparent density of which is decreased substantially by the presence of numerous cells disposed throughout its mass (21). In this article the terms cellular plastic, foamed plastic, expanded plastic, and plastic foam are used interchangeably to denote all two-phase gas—soHd systems in which the soHd is continuous and composed of a synthetic polymer or mbber. 

Eoamable compositions in which the pressure within the cells is increased relative to that of the surroundings have generally been called expandable formulations. Both chemical and physical processes are used to stabilize plastic foams from expandable formulations. There is no single name for the group of cellular plastics produced by the decompression processes. The various operations used to make cellular plastics by this principle are extmsion, injection mol ding, and compression molding. Either physical or chemical methods may be used to stabilize products of the decompression process. 

Fig. 2ab. Photomicrographs of foam cell stmcture (a) extmded polystyrene foam, reflected light, 26 x (b) polyurethane foam, transmitted light, 26 x (c) polyurethane foam, reflected light, 12 x (d) high density plastic foam, transmitted light, 50x (22).

In the presence of external forces, plastic foams in which the cells are elongated or flattened in a particular direction may be formed. This cell orientation can have a marked influence on many properties. The results of a number of studies have been reviewed (59,60). 

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

The most important stmctural variables are again polymer composition, density, and ceU size and shape. Stmctural foams have relatively high densities (typically >300 kg/m ) and ceU stmctures similar to those in Figure 2d which are primarily comprised of holes in contrast to a pentagonal dodecahedron type of ceU stmcture in low density plastic foams. Since stmctural foams are generally not uniform in ceU stmcture, they exhibit considerable variation in properties with particle geometry (103). 

Fig. 4. Load vs compression for plastic foams (149). A, polystyrene, 32 kg/m (2 lbs/fT) B, polyethylene, 32 kg/m C, latex mbber foam. To convert...

Thermal Conductivity. More information is available relating thermal conductivity to stmctural variables of cellular polymers than for any other property. Several papers have discussed the relation of the thermal conductivity of heterogeneous materials in general (187,188) and of plastic foams in particular (132,143,151,189—191) with the characteristic stmctural variables of the systems. 

Flammability. The results of small-scale laboratory tests of plastic foams have been recognized as not predictive of their tme behavior in other fire situations (205). Work aimed at developing tests to evaluate the performance of plastic foams in actual fire situations continues. All plastic foams are combustible, some burning more readily than others when exposed to fire. Some additives (131,135), when added in small quantities to the polymer, markedly improve the behavior of the foam in the presence of small fire sources. Plastic foams must be used properly following the manufacturers recommendations and any appHcable regulations. 

MoistureResista.nce, Plastic foams are advantageous compared to other thermal insulations in several appHcations where they are exposed to moisture pickup, particularly when subjected to a combination of thermal and moisture gradients. In some cases the foams are exposed to freeze—thaw cycles as well. The behavior of plastic foams has been studied under laboratory conditions simulating these use conditions as well as under the actual use conditions. 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Miscellaneous Properties. The acoustical properties of polymers are altered considerably by their fabrication into a ceUular stmcture. Sound transmission is altered only slightly because it depends predominandy on the density of the barrier (in this case, the polymer phase). CeUular polymers by themselves are, therefore, very poor materials for reducing sound transmission. They are, however, quite effective in absorbing sound waves of certain frequencies (150) materials with open ceUs on the surface are particulady effective. The combination of other advantageous physical properties with fair acoustical properties has led to the use of several different types of plastic foams in sound-absorbing constmctions (215,216). The sound absorption of a number of ceUular polymers has been reported (21,150,215,217). 

The insulating value and mechanical properties of rigid plastic foams have led to the development of several novel methods of buUding constmction. Polyurethane foam panels may be used as unit stmctural components (220) and expanded polystyrene is employed as a concrete base in thin-sheU constmction (221). 

CeUular urea—formaldehyde and phenoHc resin foams have been used to some extent in interior sound-absorbing panels and, in Europe, expanded polystyrene has been used in the design of sound-absorbing doors (233). In general, cost, dammabUity, and cleaning difficulties have prevented significant penetration of the acoustical tile market. The low percent of redection of sound waves from plastic foam surfaces has led to their use in anechoic chambers (216). 

Flammability. Plastic foams are organic ia aature and, therefore, are combustible. They vary ia their respoase to small sources of ignitioa because of composition and/or additives (255). AH plastic foams should be handled, transported, and used according to manufacturers recommendations as weU as appHcable local and national codes and regulations. 

Toxicity. The products of combustioa have beea studied for a number of plastic foams (257). As with other organics the primary products of combustion are most often carbon monoxide and carbon dioxide with smaller amounts of many other species depending on product composition and test conditions. 

The presence of additives or unreacted monomers ia certaia plastic foams can limit their use where food or human contact is anticipated. Heavy metals can also be found ia various additives. The manufacturers recommendations or existing regulations again should be foUowed for such appHcations. 

G. P. Krat2schmer, Plastic Trends, T44 PlasticFoams, Predicasts Inc., Cleveland, Ohio, 1977 W. P. Weiser, T71 U.S. Plastic Foam Markets, Part I-Industry Study, Predicasts Inc., Cleveland, Ohio, Aug., 1983. 

C. Kienzle, "Plastic Foams," paper presented at Regional Technical Conference, Buffalo, N.Y., Society of Plastics Engineers, Inc., New York, Oct. 5, 1961, p. 93. 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

In Europe total consumption of plastic foam insulation for 1992 was 29 x 10 m (>12 billion board ft) withHttle or no growth seen from 1991. AU products were expected to grow by an average of 1% due primarily to the continued effects of the economic recession. 

Plastic Foam Materials and Koofinginsulation, 1983—1989, Peter Sherwood Associates, Inc., White Plaias, N.Y., Apr. 1984 KoofingJ Siding Insul 61, 82 (Oct. 1984). 

PS Foams. The eady history of foamed PS is available (244), as are discussions of the theory of plastic foams (245). Foamable PS beads were developed in the 1950s by BASF under the trademark of STYROPOR (246—248). These beads, made by suspension polymerization in the presence of blowing agents such as pentane or hexane, or by post-pressurization with the same blowing agents, have had an almost explosive growth, with 200,000 metric tons used in 1980. Some typical physical properties of PS foams are Hsted in Table 10 (see Foamed plastics). 

Extruded Rigid Foa.m. In addition to low temperature thermal insulation, foamed PSs are used for insulation against ambient temperatures in the form of perimeter insulation and insulation under floors and in walls and roofs. The upside-down roof system has been patented (256), in which foamed plastic such as Styrofoam (Dow) plastic foam is appHed above the tar-paper vapor seal, thereby protecting the tar paper from extreme thermal stresses that cause cracking. The foam is covered with gravel or some other wear-resistant topping (see Roofing materials). 

A 2.54-cm Styrofoam plastic foam with thermal conductivity of ca 0.03 W/ (m-K) (0.21 (Btu-in.)/(ft-b°F)) is equivalent to 61 cm of gravel. Any synthetic foam having compressive strength sufficiently high and thermal conductivity sufficiently low is effective. However, the resistance of PS-type foams to water, frost damage, and microorganisms in the sod makes them especially desirable. An interesting and important appHcation of this concept was the use of Styrofoam in the constmction of the Alaska pipeline. In this case, the foam was used to protect the permafrost. 

Container Insulation Tanks containing materials above atmospheric temperature may require insulation to reduce loss of heat. Almost any of the commonly used insulating materials can be employed. Calcium silicate, glass fiber, mineral wool, cellular glass, and plastic foams are among those used. Tanks exposed to weather must have jackets or protective coatings, usually asphalt, to keep water out of the insulation. 

At very low temperatures with hquid air and similar substances, the tank may have double walls with the interspace evacuated. The weh-known Dewar flask is an example. Large tanks and even pipe hues are now built this way. An alternative is to use double walls without vacuum but with an insulating material in the interspace. Perlite and plastic foams are two insulating materials employed in this way. Sometimes both insulation and vacuum are used. 

Chlorofluoro- Used in refrigeration and production of Refrigeration, plastic foam production, Attacks stratospheric ozone layer green-... 

Fig. 25.12. When a plastic foam is compressed beyond the linear region, the cell walls bend plastically, giving a long plateau exactly like that of Fig. 25.9.

FRISCH, K. c., and saunders, j. h. (Eds.), Plastic Foams Part I, Dekker, New York (1972) GEUSKENS, G. (Ed.), Degradation and Stabilisation of Polymers, Applied Science, London (1975) HAWKINS, E. L. (Ed.), Polymer Stabilisation, Wiley-Interscience, New York (1972)... 

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