Properties of foamed plastics

The volximetric weight, a parameter characteristic for the relative contents of both the solid and gas phase in a material, is the fundamental morphological parameter of foamed polymers and is related to all relevant physical properties of foamed plastics such as strength, thermophysical and electric properties. 

All other conditions being the same (chemical composition, volumetric weight, closeness of cells), the cell size can considerably affect the properties of foamed plastics. For instance, the thermal conductivity coefficient of foams always increases when the cell size increases due to the growing number of radiation and convection paths of heat transfer (for more information see Sect. 12). An increase of the... 

The relation between the specific surface and properties of foamed plastics is discussed in a monograph... 

As noted above, the adequate simulation of the cellular structure of plastic foams is very important for the investigation of the relation between structure and properties of foamed plastics. A solution to this problem would allow to establish not... 

Later, Polyakov and Tarakanov modified the model by presenting if of a hexahedral cell having an initial curvature (eccentricity) near the rods disposed in two perpendicular directions. This model, which makes allowance for the initial anisotropy of the plastic foam, satisfactorily describes the elastic properties of flexible foams under considerable strains, but generally predicts unduly high values of the elastic properties of foamed plastics. 

Unique Properties of Foamed Plastics Structural Features of Foamed Plastics Leading Commercial Polymers Polyurethane Polystyrene Poly(vinyl chloride)... 

Unique Properties of Foamed Plastics. Foamed plastics have certain unique properties that distinguish them from solid polymers and are particularly useful in practical applications. Basically, these properties result from their composite structure—a continuous phase of polymer that is of relatively high modulus, and a gas phase of negligible modulus, which may be either dispersed as single cells in a closed-cell foam, or continuous and interpenetrating in an opencell foam. 

Polymer Composition. The properties of foamed plastics are influenced both by the foam structure and, to a greater extent, by the properties of the parent poljuner. The poljuner phase description must include the additives present in that phase as well. The condition or state of the pol5nner phase (orientation, crystallinity, previous thermal history), as well as its chemical composition, determines the properties of that phase. The pol5uner state and cell geometry are intimately related because they are determined by common forces exerted during the expansion and stabilization of the foam. 

Since the mechanical properties of foamed plastics are functions of their densities, however, and since test specimens cut from the same large sample will show variations in density, comparison of the properties of different specimens can only be made after accounting for this variation. Manufacturers literature gives mechanical properties of different types of plastic foams either as an exponential function or as... 

The properties of commercial rigid foamed plastics are presented in Table 2. The properties of commercial flexible foamed plastics are presented in Table 4. The definition of a flexible foamed plastic is that recommended by the ASTM Committee D 11. The data shown demonstrate the broad ranges of properties of commercial products rather than an accurate set of properties on a specific few materials. Specific producers of foamed plastics should be consulted for properties on a particular product (137,138,142). 

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). 

PROPERTIES OF A PLASTIC FOAM BASED ON A MIXTURE OF HDPE AND LDPE... 

While the Bingham plastic model is an adequate approximate description of foam rheology, it is by no means exact, especially at low strain rates. More detailed models attempl to relate the rheological properties of foams to the structure and behavior of the bubbles. 

To improve the properties of PLA, plasticizers, special additives such as chain-extenders, polymer blends, and composites are commonly investigated. Martin and Averous (10) have studied the effects of various plasticizers on the properties of PLA. Pilla et al. (11-12) have investigated the effects of chain-extenders on the foaming properties of PLA. In addition, a vast number of studies have been conducted to enhance the properties of PLA by blending it with various polymers such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinyl acetate, polyolefins, polystyrene, HIPS (high impact polystyrene), polyacetals, polycarbonate, and acrylonitrile butadiene styrene (ABS) (13-26). 

The initial oligomer as an aqueous solution is obtained from the reaction of urea and formaldehyde at 100°C and pH = 5.8-6 [130]. The process of polycondensation occurs in the presence of acidic catalyst and yields a tri-dimensional polymer, releasing water and formaldehyde [131]. Surfactants are added as foaming agent to the initial composition for the formation of urea polymer foams [125,130]. Various additives are employed to improve the sanitary properties of these plastics. For example, ammonium carbonate reduces the content of free formaldehyde, while addition of carbonates of alkaline metals inhibits corrosion [125]. 

The rapidly increasing use of foamed plastics has stimulated the study of their properties and of the relationship between technical properties, composition, method of preparation and the effect of environmental factors. Nevertheless, attention to the problems of ageing as a result of thermal, thermal oxidative and radiation degradation is insufficient. 

Since the electroconductivity of pure water is by several orders of m itude hi er than those of polymers, even smaU amounts of moisture markedly reduce electrical insulation properties of foams. For this reason, measuren nt of dielectric properties is a precise, rapid and non-destructive method of monitoring the kinetics and level of moisture absorption. Thus, the establishment of correlations between dielectric properties and the hygroscopicity of plastic foams makes it possible to solve two practical problems how moisture affects dielectric properties and how to determine non-electrical properties by electrical measurements ... 

In order to establish and evaluate quantitatively the relation between the morphology and the properties of foamed polymers the basic macrostructural parameters must be determined. These parameters include relative number of open and closed cells, volumetric weight or apparent density cell size, shape, wall thickness, cell distribution according to size and shape in a given volume and specific surface area of the foamed plastic material. 

These differences in physical structure are responsible for the different properties of foamed polymers containing varying proportions of open cells. In contrast to closed-cell foamed polymers, open-cell foamed plastics have a higher water and moisture absorptive capacity, a higher gas and vapor permeability, less pronounced electric and heat insulation characteristics but a stronger sound absorptive and damping power... 

However, a real foam structure is composed of cells having differing shapes, sizes and volumes. In studying the properties of foamed polymers as well as in developing and elaborating preparative processes, it is necessary to find out cell size, shape and volume distribution. The methods for calculating the respective distribution functions will be discussed in Sect. 9.2, 9.3 here, we only note that the cell size distribution function is a most comprehensive and valuable characteristic of plastic foam structures. 

Chapter 10 on test methods is an expansion of the listings of standard test methods presented in Chapter 11. The first section of this chapter lists, in alphabetic order, 130 properties of cellular plastics and elastomers and tabulates the standard test methods used for each. Only number designations are given. The second section is a somewhat detailed discussion, also in alphabetical order, of 22 foam properties tested by standard test methods. The third section is a brief invited presentation of several non-standardized test methods currently in use. Chapter 11 on standardization documents lists published specifications, test methods and other related standards used in the U.S., in addition to British standards and ISO International Standards. A total of 361 standards are covered. A glossary of 221 terms is included. 

For any or all of these reasons, foamed plastics are now growing at a much faster rate than the plastics industry as a whole. At the same time, current notions of the basic relationships between foam structure and properties must be refined considerably to facilitate the proper production and use of foamed plastics in an even greater range of applications. 

Structural Features of Foamed Plastics. The critical structural features of foamed plastics may by itemized as follows polymer, gas, density, open/closed-cel1 ratio, cell size, and anisotropy. It is important to consider how each of these structural features affects Important properties of the foamed plastic. 

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