Open-Cell Content

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. 

This method is based on Boyle s Law, which states that a decrease in volume of a confined gas results in a proportionate increase in 

Procedure A—Testing cell dimensions to correct for cells cut by sample preparation. 

Procedure B—Correcting for cells opened in sample preparation by cutting the test samples into eight smaller samples. 

Procedure C— Determination of open-cell content without cor-ecting for surface cells opened by cutting. 

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Compressive modulus of various PE foam boards, in different densities and open cell content, are measured and compared with different models. Contributory elements to compressive resistance are investigated. Agreement between test results and modelling improves significantly while only considering the struts strength parallel to compressive force. 7 refs. 

The dimensional stability of low density, water blown rigid PU foams for pour-in-place thermal insulation applications was improved by the use of a phthalic anhydride based polyester polyol containing a dispersed cell opening agent. The foam systems obtained allowed some of the carbon dioxide to be released through the cell windows immediately after filling of the cavity, and to be rapidly replaced by air. Studies were made of the flowability, density, open cell content, dimensional stability, mechanical properties, thermal conductivity and adhesion (particularly to flame treated PE) of these foams. These properties were examined in comparison with those of HCFC-141b blown foams. 21 refs. 

Microcellular foaming, bimodal cell size distributions, and high open-celled contents of molecular composites of HT-polymers were reported by Sun et al. [33], investigating blends of a rod-like polymer polybenzimidazole with an aminated PSU and poly(phenyl sulfone) by using carbon dioxide as a blowing agent. The complex foaming behavior was related to phase separation within the otherwise... 

Similar studies on foam morphology were reported by Williams et al.12,13 for polyimide foams with different densities or surface area also two different chemical formulations were used. Comparing foams with the same chemical composition, it was shown that no consistent correlation could be found between PHRR and foam density or open cell content while greater correlation is proved between the surface area and PHRR because they showed the same trend. Foams having the same density but different surface area and chemical composition show great variation of PHRR (up to 50%) both at 75 and 50kW/m2. 

Manufacturing Process. Both prepolymer and one-shot processes are available, but the polymeric isocyanate-based one-shot process is used in preference because of the easy processing due to the low viscosity of the system, relatively low toxicity of polymeric isocyanates, and fewer environmental problems. However, a disadvantage of the one-shot process is a possible risk of shrunken-foam formation due to its higher closed-cell content. In contrast, the TDI-based prepolymer process has advantages including better in-mold flowability and higher open-cell content. 

Floral Foam. In the U.S. and Western European countries the most popular use of phenol foam for floral applications is a water-soaked foam using a resol-type composition. It is important to maximize the open-cell content in order to inq)rove its water penetration. Floral foam is produced as laige block or slab by a batch process. The foam is easily cut and packaged with a minimum of loss due to waste. Floral foam is commonly used for living flowers, but is not used for dried and artificial flowers. 

Closed-cell Content (Count) (see also open-cell Content (Count))... 

ASTM Test Methods D 2842 on water absorption and D 2856 on open-cell content require knowledge of surface cell volume, which uses cell-size values in the calculation. 

ASTM D 1940, which was formerly used for open-cell content, has been discontinued. 

ASTM D 2856-87 Standard Test Method for Open Cell Content of Rigid Cellular Plastics by the Air Pycnometer, 1 pp (DOD Adopted) (FSC 9330) (YD) (Comm D-20)... 

ASTM D. 2856-94. Open-cell content of rigid cellular plastics by the air pycnometer. 

FIGURE 3.93 Determination of open cell content of rigid cellular plastics. Open cell content (approx.) = 100 (V— Vp/V, where displacement volume of specimen, cm and V, geometric volume of specimen, cm. Standard test... 

Various geometric coring patterns in polyurethanes (174,178) and in latex foam rubber (179) exert significant influences on their compressive behavior. A good discussion of the effect of cell size and shape on the properties of flexible foams is contained in References (60) and (163). The effect of open-cell content in polyethylene foam is demonstrated (173). 

Different behaviors can be seen with the data listed in Table 17.5. The concentration of gas had a limited impact on the density reduction of neat PLA at lower CO2 content. Below 5 wt% CO2, a smaller number of cells were nucleated and both the density and the cell population density remained almost constant. However, above 7 wt% CO2, lower density foams with a large number of nucleated cells were produced. The density remained constant while the cell population density slightly increased with CO2 content. Since higher gas concentration favors the nucleation of large number of cells, the results imply that a critical gas concentration of 7 wt% is required to achieve low-density foams. The highly expanded foams had a high open-cell content. 

Javni et al. (2011) studied the possibility of replacing polyol copolymers used in the preparation of flexible PU foams by the incorporation of unmodified and modified montmorillonites. While the addition of the unmodified MMT increased hardness, compression strength, and resilience of the foams, the incorporation of the organically modified MMT resulted in foams with lower modulus, hardness, and compression strength values, a direct result of the higher open-cell contents and poorer cellular structure of the resulting foams, apparently leading to the conclusion that in this specific case the addition of modified MMT led to a counterproductive result. 

The cushioning and soft-feeling properties required for some car interior applications were achieved by matrix modification as well as by the structural parameters affecting the compression-deflection of the foamed material (i.e. density, open cell content, cell size). 

The apparatus used in the test is called an air pycnometer, and is schematically illustrated in Figure 14-1. It consists of two cylinders of equal volume with a specimen chamber provided in one. Pistons in both permit volume changes. When the volumes of both cylinders are altered, the volume change for the specimen-containing cylinder is smaller than for the empty chamber because of the presence of the sample. The extent of this difference is measured and a calculation is carried out to determine the open-cell content of the foam sample. A commercially available air pycnometer is shown in Figure 14-2. ... 

D2856 Open-Cell Content of Rigid Cellular Plastics by the 4590... 

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