Closed cell content

The PP is produced by melt-kneading PP, an isoprene monomer and a radical polymerisation initiator, which has a high melt viscosity and a high melt tensile strength and is difficult to cause drawdown. Foamed articles made therefrom have a low density, high closed cell content, good appearance and excellent heat resistance. 

Most low density rigid polyurethane foams have a closed-cell content of >90%. Above 0.032 g/cm3, closed-cell content increases rapidly and is generally >99% above 0.192 g/cm3. Bun foam, produced under controlled conditions, has a very fine-cell structure, with cell sizes of 150—200 im. 

Sulfur foams with a wide spectrum of properties can be prepared by this process. Properties generally vary with density, which may range from 3 to 45 lb/ft3. However, while density is kept constant, properties such as compressive strength, flexural strength, and closed-cell content may be altered by formulation changes. Some of the more common properties are listed in Table II. 

Sensitive to hydrostatic pressure and closed cell content. 

Understanding the link between microstructure in PDMS foams and the mechanical performance is crucially important for the development of ageing trends that allow long term qualification. The plots in Figure 13 show force decay curves for a series of siloxane foam samples with varying closed cell contents. The trend demonstrates the time dependent nature of the measurement of physical properties. This creates an additional source of variation and if possible then it should be eradicated by the production of 100% open cell structures. 

Figure 13. Force decay curves for a series of siloxane foam samples with varying closed cell contents demonstrating the time dependent nature of resulting physical properties measured.

This idea has been confirmed by Lowe et aL l describing the dependence of water absorption of phenolic foams (7 = 35 kg/m ) on the fraction of closed cells this dependence recalculated for the fraction of open cells is shown in Fig. 9. It is noteworthy that an increase of d< from 10 to 98% corresponds to an increase of the maximum water absorption only from 6 to 8 g/lOOmL On the basis of these data, Lowe et al. concluded that surface chemistry is more important for foamed polymers than the closed cell content They explain the obtained data by the presence of microcells and interstices between macrocells (see Chap. 5.2). 

Fig. 9. Dependence of water adsorption of phenolic foams Pw on closed cell content (samples were stored under water for 7 days) )...

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. 

The NCO/OH equivalent ratio was 5.0 and the aliphaticity index was 0.15. The resultant foam had a cream time of 15 sec, rise time of 20 sec, foam density of 0.041 g/ml, tensile strength in the direction of foam rise of 2.0 kg/cm and a closed-cell content of 90%. 

The foam had a density of 0.0317 g/cin , closed-cell content of 96.9% compression resistance 1.8 kg/cm and brittleness (ASTM C-421) 27.4% weight loss after a 10-min tumbling test. 

Type A High closed-cell content, low thermal conductivity, high fire resistance... 

In the past, types C and D were common in resol-type foam. Type D is low in density and is an open-cell-type foam used widely. Type C is high in density and mechanical strength. Type B was introduced by the American manufacturers, Koppers, Monsanto and others. This type has a high closed-cell content and has attracted special interest recently. Since Type B is a high closed-cell it is excellent for thermal insulation. This type has been used in construction in the U.S. and Western European countries. 

In Type A, the closed-cell content is high, providing low thermal conductivity. Type A should be able to pass various types of fire tests. JIS-A-1321 (incombustibility test for internal finish material of buildings) is a Japanese fire test method said to be the strictest test in the world. Type A has successfiilly passed the Model Box Test, one of the items of this severe standard. 

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

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. 

An additional factor which may have an impact on the vacuum properties is the closed cell content of the PU foam. All the open cell PU foams so far presented in the literature for VIP applications are quoted to be 100% open cell based on pycnometric measurement. On the other hand, the accuracy of this test method is generally close to 0.2% [15], so that the possibility of a small fraction of closed cells cannot be completely ruled out and has to be considered as an additional potential source of pressure build-up in the panel. 

Values of thermal conductivity (K factor) depend on density, closed cell content, composition of the gas. Cell size and orientation also affect the K factor, which decreases with the decrease of temperatnre independent on the nature of foaming agent [39]. 

The resole closed cell foam can be classified as high closed cell content, low thermal conductivity, high fire resistance, and high closed cell content, low thermal conductivity and low fire resistance. Both these types have a density of 40 kg/cm a thermal conductivity of 0.02 kcal/m.h.°C, a closed cell content of 90%, a water absorption of 0.5 g/100 cm, but the limited oxygen index (LOI) is 50 for the first type and only 33 for the second one. Compressive strength varies between 1.6 and 2.0 kg/cm and the tensile strength is 1.1 kg/cm [45]. 

Home insulating foam in closed spaces, retains moisture on occasion for several years, mainly in cold climates. UF foam, having a density of 10-14 kg/m, is the lightest of the polymeric insulating materials. It is white, has no mechanical strength even in compression, and does not support combustion and it is very friable. Commercial products have a closed cell content of about 80%. UF foams have a thermal conductivity of 0.022-0.029 W/m.K. Its maximum service temperature is very low, around 49 °C. If not formulated properly UF foams lose their insulation efficiency. Since they are water-absorbing, they lose this characteristic at high humidity [30]. 

There is an increase in closed cell content, tensile strength and compressive strength up to an addition of 10% PFA by weight of polyurethane chemicals. 

BS5608 (1986) gives the minimum physical properly requirements for rigid polyurethane foams. The most important properly given is for closed cell content. It is this which determines the insulation value and structure of the foam. The minimum value for closed cell content in BS5608 (1986) is 85%. It can be concluded from the results of... 

Table 7 gives a sample of the results obtained from a number of trials. It was found diflicult to mix the polyurethane chemicals and catalyst thoroughly. It was necessary to increase the catalyst level in order to obtain improved closed cell content. At a catalyst level 1.5 times greater than normal (4.3%), a closed cell content was obtained within that specified in BS5608 (1986). No further significant improvement was obtained at higher catalyst levels. 

Table 7. The effect of catalyst levels on closed cell content...

Catalyst Closed Cell Content Std Closed Cell % Diff... 

Parts Blowing Agent % PFA Closed Cell Content... 

The coupling agents have decreased the closed cell content of the foam. 

Procedure C. Does not correct for cells opened during sample preparation and gives good accuracy on highly open-celled materials. The accuracy decreases as the closed cell content increases and as the cell size increases. 

Cellular plastics have the lowest thermal conductivity or K factor of any insulating material available today. This superior insulating ability depends upon many variable factors. Thermal conductivity of cellular plastics is greatly influenced by the blowing agent, cell size, closed-cell content, and density of the foam. Temperature and moisture are other factors that affect the thermal conductivity of cellular... 

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