Foam types and properties Deepak kumar samriya Yatharth gupta Ashok kumar

What is a Foam? Foam is a substance that is formed by trapping pockets of gas in a liquid or solid. E.g. a bathing sponge and foam due to soap. We will be discussing about solid foam. These are of two type Open-cell foam and Closed-cell foam.

Closed-cell Foam Open-cell Foam A closed-cell foam is one where distinct bubbles of gas are trapped individually. An open-cell foam is one where there are windows between adjacent bubbles which often create serpentine passages through the foam. The blowing agent is intact in the bubble for long term. Due to the open cell the blowing agent will quickly disperse into the atmosphere and the cell will refill with the surrounding air. Higher dimensional stability and low moisture traped as compared to open-cell foam. Immediate transfer of air or water vapor is enabled through the ‘open’ or ‘porous’ interconnected material structure. Higher plastic content and less air/gas content Lower plastic content and more air content More than 90% cell are closed (industrial definition) More than 50% of the cells are open. Closed-cell foam will be those which release less air as compared to open- cell foam More the air released, the more will it goes out of the foam and leave empty spaces.

Properties affected by the structure of foam What will happen by varying different properties of Foam?

When there are more and smaller pores the foam is a much better insulator, because the gas cannot transfer heat and the heat convection has to find its way around all the pores. More pores means more twisted path, and there is less chance of heat transfer.

Mechanical Strength Since gas has the least mechanical strength, the more the gas the less strong the foam will be, which means the lower density foams exhibit the smallest Young’s modulus. The strength is better along the direction of foaming. If the ratio of the height to width of a cell is increased by a certain amount, the compressive strength and the tensile strength of the foam also increases by the same amount. The polymer foam has to be able to resist multiple loadings, and resisting change to the structure so the loading can be repeated many times. If a foam is exposed to loads that are too large the cell walls will rupture and the amount of open-cell structure will increase and weaken the mechanical properties of the foam. https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CBwQFjAAahUKEwjszsv1kJzJAhVBWI4KHc_fACc&url=http%3A%2F%2Focw.mit.edu%2Fcourses%2Fmaterials-science-and-engineering%2F3-063-polymer-physics-spring-2007%2Fassignments%2Fpolymer_foams.pdf&usg=AFQjCNFyBsdeaQcBmtE3cUV2SgtauZScuQ&sig2=GyHrEj7qPf1Hj34xkznb8w&bvm=bv.107763241,d.c2E

Sound and impact absorption Open-cell structure are good at sound absorption. Impact absorption is a key advantage of polymer foams over the unfoamed polymer. The cells deform to absorb the energy impact and then reform when the load is removed.

Thermal Conductivity The total thermal conductivity is the sum of the conductivities of both the gas and solid phase plus the thermal conductivity due to convection and radiation. The thermal conductivity is very low for polymer foams because the amount of solid in the foam is very small. The gas phase contributes the most to heat transfer. The thermal conductivity of a gas decreases with an increase in molecular weight, so a heavy gas is more desirable as a blowing agent, although this is not always possible either because of cost or convenience. After usage and time the gas phase in the foam is replaced by air which has just a slightly higher thermal conductivity than some heavier blowing agent like CFC’s and CO2

The larger molecular weight compounds also diffuse gas much slower through the polymer matrix, and the replacement of air into the foam goes much slower, keeping the thermal conductivity as low as possible. Radiative heat transfer happen through cell walls, so a foam with many small cells will transfer less heat than the foam with a small amount of large cells. The cell size has larger impact than just changing the density. Convective heat transfer only takes place in foams with very large cell sizes.

The thermal conductivity has a minimum at a certain density The thermal conductivity has a minimum at a certain density. At lower densities the radiative component is more dominant since the cell size is larger, and so is the convective component. At higher densities the number of cell walls and the thickness of them is higher so the radiative component is less important, but the heat transfer through the solid phase becomes more dominant.

Flammability The volatile liquids that are often used as blowing agents in foam and lead to easily combustible foams. The flexible polyurethane foams are low density and have a lot of open-cell structure, which means it is permeable to air. This means that it will easily burn when exposed to a flame. To reduce the flammability, flame retardants in foams are used. E.g. Chlorinated phosphate esters, chlorinated paraffin, melamine powders and graphite are common as flame retardants.

Adding different properties to foam Changing the chemical structure of foam to alter its property.

Non Flammable Foam Non flammability in foam is added by making the foam deficient in the two most easily oxidizable (highly combustible) elements: Hydrogen and Carbon. Hydrogen is largely eliminated or replaced by a noncombustible monovalent element, such as fluorine Carbon is partially replaced by: 1. Elements that cannot oxidize by themselves, such as oxygen 2. Elements that do not support combustion such as nitrogen. 3. Oxidizable elements such as phosphorus, boron and silicon, incorporated in the polymer chain at a high oxidation state, P+5 (or P+4 ), B+3 , Si+4 . Certain metal and/or metallic compounds (oxides, hydrated oxides, carbonates) with fire retardant properties may also be incorporated into the chemical structure to further increase nonflammability. http://www.google.co.in/patents/US5514726

Inherent inflammability Increasing the interaction between the polymer or stiffening the polymer chain. Chain interactions can be enhanced by several means, such as increasing crystallinity, the introduction of polar groups, and hydrogen bonding. Chain stiffening can be accomplished by the use of aromatic or heterocyclic structures in the polymer backbone, such as in poly(p- phenylene), aromatic polyamides, and polyesters.

Mold properties Changing mold shape can also enhance foam properties. There are two types of molds: open and closed.

Open mold Closed mold Mold is open from the top. Mold is closed on the top. Gases released freely from the top. To release gases, small holes are made to the top. Corners are not occupied by the foam completely Every corners are occupied by the foam. Foam expand freely, so could be open cell Due to the back pressure of the released gas from the foam, the foam become closed cell. Density of the foam cannot be varied. Density of the foam can be varied by changing the holes diameter or closing some hole (by varying the back pressure) Foam surface is rough. Foam surface is smooth.

The foam in closed mold is smooth. Closed cell foam Due to less quantity of foam material, rough on the top part (which is bottom here) The top portion is open celled.

Different type of foams

Closed-cell foam Open-cell foam Expended polystyrene (EPS) Extruded polystyrene (XPS) Ethafoam/polyethylene foam EVA foam Polypropylene foam Neoprene rubber foam Polyether foam Polyurethane/polyester foam Memory foam (Polyurethane)

USES of Open cell foam

Polyurethane Memory foam Low density flexible foam is used in Walls of most refrigerators and freezers Low density rigid foam used in Automotive and truck seating Hard solid plastic used as electronic parts. Memory foam Generally used in bedding foam, because it changes shape according to the body shape and give most comfort.

USES of Closed cell foam

Polystyrene foam Naturally transparent, can be colored Cups and plates Expanded polystyrene is used in packaging

Ethafoam Covering sheets Packaging.

EVA foam Small volume gas trapped, make it rigid Used in soles of shoes, protective armor and impact resistance, mats