Thin Polyolefin as well as PS foamed products have been commercially used for the last two decades. They are used for labels, tamperproof seals, pearlescent wrapping paper or airline tray liners. Most of these thin foam sheets are based on Polystyrene or Polypropylene. They are usually made in thickness greater than 500 microns.

Foaming for thinner films is more difficult and therefore more expensive. The massive growth of cellular phones and PDAs over the last few years has propelled the development of thinner polyolefin foams since they are used as gaskets for these products. The developments are focused on newer grade as well as on processing technologies.

To achieve good foaming, the polyolefins are required to have excellent melt strength in order that microbubbles do not get collapsed. Recently introduced grades of PP as well as PE have long chain branchings in addition to bimodal molecular weight distribution. These long chain branchings have been developed by reactive extrusion.

Borealis had introduced Dapoly PP grades at K-2001. For PE produced by Metallocene catalysts, long chain branchings have been introduced by reacting with silanes in the reactive extrusion process. To enchance rigidity of PE foam, higher density grade is desired. To obtain softness, particularly on the surface, newer grades of Ethylene-Styrene interpolymers available from DOW could be used either in coextrusion or in monolayer. Besides, the additives in polyolefins should be carefully selected or optimised in order to prevent any interference to the formation process of microvoids. For instance, calcium stearate used as acid scavenger in production of PP acts as a defoaming agent.

PP grade required for thin foam:

Foaming can be achieved by chemical blowing agents. Both types of blowing agents (exothermic and endothermic) are used. The endothermic blowing agents are expected to provide more uniform foaming in polyolefins. Foaming can also be achieved mechanically. However, for thin foamed films, endothermic blowing agents are used in combination with Isopentanes or Flurocarbons because endothermic blowing agents provide nucleating effect and achieve finer cell structure.

Thin foamed films essentially have lower strength. Therefore they require either orientation or cross linking to improve their strength. Both-extrusion film blowing as well as cast film process can be used for thin foamed films. However, cast films are brittle in one direction, since the orientation is achieved only in one direction in this process. To produce thin foamed PP films, biaxial orientation process can also be used. Infact Bruckner has developed simultaneous biaxial orientation process known as LISIM. LISIM helps in producing stronger foamed films. To avoid bubbles break during orientation, cross linking of film is done before the orientation process.

These developments have generated thinner foamed films with thickness as low as 250-300 microns compared to the earlier thickness of 500 microns. Infact Sekisui, a world leading foam manufacturer demonstrated a 250 micron thin film based on LDPE copolymer at K-2001. These developments would lead to 0.2 - 0.3 grams/cc density at thickness as low as 200-250 microns that will be required for cellular phones,PDAs etc.