This sector refers to low density flexible foam made as slabstock either by continuous or discontinuous (batch-block or box-foam) production. Having been made in a block – say 2.5 m * 1.5 m * 1.5 m in a discontinuous box process or as a continuous slab several hundred metres long – 1.5 m wide and 1.5 m high in the continuous process, the foam is cut to the desired thickness and dimensions on proprietary foam cutting equipment. It is also common for such equipment to be able to cut contoured shapes from the foam block. Where the foam is required in a thin sheet say 3mm thick – it is generally moulded as a round block, rather than with a square cross-section, and the foam is peeled into foam sheet working from the outer circumference of the round block – in.
Most of this foam is used for mattresses – either as a fully foam product or in the form of foam sheet wrapped around a spring structure.
The other major application is in cushions for domestic furniture, where the selection of the correct foam density determines the comfort level and durability of the furniture.
The are many other varied applications for low density flexible foam – thin sheet is laminated to fabric to provide a warm layer to clothing. There is also a considerable market in the automotive sector included here where thin foam sheet is laminated to fabric or leather for interior trim applications – seat covers, door panel, headliner and other panel covers.
There are applications in household items such as scouring pads and in clothing in shoulder pads when fashion dictates. There is also a market in acoustic products to sound-proof rooms – in cars the foam sound insulation is generally moulded these days but there is still a certain amount of die-cut foam sheet used in automotive acoustic packages.
Flexible foam made in block form either by continuous or discontinuous methods is made at a variety of densities ranging from 12Kg/M3 to 80Kg/M3. Within that density range it is also possible to produce widely varying foam hardness. It is common for hardness and density to be interchanged where a high hardness foam is mistakenly referred to as a high density foam. It is quite likely that hard foams are produced at low density and likewise supersoft foam produced at relatively high density.
In Australia over 50% of the slabstock foam produced is at a density of 22Kg/M3 or less. This is perhaps more in line with developing countries compared to Western Europe for instance where an average density of 28 Kg/M3 is prevalent. In Western Europe there is a well-developed “High Resilience (HR)” foam market – where the foam density is in excess of 30Kg/M3. In Australia High Resilience foam accounts for less than 15% of the market.
The vast majority of low density flexible foam slabstock is made using TDI. Generally speaking the base polyol used is a polyether but for special requirement eg foam to be peeled to sheet form and flame laminated to textiles, polyester grades are used as the base polyol.
There is of course a considerable amount of waste foam after the cutting process and after trimming the edges off the blocks. Great efforts have been made to minimise this scrap – using computerised cutting patterns to cut the block to required dimensions most efficiently for instance. Peeling a round block is the most efficient way of obtaining a length of thin sheet. The rising foam in a block tends naturally to dome and great effort is applied in the design of continuous plant to produce a flat-top block since the foam volume in the domed block will be cut off and scrapped. In discontinuous batch-block or box-foam processes it is common to use a floating lid to flatten the top of the rising foam bun.
There is however a considerable market for scrap foam off-cuts as it is granulated to chips and rebonded using a Polyurethane binder to form reconstituted foam or chipfoam. In the United States there is a considerable market for “recon” in carpet underlay for example.
One of the major issues facing the flexible foam industry in particular with respect to domestic applications in mattresses and furniture is the behaviour of the foam in a fire. The British authorities introduced stringent fire specifications in 1980 and the Polyurethane suppliers and foam producers have met the challenge with the development of Combustion Modified High Resilience (CMHR) foams that meet these tough specifications.
Another issue facing the flexible foam slabstock manufacturing industry is the choice of blowing agent to replace CFC’s. Methylene Chloride has been the Australian selection but this is relatively short term due to its own environmental problems (that sees its use banned in Germany for instance). The most commonly suggested replacement is to use Carbon Dioxide injected into the liquid polyol or into the reacting foam mixture at the mixing head. Another alternative developed for discontinuous batch-block or box foam production is to foam at reduced atmospheric pressure in a vacuum chamber.