THE FOAMING PROCESS

The following sequence of events is a simple picture of what happens when flexible foam is made by mixing together the materials

MIXING

In the first step the foam ingredients are mixed by mean of a stirrer. Good mixing is essential to produce homogeneous foam. The silicone surfactant assistS in achieving good mixing since it lowers the surface tension of the polyol.


NUCLEATION

During the mixing air bubbles are created in the liquied. These act as nucleation point for the expansion gases. When making a box foam with simple equipment. It is not always possible to regulate the size or the number of these bubbles on a continuous slabstock machine however there are a number of way in which to ensure that there are sufficient of these initiating point of foam formation and that they are fit the right size.

After about 10 second the blowing gases (CO2 and an ABA if used) diffuse into these small air bubbles and enlarge them giving the mixture of ingredients a ‘creamy’ appearance. The time from initial mixing to change in appearance is called the cream time.

EXPANSION

As more blowing gas is generated, the bubbles expand and the foam begins to rise. During the foam rise the number of bubbles remains constant. The silicone surfactant stabilizes the bubbles preventing them from coalescing; without surfactant the mixture appears to boil and the foam collapses. At the same time as the bubbles are expanding, polymerization reaction also takes place after mixing, the gas reaction stops. The foam now occupies 30-50 time the original liquid volume. The polymer part of the foam has begun to gel in the form of gas-filled cells with thin walls and somewhat thicker struts along the edges.

CELL OPENING

In the flexible foams the cell walls are unable to contain the full gas pressure: at about the full rise time, they break and the polymer contracts into the struts. At the same time the polymer is sufficiently gelled for the struts to strong enough to stand while the gas escapes through the pen cells.



GELANATION

Continuing polymerization increases the strength of the polymer which reaches the gel time about-3 minutes after mixing.

CURING

The foam is then left for at least 24 hours to cure, during which time various slow cross-linking reactions take place to give the foam its final physical strength.

NOTE
In the foam formation just described the two types of reaction one causing gas evolution and the other polymerisation of the cell wall and struts, must be finely balanced. If the balance is wrong then various faults can occur into foam. In the simplest terms, the two main types of fault are caused by the polymerisation reaction occurring too soon or too late.

When polymerisation occurs too soon polymer strength develops too early. Some of the cell walls will not burst under the gas pressure. The result of this is that the foam will have poor resilience and feel ‘dead’, often called a ‘tight’foam: if many of the cells remain closed, then as the foam cools and the internal gas pressure falls below atmospheric, the foam shrinks.

When the polymerisation occurs too late the struts will be weak at the time when the cell walls burst. If this happens the struts can break and since the struts of one cEel are shared by all the immediately surrounding cells, the result is a series of struts breaking, forming a split. The extent of this split can be increased by the expanding gases forcing the spilt further apart and breaking even more struts.
(Thus it can be seen that the weaker the foam, i.e. the later the polymerisation, the larger the spilt.) Splits due to late polymerization usually occur ate the top edges (shoulders) of the blcok where the foam is weakest (coldest and therefore slowest to polymerize) and in the center of the blcok where the exothermic is highest (highest gas pressure).

For more information visit
http://www.polyurethanesinfo.com