Fig. 5. Variation of Impact Strength with Respect to Percentage of Foam in LLDPE
As the foaming percentage is increased from 6 to 10 the impact strength reduces this reduction in impact
strength can be attributed to two different facts. The first one being, the developed finer cells get destroyed because
of increase in foaming content. Since the foam percentage is more, the amount of gas dissolved increases resulting
in a cell growth. Adjacent cells will be in contact with each other eventually. These neighboring cells will first share
a common wall in a polyhedral shape of cells and the thickness of the wall will decrease as the void fraction
increases. In other words, the drainage of the polymer in the wall occurs as the foam density decreases. With the
polymer drainage in the cell wall, the instability of the cells to coalesce increases because the strength of the cell
wall decreases. This increased amount of dissolved foam induces a greater thermodynamic instability resulting a
courser and non uniform foam structure. This ultimately reduces the impact strength of the product. The other
reason being, since the density of foamed parts decreases and cell sizeincreases with increased foaming content.
Large size cells supported by thin cell walls collapse as they aresubjected to external impact forces. The impact
energy that foamed parts could absorb therefore decreased (Klempner and Frisch, 1991).