This is reasonable when considering the

volume expansion ratio of PE-15, namely, only 15, and its total pore volume was only half of that of PE-30 specimens.

where [N.sub.0] is the number of cells in a specified area A, and [PHI] is the

volume expansion ratio.

The effects of temperature, GMS, talc content, die geometry, and butane content on the shape of the foam extrudate, as well as on its

volume expansion ratio and cell density are shown in Figs.

The

volume expansion ratio ([PHI]) was calculated by taking the ratio of the bulk density of pure PLA material ([[rho].sub.p]) to the bulk density of foam sample ([[rho].sub.f] as follows (33):

where [V.sub.p,i] is the volume of particles in its initial state, [V.sub.T,RT] is the total volume at room temperature, [[epsilon].sub.p] is the particle

volume expansion ratio, [[phi].sub.p,i] is the polymer volume fraction at state i, [[alpha].sub.1](T) is the volume expansion coefficient for squalane [33], [m.sub.1] is the mass of polymer, [m.sub.2] is the mass of squalane, [[upsilon].sub.sp,1] is the specific volume of the polymer, and [[upsilon].sub.sp,2] is the specific volume of squalane.

The

volume expansion ratios did not increase with the addition of n-butane.

The vertical axis [t.sub.1.5] indicates the time when the

volume expansion ratio [S.sup.3]/[S.sub.0.sup.3] becomes 1.5 after the nucleation.

These graphs show that different PP based foamable compositions, initially formulated to achieve a same

volume expansion ratio, demonstrate different mold filling rates while being processed in rotational foam molding.

The purpose of this study is to develop an engineering basis for extrusion processing of low-density polypropylene foams with a large

volume expansion ratio. This paper presents an effective means for the control of cell growth to achieve a desired

volume expansion ratio using butane as the blowing agent.

It is assumed in this calculation of the maximum

volume expansion ratio that the air eventually diffuses into the vacuumed cells after the foams are cooled to room temperature and the foam shape is restored.

The development of microcellular plastic production is addressed to obtain foam materials with high cell density, small cell diameter, and high

volume expansion ratio. Generally, there are three main processing steps: gas-polymer solution formation, subjecting the solution to thermodynamically unstable state to promote nucleation of cells, and controlling the cell growth to obtain the desired cell structure.

The amounts of LLDPE and the chemical blowing agent to be processed were determined based on the desired

volume expansion ratio. The processing and material parameters such as the amount of the chemical blowing agent and the heating period were varied.