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Cellulose acetate (CA) is a bio-based polymer suitable to replace foamed polystyrene (PS) in packaging applications. Foam trays can be produced by thermoforming of extruded sheets foamed with physical blowing agents. In this paper, the effects of various process settings and the calibration of the sheet on foam morphology and surface quality of extruded CA sheets are presented. Different contact cooling options were applied in order to investigate their influence on surface roughness, density, and morphology of the sheets. By adjusting cooling parameters, blowing agent formulation, and process settings, smooth foam sheets with a surface roughness below 10 µm and a density in the range of 150 kg m−3 were produced.

Cellulose acetate is a bio‐based plastic which can be used to produce foam sheets through the extrusion process. In this study, different co‐blowing agents (BAs) were added to the foaming process in order to improve the extrusion foaming behavior and cell density of sheets foamed with HFO‐1234ze. The effect of co‐BAs on the melt viscosity was investigated with an inline extrusion rheometer. Furthermore, the resulting cell morphology of foam sheets was studied. While the plasticizing effect of the co‐BA is similar to HFO‐1234ze, the cell density of the foam sheets could be improved by adding co‐BAs in concentrations between 0.04 and 0.16 mol/kg. Adding 0.06 mol/kg of ethanol increased the cell density from approximately 1 to 12 × 105 1/cm³ at an invariant density of ∼130 kg/m³. The density reduction of the foam sheets can be calculated in good agreement from the molar concentration of the BAs. However, high amounts of co‐BA slightly reduce the BA yield.

Two types of externally plasticized cellulose acetate (CA) were chemically modified using 4,4'-methylene diphenyl diisocyanate (MDI) as crosslinking agent. Crosslinking was performed in the molten state by means of melt mixing in an internal mixer. The viscoelastic properties of the non-crosslinked, externally plasticized CA show typical temperature dependence, similar to conventional thermoplastics. A strong increase in storage modulus is observed with increasing crosslink density indicating that the crosslinked compounds exhibit predominately elastic response. The complex viscosity also increases considerably with increasing crosslink density and does not reach the typical Newtonian plateau at low radial frequencies any more. The viscoelastic properties correlate well with the data recorded online during reactive melt processing in the internal mixer. In comparison to the non-crosslinked CA, the crosslinked compounds show higher glass transition temperature, higher VICAT softening temperatures, improved thermal stability and lower plasticizer evaporation at evaluated temperatures.

This paper presents recent results of foam extrusion of thermoplastic cellulose acetate (CA) using HFO 1234ze as low global warming blowing agent and talc as nucleating agent. Foam extrusion behavior, physical foam properties, and foam morphologies were studied in detail with respect to blowing agent concentration and talc content. Depending on these parameters, thermoplastic CA exhibits excellent foam extrusion performance with good expansion behavior at the die. Talc as nucleating agent results in homogeneous fine foam morphologies with closed cells [i.e., Fig. 3(3)]. Depending on the blowing agent content and talc content, average cell size ranges from 1 to 0.12 mm and foam density ranges between 100 and 400 kg/m3.

Elongational flow properties of polymer melts are very important for numerous polymer processing technologies such as blown film extrusion or foam extrusion. Rheotens tests were conducted to investigate the influence of plasticizer content on elongational flow properties of cellulose acetate (CA). Triethyl citrate (TEC) was used as plasticizer. Melt strength decreases whereas melt extensibility increases with increasing plasticizer content. Melt strength was further studied as a function of zero shear viscosity. The typical draw resonance of the Rheotens curve shifts to higher drawdown velocity and the amplitude of the draw resonance decreases with increasing TEC content. With respect to foam extrusion, not only are melt strength and melt extensibility important but the elongational behavior at low strain rates and the area under the Rheotens curve are also significant. Therefore, elongational viscosity as well as specific energy input were calculated and investigated with respect to plasticizer content. Preliminary foam extrusion tests of externally plasticized CA using chemical blowing agents confirm the results from rheological characterization.