Vinylidene chloride copolymer-based foams

The copolymer samples went through five steps in order to generate the microcel-lular foam. First, the samples were brought up to the desired saturation temperature and pressure inside the vessel using the syringe pump and water bath. Second, the polymer samples were allowed to soak at this pressure and temperature for the duration of the desired soak time. Third, the vessel was isolated from the syringe 

Of critical importance, analysis of poly(methyl methacrylate) (PMMA) showed that at a saturation temperature, T, of 40°C, a saturation pressure, P%, of 1,500 psig (at these conditions, carbon dioxide is considered a supercritical fluid), and a saturation time, ts, of 24 h, a 1 mm thick disk absorbed 16.4 wt% carbon dioxide. Additionally, at a foaming temperature, Tf, of 120°C and a foaming time, tf, of 1 min, PMMA had a stable volumetric expansion ratio of 20. Other polymers also absorbed significant quantities of carbon dioxide, such as polystyrene (PS) and poly(vinylidene chloride-co-acrylonitrile) (P(VDC-AN)), which absorbed 8.9 and 2 wt% carbon dioxide, respectively, yet the stable foams that were formed had expansion ratios of less than 2 at the same conditions used to form the PMMA samples. Another polymer poly(vinyl methyl ketone) (PVMK) achieved an expansion ratio of 20. However, the foams were unstable, readily collapsed, and exhibited large voids ( 5 mm diameter), which are inconsistent with microcellular foams. The fact that PVMK readily collapsed after the foaming process made it difficult to determine the concentration of carbon dioxide in the sample. These results led to the eventual incorporation of the MMA monomer into the polymer formulation from the standpoint of carbon dioxide-induced microcellular foamability. 

Plasticizer studies were conducted in order to improve the foamability of the other monomeric constituents of the VDC-based copolymers. Table 4.5.2 shows the classes and formulations of the Scientific Polymer Products plasticizers used in this study. Also, water and V-methyl-2-pyrolidinone (NMP) were considered in the plasticizer studies. Studies of P(VDC-AN), poly(vinylidene chloride-co-vinyl chloride) (PVDC-VC), and poly(vinylidene chloride-co-acrylontirile-co methyl methacry- 

Polymer Saturation temperature, Ts c Saturation pressure, s,psig Saturation time, ts, h Foaming temperature, 7f,°C Foaming time, tf, min 

The use of NMP and/or plasticizers as well as varying foaming conditions was incorporated in a high-throughput experimental methodology during the downselec-tion for the polymer. For the premade VDC-based copolymer from the polymerization track, an RBl-201 VDC-based copolymer product described in Section 4.3.2 resulted in microcellular structures (Fig. 4.5.2). 

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