*Article* **Supercritical CO2 Foaming of Poly(3-hydroxybutyrate-***co***-4-hydroxybutyrate)**

**Tao Zhang 1, Yunjae Jang 2, Eunhye Lee 2, Sooan Shin <sup>2</sup> and Ho-Jong Kang 1,\***


**Abstract:** The supercritical carbon dioxide foaming characteristics of the biodegradable polymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-*co*-4HB)) are studied for environmentally friendly packaging materials. The effect of the 4HB composition of the P(3HB-*co*-4HB) copolymers on the foaming conditions such as pressure and temperature is studied and the density and the expansion ratio of the resulting P(3HB-*co*-4HB) foam are together evaluated. The increase in the 4HB content reduces the crystallinity and tan δ value of P(3HB-*co*-4HB) required for the growth of the foam cells. Therefore, the foaming temperature needs to be lower to retain a suitable tan δ value of P(3HB-*co*-4HB) for foaming. It was found that P(3HB-*co*-4HB) with less crystallinity showed better formability and cell uniformity. However, foaming is not possible regardless of the foaming temperature when the 4HB content of P(3HB-*co*-4HB) is over 50%, due to the high tan δ value. A lower foam density and higher expansion ratio can be obtained with crystalline P(3HB-*co*-4HB) of low 4HB content, compared with non-crystalline P(3HB-*co*-4HB) of high 4HB content. The expansion ratio of P(3HB-*co*-4HB) foams can be increased slightly by using a chain extender, due to the lowing of crystallinity and tan δ. This is most effective in the case of P(3HB-*co*-4HB), whose 4HB content is 16%.

**Keywords:** poly(3-hydroxybutyrate-*co*-4-hydroxybutyrate); supercritical carbon dioxide; P(3HB-*co*-4HB) foams; expansion ratio; chain extender
