Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 35

Poly(lactic acid) (PLA) is a kind of bio-based and biodegradable polymers. PLA foams with high expansion ratio have great application potential in heat insulation, adsorption and other areas. However, due to its low melt strength and slow crystallization rate, linear PLA has poor foaming ability, so it is hard to prepare PLA foams with high expansion ratio (45).

Although chain branching, blending, and other methods have been utilized to improve PLA’s melt strength and foaming ability, they easily destroy the biodegradability of PLA, cause chemical pollution, and raise production costs.

A new supercritical fluid foaming process, based on pre-isothermal cold crystallization, was proposed to prepare PLA foams with a high expansion ratio (45). To improve PLA’s melt strength and foaming ability, a pre-isothermal treatment was applied to induce sufficient cold crystallization.

SEM shows that a higher pre-isothermal temperature (Tc) leads to a larger spherulite size and higher crystal stability before foaming (46). The foaming experimental results demonstrate that, as the Tc increases, the expansion ratio and cell size first increase and then decrease. This is because proper crystallization helps to improve melt strength and promote foaming, but excessive crystallization restricts cell growth. Finally, as Tc increases, the high melting temperature crystals of the foam gradually increase, while the crystallinity of the foam first increases and then decreases, which is attributed to strain-induced crystallization.

The DSC and wide-angle X-ray diffractometer results confirm that the pre-isothermal treatment remarkably promotes the PLA’s cold crystallization, and endows the PLA sample higher crystallinity and more perfect crystalline structure (45).

Moreover, the high-pressure rheological testing results indicate that the pre-isothermal treatment improves the PLA’s melt viscoelasticity significantly. Finally, the foaming results show that the pre-isothermal treatment significantly enhances the PLA’s foaming ability. With the pre-isothermal treatment, the PLA’s maximum expansion ratio increases from 6.40-fold to 17.7-fold, and the uniformity of cellular structure is also improved obviously. The new process provides a green, flexible, and low-cost way to prepare fully biodegradable PLA foams with high expansion ratio (45).

A simple and effective methodology to improve the crystallization behaviors, rheological property, and supercritical CO₂ foaming of PLA using high-density poly(ethylene) (HDPE) as modifier was proposed (47). Here, PLA was blended with various contents of HDPE using a melt blending method.

The results demonstrated that with the blending of PLA and HDPE, the crystallization behaviors of PLA and HDPE were improved simultaneously and the rheological property of PLA gradually improved. The morphology of HDPE dispersion phase in the PLA/HDPE blends was irregular and its average size gradually became larger with the content of HDPE increasing. Then resultant PLA/HDPE blends were foamed using supercritical CO₂ as physical blowing agent in an autoclave. The cellular morphology evolution of PLA/HDPE blending foams had a relationship with the content of HDPE. When the content of HDPE increased from 0% to 5%, a complex cellular structure (CCS) appeared in the PLA/HDPE blending foams. With a content of HDPE more than 10%, the CCS disappeared gradually.

The interface between PLA and HDPE acted as heterogeneous nucleation points for both the crystallization of PLA and the cell nucleation of PLA/HDPE blends. Finally, the influence of foaming temperature on the foaming behaviors of pure PLA and PLA/HDPE blends was also investigated and the foaming mechanism on pure PLA foam and PLA/HDPE blending foams with various blending ratios was presented (47).

Nanofoams. The use of physical foaming agents to form nanofoams in semicrystalline polymers is still a tremendous challenge. The change from microcellular to nanocellular bubbles (the micro/nano transition) in PLA foam was investigated by adjusting the type of crystals induced in PLA during cold crystallization (48).

The results showed that at saturation temperatures below the micro/nano transition temperature, α′ crystals formed. Their coarser surfaces and higher crystallinity resulted in a higher cell nucleation efficiency, and thus a microcellular foam was converted to a high-cell-density nanofoam. After chain extension, the micro/nano transition temperature decreased, and the properties of the foam were improved at temperatures above the micro/nano transition temperature. A nanocellular foam with a cell density of 1015 cells cm^–3 and a cell size of approximately 30 nm was obtained (48).