利用水的相变热调控聚丙烯发泡材料的泡孔结构

Controlling Cellular Structure of Polypropylene Foams through Heat of Phase Transition of Water

为了控制发泡体内部泡孔结构,以制备聚丙烯(PP)泡沫为例,提出在CO2作为发泡剂进行物理发泡过程中,以水为助发泡剂,利用其发泡过程中汽化吸热,原位冷却发泡体内部.将PP与聚乙二醇(PEG)熔融共混制备共混物(PP/PEG),PEG的存在赋予了共混物吸水的能力.采用红外线成像仪测试发泡体表层和芯部温度,利用扫描电子显微镜表征了泡孔结构.结果表明,与纯PP发泡体相比,在同样发泡条件下,PP/PEG共混物发泡体内部温度明显降低,内部与表层泡孔结构相近,且发泡倍率提高.进一步分析讨论了PP/PEG共混物发泡体内部温度明显降低的机理.这种方法适用于不同聚合物体系和不同发泡工艺,如:模压、挤出、釜式发泡工艺.

At present, a problem in the field of polymer foaming is how to efficiently prepare polymer foamed products with large size, especially for semi-crystalline polymers （for example, polypropylene （PP））. A difficulty is that the polymer melt inside the foam samples cannot be quickly cooled after the formation of foam structure, resulting in collapse and merging of the cell structure, which may lead to form the gradient cellular structure between inside and outer layers. This problem severely restricts the development of polymer foaming technology and the application of foam products. In this work, we use PP as an example to demonstrate a novel foaming method with water as an in situ cooling medium for the first time. PP/hydrophilic polyethylene glycol （PP/PEG） blends are prepared by melt mixing, and then the blends are subjected to moulding physical foaming using CO2 as a foaming agent and water as an assistant foaming agent. The temperatures in the surface and core of the foamed samples are measured by the infrared imager. The results show that the addition of PEG makes PP has an ability to absorb water in the saturation stage. Owing to the water penetrating into the PP/PEG samples, the adsorption amount -） of foaming agent for PP/PEG blends is significantly higher than that for pure PP. In the foaming processing, water can quickly vaporize as a co-foaming agent. At the same time, the vaporization of water absorbs a lot of heat, which leads to the temperature drop in the inside of foam samples. Therefore the foam structures are cooled and shaped. As a result, under the same foaming conditions, the internal temperature of the PP/PEG foam is quickly fallen, and the foaming materials with higher expansion ratio are obtained compared with the case of pure PP. The mechanism of the obvious reduction in internal temperature of PP/PEG foam is discussed. This method has a universality, which can be applied in different polymer systems and different foaming processes such as moulding foaming, extrusion foaming and batch foaming,