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 CO

2

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 (

μ

) 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.