Coating removal from an industrial polypropylene blend by cryogenic blasting: the development of substrate damage

Cryogenic blasting is a pneumatic jet based process in which solid carbon dioxide (CO2) particles impinge upon a surface. It may be employed to remove paint layers from a variety of substrate materials. During this process, the substrate material may be damaged due to the carbon dioxide particles striking the substrate surface. For applications where the component is to be recoated, such damage is undesirable. This work describes the damage sustained by an industrial polymer blend when exposed to cryogenic blasting with solid carbon dioxide particles of ∼0.5 mm diameter impacting at mean particle velocities in the range 155-241 m s−1. The damage is recorded by profilometry and scanning electron microscopy (SEM) studies. Roughness (Ra) values are reported for blasted surfaces as a function of particle impact velocity and exposure time. At particle impact velocities below 206 m s−1 and 20 s blasting time the recorded roughness is generally less than 1 μm. At higher velocity the polymer substrate is more obviously damaged with Ra = 5-20 μm. The SEM and profilometry work shows a clear transition in damage behaviour resulting in low and high damage regimes, dependent on impacting particle velocity and duration of blasting. This transition is explained in terms of the thermo-mechanical effects of cryogenic blasting, i.e. whether the net effect of the impacting jet of particles is to heat or cool the substrate. Temperature recordings of the sample during blasting reveal the high damage regime is associated with heating of the substrate (temperatures up to ∼90 °C were observed) whereas the low damage regime is associated with cooling of the substrate. The damage sustained by the polymer substrate is shown to be temperature dependent.