Laser-induced roughening

The extensive industrial appUcation of ultrashort pulse lasers is, however, still inhibited by the currently high costs of the beam sources and the low average powers and repetition rates. Pico and especially femtosecond lasers are applied only if their cost is justified by economic or scientific added value of the relevant process step. 

Laser-induced roughening of the polymer surfaces can be observed below, near and above the ablation threshold. Below the threshold, it can be initiated by photolytically driven reactions, for example, by photo-oxidation or photobleaching. Near the ablation threshold, the change of surface topography plays an important role, for 

The physico-chemical changes of polymer surfaces upon laser treatment can be tailored for sensing applications for fast response, sensitivity, repeatability and chemical selectivity. As an example, the absorbance of PMMA thin films irradiated at 157 nm was shown to increase as a result of chemical modification of the material surface. Polymer hydrophobicity, which is responsible for the polymer volume expansion during sorption, was enhanced after irradiation. The methodology was used to enhance sensor detection sensitivity of ethanol and methanol analytes [89]. 

Femtosecond laser was also used to ablate polypropylene film with micropattems specifically tailored in size, location and number for breathable microperforated packaging films [99]. 

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The objective of the following sub-sections is to exemplify the synergetic effect of the combined processing by laser-induced siuface roughening and subsequent UV-induced modification of surface chemistry and simi-marizes experimental results from earlier studies of the authors. The first example (sub-section 13.3.1) concerns the increase of water repellence of... 

This is illustrated in the context of data recorded on the p-aramid (Kevlar ) fabrics, which are summarized in Table 13.4. The first observation is that, again, extremely high water repellence is achieved by the combined process of laser-induced surface roughening and subsequent UV-induced modification of surface chemistry. The photo-chemical modification used... 

Table 13.4 Water wetting behavior of technical p-aramid fabrics following (a) laser-roughening, (b) UV-induced grafting of perfluoro(4-methylpent-2-ene) (PFMP) and (c) combined laser roughening and photo-chemical modification. Water wettability was characterized by measurement of the drop penetration time of an aqueous dyestuff solution according to the TEGEWA procedure (see text).

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