Perfluoropolyethers used as fluids for testing in electronic field

Claim 1

Method for carrying out Thermal Shock Tests which comprises: (A) dipping the sample into a liquid perfluoropolyether having a molecular weight of at least 390, at a temperature between -75° and -55° C., and wherein said liquid is selected from the group consisting of the following seven classes: ##STR5## wherein X is equal to --F or --CF 3 ; A and A', equal to or different from each other, are --CF 3 , --C 2 F 5 or --C 3 F 7 ; the units CF(CF 3 )CF 2 O and CFXO are randomly distributed along the perfluoropolyether chain, m and n are integers, n being 0 or greater, and the m/n ratio being 2 when n≠0; ##STR6## wherein B is --C 2 F 5 or --C 3 F 7 , and m is a positive integer; ##STR7## wherein m is a positive integer A'O[CF(CF.sub.3)CF.sub.2 O].sub.m (C.sub.2 F.sub.4 O).sub.n (CFXO).sub.q --A (4) wherein A and A', equal to or different from each other, are --CF 3 , --C 2 F 5 or --C 3 F 7 ; X is --F or CF 3 ; and m, n and q are integers; CF.sub.3 O(C.sub.2 F.sub.4 O).sub.p (CF.sub.2 O).sub.q --CF.sub.3 ( 5) wherein p and q are integers equal to or different from each other, wherein the p/q ratio is between 0.5 and 2; AO--(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.m--A' ( 6) wherein A and A', equal to or different from each other, are --CF 3 , --C 2 F 5 or --C 3 F 7 , and m is an integer; and DO--(CF.sub.2 CF.sub.2 O).sub.r --D ' (7) wherein D and D', equal to or different from each other, are --CF 3 or C 2 F 5 , and r is an integer; said perfluoropolyether when ##STR8## units are present show a kinematic viscosity lower than 8.5 cSt at 20° C. and a distillation loss not higher than 10% by weight of the total at 140° C. under atmospheric pressure and at least 90% at 260° C., whereas when ##STR9## units are absent show a kinematic viscosity lower than 18 cSt and a distillation loss not higher than 10% at 140° C. and at least 90% at 280° C. (B) extracting the sample from the perfluoropolyether of operation (A) and dipping it into a second liquid consisting of the same perfluoropolyether as in operation (A) at a temperature between 125° and 210° C.