Thermal cracking of waste printed wiring boards for mechanical recycling by using residual steam preprocessing

Mechanical waste-processing methods,whichcombine crushing and separation processes for therecovery of valuable materials,have been widely appliedin waste printed wiring board(PWB)treatment.However,both the high impact toughness and the tensile and flexuralstrengths of whole PWB with a laminated structure resultin great energy consumption and severe abrasion of thecutters during multi-level crushing.In addition,the hightemperatures occurring in continual crushing probablycause the decomposition of the polymer matrix.A thermalcrackmethod using residual steam as the heating mediumhas been developed to pre-treat waste PWBs.Thistreatment reduces the mechanical strength in order toimprove the recovery rate of valuable materials insubsequent mechanical recycling.The changes of thePWBs’macro-mechanical properties were studied toevaluate thermal expansion impacts associated withchanges in temperature,and the dynamic dislocationmicro-structures were observed to identify the fracturemechanism.The results showed that thermal cracking withsteam at the temperature of 500 K can effectively attenuatethe mechanical properties of waste PWBs,by reducing theimpact,tensile and flexural strengths respectively,by59.2%,49.3%and 51.4%,compared to untreated PWB.Thermal expansion can also facilitate the separation ofcopper from glass fiber by reducing peel resistance by95.4%at 500 K.It was revealed that the flexural fracturewas a transverse cracking caused by concentrated stresswhen the heating temperature was less than 500 K,andshifted to a vertical cracking after exceeding 500 K.