Most finishing consultants have lost count of the number of times that inadequate cleaning and pretreatment was the cause of defective painting or plating. Skip plating, blistering, delamination—these are just some of the commonly found defects caused by poor cleaning.
So, without question, good cleaning is essential in almost all branches of surface finishing. Though it is money well-spent, good cleaning is not without its own cost. Over-cleaning is a waste of time, money, and machinery, in some cases. This was a point made by R.N. Miller at Lockheed Aircraft1, who used cleanliness test methods to evaluate cleaning routines prior to painting aircraft fuselages and showed that, beyond a certain point, no benefit resulted.
A great deal has been written on cleanliness testing, and $10,000 will buy a state-of-art goniometer or other sophisticated test instrumentation. At Sam Houston State University, in collaboration with our colleagues in industry, we are running an on-going program to develop more affordable cleanliness test methods. We are by no means the first with this aim. Back in the 1950s, the AESF sponsored H.B. Linford at Columbia University with this task. His research was published in Plating magazine at that time2.
Half a century later, what has changed? Not a lot in terms of the physicochemical principles that underlie cleaning, but in terms of low-cost hardware and software tools, one could say that everything has changed. Today digital photography, spreadsheets and measurement software are available and portable using the miracle that is today’s camera-equipped, network-connected cell phone. Our group has recently demonstrated the utility of cellphones in making contact angle measurements using the Bikerman method.3
Back in 1941, J.J. Bikerman4 developed a new approach to contact-angle measurement. A droplet of known volume (v, typically 5-10 µL) is placed on a horizontal surface and viewed from above. The diameter (d) of the base of the drop is measured using a calibrated microscope or similar imaging device. The ratio d3/v is a function of the contact angle as reported by Bikerman (Eq. 1).