Chapter 15
Laser Surface Profilometry: A Novel Technique for the Examination of Polymer Surfaces 1
S. A. Fairbrass and D. R. Williams
Downloaded by COLUMBIA UNIV on August 26, 2012 | http://pubs.acs.org Publication Date: December 28, 2000 | doi: 10.1021/bk-2001-0779.ch015
Particle Technology, Department of Chemical Engineering, Imperial College, London SW7, United Kingdom
Laser Surface Profilometry is a non-destructive technique that requires little or no sample preparation and takes place in ambient laboratory conditions. Samples of poly(vinyl chloride) were artificially aged by exposing diem to high levels of ultra violet light A near infrared laser was used to produce topographical images of the changes in surface morphology, both on ageing and after simple conservation cleaning techniques. Cumulative percentage graphs of R data were shown to be a sensitive indication of any changes, which had taken place. The percentage of reflected laser light was also collected and maps of surface chemical changes were produced. This technique has proved to be a valuable tool for the examination of museum objects. a
There are many, specific problems that arise out of the need to examine museum objects. It is often the case that die object itself is unique or that very little of die original remains intact In this case, die opportunity to take samplesfromthe actual artifact or to use any technique that is destructive may be forbidden. This prohibition includes such ubiquitous examination methods such as scanning electron microscopy where the sample may have to be gold or carbon coated and submitted to a high vacuum. The use of a laser, which provides an image of the surface without the need for sample preparation, has provided fresh scope for the close examination of Corresponding author. 202
© 2001 American Chemical Society
In Historic Textiles, Papers, and Polymers in Museums; Cardamone, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
203
surfaces. In addition to Ms, because die equipment allows for an item to be treated in situ, repositioning is no longer a problem and it is now possible to examine and evaluate the results of conservation treatments.
Downloaded by COLUMBIA UNIV on August 26, 2012 | http://pubs.acs.org Publication Date: December 28, 2000 | doi: 10.1021/bk-2001-0779.ch015
Experimental Samples of plasticised polyvinyl chloride) were exposed to high levels of light in order to age the surfaces artificially. Plasticised PVC was chosen for this study as it represents on of the most commonly used polymers and is also known to change and deteriorate noticeably over time. [1,2] Light ageing took place in London, where a bank of lights consisting of a mixture offlorescenttubes and ultra-violet lights were used, and also in Los Angeles where the samples were simply left outdoors in bright sunlight, high humidity and significant atmospheric pollution. The Blue Wool Scale, a simple light dosimeter produced by the textile industry to monitor dye fading [3] was used to give some indication of the light exposure in both locations. This scale ranges from 1 to 8, with wool sample #1 being the most light fugitive and sample #8 being the least. The PVC was exposed to enough natural or artificial light to fade Blue Wool Sample #4, 5 or 6. After aging, the PVC samples were imaged using Laser Surface Profilometry. The top, exposed surfaces of the aged samples were also cleaned by using either a 2% non-ionic surfactant solution or de-ionized water. Surface images were made of the samples before and after cleaning.
The Profilometer
The profilometer consisted of an infrared laser (780nm/0.2 microwatts) and three micro stages, x,y andz, that could be controlled to within an accuracy of Ο.ίμιη. The auto focusing laser was attached to the z-axis and focused onto the top surface of a sample of PVC. The laser worked within three ranges, ± 400μπι which could be used to produce an image of the surface offineglass paper, ± 40μηι which would be the range for writing paper and ± 40pm which is the range for most synthetic polymeric materials. It has a best resolution of 5nm in the z-axis over a range of ± 4μπι. The stand off distance of the laserfromthe sample was 10mm. The samples were held on a flat platform, normal to the laser light, and capable of being moved in the χ and y directions. The stages were moved so that the laser light,
