tailieunhanh - Coatings of Polymers and Plastics Part 3

Tham khảo tài liệu 'coatings of polymers and plastics part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Overview of the Automotive Plastics Market 39 Fig. 27 Electrostatic paint transfer efficiency versus substrate conductivity for plaques painted in a test laboratory. Polar materials will have a faster inherent dissipation constant than nonpolar materials because of their higher dielectric constant. Such charge dissipation processes that occur with highly resistive substrates produce characteristically low dissipation currents in comparison to those of a conductive substrate. Figure 27 plots the transfer efficiency versus part conductivity for plaques of polymers having various conductivities. For these experiments transfer efficiencies were evaluated by paint thickness theoretical yield is calculated based on percent solids in the paint and time of plaque exposure to the paint which is sprayed at a particular delivery rate . A plateau of transfer efficiency versus substrate conductivity occurs in the region of approximately 10-5 S cm which shows that paint transfer efficiency equivalent to that of metal can be observed at levels of polymer conductivity significantly below metallic 35 . Practical aspects to grounding a conductively modified part in a painting shop are of equal importance as the theory. Figure 28 illustrates that a charge will typically travel a shorter distance if it moves toward the grounding clip through the bulk of a semiconductor instead of along the surface. However it is not the physical distance but instead the total resistance of the path that is the determining feature for current distribution. The resistance of any given path is the average resistivity of the material traversed multiplied by the distance traveled. If substrate bulk conductivity is equal to or greater than that of the Copyright Marcel Dekker Inc. All rights reserved. Marcel Dekker Inc. J 270 Madison Avenue New York New York 10016 40 Babinec and Cornell Fig. 28 Charge dissipation currents divide across all available paths using relative power IR drop losses just as they .