Friction of plastic webs

This paper is about the measurement of the friction of plastic and similar flexible webs in contact with cylindrical surfaces. There are numerous ways for measuring friction coefficients of materials in relative contact, but to be meaningful, the testing a technique should simulate the tribosystem of interest. The subject of this paper is the capstan friction test, which simulates the tribosystem of a flexible material with some angle of wrap around a cylinder. It is the purpose of this paper to propose the use of the ASTM G-143 capstan friction test standard, which is the preferred technique to study web transport systems that involve a web in contact with rollers. This tribosystem is used to transport plastic sheets, rubbers, paper, fabrics, wrought metals and many industrial products. If the roller/product friction is too low, there may be transport tracking problems. If the friction is too high, it may lead to breaking of the web or other problems. Transport friction needs to be controlled. This paper presents the equations governing the test, describes the ASTM test procedure, and presents data on the effect of variables on the test. The repeatability of the test was determined to be acceptable in interlaboratory tests; the coefficient of variation was less than 10%. The effects of temperature, humidity, velocity, surface cleanliness and web tension depended on the particular tribosystem. The friction of plastic webs is affected by most of these factors when sliding against themselves or metal rollers. When the tensions in this test are high, the web surface may be damaged. If this happens, the measured friction coefficient becomes quite low (approaching 0.1); the test is no longer measuring friction of surface a sliding on surface b. It is measuring the deformation of surface a by surface b. As with all tribotests, care must be used in interpreting results, but it has been determined that this test correlates with manufacturing web/roller transport systems and its use is recommended for these kinds of application. (C) 2001 Elsevier Science Ltd. All rights reserved.