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SURFACE MODIFICATION OF INNER WALLS OF POLYETHYLENE TUBING GENERATED BY DIELECTRIC BARRIER DISCHARGE PLASMAS
thesisposted on 02.08.2021, 17:25 by Lee E OrganskiLee E Organski
Plasma treatment of polymers has been a rapidly growing area of research due to its broad applications, homogenous and repeatable surface properties, low cost, and environmental friendliness when compared to alternative techniques. Only recently have significant developments been made in the application of atmospheric pressure plasma in polymer surface treatment. The use of atmospheric pressure plasma enables further reductions in cost and mechanical complexity. Of particular interest in this work is the application of atmospheric pressure plasma for the isolated modification of the inner surfaces of small diameter polymer tubing to improve the wetting and adhesion characteristics compared to untreated polymer.
This work focuses on the development, characterization, and implementation of an atmospheric pressure dielectric barrier discharge (DBD) plasma apparatus for the treatment of the inner surface of polymer tubes. The iterative process of the development of this system is detailed, with two finalized designs established and defined. These two designs are then applied to low density polyethylene (LDPE) tubing of 0.38 mm inner diameter (ID), and characteristics for surface morphology and wettability are analyzed.
Investigation of the relationship between plasma power and treatment time with morphology characteristics of protrusion density and size and surface roughness parameter, R_a is presented. Treatment times of 5, 10, 15, 30 and 45 minutes are performed on tubing samples at a power level of 35 mW. From 5 to 15 minutes, protusion density increases rapidly, from n_p=4*10^4- 10^7 protrusions/(mm^2 ), and small variation in protrusion size, with 0.1< A_p<0.2 μm^2. At treatment times of 30 and 45 minutes, coalescence of protrusions was observed, resulting in a decrease in protrusion density, down to n≈4*10^4 protrusions/(mm^2 ), and substantial increases in mean protrusion size, up to A_P=5-9 μm^2. Plasma powers of 9, 12, 16, 25, 35, and 45 mW were also investigated, at a treatment time of 15 minutes. As power level was increased, protrusion density was observed to increase, with an inverse relationship with mean protrusion size. Protrusion density was observed to increase from n_p=2*10^5-10^7 protrusions/(mm^2 ), with diminishing increases in density observed between power levels of 35 and 45 mW. Protrusion mean size was observed to decrease from A_p= .25-0.025 μm^2, with similar diminishing reductions observed at 35 and 45 mW. Surface roughness, R_a, was observed to vary from .01-0.3 μm, or ISO roughness grades N1 to N5, in the treated samples.
Wettability characteristics were measured and characterized relative to plasma power and linear feed rate. Wettability was measured by measurement of contact angles of the meniscus formed from water introduced into the tubing volume by capillary action. On all samples treated, a duality of mechanisms for surface wetting were observed. After initial treatment, samples were observed to have a lower contact angle, indicating higher wettability, but after 12 hours samples were observed to have reduced wetting characteristics, indicating a transient mechanism for surface wetting in addition to permanent effects induced my surface morphology. Samples were treated at plasma powers of 7, 10, 15, 20, and 40 mW. At all power levels, initial contact angle was generally consistent, with 20^o< θ_0<30^o. Permanent wetting features measured on these samples indicated almost complete reversing of wettability at 7 and 10 mW, with θ_SS measured at ~75^o, comparable to the average measurement of an untreated sample of ~80^o. Conversely, at higher powers of 15, 20 and 40 mW, significant retention of wettability was observed, with 45^o<θ_SS<55^o for those samples.