Polymer surfaces, frictional characteristics

Mor Morgan, S. E., Misra, R., Jones, P. Nanomechanical and surface frictional characteristics of a copolymer based on benzoyl-1,4-phenylene and 1,3-phenylene. Polymer 47 (2006) 2865-2873. 07Ehr Ehrenstein, G. W., Pongratz, S. Bestandigkeit von Kunststoffen. Carl Hanser Verlag, Munich, 2007. 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

For example a polymer s interfacial characteristics determine chemical and physical properties such as permeability, wettability, adhesion, friction, wear and biocompatibility. " However polymers frequently lack the optimum surface properties for these applications. Consequently surface modification techniques have become increasingly desirable in technological applications of polymers. - ... 

If solid polymer objects are fluorinated or polymer particles much larger than 100 mesh are used, only surface conversion to fluorocarbon results. Penetration of fluorine and conversion of the hydrocarbon to fluorocarbon to depths of at least 0.1 mm is a result routinely obtained and this assures nearly complete conversion of finely powdered polymers. These fluorocarbon coatings appear to have a number of potentially useful applications ranging from increasing the thermal stability of the surface and increasing the resistance of polymer surfaces to solvents and corrosive chemicals, to improving friction and wear properties of polymer surfaces. It is also possible to fluorinate polymers and polymer surfaces partially to produce a number of unusual surface effects. The fluorination process can be used for the fluorination of natural rubber and other elastomeric surfaces to improve frictional characteristics and increase resistance to chemical attack. 

Organic photoreceptors can be prepared in either a flexible web or drum format. Webs are usually prepared on polymer substrates, polyethylene tere-phthalate being the most common. The substrates are between 100 to 200 pm in thickness and coated with a conducting surface layer. The substrates often contain layers on the reverse side for reduced curl, static discharge prevention, and control of frictional characteristics. The web configuration is also widely used for laboratory studies. For drums, the substrate is a metal cylinder, usually Al. Recently, however, drums of a poly(phenylene sulfide) resin doped with conductive C black have been developed (Kawata and Hikima, 1996). Drums are widely used in low- and mid-volume applications. Drums, however, are not well suited for research purposes. Thus, the preparation and characterization of drum photoreceptors is usually related to a specific application. 

Coating. PET and PEN are fairly inert polymers and for many applications the surface of the film is altered by coating or adhesive lamination to other materials, eg film for packaging will be lacquered to accept inks or adhesives, or, for photographic applications, primed to accept photosensitive overcoats. Coatings are also applied to achieve other surface effects, such as antistatic, barrier (water, oxygen, carbon dioxide, flavor), release, and frictional characteristics (8,9). 

Any of these components of tear or fatigue can play a main role or a secondary one, in function of the polymer concrete structure and the ingredients it contains, the medium nature, mechanical regime, the characteristics of the surfaces in contact with the polymer during friction. 

Velocity / Frequency Relationship. The characteristic temperature-dependent line shapes for each polymer, as determined by bulk dynamic mechanical or dielectric experiments from literature (4), were used as a comparison to our fnctional measurements. For the comparison to be valid the literature data must be adjusted to match the same frequency (time-scale) of the friction experiment. The conversion procedure of scan velocity to frequency has been described in previously (7), whereby a contact diameter was calculated to convert the scan velocity to a frequency by simple division. The contact diameter thus allows a gauge for the time the probe tip affects a point on the polymer surface. For the given radius of curvature of 20 nm, applied load=10 nN, adhesive load=15 nN, and assuming a bulk storage modulus, the contact diameter can be estimated by JKR theory to be 18.7 nm. Thus for a scan velocity of 40 pm/sec the equiv ent frequency of measurement is 2000 Hz and all tabulated tanS data used are scaled accordingly to this frequency for comparison. 

PTFE is a white solid with a waxy appearance. It is very tough and flexible, with a good electrical insulation properties. The surface energy and coefficient of friction are both very low, the latter being the lowest of any solid. The combination of low surface energy and low coefficient of friction cause PTFE to have excellent non-stick characteristics, a feature which underlies many of the everyday uses of this polymer. 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

Surface treatments involving alkali metals are sometimes used to eliminate the characteristic surface properties and promote the adhesion between polytetrafluoroethylene and other substances (Doban Nelson, Kilduff, and Benderly Purvis and Beck Rappaport). It has been shown that these treatments produce a marked increase in the polarity of the surface as measured by the contact angle with various liquids (Allan, 1957). They also increase the coefficient of friction. One interesting application of surface properties of polytetrafluoroethylene was reported by Bowden (1953, 1955) who applied the polymer to the bottoms of his skiis and thereby reduced the friction between the skiis and the snow. 

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