Surface Modification of Engineering Plastics Through Swelling In
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#2008 The Society of Polymer Science, Japan Surface Modification of Engineering Plastics through Swelling in Supercritical Carbon Dioxide By Toshimi TAKAJO,1;2 Atsushi TAKAHARA,1;Ã and Takefumi KICHIKAWA2 In order to improve the tribological characteristics of polymer materials, the authors tried to impregnate the surface of engineering plastics with the lubricating oil by using supercritical carbon dioxide. The oil impregnation ratio on crystalline polymer was influenced by both the glass transition temperature (Tg) and the degree of crystallinity. In case of using the polymer possessing lower Tg with large difference between the Tg and treatment temperature, the higher impregnation ratio was obtained, and the crystallite in crystalline polymer prevented the impregnation of lubricating oil. The results of polarized optical microscopic observations and FT-IR spectroscopy studies indicated that the lubricating oil was preferentially impregnated to the amorphous region in crystalline polymer, and the high-concentration layer of lubricating oil having the thickness of ca. 30 mm was formed at the vicinity of surface area. The result obtained in this study, which reports the preferential lubricating oil impregnation to the surface of crystalline polymer under supercritical carbon dioxide, suggests that the tribological characteristics of crystalline polymer would be improved by applying this oil impregnation method without sacrificing the bulk mechanical strength. KEY WORDS: Supercritical Carbon Dioxide / Oil Impregnation / Engineering Plastic / Tribology / Crystallinity / Many studies carried out by utilizing supercritical fluid have lubricating oil at high temperature over long periods of time to been recently reported, and the representative examples of the impregnate the product with lubricating oil. According to such studies are as follows: researches for the polymerization the method, however, it is unavoidable that the deterioration of reaction in supercritical fluid, the supercritical extraction- polymer is accelerated. Thus there was not optimum method separation technique, the supercritical fluid cleaning technol- available to impregnate polymer material with lubrication oil ogy for precision parts, the coating technique using super- so far. critical fluid on the surface of fine powder and on the inner In this study, the oil impregnation experiments for repre- wall surface of an optical cable, and the supercritical dyeing sentative crystalline and non-crystalline polymers using super- technique. critical fluid were carried out for the purpose of developing a Now the substances of water, carbon dioxide and ethanol in new technique to impregnate engineering plastics with lubri- the supercritical states are already industrially used. In the case cating oil. The influence of condition on the oil impregnation of carbon dioxide, many studies1–7 for its application have been ratio and the oil dispersion state were investigated, further- reported because carbon dioxide has the moderate critical more, the oil impregnation mechanism under the existence of conditions (the critical temperature = 304.2 K, the critical supercritical carbon dioxide was also discussed. pressure = 7.37 MPa) and is non-flammable, non-toxic and relatively inexpensive. EXPERIMENTAL Polymer materials are widely used as an engineering material for parts used in various fields. For rolling bearings, Polymers and Lubricating Oils which are important mechanical elements, the cages manufac- In order to discuss the relationship between physicochemical tured from polymer materials are used to arrange rolling properties of polymers and the oil impregnation behavior, elements such as balls equally and to prevent the metal-to- various crystalline and non-crystalline polymers, which are metal contact. The cages are usually made from engineering shown in Table I9–13 and II, were used in this study. For plastics, and the resistance against lubricating oil is required as crystalline polymers and non-crystalline polycarbonate (PC), the fundamental property of material used for the bearing cage. extrusion-molded pieces manufactured by Nippon Polypenco Recently, it has become more important to reduce the Co., Ltd. were used. For amorphous polymethylmethacrylate friction between the rolling elements and the cage due to the (PMMA), commercially produced plates were used. The pieces demand for higher fuel efficiency of automobiles. Application machined into the shapes of circular discs with outside of oil-impregnated polymer to the cage material is one of the diameters of 35 mm and thicknesses of 3 mm were used as most appropriate ways for improving its frictional property.8 specimens. As lubricating oils, hydrocarbon synthetic oil, ester- As traditional oil impregnation method for an oil-resistant based synthetic oil and ether synthetic oil, which are shown in polymer, there is a way of processing a polymer product in Table III,14 were used in this study. 1Institute for Materials Chemistry and Engineering, Kyusyu University, 744 Motooka, Nishi-ku Fukuoka 819-0395, Japan 2Materials Technology Development Department, NSK Ltd, 1-5-50, Kugenuma Shinmei, Fujisawa 251-8501, Japan ÃTo whom correspondence should be addressed (Tel: +81-92-802-2517, Fax: +81-92-802-2518, E-mail: [email protected]). 716 Polymer Journal, Vol. 40, No. 8, pp. 716–724, 2008 doi:10.1295/polymj.PJ2007212 Surface Modification of Engineering Plastics through Swelling in Supercritical Carbon Dioxide Table I. Chemical structures and thermal characteristics of crystalline polymers ÁH0/ Polymer material Abbreviation Structural formula T /K Tm/K f g kJ molÀ1 Ultra high molecular UHMWPE 153 403 8.22 weight polyethylene CH2CH2 n Polyoxymethylene POM 217 448 9.79 CH2O n Polyvinylidene 441 to PVDF 238 9.12 fluoride CH2CF2 n 453 488 to Polyamide 6 PA6 NH(CH ) CO 323 25.6 2 5 n 498 HH OO 526 to Polyamide 6,6 PA6,6 339 67.8 536 N-(CH2)6 -N-C-(CH2)4 -C n Polyphenylene PPS -S 363 558 15.8 n Sulfide O Polyetheretherketone PEEK 416 607 — O- -O- -C - n Table II. Chemical structures and glass transition temperatures Equipment for Supercritical Carbon Dioxide Treatment of non-crystalline polymers and Treatment Conditions Polymer The equipment for the oil impregnation experiments in Abbreviation Structural formula Tg/K material supercritical carbon dioxide is shown in Figure 1. The equip- CH3 ment used in this study is manufactured by Taiatsu Techno. Polymethyl Co., Ltd. The maximum allowable pressure is 29.5 MPa, the PMMA (CH -C) 378 methacrylate 2 n maximum allowable temperature is 523 K, and the capacity of our high-pressure chamber is 1:0 Â 10À3 m3. The amount O=C -OCH - 3 of lubricating oil for our oil impregnation experiments is À5 3 CH3 2:5 Â 10 m . On the basis of assumed solubility of organic 413 substance in supercritical carbon dioxide under our exper- Polycarbonate PC O- -COC - - - to 422 imental condition,15,16 it was considered that this amount n À5 3 CH3 O of 2:5 Â 10 m to the capacity of our chamber is sufficient for the purpose of maintaining a saturated concentration of lubricating oil. Table III. Chemical structures, solubility parameters and molecular weights of lubricating oils Solubility Abbre Molecular Lubricating oil Structural formula parameter, , viation weight (J/cm3)1=2 Polyalpha olefin CH2-CH PAO n 15.9 1386 (Exxon-Mobile) n=33 CH3 Alkyl diphenyl ether (Matumura Oil research ADE -O- -(C13H27)n n=1 2 16.2 443 Corp) Dioctyl sebacate CH3-(CH2)3-CH-CH2-O-C-(CH2)8-C-O-CH2-CH-(CH2)3-CH3 (Reagent, Wako Pure DOS 17.3 426.8 C H OOCH Chemical Industries Ltd.) 2 5 2 5 OCH3 O Polyolester POE CH -C=C-(CH ) -C-O-CH -C-CH -O-C-(CH ) -C=C-CH 17.9 864 (NOF Corporation) 3 2 13 2 2 2 13 3 CH2-O-C(O)-(CH2)13-C=C-CH3 Dioctyl adipate CH3-(CH2)3-CH-CH2-O-C-(CH2)4-C-O-CH2-CH-(CH2)3-CH3 (Daihachi Chemical DOA 18.3 370 C H OO CH Industry Co., Ltd.) 2 5 2 5 Polymer Journal, Vol. 40, No. 8, pp. 716–724, 2008 #2008 The Society of Polymer Science, Japan 717 T. TAKAJO,A.TAKAHARA, and T. KICHIKAWA Generally, for supercritical carbon dioxide treatment, if φ120 Liquid CO2 Test piece canister pressure is rapidly reduced, microbubbles are formed in the 86 Pressure gauge sensor polymer.17–20 In order to prevent the formations of such Lubricating oil Thermocouple microbubbles, the specimen is usually taken out after the Valve completion of the gradual cooling and pressure reduction. In Vessel Back-pressure valve Valve this study, nevertheless, in order to elucidate the influence of cooling and pressure reduction process on the oil impregnation ratio, the oil impregnation experiment in which the rapid cooling and pressure reduction process was adopted, was also Buffer (for oil separation) Thermocouple carried out. Figure 1. Schematic illustration of supercritical carbon dioxide treatment Determination of Degree of Crystallinity in Crystalline equipment. Polymer In order to determine the degree of crystallinity in crystalline polymer, the samples (20 mg each) were taken from The treatment procedure is as follows: First the weight of the center and the edge of the specimen, and differential well-dried specimen is measured and then the specimen is scanning calorimetry (DSC) measurement was conducted on placed with lubricating oil into the high-pressure chamber. The each sample by using a Seiko Instruments Inc. DSC6220 under supercritical carbon dioxide treatment is carried out after the nitrogen. The sweep rate of temperature was 10 K/min, and the completion of substitution of carbon dioxide for air in the temperature range was 303 K through 573 K. The degree of chamber, and the treatment is conducted at a specific temper- crystallinity is determined by quantifying the heat associated ature, time and pressure. After the specific time, the temper- with melting of the polymer. That is, the degree of crystallinity ature is lowered to 303 K and, the pressure is gradually is obtained by normalizing the observed apparent heat of fusion reduced.