5950
Ind. Eng. Chem. Res. 2003, 42, 5950-5953
Development of Polyurethane Antimicrobial Composites Using Waste Oil Refinery Catalyst Luciana R. M. Esteva˜ o,† Leda C. S. Mendonc¸ a-Hagler,‡ and Regina S. V. Nascimento*,† Instituto de Quı´mica - DQO, Universidade Federal do Rio de Janeiro, CT Bl A 6 andar, Cidade Universita´ ria, Ilha do Funda˜ o, Rio de Janeiro CEP 21941-590, R.J. Brazil, and Instituto de Microbiologia, Universidade Federal do Rio de Janeiro, CCS Bl I, Cidade Universita´ ria, Rio de Janeiro CEP 21941-590, R.J. Brazil
Waste oil refinery catalyst from the FCC unit was tested as a possible additive for the production of polymeric composites with antimicrobial properties. The effect of the silver ion on the growth inhibition of bacteria (E. coli and S. aureus), mold (A. niger), and yeasts (C. tropicalis and Cr. humicolus) was studied. The catalyst was ion exchanged with silver nitrate and incorporated into thermoplastic polyurethanes with different hardness ratings. The materials produced were submitted to microbial testing both in solid and liquid media, and the effect of the polymeric matrix on the rate of silver ion release from these materials was also investigated. The results obtained show that polyurethane composite containing the silver-ion-exchanged waste catalyst successfully inhibited the growth of all microorganisms under study. Introduction The development of materials that provide high and long-lasting antimicrobial effects, while maintaining low human toxicity, is a present-day challenge. Certain metal ions exert microbial activity by a number of methods, such as binding themselves to a variety of organic ligands causing denaturation of proteins, including enzymes; disrupting cell membranes; and decomposing essential metabolites.1 Silver is probably the most powerful antimicrobial metal ion having a remarkably low human toxicity compared to other heavy metal ions, and it has been used in biomedical materials, such as bone cement and artificial skin, and in the prevention of nosocomial infections related to the use of medical devices (urinary catheters, CV catheters).2 Concurrently, the decreasing of waste material discharge has been encouraged because of economic and environmental concerns. Regarding waste material from oil refineries, the fluid catalytic cracking (FCC) unit alone is responsible for the worldwide annual discharge of 300 000 tons of zeolite-based catalyst.3 The ionexchange properties of zeolites have been thoroughly investigated over the years, and a number of patents and journal articles have proposed the use of Ag+exchanged zeolites for microbial control purposes, ranging from food packaging and mouthrinse to cardboard boxes and mechanical keys.4-10 Within an expanding polymer market, attention can be drawn to polyurethanes, which have found use in furniture, automobiles, construction materials, coatings, adhesives, and medical applications, among others.11 Thermoplastic polyurethanes (TPUs) especially have been used in medicine, as their high rubberlike elasticity and ductility and high mechanical and chemical resistance are desirable properties for many applications. The mechanical characteristics of TPUs are primarily * To whom correspondence should be addressed. E-mail:
[email protected]. † Instituto de Quı´mica - DQO. ‡ Instituto de Microbiologia.
controlled by their chemical compositions given that they are dependent on the ratio between soft and hard segments. The physical cross-links formed by hydrogen bonding in the urethane groups form the hard segments, whereas the intervening regions, consisting of the polyol chain, constitute the soft segments.12 Considering that the properties of TPUs can be varied over a wide range, it is hereby proposed that they might also play a key role on the rate of ion release from within the polymeric matrix. The aim of the present study was then to undertake the quest for developing antimicrobial polymeric composite, making use of silver-ion-exchanged waste FCC catalyst, while studying the effect of the polymeric matrix on the rate of silver ion release. Experimental Procedure Because of its relatively large particles (40-140 µm), the catalyst was ball milled and wet sifted, the -635 Tyler mesh fraction (MEC) being collected and submitted to further testing. The physical properties and chemical composition of the milled and sifted waste catalyst were compared with those of the catalyst as received to detect whether any differential loosening of the catalyst’s components occurred during the process. The chemical composition of the waste FCC catalyst, as received (EC) and after milling and sifting (MEC), was determined by X-ray fluorescence analysis using a Philips PW 1480 X-ray spectrophotometer. The zeolite content of both samples was determined by X-ray diffraction, in a Philips PW 1710 instrument with a copper anode, taking into consideration absorption variations due to the presence of rare earth elements. Textural analysis was carried out using a Micromeritics ASAP 2400 accelerated surface area and porosimetry analyzer, through which the specific surface area (SABET) was obtained. The micropore area and volume (MiPA and MiPV, respectively) were calculated by the t-plot method and Harkins and Jura equation, and the mesopore area (MSA) was measured by the BJH technique.13,14 For the ion-exchange procedure, 120 g of MEC was added to 600 mL of an aqueous 1.18 M AgNO3 (Merck)
10.1021/ie0206960 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5951
solution and refluxed for 2 h under vigorous stirring. The material was then vacuum filtered and washed with 5 L of distilled water at 70 °C to ensure the removal of all nonexchanged silver nitrate. The silver-exchanged waste catalyst (MEC-Ag) was dried overnight at 90 °C. The silver content of the obtained material was determined by atomic absorption in a Perkin-Elmer 330 atomic absorption spectrometer. The samples were prepared by adding 3 mL of water and 3 mL of HF to 1 g of MEC-Ag in a platinum crucible and heating to dryness under magnetic stirring. This procedure was repeated to ensure complete silicon removal in the form of fluorides from the original sample. The residue was dissolved in 3 mL of water, 3 mL of HF, and 4 mL of HNO3, to avoid reduction of the silver ion. The solution was then diluted with distilled water to attain the optimum working range of 1-5 µg/mL. The effect of the silver ion on microbial growth was studied using two bacteria, Escherichia coli (a Gram-negative rod) and Staphylococcus aureus (a Gram-positive coccus); two yeasts, Candida tropicalis (an ascomycetous yeast) and Cryptococcus humicolus (a basidiomycetous yeast); and one mold, Aspergillus niger (a filamentous fungus). The Minimum Inhibitory Concentration (MIC) of the silver ion was determined by colorimetric measurements in a Klett-Summerson instrument. The microorganisms were inoculated at 37 °C for 48 h, the bacteria in nutrient broth and the mold and yeasts in liquid Sabouraud media, after which they were diluted in phosphate buffer solution to achieve concentrations of approximately 105 bacteria or mold cells/mL and 104 yeasts cells/mL. A volume of 0.1 mL of each the aforementioned solutions was added to 5 mL of liquid growth media (Sabouraud media for the fungi and Muller Hinton media for bacteria) containing different AgNO3 concentrations. These systems were incubated in darkness at 37 °C and 120 rpm for 72 h, after which turbidity measurements were carried out on the systems containing bacteria and yeasts. A. niger grows as a cohesive layer on the surface of the growth medium and could not be dispersed in solution even by action of a vortex mixer, thereby not giving accurate turbidity results. Hence, the results for mold growth inhibition could be obtained only by visual observation of culture growth. Qualitative microbial growth tests were carried out in solid media to observe a zone of inhibition around pressed waste catalyst disks. These disks were placed on the bottom of an empty Petri dish, and the agar growth media (Sabouraud media and Muller Hinton media) were poured, while still hot, over the disks. After cooling and solidification, 0.1 mL of the inoculum was spread over the surface, and the system was incubated at 37 °C for 48 h. Medical-grade thermoplastic polyurethanes (TPUs) with a wide range of durometer ratings, produced through the reaction of methylene diisocyanate with polyols and commercialized under the tradename Tecothane (Thermedics Inc.), were studied. The polymers’ physical data supplied by the manufacturer are presented in Table 1. The ratio between hard and soft segments was calculated from the results obtained by CHN analysis in a Thermo Finnigan 1112 Series CHN analyzer. The TPUs were pulverized using a Pulverisette 14 cryogenic grinder, operating at 10 000-12 000 rpm with liquid nitrogen. MEC-Ag was initially mixed with the
Table 1. Physical Properties of the TPUs Studied
sample identification
commercial name
durometer rating (shore hardness)
TPU-74A TPU-95A TPU-65D TPU-75D
TT-1074A TT-1095A TT-1065D TT-1075D-M
75A 94A 64D 75D
flexural modulus (psi)
ultimate elongation (%)
1300 8000 26 000 180 000
550 400 300 150
Table 2. Physical Properties of the Waste Catalyst, Both as Received (EC) and after Milling and Sifting (MEC) physical property
units
median particle diameter
EC
MEC
75.53
8.78
m2/g m2/g
210.93 217.42
224.58 231.02
cm3/g m2/g m2/g
0.073 157.19 53.73
0.077 166.19 58.38
µm
Textural Properties BET analysis multipoint surface area single-point surface area t-plot micropore analysis micropore volume micropore area external surface area
milled TPUs, at room temperature, to account for 20 wt % of inorganic material in the mixture, and then heat pressed in a Carver Press, at 200 °C, applying a 15 000 lbf load on a 12 × 12 cm area, to obtain sheets 0.8 mm thick. These sheets were once again cryogenically ground and submitted to further testing. The effect of the chemical composition of the TPUs on the rate of silver ion release was studied by adding varying amounts of the ground TPU composites to 5 mL of distilled water inside a test tube. The test tubes were closed and placed in a shaker at 37 °C at 120 rpm. After 24 h, the solutions were filtered and had their silver ion concentration determined by atomic absorption, in a Perkin-Elmer 330 atomic absorption spectrometer. The aforementioned conditions were selected to resemble those used for microbial growth. The TPU/MEC-Ag system that gave the greatest Ag+ release rate was then submitted to microbial testing. Test tubes containing 0.3 g of TPU/mL of liquid medium were inoculated and incubated using the same procedure as used for MIC determination. After 72 h, the polymer was removed from the media, and each microorganism suspension was diluted in the same manner as described before. From these diluted solutions, 0.1 mL was withdrawn and inoculated in 5 mL of liquid medium. The tubes were once again incubated at 37 °C and 120 rpm. After 48 h, turbidity measurements were carried out in a Klett-Summerson colorimeter to observe bacteria and yeast culture growth. The mold’s culture growth and inhibition were observed visually. Results and Discussion The physical properties of the exhaust catalyst or equilibrium catalyst (EC) as received and those of the -635 Tyler mesh fraction after ball milling and sifting (MEC) are presented in Table 2. Although the average particle diameter, obtained from the median of the cumulative mass plot, reveals a significant decrease due to milling and sieving, the relative surface area was only slightly altered by the process, indicating that the micropore area makes a dominant contribution. The chemical composition and zeolite content of both catalysts are shown in Table 3. An increase in the zeolite content was observed for MEC when compared to the catalyst as received from the refinery. However, this does not indicate that zeolite loosening was favored during milling, given that in a complementary analysis,
5952 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 Table 3. Chemical Composition Determined by X-ray Fluorescence and Zeolite Content Determined by X-ray Diffraction of the Catalysts component
units
EC
MEC
aluminum oxide silicon oxide nickel iron(III) oxide vanadium titanium(IV) oxide antimony lanthanum(III) oxide cerium(III) oxide praseodymium(III) oxide neodymium(III) oxide sodium oxide magnesium oxide barium oxide copper tin(IV) oxide phosphorus(V) oxide bismuth(III) oxide molar SARa zeolite content
% % mg/kg % mg/kg % mg/kg % % % % % % % mg/kg mg/kg % mg/kg
31.9 66.2
