Environ. Sci. Technol. 2002, 36, 1856-1860
Ecologically Benign Polymers: The Case of Maleic Polyelectrolytes GABRIELLE CHARLOTTE CHITANU* AND ADRIAN CARPOV Department of Bioactive and Biocompatible Polymers, Romanian Academy, “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 6600 Iasi, Romania
The paper discusses a series of results that evidence the favorable effect of maleic polyelectrolytes in different fields in which a protective action of the environment is attained: (i) the exploitation of geothermal water, which is a cleaner source of energy; (ii) the improvement or preservation of the structure of agricultural soils; (iii) the substitution of phosphates in detergents; and (iv) the reduction of chromium load of effluents from tanning plants.
Introduction Generally, synthetic polymers, along with inorganic fertilizers, pesticides, and many other synthetic organic chemicals, are considered as pollutants for the environment (1). The numerous advantages of these polymers often change to disadvantages when they are discarded, becoming litter in the environment. An examination of the polymers structure related to their uses could change this approach, evidencing that not all macromolecular compounds are dangerous for the environment. From the viewpoint of solubility and electric charge, synthetic polymers may be classified as the following: neutral, water-insoluble polymersssuch as polystyrene, polyethylene, poly(methyl methacrylate), poly(vinyl chloride); neutral, water-soluble polymersssuch as poly(vinyl alcohol), poly(N-vinylpyrrolidone), polyacrylamide; water-soluble polymers bearing ionized and/or ionizable groups, called polyelectrolytes,ssuch as poly(acrylic acid), maleic acid copolymers, and polyethyleneimine. A survey of the relationship between polymers and environment shows that the polymers of the first class are in most cases pollutants (2). Nevertheless, one should mention that a considerable research effort was dedicated to balancing the benefits brought by polymers and their limitations from the environmental point of view. Study of environmental durability, optimization of the recycling processes to aid waste minimization, and the development of biodegradable polymers are the most important ways to attain this goal (2, 3). The application of many neutral water-soluble polymers and polyelectrolytes (4-6) allows them to be considered, for most part, ecologically favorable. In our paper we will try to support this approach by the presentation of some results regarding the application of maleic acid copolymers (maleic polyelectrolytes) in the fields in which a protective action of the environment is attained. Our research effort was mainly focused on (i) the use of new cleaner sources of energy by using maleic polyelectrolytes (MP) as antiscale agents in the exploitation of geothermal water; (ii) the improvement of soils management and enhancement of the agricultural output using MP based soil conditioners; (iii) the reduction of eutrophication of stagnant water by substitution of * Corresponding author phone: +40 32 217454; fax: +40 32 211299; e-mail:
[email protected]. 1856
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TABLE 1. MPs Used as Crystallization Inhibitors
a
sample code
chemical composition of the copolymer
MW,a g/mol
AV AP AS AM
MAc-VA MAc-NVP MAc-St MAc-MMA
103 000 15 000 145 000 60 000
Molecular weight estimated from viscometric measurements.
phosphates in detergents with MP; and (iv) the avoidance of pollution with chromium ions from tanning plants effluents using MP as additives.
Experimental Section Polymers. The MPs used in our experiments were obtained in the laboratory from maleic anhydride (MAn) copolymers. The copolymers of MAn with vinyl acetate (VA), styrene (St), methyl methacrylate (MMA), or N-vinylpyrrolidone (NVP) were synthesized by radical polymerization in benzene, toluene, or methyl ethyl ketone at 70-90 °C. Under these conditions it is assumed that alternating reproducible distribution of the two monomers occurs along the chain, the copolymer with MMA excepted. MAn copolymers were characterized by conductometric titration and viscometric measurements [see for details refs 7-10]. MPs were obtained from MAn copolymers by mild hydrolysis with water, at room temperature, followed by neutralization with an aqueous solution of NaOH. Methods. The methods used in our investigations will be mentioned separately for each application. More details are given in some previous papers (7-9, 11, 12).
Results and Discussion 1. Use of MP as Antiscale Agents in Geothermal Water Systems. The formation of scale from sparingly soluble salts represents a serious drawback in industrial installations, in which natural waters and/or seawater are used without any pretreatment: secondary oil recovery, geothermal energy production, water cooling in heat exchangers, cooling towers, potable water production with reverse osmosis, etc. (13). The most commonly encountered sparingly soluble salt is calcium carbonate in form of calcite. One of the methods employed for the prevention of calcium carbonate formation is the addition of water soluble polymers or low molecular compounds which suppress scale formation (14-17). The water-soluble polymeric additives give satisfactory results even at very low concentrations (the so-called threshold effect), less than 1/100 of the total calcium concentration present in the treated water. It is considered that such additives may cause modifications of the crystal habit of the calcite particles formed, reducing their ability to adhere on the surfaces, or may decrease the rate of decomposition of Ca(HCO3)2 to CaCO3 (18, 19). In fact, the action of the polymeric additives as scale inhibitors is not still fully understood. The polymeric additives used are, in most cases, anionic polyelectrolytes. The polyelectrolyte properties are governed by electrostatic forces and by weak interactions such as hydrophobic forces. Our objective was to verify whether and how the balance between these two kinds of forces affects their performance as scale inhibitors. To this end several MP with hydrophilic or hydrophobic comonomers have been synthesized (Table 1), and their effectiveness as crystal growth inhibitors using the seeded growth method at constant supersaturation (8) has been evidenced. 10.1021/es010094s CCC: $22.00
2002 American Chemical Society Published on Web 03/01/2002
TABLE 2. Affinity Constants b for the Additive-Calcite Interfacea
FIGURE 1. Dependence of relative inhibition on polymer concentration. Crystal growth experiments, performed over a wide range (0.05-50 ppm) of polymer concentrations (Cp), showed that all MPs decreased significantly the rates of calcium carbonate crystal growth. Inhibition of the crystal growth rates, relative to the uninhibited system, was expressed by the percent relative inhibition defined as
relative inhibition (%) )
R0 - Ri × 100 Ro
(1)
where R0 and Ri are the calcite crystal growth rates measured in the absence and in the presence of the additives, respectively (8). Typical plots showing the dependence of the relative inhibition of calcite growth on the additive concentration are shown in Figure 1. In all cases examined crystal growth rates reduction was proportional to Cp, and a limiting Cp was found in which the crystal growth of the calcite seed crystals was totally suppressed. This Cp limit was dependent on the chemical structure of MP. All polyelectrolytes tested were efficient at substoichiometric proportions with respect to the total calcium concentration in solution, which suggested that inhibition was due to the preferential adsorption of the polyelectrolyte molecules on the active growth sites of calcite crystals rather than to a decrease of supersaturation through sequestration of the calcium ions. The same behavior has been reported for other water soluble polymers (17). Assuming that inhibition was due to adsorption, the kinetic data could be fitted to a model assuming that reduction of the crystal growth rate is proportional to the coverage of the surface area of the seed crystals by the additive molecules adsorbed, as predicted by Langmuir adsorption model (20). According to this model, the intercept is unity. For AM sample and for the other polymers tested in our experiments, the value of the intercept was AS > AP > AM. This order suggests that the inhibition efficiency of a MP is favored by the hydrophilic comonomers (AV > AS) and is higher when molecular weight is higher (AV > AP). It was found that the
additive
b
AV AP AS AM mellitic acid phosphate DHHPA BPDMI ETMPA
1.66 × 1010 6.71 × 108 3.44 × 109 1.74 × 108 2 × 106 b 5.84 × 107 b 1.35 × 107 b 1.59 × 107 b 1 × 107 b
a Abbreviations: DHHPA, 2-dihydroxyphosphonyl-2-hydroxypropionic acid; BPDMI, 1,3-bis[(1-phenyl-1-dihydroxyphosphonyl)methyl]2-imidazolidinone; ETMPA, ethylenediaminetetrabismethylene phosphonic acid. b Data according refs 21-23 and 27 from our ref 15.
TABLE 3. MPs Tested as Soil Conditioners sample code
chemical structure
composition, mol
Ponilit GT-1 AS 1.1 AM 46.1 ATS 27.1 ATM 10.1 ATMS 2.1
MAc-VA MAc-St MAc-MMA MAc-VA-St MAc-VA-MAc MAc-MMA-St
1:1 1:1 1:1.33 1:0.54:0.46 1:0.53:0.43 1:0.32:0.68
inhibition effect of MP is dependent, too, on the nature of the sparingly soluble salt (21). In conclusion, it has been shown that the presence of MP at very low concentrations (below 0.25 ppm) can strongly inhibit the crystal growth of calcite from calcium carbonate supersaturated solutions. Based on this effect, a MP, namely Ponilit GT-1 (22, 23), was developed and used in the geothermal field from northwestern Romania. The ecological gain is the reduction of pollution, since geothermal energy is a cleaner alternative source of energy. 2. Use of MP as Soil Conditioners. The soil structure is an important physical characteristic, which determines its fertility. An inadequate soil structure acts negatively on germination, emergence of seeds and on plants growth, mainly in the first vegetation phases. The treatment of soils with conditioners is a way to impart or preserve their structure. These chemicals are low or high molecular organic or inorganic compounds, inter alia, neutral or charged watersoluble polymers. Based on these data, several systematic trials have been performed in the laboratory and in the field on the performance of MPs in treating some Romanian types of soil, the results being more than encouraging (24). Thereafter, our interest was focused on the possible correlation between the chemical structure of a MP and its performance as a soil conditioner, once assuming that the effect of the polyelectrolyte is due to the interactions with the ions/molecules/ particles of soil. To this end, three binary and three ternary copolymers of maleic acid (MAc) with VA, St, or MMA, the synthesis and characterization of which were described elsewhere (9) have been used (Table 3). The trials were made on a Romanian Cambic Chernozem sample distributed by dry sieving in four fractions (9). To assess the effect of the polyelectrolytes on the structure of the soil samples, three parameters were measured or calculated: (a) water stability of the aggregates larger than 0.25 mm in diameter; (b) dispersion, that is, the amount of small particles with a diameter between 0.01 and 0.02 mm; and (c) structure instability, an index calculated from (a) and (b) according to Henin’s method (11). Figure 2 shows the effect of the MP chemical structure on structural instability. VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. MPs Tested as Phosphate Substitutes sample code
chemical structure
compositiona
MW,b g/mol
AH 3.2 AA 11.2 AP 21.2 AV 335.2 AV 309.2 AV 315.2 AS 1.2 ATS 20.2
Poly(MAc) MAc-AA MAc-NVP MAc-VA MAc-VA MAc-VA MAc-St MAc-VA-St
1:3 1:1 1:1 1:1 1:1 1:1 1:0.43:0.47
21000 140000 93000 9000 180000
a Moles MAc: moles comonomer. b Molecular weight estimated from viscometric measurements.
FIGURE 3. The influence of chemical structure (comonomer) on the MPs calcium binding capacity.
FIGURE 2. The effect of some MPs on the structure of a Romanian Chernozem: dependence of structural instability on polymer concentration Cp. From left to right: Cp ) 0; 2; 3; 4; 5%. The results showed that all MPs decreased the structural instability of soil. It can be noticed the favorable effect induced by the presence of hydrophobic St units in the macromolecular chain, “diluted” with hydrophilic VA units. This fact can suggest that the effect of the polyelectrolyte is due not only to the electrostatic interactions produced by the charged maleic acid units but also to some weak interactions, as van der Waals or hydrophobic forces occurring from the comonomer units, being in agreement with the current theories explaining the mechanism of interaction between synthetic polymers and soil particles (25). In conclusion, MPs have a favorable influence on the preservation or improving the structure of soils, depending on their chemical structure. The soil being an important part of the environment, the ecological gain is obvious. 3. MP as Phosphate Substitutes in Detergents. Detergent formulations are complex products, which contain different types of substances. These ingredients may be grouped into four major categories: surfactants, builders, bleaching agents, and auxiliary substances. The essential builder substances are alkalis (e.g. sodium carbonate, sodium silicate), sequestrants (e.g. sodium triphosphate), and zeolites. The discarded phosphates led to the eutrophication of stagnant water and slowly flowing rivers (26). To control such undesired phenomena, legal regulations to reduce the phosphate content in detergents have been imposed in many countries (27). Suitable products for the substitution of phosphates in detergents are zeolites, nitrilotriacetic acid (NTA), ethylenediaminotetracetic acid, sodium salt (EDTA), citrates, low molecular phosphonates and polymeric acids, called polycarboxylic acids e.g. poly(acrylic acid) (polyAA), AA copolymers, and maleic acid copolymers. The literature mentions numerous results regarding the abilities of polycarboxylic acids as phosphate substitutes and their mechanisms of action (27). 1858
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FIGURE 4. Influence of MW on the MPs calcium binding capacity. In a series of experiments, we examined the influence of the comonomer in several MPs on their ability to substitute phosphates in detergents (12). A sample of polyMAn, several binary copolymers of MAn with VA, St, NVP, acrylic acid (AA), and a ternary copolymer of MAn with VA and St have been synthesized and MP were obtained from them (Table 4). The ability of MP as a phosphate substitute was examined by one of the most convenient tests, namely the Hampshire test, in which a known amount of polyelectrolyte is added to a solution containing sodium carbonate and then titrated with 0.25 M calcium acetate solution until a durable turbidity appears. From the calcium acetate volume the calcium binding capacity is calculated as mg CaCO3/g polyelectrolyte (27, 28). For sake of comparison, NTA and EDTA were also examined. The results are presented in Figures 3 and 4. Figure 3 shows that the comonomer influences the MP capability to bind CaCO3. The hydrophobic comonomer reduces while the presence of a hydrophilic one favors this performance. The presence of a N-containing comonomer enhances such ability, yet samples AA 11.2 and AV 309.2, which contain only C, H, and O, shows also a good capability for carbonate binding. For the same MP, increase of MW reduces CaCO3 binding capacity (Figure 4).
From these data, one may observe that the MPs are able to replace the phosphates in detergents depending on the comonomer and on molecular weight. Our results are in agreement with other reports relating advanced performances of maleic, acrylic, or other anionic polyelectrolytes in this application (27, 29-31). Thus, MPs can be considered ecologically useful polymers. A comprehensive coverage of the environmental behavior of surfactants, zeolites, carboxylic polyelectrolytes, and complexing acids was recently done (32). 4. Tanning of Hides with Basic Chromium Salts Using MP as Additives. The leather industry has encountered new difficulties related to the requirements concerning the discharge of wastewater and the treatment of sludge. The main challenge is to provide high quality products ensuring, at the same time, protection of the environment. Especially the presence of chromium is not allowed by legal regulations (33). Wastewater discarded after tanning of hides have concentrations of 4-10 g Cr2O3/L compared with hides exposed to 1.8-2.2% Cr2O3, the loss being 15-30%. A recent trend to increase the efficiency of the chrome tanning process is the use of compelling agents which change the affinity and induce a better binding of the chromium ions in the hides. Dicarboxylic organic acids, which act as carriers of chrome to the carboxylic and/or amine groups of the collagen from the hide, are used to this end. Some anionic polyelectrolytes based on poly(acrylic acid) (PAA) or MAc copolymers were also developed for use as additives in chrome tanning. It was shown that the quality and the characteristics of the tanned hides depend on the chemical structure of the polymer (34, 35). From the point of view of polymer chemists, the use of MP for binding of chromium ions in the hide is justified by their increased ability of binding multivalent ions and of forming complexes with other polyelectrolytes, peptides, or proteins (4, 5). Consequently, we have tested the sodium salt of a MP (MAc-VA copolymer) with various MWs as additive in the tanning of cattle hides with chromium basic salts. Our preliminary results (some of which are covered by patent applications) showed that these polyelectrolytes reduced the use of chromium basic salts up to 0.5% Cr2O3 compared to the usual value of 2.0% Cr2O3. The chrome content in the waste liquor was also drastically decreased up to 0.2-1.0 g Cr2O3 /L. The leather obtained had physicochemical properties meeting the required quality standard (36-39). The results presented in this paper evidenced the favorable effect of MPs in several applications in which the pollution of the environment is reduced or one of its partsssoilssis protected. The use of MP facilitates the exploitation of geothermal water as a cleaner alternative source of energy, improves or preserves the structure of agricultural soils, and allows diminution of the discarded amount of some ecologically dangerous compounds such as phosphates or chromium ions. It can be mentioned that the polyelectrolytes based on MAc-VA and MAc-St copolymers tested by us were examined for toxicity and were found as belonging to the “fourth toxicity class, i.e., practically nontoxic products” (40). Other polyelectrolytes based on MAc-AA copolymers or PAA were also characterized as essentially nontoxic products (41, 42). Concerning the biodegradability of MP, we have not yet studied this aspect and consequently we cannot state they are biodegradable. Of course, they should be used only in special cases. Another MP, MAc-AA copolymer, was investigated, and it was shown that it is adsorbed on the sludge in wastewater treatment plants and partially biodegraded (41). Naturally occurring or biologically produced polymers, which are hydro-biodegradable, were generally considered environmentally acceptable. More recently synthetic polymers, particularly the polyolefins, that degradesin the
presence of prooxidantssby peroxidation followed by bioassimilation of the oxidation products (oxo-biodegradable polymers) were designated as more environmentally acceptable and called “green polymers” (3). MPs, which are used in low concentrationsin some applications at ppm levelsleading to important beneficial effects on the environment, act also as ecologically relevant polymers. In these cases, the lack of biodegradability could be balanced by the avoidance of other drawbacks. They could be proposed as another class of environmentally acceptable polymers.
Acknowledgments This paper was elaborated with the support offered by the Romanian Agency for Science, Technology and Innovation, Grant no. 6182GR/2000. We are grateful for the cooperation with Prof. P. G. Koutsoukos and co-workers, Patras University, Greece; Dr. S. Chivulete, Research Institute for Soil Science and Agrochemistry, Bucharest, Romania; and Dr. I. D. Costas, Research Institute for Leather and Footwear, Iasi Branch, Romania.
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Received for review April 2, 2001. Revised manuscript received December 28, 2001. Accepted January 16, 2002. ES010094S
