Wypych Handbook of Solvents

tured with solvents for shoes, food packaging, and textile and plastic film lamination. Current technologies use TPU in the form of a hot melt, as a reactive PUR and as a thermoplastic laminating film.3 Reactive hot-melts were first introduced in the early 1980s and since than have grown very rapidly. After application the adhesive is cured by moisture.14 These adhesives are already in use by the automotive industry (bonding carpet to door panels, tray assembly, lenses to headlamp housing, and lamination of foam to fabric) and in furniture and building products (moldings, picture frames, decorative foil, edgebanding), in bookbinding, and in the footwear industry. Polyurethane water dispersions are expected to grow 8-10%/year from the current 5,000-6,000 tones/year market in Europe.14 Applications are similar to those of hot melts.

UV-curable pressure-sensitive adhesives are the most recent application of the advancing radiation curing technology.16 Low viscosity formulations allow the use of standard application techniques with several advantages such as improved production rate, energy efficiency, improved properties of the final products, and new potential applications for pressure-sensitive additives in thicker films with mechanical performance. It is expected that radiation cured materials will expand at a rate of 10%/year.17 Adhesives constitute 16% by value and 13% by volume of radiation cured products (two major applications for radiation cured materials are coatings and inks). Henkel introduced a series of water-based laminating adhesives.18 Hot melt systems, high-solids solvent systems with a 3 times higher solids content, and water based adhesives have been introduced to textile lamination to replace traditional low-solids solvent-based adhesives.19

Odor elimination is the additional benefit which has helped to drive the replacement of solvent-based systems.20 In packaging materials, most odors are related to the solvents used in inks, coatings and adhesives. Also, coalescing solvents from water-based systems caused odors. Elimination of solvent is a priority but solvent replacement may also change the response to the odor because solvents such as toluene and xylene smell like lubricating oils or turpentine whereas isopropanol smells more like a disinfectant. Odors stem not only from solvents but also from products of the thermal and UV degradation of other components and solvents.20

In view of the above efforts, it is surprising that the majority of recent patents on adhesives are for solvent-based systems.21-26 The new inventions include a universal primer,21 an adhesive composition in which solvents have been selected based on Snyder’s polarity (only solvents which belong to group III are useful in adhesive for automotive applications to avoid a deleterious effect on paint),22 a low VOC adhesive for pipes and fittings,23 a sol- vent-containing heat-resistant adhesive based on siloxane polyimide,24 a water-based polyimide adhesive,25 and two-component solvent-free polyurethane adhesive system for use in automotive door paneling.26

REFERENCES

1R Vabrik, G Lepenye, I Tury, I Rusznak, A Vig, International Polym. Sci. Technol., 25, No.3, T/1-9 (1998).

2D Raftery, M R Smyth, R G Leonard, International J. Adhesion Adhesives, 17, No.4, 349-52 (1997).

3J B Samms, L Johnson, J. Adhesive Sealant Coun., Spring 1998. Conference proceedings, Adhesive & Sealant Council, Orlando, Fl., 22nd-25th March 1998, p. 87.

4 A Stevenson, D Del Vechio, N Heiburg, Simpson R, Eur. Rubber J., 181, 1, 2829, (1999). 5 K Menzefricke, Adhesive Technol., 15, No.1, 6-7 (1998).

6L White, Eur. Rubber J., 179, No.4, 24-5 (1997).

7J Baker, Eur. Chem. News, 69, No.1830, 20-2 (1998).

Table 14.2.1. Total releases of solvents by the aerospace industry. [Data from ref. 1]

ture difference), UV radiation (substantially higher during flight), mechanical abrasion due to the high speed of travel, exposure to salt in the atmosphere, exposure to higher level of acids and sulfur dioxide, and exposure to de-icing fluids during winter.2,3 These unusual conditions should be considered in conjunction with the mechanical movement of the coating caused by rapid changes in temperature and the flexing of aircraft elements because of changes in pressure and severe load variations on wings.2 In addition, because of their size, aircrafts must often be painted at low temperatures which requires a coating that will cure at these temperatures without leaving entrapped volatiles. These could evaporate in the low pressure conditions at high altitude and cause the formation of voids where corrosion could start. These factors make the design of an effective coating system a severe technological challenge.

Coatings are used by the aerospace industry both for OEM and maintenance purposes. In each case surface cleaning and preparation is required. A paint stripping operation is added to the task in maintenance repainting. Coatings are applied by spraying, brushing, rolling, flow coating, and dipping. Depending on the method of application, the rheological properties of coatings must be adjusted with solvents and, in some cases, with water. An alternative method of viscosity adjustment involves heating the coatings to lower its viscosity by increasing its temperature. This reduces solvent usage. Solvents are also used for equipment cleaning.

In addition to paints, sealants are also used. Sealants are mostly based on polysulfides, containing solvents as discussed in the previous section. Also, non-structural adhesives containing solvents are used as gaskets around windows and for carpeting.

Paint removal is accomplished by either chemical or blast depainting. Dichloromethane is the most common solvent used for this application. Aerospace industry estimates that 15,000 to 30,000 different materials are used for manufacturing some of which are potentially toxic, volatile, flammable, and contain chlorofluorocarbons. Some of these substances may result in air emissions, waste-waters, and solid waste.

854 George Wypych

Table 14.2.2. Total transfers of solvents by the aerospace industry. [Data from ref. 1]

Air emissions result from sealing, painting, depainting, bonding, as well as from leakage in storage, mixing, drying, and cleaning. The most common solvents involved in coatings are trichloroethylene, 1,1,1-trichloroethane, toluene, xylene, methyl ethyl ketone, and methyl isobutyl ketone. Wastewater is generated through contamination by paints and solvents used for cleaning operations. Solid waste containing solvents comes from paint overspray intercepted by emission control devices, depainting, cleanup, and disposal of unused paint. Solvents used for cleaning are usually a mixture of dimethyl-benzene, acetone, 4-methyl-2-pentanone, butyl ester of acetic acid, naphtha, ethyl benzene, 2-butanone, toluene and 1-butanol. Some solvents used for painting and cleaning are either recycled are burned to recover energy.

In 1996, 199 aerospace facilities (out of 1885 analyzed in the report) released and transferred off-site or discharged to sewers about 12,000 kg of 65 toxic chemicals (solvents in these releases are reported in Table 14.2.1 and transfers in Table 14.2.2).

Methyl ethyl ketone, 1,1,1-trichloroethane, trichloroethylene, and toluene accounted for 66% of all releases. 70% of all transfers was for recycling purposes. The aerospace industry released 10,804 tons of VOC in 1997 which constituted 0.61% of the total releases from 29 industries which were analyzed. Thirteen other industries release more VOC than the aerospace industry.

Recycling and disposal of solvents in the aerospace industry equals the purchase cost of the solvents. Therefore reduction of solvent use is very cost effective. Some chemical stripping operations are now being replaced by cryogenic stripping with liquid nitrogen. Also, supercritical carbon dioxide has been used in Hughes Aircraft Company in some cleaning applications. Solvent emissions can be reduced through control of evaporation (lids, chillers), by dedicating process equipment (reduces cleaning frequency), production scheduling, immediate cleaning of equipment, better operating procedures, reuse of solvent waste, and use of optimized equipment for paint application. There are plans to evaluate powder coatings and water-based paints.1,2 There are trials to use water as the paint thinner and to lower viscosity of paints by application of resins which have lower viscosity.2 Work is under the way to replace dichloromethane/phenol stripper with benzyl alcohol.2 The introduction of an intermediate layer between the primer and the top coat has been proposed.

This will aid the stripping action of the proposed stripping solvent, benzyl alcohol. VOC have already been reduced in several components: bonding primer (from 1030 to 850 g/l), undercoats (from 670 to 350), top coats (from 700-900 to 250-800), clear coats (from 700-800 to 250-520), surface cleaners (from 850 to 250) as well as other materials.3

REFERENCES

1Profile of the Aerospace Industry. EPA Office of Compliance Sector Notebook Project. US Environmental Protection Agency. November 1998.

2 R W Blackford, Surface Coatings International, 80, No.12, 564-7 (1997).

3R Blackford, Polym. Paint Colour J., 186, No.4377, 22-4 (1996).

14.3ASPHALT COMPOUNDING

George Wypych

ChemTec Laboratories, Inc., Toronto, Canada

Numerous construction products are formulated from asphalt and coal tar for such applications as driveway sealers, cutback asphalts, flashing cements, concrete primers, concrete cold mixes, roof cements, expansion joint fillers, patch liquids, waterproofing liquid-applied membranes, and pipeline coatings. All these products are likely to contain solvents.

The simplest formulations are mixtures of asphalt and (usually) mineral spirits used for sealing , priming, and coating of concrete. These are usually very low performance products which are used in large quantities because of their low price. They release about 40% of their weight to atmosphere during and after application. Since they do not perform well they have to be re-applied at frequent intervals. Driveway sealer is an example of a product which is used every spring, in spite of the fact that, in addition to the pollution it causes, it also produces a gradual degradation and cracking of the driveway. The only solution for elimination of this unnecessary pollution seems to be banning the product by regulation. Some of these products can be replaced by asphalt emulsions which contain water in place of organic solvents.

Several products are used for patching and joint filling purposes. These materials (flashing cement, roof cement, patch liquid, and expansion joint filler) also use solvents to regulate viscosity. The solvents are usually mineral spirits, fuel oil, or polycyclic aromatic hydrocarbons. In addition to the base components, inexpensive fillers such as calcium carbonate or limestone but also still asbestos are added. These products harden on evaporation of the solvent and fill the joints, adhere to surfaces, and provide some waterproofing. These are again, low technology materials, traditionally used because of their very low cost. Most of these products can be replaced by modern sealants which will result in higher initial cost but longer service.

The most technologically advanced products are used for waterproofing and pipeline coatings. These products are also based on dispersion of asphalt in the above mentioned solvents but reinforced with addition of polymer. The addition of polymer modifies the plastic behavior of asphalt and renders it elastomeric. Additional solvents are usually added to improve the solubility of polymeric components. Reactive polyurethanes are the most frequently used modifiers for waterproofing liquid membranes. Toluene and xylene are the

most frequently used additional solvents. These materials partially solidify because of evaporation of the solvent. Their elastomeric properties are derived from chain extension and crosslinking reactions which form an internal polymeric network which reinforces asphalt.

There is no data available on the solvent emissions from these materials but their scale of production suggests that their emissions are probably comparable with the entire rubber industry. This is one industry which should be closely monitored not only because of the emission of the above listed solvents but because some of the low grade solvents used contain large quantities of benzene and hexane. It is also cause for concern that asphalt and tar have carcinogenic components.

Recent inventions1-5 are driven by product improvement needs and environmental aspects of application of these products. Janoski’s patent1 describes a product which is an anhydrous blend of polymer and asphalt and is substantially solvent-free. This technology shows that it is possible for an ingenious designer to produce low viscosity materials without using solvents but by selecting the appropriate type and concentration of bituminous materials, polyurethane components, and plasticizers.

In another invention,2 a modifier is introduced to increase the adhesion of asphalt/water emulsions to aggregates. Emulsified asphalt is not so deleterious to the environment but its performance suffers from aggregate delamination. In yet another recent invention,3 terpene solvent, which is a naturally occurring (but never in this high concentration), biodegradable material, was used to replace the mineral spirits, xylene, trichloroethane, toluene, or methyl ethyl ketone normally used in cutback formulations (cutback asphalt is a dispersion of asphalt in a suitable solvent to reduce viscosity and allow for cold application). The two other patents4,5 discuss inventions leading to an improvement of high and low temperature properties of asphalt with no special impact on reduction of solvents used.

REFERENCES

1R J Janoski, US Patent 5,319,008, Tremco, Inc., 1994.

2P Schilling, E Crews, US Patent 5,772,749, Westaco Corporation, 1998.

3 R W Paradise, US Patent 5,362,316, Imperbel America Corporation, 1994. 4 M P Doyle, J L Stevens, US Patent 5,496,400, Vinzoyl Petroleum Co., 1996.

5M P Doyle, US Patent 5,749,953, Vinzoyl Technical Services, LLC, 1998.

14.4BIOTECHNOLOGY

14.4.1 ORGANIC SOLVENTS IN MICROBIAL PRODUCTION PROCESSES

Michiaki Matsumoto, Sonja Isken, Jan A. M. de Bont

Division of Industrial Microbiology Department of Food Technology and Nutritional Sciences Wageningen University, Wageningen, The Netherlands

14.4.1.1 Introduction

Solvents are not dominating compounds in the biosphere of our planet. Under natural conditions, their presence in appreciable amounts is restricted to specific areas. Only a very limited number of solvents is of biological origin and some may reach higher concentrations in nature. The best known example is ethanol. However, also butanol and acetone can be

formed readily by microbes and locally high concentrations may occur. In fact, in the beginning of the 20th century, very large production facilities were in operation for the microbial production of butanol and acetone. Furthermore, terpenes are natural solvents that are produced mainly by plants and locally they can reach high concentrations. For instance, limonene is present in tiny droplets in the peel of oranges. All these solvents are toxic to microbial cells. Some others as higher hydrocarbons that are present for instance in olive oil, are not toxic to microbes as will be discussed later.

With the advent of the chemical industry, this picture has changed dramatically. In polluted locations, microorganisms may be confronted with a large number of solvents at high concentrations. With a few exceptions only, it has turned out that microbes can be found that are able to degrade these compounds if their concentration is low. This degradative potential is not unexpected in view of trace amounts that may be present locally in the natural biosphere. But the exposure of cells to unnatural high concentrations of these solvents usually leads to irreversible inactivation and finally to their death.

The chemical industry is largely based on solvent-based processes. But in biotechnological processes, the microbes usually are exploited in a water-based system. This approach is quite understandable in view of the preference of microbes for water and the problems solvents pose to whole cells. Solvents often are used to extract products from the aqueous phase but only after the production process has been completed. At this stage, damage to whole cells is obviously no longer relevant. In both, chemical industry and biotechnology, organic solvents have many advantages over water because of the nature of either product or substrate. Consequently, during the last decades many possibilities have been investigated to use solvents in biocatalytic processes.1,2 The more simple the biocatalytic system, the less complex it is to use solvents.

Free or immobilized enzymes have been exploited already in a number of systems. Here, biocatalysis may take place in reversed micelles or in an aqueous phase in contact with an organic solvent.3 In a powdered state some enzymes are able to function in pure organic solvents.4 Furthermore, modified enzymes such as polymer bound enzymes5 or surfactant-coated enzymes6 have been developed so that they can solubilize in organic solvents to overcome diffusion limitation. The advantages of enzymatic reactions using organic solvents can be briefly summarized as follows:1,3,4

1)hydrophobic substances can be used;

2)synthetic reactions can take place;

3)substrate or production inhibition can be diminished and

4)bioproducts and biocatalysts can easily be recovered from the systems containing

organic solvents.

Although organic solvents have often been used in enzymatic reactions, the application of organic solvents for biotransformation with whole-cell systems is still limited. Cells might be continuously in direct contact with the organic phase in a two-phase water-solvent system during the whole production cycle (Figure 14.4.1.1).7 Alternatively, cells may remain separated from the bulk organic phase by using membrane bioreactors (Figure14.4.1.2).8 In these instances, cells encounter phase toxicity9 or molecular toxicity, respectively.

Because whole bacterial cells are more complex than enzymes, they pose by far greater problems in operating bioproduction processes when organic solvents are present. The most critical problem is the inherent toxicity of solvents to living organisms.1,2,10,11 As