Paints, Coatings and Solvents Episode 9 pptx

12.1. Clean Air Measures 269 the shape of the part being coated). Higher application efficiencies are achieved with airless and electrostatic spraying methods, and may reach 90 YO in the case of electro- static spraying. The overspray of coating powders can be largely recovered and reused directly. Problems arise with changes in color which can, however, largely be solved. The highest application efficiencies for wet paints (nearly 100 wtY0) are obtained with brushing, rolling, and pouring methods, which, however, can only be used on flat surfaces. Parts with hidden areas can be effectively coated by dipping methods. Waste Air Treatment. Two main methods are used for treating waste air in coating and paint shops : afterburning with heat recovery and adsorption with solvent recov- ery. In order to reduce expenditure on waste air treatment, the amount of waste gas should be minimized by enclosing the paint application area and recycling circulat- ing air. Water washers (scrubbers), fabric filters, and electrical separators are used to remove paint aerosols from the atmosphere. Recycling of circulating air is already current practice in modern automated paint shops (e.g., in the automobile industry). If the workforce is protected against relatively high solvent concentrations by res- pirators equipped with a fresh air supply, manual spraying zones can also be oper- ated with recycled air. If such a concentration procedure is not possible, large-vol- ume waste air streams with a low solvent content can be concentrated with a continuously operating adsorption wheel (e.g., with special activated carbon) and then treated [12.4]. The solvent from the waste air is adsorbed on one side of the rotating wheel and desorbed with a small air stream on the other side. The concen- trated waste air (10% of the original waste air stream) can then be purified either by an adsorption unit with solvent recovery, or by afterburning. The heat from the afterburning plant should be utilized; if this is not possible, newly developed thermal methods with internal heat utilization may be used [12.5]. Biological waste gas treatment methods are also suitable for purifying solvent- containing waste gases, especially slightly contaminated, large-volume waste gas streams; they have already been tested [12.6]. In biological methods organic sub- stances are degraded by microorganisms on the surface of a wet filter layer or in a scrubber. In various countries (e.g., the Federal Republic of Germany, the United States, Scandinavia, and Switzerland) regulations exist concerning the treatment of waste air from paint shops. Large paint shops (e.g in the automobile industry) are covered by these regulations. In the Federal Republic of Germany the maximum permissible emissions from automobile paint shops are limited by the amount of solvent used per square meter of car body [12.7] (see p. 266). For automated spraying zones in other paint shops the emission of organic substances in the waste gas is restricted to a maximum of 150 mg/m3. 12.2. Wastewater In industrial paint application the principal sources of wastewater are spraying cabins, wet filters, and scrubbers. Further sources of wastewater are the cleaning of apparatus, equipment, vessels, tanks, and working areas, as well as the retentate produced in the ultrafiltration of electrodeposition paints. To reduce environmental pollution, attempts should be made to minimize the amount of wastewater. Waste- water from spraying cabins may be treated by coagulating the overspray and contin- uously extracting the paint slurry, as well as by reducing the amount of water used. Continuous methods for cleaning the circulating water are used both for solvent- borne and waterborne paints. Products based on alumina, metal hydroxides, and organic fatty acid derivatives are used as coagulating agents. These auxiliaries coat and envelop the paint particles. With waterborne paints, stable dispersions or emulsions are sometimes formed in the circulating water. The coagulating agent also has to "break" these disperse systems. A very fine flocculant coagulate is often formed which has to be separated with special filters or a centrifuge, or converted into larger, more easily removable flakes by using a further coagulating agent. If the circulating water from the spray cabin has to be drained off due to high levels of contamination, it must be treated before being discharged into the sewage system or wastewater treatment plant. Treatment usually comprises flocculation, neutralization, and filtration. With cer- tain water-soluble toxic substances (eg , heavy-metal compounds), organic solvents, and additives, further purification steps may be necessary. Heavy metals can be precipitated. Methods used for organic solvents depend on their nature and concen- tration in the water; they include ultrafiltration, reverse osmosis, adsorption (e.g on activated charcoal), biological purification, and, with high solvent concentrations, distillation [12.8]. The concentration of organic substances in the wastewater is described by the chemical oxygen demand (COD) and the biological oxygen demand within 5 days (BOD,). Statutory requirements governing the preliminary purifica- tion of the wastewater can vary widely. They depend on purification facilities in the existing wastewater treatment plants. The effort and expense involved in wastewater pretreatment can be considerably reduced by avoiding the use of toxic substances (e.g., heavy metals). With waterborne paints particular attention should be paid to adequate removal of water-soluble organic substances (e.g., solvents) from the wastewater. 12.3. Solid Residues atid "osw 271 12.3. Solid Residues and Waste Considerable amounts of solid residues and other waste are produced when paints are applied, particularly by spraying. The overspray is collected as a coagulated residue from the spray cabin water. Articles such as contaminated filters, paint residues, and empty containers also have to be disposed of. For ecological reasons minimization of waste production and reutilization should take precedence over disposal methods (incineration, landfill). In the Federal Republic of Germany, for example, this principle is laid down in waste control and emission legislation (Kreis- laufwirtschafts- und Abfallgesetz, Bundes-Immissionsschutzgesetz). Up to now paint slurries were mainly disposed of in special landfills. On account of increasingly stringent requirements to prevent pollution of soil and groundwater, paint slurries will have to be disposed of in special refuse incinerators. This will inevitably lead to higher disposal costs; avoidance of waste and recycling will therefore be of economic advantage. The production of paint residues and waste can be prevented or reduced by using coating methods with high application efficiencies (dipping, brushing, rolling, and pouring). Compared with the spraying technique, these methods generally also result in lower solvent emissions (see p. 266). The amount of overspray produced in spray- ing methods can be lowered by using electrostatic application procedures. In powder coating the overspray is trapped and separated in a dust-removal filter; it can then be directly recycled, if necessary after purification. In the spray application of wet paints the overspray can also be recovered and recycled by various methods which have not, however, all been tested industrially [12.9]. The overspray can be recovered with rotating disks or circulating belts. Stable paints can be reused directly after conditioning (e.g., viscosity adjustment). In sol- ventborne paints the disk or belt often has to be wetted with solvents, the waste air should therefore be treated to prevent high solvent emission. Some waterborne paints can also be recovered by this method. In paint recycling the overspray should not be entrained with the waste air from the spray cabin; certain preconditions should therefore be observed: airless spraying guns are most suitable and the parts to be coated should not be too large or have a complex three-dimensional structure (e.g., car bodies). The overspray can be recovered from the cabin circulation water if it can be coagulated without destroying its chemical structure. After mechanical dewatering with kneaders or mixers, purification, and work-up, small amounts of this material can be added to the new paint. A new develepment is the recycling of waterborne paints through ultrafiltration [12.10]. Modern paints based on water- borne binders fulfill the demands of ultrafiltration and common quality require- ments so that direct addition to the new paint is possible. If direct recycling is not possible, the material can be separated by centrifugation into a binder solution and pigment concentrate that can be used as raw materials for paint production. Physi- cally drying paints and stoving finishes are particularly suitable for recovery. Paint coagulates can also be used as a binder constituent in the production of molded plastics and as a filler replacement in plastics dispersions. If used as a binder for molded plastics, the material must not be cross-linked; the coagulate is worked 272 12. Erivirotir?ier~ral Protection arid Toxicology up into an aqueous dispersion which is used to wet or impregnate fiber mats that are then compressed. In order to produce fillers the paint coagulate first has to be dehydrated and dried, the material becomes completely cross-linked and can be ground into a powder. The powder is used as a filler for plastics dispersions (e.g., for underbody protection in automobiles and in sealing materials). 12.4. Toxicology Many different substances are used in paints and coating materials as binders, pigments, solvents, and additives. Workers involved in painting and coating work are regularly exposed to volatile organic compounds, especially solvents. In spraying application methods the inhalation of all paint constituents in the form of aerosols should be borne in mind even if they are nonvolatile or of low volatility. Contact with the skin represents a further source of exposure to paint constituents, many of which can be absorbed through the skin. With manual application by brushing or rolling the health hazards due to solvent exposure (aliphatic and aromatic hydrocarbons, esters, ketones, alcohols, and glycol ethers) are a major factor. Solvents are predominantly absorbed via the respiratory tract. Their toxic effects depend on the nature of the solvent, its concentration, and the length of exposure. Depending on the concentration, symptoms after acute exposure include irradiation of the mucous membranes (eyes and respiratory tract), vertigo, nausea, and vomiting; narcosis symptoms are also observed which are attributed to disturbances of the central nervous system. Chronic poisoning is initial- ly undetectable, but may subsequently produce damage to organs specific for the solvent concerned. The neurotoxic effects found in painters and coaters exposed to solvents are the subject of controversy. Some studies describe subjective symptoms such as fatigue, difficulty in concentrating, and short-term memory problems in workers employed in industrial paint and coatings application. These symptoms have not, however, been observed in painters employed in the architectural and exterior-use paints sectors who mainly use waterborne paints [12.11]. During surface treatment prior to paint application, abrasive dust and pyrolysis products produced during the removal of paints and solvents may also be inhaled. The dust produced from corrosion protection agents and some older colored paints is often contaminated with heavy metals. Chlorinated hydrocarbons are still used in paint strippers. Frequent skin contact with paints and coating materials can cause skin disorders, particularly on the hands, in painters and coaters. The lipid-solubilizing properties of the organic solvents may cause or at least promote contact eczema. In particular, paints based on reactive resins (e.g., epoxy and polyester resins) may cause allergic skin disorders. Skin-sensitizing substances include residual monomers and reactive diluents (e.g., acrylates and epoxides) and paint additives (e.g., acid anhydrides, 12.4. Toxicologj. 273 peroxides, amines, as well as cobalt and zirconium in driers, and formaldehyde and isothiazolinone in biocides). Some of these paint constituents are also skin irritants. In spraying methods often employed for industrial paint application, workers are not only exposed to solvents, they may also inhale paint constituents in the form of aerosols. On account of their very small size, some aerosol components can reach and penetrate the lung virtually unhindered. Substances that have a particularly sensitizing and irritant action on the skin can thus also affect the respiratory tract. Isocyanates can have a sensitizing effect even at very low concentrations (1 pL/m3) and can cause chronic bronchial asthma in particularly susceptible persons. A liter- ature study carried out by a working group of the International Agency for Research on Cancer came to the conclusion that there is sufficient evidence for carcinogenicity due to the occupational exposure of painters [12.3 21. Occupational exposure in paint manufacture cannot be assessed however. Depending on the application method (brushing, spraying) and the paint used, technical and personal work safety measures should be adopted when applying paints and coatings. Technical measures include adequate supply of fresh air and removal of waste air (e.g., in special hoods), as well as the replacement of “haz- ardous” paints with less dangerous ones [12.13]. Many hazardous dangerous sub- stances have maximum workplace concentrations (threshold limit values) which should be strictly observed and monitored. Adequate facial and skin protection must also be ensured (e.g with masks and gloves). 13. Economic Aspects L13.11, L13.21 Paints and varnishes (coatings) have two primary functions: protection and deco- ration. Other objectives include information, identification, safety, insulation, vapor barrier, nonskid surface, and control of temperature, light, and dust. A range of product categories with a wide variety of application is therefore available: 1) Architectural (decorative) coatings include exterior and interior house paints which are normally distributed through wholesale-retail channels and purchased by the general public, painters, building contractors, government agencies, etc. 2) Product,finishes are coatings formulated specifically for original equipment man- ufacture (OEM) to satisfy application conditions and manufacturing require- ments for a wide variety of industrial and consumer products, e.g., wood and metal furniture and fixtures; automotive and nonautomotive transportation, aircraft, machinery and equipment, appliances, electrical insulation, film, paper, foil. toys, and sports goods. 3) Special-purpose coatings are formulated for special applications or extreme envi- ronments and include automotive and machinery refinishing, high-performance maintenance, road markings, marine (bridge) maintenance, crafts, metallic and multicolored coatings. The number of coatings producers worldwide was estimated at about 7500 in 1997. The total world coatings market was estimated to be ca. $55-60 x 10'; the product market sectors were as follows: Decorativt 50 Yn Auto OEM 6 % Refinishes 5 '/n Can coatings 3 Yo Coil coatings 2 % Others including industrial paints Total production 32 Yn 23 x 106 t World paint markets by region were in 1994 North America Western Europe Eastern Europe Japan Rest of Asia-Pacific South America Rest of world Total production 29.4% 25.1 Yn 12.9 Yn 7.2% 15.2 Yn 5.1 % 3.9 Yo 21 x loh t Paints, Coatings and Solvents Second, Completely Revised Edition Dieter Stoye, Werner Freitag copyright 0 WILEY-VCH Verlae CirnhH. IYYX 276 13. Economic, Aspec1.r In 1996 the top ten paint companies accounted for about 60% of the total world market, by the year 2000 they could well account for more. This development is expected because of permanent streamlining of activities by larger companies through selective acquisitions and/or divestments. Single sourcing as in the car industry (one supplier for one model), marine sector (direct availability at each ship yard), or canning industry (worldwide health and safety standards) is a key factor in this globalization. Recouping in international markets for expenditure in research and development for technically sophisticated, high added value products is the other reason for this evolution. Internationalization and the generally high standard of technical products shifts the economic importance from countries to companies. Therefore simple national per capita consumption figures are no longer indicative of productivity and standard of living. This is independent of some standard parameters such as climatic, cultural, or other impacts. The paint and coatings industry as a whole is considered as a mature industry. An overall growth of 2.5-3.0% is estimated for the 1990s assuming overall growth of the corresponding gross national product of + 3.5%. This is remarkable in view of the fact that improved techniques such as high-solids coatings and coating powders, reduction in overspray, recovery, and recycling have considerably increased the surface area covered by a given amount of paint. Raw materials account for roughly half of the production costs; prices of many of them are linked directly or indirectly to the price of crude oil. In the decorative market products are mainly waterborne and consumption is dominated by new construction work and maintenance. Higher growth rates are therefore expected in newly industrialized and developing economies. In the automo- tive sector paint supply (not necessarily production) follows the requirements of car producers. Major growth potential for packaging (food and drink cans) lies in developing countries. Industrial paint markets are characterized by replacement of solventborne paints by high-solids. waterborne, and powder coatings to reduce environmental pollution. Therefore, coil coating can also avoid the classical painting process. Solvents 14.1. Definitions Solverits are compounds that are generally liquid at room temperature and atmo- spheric pressure; they are able to dissolve other substances without chemically changing them. The liquid mixture formed on dissolving a substance (solute) in a solvent is termed a solurion. The molecules of the solution components interact with one another. Solutions are obtained by mixing liquid, solid, or gaseous components with liquids, the liquid always being termed the solvent. When two liquid compo- nents are combined, it is arbitrary which of the two components is considered to be the solvent, and which the solute; the liquid component present in excess is usually termed the solvent. Accordingly, plasticizers that are used for flexibilization in plastics processing and paint production may also be regarded as solvents. Plasticiz- ers differ from solvents, however, with regard to their technological significance. A good plasticizer should have a very low volatility and thus permanently affect the dissolved substance. An ideal solvent should, in contrast, have a high volatility, so that it can evaporate as rapidly as possible to leave the dissolved substance (e.g., in a paint film). The boundary between plasticizers and solvents is not clear cut-some high-boiling solvents of very low volatility exert a flexibilizing effect over a pro- longed period. A solvent should generally have the following properties [14.1]; 1) Clear and colorless 2) Volatile without leaving a residue 3) Good long-term resistance to chemicals 4) Neutral reaction 5) Slight or pleasant smell 6) Anhydrous 7) Constant physical properties according to the manufacturers’ specification 8) Low toxicity 9) Biologically degradable 10) As inexpensive as possible Inorganic substances (e.g., hydrogen sulfide, ammonia, sulfur dioxide, hydrogen fluoride, and hydrogen cyanide) that are used as solvents in special applications, mainly at low temperature or under pressure, will not be discussed here. Paints, Coatings and Solvents Second, Completely Revised Edition Dieter Stoye, Werner Freitag copyright 0 WILEY-VCH Verlae CirnhH. IYYX 14.2. Physicochemical Principles 14.2.1. Theory of Solutions During dissolution the solvent acts on the substance to be dissolved to increase its state of distribution. Dissolution results in the formation of real solutions, colloidal solutions, or dispersions depending on the size of the particles that interact with the solvent molecules. In real solutioris the diameter of the dissolved particles is ca. 0.1 nm, and is thus of the order of magnitude of the free molecules. Real solutions are formed by most inorganic and organic compounds of low molecular mass. They are clear, physically homogeneous liquids. In colloidal solutions the diameter of the dissolved particles is ca. 10-100 nm. Colloidal solutions are generally clear to weakly opalescent liquids, but exhibit inhomogeneities as regards some physical properties (e.g., the Tyndall effect). In dispersions the diameter of the particles is larger than in colloidal solutions. Dispersions are turbid to milky liquids consisting of at least two phases. Intermolecular Forces. During the dissolution of a substance (A) in a solvent (B) the forces of attraction between the molecules of the pure components (KA-A and K, ,) are destroyed, and new forces are simultaneously formed between the solvent and substance molecules: KA-A + K6-6 - 2K, , A substance is generally readily soluble in a solvent if the forces of attraction in the pure substance are of the same order of magnitude as the forces of attraction in the pure solvent. A substance is generally insoluble in a solvent if the forces of attraction between its molecules are significantly higher or lower than in the pure solvent. In this case more energy is required to overcome the forces of attraction in the pure components than is released on formation of the solution. This is the explanation of the rule of thumb “Like dissolves like” (sinzilia sinzilihus solvuntur). The intermolecular forces of attraction differ-they are strongest in crystalline solids, weaker in amorphous solids and liquids, and weakest in gases. Intermolecular forces are classified according to their physical nature (Table 3) [14.2]-[14.5]. Ionic (Coulomb) Forces. Forces of attraction between ions of opposite charge are termed ionic or Coulomb forces. The force with which two ions 1 and 2 attract one another depends on their electrical charges el and e, and the distance I’ between them : el . e, r2 K,, z - ~ Table 1. Intermolecular forces Type of force Interaction between Temperature dependence Ionic ions weak Ion-dipole ions and dipoles weak Directional permanent dipoles strong Induction permanent and induced dipoles weak Dispersion atomic dipoles weak Hydrogen bonds groups of molecules strong Coulomb forces are responsible for the stability of ionic crystals (e.g., NaCI). When such a compound is dissolved in a polar solvent (dipole moment p), dissocia- tion and simultaneous solvation of the ions occur. The force of attraction between the ions is now inversely proportional to the dielectric constant of the solvent, and is thus reduced. New ion-dipole forces are formed as a result of the attraction of the permanent dipoles of the solvent by the ions: The distance between the solvated ions in the solution generally changes only slightly with temperature and depends on the thermal expansion coefficient of the solution. The forces between the ions are therefore only slightly temperature depen- dent. Dipole-Dipole Forces. Dipole-dipole (directional) forces are forces of attraction between molecules with a finite, permanent overall dipole moment. The forces of attraction resulting from the dissolution of a polar molecule (p,) in a polar solvent (p2) are given by [14.6]: The distance between the dipoles depends largely on the position of the poles in the molecule (i.e., on steric molecular influences) and on thermal vibrational move- ments. The force of attraction between the dipoles accordingly decreases sharply with increasing temperature. Induction forces are produced as a result of interactions between permanent dipoles and induced dipoles. The electric field of a molecular dipole leads to charge displacement in the neighboring molecule and thus to the induction of a dipole. The magnitude of the induced dipole moment pind depends on the magnitude of the permanent dipole moment p and on the polarizability c1 of the second molecule [14.7]. Induction forces are only slightly temperature dependent: [...]... bond forces and dipole forces have little influence because both the binder and the solvent molecules are proton acceptors, and few hydrogen bonds are formed Furthermore, the dipole moments of the solvents are roughly equal In other cases, however, hydrogen bonding and dipole moments can greatly influence the degree of solvation 14.2.5 Solvents, Latent Solvents, and Non -Solvents True (active) solvents. .. alcohol Butanol Isobutanol 1.3 290 1.36 19 1.38 59 1.3772 1. 399 4 1. 396 0 ~~ Cyclohexanol Benzyl alcohol Methylbenzyl alcohol 1.4667 1S 390 1.5270 Toluene p-X ylene Tetrahydronaphthalene 1. 495 5 1. 495 6 1.5443 Ethyl glycol Ethylene glycol Diethylene glycol 1.4075 1.4310 1.4460 Dichloromethane 1.4234 The refractive index decreases with increasing temperature (Fig 7) 14.3.4 Viscosity and Surface Tension The viscosities... coefficient, K-l x 10-3 1. 29 Electrical conductivity, S/cm 2.01 2.3 1.0 2.5 22.4 18.2 4.5 9. 0 18.4 13.1 9. 2 11.0 4.34 7.6 0 .9 1.3 1.08 1.1 0. 79 1.21 1.12 1.27 1.15 0 .92 0.85 Dielectric constant (20 'C) 1 .9 x lo-'' 10-5 4.3 x l o - " 5.6 x 10-4 1 4 lo-' ~ 9. 1 x 10 -9 5 2 l~ - ' ' o 1.2 x 3.6 x lo-' 3x 4.3 x l o - ' 2 x lo-@ 4 ~ 1 0 - l ~ 0.5 x l o - * 14.3.7 Flash point, Ignition Temperature, and Ignition Limits... Dichloromethane 3.18 3.61 1. 59 2.56 3.04 4.01 2. 49 3.11 2 .93 14.3.6 Thermal and Electrical Data The dielectric constant and thermal conductivity decrease with increasing temperature, whereas the specific heat increases The thermal conductivities, cubic expansion coefficients, dielectric constants, and electrical conductivities of various solvents are listed in Table 11 Critical data of solvents and the technical... Keioiies 0 .96 6 0 .93 1 0 .91 1 0 .90 2 Acetone Methyl ethyl ketone Methyl propyl ketone Amy1 methyl ketone 0. 792 0.805 0.807 0.816 The rejructive inties n, is measured in a refractometer with a sodium vapor lamp (Na-D lines, 5 89. 0 and 5 89. 6 nm) The value of the refractive index [14.45], [14.46], [14.80] is largely determined by the hydrocarbon skeleton of the substance in question Aliphatic esters, ketones, and. .. water ratio 1 :o 9: 1 10 12 46 -9 60 19 19 57 -7 z 65 1:l 1 :9 25 24 72 -6 51 41 5 -9 tion is made between the gas ignition temperature and drop ignition temperature, depending on whether the measurement is made by determining the gas temperature or by allowing the solvent to drop onto a hot surface of known temperature [14 .93 ] The value of the ignition temperature is used to group solvents into temperature... f ) Butanol 298 14 Solvents esters and glycol ethers decrease with increasing molecular mass, whereas those of ketones and alcohols increase: Esters Methyl acetate Ethyl acetate Propyl acetate Butyl acetate Amy1 acetate Alcohols 0 .93 4 0 .90 1 0.886 0.881 0.876 G I j u d Eilier.7 Methyl glycol Ethyl glycol Propyl glycol Butyl glycol Methanol Ethanol Propanol Butanol Amy1 alcohol 0. 791 0.7 89 0.804 0.810... 3 .9 3.3 3.7 7.5 2 .9 3.2 2.0 Aliphatic hydrocarbon 0 .9 1 o 1.2 1.5 1 o 0.6 1 o 0.2 2.3 0.3 Butanol 8.4 8.2 7.5 7.0 14.2 P/zj~sic'u~.heniit'al Principles 2 89 better solvents for cellulose nitrate-synthetic resin combinations if they contain a proportion of ethanol or butanol, respectively The solubility parameters of the mixed solvents are more comparable to those of the binder than those of the pure solvents. .. solzrhilixr for the two immiscible solvents Solvents having average solubility and hydrogen bond parameters are generally suitable as solubilizers, particularly ketones and glycol ethers Butyl glycol, diglycol, and triglycol are often used because they contain hydrophilic and hydrophobic groups which effect miscibility between partners having widely differing solubility and hydrogen bond parameters Very... Dispersion forces act in all atoms and molecules Hydrogen Bonds [14 .9] -[14.13] Hydrogen bonding forces exist in substances that have hydroxyl or amino groups (e.g., water, alcohols, acids, glycols, and amines) These molecules act as hydrogen donors and thus form a bond with hydrogen acceptors (e.g., esters and ketones) Water, alcohols, and amines act both as hydrogen donors and acceptors Very weak hydrogen . South America Rest of world Total production 29. 4% 25.1 Yn 12 .9 Yn 7.2% 15.2 Yn 5.1 % 3 .9 Yo 21 x loh t Paints, Coatings and Solvents Second, Completely Revised Edition Dieter. (bridge) maintenance, crafts, metallic and multicolored coatings. The number of coatings producers worldwide was estimated at about 7500 in 199 7. The total world coatings market was estimated to. cases, however, hydrogen bonding and dipole moments can greatly influence the degree of solvation. 14.2.5. Solvents, Latent Solvents, and Non -Solvents True (active) solvents dissolve a given substance