Paints, Coatings and Solvents Episode 9 pptx

Two main methods are used for treating waste air in coating and paint shops : afterburning with heat recovery and adsorption with solvent recov- ery.. The production of paint residues a

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 ( e g , 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:

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 I Y Y X

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

Paints, Coatings and Solvents Second, Completely Revised Edition

Dieter Stoye, Werner Freitag copyright 0 WILEY-VCH Verlae CirnhH I Y Y X

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:

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

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 ( p 2 ) 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:

Dispersion (London-Van der Waals) Forces Dispersion forces are formed by

mutual induction of atomic dipoles due to the electromagnetic field between the nucleus and electrons of the atom

Dispersion forces therefore depend on the displaceability of the electrons in the atoms, the polarizability a, and the availability of the electrons, i.e., on the ionization

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 hydro- gen donors and acceptors Very weak hydrogen bonds also exist in halogens and sulfur Hydrogen bonds are highly dependent on the mutual orientation of the molecules and thus on the temperature

Thermodynamic Principles [14.14] Forces of attraction act between the molecules

of the pure components and between the different molecules in the solution If the forces of attraction in the solution are greater than those in the pure components, dissolution is accompanied by a decrease in the internal energy of the system The process is exothermic and heat is released If, however, the forces of attraction between the molecules of the pure components are greater than those in the solution, the internal energy of the system is increased with absorption of heat In a closed system, this endothermic dissolution process is accompanied by cooling In open systems heat is absorbed from the surroundings

Most dissolution processes are endothermic and are thus promoted by a temper- ature increase: the solubility has a positive temperature coefficient Exothermic dissolution processes have a negative temperature coefficient (i.e., the solubility decreases with rising temperature) Miscibility gaps frequently occur above certain temperatures (Fig 1)

Why do endothermic dissolution processes occur spontaneously? The Gibbs- Helmholtz equation provides the answer:

Figure 1 Miscibility gap for mixtures of bu-

tyl glycol and water at atmospheric pressure

Systems with a negative temperature coefficient of solubility are mainly enthalpy- determined The interaction between the partners is often due to weak hydrogen bonds, which become weaker than the forces between the pure components on increasing the temperature The result is a decrease in solubility Examples of enthalpy-determined systems with a negative temperature coefficient are summa- rized in Table 2

The change in the total energy of the system can thus be negative or positive This change in energy is termed heat of solution or mixing, or solution or mixing enthalpy [ 1 4.1 61 , [ 1 4.1 71

Systems that are miscible with one another without any change in temperature are termed athermic systems [14.18], [14.19]; they include the following solvent pairs: Benzene- trichloromethane

282 14 Solvents

Table 2 Aqueous solution systems with a negative temperature coefficient of solubility

Methyl butyl ether 1.5 (10 C)

7.7 (25 'C)

11.0(60 C)

Positive heats of mixing (i.e., cooling of the system during mixing) occur with the following solvent pairs:

Trichloromethane -carbon disulfide

Acetone -carbon disulfide

Gasoline -carbon disulfide

Cohesive Energy Density and Solubility Parameters As a result of attractive or cohesive forces the molecules in pure solvents have a cohesive energy that has to be expended in molecular separation processes (e.g., dilution, evaporation, or addition

of another substance) The cohesive energy can be calculated from the enthalpy of

vaporization AHv and the work that is required to expand the vapor against the

atmosphere (volume work) [14.20] The cohesive energy per unit volume, i.e., the cohesive energy density, is defined as [14.16], [14.21], [14.22]:

The cohesive energy density of a solvent A is altered when it is mixed with another solvent B Two interaction pairs A - B are formed from each interaction pair A - A

and B-B The new cohesive energy density of the pair A-B in solvents of very low polarity being approximately equal to the geometric mean of the cohesive energy densities of the pure components [14.16], [14.21], [14.23]-[14.25]:

e A B = ( e A A egg)'''

The square root of the cohesive energy density is termed the solubility purunteter

6 It is also a measure of the intermolecular forces in pure substances Solvents with comparable solubility parameters have similar interaction forces and are therefore readily miscible and mutually soluble In order to take account of not only the dispersion forces but also of the polar forces and hydrogen bonds, the solubility parameter is resolved into the following components [14.26], [14.27]:

6' = 6,' + 6,' + 6,'

14.2 Plij~sicochemicnl Principles 283

Figure 2 Solubility parameter diagram

S , = solvent; Sz = non-solvent; M = solvent mixture

If the three solubility parameter components for dispersion forces 6,, dipole forces 6,, and hydrogen bonds 6, are plotted on a three-dimensional space diagram (Fig 2), a system is obtained in which a vector 6 is defined for each solvent The vector describes the solvent's solubility and miscibility behavior [14.28], [14.29] Solvents that lie close to one another in this space diagram (i.e., whose vector difference A6 is small) have similar solution properties and often a similar chemical structure Solvents that are far apart on the diagram differ greatly in their chemical and physical characteristics; they are generally immiscible [14.30]-[14.32] The sol- ubility parameters as well as their components are shown for some solvents in Table 15

The predictions that can be made for solvent mixtures on the basis of the solubility diagram are not strictly valid because they involve thermodynamic simplifications and empirical parameters, and disregard temperature effects [14.29], [14.33] A

strong warning must therefore be given against the uncritical use of solubility parameters Nevertheless, description of the solvents with the aid of the solubility parameter concept often provides useful information about their solvency and re- veals similarities which can otherwise only be characterized empirically (e.g., latent solvents or dilutability)

Solvent Miscibility As a rule solvents are readily miscible if the difference in their solubility parameters A6 is 5 4-6 units Solubility is generally limited if the differ- ences are larger (Table 3)

In the case of strongly polar (ionic) forces or hydrogen bonds, the solubility parameter concept is of limited applicability For example, the following pairs of solvents are only partially miscible despite the small difference in their solubility parameters :

4.1 5.9

284 14 Solvents

Table 3 Miscibility of solvents as a function of the difference in solubility parameters Ah

dimethyl sulfoxide methanol

In some cases the resolution of the solubility parameter into the three components 6,, h,, and 6, [14.31], [14.32], [14.34] is more suitable for explaining the experimental facts For example, although the difference between the solubility parameters of the partially miscible solvent pair nitromethane ~ ethylene glycol is indeed small

(Ah = 3 3 , both solvents differ greatly as regards their solubility parameter 6, (Ah, = 22.0) Many empirical results (e.g., the good miscibility of benzene and methanol) cannot, however, be explained by this approach

Solubility of Polymers [14.35] -[14.42] Most polymers are readily soluble if their solubility parameters are comparable to those of the solvent in question The upper limit for good solubility is a difference in the solubility parameters of 6 units (Table 4) The following selected systems are not, however, mutually soluble despite the small solubility parameter difference Ah:

Poly(viny1 chloride) - trichloromethane 1.6

Poly(viny1 chloride)- toluene 2.5

Poly(viny1 acetate)-diethyl ether 3.7

Cellulose -dimethyl sulfoxide 5.3

Furthermore, poly(viny1 acetate) is readily soluble in methanol and amylose is readily soluble in water, even though the differences in the solubility parameters are large (Ah = 9.8 and 22.5 units, respectively)

14.2 Pliysicochemical Principles 28 5

Table 4 Miscibility of polymer-solvent pairs as a function of the difference in solubility para- meters AS

14.2.2 Dipole Moment, Polarity, and Polarizability

In order to describe their solubility properties, solvents are often subdivided according to their polarity, i.e., polar and nonpolar solvents Since the term polarity cannot be defined unambiguously in physical terms, such a classification is not meaningful The term polarity includes parameters such as dipole moment, hydro- gen bonding, polarizability, entropy, and enthalpy The dipole moment p of a sub- stance is a molecular property resulting from the vector sum of bond-dipole mo- ments Highly symmetrical molecules (e.g., tetrachloromethane and benzene) ac- cordingly have no dipole moment ; other aromatic hydrocarbons and dioxane exhibit only very small dipole moments Less symmetrical molecules with strong bond dipoles have dipole moments between 1.6 and 3.9 D (alcohols, esters, glycol ethers),

glycols and ketones have higher values (2.3-2.9 D) The solvents with the largest dipole moments (3.7- 5.0 D) are ethylene carbonate, nitropropane, dimethylfor- mamide, and dimethyl sulfoxide (Table 15, p 324)

The dissolution behavior of a solvent cannot be predicted solely on the basis of its dipole moment For example, dioxane (p = 0.4 D) is a very good solvent and has a comparable solvency to dimethyl sulfoxide ( p = 4.0 D)

Dipole-dipole and induction forces in solvents or solutions decrease with increas- ing molecular mass of the solvent [14.43] Since this effect is not reflected in the dipole moment of the solvent, a polarizability parameter P is used to describe the dipole-dipole interaction forces [14.34] This parameter can be calculated from the ionization potential IP, polarizability a, and dipole moment p [14.44]:

where E 2 a IP

The dipole moment p is also used in conjunction with the solubility parameter 6

as a coordinate in solubility diagrams to take account of the influence of variations

in polarity [14.44] - [14.46]

The polarizability c( of an electrically neutral compound is a measure of the displaceability of charge carriers within the molecule The greater the polarizability, the stronger are the dipoles induced by an external electromagnetic field; the magni- tude of the polarizability and strength of the dispersion forces are thus related to one another

14.2.3 Hydrogen Bond Parameters [14.47]

The strengths of hydrogen bonds in solvents have been divided into three classes 1) Solvents with weak hydrogen bonding (hydrocarbons, chlorinated hydrocar- 3) Solvents with moderately strong hydrogen bonding (ketones, esters, ethers, ani- 3) Solvents with strong hydrogen bonding (alcohols, carboxylic acids, pyridine,

This classification allows solvents to be described in terms of their solvency for polymers by means of solubility parameters, and also permits hydrogen bond forces

to be taken into account This is done by determining the solubility limits of the polymer in solvents belonging to each of the three classes Three solubility parameter regions for a polymer are thus obtained Attempts have also been made to use corrections to take account of the variations in solubility parameters caused by hydrogen bonding [14.53] -[14.56]

The hydrogen bond parameters 7 provide a relatively accurate, simple character-

ization of solvents that can form hydrogen bonds [14.57], [14.58] The parameter (Table 15) is determined from the shift of the oxygen-deuterium vibration frequency

in the IR spectrum obtained when deuteromethanol is dissolved in the relevant solvent

Solvents that undergo hydrogen bonding may act as proton donors or acceptors [14.9], [14.47]:

1) Proton donors (e.g., trichloromethane)

2) Proton acceptors (e.g., ketones, esters, ethers, aromatic hydrocarbons)

3) Combined proton donors and acceptors (e.g., alcohols, carboxylic acids, primary

4) No hydrogen bonding (e.g., aliphatic hydrocarbons)

No hydrogen bonds exist in solvent mixtures comprising solely proton acceptors; hydrogen bonds are only formed in the presence of a proton donor and result in an increase in miscibility [14.47], [14.59]

[ 1 4.481 - [ 1 4.521

bons, nitro compounds, nitriles)

line)

water, glycols, amines)

and secondary amines, water)