Sorry for the length, folks, but I wasn't happy with shrinking it, and the more I worked the unhappier I got. The formatting and graphics won't make it through, but you should get the idea. So here it is in three installments. - DCW
Have you ever wondered what was in lacquer thinner? Or what mineral spirits was, or retarder, or fish-eye controller? How about this one. Why can't you put lacquer over varnish? Easy, you say. You can't put lacquer over varnish because the varnish will wrinkle. A simple explanation to an easy question. Well, what about the next question. What causes the varnish to wrinkle? Hmm, ah, er, ah, well, it's because the lacquer thinner "attacks" the varnish. Well, what do you man by "attacks the varnish?" You're getting closer, but at this rate by the time the question is fully answered my daughter's children will be retired. Everybody who has faced these questions with uncertainty raise your hand. (My hand is way over my head). The answers to these and a host of other questions important to finishers can be found in the study of solubilities, a peculiar mix of organic chemistry and materials science.
Many of us have a mixture of knowledge, ignorance and curiosity about solvents. Mostly ignorance, I'm afraid. And given the fact that most of our coatings are used in 90 - 95% solvent mixtures, this can be a fundamental ignorance as well. Most finishers simply haven't a clue as to what they are putting on the surfaces of the furniture we finish. And this amounts to 90 -95% ignorance. This is a situation which must change if we are to improve our abilities.
I remember clearly the incident that got me started on the subject. Several years ago while working as a finisher in southern Florida, I answered the phone and heard a request to name the ingredients in lacquer thinner. A local college student needed the information for a project in a science class. Dutifully I went over to the drum, made a note of the ingredients, and then returned to the phone and repeated my findings. The caller was appropriately grateful, the matter was closed, and I pursued it no further for a long time.
At the time I gave the exchange little thought, other than to note that not only did I not know what any of those ingredients were or what they did, I wasn't even sure I had pronounced the names correctly. As the years passed I learned the importance of knowing those things. What exactly is lacquer thinner? Or, to put it more correctly, what are all those things in lacquer thinner and how do they work? Twenty-five years ago I would have said "Who knows, who cares?". Now I know better the importance of such information.
As a conservator it is frequently critical to my work to know these things.Had I known much of this when I was a custom finisher, I would have had many more options at my disposal for dealing with finish problems, and would have been a better and more skilled finisher.
Unfortunately, the whole subject of solvents and their use is so large that after a couple decades of fairly rigorous study I am just beginning to scratch the surface. I don't mean to discourage you, in fact, just the opposite. There's a lot of very useful information out there about solvents, and I think it goes, almost without saying, that the more thoroughly we understand the materials we use the better work we can do with them. Hopefully, by the time you finish this chapter you will have learned enough to have a "knowledgeable curiosity" and will pursue the subject yourself. I don't pretend to be an expert, and I certainly don't know it all. However, I have learned a little over the years, and at least now I know how to pronounce most of the names. I guess I've made some progress.
The first section of this chapter will deal with the description of solvents; what they are, where they come from, and what their characteristics are. The second portion will continue from there and discuss the concept of solubility. In other words, to repeat part of the title, why do they do the things they do?
The number of solvent chemicals is enormous, and their specific functions in the solution vary (more about this in the second part of the chapter). In 1943, the definitive technical text on the subject of finishing materials of the day, Protective and Decorative Coatings (Joseph Mattiello, editor), listed 100 solvent chemicals for nitrocellulose lacquer alone. Twenty five years later came the book Compatability and Solubility by Ibert Mellan, which was a three hundred page list of widely used solvents and resins and their characteristics. By the time the current edition of the Industrial Solvents Handbook by Ernest Flick was completed, there were literally hundreds of solvent chemicals available, most with some applicability to film-forming technologies.
As I already mentioned, a discussion of what solvents do will take place later on, so for now I will just try to tell you what they are. Unfortunately, much of this descriptive information is based on organic chemistry, with which you may or may not be familiar. Specific and exact details about their physical and chemical characteristics, along with their molecular structure, are easily ascertained from any number of reference books, so I'll try not to belabor such points. However, I will need to mention some of these characteristics as I go or you won't be able to follow me at all. While the importance of all these details may not be obvious, just trust me that I'll tie it all together by the time you reach the end of the chapter. The molecular structure is important in explaining what the solvents do, and hopefully you can make the connections to understand why some solvents do one thing and other solvents another. Needless to say, I will stick to the solvents you are most likely to encounter or need.
Solvents fall into broad categories based on either chemical or physical characteristics, e.g. structure, boiling point, volatility (how easily they evaporate), etc. For the purposes of this listing I will try to stick to groupings by chemical structure, and include physical characteristics in the descriptions when appropriate. However, the most important aspect in describing solvent chemicals is evaluating their effect on the performance of a solution. For that reason much of the comparative description will be presented in the second part.
The first of these groups is the hydrocarbons, which contain hydrogen and carbon as their basic structure as the name would suggest. Hydrocarbons are subdivided into three major groups; aliphatic, aromatic, and a special group of substituted aliphatics called chlorinated hydrocarbons. I'm not going to go into a long discourse on the descriptive differences between these designations. However, it is important to remember that chemical (not physical) characteristics within groups are similar. In other words, aliphatic hydrocarbons have similar chemical (not physical) behaviors which differ from those of aromatic hydrocarbons, chlorinated, etc. This is true for all the groups we will discuss here, not just the hydrocarbons. I hope that as the discussion continues this will become more apparent.
Aliphatic hydrocarbons can be further divided into three major groups (saturated, unsaturated, and cyclics), to which I will refer when appropriate. Saturated hydrocarbons are groups of molecules that have only C-C single bonds and no C=C double bonds.
Saturated hydrocarbons can be only carbon and hydrogen in composition, in which case they are referred to as "alkanes (or paraffins)" and are primarily the result of petroleum distillation, or some of the hydrogens can be replaced by other atoms such as nitrogen, chlorine, etc. Of the alkanes, the most important of these for us finishers are in the 6-10 carbon range.
In order of increasing size and decreasing volatility, they are hexane (6 carbons or CH CH CH CH CH CH ), heptane (7 carbons), octane (8 carbons), nonane (9 carbons) and decane (10 carbons), which are solvent ingredients in "oil-based" reactive finishes such as alkyd or polyurethanes. They may also be used for specific solvent-release finishes.
Usually these compounds are not used in pure forms, primarily for economic reasons. The distillation required to isolate a pure compound is within very exact limits and therefore expensive. A more common practice is to broaden the distillation range and simply call the by-product mineral spirits, naphtha, petroleum benzine, paint thinner, kerosene, or whatever (finally, some familiar words!). These products are also frequently defined by the percentage of hexane, the percentage of heptane, etc. An example of this distinction is the list below.
Petroleum ether is an extremely volatile liquid distilled off between 40 C and 60 C, consisting of mostly pentane and hexane. Naphtha is somewhat volatile, being distilled off between 70-90 C and containing hexane and heptane.
Petroleum benzine (mineral spirits, paint thinner, Stoddard solvent) is only moderately volatile, and in fact can be used as a retarder in certain applications. The distillation limits are between 120 C and 150 C, and the constituents are primarily octane and nonane.
One additional form of these compounds is the cyclic hydrocarbons, which are represented by any number of molecules that are basically saturated hyrocarbons that have formed a circle. The most important of these is cyclohexane, a circle of six carbons, and methyl cyclohexane, a C ring with an extra CH appendage. Cyclic hydrocarbons can also take the form of multiple rings bound together, such as turpentine, the oldest of all paintvolatiles.
Turpentine, as a biochemical compound rather than a geochemical one, is a mixture of chemicals known as terpenes, which have several different forms within the same chemical composition (C H ). Turpentine is obtained from several different processes, all based on extraction from coniferous tree resins and oils. The two main types are gum turpentine obtained from the sap of living trees, and steam or distilled turpentine, obtained in the processing of cut timbers. Since distilling is involved, the exact composition and characteristics of turpentine can vary greatly depending onthe distillation precess. In most uses turpentine has been replaced by alkane solvents. An additional aspect of turpentine is that there seems to be a greater problem of allergic sensativity to turpentine than other solvents performing similar functions.
The basis for all aromatic compounds is the benzene molecule (C H , indicated by the symbol ). The most important chemicals within this family are toluene ( CH ), xylene (CH CH ), ethyl and diethylbenzene (CH CH ,CH CH CH CH ). All of these are important ingredients in at least some of the acrylic or nitrocellulose lacquers.
Hydrocarbons in which one or more of the hydrogen atoms have been replaced by an atom from the halogen group of elements, primarily chlorine and fluorine, are generally referred to as chlorinated hydrocarbons. By far the most important of these is methylene chloride (dichloromethane, CH Cl ), the main ingredient in most solvent-type paint removers. Methylene chloride is so volatile, however, that it is generally not included in film-forming formulations.
Other important families of solvent types are those which incorporate oxygen atoms in the molecule, and include esters, glycol ethers, ketones, and alcohols. Although they aren't really hydrocarbons in the same sense that the aliphatics or aromatics are, I find it easiest to act as though they are hydrocarbons with a structural defect. So, in my descriptions of these groups I refer to them as hydrocarbons but also describe the structural difference from true hydrocarbons. I am not the only one to think of these structures in such a manner. Chemists have devised a notation for the hydrocarbon chains in these molecules and have assigned the generic designations R, R', R'', etc., to stand for these carbon chains.
Esters are hydrocarbons where the C-C chain is broken with a OC=O group and have the general structure of R-(OC=O)-R'. The structures of the specific esters tends to be too complicated for me to address here, but a quick peek at a good organic chemistry text should tell you all you want to know. Esters were among the first of the solvents for synthetic finish films. Currently they are particularly important for acrylics in addition to nitrocellulose. Widely used esters, in order of decreasing volatility, are ethyl acetate (very important in -acrylate formulations), isopropyl acetate, isobutyl and butyl acetate, and "Cellosolve" acetate, frequently used as a retarder or "flow out" agent in acrylic finishes.
The structural differences between glycol ethers and hydrocarbons is about as drastic as we will encounter in this chapter. Glycol ethers are a hydrocarbon chain terminating in an oxygen, a series of CH CH O units followed by a single hydrogen, as indicated by the formula RO(CH CH O)nH. The two ethers in greatest use are methyl and butyl "Cellosolve". "Cellosolve" is a registered trademark of the Union-Carbide for the mono-ethyl glycol ether, and the methyl or butyl indicate the hydrocarbon chains attached. Butyl "Cellosolve" is valuable in that it is slow drying, and increases flow-out and blush resistance. It should be no surprise that it is the primary ingredient in many retarders.
For contemporary lacquer finishing, no group of solvent chemicals in more important than the ketones. Ketones are hydrocarbons where one of the CH groups in the chain has been replaced by a C=O, or carbonyl. The ketones in wide use tend to be very powerful, volatile solvents. These include acetone (or methyl methyl ketone CH (C=O)CH ), methyl ethyl ketone or MEK (CH (C=O)CH CH ), and methyl isobutyl ketone (the formula is too long so forget it). Any of you even remotely familiar with the ingredients of acrylic or nitrocellulose lacquers recognize these names. The development of synthesized ketones (any other than acetone or MEK) parallelled the development of new synthetic resins beginning in the 1930's.
Alcohols are a group of pseudo-hydrocarbons where one of the hydrogen atoms has been replaced by a hydroxyl group, or a pairing of an oxygen atom with a hydrogen (OH). This substitution and the length of chain are very important, and will be discussed further in the second part of the chapter.
Most of you are familiar with the common alcohols. Methanol (CH OH) is the technical name for wood alcohol, and is the best solvent for a variety of spirit varnishes such as shellac. Because of its extreme toxicity, methanol is usually replaced by ethanol ("grain alcohol", CH CH OH). The common terms "denatured alcohol" or "reagent alcohol" indicate mixtures of ethanol with just enough methanol or other "contaminant" to make it undrinkable and thus exempt from liquor taxes. Other alcohols common in finishes are the propanols (three carbons) and the butanols (four carbons), which are volatile components in many types of solvent-release finishes.
In this first part of the chapter I have attempted to familiarize you with many of the common solvent chemicals common to the finishing room. I won't suggest for a minute that I have given you a complete listing, but rather have presented a brief glimpse at the ones most prevalent. There is no need for me to include every single possible solvent chemical. That has been done already, and I commend to your attention the references at the conclusion of the second part of this chapter. All I am trying to do is give you some insight into this information and how useful it is to you in your practice as wood finishers.
Just as a cook needs the instructions to go with the ingredient list, you need some sort of framework to make all these chemicals work best for you. Well, that's what is coming next, when we discuss the role solvents play in creating and manipulating solutions. If you can keep in mind the fact that all liquid finishing materials are some sort of solution, then the information coming next will make that much more sense.
So you persevered and made it all the way here with the exclamation "What does this do for me?" Well, that's what this half of the chapter should help you with. My goal for this chapter is to give you the information tools to understand finishing materials as liquid solutions. Once you understand this, you can begin to understand how finishes behave, whether in the liquid form as you apply them, or in the solid form when they are on the piece of furniture. Once you clear this hurdle you can begin to use your observations about their characteristics to your best advantage.
Suppose your finish isn't behaving exactly the way you would like, and you want to change it. In order to do this there are certain things you must know - there's just no way around it. For example, if you know the characteristics of a material, and you know its constituents and understand how those constituents influence the properties of that material, and you know other possible constituents which are compatible but impart different properties, then you can change the characteristics of the material by changing some of the ingredients of the original composition. ***(This is the most important single sentence of the entire chapter. Do yourself a favor and go back and re-read it. Now go back and reread it again. This time substitute the world "finish" every time you see the word "material" and see if makes more sense to you.)***
It all comes back to the same key: you have to know what something is before you can control the changes you make to it. Guessing just isn't good enough. Besides, without all the information you can't even guess very well. If you can predict rather than guess, you can save yourself a lot of money and time and provide a superior product.
Previously I mentioned some solvent chemicals we use in the finishing room, probably without knowing it. The next step is to combine that list to some chemical and physical principles and come up with useable information. We will start out this section with a review of the role solvents play in solutions.
Solvents are simply thinners, aren't they? Maybe yes, maybe no. Let's sidetrack for a while so we can be sure we are all talking the same language, and then we will decide whether solvents are simply thinners or not. A quick look at some definitions to help us clarify our terminology. Since almost everything we use is a liquid mixture of some sort, let's start with the definition of a solution as found in the Paint/Coatings Dictionary (Federation of Societies for Coatings Technology, Philadelphia).
Solution - A homogenous mixture of two or more substances of dissimilar molecular structure; term commonly applied to solutions of solids in liquids.
Solvents - Liquid, usually volatile, which used in the manufacture of paint to dissolve or disperse the film-forming constituents, which evaporates during drying and therefore does not become part of the dried film.
Fine, we now know that the solvent does the dissolving. But something has to be the material dissolved. My college chemistry book helps out here.
Simple solutions usually consist of one substance (the solute) dissolved in another substance (the solvent). The solute may be thought of as the dissolved substance and the solvent as the dissolving substance.
The picture is getting clearer, but still no mention of thinner. Returning to the Paint/Coatings Dictionary, we find
Thinner - 1. The portion of a paint, varnish, lacquer or printing ink, or related product
that volatizes during the drying process.
2. Any volatile liquid used for reducing the viscosity of coating compositions or components; may consist of a simple solvent, or diluent or a mixture of solvents and diluents.
Again, from the Paint/Coatings Dictionary
Diluent - 1. A volatile liquid which, while not a solvent for the non-volatile constituents of a coating or printing ink, may yet be used in conjunction with the true solvent, without causing precipitation.
Suddenly the picture isn't so clear. What differentiates between a solvent and a diluent? My explosion-proof cabinet is marked "solvent cabinet" not "diluent cabinet", and I order solvents rather than diluents from the chemical company.
Obviously there is more to it than has been explained up to this point, and to that charge I plead "guilty". The information missing from all of this is an explanation of a phenomenon known as solubility. Solubility is generally defined as the ability of a solvent to dissolve a solute, and the compatibility (ability to dissolve) or incompatibility (inability to dissolve) is expressed in yet another term, solubility parameter. Remember the question "Why does this solvent do what it does?" Well, the solubility parameter gives us that information and much more, like why shellac flakes become liquid when mixed with alcohol and why oils mix with naphtha but not water.
To explain the whole picture we must begin with some basic ideas, such as "How exactly does something dissolve?" You may not think such an abstract question could be important in our discussion, but consider this - all finishes (except pure oils or molten waxes, etc.) are comprised of solids dissolved in liquids. Check the description on a can of nitrocellulose lacquer. Very likely it will state something like "11% solids", which indicates a ratio of 11 parts resins (by weight) dissolved into 89 parts solvent(s). Again, the answer to how and why something dissolves is found in the solubility parameter.
There are several factors contributing to the solubility parameter, all of which are based on the molecular structure of the solvents (remember, I told you that this would start to come together!). The solubility parameter is expressed as a number which is derived from the measurement of the thermodynamic forces holding liquids together. Thus, each liquid solvent (or anything else which can be liquified and directly vaporized) has a specific amount of energy holding it together and therefore a specific solubility parameter. For solvents this is expressed as an exact number, e.g. hexane=18.7, but resins are usually given ranges, e.g. cellulose nitrate=7.8-14.7 (depending on the solvent class and the resin formulation).
If the solubility values for two materials are similar they are considered to be soluble. For example, if the solubility parameter of "solvent X" is 10.0, then resins with values close to 10.0 will dissolve in "solvent X". These solubility values have already been calculated for hundreds of chemical compounds and the information is readily available, so all you have to do is look them up.
The phenomenon described in the previous paragraph is usually given the shorthand phrase "Like dissolves like". In other words, materials with similar solvent parameter characteristics are compatible, and those with dissimilar characteristics are incompatible. This is true not only for pure solute/solvent systems but those with modifiers as well. When silicone contamination is present on the surface of a piece of wood, the application of finish can be extremely difficult because the finish is so chemically and physically dissimilar to the silicone it will be repelled. The addition of "Fish-eye remover" makes the finish much more "like" the silicone contaminant and thus compatible.
As important to knowing what is compatible (soluble) is knowing what isn't, and the things that fall in the "partially compatible" grey area. To ask the same question as in the subtitle of the first part of this chapter, "What is lacquer thinner and why does it do what it does to varnish?" Of course, by this I am referring to oil/resin varnish.
For a solid to be dissolved, the forces holding the molecules of the solid together must be broken down to allow the solvent to permeate the material, or to "dissolve" it. This occurs when the inter-atomic forces of the solvent molecules are similar to those of the solute, as I mentioned earlier.
When a solvent first begins to penetrate a solute, the solute begins to be displaced by the solvent and swells. If the conditions are right, such as parameter similarities, and adequate time to allow for dissolution, the solute will eventually dissociate completely and become liquid, a solution with the solvent. Even if things aren't exactly perfect, since solvation is a thermodynamic phenomenon (in other words it is an artifact of energy applied over time) given enough time, if there is enough similarity the solute will go into solution. However, if the conditions are not right, but only close, such as a solubility parameter that doesn't quite match or not enough time allowed for the reaction, the solute will never get past the swelling state and will either return to its original condition once the solvent evaporates or will simply remain swelled when dry. This is what you are seeing when varnishes wrinkle under lacquer thinner. The resin component in the oil/resin varnish film has swelled and now takes up more space than it did, but the surface under the varnish is the same size, so the resin "bunches up" and wrinkles. If you know the parameters of the solvent(s) and the resin(s) you can predict whether or not the film will dissolve, swell, or remain unaffected.
However, it turned out that the single number solubility values weren't adequate for explaining the ability of solvents to work in a solution, because some solvents with similar values worked differently when they were used to dissolve resins. Clearly, there were additional factors involved. About thirty years ago a system was devised to explain these differences, and the result was a further breakdown of the solubility parameter into three contributing fractions. These were defined in the chemical/thermodynamic terms of polarity, hydrogen bonding, and dispersion forces. Remember when I told you in the last issue that all the information about the molecular structure of solvents was critical? Well, here's where it enters the picture in a manner you will see as being useful. Keep in mind that all molecules act as miniature magnets (not really but work with me here, folks)and have positive and negative charges, and the three fractions are simply specific manifestations of this phenomenon.
Polarity is the term describing a positive-negative relationship in a molecule that is not necessarily defined by hydrogen-oxygen bonds, such as the case with ketones. Polarity is an expression of the imbalance or symmetry of a molecule. For a similar structural imbalance, smaller molecules are more asymmetrical than larger ones, hence more polar (more about this in a minute). It actually makes sense. If you took a penny (the structural imbalance) and placed it on one end of a balanced straw (a small molecule) what would happen? Take the same penny and place it on one end of a balanced two-by-four (a large molecule). Now what happens? The straw is experiencing much greater asymmetry than the two-by four-, and could be said to be much more polar.
In general, different classes of chemicals have generally different polarities, generally speaking. Families of solvents from most polar to least polar are
- Most Polar - water
- Ketones & Esters
- Chlorinated hydrocarbons
- Aromatic hydrocarbons
- Least polar- Aliphatic hydrocarbons
Hydrogen bonding is the term for the strong attraction between hydrogen atoms (which act partially positive) and partially negative atoms such as oxygen. It is a very specific type of polarity. The bonding of hydrogen and oxygen form a binary that looks like this; (-)O-H(+), with the symbols (+,-) indicating partial electronic/magnetic charges. In this structure the oxygen end of the couplet acts as the negative end of the magnet and the hydrogen as the positive end. This molecular "magnet" influences and "arranges" the molecules around it much like a horseshoe magnet influences and "arranges" iron filings. Hydrogen bonding is a particularly important force in alcohol and alcohol-type solvents. Most solvents are listed in categories of hydrogen bonding strengths, with the classifications being
- Class I - weak hydrogen bonds
- Class II - moderate hydrogen bonds
- Class II - strong hydrogen bonds
- Aliphatic Hydrocarbons
- Aromatic Hydrocarbons
- Chorinated Hydrocarbons
Hydrogen bonding is by far the most important of these three solubility fractions, and in many cases the description given by only the solubility parameter and the hydrogen bonding value are sufficient for defining the characteristics of solvents. An example of the importance of hydrogen bonding is that nitrocellulose has a solubility parameter of 11.9 in weakly bonded solvents but is completely insoluble in strongly bonded solvents regardless of solubility parameter.
Finally, dispersion forces are momentary positive-negative forces set up by the movement of electrons within the entire molecule, which again, help hold the molecule together. These are present in all molecules but are particularly important in paraffins and other hydrocarbons, which are very "balanced" in electronegative terms. In otter words, they are not polar, or "magnetic. "
Dispersion forces increase with increased molecular size because the positive-negative effects are spread out over a much larger area and are "diluted". For example, the dispersion force of methanol is lower than ethanol, which is in turn lower than propanol, etc. This tendency is true for all families of chemicals.
We have now identified all the key contributors to explain how chemicals are classified in regard to their ability to act as solvents. Now all we have to do is to translate all these various bits of information into a format that you can use easily. Fortunately, all of this has been done for us by the chemists who defined all these forces, and the chemical companies, who use these descriptors for their chemicals (figure 1). Through laboratory experiments, molecular analysis and mathematic models the solubility parameters for virtually every solvent and resin of interest to us has been calculated. That's right, all the hard part has been done already.
From now on, if you can read a map, you can compare and manipulate solvents and solutions.
Chemists don't like to spend their time with arcane mathematical formulas any more than you or I do, so they have come up with graphic representations for all this information. There are two main mapping systems for solubility and I will describe and show you both of them so you will be prepared for whatever system your chemical supplier uses. The first, and simplest system, uses only two bits of information plotted on an X:Y graph just like common graph paper. For this system they use the value of the solubility parameter as "X" and the value of hydrogen bonding as "Y" and simply put points on the chart indication each chemical in question (figure 2). Since these values are already known and all you have to do is look them up. You can even buy these charts from most industrial chemical companies. By combining this plot with the plot of a particular resin or its additives you can get a very accurate picture of what dissolves the resin and what doesn't.
For example, look at the map of nitrocellulose solubility (figure 3). The curved line is the delineator between soluble and insoluble. Solvents on the left don't work and solvents on the right do. By knowing this the manufacturers design systems whereby all the main ingredients (nitrocellulose, additional resins, plasticizers, retarders, etc.) will fall on the right side of the chart. If they don't, the solution just won't work very well. These charts can even describe not only if something can dissolve or not, but how it will appear once it does dissolve (figure 4). The second mapping system uses a slightly different approach. Rather than being a binary mapping system (X:Y) this system is terniary (X:Y:Z). This system maps the three contibuting forces to the solubility parameter; hydrogen bonding, polarity, and dispersion forces. The triangular map for this system is called the Teas diagram, named after Dr. Jean Teas who developed the chart (figure 5). Just like the binary system, the terniary system can visually indicate not only solvents (figure 6), but also resins and other ingredients (figures 7,8,9).
For both mapping systems it is important to remember that the lines and curves act as boundary guides rather than absolute limits. For example, imagine a resin with a Solubility Parameter of 9.0-10.0 (remember I said that parameters for resins could be ranges or specific numbers). In general, solvents with parameters between 9 and 10 will dissolve the resin, with 9.5 being the optimum. As the solubility parameter of the solvents gets further from 9.5 in either direction the solvency power will decline, and the resin will become less and less dissolveable, until a point is reached when the resin is considered insoluble in that particular solvent.
The reason why parameters are not absolute is that most solutions can tolerate a limited amount of material which isn't really soluble in the system. These insoluble ingredients are present as "suspensions". This can be particularly important when minute amounts of a specific material can impart desired characteristics to a film, but that specific ingredient is not truly soluble in the system.
One additional and extremely useful to these mapping charts is the ability to predict the solvent power of mixtures of solvents. Imagine you have a resin "A" whose primary area of solubility doesn't really fall neatly into an area of a solvent. Instead it falls about one-third of the way between the points indicating solvents "X" and "Y". By translating that into a solvent system, you can predict that a solvent mixture of one part "Y" to two parts "X" you would get a solvent capable of making the resin liquid and thus useful (figure 10). Remember in the first part of this chapter I made reference to the physical characteristics of chemicals. The structure of molecules determines what these characteristics are. The viscosity, volatility, boiling point, flash point, etc., of all of these chemicals is known and you should have this information at your fingertips. By combining this with the solubility parameter you can manipulate solutions, enabling you to impart specific characteristics to the finish material.
So, how does this get used in the finishing room? Suppose you are working with shellac on a particularly hot and dry day and the shellac varnish is drying too fast. In general you are faced with two options; wait for a more favorable atmosphere, or modify the shellac varnish. By looking at the charts and reading the descriptions of the solvents you would know that you could add butanol to the varnish to help you out. Butanol is compatable with ethanol (and thus shellac) and evaporates much more slowly (has a much lower volatility) than ethanol and would aid in the flow-out and the work ability of the varnish. In addition certain dewaxed shellacs can be brittle enough to need plasticizers. While the shellac varnish isn't soluble with oil the system can tolerate small amounts of oil as a plasticizer. Remember using oil on the french polishing rubber?
One of the most common finishing materials for contemporary finishers is nitrocellulose lacquer. Coincidently this system is one where solubility information is most critical to the formulation and function of the material. Let's return to the diagrams we just finished talking about.
Whichever mapping system you use, you will be able to understand what you are using, define its characteristics and predict how those characteristics can be altered. In fact, with a little experience you can design your own finishing systems. If the performance of your finishing material is important to you, this is an option you would be wise to consider.
Most commercially available lacquers are a compromise between ingredients which have specific characteristics and ingredients which are cheap. By designing you own lacquer system you can get exactly what you want. It is unlikely that you will develop the expertise or the facilities to actually make your own lacquers, but there are small finish manufacturers who can. Of course, the cost will be greater, but for most custom finishing shops the cost of the lacquer is one of the smallest expenses.
There are several ingredients besides cellulose nitrate in nitrocellulose lacquers. Because cellulose nitrate is a film forming material with drawbacks as well as benefits it must be altered by the addition of modifiers such as resins or plasticizers (cellulose nitrate is not a true resin even though it is a film-forming material). So, in reality the lacquer consists of at least four things; cellulose nitrate, other resin(s), plasticizer, and thinner. Cellulose nitrate comes in different grades depending on chemical or physical characteristics. Films of this lacquer can be extremely tough and durable, and have the distinct advantage of being easily manufactured and applied.
Resins are added to lacquer to build better depth in the finish, to cause better cohesion and adhesion, to increase strength, and to modify sheen. Looking at the solubility charts it is easy to determine the resins which will be compatible with cellulose nitrate. Natural resins such as shellac and dammar were among the earliest, but now the resins used are primarily alkyds, acrylics and vinyls.Plasticizers are used because cellulose nitrate tends to be extremely brittle when in a film. The function of plasticizers is to increase flexibility of the finish film. There are several "ideal" characteristics to a plasticizer. However, no single plasticizer has all of them so more than one may be required for a lacquer.
The final ingredient, and the reason for this chapter, is the "lacquer thinner". In order for cellulose nitrate finishes to be used they must be liquid, or "in solution". In addition, it must remain liquid for a specific period of time to achieve the best results (its evaporation rate must be controlled), and like the formulation of the solid film-forming materials, the thinner must be economically practical. This is critical because in many application methods 95% of the material applied will simply evaporate. With that in mind let's use all we have learned so far to look at lacquer thinners.
To begin with, thinners almost always have three major types of ingredients; active solvents, latent solvents, and diluents. Active solvents are the chemicals which actually dissolve the solids (cellulose nitrate or the modifiers). Nitrocellulose is soluble in most ketones or esters and glycol ethers, so MEK, acetone, acetates, and "Cellosolves" could be used. Latent solvents, sometimes called co-solvents, cannot dissolve nitrocellulose by themselves but can be used to enhance the solvency of the active solvent or to act as a solvent for the modifiers. Alcohols are usually used as latent solvents because they increase the solvent power of the active solvent and reduce the viscosity of the solution. If shellac orother spirit varnish is present the alcohol can also serve as an active solvent for it. Diluents are simply liquids used to dilute the thinner, making it more economical. Generally diluents are hydrocarbons, both aliphatic and aromatic. While not necessarily soluble in the varnish the strictest sense, the system can usually tolerate certain amounts of these solvents without adverse effect. I believe that with a little thought you could come up with an excellent formulation of both the solids content for a lacquer and the thinner in which to dissolve it. Think about the characteristics you want and the characteristics of the different types of chemicals we have discussed and design a solvent/solute system which matches the latter to the former.
So let's do it! I have included a chart showing the process of determining the solvents for a similar situation (figure 11). To start out with, which celloluse nitrate will you use? Yes, it comes in three different grades; RS, SS, and AS. RS is the most common in furniture lacquers so we'll go with it. We want high solvency so the active solvent will be MEK with a dash of acetone. Other ingredients would include butyl Cellosolve, which would act not only as an active solvent but also a retarder and aid flow-out. A good latent solvent would be ethanol because it works well with the ether (Cellosolve) to improve its solvency and to decrease the viscosity of the lacquer because we want to build solids quickly.
For a resin additive let's go with an acrylic to increase resistance to moisture and provide greater long term stability. The resin chosen is poly-methyl methacrylate, which is compatible and soluble in toluene. Not coincidently toluene is an excellent co-solvent for nitrocellulose and a useful hydrocarbon diluent. I would also add a little ethyl acetate to"jump-start" the solvent process and to contribute to flow-out.
Now for the plasticizer. It turns out that dibutyl phthalate is compatible with cellulose nitrate and acrylics so it's the choice here. Since it is compatible with two main ingredients, the solvents for them will suffice here.
The last thing is the addition of diluents to bring down the cost of the whole mixture. The balance is pretty important here, because cellulose nitrate RS can tolerate a great deal of aliphatic hydrocarbon but the acrylic cannot. So, we will go with a mixture of mostly toluene with a little bit of hexane and heptane.
Well, while you're not a finish chemist yet, there you have it; a very simple nitrocellulose lacquer system of your own design. Naturally there is far too much specific information about material properties and such formulations for me to cover it all here, but I hope that you now have the basic tools needed for the exercise of using materials to their greatest advantage and your greatest benefit. If you decide to contact finishing manufacturers you will now have enough knowledge to engage in knowledgeable discussions with them. The specifications you need to use all of this new knowledge, and additional information as well can be found in the following books. I have included brief notes on each for your assistance.
Predicting Resin Solubilities by Jean P. Teas (Ashland Chemical Corp.). Free booklet describing terniary mapping system and how to use it informulating solutions.
Solvents by William H. Ellis (Federation of Societies for Coatings Technology). Excellent short book (29 pages) giving chemical and physical characteristics of solvent groups and their uses. Hey, it sounds a lot like this chapter! It's available for cheap so get it.
On Picture Varnishes and Their Solvents by Feller, Stolow and Jones (available from American Institute for Conservation, approx. $12.00). One of the first scholarly works on solubility of varnish films (and still themost important), nevertheless very readable and assumes little previous knowledge. If you read this chapter you are ready for this book, and at $12.00 you have no excuse.
Industrial Solvents Handbook by Ernest Flick (Noyes Data Corp.). Expensive book but you shouldn't have to buy it. Many good libraries have it. Gives pertinent information about hundreds of commercially available solvents including all discussed in this chapter.
Hercules Nitrocellulose: Chemical and Physical Properties and Solubility Parameter Maps of Hercules Film-Formers from Hercules Incorporated (publications 450-9 and CSL-204A).
Available free from Hercules, describes the raw material, processing, solubility, formulation, etc. Anybody using nitrocellulose lacquer should have this. Uses binary solubility maps.
Technology of Paints, Varnishes and Lacquers edited by Charles Martens (Robert E.Krieger). Descriptions of all contemporary finishing systems, their components and formulations. This should be on your shelf. Today. Compatiability and Solubility by Ibert Mellan (Noyes Data Corp.).
Now out of print but widely available in libraries. Extremely valuable in formulating, hundreds of charts assigning solubility and recipes to contemporary resins.
Finally, check with your insurance agent. Lots of them provide solvent toxicity handbooks, which coincidentally contain a gold mine of information about solvent physical properties.
There are many more books and lots of excellent articles on the topic of solubility and manipulating solutions and finish films but this should get you started.
Copyrights 1987, 1998, 1999 by Donald C. Williams
Donald C Williams is Senior Furniture Conservator at the Conservation Analytical Laboratory, Smithsonian Institution, Washington, D.C. The views expressed are solely his and do not necessarily reflect the position of the Smithsonian Institution.