The Basics of … Solvents and Thinners L A Fisher FTSC
OCCA Student Monograph No. 9 Oil & Colour Chemists’Association
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The Basics of … Solvents and Thinners L A Fisher FTSC
OCCA Student Monograph No. 9 Oil & Colour Chemists’Association
Published by the Oil & Colour Chemists’ Association, 1st Floor, 3 Eden Court, Eden Way, Leighton Buzzard, LU7 4FY, UK © OCCA 1997 ISBN 0 903809 39 7 Printed by The Burlington Press, Foxton, Cambridge CB2 6SW, United Kingdom Typeset by My Word!, PO Box 4575, Rugby, CV21 9EH, United Kingdom
Foreword This Monograph is the first in a new series of Student Monographs published by the Oil & Colour Chemists’ Association, and is part of a collaborative project with its South African Division. The objective for these new Monographs is to provide a series of basic primers for newcomers to the surface coatings industry and, specifically, for operatives and trainees in Southern Africa and the Indian Sub-Continent. Companies, Trade Associations, Professional Bodies and the Authorities have identified this sector as lacking educational resources and the Oil & Colour Chemists’ Association is pleased to respond to this need. The series has been published with the assistance of a grant by the Trustees of the Ellinger-Gardonyi Fund, an educational trust, administered by the Oil & Colour Chemists’ Association and is supported from resources provided by its South African Division from the funds generated by SURFEX – Southern Africa.
The Basics of … Solvents and Thinners L A Fisher FTSC Introduction Working in the so called third world countries is an education in itself. One of the things I came to realize was that, due to lack of opportunity, there were many more people of high intelligence in the working class than there are in the first world. The reason is due to the fact that in the developed nations anybody with the brains and/or gumption can get an education. Hence by a natural selection system there is a stratification. At the same time the type of education is also at variance; going from the farm school with 30 pupils in 10 grades and one teacher, to the specialised schools with the capacity and the staff to provide a broad curriculum. A further problem is that, as we develop, the basic qualifications required also change – 50 years ago a matric was a good starting point whereas industry can, and does, now require a degree for the same job, that’s progress. The result of this is that the first – third world gap is increasing. In South Africa, as is also the case in Latin American and Asian countries, we have a strange mixture of first and third world. This means that the education tends to be fairly rudimentary for the less privileged and the Ox Wagon, Porsche gap has to be bridged. This means that industry must fill in the gaps themselves. The result of this is that, because the demand is relatively low for such disciplines as paint technology, the technical colleges cannot assist and the paint industry must provide its own education. Unfortunately the courses available from the UK assume a basic school leaving certificate with maths, physics and chemistry. To this can be added the language problems. Following on the results of the last National Matric exams, the first post-apartheid ones in which students all wrote the same exam, one official comment was to the effect that a greater emphasis on the Sciences will be required. So, we have some very bright people out there with a basic education and they need only a little help. We could throw them in the deep end, but it would be better to teach them to float first. The object of these monographs is to help our students to bridge the gap. It is hoped that, by explaining the basic concepts, in the least complicated manner, emphasising only the points they really need to know, they will have a better understanding of what is going on and be less confused. We are not targeting the higher modules of the course but only the beginners. As someone once told me ‘When you are up to your neck in crocodiles it can be difficult to remember that the objective was to drain the swamp’. At least let us attempt to extract some of the crocodiles’ teeth.
About the author Les Fisher has worked for over 40 years in a technical capacity in the paint, synthetic resin, GRP and adhesive industries in the UK, USA, Latin America, SE Asia and South Africa. Since retiring in July 1990 he has worked as a consultant in chemical technology specialising in the identification and classification of chemical preparations with regard to their safe transport, general handling, labelling, packaging, storage and use. He is currently also a tutor for the South African Paint Manufacturers Paint Technology Course in the Durban area.
Contents Part 1 – Elementary Solvent Chemistry
1
Organic Chemistry
1
Elements, Compounds and Mixtures
1
Atoms and Molecules
1
Bonding of Atoms
1
Chemical Annotation
1
Valency
2
Carbon Compounds
2
Homologous Series
2
Isomerism
3
Branched Chain Isomers
3
Aromatic Hydrocarbons
4
Other Solvent Groups
5
Physical Properties
8
Conclusions
9
Part 2 – Thinners and Solvents
10
Solvents and Dispersions
10
Class of Solvents
11
Diluents
11
General Considerations
12
Ozone - what is it?
13
1
Part I – Elementary Solvent Chemistry Organic chemistry The chemistry to be dealt with here is referred to as organic chemistry. This is a very complex science and all living organisms are covered by it as well as a host of others. The basis of all chemicals in this class is that they contain the element Carbon in combination with others. In order to keep things uncomplicated the first chemical compounds dealt with will only involve three elements: carbon, hydrogen and oxygen and, as will be shown, it is in the permutations and combinations of these that gives the differences that make up most of the chemicals called solvents and which are of concern to us at present.
Elements, compounds and mixtures The first requirement is an understanding of what is meant by an element and a compound. An element is a single substance which cannot broken down any further by normal chemistry, it would require drastic action in a nuclear reactor to change it. A compound is a chemical combination of two or more elements as opposed to a simple mixture. The properties of a compound are distinct for that compound and differ from those of the elements from which it is derived. A compound cannot be converted back to its original components by physical means. A mixture would combine the properties of the substances from which it was prepared and it will be possible to separate the components by a physical process ie dissolving one out, boiling one away or even hand sorting. For example carbon is a black solid, air is a simple gaseous mixture essentially composed of around 1 part oxygen and 4 parts nitrogen. If carbon is burnt in air a chemical reaction will take place, the oxygen present combining with the carbon to form carbon dioxide, also a gas. The result will be another mixture but this time it will be a mixture of carbon dioxide and nitrogen – the carbon dioxide being a compound and the nitrogen an element. If required the carbon dioxide can be dissolved in water and so removed from the mixture.
Atoms and molecules An understanding of the difference between atoms and molecules is now required.
An atom is the smallest particle of any single element as found in nature which can be identified as forming part of any substance and taking part in a chemical reaction. When several atoms join together chemically, either with like atoms or those of other elements, to form another stable substance or compound, this is called a molecule. In this way although an element may exist as a molecule ie two or more atoms joined together, a compound must, by definition, comprise two or more atoms and so is always a molecule. These are the stable forms which we encounter under normal conditions.
Bonding of atoms The first element to consider is carbon. Carbon acts as a single atom and it has the chemical symbol C. Hydrogen, as found in nature comes in pairs. It has the symbol H but, because of this pairing, will usually be written as H2 when shown as taking part in chemical reaction equations. Similarly with oxygen, this is never found in the natural form as an atom but as a molecule consisting of two atoms. It’s symbol is O and in chemical equations will generally be represented as O2.
Chemical annotation Water is often referred to as H2O, that is the molecule is made up from two atoms of hydrogen – chemical symbol H and one atom of oxygen – chemical symbol O. To familiarise you with the way we write chemical equations the reaction of hydrogen and oxygen would be shown as 2H2 + O2 + 2H2O This equation indicates that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water. In reality this reaction does not take place immediately and the two elements can be mixed. But, once a spark or a flame is placed in the mixture, the reaction will take place immediately – in fact it will do so with explosive violence! So take warning! This is an opportunity to caution everyone to take care when mixing chemical
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substances. Always ascertain what might happen and by first consulting someone with experience. To further understand what is happening the molecule of water, H2O, can also be shown as: H—O—H
Valency Note that an oxygen atom has two points at which other atoms can be attached while hydrogen has only one. These points are called valency bonds and we say that oxygen has a valency of two while hydrogen has a valency of one.
Carbon compounds Let us now consider carbon. This has a valency of four – that is it can join up with four other valency bonds from other elements to form compounds. Carbon dioxide consists of one atom of carbon and two atoms of oxygen, this formula is written CO2 and shown structurally, O=C=O again each bar in this example represents a valency bond. Similarly methane is made up of one atom of carbon and four atoms of hydrogen ie CH4 or, when shown structurally:
H
H note the valency bonds.
Propane C3H8
H
H
H
C
C
C
C
H
H
H
H
H
Pentane Hexane Heptane Octane Nonane Decane Unodecane
C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 C11H24
Each substance having one carbon atom and two hydrogen atoms more than the one before it. Counting up the valency bonds on all these structural formulas, will show that they are all used up or satisfied. Note that various specific arrangements keep appearing and repeating and it is these arrangements which are used to typify the various compounds into special categories. All of the above compounds have the same basic formula C(n)H(2n+2) that is the number of hydrogen atoms is equal to twice the number of carbon atoms + two. They consist of only carbon and hydrogen.
Homologous series
The first members of this series are gases and then from pentane on they are liquid. Propane and butane are marketed as Liquefied Petroleum Gases (LPG) as they are easily compressed into a liquid and sold in the familiar gas bottles.
Other similar compounds are:
Ethane C2H6
H
This is known as a homologus series and, in this case, these are the aliphatic hydrocarbons or alkanes – (note – the names all end up with -ane). You will generally meet up with these as natural petroleum products or as derivatives from the petroleum refining industry.
H C
H
H
and so on with:
that is – two hydrogen atoms are connected to one oxygen atom. This is what is known as the chemical structural formula.
H
Butane C4H10
H
H
H
C
C
H
H
H
H
H
C
C
C
H
H
H
H
As the molecules gets bigger the compounds become liquids. They then begin to be more viscous eventually becoming pastes. As the size increases further the compounds are waxy solids and these get harder as the chain length increases.
H
The liquid members of this series we meet up with as solvents, the pastes and the solid members as greases and waxes.
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It is also possible to react these compounds with others and the part which is created is then called a radical. When shown as a radical the last hydrogen atom is omitted and the result is: Methyl radical Ethyl radical Propyl radical
CH3– C2H4– C3H7–
In the following formulae these radicals can be substituted whenever an R is shown. R1, R2, and so on, indicate different radicals.
Isomerism The next consideration is that of the phenomena of isomerisation. Isomers are compounds which have the same combination of atoms but have them arranged differently. Consider Butane.
Branched chain isomers The members of the series other than the straight ones are referred to as branched chain isomers. As an indication of the manner in which the structure can change as the size of the molecule increases, consider a member of the series, Hexane, and see how the number of isomers increases. All the substances which have a straight backbone are called normal and given the ‘n-’ prefix; the branched ones are given names to indicate the structure by numbering the carbon atoms in the main backbone, naming it as it would appear in the series and then naming the attached groups and indicating to which carbon they are attached. Examples are as follows:
Isomers of Hexane
C4H10 The formula we have given above is, in fact, that of normal Butane and when shown structurally would be as follows. (We use symbols for the carbon and the hydrogen atoms this time for simplicity and clarity).
n-Butane
n-Hexane
2-Methyl Pentane
Note that all the carbons are in a straight line. It is, however, possible for the carbons to have a branched form and would be as below.
2,3-Dimethyl Butane
iso-Butane
Note that this substance has exactly the same combination of atoms as the first, (n-butane), but, although it is very similar to the first, it can react in a different way and has slightly different properties. So much for the simple members of the group but it should be appreciated that, as the number of atoms in the compound increases and the chain length gets longer and longer, all sorts of combination will be possible in much the same way as it happens with a tinker toy or Lego set.
2,2-Dimethyl Butane
If the last one was turned sideways it could then be called 2.2-Methyl Ethyl Propane.
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This way of naming these compounds is useful to chemists by indicating how they will react with other substances and what the end product will look like.
Again the valency bonds can all be accounted for. For the sake of simplicity this is often shown in forms such as those below.
At this stage the principal concern is with their physical properties and so, when solvents are referred to as being iso- or secondary or tertiary it can be understood that these are different substances. They have the same basic chemical composition as the normal but are structurally different and with some differences in their properties.
or
Commercially many solvents are supplied as mixtures of isomers as there is no useful purpose served in separating them.
The next member of this series is Methyl Benzene or, as you may also have heard it called, by it’s more familiar name, – Toluene with the formula C6H5CH3 – or structurally:
As a point of interest, in the aliphatic series of hydrocarbons, the number of possible isomers for each is as follows: C8 C9 C10 C11 C12 C13 C14 C15 C20 C25 C30 C40
18 35 75 159 355 802 1858 4347 366 319 36 797 588 4 111 846 763 62 491 178 805 831
Methyl Radical
Benzene Radical
Putting the methyl radical into simpler form -CH3 permits the showing of Toluene in a simpler way. CH3
Aromatic Hydrocarbons The next homologous series for consideration also contains only carbon and hydrogen but, instead of being arranged in open chains, the atoms form a closed loop to which other groups of atoms can be attached. They are known as the Aromatic Hydrocarbons.
Once more we find a homologous series developing. The next member is Dimethyl Benzene, or as it is usually called Xylene.
These are distinguished by having, as a basic entity, a ring structure with six carbon atoms.
Methyl Radical
The first one in the series is Benzene C6H6 and this has the following structure
Benzene Radical
Benzene C6H6
Methyl Radical Again the problem of isomerisation arises – the above structure represents ortho-xylene (o-xylene) and there are two other possibilities, meta-xylene (m-xylene) and para-xylene (p-xylene).
5
CH3
CH3
CH3 CH3
CH3 orthoxylene
metaxylene
CH3 paraxylene
Following on with the logic the next chemical in the series will be
that different radicals are involved. These could be Methyl – , Ethyl – , Propyl – , Butyl – , Pentyl – and so on. Just for a bit of confusion the Ethyl – is sometimes called the Acetyl – radical and Pentyl – the Amyl – radical. These names are of historical significance rather than technical. Functional Group
Name
Actual example
R–OH
Alcohol
CH3OH Methyl alcohol
CH3 Tri-Methyl Benzene CH3
CH3
The above can be repeated with the ethyl radical C2H5 in one or more of the positions shown and then with the higher members – C3H7, C4H9 and so on. It should now be appreciated that permutations and combinations of the radicals are possible giving Ethyl Methyl Benzene, di-Ethyl Methyl Benzene and so on. As different radicals can be tacked onto any of the three points, this should give an appreciation of how many possible combinations there can be. These substances are called Higher Aromatics. As the size of the molecule increases so will the viscosity and so it is only some of the lower members that we meet up with as solvents.
Other solvent groups Until now the chemicals which have been dealt with have contained only hydrogen and carbon, these are referred to as ‘hydrocarbons’. The next compounds for consideration also contain another element, namely oxygen. These are often referred to as the oxygenated solvents. The oxygen can be bonded into the structure in several different ways and, depending upon this basic grouping, we classify each group. Again there will be variations following the homologous series we have mentioned earlier, this should become apparent later on when considering the members of each group. The different bonds, double and single, should be easily recognised and it should be remembered that where there is an R this refers to one of the hydrocarbon radicals. To recap. R1, R2 indicates
Ester
O || CH3–C–O–CH2–CH3 Ethyl acetate
O || R–C–R
Ketone
O || CH3–C–CH2–CH3 Ethyl methyl ketone
R–O–R
Ether
CH3–O–CH2–CH3 Methyl ethyl ether
Glycol ether
CH3–O–C2H4–OH
O || R–C–O–R
R–O–R–OH
Ethylene glycol methyl ether
Alcohols The simplest member of the Alcohol family is Methyl Alcohol and it has a structure as shown below. H | H — C — OH | H Go back and look at the structure of Methane. The similarity should be apparent at once, all that has happened is that we now have an atom of oxygen introduced into the chain or, to put it another way we have added an – OH group onto a Methyl (CH3–) radical. Chemical name Short name Methanol Methyl alcohol Ethyl alcohol Ethanol Propyl alcohol Propanol Butanol Butyl alcohol Pentyl alcohol* Pentanol Hexyl alcohol Hexanol Octyl alcohol Octanol * also known as Amyl alcohol
Formula CH3OH C2H5OH C3H7OH C4H9OH C5H11OH C6H13OH C8H17OH
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Examining the Aliphatic series again we find a range of similar compounds but this time they all have an – OH Group replacing an – H or tacking itself on to a radical. Obviously there are many Alcohols and the picture is complicated by the fact that there are many possible Isomers. In fact we have many more Isomers than just those arising from the branched chain iso-compounds because, in addition to the branching of the main backbone, the -OH group itself can be attached at different positions so giving further different compounds. This is demonstrated by looking at the Butyl Alcohol range where we have secondary, and tertiary Isomers.
Butyl Alcohols CH3CH2CH2CH2OH CH3
\ CH2CH2OH ⁄
Normal Butyl alcohol
Isobutyl alcohol
CH3 CH3CH2CHCH3 | OH CH3 \ CH3 – COH ⁄ CH3
Whilst in general terms the two radicals can be any of the hydrocarbon radicals we have mentioned earlier, consideration here will be with one series where one of these R’s’ is always a Methyl group. O || R—C—O—CH3
O || CH3—CH2—C—O—CH3
Acetate
Ethyl acetate
These are called Acetates and they are the most common esters employed as solvents. Once again, there is an homologous series following the Alcohols as given earlier. Chemical name Formula Ethyl acetate CH3CO2C2H5 Propyl acetate CH3CO2C3H7 Butyl acetate CH3CO2C4H9 Pentyl acetate* CH3CO2C5H11 Hexyl acetate CH3CO2C6H13 * also known as Amyl acetate These form a very useful range of solvents and are effective on a wide range of resins.
Secondary Butyl alcohol
Tertiary Butyl alcohol
There are other Esters and you may hear of them as Lactates, Phthalates, Maleates, Propionates. Although these also have solvent powers they are not used in the thinners type of application as they do not evaporate easily.
Ketones The general formula for Ketones is
Chemists, being great lovers of shorthand, will refer to these as n-Butyl Alcohol, IBA, sec-Butyl Alcohol and tert-Butyl Alcohol and at the same time use Butanol instead of Butyl Alcohol. In the case of the other Alcohols, their is also a variety of Isomers and each set will have similar properties, but this will not usually have much consequence in their use as solvents.
Esters The next group of concern is the Esters. Again there is oxygen along with carbon and hydrogen. The active group is O || — C—O— and is between two other radicals O || R1—C—O—R2
O || R1—C—R2 There are two radicals attached to the =C=O group. Ketones are good solvents and will dissolve almost all resin systems. Some of the Ketones you may come across in the paint industry are: O || Acetone (dimethyl ketone) CH3—C—CH3
Methyl ethyl ketone
Methyl isobutyl ketone
O || CH3—C—CH2—CH3 O || CH3—C—CH—CH3 | CH3
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Other Ketones of interest Another Ketones which is frequently used because of excellent solvent properties, combined with slow evaporation rate, is cyclohexanone. H2 H2 C—C ⁄ \ H2 C C=O \ ⁄ C—C H2 H2 Note that this is not Aromatic, there are no double bonds in the ring, it is one of the cyclic compounds which are in effect molecules wrapped around so that the ends join up. Just for good measure there is one hybrid solvent worthy of mention at this stage and that is diacetone alcohol. O CH3 || | CH3—C—CH—OH | CH3 It should be easy to identify the Ketone group and the Alcohol group in this molecule.
A Word of Warning Some very low boiling aliphatic hydrocarbons are often referred to as Petroleum Ethers. As this is both incorrect and misleading the use of this term should be discouraged. These solvent fractions are also often called Benzine (note the spelling). This is also incorrect as well as misleading due to its similarity to Benzene and its use must also be discouraged. in the extraction of organic substances from vegetable matter. The Ether group is also found in the structure of certain materials called surfactants which are used in the manufacture of emulsion paints. There are, however, some higher members of the group which have very useful solvent properties and evaporation rates and so makes them very acceptable as paint solvents. These chemicals are a mixture of an Alcohol and an Ether and are known as the glycol ethers.
Glycol Ethers and Esters To illustrate these consider the simplest member: CH3— O — C2H4 — OH
Ethers The general structure of Ethers is R1— O — R2 typical members of this group are Diemethyl Ether Methyl Ethyl Ether Diethyl Ethyl
CH2—O—CH2 CH2—O—C2H5 C2H5—O—C2H5
It should be be possible to work out others which can be expected. Dimethyl Ether (DME) has found use as aerosol propellant as it is a gas at normal temperatures but compresses into a liquid very easily while at the same time it is an excellent solvent so maintaining the paint liquid in the can. Diethyl Ether (ethyl ether) is the one which is often simply called ‘ether’. This, along with Methyl Ethyl Ether, has found use as an anesthetic. Being very volatile they do not find acceptance directly as solvents in the paint industry other than as mentioned. Because of their excellent solvent powers Ethers are very important industrial chemicals particularly
This substance is known as 2-methoxy-ethanol or to give it another name ethylene glycol (mono)methyl ether (note the mono is sometimes omitted). It can also be referred to by its initials EGMME or by a multitude of trade names; the most famous of which is Methyl Cellosolve. Note however that Cellosove is the registered trade mark of the Union Carbide Co. and, by rights, only their products should be referred to by that name. However, it is quite common to hear the word used as if it was the generic name. As a guide, other trade names include Dowanol Ektasolve Oxitol Sensolve
Dow Chemicals Eastman Kodak Shell Chemicals BP (NCP)
The naming and the structure should be obvious now, a radical attached by an ether link, (—O—), to another radical to which an Alcohol group is attached. Why a glycol? A glycol is in fact another form of alcohol but one with more than one hydroxyl (OH)
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group and here, in the case of ethoxy methanol, it can be said that the parent glycol is ethylene glycol with the formula: OH — C2H4 — OH where one of the — OH groups and has been replaced with an ether link to a methyl group. Methyl alcohol
+
CH3 — OH
Ethylene glycol OH — C2H4 — OH
gives Ethylene glycol methyl ether (Methoxy ethanol) CH3 — O — C2H4 — OH + H2O This group of solvents becomes more interesting as it progresses, the next variation is the di-glycol series. Again a simplest member illustrates this: CH3 — O — C2H4 — O — C2H4 — OH Diethylene Glycol Methyl Ether or – Methoxy-Ethoxy Ethanol Note that it starts out as an ether then has another ether bridge and ends up with an alcohol group, another example of, tinker toy or Lego chemistry. This compound also has a multitude of names including DGME and, again you will often hear it referred to as methyl carbitol. Note however, that ‘Carbitol’ is a trade mark of the Union Carbide Co. but the other producers have their own names such as methyl dioxitol (Shell). As a matter of interest, due to the fact that they may have long term health problems, the use of ethyl and methyl versions of this group has been drastically reduced. Some companies have eliminated them from their range. Even the Buty – versions are suspect and attempts to replace them with the Propyl – version are being made.
Glycol Ether Esters As was the case earlier when it was shown that an Alcohol could be converted to an Ester, a similar reaction with these solvents is possible and a series of Esters is possible, the simplest examples is: CH3 — O — C2H4 — COO — CH3 Ethylene Glycol Methylether Acetate
Being Esters they have different solvent powers. The naming of these solvents is fairly complex and especially when isomers are concerned. It is common for ethylene glocol methyl ether to be named ethylene glycol monomethyl ether so as to ensure there is no confusion with the dimethyl. Just ensure that you are aware of which one it is. The present tendency especially in Europe is to use the “2–methoxy ethanol” style wherever possible.
Physical properties In paint formulation it is the physical properties of solvents which are important and not so much the chemical properties. The chemical grouping is important because, if a given resin is soluble in one particular type of solvent or combination of solvents, then generally any solvents in the particular group will show the same characteristics. Thus if a resin is soluble in acetone it will almost certainly be soluble in other ketones. As a general rule it is also the case that the lower members of any homologous series will give a lower viscosity, on a volume to volume basis, than that given by the higher members. In other words the cutting power (ability to reduce viscosity) will be greater for the lower members. At the same time the evaporation rate, (the rate at which the solvent leaves the films) will change with the size of the solvent molecules (number of carbons in the backbone) in any homologous series. Such properties as Boiling Point, and Flash Point will also follow a similar pattern but remember that this is generally confined again to the homologous series.
Specific gravity It is the custom, in many companies, to use weight in their formulations rather than volume. The reason for this lies in the fact that the specific gravity of solvents varies. That is, a litre of one solvent will not weigh the same as a litre of another. At the same time volume is temperature dependant, that is to say that on heating a liquid it will expand, this in turn means that the volume of a given weight will increase with temperature. It therefore, follows that, if volume is to be used as a measure, then the temperature will need to be controlled for all substances used. In the laboratory the weight method has generally become the preferred procedure. In the factory
9
volumes are often used, especially for solvents, some operators using a standard conversion, others relying upon the final adjustment to make up differences. In general, paints, while being sold by volume, are packed by weight for operating convenience and accuracy. Whether the volume or weight method is used is not of such great importance as that of understanding the implication and taking the correct precautions. The paint formulator must therefore bear the concept in mind when substituting solvents and allow for differences in specific gravity. Also remember that the non-volatile content of a paint in generally expressed in terms of weight and so is influenced by the specific gravity of the solvent.
Boiling point With regard to boiling point; pure chemicals have a fixed boiling point but mixtures will have a range. The mixture will usually start to boil at or around the boiling paint of the lowest one in the mix and the final (dry point) will be around that of the highest component. For any mixture of solvents these two figures will not change to any great extent and so the information is somewhat limited. The exact composition of mixtures can only be compared properly by considering the distillation range. This is given as the percentage of liquid which distils over at defined temperatures. By comparing the ranges of two mixtures the composition of two mixtures can be compared.
Evaporation rate This effect also applies to evaporation rate. The evaporation rate is a fixed figure which only really tells us the slowest member. For this data to be useful it should be read in conjunction with the distillation range. The evaporation rate may also be affected by the solvent affinity of some resins. Different resins have different release rates for certain solvents. In this case a solvent entrapment or slowing down of
speed of set of the coating can be the result. Do not be surprised if a coating does not follow the path expected from the evaporation rate. At the same time do not be surprised if a coating which appeared to dry will becomes brittle after a few days, this can be the result of solvent affinity and so what we have is a temporary plasticiser.
Flash point With regard to flash point this will generally depend upon that of the lowest member of the mixture. But a word of warning, the flash point of some mixtures can be lower than the expected one. An example where this can happen is with butanol and xylol. If this is near the cut off point used in legislation, it is better to carry out a final check on the finished paint.
Conclusions Solvents are in general the tools of the trade for paint formulators. They are hammers, screwdrivers and spanners. They do not remain in the finished coating but instead provide a way of getting the coating into the required place in the required way. Just as there are different sizes and types of hammers, screwdrivers and spanners so there are different types of solvent groups. Having decided just what type of spanner is needed to adjust a particular fastener we next need to know what size it is. Using the wrong one could damage the fastener and so weaken the whole structure. So it is with solvents, being near enough is not always good enough and, as the skilled engineer must know how to judge and pick the correct tool and apply it properly, so must the paint formulator know his solvents. Aliphatics, Aromatics, Esters, Ketones and are the hammers, screwdrivers and spanners of the paint chemist. Knowing how to use them properly is the most important skill of the paint formulator. Note that when the job is finished the tools are taken away in the same way as the solvents. Any
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Part 2 – Thinners and Solvents Liquid paints consist essentially of two parts, the solids which will be left after it has dried and the volatile portion which evaporates during and after application and thereafter plays no further part in the action. During this initial phase, as the solvent evaporates, the applied paint increases in viscosity. If this takes place too quickly it will be difficult to apply and will not flow out to give the desired finish, if this takes place too slowly sags and runs will be the result. For these reasons the volatile component has to be carefully chosen. This volatile part is referred to as the thinner or solvent and, if the paint has to be applied by brush or roller, it will generally be slower evaporating than the one needed for spray application. This thinner/solvent is only required for application purposes, is completely disposed of and hence to some extent fills the same role as the container. In other words it gets the paint from the manufacturer and onto the job where it is then thrown away. What is therefore required is the most cost effective method of getting the proper thickness of paint onto the job but, as these liquids are Volatile Organic Components (VOC’s), they also have environmental side effects which must be kept in mind. What is the difference between a solvent and a thinner? A solvent, as we know it in the paint industry is, almost without exception, a thinner. A thinner, on the other hand, is not always a true solvent. In others words if you mix a thick liquid with a thin liquid then the thin liquid can be described as the thinner. In simple terms it will reduce the viscosity. In the case of emulsion paints the thinner is water but it is in no way a solvent for the system as a whole! When a liquid can be mixed with another, but is not a solvent, it is referred to as a diluent. This is only a physical mixture of two liquids. In some cases, when further additions of the thinner are made a situation could arise when the mix begins to curdle and throw out. An example of this would be an automotive lacquer which will only take a limited quantity of white spirit before curdling. This problem can become obvious when it comes to washing the equipment. In the case of the emulsion paint the equipment can be washed with
water because the paint will accept unlimited addition of water. This is because it is in a dispersion and not in a solution! Washing equipment used to apply industrial paints calls for a careful choice of a suitable agent – the thinner might do the job easily but it is not necessarily so. There are cases where the thinner is not the best cleaner. After all the thinner was formulated with application properties in mind. A gun/equipment cleaner may be a more economical and better proposition; especially when formulated for the job.
Solutions and dispersions Generally liquid coatings are of two types, solution based or dispersion based. Solvent based coatings will be based upon a solution of polymeric or polymerisable material(s) whilst, in the case of the dispersion, the solid or semi-solid particles are suspended in another liquid. As mentioned above the objective in both cases is to make the paint sufficiently fluid so that it can be applied easily. By definition a solution is a homogeneous mixture of two or more substances. The dissolving medium is called the solvent, and the dissolved material is called the solute. In most common solutions, the solvent is a liquid, and the solute may be a solid, gas, or liquid. Syrups are solutions of sugar, a solid, dissolved in water. Soda water is a solution of carbon dioxide, a gas, in water, and Vinegar is a solution of acetic acid, a liquid, in water. A solution is not necessarily liquid, it can be solid – certain plastics are examples of this and so are some coloured glass objects – for example sunglasses. In other words the resultant mixture will appear as one and will result in a clear (coloured maybe, but transparent) product. This is referred to as a single phase system. In the case of solution materials the bigger the polymer, the bigger the individual molecules and consequently very viscous solutions are the result. This means that the amount of polymer in the solution can be low with the result that the film of the paint/lacquer, after the solvent has evaporated, will be very thin. To give an adequate
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coating under these circumstances will usually mean that several coats will need to be applied. In the case of the dispersion resins (the most common and best known are usually referred to as emulsions) relatively big molecules or particles are involved. If these were to be used in solution form, they would give high viscosity materials of no practical value. By suspending small particles of these resins in a water phase we can produce a liquid material which will permit a high solids film. In this case the resin particles usually need to have a small quantity of solvent added to them to make them stick or coalesce together. This solvent, which evaporates after the water has left the film, is referred to as a coalescent. As a point of interest, note that the term emulsion resins comes from the method of manufacture – they are not true ‘emulsions’ but are actually latices (latexes). By general definition a latex is a dispersion of a solid in a liquid while an emulsion is the dispersion of a liquid in a liquid. In the emulsion polymerisation process the monomer(s) from which the polymer is made are emulsified first and then converted into the final product. The speed at which a solvent leaves a system is something which must be given careful consideration. Resins and polymers have a variable tendency to hold on to solvents and some can be retained by the film for some time. When solvent is retained it acts as a temporary plasticiser and so the film can appear to be quite flexible but after a period the film may become brittle and lose its flexibility. Always check for this. This effect is often noticeable in the form of the smell it gives off if kept in an enclosed space. It also accounts for the solvent smell which come from certain water-based paints and which can be noticed after the room has been left closed for some time – quite a problem for hotels who want a quick turnaround of their rooms during maintenance. This affinity for solvents varies with the resin system and the solvent, in other words some classes of solvent are retained more easily by some systems than by others. Care must also be given to the problem of case hardened coatings. Often the surface of the paint can dry more quickly than the underlying layers thus trapping solvent. The use of anti-skinning aids in convertable systems often proves necessary but the use of small quantities of high boiling point (slow evaporating) solvents may also prove necessary.
Classes of solvents A true solvent is one which will satisfy the criteria in all proportions. There is sometimes a limit to the amount of substance which can go into a solution. If salt is added to water it will dissolve. If, however, the addition of salt is continued a point will be reached when no more can be dissolved; this is known as the saturation point. In the case of salt the solid would settle out as white crystals. We would then have a two phase system – one solid phase (the salt) and one liquid phase (the salt solution). This is referred to as a saturated salt solution – no more salt can dissolve in the water. In the case of liquids like oil and water they would form two phases – both liquid. In the case of certain resins these will only partially dissolve or swell when mixed with certain solvents but on addition of a second solvent a true single phase solution can result. When such a mixture of solvents is used and a true solution is obtained, these solvents are referred to as co-solvents or latent solvents. In effect what is happening is that part of the resin molecule dissolves in one solvent and another part in the other solvent or we have a mixture of two solutions which are compatible. It is also quite usual in the surface coating industry to make binders from two or more resin systems each with its own solubility parameters. This often leads to the need to use a mixture of solvents to get a clear homogenous product, or, as we term it, compatibility. This compatibility must be maintained until all the solvent/thinner has left the coating. Certain solvents and thinners can consist of a mixture of substances. One way of telling whether it is a mixture or not is to look at its boiling point. If this is a single number, or a range of two numbers close together, then it will almost certainly be a pure, or relatively pure, substance. If there are two numbers, a range, then it will be a mixture. Always consider the boiling point and the evaporation rate when comparing solvents, The evaporation rate alone, being dependant upon the higher boiling component, can be misleading.
Diluents In certain cases, once a solution has been made, then another liquid can be added and the result will be a clear homogenous solution. If, however, on further dilution, there is a separation into two phases shown either by a clouding of the mix or by a separation into two layers then the liquid
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being added is a diluent. For these reasons it is important to ensure that the diluents leave the film before the true solvents i.e. have a faster evaporating rate Diluents are sometimes referred to as a means of cheapening paints but let us be quite clear that, whilst some unscrupulous formulators may use them in this way, it is not to be accepted as a general rule. These formulators make use of the fact that diluents do not generally reduce the viscosity as quickly as solvents and so they deliver a product with a lower solid content. As it is the solids which give the protection a proper balance must be kept. Diluents, properly used, play a very important part in the formulation of thinners and their use should never be decried. When used correctly they can produce more cost effective formulations and be more environmentally friendly and less hazardous to health. As an example consider a paint which is to be applied by spray. The purpose of the thinner is to reduce the viscosity to a point where it will atomise properly. As some 30% of the solvent can evaporate between the nozzle of the gun and the surface to which it is applied, one part of the thinner can be considered to have done it’s job as soon as the paint leaves the gun. Providing this has no other side effects such as blooming, then this component of the thinner only needs to be a diluent. A fractionally heavier spray setting to get the correct film build can give compensation but, in the case of pigmented coatings, obliteration will dictate the thickness of the applied film. If the correct solvent balance is built into the original paint there should be no problem but it is advisable to buy thinners from the paint manufacturer or conduct careful tests as to their suitability. If a high proportion of diluent is already built into the original paint, thinner selection can be critical. In the case of aerosol containers the choice of propellant for paints is usually Liquefied Petroleum Gas (LPG) which can be propane or butane (in South Africa the commercial available version is a mixture). On a cost/availability basis the choice is not difficult. Other propellants are di-methyl ether or fluorocarbons but, due to environmental problems, the latter is now only used in very specialised cases. Di methyl ether is favoured, by the cosmetic industry and other specialised end users, when LPG is either undesirable or unsuitable. So far as the paint industry is concerned LPG is simply a low boiling aliphatic solvent and so serves the dual purpose of being a propellant as
well as a diluent. Although, in some cases, the aliphatic might be a latent solvent, the term ‘diluent’ is probably more appropriate here as it plays that role perfectly. The paint system used must be aliphatic compatible and this can be achieved by the incorporation of strong solvents in the orignal base paint. Compatibility tests on the resin system will be needed and, if the paint can stand dilution with pure pentane or hexane to reduce it to spraying viscosity, then it will probably will accept LPG. The LPG is usually injected into the can after the required amount of paint has been filled and the container capped but cans may also be prefilled with propellant and the paint injected into the can. This latter procedure was used for automotive touch-up paints. As a point of interest consider the Soda Water siphon which is, in effect, an aerosol. Here the propellant (a gas – carbon dioxide) is dissolved in the water and is maintained in solution by confining it to its receptacle, keeping the gas under pressure. So, as is the case with the aerosol, the pressure is sufficient to force the contents out of the container when the valve is opened.
General considerations The pigments and extenders in coatings are insoluble materials and are dispersed in the paint whether it be a solution or a dispersion resin system. They will therefore interfere with the system and it is always advisable to examine the binder system without these present to ensure a proper film is formed. it is also important that the resin systems themselves remain compatible after all the solvents/diluents have left and, for these reasons, tests must always be conducted on the unpigmented system to ensure that the residual film is compatible. Whether using a diluent or a solvent in any system the dry unpigmented film should be as clear as possible once formed or coalesced properly, if it is not, an underbound friable coating may be the result. It should be borne in mind that all solvents and thinners are VOC’s and are becoming a cause for global pre-occupation. This is due to the contribution they are making to photo-chemical smog, high atmospheric ozone levels (not to be confused with the Ozone Layer problems), and global warming. For these reasons legislation is being introduced in many parts of the world to
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regulate the quantity of these substances which is emitted to the atmosphere.
Ozone – what is it? There appears to be a certain amount of confusion concerning the role of solvents and their relationship to problems of the Ozone Layer, global warming amongst other environmental issues. These notes are aimed at clarifying the position. Oxygen can exist in two molecular forms O2 or O3 – that is either two or three atoms in the molecule. Ozone has three atoms and is relatively unstable returning to the more stable O2 form easily. As it gives up its third atom very easily it is a strong oxidising agent and, as such, a potent bleach and disinfectant. It is active in the breakdown of plastics and surface coatings. It has a distinctive smell and can be detected in a concentration of 0.01ppm in air. It is highly poisonous at a concentration of 0.1 ppm. It is about a hundred times as poisonous as carbon monoxide! Ozone is formed in the air around us by the action of ultraviolet light. In rural areas it may reach a level of 0.02 to 0.03 ppm. In cities, there is usually less because there is less sunlight – unless there are certain chemical impurities in the air. The greatest effect is from oxides of nitrogen NOx coming predominantly from exhaust emissions. This is broken down by ultraviolet rays to give nitric oxide, NO, and atomic oxygen O. The atomic oxygen then reacts immediately with molecular oxygen to form ozone. Under normal circumstances this ozone reacts again with the NO to give NO2 and the cycle keeps repeating and a stable state should result. However the presence of hydrocarbons interferes with this second reaction by scavenging the nitric oxide thus increasing the levels of nitrogen peroxideand ozone in the atmosphere. This results in photochemical smog and concentrations of ozone as high as 0.5 ppm (above the danger zone) have been reported on smoggy days. Whilst it was first thought that only hydrocarbons were responsible, it has now been found that, to a greater or lesser extent, all volatile organic substances take their part – hence the concern with solvents. It is of interest to note that many people think that the need for lead free petrol is to reduce lead pollution in the air. While this may be true to some extent, the principle role of lead free fuel is to minimise the poisoning of the catalyst used in the
exhaust system to cut down hydrocarbon emissions. When fluorocarbons first came into use it was felt that their great advantage lay in their relatively stable nature. They are not broken down in the troposphere (the first 10Kms. of air above us). Instead they migrate to the upper level of the stratosphere (50 Kms.) where the ozone layer is found. Here the ultraviolet breaks down what little oxygen there is to form ozone. The reaction of CFCs with the ozone means that the total oxygen/ozone levels are depleted and this allows the penetration of ultraviolet light to the earth’s surface. The presence of the ozone layer, by preventing ultraviolet rays penetrating, has made life, as we know it, possible. In addition to other side effects the penetration of ultraviolet rays gives rise to the high ozone at ground level as discussed earlier. The high levels of tropospheric ozone cannot cancel out the imbalance. Ozone is not only unstable but there is also no mixing of the tropospheric and stratospheric layers Ozone, carbon dioxide, methane, halogenated solvents and oxides of nitrogen, because they all retain heat, contribute to global warming and the greenhouse effect. The two ozone problems are different but the hole in the ozone layer and the greenhouse effect is interrelated. Solvents and vehicle emissions alone are not the only source of VOCs. Natural sources include pine needles, gum trees and camphor as well as natural oil and gas wells. There is also a considerable production of Methane from natural decomposition and the digestive processes of ruminants. This is not classed as a VOC. This is nevertheless, quite a cause for concern – especially when we hear of forests being burnt to provide beef pasture. Hydrocarbon emissions from automobiles have, as we have mentioned, been targeted. Efforts are being made, to a greater or lesser extent, to reduce the venting of tanks to the atmosphere. This depends, to a great extent on the smog situation and the action of pressure groups. The use of solvents could be reduced even with present day technology. One of the reasons for the slow transition being that many people look at the price rather than the cost. Our industry must face up to the fact that this problem must not be allowed to get worse – it is certainly not going to go away. ■
Titles in the Student Monograph Series
No. No. No. No. No. No. No. No. No.
1 2 3 4 5 6 7 8 9
Basic Science for Students of Paint Technology Corrosion Water-borne Resins Colour Physics Dispersion and Dispersion Equipment Additives for Water-borne Coatings Standards Water-borne Coatings The Basics of … Solvents and Thinners
Honorary Technical Education Officer: A T Hopgood FTSC Honorary Editor: P S Thukral PhD CChem FTSC Chairman Special Publications Committee: R H E Munn BSc LRSC FTSC Oil & Colour Chemists’ Association 1st Floor, 3 Eden Court, Eden Way, Leighton Buzzard, LU7 4FY, UK Tel: 01525 372530 Fax: 01525 372600 Email:
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