The Basics Of - Solvents and Thinners.pdf

8/11/2019 The Basics Of - Solvents and Thinners.pdf

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The Basics of

Solvents and ThinnersL A Fisher FTSC

Oil & Colour ChemistsAssociation

OCCA Student Monograph No. 9


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Publications

Surface Coatings International

Regional Activities

SURFEX

Professional Qualifications

Surface Coatings Handbook

Conferences

Surface Coatings International Bulletin

Oil & Colour Chemists Association1st Floor, 3 Eden Court, Eden Way, Leighton Buzzard, LU7 4FY, UK

Tel: 01525 372530 Fax: 01525 372600Email: [email protected]

OCCA


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The Basics of


Oil & Colour ChemistsAssociation

OCCA Student Monograph No. 9


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Foreword

This Monograph is the first in a new series of Student Monographs published by the Oil & ColourChemists 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 thesurface coatings industry and, specifically, for operatives and trainees in Southern Africa and the IndianSub-Continent. Companies, Trade Associations, Professional Bodies and the Authorities have identifiedthis sector as lacking educational resources and the Oil & Colour Chemists Association is pleased torespond to this need.

The series has been published with the assistance of a grant by the Trustees of the Ellinger-GardonyiFund, an educational trust, administered by the Oil & Colour Chemists Association and is supportedfrom resources provided by its South African Division from the funds generated by SURFEX SouthernAfrica.

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 KingdomTypeset by My Word!, PO Box 4575, Rugby, CV21 9EH, United Kingdom


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The Basics of


Introduction

Working in the so called third world countries is an education in itself. One of the things I came to realizewas 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 nationsanybody with the brains and/or gumption can get an education. Hence by a natural selection systemthere is a stratification. At the same time the type of education is also at variance; going from the farmschool with 30 pupils in 10 grades and one teacher, to the specialised schools with the capacity andthe staff to provide a broad curriculum. A further problem is that, as we develop, the basic qualificationsrequired also change 50 years ago a matric was a good starting point whereas industry can, anddoes, now require a degree for the same job, thats progress. The result of this is that the first thirdworld gap is increasing.

In South Africa, as is also the case in Latin American and Asian countries, we have a strange mixture offirst and third world. This means that the education tends to be fairly rudimentary for the less privilegedand 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 painttechnology, 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 lastNational Matric exams, the first post-apartheid ones in which students all wrote the same exam, oneofficial 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 ofthese monographs is to help our students to bridge the gap. It is hoped that, by explaining the basicconcepts, in the least complicated manner, emphasising only the points they really need to know, theywill 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 rememberthat the objective was to drain the swamp. At least let us attempt to extract some of the crocodilesteeth.

About the author

Les Fisher has worked for over 40 years in a technical capacity in the paint, synthetic resin, GRP andadhesive 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 theidentification 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 PaintManufacturers Paint Technology Course in the Durban area.


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Part 1 Elementary Solvent Chemistry 1Organic 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 10Class of Solvents 11

Diluents 11

General Considerations 12

Ozone - what is it? 13

Contents


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Organic chemistryThe chemistry to be dealt with here is referred toas organic chemistry. This is a very complexscience and all living organisms are covered by itas well as a host of others. The basis of allchemicals in this class is that they contain theelement Carbon in combination with others. Inorder to keep things uncomplicated the firstchemical compounds dealt with will only involvethree elements: carbon, hydrogen and oxygen and,as will be shown, it is in the permutations andcombinations of these that gives the differences

that make up most of the chemicals called solventsand which are of concern to us at present.

Elements, compounds and mixturesThe first requirement is an understanding of whatis meant by an element and a compound.

An element is a single substance which cannotbroken down any further by normal chemistry, itwould require drastic action in a nuclear reactor tochange it. A compound is a chemical combinationof two or more elements as opposed to a simple

mixture. The properties of a compound aredistinct for that compound and differ from thoseof the elements from which it is derived. Acompound cannot be converted back to itsoriginal components by physical means.

A mixture would combine the properties of thesubstances from which it was prepared and it willbe possible to separate the components by aphysical process ie dissolving one out, boiling oneaway or even hand sorting. For example carbon isa black solid, air is a simple gaseous mixture

essentially composed of around 1 part oxygenand 4 parts nitrogen. If carbon is burnt in air achemical reaction will take place, the oxygenpresent combining with the carbon to form carbondioxide, also a gas. The result will be anothermixture but this time it will be a mixture of carbondioxide and nitrogen the carbon dioxide being acompound and the nitrogen an element. Ifrequired the carbon dioxide can be dissolved inwater and so removed from the mixture.

Atoms and moleculesAn understanding of the difference between atomsand molecules is now required.

An atom is the smallest particle of any singleelement as found in nature which can be identifiedas forming part of any substance and taking partin a chemical reaction. When several atoms jointogether chemically, either with like atoms orthose of other elements, to form another stablesubstance or compound, this is called a molecule.

In this way although an element may exist as amolecule ie two or more atoms joined together, acompound must, by definition, comprise two ormore atoms and so is always a molecule. Theseare the stable forms which we encounter undernormal conditions.

Bonding of atomsThe first element to consider is carbon.

Carbon acts as a single atom and it has the chem-ical symbol C. Hydrogen, as found in nature comesin pairs. It has the symbol H but, because of thispairing, will usually be written as H2 when shown astaking part in chemical reaction equations. Similarlywith oxygen, this is never found in the natural form

as an atom but as a molecule consisting of twoatoms. Its symbol is O and in chemical equationswill generally be represented as O

2.

Chemical annotationWater is often referred to as H

2O, 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 chemicalequations the reaction of hydrogen and oxygen

would be shown as

2H2

+ O2

+ 2H2O

This equation indicates that two molecules ofhydrogen react with one molecule of oxygen toform two molecules of water.

In reality this reaction does not take placeimmediately and the two elements can be mixed.But, once a spark or a flame is placed in themixture, the reaction will take place immediately in fact it will do so with explosive violence!

So take warning! This is an opportunity to cautioneveryone to take care when mixing chemical

Part I Elementary Solvent Chemistry


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substances. Always ascertain what might happenand by first consulting someone with experience.

To further understand what is happening themolecule of water, H

2O, can also be shown as:

H O H

that is two hydrogen atoms are connected toone oxygen atom. This is what is known as thechemical structural formula.

ValencyNote that an oxygen atom has two points at whichother atoms can be attached while hydrogen hasonly one. These points are called valency bondsand we say that oxygen has a valency of two whilehydrogen has a valency of one.

Carbon compoundsLet us now consider carbon. This has a valency offour that is it can join up with four other valencybonds from other elements to form compounds.

Carbon dioxide consists of one atom of carbonand two atoms of oxygen, this formula is writtenCO

2and shown structurally,

O=C=O

again each bar in this example represents a

valency bond.

Similarly methane is made up of one atom ofcarbon and four atoms of hydrogen ie CH

4or,

when shown structurally:

note the valency bonds.Other similar compounds are:

and so on with:

Pentane C5H

12

Hexane C6H

14

Heptane C7H

16

Octane C8H

18

Nonane C9H

20

Decane C10

H22

Unodecane C11

H24

Each substance having one carbon atom and twohydrogen atoms more than the one before it.

Counting up the valency bonds on all thesestructural formulas, will show that they are allused up or satisfied.

Note that various specific arrangements keepappearing and repeating and it is thesearrangements which are used to typify the variouscompounds into special categories.

All of the above compounds have the same basicformula

C(n)

H(2n+2)

that is the number of hydrogen atoms is equal totwice the number of carbon atoms + two. Theyconsist of only carbon and hydrogen.

Homologous seriesThis is known as a homologus series and, in thiscase, these are the aliphatic hydrocarbons oralkanes (note the names all end up with -ane).You will generally meet up with these as naturalpetroleum products or as derivatives from thepetroleum refining industry.

The first members of this series are gases andthen from pentane on they are liquid. Propane andbutane are marketed as Liquefied PetroleumGases (LPG) as they are easily compressed into aliquid and sold in the familiar gas bottles.

As the molecules gets bigger the compoundsbecome liquids. They then begin to be moreviscous eventually becoming pastes. As the sizeincreases further the compounds are waxy solidsand these get harder as the chain length increases.

The liquid members of this series we meet up withas solvents, the pastes and the solid members asgreases and waxes.

H HC

H

H

C

H

H

C

H

H

C

H

H

Butane C4H10

H HC

H

H

C

H

H

C

H

H

Propane C3H8

H HC

H

H

C

H

H

Ethane C2H6

H C

H

H

H


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It is also possible to react these compounds withothers and the part which is created is then calleda radical. When shown as a radical the lasthydrogen atom is omitted and the result is:

Methyl radical CH3

Ethyl radical C2H4Propyl radical C

3H

7

In the following formulae these radicals can besubstituted whenever an R is shown. R

1, R

2, and

so on, indicate different radicals.

IsomerismThe next consideration is that of the phenomenaof isomerisation.

Isomers are compounds which have the same

combination of atoms but have them arrangeddifferently. Consider Butane.

C4H

10

The formula we have given above is, in fact, thatof normal Butane and when shown structurallywould be as follows. (We use symbols for thecarbon and the hydrogen atoms this time forsimplicity and clarity).

Note that all the carbons are in a straight line.

It is, however, possible for the carbons to have abranched form and would be as below.

Note that this substance has exactly the samecombination of atoms as the first, (n-butane), but,although it is very similar to the first, it can reactin a different way and has slightly differentproperties.

So much for the simple members of the group butit should be appreciated that, as the number ofatoms in the compound increases and the chainlength gets longer and longer, all sorts ofcombination will be possible in much the sameway as it happens with a tinker toy or Lego set.

Branched chain isomersThe members of the series other than the straightones are referred to as branched chain isomers.

As an indication of the manner in which the structurecan change as the size of the molecule increases,

consider a member of the series, Hexane, and seehow the number of isomers increases.

All the substances which have a straight backboneare called normal and given the n- prefix; thebranched ones are given names to indicate thestructure by numbering the carbon atoms in themain backbone, naming it as it would appear inthe series and then naming the attached groupsand indicating to which carbon they are attached.Examples are as follows:

Isomers of Hexane

If the last one was turned sideways it could thenbe called 2.2-Methyl Ethyl Propane.

2,2-DimethylButane

2,3-DimethylButane

2-MethylPentane

n-Hexane

iso-Butane

n-Butane


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This way of naming these compounds is useful tochemists by indicating how they will react with othersubstances and what the end product will look like.

At this stage the principal concern is with theirphysical properties and so, when solvents are

referred to as being iso- or secondary or tertiaryit can be understood that these are different sub-stances. They have the same basic chemical com-position as the normal but are structurally differentand with some differences in their properties.

Commercially many solvents are supplied asmixtures of isomers as there is no useful purposeserved in separating them.

As a point of interest, in the aliphatic series ofhydrocarbons, the number of possible isomers foreach is as follows:

C8 18C9 35C10 75C11 159C12 355C13 802C14 1858C15 4347C20 366 319C25 36 797 588C30 4 111 846 763

C40 62 491 178 805 831

Aromatic HydrocarbonsThe next homologous series for considerationalso contains only carbon and hydrogen but,instead of being arranged in open chains, theatoms form a closed loop to which other groupsof atoms can be attached. They are known as theAromatic Hydrocarbons.

These are distinguished by having, as a basicentity, a ring structure with six carbon atoms.

The first one in the series is Benzene C6H

6and

this has the following structure

Again the valency bonds can all be accounted for.

For the sake of simplicity this is often shown informs such as those below.

The next member of this series is Methyl Benzeneor, as you may also have heard it called, by itsmore familiar name, Toluene with the formulaC

6H

5CH

3 or structurally:

Putting the methyl radical into simpler form -CH3

permits the showing of Toluene in a simpler way.

Once more we find a homologous seriesdeveloping.

The next member is Dimethyl Benzene, or as it isusually called Xylene.

Again the problem of isomerisation arises theabove structure represents ortho-xylene (o-xylene)

and there are two other possibilities, meta-xylene(m-xylene) and para-xylene (p-xylene).

BenzeneRadical

Methyl

Radical

MethylRadical

CH3

BenzeneRadical

Methyl

Radical

or

BenzeneC6H6


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Following on with the logic the next chemical inthe series will be

The above can be repeated with the ethyl radical -C

2H

5in one or more of the positions shown and

then with the higher members C3H

7, C

4H

9and

so on.

It should now be appreciated that permutationsand combinations of the radicals are possiblegiving Ethyl Methyl Benzene, di-Ethyl MethylBenzene and so on. As different radicals can betacked onto any of the three points, this shouldgive an appreciation of how many possiblecombinations there can be.

These substances are called Higher Aromatics.As the size of the molecule increases so will theviscosity and so it is only some of the lowermembers that we meet up with as solvents.

Other solvent groupsUntil now the chemicals which have beendealt with have contained only hydrogen andcarbon, these are referred to as hydrocarbons.The next compounds for consideration alsocontain another element, namely oxygen. These

are often referred to as the oxygenatedsolvents.

The oxygen can be bonded into the structure inseveral different ways and, depending upon thisbasic grouping, we classify each group. Againthere will be variations following the homologousseries we have mentioned earlier, this shouldbecome apparent later on when considering themembers of each group.

The different bonds, double and single, should beeasily recognised and it should be remembered

that where there is an R this refers to one of thehydrocarbon radicals. To recap. R

1, R

2indicates

that different radicals are involved. These couldbe Methyl ,Ethyl , Propyl , Butyl ,Pentyl and so on.

Just for a bit of confusion the Ethyl issometimes called theAcetyl radical and

Pentyl theAmyl radical. These names areof historical significance rather than technical.

AlcoholsThe simplest member of the Alcohol family isMethyl Alcohol and it has a structure as shownbelow.

Go back and look at the structure of Methane. Thesimilarity should be apparent at once, all that hashappened is that we now have an atom of oxygenintroduced into the chain or, to put it another waywe have added an OH group onto a Methyl(CH

3) radical.

Chemical name Short name FormulaMethyl alcohol Methanol CH

3OH

Ethyl alcohol Ethanol C2H

5OH

Propyl alcohol Propanol C3H

7OH

Butyl alcohol Butanol C4H

9OH

Pentyl alcohol* Pentanol C5H

11OH

Hexyl alcohol Hexanol C6

H13

OHOctyl alcohol Octanol C

8H

17OH

* also known as Amyl alcohol

H|

H C OH|H

Functional Name Actual exampleGroup

ROH Alcohol CH3OH

Methyl alcohol

O O|| ||

RCOR Ester CH3COCH

2CH

3

Ethyl acetate

O O|| ||

RCR Ketone CH3CCH

2CH

3

Ethyl methyl ketone

ROR Ether CH3OCH

2CH

3

Methyl ethyl ether

ROROH Glycol ether CH3OC

2H

4OH

Ethylene glycolmethyl ether

Tri-Methyl Benzene

CH3

CH3CH3

ortho-xylene

meta-xylene

para-xylene

CH3

CH3

CH3

CH3

CH3

CH3


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Examining the Aliphatic series again we find arange of similar compounds but this time they allhave an OH Group replacing an H or tackingitself on to a radical.

Obviously there are many Alcohols and the picture

is complicated by the fact that there are manypossible Isomers. In fact we have many moreIsomers than just those arising from the branchedchain iso-compounds because, in addition to thebranching of the main backbone, the -OH groupitself can be attached at different positions sogiving further different compounds. This isdemonstrated by looking at the Butyl Alcohol rangewhere we have secondary, and tertiary Isomers.

Butyl Alcohols

CH3CH2CH2CH2OH Normal Butyl alcohol

CH3\CH

2CH

2OH Isobutyl alcohol

CH

3

CH3CH

2CHCH

3Secondary Butyl alcohol

|OH

CH3

\CH3 COH Tertiary Butyl alcohol

CH3

Chemists, being great lovers of shorthand, willrefer to these as n-Butyl Alcohol, IBA, sec-ButylAlcohol and tert-Butyl Alcohol and at the sametime use Butanol instead of Butyl Alcohol.

In the case of the other Alcohols, their is also avariety of Isomers and each set will have similar

properties, but this will not usually have muchconsequence in their use as solvents.

EstersThe next group of concern is the Esters. Againthere is oxygen along with carbon and hydrogen.The active group is

O||

CO

and is between two other radicals

O||R1COR2

Whilst in general terms the two radicals can beany of the hydrocarbon radicals we havementioned earlier, consideration here will be withone series where one of these Rs is always aMethyl group.

O O|| ||RCOCH

3CH

3CH

2COCH

3

Acetate Ethyl acetate

These are calledAcetates and they are the mostcommon esters employed as solvents.

Once again, there is an homologous seriesfollowing the Alcohols as given earlier.

Chemical name FormulaEthyl acetate CH

3CO

2C

2H

5

Propyl acetate CH3CO2C3H7Butyl acetate CH

3CO

2C

4H

9

Pentyl acetate* CH3CO

2C

5H

11

Hexyl acetate CH3CO

2C

6H

13

* also known as Amyl acetate

These form a very useful range of solvents andare effective on a wide range of resins.

There are other Esters and you may hear of themas Lactates, Phthalates, Maleates, Propionates.Although these also have solvent powers they arenot used in the thinners type of application asthey do not evaporate easily.

KetonesThe general formula for Ketones is

O||

R1CR2

There are two radicals attached to the =C=Ogroup.

Ketones are good solvents and will dissolve

almost all resin systems. Some of the Ketonesyou may come across in the paint industry are:

O||

Acetone (dimethyl ketone) CH3CCH

3

O||

Methyl ethyl ketone CH3CCH

2CH

3

O||

Methyl isobutyl ketone CH3CCHCH

3|CH

3


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Other Ketones of interestAnother Ketones which is frequently used becauseof excellent solvent properties, combined withslow evaporation rate, is cyclohexanone.

H2

H2

C C \

H2C C=O\ C CH

2H

2

Note that this is not Aromatic, there are no doublebonds in the ring, it is one of the cycliccompounds which are in effect moleculeswrapped around so that the ends join up.

Just for good measure there is one hybrid solventworthy of mention at this stage and that is di-acetone alcohol.

O CH3

|| |CH

3CCHOH

|CH

3

It should be easy to identify the Ketone group andthe Alcohol group in this molecule.

EthersThe general structure of Ethers is

R1 O R

2

typical members of this group are

Diemethyl Ether CH2OCH

2

Methyl Ethyl Ether CH2OC

2H

5

Diethyl Ethyl C2H

5OC

2H

5

It should be be possible to work out others whichcan be expected.

Dimethyl Ether (DME) has found use as aerosol

propellant as it is a gas at normal temperaturesbut compresses into a liquid very easily while atthe same time it is an excellent solvent somaintaining the paint liquid in the can.

Diethyl Ether (ethyl ether) is the one which is oftensimply called ether. This, along with Methyl EthylEther, has found use as an anesthetic.

Being very volatile they do not find acceptancedirectly as solvents in the paint industry other thanas mentioned.

Because of their excellent solvent powers Ethersare very important industrial chemicals particularly

in the extraction of organic substances fromvegetable matter.

The Ether group is also found in the structure ofcertain materials called surfactants which areused in the manufacture of emulsion paints.

There are, however, some higher members of thegroup which have very useful solvent propertiesand evaporation rates and so makes them veryacceptable as paint solvents. These chemicals area mixture of an Alcohol and an Ether and areknown as the glycol ethers.

Glycol Ethers and EstersTo illustrate these consider the simplest member:

CH3 O C

2H

4 OH

This substance is known as 2-methoxy-ethanol orto give it another name ethylene glycol(mono)methyl ether (note the mono is sometimesomitted). It can also be referred to by its initialsEGMME or by a multitude of trade names; themost famous of which is Methyl Cellosolve.

Note however that Cellosove is the registeredtrade mark of the Union Carbide Co. and, byrights, only their products should be referred toby that name. However, it is quite common tohear the word used as if it was the generic name.As a guide, other trade names include

Dowanol Dow ChemicalsEktasolve Eastman KodakOxitol Shell ChemicalsSensolve BP (NCP)

The naming and the structure should be obviousnow, a radical attached by an ether link, (O),to another radical to which an Alcohol group isattached.

Why a glycol? A glycol is in fact another form ofalcohol but one with more than one hydroxyl (OH)

A Word of WarningSome very low boiling aliphatic hydrocarbonsare often referred to as Petroleum Ethers. Asthis is both incorrect and misleading the use ofthis term should be discouraged. These solventfractions are also often called Benzine (notethe spelling). This is also incorrect as well asmisleading due to its similarity to Benzene andits use must also be discouraged.


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group and here, in the case of ethoxy methanol, itcan be said that the parent glycol is ethyleneglycol with the formula:

OH C2H

4 OH

where one of the OH groups and has beenreplaced with an ether link to a methyl group.

Methyl alcohol + Ethylene glycol

CH3

OH OH C2H

4 OH

gives

Ethylene glycol methyl ether(Methoxy ethanol)

CH3

O C2H

4 OH + H

2O

This group of solvents becomes more interestingas it progresses, the next variation is the di-glycolseries.

Again a simplest member illustrates this:

CH3

O C2H

4 O C

2H

4 OH

Diethylene Glycol Methyl Etheror Methoxy-Ethoxy Ethanol

Note that it starts out as an ether then hasanother ether bridge and ends up with an alcoholgroup, another example of, tinker toy or Lego

chemistry. This compound also has a multitude ofnames including DGME and, again you will oftenhear it referred to as methyl carbitol.

Note however, that Carbitol is a trade mark ofthe Union Carbide Co. but the other producershave their own names such as methyl dioxitol(Shell).

As a matter of interest, due to the fact that theymay have long term health problems, the use ofethyl and methyl versions of this group has been

drastically reduced. Some companies haveeliminated them from their range. Even the Buty versions are suspect and attempts to replacethem with the Propyl version are being made.

Glycol Ether EstersAs was the case earlier when it was shown that anAlcohol could be converted to an Ester, a similarreaction with these solvents is possible and aseries of Esters is possible, the simplestexamples is:

CH3 O C2H4 COO CH3

Ethylene Glycol Methylether Acetate

Being Esters they have different solvent powers.

The naming of these solvents is fairly complexand especially when isomers are concerned. It iscommon for ethylene glocol methyl ether to benamed 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 touse the 2methoxy ethanol style whereverpossible.

Physical propertiesIn paint formulation it is the physical properties ofsolvents which are important and not so much thechemical properties. The chemical grouping is

important because, if a given resin is soluble inone particular type of solvent or combination ofsolvents, then generally any solvents in theparticular group will show the samecharacteristics. Thus if a resin is soluble inacetone it will almost certainly be soluble in otherketones. As a general rule it is also the case thatthe lower members of any homologous series willgive a lower viscosity, on a volume to volumebasis, than that given by the higher members. Inother words the cutting power (ability to reduce

viscosity) will be greater for the lower members.At the same time the evaporation rate, (the rate atwhich the solvent leaves the films) will change withthe size of the solvent molecules (number ofcarbons in the backbone) in any homologousseries. Such properties as Boiling Point, and FlashPoint will also follow a similar pattern butremember that this is generally confined again tothe homologous series.

Specific gravity

It is the custom, in many companies, to useweight in their formulations rather than volume.The reason for this lies in the fact that the specificgravity of solvents varies. That is, a litre of onesolvent will not weigh the same as a litre ofanother. At the same time volume is temperaturedependant, that is to say that on heating a liquid itwill expand, this in turn means that the volume ofa given weight will increase with temperature. Ittherefore, follows that, if volume is to be used asa measure, then the temperature will need to becontrolled for all substances used. In thelaboratory the weight method has generallybecome the preferred procedure. In the factory


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volumes are often used, especially for solvents,some operators using a standard conversion,others relying upon the final adjustment to makeup differences. In general, paints, while being soldby volume, are packed by weight for operating

convenience and accuracy. Whether the volume orweight method is used is not of such greatimportance as that of understanding theimplication and taking the correct precautions.The paint formulator must therefore bear theconcept in mind when substituting solvents andallow for differences in specific gravity. Alsoremember that the non-volatile content of a paintin generally expressed in terms of weight and sois influenced by the specific gravity of the solvent.

Boiling pointWith regard to boiling point; pure chemicals havea fixed boiling point but mixtures will have arange. The mixture will usually start to boil at oraround the boiling paint of the lowest one in themix and the final (dry point) will be around that ofthe highest component. For any mixture ofsolvents these two figures will not change to anygreat extent and so the information is somewhatlimited. The exact composition of mixtures canonly be compared properly by considering thedistillation range. This is given as the percentage

of liquid which distils over at definedtemperatures. By comparing the ranges of twomixtures the composition of two mixtures can becompared.

Evaporation rateThis effect also applies to evaporation rate. Theevaporation rate is a fixed figure which only reallytells us the slowest member. For this data to beuseful it should be read in conjunction with thedistillation range.

The evaporation rate may also be affected by thesolvent affinity of some resins. Different resinshave different release rates for certain solvents. Inthis case a solvent entrapment or slowing down of

speed of set of the coating can be the result. Donot be surprised if a coating does not follow thepath expected from the evaporation rate. At thesame time do not be surprised if a coating whichappeared to dry will becomes brittle after a few

days, this can be the result of solvent affinity andso what we have is a temporary plasticiser.

Flash point

With regard to flash point this will generallydepend upon that of the lowest member of themixture. But a word of warning, the flash point ofsome mixtures can be lower than the expectedone. An example where this can happen is withbutanol and xylol. If this is near the cut off pointused in legislation, it is better to carry out a finalcheck on the finished paint.

ConclusionsSolvents are in general the tools of the trade forpaint formulators. They are hammers,screwdrivers and spanners. They do not remain inthe finished coating but instead provide a way ofgetting the coating into the required place in therequired way. Just as there are different sizes andtypes of hammers, screwdrivers and spanners sothere are different types of solvent groups. Having

decided just what type of spanner is needed toadjust a particular fastener we next need to knowwhat size it is. Using the wrong one could damagethe fastener and so weaken the whole structure.So it is with solvents, being near enough is notalways good enough and, as the skilled engineermust know how to judge and pick the correct tooland apply it properly, so must the paint formulatorknow his solvents. Aliphatics, Aromatics, Esters,Ketones and are the hammers, screwdrivers andspanners of the paint chemist. Knowing how touse them properly is the most important skill ofthe paint formulator.

Note that when the job is finished the tools aretaken away in the same way as the solvents. Any


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Liquid paints consist essentially of two parts, thesolids which will be left after it has dried and thevolatile portion which evaporates during and afterapplication and thereafter plays no further part inthe action. During this initial phase, as the solventevaporates, the applied paint increases inviscosity. If this takes place too quickly it will bedifficult to apply and will not flow out to give thedesired finish, if this takes place too slowly sagsand runs will be the result. For these reasons thevolatile component has to be carefully chosen.

This volatile part is referred to as the thinner orsolvent and, if the paint has to be applied bybrush or roller, it will generally be slowerevaporating than the one needed for sprayapplication. This thinner/solvent is only requiredfor application purposes, is completely disposedof and hence to some extent fills the same role asthe container. In other words it gets the paint fromthe manufacturer and onto the job where it is thenthrown away. What is therefore required is themost cost effective method of getting the properthickness of paint onto the job but, as these

liquids areVolatile Organic Components(VOCs), they also have environmental sideeffects which must be kept in mind.

What is the difference between a solvent and athinner? A solvent, as we know it in the paintindustry is, almost without exception, a thinner. Athinner, on the other hand, is not always a truesolvent. In others words if you mix a thick liquidwith a thin liquid then the thin liquid can bedescribed as the thinner. In simple terms it willreduce the viscosity. In the case of emulsionpaints the thinner is water but it is in no way asolvent for the system as a whole! When a liquidcan be mixed with another, but is not a solvent, itis referred to as a diluent. This is only a physicalmixture of two liquids.

In some cases, when further additions of thethinner are made a situation could arise when themix begins to curdle and throw out. An example ofthis would be an automotive lacquer which willonly take a limited quantity of white spirit beforecurdling.

This problem can become obvious when it comesto washing the equipment. In the case of theemulsion paint the equipment can be washed with

water because the paint will accept unlimitedaddition of water. This is because it is in adispersion and not in a solution!

Washing equipment used to apply industrial paintscalls for a careful choice of a suitable agent thethinner might do the job easily but it is notnecessarily so. There are cases where the thinneris not the best cleaner. After all the thinner wasformulated with application properties in mind. Agun/equipment cleaner may be a moreeconomical and better proposition; especially

when formulated for the job.

Solutions and dispersionsGenerally liquid coatings are of two types, solutionbased or dispersion based. Solvent basedcoatings will be based upon a solution ofpolymeric or polymerisable material(s) whilst, inthe case of the dispersion, the solid or semi-solidparticles are suspended in another liquid. Asmentioned above the objective in both cases is tomake the paint sufficiently fluid so that it can be

applied easily.By definition a solution is a homogeneous mixtureof two or more substances. The dissolvingmedium is called the solvent, and the dissolvedmaterial is called the solute. In most commonsolutions, the solvent is a liquid, and the solutemay be a solid, gas, or liquid. Syrups aresolutions 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, aliquid, 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 forexample sunglasses.

In other words the resultant mixture will appear asone and will result in a clear (coloured maybe, buttransparent) product. This is referred to as asingle phase system.

In the case of solution materials the bigger thepolymer, the bigger the individual molecules andconsequently very viscous solutions are the result.This means that the amount of polymer in thesolution can be low with the result that the film ofthe paint/lacquer, after the solvent hasevaporated, will be very thin. To give an adequate

Part 2 Thinners and Solvents


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coating under these circumstances will usuallymean that several coats will need to be applied.

In the case of the dispersion resins (the mostcommon and best known are usually referred toas emulsions) relatively big molecules or particles

are involved. If these were to be used in solutionform, they would give high viscosity materials ofno practical value. By suspending small particlesof these resins in a water phase we can produce aliquid material which will permit a high solids film.In this case the resin particles usually need tohave a small quantity of solvent added to them tomake them stick or coalesce together. Thissolvent, which evaporates after the water has leftthe 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 actuallylatices (latexes). By general definition a latex is adispersion of a solid in a liquid while an emulsion isthe dispersion of a liquid in a liquid. In the emulsionpolymerisation process the monomer(s) fromwhich the polymer is made are emulsified first andthen converted into the final product.

The speed at which a solvent leaves a system issomething which must be given carefulconsideration. Resins and polymers have avariable tendency to hold on to solvents and somecan be retained by the film for some time. Whensolvent is retained it acts as a temporaryplasticiser and so the film can appear to be quiteflexible but after a period the film may becomebrittle and lose its flexibility. Always check for this.This effect is often noticeable in the form of thesmell it gives off if kept in an enclosed space. Italso accounts for the solvent smell which comefrom certain water-based paints and which can benoticed after the room has been left closed forsome time quite a problem for hotels who want

a quick turnaround of their rooms duringmaintenance.

This affinity for solvents varies with the resinsystem and the solvent, in other words someclasses of solvent are retained more easily bysome systems than by others. Care must also begiven to the problem of case hardened coatings.Often the surface of the paint can dry morequickly than the underlying layers thus trappingsolvent. The use of anti-skinning aids inconvertable systems often proves necessary butthe use of small quantities of high boiling point(slow evaporating) solvents may also provenecessary.

Classes of solventsA true solvent is one which will satisfy the criteriain all proportions. There is sometimes a limit to theamount 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 bereached when no more can be dissolved; this isknown as the saturation point. In the case of saltthe solid would settle out as white crystals. Wewould then have a two phase system one solidphase (the salt) and one liquid phase (the saltsolution). This is referred to as a saturated saltsolution no more salt can dissolve in the water.

In the case of liquids like oil and water they wouldform two phases both liquid.

In the case of certain resins these will only

partially dissolve or swell when mixed with certainsolvents but on addition of a second solvent atrue single phase solution can result. When sucha mixture of solvents is used and a true solution isobtained, these solvents are referred to asco-solvents or latent solvents. In effect what ishappening is that part of the resin moleculedissolves in one solvent and another part in theother solvent or we have a mixture of twosolutions which are compatible.

It is also quite usual in the surface coatingindustry 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 ofsolvents to get a clear homogenous product, or,as we term it, compatibility. This compatibilitymust be maintained until all the solvent/thinnerhas left the coating.

Certain solvents and thinners can consist of amixture of substances. One way of telling whetherit is a mixture or not is to look at its boiling point.If this is a single number, or a range of twonumbers 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 amixture. Always consider the boiling point and theevaporation rate when comparing solvents, Theevaporation rate alone, being dependant upon thehigher boiling component, can be misleading.

DiluentsIn certain cases, once a solution has been made,then another liquid can be added and the resultwill be a clear homogenous solution. If, however,

on further dilution, there is a separation into twophases shown either by a clouding of the mix orby a separation into two layers then the liquid


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being added is a diluent. For these reasons it isimportant to ensure that the diluents leave the filmbefore the true solvents i.e. have a fasterevaporating rate

Diluents are sometimes referred to as a means of

cheapening paints but let us be quite clear that,whilst some unscrupulous formulators may usethem in this way, it is not to be accepted as ageneral rule. These formulators make use of thefact that diluents do not generally reduce theviscosity as quickly as solvents and so they delivera product with a lower solid content. As it is thesolids which give the protection a proper balancemust be kept. Diluents, properly used, play a veryimportant part in the formulation of thinners andtheir use should never be decried. When usedcorrectly they can produce more cost effective

formulations and be more environmentally friendlyand less hazardous to health.

As an example consider a paint which is to beapplied by spray. The purpose of the thinner is toreduce the viscosity to a point where it will atomiseproperly. As some 30% of the solvent canevaporate between the nozzle of the gun and thesurface to which it is applied, one part of thethinner can be considered to have done its job assoon as the paint leaves the gun. Providing this hasno other side effects such as blooming, then this

component of the thinner only needs to be adiluent. A fractionally heavier spray setting to getthe correct film build can give compensation but, inthe case of pigmented coatings, obliteration willdictate the thickness of the applied film. If thecorrect solvent balance is built into the originalpaint there should be no problem but it is advisableto buy thinners from the paint manufacturer orconduct careful tests as to their suitability. If a highproportion of diluent is already built into the originalpaint, thinner selection can be critical.

In the case of aerosol containers the choice ofpropellant for paints is usually Liquefied PetroleumGas (LPG) which can be propane or butane (inSouth Africa the commercial available version is amixture). On a cost/availability basis the choice isnot difficult. Other propellants are di-methyl ether orfluorocarbons but, due to environmental problems,the latter is now only used in very specialisedcases. Di methyl ether is favoured, by the cosmeticindustry and other specialised end users, when LPGis either undesirable or unsuitable.

So far as the paint industry is concerned LPG issimply a low boiling aliphatic solvent and soserves the dual purpose of being a propellant as

well as a diluent. Although, in some cases, thealiphatic might be a latent solvent, the termdiluent is probably more appropriate here as itplays that role perfectly. The paint system usedmust be aliphatic compatible and this can beachieved by the incorporation of strong solventsin the orignal base paint.

Compatibility tests on the resin system will beneeded and, if the paint can stand dilution withpure pentane or hexane to reduce it to sprayingviscosity, then it will probably will accept LPG. TheLPG is usually injected into the can after therequired amount of paint has been filled and thecontainer capped but cans may also be prefilledwith propellant and the paint injected into the can.This latter procedure was used for automotivetouch-up paints.

As a point of interest consider the Soda Watersiphon which is, in effect, an aerosol. Here thepropellant (a gas carbon dioxide) is dissolved inthe water and is maintained in solution byconfining it to its receptacle, keeping the gasunder pressure. So, as is the case with theaerosol, the pressure is sufficient to force thecontents out of the container when the valve isopened.

General considerationsThe pigments and extenders in coatings areinsoluble materials and are dispersed in the paintwhether it be a solution or a dispersion resinsystem. They will therefore interfere with thesystem and it is always advisable to examine thebinder system without these present to ensure aproper film is formed. it is also important that theresin systems themselves remain compatible afterall the solvents/diluents have left and, for thesereasons, tests must always be conducted on theunpigmented system to ensure that the residualfilm is compatible.

Whether using a diluent or a solvent in any systemthe dry unpigmented film should be as clear aspossible once formed or coalesced properly, if itis not, an underbound friable coating may be theresult.

It should be borne in mind that all solvents andthinners are VOCs and are becoming a cause forglobal pre-occupation. This is due to thecontribution they are making to photo-chemicalsmog, high atmospheric ozone levels (not to beconfused with the Ozone Layer problems), andglobal warming. For these reasons legislation isbeing introduced in many parts of the world to


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regulate the quantity of these substances which isemitted to the atmosphere.

Ozone what is it?There appears to be a certain amount of confusion

concerning the role of solvents and theirrelationship to problems of the Ozone Layer, globalwarming 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 themolecule. Ozone has three atoms and is relativelyunstable returning to the more stable O

2form

easily. As it gives up its third atom very easily it isa strong oxidising agent and, as such, a potentbleach and disinfectant. It is active in the

breakdown of plastics and surface coatings. It hasa distinctive smell and can be detected in aconcentration of 0.01ppm in air. It is highlypoisonous at a concentration of 0.1 ppm. It isabout a hundred times as poisonous as carbonmonoxide!

Ozone is formed in the air around us by the actionof ultraviolet light. In rural areas it may reach alevel of 0.02 to 0.03 ppm. In cities, there isusually less because there is less sunlight unless there are certain chemical impurities in theair. The greatest effect is from oxides of nitrogenNOx coming predominantly from exhaustemissions. This is broken down by ultraviolet raysto give nitric oxide, NO, and atomic oxygen O.The atomic oxygen then reacts immediately withmolecular oxygen to form ozone. Under normalcircumstances this ozone reacts again with theNO to give NO

2and the cycle keeps repeating and

a stable state should result. However thepresence of hydrocarbons interferes with thissecond reaction by scavenging the nitric oxidethus increasing the levels of nitrogen peroxideandozone in the atmosphere. This results inphotochemical smog and concentrations of ozoneas high as 0.5 ppm (above the danger zone) havebeen reported on smoggy days.

Whilst it was first thought that only hydrocarbonswere responsible, it has now been found that, to agreater or lesser extent, all volatile organicsubstances take their part hence the concernwith solvents.

It is of interest to note that many people think thatthe need for lead free petrol is to reduce leadpollution in the air. While this may be true to someextent, the principle role of lead free fuel is tominimise the poisoning of the catalyst used in the

exhaust system to cut down hydrocarbonemissions.

When fluorocarbons first came into use it was feltthat their great advantage lay in their relativelystable nature. They are not broken down in the

troposphere (the first 10Kms. of air above us).Instead they migrate to the upper level of thestratosphere (50 Kms.) where the ozone layer isfound. Here the ultraviolet breaks down what littleoxygen there is to form ozone. The reaction ofCFCs with the ozone means that the totaloxygen/ozone levels are depleted and this allowsthe penetration of ultraviolet light to the earthssurface. The presence of the ozone layer, bypreventing ultraviolet rays penetrating, has madelife, as we know it, possible. In addition to otherside effects the penetration of ultraviolet rays

gives rise to the high ozone at ground level asdiscussed earlier. The high levels of troposphericozone cannot cancel out the imbalance. Ozone isnot only unstable but there is also no mixing of thetropospheric and stratospheric layers

Ozone, carbon dioxide, methane, halogenatedsolvents and oxides of nitrogen, because they allretain heat, contribute to global warming and thegreenhouse effect.

The two ozone problems are different but the holein the ozone layer and the greenhouse effect is

interrelated.

Solvents and vehicle emissions alone are not theonly source of VOCs. Natural sources include pineneedles, gum trees and camphor as well asnatural oil and gas wells. There is also aconsiderable production of Methane from naturaldecomposition and the digestive processes ofruminants. This is not classed as a VOC. This isnevertheless, quite a cause for concern especially when we hear of forests being burnt toprovide beef pasture.

Hydrocarbon emissions from automobiles have,as we have mentioned, been targeted. Efforts arebeing made, to a greater or lesser extent, toreduce the venting of tanks to the atmosphere.This depends, to a great extent on the smogsituation and the action of pressure groups.

The use of solvents could be reduced even withpresent day technology. One of the reasons forthe slow transition being that many people look atthe price rather than the cost. Our industry mustface up to the fact that this problem must not beallowed to get worse it is certainly not going togo away.


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Titles in the Student Monograph Series

Honorary Technical Education Officer: A T Hopgood FTSCHonorary Editor: P S Thukral PhD CChem FTSCChairman 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, UKTel: 01525 372530 Fax: 01525 372600Email: [email protected]

No. 1 Basic Science for Students of Paint Technology

No. 2 Corrosion

No. 3 Water-borne Resins

No. 4 Colour Physics

No. 5 Dispersion and Dispersion Equipment

No. 6 Additives for Water-borne Coatings

No. 7 Standards

No. 8 Water-borne Coatings

No. 9 The Basics of Solvents and Thinners

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