waterborne & solvent based surface costings & their applications, vol. 1, vinyl acrylic by m. barbour

The initiator is consumed during the course of the polymerisation and the reaction mixture requires “topping up” or “spiking” with initiator at regular intervals to maintain the free rad

VOLUME I

& Solvent Based Acrylic:

End User Applications

M Barbour BSc, | Clarke GRSC, D Fone BSc,

A Hoggan BSc, R James, P Jones Grad PRI,

ér Lam PhD BSc, C Langham LTSC, K O'Hara LRSC,

P Oldring PhD BA, G Raynor BSc, | Royston BSc,

N Tuck PhD BSc, R Usher BSc

Dr P Oldring PhD BA and Dr P Lam PhD BSc

M Barbour BSc, J Clarke GRSC, D Fone BSc,

A Hoggan BSc, R James, P Jones Grad PRI, Peter Lam PhD BSc, C Langham LTSC, K O'Hara LRSC,

P Oldring PhD BA, G Raynor BSc, | Royston BSc,

0 N Tuck PhD BSc, R Usher BSc

Edited by

Dr P Oldring PhD BA and Dr P Lam PhD BSc

JOHN WILEY & SONS

CHICHESTER @ NEW YORK @ WEINHEIM @ BRISBANE ® TORONTO @ SINGAPORE

Published in association with

SITA TECHNOLOGY LIMITED

LONDON, UK

(i) Transfer to SoÌerf 10

(ili) Transfer to @ Modifier mẽ ẽaẽeaeaaA 14

6 Inhibitors 16 THE KINETICS OF THE FREE RADICAL POLYMERISATION

2 Relationship between Monomer Structure and Reactivity .csssscsccscssssssscnsee 30

3 Curing and Film Forming Reactions 32

(i) Reactions of Hydroxyl Containing Polymers with Amino

Kormaldehyd€ R€SHLS con nh HH HH gà g0 00 ngơ 32

(ii) Reactions of Hydroxyl Containing Polymers with Isocyanate .sssssssecssevees 34

(iii) Reactions of Hydroxyl Containing Polymers with Polyanhydrides - 34

(iv) Reactions oƒ Hydroxyl Contaiming Polymers with Epoxy GTOMDS 35

(v) Reactions of Hydroxyl Containing Polymers with Carboxyl Groups 3S

(vi) Reaction oƒ Carboxyl Contaừùing Polymers with Epoxy Groups 35

(vit) ReacHon oƒ Carboxyl Comtatng Polymers with Ïsocyanate GrOMps 36

(viii) Reaction of Carboxyl Containing Polymers with

Alkoxyl Methyl Ether GTOMDS <e 36 (ix) Reaction of Glycidyl Containing Polymers with Epoxy GTOHDS 37

(x) Reaction oƒGlycidyl Cormtaiming Polymers with Amine CTODS 37

(xi) Reaction of Glycidyl Containing Polymers with

Amino Formaldehyde Resins .:csssceccsseveesesseessessonevesaeesacesesenestecsenaneevesenss 37 (xii) Reaction of Glycidyl Containing Polymer with

Phenol Formaldehyd€ ÍÑGSÙTLS «so HÀ TT HT HH TH tớ HH nhện 38 (xiii) Reactions of Methylol Containing POlmFS àcerhetskrerererseseserrer 38

(xiv) Reactions of Methylol Comtaining Polymers with Epoxy GTOHMDS 39

1 Orientation of the Repeating Unit 49

(i) Electrostatic Forces

(4) Steric Himdrance cccccsccssssceseessensesssecessqeessceessuersacencnssessscsenessneecsnenessuseeuacesesaes 50

U22, 02.2 1n hố ne 30

3 Molecular Weight 52

(i) Degree of Polymerisation 55

|

Contents Ul

(i) Non Volatile Content vs Molecular Weight at Constant ViscoSify 56 () — Durablluy vs Molecular Weight (Polymethyl Methacrylate) « s 37

6 Relationship between Molecular Weight and Ÿiscosity svevstaceeceseens 57 (i) — Establishing Values for K and Œ Ăn, 58

(i) Tp öoƒ Acrylic CopolWH€FS HH HH Hào 6l

(i) —— Bulk Podymerisation 0.0 65 (ii) — Suspension PolymeriSation 66

1 Factors Influencing Solution Polymerisation 70 ( — Ñeaction TeImD€TdHF€ HH TH HH HH TH HH th ưy 70

U NHL.(( , A9 n nh .e 70 DI" nh he .- 70

(V) Chain Transfer Agents .cccccccsesccccccessecssetneneccececcceuatuanessettaceaeuatnessaetnntesaaeentnaess 71 (VE) Tromsdor ff Effect cccccccccccccccessescessnecuscesseeseesscscessaneateaessseeseecsensensessaneeeecsaeseeaes 71

2 Conversion of Monomer into Polymer 71

(i) 'L x1 h.- 72 (lì) — Continuous Additon or Drip Feed PTOCSS à SH re 73

(i) | Free Radical Generation oe " 74 (ii) Examples of Commonly Used IHÍHGFOFS- à chen key 77 UUON/Q 7 7 ,.nnnhe 78 (iv) Efect oƒ Solvent TYpD€ OFi LỤ,, Ăn TY KH HH HH HH KH Tà HH HH BI

(i) — Solubility and Solubility Parameters .0 ccccccescccssecesseveseeeeeseseeeeseneeneseetaeass 85

5 Solution Polymerisation Plant and Process 89 Formulations and Methods for the

Preparation of Solution Acrylic Resins 91 (i) | Preparation of an Acrylic Copolymer in Methyl Ethyl Ketone 92 (ti) Preparation oƒa Thermoplastic Acrylic Resim in Solution 93 (iii) Preparation of Thermosetting Acrylamide Ñe$i' c-cvceeisikeceerkee 93 (iv) Three Methods oƒ Preparing Thermosetting Acrylamide Resins 96

MECHANISM OF EMULSION POLYMERISA TION e e 105

1 Choice of Components for Emulsion Polymers 111 (i) WWG€T À GHI HT TH 02 84 th 9 TA ill (ii) Acrylic Monomers

(ii) Surfacldmts

(EV) INIT ALOT S oi eeceeeececetsensscstcsseeensetsasesceseeneesecsassnsansassseaceseesageseseeessasenseessaaaseesassess

BI NA a— (vi) ĐH̓€FS Ă.H"nH> Hà Hàn Hàn KT HH HH nhà HH rệt

VARIATIONS ON CONVENTIONAL EMULSION

POLYMERISATION 118

1 Polymerisation Technique Variations 118

DA °® ® n 118 (ii) Emulsifier Free Latices - Resin Supported SŠVSÍCT che 119 I2 0) nnnnh 120

2 Water Based Acrylic Polymers Used as Additives in Coating Formulations 120 (i) — Pigment Dispersing Agents .csccscssccsccsetsccsssesssesencensensenscotsseseseeeceseeseeseensans 12] (ii) | Carboxylated Acrylic Thickeners and Rheology Modilfiers 122 (iii) Manufachưing Process for Acrylic THỈCk€H€FS ii 123 DIỚNN: 2.2278 , 0nn Ắ 126

FILM FORMATION 128 EXAMPLES OF LATEX FORMULATIONS &

PREPARATIVE METHODS 131

2 Vinyl Acetate - Methyl Methacrylate Copolymer eesceseencesesesneescoe eeveessecsescees 132

3 Vinyl Acetate - Acrylic Terpolymer 134

6 Preparation of an Acrylate Copolymer Latex Containing Acidic Groups 138

7 Preparation of a Water Soluble Acrylic Copolymer "1 139

8 Example of Preparation Method for Stabilisers

for Surfactant Free Emulsion Polymerisation 141 DI" 19 141 (BE) EMU SIO an 14]

1 Preparation of Acid Functional Acrylic Resin .-eeeceseseessssesesseenesesasssseae

2 Preparation ofa Water Reducible Acrylic Resin «-cessesesesesseseseseessssese

3 Solvent Distillation Process for Preparing Waterborne Acrylic Resins

4 Examples of Water Reducible Resins -

VeoVa Based Water Soluble Resin (Courtesy oƒ Shell ChemiCdÏS)-««es«eesssessssessses 149

1 The Electrodeposition Process in Practice 151

2 Considerations when Formulating Electrodeposition Resins «- 158 DÀNH A/, ẽn ố 158 (l) Glass TransHion TeD€TQ[HF€ SH HH ciờ 158 UUIC S8 ,.A/,., ah n 158 DI 8 nn e 159 (V) Application Ắ ằ ẻ 162

( — ACcrylic ÑGSÙN HH nh KH H1 KT HH HT gtn tươ 162 (ii) Application of Electrodeposition Resins -

General Formulating PTÙTCÍDÏ€S cà HH nàn nhe, 165

() — Thermoplastic Acrylics Soluble in Polar SOÌV€HIS chen neeeeesve 207 (ii) Acrylic Based Polymers Soluble in Aliphatic Hydrocarbon Solvers 208 II (19 anh nhe 209 (iv) Concrete and Terrazo Š€QÌ€TS ST HH TH TH Tnhh 209

DN.( i9 ñ69s an 211 HIẾN: .0 ai an ố 212 (iii) Marine and Offshore Coating ŠÿSÍ€HHS chư, 213 (iv) High Durability Coil Coating Enamels for

Metal Cladding Poly Vinylidene Fluoride (PVFa Coatings) 214 MISCELLANEOUS APPLICATIONS FOR

THERMOPLASTIC SOLUTION ACRYLICS cccccosssessocssssesseesescenssosscers 217

MAJOR CROSS LINKING MECHANISMS sescccccscccsscccssccssccccssoscccens DOE

Contents vii 3 Carboxy Functional Acrylics 228 4 Choice of Co-reactants 229 (BE) HN2.2/ 229

(4E) — AMIO RESINS 1a Ắ 229

UUON, 1y 0n nh 232

(iv) Alkyd Modification an nn 232

(v) Other Binder Modifications 233 Tt PIGMENT SELECTION 234 1 Whites 234 2 Blacks 234 3 Blues 234 4 Greens 234 5 Reds 234 6 Yellows & Oranges 235 7 Transparent Iron Oxides 235 IV SOLVENT SELECTION 236 V CATALYSTS 237 1 Acid Catalysis 237 2 Metal Catalysis 237 VI MAJOR END USES FOR THERMOSETTING ACRYLIC RESING 238

1 Automotive Finishing 238 () — Sohuion Thermosetting ACTVÌÍCS SH HH Hàn HH HH LH ki, 239 (ii) Thermoset Non Aqueous Dispersions 239 LUON (mo nan 240

(iv) Clear Over Basecoat Systems (CO) ch Y nà ng cua, 242 ANH 1 8 i6 nan 243

2 Acrylamide Acrylics in Domestic Appliance Finishing ee.se-«.e- „248 3 Hydroxy Functional Acrylics for General Industrial Stoving Finishes

(i) High Performance OH Acrylics in Tractors and Construction Equipment ccccccccccscccscsscscsssssescscasseasesansstsacenanesunseaeessensseessessess 251 (ii) | General Purpose Stoving Acrylics for the General Industrial Market 254

4 Metal Decorating - Solvent

DA n (ii) The Use of Acrylic Resins in Metal Decorating

(iit) Why Acrylics are sed in Metal DeCOTdfUIE se SsHiH HH,

vi Contents

(iv) Vinyl Coatings for Internals

V22 (vi) Pigmentation

Chapter V TWO COMPONENT ISOCYANATE CURED ACRYLICS APPLICATIONS

CURING, APPLICATION AND FILM PROPERTIES , ccccsossoreeconsseeees 299

§ Acrylic Polyurethanes for Agricultural and

Acrylic Polyurethanes for Information Technology (IT) Finishing .00 319

7 Two-component Ísocyanate Cured Acrylics for PlasficS .-.-.«-<- 321

Vv MAJOR EUROPEAN SUPPLIERS OF HYDROXY

FUNCTIONAL ACRYLICS 324

VỊ, REFERENCES 325

Chapter VI TWO COMPONENT NON-ISOCYANATE ACRYLICS APPLICATIONS

3 Cure Aspects Related to End User 336

Vv FORMULARY & TYPICAL PROPERTIES .cses.e<<eessee „339

VI COMMERCIAL RESINS 345

Contents

VII REFERENCES 346

Chapter VII WATERBORNE ACRYLIC SYSTEMS FOR INDUSTRIAL APPLICATIONS

4 Waterborne Resins Used in Surface Coatings 361

7 General Properties of Emulsion Films 386 NEW DEVELOPMENTS FOR WATERBORNE SYSTEMG cccccsssssese 387

FORMULATIONS FOR WATERBORNE COATINGS BASED

UPON VINYL AND ACRYLIC POLY MERS .c-<<<<<<-<<essee 389

1 General Considerations in Paint Formulation 390 CE) EMULSIONS hS 39] (it) — Dispersions (Inverted SOÌHH[OPN) Ăn TH HH cư rưy 391

2 Additives Common to both Emulsions and Water Reducible Systems .« 392 (1) —_ Dispersing/Wetting Agents 392 (EE) — DC fO IMCS ec ceccccvececscssccessasessteescesssasceesseanectsuaecsuesaeesssnssaessessecsaeeseeaenetneneaess 394 (ii) Viscosity and Rheology Modifiers ¬ ÔÔÔÔÔ 394 (EV) Rust [nhibiters 0 396 (V) Urethane Modifications .ccccccccccceeieesesseeseeeeteeneeeeveeneeeeseesaeseceesntessenenesennntes 396 (Vi) — Mimo Additives c.cccccccesccsceessensecssesssessesaecsasecsessesasensesnseaeceteaersacesceesaeensnreanees 397

1 Self-Crosslinking Systems 398

DA" hanh 398 CH) — Polycarbodtirmides ccscccccccccccessccevsesecesessctenssssncssatsensenncsasseecsensceseeeesenersesseese 399 (UE) — PolyisOCYGNGLES 0 e 399 (4V) ROP ANIC SALES ceccccscensessceesnsseesscusecsesscensneetenecaseaseessenseenseaeaeesaeeeseeaeeaseeeaaes 400 2N, ai 7n - 400 (VE) 7221 400

WATER BASED ACRYLIC SYSTEMS FOR FINISHING WOOD 407

1 Components of Waterborne Wood Coatings 407 (41) ® h ae - 408

xi

X

XI

XI

Contents II N 8 7 408

II 7 7 ẽm 408

(iv) NT) 2.5 na ne 409

HH 2 gốc 7a n 409

20 2:8 m 409

2 Water Based Formulations for Wood Finishes

(i) Water Based Self-Crosslinking ñqs€COdI càng (i) A Self-Crosslinking Satin LACQM€F LH HH HH ghe UUONG, ác 7 an — 412

(iv) A Spray Applied White PTÙH€T cv nh HH HH Thành (v) White Roller Coating Basecoat (vi) Spray Applied Satin White LACQM€T HH HH HH co 415 (vii) A Formulation for Clear Flat Furniture FHlGÌ Hee 416 (viii) A Formulation for a Matt Finish for FUTHÍIHF€ cover 417 WATER BASED FORMULATIONS FOR MASONRY AND FLOOR COATINGS 418 1 Masonry and Floor Coatings 418 (1) —- Good Quality Masonry Paint .ccccccccecceeesccsetscsssnetaceaeeneceeseneeseeeeteenenstteteees 418 (ii) A Basic Sprayable Exterior Masonry Coating Formulatfion 419

(iii) Formulation for a Marble and Terrazzo Polish .cccccccscsscssseseensensceeeterectseeetes 419 (iv) Formulation for Floor Coating (v) Floor Coating for Vinyl Composite Tiles (vi) Formulation for Low Cost ConCrete S€QÌ€T, -c se se ceeeseseersrrrrerseree 423 PLASTIC COATINGS 424 1 Formulations for Waterborne Plastic CoatÏnpS eoeessesssssssesessessessnsesnen 425 (i) — A White Waterborne Plastic Coating ĐH te ng ng rrry 425 (ii) A Clear Waterborne Plastic Coating cc-cccccccrcreeiEreeriee (iii) Water Based Metallic Basecoat for Plastics (iv) Waterborne Coating for POÌySET€H HH HH Hà TH TH hy tườ GENERAL INDUSTRIAL COATINGS FOR METAL PROTECTION 429 1 Waterborne Formulations for Metal Protection sesesesessesssessesesersssessse 429 (i) Metal Primer N72 0u6, 7 ỐốỐồỐ 431

U15 718885 432

DJ A/ AtL),09 nh he ốỐố.ố.ốỒ.ồ 433

2 Two Piece Beverage Can Fabrication 440 Waterborne Acrylics in DWI Externals 441 (i) | White Water Reducible Acrylic for DWI Aiuminium Cans 444

4 Waterborne Internal Lacquers for DWI Beer and Beverage Cans 445 (1) DWI Irmternal Spray LLACQM€T Hs KH HH TH khen 449 (ii) The Spray Application of Two Piece DWI Internal Can Coating Lacquers .450

5 Evaluation and Testing of DWI Internal LaCQU€TS . «-sesesssesecsessssssessesee 455

Ha T1 ae 455 N2 ) 455 (ili) Flavour

DU ẽẽh 456

2 Nư 2 an -.4 ':.:A -"'.'.ồ'.' 456 J8 N 1: 7 .- 456

(i) Sheet Fed Water Based Lacquer for Food Can ÌnternaÌs 464

CHAPTER I

THE CHEMISTRY OF ACRYLIC RESINS

by PETER JONES Grad PRI

Edited by Peter Oldring PhD BA

I- The Chemistry of Acrylic Resins 3

e Thermoplastic polymer coatings

e Thermoset polymer coatings

The thermoplastic types are long chain polymers with high molecular weights and they film form without external chemical reaction The thermosetting types are shorter chain polymers containing reactive groups which can be ‘cured’ by the application of heat or usually by reaction with another chemical type to form a crosslinked film

Thermoplastic films harden by solvent evaporation, whereas thermoset films harden by chemical reaction

Vinyl resins are produced by the addition polymerisation of viny! monomers The term

‘vinyl monomer’ is applied to the species of molecules which contain a reactive C=C

double bond Individual molecules (termed ‘monomers’) of a vinyl compound are capable of undergoing a reaction to form long chains of monomer units linked via C-C bonds (termed ‘polymer’) The monomer units in the molecular chain may be exclusively

of one species (a ‘homopolymer’) or the polymer may comprise two or more species of monomer (‘co-polymer’, ‘terpolymer’, etc.)

This mechanism for resin formation is totally different from that for any other class of resins

Alkyds and polyesters are formed by condensation reactions which generate water of reaction Polyurethanes are formed by chemical reactions which do not involve the generation of any by-product Vinyl and acrylic resins alone are formed by addition polymerisation of the carbon carbon double bond

In this chapter, the theory, mechanisms, kinetics and types of addition polymerisation will be considered The acrylic monomers which can be used and the properties that they impart, will be discussed, as will their effects on polymerisation Crosslinking reactions and functional monomers, which can participate in them, are the basis of industrial

thermoset systems and are, therefore, also taken into account

4 1 - The Chemistry of Acrylic Resins

The four methods of polymerising acrylic monomers, namely bulk, suspension, solution

and emulsion are considered Chapter I concerns itself with the chemistry of acrylic

polymerisation and solvent based acrylics Chapter II is devoted to waterborne resins,

especially emulsion polymerisation, because of its significant differences from the other

types of polymerisation

Much of the discussion about solvent based solution polymerisation of acrylic monomers

is also relevant to waterborne systems, where the monomers are polymerised in solution

in a water soluble solvent, neutralised and then dispersed or inverted into water

However, the basis of acrylic addition polymerisation needs to be considered first

Il ADDITION POLYMERISATION

Long chain molecules can be formed by the addition polymerisation of monomers of the general formula CH, = CRX

Where R may be H, CH; or a halogen and X includes halogen, aryl, amide, ester,

substituted ester, nitrite and carboxyl groups

The polymerisation reaction can be represented as

nCH2 = CRX > [CH2-CRX}

and there are three quite distinct stages in addition polymerisation

a) Initiation: Attack on the C=C bond by an initiating species

b) Propagation: Growth of the polymer chain by successive addition of monomer

units

c) Termination: Deactivation of the polymer chain, terminating further addition of

monomer units

I - The Chemistry of Acrylic Resins 5

1 Initiation

Polymerisation may be initiated by one of three mechanisms:

a) Free Radical Initiation

RR,—>R + R,,then R” + CH, = CHX —> RCH, — CHX

radical radical + monomer radical

b) Anionic Initiation

BA +CHa=CHX > A-CH2-CHX Br c) Cationic Initiation

A Bï+CHa=CHX > B-CH2-CHX*A™

Although most vinyl! and acrylic monomers will undergo any of the above types of initiation, some of the polyether monomers can only be polymerised when ionic initiation techniques are used

In practice ionic initiation is restricted to the production of the so called "high" polymers used in structural applications Polymers for surface coating applications are normally prepared exclusively using free radical initiation techniques

The initiator is consumed during the course of the polymerisation and the reaction mixture requires “topping up” or “spiking” with initiator at regular intervals to maintain

the free radical concentration at the desired level, to ensure a high degree of conversion

of monomer to polymer

Free radicals are extremely reactive, and the monomer units in the reaction mixture have

to compete with other species for the free radical This includes competition with other free radicals

The rate of initiation is defined in terms of the rate of formation of polymer chains and is expressed as:

Rị = 2fka[l]

where: Ri is the rate of initiation

f is the frequency factor for free radicals reacting with monomer units

ka is the decomposition rate for the initiator

Ì 1s the molar concentration of the initiator

6 I - The Chemistry of Acrylic Resins

The number of chain ends present at any given time can be determined by:

Ni = 2fkafljt where: Ni is the number of initiated chains at time t

The number of chain ends per monomer unit is given by

Mp

where: Mi is the number of chain ends per monomer unit

Mm _ is the molecular weight of the monomer

Mp _ is the mass of polymer formed during time t

Example: A monomer of molecular weight 62.5 shows a conversion of 6% over a7 hour reaction time, the mass of polymer formed being 18.2 grammes from a 5 molar solution

of the monomer and 0.1 molar solution of initiator

Once the monomer radical has been formed, propagation proceeds rapidly as the number

of monomer units increases successively to produce a growing polymer chain After each successive addition, the free radical is retained on the vinyl carbon atom of the end chain unit

I - The Chemistry of Acrylic Resins 7

The ease with which a monomer forms a free radical and the reactivity of the radical, once formed, determines the rate at which propagation proceeds Each propagation step proceeds at a speed determined by the propagation rate k, which is constant for a given monomer at a given temperature The propagation rate is independent of the degree of polymerisation of the growing chain

For homopolymerisation, the reaction rate (R,) is given by the equation

Rp = kẹ [M] [M']

where: Rp _ is the reaction rate

p is the rate constant for propagation of monomer M

[Mj is the molar concentration of monomer M

[M] ¡is the molar concentration of monomer radicals M'

3 Termination

In order to terminate a growing polymer chain, it is necessary to effect the removal of the free radical from the polymer chain This may be accomplished in one of several ways,

but all involve either a “combination” reaction, a “disproportion” reaction or a “transfer”

reaction The exact mechanism will be dependent on the chemical structure of the monomer, the polymer chain and the nature of any other species present

(i) Termination by Combination

This is the simplest form of chain termination It involves two growing polymer chains which combine with the mutual extinction of the radicals

This type of termination reaction will result in a “head to head” linkage of the two

polymer chains involved

(ii) Termination by Disproportion

This occurs when the radical of a propagating chain, abstracts an H atom from another propagating polymer chain, to achieve the mutual extinction of the radicals

8 1 - The Chemistry of Acrylic Resins

In this case the two polymer chains do not combine, but remain as separate entities, one chain containing an unsaturated end unit, and the other a saturated end unit

will result in the termination of the growing chain and give rise to a C=C bond at the

position on the polymer chain where the abstraction occurred

Whether a polymer terminates by a disproportionation or by a combination reaction depends upon the configuration of the monomers involved If there are no labile H atoms available, then termination will be by a combination reaction, e.g isobutyl styrene homopolymers If labile H atoms are available, then termination may occur by both combination and disproportionation reactions, e.g acrylic monomers

When disproportionation occurs, the product wil! contain polymer chains with

unsaturated chain ends or with unsaturated sites along the polymer backbone, as a result

of the abstraction of H atoms

These C=C bonds are capable of reactivation by a free radical

Where the unsaturation is at the chain ends, propagation will continue, in the presence of sufficient monomer, as before

Where the unsaturated sites are along the polymer backbone, the restart of chain propagation will result in branching of the polymer chain

The above illustrates a branched chain polymer formed by activation of a double bond situated on a previously formed polymer chain

I - The Chemistry of Acrylic Resins 9

(iii) Termination by Transfer

This involves the removal of the radical from the propagating chain and the transfer of the radical to another chemical species

Both combination and disproportionation reactions result in the extinction of radicals

However, in the case of transfer reactions, the radical is not destroyed, but merely removed from the propagating chain and transferred to another species The new radical may, dependent upon its stability, act to initiate the propagation of another polymer chain

10 I - The Chemistry of Acrylic Resins

The species AB in the example above, can be a solvent, a polymer, a modifier added

specifically as a chain transfer agent, or an inhibitor

If the radical B’ does initiate polymerisation, then the overall rate of propagation is unaffected by the transfer reaction However, the molecular weight of the polymer formed under these conditions, will be lower than that formed without the occurrence of transfer reactions

If B’ does not initiate polymerisation, it can be considered to be an inhibitor for the polymerisation, since it has, in effect, stopped chain growth and removed a free radical from the system

There are various forms which the transfer reaction can take It is often used as a means

of modifying the length of the propagating chain, and hence, controlling the molecular weight of a polymer In general, the qualitative effect of transfer reactions on the molecular weight of the polymer is the same, by whatever mechanism it occurs The effect of unsaturated chain ends or residual unsaturation along the polymer backbone, is a source of chemical weakness in a surface coating system For example, an exterior coating made from polymer with unsaturated chain ends would be particularly susceptible to degradation from UV attack

4 Transfer Mechanisms

(i) Transfer to Solvent

Here the polymerisation solvent acts as a transfer medium

Consider the following example of polystyrene homopolymerisation in carbon tetrachloride solvent:

The solvent participates in the reaction

resulting in the termination of the

propagating chain, and the formation

of a new free radical species "

The “CCl, radical so formed, is

active and may initiate polymerisation

R tocu,- CH+CH,—-CHCI + CCI,

Figure 1-6

1 - The Chemistry of Acrylic Resins 11

CCl,+ CH = CH, ———đằ CH— CH,CCI,

a oO

Figure 1-7 The frequency with which transfer occurs will depend on the chemical structure of the

solvent, the monomer and the solvent radical

The transfer constant can be quantified in:terms of the ratio of the reactivity of a given polymer radical towards the chain transfer agent and the reactivity of the given polymer towards the monomer

ks

ZG

where: kz = transfer constant

Ks = rate coefficient for transfer to solvent

Kp = rate coefficient for propagation of the polymer radicals

The k, varies with temperature and solvent type, but in general, solvents are relatively weak chain transfer agents Typical examples of chain transfer constants are listed in the table below

TABLE 1-1: CHAIN TRANSFER CONSTANTS AT 60°C

Solvent Acrylonitrile Methaorylate Styrene any Acetone 0.000095 0.00036 0.023 0.0016 Acetonitrile 0.00017 - 0.0023 0.0013 Aniline 1.05 0.0075 0.011 0.026 Benzene 0.00021 0.0014 0.00017 0.00016 Methy! ethyl ketone 0.00055 0.00089 0.028 0.0097 Carbon tetrachloride 0.000073 0.0043 0.57 1.18 Chloroform 0.00049 0.00089 0.00345 0.02 Ethyl acetate 0.00022 0.00027 0.0091 0.0004 Ethyl! alcohol : 0.00071 0.0085 0.0033 Triethy! amine 0.155 : 0.017 0.049

The term “degree of polymerisation” is used to define the number of monomer units in the polymer

12 I - The Chemistry of Acrylic Resins

In a free radical polymerisation, where the rate of radical formation is kept constant, the degree of polymerisation is proportional to the concentration of monomer

The degree of polymerisation, multiplied by the molecular weight of the monomer gives the polymer molecular weight Chain transfer to the solvent not only reduces the molecular weight of the polymer formed initially, but it also affects the polydispersity which increases as the polymerisation proceeds

Normally, when carrying out solution polymerisation on a commercial scale, solvents with chain transfer coefficients below 0.001 would be employed, unless a low molecular weight product is required

(ii) Transfer to Monomer

Transfer to monomer can occur in one of two ways Both involve an abstraction process through which a hydrogen atom is transferred to the propagating chain The free radical

is transferred to the monomer to form a monomer radical

a) Abstraction from the Vinyl Carbon Atom

|

RTcn,~cHx} CH,—CH,—X + CH,= CX

Figure 1-8 The monomer radical formed is free to initiate the propagation of another polymer chain

CH,=CX + nCH,=CHX ——> CH,=CX tcr,—cHxt CH,CHX

n-1

Figure 1-9 The newly propagated chain will have an unsaturated end group This end group is available for re-initiation

Figure 1-10

1- The Chemistry of Acrylic Resins 13

Propagation will lead to the formation of a branched polymer chain

Figure 1-11 b) Abstraction from a Side Chain

This type of reaction is typified by vinyl acetate, but may also occur with acrylic monomers with alkyl side groups

14 I - The Chemistry of Acrylic Resins

(iii) Transfer to a Modifier

Modifiers are species that contain a labile atom (most usually hydrogen or halogen) which can be abstracted by the propagating chain

Propagation of the chain is terminated and the radical is transferred to the modifier

a) It can act to initiate the propagation of a new polymer chain The net result in this case is that the modifier has acted as an agent in the transfer of the radical from one propagating chain to another Examples of such “Chain Transfer Agents” (CTA) are carbon tetrabromide, ethane thiol and tertiary butyl mercaptan

or b) ‘It can form a stabilising free radical which will not take part in further initiation

reactions In effect, the free radical has been removed from the system, and the

modifier has acted as a polymerisation inhibitor Examples of such “Inhibitors” are hydroquinone and nitrobenzene

5 Chain Transfer Agents

The radical formed by abstraction above is available to initiate propagation of another polymer chain

I - The Chemistry of Acrylic Resins 15

The effectiveness of a chain transfer agent is measured in terms of its ability first to terminate a growing polymer chain, and then to initiate the propagation of another polymer chain

This is quantified as the transfer rate constant C,, which is temperature dependent and is different for different monomer types

In general, long chain alkyl mercaptans give the best performance as CTA’s They are extensively used in commercial polymerisations of acrylate monomers as modifiers to control the molecular weight of the final polymer

The number of chain ends arising from transfer reactions can be obtained from the equation:

Cs = rate co-efficient of transfer for the modifier

[S] = molecular concentration of modifier

[M] = molecular concentration of monomer

This has been related empirically to the molecular weight of the polymer by Mayo in his equation:

where: P degree of polymerisation in presence of modifier

Po degree of polymerisation without modifier

Cs, [S] & [M] are as before

TABLE 1-2: COMPARISON OF C,; FOR VARIOUS COMMONLY

ENCOUNTERED CTA’s Chain Transfer Agent Homopolymerisation at 60°C

Methyl Methacrylate Styrene Carbon tetrachloride 0.27 2.2

Butane thiol 0.66 22.0

Ethyl mercapto acetate 0.63 58.0

16 1 - The Chemistry of Acrylic Resins

6 Inhibitors

Inhibitors act by removing free radicals from the system Depending on the relative

reactivities of the inhibitor and the radical concerned, the free radicals may be removed

either as fragments of initiator decomposition or as very short chain radicals

Commonly encountered inhibitors include nitrobenzene

NO, AXxx(CH-CH +

and Ferric chloride

AWW CH,-CHX + FeCl, ~———» AAWVCH—CHGI + FeCl,

NWW CH CHXC] + FeCl, ———® AWW CH=CHX + HCI + FeCl,

Figure 1-16 The radical formed with benzoquinone is resonance stabilised and thus will not initiate polymerisation

I - The Chemistry of Acrylic Resins 17

Il THE KINETICS OF THE FREE RADICAL

POLYMERISATION OF ACRYLIC MONOMERS

1 Initiation

Initiation proceeds in two stages:

a) decomposition of initiator into free radicals

where: kd is the rate dissociation constant

[I] is the initiator concentration

Usually it is not every free radical, produced by initiator decomposition, which reacts with a monomer molecule and the equation is modified to become

18 1- The Chemistry of Acrylic Resins

Thus an overall equation for acrylic polymerisation can be written as

— kel PL)! = kota ally

Rp = kp MKC 2M) Ð kp[MK kt )

where: [Ï] = concentration of initiator

[M] = concentration of monomer

fkq = frequency and decomposition constants for the initiator

kt = rate coefficient of chain termination

thatis: Rpo[M] and Rpa[l]”

Plots of Rp against [M] for various monomers produce a straight line graph of the type illustrated below

1- The Chemistry oƒ Acrylic Resins 19

4 Polymer Chain Length

Kinetic Chain Length v is defined as the number of molecules reacting with a reactive centre from its formation to its termination

_ Keim?

2kt Rp

and for the steady state

Ri = Ri and

fka[l]-›

Rp = kp My (“2lly’s

it follows that the rate constant for propagation k, (in the early stages of the reaction) will

be proportional to [I]

20 1- The Chemistry oƒ Acrylic Resins

If f (the initiator frequency or efficiency) is independent of [M] then the rate of propagation is proportional to [M]

that is:

Rp o [M]

This holds experimentally where the initiator frequency f is high

Where f is low then:

“styrene with benzoyl peroxide initiator

IH —>

Figure 1-19

If the initiator concentration does not vary much during polymerisation (as for example when using a continuous addition or ‘drip feed’ process) and the initiator frequency f is independent of the monomer concentration (as is the case when f approaches 1) then

Rp a [MỊ

and the polymerisation will proceed in accordance with the principles of first order

kinetics.

I- The Chemistry of Acrylic Resins 21

An example is included here to illustrate this

Methy] acrylate is polymerised in xylene at 50°C using azo di-isobutyronitrile (AZDN)

If [1] = 1 x 10? mole litre!

Initiation rate constant ka = 1.2 x 10 sec!

Termination rate constant k; = 7.2 x 10’ litres mole? sec’!

22 I - The Chemistry of Acrylic Resins

The following table taken from the literature compares [M] and [I] for two monomers

commonly encountered in acrylic polymers The initiator is AZDN and a reaction

temperature of 50°C was quoted

Since, as we have seen Vv =

If the monomer concentration is kept constant then the kinetic chain length relationship is;

1 - The Chemistry of Acrylic Resins 23

The degree of polymerisation will be given by:

Examples for styrene in xylene using AZDN at 50°C and assuming f=1

{I}=1x10° mole! [M] =1 mole litre’

i = 186

Molecular weight of styrene monomer = 104

Therefore molecular weight of styrene polymer = 186 x 104 = 19,000

For termination by disproportionation, I=v

24 I- The Chemistry oƒ Acrylic Resins

IV CHEMISTRY OF ACRYLIC MONOMERS

1 The Major Acrylic Monomers

TABLE 1-4: THE MAJOR ACRYLIC MONOMERS

Butyl acrylate CH;—CH— COOC,H, -54

A erylate mino CH,—CH—COOCH,CH,N(CH,), — -38

I- The Chemistry of Acrylic Resins 25

TABLE 1-4: THE MAJOR ACRYLIC MONOMERS (continued)

methacrylate |

CH, n=16-20 Dimethyl amino CH;C — COOCH,CH,N(CH,), 19

26 I - The Chemistry of Acrylic Resins

Of particular importance in surface coating applications are the two series of related monomers derived from acrylic acid and methacrylic acid These monomers are termed

“acrylates” and “methacrylates” respectively and polymers derived from either species are known collectively as acrylic resins Typical acrylate and methacrylate monomers are shown in the table above

Acrylic esters have the common structure:

CH= 1"

COR

|

O Figure 1-21 Whilst methacrylic esters have the common structure:

CH,=C— CH,

i

O Figure 1-22 Typical examples are:

CH,= CH CH,= oe CO,C,H, CO,C,H;

Butyl Acrylate Butyl Methacrylate

Figure 1-23 Various types of functionality can be built into acrylic and methacrylic esters which can

be introduced into the final acrylic polymer Typical functionality includes hydroxyl, carboxyl, amide and epoxy groups