Ox didines end tetremethylxylenediisocyenete based polyurethenesin iegislation-comp|ient antkorrosion coatings GA Howa~h 4 N o r m a n d y R o a d , W o o d p l u m p t o n , P r e s t o n , PR4 0AY, U n i t e d K i n g d o m WoIR c a r r i e d o u t b y I n d u s t r i a l C o p o l y m e r s Ltd, P O B o x 3 4 7 , P r i r m ' o s e Hill, P r e s t o n PR1 4LT, U n i t e d K i n g d o m

Introduction There are only a few companies in the world that produce oxazolidines commercially. Fewer still produce low viscosity oxazolidines which are suitable as latent hardeners to enable production of single-pack moisture activated polyurethanes, There is, therefore, very little published data on their use, although the principles of manufacture have been published with suggested formulations. >~ Their use in anticorrosion coatings has also been recently investigated as part of an MSc thesis, 5 There are many papers and reviews of urethane tecl-mology6 and its use continues to expand. Urethanes are frequently used in anticorrosion coatings.7 However, the isocyanate chosen for this research, is tetramethylxylenediisocyanate (TMXDVM) which has been available commercially only since 1988. There is very little published information on polyurethanes based on this isocyanate, There is no published information about the use of TMXDVM in anticorrosion finishing coat applications. There are, however, a number of papers discussing the use of TMXDVMin water-borne polyurethane dispersions.8-12 Polyurethanes are a very" useful class of polymers which exhibit many desirable properties which those designing anticorrosive coatings can beneficially exploit, There is a large range of uses for this class of product. Examples of the diverse range of applications include their use in clear coatings for shatter-proof glass;:3 as a robber crumb binder for sports surfaces; :4 as the main binder in anticorrosive paint finishing coats;~5 the main binder in aliphatic single pack roof coatings; :6 and as encapsulanLs for electronic components, This wide range of uses continues to expand despite legislative constraints. The commercial availability of new isocyanates has also contributed to the continuing growth of the technolog:~ of pol}qarethanes.

Legislation In 1990, spurred on by legislation in the USA, the Environmental Protection Act legislated in the UK to control further the use of Volatile Organic Compounds (VOC). This had European backing as all governments in the European Union likewise passed laws aimed at improving the enviro:mmnt, This legislation has further concentrated the thinking of resin and coatings formulators. PG6/23(95) published by HMSO gives a number of VOC figures that coatings must meet to conform to legislation. :9 The alternative is to install an abatement plant to reduce the solvent emissions at source. This equipment, however, is expen-

14

sive and has quite high rurming costs. Consequently, the research and development &ive of most coatings and resin producers, is at the moment, the reduction of the solvent content of the paint and resin systems by the high solids route. ~M:other drive is towards conversion to water-borne systems, :2 The pol3n_:rethane resin industry is no exception. One method of achieving the environmental aims demanded by legislation in the pol3,qarethane coatings industry is to use oxazolidines, The low viscosity oxazolidines are particularly useful.

Polyurethanes Polyurethanes do not have quite the same degree of adhesion to metal substrates as epoxy resins, However, they have extremely high resistance to chalking and good abrasion resistance. It is, therefore, more usual to see epoxies specified for use in primer or base coats, and pdyurethanes as finishing coats. The main disadvantage of polyurethanes is that isocyanates are used in their manufacture. With the exception of MDI (Fig~e 1) and TMXDIT M (Figure 3), all isocyanates are toxic. The polyurethanes produced from them are thus perceived to be toxic also. They are not necessarily so when based on MDI and TMXDVM, However, aromatic isocyanates are relatively cheap and, hence, the polyurethanes synthesised from them are also relatively low in cost and, therefore, are very- popular. They do, however, suffer from yello~4ng due to the higl: degree of conjugation and the aromaticity present in the molecule. Aromatic polyurethanes cure very rapidly by moisture and are relatively cheap. Thus, formulators still use them in vast quantities. Th.e yellowing is tolerated because of their low cost and very useful properties. A classic example of an aromatic isocyanate used to manufacture aromatic polyurethanes is methylene diparaphenylene diisocyanate (MDI). Usually, ortho and meta isomers are also present. LW radiation is easily absorbed by the molecules because of the ease by which the energy" can be spread (delocalised) over the molecule. This gives rise to the yellowing seen when aromatic polyurethanes are used

Figure 1: Double~singlebond coniugalion in MDI

Systemof doubleand singlebondsin MDI Surface Coatings Internationd 1999 (I)

Curing mechanismsof polyurethanes

Figure2: Cornmon usagealiphaticisocyanales N{O OCN~ CH2 NCO H3C CH3 IsophoroneDi-lsocyanate IPDI

DictclohexytmetMne4,4-Di.lsocyanale

H3C ~

He•

CH2 ~NCO

HI2MDI OCN(CH2)6NCO Di-lsocyanaleHDI

Figure3: Structureof m-NXDI TM iso(yanalecuringmechanismof polyurethanes NCO H3r~CH3

'2o Tetramelhylxylylenediisocyanate m-TMXDITM

(Figure 1). To overcome this problem, their aliphatic counterparts can be used, Aliphatic isocyanates are considerably more expensive than their aromatic counterparts, Consequently, polFnarethanes made from aliphatic isocyanates are also more expensive, Aliphatic isocyanates cure comparatively m u c h more slowly, However, the yello,a4ng performance of aliphatic polyurethanes is far superior to aromatic polFnarethanes. They yellow only over a long period of time, Examples of aliphatic isocyanates in c o m m o n usage include isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI) and hydrogei~ted MDI (Hla MDI), These c o m p o u n d s are illustrated below in Figure 2, One isocyanate that has been introduced in the last ten years is TMXDV M, This isocyanate has two hindered tertia9 ~ isocyanate groups in the molecule, The structure is shown in Figure 5, It should be noted that although there is an aromatic ring present on the m-TMXDVM molecule, its behaviour and reactivity resemble very closely those of an aliphatic species. The smlcture of meta-Tetramethylxylylene diisocyanate is asymmetrical, The NCO groups are tertiary, They are hindered and shielded from the aromatic rings by the methyl groups, 8-~2 These factors have facilitated the production of TMXDV M based polyurethanes and, in particular, enable the attainment of lower viscosity pre-polymers. They also offer lower water absorption than the other aliphatic isocyanate based pol}narethanes and confer excellent Lv~; resistance and weatherability, This is g o o d for an anticorrosion finishing coat, To obtain good drying rates with aliphatic polyurethanes and thereby corrosion resistance, the use of oxazolidine technology is strongly recommended.

Figure4: Curingreadion of a polyurethanewilh moisture NCO +I120

\ /

Surface Coatings International ]999 (1)

A

C=O + r

Polyurethanes cure by reaction with atmospheric moisture fom~ing an amine and then a substituted urea with the release of carbon dioxide gas. The hydrogen atoms which are attached to the nitrogen atoms can undergo a further reaction with NCO groups on other urethane pre-polymer molecules. Eventually, a tightly cross-linked structure results. The disa&,antage with this type of polyurethane system is that carbon dioxide can get trapped in a drying paint film, Tt~s causes blisters and hence paint film voids. These then b e c o m e areas of unfavourable anode/cathode ratios and veD' high localised corrosion can result, especially where cathodic protection systems are in use or where there a r e stray currents. An explanation of this p h e n o m e n o n is worthwhile. The anodic and cathodic reactions involved in the dissolution of a metal are illustrated below. An appeal to logic will s h o w that a metal of 1 square millimetre corroding at a rate of say 10 ~5 electrons per second will dissolve completely before th.e same metal of an area of 1 square centimetre corroding at the same rate of 10 ~5 electrons per second. The corrosion rate is related to current density. Corrosion is, thus, an electrochemical reaction (Figure 5).

Figure5: Anodirand cathodicreadionsin metal dissolution n| Anodiccorrosionreaction 2 H20 + 02 + 4 e~

~,. 4 OHG

Balancingcathodicreaction in neutral/alkaliner When defects occur in paint films, on structures where there is a stray current, corrosion is greatly accelerated because all the anodic reaction occurs at the defects. This results in a veD~ large current densiD~ and pinholes can occur in the metal itself, completely weakening structural integrity. A method of simultaneously overcoming both potential carbon dioxide gassing and slow cure problems with aliphatic pol}~arethanes, is to use oxazolidine technology.

Curing mechanismof polyurethaneswith oxazolidines Oxazolidines are heterocyclic rings containing both nitrogen atoms and oxygen atoms. They are synthesised by the cyctodehydration reaction of an amino a l c o h d by an aldehyde or ketone. The reaction proceeds by the removal of water which drives the reaction forward because of Le Chatelier's Principle. Oxazolidines can be further reacted to produce a species illustrated in Figure 6. \'~q~en in the presence of an aliphatic isocyanate, the oxazolidine species is very stable and in-can stabilities of nearly two years have b e e n obse~'ed. However, w h e n a coating containing the oxazolidine species and the aliphatic polyurethane is exposed to atmospheric moisture, the water vapour reacts preferentially with the oxazolidine species. This causes the oxazolidine rings to o p e n yielding -NH and - O H functional groups. These then react with the aliphatic pol~mrett~ne species to give a highly crosslinked coating with no carbon

15

Figure 6: B[sOxazolidine reactingwith water and then subsequentproductreading with NCO terminated palyurethanepolymer chains

--/

~1

N

~H , ~ ~

H N~

+ 2 H20

I

Incorez806/007 Xylene TogaFlow300 TogaGlideZG400 Tit(injureDioxide Ircoget906 DBTL tncorez701 lncozot4 Total

0H

31.23% 17.36% 00.43% 00.43% 23.42% 04.34% 00.06% 10.41% 12.32%

trifunclionalTMXDPPUresin additive ~dditive

TR92 thickener cat(~lyst

FlexiblePolyurelhaneresin Oxazolidinelatenthardener

100.00%

Paint properties

_/ N{O

Table 1: Hard anlicarrosive finishing coat reference 91/806/AC23

N{O

N{O

N{O

dioxide gassing. A tFpical reaction sequence is s h o ~ n in Figure 6, Full reviews and p a p e r s illustrating the chemistry of these species a n d some of their applications are available, J-4,~7,~8

Oxazolidine - PoJyurethane anticorrosion paints \,g'1~en p r o d u c i n g paints using polyurethane and oxazolidine resin, the normal guidelines regarding exclusion of moisture apply. Pigments a n d solvents are dispersed in the polyurethane resin and all additives and solvents are added. The moisture content is then determined by techniques such as the Karl-Fischer procedure, Then a n u m b e r of drying tectmiques can b e employed. Vacuum with heat m a y be u s e d or chemical drying techniques. There are a n u m b e r of chemical techniques available. One m e t h o d is to use an oxazolidine specifically designed for moisture s c a v e n ~ a g such as Incozol 2 (Figure 7). s-a

PVC Wt. Solids Vol.Solids SG PersozH(irdness

18.46% 71.33% 60.82% 1.206 285

p r o d u c e d with oxazolidines. Careful examination of tl~e sm~cture p r o d u c e d from the reaction of a diisocyanate with oxazolanes, ie Incozol 4, shows that urethane linkages are p r o d u c e d which, in turn, can h y d r o g e n b o n d , H y d r o g e n b o n d i n g p r o d u c e s some excellent properties such as toughness but it also increases the viscosity of the product. This m a k e s formulation of low VOC, a n d hence, c o a t i n ~ that c o m p l y with the legislation content more difficult, The paint, s h o w n in Table 1, is legislation compliant having a VOC of 504 g/litre. However, m a w formulators w a n t an even lower VOC anticipating that further legislation is on the way, O n e w a y to accomplish this is to use the low viscosity oxazolidine, Incozol LV. O n reaction with atmospheric moisture, the Incozol LV is activated yielding -NH and -OH functional ~ o u p s wh.ich then react with the NCO ~ o u p s of the polyurethane to give a tough crosslinked s~ucture (Figure 8),

Figure 8: Incozot LV reactingwith moistureyielding NH and OH Figure 7: Incozol 20xazolidine Moisture Scavenger IncazolLV N --

O-

c, c, ococ, c, N

C4H9

+ 2 H20

I o

H~// ~CHC4He C2Hs

2-(3-heptyl)-N-bulyl-l,3-Oxazolcme Molar Mass227 Alternatively, an isocyanate m o n o m e r or a very reactive h N h NCO content resin m a y be used, One such resin is Incorez 806/007 w h i c h is a trifmactional species with an NCO content of circa 8%, b a s e d o n TMXDF M, Tl~s c o m p o u n d m a y also b e u s e d as the m a i n binder in anticorrosion finishing coats, An example of its use is p r e s e n t e d in the formulation AC23 featured b e l o w in Table 1, Incorez 701 is a relatively soft and higMy flexible prep o l y m e r which is present to provide some flexibility in the finished system. This illustrates that hard systems can be

]6

HO

NCH2CH2OCOCH2CH2N H H

+ 2 (CH3)2CHCHO OH

This species can n o w b e incorporated with the Incorez 806/007 TMXDV M b a s e d polyurethane to give an anticorrosion finishing coat w~ith e v e n lower levels of solvent and h e n c e VOC content. This p r o d u c t is AC24 a n d is shorten b e l o w in TaMe 2, TMXDI T M b a s e d pol?,narethanes, as already discussed, have extremely g o o d weathering properties a n d so UV adsorbers and Hindered ,~rnine Light Stabilisers (HALS) are not usually used, However, the QI.5 r performance of the above coating can b e e n h a n c e d e v e n further b y the addition, of these addi-

Surfuce Coatings International 1999 (|)

Table 2:AC24 polyurethanefinishingcoal basedon Incorez 806/007 and

Table 4: Guide FormulationRe[ ACP186waler-Msed two-pack epoxy primer

Incozd LV

Component

Incorez806/007 Xylene Additives Titanium dioxideTR92 Ircoge1906lhickener Dibutyl tin catalyst Incorez 701 flexible PU Incozol LV Total

41.00% 05.00% 00.94% 28.00% 04.00% 00.06% 06.00% 15,00% 100.0%

Weight

Port A Incorez 148/024 Shieldex| AC5 gayferrox red Iron Oxide Blanc Fixe N Mistron Talc 754 TegoFoamex1488 Nac0rr1651

10,00

Water

ex IndustrialCopolymersLtd ex WRGraceUd ex Bayer ex SachtlebenChemie ex CyprusMinerals ex TegoChemie ex KingIndusiTies/K&KPolymerics

419.50

Poinl properties

1200,00

PVC Wt solids PersozHardness Volumesolids VOC

Port g

13.00% 80.40% 270 70.30% 220 g/litre

BisphenotA EpoxyResin

Incorez806/007 Incorez 701 flexible PU TegoFlow300 TegoGlideZG400 HALSTinuvinTM 292 UV absorberTinuvinTM 900 Titanium dioxideTR92 Carbonblack Ircoge1906thickener Dibutyl tin catalyst Xylene Incozol LV Total

200.00

Poinl properties

Table 3:AC25 polyurelhanefinishingcoal wilh I-IALSa,d LIV adsorbers 41.00% 05.00% 00.47% 00.47% 00.60% 00.60% 25.00% 03.00% 04.00% 00.06% 05.00% 14.80% 100.00%

Poinl properties PVC Wt solids PersozHardness Volumesolids VOC QUVA Salt spray ProhesionTM

150.00 50.00 200.00 210.00 160.00 0.50

12.90% 80.60% 290 70.50% 217 g/lilTe greater lhun 10000 hours greaterthan 10000 hours greaterlhan 10000 hours

tires. In addition, if a grey coating is acceptable to the customer, then addition of small amounLs of carbon black can eiltlance the weathering performance even further. The accelerated weathering figures are obtained w h e n the pol}~.rethane coating is a p p l i e d over a two-pack water-based epox"y~ coating. Various colours a n d shades are available b y replacing the titaIlium dioxide either totally or partially with s~Tithetic red, yellow and green iron oxides. Use of a waterb a s e d two-pack e p o x y primer ensures that the system as a whole falls well within the VOC regulation limits as defined in PG6/23,19 For completeness, the formulation for this water-based epoxy" primer is s h o w n b e l o w in Table 4. It can b e demonstrated that the primer d o e s n o t have to b e completely dry- before the finishing coat b a s e d o n pol3~arethane a n d oxazolidine is applied. In fact, i m p r o v e d

Surface Coatings International 1999 (|)

PVC Weight solids Volumesolids PHR Touchdry Throughdry Mix ratio VOC

38.44% 67,51%

50.63% 75 30 minutes 5.5 hours 6:1 PartA: Part B 3 g/iitre

results are obtained.5 This is thought to be due to the strong primary covalent b o n d s b e t w e e n the coats. If the primer is n o t fully dried a n d cured, there x~dll b e residual water. In addition, there will b e residual amine, hydroxyl and e p o x y functionaliB~. These groups are all able to react with the urethane/oxazolidine finishing coat leading to excellent intercoat adhesion. However, b o t h coats are also flexible e n o u g h to ensure that little residual tensile and compressive stresses are available to induce cracking a n d peeling,

Polyurethaneprimers If a more traditional single component, aromatic, moisture cured urethane is required, then these are also available. A sample formulation is shovm b e l o w in Table 5.

Table 5: Aromalicmoislurecuredpolyurethaneprimerref. 902024/006 Incorez902/024 Xylene NacorrTM 1651 Bayferroxyellow iron oxide Talc Barytes Zinc Phosphate

26.00 % 25.97 % 1.00 % 15.42 % 12.34 % 15.42 % 3.86 % 100.00 %

lnduslTidCopolymersLtd King/K&KPolymerics Bayer CyprusMinerals Waardals

Aromolk urethone primer pointproperties PVC Wts0Iids

VOLSolids SG Touchdry Throughdry V0C

40% 68.0% 46.0% 1.47 45 minutes 3 hours 320 g/litre

17

The intercoat adhesion between this primer and the Polyurethane/Oxazolidine fu~isbing coat is excellent, However, the adhesion to a variety of substrates is marginally improved when the epoxy primer is used and also the VOC content is m u c h lower. The VOC content of the above primer, complies with the legislation,

13. Wilson MG, 'New coatings for glass', JOCCA, 11, 412~415, 1991.

Conclusion

16. Stanfield RF, 'Moisture triggered polyu~ethane: High solids systems for long term waterproofing', proc. Coatings for Africa 96; P1L~./OCCA, 1996.

The use of polyurethanes continues to expand and newer tectmolo~- has fuelled this growth. The development and commercial introduction of n e w isocyanates such as TMXDY Mhas also spurred on development of polyurethane technology. The Environmental Protection Act (1990), at first sight might seem to hamper formulators. This need not be the case as the legislation can be viewed positively, especially by those companies operating in niche markets, Oxazolidines offer an important newer technology route that formulators may utilise in combination with polyurethanes, There is little doubt that the futt~e appears rosy for polyurethanes as high solids and ware>borne versions are already available and continue to be developed. TMXDY Mis the trademark of Cytec Industries.

14. Moore DP, 'One-component moistm'e-cured urethanes as binders for sports and safety~surfaces', Europea;a Adhesives and Sealants, 35, 32, 1993. 15. ibid. ref. 3.

17. Emmons WD, Mercurio A and Lewis SN, 'Hffo~ Performance Isocyanate Oxazolidme Coatings', Journal of Coatings Technology, 4, No 631, 1977. 18. Schall DC, ~High Solids IsocTanate-Oxazolidine Coatings', Proceedings Water-borne & Higher Solids Coating Systems Symposium New- Orleans, 120, 1985. 19. PG6/23 published by HMSO. []

References 1. Howarth GA, 'Hio~ solids pol~urethane coatings using oxazolidme reactive diluents', Proceedings 3rd Nuremberg conference, Paint Research Association, 1995. 2. Howarth GA, 'New Oxazolidine Curing Agents', Paint andResin, 1, 20, 1995. 3. Howarth GA, 'Devdopments in Oxazolidine Technolog~~, Modern Paints and Coatings Journal, 49-52, 1995. 4. Howarth CA, 'Oxazolanes-old chen~stw, new applications' proceeclmgs 23rd annual international 'Waterborne, High-Solids and Powder Coatings Symposirtm' New Orleans, 1996 5. Howarth GA, MSc, thesis submitted, 'Legislation compliant coatings for the corrosion protection of engineering materials', 1997. 6. Proceedings of the Pol}~u'ethanes World Congress, Vancouver, B.C., 1993. 7. Schx~dndtJ, 'Moisture-Cured, Pol)~u'ethane Based, Surface Tolerant Coatings: An Economical Alternative for Corrosion Contiol', _/14ate~aL~Performance Jou ,~nal pub lished by NACE, 25-31, 1996. 8. Cytek Technical literature m-TMXDYM. 9. Cody RD, 'TMXDUM(meta) Aliphatic Diisocyanate, A Versatile Pol]~rethane Component', proc. Pol3~trethanes Tomorrow Conference, Milan 1988. 10. Howarth GA, Manock, H. L., 'The ChemistW and Technology of Waterborne Polyurethane Dispersions and thei, Use in Functional Coatings including Flooring and Hygiene Applications ', Proceedings of OCCA s>Tnposium at Salford ed. Stanfield R.E 11. Manock HL, 'TMXDUM- based anionic polyurethane dispersions', JOCCA, 9, 348-352, 1993. 12. Howarth GA and Manock HL, 'Zero VOC and Hi o~ Solids Waterborne Polyurethane Dispersions', Coatings Worm Jo~rnal, 18-22, 1996.

|8

Surface Coatings Inlernafional | 999 (|)