A review of significant directions in fluorosiloxane coatings

M J Owen Michigan Molecular MI 48640, USA

Institute, 1910 W Saint Andrews

Road, Midland,

Keywords Fluorosiloxanes, low-surface-energy polymers, hydrophobicity, glass-transition temperature, applications

Summaries A review of significant directions in tluorosiloxane coatings An outstanding characteristic of fluorocarbon substitution in polymers is tile potential for obtaining coatings of very low surface energy. The low degree of fluorination in the most familiar fluorosiloxale, polymethyltrifluoropropylsiloxane (PMTFPS)is insufficient to attain this end. PMTFPS is of comparable surface energy to polydimethylsiloxane (PDMS), with similar uses that particularly exploit its solvent resistance characteristics. An exciting trend in fluorosiloxanes is the increasing interest in materials with a higher degree of fhorination than PMTFPS. These polymers have lower surface energies than PDMS with potential for a variety of PDMS surface modification and novel coating ap plications.

Examen d'imporlanls d~.veloppements dans le domaine des revilements de lluorosiloxane Une remarquable caracteristique de la substitution fluorocarbone dans les polym~res est le potentiel offert dans le domaine de la production des rev~tements qui ont une tr~s basse ~nergie de surface. Le bas degre de fluorisation dans le fluorosiloxane le plus familier, polym~thyltrifluoropropylsiloxane (PMTFPS) n'est pas assez ~lev~ pour atteindre ce but. Le PMTFPS a une ~nergie de surface qui est comparable ~ celle du polydim~thylsiloxane (PDMS) et ofhe des utilisations similaires, utilisations qui exploitent surtout ses caract~ristiques dans le cadre de la r~sistance aux solvants. Un d~veloppement tr~s prometteur en ce qui conceme les fluorosiloxanes est repr~sent~ par rint~r~t qui est manifest~ pour les mat~riaux qui ont un degr~ de fhorisation plus ~lev~ que celui du PMTFPS. Ces polym~res ont une ~nergie de surlace qui est plus basse que celle du PDMS et montrent la possibilit~ de modifier la surlace du PDMS d'une vari~t~ de mani~res, tout en oflrant de nouvelles applications pour les rev~tements.

Eine Ubersicht der wichligen Entwicklungen bei Fluorosiloxanlacken

or correspondenceconla(

Eine herausstechende Eigenschaft der Fluorocarbon-Substitution in Polymeren ist die M6glichkeit, Lacke mit einer sehr niederen Oberfl~chenenergie herzustellen. Die Cngisten Fluorosiloxane, Polymethyltrifluoropropylsiloxane (PMTFPS), sind li3r diesen Zweck wenig geeignet, da sie nur einen geringen Grad der Fluorination zulassen. PMTFPS haben eine ~ihnliche Oberfl~ichenenergie wie Polydimethylsiloxane (PDMS) und haben vergleichbare Anwendungen, die vor allem ihre L6sungsmittehesistenz ausnutzen. Ein interessanter Trend bei FI uorsiloxanen ist das steigende Interesse in Materialien mit einer geringeren Oberll~chenenergie als PMTFPS. Oiese Polymere haven deutlich niedere Oberfl~chenenergien als PDMS mit dem Potential fill mehrere PDMS Oberfl~chenver~nderungen und neuen Anwendungen in Earben und Lacken.

M J Owen Michigan Molecular Institute, 1910 W Saint Andrews Road, Midland, M148640, USA Tel: +1 (989) 631-7339 Fax: +1 (989) 832-5560

Emaih [email protected]

Copyright OCCA2004

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A review of significant directions in nuorosiloxane coatings M J Owen

Introduction Within the silicone industr)~ mention of fluorosiloxanes, or fluorosilicones as they are more loosely known, has long brought solely to mind polymethyltriflLIoropropylsiloxane (PMTFPS). Available since the 1 950s as a liquid polymer of various molecLilar weights, and in crosslinked form as a gel, sealant or elastomer, it provides an alternative to polydimethylsiloxane (PDMS)in situations where low swell in the presence of organic solvents, oils and fuels is required. PMTFPS was chosen for industrial productiolf because it combines excellent solvent resistance with adequate thermal, oxidative and hydrol)4:ic stability for the minimLim degree of fluorination, single C-F bonds being unable to provide the stability and solvent resistance offered by the CF;- cluster2 In essence, PMTFPS contains the least amount of fluorine (least added cost) consistent with marked property improvement. Note that the ethylene bridge between the CF3- group and the silicon in the siloxane backbone is essential for maintaining sufficient thermal stability. Fluorine is often incorporated into organic polymers to enhance thermal stability, but in the case of PDMS this was already adequate; the synthetic design challenge that was SLICCeSSfLIIly met was to maintain this asset while adding the solvent resistance capability. Lowering surface energy was not an original aim despite the fact that one of the outstanding characteristics of fluorocarbon substitution in polymers is the potential for obtaining coatings and other materials of very low surface ener~: As with the stability,' aspects, it was sufficient to match the already low surface ener~' of PDMS. The replacement of one methyl on each silicon with a trifluoropropyl group proved to be insufficient to significantly lower the PMTFPS surface ener9 below that of PDMS. The studies of Jarvis and Zisman on the surface energy of fluoropolymers 3 showed that incorporation of longer fluorocarbon side-chains of structure CF3(CF2)n(CH2)2- would be required, but it is only in the last decade that an increasing interest has arisen in exploiting these more-highly fluorinated fluorosiloxanes such as polymethylnonafluorohexylsiloxane (PMNFHS). This exciting trend is one of several that are currently changing the fluorosiloxane arena) These trends may be summarised as: 9

72

an increasing interest in more-highly fluorinated siloxanes than PMTFPS;

9 a greater variet~ of synthetic strategies to couple fluorocarbon sidechains to the siloxane backbone; 9 an increasing diversity in the nature of the flLiorinated moiety; 9 variations in the polymer backbone providing fluorosiloxane/fluoroalkane copolymers; 9 the use of small amounts of fluorosiloxane in conventional PDMS silicone matrices; 9 a growing number of new and potential coating-related applications for these fluoropolymers.

tified as a wide variety of materials and surface ener@' characterisation techniques are involved and the reader nlust refer to the original citations to be sure of the approach used, the contact angle liquids chosen, and the conditions of the measurements. When available, critical surface tensions of wetting using n-alkanes have been selected for this table. Note also that when a range of material compositions have been examined, the lowest value reported has been selected for inclusion in the table. Advancing water contact angles (Oa) are included in Table 1 as they can usually be compared with less unceitainty than the derived surface ener~' values. Moreover, they provide an indication of relative hydrophobicity. There are several materials that correspond closely to the 122 ~ value for n-perflLIoreicosane, s This material has been inclLided as it is a solid perfluorocarbon that is claimed to be the lowest surface fl'ee ener~/based on a CF;- alignment. Unfortunatel)~ the reported value is not a critical surface tension of wetting but an C)wens/Wendt geometric mean surface ener~' calculation. There may be some further room to challenge the 'ultimate' low value assertion as the contact angle values measured are dynamic values. Neveltheless, this n-perfluoroeicosane repolt offers a basis for comparison. The several contact angle values significantly higher than 1 22 ~ in Table 1 may be real challenges to the ultimate claim or they may be due to factors such as compositional hetero-

H ighly fluorinated siloxanes The surface properties of flLiorosiloxanes were reviewed in detail in 1995.s At that time most of the materials were fluoroalkylsiloxanes of structure:

(F3(CFT),(CHT)7 I - (Si- 0 -)x I OH3 This is simply the PMTFPS structure with a longer perfluoroal194 segment. Since then a broader diversity of fluorosiloxanes has been characterised. Examples are shown in Table 1 where o~ is the solid surface ener9 and Oa the advancing contact angle of waten This table is not comprehensive; it is merely illustrative of the trend. Detailed comparisons within the table are not jus-

Table 1: Fluoropolymer surlace properlies Polymer

Solid surface energy ~ s (rnN/m)

Advancing contact angle of water e~ (deg)

P01yheptadecaflu0r0decyl0xymethylstyreneG 6 N0naflu0r0hexylsilsesqui0xane7 6.6 122 n-perfluoroeicosanes 6.7 122 117 Polymethylpropenoxyfluoroalkylsiloxar~e9 8.8 118 Fluorosilicone-acrylate copolymers/PVC1~ 9.0 124 EsteFtunctior~al fluorosilicones" 9.5 Polypentadecafluorooctyl metllacrylaten 10.6 119 Polyhexafluoropropylene~3 16.2 Polymetllylnonafluorollexylsiloxane~4 16.3 115 108 Polytetrafluoroetllylene [PTFE]~ 18.3 104 Polymethyltrifluoropropylsiloxane [PMTFPS}~6 21.4 108 Polydimethylsiloxane [PDMS]~7 22.7 CFs(CF2)9(CH2)2SJO~/2monolayeH8 6 124 CF~(CF2)gCH2OCO(CH2)lomonolayed~' 9.2 Pertluoroetllersilane/PDMS2o 8 140 Fluoroalkylsilane/PDMS2~ 140 Tridecatluorooctylsilane x-linker/PDMS22 136 CF~(CF2)m(CH2)nCOCI/PDMS23 14.5(JKR) CF3(CF2)7(CH2)2SJO~/2monolayer on PDMS24 15.5(JKR) JKR:Directsolid surface-free-energymeasurementby the Johnson,Kendalland Robertscontactmechanics approach

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A review of significant directions in fluorosiloxane coatings M J Owen

geneity or roughness effects. The latter are well known to enhance the apparent contact angle of intrinsically non-wettable materials and such effects have been exploited in a variety of 'hyper' or 'super' water-repellent material approaches. Direct solid surface-free-energy measurements by the Johnson, Kendall and Roberts (JKR)4s contact mechanics approach eliminate the unceltainties inherent in contact angle approaches. Unfortunatel,~, there has been little extension of this approach to fluorosiloxanes since the last review, s with the exception of the work of Perutz et aP~ on semi-fluorinated acid chloride grafting to hydrolysed PDMS surfaces. Table 1 also contains comparative data for other fluoropolymers with an emphasis on vel~/ low surface energy values. A variety of polymers with surface energies less than half that of PDMS and polytetrafluoroethylene (PTFE) are now available with indications that the limit in low surface ener~1 fluoropolymers can be matched by other non-silicone materials but has been effectively reached by a variety of fluorosiloxane synthesis strategies.

Synthetic strategies Tile increasing variety of synthetic strategies to couple fluorine-containing sidechains to the siloxane backbone is obvious from some of the exam pies i ncl uded in the table. Production of the original PMTFPS commercial polymer was by hydrosilylation addition of CF~CH=CH 2 to Sill so this was the obvious way to introduce longer fluorocarbon sidechains using CF3(CF2)nCH=CH 2 olefins. However, numerous other ways have since been suggested, including, for example, tile use of unsaturated perfluoroall~d esters instead of fluoroolefins for addition to Sill," and the addition of SH-functional fluorocarbons to Si-vinylcontaining siloxane polymers2 s Moreover, several other ingenious developments in synthetic strategies are emerging. Some researchers use fluorosilanes and fluorosiloxanes to modify other polymers such as the work of Kawase and co-workers 26 on the rnodification of cell u lose using allylam m o ni u m hydrochloride-fluorosilane combined systems and fluorosilanes having tributylphosphoniLIm segments. Coatings with high water and oil repellency and antibacterial activity resLiIted. Sometimes only tile chain ends of polymers are modified. Examples of this type include the use of bis[3,5-(trifluorom ethyl)phenyl]m ethylsilyl-termi nated

polybutadiene 27 and fluorosilane-terminated polysty,'renes.28A palticularly interesting example of this strate~1 has been repolted by Krska and co-workers. 29 They use perfluoroalkyl groups to terminate carbosilane dendrimers. The interest lies in tile new direction this promises. Carbosilanes are a related organosilicon polymer to siloxanes and it must be just a matter of time before the approach is extended to siloxanes. Dendritic polymers, both dendrimers and hyperbranched polymers, are characterised by the large number of chain ends that arise in this most recently recognized polymer architecture. Another chain-ending modification strate~1 is to incorporate the fluorinated moiety in the cross-linking molecule. Johnston and co-workers have reported the use of fluorinated cross-linkers such as (tridecafluoro-1,1,2,2-tetrahyd rooctyl)-trieth oxysilane in tin-catalysed alkoxysilane cross-linked siloxane networks2 2 Note that in many cases fluorosiloxane coatings are not produced from preformed fluoropolymers. Rather, the polymer is formed in situ as, for exampie, in the preparation of weather-resistant coatings by the reaction of alkoxysilanes such as CFg(CF2)7(CH2)2Si(OMe)g with orthosilicates such as tetraethox~,'silane (TEC)S).~~ The fluoropolymer and siloxane components of a coating need not even be chemically reacted together to obtain tile desired combinations of properties. It may be sufficient to simply blend the two entities to achieve such aims as highly water- and oil-repellent textile coatings. On surface energy grounds one would expect the fluorinated component to accumulate at the air interface but this need not always be the case, particularly with deliberately placed layered coatings. For example, release coatings have been described where the fluoropolymer is between the base paper and the silicone release coating to provide resistance to permeation by the release agent enabling clearer separation of tile layers during recycling. 3~

The use of branched flLioroalkyl sidechains should increase the CF~- to -CF2ratio for a given degree of flLiorination and thereby enhance the chances of achieving very low surface ener~l polyrners. This strate~1 has been proposed by Monde et a/ in their use of groups such as CF~(CF2)2[(CF3)2]C(CH2)3Si- in coatings prepared by the sol-gel process from tri ethoxy[4,4-bis(trifl uororn ethyl)-

5,5,6,6,7 ,7,7-heptafluoroheptyl]silane and tetraeth ox~,'silanes. 32 Several workers have incorporated short-chain perfluoroethers into the side-chains of polymerizable silanes33 and siloxane polymers. 2~ This is an interesting development. One of the unusual features of siloxanes is the very flexible siloxane backbone. The very low glass-transition temperatures (Tg) of most siloxane polymers testify to this marked chain flexibility Several uses of siloxanes depend paltly on this backbone flexibili~'. Howevel; incorporation of relatively bullet fluorine-containing entities compared with their hydrogen-based analogs could significantly compromise this high chain flexibility feature. For example, polytetrafluoroethylene (PTFE) has a much higher T_ than PDMS. There is some dispute in the literature as to which of the so-called o~or I~ transitions in PTFE correspond with a glass transition. Both are included in Table 2; but whichever value applies, it is significantI)i higher than that of PDMS. Interestingly, the replacement of a methyl group by either a trifluoropropyl group or a nonafluorohex~,'l group does not have too detrimental an effect on the glass transition, either substitution only raising T by approximately 50K (see Table 2). Igresumably the bulkier group disrupts chain packing as much as it inhibits chain rotation. Nevertheless, tile potential for a much increased T_,~ is ~)ossible for hi~her degrees of substitution or copolymerisation with PTFE-like entities. !

,J

The advent of fluorosiloxanes such as these perfluoroether-modified polymers,

Table 2: Fluoropolymer glass-transilion lemperalures

Polymer Polydimethylsiloxane (PDMS):~-s Polymethyltrifluoropropylsiloxane (PMTFPS)3~ Polymethylnonafluorohexylsiloxane(PMNFHS)~4 Polyoxyhexafluoropropylene (PHFPO)37 Copoly(oxytetrafluoroethylene-oxydifluoromethylene)38 Polytetrafluoroethylene (PTFE)3~'

150 2O3 198 195-219 (varying MW) -140 399 (c~,glass I) 292 (p)

MW= molecularweight

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Fluorinated side-chain diversity

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A review of significant directions in nuorosiloxane coatings M J Owen

where both the side-chain and the backbone are intrinsically flexible, low Tg entities should be important in the many release and low-soiling applications of fluorosiloxanes. Note that tile T values of the fhorosiloxanes in Tableg2 and polyoxyhexafluoropropylene (all CF3containing entities) are similar. However, copoly(oxytetrafluoroethylene-oxydifluoromethylene), with no bulky CF3groups, has a lower Tg than PDMS. Tile fluoroethers present an oppoltunib' and a challenge to fluorosiloxanes. The oppoltunity is for useful new hybrids and copolymers as shown by the work of Thanawala and Chaudhury 2~ previously described. The challenge is the potential replacement of fluorosiloxanes in established applications by fluoroethers. The two polymer systems have similar property profiles and will compete with each other much more directly than with earlier fluoropolymers. Tile balance will likely depend on other factors such as cost. An unusual fluorosiloxane has been described 4~ that has low surface ener~ trifluoromethvI groups and strong hydrogen-bonding acidic hydroxyl groups adjacent to each other at the terminus of the polymer side-chain substituent. The structure of SXFA is {[(C F3)2(OH)CCH2C H = CHIC H sSiO} n and this combination of disparate characteristics makes it useful as a coating in sensor devices.

Polymer backbone diversity In principle, any of the common fluorocarbon repeat units could be used to create fluorosiloxane/fluoroalkane alternating copolymers. Recent interest has focused on novel hybrid flLlorosiloxanes based on fluorinated telomers containing vinylidene fluoride (VDF), tetrafluoroethylene (TFE), and hexafluoropropylene (HFP). 4~,42 Structures such as [R2Si(CH2)n-RF-(CH2)nR2SiO]• have been reported, where R can be CHs, C2H4CFs or C2H4C4Fg, n is 2 or 3, and RF is (VDF)0 I(TFE)23(H FP)0:. These materials are tather sta'ble over a wide range of temperatures (Tg-- -40~ and decomposition temperature --400~

Fluoropolymer modification of PDMS surfaces From a SLirface-propel~' and applications viewpoint, it is often sufficient to modify the surface of a coating. AB or (AB)n block polymers where one segment is a fluorosiloxane, surface-active 74

block and the other is the same as the bulk material (anchor block), have been used for this purpose. 4s This picture of an anchor block is an idealised model. In practice, random copolymers can be effective and AB-type polymers can be used in a different C-coating matrix. For example, Kim et al 1~ have shown that random, block and graft fluorosiloxane copolymers synthesised from a fluorinecontaining monomer such as perfluoroalkyl ac~'late, and a silicon-containing methacrylate or vinyltrialkoxysilane can lower the surface ener~' of pol)Mnylchloride (PVC) films to 8 to 9mN/m at best. As with many other coating formulations, tile surface-modifying copolymer can be formed in situ during preparation of tile coating. Tile previously mentioned work of Thanawala and Chaudhury 2~is a case in point. Other examples include Everaert and co-workers' use of chemisorbed fluoroalkylsiloxanes on siloxane elastomer surfaces to provide a bacteria-repellent treatment for indwelling voice prostheses21 Using a little fluorosiloxane to confer fluoropolymerlike propelties on PDMS in these ways should open up significant new opportunities. Much of the commercial success of PDMS since its 1943 introduction has been due to its ability to modify' the surface properties of most common organic polymers by virtue of having a lower surface ener9 almost all hyd rocarbonbased polymers. Tile growing availability and diversity of fluorosiloxanes such as polymethylnonafluorohexylsiloxane (PMNFHS) with significantly lower surface energies than POMS should expand the market for siloxanes just as POMSbased additives and surface treatments ha\e produced new oppoltunities for organic coatings, s

Coating applications for fluorosiloxanes The following list summarises many of the applications proposed for fluorosilane and fluorosiloxane coatings in the scientific and patent literature: 9 self-assembled monolayers; 9 coupling agents; 9 fuel-resistant coatings; 9 wear-resistant coatings and lubricants; 9 release coatings; 9 low soiling treatments; 9 marine antifouling coatings; 9 anti-bacterial coatings; 9 hydrophobic/oleophobic textile and fabric treatments; 9 weather-resistant coatings; 9 ice- and snow-repellent coatings;

9 sensor coatings; 9 personal care barrier creams; 9 polishes; 9 antifoams. This list is significantly longer than that given in the author's 1995 s review. Some of tile applications in this listare

well-established uses of fluorosilane and fluorosiloxane coatings. These include fuel-resistant coatings in tile automotive and aeronautical area; release coatings, particularly for PDMS-based siloxane pressure-sensitive adhesives; low soiling treatments for optical equipment such as camera lenses; and hydrophobic/oleophobic textile and fabric treatments. Newel; developing applications where their potential has been recognised but commercial implementation has yet to occur include marine antifouling coatings and anti-bacterial coatings. Note that ice- and snow-repellent coatings are listed separately from weather-resistant coatings as the former has more to do with low surface ener9 whereas the latter area is a consequence of the high thermal and oxidative stabili~' and UV resistance of siloxane polymers in general. Self-assembled fluoroal~'lsilane monolayers are of scientific importance for the significant role they have played in understanding tile fundamentals of wetting and surface energy of fluorinated surfaces. They also represent tile ultimate in thinness of coatings. Fluorinated materials are rarely used to enhance adhesion, not surprisingly because producing low surface energies is the opposite, in terms of wettability, to what is usually required in primers and coupling agents. Nevertheless, there are specialised fluorosilane coupling agents proposed for enhancing the adhesion of fluoropolymer coatings, s Non-cross-linked coatings such as polishes and barrier creams may be of less interest to paint and coatings technolo,gists than cross-linked coatings. However, they are included because they offer advantages over conventional siloxanes in substantivity and stain repellency. Antifoams are also included in this list because they are useful in achieving high coating quality. PDMS-based siloxane antifoams and defoamers are useful in organic coatings and it is anticipated that fluorosiloxane-based antifoams will be needed for siloxane coatings. Possibly related to defoaming phenomena is the unique application for noxious vapour suppression from pool, puddles and spills etc, by a layer of glass microbubbles coated with a fluorosilane or fluorosiloxane film. '4'4

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A review of significant directions in fluorosiloxanecoatings M J Owen

C)bviously, what sets fluorosiloxanes apalt from other film and coating-forming fluoropolymers isthe presence of the siloxane bonds. This feature brings both advantages and disadvantages compared with other fluoropolymers. One drawback associated with the siloxane and all other heteroatom backbones is a susceptibility to heterol)4ic chain scission at extremes of pH. Some fluoropolymers show extreme resistance to concentrated acids and bases, but these are not areas of application open to fluorosiloxanes. The paltially polar siloxane backbone and also, to a celtain extent, the hydrocarbon component necessal)~ for adequate thermal and oxidative stability,', fundamentally limit this aspect of fluorosiloxane performance when compared with materials such as PTFE which can cope with aggressive acids such as H F and H2SO4 at elevated temperatures. On the other hand, this is offset by another great benefit of the siloxane backbone, its very high flexibility. This is vel)~ evident at low temperatLires where fluorosiloxanes are useful to -60~ fifty degrees lower than other common fluoroelastomers such as copolymers of vinylidene fluoride and hexaafluoropropylene (VO F-H FP). Such low temperatures are often encountered in aerospace environments accounting for the considerable uses of fluorosiloxane coatings, elastomers and sealants in this area. FILiorosiloxanes can also be blended with other flLIoropolymers to enhance the latter's low temperature serviceability. Moreover, the siloxane character of fluorosiloxanes brings further benefits to coating technologists not shared by other flLIoropolymer systems. Not only does the siloxane backbone confer desirable flexibility characteristics but it also brings network-forming, siliconbased chemistries to fluorosiloxane coatings, notably those based on platinumcatalysed, hydrosilylation additions of Sill groups to vinyl, allyl and other unsaturated hydrocarbon groups, and ti n-catalysed, al koxysilyl hyd rolysis/condensation reactions.

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Surface Coatings International Part B: Coatings Transactions

Vol.87, B2, 71-148, June 2004