Comparison of retarder solvents. used in high-solids automotive refinish clear coatings

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Felicity Edwards
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1 Comprison of retrder solvents used in high-solids utomotive refinish cler cotings

2 Regultory restrictions on voltile orgnic compound (VOC) emissions continue to impct industries tht use solvents. One solution to the mndted VOC requirements in the cotings industry hs been the development of high-solids (HS) cotings. Formultors choose this technology pthwy becuse of the unmtched ppliction verstility of solventborne cotings, i.e., suitbility for shop nd field conditions, precise control of evportion rte, good tolernce for mrginl surfce qulity, chnging environmentl conditions, high esthetics, nd excellent performnce properties. The chllenge to formulte regultory-complint HS formultion tht hs sme or better performnce thn previous coting technology is being ddressed by cretive resin chemistry nd more creful solvent selection. Globl VOC trend VOC requirements imposed upon cotings formultors nd solventborne utomotive refinishers hve been topic of discussion for mny yers in the U.S. The current VOC requirement for utomotive cler refinish is 420 g/l for most of the country. However, Chin hs recently begun to tke ction to improve ir qulity nd environmentl impct by following suit in limiting the mount of solvents llowed in utomotive refinish cler cots. The implementtion of this regultion will require Chinese utomotive refinish shops to exercise more diligence when selecting solvent for their formultion in order to void txes nd/or remin prcticing in certin provinces. On Jnury 26, 2015, Chin s Ministry of Finnce (MOF) nd Stte Administrtion on Txtion (SAT) relesed the Notice on Imposing Consumption Tx on Btteries nd Cotings ( Notice ). The tx took effect Februry 1, Cotings dischrging less thn 420 g/l of voltile orgnic compounds during pinting processes re exempt from the consumption tx. The rest re subject to this 4% consumption tx in the process of production, no mtter wht kind of coting (wterborne/solventborne) it is. For refinish cotings, Beijing, s cpitl of Chin, hs tken the inititive to lunch stricter regultion. The VOC content limits for cler cot must be lower thn 480 g/l strting Jnury 1, 2017, which mens: 1) if VOC < 420 there is no need to py the consumption tx; 2) if VOC is between 420 nd 470, the consumption tx is required; 3) if VOC > 480, it is not llowed in the Beijing mrket whether the consumption tx is pid or not. Since HS formultions require lower solvent loding, the criteri for selecting solvents tht enble gret film properties t lower solvent levels hve gined importnce. Methyl myl ketone (MAK) nd propylene glycol monomethyl ether cette (PMA) re two retrder solvents tht should be considered when formulting nd/or pplying high-solids refinish cotings. Both solvents exhibit trits tht re desirble for VOC-complint HS cotings. Some of these properties include low density, high solvent ctivity, nd slow evportion rte, resulting in better film formtion. Xylene will be mentioned throughout much of this document s reference due to its ubiquitous use in the industry. Tble 1. Comprison of physicl nd chemicl properties Evportion Density, Vpor Dilution Hnsen solubility prmeters rte pressure, rtio b Hydrogen MIR vlue Nonpolr Polr Solvent n-buoac = 1 Functionlity 20 C 20 C toluene bonding g O 3 /g VOC MAK 0.4 Ketone PMA 0.4 Glycol ether ester Xylene 0.7 Aromtic Estmn solvent selector chrt SOL-030A b Dilution rtio determined with RS ½-sec nitrocellulose.

3 Chllenges when developing high-solids cotings include incresing the solids content of the coting while meeting ppliction viscosity requirements, blncing cure chemistry to chieve suitble pot life with cceptble drying speed, nd meeting or exceeding mndted coting properties (durbility, chemicl resistnce, etc.) germne to specific ppliction. These chllenges cn be seen long the vlue chin from rw mteril nd intermedite production to ppliction. Resin synthesis Let-down solvent The mjor binder component in typicl industril cler coting is n crylic, polyester polyol, or combintion of the two. Thus their redesign represents the gretest opportunity to build solids content without pprecibly incresing solution viscosity. Therefore, let-down solvent selection for high-solids cotings is extremely importnt, since less solvent is vilble for controlling pint tomiztion nd rheology chrcteristics. Tble 2. Polyester resin HS-3-6T viscosity profile b Brookfield viscosity, Solvent cp MAK 808 PMA 1,930 Xylene 1,420 HS polyester resin bsed on Estmn TMPD glycol (Estmn publiction N-306). b Estmn publiction M-271C Polymeriztion solvent Low-moleculr-weight crylic resins (oligomers) re prepred by free-rdicl polymeriztion of vrious crylic monomers. The extent of monomer-to-polymer conversion nd the moleculr weight distribution (polydispersity) of the resultnt polymer depend on the monomer/solvent rtio, monomer feed rte, polymeriztion temperture, inititor type nd mount, chin trnsfer gents, nd solvent chin trnsfer ctivity. The boiling point nd chin trnsfer chrcteristics of the polymeriztion solvent re importnt vribles in chieving these types of crylic resins. Nitrogen blnketing nd sprging re recommended to reduce color formtion when using oxygented solvents. Tble 3. Acrylic resin properties Property MAK PMA Polymeriztion temperture, C Inititor Lupersol 533 b Lupersol 533 Resin moleculr weight (M n ) 3,800 4,100 Polydispersity (M w /M n ) Wt% solids Viscosity 1,960 5,460 Estmn publiction M-271C b Pennwlt

4 Coting ppliction Solvent density The density of solvent hs mjor effect on the VOC content of coting formultion. Lower density enbles formultors to dd more volume of solvent per unit weight of coting, thereby incresing free volume. The importnce of this physicl property is mgnified by VOC regultions tht limit the weight of solvent per volume of coting. Tbles 4 nd 5 show how VOC contribution is relted to density in smple 2K polyurethne formultion. Tble 4. Smple 2K polyurethne cler cot formultion Component Wt% Acrylic resin 55 Additives 2 NCO cross-linker 12 Solvent 31 Totl 100 Estmn publiction TT-70B Tble 5. Solvent effect on VOC Solvent Density, 20 C Cotings VOC, g/l MAK PMA Xylene Estmn publiction TT-70B Solvent ctivity It is essentil tht VOC-complint coting meet trgeted viscosity specifictions s required by the mode of ppliction. If not met, the coting s pplied will hve uncceptble ppernce nd poor film formtion. One importnt formulting tool for meeting pint viscosity requirements is vi the selection of retrder solvents with gret solvent ctivity cross different resin fmilies. Tble 6. Acrylic nd polyester resin solubility comprison vi Brookfield viscosity, cp t 25 C Acrylic Polyester Solvent Thermoset 70 b Wt% Prloid B Wt% Elvcite Wt% 65 Wt% c MAK PMA Xylene Insoluble 157 Resin Solubility Chrt M-282D b Acrylic resin 80 wt% solids in MAK. The crylic resin ws then diluted to 70 wt% solids with the solvent shown. The finl solvent ws 58/42 wt% blend of MAK/solvent shown. c Polyester resin 85 wt% solids in PM cette. The polyester ws then diluted to 65 wt% solids with the solvent shown. The finl blend ws 35/65 wt% of PM cette/solvent shown. The lower mounts of solvent used in high-solids cotings mke them less forgiving thn conventionl cotings. This cuses the vilble free volume for solvent to diffuse through the coting nd decrese exponentilly s the coting dries. The rte of incresing solution viscosity, s it reltes to pot life, cn determine ny djustments to be considered when sprying high-solids cotings. Proper solvent selection to compenste for this while llowing cceptble solution viscosity nd rte is importnt.

5 Tble 7. Smple 2K polyurethne cler cot formultion Component (trde nme) MAK blend PMA blend Xylene blend Acrylic resin (Setlux 1753) Additives NCO cross-linker (Desmodur 3300) Thinner solvents (butyl cette) MAK PMA Xylene Totl, wt% Solids, wt% VOC, g/l Test density, g/l TR Tble 7 highlights differences between solids content, solution density, nd VOC with respect to ech retrder solvent used in the blend. Figure 1 shows viscosity increse over time for the formultions from Tble 7. Figure 1. Viscosity rte comprison for 2K polyurethne cler cot blends (TR ) 30 MAK blend (466 g/l VOC) Din-4 viscosity, s PMA blend (465 g/l VOC) Xylene blend (544 g/l VOC) Time, h VOC-exempt solvent U.S. only The forementioned formulting principles re still vlid when blending conventionl solvents with VOC-exempt solvents. However, very few VOC-exempt solvents re suitble for coting pplictions, nd ll hve notble properties tht limit their use in high-solids cotings. Exmples include cetone, methyl cette, nd p-chlorobenzotrifluoride. These solvents cn be used to meet specified viscosity without contributing to the totl VOC of the coting. Using VOC-exempt solvents is sometimes necessry in extremely restrictive regions, such s those governed by Cliforni s South Cost Air Qulity Mngement District (SCAQMD). However, VOC-exempt solvents cn hve some drwbcks, prticulrly in undesirble or poor solvent ctivity. Tble 8 shows how retrder solvent choice cn ffect the mount of VOC-exempt solvent needed to djust to desired viscosity nd totl VOC limit of 420 g/l.

6 Tble 8. Exmple of 2K polyurethne cler cot compring VOC-exempt usge bsed on retrder solvent choice Component (trde nme) MAK blend PMA blend Acrylic resin (Setlux 1907) Polyester polyol resin (Setl 1603) 4 3 NCO cross-linker (Tolonte HDT LV) Totl solids Additives (Tinstb BL 277) Thinner solvents MAK 27 PMA 20.5 p-chlorobenzotrifluoride Totl, wt% VOC, g/l Ford 4-cup viscosity, s Conclusion More stringent globl VOC regultions re cusing formultors nd utomotive refinishers to exercise more scrutiny when selecting solvents for ech step in their process, from synthesis to ppliction. Choosing the correct retrder solvent cn: 1. Reduce the VOC content of the resin t synthesis, providing higher solids per volume to the formultor 2. Decrese shipping weight nd cost for the sme solids content 3. Increse the coverge per cot pplied due to higher solids, which reduces the overll energy expended to pply refinish coting Lower energy expense is seen not only in the synthesis nd ppliction steps but lso t the solvent production step, which results in lower greenhouse gs (GHG) emissions. Some preliminry life-cycle ssessment (LCA) studies show tht crdle-to-grve production of PMA results in more thn twice the GHG emissions of MAK, which is more thn twice tht of xylene. Tble 9. Reltive GHG impcts of solvent production Normlized GHG /L Solvent MAK = 1 b MAK 1 PMA 3 Xylene yer globl wrming potentil (GWP) is common GHG impct ssessment metric used in life-cycle nlysis. b These vlues re normlized to MAK using internl clcultions for GHG emission of Estmn mnufcturing processes nd dtbse vlues, which my not represent other suppliers. The internl clcultions from Estmn s LCA group hve not been third-prty verified nd re intended to show the order of mgnitude differences between the GWP of the different solvents.

7 When greenhouse gs impcts of solvent production, solvent efficiency improvements, nd lower VOC emissions per kilogrm re considered, the environmentl benefits cn be substntil. Regultory constrints nd globl sustinbility trends cn impct how formultors could benefit by choosing the proper retrder solvents for HS cotings tht reduce the environmentl impct without scrificing performnce, qulity, nd color-mtching excellence for HS utomotive refinish cler cots. Fctors, from solvent ctivity nd density to environmentl impcts, should be considered when evluting retrder solvents. Blncing the prmeters tht re criticl to qulity to meet tody s regultory constrints in the utomotive refinish industry is prmount. Estmn methyl n-myl ketone (MAK) hs high solvent ctivity, slow evportion rte, low density, low surfce tension, nd high boiling point. These properties mke MAK very good solvent for cellulosic lcquers, crylic lcquers, nd high-solids cotings. Becuse regultions limit the weight of solvent per gllon of coting, formultors fvor the use of low-density solvents tht help reduce the VOC content of coting. MAK is lower in density thn ester, romtic hydrocrbons, nd glycol ether solvents with similr evportion rtes. The low density nd high ctivity of MAK re significnt dvntges when formulting high-solids cotings to meet VOC guidelines. For more informtion, visit

8 Estmn Chemicl Compny Corporte Hedqurters P.O. Box 431 Kingsport, TN U.S.A. U.S.A. nd Cnd, 800-EASTMAN ( ) Other Loctions, +(1) Although the informtion nd recommendtions set forth herein re presented in good fith, Estmn Chemicl Compny nd its subsidiries mke no representtions or wrrnties s to the completeness or ccurcy thereof. You must mke your own determintion of its suitbility nd completeness for your own use, for the protection of the environment, nd for the helth nd sfety of your employees nd purchsers of your products. Nothing contined herein is to be construed s recommendtion to use ny product, process, equipment, or formultion in conflict with ny ptent, nd we mke no representtions or wrrnties, express or implied, tht the use thereof will not infringe ny ptent. NO REPRESENTATIONS OR WARRANTIES, EITHER EXPRESS OR IMPLIED, OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR OF ANY OTHER NATURE ARE MADE HEREUNDER WITH RESPECT TO INFORMATION OR THE PRODUCT TO WHICH INFORMATION REFERS AND NOTHING HEREIN WAIVES ANY OF THE SELLER S CONDITIONS OF SALE. Sfety Dt Sheets providing sfety precutions tht should be observed when hndling nd storing our products re vilble online or by request. You should obtin nd review vilble mteril sfety informtion before hndling our products. If ny mterils mentioned re not our products, pproprite industril hygiene nd other sfety precutions recommended by their mnufcturers should be observed Estmn Chemicl Compny. Estmn brnds referenced herein re trdemrks of Estmn Chemicl Compny or one of its subsidiries or re being used under license. The symbol denotes registered trdemrk sttus in the U.S.; mrks my lso be registered interntionlly. Non-Estmn brnds referenced herein re trdemrks of their respective owners. SOL-COAT /16

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