VORTEX-FREE COATING DEVICE FOR TRAVELING WEBS

Abstract: Coating devices for application of coating material to the surface of a web or a flexible substrate (18) wherein the coater may be modified to provide an air layer between the coating liquid and any lower boundary. The coater devices of the described embodiments provide two inlet channels and an outlet channel (26). The first inlet channel (12) carries the coating liquid, and the second channel (20) can be used to pump the carrier fluid such as air into the coating head to pressurize the chamber (16) and to keep the contact wetting line at the upstream section attached to the substrate. The air layer serves as a carrier fluid removing the wall shear stress on the coating liquid in the channel, and thus the coating flow for the operation of the device may proceed without flow separation from the wall at relatively low flow rates.

Description: VORTEX-FREE COATING DEVICE FOR TRAVELING WEBS Field of the Invention The present invention relates generally to a coating device for uniform coating of a traveling web of material. More particularly, the present invention relates to a pressurized coater which eliminates the captive pond associated with pressurized pond coaterε, and provides the coating material in the form of a flowing stream of liquid coating composition which flows in the same direction as the web movement in a vortex-free coater reducing wall shear stress on the coating material. Background of the Invention One of the most significant changes in light weight coated (LWC) paper production is the use of the pressurized pond coater. The pressurized pond coal θjjisuch as short-dwell coaters has enabled the paper maker to improve product); vi y while maintaining coated paper quality. The term "short-dwell" refer.- to '.lie relatively short period of time that the coating is in cor.fεu.t Λ -L i i. ' .fib of paper material before the excess is metered off by a trailing done- blade. Prior art short-dwel] < oaters consist of a captive pr nd ju t p-no: t" a do c tor blade. The pond is approximate y 5 cm in length and if- fliαli: v pressurized t.c promote adhesion of the coating to the paper web Tin ctrφ ' atiπq supplied to the sheet creates a backflow of coating. Ihir coatinq back flow provides a wetting line and thus, to some extent, excludes the boundary layer of air entering with the sheet and eliminates skip coat in';. The excess coating is typically channeled over an overflow baffle and co i l ote in a return pan before returning to tanks to be screened. While pond coaters are extensively used in coating paper webs, such coaterε suffer from a major problem. The flow in the coating chamber of the pond uprtieam of the doctor blade contains recirculating eddies or vortices which can result in coat-weight nonun formities and wet streaks or striationπ in sevei l ways. For example, these eddies can become unstable due to centrifugal forces and result in the generation of unsteady flow and rapidly t 1 c-tual i ng vortices, which deteriorate the coating uniformity and its quality Also, the vortices tend to entrap small air bubbles which result in the buildup of relatively large air inclusions in the coating liquid which tend to accumulate in the core region of the eddies. Vortex fluctuations tend to force these air inclusions into the blade gap. This adversely affects the coating quality. Usually, the presence of air inclusions results in regions of lower coat weight which are 2-4 cm wide and about 10-100 cm long, known in the industry as "wet streaks". These problems are discussed in an article "Principles of Hydrodynamic Instability: Application' in Coating Systems", C.K. Aidun, Tappi Journal, Vol. 74, No. 3, March, 1991. Previously, geometries utilizing streamlined boundaries in coating devices have been employed to eliminate the formation of recirculating eddies or vortices. See, e.g., Aidun, U.S. Patent No. 5,366,551 entitled "Coating Device for Traveling Webs," wherein curvilinear geometries are employed for the elimination of vortices and flow instability due to centrifugal forces, and foi the avoidance of harmful pressure fluctuations which could result in coat-weight nonuni for i ties . The elimination of recirculating eddies or vortices also reduces the possibility of entrapping air pockets or air bubbles in t he core of the vortices which could reach the blade gap and could result in coat weight nonuni ior i l s and wet streaks. Addition lly, the a 11 ς of the coating composition application chamber in conventional coa mq devices are considered rigid and do not deform under th' i ' f fcrr of hydi odynrimi pressure, and thus exert shear stress by t ho f 1 ow on the boundaries in contact with the coating liquid Such wall sheai li st cm t h ' j coating liquid creates flow separation from the applicator wall- in the application chamber winch also results in coat -weight nonuni formit les and wet streaks, as well as, re ircula ing eddies and vortices Pranckh, F.K., and c liven, L E , "The Physics of Blade Coating of Deformable Substrate," 19B 'oatinq Conference Proc., TΛPPI Press, Atlanta, GA , (1988) have provided a detailed arialysis of blade coating using a finite element approximation method ncludinq h p complex interactions of the boundary in addition to the solution oi the flow field and free surface location. The blade was modeled as a thin, inextensible, elastic solid and the substrate deformed due to normal stresses In Aidun, U S. Patent No 5,354,376 ent.tled "Floatation Coating Device feu 'liavelinq Webs," one of the applicator walls is designed to be a floatmq oi moving wall oi belt The effect of t e floating applicator wall is to redu e vortices hrough the use of a moving substrate, e.g. a suspended belt, as the applicator wall which moves with a given speed with the liquid to prevent 11 ow 1 separa ion and reci rculation inside the application chamber. The floatation coating device for traveling webs seeks to alleviate recirculations in a fixed domain pressurized pond coating system. The combination of a moving applicator wall and a sufficient flowrate allow for the design of a vortex- free coater configuration. Development of high speed blade coating is of particular interest in the industry to enhance production, and to reduce cost the analysis of the coating process which is complex because the governing equations of fluid motion are non-linear and the free-sur ace position is part of the unknown. Moreover, the non-linear constitutive behavior of typical coating fluids increases the complexi ty . It would be desirable to provide a coating device which has the coating advantages of a short-dwell coater, but which did not have the problems associated with recircula ing eddies or vortices and the entrapment of air pockets 01 air bubbles in the core of the vortices. It would be further desirable to provide a coating device with reduced sho.u stiess on the flowing stream of the liquid coating composition in the appl i ' at ion chamber a 1 - ho coating composition downstreams. It is another object of the present invention to provide a coating device which receives a liquid flow of a carrier fluid introduced in the direction of the travel of the web positioning the liquid flow of the liquid co. ' i'ιq composition between the carrier fluid and the web with reduced shear stiess w the flowinq stream of the liquid coating composition in thr application chamber as the coating composition downstreams. Tt is a further object of the present invention to provide a coating device which receives the flow of carrier fluid through a channel for directing air flow into the coating composition application chamber below the f 1 ow ol the liquid coating composition reducing shear stress on the flowing stream of the liquid coating composition. Accordingly, it is a principal object of the present invention to pt'ivi e voitex-free shor -dwell coating device'. These and other objects will become more apparent from the following description and the appended claims. Summary of the Invention The invention relates to coating devices for application of coating material to the surface of a web or a flexible substrate Such coatinq devices employ a pressurized channel where a flowing stream of the coating liquid comes into contact with the substrate. The coating liquid first enters at the upstream side of the channel wetting the substrate as it flows in the same direction with the substrate A doctor element is positioned at the downstream side of the channel where the excess coating in the channel follows the contour of the boundary formed by the doctor element and leaves the channel The present invention is further directed toward the study of flow pattern^ m blade coating to develop high-speed coaters, wherein the coater may be modified to provide an an layer between the coating liquid and any lowei boundary The air layer thus serves as a carrier fluid The coater devices of the desci ibed embodiments provide two inlet channels and at outlet channel The first inlet channel carries the coating liquid, and the secon channel at e used to pump the earlier fluid, r g an π'o tii' oatinq head I" pir suii e the chamb r and to keep the conta t wetting line ,H the up e tιeam T 'ID atta hed to the substrate The an rp ' tuip can vaι \ from zero to any level appropriate for the coatinq operatior The ail Id^n " five a a car i it l fluid removing he wall 'hear st res c on the c atiic 1 i 'd in the channel and I lιu he coating flow foi the operntion ol t lie device may proceed without flow separation from the wall (i e in a voi I ex free mode) at relatively 1 ov flow j at es appiopiiate for commercial appl κal lour The exec ' cortinα liquid and all of the air leave the coat i head at the outlet channel The blade s used to meter the excess coatinq f l om 'ho u s tra Accordingly the pι p s c ιιπ inside the channel may be increased above ambient pre ' ute if necessaiy in ol er to prevent air entrainment into the coatinq liquid However, the system may also opeiate at ambient pressure f air c πtr ainii ent ι< not an J f, snr The revised vortex free coater and computation : jimihit mn of the 11 ov; 11 t!ι system are presented below The computrit ion ' j imi la' l on ' SUIIK cir wn' pi *. f -> r u r ♦ in the^ 1 ay r and , t her e f or e c n i d( i he c r 'ut l no layer ju t up< t i earn of the blade Br iefly < ■ ultima i I /' the pi < <-< nt invent ion lelalcs to high s ec < i ooa t win iin ilnxb .nul <ιpp t ι i tit n r i for applying n liquid cost ng composition ot «.» w> : b of iu.iiei 1.1I as ihi web I 1.ivcLs; along a path thtuucjh the device from an upstream 1I1 led ion to a downs ream direction with a doctor element being spaced 1 rom the web and extending across the path of the web transversely ot the direction of t 1 nv l of the web. Λ coating composition application chamber receives the liqui flow of the liquid coating composition from the Upstream direction to tin clown.ti earn irect ion, and comprises an upstream interior side wall and an upstream boundary wall for directing the liquid coating composition flow into the application chamber, and the doctor element for spreading and defining the 1 1 eki n " ;'; of the i i coding composi ion on the web at the downs l 1 earn side of the cipplicat 1 on chamber. The coating composition application chamber is furthei adopted foi leceiving a liquid flow of a carrier fluid introduced at 1 tie upst ream side oi the application chamber in the direction of the travel of e wel) positioning the liquid flow of the liquid coating composition between t ho i ,i! net f luid and the web, the liquid coating composition flowing from the upst 1 e.iπi si e of the application chamber in the direction of the travel of the web iii the doet i element defining a path which t. he flowing si.1 earn ot 1 he 1 i u'd 1 luting compot 1I1011 clowns t reality in the direction of ti.ivel of 1 h« el with re uc shear stress on the flowing stream of the liquid coating ' ■ >ιii| . ii 101. Lii he application chamber as the coat lnα composition u .vw'ir.t 1 ea- - hi i e_l I'o ' ri lotion ot the Drawings F-'iciut lflif. a schema' ic cross - sec ti ona view of an embodiment of .1 short )•• l i eoit inc] device accoiding to the invention; f iq.iif IB is a schematic cross - sect ional view of another embodiment of tht . I ' : r d.-.'i 11 coating device- according to the invention; F-icπ.re IC represents a domain description in cross -sec 1 on for the ■ i. '.-jibed studies of t e short-dwell coating devices according to the 1 iqυie . ' '. represents a gap region description of the domain for short -dwell eo, 11 11 ια dev i oe , Ficpire 1 i lluεtrat.-s the ef feet of flowrate variation shown as a mesh li ..- in; r 1 present at 1 o of the domain; I iquie 1 \ i u tidos the effect of flowrate vai lation sh w as streamline- in the doπu: 111, Figure 5 illustrates the effect of flowrate variation shown as mesh of applicator channel exit, Figure 6 illustrates the effect of flowrate variation shown as streamlines in applicator channel exit, Figure 7 illustrates the effect of flowrate variation shown as pressure contour = ■ in applicator channel exit; Figure 8 lllustiαtes the effect of flowrate variation shown a? mesh of gap region, Figure 9 illustrates the effect of flowrate variation shown as streamlines in gap region; Figure 10 illustrates the effect of flowrate variation shown as velocity field in gap region; Figure 11 illustrates the effect of flowrate variation shown as pressure contours in gap region, Fig i 12 illustrates the effect of flowrate variation shown as mesh of blade ti r eαio , r i α i' c M llu 1 tiatp- the effect o fl owr o vaii i on hown a c «t i earn! me 1 j i n t 11 il lii'iiatc" the ef tcrt of 1 owra t e var i at ion shown as pi essure i o'ti π "- in blade tip I ear on FI JI it l r ill u' ' ι ales he e fect of fl owi ate variation shown at hen i 7on t a 1 ■ el J ' prof ile at mi dpn I t of b ade tip Fi juic It il lustiritc tlii effect of fl owi a te var lati on shown Λ " hot izonl rt I ΌIOI tv pinf i U t endpoinl of blade tip, l oqii' 17 ιllustιate r the effect of flowrate variation shown as horizontal i 1 K * i l > pi o l I r at I rιqiιι<- IR lilusl iri 1 e < - the effect of • lo iatP variation shown as pressure distribution along the blade, Figure l °> illustrates the effect of flowrate variation shown as pressure cli.st i ihntion a n the substrate, Ti'iiri O tl lu iicit c th> effect of flowrate variation shown as pressure d l < - 1 ι i but ion along the 3 blade tip I ici'i'i ' ι l I 1 ιι c I i a ' e the e f f e< t of f 1 owi a t t var i at i on shown a -> eυa I i n t h l kn* <■ • • ' inlet f 1 owi a t c I l gni ? ? i 1 lit' I tal c f he- c f fec 1 of J I owi te vai i a t i on hown a film flowrate vs inlet flowrate; Figure 23 illustrates the effect of flowrate variation shown as coating thickness vs thickness under web; Figure 24 illustrates the web speed variation shown as coating thickness vs web speed; Figure 2 i lustrates the web speed variation shown as coating thickness vs reynolds number; and Figure 26 illustrates the web speed variation shown as coating thickness vs capillary number. Detailed Description of the Embodiments Λs shown in Figure 1A, the short-dwell coating device 10 of the present invention includes of a first continuous channel 12 for receiving a liquid coating composition material 14 which passes through a coating application chamber 16 which is in contact with a ro l or web 18 of material which is to be coated The coεit Lng device 10 further includes of a second continuous rhiJiiπtl 20 lor leceiviπq a liquid flow of a carrier fluid such as air 2 which al=o it ~ "- c er. t hrouαh a coatinq application chamber 16 positioning the liquid flow of the liquid coaling r-ompos L t i on 14 between the carrier fluid 22 and the voe! ' lr' of material which i r. to be coated. For purposes of orientation and di cut sion , the coating chamber has an upstream side and a downstream side v..' 1 U's oft to movement of the web with the upstream side being to I left of Figure IΛ The use of thr terms " hoi i zonta 1 " and "vertical' a i c- wit}; l es oet t co a horizontal orientation of the web 18. The web 18, however, is usually suppor ed on a counter roll and has a slight curvature in the region of tht coatmσ application chamber 16. The coating devices described herein include a blade or doctor element 24 which is spaced from the web 18 for defining the thickness of the coatinq on the web 18. The doctor element 24 extends across the 18 web transversely to the direction of the web motion. The doctor element also forms a downstream boundaiy wall of the coating chamber 16 and extends downwardly for a further distance to define the downstream wall of an exit plenum or outlet channel 26 fotriied bet een the doctor element 24 and a downstream interior wall 28 in the embodiment of Figure IΛ, for the circulation of the liquid flow of the cariiei fluid, e q , air 22 which riiculates with the liquid flow of t lie liquid coating composition 14 through the coating application chamber 16 as the web 18 of ma'-etial which is coated " In figure IΛ an upstream boundary wall 30 defines the upstream side of the co it nig device 10 Ihe upstream boundary wall 30 extends downwardly foi a turthei distance to define the upstream side of an entrance plenum of the first channel 12 The upstream boundary wall 30 terminates at its uppermost end in contact with the web 18 via a contact line or wetting line 3 of the liquid coatinq composition 14, thus preventing air entrainment at the upstream section 34 Λs shown, the terminal end 36 of the upstream boundary wall 30 preferably has a curvilinear shape so that this terminus of the upstream boundary wall is subs antially tangential to the web 18 The upstream boundaiy wall 30 and its terminal end 36 also extend across the web transversely to the direction of the web motion The coating device 10 and particularly the coating application chamber 16 are represented in cic , section in Figure 1A The embodiment of Figu'-o 1A io.ee interior wall including an upstream interioi side wall < ( • ai uitti j(i top wall 40 and an downstream inteiioi de wall 42 The n ti i oi w ll 1 " " t 4 i and 42 in combination with the upstream boundaiy wall 30 and thr electee element 24 define the- coating composition application chamber 16 of t ht emb »nt liκ coating composition application chamber 16 rs further adaptor for lectriving the liquid flow of the earlier fluid 22 ε a fluid layei nit i duccd f i oπi the upstream side of the application chambei u s a ti lly rijiil I 1 ιθ ind m Liu diitction of tht^ travel of the web supporting I ht liquid floiΛ of tht liquid coating composition 14 between the fluid layci ? 2 and t ht web 1 ft 7 lit fluid 1 a^ r i opposite t lie web defines a top mteiior fluid Ifyei w ll abe *- i h< interior top w ll 40 and the fluid layer opposite the doctor blade defining a downstream interior fluid layer wall adjacent the downstream interior side wall 42 The top interior fluid layer wall of the earlier iluiu > provide a layei which substantially conveys the liquid coating composition 1 ftntit tht- tormina' inq curvilineai section of the upstream inteiior wall an the 1 d i i e e t l on of the travel of the web to t he doctor element 24 'Ihe coating <le ιcι 10 litt providi r the upslream boundary w ll 30 and the up'trean interim < i cι< wall 3P n r upwardly rnclintcl in a direct ion towaid tie oΛii I n am < i do the ownstream interioi w ll 42 and t he docfoi c lement 21 being downwardly inclined in a direction toward or away from the upstream side. Accordingly, the upstream walls 30, 38, the top interior fluid layer wall and web 18, the downstream interior fluid layer wall and doctor element 24 thus define a path in which the flowing stream of the liquid coating composition 14 downstreams in the direction of travel of the web 18 to at least reduce wall shear stress on the flowing stream of the liquid coating composition from the interior fluid layer wall as the coating composition downstreams thereon, reducing the formation of recirculating eddies and vortices in the coating composition. Figure IB shows an another embodiment of a short-dwell coating device 50 of the present invention which includes of a first continuous channel 52 for receiving the liquid coating composition material 14 which passes through a coating application chamber 56 in contact with the web 18 to be coated. The coating device 50 also includes of a second continuous channel 54 for receivinq a liquid flow of the carrier fluid, e.g, air 22 which also passes through the coating application chamber 56 positioning the liquid flow of the liquid coating composition 14 between the carrier fluid 22 and the web 18 ot material which is to be coated, as in the embodiment of Figure 1A discussed above The Figuie IB embo iment, however does not utilize the interior top wall 40 and downs t eam interior side wall 42 of Figure 1A, and thus allows the cariiei fluid 22 to exit into the open area of the coating application chamber 56, which may be provided under pressure. At an upstream opening 58 of the second continuous channel 54, the liquid coating composition material 14 is pressed as a layer against the web 18. The flow rate of the liquid coating composition material 14 is reduced in the Figure IB embodiment, with respect to the Fiquro 1A embodiment, arid an approxima ely 1 mm. thick layei' the liquid coating composition material 14 adhering to the web 18 travel 1 the 5 to 10 centimeters in the coating application chamber 56 to a doctor element 60 biased with a load 62 to spread and define the thickness of the liquid coating composition 14 on the web 18. As in the Figure 1A embodiment, the doctor element 60 also extends across the path of the web 18 transversely of the direct ion of t avel o f the web 18. Pi ssurc provided at the upstream opening 58 of the second continuous channel 54 is desirable where the liquid coating composition material 14 is layei eel against the web 18 to prevent ai i enfrainment by maintai ing the contact or wetting line of the liquid coating composition 14 with the web 18, as discussed above. Advantageously however, any pressure provided in the coating application chamber 56 of the Figure IB embodiment is reduced downstream of the opening 58, and thus the likelihood of downstream entrainment by the carrier fluid itself is reduced. The coating device 50 and particularly the coating application chamber 56 are represented in cross-section in Figure IB. The embodiment of Figure IB provides an upstream interior side wall 64 and an upstream boundary wall 66 for directing the liquid coating composition flow into the application chamber 56. The coating composition application chamber 56 also is adapted for receiving the liquid flow of the carrier fluid 22 introduced at the upstream side of the application chamber 56 in the direction of the travel of the web 18 positioning the liquid flow of the liquid coating composition 14 between the carrier fluid 22 and the web 18. The liquid coating composition 14 thus flow from the upstream side of the application chamber in the direction of the travel of the web 8 to the doctor element 60 defining a path which the flowing stream of thr> liquid coating composition downstreams in the direction of tiavel ol the- web with reduced sheai stress on the flowing stream of the liquid coating composition in the application chamber as the coating rompori t ion downs l ream . The embodiments described concern the study of modified vortex- free coater con igurations in an effort to investigate the hydrodynarnic behavior of the current system at very low flow rates. Avoidance of flow separation and rec rculation is shown in studies by way of computer modelling. The flow field and the free- surface boundary location are solved using a Ga Lei kin finite element approach for web speeds ranging from 15m/s to 30m/s and flow rates from 4 to 7 1 i toi /sec . /mete (1/s/m) . Several mechanisms of instability are present due to the complexity of the domain in coating devices. The nonlinear constitutive behavior of typical coating fluids increases the complexity. Boundaries within such high speed coating devices are typically flexible, permeable, and unknown in different regions. Accordingly, the flow is modejled as being nearly parallel throughout the majority of the domain, with the important exception of the region in which the web and the blade cniivciqo forcing some of the liquid υntlei the blade tip and the ιe:;t to curve 1 and 1 low down the 1 blade, In the gap legion, between the substrate and the blade tip, the flow is nearly parallel and experiences high shear rates. Squires theorem requires that the first instability in parallel shear flows occur due to a two- dimensional instability. In the returning flow, the possibility of centrifugal nstabilities to three-dimensional disturbances exist. The flow field of a blade? cυafei with a lower free surface is 'examined. The flow is assumed to be incompressible-?, two-dimensional and steady. The effects of flowrate and web speed variation on the design will provide insight into the optimal operating conditions. A further analysis of the stability of the resulting solutions to 2-D and 3-D disturbances will provide additional information. The velocity field, pressure field, and location of the two free surfaces of the blade coater is depicted in Figure IC with parameters detailed in Tables 1 and 2. The region of particular interest is shown in Figure 2, here the blade (G.) and the web (G 2 ), converge to form a gap with a vertical cross-section length (blade gap) of 50 microns. A portion of the fluid pumped in at the inlet (G,) proceeds through the gap and coats the substrate, while the excess is scraped off and flows nearly parallel to the blade. Table 1: Fluid Pararneteir. P density 1200 kg/rrT π ?ero shear rate 1.0 kgz(m-s) vi cosi ty μ. infinite shear rate 0.05 kg/(m-s) vi scosi ty T surface tension 0.05 kg/s *' c Carreau exponent 0.65 K time constant 0.01 s U„, i web velocity varies from 15-30 m/s f , 1 .,ι,. l center line velocity on varies from 2-5 ,m/s inlet , ..,.,, inlet flowrate vai ies from 4-7 1/s/ιιι Table 2; Geometry Parameters , n l t .r inlet length 0.0025 m L , r , gap length 50 E-6 m L ιirr applicator channel 0.5 mm exit thΛr blade thickness 1.25 mm L, ,„,, p blade length (modeled) 60.104 mm L„„, web length (modeled) 59.551 m < i ,ι a «„ angle of blade 45° C, coating thickness 0(10 μm) W, vertical distance from 0(100 Um) web to free surface at C-C he problem can be defined in a d ens on! ess manner. The inlet cross-section leπαth and web velocity are used as the length and velocity scales Table 3 relates the di ent I onl ss quantities to the parameters given in Tables 1 and ? Table 3: Diiuens i on 1 ess Quantities 1 < ~ - Reynolds Number _ pU^^L,, Ca Capillary Number _, μU Ca = web T We Weber Number ,., 1 We = ReCa pl -,,, The equations governing the flow in the coater are continuity and momentum V»v = u 1#| =0 ( i) Here O " denotes the stress tensor, is assumed to be of the form σ,,=-pδ,.+τ, Where T„ denotes the deviatoric stress tensor with the constitutive relation = 2 P e „ Where £ is the ra of strain ensor, given by The fluid fc-ii the cuirent application is assumed to be shear thinninq, the dynamic viscosity is approximated by the Carreau constitutive model (n 1),2 μ = μ» +(μ„-μ~.) 1 + K" V., (3) ni <-. I |( and (.1 denote- th mo and infinite shear rate vi cosit ies Tin pal nine t er in the Cm t PHII model are determined based on the behavior 'if I y ca 1 c< >a t i tiq colnir. 'Ilii ah ι\-e p i il IOIIΓ aio ιιo:ι d i mens i pnal i zed using the velocity of t lie web and the width of t lie inlet channel as the velocity and length scales respectively The velocity and pressure ai e scaled using the velocity and dynamic pressure 11 = — , p =-— r — Th e superscript * denotes dimens ionless variaLile. The independent variables, posit ion and time, aie scaled using the velocity and length scales X. ~ f = .ϋ The body force t is non-dimensional i zed t: = f. 'u The continuity, momentum, and cons itutive relations can respectively be expr s s d in dimenπi onl ss form as ti !4) where σ , ~ -p ' δ„ -t τ ' τ.. = 2c μ piλ 2C 'R ^ , + u i.) K ' = K u. rh> !)π irhlel boundaiy conditions lor this coatinq system are peoιi ιe-1 a< u. ink t inlet u. Ll. Lt, = 0 r -=> applicatoi channel , I „ ~ ^ bJadi IJe iinat u rendit ion^ ai applied at the outflow bourιdaπe r σ. σ„ -0 K ' - exil I , ι gap i On the free surfaces ( T- and T,, ) the kinematic condition is given by VJhen the flow is independent of time this condition reduces to ujn. =0 (7) where n, is the unit vector normal to the surface. The dynamic boundary condition requires the stress to be continuous across the interface, therefore the normal and tangential stresses are respectively given by σ„ = 2γH - p Λ ax, The fluid surface tension, γ, is constant , therefore the tangential component of the traction veeror is ?ero The above dynamic boundary condition is non di mens i onalizc-id by σ| = 0 The above non dimensional equations (4) and (5) with the consti utive relation (6) and appiopnate boundary conditions completely describe the flow field The finite element method is employed via FIDΛP to solve for the velocity and pressure at discrete points within the domain. The unknown boundary location is determined in a fully coupled manner by simultaneously requiring the condition (7) be satisfied on the free surfaces. The governing equations, constitutive relation, and boundary conditions completely define the given blade coating problem. The domain is discretized using -node 3, isoparametri , quadr lateral elements The velocity is approximated over the element using biquadtratic basis functions and the pressure with bilinear basis functions The free surface boundary is determined by satisfying the steady state kinematic and dynamic- condit ions in a lul ly i coupled manner The nonlmearity of the governing equations requires an iterative solution approach The stokes flow m the fixed domain provides an initial guess for the Newton-Raph on lteiation procedure Parameter continuation methods are u^ed to assist in the vaiiation of the parameters to reach the desired solution for given boundary conditions Convergence is achieved when the noim of the s olution change in between iterations is less than 10 J The resulting coatei configurations and streamlines are shown in Fιguιe c , and 4 for the cases listed in Table 4 A noticeable change in the free surface location is apparent as the flowrate is varied An increase in flowrate results in a larger vertical cross-section under the web a decrease in exit cross section width on G 5 , and an increase in the exit velocity magnitude on the same boundary The desire to avoid recirculating flow and minimize surface defect^ lea ^ us tr examine closely three regions where flow separation and lat ion ι c possible the meniscus -just aft of t lie applicatoi channel the corner wliei < the blade and web converge to construct the gap and the blade tip where a mcnιscu- foιm c and t lie ι c coated Ihe esh st rr HI I n an 1 pi r e-ii'e coTitotti rire plotted for t he°e three regions in Figures S 1 h c dent in t rated n t he ° e fιquιe c the results show no flow separiti t or flow l e ι r c ulat ι on A true v itey tree coating flow system exists at lov flcv rrit f> 1 (4 l c -/m) and hiqti coating speeds (20 m/s) lltf e 1 n<-ι t pi o f i 1 P 1 - in the gap leqion provi e insight ml c t l < 11 i ικι ciua 1 ty riqtnc I ' -how thr hoιι~oπtal non dimensional velocity piof ile it t 1 or 11 on Λ Λ on the ! lade tip wh i 1 e ! iqure If depic t s t ho i of i I ι at 1< at ion B the endpc>mL of the blade tip Figure 17 illustrates the effec of flowrate variation r hovn a horizontal velocity profile at I the g if o it At the static contact line it is clear that the foimation ol tie nιonιτu p slightly affects tht velocity profile The apparently linear pressure dis* i lbut ion along the blade tip, Figure 20 indicates an almost cons t ant pressure gradient in the gap that increases with the flowrate These velocity prof ile^ id re^suie distribution demonstrate α nearly Poiscuille f uette v locity dι tι llml ion the linear combination of flow between t wo v ιi it a tc ldtivc ] K 1 -) to one another and flow between < tat ion n \ il =■ \ irli ι cn t riii t pie IH qi ad IC nt thus the coating fl owi ate nel t hi c kne in ti i r f j 1 i ghl 1 ^ will t he i tin ea e in he i n 1 °ι f lowi ' e du t the iiπet pi ssure gradient, see Figures 21, 22 and 23. The portion of the coater where the blade and web form a converging channel is much more affected by the fJowrate variation. Examination of the corner region formed by web and blade, presented in Figure R , shows significant free surface shape variation with flowrate variation. Λs the flowrate is decreased the free surface migrates toward the gap threatening Lo entirely disappear into the gap with further reduction of the inlet flowrate. The corresponding streamlines are shown in Figure 9. The pressure along the blade and substrate are shown in Figures 18 and 19, all graphed quantities are non-di ensionalized . Table 6 can be used to convert all variables to dimensional quantities. Away from the gap the pressure remains fairly constant. Within the gap region the pressure peaks at the leading edge of the blade, just upstream of the gap. The maximum pressure increases ΆS flowrate increases. At higher flowrates, the pressure increases in a more gradual manner, exhibiting a more distinct plateau. Following the peak, the flow field expei ienres sub-ambien pressures and then adiusts to thr. ambient e it piosruie Th< " pi ossure contouis in the gap region, shown in fαouir 11, inrht-.iie fh<ι" r ι dι 'i prisr in f 1 owi a L e causes a latgoi piessuio ciiridieπt hut dcr-t eases the value ot the maximum pressure Table 4: Cose Study - Effect of Flowrate Variation a^c I! U Qllln ile Ca We ni πiΛs l s m l s m l/s/m μm Unt1/RcCa MR1)75 361508 27465 2084447 60 550354 4611883 27575 2590522 60 552128 5.60895 27.66575 309472 60 .553462 6.52 27.7325 354,6727 60 q " l/s/m q l/s/m q l/s/m l/s/m m/s m/s lahle 5 qιvP r result' for t e variat ion of t lie web speed for t o f InwiatP' t an i 1 ° in I h<- inui ,i" in wι_b speed is ef fect ively an inn rase n t lie t w non dπni-'i lonal piriiiip'i i ' r haraet er 17 mq the flow the Reynold' Nunihei and thr Cajn ilnr', uinhr . line w land t at as the inertia! effort 1 me magnified t li^ pies'-ui'- gradient nvreasor while the maximum pieεsuie f l" Menu t lie wι b r ι αiadual prftnip adjustment followed bi a ha*p pie' in peaV is oh'e tvcd at 1 owei Reynolds Numbers The effec t of increase in web ^-pee appeal to have a qualitative relation to the effect ' <>< " d< i f | i' l nu t r f 1 owi a t ■ < IK . I H Poj'cuil le ' u' t tr velocity profile is again present in rlκ g i i- ' i ii lι KM' IIRI . l ' J < <- t forc s ,ι qieatei amount of f luid to e it the qnp t hi i HI il \ l i. on ' in ri i runt the nt ri i 1 > c oil ' t ant iescutp T ad i ( nt ( oa t liui t hιrkne=- - mrπ ase is ob"eιved with an increase of web speed, as shown in Figures 4 , 25 and 2 The results of the present analysis exhibit qualitative agreement wit 1 lιn " r nf Pianrl & cπvui (1988) , as discussed above in connection with I lie bar ken o und of tht invention The graphical flow solution in the present 'tud, f n i n s 8-14 should be compared to those of PJ anckh & ".rπvpn foi t lie voloc t* f ir Id st i can 1 i ne' and pressure contours of thoii base cas t lh , ι ' I f , S r , i ; eπ lool" d c ι t the I n ure chsti diution along th< suhdiate f oi their base case anrl anotho case where both the Reynolds Number and flowrate were increased. In their base case Pranckh & Scriven found the pressure distribution had an inflection point, or plateau, followed by a peak just prior to the leading edge of the blade. Pranckh & Scriven found increasing the Reynolds Number and flowrate decreased the maximum pressure and eliminated the pressure plateau. In the described embodiments it is determined that the pressure profile along the substrate has a peak just prior to the gap. The slope of the pressure plateau and the dimensionless pressure peak were also found to ' decrease with increasing Reynolds Number. The described embodiments also investigate the effects of the variation of the web speed (or Re | cl=colls , and Ca| ll-r „ , ) and flowrate ( q | V f . conrt ) on the coating thickness, see Figures 24, 25 and 26. Similar to Pranckh & Scriven, it is found that the coating thickness varies nearly linearly with the increase in Reynolds Number, Capillary Number, and flowrate. While preferred embodiments of the invention has been shown and f or the apparatus and method for coating devie-es i oi travelling webs in which a f luwiiiq stream of liquid coating composition flows in the same? direction Λ Γ the web movement in a vortex-free coater reducing wall shear stress on the r i.it i n ma t r i lal. other embodimen of the present invention will be rea i ly apparent t e> theise skilled m the art from consideration of the specification and practice of the invent! tin disclosed herein. It is intended that t he spec i f i e- ( ι ion and ex mples be e-onsidered as exemplary only, with .i true scope duel spi i t of tlii' invention being indicated by the claims Appendix : Nomβncl aturβ Kronecker delta rate of strain tensor γ surface tension r, boundary η height of free surface μ dynamic viscosi ty μ zero shear rate viscosity μ. infinite shear rate viscosity p dens i y ttc^ t PIisor σ. normal component of the traction vector a, t ancient nάl component of the traction vector vi oric stress tenseji Capi 1 Idi y Numb r (Mt ιrιq thickness Caπeau exponent component of gravitational acceleration C'ausFian mean curvature of the free surface time constant apnl icator channel exit blade length (modeled) gap length I-.„ι i n 1 et 1 eπαt h I I eιι<] I h ■ < a 1 i b t| ] , blade thickness L Λ(L web length (modeled) l/s/m ( 1 r PI /sec ) -meter m/s met e /sec n, uni t normal vector p pressure p_ ambient pressure , fl owi ate exiting along blade q,,ι,„ flovMiate exiting gap q lnlI , inl t flowra e Re R ynol ^ Numb r sintiulrii i t i t πii i t l rinqiint ve < t oi {' rn ' e r 1 i ne- ve J oe 11 en j n ) et Pol en l 11 e i o f i ] »- tl ) fiKjl seal e i i ι h \ r ι i ]of l t y \ vei t it ril distance from web to tiee sui iace r ιt C ( j C'ai t esran coordinate - , , ,, angl c of b lade ' supei sci pl denotes di mensionless vaπab o 11

Claims: WHAT IS CLΛIMKI) IS: 1 Λ coating device foi applymq a liquid coating composition on a web of material as the web travels along a path through the device from an upstream direction to a downstream direction, the device comprising: a doctor element spaced from the web for spreading and defining the thickness of the liquid coating composition on the web, the doctor element extending across the path of the web transversely of the direction of travel of the web; a coating composition application chamber adapted for receiving a liquid flow of the liquid coating composition from the upstream direction to the downstream direction, the application chamber extending across the path of the web transversely of the direction of the travel of the web, the application chamber having upstream and downstream sides with the web adapted to travel from the upstream side to the downstream side of the application chambei , the coating application chamber comprising in cross-section, an ups i arn interior sick' wall, an upstream boundary wall and the doctor element, the coating composition applica ion chamber being further adapted for receiving a liquid fJov. of a caiπoi fluid a. c a fluid layer introduced from the upst ream side of the applica ion chamber substanti lly parallel to and in the direction ol the ravel of tin- web supporting the liquid flow of the liquid coatinq composi ion between the fluid layer and the web, the fluid layer opposite the v. i ' ')( 1 niπiσ a t oj inloi ιcι flui 1 aypi wa 11 and the fluid 1 dyer oppos I t e t he elect or blade defjninri a downstream interior fluid layer wall, the up c t roam boundaiy wall and the upstream mtei ιor wall being substantially parallel to the other and each having a terminating curvilinear section which are subst ntially parallel to the other, the upstream boundary wall adapted to terminate n tanαenfia] relation with tie path web, the top interior fluid layer wall sub antially conveying the liquid coating composition from the terminating curvilinear section of the upstream interior wall in the direction of the trove 1 o f the web to the downstream interior fluid layer wall and dric oi el ment, the upstream walls, the top interior fluid layer wall and web, the clown l ιeam interior fluid layer wall and doctor element defining a path which a flo ing sti' im of tht- liquid coating composition downstreams in t fie d i t ee t ion of tia -e ! of he e an at least reduces wall sheai r t res- on the flowing stream of t lie liquid coating composition from the mteiioi fluid layei 21 wall as the coating composition downstreams thereon, reducing the formation of r irculating eddies and vortices in the coating composition ? A coating device in accordance with claim 1 wherein the carrier fluid comprises an pumped nto he coating application chamber maintaining the liquid coatinq composition in contact with the web under pressure at ]ea°t at the upstream side of the application chamber preventing air entramment as the coating composition is introduced to the web 3 A coating device in accordance with claim 2 wherein the coating application chamber comprises a top interior wall opposite and substantially paia l to the web and the top interior fluid layer wall, and a downstream mteiior wall opposite and substantially paiallel to the doctor element and the downstream interior fluid layer wall defining the coating application charrtbei as a closed system for the downstream flow of the liquid coating c ompo i t i on 4 Λ r cot mil d< j ιee in a i oi dance with claim , wherein tht upstream boundaiy wn I 1 and the up<- T <=.aιn interioi side wall are upwardly inclined in a due t ion to i'd the downstream side L A cndtinπ de ic in accordance with claim wherein the down°trean l nt f ι ι or w ll and the doc t oi element ai e downwardly inc liieo in a direction f ordi oi a ay 1 rom the upst I earn side ( A (O 1 mg dex ree foi a liquid coatinq composition on n web of mn'cijr 1 as the web tiavels along a path throug h the device f r on an upstream direction to a down 1 " t ream direction, the device comprising a doctor element spaced from the web and extending across the path ot the wel r i r ii -.ver c ely of the direc tion of travel of the web, a f(.M inq composition applicat ion chamber adapted for receiving a liquid 1 ' o of i lit liquid coating composition from the upstream direction to the d< J ι tι« H diioct 'in tin appl icati n chambe extending arro " hr path oi Hie web transverse ly of he direct ion of the tinvel of the web the application t !ι IIIIIJC I havLiiq upstieain and dowi lieam s i d< i wi t h the web adapte to t i avr 1 from the upstream side to the downstream side of the application chamber, the coatinq application chamber comprising in cross-section, an upstream interioi side wall and an upstream boundary wall for directing the liquid coating composition flow into the application chamber, and the doctor element for spreading and defining the thickness of the liquid coating composition on the web at the downstream side of the application chamber, the coating composition application chamber being further adapted for receiving a liquid flow of a carrier fluid introduced at the upstream side of the application chamber in the direction of the travel of the web positioning the liquid flow of the liquid coating composition between the carrier fluid and the web, the liquid coating composition flowing from the upstream side of the application chambei in the direction of the travel of the web to the doctor element defining a path whicli the flowing stream of the liquid coating composition downstreams in the direction of travel of the web with reduced shear stress on the flowing stream of the liquid coating composition in the application chamber as the coating composition downstreams. 1 . A roaring rle f n, accordance with claim 6 wherein the upstream boundary wall and the upstream interior wa l are substantially parallel to tht other, each havinσ a tormina* i rig curvilinear section which are substantially paiallel to the other, the upstream boundary wall adapted to terminate in tangential relation with t.ho path web, reducing the formation of recirculating eddies and vortices in the coating composition. f A method oi applying a liquid coating composition on a web of material t i vr 1 i no through a patinq device comprising the steps of • idaptiru the travel of the web along a coating composition application chambei having upstream arid downstream sides extending across the path of the web transversely of the direction of the travel of the web on a path from t lie upst l earn side to the downstream side; r e cei ing a liquid flow of the liquid coating composition into the application chamber at the upstream side; ext c iidniq a do< to, element across the path of the web tr nsversely of t h< diiect ion of t i ve] of the web at the downstream side; s .icmn f|, 0 r( ( ). element from the web for spreading and defining the thickness of the liquid coating composition on the web; receiving a liquid flow of a carrier fluid introduced at the upstream side in the direction of the travel of the web; positioning the liquid flow of the liquid coating composition between the carrier fluid and the web; and pumping the liquid coating composition flow from the upstream side toward the doctor element thus defining a path which the flowing stream of the liquid coating composition downstreams in the direction of travel of the web with reduced shear stress on the flowing stream of the liquid coating composition in the application chamber as the coating composition downstreams. 9. A method in accordance with claim 8 wherein the step of receiving the flow of the liquid coating composition comprises the step of directing the liquid coating composition through an upstream interior side wall and an upstream boundary wall into the application chamber. 10. A method in accordance with claim 8 wherein the step of receiving the flow of carritji fluid comprises the step of providing a channel fejr directing air flow into the application chamber below the flow of the liquid coating compos i t ion . 11. A method in accordance with claim R wherein the upstream boundary wai 1 and the upstream interior wall are substantially parallel to the other, each having a terminating curvilinear section which are subs an ially parallel to the other, the upstream boundary wall adapted to terminate in tanςieritial • relation with the path web, reducing the formation of recircula ing eddies and vortices in the coating composition.

Inventors: AIDUN CYRUS K, AIDUN, CYRUS, K.
Applicants: INST OF PAPER SCIENCE & TECHNO, INSTITUTE OF PAPER SCIENCE & TECHNOLOGY, INC.
Application year: 1997
Main classification: B05C3
IPC codes: B05C3, B05C5, B05C11, B05D1, D21H23