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(11) | EP 0 000 241 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process and apparatus for coating a web |
| (57) Process and apparatus for applying a flowable coating medium to a travelling web
by a reverse applicator roll technique in which the peripheral velocity of the applicator
roll (U) is maintained at less than 0.4 of the forward speed of the web, and the dynamic
viscosity of the coating medium (η)is selected in accordance with the relationship
h = kη0.5UO.5, wherein k is from 0.01 to 100, such that a layer of coating medium of equilibrium
thickness (h) not exceeding 5.0 mm is entrained on the surface of the applicator roll
for transfer to the web. The technique avoids the use of a doctor blade to meter the
amount of coating medium on the applicator roll, and is suitable for the high speed
application of a heat sealable coating laver onto a polymeric packaging film. Coating
qualitv can be imoroved by stabiiising the web downstream ofthe appiicator roll. |
[0001]
of supply, subected to a process treatment and substuer rewound in roll-form or out into sheets or otherwise converted for use by a consumer Typical
include paper, and plastics films.
[0002] The term "coasing" is used througnout this specification to describe both the proccess of depositing a Liquid or flowable material, including printing inks, in a controlled manner on to a surface of a moving web, and the layer of material so deposited on that surface either in the wet state or in a subsequently dried condition.
[0003] For many years successful coating of webs, particularly of paper, has been effected by means of a transfer roll technique in which a flowable coating medium is metered onto a rotating applicator roll and thence wiped directly onto a surface of a travelling web. A reverse roll technique in which the applicator roll contra-rotates relative to the direction of movement of the web has gained wide-spread acceptance by virtue of the considerable latitude in coating viscosity and coat thickness thereby permitted.
[0004] A principle of the reverse roll technique is that the supply of coating medium to the applicator roll surface should be controlled by mechanical metering means, such as a flexible doctor blade or an associated doctor roll cooperating with the applicator roll, to ensure that the requisite volume of costing medium is entrained on the applicator roll surface and thense deposited on the web surface. However there is a tendency for the coating medium to dry out and form crusts on the mechanical metering means, for example on the edge of the doctor blade, or for particles of grit or other contaminants to accumulate on the doctoring device, with consequent disruption of coating uniformity. Reciprocation of the metering means parallel to the axis of the applicator roll has therefore been proposed to overcome this problem, but serves simply to randomize the distribution of coating defects on the web surface rather than achieving elimination of these defects.
[0005] A further principle of the conventional reverse roll coating technique is that the web should be partially wrapped around the resilient surface of a support or backing roll which rotates in the direction of movement of the web and cooperates with the applicator roll to define a "nip" through which the web passes. In an arrangement of this kind the web makes only line contact with the surface of the applicator roll and consistently uniform coating behaviour is difficult to achieve.
[0006] A still further principle of the reverse roll technique is that the deposited coating medium should be wiped onto the web surface by the applicator roll to assist the production of a smooth coating on the web. The applicator roll is therefore usually rotated at a peripheral speed at least equal to, and preferably exceeding, the linear speed of the travelling web, the latter seldom exceeding 700 feet/minute (3.5 ms-1). l..n practice, the wipe ratio, which is the ratio of the speed of the applicator roll to that of the web, is conventionally within a range of from 1:1 to 3:1. With the advent of increasingly high speed coating technology the need to maintain a positive speed differential between the applicator roll and web has increased the difficulty of achieving coating uniformity. For example, transverse ridges or "chatter" marks resulting from vibration
whereby the
defects are reduced or eliminate.
[0007] Accordingly, the present invention provides a method of coating a surface of a web comprising forwarding the web in contact with the surface of an applicator roll, supplying a flowable coating medium to the surface of the applicator roll. and rotating the applicator roll in a direction contrary to the direction of forward movement of the web, characterised in that the dynasic viscosity of the coating medium and the peripheral velocity of the applicator roll are each selected in accordance with the relationship
such that a layer of coating medium of equilibrium thickness not exceeding 5.0 mm is entrained on the surface of the applicator roll for transfer to the surface of the web, and the ratio of the peripheral velocity of the applicator roll to the forward speed of the web is maintained at a value not exceeding 0.4:1, wherein
h is the equilibrium thickness (mm) of the entrained coating medium on the applicator roll surface,
k has a value of from 0.01 to 100,
η is the dynamic viscosity (Msm-2) of the coating medium, and
U is the peripheral velocity (ms-1) of the applicator roll.
[0008] The invencion alsoprovides an apparatus for applying a liquid coasing medium to a surface a travelling web comprising an appllic ator roil rotable in a direction contrary to the direction or travel of the web a reservoir for supplying liquid coasing medium to the surface of the applicator roll, and means for forwarding the web in contact with the surface of the applicator roll characterised by guide means for maintaining the web in engagement with the surface of the applicator roll over an arc contact angle of from 5 to 90°. by the absence of means cooperating with the applicator roll surface for metering coating medium thereon, and by means for rotating the applicator roll counter to the direction of travel of the web at a peripheral velocity not exceeding 0.4 of the forward speed of the web such that, in the absence or said metering means a uniform layer of coating medium having an equilibrium thickness not exceeding 5.0 mm can be antrained from the reservoir onto the surface of the applicator roll for transfer to the web surface.
[0010] The applicator roll which may be in the form of a solid bar, or, preferably. a hollow tube with provision for circulation therethrough of a coolant medium such as chilled water is suitably formed from mild steel, the surface of the roll being sufficiently smooth to ansure the deposition of a uniform coating on the web. Preferably the surface of the roll is chromium plated and polished to a suitable degree of smoothness - for example, a surface texture with an Arithmetical Mean Deviation, Ra, (commonly referred to as Centre Line werage surface roughness ) according to British Standard 1134 of 1972 of from 0,2 to 0.05 µm (3 to 2 microinches), and preferably about C.1 un (4 microinches).
the roll projects oeyond each longitudinsi edge of the web so that the envire surface or the web may be coated, and in the case of wide webs, for example of the order of 48 inches (1.2 m), the rigidity of the roll may be improved, if desired, by the provision between the ends of the roll of at least one intermediate supporting member suen as a semi-cylindrical shell bearing, cooperating with the roll surface Suitably the supporting member comprises a self-lubricating bearing surface of a material such as the Fluon brand of polytetrafluorcethylene Fluon being a registered Trade Mark of Imperial Chemical Industries Limited,
[0011] In the absence of a doctor blade or other mechanical metering means cooperating with the applicator roll the continuous transfer to the web surface of the requisite amount of coating medium depends, inter alia, on the viscosity characteristics of the medium and on the geometry of the applicator assembly. Particularly satisfactory coating behaviour has been observed with an applicator roll of relatively small diameter compared to that employed in conventional reverse roll coating equipment, and it is therefore preferred that the diameter of the applicator roll should be within a range of from 25 to 102 mm, preferably about 50 mm.
[0012] The applicator roll rotates in a direction contrary to the direction of movement of the travelling web, and by adjusting the peripheral speed of the applicator roll to a value considerably less than the linear speed
[0013] Because of the large speed differential between the web and applicator roll, substantially all of the coating medium enntrained en tne surface of the rotating coating medium entrained en the surface of the rotating applicator roll in the region of contact with the web is transferred to the web surface, and is there spread but to provide a uniform coating of the desired thickness, on the web surface thereby elimnating the need for additional mechanical metering means cooperating with the coated surface of the web In practice the peripheral speed of e applicator roll is conveniently within a range of from 0.001 to 0.4 of the linear speed of the web although a considerable speed differential Is preferred such that ratio of the roll to web speeds is from 0.01 '1 to 0.3:1,preferably from 0.015: 1 to C0.15:1, and particularly preferably from 0.02:1 to 0,05:1, For example, a polyolefin web travelling at 1500 feet/minute (7.6 metres, sec) is conveniently coatad by an applicator roll contra-retating at a peripheral Speed of 50 feet/minute (0.25 metres/sec), i.e. a roll to web speed ratio of about 0.033:1. Desirably, the peripheral speed of applicator roll is within a range of from to 500 feet/minute to 2.5 metres/sec), range of from I to 500 feet/minute (0.005 to 2.5 metres/sec) and preferably is meintained between to and 150 feet/minute 0.05 to 0.75 :"B*cre.3/metres/sec
and the viscosity characteristics of the coating medium to ensure that the correct amount of coating medium is entrained
surface at the area of contact with the web to ansure transfer to the web surface of a coating layer of the desired thickness. The pempheral. speed of the
roll should therefere be controlled in accordance with theon in accordance with the relationship:
wherein the symbols represent the hereinperore specified the symbols represent the hereinperore specified parameters.
[0014] The thickness of the wet coating layer deposited on the web is, 9 0:, course, reduced (compared to h - the equilibrium thickness of the entrained coating medium on the applicator roll surface in proportion to the ratio of speeds cf the roll and web
[0015] Coating medium picked up from a suitable source or reservoir by the applicator roll is conveyed en the surface thereof to the junction at which the coated web disengages from the contra-rotating roill At this junction is established a distinct wedge or "bead" of coating medium supported by contact with both the web and roll. As rotation of the roll conveys coating medium towards the web there is established on, and supported only by, the roll surface; a layer of coating medium the thickness of which depends on the equilibrium balance achieved between the volume of coating medium forwarded under
forces interacting with the roll and the volume of coating medium flowing from the roll surface under gravity, It is the thickness of this Bayer. immediately before it marges into the "bead", measured in a direction radially outwards frcm the rotational axis of the applicator roll which is herein preferred to as the "equilibrium chickeness" (h) of the antrained coating medium, The thickness of layer h. which should not exceed 5.0 mm - and is preferably from 0.025 to 0.5 mm, is conveniently measured by conventional techniques - for example by means of a travelling microscope radially directed towards the applicator rell axis and focussed in turn on the roll surface and on the outermost surface of the coating layer entrained thereon, Alternatively, knowledge of the selids content and density of the coating medium and of the relative speeds of web and applicator roll enables the thickness of layer h to the derived from the measured thickness of the dry coating in the web surface by simple calculation - on the basis that all of the coating medium entrained on the roll surface is spread onto the web.
[0016] The value of k in the aforementioned relationship is from C,01 to 100 and is preferably from 0.1 to 50. In general, k is desirably maintained within a range of from 0.25 to 25, preferably from 0.5 to 10. a preferred specific value of k being 3.7
[0017] To avoid the deposition of ridged or otherwise non-uniform coatings, caused, for example, by the presence of pucker lanes in, or general lack of planarity of, the web fed to the applicator roll, it is preferred that simple line contact between the web and applicator roll is avoided during the coating operation. Preferably, therefore, the web cooperates with the applicator roll over an arcuate segment of the roll surface such that the web is spread uniformly on the roll surface. Suitably, the arc contact angle, being the angle at the rotational axis of the roll subtended by that arc of the roll surface engaged by the web has a minimum value of 5°, and may be increased to any value at which an improvement in the quality of the deposited coating is discernible. In practice, the maximum value of arc contact angle is determined by the geometry of the system and will seldom exceed 90°. Preferably, an arc contact angle of-from 8 to 45° is emplyoyed.
[0018] In conventional reverse roll coating techniques the web is partially wrapped around a resilient-surfaced backing roll cooperating with the applicator roll to define a nip through which the web travels, the separation between the nip rolls being maintained equal to the combined thickness of the web and the coating deposited thereon. Consequently, there is a high risk of damage to the web, in the mip resulting from the speed differential between the backing roll, rotates at the same speed and in the same direction as the web, and the contra-rotating applicator roll. The present invention enables reverse roll coating to be effected in the absence of a backing roll. However, to prevent vibration of the web in the absence of a backing roll, it is preferred that the path of the web be stabilised by engagement of the web with wab stabilising means positioned downstream of the applicator roll. The web stabilising
that engaging the applicator roll, the member engaging the web downstream of the point at which the webceases to contact the applicator roll and sufficiently close thereto to reduce vibration of the web to an acceptable magnitude in the vicinity of the applicator roll. The stabilising member may be a stationary rod or bar, or, preferably, a roll rotatable in the direction of travel of the web. Desirably, the diameter of the stabilising roll should exceed that of the applicator roll, the diameter ratio of stabilising to applicator roll being preferably within a range of from 1.01:1 to 4:1.
[0019] The coated web formed by transfer of the coating medium from the applicator roll to the web may be subjected to conventional treatments. Thus, if desired, smoothing of the applied coating layer may be effected by contacting the coated surface with a smoothing assembly comprising, for example, at least one polished rod or bar, or, preferably, a plurality of smoothing rolls at least some of which rotate in a direction contrary to the direction of travel of the web. Typical smoothing rolls are of mild steel with a chromium plated and polished surface.
[0020] Coatings applied in accordance with the present invention may be dried, if necessary, by conventional methods, for example - by passing the coated web through
maintained at an appropriate temperature.
which the
web floats on a current
is particularly Meltable. The speed at which webs may be coated according
to the invention depends on
factors including
roll coating techniques.
[0021] The method of the invention is applicable to the coating of a single web surface or, if desired, to the sequential application of coating medium to each of the two opposed outermost surfaces of a web. In the latter case, sequantial coating stations may be followed by a single smoothing assembly, with smoothing members cooperating with each coated surface, and, if desired, by a single even assembly for simultaneous
drying both coated surfaces of the web.
[0022] The technique of the present invention is suitably employed for the application of a variety of liquid coating media, but is particularly suitable for the application of a heat-scalable coating medium to a web surface. In particular, a heat-sealable coating medium may be applied to a polymeric web the surface of which has already been treated with a primer medium - such as an interpolymerised condensation resin of the kind described in British patent 1 134 876 or in British patent 1 174 328.
[0023] Suitable flowable, polymeric, heat-scalable coating media include copolymers of vinylidene chloride with acrylonitrile because they yield hard coatings with good heat-seal strengths, and are also resistant to moisture and have low ges permeability. It is preferred to use copolymers containing between 80% and 95% by weight of vinylidene chloride with from5% to 20% by weight acrylonitrile. whesecopolymers may contain other monomers such as acrylic acid, itaconic acid and methacrylic acid, but a particularly preferred
resin comprises a copolymer containing 88 weight % of vinylidene chloride and 12 weight % of acrynonitrile. The heat-seal coating medium may be applied to the web as a solution or a dispersion, but the solvent or dispersant should not be such that it will dissolve any resin coating already on the web surface. For economic reasons application as an aqueous dispersion is preferred. A vinylidene chloride-acrylonitrile copolymer coating medium is conveniently employed as an aqueous dispersion containing from about 30 to about 50% by weight of the copolymer based on the weight of the dispersion, i.e. a dispersion having a dynamic viscosity range at 20°C from about 0,0015 to about 0,06 Nsm-2.
[0024] Coating medium may be supplied to the surface of the the, or each, applicator roll by conventional techniques
example, by partial immersion of the rotating applicator roll in a bath containing a volume of the liquid coating medium, the roll preferably being immersed to the level of its rotational axis in the medium.
[0025] The thickness of the coating applied to the, or each, surface of the web, can be adjusted, inter alia, by appropriate selection of the rotational speed of the applicator roll. A heat-sealable resin, of the
hereinbefore described, is suitably deposited on a polymeric web to form a coating having a wet thickness, i.e. prior to drying, of the order of from 1 to 100 µm,
facilitate drying, the wet thickness of the
desirably maintained between 1 and 25 µm, preferably from 1.5 to 10 µm, and particularly preferably from 2 to 3 µm. In practice, the wet thickness of the coating may be varied over a wide range depending
solids content of the coating medium and the desired thickness of the subsequently dried coating.
thickness on the web - approximately in proportion to the ratio of the speed of
applicator roll.
[0026] Coatings applied in accordance with the present invention may contain agents, such as anti-static agents, anti-oxidants, ultra-violet light stabilisers, and the like, to control or improve the characteristics of the coated web.
[0027] Webs such as paper, paperboard, cellulosic films, metal foils, polymeric films, and laminates there of, are suitably coated or printed by the techniques of the present invention. Typical polymeric films include oriented, particularly biaxially oriented, films formed in conventional manner from polyesters such as polyethylene terephthalate and polyethylene-1,2-dipheno-yethane-4,4'-dicarboxylate, and from polymers and copolymers of 1-olefins such as ethylene, propylene, butene-1, and 4-methylpentene-1. A particularly useful thermoplastic polymer film is that formed from a high molecular weight stereoregular predominantly crystalline polymer of propylene, either in the form of a homopolymer or copolymerised with minor quantities (e.g. up to 15% by weight of the copolymer) of at least one other unsaturated monomer, such as ethylene.
[0028] Films treated according to the present invention may vary in thickness depending on the intended application, films having a thickness of from 2 to 150 microns being of general utility. Films intended for use in packaging operations are suitably within a thickness range of from 10 to 50 microns.
[0029] the technique of the invention is capable of industrial
and in a preferred
sealable polyolefin film is prepared by forwarning an oriented polyolefin film in contact with the surface of an applicator roll, supplying a solution or dispersion of a heat sealable poating medium to the surface of the applicator roll. rotating the applicator roll in a direction contrary to the direction of forward movement of the film to coat the film, and drying the coated film, characterised by maintaining the film in contact with the applicator roll surface ever an are act angle of from 5 to 90°, by supplying to the applicator roll surface a solution or dispersion of a heat sealable coating medium having a dynamic viscosity of from 0.001 to 0.1 Nsm-2 at 20°C, and by maintaining the peripheral velocity of the applicator roll within a range of from 0.001 to 0.4 of the forward speed of the film such that a layer of coating medium of squilibrium thickness not exceeding 5.0 mm is entrained on the surface of the applicator roll and thence transferred to a surface of the film.
[0030] The invention is illustrated by reference to the accompanying drawings in which:
Figure 1 is a schematic side elevation of a reverse roll coating assembly.
Figure 2 is a schematic side elevation of a similar assembly incorporating a web stabilising roll, and
Figure 3 is a schematic elevation, to a different spale, depicting sequential reverse
roll coating of each surface of a web.
[0031] a polymeric film web 10
from a supply source
into contact with
applicator roll 12, and is removed therefrom eround idler roll 13 so positioned that the zone of contact between the web and applicator roll surface subtends an are contact angle α of the order of 10° at the rotational axis 14 of the applicator roll.
[0032] Applicator roll 12 rotates, as indicated by arrow A, in a direction contrary to the direction of movement of web 10, and at a peripheral speed less than the linear speed of the web.
[0033] A bath 15 contains a volume of liquid coating medium 16 maintained at a depth such that the applicator roll is immersed therein to the level of its rotational axis 14. Surplus coating liquid overflowing from the bath can, if desired, be collected and recycled to the bath by means of suitable pumping equipment (not shown)
[0034] Rotation of applicator roll 12 creates a dynamic meniscus 17 of coating liquid which progressively forms an equilibrium layer 18 of relatively constant thickness at the roll surface as a balance is achieved between the amount of liquid entrained on the roll by surface tension and liquid inertia and that flowing back into the bath by virtue of gravity and viscosity factors.
at which the opposite surface of the web ceases to contact applicator roll 12. The proximal positioning
to applicator roll 12 reduces vibration of web 10 thereby improving the stability of the wedge19
medium and ensuring that the latter is transferred smoothly to web 10 to form a highly uniform coating lever 20 on the web surface Stabilising roll 21 may be independently driven at a suitable speed or may idle in contact with the web, and is suitably fabricated from a light metal such as aluminium.
[0035] As shown in Figure 3, a polyolefin web 50 is unwound from a supply roll 51 and fed around idler guide roll 52, throught tension-controlling nip rolls 53, 54 and around idler roll 50 to a reverse roll coating assembly generally designated 56 comprising a feed roll 57, a relatively arrow diameter applicator roll 58, contra-rotating in a bath 59 of liquid coating medium 60, and a stabilising roll 61 of diameter exceeding that of applicator roll 58. Stabilising roll 61, being in contact with the dry surface of web 50 is undriven, and allowed to co-rotate by frictional contact with the web.
[0036] The unilaterally coated web then traverses vacuum
slight frictional engagement therewith, is driven at a speed substantially equal to that of the web, and preferably in a direction contrary to the direction of movement of the web to smooth, and eliminate bubbles from the initially deposited coating layer.
[0037] The web, now coated on each surface, is fed through a bank 69 of smoothing rollers 70, 71 rollers 70 contra― rotating in contact wiht one surface of the coaled web, and rollers 71 contra―rotating in contact with the other coated surface.
[0038] Subsequently, the coated web traverses an air float drying oven 72 in which the web is suspended on currents of warm air to ensure that the smoothed coatings are dried before making contact with a solid roll surface.
[0039] After traversing idler guide roll 73, the dried web passes through the nip between guide roll 74 and the first of two internally-refrigerated cooling rolls 75, 76, the web being cooled by intimate contact with the surfaces thereof, and is then fed between tension-controlling nip rolls 77, 78 and onto wind-up roll 79.
[0040] If desired, one or more heated rolls (not shown), maintained at a temperature between the glass transition temperature and melting temperature of the polymer, may be interposed in the web path between guide roll 73' and cooling roll 75 to flatten the web, integrate the coating and effect tensioned thermal relaxation of the web in the manner disclosed in UK Patent 1 457 940.
EXAMPLE 1
[0042] The web to be coated was a biaxially oriented, heat-set and discharge treated polypropylene film 24 micron. (µm) thick and 28 inches (711 mm) wide, having on each surface thereof a gravure coated ancher layer of about
101 feet/minute (0.51 m/s) i.e. application to web speed ratio of 0.335: 1 , the uniformity of the coating applied to the web began to deteriorate and when the peripheral speed of the applicator roll was increased to above 120 feet/minute (0.61 m/s), i.e. applicator to web speed ratio of 0.4:1, surplus coating medium began to accumulate on the web surface upstream of smoothing rolls 70.
6. A method according to any one of the preceding claims characterised in that the
wet thickness of the coating deposited on the web surface is from 0.1 to 25 µm.
7. A method according to any one of the preceding claims characterised in that the
web cooperates with the surface of the applicator roll over an arc contact angle of
from 8 to 43°.
8. A method according to any one of the preceding claims characterised in that the
path of the web is stabilised against vibration downstream of the applicator roll.
9. A method according to any one of the preceding claims characterised in that the
coating medium deposited on the web surface is heat sealable.
10. A method according to any one of the preceding claims characterised in that the
web is an oriented polymeric film.
11. A method of producing a heat sealable polyolefin film comprising forwarding an
oriented polyolefin film in contact with the surface of an applicator roll, supplying
a solution or dispersion of a heat sealable coating medium to the surface of the applicator
roll, rotating the applicator roll in a direction contrary to the direction of forward
movement of the film to coat the film, and drying the coated film. characterised by
maintaining the film in contact with the applicator roll surface over an arc contact
angle of from 5 to 90°, by supplying to the applicator roll surface a soltution or
dispersion of a heat sealable coating medium having a dynamic viscosity of from 0.001
to 0.1 Nsm-2 at 20°C, and by maintaining the peripheral velocity of the applicator roll within
a range of from 0,001 to 0,4 of the forward speed of the film a layer of coating medium
of equilibrium thickness not exceeding 5.0 mm is entrained on the surface of the applicator
roll and thence transferred to a surface of the film.
12. An apparatus for applying a liquid coating medium to a surface of a travelling
web comprising an applicator roll rotatable in a direction contrary to the direction
of travel of the web, a reservoir for supplying liquid coating medium to the surface
of the applicator roll. and means for forwarding the web in contact with the surface
of the applicator roll characterised by guide means for maintaining the web in engagement
with the surface of the applicator roll over an arc contact angle offrom 5 to 90°
,by the absence of means means cooperating with the applicator roll sufrace for metering
coating redium thereon, and by means for rotating the applicator roll counter to the
direction of travel of the web at a peripheral velocity not exceeding 0.4 of the forward
speed of the web such that in the absence of said metering means, a uniform layer
of coating medium having an equilibrium thickness not exceeding 5.0 mm can be entrained
from the reservoir onto the surface of the applicator roll for transfer to the web
surface,