Abstract:
A method is taught for that allows for controlling and optimizing the level of adhesion between a multilayer composite applied to a support web. The method comprises the steps of selecting the support web with or without a subbing layer applied thereto; selecting an organic solvent that is substantially free of other constituents as a carrier layer; forming a multilayer composite including the carrier layer, the carrier layer being the lowermost layer thereof; applying the multilayer composite to the support web or the subbing layer; drying the multilayer composite on the support web, the carrier layer evaporating from the multilayer composite; testing the adhesive strength between the support web or subbing layer and the multilayer composite after drying; and repeating these steps until a desired level of adhesion is found.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to methods for coating a moving web and, more particularly, to controlling adhesion levels between the moving web and the coating applied thereto, and most particularly, to controlling adhesion levels between photographic film base web and the multilayer coatings applied thereto.  
         BACKGROUND OF THE INVENTION  
         [0002]    Multilayer coating is well known in the prior art as described, for example, in U.S. Pat. No. 2,761,791 to Russell. In the method typically referred to as slide bead coating, a multilayer composite comprised of superimposed individual liquid layers is delivered to the moving substrate through the use of a coating die. At the end of the coating die, the layers form a continuous liquid bridge or coating bead between the die and the moving substrate. The slide bead coating method is useful for making thin, highly uniform, composite elements suitable for numerous applications, including photographic, thermographic, x-ray, and photoelectric films, among others.  
           [0003]    Various methods are known in the prior art to promote adhesion between the support and applied layers. In most cases, an adhesive layer or subbing layer is required on a support prior to application of the functional layers. There are a number of disclosures describing unique chemical compositions suitable for subbing layers. U.S. Pat. No. 4,748,150 to Vanier teaches the use of a vinylidene chloride copolymer as a subbing layer to promote the adhesion of a subsequent layer containing a polycarbonate. Alternatively, a support may be subjected to electrical discharge pretreatments including corona discharge as described in U.S. Pat. No. 3,376,208 to Wood, glow discharge as described in U.S. Pat. No. 5,425,980 to Grace, or electron-beam irradiation U.S. Pat. No. 4,594,262 to Kreil. More commonly, a substrate requires a combination of electrical treatment and the application of a subbing layer to achieve good adhesion such as, for example, described for example in U.S. Pat. No. 4,128,426 to Ohta, U.S. Pat. No. 4,689,359 to Ponicello, and U.S. Pat. No. 5,232,825 to Hattori.  
           [0004]    The equipment required for electrical treatment of substrates, such as those described above, is expensive and adds complexity to the manufacturing process. Moreover, such treatments may not be suitable for some coatings when layers are sensitive to chemical alteration of the support by electrical treatment. Similarly, the materials required for an effective subbing layer may be expensive or difficult to identify. There are no known coating methods describing enhancement of adhesion of a functional layer to the support.  
           [0005]    Finally, it is sometimes desirable to attenuate the adhesive strength of a functional layer to the support depending on product requirements. In most cases, strong adhesion between applied layers and support is needed. However, some products, such as those designed to delaminate during usage, may require only weak adhesive strength. Continuous modification of the chemistry of the subbing layer to achieve such aims may be expensive and time consuming.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a method for controlling the level of adhesion between a web and a coating applied thereto.  
           [0007]    It is a further object of the present invention to provide a method for applying multilayer coatings to moving webs which allows for modification of adhesion levels between the moving web and the multilayer coating.  
           [0008]    Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by selecting the support web with or without a subbing layer applied thereto; selecting an organic solvent that is substantially free of other constituents as a carrier layer; forming a multilayer composite including the carrier layer, the carrier layer being the lowermost layer thereof; applying the multilayer composite to the support web or the subbing layer; drying the multilayer composite on the support web, the carrier layer evaporating from the multilayer composite; testing the adhesive strength between the support web or subbing layer and the multilayer composite after drying; and repeating these steps until a desired level of adhesion is found. The method of the present invention may be practiced in conjunction with slide bead coating apparatus, extrusion hopper coating apparatus, and curtain coating apparatus. In a slide bead coating operation, the multilayer composite flows down the slide surface of the slide bead coating hopper and over a coating lip of the coating hopper to form a coating bead bridging the gap between the lip in the moving web. In this manner, the multilayer composite is applied to the support web or the subbing layer. The carrier layer preferably has a viscosity in the range of 1-10 cp and is applied at a wet thickness in the range of 1-20 μm; flowing the multilayer composite down the slide surface and over a coating lip of the coating hopper; and forming a coating bead between the coating lip and the web.  
           [0009]    In the practice of the method of the present invention, one or more of the upper layers preferably has a viscosity that is greater than the viscosity of the carrier layer. When the carrier layer is formed of a blend of organic solvents, preferably at least two of the following components: methanol, ethanol, isopropanol, n-butanol, acetone, methylethyl ketone, methylisobutyl ketone, toluene and methylene chloride are contained in the blend. It is also preferred that the layers above the carrier layer contain at least one of the following polymeric materials: cellulosics, polyvinylbutyrals, polycarbonates, polyurethanes and polyesters. Further, in the practice of the method of the present invention, it is preferred that the layers above the carrier layer have a combined wet thickness in the range of range of 1-500 μm and at least one of such upper layers has a viscosity in the range of 10-5,000 cp. In addition, one or more of such upper layers may include water.  
           [0010]    As mentioned above, the present invention allows for the control of the level of adhesion between the multilayer composite and the web such that the physical bond therebetween is strong or weak. In particular, the use of the present invention is shown to substantially improve the adhesion of a functional layer to the substrate. Moreover, this advantage is achieved without chemical modification of the support through the use of electrical discharge treatments. Finally, this advantage is achieved without the use of permanent chemical additives in the coated layer. Such additives may be expensive or detrimental to the properties of the final film.  
           [0011]    Although the present invention is discussed herein with particular reference to a slide bead coating operation, those skilled in the art will understand that the present invention can be advantageously practiced with other coating operations. For example, the control of the level of adhesion should also be achievable with multilayer extrusion hopper coating operations and multilayer curtain coating operations. In addition, those skilled in the art will recognize that the present invention can be advantageously practiced with carrier layers containing additives such as polymers, surfactants, and etching agents. Practical applications of the present invention include photographic, thermiographic and x-ray films as well as photographic, thermographic and inkjet papers, among others. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic of an exemplary multi-slot slide bead coating apparatus which may be used in the practice of the method of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    Turning to FIG. 1, there is shown a schematic of an exemplary and well known multi-slot slide bead coating apparatus  10  suitable for practicing the method of the present invention. The multi-slot slide bead coating apparatus  10  is typically used to deliver and coat multiple coating compositions simultaneously as a stacked composite of layers. Coating hopper  10  is shown as having only four slots, but multiple slot hoppers  10  may have fewer than four slots and are also known to deliver a composite layer comprised of five or six (or even more) coating composition layers.  
         [0014]    Coating hopper  10 , shown in side elevational cross-section, includes a front section  12 , a second section  14 , a third section  16 , a fourth section  18 , and a back plate  20 . There is an inlet  22  into second section  14  for supplying coating liquid to first metering slot  24  via pump  26  to thereby form a lowermost layer or carrier layer  28 . There is an inlet  30  into third section  16  for supplying coating liquid to second metering slot  32  via pump  34  to form layer  36 . There is an inlet  38  into fourth section  18  for supplying coating liquid to third metering slot  40  via pump  42  to form layer  44 . There is an inlet  46  into back plate  20  for supplying coating liquid to fourth metering slot  48  via pump  50  to form layer  52 . Each slot  24 ,  32 ,  40 ,  48  includes a transverse distribution cavity. Front section  12  includes an inclined slide surface  54 , and a coating lip  56 . There is a second inclined slide surface  58  at the top of second section  14 . There is a third inclined slide surface  60  at the top of third section  16 . There is a fourth inclined slide surface  62  at the top of fourth section  18 . Back plate  20  extends above inclined slide surface  62  to form a back land surface  64 .  
         [0015]    Residing adjacent the hopper  10  is a coating backing roller  66  about which a web  70  is conveyed. Typically, the hopper  10  is movable from a non-coating position toward the coating backing roller  66  and into a coating position.  
         [0016]    In the practice of the method of the present invention, the lowermost or carrier layer  28  is an organic solvent or blend of organic solvents that is substantially free of other constituents. The term “substantially free of other constituents” as used herein is intended to mean that the organic solvent or blend of organic solvents have a purity level of at least 98% and that any contaminants or additives present do not affect the viscosity of the carrier layer  28 . This lowermost layer or carrier layer  28  which is metered through the first metering slot  24 , moves down the first slide surface  54 , and wets the moving web  70  at the point where the coating bead  72  contacts the web  70 . In a preferred embodiment of the present invention, the lowermost layer  28  is an organic solvent or blend of organic solvents having a viscosity of less than 10 cp., a surface tension of less than 40 dynes/cm, and a wet thickness range of from about 1 to about 20 μm on the moving web  70 .  
         [0017]    As mentioned above, the lowermost layer  28  may be comprised of a single organic solvent. Examples of suitable organic solvents at 20° C. include methanol (0.6 cp.), acetone (0.3 cp.), methylethyl ketone (0.4 cp.), methyl isobutyl ketone (0.6 cp.), methylene chloride (0.4 cp.), toluene (0.6 cp.), methyl acetate (0.4 cp.), ethyl acetate (0.5 cp.), isopropyl acetate (0.5 cp.), n-propyl acetate (0.6 cp.), ethanol (1.2 cp.), n-propanol (2.3 cp.) and n-butanol (3.0 cp.).  
         [0018]    Alternatively, the lowermost layer  28  may be comprised of a combination of two or more organic solvents. Examples of suitable organic solvent blends at 20° C. include 1.1 methanol:methylene chloride (0.6 cp.), 1:1 acetone:methanol (0.4 cp.), 1:1:1 acetone:methanol:methylene chloride (0.5 cp.), 1:1 acetone:n-propanol (0.6 cp.), 1:1 acetone:ethanol (0.5 cp.), 1:1 methanol:ethanol (0.7 cp.), 1:1 methylene chloride:ethanol (0.8 cp.), 1:1:1 acetone:methylene chloride:ethanol (0.5 cp.), 1:1:1 methanol:methlylene chloride:n-butanol (0.8 cp.), 1:1:1 acetone:ethanol:n-butanol (0.8 cp.), 1:1:1 methanol:methylene chloride:ethanol (0.8 cp.), and 1:1:1:1 acetone:methanol:methylene chloride:n-propanol (0.6 cp.). For these examples, solvent ratios are by weight.  
         [0019]    The second liquid layer  36  which is metered through a second metering slot  32 , moves down the second slide surface  58 , and is accelerated by the carrier layer  28  down the first slide surface  54  to the coating bead  72 . The second layer  36  must be miscible with lowermost layer  28  and is therefore preferably organic, but may contain water. As noted by layers  44 ,  52  in FIG. 1, additional upper layers may also be applied using the slide bead coating apparatus  10 . These additional upper layers may be of a distinct composition relative to the second layer  36  or of the same composition. Similarly, the number of upper layers may also be further increased beyond three by extension of the number of die slots (not shown explicitly in FIG. 1). In a preferred embodiment of the present invention, the upper layers have a combined wet thickness in the range of from about 1 to about 500 μm, and at least one of the upper layers has a viscosity greater than 10 cp.  
         [0020]    Because the method of the present invention may involve application of highly volatile organic solvents, the temperature at which coating is performed is preferably less than or equal to 25° C. to avoid non-uniformities due to streaks and mottle. Methylene chloride, acetone, methyl acetate and methanol are examples of highly volatile organic solvents having a vapor pressure above 100 mm Hg at 25° C.  
         [0021]    The method of the present invention is suitable for application of multilayer coatings to a variety of substrates such as polyethylene terephthalate (PET), cellulose acetate (CA), polycarbonate, polystyrene, and other polymeric films. Additional substrates may include paper, laminates of paper and polymeric films, glass, cloth, aluminum and other metal supports. In some cases, substrates may be pretreated with subbing layers or electrical discharge devices. Substrates may also be pretreated with functional layers containing various binders and addenda.  
         [0022]    The advantages of the present invention are demonstrated by the following practical examples given below.  
       EXAMPLE 1  
       [0023]    The apparatus  10  illustrated in FIG. 1, was used to apply three organic layers to a moving substrate  70  of untreated polyethylene terephthalate (PET). All coating fluids were comprised of a polyvinylburyral (hydroxyl content of 12%) dissolved in 1:1 methanol:ethanol. The lowermost layer  28  was a variety of organic solvents having a wet thickness in the range of 2.5-7.5 μM. The organic solvents used as carrier layer  28  in this Example included ethanol, methylethyl ketone, and methylene chloride. The second and third layers each had a viscosity of 600 cp. and a combined final wet thickness of 30 μm on the moving web  70 . Coatings were applied at a temperature of 23.9° C. The gap between the coating lip  56  and the moving web  70  was 200 μm. The pressure differential across the coating bead  72  was adjusted between 0-10 cm of water to establish a uniform coating. For Example 1, the substrate was untreated polyethylene terephthalate (PET). Coatings were made at substrate speeds of 100 cm/s without defects resulting from entrained air or streaks. Finished samples were analyzed for adhesive strength. Results are summarized in Table 1. As shown in Table 1, adhesive strength is greater on untreated PET when films were prepared with methylene chloride or methylethyl ketone as the carrier layer. In some cases, adhesive strength was more than doubled. For example, the adhesive strength of the polyvinylbutyral layer increased from 2 to 5 N/m on an untreated PET support having no subbing layer when the organic solvent used in the lowermost layer  28  was changed from ethanol to methylene chloride.  
       EXAMPLE 2  
       [0024]    The conditions were identical to those described in Example 1, except that the PET substrate was modified by various subbing layers. In Example 2, PET was precoated with a subbing layer of either cellulose acetate butyrate (CAB) having 38% butryl content or nitrocellulose having 12% nitrogen as noted in Table 1. A coating of polyvinylbutyral was subsequently coated on to the modified PET, dried and tested for adhesive strength as described in Example 1 above. The results are summarized in Table 1. As shown in Table 1, adhesive strength was greater on PET supports having a subbing layer of CAB when films are prepared using methylene chloride or methylethyl ketone as the carrier layer. In some cases, adhesive strength was improved by more than ten fold. For example, the adhesive strength increased from 10 to 160 N/m on a PET support having a subbing layer of CAB when the organic solvent used in the lowermost layer was changed from ethanol to methylene chloride.  
       EXAMPLE 3  
       [0025]    The conditions were identical to those described in Example 1, except that the substrate is changed to untreated cellulose acetate (CA). A coating of polyvinylbutyral was coated on to the cellulose acetate substrate, dried and tested for adhesive strength as described in Example 1 above. The results are summarized in Table 1. As shown in Table 1, adhesive strength was greater on cellulose acetate supports when films were prepared using methylene chloride or methylethyl ketone as the carrier layer. In some cases, adhesive strength was improved by more than ten fold. For example, the adhesive strength increases from 9 to 160 N/m on a cellulose acetate support when the organic solvent used in the lowermost layer was changed form ethanol to methylene chloride.  
                               TABLE 1                                   Polymer   Adhesive       Carrier Solvent   Support   Sub Layer   Compatibility   Strength                   Ethanol   PET   None   No   2 N/m       Methylethyl ketone   PET   None   No    3       Methylene chloride   PET   None   No    5       Ethanol   PET   CAB   Partially   10       Methylethyl ketone   PET   CAB   Partially   68       Methylene chloride   PET   CAB   Partially   160        Ethanol   PET   Nitro-   Yes   unable to               cellulose       peel*       Methylethyl ketone   PET   Nitro-   Yes   unable to               cellulose       peel       Methylene chloride   PET   Nitro-   Yes   unable to               cellulose       peel       Ethanol   CA   None   Partially    9       1:1 Methanol:ethanol   CA   None   Partially   13       Methylethyl ketone   CA   None   Partially   29       Methylene chloride   CA   None   Partially   160                           
 
         [0026]    For the data presented in Table 1, multilayer coatings of polyvinylbutyral were applied to various supports using organic solvents as the carrier layer with the slide bead die illustrated in FIG. 1. Final film samples were tested for adhesion by measuring the force required to peel the polyvinylbutyral layers from the support. Results are given in Newtons per meter (N/m). Results are for conditions described in Example 1.  
         [0027]    The following test was used to determine the level of adhesion data presented in Table 1. The adhesion strength of the coated samples was measured using a modified 180° peel test with an Instron 1122 Tensile Tester with a 500 gram load cell. First, 0.0254 m (one inch) wide strips of the coated sample were prepared. Delamination of the coating at one end was initiated using a piece of 3M 810 Magic Tape. An additional piece of tape was then attached to the delaminated part of the coating and served as the gripping point for testing. The extending tape was long enough to extend beyond the support such that the Instron grips did not interfere with the testing. The sample was then mounted into the Instron 1122 Tensile Tester with the substrate clamped in the upper grip and the coating/tape assembly clamped in the bottom grip. The average force (in units of Newtons) required to peel the coating off the substrate at a 180° angle at a speed of 2 inches/min (50.8 mm/min) was recorded. Using this force value the adhesive strength in units of N/m was calculated using the equation: 
           S   A   =F   p (1−cos θ)/ w   
         [0028]    wherein S A  is the adhesive strength, F p  is the peel force, θ is the angle of peel (180°), and w is the width of the sample (0.0254 m).  
         [0029]    From the foregoing, it will be seen that this invention is one well adapted to obtain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the apparatus.  
         [0030]    It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.  
         [0031]    As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.  
                                         PARTS LIST:                                10   multi-slot slide bead coating apparatus/hopper       12   front section       14   second section       16   third section       18   fourth section       20   back plate       22   inlet       24   first metering slot       26   pump       28   carrier layer       30   inlet       32   second metering slot       34   pump       36   layer       38   inlet       40   third metering slot       42   pump       44   layer       46   inlet       48   fourth metering slot       50   pump       52   layer       54   first inclined slide surface       56   coating lip       58   second inclined slide surface       60   third inclined slide surface       62   fourth inclined slide surface       64   back land surface       66   coating backing roller       70   moving web       72   coating bead