Abstract:
The present invention relates to a thermally printable paper article with an elastomeric underlayer, which imparts improved printing performance.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a thermally printable paper article with an elastomeric underlayer. The article of the present invention provides improved printing performance by virtue of the underlayer. 
         [0002]    In direct thermal printing, a thermal printhead comes in direct contact with paper to heat the paper and produce an image. When the paper does not contact the printhead completely, the heat conveyed to the paper tends to diffuse, resulting in unfavorably low energy efficiency. Conventionally, thermal papers are produced with high smoothness to achieve better contact between the printer and the paper; nevertheless, the match is imperfect and, consequently, defects are manifested in the image in the form of missing dots. These missing dots, which are voids found in, for example, bars of a barcode or spots found in spaces of the code that are read as irregularities in the reflectance profile, result in poor barcode readability. 
         [0003]    It would therefore be an advantage in the art of thermal printing to find a way to improve print performance by improving contact between the printhead and the paper. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention addresses a need in the art by providing, in a first aspect, a coated paper article comprising: 
         [0000]    a) a 40-μm to 500-μm thick paper substrate;
 
b) a 3-μm to 20-μm thick elastomeric layer having a compressive modulus in the range of from 10 3  Pa to 10 8  Pa disposed over the paper substrate;
 
c) a 2-μm to 10-μm thick pigmented heat insulating layer comprising insulating particles selected from the group consisting of hollow sphere polymer particles, clay particles, and zeolite particles disposed over the elastomeric layer; and
 
d) a 1-μm to 10-μm thick thermosensitive recording layer disposed over the pigmented heat insulating layer.
 
         [0005]    In a second aspect, the present invention is a coated paper article comprising: 
         [0000]    a) a 40-μm to 500-μm thick paper substrate;
 
b) a 3-μm to 20-μm thick elastomeric layer of interconnecting polymer particles disposed over the paper substrate, wherein the polymer particles have a core-shell morphology wherein the weight-to-weight ratio of the core to the shell is in the range of from 80:20 to 98:2; wherein the core comprises, based on the weight of the core, from 90 to 99.9 weight percent structural units of a monomer selected from the group consisting of ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and 2-propylheptyl acrylate, and from 0.1 to 10 weight percent structural units of a multiethylenically unsaturated monomer;
 
c) a 2-μm to 10-μm thick pigmented heat insulating layer comprising insulating particles selected from the group consisting of hollow sphere polymer particles, clay particles, and zeolite particles disposed over the elastomeric layer; and
 
d) a 1-μm to 10-μm thick thermosensitive recording layer disposed over the pigmented heat insulating layer.
 
         [0006]    The article of the present invention provides a way to improve print performance by mitigating the adverse effects of pressure applied to the paper. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    In a first aspect, the present invention is a coated paper article comprising: 
         [0000]    a) a 40-μm to 500-μm thick paper substrate;
 
b) a 3-μm to 20-μm thick elastomeric layer having a compressive modulus in the range of from 10 3  Pa to 10 8  Pa disposed over the paper substrate;
 
c) a 2-μm to 10-μm thick pigmented heat insulating layer comprising insulating particles selected from the group consisting of hollow sphere polymer particles, clay particles, and zeolite particles disposed over the elastomeric layer; and
 
d) a 1-μm to 10-μm thick thermosensitive recording layer disposed over the pigmented heat insulating material layer.
 
         [0008]    The article of the present invention is advantageously prepared by applying an elastic layer, then an insulating layer, and then a thermosensitive recording layer to the paper by sequential drawdowns of aqueous coating formulations. In a preferred method of applying the elastic layer, an aqueous dispersion of polymer particles having a compressive modulus in the range of from 10 3  Pa, preferably from 10 4  Pa, more preferably from 10 6  Pa to 10 8  Pa is applied to the paper substrate using a wire-wound rod at controlled speed on a drawdown machine. The coated paper is then advantageously dried at advance temperatures before the next layer is applied. 
         [0009]    The polymer particles are preferably characterized by a core-shell morphology, wherein the core comprises from 80, more preferably from 85, and most preferably from 90 weight percent, to preferably 98, and more preferably to 96 weight percent of the polymer particles, and the shell comprises preferably from 2, more preferably from 5 weight percent, to preferably 20, more preferably to 15, and most preferably to 10 weight percent of the polymer particles. 
         [0010]    The core preferably comprises, based on the weight of the core, from 90, more preferably from 95, and most preferably from 98 weight percent, to preferably 99.9, more preferably to 99.8, and most preferably to 99.5 weight percent structural units of a monomer selected from the group consisting of ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and 2-propylheptyl methacrylate. The core preferably further comprises, based on the weight of the core, from 0.1, more preferably from 0.2, and most preferably from 0.5 weight percent, to preferably 10, more preferably to 5, and most preferably to 2 weight percent structural units of a multiethylenically unsaturated monomer. Preferred multiethylenically unsaturated monomers are diethylenically unsaturated monomers such as allyl methacrylate, divinyl benzene, butylene glycol diacrylate, ethylene glycol diacrylate, butylene glycol dimethacrylate, and ethylene glycol dimethacrylate. 
         [0011]    The shell preferably comprises structural units of at least one monomer selected from the group consisting of methyl methacrylate, styrene, acrylonitrile, and t-butyl methacrylate. Preferably, at least 90%, more preferably at least 95%, and most preferably at least 98% of the core comprises structural units of butyl acrylate and allyl methacrylate; preferably at least 90%, more preferably at least 95%, and most preferably at least 98% of the shell comprises structural units of methyl methacrylate. 
         [0012]    The preferred thickness of the elastomeric layer is from 5 μm to 15 μm (˜5 g/m 2  to 15 g/m 2 ). 
         [0013]    An insulating layer is formed by applying an aqueous dispersion or hollow sphere polymer particles or an aqueous suspension of clay or zeolite particles to the coated paper and drying applied coating. Commercially available aqueous dispersions of hollow sphere polymer particles include ROPAQUE™ TH-2000 Hollow Sphere Polymer, ROPAQUE™ AF-1055 Hollow Sphere Polymer, and ROPAQUE™ Ultra E Opaque Polymer. (A Trademark of The Dow Chemical Company or its Affiliates.) The particle size of the hollow sphere polymers is typically in the range of from 275 nm, more preferably from 350 nm, to preferably 2 μm, more preferably to 1.8 μm, and most preferably to 1.6 μm. Preferably, thickness of the insulating layer is in the range of from 4 μm to 8 μm (corresponding to ˜1.4 g/m 2  to 10 g/m 2 , depending on the density of the insulating material.) 
         [0014]    A solution of a thermosensitive recording material is then advantageously applied to the paper coated with the elastomeric and insulating layers and dried. The thermosensitive recording material typically comprises a leuco dye and a color developer (see U.S. Pat. No. 4,929,590) and may also comprise a variety of other additives including binders, fillers, crosslinking agents, surface active agents, and thermofusible materials. 
         [0015]    As the following examples demonstrate, the article of the present invention shows an improvement in optical density, which is an indicator of print quality, over coated paper that does not include an elastomeric layer. 
         [0016]    For Example 1, the polymer particles that form the elastomeric layer are characterized as shown in Table 1. BA refers to butyl acrylate, ALMA refers to allyl methacrylate, and MMA refers to methyl methacrylate. Compressive Modulus was calculated as described in the section titled Calculation of Compressive Modulus. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Characterization of Polymer Particles 
               
               
                 forming the Elastomeric Layer 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Core:Shell wt/wt ratio 
                 94:4 
               
               
                   
                 Core (wt %) 
                 Copolymer of BA(99.3)/ALMA(0.7) 
               
               
                   
                 Shell (wt %) 
                 Poly(MMA) 
               
               
                   
                 Compressive Modulus 
                 2.1 MPa 
               
               
                   
                   
               
             
          
         
       
     
       Example 1—Preparation of a Coated Paper Article with an Elastomeric Underlayer 
       [0017]    An aqueous dispersion of the core-shell elastomeric polymer particles (119.9 g, 51.3% solids, particle size 170 nm) was combined with RHOPLEX™ P308 Binder (a Trademark of The Dow Chemical Company or Its Affiliates, 10.1 g, 49.8% solids), and water (31.6 g) with stirring. A coating was applied to the paper substrate using a wire-wound rod at a controlled speed on a drawdown machine; the coated paper was then transferred to a convection oven set at 80° C. to dry for 1 min. The density of the elastomeric layer was found to be 3.7 g/m 2  as determined by cutting a known area of coated material and weighing the sample. 
         [0018]    A solution of ROPAQUE AF-1055 Hollow Sphere Polymer (71.7 g, 26.7% solids), RHOPLEX P308 Binder (8.8 g, 49.8% solids), polyvinyl alcohol (obtained from Kremer Pigmente, 3.9 g, 14.5% solids), and water (117.5 g) was prepared; the pH of the mixture was adjusted to 7.5 and the viscosity adjusted to 400 cPs with RHOPLEX RM232D Rheology Modifier. A portion of this mixture was then applied and dried as described above. The density of the applied coating was 3.5 g/m 2 . 
         [0019]    The thermosensitive recording formulation was prepared by mixing together water (5.7 g) and a dispersant (0.03 g) with stirring. Calcium carbonate powder (4.4 g, Tunex-E from Shirashi Kogyo Kaisha, Ltd.) was then added slowly and stirring was continued for 5 min before silica powder (3.7 g, Mizucasil P-603 from Mizusawa Kagaku K.K.) was added slowly to the mixture. Stirring was continued for an additional 5 min during which time an aqueous dispersion of 4-hydroxy-4′-isopropoxydiphenylsulfone (8.8 g, 50% solids) was slowly added, followed by the addition of an aqueous dispersion of 2-benzyl-oxy-napthalene (7.3 g, 40% solids), followed by addition of an aqueous dispersion of zinc stearate (3.1 g), then an aqueous dispersion of 2-anilino-6-(dibutylamino)-3-methylfluoran (5.2 g, 35% solids). Then, defoamer (0.007 g) was added and the mixture was allowed to stir for an additional 5 min. Finally, a solution of fully hydrolyzed polyvinyl alcohol (14.7 g) was slowly added and stirring continued for an additional 5 min. The density of the applied coating was 3.5 g/mm 2 . 
       Comparative Example 1—Preparation of a Coated Paper Article without an Elastomeric Underlayer 
       [0020]    The article of the comparative example was prepared essentially as described in Example 1 except for the absence of elastomeric layer step. The optical densities of the two samples were measured at 8 mJ/mm 2  in accordance with ASTM F1405 using an Atlantek M200 thermal printer and an X-Rite optical densitometer. The coated substrate of Example 1 was found to have an optical density of 1.19 AU while the coated substrate of Comparative Example 1 was found to have an optical density of 0.86 AU. The higher optical density observed for the example of the invention correlates with significantly higher print quality. 
       Calculation of Compressive Modulus 
       [0021]    Thermal Mechanical Analysis was carried out using a TA Q400 Thermomechanical Analyzer equipped with a compression sample fixture. Samples of dried coating slab were prepared by pouring a 1-mm thick aqueous coating formulation onto a smooth Teflon petri dish and drying the sample in vacuo at 50° C. The dried specimen was removed from the Teflon surface and released as a free standing pellet. On the TA Q400 instrument with probe tip fixture, the force was ramped from 0.05 N was ramped to 0.5 N, while at the same time the dimensions of the coating pellet sample were measured. The dimension and force were then calculated to yield stress and strain according to the formula: 
         [0000]    
       
         
           
             
               σ 
               = 
               
                 F 
                 A 
               
             
             , 
           
         
       
     
         [0000]    where σ is stress, F is the force applied from the probe, and A is the area of the probe in contact with the sample surface. 
         [0000]    
       
         
           
             
               ɛ 
               = 
               
                 
                   l 
                   - 
                   
                     l 
                     0 
                   
                 
                 
                   l 
                   0 
                 
               
             
             , 
           
         
       
     
         [0000]    where ε is strain, calculated from measured real time thickness of specimen l, and original thickness of specimen l 0  before force was applied. When strain versus stress is plotted, the slope of the strain stress curve gives the compressive modulus of the test specimen.