Abstract:
The opacity and brightness of a sheet of paper coated with a composition comprised essentially of a pigment and a latex of a film-forming polymer are improved when the paper after being coated is dried under conditions adapted to prevent coalescence of the polymer particles of the latex during the drying step and then subjected to a treatment adapted to cause coalescence of the polymer particles, without subjecting the coating to compressive forces during this treatment step.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the coating of paper with latex-based coating compositions. More particularly, it relates to a process for obtaining a brighter and more opaque coated paper and to paper coated in this way. 
     In order to provide good printing surfaces, it is normal to coat paper with aqueous-based compositions which have been formulated for this purpose. Among the compositions which have been used are coatings comprised essentially of a major proportion of a mineral or organic pigment and a minor proportion of a binder in the form of a latex of a film-forming polymer. Suitable pigments have included finely divided clay, calcium sulfoaluminate also known as satin white, oxides of titanium, aluminum, silicon and zinc, calcium carbonate and microsized particles of high softening point polymers which are insoluble in the binder. Suitable binder polymers have been those which are film-forming at ambient and higher temperatures. The coating is spread over the paper surface by a roll coater, trailing blade, air knife, brush or other known means, after which it is dried. 
     The method of drying the coated paper has generally involved heating it to a sufficiently high temperature to evaporate the water and cause coalescence of the polymeric binder particles. The particles of the binder polymer will coalesce when they are dried above the minimum film-forming temperature (MFT) of the polymer. Heating can be carried out by passing the coated paper through a hot air circulating oven or by contacting it with the surfaces of heated rolls or both. It is also known to dry the coating at a temperature below the minimum film-forming temperature of the binder particles to avoid coalescence of these particles and then subjecting the dried coating to a hot calendering treatment to cause coalescence of the particles and produce a glossy surface on the paper. For more details regarding the foregoing procedures see U.S. Pat. Nos. 3,399,080 and 3,873,345 and TAPPI (Technical Association of the Pulp and Paper Industry) Monographs 7, 9, 20, 22, 25, 26, 28 and 37. While coatings of acceptable opacity and brightness can be obtained by these known procedures, it is desirable to obtain coatings in which these and other properties are enhanced. For example, improvement in ink receptivity and gloss are also an ever present goal in the industry. 
     SUMMARY OF THE INVENTION 
     It has now been found that improvement in brightness, opacity and other properties can be obtained at equivalent coating weight in a paper coating containing a latex of a film-forming polymer as the binder and a pigment by a process comprising spreading a thin layer of the coating composition over a web of paper by one of the known means, drying the coating under conditions adapted to prevent coalescence of the polymer particles of the latex during the drying step and then subjecting the dried coating to a treatment designed to cause coalescence of the polymer particles of the latex without subjecting the coating to a compressive force. Other advantages of the process include the obtainment of equivalent optical properties at a reduced coating weight, possibly higher paper stiffness at equivalent coating weight (since the coating is more bulky) and higher uncalendered gloss. Higher uncalendered gloss means less calendering is required when increase in gloss is desired which in turn means less loss in opacity on gloss calendering since loss in opacity increases as the amount or degree of calendering is increased. The final coatings are also characterized by good pick resistance. 
     DETAILED DESCRIPTION 
     Coalescence of the binder polymer particles of the latex can be prevented during the drying process by maintaining the temperature below the minimum film-forming temperature (i.e. MFT) of the binder polymer. After the drying step has been completed, the coalescence of these particles can be caused to take place by heating the coating at a temperature above the MFT of the binder polymer. Coalescence can also be induced by other means such as by treating the dried coating with a solvent for the polymer, such as benzene for styrene-butadiene copolymers, for a time sufficient for coalescence to take place. To obtain the advantages of the present invention the application of compressive forces, for example calendering, must be avoided while carrying out the coalescence step. On coalescing, the polymer particles will not only fuse with each other, they will also bond with the other components in the coating composition and with the paper substrate. 
     The lactices which may be used for preparing the coating compositions are those known to be suitable for this purpose. The polymers may be homopolymers of C 4  -C 10  dienes such as butadiene, 2-methyl butadiene, pentadiene-1,3, 2,3-dimethyl pentadiene-1,3, 2,5-dimethyl hexadiene-1,5, norbornadiene, ethylidene norbornene, dicyclopentadiene and halo-substituted derivatives of these compounds. The polymers also may be copolymers of the C 4  -C 10  dienes with each other or with one or more copolymerizable monomers containing a CH 2  ═C&lt; group. Examples of these monomers are acrylic acid and its esters, nitriles and amides such as methyl acrylate, methyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, methylol acrylamide, acrolein, alpha and beta methyl acroleins, alpha-chloroacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, cinnamic acid, cinnamic aldehyde, vinyl acetate, vinyl chloride, vinylidene chloride, isobutylene, divinyl benzene, and methyl vinyl ketone. The polymers can also be homopolymers or copolymers of other copolymerizable monomers containing the CH 2  ═C&lt; group, e.g. vinyl alcohol, copolymers such as ethylene-vinyl acetate, ethylene-vinyl chloride, vinyl acetate-methyl methacrylate-acrylic acid-styrene, styrene-vinyl pyrrolidone, ethyl acrylate-vinyl pyrrolidone methyl methacrylate-butyl acrylate-acrylic acid, methyl methacrylate-ethyl acrylate-itaconic acid or any of the other polymers proposed as binders for paper coating applications. If desired, rubbery polymer latices may be blended with minor proportions of latices of hard or resinous polymers having a high MFT such as polystyrene, polyacrylonitrile, polymethyl methacrylate, copolymers of the monomers of these resinous polymers such as styrene-acrylonitrile resins and resinous copolymers of these monomers with other copolymerizable monomers such as copolymers of styrene with butadiene in which styrene forms more than 70 weight % of the polymer. Preferred are latices in which the copolymer is composed of about 0-60 weight % of a C 4  -C 6  conjugated diolefin, 99.9-40% of a styrene and 0.1-5% of a polymerizable unsaturated monomer having a functional group in its structure, e.g. a C 3  -C 6  mono- or dicarboxylic acid, the total of the percentages adding up to 100. The total solids content of the latices should be over 20% by weight and normally about 50% or more prior to compounding. 
     In addition to the pigment and latex binder components, the usual and known other additives may be included in the paper coating composition as required. Thus minor amounts of dispersing agents, e.g. sodium hexametaphosphate, other binders, e.g. starches and proteins, viscosity modifiers, e.g. sodium polyacrylate, defoamers, pH modifiers and other film-forming latices, etc. may be included. 
     The following examples are provided to further illustrate the invention. In these examples all parts are by dry weight unless specified otherwise. 
     The light scattering coefficients (LSC) were calculated using the Kubelka-Munk theory, from reflectance measurements performed at a wavelength of 458 nm over a black background and over a background of known reflectance. A description of the method and of the correction for the reflectance of the polyester film is given in J. Borch and P. Lepoutre, TAPPI 61 (2) 45 (1978). The light-scattering coefficients are expressed in units of reciprocal coat weight, as done customarily in the paper trade. The higher the LSC, the higher is the opacity at a given coat weight. 
     Brightness is the reflectance of an infinitely thick coating at a wavelength of 458 nm. It is not measured but calculated from the light-scattering and light-absorption coefficient of the coating--see J. V. Robinson, TAPPI 58 (10) 152 (1975). 
     Opacity is determined by TAPPI Standard Method T425. 
     75° gloss is determined by TAPPI Standard Method T480. 
    
    
     EXAMPLE 1 
     A coating composition composed of 100 parts of mechanically delaminated clay (alphaplate) and 20 parts of a latex of a carboxylated copolymer of 22 parts of butadiene and 76 parts of styrene having an MFT of 42° C. and an average particle size in the range of 150 nm-200 nm was spread by means of a wire wound rod over the surface of paper in an amount of 20 grams per square meter of paper. The coated paper was dried at room temperature, i.e. below the MFT of the polymer and the opacity of the coated paper was determined. Part of the dried paper was heated in an oven for 5 minutes at 100° C., i.e. above the MFT of the polymer to cause the polymer particles to coalesce while another part was passed through a gloss calender at a pressure of 500 pounds per linear inch (90 kN/m) and a temperature of 150° C. to dry the coating and cause coalescence by pressure and heat. The sheets were in contact with the hot roll of the gloss calender for about 5 seconds. After cooling, opacities were determined on the heat-treated coatings. The results are recorded in Table I. 
     
                       TABLE I______________________________________Conditions                  Opacity______________________________________Uncoated paper                  83.0Coated paper     dried below the MFT   92.0     dried below the MFT then gloss     calendered at 150° C. and 90 kN/m                           93.6     dried below MFT and heated 5 min.     in oven at 100° C. without calendering                           95.2______________________________________ 
    
     These results show that a significant improvement in opacity is obtained by avoiding calendering during the heat treatment. 
     EXAMPLE 2 
     A number of coatings composed of 100 parts of mechanically delaminated clay and 20 parts of the latex of Example 1 were spread over polyester films in an amount of 30 grams of coating per square meter of film and dried at room temperature. The dry coatings were then heated in an oven held at 45°, 52° and 90° C. to cause coalescence of the polymer particles. Light scattering coefficients were determined after various heating times. The results are recorded in Table II and show the effect of increasing the time and temperature of the heating step. 
     
                       TABLE II______________________________________Heating Temp.        Heating Time°C.   Minutes       LSC(cm.sup.2 /g)______________________________________Unheated     --            110045           10            120045           20            135045           60            146045           200           150052            2            147052            5            165052           10            165090            2            167090            5            182090           10            1820______________________________________ 
    
     EXAMPLE 3 
     A number of coating compositions were prepared by mixing mechanically delaminated clay with various amounts of the carboxylated polymer latex of Example 1. The coatings were each spread over polyester films in amount of 30 grams per square meter of film and dried. One sample of each coating was dried at room temperature. Another sample of each coating was dried at room temperature and then heated for 10 minutes in an oven at 90° C. while a third sample of each coating was dried by placing it on a hot plate maintained at 90° C. Brightness, LSC and 75° gloss determinations were then made on each coating. The results are recorded in Table III and show the effect of varying the clay/polymer ratio. They also show large improvement in the brightness, 75° gloss and light scattering coefficient obtained by drying at below the MFT of the polymer before subjecting it to a temperature above its MFT without calendering, as compared to the results obtained with the conventional process i.e. by drying the coating at a temperature which is above the MFT of the polymer. 
     
                       TABLE III______________________________________                          Dried at RoomParts Latex    Dried at   Dried at   Temp. ThenPer 100 Parts    Room       90° C. on                          Heated at 90° C.Clay     Temperature               Hot Plate  For 10 Mins.______________________________________  BRIGHTNESS10       0.810      0.817      0.83920       0.826      0.781      0.85730       0.834      0.630      0.86440       0.837                 0.860  75° GLOSS 0       65 5       72         59         6910       73         55         7220       72         34         7130       73         30         7140       74                    65  LSC (cm.sup.2 /g) 0       1000 5       1000       1110       120010       1000       1100       140020       1100        850       190030       1200        150       196040       1200                  1790______________________________________ 
    
     EXAMPLE 4 
     A coating composition was prepared by mixing 20 parts of the latex of Example 1 with 100 parts of the delaminated clay. The composition was spread over a polyester film in amount of 30 grams per square meter of film, dried at room temperature and the light scattering coefficient of the coating was measured at a wavelength of 458 nm. The coating was then exposed to benzene vapours in a closed container for two hours at room temperature. After removal from the container they were conditioned for one week at room temperature and pressure, then the LSC of the coating was again measured. The results are recorded in Table IV and show the large increase in the LSC that is obtained by coalescing the polymer particles without calendering by exposure to a solvent. 
     
                       TABLE IV______________________________________                   LSC                   (cm.sup.2 /g)______________________________________Dried coating - before exposure to solvent                     1100Dried coating - after exposure to solvent                     1700______________________________________ 
    
     EXAMPLE 5 
     Two sets of coating compositions were prepared from two carboxylated polystyrene latices--LYTRON* 2102 and 2203, by adding to samples of a 60% dispersion of delaminated clay in water, 5, 10, 20, 30 and 40 parts of these latices. The average particle sizes of these latices were about 100 nm and 200 nm and each polymer had a glass transition temperature of about 100° C. The coatings were spread over polyester films in amounts of 30 grams per square meter of film and the coated films were dried at room temperature. Light scattering coefficients were then determined on these coatings. 
    
     The coatings were next heated for 5 minutes in an oven held at 150° C. following which the light scattering coefficients of the coatings were again determined. The results are recorded in Table V and show the large increase in opacity that is obtained by coalescing the polymer particles by the process of the present invention. They also illustrate the effect of particle size on opacity enhancement. 
     
                       TABLE V______________________________________  Light Scattering Coefficient - cm.sup.2 /gPolystyrene    Dried at room   Dried at room temp.parts per 100    temp. but       then heated at 150° C.of clay  not heated      for 5 min.______________________________________  Particle Size Particle Size    100 nm    200 nm    100 nm  200 nm 0       1050      1050      1050    1050 5       950       1080      1360    140010       870       1110      1480    160020       550       1170      1470    183030       500       1240      1360    196040       470       1300      1200    1980______________________________________