This disclosure relates to coated paperboard having good smoothness and printability at low coat weights.
Paper and paperboard are used for many printing and packaging applications. Paperboard grades are heavier than paper grades, and are typically characterized as having a caliper (thickness) of at least 10 mils (0.010″; 254 μm) or 12 mils (0.012″; 305 μm); such calipers are also commonly called 10 point (10 pt) or 12 point (12 pt). It is often desirable for paperboard to have a surface well suited for printing, which may be characterized by various properties including smoothness, gloss, ink receptivity, and other measurements.
Commonly-owned U.S. Pat. No. 8,142,887 discloses a paperboard substrate with a basecoat including calcium carbonate and hyperplaty clay, with at most about 60 percent of the calcium carbonate having a particle size smaller than 2 microns, and with the hyperplaty clay having an average aspect ratio of at least about 40:1. The disclosed paperboard has good smoothness. However, to achieve superior print quality (e.g. Parker Print Surf below 2 microns), paperboard having been base coated is often given one or more additional coats. It would be advantageous to achieve superior print quality with only a single coat, preferably using a relatively low coat weight. It would also be advantageous to achieve superior print quality with a base coat that does not require hyperplaty clay in its formulation.
In the present work, certain inventive coatings are able to provide superior smoothness and printability with a single layer of coating applied at remarkably low coat weight compared with the typical total coat weight of double coating. Parker Print Surf smoothness values of 2.5 microns and lower are achieved with a single layer of the inventive coatings having coat weights of 6 lbs/3000 ft2 (9.8 g/m2) and higher. In other embodiments, certain inventive base coats are disclosed which may be used with various top coats to achieve superior smoothness and printability.
Eventually the web is carried by a transfer felt or press felt through one or more pressing devices such as press rolls 130 that help to further dewatering the web, usually with the application of pressure, vacuum, and sometimes heat. After pressing, the still relatively wet web 300 is dried, for example using dryer or drying sections 401, 402 to produce a dry web (“raw stock”) 310 which may then be run through a size press 510 that applies a surface sizing to produce a sized “base stock” 320 which may then be run through additional dryer sections 403 and (on
The base stock 320 may then be run through one or more coaters. For example, coater 530 may apply a first coat (“BC”) to a first side (“C1”) of the web, and the first coat may be dried in one or more dryer sections 404. Coater 540 may apply a second coat (“TC”) to the first side of the web, and the second coat may be dried in one or more dryer sections 405.
If the web is to be coated on two sides, coater 550 may apply a first coat to the second side (“C2”) of the web, and this coat may be dried in one or more dryer sections 406. Coater 560 may apply a second coat to the second side of the web, and this coat may be dried in one or more dryer sections 407. The order of coaters 540, 550 may be swapped, so that both sides C1 and C2 are first given a first coat, and then one side or both sides are given a second coat. In some instances, only one side will be coated as shown in
Instead of applying coating by on-machine coaters as shown in
Various types of coating devices may be used. The coaters illustrated in
The particular materials used in the coatings may be selected according to the desired properties of the finished paperboard. For example, the coating(s) may provide desired printability, as indicated by various measurements including smoothness, gloss, ink hold out, etc.
Following the coaters, there may be additional equipment for further processing such as additional smoothening, for example gloss calendaring. Finally, the web is tightly wound onto a reel 570.
The general process of papermaking and coating is outlined at a high level in the preceding description and with
In contrast, the current invention as described in PART I below is a method for producing a quality printing surface using only a single layer of coating. In another embodiment as described in PART II below, the current invention is a method for producing a quality printing surface using a specialized base coat over which a top coat may be applied.
Two key performance parameters for coated board are smoothness and printability. There are a wide variety of methods for measuring both properties. Here, Parker PrintSurf (PPS) smoothness is used as the smoothness test, with 10 psi (68.9 kPa) pressure and a soft backing. For printability measurements, a Prufbau printability tester was used to apply a uniform layer of cyan ink. 15 μl of Prufbau cyan ink to the inking roller for each sample. The printing pressure was 1100 N, and the speed was 2.5 m/sec. Print gloss was measured using the standard TAPPI gloss method.
Paperboard samples were made using solid bleached sulphate (SBS) substrate with a caliper of 10.5 pt (0.0105″; 267 μm). The samples were coated on one side using a pilot blade coater with either one layer or two layers of coating. The pilot results are expected to be representative of results that might be achieved on a production paper machine or a production off-machine coater.
A series of coating formulations were applied to paperboard using a blade coater. The pigments had a wide range of densities, so the coatings were formulated based on volume percent. The inorganic pigments were:
Low density organic pigments (LDOP) were also used. These were hollow sphere plastic pigments from Dow, but there are other pigments that fall into the category. The LDOP pigments tested here did not include pigments that substantially expand during drying. By non-expanding pigments is meant that the pigments do not expand more than 10% by volume during drying of the coating.
The non-expanding LDOP pigments used were:
The binder used in all coatings was Basanol X497AB, a styrene acrylate latex from BASF. The addition level of this latex binder was the same for all coatings, and was 26.4% based on total dry pigment volume. When calculating the amount of LDOP to be each in a formulation, it was assumed that the empty spaces within the LDOP pigments were filled with air. The experimental design was based on pigment blends and ratios, so in the following tables, only the pigment portion is presented. All pigments total 100% for each formulation.
Coating formulations A-P are shown in Table 1. In addition, a double coated sample was made using approximately 8 lb/3000 ft2 (13.0 g/m2) of coating A as a basecoat and 6 lb/3000 ft2 (9.8 g/m2) of coating B as a topcoat. The coatings were applied onto solid bleached sulfate paperboard which had an initial (uncoated) basis weight of 103 lb/3000 ft2 (167 g/m2) and a PPS value of 7.7μ. Coatings were applied at 800 fpm (4.1 m/sec) using a bent blade configuration. For each coating multiple coat weights were applied to the board. Other than double-coated samples used as references, the samples were single-coated with range of coat weights from approximately 6-9 lb/3000 ft2 (9.8-14.6 g/m2) being run for each sample.
Table 2 shows ink gloss data for calendered single-coated samples with coat weights closest to 7 lb/3000 ft2 (11.4 g/m2). Ink gloss is reported as a percent of the reference standard. Measurements were made using a Glossmeter Model T480A from Technidyne Corporation.
Table 3 shows PPS smoothness for single-coated samples after they were hot soft roll calendered at 300 fpm (1.5 m/sec), 225° F. (107° C.) and 125 pli (21900 N/m). PPS Smoothness was measured using a Technidyne Profile Plus instrument.
In this Example, four coating formulations were selected from the list shown in Table 1. A typical coarse carbonate basecoat (formulation A) and a typical clay/carbonate topcoat (formulation B) were applied (to separate samples) as single coats. An improved basecoat (formulation C) based on hyper platy clay, in accordance with U.S. Pat. No. 8,142,887, was evaluated, and also an improved coating (formulation G) containing hyper platy clay and a LDOP.
This experiment explored the effect (single-coated samples after calendering) on smoothness and ink receptivity of LDOP particle size over a diameter range from 0.4 to 1.5μ (coatings D-H). All coatings were a 50/50 blend of clay and LDOP.
This experiment explored the effect of LDOP addition to coating containing hyperplaty clay. The control formulation had 100% platy clay as the pigment (I), that is, 0% LDOP. The LDOP level was varied between 12% and 57% by volume (Coatings F, I-N).
Experiments were performed to measure the pigment packing behaviour of pigment blends containing LDOP. Because of the density differences between LDOP and inorganic pigments, a method other than sedimentation had to be used. A method was devised using the absorption of mineral oil into layers of pigment blends to measure the void volume within packed pigments. All pigment blends were formulated based on volume. Because the films needed to maintain their integrity when oil was applied, a controlled volume of latex binder was added to each blend. The method was as follows. Pigment blends were applied to Mylar film using a Byrd bar with a 10 mil gap. Each film was air dried, then placed in an oven at 160° F. (71° C.) for 20 minutes. A die cutter was used to cut a 3″×6″ (7.6 cm×15.2 cm) area from both the coated and uncoated portion of the Mylar. These coupons were weighed to determine the weight of coating applied. The coated coupon was then saturated with mineral oil, then the excess was wiped away. The oil-saturated coupon was then weighed to determine the amount of oil picked up. The void volume can be calculated using the formulation, the weights, the densities of the components and the density of the oil. The volume of the binder was added to give the final void volume value. Pigment blends were initially made with 8% binder added. The coatings comprised of LDOP without any other pigment crazed and were not testable. The binder level was raised to 20% for these coatings and the coatings with LDOP of 1.0μ diameter or greater were testable. However, coatings with LDOP less than 1μ in diameter were still not testable. These two formulations were not tested. Table 4 contains the formulations and the results.
Table 5 has data for pigments blends containing both coarse GCC and LDOP with hyperplaty clay.
In summary, the results of PART I show that single-coated paperboard with good smoothness and printability is achieved by a single application of the inventive coating at weights of 6 lb/3000 ft2 (9.8 g/m2) or more, providing Parker PrintSurf values of 2.5μ or less and with printability similar to a conventional double-coated product typically having greater total coat weight.
Paperboard samples were made using solid bleached sulphate (SBS) substrate with a caliper of 10.5 pt (0.0105″; 267 μm). The samples were coated on one side using a pilot blade coater to apply a base coat, followed by a top coat. The pilot results are expected to be representative of results that might be achieved on a production paper machine or a production off-machine coater.
A series of coating formulations were applied to paperboard using a blade coater. The pigments had a wide range of densities, so the coatings were formulated based on volume percent. The inorganic pigments were:
A low density organic pigment (LDOP) was used in the basecoat only. One LDOP was used, which does not expand substantially during drying. There are other LDOP pigments that fall into the “non-expanding” category. By non-expanding pigments is meant that the pigment does not expand more than 10% by volume during drying of the coating.
The non-expanding LDOP pigment used was:
The binders used were:
The binder levels (based on 100 parts of pigment) were about 18-21 parts for the various basecoat formulations, and about 14 parts for the topcoat formulation.
When calculating the amount of LDOP to be each in a formulation, it was assumed that the empty spaces within the LDOP pigments were filled with air. The experimental design was based on pigment blends and ratios, so in the following tables, only the pigment portion is presented. All pigments total 100% for each formulation.
Base coat formulations Q through T are shown in Table 6, which include a “standard” formulation Q (no LDOP), a 25% (volume) LDOP formulation R, a 41% (volume) LDOP formulation S, and a “platy-clay” formulation T (no LDOP).
Table 7 gives the ingredients for a single formulation used as a top coat as will be explained below.
The amount (weight) of LDOP to give a desired volume percent in the base coating is determined as follows. Although the density of calcium carbonate varies slightly due to impurities, a density value of 2.6 g/cc was used here for the Hydrocarb 60. The Ropaque 1353 LDOP, as specified by the manufacturer, has a void volume of 53% giving it an equivalent density of 0.484 g/cc. Assuming we want 25% by volume of LDOP, our calculations will be as follows:
75 cc×2.6 g/cc=195 g Hydrocarb 60 calcium carbonate
25 cc×0.484 g/cc=12.1 g Ropaque 1353 LDOP
207.1 g Total weight
Assuming we want 50% by volume of LDOP, our calculations will be as follows:
50 cc×2.6 g/cc=130 g Hydrocarb 60 calcium carbonate
50 cc×0.484 g/cc=24.2 g Ropaque 1353 LDOP
154.2 g Total weight
On the other hand, the percent volume of LDOP associated with a particular weight of LDOP is determined as follows. Assuming twice as much (by weight) of LDOP would be used as in the first example (i.e., 11.6% by weight instead of 5.8% by weight) yields the following example (now assuming 100 g total=11.6 g LDOP and 88.4 g calcium carbonate):
11.6 g of LDOP divided by its density 0.484 g/cc=24.0 cc volume of LDOP
88.4 g of carbonate divided by its density 2.6 g/cc=34 cc volume of calcium carbonate
24 cc of LDOP divided by (24+34 cc total)=0.414=41.4% LDOP by volume
The base coat formulations were applied onto solid bleached sulfate paperboard which had an initial (uncoated) basis weight of 103 lb/3000 ft2 (167 g/m2). The uncoated paperboard has a PPS smoothness of 7.7μ and a Sheffield smoothness of 200. The base coatings were applied at 1500 fpm (7.6 m/sec) using a bent blade configuration. For each formulation, a single base coat was applied, with the basecoat weight ranging from 5 to 10 lb/3000 ft2 (8.1 to 16.2 g/m2). After drying, the samples in uncalendared condition were tested for Parker PrintSurf (PPS) smoothness and Sheffield smoothness.
PrintSurf results are listed in Table 8 and illustrated in
Particularly with the formulations containing LDOP, the smoothness improved (PPS decreased) as coat weight was increased. The samples with the basecoats containing LDOP were smoother than the standard or the platy-clay base coats. At coat weights above 6 lbs (9.8 g/m2), the LDOP samples were about 1.5μ smoother than the samples with standard basecoats, and about 0.7μ smoother than samples with platy-clay basecoats.
For the same samples, Sheffield smoothness results are listed in Table 8 and illustrated in
The smoothness improved (Sheffield decreased) as coat weight increased. The samples with the basecoats containing LDOP were smoother than the standard or the platy-clay base coats. At coat weights above 6 lbs (9.8 g/m2), the LDOP samples were about 30-35 Sheffield units smoother than the samples with standard basecoats, and about 8-15 Sheffield units smoother than samples with platy-clay basecoats.
Samples having been base-coated with the various formulations of Table 6 were then top coated with the single formulation of Table 7, at 400 fpm (2.0 m/sec) using a bent-blade coater. For each of the four base coat formulations, several base coat weights and several top coat weights were run. For the resulting (uncalendared) top-coated samples, Table 9 shows the Parker PrintSurf (PPS) results. To best compare the samples, the results were regressed to calculated PPS values normalized to 8 lb/3000 ft2 (13.0 g/m2) base coat and 6 lb/3000 ft2 (9.8 g/m2) top coat. The results are given on
In summary, the results of PART II show that a based-coated paperboard with improved smoothness relative to typical basecoats or platy-clay basecoats is achieved by the inventive coating. When top-coated, the improvement in smoothness is maintained. Presumably the improvement in smoothness would be maintained if more than one coating is applied over the base coat (for example, a second coat and a third coat).
Based on the results of PART I and PART II, it appears that coatings with high void volumes give improved smoothness.
Likewise,
The tests described above used a blade coater to apply coating. As previously discussed, various types of coating devices may be used.
Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims.
While preferred embodiments of the invention have been described and illustrated, it should be apparent that many modifications to the embodiments and implementations of the invention can be made without departing from the spirit or scope of the invention. It is to be understood therefore that the invention is not limited to the particular embodiments disclosed (or apparent from the disclosure) herein, but only limited by the claims appended hereto.
This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. provisional applications Ser. No. 62/420,586 filed on Nov. 11, 2016 and Ser. No. 62/450,191 filed on Jan. 25, 2017, both of which are hereby incorporated by reference in their entirety.
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