Food or food service packages using paper or paperboard often require enhanced barrier properties, including oil, grease, water, and/or moisture vapor barrier. Additionally, many paper or paperboard packages, for example, paper or paperboard cups for food or drink services, also require the paper or paperboard be heat sealable, making it possible to form cups on a cup machine. Polyethylene (PE) extrusion coated paperboard currently still dominate in such applications by providing both required barrier and heat seal properties. However, packages including paper cups using a PE extrusion coating have difficulties in repulping and are not as easily recyclable as conventional paper or paperboard, causing environmental concerns if these packages go to landfill. There are increasing demands for alternative solutions including coating technologies to replace paperboard packages that contain a PE coating or film layer.
Repulpable aqueous coating is one of the promising solutions to address this need. However, most polymers in aqueous coatings are amorphous and do not have a melting point as PE. Therefore, binders or polymers in aqueous coatings often gradually soften or become sticky at elevated temperature (even at, for example, 120-130° F. (48.9-54.4° C.) and/or pressure in production, storage, shipping, or converting process of aqueous coated paperboard, causing blocking issue of the coated paperboard, which usually does not occur with PE coated paperboard in practical applications. This blocking issue becomes even more critical for aqueous barrier coated paperboard that requires high barrier properties and also needs to be able to heat seal in converting packages such as cups.
The invention is directed to a method of making a paper or paperboard with barrier properties that are provided by an aqueous coating that is also heat sealable. Typical aqueous coatings used for such purposes may contain a high level (or even pure) binder or specialty polymer, that can end up blocking when stored or shipped under elevated temperature, humidity, or pressure. The blocking behavior is an even greater problem with materials that are designed to be heat sealable.
In the inventive paperboard, a heat sealing layer is provided by an aqueous coating whose binder (or polymer) component has a relatively high glass transition temperature (Tg). The inventive board offers heat seal capability and provides barrier properties without the usual blocking problems.
The invention provides a paperboard coated with an aqueous barrier coating, providing barrier properties and being heat sealable, but with minimal tendency to block.
As shown in
A barrier coating 120 may be applied to either side of substrate 100 (in
If the barrier coating is applied as a single coat, a suitable coat weight may be, for example, from 6 to 15 lb/3000 ft2 (9.8-24.5 g/m2), or about 8 to 12 lb/3000 ft2 (13.1-19.6 g/m2).
If the barrier coating is applied as two coats, a suitable coat weight for the base coat may be, for example, from 6-10 lb/3000 ft2 (9.8-16.3 g/m2), or about 7-9 lb/3000 ft2 (11.4-14.6 g/m2). A suitable coat weight for the top coat may be, for example, from 5-8 lb/3000 ft2 (8.2-13.1 g/m2), or about 6-7 lb/3000 ft2 (9.8-11.4 g/m2).
A variety of coatings were applied on a paperboard substrate 100 using a pilot blade coater. The substrate was solid bleached sulfate (SBS), specifically 13 pt (330 μm) cupstock. The coatings used these pigments:
“Clay” kaolin clay, for example, a No. 1 ultrafine clay
“CaCO3” coarse ground calcium carbonate (particle size 60%<2 micron)
The coatings used commercial binders based on styrene-acrylate (SA) but with different glass transition (Tg) temperatures as shown in Table 1.
The coating formulations are listed in Table 2, differing chiefly in the glass transition temperature of the styrene-acrylate (SA) binder. Pigment and binder were equal by weight (100 parts each), with the pigment split equally (50/50 parts each by weight) between clay and CaCO3. Approximately 7.5-8 lb/3000 ft2 (12.2-13.1 g/m2) of the coating was applied by a pilot blade coater. The coated samples were tested for blocking using a method described later herein, and with ratings as listed in TABLE 3.
As shown in Table 2 and in
Based on the promising results as seen in Table 2 with the glass transition temperature of 39° C., additional tests were run using the formulations seen in Table 4 below, in which the amount of SA binder was varied (100 parts, or 125 parts, or 150 parts), and the coatings were applied in either one or two layers. The single or base-coat weight was around 8.5 lb/3000 ft2 (13.9 g/m2), and the top coat (if used) was around 6.3 lb/3000 ft2 (10.3 g/m2). Blocking results again were good (ratings of 1.3 to 1.5).
As shown in TABLE 4, heat seal testing (after sealing with a 400° F. (204° C.) tool) gave 98% to 100% fiber tear. Repulpability ranged from 99.5% for a single-coat using 100 parts of SA binder, down to 92.1% for a double-coat using 150 parts of the SA binder. All conditions gave 2-minute-water-Cobb ratings of less than 5 g/m2.
With a single coat, coatings using 39° C. SA binder gave 3M Kit ratings of 7+(not shown in Table 4), and 30-minute-oil-Cobb ratings of less than 1 g/m2. Water vapor transmission rates (WVTR) of 820-860 g/m2-d were achieved.
With a double coat, 30-minute-water-Cobb ratings were from 52 to 28, with the best (lowest) value for 150 parts SA. Water vapor transmission rates (WVTR) as low as 445-474 g/m2-d were achieved.
For the SA binders with Tg of 39° C., a seal bar temperature of 300° F. (149° C.) gave no fiber tear (0%), while seal bar temperatures of 350 and 400° F. (177 and 204° C.) gave 90% and 100% fiber tear, respectively.
The blocking behaviour of the samples was tested by evaluating the adhesion between the barrier coated side and the other uncoated side. A simplified illustration of the blocking test is shown in
The test device 200 includes a frame 210. An adjustment knob 212 is attached to a screw 214 which is threaded through the frame top 216. The lower end of screw 214 is attached to a plate 218 which bears upon a heavy coil spring 220. The lower end of the spring 220 bears upon a plate 222 whose lower surface 224 has an area of one square inch (6.5 square centimeters). A scale 226 enables the user to read the applied force (which is equal to the pressure applied to the stack of samples through the lower surface 224).
The stack 250 of samples is placed between lower surface 224 and the frame bottom 228. The knob 212 is tightened until the scale 226 reads the desired force of 100 lbf (100 psi applied to the samples). The entire device 200 including samples is then placed in an oven at 50° C. for 24 hours. The device 200 is then removed from the test environment and cooled to room temperature. The pressure is then released, and the samples removed from the device.
The samples were evaluated for tackiness and blocking by separating each pair of paperboard sheets. The results were reported as shown in Table 3, with a “0” rating indicating no tendency to blocking.
Blocking damage is visible as fiber tear, which if present usually occurs with fibers pulling up from the non-barrier surface of samples 254. If the non-barrier surface was coated with a print coating, then blocking might also be evinced by damage to the print coating.
For example, in as symbolically depicted in
The coated paperboard samples were evaluated for heat sealability. As depicted in
Repulpability was tested using an AMC Maelstom repulper. 110 grams of coated paperboard, cut into 1″×1″ (2.5 cm×2.5 cm) squares, was added to the repulper containing 2895 grams of water (pH of 6.5±0.5, 50° C.), soaked for 15 minutes, and then repulped for 30 minutes. 300 mL of the repulped slurry was then screened through a vibrating flat screen (0.006″ (152 um) slot size). Rejects (caught by the screen) and fiber accepts were collected, dried and weighed. The percentage of accepts was calculated based on the weights of accepts and rejects, with 100% being complete repulpability.
Moisture resistance of the coatings was evaluated by WVTR (water vapor transmission rate at 38° C. and 90% relative humidity; TAPPI Standard T464 OM-12) and water Cobb (TAPPI Standard T441 om-04).
The oil and grease resistance (OGR) of the samples was measured on the ‘barrier side’ by the 3M kit test (TAPPI Standard T559 cm-02). With this test, ratings are from 1 (the least resistance to oil and grease) to 12 (excellent resistance to oil and grease penetration).
In addition to 3M kit test, oil absorptiveness (oil Cobb) was used to quantify and compare the OGR performance (oil and grease resistance), which measures the mass of oil absorbed in a specific time, e.g., 30 minutes, by 1 square meter of coated paperboard. For each condition tested, the sample was cut to provide two pieces each 6 inch×6 inch (15.2 cm×15.2 cm) square. Each square sample was weighed just before the test. Then a 4 inch×4 inch (area of 16 square inches or 0.0103 square meters) square of blotting paper saturated with peanut oil was put on the center of the test specimen (barrier side) and pressed gently to make sure the full area of oily blotting paper was contacting the coated surface. After 30-minutes as monitored by a stop watch, the oily blotting paper was gently removed using tweezers, and the excess amount of oil was wiped off from the coated surface using paper wipes (Kimwipes™). Then the test specimen was weighed again. The weight difference in grams before and after testing divided by the test area of 0.0103 square meters gave the oil Cobb value in grams/square meter.
Number | Date | Country | |
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62463857 | Feb 2017 | US |
Number | Date | Country | |
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Parent | 15902166 | Feb 2018 | US |
Child | 17464395 | US |