Barrier Coatings for Paper: Moisture – OTR – Oil – Grease Resistant
Moisture and Water Resistance – #8244E
There are several applications that require the functional coating to perform as a moisture barrier to seal in freshness and keep out moisture or humidity.
The MVTR measures the effectiveness of the moisture barrier to prevent water vapor from being transmitted from one side of the substrate to the other.
The value calculated is in either grams/100 in2/day or the metric equivalent in grams/m2/day.
The TAPPI 464 test method to calculate typical MVTR values. TAPPI 464 addresses high heat (100° F) and high humidity {90% RH) conditions (“jungle” conditions)
The lower the MVTR, the better the moisture barrier properties.
Comparing MVTR before and after the Functional Coating is applied:
Water resistance is measured using the Cobb test. This is a test method to measure how much water is absorbed into the paper or paperboard substrate. The unit of measure is grams per square meter measured over 10 minutes.
Excellent moisture and water resistance are very dependent upon a continuous and pinhole-free coating on a variety of substrates.
There are a lot of variables to be considered to achieve the optimum MVTR and Cobb values for a specific application. Some of the critical variables are noted below:
- Porosity, smoothness and density of the base stock
- Total coating weight
- Number of coats or “bumps”
Moisture and Water Resistance
MVTR and Cobb values are quite dependent on how much functional coating is applied.
Figure 1 illustrates a single coat, or bump, of a high performance moisture barrier compounded using 8244E on a highly porous kraft material.
The MVTR value decreases as coat weight increases. Excellent Cobb values (<1) can be achieved at lower coat weights. In this example, to achieve excellent moisture vapor barrier, 7 or 8 dry pounds would be recommended.
If water resistance were the key requirement, 4 to 6 dry pounds would only be required.
Multiple coats of 8244E can improve the moisture and water resistance without increasing the total coat weight. Figure 2 illustrates how one 3.3 dry lbs/3000 ft2coating of compound on kraft linerboard results in a MVTR of almost 3.
By first applying a primer coat, then an overcoat, maintaining the same 3.3 lb total coat weight, the MVTR improves dramatically to less than 1. Again, Cobb values are not as sensitive, in this case, to one coat versus two.
Different applications will require certain base stocks, which have varying smoothness, porosity and density characteristics. These will have an affect on how the barrier coating performs as a moisture barrier.
Figure 3 illustrates MVTR and Cobb values across different base stocks when applying the same coat weight of a moisture vapor resistant compound based on 8244E.
The smoother, less porous substrates, such as machine-glazed kraft or free sheet, yield the lower (better) MVTR barrier property than SBS or Kraft linerboard. Cobb values are not as sensitive to base stock.
Figure 3 illustrates excellent Cobb values (<1) across all substrates listed.
Grease and Oil Resistance
Grease and oil resistant barriers for packaging applications share many of the same requirements as moisture and water resistant barriers.
Substrate, coating weight and number of coats play important roles to obtain desired results. It is critical to have a continuous, pinhole-free coating to prevent wicking or staining of the substrate.
There are applications when grease resistance is critical, but water vapor must be allowed to escape to prevent the food product from getting soggy. For other applications, it is also critical that good moisture resistance is built into the functional coating.
Test methods for oil, grease and solvent resistance include the 3M kit test and the TAPPI Turpentine Test. Both tests are good for checking for pinhole-free coatings, wicking, staining and strike-through.
Here the 3M kit values are plotted at different coat weights using a double coat of 8769, neat, on xerobond standard copy paper. Kit values greater than 7 represent very good, pinhole-free barrier film properties for grease resistance.
In this example, kit values, at less than 2 dry lbs/3000 ft2 of coating using a double coat, are lower suggesting the barrier film is not pinhole-free. At greater than 2 dry lbs/3000 ft2using a double coat, the kit values are very good and become excellent at coat weights of 3.4 dry lbs/3000 ft2and greater.
When only one coating of 8769 is applied, kit values are lower at the same respective total coat weights.
By using two coats instead of one, an excellent grease barrier is developed at lower total coat weights.
8769 and 8768 are excellent grease and oil resistant barrier products. They work on all types of base stocks depending upon how much is applied.
Figure 5 illustrates using 8769 as the latex in a grease resistant barrier coating on two types of substrates: kraft linerboard and xerobond standard copy paper.
Two coats of the neat latex were applied on both substrates. Since the xerobond standard copy paper is smoother and less porous, less coat weight is needed to obtain a high kit value.
Technical Overview
8244E is a barrier coating and adhesive designed for systems requiring an excellent balance of water and oil resistance.
It is the workhorse product in the barrier coatings marketplace due not only to its excellent barrier properties but also to its ability to give excellent coating rheology and coater runnability in highly pigmented barrier coatings.
9067, a firmer version of 8244E, provides better water resistance but less oil resistance than 8244E. 9067 also improves the blocking resistance and repulpability of coatings.
8861 is primarily for barrier coating requiring good water resistance. For many applications, it proves to be the most cost effective solution, particularly for those applications not requiring high levels of oil resistance. It also has broad global food compliance.
8769 is an all acrylic resin supported emulsion (RSE) developed for excellent oil and grease resistance for a variety of applications. It also has good water resistance leading to excellent Cobb values.
8768 is an RSE styrene acrylic that has good oil and grease resistance and water resistance leading to a balance of good MVTR and Cobb resistance. Due to the unique chemistry of both they also are excellent film formers with good block resistant properties and high gloss.
Product polymer Tg (QC) properties:
8244E Styrene- butadiene +12 Excellent pigment compatibility, coater runnabilty. Good balance of water, vapor, and oil and grease resistance. A standard in the barrier coatings industry.
9067 +18 Excellent pigment compatibility, coater runnability. Excellent water and water vapor resistance. Low blocking. Excellent repulpability.
8861 Styrene-butadiene +14 Good water and water vapor resistance with fair oil and grease resistance.
8768 RSE Styrene-acrylic +20 Good oil and grease resistance balanced with good water and water vapor resistance.
These latexes are designed for extremely fast formation of a continuous film at low coat weight. In addition, these products are compounding compatible and coater runnable.
The result is a high strength continuous film that is extemely resistant to the penetration of water and grease.
The below charts show comparative performance properties of the neat latex coated onto 42- lb Kraft liner board, double bump, with an average coat weight of 21-24 gsm.
