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Formulating Water Repellent Varnishes

Introduction

The subject of water repellency often comes up in printing ink circles because there are a multitude of different needs and applications for those coatings that can impart varying degrees of water resistance to a substrate. Such uses include kraft paper bags, labels for beverages and other packaging containing detergent or other moisture sensitive food or chemicals. As we know, moisture can affect the shelf life of the contents of packaging, the freshness of the contents and the integrity of laminated packaging. In this article I'm going to focus on two main areas in the development of water based, water repellent varnishes: those coatings needed for water sensitive paper packaging for things like food and animal feed, and secondly those coatings needed for what are considered MVTR (or WVTR - Water vapor transmission rate) applications, which include items like detergent packaging.

MVTR is a measure of how fast water vapor will pass through a material under specified conditions. Specifically it is the measure of a specific volume of water vapor that will pass through a unit thickness of material per unit area per unit time per unit barometric pressure. Water repellency is the requirement of a coating to shed or repel any water that comes into contact with it.

Water Repellency and MVTR

A water repellent varnish is not necessarily a good MVTR varnish, but both share a similar requirement and that is to prevent moisture from penetrating the substrate, which in most cases for this discussion are porous papers and coated boards. Here we'll talk about water repellency and MVTR interchangeably, agreeing that MVTR coatings generally have more stringent and measurable barrier requirements. We should also keep in mind that coatings may be required to prevent moisture from penetrating the substrate from both sides.

Factors Affecting Moisture Barrier

The following list shows those factors that affect moisture transmission:

• Substrate type - porous substrates will absorb more moisture than non porous.
• Substrate thickness - thicker substrates will prevent more moisture transmission than thinner gauges.
• Coating formulation - the raw materials used in the coating will impact moisture transmission. Generally, moisture permeability increases as solubility of the coating increases.
• Coating application method - how the coating is applied (including how thick it is applied) will affect moisture transmission.
• Environmental conditions - It is generally agreed that as temperature increases, moisture permeability increases, and as humidity increases, moisture permeability increases.

Key Raw Materials

Probably the most widely used products for MVTR and other barrier coatings formulations are Vinylidene Chloride copolymers. Despite concerns over chlorine and its impact on the environment, these have been in use for well over 50 years (having being developed by Dow in the 1950s and introduced into the market in the 1960s) and still do an excellent job today. Some examples of Vinylidene chloride copolymer emulsions and their other derivatives include the Polidene (Scott Bader), Vycar and Permax (Lubrizol) and Solvin (Solvay Plastics) to name a few. These exhibit excellent MVTR properties and the only modifications you need to make to these are to add some viscosity adjusters (thickeners), some defoamer and some wax additive.

Outside of the PVDC arena are the styrenated acrylic emulsions (self-crosslinking and non crosslinking) which are seeing greater use today because of their ease of use and relatively low cost. In an effort to provide chlorine-free formulations, these will form the basis for the examples that follow.

Another key raw material is the choice of wax to use. In most cases the polyethylene, PTFE and polypropylene waxes are used to impart abrasion resistance, but they don't do a very good job of repelling water. The solution therefore is to incorporate a small amount of paraffin wax emulsion into the formula, which assists in making the surface of the overprint varnish or coating more hydrophobic, giving you the water-beading effect.

Formulations

What follows are some examples of water repellant varnishes, which can be used as starting points for your own developments. You may need to modify these further to build in job-specific characteristics for your needs. Something that you will notice is the high percentage of the emulsion. This is the best way to achieve the highest gloss and the best water barrier. As a cost saving exercise you could choose to reduce the emulsion and substitute a thickener and extra water, but you will compromise quality in doing so.

Illustrations

Fig 1: Left: Kraft paper without water repellent varnish; Right: Kraft paper with water repellent varnish (Formula example 1)

In the two illustrations above you can see the effect water has on a kraft paper surface. In figure 1 (left), the water penetrates the surface of the uncoated paper quite readily. In the figure on the right, the presence of a hydrophobic coating renders the paper virtually impermeable to water under normal conditions.

Application

The formulations mentioned above can be applied via gravure press or by a rod coater. The higher the application film weight - the better the water resistance will be. Some of my clients have perfected a flexo process where they apply the coating in two units using very high anilox volumes (20 bcm). If you are going to try these coatings on a flexo machine, a two-roll doctoring system is preferred over a bladed system simply because you can lay down a thicker coating weight. On substrates like kraft paper, it's important to be able to seal the substrate as best possible and this quite often means applying a thick enough film to cover all of the voids in the substrate surface. This is where applying two coats can have an advantage. The first coat seals most of the surface and then a second coat takes care of any remaining surface imperfections.

Notes

Here are a few notes about the raw materials used in the formulations.

Thyon SF10 - This is a self crosslinking emulsion that has very good water resistance.
Texicryl 13-818 is modified acrylic copolymer, also with very good water resistance.
Neocryl A2092 is an acrylic/styrene copolymer with good water resistance. I find that the self-crosslinking technology is a little better in terms of versatility in these formulations.

Aquacer 539 and Michem Emulsion 62330 - These are paraffin wax emulsion blends which improve the MVTR properties of the coatings. I find that both perform equally well, with not a lot of difference between the two in the context of these formulations.

Defoamer Dehydran 1293G - this is a solvent-free, modified polysiloxane defoamer that works well in coatings. It must be well dispersed in the formula to prevent fish eyes or cratering. It is possible to replace this defoamer with others of your choice without affecting water resistance. Another good choice is Thyon AF207 (from Diransa), which is a mineral oil based defoamer.

Iso-propyl alcohol is added to help improve the print quality, film formation and drying speed. All of the emulsions mentioned will easily form films at room temperature without extra coalescing agents, but in the presence of hydrophobic waxes a little alcohol helps considerably.

Conclusions

I hope that through this brief article I've given you some useful ideas for the formulating of water repellent varnishes. In summary, the vinylidene chloride copolymers have a long history of effective applications in MVTR coatings, but if you wish to pursue chlorine-free options, the self-crosslinking emulsions in combination with a paraffin wax emulsions offer an effective alternative.

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