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FLEXO Magazine : End of Year 2008
Sustainable FLEXO YEAR END 2008 www.flexomag.com A BETTER OPTION? So what in fact are the benefits of using soy in a flexographic ink? Let's begin with the easiest one. The soybean can, without a doubt, be considered an abundant and renewable resource, and therefore so can its derivatives. Soybeans are widely available at a low cost, with almost half of the soybeans produced in America needing no irrigation. Soy protein polymers used in the manu- facturing of water-based soy inks are indeed renewable and re- cyclable, which are strong attributes, and should be incorporated wherever possible. The key words in the last sentence are wherever possible. We will come back to this later. Let's go on to other benefits soy can provide to a water-based ink system. Soy protein offers excellent heat resistance, making its use ideal for preprint and multi-wall applications. A preprinted substrate must withstand the heat and pressure of the corruga- tion process, in which paper that has been printed on one side is glued to a series of flutes, and then glued to a second paper liner via hot plate. In addition to offering superb heat resistance, soy protein can help to provide excellent resolubility and re-wet characteristics to an ink. These traits can be essential to maintain- ing clean printing plates for long periods, especially when run- ning fine-line process jobs. They also help to promote easy plate cleanup, reducing related downtime. Most standard heat-resistant acrylic inks will tend to fall quite short in this area. Lastly, inks made with soy are better for the paper recycling process than standard inks. During the de-inking process, soy inks can be removed more easily, resulting in less damage to the paper fibers and making recycling less energy intensive and more cost effective. It is worth noting, though, that this attribute applies more to the newspaper industry, offset printing, than to flexo- graphic printing inks. Now we must look at the possible concerns when incorporat- ing soy into water-based inks. There are two shortfalls in par- ticular that, above all others, currently restrict the amount of soy protein that can be put into a water-based flexographic soy ink: The first is soy's inability to transfer and develop color as well as the acrylic resins in a typical flexographic ink. Many of you may have heard the opposite; there is some truth to that, but you have to go back to soy oil inks versus petroleum-based inks. When lithographic soy oil inks were introduced to compete with petroleum-based inks, they did have better transfer and color development. But, when incorporating soy protein into an acrylic water-based flexographic ink, the game changes dramatically. Acrylic resins have substantially better transfer and color develop- ment properties than soy protein polymers. An ink that replaces too much acrylic resin with soy protein vehicle could pose a problem when running inks on finer line anilox rolls that might already make the achievement of required densities a struggle. The second shortcoming of a soy protein vehicle is that it does not exhibit the same water resistance properties as a pure acrylic resin system. Because of this, we are limited on the amount of soy protein used for applications where moisture will come in contact with the printed surface. Of course a lack of water resis- tance can generally be surmounted by overprinting the inks with a water-resistant varnish, either UV or acrylic, but that could ulti- mately increase the job's cost. A few other concerns are not as restricting, but nonetheless limit the amount of soy protein used in inks for certain applica- tions. For example, color development can be an issue. Because of differences in light refraction, an ink made with large amounts of soy protein can tend to make a pigment appear slightly "dirtier" than it is, meaning it might look like someone added a touch of black to it. This could pose a challenge when trying to achieve very clean colors such as rhodamine. Another concern is that in large quantities, soy protein could cause an ink to dry slower than standard water-based acrylic inks. Finally, soy protein is flat and will exhibit less gloss than most acrylic polymers (see Table 1). Depending on which acrylic polymer it is being blended with, the difference can be quite significant. Like the previously stated is- sue with water resistance, this can be alleviated by use of an over- print varnish, either UV or acrylic. The final concern worth mentioning is that bacteria have an affinity for soy protein. A water-based soy polymer is far more susceptible to bacteria growth than an acrylic polymer, leading to the possibility of very unpleasant odors. This can be overcome by the incorporation of proper biocides in the finished product, and by exercising good hygiene practices at the manufacturing and printing sites. If what you have read so far makes you feel less compelled to use water-based soy inks, I urge you to hold judgment a little longer. First, when a soy vehicle is used in appropriate percent- ages, the negatives mentioned above such as transfer and color development are virtually negligible (see photos). Secondly, even though the performance shortcomings of straight acrylic inks compared to water-based soy inks are few, one negative stands out more than any other: Acrylic resins/polymers are bi-products of crude oil and natural gas. Acrylic monomers such as acrylic acid, methacrylic acid, styrene, and various acrylates that compose acrylic polymers are direct derivates of oil and gas. This combina- tion of ingredients grants acrylic inks great flexibility in terms of performance, but unfortunately it also gives them a big black-eye. For all of their grandness on press, they cannot lay claim to be- ing sustainable or renewable. For this reason, any amount of soy protein included in an acrylic ink will improve its percentage of sustainable materials. We also must remember that we are in the early stages of a de- veloping technology curve regarding the implementation of soy in water flexo inks. In some ways this can be compared to other major changes in flexographic technology that have taken place due to environmental concerns. For example, water-based flexo inks were initially developed as an environmentally friendly alter- native to solvent-based inks. Likewise, the need to reduce heavy metals in inks was the impetus for the development of a number of newer, more environmentally friendly pigments.