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Coating Basestock

Coating basestock is the medium upon which the ink jet receptive coating is applied. In the context of ink jet media it can be paper, film, vinyl, cloth, tyvek, or a non-woven composite material. The application method can be done as part of an "in-line" process such as on a paper machine, or on an "off-line" coating machine. The majority of ink jet receptive coatings are aqueous based and therefore any material which can "holdout" or support most of the coating on the substrate surface during and after the drying process will serve as an adequate basestock.

Of course, just because a material is able to holdout coating, it doesn't mean that the material will make a good ink jet coating basestock. There are several attributes of coating basestocks which are important to good ink jet coating application and printability. These attributes include surface energy, surface smoothness, and water absorptivity. The importance of the relationship between the quality of the ink jet print and the basestock properties cannot be overemphasized. The ink jet receptive coating cannot do all of the work, a premium quality basestock is the other half of a premium quality coated ink jet product.

Surface Energy

Surface energy is a measure of the ability of the basestock surface to permit the coating to wet the surface and flow uniformly over it. The surface energy of the basestock must be greater than the surface tension of the coating to allow this to occur. This principle is analogous to water beading up on the hood a freshly waxed car. The surface energy of the waxed hood is lower than the surface tension of the water and so the water forms beads which won’t lay flat and "wet out". If the surface energy of your basestock is similar to the surface energy of the waxed hood, the aqueous ink jet coating is likely to form beads and puddles instead of a uniform coating. This is the reason why many films and vinyls come with a hydrophilic (water loving) pre-coat.

Surface Smoothness

Surface smoothness is an important attribute of most ink jet coating basestocks. The impact of surface smoothness is most easily understood from the standpoint of printed image clarity. The smoother the printing surface is, the more detailed and clear your ink jet printed images will be. Ink jet printing is not like fine art in the sense where the texture of the painting can be as important and distinctive as the image being portrayed.

Ink jet media must be as smooth and uniform as possible to maximize the clarity and detail of the printed image. Another benefit provided by a smooth basestock is the increased efficiency of the coating process. A very rough, uneven surface will require a a heavier coating to "fill in the valleys" and "cover the mountain tops". However, other properties, such as strength or water resistance may be more important than smoothness and will be a determining factor of the basestock. But, in the end, the smoother the surface, the better the ink jet media.

Water Absorptivity

Water absorptivity is an important fundamental property of ink jet receptive coatings on all basestocks. In the case of vinyl, film, and tyvek, the water and other polar solvents contained in the ink vehicle are absorbed entirely by the coating itself. With paper, cloth, and cellulose containing non-woven composites, the basestock can and does serve a functional role by absorbing the ink vehicle. This absorption is usually a controlled process and is designed into the material. Of the three cellulose containing basestocks mentioned, paper has received the most attention with respect to the design of water (ink vehicle) absorptivity.

Two useful measurements of water absorptivity in paper are the Hercules Sizing Test and the Cobb Sizing Test. Of the two, the Hercules Sizing Test (HST) is usually most often specified. Complete details of the Hercules Sizing Test can be reviewed by contacting the Technical Association of the Pulp and Paper Industry (TAPPI), Technology Park/Atlanta, P.O. Box 105113, Atlanta, Georgia 30348 and asking for test method T-530 pm-89.

In the case of paper basestocks, Hercules Sizing values in the range of 30 - 70 seconds seem to provide the optimum amount of basestock sizing for good ink jet printability with most pigmented coating formulations. The required level of internal sizing of the basestock will vary with the components in your coating and the amount of coat weight being applied, so preliminary laboratory evaluation is always a good idea. The Hercules Size value is also important in the case of cloth and cellulose containing composite basestocks, although it may be difficult or impossible to measure. Regardless of the method used to measure water absorptivity or rather the resistance to it, it is important that your basestock allow for a controlled rate of ink vehicle (mainly water and other polar solvents) absorbtion.

 

Wet Strength

Over the past year many manufacturers have introduced water resistant ink jet coatings designed for short (15 - 30 days) to long (30 days plus) term outdoor applications. Inherent in the paper media designed for these applications is a moderate to high level of wet strength. Wet strength is usually measured as the percentage of a given strength property, i.e. tensile strength, that is retained when the media is wet relative to the dry tensile strength of the media. The Technical Association of the Pulp and Paper Industry has defined such a test which measures the wet and dry tensile strength of paper. There are two levels of wet strength generally defined by the industry, full (>20% wet strength) and partial (10 - 15% wet strength).

In the vast majority of outdoor applications only a full wet strength paper is suitable. In this case, a level of 25 - 35% wet strength is usually required for true outdoor durability of a paper media. However, when specifying wet strength paper media it is advisable to determine if it is an acid or alkaline based paper. This is important because alkaline wet strength papers, because of the chemistry used to impart wet strength, usually do not contain as high a level of wet strength as acid based papers. The higher the level of wet strength inherent in the paper, the more durable and longer lasting the media will be once exposed to the elements.

In addition, alkaline papers, again due to the chemistry involved,  usually do not have as high a level of internal sizing (water penetration resistance) as acid based papers. Internal sizing of wet strength paper media is extremely important for long term durability, and in fact, this type of media fails due to insufficient internal sizing just as frequently as due to insufficient wet strength. The billboard industry, a large consumer of wet strength paper media, can attest to the requirements for water resistance and wet strength in outdoor applications.