By: Erik Vlietinck
Mon, May 03, 2004 - 12:13:00

With hundreds of inkjet patents worldwide, HP has developed a broad range of products related to photo printing since the introduction of its first photo inkjet printer. Many of the advancements in HP photo printing can be directly traced to innovations in the ink itself—the center of any inkjet printing system. 

Ink formulation—something HP knows for more than 20 years—is a complex, sophisticated science. HP photo ink formulations include carefully developed and often unique ingredients. Among these are colorants, which provide the colour essential to HP inks and are a primary factor in fade resistance.

The non-colorant portion or ‘vehicle’ of the ink starts with water—chosen as the base for its environmental and performance benefits. Humectants are added to the aqueous base to dissolve the colorants and to prevent the nozzles in inkjet print cartridges from drying out. Surfactants are then chosen to control dry time and prevent printed areas from ‘bleeding’ into adjacent areas. Stabilizers are added to keep the ink in the cartridge fresh and ready for use. Each ingredient must be carefully selected to achieve the optimal balance. 

Types of Ink

There are two main types of colorants: dyes and pigments. A pigment is a solid material comprised of many colorant molecules tightly bound together in a single particle. In most liquids, pigments remain in a solid state, dispersed throughout the liquid. Special additives are used to prevent the particles from clumping together and sinking to the bottom of the liquid.

Dyes are comprised of extremely small, individual colorant molecules derived from either natural or synthetic processes. In the appropriate liquid, the colorant completely dissolves and becomes part of the liquid.

Pigment particles form a thick film on the surface of the paper coating, creating irregular surfaces that cause gloss non-uniformities. Too large to penetrate the coating, the pigment particles adhere to it, much like paint adheres to a wall. As with paint, special binding agents are used to make sure the particles “stick” to the paper. 

The dye molecules, by virtue of their small size, have penetrated below the paper coating. The coating on the paper is essentially transparent, so we ‘see’ the dye molecules even though they have penetrated below the surface.

Within current ink technologies available today, the best choice for photo printing is dye-based ink. There are several reasons for this.
Unlike pigmented inks, which remain on top of the paper, often forming small bumps on the surface, dye-based inks penetrate below the surface to provide rich colour depth, minimal dot visibility, and uniform gloss—all important attributes of image quality. 

Glossiness and Distinction

Dye-based prints can exhibit outstanding DOI (Distinctness of Image), a measure commonly used in the auto industry to reflect the glossiness of the painted surface or finish. DOI, an important component of perceived gloss, is what enables dye-based ink to produce the glossy three-dimensional “wet” look typical of high-quality photo prints.

Prints made with pigmented inks often display poor DOI and undesirable variations in gloss that result from the different amounts of ink used to produce light and dark colours.

In addition to image quality—the most important attribute for photo printing—image permanence and durability (scratch resistance and water resistance) are also critical attributes. Since dye-based inks penetrate the paper’s surface, dyes can deliver good scratch resistance and good water resistance on the appropriate media—the only limitation is the paper itself.

The dye-based inks used in the HP No. 85 ink cartridges are specifically formulated to resist fading. Advancements come from over 4 years of molecular engineering light fade resistance to dyes.  These dyes are patented by HP. 

Preliminary results from tests conducted by Wilhelm Imaging Research indicate that prints made with HP No. 85 ink cartridges and HP Premium Plus Photo and Proofing Gloss paper may be displayed indoors under glass for approximately 70 years before noticeable fading and staining will occur.

The dye-based inks used in the HP No. 85 ink cartridges offer comparable fade resistance to the newest generation of pigment printers.

Given current technological advancements and colorant performance characteristics, HP chooses to use dye-based inks for photo printing applications. HP is constantly identifying and co-developing dye solutions. 

Will dye-based inks always be HP’s colorant of choice for photo inks? HP is continually developing new versions of all colorant technologies (dye and pigment) to expand customers’ options. Future developments in HP photo printing research will likely offer additional choices to further enhance the printing experience.

Having selected dye as the best colorant for photo inks, HP turns its attention to the specific dye chemistry. There are numerous types of dyes—each with unique colour-producing structures. Among the key properties HP considers for each dye are colour, permanence, and suitability for ink designs that ensure reliable cartridge operation.