Print Permanence
- An Epson White Paper -

4. Dye Inks and Pigment Inks

Inkjet manufacturers either use dye-based inks that are similar in some respects to those used in traditional photographic prints and/or they use pigment-based inks similar to the colorants used in automotive paints. In the past the majority of inkjet printers utilized dye-based inks, which at the time had a significantly wider color gamut than pigment-based inks. This wider color range produced excellent quality prints with more vivid colors. At the same time, pigment-based inks which have excellent longevity characteristics were primarily used in conjunction with one or two specialty papers or media for specific non-photo applications, such as outdoor signage, because of the limited color gamut.

In the past several years, Epson developed a unique micro-encapsulated technology for pigment-based inks in which each pigment particle is coated in resin, yielding an increased color gamut while retaining the excellent longevity characteristics of pigment inks. This technology has resulted in an inkset called UltraChrome™, which has become the de-facto standard for the most discerning professional photographers who seek the highest image quality combined with the greatest print permanence. Epson also markets a similar micro-encapsulated pigment inkset for consumers called DuraBrite^®. In general, pigment-based inks have greater lightfastness, greater resistance to ozone, and greater resistance to humidity and water than dye-based inks. In addition, UltraChrome inks deliver outstanding image quality and print permanence on a wide variety of papers, allowing end-users more flexibility and greater creative expression. Other manufacturers may promote a single ink and media combination that achieves a high lightfastness rating; but this high rating is often achieved with only one or two specific papers, and the prints, especially those made with a swellable paper, may be susceptible to damage with normal handling.

4-2. Papers

Inkjet prints can be made on a wide variety of coated papers. High quality photographic prints are often made with coated papers in the following categories:

• Swellable

• Porous

• Cotton Rag

>Swellable Paper
As its name implies, the surface of this type of coated inkjet paper swells in the presence of the moisture in the ink, allowing colorants to penetrate the top layers. Swellable papers typically have three layers: a protective top layer; a layer that fixes the ink droplets in place; and below that, a layer that absorbs additional ink components. The paper base is sandwiched between two polyethylene layers (similar to traditional silver-halide RC photographic papers) and backed by an anti-curl coating and an anti-static layer.

The polymer coating on swellable papers not only acts to stabilize the inks by keeping the dyes from spreading, it also helps protect the image from fading due to light and atmospheric pollutants. However images printed on swellable paper often require extended drying times and are susceptible to image degradation caused by exposure to water, moisture via fingerprints and residue from a variety of vapor sources such as sneezes, cooking fumes and aerosol sprays. Swellable papers are generally suitable only for dye-based inks. Contrary to one-size-fits-all marketing claims made by some manufacturers, the use of a swellable paper produces poor image quality with pigment-based ink because the pigments are not totally absorbed into the swellable ink receptive layer. This type of ink and media mismatch produces prints that not only suffer in image quality, but that rarely dry properly. For example, Eastman Kodak promotes its swellable Ultima paper for use with both pigment-based or dye-based inks [5], but Epson's internal tests and reports from customers show that this relatively expensive paper marketed as a product to improve quality actually yields poor image quality when combined with Epson's UltraChrome or DuraBrite pigment inks.

>Porous Paper
The surface of a porous paper is coated with microscopic inert particles that create cavities in which ink is deposited. These cavities prevent the ink from spreading. Porous paper has a higher resistance to moisture and humidity and is often referred to as "instant dry paper."

As a result, these papers require minimal drying time, so the print can usually be handled immediately with less concern about smudging. However, because this type of print has no protective polymer layer, the colorants are susceptible to attack by atmospheric pollutants such as ozone and oxides of sulfur and nitrogen. Porous paper is the preferred paper to use with pigment-based inks, which are less affected by atmospheric contaminants than dye-based inks. In addition, pigment-based inks generally have much better lightfastness characteristics and ozone resistance on porous papers than dye-based inks.

>Cotton Rag Paper
100% cotton fiber matte coated papers are most suitable for fine art printing. Cotton papers are generally acid free and lignin free. Some manufactures add alkaline buffers to cotton papers for increased protection from atmospheric contaminants. When used in conjunction with pigment-based inks, these papers provide excellent image quality and the longest overall print life on the market.