ColorLok® technology represents a new more advanced standard for better printing to papers without it1. Papers with ColorLok® Technology provide higher inkjet printing and optimal results on all printing devices. Whether at home or in the office, once you've used ColorLok® papers, no ordinary paper will do.

Pigment particles are the colorant used in many inkjet inks for home and office printers. Pigments are suspended in a colorless liquid, called the ink vehicle. Most of these pigments are anionic, that is they are stabilized by negative surface charges. When pigments come into contact with a certain cationic, or positively charged, salt2 they rapidly come out of suspension or flocculate. When the salt is added during paper manufacture, it provides a means to rapidly immobilize pigments at the paper surface. This allows the ink vehicle to penetrate the paper without transporting the pigment particles into the paper. Rapid pigment immobilization also reduces "feathering" from capillary forces that cause ink vehicle to wick along surface fibers.

Figure 1 shows photomicrographs of cross-sections of HP Multipurpose Paper without ColorLok® Technology (left) and with ColorLok® Technology (right).3 Note that without ColorLok® technology, black pigment particles are carried deep into the porous sheet reducing optical density and increasing "strikethrough", where the image printed on the front of the sheet is visible on the back.4 With ColorLok® Technology, pigments remain at and near the paper surface for high optical density and reduced strikethrough.

Figure 1. Cross sections of HP Multipurpose Paper without ColorLok® Technology (left) and with ColorLok® Technology (right).
Image source: HP

Figure 2 shows how ColorLok® Technology reduces feathering to provide higher edge sharpness. HP Multipurpose Paper without ColorLok® Technology is shown on the left; HP Multipurpose Paper with ColorLok® Technology is shown on the right. Flocculating the pigments significantly reduces colorant transport along paper fibers on the surface from printed into unprinted areas. In addition, an increase in black optical density is apparent in the sample with ColorLok® Technology.

Figure 2. Improved Edge Sharpness with ColorLok® Technology (right).
Image source: HP

The pigment flocculation process offers several important benefits for inkjet printing on porous, uncoated papers:

Pigments remain near the paper surface instead of penetrating the sheet, compared to papers without ColorLok® Technology.
>   improving optical density1
>   improving color saturation1
>   increasing color gamut1
>   reducing strikethrough1

Pigments are rapidly immobilized
>   reducing feathering1
>   improving edge sharpness in text and graphics1
>   reducing color-to-color-bleed1
>   reducing ink transfer1
>   reducing smearfast dry time1

ColorLok® standards also include specifications for paper surface electrical resistivity, filler particle size limits, sheet moisture content, and surface smoothness. Control of these properties enhances performance and reliability of home and office laser printers.

1 Independent testing by SpencerLab Digital Color Laboratory, commissioned by HP. The full October 2009 report, "Print Quality Analyses ColorLok Media Evaluation: AiO Printers: Brother MFC-5490CN, Canon PIXMA MX860 & MX7600, Epson WorkForce 600, HP Officejet 6500 and Officejet Pro 8500," is available for download at
2 Different divalent salts have been shown to flocculate pigments.
3 Printed with HP black pigment ink using an HP Photosmart Pro B9180 printer with default plain paper print mode settings.
4 In extreme cases, colorant may be carried completely through the sheet to the back side.