Precision Colour and Gloss Measurement for Pulp and Paper

Suppose you get a clean ream of printer paper. The top sheet will be crisp white. As you go on, the paper gets a slight yellow tint after 10 sheets. Imagine a high-end perfume case with gold foil lettering, and the white paperboard is a pale grey with store lighting.

The optical behaviour of paper is very complicated, but simple. Raw wood fibers, recycled pulp, bleaching chemicals, water, fillers and coatings are whisked through the paper mills in a matter of blistering speeds. Whiteness, opacity and surface gloss change instantly with a subtle change in pulp bleaching, filler mix or press roll pressure.

On fast paper lines, visual inspection is unable to detect. Bright white sheets are difficult for the human eye to maintain a gaze on. Our vision is deceived by room lighting as well. Objective optical data can ensure consistency from roll to roll, ream to ream and packaging box to packaging box. With this in mind, 3nh precision spectrophotometers and gloss meters come into play.

Pulp Bleaching and Brightness: Quality at the Source

The quality of paper starts at the heart of the pulp mill. In any pulp mill, whether they are working virgin wood fibers or recycled cardboard, it is essential for pulp masters to always check the pulp's brightness.

Pulp brightness is the percentage of blue light reflected from a wet pulp pad or dried handsheet at a particular wavelength (457 nanometers). The ISO Brightness and TAPPI Brightness are standardized by tests such as ISO 2470 and TAPPI T452. Loss of pulp whiteness of 0.5 point will make it impossible for the paper mills to achieve the desired whiteness level without excessive consumption of expensive optical dyes.

Stable reflectance spectrophotometers are used to measure pulp sheets. Fiber coarseness, moisture content, and surface irregularities are averaged in benchtop instruments, which have a large measuring aperture.

The 3nh benchtop spectrophotometers offer high accuracy blue-reflectance measurements directly in the mill lab. The chemists are able to keep an eye on the bleaching efficiency in real time, which helps to reduce the amount of chemical waste and locks the baseline pulp quality in before the paper production begins.

Whiteness, Yellowness, and OBA Control

For modern office paper and marketing materials, it is essential that they can be cleanly white! For this, paper makers use Optical Brightening Agents (OBAs), or Fluorescent Whitening Agents (FWAs).

The principle behind OBA is that it absorbs the invisible ultraviolet (UV) radiation and re-emits the radiation as bright blue light. This optical phenomenon creates the illusion of a paper sheet being whiter and cleaner than it actually is. But there are significant measurement difficulties with OBAs.

Standard light bulbs in older instruments give off too much UV and cause paper readings to move around. To get accurate whiteness measurement, UV filter in the spectrophotometers must be adjustable and automated.

In order to meet global standards such as CIE Whiteness (W10/D65), strict UV calibration is required. When the UV output from the instrument is too high, the instrument will over-report whiteness. When there is no UV energy, the reading will not recognize the brightener.

3nh high precision spectrophotometers come with automated UV control. The modes of UV Included, UV Excluded and UV Calibrated can be quickly switched by the operator. This provides a true CIE Whiteness Index, Tint and Yellowness Index (YI) consistent with the mill standards and buyer specifications worldwide.

Opacity and Sheet Density: Preventing Print Show-Through

Place a thin book paper against a window.Place thin book paper against window. Low opacity paper has clearly visible text on the back that is visible. Low opacity compromises the reading experience and product presentation for book publishers, notebook makers, catalog printers, and food packaging assembly companies.

Opacity is a measure of the amount of light that is absorbed by the paper when light is passed through it. Opacity is managed in the paper-making process through varying sheet thickness, fiber refining and the addition of mineral fillers such as titanium dioxide and calcium carbonate.

Double reading procedures are needed to measure the opacity of paper. A paper sheet is first laid over a black light-trap cavity or black ceramic tile by the operator to measure. Then, the operator measures the same sheet, supported by a thick pile of white paper sheets or a white reference tile. The ISO or TAPPI opacity percentages are automatically calculated by software.

The 3nh benchtop and portable spectrophotometers feature automatic opacity measurement mode. Paper stock is tested in seconds by quality teams, which allows you to make sure that the paper has high opacity while not adding too much material weight and production cost.

Gloss Control for Coated and Packaging Board

Colour is just one of the visual equations. Surface gloss refers to a coated magazine stock, glossy packaging, matte art paper, and textured cardboard.

A high-gloss magazine cover requires a high level of gloss. Matte photo paper doesn't have any glare and thus can help to avoid eye strain. If the gloss in a folding carton box is different on one side than the other, the packaged product will appear inexpensive on the store shelves.

Gloss measurement is based on a precise angle of specular reflection. Paper and paperboard standards such as TAPPI T480 have a 75 degree measurement angle, optimized for paper gloss. A 60 degree angle is used for general matte-coated or semi-gloss items while a 20 degree angle is used for high-gloss coated paper.

3nh precision gloss meters provide accurate surface shine measurement results instantly. Single-angle and multi-angle versions are available in hand-held form for quality control personnel to scan paper webs, coated rolls, and printed cardboard, directly on the mill floor. They determine the uneven coating distribution prior to the rolls going to the converting plant.

Managing Recycled Fiber and Packaging Shade Drift

In an effort to meet sustainability goals, large volumes of recycled pulp, brown Kraft paper and corrugated board are being used. Recycled fibers also contain naturally occurring variations in colour, some leftover printing inks and different amounts of lignin.

Spectral tracking is necessary to control brown Kraft box shades or recycled paper tissue. The 3nh portable spectrophotometers enable warehouse auditors to quickly and accurately inspect paper rolls coming in to the warehouse in seconds. The CIELAB coordinates and Delta E differences are quickly captured by the inspectors, right beside the reel.

Physical measurements are related to 3nh SQCX software which collects the spectral data in pulp mills, paper mills, packaging converters and brand buyers. Digital colour standards are used to replace physical paper standards that yellow, soil and degrade over time in storage. In seconds, mill managers in Asia can send digital spectral data to buyers in Europe, thus avoiding the delays associated with approval.

Building Lasting Paper Quality with 3nh

The paper industry is a low profit, high-volume enterprise. The one out of spec paper roll causes a lot of waste and loss of customer confidence.

Whether it's raw pulp bleaching or fine printing paper to high gloss packaging board and recycled box stock, 3nh optical measurement solutions provide total control. The whiteness, brightness, opacity and surface shine of each roll is accurately measured by precision spectrophotometers and gloss meters.