Many paper mills send out a certificate of analysis, or “CofA,” with each shipment of newsprint.

The certificate contains the pressroom’s contact information, lot and job numbers and the grade’s specifications, brand and mill origin. Rolls are also individually labeled with a bar code for inventory management. All of the data is distributed to newspapers to help them assess quality before the printing process begins.

At newspapers with TQMS, or total quality management systems, in place, the pressroom will collect the data and match it with press conditions, customers, destinations and press performance.

Testing machines help users define roughness 
and brightness of newsprint.

If a problem occurs or a complaint is logged, press operators retrieve the data to determine the culprit and take steps to eliminate future problems.

With this approach, TQMS help all constituents in the supply chain as well as providing a well-respected service to customers.

Let’s examine the current specifications commonly found on the CofA and match them to the importance of pressroom operations and desired quality for advertisers and readers.

Optical properties lead interest

In the past 10 years, attention has focused on newsprint’s brightness, whiteness and shade.

Before color became an everyday occurrence in newspapers, newsprint brightness was not a factor. Newsprint was composed of pulp fibers that were mechanically separated and cleaned, then reinforced with bleached kraft pulp for strength. As a result, newsprint boasted a range of 60 to 65 GE (a U.S. standard) and yellowed quickly under sunlight. Quality color reproduction was all but impossible.

The debut of USA Today in 1982 changed everything. Optical qualities had to be improved and uniform for every pressroom. Delivering a quality product to every hotel and airline, a tact USA Today initiated, challenged North American paper mills to make a brighter, stronger sheet.

In response, mills installed bleaching lines (whitening fibers), new forming equipment (to take away two-sidedness), added high brightness minerals and finishing equipment (for calendaring). All these enhancements bolstered optical and surface qualities.

Shade factor

Equally important as brightness and whiteness is shade. The paper industry defines shades using the CIE L*a*b* color specification. The spec is a three-dimensional model of the visual light spectrum, where “L” represents whiteness, or the total amount of light reflected off the paper’s surface; “a” represents the green-red axis and “b” represents the blue-yellow axis.

Typically, when marketing a particular sheet, paper mills offer sheets based on their “b” value. Sheets with a “b” value of less than zero, for example, exhibit a blue shade while those with a “b” value of more than zero exhibit a yellow shade (cream or white).

Furthermore, a blue-white sheet will also exhibit red hues while warm white sheets exhibit shades of green. A printer must compensate for these shades to achieve accurate color reproduction, because most inks are transparent.

Mills use fiber, chemicals (minerals, tinting agents, fluorescence) and mechanical equipment (calendaring stacks) to hit a target shade and brightness.

2 standards govern

Brightness is measured by two standards today, one set by the International Standards Organization  (2469, 2470, 2471, 5631.1, and 9416) and the other by TAPPI, the paper industry’s technical association (T452 and ASTM D985).

Unfortunately, due to technical reasons, there is no way to compare or establish a linear relationship between the two. The ISO standard appears to be graining ground because its international orientation more accurately reflects the global newsprint trade.

Brightness values for standard news products (30 lb. or 48.8 gsm) range between 55 and 60 ISO, depending on wood fiber types, kraft pulp content and fiber processing techniques. Mills use devices from Testing Machines, Datacolor and Technidyne, among others, to gauge brightness.

As mills bleach fiber, add high brightness market pulp, fill the sheet with white minerals (calcined clays and calcium carbonate) and use hot calendaring to finish the sheet, the sheet gets brighter.

Abitibi-Consolidated’s Abibrite line, for example, uses processing that raises sheet brightness to as much as 65 and 75 ISO.

The smooth factor

Even as mills and newspapers assess brightness and shade, newsprint’s smoothness has also become an important consideration, particularly for flexographic and letterpress users.

To understand smoothness, you first have to understand roughness. Roughness is defined as the deviation of the surface of interest (newsprint) from the surface of a reference plane (metal or glass plate, or electrical signal), and smoothness is its inverse.

For decades, pressrooms ran letterpress machines and struggled with mid-production corrections stemming from changes in the roughness of the paper.

The industry, searching for a way to predict newsprint performance, found a solution in the mid-1960s, when Bowater scientist Dr. John Parker invented the Parker print-surf roughness tester.

The PPS, which used a different means to measure roughness than prevailing tools did, provided quick and understandable results.

In a nutshell, PPS calculates roughness as the average depth of pits in the paper’s surface, measured in micrometers. Press operators use PPS to determine blanket specifications in offset printing, thus improving print performance.

Mills, too, use PPS to reflect newsprint’s smoothness. Before paper makers eliminated two-sidedness, a PPS was reported for both the top side and bottom side. Typical PPS readings for standard newsprint is between 3 and 4.4 while value-added grades usually boast a PPS of below 3. Lightweight-coated and supercalendared grades have PPS scores of well below 3.

PPS is not the only way to monitor smoothness. Some mills use another standard known as Sheffield smoothness.

Modern offset web presses aren’t as vulnerable to variations in smoothness. With these machines, other approaches, such as varying drive speeds can be used to hold the sheet.

But in operations where a single shaft pulls the sheet through the units or tower, variations in the sheet can cause it to wander, thus causing misregistration to occur.

Still, for the most part, adaptations in pressroom equipment have created a working environment where paper smoothness is not as critical if it hits close to a target. In fact, acceptable smoothness ranges have widened a bit, as new pulps including recycled content, bleached chemithermo mechanical pulp and even eucalyptus are used as furnish materials for newsprint grades.

Smoothness is often used today to control the calendaring process at mills. PPS and other measurements are taken to control the zones of the calendaring roll (in the case of soft-nip) or rolls, for both temperature and pressure.

If brightness is king, basis weight is queen. A sheet’s dimensions will have the greatest impact on throughput, folding and stacking, not to mention cost. Mills report basis weight and caliper on the CofA but also brand definition and model number of grade.

Caliper, or thickness, also influences smoothness. If the sheet runs at higher rates, it can be rougher. That might help ink application but detract as far as mottle is concerned.

Conversely, smoother sheets, which may be thinner, can slip and wander, causing misregistration, jams or breaks.

Basis weight also key

Basis weight also influences opacity. Thicker sheets reduce show-through and strike-through. Mills will use more filler and shorter fibers to bulk up the sheet and make it brighter and more opaque. Press operators must compensate for the bulkier web, however, by tightening controls throughout the press.

Thicker sheets are stiffer (an empirical relationship), in the process jamming less. But they can run slower or alter tension settings. Stiffness is usually perceived as advantageous, but nip pressure change, packing and ink tact should be monitored to assure optimal performance and economics.

Source: Manufacturers’ data sheet

Michael Ducey is a writer and researcher in the pulp, paper and printing industries. He contributes to a variety of technical and business journals about paper, printing, packaging and converting, and publishes market research reports for a worldwide client base. He can be contacted by e-mail at paperinfo@excite.com.