Aspen BCTMP: proven performance: new pulps and papermaking technology blur the line between groundwood and freesheet.

A century ago, the divide between wood-containing and woodfree papers was clearly marked. Wood-containing papers, such as news-print, were made primarily with mechanical pulps using spruce stone groundwood, while woodfree papers, such as fine writing papers, were made from rag and sulphite or kraft chemical pulps.

Today, thanks to advances in pulp manufacturing, the distinction between so-called "woodfree" and "wood-containing" papers has blurred significantly. Millar Western has promoted its aspen bleached chemithermomechanical pulp (BCTMP) not as mechanical pulp but as a substitute for chemical pulps. Aspen BCTMP's high brightness and low coarseness at high freeness levels make it ideally suited for use in traditional woodfree applications, which is where virtually all of the company's aspen pulp is used.

With the notable exception of the United States, markets around the world have embraced BCTMP in a number of high-end applications, including coated woodfree papers, copy papers, and carbonless and other specialty papers. U.S. papermakers are reluctant to use higher volumes of BCTMP for several reasons, including restrictive paper standards, a high level of integration, and a lack of familiarity with recent BCTMP advances.

CHANGING PAPER STANDARDS

Despite BCTMP's proven performance in a variety of applications, restrictive paper standards continue to limit BCTMP usage in many different applications. This has prompted calls for reform of paper standards. In essence, the paper standards debate revolves around the longevity of the paper products. Acid-based papers have a relatively short life span and are not recommended for archival documents. As they age, these papers turn yellow and eventually disintegrate. Lignin-containing papers have also been observed to yellow faster, with lignin thought to accelerate strength loss. However, recent research has demonstrated that this is not the case, as discussed below.

Based on historic misunderstandings, many paper standards have limited the amount of lignin in certain permanent grades of paper to 1% or less. Because aspen BCTMP contains only 17% lignin, its usage has tended to be limited to around 3% of the fiber furnish. Some standard-setting agencies have extended BCTMP content limits to non permanent papers, despite the fact that there is no evidence that BCTMP compromises their quality.

Some agencies have recently begun to take a second look at BCTMP. The United States Government Printing Office, in recognizing the acceptability of BCTMP in non-permanent paper grades, wrote the following in its Government Paper Specification Standards, No. 11 of February 1999:

"Barriers to the use of 'chlorine-free' pulps and BCTMP in the paper manufacturing process have been removed from the specification where the intended use of the paper is not affected." The printing standards go on to define BCTMP as a semi-chemical pulp and specifically exclude it from the definition of groundwood.

New research is also challenging the notion that lignin contributes to loss of paper strength, which may lead to greater usage of BCTMR A recent Canadian research project concluded that paper acidity--not lignin content--is the real cause of paper decomposition. Subsequent to these findings, the Canadian Standards Board removed the limitation on lignin content in permanent papers except for those grades requiring optical stability.

With obstacles like these disappearing, U.S. papermakers are more interested in BCTMP due to its excellent performance in a broad range of applications.

COATED WOODFREE PAPERS

Coated woodfree paper is now Millar Western's largest end-use market for aspen BCTMP, representing about 37% of its production. Using a typical addition rate of 15%, approximately four million air-dried metric tons (ADMT) of coated woodfree paper are made with Millar Western aspen BCTMP each year.

The high usage of BCTMP in coated wood free papers today reflects many years of grade development. Ten years ago, a high-tensile version of the 250-85-100 grade (freeness-brightness-percent aspen) was most commonly used, but the freeness level was considered too low for higher substitution rates. Over time, Millar Western has been able to increase freeness while maintaining low debris levels. Now, 325-85-100, 450-85-100 and the higher-bulk 250-85-100 HB aspen grades are most often used in woodfree coated papers.

For high-gloss coated paper grades produced on supercalenders, Miller Western produces a higher tensile grade, 325-85-100. A higher-hulk grade such as 250-85-100 HB is generally not recommended in this application because older style supercalenders tend to pound and densify the sheet as it passes through multiple nips, which collapses the thin aspen fiber walls. A higher-bulk grade is advised, however, for lower-gloss matte-finished or lightly calendered paper grades because of its positive impact on sheet bulk, stiffness and opacity.

This means mills can produce similar paper properties at a lower basis weight, which offers advantages to the paper printer or consumer. New calendering techniques such as the Optiload, Janus, or Hot Soft Nip calenders, when combined with new coating formulations, are allowing the preservation of base sheet bulk, stiffness and opacity while still achieving high gloss and printing characteristics. This development is leading to the creation of new paper grades based on higher-bulk, high-yield fibers like aspen BCTMP.

New users of high-bulk BCTMP are sometimes concerned about a drop in the tensile of the refined mixture of BCTMP and hardwood BKR. However, low tensile results in the lad have not translated into poor machine runnability, even on open-draw machines.

COPY AND OFFSET PAPERS

Aspen BCTMP has been used by Asian, European, and North American copy paper producers for about 12 years, typically at addition rates of 5% to 20%. Approximately 200,000 ADMT of high-bulk aspen BCTMP pulps are used by copy/offset/ink jet uncoated paper producers each year.

For much of the past decade, the main reason for using aspen BCTMP had been to reduce furnish costs. The characteristics of older BCTMP pulps tended to be closer to those of refined hardwood bleached kraft pulp (BKP), with bulks of 1.7 to 1.9 [cm.sup.3]/g. Over the past three years, however, a growing number of copy paper producers have shifted to aspen BCTMP pulps with bulks typically in range of 2.0 to 2.5 [cm.sup.3]/g. The main reasons behind this growing shift are as follows:

* Increased use of fillers such as PCC, which reduces copy paper bulk and stiffness;

* More recycled pulps, which reduce copy paper bulk;

* Higher copier printer speeds, which have increased demands for stiffer copy papers; and

* New blade or rod metering size presses that can better control paper surface properties.

New paper machinery has also facilitated the use of high bulk aspen BCTMP, allowing lower-strength, higher-bulk pulps to be used at higher addition rates without compromising runnability and quality. Some of the fastest, largest paper machines in the world are running fiber furnishes containing at least 15% aspen BCTMR In cut size copy papers, strength is not as important as stiffness, caliper, surface characteristics and printability. In offset grades sold as reels, runnability on the paper machine has not been an issue.

Figure 1 (next page) shows the improvement in bulk when 30% 250/85 HB aspen is co-refined with 70% U.S. southern mixed hardwood BKP in a PFI mill. It also shows a lower-bulk, higher-tensile 400/85 aspen BCTMP for comparison. Figure 2 shows a similar improvement in opacity.

[FIGURE 1-2 OMITTED]

Bulk and opacity improvements have been demonstrated whether paper machines are producing 20 lb. copy paper, 70 lb. reply card stock or offset grades.

This high bulk approach is not for every paper machine or sheet, however. The following limitations in copy or offset papers have been revealed over time:

* Older fourdriniers, as well as open draws, have more bonding and linting issues, and high-bulk pulps typically have lower bonding;

* Puddle type size presses have more surface strength issues;

* Feathering or wicking effects can increase on ink jet papers with higher-bulk pulps; and,

* The lignin in BCTMP adsorbs ultraviolet light and interferes with fluorescent whitening agents (FWA) or optical brightening agents (OBA).

Brightness reversion or yellowing has not been an issue in copy papers when BCTMP content is less than 20% of the furnish. If yellowing occurs at this addition rate, excessive FWA or OBA use is the likely cause.

CARBONLESS PAPERS

Technical specialty papers--specifically carbonless or no-carbon required (NCR) papers--present another significant opportunity for BCTMP. Approximately 10% of Millar Western's production goes to this market. While not as large as the printing and writing and copy paper markets, this market's demand is consistent. European and North American technical specialty papermakers have been using BCTMP for the past decade or more. In fact, most major producers of carbonless base sheet use some hardwood BCTMP in their furnish.

Due to its relatively short fiber length (about 0.8mm) and high fines content (17% to 25%), aspen BCTMP offers superior formation compared to most kraft pulps. BCTMP also can have lower porosity than hardwood kraft pulp, which helps keep the coatings on the surface. Finally, in some cases, BCTMP can also improve sheet surface smoothness.

SPECIALTY PAPERS

European and Japanese papermakers have been using hardwood BCTMP in silicone release paper base sheets, glassine, label and similar grades of specialty papers for almost 10 years. Millar Western produces about 20,000 to 30,000 ADMT/year range for this use. Several functional advantages drive the use of BCTMP in these specialties:

* Refining energy, requirements to achieve freeness or porosity targets at the average proportions in the furnish can be reduced by as much as 30%

* Compared to birch BKP, BCTMP delivers improved sheet strength, drainage and drying

* BCTMP contributes to superior paper smoothness alter calendering, which allows For lower rates of silicone usage resulting from more even application

* Using BCTMP in the furnish permits a lower glass transition temperature for hot soft calendered grades, allowing for lower calender temperature and pressure

* BCTMP helps achieve higher bulk, an advantage for the conversion stages of release papers.

The portion of aspen BCTMP typical]y used in specialty paper furnishes ranges from 15% to 30%, depending on the paper grade. Pulps are normally co-refined to the 50-70 degrees SR (100-200 csf) level, depending on the grade and basis weight of the finished paper.

In one-side coated label papers, aspen BCTMP has been used primarily to improve formation and dimensional stability, or wet stretch, which is especially important for label paper applications. Lab tests and paper machine results show that BCTMP can achieve better dimensional stability than BKP.

FUNCTIONAL ADVANTAGES

Two factors--new papermaking technology and the functional advantages of hardwood high yield pulp--have led to the increased use of lower-strength, higher-bulk aspen BCTMP pulps. Millar Western has marketed aspen BCTMP as a white paper pulp to replace bleached chemical pulps. Use of aspen BCTMP has grown most notably in coated woodfree papers. The performance of BCTMP in various applications is encouraging changes in paper standards. Attitudes are clearly changing, paving the way for even greater BCTMP use.

About the authors: Ran J. Reis is senior vice president, engineering and technology, and Gregg Nielsen is manager, pulp sales administration for Millar Western Forest Products Ltd., in Edmonton, Alberta, Canada; email Reis at [e-mail] or Nielsen at [e-mail].