Woodfree Paper.

Uncoated Woodfree — UWF in the trade, holzfrei in German mill specs — is the chemically-pulped printing-and-writing paper that the modern office runs on. It is the A4 ream in the copier tray, the envelope in the mail, the interior pages of the trade paperback, the letterhead under the corporate logo, and the exercise book in the schoolbag. Roughly 30 million tonnes move through global markets every year, making UWF the largest single printing-paper category by both tonnage and commercial value WPI-g-000875. What distinguishes it from its wood-containing cousin WPI-g-000869 is not whether the fibre came from a tree — it did, in both cases — but whether the lignin was chemically dissolved out of that fibre before sheet formation. Woodfree means lignin-free, not wood-free: the phrase is a nineteenth-century coinage that has outlived its accuracy and become a technical term of art. The lignin removal is the entire point — it is what gives UWF its high brightness, its archival stability measured in centuries rather than months, and its compatibility with every printing and writing technology invented since 1900.

What it's used for

The single largest application is office copy paper — A4 in every market outside North America, Letter (8.5 × 11 in) in the US and Canada, typically 75 or 80 g/m², packed 500 sheets to a ream and sold by the pallet. Office copy paper alone accounts for roughly 40% of UWF tonnage. The sheet has to charge electrostatically for xerography, accept liquid ink for inkjet, absorb toner without curling, and feed reliably through automatic document handlers at 60–100 pages per minute. UWF is the only grade that does all four acceptably at commodity prices.

Commercial printing is the second-largest bucket at roughly 25%. Sheet-fed offset presses run UWF at 70–120 g/m² for invoices, direct mail, brochures without photographic imagery, annual reports with an uncoated finish, product manuals, and the transactional print that still moves through enterprises despite thirty years of digitisation forecasts.

Book publishing is 15% and remains the grade's most visible cultural use. Trade paperbacks run on 70–90 g/m² UWF, often cream-tinted to reduce eye strain; hardback fiction interior pages use 80–100 g/m² at higher bulk (1.2–1.5 cm³/g) to give a 300-page novel the spine thickness buyers expect; mass-market paperbacks pinch down to 55–65 g/m². The grade's archival stability matters here in a way it does not for office paper: books are meant to last.

Envelope paper accounts for roughly 10% of the category and has its own specification dialect — tighter formation uniformity for window patches, higher tear resistance for machine insertion, sometimes lighter weight (60–80 g/m²) than general UWF.

The remaining 10% is specialty bases: thermal paper base before coating, lottery ticket substrate, cheque paper, school textbook interiors, notebook and exercise-book paper, preprint paper for later over-printing. UWF is the default answer whenever a converter needs a bright, stable, printable, writable sheet and cost matters more than coated-grade gloss.

Origins and history

UWF is the industrial descendant of rag paper, but it could not exist without the nineteenth-century breakthrough in chemical pulping. Until the 1860s, all fine paper — the kind used for books, writing, and government records — was made from cotton and linen rags. Rag supply was inelastic. Industrial printing demand was not. The gap between the two was the commercial problem that chemical pulping was invented to solve.

Benjamin Tilghman, a Philadelphia-born chemist, patented the sulphite process in 1866 in the UK and 1867 in the US (US Patent 70,485), cooking wood chips in acidic bisulphite liquor to dissolve lignin and liberate bleachable cellulose fibre. Sulphite mills multiplied across Europe and North America through the 1880s and became the backbone of fine-paper manufacture for the next seventy years — the fine book and writing papers of the late Victorian and Edwardian periods were almost entirely sulphite UWF.

Carl Ferdinand Dahl, working at Danzig in 1879 and patenting in 1884 (German patent DRP 25,629), developed the alkaline sulphate process — later renamed kraft after the Swedish word for strength. Kraft was initially used for brown packaging where its superior fibre strength mattered more than bleachability. Through the 1930s–50s, improvements in kraft bleaching let bleached kraft pulp match sulphite for brightness while retaining its strength advantage. By the 1960s, bleached kraft had displaced sulphite as the dominant fibre for UWF, and most sulphite mills had converted or closed.

Two twentieth-century events turned UWF from a large grade into the largest grade. The Xerox 914, launched September 1959, was the first plain-paper photocopier — it required an uncoated, chemically-pulped sheet that could accept electrostatic charge, which is to say, it required UWF. By 1970 the office-copy market was consuming more UWF annually than the entire book-publishing industry ever had. ISO 216, adopted in 1975, standardised the A-series (A4 = 210 × 297 mm) and gave mills a single global copy-paper SKU to run at industrial scale.

The 1990s brought the transition from acidic rosin sizing to alkaline papermaking with AKD or ASA sizing and calcium-carbonate filler. Alkaline chemistry was cheaper (CaCO₃ is less expensive than kaolin), allowed higher filler loadings, and dramatically extended archival life by eliminating the slow acid hydrolysis that had embrittled twentieth-century books and documents. Post-1995 UWF meets the ISO 11108 permanent-paper standard by default where mid-century paper did not. Global UWF demand peaked around 2010 at roughly 35 million tonnes and has declined about 3% annually in developed markets since, with emerging-market growth in India, Southeast Asia, and sub-Saharan Africa offsetting roughly half the Western decline. Major producers today are UPM (Finland, the world's largest UWF maker), Sappi (South Africa and global), Stora Enso, Metsä Group, Domtar in North America, Asia Pulp & Paper in Indonesia, and Nine Dragons in China.

How it's made

Modern UWF begins with bleached chemical pulp — typically a blend of 60–70% short-fibre hardwood (eucalyptus in Brazil and Iberia, birch and aspen in Nordic mills, mixed temperate hardwoods in North America) for formation, smoothness, and opacity, combined with 30–40% long-fibre softwood (Scandinavian spruce and pine, North American loblolly) for tensile backbone and runnability on high-speed machines. The pulp is bleached to 88–92% ISO brightness at the pulp mill using multi-stage ECF (elemental-chlorine-free, ClO₂-based) or TCF (totally-chlorine-free, O₃/H₂O₂) sequences before it ever reaches the paper machine.

At the paper mill, pulp is refined in disc refiners to develop inter-fibre bonding, then blended with calcium carbonate filler at 15–30% by weight. Filler is the economic lever of modern UWF: it is cheaper than fibre, improves opacity and brightness, smooths the sheet surface, and reduces drying energy. Optical brightening agents (OBAs) — stilbene-based fluorescent compounds that absorb UV and re-emit blue light — are added to push perceived whiteness above what reflectance alone could deliver.

The stock hits the wire of a Fourdrinier or gap former at 0.3–0.8% consistency and runs at 1,000–1,800 m/min. Internal sizing with alkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA) is metered into the wet end; both are reactive sizes that bond chemically to cellulose hydroxyls, making the sheet moderately hydrophobic without the pH limits of older rosin sizing. Surface sizing with an oxidised starch solution is applied at the size press midway through the dryer section. The sheet is calendered through smooth steel-and-polymer rolls to hit target smoothness, then wound to parent rolls and cut to A4, A3, Letter, and Legal for 500-sheet reams.

The pH profile matters more than the speed does. Pre-1990s UWF was acidic (pH 4.5–5.5) because rosin sizing required it; the paper embrittled over 50–80 years as residual acid hydrolysed the cellulose backbone. Modern UWF runs pH 7.5–8.5, carries calcium-carbonate buffer, and is specified as permanent paper to ISO 11108. The chemistry is what makes today's UWF archival; the speed is what makes it cheap.

Specs that distinguish it

The numbers a buyer should verify on a UWF spec sheet:

• Basis weight — 60–120 g/m² is the commercial range; 75–80 g/m² is the office copy standard; 90–100 g/m² is the premium correspondence and trade-paperback range. Measured to ISO 536, tolerance ±3%.
• Caliper / thickness — 95–115 µm at 80 g/m², measured to ISO 534. Bulk 1.15–1.35 cm³/g for office grades; book-bulky UWF reaches 1.4–1.5 cm³/g.
• ISO brightness — 92–106%. Yes, above 100 is possible and common: OBAs fluoresce under UV, and ISO 2470-1 measures under a D65 illuminant that includes UV. Standard office copy runs 94–98; premium "bright white" brands push 102–106.
• CIE whiteness — 140–165 is the modern commercial range. CIE whiteness weights the full visible spectrum including the OBA fluorescence peak and is the number marketers put on the ream wrapper.
• Opacity — 88–95% at 80 g/m² (ISO 2471), determined by filler loading and formation. Below 88% means visible show-through on double-sided print.
• Tensile index (MD) — 55–75 N·m/g measured to TAPPI T-494 or ISO 1924-2. The softwood component carries the tensile; drop below 50 and the sheet jams in automatic document handlers.
• Moisture — 4.5–5.5% at equilibrium. This is tighter than most grades because moisture drift causes curl and jams in laser fusers.
• Surface roughness (Bekk) — 30–60 seconds for standard uncoated grades; calendered premium grades run higher. By definition UWF is uncoated, so Bekk never approaches the 200+ seconds of a coated woodfree sheet.
• Formation — measured by Beta-ray or optical methods; uniformity is critical for toner adhesion in laser printing and line quality in inkjet. Poor formation reads visually as cloudy look-through and performs as mottled print.

Variants and family

UWF is an umbrella grade; its commercial variants are differentiated by end use rather than chemistry:

• Office copy paper / laser paper — 75–90 g/m², OBA-loaded, tight moisture and curl specs, the dominant subcategory by volume.
• Offset printing paper — 70–120 g/m² for sheet-fed offset; broader weight range and slightly looser tolerance than copy paper. See the dedicated article on WPI-g-000890.
• Bond paper — premium UWF historically containing rag cotton content for letterhead and legal documents; today often all-wood-pulp but specified to higher whiteness and watermarked standards. See WPI-g-000858.
• Envelope paper — specific formation, tear, and folding-endurance requirements; usually 70–90 g/m².
• Book paper — lightly calendered, often cream-tinted, higher bulk (1.3–1.5 cm³/g) to generate spine thickness at lower weights.
• Preprint paper — UWF base destined for secondary offset over-printing; specified for dimensional stability through multiple press passes.
• Permanent paper — UWF meeting ISO 11108 for archival and government records; alkaline reserve, no groundwood, guaranteed minimum fold endurance.
• 100% woodfree — the all-chemical-pulp variant without mechanical fibre at any substitution level. See WPI-g-000022.

The grade's hard boundary is the chemical-pulp requirement. Any mechanical fibre — groundwood, TMP, CTMP — disqualifies a sheet from being woodfree regardless of how white it looks. That distinction, and why it matters, is the compare below:

Buying notes

Three red flags worth catching before a purchase order goes out.

OBA overload for archival briefs. Optical brighteners push ISO brightness above 100 and CIE whiteness above 160, which looks striking under fluorescent light but fails ISO 11108 permanent-paper certification: OBAs degrade on UV exposure and leave the underlying sheet visibly yellower than when it was new. For records retention, legal archive, or library use, ask for "ISO 11108 permanent paper" explicitly and accept CIE whiteness in the 140–150 range rather than chasing the 165 that office marketing favours.

Moisture drift outside 4.5–5.5%. UWF that arrives at 3.8% or 6.2% will jam laser printers, curl in multifunction devices, and produce dimensional drift on offset presses. The cause is usually storage — UWF left on a humid dock for a week absorbs atmospheric moisture through the ream wrapper — but mill-side drift also occurs when the dryer section is mis-tuned. Measure incoming pallets with a probe; reject lots outside spec rather than fighting the jams downstream.

Mottled print / linting. Both are formation and fibre-release problems. Specify a TAPPI dust test (T-230 or T-437) on critical applications; inkjet line quality and toner adhesion both suffer when the sheet sheds fibre under the fusing roll or under the print head.

On certification: FSC Mix 70 is the practical retail baseline for UWF across European and North American markets, with FSC 100% available at a 5–10% premium from Nordic and Iberian mills. PEFC is accepted by many supply chains but not treated as FSC-equivalent by all downstream brands — check your requirement before buying on price.

Related reading

• Offset paper — the printing-oriented subset of UWF, with more detail on the sheet-fed lithography workflow.
• Coated woodfree paper — the clay- or carbonate-coated sibling grade for high-quality colour reproduction.
• Bond paper — the premium UWF dialect for letterhead, legal documents, and correspondence.
• Newsprint — the wood-containing counterpart; same raw material (wood), different pulping chemistry, entirely different ageing behaviour.
• Chemical pulp — the kraft and sulphite processes that make the "woodfree" in UWF meaningful.

Sources

• Tilghman, B. C. — US Patent 70,485 (1867): Treating Vegetable Substances for Making Paper-Pulp. https://patents.google.com/patent/US70485A
• Dahl, C. F. — German Patent DRP 25,629 (1884): Verfahren zur Bereitung von Zellstoff. https://depatisnet.dpma.de/DepatisNet/depatisnet?action=bibdat&docid=DE000000025629A
• ISO 2470-1:2016 — Paper, board and pulps: Measurement of diffuse blue-reflectance factor (ISO brightness). https://www.iso.org/standard/67589.html
• ISO 534:2011 — Paper and board: Determination of thickness, density and specific volume. https://www.iso.org/standard/53060.html
• TAPPI T-494 — Tensile properties of paper and paperboard. https://www.tappi.org/content/SARG/T494.pdf
• ISO 11108:1996 — Archival paper: Requirements for permanence and durability. https://www.iso.org/standard/21063.html
• Smook, G. A. — Handbook for Pulp and Paper Technologists, 4th ed. (TAPPI Press, 2016), Ch. 18: Printing Papers.
• Blechschmidt, J. (ed.) — Taschenbuch der Papiertechnik, 3. Aufl. (Hanser, 2019), Kap. 12: Holzfreie Druckpapiere.
• CEPI — Key Statistics 2023: European Pulp and Paper Industry. https://www.cepi.org/key-statistics-2023/
• Xerox Corporation — Xerox 914 product history (1959). https://www.xerox.com/en-us/about/history