HP Paper - Acid-Free and Lignin-Free Paper

The field of paper preservation is constantly changing as new papers are developed and paper manufacturing evolves. Developing new and specialty papers for inkjet printers has created a need to understand the factors that determine if the paper can be archived.

Many consumers are becoming aware of buzzwords like “acidity” and “lignin” content (see Papermaking terms and definitions), and they understand that these words refer to factors that somehow reduce the longevity of paper. Consumers are also confused by the conflicting messages from paper manufacturers regarding how acid and lignin affect the preservation of paper and the text or images printed on the paper. In addition, a multitude of paper industry organizations use varying standards regarding the effects of acid and lignin on paper preservation. As a result, consumers have many questions about claims that paper is acid-free and lignin-free, including:

• What are these claims based on?
• How long will the document or photograph remain unaffected?
• What other factors influence the permanence of documents and photographs?

As the leader in the field of photo and specialty papers for inkjet printers, Hewlett-Packard’s (HP) position on acidity and lignin content is needed to provide clarity in a confused marketplace. HP recognizes the traditional claims made by the paper industry concerning acid-free paper are based on the alkaline process used to make the base paper (substrate), but does not consider coating that is added to the paper.

In the past, many pulping processes used strong acid treatments to separate the lignin from the cellulose fibers; this created papers that degraded rapidly. As a result, the traditional definition of acid-free paper began to be used to describe papers made by non-acidic papermaking processes; these acid-free substrate papers were less likely to degrade with age. Acid-free materials were considered safe for contact with other materials since they would not cause acid-related degradation. Therefore, “acid-free paper” became a traditional term synonymous with long-lasting or archival-quality paper.

Unfortunately, the traditional definition of acid-free paper may have limited application to many modern coated papers designed for printing purposes and image preservation. Modern papers often use no strong acids in the papermaking process, but may use weak acids in the coating process. These weak acids do not necessarily increase paper degradation, but they do affect the pH value (see Papermaking terms and definitions below) of the paper. Therefore, modern papers may or may not meet the traditional definition of acid-free, but could still be suitable for archival purposes.

A vast majority of HP photo and specialty papers for inkjet printers meet the traditional definition of acid-free and lignin-free because their substrates are produced by alkaline processes and they contain no more than one percent lignin (see table below). While these papers provide a competitive solution for archiving photos and valued documents, HP believes the question of archivability is complex and has not been treated with the proper level of sophistication. HP also concurs with the recommendation of the International Standards Organization (ISO) that paper marketed as lignin-free should contain no more than one percent lignin.

In an effort to help the industry redefine or replace the term “acid-free,” HP is continuing aging-simulation studies to provide a better understanding of how modern paper permanence is affected by lignin, various types and amounts of coating acids, and other archival factors. HP’s dedication to paper permanence research is a reflection of their commitment to produce the best photo and specialty papers for inkjet printers.

The following table is a comparative listing of the acid-free papermaking process and lignin content for many of HP’s photo and specialty papers for inkjet printers. For an explanation of some of the key papermaking terms, see Papermaking terms and definitions below.

Long before modern printing technologies were developed, people noticed that different types of paper suffered various types of aging such as discoloration and loss of strength. At that time, the difference in aging behavior was largely caused by the different types of pulping methods that were used to convert the raw material (wood, cotton, or other materials) to a pulp that was then formed into paper.

In determining the aging of modern papers, the pulping process is only one consideration. Other key factors—including the acidity of the ink, substrate and coating; the storage conditions; and the lignin content of the paper—also affect the longevity of modern papers. This report deals primarily with two of these key variables associated with paper permanence:

• What percentage of the lignin remains in the paper?
• What residual acids, if any, remain in the paper substrate and coating layers?

This report also discusses key papermaking terms, paper preservation standards, pH testing, and the role of lignin and acid content in paper archivability.

This section defines key papermaking terms.

• Acid, as it applies to papermaking, is a water-soluble compound that can be present in paper as a result of the papermaking process. In the presence of moisture, acid may cause paper to degrade.
• pH is a symbol that represents the acidity or alkalinity of a substance. A neutral solution has a pH value of seven. The pH value drops toward zero as the acidity increases and rises toward 14 as the alkalinity increases.
• Coated paper is any type of paper to which a surface coating has been applied in order to enhance appearance and printability.
• Kraft (alkaline) process is the most common chemical pulping process. It uses alkaline-based chemicals—rather than acid-based chemicals—to separate the lignin from the wood fibers.
• Lignin is a brown organic substance that binds to cellulose fibers and hardens and strengthens the cell walls of plants. Lignin is the chief non-carbohydrate constituent of wood and is also a major contributor to the chemical degradation of paper. As it deteriorates, lignin gives off acids that cause paper to become brittle and to discolor on exposure to heat and light.

The paper industry and paper consumers are interacting constituencies whose goals and roles are quite different, yet both are trying to keep up with a complex and evolving industry. This section explains the present outlook of the paper industry and consumers in regards to paper preservation.

The paper industry, including paper manufacturers, has created many organizations to establish paper specifications, standards, and test methods. These organizations sometimes undermine the validity of one another by using different criteria that yield dissimilar conclusions.

Consumers simply want long-lasting documents and photographs, and seek what they hope is helpful information by asking questions like, “Is this acid-free?” Industry leaders such as HP have a responsibility to not only provide answers to questions like this, but perhaps more importantly, to educate the consumers about other information regarding storage conditions, types of ink, and coatings that may be more relevant to the fundamental question “How long will my documents and photos last?”

The facts regarding paper permanence can dissolve into a complex morass of marketing claims. HP’s competitors have made claims based on varying standards set by the paper industry. Consumers make paper-purchasing decisions based on these claims with only limited knowledge regarding the complex subject of modern papermaking and paper permanence.

Although paper manufacturing methods have evolved, and new specialty papers for inkjet printers have augmented the paper selection beyond uncoated papers, consumers still associate any preservation issues with buzzwords such as “acid-free” and “lignin-free.” However, since these terms originated before the advent of modern coated papers, the terms have diminished relevance to the subject of modern paper archivability.

The uncertainty that exists among both consumers and suppliers regarding paper permanence has to do with the following factors:

• Varying industry standards
• Ambiguous marketing claims
• The complex interaction of new inks
• Changing papermaking techniques
• The impact of environmental factors such as storage temperature and humidity
• The various types of acid, including very weak acids, used in modern paper coatings
• The ramifications of acids on the archivability of paper

Consumers are understandably very devoted to preserving their valuable documents and photographs. According to the Photo Marketing Association International (PMAI), in 2002, Americans took approximately 55 million photos per day or more than 20 billion photos per year. Over 75 percent of Americans try to preserve their memorable photographs by putting them into scrapbooks and albums.

For millions of consumers, the introduction of complex contemporary photo and specialty papers for inkjet printers has complicated the issue of paper archivability.

HP’s position on acid and lignin content is needed to help clarify the current confusion in the marketplace. Therefore, HP lists its own papers in a paper permanence table (see table above) and issues the following position on paper permanence:

• Lignin-free paper — HP supports the recommendations of the ISO/ANSI Z39.48 standard that paper marketed as lignin-free should contain no more than one percent lignin.
• Acid-free paper — Since there is no definitive scientific linkage between surface coating pH results and paper archivability, HP believes the traditional term “acid-free” needs to be redefined or replaced by another measure that better predicts archivability. As an interim solution, HP is providing the information contained in the table above.
• Commitment to testing — HP is committed to serving its customers by conducting ongoing aging simulations for several representative papers in its product line.
• Commitment to consumers — HP is committed to providing consumers with a clear message and tested results on the effects of acidity, lignin content, and all other archival factors regarding the preservation of current and future papers as well as educational documents such as this report.
• Commitment to development — HP is dedicated to exploring new paper technologies in order to produce the best photo and specialty papers for inkjet printers.

To understand how to interpret display-permanence predictions and the factors that affect how long a print will last, see HP Inkjet Photo Prints: Memories for Generations.