Acid free paper- why? And what is lignin anyway?

Whether you are a die hard scrapbooker or a collector of something (for example, Discworld stamps), you have probably encountered the term ‘acid free’ or ‘lignin free’ paper.  These tell the user that the paper is suitable for long term use.  But what do they mean?

For that we need to take a big step backwards to look at the history of paper itself.  Papermaking has been around for a long time- in a 2006 World Archaeological report fragments of linen paper from China dating back to 8 BCE have been reported.  While paper can be made from a wide range of materials including mulberry, flax and hemp, both directly from plants and from old worn out materials like rope, rags and hemp waste, most paper commercially made today is made from wood pulp.

We can use these materials, including wood, due to the nature of the plant material.  Plant cells are surrounded by a special structure called a cell wall.  The cell wall is a strong, flexible structure that helps plants to keep their shape- whether that means a leaf of 5-6 cells thick or a massive eucalypt tree trunk that must support the weight of the branches above.  It does this by stopping cells from expanding too much under water pressure.   Think of a cell like a balloon.  If you keep adding water, it will continue to expand.  However, the cell wall acts like a flexible but non-expanding sock around that balloon- like a fabric bag around the balloon.  While the balloon is smaller than the tube, it can expand.  But once it has expanded to touch the fabric material, it must hold it’s shape.  It can’t expand out because the fabric will not stretch.  If you made the fabric bag long and skinny, the balloon could stretch to a long and skinny shape- and if the pressure of the water is enough, then it will hold that shape and be able to support itself.  Conversely, if you take the water away it will droop- just like plants can droop if they don’t have enough water.

Cell walls are made of lots of different polymers and other compounds in several separate layers.  The main component is cellulose (35-50%) which is made of thousands and thousands of glucose molecules linked together to make a long chain.  This is the stuff that we are interested in to make paper.  The long chains of cellulouse from wood (or the plant material of choice) are processed form fibres suspended in liquid.  These fibres are then passed through a fine screen so that the water drains away, leaving a thin layer of randomly tangled mat of fibres.  When this dries, we have our piece of paper.

But cellulose is not the only component of cell walls.  One of the other major components is lignin.  Lignin is a complex polymer (for just how complex, see the Wikipedia entry for it) that makes up 10-25% of the cell wall.  In the plant it acts to strengthen the cell wall.  But if left in paper it will break down over time.  Newsprint has most of the lignin still in it.  Over time lignin will react with oxygen in the air and form a yellow colour- this is why newspapers yellow over time and why those making scrapbook paper art don’t want lignin in their papers.

Paper can be chemically treated in the production process to remove most of the lignin but the process is acidic.  The downside of this process is that the paper produced is acidic (pH less than 7).  Over time the acid reacts with the cellulose and degrades it- breaks it down.  This is a bad thing if you want to store information in a book, wrap up a precious object in tissue paper (as the acid will also act on the object, particularly if it is a natural material like cotton or linen), or have something precious in contact with it, like stamps in an album.

So to sum up.  If you are making a book, scrapbooking old family photos, storing your stamps or wrapping up your baby’s first outfit to save for your grandchildren, you might want to consider the paper you are using.  Acid free means that the paper will not degrade over time, nor will it damage anything it is in contact with over time.  Lignin free means that it will not yellow over time.  If you have some paper that you don’t know is acid free or not, pH pens exist that you can test your object with.  These are textas (or markers) that change colour based on the pH of the paper.  Just put a dot on an unobtrusive section and compare the colour to the chart that will come with the pen.  If you have something that is acidic that you want to preserve, there are pH neutralisers available in spray cans that should work on all papers (your results may vary, etc etc).

And if you just want to print out an assignment for uni, don’t worry about it.  No one will, including you, will ever want to look at that piece of paper again so just use whatever is cheap and handy and don’t forget to recycle it after!