Stone Paper vs. Traditional Paper: The Difference at Microscopic Level
The Pebble Printing Group is dedicated to printing on stone paper. This is because stone paper is not just a sustainable solution to printing, but a fundamentally different material than traditional fiber-based paper. To demonstrate the difference (and arguably superiority) of stone paper to traditional paper, the Pebble Printing Group partnered with Shenzhen University to investigate stone paper under a scanning electron microscope. In the investigation, traditional coated offset paper and stone paper were compared under four different conditions (offset paper left and stone paper right):
- Plain Surface
- Ink Coverage
At this level, the coating of offset paper is quite apparent. Even though traditional paper is based on interwoven tree-fibers, a myriad of additives form a thick layer across the surface to increase the apparent quality and feel. This is where the whiteness of graphic papers comes from.
Meanwhile, the uncoated nature of stone paper can be seen in the scales of calcium carbonate. Despite a lack of coating, stone paper maintains a very high quality appearance and feel. This is one of the reasons stone paper is less wasteful and less toxic than old-fashioned paper.
Here one can see the individual particles that make up the coating of graphic offset paper. In this maze of minerals, the main ingredient of stone paper, calcium carbonate, can also be found. Calcium carbonate improves the whiteness of high-quality fiber-based paper. One other thing to note is the porous structure of the field. This is why solvent in printing inks can bleed through, making precise lines (dot gain) more difficult to control.
At this level, stone paper reveals its very simple composition: calcium carbonate held together by HDPE. Here it’s even more obvious that 80% of stone paper’s weight is calcium carbonate, with visible crystals throughout the image field. One may also note that there are far fewer holes between the particles, and that the whole structure has a far smoother surface than traditional paper. This is why stone paper is waterproof.
The fibers of coated offset paper are covered by a rigid layer of minerals that break when the paper is folded. This is characteristic of the stiffness of fiber-based paper, and without proper finishing, it can produce unsightly white cracks in a printed image.
Stone paper in comparison is more-or-less unchanged by folding. For a printing engineer, this is a dramatic difference that can dictate how machinery runs. There is no grain direction and higher resistance that could cause the paper to bounce back. But it does mean more elegant folding appearance and less cracking.
Traditional paper has far less resistance to tearing than stone paper. The cause for this is apparent at 500x magnification. Traditional paper is made of a sparse lattice of cellulose fibers that hold the paper together. When it is torn, it is only those fibers that are pulled apart- the thick coating (seen on top of the fibers) serves no purpose to structural integrity.
Stone paper stretches before tearing. This is because stone paper is very dense, with far more layers that all serve to hold the paper together. The HDPE component also gives stone paper better forgiveness under pulling stress.
A closer look at the previous image reveals just how many connections are broken when traditional paper and stone paper are torn. Traditional paper looks like the roots of a tree splayed out while stone paper shows a huge number tendril-like outcroppings that, from top to bottom, all hold the paper together and give it significant strength. The calcium carbonate “stones” that make up stone paper are also visible at this magnification.
The fine pores in traditional paper absorb a large part of solvent-based printing ink. This is apparent by the still-visible texture inside of the dark center of the printed dot. It is also the reason that images printed on traditional paper appear lighter and less saturated.
On the stone paper side, the texture of the underlying calcium carbonate particles is less visible. This is because the ink (in this case a solvent-based ink and not UV ink) sits completely on top of the waterproof surface. Colors on stone paper are particularly vibrant, and lines are sharper for this reason. The movement of the ink is basically easier to control. Finally, the crests and valleys of the stone paper printing dot seem much smoother than the offset printed dot. This is because this dot was covered in varnish to fasten the printed image. This low-impact method, in contrast to the more typical UV printing process, has been perfected by the Pebble Printing Group.