Stone Paper

Characteristics

Due to stone paper’s fundamentally different structure, it behaves differently as a material than traditional paper. Here are a few of the interesting features of stone paper (some examples compared to regular office paper).

Waterproof

Due to its impermeable structure of stone powder and HDPE, stone paper is completely waterproof. Water simply sits on the surface of the paper and can be wiped off. The waterproof nature of stone paper doesn’t only ensure durability, but opens new creative applications in both publishing and packaging. It also has an effect on the printing performance of the paper.

Natural Whiteness

One of the most stunning differences between stone paper (right) and traditional paper (left) is the natural whiteness. A piece of traditional office paper can be seen on the left- it appears much bluer than stone paper. This is because office paper, and many other types of papers, are mixed with brighteners. In this case, traditional office paper is often mixed with something called an “optical brightener” that converts invisible UV light into visible blue light, making the paper appear whiter to the human eye. Stone paper requires no additives to show a pure white appearance. In fact, calcium carbonate, the main component of stone paper, is a common filler used for traditional paper to increase volume and whiteness. This is partly because calcium carbonate is also cheaper than fresh wood fiber.

Tear Resistance

Stone paper (right) is much more difficult to tear than traditional paper. It has a much more forgiving response to a pulling force, stretching significantly before failing. Traditional paper (left) tears much more easily, particularly along its “grain direction.” The result of tearing this paper makes clear that stone paper does not have a fibrous structure like regular paper. Along with being waterproof, this ability also ensures high durability and long life.

Low Flammability, Low Heat Resistance

Stone paper does not burn readily. Exposure to flame causes the paper to revert back to its principal component calcium carbonate (right). As HDPE is one of the cleanest forms of plastic, stone paper also does not release toxic gases when burned. Fire breaks stone paper down into water, carbon dioxide, and calcium carbonate. This is another striking difference with traditional paper ash (left). There are a number of toxic gases released when paper is burned, including carbon monoxide. The resulting ash also has little utility, unlike the calcium carbonate from stone paper, which can be recycled into more stone paper, used for construction materials (such as concrete), or fertilizer.

Stone paper is sensitive to heat above 120° Celsius (248° F). At this temperature, stone paper begins to deform as the long chains of HDPE molecules begin to slip around each other. For this reason, stone paper is currently not suitable for laser printing.

UV Light Degradation

Although incineration and recycling are both sustainable methods of disposal for stone paper, UV light emitted by the sun can also break stone paper down into calcium carbonate. The molecules of HDPE in the stone powder structure begin to fall apart after an estimated 6 months of exposure to UV rays. The result is then similar to stone paper after incineration, a brittle shell of calcium carbonate dust.

Other Interesting Features

• Does not cause paper cuts
• Non-toxic
• Soft touch
• Complies with FDA food-grade and ROHS standards

The Main Ingredient: Calcium Carbonate (60-80% by Weight)

Traditional paper is a tapestry of organic plant fibers (mostly from trees) that interlock due to some chemistry and physics between close-proximity molecules. These microscopic fibers can differ in length, thickness, so-called lignin content, and many other variables that determine the final appearance and behavior of paper. Stone paper is fundamentally different from traditional paper in this aspect. Stone paper is largely composed of particles of calcium carbonate, one of the most common mineral compounds on Earth. Calcium carbonate can be harnessed from limestone, marble, and a number of other sources. Currently, the calcium carbonate for stone paper is produced from recycled construction and mining waste.

The Binding Agent: High-Density Polyethylene (HDPE) (20% by Weight)

The second component of stone paper, high-density polyethylene, binds the calcium carbonate particles together into sheets. Many people are initially skeptical of the use of a petroleum product in paper manufacturing, but there are many reasons why HDPE is more sustainable than traditional paper production. Firstly, fresh fiber and recycled paper production use enormous amounts of fresh water in manufacturing. It is estimated that around 10 liters (2.6 gallons) of fresh water are required for a single A4 sheet of office paper. This water, used to suspend plant fibers in a workable pulp (or for washing in recycled paper), must also be dried out of the pulp to form sheets. This step requires industrial dryers along the entire web of paper, and accordingly, a lot of electricity. Finally, as many paper manufacturing loops are not circular, the dirty water must then be released into the environment, causing pollution of fresh water sources. Because stone paper is bound with HDPE, manufacturing is virtually waterless- after mixing the calcium carbonate and HDPE, the “pulp” can be stretched on rollers to create thin sheets. There is no drying process and no release of water into the environment.

…when scaled up to the European book-paper supply, this means that 2,947,063 tons of oil would be required to maintain the status quo of 72.9% virgin fibers and 27.1% recycled papers… For virgin [non-recycled] HDPE in a stone paper supply, this number would be 1,897,727 tons of oil.

The Sustainability of Stone Paper in European Book Paper

HDPE is also more recyclable than traditional paper, which can only be recycled about seven times before the fibers lose too much mass to produce usable paper. A sheet of stone paper could potentially be used for around 200 years, or the estimated usable recyclable lifetime of HDPE.

Coating (1-5% by Weight)

The coating of stone paper is the secret sauce that makes it perfect for printing applications, and composes about 1-5 percent of the paper by weight. Stone paper coating is proprietary and depends on the manufacturer. There are a number of different patents for stone paper coating- but they largely serve the functions of lubrication, dispersion, softening, heat resistance, and reduction of static.