Resolute Forest Products recently announced a $27 million investment to build a 21 tonne per day cellulose filaments plant at its Kénogami paper mill, to start up in 2021.
Many readers probably don’t know what a cellulose filament is, so I thought I’d tell the story of how this nanocellulose product was developed from a lab experiment around 2008 at FPInnovations in Pointe Claire, Quebec. The properties of this product can be better understood using a little physics, chemistry, and math, as well as familiarity with the nature of string cheese!
Wood is composed of cellulose fibers, which for the purposes of the little math problem to be discussed in a few moments, we’ll consider as cylinders with diameter d and length h. The kraft pulping process removes the lignin-based “glue” holding the fibers together in a tree, isolating these fibers which, in a typical softwood fiber, have dimensions d = 40 µm and h = 3 mm.
String cheese uses a process where cheese is pulled into strings of aligned protein chains, and individual cheese “fibers” can be easily peeled away from the main tube of cheese. In a similar way, the manufacture of cellulose filaments (CF for short) peels narrow filaments from cellulose fibers. For our math problem, we’ll use d = 300 nm and h = 0.5 mm, although in reality the filaments have quite a wide range of dimensions.
Softwood kraft pulp is often used to reinforce weaker pulp furnishes. An example is its use in tissue paper where most of the furnish is made of weaker hardwood fibers.
Basic fiber physics teaches that a high aspect ratio – the ratio of the length to the diameter – is one of the key criteria for strengthening purposes because it promotes the degree of bonding of a network while providing flexibility, thereby improving its mechanical properties. CF have a much higher aspect ratio than kraft pulp fibers: using the dimensions above, the aspect ratio is 1667 for CF (0.5×10-3 m/300×10-9 m) versus 75 for kraft fiber (3×10-3 m/40×10-6 m), so the strengthening properties of CF are extraordinarily better.
Cellulose is a polymer made up of glucose units (C6H12O6) and one of its properties is an ability to form hydrogen bonds, due to an attraction between positively-charged hydrogen atoms and negatively-charged oxygen atoms. When mixed with other materials containing oxygen groups, –OH groups on the surface of the cellulose can also form hydrogen bonds with those materials, holding the mixture together. The availability of cellulose –OH groups on the surface of the fiber or filament (as opposed to buried inside) determines its bonding power.
Ok, so now let’s use some high-school geometry. If the radius of a cylinder is r and its length is h, its volume is , and its surface area is (ignoring the surface area of the two ends, because h >> r). The surface area to volume ratio is therefore = 2/r. When a kraft fiber of 40 µm diameter is peeled into 300 nm diameter strips, the surface area of the same amount of cellulose therefore increases by the ratio rKraft/rCF = 20×10-6 m/150×10-9 m = 133. In other words, means CF has 133 times as many OH bonds on the surface of the cellulose for the same weight of material.
The first experiments
In the early 2000s, there was a project at FPInnovations to make paper containing high (up to 50%) amounts of filler such as calcium carbonate. The filler tends to get between the cellulose fibers and prevent them from being close enough to form hydrogen bonds, so one of the strategies developed to counter this effect was to make “super-refined” fibers with high surface area. Experiments were carried out on a dilute kraft pulp suspension in water sheared vigorously with sharp blades with or without filler for several hours in a Waring blender (similar to the equipment you’d use to make a smoothie!). The results showed that a high surface area pulp could be produced, with unprecedented levels of reinforcing strength, but the conditions were difficult to scale up, due to the nature of the method and the fact that the product had a very high water content.
The challenge was to find a way of developing the surface of the fibers using scalable technology at lower final water content. Because disk refining is a mature technology for making mechanical pulp from wood chips with little water added, Keith Miles and Reza Amiri, mechanical pulping experts at FPInnovations, were therefore added to the team of Makhlouf Laleg and Xujun Hua, involved in the previous project. A series of experiments was carried out using a disc refiner to process kraft pulp at high consistency. The secret, revealed in the patents they subsequently were granted (US Patent 9,051,684, among others), was to control the conditions of the process so that the fibres were not substantially reduced in length, but “peeled” longitudinally, reducing their diameter by up to 1,000-fold.
As with any new technology, the road from lab to commercialization has been a long one. Many other researchers at FPInnovations worked on the technology over the next several years, working out not only how to optimize the technology and how to measure and control the properties of the new material, but also to explore its potential applications in various fields. Companies that are members of FPInnovations have the first rights to use the technology, and in 2014 one such member, Kruger, built a demonstration plant to produce 5 tonnes per day of CF at its Trois-Rivières facility, as reported by PaperAdvance in 2015, where they proceeded to learn the challenges of scale-up.
Kruger, being a producer of diverse products within the pulp and paper field, has been able to develop an internal market for CF over the last few years. At their Bromptonville mill, for example, they are able to integrate CF into their 100% post-consumer recycled packaging paper grades to improve strength and performance at a lower basis weight. In 2019, with the help of government funding, they upgraded the plant to a commercial facility able to run 24/7, and at a nominal capacity of 6,000 tonnes/year. Kruger has also been exploring markets such as polymer composites, cement and concrete products, cosmetics, paints and coatings, sealants, and adhesives.
Meanwhile, Resolute and Mercer, also FPInnovations members, formed a joint venture in 2014 called Performance Biofilaments, focused on the development of commercial applications for CF. According to their website, they have developed applications in four key areas: strengthening concretes, mortars and cements; enhancing rheology of coatings and industrial fluids; strengthening nonwovens, filter media and construction materials; and reinforcing polymers, composites and foams.
Cellulose filaments are made from sustainably harvested wood fiber that is mechanically processed without chemicals or enzymes, and the energy used is renewable hydro-electricity at both the Kruger and Resolute sites, so they have a low carbon footprint. As a sustainable, biobased additive that enables lighter weight and lower carbon footprint products for many diverse applications, cellulose filaments should have a great future!
Martin Fairbank has worked in the forest products industry for 31 years,
including many years for a pulp and paper producer and two years with
Natural Resources Canada. With a Ph.D. in chemistry and experience in
process improvement, product development, energy management and lean
manufacturing, Martin currently works as an independent consultant,
based in Montreal. He is also an author, having recently published
Resolute Roots, a history of Resolute Forest Products and its
predecessors over the last 200 years.
Martin Fairbank Consulting
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