Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 97

4.4 Friction Phenomena in Cellulose Systems

Friction is an important surface phenomenon that is strongly influenced by molecu-
lar adsorption. Inter-fiber friction plays an important role in flocculation and network
strength of paper (Zauscher and Klingenberg 2001). Relevant work related to the mea-
surement of friction in cellulose systems can be found in several references (Bogdanovic,
Tiberg et al .

2001; Zauscher and Klingenberg 2001; Theander, Pugh et al . 2005;

Stiernstedt, Brumer et al . 2006; Stiernstedt, Nordgren et al . 2006).
Friction, lubrication and wear have long been of both technical and practical interest
since the operation of many mechanical systems depends on these surface phenomena
(Dowson 1998). They have received increased attention in response to the inordinate
waste of resources that has resulted from high friction and wear. In fact, estimates
indicate that proper attention to tribology issues could lead to economic savings up to
1.3 to 1.6% of the Gross National Product (Jost 1990). Beyond industrial applications
tribology is critical in the performance of body implants, cell adhesion, and interfacial
phenomena in composite materials.
Fibrous polymeric materials go through different processing stages including pretreat-
ment, dyeing, printing and finishing before they are finally assembled into end products
(woven and nonwoven webs, composites, etc.). Machinery and equipment are inevitably
involved in handling fibers at high rates of deformation. Fibers and related materials are
also subjected to destructive abrasive forces that may result in mutual abrasion between
fibers and/or between fibers and equipment surfaces. In order to control friction and
reduce wear between fibers and between fibers and solid surfaces, surface modifica-
tion treatments are necessary. Fiber finishes are commonly used during the production
of many different fiber grades (Proffitt and Patterson 1988) and a myriad of different
finishing formulations exists depending on the intended use of the fibers and the fiber
processing operation conditions. In general four general classes of boundary lubricants
can be identified:

1. high molecular weight, water dispersible products - significantly reduce abrasion
damage to fibers in aggressive processes and seem to function most effectively in
dynamic, higher speed situations;
2. waxy materials - traditional boundary lubricants that function in both low speed (fiber
to fiber) and high speed (fiber to metal, fiber to ceramic) processing conditions;
3. low molecular weight polymers that have high affinity for the surface of the fiber
and tend to self-assemble depending on the chemical interactions with the modified
substrate;
4. silicone based materials - tend to have high affinity for the surface of many of the
fiber forming polymers.

Recent technological developments in fiber processing trend towards higher speed pro-
cessing making the dynamics of the adsorption process and the durability of the adsorbed
layer even more relevant. A need to continuously develop high performance finishes for
surface modification is required in order to meet the increasing requirements of modern
fiber processing operations.