Chapter 8: Physical Properties of Renewable Composites Moisture is Everything…Nearly • Natural fibers are used by plants for two purposes: provide structure and transport water. – We seek these fibers for the first purpose, but its affinity for water governs most of its use and properties. • Moisture content in a tree can range from about 30% to more than 200%. – Water is contained in basically two locations: within the pore structure of the wood (lumens) or absorbed by the wood polymers via hydrogen bonding. The absorbed water is usually about 30%. – Absorbed moisture is termed bound water, and the water in the pores is termed free water. – When drying natural materials, bound water requires much more energy to remove than free water. Additionally, there may be a small adsorbed moisture layer on the natural fiber surface that requires more energy to remove than free water but less than bound water. Relationship Between Temperature and Moisture Source: Wood Handbook Moisture Property Relationships • • • Natural materials, notably wood, have anisotropic physical properties along with mechanical properties. Anisotropic shrinking and swelling is a large problem to control in service and when drying for wood and other natural materials. Swelling and shrinking are controlled in composites and wood by – Barriers and coatings that limit moisture sorption such as paint and laminates – Adding more resin to composites to resist swelling – Adding waxes to coat fibers and make them more hydrophobic – Chemical modification of wood polymers, such as acetylation, to replace hydrophilic hydroxyl groups with more hydrophobic chemical groups. Source: Wood Handbook Composite Properties are Affected by Moisture in the Panel In general, the denser the product the more it will be affected by moisture. Moisture absorbed impacts the performance of panel products. In these figures, MBL (wet process medium density fiber board), MDF (dry processed medium density fiber board), and HB (hard board) were tested. Source: Bekhta and Niemz. 2009. European Journal of Wood Products. 67: 339-342. Relationship Between Temperature and Moisture in Composites • Moisture relationships can be easily modeled in WPCs • Fickian behavior F Dm m / t F / x • Water diffusion (Arrhenius) D D 0e EA RT Density • Density more than any other characteristic will influence other properties such as moisture sorption, thickness swell, thermal properties, and mechanical properties. • Wood’s density mostly varies between 320 and 720 kg/m3 but can be 160 kg/m3 for balsa to 1,040 kg/m3 for some tropical species. Density may vary more than 10% within a species. • Agriculture fibers density – Straw has a low bulk density of approximately 134 kg/m3. – Cellulose fiber by itself may have a density of 1,500 kg/m3 – Handling and dispersing fibers evenly in a matrix and transporting them becomes an economic problem. • Acoustic properties are related to density. Friction • Friction is an important property for many construction applications such as decking, flooring, stairs, etc. It is another property that is heavily dependent on moisture content. A fresh, dry, smooth wood surface coefficient of friction will be 0.3 to 0.9 for a surface near the fiber saturation point. Other coefficients of friction are 1 for a tire, 0.8 for steel, and 0.2 for polyethylene. Thermal Properties Material a1 (10-6 m/m)/K a2 (10-6 m/m)/K Thermal conductivity (W/mK) Specific Heat (kJ/kg K) Graphite/epo xy 0.88 31.0 NA NA E-glass/epoxy 6.3 20 NA NA epoxy 55 55 0.35 NA 1020 steel 12 12 43 0.49 Concrete 14.5 14.5 1.7 0.75 Wood (oak) 4.9 5.4 0.17 2 Polyethylene 200 200 0.5 1.67 Source: http://www.engineeringtoolbox.com/ The coefficients of thermal expansion (a) in two principle material directions. This relates to how much the material will deform if the temperature raises 1K. Thermal Properties Thermal stability of Switchgrass • • Natural fibers start to thermally degrade above 100°C, but undergoes significant degradation above 200°C. The amorphous polysaccharide degrade first. This property has significant implications on processing conditions and end use of materials. Thermal degradation is a kinetic process, meaning that it depends on time and temperature. Combustion properties have tremendous importance for fire protection and bioenergy. – Autoignition temperature for wood and coal are 300 and 400°C respectively. – The combustion energy contained in wood (hardwood), switchgrass, and coal (anthrocite) are 8500, 7990, and 14,500 Btu/lb. Electrical Properties • The resistivity of steel is 10-7 W m. • Wood is a very good electrical insulating material, but insulating polymers are on the order of 1016 W m. • Cellulose crystals are weakly piezoelectric, which means that they will deform when subjected to an electric current. Quartz is an example of a piezoelectric material. Electrical resistance varies with moisture content and is how many hand-held moisture meters work. Source: Wood Handbook Electromagnetic Spectrum • • Electromagnetic radiation interacts with natural materials in different ways and may serve different purposes. Radio and microwaves (non-ionizing radiation) – – • Interact with water and other polar molecules in a rotating electromagnetic field to cause dielectric heating Useful in curing adhesives (e.g. Parallam) and in drying wood Ionizing radiation (gamma, x-ray, electron beam, UV) – – – – Interacts with oxygen, unsaturated carbon bonds, and aromatic rings Can penetrate into materials Generates free radicals that can degrade natural materials UV exposure although not high enough energy to penetrate deep into natural materials will degrade the surface causing oxidation and degradation of the material surface. Coatings, paint, and other additives can be used to preserve the surface. Source: http://mc2.gulf-pixels.com/wp-content/uploads/2009/07/Electromagnetic-Spectrum2.jpg Radiation and Natural Materials • Ionizing radiation interacts with matter • Intensity interacts with a material according to the relationship – I=I0exp(-mx) where I is the intensity in the material as a function of and I0 is the incident, and m is the linear absorption co-efficient of the material, which depends on the material and the type of radiation. Values for of m for wood depend on density, such as 0.065 to 0.11 cm-1 for poplar over its density range for g radiation and around 3.0 cm-1 for b radiation (Wood Handbook). Summary • Natural materials have a number of appealing physical properties that make it very desirable for use in composites, such as low density to strength, insulating, reactive surface, etc. • Care still is needed when using natural materials so that they do not degrade. This requires protection from moisture, thermal degradation, and UV degradation.