Presentation on theme: "Chapter 8: Physical Properties of Renewable Composites"— Presentation transcript:
1Chapter 8: Physical Properties of Renewable Composites
2Moisture 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.
3Relationship Between Temperature and Moisture These values reflect the average expected moisture content for wood given a temperature and relative humidity. To reach equilibrium, it may take several days to weeks at these conditions depending on the size of the member.Source: Wood Handbook
4Moisture 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 byBarriers and coatings that limit moisture sorption such as paint and laminatesAdding more resin to composites to resist swellingAdding waxes to coat fibers and make them more hydrophobicChemical modification of wood polymers, such as acetylation, to replace hydrophilic hydroxyl groups with more hydrophobic chemical groups.Source: Wood Handbook
5Composite 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 European Journal of Wood Products. 67:
6Relationship Between Temperature and Moisture in Composites Moisture relationships can be easily modeled in WPCsFickian behaviorWater diffusion (Arrhenius)
7DensityDensity 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 densityStraw has a low bulk density of approximately 134 kg/m3.Cellulose fiber by itself may have a density of 1,500 kg/m3Handling and dispersing fibers evenly in a matrix and transporting them becomes an economic problem.Acoustic properties are related to density.
8FrictionFriction 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.
9Thermal Properties Material a1 (10-6 m/m)/K a2 Thermal conductivity (W/mK)Specific Heat(kJ/kg K)Graphite/epoxy0.8831.0NAE-glass/epoxy6.320epoxy550.351020 steel12430.49Concrete220.127.116.11Wood (oak)18.104.22.168Polyethylene2000.51.67Wood is very dimensionally stable over a wide range of temperatures. This holds true for most natural fibers compared to other materials. Natural materials also have good insulating properties as can be observed from the low thermal conductivity and high specific heat.Source: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.
10Thermal 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.
11Electrical 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
12Electromagnetic 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 heatingUseful in curing adhesives (e.g. Parallam) and in drying woodIonizing radiation (gamma, x-ray, electron beam, UV)Interacts with oxygen, unsaturated carbon bonds, and aromatic ringsCan penetrate into materialsGenerates free radicals that can degrade natural materialsUV 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:
13Radiation and Natural Materials Ionizing radiation interacts with matterIntensity interacts with a material according to the relationshipI=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 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).
14SummaryNatural 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.