Definition1

Flexural strength
- bending str

Impact strength
- ability to resis suddenly applied load and is measures in food-founds

Impact test
- test for toughness

Elongation-to-break
- when stress is given, sample is simply change in shape.
- elongation = [length (L) / original length (Lo) ] x 100%

Young modulus
- ratio of stress / strain

Modulus
- material ability to resist deformation

Toughness
- energy can absorb before deform
- area below the graph of stress/ strain

Strength
- load / cross section area
- unit: N/mm^2 or MPa

Stiffness
- strength / strain
- unit: N/mm^2 or MPa

Strain
- percentage of wood elongation

Fiber Flexibility vs Paper Flexibility

There is not direct relation between fiber flexibility and paper flexibility
fiber flexibility ↑ , inter-fiber bonding ↑, density of paper ↑, rigidity ↑, paper porosity ↓.
- Flexibility of fibers only affect the density, porosity and rigidity of paper, but not the flexibility of papers.

How to increase the flexibility of fibers?
- beating process.
- beating process will cause internal fibrillation (rupture of s2 microfibril layer, flexibility ↑) and external fibrillation (peeling effect on cell wall, ↑ surface area, ↑ bonding area)

Paper Tear Strength

Paper tear strength depends on the
(1) no of bonding of fibers,
(2) the length of fibers, and the
(3) no of fibers in the paper (density).


Q1 Why fibers easy to break after beating process?
ans:
1- this is due to shorter fiber will lower the fiber strength
2- the destruction of fibers become weak after beating
3- low in energy transfer cause fibers to break


Q2 Slightly beating helps to increase paper tear strength, why?
ans:
1- there is no rupture of fibers occurred in slightly beating process, which is, the length of fibers not really affected. Slightly beating process will cause internal and external fibrillation which will increase the bonding between fibers and the flexibility of fibers.
2- when flexibility ↑, bonding ↑, no change in fiber str, shear str ↑, therefore the tear str also ↑


Q3 Why fine beating will lower the tear strength?
ans:
1- fine beating will ↓ the length of fibers. length of fibers ↓ will cause the strength of fibers ↓. Strong bonding strength will not break easily, but weak fiber str wil cause the rupture of fibers.
force that need to break short fibers are lower than the force that need to break fiber-bond.
2- fiber length ↓, fiber strength ↓, fiber bonding ↑↑, force only concentrated on one spot, energy transfer ↓, fiber easy break, tear str ↓

Effect of Hysteresis on Paper

what is hysteresis?

Hysteresis represent the history dependence of physical systems. If you push on something, it will yield: when you release, does it spring back completely? If it doesn't, it is exhibiting hysteresis, in some broad sense.

same like paper, when the moisture equilibrium achieve, by increasing/decreasing the Rh (Relative Humidity) will follow the different trend.
refer to the graph shown below;
it means the rate of swelling from dry to wet, and wet to dry is different
this is cause by the effect of hysteresis.

for wet to dry, it will follow the upper line swelling rate,
and for dry to wet, lower line is used.

iwk205 - Chemical Additives and Paper Properties
lecturer: Dr Leh

Fiber Composite

what is composite?
- composite is a material that is made from several different substances.
for examples, plastic-wood composite is a material that made from plastic and wood.
in between this 2 component there are a region that we call interface region.

what is interface region?
- interface region is a region that stay in between two different component in one composite
- interface region is important to determine the performance of a composite
- interface region is responsible to transfer stress from one component to another.
- in another words, good interface region properties will increase the stress transfer efficiency

properties of interface region can be improve/modify by adding coupling agents or undergoes surface modification (physically or chemically).

what is coupling agents?
- it is a chemical reagent with 2 characteristic.
- coupling agents normally also act as interface region

For example; The properties of wood is hydrophilic (like water) and the properties of plastic is hydrophobic (anti-water). To make a plastic-wood composite, both components cannot form strong bond. Therefore, coupling agents is added as a interface region and strongly hold these two component together.

iwk306 - Fiber and Lignocellulosic Composite
lecturer: Dr Tay