Introduction and aims Flashcards
What do paper properties determine
converting properties
Quality and functionality
Price and grade differentiation
Why do we need paper testing?
Get a description of paper properties.
Important as they determine:
Converting properties
Quality + functionality
Price + grade differentiation
Testing helps you achieve quality and process control
What does paper testing help you control
Process control and quality control
What do paper properties depend on
The stock - fibre type and additives
Process - stock preparation, formation, pressing and drying
Converting and finishing - calendaring, coating and printing
Important paper properties(not just strength!)
Weight and thickness
Strength (tensile, burst, tear, z-direction)
Structure ( Density and porosity)
Optical (brightness, opacity)
Surface (Roughness)
Stiffness
How can stock influence paper properties
Pulp type - longer fibres have higher tear and tensile strength. Shorter fibres increase opacity and smoothness
How pulp was created sgw weaker than chemical and recycled fibre also have varying properties
Additionally the additives can make a big difference - cross linkers, brightening agents
How can the process influence paper properties
the stock preparation, eg if there is a refiner
sheet formation - dilution and shear in headbox which influence flocs
drying - too rapidly can compromise bonding
How can converting/ finishing influence paper properties
If there is a calender will make the paper smoother and denser.
Coating can change water absorption or strength
What properties do hardwood fibres contribute and what is the average length
Hardwoods are shorter than softwoods. with an average length of 1-1.5mm
Small fibres increase smoothness and printability
(Birch, eucalyptus, beech)
why is fibre length important
Longer fibres contribute to tensile strength and stiffness, they also increase tear and fold endurance
Whereas shorter fibres contribute to smoother surface therefore printability. They also increase opacity
What benefits do long fibres bring in pulp mix
Increase tensile strength + stiffness, tear & folding endurance
What benefits do short fibres bring in pulp mix
Contribute to smoother surface, therefore printability. They also increase opacity by scattering more light
What is fibrillation?
the partial breakdown of fibre in order to expose fibrils from the S2 layer. It increases surface area and exposes more bonding sites
How can fibrillation be measured?
Drainage measurements such as schopper reigler or standard canadian freeness.
This is because increased fibrillation will decrease drainage.
You can also measure opacity - as fines and fibrils will be created which increase light scattering
Define hygroscopic
substances that easily absorb water from the air
Why is paper being hygroscopic important to note
Because it can absorb water from the air, the presence of more water will impact properties
How does moisture content affect properties
Increased moisture content will decrease strength properties.
Extra water will compete for hydrogen bonding sites, thus reducing bonds making paper weaker
What is a hydrogen bond
It is when a hydrogen is bound to an electronegative atom (Like oxygen in the hydroxyl groups of cellulose) this creates a partial positive on the hydrogen allowing it to bond to neighboring oxygen atoms
Why do we need to standardise testing
Need consistent conditions so that the moisture content does not impact the results. Needs to be comparable
(23C 50%)
When is the paper conditioned?
When it has reached equilibrium in those conditions - this means no moisture is gained or lost.
What does increasing temperature do to relative humidity?
Also increases it. Rel.Humidity is measure of how much moisture the air can hold without condensing it can hold more with more energy so increases with temp
Why do we need to condition paper before testing?
Because it is hygroscopic - meaning it will absorb water so the rel humidity needs to be kept constant
What is the hysteresis effect?
Moisture content depends on whether paper is conditioned from the dry side of equilibrium or the wet one. One starting from dry could have 1% lower moisture content. So could show as having higher strength results
Why does the hysteresis effect occur?
Cellulose chains are linked via hydrogen bonds, when the paper is dried hydroxyl groups are freed from water and rearrange themselves in a different geometry. When water is reintroduced these bonds need to first be broken and this costs energy thus making it harder to absorb water
How can you differentiate between cross and machine direction?
Visual - fibres aligned in machine direction due to efflux ratio
Bending stiffness higher MD bc more long fibres aligned
Tensile strength higher MD can distribute load
Tensile stretch and tear higher in CD as random orientation disspate force rather than tearing with grain
Which properties higher in CD
Stretch and tear as random alignment can disspate force
Which properties higher in MD
Tensile strength
Stiffness
Fibres are longer and aligned thus more load bearing
How can you differentiate top side vs wire side
Wire marks using oblique lighting,** wireside is roughest **
how the paper curls - more fibres on the bottom, fillers dont shrink so more shrinkage on wire side causing paper to curl inwards
What are considered basic properties? (6)
Moisture content
Thickness (micrometers)
Grammage (GSM) (Mass/Area)
Density (grammage/thickness)
Bulk (1/density) thickness relative to mass
Filler content
What is density?
it is mass in a given volume
grammage/thickness
Describes how tightly packed material is
What is bulk?
How much space paper takes relative to its weight - high bulk means paper is thicker for the same weight. Low bulk = more compact
How are density and bulk related?
Low bulk means paper has low thickness compared to weight - it will have high density as material is compacted
How do you measure moisture content?
Initial - final/ initial x100
Weigh - dry - weigh at 105C until constant weight
what is the difference between ash content as 525C and 900C
Calcium carbonate decomposes at 600C so doing both methods will allow to work out what % of inorganics is calcium carbonate
What parameters can affect paper strength?
Raw material - length, recycled, how created (SGW or chemical)
Chemicals - cat starch or filler
Stock prep - refiner, mixing
Process - headbox dilution, drying
environment - humidity
Surface properties
Which bonds occur in paper?
Covalent bonds
Hydrogen bonds
vander waals
Which bonds contribute most to strength?
When wet atomic bonds
When dry hydrogen
List some strength properties
Tensile - force per width until breaking
Zero span tensile - individual fibre strength
tear - force to continue initial cut
burst - pressure until rupture
z direction - ply bond strength
folding endurance
Picking strength
Which direction do you have higher tensile and why?
highest in MD
Fibres are aligned and distribute load efficiently. work together rather than randomly orientated CD
Which direction do you have higher tear and why?
Tear is highest in cross direction. As the fibres are randomly orientated, when tearing in machine direction it is easy to follow the ‘grain of the fibres’ tearing in cross direction means tearing against it which will dissipate the force
What do you need to calculate the e modulus and how is it done?
Need a tensile graph from this stress (force/area) and strain (change in length/original). Dividing these numbers will give the e modulus.
The numbers need to be taken from the linear part of the graph
What is the e modulus
A measure of how a material deforms when force is applied. A low e modulus means flexible and and springy
What does high e modulus mean?
Stiff little stretch when deforming
What is tensile index?
It relates tensile strength to the weight of the sample. Helps you directly compare sheets
What is breaking length
Length of paper strip which can hold its own weight before rupture
What is tear strength?
The mean force required to continure tearing along an initial cut. It is energy absorption via pendulum motion (higher in CD)
What is tear affected by
Fibre length, fibre strength, degree of bonding
Why is tear not realistic measurement?
On the PM breaks dont have an initial cut
What are the main influences on tensile strength?
Fibre alignment/orientation (Higher MD)
Fibre length and strength
Degree of bonding
Rate of loading - faster = higher results as paper has less time deforming
Moisture content
What are the main influences on tear strength?
Fibre alignment/orientation (higher CD)
Fibre length/strength
Degree of refining and bonding
What is burst strength?
Maximum pressure paper can resist before breaking
Measured in KPa
Common - Mullen tester
Relates somehow to tensile and elongation
What is z direction strength?
The papers ability to resist tensile loading in a direction vertical to the paper plane. Measure of strength between layers of paper.
Common is scott bond
What is the test for z direction?
The scott bond tester
Double sided tape on instrument - sample on top - more double tape - metal L plates - pressure - L plate to sample holder - knocked swinging hammer - force recorded
What is folding endurance
papers ability to resist breaking when folded under a certain load. Expressed in number of double folds it can withstand
What is surface strength?
Resistance to fibres being pulled from the surface.
Important as this can cause runnability and printability issues
Picking method used to test this (tacky oil applied to paper at increasing speed, speed at which picking occurs is recorded)
What is the picking test?
Test for surface strength.
Tacky oil continuously applied at increasing speed - speed which picking occurs is measured
Which direction do you have higher tensile strength?
higher in MD
fibres are orientated together/aligned therefore they can distribute the load evenly. Also as they are aligned hydrogen bonds formed
In which direction do you have higher tear strength?
In CD
Going against the grain of the fibres - going across the fibres. Not inbetween them as with MD
What direction do you have higher elongation?
In CD
There are more gaps within the mat these gaps help with the elongation.
In cross direction you have a lot of fibres laying across each other it is this mechanical linking
What is the most important factor when wanting to increase tear strength?
Fibre length. Increase in fibre length increases the mechanical interaction
How do you calculate e modulus?
From a tensile graph using only the linear region - as this is the elastic region
Divide stress over strain
in other words
(∆Force/Area)
/
(∆length/ original length)
