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Paper physics > Introduction and aims > Flashcards

Introduction and aims Flashcards

1
Q

What do paper properties determine

A

converting properties

Quality and functionality

Price and grade differentiation

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2
Q

Why do we need paper testing?

A

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

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3
Q

What does paper testing help you control

A

Process control and quality control

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4
Q

What do paper properties depend on

A

The stock - fibre type and additives

Process - stock preparation, formation, pressing and drying

Converting and finishing - calendaring, coating and printing

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5
Q

Important paper properties(not just strength!)

A

Weight and thickness

Strength (tensile, burst, tear, z-direction)

Structure ( Density and porosity)

Optical (brightness, opacity)

Surface (Roughness)

Stiffness

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6
Q

How can stock influence paper properties

A

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

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7
Q

How can the process influence paper properties

A

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

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8
Q

How can converting/ finishing influence paper properties

A

If there is a calender will make the paper smoother and denser.
Coating can change water absorption or strength

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9
Q

What properties do hardwood fibres contribute and what is the average length

A

Hardwoods are shorter than softwoods. with an average length of 1-1.5mm

Small fibres increase smoothness and printability

(Birch, eucalyptus, beech)

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10
Q

why is fibre length important

A

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

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11
Q

What benefits do long fibres bring in pulp mix

A

Increase tensile strength + stiffness, tear & folding endurance

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12
Q

What benefits do short fibres bring in pulp mix

A

Contribute to smoother surface, therefore printability. They also increase opacity by scattering more light

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13
Q

What is fibrillation?

A

the partial breakdown of fibre in order to expose fibrils from the S2 layer. It increases surface area and exposes more bonding sites

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14
Q

How can fibrillation be measured?

A

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

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15
Q

Define hygroscopic

A

substances that easily absorb water from the air

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16
Q

Why is paper being hygroscopic important to note

A

Because it can absorb water from the air, the presence of more water will impact properties

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17
Q

How does moisture content affect properties

A

Increased moisture content will decrease strength properties.

Extra water will compete for hydrogen bonding sites, thus reducing bonds making paper weaker

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18
Q

What is a hydrogen bond

A

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

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19
Q

Why do we need to standardise testing

A

Need consistent conditions so that the moisture content does not impact the results. Needs to be comparable

(23C 50%)

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20
Q

When is the paper conditioned?

A

When it has reached equilibrium in those conditions - this means no moisture is gained or lost.

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21
Q

What does increasing temperature do to relative humidity?

A

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

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22
Q

Why do we need to condition paper before testing?

A

Because it is hygroscopic - meaning it will absorb water so the rel humidity needs to be kept constant

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23
Q

What is the hysteresis effect?

A

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

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24
Q

Why does the hysteresis effect occur?

A

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

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25
Q

How can you differentiate between cross and machine direction?

A

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

26
Q

Which properties higher in CD

A

Stretch and tear as random alignment can disspate force

27
Q

Which properties higher in MD

A

Tensile strength

Stiffness

Fibres are longer and aligned thus more load bearing

28
Q

How can you differentiate top side vs wire side

A

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

29
Q

What are considered basic properties? (6)

A

Moisture content

Thickness (micrometers)

Grammage (GSM) (Mass/Area)

Density (grammage/thickness)

Bulk (1/density) thickness relative to mass

Filler content

30
Q

What is density?

A

it is mass in a given volume

grammage/thickness

Describes how tightly packed material is

31
Q

What is bulk?

A

How much space paper takes relative to its weight - high bulk means paper is thicker for the same weight. Low bulk = more compact

32
Q

How are density and bulk related?

A

Low bulk means paper has low thickness compared to weight - it will have high density as material is compacted

33
Q

How do you measure moisture content?

A

Initial - final/ initial x100

Weigh - dry - weigh at 105C until constant weight

34
Q

what is the difference between ash content as 525C and 900C

A

Calcium carbonate decomposes at 600C so doing both methods will allow to work out what % of inorganics is calcium carbonate

35
Q

What parameters can affect paper strength?

A

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

36
Q

Which bonds occur in paper?

A

Covalent bonds

Hydrogen bonds

vander waals

37
Q

Which bonds contribute most to strength?

A

When wet atomic bonds

When dry hydrogen

38
Q

List some strength properties

A

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

39
Q

Which direction do you have higher tensile and why?

A

highest in MD

Fibres are aligned and distribute load efficiently. work together rather than randomly orientated CD

40
Q

Which direction do you have higher tear and why?

A

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

41
Q

What do you need to calculate the e modulus and how is it done?

A

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

42
Q

What is the e modulus

A

A measure of how a material deforms when force is applied. A low e modulus means flexible and and springy

43
Q

What does high e modulus mean?

A

Stiff little stretch when deforming

44
Q

What is tensile index?

A

It relates tensile strength to the weight of the sample. Helps you directly compare sheets

45
Q

What is breaking length

A

Length of paper strip which can hold its own weight before rupture

46
Q

What is tear strength?

A

The mean force required to continure tearing along an initial cut. It is energy absorption via pendulum motion (higher in CD)

47
Q

What is tear affected by

A

Fibre length, fibre strength, degree of bonding

48
Q

Why is tear not realistic measurement?

A

On the PM breaks dont have an initial cut

49
Q

What are the main influences on tensile strength?

A

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

50
Q

What are the main influences on tear strength?

A

Fibre alignment/orientation (higher CD)
Fibre length/strength
Degree of refining and bonding

51
Q

What is burst strength?

A

Maximum pressure paper can resist before breaking
Measured in KPa
Common - Mullen tester
Relates somehow to tensile and elongation

52
Q

What is z direction strength?

A

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

53
Q

What is the test for z direction?

A

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

54
Q

What is folding endurance

A

papers ability to resist breaking when folded under a certain load. Expressed in number of double folds it can withstand

55
Q

What is surface strength?

A

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)

56
Q

What is the picking test?

A

Test for surface strength.
Tacky oil continuously applied at increasing speed - speed which picking occurs is measured

57
Q

Which direction do you have higher tensile strength?

A

higher in MD
fibres are orientated together/aligned therefore they can distribute the load evenly. Also as they are aligned hydrogen bonds formed

58
Q

In which direction do you have higher tear strength?

A

In CD
Going against the grain of the fibres - going across the fibres. Not inbetween them as with MD

59
Q

What direction do you have higher elongation?

A

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

60
Q

What is the most important factor when wanting to increase tear strength?

A

Fibre length. Increase in fibre length increases the mechanical interaction

61
Q

How do you calculate e modulus?

A

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)

62
Q
A

Paper physics (5 decks)

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