Effect of Surrounding Conditions on Paper Properties

8/12/2019 Effect of Surrounding Conditions on Paper Properties

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Effect of Surrounding Conditions

on Paper Properties

Standard Conditions of Measurement


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Strength and Surrounding Conditions

Strength of paper may be affected very significantlydue to variation of temperature and relativehumidity of atmosphere

Consequences :- Paper/board suitable at one place may

not serve the same purpose at another

place and may fail.

- Measurement of properties at some

specified temperature and RH conditions.


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Variation of moisture content in paper due to

temperature and RH of atmosphere.

Effect on fiber-fiber bonding due to moisture

content in paper


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Variation of moisture content is

due to hygroscopic nature of

cellulose fiber

25-65% RH

-1% increase per 10%

change in RH

RH>65%, faster increase

RH~100%,- Moisture ~ 27-30%


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Moisture Content vs Conditioning Time

Rate at which paper achieves moistureequilibrium depends upon :

- Type of fiber,

- Process conditions e.g. pulping, beating etc.

- degree of sizing,

- type and amount of coating,

- basis weight,

- type and amount of fillers.


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Effect of conditioning time on equilibrium moisture in 60 lbkraft paper. Conditioned from (A) 35-85% RH (B) 85-35% RH

Moisture content changes very rapidly at first and thenslows down.

Time to reach equilibrium moisture content- 15 minutes for lightweight papers

- 24-48 hours for paper board and heavily sized papers


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Measurement of Moisture Content

Most common is Drying Method.

Dry a weighed specimen in a circulating air ovenat 105C.

Moisture content =wt. loss on drying(gm)x100/original wt.(gm)

On a cold winter day, oven has low humidity. Thisresults in 0.51% higher moisture content than

on a hot, humid, summer day. Vacuum removes 0.51% more moisture than an

ordinary oven.


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Basis Weight Variation

Increased moisture content, due to enhanced

RH conditions, increases basis weight.

Normally, Moisture content ~ 6-7%

Change in basis weight varies from paper to

paper and depends upon

- Type of fibers and process conditions.- Type and amount of fillers


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RH, % -cellulose

and rag paper%

Average

Papers%

Groundwood

Papers%

20 -1.5 -2.5 -3.3

30 -1.0 -2.0 -2.7

40 -0.5 -1.0 -1.3

50 0 0 0

60 +0.5 +0.7 +1.2

65 +1.0 +1.5 +2.2

70 +1.7 +2.5 +3.7

75 +2.5 +4.0 +5.2

80 +3.5 +5.0 +7.2

Basis Weight Variation with Moisture Content

(w.r.to 50% RH Value)

1. Least variation in rag pulps due to lowest amount of hemicellulose

2. Highest variation in Groundwood due to highest amount of hemicellulose


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Consider a bond paper of 80 gsm.

At 50% RH, its moisture content will be 6%

On taking it to 80% RH (hot and humid environment

i.e. places near sea shores)

Increase in basis weight 5% i.e. 5% of 80 = 4 gsm

Or This bond paper will now have 84 gsm basis weight

Considering this aspect one can sell this paper at a higher

cost with around 6% less fiber whereas actually

this paper will not give any advantage of paper with

higher basis weight e.g. improved strength, opacity, stiffness

In fact, all these properties will deteriorate (on account of less

fiber content and deterioration in above props with higher

moisture content.)


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Effect on various strengthparameters due to variationin Relative humidity

Stretch - increases Burst- TS and stretch

foldTS and flexibility

Tearhigher fr. of fibers

come out after breakingfiber bonds


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Specifications for Measurement

BIS RH = 652%, Temp.=272C

Sample conditioning

- RH=25-35%,Temp.=40C,Time=24hrs

Condition at BIS specified conditions

- Time ~ 4 hours (normal paper)

- Time ~ 24 hours (hard sized papers)

Difference between successive weighing

< 0.25% measured at an interval of 1 hour


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Ex:

For a sheet (22x30 cm) of 70 gsm

Weight of sheet = 4.6 gm 0.25% of weight of sheet = 0.01gm

Weight 4.64.61 gm (indication of

conditioned sheet)


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After conditioning :

Paper should not be kept near

- Window (avoid sunlight)

- Radiator (avoid heat)

- Open space during rainy season

Not touched by hands (to avoid grease and moisture

Avoid water marks, edges, fold, crease, torn part


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Tissue and Towelling

Tissue products e.g. toilet tissue and towels

are sold in roll form (in some cases with

perforations provided for separating small

pieces from roll).

Term sheet has no relevance in these cases.

Important is the Roll from which portion of

material for testing is being taken.


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Sampling

Tissues have high level of creping, to improvepliability and compressibility.

They get stretched easily when test pieces are

prepared which in turn affects the properties oftissue.

When preparing test pieces from rolls, theproduct be unrolled while supported by laying it

on a lab bench. Pulling sheets vertically from a suspended roll

must be avoided to minimize stretching.


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Conditioning (curing)

Tissue products have wet strength, which in turn isprovided by adding wet strength chemicals in pulpfurnish.

Wet strength is achieved after these chemicals aregiven sufficient time for ageing/curing which could varyfrom few minutes to several weeks for different wetstrength chemicals.

Recently manufactured tissue products, therefore, be

cured before they are tested for different properties.

Measured properties will be indicative of thoseexperienced while the product is in use.


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Curing is done by exposing the sample at highertemperature for a specified duration.

It is simply a process by which paper manufacture

determine the wet strength performance ofpaper several days/weeks before the paper isused.

(paper maker would like to determine the

possible performance of the product severalweeks after the manufacture, by measurement

just after paper has been made)


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Generally for curing the conditions are :

SCAN-57

Temperature 80C, Time 30 min or 24 hour European Std. EN 12625-5

- Temp. 80C, Time 30 min or

- Temp. 105, Time 15 min (for productioninspection to enable correction, if required)


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Important Consideration :

To determine the properties of tissue product,

as experienced by the end user on product

purchased from store, test the sample without

curing (The store sample is likely to have been

manufactured at-least few weeks earlier so no

need for curing)


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Ageing Characteristics

Determines whether a paper sample will age

more rapidly as compared to others.

Important for record papers, ledgers, map etc. In

general any paper which is meant to keepdata/records for long duration.

The effect of ageing is due to breaking of fiber-

fiber bonds due to gradual drying of sample in airwith time. This results in deterioration of strength

of paper. Ultimately paper becomes brittle.


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The aging characteristics of paper is determinedby finding decrease in double fold value of asample when it is given aging treatment.

The aging process is simulated by heating papersample at a temperature varying between 60125C.

Change of temperature by 20C alters rate ofdeteroration by a factor of 7.5. Accelarated aging

temp. must be rigorously controlled. In addition, the paper moisture content must be

held constant during accelerated aging.


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Effect of Surrounding Conditions on Paper Properties

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