MWV 5460 TB2004 2 Moisture - Glatfelter

1 Technical BulletinPaperMoisture and Relative HumidityIssue 2005 2 The major component of paper, cellulose fiber, ishygroscopic in nature. A material that is hygroscopicwill absorb Moisture from a damp atmosphere orrelease Moisture to a dry atmosphere and, after sufficient exposure time, will come into equilibrium(balance) with its atmospheric paper encountered in normal situations containssome Moisture since it is composed of cellulosefibers. This normal Moisture content will generallyrange between 2% and 10% depending on the typeof paper, its past Moisture history and the atmos-pheric conditions to which it is exposed. Whensubjected to extreme conditions the Moisture contentof a paper may range from as low as to as highas 13%. Many basic properties of paper are greatlyaffected by its Moisture content. Paper dimensions,flatness, conductivity, strength and fold are amongthe more significant properties influenced by its Moisture content.

2 These paper properties can bevery critical to the proper performance of paper andtherefore, effective control of paper Moisture level and Moisture uniformity is essential for efficient printingand converting Moisture CONTENT AND RELATIVEHUMIDITY: DEFINITIONSM oisture contentof paper is the percentage ofthe total paper weight, which is of paper is determined by weighing a papersample and, after oven drying, weighing it again. The difference in the two weights is calculated andexpressed as a percentage of the original weight. It is the level of Moisture in a particular paper that influ-ences stability of the critical properties of dimensions,flatness, conductivity and strength of that , since it is hygroscopic, will exchange moisturewith its environment until equilibrium with that envi-ronment is achieved. The Moisture level of paper isroutinely referred to in relation to the Moisture level of the environment with which the paper is in equilib-rium (Figure 1).

3 The Moisture level of an environmentcan be measured in two ways: absolute humidity andrelative humidity (RH).Absolute humidityis a measure of the number ofgrains of water per pound of dry air(7,000 grainsequals one pound). Absolute humidity is independentof air humidity (RH)is a measure of the amountof water in the air, at a specific air temperature,expressed as a percentage of the maximumamount of water the air can hold at that 1. Typical relationship between the Moisture content of cellulose-based paper and ambient relative humidity % Relative humidity of AirLowHigh% Paper MoistureHighLowTechnical Bulletin2As the temperature of the air increases, its ability tohold Moisture increases. Air, when heated, will retainthe same absolute humidity , but its relative humiditywill be lowered (Figure 2). Conversely, as the temper-ature of air decreases, its ability to hold Moisture isreduced. Air when cooled therefore, also will retainthe same absolute humidity , but its relative humiditywill be relative humidity of the paper s environment is the primary atmospheric factor that affects thepaper s Moisture content.

4 When the relative humidityof the surrounding air is high, hygroscopic materialssuch as paper gain Moisture from it. Paper, exposedto conditions of low relative humidity , will lose mois-ture to the air. Therefore, it is the relative humidity ofsurrounding air that controls the actual moisturecontent of hygroscopic materials such as paper. It isalso the relative humidity of air surrounding exposedpaper which affects those properties of paper thatvary as its Moisture content Pointis the temperature at which Moisture in the air will condense on to and tables are available for the conversion ofrelative humidity to absolute humidity and absolutehumidity to relative humidity . The absolute humidityvalue is sometimes needed for various calculationsand measurements. However, it is not of great impor-tance in its influence on paper Moisture RELATIVE HUMIDITYR elative humidity , since it is the major factor that influ-ences the Moisture content of exposed paper, is theenvironmental Moisture level measurement used inreference to paper Moisture level.

5 Paper, therefore, iscommonly referred to as having a relative humidity (RH).Paper relative humidity (paper RH)is defined asthe relative humidity of the air with which that paperis in Moisture equilibrium(Figure 1).In other words,the relative humidity of an environment in which thepaper will neither give off nor take on ideal relative humidity of paper as supplied fromthe paper manufacturer has often been debated. Therelative humidity of pressrooms and binderies in theUnited States has a known range each year from aslow as about 10% to as high as 90%. This fact,together with the differences in average relativehumidity in various geographic areas, is largelyresponsible for the difference of opinion regardingpaper relative 2. Relationship between relativehumidity versus temperature for a fixed absolute humidityFigure 3. The hysteresis effect and its impact on paper Moisture as the relative humidity changesTemperature, FRelative humidity 8070605040302060657075808590 Relative humidity vs.

6 Temperaturewhen RH = 50% @ 70 F% Relative humidity of AirLowHigh% Paper MoistureHighLowENDSTART Glatfelter 2005 Paper manufacturers cannot produce papers with RHlevels that will allow a given paper to function ideallyunder all conditions. Therefore, the RH produced bythe papermaker will be a compromise, selected toaccommodate the majority of all environments inwhich the paper will be processed. Additional factorsinfluencing the RH selected by the paper manufac-turer for a particular paper are characteristics desiredin the finished product and limitations that exist withinthe papermaking papers cannot be manufactured consistentlyat Moisture levels which would generate values above50% RH in the paper. Appearance characteristics aresignificantly deteriorated at elevated Moisture high level of Moisture in paper during certainprocessing operations causes both brightness andopacity to decrease and shade to vary.

7 Appearancecharacteristics are more easilycontrolled and main-tained during these processing operations whenpaper RH is approximately 40%.The desired characteristics of the finished productand technological limitations of the papermakingprocess result in practical paper Moisture levelsallowing paper to be produced with relative humidityranging from about 25% to 50%. It has been deter-mined that a Moisture level generating an RH rangeof 38% to 45% is generally satisfactory for uncoatedpapers and that a slightly higher range, 40% to 45%,is satisfactory for coated paper which is produced insheets. Coated papers produced for use in heatsetprinting operations are commonly manufactured to have an RH of approximately 30% to aid in theprevention of blister problems during the processingof the paper on such , if paper is exposed to an environmentwhich is significantly above or below the RH of themanufactured product, problems may occur depending on the time and conditions of exposure,requirements of the particular job, PAPER Moisture CONTENT DEPENDSON PAPER CONSTRUCTIONP aper should be in Moisture equilibrium with anatmosphere that has a relative humidity of 40-50% at 72 degrees Fahrenheit.

8 However, the actual mois-ture content of various papers will differ under theseconditions depending on the individual constructionof each paper. Similar papers may be encounteredwhich, when in equilibrium with the same relativehumidity environment, have a considerable differencein their Moisture content. The actual Moisture presentin a particular paper (at a specified RH) is determinedby the following: fiber (type, purity and quantity), filler(type and quantity), adhesives/binders (quantity andnature), manufacturing techniques (degree of fibertreatment), and the past Moisture history of theparticular paper (Figure 3).Paper is routinely manufactured to a Moisture contentspecification since the measurement of moisturecontent is more accurate, reliable, and convenient asa means of production control. However, the relation-ship between Moisture content and relative humidityof the particular paper must be established in orderto produce the finished product at the proper relativehumidity.

9 This relationship is determined by allowingthe paper to reach equilibrium in a series of differentrelative humidity environments and, in each situation,measuring the Moisture content of the paper afterequilibrium has been established. Graphs producedusing the data points obtained are referred to asmoisture equilibrium curves. Once the proper mois-ture equilibrium curve has been developed for aparticular paper, that paper can be manufactured tobe in equilibrium with a specified relative humidity bycontrolling the Moisture content of the final paper manufacturers have automatic onlinemonitoring/controlling devices to ensure proper anduniform Moisture content as their products areproduced on paper machines and coating paper is processed in areas that are monitoredand/or controlled and then packaged using materialsthat protect the product from its environment duringshipping and warehousing. Paper produced today is3 Technical Bulletin4more consistent and uniform with respect to moisturecontent than ever before in the history of papermanufacturing.

10 When properly packaged and stored,paper will retain a stable Moisture content for wellover two SHOULD BE TAKEN TO PROTECTPAPER Moisture CONTENTC ertain routine precautions should be taken to mini-mize the opportunity for paper to exchange moisturewith an environment where the relative humidity of thepaper and the environment may differ thus reducingthe possibility of problems related to paper Moisture :1. Paper removed from a truck, rail car, or an uncon-ditioned warehouse should be allowed to reachthe temperature of the area where it is to beprocessed before being Paper should not be stored near any type of heatingor cooling unit, in an air stream of heated or cooledair, or in the path of a humidifying stream of Paper should remain in wrappers until immediatelyprior to being processed (this applies to rolls,skids, cartons and reams).4. Paper being held between processing stepsshould be protected by vapor-proof wrapperswhile awaiting additional processing heatset printing the oven-dried paper can be left unwrapped for several days, allowing thepaper to regain lost PAPER PROPERTIES ARE ALTEREDAS Moisture CONTENT OF PAPER CHANGESThe most significant paper properties affected bychange in Moisture level or the development of anon-uniform Moisture level within the paper, as stated earlier, are: dimensions, flatness, conductivity,strength and AND PAPER DIMENSIONSP aper dimensions increase with higher moisturecontent and decrease with lower Moisture cellulose fibers will change in dimension as their Moisture content changes.