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Tissue creping training

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  • Hi.
  • 1. Increase sheet bulk: debonding the sheet increases its bulk. 2. Increase sheet softness: sheet softness is a function of of bulk, limpness, and smoothness. Creping increases bulk and makes the sheet more limp (weaker) because of fiber debonding). 3. Increase sheet absorbency: debonding the sheet opens up the sheet structure, making it easier for water to enter the sheet. 4. Increase sheet stretch: debonding (up to a point) increases stretch by giving the sheet more “give”. However, too much debonding will decrease stretch because stretch is measured as the amount of sheet elongation at sheet failure. The sheet will fail sooner if it is too weak.
  • Percent crepe is the oldest method of measuring sheet stretch. It is still used to give an approximation of what is going on at the creping blade. Percent crepe is also important because most tissue products are sold by area, not by weight. Therefore, the more a sheet is creped, the less final product will be produced per tonne of base paper.
  • 1. If the sheet is not held against the dryer by some adhesive force, it will release before it contacts the creping blade- you will be making “machine glaze” paper (wax paper, butcher wrap, burger wrap) instead of tissue. 2. The sheet still needs to be pulled off the dryer properly; otherwise it will tend to break at the creping blade. 3. Dry creped sheets (about 90% sheet solids) typically lose 50-70% of their non-creped strength. Wet creped sheets lose much less, but tend to shrink in the after dryer section and are much less soft. Wet creping is typically used for commercial and industrial towel grades.
  • 1. The Yankee coating prevents the creping and cleaning blades from coming into direct contact with the dryer surface, which will tend to damage the dryer. 2. The way the sheet is held against the dryer during creping is a major factor in how the sheet is creped. 3. Having a good, even coating on the Yankee helps produce an evenly built roll at the reel. Sheets that are unevenly creped tend to have soft and “tight” sections. The tight sections are much less bulky than the soft sections, creating low spots in the reel (corrugations). An extreme case of dryer streaking will cause carbon marking of the sheet.
  • Some information on the meaning of the various angles: 1. Cutting angle: Lower cutting angles are harder on the sheet; higher cutting angles are easier on the sheet. 2. Blade bevel angle: Beveling the creping blade reduces the “shelf” on the creping blade, producing a finer crepe. However, beveled blades are easier on the sheet and do not “work” it as much. Therefore, beveling the blade increases sheet strength and stretch, while reducing bulk and softness. 3. Blade angle: Lower blade angles produce a finer crepe; higher blade angles are more likely to cut the sheet and remove more coating from the dryer. 4. Take off angle: A take off angle that is too high will tend to pull the sheet off the Yankee too easily, interfering with good creping. One that is too high will tend to cause the creping blade to pick the sheet, causing holes and breaks.
  • 1. Cross linking: In this stage, carrier water is evaporated and the polymer will crosslink with itself, creating a continuous film on the dryer surface. 2. Glass transition: In this stage, the polymer film begins to melt, becoming tacky and flowable. The glass transition temperature of the particular polymer is important, as it will determine what type of polymer is suitable for a particular situation. For instance, crepe aids that are suitable for “through air dryers” are generally not suitable for Yankee dryers because they have a higher glass transition temperature and wouldn’t become tacky enough in conventional creping. 3. Rewetting: In this stage, the polymer film picks up water from the sheet, becoming hydrated (it was originally dehydrated during the cross linking phase). It is important for the polymer to be able to accept water from the sheet and have intimate contact with the sheet. 4. Setting: In this stage, the polymer dehydrates again, forming a strong film that holds the sheet against the dryer surface. Good contact with the dryer surface aids even, faster drying of the sheet and allows for good sheet creping. 5. Doctoring: This is where the coating is partially removed from the dryer surface by the doctor blade (and cleaning blade, if one is used). 6. Cured: On this part of the dryer, only the hard, fully cured coating is present. This hard coating protects the dryer surface from damage.
  • 1. The type of metallized coating on the dryer impacts how easily a coating is formed. Cast iron and molybdenum (“moly”) coat easily, while stainless steel coats less readily. 2. The inorganic substrate helps the organic coating stick to the dryer surface. Usually the inorganic portion of the coating is provided by the hardness in the water (stock and spray boom). However, sometimes mills that have very soft water add MAP or TSP to promote a good inorganic layer on the Yankee. 3. The cured organic coating is what protects the Yankee surface from the creping and cleaning blades. 4. The soft organic coating is where the creping blade “rides”. It is removed every revolution of the Yankee, so it must be reapplied continuously. The soft organic coating is modified by the release oil that is fed in conjunction with the crepe aid. The release oil interferes with the integrity of this portion of the coating, keeping it softer than the cured portion.
  • 1. Furnish: hemicellulose is a compound that is found in wood. It is a natural adhesive, provided some level of coating on the Yankee. The more hemicellulose that is left in the stock, the heavier the natural coating on the dryer. Hemicellulose can be washed out of the stock: high yield, unwashed furnish has much more hemicellulose than lower yield, washed furnishes. Recycle fiber contains little hemicellulose. 2. Dryer surface: a. cast iron and moly coated dryers develop a coating more easily than a stainless steel coated dryer. b. A worn spot on the dryer will provide poor contact with the sheet, leaving it wetter- that part of the dryer will not develop a coating as easily as the rest of the surface. 3. Blades: a. The more heavily the cleaning blade is loaded, the more coating is removed each revolution of the dryer. Cleaning blades are generally used to even out the coating left on the dryer b. It is important to oscillate doctor and cleaning blades. Otherwise, they will tend to damage the dryer surface, leaving narrow, worn bands on the surface. c. Beveled crepe blades are gentler on the sheet than square blades. They are good for preserving sheet strength at the expense of bulk and softness development. 4. Dryer surface temperature: a portion of the dryer with poor condensate removal will be cooler than the rest of the dryer, and will develop a relatively poor coating. 5. Uneven moisture profiles are one of the most common causes of uneven coating development. It is very important to minimize moisture profile problems. 6. Too much water at the pressure roll nip makes it harder for the crepe aid to crosslink and form a good coating on the dryer.
  • This one should be pretty much self-explanatory.
  • 1. It is very important to avoid damaging the Yankee. Once the Yankee surface is damaged, it must be reground before the defect will go away. You may develop a permanent wet streak in the sheet by damaging the Yankee. 2. It is important to make salable paper. It is difficult to sell paper that is full of holes or has black streaks in it. 3. Of course, it is also important to make paper that conforms to specifications.
  • 1. If you can see yourself in the Yankee, you need more coating. 2. If you have an uneven coating on the Yankee, you have two basic choices: a. Have too much coating on the high spots, and the right amount of coating on the low spots. You will be prone to sheet holes and breaks. b. Have the right amount of coating on the high spots, and not enough on the low spots. You will be prone to sheet corrugation, carbon marking, and Yankee damage.
  • 1. Dry end holes generally exhibit one of the following two characteristics: a. The hole is very “clean” around the edges- there is virtually no fiber going into the hole from the edges. b. The hole has a flap on the trailing edge. If there are relatively long fibers going into the hole from the edges, the hole is from the wet end. Wet end holes also may exhibit a disruption of the sheet around the hole: a ring of fibers (wire deposit) or crushing (press section sheet disruption). 2. Carbon marks come from direct metal to metal contact between the creping blades and the dryer surface. You will probably smell “burning metal”- the odor is very distinctive.
  • 1. In general, to increase sheet strength: a. Improve fiber to fiber bonding (formation, fiber fibrillation, wet pressing) b. Improve sheet formation c. Minimize sheet debonding during creping d. Use a “stronger” furnish, especially well-refined softwood
  • 1. Increasing % crepe increases the stretchiness of paper. 2. Remember that stretch is measured at the point of failure- if the sheet is very weak, it will fail early and not provide a good stretch value. Reducing the intensity of creping will leave the sheet stronger, and may give the sheet a higher stretch value.
  • 1. In general, bulk is developed by preventing good fiber to fiber bonding. Therefore, do the opposite of what you would normally do to increase strength. However, you should still avoid calendering, as it compresses the sheet.
  • 1. Softness is a combination of “bulk softness” and sheet smoothness. Bulk softness is developed by maximizing bulk and minimizing sheet bonding. Sheet smoothness is developed by maximizing formation and calendering. Note that calendering will reduce bulk while improving smoothness, so it is usually not done, or is done lightly, when making consumer grade tissue (bath or facial tissue).
  • 1. Here are the three basic strategies. Among them, the felt-supported sheet is the least often seen in my experience).
  • 1. All you need is a couple of feed pumps to feed the products to the fan pump suction.
  • 1. An atomizing shower (expensive and prone to plugging) is common for this type of application.
  • 1. It is critical to have a good, double coverage spray bar to give even coverage across the dryer. 2. Use the smallest volume nozzles possible given the cleanliness of the fresh water used as carrier water. It is best to heat the shower water to around 60 o C. It is important to have a filter in line after the static mixer to catch gels that might be present .
  • This setup is generally preferable to using a run tank for the coating/release mixture. Run tanks are prone to deposition; they also introduce lag time when making feed rate adjustments.

Tissue creping training Tissue creping training Presentation Transcript

  • Tissue Machine Operator Training
  • Why Crepe Tissue?
    • Increase sheet bulk
      • Sheet thickness
    • Increase sheet softness
      • Bulk
      • Flexibility (limpness)
      • Extensibility (extension under a specific load)
      • Surface smoothness (roughness and coefficient of friction)
    • Increase sheet absorbency
    • Increase sheet stretch
      • Elongation at sheet failure
  • What is Percent Crepe?
    • Percent Crepe = Yankee Speed - Reel Speed X 100% Yankee Speed
    • Percent Crepe is one way of measuring the stretch(su căng ra) in tissue
  • What Happens During Creping?
    • The sheet is held against the dryer by the coating (crepe aid)
    • The sheet is pulled off the dryer by the reel
    • The sheet is “blown apart” (debonded) at the creping blade, losing >50% of its strength
    Doctor Blade Yankee Dryer Paper Soft Coating (Blue) Hard Coating (Green)
  • Why Apply Coating and Release?
    • Protect the Yankee dryer surface against damage from the creping (doctor) blade and cleaning blade
    • Hold the sheet against the dryer during creping- the coating acts as an adhesive (glue)
    • Help build a good roll at the reel
  • Creping Mechanics
    • A = Cutting angle
    • B = Blade bevel angle
    • C = Blade angle
    • D = Take off angle
    • R = Reel speed
    • Y = Yankee dryer speed
    • % Crepe = Y - R X 100% R
    Yankee Dryer Creping (Doctor) Blade A B C D Y R
  • Stages in Coating Formation Pressure roll Spray bar Creping Blade Yankee Dryer CROSSLINKING CURED GLASS TRANSITION REWETTING DOCTORING SETTING
  • Coating Layers Dryer Surface: Iron Dryer Surface: Metallized coating Inorganic Substrate Cured (Hard) Organic Coating Soft (Modified) Organic Coating Paper Sheet Creping Blade
  • Factors Influencing Creping
    • Furnish: Hemicellulose content
    • Water: Hardness/pH/chlorine
    • Wet end additives (especially wet strength and surfactants)
    • Dryer surface
      • Cast iron vs. metallized
      • Physical damage
    • Blades
      • Cleaning blade loading
      • Oscillation
      • Beveling
      • Uneven wear
    • Dryer surface temperature
      • Steam pressure
      • Uneven condensate removal
    • Moisture profile
      • Uneven sheet basis weight
      • Press section: uneven showering, filled/damaged press felts
      • Uneven crepe/release shower bar coverage
    • Moisture content
      • Sheet solids at pressure roll
      • Crepe/release spray bar water volume
  • Common Problems
    • Too much coating
    • Sheet picking at the creping blade (excessive dust and/or sheet holes)
    • Sheet can’t release from the dryer (sheet breaks)
    • Severe creping (low sheet strength and stretch)
    • Too little coating
    • Not enough creping (low softness and bulk)
    • Corrugated roll
    • Carbon marking of the sheet
    • Yankee dryer damage
  • How to Adjust Crepe & Release Aids
    • 1st priority: protect the Yankee dryer surface
    • 2nd priority: build a good roll at the reel
    • 3rd priority: attain sheet specifications
  • Protecting the Yankee Surface
    • Look for a hazy coating on the Yankee; the metal surface should not look like a mirror when a flashlight is shined on it
    • Increase the coating feed rate until a hazy coating is present
    • Make sure the coating is even across the dryer; if not:
      • Make sure the Yankee spray bar gives even coverage
      • Compare the basis weight profile with dryer streaks
      • Compare press felt streaks with dryer streaks
      • For narrow streaks (<1cm across): if they move, check needle showers; if not, the problem may be poor condensate removal
  • Building a Good Roll at the Reel
    • When increasing the creping aid (coating) or decreasing the release aid feed rate:
      • Look for (feel for) pin holes in the sheet
      • Make sure the holes are “dry end” holes
      • If you find “dry end” holes, there is too much coating on the dryer
    • When decreasing the creping aid (coating) or increasing the release aid feed rate:
      • Watch for roll corrugation, carbon marks, or sparks on the Yankee
      • Check for the odor of burning metal
      • If you find any of the above, there is not enough coating on the dryer
  • Attaining Sheet Specifications: Strength
    • Mechanical Options
    • Increase refining (brushing, not cutting)- up to a point
    • Increase wet pressing
    • Decrease calendering
    • Reduce headbox consistency (to improve sheet formation)
    • Use a beveled creping blade
    • Use more long fiber pulp
    • Chemical Options
    • Increase release aid feed rate
    • Decrease coating feed rate
  • Attaining Sheet Specifications: Stretch
    • Mechanical Options
    • Increase % crepe
    • Use a beveled creping blade
    • Chemical Options
    • Increase release aid feed rate
    • Decrease coating feed rate
  • Attaining Sheet Specifications: Bulk
    • Mechanical Options
    • Decrease refining
    • Decrease wet pressing
    • Decrease calendering
    • Use more short fiber pulp
    • Chemical Options
    • Decrease release aid feed rate
    • Increase coating feed rate
  • Attaining Sheet Specifications: Softness
    • Mechanical Options
    • Decrease refining
    • Decrease wet pressing
    • Decrease/increase calendering (depends if you need more bulk or smoothness)
    • Use more short fiber pulp
    • Decrease headbox consistency (to improve sheet formation)
    • Use a square creping blade
    • Chemical Options
    • Decrease release aid feed rate
    • Increase coating feed rate
    • Use a softening agent
  • Coating/Release Feed Strategies
    • Add coating/release to the wet end
    • Add coating/release to the felt-supported sheet
    • Add coating/release to the Yankee dryer
  • Wet End Addition
    • Advantages
    • Simple, low cost equipment setup
    • Product addition is fairly even across the sheet width (depends on basis weight profile)
    • Disadvantages
    • Product is not applied outside the sheet edges, leading to dubbing of the dryer
    • High feed rates (around 10 times as much as through a spray bar)
    • Longer lag time when adjusting coating/release ratio
  • Felt-Supported Sheet
    • Advantages
    • More cost effective than wet end addition
    • Minimal lag time when adjusting coating/release ratio
    • Independent of basis weight profile
    • Disadvantages
    • Product is not applied outside the sheet edges, leading to dubbing of the dryer
    • Requires a spray boom (more expensive equipment setup)
    • Possibility of uneven product application (worn/plugged nozzles or poor shower design)
  • Yankee Surface
    • Advantages
    • More cost effective than wet end addition
    • Treats the entire Yankee surface, not just the part that contacts the sheet
    • Minimal lag time when adjusting coating/release ratio
    • Independent of basis weight profile
    • Disadvantages
    • Requires a spray boom (more expensive equipment setup)
    • Possibility of uneven product application (worn/plugged nozzles or poor shower design)
  • Typical Yankee Application Setup Quick Disconnect For Pressure Equalization of Home Made Shower Bars- Not Needed for Professionally Engineered Shower Bars Quick Disconnect (or Flush Line) Double Coverage Shower Bar Static Mixer Fresh Water Source Crepe aid Release aid Filter
  • Creping Trial: Overall Trial Plan Initial feed rates need to be determined for each trial individually; double click on the chart at left to determine crepe aid flow
  • Protect the Yankee Surface
  • Build a Good Roll at the Reel- Coating/Release
  • Build a Good Roll: Coating/Modifier/Release
  • Attain Sheet Specifications
  • Yankee-Applied Softener Trial