STARCH SETBACK AND PAPERMAKER’S AMYLOSE

Enzymatic conversion of starch for size press application can offer fully acceptable and satisfactory results at an advantageous cost as pointed out in the previous discussion (Size Press Starches).

• The cost benefit derives from using unmodified starch compared to the cost of modified starches. 
• The performance benefit can be achieved by producing starch that is tailor-made for specific needs defined by grade or target property rather than forcing one starch to work across a wide spectrum of grades and objectives.  The ability to control the degree of conversion allows for variable starch chain length and solids to favor internal strength development or surface properties and total starch pick-up.

Despite these advantages, potential problems are possible, but can easily be avoided.  The most prominent issue is starch retrogradation that can result in either high viscosity gelled starch (setback) or papermaker’s amylose.

a)    Setback is simply the gelling of the cooked starch mass that occurs as straight chain amylose starch molecules begin to realign and form a stable gel structure. 

• This condition is readily avoidable by keeping the starch mass under gentle agitation and stored at a temperature of 60°-65° C (140°F -150°F).  Once the mass has begun to set back, it is not readily or totally reversible. 
• Setback usually occurs when the starch is allowed to cool excessively.  Thus, maintaining starch temperature at 60°C-65°C is desirable. 
  • Long un-insulated starch pipe runs, especially in cold environments can be a significant cause of starch setback.  Starch gel may build up in the pipes and eventually lead to pipe plugging. 
  • Before pipe plugging occurs, the protected environment below the starch gel can become a haven for significant bacterial activity that can significantly degrade the starch and cause corrosion of the pipe.

b)   Papermaker’s amylose, also referred to as RAPs (reformed amylose particles) or amylose crystallization is a result of the re-association of linear amylose or straight-chain fragments.  Papermaker’s amylose can lead to a number of paper making problems including hard drying, picking, dusting, low porosity, and a weak sheet.  Amylose can be especially difficult in coating colors and coated paper manufacturing and can cause streaks on a coater. 

There are four primary and controllable causes for the formation of papermaker’s amylose:

1. The biggest issue is to avoid storing cooked starch between 67°C and 89°C.  The old rule of thumb is to avoid 80°C ± 10°C (or 170°F ± 20°F).  It is important to keep the starch hotter than the upper limit, or cooler than the lower limit (but above 60°C to avoid the set back problem).  The optimal formation of amylose occurs at 77° C (170° F).
2. Second, amylose formation is enhanced in slightly acid conditions ?pH 6.5.  Thus, a higher pH of 7.0 to 7.5 will help to avoid the formation of papermaker’s amylose.
3. Keeping starch moving under gentle agitation (but without a vortex to entrain air) will slow the tendency for amylose chains to re-associate.
4. Finally, contamination in the storage tank can seed new amylose crystals.  It is important that the storage vessel and piping be clean and free of contaminants such as previous amylose-containing starch left in the storage tank, fiber contamination that can be washed out of the sheet at the size press and into the run tank, and fatty acid materials like sizing agents, surfactants and coating lubricants.

SYMPTOMS AND IDENTIFCATION OF AMYLOSE

If the conventional size press is running poorly (jumping or splashing) or is limited in solids, there may be an amylose problem. 

In the case of rod-metered size presses, larger amylose crystals may actually pass through round wire grooves and deposit onto the sheet. 

Poor print density, low porosity, dusting, or other surface defects without explanation may indicate a possible amylose problem.

Sludge, either in the starch storage tank, but especially in the size press run tank may also indicate a problem with papermaker’s amylose.  A clean running size press starch might normally have 3% to 5% sludge by volume spun down in a centrifuge, from sheet washing.  Higher levels of sludge should be checked under the microscope for the presence of papermaker’s amylose.

Amylose can be positively identified in a number of ways:

• By microscopic exam.  This is the surest way to confirm amylose.   Amylose will be apparent in one or both forms, Type I (very small) and Type II (large), and sometimes with intermediate size crystals.
  • Type I crystals are very small (much smaller than uncooked starch granules), typically linear-appearing contamination (100X) within the cooked starch solution.
  • Type II amylose will be apparent as either single, large round crystals, or often as a bi-valve, two half-round crystals, appearing stuck together. Smaller round spheres, single or sometimes linked 2, 3, or 4 together of intermediate sizes are also sometimes seen.
  • The large amylose crystals can sometimes be confused with swollen, incompletely pasted starch granules.  It may take practice to distinguish between the two, but neither is desirable. 

If your mill suffers from inconsistent size press performance, unidentified “particulates” in the starch, high sludge content, after-section picking or hard drying,  low porosity, or dusting problems call STARCH PERFORMANCE SERVICES today for a complete system audit.