Starch is a natural carbohydrate found in plants containing chlorophyll in the form of dense packed, tiny granules especially in cereals such as maize and wheat. It can also be extracted from sources such as potato and tapioca.

Starch has found favor in many industries such as adhesives, because it has advantages like
– Low cost material
- Bio degradable
- Renewable source

• Viscosity
• Solids content
• pH
• Gel temperature

This will normally range between 18 % - 30 % on a commercial basis or 16 % - 26 % on a dry solids basis.

• This is considered to be the temperature at which swelling of starch granules commences and visibly signs of thickening will be apparent
• The values will be of the range of 53-63 degree C, and this value can be measured by heating the sample of starch until the thickening starts to occur. The temperature at this point is considered as the gel temperature

• This can be very easily controlled but often overlooked
• Consistent bond quality can be maintained only if this temp is adequately controlled
• The storage temperature, formulation of adhesives used and its gel temperature are very important

Bonding process

• Glue application
• Glue penetration
• Gelatinization
• Green and final bond

• The first stage of bonding process is the application of the adhesives to the flute tips
• Application depends on the uniformity of the glue film and maintaining a constant glue film thickness
• Glue must be applied in the correct quantities to the flute tips
• Application and quality of the glue must remain constant at all corrugators speed
• Shorter the texture of the glue less the stringy

•  It is critical for good bonding that the glue achieves a degree of penetration on the paper.

• Over penetration is as bad as under penetration.

• The factors affecting the glue penetration are
  • adhesive pH
  • paper moisture
  • porosity of the paper

  Glue penetration can be affected during the preparation of controlled use of chemicals.
  The simplest form of penetration control is by preconditioning the machine.

• Is the rapid swelling of starch granules as they absorb water and is critical to good adhesion and bond strength (PAT)
• Gel point can be adjusted by modifying the starch and addition of caustic

Green Bond and Final bond

• Green bond is formed during gelatinization but it is uncured joint, the board should not be subjected to any disruptive stress during this phase and is allowed to dry out.

Bonding problems

• Zip bonding
• White bond
• Crystallize bonds

• Insufficient glue penetration, because the adhesive has not remained fluid for long enough to properly penetrate the paper. Hence the fibers of the papers are not tied to each other by the starch and the board will not pull apart and there will be no sign of fiber tear.
• The reasons :
• Over heating of paper
• Incorrect formulation (checks the viscosity and gel point)
• Insufficient application of the glue

• The appearance of the starch lines when the board is separated
• The problem is caused by the incomplete gelling of starch
• The water in the adhesive , which absorb prematurely into the paper, leaving insufficient water available for complete gelatinization of starch granules
• Reasons
  1. Low heat transfer
  2. Low adhesive viscosity transfer
  3. Excessive paper moisture

• Resulting in poor bond strength, blister or crystallization of glue bond occurs due to poor contact of the paper at the substrate tips
• Visibly glass like appearance to the glue line , this problem is only in the double facer

Moisture control in the sheet and board

• The moisture of 2 ply sheets has to be kept under 6% moisture level to give minimum curing time of 30 min
• The adhesive application at the sheet pasting should be maintained between 50-60 grams/ sq.m for more than 200 gsm and 30-40 grams/sq.m for less than 150 gsm

• For lower gsm < than 140 gsm Solids between 20 and 25 % and viscosity 25 to 28 secs
• For higher gsm > than 200 gsm Solids between 25 and 28 % and viscosity 30 to 32 secs

• To enhance the quality of the board such as PAT, ECT, and BCT certain starch additives are added during the preparation of the glue.
• These additives help to control the moisture in the board and improve the bonding of the substrates.
• This also helps in producing the moisture resistance or the water resistance boards.

Trial results

• Starch additives glue and the ordinary glue
• PAT with the common adhesive for 420 X 200 gsm combination is 100.
• PAT with additive based adhesive 420 X 200 gsm combination is 128.

This shows an increase of 28 %, the test has been conducted immediately after the formation of 2 ply sheets

Additive based adhesives

• The additive based adhesive controls the moisture of the 2 ply sheets
• There is no absorption of moisture from the atmosphere on the glue line; also the moisture is not absorbed while sheet pasting.
• The bursting strength and the compression improved by 8 % - 10 % (27 +/- 1 deg C and 66% RH).

Biocides

• It is necessary to add suitable biocides in the starch adhesives to control the fungal and bacterial growth.
• Regular cleaning and washing of glue tank and the glue tray is essential.