Viscosity :
- The Viscosity of a substance can be defined as its resistance to flow.
- This property can be measured with the use of viscosity cup (Stein Hall, BIS cup number 4) or by Brookfield viscometer.
- Viscosity measurement should always be checked against the temperature at which the reading was taken.
Solid Content:
- This will normally range between 18% 30% on a commercial basis or 16% 26% on a dry solids basis.
Gel temperature :
The values will be of the range of 53-63 degree C, and this value can be measured by heating the sample of starch glue until the thickening starts to occur. The temperature at this point is considered as the gel temperature.
Storage or circulation temperature :
This can be very easily controlled but often overlooked. Consistent bond and 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.
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 minutes
The adhesive application at the sheet pasting should be maintained between 50-60 grams/sq.m for more than 200 gsm and 40-45 grams/sq.m for less than 150 gsm
Biosides :
It is necessary to add suitable biosides in the starch adhesives to control the fungal and bacterial growth.
BONDIGN PROCESS :
Gelatinization :
It 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.
Green 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.
Glue penetration:
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
Adhesives 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
Glue application :
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
Zip bond :
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.
White bond :
The appearance of the starch lines when the board is seperated
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
Crystallized bonds :
Resulting in poor bond strength blister or crystallization of the glue bond occurs due to the poor contact of the paper at the substrate tips
Visibily glass like appearance to the glue line, this problem is only in the double facer