Cold Corrugating Process

There are quite a few basic differences between old and hot (regular) processes, but extremely important differences are in:

1) Type of adhesive, its application and setting temperature.

In conventional hot-corrugation process uncooked starch is dispersed in semi cooked solvent with additives. At elevated temperatures, the starch granules swell by water absorption and gelatinisation occurs. This slurry gets to the pores and voids of paper and solidifies there by making a bond. In cold corrugation process something like the reverse happens; i.e. the adhesive develops high viscosity during cooling process (instead of gel formation in heating during hot-corrugation). The adhesive is made up of fully cooked starch which is applied at high temperature to the cold paper, with very low viscosity; then it undergoes cooling. The adhesive is chemically made by thermo chemical modification of corn-starch. Cooking is carried out at a temperature of about 130/150⁰ C in the presence of ammonium persulfate. Sodium Hydroxide is then added at a high pH (9-10) which helps to increase alkalinity, stabilise the solution and bonding. Boric acid can also be added to improve viscosity.

On application of the adhesive which is at a high temperature, 90⁰ C, the molecules of starch get into the pores and voids. This happens by absorption process due to the paper (at room temperature) trying to establish equilibrium with hot glue and in the process heat transfer, water absorption and molecule migration into paper occurs. Further, the process is aided by high pressure (about 25% more than what is applied in hot process); applied on corrugating rolls. The absorption of glue under high pressure takes place in a slip second time as the production process operates at 300-350 m/min. The small portion of glue that goes into paper develops "green bond" which is just adequate to take the paper lines and fluting off the labyrinth. Subsequently a few seconds later, as the glue cools down, its viscosity increases more and more and the bond solidifies.

2) Tension control

If paper enters labyrinth at a very high speed the subsequent speed of its travel through the nip line needs to be much more. Higher speeds than desirable levels at labyrinth would lead to poor corrugation due to inadequate time available for paper to form flutes and such flutes to pick up gum. This would also result in poor bonding and resultant delamination.

Web tension is much more critical. Under high tension, the corrugations would develop cracking at the tips because in cold process the paper is not heated and moistened to make cellulose and lignin more pliable. In cold condition the paper is rather brittle or less plastic in nature and when such paper is fluted under high tension it is bound to crack on flute tips. Hence in the cold process it would be necessary to provide pre feeder to control tension and keep speed. Adjustments of regular break tension may also be given as required, to keep medium entering the fluting nip line at constant tension. The pre-feeder and break-tension adjustments would also help to even out any piping in paper which if continues up to labyrinth, will surely develop creasing and high-low flutes. Tension adjustment of medium is also required to even out wobbling of paper due to bad winding of reel, besides providing initial tension required to feed paper properly just at the entrance of labyrinth along upper corrugating roll.

3) Frictional coefficient of paper

Co-efficient of friction can be reduced by use of chemical agents (like paraffin wax), by keeping higher moisture in medium, by less wrap of medium on corrugating roll and by adjustments on pre-feeders.

The latest in metallurgical sciences can help to manufacture rolls with less co-efficient of friction and very high smoothness so that paper under tension and speed, can move smoothly on such rolls. Nevertheless it is important to see that friction is not very low as to avoid slippage of paper which is much more dangerous.

4) Picking and its control

Picking relates to accumulation of loose fibres detached from paper surface. In hot process such dust or fluff is reduced by release properties of paper roll-surface due to high heat. In the cold process the loose fibres can be blown off by air-jet web cleaner suitably designed. Certain chemical agents can be also used to treat medium against picking.

5) Wrap on top corrugating roll

As the flutes are continuously formed, the medium develops wrap and it wraps itself around top corrugating roll. This needs to be reduced so that fluff-out, i.e. detachment form the roll and movement to take liner is easy. This may be done by finger guide adjustment or by vacuum pull.

6) Medium moisture

7) Forming pressure

Forming pressure is one important factor which helps in glue absorption mentioned above as well as flute forming. In hot process the paper is flexible and therefore greater nip pressures are not required to bend the paper to corrugate. In cold process, the paper is less flexible and hence a 25% more pressure than usual would greatly help to form perfect flutes.

8) Pre-treatment of paper

9) Finger guides and/or vacuum handling of web

10) Double-backer and bond formation there.

Adhesive application at the double facer is rather demanding. It has to be put on the fluted paper flute-tips when it is not supported by any back-up roll. There is a much open time between glue application and double-facer joining. Hence the double facer-glue needs to be slightly different so as to take into account the transport time. If the adhesive solidifies before joining of liner then there is no bonding. If may help, to pre-heat the liner at about 40-50⁰C (or slightly above room temp.) so as to allow glue to penetrate liner easily. This preheating is required only for double-backer.