1. We use poly vinyl alcohol adhesive for our thin walled paper tubes. We’re loking to add a plasticizer to make the tube a bit pliable. What can be added and how?

pVOH can be softened by adding glycerin or propylene glycol after cooking and cooling. Addition rate is typically 5-10% of the dry pVOH weight in the cook. The tradeoff is set speed. Both glycerin and pg act as humectants (materials which hold onto water, and slow set), which can be detrimental to winding operations. Water resistance of the finished tubes could be adversely affected as well.

Another option is to purchase a plasticized resin emulsion from an adhesive supplier. The level of addition to the pVOH adhesive would be determined by the amount of flexibility required. In addition to providing flexibility, the resin base would improve specific adhesion (thus stronger, deeper bonds). Emulsion increases wet-pound cost of the adhesive mixture, but you will usually be able to apply a thinner film of adhesive with higher emulsion content (more active adhesive solids), which will mitigate the increased cost and help further with flexibility. Emulsion should be added to the pVOH cook after it has cooled below 120°F, and thoroughly blended.
I would recommend adding an (EVA) Ethylene-Vinyl Acetate Co-Polymer to the adhesive. Recommend maybe 15-20% on a wet basis. This should soften the glue line and make the dry film more pliable.

2. To reduce all costs I want to prepare the glue in my mill. What is the best way to do that? And what is the best glue: poly vinyl alcohol, acetate, yellow dextrin or silicate?

This is a very hard question to answer without more detail on the processing and end-use requirements for the tubes. The properties of the different adhesive types you mention can be vastly different. For example: Poly vinyl acetate (PVA) emulsions typically have very fast set speed and excellent adhesion, but rigidity is generally less than the other types mentioned, and wet-pound cost is usually higher.

Poly vinyl alcohol (pVOH)/clay cooks have moderate cost and are fairly versatile, but have more limited specific adhesion (ability to adhere difficult papers) than PYA, and tend to add more moisture into the finished tubes (dimensional stability, drying/staging time, moisture content issues). Dextrins are moderate in cost, have good rigidity, and add moderate moisture to the finished tubes, but they have limited specific adhesion and generally slower set speed (trimmer issues).

Silicates have the lowest wet-lb. cost and a very rigid dry film, but have the most limitation in terms of set speed, specific adhesion, and contribution of moisture to the finished tubes. Inner and outer plies often require a different type of adhesive applied to hold the assembly together until silicate can dry. The best starting point may be to prepare a clay/pVOH cook, and post-add some PYA resin emulsion ‘sweetener’.