AKD Emulsion EXPEL Series

The driving forces for papermakers to change from acid to neutral papermaking are not always easy to establish, but probably the most important one is that neutral pHs allow the use of calcium carbonate, a relatively inexpensive and good quality filler. However, calcium carbonates cannot, for obvious reasons, be used in the acidic rosin-alum environment. Other important advantages of neutral papermaking which are often quoted are improved permanence, reduced machine corrosion and increased paper strength resulting from the increased fiber bonding obtained at higher pH.

A large number of cellulose reactive sizes have been considered over the years as neutral sizing agent, but they have found no commercial use. The most successful of neutral and alkaline sizes are the alkyl ketene dimmers (AKD) and the alkenyl succinic anhydrides (ASA). These are at opposite ends of the spectrum of reactivity and hydrolytic stability. AKDs are rather unreactive towards cellulose but have good hydrolytic stability. The reverse is true for ASAs.

EXPEL series, the neutral and alkaline sizing agents of TGchem, are emulsion products of AKD.

AKDs, which have known to be cellulose-reactive sizes, can react with the cellulose during the drying of the paper with the formation of beta-ketoesters. The ring-opening reaction always takes place at the acyl oxygen. AKD can also be hydrolyzed, giving an unstable beta-ketoacid, which decarboxylates to the corresponding ketone (Fig. 1).

In order to obtain good size degree with AKD, the following is required: Internal sizes are introduced at the wet end of the papermaking system, usually as colloidal suspensions, which are retained in the fiber network during sheet formation. During drying, and to some extent afterwards, they are able to spread and migrate, and undergo some form of interaction with the surface which allows orientation of the hydrophobic group and finally creation of a low-energy surface (Fig. 2).

As for AKD sizing, consequently, the sizing takes place in three consecutive steps; deposition-spreading-orientation or interaction. The orientation of AKD molecules is a rate-determining step in the development of size degree. Thus, one of the disadvantages in AKD sizing, delayed size development is due to the orientation of AKD.

AKD dispersions have considerable shelf life and hydrolysis can usually be avoided during storage. Low level of retention and high stock temperature are the main causes of hydrolysis of AKD in commercial use.

AKD is used primarily when the requirements are for hard sizing, e.g., liquid board. AKD gives a good resistance to fruit juice, lactic acid, etc. Another important area of use is CaCO3 containing fine papers. The addition level is usually between 0.06 and 0.2% calculated on dry pulp.

So-called sizing accelerators play a very important role in sizing with AKD. The reaction with cellulose can be catalyzed with additives so that the rate of reaction can be increased by a factor of at least 20. The important catalysts are HCO3- and basic polymers with amine groups.

The HCO3- ion has a unique ability to catalyze the reaction between AKD and cellulose. HCO3- ions can exist in a natural state in the system, e.g., when CaCO3 is used as filler, but it is also general practice to add Na2CO3 or NaHCO3 to the stock to increase the alkalinity.

Polymeric amines having amino groups with a free electron pair are classic sizing accelerators for AKD. Several different types of condensation polymers occur in commercial contexts. The polymeric sizing accelerator is added either to the AKD dispersion (rapid curing dispersion) or used separately as combined accelerator and retention aid. The effects of HCO3- and polymeric accelerators are synergistic.

EXPEL series can be divided into three categories: EXPEL-D series is classical AKD emulsion, which is very effective size used with fixing agent and retention aids, having an excellent thermal stability. Second is polymer-modified AKD emulsion, EXPEL-F series. EXPEL-F series can develop a sufficient size degree alone because the sizing accelerators are involved in the AKD emulsion. other one is modified by special agent, EXPEL-X series. EXPEL-X series can extremely high size degree, fast-cure, self-cure. Above all, EXPEL-X series can develops size degree at size press.

1. Typical Properties

2. Packaging

3. Heath and Safety

We know of no ill effects that could have resulted from using EXPEL-200D for the purpose for which it is intended and from processing it in accordance with current practice. EXPEL-200D does not exert any harmful effects on health, provided it is used properly.

These data are given for information only. They do not constitute a specification and no liability is assumed, nor freedom from any existing patents.
We do not guarantee the applicability or the accuracy of this information and the suitability of our products in any given situation, since we cannot anticipate the many different conditions under which our products may be used.