PAE resins

Another application of Py-GC in paper analysis has been the determination of wet strength polyamide-epichlorhydrin (PAE) resins [89]. 

Pyrolysis of paper cross-linked with (PAE) resins generate compounds that are easily distinguished from those of cellulose. Table 16.4.2 shows several compounds in the pyrolysate of PAE. 

The treatment of this lower molecular weight polyamide with epichlorohydrin, principally by reaction at these secondary amine sites, to form the key cationic, reactive functionality 3-hydroxyazetidinium chloride and to develop further the molecular weight of the PAE resin... 

Using epichlorohydrin to develop the structural and functional properties of the resin also results in the production of residual by-products dichloropropanol (DCP) and chloropropandiol (CPD). Over the years, these residual by-products have come under scrutiny from various regulatory bodies. The efforts by chemical manufacturers to reduce the levels of DCP and CPD can be used to classify the different grades of PAE resins available. 

Irrespective of its generation, the ability of a PAE resin to impart wet strength to paper is dependent upon two properties ... 

A PAE resin mnst be retained on the surface of the paper fibre before it can impart wet strength to the paper. The cationic nature of PAE resins makes them naturally substantive to the anionic sites of paper fibres. Any factors that interfere with this initial mechanism will resnlt in poor retention of the PAE resin and, conseqnently, lower than expected levels of wet strength in the paper. 

Once on the paper fibre, the PAE resin must do at least one of four things ... 

When using PAE resins, two mechanisms explain the development of wet strength in paper (Fig. 8.3). The first is a preservation or restriction mechanism but is more often referred to as homo-CTOSslinking . After adsorption onto the fibre surface, the azetidnium functionality of resin reacts with amine sites elsewhere on the backbone of the resin, effectively forming a self-crosslinked network that physically... 

To control dosing of the PAE resin to the papermaking system, metering pumps and mini magnetic flow or mass flow meters fabricated from 316L stainless steel and fitted with Teflon or Viton-wetted seals are most preferred. Progressive cavity or centrifugal pumps are preferred because they are low shear and impart less heat to the PAE resin. 

To provide wet strength, a PAE resin must be retained on the surface of the paper fibre. The principle mechanism of this retention is the ionic attraction between the anionic carboxylate groups on the fibre surface and the cationic azetidinium groups... 

Refining the pulp increases the surface area of the fibre, exposing more carboxylate sites capable of attracting the PAE resin. This allows a greater level of PAE resin to be adsorbed onto the fibre before reaching the maximum level of PAE resin. The effect is most pronounced in very responsive pulps such as UBK. However, refining also increases the levels of fines in a paper furnish. PAE resins absorb equally well onto... 

Fig. 8.4 Effect of pulp carboxyl content on PAE resin retention...

Aside from low wet strength values, the consequences of poor retention of the PAE resin onto the fibre or of fines that have absorbed the PAE resins are ... 

Foam - The chemistry and cationic nature of PAE resins provide a very good surfactant property for the formation and stability of foam. Overuse of PAE resins to attain a desired strength target often exacerbates this problem and results in excess resin in the white-water system. The presence of foam is often a sign of poor PAE resin retention. 

The quality of the water in the wet-end of the papermaking system can have a large bearing on the performance of the PAE resin. Whilst dilution of the resin from the storage tanks to the chosen point of addition requires fresh water, recycled water is used for preparation of the thick stock and subsequent dilutions to the thin stock prior to formation of the wet web. The properties of the recycled water can determine the effectiveness of the PAE resin. 

The pH value of the water can have one of the biggest effects on the resins effectiveness. PAE resins work best when the value of the papermaking system pH is between 6.5 and 8.5. No beneficial effect on wet strength performance occurs... 

The total alkalinity of the water, in the form of carbonate and bi-carbonate ions, has a beneficial impact on the performance of PAE resins. Increased alkalinity improves fibre swelling, which exposes more reactive carboxylate sites, improves overall resin retention onto the fibre and iuCTeases the rate of the co-crosslinking reaction. Furthermore, carbonate and bi-carbonate act as catalysts for the ringopening reaction of the azetidininm groups iuCTeasing reactivity. 

The hardness of the water can be problematic for PAE resins. Divalent cations such as calcium and magnesium can tie up pulp anionic (carboxyl) sites and inhibit retention of the cationic PAE wet strength resin. Hardness levels up to 150 ppm usually present no problems. 

S.2.3.5 Using Additional Polymers to Enhance PAE Resin Performance... 

Drying conditions, which determine the rate of reaction of the PAE resin... 

Using additional polymers can overcome some of the factors that impede wet strength development. These polymers can even boost the performance of a PAE resin in ideal conditions. 

Greater molecular weight of coacervate spans greater distances between fibres, allows PAE resin to be more effective across the sheet... 

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