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Wet Strength Resins (WSR)

[2, 9–11, 21–24] Certain types of paper can only fulfil their purpose if they have adequate wet strength. Such papers include, for example, filter papers, hygienic papers, papers for bags, label papers, wallpapers, laminate base papers, packaging papers for moist goods and all papers which, in the course of further processing and use, risk breaking when rewetted. The required wet strength (up to 50 % of the dry paper strength can be retained) is obtained with the aid of wet-strength resins. For ex¬tremely high wet-strength properties the most common WSR are urea formal¬dehyde resins (UF-resins) and melamine formaldehyde resins (MF-resins), These chemicals need acid pH conditions and the presence of alum in the papermaking process. For neutral pH conditions polyamide-epichlorohydrin resins (PAE-resins) are mainly used (e. g. for hygiene and laminate papers); polyethylenimine products are used for specialty papers such as industrial filter papers and shoe board.
The total consumption of wet strength resins, together with insolubilizers for coating (see, accounts for about 0.07 % of the worldwide paper produc¬tion or 7 % of all specialty chemicals, calculated on the active ingredient (Fig. 3.3). Over the past years, consumption of PAE-resins has increased overproportionately. This is partly due to the trend from acid manufacturing conditions to the neutral pH range, where polyamide-epichlorohydrin resins are more effective than urea formaldehyde and melamine formaldehyde resins. However, the increasing im¬portance of PAE-resins is no doubt also largely a result of the formaldehyde con¬troversy of the early eighties. PAE-resins account today for about 45 %, urea for¬maldehyde resins 15 %, melamine formaldehyde resins 10 %, glyoxal resins 15 % and the remaining 15 % are others, e. g. ammonium zirconium carbonate (as in¬solubilizer) and newly developed products e. g. polyvinylamines.

There are two theories regarding the mechanism of wet strength. The first states that the wet strength effect is due, at least in part, to a reaction between the resin and the cellulose, which leads to the formation of ether bonds. The second theory assumes that the wet-strength resins crosslink on exposure to heat in the dryer section to form a three-dimensional network, wrap themselves around the points where the fibers intersect and thus protect the points of intersection from water penetration and swelling. Given the short contact times with the steam-heated cylinder surfaces (less than a second in the case of the yankee cylinder used in hygienic paper production), wet-strength agents require a high level of reactivity to allow crosslinking to take place and bonds to form. At the same time, the wet-strength resins have to have a selective effect if they are not to react with the surplus hydroxy groups in the paper stock suspension. Therefore a healthy balance between reactivity and selectivity has to be found, so that the chemical reactions (crosslinking, formation of covalent bonds) are not completed at the end of the paper manufacturing process but continue during storage until maximum wet strength is reached one to three weeks later. This gradual curing should not be looked upon as a disadvantage as it is essential for good recycling of the paper machine broke. Melamine-Formaldehyde Resins
In most instances, melamine is made from a basic product such as cyanamide. The melamine molecule is then condensed with formaldehyde to form a series of methylol melamines, e. g. monomethylol melamine and hexamethylol melamine. On introduction to a papermaking system, the melamine formaldehyde product can crosslink with itself forming an ether link or a methylene link, as well as crosslinking with a cellulose carboxyl group to form the covalent bond, both of which contribute to wet strength. The advantages of the melamine-formaldehyde resins are that they lead, with similar addition rates to UF-resins, to wet tensile strength levels up to 50 % and to even higher wet bursting strength. MF-resins also provide a very high alkaline resistance, therefore such products are mainly used for label papers and banknote base papers. Urea-Formaldehyde Resins
The formation of aqueous solutions of urea-formaldehyde condensation products involves the stepwise reaction of urea with formaldehyde, and the first step is undertaken at pH 7–8 to form dimethylol urea. Further reaction to a controlled degree with formaldehyde forms a condensation product in aqueous solution, which can be stored and transported. Urea-formaldehyde and melamine-formal-dehyde resins are delivered in aqueous solutions with solid contents of 12 % (MF) and 40 % (UF) as well as in powder form (MF). They are mainly applied at the wet end, but they can be also used via surface application in the paper machine. Suita¬ble feeding positions for the continuous wet-end addition are between the stock consistency controller and the mixing pump, shortly before final stock dilution, where an optimum of mixing is guaranteed. The UF-resins are the least expensive ones. An important area of application is in the manufacture of sack paper for shipping and cement packaging. With addition rates of 0.5 to 3 %, calculated on dry paper stock, a wet-strength level of up to 40 % of the dry-strength figure can be achieved. Epoxidised Polyamide Resins
The chemistry of the production of polyamide resin is very similar to the original process by which nylon was produced. In the Nylon 66 process a dicarboxylic acid, such as adipic acid is reacted with a six carbon amine, for example hexamethylene¬diamine, to produce a synthetic fiber. In the case of polyamide resin, a dicarboxylic acid is reacted or condensed with an amine such as diethylenetriamine to form an amino polyamide. The secondary amine groups of this water soluble polymer are then reacted with epichlorohydrin to form the aminopolyamide epichlorohydrin intermediate. This is then crosslinked to build molecular weight whilst maintain¬ing solubility. The polymerization reaction is terminated by dilution and acidification.

The amount of polyamide-epichlorohydrin resin required in hygienic paper pro¬duction is between 0.1 and 4 % dry substance, calculated on the paper. These resins are supplied in the form of aqueous solutions with a solids content of 12 to 25 %. They are effective in the pH range 5 to 8, although the best wet-strengthen-ing effect is obtained in the neutral or slightly alkaline range; they are therefore often called neutral wet-strength resins. In the majority of cases, PAE-resins are added to the stock, preferably just before the last stock pump in front of the head-box. This ensures that the fiber/resin bond is not impaired by high shear forces. Depending on the amount added, the relative wet strength can be increased to over 35 % without significantly reducing the absorbency of the paper. Wet-strength res¬ins in general also increase the dry strength of paper (i. e. tear and burst). They also have favorable effects on the dry and wet abrasion resistance of paper. Additionally the retention of fillers and fines is increased. A special effect of PAE-resins, even with small quantities, is to form a coating on the crepe cylinder of a hygiene paper machine to control the adhesion of the paper web on the cylinder.

The disadvantages are that the degree of whiteness is less stable than with UF and MF resins, and the AOX problematic. Much effort was put into developing “low-AOX” and recently also “AOX-free” polyamide-epichlorohydrin resins with no detectable amount of byproducts. At the same time, chlorine-free wet-strength agents have also been developed, e. g. modified glyoxals and polvinylamines, but often with higher costs. In the longer term, workplace safety aspects could lead to the application of these new products becoming successfully established in the market. Glyoxalated Polyacrylamide Resins
These products are prepared by crosslinking a low molecular weight polyacryla¬mide (PAM) with glyoxal. The PAM is normally a copolymer of acrylamide and a quaternary ammonium cationic monomer which is prepared in aqueous solution. This results in a cationic polymer which is attached to pulp fibers. The cationic backbone is then crosslinked with sufficient glyoxal to react with most, but not all, of the PAM backbone amide groups. On storage, the resin continues to crosslink and can ultimately gel. In order to achieve the desired stability, paper mills dilute the resin on receipt. At 25 °C, a 10 % solution will gel in about 8 days, whereas a 6 % solution will take about 65 days to gel at room temperature.
There is strong evidence that glyoxalated PAM imparts wet strength primarily through covalent bond formation between the resin and the fibers. It can be taken for granted that there is some intermolecular crosslinking within the resin but, in order to function, there needs to be at least some fiber-resin-fiber bonds within the fiber-fiber bonded area. The reaction of glyoxalated PAM with cellulose is rapid at neutral pH 6–8 and even more rapid at acidic pH 4–6, resulting in 80–100 % of the potential wet strength. Ageing or curing of the paper gives little or no additional wet strength. The reaction is reversible in the presence of water and a resin-treated paper gradually loses wet strength on prolonged soaking. This temporary wet strength can be sufficient for some paper grades, e. g. paper towels, and also dur¬ing the paper manufacturing process when the the sheet is passing through a size press or coater. Glyoxalated PAM resins also contribute significantly to the dry strength of treated paper. Other Wet-Strength Resins
. • Polyethylenimine (PEI) was the first effective WSR used under neutral/alkaline pH conditions in papermaking without influencing the absorbency of the paper. The PEI manufacturing process is described in Section The effective mechanism of PEI formation is somewhat different from the resins. PEI devel¬ops wet strength without curing the paper and the level of wet strength that can be attained is less than with the thermosetting resins. It has been proposed, that PEI functions by creating stronger interatomic bonding, rather than by forming homo- or co-crosslinked networks. The amine cationic groups responsible for wet strength have dissociation constants of around 12 and the retention and performance of PEI is best at pH 7–9. The reasons why PEI has not been used more extensively as a wet-strength resin are higher costs than with thermoset¬ting resins and because it causes yellowing and loss of brightness in white print¬ing and writing papers.
. • Polyvinylamines are a relatively new group of wet-strength resins. These products are environmentally friendly, their use does not result in any negative ecological impact (see also Section 3.6.5 on Dry Strength Resins). Their cost-performance ratio is at present less favorable than the conventional WSR in most cases.
. • Polyisocyanate is another new type of WSR, up to now with very little practical use.
. • Dialdehyde Starch (DAS) also has the potential for crosslinking cellulosic hydroxy groups in paper to give temporary wet strength. DAS is essentially a highly modified starch in which the vicinal hydroxy groups (at the C-2 and C-3 carbons) are selectively attacked by periodic acid, severing the C-2 to C-3 bond to form dialdehyde starch. The aldehyde groups are not present to any extent as free aldehydes, but rather as hemiacetals or as hemialdals. Since the linkages in these compounds are weak, dialdehyde starch reacts as if the aldehydes were free, permitting its use as a reactive polyaldehyde capable of reaction through hydroxy amino or imino groups.

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