The ability to form paper of superior strength at minimal cost is important to the manufacture of paper products. Paper strength is dependent upon a number of factors, including choice of fibers, refining methods, press loading, and chemical additives employed, and can be altered by adjusting these variables. Trends in the industry including increased use of lower quality fiber sources and increased closure of mills' water systems has generated a greater need for the development of exemplary and cost-effective strengthening processes. For example, the use of inexpensive low-quality fibers may necessitate strategies such as increased refining, greater press loads, or chemical additives. Unfortunately, greater refining often leads to undesirable paper properties such as increased paper density, reduced tear, and decreased porosity, as well as slower production times. Increasing press loads have mechanical limitations such as sheet crushing, and can also lead to inefficient paper production. Thus, chemical additives are commonly added to the papermaking process to enhance the properties of paper, and can be utilized to increase the compressive strength, bursting strength, and tensile breaking strength of the paper product.
The papermaking process involves taking a slurry of papermaking raw materials at a consistency (weight percent solids) in the range of 0.1 to 1.5 weight percent and dewatering it to form a sheet with a final consistency of about 95 weight percent. The rate of paper production or, equivalently, the machine speed is dictated by the rate at which the water can be removed, and consequently, any chemical treatment which can increase the rate of water removal has value for the papermaker. An ideal chemical additive for papermaking would impart superior paper strength, promote good dewatering (e.g., press dewatering), and would be less costly than alternative strategies.
A variety of polymeric additives act as strengthening agents, and can be used to obtain high-strength paper products, including natural polymers (e.g., starch, cellulose gum) and synthetic resins (e.g., polyacrylic esters, polyacrylamides, glyoxalated polyacrylamides). Strengthening agents are believed to increase the strength of paper by forming covalent bonds with the cellulosic material in paper. Optimal strengthening agents form strong bonds to cellulose fibers, and impart increased strength per unit of bonded area of the paper. Synthetic resins offer unique benefits, such as ease of use and structural versatility.
Polyacrylamides are well-known synthetic resins used to generate dry strength as well as aide in press dewatering, and often comprise a small amount of cationic monomer incorporated into the backbone of the polymer to increase association of the polymer with the anionically charged cellulose fibers. In particular, glyoxalated polyacrylamides (i.e., GPAMs) are a class of modified polyacrylamides that are generally formed from glyoxalation of a polyacrylamide, and are well-known to deliver enhanced paper strength as well as press dewatering efficiency.
The properties of glyoxalated polyacrylamides depend on the monomers and the percent content of mono-reacted amide and di-reacted amide present in the modified polymer. The mono-reacted and di-reacted amide species each have distinct reactivities. The amount of mono-reacted amide and di-reacted amide indicates the amount of free aldehyde available in the polymer for bonding, and is highly dependent on the conditions of the aldehyde-functionalization reaction (e.g., temperature, reactant/reagent ratios). The amount of mono-reacted and di-reacted amide present in the aldehyde-functionalized polymer is often represented as a ratio between the two species.
The conventional wisdom in the papermaking field is that polymeric additives (e.g., GPAMs) provide paper of higher dry strength when the molecular weight of the polymer is relatively high, e.g., in the range of hundreds of thousands to a few million g/mole. It is generally believed that paper strength deteriorates when the molecular weight is too low, and thus, many polymeric additives of high molecular weight have been developed in the industry. However, high molecular weight polymers can be more difficult to manipulate due to viscosity, solids, and stability requirements. High molecular weight polymers can also be more cost prohibitive to use than the corresponding low weight polymers.
A more economical and simple chemical composition and method for enhancing the strength and dewatering properties of the papermaking process would be a valuable contribution to the field.