One of the key properties of paper is its breaking length, which is usually controlled in many paper products. To achieve this, several natural and synthetic polymers are used in paper industries in accordance with paper grades and customer needs. In this study, the combination of cationic starch (CS) and/or polyacrylamide (CPAM) as common additives, and cellulose nanofibers (CNFs), were added to a short-fiber pulp suspension to investigate the reinforcement effects and to compare such properties with those of paper prepared with 20% softwood long-fiber. The breaking length was measured on the prepared handsheets. The results showed that adding 1% CS significantly improves paper breaking length, which was well comparable to the handsheets reinforced with 15% softwood pulp. The results showed that adding less CS (0.5%) along with 3% CNFs significantly increased the paper breaking length, while reducing process difficulties associated with CS. The same result was also achieved adding 3% CNFs along with 0.03% CPAM. Furthermore, a triple system of CNFs, CS, and CPAM additives significantly enhanced the paper breaking length and surpassed the breaking length of paper made with 20% softwood pulp. Using this triple system led to the least changes in handsheet thickness as well. Therefore, this triple system of additives can replace softwood pulp, thereby significantly expanding the spectrum of paper products for the countries with limited softwood pulp sources.

The influence of rosin (0.1-0.3%), alum (0.4-0.6%), polyaluminum chloride (0.3-0.7%), and starch (0.5-1.5%) in the making of paper from old corrugated container (OCC) pulp on the freeness, breaking length, tear index, and burst index of pulp and paper sheets was studied. Using a full factorial design to identify the optimum operating conditions, equations relating the dependent variables to the operational variables of the chemical additives were derived that reproduced the former with errors lower than 5%. Using a high starch (1.5%), high PAC (0.7%), low alum (0.4%), and low rosin (0.1%) combination led to pulp that was sufficient to obtain paper with good strength properties (breaking length 5720m; burst index: 3.1 kPam2g-1; tear index: 6.2mNm2/g; Cobb test: 94; fold endurance: 1.52). SEM analysis showed increasing bonding between fibers together at this level of additives. The influence of starch on Cobb test values was not significant.