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In the papermaking process, it can be difficult to maintain specified mechanical-strength levels while minimizing fiber costs and meeting specified targets. If significantly increased, bonding could be achieved without as much refining of the fibers, just adding a biopolymer, then it might be possible to achieve important paper-property targets at lower basis weights. The incorporation of a biopolymer in the papermaking process changes positively the relations between important pulp properties, rendering its application differentiated and advantageous, as these gains will be immediately passed on to the paper obtained. Thus, this study aims to evaluate the development potential of the physical-mechanical properties of Sack Kraft paper through the application of biopolymers in two types of cellulose pulp from Pinus spp. (UKP - Unbleached Kraft Pulp and DKP - Delignified Kraft Pulp), allowing for a possible basis-weight reduction in these papers. Two different biopolymers were developed according to the specific type of cellulose pulp and production process conditions of the present study, B1 and B2. The applications and analyses contemplated two scenarios: 1) cellulose pulp without refining, with 120 minutes of retention time for both pulps; and 2) industrially-refined cellulose pulp, with 23 minutes and 20 minutes retention time for UKP and DKP, respectively. For scenario 1, the cellulose pulp was refined in laboratory after application of the biopolymers, considering the following refining degrees: initial, 15, 20 and 30°SR. For both scenarios, laboratory handsheets were formed to carry out physical and mechanical tests in order to assess properties of the pulp itself, which were: tensile strength, stretch, TEA (Tensile Energy Absorption), tearing resistance and burst strength. Overall, the industrially -refined cellulose pulp with subsequent application of Biopolymer B2 showed the best performance for both types of cellulose pulp, with a substantial increase in tensile index (9% - 16.5%), stretch (9% - 16%), TEA (21% - 30%) and burst index (13% - 14%) for UKP and DKP cellulose pulp, respectively. The results show that applying the biopolymer after refining allows for the gains obtained in cellulose pulp to be passed on to the end product, resulting in expressive gains in Sack Kraft paper quality and production. [ABSTRACT FROM AUTHOR]