Abstract: The influence of fiber size on the yield stresses of fiber suspensions was discussed in same ranges of mass fraction and crowding number with the bleached softwood and hardwood kraft pulp as raw materials, respectively. Then these two pulp suspensions were mixed with different mass ratio, and the relationships between the fiber size and the yield stress of the mixed suspension were analyzed. The results showed that the suspension of the softwood pulp showed high yield stress because of its large size on the same mass fraction range. And with the increase of fiber size, the multiplier term coefficient of the power law relationship between the yield stress and the mass fraction increased, but the corresponding exponential term coefficient decreased. The yield stress cannot be expressed by the function only with the crowding number, and it is inverse proportional to the exponential function of the shape factor as the formula, τ_y≈0.003·N^2.2/β^0.6. Therefore, in the same crowding number range, the suspension of the hardwood pulp with the low shape factor showed the high yield stress. The values of the shape factors' power with the constant exponent followed the linear rule as the formula, β_AB^1.6≈β_B^1.6+(β_A^1.6-β_B^1.6)/100·x, so the yield stresses of the mixed pulps also presented the similar rule with the mixing ratio changed as the formula, τ_y(AB)≈τ_y(B)+(τ_y(A)-τ_y(B))/100·x.

Key words: bleached kraft wood pulp, fiber suspension, yield stress, crowding number