3122 Chapter 25 



Operation and to soften the wood being ground. Experiments on softwoods 

 indicate such pressurization significantly improves stoneground pulp properties 

 with no change in power consumption (Aario et al. 1980). Work by R. Marton 

 (TAPPI 1980) with fairly dense hardwoods, e. g. Betula sp., indicates that 

 addition of caustic soda and hydrogen peroxide to the pressurized stone grinding 

 process yields excellent pulps for corrugating medium and cardboards. No data 

 are published on southern hardwoods pulped by this process. 



Readers interested in process control of stone grinding will find useful Paula- 

 puro (1977ab), Domfeld et al. (1978), and DeVries et al. (1978). See also 

 references page 2238. 



REFINER MECHANICAL PULP (RMP) 



Resource managers seeking commodity uses for pine-site hardwoods are 

 frequently thwarted by the small size and low quality of the stems, and by the 

 diversity of density, anatomy, and color of wood of these species. One method 

 of homogenizing the raw material involves chipping the wood (sometimes with 

 bark included) and defibrating the chips in a disk refiner (see sec. 23-6 and fig. 

 18-282 and related discussion). Metering wet chips of the major species groups 

 to disk refiners in carefully controlled species proportions can produce me- 

 chanical fiber with predictable properties. 



Southern hardwood fibers prepared by disk refiners are much used to make 

 hardboard, medium density fiberboard, and insulation board. The technology of 

 making refiner mechanical pulp for such products is described in section 23-6. 



Even with control of species mixes, however, dense hardwood chips me- 

 chanically pulped in a disk refiner may be difficult to use in paper and paper- 

 board because of their short thick-walled fibers, high proportion of parenchyma 

 cells and vessels, and color. The vessel elements of hardwoods — especially the 

 short, wide, barrel-shaped type present in oaks — do not bond readily into a fiber 

 network, and when present on the surface of print-grade papers cause picks on 

 the printing press from tacky inks (Byrd et al. 1967). 



THERMOMECHANICAL PULP (TMP) 



Thermomechanical pulps are produced by heating wet woodchips prior to disk 

 refining (fig. 25-21 and sec. 23-6). Under proper conditions, the chips are 

 softened, defibrating more completely and with less power (fig. 25-20) than if 

 defibrated cold; the resulting pulp is not discolored (West 1979). Most often, 

 heating and refining are conducted under pressure; some systems, however, heat 

 chips under pressure and refine them at atmospheric pressure. Additional atmo- 

 spheric refining stages are generally, but not always employed. Even with these 

 process improvements, however, dense southern hardwoods require excessive 

 power to defibrate and generally do not yield paper grades of pulp equal in 

 strength to refiner groundwood from preferred softwoods. 



