Kraft sheet pressure

Europe has over 40% of the 10 billion m global market for panel surfacing materials, with market share for low pressure melamine (51%), veneer (18%), paper foils (13%), high pressure laminates and paint (7% each) etc. (O Carroll, 2001). The prime substrate is particleboard and MDF, except for the uses discussed above, e.g. formwork and floating floors where plywood excels. The laminating paper is typically an absorbent kraft sheet that is saturated with resin. 

Figure 5. Effect of pressure on Kraft sheets ( ) sheets pressed and rewetted (%) pressed, dried, and rewetted...

The decorative plastic laminates widely used for countertops and cabinets are based on melamine—formaldehyde resin (see Laminates). Several layers of phenohc-saturated kraft paper are placed in a press and a sheet of a-ceUulose paper printed with the desired design and impregnated with melamine—formaldehyde resin is placed over them. Then a clear a-ceUulose sheet, similarly impregnated with the resin, is placed on top to form a clear, protective surface over the decorative sheet. The assembly is cured under heat and pressure up to 138°C and 10 MPa (1450 psi). A similar process is used to make wall paneling, but because the surfaces need not be as resistant to abrasion and wear, laminates for wall panels are cured under lower pressure, about 2 MPa (290 psi). 

Extraction of HemiceUuloses from Various Pulps. The extraction procedure followed was that of Giertz and McPherson (6). Fifteen grams of unaged pulps [four kinds in all bleached kraft pulp (BP), unbleached kraft pulp (UBP), unbleached groundwood pulp (GP), and chlorited UBP] was treated with 290 mL of 10.9 lithium hydroxide solution for 1 h at room temperature and was stirred intermittently (see Table I for the characteristics of the pulps). After the samples were diluted to about 1 L, the insoluble material was filtered off. The extract (filtrate) was neutralized to pH 6 with 2 M phosphoric acid (300 mL) and was allowed to stand overnight. The precipitated lithium phosphate and /3-cellulose were separated by filtration. The filtrate was then concentrated to about 175 mL under reduced pressure at 30-40 °C. Salts present in the system were further precipitated with methanol and separated by filtration. The filtrate was again concentrated to about 60 mL. The filter paper was immersed into this concentrate to saturate the test sheets. 

Schematic diagrams of (top) stacked dried PF resin-soaked kraft paper (brown) plus a metal foil (optional), a pattern or veneer sheet (red) and a translucent sheet (blue) (middle) compression molding between highly pol-ished heavy steel platens under high temperature and pressure (bottom) a sheet of Formica laminate.

Effect of Pressing on Non-freezing Water. Figures 4, 5, and 6 show that the non-freezing water Initially Increases on pressed rewetted sheets and on pressed, dried, and rewetted handsheets for sulfite, kraft, and groundwood pulps. However, at higher pressures, the non-freezing water values decrease. This effect varies with the type of pulp. 

Steam Supply and Condensate Removal All cylinder dryers are heated by the steam condensing inside the sheet because of its efficiency (heat-transfer coefficient for film condensation of steam ranges from 5,700 to 17,000 W/m K (1,000-3,(XX) Btu/h ft °F). In the early days of papermaking, steam pressures as low as 14-21 kPa (2-3 psi) were used after World War n, the new machines for Kraft papers were made... 

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