Self Bonding

The first industrial hardboard was developed by W. Mason in the mid-1920s he found that a mat of wet fiber pressed in a hot press would produce a self-bonded flat panel with good strength, durabiUty, and stabiUty. The product was patented in 1928, trademarked as Masonite, and commercial production began. Over time several other processes for producing hardboards have been developed from modifications of the original process. Brief descriptions of these processes foUow and a flow chart of the process is shown in Figure 5. 

Neste patented an industrial route to a cellulose carbamate pulp (90) which was stable enough to be shipped into rayon plants for dissolution as if it were xanthate. The carbamate solution could be spun into sulfuric acid or sodium carbonate solutions, to give fibers which when completely regenerated had similar properties to viscose rayon. When incompletely regenerated they were sufficientiy self-bonding for use in papermaking. The process was said to be cheaper than the viscose route and to have a lower environmental impact (91). It has not been commercialized, so no confirmation of its potential is yet available. 

In gel-forming processes, the reactive aluminosibcate gel is first formed into a pellet which reacts with sodium aluminate solution and caustic solution. The 2eobte crysta11i2es in situ within an essentiaby self-bonded pellet, or as a component in an unconverted amorphous matrix. 

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

Self-bonded reaction-sintered Si3N4 parts... 

Fig. 2. Flow sheet for the manufacture of self-bonded, reaction-sintered sihcon nitride. Courtesy of Annawerk, Ceranox (Roedental, Germany).

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

The self-bonding tendency of B results in strong boron frameworks, which leads to predictions of defect structures that primarily involve changes in the fraction of the metal sites occupied. 

E. W. Hoyt, J. Chrone, Preparation of Self-Bonded Borides, Report USAEC-GEAP-3332, 1960 Chem. Abstr., 55, 1338c (1961). 

Self-baking electrodes, 12 305, 755 Self-bonded reaction-sintered silicon nitride, 17 210, 211 Self-catalyzed polyols, 25 464 Self-cleaning materials, 22 108-127 problems and outlook for, 22 123-124 surface characteristics of, 22 108-109... 

Chemical modification with a bifunctional reagent Polymer grafting, self-bonding, stability to weathering... 

Bonding of a reagent to the wood surface in order to thermoplasticize the surface for self-bonding. 

Surface Chemical Modification for Self-bonding by Thermoplasticization... 

Chemical modification of wood surfaces can be employed in order to provide active sites to allow for self-bonding directly, or to allow for covalent bonding between wood... 

Fenton s reagent chemistry has been used for the direct self-bonding of wood particles. Indirect methods, involving the use of surface-activated fibres in combination with other bonding agents such as furfural and lignosulphonates, have also been studied (Philippou, 1982 Philippou etal, 1982). Since the use of Fenton s reagent requires the presence of... 

Kharazipour etal. (1998) used a peroxidase enzyme in combination with H2O2 to activate the surface of TMP fibres for self-bonding. Fibres were activated in a wet system, then dewatered and fluffed out before pressing at 190 °C for 5 minutes. The best IBS recorded for boards of 5 mm thickness made from the activated fibres was 0.55 MPa. The IBS was found to be dependent upon the pH of the treatment solution, the time of treatment and the board density. The authors noted that phenoloxidase gave comparable results to laccase, which was unexpected, since it was thought that the phenoloxidase would lead only to depolymerization of the lignin. 

Kiguchi, M. and Yamamoto, K. (1991). Dimensional stability and decay resistance of hot-melt self bonded particleboard by surface benzylated pine chips. International Research Group on Wood Preservation, Doc. No. IRGAVP 3652. 

Kiguchi, M. and Yamamoto, K. (1992). Chemical modification of wood snrfaces by esterification III. Some properties of self-bonded benzylated particleboard. Mokuzai Gakkaishi, 38(2), 150-158. 

Figure 2 is a schematic diagram of the two-section reactor body and accessories. The fluid bed section was made of self-bonded silicon carbide, 16 inches high by 6% inches o.d. with a recessed flange. The recess accommodated a 120-mesh porosity silicon carbide gas distribution plate. The nickel manifold assembly was topped by a heavy support flange. This manifold sup-... 

Hoyt and Chorne [230] have recently reported the preparation of several self-bonded dense borides. EuBe (90%) was made by hot vacuum pressing in graphite dies at 1800—2000° C. Temperature, pressure and time are important variables. Unsuccessful attempts have been made to prepare EUB4. 

Silicon Nitride. Silicon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, silicon nitnde parts. 

One year later, Burrows published results from a comprehensive series of experiments using 100% Douglas-fir bark as furnish for particleboard (35). No binder was used he relied upon the "self-bonding" properties of Douglas-fir bark. Variables included bark particle size, mat moisture content, pressing pressure, and use of various overlays. Additional boards were made from ponderosa pine, western hemlock, and white fir barks. A pilot-plant-size run was made using results gathered from the study. 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

Method 2 produces what is usually referred to as self-bonded SiC by a process essentially the same as that developed for nuclear applications [2] and known as Refel silicon carbide. Refel silicon carbide contains a finely dispersed residual silicon phase accounting for between 5 and 10% of the total volume, the precise amount depending upon the porosity of the original preform. In the case of self-bonded silicon carbide elements the residual silicon is volatilized away by a final heat-treatment at over 2500 °C when recrystallization occurs resulting in grain growth and further bonding between primary and secondary carbide phases. 

Forrest, C.W., Kennedy, P. and Shennan, J.V. (1972) The fabrication and properties of self-bonded silicon carbide bodies, in Special Ceramics 5, The British Ceramic Research Association, Stoke-on Trent, UK. 

Phosphate bonded ceramics have several advantages over their cements. Unlike polyalkenoate cements, phosphate based ceramics are entirely inorganic and nontoxic. Unlike Portland cement, which is formed entirely in an alkaline solution, these are acid-base cements, and are neutral. They are stable in a wider range of pH, and since they are made from natural minerals, the raw materials needed for their manufacture are readily available. For the same reason, they are also less expensive compared to other acid-base cements. They are self-bonding, i.e., a second layer will bond intimately to a first set layer. These attributes motivated further research into phosphate bonded materials for... 

---