Resins compression

Southern pine, Douglas-fir, and yellow poplar stakes were impregnated with phenolic resin and cured (impreg) or impregnated with phenolic resin, compressed, and cured (compreg). Separate samples were treated with urea-formaldehyde and cured. These samples were placed in the ground and their average lifetime determined. The results are shown in Table I (18). 

The aged lubricated resin without lumps is loaded into the preform cylinder and is evenly distributed around the core mandrel to ensure uniform compaction throughout the preform. The pusher is put on top of the cylinder and compaction commences. Resin compression can begin at a fairly rapid rate but has to be reduced at the latter stages of compaction. This rate reduction is size dependent (Table 5.4) and is aimed at the prevention of air entrapment, otherwise the preform may crack. 

Apart from the wet processes it is also possible to make pultruded sections from prepregs. The forming procedure is the same as that used with wet resin systems. The prepreg is drawn through a heated die which melts the resin, compresses the prepreg into the required shape and cures the resin. This is a somewhat cleaner process than that using a resin bath. 

The silicone oils and silicone resins find application as (i) lubricants (their change of viscosity with temperature is small), (ii) hydraulic fluids (they are unusually compressible), (iii) dielectric fluids, (iv) for the pro duction of water-repellant surfaces, and (v) in the electrical industry (because of their high insulating properties). 

Fabrication. Acetal resins are most commonly fabricated by injection mol ding. A homogeneous melt is essential for optimum appearance and for performance of injection molded parts. A screw compression ratio of no less than 3 1 is advised and the size of the injection molded shot should be 50 to 75% of the rated capacity (based on polystyrene) of the barrel. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

During the press operation, which is actually a form of compression mol ding, the resin-treated laminate pHes are heated under pressure and the resins cured. The initial heating phases cause the resin to melt and flow into voids in the reinforcing ply and bond the individual pHes together. The appHed heat simultaneously causes the resin to polymerize and eventually to cross-link or gel. Therefore, resin viscosity reaches a minimum during the press cycle. This is the point at which the curing process becomes dominant over the melt flow process. Dynamic mechanical and dielectric analyses (11) are excellent tools for study of this behavior. 

Mechanical Properties. The performance of various polyester resin compositions can be distinguished by comparing the mechanical properties of thin castings (3 mm) of the neat resin defined in ASTM testing procedures (15). This technique is used widely to characterize subtle changes in flexural, tensile, and compressive properties that are generally overshadowed in highly filled or reinforced laminates. 

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