Phenolic Resins with Fillers

For the MI and HI compounds, fibers are added to increase the impact strength. Cotton fibers and paper pulp fibers increase the impact strength of the MI PF by five times. Rayon yard, nylon filament, or glass fibers are used to increase the impact resistance of the HI PF up to a factor of 20. However, the fiber-reinforced compounds suffer from poorer surface finish and they require higher molding pressures. The composition of a typical PF mixture is listed in Table 7.1.  

Source Adapted from Milby, R. V., Plastics Technology, McGraw-Hill, New York, NY, 1973, 581pp. 

The Society of Plastics Industry (SPI) has adopted an SPI grade number or notation for PF illustrated as follows for the following example  

Pot handles, knobs, camera parts, iron handles, closures, appliances, 

Impact resistant Gears, gunstocks, portable tools, pulleys, welding rod holders 

---

In the past, phenolic mixes were simply a combination of the phenolic resin with a filler such as walnut shell flour or pecan shell flour. These type mixes are still used in some hardwood exterior plywood. Todays phenolic glue mixes for softwood plywood involve mixing phenolic resin with water, filler, extender and sodium hydroxide (usually 50 percent). 

Fire resistance is an important property of phenolic resins. The combination of phenolic resin with Expancel expandable microspheres leads to many useful products. Composites for high speed train interiors take advantage of the light weight, excellent fire rating, and very low thermal conductivity. Polyester filled with aluminum hydroxide is an alternative solution for train interior materials. The resin and filler can be easily processed when viscosity regulating additives are added. 

Resin Filler mixture Phenolic resin with Wood flour and Cellulose and Cellulose and... 

Reclaimed fillers include macerated fabric and cord. These fillers lead to compounds with 10 or more times the impact strength of corresponding wood flour-filled compositions. Reclaimed rubber, such as nitrile rubber, improves flexibility as well as impact strength in phenolic resins. Thermoplastic fillers are also used in thermoset resins for the purpose of increasing toughness and impact strength. 

Another approach is slurry molding and this technique was firstly used to manufacture carbon-carbon composites by Besmann et al. (2003). This process mixes phenolic resin with carbon fillers in water to create slurry which is fed out and vacuum molded into a preform. A second process called carbon chemical vapor infiltration (CVl) is then used to seal the plate for gas impermeability and for improvement of electrical conductivity. This process has been further developed by Huang et al. (2005) to reduce the cost caused by the CVl process and by Cunningham et al. (2007) to improve the properties of the bipolar plates. However, the mechanical properties of the bipolar plates were not found to be as high as the solely wet-lay-based plates (see Figures 6.7 and 6.8). 

Figure 14.19 Relative Shore D surface hardness response for phenolic resin with varying percentages of biobased fillers.

Bulk Molding Compoimd, BMC, (Dough Molding Compound in Europe) is produced by first mixing pre-catalyzed liquid resin with fillers, mainly calcium carbonate and talc, in a heavy duty low speed sigma blade mixer. This is compression molded at 500 psi and 300 to 400°F. The resin most commonly used is unsaturated styrene-diluted polyester. Other BMC resins are alkyds, phenolics, urea, melamine, diallyl phthallate, silicones and epoxy. All are highly filled with calcium carbonate, talc, mica or alumina to improve mechanical properties and reduce shrinkage. 

Whilst the injection moulding process has now been widely accepted for phenolics the transition from compression moulding has been less extensive with U-F materials. The basic reason for this is that the U-F materials are more difficult to mould. This has been associated with filler orientation during moulding, which can lead to stress peaks in the finished product which the somewhat brittle resin in less able to withstand than can a phenolic resin. 

The second path in Fig. 3 outlines the approach to a more robust tape designed by Drew [21]. Here the milled rubber and filler are combined with tackifiers and other additives/stabilizers in an intensive dispersing step, such as a Mogul or Banbury mixer. Next, a phenolic resin or an alternative crosslinker is added and allowed to react with the rubber crosslinker to a point somewhat short of crosslinking. The compounded mixture is then charged to a heavy duty chum and dissolved in a suitable solvent like mineral spirits. To prepare a masking tape. 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

Wood flour was one of the first fillers used with phenolic resin. 

By far the most important phenolic resins are those made from phenol and formaldehyde. They exhibit high hardness, good electrical and mechanical properties, and chemical stability. Very often they are used in combination with (reactive) fillers like sawdust, chalk, pigments etc. 

The high frictional coefficient (0.4 to 0.5 compared with < 0.1 for glass fibers) of asbestos fibers is crucial to its utilization in the frictional lining sector. In the manufacture of brake and clutch linings 20 to 60% asbestos is incorporated together with fillers, metal chips and preferably phenol resins and rubber into a composite material, which has to satisfy many requirements. Currently there are asbestos-free so-called semimetallic brake linings, which consist of mixtures of metal fibers, metal powders, cellulose fibers, aluminum silicate fibers and mineral wool bonded with synthetic resins. 

Current research indicates that there is a growing interest in natural fibers. Natural fibers Ifom jute were tested in thermosetting and thermoplastic resins. Lignin fillers were used in phenol-formaldehyde, SBR, SBS, and S1S ° and with good results. The opportunities for applications of natural fibers in industrial products have been the subject of recent reviews. Cellulose whiskers with a high reinforcing value were obtained from wheat straw. " Wood fibers were found applicable to such diverse materials as polypropylene... 

Lignin fillers decreased the cure rate of phenol-formaldehyde resin. Here, the filler acts as a diluent and does not have the ability to affect the reaction kinetics by interaction with the polymer. Glass fibers also decreased the rate of cure of a phenolic resin in another study. 

Methods of filler pretreatment lignin treated by methylolation decreases the rate of cure of phenolic adhesives " carbon fiber was anodicaUy oxidized and subjected to various treatments with coupling agents to improve interfacial interaction with phenolic resins and oxidative stability of carbon fibers titanate coupling of oxidized fibers resulted in improved adhesion to matrix and enhanced thermal stability of fibers ... 

Typical systems are polyester and phenolic resins filled with glass fibres and mineral fillers. 

---