Phenol Formaldehyde Resins (PF): are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Phenolic resins are mainly used in the production of circuit boards. They are better known however for the production of molded products including pool balls, laboratory countertops, and as coatings and adhesives. In the form of Bakelite, they are the earliest commercial synthetic resin. Phenol-formaldehyde resins, as a group, are formed by a step-growth polymerization reaction that can be either acid- or base-catalysed. Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene glycol oligomers, the concentration of the reactive form of formaldehyde depends on temperature and pH.
- Pure phenol crystallizes in long colourless needles, which melt at 41℃.
Xylenols: Of the six isomers only 3-5-xylenol has the three reactive positions necessary for cross-linking and thus mixtures with a hUpigh proportion of this isomer are generally used.
PREPARATION OF FORMALDEHYDE:
- These materials will then slowly react with further formaldehyde to form methylol derivatives which in turn rapidly react with further phenol to produce higher polynuclear phenols with 5-6 benzene rings per molecule.
- Novolak resins themselves contain no reactive methylol groups and do not form cross-linked structures on heating.
The end point is checked by noting the extent of flow of a heated pellet down a given slope, melting point,alcohol solubility, free phenol content and gelation time with 10% hexa.
- Above 160°C additional cross-linking reactions take place involving the formation and reaction of quinone methides by condensation of the ether linkages with the phenolic hydroxyl groups.
- It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic compression mouldings.
HISTORICAL DEVELOPMENT:
- Phenol formaldehyde polymers are formed by the reaction product of phenol, or mixture of phenols, with formaldehyde
- PF resins considered to be the first polymeric products produced commercially
- In 1872 Von Bayer obtained a colourless non-crystallizing resinous product from the reaction of phenol with formaldehyde, while he was investigating phenol based dyes.
- Similar materials in an acidic medium was also reported by ter Mer, Claus and Trainer, Claisen.
- In 1894 similar material in alkaline pH range was first reported.
- In 1907, Baekeland developed an economical method to convert these resins into mouldable compositions
- The first commercial phenolic resin plant, Bakelite GmbH, was started on 25th May 1910 by Rutgerswerke AG at Erkner near Berlin.
- PF resins are used as moulding powders, laminating resins, adhesive, binders, surface coatings and impregnants.
PREPARATION OF PHENOLS:
- Cumene process is the most important process and accounts for more than 95% of phenol capacity.
- In this process liquid propylene, containing some propane is mixed with benzene and passed through a reaction tower containing phosphoric acid on kieselguhr as catalyst.
OTHER PHENOLS:
Cresol: Only the m-cresol has the three reactive positions necessary to give cross-linked resins and as so this is normally the desired material.
- Formaldehyde is produced by dehydrogenation of methanol, over either an iron oxide/molybdenum oxide catalyst or over a silver catalyst.
- Because of the hazards in handling mixtures of pure oxygen and methanol, air is used as the oxidizing agent.
- Oxygen is used to burn the hydrogen by-product.
- Formaldehyde is a gas with a boiling point of –21℃. It is usually supplied a stabilized aqueous solution known as formalin
- Furfural is occasionally used to produce resins with good flow properties for use in moulding powders.
TYPES OF PHENOL FORMALDEHYDE RESINS
There are two types of phenol formaldehyde resins
1. Novolak Resins
2. Resol Resins
Novolaks:
- Prepared by reacting phenol with formaldehyde in a molar ratio I : 0.8 under acidic conditions.
- There is a slow reaction of the two reactants to form the Ortho and para hydroxymethylphenols.
- These then rapidly condense to form bis(hydroxyphenyl)methane (HPM).
- Three possible isomers, 2,4'- and 4, 4'-HPM compounds are the main products.
- When mixed with compounds capable of forming methylene bridges, e.g. hexamethylenetetramine (hexa) or paraformaldehyde, cross-link on heating to form infusible, 'thermoset' structures
Resols:
- Prepared by reacting phenol with excess of formaldehyde in a molar ratio I : 1.5 - 2 under basic conditions.
- Resulting polynuclear polyalcohols are low molecular weight liquid with less than two benzene rings per molecule, while a solid resol will have three to four.
- Heating the resins result in cross-linking via the uncondensed methylol groups or by more complex mechanisms.
- Resols are referred as one-stage resins since cross-linked products may be made by adjusting the pH.
- Novolaks on the other hand referred as two-stage resins as it is necessary to add some agent to enable additional methylene bridges.
RESIN MANUFACTURE:
Both novolaks and resols are prepared in similar reactor
Novolak resins : A typical charge ratio would be:
Phenol 100 parts by weight
Formalin 33 % w/w 70 parts by weight
Oxalic Acid 1.5 parts by weight
MANUFACTURE OF RESOL RESINS:
Typical charge for a laboratory-scale preparation would be:
Formalin (40%) 112 cm3 (1.5 moles formaldehyde)
Phenol 94 g (1 mole)
0.88 ammonia 4 cm3
- Reaction is followed by melting point, acetone or alcohol solubility, free phenol content or loss in, weight on stoving at 135°C.
- Two classes of resol distinguished, water-soluble resins prepared using caustic soda as catalyst, and spirit-soluble resins catalysed by addition of ammonia.
- Water-soluble resols are used for mechanical grade paper, and in decorative laminates.
- Spirit-soluble resins have good electrical insulation properties
CURING OF PHENOLIC RESINS:
- Novolaks and resols are soluble and fusible low molecular weight products.
- At temperatures below 160°C cross-linking occurs by phenol methylol-phenolmethylol and phenolmethylol-phenol condensations.
- As these reactions occur at the two ortho and the para positions in phenol, m-cresol and 3,5-xylenol, cross-linked structures will be formed.
Phenol is reactive towards formaldehyde at the ortho and para sites (sites 2, 4 and 6) allowing up to 3 units of formaldehyde to attach to the ring. The initial reaction in all cases involves the formation of a hydroxymethyl phenol:
HOC6H5 + CH2O → HOC6H4CH2OH
The hydroxymethyl group is capable of reacting with either another free ortho or para site, or with another hydroxymethyl group. The first reaction gives a methylene bridge, and the second forms an ether bridge:
HOC6H4CH2OH + HOC6H5 → (HOC6H4)2CH2 + H2O
The diphenol (HOC6H4)2CH2 (sometimes called a "dimer") is called bisphenol F, which is itself an important monomer in the production of epoxy resins. Bisphenol-F can further link generating tri- and tetra-and higher phenol oligomers.
2 HOC6H4CH2OH → (HOC6H4CH2)2O + H2O
PHENOLIC MOULDING POWDERS:
Phenol-folmaldehyde moulding powder will contain the following ingredients:
1. Resin.
2. Hardener (with Novolaks).
3. Accelerator.
4. Filler.
5. Lubricant.
6. Pigment.
7. Plasticiser (not always used).
Phenolic Resins: In addition to the selection of phenol used and the choice between novolak and resol there is a number of further variations possible in the resin. Eg Phenol/formaldehyde ratio, residual volatile content
Hardener: Hexa is used almost universally as the hardener, 10 to 15 parts of hexa are used in typical moulding compositions.
Accelerator: Basic materials such as lime or magnesium oxide increase the hardening rate of novolak-hexa compositions.
Fillers-Woodflour: Fine sawdust is most commonly used filler. It is not only an effective diluent for the resin to reduce exotherm and shrinkage, but it is also cheap and improves the impact strength.
Coconut shell flour: Incorporated into the moulding composition in large quantities and these results in cheaper mixes than the woodflour.
Coconut shell flour-filled mouldedings have poor mechanical properties and hence the filler is generally used in conjunction with woodflour.
Cotton Flack: For better impact strength cotton flock, chopped fabric or even twisted cord and strings may be incorporated.
The cotton flock-filled compounds have the greatest mouldability but the lowest shock resistance whilst the twisted cords and strings have the opposite effect.
Other Fillers: Nylon fibres and fabrics are used to confer strength and flexibility and glass fibres are used for strength and rigidity.
Asbestos are used for improved heat and chemical resistance and silica, mica and china clay for low water absorption grades (Now a days asbestos fillers is not recommended )
Iron-free mica powder is particularly useful for electrical insulation characteristics
Moulding compositions of enhanced heat resistance are developed by the use of heat-resisting polymers used in conjunction with asbestos and other mineral fillers.
Lubricants: Stearic acid and metal stearates such as calcium stearate are used as lubricants at a rate of 1-3% on the total compound
Pigments: Since the formation of quinone methides and their derivatives which impart a dark colour to the resin, the range of pigments available is limited to blacks, browns dark blues, greens, reds and oranges.
Plasticisers: Naphthalene, furfural and dibutyl phthalate are used as plasticisers or more strictly as flow promoters. They are particularly useful where powders with a low moulding shrinkage are required.
Typical formulation phenolic moulding composition
There are two types compounding process : Dry process and Wet process.
1. Dry Process:
- Finely ground resin is mixed with the other ingredients for about 15 minutes in a powder blender.
- This blend is then fed on to a heated two-roll mill. The resin melts and the powdery mix is fluxed into a leathery hide
- The hide from the mill is then cooled, pulverised with a hammer-mill and the resulting granules are sieved
- Extrusion compounders such as the Buss Ko-Kneader have been used for mixing phenolic resins. It is claimed that they produce a better product and are more economical than mill-mixers.
2. Wet Process:
- High-shock grades cannot be processed on mills or other intensive mixers without destroying the essential fibrous structure of the filler.
- In these cases a wet process is used in which the resin is dissolved in a suitable solvent, such as industrial methylated spirits, and blended with the filler and other ingredients in a dough mixer.
- The resulting wet mix is then laid out on trays and dried in an oven.
Properties of Phenol-Formaldehyde:
1. High-Strength Glass Fiber Reinforced
2. Relative Density 1.69-2.0
3. Water Absorption 24h(%) 0.03-1.2
4. Melting Temperature (◦c)
5. Thermo set Processing Range (◦F) C:300-380 I:330-390
6. Molding pressure I-20
7. Shrinkage 0.001-0.004
8. Tensile Strength 7000-18000
9. Compressive Strength 16,000-70,000
10. Flexural Strength 12,000-60,000
11. Izod Impact(ft-lb/in) 0.5-18.0
12. Linear expansion 8-21
13. Hardness Rockwell E54-101
14. Flammability V-0
15. Hard, stiff & brittle Does not melt or soften on exposure to heat
16. Excellent creep resistance
17. Good ageing properties
18. Excellent flexural fatigue resistance
19. Heat resistant
20. Good dimensional stability
21. Very good shear strength
22. Good chemical resistance
23. Good resistance to staining
24. Good insulation properties
25. Easy to mould
26. High dimensional stability
Bis-maleimides: This is the most recent polymer this polymer is produced by the condensation reaction of a diamine with maleic anhydride. It can be processed basically like the epoxy (350 °F (177 °C) cure). After an elevated post-cure (450 °F (232 °C)), it will exhibits superior properties. These properties are influenced by a 400-450°F continuous use temperature and a glass transition of 500 °F (260 °C). This polymer is merged into composites as a prepreg matrix used in electrical printed circuit boards, structural aircraft – aerospace composites, etc. It is also used as a coating material and as the matrix of glass reinforced pipes, particularly in high temperature and chemical environments.
PHYSICAL PROPERTIES
Mechanical and Thermal Properties
- Mechanical and thermal properties are strongly dependent on the type of filler used.
- Retention of physical properties at elevated temperature sets phenolic apart from other plastic materials.
- Phenolic mouldings have good flexural strength retentions in a wide range of temperatures
- Phenolic moulding compounds filled with asbestos and special fillers have a higher continuous service temperature range of 250 to 280℃.
- Impact resistance and hardness is a function of filler and resin type as well as degree of cure.
- Industrial/Mechanical grinding wheels, bobbins and sleeves, rice rollers, carbonless copy paper, adhesive paints, tool handles, industrial items, textile components, textile drums, welding torches and picker bushes.
Electrical properties
- As the mouldings are polar, the electrical insulation properties are not outstanding but are adequate for many purpose.
- One disadvantage of phenolic compared with aminoplastics and the alkyd resins is their poor tracking resistance under conditions of high humidity.
- Electrical/Electronics Fuse boxes, holders, connectors, lamp sockets, heat resistant lamp holders, fan regulator covers, telephone handsets, cyclostyling drums, relay base covers.
- Phenolic commutators are widely used in starter motors, hand-held appliances, vacuum cleaners, and fractional horse-power motors
- Electrical components such as circuit breakers, motor control parts, solenoid covers, relays, coil formers or bobbins.
CHEMICAL PROPERTIES
Resistance to chemicals
- PF materials are readily attached by aqueous sodium hydride solution but cresol- and xylenol – based resins are more resistant.
- PF mouldings are resistant to acids except 50% sulpuric acid, formic acid and oxidizing acids.
Weathering Resistance
- On continued weathering the phenolic mouldings retain their properties, however the surface becomes rough and matt
- The yellow-brown self colouration of phenolic moulded materials darkens on exposure to heat and light.
Resistance to High Energy Radiation
- Phenolic resin is very resistant to radiation.
- The best are the asbestos reinforced moulding compounds.
Flammability
- Flammability of PF moulding compounds depends on the basic substance and the additives.
Toxicological Assessment
- If good ventilation is provided,no health risk in the manufacture and processing of PF moulding compounds.
- Utensils made from PF moulding compounds which come into contact with any type of food or condiments are not permitted.
PROCESSING OF PHENOLIC COMPOUNDS
- Phenolic moulding compounds are processed by compression moulding, transfer moulding, injection moulding and ram extrusion with simultaneous application of heat and pressure.
Compression moulding
- Compression moulding yields the strongest product in that there is less damage to any fibrous filler compared with the other process.
Transfer moulding
- In transfer moulding process, pre-weighed, pre-heated compound is pressed by a plunger through a gate into the mould.
Injection moulding
- Injection moulding of phenolic compound is as same as the thermoplastic materials
- The advanced technique RTC (Runnerless Injection Compression) method has been developed for fast curing and improved cycle time.
Phenolic Laminates
Impregnation plant fitted with vertical drying oven.
Properties of a phenolic laminate will obviously depend on
- Type of resin used, catalyst, the concentration of methylol groups and the average molecular weight.
- Properties of the varnish, such as the nature of the solvent and the viscosity and resin content of the varnish.
Type of reinforcement.
- Moulding conditions, i.e.moulding pressure, temperature and time.
AVAILABILITY
- A variety of thermoset phenolics moulding compounds are available
- Asbestos-free products are available for various applications.
- There are special grades for the electrical and automobile industries
- Phenolic moulding compounds are supplied as powder, granules for compression, transfer and injection moulding.
- Fibrous and chipped moulding compounds are also available.
TYPICAL APPLICATIONS
Appliance
Coffee-pot bases and handles, knobs, stick handles and toaster ends are all made up of PF moulding compounds. Handles of Iron, Pressure cooker, Ovens, heat proof buttons, bottle closures, novelty items, mixie bases.
Household: Particle boards, glues, laminating wood and water outlets, bathroom fixtures, toilet seats, fibre glass & paper impregnation.
Automotive:
Phenolics are used in all power-assist break systems because of their excellent flexural fatigue, heat-resistance, ability to hold close dimensions, and low manufacturing cost. Auto coil tops, disk brake pistons, disconnect boxes, propellers, break linings, shell modelling, gear, clutch disks, lamp sockets, ignition parts, castor wheels, engine blocks and intake manfolds, Brake Liner.
- Transmission parts as thrust washers, spacers, and converter reactors are moulded from heat-resistance phenolic compounds
- Carburetor spacers, . Ignition parts, speedometer housings, solenoid covers, connectors and numerous other under-the-hood parts are moulded from heat-resistant and high impact grades of phenolics
Miscellaneous
Phenolic resin-paper laminates are extensively used for high-voltage insulation applications.
Cotton fabric laminates are used in the manufacture of gear wheels which are quiet running and which withstand shock loading.
Phenolic laminates have also been used in aircraft construction and in chemical plant.
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