FIELD OF INVENTION:
This invention relates to a process for the preparation of phenolic resin sphere which is used as precursor material for the preparation of activated carbon spheres.
PRIOR ART
It was noticed by A. Bayer in 1872 that Phenol and formaldehyde react to give resinous substance. However it was the work of the Fleming. L. H. Bakeland, Lebach and others, who provided the impetus for the industrial production of phenolic resins. However, the prepolymers obtained in the first stages, whether novolac or resole, are thermoplastic resins of low molecular weights and they must be further polymerized to become thermosetting resins.
Since the discovery of phenol-formaldehyde resin in 1907, numerous reports have been published about modification of the original reaction of Bakelite. Bakelite was the product obtained by base-catalyzed reaction between phenol and formaldehyde, which involves two separate operations. The first step involves the formation of a low molecular weight fusible resin soluble in common solvents and the second operation involves curing operation, which is result of a controlled reaction between a phenol and formaldehyde to give polymeric products. Some researchers tried to use other aldehydes than formaldehyde, whereas others used different phenols and also tried to modify polymerization conditions.
Cross-linked polymer resins are useful in a wide range of chemical applications. In many applications, it is advantageous to produce cross-linked resins in the form of regular microspheres of controlled size, which can be achieved by using heterogeneous polymerization techniques, including suspension, dispersion and emulsion polymerization. Suspension polymerization technique is useful for preparation of cross-linked polymer beads in the size range 10-2000 pirn. A typical oil-to-water (0/W) procedure involves the suspension of an immiscible, oil-soluble monomer in water.
These particles are useful in many applications such as preparation of ion selective resins. Another important application is in preparation of activated carbon in spherical form (ACS) having superior adsorption capacity and mechanical properties. The carbonized microspheres of phenolic resin may be used be as electrodes in ii-ion batteries. The ACSs are new generation materials with potential for wide range of novel application in personal protective clothing/mask against toxic gases in chemical and biological warfare. ACS also find applications in the field of purification of chemicals and drugs, pollution control, treatment of potable water, solvent recovery and respirators meant for removal of toxic chemicals.
The polymerization technique, recently, has been shown to be useful for the synthesis of phenolic resin spheres in a Japanase Patent (No 2000,53,738). According to the process, as disclosed in this patent, phenol reacts with formaldehyde in aqueous solvent in the presence of TEA and PVA to give spherical particles having average particle size 160 j.tm.
Japanese Patent No 11,116/648 also discloses a process for the formation of spherical phenolic resin, especially those with wide diameters 10% (<212 |im) 36% (212-1000 j^m) 54% (>1000 urn).
The main disadvantage of the above process is that the broadening in the particle size distribution may be caused either by the extended particle nucleation or the initial formation of broadly dispersed nuclei that preserve their broad distribution during the growth. Which depends upon the type and concentration of stabilizer, the type and agitation rate, and monomer-to-water ratio.
Another disadvantage of the above process is that it is difficult to control the structural properties of phenolic resin spheres, which depends on many factors involved in the synthesis of the polymer, including pH value, nature and amount of the catalyst, initial formaldehyde-to-phenol ratio (F/P).
Yet another disadvantage of the above process is that the yield of the particles in this process is very low.
The present invention relates to an improved process and particle size distribution by changing concentration of stabilizer, agitation speed and monomer-to-water ratio in the reaction. The use of this suspension polymerization process is to form desired particle size distribution in cross-linked beads ranging from 0.1-2.0 mm in diameter with higher yield. In addition, in this invention, phenolic resin spheres of uniform diameter 1.1-1.3 mm can be synthesized by controlling the reaction parameters.
OBJECTS OF THE INVENTION:
The primary object of the invention is to provide a process for the preparation of phenolic resin spheres, which can be used as precursor material for preparing activated carbon spheres.
Another object of the invention is to provide a process for the preparation of phenolic resin spheres in which dispersion of monomer polymerization process is carried out in one step by suspension polymerization using water, a cheaper and easily available solvent, as a solvent,
Yet another object of the invention is to provide a process for the preparation of phenolic resin spheres, in which poiymer collection can be done easily in the form of beads.
Still further object of the invention is to provide a process for the preparation of phenolic resin spheres which uses PVA as stabilizer, a cheaper and easily available raw material in India.
Yet further object of the invention is to provide a process for the preparation of phenolic resin spheres, which uses commercially available phenolic resin.
^till further object of the invention is to provide a process for the preparation of phenolic resin spheres having spherical shape and higher yield.
Yet further object of the invention is to provide a process for the preparation of phenolic resin spheres, wherein the size of particles as well as size distribution of the particles can be controlled by changing concentration of stabilizer, agitation rate and monomer-to-water ratio.
DESCRIPTION OF INVENTION
According to this invention there is provided a Process for the preparation of phenolic
resin spheres comprising the steps of :-
(i) preparing a monomer: methylol phenol by mixing phenol and formaldehyde in the presence of basic catalyst, agitating the mixture at 300-100 rpm for 2-10 hrs. and maintaining the temperature of the reaction vessel in the range o 20-50°C;
(ii) dispersing monomer methylol phenol, obtained from step (i) in water by adding 50-100 parts (by weight) of distilled water to the said reaction vessel and agitating the mixture at 450-700 rpm for 1-2 hr and maintaining the temperature in the range of 20-50°C;
(iii) stablizating the monomer spheres obtained from step(ii) by adding 0.7-2.0 part of a stabilizer such as polyvinyl alcohol (PVA) to the said reaction vessel, raising the ambient temperature to 95°C at the rate of 5°C per minute, maintaining the temperature at 90-98°C for 20-90 minutes while maintaining the agitation rate in the range of 450-750 rpm;
(iv) thermosetting the stabilized monomer spheres obtained from step (iv) by adding 0.5-1.5 parts of a cross linker such as hexamethylenetetaamine (HMTA) and maintaining the temperature and agitation rate to the level of steps (iii) for another 2-10 hrs;
(v) separating and drying the phenolic resin spheres by cooling the slurry obtained from step (iv) to room temperature, washing the filtrate comprising phenolic resin spheres with water, methanol and acetone to remove protective colloid and other impurities and drying.
The sizes of particles (diameter) are also to be dependent on the type and concentration of stabilizer and monomer-to-water ratio. In the present invention, the monomer polymerization process is carried out in one step by suspension polymerization using water, a cheaper and easily available solvent, as a solvent. Further, the process uses PVA as stabilizer, a cheaper and easily available raw material in India. In the present process, the polymer collection can be done easily in the form of beads. Further, in this process, each monomer droplet is isolated and is independent of the other droplets. The continues aqueous phase separating the monomer droplets acts as an efficient heat transfer medium and hence, exothermically is well controlled.
In suspension polymerization, the polymerization is conducted in 1000 ml round bottom four-neck reaction vessel fitted with a Teflon stirrer, a reflux condenser and thermocouple. In this invention, the condensation of phenol and formaldehyde is carried out in a respective molar ratio by preference 1:1 to 1:4.5 in the presence of TEA as alkaline polymerization catalyst and maintained the pH value 7-12. The reactor is charged with a mixture of phenol and formaldehyde, initiator TEA and aqueous solution of stabilizer poly (vinyl alcohol). The PVA concentration is varied between 1.0 to 30.0 wt percent heated for 15 to 90 minutes at temperature 15-60°C. In the next step, cross linking agent (HMTA) is added to reaction vessel and the polymerization is carried out between 70-98°C temperature range, and agitation speed 300-800 rpm for 2 to 10 hrs at fixed speed to produce the desired particle size distribution.
At the end of the reaction, the reactor is cooled and reaction product isolation becomes easy, as this involves only filtration. The spheres so obtained are washed with water and methanol for removal of surface-active agent. The spheres are finally washed with acetone before drying. The polymer conversion is determined gravimetically. The weight of the stabilizer is subtracted from the total mass of product to obtain the weight of phenolic resin spheres.
DERAILED DESCRIPTION OF THE INVENTION:
The process of the present invention for the synthesis of phenolic resin spheres, uses a 1-5L round bottom four neck reactor vessel fitted with a Teflon stirrer, a reflux condenser and thermocouple. In the first stage, formation of monomer(s) takes place from a mixture of phenol and formaldehyde as precursor materials, and TEA as initiator at agitation speed 500 jpm at temperature 20 to 60°C. In the second stage, the monomer is dispersed in water. In this process the water insoluble monomer(s) (e.g. phenol-formaldehyde) is dispersed in a non-solvent (usually water) by the action of mechanical force (stirring) and is polymerized in this form. Under the effect of interfacial tension, the monomer is converted into spherical droplets.^ In the last and the third stage, polymerization of reaction mixture to final product takes place. Under the effect of interfacial tension, the monomer forms spherical droplets, which are stabilized by use of suspension stabilizer such as, poly(vinyl alcohol). This substance brings an increase in viscosity, \vhich reduces the probability of the agglomeration of particles. In addition, it increases the interfacial tension between monomer droplets and water and reduces the difference in density of the two phase, thereby presenting an additional hindrance to coalescence of droplets. In this invention, preferably an organic precursor material is used, which can be properly mixed with the help of stirrer. A precursor for such a resin is understood to be a monomer or a prepolymer, possibly mixed with at least one polymerization catalyst.
In this invention, the condensation of phenol and formaldehyde is carried out in a respective molar ratio by preference 1: 1 to 1: 4.5 in the presence of TEA as alkaline polymerization catalyst and maintained the pH value 7-12.. It is equally possible to use a phenol formaldehyde resin of the novolak type. It is also possible to use other resins for example phenol furfural, phenol-acetaldehyde etc. A suspension stabilizer is also introduced to the mixture in order to prevent coalescence of the particles during the expansion of the particles while the mixture is still unhardened. The stabilizers are surface-active substances, which contains combining groups, which form a bond to the water phase and monomer phase even at a low concentration. These substances form a thin film at the phase boundary (i.e. on the surface of the droplets) with the consequence that the highly viscous particles repel each other, and coagulation is avoided. When selecting stabilizers, it must be ensured that they can be separated from the polymer particles again after polymerization attachment points. At least one cross-linking agent is also added to the mixture in order for setting the chemicai links between molecular chain to form a three dimensional network of connected molecules
In this process, droplets of the spherical shape form under the effect of interfacial tension with the help of stirrer. In addition, the interfacial tension between monomer droplets and native increase by stabilizer and reduce the difference in density of the two phase, thereby presenting an additional hindrance to coalescence of the droplets. The particle size and their size distribution can be influenced over a broad range by appropriately adjusting the stirrer conditions, speed and design of stirrer, and reactor shape. In suspension polymerization, the particle has approximately the same diameter as the original monomer droplets. The size of monomer droplets also depends on
mechanical force acting across the drops. The agitation rate is affecting the size and shape of particles. So an appropriate agitation speed rates about 500 - 600 rpm (in our study) is a necessary for preparation of phenolic resin spheres. Since polymerization takes place in large number of small monomer droplets with diameters of 10 to 5000 pun, the heat of polymerization can be dissipated with much greater ease. It is, therefore, possible to carry out this type of polymerization in much shorter time i.e. 4.5 hrs.
The process of the present invention comprises of following steps: (i) Preparation of monomer methvlol phenol
The monomer methylol phenol is formed by mixing of phenol and formaldehyde in the presence of basic catalyst. 10-15 parts (by weight) phenol, 7-35 parts (by weight) formaldehyde and 0.4-1.5 parts (by weight) triethylene amine are mixed to a four-neck reaction vessel of 1000-5000 ml capacity. The reaction vessel is fitted with Teflon stirrer, reflux condenser and a thermocouple. The pH value of the mixture is maintained in the range of 9-11. The mixture is agitated at 300-100 rpm for 2-10 hrs. and the the temperature of the reaction vessel is maintained in the range of 20-50°C. At the end of this step, monomer methylol phenol is obtained.
(ii) Dispersion of monomer methvlol phenol
In this step, the dispersion of the monomer, prepared in step (i) in water takes place. 50-100 parts (by weight) of distilled water is added to the reaction vessel used in step 1 containing monomer. The mixture is agitated at 450-700 rpm for 1-2 hr for better dispersion of the monomer. The temperature of the reaction vessel is maintained in the range of 20-50°C during dispersion. . In the process of agitation, formation of monomer spheres takes place.
(in) Stabilization of the monomer spheres:
In this step, 0.7 -2.0 part of a stabilizer such as polyvinyl alcohol (PVA) is added to the reaction vessel containing the monomer spheres. Alternatively, gelatin, and methyl cellulose can also be used as stabilizer The temperature of the reaction vessel is raised from ambient temperature to 95 °C at the rate of 5 °C per minute and the temperature of the reaction vessel is maintained at 90-98 °C for 20-90 minutes. The stirring of the mixture is continued while maintaining the agitation rate maintained in the range of 450-750 rpm. The stabilization of the monomer takes place in the presence of PVA, a stabilizing agent
(iv) Tnermosetting of the stabilized monomer spheres:
In this step, 0.5-1.5 parts of a cross linker such as Hexamethylenetetraamine (HMTA) is added to the reaction vessel containing the stabilized monomers. The temperature and agitation rate used in step 3 is maintained for another 2-10 hrs. In this step, hardening of the monomer spheres take place due to the crosslinking . At the end, slurry containing hard phenolic resin spheres (PRS) is obtained.
(v) Separation and drying of PRS:
The reaction vessel containing PRS in the form of slurry is cooled to room temperature. The slurry is filtered and PRS are obtained as filtrate. The filtrate consisting PRS is washed with water, methanol and acetone subsequently to remove protective colloid and other impurities. Finally, the PRS, thus obtained, are dried which are used as precursor for preparation of active carbon spheres.
The invention will now be illustrated with working examples, which are intended to be illustrative examples, and are not intended, to be taken restrictively to imply any limitation on the scope of the present invention.
Working Example 1
In suspension polymerization, the polymerization was conducted in 1000 ml round bottom four-neck reaction vessel fitted with a Teflon stirrer, a reflux condenser and thermocouple. For typical polymerization, 50g of phenol with 24.5 g of formaldehyde and 1.5 ml of TEA were mixed at temperature 30°C and pH value of polymerization initiation was 10. The monomer obtained in above step was dispersed in 210 ml of distilled water with the stirring. Then 2.5 g of PVA was added to reaction vessel and stirring was continued for 30 minutes at 95 - 97°C. In the next step, 2.7 g of HMTA was added and polymerization was carried out at temperature 95 - 97°C at agitation speed (550 rpm) continued for 4.5 hrs. At end of the reaction, the reactor was cooled and isolation of the spheres became easy as this involved only filtration of spheres and removal of the protective colloids by water washing. Finally spheres were washed with methanol and then acetone before drying obtained as spherical resin spheres. The polymer conversion was determined gravimetrically.
The properties of end product obtained are as follows:
The particle size distribution : 1..0 - 1.4 mm
Yield of phenolic resin spheres : 81.5%
Density : 0.8 g/cm3
Spherical shape : 95 %
The free flowing powder of size below 100 urn (11.3%) is obtained, which is not taken into account for total yield calculation.
Working Example 2
The reactor was charged with 50 g of phenol and 24.5 g of formaldehyde with 1.5 g of TEA treated to polymerization, was carried out with 11.0 g of PVA and 3.7 g of HMTA at temperature 95 - 97°C agitation speed 550 rpm and for 4.5 hrs. The rest of the conditions were same as that provided in working example-1 above.
The properties of end product are as follows:
The particle size distribution : 0.1 - 0.02 mm
Yield of phenol resin spheres : 91.2 %
Density : 0.85 g/cm3
Spherical shape : 95%
The present embodiment of the invention, which has been set forth above, was for the purpose of illustration and is not intended to limit the scope of the invention. It is to be understood that various changes, adaptations and modifications can be made in the invention described above by those skilled in the art without departing from the scope of the invention, which has been defined by following claims

WE CLAIM:
1. A process for the preparation of phenolic resin spheres for the preparation of activated carbon spheres comprising the steps of:-
i) preparing a monomer methylol phenol by mixing 10-15
parts by weight of phenol, 7-35 parts by weight of formaldehyde in the presence of 0.4-1.5 parts by weight of basic catalyst as herein described, agitating the mixture at 300-100 rpm for 2-10 hrs. and maintaining the temperature of the reaction vessel in the range of 20-50°C characterized by,
ii) dispersing monomer methylol phenol, obtained from step (i) in water by adding 50-100 parts (by weight) of distilled water to the said reaction vessel and agitating the mixture at 450-700 rpm for 1-2 hr and maintaining the temperature in the range of 20-50 °C;
iii) stabilizating the monomer spheres obtained from step (ii) by adding 0.7-2.0 part of a stabilizer such as polyvinyl alcohol (PVA) to the said reaction vessel, raising the ambient temperature to 95 °C at the rate of 5 °C per minute, maintaining the temperature at 90-98 °C for 20-90 minutes while maintaining the agitation rate in the range of 450-750 rpm;
iv) thermosetting the stabilized monomer spheres obtained form step (iv) by adding 0.5-1.5 parts of a cross linker such as hexamethylenetetaamine (HMTA) and maintaining the temperature and agitation rate to the level of steps (iii) for another 2-10 hrs;
v) separating and drying the phenolic resin spheres by cooling the slurry obtained from step (iv) to room temperature, washing the filtrate comprising phenolic resin spheres with water, methanol and acetone to remove protective collocid and other impurities and drying.
2. A process for the preparation of phenolic resin spheres as claimed
in preceding claims, wherein said basic catalyst is selected from
sodium hydroxide, ammonia or triethyleneamine.
3. A process for the preparation of phenolic resin spheres as claimed
in preceding claims, wherein said stabilizer is selected from gelatin
and methyl cellulose.
4. A process for the preparation of phenolic resin spheres as claimed in any one of claims 1 to 3, wherein the size of the said active carbon spheres is between 0.01-2.0 mm.
5. A process for the preparation of phenolic resin spheres substantially as described and exemplified herein.