S. D. Bruck The Johns Hopkins University Applied Physics Laboratory
Silver Spring, Maryland.
Thermally Stable Polymeric Materials
Synthetic polymeric materials that are stable a t high temperatures are of great importance in a variety of applications such as protective coatings, nose cones for missiles, ablative heat shields for satellites, electrical insulators, and other aero-space uses. These recent technological demands have stimulated vigorous research and develop~nentefforts aimed a t the synthesis and physico-chemical characterization of high polymeric materials capableof reliably performing under extreme environmental conditions. Such important technical challenges, however, can be met successfully in the long run only if enough fundamental information is available to guide the synthesis of new polymeric materials and the optimum utilization of both the new materials and those already available. The object of this article is to discuss some of the factors responsible for high thermal stability in polymers, to review briefly a few of the most important advances that have recently been made in the synthesis of such materials, and to present some of the results of the thermal degradation of an aromatic polyimide which is under investigation in the author's laboratory. The Thermal Stability of Polymers
The thermal degradation of addition polymers follows largely free-radical initiated chain reactions. Condensation polymers, on the other hand, may be influenced by traces of moisture and acidic catalysts present in the material which promote hydrolytic breakdown, thus
According to Sirnha and Wall (I), freeradical initiated chain reactions involve the following elementary steps: initiation, depropagation, intermolecular or intramolecular transfer, and termination. The initiation step may be brought about by thermal, radiation, or chemical means; this primary attack may occur a t random, or a t weak points in the polymer chain. Unlike gas-phase reactions where depropagation is favored, most solid-phase denadation of polymers The degradation of polymers is governed by the dissociation energies of the bonds in the chain. I n gasphase degradation, the free radicals that are formed as the result of bond rnpture are able to combine freely (the activation energy of this recombination is nea& zero) but in the condensed-phase such as in the case of solid polymers, the free radicals must overcome an 18 / Journal of Chemical Education
additional energy barrier due to the viscous medium in which they find then~selves. This latter phenolnenon is the so-called "cage-effect" (8) and is responsible for the higher activation energies needed for bond rupture than are usually observed in gas-phase reactions. During the degradation of polymeric materials, two processes can take place: scission (rupture) of the bonds and crosslinking. Polymers in which the "backbone" undergoes mainly scission usually vaporize completely without leaving any appreciable quantity of residue, if heated (pyrolyzed) for prolonged periods. As a consequence, the molecular weight and strength of the material gradually decrease until all the polymer is vaporized. This is not true of those polyn~ersin which the formation of crosslinks is promoted. These materials become stabilized due to the formation of a rigid insoluble network, exhibiting increased toughness and having no melting point. Both chain scission and crosslinking follow a free radical mechanism and are initiated by the abstraction of hydrogen atoms from the polymer chain. The incorporation of aromatic rings into the polymer chain, however, tends to increase the resistance of the system to free radical attack, since much higher energy is required for the removal of a hydrogen atom from a benzene ring. The thermal stability of a polymer can be increased by the replacement of the hydrogen atoms by fluorine as the side groups of the polymer backbone; an example of this is poly(tetrafluoroethylene) or Teflon.' The high thermal stability of Teflon is due mainly to the "shielding" of the carbon-carbon links of the polymer backbone from oxidative cleavage by the overlapping orbitals of the fluorine atoms, and to chain stiffness as the result of close packing of the fluorine atoms. Another approach for iucreased thermal stability involves the synthesis of polymers in which the polymer backbone is composed of inorganic elements such as Ti, Al, P, B, etc. instead of carbon. For example, the polymer polysiloxane
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has additional aluminum atoms in the chain. By replacing some of the silicon atoms by a metal the thermal stability of the polymer chains is increased because polysiloxanes tend to undergo rearrangement which splits the chains. Thermally stable boron and silicon containing polymers with carborane (>C;BIOHIO) groups have recently been reported having the following structure
At least two other important discoveries have very recently been made concerning ordered polymer systems. In one group of inorganic coordination polymers reported by Block and associates (4),beryllium, chromium, zinc, and other metal atoms are linked together by pairs of diphenylphosphinate groups, forming a double chain metallophosphinate polymer:
In this polymer, a nucleus of ten boron atoms is bridged by two carbon atoms. The proposed structure of the carborane nucleus is that of %sphere having an effective van der Waals radius of 4A. These carborane-silicon polymers are stable above 400°C, and are fusible and soluble in organic solvents; such properties are important in assessing the ultimate industrial usefulness of the polymers. Linear and random network polymers do not exhaust the range of possibilities. Network polymers with varirms degrees of order, (stereospecificity) can he made which exhibit extremely high thermal stability. Considering the naturally occurring polymers, probably the best known example of an ordered network is asbestos:
Among synthetic organic polymers that have a t least some ordering in a double chain arrangement is "Black Orlon," obtained by catalytic heating of a polyacrylonitrile fiber (Orlon)' in the presence of controlled quantities of oxygen. A possible mechanism of this reaction is: H
The beryllium-containing polymer may be heated to 530°C without loss of weight, whereas the polymers with Cr and Zn atoms are somewhat less stable. Another interesting synthetic ordered-network polymer having very high thermal stability has recently been reported by Brown and colleagues (5).
These polymers contain phenylsilsequioxane units, which are joined together in such a way as to give a ladder-like, stereo-regular linear network structure. It has been reported that these materials could withstand intimate contact with a white-hot electrically heated wire without degradation. So-called "semi-ordered" network polymers have also been recently prepared (6). In one case, a partly oriented linear polyamide fiber was cross-linked with formaldehyde utilizing specific sites (in this case amide nitrogens) in the amorphous regions of the polymer. In a related study, methyleue sulfide and methylene disulfide cross-links were introduced at specific sites of the partially oriented polymer. Among the novel properties of these systems were theunexpected helical coiling and increased thermal and ultraviolet-radiation stability. Very recently, organic polymers containing benzene and/or heterocyclic ring structures that are thermally stable above 400°C, have been synthesized. Among these are p-polyphenyl (7)
This material may be heated with an oxygen toroh without decomposition.
and aromatic polyimides, of which poly [N,N1-(p,ploxydiphenylene)pyromellitimide] (9, 10) is an example: Volume 42, Number 1 , Janwry 1965
RIauy other pyromellitiniides have also been prepared by using aliphatic and substituted aromatic diamines. Therrnogravimetric Studies
IZates and Activation Energies. Poly [N,N'-(p,pfoxydiphenylene)pyromellitimide] reportedly exhibits excellent inecha~iicaland dielectric properties iu a wide temperature range (9). To obtain more basic information on the thermal properties of this material, thermogravimetric studies were conducted with one-mil (0.001-inch) samples under isothermal conditions using a Cahn RG electrobalance (11, 12). A schenlatic illustration of the experimental set-up is giver1 in Figure 1. The first series of experiments was carried out in air a t atmospheric pressure. In Figure 2 the rates of volatilization are plotted versus percent volatilization. These rates were calculated from the slopes of the volatization-time curves with the aid of an electrouic computer. Essentially complete volatilization takes place in the temperature range of 468"C485"C after approximately six hours of heating. Below 400°C the polymer shows practically no weight loss. The second series of experiments was conducted in vacuum (lo-? to mm of Hg). Figure 3 illustrates the rate curves for the experiments carried out between 570°C and 601°C. The data are in contrast to the thermal degradation in air where essentially 100yo volatilizatiori was observed. Thermal degradation in vacuum leaves a brittle dark-gray carbonized residue which retains the general shape of the original sample. During the heat degradation studies on unpurified "Hn-film a pale ycllow material appeared in the combustion tube a t a place immediately next to the hot zone. This material formed at an early stage of the pyrolysis experiments in vacuum, in the temperature range of 200°C to 250°C, and was partially soluble
The efecfrobolance assembly lschematic).
Journal of Chemical Edvcafion
Rates of thermal degradation of "W-film In oir.
in dimethylformamide. In all attempt to remove or diminish this impurity, the "H"-film sanlples were extracted a t room temperature with dimethylformamide, and thermogravimetric experimemts were then conducted on this purified film. The nature of the pale yellow degradation product call best be discussed later, following the presentation of the results on the thermogravimetric experiments. Using this purified material, thermal degradation experiments were carried out in air. From the volatilization-time curves the corresponding rates were calculated. Significantly, the thermal degradation of the purified material proceeds a t rates that are lower by approximately 40% than the rates of the unpurified product. Although the rates are lower, the rate curves are essentially similar to those shown in Figure 2. The rates of degradation of purified "Ha'-film in uaeuum show the same general form as those for the unpurified material, (Fig. 3) but the extrapolated "apparent initial" rates are slower by a factor of approximately four.
Rote, of thermal degradation of "H-film in vacuum.
Usually, the activation energies for the degradation of polymers can be obtained from either extrapolated "apparent initial" rates or from maximum rates (IS), unless the degradation follows simple kinetics from which the order and rate constants of the reaction can be obtained. Tetrafluoroethylene, for example, follows a simple first order reaction (14), but many other polymers exhibit very complex rate patterns. The activation energies for the thermal degradation in air and vacuum of the p u d e d and unpurified "H"film were calculated from the well-known Arrhenius relationship (k = SecEIRT) by plotting either the logarithm of the maximum rates (in the case of the degradation in air), or the logarithms of extrapolated "apparent initial" rates (in the case of the degradation in vacuum) against the reciprocal of the absolute temperatures. From the slopes, the calcu'ated activation energy for the vacuum pyrolysis is 74 and 73 kcal/ mole for the purified and unpurified material, respectively. The activation energy for the air degradation is 33 and 31 kcal/mole for the purified and unpurified polymer, respectively. The structure of the polymer indicates stiff benzene rings which hinder chain mobility and rotation and hence exert a stabilizing influence similar to cross-links. As a result, a carbonized residue forms upon vacuum pyrolysis where oxidative processes are minimized or eliminated. The fact that "H"-film has no melting point and no glass transition temperature below 50O0C substantiates this argument. A comparison of the activation cnerglei for the vacuum pyrolysis of selt.t~ted onlvnitm nmv Oe obtained fnm Tahle 1. which it~tlimm that this material has the highest adtivation energy among hydrogen polymers studied so far. Programmed Temperature Studies. In a programmed thermogravimetric experiment, the temperature of the sample is raised linearly while the loss of weight of the materid is recorded. A linearly increasing temper* ture profile can be obtained with a motor-driven cam mechanism operating an electronic proportional controller. Programmed temperature thermogravimetric analyses were carried out both in air (atmospheric conditions) and in vacuum, as illustrated in Figure 4. As indicated, the polymer has good short-range heat stability up to approximately 600°C under relatively rapid rates of heating. On the other hand, as shown earlier, prolonged isothermal heating a t the lower temperature of 485T in air caused virtually total volatilization of the polymer in five hours. It is thus apparent that the rate of heating has profound effect on Table 1.
Thermal degradation of "ti"-film in air and vacuum by pro-
the relative thermal stability of a polymer. Recently a new mathematical interpretation of some programmed thermogravimetric curves was published from which the activation energy of the degradation can be calculated directly by plotting a function of the residue against the temperature (15). The applicability of this method was demonstrated for several polymers; good agreement was observed between the activation energies obtained from isothermal thermogravimetric experiments and those calculated from programmed temperature curves. Many polymers, however, do not degrade according to a smooth curve and in such cases the use of programmed thermogravimetrie analyses for the calculation of the activation energies is not readily applicable. Degradation Products
This discussion will be limited to the degradation products of the vacuum pyrolysis. Since the degradation of the polyimide in air undoubtedly involves the oxidative cleavage of the chains with essentially total volatilization of the sample, the gaseous degradation products formed during pyrolysis in air are expected to be quite diierent from those resulting from pyrolysis in vacuum. In discussing the nature of the degradation products of the vacuum pyrolysis, it is desirable to differentiate between those that arise from impurities and those that originate from the degradation of the main polyimide chains. While it is not as yet possible to
Activation Energies for the Thermal Degradation of Selected Polymers in Vacuum
Polymer Polypyromellitimide ("Hn-am) Polytrivinylbenzene Polyrnethylene (Linear) Poly(m-methylstyrene) Polypropylene (linear) Polystyrene (M.W. = 230,000) Polybenzyl Polyisobutylene Poly(methylrnethacrylrtte) Polvca~rolactarn(Nylon-6) Volume 42, Number I, January 1965
clearly identify the originof each component, the following discussion will attempt to illuminate the problem. The infrared spectra of undegraded "H"-film are shown in Figures 5 and 6. The significant bands for the purposes of this discussion are the imide hands a t 1780 cm-1 and 720 cm-', the aromatic =CH stretching band a t 3080 cm-', and the -NH stretching hand a t a pure 3360 cn1-I. substituted This latter imide.band The would bands notbetween be present 3450 in em-' and 3650 em-' may be due to -OH or -NH groups.
Infrared absorption spectrum of undegroded "H"-fllm between 2.6 and 3.4,' (1-mil sample, Beckman DK-?A).
To account for the -NH stretching band a t 3360 cm-1, an examinationof thesynthetic processisnecessary (9, 10, 16). I n the first step, pyromellitic dianhydride reacts with bis(4-aminophenyl) ether to form the intermediate polypyromellitamic acid (also called polyamic or polyamide acid) which is soluble in dimethylformamide and dimethylacetamide. This is followed by a second step in which the polypyromellitic acid is heated (while in solution) to undergo an intramolecular cyclodehydration (imidization) reaction yielding the insoluble polyimide. The two-step reaction scheme may be written as follows:
where. n >> x. The -NH groups - of (I) would then be responsible for the absorption peak at3360 em-'. The bands between 3450 om-' and 3650 cm-' may be due to amino-impurities, especially since aromatic amines are known to exhibit similar absorption peaks in this region, and/or to - OH(H20)groups. Examination of the infrared spectrum of the pale yellow degradation product, shown in Figure 7, adds further weight to this argument. As seen, a strong band appears a t 3400 cm-' and a smaller one a t 1680 em-', in addition to the imide absorption peaks (1780 em-' and 720 cm-I). The band a t 3400 em-' represents the -NH stretching mode, whereas the hand a t 1680 em-' is probably due to carbonyl groups of amide units. These latter features of the infrared spectrum are similar to those of polyamic acid (16). I n this connection, it is interesting to note that it is apparently possible to facilitate the complete conversion of the polyamic acid intermediate into the polyirnide under special conditions. I n a recent paper (16) it was shown that a 0.1-mil thick polyamic acid film could apparently be completely converted to the polyimide, as evidenced by the disappearance of the NH-band at 3400 cm-' and the appearance by the imide bands a t 1780 em-' and 720 em-'. However, this sample might have been prepared under more stringent conditions than are employed in the production of "H"-film, which could account for the presence of small amounts of polyamic acid in some of the samples.
It is possible that the intramolecular cyclodehydration reaction is incomplete, giving rise to some polymer chains of randomly alternating units of (I), and (II), rather than being composed of only polyimide units (11). Hence (to account for the presence of -NH groups) it it suggested that the unpurified sample of "Hn-film probably contains snlall amounts of uncyclized polyamide acid units (I), either as a mixture or
R )-g:I$+>$)N !(
perhaps in the form of a copolymer. Such a copolymer may he written as follows:
Journal o f Chemical Education
Further evidence as to the nature of the pale yellow material arises from the mass spectrometric analysis of the volatile degradation products collected a t liquid nitrogen temperature. Table 2 summarizes the results of the thermal degradation of both unpurified and purified polymer in vacuum (600°C, 4 hr). In the case of the unpurified polymer (Column A ) , the volatile degradation products showed carbon dioxide and water
W h V I NUMBER IN CY-'
Infrared absorption spectrum of yellow degradation product Beckmon IR-7, KBr
as the major constituents, with some ammonia and hydrogen cyanide, and lesser amounts of olefins, benzene, phenol, and henzonitrile. Purification of the polymer in dimethylformamide decreased the production of carbon dioxide, and eliminated ammonia and hydrogen cyanide from the gaseous degradation products (Column B). I t should be noted that the mass spectrometric data do not include decomposition products such as carbon monoxide and hydrogen, since these will condense below the temperature of liquid nitrogen a t 10-amm of Hg. However, it is expected that the pyrolysis of the polyimide chains would give rise to large quantities of carbon monoxide, as will he discussed later. Consequently, it appears that the products collected a t liquid nitrogen temperature reflect primarily the degradation of impurities, such as uncyclized polyamic acid units, rather than the main degradation of the polyimide chains. Such polyamic acid units are expected to d e carboxylate a t elevated temperatures and, in addition, undergo free radical as well as hydrolytic scissions. The hydrolytic scission and the thermal decarhoxylation steps may he written as follows:
The hydrolytic scission of polyamic acid could arise from catalytic amounts of absorbed water present in the polymer that could not he removed under normal drying conditions (120°C, 12 hr, lo-= mm of Hg). It has been shown by several investigators that the complete removal of a11 water in polyamides is extremely difficult, if not impossible, below the melting point of the polymer, and that the pyrolysis of polyamides r e sults in the production of large quantities (55 to 60
mole %) of carbon dioxide in even well-dried samples (17-19). If, however, only a thermal free-radical mechanism were operative, carbon dioxide would not he expected in appreciable amounts in the degradation products. I n the case of "H"-film, the small quantity of polyamic acid impurity could thus undergo hydrolytic scission (in addition to the thermal cleavage of its free carboxyl groups), resulting in the production of carbon dioxide. Table 2 indicates that in the case of the unpurified "H"-film, 38 mole% carbon dioxide was produced during vacuum pyrolysis. The decreased production of 24.5 mole% carbon dioxide in the case of the purified "HJ'-film indicates the partial removal of the polyamic acid impurity from the polymer. The over-all results are thus in conformity with the data of other investigators on the thermal degradation of polyamides. Once hydrolytic scission occurs, the amine containing part of the ruptured chain could further degrade to produce the nitrogenous fractions observed by mass spectrometry. It is significant that the formation of nitrogenous products was limited to the pyrolysis of the unpurified polymer, suggesting that they may originate from species other than the imide groups, in accordance with the above scheme. Table 2. Mass Spectrometric Analysis of the Gaseous Degradation Products of "H"-Film Collected at Liquid Nitrogen Temperature (Vacuum Pyrolysis, 60OoC,4 hr, mm -.....of -. Hal .-z.
Com~onent Carbon dioxide Water Ammonia Hydrogen cyanide Hydrocarbons Aniline Benzene Phenol Benzonitrile
A Unpurified "H"-film Mole '7, .. 38.0 53.0 2.1 5.6 0.8 0.05 0.6 0.04 0.06
B Purified "H"-film Mole % .. 25.4 72.6 None None 1.0 0.05 0.4 0.2 0.1
Although the presence of water in the degradation products of "H"-film may be due to absorbed moisture that could not he removed under normal drying conditions, it may also arise from the further imidization (cyclodehydration) of some of those unreacted polyamic acid units which do not suffer decarhoxylation and hydrolytic scission during the extended thermal exposure. Thus, the small quantity of polyamic acid impurity could undergo a t least four types of reactions: (1) thermal decarboxylation, (2) hydrolytic scission, (3) thermal free-radical degradation, and (4) further imidization. Reactions (1) and (2) will yield carbon dioxide, whereas reaction (4) produces water. It is quite possible that all four of these proceed to some extent simultaneously during pyrolysis. The elimiuation of polyamic acid impurities could he important in certain aero-space applications in which surfaces of electronic components might become contaminated by impurities escaping under space conditions. An exact mechanism of the main thermal degradation of the polyimide chains can not be written a t this time. However, some evidence for the main degradation path Volume 42, Number
1, Jonuory 1965
may be gained from the elemental analyses of the uudegraded "H"-film and of the residue remaining after pyrolysis in vacuum (600°C, 4 hr, mm of Hg). The results which are summarized in Table 3 indicate Table 3. Elemental Micro Analyses of Undegraded "H"Film and of Residue Remaining After Pyrolysis in Vacuum mm of Hg) [600°C, 4 hr, Element Carbon Hydrogen Nitrogen Oxveen - "e
"Hn-film (unpurified) Found Theoretiedo (%) (%I 67.5 2.8 7.9 21.0
69.1 2.6 7.3 20.6
Residue ---. Theoretical6
81.0 2.7 8.3 4.2
80.0 3.6 10.3 5.5
Based on 100% poly[N,N'-(p,p'-oxdiphenylene)pyromellit-
Assuming the loss of four CO groups per repeat unit
that the degraded material contains almost the same percentage of nitrogen as the uudegraded material. It appears that heat degradation in vacuum results in the loss of CO from the imide groups, and the simultaneous recombmation and retention of the imido-nitrogen atoms, with the formation of a condensed product, thus:
energies indicate that the imide bond is the weakest. Hence, an activation energy of 74 kcal/mole for the vacuum pyrolysis suggests that the primary scission of the chain occurs at this linkage, most likely followed by a secondary cleavage resulting in elimination of CO groups. literature Cited (1) S m w , R. AND WALL,L., J. Phys Chem., 56,707-15 (1952). E., Trans. Faraday Soe., 30, (2) J. AND RABINOWITCH, . . FRANCK, 12(t3i (1934). (3) GREEN,J., ET AL., "Proceedings of the Battelle Symposium on Thermal Stahilitv of Polvmers." Calumhus. Ohio. December 5, 6, 1 9 6 3 , " ~R~ .1 to R-9. (4) BLOCK, B. P., SIMKIN, J., AND CONE, L. R., J. Am. Chem. Soc., 84, 174%50 (1962). J. F., ET AL.,J. Am. Chem. Soc., 82, 6194-5 (1960). (5) BROWN, (6) BRWCK, S. D., J. Research Nat. Bur. Standards, 65A, 489-94, (1961): ibid., 66A, 77-81 (1962); ibid., 66A, 489-95 (1962): (7) KOVACIC, P. AND KYRIAKIS,A,, J. Am. Chem. Sac., 85, 45&8 (1963). (8) FEAZER,A. H., KANE, J. J. AND WALLENBERG, F. T., "Technical Documentary Report No. ASO-TDR-62-679," July, 1962 (Directorate of Materids and Processes, Aeronautical Syatems Division, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio). L. E.. Ind. Eno. Chem. Prod. Res. D e v e h . , 2, (9) AMBORSKI. 189-94 (19631.' (10) FROST,L. W. AND BOWER,G. M., Abstract of Papem, No. 49, p. 190, 144th ACS Meeting, Los Angeles, California, March 31-April 5, 1963. (11) BRUCK,S. D., Polvmw 5, 435(1964); ibid., in press. (12) BRUCE,S. D., Polymer Preprinta, p. 148, 147th ACS Meeting, Philadelphia, Pennsylvania, April 5-10, 1964. (13). MADORSKY. S. L.. J. Research Natl. Bur. Standa~ds,62, 219 (1959). (14) MADORSKY, S. L?, ET AL., J. Research Natl. BUT.Standards, 1--, . 327 (19%). ~----,~ (15) HOROWITZ, H. H., AND METZGER,G., Anal. Chem., 35, 1465-8 (1963). (16) SROOG, C. E., ET AL., Polymer Preprinta, p. 132, 147th ACS Meeting, Philadelphia, Pem~ylvanirt,April 5-10, 1964. (171 S.. AND WALL.L. A,. J . Reseavch Natl. BUT.Stand. . STRAWS. a d s , 6 3 ~269 , (1959). (18) STRAUB, S., AND WALL,L. A., J. Reseaxh Natl. Bur. Standards, 60,39 (1958). (19) AcEH~MMER, B. G., REINHBT, F. W., AND KLINE,G. M., J. Reseaveh Nall. But. Standad?, 46, 391 (1951). S., J . Research Natl. BUT. (20) MADORSKY, S. L., AND STRAWS, Standards, 63A, 261-8 (1959). (21) S. L., J . Resewch Natl. Bur. Standards, 62,219. . MADORSKY. 28 (1959j. (22) BROWN, D. W., AND WALL,L. A,, J. Phys. Chem., 62,84&52 1195R). ~-...,~ (23) S a n s , S., AND WALL,L. A,, J. Research Nall. Bur. Standards, 63A, 26%73 (1959).
Hence the bulk of the nitrogenous degradation products indicated by mass spectrometric analysis probably arises from the degradation of small quantities of amide groups, rather than from the much more stable imide groups, since the polymer c h i n is expected to cleave preferentially a t the points where isolated and thermally less stable units occur. Since the molecular weight of the repeat unit of poly[N,N1-(p,pl-oxydiphenylene)pyromellitimide] is 382,the loss of four CO groups per repeat unit would represent approximately 30% weight loss based on a 100% pure polyimide. This is in fairly good agreement with the 7 ~ loss) in vacthermogravimetric results ( ~ 4 0 weight uum, especially when considering the additional volatilization arising from the degradation of polyamic acid impurities. The approximate bond dissociation
/ Journal of Chemicol Education