Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 163-171
103
REVIEWS SECTION Chromatographic Analysis of Some Condensation Polymers after Alkali Fusion John K. Haken Department of Polymer Sclence, University of New South Wales, Kenslngton, Austral& 2033
With the increasing complexity of polymer materials such that hybrid materials containing multiple functional groups are becoming commonplace, the difficulty of reliable analysis is increased. A technique is outlined applicable to many condensation polymers and certain other types where analysis is facilitated by preliminary chemical degradation and subsequent individual identification of the initial reactants or their derivatives using a variety of chromatographic procedures.
Introduction Many polymers or polymer systems are not readily analyzed by conventional means due to their complexity, their intractable nature, or the variable response of spectrophotometric techniques. The samples may be compounded mixtures where the major component may be readily determined but the presence of a range of minor materials which are often of great importance is not obvious. The polymer may be cross-linked either to a limited degree, as in a rubber or elastomer, or highly cross-linked, as in a laminate, and in either situation the possibility of achieving sample solution is restricted. The polymer product may be filled with carbon black or silica such as in rubbers or with pigments and extenders in coatings, and preliminary removal of these materials is a prerequisite to examination by spectroscopy. While infrared spectrophotometry is the usual first method of examination of polymers, the limitations of the technique must be appreciated. The discriminating power of infrared with members of homologous series of compounds is very poor. If a mixture of acrylic esters, either as a monomer mixture or in a copolymer, or alkyl phthalate esters is examined, the possibility of reliable individual identification is remote. The intensity of absorption bands in the infrared region is greatly variant, and masking of components is common. The possibility of even detecting significant contamination with an aliphatic oil in an intensely aromatic polymer solution is unlikely. Polymers are amenable to thermal degradation, and the pyrolysis products are readily separated by chromatographic techniques as a preliminary to reliable identification. The pyrolysis products are frequently difficult to relate to the initial composition even with considerable experience due to the predominance of secondary reactions. A common acrylic copolymer will contain three or four monomers, and on pyrolysis gas chromatography some 30-50 component peaks of various oligomers will be obtained. To an experienced worker such fragments are of assistance, but the quantitative results are frequently poor. 0196-432 118611225-0 16380 1.SO/ 0
Chemical cleavage using reactions well-known in organic chemistry are applicable to polymers containing various functional groups in both the main chain and in pendant groups. Such degradation may employ prolonged refluxing and heating, and due to the stability of many polymers, fusion reactions have been employed with considerable success. The application of chemical cleavage as a preliminary to the analysis of polymers generally was reviewed some years ago,l as was a reaction chromatographic procedure applicable to various amenable compounds, including some of high molecular weight.2 The present work integrates a number of reports of the analysis of various types of industrially important condensation polymers after an alkali fusion technique using gas, liquid, or gel permeation chromatography with postfusion chemical reaction or derivative formation as appropriate to allow identification of all of the polymer reactants.
Alkali Fusion Historically, chemists have looked upon fusion reactions as being crude and similar to pyrolysis, but fusion reactions can be clean, quantitative, and stoichiometric, provided two precautions are taken. The fusion temperature must be kept below the compound's pyrolysis temperature, and, for some compounds, oxygen and carbon dioxide must be excluded. This type of vigorous chemical reaction applied to gas chromatography has been described as fusion reaction gas chromatography,2 and while requiring a much longer period of time than a simple infrared scan or pyrolysis gas chromatography, quantitative analyses are anticipated. Solution reactions are restricted to solubility and temperatures achieved by refluxing, and the use of fusion is indicated. Fusion reactions use solid reagent mixed with a small amount of sample, usually at mole ratios of 30:l and sometimes as high as 50:l. The fusion-reaction temperature must be above the fusion reagent melt temperature but held below the thermal-decomposition temperature of the sample components, and for many organic compounds fusion temperatures of 300-350 "C are used. Few compounds will remain unreacted under these conditions. Two factors control the resistance of a compound to chemical reaction: the inherent stability of the molecular bonds and the steric configuration of the molecule. For example, primary amides are hydrolyzed rather easily with alkali, while tertiary amides are fairly resistant to hy0 1986 American Chemical Society
164
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
drolysis. Polymeric esters, where the ester groups are pendant from a hydrocarbon polymer, are very difficult to hydrolyze completely in ~ o l u t i o n .Poly(methy1 ~ methacrylate) saponifies to only 30% with 1 N potassium hydroxide in amyl alcohol (bp 137 "C) after 90 h at reflux. Alkali fusion with potassium hydroxide, however, proceeds rapidly and completely with both these types of compound~.~ Reagents for fusion reactions fall into four categories: alkaline, acid, reductive, and oxidative, with alkaline and acidic conditions having been employed by Siggia and his co-workers2 for fusion reaction gas chromatography. For alkali fusion, sodium hydroxide, potassium hydroxide, and alkali-metal hydroxides have been used. Potassium hydroxide is much preferred because of its lower melting point and the significantly higher solubility of organic compounds in a potassium hydroxide melt compared with a sodium hydroxide melt. Commercially available potassium hydroxide contains about 15% water (w/w, present as the hemihydrate) and works well for fusion. The water is essential, since it lowers the potassium hydroxide melting point to about 125 OC (compared with 360 "C for pure KOH) and contributes to the hydrolysis reaction. The water of hydration is driven off very slowly, causing a beneficial microstirring action during the fusion reaction. The water is released completely only at temperatures higher than 400 "C. To improve the potassium hydroxide fusion character for some applications, experiments have shown that the addition of a fluxing agent (1-10 wt %), such as sodium acetate, aids the achievement of a homogeneous melt during the fusion reaction. Acid fusion reagents include sodium hydrogen sulfate and crystalline orthophosphoric acid. The anhydrous form of sodium hydrogen sulfate has a melting point of 315 "C, and the monohydrate melts at 58.5 "C. The water associated with the hydrated form is needed for this application since the reaction of interest is usually hydrolysis. The amount of water, in addition to the melt temperature, can be regulated by mixing the monohydrate with the anhydrous form. Sodium hydrogen sulfate, however, has two properties that limit its usefulness. First, oxidation and dehydration of the organic matter can occur, since the reagent is used in a hot, concentrated form. Second, the reagent undergoes a series of thermal degradation, causing a continual change in its chemical composition during reaction. Orthophosphoric acid with a melting point of 42 "C is a better reagent because it is stable and shows little tendency to oxidize materials. Reductive fusion gas chromatography employs a highly concentrated organic reagent, solvent free, that is either a strong reducing agent by itself or slowly decomposes to release the reducing agent for reaction. Hydrazine can be used but is volatile and unstable in air. Its use, however, is described for some reactions in solution, but extended reaction periods limit its application. Reagents for oxidative fusion have been investigated, and possible oxidative fusion reagents include potassium metaperiodate (582 "C), sodium metaperiodate (300 "C), lead tetraacetate (174 "C), potassium dichromate (398 " C ) , sodium dichromate (357 "C), and chromium trioxide (196 "C). Fluxes are incorporated in the fusion reaction for two reasons. The flux controls the melting point of the fusion reagent. If a fusion-reagent melting point is sufficiently high, it may cause thermal decomposition of the sample before fusion occurs, the flux material depressing the melting point to a lower, more acceptable temperature. A
Table I. Fusion Gas Chromatography Using Pyrolysis Device unidentified product or comod Droduct examined reaction nil arylsulfonic acids and phenol and/or sulfiten salts sodium salt carboxylic esters alcohol (phthalates) alcohol sodium salt poly(methacry1ates) sodium salt alcohol poly (a-chloroacrylates) sodium salt amine urea compounds sodium salt amides nylon 66 diamine sodium salt nylon 610 diamine polyacrylamide b ammonia polyacrylonitrile b ammonia b cellulose esterse acetic acid sodium salt unsubstituted imides ammonia sodium salt substituted imides primary amine b polysiloxanes hydrocarbon (aliphatic and aromatic) b polyvinyl esters carboxylic acid sodium salt diamine polyamides imides diamine sodium salt carbamates diamine sodium salt polyurethane esters b polysiloxanes hydrocarbon poly(carboranesi1oxanes) amine and diamine nil azo compoundsd amine and diamine nil nitro compoundsd organic fragment sulfonatesd nil
ref 4 6 6 6 7 7 7 I 7 8 10 10 11
12 13 14 14 15 9 9 9
Determined chemically by titration. *Pendant group cleaved from polymer chain. Fused with orthophosphoric acid. Fused with carbohydrazide.
flux can also be used to increase the fusion temperature if the melting point of the flux is higher than that of the fusion reagent. Fluxes also promote sample solubility in the melt to achieve a more homogeneous fusion mixture, facilitate reaction completeness in the shortest time, and increase the precision of the analysis. Sodium acetate is the most useful flux. Professor Siggia at the University of Massachusetts constructed a device from an obsolete pyrolyzer for attachment to the injection port of a gas chromatograph. The fusion reaction apparatus is a modification of a furnace pyrolyzer5 formerly marketed by the Perkin-Elmer Corp., Norwalk, CT (Pyrolysis Accessory 154-0825),and has been described elsewhereS2For volatile degradation products a cold trap is inserted between the fusion device and the chromatograph. This is immersed in liquid nitrogen during the fusion and then heated, and the contents are swept directly into the gas chromatograph. The apparatus has been extensively used over the last decade by a number of workers. The classes of compounds studied are shown in Table I together with the product examined and, where appropriate, the product remaining in the reactor. Since 1976 alkali fusion has also been developed in this laboratory, extending the work of Siggia such that any advantage of the in situ fusion is far outweighed by the flexibility of separate microfusion. The procedure developed has allowed (1)more rapid and efficient fusion, as water necessary for the reaction remains in the reaction environment rather than tending to be preferentially swept into the cold trap; (2) multiple fusions to be carried out in an external heater without restricting the use of the gas chromatograph or, more importantly, restricting examination to this instrument; (3) materials that might normally be retained in the chromatograph, Le., as soaps or
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
low volatility materials, to be examined after suitable chemical reaction and/or derivatization; (4) other chromatographic procedures, i.e., LC or GPC to be employed; and (5) analysis of the total molecular rather than part to be conducted.
Polyamides The procedure was developed with linear polyamides, our interest resulting from the examination of synthetic sutures. Infrared spectra identical with those of commercial nylon products were produced, but the physical properties were widely variant of materials of the molecular weight indicated. Prolonged analysis clearly showed that the products were copolyamides. Acidic hydrolysis of certain polyamides allows satisfactory analytical procedures for the reactants, but prolonged reaction periods or elevated pressures are necessary in order to carry the sluggish reactions toward completion. The hydrolysis of nylon samples resulting from diamine-dicarboxylic acid condensation required refluxing for 24 h with 6 N hydrochloric acid at atmospheric pressure or for 4 h at 40 psi. Anton16 divided the hydrolysate, and the diamines were extracted with 1-butanol from alkaline solution for examination. The methyl esters of the dicarboxylic acids were prepared with methanolic boron trifluoride and chromatographed. Nylon 6, yielding waminocaproic acid, was not resolved, whereas with the column conditions used nylons 610 and 6T could not be identified due to coincident peaks of dimethyl sebacate and dimethyl terephthalate. The same acid hydrolysis was conducted by Mori et al.17 at 130 "C with an additional derivative step allowing separation of the products. Nylon 6 required 2 h, nylon 66 4 h, and nylons 11 and 12 8 h of digestion to effect cleavage. The hydrolysis products were esterified with hydrochloric acid and methanol and then subjected to reaction with trifluoroacetyl anhydride. A modified procedure by the same workers18 involved a single derivative step. The method involved trimethylsilation of the diamine hydrochlorides, the dibasic acids, and the w-aminoalkanoic acid hydrochlorides from the hydrolysate. The dried products were directly silylated with bis(trimethylsily1)acetamine (BSA) and chromatographed. Frankoski and Siggia applied alkali fusion to esters that were difficult to saponify6 and later to amides, anilides, and ureas7 (Table I). The fusion involved heating the sample with solid alkali in mole ratio 50:l at 360 "C for 30 min to liberate the diamine. The method was extended by Glading and Hakenlg with both the amine or the methyl esters of the dicarboxylic acids and the w-aminoalkanoic acids to be determined in about 45 min. The alkali fusion was carried out by using 0.01 g of polymer and 0.1 g of prepared potassium hydroxide containing 0.5% sodium acetate.2 The reaction was carried out in 6-mm 0.d. borosilicate tubes (total length about 10 cm) that were sealed under vacuum. Three identical tubes were heated in a cylindrical block of stainless steel containing four holes of suitable diameter. The block was heated by a resistance heater about the circumference, with input current regulated to produce a temperature of 260 "C. The temperature was monitored by placing a thermometer or thermocouple in one of the four holes. The reactants were heated for 0.5 h, and the reaction mixture was allowed to cool before further treatment. It is essential that aJl heating be carried out in a suitable fume chamber to eliminate any possibility of injury by glass fragments due to improper sealing or gaseous degradation products
185
Table 11. Quantitative Determination of 1,6-Hexanediamine and Methyl Esters of Dicarboxylic Esters in Nylon Samples av w t of av w t of nylon
diamine recovered
error, %
ester recovered
66 69 610 612
0.04895 0.0491 0.0489 0.04906
2.1 1.87 2.21 1.88
0.0490 0.0495 0.0488 0.0486
error, % 2.0
1.0 2.4 2.8
Table 111. Analyses of Copolyamides copolyamide nvlon 66/610 nilon 69)612
actual 60:40 60:40
composition, % analysis averages 61.3:38.7 60.66:39.34
error, % 2.16 1.10
such as with certain polyurethanes which eliminate carbon dioxide. Analyses were carried out on four polyamide samples produced by diamine-dicarboxylic acid condensation. The samples of nylon 66,69, 610, and 612, which are condensation products of 1,6-diaminohexanewith hexanedioc acid (adipic acid), nonanedioc acid (azelaic acid), decanedioic acid (sebacic acid), and dodecanedioic acid, respectively, were examined after fusion for 30 min. The analyses were essentially quantitative. The conversions to diamine and of the dimethyl esters are shown in Table 11. The values refer to an initial weight of polymer of 0.05 g, and the diamine assay is consistent with that of Frankoski and Siggia,I who reported near-quantitative conversions. Table I11 shows similar analytical results from several c~polyamides.'~ Nylon 6T, the condensation product of 1,6-diaminohexane and terephthalic acid, was readily cleaved, but the reaction was incomplete after 0.5 h of heating, and 1.0 h was necessary for complete hydrolysis. Analyses of w-aminoalkanoic acids as the methyl esters were carried out on nylon 6 (w-aminocaproicacid), nylon 11 (w-amino-n-undecanoic acid), and nylon 12 (w-aminon-dodecanoic acid), the percentage error being 1.8,2.2, and 2.0, respectively. The particular value of the procedure was immediately evident, as a complete analysis was possible in a fraction of the time necessary for the procedure of Anton,16while with the method of Siggia7all of the polymers and copolymers shown in Tables I1 and I11 would give the same product, Le., 1,6-diaminohexane. Resinous or Fatty Polyamides. During 1976 a working party of the Macromolecular Division of IUPAC reported the results of a cooperative study on five resinous polyamides.20 The polyamides were characteristic of the principal types of industrial products and were both of the reactive and nonreactive type. The reactive polyamides are of considerable importance, forming the major group of cross-linking agents for epoxide systems; containing an excess of amino groups, they may have terminal primary and secondary amino groups, both of which are reactive toward the epoxide group. The nonreactive polyamides have essentially stoichiometric amounts of acidic and amino reactants and are used in printing inks, in coatings, and as the modifier to achieve the thixotropic character of alkyd enamels. The work highlighted the lack of a reliable systematic method of analysis for fatty polyamides, and a further study of the literature failed to disclose any analytical procedures for the materials. The procedures indicated in the report are in part repetitive, extremely time consuming, of limited accuracy, and generally unsuitable for
166
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
T L 45 50 5 5 60 Eluate v o l u m e lml)
Figure 1. Gel permeation chromatogram of dimer acid methyl esters.
/
POTASSIUM HYDROXIDE
\
CHLOROFORM
DIAMINE T H F
POTASSIUM SALTS
~
i
(ORGANIC LAYER)
(AOUEOUS LAYER)
1
I
O R G A N I i LAYER
I
I
I
I
8
I
0
2
Ir
6
8
T i m e
10
I
BORON TRIFLUORIDE
Figure 3. Analytical scheme for resinous polyamides.
(minl
Figure 2. Gas chromatogram showing separation of TFA derivatives of l,6-diaminohexane and ethylenediamine.
any type of routine characterization. While some information was obtained by preliminary chemical and spectroscopic examination, prolonged acid hydrolysis was necessary to obtain the reactant acid for chromatographic analysis and the amine compounds for spectrophotometry. The IUPAC study considered five fatty polyamides of known composition, these being the usual type of condensation products of low molecular weight polyfunctional amines with difunctional dimer acid materials. Samples remaining from the IUPAC study or materials made to the same formulas were reexamined by using the procedures developed, and much more satsifactory results were obtained in a fraction of the time.21 The fusion procedure used was as for the nylon samples.ls The diamines liberated were examined as trifluoroacetyl derivatives and the low molecular weight dibasic acids and monobasic acid as methyl esters by gas chromatography and the dimer acids (C18HNCM)as methyl esters by gel permeation chromatography. Figures 1and 2 show the gel permeation chromatogram of dimer acids and gas chromatogram of the diamines from sample C. A general procedure for the rapid analysis of resinous polyamides was subsequently reportedz2in which a wider range of reactants was considered, and several complications encountered since the earlier work were discussed. Free diamines were also separated, being of interest in the analysis and for residual monomer determinations. The fusion and subsequent separatory procedure is shown in Figure 3. The chromatograms, Figures 4-7, show separations of simple aliphatic esters, dibasic esters, diamine TFA derivatives, and free diamines, respectively. It is shown later that the isomeric phthalate esters (Figure 8) are readily separated on a highly polar stationary phase.
Time
lminl
Figure 4. Chromatogram showing separation of the methyl esters of aliphatic dicarboxylic acids.
I
I
,
I
I
I
/
I
/
2 3 4 5 6 7 8 9 1 0 T i m e lminl
Figure 5. Chromatogram showing separation of acidic modifiers on an OV-1 column programmed between 160 and 250 “C at 10 OC/min.
While the resinous polyamides are more readily hydrolyzed than the simple nylons, the subsequent examples are of considerable stability and extremely resistant to solution hydrolysis. Aramid Fibers Aramid fibers were defined by the U.S.Federal Trade Commission as a “manufactured fiber in which the fiberforming substance is a long-chain polyamide in which at
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
poly@-phenyleneterephthalamide) (II), currently marketed as Kevlar, both developed by Du Pont. The polymers in general have extremely high chemical and thermal resistance, and reaction to cleave the amide links to allow analysis is restricted. Analytical procedures were not available, despite some use in composites for military purposes. Nomex is generally prepared by reaction of the diamine, m-phenylendiamine (III), with the acid chloride, isophthaloyl chloride (IV).In the presence of an acid acceptor rapid reaction occurs under mild conditions either in suspension or solution.23 Kevlar is similarly prepared by reaction of the diamine V with the acid chloride VI in hexamethylenephosphoramide and N methylpyrridolidone solution (2:l) at -10 "C.
U y . 4 I
l
I
I
1
I
l
I
-
&
,
I
I
167
t
1
0
0
0 1 2 3 6 5 6 7 8 9 T irne (min)
Figure 6. Gas chromatogram of polyamide diamine-TFA derivatives.
(VI 1 -?HCI 1
8
4
I
I
I
I
l
0 2 4 6 8 I01214 Time l m i n l
t
Figure 7. Gas chromatogram of polyamide diamines. (11)
li
T i m e (minl
Figure 8. Chromatogram showing the separation of isomeric phthalate eaters and common aliphatic dibasic eaters on 10% SILAR 10 CP programmed from 170 to 220 OC at 4 OC/min. Peaks: (1) solvent; (2) dimethyl maleate, (3) dimethyl succinate; (4) dimethyl adipate; (5) dimethyl azelate; (6) dimethyl sebacate; (7) dimethyl terephthalate; (8) dimethyl isophthalate; (9) dimethyl o-phthalate.
least 85% of the amide linkages are attached directly to two aromatic rings". This definition does not exclude the possibility of the presence of other functional groups that may be associated with the aromatic group. Thus, within the group may be included the poly(amide hydrazides) and related copolymers developed by Monsanto and the poly(quinazo1inediones)developed and formerly marketed by Bayer. The first polymers of the class examined were poly(mphenyleneisophthalamide) (I),currently marketed as Nomex and developed several decades ago, followed by
The three phenylenediamine isomers were readily separated on a nonpolar OV-1 column, the peaks being essentially ~ y m m e t r i c a l . ~The ~ three isomeric methyl phthalates are readily separated by gas chromatography using a polar cyanoalkylsiloxane stationary phase.25 The phenylenediamines are also separated by HPLC, but separation of the three phthalate esters has not yet been reported, although separation using an amine column would be expected to be successful. The polymers described were linear condensation products of dicarboxylic acids and diamines. A representative polymer (VII) is shown below where R and R'
IVII)
are methylene groups or aromatic rings and may be more complex coupled aromatic systems, as shown by several examples. In addition to the polyamide linkage -C(= 0)"-, it was shown by Schlueter and Siggia13that the polyamide linkage -C(=O)N< is cleaved by the same hydrolytic reaction. The procedure was of limited value, however, as with increasing molecular size, the amines formed were less amenable to gas chromatography. Aromatic Polyamides/Polyimides With acids of functionality greater than two polyimides are often produced through the polycondensation of
168
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
1,2,4,5-benzenetetracarboxylic dianhydride (VIII) (pyromellitic anhydride) with 4,4'-diaminodiphenyl ether (IX) to form a polyamide carboxylic acid intermediate X, which is converted by dehydration and ring closure into the polyimide XI. This polymer is representative of the Du Pont 0
alkaline fusion and subsequent examination of the fragments of the c l e a ~ a g e . ~ ~ ~ ~ ~ The polymer H-22 was produced from terephthaldihydrazide (XIV) and terephthaloyl chloride (VI) to form polyterephthalhydrazide (XV).
0
N Hp- N H -F;
+
$-NH-NH1
C1-T;
0
0
1
IXIVI
0 (VI)
-?HCI
t I XVI
0
0
IX)
The polymer H-20 was produced from oxalic dihydrazide (XVI) and terephthaloyl chloride (VI) to form the alternating polyhydrazide of oxalic acid and terephthalic acid (XVII).
I
-2 H20
I
+ C 1 - C G C - C
N H 2 - N H - C - CI, - N $H1 - N H 2
a
3
- 2 HCI
IXVII
0
I VI)
0
(XI)
material available in various forms as Vespel and Kapton. The reaction of 1,2,4-benzenetricarboxylic acid (XII) (trimellitic anhydride) in place of pyromellitic anhydride (VIII) leads to the formation of poly(amide imides) (XIII).
(XVIII
The polymer H-202 (XVIII) is a random copolymer produced from oxalic dihydrazide (XVI), terephthaldihydrazide (XIV), and terephthaloyl chloride (VI). N H 2- NH- C - C - N H -N H
r,
(XIIII
,I
I/
0
0
+
IXIVI
N H - NH - 5- F, - N H - NH - $ 0 0 0
/(VI)
rn )i
f /2"1
U
The Amoco AI polymers are aromatic homopolymers of this type. Two basic polymers are involved, designated as A1-10 and Al-11, and 4,4'-methylenedianiline and mphenylenediamine are the diamines used. The diesters and triesters recovered from a series of polymers were separated as the methyl esters by gas chromatography on an SE-30 column, while triesters and tetraesters were more successfully resolved by using HPLC on a Bondpack CI8 column with elution using methanolwater (65:35)26 The amines have been determined by gas chromatography as free amines2s on an FFAP column or as trifluoracetyl derivatives by using HPLC,26as detailed for the higher molecular weight esters.
Polyhydrazides A series of high-modulus fibers developed by the Monsanto Co. comprising both aromatic polyamides and aromatic amide hydrazides and alternating oxalyl/arylene polyhydrazides were examined for the first time by using
IXVIII1
The polymer PABH-TX-500 is an ordered poly(amide hydrazide) (XIX) formed by the reaction of p-aminobenzhydrazide (XX) with terephthaloyl chloride (VI).
IXXI
(XIXI
The fusion reactions carried out by Siggia et a1.2 and Haken et al.19 were conducted in sealed systems. When the polyhydrazide or their copolymers were heated as indicated, i.e., in borosilicate tubes that had been sealed under reduced pressure, violent explosions frequently occurred due presumably to some trivial degradation of the hydrazine liberated at the temperature used (250-300 "C). The analyses were conducted instead by placing the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 169
ALKALINE FUSION
TEREPHTHALIC ACID
t
BENZENE OR TOLUENE
2 TFAA
WATER
POTASSIUM SALTS
l
l
2
3 4 5 6 T i m e (mini
l
l
l
l
7
Figure 10. Chromatogram showing in increasing elution order the separation of trifluoroacetyl derivatives of neopentyl glycol, propane-1,2-diol, propane-1,3-diol, ethylene glycol, dipropylene glycol, butane-1,4-diol, and diethylene glycol.
DIL H C I
I
I &&I
HYDRAZINE-TFAA
TOLUENE
I I ORQANIC LAYER
BF3 I N M E T H A N O L
0 DIMETHYL OXALATE
DIMETHYL TEREPHTHALATE
Figure 9. Analytical scheme for H-202.
reactants in a suitable microflask filled with an air condenser, or more economically and conveniently in 25 cm X 6.0 mm borosilicate tubes sealed at one end and heated in the steel block with electric resistance heaters as described p r e v i o ~ s l y . ~ ~ After fusion at the refluxing temperature of the mixture, i.e., ca. 180 O C , all of the polymer had dissolved, and after the mixture cooled, the melt was separated according to the scheme shown in Figure 9. The hydrazine cleaved from the polymers was estimated either by gas or liquid chromatography on the polyester25 or Bondpak C18 columns26 used previously as the trifluoroacetyl derivative. The diesters of oxalic and terephthalic acids were separated on an OV-1 column24or in the presence of other isomeric phthalates on a cyanopropylsiloxane column.26 The paraaminobenzoic acid is conveniently identified as the trifluoracetyl d e r i ~ a t i v e . ~ ~ As certain of the intermediates are not available, in very high purity forms the quantitative aspects of the procedure have not been completely resolved. Studies would indicate that the reactants are of high purity, and near quantitative results are achieved. The errors are suggested to be 2-3%, which is comparable with our earlier re~ults.’~ Some minor degradation of aromatic amines was shown by Schlueter and Siggia13by identification of ammonia. In this work it was evident that little ammonia was produced, although traces of other products were evident.
Unsaturated Polyester Resins The analysis of unsaturated polyester resins both in the liquid or uncured form and in the laminate form have recently been reported.25 The materials are linear polymers formed by the condensation of aromatic and aliphatic dibasic acids with diols in solution in a polymerizable solvent, usually styrene. Free-radical polymerization is used to form the final laminate, which is often reinforced with fiberglass or materials with other properties for some
1
2
3 4 5 T i m e lminl
6
Figure 11. Chromatogram showing separation of trifluoroacetyl derivatives of ethylene and propylene glycols.
military purposes. The polyester and the styrene are copolymerized to a controlled degree in the final laminate by free-radical polymerization of the ethylene double bond in maleic or fumaric acids usually present in the polyester. The analysis of simple polyesters has been extensively studied,30although analysis of cross-linked products has been of limited success. In this analysis the reactants are recovered with the exception that the maleic acid styrene linkage is not broken. Separation of the aliphatic dicarboxylic esters has been shown for the nylon analyses, while the phthalates have also been shown in Figure 8. Figure 10 shows a separation of a series of glycols experienced in such resins. It is well-known that the fiber polyester ethylene terephthalate is difficult to saponify and requires hydrolysis under pressure, although with alkali fusion the material is readily cleaved. Figure 11shows a composite chromatogram of ethylene and butylene glycol, produced from ethylene terephthalate (Dacron) and butylene terephthalate, both produced by Du Pont. Other analyses of this newer polymer have not been reported.
Polyurethane Systems Polyurethane materials are of considerable industrial importance, a wide range of types being used in various physical forms for a large number of commercial purposes. The analyses of such materials have expectedly been extensively studied, and a large number of procedures are shown in a recent report31 from which it is evident that satisfactory results are frequently not achieved. The first difficulty with the analysis of polyurethane is its resistance to hydrolysis, with the polyether types being more resistant than the polyester types. It has been shown that alkaline h y d r o l ~ s i susually , ~ ~ in a “Parr” bomb, is superior to acidic hydrolysis and that the reaction is facilitated by alkaline fusion.14 A simple, rapid, and reliable analytical procedure for the complete analysis of polyurethane foam based on polyether
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
170
OCN
-
f--O
,& -
NH
Adipic Acld
CH3
I
NCO
t n HOCCH,-
&
CH,-
O$;--CCHZ-
CH
-
Oiethyienetriamine
1
O t H
[C O(CH2 I, CONHCH2CH2 N H CH2 C H2 N H I n
NH
-
iC
I
O+CH,-
I
CH, CH;
I
OFCH,-
CH
-
O-t;-f,
I
Alkali fusion ( KOH/H,O )
[-COICH21& Where CH 3
CH 3
6
I ( CF, CO )20
t
I 0 -HCH2- CH -
CH,-
.CH,-
n
CF
+
pyridine
t
CH,CO
)2
X =>NH-CH2CH-CH2Ci\I
0
O+;H
0
Figure 13. Preparation and structure of the polyamide-epichlorohydrin resins.
S03H
-
CO N H C H2CH2-X-CH2 CH2 NH],
n
0
n CH,nx
I
CH,-
0
-
1I
C
-
CH,
CH t
0
-
C
II
-
CH3
o
y
- CH-
3 1
CH
-
II 0 - C - CHI 0
- C - CH I1
0
Figure 12. Reactions involved in acid and alkali fusion of polyether-based polyurethanes.
type alcohols and diisocyanates has been reported.31 The structure of the polyhydroxy ether compound has a major effect on the properties of the resultant foam. The first polyether designed specifically for polyurethane manufacture was poly(oxytetramethy1ene Polyether compounds derived from propylene oxide and/or ethylene oxide are of considerable importance, and block copolymers are widely used. Poly(oxypropy1ene triols), using as a basis of synthesis the low molecular weight triols such as glycerol, trimethylolpropane, and 1,2,6-hexanetriol rather than propylene glycol, are also of importance. As reported,31the alkaline fusion was carried out on a larger scale than previously reported to allow the application of other reactions, although it should equally be applicable on a smaller scale. The polymer (500 mg) with 10 g of the fusion reagent (prefused potassium hydroxide containing 5 % sodium acetate) was reacted in a small stainlesssteel reactor equipped with an air condenser. The reaction was allowed to reflux at 250 "C for 1 h under an inert nitrogen atmosphere. The cooled reaction mixture was dissolved in chloroform and the filtrate extracted with 5 N hydrochloric acid. The diamines were extracted from the alkaline solution with chloroform and examined as their trifluoracetyl derivatives. The polyether was then recovered from the original chloroform solution and examined by gel permeation chromatography, or the ether links were cleaved by reaction with toluene sulfonic acid and acetic anhydride. The reactions involved in the formation of conventional polyether-based polyurethane, of the subsequent products of alkali fusion, and of the acidic cleavage of the ether links are shown in Figure 12. Other polyurethanes possess considerable cross-linking due to the uae of polyfunctional reactants, while others are employed as elastomers and others such as the Spandex materials incorporate hard polyurethane segmenta and soft polyester fragments frequently based on adipic acid. The basic chemistry and functional groups present are the same, and while the ease of hydrolysis by conventional
means may be very low, alkali fusion is suggested to be of general utility. The analysis of complex polyurethane encapsulating agents for medical implants is currently under Possibilities of Alkali Fusion The use of alkaline fusion with a variety of condensation polymers has been described. Several other reports of both polymers and low molecular weight compounds are shown in Table I. In polymer chemistry it is clear that the complexity of products is increasing, within many cases, not new products but complex terepolymers of existing materials utilizing widely variant functional groups that would not have been thought possible a decade or so ago. With many of these products, and more so when filled or cross-linked, examination of the total polymer or systems becomes more complex and less rewarding. Alkali fusion with many of these systems, which have functional groups amenable to hydrolytic cleavage, appears to be extremely attractive as an analytical procedure. While the number of organic compounds is seeminly limitless, the number of tonnage intermediates is probably a few thousand, and such fragments can usually be readily identified by mass spectrometry as a preliminary to the development of a separatory scheme. Systems where analysis is likely include silicone polyesters. These are of considerable utility and would seem appropriate, as the ester component is of the same general type as that already reported.% Polysiloxanes were studied by Sarto.I5 A recent report of the analysis of polyols in silicone alkyds35shows the limited methods of analysis available. The utility of the procedure after cross-linking with amino resins is very low. Polysiloxanes are also used in a medical polyurethane elastomer, and analysis using alkali fusion is being investigated.34 Epoxide systems are widely used, and preliminary studies suggest that some products are susceptible to alkali cleavage.36 Products include epoxy polyester hybrids and epoxy acrylic copolymers. Compounded materials for aerospace and military use employ epoxy adhesives filled with Kevlar, graphite, etc., and the analysis might be improved by preliminary fusion. Cationic polyamide-epichlorohydrin resins prepared by the reaction of epichlorohydrin with polyamides derived from adipic acid and diethylenetriamine, as shown in Figure 13, are widely used as wet-strengtrh additives in paper37 and to shrink-resist woo1.38,39 Gel permeation chromatography of these polymers has been reported by
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 171-178
171
References HO C H 0”-COONGO ~
O ~N - O - F , - N
O
~
N
O
li
I
0
t
HO C N ~ C H ~? N
C HO O N ~ O
It HOGNO
It O
N
~
O
H
/
CROS2LINK
1 HP-Q
/
Alkal, +usion
CHZ
O”*
F i g u r e 14. Mechanism of cross-linking w i t h urethane cross-linking agenta and alkaline fusion.
Guise and Smith,4O and alkali fusion would seem to be applicable as a more detailed method of analysis. Other possible amide type applications concern the analysis of poly(l,4-benzamides) and their chlorine-substituted derivatives41and a wide variety of polyamides that exhibit liquid-crystalline proper tie^.^^ The increasing complexity of polymer produds is further illustrated by the recent commercial development of a transparent engineering nylon and a thermoplastic elastomer based in part on 4,4‘-methylenediphenyl diisocyanate by the Upjohn Co. The analysis of such products is very likely to be successfully conducted by using alkali fusion technique^.^^ A slightly different application has been reportedu with a compounded natural rubber cross-linked with a proprietary polyurethane cross-linking agent, i.e., Novor 924. The polyurethane and its method of cross-linking and subsequent cleavage are shown in Figure 14. Here the aromatic nature of the cross-linker present as a very minor component was readily established. The presence of other minor additives in rubber is possible as should be the recently introduced aliphatic polyurethane cross-linker, Le., Novor 950.
Conclusion The use of alkaline fusion as the preliminary step in the analysis of condensation polymers as conducted in these laboratories is described; other studies are tabulated. Current studi- on other systems are indicated, and further utility of the technique with several additional systems is suggested.
Haken, J. K. Pfog. Org. Coat. 1979, 7 , 209-252. Whltlock, L. R.; Siggla, R. Sep. furif. Methods 1974, 3, 299-337. Smets, G.; De Loecker. W. J. pOrVm. Sci. 1959, 4 1 , 375-380. Siggla, S.; WhRlock, L. R.; Tao, J. C. Anal. Chem. 1989, 4 7 , 1387-1392. ( 5 ) Ettre, K.; Varadl, P. F. Anal. Chem. 1983, 35, 69-73. (6) Frankoskl, S. P.; Siggla, S. Anal. Chem. 1972, 4 4 , 507-511. (7) Frankoskl, S. P.; Slggla, S. Anal. Chem. 1972, 4 4 . 2078-2088. (8) Williams, R. J.; Siggia, S. CRed In Sep. furif. Methods 1974, 3 , 299-337. (9) Rahn, P. C.; Slggia, S. Anal. Chem. 1973, 4 5 , 2336-2341. (10) Schlueter, D. D. Thesis, University of Massachusetts, Amherst, MA, 1976. (11) Schlueter, D. D.; Slggia, S. Anal. Chem. 1977, 49, 2343-2348. (12) Williams, R. J.; Slggla, S. Anal. Chem. 1977, 49, 2337-2342. (13) Schlueter, D. D.; Slggia, S. Anal. Chem. 1977, 49, 2349-2353. (14) Gibian, D. 0. Thesis, University of Massachusetts, Amherst, MA, 1979. (15) Sasto, L. G., Jr. Thesis, University of Massachusetts, Amherst, MA. 1982. (16) Anton, A. Anal. Chem. 1988, 4 0 , 1116-1118. (17) Morl, S.; Furusawa, M.; Takeuchl, T. Anal. Chem. 1970, 42, 138- 140. (18) Morl, S.; Furusawa, M.; Takeuchl. T. Anal. Chem. 1970, 42, 959-961. (19) Gladlng, G. J.; Haken, J. K. J. Chromatogr. 1978, 757, 404-409. (20) O’Nelll, L. A.; Christensen, G. J . Oil Colour Chem. Assoc. 1978, 59, 285-290. (21) Haken, J. K.; Obita, J. A. J. Oil Colow Chem. Assoc. 1980, 6 3 , 200-209. (22) Haken, J. K.; Ob&, J. A. J. Chromatogr. 1981, 213, 55-62. (23) Lee, H.; Stoffey, D.; Neville, K. ”New Linear Polymers”; McGraw-HIII, New York, 1967; Chapter 6-6. (24) Haken, J. K.; Obita, J. A. J. Chromatogr. 1982, 244, 265-270. (25) Haken, J. K.; Rohanna, M. A. J. Chromatogr. 1984, 298, 263-272. (26) Haken, J. K.; Obita, J. A. J. Chromatogr. 1982, 244, 259-263. (27) Preston, J. US. Patent 3376269, 1966. (28) Preston, J. US. Patent 3484407, 1969. (29) Haken, J. K.; Oblta, J. A. J. Chromatogr. 1982, 239, 377-384. (30) Haken, J. K. “The Gas ChromatoaraDhy of Coating Materials”; Dekker. New York, 1974. (31) Vlmaiaslrl, P. A. D. T.; Haken, J. K.; Burford, R. P. J . Chromatogr. 1985, 379, 121-130. (32) Matuszak, M. L.; Frisch, K. C.; Reegen, S. L. J. f o k m . Sci. 1973, 1 7 , 1683- 1690. (33) Barringer, C. M. Teracel30 Polyalkylene Ether Glycol Bulletin No. 11R1-1956, Du Pont, Wllmlngton, DE, 1956. (34) Haken, J. K.; Burford. R. P.; Vlmalaslrl, P. A. D. T. Advances in Chromatography; Elsevier: Amsterdam, 1985; pp 347-356. (35) McFadden, J.; Scheulng, J. Chromatcgr. Sci. 1984, 2 2 , 310-312. (36) Haken, J. K., unpubllshed results, 1984. (37) Hercules Inc. U.S. Patent 2926 154, 1960. (38) Earle, R. H., Jr.; Saunders, R. H.; Kangas, L. R. Appi. folym. Sci. Symp. 1971, 18, 707-714. (39) Smith, P.; Mills, J. H. CH€M€CH 1973, 3 , 748-755. (40) Guise, 0. B.; Smith. G. C. J. Chromatogr. 1982, 235, 365-376. (41) Morgan, P. W. U.S. Patent 3943 110, March 9 1976. (42) Morgan, P. W. CH€MECH 1979, 9 , 316-326. (43) Chem. Eng. News July 9, 1984, 62(28), I O . (44) Burford, R. P.; Haken, J. K.; Obita, J. A. J. Chromatogr. 1983, 268, 515-521, (1) (2) (3) (4)
Receiued for review
December 31, 1984 Accepted December 27, 1985
Desirable Catalyst Properties In Selective Oxidation Reactions Harold H. Kung Chemical Engineering Department and the Ipatieff Laboratoty, Northwestern University, Evanston, Illinois 6020 1
Heterogeneous oxide-catalyzed selective oxidation reactions can be classified into dehydrogenation and dehydrogenation with oxygen insertion. The oxide properties that are important In each of the steps in these reactions are discussed. The breaking of the C-H bonds in alkanes is facilitated by weakly adsorbed oxygen. The C-H bond breaking,of alkenes is enhanced by strongly basic surface lattice oxygen and cations that 01 96-4321/86/1225-017 1$01.50/0
are soft acid and undergo redox readily. Desorption of alkenes and dienes is enhanced by cations that are hard acid. The selective CO bond formation Is controlled by the number and the ease of removal of the available lattice oxygen, while the combustion reaction can be minimized by shortening the residence time of the surface intermediates, weakening the adsorption of the desired products and minimizing the amount of
0 1986 American Chemical Society
