I
A COMPARATIVE STUDY OF
A C O M P A R A T I V E STUDY
OF
S O M E P O L Y A M I D E S
PROEFSCHRIFT
TER V E R K R I J G I N G VAN DE GRAAD VAN D O C T O R IN DE T E C H N I S C H E W E T E N S C H A P P E N AAN DE T E C H N I S C H E H O G E S C H O O L TE DELFT OP G E Z A G VAN DE R E C T O R M A G N I F I C U S IR. H. J. DE WIJS, H O O G L E R A A R IN DE A F D E L I N G DER M I J N B O U W K U N D E , VOOR EEN COMMISSIE
U I T DE SENAAT TE V E R D E D I G E N OP W O E N S D A G 3 MEI 1967 OM 16.00 UUR
D O O R
JULIANUS LEONARDUS ARNOLDUS JANSEN
S C H E I K U N D I G I N G E N I E U R GEBOREN TE H I L L E G E R S B E R G
iSiOZ
o Pria
Dit proefschrift is goedgekeurd door de promotor: Prof. Dr. D. W. Van Krevelen
Aan mijn ouders Aan mijn wouw
DANKBETUIGING
Aan de Raad van Bestuur van de Algemene Kunstzijde Unie N.V. en aan de Directie van het Centraal Researchinstituut van de Algemene Kunstzijde Unie N.V. en daar-mee verbonden ondernemingen te Arnhem, betuig ik mijn dank voor de toestemming tot publicatie van dit onderzoek.
Gaarne betuig ik mijn dank aan de heer E. H. Boasson voor de waardevolle ad-viezen die hij mij tijdens het schrijven van dit proefschrift gegeven heeft.
De heren A. Bezemer, M. van Loon, P. F. de Maat en R. Timmer wil ik gaarne danken voor hun hulp en raad tijdens het onderzoek en bij de afwerking van de tekst. Tenslotte stel ik er prijs op in mijn dankbetuiging te betrekken allen die op enigerlei wijze hebben meegewerkt bij het tot stand komen van dit proefschrift.
C O N T E N T S
I . Introduction 13 II. The polymerization of co-lactams in the presence of water:
Review of literature 17 II. 1. Reaction mechanisms in the formation of polyamides 17
II. 1.1. Condensation reactions 17 II. 1.2. Addition reactions 19 II. 1.3. The hydrolytic polymerization of caprolactam 20
II.2. Structural effects in the polymerization of cu-lactams 22
I I . 2 . 1 . Chain length of the monomeric unit 22
II. 2.2. Influence of substituents 23
II. 3 Conclusions 25 III. Polymerization ofw-lactams in the presence of water:
A mathematical model of the kinetics 27 III. 1. Formulation of conversions due to reactions during polymerization. 27
III. 2. Evaluation of equilibrium and rate constants 32 III. 2.1. Equilibrium and rate constants in the polymerization of caprolactam. 32
III. 2.2. Equilibrium and rate constants in the polymerization of
enantho-lactam 41 III. 2.3. Equilibrium and rate constants in the polymerization of
dodecano-lactam 43 III. 2.4. Equilibrium and rate constants in the polymerization of e-cyclohexyl
caprolactam 48 111.3. Comparison between values of reaction constants 51
I I I . 3 . 1 . Industrial polymerizability of the lactams 51 III. 3.2. Structure of monomer and values of constants 53
111.4. Conclusions 54 IV. Economic aspects 55 IV. 1. Procedure of price estimation 55
IV. 1.1. Cost price and sales price 56 IV. 1.2. Costs of equipment and installation 60
IV. 1.3. Procedure of price estimation 63 IV. 2. Price estimation of nylon 6 65 IV. 2.1. Preparation of caprolactam from phenol 66
IV. 2.2. The price of caprolactam 70 IV. 2.3. Preparation and price of nylon 6 from caprolactam 72
IV.3. Price estimation of nylon 12 75 I V . 3 . 1 . Preparation of dodecanolactam from butadiene 76
IV. 3.2. The price of dodecanolactam 82 IV.3.3. Preparation and price of nylon 12 from dodecanolactam . . . . 83
IV. 4. Price estimation of CH-nylon 6 85 IV.4.1. Preparation of CH-caprolactam from phenol 85
IV. 4.2. The price of CH-caprolactam 90 IV.4.3. Preparation and price of CH-nylon 6 from CH-caprolactam. . . 90
IV. 5. Price estimation of nylon 7 95 IV. 5.1. Preparation of methyl-7-aminoheptanoate from phenol . . . . 95
IV.5.2. The price of methyl-7-aminoheptanoate 101 IV.5.3. Preparation and price of nylon 7 from methyl-7-aminoheptanoate . 104
IV.6. Comparison of the polymer prices 106
IV.7. Conclusions 107 V. Some comparative properties 109
V . l . Melting temperatures 109 V.2. Moisture absorption H I V.3. Density and crystallinity 112 V.4. Solution viscosity in m-cresol 112
V.5. Melt viscosity 113 V.6. Mechanical properties 117
V.7. Processability 118 V.8. Conclusions 119 Appendix: Experimental part
A . l . Synthesis of some lactams and their intermediates 121
A. 1.1. Hydrogenation of some 2-alkylphenols 122 A . 1.2. Oxidation of some 2-alkylcyclohexanols 122 A. 1.3. Oximation of some 2-alkylcyclohexanones 123 A . 1.4. Beckmann rearrangement of some 2-alkylcyclohexanone oximes . . 124
A. 2. Polymerization of lactams 125 A.2.1. Laboratory experiments 126 A.2.2. Experiments on a pilot-plant scale 127
A.3. Analyses of polymers 128 A.3.1. Determination of carboxyl end groups 128
A.3.2. Relative solution viscosity in m-cresol 128
A.3.3. Meh viscosity 129 A.3.4. Determination of the content of low-molecular materials . . . . 130
A.3.5. Thin-layer chromatography of low-molucular materials from poly-e
cyclohexylcaprolactam 131 A.4. Procedure for estimating the values of equilibrium and rate constants. 134
Sunmiary 138 Samenvatting 139
I. I N T R O D U C T I O N
The importance of synthetic fibres is steadily increasing. The production of this class of fibres as well as its relative contribution to the total fibre production are growing vigorously, as is illustrated in Table 1.1 and Fig. 1.1.
T A B L E I . l
PRODUCTION OF RAW MATERIALS FOR TEXTILES (MILLION TONS)l
Year 19S0 1955 1960 1961 1962 1963 1964 1965 Natural fibres: wool, cotton, silk
Man-made fibres from natural polymers: rayon acetate Fully synthetic fibres 1.13 (82%) 10.80 (81%) 11.60 (78%) 11.34 (76%) 11.98 (75%) 12.51 (74%) 12.82 (72%) 12.91 (71%) 1.61 (17%) 2.28 (17%) 2.60 (17%) 2.68 (18%) (18%) (18%) (18%) (18%) 2.86 3.05 3.28 3.33 0.07 ( 1%) 0.26 ( 2%) 0.70 ( 5%) 0.83 ( 6%) 1.08 ( 7%) 1.33 ( 8%) 1.69 (10%) 2.02 (11%)
0.
0"
1950 1955 / u . 9 \ I 18 2 ^ ^ k - ^ ^ 5'/. ^ ^ ^ ^ 11"/. 1950 1965Figure I . l . The growth of the production of textile raw materials (million tons) and the contribution of fully synthetic fibres.
Advantages such as the relatively low specific gravity, high strength and good durabili-ty of synthetic fibres can be taken into account by expressing the production of these fibres in cotton equivalent weights.
For 1965 this results in the following approximate contributions: natural fibres 6 1 % man-made fibres from natural polymers 22 %
fully synthetic fibres 17%
These figures underline the growth of the production of fully synthetic fibres as com-pared with the situation in, e.g., 1955. The point of break-through for the development of these fibres was the appearance of the first samples of the polyamide nylon 66 on the market in 1938. In 1941/1942 the first nylon 6 was produced in Germany. These two types still constitute more than 50 per cent of the market of fully synthetic fibres:
TABLE 1.2
APPROXIMATE DISTRIBUTION OF THE PRODUCTION OF FULLY SYNTHETIC FIBRES* Nylon 6 Nylon 66 Polyester Polyacryl Other types 2 4 % 3 0 % 2 0 % 18% 8%
Nylon 6 and 66 are used not only for textile purposes but also as moulding resins for the plastics industry. In the U.S.A., 8 per cent of the polyamides consumed (approximately
1/3 of the world production) went into plastics^.
Thanks to inherent properties and as a result of the choice of processing conditions and (if desired) the choice of thermomechanical treatment of fibres or fabrics, the existing polyamides fulfil quite a number of requirements. Nevertheless, they possess some real disadvantages in a number of typical applications, e.g., flat-spotting in tires (canvas), insufficient dimensional stability in industrial plastics (gearings), electrostatic charge in carpets, etc.
Therefore the search for well-processable polyamides with improved properties for special purposes and/or based on potentially cheap raw materials is an object of study in many laboratories. Nylon 6-10 and nylon 11 are examples of polyamides which were developed later and which are now being produced commercially. An example of a new type of polyamide is nylon 12, which is being taken into production in Europe*. This polyamide, whose monomer is prepared from butadiene, has properties almost equal to those of nylon 11, which is appHed both as a fibre and as a plastic. A dis-advantage of nylon 12 is the low melting point, abt. 180° C, as compared with abt. 220 ° C for nylon 6 and abt. 255 ° C for nylon 66. Nylon 7 is in a still earlier phase of development; the properties of this polymer are good, but in view of its price it is not certain whether it will have a chance or not. A new polyamide, poly-e-cyclohexyl e-caprolactam, further referred to as CH-nylon 6, was recognized as an interesting product in view of its unexpectedly high melting point, abt. 250° C, as compared with unsubstituted nylon 6.
This new polyamide shows resemblance to nylon 6 in regard of the polymer back-bone and to nylon 12 as regards the number of C atoms per amide group in the repeating unit: ro HI HO— Lc—CH2—CHa—CHj—CH,—CHa—NJ g-H nylon 6 (poly-«-caprolactam) ro HI II I C—CH2—CH2—CH2—CH2—CH2—CH2—C-H2—CHa—CH2—CHg—CHg—NJ ^—H nylon 12 (poly-x-dodecanolactam) HO— O H HO— C—CH2—CH2—CH2—CHj-CH—N —H ri2C CH2 I I H2C CH2 > 1 l 2 CH-nylon 6 (poly-£-cyclohexyl-«-caprolactam)
The object of the present study is a technological and economic comparison of the polylactams, especially nylon 6, nylon 7, nylon 12 and CH-nylon 6.
The study includes:
• An investigation of the polymerization process. Mathematical models which may contribute to the choice of optimum process conditions and apparatus will be developed.
• A procedure for estimating the cost prices of chemical products; this procedure will be developed and will be applied to approximate the cost prices of the materials concerned.
• A comparative study of some properties which can be determined with suflScient reliability on small samples of polymer.
Prior to these studies some expectations, as given in Table 1.3, can be made on the basis of existing knowledge.
T A B L E 1.3
EXPECTATIONS REGARDING POLYMERIZABILITY, PROPERTIES AND ECONOMICS OF SOME POLYAMIDES
Polymer Ease of polymerization Properties Melting point Dimensional* stability Mechanical Economics properties Nylon 6 Nylon 7 Nylon 11** Nylon 12 CH-nylon 6 0 -h ? 7 0
+
+
0 0+
+
9* Derived from moisture absorbtion. ** For comparison.
In this table 0 = standard + = good — = poor.
It appears from this table that special attention has to be paid to - comparative studies of the polymerization,
- comparative studies of the economics of the polymers, - study of the properties of CH-nylon 6.
Literature 1 Textile Organon, 37 (1966) 97.
2 D.W. VAN KREVELEN, Chem. Weekbh, 62 (1966) p. 45. 3 L . F . HATCH, Petr. Ref., 42 (1963, April) p. 157. 4 K.DACHS et al., Angew. Chemie, 74 (1962) p. 540.
II. T H E P O L Y M E R I Z A T I O N OF «-LACTAMS IN THE
P R E S E N C E OF WATER: REVIEW OF L I T E R A T U R E
The reaction mechanism of the polymerization of cj-caprolactam in the presence of water has already been thoroughly investigated '"^. Other studies relate to the influence of the structure of the monomer on the polymerizability. In this respect the influence of substituents to the lactam ring and of the number of C-atoms in the ring was inves-tigated. A review of these studies will be given in this chapter.
II. 1. Reaction mechanisms in the formation of polyamides
Historically, polymerization reactions are divided into two groups:
the addition polymerization, with a chain reaction mechanism, which is generally characterized by fast reactions, and the condensation polymerization, with
step-by-step reactions, which are slow compared with chain reactions.
In later investigations polymerization processes have been developed which cannot be classified in such a simple way, but which have aspects of each of these groups, depend-ing on the criteria applied. Examples of such criteria are given in Table 11.1.1. Their application to some polymers is shown in Table II. 1.2.
TABLE I I . 1.1
SOME CRITERIA IN THE CLASSIFICATION OF POLYMERS
. . Typical of Typical of addition pol. (a) condensation pol. (c) Type of reaction Addition reaction Condensation reaction Reaction mechanism Ionic or radical Molecular Reaction rate Fast Slow
(minutes or shorter) (hours or longer)
Bonds between monomer units are hydrolyzable N o Yes Interchange of segments between pol.chains occurs
during polymerization No Yes Polymerization is an equilibrium reaction N o Yes
Hence, polyamides, which are typical condensation polymers from the historical point of view, can be formed in condensation reactions as well as in addition reactions. In the hydrolytic polymerization of caprolactam both types of reactions occur. 11.1.1. Condensation reactions
a The condensation reaction occuring most frequently in polyamide chemistry is
T A B L E 11.1.2
CLASSIFICATION OF SOME POLYMERIZATION REACTIONS AND POLYMERS
Polymerization of Styrene Acrylamide Ethylene oxide Acrylamide (anionic) Caprolactam (anionic) Caprolactam (hydrolytic) Diamines with diacids Repetitive unit — C H 2 - C H — C.H5 — C H 2 - C H — C O N H 2 —CH2—CH2—0— —CO—(CH2)2—NH— —CO—(CHa)^—NH— —CO—(CH2)5—NH— — C O — R — C O — N H — R " — N H — Type of reaction addition (a) addition (1-2) (a) addition (a) addition (1-4) (a) addition (a) addition (mainly) (a) condensation (c) Criteria Reaction mechanism radical-ionic (a) radical (a) molecular-ionic (a, c) • ionic (a) ionic (a) molecular (c) molecular (c) Reaction rate fast (a) fast (a) fast (a) fast (a) fast (a) slow (c) slow (c) Hydrolyzable Interchange bonds of segments N o (a) No (a) No (a) Yes (c) Yes (c) Yes (c) Yes (c) N o (a) No (a) No (a) No (a) Yes (c) Yes (c) Yes (c) Equilibrium polymerization No (a) N o (a) No (a) N o (a) Yes (c) Yes (c) Yes (c)
The commercial polymers nylon 6-6, nylon 6-10 and nylon 11 are obtained in this way, nylon 6-6 from hexamethylene diamine and adipic acid:
n HjN-(CHj)e—NHa + n H O O C — ( C H j ) * - C O O H -*
H—[HN—(CHa),—N H C O — ( C H a ) ! — C O J H - O H + (2n - 1 ) H 2 0
nylon 6-10 from hexamethylene diamine and sebacic acid:
n H j N - ( C H a ) 8 — N H a + n HOOC—(CH2)8—COOH ->
H—[HN—(CH2)6—NHCO—(CHa),—COls-OH + (2n - DHjO
and nylon 11 from co-amino undecanoic acid:
n H2N—(CH2)io—COOH ->
H—[HN—(CHa)io—CO]s-OH + ( n - l)HaO.
b The amide bonds can also be formed in reactions between amino groups and ester groups. An example of such reactions is the preparation of the experimental polymer nylon 7 from cü-amino-ethyl heptanoate'^:
n HaN—(CHa)»—COOC2H5 -*
H—[HN—(CHa)6—COln-OCaHs + (n - l)CaH50H
Polyoxamides, such as nylon 6-2, can be obtained in a similar reaction in the presence of catalysts like SbFj and AS2O3 ' :
n H2N—(CHa)»-NHa + n HjCjOOC—COOC2H5 -^
H—[HN—(CH2)6—NHCO—COJg^OCaHs + (2n - ^CaHjOH.
c In the so-called interfacial condensations immiscible solutions of diamines and dicarboxylic acid chlorides are combined. In the "interface" a polyamide is formed. The evolved hydrogen chloride is usually bound with an acid acceptor, for instance caustic soda or a tertiary amine*:
n H a N — ( C H a ) , - N H a + n ClOC—CeH4—COCH-2n R ' R ^ R ' N + H a O -> n H—[HN—(CHa),-NHCO—CeH,—CO]ir-OH+2n RiR'^R'N.HCI.
It is assumed that the High-Temperature yarn "NOMEX" is based on a polymer prepared in this way^' *.
II. 1.2. Addition reactions
All the condensation reactions described before are very similar, whereas the known addition reactions may differ completely from one another.
a As will be shown later, the addition of a lactam to amino groups is the most im-portant reaction in the conversion of caprolactam into nylon 6:
n p [ N H — ( C H a ) 5 — C O ] - | +H2N—(CH2)6—COOH -> H—[HN—(CH2)6-CO]^,^^rï)-OH.
b Nylon 3 can be obtained by anionic polymerization of acrylamide^. Formally this reaction can be regarded as a 1-4 addition:
n CHa=CH—CO—NHa -* n[CHa—CHa—CO—NH] -* CHa=CH—CO—[NH—(CHa)a—CO](„_^) NHa.
c A remarkable reaction is the formation of polyamides from alcohols and car-bonitrils^°. In this reaction an addition followed by a rearrangement must be assumed:
N H
II
H O — R i — O H + N = C — R ^ — C = N -> H O — R i — O — C — R ^ - C = N ->• H O — R ' — N H — C O — R ^ — C = N ,
etc. Such a mechanism is similar to the hydrolysis of carbonitrils to acid amides via the hypothetical hydroxy-imine:
R R I + H a O -^ C = N H -^ C—NH
C = N I II
^ H ^
II. 1.3. The hydrolytic polymerization of caprolactam
Before sufficient analytical and kinetic data were available, it was assumed that nylon 6 was formed from caprolactam in two reactions'':
hydrolysis of caprolactam to aminocaproic acid, and stepwise condensation of the latter to nylon 6.
Later studies ('•^'•'> have indicated that most of the amide groups in the polymer are formed by addition of caprolactam to amino groups. The first amino groups are formed by hydrolysis of caprolactam to aminoproic acid. When the conversion of monomer into polymer has been nearly completed by addition, a sufficiently high degree of polymerization is reached by condensation of most of the remaining end groups.
This results in the following scheme of reactions: 1) Ring opening of caprolactam with water:
r [ N H — ( C H a ) 5 — C O ) - , + H j O F ^ HaN—(CH2)5—COOH
Actually this reaction can also be applied to cyclic dimer, trimer and other oligomers:
r-[NH—(CHa)5—CO]-j+HaO ^ ^ H — [ H N — ( C H a ) 6 — C O J H - O H .
I 1 k ' h . n
In technical polymers the cyclic oligomers up to the nonamer could be detected*^. 2) Polyaddition of caprolactam to amino end groups:
p [ N H — ( C H a ) 6 — C O ] - , + H - [ H N — ( C H a ) 5 — C O ] B - O H ? = ^ H—[HN—(CHa)6—COlj^ppj) OH
This reaction can also be applied to cyclic oligomers:
The interchange of polymer chains by transamidation may be due to a similar reaction: H—[HN—(CHa)^—COis—i _ _ ^ . . k. HO—[CO—(CHa)5—NH]= + H—[HN—(CHa)6—CO]—OH ^ H—[HN—(CHj)5—CO]—[HN—(CHa),-CO]—OH+ HO—[CO—(CHa)6—NH]—H 3) Polycondensation.
The amino and carboxyl groups formed in the hydrolysis of caprolactam and grown into polymer end groups by addition of caprolactam will finally react, with reformation of water:
H—[HN—(CHa)6—CO]—OH + H—[HN—(CH2)6—CO]—OH ? = ^
k c
H-[HN-(CHa)5-CO](jpp^)OH.
There are strong indications that nearly all the reactions in the caprolactam polymeriza-tion are catalyzed by the carboxyl groups present, so that the reacpolymeriza-tion rate constant may be split up into a carboxyl-catalyzed and an uncatalyzed part:
k „ = •'k„+c-'=k„ (H.l)
where:
kn = reaction rate constant "kn = "uncatalyzed" part "kn = "catalyzed" part
c = carboxyl group concentration
n = number of monomer units of reactant (1,2, etc., for monomer, dimer, etc.). For the concentrations of functional groups and of compounds, expressed in mols per g, the following nomenclature is introduced:
Xi for the concentration of caprolactam a for the concentration of amino end groups c for the concentration of carboxyl end groups w for the concentration of water
z for the concentration of amide groups of polymer u 1 for the concentration of aminocaproic acid x„ for the concentration of cyclic n-mer u„ for the concentration of linear n-mer
This nomenclature is used in the definition of the equilibrium and rate constants given in Table II. 1.3.1.
A non-conventional (but uniform) definition of the equilibrium constants has been chosen to obtain correspondence, as far as possible, with the existing literature.
T A B L E I I . 1 . 3 . 1
EQUILIBRIUM AND RATE CONSTANTS IN THE POLYMERIZATION OF CAPROLACTAM Equilibrium Rate constants, Reaction . . T^- I J / /
constant, K k and k Ring opening K^ „ = x„.w / u„* TiL^„= k\„ / k^ „ Polyaddition K^ „ = a.x„ / a = x„** }L^„= \i\„ / k^ „ Polycondensation K^ = a.c / z.w K^. = k'^ / k^ * It can be proved that for sufficiently high average degrees of polymerization Kh n ~ ^ a n / ^c-** For this expression it is assumed that the concentration of an n-mer is almost equal to tiie concen-tration of an (n-f- l)-mer. This requires a sufficiently high average degree of polymerization.
The reaction mechanism discussed before does not account for ionization of amino and carboxyl end groups. In one of the latest publications on the reaction mechanism of caprolactam polymerization' ^ the effect of possible ionization on the rate of poly-addition is discussed. The degree of ionization changes during the reaction as a result of a change in dielectric constant of the reaction mixture due to conversion of monom-er into polymmonom-er
It will be shown in chapter 111 that the rate of end group condensation is evidently influenced by the composition of the reaction mixture for caprolactam and enanthol-actam polymerization.
II. 2. Structural effects in the polymerization of c»-Iactams
In the series of lactams which will be discussed here, two structural effects are present. Firstly, the numbers of methylene groups in the main chain of the monomeric units differ for caprolactam, enantholactam and dodecanolactam.
Secondly, a substituent is present in the e-substituted isopropyl, tert. butyl and cyclo-hexyl caprolactam and in a-isopropyl caprolactam.
II. 2.1. Chain length of the monomeric unit
In the series of homologue lactams the amide groups were found to have different configurations'*. Caprolactam and enantholactam have cis-amide groups, whereas dodecanolactam has a trans-amide group. This is reflected in the change of the dielec-tric constant with the concentration of solutions of the lactams in non-polar solvents. For lactams with a cis-amide configuration the dielectric constant decreases with in-creasing concentration as a result of "neutralization" on dimerization:
For lactams with a trans-amide configuration the influence of concentration on the dielectric constant is the reverse.
In linear chain amides the amide group has a trans-configuration ' ' • '*. So in the polymerization of caprolactam and enantholactam cis-amide groups are converted into trans-amide groups, whereas in the polymerization of dodecanolactam the con-figuration of the amide groups does not change. On this basis it may be expected that a possible change of the dielectric constant of polymerizing lactams will be different for caprolactam and enantholactam than for dodecanolactam. This effect has been dis-cussed for caprolactam in '^. As will be described in the Appendix, the amide group of e-cyclohexyl caprolactam has a cis-configuration just as the amide group of caprolac-tam, whereas in the cyclic dimers of both caprolactam and e-cyclohexyl caprolactam the amide group has a trans-configuration, as was to be expected on the basis of the afore-mentioned observations.
II. 2.2. Influence of substituents
The influence of the presence of substituents on the polymerization of w-lactams has many times been a subject of investigation. Mostly, the polymerizability of substituted lactams has been studied for equilibrium situations'^"^'. A recent study compares some rates of polymerization on the basis of direct calorimetric measurements^^. Unfortunately, different procedures for determining residual monomer (by extraction) have been used and in some cases they have been described insufficiently. This is a source of confusion in the judgment of the polymerizability and in comparisons. The following examples may illustrate this. In one instance e-isopropyl and e-tert. butyl caprolactam were found to have no tendency towards polymerization^", whereas elsewhere'* polymerization up to respectively 63% and 26% was observed. Another example of different results by different extraction techniques is given in the
experimen-tal part (Appendix). It was found that by benzene extraction only monomer could be removed from polymers of e-cyclohexyl caprolactam, whereas by prolonged methanol extraction also dimer was found to be extracted.
In Table II.2.2.1. the most interesting data on the polymerization of y- and e-substituted caprolactams are collected.
TABLE II.2.2.1
POLYMERIZABILITY OF y- AND S-SUBSTTTUTED CAPROLACTAMS
Substituent y-methyl y-methyl y-methyl y-ethyl y-ethyl y-n-propyl y-i-propyl y-tert.butyl y-tert.butyl y-cyclohexyl y-phenyl £-methyl E-methyl £-ethyl £-propyl e-isopropyl £-isopropyl e-tert. butyl e-cyclohexyl e-cyclohexyl Conv. into polymer ( %) 72 91 67 33 70 59 63 0 26 50 0 90 82 76 75 0 < 1 0 , > 0 < 1 0 , > 0 abt. 55** abt. 25 Pol. temp.
r
c)
250 254 240 250 254 254 254 250 254 254 80* 254 240 240 240 250 260 260 260 260 Extractant water benzene benzene water benzene benzene benzene water benzene benzene water/acetone benzene benzene benzene benzene water benzene benzene benzene methanol Literature referance 20 18 22 20 18 18 18 20 18 18 17 18 22 22 22 20 this study this study this study this study * anionic polymerization.** mol.wt. of monomeric unit is 195, so equilibrium polymer content is (1000-also III.
1.3 X 195) mg/g. See
The conclusions to be drawn form Table II.2.2.1. are:
- Increasing steric hindrance due to the presence of a substituent results in decreas-ing polymerizability.
- Only a slight influence, if any, of the place of the substituent can be estabHshed for y- and e-substituted lactams.
- Apparently différents results obtained in polymerizations must sometimes be attrib-uted to differences in polymer extraction technique.
Explanations of the influence of substituents on the polymerizability have been given in terms of rotational isomerism ' ^' '^' ^'. The enthalpy of ring closure can be related to the increase of the number of gauche interactions between adjacent C-atoms. Intro-duction of a substituent reduces the increase of this number of gauche interactions and therefore results in a lower contribution to the enthalpy of the ring closure reaction.
It may be assumed that the presence of a substituent has little or no influence on the
entropy of the ring compounds, whereas the entropy of substituted chain compounds
is lower than that of unsubstituted chains owing to hindrance of rotation of the chain. So the entropy difference between corresponding chain and ring compounds increases by the introduction of substituents, as is illustrated below; the more bulky the sub-stituent, the more pronounced the effect.
increasing S
' * ring (substituted and unsubstituted) chain (unsubstituted)
chain (substituted)
The entropy of ring and chain compounds in the polymerization of lactams.
These two effects of substituents, a lower reaction enthalpy and a higher reaction
entropy on ring closure, shift the equilibrium towards the ring structure.
A more quantitative treatment on the basis of free energy of reaction is given in'^. In '* the rates of polymerization have been determined for some substituted capro-lactams and enanthocapro-lactams. Unfortunately, no comparison could be made between the polymerization rate of caprolactam itself and those of the substituted caprolac-tams, because the latter lactams were polymerized in the presence of phosphoric acid, which is a very effective catalyst in the hydrolytic polymerization of lactams. For the same reason it is also impossible to make a comparison with the polymerization rate of e-cyclohexyl caprolactam.
II.3. Conclusions
1. The hydrolytic polymerization of caprolactam is a result of 3 reactions, viz:
- hydrolysis of caprolactam (and cyclic oligomers)
- addition of caprolactam (and cyclic oligomers) to amino end groups of polymer.
- condensation of end groups of polymer. (References 1,2, 3.)
2. The monomer content may be expected to have an influence on the reaction rates in the polymerization of caprolactam and enantholactam, as a result of a change in the configuration of the amide groups during polymerization.
(Referencesl3,14,15, 16.)
3. The polymerizability of a lactam decreases with increasing bulkiness. Only a slight influence of the place of the substituent can be established.
Literature
1 F . WiLOTH, Z.f Phys. Chem., N F 11 (1957) 78. Kolloid Z., 160 (1958) 48. 2 P. H. HERMANS et al., J. Pol. Sci., 30 (1958) 81.
3 C H . A . KRUISSINK et al., J. Pol. Sci., 30 (1958) 67. 4 C. F. HORN et al., Angew. Chemie, 74 (1962) 531. 5 S. D . BRUCK, Ind. & Eng. Chem., 55 4 (April, 1963) 9. 6 H. MARK, Makromol. Chem., 35 A 3 (1960) 49. 7 L. K. Mc. CUNE, Text. Res. J., 32 (1962) 762. 8 A N . , Chem. Week, 88 15 (1961) 91.
9 W. R. SORENSEN et al., "Preparative Methods of Polymer Chemistry," Interscience Publ., New York (1961).
10 H. LAUTENSCHLAGER (BASF), German Patent 51239 (priority November 28, 1958). 11 H. HOPE et al., "Die Polyamide", Springer Verlag, Berlin (1954).
12 M. ROTHE, J. Pol. Sci., 30 (1958) 227.
13 C H . A . KRUISSINK, Chem. Weekbl., 56 (1960) 141. 14 R. HUISGEN, Angew. Chemie, 69 (1957) 341. 15 N . OGATA, / . Pol. Sci., A I (1963) 3151.
16 M. TsuBOl, Bull. Soc. Chem. Japan, 22 (1949) 215. 17. R. CuBBON, Makromol. Chem., 80 (1964) 44. 18 L. E. WOLINSKY et al., J. Pol. Sci., 49 (1961) 217. 19 H . YUMOTO, J. Chem. Phys., 29 (1958) 1234. 20 W. ZiEGENBEiN et al., Chem. Ber., 88 (1955) 1906. 21 H. K. HALL, J. Am. Chem. Soc, 80 (1958) 6404. 22 O. B. SALAMATINA et al., Pol. Sci. USSR, 1 3 (1965) 538.
III. P O L Y M E R I Z A T I O N OF «-LACTAMS IN
T H E P R E S E N C E OF WATER: A M A T H E M A T I C A L
M O D E L OF T H E K I N E T I C S
A mathematical model for a chemical process comprises a set of equations describing conversions (due to the reaction), mass transfer and heat transfer, together with values or expressions for the independent process parameters. Such a set of equations, values and expressions is necessarily a simplification of reality. In general it is desirable to simplify a model as far as possible without significantly affecting the results of calcula-tions.
In this study the results of laboratory experiments carried out in closed glass tubes will be described. The contents of the tubes are regarded as a homogeneously reacting mass, which implies that the set of equations may be reduced to equations describing the conversion due to chemical reactions exclusively.
These assumptions have been made in view of the following considerations:
- results of reactions carried out in glass tubes with strirring of the reaction mixture ^'*' do not show any significant difference from results obtained in glass tubes without stirring " ' ;
- results of experiments in autoclaves with stirred contents are in agreement with laboratory experi-ments in glass tubes without stirring.
The process parameters can be distinguished into rate and equilibrium constants and process conditions, such as temperature and initial concentrations of reactants. In most cases the reaction times are long compared with the time necessary for heating up, cooling
down and dissipation of the heat of polymerization. Therefore isothermal conditions are usually assumed. Some experiments with caprolactam are an exception to this rule. In such cases the overall effects of heating up, cooling down, and evolution of heat of plymerization have been taken into account.
When the values of the rate and equilibrium constants are known, the conversions under fixed conditions can be predicted. On the other hand, the values of the reaction constants can be derived by fitting curves for the conversion of reactants to the ex-perimental data. Values of constants found in this way need not necessarily be the true values of the constants: especially when more than two constants have to be deter-mined, the values of these constants may be mutually adabtaple, so that more than one set of values for the constants give a good fitting of calculated to experimental data. III. 1. Formulation of conversions due to reactions during polymerization
The lactams considered in this study are very similar in structure, so that, apart from quantitative aspects, the set of reactions occurring may be expected to be identical.
For this reason the mathematical model of the polymerization of lactams, as used in this study, will be based on the reaction mechanism for the polymerization of capro-lactam discussed in section II. 1.3. In this reaction mechanism not only monomer reactions but also oligomer reactions were mentioned.
In the polymerization of caprolactam and dodecanolactam the results of calcula-tions are not essentially different if only monomer reaccalcula-tions are taken into account. In the polymerization of caprolactam the only effect of the introduction of oligomer reactions is a slightly lower concentration of polymer amide groups and, consequently, a somewhat lower degree of polymerization. In the case of dodecanolactam polymer-ization the content of low-molecular materials in equilibrium is so small that even this effect may be neglected. In the polymerization of e-cyclohexyl caprolactam, how-ever, the concentration of cyclic dimer is considerable, so that in this case the dimer reactions have to be taken into account.
Therefore two mathematical models will be developed: one restricted to monomer reactions and one extended to dimer reactions. In the models the conversions of linear and cyclic monomer (and oligomers) and carboxyl end groups will be given as differential equations. The concentrations of the other reactants, viz: polymer amide groups and amino end groups, and of water are linearly related to the concentrations of the reactants mentioned before. The conversions of the reactants, R, are given in mols per unit time and unit weight.
Conversion of cyclic monomer and dimer
The conversion of lactam is given by:
Rxi = - k h , i ( x i - w - K h , x - U i ) - k a , i ( a - X i - K a , i ( a - U i ) ) hydrolysis of monomer addition of monomer
The term Uj in the addition of monomer has been introduced because no lactam can be formed from linear monomer in this reaction.
A similar equation describes the conversion of cyclic dimer:
Ri8 = -kh.aCxj • w - K h , , • u , ) - k a , s ( a • X 8 - K a , j ( a - U i - U j ) ) hydrolysis of dimer addition of dimer
Conversion of linear monomer and dimer
Differential equations for the conversion of linear monomer and dimer, including restriction of cyclic compounds to monomer and dimer respectively, are:
Rui = kh,i(xi • w K i , , i • u , ) k a , i ( X i • u , K a , i • U j ) -hydrolysis and addition of cyclic monomer
(III 3) - k e ( u i ( a - h c ) - K o - w ( c - f - a - 2 u O )
In this equation the last term involves hydrolysis of the amide groups nearest to the chain ends:
Polymer - CONH - (CH j)6 - COOH Polymer - NHCO - (CH^)» - N H^.
The similar equation for linear dimer is given by:
Rua = l'h,2(X2 • W —Kh,2 • Ua) —ka,2(X2 • U2 —Ka,2 ' u j
hydrolysis of dimer addition of cyclic dimer
-ka,i(Xi • Ua-Ka,i • Us)-|-ka,i(Ui • X j - K a , i • U2)
addition of cyclic monomer + kc(ui • U i - K c • w • Ua)
condensation of linear monomer with itself — kc(ua(c-|-a) —Kc • w(c + a —2u2—2ui))
condensation of linear dimer with chain ends
(III. 4)
In equation III.3, for linear monomer, there occurs a term containing Uj, while in equation III.4, for linear dimer, terms with U3 and U4 are present. Generally, terms for the concentrations of "higher" oligomers are introduced into all equations describ-ing the conversion of a linear n-mer. To avoid an infinite number of equations it is necessary that the concentrations of oligomers "higher" than the "highest" oligomer under consideration are somehow related to the concentrations of the "lower" oligo-mers. Such a relation could be based on the well-known "Flory distribution" for condensation polymers'. Actually, this distribution of the degrees of polymerization of polymer chains only holds for polymers in condensation equilibrium. Computations based on the model developed have shown, however, that, long before this equihbrium is reached, and even in the beginning of the polymerization of lactams, the assumption of a "Flory distribution" is justified in view of the accuracy of the rate equations. This distribution relates the concentrations of linear n-mers to each other on the basis of the extent of the condensation reaction p:
Un = P • Un-i = P ' • U„-2 = P<"-»> • Ui (III.5)
where:
p = z/(a-hz) (III.6)
So U2 in equation III. 2 can be replaced by
and U3 and U4 in equation III .4 by
u. = p U l = p» = p • U 2 U j (III. 7) (III.8) (III .9)
In the latter case the substitution according to equation III. 7 has to be omitted in equation III. 3.
Conversion of carboxyl end groups
The conversion of the carboxyl end groups is given by equation III. 10 or 11. When only monomer is taken into account,
Re = kh,i(xi • w—Kh,, • U i ) - k c ( a -c—Kc • w • z)
hydrolysis of monomer condensation of end groups (III. 10)
When monomer and dimer are taken into account,
Re = kh,i(Xi w - K h , ! • Ui) + kh,2(x2 • w - K h , 2 - Ua) hydrolysis of monomer and dimer
— kc(a • c—Kc • w • z) condensation of end groups
(III. II)
Conversion of other reactants
Finally, for a closed system the conversions of water, amino end groups and amide groups are expressed by the conversions of carboxyl end groups and cyclic monomer (and dimer):
Rw = - R a = - R c (III. 12)
and
Rz = - R x i - R c (III.13)
or, when also dimer is considered,
Rz = — Rxi—2 • Rx2—Re (111.14)
Addition of the subscript o to the initial concentrations (ZQ, XJO, X2_O,CO, Wo,andao) and substitution on the basis of the integral values of equations 111.12 through 111. 14 yields for w and a:
for z:
or,
Wo —w = —ao+a = —Co+c
Zo-z = - ( x i , o - x , ) + (co-c)
Zo^Z = —(Xi,o —Xi) —(Xj.o —X2) + (Co—c)
(III. 15)
(III. 16)
(III. 17)
Summarizing, the mathematical model of the polymerization of lactams comprises a set of equations for the conversion of the reactants and their initial concentrations. A model has been developed for two cases in which:
- reactions of low-molecular compounds are restricted to monomer,
- reactions of low-molecular compounds are restricted to monomer and dimer. The equations necessary for a balanced description are collected in Table III. 1.1.
T A B L E I I I . 1 . 1
EQUATIONS IN THE MATHEMATICAL MODEL OF THE POLYMERIZATION OF LACTAMS , Only monomer Monomer and Reaction of ,. j - .•
reactions dimer reactions Cyclic monomer
Cyclic dimer Linear monomer Linear dimer Carboxyl end groups Amino end groups and water Amide groups of polymer
111.1 I I I . 3 + I I I . 7 111.10 III.15 111.16 I I I . l III. 2 III. 3 I I I . 4 + I I I . 8 + III.9 III.11 III.15 III.17
Simulations of the polymerization can be made by numerical integration of the system of differential equations. In this study the numerical integration procedure of Runge Kutta was applied^. Application of the given model to the polymerization of caprolactam and enantholactam did not result in fully acceptable description, as will be shown later. To get around this difficulty the model was extended by an empirical relation accounting for a possible influence of the composition of the reaction mixture (dielectric constant!) on the reaction rates as discussed in chapters III. 1 and II. 1.2 (Lit: Kruissink ref. 11.13). Fundamental relations describing the ionization of end groups were considered too speculative.
For the conditions studied well-fitting curves could be obtained by the introduction of:
lCe(l-(x/Xo)°) (111.18)
where:
v.* and kj are rate constants of end group condensation with and without introduc-tion of the influence of the composiintroduc-tion of the reacintroduc-tion mixture,
n is a parameter whose value has to be chosen.
The influence of the value of n can be illustrated as follows:
kc* -* kc when n -> -f cv^
kc* ->• 0 when n -> « b e c a u s e x/x» < 1
Only values of n > 0 are acceptable from the kinetic point of view.
With dodecanolactam and e-cyclohexyl caprolactam the introduction of 111.18 is not necessary, as will be shown later. In the case of dodecanolactam this is plausible, because, owing to the conversion of monomer into polymer, the configuration of the amide group does not change, whereas it does change in the case of caprolactam and enantholactam (cf. section III. 1). For e-cyclohexyl caprolactam the calculations were not pursued to such an extent that a significant answer to this question could be ob-tained. It is expected, however, that the influence of the substituent will overshadow the effect.
I l l . 2. Evaluation of equilibrium and rate constants
The level of knowledge of the numerical values of the reaction constants occurring in the mathematical models of the polymerization varies with the lactams studied. The polymerization of caprolactam has been investigated thoroughly ^~*, yet the knowledge of the values of the constants is distinctly insufficient. Although no values for reaction constants of dodecanolactam have hitherto been published, an impression can be obtained from some reported experimental work'. Data concerning the poly-merization of e-cyclohexyl caprolactam have not yet been published in the literature. An extension of the data on the above-mentioned polymerizations will be furnished. Moreover an interpretation of reported work on enantholactam* will be given. The values of rate and equilibrium constants are not completely independent of the com-position of the reaction mixture. When necessary, reference to this effect will be made in the following considerations.
I l l . 2 . 1 . Equilibrium and rate constants in the polymerization of caprolactam
In the early fifties the polymerization of caprolactam was studied in detail by at least three groups of scientific workers^"*. Additional studies dealing with certain aspects of the polymerization'''° extended the knowledge of numerical values of the reaction constants. In spite of these investigations a complete set of kinetic data is only available for temperatures of about 220 °C, and it does not include data on oligomer reactions. For some of the constants the temperature dependence is not known at all, while for most of the other constants it is derived from values at only two temperatures, about 220° C and 250 ° C. The numerical values known up to now are collected in Table 111. 2 . 1 . 1 . Extension of the data given in Table I I I . 2 . 1 . 1 . was obtained in the following ways:
1. Determination of the equilibrium constant for condensation K c at higher tem-peratures.
2. Interpretation of polymerization experiments to derive the values of rate con-stants for the hydrolysis of caprolactam and to dertermine the influence of the compos-ition of the reaction mixture on the value of the rate constants for condensation. Equilibrium constants for polycondensation
Additional values for the equilibrium constant for polycondensation were determined at 280, 295 and 310° C. For this purpose polymers with known concentrations of end groups and water (0.05-0.15% w/w) were equilibrated in a closed system in the pre-sence of some phosphoric acid as a catalyst. From the degree of polymerization in equili-brium the values of Kc could be derived which (at the low levels of water content applied) are not, or only slightly, dependent on the initial water content, viz:
TCP K^
280 2.80 • I0-» 295 3.30 • 10-» 310 3.94 • 10-»
T A B L E I I I . 2 . 1 . 1
PUBLISHED KINETIC DATA ON THE HYDROLYTIC POLYMERIZATION OF CAPROLACTAM
Constants 221.5 "C Values at 254 "C Other temperatures Ringopening (hydrolysis) X _ , (g s e c - i m o l - i ) "^•«h 1 (S* sec"' mol'^) Kt'_i (mol g->) (K^,! = K3 ,/K,) 2.2 • lO-* (3) 4.7 • 10+1 (3) 4.1 • lO-i (4, 19) Polyaddition "k. 1 (g see-» mol-i) 2.8 • 10"' or 1 (3) % 1 (g" sec-i mol->) 5.8-10+»' 8.9-10+»' K.',(molg->) 5.5 lO-* (4,11) 0 0 < ' ' k < 2 . 5 - 10-1(4) 2.14- 10+* (3) 7.2- 10-* (4, II) 230 °C 5.8 • 10-* (9) 250 "C 6.9 • 10-* (9) 270 "C 1.9 • I0-* (9) Polycondensation "kj (g sec-i mo|-i) % (g' sec-i mol-») K c ( - ) 1.33 • 10-» (3, 4, 5, 10) 1.90- 10-» ( 3 , 4 , 5, 10) 229 °C 243 "C 256 °C 273 °C 4.4- 10-1 (5) 7.9 • 10-1 (5) , 2 6 (5) 2.21 (5) l.I • I0+* (5) 1.5-10+* (5) 2.0-10+* (5) 2.9 • 10+* (5)
-InKct
1.90 2.00 Tm 2.10
-^ T - i - 1 0 + » r K - i )
Figure III. 2.1 . l . T h e equilibrium constant for condensation K,, as a function of temperature at low moisture content.
Together with already known values of low moisture content these data are repre-sented in the Arrhenius plot constructed in Fig. 111.2.1.1.
The value of In (Kc) deviates from a straight line at 221.5 ° C. Values of Kc at lower temperatures are not known.
A similar deviation has been found for the equilibrium constant of polyaddition Ka, 1 ' ^, as is shown in Fig. I l l . 2.1.2.
An explanation of these phenomena may be that, owing to the phase transition in the melting range, the entropy and enthalpy of reaction become more dependent on temperature. lnKa,,t 8.5 6D 7.5 7.0 < -U , 9 , 1 1 ] ^ , ^ . 1 t 1 [12] ^ X
1
^1
, i ,
17 1.9 2.0 Tm 2.1 2.2 —> T - i - 1 0 + » ( ° K - i )Similar considerations were applied to the low temperature polymerization of cer-tain monomers in relation to the ceiling temperature, which is the temperature above which no polymerization can occur'^.
The values of Kc presented in Figure I I I . 2 . 1 . 1 . relate to polymers of low moisture content. It was found earlier'* that the value of Kj as defined before is influenced by the concentration of water in the polymer. This can be explained on the basis of thermodynamic considerations' *.
Rate constants for caprolactam hydrolysis and end-group condensation Rate constans at 220 ° C.
As shown in Table I I I . 2 . 1 . 1 , the values of the rate constants for hydrolysis of caprolactam were known at only one temperature, abt. 220° C. For this temperature the mathematical model was checked against results of experiments by Hermans^ and Wiloth'*, applying values for reaction constants as given in Table 111.2.1.1. With the values as such no convenient curve fitting could be obtained.
In section II. 1 it was mentioned that the composition of the reaction mixture prob-ably exerts an influence. This influence was empirically quantified in Eq. I l l . 18:
kc* = kc[l-(x/x„)°] (III. 18)
In Figure III. 2.1.3. the influence of the value of n on the results of calculations is, illustrated for values of n of 0.1, 0.6 and 10,000. With n = 10,000 (given for compari-son) the curve is almost identical to a curve which would have been obtained without application of Eq. 111. 18. I (mmls/g) 8 8 i 2 2 « E e 10 —> time (h)
Figure III. 2.1.3. Polymerization of caprolactam at 221.5° C. (Hermans», w^ = 0.87 mmols/g.)
(meq/g) 0.15 O.10 -OOS ~«~^ • ^ • \ • / J ^ ET
\ I
_ ' N ^ • • curve n I lOOOO I Of m 0.1 c ^ XAs the value of n is assumed to be related to the degree of ionization of end groups, an influence of water content and temperature on this value could be expected. This is illustrated by reference to similar simulations of experiments originating from Wiloth'*, in which the water content was varied this time at a temperature of 220° C.
For these experiments an acceptable description, as presented in Figs. 11.2.1.4 through 7, was obtained with the following values of the exponent n:
Initial water content (molsjg) 0.089 • 10-» 0.177 • 10-» 0.354 • 10-» 0.710- 10-» Value of n 0.15 0.40 2.0 10.0
However, the value of the rate constant of uncatalyzed monomer hydrolysis best suited to the experiments of Wiloth has to be twice the value given in (3).
In view of the level of accuracy of the measurements and of the insensitiveness of the calculations to n at higher values (see Fig. 111.2.1.3) the accuracy of the values of n will not be very high. The conclusion will therefore be restricted to a qualitative one, viz: at higher water contents the value of n tends to increase.
It will be shown in Section III. 2.2 that this approach to the influence of the composition of the reac-tion mixture is also applicable to enantholactam.
Rate constants of hydrolysis at temperatures higher than 220° C
To estimate the values of the rate constants of hydrolysis of monomer as a function of temperature two series of experiments were made. In the presence of 0.024 meq of benzoic acid per g and 0.20 mmols of water per g, caprolactam was heated in an oven to 240 or 280 ° C and subsequently polymerized for a certain time. After homogene-nation of the polymer the contents of monomer and of carboxyl end groups were deter-mined. A detailed description of the polymerization procedure and of the analyses is given in the appendix. The experimental results are given in Table III.2.1.2.
T A B L E I I I . 2 . 1 . 2
POLYMERIZATION OF CAPROLACTAM AT 240° C AND 280° C, MONOMER CONTENT AND CARBOXYL END GROUP CONTENT (c^ = 0.024 mcq/g, WQ = 0.20 mmols/g)
Pol. time (h) 0.5 1 2 4 6 9 240° Monomer (mmols/g) 8.81 8.58 7.47 4.37 2.47 1.66 C -COOH fmeq/g) 0.040 0.075 0.101 0.118 0.106 0.082 280' Monomer (mmols/g) 8.61 5.74 2.55 — 1.33 1.30 C -COOH (meq/g) 0.072 0.122 0.092 0.077 0.076 0.075
(mmols/g)
20 40 60 60 100 120
—> time (h)
Figure 111.2.1.4. Polymerization of caprolactam at 220° C (16); w„ = 0.089 mmols/g.
meq/g I p (mmols/g)
me q/gT 0.10 0.08 0.06 0.04 ao2 -^ - / - /
f
\ \ \ • \ \ 1 • ^ ^ -._._»^_ 1 / X -_ -1 (mmols/g) - 4 - 2 S 10 15 20 25 30 —> time (h)Figure III.2.1.6. Polymerization of caprolactam at 220° C (16); w^ = 0.354 mmols/g
meq/g I aio 0.08 0.06 0.04 0.02
• p.
\ ƒ \ •7 \
/ 1 ^ _ \ . ^ 1 c • ^x 1 • • ~ 1 -• ->'A-H V — ^ t " I (mmols/g) - 4 10 15 20 28 —> time (h)meq/g T 0.12 - (mmols/g)
8 10
—> time (h)
Figure III. 2.1.8. Polymerization of caprolactam at 240° C; w„ = 0.20 mmols/g, c„ = 0.024 meq/g.
meq/g
T
0.12 0.10 0.08 0.06 0.04 0.02}
N-1
/ \ \ • ____. • / /" 14
• }j
• • 1 (mmols/g) Ai 10 time (h)It was necessary in this case to use a mathematical model into which the temperature changes (a) during heating, (b) as a result of evolution of heat of polymerization, and (c) during cooling, had been incorporated, especially in view of the rapid conversions in the experiments at 280° C.
The first estimate of the values of 'k^^ i and "k^^ i (the rate constants for catalyzed and uncatalyzed hydrolysis) at 240 and 280° C was based on the values for these constants at 221.5° C and extrapolation, using the same temperature dependence for ring opening of caprolactam by hydrolysis as for ring opening of caprolactam by carboxyl-catalyzed polyaddition. The values found in this way were too low to yield an acceptable fitting of the curves.
For both temperatures a much better agreement between calculated and experimen-tal data was obtained with values of "^k^, i and "k^, i which were five times as high as those given in Table I I I . 2 . 1 . 1 . The ratio of the rate constants for the catalyzed reac-tion to those for the uncatalyzed reacreac-tion was equal to the ratio of these values at 221.5° C (Table III .2.1.1.) In fact, values of this ratio ranging form 0 to infinity do not give significant differences in the results of calculations, provided that the values themselves have been chosen appropriately. This phenomenon will further be referred to as mutual adaptability of values of constants. The value of the exponent n in Eq III. 18 appeared to be different, viz: 0.05 at 240° C and 0.30 at 280° C.
It may be a result of the influence of phase transition (as discussed earlier in this section) that the values of "k^^x, "k^i and n at 220° C cannot be obtained by means of extrapolation of the values of these constants at 240° C and 280° C. More definite relations between the value of n and process conditions such as water content and temperature have not yet been developed.
The results of the calculations are compared with experimental data in Figs. I l l . 2 . 1 . 8 and 9.
After introduction of the temperature dependence by means of Arrhenius equations the following set of constants is suitable for describing caprolactam polymerizations at low water content, as applied commercially, and at temperatures ^ 230° C
"kh.l % . l Kh.i "k,! % . i Ka.l "k, = = exp = exp = (K,. = 0 = exp = exp = exp (-.(-f7.49 (+19.76 I / Kc) = (+23.38 (3.45M 9 . 7 0 -- 7070 /T) - 7070 /T) exp (-4.55 - 7070 /T) 2000 /T) 10300 IT) + 1870 IT) 'kc = exp (+21.63 - 6200 /T) Kc = e x p ( + 1.10- 3870/T)
T A B L E I I I . 2 . 1 . 3
VALUES OF RATE AND EQUILIBRIUM CONSTANTS IN THE POLYMERIZATION OF CAPROLACTAM
Constants Ring opening (hydrolysis) "k^ , (g sec-i mol-i) •^k^ , (g» sec-i mol-»)
Kh,i (mol g-i) Polyaddition " k , j ( g sec-imol-i) ^ k / i (g» sec-i mol-») K ; ^ (mol g-i) Polycondensation "k^(g sec-i mol-i)** 'k^ (g» sec-i mol-»)** K c ( - )
Rate constants for c = 2.10" k^_, (g sec-i mol-i) kj'j (g sec-i mo|-i) k^' (g sec-i mol-i)** 220° C 0.2- 10-»* 0.5 - 10+2* 4.7 • 10-1 0 0.8 - 10+* 5.5 • I0+* 0.3 0.9 - I0+* 1.2- I0-» *eq/g 0.5 • 10-1 1.7 2.0 230° C 1.4- I0-» 3.0 - 10+2 4.4 • 10-1 0 1.1 • 10+» 6.0 • 1 0 - ' 0.5 1.1 • 10+« 1.4- I0-» 0.6 • 10-1 2.2 2.7 240° C 1.9- I0-» 4.0 - I0+» 4.1 - 10-1 0 1.5- I0+* 6.4 • 10-* 0.7 1.4- 10+* 1.6- I0-» 0.8 - 10-1 2.9 3.5 Values at 250° C 2.4 - 10-» 5.1 - 10+2 3.8 - 10-1 0 1.9 • 10+* 6.9 • 10-* 1.0 1.8- 10+* 1.8 • 10-» 1.0- 10-1 3.8 4.5 260° C 3.1 - I0-» 6.6 • 10+2 3.5 - 10-1 0 2.5 • 10+* 7.5 • 10-* 1.5 2.2 - 10+* 2.1 • 10-» 1.4- 10-1 4.9 5.9 270° C 4.0 - I0-» 8.5 • 10+2 3.3 • 10-1 0 3.2- 10+* 8.0 - 10-* 2.1 2.7 - 10+* 2.4 - 10-» 1.7 • 10-1 6.3 7.5 280° C 5.0 -10-» 10.7 - 10+» 3.1 - 10-1 0 4.0 - I0+* 8.5 - 10-* 2.9 3.4 - 10+* 2.7- 10-» 2.2 • 10-1 8.0 9.6 * The values of Table III. 2 . 1 . 1 have been used for this temperature, at which the formulae are not applicable. ** For monomer content 0 ; for other monomer contents a correction according to Eq. I I I . 8 has to be applied.
III.2.2. Equilibrium and rate constants in the polymerization of entholactam The derivation of constants in the polymerization of enantholactam was based on experimental data published earlier*. In these experiments enantholactam was poly-merized at 230° C in the presence of 4% by weight of water. The end-group content and monomer content were determined for reaction times up to 4 hours. In a great number of calculations (abt. 200) it was found that an acceptable curve fitting could not be obtained with the conventional model. As described in section 111.2.1 for caprolactam, introduction of the influence of the composition of the reaction mixture by:
kc* = kc(l-(x/x„)'"), k c ^ O , n > 0 (111.18)
was sufficiënt to obtain a convenient fitting of the curves. It was found that a number of combinations of the values of the exponent n and the rate constant kc yield "ap-propriate" values for the end-group content in the top region of the curve. In Fig. Ill. 2.2.1 this is illustrated for the following values of the rate constant for condensa-tion: k* (g sec-i mo|-i) A B C 0.05 0.5 3.55 80.5 9.17 0 . 5 3 + c - 1 . 0 5 - 1 0 * (5)
In this figure the monomer curve is only given for the best model, viz. case B.
T
c (meq/g) Q2S 0.20 0.15 0.10 0.05 -2Str.
(% by weight) 4 —» time (h)Figure III. 2 . 2 . 1 . Polymerization of enantholactam, an interpretation of earlier work».
The values of the other constants are collected in Table III. 2.2.1.
T A B L E I I I . 2 . 2 . 1
VALUES OF CONSTANTS IN THE POLYMERIZATION OF ENANTHOLACTAM AT 230° C Constants Ring opening (Hydrolysis) X j ( g s e c - i m o l - i ) <^kh , (g2 sec-i mol-2) Kh_. (mol g-i) Polyaddition " k 3 i ( g sec-imol-i) ''k^ J (g2 sec-i mol-2 K,,, (mol g-i) Values at 230° C 5.6 • 10-* 5.3 • 10+1 8.9 • 10-2 1.07 2.0 • 10+» 1.6 • 10-* Constants Polycondensation % ( g s e c - i m o . - i ) ' k , (g2 sec-i mol-2) K c ( - )
Rate constants for c = 2.10-* eq/g k^ J (g sec-i mol-i) kg , (g sec-i mol-i) kc (g sec-i mol-i) Values at 230° C * * 1.8 • 10-» 1.1 - 10-2 1.5 *
* see Figs. I I I . 2 . 2 . 1 and 2.
NOTE The values given in ^^' were determined in postcondensation experiments in which the conden-sation of end groups of a dried polymer was studied. Such a situation is comparable with the reaction occurring during the melt spinning of polymer.
The values of n and k^ found for the polymerization cannot be applied to the postcondensation of monomer-free polymer as occurs during melt spinning, because application of Eq. 111.18 results in unrealistically high values for k* in cases A and B for monomer-free polymer compared with the
values for this situation according, to C, obtained from condensation of monomer-free polymer. This is illustrated in Fig. III. 2 . 2 . 2 , where the values of k^,* are given as a function of the monomer content. For this figure the carboxyl content was fixed at 2 • I0-* to obtain values of k^* according t o C . kc* t I ' 85 ^ 10 e 6 2 0.2 0.4 0.6 0.8 1.0 —* x/x„
Figure III. 2 . 2 . 2 . Values of the rate constant for condensation in the polymerization of enantholactam according to k^* = k^ (1 —(x/x^)").
A : n = 0.05 B : n = 0.5 C : n = 3.55
III. 2.3. Equilibrium and rate constants in the polymerization of dodecanolactam As the commercial production of nylon 12 was announced only recently, it is not surprising that only little information on the kinetics of the polymerization has become available until now.
The equilibrium monomer content is of the order of 2% by weight'"', which indi-cates a value of about 0.1 • 10~^ mols/g for K^i, the equilibrium constant for poly-addition. Moreover, results of polymerizations of dodecanolactam in the presence of phosphoric acid have been described^. In these studies conversions of nearly 100% were observed. Depending on the amount of phosphoric acid added (in the absence of water), it took 24-48 hours before "complete" conversion was reached at 260-280° C. Activity of catalysts
In a patent concerning the hydrolytic polymerization of dodecanolactami» it is stated that at moderate temperatures (abt. 280° C) short polymerization times (abt. 6h) can only be realized if fatty acids with chains of more than 4 carbon atoms have been added as catalysts. This is in contradiction with results obtained during this study. The activity of some catalysts was tested in experiments where dodecano-lactam was polymerized in the presence of 10% water w/w. on monomer and 0.5 meq acid per 100 g monomer at a temperature of 260° C. The results of these experiments are summarized in Table I I I . 2 . 3 . 1 .
T A B L E I I I . 2 . 3 . 1
ACTIVITY OF CATALYSTS IN THE POLYMERIZATION OF DODECANOLACTAM. ( T = 260° C, WO = 5.06 mmols/g)
Catalyst (4.55 • 10-" meq/g) % Conversion into polymer after 4 hours 8 hours
Benzene sulfonic acid 61 95.9 1,2-ethane dusulfonic acid 70 95.6 Benzoic acid 72 96.2 Terephtalic acid 75 96.3 Sebacic acid 72 96.2 Caproic acid 70 95.4 Acetic acid 65 95.9 Phosphoric acid (9.3 • 10-» mmols/g) 77 96.0 Model calculation (to be explained later) 84 96.5 It may be concluded from this table that there are only small differences in activity between the catalysts.
Probably the requirement of "long-chain" fatty acids represents the requirement of non-volatile acids under the conditions applied in the patent mentioned before.
Kinetic experiments and their interpretation Experimental data
To determine the values of equilibrium and rate constants, experiments were made in which dodecanolactam was polymerized under the conditons stated in Table 111. 2.3.2 for 4, 8 and 16 hours.
T A B L E I I I . 2 . 3 . 2
POLYMERIZATION CONDITIONS IN THE KINETIC EXPERIMENTS Temperature (° C)
Exp. series Initial water content (mmols/g) Initial carboxyl content (meq/g)
1.1 1.612 0.0287 240 and 280 1.2 2.1 3.133 1.607 0.0279 0.0574 2.2 3.130 0.0557
The polymers were characterized by their monomer content and their carboxyl end-group content, determined in accordance with the procedures given in the appendix.
The results are collected in Tables III. 2.3.3 and III. 2.3.4.
T A B L E I I I . 2 . 3 . 3
POLYMERIZATION OF DODECANOLACTAM AT 240° C; MONOMER CONTENT AND CARBOXYL END-GROUP CONTENT Series Pol. time (h) 4 8 16 1.1 Monomer -mmols/g 4.17 3.30 1.70 -COOH meq/g .127 .135 .136 1.2 Monomer -mmols/g 3.45 1.93 .73 'COOH meq/g .173 .178 .178 2.1 Monomer -mmols/g 3.96 2.78 1.42 -COOH meq/g .148 .156 .153 2.2 Monomer -mmols/g 3.34 1.84 .70 -COOH meq/g .184 .193 .193
T A B L E I I I . 2 . 3 . 4
POLYMERIZATION OF DODECANOLACTAM AT 280° C; MONOMER CONTENT AND CARBOXYL END-GROUP CONTENT Series Pol. time (h) 4 8 16 1.1 Monomer -mmols/g .116 .169 .144 -COOH meq/g .173 .167 .165 1.2 Monomer -mmols/g .288 .147 .150 -COOH meq/g .223 .212 .211 2.1 Monomer -mmols/g .665 .150 .130 'COOH meq/g .187 .189 .182 2.2 Monomer -COOH mmols /g meq/g .261 .237 .158 .236 .159 .234 Interpretation
The curve fitting was performed separately for the experiments at 240 and at 280° C. The procedure is given in the experimental part, section A. 4.
The sets of constants presented in Table III. 2.3.5 were found useful.
T A B L E I I I . 2 . 3 . 5
V A H ; E S OF RATE AND EQUILIBRIUM CONSTANTS IN THE POLYMERIZATION OF DODECANOLACTAM
Constants Values at 240° C 280° C Ring opening (Hydrolysis) "k^ , (g sec-i mol-i) 1.3 - 10"» 4.2 • 10-» ' k ^ ' , (g» sec-i mol-2) 3 5 . I Q - I 6.9 - 10 " Kh_, (mol g-i) 4.0 - 10-2 4 5 . 10-2 Polyaddition
"ka_i (g sec-i mol-i) I.l • 10-» 2.5 • lO-i ' k / j (g2 sec-i mol-2) 1.4 • 10+» 4.9 • 10+» K^_, (mol g-i) 8.0 • 1 0 - ' 1.4 - 10-* Polycondensation
% (g sec-i mol-i) I.I - 10-1 I.4-IO-1 % (g» sec-i mol-2) 3.1 - I0+» 3.5 • I0+»
K c ( - ) 2.0 10-» 3.1 - I0-» Rate constants for c
= 2.10-* eq/g
k^ , (g sec-i mol-i) 1.4 • 10-» 5.6 • 10-» k^', (g sec-i mol-i) 2.9 • lO-i 12.3 - lO-i k / (g sec-i mol-i) 7.3 - lO-i 8.4- lO-i
In view of the mutual adaptability of the values of the rate constants for COOH-catalyzed and unCOOH-catalyzed reactions, the values of the rate constants have been com-pared at a carboxyl end-group concentration of 2.10"* eq/g, around which the experi-mental data are grouped. Application of formula 111.18, which accounts for the in-fluence of the monomer content on the rate constant of condensation, appears to be unnecessary for this monomer.
This may be due to the fact that in the polymerization of dodecanolactam the struc-ture of the amide group remains unchanged, as was discussed in section 11.2.1.
In Figs. 111.2.3.1 through 4 the calculated concentrations of monomer and carbo-xyl end groups (drawn lines) are compared with experimental data (dots).
c I
(meq/g) 0.20 0.15 0.10 0.05 ^-KvL
/ \^'^^.* / / \ \7/ \
/ \ 1 ' b a \ . a \ . b -C * -^ . X <t "
(mmols/g) - 4 4 8 12 16 —> time (h)Figure III. 2.3.1. Polymerization of dodecanolactam at 240° C. a — Wo = 1.612 mmols/g (3% w/w on monomer) series I-l. b — w„ = 3.133 mmols/g (6% w/w on monomer) series 1-2.
c I
(meq/g) 0.20 0.15 0.10 OAS N / \ / » / \ " \.—/A
-// ^ ' • \ \ \ 1 b a \ a \ b C • X 1-t ""
(mmols/g) - 2 8 12 16 —> time(h)Figure III.2.3.2. Polymerization of dodecanolactam at 240° C. a — w„ = 1.607 mmols/g (3% w/w on monomer) series 2-1. b — w„ = 3.130 mmols/g (6% w/w on monomer) series 2-2.
(meq/g) 0.20 0.15 0.10 0.06
"fV^
•-—^i^-T T , " • ^'' 1T "
(mmols/g) 4 8 12 16 —* time (h)Figure III.2.3.3. Polymerization of dodecanolactam at 280° C. a — WQ = 1.612 mmols/g (3% w/w on monomer) series 1-1. b — w = 3.133 mmols/g (6% w/w on monomer) series 1-2.
(meq/g)
t "
(mmolc/g)
4 8 12 16
—> time(h)
Figure III. 2.3.4. Polymerization of dodecanolactam at 280° C. a — Wg = 1.607 mmols/g (3 % w/w on monomer) series 2-1. b — Wo = 3.130 mmols/g (6% w/w on monomer) series 2-2.
