Mechanism of the interaction in a vinylidene fluoride copolymer+aminoalkoxysilane+metal system

PolymerScience Vo|. 33, No. 2, pp. 230-237, 1991

0032-3950/91 $15.00 + .00 ~ 1992 Pergamon Press pie

Printed in Great Britain.

M E C H A N I S M OF THE INTERACTION IN A VINYLIDENE FLUORIDE C O P O L Y M E R + AMINOALKOXYSILANE + METAL SYSTEM* YE. V. K U Z N E T S O V A , A. G. LiPsON, D. M. SAKOV and N. I. MOROZOVA Institute of Physical Chemistry, U.S.S.R. Academy of Sciences

(Received 12 February, 1990)

The mechanism of interaction in a vinylidene fluoride copolymer + aminoalkoxysilane + metal system has been studied by IR spectroscopy and thermostimulated depolarization. It is shown that the role of aminoethoxysilanc is to carry out intermolecular reactions, on one hand, with the metal with formation of delocalized bonds and, on the other, with the polymer matrix leading to the formation of a spatially crosslinked structure.

IN THE last few years the number of publications dealing with fluoropolymers has steadily grown.

Such interest in the problem of studying and applying fluoropolymers is dictated by a number of unique properties peculiar to this class of polymers [1]. However, despite the complex of valuable qualities, their use as adhesive coatings on metals turns out to be very limited. The reason is that the chemical passivity of fluorocopolymers necessary in terms of the protective properties of the coatings based on them leads to adsorption inactivity of the polymer which, in turn, adversely affects the strength of the metal-polymer adhesive contact. Our previous investigations had shown that chemical modification of the metal surface with compounds of the aminoalkoxysilane class sharply raises the adhesive strength (by 2-3 orders) in the system vinylidene fluoride copolymers + metal, an important feature being that in this series the most active in raising the strength of adhesive contact and the protective properties of the coatings are compounds with a high content of amine groups in the molecule [2, 3]. Detailed study of the mechanism of interaction of vinylidene fluoride copolymers with aminoethoxysilane and the latter with metal forms the subject of the present work. We investigated the adhesive contact of the copolymer of tetrafluoroethylene with vinylidene fluoride (FP) with a steel support (St. 3) using a non-fractionated sample with M = 1.5 x 10s and the monomer ratio 3 : 7 respectively. To ensure dosed adhesion to the copolymer solution was added an oligomeric modifying additive (the polycondensation product of ),-amino-propyltriethoxysilane) containing eight amine and six ethoxyl groups (ASOT). As organic solvents we used acetone and butyl acetate in the ratio 1 : 1 [3]. The adhesive strength of the coatings was determined by the method of normal detachment [4], the chemical interactions between the components by IR spectroscopy using the Perkin-Elmer spectrophotometer in the frequency interval 400-4000 cm -~, the samples were prepared as thin films on KBr prisms. The films of the modifier were formed from solutions in cyclohexanone. The electrical parameters of contact were determined by thermostimulated depolarization (TSD) *Vysokomol. soyed. A33" No. 2, 304-310, 1991.

230

Vinylidene fluoride copolymer + aminoalkoxysilane + metal system

231

[5]. In the experiments with TSD we used samples of steel with a diameter 20 and thickness 2 mm on which the coatings were formed from a 15% FP solution in the mixture acetone: butyl acetate -- 1 : 1 with and without modifier. The content of the modifier introduced into the polymer solution varied from 0.5 to 2.5% per dry polymer residue. The thickness of the coatings was held at 50 + 1/~m, and they were subjected to vacuum drying for 15 days at 25°C. Octadecylamine (ODA) was adsorbed from solutions in toluene, ASOT in propyl alcohol. The presence of a spatial crosslink in the composition was judged from the value of equilibrium swelling of the free films. The initial act of the process of chemical modification of the substrate surface is adsorption of the modifier. From the formula for ASOT it follows that its molecule contains two types of adsorption-active functional groups--ethoxyl and amine and, consequently, the interaction in the metal + modifier subsystem may be due to the participation either of the ethoxy [6, 7] or the amine groups, which is linked with the adsorption-inhibiting action of the modifying additives [8, 9]. To solve the problem of the role played by these groups in surface-chemical modification we ran comparative experiments with adsorption of ASOT and the reference compound O D A on powdered iron. Figure 1 presents the adsorption isotherms of these compounds. The limiting adsorption of ASOT is 26 and of O D A 17 mg/g. After washing with active solvent in a Soxhlet apparatus the adsorption level in both cases changed insignificantly, amounting to 24 for ASOT and 14 mg/g for O D A indicating the predominantly chemosorption nature of the interactions in both cases. The chemosorption activity of O D A having in its molecule only amine functional groups suggests that the interaction of ASOT with metal is also realized, in the main, through the amine groups. These findings are confirmed by the results of reference [10] according to which the ASOT molecule belongs to the syndiotactic variant of isomerism, i.e. the silanol groups are screened by longer ,~, mg/g

26

1H

o

9

o

o

2 j._

10,

I

I

I

0,1

0.5

0.9

I

O, Q/100 rnl

FIG. 1. Adsorption isotherms of ASOT (1) and ODA (2) on powdered iron. Broken line indicates the level of chemosorption. PS 35:2-E

232

YE. V. KUZNETSOVAet al.

radicals with an amine group and, therefore, the interaction via the silanol (residual ethoxy groups) groups is sterically hindered and the relative contribution to the surface interactions of the silanol groups is far smaller. Confirmation that the linkage of the modifier with metal is realized through the ASOT amine groups was obtained from analysis of the IR spectra of the initial modifier and a polydisperse iron powder treated with modifier. The spectrum of the film of the starting oligomer was characterized by the presence of absorption bands in the region of the deformation vibrations 8 of the NH-group (1659 cm -1) and the valent vibrations v of the NH-group (3000-3200 cm -1). The spectrum of the iron powder treated with ASOT also contains the characteristic bands of the modifier indicated by the presence in the spectrum of the adsorbent of bands characteristic of the oligomer. However, the intensity of ~ of NH (1659 cm -1) falls considerably relative to that of the band ~ - O (1715 cm -~) and the absorption band of NH also shifts to the region of lower frequencies suggesting the advent of linkage of the amine groups with the metal. From the strong donor activity of the amine groups one may postulate the advent of a double electric (DEL) on formation of contact, i.e. the realization of the electrical concept of adhesion [11]. To determine the parameters of the electrostatic component of adhesion we employed the TSD method based on thermal decompensation of the DEL charges. Reference [5] demonstrated the possibility of using this method to determine the quantitative characteristics of the energy parameters of metal + polymer adhesive contact. This approach was also used in the present work, in particular, for evaluating the bonding energy of the donor-acceptor pair on adhesive contact from the activational energy of the TSD process. The TSD spectrum of the metal + polymer adhesive system (Fig. 2, curve 1) has a peak with a temperature of the maximum at 458 K, the TSD current having negative polarity indicating negative charging of the polymer on decompensation of DEL [11], i.e. on the acceptor properties of the

Ix 108 10-

6

2 300 ~

7OO7;,t(

-Z

-6

FIo. 2. TSD spectra of metal+polymer adhesive contact (I), the free film of the polymer (2) and metal+ ASOTmodifiedadhesivecontact(3).

Vinylidene fluoride copolymer + aminoalkoxysilane + metal system

233

polymer through peroxide group impurities [11]. The peak at T = 310 K (Fig. 2, curve 2) shows up in all three spectra, according to the published data [5, 12] it is linked with the ferroelectric transition in the fluoropolymer and will not hereafter be considered. The situation changes sharply on introducing a modifier (Fig. 2, curve 3). The TSD current in this case assumes positive polarity and its intensity rises by an order as compared with the polymer + metal system. An intense peak appears with the temperature of the maximum 520 K. The positive polarity of the TSD current indicates the donor chracter of the polymer in relation to steel which is probably due to the high concentration of the NH2 groups in the modifier (-1020 cm-3). The TSD curves were treated and from the initial portion of the curves (rise in current) we determined the activational energy of the process for the metal + polymer and the metal + modified polymer adhesive contacts for different concentrations of the modifier introduced [5]. In similar contacts we determined the adhesive strength by the normal detachment method. The values of the activation energy and the adhesive strength of the contacts of the modified and non-modified polymer with metal are given in Table 1. Rise in the activation energy of the TSD process with increase in the concentration of modifier introduced into the polymer solution from 0.75 to 3.1 eV which may be regarded as the bonding energy of the donor-acceptor pair on the adhesive contact 8 indicates gradual transition from physical adsorption to chemical binding with increase in adhesion since as Table 1 shows rise in the concentration of modifier led simultaneously to rise in the adhesive strength. We would note that increase in the activation energy (bonding energy of the donor-acceptor pair) with rise in the concentration of modifier is obviously connected with the presence of closer contact between metal and polymer with increase in the concentration of NH2 groups. This process must promote convergence of the donor and acceptor atoms and greater asymmetry of electron density and, thus, enhancement of the interaction between them. Using the observed correlation function between the activation energy of the TSD process and the value of adhesive strength on the metal + polymer contact and also determining the size of the charge of DEL Q one may calculate the contribution of the electrostatic component of adhesion to the adhesive strength [5, 13]. These magnitudes are also indicated in Table 1. It is not hard to see that in the case of weak adhesive strength (4 MPa) the contribution of the electrostatic component (0.04 MPa) does not exceed 1% of the total value of adhesive strength. The situation radically changes on introducing donor NH2 groups into the polymer and for the maximum concentration of modifier (2.5%) introduced we obtained the contribution of the electrostatic component Fel = 21 MPa, while the adhesive strength of contact F = 30 MPa. Thus, the contribution of the electric forces in this case already amounts to 70%, i.e. is decisive. From this it is clear that since in this case the contribution of the electrostatic forces prevails the values of the bonding energy of the TABLE 1. P A R A M E T E R S OF METAL "F MODIFIED POLYMER CONTACT EVALUATED BY T H E METHOD OF NORMAL DETACHMENT AND CALCULATED FROM THE

TSD

SPECTRA FOR DIFFERENT CONCENTRATIONS OF MODIFIER

N*,,.a x 10-18,

Content of modifier, %

3, eV

Q x 106, Cl

cm-3

0 0.5 1.0 2.0 2.5

0.75 1.30 1.90 2.60 3.10

2.2 4.8 8.1 20.0 50.0

3.0 7.0 20.0 80.0 300.0

*Number of donor (acceptor) centres in polymer.

Adhesion, kg/cm2 Electrostatic Complete 0.4 20.0 60.0 140.0 210.0

40 120 185 260 300

234

YE. V. KUZNETSOVAet aL

donor-acceptor pairs obtained from the TSD experiment must be closest to their real values. As well as the electrostatic forces a contribution to the value of adhesive strength will also be made by the factor associated with the molecular component and with the spread of the adhesive into the microdefects of the substrate. Treatment of the TSD spectra established that the advent of a strong adhesive link involves donor groups which in the metal + modified polymer system can only be the amine groups since in their donor activity they far exceed the O H groups of silanol [11] clearly indicated by the bonding energy of the donor-acceptor pair on adhesive contact which for a content of 2.5% modifier in the polymer matrix amounts to 3.1 eV far exceeding the energy of the hydrogen bond (~1 eV). From the fact that the metal+ modified polymer adhesive contact is realized through the advent of the bond of the NH2 groups of the modifier with the metal and the fact that the bonding energy varies from 1.5 to 3.1 eV, with increase in the concentration of modifier from 0.5 to 2.5% one may postulate the formation of a delocalized semipolar Mt--NH2 bond. Such compounds may be complexes with charge transfer of the Fe--NH2 type at the metal + modified polymer interphasic boundary. In addition, work on the chemosorption of amines on iron and steel has established that the bonding energy of the NH2 groups with metal is 1.73--2.60 eV [8, 9] which is in good agreement with the bonding energy of the donor-acceptor pair obtained by us. Thus, the assumption that the formation of adhesive contact occurs with the participation of the amine groups of the modifier advanced on the basis of the experimental data on adsorption and IR spectroscopy presented above finds quantitative confirmation in experiments on the TSD of adhesive contact. It is known that the realization of strong adhesive contact in the polymer + modifier + metal system requires not only the metal + modifier interaction already considered but also the i n t e r a c t i o n of modifier with the polymer matrix. Study of the kinetics of swelling (Fig. 3) of modified (with a different percentage content of ASOT) and unmodified films in acetone-active solvent vapour showed that with increase in the content of the modifier in the film its degree of swelling diminishes while the unmodified film completely dissolves within 26 h. Such a character of swelling of the films suggests that the introduction of modifier into the polymer solution will lead to the appearance of a spatial network

o:%

o°i j qO 2

0 /

2

IO

gO

150 r, h

Fio. 3. Kinetics of swelling in acetone vapours of a film of the polymer (1) and a film of polymer modified

with ASOT (2).

Vinylidene fluoride copolymer + aminoalkoxysilane + metal system

235

during transition from the liquid to the solid state, the density of the network rising with the content of modifier in the system. To elucidate the mechanism of interaction in the vinylidene fluoride copolymeraminoethoxysilane system the samples of the films were investigated by IR spectroscopy. The spectra of the starting products (copolymer of tetrafluoroethylene with vinylidene fluoride, ASOT) and the products of their interaction differ significantly. In the region 1500-1750 cm -1 unlike the spectrum of ASOT in a film of modified polymer shift in the band 1660 to 1630 cm -1 is observed with the appearance of an additional maximum in the region 1520 cm -1. These maxima are due to the valency and deformation vibrations of NH in the C--NHa+F ion. Apparently the interaction of modifier with polymer involves the NH2 groups of ASOT. This is indicated by the changes in the region 2800-3600 cm -1 where there is complex absorption with several maxima due to the NH3 + salts. With the deformation vibrations of these groups is also associated absorption in the region 1600 cm -1 and 1320 cm -1 and also 730 cm -~ (pendulum vibrations of NH3 ÷ ). In the region 1000-1200 cm -1 fall in the intensity of the bands 1185 and 1168 cm -1 is observed as compared with the unmodified polymer due to the vibrations of the CF2-groups. The above data allow one to make an assumption on the mechanism of interaction of the ASOT modifier with vinylidene fluoride copolymers. In presence of primary polyamine the hydrogen fluoride splits off from the copolymer molecule by the reaction OC2H5

OC2H5

R--(--CH,-- CF~--)n--R~- -- (--~i.- O--)--

, R--(--CH=CF--)n--R ~ (--~i--O)

(CHs),

(cn2)a

NH2 [

~H~.HF

with the formation of amine salts which show up in the spectrum. Fall in the intensity of the absorption bands corresponding to the vibrations of the CF2 groups indicates that they are consumed in the course of the reaction. At the second stage of the reaction via the polar (as a result of the presence of the fluorine atom) bond there is attachment of the amino groups of the modifier.

R l F--C

l , NH~--(CH2)a--Si--R [

li +

0

H--C

,

t

I

R i C--F + II C--H

R --Si--(CH~)a--NH~

]

o l

R

O I

F-- ~--NH--(CH~)a--Si--R' I

L

CH~

0

R

I

I

I

R

R'--Si--(CH2)a-- NH--C--F, I () I

[ CH~ I

R

236

YE. V. KUZNETSOVAet al.

where R' =---OC2H5 (OH in the case of hydrolysis); R = (---CF2----CFz---), (---CF2----CCIF--), (---CF2---CFz--CFz---), etc. Since the reaction involves vinylidene fluoride units this mechanism of interaction may also be realized for its other copolymers and together with these reactions on the metal surface crosslinks will form between the molecules of aminoethoxysilane through the reaction of the amine groups with the silanol or residual ethoxy groups [13]. OR

OR

--O--Si--O-(~H,)s

--O--Si--O--, {~H,),

_O_SIi_O_ l where R = H, C2H5. The occurrence of such a process is indicated by fall in the refractive index of ASOT on the surface (n = 1.26) as compared with the refractive index in volume (n = 1.46) [10]. Thus, one may postulate the formation of a rarefied spatial network on the surface of the adsorbent through the reaction of polycondensation with the participation of the ethoxy groups (silanol) of the modifier. If the modifier layer on the surface is a high molecular mass partially crosslinked polysiloxane then it must be resistant to desorption in view of its low solubility. In addition, its numerous polymer segments may in toto provide a strong bond with the metal surface which apparently also occurs in the case of chemosorption carried out on steel powder. Thus, from the results it may be concluded that the role of the modifier evidently adds up, on the one hand, to the formation on the metal surface of a delocalized bond of the Mt--NH2 type and, on the other, to the fact that it raises the activity of the surface in relation to a quite inert fluorocopolymer since it contains primary amine groups via which the chemical interaction with the polymer may take place. The ethoxy groups (silanol) of ASOT are responsible for the formation of the network on the metal surface which also leads to increase in adhesion in the metal-polymer system. Translated by A. CROZY

REFERENCES 1. Ftorpolimery (Fluoropolymers) (Translated from the English) (Edited by I. L. Knunyants). 448 pp., Moscow, 1975. 2. N. I. MOROZOVA, Ye. V. ZONTOVA and I. I. SHIGORINA, Lakokrasoch. material, i ikh primeneniye No. 2, 25, 1986. 3. Ye. V. ZONTOVA, N. I. MOROZOVA and P. I. ZUBOV, Ibid. No. 3, 31, 1985. 4. A. D. ZIMON, Adgeziya plenok i pokrytii (Adhesion of Films and Coatings). 273 pp., Moscow, 1977. 5. A. G. LIPSON, D. M. SAKOV and Yu. P. TOPOROV, Pis'ma v ZhTF 15: No. 21, 1989. 6. E. PLUDEMANN, Kompozitsionnye materialy (Composite Materials). Vol. 6, p. 181, Moscow, 1978. 7. E. P. PLUDEMANN, Progress in Organic Coatings 11: 205, 1983. 8. E. Kh. YENIKEYEV, I. L. ROZENFEL'D, A. A. GONIK and K. V. ZVEZDINSKH, Zashchita metallov 11: 706, 1975. 9. K. ARAMAKI and N. J. HACKlgRMAN,Elektrochem. Soc. 116: 205, 1969. 10. Ye. V. ZONTOVA, V. A. KOTIgNEV, N. I. MOROZOVA and V. A. OGAREV, Kolloid. zh., No. 2,342, 1987.

Isothermal crystallization of isotactic polypropylene

237

11. B. V. DERYAGIN, N. A. KROTOVA and V. P. SMILGA, Adgeziye tverdykh tel (Adhesion of Solids). P. 296, Moscow, 1973. 12. O. IOSHIO, K. NACKAZU and I. MURATA, J. Polymer Sci. Polymer Phys. Ed. 24: 2059, 1986. 13. E. PLUDEMANN, Itog. nauk. i tekhn. Khimiya i tekhnologiya VMS (Chemistry and Technology of High Molecular Weight Compounds). Vol. 19, Moscow, 1984.

PolymerScienceVol.33, No. 2, pp. 237-241,1991

0032-3950/91$15.00+ .00 ~) 1992PergamonPressplc

Printedin GreatBritain

STUDY OF THE THERMODYNAMICS OF THE MELTING AND KINETICS OF ISOTHERMAL CRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE AT RAISED PRESSURES* V. M. BARANOVSKII, A . M. TARARA, A . A . KHOMIK, V. YA. BULGAKOV a n d V. N. KESTEL'MAN Gorkii State PedagogicalInstitute, Kiev (Received 21 February 1990)

Volumetric dilatometry under pressure has been employed to investigate the dependence of the temperature and enthalpy of melting, the crystallizationrate and the energy parameters of nucleation of polypropyleneon hydrostatic pressure on crystallizationfrom the melt. An abrupt fall was found in the free surface energy of the end phases of the crystallization seeds with fall in pressure from 30 to 20 MPa considerablyincreasing the rate of crystallizationof the polymer from the melt. The phenomenon observed is due to passage of the macromoleculesfrom a folded to a more straightened conformation in the crystal (topomorphism). POLYPROPYLENE falls into the category of polymers gaining wide acceptance in the national economy. Although industrial product processing of PP is essentially effected by pressing or injection moulding, the literature provides no scientific validations of the choice of the processing parameters. This applies particularly to the pressure of pressing Ppr prompting investigation of the thermodynamics of melting and the kinetics of isothermal crystallization of PP over the pressure range 10-40 MPa. We investigated commercial PP of grade 06P10/040. Cylindrical samples for the investigations (h = 8 x 10 -3 m, d = 6 x 10 -3 m) were pressed at 473 K and at a pressure 20 MPa from granulated unmodified PP. The investigations were carried out with the apparatus previously described [1] and improved by us. Measurement of the specific volume in the course of stepped heating and investigation of the kinetics of isothermal crystallization were based on volumetric dilatometry by the technique of reference [2]. The first results of the investigations are given in Fig. 1. Using a set of heating isobars we *Vysokomol. soyed. A33: No. 2, 311-315, 1991.