Synthesis and characterization of diisocyanate-cured organotin polymers

Progress

in Organic Coatings,

19 (1991)

245-255

245

Synthesis and characterization of diisocyanate-cured organotin polymers R. R. Joshi and S. K. Gupta* Centre for Environmental Science and Engineering, Powai, Bombay-400 (Received

October

Indian Institute of Technology,

076 (kdia) 16, 1990; in revised form December

11, 1990)

Abstract Organotin copolymers containing hydroxy functional groups have been crosslinked with four different diisocyanates in different molar proportions. The crosslinked polymers were characterized by IR and XRD spectroscopy, swelling properties and thermal studies. The biocidal effects of the cured copolymers have been studied against two bacterial species and a larva.

Introduction The controlled release of biological agents is a concept that has gained considerable attention in the past few decades. The basic concept of this technology is concerned with the controlled release of the biological agent, slowly and continuously, with precision at a predetermined site. This technique has gained considerable importance in various fields such as antifouling coatings for marine structures, in the pharmaceutical industry for the controlled release of drugs or medicines, and in the agriculture industry for the controlled release of fertilizers and pesticides. Tributyltin polymers find wide application in the shipping industry in antifouling coatings. However, the uncontrolled release of the biocide, Le. the tributyltin moiety, from these coatings can be a serious deficiency. A more recent development is the synthesis of polymeric coatings containing organotin compounds chemically bonded to the polymer chain as pendant groups. It is well known that polymers of tributyltin acrylate and tributyltin methacrylate do not possess satisfactory film-forming ability and that they show uncontrolled release of the biocide and poor adhesion to the metal surface. This results in wastage of the biocide and causes environmental pollution. In order to increase the effective life of these coatings, a number of copolymers and terpolymers of the above monomers have been evaluated by several workers [l-3]. Although many of these polymeric systems have shown promise of offering longer-lasting antifouling activity, better control *To whom all correspondence

0033-0655/91/$3.50

should be addressed.

0 1991 -

Elsevier

Sequoia, Lausanne

246

of the leaching rate is still required. One of the methods used to achieve the controlled release of the biocidal moiety from the tributyltin polymer matrix involves crosslinking of the polymeric system. The literature on the crosslinking of organotin polymers is modest. Subramanian et al. [4] have reported some crosslinking studies using copolymers of tributyltin methacrylate (TBTMA) with glycidyl (meth)acrylate. They used aliphatic amines and uranyl nitrate as crosslinkers and the strengthening agent. Recently, Dharia [5] has reported crosslinking studies using copolymers of TBTMA and hyclroxy a&y1 methacrylates with three different diisocyanates, viz. tolylene diisocyanate, trimethylhexamethylene diisocyanate and isophorone diisocyanate. He reported crosslinking at different mole ratios of OH to NC0 groups obtained by adding the requisite amount of the diisocyanate into the copolymer solution (20%) in a mixture of toluene and chloroform (3:l v/v) at room temperature. Swelling measurements of the crosslinked polymers in different solvents showed maximum swelling in chloroform and least in benzene. The swelling was also found to decrease with the increase in the amount of crosslinking agent in the polymer. The nature of crosslinking agent had little effect on the thermal stability of the crosslinked polymer. The biocidal effects of cured polymers were found to decrease upon crosslinking. The present investigation deals with the synthesis, characterization and application of diisocyanate-cured copolymers of tributyltin cY-chloroacrylate (TBTCA) (M,) with 2-hydroxy ethyl methacrylate (HEMA) (M2) and 2-hydroxy propyl methacrylate (HPMA) (M,). Aliphatic and aromatic diisocyanates were used as crosslinkers to crosslink the hydroxy functional groups in the above copolymers, resulting in urethane-type linkages. The cured copolymers were characterized by IR and XRD spectroscopy and by swelling measurements. The thermal stability of the cured copolymers was also studied. The biocidal effects of the cured polymers were assessed against two bacterial and one larval species.

Experimental Materials Tri-n-butyltin oxide (TBTO), tolylene diisocyanate (TDI) (80:20 mixture of 2,4- and 2,6-isomers), isophorone diisocyanate (IPDI), methylene bis(pphenylene diisocyanate) (PDI) (all F’luka A.G. make) and trimethyl hexamethylene diisocyanate (TMDI) (a 1:l mixture of 2,2,4- and 2,4,4-isomers) (Chemish, Germany) were used as received without further purification. Methyl acrylate, 2-hydroxy ethyl methacrylate (HEMA) and 2-hydroxy propyl methacrylate (HPMA) (all Fluka A.G.) were purified and dried according to standard procedures [6]. Azobis(isobutyronitrile) (AIBN) initiator was recrystallized from methanol and a solution prepared in tetrahydrofuran (THF’). The various solvents used during the synthesis were rigorously dried according to standard procedures [ 61.

247

Synthesis of TBTCA monomer The synthesis of TBTCA involves three steps, viz. (i) chlorination of methyl a&ate to methyl 2,3-dichloropropionate (MDP), (ii) dehydrohalogenation of MDP to cY-chloroacrylic acid (CAA) and (iii) esterification of CAA to TCA. The chlorination of methyl acrylate and dehydrohalogenation of MDP to obtain CAA was performed according to the procedure described by Marvel et al. [7]. Esterification of CAA was carried out according to the method described for the synthesis of TBTMA [8]. CAA and TBTO (2:l molar ratio) were mixed in dry benzene and the water formed during the reaction was removed by azeotropic distillation under reduced pressure. The crude monomer was dried and then recrystallized from dry hexane. Synthesis of copolymers The copolymerization reaction was carried out by charging the required amounts of TBTCA, 2-hydroxy alkyl methacrylate (HEMA or HPMA), AIBN (0.1% w/v) and dry THF into the polymerization tubes. The tubes were sealed following repeated degassing and freeze-thawing cycles and placed in a water bath whose temperature was adjusted at 55 f 0.1 “C. After the desired conversion ( < 10%) the reaction was stopped, the tubes opened and their contents poured into dry n-hexane and precipitated. The copolymers were further purified by repeated reprecipitation from THF into n-hexane so as to remove any unreacted comonomers and initiator species. The resulting copolymers were vacuum dried at 35 “C and characterized [ 91. Crosslinking reaction Fresh samples of HEMA and HPMA copolymers were synthesized, purified and characterized by tin analysis [lo], IR and PMR spectroscopy to ensure the absence of traces of monomer and of other impurities. The copolymers of HEMA and HPMA crosslinked with TDI are designated as HETDl and HETD4, HPTDi and HPTD4, where the subscripts 1 and 4 indicate increasing molar proportions of TBTCA (M,) and decreasing molar proportions of the hydroxy comonomer (M2) in the crosslinked polymer as listed in Table 1. A similar notation has been used to designate other cured polymers. Since the proportion of isocyanate crosslinker is stoichiometrically related to the hydroxyl content of the comonomer, appropriate molar proportions of the diisocyanates were added to each polymer solution. For example, the amount of crosslinker decreased in going from HETDi to HETD4 and a similar procedure was followed for the other systems (Table 1). The solutions were then thoroughly mixed in a homogenizer and poured on to a glass plate or cast on microscope slides. The solvent was allowed to evaporate at room temperature and the films stored in an oven for 30 min at 60 “C to ensure complete crosslinking. Characterization Infrared spectra of the cured and uncured polymers were recorded on a Perkin-Elmer model 621 IR spectrometer. The spectra of the cured polymers

248 TABLE

1

Composition Polymer code”

of cured organotin

copolymers

TBTCA

(TBT)b

HETDz.5 HEIPz HEIP, HETM, HETM4 HEPDz HEPDl

0.073 0.323 0.175 0.576 0.073 0.576 0.175 0.576

Polymer code”

Mole fraction of monomers in polymer

HETD,

TBTCA HPTD, HPTD, HPIP, HPIP, HPTM, HPTM* HPPD, HPPD,

HEMA

Mole of crosslinker/ mole of comonomer (M,)

0.927 0.677 0.825 0.424 0.927 0.424 0.825 0.424

0.58 0.45 0.70 0.46 0.54 0.35 0.53 0.35

HPMA

Mole of crosslinker/ mole of comonomer (M,)

0.921 0.661 0.921 0.442 0.921 0.442 0.815 0.442

0.46 0.35 0.70 0.40 0.54 0.31 0.53 0.30

Mole fraction of monomers in polymer

0.079 0.339 0.079 0.055 0.079 0.558 0.185 0.558

(0.05) (0.18) (0.13) (0.43) (0.05) (0.43) (0.43) (0.13)

(TBT)b (0.06) (0.29) (0.06) (0.46) (0.06) (0.46) (0.06) (0.46)

“Abbreviations: HETD, TDI-cured TCHE copolymer; HEIP, IPDI-cured TCHE copolymer; HETM, TMDI-cured TCHE copolymer; HEPD, PDI-cured TCHE copolymer; HF’TD, TDI-cured TCHP copolymer; HPIP, IPDI-cured TCHP copolymer; HPTM, TMDI-cured TCHP copolymer; HPPD, PDI-cured TCHP copolymer; TCHE, copolymer of TBTCA and HEMA; TCHP, copolymer of TBTCA and HPMA. VBT = mole fraction of TBT in polymer concerned.

were recorded using the KBr pellet method whereas those of the uncured polymers were recorded using f%lms cast from CHCl, solvent. Swelling measurements were performed at 26 “C after immersion for 24 h in five different dry organic solvents and in artificial sea water. The weights of the cured polymers after storage in the solvents were noted at intervals of 15, 30, 45, 60 and 75 s, respectively. The initial weight at zero time was determined by extrapolation and the percentage swelling calculated from this. Thermal analysis of the cured polymers was determined using a V2.2A DuPont 2000 thermal analyzer employing a nitrogen atmosphere and a heating rate of 10 “C min-‘.

249

XRD spectra of the cured polymers were recorded using a Phillips Xray diffractometer by coating the sample on a glass slide and using ‘Cu’ as the target. The biocidal effects of the cured copolymers were assessed against two species of bacteria, viz. Pseudomcmas aerugimsa and Sarcina lutea, via a modified culture plate method, and against Artemia saliruz nauplii II larvae by determining L& values. The procedure adopted has been described elsewhere [ 111.

Results

and discussion

The diisocyanate-cured copolymers were smooth, hard, brittle and glossy in nature, the crosslinking of the hydroxy functional group of the comonomers giving rise to a urethane linkage. By analogy with other urethane-containing polymers, these materials might be expected to show good abrasion resistance, good adhesion to metallic surfaces, good chemical resistance and a selfpolishing ability. As these types of coating often have a low energy surface tension [ 121, the adhering of the fouling organism should be very small. Thus the life of the coating should increase. Infrared spectra of the cured and uncured copolymers are depicted in Figs. 1 and 2, respectively. Examination of these spectra indicate recognizable changes both in the fingerprint region and at 3600 cm-‘; the broad peak due to the hydroxyl group in the uncured copolymers reduces to a small peak and is shifted to slightly lower wavenumbers. This confirms the reaction of the diisocyanate with the hydroxy functional group leading to crosslinking of the polymer. The fingerprint regions for the cured and uncured polymers also differ from each other indicating that some structural changes must take place upon crosslinking. The hydrophilic OH functional groups of the comonomerS are capable of absorbing large amounts of polar solvents into the polymer matrix [ 131. With the uncured organotin copolymers the rate of hydrolysis of the toxicant is very high, most of the toxicant being rapidly removed and hence being

Wavenumber Fig.

(cm’)

1. Infrared spectra of uncured copolymers of TCHE3 (-)

and TCHP2.6 (- -

-1, respectively.

250

2.5 .^^

0

3

I

3500

4

I

I

2500

5Microns

(f)

7

8

12 , 14,( 11

9’p

I

I

1800 Wavenum

ber

Fig. 2. Infrared spectra of cured copolymers

1400

1000

6 0

(cm’)

of HETD, (-)

and HF’TD, 5 (- - -), respectively.

not available to prevent fouling. On crosslinking with various diisocyanates, the absorbing capacity of the polymers should decrease drastically and the aflinity of the polymer matrix towards water should be reduced. In order to study this property, swelling measurements were carried out in five different dry solvents, viz. benzene, toluene, THF, CHCla, acetone and artificial sea water. The resulting swelling data in these various solvents are summarized in Table 2. All the cured polymers swell to the greatest extent in chloroform and to the least extent in benzene. Dharia [5] has reported similar results with TBTMA, Le. hydroxy alkyl methacrylate copolymers crosslinked with diisocyanates. Swelling in the various organic solvents decreased as the amount of crosslinking agent increased in the polymer. This is because the increase in crosslinking density prevented penetration of the solvent molecule into the polymer matrix. The swelling percentage also increased with an increase being in the polarity of the solvent, with the order of swelling CHCl, > THF > acetone > toluene > benzene, In all cases cured HEMA copolymers exhibited a greater percentage swelling than the corresponding HPMA polymers. Although the structure and nature of crosslinker often plays an important role in the swelling of cured polymers, no structure-swelling relationship could be established in this case. Whenever the polymers studied were actually tested in an artillcial sea water environment, they exhibited encouraging swelling results. HEMA copolymers swelled to a greater extent relative to HPMA in all the solvents, the extent of swelling decreasing as the amount of the crosslinker increased. The order of swelling with respect to diisocyanate was found to be TDI > IPDI > PDI > TMDI.

251 TABLE

2

Swelling properties Polymer code

(a) Tolylene HETD, HETD, HPTD, HPTD, (b) Imp&one HEIP, HEIP, HPIP, HPIP,

of cured polymers

% Swelling in Benzene

Toluene

diisocyanate

(TDI)

diisocyanate

hexamethykne 9.13 6.80 8.48 5.54

Artificial sea water

94.21 71.50 46.87 42.03

51.01 40.52 31.77 29.28

60.65 41.47 41.37 11.21

37.24 20.20 6.75 3.07

87.75 38.93 40.30 38.46

27.90 15.33 23.40 13.31

77.19 75.12 72.98 57.70

32.36 20.16 10.00 3.61

42.57 25.87 18.05 16.82

73.92 66.93 63.30 59.16

7.47 5.81 6.68 5.70

diisocyanate) (PDI)a 83.25 70.45 77.87 55.90 53.37 53.80 50.54 27.71

90.14 63.43 81.91 51.91

29.78 23.40 6.91 3.17

(IPDI)

diisocyanate (TMDI)= 68.98 38.73 49.92 31.53

24.13 15.44 21.56 6.69

(d) Methylene

bis(p-phenykne

HEPDz HEPD, HPPDz HPPD,

8.40 6.54 6.80 6.19

“Crosslinking

Chloroform

34.52 11.76 29.70 4.78

14.94 5.63 11.56 4.81

(c) Trimethyl

Acetone

21.02 15.06 13.25 8.29

6.24 4.11 4.39 2.68

HETM, HETM, HPTM, HPTM,

THF

23.52 21.77 21.95 18.47

agent.

Temperature

(“C)

Fig. 3. TGA plots for various crosslinked copolymers HEIP2; and -- -, HETM2.

studied: -,

HETD2; -. .-

HEPD,; -.

-*

The thermal stability of the cured copolymers was also examined by thermogravimetric analysis methods, the data for the thermal stability of the cured polymers being listed in Table 3 and Pigs. 3, 4 and 5 illustrating the TGA curves for the cured copolymers. The integral procedure decomposition temperature (IPDT) has been used as the criterion for thermal stability. Prom the IPDT values it is clear that all these polymers exhibited good thermal

252 TABLE

3

TGA data for the cured copolymers Polymer code

(a) Tolylene HETD, HETD, HPTD, HPTDz (b) I~op~o~ HEIPt HEIP, HPIP, HPIPl

IDTa (“C)

diisocyanute (TDI)b 240.60 233.45 240.42 275.75 d~~o~a~te 90.25 102.10 230.15 175.30

(c) Trimethyl HETMl HETM, HPTM, HPTM.,

hexamethylene 115.15 112.25 260.25 210.15

(d) Meth&m? HEPDz HEPD, HPPDz HPPD,

bti(p-phenykrze 110.30 105.15 250.55 250.05

Temp. at 50% wt. loss (“C)

IPDT”

FDT

(“C)

(“C)

330.30 325.60 335.25 330.35

345.50 320.20 375.75 342.31

410.05 425.25 425.75 460.21

350.75 325.50 375.20 340.25

295.34 280.60 359.45 338.45

465.90 437.15 450.30 425.25

295.07 283.62 385.50 375.75

368.75 475.00 490.05 495.75

310.40 301.65 359.05 355.25

405.90 425.25 501.05 495.95

(IPDI)”

diisocyanate (TMDI)’ 350.25 329.13 375.15 360.20 diisocyanute) (PDI)b 360.10 325.75 380.35 350.15

YDT, initial decomposition temperature; FDT, final decomposition temperature. “Crosslinking agent.

IPDT, integral procedure

Fig. 4. TGA plots for various crosslinked

polymers

studied: ---,

decomposition

HPPDI;

temperature;

and -,

HPTD,.

stability but that the stability was lower than that reported by Dharia. The values range from 301-385 “C which is higher than for the uncured polymers [ 9 1. TDI-cured copolymers showed the highest thermal stability whereas those prepared with IPDI showed the lowest stability. No structural relationship between the thermal stability and the diisocyanate employed could be established. In general, it was observed that cured HEMA copolymers exhibited

253

Effect

of cured organotin

Polymer

copolymers

on test microorganisms

Inhibition

zone size after 96 h (cm)

code

PS&

??zcwlaq f_-LemQi?wsa

Sarcinu lutea

10 10

4 6

%tandard deviation for the two microorganisms observed. bCrosslinking agent.

Fig. 5. TGA plots for various crosslinked

was within

copoIymers

kO.047;

studied: -

-, inhibition zone not

HPIP2; aSId-

-7

wPTM2.

a lower thermal stability than HPMA polymers. The stability decreased with both an increase in the diisocyanate and TCA contents. The XRD spectra of the cured polymers showed no change in the d~~ct~o~ pattern, indicating that the polpers are amorphous in nature with cryMinity being induced by erosslinkig. The biocidal effects of all the cured copolymers relative to two species of bacteria, viz. P. Sousa and S. lutea, and relative to A. satina nauplii

254 TABLE Effect

5 of cured organotin

Polymer code

(a) Tolylew HETD, HETDa 5 HPTD, HPTDa

copolymers

Mole fraction of TBT in compound diisocyanate 0.05 0.18 0.06 0.29

on A. salina ra

log Y=nlz+c

48 h LC,a value [pg (100 ml)-il

(TDI)b

(b) Isophmone HEIP, HEIP, HPIP, HPIP,

ditiocyanate 0.13 0.43 0.06 0.46

(c) Trimethyl HETM, HETM_, HPTM, HPTM,,

hexarnethykne 0.05 0.43 0.06 0.46

(d) Methylene HEPDa HEPD., HPPD, HPPD4

bis(p-phenykne 0.13 0.43 0.06 0.46

0.99 0.96 0.95 0.96 (IPDZ)’ 0.99 0.98 0.97 0.95 diisocyanate 0.95 0.99 0.99 0.98 diisocyanute) 0.94 0.99 0.99 0.99

5.0x 5.8x 2.4~ 4.3x

lo-%z+ 1.53 lo-%+ 1.56 10-%-r+ 1.12 lo-3x+0.73

1.9x lo-%+ 1.29 1.19x lo-%+ 1.60 1.2x lo-%+ 1.59 9 x lo-‘s+ 1.63

282 240 241.5 225.50 215.5 83.75 91 77

(TMDI)b 0.16x lo-%+ 1.33 4.0x lo-3x+0.775 1.3x 1O-3x+ 1.23 214x 1O-3~+ 1.09

318.5 231 361 254

(PDl)b 7.0x 1o-4x+ 1.39 1.3x lo-“x+1.23 0.3x 1o-3x+ 1.07 1.32x10-3~+1.11

442 361 484.5 446.5

al.= correlation coefficient. bCrossIinking agent.

II larva are listed in Tables 4 and 5. The results show that cured copolymers of HETD, HPTD, HETM and HEPD were toxic towards P. czerugimsa although the size of the inhibition zone was small. Cured copolymers of HETM and HEPD showed relatively higher toxicity towards A. salina and demonstrated by the large size of the inhibition zone. All the cured copolymers showed toxicity towards A. salina with the LCSOvalue decreasing as the TBT content increased. A comparison of the toxicity values for the cured copolymers with those for the uncured copolymers [ 111 showed that the toxicity of the cured polymers decreased upon crosslinking. Hence it may be concluded that the release of TBT toxicant from the copolymer may be controlled by incorporating appropriate proportions of crosslinker in the copolymer. References 1 J. C. Montermoso, T. M. Andrews and L. P. Marine& J. Polywz. Sci., 32 (1958) 523. 2 R. V. Subramanian and R. N. Somasekharan, .J. Macromol. Sci., Chem., A16 (1981) 73. 3 N. A. Ghanem, N. N. Messiha, N. E. Ikladlous and A. F. Shaaban, Eur. Polym. J., 15 (1979) 823.

255 4 R. V. Subramanian,

5 6 7 8 9 10 11 12 13

B. K. Garg and J. Corredore, in J. E. Carraher, J. E. Sheats and C. E. Pitman (eds.), Organometallic Polymers, Academic Press, New York, 1979. J. R. Dharia, Ph.D. Thesis, Indian Institute of Technology, Bombay, 1989. D. D. Pen-in, W. L. F. Armarego and D. R. Perrin, PuriJicaticm of LaboratoqJ Chemicals, 2nd edn., Pergamon Press, New York, 1983. C. S. Marvel, J. Dee, H. G. Cook, Jr. and J. C. Covan, J. Am. Cha. Sot., 62 (1940) 3495. J. A. Montemarano, B. J. Dyckman and E. C. Fischer, Nav. Eng. J., 85 (1973) 33. R. R. Joshi, S. K. Gupta and J. R. Dharia, Angew. Makromol. Chem., in press. H. Gilman and D. Rosenberg, J. Am. Gem. Sot., 75 (1953) 3592. R. R. Joshi and S. K. Gupta, Toxicity Assessment, 5 (1990) 180. A. Milne and M. E. Gallow, Non-biocidal Antifouling Processes, Trans. Inst. Mar. Eng. (C), 97 (1984) Conf. 2, Paper 37. A. D. Deshpande, Ph.D. Thesis, Indian Institute of Technology, Bombay, 1982.