- Email: [email protected]
Formulation and antifouling activity of marine paints: a study by a statistically based experiments plan
Progress in Organic Coatings 44 (2002) 85–92
Formulation and antifouling activity of marine paints: a study by a statistically based experiments plan Muriel Thouvenin a , Jean-Jacques Peron a , Valérie Langlois b , Philippe Guerin b , Jean-Yves Langlois c , Karine Vallee-Rehel a,∗ a b
Laboratoire de Biologie et Chimie Moléculaires, Université de Bretagne-Sud-Rue de St-Maude, 56325 Lorient, France Laboratoire de Recherche sur les Polymères, UMR CNRS, Université Paris XII-2-8 rue H Dunant, 94320 Thiais, France c NAUTIX, ZI des 5 Chemins, 56520 Guidel, France Received 5 July 2001; received in revised form 4 September 2001; accepted 6 September 2001
Abstract To produce the new generation of protective marine paints with reduced amount of biocides, it is necessary to understand the phenomenon of the antifouling activity. It would require to define all the mechanisms which are involved (diffusion of water and biocides in the matrix, hydrolytic degradation of the matrix), the resulting properties (erosion, presence of biocides), the influential factors (formulation, nature of the binder) and the links between factors and properties, which is totally unrealisable. In a first step, the antifouling activity is tackled by considering only three selected properties which are essential for the consumers: the toughness, the erosion and the release of biocides. A statistically based experiments plan is established in order to progress systematically in the study of the properties of antifouling paints and to define the influential factors on antifouling activity. The obtained results enabled us to exhibit two discriminating factors: the nature of the binder and the biocide. The amount of released biocides is largely inferior to the lethal doses for bacteria. These results lead to ask new questions about the antifouling activity and to produce efficient paints with restricted amounts of biocides. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Antifouling; Biocides; Release; Experiments plan
1. Introduction Paints are complex materials with regard to the large number of compounds introduced in their formulation. The optimisation of the protective activity of paints is an economical and ecological challenge which requires the understanding of the precise function of each compound. The technological and juridical aspects of the history of antifouling paints illustrate this challenge. The short-dated withdrawal of the organostannic molecules leads to the research of paints with minimised impact on the marine environment [1–3]. Nevertheless, the bibliographic references about the mechanism of action of these coatings are scarce or incomplete [4–6]. In the case of erodable paints, the antifouling activity is assumed to be obtained by the release of toxic molecules, called biocides, from a polymeric matrix. The understanding of antifouling activity would require to define the mechanisms which are involved (e.g. diffusion of water and biocides in the matrix, hydrolytic degradation of the matrix), the resulting properties (erosion, presence ∗ Corresponding author. E-mail address: [email protected] (K. Vallee-Rehel).
of biocides) and the influential factors (chemical composition: formulation, nature of the binder). It would especially require the study of the links between all these factors and properties: this exhaustive study is unrealisable. So, we have selected some factors for which the study seems to be essential and we have established a statistically based experiments plan in order to progress systematically in the study of the properties of antifouling paints. The benefits of statistically based experiments plan is to test a large number of factors by using minimum experiments and to organise the assays to make easier the analysis of experimental results. The aim is to define the factors which could be considered influential on antifouling activity.
2. Assumption Our assumption is that the antifouling activity of paint depends on its properties of erosion, toughness of the dry film (a soft film is fragile and will prevent from fouling during a short period) and leaching of biocides. The other main factors which could be considered are the chemical structure of the binder and the nature and the amount of
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 2 4 7 - 8
86
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
Fig. 1. Assumption.
biocides and pigments. This group of intrinsic factors of the composition constitute the formulation of the paint. The mechanisms (hydration, degradation, and diffusion) which are involved in the efficiency of the marine coatings could be analysed for a more complete but intricate study of the activity of antifouling paints (Fig. 1).
3. Experiments plan The optimisation of the formulation of antifouling paints is proving difficult and time consuming because of the large number of factors to consider and also the large number of properties to study. Fig. 2 illustrates the analytical protocol developed to obtain information about the evolution of paints during immersion and to explain their final properties [7]. All the following parameters are analysed simultaneously: • • • • •
permeability of the paint film; hydrolytic degradation of the binder; erosion, toughness and aspect of the film; release of the biocides mixed in the formulation; antifouling activity during immersion in natural sea water.
The aim of this study is to define the factors which could be influential on the antifouling activity of a paint. We study a
large number of parameters without knowing precisely their function in the formulation. We use a fractional plan 26 − 3 built from a base plan 23 study six factors by performing eight assays. A complete plan would require 26 = 64 assays. All the fractionnal plans imply the same issue for the analysis because all the experimental values necessary to a strict interpreting are not available on the contrary to the complete plan [8,9]. To analyse the results, we have considered that interactions between factors could be neglected. A factor is considered influential if its effect on a property is superior to the experimental error. The studied factors are the following: • Factor 1. The nature of copper derivative, copper thiocyanate or cuprous oxide. • Factor 2. The concentration of polymer, 12 or 16 wt.%. • Factor 3. The nature of organic biocide, biocide B1 (dimethylurea) or biocide B2 (sulphamide). • Factor 4. The amount of copper, 20 or 30 wt.%. • Factor 5. The amount of biocide, 2 or 5 wt.%. • Factor 6. The amount of the pigment number 1 (zinc oxide), 3 or 8 wt.%. Some products are kept constant in the formulation: • solvent: 40 g, • pigment (blue): 2 g,
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
• • • •
87
thixotroping compound: 1.2 g, additive (dertol): 0.5 g, plastifing compound (butylphthalate): 2 g, pigment number (titanium oxide): 4 g.
The three factors 4–6 are studied under the signs of the interactions of the base plan following the aliase theory: 4 = 123,
5 = 13,
6 = 12
Various chemical structures can be used as binders in erodable antifouling paints. Three different binders were chosen: • Two experimental binders (called PLA/t-Bu and HPA/t-Bu/DMAM). These acrylic erodable binders are prepared in the laboratory. They are copolymers with pendant hydrolysable functions and a variable hydrophobic/hydrophilic balance [10,11]. The different monomer units are described in Table 1. The hydrophilic repeating units are introduced in order to make easier water penetration in the film. Indeed, the penetration of water is essential to the biocides release and the erosion phenomenon. The hydrophobic units are introduced in order to obtain the solubility in solvents as xylene or naphta and to control the water penetration in the film. The hydrophobic units enhance mechanical properties of the paint film. The hydrolysable functions lead to a progressive degradation of the immersed binder and to the initiation and the control of erosion. • A commercial binder called PAC. It is a blend of a copolymer PMMA–PBMA (polymethylmethacrylate–polybutylmethacrylate) with rosin (60 wt.%).
Fig. 2. Analytical protocol.
In order to adjust the formulation to 100%, each formulation is completed with an extender (barium sulphate). We have proven experimentally that it is an inert filler (towards hydration, degradation, release, erosion and antifouling activity). The experiments matrix is presented in Table 2.
Table 1 Chemical structures of the monomers Monomer unit
Chemical structure
Function
tert-Butylacrylate (t-Bu)
Hydrophobic
Hydroxypropylacrylate (HPA)
Hydrophilic
Dimethylaminoethylmethacrylate (DMAM)
Hydrophilic
Polyacid lactic oligomers (PLA)
Hydrolysable
88
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
Table 2 Experiments matrix of the fractional plan Formulation number
Nature of copper (1)
Concentration of polymer (2)
Nature of biocide (3)
Amount of copper (4) 123
Amount of biocides (5) 13
Amount of pigment 1 (6) 12
1 2 3 4 5 6 7 8
− + − + − + − +
− − + + − − + +
− − − − + + + +
− + + − + − − +
+ − + − − + − +
+ − − + + − − +
Level − Level +
CuSCN Cuprous oxide
12 16
B1 B2
20 30
2 5
3 8
4. Experimental part 4.1. Materials tert-Butylacrylate (t-Bu), dimethylaminoethylmethacrylate, hydroxypropylacrylate were distilled before use. The methacrylate of lactic acid oligomers was prepared by condensation between methacrylic acid and oligolactic acid in anhydrous CH2 Cl2 with DCC and DMAP [10]. 4.2. Copolymerisation Copolymerisations were carried out in anhydrous dioxane at 70 ◦ C during 24 h using AIBN (1%). Copolymers were recovered by precipitation in water or petroleum ether and dried in vacuum at 60 ◦ C. 4.3. Preparation of paints Polymers were formulated in paint. All the ingredients were dispersed under vigorous agitation (2000 rpm) during 1 h. Then the paints were filtered through a sifter (200 m). 4.4. Analytical methods • The degradation of PLA/t-Bu has been controlled by using enzymatic kit (SIGMA ref735-10). • Copper was quantified by inductively coupled plasma. Samplings were acidified with a solution of nitric acid (69%) (5 ml of acid for 1-l water) in order to dissolve all degradation products. Standard solutions used were prepared in the same conditions. Wavelengths used were 327,395 and 324,754 nm. The spectrometer was a Varian Liberty Series II. • The release of propane-1,2-diol (product resulting from the degradation of HPA units) has been studied by GC-FID (Varian 3300). ◦ column: Nukol (Supelco), 30 m × 0.53 mm × 0.5 mm; ◦ injector: 60 ◦ C, 10 ◦ C/min to 180 ◦ C;
◦ column: 60 ◦ C (1 min), 10 ◦ C/min to 150 ◦ C (5 min); ◦ detector: 180 ◦ C; ◦ gas: nitrogen; ◦ injected volume: 1 l. • Coulometry. Painted plates were immersed in synthetic sea water. Pieces of films were cut off in order to quantify water amount present in the film by Karl-Fisher titration. ◦ Coulometer Metrohm KF 737; ◦ Oven Metrohm KF 707 (T = 150◦ C); ◦ Gas: nitrogen (N50), flow 200 ml/min; ◦ Reactant: hydranal-coulomat AG. • Release of biocides. The protocol used is described in the European standard ISO 15181. The release of organic biocides B1 and B2 have been quantified by HPLC–UV. A preliminary calibration was carried out with solutions for which concentrations varied from 0.01 to 10 mg/l. ◦ mobile phase A (40%, v/v): 1-l water HPLC + 100 ml acetonitrile + 1 ml H3 PO4 (85%); ◦ mobile phase B (60%, v/v): 1-l acetonitrile + 100 ml water HPLC + 1 ml H3 PO4 (85%); ◦ elution speed: 1 ml/min; ◦ column: ODS, 5 m × 250 mm × 4.6 mm; ◦ wavelength: 249 nm; ◦ injected volume: 100 l; ◦ detector UV: Varian series 9050 and data analysis software Borwin.
5. Results and discussion For the three polymers, eight formulations are tested. The properties of the 24 paints (toughness, erosion, and antifouling activity) are analysed (Table 3). The hydration, degradation and the release of biocides are only determined for the binders PAC and PLA/t-Bu (Tables 5–9). The study of the effect of formulation on the properties of paints requires at least two distinct works: the study of the effect of the pigments and the biocides by the experiments plan and the study of the effect of the main compound of the formulation, the binder.
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
89
Table 3 Observations of paints after 5 and 15 months of immersion Binder
Blend Blend Blend Blend Blend Blend Blend Blend
Formulation
(PAC) (PAC) (PAC) (PAC) (PAC) (PAC) (PAC) (PAC)
Toughness
Erosion
Antifouling
5 months
15 months
5 months
15 months
5 months
15 months
1 2 3 4 5 6 7 8
Tough Tough Tough Tough Tough Tough Tough Tough
Soft Soft Soft Soft Soft Tough Tough Tough
Erodable Erodable Erodable Erodable Erodable Erodable Erodable Erodable
Erodable Too erodable Too erodable Erodable Erodable Erodable Erodable Erodable
Little dirty Clean Clean Clean Clean Clean Clean Clean
Dirty Dirty Dirty Dirty Dirty Clean Clean Clean
Too Too Too Too
erodable erodable erodable erodable
Little dirty Very dirty Dirty Dirty
Very Very Very Very
Erodable Erodable Too erodable Erodable Too erodable Erodable Too erodable Erodable
Dirty Dirty Clean Dirty Little dirty Clean Dirty Little dirty
Dirty Dirty Very dirty Very dirty Very dirty Clean Dirty Dirty
Copolymer Copolymer Copolymer Copolymer
(HPA/t-Bu/DMAM) (HPA/t-Bu/DMAM) (HPA/t-Bu/DMAM) (HPA/t-Bu/DMAM)
5 6 7 8
Soft Soft Soft Soft
Soft Soft Soft Soft
Too Too Too Too
Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer
(PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu)
1 2 3 4 5 6 7 8
Tough Tough Tough Tough Tough Tough Tough Tough
Soft Soft Soft Soft Soft Tough Tough Tough
Erodable Erodable Too erodable Erodable Too erodable Erodable Erodable Erodable
5.1. Effect of the formulation on the properties: toughness, erosion and antifouling activity Table 3 summarises the observations realised on natural site after 5 months of immersion (the testing period includes one summer) and after 15 months of immersion (the testing period includes two summers). The results obtained at these two dates differ. This discordance illustrates the difficulty of the study of antifouling paints. The tests of efficiency can only be performed on natural site during seasons propitious to the settlement and the development of micro and macro living species. These tests must be performed over long periods: 1 or 2 years. After 5 months of immersion, the formulation does not modify most of the properties. Only one effect of the formulation is displayed: the incompatibility of biocide B1 (formulations 1–4) with the binder HPA/t-Bu/DMAM. All the paints containing these two compounds delaminate. The study of the effect of the formulation on the antifouling properties of HPA/t-Bu/DMAM cannot be achieved by this experiments plan because the half of the results is not available. For the three binders, toughness and erosion depend on the nature of the binder. The paints based on binders PAC and PLA/t-Bu were tough contrary to those with HPA/t-Bu/DMAM, whatever the formulation. The nature of the polymer also defines its ability to erode. All the chosen polymers were erodable, the formulation only modulates this characteristic. After 15 months of immersion, the observations made for a binder differ following the formulation used. A study is possible for the two binders with no incompatibility with
erodable erodable erodable erodable
dirty dirty dirty dirty
the biocide B1. Table 4 presents the scale of marks used to evaluate the paints after 15 months of immersion. It is admitted that the experimental error was equal to 0. The calculation matrix (Table 5) for the property toughness (estimated by touching) after 15 months of immersion showed that, for the two binders, only one factor was influential: the nature of the biocide. When the biocide B2 was used, the corresponding films were tough. These results confirm the unfavourable effect of the biocide B1 on the toughness of the films, already shown in the case of HPA/t-Bu/DMAM. For erosion (Table 5), a change of the characteristics of PAC is observed after 15 months of immersion: the erosion is more important. Another time, the nature of the biocide is a discriminating factor: a controlled erosion can be achieved with the biocide B2 contrarily to the biocide B1. In the case
Table 4 Scale of marks used to evaluate the paints on natural site Fouling Clean
Little dirty
Dirty
Very dirty
1
2
3
4
Erodable
Too erodable
Non-erodable
Little erodable
1
2
3
3
Tough
Soft
Delaminated
1
2
3
Erosion
Toughness
90
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
Table 5 Calculation matrix of effects Response
Binder
Mean
Factors 1
2
3
4
5
6
Toughness
PAC PLA/t-Bu
1.625 1.625
−0.125 −0.125
−0.125 −0.125
−0.375 −0.375
0.125 0.125
−0.125 −0.125
0.125 0.125
Erosion
PAC PLA/t-Bu
1.625 1.625
−0.125 −0.375
−0.125 0.125
−0.375 −0.125
0.125 −0.125
−0.125 −0.125
0.125 0.125
Antifouling activity
PAC PLA/t-Bu
2.25 3.12
−0.25 −0.375
−0.25 0.375
−0.75 −0.375
0.25 0.375
−0.25 −0.375
0.25 0.375
Hydration
PAC PLA/t-Bu
0.51 3.09
0.03 −0.16
Degradation
PLA/t-Bu
21.12
−3.125
−6.375
−6.625
Copper release
PAC PLA/t-Bu
−0.11 0.07
−0.09 0.04
Organic biocides release
PAC PLA/t-Bu
−1.375 2.375
−6.375 1.375
0.755 0.83 20.12 22.12
of PLA/t-Bu, the nature of copper also changes the erosion which is estimated as “good” in the presence of cuprous oxide. The calculation matrix for the antifouling activity (Table 5) shows that after 15 months of immersion, the binder PAC associated with the biocide B1 is not efficient: the paints are colonised rapidly: green algae have settled. In the case of the binder PLA/t-Bu, all the studied factors have little effect on antifouling activity. Nevertheless, an unexplained result is obtained in the case of the formulation number 6 for which no colonisation is observed. This result illustrates the restrictions of the interpretation of the results of an fractional experiments plan. This interpretation reveals the unfavourable effect of the biocide B1 on all the properties. 5.2. Effect of the formulation on the hydration of films The hydration of paint films is determined by Karl-Fisher titration [12]. The results are presented in Table 4. The selected binders possess different behaviours during immersion. Polymers like PAC are hydrophobic: the amount of absorbed water after 3 months of immersion is inferior to 1%. HPA/t-Bu/DMAM and PLA/t-Bu are more hydrophilic: e.g. a paint based on PLA/t-Bu absorbs about 5 wt.% of water after 3 months of immersion. The calculation matrix of the effects are presented in Table 5. The experimental error is estimated equal to 10%: the coulometric error represents 5%, the other errors come from the weighing and the method of drying (in order to remove the surface water). The nature of the binders determines in a large part the water uptake of the paint films. The commercial paints based on PAC contain small degrees of water which keep being constant during the immersion. These amounts are relatively
0.025 0.53
0.15 −0.07
0.05 −0.06
0.03 −0.06
0.04 −0.31
7.125
−4.875
0.375
−0.12 0.04
−0.14 −0.083
+0.033 −0.016
0.008 −0.016
−20.125 −22.125
0.375 −3.125
1.375 −2.375
−0.375 3.125
independent from the formulation. The calculated effects are similar to the experimental error and so no factor is considered as influential. In the case of the experimental binder PLA/t-Bu, the water uptakes are more important: the amount of absorbed water varies from 2 to 5 wt.% following the formulation. Two factors have an effect on the water absorption: the concentration of polymer and the amount of pigment. The water uptake is greater when the percent of polymer is high (equal to 16) and when the amount of pigment is low (equal to 3). 5.3. Effect of the formulation on the hydrolysis The acrylic binders used are elaborated in order to degrade. The aim of the preliminary study is the identification and the choice of the molecules which can be used to follow the degradation: they will be indicatory molecules. The binder PAC is a blend of an acrylic resin polybutylmethacrylate–co-methylmethacrylate (BMA–MMA) with rosin (a natural compound containing abietic acid). The acrylic resin does not lead to erosion because of its strong hydrophobicity. The possible degradation of PAC can be studied by the determination of resinic acids contained in rosin. The use of the HPLC coupled with detectors such as UV, fluorimetric and evaporating light scattering detection does not enable the detection of the products resulting from the degradation of rosin. A more sensible method like HPLC-mass spectrometry should be considered. In the case of the experimental polymer PLA/t-Bu, two units are capable of hydrolyse: the t-Bu (with the formation of tert-butanol) and the PLA units (with the formation of lactic acid). In order to quantify the amount of tert-butanol in the medium the GC-FID is used: its limit detection is 0.1 mg/l. For all the tests, no product is displayed. These different assays of quantification show that the t-Bu does not
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
91
Table 6 Water uptakes of paints after 3 months of immersion (wt.%)
Table 8 Amounts of copper released (%) from paints after 3 months of immersion
Formulation number
Responses for PAC
Responses for PLA/t-Bu
Formulation number
Responses for PAC
Responses for PLA/t-Bu
1 2 3 4 5 6 7 8
0.40 0.42 0.44 0.42 0.62 0.53 0.49 0.82
1.63 2.18 4.92 3.95 3.20 3.24 3.29 2.34
1 2 3 4 5 6 7 8
1.358 0.762 0.696 0.711 0.577 0.690 0.839 0.410
0.740 0.784 0.671 0.982 0.668 0.928 0.978 0.891
degrade in such environment. The hydrolysis of PLA results in the formation of lactic acid which can be quantified by the use of an enzymatic kit (limit detection 0.5 mg/l). Lactic acid is displayed in numerous samples. In the case of HPA/t-Bu/DMAM, the degradation is very low: 1% of hydrolysed functions after 2 months of immersion. The results, summarised in Tables 5–7, show a great effect of the formulation on the hydrolysis of the binder PLA/t-Bu. The experimental error is estimated to 20%. In all the cases, the effects of the factors are superior to the experimental error and so are influential. Only the factor number 6 (the amount of ZnO) has a negligible effect. The degradation of PLA/t-Bu is enhanced in the presence of • 30% of copper thiocyanate, • a concentration of polymer equal to 12, • 2% of biocide B1. These observations confirm the effect of the pigments and biocides on the properties of paints. The formulation changes the degradation by limiting the accessibility and the reactivity of the ester functions towards water and/or by decreasing the diffusion of the products resulting from the degradation. 5.4. Effect of the formulation on the release of biocides The degradation shows strongly different kinetic profiles: the nature of the binder has a great effect on the release of cuprous oxide and organic biocide B1 (data not shown). Table 7 Amounts of lactic acid released (percent of hydrolysable functions) from paints based on PLA/t-Bu after 3 months of immersion
The reason why the nature of the binder is a critical factor for biocide release can be found by considering different phenomena which are generally involved in controlled release of actives molecules (like drugs) from polymeric delivery systems: hydration, degradation and erosion of the matrix [13–16]. This study has been carried out but no link was made between these phenomena and the leaching of biocides. In all the cases, the amounts of copper released by the different formulations are weak (cf. Tables 5 and 8): its release was limited to 1 or 2% of the total amount. For the two binders, the studied factors are inferior or similar to the experimental error and so are considered no influential. For all the paints, the biocide B1 is released faster and in greater amounts than the biocide B2 (Tables 5 and 9). These observations can be explained by their different solubilities: the biocide B1 is more soluble in water (35 mg/l at 20 ◦ C) than the biocide B2 (0.9 mg/l at 20 ◦ C). The mean release rate for this biocide is 0.3 g/J cm2 . The biocide B2 is never totally released from the film: the leaching keeps being incomplete. The binders release small quantities of B2, these amounts are inferior to the limit detection which is equal to 0.25 mg/l. To calculate the values of effects, a zero value is attributed to the response “inferior to limit detection”. The mean release rate for this biocide is 0.01 g/J cm2 . The water solubility of the biocide is the preponderant factor and has the same effect for the two binders. In all the cases, the release of mineral or organic biocides was inferior to the expected values from literature [17], respectively, 10–50 and 2–5 g/J cm2 . Table 9 Amounts of biocides released (%) after 3 months of immersion (
Formulation number
PLA/t-Bu
Formulations (nature of biocide)
Responses for PAC
Responses for PLA/t-Bu
1 2 3 4 5 6 7 8
20 37 32 22 42 11 3 2
1 2 3 4 5 6 7 8
55 51 31 24
43 40 36 58
(B1) (B1) (B1) (B1) (B2) (B2) (B2) (B2)
92
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
6. Conclusion The analytical protocol used enables us to study the main factors which can modify the antifouling activity and the mechanisms involved in the protection. Three properties, toughness, erosion and biocides release, were selected to simplify the study. These properties are commonly used to estimate the efficiency of antifouling paints. The organisation of assays by the mean of statistically based experiments plan enables an easier and closer interpretation of the results. This preliminary study of the factors which influence the antifouling activity show that three main factors have to be considered: the nature and the concentration of the binder and the nature of the biocide. Among them, the nature of the binder is preponderant because it determines the erosion and the toughness. About the effect of the extenders and additives, there is a distinct behaviour between the commercial and the experimental binders. The formulation has few effects on the properties of commercial paints. The relative impermeability and the restricted degradation of these binders enable the obtention of a controlled erosion associated with an antifouling activity. In the case of the experimental binder, the effect of the extenders is stronger on the phenomena involved in the antifouling activity: water diffusion, hydrolysis, erosion and biocides release. The amounts of biocides released keep being largely inferior to the values generally admitted and probably to the lethal dose for marine bacteria. These results raise the question of the mechanism of action of the biocides. What is the function of the biocide present at the surface of the paint film? In order to realise less toxic paints, new formulations with few amount of biocides have been tested. The preliminary results showed that cuprous oxide and organic biocides are necessary in greater amount than the released quantity to
obtain a good protection. However, it is possible to reduce drastically the mixed quantity.
Acknowledgements We greatly acknowledge La région Bretagne and NAUTIX for the financial support of M. Thouvenin. References [1] P.L. Layman, Chem. Eng. News 1 (1995) 23. [2] C. Alzieu, J. Sanjuan, P. Michel, Mar. Pollut. Bull. 20 (1) (1989) 22. [3] C. Alzieu, J. Sanjuan, P. Michel, Mar. Pollut. Bull. 17 (11) (1986) 494. [4] M.M.H. Ayoub, M.M.A. Malek, N.N. Messiha, Pig. Resin Technol. 10 (1990) 4. [5] M. Camail, B. Loiseau, A. Margaillan, J.L. Vernet, Double Liaison-chimie des Peintures 347 (1984) 377. [6] E. Haslbeck, A. Valkirs, P. Seligman, A. Zirino, I. Rivera, J. Caso, E. Chen, J. Montemarano, Polym. Mater. Sci. Eng. 83 (2000) 349. [7] M. Thouvenin, K. Vallée-Réhel, J.J. Péron, V. Langlois, P. Guérin, Polym. Mater. Sci. Eng. 83 (2000) 357. [8] J. Goupy, La méthode des plans d’expériences, Dunod, 1988. ISBN 2040187324. [9] J. Goupy, Revue Statistique Appliquée 24 (4) (1990) 5. [10] K. Vallée-réhel, V. Langlois, Ph. Guérin, L. Borgne, J. Environ. Polym. Degrad. 7 (1) (1999) 27. [11] K. Vallée-Réhel, V. Langlois, Ph. Guérin, J. Environ. Polym. Degrad. 6 (4) (1998) 175. [12] K. Vallée-Réhel, B. Mariette, P.A. Hoarau, P. Guérin, Analusis 26 (1) (1998) 1. [13] A. Adrover, M. Giona, M. Grassi, J. Membr. Sci. 113 (1996) 21. [14] A. Göpferich, R. Langer, Macromolecules 26 (1993) 4105. [15] A. Göpferich, R. Langer, Biomaterials 17 (1996) 103. [16] R. Langer, Nature 392 (1998) 5. [17] CEPE Report, CEPE No. 96/559/3040/DEB/E2.
Formulation and antifouling activity of marine paints: a study by a statistically based experiments plan Muriel Thouvenin a , Jean-Jacques Peron a , Valérie Langlois b , Philippe Guerin b , Jean-Yves Langlois c , Karine Vallee-Rehel a,∗ a b
Laboratoire de Biologie et Chimie Moléculaires, Université de Bretagne-Sud-Rue de St-Maude, 56325 Lorient, France Laboratoire de Recherche sur les Polymères, UMR CNRS, Université Paris XII-2-8 rue H Dunant, 94320 Thiais, France c NAUTIX, ZI des 5 Chemins, 56520 Guidel, France Received 5 July 2001; received in revised form 4 September 2001; accepted 6 September 2001
Abstract To produce the new generation of protective marine paints with reduced amount of biocides, it is necessary to understand the phenomenon of the antifouling activity. It would require to define all the mechanisms which are involved (diffusion of water and biocides in the matrix, hydrolytic degradation of the matrix), the resulting properties (erosion, presence of biocides), the influential factors (formulation, nature of the binder) and the links between factors and properties, which is totally unrealisable. In a first step, the antifouling activity is tackled by considering only three selected properties which are essential for the consumers: the toughness, the erosion and the release of biocides. A statistically based experiments plan is established in order to progress systematically in the study of the properties of antifouling paints and to define the influential factors on antifouling activity. The obtained results enabled us to exhibit two discriminating factors: the nature of the binder and the biocide. The amount of released biocides is largely inferior to the lethal doses for bacteria. These results lead to ask new questions about the antifouling activity and to produce efficient paints with restricted amounts of biocides. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Antifouling; Biocides; Release; Experiments plan
1. Introduction Paints are complex materials with regard to the large number of compounds introduced in their formulation. The optimisation of the protective activity of paints is an economical and ecological challenge which requires the understanding of the precise function of each compound. The technological and juridical aspects of the history of antifouling paints illustrate this challenge. The short-dated withdrawal of the organostannic molecules leads to the research of paints with minimised impact on the marine environment [1–3]. Nevertheless, the bibliographic references about the mechanism of action of these coatings are scarce or incomplete [4–6]. In the case of erodable paints, the antifouling activity is assumed to be obtained by the release of toxic molecules, called biocides, from a polymeric matrix. The understanding of antifouling activity would require to define the mechanisms which are involved (e.g. diffusion of water and biocides in the matrix, hydrolytic degradation of the matrix), the resulting properties (erosion, presence ∗ Corresponding author. E-mail address: [email protected] (K. Vallee-Rehel).
of biocides) and the influential factors (chemical composition: formulation, nature of the binder). It would especially require the study of the links between all these factors and properties: this exhaustive study is unrealisable. So, we have selected some factors for which the study seems to be essential and we have established a statistically based experiments plan in order to progress systematically in the study of the properties of antifouling paints. The benefits of statistically based experiments plan is to test a large number of factors by using minimum experiments and to organise the assays to make easier the analysis of experimental results. The aim is to define the factors which could be considered influential on antifouling activity.
2. Assumption Our assumption is that the antifouling activity of paint depends on its properties of erosion, toughness of the dry film (a soft film is fragile and will prevent from fouling during a short period) and leaching of biocides. The other main factors which could be considered are the chemical structure of the binder and the nature and the amount of
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 2 4 7 - 8
86
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
Fig. 1. Assumption.
biocides and pigments. This group of intrinsic factors of the composition constitute the formulation of the paint. The mechanisms (hydration, degradation, and diffusion) which are involved in the efficiency of the marine coatings could be analysed for a more complete but intricate study of the activity of antifouling paints (Fig. 1).
3. Experiments plan The optimisation of the formulation of antifouling paints is proving difficult and time consuming because of the large number of factors to consider and also the large number of properties to study. Fig. 2 illustrates the analytical protocol developed to obtain information about the evolution of paints during immersion and to explain their final properties [7]. All the following parameters are analysed simultaneously: • • • • •
permeability of the paint film; hydrolytic degradation of the binder; erosion, toughness and aspect of the film; release of the biocides mixed in the formulation; antifouling activity during immersion in natural sea water.
The aim of this study is to define the factors which could be influential on the antifouling activity of a paint. We study a
large number of parameters without knowing precisely their function in the formulation. We use a fractional plan 26 − 3 built from a base plan 23 study six factors by performing eight assays. A complete plan would require 26 = 64 assays. All the fractionnal plans imply the same issue for the analysis because all the experimental values necessary to a strict interpreting are not available on the contrary to the complete plan [8,9]. To analyse the results, we have considered that interactions between factors could be neglected. A factor is considered influential if its effect on a property is superior to the experimental error. The studied factors are the following: • Factor 1. The nature of copper derivative, copper thiocyanate or cuprous oxide. • Factor 2. The concentration of polymer, 12 or 16 wt.%. • Factor 3. The nature of organic biocide, biocide B1 (dimethylurea) or biocide B2 (sulphamide). • Factor 4. The amount of copper, 20 or 30 wt.%. • Factor 5. The amount of biocide, 2 or 5 wt.%. • Factor 6. The amount of the pigment number 1 (zinc oxide), 3 or 8 wt.%. Some products are kept constant in the formulation: • solvent: 40 g, • pigment (blue): 2 g,
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
• • • •
87
thixotroping compound: 1.2 g, additive (dertol): 0.5 g, plastifing compound (butylphthalate): 2 g, pigment number (titanium oxide): 4 g.
The three factors 4–6 are studied under the signs of the interactions of the base plan following the aliase theory: 4 = 123,
5 = 13,
6 = 12
Various chemical structures can be used as binders in erodable antifouling paints. Three different binders were chosen: • Two experimental binders (called PLA/t-Bu and HPA/t-Bu/DMAM). These acrylic erodable binders are prepared in the laboratory. They are copolymers with pendant hydrolysable functions and a variable hydrophobic/hydrophilic balance [10,11]. The different monomer units are described in Table 1. The hydrophilic repeating units are introduced in order to make easier water penetration in the film. Indeed, the penetration of water is essential to the biocides release and the erosion phenomenon. The hydrophobic units are introduced in order to obtain the solubility in solvents as xylene or naphta and to control the water penetration in the film. The hydrophobic units enhance mechanical properties of the paint film. The hydrolysable functions lead to a progressive degradation of the immersed binder and to the initiation and the control of erosion. • A commercial binder called PAC. It is a blend of a copolymer PMMA–PBMA (polymethylmethacrylate–polybutylmethacrylate) with rosin (60 wt.%).
Fig. 2. Analytical protocol.
In order to adjust the formulation to 100%, each formulation is completed with an extender (barium sulphate). We have proven experimentally that it is an inert filler (towards hydration, degradation, release, erosion and antifouling activity). The experiments matrix is presented in Table 2.
Table 1 Chemical structures of the monomers Monomer unit
Chemical structure
Function
tert-Butylacrylate (t-Bu)
Hydrophobic
Hydroxypropylacrylate (HPA)
Hydrophilic
Dimethylaminoethylmethacrylate (DMAM)
Hydrophilic
Polyacid lactic oligomers (PLA)
Hydrolysable
88
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
Table 2 Experiments matrix of the fractional plan Formulation number
Nature of copper (1)
Concentration of polymer (2)
Nature of biocide (3)
Amount of copper (4) 123
Amount of biocides (5) 13
Amount of pigment 1 (6) 12
1 2 3 4 5 6 7 8
− + − + − + − +
− − + + − − + +
− − − − + + + +
− + + − + − − +
+ − + − − + − +
+ − − + + − − +
Level − Level +
CuSCN Cuprous oxide
12 16
B1 B2
20 30
2 5
3 8
4. Experimental part 4.1. Materials tert-Butylacrylate (t-Bu), dimethylaminoethylmethacrylate, hydroxypropylacrylate were distilled before use. The methacrylate of lactic acid oligomers was prepared by condensation between methacrylic acid and oligolactic acid in anhydrous CH2 Cl2 with DCC and DMAP [10]. 4.2. Copolymerisation Copolymerisations were carried out in anhydrous dioxane at 70 ◦ C during 24 h using AIBN (1%). Copolymers were recovered by precipitation in water or petroleum ether and dried in vacuum at 60 ◦ C. 4.3. Preparation of paints Polymers were formulated in paint. All the ingredients were dispersed under vigorous agitation (2000 rpm) during 1 h. Then the paints were filtered through a sifter (200 m). 4.4. Analytical methods • The degradation of PLA/t-Bu has been controlled by using enzymatic kit (SIGMA ref735-10). • Copper was quantified by inductively coupled plasma. Samplings were acidified with a solution of nitric acid (69%) (5 ml of acid for 1-l water) in order to dissolve all degradation products. Standard solutions used were prepared in the same conditions. Wavelengths used were 327,395 and 324,754 nm. The spectrometer was a Varian Liberty Series II. • The release of propane-1,2-diol (product resulting from the degradation of HPA units) has been studied by GC-FID (Varian 3300). ◦ column: Nukol (Supelco), 30 m × 0.53 mm × 0.5 mm; ◦ injector: 60 ◦ C, 10 ◦ C/min to 180 ◦ C;
◦ column: 60 ◦ C (1 min), 10 ◦ C/min to 150 ◦ C (5 min); ◦ detector: 180 ◦ C; ◦ gas: nitrogen; ◦ injected volume: 1 l. • Coulometry. Painted plates were immersed in synthetic sea water. Pieces of films were cut off in order to quantify water amount present in the film by Karl-Fisher titration. ◦ Coulometer Metrohm KF 737; ◦ Oven Metrohm KF 707 (T = 150◦ C); ◦ Gas: nitrogen (N50), flow 200 ml/min; ◦ Reactant: hydranal-coulomat AG. • Release of biocides. The protocol used is described in the European standard ISO 15181. The release of organic biocides B1 and B2 have been quantified by HPLC–UV. A preliminary calibration was carried out with solutions for which concentrations varied from 0.01 to 10 mg/l. ◦ mobile phase A (40%, v/v): 1-l water HPLC + 100 ml acetonitrile + 1 ml H3 PO4 (85%); ◦ mobile phase B (60%, v/v): 1-l acetonitrile + 100 ml water HPLC + 1 ml H3 PO4 (85%); ◦ elution speed: 1 ml/min; ◦ column: ODS, 5 m × 250 mm × 4.6 mm; ◦ wavelength: 249 nm; ◦ injected volume: 100 l; ◦ detector UV: Varian series 9050 and data analysis software Borwin.
5. Results and discussion For the three polymers, eight formulations are tested. The properties of the 24 paints (toughness, erosion, and antifouling activity) are analysed (Table 3). The hydration, degradation and the release of biocides are only determined for the binders PAC and PLA/t-Bu (Tables 5–9). The study of the effect of formulation on the properties of paints requires at least two distinct works: the study of the effect of the pigments and the biocides by the experiments plan and the study of the effect of the main compound of the formulation, the binder.
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
89
Table 3 Observations of paints after 5 and 15 months of immersion Binder
Blend Blend Blend Blend Blend Blend Blend Blend
Formulation
(PAC) (PAC) (PAC) (PAC) (PAC) (PAC) (PAC) (PAC)
Toughness
Erosion
Antifouling
5 months
15 months
5 months
15 months
5 months
15 months
1 2 3 4 5 6 7 8
Tough Tough Tough Tough Tough Tough Tough Tough
Soft Soft Soft Soft Soft Tough Tough Tough
Erodable Erodable Erodable Erodable Erodable Erodable Erodable Erodable
Erodable Too erodable Too erodable Erodable Erodable Erodable Erodable Erodable
Little dirty Clean Clean Clean Clean Clean Clean Clean
Dirty Dirty Dirty Dirty Dirty Clean Clean Clean
Too Too Too Too
erodable erodable erodable erodable
Little dirty Very dirty Dirty Dirty
Very Very Very Very
Erodable Erodable Too erodable Erodable Too erodable Erodable Too erodable Erodable
Dirty Dirty Clean Dirty Little dirty Clean Dirty Little dirty
Dirty Dirty Very dirty Very dirty Very dirty Clean Dirty Dirty
Copolymer Copolymer Copolymer Copolymer
(HPA/t-Bu/DMAM) (HPA/t-Bu/DMAM) (HPA/t-Bu/DMAM) (HPA/t-Bu/DMAM)
5 6 7 8
Soft Soft Soft Soft
Soft Soft Soft Soft
Too Too Too Too
Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer
(PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu) (PLA/t-Bu)
1 2 3 4 5 6 7 8
Tough Tough Tough Tough Tough Tough Tough Tough
Soft Soft Soft Soft Soft Tough Tough Tough
Erodable Erodable Too erodable Erodable Too erodable Erodable Erodable Erodable
5.1. Effect of the formulation on the properties: toughness, erosion and antifouling activity Table 3 summarises the observations realised on natural site after 5 months of immersion (the testing period includes one summer) and after 15 months of immersion (the testing period includes two summers). The results obtained at these two dates differ. This discordance illustrates the difficulty of the study of antifouling paints. The tests of efficiency can only be performed on natural site during seasons propitious to the settlement and the development of micro and macro living species. These tests must be performed over long periods: 1 or 2 years. After 5 months of immersion, the formulation does not modify most of the properties. Only one effect of the formulation is displayed: the incompatibility of biocide B1 (formulations 1–4) with the binder HPA/t-Bu/DMAM. All the paints containing these two compounds delaminate. The study of the effect of the formulation on the antifouling properties of HPA/t-Bu/DMAM cannot be achieved by this experiments plan because the half of the results is not available. For the three binders, toughness and erosion depend on the nature of the binder. The paints based on binders PAC and PLA/t-Bu were tough contrary to those with HPA/t-Bu/DMAM, whatever the formulation. The nature of the polymer also defines its ability to erode. All the chosen polymers were erodable, the formulation only modulates this characteristic. After 15 months of immersion, the observations made for a binder differ following the formulation used. A study is possible for the two binders with no incompatibility with
erodable erodable erodable erodable
dirty dirty dirty dirty
the biocide B1. Table 4 presents the scale of marks used to evaluate the paints after 15 months of immersion. It is admitted that the experimental error was equal to 0. The calculation matrix (Table 5) for the property toughness (estimated by touching) after 15 months of immersion showed that, for the two binders, only one factor was influential: the nature of the biocide. When the biocide B2 was used, the corresponding films were tough. These results confirm the unfavourable effect of the biocide B1 on the toughness of the films, already shown in the case of HPA/t-Bu/DMAM. For erosion (Table 5), a change of the characteristics of PAC is observed after 15 months of immersion: the erosion is more important. Another time, the nature of the biocide is a discriminating factor: a controlled erosion can be achieved with the biocide B2 contrarily to the biocide B1. In the case
Table 4 Scale of marks used to evaluate the paints on natural site Fouling Clean
Little dirty
Dirty
Very dirty
1
2
3
4
Erodable
Too erodable
Non-erodable
Little erodable
1
2
3
3
Tough
Soft
Delaminated
1
2
3
Erosion
Toughness
90
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
Table 5 Calculation matrix of effects Response
Binder
Mean
Factors 1
2
3
4
5
6
Toughness
PAC PLA/t-Bu
1.625 1.625
−0.125 −0.125
−0.125 −0.125
−0.375 −0.375
0.125 0.125
−0.125 −0.125
0.125 0.125
Erosion
PAC PLA/t-Bu
1.625 1.625
−0.125 −0.375
−0.125 0.125
−0.375 −0.125
0.125 −0.125
−0.125 −0.125
0.125 0.125
Antifouling activity
PAC PLA/t-Bu
2.25 3.12
−0.25 −0.375
−0.25 0.375
−0.75 −0.375
0.25 0.375
−0.25 −0.375
0.25 0.375
Hydration
PAC PLA/t-Bu
0.51 3.09
0.03 −0.16
Degradation
PLA/t-Bu
21.12
−3.125
−6.375
−6.625
Copper release
PAC PLA/t-Bu
−0.11 0.07
−0.09 0.04
Organic biocides release
PAC PLA/t-Bu
−1.375 2.375
−6.375 1.375
0.755 0.83 20.12 22.12
of PLA/t-Bu, the nature of copper also changes the erosion which is estimated as “good” in the presence of cuprous oxide. The calculation matrix for the antifouling activity (Table 5) shows that after 15 months of immersion, the binder PAC associated with the biocide B1 is not efficient: the paints are colonised rapidly: green algae have settled. In the case of the binder PLA/t-Bu, all the studied factors have little effect on antifouling activity. Nevertheless, an unexplained result is obtained in the case of the formulation number 6 for which no colonisation is observed. This result illustrates the restrictions of the interpretation of the results of an fractional experiments plan. This interpretation reveals the unfavourable effect of the biocide B1 on all the properties. 5.2. Effect of the formulation on the hydration of films The hydration of paint films is determined by Karl-Fisher titration [12]. The results are presented in Table 4. The selected binders possess different behaviours during immersion. Polymers like PAC are hydrophobic: the amount of absorbed water after 3 months of immersion is inferior to 1%. HPA/t-Bu/DMAM and PLA/t-Bu are more hydrophilic: e.g. a paint based on PLA/t-Bu absorbs about 5 wt.% of water after 3 months of immersion. The calculation matrix of the effects are presented in Table 5. The experimental error is estimated equal to 10%: the coulometric error represents 5%, the other errors come from the weighing and the method of drying (in order to remove the surface water). The nature of the binders determines in a large part the water uptake of the paint films. The commercial paints based on PAC contain small degrees of water which keep being constant during the immersion. These amounts are relatively
0.025 0.53
0.15 −0.07
0.05 −0.06
0.03 −0.06
0.04 −0.31
7.125
−4.875
0.375
−0.12 0.04
−0.14 −0.083
+0.033 −0.016
0.008 −0.016
−20.125 −22.125
0.375 −3.125
1.375 −2.375
−0.375 3.125
independent from the formulation. The calculated effects are similar to the experimental error and so no factor is considered as influential. In the case of the experimental binder PLA/t-Bu, the water uptakes are more important: the amount of absorbed water varies from 2 to 5 wt.% following the formulation. Two factors have an effect on the water absorption: the concentration of polymer and the amount of pigment. The water uptake is greater when the percent of polymer is high (equal to 16) and when the amount of pigment is low (equal to 3). 5.3. Effect of the formulation on the hydrolysis The acrylic binders used are elaborated in order to degrade. The aim of the preliminary study is the identification and the choice of the molecules which can be used to follow the degradation: they will be indicatory molecules. The binder PAC is a blend of an acrylic resin polybutylmethacrylate–co-methylmethacrylate (BMA–MMA) with rosin (a natural compound containing abietic acid). The acrylic resin does not lead to erosion because of its strong hydrophobicity. The possible degradation of PAC can be studied by the determination of resinic acids contained in rosin. The use of the HPLC coupled with detectors such as UV, fluorimetric and evaporating light scattering detection does not enable the detection of the products resulting from the degradation of rosin. A more sensible method like HPLC-mass spectrometry should be considered. In the case of the experimental polymer PLA/t-Bu, two units are capable of hydrolyse: the t-Bu (with the formation of tert-butanol) and the PLA units (with the formation of lactic acid). In order to quantify the amount of tert-butanol in the medium the GC-FID is used: its limit detection is 0.1 mg/l. For all the tests, no product is displayed. These different assays of quantification show that the t-Bu does not
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
91
Table 6 Water uptakes of paints after 3 months of immersion (wt.%)
Table 8 Amounts of copper released (%) from paints after 3 months of immersion
Formulation number
Responses for PAC
Responses for PLA/t-Bu
Formulation number
Responses for PAC
Responses for PLA/t-Bu
1 2 3 4 5 6 7 8
0.40 0.42 0.44 0.42 0.62 0.53 0.49 0.82
1.63 2.18 4.92 3.95 3.20 3.24 3.29 2.34
1 2 3 4 5 6 7 8
1.358 0.762 0.696 0.711 0.577 0.690 0.839 0.410
0.740 0.784 0.671 0.982 0.668 0.928 0.978 0.891
degrade in such environment. The hydrolysis of PLA results in the formation of lactic acid which can be quantified by the use of an enzymatic kit (limit detection 0.5 mg/l). Lactic acid is displayed in numerous samples. In the case of HPA/t-Bu/DMAM, the degradation is very low: 1% of hydrolysed functions after 2 months of immersion. The results, summarised in Tables 5–7, show a great effect of the formulation on the hydrolysis of the binder PLA/t-Bu. The experimental error is estimated to 20%. In all the cases, the effects of the factors are superior to the experimental error and so are influential. Only the factor number 6 (the amount of ZnO) has a negligible effect. The degradation of PLA/t-Bu is enhanced in the presence of • 30% of copper thiocyanate, • a concentration of polymer equal to 12, • 2% of biocide B1. These observations confirm the effect of the pigments and biocides on the properties of paints. The formulation changes the degradation by limiting the accessibility and the reactivity of the ester functions towards water and/or by decreasing the diffusion of the products resulting from the degradation. 5.4. Effect of the formulation on the release of biocides The degradation shows strongly different kinetic profiles: the nature of the binder has a great effect on the release of cuprous oxide and organic biocide B1 (data not shown). Table 7 Amounts of lactic acid released (percent of hydrolysable functions) from paints based on PLA/t-Bu after 3 months of immersion
The reason why the nature of the binder is a critical factor for biocide release can be found by considering different phenomena which are generally involved in controlled release of actives molecules (like drugs) from polymeric delivery systems: hydration, degradation and erosion of the matrix [13–16]. This study has been carried out but no link was made between these phenomena and the leaching of biocides. In all the cases, the amounts of copper released by the different formulations are weak (cf. Tables 5 and 8): its release was limited to 1 or 2% of the total amount. For the two binders, the studied factors are inferior or similar to the experimental error and so are considered no influential. For all the paints, the biocide B1 is released faster and in greater amounts than the biocide B2 (Tables 5 and 9). These observations can be explained by their different solubilities: the biocide B1 is more soluble in water (35 mg/l at 20 ◦ C) than the biocide B2 (0.9 mg/l at 20 ◦ C). The mean release rate for this biocide is 0.3 g/J cm2 . The biocide B2 is never totally released from the film: the leaching keeps being incomplete. The binders release small quantities of B2, these amounts are inferior to the limit detection which is equal to 0.25 mg/l. To calculate the values of effects, a zero value is attributed to the response “inferior to limit detection”. The mean release rate for this biocide is 0.01 g/J cm2 . The water solubility of the biocide is the preponderant factor and has the same effect for the two binders. In all the cases, the release of mineral or organic biocides was inferior to the expected values from literature [17], respectively, 10–50 and 2–5 g/J cm2 . Table 9 Amounts of biocides released (%) after 3 months of immersion (
Formulation number
PLA/t-Bu
Formulations (nature of biocide)
Responses for PAC
Responses for PLA/t-Bu
1 2 3 4 5 6 7 8
20 37 32 22 42 11 3 2
1 2 3 4 5 6 7 8
55 51 31 24
43 40 36 58
(B1) (B1) (B1) (B1) (B2) (B2) (B2) (B2)
92
M. Thouvenin et al. / Progress in Organic Coatings 44 (2002) 85–92
6. Conclusion The analytical protocol used enables us to study the main factors which can modify the antifouling activity and the mechanisms involved in the protection. Three properties, toughness, erosion and biocides release, were selected to simplify the study. These properties are commonly used to estimate the efficiency of antifouling paints. The organisation of assays by the mean of statistically based experiments plan enables an easier and closer interpretation of the results. This preliminary study of the factors which influence the antifouling activity show that three main factors have to be considered: the nature and the concentration of the binder and the nature of the biocide. Among them, the nature of the binder is preponderant because it determines the erosion and the toughness. About the effect of the extenders and additives, there is a distinct behaviour between the commercial and the experimental binders. The formulation has few effects on the properties of commercial paints. The relative impermeability and the restricted degradation of these binders enable the obtention of a controlled erosion associated with an antifouling activity. In the case of the experimental binder, the effect of the extenders is stronger on the phenomena involved in the antifouling activity: water diffusion, hydrolysis, erosion and biocides release. The amounts of biocides released keep being largely inferior to the values generally admitted and probably to the lethal dose for marine bacteria. These results raise the question of the mechanism of action of the biocides. What is the function of the biocide present at the surface of the paint film? In order to realise less toxic paints, new formulations with few amount of biocides have been tested. The preliminary results showed that cuprous oxide and organic biocides are necessary in greater amount than the released quantity to
obtain a good protection. However, it is possible to reduce drastically the mixed quantity.
Acknowledgements We greatly acknowledge La région Bretagne and NAUTIX for the financial support of M. Thouvenin. References [1] P.L. Layman, Chem. Eng. News 1 (1995) 23. [2] C. Alzieu, J. Sanjuan, P. Michel, Mar. Pollut. Bull. 20 (1) (1989) 22. [3] C. Alzieu, J. Sanjuan, P. Michel, Mar. Pollut. Bull. 17 (11) (1986) 494. [4] M.M.H. Ayoub, M.M.A. Malek, N.N. Messiha, Pig. Resin Technol. 10 (1990) 4. [5] M. Camail, B. Loiseau, A. Margaillan, J.L. Vernet, Double Liaison-chimie des Peintures 347 (1984) 377. [6] E. Haslbeck, A. Valkirs, P. Seligman, A. Zirino, I. Rivera, J. Caso, E. Chen, J. Montemarano, Polym. Mater. Sci. Eng. 83 (2000) 349. [7] M. Thouvenin, K. Vallée-Réhel, J.J. Péron, V. Langlois, P. Guérin, Polym. Mater. Sci. Eng. 83 (2000) 357. [8] J. Goupy, La méthode des plans d’expériences, Dunod, 1988. ISBN 2040187324. [9] J. Goupy, Revue Statistique Appliquée 24 (4) (1990) 5. [10] K. Vallée-réhel, V. Langlois, Ph. Guérin, L. Borgne, J. Environ. Polym. Degrad. 7 (1) (1999) 27. [11] K. Vallée-Réhel, V. Langlois, Ph. Guérin, J. Environ. Polym. Degrad. 6 (4) (1998) 175. [12] K. Vallée-Réhel, B. Mariette, P.A. Hoarau, P. Guérin, Analusis 26 (1) (1998) 1. [13] A. Adrover, M. Giona, M. Grassi, J. Membr. Sci. 113 (1996) 21. [14] A. Göpferich, R. Langer, Macromolecules 26 (1993) 4105. [15] A. Göpferich, R. Langer, Biomaterials 17 (1996) 103. [16] R. Langer, Nature 392 (1998) 5. [17] CEPE Report, CEPE No. 96/559/3040/DEB/E2.