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Progress in Organic Coatings 69 (2010) 45–48
Contents lists available at ScienceDirect
Progress in Organic Coatings
journa l homepage: www.e lsev ier .com/ locate /porgcoat
olecular release from painted surfaces: Free and encapsulated biocides
ars Nordstiernaa,∗, Atta Alla Abdallaa, Mariam Masudaa, Gunnar Skarnemarkb, Magnus Nydéna
Division of Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, SwedenDivision of Nuclear Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
r t i c l e i n f o
rticle history:eceived 15 October 2009eceived in revised form 19 March 2010ccepted 6 May 2010
a b s t r a c t
The current standard way of using biocide in coatings for protection against micro-organic surface growth,so called biofouling, is insufficient as the biocide leaks out from the coating too fast. In this article, weinvestigate a method for prolongation of the coating protection by slowing down the release rate of
eywords:ontrolled releaseainticrocapsulesicrospheres
biocides in a controlled way. The biocide is placed into micrometer-sized reservoirs, called microspheres,from where it is slowly distributed into the coating. By different microscopic techniques the microsphereswere found to be compatible (i.e. no phase separation was observed) both with the coating material andthe paint. Biocide release from the coating is recorded by liquid scintillation counting and it was clear thatthe release is considerably slower from coatings with microspheres compared to an ordinary formulationwith freely dispersed biocides. Microspheres might thus be a beneficial tool for the development of
otect
coatings with a longer pr. Introduction
During the last decades a growing awareness of environmen-al issues has led to major changes in the production of paint andoating materials. The ambition to reduce environmental pollutionmplies that several toxic preservatives nowadays are prohibited.his has however caused severe problems with micro-organic sur-ace growth. Mold and algae can be found on many painted outdooralls. House paint is one example where current protections of
he product are insufficient. Another major use for protective coat-ngs is in the marine industry; barnacles and other organisms causenwanted fouling on ships and other marine constructions. The sur-ace of paint coatings is damaged, and loss in durability is costly,oth in economic and ecologic terms.
There is a growing demand for surface protection technologieshat meet the requirements of material durability and environ-
ental sustainability. Concerning protective coatings formed fromaints, the current applications of anti-growth agents, i.e. biocides,ungicides etc, provide protection only during a short period. It isowever not necessarily the specific efficiency of the biocides that
s poor. The biocides are most often organic molecules with a lowolecular weight and the small size renders a rapid leakage out
rom the coating. The leakage is caused by diffusion through theolymer matrix that constitutes the main fraction of the paint. Ifiocides diffuse out, water molecules must at the same time replacehem by diffusing into the matrix from the paint surface. The rea-
∗ Corresponding author.E-mail address: [email protected] (L. Nordstierna).
300-9440/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2010.05.002
ion against biofouling.© 2010 Elsevier B.V. All rights reserved.
son for the biocides and water diffusing at all is that the coatingmaterials fall under the category soft material although it also con-tains solid fractions. The soft fraction consists mostly of polymersand even though their flexibility is strongly restricted compared tothe situation in a solvent it is still high enough for a small moleculeto be able to move through the coating. As most of today’s biocidesare small molecules, the diffusion-driven flux makes the fungi andmold protective properties of the coating decay much more rapidlythan the loss of mechanical properties of the coating (for instancedue to UV radiation). With the early leakage of biocides, mold pro-tection is lost and the surface becomes discolored after merely acouple of years. The lifetime of exterior wall coating, in the absenceof surface growing organisms, may be typically 3–5 times longer.
A couple of recent reviews cover the historical and contempo-rary development of paints/coatings [1,2]. The scientific interestseems to be directed towards marine applications where the pro-tective coating is central as the problem of corrosion is alwayspresent. A non-protective coating also gives rise to biofoulingwhere marine organisms such as barnacles settle on the ship hull.Biofouling on the hull surface is known to increase the frictionaldrag and thereby significantly increase fuel consumption. Anotheraspect with biofouling is the risk to introduce new organisms toeco-systems where they normally do not belong.
The formulation of the paint is decisive in order to retain the bio-cides in the coating over extended time-periods [3]. One approach,
described by Handa et al. [4], to delay the release of biocides fromthe paint is to use a polymer material into which the biocide isstrongly adsorbed and thereby causing a slower diffusion in thecoating material. Another possibility is to adsorb the anti-growthagents on nanoparticles dispersed in the paint matrix [5]. Fay et al.4 n Orga
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6 L. Nordstierna et al. / Progress i
6] discuss the relation between biocide leakage rate and polymerlm stability for self-polishing coatings. For these types of coatingsolymer surface erosion rate plays an important role in the releaseinetics of biocides.
This work explores the possibility to prolong the protection ofoatings by encapsulating the biocides in micro-sized capsules orpheres. Thereby, the biocides can be slowly distributed into theaint by a controlled release mechanism. The biocide concentration
n paint is normally ∼0.5 mass% so in theory the protection couldimply be prolonged by increasing the biocide concentration. Thatill, however, most often alter imperative mechanical properties
f the coating and sometimes also the esthetic appearance. Theaximum concentration of a dispersed biocide in the coating is
hus often limited. The main idea with our research presented heres to put biocides into micrometer-sized reservoirs with a surfacehat is compatible with the paint/coating components. Biocides arehen slowly distributed from the reservoir to the rest of the painthere it then reaches the coating surface by diffusion. Moreover,substantially higher concentration of biocide can be employedithout destroying the paint or the coating which means that therotection will be substantially prolonged. We denote these smalleservoirs microcapsules or microspheres.
A microcapsule consists of a spherical polymer shell and a corer oil, e.g. some simple alkane, in which the biocide is dissolved. Theicrosphere [7], on the other hand, we define as a homogeneous
pherical particle where the biocide is uniformly, or non-uniformly,issolved or dispersed in a polymer matrix. In this article we onlyeal with microspheres. There are some important demands thateed to be fulfilled when microspheres are used in a paint system,i) a high loading ratio of biocide and (ii) a polymer that is compat-ble with the paint latex but that has a glass transition temperaturee.g. above 80 ◦C for PMMA) high enough to prevent film formation.
The objective of this paper is to investigate the use of micro-pheres in coating systems for controlled release of biocidesltimately for prolonging biofouling protection. We therefore studyhe release of the biocide medetomidine from a coated surface ande compare the release profiles where the substance is (i) freelyispersed in the matrix with (ii) encapsulated in microspheres.ased upon the release profiles, we emphasize the fundamentalroperties of the coating and microspheres that govern the biocideelease.
. Experimental
All the chemicals, acetone (Merck, ≥99.5%), CDCl3 (Aldrich,9.8 at.% D), ethanol (Kemetyl, ≥99.5%), ethyl acetate (Acros Organ-
cs, ≥99.6%), medetomidine (Fermion, Finland), 14C-medetomidine4.14 g/l in ethanol solution, 4831.8 MBq/g, 14C-labelled on the-imidazole carbon, Izotop, Hungary), poly(methyl methacrylate)Aldrich, MW 3,500,000), poly(vinyl alcohol) (Acros Organics, 95%ydrolyzed, MW 950,000) and Ultima GoldTM (PerkinElmer) weresed as received. All water was of MilliQ purity.
The preparation of microspheres is identical to that presentedreviously [8] and similar to that by Loxley and Vincent [9].he microsphere polymer consists of poly(methyl methacrylate)PMMA) in which the medetomidine is dispersed.
The organic solution was prepared from 57 ml ethyl acetate,.0 g PMMA, 3.8 ml acetone (co-solvent), 0.90 g normal medeto-idine, and 5.0 ml ethanol containing 4.14 g/l 14C-medetomidine.
he aqueous solution contained 2 mass% poly(vinyl alcohol) and
ml ethyl acetate in water at a total of 80 ml and was placed inwater-cooled reactor. 9 ml of ethyl acetate in the aqueous solu-ion correspond to the saturation in water [10]. A homogenizer ofype Silent Crusher M tool 22F (Heidolph Instruments) was usedo shear the aqueous solution at 5000 rpm. The organic phase was
nic Coatings 69 (2010) 45–48
gently added to the aqueous solution over a 5 min period and thehomogenizer was left to stir for 1 h. The formed emulsion then waspoured into 120 ml aqueous solution with 2 mass% poly(vinyl alco-hol). Ethyl acetate was then, during magnetic stirring at 600 rpm,evaporated from the open beaker in a fume hood. During the evap-oration of ethyl acetate, the emulsion undergoes internal phaseseparation and forms an aqueous suspension of micro-sphericalparticles with PMMA and medetomidine.
The quality and size distribution of the microspheres in thesuspension were examined by optic microscopy (Olympus BH-2) equipped with a digital camera system (Olympus DP12). Thesuspension was then left to sediment in a closed beaker for 1week leaving the microspheres at the bottom. The small fractionsuspended in the supernatant was carefully decanted. The watercontent in the suspension sediment was determined by placing asmall precise weight in a 105 ◦C oven in order to evaporate thewater. The water content was 42% by weight. The dried test sub-stance was then dissolved in CDCl3 and measured by quantitative1H NMR [11] on a JEOL 400 MHz spectrometer to affirm that themicrosphere composition was 90% PMMA and 10% medetomidine,which was what was originally added in the synthesis.
Two different types of paint were then prepared. One water-based standard white exterior wall paint and one solvent-basedstandard white exterior wall paint, both biocide-free. Two sam-ples of 30 g with water-based paint and two samples of 30 g withsolvent-based paint were prepared. To each sample, a total of0.40 mass% medetomidine was added, either in free form (dissolvedin small amount of ethanol) or encapsulated in microspheres. Fromnow on, these samples are denoted WF, WS, OF, and OS paintsrespectively where the abbreviation capital letters are given by“Water”, “Organic”, “Free”, and “Sphere”. Concerning the combinedamount of non-labeled and 14C-labeled medetomidine, the portionof labeled material was 2.3% with respect to the total amount ofmedetomidine. Stirring the paint with an electrical mixer assuredhomogeneous dispersion of the added substances.
Small amounts of the paints were applied on glass plates. Afterdrying, a needle scratch was made through the coating. The sampleswere then sputtered (JEOL JFC-1100E) with a 10 nm layer of goldand analyzed with a Leo Ultra 55 FEG scanning electron microscope.
For the release studies, approximately 1 g of coating (i.e.dry weight) were applied on 100 mm × 100 mm × 10 mmpoly(propylene) plates. A paint applicator (Elcometer, birdfilm applicator) provided ∼200 �m wet film thickness and89 mm × 89 mm area. Each system was painted on triplicatesamples. After drying and weighing, each plate was verticallypositioned in a closed container containing 1000 ml water thatcovered the plate. The containers were placed on a mechanicalshaker and a gentle speed was applied. At various times 1000 �lwater was pipetted out from each system and transferred to 20 mltubes. After addition of 10 ml Ultima GoldTM, the radioactivitywas recorded by a liquid scintillation counter (PerkinElmer WallacGuardian 1414). It was necessary to assure that the release rate didnot decrease as a consequence of equilibrium between the paintand the surrounding water reservoir. Therefore, the whole 1000 mlwater content was replaced with fresh water two times during theexperiment. However, this action had no effect on the release rateon any paint system. We may thus consider the experimental setupto provide a release profile not affected by the water solubility ofmedetomidine since this never will be the case in the sea.
3. Results and discussion
From optic micrographs, the microsphere diameter was calcu-lated to be 12 ± 5 �m. A representative optic micrograph of thesuspension of PMMA/biocide-microspheres is shown in Fig. 1.
L. Nordstierna et al. / Progress in Organic Coatings 69 (2010) 45–48 47
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ig. 1. An optic micrograph of microspheres containing 90% polymer and 10% bio-ide.
Electron micrographs of the area inside exposed paint matricescreated by scratching the coating with a thin needle) are displayedn Fig. 2. It is evident that the microspheres are homogeneouslymbedded in the paint and that their geometry is unaffected byhe surrounding chemicals, the drying of the paint or by the elec-rical mixer used to formulate microspheres into the paint. Onebservation was that the initially white OF paint became slightlyink immediately after the free medetomidine addition. The mostrobable explanation is that the red complex formed in the solvent-ased paint originates from the imidazole group of medetomidineinding strongly to metal ions in the paint [12,13]. On the con-rary to the OF paint, the white color of the OS paint remainednchanged after addition of medetomidine-filled microspheres.he visual difference between OF and OS is a clear demonstrationf the encapsulation effect where the biocide can be protected fromhe surrounding substances.
Fig. 3 displays the release data demonstrating the amount ofeleased biocide from the coated surface to the water reservoir.rror bars are calculated from standard deviation obtained fromriplicate samples of each coating system. The data in Fig. 3a arelotted with linear scale while Fig. 3b shows the same data plottedn logarithmic scale for clarity.
The first observation that can be drawn from the data is that theelease from water-based coating is faster than from solvent-basedaint. The water-based coating is more hydrophilic and thereforewells to a larger extent when placed in aqueous environment. Thewelling brings water into the coating, which increases the mobilityf the releasing biocide. In terms of comparison release rate behav-or a more appropriate examination is therefore to study the twoypes of paints as two separate cases: WF versus WS and OF versusS.
It is clear that free biocide is released from the coating fasterhan encapsulated biocide, both for water-based and solvent-basedoatings. The release from WF is about two times faster than thatrom WS. For the solvent-based paint, a similar comparison reveals10 times faster release from OF than from OS. The slower release isue to the microsphere and the “protection” it offers for the biocide.
n order for the biocide to leave a microsphere the space occupiedy the biocide needs to be replaced by another molecule. By neces-ity this molecule needs to be small enough in order to penetrate
he very crowded polymer network in the microsphere. If the sur-ounding coating matrix contains mostly large molecules, such asolymers, the release rate of biocides is most likely strongly affectedy the rate by which water can replace the biocide. Due to the veryydrophobic character of PMMA the microsphere swells slowlyFig. 2. Electron micrographs of the paint matrix (scratched by a needle) of (a) water-based paint with free biocide, (b) water-based paint with encapsulated biocide, (c)solvent-based paint with free biocide, and (d) solvent-based paint with encapsulatedbiocide. Arrows point out the presence of microspheres.
in water (it should be noted that this material typically swells to1–1.5% in water). In the case of free biocide the rate of release ispresumably also correlated to the penetration of water throughthe coating. In the WF and OF samples,the biocide is formulated
48 L. Nordstierna et al. / Progress in Orga
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[9] A. Loxley, B. Vincent, J. Colloid Interf. Sci. 208 (1998) 49–62.
ig. 3. The release of biocides, on (a) lin–lin scale and (b) log–log scale, from (�)ater-based paint with free biocide, (�) water-based paint with encapsulated bio-
ide, (�) solvent-based paint with free biocide, and (�) solvent-based paint withncapsulated biocide. Intersecting lines a solely for clarification.
nto the coating without the microsphere “protection”. Hence, the
iocide is more likely to be surrounded by more hydrophilic com-ounds in the paint matrix, which then swells more by the water,esulting in faster release of biocide.Another observation that can be made for all release profiles ishe initial slope of the curves that indicates a fast release. After a
[[[[
nic Coatings 69 (2010) 45–48
couple of days the slopes decrease to display a more modest releaserate. We interpret the period during the steep slope (30–100 h)as the stage where the paint film becomes swelled by water. Theswelling causes faster release since a small fraction of biocideis directly exposed to the surrounding. As mentioned above theswelling of the hydrophilic water-based paint is more notable andthereby provides a larger extent of biocide leakage.
4. Conclusions
The results bring us to the conclusion that release from coatedsurfaces is slower when biocides are formulated into the paint sys-tem in microspheres, compared to the ordinary formulation byformulating the biocides directly into the paint. The microspheresare compatible with both the paint and the coating and renderno visual or mechanical disturbance with regard to stability andapplication of the paint. The slower leakage obtained when usingbiocides in microspheres is a quality that might provide a benefit. Inthe search for paint formulations, which offer a prolonged lifetimeand more robust protection against surface growing organisms, adecreased biocide leakage rate is required.
Acknowledgments
We would like to thank Kurt Löfgren for providing thepoly(propylene) plates. We also would like to thank people at theMarine Paint program and Georgia Kourouklidou and Robert Svens-son at Capeco AB for useful discussions. Mark Foreman is thankedfor experimental assistance. This study was supported by MISTRA(Swedish Foundation for Environmental Research) and VINNOVA.
References
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Prog. Org. Coat. 44 (2002) 75–83.[4] P. Handa, C. Fant, M. Nydén, Prog. Org. Coat. 57 (2006) 376–382.[5] L. Shtykova, C. Fant, P. Handa, A. Larsson, K. Berntsson, H. Blanck, R. Simonsson,
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