Research Article

626

Received: 22 July 2008, Revised: 11 September 2008, Accepted: 18 September 2008, Published online in Wiley InterScience: 17 December 2008

(www.interscience.wiley.com) DOI: 10.1002/pat.1311

Optical behavior of antibiofouling additives inenvironment-friendly coverglass materials forbio-sensors and solar panels

Chad Bootha*, Phil Wheelera, Jesse Hancocka, Ray Ximenesa

and Donald E. Pattersonb

Solar panels and bio-optical sensors play a signific

Polym. Adv

ant and growing role in a number of applications that are ofimportance tomany organizations. Many of these instruments require a high transmission of radiation into the devicefor it to work properly. A major issue faced is that harsh marine environments often aid in the growth or developmentof fouling on the coverglass used to protect the instruments. Over a period of time in an ocean environment, someplant or animal may attach itself to the coverglass, ultimately obscuring the glass and rendering the instrumentuseless. As such, an antifouling mechanism is needed for these instruments that is inexpensive, long-lasting, andenvironment friendly. The approach discussed herein involves the use of known antifouling chemicals which havebeen incorporated into the polymer matrix. Polymethylmethacrylate (PMMA), bisphenol A polycarbonate (Bis A PC),and a co-polyterephthalate (CPTE) were examined. The plaques are optically transparent and previous work hasshown that, for most samples, the materials display a minimal decrease in mechanical behavior upon the addition ofthe algaecides. This paper will discuss the effects on the materials’ optical properties when exposed to both harshmarine conditions as well as high intensity UV light. Specifically, the decrease in transmission of visible light wasexamined over a 6 month period of time. Copyright � 2008 John Wiley & Sons, Ltd.

Keywords: transparency; compounding; optics; antibiofouling; marine environment

* Correspondence to: C. Booth, Department of Chemistry & Biochemistry, Texas

State University, San Marcos, Texas 78666, USA.

E-mail: cb31@txstate.edu

a C. Booth, P. Wheeler, J. Hancock, R. Ximenes

Department of Chemistry & Biochemistry, Texas State University, San Marcos,

Texas 78666, USA

b D. E. Patterson

Nanohmics, Inc., 6201 E. Oltorf St. #400, Austin, Texas 78741-7511, USA

Contract/grant sponsor: National Oceanographic and Atmospheric Adminis-

tration (NOAA).

Contract/grant number: #DG133R06CN0202

INTRODUCTION

A coverglass or window is used to protect almost all solar cellsand optical sensors. The coverglass ideally has a hightransmission of UV, visible, and/or IR light (depending on theultimate function of the device) and is durable (impact andscratch resistant, chemically inert, temperature stable, UV stable,etc.). The most commonly used materials for these protectingwindows are borosilicate glass and fused silica. Borosilicate glass(typically 5% Co-doped for radiation hard solar panels) has up to93.5% transmission in the 350–4000 nm range,[1] and for fusedsilica (quartz) it can be as high as 96%.[2] For special applications,coverglasses have been made of such exotic materials assapphire and even diamond.Another important class of coverglass and protective windows

is the optically transparent polymers. Chief among these arebisphenol A polycarbonate (Bis A PC), polymethylmethacrylate(PMMA), polystyrene (PS), and various polyesters. Even thoughPMMA has slightly better transmission properties, polycarbonateis more commonly used due to its combination of optical,mechanical, and chemical properties.[3] One of the mainproblems associated with the use of polycarbonate and othersimilar aromatic polymers as a coverglass material is that theyturn yellow over time due to UV exposure, lessening theirtransmission properties. This degradation has been linked tophoto-Fries rearrangements, which produce benzophenones andrelated species, and due to photooxidative cleavage.[4] Various

. Technol. 2009, 20 626–630 Copyright �

compounds are added to these materials to improve their UVstability and improve their performance over time.More recently, a new more advanced polymer has been of

great interest.[5,6] This polymer, shown in Fig. 1, is a copolymer of2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO), 1,3-propanediol(PDO), and dimethyl terephthalate (DMT). This material hassuperior impact resistance, transparency, and UV stability. Theamorphous co-polyterephthalate (CPTE) can be prepared with alarge range of physical properties by simply varying the ratio ofCBDO to aliphatic glycol.A number of antifouling agents are routinely used in marine

environments. The largest use for these materials has been incoating the hulls of waterborne vessels to reduce or eliminate thegrowth of unwanted biological species on the hulls. An ideal

2008 John Wiley & Sons, Ltd.

Figure 1. Structure of the amorphous CBDO–PDO co-polyterephthalate.

OPTICAL BEHAVIOR OF ANTIBIOFOULING ADDITIVES

antifouling agent controls the growth ofmarine organisms on thesurface to which it is applied; however, the antifouling agentmustalso be relatively benign to the environment. In other words, asuitable antifouling agent must degrade rapidly in a marineenvironment or have controlled release partitioning in theenvironment resulting in limited bio-availability to marineorganisms. The antifoulant must result in minimal accumulationof toxic compounds (i.e. toxic antifoulants must chemicallydegrade rapidly in the environment), and must have limitedtoxicity to marine organisms in the local environment surround-ing the protective coverglass at standard concentrations. Theprimary antifouling agent used extensively over the past severaldecades has been the family of organotins, and the chief amongthese being the tributyltin (TBT) compounds.[7] While thesecompounds were extremely effective as antifoulants, they haveproved to be too toxic and too long-lived. As a result, TBTs werebanned by the International Maritime Organization, effective 1January 2003.Less toxic antifouling agent alternatives have since been

approved and put into use. These include: various coppercompounds (copper (I) oxide, copper thiocyanate,metallic copper,copperbronze, coppernapthanate, andother copper compounds,SeaNine 211 (4,5-dichloro-2-m-octyl-4-isothiaz-olin-3-one; Rohmand Haas) and Irgarol1 1051 algaecide (Ciba Specialty Chemical).While all of these materials are attractive as being moreenvironment friendly than TBT, they still are not completelyenvironmentally benign.This paper will focus on the optical characterization of the

materials studied after prolonged exposure to both UV andmarine algae. Specifically, the percentage loss in transmission wasevaluated after a 6 month period for promising samples as well asthe leach rate of algaecide into the surrounding marineenvironment.

EXPERIMENTAL

PMMA and PC were purchased from TDL Plastics (Corpus Christi,TX) and used without further purification. The CBDO based CPTEwas donated by Eastman Chemical and used as provided withoutfurther purification. Irgarol1 1051 (Fig. 2) was provided by Cibafree of cost, while the Diuron (Fig. 2) was purchased from AldrichChemical; both were used as received. Polymer composites weremade using a previously reported procedure.[8,9] Based on themanufacturer’s recommendations, composite samples using 0, 2,

Figure 2. (a) Irgarol1 1051 and (b) Diuron.

Polym. Adv. Technol. 2009, 20 626–630 Copyright � 2008 John

and 4% Diuron by weight and 0, 0.25, and 0.5% Irgarol by weightwere fabricated for the tests.Polymers were examined for both impact values (Dart impact)

as well as typical mechanical values such as peak load, peakstress, percentage of strain at break, and modulus (Sintech 1/D).The techniques and instruments used as well as the results ofthese experiments have been previously reported.[8,9]

A 30 gallon salt water aquarium was used for in-house testingof antifouling properties. A strong bloom of Chlorella capsulata (aknown aggressive biofouling algae) was established in theaquarium. The aquarium was kept at optimal growth conditionsfor the algae (26.5� 0.58C, pH¼ 8.3� 0.1, specific grav-ity¼ 1.022� 0.002, and constant exposure to light). Algae growthwas further enhanced by supplying the tank with KentPro-Culture algal culture formula. Both algaecide-impregnatedpolymer samples and controls were placed into the tank andevaluated after 6 months.UV aging tests were carried out on both treated samples and

controls for 22 weeks using continuous exposure to UVB light inan enclosed testing chamber. This chamber supplies constantexposure from two 15W USHIO UVB fluorescent lamps providinga luminous intensity of 550 Lux.An Ocean Optics SD2000 UV–visible spectrometer was used to

test for optical transmission through the various sample plaquesbefore and after both algae growth and UV aging tests. Astandard tungsten/halogen light source was used, and thesamples were tested against an air reference.

6

RESULTS AND DISCUSSION

In addition to the impact/tensile data previously reported,[8,9]

optical studies were carried out to evaluate the effectiveness ofthe antibiofouling windows. The averaged results for the polishedsamples are tabulated in Table 1. The test shows that there issome degradation of the optical properties of the polymersacross the board upon inclusion of the algaecide. The tests alsoshow that a reduction in transmission occurs with the inclusion ofeither of the studied algaecides.The PC sample with 0.25% by weight Irgarol1 added retains

the highest percentage transmission (a loss of >1%) uponinclusion of the algaecide. When examining the samples for thegreatest overall benefit, PMMA samples with incorporatedIrgarol1 1051 show an approximate 50% improvement intransmission properties (loss of�4.3% transmission with Irgarol1

vs. �7.7% without Irgarol1) when compared with controlsamples. The algaecide-impregnated samples of CPTE actuallydisplayed a loss in percentage transmission when compared tothe neat samples. Likewise, the PC samples, with the exception ofthe above mentioned 0.25% Irgarol1 samples, display apercentage loss of transmission with increasing additive. A setof representative spectra for some of the PC samples is shown inFig. 3 and it graphically illustrates the loss of optical transmissionfor the samples not containing Irgarol1.

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Table 1. Average optical transmission values for algaecide-impregnated samples after algae exposure

Sample Transmission at 600 nm (%) % transmission after algae growth % loss during transmission

Neat CPTE 89.4 85.8 4.010.25% Irgarol/CPTE 86.8 80.8 6.850.5% Irgarol/CPTE 86.7 81.2 6.352% Diuron/CPTE 84.5 75.6 10.594% Diuron/CPTE 81.5 73.4 9.89Neat PMMA 92.2 84.7 7.740.25%Irgarol/PMMA 90.3 86.4 4.250.5% Irgarol/PMMA 90.9 86.9 4.362% Diuron/PMMA 90.3 83.6 7.394% Diuron/PMMA 89.7 84.6 5.70Neat PC 89.3 88.2 1.200.25% Irgarol/PC 90.1 89.3 0.940.5% Irgarol/PC 90.4 88.8 1.782% Diuron/PC 90.3 88.0 2.554% Diuron/PC 89.5 87.4 2.40

Figure 3. Optical transmission spectra of algaecide-impregnated poly-

mers after 6monthsof algaegrowth: algae resistance for (a) CPTE, (b) PMMA,and (c) PC. Untreated samples (neat) are included for reference. This figure

is available in color online at www.interscience.wiley.com/journal/pat

www.interscience.wiley.com/journal/pat Copyright � 2008

C. BOOTH ET AL.

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When examining the data obtained from the UV studies (datashown in Table 2 and actual plaques shown in Fig. 4) carried outon the samples, it becomes apparent that, with the exception ofthe PC samples with Irgarol1 added, all samples display a loss inpercentage transmission as the quantity of additive is increased.The PC samples with Irgarol1 additive actually display apercentage transmission loss which is less than (�65%) theneat sample. PC samples with incorporated Diuron were notmeasured due to the brittle nature of the composite (thesesamples lost all mechanical integrity with UV exposure) whichmade it unsuitable for further evaluation as a window material.The CPTE samples with both the Irgarol1 and Diuron additivesdisplay an �50% increase in loss of percentage transmission.Likewise, the PMMA samples with Irgarol1 additive display an�70% increase in loss of percentage transmission while the valueof the same for the Diuron samples is �90%.In addition to the optical transmission studies, an examination

of the leach rate of Irgarol1 1051 from the PMMA samples wasconducted. In these tests, a set of 1 cm2 sample coupons of 0.5%Irgarol1 in PMMA were placed into a series of three 100mlcontainers of deionized water and allowed to sit for severalmonths at ambient temperatures. The leach rate of Irgarol1 intothe water was determined by obtaining the UV spectrum ofaliquots of the solution at various study times using a simpleBeer–Lambert law. The UV–visible spectrum of Irgarol1 1051 isshown in Fig. 4. For the UV spectra, the Ocean Opticsspectrometer was again used; however, a deuterium light source(Ocean Optics DT-Mini-2-GS) was used for sample illumination. Aseries of calibration tests resulted in a molar absorptivity of theIrgarol1 1051 of 8697.2mol/L (R2¼ 0.9946) at Irgarol’s maximumabsorbance of 234 nm.After running the tests and aging for almost 2 years (687 days),

sample aliquots of the solutions failed to show any evidence ofIrgarol1 1051 down to a spectrometer detection limit of 10�7M.This result effectively gives us a leach rate of less than 1.84 ng/cm2/day.Similarly, in a concentrated sample leach test, a large quantity

of 0.3% by weight Irgarol/PMMA pellets were placed into 100mlof deionized water. The pellets have an effective surface area of�310 cm2. After aging for 79 days, there was still no evidence of

John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 626–630

Table 2. Average optical transmission values for algaecide-impregnated samples after UV exposure

Sample Transmission at 600 nm (%) % transmission after UV % loss during transmission

Neat CPTE 90.4 88.1 2.570.25% Irgarol/CPTE 90.1 87.1 3.290.5% Irgarol/CPTE 90.3 86.4 4.372% Diuron/CPTE 88.54 84.7 4.334% Diuron/CPTE 86.74 81.3 6.29Neat PMMA 88.6 87.9 0.780.25% Irgarol/PMMA 90.9 89.7 1.670.5% Irgarol/PMMA 91.0 89.1 2.102% Diuron/PMMA 90.4 84.4 5.974% Diuron/PMMA 90.2 82.6 7.40Neat PC 90.9 88.8 2.230.25% Irgarol/PC 90.8 89.7 1.240.5% Irgarol/PC 90.2 89.0 1.272% Diuron/PC Not measured Not measured Not measured4% Diuron/PC Not measured Not measured Not measured

Figure 4. Test plaques after UV exposure. This figure is available in color online at www.interscience.wiley.com/journal/pat

OPTICAL BEHAVIOR OF ANTIBIOFOULING ADDITIVES

Irgarol in the aqueous solution. These findings indicate a low levelof environmental impact for the Irgarol/PMMA antibiofoulingpolymers.Leach rates for the CPTE and PC were not determined. At this

point during the evaluation it was decided that the loss ofmechanical properties for both the CPTE and PC samples weretoo great to warrant further evaluation as optical windows. Thesamples were, therefore, not subjected to leach rate studies.

6

CONCLUSIONS

On examining the results obtained from the percentagetransmission studies on samples placed in marine tanks for a6 month period, it became apparent that Irgarol1 1051 samplesperformed the best. The samples containing Diuron show agreater loss of transmission even though these samples contain ahigher percentage of algaecide. This is evidence that Irgarol1

1051 is a better choice for protecting polymeric materials againstthe growth of aggressive types of algae.The UV studies show that samples containing Diuron display a

rapid loss in light transmission. The PC/Irgarol1 samples display a

Polym. Adv. Technol. 2009, 20 626–630 Copyright � 2008 John

percentage loss in transmission which is lower than that of theneat sample. It is believed that this is due to the ability of Irgarol1

1051 to absorb the UV rays which translates to less UV absorptionby the PC itself. All other samples show the expected trend ofincreased percentage loss during transmission with increasingexposure to UV light.As previously reported,[8,9] with respect to the impact and

mechanical properties, Irgarol1 1051 samples again had themost desired properties for an antibiofouling, transparentpolymeric window material. Previous data[8,9] show that PMMAexhibits nil to minimal loss in impact and mechanical propertieswith the incorporation of either Irgarol1 or Diuron. The CPTEsamples with Irgarol1 displayed an �10% loss in impactresistance while the Diuron samples displayed an �25% lossin impact resistance. The PC samples with Irgarol1 displayed an�20% loss in impact resistance while the impact values for theDiuron samples fell to below those of the PMMA samples (a>99% loss in impact resistance).When subjected to mechanical evaluations, we examined peak

load, peak stress, percentage strain at break, and modulus, thedata show[8,9] that both PMMA and CPTE samples are stiffenedand, to a lesser extent, toughened by the addition of the

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C. BOOTH ET AL.

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algaecides. The PC samples, on the other hand, displayed adecrease in all of the above mechanical properties. These datadirectly parallel the observations reported in this paper, in thatthe PC samples do not perform well as optical windows after theaddition of the algaecides.Impact resistance is an important parameter required in many

applications. If an application requires the maximum value ofimpact resistance then the CPTE/Irgarol1 samples are the mostappropriate. These samples still maintain Dart impact values of�900 J/m but do display more loss of transparency, whencompared to the PMMA samples. If standard impact resistance isacceptable, then the PMMA samples are adequate (�5 J/m). Inneither case is there an unacceptable loss of impact resistance,while this is not true for the PC samples.If one considers the impact/mechanical properties, UV study,

and the algae exposure study, the best properties are achievedwith the PMMA/Irgarol1 combination. This combination typicallyresults in better than 90% transmission across the visiblespectrum, the smallest decrease in percentage transmission withprolonged exposure to a UV source, no loss to the material’simpact resistance, and a slight stiffening of the materials’mechanical properties.[8,9]

www.interscience.wiley.com/journal/pat Copyright � 2008

REFERENCES

[1] Product literature for Thales-Optics coverglass. www.thales-optics.co.uk

[2] Product literature for OCLI coverglass. www.ocli.com[3] K. D. Drechsler, C. L. Schultz, U. Wollborn, M. Moethrath, M. Erkelenz,

Advances in Polycarbonates, Symposium Series, vol. 898: Mechanical andMorphological Properties of Copolycarbonates of Bisphenol A and4,40-Dihyroxydiphenyl (Eds. D. J. Brunelle M. R. Korn), American Chemi-cal Society, Washington, DC, 2005.

[4] S. Pankasem, J. Kuczynski, J. K. Thomas, Macromolecules 1994, 27,3773. DOI: 10.1021/ma00092a016

[5] C. J. Booth, M. Kindinger, H. R. McKenzie, J. Hancock, A. V. Bray, G. W.Beall, Polymer 2006, 47, 6398–6405. DOI: 10.1016/j.polymer/2006.06.056

[6] G. W. Beall, C. E. Powell, J. Hancock, M. Kindinger, H. R. McKenzie, A. V.Bray, C. J. Booth, Appl. Clay Sci. 2007, 37, 295–306. DOI: 10.1016/j.clay.2007.03.011

[7] A. Terlizzi, S. Fraschetti, P. Gianguzza, M. Faimali, F. Boero, Aquat.Conservat. Mar. Freshwat. Ecosyst. 2001, 11, 311–317. DOI: 10.1002/aqc.459

[8] C. J. Booth, P. Wheeler, J. Hancock, R. Ximenes, D. E. Patterson, Int. J.Polym. Mater. 2008, 57, 452–462. DOI: 10.1080/00914030701729628

[9] P. Wheeler, J. Hancock, R. Ximenes, D. E. Patterson, C. J. Booth, ACSPolym. Pre. 2008, 49(1), 408.

John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 626–630