Espessura de Filme FTIR

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PACKAGING ANO CONTAINERSJames S. Swinehart and Robert W. HannahThe Perkin-Elmer Corporation

Infrared spectroscopyhas a significan t role to playin the continuing quest of the packaging and containerindustry for materiais to meet demands for betterproduct protection, better appearance, and lower costs.Not only will infrared spectra provide large amountsof information about materiais to be used in packaging ,but such spectra can be obtained rapidly and easily

..., with the aid of a variety of available sampling techniqueswhich are applicabte to substances having widelydiffering physical properties.

Among the many uses of infrared spectroscopyin the packaging industry are such applica tions as thedetection of impur ities in the raw materiais used in a

that the spectrum of the unknown film was the samas the spectrum of polyethylene.

Other alkenes, such as polypropylene. have spectrasimilar to that of polyethylene. Certain differenees,however, allow one to distingui sh easily among thevarious members of the series. For example, the speof polyolefins other than polyethylene have onemore moderately st rong absorpt ions between 1300 and80 em- due to ehain branehing. Furthermore, thespectra of almost ali polyolefins other than polyehave a mueh stronger absor ption at 1380 em- l ththat shown in Figur e 1. Also. no other alkene polymhas a strong doublet at 725 em- l .


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REFLECT NCE TECHNIQUES

The foregoing example illustrates identification ofa transparent sample. Most spectra are obtained ontransparent samples; however, techniques are availablefor obtaining spectra on opaqu e samples. Figures 2

and 3 are spectra of opaque samples obtained byusing reflectance techniques.

Figure 2. Spec lrum of unknown opaque sample by FMIR.

Figure 3. Spaclrum of Epon on IInplale by specular refleclance.

at 1432 em- 1 asymmetrie methyJ bend next to acarbonyl). The data in dicate that the coating isaliphati c acetate with a high molecular weight, propolyvinyl acetate or one of its copolymers. Finaconfi rmation that the coating is indeed PVA is eas

obtained by comparison of the spectrum with areferenee speetrum of polyvinyl acetate.

Figure 3 is the speetrum of Epon epoxy resinon tinplate. Even though the thiekness of the eoatinwas less than 0.5 mil, a useful spectrum was obtaineusing a Perkin-Elmer Specular Reflectance Accesspart number 186-0188), This accessory focuses th

sample beam of the spectrophotometer on a sam

placed on the aceessory sample holder so that thesample refleets radiation baek into the instrument.If the sample is a coating which is partially ortransparent to inf rared radiation and is on a refleetsubstrate. the infrared radiation passes through thecoat ing and is refleeted back to the spectrophotomA transmission spectrum of the coating is thus obtSi nce the beam is very small at the sample poi

the reflecting surface can be either flat orsomewhat curved.

ME SUREMENT OF FILM THICKNESS

Whenever a beam of radiation travels from a me


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The relation between film thickness, d, and wavelengthwill determine whether there is reinforcement orattenuation, and as the spectrophotometer scansthrough successive wavelengths a series of "fringes"

). will be produced as reinforcement and attenuation

alternate with each other.Figure 5 is a transmission spectrum of a filmof

Saran, and exhibits typical interference fringessuperimposed on the characteristic absorptionspectrum. Fringe maxima are labeled "r" (forreinforcement), and fringe mnima "a" (for attenuation).

Figure 5. Sa an.

Interference fringes can usually be recognized anddifferentiated from absorption bands because of theiruniform appearance and because their peaks andvalleys are evenly spaced on a linear frequency scale

REFEREN ES

Further applications of infrared spectroscopy thatmay be of interest to technical personnel in thepackaging and container industry are found in:1. J Fahrenfort, "Attenuated Total Reflection, a N

Principie for the Production of UsefullnfraredReflection Spectra of Organic Compounds."Spectrochimica Acta 17,698-709 (1961).

2. R. W. Hannah, J. S. Swinehart, "Determining theThickness of a Sealed Cell and Polymer Film,"Experiments in Techniques 01 Inlrared Spectrospp.3.1 - 3.6, Perkin-Elmer Corporation,Norwalk, Connecticut (1968).

3. N. J Harrick, "Total Internai Reflection and ItsApplication to Surface Studies," Annals 01 lheAcad. of Scences 101,928-959 (1963).

4. Walter Morris. Jr "'dentification of PackagingMaterial by Infrared Reflectance Techniques,"Journal 01 Assn. Df Ofticial Agricultural Chemis48,497-501 (1965).

5. Richard A. Nyquist, "Infrared Spectra Df Plastic

Resins," Dow Chemical Company, Midland, Mi6. o ~ i sH. Sharpe. "Observation of MolecularInteractions in Oriented Monolayers by InfraredSpectroscopy Involving Total Internai ReflectionProceedings of the Chem Soc December 1961


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P a g e 3-1

E X P E R I M E N T

Dete rm. in ing the T h i c k n e s s of a Sea led Ce l l and of a Polym.e r Fi lm.

O B J E C T I V E

T o de te rm. ine t he t h i c k n e s s of a s e a l e d l iquid c e l l and o f a polyrn.er

film. hy the i n t e r f e r e n c e f r i n g e t echn ique .

MATERIALS

A s e a l e d c e l l t h i ckness 0.015 to 0 . 3 rn.rn. and the p o l y s t y r e n e c a l i b r a -t i on film..

INTRODUCTION

T h e t h i c k n e s s of s e a l e d c e l l s i s one o f t he b a s i c q u a n t i t i e s r e q u i r e dfo r quan t i t a t i ve a n a l y s i s . The va lue shou ld b e v e r i f i e d f rom. t im.e to t im.es ince the th icknes s m.ay change due to a g r a d u a l e r o s i o n of t he i n t e r n a l s u r -
http:///reader/full/Polym.erhttp:///reader/full/polyrn.erhttp:///reader/full/Polym.erhttp:///reader/full/polyrn.er


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P a g e 3- 2

-_ c - - - -

REFLECTEO - - - - REFlECTEDTRANSMITTED TRANSMITTED

Fig . 3-1 - P a t h of r a d i a t i o n be tween F i g . 3 - 2 - Wave p a t t e r n s fo r t r a nthe i n n e r s u r f a c e s of a filITl o r ce l l . r n i t te d a n d r e f l e c t e d p o r t i o n s of rP a t h of r e f l e c t e d r a d i a t i o n i s d r a w n t i on when c e l l t h i c k n e s s , d, i s s ua t a n ang1e to s e p a r a te i t froITl tha t tha t 2d ITl t he i n - p h a s e cond i tt r ans r r t t ed . fo r a f r i n g e ITlaxiITluITl. T h e r e f l e

r a d i a t i o n , a s f ina l ly t r ans rn i t t ed , i sin p h a s e with t h a t t r a n s r n i t t e d d i r e c t l


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P a g e 3-

P R O C E D U R E

P a r t I - Thicknes s of a S e a l e d Cel l

P l a c e a n e m p t y s e a l e d c e l l nomina l t h i c k n e s s 0.015 to 0 . 3 m m ) , ingood condi t ion, in the s a m p l e b e a m and obta in the s p e c t r u m f r o m 4000 to650 e m I . The s p e c t r u m p r o d u c e d shou ld b e s i m i l a r to tha t shown in F i g u r3- 3 e x c e p t fo r the spac ing be tween m i n i m a o r m a x i m a . The ca lcu la t ion , bon Equa t ion 1, i s d e m o n s t r a t e d on the f igure with app rop r i a t e va lues of .o.mvI and vz. Note tha t a s m a n y f r inges a s p o s s i b l e a r e coun ted in o r d e r to r educe the effec t of e r r o r s in m e a s u r i n g v l a n d vz.

P a r t I I Thickness of a P o l y m e r F U m

P l a c e the p o l y s t y re n e c a l i b r a t i o n f i lm in the s a m p l e b e a m o the ins t r u m e n t . Obtain a s p e c t r u m i r o m 4000 to 650 e m I . The c u r v e shou ld bes i m i l a r to tha t shown in F i g u r e 3-4 . The i n t e r i e r e n c e i r inges u s e d in theca lcu la t ion a r e noted on the c u r v e . In th is ca se , Equa t ion Z shou ld be used .A r e f r a c t i v e index va lue of 1. 6 w a s u s e d f o r polys tyrene . The o p e r a t o r shob ab le t v e r i f y tha t hi p o l s t r e n e f i l m i s a p p r o x i m a t e l y th th ickne


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Page 3-4

FREQUENCY (CM ')4 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650

100_; l

90 rii

' ,70 ~ r J . .

- l1 I

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