An Instrumental Method for Measuring the ... - ACS Publications

8 A n Instrumental Method for Measuring the Sterilization Resistance (Blushing) of Can Coatings W. RAUDENBUSCH, H. C. J. VENSELAAR, and D. M. PAUL

Downloaded by NORTHWESTERN UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch008

Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Badhuisweg 3, 1031 CM Amsterdam-Noord, The Netherlands Of the various coating materials used for food- or beverage-containing cans epoxy resins are some of the most important. Worldwide they account for about one third of the total coating materials employed in this market. While epoxies are important to the can-coating market, the reverse is also true: In 1976, epoxy resin consumption for can and container coatings was around 10 000 metric tons both in the US (1) and in Western Europe, making this the largest single outlet for epoxies in the entire coating field. Therefore, as a major producer of epoxy resins, we are devoting a considerable effort to this application. One result of this development work is an instrument for accurately measuring a key property of can coatings, the s t e r i l i sation resistance. First, however, a brief description of can coatings and some of their properties is in order. The discussion will be limited to coatings to be applied to the interior of food-or beverage-containing cans since this application requires the highest chemical resistance. Most of the work described has been carried out with can lacquers of the solid epoxy resin/phenolicformaldehyde (E/PF) type as these are the predominant type used in Western Europe. Can Coatings

and Some o f T h e i r P r o p e r t i e s

E/PF can lacquers u s u a l l y are unpigmented and a p p l i e d at dry f i l m thicknesses around 5 (0.2 m i l ) . On s t o v i n g at or above 200 C glos sy, c l e a r coatings with a yellow-golden colour ("gold l a c q u e r s " ) are formed. One o f the most important requirements which can l i n i n g s must meet i s a high s t e r i l i s a t i o n r e s i s t a n c e . This implies that the l i n i n g s should not be damaged by the food contents during a steam s t e r i l i s a t i o n (or p a s t e u r i s a t i o n ) process a p p l i e d a f t e r f i l l i n g and c l o s i n g o f the cans. U n s a t i s f a c t o r y s t e r i l i s a t i o n r e s i s t a n c e u s u a l l y shows up as a haziness or c l o u d i ness i n the c o a t i n g , a phenomenon which i s c a l l e d "blushing". More pronounced b l u s h i n g i s u s u a l l y accompanied by some l o s s o f surface gloss while i n even more extreme cases the coatings become rough and milky-white. Only s l i g h t b l u s h i n g , however, i s s u f f i c i e n t to cause the r e j e c t i o n of the corresponding lacquer by the can-coating i n d u s t r y . 0-8412-0525-6/79/47-114-091$05.00/0 © 1979 American Chemical Society

Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

EPOXY RESIN CHEMISTRY

92 Nature and Assessment o f

Blushing


Some years ago we i n v e s t i g a t e d the nature o f the b l u s h i n g phenomenon. To t h i s end, E/PF can l i n i n g s on t i n p l a t e panels were exposed t o water and steam at 121 °C f o r 90 minutes. The panels were then examined by o p t i c a l and scanning e l e c t r o n microscopy. From t h a t work the f o l l o w i n g conclusions c o u l d be drawn: 1. The b l u s h i n g e f f e c t i s caused by a l a r g e number of voids present throughout the coatings. 2. The i n t e n s i t y o f b l u s h i n g i s p r o p o r t i o n a l t o the number of v o i d s . 3. Most o f the voids are s p h e r i c a l , with a diameter o f around 1 ym (0.0Î+ m i l ) , and t h e i r s i z e d i s t r i b u t i o n i s r a t h e r narrow. k. Voids near the surface tend t o r e s u l t i n bubble and/or c r a t e r formation. Coalescence o f s e v e r a l c r a t e r s may l e a d t o l a r g e r c a v i t i e s . This e x p l a i n s the l o s s o f gloss and the surface roughness observed on h e a v i l y blushed specimens. 5. Blushing only becomes v i s i b l e during c o o l i n g of the t e s t panels t o ambient temperature. Our theory regarding the formation of these voids i s that they are caused by excess water absorbed homogeneously ( i . e . i n u n i t below the l i m i t o f v i s i b i l i t y , CHANGE BACKLENS J SCATTERING ANGLE

DIGITAL VOLTMETER

Figure 1.

System components of experimental apparatus

-SCATTERING PLANE

Figure 2.

Scattering geometry


RAUDENBUSCH ET AL.


8.

Sterilization Resistance of Can Coatings

95

This l a b o r a t o r y set-up permitted accurate v a r i a t i o n s of both the angle of incidence (θ) and the s c a t t e r i n g angle (φ). The t e s t panels were mounted m a g n e t i c a l l y and could be s h i f t e d two-dimens i o n a l l y t o allow scanning of t h e i r e n t i r e surface. With t h i s apparatus a set of 15x6 cm t e s t panels (A-F, 5 ym E/PF coatings on t i n p l a t e , stoved at 200 °C f o r 10 min, and s t e r i l i s e d i n water at 121 C f o r 90 min) showing v a r i o u s degrees o f b l u s h i n g was evaluated. At f i x e d angles o f i n c i d e n c e (θ) the s c a t t e r e d l i g h t was measured over a range of angles φ from 10° t o 80° i n one quadrant of the backward s c a t t e r i n g plane (defined by M, A and Β i n F i g . 2). A t y p i c a l p l o t i s shown i n F i g . 3, from which i t i s c l e a r t h a t , i n accordance with the theory, there i s a b a c k - s c a t t e r i n g maximum l y i n g between 70 and 85°. For t e c h n i c a l reasons we adopted a standard b a c k - s c a t t e r i n g angle φ of 70° i n our subsequent work. In a s i m i l a r manner we v a r i e d the angle of i n c i d e n t l i g h t θ and found a maximum of s c a t t e r i n g at about 60° ( F i g . 3). With a f i x e d o p t i c a l geometry (θ = 6θ°, φ = 70°) the set of t e s t panels A-F was then measured s y s t e m a t i c a l l y . On each panel about 50 i n d i v i d u a l measurements were made at d i f f e r e n t spots, c o v e r i n g the whole surface. The mean instrument readings ( i n mV) correspond very w e l l with the b l u s h l e v e l s determined v i s u a l l y by two expert observers (see the t a b l e below). In p a r t i c u l a r , the b a c k - s c a t t e r i n g method appeared s u f f i c i e n t l y s e n s i t i v e t o d i s t i n g u i s h between the various degrees o f weak b l u s h i n g , the range where a l t e r n a t i v e o p t i c a l d e t e c t i o n methods a l l had f a i l e d . Test panel

No. o f points measured

Mean reading, mV

A

hi

21

Β

kQ

33

C

52

D

1+8

^9 70

Ε

52

F

6k

1U8 61U

Visual blush assessment

Visual rating

h

trace

U-5

5 16

very weak

U

weak-very weak

3-1+

2k

weak

k2

blushing

3 2

113

heavy

1

Standard deviation, mV

In f u r t h e r work the 15 mW l a s e r was r e p l a c e d by a smaller and l e s s expensive one having an output o f only 2 mW. S i m i l a r good c o r r e l a t i o n with v i s u a l assessment of b l u s h i n g was found although the a c t u a l mV readings o f course were lower. Based on the experience gained with the l a b o r a t o r y set-up, a prototype instrument was then b u i l t , the S h e l l "Blushmeter" ( F i g . U). A 2 mW He-Ne l a s e r i s used and the o p t i c a l arrangement i s essen t i a l l y the same as d e s c r i b e d above, with φ and θ f i x e d at 70° and 60°, r e s p e c t i v e l y . F l a t t e s t panels are clamped m a g n e t i c a l l y onto a h o r i z o n t a l sample t a b l e which can be d i s p l a c e d (at 0.2 cm/s) i n the X and Y d i r e c t i o n s by means of two e l e c t r i c stepping motors. D i g i t a l read-out o f the a c t u a l p o s i t i o n permits accurate and repro d u c i b l e scanning o f the e n t i r e panel s u r f a c e . Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

96


INTENSITY, mV 500


r

ANGLE

Figure 3.

Dependence of scattered light intensity on back-scattering angle φ and incident beam angle θ

Figure 4.

Prototype blushmeter: (1) laser; (2) beam expander; (S) mirror; (4) panel on sample table; (5) lens; (6) photoamplifier


8. RAUDENBUSCH ET AL.

97

Sterilization Resistance of Can Coatings

For the time being, scanning i s c a r r i e d out by manual c o n t r o l of the stepping motors; between 10 and 50 i n d i v i d u a l readings are taken over the whole panel s u r f a c e , depending on the u n i f o r m i t y o f the b l u s h i n g e f f e c t , i n order t o obtain a s t a t i s t i c a l l y sound value. Since t h i s i s somewhat time-consuming, the instrument i s c u r r e n t l y being modified t o allow automatic scanning and data processing. T y p i c a l readings obtained with t h i s instrument f o r various degrees o f b l u s h i n g were the f o l l o w i n g :


Visual rating

Blushmeter reading, mV

No b l u s h i n g

5

1-

Very weak b l u s h i n g

h

6- ^k

5

Weak b l u s h i n g

3

28- 35

Blushing

2

6U-

Heavy b l u s h i n g

1

120-13U

Very heavy b l u s h i n g

0

226-278

69

In our experience the prototype "Blushmeter" allows accurate numerical assessment o f both weak and heavy b l u s h i n g . The only requirement f o r a comparison o f the performance o f d i f f e r e n t coatings i s that they be a p p l i e d (at uniform t h i c k n e s s ) t o f l a t substrates o f the same type and q u a l i t y . The operation o f t h e Blushmeter i s simple and does not r e q u i r e s p e c i a l s k i l l s . An i n t e r e s t i n g observation i s that the readings a r e i n s e n s i t i v e t o c o l o u r changes o c c u r r i n g during the s t e r i l i s a t i o n t e s t . In our Amsterdam l a b o r a t o r y the prototype Blushmeter has already helped us t o define c e r t a i n parameters l e a d i n g t o b l u s h i n g i n can lacquers. As a consequence we have been able t o adapt c e r t a i n grades o f s o l i d epoxy r e s i n t o b e t t e r meet the needs o f the cancoating industry.

Abstract Internal coatings for food and beverage cans are an important application for epoxy resins. Such coatings must resist the heat sterilisation (or pasteurisation) process to which the filled cans are eventually subjected. Unsatisfactory sterilisation resistance appears as haziness in the otherwise clear coatings, an effect called "blushing". In typical cases the blushing effect was found to be caused by many small (~1 µm) voids inside the coatings. Blushing is customarily assessed by visual methods that require considerable experience and are difficult to quantify. Based on the back-scattering of laser light, an instrument has been developed with which accurate measurements of various degrees of blushing can be made. A 2 mW laser beam is directed onto a flat test panel at an angle of 60°. Back-scattered light is detected by a silicon photodiode, amplified and read out on a digital voltmeter. The


98


instrument allows reproducible scanning of the whole panel surface. Instrument readings on coatings showing widely differing degrees of blushing agreed well with visual ratings. The instrument permits objective, accurate and reproducible assessments to be made by inexperienced persons. Moreover, instrument readings were found to be insensitive to colour changes frequently occurring during sterilisation.


Acknowledgement The f o l l o w i n g colleagues have contributed t o t h i s work: P. de C a r p e n t i e r , D. Cooke, W. F l e i s c h e r , G.A. Pogany, J.M. Sketchley and F.A. Smid.

Literature Cited 1. Chemical Economics Handbook, Stanford Research Institute, March 1978. 2. Crank, J.; Park, G.S. "Diffusion in Polymers": Academic Press, London, 1968. 3. Pogany, G.A. Polymer, 1976, 17, 690. 4. Born, M.; Wolf, E. "Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light": Pergamon Press, Oxford, 1975. RECEIVED May 21, 1979.


An Instrumental Method for Measuring the ... - ACS Publications

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