Barrier Polymers and Structures - American Chemical Society

Chapter 11

Performance of High-Barrier Resins with Platelet-Type Fillers Effects of Platelet Orientation

Downloaded by PURDUE UNIV on November 3, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch011

T. C. Bissot Polymer Products Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, DE 19880-0323

The use of platelet-type f i l l e r s , preferably fine particle size mica, in ethylene vinyl alcohol (EVOH) copolymers increases oxygen barrier performance approximately threefold. The benefit is ascribed to the increased diffusion path length (tortuous path) produced by the overlapping platelets. Typical polymer processing operations used in packaging lead to the preferred orientation with the platelets aligned parallel to the surfaces. An exception was found in a blow molded bottle test where nonparallel alignment gave minimum barrier improvement. This result is explained by the mismatched die swell between the structural and barrier layers of the multilayer parison. Changes in polymer rheology or the type of machine reduced or eliminated the problem and gave the expected barrier performance.

P l a s t i c s continue to expand into food packaging applications t r a d i t i o n a l l y served by metal and glass containers. The oxygen b a r r i e r properties of the p l a s t i c food container i s frequently a major consideration affecting i t s s u i t a b i l i t y for a s p e c i f i c application. Polymers used i n packaging films and containers can be c l a s s i f i e d by their relative permeation to oxygen. Of the many classes of polymers used i n packaging, only three can be designated as high barrier materials, i . e . , those having an oxygen permeation value of less than 1 cm -mil/100 in -day-atm (Table I ) . 3

2

0097-6156/90/0423-0225$06.00Α) © 1990 American Chemical Society

In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

226

BARRIER POLYMERS AND

Table I.

EVOH EVOH PVDC AN -

Melt Processible

Resin - 30 mole % E - 44 mole % Ε - high barrier extrudable copolymer (BAREX)

3

STRUCTURES

High Barrier Resins Oxygen Permeation Value* Dry, 25°C 80% RH, 25°C "TOI ΓΠ .09 .24 .15 .15 .8 .8

2

*cm 0 -mil/100 in -day-atm


2

Among the high barrier resins, the ethylene v i n y l alcohol (EVOH) copolymers are showing the most rapid growth. Familiar containers relying on EVOH oxygen barrier are the squeezable ketchup bottle and the single serving, shelf-stable and microwaveable entree container. Introductory guides to the selection and use of f i l l e r s (1,2) generally s t a r t by c l a s s i f y i n g f i l l e r s according to shape classes such as spheres, cubes, blocks, flakes and f i b e r s . The f i r s t three classes have shape or aspect ratios close to 1 and, therefore, cannot display orientation i n polymer systems. Only flakes (or p l a t e l e t s ) and f i b e r s (needles) with s i g n i f i c a n t aspect ratios can show orientation. The most common reasons for adding f i l l e r s , e s p e c i a l l y high aspect r a t i o f i l l e r s , to polymers i s to improve physical properties such as increased modulus (stiffness) or reduced creep. In addition to t h i s major use to improve mechanical properties, high aspect r a t i o flake-type f i l l e r s have been added to polymers for a v a r i e t y of other purposes. They include improved thermal s t a b i l i t y (3), high voltage resistance (4), e l e c t r i c a l conductivity, radiation shielding (5) and o p t i c a l and aesthetic e f f e c t s (6). High aspect r a t i o flakes or p l a t e l e t s have also been previously used to improve the gas barrier properties of low and medium b a r r i e r polymers (7,8). The d i f f u s i o n of small molecules i n a polymer f i l m can be described by Fick's f i r s t law of d i f f u s i o n J = -Ddc/dx

(1)

which simply states that the flux of a gas permeating the f i l m at a constant concentration or pressure d i f f e r e n t i a l w i l l be inversely proportional to the distance the d i f f u s i n g gas must t r a v e l . The addition of p l a t e l e t f i l l e r s distributed i n the polymer f i l m can greatly increase t h i s d i f f u s i o n distance by creating a tortuous path for the d i f f u s i n g species. A recent paper by Cussler, et a l . , (9) uses the more picturesque term "wiggle length" to describe segments of t h i s pathway. Several attempts have been made i n the l i t e r a t u r e to model t h i s system of p l a t e l e t s dispersed i n a polymer f i l m i n order to predict the r e l a t i v e reduction i n flux i n a p l a t e l e t f i l l e d system, J / J , where J i s the flux measured with an u n f i l l e d f i l m and J i s the flux measured with the p l a t e l e t - f i l l e d f i l m . Q

o

n


n


11. BISSOT

227

High-Barrier Resins wUh Platelet-Type Fillers

An e a r l y model p r o p o s e d b y P r a g e r (10) assumed random o r i e n t a t i o n o f f i l l e r s o f v a r i o u s shapes. H i s p r e d i c t i o n s showed t h a t w h i l e p l a t e l e t s would g i v e a g r e a t e r improvement t h a n c y l i n d e r s o r s p h e r e s , t h e r e l a t i v e improvement w i t h a 20 volume % l o a d i n g o f randomly o r i e n t e d p l a t e l e t s would y i e l d o n l y about a 40% r e d u c t i o n in permeability. T h i s would be a J / J o f 1.67. T h e r e have a l s o been models based"on v e r y r e g u l a r a r r a y s o f t h e p l a t e l e t s d i s t r i b u t e d i n a f i l m matrix. An e a r l y model p r o p o s e d b y B a r r e r (11,12) modeled the system a s a u n i f o r m d i s p e r s i o n o f a l a t t i c e o f r e c t a n g u l a r p a r a l l e l e p i p e d s . U s i n g t h i s model, Murthy (13) c a l c u l a t e d t h a t a l a r g e r e d u c t i o n i n p e r m e a t i o n c o u l d be a c h i e v e d w i t h a p l a t e l e t f i l l e r volume o f 20%. A sample c a l c u l a t i o n from d a t a i n the l a t t e r paper p r e d i c t e d t h a t 20 volume % o f a p l a t e l e t f i l l e r h a v i n g a l o n g d i m e n s i o n o f 5 m i c r o n s and a t h i c k n e s s o f 0.1 m i c r o n would y i e l d 144 l a y e r s o f p l a t e l e t s i n a 50 m i c r o n membrane. There would be a s p a c i n g o f .32 m i c r o n between l a y e r s o f p l a t e l e t s w i t h a 1.0 m i c r o n s e p a r a t i o n between the r e c t a n g u l a r p l a t e l e t s i n a l a y e r . The c a l c u l a t e d r a t i o o f the d i f f u s i o n c o e f f i c i e n t o f the f i l l e d composite t o t h a t o f the m a t r i x i s 0 . 0 4 . This represents a J / J o f 25. A r e c e n t paper b y C u s s l e r , e t a l . , (9) u s i n g a r e g u l a r a r r a y model a l s o p r e d i c t e d q u i t e h i g h b a r r i e r improvements. T h e i r equa t i o n ( E q u a t i o n 2) p r e d i c t e d t h a t a s p e c t r a t i o and volume f r a c t i o n o f f i l l e r would be the major v a r i a b l e s . They i n c o r p o r a t e d a u n i v e r s a l c o r r e c t i o n designated, as a geometric f a c t o r t o c o r r e c t f o r t h e r e a l i t y t h a t a v a i l a b l e p l a t e l e t f i l l e r s were n o t shaped l i k e u n i f o r m r e c t a n g u l a r p a r a l l e l e p i p e d s o f u n i f o r m s i z e and shape. 0

n

J /J o

2

n

2

= 1 + /να (φ /(1-Φ))

(2)

S u b s t i t u t i n g v a l u e s i n the C u s s l e r e q u a t i o n , w h i c h we w i l l show l a t e r t o be i n a p r a c t i c a l range, o f 12 volume % l o a d i n g ( φ ) o f a f i l l e r h a v i n g a n a s p e c t r a t i o o f 23 («) and assuming a n i d e a l g e o m e t r i c f a c t o r o f 1 would p r e d i c t a r e l a t i v e b a r r i e r improvement, J / J o f 10. ο η There a r e v e r y few r e p o r t s i n the l i t e r a t u r e documenting the improvement i n gas b a r r i e r p r o p e r t i e s t h a t can be a c h i e v e d u s i n g commercially a v a i l a b l e p l a t e l e t - t y p e f i l l e r s i n melt processed polymer f i l m s and none showing the improvement i n t h a t c l a s s o f polymers p r e v i o u s l y i d e n t i f i e d as h i g h b a r r i e r polymers. Some o f the b e s t d a t a on a p r a c t i c a l system i s t h a t o f Murthy, e t a l . , (13) based on b l e n d s o f t a l c i n p o l y e t h y l e n e and n y l o n films. T h e i r d a t a demonstrated r e l a t i v e b a r r i e r improvements ( J / J ) o f 1 . 8 - 1 . 9 u s i n g f i l l e r l o a d i n g s i n a p r a c t i c a l range o f about 12 volume %. o

n

EXPERIMENTAL There a r e b a s i c a l l y o n l y t h r e e t y p e s o f p l a t e l e t - t y p e f i l l e r s w h i c h can be c o n s i d e r e d f o r u s e i n t h i n b a r r i e r f i l m s . These a r e aluminum f l a k e , mica and t a l c ( T a b l e I I ) . Other t y p e s o f p l a t e l e t s , such a s g l a s s , s t a i n l e s s s t e e l o r b r a s s f l a k e s and c e r t a i n aluminum s i l i c a t e m i n e r a l s , such a s k a o l i n c l a y , a r e e i t h e r too l a r g e i n p a r t i c l e s i z e o r have t o o low an a s p e c t r a t i o t o be u s e f u l . W i t h these t h r e e


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BARRIER POLYMERS AND STRUCTURES

preferred f i l l e r s , comparable p a r t i c l e size products can be obtained by s e l e c t i o n of appropriate grinding and size c l a s s i f i c a t i o n techniques. The 7 micron average p l a t e l e t diameter i s a good median p a r t i c l e size for blending into polymers designed for extrusion into t h i n f i l m s and layers i n coextrusions.

Table I I .


Type Supplier Grade Screen Analysis Average Part Diameter //m (D)

P l a t e l e t Thickness /ym (T) Aspect Ratio D/T Density, g/cc Price, $/lb Price/EVOH Volume Equivalent

Representative Platelet-Type F i l l e r s

A l Flake Reynolds Metal Co. 40XD 99.5%

Barrier Polymers and Structures - American Chemical Society

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