Fluorinated High-Density Polyethylene Barrier Containers - American

Chapter 15

Fluorinated High-Density Polyethylene Barrier Containers Performance Characteristics

Downloaded by UNIV OF PITTSBURGH on June 13, 2016 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch015

J. P. Hobbs, M. Anand, and B. A. Campion Air Products and Chemicals, Inc., 733 Broad Street, Emmaus, PA 18049

Surface fluorination of high density polyethylene (HDPE) is very effective in improving the barrier properties toward hydrocarbon solvents. Containers prepared by in situ fluorination of HDPE were found to have a two-order-of-magnitude decreased toluene permeability, compared to untreated containers, both at room temperature and 50°C. The fluorination process is also effective in eliminating container distortion or paneling which typically occurs in HDPE containers f i l l e d with hydrocarbon solvents. The paneling of untreated HDPE containers is due to a simultaneous deterioration in mechanical properties of the HDPE and the development of a weak vacuum in the containers, both resulting from the toluene sorption by the polymer. Fluorination of HDPE reduces solvent sorption into the container walls, thus preserving the polymer mechanical properties. High density polyethylene (HDPE) i s extensively used in fabricating both industrial and household containers for containing a wide range of chemicals. When such containers are used to package hydrocarbon solvent-based systems, i t i s widely recognized that a barrier treatment (1^1) i s desirable to reduce product loss due to permeation or absorption. This loss of product i s t y p i c a l l y accompanied by container d i s t o r t i o n often referred to as paneling or buckling. This paneling may be manifested as a loss of roundness, or in extreme cases can cause the container to f a l l over, or collapse under top loading. The structural c h a r a c t e r i s t i c s of surface fluorinated HDPE have been studied by several authors ( ^ β ) . Other authors (7-14) have examined the effects of the fluorinated HDPE barrier on the absorption and permeation of organic compounds. Likewise, there i s a large body of information on the sorption, d i f f u s i o n , and permeation of organic compounds in HDPE (15-18) and on the effects 0097-£156/90/0423-O280$06.00/0 © 1990 American Chemical Society

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

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Fluorinated High-Density Polyethylene Barrier Containers 281

of solvent exposure on the morphological structure and physical properties of HDPE (19-21). The fundamental aspects of paneling in HDPE containers caused by the exposure to solvents has not been addressed in the l i t e r a t u r e . The objective of t h i s paper is to elucidate some of the processes that result in paneling, and the role of the AIROPAK Process i n - l i n e fluorinated barrier layer in preventing paneling in HDPE containers.


Experimental Sample Preparation. The experimental containers were 16 oz. c y l i n d r i c a l bottles prepared by extrusion blow molding of Rexene B54-25H grade HDPE resin from Soltex Corp. (0.954 g/cc density). The melt temperature of the extrudate was 210°C and the bottles were formed by i n f l a t i o n of the parison to 0.72 MPa in a 10°C water-chilled mold. The containers, weighing approximately 41.5 grams, had a nominal wall thickness of 0.1 cm and a surface area of 408 cm^. One-half of the containers were prepared untreated using nitrogen as the i n f l a t i o n gas. The other half were in s i t u fluorinated by the AIROPAK Process using a nitrogen-diluted fluorine as the i n f l a t i o n gas (Figure 1), as described by Dixon (7) and K a l l i s h , et al (22). Test Procedures. Blow molded fluorinated and untreated bottles were f i l l e d with 370 grams of reagent grade toluene and heat sealed with a low density polyethylene-coated aluminum s e a l , and capped with a polypropylene closure. Ten sample containers of each type were tested at 50 ± 2°C (50°C) and at room temperature (RT) for permeation and the onset of paneling. Empty containers, both treated and untreated, were also stored with the f i l l e d containers to serve as controls. Over the six-week period of permeation t e s t i n g , sample containers from each type and temperature set were randomly selected and used for t e n s i l e t e s t i n g . The t e n s i l e specimens were prepared and tested following the procedures of ASTM Test Method D1708-79 with the following modifications: (i) the bottles were drained, and f i v e specimens were cut p a r a l l e l to the long axis of the cylinder (machine d i r e c t i o n ) , ( i i ) the specimens were tested immediately after blotting to remove any surface solvent, ( i i i ) a test speed of 5.08 cm/min. (2 i n . / m i n . ) was used, and (iv) the t e n s i l e modulus of e l a s t i c i t y was calculated from the i n i t i a l slope of the load-extension curve. Upon emptying the containers, three additional specimens were cut, blotted on f i l t e r paper to remove surface solvent, weighed and dried at 60°C in an a i r c i r c u l a t i n g oven. The solvent loss of these specimens in (g. toluene/g. dry polymer) was defined as the dynamic toluene sorption for the tested container. Equilibrium toluene sorption was determined at RT and 50°C by immersion to constant weight of replicated cutouts from control b o t t l e s . A d d i t i o n a l l y , t o l u e n e - f i l l e d b o t t l e s , both untreated, and in s i t u f l u o r i n a t e d , were f i t t e d with pressure gauges and monitored for pressure changes in the containers. Also, a set of untreated and in s i t u fluorinated containers was f i l l e d with toluene, but not sealed, and monitored for paneling at RT.



Figure 1. In s i t u f l u o r i n a t i o n of blow molded container by the AIROPAK Process.


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Fluorinated High-Density Polyethylene Barrier Containers


Results and Discussion Permeation Properties. The data shown in Figure 2 are the toluene permeation rates of the fluorinated and untreated containers; g. toluene/container per day are plotted v s . the time of toluene exposure on a logarithmic s c a l e . These cumulative permeation rates were calculated based on the cumulative weight loss over the time of toluene exposure, as opposed to the d i f f e r e n t i a l permeation rates based on the d i f f e r e n t i a l weight loss over each time interval. The room temperature permeation rates for the i n - s i t u fluorinated containers were less than 0.01 g./day; and, hence, have been rounded up to 0.01 g./day for i l l u s t r a t i v e purposes. In Figure 2, t h e , f l a t portion of the curves for the untreated containers yielded the steady state permeation rates. From these values, the permeability c o e f f i c i e n t s (P) for the untreated containers were calculated using Equation 1. p

=

(AW)(L)(22400) (MW)(A)(AP)

(

_

)

cc(STP)cm cm sec.(cm Hg) 2

(1)

where AW i s the steady state permeation rate in grams per second, MW i s the gram molecular weight of the penetrant (92.15 g . / g . mol), L i s the wall thickness (0.07 cm), A i s the surface area of permeation (408 c m ) , and Δ Ρ i s the pressure gradient of the penetrant across the w a l l . Under the test conditions employed, the value of Δ Ρ i s equal to the vapor pressure of toluene at the test temperature. The calculated values of Ρ are shown in Table 1. 2

It may be noted in Figure 2 that the i n - s i t u fluorinated containers exposed to toluene at 50°C appear to be just approaching steady state after 1000 hours of solvent exposure. Thus, the cumulative permeation rate of Figure 2 w i l l result in a s l i g h t underestimation ("10%) of the steady state permeability c o e f f i c i e n t . This underestimation was corrected for in Table 1 by using the d i f f e r e n t i a l weight loss rate in Equation 1. It may be correspondingly noted that the i n - s i t u fluorinated containers exposed to toluene at room temperature would s t i l l be far from steady state after 1000 hours of solvent exposure. The underestimation of the room temperature toluene permeability c o e f f i c i e n t for the i n - s i t u fluorinated containers was minimized in Table 1 by using the d i f f e r e n t i a l weight loss rate in Equation 1. Table I shows that the application of an in s i t u fluorinated barrier has resulted in an approximately two-order-of-magnitude reduction in the steady state toluene permeation rate r e l a t i v e to the untreated containers. Paneling C h a r a c t e r i s t i c s . Indicated by v e r t i c a l arrows in Figure 2 are the times at which the onset of paneling, or buckling, of the untreated containers was observed. The paneling of the untreated containers progressed from a v i s u a l l y and t a c t i l e l y detectable loss of roundness observed between 8 and 16 hours of solvent exposure to a severe d i s t o r t i o n of the sidewalls within 24 hours of f i l l i n g .


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BARRIER POLYMERS AND STRUCTURES


284

Figure 2. Toluene permeation rate for HDPE containers versus solvent exposure time. Paneling time indicated by v e r t i c a l arrows.


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TABLE I.

STEADY STATE PERMEATION


Wt. Loss Rate (g. Toluene/Day) Untreated HDPE RT 50°C Fluorinated HDPE RT 50°C

Toluene Permeability* Coefficient Ρ _ ccl(STP)cm cm sec.(cm Hg) 2

1.3 7.2

2.2 χ 10" 3.7 χ 10"

Fluorinated High-Density Polyethylene Barrier Containers - American

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