In-line fluorinated and ultra-clean HDPE containers for solvent-based, d-limonene-based and ultra-pure liquids

Introduction

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The many functional and economical benefits of using plastic containers in lieu of metal or glass are well documented. The packaging industry is validating these claims through its ongoing trend to use plastic containers to enhance the marketability of old products or to economically and attractively market new products. One technology that has contributed significantly to the introduction of plastic containers in applications previously served by metal or glass is in-line fluorination of blow-molded containers. By using a dilute mixture of fluorine (F2) in nitrogen as a substitute for air as the blow gas, in-line fluorination causes a chemical modification of the surface of the polyolefin container, thereby enhancing the solvent barrier properties of many polyolefins such as HDPE. As a result, a significant reduction in the permeability of most nonpolar solvents is achieved. In-line fluorination enables a packager to use plastic containers such as HDPE to contain solvents that otherwise would require a metal or glass container. It has opened new markets to HDPE and blow molding technologies such as: insecticides, herbicides, cleaning solvents and petroleum- based products.

Permeation Chemistry
In order to appreciate the potential of using HDPE and other polyolefins to contain nonpolar solvents, a brief review of solvent permeation and fluorine chemistry is appropriate. The mechanism for solvent permeation in a polymer is a stepwise process which includes: solvent wetting of the polymer surface, dissolution of the solvent into the polymer, diffusion of the solvent through the polymer, and evaporation of the solvent. Since the chemical structure of certain solvents (nonpolar solvents) is similar to many polyolefin container materials (HDPE, polypropylene, etc.), the solvent and the container are compatible, i.e., the solvent permeates the container walls via surface wetting, solvent dissolution and diffusion. The fluorination process is based on the formation of a fluorocarbon barrier layer on the polymer surface. The basic reaction which occurs with HDPE is a substitution of fluorine in place of hydrogen atoms on the polymer backbone as shown in Figure I.

FIGURE I

The fluorocarbon barrier changes the surface characteristics of the polymer in terms of polarity, cohesive energy density, and surface tension. This in turn has a major effect in reducing the wetting, dissolution, and diffusion of nonpolar solvents relative to the polymer, as shown in Figure II. Thus, fluorination is effective in minimizing the permeability of nonpolar solvents through a polymer surface. Since inline fluorination modifies only those polymer molecules near the surface, there is no measurable change to the bulk properties such as tensile strength and impact resistance.

FIGURE II

In-line Fluorination Process
In-line fluorination is a process adapted to standard blow-molding technology to produce an effective permeation barrier at the inner walls of plastic containers. Unlike post treatment processes, which require container reheating and may result in distortion or dimensional changes, barrier treatment occurs during the normal blow-mold cycle as the container is being formed. The in-line fluorination process results in a permanent chemical modification of the container surface. It does not leave a residue or cause discoloration to the container. The treated surface is highly resistant to deterioration from chemical attack or mechanical abrasion. Where hot filling is possible, the barrier integrity is maintained.

Study of Permeation Barrier Effectiveness

TABLE I
Selected Properties of Various Container Materials
(H=High, M=Medium, L=Low)	HDPE	Metal	Glass	PVC
Weight	L	M	H	L
Cost	L	H	H	H
Stress Crack Resistance	H	H	L	M
Drop Impact Strength	H	M	L	M
Processability/Design Flexibility	H	M	M	M
Solvent Resistance	L	H	H	M
Clarity	L	L	H	H
Moisture Resistance	H	H	H	M

Many container markets have expressed interest in using polymer materials such as HDPE if these materials could provide an effective barrier to solvent permeation. HDPE has numerous desired characteristics when compared to metal, glass and PVC such as Study of Permeation Barrier Effectiveness low cost, light weight, good stress crack resistance, good drop impact strength, and design flexibility, as Table I demonstrates. An effective solvent barrier would make it ideal for packaging solvent based materials.

Therefore, the major emphasis of in-line fluorination has been to enhance the barrier properties of HDPE. The studies that are reported herein examine the effectiveness of in-line fluorination to reduce solvent permeability in treated HDPE containers when compared to untreated (air-blown) HDPE containers.

TABLE II
Permeation Test Data for Hydrocarbon-based Solvents packaged in HDPE
	Control Container % Weight Loss	Airopak Products Container % Weight Loss	Relative Barrier Improvement
Carbon Tetrachloride	28.26	0.05	565
Pentane	98.10	0.21	467
Hexane	61.29	0.19	323
Heptane	24.26	0.08	303
Xylene	42.52	0.21	203
Iso-Octane	4.54	0.03	151
Cyclo-Hexane	22.34	0.15	149
Toluene	61.90	0.52	119
Paraxylene	59.20	0.54	110
1,3,5 Trimethyl-benzene	15.85	0.18	88
Trichloroethylene	5.70	0.30	19
Benzene	36.68	3.65	10
Chlorobenzene	32.05	5.41	6
1,2 Dichloreathane	11.55	2.89	4

A generally accepted method to study solvent permeation involves high temperature exposure of contained material over a set time period to examine the amount of solvent loss due to permeation in that time. Known amounts of solvents are contained in a barrier treated container and an un-treated (air-blown) container. These containers are then exposed to a temperature of approximately 50°C (122°F) for 28 days, which is representative of the permeation that can be expected after 180 days of normal shelf-life. After 28 days, a comparison of percent weight loss in the treated and untreated containers will provide an indication of permeation barrier effectiveness. Table II lists the results of permeation studies performed for various common chemicals.

The data in Table II provides strong support of the effectiveness of in-line fluorination in reducing the permeability of hydrocarbon-based solvents in HDPE containers. The 15 solvents listed are found in a large percentage of industrial, agricultural and household products. Thus, not only is fluorinated HDPE suitable for packaging of these basic solvents, but any commercial formulation that contains these or similar solvents may be amenable to packaging in a container produced by in-line fluorination.

It should be noted that combinations of certain chemicals in a specific formulation may cause a reduction in the effectiveness of a fluorinated barrier. Therefore, it is recommended that all formulations be thoroughly tested by the method previously described prior to the final choice of packaging material.

End Use Applications

TABLE III
Permeation Test Data for Commercial Products Packaged in HDPE
	Container % Weight Loss
	Control Container	Airopak Products
Pesticides
Lasso®	16.41	0.13
Raid®	13.42	0.43
Ortho Crab Grass Killer®	0.62	0.22
Thiodan®	26.12	0.35
Super BK32®	6.66	0.11
Kelthane®	15.30	0.21
Borer Spray®	13.91	0.13
25% Diazonon®	10.92	0.23
Cygon 2-E®	8.23	0.24
50% Malathion®	3.99	0.26
Banvel	1.73	0.04
Carbyne	4.24	0.09
Chlordane®	1.25	0.05
Killmaster®	26.90	0.09
Termide®	2.95	0.08
Petroleum-Based Products
Gulflite™ Charcoal Lighter Fluid	4.65	0.13
Kerosene	9.21	0.38
Paint-Related Products
Lacquer Thinner	16.50	3.80
Mineral Spirits	15.07	0.12
Varnish	6.78	0.01
Turpentine	3.92	0.00
Automotive-Related Products
Engine-Cleaner	2.30	0.00
Snowmobile Motor Oil	0.30	0.00
2-Stroke Plus Motor Oil	7.10	0.44
STP® Gas Treatment	9.10	0.00
Miscellaneous Cleaning and Household Products
Bruce One-Step Floor Cleaner	24.55	0.20
Pine Sol	6.57	0.40
General Pine Oil Cleaner	6.30	0.00
Pledge®	2.67	0.34
Butyl Caulking	1.70	0.00

In-line fluorination is used to economically produce millions of blow-molded containers annually for use in the packaging of various products. Applications that have produced the greatest demand for fluorinated containers are: insecticides, herbicides, petroleum-based products such as gasoline and kerosene, cleaning solvents, automotive additives, penetrating oils, and paint-related products. Table III lists an assortment of consumer-related products that have proven amenable to Airopak products. Test data is included as evidence of the reduction in solvent permeation that is provided by treated Airopak products versus the permeation that occurs in an untreated container. HDPE containers were used to contain the solvents at 50°C for 19 to 28 days.

In the pesticide market, both agricultural and home use products were tested. Agricultural-grade products such as LASSO® and DIAZINON ® showed permeation losses in untreated HDPE on the order of 10% to 16%. The use of a container produced by in-line fluorination reduced these losses to below 0.25%, a reduction of over 98% in permeability losses. Home use products such as ORTHO® and RAID® produced similar results. Another significant difference between the treated and untreated containers was in the degree of distortion to the container wall, or paneling, which is a secondary effect of diffusion of solvent through the container walls. This phenomena is exhibited in Figure III. The untreated HDPE containers panel severely over time due to solvent permeation into the container walls. The paneling has a detrimental effect on container aesthetics and, in turn, consumer acceptance, since the container appears damaged and often has an oily surface. In contrast, the barrier treated containers exhibited minimal or no paneling compared to untreated containers. Thus, in-line fluorination maintains fill volume as well as container aesthetics over prolonged shelf life.

Petroleum-based products such as charcoal lighter fluid and kerosene were found to permeate untreated HDPE. In-line fluorination reduced permeation of these products by 96% or more. Five different blends of gasoline were also tested to examine the amenability of in-line fluorination for fuel tank applications and to determine if different combinations of gasoline additives have a detrimental effect on barrier performance. Varying amounts of additives such as methanol, ethanol, water, and toluene were tested. In all cases the HDPE container produced by in-line fluorination outperformed an untreated HDPE container.

FIGURE 3

Various paint sundries such as thinner and turpentine tend to be packaged in metal containers because they permeate HDPE readily. By containing various paint related products in barrier treated containers, solvent weight loss was significantly reduced and container paneling was virtually eliminated.

Automotive additives and motor oils also permeate HDPE readily and as a result, paneled, oily containers can be found on store shelves. In-line fluorination was found to reduce permeation of various automotive products by greater than 94%, eliminating container paneling.

The permeability of several other household items such as floor cleaner and pine oil was examined to determine if a fluorinated HDPE container could replace the metal or glass container presently in use. Of the 5 items tested, each permeated untreated HDPE containers. But blow molded containers produced by the in-line fluorination process were found to virtually eliminate solvent permeation (< 0.40%).

Many of the products listed in Table III are now commercially available in NOVAPAK's Airopak products. Others are undergoing further analysis by the manufacturer. A considerable number of other products are packaged in containers produced by in-line fluorination, but permeation data was unavailable for publication.

Many major chemical manufacturing companies are users of NOVAPAK's Airopak containers, and the list is rapidly growing every day, as the economical and functional benefits of fluorinated containers become evident.

Summary
In-line fluorination of blow molded containers provides a proven solvent barrier to HDPE containers. The total volume of solvents packaged in NOVAPAK's Airopak containers continues to increase as fluorinated HDPE containers are chosen for new solvent based products and as a substitute for metal and glass containers. The in-line fluorination process allows for production of blow-molded containers with barrier properties approaching that of metal and glass at a cost competitive with other packaging alternatives.