Originally Published January 2001

Robert Kelch

High-frequency (HF) sealing technology has long been recognized for producing superior weld integrity in a variety of critical applications. As a result, the medical industry has used HF sealing—which is also known as radio-frequency (RF) sealing or dielectric sealing—for the manufacture of fluid-delivery and fluid-collection bags, inflatable devices such as air mattresses and air splints, and for sealing of both flexible and rigid packaging. Flexible poly- vinyl chloride (PVC) has been the predominant HF-weldable film substrate for many years. Recently, however, there has been a growing desire in the medical industry for alternative plastic films that could be sealed with HF energy. Thermoplastic polyurethane (TPU) and ethylene-vinyl acetate (EVA) resins have increasingly been used to produce films that could be readily HF welded.

Collection bag used to develop HF-weldable polyolefin films.

While both TPU- and EVA-based films contain no halogen or added plasticizer and exhibit lower density than PVC, which results in more film area per kilogram yields, both also have limitations. TPU films exhibit excellent strength, toughness, and high water-vapor permeability, as well as rapid HF seal rates. However, TPU films are generally from three to six times more expensive than PVC films on a weight basis, and may be over-designed for some applications. EVA films offer a 25% yield improvement due to their lower density compared with PVC, but typically exhibit much lower HF activity than PVC films and thus take longer to HF seal or weld. Increasing the percentage of polar vinyl-acetate comonomer in the EVA will increase HF activity, although this typically reduces the melting point of the EVA resin and film, reduces the film strength, and increases the tackiness or tendency of the film to block.

Coextruded polyolefin films have been developed in recent years to replace PVC for medical infusion bags and blood storage containers. These films are typically based on coextrusions of polyolefin skins—such as polypropylene (PP) or linear-low-density polyethylene (LLDPE)—with HF-active core layers, such as polyester (PET) or nylon. Additional PP films comprising blends or coextrusions with styrene-ethylene/butylene- styrene (SEBS) or TPU have also been developed.^1–4 Although these films exhibit good physical properties, barrier properties, HF weldability, and, in some cases, autoclaveability, they have apparently not yet found widespread commercial use because of their high cost.

The development of metallocene single-site-catalyst technology over the past decade has resulted in the production of polyolefin resins with very low densities and narrow molecular-weight distributions.⁵ Films made from these resins have demonstrated high toughness, impact strength, clarity, elasticity, and heat sealability, which make them quite suitable for many medical device and packaging applications.^6,7 These nonpolar resins, however, have insufficient dielectric properties to render them HF active, although they can be readily heat sealed. As a result, the metallocene films inherently cannot be sealed with existing HF welding equipment. It is possible to HF weld these metallocene polyethylene (mPE) films using "catalyst" or buffer technology, wherein a reusable HF-active catalyst film is employed to essentially convert HF welding equipment into a thermal sealer so that the metallocene film can be welded.⁸

It was the desire to achieve the performance properties of polyethylene films with HF activity comparable to that of flexible PVC that has led to the development of a new family of HF-weldable polyolefin films. The term "family" is used to indicate that a variety of films—including different structures (asymmetric and symmetric film structures), different film chemistries, and different performance attributes—are being developed to meet a multitude of end-use applications. The films described in this article are developmental products that are available from The Dow Chemical Co. (Midland, MI), and will be commercialized as Covelle HF-weldable polyolefin films. Additional films with property characteristics similar to those of TPU, as well as rigid film and sheet with characteristics similar to those of glycol-modified polyester (PETG), are also being developed so as to provide a full complement of HF-active products.

POLYMER PROPERTY EVALUATION

The potential of a material to be HF weldable is based on properties inherent in the polymer. During HF welding, high- frequency electromagnetic energy in the form of an alternating electric field will cause polar molecules to oscillate very rapidly. This molecular motion can result in molecular friction, with the resulting generation of heat. If sufficient heat is generated within the polymer, the material will melt.⁹ The amount of heat generated by the HF energy is determined by the polymer dielectric loss factor (DLF), which can be used as a screening tool in evaluating resins, blends, and compounds for HF weldability.

In general, materials with a DLF of less than approximately 0.05 are considered to be HF inactive, while those with a DLF from 0.05 to approximately 0.1 are considered to be weakly HF active. Materials with a DLF greater than 0.1 are HF weldable, with those exhibiting values greater than approximately 0.2 being very active, and thus potentially quite weldable. Although the high-frequency range is typically considered to encompass from 0.1 to 10,000 MHz, the predominant frequencies of interest in sealing fall within the RF range of roughly 1 to 300 MHz, with most commercial sealers operating at 27.12 MHz.^9,10

During the course of the investigation described in this article, the relevant properties of many materials were evaluated to determine their potential as HF-weldable film components. Additional consideration regarding film structure and issues of composition design versus cost was required in order to balance performance properties with commercial viability. Current film development reported herein was directed toward achieving HF weldability greater than that of EVA and comparable to that of PVC and TPU films.

HF-WELDABLE FILM DEVELOPMENT

One-Side-Weldable (Asymmetric) Film. Initial HF-weldable polyolefin films were developed for a medical collection-bag application. The goal was to produce a polyolefin film with HF weldability at a cost comparable to that of commercial flexible PVC films that were being utilized in collection-bag manufacture. A commercially available EVA-based film that was being converted into collection bags was also used for comparative purposes.

The initial film developed was a multilayered film with one-side HF weldability. This asymmetric film structure requires that two plies of film (from two rolls of film) be brought together with the seal side of one ply toward the seal side of the second ply, such that the two film plies can be HF welded together (Figure 1). This 4.0-mil (100-mm) film, denoted as HF Polyolefin (HF PO), exhibited physical properties comparable to those of the 4.0-mil (100-mm) PVC film and the 6.0-mil (150-mm) EVA film that were being used in collection-bag structures (Table I). The asymmetric HF polyolefin film exhibited much lower water-vapor permeability (better water-vapor barrier) than both the EVA and PVC films and lower oxygen permeability (better oxygen barrier) than the EVA film. Although the HF PO film was the same gauge as the flexible PVC, the olefin exhibited an approximately 25% reduction in density, or 25% improvement in yield per square meter.

Figure 1. HF welding of one-side-weldable films.

Table I. Comparative properties between HF-weldable polyolefin, PVC, and EVA films.

The HF polyolefin film was successfully run on a medical collection-bag welding line at lower power and faster rates than the EVA-based film. The line was fitted with a slightly heated bag die and included a tube-welding attachment. With a slightly lower power setting on the HF welding generator, a 20% reduction in seal time versus EVA film was achieved. In addition, the plate (or anode) current reading was approximately 20% lower with the HF PO film versus EVA.

The finished bags, shown on page 82, passed all standard quality control tests. No air leakage at sealed seams or at the tube seal was observed when the bags were inflated with air to 125 mbar (1.8 psi) and immersed in water; the inflated bags were also subjected to a 75-kg platen-press burst test, without rupture of the inflated bag. The bags then underwent ethylene oxide (EtO) sterilization prior to testing according to the ISO 8669-2 urine collection-bag standard. (Although the film can be sterilized by EtO, it is not autoclave or steam sterilizable.) Bags made from the HF PO film passed all prescribed test requirements. The outer surface of the film was successfully printed on a commercial flexographic printing line using conventional inks suitable for polyolefin printing.

Several one-side HF-weldable polyolefin films have now been developed. One such film is described in Table II as XU 66130. The toughness and high tear strength of the HF PO film and XU 66130 result in less "tear-sealability" versus PVC and EVA films at welded edges. In an effort to develop better tear-seal characteristics and lower film modulus, the XU 66127 film was developed for general-purpose packaging structures (Table II). Both XU 66130 and XU 66127 films have asymmetric structures and are one-side HF weldable or sealable—that is, HF energy will only activate one side of the film to provide weldability.

Table II. Physical properties of XU 66130 and XU 66127 polyolefin packaging films.

A 2.0-kW Callanan HF sealer fitted with a 0.125 x 8-in. brass bar seal die operating at ambient room temperature (72°F) was used to seal two 7.0-mil plies of film together, with the adhesive side sealed to the adhesive side of the adjacent ply. Using a 1.0-second seal time with 50% power setting resulted in weld strengths in excess of 8.5 lb/in. when tested according to ASTM D903, with films breaking prior to bond failure. It should be noted that all films can also be thermally welded with conventional heat-seal techniques.

A very-low-modulus, "soft" HF-weldable film was developed for applications needing greater flexibility and improved tear-seal properties (Table III). XU 66133 is a coextruded film based on metallocene polyethylene and exhibits many of the properties of MDF 7200 and similar metallocene films. XU 66133 features a 2% secant modulus that is about one-third the value of previously discussed films, as well as very good impact strength. The film exhibits good clarity, although somewhat lower barrier properties than films such as XU 66127 and XU 66130. HF welding using a 2.0-kW Callanan HF sealer resulted in excellent weld strength in as little as 1.0 second, with adhesion strength in excess of 8.0 lb/in. During HF welding, the film demonstrates good tear-seal properties when appropriate tooling dies are used.

Table III. Physical properties of XU 66133 low-modulus, soft HF-weldable film.

Two-Side Weldable (Symmetrical) Film. In many cases, it is desirable to be able to weld both sides of a film, either as a film-to-film weld or for attaching fitments to the film. A two-side, HF-active film was developed to provide this versatility, which is typically inherent in monolayer EVA and PVC films. The XU 66126 film is a symmetrical film, with both sides exhibiting good HF weldability as well as thermal sealability (Table IV). The film can be directly fabricated into bag or packaging applications. In addition, the film has been formulated such that both sides can be thermally adhered to a variety of other substrates when heated above approximately 212°F (100°C), the minimum adhesive-activation temperature.

Table IV: Physical properties of XU 66126 polyolefin packaging and lamination film.

Conventional hot-roll lamination, flame lamination, or heated- press lamination can be used to thermally bond the XU 66126 film to substrates such as paper or cellulosics; polyolefins; most polar polymers such as nylon, urethane, or polyester; nonwovens or fabrics; and cellular-foam materials (Figure 2). Thinner film gauges (2.0-mil, 50-mm) can be first laminated to a porous or breathable substrate such as a nonwoven, cloth fabric, or foam in such a fashion as to melt the film sufficiently to develop porosity, which can be beneficial when moisture or air breathability is desired. Thicker films can be used to laminate if a nonporous barrier is desired. At ambient room temperatures, the film exhibits low tack and little blocking (or cling).

Figure 2. Thermal lamination of film to substrate.

Non-HF-active substrates or materials that otherwise have no melt-weldable characteristics can be thermally laminated with HF-active adhesive to permit welding. A spunbonded, polypropylene nonwoven of 1.0 oz/square yard (34 g/m²) basis weight was thermally laminated with 2.0-mil XU 66126 adhesive film at a temperature of 300°F (149°C). The film was well adhered to the nonwoven and attempts to peel the film from the spunbonded nonwoven resulted in cohesive destruction of the nonwoven. Similarly, a soft, open-cell polyurethane foam of 0.04 g/cm³ density was laminated with 2.0-mil XU 66126 adhesive film. The film also exhibited good adhesion to the foam, and attempts to delaminate or peel the film from the foam resulted in cohesive failure (or destruction) of the foam.

Both laminates were then subjected to HF welding using a 2-kW Callanan sealer fitted with a nonheated (ambient room temperature) 0.5 x 8-in. brass flat seal die. Two plies of each laminate were sealed, with the film side to the film side of the respective materials (Figure 3). As described in Table V and shown in Figure 4, HF welds were achieved for each of the three laminate combinations. By comparison, even with longer weld times, neither the original PP spunbonded nonwoven nor the polyurethane foam substrates (which had not been laminated with the adhesive film) could be HF welded.

Figure 3. Laminate structures of non-HF-active substrates with XU 66126 HF-weldable film.

 
Table V. Laminate HF welding results. Comparison of PP nonwoven and PU foam substrates with and without prelamination of XU 66126.

Figure 4. HF-welded laminate structures.

A similar evaluation was made using paper. A photocopy grade of coated paper of 20-lb bond weight was thermally laminated with 2.0-mil XU 66126 adhesive film at a temperature of 230°F. The 2-kW Callanan HF press was used to seal different composites of the adhesive film–laminated paper and the original nonlaminated paper (Table VI). HF seals were readily accomplished with a ply of laminated paper sealed to a ply of nonlaminated paper, and also with two plies of film- laminated paper sealed together. Weld times of 0.7 seconds were achieved. By comparison, paper is not HF weldable, as it cannot melt or fuse. HF welding of nonlaminated paper actually resulted in burning of the paper.

Table VI. Laminate HF welding results. Comparison of paper substrate with and without prelamination of XU 66126.

The use of HF-active adhesive lamination films that can be applied to another substrate by heat or HF, and then subsequently reactivated by heat or HF, provides great potential in the medical device, apparel, textile, and packaging industries. In such a manner, non-HF-responsive materials can be welded together. The low melting-activation temperature of the XU 66126 allows it to be thermally applied to one surface of many substrates without thermal degradation, melting, or deorientation of the substrate. As HF energy is subsequently applied through the entire composite structure, the HF will generally not affect the non-HF-active substrate, thus concentrating the HF energy and thermal generation at the film-to-film interface where it is desired to cause bonding. By comparison, conventional heat sealing of similar substrates would require heat energy to be passed through the entire thickness of the substrate, thus potentially damaging, degrading, or melting the material.

In the lamination trials cited above, neither the spunbonded PP nonwoven textile nor the polyurethane foam were distorted or melted. Indeed, attempts to heat seal these materials together with sufficient temperature to affect melting at the interface would result in melting of the entire substrate thickness.

Pouch Testing of Film HF Weldability. Various developmental films were subjected to independent testing by an industry consultant to evaluate their HF weldability compared with that of commercially available films of other polymers, including flexible PVC, EVA, ethylene-methyl acrylate (EMA), and TPU. A 4-kW Thermex Thermatron welder fitted with a one-liter rectangular pouch (bag) die and preheated to 150°F (65°C) was used in the study. All HF welding- process parameters were kept constant except for power level and HF seal time, which were varied in order to obtain optimum welds. The power setting was initially started at typical PVC seal conditions, with the power level gradually increased in order to achieve a strong pouch seal. Optimum conditions were determined by taking each welded pouch and manually inflating it with an 80-psi air hose to determine if any leakage or debonding occurred. If the pouch exhibited good edge or seam bonding, then the power setting was maintained and the seal time reduced until minimum weld parameters (optimum in terms of shortest time and lowest power to achieve a strong pouch seal) were obtained.

Comparison of optimum weld parameters of six developmental weldable polyolefin films (5.0- and 7.0-mil XU 66127, 7.0- and 10.0-mil XU 66133, and 2.0- and 5.0-mil XU 66126) and five different commercially available films (6.0-mil flexible PVC "A," 11.0-mil flexible PVC "B," 4.5-mil TPU, 11.5-mil EVA, and 7.5-mil EMA) is provided in Table VII. The XU developmental films required less power than the TPU, EVA, and EMA films and lower current than all of the comparative films in order to achieve a strong bond. While PVC films required slightly lower power settings than did the developmental HF-weldable XU films, all of the XU films sealed in equal or less time compared with PVC, with less current flow required. The EVA film necessitated the longest weld time (6 seconds total) of any film, and required significant power and current to achieve a good weld. The TPU film was quite HF active, and could be sealed in only 2 seconds total seal time. The EMA film was also sealed readily in 2 seconds, but required the greatest power and current flow through the plate of any film to achieve this seal.