What are VCI bags made of, and how is the inhibitor built in?

How is VCI built into a polyethylene bag?

VCI is built into a polyethylene bag in one of two ways: the inhibitor is compounded into the resin and extruded with it, or a resin coating that carries the inhibitor is applied to finished film. In a Cortec patent on biodegradable polyester films, not polyethylene, VCI is first blended into a masterbatch at 10% to 20%, and the masterbatch is diluted with film resin so that the extruded film preferably contains 1% to 3% inhibitor (Cortec, US 6,156,929, 2000). The same patent notes that powder fed straight into the extruder may need a higher percentage than the masterbatch route, which disperses the powder better. In its coated example, 1 mil Bionolle 3001 biodegradable polyester film was coated with 3 to 5 grams per square meter of a VCI formulation in an acrylic latex (Cortec, US 6,156,929, 2000). Read them as one maker's examples, not polyethylene limits.

The inhibitors themselves are salts and small molecules. The patent names amine salts, ammonium benzoate, alkali metal molybdates, alkali nitrites, alkali dibasic acid salts and triazole compounds, with triazole used for copper (Cortec, US 6,156,929, 2000). Once the bag is closed, the inhibitor leaves the film as vapor. Cortec describes the vapor saturating the enclosed airspace within hours and condensing on metal surfaces as a thin protective layer (Cortec, 2014).

Which VCI bag constructions are there?

VCI bag constructions fall into five groups: extruded VCI polyethylene bags, military transparent VCI barrier material, VCI stretch film, VCI bubble film and VCI foil barrier laminates. They share the inhibitor but differ in thickness, cushioning and how well they stop water vapor. The table below shows which construction fits which shipment and what MIL-STD-2073-1E says about each.

The key row for long storage is the foil laminate: its water vapor transmission rate is below 0.0006 g/100 in2/24 h at 73 °F and 90% RH (Cortec, 2017), while an extruded VCI polyethylene bag relies on the inhibitor and an outer container.

Mono-extruded and co-extruded VCI bags differ in build, not in purpose: co-extrusion joins several polymer layers into one film, and one maker offers custom resin blends for puncture resistance or tear strength (Cortec, 2013). VCI stretch film and VCI bubble film are wraps and cushions rather than bags, and the maker describes the inhibitor vapor saturating an enclosed airspace (Cortec, 2014), so they work best inside a closed outer package. The MIL-PRF-22019 row matters when a contract names MIL-STD-2073-1E preservation methods.

How thick should a VCI bag be?

A VCI bag should be thick enough for the part's weight, edges and handling, and published VCI bag films run from about 50 to 150 µm, or 2 to 6 mil (Cortec, 2013). Thickness mainly sets puncture and tear strength; the corrosion protection comes from the inhibitor in a closed package, so gauge alone does not tell you how well a bag protects.

MIL-PRF-22019E uses 0.005 inch (127 µm, 5 mil) as its class line for transparent VCI barrier material. Type I Class 1 heat-sealable material is 0.005 inch or thicker and must resist a puncture of at least 10 lb; Class 2 is thinner and must resist at least 6 lb (MIL-PRF-22019E, 2010). Wrapping films are thinner: one VCI machine-grade stretch film comes in 25, 38 and 50 µm (Cortec, 2019).

A practical reading of these figures, not a rule from any standard: small, smooth parts packed inside a carton sit at the low end of the bag range; heavy parts, sharp edges and bags shipped without an outer box call for the 127 µm Class 1 range or more (MIL-PRF-22019E, 2010), or a foil laminate. Ask the supplier for puncture and tear values measured on the film you will buy, not only the gauge.

Does a VCI bag act as a moisture barrier?

A polyethylene VCI bag slows water vapor but is not a watervaporproof barrier; military packaging gets that protection from a heat-sealed barrier bag instead. One VCI LDPE bag data sheet gives a water vapor transmission of 5 g/m2/24h for a 100 µm sample, without stated test conditions, and strongly recommends indoor storage or external containers such as boxes, cartons or wooden crates (Intertape Polymer Group, 2020).

A VCI foil barrier laminate is far tighter. Its data sheet gives a WVTR below 0.0006 g/100 in2/24 h at 73 °F and 90% RH by TAPPI T557 (Cortec, 2017), which is below 0.01 g/m2/24h by our conversion. The polyethylene figure has no stated test conditions, so compare the order of magnitude, not the exact ratio. The same sheet states that the foil layer prevents the VCI vapor from escaping the package (Cortec, 2017).

MIL-STD-2073-1E keeps the two jobs apart. Method 40 items are heat sealed within a watervaporproof enclosure, Method 41 uses a heat-sealed MIL-DTL-117 bag, and Method 50 adds activated desiccant and a humidity indicator (MIL-STD-2073-1E, 2019). When VCI is used with Method 41, the item is wrapped in VCI-treated material to MIL-PRF-3420 or MIL-PRF-22019 and then enclosed in the barrier bag (MIL-STD-2073-1E, 2019).

VCI bag vs plain polyethylene bag: what is the difference?

The difference between a VCI bag and a plain polyethylene bag is the inhibitor: both are polyethylene film, but the VCI bag releases inhibitor vapor that condenses on the enclosed metal, while a plain bag keeps dust and liquid water off the part. The inhibitor does not turn the film into a watervaporproof barrier, so the VCI bag still needs a closed package.

Four checks follow from that difference. First, the metals: one co-extruded film is sold as a multimetal system (Cortec, 2013) and lists carbon steel, stainless steel, aluminum, copper, brass, galvanized steel and solder among others (Cortec, 2013). Second, compatibility: MIL-STD-2073-1E asks for item compatibility with VCI to be considered under MIL-I-8574 and states, "When selecting use of a VCI, application of a contact preservative is prohibited" (MIL-STD-2073-1E, 2019). Third, the film's own life: one maker gives a shelf life of 2 years for its VCI film (Cortec, 2013), and another 24 months in the original package below 50 °C (Intertape Polymer Group, 2020). Fourth, electronics: antistatic VCI film grades exist (Cortec, 2013), and for electrostatic discharge sensitive items Method 41 names a MIL-DTL-117 Type I Class F bag (MIL-STD-2073-1E, 2019). When your own parts list, data sheets and storage history have to be matched to one construction, Lubinpla can work from those documents and return one cited document for your review.

Related terms

• Volatile corrosion inhibitor (VCI), also vapor phase corrosion inhibitor: a compound that vaporizes and condenses on metal to slow corrosion.

• Masterbatch: a resin concentrate with a high additive content, diluted with plain resin before film extrusion.

• Watervaporproof: the military packaging term for barrier protection against water vapor, used in MIL-STD-2073-1E Methods 40 and 50.

• WVTR: water vapor transmission rate, the mass of water vapor passing through a film area in 24 hours under stated temperature and humidity.

• Co-extrusion: extruding several polymer layers together into one film.