Want in on the MAP action?

You have to know your resin choices and how to combine them into 'breathable' films before you can bite into the hot growth market for modified-atmosphere packaging.

* Innovative resins have created a new food-packaging market of "breathable" films for fresh-cut produce. Also known as modified-atmosphere packaging, or MAP, these films let growers offer consumers precleaned, cut, and ready-to-eat fresh produce. But engineering films for MAP is not simple. It requires understanding the biology of how a particular type of produce "breathes." Then that biological knowledge must be matched with plastics know-how to select the right resin or combination of resins and put them together in a cost-effective blend or multi-layer structure.

MAP is radically different from traditional packaging for fresh produce, which is simply well ventilated. Instead, MAP hermetically seals the cut and washed produce, reducing its respiration rate by controlling the transfer of gases in and out of the package. MAP's goal is to slow the aging of produce by reducing respiration without cutting it off entirely.

How produce breathes

Produce continues to breathe after it is harvested, consuming oxygen and giving off carbon dioxide and water. MAP film should reduce, but not completely inhibit, respiration. It requires selective barrier properties that provide controlled oxygen transmission rate and controlled oxygen content inside the package, so that a limited amount of aerobic respiration can occur.

If there's no oxygen present, anaerobic respiration takes place, followed by rapid spoilage. So a fine-line exists between extending shelf life and creating an anaerobic atmosphere in which produce suffocates. That's why high-barrier packages, which prevent most transmission of gases, aren't suitable for long-term packaging of fresh produce.

Among the factors that complicate MAP technology is that different types of produce have different respiration rates. For example, not only do lettuce and apples breathe differently, but romaine lettuce breathes faster than iceberg lettuce, and Red Delicious apples faster than Granny Smiths.

Besides respiration, other chemical reactions occur in cut produce. Some fruits give off ethylene, which accelerates ripening in some cases.

The U.S. Department of Agriculture and many university food-science departments can provide information on post-harvest physiology and can verify the respiration rate of produce in a temperature-controlled chamber where actual gas exchange is measured.

MAP is a complex package

There are many variables to consider when designing a film for MAP. First, understand the produce and package requirements. What mechanical strength, oxygen transmission rate, optics, hot-tack strength, and heat-seal initiation temperature are needed? These properties must be considered in relation to the dimensions of the package - volume, surface area, and the weight of its contents.

MAP packages are typically designed around the correct oxygen transmission characteristics. But it is also good to make sure the C[O.sub.2] transmission rate is right for that produce so that C[O.sub.2] does not build up. Elevated C[O.sub.2] levels can have desirable effects - slowing down respiration, inhibiting growth of microbes, and even killing fungus in some foods - but excessive C[O.sub.2] can also damage some foods. Also, moisture transmission must be considered to ensure that the produce does not dry out.

It's also important to anticipate storage conditions for the package, since temperature is the key to respiration rate. At room temperature, most produce respires rapidly. At 40 F refrigeration temperature, it respires much less. Temperature also affects gas transmission rates through plastic films. Every 1 [degrees] C drop in the film's temperature reduces its oxygen transmission rate by 6%. But standard commercial laboratory equipment is designed to test [O.sub.2] transmission of film samples only at room temperature. Some resin companies and universities can test film permeability at low temperatures.

Film used in automatic packaging lines also requires sufficient modulus, proper coefficient of friction, and low seal initiation temperature for increased line speeds, as well as high hot-tack strength and seal integrity. Retail sale also requires high gloss, low haze, good laminated print quality, and antifogging characteristics. Other requirements include no off odor or taste.

So many resin choices

Potential resin choices for MAP films include polyolefin plastomers (POPs), EVAs, ultra-low-density polyethylene (ULDPE), LLDPE, LDPE, polypropylene, and styrene-butadiene (SB) copolymers. Films can be monolayer, coextruded, or laminated, and any layer can involve blends of different resins. Total thickness of the film and of individual layers are key performance factors. Slip, antiblock, and antifog additives are frequently needed, but may affect other properties. Slip, for example, may reduce toughness.

Polymers that provide high oxygen permeability for produce with high respiration rates include POP, EVA, and ULDPE. Their [O.sub.2] transmission ranges from 500 to about 1500 cc/mil per 100 sq. in. per day at 25 C and one atmosphere.

* POPs, which are made using a single-site catalyst, feature narrow molecular-weight distribution and uniform incorporation of-octene, butene, or hexene comonomer. They can also have long-chain branching for improved processability. POPs offer the highest oxygen transmission rate and the lowest heat-seal initiation temperature of any polymer. They also offer very high seal strength, low moisture transmission rate, excellent optics, and toughness.

As comonomer content of POPs increases, crystallinity and density decrease. And as crystallinity and density decrease, oxygen permeability increases, allowing it to be precisely tuned by controlling density. As density drops, optics improve, and melting point, heat-seal initiation temperature, and modulus decrease.

Disadvantages of POPs are tackiness, low modulus, and higher cost. The cost can be moderated by blending 10-40% of LDPE or LLDPE with POP in a monolayer film or heat-seal layer of a multilayer film. But slip and antiblock will still be needed to counteract stickiness.

* EVA copolymers are typically blended into MAP films because of their high oxygen transmission rates, low seal initiation temperature, and good optics. As with POPs, increasing comonomer (VA) content correlates with decreasing crystallinity, increasing oxygen transmission, and lower melting point, seal initiation temperature, and modulus. Density, however, increases with higher comonomer content.

EVAs have disadvantages in taste and odor, as well as lower hot-tack strength than POPs and higher water-vapor transmission. EVAs also suffer from tackiness and low modulus.

* ULDPEs are copolymers of ethylene and one or more comonomers such as octene, hexene, or butene. Better suited to MAP than EVAs, ULDPEs offer advantages of high oxygen transmission rate, low heat-seal initiation temperature, high seal strength, good optics, low moisture transmission rate, and excellent toughness. ULDPEs, however, lack uniform molecular weight and comonomer distribution. They have low modulus, which limits the speed at which they run in downstream equipment, such as vertical form-fill-seal lines. They're also tacky and have poorer optics than EVAs or POPs.

A number of other polymers are often used as ingredients in coextrusions, blends, or laminations for MAP. SB copolymers, PP, LLDPE, and LDPE, have only moderate oxygen permeability but offer other benefits. Polystyrene and HDPE are also occasionally used in MAP to increase stiffness or heat resistance.

* SB copolymers offer higher modulus, excellent optics, and a crinkly feel, which communicates "freshness" to the consumer. But their high modulus makes them prone to stress cracking. They are also somewhat harder to process than polyolefins. Multilayer scrap containing SB resin can't be reclaimed, so that adds cost.

* Polypropylene, often used for produce packaging in the past, has the best heat resistance among resins with moderate oxygen permeability. It also has very high modulus and low cost. PP's oxygen transmission rate is suitable for slow-respiring vegetables but is too low for many kinds of produce. PP also has poor melt strength, and in blown film shows poor optics and poor low-temperature toughness.

* LLDPEs are popular in MAP films, generally to improve heat resistance in blends and coextrusions with EVA or POP. However, LLDPE requires higher heat-seal temperatures than do ULDPEs, EVAs, or POPs.

Alternative structures

Blown, cast, and biaxially oriented films are all used in MAP. Choices depend on film properties, cost, and customer preference. Blown film is generally the most cost-effective method. Monolayer films - which can be complex blends of ingredients-are the least expensive option for basic applications. However, their limited seal performance, limited range of oxygen transmission, and inferior print quality make monolayer films a poor choice for highly decorative retail packaging. Monolayer films, however, can be used in laminated structures.

Cast films can be oriented in the machine direction or biaxially to provide greater strength and clarity. Then they can be laminated.

Both coextrusion (typically of three to seven layers) and lamination permit more complex functionalities. Each layer of a structure can also contain multiple ingredients. Laminated MAP films have better optics than coextruded or monolayer films because lamination enables printing to be trapped under a clear gloss layer. This protects the print and enhances its sharpness. Lamination also allows you to combine materials that are not compatible for coextrusion. However, the additional processing step adds cost and complexity, and can result in variations in oxygen transmission that may be difficult to predict or control.

Oxygen transmission through multilayer films is controlled primarily by the resin with the lowest permeability. Overall permeability and other properties can be controlled by adjusting thickness ratios of the different layers. Less breathable layers are usually kept thin. For example, a MAP film might have 20% of a PP layer for stiffness and 80% of a POP layer with higher oxygen transmission.

Choosing from all these options ultimately depends on the needs of the application. Appearance is extremely important in retail fresh-cut packaging. For best optics, the choices might be oriented PP laminated to EVA or POP. Another option would be SB copolymer coextruded with a POP blend. For more utilitarian food-service packaging, high toughness is more important than cosmetics. Suitable MAP structures might be coextruded LLDPE/plastomer or PP/plastomer. A monolayer film could be of ULDPE or ULDPE/EVA blend.

Any performance criterion for MAP can be attained by different routes. For example, a 2-mil film could have 0.2 mils of PP (for heat resistance and stiffness) coextruded with 1.8 mils of LDPE (for bulk and sealability). That structure would have the same oxygen transmission rate as a 2-rail film consisting of 1 mil each of PP and POP. The latter film would be stiffer, more machinable, and more heat resistant. It would also have lower seal-initiation temperature, higher seal strength, and better optics.