Barrier polymers Polyamide, EVOH and PVDC

Polyamide

Polyamides (PA) or nylons are condensation polymers, linear, thermoplastic polyamides that contains amide group as a recurring part of the polymer chain.  In general they are clear, thermoformable, strong and tough over a broad range of temperatures.

They can be made from the condensation of diamines and dibasic acids or from the condensation of amino-acids which contain both amine and acid functional groups in a single molecule.  Nylons are identified by numbers corresponding to the number of carbon atoms in the monomers. Two numbers are needed in the case of the condensation of diamines and dibasic acids and only one number indicates how many C atoms are in the amino acid.  For example:

 

Number of C atoms in the diamine                6	Number of C atoms in the diacid 6	Name of the nylon Nylon 66
Number of C atoms in the amino acid 6		Name of the nylon Nylon 6

 

Polyamides (PAs) nylon 6 and nylon 66 are widely used as gas barrier materials in flexible packaging applications.  They are not often used as a gas barrier in rigid plastics packaging applications.  The gas permeability of nylon 6 and nylon 66 increases at high humidity.  They are also chemical resistant raw materials and suitable for solvent barrier applications.  Therefore, both nylon 6 and nylon 66 are used in multilayer HDPE bottles for agricultural chemicals and other solvent based chemicals. 

 

Polyamides have strong intermolecular forces.  These forces are combined with crystallinity to yield tough, high melting thermoplastic materials (For example, Nylon 66 has a melting point of 269°C).

Nylons can be processed using conventional extrusion process.  Films can be produced either the cast film or blown film processes.  During the film production, different degrees of crystallinity are obtained depending on the temperature and rate of quenching.  When the cooling rate is increased, a less crystalline nylon is obtained since the polymer was not given sufficient time to form crystals.  The decrease in crystallinity produces a more transparent and more thermoformable film.  Biaxial orientation of nylon films provides better crack resistance, mechanical and barrier properties.     

Blow molding is used to produce industrial containers, fuel tanks and oil reservoirs, as well as some other containers.  Thermoformed polyamides are also used for disposable pharmaceutical packs, and meat and cheese packaging.

 

Polyamides are used in coextrusion with other plastic materials.  Polyolefins are commonly used in the coextrusion processes to provide heat sealability and moisture barrier, and also to reduce cost.  Multilayer films containing nylon layer are used in vacuum-packing of processed meats.  Also, PVDC coating on PA is available for improved gas barrier properties.  PA is also used to extrusion coat paperboard to get heavy duty paperboard. 

 

 

EVOH

Ethylene vinyl alcohol (EVOH) is produced by a controlled hydrolysis of ethylene vinyl acetate copolymer.  The highly polar OH groups increase the intermolecular forces, while the ethylene groups maintain molecular mobility.  The polymer has randomly distributed ethylene and vinyl alcohol units.  The ratio of ethylene to vinyl alcohol determines the end product characteristics.  The lowest ethylene or the highest vinyl alcohol content can be used to get very high barrier properties whereas the highest ethylene content or the lowest vinyl alcohol leads to better flexibility.  Processors should decide the desired processing characteristics in order to decide on the ethylene/vinyl ratio that is suitable to their needs.

 

 

 

Structure of EVOH

 

EVOH is a moisture sensitive polymer.  The presence of water decreases the oxygen barrier properties of the material.  Therefore, EVOH is usually incorporated into packaging structures as a buried inner layer in a coextrusion surrounded by polyolefins or other good water vapor barrier polymers.  These structures contain an adhesive or tie layer between the EVOH and the polyolefin to provide adequate adhesion between EVOH and polyolefin. 

 

 

Properties of some EVOH copolymers

 

 

Property	EVOH 32 %, Ethylene	EVOH 44%, Ethylene
Density, g/cm3	1.19	1.14
Tensile Strength, MPa	88	68
Tm, °C	181	164
Tg, °C	70	55
Heat Seal Temperature, °C	179-238	177-238
Oxygen Permeability, cm3 µm2 day atm 0% RH 65% RH	0.004 0.013	0.24 0.045
WVTR, g µm/m2 day at 38°C 90% RH	2.5	2.8

 

Processing and applications of EVOH

Processing method	Sample application	Sample structure
Cast coextrusion	Processed meat, cheese	PET/EVOH/EVA
Blown extrusion	Red meat	LLDPE/EVOH/LLDPE
Lamination	Condiments	OPP/EVOH/LDPE
Coextrusion coating	Aseptic packaging	LDPE/paperboard/EVOH/Ionomer
Thermoforming	Yogurt	PP/EVOH/PP
Coextrusion blow molding	Ketchup	PET/EVOH/PET

 

PVDC

Polyvinylidene chloride (PVDC) is a transparent and almost colorless polymer.  PVDC can be obtained directly ethylene and chlorine or by the further chlorination of vinyl chloride with removal of hydrogen chloride by alkali treatment.  It is polymerized in suspension or emulsion processes.  The chemical formula CH₂=CCl₂, polymer repeating unit structure -[CH₂-CCl₂-]

Vinylidene chloride homopolymers and copolymers were first produced as Saran, a registered trademark of Dow Chemical.  Their low water vapor and gas permeability make them ideal for food packaging.  The polymers are based on vinylidene chloride (VCD) and comonomers such as vinyl chloride, methyl acrylate and vinyl nitrile. 

 

PVDC has a melting point around 400°C but it decomposes at 205°C, producing hydro chloric acid HCl in a manner similar to PVC.  Therefore, these conditions make PVDC homopolymer impossible to melt process.  By adding comonomers, the melting point is decreased to a range of 140-175°C, making melt processing feasible. 

Addition of comonomers also reduces crystallinity and the crystalline melting point, permitting processing at lower temperatures.  For example, vinyl chloride and methyl acrylate are used for extrudable resins, in amount from 6 – 28 %.  All commercially available PVDC resins are copolymers.           

 

The main applications of PVDC are in food packaging as barrier materials to moisture, gases, flavors and odors.  PVDC is also used in pharmaceutical and cosmetic packaging.  The structures containing 10 – 20 % of PVDC copolymer are used as shrinkable films.  PVDC is also used as a barrier material in thermoforming applications.    

 

 

 

 

 

Bariyer polimerleri Poliamid, EVOH ve PVDC

Poliamid

Poliamidler (PA) ya da naylonlar polimer zincirinin tekrarlanan bölümü olarak amid grubu içeren kondenzasyon yolu ile sentezlenmiş lineer, termoplastik polimerlerdir.  Bunlar genellikle berraktırlar ve ısıyla şekillendirilebilirler.  Güçlüdürler ve geniş bir sıcaklık aralığında sağlam kalırlar.

PA’lar diaminlerin ve dibazik asitlerin reaksiyona girmeleriyle, ya da tek bir molekülde hem amin hem de asit fonksiyonel gruplarını içeren aminoasitlerin reaksiyona girmesiyle elde edilebilirler.  Naylonlar monomerlerdeki karbon atomlarının sayısını ifade eden sayılarla isimlendirilirler.  Diaminlerin ve dibazik asitlerin bir araya gelmesiyle oluşanlar, iki ayrı rakamla ifade edilirken; sadece bir rakamla isimlendirilen naylonlar, üretimleri için kullanılan aminoasit kökünde kaç tane C atomu olduğunu gösterir.  Örneğin:

 

Daimindeki C atomu sayısı 6	Diasitteki C atomu sayısı 6	Naylonun adı Naylon 66
Aminoasitteki C atomu sayısı         6		Naylonun adı Naylon 6

 

Naylon 6 ve naylon 66 poliamidleri, esnek ambalaj imalatında, gaz bariyeri olarak sıkça kullanılırlar.  Sert plastik ambalaj uygulamalarında ise pek tercih edilmezler.  Naylon 6 ve naylon 66’nın gaz geçirgenliği nemli ortamlarda artar.  Bunlar aynı zamanda kimyasallara dirençli hammaddelerdir ve solvent geçirgenliğini engellemek için uygundurlar.  Bu nedenle, hem naylon 6 hem de naylon 66 zirai kimyasalların ve diğer solvent bazlı kimyasalların konduğu çok katmanlı HDPE şişelerde kullanılmaktadırlar.

 

Poliamidlerin molekülleri arasındaki kuvvetler güçlüdür. Bu kuvvetler kristal yapı ile birleşince ortaya erime noktası yüksek, katı termoplastik malzemeler çıkar (örneğin, Naylon 66’nın erime noktası 269°C’dir).

 

Naylonlar geleneksel ekstrüzyon prosesi ile işlenebilirler.  Filmler cast film ya da şişirme yöntemi ile üretilebilir.  Film üretimi sırasında, elde edilen kristallik derecesi soğutma sıcaklığına ve hızına bağlı olarak değişebilir.  Soğutma hızı artınca, daha az kristalli bir naylon elde edilir çünkü polimerin kristal oluşturacak zamanı olmaz.  Kristallikteki azalma ile daha şeffaf ve ısıyla daha kolay şekillendirilebilen bir film elde edilir.  Naylon filmlerin çift yönde gerdirilmesiyle yırtılmaya daha dirençli, iyi mekanik özellikler ve bariyer özellikleri elde edilir.    

 

Şişirme ile kalıplama yöntemi ile, endüstriyel konteynır, yakıt tankı ve petrol rezervuarı ve benzeri ambalajların imalatında kullanılmaktadırlar.  Isıyla şekillendirilen (termoform) poliamidler tek kullanımlık ilaç ambalajlarında ve et ve peynir gibi ürün ambalajlarında da kullanılırlar.

 

Poliamidler diğer plastik hammaddelerle koekstrüzyon prosesi ile birleştirilirler.  Poliolefinler koekstrüzyon süreçlerinde sızdırmazlık ve nemi engelleme özelliği sağlamaya ve maliyeti azaltmaya yarar. Naylon katman içeren çok katlı filmler işlenmiş etlerin vakumla ambalajlanmasında kullanılır. Ayrıca, PA üzerine PVDC kaplama yapılarak gazı engelleme özellikleri iyileştirilebilir. PA ile ayrıca karton üzerine ekstrüzyonla kaplama yapılarak ağır hizmet kartonu elde edilir. 

 

 

EVOH

Etilen vinil alkol (EVOH) etilen vinil asetat kopolimerinin kontrollü hidrolizi ile üretilir. Polar özelliği güçlü olan OH grupları, moleküllerin arasındaki kuvvetleri arttırırken; etilen grupları moleküler hareketliliği sağlar. Polimerde rasgele dağılmış etilen ve vinil alkol birimleri vardır.  Etilenin vinil alkole oranı son ürünün özelliklerini belirler.  En düşük etilen ya da en yüksek vinil alkol içeriği kullanılarak çok güçlü bariyer özellikleri elde edilebilinir, öte yandan en yüksek etilen içeriği ya da en düşük vinil alkol içeriği esnekliği arttırır. Üreticiler kendi ihtiyaçlarına uygun etilen/vinil oranını bulmak için istenen işlenme özelliklerine karar vermelidir.

 

EVOH’un yapısı

 

EVOH neme duyarlı bir polimerdir. Suyun varlığı malzemenin oksijeni engelleme gücünü zayıflatır. Bu nedenle, EVOH ambalaj yapılarında genellikle poliolefinlerle veya su buharını engelleyen diğer polimerlerle çevrelenmiş koekstrüzyon içinde kalmış, bir iç katman olarak ambalaja dahil edilir.  Bu yapılarda EVOH ile poliolefin arasında bulunan yapışkan katman ya da bağ katmanı EVOH ile poliolefinin birbirine yeterince yapışmasını sağlar. 

 

Bazı EVOH kopolimerlerinin özellikleri:

 

Özellik	EVOH %32, Etilen	EVOH %44, Etilen
Yoğunluk, g/cm3	1.19	1.14
Gerilme Direnci, MPa	88	68
Tm, °C	181	164
Tg, °C	70	55
Isıl Yapışma Sıcaklığı, °C	179-238	177-238
Oksijen Geçirgenliği, cm3 µm2 gün atm % 0  bağıl nem % 65 bağıl nem	0.004 0.013	0.24 0.045
Su buharı geçirme oranı, 38°C % 90 bağıl nemde, g µm/m2 gün	2.5	2.8

 

Proses ve ürün uygulamaları:

Proses	Örnek uygulama	Örnek yapı
Cast koekstrüzyon	İşlenmiş et, peynir	PET/EVOH/EVA
Şişirmeli koekstrüzyon	Kırmızı et	LLDPE/EVOH/LLDPE
Laminasyon	Soslar	OPP/EVOH/LDPE
Koekstrüzyonla kaplama	Aseptik ambalaj	LDPE/karton/EVOH/Ionomer
Isıyla şekillendirme (termoform)	Yoğurt	PP/EVOH/PP
Koekstrüzyonlu şişirmeli kalıplama	Ketçap	PET/EVOH/PET

 

PVDC

Poliviniliden klorür (PVDC) şeffaf ve neredeyse renksiz bir polimerdir. PVDC doğrudan etilenden veya klordan elde edilebilir ya da alkali işlemiyle hidrojen klorür giderilerek vinil klorürün tekrar klorlanmasıyla elde edilebilir.  Süspansiyon veya emülsiyon prosesleri ile polimerleşebilir.  Kimyasal formül CH₂=CCl₂ dir ve polimerin tekrarlayan birimi  ise -[CH₂-CCl₂-] dir.

 

Viniliden klorür homopolimerler ve kopolimerler ilk kez Dow Chemical firmasının tescilli markası Saran olarak üretilmiştir.  Su buharını ve gazı fazla geçirmedikleri için gıda ambalaj uygulamalarında idealdirler.  Kopolimerler ise viniliden klorür (VCD) ve vinil klorür, metil akrilat ve vinil nitrat gibi komonomerlerden oluşmaktadırlar. 

 

PVDC’nin erime noktası 400°C civarıdır ama 205°C’de ayrışarak PVC’ye benzer biçimde hidroklorik asit (HCl) açığa çıkarır.  Bu nedenle, bu koşullarda PVDC homopolimerini erime sürecinde kullanmak imkansızdır. Komonomerler eklenerek erime noktası 140 – 175°C aralığına düşürüldükten sonra erime süreci mümkün olur. 

 

Komonomerlerin eklenmesi kristalliği azalttığı ve kristalin erime noktasını düşürdüğü için düşük sıcaklıklarda işlem yapmak mümkün olur. Örneğin, vinil klorür ve metil akrilat kalıptan geçirilebilinen reçinelerde % 6 ile 28 oranında kullanılır. Piyasada bulunan bütün PVDC hammaddeleri kopolimerlerdir.         

  

Nemi, gazları, tatları ve kokuları engelleyen bir malzeme olarak PVDC’nin başlıca uygulamaları gıda ambalajlarıdır.  PVDC ayrıca ilaç ve kozmetik ambalajlarında da kullanılır.  % 10 – 20 oranında PVDC kopolimer içeren yapılar büzülebilen film uygulamalarında kullanılırken; ısıyla şekillendirme – termoform uygulamalarında bariyer malzemesi olarak da kullanılmaktadırlar.

 

 

 

 

 

 

Packaging Requirements for Fresh Fruits and Vegetables



Packaging fresh fruits and vegetables is one of the most important steps in the long and complicated journey from grower to consumer.  Bags, crates, hampers, baskets, cartons, bulk bins and palletized containers are convenient containers for handling, transporting, and marketing fresh produce.  More than 1,500 different types of packages are used for produce in developed countries and the number continues to increase as the industry introduces new packaging materials and concepts. Although the industry generally agrees that container standardization is one way to reduce cost, the trend in recent years has moved toward a wider range of package sizes to accommodate the diverse needs of wholesalers, consumers, food service buyers, and processing operations.

Packing and packaging materials contribute a significant cost to the produce industry; therefore it is important that packers, shippers, buyers, and consumers have a clear understanding of the wide range of packaging options available. This fact sheet describes some of the many types of packaging, including their functions, uses, and limitations. Also included is a listing, by commodity, of the common produce containers standard to the industry.

A significant percentage of produce buyer and consumer complaints may be traced to container failure because of poor design or inappropriate selection and use. A properly designed produce container should contain, protect, and identify the produce, satisfying everyone from grower to consumer.

Containment

The container must enclose the produce in convenient units for handling and distribution. The produce should fit well inside the container, with little wasted space. Small produce items that are spherical or oblong (such as potatoes, onions, and apples) may be packaged efficiently utilizing a variety of different package shapes and sizes. However, many produce items such as asparagus, berries, or soft fruit may require containers specially designed for that item. Packages of produce commonly handled by hand are usually limited to 50 pounds. Bulk packages moved by fork lifts may weigh as much as 1,200 pounds.

Protection

The package must protect the produce from mechanical damage and poor environmental conditions during handling and distribution. To produce buyers, torn, dented, or collapsed produce packages usually indicate lack of care in handling the contents. Produce containers must be sturdy enough to resist damage during packaging, storage, and transportation to market.

Because almost all produce packages are palletized, produce containers should have sufficient stacking strength to resist crushing in a low temperature, high humidity environment. Although the cost of packaging materials has escalated sharply in recent years, poor quality, lightweight containers that are easily damaged by handling or moisture are no longer tolerated by packers or buyers.

Produce destined for export markets requires that containers to be extra sturdy. Air-freighted produce may require special packing, package sizes, and insulation. Marketers who export fresh produce should consult with freight companies about any special packaging requirements. Additionally, state export agencies may be able to provide specific packaging information.

Damage resulting from poor environmental control during handling and transit is one of the leading causes of rejected produce and low buyer and consumer satisfaction. Each fresh fruit and vegetable commodity has its own requirements for temperature, humidity, and environmental gas composition. Produce containers should be produce friendly - helping to maintain an optimum environment for the longest shelf life. This may include special materials to slow the loss of water from the produce, insulation materials to keep out the heat, or engineered plastic liners that maintain a favorable mix of oxygen and carbon dioxide.

Identification

The package must identify and provide useful information about the produce. It is customary (and may be required in some cases) to provide information such as the produce name, brand, size, grade, variety, net weight, count, grower, shipper, and country of origin. It is also becoming more common to find included on the package, nutritional information, recipes, and other useful information directed specifically at the consumer. In consumer marketing, pack- age appearance has also become an important part of point of sale displays.  Bar codes may be included as part of the labeling

Types of Packaging Materials

Pallets

Literally form the base on which most fresh produce is delivered to the consumer. Pallets were first used during World War II as an efficient way to move goods.  Because many are of a non-standard size, the pallets are built as inexpensively as possible and discarded after a single use. Although standardization efforts have been slowly under way for many years, the efforts have been accelerated by pressure from environmental groups, in addition to the rising cost of pallets and landfill tipping fees.

Standardization encourages re-use, which has many benefits. Besides reducing cost because they may be used many times, most pallet racks and automated pallet handling equipment are designed for standard-size pallets. Standard size pallets make efficient use of truck and van space and can accommodate heavier loads and more stress than lighter single-use pallets. Additionally, the use of a single pallet size could substantially reduce pallet inventory and warehousing costs along with pallet repair and disposal costs. The adoption of a pallet standard throughout the produce industry would also aid efforts toward standardization of produce containers.

Depending on the size of produce package, a single pallet may carry from 20 to over 100 individual packages. Because these packages are often loosely stacked to allow for air circulation, or are bulging and difficult to stack evenly, they must be secured (unitized) to prevent shifting during handling and transit. Although widely used, plastic straps and tapes may not have completely satisfactory results. Plastic or paper corner tabs should always be used to prevent the straps from crushing the corners of packages.

Plastic stretch film is also widely used to secure produce packages. A good film must stretch, retain its elasticity, and cling to the packages. Plastic film may conform easily to various size loads. It helps protect the packages from loss of moisture, makes the pallet more secure against pilferage, and can be applied using partial automation. However, plastic film severely restricts proper ventilation. A common alternative to stretch film is plastic netting, which is much better for stabilizing some pallet loads, such as those that require forced-air cooling. Used stretch film and plastic netting may be difficult to properly handle and recycle.

A very low-cost and almost fully automated method of pallet stabilization is the application of a small amount of special glue to the top of each package. As the packages are stacked, the glue secures all cartons together. This glue has a low tensile strength so cartons may be easily separated or repositioned, but a high shear strength so they will not slide. The glue does not present disposal or recycling problems.

Wooden Baskets and Hampers

Wire-reinforced wood veneer baskets and hampers of different sizes were once used for a wide variety of crops from strawberries to sweetpotatoes. They are durable and may be nested for efficient transport when empty. However, cost, disposal problems, and difficulty in efficient palletization have severely limited their use to mostly local grower markets where they may be re-used many times.

Corrugated Fiberboard

Corrugated fiberboard is manufactured in many different styles and weights. Because of its relativity low cost and versatility, it is the dominant produce container material and will probably remain so in the near future. The strength and serviceability of corrugated fiberboard have been improving in recent years.

Most corrugated fiberboard is made from three or more layers of paperboard manufactured by the kraft process. To be considered paperboard, the paper must be thicker than 0.008 inches. The grades of paperboard are differentiated by their weight (in pounds per 1,000 square feet) and their thickness. Kraft paper made from unbleached pulp has a characteristic brown color and is exceptionally strong. In addition to virgin wood fibers, Kraft paper may have some portion of synthetic fibers for additional strength, sizing (starch), and other materials to give it wet strength and printability. Most fiberboard contains some recycled fibers. Minimum amounts of recycled materials may be specified by law and the percentage is expected to increase in the future. Tests have shown that cartons of fully recycled pulp have about 75 percent of the stacking strength of virgin fiber containers. The use of recycled fibers will inevitably lead to the use of thicker walled containers.

Double-faced corrugated fiberboard is the predominant form used for produce containers. It is produced by sandwiching a layer of corrugated paperboard between an inner and outer liner (facing) of paper-board. The inner and outer liner may be identical, or the outer layer may be preprinted or coated to better accept printing. The inner layer may be given a special coating to resist moisture. Heavy-duty shipping containers, such as corrugated bulk bins that are required to have high stacking strength, may have double- or even triple-wall construction. Corrugated fiberboard manufacturers print box certificates on the bottom of containers to certify certain strength characteristics and limitations. There are two types of certification. The first certifies the minimum combined weight of both the inner and outer facings and that the corrugated fiberboard material is of a minimum bursting strength. The second certifies minimum edge crush test (ETC) strength. Edge crush strength is a much better predictor of stacking strength than is bursting strength. For this reason, users of corrugated fiberboard containers should insist on ECT certification to compare the stackability of various containers. Both certificates give a maximum size limit for the container (sum of length, width, and height) and the maximum gross weight of the contents.

Both cold temperatures and high humidities reduce the strength of fiberboard containers. Unless the container is specially treated, moisture absorbed from the surrounding air and the contents can reduce the strength of the container by as much as 75 percent. New anti-moisture coatings (both wax and plastic) are now available to substantially reduce the effects of moisture.

Waxed fiberboard cartons (the wax is about 20 percent of fiber weight) are used for many produce items that must be either hydrocooled or iced. The main objection to wax cartons is disposal after use— wax cartons cannot be recycled and are increasingly being refused at landfills. Several states and municipalities have recently taxed wax cartons or have instituted rigid back haul regulations. Industry sources suggest that wax cartons will eventually be replaced by plastic or, more likely, the use of ice and hydrocooling will be replaced by highly controlled forced-air cooling and rigid temperature and humidity maintenance on many commodities.

In many applications for corrugated fiberboard containers, the stacking strength of the container is a minor consideration. For example, canned goods carry the majority of their own weight when stacked. Fresh produce usually cannot carry much of the vertical load without some damage. Therefore, one of the primarily desired characteristics of corrugated fiberboard containers is stacking strength to protect the produce from crushing. Because of their geometry, most of the stacking strength of corrugated containers is carried by the corners. For this reason, hand holes and ventilation slots should never be positioned near the corners of produce containers and be limited to no more than 5 to 7 percent of the side area.

Interlocking the packages (cross stacking) is universally practiced to stabilize pallets. Cross stacking places the corner of one produce package at the middle of the one below it, thus reducing its stacking strength. To reduce the possibility of collapse, the first several layers of each pallet should be column stacked (one package directly above the other). The upper layers of packages may be cross stacked as usual with very little loss of pallet stability.

There are numerous styles of corrugated fiberboard containers. The two most used in the produce industry are the one piece, regular slotted container (RSC) and the two piece, full telescoping container (FTC). The RSC is the most popular because it is simple and economical. However, the RSC has relatively low stacking strength and therefore must be used with produce, such as potatoes, that can carry some of the stacking load. The FTC, actually one container inside another, is used when greater stack- ing strength and resistance to bulging is required. A third type of container is the Bliss box, which is — constructed from three separate pieces of corrugated fiberboard. The Bliss box was developed to be used when maximum stacking strength is required. The bottoms and tops of all three types of containers may be closed by glue, staples, or interlocking slots.

Almost all corrugated fiberboard containers are shipped to the packer flat and assembled at the packing house. To conserve space, assembly is usually performed just before use. Assembly may be by hand, machine, or a combination of both. Ease of assembly should be carefully investigated when considering a particular style of package.

In recent years, large double-wall or even triple- wall corrugated fiberboard containers have increasingly been used as one-way pallet bins to ship bulk produce to processors and retailers. Cabbage, melons, potatoes, pumpkins, and citrus have all been shipped successfully in these containers. The container cost per pound of produce is as little as one fourth of traditional size containers. Some bulk containers may be collapsed and re-used.

For many years, labels were printed on heavy paper and glued or stapled to the produce package. The high cost of materials and labor has all but eliminated this practice. The ability to print the brand, size, and grade information directly on the container is one of the greatest benefits of corrugated fiberboard containers. There are basically two methods used to print corrugated fiberboard containers:

Paper and Mesh Bags. Consumer packs of potatoes and onions are about the only produce items now packed in paper bags. The more sturdy mesh bag has much wider use. In addition to potatoes and onions, cabbage, turnips, citrus, and some specialty items are packed in mesh bags. Sweet corn may still be packaged in mesh bags in some markets. In addition to its low cost, mesh has the advantage of uninhibited air flow. Good ventilation is particularly beneficial to onions. Supermarket produce managers like small mesh bags because they make attractive displays that stimulate purchases.

However, bags of any type have several serious disadvantages. Large bags do not palletize well and small bags do not efficiently fill the space inside corrugated fiberboard containers. Bags do not offer protection from rough handling. Mesh bags provide little protection from light or contaminants. In addition, produce packed in bags is correctly perceived by the consumer to be less than the best grade. Few consumers are willing to pay premium price for bagged produce.

Plastic Bags. Plastic bags (polyethylene film) are the predominant material for fruit and vegetable consumer packaging. Besides the very low material costs, automated bagging machines further reduce packing costs. Film bags are clear, allowing for easy inspection of the contents, and readily accept high quality graphics. Plastic films are available in a wide range of thicknesses and grades and may be engineered to control the environmental gases inside the bag. The film material "breathes" at a rate necessary to maintain the correct mix of oxygen, carbon dioxide, and water vapor inside the bag. Since each produce item has its own unique requirement for environmental gases, modified atmosphere packaging material must be specially engineered for each item. Research has shown that the shelf life of fresh produce is extended considerably by this packaging. The explosive growth of precut produce is due in part to the availability of modified atmosphere packaging.

In addition to engineered plastic films, various patches and valves have been developed that affix to low-cost ordinary plastic film bags. These devices respond to temperature and control the mix of environmental gases.

Shrink Wrap. One of the newest trends in produce packaging is the shrink wrapping of individual produce items. Shrink wrapping has been used successfully to package potatoes, sweetpotatoes, apples, onions, sweet corn, cucumbers and a variety of tropical fruit. Shrink wrapping with an engineered plastic wrap can reduce shrinkage, protect the produce from disease, reduce mechanical damage and provide a good surface for stick-on labels.

Rigid Plastic Packages. packages with a top and bottom that are heat formed from one or two pieces of plastic are known as clamshells. Clamshells are gaining in popularity because they are inexpensive, versatile, provide excellent protection to the produce, and present a very pleasing consumer package. Clamshells are most often used with consumer packs of high value produce items like small fruit, berries, mushrooms, etc., or items that are easily damaged by crushing. Clamshells are used extensively with precut produce and prepared salads. Molded polystyrene and corrugated polystyrene containers have been test marketed as a substitute for waxed corrugated fiberboard. At present they are not generally cost competitive, but as environmental pressures grow, they may be more common. Heavy-molded polystyrene pallet bins have been adopted by a number of growers as a substitute for wooden pallet bins. Although at present their cost is over double that of wooden bins, they have a longer service life, are easier to clean, are recyclable, do not decay when wet, do not harbor disease, and may be nested and made collapsible.

As environmental pressures continue to grow, the disposal and recyclability of packaging material of all kinds will become a very important issue. Common polyethylene may take from 200 to 400 years to breakdown in a landfill. The addition of 6 percent starch will reduce the time to 20 years or less. packaging material companies are developing starch-based polyethylene substitutes that will break down in a landfill as fast as ordinary paper.

The move to biodegradable or recyclable plastic packaging materials may be driven by cost in the long term, but by legislation in the near term. Some authorities have proposed a total ban on plastics. In this case, the supermarket of the early 21st century may resemble the grocery markets of the early 20th century.

Standardization of Packaging

Produce package standardization is interpreted differently by different groups. The wide variety of package sizes and material combinations is a result of the market responding to demands from many different segments of the produce industry. For example, many of the large-volume buyers of fresh produce are those most concerned with the environment. They demand less packaging and the use of more recyclable and biodegradable materials, yet would also like to have many different sizes of packages for convenience. packers want to limit the variety of packages they must carry in stock, yet they have driven the trend toward preprinted, individualized containers. Shippers and trucking companies want to standardize sizes so the packages may be better palletized and handled.

Produce buyers are not a homogeneous group. Buyers for grocery chains have different needs than buyers for food service. For grocery items normally sold in bulk, processors want largest size packages that they can handle efficiently - to minimize unpacking time and reduce the cost of handling or disposing of the used containers. Produce managers, on the other hand, want individualized, high quality graphics to entice retail buyers with in-store displays.

Selecting the right container for fresh produce is seldom a matter of personal choice for the packer. For each commodity, the market has unofficial, but nevertheless rigid standards for packaging; therefore it is very risky to use a nonstandard package. packaging technology, market acceptability, and disposal regulations are constantly changing. When choosing a package for fresh fruits and vegetables, packers must consult the market, and in some markets, standard packages may be required by law.