Freezing Technology

7.2.1.2 Flexible films

1. Flexible films 

          A variety of flexible films is used in frozen food packaging as bags, pouches and over wraps. Almost all of these films are made from organic polymers. Some of the natural polymers such as starch, cellulose and rubbers are also useful in food packaging. 

Polyethylene (PE):  Polyethylene is a long chain polymer of ethylene, a gas derived from natural gas or petroleum. It is an useful film in frozen food packaging. It is low in cost, has excellent clarity and gloss, is resistant to tearing, has relatively low WVTR, is flexible at low temperatures and is heat sealable. Because of the last property, it is frequently used as coating or as layer in a laminate with heat sealable films. Important properties such as heat seal ability, moisture resistance and flexibility depend on the density of the polyethylene resin from which it is made.  Polyethylene is classified into three density ranges and expressed as g/cc as follows.

(1) Low – 0.910 to 0.925

      (2) Medium – 0.926 to 0.940

(3) High – 0.941 to 0.965

          Most polyethylene films used in frozen food packaging are in the low and medium density ranges. 

Nylon (Polyamide PA): It is a group of long chain polymers with re occurring amide groups as a part of the chain. Nylon films have excellent properties of toughness, tear and breaking strength. They are useful both as single films and in many laminates. 

 

Plio films: It is the trade name for rubber hydrochloride film. It was the first transparent film which could be heat sealed. 

 

Polypropylene (PP): It is relatively new film in frozen food packaging, but its clarity, strength at low temperatures and low WVTR make it useful in laminates, and in coating paper or aluminum foil. It is heat sealable, when it is heated with a variety of coatings. 

 

Cellophanes: Cellophane is made from regenerated cellulose. When it is coated with other plastic resins such as polyethylene or saran, a wide variety of cellophanes result, each designed for a specific use. These make excellent packaging films. They are strong, moisture proof, heat sealable and flexible at low temperatures. 

 

Poly vinylidene chloride (PVDC): Polymers of vinylidene chloride are better known as saran and are available not only as films, but as water based emulsions in which form they are frequently used as coating resins for paper substrates. Saran is superior to polyethylene as a water vapour barrier material and has low oxygen permeability. The film is not heat sealable. The film is excellent for use as a wrap around odd shaped meat (shrink wrapping). 

 

Polyesters (PES): This strong film is well known under the trade name of Mylar. It is frequently used in lamination with other films, especially with polyethylene to make pouches which can be filled and heat sealed. These laminates are now used for boil in the bag pouches. Polyester films are strong, durable, transparent, non-toxic, inert and easily stretched. They also have the advantage of being flexible at very low temperatures (-71⁰C).  They are sometimes known as nylon type films and because of their roughness,they are useful in packaging sharp pointed, irregular foods such as lobsters.  

 

2. Plastic Pouches     

            The pouch is usually made from a laminate of polyethylene, nylon and Mylar films. One of the weakest links in plastic bags is the seal, both from the stand point of vapor leakage and that of bag breakage at the seal.