CHEMICALS
Grace Puts Snap into Polyethylene Electron-irradiated shrinkable polyethylene film for form-fitting packaging goes commercial A strong new contender for packaging honors got its official send-off last week from W. R. Grace's Cryovac division. The company bills it as the "first successful shrinkable polyethylene." Its name—Cryovac Type L. The new item is a low density polyethylene with a built in two-way stretch. At 180° F. or above, the film starts to shrink equally in all directions back to its original size. The trick: cross-linking by Grace's new electron beam process. The process, according to Grace, can start with any polyolefin filmeither high or low density polyethylene, polypropylene, or various copolymers. With Cryovac L, starting material is extruded, low density polyethylene film. The film passes under a General Electric Electron Beam Generator which bombards it with up to two million electron volts. The electron stream jolts hydrogen atoms free. Monatomic hydrogen atoms combine to go off as gas. The active sites remaining on the carbon atoms then
couple together to form cross-links with adjacent chains. This cross-linking boosts the film's tensile strength by more than 500%, the company claims. And, with the added strength, the film then can be stretched more than 200% in both directions (biaxially). Upon subsequent heating, the film reverts to its prestretched size. The process, says Grace, is steeped in patents, both granted and pending. Cryovac L is a strong, transparent, "breathing-type" film with excellent printing qualities and a high gloss and clarity unequaled in most films at comparable prices, says Grace. Its tensile strength ranges between 8000 and 16,000 p.s.i., depending upon thickness, as against the 1350 to 2500 p.s.i. recorded for the film before irradiation. And, once started, the new film tears easily (10 to 15 grams/mil on the Elmendorf Tear Tester), paving the way to tear-open packaging uses. Not so with other polyethylene films, where tear strength is in the 150
HIGH ENERGY RADIATION. To irradiate the film, Grace passes it down through and under the eye of GE's Electron Beam Generator, housed in a thick concrete vault. Dome on the right houses all the electronic components under pressure
to 300 grams/mil range, the company points out. The ding's the Thing. Main target for the new film is the booming overwrap food field as an attractive, form-fitting wrap for such items as cut-up chicken, frankfurters, fruits, vegetables, and bread. Here, eye appeal and low cost, rather than extended shelf life, are the prime requirements. Already Grace has testmarketed Cryovac L for over a year in these applications, calling it only a "new type" of film. It will run up against other new shrinking films, such as Goody ear's Videne (laminate of polyester film with Pliofilm, a material based on rubber hydrochloride) and Reynolds' Reynolon (polyvinyl chloride). The new development stems from attempts to broaden the market for Grace's Cryovac line. This polyvinylidene chloride copolymer is the skin-tight wrap used on a wide range of perishable meat and poultry products—turkey, most notably. But its price, $0.04 per thousand square inches of 60-gage (0.6 mil) film—acceptable where the film's low oxygen permeability is needed to cut down spoilage—rules out its use as a general packaging material. Cryovac L, on the other hand, logs in at $0.03 per thousand square inches at 1 mil thickness—about in the same price bracket as cellophane and just a shade more expensive than standard polyethylene films. Commercial production of the new film is now under way at Simpsonville, S.C.
FOUR-COLOR PRINTING. A strong, transparent, "breathing" film, Cryovac L can be printed easily, according to Grace. After stretching it in both directions, the company prints the film before shipping it to food packagers. Here, workmen at Grace's Simpsonville, S.C, plant watch presses put a four-color pattern on the polyethylene film
