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Polyethylene Mulch Films - What’s in a Name?

More and more names are being tossed about these days, especially in the discussions concerning methyl bromide (MeBr) rate reductions with various plastic mulch films. LDPE, LLDPE, HDPE, High Barrier, and VIF - - just how do they differ? To begin with, the polymer resins used to produce the mulch film determine the physical characteristics of the finished product. Traditionally, two major types of polyethylene resins are used to produce plastic mulch films, each having a different polymer chain structure. High-density polyethylene (HDPE) is a more linear polymer and results in a stiffer and stronger plastic. Low density polyethylene (LDPE) is a more highly branched polymer, resulting in a more flexible plastic. Because HDPE is stronger than LDPE, it’s possible to get a thinner gauge plastic, allowing for more plastic on the roll which reduces costs and speeds up field operations. The down side is it may require modification of equipment to accommodate the stiffer plastic. It does not hug the bed as tightly as LDPE. More recently, linear low-density polyethylene (LLDPE) has come on the market. Reportedly, greater film strength allows for high puncture and tear resistance; however, it is difficult to find anyone in Florida who has experience with LLDPE. One comment heard is that it ‘balloons’ out too much, probably due to the greater flexibility or ‘stretchiness’. A blend of LLDPE and LDPE reportedly can give the desirable amount of stretch and is being used by at least one manufacturer.

Confusion exists when the terms ‘high barrier’ and virtually impermeable film (VIF) are used interchangeably, implying they are the same product. This is not the case. High barrier films refer to polyethylene films that are less permeable to MeBr than standard LDPE. Reductions in fumigant emissions typically range from 20% to 60% but are dependent upon soil and ambient temperatures. Permeability increases 1.5 to 2 times for every 18°F increase in air temperature. Virtually impermeable films are multilayered films that include polymers other than polyethylene in the center layer that are gas impermeable. Polyamide, or nylon, is commonly used in VIF films to create the impermeable barrier. The standard for MeBr permeability of VIF is provided by the French government, who state that to be classified as VIF, the film must have a permeability factor of no more than 0.2 grams of MeBr per square meter of film per hour. By contrast high barrier films have a permeability factor between 5 and 8 grams MeBr per square meter of film per hour, according to the California Environmental Protection Agency. The use of the term ‘high barrier’ was initiated in California in response to efforts to force the agricultural industry to convert to VIF. In actuality, the permeability differences between HDPE and LDPE are small when compared to the permeability of VIF. Reportedly, Telone emission rates under LDPE and HDPE are nearly identical. In testing done in Florida, vapor studies compared the percent transmission across a film in a given period of time. Preliminary results indicated the standard black LDPE allowed 45% transmission in 81 hours compared to 38% for metallized mulch film and 4% for Klerck’s VIF over the same time frame. Similar work by Yates with USDA/ARS in California showed that MBr emission in bare soil was 87%, emission with HDPE was reduced to 67%, while emission under VIF was less than 5%. In work at Bradenton, metallized mulch was slightly more retentive than high barrier by the same company, and gave slightly better nutgrass control. In addition, soil temperature was lower and air temperature higher with metallized mulch, but there was no difference in plant growth. Growers have complained about the handling characteristics of VIF materials. Some of the newer VIF materials coming out do allow faster laying speeds without sheer and are “whiter”, more like the standard white on black LDPE. Benefits from the use of VIF are the ability to significantly reduce, by at least 50%, the amount of MeBr used, a feature which will become more important as the industry works within the availability and regulatory constraints of MBr issues, and the ability to improve the performance of alternative fumigants, such as Telone C-35. As with any new technology, adoption rates of VIF will depend on how a grower perceives the need. Once the benefits are substantial enough, ways will be found to make it work.

(Phyllis Gilreath, Manatee County, Jim Gilreath, GCREC-Bradenton,
Dan Chellemi, USDA/ARS, Ft. Pierce - Vegetarian 04-08)