Plastics may be getting a bad press in some areas, but in agriculture they are contributing to feeding the world. These films extend the growing season and raise yield, which is critical as the population grows but the area being farmed is static. Demand for plastics for agricultural uses in the film area was 3.6 million tonnes in 2008 according to Andrew Reynolds of Applied Market Information: Asia holds by far the largest share of the market at 60%. Reynolds was speaking at the 2009 AMI conference on Agricultural Film, held in February in Barcelona., Spain. CSIC puts the area under film in the Mediterranean at 400,000 hectares, with 80,000 hectares of vegetables in Spain (tomatoes, peppers and lettuce).
Pliant Corp. estimates the agricultural film market in the USA at 70mm lb. and around 20% is barrier film, which is mainly used for pest control and is produced from materials such as EVOH and nylon. Permeability to fumigation gasses is the critical property here, and can vary with relative humidity - there are no official standards at this time. Semi-permeable films include a LDPE sandwich containing HDPE; virtually impermeable films (VIF) typically comprise a LLDPE sandwich containing nylon. Mitsui and Kuraray have collaborated to produce a totally impermeable film (TIF) comprising a polyethylene sandwich containing two adhesive layers (“Admer”) and a central layer of ethyl vinyl alcohol (“EVAL”), which provides a very effective fumigant barrier. In tests in January 2009 in Argentina on a pepper crop, the methyl bromide fumigant dose could be cut by 50%, due to the low permeability. It has good tear resistance (important for installation) and good overlap adhesion.
Kafrit Industries has looked at stabilisation of film under severe treatment with pesticides. Expectations are growing and greenhouse materials are now expected to last 5 years: durability is affected by many factors including wind, sun, temperature, effects of structural supports, surface coatings, etc. Pesticides like sulfur can have a big effect on film, for example, they can deactivate light stabilisers. Kafrit Industries has been testing different combinations of stabilisers with sulfur and weathering. Nickel phenolate and UVA showed the best performance. In mulch film stabilization with exposure to both sulfur and chlorine, NOR-HALS was best. All films are affected by the pesticide treatments.
Around 2.5% of the film used in the US is biodegradable. Limagrain/Ulice manufactures this type of film in France.
Currently standards such as EN 13432 are in force, but stricter ones are being developed for agriculture such as NF U 52001, which looks at degradation in soil. The product, “biolice”, was compared with PE film and average temperatures in mulch were the same. It has been used in short-term crops such as maize, cotton and salad; longer term films are under development for tomatoes and melons.
Telles is a joint venture between ADM and Metabolix to produce the Mirel range of bioplastics from corn sugar. These are aliphatic polyesters with chemical resistance similar to PET. In tests, the mulch film produced had completely degraded in soil by the next growing season.
Campo Tecnico is a Japanese company specialising in biodegradable products including mulch films. In 2006, 1425 tonnes were produced in Japan and the annual growth rate is around 10%. Biodegradable film can reduce the work for the farmer because it does not need to be recovered, and it cuts soil pollution. However, it is 3-4 times more expensive. Campo Tecnico is making blends of polymers to give a longer degradation time. The base resin has a low melting point and the added polymer has a high melting point: the film from this blend has better mechanical properties than LDPE.
Plastika Kritis produces greenhouse film in Greece, France and China (Shanghai HiTec Plastics). It describes the thermic effect as increasing night temperature and evening out temperature variations, and absorption of long infra-red waves. A good film will also diffuse light, reducing plant damage and reflecting some near infrared to offer a moderate cooling effect. In a new development, aluminium particles can be added to increase the cooling effect. Anti-dripping properties can be provided by surface migrating additives, which last around 24 months.
Failure of greenhouse film can be due to too high a processing temperature, incorrect use of stabilisers, aggressive agricultural chemicals, heat, antagonistic effects of pigment, and contamination during film production (for example introducing pro-degradant iron ions). This has been researched by Cytec, a company which specialises in stabilisers for greenhouse and mulch films. In one project in Dhahran, Saudi Arabia, the greenhouse film retains 70% elongation after 3 years. The company has also tested light transmission, which is a key factor in colour development in flowers. In separate research, Uri Peled has worked on a multimaterial film showing improved plant growth compared with conventional PE products.
Heat control has been examined by scientists at BASF – the main markets for agricultural film are in the tropics and sub-tropics, where the summer months are too hot for cultivation. Improving greenhouse film properties can extend the growing period, the additive Lumogen IR 1050 works by reflecting infrared and allowing visible light to pass for photosynthesis.
Davis Standard has developed versatile film production equipment to allow for seasonal variation in demand. Agricultural film ranges from 20 to 300 microns thickness and up to 20 metres width, which demands large extruders and in turn means that temperature control and cooling are key factors. Greenhouse film needs good bubble stability because of the large size and often short lengths are needed (200m) so that the film is easier to handle in the field. Winding is another challenge.
Horticultural film is being recovered and recycled at an increasing rate. Previero producers grinders, shredders and pulverisers to break down material for recycling. Sorema works with Proviero to provide complete recycling systems. Agricultural film is contaminated with heavy matter such as sand, stones and soil, together with light residue such as plant matter and other polymers from irrigation. The film is washed prior to processing. Gestora Catalana de Residuos sells both virgin and recycled LLDPE into the agricultural film market: the recyclate can cut costs and meet appropriate standards in the field.
Alberto Fereres of CSIC has studied the use of selective films and nets for pest control. Common problems include aphids, white fly, thrips, powdery mildew, viruses and bacteria. Insects act as vectors in many cases, so preventing access for the vector can protect the crop and reduce the need for pesticides. Blocking transmission of UV radiation has long been known to reduce the number of insects; it affects their flight behaviour. It also reduces sporulation of Botrytis cinerea. In a study on lettuce in Spain, the aphid and silver Y moth infestation was greatly reduced. One disadavantage is that some ornamental plants need UV light to develop colour.
Agricultural Film 2009 attracted delegates from around the globe and generated extensive debate on ways to develop the best film technology for the horticultural industry.