Technical consultation on specifications and quality control of netting materials and mosquito nets
4.3 Filament texture and filament count of yarn
Evidence (WHO unpublished results) indicates that texturization of multi- filament polyester yarn results in slightly higher water absorption (2 to 5 %) during field treatment with insecticide formulation diluted in water. Nets made from texturized yarn usually have the same range of bursting strength, a slightly lower shrinkage and a softer feel than netting made of flat or round yarn. Both materials are suitable for use in mosquito nets.
The number of filaments which the yarn is made influences physical characteristics of the netting: such as water uptake; the surface area to which insecticide can bind; and hence the final concentration of insecticide per unit area of net. The cross-sectional shape and surface texture of filaments also influence the same characteristics: flat, irregular-shaped and textured filaments have a greater surface area than round filaments of the same cross- sectional area.
In 2000, a minimum filament count of 36 was agreed for the yarn to be incorporated into polyester netting material. Netting made from polyester multi-filament yarn with filament counts above 30 has proven acceptable for insecticide treatment of nets at field level. Monofilament yarn is suitable for netting made of high density polyethylene. The meeting agreed that it was more important to ensure that the net absorbs the expected volume of insecticide formulation diluted in water, than to maintain 36 as minimum filament count.
a. Further research should be undertaken by commercial partners on the rela- tionship between filament count, physical strength and insecticide uptake, both at laboratory and at field level.
4.4 Linear density
Linear density is a characteristic of the yarn and is expressed in units of denier (grams per 9000 metres) or decitex (grams per 10 000 metres). Denier and decitex are a description of yarn, not a minimum characteristic of the netting, although the values influence the fabric weight per unit area. Linear density is difficult to measure in any type of finished netting product and impossible to measure in warp-knitted netting. The denier/decitex of yarn is one element contributing towards the bursting strength of netting but bursting