Developing stronger nets and testing them in the field



A study conducted by:

R. Bosselmann and M. Ivic, TANA Netting, UAE

D. Syafruddin, Eijkman Institute for Molecular Biology, Indonesia

D. Dang, Vegro Lab, Vietnam

O. Skovmand, Intelligent Insect Control, France

This study aimed at evaluating the impact of changing knitting pattern and denier but not square meter weight in mosquito nets to show it is possible to provide stronger nets without increasing cost as previously predicted (1) based on laboratory stories. However, so far little correlation has been found between laboratory strength test and field measured net durability as measured in attrition rate and hole index. Therefore, a three year study was started last year on Sumba Island, Indonesia to follow these two parameters, insecticide decline and bioassay efficacy from three minute exposures of fully susceptible mosquitoes in cones attached to net samples taken from nets in the field. Six months’s data are provided.

Challenge (developing mosquito nets with two insecticides)

When developing mosquito nets with two insecticides, and especially so when one is a synergist to the other, it is important that the ratio of the two at the surfaces of the yarn remains constant. Surface concentrations are low on mosquito nets, and the method accepted now is to use loss rates of insecticide after washing as a substitute number. The wash resistance index represents such a method (WRI). Using this method, the WRI for the two index should be the same. To obtain that the two insecticides have the same release rate, the two insecticides can be placed one in each of the two yarns used for knitting the net, and the release rate can be maintained independently by modifying the polymer composition.

Standard knitting on nets provide very asymetric use of the two yarns used for knitting. One will provide 75 % of the net and the other the 25 %. This can complicate the goal of obtaining parallel release rate and we therefore studied if other knitting patterns based on 50:50 ratio of the two yarns would provide nets with similar strength or better.

TANA Netting | The knitting machine above shows the chain drives for the two sets of rollers, one of these rows are seen on the photo to the right.

The knitting machine above shows the chain drives for the two sets of rollers. One of these rows are seen on the photo to the right.

Method

118 Denier PE yarns were knitted in standard 25:75 pattern or in three different symmetric knittings. TANA Netting provided yarns to two nets for standard knitting and for three nets with symmetric knitting that was carried out as a service by Karl Meyer, a knitting machine company.

All nets were sent to the WHO textile reference laboratory CITEVE in Portugal that measured tension strength in two directions, classic bursting strength and wounded bursting strength all according to ISO protocols. The wounded net protocol does not state which of the two yarns are to be cut before measuring bursting strength, therefore CITEVE made one series for one yarn and another for the other. All measurements were made on five different pieces of net.

Net code Net description Tensile strength Burst strength
Wrap Weft STD 1st yarn cut 2nd Yarn cut
8881PES OA STD Knit 210 98 412 AB 123 C 134 A
8886PES OB STD Knit 190 85 432 A 164 B 146 A
88872006-45 Symmetric 180 94 380 B 227 A 155 A
88872006-49 Symmetric 230 82 452 A 110 C 137 A
88872006-50 Symmetric 220 72 441 A 181 B 162 A

Results

The two standard nets are made the same way so the results illustrate variations for one type of net. Tensile strength direction warp (along the net), two grabs (ISO 13934-2:2014) varies from 180 to 230, symmetric 45 is lower than the rest. In weft direction, symmetric 50 is lower.

Bursting strength varies between 380 and 452 kPa, but only symmetric 45 is significant lower than the rest (letters A, B and C indicates groups). However, when 1st yarn is cut, symmetric 45 loses much less strength than the others and becomes the strongest.

Finally, there is not a correlation between bursting strength and wounded bursting strength (P=0.2 for 1st yarn and 0.7 for second yarn).

Conclusion

Field data registering holes show that all nets get holed and fast. The wounded bursting strength has been indicated to show resistance to hole expansion once a small hole is made.

The RBM textile expert S. Smidt has shown that this parameter is better related to long term net damage than bursting strength.

Wounded bursting strength cannot be predicted from bursting strength and has to be measured independently.

Challenge (various ways that mosquito nets can be made stronger)

Stronger nets can be made by increasing yarn diameter and net weight, but thus become more expensive. Increasing yarn diameter, but at the same time making the mesh bigger can result in a net with the same square meter weight and thus roughly the same price, but will it be stronger? If so, will that show in the field?

All tests conducted at CITEVE 150D 100D Unit 150D/100D
GSM 42.1 42.7 g/m² 98.59%
Tear strength length 19 14 N 135.71%
Tear strength width 13 13 N 100.00%
Bursting strength 535 448 Kpa 119.42%
Bursting strength seams 633 496 Kpa 127.62%
Wounded bursting strength 259 245  Kpa  105.71%
Tensile strength length 210 185  N  11.51%
Tensile strength width 130 85  N  152.94%
Tensile strength W hooks length 46 39  N  117.95%
Tensile strength W hooks width 36 25  N  144.00%
Snag test length 21  N
Snag test width 31  N

Method

Net in polyester yarn 100 D and 150 D from the same factory and same basic polyester quality was standard mosquito net knitted. Nets from each knitting were sent to CITEVE for comparative strength tests and determination of mesh and square meter weight.

Hamlet 150 D 100 D
1 33 41
2 19 16
3 27 19
4 21 24
Total 100 100

Nets were sent to Sumba Island, Indonesia, 300 of each type were distributed randomly in villages and after every six months, attrition rate is followed, net damages noticed and 10 nets are redrawn and examined for insecticide dosage  and bioefficacy.

Results from CITEVE

The table shows the parameters measured for the two types of nets and then the ratio of the 150 D net/100 D net in percentage. It can be seen that despite the 150 D net is a few percent lighter, it is stronger in all parameters except tear strength width, which is exactly equal.

Insecticidal activity

Survey (month) 150 D 100 D
KD 1hr Mortality KD 1hr Mortality
0 91.8% 100% 92.6% 100%
6 99.3% 100% 94.6% 100%

Chemical analysis

Survey (month) 150 D 100 D
No. of nets Dosage ai/kg No.of nets Dosage ai/kg
0 5 1.85 5 1.90
6 5 0.97 5 0.92

Attrition rate of bed nets

Survey (month) 150 D 100 D
Followed Present Missing Attrition rate Followed Present Missing Attrition rate
0 300 300 0 0.0% 300 300 0 0.0%
6 282 256 23 9.2% 270 270 32 11.9%

Fabric integrity

Survey (month) 150 D 100 D
Hole index Hole area Hole index Hole area
0 Mean (SD) Median (IQR) Mean (SD) Median (IQR) Mean (SD) Median (IQR) Mean (SD) Median (IQR)
6 512 (881) 196 (610) 692 (1080) 240 (750) 615 (993) 196 (724) 734 (1218) 240 (889)

The integrity results indicate a small difference in mean hole index but not in median hole index. Note that standard deviations are bigger than mean values clearly showing that holes are not normal distributed but clumped. Some nets have much more and bigger holes than others. However, six months is far too early, and net development will be followed for three years.

Conclusion

Stronger-nets---TANA Netting

Traditional Sumba houses are usually made from bamboo with high roof and thatched walls enabling free mosquito entrance.

There are various ways that mosquito nets can be made stronger. Changing knitting pattern does not change speed of knitting; however, strengthening one parameter influence other parameters positively or negatively and the net producer must make a choice of which parameters are  most important.

As predicted (1) increasing yarn diameter while keeping square meter weight increases net strength in nearly all parameters and decreases in none. The six month data start showing tendency that may confirm that increased strength may give higher attrition rate and fewer big holes.

 

To download the leaflet PDF version of this publication, click here.

 

References:

Skovmand, O and Bosselman R:  Strength of bed nets as function of denier, knitting pattern, texturizing and polymer. Mal J, 2011: 10:87.

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