Geotextiles and Geomembranes 22 (2004) 255–272
Natural weathering of textiles used in agricultural applications
W.Dierickx
a,*, P.Van Den Berghe
baDepartment of Mechanisation, Labour, Buildings, Animal Welfare and Environmental Protection, Agricultural Research Centre, Van Gansberghelaan 115, 9820, Merelbeke, Belgium
bBonar Technical Fabrics nv/sa, Weverslaan 15, 9610, Lokeren, Belgium
Received 6 September 2003; received in revised form 4 February 2004; accepted 7 March 2004
Abstract
In-field research was carried out on five agrotextiles which were exposed to natural weathering for 5 years.Prior to weathering and each subsequent year of exposure, samples were taken to evaluate tensile strength, strain at break, thickness and mass per unit surface.
The results show that the effect of weathering is material dependent.The two polyvinyl chloride (PVC)-coated polyester materials exhibited a distinct decrease in tensile strength, especially after the first year of exposure, whereas the three other polyethylene materials, which were ultraviolet (UV) stabilised, showed only a continuously slight decrease in tensile strength with exposure time.Strain at break presented more or less the same tendency as the tensile strength.Material thickness was not affected by weathering.A decrease in mass was only observed for the two PVC-coated materials, whereas weathering had no effect on the mass of the UV-stabilised materials.The tensile data led to a logarithmic-model to predict the long-term weathering behaviour.However, the tensile properties prior to exposure do not fit with this model unless it is assumed that they do not change during the early stages of weathering.Three short periods of exposure with duration of, respectively, 15 days, 7 days and 1 day, within which it was assumed that the initial tensile properties remained unaltered, were used to investigate the effect of this assumption on the long-term prediction of the tensile properties.The accepted period was, however, of crucial importance to the long-term prediction and could not be determined unequivocally.Therefore, it was decided to exclude the initial values prior to exposure in deriving the logarithmic-model.
r2004 Elsevier Ltd.All rights reserved.
Keywords:Textiles; Agrotextiles; Geotextiles; Natural weathering; Tensile strength; Strain at break
*Corresponding author.Tel.: +32-9-272-2800; fax: +32-9-272-2801.
E-mail address:[email protected] (W. Dierickx).
0266-1144/$ - see front matterr2004 Elsevier Ltd.All rights reserved.
doi:10.1016/j.geotexmem.2004.03.001
1. Introduction
Textiles, composed of synthetic polymers, are applied in a wide variety of circumstances where various agents may attack them.Synthetic polymers, however, are relatively inert to the action of a great number of chemicals and environmental effects (Veldhuijzen van Zanten, 1986), yet they also possess a number of weaknesses.They are particularly vulnerable to natural weathering leading to deterioration of tensile strength and elongation or strain at break, and to increased brittleness (Koerner, 1986).Used as geotextiles in geotechnical applications, textiles will mostly not be exposed to natural weather conditions, except for a restricted period during construction.Agrotextiles or textiles used in agriculture and horticulture as windbreak, groundcover, insect screen, bird netting, ventilation screen, animal barrier and hail netting among other things are exposed to natural weathering.In all these applications and also in some geotechnical and civil engineering works where textiles are subjected to climatic influences for longer periods, the effect of weathering on their properties is of paramount importance.The investigation into the effect of natural weathering on mechanical properties of textiles is a complex problem.The rate of ageing depends on various external factors such as intensity of radiation, temperature, humidity, air pollution, etc.(Rankilor, 1990; Schneider, 1990).Chemical compounds are added to the bulk polymer, not only to facilitate the processing but also to improve the performance of the final product.Antioxidants are used to prevent oxidation at higher temperature during the processing phase as well as to minimise the long-term oxidation during practical application.Ultraviolet (UV) stabilisers are added in the processing phase of synthetic products that are exposed for part or the whole of their working life to natural weather conditions.Polymers used in textiles differ considerably in their intrinsic resistance to thermo-oxidation and photo-oxidation (Raumann, 1982;
Rankilor, 1990).Polypropylene (PP), for instance, is particularly sensitive to both oxidation and UV radiation.Therefore, each polymer requires a specifically composed stabilisation system (Veldhuijzen van Zanten, 1986).Furthermore, depending on the stability required with respect to durability, various mixtures and concentrations of these additives may be used.It is also imperative that water does not leach additives in the long term.From the above it is obvious that the long- term performance of a textile depends on the types of additives used.This suggests that data from durability trials may be product specific (Brady et al., 1994).Various kinds of durability tests for geosynthetic materials can be found inASTM D5819 (1999)and appropriate tests in accordance with the application can be selected.
As in-field weathering tests are of long duration and site specific, accelerated weathering devices for nonmetallic materials have been developed (ASTM G151, 2000; ASTM G152, 2000; ASTM G153, 2000; ASTM G154, 2000; ASTM G155, 2000;EN 12224, 2000;ISO 4892, 1994) as well as standards to evaluate deterioration (ASTM D4355, 2002;EN 12226, 2000).Artificial weathering results depend on the testing device, light source and test conditions.Accelerated tests with specific artificial light sources are based on a maximum outdoor exposure of 6 summer months in Northern Europe, which is accepted to cause a broad variability of results.
However, the accelerated test cannot guarantee this and therefore it remains an index test, which can differentiate various products in accordance with their resistance to weathering exposure.Moreover, it is extremely difficult to transfer the artificial weathering to life expectancy under natural weathering conditions.Experience has shown that the correlation between laboratory results obtained with light sources and results in natural daylight is not very accurate (Davis, 1983; Schneider, 1990), material dependent and differs for various types of products (EN 12224, 2000).The correlation applies only to that specific type and formulation of material and for the particular properties evaluated.Moreover, accelerated test conditions should be chosen as close to the real situation as possible (Van Langenhove, 1990).
In spite of its disadvantages, in-field testing over a long period seems to be the most appropriate method of estimating the life expectancy of textiles or for predicting whether or not a textile will continue to fulfil its design function after a premised lifetime.Solar radiation is the primary factor that affects the properties of the synthetic material that constitutes most textiles.The quality and intensity of solar global radiation vary with climate, location and time, but the average effect on ageing during an entire year does not differ very much from 1 year to the other at a given location (Veldhuijzen van Zanten, 1986).Besides solar radiation, other important environmental factors such as temperature, temperature cycling, humidity and chemical pollution affect the ageing process (EN 12224, 2000), and are automatically included in the in-field testing but cannot always easily be simulated in laboratory experiments.
Whatever the application, textiles may be vulnerable to degradation when exposed to natural weathering.Their mechanical properties, especially the tensile strength and strain at break, which are important properties for textiles, may decrease as a result of natural weathering exposure.They can be tested according toASTM D5970 (2002)which is directed towards evaluating the deterioration in tensile strength after outdoor exposure, althoughASTM D4355 (2002)andEN 12226 (2000)can also be used for this purpose.To check possible alteration of the mechanical properties five textiles, used in agriculture and horticulture, were exposed to natural weathering under humid temperate climatic conditions during a period of 5 years.Not only the effect of natural weathering exposure on tensile strength and strain at break was evaluated but also thickness and mass were investigated.This paper represents the results of the exposure to natural weathering and an attempt to predict future strength and strain at break properties by extrapolation.
2. Materials and methods
The characteristics of the five agrotextiles studied are given in Table 1.Two materials were manufactured from polyester (PET) and the other three from polyethylene (PE).Four materials were woven and one material was knitted.
Materials M1and M2, both from the same manufacturer, were coated with polyvinyl chloride (PVC).Materials M3and M4were produced by another manufacturer and material M5originally came from a third manufacturer (who has now merged with
the manufacturer of materials M3and M4).These materials were protected against UV-degradation by specific stabilisers added to the bulk polymer.
All materials were vertically exposed to natural weathering.Therefore, a piece of 1.55.0 m2of each material was clipped on tile laths screwed onto a frame of six wooden poles spaced 1.0 m apart (Fig.1).The distance between two adjacent screens was 10 m.They all stood perpendicular to the north–south direction to expose them as much as possible to solar radiation.
First of all, an unexposed test sample of 1.51.0 m2of each of the five materials was used to determine the physical properties prior to natural weathering.
Furthermore, during 5 consecutive years a test sample of 1.51.0 m2 between two adjacent poles of each of the five materials was removed and evaluated.The evaluation of the effect of natural weathering on the exposed materials consisted of
Table 1
Identification of the products investigated
Code Trade name Material type Polymer type Yarn type (warp–weft) Yarn width (mm)A(%) M1 Vervaeke 1-B Woven PET Multi–multi (PVC coated) 1000/600 35.7
M2 Vervaeke 2-G Woven PET Multi–multi (PVC coated) 1000 25.8
M3 Bonar T42 Woven PE Twisted tapes–tape 600 49.1
M4 Bonar AT NET-38B Knitted PE Mono 250 57.5
M5 Bonar AT 1410 Woven PE Mono–mono 200 59.1
Fig.1. Experimental set-up to expose textiles to natural weathering (dimensions in mm).
the determination of tensile strength, strain at break, thickness and mass per unit area on randomly taken specimens of each of the test samples of the five materials.
For tensile strength and strain at break, five test specimens in both the machine (warp) and the cross (weft) direction were randomly taken from each of the test samples.Each specimen had a length of about 30 cm and was 5 cm wide.The number of yarns was counted so that, for a given direction, each specimen of a given material had the same number of yarns for the test prior to exposure and for each of the tests of the 5 consecutive years of exposure.The thus prepared specimens were conditioned in a standard atmosphere at a temperature of 2072C and a relative humidity of 6572% (EN 20139, 1992) and then subjected to the tensile test at a constant speed of 100 mm/min to determine breaking loads and strains at break.No specific standard (ASTM D5970, 2002;ASTM D4355, 2002;EN 12226, 2000) was used to determine tensile strength and strain at break but each year the same described procedure was used to evaluate possible deterioration.
Thickness under a pressure of 2 kPa (EN 964-1, 1995) was determined on 10 randomly punched circular specimens with a diameter of 7.3 cm. The same specimens were used to determine the mass per unit area according toEN 965 (1995)with the exception that the minimum surface was only 41.85 cm2instead of 100 cm2.As the result of the exposure to the natural environment, dust sticking on the materials could be observed visually.Therefore, each specimen of the exposed materials was weighed again after being washed and dried, to quantify the effect of dust on the mass of the specimens.
3. Results
The results of tensile strength Ts as the mean value of five randomly taken specimens of each of the investigated materials as a function of the time of exposure te are given in Fig.2.To illustrate the variability of the tensile strength of the specimens, the 95% confidence limits are indicated for the materials M1 and M3. These limits were of the same order of magnitude for the other materials but are not shown for clarity of presentation.Both the tensile strength in warp (Fig.2a) and in weft (Fig.2b) direction exhibit the same tendency, although tensile strength for corresponding materials is, in general, somewhat greater in warp than in weft direction.The tensile strengths of both PVC-coated materials M1and M2show a substantial decrease, ranging between 35% and 46%, of the initial tensile strength after the first year of exposure and proceed to decrease continuously, although to a lesser extent, during the following years of exposure.The outdoor exposure tests of Grubb et al.(2001) on a woven multifilament PET geotextile, for a period of 12 months, carried out in two completely different environments, also showed an average retained tensile strength that did not exceed 60%.Here, it is not clear whether the decrease in tensile strength is due to deterioration of the PVC coating, the PET fibres or both.The materials M3, M4and M5which are UV stabilised do not exhibit such a substantial decline in tensile strength with time and certainly not after the first year of exposure.At worst, a slight decrease in tensile strength with time can
be observed with materials M3and M5 in both the warp and weft direction.The tensile strength of material M4remained rather constant over the years.
It may be expected that the more the tensile strength decreases and consequently the more the brittleness increases, the more the strain at break will be reduced.Fig.3 shows how the strain at break Sb; as the mean value of five randomly taken specimens of each of the investigated materials, evolves with exposure timete:Here also, the 95% confidence limits are only indicated for some of the materials (M2, M3, M4) for the sake of clarity.In the warp direction (Fig.3a) as well as in the weft direction (Fig.3b), the decrease in strain at break of both materials M1and M2is the
0 5 10 15 20 25 30 35 40 45
0 1 2 3 4 5
0 5 10 15 20 25 30 35 40 45
Time of exposure te (years) Tensile strength Ts (kN/m)
(b) (a)
Fig.2. Mean tensile strengthTsof the various fabrics as a function of time of exposuretefor (a) the warp and (b) the weft direction with the 95% confidence limits of the materials M1and M3;n, M1;’, M2;&, M3;m, M4;K, M5.
0 5 10 15 20 25 30 35
0 1 2 3 4 5
45 50 55 60 65 70 75 80 0 5 10 15 20 25 30 35 40
Time of exposure te (years)
Strain at break S (%)
(a)
(b)
Fig.3. Mean strain at break of the various fabrics as a function of time of exposuretefor (a) the warp and (b) the weft direction with the 95% confidence limits of the materials M2, M3and M4;n, M1;’, M2;&, M3;m, M4;K, M5.
greatest after the first year of exposure, just as for their tensile strength.During the following years, a continuously slight decrease in strain at break is observed with these materials.Material M3also exhibits a distinct decrease in strain at break in the warp direction during the first 2 years of exposure but not in the weft direction.
Thereafter, a continuously small decrease with time is observed as for materials M1
and M2.In the weft direction, material M3shows a distinct increase in strain at break after the first year of exposure.The obtained level was maintained after the second year but a substantial decrease in strain at break was observed after the third year.
The confidence limits of material M3are substantially smaller in the warp than in the weft direction because of two twisted tape yarns in the warp and only one twisted tape yarn in the weft direction.Because of its stretchable nature material M4exhibits quite large confidence limits compared to the other materials, yet a limited continuous decrease in mean strain at break was observed for both directions.The strain at break of material M5remained fairly constant with time in both warp and weft direction.
Weathering did not affect the thickness of the tested materials but did affect the mass of some materials.The PVC-coated materials M1and M2exhibited an almost systematic reduction in mass during exposure while material M3showed a rather slight increase in mass with exposure time.The mass of the materials M4 and M5
remained fairly constant for the duration of exposure.The dust, adhered onto the exposed materials, did not significantly affect the mass as could be observed from the mass of unwashed and washed test specimens.
4. Data processing
The evolution of the tensile strength during this short-term field research of 5 years should enable either the prediction of the long-term tensile strength or the life expectancy of a given fabric.A mathematical model that fits very well the obtained data of tensile strength with the time of exposure is the log-model
Ts¼alnteþb; ð1Þ
with Ts the tensile strength (kN/m); te the time of exposure (years); and a and b fabric-dependent coefficients.The power-model
Ts¼ctde; ð2Þ
withcandd the fabric-dependent coefficients, is another mathematical model that fits very well the tensile strength data.The only problem is that the data obtained at the time of exposurete¼0 are not very useful for these mathematical models unless it is accepted that a short exposure time does not affect the strength properties of the investigated fabrics.The best-fit curves through the measured points assuming that the obtained tensile strength did not change during the first 2 weeks of exposure (upto te¼0:0411 year) are given in Fig.4 for the log-model (Fig.4a) and the power-model (Fig.4b).The coefficientsaandbof the log-model andcanddof the
power-model are given in Table 2 together with their respective coefficients of determination.
If, alternatively, it is assumed that the time of exposure that did not yet influence the tensile properties was only 1 week (te¼0:0192 year), then different values of the coefficientsaandbof the log-model and the coefficientscanddof the power-model with their respective coefficients of determination are obtained as given inTable 2.
Obviously, the choice oftemodified the mathematical models.Therefore, the time was further reduced to 1 day or 0.00274 year and the coefficients of the mathematical models for this situation are also given inTable 2.These mathematical models were used to predict the tensile strength by extrapolation to 20 and 30 years of service,
0 5 10 15 20 25 30 35 40 45
0 5 10 15 20 25 30 35 40 45
0 1 2 3 4 5
Time of exposure te (years) Tensile strength Ts (kN/m)
(b) (a)
Fig.4. Logarithmic-model (a) with the 95% confidence limits of the individual (- - - -) and the mean (– – – –) values of materials M1(,n) and M3(J,&), and power-model (b) fitting the tensile strength values in warp direction, assuming that the initial tensile strength prior to exposure was not altered after 15 days (te¼0:0411 year) of exposure;n, M1;’, M2;&, M3;m, M4;K, M5.
which may be the life expectancy of agrotextiles.The results of these calculations are given inTable 3.They confirm that the predicted long-term tensile strength will be greater when the time of exposure which should not influence the tensile strength and to which the tensile strength prior to exposure corresponds is shortened.
Comparing the coefficients of determination, it may be concluded that the log- model fits slightly better the measurements than the power-model.Furthermore, the predicted tensile strengths after 20 and 30 years of service obtained with the log- model are more conservative and consequently more reliable than those of the power-model.Therefore, the log-model was retained for further analyses.The results of the log-model, applied to the weft direction, supposing the tensile strength values prior to exposure did not change after 15 days of exposure, are shown inFig.5.The coefficientsa andb, and the coefficients of determination accepting tensile strength values prior to exposure as unchanged after, respectively 15 days, 7 days and 1 day, are given inTable 4.The results are similar to but not identical with those of the warp direction because of differences in yarns and initial tensile strength.Obviously, the predicted tensile strength after 20 and 30 years of service, as presented inTable 5, will differ as well.
The initial tensile strengthTsiprior to exposure corresponds with the limit of the tensile strengthTswhen the time of exposuretetends to zero, hence
Tsi¼te-0limTs ð3Þ
Table 2
Coefficientsa and bof the equation Ts¼alnteþb and c and dof the equationTs¼ctde with the coefficient of determinationr2for the warp direction, assuming that weathering does not change the initial tensile strength (te¼0:0411 year or 15 days;te¼0:0192 year or 7 days andte¼0:00274 year or 1 day
Material a b r2 c d r2
te¼0:0411
M1 4.322 26.25 0.996 25.28 0.148 0.995 M2 4.705 21.96 0.997 20.62 0.192 0.979 M3 0.678 10.10 0.631 10.00 0.066 0.631 M4 0.098 14.79 0.109 14.78 0.007 0.112 M5 0.660 19.50 0.814 19.46 0.033 0.799 te¼0:0192
M1 3.710 25.62 0.998 24.73 0.127 0.989 M2 4.027 21.27 0.993 20.04 0.163 0.964 M3 0.563 10.00 0.592 9.90 0.055 0.591 M4 0.077 14.77 0.091 14.76 0.005 0.094 M5 0.556 19.40 0.784 19.36 0.028 0.768 te¼0:00274
M1 2.697 24.60 0.992 23.89 0.091 0.970 M2 2.915 20.17 0.978 19.17 0.117 0.932 M3 0.387 9.85 0.527 9.76 0.038 0.525 M4 0.048 14.75 0.066 14.74 0.003 0.068 M5 0.391 19.25 0.730 19.21 0.020 0.712
The time of exposuretewithin which it is supposed that tensile properties are not yet affected is crucial for the prediction of tensile strength after 20 or 30 years.Tables 3 and 5 clearly illustrate that the tensile strength increases as te approaches zero.
Moreover, the mean and the individual values of the tensile strength, supposing the initial tensile value is not changed after 15 days of exposure, do not always fall within
Tensile strength Ts (kN/m)
0 5 10 15 20 25 30 35 40
0 1 2 3 4 5
Time of exposure te (years)
Fig.5. Logarithmic-model fitting the tensile strength values in weft direction, assuming that the initial tensile strength prior to exposure was not altered after 15 days (te¼0:0411 year) of exposure;n, M1;’, M2;&, M3;m, M4;K, M5.
Table 3
Predicted tensile strength (kN/m) in warp direction after 20 and 30 years based on the logarithmic- and power-model, assuming that weathering does not change the initial tensile strength (te¼0:0411 year or 15 days;te¼0:0192 year or 7 days andte¼0:00274 year or 1 day)
Material Log-model Power-model
te¼0:0411 te¼0:0192 te¼0:00274 te¼0:0411 te¼0:0192 Te¼0:00274 20 years
M1 13.3 14.5 16.5 16.2 16.9 18.2
M2 9.8 9.2 11.4 11.6 12.3 13.5
M3 8.1 8.3 8.7 8.2 8.4 8.7
M4 14.5 14.5 14.6 14.5 14.5 14.6
M5 17.5 17.7 18.1 17.6 17.8 18.1
30 years
M1 11.5 13.0 15.4 15.3 16.1 17.5
M2 8.1 7.6 10.3 10.7 11.5 12.9
M3 7.8 8.1 8.5 8.0 8.2 8.6
M4 14.5 14.5 14.6 14.4 14.5 14.6
M5 17.3 17.5 17.9 17.4 17.6 18.0
the 95% confidence limits as illustrated for the warp direction of the materials M1
and M3(Fig.4a).For these reasons the initial tensile strength was excluded from the measurements and the log-model applied to the values of the tensile strength obtained during the 5 consecutive years of exposure.The results for the warp and weft direction are shown in Fig.6.The 95% confidence limits of the mean and individual values for the warp direction of the same materials M1 and M3 but without the initial tensile strength clearly show that the thus obtained models are much more reliable to predict the tensile strength.It is, however, evident that still always 5% of each of these values may fall beyond these limits.The coefficientsa andbof this log-model and the coefficients of determinationr2are given inTable 6.
Table 7gives the predicted tensile strength after 20 and 30 years of service.
The same procedure was applied for the strain at break Sb: The results of the strain at break in the warp and weft direction are given inFig.7for all investigated materials.The material-dependent coefficientsa1andb1of the equation
Sb¼a1lnteþb1; ð4Þ
withSb(%) the strain at break, are given inTable 8.The predicted values of strain at break after an exposure time of 20 and 30 years are represented inTable 9.
Table 4
Coefficientsaandbof the equationTs¼alnteþbwith the coefficient of determinationr2for the weft direction, assuming that weathering does not change the initial tensile strength (te¼0:0411 year or 15 days;te¼0:0192 year or 7 days andte¼0:00274 year or 1 day
Material Te¼0:0411 te¼0:0192 te¼0:00274
a b r2 a b r2 a b r2
M1 3.829 20.26 0.996 3.288 20.06 0.999 2.394 19.16 0.995 M2 4.845 18.82 0.998 4.156 18.11 0.998 3.018 16.97 0.990 M3 0.684 5.24 0.868 0.577 5.14 0.842 0.408 4.98 0.720
M4 0.100 4.14 0.133 0.093 4.15 0.155 0.075 4.18 0.191
M5 0.365 11.64 0.572 0.302 11.59 0.535 0.207 11.51 0.473
Table 5
Predicted tensile strength (kN/m) in weft direction after 20 and 30 years based on the logarithmic-model and assuming that weathering does not change the initial tensile strength (te¼0:0411 year or 15 days;
te¼0:0192 year or 7 days andte¼0:00274 year or 1 day
Material 20 years 30 years
te¼0:0411 te¼0:0192 te¼0:00274 te¼0:0411 te¼0:0192 te¼0:00274
M1 9.2 10.2 12.0 7.6 8.9 11.0
M2 4. 3 5. 7 7. 9 2. 3 4. 0 6. 7
M3 3. 2 3. 4 3. 8 2. 9 3. 2 3. 6
M4 4. 4 4. 4 4. 4 4. 5 4. 5 4. 4
M5 10.4 10.7 10.9 10.4 10.6 10.8
5. Discussion
Differences between the log-model and the power-model are small (Fig.4).The reason for choosing the log-model is based on the coefficient of determinationr2, which is slightly better for the log-model than for the power-model, except in those cases where the association between tensile strength and time of exposure is small (Table 2).In addition, the predicted tensile strength after 20 and 30 year of exposure (Table 3) is higher and consequently less reliable when applying the power-model.
0 5 10 15 20 25 30
Time of exposure te (years) 0
5 10 15 20 25 30
0 1 2 3 4 5
Tensile strength Ts (kN/m) (a)
(b)
Fig.6. Logarithmic-model fitting the tensile strength values in warp direction (a) with the 95% confidence limits of the individual (- - - -) and the mean (– – – –) values of materials M1(,n) and M3(J,&), and in weft direction (b), ignoring the initial tensile strength prior to exposure;n, M1;’, M2;&, M3;m, M4;K, M5.
Theoretically, the log-model for which the initial tensile strength values prior to exposure are supposed to be the same as those after 1 day of exposure should be the most correct model because the chance that the material is affected by weathering after 1 day is much less than after 15 days.The coefficients of determination, however, show that the curves generally fit better with the model for which the tensile strength values prior to exposure correspond with an exposure time of 15 days (Tables 2 and 4), again with the exception of the cases where the association between tensile strength and time of exposure is small.
The model, which does not consider the initial tensile strength prior to exposure, seems to be the best approach to predict the tensile strength after 20 and 30 years of exposure to weathering.In general, the obtained values for both the warp and weft direction (Table 7) are smaller, and consequently more reliable than the values of the models which consider the initial tensile strength (Tables 3 and 5) with the exception of material M1for the warp direction atte¼0:0411 year and for the weft direction at te¼0:0411 year and te¼0:0192 year, and material M2 for the weft direction at te¼0:0411 year.The time of exposure, which does not exert a noticeable influence on the tensile properties of a material is difficult to fix and may differ from one material to another.Therefore, it is better to omit the initial values.Although additional measurements after, for instance, 3 and 6 months of exposure can be
Table 6
Coefficientsaandband coefficient of determinationr2of the equationTs¼alnteþbfor the warp and weft direction, ignoring the initial tensile strength prior to exposure
Material Warp direction Weft direction
a b r2 a b r2
M1 3.767 25.68 0.974 3.174 19.95 0.994 M2 4.915 22.18 0.975 4.452 18.42 0.988 M3 1.973 11.41 0.832 1.302 5.88 0.860 M4 0.611 15.32 0.433 0.403 4.66 0.307 M5 1.386 20.25 0.828 1.103 12.41 0.764
Table 7
Predicted tensile strength (kN/m) in warp and weft direction after 20 and 30 years based on the logarithmic-model and ignoring the initial tensile strength prior to exposure
Material Warp direction Weft direction
20 years 30 years 20 years 30 years
M1 14.4 12.9 10.4 9.2
M2 7.5 5.5 5.1 3.3
M3 5.5 4.7 2.0 1.5
M4 13.5 13.2 3.5 3.3
M5 16.1 15.5 9.1 8.7
done, the results will largely depend on whether exposure is started in winter or summer.
The coefficients of determinationr2(Table 7) indicate the existence of a distinct association, in both warp and weft direction, between tensile strength and duration of exposure for materials M1and M2.This also holds for materials M3and M5. A
0 5 10 15 20 25 30 35
0 5 10 15 20 25 30 35
0 1 2 3 4 5
60 65 70
Time of exposure te (years) Strain at break Sb (%)
(b) (a)
Fig.7. Logarithmic-model fitting the values of strain at break in (a) warp and (b) weft direction, ignoring the initial strain at break prior to exposure;n, M1;’, M2;&, M3;m, M4;K, M5.
rather weak association for material M4was found.This means that materials M1
and M2are severely influenced by weathering, as well as materials M3and M5while material M4is much less affected.
The negative coefficientaindicates that the tensile strength decreases with the time of exposure, while its magnitude is a measure of the rate of decrease.The coefficient b is an indication of the tensile strength after 1 year of exposure to weathering.
Comparison of these values with the predicted values after 20 and 30 years gives an indication of their rate of decrease with the time of exposure.
The same deduction can be made for the coefficientsa1and b1for the strain at break (Table 8), although the positivea1-values in the warp direction for materials M4and M5mean that the strain at break increases with the time of exposure.
6. Conclusions
Weathering clearly affects tensile properties (tensile strength and strain at break) and mass of the PVC-coated materials while thickness remains unaltered.Tensile properties of UV-protected textiles are much less affected by weathering and the mass and thickness of these materials did not change with time.The reduction in tensile properties is the largest during the first year of exposure and can be approached by a logarithmic-model for all materials.However, the model differs
Table 8
Coefficientsa1andb1and coefficient of determinationr2of the equationSb¼a1lnte+b1for the warp and weft direction, ignoring the initial value of strain at break prior to exposure
Material Warp direction Weft direction
a1 b1 r2 a1 b1 r2
M1 1.064 16.12 0.971 1.211 17.47 0.968 M2 1.572 14.63 0.950 2.325 16.99 0.987 M3 1.770 21.58 0.846 3.523 27.80 0.733
M4 0.042 27.83 0.002 2.552 66.45 0.262
M5 0.408 20.19 0.632 0.005 22.93 0.000
Table 9
Predicted strain at break (%) in warp and weft direction after 20 and 30 years according to the logarithmic-model, ignoring the initial value of strain at break prior to exposure
Material Warp direction Weft direction
20 years 30 years 20 years 30 years
M1 12.9 12.5 13.8 13.4
M2 9.9 9.3 10.0 9.1
M3 16.3 15.5 17.2 15.8
M4 28.0 28.0 58.8 57.8
M5 21.4 21.6 22.9 22.9
according to the material and even between warp and weft direction within a material because of difference in nature.
Initial tensile properties prior to exposure (te¼0) do not fit in the log-model unless they are considered unaltered after a short period of exposure.The problem is, however, to define that short period of exposure.It can be expected that the shorter the period of exposure, the better the log-model should fit but that is not so.
Therefore, the initial values prior to exposure have been excluded for defining the log-model to obtain what the authors consider to be a more accurate and reliable prediction of the tensile properties after a long period of exposure.
Since the largest decrease in tensile properties occurs within the first year of exposure, it would be interesting to assess the change over this period.However, it should be recognised that if this was done results could be expected to depend on the time of the year the exposure is started.
Prediction of tensile strength and strain at break for a period of 20–30 years using the data of 5 years observation time involves some risk.Testing over longer periods can reduce potential errors in these predictions and further experimental verification is required.
References
ASTM D4355, 2002.Standard test method for deterioration of geotextiles by exposure to light, moisture and heat in a xenon arc type apparatus.ASTM Book of Standards, Vol.04.13, 2003.ASTM, Philadelphia, PA.
ASTM D5819, 1999.Standard guide for selecting test methods for experimental evaluation of geosynthetic durability.ASTM Book of Standards, Vol.04.13, 2003.ASTM, Philadelphia, PA.
ASTM D5970, 2002.Standard practice for deterioration of geotexiles from outdoor exposure.ASTM Book of Standards, Vol.04.13, 2003.ASTM, Philadelphia, PA.
ASTM G151, 2000.Standard practice for exposing nonmetallic materials in accelerated test devices that use laboratory light sources.ASTM Book of Standards, Vol.14.04, 2003.ASTM, Philadelphia, PA.
ASTM G152, 2000.Standard practice for operating open flame carbon arc light apparatus for exposure of nonmetallic materials.ASTM Book of Standards, Vol.14.04, 2003.ASTM, Philadelphia, PA.
ASTM G153, 2000.Standard practice for operating enclosed carbon arc light apparatus for exposure of nonmetallic materials.ASTM Book of Standards, Vol.14.04, 2003.ASTM, Philadelphia, PA.
ASTM G154, 2000.Standard practice for operating fluorescent light apparatus for UV exposure of nonmetallic materials.ASTM Book of Standards, Vol.14.04, 2003.ASTM, Philadelphia, PA.
ASTM G155, 2000.Standard practice for operating xenon arc light apparatus for exposure of nonmetallic materials.ASTM Book of Standards, Vol.14.04, 2003.ASTM, Philadelphia, PA.
Brady, K.C., McMahon, W., Lamming, G., 1994. Thirty year ageing of plastics. In: Karunaratne, G.P., Chew, S.H., Wong, K.S. (Eds.), Proceedings of the Fifth International Conference on Geotextiles, Geomembranes and Related Products.SEAC-IGS, Singapore, pp.1217–1222.
Davis, A., 1983.Weathering of polymers.Reprinted in 1986.Elsevier Applied Science Publishers Ltd, Essex, UK.
EN 964-1, 1995.Geotextiles and Geotextiles-Related Products—Determination of Thickness at Specified Pressures—Part 1: Single Layers.CEN, Brussels.
EN 965, 1995.Geotextiles and Geotextiles-Related Products—Determination of Mass per Unit Area.
CEN, Brussels.
EN 12224, 2000.Geotextiles and Geotextile-Related Products—Determination of the Resistance to Weathering.CEN, Brussels.
EN 12226, 2000.Geotextiles and Geotextile-Related Products—General Tests for Evaluation Following Durability Testing.CEN, Brussels.
EN 20139, 1992.Textiles—Standard Atmospheres for Conditioning and Testing.CEN, Brussels.
Grubb, D.G., Diesing III, W.E., Cheng, S.C.J., Sabanas, R.M., 2001. Comparison of the durability of geotextiles in an alkaline mine tailings environment.Geosynthetics International 8 (1), 49–80.
ISO 4892, 1994.Plastics—Methods of Exposure to Laboratory Light Sources: Part 1: General Guidance;
Part 2: Xenon-Arc sources; Part 3: Fluorescent UV Lamps; Part 4: Open-Flame Carbon-Arc Lamps.
Gen"eve, Switzerland.
Koerner, R.M., 1986. Designing with Geosynthetics. Prentice-Hall, Englewood cliffs, NJ, USA.
Raumann, G., 1982. Outdoor exposure tests on geotextiles. Proceedings of the Second International Conference on Geotextiles, Las Vegas, NV, August 1–6, 1982, IFAI, pp.541–546.
Rankilor, P.R., 1990. The weathering of fourteen different geotextiles in temperate, tropical, desert and permafrost conditions.In: den Hoedt, G.(Ed.), Fourth International Conference on Geotextiles, Geomembranes and Related Products.Balkema, Rotterdam, The Netherlands, pp.719–721.
Schneider, H., 1990. Durability testing and prediction of geotextiles and related products. In: den Hoedt, G.(Ed.), Fourth International Conference on Geotextiles, Geomembranes and Related Products.
Balkema, Rotterdam, The Netherlands, pp.723–726.
Van Langenhove, L., 1990. Conclusions of an extensive BRITE-research programme on ageing. In: den Hoedt, G.(Ed.), Fourth International Conference on Geotextiles, Geomembranes and Related Products.Balkema, Rotterdam, The Netherlands, pp.703–707.
Veldhuijzen van Zanten, R., 1986. Geotextiles and Geomembranes in Civil Engineering. Balkema, Rotterdam, The Netherlands.
