High-Tech Fibrous Materials - American Chemical Society

Chapter 20

Polyester Geotextiles Durability of Polyethylene Terephthalate Fibers and Fabrics 1

2

C. J. Sprague and G. W. Davis 1

Nicolon Corporation, Norcross, GA 30092 Hoechst Celanese Corporation, Charlotte, NC 28209

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2

In the last decade, the use of geotextiles has evolved from specialty "engineering fabrics," considered state-of-the-art in unique geotechnical designs, to commonly used construction materials, considered state-of-the-practice in many civil engineering applications. This relatively quick acceptance of geotextiles can best be explained by their proven track record. Geotextiles have generally performed as expected over their designed service life. Performance of geotextiles is commonly termed, "durability." Durability can be thought of as relating to changes over time of both the fiber microstructure and the fabric macrostructure. The former involves molecular polymer changes and the latter assesses fabric property changes. This paper takes a look at each of these components of durability as they relate to the use of polyethylene terephthalate (P.E.T.), commonly called polyester, fibers in geotextiles.

For more than two decades, t e x t i l e materials constructed of synthetic polymer f i b e r s , have been u t i l i z e d i n the construction of roads, drainage systems, and other c i v i l engineering projects. These materi a l s primarily composed of polyester or polypropylene have become known as "geotextiles" because they are t e x t i l e materials used i n conjunction with the ground (hence "geo-"). Geotextiles are designed to perform a function, or combination of functions, within the s o i l / geotextile system. Such functions as f i l t r a t i o n , separation, planar flow, or reinforcement are expected to be performed over the l i f e of the i n s t a l l a t i o n , which i s often 50 to 100 years, or more. Geotextiles are accepted construction materials and, l i k e a l l other materials, they have l i m i t a t i o n s .

0097-6156/91/0457-0304$06.00/0 © 1991 American Chemical Society

In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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While woven geotextiles may be more suitable for r e i n f o r c i n g than are nonwoven geotextiles, they may also be more prone to clogging or p i p i n g . S i m i l a r l y , polyester geotextiles may provide better strength c h a r a c t e r i s t i c s but poorer chemical resistance than polypropylene geotextiles. A l l polymeric materials are susceptible to degradation. For example, polyolefins such as polypropylene and polyethylene undergo oxidative degradation, whereas polyethylene terephthalate ( P . E . T . ) , can be hydrolyzed, and polyamides degrade by both hydrolysis and oxidation. However, i t must be emphasized that these reactions can be retarded by the use of suitable additives. Although these processes may be quite slow at ambient temperature, anticipated lifetimes for geotextiles of up to 100 years have been proposed. Furthermore, the degradative processes are r e a d i l y catalyzed by, for example, t r a n s i t i o n metals i n the case of oxidations and by low pH i n the case of polyester hydrolysis [1]. Therefore, an understanding of the geotextile f a b r i c structure and f i b e r molecular make-up and how these are affected by the in-service environment are necessary to select the appropriate material.

R e s i s t a n c e t o F i b e r Degradation Table 1 l i s t s the p o t e n t i a l mechanisms of polymer degradation. Table 2 rates the i n t r i n s i c resistance of common polymers to degradation. The most predominant degradation concern, e s p e c i a l l y for polypropylene and polyethylene, involves oxidation. Conversely, s o l v o l y s i s i n the form of hydrolysis i s a more pronounced mechanism of degradation for polyesters and polyamides. Both oxidation and solvolysis are forms of chemical degradation. Generally, s i g n i f i c a n t chemical degradation i s observed only at elevated temperatures because the a c t i v a t i o n energy for these processes i s high [2] . The moderate temperatures associated with most i n s t a l l a t i o n environments would, therefore, not be expected to promote degradation. A d d i t i o n a l l y , the majority of synthetic polymers i s rather i n e r t towards b i o l o g i c a l enzymatic attack [2].

Fabric

Performance

Fabric performance i s most obvious to the geotextile user. Table 3 l i s t s several f a i l u r e mechanisms which can cause unsatisfactory performance of the geotextile. In general, long-term piping and clogging resistance, as well as t e n s i l e and compression creep resistance, are the most common f a b r i c properties related to d u r a b i l i t y .

Aging The exposure environment w i l l generally be characterized by complex air, s o i l and water chemistry as well as unique r a d i a t i o n , hydraulic and stress conditions. The effect of this combination of exposures, over time, i s termed aging. Aging therefore includes both polymer degradation and reduced f a b r i c performance and i s dependent on the s p e c i f i c a p p l i c a t i o n environment. D u r a b i l i t y refers to a geotext i l e 's resistance to aging.


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HIGH-TECH FIBROUS MATERIALS


Table 1 Mechanisms o f Polvmer Dearadationt Chemical Rx

• • • • •

Oxidation Ozonization (oxidation i n i t i a t o r ) Solvents Solvolysis (hydrolysis, i o n i c reactions) Trace Metals (oxidation catalyzer)

Thermolysis

(oxidation

initiator)

Mechanical Stress

(oxidation

initiator)

Photolysis

(oxidation

initiator)

Radiolysis

(oxidation

initiator)

B i o l o g i c a l Attack

• Enzyme (microorganisms) • Mechanical (rodents, insects)

f After Schnabel [2]

Table 2 I n t r i n s i c M a t e r i a l R e s i s t a n c e A g a i n s t Degradation Mechanisrost PA

PE

Photo-oxidation

+

Thermo-oxidation

+

o>

Hydrolysis

o

+

Chemical Rx - a c i d

o

- alkaline Ratings: improved relative out. resistance

PET ++

X

++

PP 0

D4)

.1)4)

-/o >

3

+

++

+

+

+

++

0

+

NOTES:

^ Additives can improve the i n t r i n s i c resistance provided they do not leach 2)

The resistance of a l l materials is influenced by material thickness [4]. 3) Hydrolysis i s primarily a result of exposure to a strongly alkaline, high temperature environment. 4) Exposure to atmosphere or contact with iron can increase the rate of degradation

t After den Hoedt [3]



Clogging of the geotextile

Reduced t e n s i l e r e s i s t i n g r e s i s t i n g force.

Unacceptable deformation of the s o i l / g e o t e x t i l e structure

Reduced in-plane flow capability

Reduced resistance puncture

Filtration

Reinforcement

Re inforcement

Planar-Flow

Cushioning

Excessive compression creep of the f a b r i c .

Excessive compression creep of the f a b r i c .

Excessive t e n s i l e creep of the f a b r i c .

Excessive t e n s i l e stress relaxation of the fabric.

Permeability of the fabric i s reduced as a r e s u l t of p a r t i c l e build-up on the surface of or within the f a b r i c . Openings may have been compressed as a r e s u l t of long-term loading.

Openings i n the f a b r i c are incompatible with retained s o i l . Openings may be enlarged as a r e s u l t of i n - s i t u stress or mechanical damage.

POSSIBLE CAUSE

* These f a i l u r e mechanisms do not include polymer degradation-related causes.

to

Piping of s o i l s through the geotextile

FAILURE MODE

Separation/Filtration

FUNCTION

Table 3 G e o t e x t i l e F a i l u r e Mechanisms*



308


Do geotextiles commonly encounter problem s o i l environments? Not according to a 1986 study by the U.S. Army Engineer Waterways Experiment Station which concluded that no cases of f a b r i c f a i l u r e because of attack from chemicals present i n a natural s o i l environment were found i n the l i t e r a t u r e [5], However, i n cases of f a b r i c b u r i a l i n s o i l s having a very low or very high pH, consideration should be given to the composition of the geotextile selected. This should be a rare occurrence because most s o i l s have a pH i n the range of 3 to 10 [6]. Geotextile composition should also be considered i n cases of complex chemical exposure ( i . e . , leachate), b u r i a l i n metal-rich s o i l s , and extended exposure to sunlight. In order to evaluate these unique exposure conditions, tests which c l o s e l y simulate actual exposure conditions on the geotextile selected are recommended. When testing i s not possible, most manufacturers can provide data from exposure testing of t h e i r s p e c i f i c fibers or fabrics. Do geotextiles commonly encounter s o i l conditions which would be expected to cause reductions i n fabric performance? Almost always. But, whether i t s a gap-graded s o i l which could lead to f a b r i c clogging, or large embankment loads which must be r e s i s t e d with l i t t l e f a b r i c creep, f a b r i c properties can be selected to protect against excessive reductions i n performance.

PET F i b e r s and F a b r i c s Long-term performance of a geotextile depends on how the s p e c i f i c a p p l i c a t i o n environment affects the geotextile's fibers (polymer) and fabric structure. Therefore, i t i s very important to understand not only the s p e c i f i c environment but also the properties of the s p e c i f i c geotextile selected. The remainder of this paper provides detailed information on P . E . T . f i b e r and f a b r i c properties. When the term polyester f i b e r i s used, i t generally refers to poly (ethylene terephthalate) which has been shortened to PET i n the trade. PET i s the condensation homopolymer of terephthalic a c i d (or i t s dimethylester) and ethylene g l y c o l as shown i n Figure 1. PET i s a long-chain l i n e a r polymer with a t y p i c a l number-average molecular weight between 15,000 and 30,000. The polymer i s thermop l a s t i c and may be converted to a fiber by melt spinning. Chemical f u n c t i o n a l i t y i s derived from the ester group. Manufacture There are two major processes ( F i g . 2) used to manufacture polyester geotextiles. These are: (1) the spuribond filament process for producing nonwoven fabrics and (2) the high tenacity filament process for yarns used i n producing woven f a b r i c s . Terephthalic a c i d and ethylene g l y c o l form a diester monomer which i s polymerized to PET. The polymer i s melted, extruded, and spun through a spinneret with small holes, forming continuous filaments that are s o l i d i f i e d by cooling i n a i r . For spunbond f a b r i c s , polymers with molecular weights of about 20,000 form these filaments which are l a i d down to form a web, then bonded, i n t h i s case by needlepunching, to interlock the filaments to give the spunbond i t s c h a r a c t e r i s t i c t e n s i l e strength and dimensional s t a b i -


20. SPRAGUE & DAVIS

Polyester Geotextiles

-C00H

HOOC-

+

H0CH CH 0H 2

2

Ethylene Glycol

Terephthalic Acid

C00CH CH 0) H n

H0(0C

z

9

z

9

+

P o l y ( E t h y l e n e T e r e p h t h a l a t e ) PET Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch020

Fig.

1 - Chemical F o r m u l a t i o n o f PET

TA

EG

PET Polymer

Melt Intrusion

Spinning

Web

Drawing Heatset

Forming

Needle

Bobbin

Punching

Winding

Bonding

Beaming

Winding Roll Packaging

Slashing Weaving

Nonwoven

Woven Fabric

Fabric

Spunbond F i l a m e n t Process

Fig.

2

-

Polyester

High Tenacity

Filament

Process

F i b e r and F a b r i c Flow

Diagrams


310



lity. Chemical and thermal bonding can also be used. The formed f a b r i c i s then wound onto a core package and cut into standard lengths. For high tenacity filament woven f a b r i c s , the as-spun f i l a ments, which are formed from about 30,000 molecular weight polymer, are drawn by heating and stretching them to several times t h e i r o r i g i n a l length, followed by heatsetting to form an oriented semic r y s t a l l i n e structure and impart the desired physical properties of high strength, toughness and dimensional s t a b i l i t y . The treated filaments i n yarn form are wound onto a bobbin, then rewound from many bobbins onto a beam for slashing, followed by weaving on a loom to the specified f a b r i c . Characteristics The c h a r a c t e r i s t i c s that account for the the great v e r s a t i l i t y of polyester fibers are t h e i r high strength, high modulus, toughness, dimensional s t a b i l i t y , low shrinkage and h e a t - s e t t a b i l i t y , low moisture regain and quick drying a b i l i t y , thermal s t a b i l i t y , and r e s i s tance to stretching and shrinking, most chemicals, abrasion, wrinkles, cuts, weather, and s o i l - - a l l t h i s with acceptable economics . The enormous success of polyester i n t e x t i l e end uses such as apparel and home furnishings, i n i n d u s t r i a l end uses such as t i r e s and roofing, and i n carpet markets i s attributed to these outstanding c h a r a c t e r i s t i c s . In the following sections, these c h a r a c t e r i s t i c s w i l l be d i s cussed and related to polyester geotextiles. Most of the data, unless s p e c i f i c a l l y stated otherwise, has been taken from TREVIRA® polyester in-house studies.

PET R e s i s t a n c e t o F i b e r Degradation Thermal Resistance Polyester fibers have outstanding thermal properties. They melt i n the range of 4 7 8 - 4 9 0 ° F , c r y s t a l l i z e at 2 5 0 - 2 6 5 ° F , have a glass t r a n s i t i o n temperature of 150-165 F--all well above normal conditions encountered i n geotextile applications. In addition, polyester fibers maintain excellent f l e x i b i l i t y and strength at temperatures below freezing. P

Chemical Resistance - General Another property c r i t i c a l to geotextile performance i n some a p p l i c a tions i s chemical resistance. Fortunately, many diverse environments have been tested and the results are a v a i l a b l e . A review of some of that data with the intent of showing general resistance to classes of chemicals as well as the effects of temperature, time, and concentrat i o n i n select cases follows. Generally speaking however, polyester geotextiles have excellent s t a b i l i t y to many chemical classes such as water, s a l t s , organic acids, organic solvents, dry cleaning solvents, o x i d i z i n g agents (bleaches), reducing agents, gases and fuels (petroleum). They are susceptible, under c e r t a i n conditions, to chemical classes such as inorganic acids, halogenated organic acids, inorganic and organic bases, benzyl a l c o h o l , and halogenated phenols, while the effects of leachates and "buffered solutions" are dependent on t h e i r chemical composition. In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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Chemical Resistance - Water Polyester i s v i r t u a l l y unaffected by r a i n , seawater or other forms of water. Polyester's resistance to the effects of water i s e a s i l y explained. Moisture absorption of polyester i s only 0.4% at room temperature and 65% r e l a t i v e humidity. Even at 95% humidity, less than 0.6% moisture i s absorbed. Because of i t s low moisture absorpt i o n , polyester geotextiles w i l l not get heavy when wet, w i l l not mildew, and w i l l dry very quickly. Most importantly, polyester's strength i s unaffected by water under normal conditions. The mechanical properties of wet polyester geotextiles are the same as dry. There i s no strength loss as mentioned e a r l i e r ; stretch to break (elongation) i s not affected; there i s no change i n abrasion resistance; and there i s no growth or shrinkage ( i . e . , e l a s t i c i t y i s not affected). Polyester fibers can be p l a s t i c i z e d by water, but t h i s p l a s t i c i zation i s very slow. The water enters the amorphous regions of the polymer and enhances molecular mobility. The r e s u l t i s a small loss i n modulus and strength, but the effect i s completely reversible when the water i s removed. PET fibers are only affected by water i f conditions are such that hydrolysis of the ester group takes place. The rate of hydrolys i s i s dependent on temperature and i s most sensitive to strong acid or base conditions which can catalyze the hydrolysis reaction. Normally encountered s o i l temperature and chemistries have not been known to cause h y d r o l y s i s . Chemical Resistance - Acids Polyester i s highly resistant to most mineral and organic acids. Some of them are quite strong, but s t i l l have no effect on polyester after a one year exposure as shown i n Table 4. Polyester's long-term resistance to s u l f u r i c a c i d makes i t excellent for use i n a c i d r a i n conditions which become more severe each year. While polyester i s highly resistant to most acids, i t i s affected by high concentrations of some very strong acids. Generalizations are not appropriate when looking at the effect of pH on acid degradation. In some conditions, polyester can withstand pH's below 0.1. Acid catalyzed hydrolysis i s the more important mechanism. C l e a r l y , s p e c i f i c exposure conditions must be known before assessing the effects of acids on polyester geotextile d u r a b i l i t y . Chemical Resistance - A l k a l i s In contrast to excellent acid resistance, polyester has somewhat l i m i t e d resistance to a l k a l i s , as shown i n Table 5. However, again, generalizations aren't always appropriate. In f a c t , some a l k a l i e s only s l i g h t l y affect polyester, even after one year of exposure. As i n the case of acids, a l k a l i n e degradation increases with higher temperatures, higher concentrations and longer exposure times. Also, as with a c i d , the effect of pH on a l k a l i n e degradation should not be generalized. pH determines hydroxide ions, but doesn't determine the strength of the hydroxide ions or other nucleophiles (negatively charged ions or fragments), which attack the carboxyl group of an ester. The strength of the nucleophile roughly p a r a l l e l s b a s i c i t y but i s dependent also on the nature of the leaving group displaced and a c c e s s a b i l i t y to the ester group. Therefore, strength



312

Table 4 A c i d R e s i s t a n c e o f PET g e o t e x t i l e s


Compound A c e t i c , concentrate Benzoic Boric Chlorosulfonic C i t r i c , 25% Formic, cone. Hydrochloric, 15% Hydrochloric, 20% Hydrofloric, 10% L a c t i c , concentrate N i t r i c , 15% Phosphoric, 20% S u l f u r i c , 38% (Battery)

DH

0.1

-

3.5

-

1.2 0.1 0.1 0.1

-

0.7 0.1 0.6 0.1

% Strength Retained After One Year at \ 100 100 100 Degraded 100 100 100 Degraded Degraded 100 100 Degraded 100

Table 5 A l k a l i n e R e s i s t a n c e o f PET G e o t e x t i l e s

Compound

DH

Ammonium hydroxide Ammonium hydroxide, 2% Calcium hydroxide, 15% Diethylamine Hydrazine, 2% Hydrazine, 5% Potassium hydroxide, 0.1% Potassium hydroxide, 2% Sodium hydroxide, 0.1% Sodium hydroxide, 2% Triethanolamine Urea

8-10 11.4 12.4 13.5 10.6 10.8 12.5 13.4 12.1 12.8 13.3 10.4

•Severely

% Strength Retained After One Year 88 SD* SD SD 76 SD 90 SD 94 SD 66 91

Degraded.


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loss i n some cases at pH's above 12 i s small. Once again, actual exposure conditions must be known before determining the effect of a l k a l i on polyester geotextiles.


Chemical Resistance - Inorganic Salts Aside from i t s s u s c e p t i b i l i t y to a c i d catalyzed hydrolysis and selective nucleophilysis, polyester i s resistant to most other chemicals. These include a wide range of inorganic s a l t s (and organic s a l t s as well) which do not affect polyester after a f u l l year of exposure (Table 6). There are a few exceptions, but again, pH i s not necessarily a determining factor. Chemical Resistance - Organic Solvents Polyester i s insoluble i n most organic solvents. The l i s t (Table 7) i s very long for organic solvents, including commonly used organic chemicals, fuels and p l a s t i c i z e r s , which do not affect polyester strength after a f u l l year of exposure. Chemical Resistance - F e r t i l i z e r s As mentioned, the effect of f e r t i l i z e r on polyester i s dependent on both the chemical composition and the moisture content of the f e r t i l i z e r (Table 8). Materials which form acids or bases can lead to varying amounts of h y d r o l y t i c degradation. U l t r a v i o l e t Resistance. For up to 400 hours of d i r e c t sunlight (two-six months aging i n calendar time), untreated polyester w i l l r e t a i n greater than 90% of i t s strength ( F i g . 3). In the long term, the u l t r a v i o l e t degradation of polyester slows down and can l e v e l out. Half the strength remains after 4000-5000 hours. Actual levels of deterioration vary, depending on the number of sunlight hours and sunlight i n t e n s i t y , which vary each year and depend greatly on geographic location and climatic conditions. For this reason, such conditions should be considered i n any p r e d i c t i o n of the effects of sunlight on any organic material. Further, note that polyester i s only sensitive to the u l t r a v i o l e t region between 3000-3300 angstroms. It i s very easy to block these rays, allowing a long l i f e , with no effect on strength. In fact, the outer layers of the f a b r i c i t s e l f have the a b i l i t y to f i l t e r these rays from inner layers. Therefore, deterioration i n thick, heavy fabrics i s s l i g h t r e l a t i v e to that i n yarn. This accounts for the l e v e l i n g effect over time, as shown i n F i g . 3.

PET F a b r i c Performance P r o p e r t i e s Fabric Properties When discussing f a b r i c properties, the focus w i l l be on those of polyester fibers and fabrics t y p i c a l l y used i n geotextiles. Fabrics can be s e l e c t i v e l y engineered to give very high strength and low elongation with wovens, or moderate strength and high elongation with needlepunched nonwovens, plus quite a range of other properties, to meet a v a r i e t y of geotextile requirements. V i v i d examples of this f l e x i b i l i t y can be seen by looking at the properties of different geotextile f a b r i c s . Polyester spunbond, needlepunched nonwoven fabrics of weights ranging from 3.5 to 16 o z / y d have a range of properties as shown i n Table 9. 2



314


Table 6 Inorganic S a l t Resistance

o f PET G e o t e x t i l e s

No strength loss after one year at 70°F Compound Aluminum sulfate Ammonium chloride Ammonium n i t r a t e Ammonium sulfate Calcium chloride Calcium n i t r a t e Copper sulfate F e r r i c chloride Ferrous sulfate Magnesium chloride Magnesium sulfate Nickel sulfate Potassium bichromate Potassium bromide Potassium carbonate Potassium chlorate Potassium chloride Potassium chromate

DH

2..9 5..1 4,.8 4,.6 7..2 3,.9 3,.5 0,.8 3 .0 4,.0 6 .6 6 .6 3 .7 6 .5 13 .1 6 .9 8 .0 9 .4

Compound

pH

8.8 Potassium n i t r a t e 9.9 Potassium perchlorate 9.7 Potassium permanganate 7.5 Potassium sulfate 4.6 Silver nitrate Sodium ammonium hydrogen 8.2 sulfate 7.8 Sodium bicarbonate 11.2 Sodium carbonate 7.4 Sodium chlorate 7.4 Sodium chloride 8.3 Sodium n i t r a t e 5.8 Sodium perchlorate 5.4 Sodium sulfate 9.3 Sodium tetraborate 5.4 Sodium thiosulfate 2.4 Zinc chloride 4.0 Zinc sulfate

Strength Loss Sodium b i s u l f i t e Ammonium sulfide

4 .1 9 .6

(94% after one year) (Destroyed after s i x months)


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Table 7 Organic S o l v e n t R e s i s t a n c e o f PET G e o t e x t i l e s


No strength loss after one vear at 70°F Acetone Amyl acetate Aniline Asphalt Benzaldehyde Benzene Butanol Butyl acetate Carbon tetrachloride Chloroform m-Cresol Cyclohexanone Diesel Fuel Dimethyl formamide Dimethyl sulfoxide Epichlorohydrin Ethanol Ether Ethyl acetate Formaldehyde, 30% Formamide Gasoline Glycol

Hydroquinone Isopropyl alcohol Isooctane Jet propellant Methyl acetate Methyl alcohol Methylene chloride Methyl ethyl ketone Mineral o i l Nitrobenzene Phenol m-Phenylene diamine 2-Phenylethylalcohol Pyridine Resorcinol Styrene Toluene Trichloroethylene Trimethy1amine Turpentine White s p i r i t Xylene

Strength Loss Benzyl alcohol (Dissolved) A l k y l amines (Degraded) Tetrachloroethane (92% after one year)

Table 8 Resistance of Polyester t o F e r t i l i z e r s D E G R A D A T I O N 12 Months 1 Month Ammonium n i t r i t e Ammonium sulphate Calcium n i t r a t e Calcium cyanamide, dry Calcium cyanamide, wet Lime, slaked Lime, slaked, moist 50% NPK Thomas Mead Thomas Mead, moist 50% Urea

None None None None None None None None None Slight None

None Slight None None Degraded Slight Destroyed Slight Slight Appreciable None


316


100


Strength Retention (%)

6

0

0

250

500

1000 2000 3000 4000 Exposure Time (hours) (Simulated outdoor exposure using Xenon Arc or Carbon Arc)

F i g . 3 - R e s i s t a n c e o f PET F i b e r s t o D i r e c t S u n l i g h t Exposure Table 9 G e o t e x t i l e P r o p e r t i e s o f PET Sounbond F a b r i c s 2

Weight, o z / y d Thickness, mils Grab t e n s i l e strength, lbs Puncture strength force, lbs Mullen burst point, p s i Trapezoid tear strength, lbs Elongation at break^ % P e r m i t t i v i t y , sec Normal permeability c o e f f i c i e n t , cm/sec V e r t i c a l water flow, gpm/ft Apparent opening s i z e , sieve size Thermal shrinkage, % (400°C) Moisture regain, % (65% RH, 72°F) Specific gravity 2

3.5 60 110/90 50 180 50/40 70/85 2.04 0.3 150 70-100 2.9 0.4 1.36

16 210 625/560 240 840 205/200 90/95 0.75 0.4 55 100-170 2.9 0.4 1.36


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In the case of high tenacity polyester filament yarn, fabrics may be woven i n various widths and may vary greatly i n weights and thickness. This i s accomplished by changing the denier (thickness) of the feeder yarns, the number of yarns i n the warp and f i l l i n g directions on the loom, and the pattern of the weave. Commercial PET woven geotextiles offer a range of wide width t e n s i l e strength from 1000 l b s / i n c h i n both the warp and f i l l d i r e c t i o n up to 3790 and 1180 l b s / i n c h , respectively, i n the warp and f i l l direction. The extension at f a i l u r e conversely ranges from 15% to 11% for these fabrics [7], The important areas are strength and low elongation. A t y p i c a l polyester high tenacity filament yarn for this a p p l i cation has the t e x t i l e properties given i n Table 10. Creep Resistance - Reinforcement For a f a b r i c to be dimensionally stable, i t not only has to be s t i f f , have low stretch and shrinkage, but i t should have good recovery properties. Elongation which occurs due to long-term loading versus instantaneous elongation i s referred to as creep. Data i n F i g . 4 suggests that polyester can support a load over 50% of i t s breaking strength with minimal creep for an extended service l i f e . This excellent creep resistance assures that polyester geotextiles maint a i n acceptable s t r a i n levels under load for extended periods. Creep Resistance - S e p a r a t i o n / F i l t r a t i o n . Planar Flow, and Cushioning. The proper design of a geotextile for drainage and f i l t r a t i o n e n t a i l s selection of a geotextile which i n h i b i t s the migration of fines into the core of the drain and induces the formation of a stable f i l t e r cake i n the s o i l [8]. Needlepunched nonwovens have been shown to provide an effective balance of thickness, porosity, and pore size d i s t r i b u t i o n to r e t a i n fine s o i l s , while inducing a stable f i l t e r cake without clogging [10]. PET needlepunched nonwovens exhibit t e n s i l e and compression creep resistance that prevents changes i n thickness, porosity and pore size d i s t r i b u t i o n over time which can lead to changes i n f i l t e r behavior. The a b i l i t y of PET needlepunched nonwovens to r e s i s t compression creep protects t h e i r long-term planar flow and cushioning a b i l i t i e s .

Aging o f PET Most of the effects discussed i n this presentation have dealt with general properties of polyester fibers and fabrics exposed to a v a r i e t y of i n d i v i d u a l conditions. I t i s not possible to deal with a l l the factors, nor combinations of factors which could affect the aging and d u r a b i l i t y of polyester geotextiles. The following sections b r i e f l y describe PET's resistance to aging i n the most common application environments: atmospheric, s o i l , and leachate exposure. Atmospheric Resistance Polyester's d u r a b i l i t y i n most atmospheric conditions i s quite good. This i s especially true for normal weather with respect to r a i n , heat, sunlight and r a d i a t i o n . The excellent water and heat r e s i s tance has already been discussed. Sunlight ( u l t r a v i o l e t ) resistance i s a more controversial area, since untreated polyester can experience slow u l t r a v i o l e t degradation though much superior to untreated polypropylene. Polyester properties are not affected by moderate doses of high energy r a d i a t i o n . In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

318

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Table 10 T e x t i l e P r o p e r t i e s o f PET High

Tenacity


FUamsnt Yarn Denier/Fils DPF Breaking tenacity, gpd Breakload, lbs Load at 5% elongation, lbs Breaking elongation, % E l a s t i c recovery at 5% elong, % I n i t i a l modulus, gpd Toughness, g cm Moisture regain, % (65%, RH, 72°F) Specific gravity Hot a i r shrinkage, % ( 3 5 0 ° F ) B o i l i n g water shrinkage, %

Total Strain, (%)

13 12 10 9 8 7 6 5 4 3 2 1

60% break load 40% break load 20% break load

0.1

Fig.

4

1000/192 5.2 8.9 19.8 10 13 90 110 0.7 0.4 1.39 13.5 6

10 100 1000 Duration, (hours)

- Creep R e s i s t a n c e

100000

o f PET F i b e r s

S o i l Resistance In the case of s o i l exposure, polyester d u r a b i l i t y to moisture, microorganisms, dry f e r t i l i z e r s , dry concrete, asphalt, and bitumen i s excellent. However, as discussed e a r l i e r , f e r t i l i z e r s can attack polyester under c e r t a i n moist conditions. Research on concrete i n moist conditions i s underway. In an independent study by G. C o l i n and co-workers [1], polyester nonwoven fabrics buried i n s o i l for seven years showed no s i g n i f i c a n t decrease i n bursting strength. S o i l b u r i a l exposure was c a r r i e d out i n a moist, organically r i c h s o i l maintained at 86°F and 85-90% r e l a t i v e humidity. Leachate Resistance Leachate i s generated as a r e s u l t of l i q u i d flowing through the s o i l , forming solutions of both dissolved and suspended materials. The composition of the leachate i s highly dependent on the s o i l and


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weather conditions and is difficult to predict since there are so many variables which affect this solubilization of organic and inorganic constituents. The characteristics of leachate, however, generally vary greatly in solid waste landfills. In the case of polyester, it is important to determine if hydrolysis can occur. Equally important in practical terms is to determine if the leachate forms a buffered solution which will inhibit hydrolysis. That is, the acids and bases will react with each other preferentially to form salts instead of attacking the polyester. Again, because of the variance in composition, caution should be used when describing a typical leachate. Therefore, it is best to characterize the actual leachate before designing the geotextile for a given location. Summary and Conclusions Polyethylene terephthalate, more commonly called PET, or just plain polyester, offers great versatility for selective engineering which yields a broad range of geotextile products with excellent strength, toughness and dimensional stability. This is the case in wet or dry conditions. In addition, the weight of the fabric is affected to only a very limited extent by water absorption. Furthermore, resistance to permanent deformation (creep) under long-term loading is excellent. Also, polyester is inert to a wide range of chemical classes encountered in soil. And, superior sunlight resistance allows polyester geotextiles to suit most construction installation requirements. Finally, polyester geotextiles are not affected by microorganisms in soil. Because of polyester's excellent chemical and physical properties, this material can be used with confidence and is the material of choice for most geotextile applications. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Colin, G., Mitton, M. T., Carlsson, D. J . , and Wiles, D. M., Geotextiles and Geomembranes, 1986, Vol. 4, No. 1; p 2. Schnabel, W., Polymer Degradation: Principles and Practical Applications, Hanser Int'l. 1981. den Hoedt, G., Proceedings from Durability and Aging of Geosynthetics, 1988. Plasticization and Hydrolysis: The Effects of Water on Polyethylene Terephthalate in Geotechnical Applications. ITW Enterprises, In-house report, 1988. Geotextiles for Drainage, Gas Venting, and Erosion Control at Hazardous Waste Sites, Environmental Protection Agency, 1986. Encyclopedia of Chemistry, Van Nostrand Reinhold, 1984, p. 706. Paulson, J. N., Proceedings From The Geosynthetic Research Institute Seminar, Very Soft Soil Stabilization Using High Strength Geosvnthetics, 1987, p. 239. Williams, N. D., and Abouzakhm, M. A., Geotextiles and Geomembranes. Vol. 8, No. 1; p. 5.

RECEIVED March

14, 1990


High-Tech Fibrous Materials - American Chemical Society

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