emerging methods for geomembrane seaming

Geotextiles and Geomembranes 9 (1990) 357-367

Emerging Methods for Geomembrane Seaming

A r t h u r E. L o r d , Jr, Yick H. Halse & R o b e r t M. K o e r n e r

Geosynthetic Research Institute, Drexel University, West Wing -- Rush Building, Philadelphia, Pennsylvania 19104, USA

A B S T R A C T

Selected geomembrane sheet seaming methods which are not widely used at the present time are discussed in this paper. These include ultrasonic bonding, electrical conduction bonding and electromagnetic induction bonding methods.

The physical principles of each of these techniques are discussed, as well as their current degree of development and implementation. The potential advantages and disadvantages of the methods are discussed.

I N T R O D U C T I O N

The fabrication of geomembrane seams is a most critical area. While 'conventional ' bonding techniques have been developed for specific geomembrane materials, 1 there is always room for improvement in the methods and possibly for the introduction of new methods. Therefore it is of interest and importance to explore new bonding approaches. The emerging techniques felt to be appropriate which will be described here are ultrasonic bonding, electrical conduction bonding and electromagnetic induction bonding.

U L T R A S O N I C SEAMING

Ultrasonic methods are used by various industries in a variety of ways. These include product cleaning, thickness gauging, nondestructive flaw

357 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~) 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

358 Arthur E. Lord, Jr, Yick H. Halse, Robert M. Koerner

detection, hardness testing, exotic machining, emulsification, sewing, biological cell disruption, bonding of quite dissimilar materials (as in the microelectronics area) and, of course, joining plastics. Thus the technique is well advanced and fully implemented in areas other than geomembrane seaming.

In ultrasonic seaming, an intense local vibration is induced at the material interface by means of a piezoelectric, or magnetostrictive, driven horn (see Fig. 1). The exact mechanism of the bonding is not completely understood, 2 but it involves friction-driven melting (at least locally) of the plastic and subsequent solidification and bonding. Pressure is usually applied to the interface during the bond's formation. It is also suggested 2'3 that the breaking of nonresin materials (e.g. oxides) at the interface may be inherent to the bonding process. The frequencies of vibration in ultrasonic welding are usually in the tens of kilohertz range. The weld time is typically very short; of the order of a few seconds.

I

Y Plas t i c sheets

T (a)

Horn

Anvil

~ Horn

' '

M e t a l sh Anv i l

(b)

Fig. 1. Schematic diagrams of ultrasonic welding of plastic (and metal) sheets (from ref. 2).

Applied Ultrasonics Inc. of Bethel, Connecticut, markets an ultrasonic welder for geomembrane sheet. The system can be used for factory seams and for field seams. Other papers in this book describe such activity. Figure 2 is a schematic drawing of the ultrasonic seaming process. The sheet material to be seamed is fed between the two rollers and between the sheets is located the ultrasonic horn. The knurled horn is shown in more detail in Fig. 3. The horn vibrates longitudinally at 40 000 Hz (cycles/s) and works in the squeeze roller nip areas directly against the two surfaces to be joined. The vibration peak-to-peak amplitude is about one and one-half thousandths of an inch. The vibrating action works with the frictional characteristics of the material to produce the heat for melting and bonding the membrane. Materials with higher frictional coefficients produce heat more rapidly. Knurled surfaces are usually incorporated into the working areas of the horn to better engage the material, to disrupt any surface contaminants, to concentrate the energy and to provide for mixing

Emerging methods for geomembrane seaming 359

Fig. 2. Schematic diagram of rollers, ultrasonic horn and geomembrane sheets in the ultrasonic seaming process. The method is called the 'Ultrascanner' by the developers.

(Redrawn from Ref. 4.)

Fig. 3 Close-up schematic diagram of the ultrasonic horn in the ultrasonic seaming process (Redrawn from Ref. 4.)

of the molten polymer just as the sheets are entering the squeeze portions of the rollers. The unit can seam thermoplastic liner material from 2.56 × 10 -4 to 3.2 × 10 -3 m (10-125 mils) thick, at rates of 0-9-1-5 m/min (3-5 ft/min). Shear and peel values of the comple ted seams are repor ted to be as good as with traditional seaming techniques.

The manufacturer emphasizes certain advantages of the ultrasonic seamer:

• no pregrinding or pretacking is necessary; • the entire process is almost completely under machine control, which

reduces the dependence on opera tor skill and attention;


• significant labor reduction (claimed as high as 50%) over more conventional methods;

• can be tooled for dual seam fabrication with an air-space between the seamed tracks;

• real-time feedback of welding conditions can be made so that welding power can be adjusted to prevent overheating, or underheating.

To date, the ultrasonic seaming technique has not had universal acceptance by the liner industry. A series of successful operations in the field will, of course, help its acceptance. Of interest are two technical notes by Obeda, 5"6 which describe some experiences with ultrasonic welding of geomembranes.

E L E C T R I C A L C O N D U C T I O N SEAMING

In the electrical conduction seaming technique for plastics, electric current is passed through wires embedded in (or placed between) the vicinity of the parts to be joined. The temperature of the wires rises via ohmic heating and the heat is transferred to the plastic which melts in the vicinity of the wires. Upon solidifying the parts are joined. Pressure is usually applied, either physically, or indirectly by differences in thermal expansion of the parts. Both AC and DC currents have been used. Welding times are typically the same as those common in the geomembrane thermal techniques, for it is essentially a thermal technique. This particular seaming method is widely used in the natural gas plastic pipeline area and is usually called the 'electro-fusion' technique. 7"8 Figure 4 gives a schematic diagram of the electro-fusion process which indicates that certain properties are measured in real welding time, and fed back to the control panel to readjust and optimize welding conditions. Figure 5 indicates the advanced research (finite element heat distributions and strength versus welding power) that has been done in the electro-fusion a r e a . 9

The initial adaptation of this technique to geomembranes was by E. O. Butts Ltd of Nepean, Ontario, Canada. 12 HDPE sheet of density 0-945 g/cm 3 and thickness 1.5 mm (0.060 in) was used. Stainless steel (400 series) wires with diameters ranging from 0.01 cm to 0.05 cm (0-004 in to 0.020 in) were coiled or braided around HDPE cylindrical cores (of the same density as the sheet) of either 3 mm or 5 mm diameter. The wires were actually braided onto the cores with a specially adapted braiding machine. 12 The wires were embedded into the surface of the plastic cores via a heat treating process at the end of the braiding process. The final wire


Fig. 4. Schematic diagram of an electro-fusion pipe coupling process. (From Ref. 9.)

M O D E L L I N G "HEAT T R A N S F E R

U S I N G F IN ITE E L E M E N T S

25 ,00~

20 ,00~

15,00~

10.000

Rupture Stress (N)

Load at Rupture

Coil Temperature at tt~e oncl of

Wek~pn PhL~o

Interface Temperature at the end of

ing Phas,

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I I I I I I I 3000 4000 5000 8000 7000 8000 9000 10,000

Temperature ("C)

400

' 300

' 200

'100

|

Supplied Energy (J)

B E H A V I O R OF A W E L D E D JOI.NT

Fig. 5. Heat transfer in and mechanical strength of an electro-fusion welded joint. (Redrawn from Ref. 9.)


EOMEMBRANE ,~,'~ ~" ,

0 SOURCE 0 CURRENT

Fig. 6. Schematic diagram of the electrical conduction method of joining geomembrane sheet (this method is called 'Fustich' by the developers).

250

200

LU iSO

I.~

I00 t.u

LU 50

core

/ /~ - - - - sur foce q- - / S . . . . .

p o i n / / / / ,.. vo,Ts [

/ / 8 AMPERES

I i i I

o so Ioo Iso 200 2so

TIME (seconds)

i

30O

Fig. 7. Typical core and sheet surface temperatures during the electrical conduction welding process. (From Ref. 12.)

wound cores had an electrical resistance of about 3-6 ohms/m (1-2 ohms/ft).

The sheet surfaces were cleaned with isopropyl alcohol before the joining process. It was found that any grinding before assembly greatly deter iora ted the final weld strength. The coiled core was then placed be tween the sheets. Figure 6 is a schematic diagram of the fabrication process. A force was applied normal to the sheets with the braided wire between. Electrical current (AC) of 5-10 A was passed through the wires. The wires heat due to ohmic effects and melt the core and adjacent sheet material. Typical core and sheet surface tempera tures are shown in Fig. 7.


250

zoo

[~1 1 5 0

I 0 0

50

co o

mel t ing . . . . . . . . . . . . ~ ~ t -

I I I I I

2 I 0 I 2

D/STANCE FROM CORE CENTER (cm )

Fig. 8. Typical lateral sheet surface temperature gradients in the electrical conduction welding process. (From Ref. 12.)

Core temperature was measured with an embedded thermocouple. Surface temperature was measured with an infrared pyrometer. Surface temperature gradients are shown in Fig. 8. The current was stopped, after a prescribed time, and the material subsequently solidified, thereby bonding the sheets together. Under the present experimental circumst- ances, maximum weld lengths of about 1 m (3 ft) can be made with a single application of the current electrodes.

Figure 9 shows a polished cross-section of the wires in a finished electrical conduction weld. There is no indication of large voids in the immediate vicinity of the interface between wire and polymer.

Standard shear and peel tests were performed under ambient conditions on 25 mm wide samples at an extension rate of 5 cm/min (2 in/min). In the electrical conduction welds, shear strengths of 90% (of sheet) and peel strengths of 80% (of sheet) are consistently realized. These values are comparable to other seaming techniques currently available. Seams which gave somewhat lower values than those given above usually showed exposed wires after failure. Good mechanical strengths were obtained with input electrical energy (voltage × current × time) between 7000 and 10 000 J. The average power was between 40 and 70 W. These figures apply for a 1/3 m (1 ft) length of seam.

Two types of long-term mechanical tests are being evaluated. In the first type, 12 12 mm wide, hand-cut seamed samples were put under an initial stress of 7000 kPa (1000 psi) and cycled between -20°Cand +50°C once daily. The stress did relax somewhat during certain of these tests. In other


(a)

(b)

Fig. 9. Cross-section of the electrical conduction welds (a) 50x (b) 500x. (From Ref. 12.)


tests, the stress was kept reasonably constant. After 6 months of testing, no seam failures have occurred in the four samples of electrical conduction welds. In the second type of long-term testing, a fixed stress of 7000 kPa is applied to 12 mm wide, die-cut samples of seamed materials under ambient conditions. After 4 months of ambient temperature testing, no failures have occurred in the electrical conduction weld (or in any of the other three commonly used types of weld also being tested). More extensive long-term tests on electro-fusion bonds indicate that the presence of these foreign bodies (i.e. metal wires) does not shorten the lifetime of the joints. 7"s

ELECTROMAGNETIC INDUCTION SEAMING

In electromagnetic induction seaming a conductor and/or hysteretic material (in the form of wires, particles, strips, etc.) is placed at the interface to be joined. A noncontacting induction coil containing high- frequency electric current passes over the area to be seamed. The time-varying magnetic field caused by the current in the coil induces eddy currents and/or hysteresis loss in the embedded materials. Hence the area is heated, melts, solidifies and bonding takes place. Pressure is usually applied to the interface. Frequencies mentioned range from 3-7 MHz ~1 to 80-320 kHz, 12 depending on the application. A wide variety of plastic assembly and sealing applications has been performed. ~ The electromagnetic induction method has been mentioned briefly in the natural gas pipeline literature, 8 but with no details as to its use.

The initial adaptation of this technique to geomembranes was also by E. O. Butts Ltd. 12 Fig. 10 is a schematic diagram of the fabrication process of electromagnetic induction seaming. As with the electrical conduction method described previously, HDPE sheet of 0.945 g/m 3 density and 1.5 mm thickness was used in the tests. The braided core, made by the same process as described for the electrical conduction method, is placed between the two (cleaned) sheets and force is applied normal to the sheet. An electromagnetic coil carrying high-frequency alternating current of about 200 kHz is passed directly over the braided core. No contact whatsoever is made between the electromagnetic coil and the sheet. The coil is about 1 cm above the top geomembrane sheet. Eddy currents are induced in the embedded braided wire by the time-varying magnetic field. This results in ohmic heating which melts the core and a certain amount of the adjacent sheet material. After the coil has passed, the eddy currents cease and the material solidifies and bonds the sheets together. Rates of about 1/3 m/min (1 ft/min) have been achieved at this time. Preliminary

366 Arthur E. Lord, Jr, Yick H, Halse, Robert M. Koerner

NIGN FREQUENCY ~ 'P'ALTE RNATIN G CURRENT

/GEOMEMBRANE// , . .

COIL--~ fl'~ "t~" INDUCED

CORE P~R E ~ ESSUR

Fig. 10. Schematic diagram of the electromagnetic induction method of joining geomembrane sheet.

results of mechanical testing of the seams give about 90% of sheet value for shear but are very poor in peel. By optimization of the welding parameters, this situation may improve.

CONCLUSIONS

A group of possible emerging geomembrane seaming techniques have been described and discussed here. They are the ultrasonic, electrical conduction and electromagnetic induction methods.

The ultrasonic method is clearly the more highly developed of the three with actual field seaming projects in operation (refer elsewhere in these Proceedings). The producers of the ultrasonic seaming approach mention that the method produces seams of the same mechanical strength as the more widely used methods. It is also claimed that the method is more 'user-friendly' than currently available seaming techniques and is more automatic, hence less prone to operator error. Real-time feedback of welding conditions allows for adjustment of input power to lessen chances of improper heating.

The electrical conduction method, although widely used in the natural gas pipeline industry, is in its formative stages as a seaming method for geomembranes. Although good mechanical properties of geomembrane seams can be achieved, only short lengths of geomembranes can be seamed.~2 Furthermore no field trials have as yet been performed.

The electromagnetic induction method has been used to join relatively small parts, 11 but has not been used to any extent in the natural gas pipeline and not at all in the geomembrane joining areas as far as the


authors are aware. Preliminary work done in this regard on geomembrane sheet '2 indicates some potential for the method. There are certain advantages in the electromagnetic induction method. These include:

• The ability to go back over the weld and reheat if bonding was incomplete on the first pass.

• Real-t ime readout of the welding parameters so that power adjust- ments can be made.

• Multicore, i.e. multiseam welds are essentially the same as one seam.

It is hoped that this discussion of emerging seaming methods will be of help to workers in the field in gaining more perspective of the entire area of plastics seaming and where technology transfer can be used to aid in geomembrane seaming.

R E F E R E N C E S

1. Koerner, R. M. Designing with Geosynthetics. Prentice Hall, Englewood Cliffs, N J, 1986, pp. 292-6.

2. Thomas, R. H. Sr Ultrasonics in Packaging and Plastics Fabrication. Cahners Books, Boston, MA, 1974.

3. Herman, G. G. (ed.) Microelectronic Ultrasonic Bonding. Nat. Bur. Std Spec. Publ. 400-2, 1974.

4. Obeda, E. G. United States Patent No. 4,834,827, May 30, 1989, Apparatus and method for ultrasonically joining sheets of thermoplastic materials.

5. Obeda, E. G. Tap into ultrasonic technology. Presented at IFAI Session (no date given).

6. Obeda, E.G. Ultrascanner advances textile technology. Presented at IFAI Session (no date given).

7. Ewing, L. & Maine, L. The electrofusion of PE gas pipe systems in British Gas. Proc. 8th Plastic Fuel Gas Pipe Symp., Nov. 29-30, Dec. 1, 1983, New Orleans, LA, pp. 102-9.

8. Ewing, L. & Richardson, W. Polyethylene gas pipe systems: An appraisal of joint design and construction methods. Proc. 7th Plastic Fuel Gas Pipe Syrup., Nov. 12-14, 1980, New Orleans, LA, pp. 25-31.

9. Usclat, D. Producing a good joint with electrofusion fitting. Proc. 9th Plastic Fuel Gas Pipe Symposium, Nov. 12-14, 1985, New Orleans, LA, pp. 57-69.

10. Journay, J. P. & Usclat, D. The construction of plastic distribution networks in Gaz de France. Proc. 8th Plastic Fuel Gas Pipe Syrup., Nov. 29-30, Dec. 1, 1983, New Orleans, LA, pp. 148--62.

11. Literature from Emabond Systems, Norwood, NJ. 12. Welding Thermo Plastics, IRAP Collaborative Grant, CA-103-8-1277,

Interim Technical Report, submitted to the National Research Council of Canada, Dec. 30, 1988 by E. O. Butts Consultants Ltd, Nepean, Ontario, Canada.

emerging methods for geomembrane seaming

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