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Extrusion fillet welding of geomembranes
Geotextiles and Geomembranes 9 (1990) 281-293
Extrusion Fillet Welding of Geomembranes
F r e d Struve Gundle Lining Systems Inc., Houston, Texas 77073, USA
ABSTRACT This paper focuses on some of the basic principles that underly the fillet extrusion welding process. It explains why mixing tips in the melt enhance heat transfer and why peel test results of extrusion fillet welds are not directly comparable with results obtained by other welding methods.
1 INTRODUCTION 'Welding' two physical objects together requires a molecular interpenetration of the two objects at their interface. To achieve this, the materials at the interface need to be molten. After welding there is no longer a distinct recognizable interface. Joints without molecular interpenetration are usually referred to as 'laminates' in the plastics industry (and are similar to brazed or soldered metal joints). All of these non-welded joints still have a recognizable distinct interface. Analysis of geomembrane welding requires consideration of how energy (heat) is provided and transferred to the surface areas which are to be welded. The two most effective methods used today are: (a) Hot wedge welding which utilizes conduction of heat from a metal wedge to the geomembrane surfaces. This process can readily be adapted to self-propelled semi-automatic machines which perform very well on straight long seams. 281 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~) 1990 Elsevier Science Publishers
Ltd, England. Printed in Great Britain
282
Fred Struve
(b) Extrusion welding which utilizes the deposition of a bead of molten polymer on the geomembrane surfaces to provide heat. Extrusion welding can be performed by depositing a bead between the overlap of two geomembranes. This method, known in the industry as 'flat welding', is used only for long straight seams. It can also be performed by depositing a bead over the top of the upper and lower geomembranes at and adjacent to the edge of the upper geomembrane. This is known as an extrusion fillet weld. Figure 1 shows a typical extrusion fillet weld cross-section. Extrusion fillet welds are universally used for welding penetration details, cross-joints, patches and repairs, as well as being used for complete installations in some cases. Extrusion fillet welding is the one method used by all installers.
Overlap (Redundant)
Lower Sheet
Fig. 1. Typical cross-section of fillet extrusion weld. - - - Represents the sheet surfaces before welding. The shaded portion is the effective sheet continuum.
2 E X T R U S I O N F I L L E T W E L D I N G OF P O L Y O L E F I N S 2.1 Surface preparation All polyolefins are made up of molecules of varying molecular weights. They all contain some very low molecular weight molecules which are oily or waxy. A typical molecular weight distribution curve of a gas pipe grade High Density Polyethylene (Phillips T R 400) is shown in Fig. 2. Some of the very low molecular weight material tends to bloom to the surface of the geomembranes, forming a waxy layer. This layer should be removed by abrasion before extrusion welding as it tends to behave like a release layer between the geomembrane and the extrudate bead. 2.2 Heat considerations For a simplified analysis of an extrusion fillet weld the sizes and conditions shown in Fig. 3 will be assumed based on typical field weld conditions and
Extrusion fillet welding of geomembranes
283
Normalized Mass Distribution
.61 .5
"~.3
c5 ~.2
.1 0 2
3
4
5
6
7
Log(M)
Fig. 2. Molecular weight distribution of typical geomembrane HDPE.
=
1.00 INCH
i Upper And Lower Sheel 40°C (Without Pre-Heat)
=' /
I
Extrudate Bead
Shaded Area Must Reach 200"C To Melt And Weld To The Bead
Sheet thickness 0.080 in Sheet area to be melted 1.1 x 0.002 = 0.0022 in2 ~ m Area of extrudate bead = 0-176 in 2 ~ M M = 80 m (approximately) Fig. 3. Typical weld conditions. (One inch = 2.54 cm.)
b e a d sizes. T h e c r o s s - s e c t i o n a l a r e a s o f b o t h the e x t r u d a t e b e a d a n d the p o r t i o n o f the s h e e t s to be m e l t e d are p r o p o r t i o n a l to t h e i r r e s p e c t i v e mass p e r unit lengthl T h e a m o u n t o f h e a t r e q u i r e d to melt the s u r f a c e o f the s h e e t u n d e r the b e a d is ( a s s u m i n g n o h e a t loss) h (required) = m x S H x A T m = mass of sheet to be melted S H = specific heat of H D P E A T = temperature gradient: in this case (200 - 40) = 160°C h (required) = m x S H x 160
(1)
284
Fred Struve
The a m o u n t of heat available from the bead is: H
(available)
=
M×SH×
A T
M = mass of the extrudate bead specific heat of HDPE temperature gradient: in this case (260 - 200) = 60°C
SH= AT=
H (available) = M x
× 60
SH
Substituting 80 m for M (see Fig. 3) H (available) = 80 m x
x 60 × 4800
SH
m x SH
(2)
Dividing (2) by (1) we have: H (available) h (required)
4800 160
- -
-
30
We have approximately 30 times m o r e heat than we need. Note that at the time of deposition of the extrudate bead there is no heat loss through the top of the bead as it is covered by the extrusion welder's heated nozzle. The only heat loss at that time is due to conduction through the weld layer into the remaining thickness of the g e o m e m b r a n e sheets and ultimately the subgrade. Since H D P E has a relatively poor coefficient of conductive heat transfer (0.001 cal cm/s/cm a C) the simplified analysis presents a valid conceptual picture. Despite the enormous surplus of heat indicated, field experience demonstrates that the above listed conditions will not generally result in a weld. The efficiency of heat transfer from the extrudate bead into the sheet is very low. Two methods of overcoming this problem are generally used: (a) Preheating the sheets with hot air to approximately 100°C is one m e t h o d used to reduce the a m o u n t of heat required (h required), and in addition an increased size of the extrudate bead is used to provide m o r e heat ( H available). Equation (1) becomes: h (required) = m x
SH x A T = m x S H x (200 = mxSHx 100
- 100) (la)
Extrusion fillet welding o f geomembranes
285
and eqn (2) becomes: H (available) = M x S H x A T = lOOm×SHx60
(assuming a 25% increase in mass) = m x S H x 6000 Dividing (2a) by (la) we have:
(2a)
H (available)_ 60 h (required) With sixty times the required heat the inefficiency is overwhelming. However preheating is not very easily controlled, and the larger weld bead causes a larger undesirable change in section (bad for stress concentration).1 In addition the whole process does not lend itself to welding of thinner geomembranes. (b) The second m e t h o d used to overcome the heat transfer efficiency problem is one which directly reduces the inefficiency itself. This m e t h o d mechanically stirs the extrudate bead while it is in contact with the g e o m e m b r a n e sheet. The reasons why this m e t h o d is successful are analysed as follows: Heat transfer can be accomplished by three methods: (1) Radiation--which is of no relevance to this problem. (2) Conduction--which in the absence of stirring is the only means of heating the sheets. (3) Convection--which in the absence of stirring is zero (because a highly viscous extrudate bead is static). The two steady state equations describing conductive and convective heat transfer rates are: A T1 QI = k A - Ax
-
time rate of conductive heat flow
(3)
time rate of convective heat flow (4) surface area temperature gradient across the heat barrier thickness of the heat barrier thermal conductivity of the heat barrier difference in temperature between the solid surface and the bulk temperature of the fluid flowing over it h = convective heat transfer coefficient (which is a complex function of the fluid properties and characteristics of the flow).
Q2 = h A A T2 = A= AT1= Ax= k= AT2=
286
Fred Struve
In the case of a static extrudate bead on the sheet surface: (a) Q~ (rate of conductive heat flow) is a time-dependent function since the temperature gradient is continuously decreasing over any thickness element we may define at the surface of the sheet. (b) Q2 (rate of convective heat flow) is zero. If the extrudate bead is stirred, however, the values of both Q1 and Qe are increased as follows: (a) Q1 (rate of conductive heat flow) is increased as a result of the temperature at the sheet surface being maintained at the bulk temperature of the extrudate bead (which does not reduce as quickly as a static layer's temperature would). (b) Qa (rate of convective heat flow) becomes a real factor as the stirring creates a genuine mass flow across and in contact with the surface of the sheet. A detailed analysis of the heat flow rate in a dynamic system is an extremely complex problem in fluid dynamics involving boundary layers and turbulent flow as well as the analogous thermodynamic considerations (see Refs 2-4). For our purposes the analogy with heating a pot of mashed potatoes on a hot plate is a good one (although the heat flow is in the opposite direction). When placed on a hot plate the cold viscous static mashed potatoes will rapidly heat (and burn) at the bottom of the pot while the top surface of the potatoes will not perceptibly heat up for a long time (waiting for the slow conduction of heat through the static mass of mashed potatoes). If the mashed potatoes are stirred creating a mass flow of mashed potatoes across the heat source more heat can be removed from the heat source and the time taken to heat the complete mass of potatoes is reduced (efficiency increased). Figure 4 shows the most common configuration of an extrusion fillet weld with a rotating stirring element directly coupled to the rotating extruder screw. 8 Since the rate of heat transfer is increased by the stirring, melting of the geomembrane surface occurs rapidly while the nozzle is still over the weld zone. This process has proved its effectiveness: (a) By eliminating the need for preheating. (b) By making it possible to weld thinner geomembranes (without melting through them). (c) By allowing the use of a desirably small weld bead (minimum change of section and minimum total heat history of the weld zone). (d) By making it possible to weld consistently in cold and variable
Extrusion fillet welding of geomembranes
287
Extruder Screw
Extruder
Barrel
Electric Heater
Extrudate Bead I i
Geomembrane
1
I
Extrudate Deposited Ahead OI Rotating Element After Which It Is Rapidly Stirred Which Increases Heat Transfer Efficiency
Fig. 4. Schematic view of fillet extrusion welder (with rotating element).
ambient conditions since the extruder/extrudate conditions can be well controlled.
3 E X T R U S I O N FILLET W E L D I N G USED TO R E P A I R B E L O W SPECIFICATION HOT W E D G E W E L D S Extrusion fillet welding is performed on two geomembranes which are held in close proximity to one another by a 'tack weld' (double-sided adhesive tape is also used). This 'tack welding' is commonly done using either hot-air or hot wedge welders. (Since this is only a holding together function, the 'tack welds' are usually only laminates and not true welds.) Accepting the fact that extrusion fillet welding is required on all sites for close details and patches (and is therefore approved for all sites) it is logical to regard a below specification true hot wedge welded seam as a 'tack weld'. That seam can then be extrusion welded in the conventional manner to provide an acceptable seam (with proper QC testing of course). The alternative is to 'cap strip' the bad seam which requires two additional welds to be performed (and results in one additional seam in the job). Concern has been expressed about using extrusion fillet welds over a hot wedge weld if the extrudate bead is deposited over one of the hot wedge tracks. The 'double heat history' of the sheet at that point has been regarded with suspicion.
288
Fred Struve
Analysis of this type of seam repair is in progress as follows: (a) An analysis of molecular weight distribution thermal properties and rheology of the bulk sheet polymer, and the 'double heat history' polymer has been made by Phillips Chemical Company (Appendix). The results show 'no significant differences in molecular weight distribution, rheology or thermal properties'. (b) A hot wedge welded seam that has already successfully passed a 1000-h test for constant load environmental stress cracking by the Geosynthetics Research Institute (GRI) has had an extrusion fillet weld performed over it. This seam is currently being retested by the GRI with no results available at the time of writing this paper. All indications at this time are that extrusion fillet welding repair of hot wedge welds is good practice.
4 P E E L TESTING E X T R U S I O N FILLET WELDS--G E O M E T R I C FACTORS A peel test of a geomembrane weld is the most severe and revealing QC test the weld can be subjected to. Peel tests are therefore widely used even though welds are never designed to function in a peel configuration. There is a justifiable move in the industry to add to the 'Film Tear Bond' (FTB) criteria, specific bond strength values. The FTB criteria have a certain elegance in that the strength of the bond is compared with the actual strength of the two sheets joined (but only if the sheets have no significant scratches or imperfections). Thus the bond is assessed relative to the strength of the sheets it is joining and not relative to some theoretical minimum value (NSF or other). Assigning definite minimum load values that a seam should withstand in a peel test is however sound practice. The logical extension of this process of formalizing and quantifying peel test results, is to establish peel load criteria that should be met irrespective of the seaming method used. This is not correct! Comparison of extrusion fillet and wedge welds (for example) during testing shows significant configuration differences. The geomembrane sheet material (not the seam) is subjected to a totally different test. Figures 5, 6 and 7 show the actual configuration of 80 mil weld peel samples at different times during the course of tensometer peel tests. The theory of bending a rectangular beam (within the proportional limit of the material it is made from) teaches 5~7that the level of stress within the beam is proportional to the distance from the neutral axis (see Fig. 8(a)).
Extrusion fillet welding of geomembranes
t
l
289
LargeRadius
/~
(a) ExtrusionFilletWeld
L_J
~
..... (b;Hot=Wedg:Wdel
L_J ~rgegRadeiBsends
1 Fig. 5. Peel test samples when clamped in tensometer before loading.
/ BendRadiusDecreases/ AngleBecomesGreater\ Than90 Degrees
l (a) ExtrusiTilciill~icW~id
oesNotBend
~.
\
TailOfWeld es Down
L
90 Degree , ..... ~ Bends l (b) HotWedgeWeld TopAndBottomSheets BendIdentically Fig. 6. Peel test samples as initial load is applied (approximately 10 lb/in (1.8 kN/m) width).
290
Fred Struve
"l (a) Extrusion Fillet Weld Extremely Tight Radius pproximately 150 Degrees
l
l(b) Hot Wedge Weld I I '~ ~_ Degl'ee Bends I I • (Sometimes The Lower Sheet Bends I I As Much As 110 Degrees)
1 Fig. 7. Peel test samples as a load of 60 lb/in (10-8 kN/m) width is approached.
o
. . . . .
"-
(a)
(b)
Fig. 8. Stress diagrams of a rectangular beam in bending. (a) Outer fibre at yield point stress and (b) outer fibres plastically deformed.
When bending forces are increased to the point that the yield stress of the material is exceeded at the outer fibre, the outer fibre begins to deform plastically 6 (Fig. 8(b)). The tighter the radius and the greater the angle of bending the more the outer fibres are elongated. At the same time less and less material is left that has not been overstressed (Fig. 8(b)). This is due entirely to bending at a time when the peeling load on the weld interface is still relatively low (approximately 10 lb/in (1-7 kN/m) width on 80 mil (2.0 mm) versus a typical material yield strength of 190 lb/in (33 kN/m)).
Extrusionfillet weldingof geomembranes
291
The result of this is that extrusion fillet weld peel test results (of top-quality welds) are generally quantitatively lower than hot wedge weld test results by approximately 25%. This is due to sheet failures initiated prematurely in the overstressed portion of the severely bent tail of the weld where short lengths of the outer fibres are soon elongated and stressed beyond breaking strength. This difference must be recognized when specifying minimum values for peel strength. Note: Since the stress is proportional to distance from the neutral axis (and therefore thickness), this phenomenon is noticed less with thin sheet and becomes more significant as thickness is increased (it is already highly significant for 60 mil (1.5 mm) sheet).
REFERENCES 1. Timoshenko, S. Strength of Materials Part H. D. Van Nostrand, New York, 1962, pp. 324-9. 2. Purday, H. F. P. Streamline Flow. Constable, London, 1949, pp. 117-28 and 135-40. 3. Peck, W. J. & Richmond, A. J. Applied Thermodynamics Problems for Engineers. Edward Arnold, London, 1961, pp. 326--39. 4. Jones, J. B. & Hawkins, G. A. Engineering Thermodynamics. John Wiley, New York, 1960, pp. 661-72. 5. Timoshenko, S. Strength of Materials Part I. D. Van Nostrand, New York, 1960, pp. 346-52. 6. Timoshenko, S. Strength of Materials Part H. D. Van Nostrand, New York, 1962, pp. 346-52. 7. Case, J. & Chilver, A. H. Strength of Materials. Edward Arnold, London, 1959, pp. 151-6. 8. Struve, F. US Patent No. Re 32,103, April 1986. Welding of Plastic Material. APPENDIX September 13, 1989 PHILLIPS P E T R O L E U M COMPANY INTER-OFFICE CORRESPONDENCE/ SUBJECT: Bartlesville, Oklahoma From: K. W. Rollmann 153 CPL To: J. R. Burkinshaw 209 PTC
TR-400 Gundle Sheet Seams Ro11-13-89
Fred Struve
292
We tested a sample of Gundle pond liner (Fig. A1) made from TR-400 in order to determine if the resin in the weld seams is degraded in any way, causing it to be different from the sheet. The data in the following table and Figs A2 and A3 indicate that the welding results in no significant differences in molecular weight distribution, rheology, or thermal properties.
Sample No.
AD89-25 X sheet
Molecular weight Mw/1000 Mn/1000 HI IV (calc.) Melt rheology (RMS) Gx* 10 -5 (dyn/cm 2) RDI Vis*10 -5 @ 0.1 s -1 Vis*10 -4 @ 100 s -1 tan d @ 0.1 s -~ tan d @ 100s -1 Morphology Tm(°C) del Hf (J/g) Tmc(°C) del Hc (J/g) Tm (rescan) (°C) del Hf (rescan) (J/g)
Initial crystallinity (%)
191 10.1 18.8
AD89-25 Y hot wedge seam
AD89-25 Z hot wedge + fillet weld
210 10.2 20.6
211 10.0 21.1
2.09
2.20
2.21
3-75 1.83 4.73 1.73 1-50 0-79
3.64 1.85 4-83 1.73 1.46 0.79
3-83 1.84 5.08 1.82 1-45 0-79
129.4 178.2 112"3 165-3 129.1 176.0
128.2 178.4 113"1 167"3 128.5 176.3
130-4 179-2 111"8 106.9 130.3 174.2
62.3
62.4
62.6
Hot wedge weld
Hot w e d g e and f i l l e t weld
Sheet
(80 rail)
Fig. AI. Gundle weld study sample diagram. (80 mil = 2.0 mm.)
Extrusion fillet welding of geomembranes 0.6
|
i
3
4
293
|
!
5
6
0.4 t~
E
0~2
0
2
Log M
Fig. A2. Normalized mass distribution.
1 0
%
7
-
"
•
, l l l l l
-
.
•
•
i l i l l l
.
"
'
I I I I I I
"
•
'
l i l l l l
"
l i i l l J
_
•
•
l i l l l .
10 6
u
"o
10 5 O_
1010" 2
10-I
.
•
= i l l i l
.
•
,
10 °
101
,
•
l i l t
102
Frequency (radian Is)
Fig. A3. Mechanical properties. , VL16018 AD-89-25X (frequency sweep 190); , VL16019 AD-89-25Y (frequency sweep 190); , VL16020 AD-89-25Z (frequency sweep 190).
Extrusion Fillet Welding of Geomembranes
F r e d Struve Gundle Lining Systems Inc., Houston, Texas 77073, USA
ABSTRACT This paper focuses on some of the basic principles that underly the fillet extrusion welding process. It explains why mixing tips in the melt enhance heat transfer and why peel test results of extrusion fillet welds are not directly comparable with results obtained by other welding methods.
1 INTRODUCTION 'Welding' two physical objects together requires a molecular interpenetration of the two objects at their interface. To achieve this, the materials at the interface need to be molten. After welding there is no longer a distinct recognizable interface. Joints without molecular interpenetration are usually referred to as 'laminates' in the plastics industry (and are similar to brazed or soldered metal joints). All of these non-welded joints still have a recognizable distinct interface. Analysis of geomembrane welding requires consideration of how energy (heat) is provided and transferred to the surface areas which are to be welded. The two most effective methods used today are: (a) Hot wedge welding which utilizes conduction of heat from a metal wedge to the geomembrane surfaces. This process can readily be adapted to self-propelled semi-automatic machines which perform very well on straight long seams. 281 Geotextiles and Geomembranes 0266-1144/90/$03.50 (~) 1990 Elsevier Science Publishers
Ltd, England. Printed in Great Britain
282
Fred Struve
(b) Extrusion welding which utilizes the deposition of a bead of molten polymer on the geomembrane surfaces to provide heat. Extrusion welding can be performed by depositing a bead between the overlap of two geomembranes. This method, known in the industry as 'flat welding', is used only for long straight seams. It can also be performed by depositing a bead over the top of the upper and lower geomembranes at and adjacent to the edge of the upper geomembrane. This is known as an extrusion fillet weld. Figure 1 shows a typical extrusion fillet weld cross-section. Extrusion fillet welds are universally used for welding penetration details, cross-joints, patches and repairs, as well as being used for complete installations in some cases. Extrusion fillet welding is the one method used by all installers.
Overlap (Redundant)
Lower Sheet
Fig. 1. Typical cross-section of fillet extrusion weld. - - - Represents the sheet surfaces before welding. The shaded portion is the effective sheet continuum.
2 E X T R U S I O N F I L L E T W E L D I N G OF P O L Y O L E F I N S 2.1 Surface preparation All polyolefins are made up of molecules of varying molecular weights. They all contain some very low molecular weight molecules which are oily or waxy. A typical molecular weight distribution curve of a gas pipe grade High Density Polyethylene (Phillips T R 400) is shown in Fig. 2. Some of the very low molecular weight material tends to bloom to the surface of the geomembranes, forming a waxy layer. This layer should be removed by abrasion before extrusion welding as it tends to behave like a release layer between the geomembrane and the extrudate bead. 2.2 Heat considerations For a simplified analysis of an extrusion fillet weld the sizes and conditions shown in Fig. 3 will be assumed based on typical field weld conditions and
Extrusion fillet welding of geomembranes
283
Normalized Mass Distribution
.61 .5
"~.3
c5 ~.2
.1 0 2
3
4
5
6
7
Log(M)
Fig. 2. Molecular weight distribution of typical geomembrane HDPE.
=
1.00 INCH
i Upper And Lower Sheel 40°C (Without Pre-Heat)
=' /
I
Extrudate Bead
Shaded Area Must Reach 200"C To Melt And Weld To The Bead
Sheet thickness 0.080 in Sheet area to be melted 1.1 x 0.002 = 0.0022 in2 ~ m Area of extrudate bead = 0-176 in 2 ~ M M = 80 m (approximately) Fig. 3. Typical weld conditions. (One inch = 2.54 cm.)
b e a d sizes. T h e c r o s s - s e c t i o n a l a r e a s o f b o t h the e x t r u d a t e b e a d a n d the p o r t i o n o f the s h e e t s to be m e l t e d are p r o p o r t i o n a l to t h e i r r e s p e c t i v e mass p e r unit lengthl T h e a m o u n t o f h e a t r e q u i r e d to melt the s u r f a c e o f the s h e e t u n d e r the b e a d is ( a s s u m i n g n o h e a t loss) h (required) = m x S H x A T m = mass of sheet to be melted S H = specific heat of H D P E A T = temperature gradient: in this case (200 - 40) = 160°C h (required) = m x S H x 160
(1)
284
Fred Struve
The a m o u n t of heat available from the bead is: H
(available)
=
M×SH×
A T
M = mass of the extrudate bead specific heat of HDPE temperature gradient: in this case (260 - 200) = 60°C
SH= AT=
H (available) = M x
× 60
SH
Substituting 80 m for M (see Fig. 3) H (available) = 80 m x
x 60 × 4800
SH
m x SH
(2)
Dividing (2) by (1) we have: H (available) h (required)
4800 160
- -
-
30
We have approximately 30 times m o r e heat than we need. Note that at the time of deposition of the extrudate bead there is no heat loss through the top of the bead as it is covered by the extrusion welder's heated nozzle. The only heat loss at that time is due to conduction through the weld layer into the remaining thickness of the g e o m e m b r a n e sheets and ultimately the subgrade. Since H D P E has a relatively poor coefficient of conductive heat transfer (0.001 cal cm/s/cm a C) the simplified analysis presents a valid conceptual picture. Despite the enormous surplus of heat indicated, field experience demonstrates that the above listed conditions will not generally result in a weld. The efficiency of heat transfer from the extrudate bead into the sheet is very low. Two methods of overcoming this problem are generally used: (a) Preheating the sheets with hot air to approximately 100°C is one m e t h o d used to reduce the a m o u n t of heat required (h required), and in addition an increased size of the extrudate bead is used to provide m o r e heat ( H available). Equation (1) becomes: h (required) = m x
SH x A T = m x S H x (200 = mxSHx 100
- 100) (la)
Extrusion fillet welding o f geomembranes
285
and eqn (2) becomes: H (available) = M x S H x A T = lOOm×SHx60
(assuming a 25% increase in mass) = m x S H x 6000 Dividing (2a) by (la) we have:
(2a)
H (available)_ 60 h (required) With sixty times the required heat the inefficiency is overwhelming. However preheating is not very easily controlled, and the larger weld bead causes a larger undesirable change in section (bad for stress concentration).1 In addition the whole process does not lend itself to welding of thinner geomembranes. (b) The second m e t h o d used to overcome the heat transfer efficiency problem is one which directly reduces the inefficiency itself. This m e t h o d mechanically stirs the extrudate bead while it is in contact with the g e o m e m b r a n e sheet. The reasons why this m e t h o d is successful are analysed as follows: Heat transfer can be accomplished by three methods: (1) Radiation--which is of no relevance to this problem. (2) Conduction--which in the absence of stirring is the only means of heating the sheets. (3) Convection--which in the absence of stirring is zero (because a highly viscous extrudate bead is static). The two steady state equations describing conductive and convective heat transfer rates are: A T1 QI = k A - Ax
-
time rate of conductive heat flow
(3)
time rate of convective heat flow (4) surface area temperature gradient across the heat barrier thickness of the heat barrier thermal conductivity of the heat barrier difference in temperature between the solid surface and the bulk temperature of the fluid flowing over it h = convective heat transfer coefficient (which is a complex function of the fluid properties and characteristics of the flow).
Q2 = h A A T2 = A= AT1= Ax= k= AT2=
286
Fred Struve
In the case of a static extrudate bead on the sheet surface: (a) Q~ (rate of conductive heat flow) is a time-dependent function since the temperature gradient is continuously decreasing over any thickness element we may define at the surface of the sheet. (b) Q2 (rate of convective heat flow) is zero. If the extrudate bead is stirred, however, the values of both Q1 and Qe are increased as follows: (a) Q1 (rate of conductive heat flow) is increased as a result of the temperature at the sheet surface being maintained at the bulk temperature of the extrudate bead (which does not reduce as quickly as a static layer's temperature would). (b) Qa (rate of convective heat flow) becomes a real factor as the stirring creates a genuine mass flow across and in contact with the surface of the sheet. A detailed analysis of the heat flow rate in a dynamic system is an extremely complex problem in fluid dynamics involving boundary layers and turbulent flow as well as the analogous thermodynamic considerations (see Refs 2-4). For our purposes the analogy with heating a pot of mashed potatoes on a hot plate is a good one (although the heat flow is in the opposite direction). When placed on a hot plate the cold viscous static mashed potatoes will rapidly heat (and burn) at the bottom of the pot while the top surface of the potatoes will not perceptibly heat up for a long time (waiting for the slow conduction of heat through the static mass of mashed potatoes). If the mashed potatoes are stirred creating a mass flow of mashed potatoes across the heat source more heat can be removed from the heat source and the time taken to heat the complete mass of potatoes is reduced (efficiency increased). Figure 4 shows the most common configuration of an extrusion fillet weld with a rotating stirring element directly coupled to the rotating extruder screw. 8 Since the rate of heat transfer is increased by the stirring, melting of the geomembrane surface occurs rapidly while the nozzle is still over the weld zone. This process has proved its effectiveness: (a) By eliminating the need for preheating. (b) By making it possible to weld thinner geomembranes (without melting through them). (c) By allowing the use of a desirably small weld bead (minimum change of section and minimum total heat history of the weld zone). (d) By making it possible to weld consistently in cold and variable
Extrusion fillet welding of geomembranes
287
Extruder Screw
Extruder
Barrel
Electric Heater
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Geomembrane
1
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Extrudate Deposited Ahead OI Rotating Element After Which It Is Rapidly Stirred Which Increases Heat Transfer Efficiency
Fig. 4. Schematic view of fillet extrusion welder (with rotating element).
ambient conditions since the extruder/extrudate conditions can be well controlled.
3 E X T R U S I O N FILLET W E L D I N G USED TO R E P A I R B E L O W SPECIFICATION HOT W E D G E W E L D S Extrusion fillet welding is performed on two geomembranes which are held in close proximity to one another by a 'tack weld' (double-sided adhesive tape is also used). This 'tack welding' is commonly done using either hot-air or hot wedge welders. (Since this is only a holding together function, the 'tack welds' are usually only laminates and not true welds.) Accepting the fact that extrusion fillet welding is required on all sites for close details and patches (and is therefore approved for all sites) it is logical to regard a below specification true hot wedge welded seam as a 'tack weld'. That seam can then be extrusion welded in the conventional manner to provide an acceptable seam (with proper QC testing of course). The alternative is to 'cap strip' the bad seam which requires two additional welds to be performed (and results in one additional seam in the job). Concern has been expressed about using extrusion fillet welds over a hot wedge weld if the extrudate bead is deposited over one of the hot wedge tracks. The 'double heat history' of the sheet at that point has been regarded with suspicion.
288
Fred Struve
Analysis of this type of seam repair is in progress as follows: (a) An analysis of molecular weight distribution thermal properties and rheology of the bulk sheet polymer, and the 'double heat history' polymer has been made by Phillips Chemical Company (Appendix). The results show 'no significant differences in molecular weight distribution, rheology or thermal properties'. (b) A hot wedge welded seam that has already successfully passed a 1000-h test for constant load environmental stress cracking by the Geosynthetics Research Institute (GRI) has had an extrusion fillet weld performed over it. This seam is currently being retested by the GRI with no results available at the time of writing this paper. All indications at this time are that extrusion fillet welding repair of hot wedge welds is good practice.
4 P E E L TESTING E X T R U S I O N FILLET WELDS--G E O M E T R I C FACTORS A peel test of a geomembrane weld is the most severe and revealing QC test the weld can be subjected to. Peel tests are therefore widely used even though welds are never designed to function in a peel configuration. There is a justifiable move in the industry to add to the 'Film Tear Bond' (FTB) criteria, specific bond strength values. The FTB criteria have a certain elegance in that the strength of the bond is compared with the actual strength of the two sheets joined (but only if the sheets have no significant scratches or imperfections). Thus the bond is assessed relative to the strength of the sheets it is joining and not relative to some theoretical minimum value (NSF or other). Assigning definite minimum load values that a seam should withstand in a peel test is however sound practice. The logical extension of this process of formalizing and quantifying peel test results, is to establish peel load criteria that should be met irrespective of the seaming method used. This is not correct! Comparison of extrusion fillet and wedge welds (for example) during testing shows significant configuration differences. The geomembrane sheet material (not the seam) is subjected to a totally different test. Figures 5, 6 and 7 show the actual configuration of 80 mil weld peel samples at different times during the course of tensometer peel tests. The theory of bending a rectangular beam (within the proportional limit of the material it is made from) teaches 5~7that the level of stress within the beam is proportional to the distance from the neutral axis (see Fig. 8(a)).
Extrusion fillet welding of geomembranes
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l
289
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/~
(a) ExtrusionFilletWeld
L_J
~
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1 Fig. 5. Peel test samples when clamped in tensometer before loading.
/ BendRadiusDecreases/ AngleBecomesGreater\ Than90 Degrees
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oesNotBend
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90 Degree , ..... ~ Bends l (b) HotWedgeWeld TopAndBottomSheets BendIdentically Fig. 6. Peel test samples as initial load is applied (approximately 10 lb/in (1.8 kN/m) width).
290
Fred Struve
"l (a) Extrusion Fillet Weld Extremely Tight Radius pproximately 150 Degrees
l
l(b) Hot Wedge Weld I I '~ ~_ Degl'ee Bends I I • (Sometimes The Lower Sheet Bends I I As Much As 110 Degrees)
1 Fig. 7. Peel test samples as a load of 60 lb/in (10-8 kN/m) width is approached.
o
. . . . .
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(a)
(b)
Fig. 8. Stress diagrams of a rectangular beam in bending. (a) Outer fibre at yield point stress and (b) outer fibres plastically deformed.
When bending forces are increased to the point that the yield stress of the material is exceeded at the outer fibre, the outer fibre begins to deform plastically 6 (Fig. 8(b)). The tighter the radius and the greater the angle of bending the more the outer fibres are elongated. At the same time less and less material is left that has not been overstressed (Fig. 8(b)). This is due entirely to bending at a time when the peeling load on the weld interface is still relatively low (approximately 10 lb/in (1-7 kN/m) width on 80 mil (2.0 mm) versus a typical material yield strength of 190 lb/in (33 kN/m)).
Extrusionfillet weldingof geomembranes
291
The result of this is that extrusion fillet weld peel test results (of top-quality welds) are generally quantitatively lower than hot wedge weld test results by approximately 25%. This is due to sheet failures initiated prematurely in the overstressed portion of the severely bent tail of the weld where short lengths of the outer fibres are soon elongated and stressed beyond breaking strength. This difference must be recognized when specifying minimum values for peel strength. Note: Since the stress is proportional to distance from the neutral axis (and therefore thickness), this phenomenon is noticed less with thin sheet and becomes more significant as thickness is increased (it is already highly significant for 60 mil (1.5 mm) sheet).
REFERENCES 1. Timoshenko, S. Strength of Materials Part H. D. Van Nostrand, New York, 1962, pp. 324-9. 2. Purday, H. F. P. Streamline Flow. Constable, London, 1949, pp. 117-28 and 135-40. 3. Peck, W. J. & Richmond, A. J. Applied Thermodynamics Problems for Engineers. Edward Arnold, London, 1961, pp. 326--39. 4. Jones, J. B. & Hawkins, G. A. Engineering Thermodynamics. John Wiley, New York, 1960, pp. 661-72. 5. Timoshenko, S. Strength of Materials Part I. D. Van Nostrand, New York, 1960, pp. 346-52. 6. Timoshenko, S. Strength of Materials Part H. D. Van Nostrand, New York, 1962, pp. 346-52. 7. Case, J. & Chilver, A. H. Strength of Materials. Edward Arnold, London, 1959, pp. 151-6. 8. Struve, F. US Patent No. Re 32,103, April 1986. Welding of Plastic Material. APPENDIX September 13, 1989 PHILLIPS P E T R O L E U M COMPANY INTER-OFFICE CORRESPONDENCE/ SUBJECT: Bartlesville, Oklahoma From: K. W. Rollmann 153 CPL To: J. R. Burkinshaw 209 PTC
TR-400 Gundle Sheet Seams Ro11-13-89
Fred Struve
292
We tested a sample of Gundle pond liner (Fig. A1) made from TR-400 in order to determine if the resin in the weld seams is degraded in any way, causing it to be different from the sheet. The data in the following table and Figs A2 and A3 indicate that the welding results in no significant differences in molecular weight distribution, rheology, or thermal properties.
Sample No.
AD89-25 X sheet
Molecular weight Mw/1000 Mn/1000 HI IV (calc.) Melt rheology (RMS) Gx* 10 -5 (dyn/cm 2) RDI Vis*10 -5 @ 0.1 s -1 Vis*10 -4 @ 100 s -1 tan d @ 0.1 s -~ tan d @ 100s -1 Morphology Tm(°C) del Hf (J/g) Tmc(°C) del Hc (J/g) Tm (rescan) (°C) del Hf (rescan) (J/g)
Initial crystallinity (%)
191 10.1 18.8
AD89-25 Y hot wedge seam
AD89-25 Z hot wedge + fillet weld
210 10.2 20.6
211 10.0 21.1
2.09
2.20
2.21
3-75 1.83 4.73 1.73 1-50 0-79
3.64 1.85 4-83 1.73 1.46 0.79
3-83 1.84 5.08 1.82 1-45 0-79
129.4 178.2 112"3 165-3 129.1 176.0
128.2 178.4 113"1 167"3 128.5 176.3
130-4 179-2 111"8 106.9 130.3 174.2
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Fig. AI. Gundle weld study sample diagram. (80 mil = 2.0 mm.)
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