Environmental Protection, Inc.

---

THERMALLY BONDED PVC SEAMS

Phase II - The Effects of Welding Speed, Welding Temperature, and Sheet Temperature on the Peel Strength and Burst Strength of 30 mil and 40 mil PVC Double-Track Fusion Seams

Author:  Richard W. Thomas, TRI/Environmental, Inc.

Summary: Test welds were made with two types of welding machines, at two different sheet temperatures, on two thicknesses of sheet, at three set point temperatures and at three speeds.  The 72 seams were evaluated by the peel test at room temperature and by burst tests performed at three different temperatures. The results showed the importance of welder set point and speed.  The results also showed that there is a strong relationship between peel and burst and that a non-destructive burst test, performed in the field, could be used to ensure the strength of installed seams.

INTRODUCTION

The objective of Phase II was to learn more about the thermal welding process of PVC geomembranes and to develop a window of appropriate conditions for welding.  More specifically, a wider range of welder temperatures and speeds were to be evaluated along with the effects of sheet temperature.  There was also interest to further explore the relationship between bursting the seams from the air channel out and peeling the seams from the outside in.  If this relationship is known, one can eliminate the practice of cutting holes in seams to determine seam strength.

SEAM PREPARATION

The 72 prepared seams were made in a single day in Austin, Texas on an asphalt subgrade.  There were two crews, one using a hot air welder and the other a hot wedge welding machine.  The hot air machine was the same one used in Phase I, namely a Leister Twinnie Model CH6056.  The hot wedge machine was a “Mini-Wedge” made by Plastic Welding Technologies (formerly Columbine).

The crews each used three welder set points and three welder speeds based on their “normal” conditions, and their experience.  Each crew made a set of 30 mil and 40 mil seams in the shade in the morning. Then, they each made an identical set of seams in the sun in the afternoon.  The sheet temperatures range from 50 to 100°F.  The temperature was monitored by a thermocouple attached to the sheet.  The effect of nip roller pressure on seaming was not examined during this study.  Both welders had a typical pressure pre-set and this was maintained throughout the seaming operation.  The following table shows the different conditions used.

                                                   Table 1 - Seaming Parameters Used

Welder Type	Sheet      Thickness (mil)	Sheet     Temperature (°F)	Welder       Speed (ft/min)	Welder     Temperature (°F)
Hot Air	30                 40	50,80               60, 90	4, 7, 10             4, 7, 10	608, 680, 734         680, 734, 824
Hot Wedge	30                 40	50, 90              60, 100	3, 10, 19            3, 10, 19	700, 800, 900         750, 825, 900

 

SEAM EVALUATION

The seams were evaluated by the standard peel test at 20 in/min at 73°F and by a burst test developed for this project.  The burst test was performed by sealing off one end of a seam length and pressurizing the other end with compressed air.  The basic procedure was to select a starting pressure, hold there for 30 seconds, then ramp 5 psi at a time, holding for 30 seconds for each 5 psi step.  The 5 psi was applied in a 5 second time period.  This went on until failure occurred.  Most of the failures were peels that occurred during the 30 second soak.  However, there were some seams that burst during a 5 psi step.

The burst test done at room temperature was done on a 6 feet length of seam.  More seam length was useful to determine the relationship between peel and burst.  The burst test was also performed at two higher temperatures.  These tests were performed in a constant temperature room set for 100°F and 120 °F.  The actual sheet temperatures were 95°F and 116°F.  These elevated temperature tests were performed on 4 feet lengths of seam. The seam was clamped in the center, then both 2 feet halves tested to produce duplicate results.  None of the individual test strips that were pressurized were also peel tested. All tests were performed on as-made strips from the original 30 ft length.

Ø Click for Slideshow 

RESULTS

The results from testing all of the 72 seams for peel strength and for burst strength at three temperatures are given in Tables 2-9.  The two values listed for peel strength are the two weld tracks.

The burst values at room temperature were done once while the higher temperature bursts were done in duplicate. The values presented are the averages of the two burst tests.  Typically, the two results were within 5 psi of one another.  Also, in the hot air seams (“A” series) the numbering is not consistent from 1 to 36, as it is in the hot wedge seams (“W” series).

There were two instances where a particular seam burst at a very low value due to specific weak spot in the seam.  They occurred at the temperatures of 800° F and 900° F, and looked like “burn through”.  This is the type of thing that will occur in the field since 100% of the seams will be tested.  In the field, these weak spots would be locations for a patch.

Otherwise, the burst behavior was as one might predict.  As more heat got into the seam, the peel  and burst values increased.  Of course, more heat gets into a seam from higher set point temperatures, slower speeds, or an increased sheet temperature.  Experienced welders will often adjust their welding speed to the ambient conditions.  For example, they might increase their speed during the day as the temperature gets warmer.  Or, if clouds suddenly appear, they may slow down because the sheet is cooler in the shade.

The strongest seams exceeded the pressure gauge used.  It had a scale up to 120 psi, so above this pressure, the operator made an estimate of the pressure.  The pointer still went higher, but there was no scale to read above 120 psi.

Table 2.  Peel and Burst Results of 30 mil Seams

Made with a Hot Air Welder at 50°F

Seam    Number	Welder Temp (°F)	Speed     (ft/min)	Peel Str.    At 73°F       (ppi)	Burst Str.    At 73°F       (psi)	Burst Str.    At 95°F       (psi)	Burst Str.   At 116°F       (psi)
A1	608	3.9	24/35	65	45	39.5
A2		6.9	18/24	33	25	20
A3		9.8	12/13	20	15	10
A4	680	3.9	39/40	75	52.5	40
A5		6.9	28/29	45	32.5	27.5
A6		9.8	13/14	30	17.5	15
A7	734	3.9	43/45	70	55	40
A8		6.9	27/32	60	42.5	35
A9		9.8	17/25	33	22	19.5

 

Table 3.  Peel and Burst Results of 30 mil Seams Made with a Hot Air Welder at 80°F

Seam    Number	Welder Temp (°F)	Speed     (ft/min)	Peel Str.    At 73°F       (ppi)	Burst Str.    At 73°F       (psi)	Burst Str.    At 95°F       (psi)	Burst Str.   At 116°F       (psi)
A10	608	3.9	35/37	65	50	37.5
A11		6.9	29	35	27.5	22.5
A12		9.8	9/12	20	15	10
A13	680	3.9	41/47	75	55	40
A14		6.9	18/27	40	32.5	22.5
A15		9.8	15/17	35	25	20
A16	734	3.9	45/45	79	55.5	42.5
A17		6.9	32/33	50	40	30
A18		9.8	16/19	30	27.5	20

 

Table 4.  Peel and Burst Results of 40 mil Seams Made with a Hot Air Welder at 60°F

Seam    Number	Welder Temp (°F)	Speed     (ft/min)	Peel Str.    At 73°F       (ppi)	Burst Str.    At 73°F       (psi)	Burst Str.    At 95°F       (psi)	Burst Str.   At 116°F       (psi)
A37	680	3.9	43/46	100	65	50
A38		6.9	22/25	80	30	19
A39		9.8	15/15	35	19	14.5
A40	734	3.9	44/44	100	70	51
A41		6.9	29/34	55	37.5	27.5
A42		9.8	18/19	42	25	17.5
A43	824	3.9	40/49	115	77.5	60
A44		6.9	39/42	80	55	37.5
A45		9.8	26/27	54	25	25

 

 Table 5.  Peel and Burst Results of 40 mil Seams Made with a Hot Air Welder at 90°F

Seam    Number	Welder Temp (°F)	Speed     (ft/min)	Peel Str.    At 73°F       (ppi)	Burst Str.    At 73°F       (psi)	Burst Str.    At 95°F       (psi)	Burst Str.   At 116°F       (psi)
A28	680	3.9	45/48	110	75	52.5
A29		6.9	19/23	55	35	32.5
A30		9.8	10/10	25	25	15