Analysis of Thermoplastics Used in Fold-and-Form PVC Pipe Liners
by E. R. Griffin, Senior Technical Specialist, DuPont
Of the 200,000 miles of wastewater (sewer) pipe in need of repair, more than 20,000 miles will be repaired in the next 5 years. PVC fold-and-form pipe had about 7 percent of the total rehabilitation market in 1995 and is expected to grow to 27 percent of the total by 2000.
The objective of this paper is to relate plastic material properties to installation and performance properties of pipeline rehabilitation materials. The work will concentrate on plastics used in the area of fold-and-form or deform/reformed pipe liner. These plastics include polyvinyl chloride (rigid PVC), PVC modified with DuPont™ Elvaloy® ketone ethylene ester (KEE) resin (modified PVC), and high-density polyethylene (HDPE).
This paper will show that PVC modified with Elvaloy®:
- Has a wider forming temperature window and installs faster and easier than polyethylene or rigid PVC;
- Provides a snug fit with less annular space than polyethylene or rigid PVC;
- Creeps less than polyethylene over the life of the liner; and
- Has less tendency than rigid PVC to split or crack.
"No-Dig" Fold-and-Form PVC Pipe Liners
Rigid PVC, PVC modified with Elvaloy® , and high-density polyethylene are three principal thermoplastics used for trenchless ("no-dig") rehabilitation of wastewater pipes. Liners made of these plastics are manufactured with a cross-sectional area, then wound on large spools to be shipped to installation sites. In the field, the new liner is pulled through the host pipe, entering and exiting via existing manholes. The liner is then reformed in place using heat and pressure.
This paper addresses properties of plastic that allow the material to be thermoprocessed and reduce the effects of stress introduced by thermoprocessing. Properties of interest include the liner's ability to be flexible while being installed, to be stretched and formed into the shape of the host pipe, and to maintain its strength and shape over time.
Plastic Forming or Reforming
Forming the pipe liner inside a host pipe is similar to plastic thermoforming, where plastic sheet is heated to a forming temperature and then deformed or stretched to a desired shape using vacuum or pressure. Often, the thermoformed shape is defined by a mold. Examples include blister packaging, cups, skylight bubbles and refrigerator door liners.
In the pipe liner forming process, the plastic liner is heated and formed or stretched into the shape of the host pipe. The forming quality depends on the temperature, pressure, and time used by the operator, and on the forming temperature and pressure window allowed by the plastic.
The forming window is determined by the polymer morphology, and by physical properties such as tensile strength and elongation. Properties important to various pipe liner processes are listed in Table 1.
TABLE 1. PVC PIPE LINERS FOR REFORMING PROCESS
|Pipe Liner Process||Plastic Property|
|Reducing internal stresses during forming and reforming||Modulus, molecular mobility and elongation at the forming temperature|
|Stretching over joints and obstacles in the host pipe||Tensile strength and elongation at the forming temperature|
|Shrinkage, maintaining a snug fit after cooling and leaving a small annular space||Elongation at the forming temperature|
|Relieving internal stresses and withstanding stress during finishing and trimming||Elongation and impact resistance at ambient temperature|
|Strength and resistance to deformation under load over time||Viscoelastic properties and creep resistence|
The plastic properties of strength, elongation, molecular mobility, volume versus temperature, and viscoelastic properties, are functions of the crystallinity or lack of crystallinity of the polymer, discussed below.
Crystalline and Amorphous Polymers
There are two broad categories of polymers: crystalline and amorphous. Polymers are considered crystalline if their molecules arrange in an orderly, laminar configuration. More accurately, these polymers are referred to as semicrystalline, because only a portion of their molecules are in a crystalline form.(5) In contrast, amorphous polymers are those that have no known order or pattern.
As polymers are heated, the polymer chains gain mobility and the polymer properties go through notable transitions. One of the most significant thermal transitions is the glass transition, which occurs over a temperature range starting at the glass transition temperature (Tg). The temperature range for this transition is unique for each polymer.
This glass transition relates only to the amorphous (noncrystalline) portion of polymers. At temperatures below this transition, the polymer is glasslike: It has a high flexural modulus or high stiffness. As temperatures increase through the glass transition region, the amorphous portions gain molecular mobility and change from a high-modulus (rigid) state to a lower-modulus (rubbery) state. The rate of this transition is unique for each polymer.
After the glass transition, the rate of change of modulus versus temperature returns to a very flat curve. The temperature continues to increase to the crystalline melting temperature.
Highly crystalline polymers, such as high-density polyethylene, remain somewhat stiff as their small amorphous regions go through their glass transition. The crystalline regions of the polymer hold the molecules in place. Because the polymer remains stiff, it can be used above its Tg without losing its form. However, as the temperature of a highly crystalline polymer continues to rise, its crystalline regions transition to a low-modulus liquid at the crystalline melt temperature (Tm).
Tm is usually much higher than Tg. For example; for HDPE Tg is a very low -160°C and Tm is 134°C. For PVC, Tg is 87°C and Tm is considered 200°C. At temperatures above Tg (in the case of HDPE, anything above -160°C), a crystalline polymer can and does deform under load . . . more easily than if it were at temperatures below its Tg.
As it relates to pipe liner installation, the thermal transitions and amount of crystallinity greatly affect how easily the material forms at the forming temperature, how well the internal stresses are relieved, how much the polymer shrinks, and how much the pipe liner deforms over time, or creeps.
Forming the Polymer
As plastic is heated, the polymer chains gain mobility and can be reformed into a different shape. Above their Tg, most amorphous polymers (including PVC) can begin to flow if there is sufficient pressure or load. With increasing temperatures, the polymer chains gain more mobility and can flow using less pressure. Amorphous material can retain its new shape after the temperature is dropped below Tg and the load is removed.
For crystalline polymers at temperatures between Tg and Tm, chain mobility is constrained by the crystalline regions of the polymer. The crystalline structures are not fully mobile until the temperature exceeds the crystalline melting point (Tm).
Polyethylene is formed mostly by melting the crystalline regions and reforming them into the new shape. The crystalline melting region for polyethylene has a wide temperature range, with a very slow increase in flow as temperature is increased. To completely reform polyethylene and relieve all the stress of the original shape, the polymer must be completely melted to a liquid by heating to 284°F (140°C) or higher.
However, as polyethylene is heated above its Tm of about 273°F (134°C), it begins to flow like a liquid. Its melt viscosity at such temperature is too low for thermoforming and pipe liner forming. Thus, HDPE pipe liner forming requires a difficult balance: The polymer needs to get hot enough to relieve stress and reform crystalline regions, but stay cool enough to have sufficient melt strength to maintain pipe shape and not become a liquid. Thus the forming window is very narrow and must be carefully balanced during pipe liner forming. Complete stress relief is not possible while melt strength is maintained.
In contrast, PVC is formed mostly while it is in a rubbery state, above Tg and below Tm. The high melt strength of the rubbery state makes PVC easy to thermoform. PVC modified with Elvaloy® is even easier to thermoform because it has a wider temperature window and increased melt strength.
One way to study thermal transitions and polymer flow is dynamic mechanical analysis (DMA). DMA measures the storage modulus (E'), the loss modulus (E") and their ratio (E"/E'), known as tan delta of a polymer over a temperature range. The storage modulus can be thought of as the stiffness of the polymer, like flex modulus or tensile modulus. Figures 1, 2, and 3 show the DMA data for PVC, PVC modified with Elvaloy® , and HDPE.
Figure 4 compares E' of PVC, PVC modified with Elvaloy® , and HDPE as temperature is increased. The modulus of HDPE is lower than both PVC types at room temperature, and then slowly curves down as the temperature increases. Because the test cannot hold a liquid sample, the HDPE test is terminated before the polymer melts at 273°F (134°C).
The moduli of PVC and PVC modified with Elvaloy® are high until the resins reach their glass transition temperatures (about 180 and 155°F, respectively). The modulus drops as the temperature increases through the Tg region, and finally flattens in the rubbery state. The PVC samples are still rubbery at the melt temperature (Tm) of HDPE.
It is instructive to examine the modulus of the polymers at the pipe liner forming temperatures described in ASTM standard sample preparation methods for polyethylene and PVC pipe liner. (See Table 2.)
TABLE 2. PIPE LINER PREPARATION METHODS(1,2)
|SAMPLE FORMING PROCEDURE||PVC||PE|
Step 1 Temp. °F (°C)
Step 2 Temp. °F (°C)
Step 3 Temp. °F (°C)
Step 4 Temp. °F (°C)
The minimum forming temperature can be defined as the temperature listed in Step 2. The maximum temperature is Tm. Note the modulus E' of the plastics at the forming temperatures 250 to 273°F (121 to 134°C) of HDPE, as plotted in Figure 4. PVC and PVC modified with Elvaloy® have lower modulus and flat curves of the rubbery state at these temperatures. The modulus of the HDPE is still very high and continues to drop as it reaches its melting point.
One can assume that the modulus of the pipe liner during forming must be as low as the modulus of HDPE at 250°F (121°C). Therefore, Figure 4 includes a line indicating that modulus. The PVC and the PVC modified with Elvaloy® will reach the same modulus at lower temperatures: 195°F and 180°F (90°C and 82°C). The modulus of polyethylene must stay above the liquid stage at Tm 273°F (134°C).
This results in a narrow forming window for the HDPE: 250° to 273°F, at a higher temperature and pressure. A narrow forming temperature window leaves little room for uncontrollable variables like groundwater temperature, water in the host pipe, and thermocouple error. The PVC-based liners have much broader temperature windows: 180 to 273°F for PVC modified with Elvaloy® and 195 to 273°F for PVC. The PVC based liners can be formed at lower pressure since the modulus can be lowered by increasing the temperature to the rubbery stage. This leaves more room for those uncontrollable variables.
In addition, because the crystalline melting temperature of HDPE is not reached during forming, not all of the crystalline regions are reformed. Thus, these crystalline regions will tend to revert back to their original form and shape, which is significant for the highly crystalline HDPE. Because the temperatures during forming are above the glass transition temperatures, the amorphous regions of the polymers are relieved of internal stress and reformed. This is more significant for the amorphous PVC systems, including PVC modified with Elvaloy®.
Creating a Snug Pipe Liner
The PVC modified with Elvaloy® maintains a snug fit after cooling and does not move during temperature fluctuations. The reason for this is found in the way plastics cool.
The molecules of crystalline polymers such as polyethylene tend to move closer to each other as they cool. This is the nature of crystallinity, giving the polymer a tight molecular matrix and some rigidity at temperatures above Tg. In contrast, amorphous polymers such as PVC do not pack closer as they cool. They gain rigidity by locking the amorphous regions into place at temperatures below their Tg.
Polymer volume change during cooling is studied using pressure-volume-temperature (PVT) data, or volume expansivity. The specific volumes of the polymers were measured as the temperature was increased over the range of 86 to 375°F (30 to 190°C). Linear thermal expansion is one dimension of specific volume. Figure 5 is a plot of the specific volumes (cm3 per gram) of PVC modified with Elvaloy® , of rigid PVC dry blend, and of polyethylene pipeliner. The polyethylene curves show a dramatic change in specific volume near 266°F (130°C), which is where the polymers begin to melt.
Table 3 shows the change in specific volume during cooling of HDPE and PVC samples, using the temperatures suggested in the standard sample preparation method.
TABLE 3. Specific Volume at Forming and Cooled Temperatures
|SPECIFIC VOLUME||PVC WITH ELVALOY®||PVC||HDPE|
|At forming temperature||
0.8 cm3/g at 200°F (93°C)
0.75 cm3/g at 200°F (93°C)
1.16 cm3/g at 250°F (121°C)
|At 100°F (38°C)||0.779 cm3/g||0.736 cm3/g||1.062 cm3/g|
|Change in specific volume||2.6%||1.5%||8.4%|
Note that the volume change of the HDPE is five times greater than the volume change of PVC and three times greater than the volume change of PVC modified with Elvaloy®. HDPE shrinkage is based on the cooling rate and change in temperature. If the cooling happens quickly (i.e., the cooling temperature is much lower than the hot HDPE), then the crystalline structure may not form well. This increases the potential for creep over time, and can lead to further shrinkage if the liner is annealed (reheated and cooled).
The high rate of HDPE shrinkage can create annular space between the liner and the host pipe. The dimensional change also forces installers to wait several hours before reinstating lateral connections. This helps prevent shifts or breaks in the lateral connections as the liner finishes cooling. Often, a subsequent workday may be scheduled to complete these connections.
PVC and PVC modified with Elvaloy® don't shift the way HDPE does, because the molecular structure of PVC is not crystalline and the molecules do not continue to compact. In addition, the PVC with Elvaloy® has higher melt strength and ultimate elongation at the forming temperature than rigid PVC. This allows the liner to stretch outward and conform tightly to the grooves and ridges of the host pipe; it "grabs on" to these features, helping maintain its position as it cools. The pipe designer can balance the properties of "grab on" and shrinkage by balancing the amount of Elvaloy® modifier.
PVC with Elvaloy® will creep less than HDPE.
Flexural Modulus and Creep
Flexural modulus is a measure of the rigidity of a material in the flex mode. For plastics, this is measured under ASTM guidelines D790. When designing a pipe liner, the engineer uses flexural modulus to determine the liner stiffness and the critical buckling pressure. There are many references to this calculation and it will not be discussed here. This paper will address the flexural (or flex) creep modulus, flexural (or flex) creep, and their effect on the pipe liner.
Flex creep is the deformation of a material over time, under flexural load. It refers to the deformation or strain of the plastic with a flexural load. Flex creep is measured using ASTM D2990: A standard flexural test sample bar is placed in horizontal clamps and constant stress or load is applied. The deflection or strain of the bar is measured at specific time intervals.
Flex creep modulus is calculated from this strain versus stress data. It's a ratio of the constant stress load applied at the beginning of the test, divided by the deflection strain at the given time. Creep modulus -- whether under flexural, tensile or compression load -- is not a measure of the modulus of the material at the time the constant stress vanishes. It has been noted(3) that if a PVC tensile creep sample were to be taken off test after a period of time and tested in a tensile tester, the strength of the sample would be greater than the initial strength and the slope of the stress strain curve (modulus) would also be equal to or greater than the original slope.
In order to compare the creep of materials, a design engineer often uses isochronous (equal time) creep stress strain curves. Figure 6 plots the stress versus strain at 1000 hours using literature values(9) for PVC and HDPE and data for the PVC modified with Elvaloy® . As shown, over time -- given the same stress or load -- HDPE will deform more than the PVC with Elvaloy® , and much more than the PVC.
Internal Stress In Plastic Pipe Liners
Fold-and-form pipeline rehabilitation requires a material that can withstand or relieve internal stress from the many operations it undergoes. The material is stressed again and again as it is extruded, folded, wound for shipping, pulled through the host pipe, reformed, and finally cut through to make lateral connections. It would be impractical to study and predict all the forces and stresses on the pipe liner. Therefore the material used to make the pipe liner should be designed to relieve high levels of stress.
Stress relaxation and creep are often studied together. While creep is the deformation due to an applied load over time, stress relaxation is the reduction of stress of a deformed material over time under constant strain. Materials with more creep tend to reduce more applied external stress.
One way to predict the ability of a plastic to relieve stress is by studying the ratio of the plastics energy loss to the plastic's energy stored. On DMA curves, these energies are referred to as the loss modulus (E") and the storage or elastic modulus (E'). The ratio E"/E', referred to tan delta, is plotted on the DMA curves in Figures 1,2 and 3.
Pipe liner materials should balance E' and E" to optimize the properties needed to form the liner and relieve stress. If the loss modulus E" is too low during forming, the viscosity will be low and the material will be too weak to deform evenly. If the elasticity E' is too high during forming, there will be too much memory and higher-than-desired levels of molded-in stress.
At liner forming temperatures, the PVC and the modified PVC liners are above Tg and in their elastomeric state, long before the PVC melts. (When melted E" is very low.) While cooling, the plastic goes through a gradual transition between the elastomeric and viscous phases (as indicated by the tan delta), transitioning from above Tg to below Tg. A slow transition gives the material time to settle into an unstressed condition. If the transition is too sudden, the molecules do not have as much time to relax. The broad tan delta for PVC modified with Elvaloy® indicates that this material has more time to relax than does the rigid PVC.
Since HDPE is not cooled to below Tg, this argument does not apply. E' and E" are both high before the crystalline regions melt. Optimum stress relaxation occurs when the crystalline regions are melted and reformed. As mentioned earlier, HDPE will continue to relax and creep after the liner is cooled.
Another way to measure of the plastics' ability to relieve stress in the operations of deforming and reforming is to measure elongation at break. Figure 7 shows the elongation measured for a rigid PVC, a PVC modified with Elvaloy® , and HDPE at room temperature and at 150°F (65°C). As shown, adding Elvaloy® increases the ability of the PVC to elongate. This makes it easier to process the liner through folding/forming, winding on a reel and pulling. This increase in elongation also shows that Elvaloy® helps to relieve the internal stress in the pipe liner and avoid brittle cracks of rigid PVC.
The very crystalline HDPE has high strength and elongation at 150°F (65°C). This shows that the HDPE can relieve much of the stress placed on it, if the crystallites are not melted and reformed.
Lastly, impact force is placed on the pipe liner as cuts are made for lateral line connections. Higher Izod impact properties indicate the strength to avoid splitting while being cut. Izod impact data for rigid PVC, HDPE, and PVC modified with Elvaloy® are shown in Figure 8. HDPE and PVC modified with Elvaloy® have very good impact properties compared to rigid PVC.
Of course there are many factors that effect cracking. But using Elvaloy® gives these systems more ability to elongate, to relieve stress, and to withstand the impact of cutting.
Adding Elvaloy® to a PVC pipe liner compound helps balance the liner's material properties of stiffness and stress relief. The liner installation can be completed faster, at lower reforming temperatures and pressures than when using rigid PVC or HDPE. Because Elvaloy® creates a wide operating window, the rehabilitation project is less susceptible to variations in temperatures and pressures of the process steam, or varied groundwater conditions. Modifying PVC with Elvaloy® also helps the pipe liner maintain its integrity during processing and over time, by relieving the stress and avoiding cracking, splitting and stress concentrations. PVC liners made with Elvaloy® have the widest forming window, relieve the most stress, closely conform to the host pipe, and provide a snug fit that stays properly sized and positioned inside the rehabilitated pipe.
The author acknowledges the assistance provided by laboratory personnel, especially J. Behm for diligence in testing samples and providing data; and the valuable assistance of Drs. G. T. Dee, W. H. Tuminello, and M. Y. Keating in consulting on polymer physics.
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