Tank Linings

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Corr-Tite

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Materials

PVC - Information & PVC Study
Uses, Reliability, Resistance & Manufacturing Process

PVC Plastics

PVC can be used in recreational sites, waste pits, canals, water storage tanks, floor coverings, plating tanks, fuel, oil storage and containment, landfills, galvanizing tanks, anodizing tanks, fish ponds, chemical tanks, reservoirs, mining applications, floating covers, lagoons, construction ponds, mud pits, fish hatcheries, secondary containment, remediation sites and many other applications.

►Polyvinyl chloride has excellent resistance to inorganic acids and alkalis, as well as a wide range of corrosive inorganic chemicals.
 
►The combination of chemical resistance and good physical properties has led to usage in numerous industrial applications
exposed to corrosive conditions, such as plating baths, coatings for tanks and pipes, protective clothing, aprons,
tarpaulins, splash guards, drum linings and numerous others. 

►Water resistance and low water absorption permit use for sewer pipe (see Corr-Tite) and coatings for ocean going ships. 

►Resistance to attack from oxygen and ozone is very good, with PVC being resistant to oils, alcohols, aliphatic hydrocarbons and household detergents.  The resistance of PVC to chemicals begins to drop off above 140 degrees F, as the softening point of the resin is approached.   However, High Temperature PVC is used to contain fluids with a sustained temperature up to 200 degrees F., dependent on the fluids to be contained.

Fabrication

Liners are fabricated for recreational sites, waste pits, canals, potable water storage tanks, floor coverings, plating tanks, fuel  and oil storage, landfills, galvanizing tanks, anodizing tanks, fish ponds, chemical tanks, reservoirs, mining applications, floating covers, lagoons, construction ponds, mud pits, fish hatcheries, secondary containment, remediation, agriculture covers and tarps, foodstuffs,  hazardous materials, acids, earthen pits, berms and sumps.   Materials range in thickness from 8 mil to 3/16 thick and some can hold solutions up to
200 F.

Most PVC geomembranes are either black or shades of gray. Carbon black and titanium dioxide are used to make these colors, with carbon black being an excellent UV protector, absorbing most of the UV radiation that strikes the geomembranes, converting it to heat. Titanium dioxide reflects almost all UV radiation, so together they offer excellent UV protection.

Geomembranes can be exposed for years with minimal UV degradation. Miscellaneous raw materials that are used in geomembranes are not necessarily in every formula. These would include biocides, UV additives, process aids and impact modifiers. Biocides are added to resist any biological attack that the geomembranes may experience in the field and to meet soil burial requirements for NSF standard 54.

UV additives are added to PVC geomembranes specifically designed for outdoor, exposure. Impact modifiers may be added to improve low temperature resistance, for low temperature applications.

The Process of Manufacturing Plastics

Chemical Nature

The chemical nature of a plastic is defined by the monomer (repeating unit) that makes up the chain of the polymer. For example,  polyolefin's are made up of monomer units of olefins, which are open-chain hydrocarbons with at least one double bond. Polyethylene is a polyolefin; its monomer unit is ethylene.

Other categories are acrylics (as poly- methylmethacrylate), styrene (such as polystyrene), vinyl halides (such as polyvinyl chloride),   polyesters, polyurethanes, polyamides (such as nylons), polyether, acetyls, phenolics, cellulosics, and amino resins.

Synthesizing the Polymer

The first stage in manufacturing plastic is polymerization.   As noted, the two basic polymerization methods are condensation and addition reactions. These methods may be carried out in various ways. 

In bulk polymerization, the pure monomer alone is polymerized, generally either in gaseous or liquid phase, although a few solid-state polymerizations are also used. In solution polymerization, an emulsion is formed and then coagulated. In interfacial polymerization, the monomers are dissolved in two immiscible liquids, and the polymerization occurs at the interface of the two liquids.

Shaping and Finishing

The techniques used for shaping and finishing plastics depend on three factors: time, temperature, and flow (also known as deformation). 

One of the most widely used operations is that of extrusion. An extruder is a device that pumps a plastic through a desired die or shape. Extrusion products, such as pipes, have a regularly shaped cross section. The extruder itself also serves as the means to carry out other operations, such as blow molding and injection molding. 

In extrusion blow molding, the extruder fills the mold with a tube, which is then cut off and clamped to form a hollow shape called a parison. This hot, molten parison is then blown like a balloon and forced against the walls of the mold to form the desired shape.

In injection molding, one or more extruders are used with reciprocating screws that move forward to inject the melt and then retract to take on new molten material to continue the process. In injection blow molding, which is used in making bottles for carbonated beverages, the parison is first injection molded, then reheated and blown. 

Compression molding uses pressure to force the plastic into a given shape.

Another process, transfer molding, is a hybrid of injection and compression molding: The molten plastic is forced by a ram into a mold.

Other finishing processes include calendering, in which plastic sheets are formed, and sheet forming, in which the plastic sheets are formed into a desired shape. Some plastics, particularly those with very high temperature resistance, require special fabrication procedures. For example, polytetrafluoroethylene has such a high melt viscosity that it is first pressed into shape and then sintered-exposed to extremely high temperatures that bond it into a cohesive mass without melting it. Some polyamides are produced by a similar process.

Manufacturing
The manufacturing of plastic and plastic products involves procuring the raw materials, synthesizing the basic polymer, compounding the polymer into a material useful for fabrication, and molding or shaping the plastic into its final form, in this case flexible, semi-rigid and rigid plastic sheeting.

Additives
Chemical additives are often used in plastics to produce some desired characteristic. For instance, antioxidants protect a polymer from chemical degradation by oxygen or ozone; similarly, UV stabilizers protect against weathering. Plasticizers make a polymer more flexible, lubricants reduce problems with friction, and pigments add color. Among other additives are flame retardants and antistatics.

Many plastics are manufactured as composites. This involves a system where reinforcements (usually fibers made of glass or carbon) are added to a plastic resin matrix. Composites have strength and stability comparable to that of metals but generally with less weight. Plastic foams, which are composites of plastic and gas, offer bulk with low weight.


PVC comes in a wide range of different types of materials such as:

Reinforced Material, Potable Water, Fuel Grade, Chemical Resistant, High Temperature, Fish & Aquatic Safe and Agriculture Grade.

 

Case Study of PVC Geomembrane Durability

It is widely believed by some within the geosynthetic community that PVC geomembrane degrades after installation. While it is true that PVC may degrade when in contact with some chemicals, the same holds true for other geomembrane such as polyethylene. There is no perfect geomembrane in terms of chemical compatibility. For example, neither PVC nor polyethylene is compatible with benzene; however, urethane geomembrane are available for retaining this chemical.

PVC resins by themselves are hard, brittle compounds due to the strong attraction bonds between hydrogen and chlorine atoms of adjacent polymer chains, resulting in secondary bonding between the polymer chains. In order to facilitate the processing of geomembrane, plasticizers are added to the resin to increase low temperature properties, elongation, and flexibility. Plasticizers are clear, organic liquids that improve process ability and provide the physical properties associated with PVC geomembrane. These compounds fall into two categories based on their compatibility with the resin; primary plasticizers, those that have a high degree of compatibility with the PVC resin matrix, and secondary plasticizers, which are used in some markets other than geomembrane to lower the overall cost. There are many different types of primary plasticizers used in PVC, of which phthalates are the most common in PVC geomembrane production, because they provide the geomembrane with the best balance of properties.

Typical PVC geomembrane contain 30 to 35 percent plasticizers per weight. Plasticizers on the surface of the geomembrane are subject to migration out of the product. The plasticizers within the sheet that are secondarily bonded to the PVC chains require encouragement to migrate. The plasticizer loss is a function of plasticizer type, temperature, sheet thickness, environmental conditions, and exposure time. The worst case for plasticizer loss is when there is a large gradient of organic compounds between the geomembrane and the surrounding environment. Here the gradient must have sufficient energy to overcome the Vander Waals bonding of the ester group of the compound, allowing the linear group to separate the chains and provide migration paths between the chains. As the percentage of plasticizer is reduced, secondary bonding between the polymer chains increases, "locking in" the remaining plasticizers. Studies by the U.S. Bureau of Reclamation on 10-mil PVC geomembrane used in canal linings show that 54 percent of the initial plasticizer content remained after 19 years of service. In this application, the organic gradient was very high due to running water within the canals minimizing organic concentrations from the geomembrane surface. Even with a 46 percent reduction in plasticizer, the geomembrane still met the original design specifications.

Case studies of geomembranes under actual conditions contain some of the most important information that can be gathered to determine long-term performance. Opportunities to evaluate the performance of the geomembrane under these conditions are limited because of the expense of excavating the geomembrane and the fear of disturbing the geomembrane in an attempt to obtain a sample. This leaves only limited opportunities to investigate the durability, such as when sites are being expanded or require modification. To increase the number of case studies the PGI has initiated a research project with the Minnesota Department of Natural Resources. The main objective of the project is to investigate the long-term (30 year) durability of PVC geomembrane and seams. Samples of different PVC geomembrane and seams are being obtained annually from a double lined settling basin that contains mine drainage. Since this project is in the second of the 30-year duration, other case histories were sought to provide an insight to the long-term durability of PVC geomembrane. The following paragraphs describe such a case history.

In 1993 a golf course pond was being enlarged and the existing PVC geomembrane was excavated in the process. The site was at the Lake of the North Golf Course located in the northern part of the lower peninsula of Michigan. According to Jerry Matthews, the golf course architect who originally designed the project, the PVC geomembrane was installed in the summer of l968. The material, a 10-mil PVC geomembrane, was originally covered by twelve inches of sand. Approximately six to eight inches of silt had accumulated over the sand during the 25-year period from 1968 to 1993.

The climate is harsh with winter temperatures falling well below 0° F and summer temperatures rising to above 90° F. Also based upon some other previous studies of plasticizer extraction, rainwater may be more severe than a typical municipal landfill leach ate. This phenomenon is due to the fact that there is a larger gradient for plasticizer migration in water than leachate, due to the lack of organic compounds in the water. Lastly, this geomembrane was only 10 mils thick. Changes to a PVC geomembrane will occur more quickly with this gauge than the thicker gauges, 20 to 40 mils, which are typically used on projects today. For all of these reasons, this site provided some meaningful information relating to the long-term performance of PVC geomembrane.

Samples were taken to evaluate the physical properties of the parent material and the factory and field seams. In 1968, all seams were made using a chemical fusion weld. Physical testing according to NSF Standard 54 for PVC geomembrane was conducted, along with chemical analysis of the film. The specific tests that were conducted included thickness, specific gravity, tensile, elongation, 100% modulus, and tear resistance. Peel and shear tests were conducted on both the factory and field seams. The test results are summarized in Figures 1 and 2. There are several things that are immediately apparent from this data.

The physical properties still exceed the requirement of NSF Standard 54 - 83 even after 25 years. In fact they exceed the NSF 54 Standard by a large margin.

There was no deterioration of the seams by the peel and shear values in either the field or factory seams. All of the peel tests on the factory and field seams resulted in a film-tearing bond.

The analytical tests confirm that the formulation of this PVC geomembrane has changed very little over the 25-year period. Existing PVC geomembrane formulas have about 30% plasticizer, versus the 27.8% found in this geomembrane. (An unexposed sample of the geomembrane was not available for comparative purposes so the remaining plasticizer content was compared to the formulation of a current geomembrane, i.e., 30% plasticizer.) It may be concluded that the geomembrane had reached a steady state with the surrounding harsh environment and it was not losing any additional plasticizer.

The samples themselves were still very flexible with no sign of deterioration or cracking of the surface. There appeared to be no physical signs that would indicate that the geomembrane had not functioned as designed for the 25 years it was in service.

This study only adds to the growing amount of information that suggests PVC geomembranes are a viable choice for a wide range of applications. The fact that they do contain plasticizer is not the problem that some people would believe, but gives the geomembrane the flexibility that is important in geosynthetic design, installation, and long-term service.

You can obtain an original copy of this bulletin from the PVC Geomembrane Institute, PGI - Technology Program, University of Illinois, 2215 Newmark Civil Eng. Lab, 205 North Matthews Ave., Urbana, IL 61801, or by phoning the PGI at 217-333-3929, or by email at pgi-tp@uiuc.edu

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