COMPARISON OF WATER PIPING
PVC and PEX

Extended Research Task

Introduction
Polymers are nearly everywhere we look in present-day society. We use them for many different purposes as certain polymers can have certain characteristics which determine its applications. These characteristics depend on the very small intra and inter molecular bonding as well as their individual functional groups that greatly contribute to their properties. PVC and PEX are two very common polymers used to transport cold water in residential buildings; however their chemical and microscopic structures differ, giving each polymer its own unique properties. The purpose of this investigation is to compare the properties of each polymer determined by their inter and intra molecular structures. Properties needed in pipes for cold water transportation include flexible, lightweight, cost effective and safe for household/buildings.

Polyvinyl Chloride
Polyvinyl chloride (PVC) has many uses; one of its uses is water piping in residential buildings, due to its low cost, light weight , good insulation and low maintenance (“Polyvinyl chloride,” 2015) it suits the application perfectly. PVC is made from the polymerisation of the monomer vinyl chloride (VCM) displayed in Image 1, and achieves these properties due to its molecular structure.
Image 1: Monomer vinyl chloride (VCM)

PVC is mainly manufactured via suspension polymerisation; this process is explained in image 2.
Image 2: Suspension Polymerization

Suspension Polymerization is the process whereby the vinyl chloride monomers are dispersed into water by the collective action of surface active agents and agitation shown in the image above. The vinyl chloride monomers then polymerize into polyvinylchloride (“PVC chemistry and production,” n.d.). The polymers are then fed into a die and moulded into piping with specified dimensions.

Image 3: Polyvinylchloride molecule
The PVC polymer is a continuous chain of the monomer vinyl chloride. The double bond between the two carbon atoms in the vinyl chloride monomer in image 1 is broken and bonded to a carbon atom of a nearby VC monomer(“A Study of Poly(Vinyl Chloride) Microstructure,” , 1997). The chain is seen below in image 3. This results in a continuous pattern between the chlorine and hydrogen atoms. The functional group found in PVC is vinyl; this group consists of the three hydrogen atoms, seen below, minus the chlorine atom.

The PVC polymer structure has low branching properties as its molecules, seen in image 3, do not fork out away from the carbon backbone; this means the intermolecular forces (such as Van der Waals force) would be stronger allowing less creep and fewer gaps in the polymer. The hydrogen atoms in the polymer form dipole-dipole bonds, which would also contribute to greater intermolecular forces. This gives rise to its low creep resistance and impermeable qualities (Titow, 1984). The vinyl group discussed in the paragraph above results in properties such as flexibility and the plastic feel, all thanks to PVC’s molecular structure.

As PVC is amorphous, it provides the flexible qualities of the piping. If the polymer where crystalline the piping would be too brittle and increase the chance of it shattering under strain from water cavitation or movements in the pipes. Within the PVC polymers the chlorine atoms are bonded to the carbon chain backbone, this causes the polymer to attain a high resistance to oxidative reactions such as corrosion. Therefore increasing PVC’s durability, as PVC’s average lifespan is around 100 years(“Vinyl (Poly Vinyl Chloride),” n.d.). Since the chlorine atoms in the molecule are very polar it causes the PVC to achieve superior fire retardant properties, thus making PVC safer in the emergency of a house fire(“PVC’s Physical Properties - PVC,” n.d.). The disadvantages for PVC are; any water or direct heat over 65°C causes changes to the intermolecular forces of the PVC. The bonds become weaker; therefore the PVC piping loses its firmness affecting it’s transportation of water (“Pros and Cons of Using PVC Pipes,” 2014).

Cross-linked Polyethylene
PEX is the term given to cross-linked polyethylene also known as XLPE. PEX is most commonly used for residential water piping and is made from high density polyethylene (HDPE) that contains cross linked bonds throughout it’s polymer structure. The production of PEX is described in image 4.
Image 4: Peroxide Crosslinking Machine

Cross linked polyethylene can be made from a process called Engel cross-linking also known as the peroxide method. With this method high density polyethylene is mixed with a peroxide based chemical and then quickly melted with controlled temperatures. The mixture is shaped into a pipe with standard dies where it is then heated under pressure to allow the peroxide to decompose and cross-link the polymer. These processes are carried out by the machine above (“Peroxide Cross-linking Pipe Set-- Plastics Machinery Co., Ltd.,” n.d.).

Crosslinking is the process whereby carbon atoms of the same or different polyethylene chains are linked together to create a three dimensional network structure (PEX). Cross linking essentially enhances the physical properties of the polymer and can produce an elastomer that is both elastic and more durable(“The Crosslinking Page,” n.d.). When HDPE is cross-linked the polymer changes from a thermoplastic to a thermoset (“Cross-linked polyethylene,” 2014). The carbon and hydrogen atoms in the PEX molecule are linked at certain points (seen in image 5), this brings the polymer chains closer together and thus reduces the amount of gaps in the polymer giving rise to it’s impermeable properties. Cross-linking also reduces the melt index as well as improving creep resistance and impact strength, however the density is not greatly affected (“CROSS-LINKING METHODS,” 2009). PEX, like PVC is also amorphous which further contributes to its flexibility by allowing the pipe to bend around corners and require fewer fittings than PVC, thanks to how the polyethylene is constructed when cross-linked.
Image 5: Cross-linked Polyethylene Molecule

Secondary Data Table 1: Correlations of properties between PVC and PEX water piping Properties | Polyvinyl Chloride PVC | Cross-linked Polyethylene PEX | Density | 1.42 g/cm^3 | 0.93 g/cm^3 | Melting Point | 80°C | 120-150°C | Elongation at Break | 100-130 | 200-450 | Flash Point | 435-557°C | 300°C + | Cost | $0.20/ft | $0.46/ft | Analysis The secondary data displayed in table 1 shows that PEX’s melting point (120 degrees C) is greater than PVC’s, indicating PEX could also be used for hot water systems in residential buildings. As the density of PVC (1.42g/cm^3) is greater than PEX (0.93g/cm^3) the mass for a length of PVC piping is expected to be greater than the same length of PEX piping. The elongation at break and flash point are properties that do not greatly contribute to the piping being used residential buildings unless there was a fire, in which case the PVC would prove safer due to its low flash point and greater fire retardant properties. Another aspect seen in the table is the cost of PEX turns out nearly twice as expensive as PVC, indicating it’d be more economical to use PVC piping instead. Comparison Collectively PVC and PEX piping have many pros and cons between them. Determining the choice between PVC or PEX piping for use as residential water piping is dependent on the similarities and differences of the two pipes. Some of the advantages of PVC are; it’s inexpensive and durable with low maintenance. PVC also provides great insulation, proving perfect for water piping as minimal heat is lost. The disadvantages for PVC are; any water or direct heat over 65°C causes changes to the intermolecular forces of the PVC. The bonds become weaker; therefore the PVC piping loses its firmness affecting it’s transportation of water (“Pros and Cons of Using PVC Pipes,” 2014). Other disadvantages are; PVC has a larger density than other plastics and can be very brittle in cold climates, cracking if mishandled. Advantages of using PEX piping are; it can transport both hot and cold water with a maximum temperature of up to 100°C making it perfect for hot water systems. When used for water piping it needs less fittings as the pipe is more flexible which reduces the possibility of leaks. PEX does however have some disadvantages, these include; it cannot be left outdoors for long time periods because of UV damage to the pipes and rodents can bite into PEX causing a loss in water pressure and leaks. The piping also contains an impermeable membrane that results in the possibility of contaminated water. Conclusion Overall, based on the properties and data observed in both polymers, PEX is more suitable for the application of water piping due to its numerous properties. Cross-linked PE is both versatile and more suitable as water piping as it requires fewer fittings due to its flexibility and durability, reducing the chance of leakage. Although PVC is inexpensive it is not very flexible requiring more fittings, increasing chances of leakage and requiring higher maintenance. Therefore PEX water piping proves more suitable in residential households than PVC, due to its unique molecular and intra molecular characteristics giving rise to its unique properties.

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