Plastic pipe types in mechanical services explained, By Martin Stirling

What is plastic anyway?

We often find ourselves using products that we know only as an acronym without really understanding much about them, other than their letters. The plastic pipes that we come across in mechanical services trades are no exception - we've all worked with PVC, but what is it? How about some of the newer pipe types such as PE-X?

By gaining an understanding of the different pipe types and their properties, we'll know what to look for in the fastenings we use with them and how to firestop penetrations that involve them. In this blog, we'll look at the history of plastic pipe products, the different types that are commonly used and how to fasten and firestop them.

Back to basics

Let's start with the basics. 'Plastic' refers to a group of naturally occurring and synthetic organic polymers, but can also consist of other substances that enhance the material's properties.

Polymers are made by linking single 'monomers' (single molecules) into long chains by using a catalyst, which produces millions of these chains simultaneously. This process is known as 'polymerisation,' and these single molecules have constituent atoms such as carbon, oxygen and hydrogen. The mass that's created is known as a resin and takes the form of either a powder or small pellets, ready to be mixed with other additives prior to forming the pipe or fitting. There are two main categories of polymers - known as 'thermoplastics' and 'thermosets'.

Although both PVC and PE fall into the group of 'plastic materials,' they are quite different in terms of strength, hardness, toughness and thermal properties.

Thermoplastics have long chains which are only weakly connected to other chains. Due to these weak links, when a thermoplastic is heated the bonds are easily broken, melting the plastic to a liquid state. Thermoplastics are recyclable and have generally lower strengths at high temperatures.

Thermosets are thermoplastics that are crosslinked together - mostly through the addition of a chemical before forming, which is then activated under high heat during forming. The amount of crosslinking is proportional to the stiffness of the polymer. The more crosslinks, the stiffer the thermoset. Thermosets are generally able to withstand higher temperatures than thermoplastics and will hold their shape when heated.

Polyvinyl Chloride (PVC)

'Vinyl' is a molecule consisting of two carbon and four hydrogen atoms. 'Poly' means 'many', so 'polyvinyl' describes the polymer chain. The 'chloride' part refers to a chlorine atom that is used instead of a hydrogen atom, so 'Polyvinyl Chloride' consists of chains of molecules that contain two atoms of carbon, one of hydrogen and one of chlorine. Knowing that makes it easy to figure out why a burning PVC pipe is noxious! PVC can be made rigid or soft and is a thermoplastic.

Polyethylene (PE)

'Methylene' contains molecules with one carbon and two hydrogen atoms. The polymerisation process joins these molecules together to form 'Polyethylene' (the 'm' is dropped from methylene). The substance has low strength and hardness, but is ductile and solid under impact. PE's can be cross-linked with a range of additives to form a number of plastic types with advanced mechanical, thermal and chemical properties. For example, in use as high strength plastic pipes carrying hot and cold drinking water.

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PVC and PE. Two very different polymers.

Although both PVC and PE fall into the group of 'plastic materials,' they are quite different in terms of strength, hardness, toughness and thermal properties. As such, they are used in different mechanical services applications.

PVC was the first plastic pipe material to be used in mechanical services trade applications. The type that is commonly used is rigid, but PVC can also be made in a soft 'plasticised' version. Rigid PVC (which is what we use for pipes) is often referred to as 'uPVC' or 'un-plasticised' PVC, and its polymer chains are straight and strong.

PVC pipes almost always contain a range of different additives which give them some unique properties. You might have come across 'cPVC': This is 'chlorinated' PVC which is a strange name as the standard substance is already made up of 57 per cent chlorine! 'This variant has even higher chlorine content - up to 67 per cent - giving it better strength at high temperatures which means it can be used for hot drinking water as well as cold.

PE is different from PVC in that it can be thermoset or cross-linked in a range of conditions and with a variety of additives to form a number of plastic types with advanced mechanical, thermal and chemical properties. A cross-linked PE is commonly referred to as a 'PE-X' (or just PEX).

PE can also be formed into 'high density polyethylene' or 'HDPE'. The irony here is that HDPE is only 3 per cent denser than PE, with the significant difference being that HDPE is stronger. In essence, this is because the structure of the polymer chains is a little less chaotic, and the bonds between them are more numerous. HDPE can also withstand higher temperatures and is resistant to many solvents which makes it good in chemically hostile environments.

What's all this about cross-linking?

Let's talk a bit about cross-linking. This can be done in one of three ways involving a combination of heat, pressure and mixing an additional chemical into the thermoplastic which then binds itself to reactive ends of polymer chains, locking them together in a more rigid 3D shape. Cross-linking locks up to 90 per cent of the polymer chains so there is a big increase in strength. A typical PE-X product is about 20 per cent stronger than PE at room temperature but the big difference is that it holds its strength much better at higher heats. At 70 degrees Celsius, it is only 20 per cent weaker, whereas standard PE has lost 80 per cent of its strength.

With the increase in strength over PE as well as being able to hold that strength over a wide range of temperatures, PE-X can be used for a greater range of applications in mechanical services. The latest development in PE-X piping involves making a composite sleeve between the plastic and a metal alloy (usually aluminium) to greatly boost the strength of the pipe so it can carry high pressure gas. These products are called 'PE-X-Al' or 'PEX-Al-PEX' and are in the process of completely changing the way we distribute gas inside buildings.

The range of polymer properties

Although PVC and PE fall into the broad category of 'plastic materials' they are really quite different in terms of strength, hardness, toughness and thermal properties. As such they are used in different mechanical services applications. They also respond differently to temperature. For example, PVC has a significant fall off in strength with increasing temperature, whereas as PE-X does not.

Perhaps the easiest way to visualise the difference in properties between these materials is by comparing some nominal values for commonly used types in a table. We've shown a copper pipe alloy as a comparison, because this is the material that PE-X is displacing as an alternative technology.

Density (kg/m3)
Strength at room temp (MPa) Strength at 60ºC (MPa)
Stiffness (GPa)
Max service temp (ºC)
Thermal expansion co-efficient (10-5 / ºC)
Thermal conductivity (W / mK)
1,400 55 12 3.4 60 5.2 0.2
1,530 55 28 3.1 93 7.0 0.2
955 22 9 0.8 80 22 0.5
940 26 18 0.6 100 16 0.4
8,900 130 130 117 700 1.7 385

An important characteristic of any building material that we haven't yet covered is how it transmits sound. The properties that determine how well sound is or is not transmitted can be derived from stiffness and density figures. Take the square root of the stiffness and divide it by density, and you'll see that the speed of sound is highest in the PVC polymers, followed by PE and then Copper.

Who is concerned with which properties?

All of these properties mean different things to different stakeholders when it comes to a building's design. The minimum requirement is that the pipes used must be able to withstand the pressures, loads and temperatures of any liquids that they are carrying (let's just keep things simple for the moment and consider water only).

As society has evolved, we now have progressively more demanding ergonomic requirements. Water-borne noise needs to be minimised and we also have to maximise energy efficiency by reducing heat loss. This can be done by using thick PE-X plastic pipes that have a service temperature in excess of the maximum allowable hot water supply temperatures (50 degrees Celsius) and that don't transmit sound all that well.

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In conclusion

It's clear that all of the plastic pipe materials that are in use today have a wide range of properties. So it stands to reason that we can't treat them the same in terms of how we install them (attaching fittings, hanging pipes) and how we firestop penetrations that they pass through.

To find out more, get in touch with the team at Hilti today.



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