The polymer alphabet - A, B, C and PERT

In the article "Marking of plastic pipes.Pipes based on polyethylene " I told about what the producers write on their products. There was superficially touched on the topic of materials. Today we will go into this topic.

Pict.1 Cross-linked polyethylene pipes

So, the main types of materials that are used for the production of plastic and metal-plastic pipes are:

- Polyethylene PE;

- High density polyethylene PE-HD;

- cross-linked polyethylene PEX-a;

- cross-linked polyethylene PEX-b;

- cross-linked polyethylene PEX-c;

- polyethylene of raised temperature PE-RT.

 The first two materials do not have the qualities of heat resistance. Polyethylene has a branched molecular structure. And under the influence of temperature, its crystal lattice will collapse. Therefore, pipes made of such materials cann’t be use in systems with a working medium temperature higher than 70 ° C.

Pict.2 Molecular structure of polyethylene PE

To ensure that the pipes can be install in high-temperature heating systems, it is necessary to use more heat-resistant materials. There are two types: cross-linked polyethylene and polyethylene of raised temperature PERT.

Cross-linked polyethylene

By "cross-linking" polyethylene is mean the creation of molecular bonds between macromolecules of polyethylene. Three methods of cross-linking are use for the production of pipes:

- peroxide (RE-Xa);

- silane (PE-Xb);

- radiation (PE-Xc).

The main quality indicator (in addition to the direct manufacturing process) is the degree of cross-linking. It shows the number of bound macromolecules among themselves. As a percentage of the total volume of the material. For example, if the material has a degree of cross-linking of 65%. 100% of polyethylene entered the chemical process and 65 of them "stitched together", the rest - no.

For different methods, this parameter is different:

- PE-Xa - 70-75%;

- PE-Xb - 60-65%

- PE-Xc - 55-60%

Now let us touch on a few methods of manufacturing.

The cross-linked PE-Xa polyethylene is prepared by adding peroxides to the starting material PE. With the help of free radicals, peroxides split off hydrogen atoms from polyethylene macromolecules. After that, a bond formed between them via carbon atoms C.

Thomas Engel invented this method. Sometimes this species is also call by his name - Engel's method.

Pict.3 The process of stitching by the method of Engel. To the right - the structure of cross-linked PE-Xa

Cross-linked polyethylene pipes have the highest degree of cross-linking. They are more elastic and soft. The cross-linking of polyethylene occurs evenly throughout the pipe.

But there is only one drawback - the speed of production. For example, for one unit of time, PE-RT pipes will be produced twice as much as PE-Xa. Accordingly, this affects the cost of the product and its price.

The silane PE-Xb method is slightly simpler. Silane-based silanides are add to the polyethylene here.

Pict.4 Crosslinking process by the method b

The producers who provide this product in the CIS are much larger than PE-Xa. There are several reasons for this.

The first and for a long time the only method of producing a pipe made of cross-linked polyethylene was Engel's method. Because the process itself is complicated and requires special extruders, there have been few manufacturers of such pipes - Rehau, Golan, etc.

Therefore, the silane method of obtaining the tube appeared as a counterweight to the peroxide. Here, conventional extruders are used, the process itself is simpler, and the pipe is cheaper. Though not much.

But if the PE-Xa method, the pipe from the line is already ready, then this method is not available. The degree of cross-linking of the pipe after extrusion is 15-20%. Sometimes it happens less. In order for the pipe to "finish", it must be place in a special chamber. Where it is expose to temperature and pressure for an extended period. The cost of such a camera is not small. Many factories do not "see meaning" in it. Argumenting this by the fact that the pipe "will reach" the required degree of cross-linking after installation in the heating system.

However, it is worth remembering that this "additional crosslinking" should be do no later than 2-3 months after the production of the pipe. Let try to calculate. In our market, a lot of pipes of Chinese origin. The way from China to Ukraine to the warehouses of importers is from 30 to 50 days. After that, it moves to the warehouses of distributors. After in the outlets. It's still 10-30days. Then there is a sale to the client or installer. Here already as will carry. Still take into account the installation time and start time of heating systems. It turns out 4-6 months. This is at best.

In addition, there are questions on the environmental friendliness. Since silanides are a toxic material. And the use of such pipes in drinking water systems remains questionable.

In this regard, the German DIN Quality Institute does not recommend the use of PE-Xb pipes.

I believe that such a pipe can only be purchase from proven manufacturers from Europe. Well, of course with the inscription "made in ...".

The PE-XC pipe is produce using a physical method. The macromolecules are join by irradiating the tube with electrons. Such a process does not require the addition of any catalysts (silanides or peroxides).

Pict.5 The physical method of cross-linking polyethylene

This pipe also has good elasticity. But it can have uneven stitching along the length. So much depends on the correct position of the electrode and the adjustment of the line itself.

The degree of cross-linking by method C is the smallest.

Polyethylene of raised temperature PE-RT

This kind of polyethylene does not require cross-linking. It has a high resistance to temperature due to its branched molecular structure. Where the spatial lateral branches of the octane group create reliable interweaving.

Pict.6 Molecular structure of high resistant polyethylene PE-RT

The process of producing such pipes is distinguish by its simplicity and speed. In addition, the price of such pipes is lower.

Also PERT is a thermoplastic, like polypropylene. Therefore, the PERT system of pipes and fittings has appeared on the market, which are connect by the type of polypropylene using welding machines.

They also have a greater resistance to ultraviolet than PEX.

Thanks to these and other advantages, European manufacturers increasingly use this material. For the production of both homogeneous and metal-plastic pipes.

According to statistics for 2007, heat-resistant polyethylene PE-RT was use in 58% of cases for the manufacture of plastic pipes in Europe. PE-Xc only 14%.

I hope that the information for you was available and useful. 

Answer in comments. wich pipes do you use?