How to obtain a correct internal wall of a corrugated pipe with a regenerated LDPE granule

By producing flexible corrugated pipes in rolls or small double-walled rigid diameters, the problem of making a good quality inner layer has led manufacturers to frequently use virgin polymers due to the difficulty of generating a correct and durable wall with the material regenerated.

In reality the inner layer of the pipes, due to its characteristics, needs special attention due to the small thickness of the wall, the tensions that are created in the co-extrusion phase and the different thermal movements with the external wall.

The choice of the raw material normally falls on the LDPE whose main characteristic required is elasticity and good adhesion to the external HDPE layer.

If you want to use a regenerated LDPE granule, keep in mind and analyze some important production factors to be able to choose a LDPE granule of quality suitable for the purpose.

When we talk about regenerated granules, it is not sufficient to verify whether the product offered to us has a “pipe” grade, as it is sometimes mistakenly sold, since the inner wall of a corrugated tube requires a granule with well-defined characteristics.

First of all we have to ascertain the origin of the input of the material that constitutes the granule, starting to understand if it comes from a post-industrial supply chain and from post-consumption. These two families, we will see later, have very different characteristics that will influence the production of the pipe in different ways.

Secondly, we need to verify from which product the input is made to understand the history of the material being recycled and the possible problems it has encountered in its recycling life.

The third thing is to verify the technical values, then the melt index, the DSC and the density of the material that will make us understand exactly how the granule we will use for the inner wall of the corrugated tube is made.

The fourth thing is to know the production process of the granule proposed in particular how the selection of the waste is done, the washing and the extrusion to have more data that help us to choose the most suitable product.

The last thing, very important for the granule that comes from post-consumption is to understand the degree of humidity present in the product at the time of purchase as a high value will affect the quality of the wall if no appropriate measures are taken.

It is obvious that the points listed above are not completely exhaustive in the technical analysis of a granule, but I can say that for the application we are talking about today, they are a good starting point considering that they are data that are not difficult to find.

If we want to go into the above points, we will start by talking about the input families that can be used for the production of the inner wall of the corrugated tube. We have seen that it is possible to produce a granule with material coming from separate collection or from industrial waste.

The post-consumer supply chain makes it possible to have a quantitative source much greater than that coming from industrial waste and therefore it would seem the best way to satisfy production requirements, but the technical characteristics that the production of the internal LDPE wall of a pipe requires corrugated puts stakes to its use.

By its very nature, the LD that comes from separate collection, despite a good selection and washing, presents a percentage of foreign materials (PVC, poly-coupled, PP, etc. ..) that behave in contrast to what we expect from the point qualitative view.

The wastes that come instead from the production of LDPE articles are normally virgin or off grade materials, which by their nature are composed of mono-plastics and therefore do not contain impurities. There is usually no need to wash them and they have very specific technical characteristics.

There are also LDPE Compounds made on the market using post-consumer and post-industrial portions, combining a selection of materials suitable for the production of the internal wall.

If the verification of the origin of the post-industrial input does not involve great commitment, for the other two categories more attention must be paid.

For post-consumption it is advisable to privilege material such as the film but which has not come into contact with domestic separate waste collection, for example garbage bags or food packaging, which carry pollutants that are difficult to completely eliminate.

Another advisable source is the irrigation pipes which however need very accurate washing cycles as they contain a fraction of sand that compromises the quality if not completely removed.

For the production of mixed post-consumer / post-industrial compounds, films from industrial packaging are normally used which have a collection chain separate from household waste, maintaining higher quality characteristics.

As far as the quality control of the product granule is concerned, there are some essential tests I would say. The calculation of the MFI tells us if the material is suitable for the extrusion operation of our wall, this value should be between 0.5 and 1 at 190 ’/ 2.16 Kg.

The second test is the DSC which gives us the radiography of our granule, an essential test especially if you want to use a post-consumer source.

This test tells us how much LDPE in % is contained in the recipe and how many and which other components are present. The SDC, in particular, tells us if a granule can be suitable to create thin, homogeneous and smooth walls.

Once the DSC test is done, it is easier to guess the result of the density value which is influenced, with respect to the standard value of the LDPE, by materials other than the primary one.

A good rule for the evaluation of the quality of the granule to be chosen would be to know the history of the recycling that led to the birth of the same.

After discussing the choice of input it is a good rule to know the recycling method that the supplier adopts. In particular, the type of washing significantly influences the presence of pollutants with high density in the waste, therefore, if the operation is carried out in short tanks or / and with a transit speed of the same high, or with a high concentration of pollutants in the wash water due to its low turnover, the probability of having a high accumulation of gas or rigid parts inside the granule is very likely.

The second thing to check is the quality of filtration which is very much influenced by the quality of the washing. We could say that an increase in attention during washing can correspond to a lower performance requirement of filtering systems.

In reality, correct washing in terms of tank dimensions, input transit speed and water quality are not topics that are very popular among recyclers as everything translates into higher production costs and sometimes granule prices from post-consumption are definitely compressed due also to the presence on the market of a low quality offer at low prices.

In any case, if you want to make a good granule for the inner wall of the flexible corrugated pipe, these precautions should be respected including the correct filtering operation that would foresee the use of continuous or scraping plants with progressive filters up to 50 microns.

As a last indication in terms of raw material I suggest an attention to the degree of humidity present in the LDPE big bag that is purchased because the presence of this involves a micro deformation of the surface film that makes up the wall of our pipe and a greater difficulty in terms of extruder speed. The excessive humidity creates that orange peel effect on the walls which is a kind of unsightly and non-functional roughness.

However, the consequences of humidity, moreover normally solvable during the extrusion of the pipe, is not to be confused with the negative result produced by an accumulation of gas inside the granule, for which there are few weapons available.

See more info about LDPE recycling

Automatic translation. We apologize for any inaccuracies. Original article in Italian.