How to Work with Recycled Plastic Wood

What is Filcris Recycled Plastic?

Nearly all the recycled plastic supplied by Filcris Ltd is HDPE or High Density Polyethylene. The only exception to this is Stokbord, which is LDPE or Low Density Polyethylene and the mixed plastic used in the Ecohoarding sheet.

Polyethylene is a very tough, durable plastic which has high impact strength and is resistant to degradation by most acids and rot, as well as being maintenance free and chemically inert, so it is ideal for use in exterior, and especially wet or underground applications. It is tough, but very flexible, and it has a high coefficient of thermal expansion (it expands and contracts a lot with changes in temperature). Recycled HDPE can vary from manufacturer to manufacturer in its exact properties but these general rules still apply.

We have over 20 years of experience working with this material and this guide is designed share some of this experience.

Dimensions

As a consequence of the manufacturing processes used in the production of recycled plastic and the relatively high coefficient of thermal expansion of the material is is made from. the material dimensions can vary by up to 3% from the nominal sizes stated in the product description.

Colour

All Filcris Recycled Plastic products are UV stabilised. The standard pigment used for this purpose is carbon black, which is why most of the posts and planks are black. However there is a strong demand from our customers for more natural looking colours and we also supply brown and grey profiles, although some manufacturers charge a premium for colours other than black hence the difference in price.

If the source material is of a mixture of colours (like most) it can be relatively easily turned into a black a black or dark brown product. However, the lighter the colour of a product, the more expensive the input material will be, and a light coloured source material is required if it is to be coloured other than dark grey, brown or black. Unfortunately because it is a low energy polymer paint does not generally bond to it very successfully and we do not recommend painting the material.

Longevity

We are often asked how long our products will last. This is not actually an easy question to answer! Most testing facilities are only able to test for up to 15 years and when this material emerges after such a test it is unchanged. We have a number of installations nearby from our very first days fabricating this material, and they have not degraded in full sunlight. The literature says that over a 50 year period the top tenth of a millimetre can fade due to UV degradation, but this only causes slight fading and does not have any structural impact.

How to Work with Recycled Plastic

In principle Filcris Recycled HDPE can be treated like wood. All treatments possible for wood, such as drilling, sawing, milling and planing can be applied. The general rule is not to choose cutting speeds which are too high to prevent the material from melting. It is also a good idea to use a large toothed blade to ensure the waste material is shed efficiently. However the different recycling processes used by different manufacturers can result in slightly different properties so some trial machining / cutting / drilling on an offcut is sometimes a good idea – we will happily provide some free offcuts for this purpose if you ask, subject to availability.

In cold weather conditions Filcris Recycled HDPE will be slightly stiffer and more brittle but it will show be tough up to at least 20 degrees below zero. In warm weather condition the material tends to become slightly tougher and more flexible.

Drilling

Use metal or wood bits (HSS or HM type) Spiral or speed bits are both possible.

Don’t choose too high cutting speeds.

Sawing

This can be done with a handsaw, circular saw, chainsaw, table saw etc.

Ripping, Milling, Planing, Routing Etc.

It is certainly feasible to plane, rout and mill Filcris Recycled HDPE and rip down planks. However, bear in mind that planks and posts tend to have a core which is less dense, (often described as ‘honeycomb’) and that most of the strength is located in the wall of the product. If this is disrupted the material may warp or curl in towards the cut face. We recommend that you seek specific advice from us relating to this. (Note that this is not an issue with recycled plastic sheet including Stokbord which can be cut into complex shapes without this problem occurring.)

Fixing Recycled Plastic Products

We recommend using mechanical fixings where possible.

Screwing

Standard wood or chipboard screws are suitable, but we generally screw smaller joints using stainless steel countersunk screws such as these. Larger joints, such as bearers, should be bolted.

If screwing, we strongly recommend pre-drilling the screw holes, countersinking may be necessary (high quality screws such as the ones shown above can self countersink but this depends on the density of the material). The diameter of the screw hole should be slightly less than the diameter of the screw. Bolt holes should be slightly larger than the diameter of the bolt (0.5mm – 1mm).

Nailing

Recycled plastic materials can be nailed but it is a dense material and but practice shows that it’s better to pre-drill unless a nail gun is used. In either case galvanised or stainless steel RINGSHANK nails should be used to reduce the risk of the nails pulling out.

Welding

Recycled plastic can be welded by melting, for which special extrusion welding devices exist. The results of this welding can be quite variable and will depend on the specific product being welded, and the material used as the weld. A good weld can reach 30 – 50% of the strength of the original material at the most. Without an extrusion welding device the material can also be melted and welded with other heat sources, with more variable results.

Gluing and other adhesives

The received wisdom is that recycled plastic, if made from polyethylene (which the majority is) cannot be glued or bonded with tape since it is a low energy polymer with a slightly greasy surface which repels adhesives. This is true, but there are now a number of specialist adhesive products which allow the material to be bonded together or to other materials. These are expensive and the bond is not in our opinion sufficiently strong for structural applications (although the manufacturers claim that it is, and we have not seen evidence to the contrary) but we use it very successfully for sign making and other light duties.

General Construction Rules

Recycled plastic can be used for many applications. In principle it is possible to use it anywhere where wood can be used. However, plastic is a different material and behaves differently from timber. In general recycled plastic is more flexible

than wood. This means it is not possible to apply the same span used for wood constructions in a 1:1 way. Generally either the span will have to be smaller or

a thicker plank will have to be used. You can either use the data in Table 1 to calculate the correct thickness, or discuss your requirements with us – we are always happy to assist.

Expansion/Shrinkage Due to Temperature Fluctuations

As it gets warmer plastic expands and as it gets colder it will shrink. The construction will have to allow the material to expand or shrink; especially in the case of large constructions, such as fencing along dozens of metres. Due to the long length expansion and shrinkage will become too large to be intercepted by the flexibility of the construction. In the case of smaller constructions it depends a little on how much the construction can transform and by doing so can intercept expansion/shrinkage by itself. Nevertheless we advise to always take this material behaviour into account.

Recycled plastic will expand to a maximum of 2 mm per meter plank at 10°C. In the case of outdoor applications we assume that mounting normally takes place at a temperature between 10 and 20°C. We assume that -20°C is the minimum temperature. In summer, when the sun is shining on the planks, temperature can rise to approximately 50°C due to their black colour. The largest possible temperature difference will then be + or -40°C. Therefore it has to be taken into account that the length of the plank can become up to 8 mm longer or 8 mm shorter per span meter.

Creep

If you place a load on plastic, it will show a certain deflection. How far this deflection will go can be calculated with the Elasticity modulus. If the load is maintained and the deflection is calculated again later on, you will find an increased deflection in plastics. This phenomenon is called creep. The material creeps away as it were when permanently stressed. Therefore when calculating stress (load) situations a distinction has to be made between the Elasticity modulus, relevant for short term stress, and the creep modulus, showing the relation between stress and deflection in long term stress situations. For example stress caused by own weight will rest on the construction for the rest of its life. This means that deflection caused by this will have to be calculated by means of the creep modulus. Most of the creep will appear during the first few months, after that the process will slow down. After a couple of years there will hardly be increased deflection due to creep. The short term strength can equally not be used for tensile strength or bending strength in long term stress situations. Breakage behaviour of plastics in long term applications however, is rather complex.

Maintenance

One of the benefits of recycled plastic is that it requires no maintenance to preserve it. Over time lichens and possibly algae can grow on the surface of the material, and dirt or dust from the surrounding environment can make it appear dirty. The only thing we advise you to do is to clean it with a high-pressure hose now and then. Plain clean water, without any additions and normally applied pressures. Unless a high-pressure hose fails to clean sufficiently, we don’t recommend cleaning with a steam cleaner, as the material’s surface may become somewhat fibrous. Provided that graffiti hasn’t been on the material too long a high-pressure hose should also be able to remove this to a reasonable extent.

Fire Conduct

All plastics will burn when in contact with a flame; they are combustible as are most materials. However, there are significant differences in behaviour once the flame is removed. Some materials keep on burning, while others extinguish.

NEN 6065

In NEN 6065, a norm used in the construction industry, a number of tests are being performed, thus judging flame extension and flame spreading. Then the material is classified, ranging from class 1 for the least combustible up to class 5 for the most combustible materials. Recycled plastic is classified in class 3.