Wood Flour PVC Formulation: Coupling, Moisture and Die Wear

What wood flour does to PVC

Wood flour is a filler that is not inert. It is a lignocellulosic material with hydroxyl groups on the surface, and those groups are polar in a way that PVC is and polyethylene is not. That is why wood flour disperses in PVC more easily than in a polyolefin, and why PVC is the natural matrix for a wood plastic composite in Europe.

The mechanical effect is the same as any rigid filler: the modulus rises, the elongation falls, and the impact strength falls faster than either. A composite at forty percent flour is roughly twice as stiff as the unfilled compound and has a fifth of the impact strength, which is why wood plastic composite profiles are used for decking and cladding rather than for anything that gets hit.

The processing effect is more subtle. Wood flour raises the melt viscosity, so the screw has to work harder and the melt temperature rises. It also lowers the thermal stability window, because wood starts to degrade at around 200 degrees and the degradation products are acidic, which is the last thing a PVC compound needs.

So the formulation is a balance between three things that pull in different directions: enough flour to get the stiffness and the cost, enough lubricant and stabiliser to keep the melt processable, and enough coupling to keep the flour bonded to the matrix.

Coupling and the interface

A wood particle in a plastic matrix is a rigid inclusion with a weak boundary. Load applied to the composite has to transfer from the matrix to the particle across that boundary, and if the boundary is weak the particle debonds and becomes a void. The composite then fails at the interface rather than in the material.

Coupling agents exist to fix that. For polyolefins the standard is maleic anhydride grafted polypropylene, which bonds to the hydroxyl groups on the wood and entangles with the matrix. For PVC the chemistry is different and the common route is an aminosilane or a titanate, sometimes combined with a chlorinated polyolefin.

The effect of coupling is measurable and it is large. A well coupled composite at fifty percent flour can retain sixty percent of its tensile strength; an uncoupled one retains forty and loses most of its wet strength. The coupling agent is one to three percent of the formulation and it is the difference between a product and a sample.

The interface also decides the water resistance. Water enters a composite through the interface, not through the wood itself, and a coupled interface is a narrower path. That is why a coupled composite swells less in a humid test even though the flour content is the same.

Moisture, the first enemy

Wood flour arrives at five to ten percent moisture and it has to be at one percent or below before it goes into the extruder. The water does not just make bubbles: at extrusion temperature it flashes to steam, cools the melt locally, and produces voids, silver streaks and a rough surface that no downstream operation can fix.

Drying wood flour is not the same as drying resin. It is a hygroscopic, insulating material with a low thermal conductivity, so the moisture has to diffuse out of the particle rather than evaporate from the surface. A dryer that works for pellets will leave the centre of a wood particle wet.

The practical answer is a dedicated dryer at 105 to 120 degrees with a residence time of two to four hours and a dew point below minus twenty. A vacuum dryer is faster but the throughput is lower. What matters is that the flour is dry at the hopper, not that it was dry when it left the dryer.

The second moisture problem is the one after the part is made. A composite in service absorbs water through the interface and swells, and the swelling is anisotropic because the wood particles are not randomly oriented in an extruded profile. That is why a decking board grows in width more than in length, and why the fixing holes have to be slotted.

Loading, and where the properties cross

The chart on the left plots property retained against wood flour loading. Flexural strength holds 96 percent at ten percent flour, 90 at twenty, 82 at thirty, 74 at forty, 65 at fifty and 56 at sixty. Notched impact holds 82, 64, 48, 36, 27 and 20 over the same range.

The two curves diverge from the first percent of flour, and that divergence is the design problem. At thirty percent the composite still has 82 percent of its flexural strength and less than half of its impact strength, which is a material that will carry a load and will not survive a knock.

The usual compromise is forty to fifty percent for a decking profile, where stiffness matters and impact does not, and thirty to forty for a profile that has to be handled or fixed with screws. Above sixty percent the melt becomes difficult to process and the water absorption becomes the dominant property.

The chart on the right shows what actually goes wrong in service. Moisture swelling accounts for 34 percent of failures, interfacial debonding 26, die wear 13, colour and odour 11, screw slip and feed 10, and everything else 6. Sixty percent is water and the interface, which are the same problem seen from two sides.

Die wear and the profile

Wood flour is abrasive. It is not as abrasive as glass fibre, but it is abrasive enough to wear a die over a few hundred tonnes, and the wear is not uniform. The die land wears fastest where the melt velocity is highest, which changes the flow balance and eventually the profile dimensions.

The practical consequences are a die that has to be replaced or reworked every year or two, and a profile that drifts in dimension over its production life. Tooling for wood plastic composite is usually made in a harder steel than for unfilled PVC, and the land length is often shorter to reduce the pressure drop across the worn section.

The screw and barrel wear too, and the wear shows up as a loss of output at constant screw speed. A machine that has run wood composite for a year will not deliver the same rate on unfilled compound, because the clearances have opened. That is worth knowing before the machine is scheduled for a precision job.

The profile design has to account for the material's low elongation. A wood plastic composite does not stretch, so a profile with a sharp internal corner will crack at the corner during cooling or during a bend test. Generous radii, uniform wall and a slow cooling rate are the design rules, and they are the same rules as for any brittle material.

Table 1 — A starting formulation, phr against 100 resin

A formulation that runs

A formulation that runs has four properties. It dries to under one percent without case hardening, it processes at a melt temperature below 190 degrees, it produces a profile with a smooth surface and no voids, and it survives a boiling water test without delamination.

The melt temperature limit is the one that catches people. Wood starts to degrade at 200 degrees and the degradation is exothermic once it starts, so a compound that runs at 195 is a compound with no margin. The lubricant package and the screw design both exist to keep the melt below that limit at a useful output.

The surface is the visible test. A wood composite profile with a rough or streaky surface is usually a moisture problem or a dispersion problem, and the two look similar. Drying the flour properly and checking the dispersion with a thin section will separate them.

The boiling water test is the acceptance test for the interface. A sample immersed in boiling water for an hour should show no blistering, no delamination and a weight gain under five percent. A sample that fails has a coupling problem, not a formulation problem, and no amount of filler adjustment will fix it.

Table 2 — Six faults and where to look first