This application is a U.S. National Phase under 35 U.S.C. § 371 of International Application No. PCT/IB2018/060228, filed Dec. 18, 2018, which claims priority under 35 U.S.C. §§ 119 and 365 to Swedish Application No. 1751603-0, filed Dec. 21, 2017.

FIELD OF THE INVENTION

The present invention relates to an improved process for manufacturing an extruded composite product comprising natural fibers and a thermoplastic polymer. According to the present invention, mixing as well as reactive crosslinking takes place inside an extruder. The natural fibers may be provided for example in the form of pulp or wood particles.

BACKGROUND

In the field of extrusion and extrusion compounding of wood-polymer composite products, crosslinking between hydroxyl groups of the fibers is important to achieve desirable properties of the final product. At the same time, the manufacturing method needs to be efficient.

It is desirable to be able to manufacture composite products with a reduced amount of thermoplastic matrix, i.e. a product that contains as much natural fibers as possible. Generally, composite products become more brittle as the amount of thermoplastic matrix is reduced, said brittleness being a property which is typically undesirable.

For standard wood fiber polymer composites (WPC's), a common problem is moisture uptake and creep of the composite material. This problem often limits the use of standard wood fiber polymer composites in more detailed and challenging outdoor applications, such as in cladding, decking and joinery products. This problem also applies to injection moulded products that are exposed to challenging outdoor applications and humidity.

There is thus a need for more efficient processes for achieving composites having reduced moisture uptake, yet maintaining adequate mechanical properties of the composite product.

SUMMARY OF THE INVENTION

It has surprisingly been found that the method according to the present invention provides more efficient reactive crosslinking which leads to several advantageous properties of the extruded composite product, including high dimensional stability and stiffness, as well as reduced moisture uptake. The present invention also allows the use of high amounts of natural fibers, which is beneficial not only in terms of being a renewable material, but also facilitating recycling and contributing to a light-weight extruded composite product. In addition, it has been found that extruded composite products manufactured by the method according to the present invention exhibit surprisingly low creep compared to traditional wood-plastic composites. The extruded composite products obtained with the method according to the present invention also have a high biological durability.

Thus, the present invention is directed to a process for manufacturing an extruded composite product comprising the steps ofa) introducing natural fibers and particles of a thermoplastic polymer and optionally 0.5-5 wt-% of coupling agent and optionally 0.5-4 wt-% of lubricant into an extruder, wherein the moisture content of the natural fibers and thermoplastic polymer particles introduced into the extruder is less than 1 wt-%; wherein the natural fibers have either been mixed with 1-20 wt-% of crosslinking agent prior to being introduced into the extruder or the crosslinking agent is introduced into the extruder via another inlet than the inlet through which the natural fibers and thermoplastic polymer particles are introduced, and wherein the temperature in the extruder at the point where the crosslinking agent is introduced is less than 150° C.;b) if the coupling agent and lubricant was not introduced in step a), introducing 0.5-5 wt-% of coupling agent and optionally 0.5-4 wt-% of lubricant into the extruder via an inlet which is separate from where the natural fibers, thermoplastic polymer particles and optionally cross-linking agent is introduced into the extruder; mixing, the temperature in the extruder during mixing being at least 170° C. and wherein the pressure exerted on the introduced materials inside the extruder during mixing is at least 50 bar; andc) extruding a composite product through a die.

The composite product manufactured according to the present invention may for example be an extruded shaped product or be further processed into composite granules.

