Process for manufacturing composite product
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 crosslinking takes place inside an extruder. The natural fibers may be provided for example in the form of pulp or wood particles.
1. A process for manufacturing an extruded composite product comprising the steps of
a) 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 a moisture content of the natural fibers and thermoplastic polymer particles introduced into the extruder is less than 1 wt-%, and 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 different than the inlet through which the natural fibers and thermoplastic polymer particles are introduced, and wherein a temperature in the extruder at the point where the crosslinking agent is introduced is less than 150° C.; and
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 different from where the natural fibers, thermoplastic particles and optionally crosslinking agent are introduced into the extruder and mixing, a temperature in the extruder during mixing being at least 170° C. and wherein a pressure exerted on the introduced materials inside the extruder during mixing is at least 50 bar; and
c) extruding a composite product through a die.
2. The process according to claim 1 , wherein the natural fibers are provided in the form of wood flour.
3. A process according to claim 2 , wherein the wood flour has been thermally or chemically modified prior to step a).
4. The process according to claim 1 , wherein the natural fibers are provided in the form of pulp.
5. The process according to claim 1 , wherein the crosslinking agent is citric acid.
6. The process according to claim 1 , wherein an amount of natural fibers is 45-90 wt-%, by weight of the extruded composite product.
7. The process according to claim 1 , wherein an amount of polymer particles is less than 50 wt-%, by weight of the extruded composite product.
8. The process according to claim 7 , wherein the amount of polymer particles is less than 30 wt-%, by weight of the extruded composite product.
9. The process according to claim 1 , wherein the cross-linking agent is introduced into the extruder via another inlet different than the inlet through which the natural fibers and thermoplastic polymer particles are introduced, wherein the temperature in the extruder at the point where the crosslinking agent is introduced is less than 150° C.
10. An extruded composite product obtained by the process of claim 1 .
11. The extruded composite product according to claim 10 , wherein the composite product is further processed into a composite granule or extruded to sheets that may be thermoformed in a process, optionally with additional layers, such as packages, trays or other thermoformed items.
12. The extruded composite product according to claim 11 , wherein the sheets are thermoformed with additional layers.