NL2008059C2

NL2008059C2 - MOLDING PROCESS FOR FIBER CLEANING FORCED THERMOPLASTICS.

Info

Publication number: NL2008059C2
Application number: NL2008059A
Authority: NL
Inventors: Jozef Gerardus Antonius Lindert; Peter Floris Evert Boer
Original assignee: Cato Composite Innovations B V
Priority date: 2011-12-30
Filing date: 2011-12-30
Publication date: 2013-07-03

Description

Moulding Process for Fibre-reinforced Thermoplastics

The present invention relates to a process for moulding articles of fibre reinforced thermoplastic materials and more in particular to the use of recycled materials in such 5 processes. The invention also relates to products formed by such processes.

Background to the Invention

Procedures for moulding components using fibre-reinforced thermoplastic resin based materials are known. An advantage of thermoplastic resins is that they may be 10 repeatedly softened and reformed allowing advantageous multi-stage moulding or forming procedures to be carried out. Their properties may be optimised by the choice of resin and the selection, orientation and size of the fibres used. Components may be moulded by laying up the fibre material and subsequently adding an appropriate quantity of thermoplastic resin to the mould. Alternatively, pre-impregnated fibrous 15 material may be used in which the fibres are supplied, impregnated with the matrix resin in the correct proportions.

Depending upon the materials used, pre-impregnated fibre-reinforced thermoplastic materials may be relatively expensive. High-strength unidirectional carbon fibre based materials are particularly expensive. The fibre materials may be laid up as individual 20 layers to form a uniaxial, bi-axial or multiaxial structure. Alternatively, fibre reinforced consolidated sheet material (consolidated laminates) may be prepared in advance.

Another form of moulding for fibre-reinforced thermoplastic material is known as Long Fibre Thermoplastic (LFT) moulding. LFT uses fibres of limited length mixed into a 25 thermoplastic matrix. The fibres and matrix are ejected into a mould and formed to the desired shape by pressure. This procedure is particularly useful for mass production of lightweight semi-structural components such as car body parts. Due to the shorter fibre lengths, the lower fibre volume content and the isotropic orientation, the strength of such components is generally lower than that of a corresponding part made using 30 continuous fibres.

During the manufacturing of thermoplastic composite components from continuous pre-impregnated fibre reinforced material or from consolidated laminates, waste material is generated. On the one hand, the blanks required for the forming process are 2 trimmed from a larger sheet leading to nesting losses. In addition, excess material is trimmed from the formed and shaped three dimensional components, which also leads to further material waste. At present, such waste material is incinerated as waste or finely ground and re-used in injection or extrusion moulding processes. In such 5 processes, after grinding, the pre-impregnated fibre material is limited in length to around 1-2 mm and compounded with additional virgin resin in order to reduce the fibre content. Once heated to the moulding temperature of the thermoplastic resin, it can easily be injected or extruded into various shapes in an injection mould or through an extrusion die. Nevertheless, due to the short fibre lengths, the resulting product is of 10 considerably lower strength than a component produced using continuous fibres and also generally weaker than those produced using LFT procedures.

It would therefore be desirable to find a use for this waste material, while profiting from its excellent mechanical properties. It would be particularly desirable to leave the fibre length as long as possible.

15

Brief Description of the Invention

According to the invention, there is provided a method of forming a fibre-reinforced thermoplastic component comprising the steps of: providing a quantity of recycled resin-impregnated fibre flakes wherein the flakes are of different shapes and sizes; 20 locating a sufficient quantity of the flakes in a mould; heating the mould and applying pressure to conform the flakes to the shape of the mould to form the component and to evacuate air from the mould; and removing the component from the mould. The resulting product is isotropic and has been found to have excellent mechanical properties and fills a gap between injection moulded and LFT components on one hand 25 and the continuous fibre reinforced components on the other hand.

In a preferred aspect of the invention, the pre-impregnated flakes are ground, resulting in a maximum dimension, preferably of less than 30 mm, more preferably of less than 25 mm, prior to the moulding procedure. In case the thermoplastic consolidated waste materials consist of more than one layer, those layers will be (partially) delaminated 30 resulting in flake thicknesses of less than or equal to the original material thickness. The precise dimension will depend on the intensity of the grinding process and will depend to a certain extent on the flake size and the mechanical properties which are desired by nature of the component being produced. The skilled person will understand 3 that a larger size flake may be appropriate for components requiring higher structural properties and which are larger in size, while smaller sized flakes may be preferred for components having a fine structure with sharp angles and/or which require lower mechanical properties. Preferably, the flakes are at least 3 mm in their largest 5 dimension. The flakes may have been sieved and also have been filtered to remove fine particulates and powder. This is especially desirable in open systems where the material is to be handled and the presence of powders may raise safety and health issues.

Waste material from a pre-impregnated continuous fibre based process may be relatively thick. Advantageously, the flakes are processed to a thickness that is 10 appropriate to the component to be moulded.

According to the method the flakes may be produced by granulating, shredding, grinding or otherwise cutting off-cuts from a continuous fibre reinforced thermoplastic component. In this sense it is understood to include off-cuts from the component or consolidated laminate used to produce such component. The cutting step may take 15 place prior to the moulding process and/or be integrated as part of the moulding process. Pre-impregnated thermoplastic composite consolidated waste material may have to be cut to a size to fit the filler of the grinding equipment.

The grinding equipment may shred the materials at room temperature. Alternatively, lower temperatures may enhance the cutting process because the thermoplastic matrix 20 material will become stiffer and more brittle making the material easier to cut. During this process, delamination of the individual fibre layers of the consolidated laminates may occur. The grinding process is continued in time until a satisfactory homogenous flake size and thus fibre length is obtained. Different cutting knife settings or different knife blades may be used as appropriate in order to obtain the desired flake size. Flakes 25 larger then a certain size may be kept within the grinding process by using a sieve.

From the smaller particles which pass the sieve, particles smaller than a certain minimum size may be filtered out. This grading process results in a homogenous flake size distribution.

The skilled person will be aware that the invention is applicable to all types of mixtures 30 of thermoplastic resins with fibre reinforcement including but not limited to glass, carbon, alumina, alumina-silica, silicon carbide and aramid fibre in bundles or single layer fibre structure such as weaves, uni-directionals, multidirectional or multilayer layups out of those. In general, the benefit of the invention lies in preserving the desirable 4 material properties of the pre-impregnated fibres by leaving the fibre length as long as is possible to manufacture a certain product out of a certain material and preventing the flakes to be mixed with virgin resin, retaining the fibre volume content intact. Another benefit of the invention is the use of resin pre-impregnated fibres which allow the 5 moulding process to be conducted quickly because an impregnation of the fibre in the mould is no longer necessary. Such an impregnation is needed when using a non-impregnated mixture of monofilamentous fibres and thermoplastic resin, in order to obtain the desired high mechanical properties of the moulded component.

The resin matrix material may be any suitable thermoplastic resin. Suitable resins 10 include but are not limited to: crystalline resins such as polyesters (e g., polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), and liquid crystal polyester), polyolefins (e g., polyethylene (PE), polypropylene (PP), and polybutylene), polyoxymethylene (POM), polyamide (PA), polyphenylene sulfide (PPS), polyketone (PK), polyetherketone 15 (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyethernitrile (PEN), fluorine-based resins (e g., polytetrafluoroethylene), and liquid crystal polymer (LCP); amorphous resins such as styrene-based resins, polycarbonate (PC), polymethylene methacrylate (PMMA), polyvinyl chloride (PVC), polyphenylene ether (PPE), polyimide (PI), polyamideimide (PAI), polyetherimide (PEI), polysulfone 20 (PSU), polyethersulfone (PES), and polyallylate (PAR); other thermoplastic elastomers including, polystyrene, polyolefin, polyurethane (TPU), polyester, polyamide, polybutadiene, polyisoprene, fluorine, acrylonitrile, and the like; and thermoplastic resins selected from among copolymers and denatured resins of the above examples. Although in general, the flakes will be sourced from a single source and have the same 25 average composition, according to a beneficial aspect of the invention the flakes may be different from each other in terms of fibre composition. In this way, the resulting material strength may be determined by mixing appropriate amounts of flakes having different properties e.g. combining glass fibres, carbon fibres and aramid fibres as desired. Additionally, the flakes may be different from each other in terms of resin 30 composition.

An advantage of the invention over known procedures for using waste materials is that there is no requirement to add additional virgin resin in order to achieve a processable mass. Nevertheless, additional resin may be added to the mould in order to lower the 5 fibre-matrix ratio if desired. It will be understood that the material may be reinforced with virgin resin impregnated continuous reinforcement consolidated fibre materials in order to enhance component performance locally. In general the content of reinforcing fibres within the matrix will be between 20-90% by volume, most preferably between 5 30-50%.

The invention also relates to a fibre-reinforced thermoplastic component comprising a resin matrix reinforced with fibres, wherein the fibres are present in a plurality of domains of different shapes and sizes. Examples of components that may be covered by the invention include toe caps for safety shoes, aircraft interior components, cell phone 10 interior frames, structural frames of electronic components, bicycle components, automobile components and any other metal-replacement where weight reduction with a high stiffness and/or strength is required.

The component may be manufactured as described above.

15 Exemplary Embodiment 1

Waste material offcuts from a continuous fibre based process are collected. The material comprises a mixture of carbon fibres in a matrix of TPU having a fibre fraction of 35-45% % by volume. The offcuts initially have a thickness range between approximately 0,25 mm or 3,0 mm and vary in size between 1 cm and 25 cm in terms 20 of their maximum dimension. The waste material is placed in a granulator and ground into flakes with a specific size between 3 mm and 25 mm. In this case the fibre length lies between 4 mm and 8 mm. The final thickness of the flakes is between 0,25 mm and 10 mm. The flaked material is selectively sieved and filtered and graded to remove fine particulates and powder and flakes larger than 25 mm.

25 An amount of 120 g by weight of the flakes is measured by using a scale and placed in the mould cavity of a compression moulding machine. The mould is then closed and heated to 220 °C. This is 60 °C above the melting temperature of the TPU thermoplastic resin material. Pressure of 90 bar is applied to the mould, causing the material to flow within the cavity. A small air gap of 0,05 mm between the two mould-30 halves avoids the resin from flowing out of the mould but makes sure then any air can be evacuated from the mould resulting in < 5% void content of the component. A component measuring 20 cm by 20 cm by 2,0 mm is formed and after a short cooling 6 cycle of 30 seconds removed from the mould, is cut and tested for bending modulus and bending strength, according to ISO 178.

The process was repeated using recycled Glass/TPU with resin content of 34% with fibre length between 4 and 8 mm. All other process parameters were kept constant.

5 The results are given in Table 1 below compared with similar results for tokens taken from commercial grade thermoplastic composite formed from a unidirectional Carbon/PA12 tape cut in 10 mm fibre lengths and having a fibre volume fraction of 50 % material. Another commercial material is a LFT grade of Carbon/TPU. These materials are processed in a similar manner and with a fibre length of 10 mm.

10 3pt bending 3pt bending Tensile Tensile stiffness ace. ISO strength modulus strength 178 acc. ISO 178 according to according to (GPa) (MPa) ISO 527 ISO 527

Example Component from 18,1 271 9,5 76 recycled Carbon/TPU with resin content of 40% with fibre between 4 and 8 mm

Example Component from 5,0 125 4,0 37 recycled Glass/TPU with resin content of 34% with fibre length between 4 and 8 mm

Pie-impregnated 17,6 261 21,2 115

Unidirectional Carbon tape with PA12 with fibre volume content of 50% and fibre length of 10 nun

Unidirectional 12,9 227 11,0 45

Carbon/TPU LFT pellets with PA12 with fibre volume content of 40% and fibre length of 10 mm

Table 1 7

As can be seen, the present process may be applied to the manufacture of 3D composite structures with properties above those of LFT materials and injection moulded components, especially when the fibre volume content of the original pre-impregnated consolidated sheet are higher than those of these samples. Such composite components 5 can replace traditional milled steel, aluminum components or injection moulded components offering higher stiffness and strength at the same weight or similar stiffness and strength at a weight reduction. Components also exhibit improved fatigue properties and non-plastic behaviour. The component offers a non-metallic solution to many manufacturing problems and has isotropic material properties since the fibres lie 10 in all directions. The properties of the material fill a gap in the availability of thermoplastic composite components between injection molding materials and LTF on one hand and continuous fibre reinforced materials on the other hand. The components may be partially reinforced with virgin continuous fibre reinforced thermoplastic composite materials as well to locally enhance the component performance further.

15

Claims

A method for forming a fiber-reinforced thermoplastic component, comprising the steps of: 5. providing a quantity of recycled resin-impregnated fiber flakes, the flakes having different shapes and dimensions; • placing a sufficient amount of the flakes in a mold; Heating the mold and applying pressure to make the flakes similar to the shape of the mold, to form the component and to remove air from the mold; • removing the component from the mold.

The method of claim 1, wherein the mold is shaped to provide a discontinuous thickness distribution of the fiber-reinforced thermoplastic material along the component.

3. Method as claimed in any of the foregoing claims, wherein the component is reinforced with pure resin-impregnated continuous fiber-reinforced thermoplastic composite material, to further increase the local rigidity and strength of the component.

A method according to any one of the preceding claims, wherein the flakes are sorted to have a maximum dimension, preferably of less than 30 mm, and in particular of less than 25 mm. 25

A method according to any one of the preceding claims, wherein the flakes have a thickness between 0.1 mm and 1 mm, and preferably between 0.25 and 0.50 mm.

6. A method according to any one of the preceding claims, wherein the flakes 30 are made by shredding, grinding, or otherwise cutting remnants of a continuous fiber-reinforced thermoplastic component.

The method according to any of the preceding claims, wherein the flakes comprise pre-impregnated carbon fibers.

8. Method according to any of the preceding claims, wherein the flakes 5 comprise unidirectional fibers, weave structures, or multiple layers with different orientations of such structures.

A method according to any one of the preceding claims, wherein the flakes differ from each other in fiber composition. 10

The method of any one of the preceding claims, wherein the flakes differ from each other in resin composition.

11. Method as claimed in any of the foregoing claims, further comprising of adding additional resin to the mold.

A fiber-reinforced thermoplastic component comprising a resin matrix reinforced with fibers, the fibers being present in a plurality of unilateral domains. 20

A fiber-reinforced thermoplastic component according to claim 12, manufactured in accordance with the method according to any of claims 1-11.