Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2001-10-15
2003-08-12
Thomas, Alexander S. (Department: 1772)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S318800
Reexamination Certificate
active
06605329
ABSTRACT:
The invention relates to a process for producing a thermoplastic moulded part, which comprises the following steps:
a) dispersing reinforcing fibres in a melt of a polyolefin or a polycondensation polymer,
b) injecting the polymer composition thus obtained into a closed mould by means of an extruder or an injection-moulding machine.
Such a process is known from JP-A-5-17631. JP-A-5-17631 describes a process for injection-moulding a flat plate of fibre-reinforced polypropylene.
JP-A-7-16933 discloses a similar process; the specific modulus and specific strength of a fiber reinforced article so made needs improvement.
A drawback of the method described in JP-A-5-17631 is that the fibre-reinforced polypropylene described herein results in a specific modulus and a specific strength which are lower than desired.
The aim of the invention is to eliminate this drawback This aim is achieved according to the invention in that the melt is injected through a nozzle into the mould, the mould is partly opened when at least part of the surface of the moulded part has cooled to below the softening temperature of the polyolefin or the polycondensation polymer while the centre of the moulded part has a temperature above the said softening temperature, and 1 to 60 wt. % of the mixture consists of reinforcing fibres with an average length of between 0.8 and 15 mm.
The moulded part surprisingly expands during the opening of the mould and a thermoplastic moulded part with a porous centre is obtained. The expanded moulded part obtained by the process according to the invention has a better specific modulus and strength than the known moulded part.
Partly opening the mould in this specification is understood to mean the opening of the mould over a certain path length until the distance between both mould halves is equal to the desired thickness of the moulded part.
In the case of moulded parts, the requirements with respect to stiffness and strength determine the moulded part's thickness, and hence also its price. The so-called specific modulus and strength are used to enable comparison of the modulus and strength of materials for moulded parts. They are index numbers that are measures of the resistance offered by a moulded part to deformation and rupture, respectively, under the influence of a bending load per unit of density. The specific modulus and strength are used especially to compare the modulus and strength of materials having different densities when searching for the lightest material offering the greatest stiffness or strength for a particular shape. A detailed description of these index numbers is given in the “Materials Selector: guidelines for minimum weight design”, Chapman & Hall, London.
Preferably, a converging nozzle is used in the process according to the invention, for this surprisingly causes better expansion of the moulded part during the opening of the mould. The use of a converging nozzle is known from WO-A-94/11177, but here a converging nozzle is used to obtain an orientation of the plastic and/or the fibre reinforcement in the moulded part, as a result of which this may possess a higher stiffness and strength in a particular direction. WO-A-94/11177 nowhere mentions the fact that a converging nozzle could lead to expansion of the moulded part, while the process according to the invention causes no or virtually no anisotropy in the moulded part.
A converging nozzle can be obtained with a conically ending nozzle, but also by for example placing a breaker plate (a plate with a number of openings) in front of the nozzle.
The merits of the invention are brought out very clearly if the process according to the invention is used for the production of dish-shaped moulded parts.
Such a moulded part possesses two dish surfaces, which are in this description understood to be the two, usually almost parallel, surfaces lying opposite one another, whose length and width are larger than the thickness of the moulded part lying between these surfaces. The dish surfaces need not only be flat, but may for instance also be curved or doubly curved.
Partly opening the mould only when at least one dish surface has cooled to below the softening temperature of the polyolefin or the polycondensation polymer results in a moulded part with at least one dish surface that is not porous, that is, with at least one surface free of pores. The presence of at least one dish surface that is not porous proves to result in an improvement of the specific modulus and strength with respect to a non-expanded moulded part. In addition, such a surface can suitably be painted.
Preferably, the mould is partly opened only when both dish surfaces have cooled down.
As a result, both dish surfaces are non-porous so that a moulded part with a sandwich structure is obtained. A sandwich structure gives a dish-shaped moulded part an extra high stiffness and strength. A weak point in sandwich structures is often the adhesion between the centre and the dish surface. In known sandwich structures this adhesion is often improved by using a so-called 3D fabric, which is understood to be a three-dimensional fibre structure in the form of a fabric. A characteristic of a 3D fabric is that some of the fibres that reinforce the centre continue into the dish surface, which promotes the adhesion between the centre and the dish surface. 3D fabrics and composites made therefrom are described by A. Schrauwers in “Kunststof Magazine”, 1993, page 16. Drawbacks of using a 3D fabric are that it has to be made to measure beforehand and that it must separately be placed in the mould for each injection. A 3D fabric can moreover only be used in combination with thermosetting plastics with a very low viscosity. It has surprisingly been found that with the process according to the invention at least a portion of the reinforcing fibres is present partly in the centre and partly in the non-porous dish surface, as a result of which it is not necessary to use a 3D fabric. It has also been found that the fibres in the centre of the moulded part form a three-dimensional network and lie parallel to the surface at the moulded part's dish surface. With this the process according to the invention for the first time offers the possibility of using the injection-moulding technique to produce (half) a fibre-reinforced sandwich from a thermoplastic fibre-reinforced plastic, in which the fibres of the centre continue into the surface. This structure contributes to the excellent specific modulus and strength, even if only one dish surface is non-porous.
The melt flow index (MFI) is in this description understood to be the melt flow index measured according to ISO 1133. For polypropylene the melt flow index is measured at 230° C. under a weight of 2.16 kg.
The melt flow index of the polyolefin to be used in the process according to the invention is preferably higher than 30 g/10 min, even more preferably higher than 50 g/10 min. It has been found that at such a melt flow index better expansion of the moulded part takes place when the mould is opened. Preferably, the melt flow index is lower than 700 g/10 min.
The number average molecular mass (M
n
) of the polycondensation polymers to be used in the process according to the invention is preferably higher than 5000 g/mol. The process proves to be effective for all the polycondensation polymers so far available. With the present polymerization technologies approximately 90,000 g/mol is the upper limit of the molecular mass of available polycondensation polymers. It may be expected that polycondensation polymers with higher molecular masses, if they become available, can be processed with the process according to the invention, up to a molecular mass of approximately 200,000 g/mol.
The ‘average fibre length’ is in this description understood to be the number average fibre length. This can be determined in the moulded part by measuring the length of the fibres with the aid of a light microscope after the polymer matrix has been removed, for example by burning out the polymer.
If the polymer composition contain
Bulters Markus J. H.
Geesink Johannes H.
Stokman Petrus H. M.
DSM N.V.
Thomas Alexander S.
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