Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2001-11-19
2004-09-28
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S219000, C428S220000, C428S372000, C428S213000, C427S407100, C427S532000
Reexamination Certificate
active
06797379
ABSTRACT:
The invention relates to a sheet-shaped product processable by means of flow molding (hereinafter also referred to as “sheet molding compound” or “SMC”), comprising carbon fibers and a thermosetting resin mixture based on a radical-curable resin as the matrix. The invention also relates to a process for the production of such a sheet-shaped product and to processes for producing net end products from such a sheet-shaped product.
As intended in this application, radical-curable resin is understood to be any resin containing an unsaturation and being capable of curing by radicals.
Sheet-shaped products processable by means of flow molding which contain carbon fibers and a thermosetting resin mixture based on a radical-curable resin as the matrix are known. Such an SMC is for example described in CA-A-2199638. This reference however refers exclusively to the preparation of SMCs that are filled with fibrous material obtained via the chopped-strand technology. In practice, no such SMCs containing carbon fibers have however ever been marketed because such compounds cannot be produced via the normal compounding lines for glass-filled SMCs. The most important reason for this is that, after the chopping, the carbon fibers do not show suitable distribution behaviour, as a result of which the compounds have an inhomogeneous fiber distribution and show non-optimum flow. In addition, an undesired amount of conductive dust is formed in chopping carbon fibers, with all the associated problems.
The aforementioned reference does describe sheet-shaped products consisting of a thermosetting resin mixture based on a radical-curable resin and fibrous material, including carbon fibers, but there are no indications that sheet-shaped products containing carbon fibers have actually been produced.
The aim of the present invention is to provide a sheet-shaped product comprising a thermosetting resin mixture based on a radical-curable resin and carbon fibers which does not possess the aforementioned drawbacks.
Surprisingly, a sheet-shaped product processable by means of flow molding comprising carbon fibers and a thermosetting resin mixture based on a radical-curable resin as the matrix is obtained when the carbon fibers are present in the form of mats that consist substantially of carbon fibers with lengths of more than 1 cm, the volume percentage of the carbon fibers relative to the resin being less than 70% and the fibers moving freely relative to one another in the mat when the sheet-shaped product is in a mold subjected to a pressure so that, at that pressure and the lay-up percentage employed in the mold, a net end product with a homogeneous fiber distribution is obtained.
Using flow molding for sheet-shaped products containing mats consisting of carbon fibers with lengths of more than 1 cm in a matrix of radical-curable resin is not obvious. Although SMC technology in general has long been known, and mats consisting of carbon fibers have long been known per se, SMCs based on carbon mats have hitherto never been marketed.
In the case of SMCs based on aminoplasts, by contrast, using mats is assumed to be advantageous because the compounds have to be dried before the processing, which can for example be done by placing the impregnated mats on a chain conveyor.
Mats consisting of radical-curable resin containing (carbon) fibrous material are incidentally used in so-called hand-lay-up (HLU) or resin-transfer-molding (RTM) techniques. Those techniques however involve the disadvantage that the required cycle times are relatively long and that only series of a limited size can be produced with them. An additional advantage of the present invention is that the SMCs according to the invention can be processed with cycle times like those that are customary in processing chopped-strand glass SMCs.
Wherever this application refers to “mats”, they are understood to comprise both isotropic and anisotropic mats.
An ‘isotropic mat’ is understood to be a mat in which the orientations of the fibers show no regularity, but there is a random distribution of orientations. The carbon fibers in the isotropic mats generally have lengths of at least 1 cm.
An ‘anisotropic mat’ is understood to be a mat in which the orientations of the fibers show a certain amount of ordering. The fibers in an anisotropic mat may for example be grouped in bundles that cross one another, for example perpendicularly. This also includes unidirectional (UD) mats. In the case of unidirectional carbon fiber reinforcement the use of a dispersed carbon fiber roving (or optionally of a number of dispersed carbon fiber rovings placed next to one another) is also regarded as “mats” as intended in the invention. A special case of a UD mat is a mat consisting of parallel fiber bundles, optionally of varying lengths, which moreover may or may not be staggered longitudinally with respect to one another.
In particular, continuous fibers are used in anisotropic mats. ‘Continuous fibers’ are understood to be fibers which substantially have a length that is larger than the mat's largest width. The maximum length of the fibers, in particular of the carbon fibers, is then determined by the maximum dimensions within the mat.
If the anisotropic mat consists substantially of continuous fibers, it may be advantageous to include a small amount of shorter fibers, for example fibers shorter than 6 cm, preferably shorter than 4 cm, in particular shorter than 2 cm, in the sheet-shaped product to obtain an even better distribution of fibers on bosses, ridges and rims. It has surprisingly however been found that even when the fibers substantially have lengths that are larger than the mat's greatest width, and in particular even when almost 100% of the fibers have lengths that are larger than the mat's greatest width, excellent 3-D end products with bosses, ribs, rims, etc. can still be obtained.
It is to be noticed that in EP-A-0768340, in a paragraph presenting a general description of possible reinforcement materials which may be added to the very specific unsaturated polyester resin (SMC or BMC) compositions as are being taught in said reference, mentions carbon fibers as the last possibility in a long list of many other fibrous materials, and also presents a list of possible forms of such fibrous materials, including the form of chopped strand mats. There is no teaching whatever in EP-A-0768340 that indeed chopped strand mats are being used conveniently as the reinforcement material, and so there is even less suggestion that carbon fiber chopped strand mats would be used. In fact said reference rather would suggest the use of chopped strand fibers (see page 11, lines 56-59). Moreover, in all examples, comparative examples and figures of EP-A-0768340 where fiber reinforcement material is being used, such material consists glass fiber (rovings), which is being chopped.
It is to be noticed, moreover, that the object of EP-A-0768340 is to provide (reinforced) unsaturated polyester resin compositions, which can be molded at a low temperature (40 to 100° C.) under a low pressure (0.1 to 10 kg/cm
2
). Said aim is achieved by providing a specific mechanism of thickening (namely an alternative for metal oxide thickening): component (B) of EP-A-0768340 is a thickening agent containing powder of a thermoplastic resin as the effective component (in an amount of 20-120 parts by weight per 100 parts by weight of the unsaturated polyester). Thus, the compositions of EP-A-0768340 are necessarily different from the compositions according to the present invention, which contain metal oxide thickeners or thickeners equivalent therewith (usually in amounts of 10 parts by weight or less per 100 parts by weight of the unsaturated polyester). In other words, EP-A-0768340 teaches away from the present invention by replacing metal oxides (for instance, magnesium oxide) as thickener by another.
Besides the carbon fibers, the mats may also contain other fibrous materials. Examples are mats consisting of carbon fibers and (optionally metal-coated) glass fibers, carbon fibers and a
Dijkink Peter
Hornman Hans H. H.
Van Dijk Hans K.
Van Voorst Bastiaan
Dixon Merrick
DSM IP Assets B.V.
Mayer Brown Rowe & Maw LLP
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