Process for the manufacture of milled glass fibers

Details Process for the manufacture of milled glass fibers Process for the manufacture of milled glass fibers

1994-06-30

2001-10-16

Hoffmann, John (Department: 1731)

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

Composite fiber matrix

C065S021100, C065S215000, C241S004000, C241S023000, C241S017000

Reexamination Certificate

active

06301935

ABSTRACT:

The present invention relates to milled glass fibers and to a simplified process for the manufacture of these milled glass fibers.
BACKGROUND OF THE INVENTION
Glass fibers have hitherto been processed to sized or unsized cakes in the spinning and winding process. These cakes, which have a moisture content of approximately 12% by weight, are then dried in a forced-air oven, precomminuted with guillotine shears and subsequently ground to so-called milled fibers in cross hammer mills.
The disadvantage of this process carried out hitherto is that, after the glas fibers bundle has been cooled and, if appropriate, coated with a size, it is first wound on to a cardboard tube in order then to be dried as so-called wet cake, which has been taken off the winding spindle. After drying, the cardboard tube is removed from the cake by hand and the cake is then extended to form a loose bed and fed to guillotine shears for precomminution. The precomminuted product is then passed into a hammer mill, where it is ground into milled glass fibers.
The milled glass fibers manufactured in this way have the disadvantage of a low apparent density coupled with correspondingly small average lengths.
The object is therefore to provide improved milled glass fibers and an improved and simplified process for the manufacture of these milled glass fibers without the quality of the milled glass fibers being adversely affected.
This object could be achieved by the glass fibers according to the invention and the process according to the invention.
SUMMARY OF THE INVENTION
The invention relates to milled glass fibers which are characterized in that they have an average length of 100 to 400 &mgr;m and at the same time an apparent density of 1 to 0.2 g/cm
3
.
The milled glass fibers have an average length preferably of 150 to 300 &mgr;m, most preferably 180 to 250 &mgr;m, and at the same time an apparent density of 0.8 to 0.3 g/cm
3
, most preferably 0.6 to 0.4 g/cm
3
.
The milled glass fibers according to the invention are distinguished by high apparent densities coupled with correspondingly large average lengths.
In general, the diameters of the glass fibers are between 8 and 24 &mgr;m, preferably between 10 and 14 &mgr;m.
The physical characteristics of the glass fibers were determined by the following procedures:
The apparent density of the glass fibers is defined as the amount, in grams, which occupies 1 cm
3
after loose pouring into a vessel, The vessel (250 ml measuring cylinder) is first weighed empty. It is then filled loosely with glass fibers until a volume of 250 ml is reached. The filled measuring cylinder is reweighed. The procedure is repeated several times and the values are averaged. The error in the apparent density data is ±0.02 g/cm
3
.
To determine the fiber length, 1-2 drops of a water/glycerol mixture (1:1) are placed on each of 3 microscope slides. Approximately 20 mg of fibers are then added carefully, finely divided. Under the microscope, the fibers in the corresponding field of vision are then counted and measured; the field should not contain more than 250 fibers if perfect counting and measurement of the fibers is to be guaranteed. The average fiber length is determined by measuring at least 600 fibers. The error in the fiber length determination is 5%.
The invention further relates to a process for the manufacture of the milled glass fibers according to the invention, which is characterized in that, after cooling and, if appropriate, after sizing, the glass fibers obtained at the spinning stage are cut directly, without previous drying, and are then ground to milled glass fibers in a high-efficiency mixer, with the addition of liquids, especially water, and/or sizing agents if appropriate, and then dried if appropriate.
The liquid or water and/or, if appropriate, size are preferably added in amounts such that at least 10% and at most 40%, preferably at least 22% and at most 28%, of water is present based on the total amount in the mixer.
Prior to grinding, the glass fiber bundles are preferably cut to an average length of 3 to 55 mm.
In a preferred embodiment of the process, the cut glass fiber bundles are ground in a solid mixer with a Froude number Fr of
Fr
=
R
·
w
2
g
⪢
1
where R=radius of the mixing elements in the mixer
w=angular velocity of the mixing elements
g=gravitational acceleration.
In the process according to the invention, the sized or unsized glass fibers are processed directly at the spinning stage to so-called chopped strands and then, whithout previous drying, are ground to milled fiber in a high-efficiency mixer, with the addition of, e.g., water.
Chopped strands, which are produced in conventional manner via an intermediate so-called cake, can also be ground to milled fibers by the novel process.
The moist ground material (water content up to approximately 30% by weight) is then dried in a drying step. Examples of drying units which can be used are forced-air ovens, HF ovens, band dryers and vibrating conveyor dryers.
Milled fiber products with different average fiber lengths can be manufactured by varying the grinding times and the amount of products and water.
Conventionally, the sizes which may be used consist of film-forming agents (e.g. based on polyurethane, epoxy resin, polyvinyl acetate or mixtures thereof), coupling agents (e.g. organofunctional silanes), lubricants (e.g. polyethylene glycol, polypropylene glycol or quaternary fatty acid amides) and, if appropriate, antistatic agents (e.g. lithium chloride, ammonium chloride or sodium hydrogen phosphate).
These milled fiber products according to the invention differ from the products manufactured by the former process especially in that the apparent densities achieved can be twice as high for comparable average fiber lengths (Table 1). Thus it is also possible to manufacture milled fiber products with an average fibre length of 0.3 mm, which can even be processed in the PUR-RIM process (Poly-urethane-Reinfoiced Reaction Injection Molding Process).


REFERENCES:
patent: 3766003 (1973-10-01), Schuller
patent: 3869268 (1975-03-01), Briar
patent: 3962172 (1976-06-01), Wurmb
patent: 4039718 (1977-08-01), Kallenborn
patent: 4145202 (1979-03-01), Grodin et al.
patent: 5007947 (1991-04-01), Kenmoti
patent: 5087518 (1992-02-01), Shimada
patent: 5417889 (1995-05-01), Akiba
patent: 0334171 (1989-09-01), None
patent: 2578833 (1986-09-01), None
patent: 0668700 (1979-06-01), None
Patent Abstracts of Japan, vol. 7, No. 185 (C181), one page; JP58088137, Feb. 26, 1983; Nitto Boseki KK, “Preparation of Glass Powder”.

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