Method of producing a thermoplastically moldable fiber-reinforced semifinished product

Abstract

A continuous method for producing a thermoplastically moldable semifinished product of a thermoplastic material and reinforcing fibers, comprises blending thermoplastic fibers and reinforcing fibers together to form a nonwoven blend, consolidating the nonwoven blend by needling or by a thermal treatment, heating the consolidated nonwoven blend to a temperature above the softening temperature of the thermoplastic, compressing the consolidated nonwoven blend successively in a heated compression mold and in a cooled compression mold at a pressure of less than 0.8 bar for at least 3 seconds, and optionally applying functional layers to the semifinished product. The preferred product is a thermoplastically moldable semifinished product of a thermoplastic material and reinforcing fibers with an average length of 20 to 60 mm and an air pore content of 35 to 65 vol %.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. application Ser. Nos. 10 / 472,530, filed Oct. 24, 2003; U.S. Ser. No. 10 / 470,969, filed Dec. 4, 2003; European Patent Application No. EP 05 007 391.5 filed Apr. 5, 2005; and European Patent Application No. EP 05 009 770.8 filed May 4, 2005, priority to all of which are hereby claimed.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention relates to a method for producing a thermoplastically moldable fiber-reinforced semifinished product from a mixed nonwoven containing thermoplastic fibers and reinforcing fibers. [0004] 2. Background Art [0005] Thermoplastically moldable semifinished products containing reinforcing fibers, in particular glass fibers, are being used to an increasing extent for the production of moldings, in particular for automotive parts. Such “plastic panels” are characterized by high strength and toughness. GMT semifinished products are manu...

AI Technical Summary

Benefits of technology

[0015] Therefore, an object of this invention is to provide a simple continuous method of producing a distortion-free semifinished product of thermoplastic material and reinforcing fibers which contains air pores with a uniform distribution, and which can be readily reshaped by thermoforming to provide finished parts having excellent and highly reproducible properties in all directions. This and other objects are achieved by the inventive method, wherein individual thermoplastic fibers and individual reinforcing fibers are blended to form a mat of blended fibers, the mat is consolidated, preferably by needling, and is compressed twice, first in a heated compression mold, followed immediately by a cooled compression mold, in both cases at a pressure of less than 0.8 bar, to form an intermediate product which contains a uniform distribution of reinforcing fibers, thermoplastic, and air pores, the latter exceeding 25% by volume.

Problems solved by technology

This procedure consumes a substantial amount of energy, because the viscous melt must be pressed into the mat at pressures far above 1 bar.

It is thus exceptionally difficult, in practice, to achieve a fiber content greater than 45 wt % and an areal weight below 2000 g / m2 by this method.

Since the reinforcing fibers in the reinforcing fibers in the glass mats are generally in the form of fiber bundles or “strands”, impregnation with thermoplastic is never entirely complete and uniform, and therefore microscopically heterogeneous regions are present, thus resulting in a high standard deviation in the mechanical properties.

Complete impregnation is very difficult to obtain, especially with components having a complex shape, so that the mechanical properties of the moldings leave much to be desired.

Production of mixed strands of reinforcing fibers and thermoplastic fibers is difficult due to the differing tensile elongations and modulus of the different fibers, and only a limited selection of blends is commercially available.

Sheet products produced in this manner are dense and have surface irregularities such as waviness.

However, it has been found that at such high pressures the air pores are forced almost completely out of the softened nonwoven blend and the melt flows apart in length and width, resulting in an uncontrolled variation in areal weight and in distortion of the semifinished product, with the result that the boundaries of the semifinished product are wavy rather than smooth and straight.

Similar problems arise in the methods according to EP-B 593 716 and U.S. Pat. No. 4,978,489 in which the nonwoven blend is compression molded by pressure rollers facing each other.

The compression of mixed nonwovens in calenders or by pressure rollers has the further disadvantage that only low production speeds can be used and that a bulge is formed by the abrupt compression in the gap between rollers, which may result in strong distortion und even the formation of holes.

“Pseudo-foamed composite sheets” are said to result from this process, but the air pore content is necessarily very low, and the high pressure will cause distortion of the non-woven and non-homogenous pore distribution.

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