PROCESS FOR MAKING CURABLE, MULTI-LAYER FIBER-REINFORCED PREPREG

Abstract

Multilayer prepregs are formed by stacking layers of unidirectional prepregs in which continuous reinforcing fibers are aligned in a single direction. The unidirectional prepregs are made by impregnating aligned tows of a reinforcing fiber with a solid thermosetting resin that softens at a temperature below its cure temperature. The layers of the stack are bonded together by softening and then re-cooling the resin portion of the individual prepreg layers. The multilayer prepreg is then molded and cured to form a composite part.

Claims

1. A method for making a curable multiaxial prepreg, comprising A) impregnating multiple aligned fiber tows with a heat-softened, thermosetting resin composition without curing the thermosetting resin composition, wherein the thermosetting resin composition is solid and non-tacky at 25 C., has a curing temperature of at least 80 C. and a Mettloff softening temperature below its curing temperature, and then cooling the composition to below its Mettloff softening temperature to form a unidirectional prepreg which is non-tacky at 25 C. and in which the resin infusion is at least 70%, the prepreg containing up to 500 g/m.sup.2 of unidirectional reinforcing fibers; B) stacking two or more layers of the unidirectional prepreg to form a multilayer stack in which the orientation of the unidirectional fibers in at least one prepreg layer is different from the orientation of the unidirectional fibers in at least one other prepreg layer to form a multilayer stack containing at least 500 g/m.sup.2 of reinforcing fibers; C) heat-softening at least a portion of the thermosetting resin composition of at least some of the stacked prepreg layers without curing the thermosetting resin composition, and then cooling and thereby re-hardening the thermosetting resin composition while maintaining the adjacent layers of the stack in contact with each other to form adhesive bonds between the layers of the stack and produce the curable multiaxial prepreg.

2. The method of claim 1, further comprising D) curing the curable multiaxial prepreg to form a composite.

3. The method of claim 1, wherein in step A), the resin infusion is at least 75%.

4. The method of claim 1, wherein in step A), there is no crossing of the tows in the unidirectional prepreg.

5. The method of claim 1, wherein in step A), the multiple aligned tows form a monolayer.

6. The method of claim 1, wherein the unidirectional layers in the multilayer stack are not mechanically affixed to each other, other than through the formation of interlayer adhesive bonds in step C).

7. The method of claim 1, wherein the unidirectional prepreg formed in step A) has an areal weight of fibers of 40 to 300 g/m.sup.2.

8. The method of claim 1, wherein 3 to 12 layers are stacked in step B) and the areal weight of the fibers in the multilayer stack produced in step B) is 750 to 2000 g/m.sup.2.

9. The method of claim 1, wherein in step C), the thermosetting resin composition is locally heat-softened in discrete areas and upon cooling and rehardening discrete and localized points of adhesion form between the unidirectional prepreg layers.

10. The method of claim 1, wherein in step C) the thermosetting reins composition is heat-softened by applying ultrasound energy.

Description

EXAMPLE 1 AND COMPARATIVE SAMPLE A

[0072] A unidirectional prepreg is made by impregnating tows of Aksaca 24K A-42 carbon fibers (DowAksa Ileri Kompozit Malzemeler Sanayi Ltd, Sti) with a thermosetting resin composition having a cure temperature of greater than 90 C. and a Mettloff softening temperature of 40 to 75 C. Bobbins containing the fibers are mounted across a width of 508 mm, and the tows are pulled sequentially through a spreading station, a heating station, a resin application station, a compacting station, a second heating station, and a second compacting station. The tows are all parallel to each other, form a single layer, and are spaced such that the fiber areal weight is about 225 g/m.sup.2.

[0073] In the first heating station, the fibers are heated to about 80 to 120 C. The thermosetting resin composition is applied to the heated fibers as a particulate solid, which is mechanically pressed into and between the fibers in the first compacting station. In the second heating station, the thermosetting resin composition is heat-softened by heating it to 80 to 120 C., and in the second compacting station the heat-softened resin composition is again mechanically pressed into the fibers. The thermosetting resin composition is then cooled to below its Mettloff softening temperature to form a unidirectional prepreg. The areal weight of the unidirectional prepreg is about 385 g/m.sup.2. The unidirectional prepreg contains about 41% by weight of the thermosetting resin composition and 59% by weight of the fibers. The % resin infusion is approximately 90%. Essentially no curing of the resin occurs during the heating steps since the heat is only applied to the resin for a short period of time.

[0074] A portion of the unidirectional prepreg is cut into 508 mm762 mm sections. Two stacks of four prepreg layers are formed. The fiber orientation in the layers (from bottom to top) is 0, 45, 90 and +45. The stacks are separately heated to 60 to 80 C. under pressure to soften the thermosetting resin composition and then cooled to produce two multilayer prepregs in which the individual unidirectional prepreg layers are bonded together by the thermosetting resin composition. Again, essentially no curing of the thermosetting resin occurs. The stacks are then cut to produce a square multilayer prepreg about 305 mm on a side.

[0075] One of the multilayer stacks is inverted and placed atop the other in a preheated, dome-shaped laboratory mold. The stacks are molded under pressure at 150 C. for about three minutes to cure the resin and form a shaped composite, which is designated Example 1. Test coupons are cut from shaped composite and tested for tensile strength at break, elongation at break, tensile modulus and compressive strength according to ASTM D3030 and ASTM D3410. Results are as indicated in Table 1 below.

[0076] To produce Comparative Sample A, a four-layer fabric is formed from the same fibers. The four layers are laid atop each other with fiber orientations of 0, 45, 90 and +45, respectively. Each layer has an areal weight of about 200 g/m.sup.2. The layers are sewn together. The four-layer fabric is impregnated with the thermosetting resin composition in the same manner described with respect to Example 1, to form a prepreg having an areal weight of 1295 g/m.sup.2, which corresponds to only 36.4% resin and 64% fibers. The % resin infusion is only 45%. This prepreg (Comp. Sample A) is cut into sections as described with respect to Example 1, and two of the sections are stacked and molded as described in Example 1. Physical properties are measured as before, with results as indicated in Table 1.

TABLE-US-00001 TABLE 1 Property Ex. 1 Comp. A Tensile Strength, MPa 445 438 Elongation, % 1.33 1.45 Tensile Modulus, GPa 42.6 <30 Compressive Str., MPa 300 <100

[0077] These results demonstrate the very substantial advantages of this invention. While tensile strength and elongation is similar between the two samples, the tensile modulus of Example 1 is almost 50% greater than the Comparative, and compressive strength is more than tripled.