Process for producing fiber composite moldings

Abstract

The invention relates to a process for producing fiber composite moldings comprising (i) a thermally crosslinkable fiber composite layer as backing layer, and (ii) a thermoplastic fiber composite layer as overlayer, by mutually superposing the fiber composite layer (i) comprising a thermally crosslinkable binder in the unhardened state and the thermoplastic overlayer (ii), and, in a molding press, converting them to the desired form and thermally crosslinking the same, which comprises a higher temperature of the first contact area of the molding press, where this has contact with the backing layer, than of the second contact area of the molding press, where this is in contact with the overlayer.

Claims

1. A process for producing a fiber composite molding, wherein the fiber composite molding comprises: (i) a thermally crosslinkable fiber composite layer as a backing layer, wherein the thermally crosslinkable fiber composite layer (i) comprises fibers and a thermally crosslinkable binder, and (ii) a thermoplastic fiber composite layer as an overlayer, wherein the thermoplastic fiber composite layer (ii) comprises fibers and a thermoplastic binder, wherein the process comprises mutually superposing the fiber composite layer (i) comprising a thermally crosslinkable binder in an unhardened state and the thermoplastic fiber composite layer (ii), and, in a molding press, converting them in one step to the desired form and thermally crosslinking the same, wherein the molding press has a first contact area and a second contact area, the first contact area of the molding press is in contact with the thermally crosslinkable fiber composite layer (i), the second contact layer of the molding press is in contact with the thermoplastic fiber composite layer (ii), the first contact area of the molding press has a higher temperature than the second contact area of the molding press, the thermally crosslinkable fiber composite layer (i) and the thermoplastic fiber composite layer (ii) are in direct contact with each other, the temperature of the first contact area of the molding press is in the range from 170 to 220 C., the temperature of the second contact area of the molding press is in the range from 100 to 170 C., and the temperature of the first contact area of the molding press is higher by at least 30 C. than that of the second contact area of the molding press.

2. The process according to claim 1, wherein the thermally crosslinkable fiber composite layer (i) comprises natural fibers.

3. The process according to claim 1 or 2, wherein the thermoplastic overlayer (ii) comprises natural fibers.

4. The process according to claim 2, wherein the natural fibers have been selected from the fibers of wood, cotton, sisal, flax, hemp, linseed, kenaf, and jute.

5. The process according to claim 1 or 2, wherein the thermally crosslinkable binder of the thermally crosslinkable fiber composite layer (i) is a thermoset binder.

6. The process according to claim 5, wherein the thermoset binder is selected from thermally crosslinkable acrylic acid, thermally crosslinkable acrylic acid/maleic acid copolymers, thermally crosslinkable polyurethanes, and thermally crosslinkable epoxy resins.

7. The process according to claim 1 or 2, wherein the overlayer (ii) is a temperature-sensitive decorative layer.

8. The process according to claim 1 or 2, wherein an additional decorative layer (iii) is laminated onto the overlayer (ii) opposite to the thermally crosslinkable fiber composite layer (i).

9. The process according to claim 1 or 2, wherein the weight per unit area of the backing layer (i) in the dry, unhardened state is in the range from 600 to 1000 g/m.sup.2, and the weight per unit area of the overlayer (ii) in the dry state is in the range from 200 to 600 g/m.sup.2.

10. The process according to claim 1, wherein the thermoplastic fiber composite layer (ii) comprises polypropylene fibers.

11. The process according to claim 1, wherein thermally crosslinkable fiber composite layer (i) comprises at least one fiber selected from the group consisting of wood fibers, cotton fibers, sisal fibers, flax fibers, kenaf fibers, hemp fibers, linseed fibers and jute fibers.

12. The process according to claim 1, wherein the thermoplastic fiber composite layer (ii) comprises at least one fiber selected from the group consisting of flax fibers, hemp fibers and kenaf fibers.

13. The process according to claim 1, wherein thermally crosslinkable fiber composite layer (i) comprises at least one fiber selected from the group consisting of wood fibers, flax fibers, kenaf fibers and hemp fibers.

14. The process according to claim 1, wherein the thermoplastic fiber composite layer (ii) comprises at least one fiber selected from the group consisting of flax fibers, hemp fibers, kenaf fibers, cotton fibers and sisal fibers.

15. The process according to claim 1, wherein the thermoplastic binder is selected from the group consisting of film-forming thermoplastic polyurethane dispersions, styrene-acrylate dispersions, and straight acrylate dispersions.

16. The process according to claim 1, wherein the temperature of the first contact area of the molding press is higher by at 30 C. to 70 C. than that of the second contact area of the molding press.

17. The process according to claim 1, wherein the fiber composite molding has a weight per unit area from 1000 to 2000 g/m.sup.2.

18. The process according to claim 1, wherein the fiber composite molding has a modulus of elasticity of from 2500 to 9000 N/mm.sup.2.

19. The process according to claim 1, wherein the weight ratio of (i) to (ii) in the fiber composite molding is 5:1 to 2:1.

20. The process according to claim 1, wherein the thermally crosslinkable binder of the thermally crosslinkable fiber composite layer (i) is a thermoset binder and comprises a thermally crosslinkable acrylic acid/maleic acid copolymer.

21. The process according to claim 20, wherein the thermally crosslinkable binder of the thermally crosslinkable fiber composite layer (i) comprises the thermally crosslinkable acrylic acid/maleic acid copolymer and triethanolamine as a crosslinking agent.

22. A process for producing a fiber composite molding, wherein the fiber composite molding comprises: (i) a thermally crosslinkable fiber composite layer as a backing layer, wherein the thermally crosslinkable fiber composite layer (i) comprises fibers and a thermally crosslinkable binder, and (ii) a thermoplastic fiber composite layer as an overlayer, wherein the thermoplastic fiber composite layer (ii) comprises fibers and a thermoplastic binder, wherein the process comprises mutually superposing the fiber composite layer (i) comprising a thermally crosslinkable binder in an unhardened state and the thermoplastic fiber composite layer (ii), and, in a molding press, converting them in one step to the desired form and thermally crosslinking the same, wherein the thermally crosslinkable binder of the thermally crosslinkable fiber composite layer (i) is a thermally crosslinkable acrylic acid/maleic acid copolymer, the molding press has a first contact area and a second contact area, the first contact area of the molding press is in contact with the thermally crosslinkable fiber composite layer (i), the second contact layer of the molding press is in contact with the thermoplastic fiber composite layer (ii), the first contact area of the molding press has a higher temperature than the second contact area of the molding press, the thermally crosslinkable fiber composite layer (i) and the thermoplastic fiber composite layer (ii) are in direct contact with each other, the temperature of the first contact area of the molding press is in the range from 170 to 220 C., the temperature of the second contact area of the molding press is in the range from 100 to 170 C., and the temperature of the first contact area of the molding press is higher by at least 30 C. than that of the second contact area of the molding press.

Description

EXAMPLES

(1) Natural fiber composite sheaths with an Emuldur-bonded overlayer for improving mechanical properties.

Inventive Example 1

(2) Production of a sandwich structure made of an Acrodur-bonded natural fiber nonwoven as backing and of an Emuldur-bonded lower-weight overlayer.

Inventive Example 1a

Impregnation of a Natural Fiber Mat to Produce Backing

(3) Natural fiber mat: hemp/kenaf 30:70 with weight per unit area of 1000 g/m.sup.2, binder: Acrodur DS 3515 (acrylic acid/maleic acid copolymer with triethanolamine as cross-linking agent)

(4) Binder concentration: 50% by weight in water

(5) Binder application rate: specified as 28%, based on dry mass

(6) Binder density (foam): from 450 to 500 g/L

(7) The nonwovens for impregnation are cut to a size of 3428 cm, and weighed. A corresponding amount of binder is prepared in a Kenwood Major mixer and foamed by agitation to a density of from 450 to 500 g/L. The density of the foam is checked by using a beaker of volume 100 cm.sup.3. The beaker is tared, and then completely filled with the foam, and again weighed.

(8) To determine the weight of the foam, the value read on the balance is multiplied by ten. The resultant stable foam is applied to the mat by using a horizontally operated HVF roll mill from Mathis. For this, the rolls are brought together until a defined separation is reached and are subjected to a counterpressure of from 4 to 6 bar. The binder foam is charged to the nip, and the rolls are driven at a velocity of 2 m/min. The pieces of mat for impregnation are introduced vertically into the gap from above and are transported through the gap by the rotating rolls. The binder is thus forced uniformly into both sides of the mat. The amount applied can be adjusted by adjustment of the gap and of the pressure applied.

(9) The resultant semifinished productnatural fiber mat and binderis weighed, and the amount of wet binder absorbed is determined. The product is dried at a temperature of 90 C. in a convection drying oven until the residual moisture level is 17%.

Inventive Example 1b

Impregnation of a Natural Fiber Mat to Produce the Overlayer

(10) Natural fiber mat: 100% flax with a weight per unit area of 220 g/m.sup.2

(11) Binder: Emuldur (thermoplastic polyurethane dispersion)

(12) Binder concentration: 40% in water

(13) Binder application rate: specified as 25%, based on dry mass

(14) Binder density: unfoamed material

(15) As described above, but the unfoamed binder is charged to the roll mill nip. The two rolls have been brought together so as to be in contact. The pressure applied to the rolls is 4 bar. After determination of the amount applied, these semifinished products are dried to a residual moisture level of 0%.

Inventive Example 1c

Production of the Sandwich Structure Made of Backing and Overlayer

(16) A backing nonwoven and an overlayer, each in 3428 cm format, are pressed in one step in a press from Vogt to give a component of thickness 2.2 mm. The temperature of the contact area for the backing is 200 C., and the temperature of the area for the Emuldur-bonded surface is 150 C. The materials are pressed for 45 seconds after an aeration cycle lasting 5 seconds, the pressures applied to the areas being about 30 bar. The resultant sheet can be removed in dimensionally stable form without any sticking.

Comparative Example

Backing Layer without Overlayer

(17) For comparison, a natural fiber mat made of hemp/kenaf in the ratio 30:70 was coated using a weight per unit area of 1000 g/m.sup.2 as in inventive example 1a, and dried, and pressed as described in inventive example 1c, but without overlayer.

Inventive Example 2

Determination of Mechanical Properties

(18) To determine mechanical properties, appropriate test specimens were prepared and were tested after 24 hours of aging under standard conditions of temperature and humidity. The structure according to the invention gives markedly higher impact resistance values without any alteration of high modulus of elasticity. The following properties were found for a component density of 0.9 g/cm.sup.3 and a thickness of 1.8 mm:

(19) Impact resistance [Charpy ISO 179]

(20) Comparative example=9 kJ/m.sup.2

(21) Novel sandwich structure=25 kJ/m.sup.2

(22) Modulus of elasticity [DIN 14125]

(23) Comparative example=4500 N/mm.sup.2

(24) Novel sandwich structure=4700 N/mm.sup.2

(25) Water absorption [DIN 52364] after 24 hours

(26) Comparative example=45%

(27) Novel sandwich structure=30%

(28) Swelling [DIN 52364] after 24 hours

(29) Comparative example=26%

(30) Novel sandwich structure=10%