WO2008008445A2

WO2008008445A2 - Injection polymerization method

Info

Publication number: WO2008008445A2
Application number: PCT/US2007/015913
Authority: WO
Inventors: Joachim Gloerfeld
Original assignee: Thetford Corporation
Priority date: 2006-07-13
Filing date: 2007-07-12
Publication date: 2008-01-17
Also published as: WO2008008445A3

Abstract

The invention includes a method for manufacturing a composite material by injecting a reaction medium such as steam or hot air into a sandwich composite to bond the layers of the sandwich composite. The sandwich composite is comprised of a core material (3), outer layers (2, 4), and covering surfaces (1,5). Pressure, temperature, and flow of the reaction medium are controlled during the bonding process.

Description

INJECTION POLYMERIZATION METHOD

FIELD OF THE INVENTION

[0001] The present invention generally relates to methods for making composite materials by injection polymerization.

BACKGROUND OF THE INVENTION

[0002] Many industries, including the automotive industry, desire cost- efficient, lightweight structures that have heavy load-bearing capabilities. Sandwich composites, which are multilayered composites with a lightweight, low- density core, offer manufacturers the ability to create lightweight structures with high stiffness to weight ratios.

[0003] Typical methods for manufacturing sandwich composites comprise placing into a heated mold a core material covered on one or both sides by pol'yurethane-impregnated mats made of fiberglass or natural fibers. If the materials do not contain water, the mold is generally heated from 60° to 100⁰C while compressing the surface of the sandwich composite. The heat and pressure polymerize the polyurethane in the mats, thus binding the mats to the core material. The cycle time needed to complete this process is rather long, for example, four to seven minutes. Furthermore, the mold cannot completely encapsulate the sandwich composite because the internal pressure created in the core material may cause the composite to explode when removed from the mold.

[0004] Another method for manufacturing sandwich composites is similar to the process above, but the mats are instead impregnated with polyurethane from the Baypreg F® chemical system. See, for example, Cageao et al., "Studies of composites made with Baypreg F®: component selection for optimal mechanical properties," Polyurethanes 2004, October 18-20, 2004, the disclosure of which is herein incorporated by reference. Furthermore, the mold is heated at a higher temperature, for example, from about 90° to 150⁰C, which shortens the cycle time to less than one minute. Although this is a significant decrease in cycle time, the benefits of increased production are off-set by problems caused by the increased thermal load. For example, the types of products generated by this method are generally limited because some surface materials cannot withstand the increased thermal load. Further, the molds require more time and energy to reach the appropriate temperatures, and also require additional safety devices to protect the equipment and workers. Lastly, the higher temperatures result in a shorter life-span for the molds.

[0005] A similar method for manufacturing a sandwich composite is shown in U.S. Patent No. 6,761 ,953, the disclosure of which is incorporated herein by reference. [0006] A drawback shared by the above methods is that the polyurethane adhesives polymerize inward, i.e., from the outer surfaces toward the core material. This inward polymerization results in greater shrinkage, which may result in protruding fibers or core material.

[0007] As more and more parts are substituted with sandwich composites, manufacturers desire more parts with smoother surfaces. Although ^■ existing technology allows for some sandwich composites to be manufactured cost-effectively; a method for making smoother surfaces is desirable. Furthermore, cost-efficient alternatives to the methods mentioned above are still desired. Lastly, new materϊarcombinations and applications are now possible.

SUMMARY OF THE INVENTION

[0008] The present teachings provide a method for preparing a composite material, the method comprising: forming a sandwich in a low- temperature mold, wherein the sandwich comprises a permeable core material between a first outer layer and a second outer layer, each of the first and second outer layers being impregnated or saturated with an adhesive, and a first covering surface adjacent to the first outer layer, and a second covering surface adjacent to the second outer layer; injecting a reaction medium into the permeable core material; and controlling the pressure, temperature, and flow of the reaction medium to polymerize or cure the adhesive.

[0009] The present teachings also include a method for preparing composites made of multilayered core materials. In other words, the present teachings may be used for a sandwich composite comprised of several core material layers (which are separated by the outer layers) and ultimately covered by covering layers.

[0010] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS [0011] The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0012] FIG. 1 illustrates an arrangement of materials within a mold according to the present teachings.

[0013] FIG. 2 is a cross-sectional view of a mold with the materials of FIG. 1 and illustrates the injection process for making a composite according to the present teachings.

[0014] FIG. 3 is a more θefaϊlecfvϊeW of tHe materials in FIG. 1 within the mold of FIG. 2 and during the injection process according to the present teachings. DETAILED DESCRIPTION

[0015] The following description is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0016] In accordance with the present invention, Applicants have discovered a method for making composite materials through injection polymerization wherein the resulting composite material has a more uniform surface as compared to composite materials prepared using traditional compression molding techniques. For example, without being held to a particular theory, Applicants have discovered that by reversing the direction of polymerization through the method described herein, sandwich composites are provided having a more uniform surface thickness and coverage of the underlying layers. [0017] In one embodiment, the injection polymerization method for making composite materials involves placing or forming a sandwich structure in a low temperature mold, wherein the surfaces of the mold conform to covering surfaces of the sandwich structure. The sandwich structure is provided having two outside surfaces and containing a permeable core and a first and second outer layer in contact with the core, the outer layers being saturated or impregnated with an adhesive. As noted below, the sandwich structure can further comprise second and further permeable cores, each with adjacent outer layers impregnated with adhesive forming alternating core outer layer structures. The sandwich structure also contains first and second covering surfaces forming the outside surfaces of the sandwich structure. The first and second covering surfaces, which become the two outside surfaces of the composite-material upon _. cure, are directly in contact with one of the outer layers that are impregnated with adhesive. In the low temperature mold, a warm gaseous reaction medium is injected into the permeable core of the sandwich structure. Heat exchange from the reaction medium initiates cure of the adhesive in the outer layers. The low temperature mold is at a temperature such that a heat exchange from.the. mold to the adhesive in the outer layers does not significantly cure the adhesive in relation to the amount of cure provided by heat exchange from the reaction -medium.

[0018] In various embodiments, the reaction medium is hot air, hot dry air, or steam. In a preferred embodiment, the reaction medium is injected at a first pressure and heat exchange occurs for a first time period to initiate the curing; thereafter, the pressure is raised to a second pressure and curing proceeds for a further time at the second pressure. In preferred embodiments, the first and second pressures range from about 1 bar to about 6 bar. In one preferred embodiment, the temperature of the reaction medium is from about 70⁰C to about 150⁰C; the reaction medium is hot air, hot dry air, or steam; the permeable core is a perforated honeycomb or an at least partially open-celled foam; and the pressure of the reaction medium is varied between 1 and 6 bar during cure. Preferably, at least one of the covering surfaces of the composite structure is a class A surface after cure. [0019] Referring how to FIGS. 1 and 2, a method of the present invention is more particularly described. As shown in FIG. 1 , a sandwich structure used with the present invention comprises a permeable core material 3 between a first outer layer 2 and a second outer layer 4. Both the first outer 5 layer and the second outer layer are impregnated or saturated with an adhesive. A first covering surface 1 and a second covering surface 5 are adjacent to the. fist outer layer 2 and second outer layer 4, respectively

[0020] Suitable core materials include paper, metal, or plastic honeycombs; open-pored (open-celled) plastic foams; thermoformable 10 polyurethane foams; open celled polyurethane foams; warp-knitted fabrics made of fibers; permeable glass materials or mineral materials; organic structures; parts or webs; reinforcing or elastic fillings of different states of aggregation; or mixtures of any of the above. In various embodiments, the permeable core

- material is moisture-resistant and/or microbe-resistant.

15 [0021] The first outer layer 2 and second outer layer 4 are materials

- that can be impregnated or saturated by an adhesive and provide further support ^"--*- ^'-- and strength-.to the sandwich composite. Suitable outer layers include mats made of natural fibers or renewable raw materials; films; fabrics or cloths; .textiles; fiberglass mats; chopped fiber glass strand mats; random layers on fiber

20 glass; cut or ground glass or mineral fibers; knitted fabrics; fiberglass fabric; non- woven and knitted fabrics based on polymers; carbon or Kevlar fiber fabrics; or mixtures thereof. Furthermore, the first outer layer and second outer layer are not necessarily the same material.

[0022] Suitable adhesives for impregnating the first outer layer 2 and

25 second outer layer 4 generally include any material that is stronger than the tensile strength of the core material. For example, the adhesive can be polyurethane, such as the polyurethane resin described in U.S. Patent No. 6,761 ,953; Baypreg^®; or Baypreg F^®. Furthermore, the adhesives of the first outer layer and second outer layer are not necessarily the same adhesive.

30 [0023] The first covering surface 1 and second covering surface 5 can be metal foils or sheets, or compact thermoplastic composites, and may be painted or colored. Suitable compact thermoplastic composites include those made of PMMA (polymethyl methacrylate), ASA (acrylic ester modified styrene acrylonitrile terpolymer), PC (polycarbonate), PBT (poly butylene terephthalate), and PPO (poly phenylene oxide).

[0024] Referring now to FIG. 2, the permeable core material 3, and the first outer layers 2 and second outer layer 4 are placed into the low-temperature mold 6 separately, or together, with the covering surfaces 1 and 5. The mold 6. is closed or compressed and a reaction medium is injected into the core material through one or more inlets 7. The pressure of the reaction medium is controlled by the inlet(s) 7 and one or more outlets 8. [0025] The core 3 is permeable and allows the reaction medium to flow throughout the volume of the core so that efficient heat transfer and application of pressure is delivered to the interface between .the core 3 and the outer layers 2 and 4. This means the core is "permeable" not-only in the "inside-out" direction indicated by the arrow in Figure 3, but also in the transverse or 90° "left to right" direction. This free moverhent'of the reaction^' medium in the permeable core is illustrated by arrows 9 in Figure 2: -"^■-^•" ^■■•■—^{■ ■■}-^{■ •} - ^•.

[0026] ^{•■ ■} Thus, if -th^"e^'-core-risc-:made of--a;;.-foam, the- foam should be sufficiently open-celled to allow the reaction medium to permeate the core. If the core is a honeycomb structure, the honeycomb structure should be perforated to allow the flow of the reaction medium.- -

[0027] In one embodimentrthe :reactionvmedium is -a gas such as heated air. If the adhesive is moisture sensitive, the gas is preferably dry. Alternatively, where the materials (core, fibres, etc.) are water resistant, the reaction medium may include steam, water and other suitable fluids and/or gases. In certain applications, it may be desirable to fill in the panel with a medium having special characteristic, including but not limited to flame resistance and noise absorbency.

[0028] Polymerization or cure of the adhesive is initiated and controlled by the temperature, pressure, and composition of the reaction medium. Thermal cure is initiated when heat transfer from the reaction medium increases the temperature of the adhesive in the outer layers to a suitable cure temperature. If cure or polymerization is moisture activated, the reaction medium can contain suitable amounts of water. Steam can be used as the reaction medium to provide both moisture for cure and large heat reservoir for efficient heat transfer. [0029] Advantageously, the direction of polymerization is from the core material inside the mold toward the outer layers and covering surfaces. In a preferred embodiment, the pressure of the reaction medium is modulated to provide a desired surface finish in the covering layers. As illustrated in Figure 3, . the inner pressure P is exerted in a direction from the center of the sandwich composite in a direction "outward" toward the outer layers 2 or 4 and the covering layer 1 or 5. The pressure tends to push the covering layer 1 and .5 . against the mold 6 to conform to the mold surface. The inside-out polymerization or cure of the adhesive also tends to provide support structure for the covering layers that does not shrink or pull back, contributing further to the quality of the surface. • Z-- --- --- - - - ; -^■- -- ^■

[0030] In a non-limiting embodiment, a reaction medium is introduced into the core of a sandwich composite at a first pressure. The -adhesive of the - outer layers begins to polymerize or cure. After the polymerization has started,- - the pressure can be increased to counter any tendency of the^cured^outer-layers— - -n- to shrink or pull away from the mold surface. For example,- the -increased pressure, which can be controlled or varied according ..to. the .individual _ characteristics and needs of the system at hand. can press- the -outside foil, say, of the covering layer against the mold surface. Advantageously— the-mold- — . — : surface is of sufficiently low temperature that natural textiles, wood. leather, and other sensitive materials are not damaged in the process.

[0031] While the Figures illustrate the invention and show a flat or planar interface between the mold tool and the composite, it is to be understood the invention is not limited to production of flat sandwich panels. Rather, composites having curved surfaces are readily made by providing flexible core materials and appropriately shaped molds and mold surfaces. In this way, a wide range of lightweight composite articles can be produced. [0032] Thus it is seen by controlling the temperature, pressure, and composition of the reaction medium throughout polymerization, one can create products with smoother surfaces. In preferred embodiments, products are produced with "class A" surfaces.

[0033] In illustrative embodiments, the inner pressure is in the range of

1 to 6 bar. The temperature can vary and is dependent on the materials. For 5 applications involving BayPreg, the temperature is preferably between approximately 70 and 150 degrees Celsuis. A greater temperature will make the. reaction faster. For applications involving a heat sensitive surface like ^a foil or a substantial amount of natural materials, a cold tool or low heated tool may be combined with a fast reaction through high temperature inside the panel. 10 [0034] Accordingly, it will be appreciated that the present disclosure

" • provides a combination that not only functions to achieve a smooth surface, but also opens up more possibilities for the creation of products with sensitive materials on the exterior. If the tools are cold, preheating of the tools in the process is not required and the associated preparation time is eliminated.

15 Additionally, the temperature surround is more friendly for the workers, dangers

— -:-- associated with accident are reduced, the tools are less expensive, the materials

W-Ir-- of tools-are less expensive, the lifetime of tools increases as there is no thermal cycling, and the process is less expensive as energy costs are reduced. [0035] The foregoing discussion discloses and describes merely

20 ^• exemplary arrangements of the present invention. One skilled in the art will

^{^}: rrrr readily recognize- from such discussion, and from the accompanying drawings

• and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.

Claims

What is claimed is: 1. A method for preparing a composite material, the method comprising: forming a sandwich in a low-temperature mold, wherein the sandwich comprises a permeable core material between a first

^• outer layer and a second outer layer, each^' of the first and second outer layers being impregnated or saturated with an adhesive, and a first covering surface adjacent to the first outer layer, and a second covering surface adjacent to the second outer layer; ^{■ "*'•} injecting a reaction medium into the permeable core material; and controlling the pressure, temperature, and flow of the reaction medium to polymerize the adhesive.

2. ^" The^" method accόrdirig tσ claim 1 , wherein the low-temperature mold has a temperature below an ambient temperature.

3. ^" The^" rhef. hod according to claim 1 , wherein the low^:temperature mold has a temp^"eratϋ^"re^~abbVe^h^~ambleήt temperature.

4. The method according to claim 1, wherein the temperature of the reaction medium ranges from approximately 70° to 150°.

5. The method according to claim 1 , wherein the pressure of the reaction medium varies from approximately 1 to 6 bar.

6. A method according to claim 1 , wherein the reaction medium is air or dry air. .

7. A method according to claim 1 , wherein the reaction medium is steam.

8. An injection polymerization method for making composite materials, comprising providing a sandwich structure having two outside surfaces, the sandwich structure comprising a permeable core, a first outer layer and a second outer layer in contact with the core, and first and second covering surfaces forming the outside surfaces of the sandwich structure, wherein the first and second outer layers are impregnated with an adhesive; placing or forming the sandwich in a low temperature mold, wherein the surfaces of the mold conform to the covering surfaces; and injecting a warm gaseous reaction medium into the core; wherein heat exchange from the reaction medium is sufficient to initiate cure of the adhesive, and wherein the mold is at a temperature such that heat exchange from the mold to the adhesive does not significantly cure the adhesive.- ■

9. A method according to claim 8, whφreinJhe.r.eactjoD^medjum is hot air.

10. A method according to claim 8, wherein the reaction medium is steam.

11. A method according to claim 8, comprising injecting the reaction medium at a first pressure for a first time period to initiate curing, and thereafter raising the pressure and curing for a further time at the raised pressure.

12. A method according to claim 11, wherein the first and second pressures are from about 1 bar to about 6 bar.

13. A method according to claim 8, wherein the core is perforated honeycomb.

14. A method according to claim 8, wherein the core is an at least partially open-celled foam.

15. A method according to claim 8, wherein the adhesive is a polyurethane.

5 16. A method according to claim 8, wherein the temperature of the reaction medium is from 70-150⁰C.

17. A method according to claim 8, wherein the temperature of the reaction medium is 70-150⁰C, the medium is hot air, hot dry air, or steam;

10 the core is a perforated honeycomb or an at least partially open-celled foam; the adhesive is polyurethane; and :, . the pressure of the reaction medium is varied between 1 and 6 bar.

18. A method according to claim 8, wherein at least one of the covering 15 surfaces is a class A surface after cure.

19. A method according to claim 8, wherein the sandwich structure contains two or more permeable cores.