Method of making a foam core article

Details Method of making a foam core article Method of making a foam core article

1998-05-07

2001-01-02

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Pore forming in situ

Composite article making

C264S257000, C264S263000, C264S331150

Utility Patent

active

06168735

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a plastic article consisting of a foam core and a fiberglass reinforced polymer layer surrounding the foam core. More specifically, the invention relates to an improved foam formulation that promotes chemical bonding between the resin and the foam core.
2. Description of the Related Arts
Fiber reinforced plastic (FRP) articles are well known. One example of a material using FRP technology is a sandwich structure, which consists of one or more fiber layers surrounding a core. Using a process called resin transfer molding (RTM), a polymeric resin impregnates the fibers and is cured or cooled to form the article. Foam is particularly suitable for FRP cores because it is lightweight, low-cost, and generally easily manufactured into the desired shape. Polyurethane foam has been widely used in the manufacture of core materials for FRP articles because it exhibits the best balance of these desired properties.
When the FRP application requires increased rigidity, such as for structural applications or applications where delamination is unacceptable, the surface of the foam core may be abraded or scored to provide sights for mechanical attachment between the foam and resin. In this case, the resin enters the surface irregularities or pores of the foam and cures or solidifies. For laminating resins and foam systems that are incompatible, the resin generally does not chemically bond to the foam, but rather forms a mechanical attachment only in the area of the surface irregularity. The mechanism for this mechanical interlock involves the micro-impregnation of uncured polymeric resin into irregularities/pores on the foam surface. This type of mechanical interlock results in a stiff structure which, when stressed to failure, results in cohesive failure (failure in the foam). RTM articles that do not use either a mechanical or chemical surface abrasion of the foam core are more subject to adhesive failure (failure at the bond line). Adhesive failure results from stressing structures containing a discrete boundary layer between that foam and resin, where the foam and resin are unbonded. Experiments performed to assess adhesion for RTM articles found that there is a greater than 200% improvement in the adhesion strength for the mechanically abraded foam core, as compared to a non-abraded foam core.
There are several disadvantages with this mechanical attachment including the complexity of providing these surface irregularities on the foam for attachment by the resin. When a high viscosity resin is used, its ability to penetrate into the surface irregularities to form a mechanical attachment to the foam is reduced. Another disadvantage of this system is that the mechanical attachment only exists on the areas of the foam that contain the surface irregularity. Therefore, the foam surface abrasion operation becomes particularly critical inasmuch as the adhesion will only be adequate in areas where the abrasion operation is adequate. In addition, excess abrasion may weaken the foam core and, consequently, the RTM article and it is difficult to reduce part variability because the mechanical abrading process (sanding) is not easily repeatable.
These mechanical attachments between the foam and resin are needed because the foam and resin normally used in FRP articles do not chemically bond to one another. For example, urethane foam cores made from saturated polyol and isocyanate form a saturated polyurethane foam having these species in a cross-linking network. Typical saturated foam cores include urethanes, ureas, allophanates, biurets, isocyanurates, uretidinedione, carbodiimide and other rigid or semi-rigid species. The hydrocarbon functional groups in these foams result from both the original saturated polyol and isocyanate precursors. Therefore, the foams currently used in RTM have no double bonds that are capable of bonding with the resin.
Resins normally used with FRP articles include unsaturated polyester or vinyl ester compounds that use a free-radical reaction mechanism to join the various polymer chains and cross-link. The unsaturated resin contains C═C double bonds that provide locations for the resin to cross-link and cure.
None of the species or bonds existing in saturated polyurethane foam can form chemical bonds with the species present in the vinyl ester resin system. Although there is physical interaction between these two polar materials, it is not strong enough to provide good adhesion. Therefore, the vinyl ester thermosetting resins typically do not adhere to the saturated polyurethane foam in an RTM process.
One way of making the resin and foam bond to one another was to make the resin and foam from the same material using a process as described in U.S. Pat. No. 4,543,289. Both the foam and resin are made from vinyl chloride copolymer. A core material that expands when heated is placed between thermoplastic polymer sheets and reinforcing layers. The sandwich is placed within a mold and heated. The heat causes the core to foam and fuse to the thermoplastic polymer sheets. Because the core and foam are made from the same resin, the article does not suffer from the compatibility problem described above. While this solution eliminates the incompatibility problem, it restricts the materials available to make FRP articles to thermoplastic resins and foams. However, thermoplastic materials do not provide the thermal properties required for many applications. Also, due to their high viscosity, thermoplastics cannot attain the same fiber reinforcement levels as thermosets and are, therefore, substantially weaker.
Another method of increasing the adhesion between vinyl resin and polyurethane is to apply an aqueous acetic acid solution to the vinyl resin. U.S. Pat. No. 4,264,643, teaches a method of making a laminated floor having a polyurethane top layer and a vinyl resinous inner layer. The vinyl resin inner layer is activated with acetic acid solution. A polyurethane layer is applied to the activated vinyl resinous layer and cured using UV light. The polyurethane layer was found to bond to the vinyl resin inner layer. The method thus described was not used to bond a liquid resin to a polyurethane foam nor can the method be used in resin transfer molding because it requires that the vinyl resin layer be cured prior to application of the acetic acid solution. It is desirable to provide a method that enables a liquid uncured vinyl ester material to be bonded to a polyurethane foam core.
By recognizing the failure mechanism between traditional foam cores and resins, the present invention enables the selection of compatible materials that adhere and produce stronger composite sandwich structures. These deficiencies and problems are overcome by the present invention.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of manufacturing a fiber-reinforced article wherein-the laminating layer is bonded to the foam core without the need of a mechanical attachment that resulted from either a chemical or mechanical abrasion of the foam surface and without the use of a chemical compatiblizer intermediate layer. The method is practiced by using a foam material having an unsaturated polyol that is available to react with the uncured resin. The method comprises a series of steps including providing a foam core made from a polyol containing at least one unsaturated C═C group and a hydroxyl functional group. A fiber-reinforcing layer is placed adjacent the foam core. An uncured liquid resin wets the fiber reinforcing layer and the outer surface of the foam core. The resin reacts with t ,he unsaturated C═C group of polyol by a free radical reaction. The resin also cross-links and cures and forms a solid polymer encasing the fiber reinforcing layer that also bonds the fiber-reinforcing layer to the foam core. The fiber reinforcing layer, foam core and resin form a composite sandwich structure that is stronger and more resistant to delamination because the resin is chemi

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