Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-10-26
2002-11-05
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S135000, C521S178000, C521S057000, C523S218000, C523S219000
Reexamination Certificate
active
06476087
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to syntactic foams and more particularly to an improved method of manufacturing syntactic foam.
BACKGROUND OF THE INVENTION
Syntactic foams are composite materials whose resinous matrix is embedded with preformed particles such as glass or ceramic microspheres. Syntactic foams distinguish themselves from other foams by the fact that hollow or solid spheres of a predetermined size and packing composition are used to control the density of the foam.
Syntactic foams have been used for purposes which require a low density packing material such as undersea/marine equipment for deep-ocean current-metering, anti-submarine warfare, sandwich composites, the aerospace industry and the automotive industry.
Examples of syntactic foams in the prior art include for example U.S. Pat. No. 5,120,769 which discloses syntactic foams having an insoluble matrix, and U.S. Pat. No. 3,832,426 which discloses foam having an insoluble matrix and carbon microspheres. Syntactic foams having a soluble polymer matrix are disclosed in U.S. Pat. No. 5,432,205.
Difficulties have been experienced, however, in producing syntactic foams that have a density which is comparable to conventional foams. Typical densities of syntactic foams vary between 0.3 and 0.5 g/cm
3
, whilst conventional foams typically vary between 0.01 and 0.1 g/cm
3
. The density of syntactic foams has generally been restricted by the limited porosity of the foams. Porosity is a measure of the total void volume of the syntactic foam, and constitutes the sum of the void volume of the microspheres and the interstitial void volume. Using current methods of syntactic foam manufacture, the void volume provided by the microspheres is greater than the void volume provided by the interstitial spaces. Thus, the density of syntactic foams have been limited by the void volume of the microspheres. As such, the application of syntactic foams have been limited.
SUMMARY OF THE INVENTION
An aim of the present invention is to provide a novel method of manufacturing syntactic foam which is ideally suitable for manufacture of low density foams and which is simple and inexpensive.
Accordingly, in a first aspect of the invention, there is provided a method of forming a syntactic foam including the steps of:
a) combining a polymer, microspheres and a solvent to form a slurry;
b) thereafter removing at least a portion of the solvent through a porous wick; and
c) applying conditions which substantially solidify the polymer.
The applicant has found that by manufacturing syntactic foams in accordance with the method of the invention, it is possible to provide adequate coating of the microspheres to form the syntactic foam using a high microsphere mass fraction. This occurs primarily by the solvent reducing the viscosity of the polymer thereby allowing less polymer to be used to coat the microspheres and by causing at least a portion of the solvent to be removed from the composition. The applicant has found that using this method the porosity of the interstitial void volume of the syntactic foams can be greater than the void volume of the microspheres. This enables syntactic foam densities of less than 0.15 g/cm
3
to be produced. Further, the technique is simple and is ideally suited for mass production and easily applicable for manufacturing large items.
In a preferred form, the method further includes the steps of combining the polymer and the microspheres to form a paste, and thereafter adding the solvent to the paste to form the slurry and mixing the slurry to achieve dispersion of the microspheres prior to removal of the solvent. Even more preferred is that a final portion of microspheres necessary to maintain the desired mass fraction is added alternatively with the solvent. This helps to maintain a consistency which allows easy pouring.
In a preferred aspect of the invention, the solvent is removed from the slurry while the slurry is in a mould such that the syntactic foam forms a desired physical conformation upon removal of the solvent. In an especially preferred aspect of the invention, the syntactic foam is made by a method in which the mould is the porous wick for removal of the solvent. The porous wick itself may be made from any material that is capable of soaking up or removing the solvent. The most preferred porous wick is made of plaster. Alternatively, the porous wick may be made of clay, ceramic, cement, concrete, finely perforated plastic or metallic sheets with absorbent backing, fine metallic mesh/gauze with absorbent backing, rigid porous foams or sponge-like material.
An advantage of this arrangement is that the removal of the solvent through the mould walls does not cause any major disturbance of the distribution of the microspheres. As such, the resulting syntactic foam is able to maintain an even density and tactile strength.
To allow for ease of moulding and speed of manufacture, the amount of solvent added is sufficient to form a slip by maintaining a soupy consistency. The mixture can then be quickly poured into the desired mould.
It is a preferred aspect of the present invention that the solvent is a volatile alcohol or ketone. The most preferred alcohols are butyl alcohol, isopropyl alcohol, ethyl alcohol or methyl alcohol. Even more preferred is that the solvent is the ketone acetone. Alternatively, the solvent may be water or a mixture of water and alcohol or water and acetone. In general, a slip made using water requires a lower ratio of solvent to polymer and hardener. The volume ratio of water to polymer is generally between 9:1 and 6:1. The volume ratio of acetone to polymer is between 40:1 and 9:1. The volume ratio of water to polymer and acetone to polymer varies with the ratio of microspheres to polymer. The greater the ratio of microspheres to polymer, the greater the volume of solvent required to maintain a consistency necessary to allow pouring of the mixture. Ideally, the volume ratio of solvent to polymer is sufficient to maintain a slip.
The polymer may be dissolved in the solvent, or dispersed throughout the solvent as a suspension. In a preferred aspect of the invention, the polymer is dissolved in the solvent. It is especially preferred that the polymer is dissolved in acetone. Alternatively, the polymer is homogenised or dispersed throughout the solvent as an insoluble suspension.
The polymer used in the method of making the syntactic foam of the present invention may be any resin that is effectively employed in syntactic foam manufacture. The classes of resins used for syntactic foam manufacture may include but is not restricted to amino resins, epoxide resins, phenolic and other tar acid resins, urea and melamine resins, vinyl resins, styrene resins, alkyd resins, acrylic resins, polyethylene resins, polypropylene resins, petroleum polymer resins, polyamide resins, polycarbonate resins, acetal resins, flourohydrocarbon resins, polyester resins and polyurethane resins. Further, the polymer may be water soluble copolymers such as poly(N-vinylpyrrolidone-vinyl acetate). A particularly preferred resin is a phenolic resin.
Depending on the nature of the polymer, the polymer may include a crosslinking agent or hardener which causes solidification of the polymer material. In cases where the polymer includes a cross-linker to solidify, it is preferred that the weight ratio of hardener to polymer is 1:10. However, it is envisaged that the ratio of hardener to polymer will vary depending on the type of polymer and the degree of crosslinking that is desired. For example, increased amounts of hardener will allow low temperature curing and will speed up gelation. Decreasing the relative amounts of hardener to polymer to a weight ratio as low as 5:100 may be possible, but higher curing temperatures and longer gelation times may be required.
Preferably, the syntactic foam of the present invention is a low density syntactic foam. To reduce the density of the syntactic foam, the microspheres are advantageously hollow gas filled spheres. They are generally made of soda-l
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