AU4268000A - Production of syntactic foams - Google Patents

Description

*4 *I

AUSTRALIA

Patents Act 1990 Richard Charles Louis Jouault

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Production of syntactic foams The following statement is a full description of this invention including the best method of performing it known to us:a

S.

S

Field of the Invention The present invention relates to a method of producing syntactic foams and syntactic foams produced thereby.

Background to the Invention Conventional foams, such as polystyrene foam or polyurethane foam, are produced by generating gas bubbles in a liquid material which is then made to set by cross-linking. Syntactic foams, on the other hand, differ from blown foams in that the cells in syntactic foams are provided by small preformed spheres, commonly known as microspheres or microballoons, that are in relatively close contact with each other and are bonded with a minimum amount of binder or matrix material.

Syntactic foams arose from developments attempting to replace some of the heavier and more expensive binder materials used in particular applications, such as epoxy resin, with a cheaper and lighter filler material that would not affect the flow properties of the resin. Microspheres were chosen as a possible suitable filler for a number of reasons with the main reason being the isotropic properties of the spherical shape of the microspheres and the minimum effect created on the flow properties of the resin by the microspheres. The isotropic properties meant that the direction 20 of loading did not have to be considered as the sphere has the same properties in every direction.

Two-phase foams were the first syntactic foams to be developed.

When developed, the mixture of microsphere filler and binder was, however, *...-limited to approximately 30% filler volume. Above this and uncontrolled 25 viscosity increases in the mixture restricted the applications of the foam.

The properties of the foam mainly reflected those of the resin, given this was the major volume fraction. The result was a foam that was highly flammable, giving off toxic gasses as it burned, as well as having poor compressive/tensile strength. Because of the relatively low content of filler, S 30 the microspheres provided no structure within the foams and had little if any effect on the foam's physical properties. Mostly the foams were produced with 10 to 15% filler and simply used as a density reducing material for resin when the resin was being used in the formation of other composites.

The second significant step in the development of syntactic foams was the developments of processes designed to overcome the uncontrolled viscosity increases when forming the mixture. In this regard, viscosity reduction agents such as the styrene monomers were used to enable filler additions to the resin system to be as high as 40 to 60% by volume.

Unfortunately, air bubbles formed in the foam structure when using such agents proved very difficult and expensive to control or remove. This in turn lead to problems with developing on-line production systems.

Despite the difficulties, the foams produced in this way did at least have physical properties that were influenced by the filler volumes. For example, foams incorporating solid glass microspheres proved to have very high compressive strength. Foam products produced in this manner are used in deep-sea buoys and submarines and as fillers for repairing damage to maritime vessels and piers. Such two-phase foams have also formed an ideal filler material in sandwich composites, with the tensile strength properties usually provided by the skin laminate. These applications were somewhat limited, however, due to the competitive option of polymer foams. The polymer foams offered the necessary strength at a reduced cost and weight for standard foam sandwich applications. But the syntactic foams were unmatched in the high compressive strength applications.

Two-phase syntactic foams have remained at about this point in development for some years. The slow development of these foams as a 20 stand-alone product is mainly due to the following two reasons: 0 Flammability Most structures destined for use in internal environments must be resin free or used in low quantities in a reinforcing skin laminate due 25 to the known flammability of this material. This severely limited the applications of the foams as they fail both internal and external fire safetystandards.

Viscosity of the Mixture The addition of the filler material beyond 30% lead to uncontrolled increases in viscosity of the system making the resin unusable. When cast the structure is very hard to control with the appearance of air voids throughout.

Currently syntactic foams are produced as two different types, twophase and three-phase. Two-phase foams have a resin phase and a filler phase, while three-phase foams include an air void phase as well. Threephase foams are a relatively new invention requiring higher filler volumes.

Three-phase foams can be difficult to produce and are therefore more expensive than two-phase foams. They involve very high filler volumes and air voids as well as resin. To produce a structure within which the air voids are evenly distributed while still maintaining the bond strength of the composite is extremely difficult. These problems are partly overcome with powdered resin systems but bond strength is still usually poor. To date, three-phase foams have been limited to unique and costly applications, such as the phenolic tiles on the space shuttle.

The following methods are commercially used to produce syntactic foam: Method 1 Epoxy resin and hollow glass spheres.

Mix resin and curing agent.

Add spheres stepwise.

Results in a thixotropic putty-like mixture.

*..Poured into compression moulds.

20 Cured in oven (approx. 120'C) for one hour.

De-moulded to cool.

go Method 2 Phenolic resins and spheres.

Powdered resin and spheres are blended together to form a flowable mixture.

Poured into a compression mould.

Cured at 120 0 C for two hours.

De-moulded to cool.

Method 3 Polyimide based foam.

Polyimide solution and glass spheres are mixed together.

The mixture is poured into moulds and compressed at a pressure of 100 psi or 0.69 MPa.

The wet mouldings are removed from the mould and placed in an oven for three to four hours at 100 120 0

C.

The temperature is raised to 270 280 0 C over the next hour.

Cured at 270-280 0 C for two hours.

Removed to cool.

Method 4 Polystyrene-Epoxy Syntactic Foam The foams are made by two methods: The hollow microspheres are pre-expanded with exposure to steam and added to a preheated mould The mixed epoxy resin system is then poured over the top of the expanded spheres and allowed to cure. During curing a further expansion of the spheres will take place, due to the exothermic reaction, to complete filling of the mould.

(ii) The polystyrene-solvent polymer powder is added to the resin system and expands to spheres during the exothermic curing of the resin. Close temperature control is required in this method to obtain 20 reproducible results, as the expansion of the polystyrene is a function of temperature.

Method can be used to produce foams of density as low as 80 kg/m 3 The polystyrene microspheres have the same physical and electrical 25 properties as those obtained by other organic syntactic foam fillers.

*The present invention provides a process that can be used to optimise the formation of two-phase and three-phase syntactic foams.

Summary of the Invention 30 According to a first aspect, the present invention consists in a method of forming a syntactic foam, the foam being a composite of at least a plurality of microspheres and a binder material, the method including the steps of: determining the void fraction of a pressed sample of the microspheres; (ii) using the determined void fraction to calculate the required volume fraction of binder material to at least partially occupy the void fraction of a quantity of the microspheres; (iii) mixing the quantity of the microspheres with said required volume fraction of uncured binder material to form a batch of mixed microspheres and binder material; and (iv) exerting pressure on the batch for at least a portion of the curing time of the binder material to form the foam.

In a preferred embodiment, the void fraction of the pressed sample of the microspheres is determined when the sample of the microspheres is pressed to a state of maximum packing. The pressed sample of the microspheres is preferably formed by compressing the microspheres in a mould or sample cup.

To determine the void fraction, the specific volume can be firstly determined using a pycnometer which calculates the specific volume of the sample of the microspheres in cubic centimetres by differential pressure measurements. The sample of the microspheres is then weighed in a sample cup, with the measured weight representing the mass of microspheres and air contained within the sample of the microspheres.

20 Knowing the volume of the microspheres, the volume of the sample cup, the relative humidity and therefore the density of the air, the mass of :i the microspheres can be calculated and hence the specific density determined. From this determination of the specific density and volume of the pressed sample, a calculation of the void fraction of any pressed quantity 25 of the microspheres can be made.

The presence of moisture in the microspheres can present a problem to their use in syntactic foams as their usually high affinity to water can result in large quantities of moisture becoming trapped in the interstitial gaps of a mixture of the microspheres. Any moisture in the pressed sample of the 30 microspheres may also prevent a correct determination of the void fraction of the sample.

Accordingly, prior to step the method can include an additional step of drying the microspheres to substantially or totally remove any moisture content. The microspheres can be dried in an oven. The time period of drying will depend on the type of microspheres being used in the method. During drying, the moisture content of the sample of the microspheres can be tested on a regular or irregular basis. The moisture content can be determined by determining the specific density of the microspheres and comparing this determined value against the manufacturer's specification. The microspheres can be considered dry when the calculated values for specific density are within the specified tolerance of the manufacturer's specification.

Once the void fraction of the sample of the microspheres is determined it can be converted to appropriate units of volume ratio for the size of the batch of mixture of microspheres and binder to be made. For example, if the void fraction of the sample of the microspheres at maximum packing is 3 of a 50cm 3 sample cup then the void fraction is 20%. Accordingly, of the batch volume is allocated to binder volume and this amount can be calculated. The remaining volume fraction of the batch is calculated into mass (based on the determined specific density of the microspheres) and sets the quantity of microspheres required to be added to make up the desired batch quantity.

Where steps are being taken to minimise moisture content in the batch, the measured quantity of microspheres should be returned to the oven until required. Alternatively, the microspheres can be weighed in a dry 20 environment.

The binder material can be a thermosetting resin. Examples of suitable materials include epoxy resins, polyester resins, polystyrene resins and polycarbonate resins. Epoxies are normally supplied in a partially polymerised state and are mixed with an catalyst/hardener just prior to use so 25 that polymerisation is completed in situ. The gelation time of the binder material following addition of the catalyst/hardener is preferably sufficient to allow sufficient time for the steps of mixing of the batch of binder material and microspheres and the exertion of pressure on the batch. In one embodiment, the gelation time can be about 40 minutes. Gelation times of 30 less than or greater than 40 minutes would, however, also be possible.

One example of a suitable resin is sold under the name Derakane 441- 400. This resin has a typical gelation time of between 40 and 60 minutes.

Suitable microspheres for use in the mixture includes ceramic microspheres, glass microspheres, polymeric microspheres, carbon microspheres and glass-reinforced thermoplastic microspheres. Suitable polymeric microspheres include phenolic microballoons. The microspheres can be hollow or solid. The microspheres typically have a diameter of less than 300tm and more preferably a diameter between about 20ptm and 150tm.

While known in the art as microspheres, it will be appreciated that the microspheres do not need to be exactly spherical and may in fact only be substantially spherical.

Suitable microspheres are readily available from a number of suppliers. For example, Minnesota Manufacturing and Mining Co market a number of glass and ceramic microspheres. Suitable microspheres are also sold under the trade mark "Eccospheres SI" by W.R. Grace Co.

The temperature of the binder material prior to mixing with the quantity of the microspheres is preferably about the same as that of the quantity of the microspheres. Accordingly, where the microspheres have been oven dried, the binder material should also be heated to at least about the same temperature. This will ensure that the microspheres do not absorb moisture out of the atmosphere so forming a condensate as they rapidly cool from the oven temperature to a lower binder temperature. Additives as are known in the art can be added to the binder material, preferably prior to the addition of the catalyst/hardener, to prolong gelation time and/or raise the '**temperature of the binder material.

20 Before and preferably during the mixing step, the binder material may .:be kept at a temperature greater than room temperature. This can be achieved by placing the binder material in a container on a hot plate, the hot plate warming and preferably maintaining an elevated temperature of the batch.

25 The step of mixing the mixture preferably includes a process of shearing the mixture to ensure coating of all of the microspheres in the mixture with the binder material. The step of mixing may be done manually or mechanically. The mixture is preferably mixed until the mixture is in the form of a thixotropic putty.

30 Once mixed, pressure is preferably exerted on the mixed batch until the binder material is at least partially cured and, more preferably, fully cured. The step of exerting pressure is preferably undertaken by placing the batch in a mould and exerting pressure on the mould. Suitable moulding techniques for exerting pressure on the batch as it cures can be readily envisaged. The moulds used in this step can be adapted to produce desired shapes of the syntactic foam ready for use or further modification.

The pressure applied to the batch preferably compresses the batch towards and preferably to maximum packing of the microspheres in the formed foam.

The process, according to the present invention, can include a step of determining the appropriate pressure to apply to the mixed batch. This step can be important as, in the case of using hollow microspheres, too little pressure will not result in maximum packing and too high pressure will result in crushing of the microspheres. Different types of microspheres will crush at different pressure levels so it is necessary to determine the appropriate pressure to reliably obtain cured syntactic foam in which the microspheres are close to or at maximum packing while not being crushed.

To determine the appropriate level of pressure to achieve maximum packing without crushing of the microspheres of a batch, batch samples should be compressed at varying pressures and then appropriately analysed to assess the structure of the resulting foam. The analysis can involve imaging the structure of the foam, for example, by using a confocal microscope or scanning electron microscopy (sem).

If maximum packing of the microspheres is not achieved before destruction of the microspheres, then this is indicative that the binder 20 content in the batch is too high and that the void fraction calculation step (ie.

.step has not been done at maximum packing of the sample of the microspheres. If this is so, it is necessary to repeat steps and (ii) to ensure that the void fraction is determined at maximum packing if a syntactic foam with maximum packing of hollow microspheres is desired.

25 Where solid microspheres are being used to form the syntactic foam, the solid microspheres will not crush on the application of excess pressure.

Instead, what is observed is that as the sample is pressed, the pressure applied by the press rises relatively sharply. As with the hollow microspheres, if this is the case, it is necessary to repeat steps and (ii) to 30 determine the correct void fraction of the batch if a syntactic foam with maximum packing of solid microspheres is desired.

When the correct void fraction and the appropriate pressure value for maximum packing are both determined for a syntactic foam produced using a particular batch of microspheres and binder, it is then possible to subsequently routinely produce syntactic foam samples from that batch of microspheres in which the microspheres are at maximum packing with the void fraction entirely filled with binder material. The resulting syntactic foam is a 100% closed cell two-phase foam.

If it is desired to produce a partially open cell three-phase foam, all that is required is to allocate less of the void fraction to the binder volume.

By reducing the binder volume but ensuring good mixing, interstitial gaps are formed between microspheres whilst appropriate binder thickness is maintained at points of contact between the microspheres.

Any reduction in binder volume will cause a reduction in the binder thickness constituting bonds between the microspheres. At a certain low level of binder content, the quantity of binder will be insufficient to bind adjacent microspheres so leading to a reduction in the strength to weight ratio of the foam produced by the method.

When syntactic foam is to be produced from a new batch of microspheres, it is appropriate to again repeat the process according to the present invention to determine the correct void fraction and appropriate pressure value for maximum packing. This is important as different batches of what are sold or supplied as identical microspheres will often vary quite see* *significantly in diameter about some mean. By repeating the process 0*00* according to the present invention, any variation in void fraction of a pressed sample of the microspheres can be accounted for in subsequent production of syntactic foam using that batch of microspheres.

According to a second aspect, the present invention consists in a method of forming a syntactic foam, the foam being a composite of at least a plurality of microspheres and a binder material, the method including the 25 steps of: pressing a sample of microspheres to a state of maximum packing "of the microspheres; (ii) determining the void fraction of the sample of the microspheres; (iii) using the determined void fraction to calculate the required 30 volume fraction of binder material to at least partially occupy the void fraction of a quantity of the microspheres; (iv) mixing the quantity of the microspheres with said required volume fraction of uncured binder material to form a plurality of batches of mixed microspheres and binder material; exerting a different pressure value on each of the batches for at least a portion of the curing time of the binder material to form foam samples of different microsphere packing levels; (vi) analysing the foam samples to determine the pressure value that forms a syntactic foam having maximum microspheres packing; and (vii) using the determined void fraction value and the maximum packing pressure value to subsequently produce syntactic foam in which the microspheres are at maximum packing and the void fraction is at least partially occupied with binder material.

The pressed sample of the microspheres in step of the second aspect is preferably formed by compressing the microspheres in a mould or sample cup.

To determine the void fraction in this aspect of the invention, the specific volume can be firstly determined using a pycnometer which calculates the specific volume of the sample of the microspheres in cubic centimetres by differential pressure measurements. The sample of the microspheres is then weighed in a sample cup, with the measured weight Goof. representing the mass of microspheres and air contained within the sample &of the microspheres.

S 20 Knowing the volume of the microspheres, the volume of the sample 9 cup, the relative humidity and therefore the density of the air, the mass of the microspheres can be calculated and hence the specific density determined. From this determination of the specific density and volume of the pressed sample, a calculation of the void fraction of any pressed quantity 25 of the microspheres can be made.

The mixing step at step (iv) preferably includes a process of shearing :iii each of the mixtures to ensure coating of all of the microspheres in each of the batches with binder material. The step of mixing may be done manually or mechanically. Each mixture is preferably mixed until the formed batch is in the form of a thixotropic putty.

Once mixed, the step of exerting pressure on the batch is preferably maintained until the binder material is at least partially cured and, more preferably, fully cured. The step of exerting pressure on each of the batches is preferably undertaken by placing the batch in a mould and exerting pressure on the mould. The moulds used in this step can be adapted to produce desired shapes of the syntactic foam ready for analysis, use or further modification.

The step of analysing the foam samples to determine if the sample has been pressed to maximum packing of the microspheres can include a visual inspection of the foam samples. Preferably, the visual inspection is done under magnification using, for example, a confocal microscope.

In this aspect, the presence of moisture in the microspheres can present a problem to their use in syntactic foams as their usually high affinity to water can result in large quantities of moisture becoming trapped in the interstitial gaps of a mixture of the microspheres. Any moisture in the pressed sample of the microspheres may also prevent a correct determination of the void fraction of the sample.

Accordingly, prior to step the method can include an additional step of drying the microspheres to substantially or totally remove any moisture content. In this step, the microspheres can be dried in an oven.

The time period of drying will depend on the type of microspheres being used in the method. During drying, the moisture content of the sample of the microspheres can be tested on a regular or irregular basis. The moisture content can be determined by determining the specific density of the microspheres and comparing this determined value against the manufacturer's specification. The microspheres can be considered dry when *the calculated values for specific density are within the specified tolerance of the manufacturer's specification.

The step of subsequently producing syntactic foam using the 25 determined void fraction value and the maximum packing pressure value (ie.

step (vii)) firstly includes the step of mixing a quantity of microspheres with said required volume fraction of uncured binder material. This mixing step preferably includes a process of shearing the mixture to ensure wetting of all microspheres with binder material. This step of mixing may be done manually or automatically. The mixture is preferably mixed until the formed batch is in the form of a thixotropic putty.

The step of subsequently producing syntactic foam using the determined void fraction value and the maximum packing pressure value secondly includes the step of exerting pressure on the batch, with the pressure preferably maintained until the binder material is at least partially cured and, more preferably, fully cured. The step of exerting pressure is preferably undertaken by placing the batch in a mould and exerting pressure on the mould. The moulds used in this step can be adapted to produce desired shapes of the syntactic foam ready for analysis, use or further modification.

The step of subsequently producing syntactic foam using the determined void fraction value and the maximum packing pressure value can also include an additional drying step, where the quantity of microspheres is dried prior to its mixing with said required volume of binder material. In this case, the microspheres can be dried in an oven. The time period of drying will depend on the type of microspheres being used in the method. During drying, the moisture content of the sample of the microspheres can be tested on a regular or irregular basis. The moisture content can be determined by determining the specific density of the microspheres and comparing this determined value against the manufacturer's specification. The microspheres can be considered dry when the calculated values for specific density are within the specified tolerance of the manufacturer's specification.

According to a third aspect, the present invention consists in a syntactic foam produced using the method according to the first aspect of the present invention.

According to a fourth aspect, the present invention consists in a syntactic foam produced using the method according to the second aspect of the present invention.

The syntactic foam of the third or fourth aspects of the present invention can be a two-phase or three-phase foam. The syntactic foam can 25 be produced as an open cell or closed cell foam.

The microspheres in the foam of the third or fourth aspects can be ceramic microspheres, glass microspheres, polymeric microspheres, carbon microspheres or glass-reinforced thermoplastic microspheres. Suitable polymeric microspheres include phenolic microballoons. The microspheres 30 can be hollow or solid. The microspheres typically have a diameter of less than 300 tm and more preferably a diameter between about 50utm and 150ptm.

The binder material in the foam of the third or fourth aspects can be a thermosetting resin. Examples of suitable materials include epoxy resins, polyester resins, polystyrene resins and polycarbonate resins.

In a fifth aspect, the present invention comprises a method of mixing a plurality of microspheres with a binder material, the method including the step of shearing the mixture to ensure coating of all of the microspheres in the mixture with the binder material.

In a preferred embodiment, the mixture is placed in a mixing container and is sheared between a blade member and a wall of the mixing container.

The step of mixing can be performed either manually or mechanically. A spatula can be used to shear the mixture against the mixing container wall in one embodiment.

In a sixth aspect, the present invention comprises a syntactic foam produced from a mixture of a plurality of microspheres and a binder material, the microspheres and binder material being mixed according to the method of the fifth aspect of the present invention.

The syntactic foam as defined herein has a number of potential applications, such as follows: Structural Foam Structural foams, also called integral skin foams, are characterised as polymer structures with nearly uniform-density foam cores and integral near-solid skins. The Structural Foam Division of the Society of the Plastics Industry Inc defines structural foam as a plastic product with integral skins, a cellular core and enough strength to weight ratio to be classed as structural.

Structural foams as defined in the ASTM D1556 for rigidity and oooo stiffness, are generally closed-cell foams with a high density. The performance of structural foams is influenced by cell morphology.

The use of solid microspheres, therefore provides the required compressive properties to meet structural guidelines. The choice of *o the type of solid microspheres will depend upon other factors such as cost. Syntactic foams of maximum packing and solid microspheres have the ability to withstand large compressive forces. Yield 30 compressive strengths are ten times greater than concrete therefore it fulfils the ASTM requirements, while the densities remain less than half that of concrete.

Thermal Insulation Good insulating foam has the following properties: Small cell size.

Low thermal conductivity.

Closed cells.

Low density, to take advantage of the low conductivity of gas.

Structural integrity.

Low permeability of the gas through resin films so that conductivity does not change over time.

A good comparison of the thermal conductivity of insulating foams is the 75°F thermal conductivity coefficient or the k-factor. The use of hollow mnicrospheres, especially ceramic microspheres, provide excellent thermal insulation properties. The ability to combine small closed cells and low density, while maintaining structural integrity make syntactic foam ideal for thermal insulation. Maximum packing within two-phase structured foam would provide the structural integrity while the hollow ceramic microspheres would provide insulation.

As an alternative, solid metal microspheres could be used with a thermally conductive resin to provide foam capable of conducting heat. A combination of two foams could be used to provide a surface 20 that on one side conducts heat and stores it, while the exterior surface 000 is an insulating one containing the heat in the inner foam. This situation, along with the compressive strength of the foams would be ideal for injection moulding platens.

25 Sound Insulation Foam for sound insulation is generally lightweight and open celled. It has been noted that sound performance is relatively insensitive to density, with small open cells providing better insulation.

30 Syntactic foams have the ability to produce a variety of excellent acoustic properties, some matching those of seawater making the foam ideal for use in radar applications and radomes. The ability to vary the acoustic properties is a result of the variety of filler materials available.

Producing partially open cells of irregular shape, interconnected with other cells provides a myriad of tunnels and paths down which the sound is absorbed. The size of the tunnel is a function of the microspheres diameters and therefore can be varied to suit a particular bandwidth of frequencies. With this in mind the foams can be designed for specific applications within which the foams can be used to filter a particular frequency or deaden all sounds.

Flotation Foams are unrivalled in the field of buoyancy. The buoyancy of foam is determined by the buoyancy factor: Pw Pf Pw where: pw is the density of water; and pf is the density of the foam.

Typical flotation foams have a buoyancy factor of 0.96. Polymer foams used for flotation must have the following properties.

Low density.

Closed cell structure.

Low or zero water absorption rates.

Chemical resistance.

For syntactic foams a combination of inorganic microspheres and r polyester or epoxy resins, can provide low densities with a very high resistance to hydrostatic pressure. Virtually all the syntactic foams ,o used for sub-sea buoyancy are based on glass spheres and epoxy resins, for the ease with which the exothermic reaction can be 30 controlled and the range of formulations available between resin and hardener. For pure syntactic foams operating at great depths it is important that the bulk modulus of the foam be similar to that of seawater as a lower modulus will result in buoyancy loss while a higher modulus will result in increased buoyancy at depth. The bulk modulus of seawater varies with depth, therefore it is desirable that the bulk modulus of the foam vary similarly. The bulk modulus of the foam can also be varied along its length, making it ideal for flotation.

One of the major applications for syntactic foams is the manufacture of riser modules to provide 98% buoyancy for the main steel riser of offshore oil platforms operating at various depths. Other applications include use as structural buoyancy elements in Remotely Operated Vehicles (ROV) and Deep Submersible Vehicles (DSV) at depths greater than six thousand metres.

Energy Absorption A range of syntactic foams have been tested in high impact energy tests. The tests have identified potential uses of the foams for the containment of explosions, including underwater explosions, and marine vessel protection during collisions, grounding and impacts at sea. For example, the foams can also be used to fill the double bottom hull of tankers designed to transport crude oil and other liquids. The foams also have application as armour for ballistics and for use in vehicles of all kinds.

In respect to energy absorption the foams behave, at a macro 20 level, like an isotropic material independent of direction of loading while deformations are still in the linear region. In the non-linear deformation region, the syntactic foam behaves differently from metals and herein lies its energy absorbing efficiency. When a fracture is initiated in the syntactic foam layer, the fracture does not propagate easily because of the presence of the microspheres which act as discontinuities in the foam. To overcome the discontinuity of microspheres, each layer of spheres have to fail, which increases the "energy absorption capacity of the foam.

(f Fire Barrier Syntactic foam is comprised of resin and microsphere filler material. Most resins are flammable (except phenolics) and produce noxious gasses when burning. To overcome these problems phenolic resin can be used but it lacks the binding strength for most structural applications. Alternatively, the filler volume of the foam can be increased. Increasing the filler volume reduces the resin content and limits the resin surface area in contact with air or flame. As such, foams of high packing densities have the ability to retard fire development, as they themselves are inflammable.

Other properties of the syntactic foam that can be made according to the present invention include: a very high compressive strength to weight ratio. For example, compressive strengths can vary from 5.17MPa to greater than 300MPa.

In contrast, the compressive strength of medium strength concrete is only about 28MPa and about 40MPa for high strength concrete; a density that can vary anywhere from 20 kg/m 3 to 1400 kg/m 3 a hydraulic crush point of up to 1200 kg/cm 2 which is equivalent to the pressure at 12000m depth in the ocean. The hydraulic crush point determines the point at which the foam suffers rapid failure via high water absorption. Therefore, water absorption of good quality foams should not exceed 3% after six weeks of exposure to ultimate hydrostatic strength. It is generally considered that in order to avoid collapse of a syntactic structure, they should be designed to ensure that the volume changes remain below It is therefore both 2 advisable and prudent to relate the service pressure to bulk modulus, so that the volume reduction never exceeds 1 to 2% depending on service; a flame penetration rate as low as about 1.1 hr/in; a high temperature range of serviceability (eg. from about -70 0 C to 25 about +450 0 and a structure that can be drilled, sawn and machined and can accept fasteners, such as nails and screws.

Because of its properties, syntactic foams have a wide field of 30 application beyond those mentioned above. For example, syntactic foams produced according to the methods of the present invention could be used as: Exterior tiles for space vehicles; Interior floor panels and wall, roof and ceiling linings; Nose cones; Fins; Rocket, missile bodies and launch tubes; Radomes or sonar windows as the properties of some of the foams are similar to seawater: Ship moorings and buoys; Deep submersible vehicles and remotely operated vehicles; and Structural sections as well as interior fittings in rail and road vehicles and buildings.

Brief Description of the Drawings By way of example only, a preferred embodiment of the invention is now described with reference to the accompanying drawing, in which: Figs. are examples of cross-sectional views of two-phase and three-phase syntactic foam produced according to the present invention; Fig. 2 is a representative depiction of the mixing of the mixture of microspheres and binder according to the present invention; and Figs. 3 to 5 are scanning electron microscope (sem) images of a fracture surface of three phase syntactic foams produced by the method according to the present invention.

Preferred Mode of Carrying Out the Invention The present invention comprises a process for forming syntactic foam, the syntactic foam being a composite of a plurality of microspheres and a 20 binder material. In a preferred embodiment, the process can be used to form a syntactic foam in which the microspheres are at maximum packing within the composite. With the microspheres at maximum packing, it is possible for the particular type of microspheres being used, to determine the void fraction of any foam formed using these microspheres. From a determination of the 25 void fraction of a sample of microspheres, it is then possible to calculate the maximum required quantity of binder material to fill the void fraction of any quantity of microspheres compressed to maximum packing and so form a :.closed cell two-phase syntactic foam (see Fig. In those cases where an open cell three-phase syntactic foam is required (see Figs. l(b) and it is 30 possible to determine what is the maximum reduction in volume fraction of binder that can be made before failure of the binder between adjacent microspheres in the formed syntactic foam.

The first step in the process is preferably the removal of moisture from the microspheres. This can be important as microspheres typically have a high affinity to water resulting in large quantities of moisture becoming trapped in the interstitial gaps of the granular material.

To overcome any problem created due to moisture absorption, the microspheres can be oven dried. The time period of exposure to heat in the oven will depend upon the type of microsphere.

The moisture content of the sample can be tested periodically during the drying process. By using a pycnometer to test the particle volumes, then weighing the samples, it is possible to calculate the particle density of the microspheres. The pycnometer calculates the specific volume of a sample of granular material in cubic centimetres by differential pressure measurements. The sample is then weighed, the weight representing the mass of spheres and air contained within the sample. Knowing the volume of the spheres, the volume of the sample cup, the relative humidity and therefore the density of the air, the mass of spheres can be calculated and hence their specific or particle density found. This figure can be checked against the manufacturer's specifications. The sample can be considered dry when the calculated results of the particle density are within a specified tolerance of the manufacturer's specifications. At this point, the moulds that are to be used later in the process for moulding the foam should also be 20 placed into the oven for heating where they will remain until the mixture is to be added.

:i The specific or solid volumes and densities of the microspheres have to be calculated at maximum packing, by the use of a pycnometer or similar.

From this information accurate estimates of the void fraction of the 25 component spheres can be calculated. It is preferable to ensure that the microspheres are at a maximum packing arrangement when being tested for the void fraction. If they are not then the incorrect amount of resin will be added to the mixture and the final structure will not be one of maximum packing.

30 From the information calculated above, namely the void fraction and the particle densities, the initial mixture ratios can be formulated. The void fraction value (in cm 3 from the pycnometer results is converted to appropriate units of volume ratio for the size of the batch. For example if the void fraction of the microspheres at maximum packing is 10cm 3 of a 50cm 3 sample cup, then the void fraction is 20%. So 20% of the mould volume is allocated to resin volume and this amount is calculated. The remaining volume fraction of the mould is calculated into mass. This involves the use of the particle density calculated above. The reason for using the particle density at maximum packing is to calculate the actual amount of microspheres required to fill the remaining volume of the mould at maximum packing. If the bulk density of the microspheres is used then the remaining mould volume will be replaced with microspheres and void, something which is to be avoided as the void fraction has already been allocated to the resin volume. Once the correct mass of microspheres is made, the microspheres are preferably placed back into the oven lntil required.

The binder, such as an epoxy resin, will preferably have an extended gelation time (eg. at least 40 minutes). This extended gelation time is to allow for adequate mixing time to ensure that all of the microspheres are properly coated with resin and that compaction within the mould can take place prior to cross-linking of the resin bonds.

Another consideration is the resin temperature. The temperature of the resin system should be as close to that of the oven drying temperature.

This will ensure that the microspheres do not absorb moisture out of the *0s atmosphere forming condensate when rapidly cooled from oven temperatures 20 to resin temperatures. This may mean a lower temperature and longer drying time in the oven. The resin formulation may require other additives to prolong gelation and raise the formulation temperature.

The system will normally require some form of heating during the :.formulation and mixing. This is to maintain the formulation temperature at a 25 constant level ready to be packed into the oven-heated moulds. The resin system should be prepared in an orderly fashion with the additives mixed thoroughly throughout the resin prior to the addition of the hardener. Also the resin system should be placed on the heating plate and brought up to temperature prior to the addition of the hardener.

Once the resin is at operating temperature, the hardener can be added and thoroughly mixed through the resin. The selected microspheres are then added to the resin system. Assuming the mixing is a manual operation, the mixture should be briefly stirred until all the resin is absorbed from the surface of the mixing bowl. At this stage, the mixture will appear to be a fine powder with a few clumps of resin soaked microspheres that continue to roll round and around the mixing bowl. It is then necessary to disperse the resin throughout the entire system. This is done by shearing the clumps through the remaining microspheres. As is depicted in Fig. 2, this is done manually by tipping the mixing bowl 10 on its side and, using a large flat spatula blade 11, squashing the resin clumps against the side wall of the bowl 10. This will form a flat paste surrounded by dry powder filler. Then with a continuous motion the dry powder is sheared through the paste up the side wall. As the mixture is piled up the side wall it will tumble back over itself to be sheared through again. The clumps are sheared through the dry powder until successfully wetting the remaining microspheres. The mixture should now form thixotropic putty. Upon closer inspection of the mixture, strands of resin should be seen stretching between the outer microspheres as they are being sheared past each other. This clearly indicates that the shearing technique provides all the microspheres with contact to the resin. Their own surface friction ensures that the microspheres roll past each other coating themselves in resin.

While a manual mixing operation is described above, it will be appreciated that suitable mixing systems, such as mechanical mixers, could be utilised to achieve the necessary mixing of the microspheres through the resin.

4.

20 Where hollow microspheres are utilised in the process, there is a fine line in exerting pressure on the mixed batch as the binder cures in the mould. The difference in the sides of the line will be in one case, the formation of a foam with maximum packing of the microspheres and in the other case, the destruction of the microspheres. The application of the *6* 25 pressure will compress the mixture towards a maximum packing density which should provide minimum density two-phase foams (Fig. la) if the right

S..

amount of pressure is applied. If too much pressure is applied then the se.

microspheres will crush. The determination of the correct pressure will vary for each type of microsphere used to form a syntactic foam.

.i 30 To determine what is the correct pressure to achieve maximum packing without destruction of the microspheres, samples of mixed batches need to be compressed at a range of different pressures and then analysed using, for example, a confocal microscope. If maximum packing factors are achieved then the correct amount of pressure has been applied and subsequent foam production should be carried out at this pressure.

If maximum packing factor cannot be achieved prior to the destruction of microspheres then the resin content is too high and the initial calculations to determine void fraction have in fact not been carried out when the microspheres sample was at maximum packing.

If using solid microspheres then the determination of the appropriate pressure level is much the same as that described above. The key difference is that the solid spheres will not crush and nor will the resin. The sample will simply press solid and the pressure applied by the press will rise sharply. If at the point where the applied pressure rises sharply, maximum packing is not achieved then the original void fraction calculations were incorrect.

In either of the above cases, the determination of void fraction of a pressed sample of the microspheres should be repeated at a greater pressure with a view to ensuring that the void fraction is calculated at maximum packing of the microspheres.

With correct void fractions calculated and the compaction pressures found, production of the syntactic foam using the tested microspheres can begin. This will produce a solid sample of resin and microspheres. The microspheres will be at a maximum packing density with the void fraction of their bulk density filled with resin. The result are syntactic foams which are 100% closed cell two-phase foams (Fig. la).

To produce partially open cell three-phase foams it is simply a matter S.of allocating less of the void fraction to the resin volume. The mixing S' method provides good wettability of all the microspheres, which means the resin reduction produces a hollowing of the interstitial gaps while bond thickness is maintained at the points of contact (Figs. lb and ic) For a slight air void fraction in the foams, the microsphere volume fraction need simply be kept constant while reducing the resin volume fraction. For a larger *increase in air void fraction, the microsphere volume can be increased by the corresponding resin reduction volume.

The reduction in resin and corresponding increase in microsphere volume will cause a reduction in the resin bond thickness between the microspheres. This will have a limit depending upon the application of the foams and can be found experimentally. Testing the resulting samples for strength to weight ratios will show at first a decrease in the weight while the strength remains constant. Then a point will be reached where the bonds are too thin and a rapid decrease in strength occurs. This point is the minimum resin content for that sample of syntactic foam.

It will be appreciated from the above discussion that the process can be repeated for different types of microspheres and binder materials.

The present inventor has undertaken a number of experiments with respect to producing samples of syntactic foam using the method according to the present invention.

The experiments were undertaken using a variety of types of microspheres, as follows: SL75 Hollow Ceramic Microspheres SLG Hollow Ceramic Microspheres Q-Cel Solid Glass Microspheres Phenolic Microballoons The following details are provided with respect to the experiments undertaken using SL75 Hollow Ceramic Microspheres. From pressure trials and confocal microscope imaging, the best structure and packing densities were determined to occur at a pressure of 827.37 kPa. With this in mind, the following trials were set up for further investigation. The trials involved varying the resin to microsphere content as follows: 20:80, 18:82, 16:84, 14:86 20 and 10:90. The pressures of compaction included 827.37 kPa, 965.27 kPa, and 1103.16 kPa.

S1, S2 S3 80:20 Mixture Mould: Type 3 Piston mould Volume: 5.973 x 10- 5 m 3 Internal Dimensions: 39mm dia x 50mm height Mixture Volume: 3.58 x 10 5 m 3 (39mm dia. x 30mm height) volume 7.16x10 6 m 3 7.16 ml Resin volume 2.864x10 5 m 3 28.64 ml Microspheres Resin: Derakane 441-400 Gelation Time 40-60 min Formulation Temperature: Hotter thirties Density 1070 kg/rn 3 Microspheres: 'SL75 Eccospheres' Particle Density 600 kg/Mn 2.864x10- 5

M

3 x 600 Applied Compaction Pressure: kg/rn 3 17.18xl10 3 kg 17.18g 8 27.4 kPa 965.3 kPa 1.103 MIPa *fl.

9* S6 82: 18 Mixture Mould: Type 3 Piston mould Volume: 5.973 x105 M 3 Internal Dimensions: 39mm dia. x 50mm height Mixture Volume: 3.58x10, 5

M

3 (39mm dia. x 30mm height) 18% volume 6.44x10' M 3 6.44 ml 82% volume =2.93x10 5 m' 29.3 ml Resin: Derakane 441-400 Gelation Time: 40 60 min Formulation Temperature: Hotter thirties Density 1070 kg/rn 3 6.44 ml resin Microspheres: Density 600kg/in 3 2.9 3x10- 5

M

3 17.6X10- 3 k 17.61 g Applied Compaction Pressure: S4 8 27.4 kPa 965.3 kPa 1. 10 3 MPa S7. S8 S9 84 16 Mixture Mould: Type 3 Piston mould Volume: 5.973x 10- 5

M

3 Internal Dimensions: 39mm dia. x 50mm height Mixture Volume: 3.58x10, 5

M

3 (39mm dia. x 30mm height) 16% volume 5.728x10 6 c In 3 5.728 ml 84% volume 3.00 72x10 5 m' 30.0 72 ml Resin: Derakane 441-400 Gelation Time: 40 -60 min Formulation Temperature: Hotter thirties Density 1070 kg/in 5.728 ml resin 20 Microspheres: Density 600 kg/in S3.0072x10- 5

M

3 =18.04x 10- 3 kg =18.04 g Applied Compaction Pressure: S7 827.4 kPa S8 965.3 kPa *.SSS9 1.103MWa S10, S11, S12 86 14 Mixture Mould: Type 3 Piston mould Volume: 5.973x10-5 M 3 Internal Dimensions: 39mm dia. x 50mm height Mixture Volume: 3.58x10- 5

M

3 (39mm. dia. x 30mm height) 14% volume 5.012x 10- M3 5.012 ml volume 3.0788x 10 5

M

3 30.788 ml Resin: Derakane 441-400 Gelation Timre: 40 -60 min Formulation Temperature: Hotter thirties Density 1070 kg/rn 3 5.012 ml resin Microspheres: Density 600 kg/rn 3 3.0788 x 10- M 3 =18.47 x 10- 3 kg =18.47g Applied Compaction Pressure: S10 827.4 0000 0 S.0.0 *000* *.0 0. kPa sil S12 965.3 kPa 1. 10 3 MPa S13. Sl4 and 10 Mixture Mould: Type 3 Piston mould Volume: 5.973 x10- 5 M3? Internal Dimensions: 39mm dia. x 50mm height Mixture Volume: 3.58x10- 5

M

3 (39mm dia. x 30mm height) volume 3.5 8x10 6 m' 3.5 8 ml volume 3.222x10, 5 Mn 3 32.22 ml Resin: Derakane 441-400 Gelation Time: 40 60 rmin Formulation Temperature: Hotter thirties Density 1070 kg/m 3 3.58 ml resin Microspheres: Density 600 kg/m 3 3.222x10 5 m 3 19.332x10-3kg 19.332 g Applied Compaction Pressure: S13 87.4 kPa S14 965.3 kPa 1.103MPa Initial Observations Stirring the mixture at first formed clumps of resin-soaked microspheres in a dry powder that continued to roll around. It appeared that the resin was not going to spread evenly throughout the system with this method of mixing. By providing a shearing action between the spatula and the sidewalls of the mixing pot, the resin clumps were dispersed throughout the mixture. The clumps were sheared through the dry powder and in doing 20 so successfully wet all of the remaining microspheres. The mixture now formed a thixotropic putty. Upon closer inspection of the mixture strands of resin could be seen stretching between the outer microspheres as they were being sheared past each other. This clearly indicated that the shearing technique would provide all the microspheres with contact to the resin.

Their own surface friction ensures that the microspheres roll past each other coating themselves in resin.

Results Sample Series S1 From the pressure trials and the imaging the best structure and packing densities occurred at the pressure of 827.37 kPa. With this in mind the following trials were set up for further investigation. The trials involve varying the resin to microsphere content as follows 20:80, 18:82, 15:84, 14:86 and 10:90. The pressures of compaction included 827.37kPa, 965.27kPa, and 1103.16kPa.

29 Finally having achieved a three-phase structure in the material means that the air void of the material will give a difference between the bulk and specific densities of the sample. The specific density was recorded using the pycnometer to calculate the volume of the solid material (spheres and resin), then the weight was taken. The weight of the sample will remain the same for both density calculations simply because as seen earlier in the bulk and specific density calculations for the microspheres themselves, the weight of the air in the voids is insignificant. It is only the volume that is changing.

The following method has been used to calculate the bulk densities of the samples.

PC

Vc Where: c composite p density M mass V- volume The volume of the sample was calculated from the varying height measurements, as the diameters were constant and equal to that of the mould.

The mass fractions of each sample are as follows: Series 80:20 Vresin 7.16 x 10 6 m 3 Presin 1070 kg/m S: Vsphere 2.867 x 10 5 m 3 Psphere 600 kg/m 3 Mresin 0.0077 kg Msphere 0.0172 kg Msample 0.0249 kg Mresin I resin MKemple Msphere msphere Msampie Where: rnx is M is Min 0.0077 0.0249 'Mresin =0.309 the mass fraction of x the actual mass of x Ms 1 )here 0.01764 0.0249 mshr 0.691 Series 82:18 Vresin= 6 .45 X 10.63' Vsphere 2.94 X 10-iM Presin 1 0 70 kg/rn 3 Psphere 600 kg/rn 3 Mresin 0.0069 kg Msphere =0.01764 kg Keample =0.02454 kg NMresin In resin Msample Msphere Insphere Msampie Min 0.0069 0.02454 msphere 0.1764 0.02454 rnresi 0.281 Series 84 :16 Vresin :-:5.73x 10- M' Vsphere 3.01 X 10-'rn msphere 0.719 Presin 1070 kg/rn 3 Psphere 600 kg/rn 3 Mresin=0 .00614 kg Msphere 0.0181 kg Msampie 0.0242 kg Mresin rnresin M~sample Msphere rnsphere Msample &4 94

S

S*

S*

'IS 9 .5 9 S S S ~S rnresin 0.00614 0.0242 94 94 4 9 4 4945 5* 99 S 55.5 .5 4 *9 1 ff1resin 0.25 Series 86:14 Vresin 5.02 x10' rn' Vsphere 3.08 x10-' M' Msphere 0.0181 0.0242 msphere 0. Prn= 1070 kg/rn 3 Psphere 600 kg/rn 3 Mrei 0.00537 kg Mshr 0.01848 kg Msarnple 0.02385 kg Mrei M resinmsainpie Msphere rnsphere Iv!sample Mresi= 0.00537 0.02385 rn =i 0.225 Inisphere =0.01848 0.02385 1 Thphere =0.775 Series 90:10 Vresin 3.584 x10-' m' Vsphere 3.225 X 10nM Presin 1070 kg/rn 3 Psphere 600 kg/rn 3 Mresi= 0.00383 kg Msphere 0.01935 kg Msanpie 0.02318 kg Miesin rnresin IMiampie Msphere 1 Mspbere Msampie rnrei, 0.00383 0.02318 M~resin 0.165 Insphere 0.01935 0.02318 M~sphere 0.835 The following densities were recorded: Series 6 Volume Mass Specific Bulk Mass Fractions (cc) (grams) Gravity Density (kg/m 3 (kg/m 3 Resin Spheres 51 2.7 2 740.7407 687.5 0.309 0.691 S2 5.7 4.9 859.6491 691.2 0.309 0.691 S3 7.2 6.35 881.9444 701.62 0.309 0.691 S4 3.91 3.2 818.4143 667.4 0.281 0.719 9.08 7.86 865.6388 674.11 0.281 0.719 S6 14.28 12.85 899.8599 689.6 0.281 0.719 S7 5.6 4.75 848.2143 647.06 0.25 0.75 S8 6.24 5.55 889.4231 653.80 0.25 0.75 S9 11.55 10.7 926.4069 664.21 0.25 0.75 13.03 11.22 861.0898 622.5 0.225 0.775 S11 9.91 8.85 893.0373 635.58 0.225 0.775 S12 7.6 7.2 947.3684 645.13 0.225 0.775 S13 5.27 4.61 874.7628 591.9 0.165 0.835 S14 10.36 9.4 907.3359 600.4 0.165 0.835 S15 11.73 12 1023.018 618.3 0.165 0.835 Using the bulk densities and the mass fractions the void fraction can be calculated: 0 0.00* 0. 0 00.0.

0.0

O*

00 Vvoid 1 Pc (mf/pf mm/Pj The volume fractions of the material can be found as follows: pfVf PmVm Pc PcV

VC

The volume of the constituent materials is constant. This volume is that of the mixture volume, ie 3.584x10 5 m 3 The volume of the resin is equal to that of the original mixture. Because only the constituent materials are being measured, the specific or particle densities calculated with the use of the pycnometer are used to calculate the volume fractions.

Void Fractions Ser. Bulk Mass Fraction Void Vol of Volume Ratios Filler 6 Density Fraction Void Volume (kg/m 3 (m 3 Resin Filler Resin Spheres S1 687.5 0.309 0.691 0.009689 0.968945 1 4.396403 3.1478E-05 S2 691.2 0.309 0.691 0.00436 0.435978 1 5.388414 3.8581E-05 S3 701.62 0.309 0.691 -0.01065 -1.06497 1 5.574415 3.9913E-05 S4 667.4 0.281 0.719 0.024962 2.496187 1 5.599682 3.6118E-05 674.11 0.281 0.719 0.015159 1.515889 1 6.037027 3.8939E-05 S6 689.6 0.281 0.719 -0.00747 -0.74712 1 6.353948 4.0983E-05 S7 647.06 0.25 0.75 0.039993 3.999276 1 6.613962 3.7898E-05 S8 653.8 0.25 0.75 0.029993 2.999299 1 7.043549 4.036E-05 S9 664.21 0.25 0.75 0.014548 1.454825 1 7.429094 4.2569E-05 622.5 0.225 0.775 0.065038 6.503797 1 7.702609 3.8667E-05 S11 635.58 0.225 0.775 0.045392 4.53925 1 8.082755 4.0575E-05 S12 645.13 0.225 0.775 0.031049 3.104891 1 8.729244 4.3821E-05 S13 591.9 0.165 0.835 0.084998 8.49982 1 11.01669 3.9484E-05 S14 600.4 0.165 0.835 0.071858 7.185829 1 11.55958 4.143E-05 618.3 0.165 0.835 0.04187 4.418717 1 13.48761 4.834E-05 Figs. 3-5 provide scanning electron microscopy images of syntactic foams produced in the abovementioned experiments. In these Figures, it can be observed that some of the spheres at the fracture surface are broken. This breakage is taken to have occurred on formation of the fracture surface in preparation of the sample for imaging. Fig. 5 is a magnified view of the surface of Sample 10 produced by the present inventor. This view depicts an interstitial gap between three bonded microspheres.

Summary By starting the production with the volume ratios required then developing a method to manufacture the foams without the hurdles encountered up till now; the maximum packing structure has been achieved.

This method achieves the required ratios of resin to filler in order for the filler properties to have a dominant effect on the properties of the foam itself as well as providing the internal structure of a granular pile. The resin content is now purely a binding agent. From images taken of the samples, it was possible to determine that the structure was uniform without discontinuities and at maximum packing. The structure in the open cell foams shows a hollowing of the interstitial gaps while maintaining the bond thickness at the points of contact.

The bulk densities of the foam samples are close to the particle densities of the microspheres themselves. The difference between the bulk and specific densities clearly indicates the presence of an air void, and the fractions calculated are about what is expected. Due to the resin content in the lower numbered samples we see that there is approximately no air void producing a minimum density, two-phase sample. This was also confirmed by checking SEM images.

The results are repeatable with the ideal structure and varying resin content. The varying resin content allows for a variation in the strength to weight ratios of the foams as well as providing foams of 100% closed cell or partially open cell. The highest numbered samples being the lightest with 20 the least amount of resin are the open cell foams as can be seen from the air void calculations.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

o*

Claims (27)

1. A method of forming a syntactic foam, the foam being a composite of at least a plurality of microspheres and a binder material, the method including the steps of: determining the void fraction of a pressed sample of the microspheres; (ii) using the determined void fraction to calculate the required volume fraction of binder material to at least partially occupy the void fraction of a quantity of the microspheres; (iii) mixing the quantity of the microspheres with said required volume fraction of uncured binder material to form a batch of mixed microspheres and binder material; and (iv) exerting pressure on the batch for at least a portion of the curing time of the binder material to form the foam.

2. The method of forming a syntactic foam as defined in claim 1 wherein the void fraction of the pressed sample of the microspheres is determined when the sample of the microspheres is pressed to a state of maximum packing.

3. The method of forming a syntactic foam as defined in claim 1 or claim 20 2 wherein the pressed sample of the microspheres is formed by compressing the microspheres in a mould or sample cup.

4. The method of forming a syntactic foam as defined in any one of the preceding claims wherein step includes the steps of: determining the specific volume of the pressed sample of microspheres using a pycnometer which calculates the specific volume of the sample of the microspheres by differential pressure measurements; weighing the sample of microspheres in a sample cup, the measured weight representing the mass of microspheres and air contained ,within the sample of the microspheres; calculating the mass of the sample of microspheres; determining the specific density of the sample of microspheres; and from the determination of the specific density and volume of the pressed sample, calculating the void fraction of any pressed quantity of the microspheres.

The method of forming a syntactic foam as defined in any one of the preceding claims wherein prior to step the method includes a step of drying the microspheres to substantially or totally remove any moisture content.

6. The method of forming a syntactic foam as defined in any one of the preceding claims wherein the binder material is a thermosetting resin.

7. The method of forming a syntactic foam as defined in any one of the preceding claims wherein the microspheres are selected from a group including ceramic microspheres, glass microspheres, polymeric microspheres, carbon microspheres and glass-reinforced thermoplastic microspheres.

8. The method of forming a syntactic foam as defined in claim 7 wherein the polymeric microspheres include phenolic microballoons.

9. The method of forming a syntactic foam as defined in claim 7 or claim 8 wherein the microspheres are hollow or solid.

The method of forming a syntactic foam as defined in any one of claims 7 to 9 wherein the microspheres have a diameter of less than about 300ptm.

11. The method of forming a syntactic foam as defined in claim 20 wherein the microspheres have a diameter between about 20m and about 150tm.

12. The method of forming a syntactic foam as defined in any one of the preceding claims wherein the temperature of the binder material prior to mixing step (iii) is about the same as that of the quantity of the microspheres.

13. The method of forming a syntactic foam as defined in any one of the preceding claims wherein mixing step (iii) includes a process of shearing the batch to ensure coating of all of the microspheres in the batch with the binder material.

14. The method of forming a syntactic foam as defined in claim 13 wherein in step (iii), the batch is mixed until the batch is in the form of a thixotropic putty.

The method of forming a syntactic foam as defined in any one of the preceding claims wherein in step pressure is exerted on the mixed batch until the binder material is at least partially cured.

16. The method of forming a syntactic foam as defined in claim wherein in step pressure is exerted on the mixed batch until the binder material is fully cured.

17. The method of forming a syntactic foam as defined in any one of the preceding claims wherein the method further includes a step of determining the appropriate pressure to apply to the mixed batch in step (iv).

18. The method of forming a syntactic foam as defined in claim 17 wherein the method includes a step of determining the appropriate level of pressure to achieve maximum packing without crushing of the microspheres in the batch.

19. The method of forming a syntactic foam as defined in claim 18 wherein to determine the appropriate level of pressure to achieve maximum packing without crushing of the microspheres, the batch samples are firstly compressed at varying pressures and then secondly analysed to assess the structure of the resulting foam. The method of forming a syntactic foam as defined in claim 19 wherein the analysis involves imaging the structure of the foam.

I.

21. A method of forming a syntactic foam, the foam being a composite of at least a plurality of microspheres and a binder material, the method 20 including the steps of: pressing a sample of microspheres to a state of maximum packing of the microspheres; (ii) determining the void fraction of the sample of the microspheres; S.'(iii) using the determined void fraction to calculate the required volume fraction of binder material to at least partially occupy the void fraction of a quantity of the microspheres; (iv) mixing the quantity of the microspheres with said required volume fraction of uncured binder material to form a plurality of batches of mixed microspheres and binder material; exerting a different pressure value on each of the batches for at least a portion of the curing time of the binder material to form foam samples of different microsphere packing levels; (vi) analysing the foam samples to determine the pressure value that forms a syntactic foam having maximum microspheres packing; and (vii) using the determined void fraction value and the maximum packing pressure value to subsequently produce syntactic foam in which the microspheres are at maximum packing and the void fraction is at least partially occupied with binder material.

22. A syntactic foam produced using the method of any one of claims 1 to

23. A syntactic foam produced using the method of claim 21.

24. A method of mixing a plurality of microspheres with a binder material, the method including the step of shearing the mixture to ensure coating of all of the microspheres in the mixture with the binder material.

The method of claim 24 wherein the mixture is placed in a mixing container and is sheared between a blade member and a wall of the mixing container.

26. A syntactic foam produced from a mixture of a plurality of microspheres and a binder material, the microspheres and binder material being mixed according to the method of claim

27. A method of forming a syntactic foam substantially as described with reference to the accompanying drawings. *s Dated this twenty-sixth day of June 2000 Richard Charles Louis Jouault Patent Attorneys for the Applicant: P F B RICE CO a0067spc.bal P. 9 P« 9