Project Mohole

A History of Syntactic Foam – Part II

[Part 2 of a 3-part series on the origins of syntactic foam]

As Lou mentioned in his earlier recollections, syntactic foam was a novel material development following World War II.  While polymer systems, including epoxy and polyester resins came a long way very quickly, it was the introduction of hollow glass spheres that proved to be a game-changer in the industry.  Looking at the available literature, it appears that the big bang moment arrived sometime in 1953 when Standard Oil Co. applied for their first patent related to hollow spheres called, “Process of Producing Hollow Particles and Resulting Product” ( US Patent 2797201 A, Franklin Veatch and Ralph Burhans).  They followed that in 1957 with another patent titled, “Method of Producing Hollow Glass Spheres”, (US2978339 A, Veatch, Alford and Croft) and the floodgates opened.  Other important discoveries followed.  Standard Oil stayed very active in the area with numerous follow-on and derivative patents.  Other companies became players in the early days with their own developments such as Exxon Research Engineering, Corning Glass and Potters Industries.  A significant breakthrough came in 1963 when 3M patented a glass bubble method under US patent 3365315A, “Glass Bubbles Prepared by Reheating Solid Glass Particles”.  In later years (1970’s, 80’s and 90’s) organizations such as WR Grace, PQ Corp and Saint Gobain Vitrage all produced hollow glass spheres..  3M became the dominant manufacturer of glass bubbles and remains so today.

As the hollow glass bubble industry began to percolate and form, so too did the variety of areas where the bubble could be used.  Scientists and engineers found lots of opportunities to mix this new material into their formulations to enhance the properties.  Most saw the potential to reduce weight and cost by replacing some of their matrix material with hollow glass.  Others saw the potential to improve their products or processes by reducing shrinkage, improving dimensional stability, or reducing exotherm without significantly diminishing the physical properties of the original products.  Hollow glass spheres began to show up in concrete, plaster, paint, wall panels, and even bowling balls.  Quietly, the Navy and a few clever engineers and scientists began to experiment with these new hollow spheres.  Their interest was in exploiting the low density, high strength and stiffness that could be attained to produce buoyant materials.  It was here that syntactics as we now know them really began to take off.

The Navy’s primary interest was buoyancy for subsea vehicles.  In 1958, the bathyscaphe Trieste was purchased by the Navy for deep water research under the Office of Naval Research.  In 1960, Treiste dove to the bottom of the Mariana Trench (10,910 meters) utilizing huge volumes of gasoline for buoyancy.  The use of this incompressible liquid with a lower density than seawater was certainly successful, but it was a less-than-elegant solution.  The Navy also had several other Deep Submergence Vehicles (DSV) on the drawing board or in early production that could use a buoyant material capable of survival in the deep.  This was the challenge that drove a flurry of activity and new developments.  In 1962, the US Bureau of Ships began to investigate alternative buoyancy solutions related to these vessels.

Most of the Navy’s work with syntactics in the early 1960’s appears to have come from the US Naval Applied Science Laboratory (ASL) out of Brooklyn, NY and Electric Boat (EB) in Groton, CT (EB).  One name that comes up quite frequently is Israel Resnick from ASL who appears to have spearheaded much of the development.  Some of the work was presented in government-sponsored sessions, one of the earliest being, “Syntactic Foam for DeepSea Engineering Applications” presented in 1965 at the Second US Navy Symposium on Military Oceanography.  In cooperation with industry, Resnick and his colleagues utilized the glass bubbles and resin systems available at the time to manufacture their syntactics.  They also pioneered the earliest development of tests and standards for this new material, including a new ASTM committee on syntactic foam.  Substantial testing was conducted to characterize the long term and cyclic hydrostatic performance as well as mechanical properties.  E.C. Hobaica from Electric Boat published “Buoyancy Systems for Deep Submergence Structures” in the Naval Engineers Journal in October of 1964.  Allen Winer from the Bureau of Ships published a paper in the Journal of Cellular Plastics in May of 1966 titled, “Syntactic Flotation Material for Deep Submergence Vehicles”.

While these teams worked on some of the Navy’s interests in this new material, there was another group working on a completely different project that would greatly change the oil and gas industry.  In 1961, the first phase of Project Mohole was executed off the coast of Mexico.  The ultimate goal of this ambitious project was to drill into the earth’s mantle to better understand the planet’s history and formation.  While the project was not successful in its ultimate mission, it yielded several new critical technologies that would later form the basis for the offshore drilling and oceanographic industries.  It was the first use of dynamic positioning which became a critical component for drill ships.  It was also the first use of syntactic foam subsea buoys for these positioning systems, introducing this new material to a group of engineers and scientists who would change the oceanographic and offshore oil worlds.

buoy float solutions

A History of Syntactic Foam – Part 1

A History of Syntactic Foam

By Thom Murray, Principal, Engineered Syntactic Systems

We are often asked how and why the syntactic foam industry seemed to emerge from one particular region of the US. Many of the true pioneers in the field started in the Northeast US and planted the technological and business seeds that gave rise to a unique material. We thought it would be interesting to look back at how the industry evolved and record some of the significant people and events that shaped it.  The three principals at CMT Materials and Engineered Syntactic Systems share some of this history, each with between 25 to 30 years of experience making syntactic foam.  The industry really got its start back in the 1950’s, however, and matured as the constituent materials used to make syntactic foam improved.  Like many industries, growth was fueled by a combination of market needs and technological breakthroughs.

There are three names that continually pop up when describing the early history of syntactic materials: Dave Cook of Flotation Technologies, Lou Watkins, and Bill Cumings, both of Cuming Corporation.  Each had a distinct influence and contributed greatly to the growth of syntactic materials in a wide variety of applications.   Bill passed away in 2009, still working and creating as he had for so many years.  Dave and Lou are both officially retired from the day-to-day business, but they are known to share their experience and knowledge.  To get started, we reached out to Lou and asked for some of his early recollections.  He graciously provided us with a starting point.

Part I: Beginnings, by Lou Watkins

“It’s exciting when new materials arrive at just the right moment to enable another revolutionary technical advance. That was the case as syntactic foam buoyancy became available to the fledgling offshore oil business. The time was 1968 and the place was the Santa Barbara Channel, off the coast of California. The vessel was the Bluewater II, one of the first true semi-submersible floating drill rigs, operating in 842 feet of water. This is the story of how that first deep-water well gave rise to a major industry that has made worldwide exploration for oil and gas at sea possible, even in water depths approaching 12,000 feet and beyond.

The term “syntactic foam” refers to a lightweight/high strength composite material made by mixing tiny hollow glass microspheres into a plastic binder. Syntactic foam was one of many new materials created during the period of explosive growth that followed World War II. Crucial to this development were the twin inventions of liquid epoxy resins and microscopic glass bubbles. It was soon discovered that the glass microspheres were greatly reinforced when encapsulated in the rigidly cured epoxy plastic. The result was a synergistic composite in which the strength of the whole was much greater than the strength of any of its parts. Even today, syntactic foam is the strongest material for its weight known to man.

In the 1950s, the United States Navy used syntactic foam to make buoyancy blocks to support its fleet of remotely-operated subsea vehicles. That soon led General Dynamics’ Electric Boat Division in Groton, Connecticut, to add buoyant void fillers to military submarines. A young chemical engineer named Dave Cook left Electric Boat in early 1968 to join Data Packaging, a plastics molding company located in Cambridge, Massachusetts. The Cambridge firm had received a contract from the Navy to develop recovery systems for sunken ships. In fact, the customer was actually the CIA and the real objective was to raise the K-129, a Russian navy submarine that sank in 16,000 feet of water in the Pacific Ocean. The Data Packaging project moved too slowly to suit the CIA and eventually the submarine was raised by Howard Hughes’ Glomar Explorer. But the seeds of a new technology had been planted.

At the same time the CIA was searching for the K-129, Exxon (then Humble Oil Company) was preparing to drill exploratory wells in the Santa Barbara Channel. The problem was that the 800-ft-plus water depth was too deep for conventional offshore drilling techniques. Stresses on the riser pipe, the steel conduit connecting the rig on the surface to the ocean floor, were considered excessive. Exxon approached Data Packaging to see if their new syntactic foam technology could provide enough buoyant lift to make safe drilling possible. The project was assigned to Dave Cook and his new protégé, Lou Watkins, who had recently relocated to Boston from North Carolina. The prototype modules were successful, and the first of several purchase orders for drilling riser buoyancy were placed by Exxon.

Unfortunately, a blowout and major oil spill near Santa Barbara in early 1969 (the Bluewater II was not involved in any way in the spill) led to a moratorium on drilling off the coast of California. But the need for riser buoyancy continued to grow in the Gulf of Mexico and in the North Sea. New orders were received from Shell Oil, Sedco, and the Offshore Company. The Flotation Products business quickly expanded and in 1970 moved into larger quarters in West Warwick, Rhode Island, close to its other customers at Woods Hole Oceanographic Institute (Falmouth, Massachusetts) and the Navy facilities in New London, Connecticut.”


Next installment: early developments in a growing industry.

engineered syntactic products

Acoustic Properties of Syntactic Foam – Part 3

Enhanced Density Products

The following measurements were made on our low density syntactic foams:

Depth Rating (meters) Average      Density (Kg/m3) Average

Speed of Sound (m/s)



1000 385 2,186 0.84
2000 400 2,461 0.99
3000 432 2,698 1.17
4000 457 2,767 1.26
4000 515 2,675 1.38
4500 490 2,922 1.43
5000 496 2,968 1.47
6000 552 2,952 1.63
7000 545 3,100 1.69

As noted, even though the hollow glass spheres and resin systems were different, the speed of sound and impedance measurements tracked well with depth rating.

It should be noted that these measurements were made on what we consider to be some of our standard buoyancy products, not materials tailored specifically to provide specific or targeted acoustic properties.  For example, formulations made to Navy specifications have the following properties:

Formulation 1

Density: 690 kg/m3 ± 16

Speed of Sound: 2,850 m/s ± 100

Acoustic Impedance: not specified

Formulation 2

Density: 380 kg/m3 ± 32

Speed of Sound: 2,595 m/s

Acoustic Impedance: 0.940 MRayls

In these instances, specific combinations of resins and microspheres were used to attain the desired properties.  This approach is always an option, though it does require some development and input from the end user.

Properties of CMT Materials Standard Tooling Products

The following measurements were made on our standard plug-assist tooling material (HYTAC®)

Product Name Avg Density (Kg/m3) Avg Speed of Sound (m/s) Impedance


HYTAC-W 690 2,151 1.48
HYTAC-B1X 708 2,568 1.82
HYTAC-C1R 747 2,579 1.93
HYTAC-XTL 755 2,697 2.04
HYTAC-FLX 838 2,749 2.30
HYTAC-WF 838 2,788 2.34
HYTAC-FLXT 897 2,647 2.38
HYTAC-WFT 968 2,700 2.61


While HYTAC materials are not optimized in any way around acoustic properties, the information is interesting nonetheless.  HYTAC-W, for example, is very close to a match for seawater with minimal inconsistencies in terms of density stratification.  As with all of the plug assist tooling products, HYTAC-W is available in a wide range of sizes and shapes.  For example, 2” diameter rods are maintained as a stock item.  This would yield an immediate ~22% savings on material for a given diameter compared to the same amount of material taken from cut blocks if the application required cylindrical shapes.  Sheet sizes starting at one-inch thick up to six-inch thick are also standard, at 0.5-inch increments.  Again, this could mean a significant material savings.

There are several additional observations of note from these test results.  Two sets of the products have derivatives that include polytetrafluoroethylene (PTFE) in syntactic.  (HYTAC-FLX => HYTAC-FLXT and HYTAC-WF => HYTAC-WFT). The PTFE is added to improve the slip at the surface of the material during the thermoforming process and to prevent sticking and material build-up (for more information on thermoforming, visit  The addition of PTFE increases the density but results in a decrease in the speed of sound.  Correspondingly, each material also shows a slight decrease in modulus.  It is also interesting to note the difference we see when making a dramatic change in the matrix material.  There is only a very subtle difference in density between HYTAC-W and HYTAC-B1X, yet the average speed of sound is significantly different due to the change from an epoxy matrix (W) to a thermoplastic matrix (B1X).

In summary, these simple observations illustrate the flexibility available to us as formulators.  The wide range of end-products allows the user to select an off-the-shelf product that may just as easily meet their needs as a complex, unique formulation.

syntactic foam

Acoustic Properties of Syntactic Foam – Part 2

Comparative Testing Procedures

Tests were done on two formulations with densities of 24 lb/ft3, the lowest density that we can currently provide for a microsphere syntactic foam. One formulation was rated for 1000 meters (MZ grade) and one was rated for 2000 meters (BZ grade). The formulations were designed for the specific depths using different resin systems and glass bubbles, so we expected to see some significant differences. The lower strength and stiffness MZ material had a speed of sound of 2,186 m/s while the BZ sample had a speed of sound of 2,489 m/s. This is interesting because the values nearly mimic the average modulus difference between the two products. The compressive modulus of BZ is about 1.15 times that of MZ. It is impossible, however, to separate whether the stiffness difference is due solely to the higher modulus resin or the different glass bubbles.

Overall we can report the following general information from the test:

Sample Density (Kg/m3) Speed of Sound (m/s) Impedance (MRayls)
MZ 385 2,186 0.84
BZ 385 2,489 0.96

The process for manufacturing low density syntactic foam does not lend itself to producing a product that is perfectly consistent in terms of density. This density difference can appear from block-to-block as well as within a block. The natural size and density variation of the hollow glass spheres that make up ~ 70% of the syntactic structure are the main reason for these differences. As made, the hollow glass spheres can vary by ±15 % in density in a single batch. This results in a density acceptance standard of ± 2 lb/ft3 (± 32 Kg/m3) for each block. While the speed of sound did not change dramatically with the density variations, the overall impedance could be greatly varied due to the wide density swing. For example, this could translate to rough impedance differences of between 0.80 and 0.91 on the MZ product. Due to the manner in which these enhanced density materials are made, the stratification between the top and bottom of a block can be even greater than the 2 lb/ft3 difference between individual blocks. Material cut from the bottom or top of a block may show these dramatic differences while the average block density is within specification. For MZ this may mean a 10 – 20% difference in both the speed of sound and impedance through the block thickness. Therefore it is crucial that the designer understands these variations while making material choices. There are ways to preselect and/or classify the products to minimize these variations if a very narrow impedance range is needed and we invite designers to have discussion as early as possible in their process.

Given these differences, we continued to explore the acoustic properties, but now we looked at them from two views. Our first goal was to measure the average properties of our high performance materials (lowest density per depth) over the established density and depth range. In this way we could determine the lowest attenuation levels for a given operating depth. Our second area of interest was in the measurement of our tooling materials (HYTAC®). We took this unusual step because the driving force behind the design of the tooling product line was not to achieve the lowest density possible. This allowed us much more flexibility in the manufacturing process allowing for a product that can be directly cast to different shapes and sizes, such as small (2” / 51mm) diameter rods. The cost advantage in utilizing a wider range of starting shapes is significant. Lower machining and material yield losses directly result in lower production costs for our customers. Further, these products do not have the same wide density variation as the enhanced density products offering better consistency in acoustic performance.

In the third and final installment, we will review material properties of our HYTAC line of syntactic foams to illustrate differences in matrix material.

AI-24 syntactic foam

Acoustic Properties of Syntactic Foam – Part 1

One of the great things about syntactic foam is that one can make an almost infinite amount of changes to the types and levels of the constituent materials in order to impart particular properties to the final part.  The types of matrix materials, hollow spheres, additives, and combinations thereof offer materials engineers a wide range of tools to determine end-properties.  Unfortunately, because the possibilities are nearly endless, it is difficult to stop making changes in order to perfect a particular property.  This holds especially true in the area of acoustic properties.  As part of our investigation into the development of products for industries interested in the acoustic properties of syntactic foams, we have discovered a number of interesting areas for exploration.

Acoustic Properties

When we think about acoustic properties, we generally think about whether a material absorbs or reflects sound.  For this discussion, we are thinking about syntactic foam as more of a window that sound waves can pass through.  In very general terms, the degree to which sound is reflected or absorbed by a material is dependent upon the acoustic impedance of the material and the media through which the sound is traveling (in the case of buoyancy materials, this is mainly water).  Other factors, such as wave frequency and incident angle can also play a role, but these are not within our control.  In most cases we are simply trying to match the acoustic impedance of water.  The acoustic impedance of seawater is ~1.50 MRayls[i] and is defined as the speed that sound can travel through the media (~1,450 m/sec) times the density (~1025 kg/m3).  These values will vary depending on things like temperature, water salinity and depth.  Variations can be as great as 10% or more.

Using 1.50 as an initial target, we set out to understand how close we could come with our standard syntactic products.  Measurements were performed on the different types of syntactic foams with a 1 MHz transducer.  Sample densities were also recorded.  Speed of sound and the calculated acoustic impedance in MRayls were reported for each material.  Overall speed of sound was found to be mainly dependent on density but was not independent of the modulus of the matrix material used.  Also, the variation in density through the thickness of the material was found to be very important.

In the next installment, we discuss comparative testing and data for multiple syntactic materials.

[i] MRayl, or Rayleigh, is a unit of measure used to describe characteristic acoustic impedance.

Subway ventilation panels

Engineered Syntactic Foam as Structural Material

Syntactic foams have long been used as buoyancy materials in subsea applications due to their extremely high hydrostatic strength and stiffness at relatively low densities.  This unique combination provides designers a source of lift for vehicles and structures operating in the deepest ocean environments.  Manned submarines, AUVs (Autonomous Underwater Vehicles) and ROVs (Remotely Operated Vehicles) all rely on syntactic foam in performance of their missions.

There are many other applications where the distinct properties of syntactic foam have also been employed.   Syntactics are excellent as low-to-moderate weight core materials for composite structures. The cellular structure of the material also makes them excellent thermal insulators, especially in situations where high strength may be required. Syntactic materials are utilized in transducers because the dielectric properties remain constant at depth.    As with most highly filled substances, the material is also dimensionally stable over a wide temperature range making it an ideal candidate as tooling for polymer or composite processing.

Challenging environments in the building and construction sectors are also areas where syntactic foam is now being used.  For example, syntactics are well-suited to the specific requirements of subway emergency ventilation panels.  Such panels must perform structurally but also be lightweight for ease of installation.  With the same strength as concrete at 20% of the weight, syntactic panels are cost-effective and highly efficient. The panel material must also be non-corrosive and resistant to water absorption over its lifetime.   Two additional properties are critical in this environment.  First, because the panels are used in an enclosed environment with direct human access, the material must pass stringent fire standards, most unusual for a product typically found hundreds or thousands of meters below the ocean surface.   Second, the need for clear radio communication in emergency tunnels is paramount for safety.  RF communication signal loss has been specified at less than 1.5 db over a wide frequency range. Syntactic panels are radio frequency wave transparent, thus meeting signal loss criteria.

We have been working on Low Flame, Smoke and Toxicity (LFST) products for use in civil infrastructure applications with new materials that have been tested and approved using ASTM methodology. Click here to see our time-elapsed video or get in touch with us to discuss your specific application.