UNDERSEA TECHNOLOGY

ACKNOWLEDGMENT This article by Warren A. Waite, Mark L. Waldron and Ardushes Na- habediun is reprinted from the June 1968 issue of Undersea Tech- nology with permission of Compass Publications, Znc.

AN WORNLOUS AMOUNT of work is being done on syntactic plastic foams for marine applications. Syn- tactic foams are low density materials that are made by mixing hollow spheres into thermosetting bind- ers such as urethanes, epoxies and polyesters. The approach selected for this paper was the use of thermoplastic resins instead of thermosetting ones as the binder material for hollow microballoons.

Such foam systems are distinct and separate from the currently used thermosetting syntactics; a new class of foams which has many applications, one of the most important being a structural, load bearing, void Wer, for submarine hulls and as a buoyant material for raising sunken vessels. Such an a p plication requires excellent water resistance and good mechanical strength over a temperature range of 25 to 150° F. These unique foams have several significant ad-

vantages over conventional thermosetting ones. A suitable thermoplastic syntactic foam can be made by mixing hollow beads into a molten thermoplastic resin. This hot liquid blend can then be pumped

into the void and allowed to cool. On simple cooling (not curing) the material attains its full mechanical strength. Thermoplastic systems do not have pot life or exothenn problems. No systemic limitation on the size of the pour or batch exists. Unpoured ma-

COST E R POUND OF BUOYANCY IN DOLLARS

Graph dating density to coot per pound of buoyancy of a polyethylene-glass microballom eompositbn.

THERMOPLASTIC FOAM UNDERSEA TECHNOLOGY

terial may be kept molten or permitted to cool and be remelted for use at a later date.

The fillers evaluated for these syntactic foams were mostly hollow glass or plastic spheres, ranging from 20 to 500 microns in diameter. Additional foams were prepared by using microspheres, 0.1 to 0.5 inches in diameter; however, for the most part these materials were low in strength and investi- gation of their properties was limited. Still other low density fillers, such as ground cork and saw- dust, were investigated during this study.

Those evaluated were: microballoons, 20-50 mi- cron dia., made of silica, glass, ceramic, phenolic, and ureaformaldehyde; macroballoons, 0.1-0.5 inches dia. made of epoxy, aluminum oxide, clay; and low density fillers such as ground cork and sawdust.

The materials which proved most satisfactory were the phenolic and glass microballoons. "he others for the most part were either too expensive, too heavy, or too fragile for best performance. The most economical systems were blends of ground cork with wax.

The thermoplastic resins which were screened in the study were wax, polyethylene, polypropylene, polycarbonate, polyamides and noncuring phenolics. The polyethylene, the polyamide and the non-curing phenolic resins processed easily and several poly- mers from each of these families were screened. The foams made from wax and polyethylene when loaded with glass microballmns, or cork, appeared to be most suitable for flotation use at moderate depths. The polyamide and phenolic polymers ap- peared to show the most promise for use as struc- tural materials at intermediate to high hydrostatic pressures.

To derive a reference set of values a typical urethane polyester foam was made at about 42 Ibs./ cu. ft. density. Blocks of various sizes were cut from the large block and prepared for immersion.

These samples were immersed in water in a pres- sure vessel at loo0 psi. After immersion of a given duration, the samples were removed, weighed and returned to the chamber, and the chamber repres- surized. At the end of each interval the process was repeated until the cumulative total of the time at the pressure was 48 hours. Several observations were made on the basis of data from this experi- ment:

Over fifty percent of the total water absorbed in 48 hours occurs in the first fifteen minutes of the test.

Regardless of the sample size the weight percent pick-up (9-15%) did not vary too greatly for these conditions and in this specimen size range.

From this experiment we concluded that any reasonably sized sample, if immersed for thirty minutes or more, would give a relatively accurate picture of the foam's general water sorption charac- teristics.

Some of the first themplastic syntactic foam systems tested employed polyethylene or blends of polyethylene and parffi wax as the binder for glass or plastic beads. These resins were readily melted and made good low viscosity mixes even at 150 to 250 F.

The data showed that the polyethylene resin can be extended with wax for use at low pressures. Such materials would probably be adequate for buoyancy applications in the raising of sunken ships etc. at pressures from 250 psi gage (about 575 ft. deep). In this area a much more intensive investigation

with detailed long time water sorption data would be necessary before a final assessment could be made. The use of low cost filters however, makes such compositions inexpensive as well as easy processing.

This low cost per pound of buoyancy becomes even more intriguing when in addition to its prob- able ability to be used at depths of from 200 to 600 ft., one considers the fact that such materials may be recovered and stored for remelting and reuse at a later date. See graph for cost comparison.

Experiments were made with low and medium molecular weight polyethylenes filled with glass microballoons. These compositions demonstrate the rather large effect of polymer molecular weight on both mechanical properties and water sorption.

The advantages of such a system for raising sub- merged vessels would be numerous. In addition to the material being inexpensive, the equipment re- quired to perform the operation of pumping the syntactic foam to the vessel would be easy. to manipulate, inexpensive and uncomplicated.

A laboratory scale apparatus was fabricated in order to prove the feasibility of such a system. When the feed line carrying molten material was introduced into a simulated submerged vessel the extruded foam displaced the water and floated the vessel to the surface.

In actual practice, the material could be delivered in one of two ways. If the submerged vessel were located in shallow water, air pressure alone would be necessary to force the material through a heated line. In deeper water, a pump would be required, since air pressure would be excessively high. Some advantages of such a system are:

The material is easily removed from the refloated vessel with a steam hose.

The material is reusable. The material has a low moisture absorption and

will retain its physical properties immersed in water for long periods of time.

The system can be varied to give changes in vis- cosity, density and strength by varying filler types, resin types and/or both.

The use of polypropylene as the resin binder was given a very limited investigation. Three types were run through melt-viscosity checks but all gave ex-

96 Navel Cnginmon Journal, February 1969

UNDERSEA TECHNOLOGY THERMOPLASTIC FOAM

cessively high viscosities at temperatures below 400 to M O O F. Polypropylenes should merit additional work as they have the following advantages:

Good thermal stability. Relatively high mechanical strength. Lowest density of any of the common plastics

(0.90) . Relatively low in price. A number of polyamide resins were investigated.

Satisfactory syntactic foams were made from most of these; however, high resin viscosity would only permit limited loading of microballoons giving den- sities in the forty to forty-five pound per cubic foot range. A special low viscosity resin was developed to allow increased microballoon concentrations. This resin worked out very well.

From a study of various grades of glass micro- balloons, the following observations were made:

All the beads evaluated had a tendency to pick-up water unless stored in a specifically conditioned area.

“Wet” beads could be dried slowly in a heated storage area (120-150 F and less than 20 percent RH) or rapidly under a combination of heat and vacuum.

Most commercial grades of beads have a small to medium percentage of broken beads.

Even in small batches the light beads tend to float to the top of the container, as cream does on un- homogenized milk. Containers must be tumbled before sampling or using the beads to ensure uni- form density.

Glass microballoons from several manufacturers were added to a low viscosity polyamide resin. The results of the study are listed in Table I.

This study illustrates the wide variations in prop- erties which can occur within one formulation. It is particularly interesting to note that while all the moduli were in the same range, the highest modulus was possessed by the foam with the highest water sorption but, both the ultimate compressive strength and the stress at 0.2 percent offset indicated the

foam with the lowest water absorption. As Bead “A” had low water absorption and high

strength it was used in a series of tests in which the percentage of microballoons was increased incrementally.

The reduction of density with increasing bead loading does not appear as regular as would be ex- pected. The large decrease in density between the 37.5 parts loading and the 50 parts loading is most likely due to the entrapment of air in the material as the mix shows a rather large increase in viscosity in this area.

In view of the excellent water resistance of the base resin it appears that most of the water pick-up must come from either the filling of voids from entrapped air and/or the filling of beads that are crushed as the hydrostatic pressure is increased. As several of these materials appeared satisfac-

tory for use at pressures up to 1,000 psi, one of the lower density formulations was selected for further testing. Three four inch cubes of the following composition were prepared: polyamide resin-100 pads by weight; glass microballoons--50 parts by weight.

Some difficulty was experienced fabricating these blocks in the laboratory. Samples were machined from slightly oversized blocks, and after checking for density and compressive strength, were tested.

When the material was tested at 1,000 psi hydre static pressure, it was found to have exceptional water resistance, picking up about 0.04 percent by weight compared to 0.4 percent for most other cur- rent materials. Because of its excellent per- formance, the material was evaluated at 2,000 psi hydrostatic pressure where it failed in crushing.

Thermoplastic syntactic foams based on polyam- ide resin “C” show a great deal of promise for use as structural void fillers. The tests show that the water resistance of these foams are equal to or bet- ter, at these pressures, than some of the currently used thermosetting systems.

Table I meet of Bead Quality in Polyamide Resin “C”

Formulation

Polyamide Resin “C” Glass Microballoons Glass Bead Type A B C Density (p.c.f.) 40.17 48.8 42.0 Ult. C m p . Strength 1900 psi 1740 psi 1535 psi % Total Strain 5.60 6.00 540 stress @ 0.2% Of€set mpsi 1470 psi 1340 psi Modulus, (psi) 56,000 48,ooo 43,200

100 parts by weight 25 parts by weight

*Hydrostartic Test (1 hr. @ each pressure)

500 psi 0.3% 03% 0.4% lo00 psi 0.3% 0.3% 05% 2000 psi 0.5% 1.4% 2.490 3OOO psi 1.6% 3.7% 9.0%

+The tabulated percent water pick-up is the cumulative

D 40.2 1500 psi 5.00 1385 psi 69m

0.4% 0.7% 5.4%

14.4%

Naval Enpinwn Journal, kbruaq I%% 97

THERMOPLASTIC FOAM UNDERSEA. TECHNOLOGY

In the next phase of the study phenolic resins were chosen because of their high modulus, as the matrix material for syntactic foams. While not normally considered thermoplastic resins, they may, by simply omitting the curing agent, be investigated as thermoplastic resins.

The viscosity of a number of phenolics was checked and the one with the lowest viscosity was selected for further evaluation. The resins selected came in several molecular weights and provided a chance to investigate some parameters as a func- tion of molecular weight.

A series of foams was made up with five materials using a loadmg of 30 parts of glass microballoons. As the density of the phenolic resins themselves was somewhat higher than previous base resins, the density of the resultant foams ran 3-5 pounds per cubic foot higher.

This study resulted in the following observations and conclusions:

The viscosity of the mix increased markedly with increasing molecular weight.

The modulus and ultimate strength of the foam increased with increasing molecular weight.

The water resistance improved with increasing molecular weight.

The total strain to failure decreased with increas- ing molecular weight.

This exploratory study has shown that some members of the set of phenolic based syntactic

foams have excellent compressive strength and very low water absorption. While these materials appear rather high in density, lower density SYS- tern having equal or better properties could be prepared with additional effort.

This effort should be concentrated on (1) treat- ment of the beads to improve their strength and (2) the selection of phenolics giving lower melt viscosity. The biggest obstacle to lowering the density of these syntactic materials is their rapid increase in viscosity at the higher filler loadings. It is possible that treatment of the beads, blends of polymers or control of bead sizes would produce sigmficant reductions of existing foam densities.

Summary. The purpose of our investigation was to de-

termine the feasibility of using thermoplastic matrices for syntactic foams. The results of this ex- ploratory investigation clearly indicate that syn- tactic foams compounded from polyoleh, poly- amide or phenolic resins and glass phenolic or silica microballoons yield materials which are me- chanically adequate for many purposes.

The limited cost studies made thus far prove these foams will be competitive economically with their thermosetting counterparts, however consid- erable effort would be necessary to develop the optimum foam for specific applications.

J J HENRY *CO*INC* . n n v n ~ nRcHitEcts mnrtnt EnGinEERs m w n g SURVEYORS

New York Philadelphia Boston 90 w.r) S h o t 401 North Broad Street 430 South Main Street New York. N.Y. 10006 Philadelphia, Pa. 19108 Cohariet, Mars. 02025 WHitehall 3-2870 WAlnut 5-1755 EVergreen 3.9200

Cable: Honrycoinc ~~~ ~

98 Naval Enqlnaarr Journal, February 1969