Resins for the Red Planet

One approach to building houses and storage tanks on Mars is to construct them out of stone blocks. For erecting such houses we need to bind the blocks together and fill any spaces between them. The principal problem is that Mars exhibits very low air pressure, which is about 0.5 psi. On Earth, we build with mortar, a mixture of cement and water, which is applied to the sides of bricks or stone blocks to bind them together and fill any cracks. In the Mars near-vacuum environment, these cementitious products based on reactions in a water mix will not work. Exposing the mix to the atmosphere will lead to rapid sublimation of the water so that the mortar mix does not have time to “set.” It will crumble uselessly.

Realizing this problem, IonMars has been searching for a better solution. In the original article “Pioneer House Building.” he proposed sintering or melting together the edges of stone blocks to create an airtight habitation. Recently, he participated in a forum under the auspices of NASApacelight.com entitled Mars Development. Planning.  On August 4, 2014, a participant named KenM pointed out that heating of rocks could split them apart. In fact, rapid heating is a technique for doing just that. IonMars responded that slow and moderate heating of the blocks to be bound together and the surrounding area would ameliorate the problem; however, this is an unproven procedure.

Not being expert in the field of resins, IonMars has sent inquiries to a number of companies and institutions, seeking a better solution. So far (Sept 2014), three companies have suggested epoxy resin products, or a selection of products, that may be applicable to construction work on Mars. These companies are Pro-Set, Resin-Research and Sika Corporation.

 

Pro-Set

In reply to our inquiry, a representative of Pro-Set stated that their company was the supplier of epoxy resins to Scaled Composites, the company that built Spaceship One. He referred to their website www.prosetepoxy.com  for technical data sheets that allow a consumer to select an appropriate product, even for Mars conditions. The company distributes a series of epoxy products based on a particular resin, called ADV-175, which when combined with various hardeners, create slow, medium or fast-setting epoxy resins. All share certain properties. They are all sold as thixoptropic gels, which means they become more liquid when agitated and require a certain time period to resettle to a semisolid state. While not clearly stated, one must assume this applies to the hardener. They are considered high performance epoxy adhesives that adhere to many different metals, concrete or stone.

The tensile strength of the hardened resin product is of immediate interest. It ranges from 5330 psi (at 22 degrees C) for the fast-curing product to 7070 psi for the slow-curing product.  Note that any of these resins has more than enough tensile strength to contain the air pressure inside a Mars house, which will present a force of 29 psi, at most. The tensile strength of the hardened resin is even greater than the basalt stone itself, which is estimated at 2100 psi. This represents a very large safety margin.

The Pro-set epoxy resins are sold in cartridge sets that each dispenses 47 feet of ¼-inch bead or 17 feet of ½ inch bead. Alternatively, the cartridge set covers 12 square feet when spread with a 1/8 inch notched spreader. One floor block or one wall block for a Mars house presents two 30 by 60 cm surfaces to be treated with epoxy resin before joining with two neighboring blocks (one “joining”), which is a total of 3600 cm2 (3.88 sq. ft.).  Therefore, if epoxy resin is spread over these entire surfaces with a 1/8 inch notched spreader, then one cartridge set will provide epoxy resin for 3 joinings. If the stone blocks could be cut with perfect accuracy and precision, then this would be the preferred method of applying resin.

 

Ben’s Beads

A Ben Franklin approach to applying the epoxy resin will combine a thrifty use of epoxy resin with a high degree of assurance that cracks will be filled and sealed airtight despite small inaccuracies in cutting. The procedure is to apply the product as a ¼ inch bead, but only to those areas where it is actually needed. Given the high tensile strength of the product, it is apparent that the entire surface of a block does not need to be joined, but only a sufficient area. The sketch below shows a 1/4 inch Ben's Beadsbead applied across the entire length of a block and one inch from the inside edge (the edge facing the inside of the house). The one-inch margin is to avoid joining a block with one of the scaffold frames. A moderate size bead will spread out under the pressure of a forklift pushing the two blocks together.  This compressed bead will spread into any irregularity in the surface up to ¼ inch deep and any small differences in accuracy of the block surfaces. In case of uncertainty a second bead should be placed over the first. It will provide an airtight seal along one side of the block. Another bead of the same length is applied on the exterior edge of the block to provide a second line of airtight seal. The other two lines of bead will provide an airtight seal along the left and right edges of the same block surfaces. These two beads will combine with like beads on other blocks to doubly ensure these seals.

Using this Ben Franklin procedure will require 360 cm (11.81 linear feet) of bead to join one block to two neighboring block. Using a ¼ inch bead, one cartridge will supply 3.98 joinings, assuming zero waste.  A more reasonable expectation may be 3 joinings, the same as the previous method.

How much resin will be required to construct a standard Mars house? Using Ben’s beads, the 136 floor blocks will each require 600 cm of bead for a total of 78,000 linear cm. Two sidewalls will require 264 wall blocks will each consume 360 linear cm of bead for a total of 95,000 cm. The two end walls will require 86 wall blocks (some cut at an angle) with approximately 369 cm per block for a total pf 30,960 cm. But the little ceiling blocks present the most junctions between blocks and 2015 of them will be required to finish the ceiling. At 224 cm per block, it will require 451, 360 cm of bead for the ceiling.  The total for constructing the house will be 446, 629 linear cm of bead, which require 312 cartridges to dispense.

As an alternative to cartridges, the resin and the hardener can be purchased in Resin Gunsgallon containers and applied through a dispenser. Pro-Set sells a high-capacity metering pump that uses two separate pump systems, one for resin and one for hardener. The two are synchronized so that one full stroke of a lever delivers the correct ratio of resin to hardener through a dispensing gun to the ¼ inch bead.. The two chemicals meet each other at the bead to begin an exothermic reaction, which initiates the process of hardening and curing. The system reservoir contains two gallons of resin and one gallon of hardener.

 

Working Time versus Curing

A critical difference between the fast-curing and slow-curing versions of these resin hardeners is the working time for applying the product. When applying fast-curing product as ¼ inch beads, the working time is only 20 minutes at 72 degrees F. The outdoor temperature will be much less but the stone blocks to be joined will be kept warm in an oven until the moment they are to be joined. The heat deflection temperature (the temperature at which the uncured resin begin to sag) is 113 degrees F so the blocks should be kept at or below this temperature prior to joining. As they are joined the temperature will begin to drop, but whether the period of higher temperature will create a short period of faster curing is unknown or unstated in the technical data sheet. As they are joined the blocks should be kept at or above 72 degrees, if possible, where the product is known to gel in 3 to 4 hours. The minimum curing temperature is 45 degrees but the time to cure at this temperature is not known, or not stated.

Is the working time of 20 minutes enough time to join 3 stone blocks with the other blocks before the product becomes useless? The pot life of 100 gm. of this material is even less, only 10 minutes at 72 degrees F. A fast, experienced crew on Earth could probably achieve this high rate of construction, but on Mars the crew must deal with plunging temperatures, difficult space suits or the limitations of external manipulators, and the unknown accuracy of the cut stones. It is doubtful that 20 minutes working time (or 10 minutes pot life) is sufficient.

The slow-curing resin hardener, on the other hand, provides a working time of 150 minutes at 72 degrees F and a pot life of 50 minutes. A bead will gel in 16 to 17 hours and the minimum cure temperature is 65 degrees F.  The pot life of 50 minutes is probably more than enough time to join 3 blocks, so this option would provide enough time for the actual setting of blocks. The problem would be the curing time because 17 hours is a very long to maintain block temperatures above 72 degrees F while the ground temperature plunges to -50 degrees overnight.

The best available choice of hardeners may be a compromise between working time and curing time. Pro-set have produced a “super-toughened” adhesive epoxy that exhibits a working time of 90 minutes (30 minutes pot life) and a curing time for a ¼ inch bead of 10 to 11 hours. Coverage is the same as the other epoxy resins and the tensile strength is 5330 psi.

While the above analysis by IonMars may serve as a beginning point for determining the best epoxy resin to employ, an investigator for this problem will want to conduct his/her own inquiries. The following may be of assistance.

 

Resin Research

The headquarters for Resin Research Epoxy Systems is located in Indian Harbor Beach, Florida, but they also have a manufacturing center in Tucson, Arizona. They list 11 major independent customers/distributors for their products who are located in the US and seven other countries.

The company literature boasts of their composite innovations, which include cycloaliphatic curing agents (1981), the first sanding additive, a mull modulus resin system (1990s), the viscosity resin infusion system (1990s), Kwik Kick fast curing system (2008), and sustainable manufacturing system (2013). Rather than just producing a standard line of products, the company specializes in working out solutions for individual corporate or institutional customers.  NASA and SpaceX may wish to avail themselves of this service (if they have not done so already).

Contact:

Greg Loehr, Resin Research West

Phone: (321) 223-5276

Email: gl10@aol.com

In reply to the author’s inquiry about resins to use for construction on Mars, Mr. Loehr said “Sounds interesting.  The resins we have are solvent free so they should function the same, or close, no matter what the air pressure.  We also do custom formulations so any application can be made suitable.”

 

Sika Corporation

Sika was the first corporation to respond to our request for any type of product that could be used as an adhesive in the severe Martian environment. They specialize in various types of cement or mortar repair products, but they acknowledged that no water-based cementitious product would work in an extreme low-pressure environment. They did, however, propose that their epoxy resin product Anchorfix 2 Arctic be tried out for this purpose.

Contact:

Elvis Torres

National Technical Service Specialist

Sika Corporation

Phone: (201) 508-6749 Fax: (201) 507-7107

Email: torres.elvis@us.sika.com


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