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Q-talk 100 - Epoxy for Homebuilts Part 2

(Continued from issue 99)

System 285 (Max. Tg 105 C -110 C)

Mixing ratioPot lifeMixed Viscosity
100:50 by volumeH285-F 40 min300-500 cp
100:43 by weightH287-S 4 hours
H285 : H287 40 : 60 2 hours*

System 335 (Max. Tg 75 C - 80 C)

Mixing ratioPot lifeMixed Viscosity
100:45 by volumeH335-F 15 min800 cp
100:38 by weightH340-S 6 hours400 cp
H335 : H340 50 : 50 1.5 hours 20 : 80 4 hours*

*Note: It took me a while to understand this, but it's apparent that this is the option that is available to mix the available hardeners to achieve different times for pot life.

There are a couple of points that concern me in this summary. First, I want to point out some things to look for when one compares data of these different materials. There is a warning that one cannot mix the two families of the MGS material systems together. This is not so significant for the most part, but I've been in a situation where I have run out of ProSet epoxy on a U.S. Navy job and had to finish a time critical lamination at the top of an ship's antenna mast at nine o'clock at night. We successfully finished the job with WEST System, of all things, as it was available in a shipboard repair kit. The two epoxy systems were fully compatible. It is nice to be able to have a resin system that is not so particular in an occasional situation like this.

Second, the claim is that these systems have been approved for several commercial aircraft manufacturers. The only approval that I have been able to confirm is that of the German Federal Aviation Authority. I am confident that if the Diamond Aircraft Company is using the resin system for construction, then the Canadian Aviation Authority probably has approved it, but this is not confirmed at the writing of this article.

Last, there is a claim for physiological friendliness of the MGS resin, but ALL epoxies need to be handled with caution and should be used with proper protection such as disposable nitrile or latex gloves.

The Properties to Consider

Hardness is an indication of just that. The caution on hardness is that there is a potential to lose toughness in the name of developing greater hardness. Hardness is classically achieved by increasing the cross-link density, but what results is that elongation, or strain to failure, is lowered. The outcome is a more brittle laminate.

Elongation is desirable for applications like wing skins that need to flex under the loads. This is especially true for those of us the have decided to keep the main gear at the end of the canard. One wants good compression properties along with good elongation owing to the loading on the upper wing skins of the canard.

Tension is never the limiting issue as the fiber is dominants in this type of structural loading. The resin matrix is important in the compression loading as the resin holds the small 'columns' of glass fiber together preventing buckling of the fibers.

Compression is of great interest owing to the fact that all composites in aircraft application are subject to bending loads under normal operation and have to perform well in this aspect. The majority of the aircraft structure is seeing bending loads at all times. Another key parameter, which is not reported from any of the manufacturers, is the performance of the compression properties under elevated temperatures and / or hot-wet conditions. When the resin operates under elevated temperature, and humidity, these conditions cause the decrease in material properties. It's a fact that the former Saf-T-Poxy system lost half of its compression strength at 145?F. This is why the glass transition temperature (Tg) is so important. Most of the data that is reported by resin manufacturers is from resins or composites that have been post cured at elevated temperatures. This is why caution has to be exercised when making the comparisons. Much to MGS's credit, they did have detailed graphs showing the various "heat treatments", or postcures, that they performed on the product. Most of us would never postcure our airplane structures to get these results commercial aircraft manufacturers would.

EPOXY PROPERTIES (neat resin)

Test MethodGougeonPTM&W*MGS**
ProSet 125/226PR2032/PH3660L160/H160
Mix Ratio by Weight100:27100:27100:28
Hardness (Shore D)ASTM D-2240808680-85
Compression yield (ksi)ASTM D-695151318-20
Compression ModulusNA430,365NA
Tensile Strength (ksi)ASTM D-63881010-11.5
Tensile Elongation (%)3.925-6.5
Tensile Modulus (ksi)5.30E+054.18E+054.8 - 5.1E+05
Flexural Strength (ksi)ASTM D-790131716-20
Flexural Modulus (ksi)5.30E+054.63 E+05NA
Glass Transition Temp.DSC134133165-175

*AKA PTM&W Aeropoxy

*AKA PTM&W Aeropoxy

**Typical data according to 5.3203 WL 2 and 1 Parts 1 and 2 of the German Aviation Materials Manual not ASTM



You can order a printed copy of Q-talk #100 by using the Q-talk Back Issue Order Page.