MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization 4.2.2 Thermoset polymers Thermoset polymers of interest,i.e.,those used as matrices in composites,are typically of sufficiently low viscosity at some point during the cure process to flow.Thus,they may be cast into plate forms to provide blanks from which finished specimens can be machined,or molded into even more complex ge- ometries if necessary to create net-dimension specimens directly. When casting neat(unreinforced)polymers for use as mechanical test specimens,it is critical that voids,inclusions,and similar defects be minimized,both in size and number.Most thermoset polymers used as matrices,even those considered to be toughened,tend to be relatively brittle,and thus their ulti- mate strengths are strongly dictated by critical flaw size. Inclusions can be present in impure resin as obtained from the supplier,or introduced during the fab- rication process (e.g.,inadequately cleaned molds,airborne dirt particles,inadequate mixing of compo- nents,etc.).Caution also must be exercised when using release agents,to avoid contamination of the polymer. Defects can be in the form of surface scratches,edge chips,and mold marks.Voids are typically caused by trapped volatiles which evolve during the initial stages of the curing process.The evolution of volatiles can be suppressed,or at least minimized,by subjecting the polymer to pressure during the cur- ing process.However,it is more common to apply a vacuum during the initial stage of the cure cycle, either while the polymer is still in the mixing container or already in the mold.This is done at one or more points in time as the temperature is being elevated,and while the viscosity is at its lowest.Thus,a vac- uum oven is useful. The vacuum can evoke a strong evolution of volatiles,requiring that the container or mold have suffi- cient volume to contain the frothy polymer until the gas bubbles burst.If a single flat panel is to be fabri- cated,a simple box mold consisting of five steel plates,viz.,a bottom and four sufficiently high sides,held together with screws,works well.This box can be disassembled after cure,for ease of polymer matrix plate removal,and easy clean-up.Individual strips of polymer can also be made in this manner,by plac- ing thin steel strips of width equal to the desired polymer matrix specimen width upright on one long edge, spaced apart to the desired polymer specimen thickness. Since volatiles are being evacuated,the vacuum pump itself should be protected,by the use of a cold trap to condense these vapors before they pass through the pump. If a cavity mold is being used to produce individual specimens of net dimensions,an elastomeric fun- nel works well to contain the volume of volatiles;the polymer will flow back down into the mold as the bubbles collapse.The funnel can then be left in place during the remainder of the cure.During clean-up, the funnel can be flexed to easily remove the cured polymer residue on it. The individual specimen cavity molds can be fabricated of metal,usually steel rather than aluminum because of its lower thermal expansion and higher surface hardness.These are typically two-piece split molds,to permit cured specimen removal.Elastomeric molds,themselves easily fabricated by casting around a permanent pattern,are an attractive alternative.The cured mold can be slit along its length to remove it from around the pattern,this slit also permitting it to be later pried open to easily remove the polymer specimen cast in it.In any case,the individual specimen molds are typically ganged together for efficiency.The as-molded specimens are ready for testing with little or no further preparation.At most, and primarily for aesthetic reasons,the mold seam(s)may be lightly sanded off. If vacuum is not being subsequently used to remove volatiles,the molds can be filled from the bot- tom,to minimize trapped air,but this adds complication and is usually not necessary.Likewise,if the vis- cosity of the polymer is too high for gravity fill,pressure can be used to force it into the mold.Again,this is not usually necessary considering the composite processing requirements of these polymers as matrix materials anyway. 4-2MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-2 4.2.2 Thermoset polymers Thermoset polymers of interest, i.e., those used as matrices in composites, are typically of sufficiently low viscosity at some point during the cure process to flow. Thus, they may be cast into plate forms to provide blanks from which finished specimens can be machined, or molded into even more complex ge￾ometries if necessary to create net- dimension specimens directly. When casting neat (unreinforced) polymers for use as mechanical test specimens, it is critical that voids, inclusions, and similar defects be minimized, both in size and number. Most thermoset polymers used as matrices, even those considered to be toughened, tend to be relatively brittle, and thus their ulti￾mate strengths are strongly dictated by critical flaw size. Inclusions can be present in impure resin as obtained from the supplier, or introduced during the fab￾rication process (e.g., inadequately cleaned molds, airborne dirt particles, inadequate mixing of compo￾nents, etc.). Caution also must be exercised when using release agents, to avoid contamination of the polymer. Defects can be in the form of surface scratches, edge chips, and mold marks. Voids are typically caused by trapped volatiles which evolve during the initial stages of the curing process. The evolution of volatiles can be suppressed, or at least minimized, by subjecting the polymer to pressure during the cur￾ing process. However, it is more common to apply a vacuum during the initial stage of the cure cycle, either while the polymer is still in the mixing container or already in the mold. This is done at one or more points in time as the temperature is being elevated, and while the viscosity is at its lowest. Thus, a vac￾uum oven is useful. The vacuum can evoke a strong evolution of volatiles, requiring that the container or mold have suffi￾cient volume to contain the frothy polymer until the gas bubbles burst. If a single flat panel is to be fabri￾cated, a simple box mold consisting of five steel plates, viz., a bottom and four sufficiently high sides, held together with screws, works well. This box can be disassembled after cure, for ease of polymer matrix plate removal, and easy clean-up. Individual strips of polymer can also be made in this manner, by plac￾ing thin steel strips of width equal to the desired polymer matrix specimen width upright on one long edge, spaced apart to the desired polymer specimen thickness. Since volatiles are being evacuated, the vacuum pump itself should be protected, by the use of a cold trap to condense these vapors before they pass through the pump. If a cavity mold is being used to produce individual specimens of net dimensions, an elastomeric fun￾nel works well to contain the volume of volatiles; the polymer will flow back down into the mold as the bubbles collapse. The funnel can then be left in place during the remainder of the cure. During clean-up, the funnel can be flexed to easily remove the cured polymer residue on it. The individual specimen cavity molds can be fabricated of metal, usually steel rather than aluminum because of its lower thermal expansion and higher surface hardness. These are typically two-piece split molds, to permit cured specimen removal. Elastomeric molds, themselves easily fabricated by casting around a permanent pattern, are an attractive alternative. The cured mold can be slit along its length to remove it from around the pattern, this slit also permitting it to be later pried open to easily remove the polymer specimen cast in it. In any case, the individual specimen molds are typically ganged together for efficiency. The as-molded specimens are ready for testing with little or no further preparation. At most, and primarily for aesthetic reasons, the mold seam(s) may be lightly sanded off. If vacuum is not being subsequently used to remove volatiles, the molds can be filled from the bot￾tom, to minimize trapped air, but this adds complication and is usually not necessary. Likewise, if the vis￾cosity of the polymer is too high for gravity fill, pressure can be used to force it into the mold. Again, this is not usually necessary considering the composite processing requirements of these polymers as matrix materials anyway