Plastics Engineering - May 2014 - (Page 33)

Figure 2: Structure-property relationships of polyamide adhesives Figure 3: Melting points of x,18 polyamides from C18 diacid4 facturers create polymers with exceptional solvent resistance, hydrolytic stability, optical clarity, and toughness that will benefit a variety of markets. Polyamide hot-melt adhesives made from shorter chain lengths exhibit the best adhesion to surfaces due to the higher polarity of the molecule but, for the same reason, these adhesives are more susceptible to moisture pickup and can delaminate in high humidity environments. the less polar, longer chain lengths (c36) have lower moisture uptake, but also can have fewer amide linkages in the chain, and therefore lower overall adhesion. (this is depicted as the performance range in Figure 2.) With the use of c18 diacid in hot-melt adhesives, this performance gap would be overcome. Specifically, using a c18 mid-range diacid to make the hot-melt polyamide should impart a combination of both higher polarity and higher adhesion due to increased amide linkages and lower moisture uptake. high-use temperature for these c18-based polyamides. copolyamides of PA6,18 with other monomers have been reported in the literature. PA6,18 was co-polymerized with PA6 for use in molded and extruded thermoplastics. the resulting polyamide was reported to be more resistant to salt stress corrosion cracking and to have a lower melting point than PA6,6 and PA6,10.4 Hot-melt adhesives containing PA6,18 have been reported in the manufacture of filters. Incorporation of the long-chain diacid is reported to decrease water absorption (the Achilles' heel of polyamides) and to provide significant increases in chemical and solvent resistance of the polyamide, including resistance to gasohol.5,6 the cycloaliphatic polyamide of bis(2-methyl-4-aminocyclohexyl)methane and c18 diacid was synthesized and shown to be a moldable amorphous polymer with lower density, increased flexibility, better chemical resistance and reduced clouding as compared to the corresponding polymer derived from dodecanedioic (c12) acid. In addition, the optical transparency was improved over dodecanedioic acid-equivalent to PMMA and superior to polycarbonate and polystyrene.7 Block copolymers of polyamides and polyethers, also known as polyetheresteramides, have been formed into shaped articles such as fibers, fabrics, films, sheets, rods, pipes, injection-molded components, or shoe soles. the polyetheresteramides that utilize c18 diacid afford a product with improved optical properties as compared to its shorter-chain homologues.8 Polyurethanes are typically synthesized via condensation polymerization of a di-isocyanate (typically MDI), a chain extender (typically butane diol), and a longer chain polyol (typically polyester or polyether). longer-chain diacids (such as c18) can also be used to make polyester polyols that make up the soft segment in polyurethanes. the use of the longer hydrophobic chain in the polyols is expected to result in a new class of polyurethanes with a very flexible, less polar soft seg- Properties of Materials Made from C18 Diacid noteworthy products that can be made using c18 diacid include polyesterification products and polyamides. Aliphatic polyamides based on the diacid (from PA2,18 to PA12,18) have been synthesized via melt condensation.3 note the trend in Figure 3 in the polyamide series as the spacing between amide groups increases with the longer diacids. the resulting polyamides are still highly crystalline; however, the melting point decreases as the length of the amide repeat unit increases. With higher aliphatic content, the polyamides become more resistant to moisture and organic solvents. Interestingly, even the long-chain and highly aliphatic PA4,18 and PA6,18 polyamides exhibit very high melting points, greater than both PA10 and PA11, enabling a very www.plasticsengineering.org | www.4spe.org | MAY 2014 | PlAStIcS EngInEERIng | 33 http://www.plasticsengineering.org http://www.4spe.org

Plastics Engineering - May 2014

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