Acetylenic compound having at least two conjugated triple bonds in the molecule and polymerizable in the cyrstalline solid state to produce coloration, but relatively stable in liquid form, and having substituents of the group consisting of urethane, hydroxy, carboxy, carboxylic acid, carboxylic ester, carbazolyl and sulfonate, are used as printing ink. Preferred compounds are diynes, especially urethanes, diols and arylsulfonates melting in the range 40-150°C. They are employed as a melt at 40°-200°C adhering to the printing surface, whence they are transferred by contact to a substrate; and developed by cooling and exposure to a source of thermal or radiation energy.
A method of imparting color to a plastic substrate comprising applying to the substrate, or incorporating within the substrate, a diacetylene compound of general formula (I) wherein n=1 to 20; R1=an optionally substituted C1-20 alkyl group which may contain heteroatoms; T=H, an optionally substituted C1-20 alkyl group which may contain heteroatoms or —(CH2)m—C(═O)-Q2R2; Q1=NH, CO, NHCONH, OCONH, COS, NHCSNH or NR3, wherein m, Q2 and R2 are independently selected from the same groups as n, Q1 and R1 respectively; R3 is an optionally substituted C1-20 alkyl group which may contain heteroatoms; and irradiating the substrate to impart color to the substrate is described.