Surface Characteristics of PU Adhesive Formulations

The major portion of this work used as reference a two-part soft-segment PU formulation. A prepolymer of methylene his-4-cyclohexylisocyanate (MDI) with a large amount of 2,4-isomer constituted Part A. Part B was a mixture of two tri-functional polyols with molecular weights of 900 and 500. These were used in weight ratios of 70/30. Part B also contained about 0.08 wt% of a tin catalyst (UL-28, Witco Chemicals). 

Following initial surface energy evaluations, the PU films were immersed in water at controlled temperatures and for controlled times. Following immersion, films were dried under vacuum (room temperature, 24 hours) and surface energies were redetermined. 

Analyses of chemical structure in cured films, before and after immersion, were carried out by IR spectroscopy, utilising a Bonem-Michelson-100 FTIR instrument. 

Positive values of Kj designate a surface which is predominantly basic. Predominant surface acidity will produce a negative Kj. Generally, polymers show finite values of both AN and DN, with different groups of the surface region acting as electron donors and acceptors. A surface is considered to be amphoteric when AN and DN are finite and of similar values, with K 0. When both AN and DN are zero, the surface is considered to be neutral. 

For IGC work with PU, the polymer was cured against Teflon sheet, peeled off, and powdered by pestle and mortar. About 1.0 g of powder was packed into previously washed and dried steel columns, 15 cm long and 1.2 cm in diameter. Polymer substrates, such as PVC and ABS, were deposited onto Chromosorb support (acid-washed, 60/40 mesh diatomaceous earth) following well-established procedures [6]. Small, nanoliter amounts of the volatile phase were injected into the chromatograph, with helium as the carrier gas at a flow rate of 10 cmVmin. 

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