Influence of the Caprolactam Quality on the Characteristics of Polycaproamide Thereof

Abstract

The influence of the caprolactam quality on the characteristics of polycaproamide thereof was elucidated.Among the main factors governing the quality of polycaproamide (PCA) is the quality of the raw material, caprolactam (CL) [1].We have found earlier [2] that caprolactam undergoes major changes during preparation for polymerization. Accumulation of the oxidation products begins even during transportation of liquid CL, as well as during melting of crystalline CL and storage of CL in the liquid state [3].It was of interest to elucidate how the quality of commercial CL 1 satisfying the requirements imposed by the appropriate standard affects the characteristics of PCA thereof, using high-sensitive instrumental methods of analysis along with the conventional techniques.First, we examined samples of commercial crystalline caprolactam, CL-1, prepared using phenol 2 : reference CL-2 sample prepared by recrystallization of CL-1 from dilute aqueous solution at room temperature; and sample of preoxidized caprolactam, CL-3, prepared by heat treatment of CL-1 in air at 90oC for 24 h.The CL samples were polymerized at 260 + 1oC in sealed glass ampules heated in an aluminum unit for 24 h. This time is sufficient equilibrium in molecular weight to be attained. Before being sealed the ampules containing CL were purged with nitrogen for a long time. The AG salt (3%) was added to the samples as a polymerization activator. No H 2 O was introduced as activator and no acid, as chain-terminating agent, ÄÄÄÄÄÄÄÄÄÄ 1 Bulgarian State Standard BDS 10780 !88.2 Agrobiokhim Company, EAD, Stara Zagora. since they create superpressure and cause ampules to break. After polymerization was the complete, the ampules were cooled, and the polymer was crushed manually.The CL samples under study were characterized by such parameters as compliance with the BDS 10780 3 88 requirements, content of carbonyl compounds (CCs) [4], and fluorescence intensity. The PCA samples werecharacterized by relative viscosity, Huggins constant K H [5] characterizing the content of branched and cross-linked PCA structures, content of the abnormal acylation product (1,11-diamino-6-ketoundecane, DAKU) [6] and of carbonyl compounds, optical density [7], fluorescence intensity, and content of terminal carboxy groups.We measured the relative viscosity of a 1% (by weight) PCA solution in H 2 SO 4 (95.6 wt %) at 25oC using an Ubbelohde viscometer at the solvent outflow time of over 100 s. The CC and DAKU content in CL and PCA was determined polarographically [7]. The optical density of the solutions was measured on an SF-16 spectrophotometer (according to BDS 10 780 3 88), and the fluorescence intensity, on a KFL-2-1 instrument. The optical density of the sulfuric acid solutions of PCA was measured by the modified Sbrolli technique [7] (300 mg PCA per 25 ml of 60 wt % H 2 SO 4 , wavelength 245 nm, rectangular cell with the absorbing layer thickness of 10 mm). The results obtained are listed in the table.The table shows that the samples of commercial and additionally crystalliz...