Effects of processing conditions on the curing of a vinyl ester resin

The effects of temperature, initiator, and accelerator levels on the curing of an epoxy bisphenol‐A vinyl ester resin Derakane® 411‐45 (formulated with styrene) were investigated by gel‐time and exotherm‐peak measurements on bulk samples. It was observed that the gel time was reduced as the initiator or accelerator ratio increased. Except at higher contents of the accelerator, a small kinetic plateau was seen in the gel curve and a shift of the maximum exotherm toward high temperatures in the DSC curves. This was explained by the dual role played by the accelerator species. A regression analysis of all gel‐time data showed a dependence of 3/2 order in the accelerator and first order in the initiator concentrations. Thus, for this polymerization initiation system, the gel time can be predicted for any initiator and cobalt levels and at any temperature within the ranges studied. The effect of the initiator on the unreacted styrene and vinyl ester was also examined. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1146–1154, 2002; DOI 10.1002/app.10403     

Use of molecular modeling to predict ligand efficiency in thermal stabilizers

Molecular modeling software is employed to correlate the thermal stability of an organotin molecule in PVC and molecular structure. Synthesizing model organotin compounds, measuring dynamic properties in PVC, and calculating structural and electronic properties yield an empirical relationship between the stabilizing ability of the tin molecule and electron density of the heteroatom.     

Truncated human endothelin receptor A produced by alternative splicing and its expression in melanoma

PURPOSE: Traumatic sphincter disruption frequently is associated with a rectovaginal fistula, but the effect of a persistent sphincter defect on the outcome of rectovaginal fistula repair is poorly documented. We analyzed the outcome of rectovaginal fistula repairs based on preoperative sphincter status. PATIENTS AND METHODS: We identified 52 women who underwent 62 repairs of simple obstetrical rectovaginal fistulas between 1992 and 1995. Fourteen patients (27 percent) had preoperative endoanal ultrasound studies and 25 (48 percent) had anal manometry studies. Follow-up was by mailed questionnaire in 36 patients (69 percent) and by telephone interview in 12 (23 percent), for a total response rate of 92 percent. Median age was 30.5 (range, 18-70) years, and median follow-up was 15 (range, 0.5-123) months. Twenty-five patients (48 percent) complained of varying degrees of fecal incontinence before surgery. There were 27 endorectal advancement flaps and 35 sphincteroplasties (28 with and 8 without levatoroplasty). RESULTS: Success rates were 41 percent with endorectal advancement flaps and 80 percent with sphincteroplasties (96 percent success with and 33 percent without levatoroplasty; P = 0.0001). Endorectal advancement flap was successful in 50 percent of patients with normal sphincter function but in only 33 percent of patients with abnormal sphincter function (P = not significant). For sphincteroplasties, success rates were 73 vs. 84 percent for normal and abnormal sphincter function, respectively (P = not significant). Results were better after sphincteroplasties vs. endorectal advancement flaps in patients with sphincter defects identified by endoanal ultrasound (88 vs. 33 percent; P = not significant) and by manometry (86 vs. 33 percent; P = not significant). Poor results correlated with prior surgery in patients undergoing endorectal advancement flaps (45 percent vs. 25 percent; P = not significant) but not sphincteroplasties (80 vs. 75 percent; P = not significant). CONCLUSIONS: All patients with rectovaginal fistula should undergo preoperative evaluation for occult sphincter defects by endoanal ultrasound or anal manometry or both procedures. Local tissues are inadequate for endorectal advancement flap repairs in patients with sphincter defects and a history of previous repairs. Patients with clinical or anatomic sphincter defects should be treated by sphincteroplasty with levatoroplasty.     

Benefits of toughening a vinyl ester resin matrix on structural materials

A commonplace vinyl ester resin blended with a core-shell polymer additive has been used as a matrix for some typical structural commercial materials to determine the benefits of increased matrix toughness. In a simulation of polymer concrete, the tougher matrix was found to increase the toughness of the concrete by a small amount. Unexpectedly the flexural strength was increased by 30% which has been ascribed to the greater damage tolerance of the matrix. In composites with fibre glass cloth, the interlaminar toughness is also improved. An application of extreme value statistics showed that the change in resin toughness due to blending was fully transferred to the composite, and also allowed an estimation of the effect of the reinforcement on toughness.     

Matched filter bound for OFDM on Rayleigh fading channels

A matched filter bound (MFB) analysis is presented, in which the normalised Doppler rate is unrestricted, and thus applicable to OFDM signalling. In contrast to the static channel case, the optimal matched filter receiver is shown to be time varying and the probability of error is shown to depend on the transmitted pulse shape     

Effect of absorbed water,temperature and strain rate on the yield strength of two methylmethacrylate-based plastics

The yielding behaviour in compression has been determined for two poly (methylmethacrylate) (PMMA)-based materials when dry or saturated with water. Measurements were made over the temperature range 0 to 60°C and at test speeds varying by 1000-fold. Wet material, as expected, shows a lower yield strength but this cannot be predicted from the strength of dry material and the change in the glass transition temperature. Yield strengths of wet material may be just as accurately calculated from that of dry material by subtracting 10 MPa for each per cent of water absorbed. A data analysis method for superimposing yield strength values at various strain rates to different temperatures is given which does not rely on a mechanical shifting of curves.     

Investigation of nitrogen compounds in coal tar products. 1. Unfractionated materials

The direct application of gas chromatography (g.c.) with a nitrogen-selective alkali-flame detector (AFD) and mass spectrometry to the identification of nitrogen compounds in coal tar products without the need for prior separation of nitrogen-rich fractions is described. Accurate mass measurement has allowed the assignment of atomic compositions to many nitrogen heterocycles despite their presence as minor components in complex aromatic mixtures, but some interference from hydrocarbons was encountered. Boiling point characteristics were used to assist in distinguishing between the various possible isomeric compound types. The AFD was used successfully for the quantitative g.c. determination of a range of nitrogen heterocycles in the unfractionated samples.