Key Aspects of Novel Catalyst Systems in all-Water Blown Integral Skin Foams-Related Catalytic Activities in the Isocyanate Reaction

This paper will focus on the generalization of catalytic activities of a variety of typical amine and tin catalysts in the isocyanate reaction of gelation, blowing and crosslinking Secondly the relationship between catalytic activities and foaming behavior, as well as foam properties, will be discussed from the stand-point of developing novel catalyst systems for all-water blown integral skin foams (ISF).

Catalysts play an important role in the control and balance between the gelation and blowing reactions. Especially in ISF systems, this control becomes increasingly important for the improvement of surface skin formation in all-water blown systems, which are now required for eliminating CFC. That is to say, skin formation and cure become inferior when using water as a blowing agent, primarily because the role of CFC is to aid in skin formation. Moreover, other than the two primary reactions, it will be considered that the catalytic activity of the crosslinking reactions such as allophanate, biuret and isocyanurate formation will have some effect on foaming behavior as well as foam properties. Therefore, it will be important to know the crosslinking activities for the design of novel catalyst systems.

Concerning the catalytic activities for the primary gelation and blowing reactions, many reports have been presented in the past where the relationships between catalytic activities and foaming behaviors, as well as foam properties, have been reported. On the other hand, several reports have been recently presented on the crosslinking activities, based on the method using HPLC analysis techniques in a model reaction. Few reports, however, describe the reaction kinetic analysis of the catalytic crosslinking activities and the systematic examination of its relation to foaming behavior and foam properties.

In this paper, the catalytic activity of a variety of typical amine and tin catalysts will be generalized with a review on gelation and blowing activities and a reaction kinetic analysis of the crosslinking activities using HPLC analysis in a model reaction.

In addition, the relationship between these catalytic activities and foaming behaviors, as well as foam properties, will be discussed in an all-water blown ISF system. As to the foaming reaction, the relationship between viscosity rise profiles, which may have a large relation to cure, will also be discussed. Concerning foam properties, the relation to cure and skin formation will be discussed. As a result, it will be shown that strong gelling catalysts with high crosslinking activities provide improvement in skin formation. This report will intro-duce novel catalyst systems in all-water blown ISF systems by suggesting the effectiveness of catalytic activities in the selection of a suitable catalyst system.

An Insight into the Characteristics of a Nucleation Catalyst in CFC-Free Rigid Foam Systems

The establishment of CFC-free polyurethane foam systems, aimed at total CFC elimination by the year 1995, is of paramount importance in the present day polyurethane foam technology. Especially in rigid foam systems, the attempts to apply a variety of alternative blowing agents such as HCFC-22, -141b, and -142b, HFC-134a and -356, and hydrocarbons such as n-pentane, isopentane and cyclopentane, as well as all-water blown systems, are being examined.

In all cases of alternative blown systems, however, there exist differences in foaming behavior as well as inferior foam properties compared to traditional CFC-11 blown systems. Especially in HCFC-141b and cyclopentane blown systems, which have gained the greatest interest among the above-mentioned options, the following three subjects are of major concern. The requirements involved in achieving the resolution of these problems are dependent not only on the development of major raw materials but also on the selection of suitable auxiliary intermediates such as catalysts and foam stabilizers. (1) Thermal Conductivity; HCFC-141b, cyclopentane and carbon dioxide, which is generated from the reaction of water and isocyanate, have high thermal conductivity compared to CFC-11, thereby causing inferior insulation performance of the foam. Fine cell technology is now being examined in order to improve the thermal conductivity. For the achievement of the fine cell structure, the selection of suitable amine catalyst systems is important, although the effect of foaming stabilizers has an especially large contribution. (2) Dimensional Stability; not only in all-water blown systems, but also in HCFC-141b and cyclopentane blown systems which use water in high concentration, dimensional stability becomes a large problem due to the diffusion of carbon dioxide gas from the foam cell. For the improvement of dimensional stability, there exists the option to increase the foam strength; moreover, it is important to improve the isotropy of cell structure by adjusting amine catalyst systems. (3) Foam density; since 141b and cyclopentane have relatively higher boiling points and less blowing efficiency, lowering the foam density becomes rather difficult. Moreover, lower foam density normally provides poor dimensional stability. It can be said that the catalytic activity ratio in blowing/gelling of amine catalysts play an important role for the lowering of foam density with improved dimensional stability.

It is very difficult to improve these three factors simultaneously. In this report, however, the improvements of these subjects are discussed from a standpoint of amine catalysts; also special newly developed nucleation catalyst systems are introduced for cyclopentane and HCFC-141b blown systems.

[Rheological characteristics of the aneurysm at the middle cerebral artery bifurcation]

We performed a retrospective clinical study to estimate the morphological index such as aspect ratio (aneurysm depth/neck) and area ratio (ratio of the cross-sectional area of bifurcated arteries) of 64 cases with saccular aneurysms (ruptured aneurysms: 41, unruptured aneurysms: 23) at the bifurcation of the middle cerebral artery (MCA) and made a flow visualization study using a flat three dimensional acrylic aneurysm model to define the flow characteristics in conditions similar to those obtained from the clinical study. The mean aspect ratio and area ratio of the ruptured and unruptured aneurysm cases were 2.81 +/- 1.45, 2.52 +/- 0.91 and 1.56 +/- 0.59, 1.73 +/- 0.38. These respective values differ statistically. The area ratio of the MCA bifurcation with no aneurysm was 1.25 +/- 0.35. This value was also statistically different from the ruptured aneurysm case. The visualization study was performed changing the neck size and flow ratio into the peripheries to simulate various aspect ratio and flow ratio into peripheries. The results showed that intraanerysmal flow was definitely influenced by aspect ratio and area ratio. The aspect ratio over 2.0 and area ratio below 2.0 failed to drive the inflow inside the aneurysms and developed as a stagnant area in the dome side. The morphological index of aspect ratio and area ratio reflected the flow characteristics in the aneurysm and was statistically different in ruptured and unruptured aneurysm cases. The unruptured aneurysms with an aspect ratio of more than 1.6 and an area ratio of more than 2.0 must be followed up, because of the risk that they might rupture in the near future.

Is the aspect ratio a reliable index for predicting the rupture of a saccular aneurysm?

OBJECTIVE

The present retrospective study was undertaken to prove the reliability of the aspect ratio (aneurysm depth to aneurysm neck width) for predicting an aneurysmal rupture. The aspect ratio is considered a better geometric index than aneurysm size for determining the intra-aneurysmal blood flow.

METHODS

We measured the aspect ratios and the sizes of aneurysms, as determined by examining angiographic films magnified 1.4x, in 129 patients with ruptured aneurysms and in 72 patients with 78 unruptured aneurysms. After categorizing the aneurysms into four groups on the basis of their locations (aneurysms of the anterior communicating artery, middle cerebral artery, internal carotid artery-posterior communicating artery [ICA-PComA], and other aneurysms), a statistical analysis of ruptured and unruptured aneurysms was performed.

RESULTS

The mean aneurysm size was found to be statistically significant in the aneurysms at the ICA-PComA and in locations excluding the anterior communicating artery, the middle cerebral artery, and the ICA-PComA. However, the mean aspect ratio was statistically significant at all four locations. In patients with ruptured aneurysms, no ruptured aneurysms with an aspect ratio of less than 1.0 were found. The distribution of the ruptured group versus the unruptured group with an aspect ratio of less than 1.6 at each location was 13 versus 79%, respectively, at the anterior communicating artery, 11 versus 58% at the middle cerebral artery, 11% versus 85% at the ICA-PComA, and 7 versus 81% at other locations.

CONCLUSION

The aspect ratio between ruptured aneurysms and unruptured aneurysms was found to be statistically significant, and almost 80% of the ruptured aneurysms showed an aspect ratio of more than 1.6, whereas almost 90% of the unruptured aneurysms showed an aspect ratio of less than 1.6. This study therefore suggests that the aspect ratio may be useful in predicting imminent aneurysmal ruptures.

Haemorrhagic complication after total extirpation of huge arteriovenous malformations

Two cases with huge arteriovenous malformations (AVMs) who developed haemorrhagic complications after surgery are described. The cause of the postoperative haemorrhage was considered to be the normal perfusion pressure breakthrough phenomenon and/or occlusive hyperaemia. These two haemodynamic insults possibly occur simultaneously and induce life-threatening haemorrhage. It was concluded that a huge high-flow AVM with a large venous ampulla in its deep drainers has a high risk of a postoperative intravenous thrombosis, resulting in haemorrhage together with normal perfusion pressure breakthrough phenomenon.

[Postoperative hemorrhage due to normal pressure hyperperfusion breakthrough after a trapping of VA-PICA dissecting aneurysm]

We described a case of cerebellar hemorrhage after trapping of a vertebral artery dissecting aneurysm. A forty-eight-year-old man had suffered from severe headache, vomiting and disturbance of consciousness. He was transferred to our hospital in an ambulance. Emergency CT scan showed subarachnoid hemorrhage in the posterior fossa predominantly, intraventricular hemorrhage and hydrocephalus change. Chest X-ray showed radiological evidence of pulmonary edema. The initial blood-gas determinations demonstrated a marked reduction in PaO2 and increased PaCO2. Five days after admission, the patient's condition was improving. Cerebral angiography was performed using the Seldinger method. It revealed a right vertebral artery dissecting aneurysm just distal to the posterior inferior cerebellar artery. We performed an operation to trap the VA dissecting aneurysm. Blood pressure was well controlled under 140 mmHg during the operation and he recovered from anesthesia completely. On the day after the operation, suddenly the patient's consciousness began to deteriorate. Emergency CT scan was performed and it showed SAH, cerebellar hemorrhage and diffuse swelling of the cerebellum on the same side as the operation. We suspected rebleeding of the aneurysm due to a clip's having slipped. Reoperation was performed, but the clip was not displaced and there were no definite bleeding vessels on the operative field. Consequently only external decompression and resection of the right cerebellum were performed. We discuss pathogenesis of the occurrence of hemorrhage in this particular case after trapping. We also review the relevant literature.

[Non-traumatic dissecting aneurysms on the intracranial internal carotid artery: report of three cases]

Subarachnoid hemorrhage (SAH) due to non-traumatic dissecting aneurysms is uncommon. Most of such cases are reported to occur in the posterior circulation. We encountered three cases of non-traumatic dissecting aneurysms on the intracranial internal carotid artery. Two cases developing SAH from the dissecting aneurysms were surgically treated by such means as proximal ligation and trapping. Two cases, which were treated surgically during the acute phase, showed poor surgical outcome, because a large cerebral infarction took place during the course of vasospasm. Trapping of the ICA or the contralateral ICA with the dissecting aneurysm failed to keep enough blood supply despite hypervolemic hypertensive therapy. Operative results seemed to depend on the collateral circulation during vasospasm, so appropriate bypass surgery was recommended in cases of dissecting aneurysm on the ICA manifesting SAH. Furthermore, the mechanism of initiation of a dissecting aneurysm on the intracranial carotid artery was discussed from the biorheological aspect.

Sudden death in a rat subarachnoid hemorrhage model

The pathogenesis of sudden death during subarachnoid hemorrhage (SAH) still remains to be elucidated. A new rat common carotid artery-prechiasmal extracorporeal shunt model was designed to study the effect of different severities of SAH on intracranial pressure (ICP), regional cerebral blood flow (rCBF), and mortality. Different severities of SAH were induced by controlling the bleeding period (from 30 to 90 sec) and number of bleedings (one or three times). SAH caused a dramatic increase in ICP and immediate depression of rCBF, which recovered slowly to a certain extent. ICP increased sharply within the first 30 seconds and reached a plateau concomitant with nearly zero rCBF, which suggested the occurrence of cerebral circulation arrest. Bleeding of more than 60 seconds and increased ICP over 80 mmHg were directly correlated with the mortality. Respiratory arrest was the first sign of death, immediately followed by cardiac depression resulting in sudden death. This model combines arterial bleeding with systemic blood pressure and controlled bleeding time to simulate the acute period of SAH.

[Spontaneous resolution of idiopathic spinal subdural hematoma: case report]

A 32-year-old female presented with about a week's history of dull pain in the head and neck pain. On the day of admission, she experienced a sudden onset of severe pain, extending from the occipital to the lower back region and hypesthesia and paresthesia of the right C8-Th3 dermatome. On admission, she was alert but there was a band of hypesthesia and paresthesia at the level of C8-Th3. Cervical X-ray, computed tomography scan (CT) and magnetic resonance images (MRI) of the head revealed no definite abnormalities. The peripheral blood picture was normal, and the bleeding and clotting times were in the normal range. The cerebrospinal fluid (CSF) was bloody and the CSF pressure was 220 mm H2O. An emergent 4-vessel intracranial angiography disclosed no abnormal findings. MRI and myelography revealed an intradural extramedullary lesion extending from the Th2 to the Th5 levels, ventral to the spinal cord. A T1-weighted MR image showed iso-low intensity and a T2-weighted MR image showed low intensity at these lesions. Spinal angiography revealed no abnormal vessels. In one month, spinal compression cleared gradually without surgery and her clinical symptoms disappeared about 40 days after admission. Spontaneous spinal subdural hematoma is uncommon and only 28 cases have been reported. 15 cases have been treated with surgical procedure, but spinal compression cleared spontaneously in this case. This spontaneous recovery is such a rare event that only 3 such cases have been reported in the literature. However, with the prevailing MRI, this disease will be detected more frequently in the future.

[Usefulness of positron emission tomography to STA-MCA anastomosis for the case with cerebral infarction]

The ischemic area surrounding the cerebral infarction in the eloquent area was salvaged by STA-MCA bypass surgery. Both the misery perfusion area evaluated by positron emission tomography (PET) using the [15O] gas inhalation steady-state method and clinical symptoms improved within a year after surgery. To confirm the ischemic area and select the suitable recipient artery was important for successful bypass surgery, because only an artery covering the ischemic area is expected to have low resistance. In this case a large ischemic area with disturbed, vasodilatation by Diamox was detected on the SPECT. However, PET clearly exposed the localized misery perfusion area in the overestimated ischemic area by SPECT. We describe our experience and discuss the technique and efficacy of PET for STA-MCA anastomosis surgery.