Ricky L. Tabor

The effect of polymer structure on the gas permeability of model polyurethanes

In the face of impending CFC elimination, a large variety of low-boiling alternate blowing agents have come under scrutiny for use in rigid foam insulation applications. The permeability of both the alternate blowing agent and atmospheric gases through the rigid foam thermoset polymer contributes to the rate at which the k-factor of the resulting foam deteriorates. In this study, a compression molding method for synthesizing polymer films from polyurethane formulation components (a variety of diols with pure and polymeric methylenediphenyl diisocyanate) was developed and utilized to produce films containing dinstinct structural features

Reduced Density Carbon Dioxide Blown Foam Based on a Novel Polyol Technology

In view of increasing pressure to use environmentally acceptable, nonflammable blowing agents with zero ozone depletion potential in the manufacture of rigid polyurethane foams, there is greater interest in 100% carbon dioxide blown technology. When trichlorofluoromethane (CFC-11) or 1,1-dichloro-1-fluoroethane (HCFC-141b) is replaced by carbon dioxide as the cell gas, the resulting foam, in general, suffers from higher thermal conductivity (k-factor), poorer adhesion and worse flowability leading to higher density. The water level in the formulation can be increased to improve flowability of these systems, but foam with poorer dimensional stability is obtained due to rapid diffusion of carbon dioxide out of the foam. In order to maintain adequate dimensional stability, similar to what is achieved in CFC-11/HCFC-141b blown systems, the water level has to be reduced. This leads to unacceptably higher foam density. In addition, the higher k-factor of the foam is primarily due to higher gas k-factor of carbon dioxide compared to those of CFC-11 and HCFC-141b. This is partially offset by lower radiative contribution arising from finer cell structure of carbon dioxide blown foams. Certain applications, however, are less sensitive to the energy requirement, and a foam with higher k-factor may be acceptable. This paper deals with a design of experiments to yield a foam with good processability and excellent dimensional stability in a variety of conditions, while maintaining the in-place density usually obtained with CFC-11/HCFC-141b blown systems. The key to the success was the development of a novel polyol that led to dimensionally stable foams at higher levels of water. The commercial viability of this technology has been demonstrated by producing actual parts without any equipment modifications.

Microcellular polyurea xerogels for use in vacuum panels

The refrigerator industry is faced with regulatory pressure on the one hand to replace CFCs as the blowing agent used in insulative foams and on the other to dramatically increase the energy efficiency of refrigerators by 1998. One of the solutions proposed is the use of vacuum panels as insulating components in the walls of the refrigerator. A variety of materials have been proposed as filler materials for the interior of such panels. A novel class of materials recently developed by Dow Plastics for this application is microcellular polyurea xerogels. These xerogels have been prepared by polymerizing polymeric MDI in solution to provide microcellular materials with pore sizes of about 10 microns and surface areas of about 98 sq meter/gram. Conventional open cell rigid foams, by contrast, have cell sizes of about 120 microns and surface areas of about 0.1 sq meter/gram. Polyurea xerogels represent an alternative filler for vacuum panels to precipitated silica or other inorganic powders, providing lower thermal conductivity under vacuum, lower density (6.5 to 8 pcf versus 12.5 pcf for precipitated silica), and potentially reduced vacuum panel fabrication costs (due to their monolithic form and elimination of powder handling). These microcellular materials were characterized by DMS (dynamic mechanical spectroscopy), DSC (differential scanning calorimetry), TGA (thermal gravimetric analysis) and SEM (scanning electron microscopy). Laboratory scale vacuum panels have been fabricated and thermal conductivities measured. Greater flexibility in vacuum panel fabrication and part integration may be possible due to the ability of these materials to be machined or potentially molded into shapes. Additionally, unlike the technically mature area of open cell rigid foams, which have historically exhibited a lower pore size limit of approximately 50 microns, the potential exists for further pore size reduction in polyurea xerogels (currently at about 10 microns). As pore size is reduced in these novel materials, the corresponding thermal conductivity performance will improve.

Low Permeability Polyols for Rigid Foam Applications

centage of acetoacetylation, and the more the polyol is branched, the more the polyol viscosity will be reduced. It should be noted that this process neither reduces the functionality of the polyol nor does it increase the amount of oxide added to the polyol initiator. It does, however, significantly reduce the viscosity. Rigid foams produced using these acetoacetylated polyols show little difference in reactivity or physical properties when compared with foams made from the standard, commercially available polyols.