Cell opening characteristics of a polyurethane foam (PUF) with conventional formulation for a closed-cell rigid PUF was studied using lithium salt of 12-hydroxystearic acid (Li-12HSA) as a cell-opening agent. The cell-opening agent, Li-12HSA, was properly prepared in the surfactant silicone oil, in which it existed as uniformly distributed in nanoscale. Cell opening content of nearly 100% could be obtained for the sample with ca. 1.0 phr of Li-12HSA. As the results, it showed that a fully open-cell rigid PUF could be obtained by introducing Li-12HSA as a new reactive cell opener, having a functional group which is able to form a bulky flexible side-chain on PU main chains. Furthermore, the formed open cell rigid PUF showed desirable cell size, bulk density, and thermal conductivity without severe loss of mechanical properties compared to those of the closed-cell PUF which was made without Li-12HSA.

일반적인 독립 셀 구조의 경질 폴리우레탄 폼을 제조하기 위한 조성비에 12-hydroxystearic acid의 리튬 염(Li-12HSA)을 셀 개방제로 사용하여 개방 셀 구조를 가지는 경질 폴리우레탄 폼을 제조하기 위한 연구를 진행하였다. Li-12HSA는 일반적인 폴리우레탄 폼 조성에 사용되는 실리콘 계면활성제를 기유로 사용하여 합성되었고 기유내에 나노스케일로 잘 분산되도록 하였다. 실험의 결과로서 약 1.0 phr의 Li-12HSA가 사용된 샘플 폼에서 거의 100%의 셀 개방률을 나타내어 Li-12HSA가 적절한 셀 개방제로 사용될 수 있음을 보여주었다. 이는 폼 생성 과정중에서 폴리우레탄의 주사슬에 Li-12HSA가 곁사슬로 형성되어 셀 개방에 크게 영향을 미치기 때문인 것으로 생각되었다. 또한 생성된 개방 셀 구조의 경질 우레탄 폼은 독립셀 구조의 폴리우레탄 폼의 기계적 특성을 크게 저해하지 않으면서도 바람직한 셀 크기, 벌크 밀도, 및 열전도 특성을 가지는 것으로 나타났다.

Keywords: open-cell, rigid polyurethane foam, lithium 12-hydroxystearic acid, cell opening agent

Polyurethane foams (PUFs) are prepared by the condensation polymerization of component A and B, in which component A is mainly composed of a polyol, a catalyst, a foam stabilizer, and a blowing agent and component B is mainly a polymeric isocyanate. The components are mixed together so that reaction is carried out with foaming process. PUF is generally classified into three groups according to foam types, which are flexible, semi-flexible, and rigid. Among them, closed-cell rigid PUF is widely used as the thermal insulating material in refrigerators, buildings, storage tanks, and pipings etc. because of its low thermal conductivity and good processibility. ^1-4
Meanwhile, development of more energy efficient materials having better thermal insulation properties becomes an essential task due to the recent trends of global warming or drastic decrease of natural resources.^5,6
Since thermal conductivity of closed-cell rigid PUF is greatly dependent on the blowing gas such as chlorofluorocarbons (CFCs) or cyclopentane (CP), it is impossible to make a closed-cell rigid PUF of which thermal conductivity is smaller than that of the blowing gas.^7,8
Vacuum insulation panel (VIP) is, therefore, recently getting attention as one of the alternative solutions to this problem, which is composed of a core material, getter, and an airtight metal-laminated plastic film packaging.^9-12
Open-cell rigid PUF as the core material of VIP has many advantages compared to the inorganic core materials such as glass fibers or perlite in viewpoints of workability, density, and cost.^13-15
There are two critical problems to be solved for utilizing the open-cell rigid PUF as the VIP core material. One of them is that PUF should have very small-size cells to retain the low thermal conductivity for the long service time, because the thermal conductivity of PUF is proportionally dependent on the cell size.^16,17 The other more important one is that it should have no closed cells. If the foam has even a small amount of closed cells, the blowing gas in those cells seeped out gradually into the VIP as time passes so that the pressure inside VIP rises, reducing greatly in the thermal insulating performance of the VIP.^16-18
Cell opening during foaming process of PUF is carried out through the complex mechanisms.^19-24 Among the various cell opening mechanisms proposed by many researchers, cellopening by urea precipitation is one of the generally approved mechanism, which is originally proposed by Rossmy and coworkers.²² They proposed that the cell opening always occurs after urea precipitation regardless of the formulation used, as is consistent with the classical particle defoaming mechanism. Neff also proposed that the precipitated urea would trigger the cell-opening process due to the rapid increase in the foam matrix viscosity, since the particles act as physical cross-links which make the system unstable by hindering of the surfactant diffusion.²⁰
Various liquid or solid types of cell opening agent are conventionally used for cell opening in the rigid PUF during foaming process.²⁵ Typical examples of liquid type are polydimethylsiloxanes or the derivatives of high molecular weight poly(oxyalkylene).²⁶ However, the main disadvantage with use of such a liquid type cell opener is that the cell openning tends to occur early in the foaming reaction, thus, making a coarse cell structured PUF and consequently unattractive thermal insulation properties. On the other hand, solid types such as divalent metal salts of fatty acids, typically calcium stearate, or polymeric solid powders can be also used as cell openers.^13,14 Another adverse effects, however, may be caused in this case such as the mechanical wear due to the use of solid inorganic powders or non-uniform dispersion.
In the present study, therefore, we tried to find a new method to overcome the above mentioned problems by using the lithium salt of 12-hydroxystearic acid (Li-12HSA) as a reactive cell opening agent. A grease, in which Li-12HSA is finely dispersed in a silicone base oil, was prepared by the reaction of LiOH with 12HSA through the conventional method of grease fabrication.²⁷ Because –OH functional group of Li-12HSA can react with isocyanate to make a bulky and rod-like side chain during PUF formation, it was expected that they can play an efficient role on the cell opening process. The focus of the work, therefore, was on the cell opening effect of Li-12HSA during the PUF foaming process using a conventionally optimized formulation for a closed-cell rigid PUF. Open-cell content, cell morphology, thermal conductivity, and mechanical properties of resulted samples were also measured.