Zero-ODP (Ozone Depletion Potential) is no longer the only barrier for PUR foamers, as they are being asked to consider blowing agents with �low-GWP' (global warming potential). Just when the multi-year conversion to urethane foam blowing agents with Zero-ODP seemed almost complete, amplified concern over climate change is spurring to adopt another new generation of blowing agents that are said to have lower �global warming potential� (GWP). Concern for the environment has also cultivated rapidly growing interest in natural-oil polyols (NOPs) made from renewable raw materials for both rigid and flexible foams. These were major themes of the recent 51st annual conference of the Center for the Polyurethanes Industry (CPI) in the American Chemistry Council, as per a report in Plastics Technology.
Conversion to zero-ODP blowing agents for rigid foams has left North American PUR processors with several choices: chemical blowing with water or methyl formate; physical blowing with HFC-245fa, HFC-134a or hydrocarbons; or combinations of any of these. However, each of these has some degree of GWP, spurring the industry to seek �fourth-generation� blowing agents characterized by zero-ODP and lower GWP values. The trend of current environmental regulations indicates the need for low-GWP blowing agents as well as refrigerants and solvents. In fact, HFC-134a will soon be phased out in Europe as a refrigerant for mobile air-conditioning units, which could foreshadow a review of foam blowing agents by global regulatory agencies in the not-so-distant future.
Arkema reported its investigation of a range of new low-GWP blowing agents for most rigid PUR applications -appliances, pour-in-place, spray and PIR boardstock. This AFA series includes both liquid and gaseous products that possess very low GWP (<15). This compares with GWP of 11 for cyclopentane, 1300 for HFC-134a, and 950 for HFC-245fa. Industry sources note that the relatively high GWP values of HFC-134a and HFC-245fa could lead the U.S. EPA to decide that they do not have an indefinite lifespan of use in the PUR industry. Also, the high smog formation potential (SFP) of the pentane isomers could lead to restrictions on their use in heavily developed areas prone to air pollution. Arkema's results show that gaseous AFA-G1 can compete with HFC-134a in terms of polyol solubility, foam dimensional stability, and k-factor. Even more interesting were results for the liquid AFA candidates in replacing the more popular HFC-245fa and hydrocarbons. AFA-L1 shows similar blowing efficiency, some improvement in dimensional stability, and significantly better k-factor.
DuPont Fluroproducts has a novel blowing agent for polyurethane foams used in construction and appliance insulation. It is said to show low vapor thermal conductivity, non-flammability, low toxicity, zero ODP and a very low GWP value of 5. This product also offers chemical and thermal stability, polyol solubility, low diffusion rate, and favorable economics.
Honeywell Intl. (sole North American producer of HFC-245fa) reported on its fourth-generation foam blowing agent with very low GWP of 6, which conforms to the European Union F-Gas Regulation.
Progress on the use of NOPs in rigid PUR foams was addressed in several papers. While they have done well in flexible foams, NOPs have had some drawbacks in rigid. Since most NOP technology is based on fatty-acid triglycerides, they have very different solubility characteristics than polyether or polyester polyols. Their fatty acid portion increases their solubility with hydrocarbon blowing agents but at the cost of limited compatibility with conventional polyols.
One highlight of the conference was the introduction of a new tri-functional NOP from BioBased Technologies, whose Agrol NOPs have achieved wide acceptance in spray foam insulation, automotive uses and carpets. Agrol Diamond was developed specifically for rigid foams, where the search is for NOPs with required reactivity and hydroxyl number while maintaining a high biobased content. A soy-based amber liquid, it has a hydroxyl number range from 320-350 mg KOH/g and 86% biobased content. In addition to secondary hydroxyl groups, this NOP contains primary hydroxyl groups that have been shown to dramatically increase its reactivity and ability to form rigid PUR networks. It is completely miscible with a wide variety of NOPs, traditional polyether polyols and hydrocarbon blowing agents, making it suitable for many biobased rigid PUR foams.
Bayer described the development of rigid PIR foam based on a novel, low-viscosity NOP for use in insulated metal building panels. The foam reportedly met ASTM E84 Class I burn requirements. This NOP is a product of new Bayer technology that converts natural plant oils via a one-step process into NOPs with low viscosity and high renewable content at no sacrifice in performance. For this PIR foam system, Bayer researchers developed NOPs that mimic the structure of a polyester polyol that is used in metal-faced panels. The polyester polyol had a hydroxyl number of 240 and functionality of 2, giving it a molecular weight of 470. The researchers prepared two NOPs with functionality of 2.1 and hydroxyl number of 210. Since some higher-functional polyols (ie crosslinking types) are generally necessary to improve mechanical properties, a third NOP with a functionality of 3 and hydroxyl content of 290 was prepared which simulated a mixture of polyester polyol and crosslinking polyol. All three NOPs showed excellent solubility with pentane blowing agents, suggesting storage-stable blends could be obtained.
Also addressing rigid PIR foam formulations was Stepan Co. which has developed NOPs using its proprietary SP3 polyol process, which has been shown to mitigate property deterioration that occurs when simply blending oils in PIR foam formulations. Researchers noted that the use of NOPs in PIR boardstock generally has a negative impact on physical properties, including initial (green) and fully cured compressive strengths, dimensional stability, and insulation properties (R-value). Stepan's SP3 approach of introducing NOPs at 20% or 25% levels in combination with aromatic polyester polyols minimized the deterioration of properties. Developmental �KP� and �MP� NOPs showed improved blowing efficiency, enhanced thermal stability, reduced polyol and B-side viscosity, and less catalyst needed to achieve foam reactivity similar to other commercial NOPs. The new polyols also processed well and with minimal adjustments on commercial laminate board lines.
Bayer reported on two new slabstock foam polyols, Multranol R-3524 and R-3525, which contain 20% renewable content. The products are made by direct alkoxylation of castor oil, soybean oil, and glycerin, converting them into polyether polyols of around 3000 MW. According to Bayer's tests, these polyols are suitable substitutes for up to 100% of conventional polyols, like Bayer's Arcol F-3040, a 56-hydroxyl polyol. They have the same hydroxyl number, functionality and foaming reactivity as 3000-MW commodity polyols, as well as similar MW distributions, viscosities, glass-transition temperatures, flash points and compatibility with water and other polyols.
Foam processing studies show that new R-3524 provides essentially the same latitude as standard F-3040, with minimal changes required in formulation or processing conditions. New R-3535 is said to process acceptably in most formulations, but requires some optimization with high-pressure liquid-CO2 processing to achieve the same cell structure.
Honeywell's new HBA-1 offers very low GWP. One-component foam samples blown with HBA-1 (left) offer better cell structure and uniformity than samples blown with HFC-134a (right).
Foams made with both of these polyols are reported to closely match the density and air flow of foams produced with F-3040. Also comparable are foam durability, strength, surface liveliness, modulus-temperature profiles, VOC emissions, odor and fogging tendency. TGA indicates these foams are more stable to thermal breakdown in air and nitrogen than standard F-3040 foams.
Cargill reported on a new NOP designed specifically for high-resilience flexible slabstock. The most common way to increase load-bearing or hardness properties of HR formulations has been to use a solids-containing copolymer polyol. But Cargill's BiOH Experimental Load Bearing Polyol is a solids-free option that is said to deliver at least 97% renewable content, wider processing latitude, and foam physical properties comparable to conventional HR foams, but with enhanced load-bearing capacity compared with SAN copolymer polyols. Dow reported on the development of NOPs for viscoelastic (memory) foams using its Renuva technology, a proprietary method to produce polyols from renewable seed oils. Launched commercially last year, this technology is said to eliminate past issues with NOPs of color, odor, reproducibility and reactivity control. A key difference in Dow's process is that the seed oil is disassembled into constituent parts, functionalized and reassembled to make novel polyol structures. The process is said to allow control of functionality and equivalent weight.
Dow evaluated viscoelastic foams made with 18-42% NOP content in both TDI and MDI systems. Results showed the physical properties and viscoelastic response of these foams can be controlled by tailoring the structure of the NOP and by adding copolymer polyol to the formulation. Dow's seed-oil-based polyols reportedly show good oxidative and UV stability, resulting in odorless polyols and foams as well as fine cell structure for good hand and touch. Also reported are improved IFD characteristics or comfort levels, a broader glass-transition region, which translates to viscoelasticity over a wide temperature range and controllable molecular structure, resulting in control over mechanical properties.