Bioplastic Refrigerator Liners Can Lower Appliance Energy Consumption By 7-13 Percent Annually
Replacing high impact polystyrene (HIPS) refrigerator liners with liners made of polylactic acid (PLA) biopolymer can lower the energy consumption of a refrigerator-freezer by 7 to 13 percent annually over the 15-year projected life of the appliance. This energy savings information comes from peer reviewed research published in the February 2019 issue of Applied Energy Journal.
A 13 percent annual savings over the life of a refrigerator-freezer would cover the lighting cost of an average-sized home for a year. If all new refrigerators sold in the U.S. in 2018 – roughly 10 million units – had PLA liners, the energy savings would be equivalent to three years energy generation at an average sized powerplant.
Polyurethane foam loses insulating properties over time
It is a well-known phenomenon that closed-cell polyurethane (PU) foam insulation loses its insulating properties over time. During the multi-year study conducted by NatureWorks and Oak Ridge National Laboratory in collaboration with BASF Polyurethanes GmbH the researchers measured the insulation degradation of PU foam sandwiched by HIPS liners, the incumbent material, and by PLA liners. PLA is a biopolymer made from plant sugars that exhibits both barrier properties and resistance to food oils. BASF measured the thermal conductivity of both HIPS- and PLA-lined foam-sandwich structures for more than two years. Figure 1 illustrates the polyurethane sandwich structures studied.
Figure 1: Foam sandwich with plastic liners as used in the aging study
Measurements of insulating properties were conducted at an average temperature of 10°C. Table 2 summarizes the thermal conductivity measurements for both HIPS and PLA. Each value represents an average from three specimens. The thermal conductivity of the PU foam with the HIPS liner increased by more than 30 percent in two years, whereas that of the PLA liner increased by about 5 percent over the same period.
Table 2. Measured thermal conductivity of the PU foam with plastic liners.
Based on thermal conductivity data, the researchers calculated the thermal resistivity over a 2.4-year period. Details of the calculations are described in . Thermal resistivity of the PLA liner barely moves over a two-year period while the HIPS liner shows a 23.5 percent decrease. See Figure 2.
Figure 2. Resistivity over 2.4 years
What is causing the HIPS/PU foam to decrease in insulating value over time?
Ambient air diffusion into & Cyclopentane diffusion out of the PU foam
To achieve high thermal resistance for PU foams, low-thermal-conductivity gases are introduced into the foam cells during production. These gases are called blowing agents and one of the most common is cyclopentane. As the foam ages, atmospheric gases diffuse into the cells and low-conductivity gases diffuse out, resulting in a decrease in thermal resistance. The process can be broken down into several stages . First, the gas within the cells dissolves into the cell wall or the strut material. Second, the gas is transported across the thickness of the cell wall or strut material. Third, the gas is released into the bounding space, which could either be the adjacent cell or the open surrounding environment. It is generally accepted that the three-step process within a homogeneous foam can be described by Fick’s law as given in equation 1 .
C is the gas concentration as a function of time in moles/m3, and Deff is the effective diffusivity. The diffusion process is driven by the gradient in the partial pressure of each gas independent of the other gases present. Thus, the rate of diffusion is directly proportional to the gradient of partial pressure for each gas under consideration [3, 4].
The diffusion of low-conductivity gases (such as cyclopentane) from PU foam is also dependent on the chemical affinity – the interaction between the gases and the plastic liner. Figure 3 shows an image of the HIPS and PLA sheets exposed to cyclopentane at room temperature for two hours. Overall, the HIPS sheet, shown top right, has a significant level of interaction with cyclopentane, as evidenced by the blisters on the sheets. The PLA sheet, bottom left, showed no deformation.
Figure 3: HIPS and PLA strips before and after exposure to liquid cyclopentane at room temperature for 2 hours.
The weight change of polymer samples after exposure to cyclopentane and its vapors at 50°C for 24 hours is shown in Table 2. The cyclopentane and its vapors caused the HIPS sheet to turn into a gummy residue – so that weight gain could not be measured after 24 hours. The PLA sheet showed no significant weight gain i.e., .0199 g. These results suggest there is minimal or no interaction between cyclopentane and PLA, whereas there is a significant amount of interaction between cyclopentane and HIPS.
Table 2. Weight gain analysis: Exposure of the two polymers to cyclopentane at 50°C for 24 hours shows significant interaction with HIPS and virtually no interaction with PLA.
The slow diffusion of cyclopentane through PLA may be associated with a lower level of affinity between the two materials and PLA barrier properties. In addition to the low permeability of cyclopentane through PLA, it has been shown that N2 and CO2 permeance through PLA is lower than through polystyrene [5,6]. The combined effects of low atmospheric gas permeability and low cyclopentane affinity with PLA may account for the improved liner energy savings performance of PLA liners over HIPS liners.
Predicting energy savings over 15 years operation
What energy savings can appliance manufacturers, consumers, and electric utilities expect from a materials substitution from HIPS to PLA? The United States Department of Energy at Oak Ridge National Laboratory researchers, utilizing the Energy-Efficient Refrigerator Analysis (ERA) Program, set out to answer this question. Since the thermal conductivity of the foam sandwiches were measured for just 2.4 years, the Oak Ridge team created two different scenarios to predict the behavior of the thermal resistivity over the life of a refrigerator. One scenario would calculate an upper range of potential energy saving and the other the lowest expected savings.
Scenario “PLA Unchanged (UC)” (see Figure 4) assumed that PLA liners are an impervious gas barrier. This would provide the upper limit of energy savings. Scenario “PLA Exponential (Exp.)” assumed that PLA would exponentially lose its barrier properties over time, a similar but lower energy loss curve to HIPS. Figure 4 shows both curves along with HIPS. The higher the curve the more energy the appliance consumes annually. The PLA UC curve shows the annual 13 percent savings, while the PLA Exp illustrates the 7 percent annual savings as compared to HIPS at the top of the energy consumption curve.
Figure 6. Simulation-predicted total energy use.
PU foam insulation commonly used in refrigerator-freezer cabinets loses its thermal resistance over time because air components diffuse into the foam cells and low-thermal-conductivity gases used as blowing agents during foam manufacturing diffuse out. Affinity between HIPS and cyclopentene blowing agent causes the refrigerator-freezer to use more energy annually during its later years. Applying a PLA liner which exhibits low gas permeability and low affinity can retard diffusion rates, slowing the degradation of thermal resistance and improving energy efficiency.
Separately from the energy savings study, NatureWorks demonstrated the manufacturability through extruding 3-4 mm sheets that were then successfully thermoformed in a typical production environment without making significant process changes. Further these formed cabinets were successfully foamed using standard foam formulations. Details on the formulations and process conditions can be found in [7-8] A full copy of the research report can be requested by writing to email@example.com with subject line Refrigerator Liners.
PLA is sourced from greenhouse gases via plant sugars
Trees, plants, and coral reefs are made from atmospheric carbon. NatureWorks looks at greenhouse gases, like atmospheric carbon, as a polymer feedstock. Plants such as corn, are used to capture and sequester CO2 transforming it into long-chain sugar molecules. The plants are milled and starch (glucose) is extracted. Enzymes are added to convert the glucose to dextrose via hydrolysis. Microorganisms then ferment this dextrose into lactic acid, a naturally occurring substance.
A proprietary process transforms lactic acid molecules into lactide, which is a valuable chemical on its own. In the process of polymerization, lactide rings are opened and linked together to form the long chain of polylactide polymer. These long chains of PLA are formed into pellets. The pellets are transported globally and then converted into a wide-range of innovative products, including 3D printing filaments, coffee capsules, yogurt cups, baby wipes, and appliance components. The manufacturing process for PLA at the NatureWorks production facility in Blair, Nebraska, emits 72 percent less greenhouse gases and uses approximately 51 percent less energy than traditional polymers such as polystyrene.
--Osei A. Owusu, Ph.D., senior scientist, NatureWorks, LLC