Making spray foam work with new blowing agents

Low-GWP blowing agents are going to be introduced to the market in the next few years and it will be necessary to optimise spray foam systems to use these ingredients.

By Christian Eilbracht, Carsten Schiller, Robert Tauchen and Wiley Rowe of Evonik.

Spray foam formulations are unusual among polyurethane systems; they must have a long shelf life and they are complex mixtures of ingredients. Surfactants can have a significant impact the shelf life, chemical compatibility and physical properties, of these formulations.

We have found that surfactants play an important role in terms of spray-foam physical properties, flow, appearance and formulation shelf-life.

Additionally, this work shows that our Tergostab B84715 material gives good all-round properties in a standard formulation but that other surfactants may be more suitable if a high R-value or improved FR are required in different formulations.

To reach this conclusion we tested ten Evonik surfactants.

Environmental improvements

In July 2015, the EPA announced final regulations changing the status of several products with high Global Warming Potential (GWP) used in polyurethane foam. As a result, users have been instructed to stop using hydrofluorocarbons (HFCs) in some applications and switch to low GWP alternatives like 4th generation blowing agents, hydrofluoroolefins (HFOs). The GWP of the new HFOs is typically 1% of HFCs.

The spray foam industry has to face this change and it will be necessary to move to systems with different blowing agents.

Formulating for success

Testing took place in two parts. First, each system was compounded and sprayed shortly afterwards and physical property and appearance data was gathered.

Then samples of each liquid spray foam system were placed in a non-climate controlled environment for three months. After that time, the systems were re-sprayed several times and again tested for physical properties and appearance.

Each initial set of samples were sprayed on cardboard, and each aged set were sprayed on oriented strand board (OSB).

The foam properties of the aged and unaged resin samples were compared to assess differences in stability between the systems.

The surfactants were sprayed following the sampling schedule below.

Surfactants Tested

We tested standard and a few experimental spray-foam surfactants. The surfactants tested had a wide range of silicone content, molecular weight, and polyether composition. The differences between the ten products are listed in Table 2.

Table 2. Surfactants Selected for evaluation.

Figure 1 shows how the chemical structures of different surfactants can vary. The black line represents the silicone backbone and the purple lines are the polyether pendants. The left figure shows surfactants with short siloxane chains, high degree of backbone modification, and low polyether chain length. The middle figure shows medium siloxane chain length, low modification, and medium polyether chain length. Finally, the figure on the right illustrates long siloxane chain length, medium backbone modification, and high polyether chain length.

Formulation

For this project, a standard 2.0 lb cuft (kg/m3) closed cell formulation was used. Opteon 1000, formerly, Formacel 1100 is used as the 4th generation blowing agent. Polymeric methylenediphenyl diisocyanate (pMDI) was sprayed in a 1:1 ratio with polyol.

Table 4. pMDI properties.

A Graco E-20 Reactor with a Graco Fusion Air-Purged gun was used to spray the foam in Evonik’s lab in Hopewell, Virginia. The machine conditions are listed in Table 5 below.

After evaluating different chemical temperature and hose temperatures we chose 120°F (49°C) as the optimum chemical temperature.

Physical Property Testing

For each surfactant, R-value, density, compressive strength, knit line density and flammability were measured.

RESULTS

Initial R-value

Aged R-value

The reaction profile of the aged system appeared to be 3 – 4 seconds slower and was sprayed onto OSB instead of cardboard. In the spray foam industry, six-month shelf life stability is typically required; so although the three month test is a guide it is important to age the systems for six months to ensure adequate system stability. The initial R-value compared to the R-value of the aged system is shown in Figure 3.

Knit Line Density

A low knit line density is desired when spraying two passes of closed cell foam to ensure consistent foam structure and density.  In a number of candidates, the initial knit line density was low, but knit line density of aged systems was significantly higher. For the recommended product, Surfactant C, the knit line density was 2.01 lb/ft3 and 2.03 lb/ft3 for the initial and aged values respectively. The formulation made with surfactant C provided much more consistent knit line densities than most of the other products tested.

We also visually inspected a cross section of foam. A number of the formulations had a much more visible knit line than the formulation made with surfactant C.

Figure 5. Left picture, Surfactant C knit line. Right picture, much more visible knit line of Surfactant G.

Compressive Strength

The initial and aged compressive strength values were measured for each of the surfactants tested. In the case of formulations with high initial compressive strength, the compressive strength decreased significantly after aging, but still stayed close to 25 psi (172 MPa). The compressive strength data for Surfactant J was much lower than all other surfactants. This kept it from being a top candidate in this experiment. The initial and aged compressive strength numbers for Surfactant C were 36.4 psi (250 MPa) and 25.7 psi (177 MPa) respectively. All of the foams produced compressive strengths for initial and aged values that were above 20 psi. The results are shown in the graph below.

Wall Cavity Comparisons

However, the formulation made with surfactant C filled the cavity without leaving gaps on either side. Foam made with surfactant A is shown in the picture on the right leaving significant gaps between the foam and wall cavity. The difference in flow is illustrated in the following comparison.

Figure 7. Left picture, Surfactant C fills cavity. Right picture, Surfactant A has gaps on the sides.

Wall cavities were also sprayed in two passes to observe flow and surface quality. Many of the candidates had even less sufficient flow when sprayed in two passes compared a single pass.

Aged Flat Board Comparison

Flat samples were sprayed in order to test physical properties as well as visual flow. For the aged samples, this was done on OSB. Spraying on OSB allowed for less warping and a more representative surface of field conditions.

The recommended surfactant, Surfactant C, consistently filled out the width of the board and did not leave any gaps on the edges. The other surfactant pictured, Surfactant B, does not reach the edges of the board.

B2 Flammability Test

The B2 flammability test is a test used in Europe to determine flammability classifications. In order to reach B2 classification, the flame height cannot exceed 150 mm in 20 seconds of burning the specimen. Although Surfactant C was not the top candidate for flammability, it did perform better than a number of the other surfactants. Below are the results from the B2 test.

Based on these results, it may be necessary to increase or change the flame retardants used in this formulation in order to achieve a Class 1 rating in the US. However, the results do prove that surfactant selection can have an impact on flammability. If flame performance is the most important criteria for the system, a different surfactant may need to be considered.

Surfactant Structural Analysis

Typically, a high degree of backbone modification is correlated with high R-value. In this case, we see that products such as Surfactant A and Surfactant B with high backbone modification (see Table 2) provided the highest R-value, but not necessarily adequate flow. The high molecular weight and high polyether chain length of Surfactant C appears to better utilize the blowing agent and lead to better appearance and better lateral flow. For further R&D, the Surfactant C molecule could be revised with a higher degree of backbone modification to investigate whether a higher R-value is achieved.

In conclusion There are often trade-offs with surfactants: some products are designed to provide superior performance in one area but are not as ideal for another area.

Surfactant C was chosen as Evonik’s recommendation because our tests showed it has superior flow characteristics and consistent properties.

This is an edited version of a paper which was presented at the CPI Technical Conference in Meeting in Florida, October 2015.

Biographies

Dr. Christian Eilbracht received his Ph.D. in Chemistry with an emphasis on solid state chemistry at the University of Dortmund in 1997. He then worked for the Clariant Pigment and Additive Division and was in charge of the R&D activities on flame retardants for flexible polyurethane foams. He joined the Degussa Goldschmidt PU Additive business line in 2001. As a technical director, he is globally responsible for product development and technical service.

Dr. Carsten Schiller received his Ph.D. in Chemistry at the University of Bochum with a thesis on biomaterials for bone substitution in 2003. He changed to the University of Essen and continued his research activities in the department of Inorganic Chemistry. Since the beginning of 2005, he joined Evonik Goldschmidt and is currently responsible for development of additives for rigid foam applications.

Robert Tauchen received his BS in Chemical Engineering from Virginia Tech in 2010. In June 2010, he joined the Evonik Rigid Technical Team and is responsible for technical service and development for rigid foam additives in North America.

Wiley Rowe received her BS in Chemical and Biological Engineering from the University of Alabama in 2009. She joined Evonik in July 2014 as a Technical Service Representative for rigid foam specifically focusing on spray foam and polyisocyanurate.

This is an edited version of a paper presented at the CPI meeting in Florida 2015.