Blowing Agent Permeation Properties of Bisazide Treated Styrenic

Jul 31, 2018 - Thermoplastic cellular foam insulation performance is governed by the choice of resin, blowing agent composition, processing additives,...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 11014−11019

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Blowing Agent Permeation Properties of Bisazide Treated Styrenic Polymers Scott T. Matteucci, Lawrence S. Hood,* and Jacob M. Crosthwaite

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The Dow Chemical Company, Midland, Michigan 48674, United States ABSTRACT: Thermoplastic cellular foam insulation performance is governed by the choice of resin, blowing agent composition, processing additives, and processing conditions. Thermal insulation performance benefits when the cellular voids comprising the foam contain a gas composition exhibiting thermal conductivity below that of air. Maintaining this gas composition is crucial to long-term thermal insulation performance. However, these gases will diffuse through the thermoplastic cell structure over time. Air will likewise diffuse into the cells. The result is an unfavorable increase in thermal conductivity over time. One means to reduce diffusion of gases is to reduce chain mobility via cross-linking. Bisazides such as diphenyl-4,4′-di(sulfonyl azide) can be used for generation of such a structure. The results reported herein demonstrate significant permeability reduction for industrially relevant blowing agents in films. Furthermore, compositions with low permeability have been tested to determine thermal conductivity after aging experiments.

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INTRODUCTION Thermoplastic cellular foam products sold into the building and construction market for thermal insulation purposes must meet many criteria: compressive and flexural strength, dimensional stability, flame performance, and low water transport properties, as well as the aforementioned thermal insulation performance. These properties are dictated by the choice of resin, blowing agent(s), processing additives, and control of expansion. These foam products offer improved thermal insulation performance when the cellular voids contain a gas having lower thermal conductivity than air. Containment of this gas is crucial to maintaining desired thermal insulation properties. The challenge is that all gases tend to seek molar equilibrium; thus these gases will diffuse from the cellular structure over time. Additionally, air will diffuse into the cells. Both effects induce an increase in thermal conductivity and a corresponding decrease in thermal insulation performance. Both the structure of gas molecules and the chemical and physical natures of the polymer dictate gas transport properties. Numerous techniques to improve thermal insulation properties have been reported in the literature: use of infrared attenuators to reduce infrared heat transfer,1,2 promotion of nucleation to reduce gas thermal conductivity (Knudsen effect),3 and use of low permeable films on the exposed foam surface (reduce infiltration of higher thermal conductivity atmospheric gases),4 to name a few. These techniques advantageously reduce thermal conductivity, but negatively affect other properties such as density, solar exposure stability, and cost to produce. In particular the use of external films is disadvantaged when foams are adhered to structures by means of mechanical fasteners such as nails as the nails would penetrate the film. An additional disadvantage with films is the need to mechanically remove the film, if it is not chemically © 2018 American Chemical Society

compatible with the foam resin, during recycling. In certain countries, foams are sold based on long-term thermal resistance (LTTR), a test in which foams are sliced, aged, and then reassembled for measurement of thermal conductivity. Barrier films would not be beneficial in such a test; thus a means to reduce gas transport throughout the totality of the structure is of value. Chemical means to reduce gas transport in styrenic foams have been shown to be effective. Hood and Matteucci have shown that, by careful selection of comonomers, diffusivity of fluorinated gases can be reduced.5 Nonspecific means to reduce gas permeation are even more desirable as a broader class of gases including high thermal conductivity atmospheric gases would be affected. Manufacturing of extruded thermoplastic cellular foams involves mixing of solid resin and additives, melting and pumping of this mixture, injection and mechanical dispersion of physical blowing agents, cooling of the resulting mixture, and controlled expansion into the atmosphere.6 Foams are then allowed to cool until they are dimensionally stable and mechanically shaped per customer requirements. There are many considerations in designing such a process. In particular, two considerations are directly affected by permeation of gases. First, during cooling of the polymer melt in the extrusion line, there must be means to supply enough time for the blowing agent to solubilize into the melt. This is achieved by using a production line of sufficient internal volume such that the residence time exceeds the dissolution time. A reduction in melt phase permeation would equate with Received: Revised: Accepted: Published: 11014

May 10, 2018 July 14, 2018 July 19, 2018 July 31, 2018 DOI: 10.1021/acs.iecr.8b02057 Ind. Eng. Chem. Res. 2018, 57, 11014−11019

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Industrial & Engineering Chemistry Research a required increase in equipment size. Another consideration is the time required for the foamed product to internally equilibrate before it can be cut to its final dimensions. If atmospheric gas permeation was reduced, more factory floor space would be required for aging of the foam prior to cutting. Thus, a means to activate reduction of gas permeability in foams at a time of one’s choosing is highly desirable. Numerous researchers have demonstrated that reducing free volume and chain mobility in polymers reduces overall permeability of penetrant molecules. In the extreme case for chain mobility, Michaels7 demonstrated crystalline polyolefins have significantly lower permeability than their amorphous analogues. More recently Lin8 has shown similar behavior for crystalline and amorphous poly(ethylene oxide)s. Within the amorphous phase, both reductions in free volume and chain mobility combine to drive permeability lower, as shown by Muruganandam for substituted polycarbonates and separately by McHattie for substituted polysulfones.9,10 One way to reduce chain mobility is to cross-link polymer chains. Lin8 has demonstrated by increasing cross-linking density in poly(ethylene oxide)s that fractional free volume is reduced, which also reduced permeability. Bisazides, specifically [diphenyl-4,4′-di(sulfonyl azide)] (BSA; Figure 1), have

discusses using UV to initiate nuclei in CO2-rich polystyrene films. They claim reduced diffusion from the UV exposed sample (foam having a 0.02 mm cellular structure) versus an unexposed sample, stating “cross-linking suppresses polymer chain mobility, which decreases diffusivity and increases viscosity”. Malwitz19 explored UV initiation of cross-linking prior to foaming. He used a transparent body between an extruder and die to expose a mixture of low-density polyethylene (LDPE), photoinitiator, cross-linker, and isobutane to UV prior foaming. Lower densities and finer cells were obtained for this flexible foam. Many groups have also shown that cross-linking polymers can significantly influence molecular transport. Patil20 shows that cross-link density can affect gas diffusivity while only slightly affecting solubility in amine-modified epoxy resins. Matteucci21 shows that thermally induced BSA cross-linking can yield membranes not affected by CO2 plasticization, thus useful for selective separation of hydrocarbons. Jia, and separately Kelman, had shown that BSA cross-linked polyacetylenes exhibited improved plasticization resistance, but also a loss of permeability.6,22 Although there are numerous studies in the open literature discussing blowing agent influence on closed cell foam insulative properties, with the exception of CO2, there are very few examples of actual permeation testing of blowing agent permeability in films. The results discussed in this paper describe the permeability of many common blowing agents in dense polymeric films and a method for decreasing blowing agent permeability using UV initiated bisazide cross-linking of polystyrene and styrene−acrylonitrile copolymers (SAN).

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Figure 1. Structure of diphenyl-4,4′-di(sulfonyl azide) (BSA).

EXPERIMENTAL SECTION Raw Materials. Polystyrene is nominal Mw = 195 kDa, with Mw/Mn = 2.6, and is produced by The Dow Chemical Co. SAN (8 wt % AN) is Mw = 141 kDa, with Mw/Mn ∼ 2.2; SAN (15.5% AN) is Mw = 141 kDa, with Mw/Mn ∼ 2; SAN (27 wt %AN) is Mw = 87 kDa, with Mw/Mn ∼ 2; all are produced by The Dow Chemical Co. Diphenyl-4,4′-di(sulfonyl azide) (CAS No. 7456-68-0) is produced by The Dow Chemical Co. Molecular Melt is DPO-BSA/1010 Molecular Melt produced by Dynamit Nobel GmbH Novasep Synthesis. This product is a blend of BSA and Irganox 1010. Irganox 1010 is added as UV and thermal stabilizer. Per SDS, 10 wt % ≤ BSA loading ≤ 35 wt %. For convenience, it is assumed Molecular Melt is 25 wt % BSA. Blowing agents for these experiments include HFC-134a [1,1,1,2-tetrafluoroethane] sourced from Arkema, HFC-152a [1,1-difluoroethane] sourced from Chemours, and HFO1234ze(E) [trans-1,3,3,3-tetrafluoroprop-1-ene] sourced from Honeywell. Polystyrene is nominal Mw = 195 kDa, with Mw/Mn = 2.6. INEOS Styrolution Lustran DN59 SAN is 29.5% AN with a melt flow rate of 25 g/10 min based on ASTM D1238 (230 °C/3.8 kg). HBCD is Great Lakes HP-900 hexabromocyclododecane. ECN is Huntsman Epoxy Cresol Novalac resin ECN-1280. Physical properties for the blowing agents are listed in Table 1. Thin Film Penetrant Studies Using HFC-134a, HFC152a, and HFO-1234ze(E). Samples of polymer (5 g) and, when appropriate, a target amount of BSA were dissolved in 20 mL of chloroform solution and stirred for up to 18 h. The

been demonstrated to reduce fractional free volume, chain mobility, and permeability of penetrant gases in disubstituted polyacetylenes. Kelman11 proposed a reaction where the activated azides lost N2 and the resulting nitrene structure stripped a hydrogen atom from poly(1-trimethylsilyl-1propyne) such that the internitrene groups bridged between two polymer chains. The resulting polymer exhibited a permeability reduction of around 25% for CH4 and similar reductions in permeability toward other gases. Monsanto patents US2518249, US2532242, and US2532243 discuss using BSA as a chemical blowing agent for cellulose acetate, styrenic, and polyethylene foams, respectively.12−14 They teach that cellulose acetate, when mixed with 4 wt % BSA used as chemical blowing agent and heated in an open mold at 140 °C for several hours, produced a foam that was insoluble in acetone. Polystyrene (PS), when mixed with 4 wt % BSA and heated at 145 °C for 1 h, produced a yellow foam with a specific gravity of 0.4, and polyethylene mixed with 4 wt % BSA produced a gray foam after heating in an open mold at 140 °C for 80 min. There are numerous examples of using cross-linking agents, and specifically BSA, to modify molecular weights of resins. Rubens15 discusses use of difunctional divinylbenzene as a cross-linker for polystyrene, and shows divinylbenzene affects both MW properties of the resulting polymer and swelling in toluene as polymerization and cross-linking are occurring simultaneously. Chaudhary generates extruded polystyrene (XPS) foam examples containing 0.1 wt % BSA to thermally induce cross-linking.16 Silvis et al.17 use a cyclic phosphazene azide to rheologically modify a syndiotactic polystyrene. Kajii18 11015

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Industrial & Engineering Chemistry Research Table 1. Physical Properties for the Blowing Agents23−25 blowing agent

critical vol (dm3/kg)

critical temp (°C)

HFC-152a HFC-134a HFO-1234ze(E)

2.74 1.94 2.05

113.5 101.1 109.4

After a day the sample is sufficiently solid that it can be removed from the dish. The unit used for UV exposure was a QUV Weathering Tester Model QUV/se. Foams were placed 5 cm from the surface of the bulb and mounted parallel to the plane of the bulbs. Bulbs are Q-Lab Corp. number UVB-313 EL. O2 permeation was measured according to ASTM D3985 using Mocon Ox-Tran 2/20 units with 0% relative humidity. Preparation of Molded Cellular Articles. Expansion of beads within molds is one means to produce foam boards for the building and construction market. These products are referred to as expanded polystyrene (EPS). Production of EPS involves several steps: • imbibing of blowing agent (typically n-pentane) within a polymer matrix by either solution polymerization or extrusion process (beads produced are typically 0.5−1 mm in size.) • aging of the beads • pre-expansion of the beads to a nominal 1−2 mm size • aging of the pre-expanded beads to allow for infiltration of atmospheric gases • molding of pre-expanded beads to the desired shape The below methodology was followed to produce molded cellular articles containing Molecular Melt. Sieve the above PS ground material using a series of nested screens to collect particles having a particle size between 0.5 and 1 mm. A 75 g sample of these sieved materials was then placed into a 2 L Parr-type reactor maintained at 25 °C via an oil bath. The vessel was sealed and air was evacuated. About 1000 mL of deionized water was loaded into the vessel through a feed port, and about 21 g of n-pentane (nC5) was added after the agitator was initiated. Sufficient carbon dioxide was added to achieve a reactor pressure of ∼100 psi (690 kPa). After 1 day the system was vented while under agitation. The ground material was collected using a strainer and placed between two screens to dry using an air nozzle. Product was stored in a refrigerator until use. The imbibed materials were further sized using a Glen Mills P-15 knife mill equipped with a 1 mm screen, where dry ice was used to avoid heat generation within the mill. An annealing step was added to improve cellular nucleation due to any CO2 which might be solubilized within the resin. A stir bar and deionized water were added to a large beaker. Milled material was added to the beaker with water preheated to between 60 and 70 °C while agitating. The imbibed resin was allowed to anneal for 45 min. The annealed materials were then strained, cooled with deionized water, and dried between two sieves using an air nozzle. The desired mass of annealed material was added to a large lidded vessel of deionized water maintained at greater than 97 °C equipped with a fine mesh basket, and the container lid was closed. After the desired time, the lid and basket were removed. Pre-expanded material was placed into a large fiberboard box and allowed to equilibrate for no less than 12 h. A 15.2 × 15.2 × 5 cm steam mold was loaded with preexpanded material until the mold was full. The mold was closed and steam injected to yield an internal mold temperature greater than 101 °C. After 1.5 min, steam flow was ended and the mold was spray-quenched with water. Molded articles were removed from the mold and allowed to equilibrate for 1 day. Finally a band saw was used to remove surface skins of molded articles. Foam slices were exposed to UV-B for 2 min with an irradiance of 1.23 W/m2 using a QUV Weathering Tester

solution was poured into a clean, dry, level PTFE Petri dish and allowed to dry at ambient conditions in a fume hood. To slow down the drying process, the Petri dish was partially covered by a second PTFE Petri dish. Generally samples were dry after 18 h of casting. Films were exposed to UV-C (254 nm) for 60 s using a Spectroline Model XX-15F UV-C lamp. Both unexposed control films and UV-C exposed films were kept in light-blocking envelopes. A constant volume/variable pressure permeation described elsewhere,26,27 was used to determine pure gas permeability for all test gases. All samples were exposed to vacuum for at least 16 h at 35 °C to degas. After exposure to vacuum, the leak rate was determined by closing both the upstream and downstream volumes to vacuum and feed gases. The rate of pressure increase was determined over a period of 5 min after the cell had been isolated for at least 1 h. Acceptable leak rates were approximately 3 × 10−3 Pa/s or below. After an acceptable leak rate had been obtained, samples were exposed to a single blowing agent at 41 kPa until the rate of pressure increase had reached steady state (i.e., less than 3% change in pressure increase over a period of at least 30 min). Between gases the upstream and downstream volumes were evacuated using a vacuum pump for at least 16 h to degas. Results are reported as fractional flux (FF): P FF = A PB where PA is the permeability after UV exposure and PB is the permeability before UV exposure. Values less than 1 denote reduction in permeation upon UV exposure. Thin Film Penetrant Studies Using Oxygen. Typical atmospheric gases have higher thermal conductivity than the blowing agents described in Table 1. Atmospheric gases penetrate foam due to pressure imbalance or chemical potential gradients between the cellular structure and atmosphere. Thus, it is desirable to understand oxygen mobility as related to UV activated BSA in both PS and SAN films. Into a 1 L Erlenmeyer flask add 500 mL of methylene chloride (MeCl2), 0.81 g of HBCD, and 2 g of Molecular Melt. Agitate until materials are dissolved. Add 0.5 g of ECN and 100 g of polymer. Agitate until dissolved. Pour contents into PTFE pans. Allow MeCl2 to evaporate overnight. Flip sample and place the sample in a vacuum oven set at ambient temperature. Evacuate oven (minimum of 85 kPa of vacuum) and allow the remainder of MeCl2 to evaporate over 20 h. Grind the dried product using a Wiley Mill equipped with a 3 mm screen. The procedure to produce thin films from the above ground material is to dissolve the ground material into tetrahydrofuran (THF). This is done by placing ∼1 g of the blend material into a small glass bottle and then adding 12 mL of THF. The bottle is then placed on a shaker table for several hours of agitation. The material is then poured into a PTFE Petri dish. A watch glass is placed on top to control the rate of THF evaporation. 11016

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Industrial & Engineering Chemistry Research Model QUV/se. Foams were placed 5 cm from the surface of the bulb and mounted parallel to the plane of the bulbs. Bulbs are Q-Lab Corp. number UVB-313 EL. Thermal conductivity (λ) was measured with a heat flow meter (Lasercomp, TA Instruments) according to ASTM C518-10 at 14 and 180 days. A typical thermal conductivity value for an expanded polystyrene foam (EPS) is ∼32 mW/ (m·K). Typically thermal conductivity increases as air infiltrates the foam structure.

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RESULTS AND DISCUSSION In order to have an estimate of the mass transport behavior of blowing agents in foamed systems, the pure gas permeabilities of blowing agents in styrenic based polymers were determined for differing levels of BSA, as shown in Figure 2, and the Figure 3. Fractional flux vs AN content at constant BSA loading of 0.25 wt %, 35 °C, and 41 kPa.

although the reaction is still able to have significant influence on permeability even when AN is not present. This data demonstrates the potential for reducing blowing agent permeation in PS and styrene−acrylonitrile based foams. Based on these results, it is plausible to suggest the nitrene insertion reaction favors the carbon neighboring the acrylonitrile group rather than the carbon neighboring the phenyl group (Ph). Possible structures for the resulting crosslinked polymer are shown in Figure 4, where Figure 4a

Figure 2. Fractional flux vs bisazide loading in SAN (8 wt % AN) at 35 °C and 41 kPa.

permeability data for the pure polymer are presented in Table 2. As defined above, fractional flux (FF) is normalized by the Table 2. Permeabilities of Common Blowing Agents for Untreated Polymers in SAN (8 wt % AN) at 35 °C and 41 kPa blowing agent

permeability (barrer)a

HFC-152a HFC-134a HFO-1234ze

4.0 6.0 6.6

1 barrer = 10−10 (cm3(STP) cm)/(cm2 s cmHg).

a

Figure 4. Possible structures of bisazide cross-linked (a) SAN and (b) polystyrene.

blowing agent permeability exhibited by the BSA filled film before UV treatment. At the UV treatment conditions discussed above, FF was observed to decline by almost 90% even at BSA loadings of 0.0625 wt %. The influence of BSA loadings decreases at higher concentrations, where the FF ranges from about 0.01 to 0.03, depending on blowing agent. Interestingly, the extent of FF reduction does not appear to relate to blowing agent size or condensability, but seems rather uniform across these materials. Figure 3 presents the behavior of the blowing agents with a variation in acrylonitrile (AN) content while BSA content is held at 0.25 wt %. In pure PS the influence is moderate, with a FF around 0.2. However, as the AN content increases the FF decreases to around 0.03−0.04 at 27% AN content. These data suggest the BSA reaction is influenced by the presence of AN,

proposes a structure for cross-linking where acrylonitrile is the neighboring pendant group, and Figure 4b, where the phenyl group is neighboring the nitrene insertion. Regardless of how the cross-linking structure actually occurs, the cross-linking of both SAN and polystyrene has demonstrated reduced permeability of common blowing agents in styrenic polymers. Atmospheric gases such as N2 and O2 have thermal conductivities greater than typical blowing agents. It is advantageous to likewise minimize infiltration of these gases. Table 3 shows that, for both PS and SAN films containing BSA, oxygen fractional flux can be reduced by UV exposure. However, after an initial 30 s exposure to UV at 0.35 W/m2, 11017

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Industrial & Engineering Chemistry Research Table 3. Oxygen Specific Fractional Fluxa resin system

film thickness (μm)

UV exposure (min)

irradiance (W/m2)

cumul UV exposure (min)

oxygen FF

PS PS PS SAN SAN SAN SAN SAN SAN SAN

76 76 76 53 53 60 60 60 60 60

none 0.5 0.5 none 0.5 none 0.5 0.5 0.5 0.5

N/A 0.35 0.35 N/A 0.35 N/A 0.35 0.35 1.23 1.23

N/A 0.5 1 N/A 0.5 N/A 0.5 1 1.5 2

1 0.61 0.67 1 0.73 1 0.71 0.68 0.65 0.59

gases such as HFC-134a and HFO-1234ze(E). It would thus be expected that the phenomenon revealed herein would be more substantial with these larger molecule gases.

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CONCLUSIONS This body of work reveals that permeation of gases typically used for EPS and XPS are retarded by inclusion of crosslinking agents that are active near the acrylonitrile or phenyl pendant groups. In this case the cross-linking agent is diphenyl4,4′-di(sulfonyl azide) which was activated by UV-B and UV-C types of radiation. The UV-B readily penetrated 13 mm of extruded foam (XPS) and still activated the permeation retardation mechanism. Furthermore, we have shown a retardation of the thermal conductivity increase typically observed in cellular materials upon activation of the BSA.

a

Error in FF is expected to be around 10% of experimental value.

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FF appeared to stabilize. For example, films exposed to UV did not exhibit a significant FF trend with increasing exposure time at a given irradiance. Even when the SAN film was exposed to an additional 0.5 min at almost 3-fold increase in irradiance, the FF did not significantly change. As such, it is reasonable to believe the 30 s UV treatment at 0.35 W/m2 is sufficient to initiate all or nearly all of the BSA available for reaction. For a foam application, it is desirable that the UV permeate the foam so that azide modification can occur throughout the bulk. To determine if the UV has sufficient penetrability into a styrenic foam, film samples were masked by 13 mm thickness of extruded styrenic foam product (XPS) prior to fluxing. Table 4 shows that that permeation of the film can be affected

Corresponding Author

*Tel.: 989-636-2969. Fax: 989-636-0194. E-mail: LSHood@ dow.com. ORCID

Lawrence S. Hood: 0000-0001-5137-2267 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Robert A. Gunther of The Dow Chemical Co. for performing the oxygen fractional flux measurements. Additionally the authors acknowledge the financial support of The Dow Chemical Co. and permission to publish this work.

Table 4. Oxygen Fractional Flux for Samples Masked by XPS resin system

film thickness (μm)

UV exposure (min)

irradiance (W/m2)

cumul UV exposure (min)

oxygen FF

SAN SAN SAN

53 53 53

none 2 2

N/A 1.23 1.23

N/A 2 4

1 0.76 0.44

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Table 5. Change in Thermal Conductivity (λ) for an EPS Foam Containing BSA λ at 14 days (mW/m·K)

λ at 180 days (mW/m·K)

absolute change in λ (mW/m·K)

none UV

33.77 34.06

33.79 33.47

0.02 −0.59

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even when it is masked by XPS. These results can be compared to the SAN sample in Table 3 (i.e., cumulative UV exposure 2 min at 1.23 W/m2), which was unmasked. This sample had an O2 FF of 0.59, which was lower than the closest equivalent masked samples, which exhibited O2 FF of 0.76. Clearly, some of the potency UV initiation is lost by masking. However, with sufficient UV treatment time the FF can fall below that of the unmasked sample. These results suggest articles greater than the thickness of a film can benefit from this azide−UV modification. Table 5 shows a favorable reduction in thermal conductivity (λ) upon UV exposure. Based on the aforementioned thin film studies, it would be expected that retardation of thermal conductivity increases would be greater for SAN-based resins. n-Pentane is a smaller molecule than low thermal conductivity

UV

AUTHOR INFORMATION

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