Grantee Research Project Results

2013 Progress Report: Novel 'Greener' Routes to Halogen-Free Flame Retardant Materials

EPA Grant Number: SU835071
Title: Novel 'Greener' Routes to Halogen-Free Flame Retardant Materials
Investigators: Nagarajan, Ramaswamy , Kumar, Jayant , Ravichandran, Sethumadhavan , Bouldin, Ryan , Kiratitanavit, Weeradech , Xia, Zhiyu
Institution: University of Massachusetts - Lowell
EPA Project Officer: Page, Angela
Phase: II
Project Period: August 15, 2011 through August 14, 2013 (Extended to August 14, 2014)
Project Period Covered by this Report: August 15, 2012 through August 14,2013
Project Amount: $75,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2011) Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Chemical Safety , P3 Awards , Sustainable and Healthy Communities

Objective:

• Scale-up of polyphenol synthesis more specifically the synthesis of polycardanol.
• Incorporation/blending of polycardanol with polyolefins and evaluation of thermal stability and flame retardancy using TGA and PCFC, respectively.
• Development of new synergistic FR, combining polyphenols with organically modified nano-clays and metal oxide for efficient gas phase and condensed phase action in polyolefins.
• Study of degradation kinetics of polyphenols using Thermogravimetric Analysis - Fourier Transform Infrared Spectroscopy (TGA/FTIR).
• Evaluation of toxicity of polycardanol.

Progress Summary:

i)    Reaction scale up

While designing a pilot-scale reactor for the synthesis for the scaling up of a lab scale procedure, the ratios of the reactants and oxidants do not remain the same at higher reaction volumes. This in turn leads to a reduction in the final yield. This may also result in a change in the structural and thermal properties of the final product. Figure 1(Left) shows the yield versus oxidant/monomer ([O]/[M]) ratio in 5 Liter reactor vessel [Figure 1. (right)] for the synthesis of polycardanol. An oxidant to monomer ratio of 2 was employed for all 5 L reactions to obtain maximum yields. Purification of the final product was done by soxhlet extraction for 24 hours. After drying the product, was ground using a custom made household nut blender into micron-sized particles prior to blending. Polycardanol after particle size reduction was used for all blending studies.

Figure 1. (Left) Scale-up reaction optimization; (Right) Scale-up reaction set-up

(ii)   Blending of polycardanol with polyolefins

Polycardanol was compounded with polyolefins using a CW Brabender type 6 mixer. At optimized blending conditions, the barrel was maintained at a constant temperature of 185 and 195°C for LDPE and PP resins respectively. Blends containing 0, 1, 5, 10 and 15 weight % of polycardanol were prepared by thermal blending for 10 min. at mixing speed 60 rpm. Thermal characterization was done on these compounded samples.

TGA: Thermogravimetric analysis of polycardanol/polyolefin blends is presented in Figure 2. Virgin PP and PE undergo thermal degradation under nitrogen atmosphere at around 463^oC and 473^oC. When polycardanol is added into these systems, there is an appreciable increase in the temperatures (T_d) at which decomposition occurs and a marginal increase in char forming capability of these blends. In case of LDPE, there is up to a 30^oC increase in T_d and 3.2% char yield upon 15 weight % addition of polycardanol. The increment in char yield for PP blended with polycardanol is slightly higher than that for LDPE blends, with a 35^oC increase in T_d and 4.2%, increase in char yield. In both cases, the beneficial effects start plateauing after 10-weight % additive loading. At 15 weight % loading of polycardanol in PP, there is a deterioration of thermal properties in comparison to 10 weight %. It was surmised that 10 weight % loading is the optimum weight/performance ratio for polycardanol/polyolefin combinations.

Figure 2. TGA thermogram of PP and LDPE blends with polycardanol

PCFC: PCFC or micro-scale combustion calorimetry was used to calculate total hear release (THR) and Heat release capacity (HRC) values which are predictors of polymer flammability.

a.     Total heat release: Polycardanol/Polyolefin blends show a 9% and 7% reduction in total HR for LDPE and PP, respectively. This amount of reduction is comparable to previously reported values for the effect of phosphorous based flame retardants on polyolefins.[1] However, at this point, the type and nature of the condensed phase reactions in polyolefin/polycardanol blends which leads to THR increase is not very well understood.

b.     Heat Release Capacity: In PP/polycardanol and LDPE/polycardanol blends, maximum degradation of the virgin material occurs at its peak heat release rate (pHRR) of 471^oC and 480^oC respectively. However, with increase in polycardanol incorporation, there is a decrease in pHRR as well as HRC (except LDPE with 15% polycardanol that may have phase separation problem so HRC and pHRR of this blend is decreased). It is hypothesized that the polycardanol strongly influences the decomposition of polyolefins either in the gas and/or condensed phase increasing the peak degradation temperature. Polycardanol also delayed the complete decomposition of polyolefins to a higher temperature. This kind of behavior is analogous to those previously observed for lignin/PP blends.[2] Analysis of thermal decomposition products of these blends will be beneficial in understanding the reason for FR efficacy with polycardanol incorporation. This analysis is currently being performed using TGA-FTIR. 

Polymer	THR (KJ/g)	HRC (J/g-K)	% Decrease in HRC
LDPE	45.7	1273	-
LDPE + 1PC	44.9	1173	7.9
LDPE + 5 PC	43.0	1145	10.1
LDPE + 10 PC	41.0	1115	12.4
LDPE + 15 PC	41.6	1149	9.7
PP	40.6	1288	-
PP + 1PC	42.5	1151	9.8
PP + 5 PC	39.7	1100	14.6
PP + 10 PC	39.7	1056	18.0
PP + 15 PC	37.8	986	23.4

Table 1. PCFC results summary of polycardanol/polyolefin blends

Figure 3. Comparison of HRC of polycardanol/PP blends with DBDPE/PP and HBCD/PP blends

Figure 3 shows HRC comparison of polycardanol, commercially available brominated FR decabromodiphenylether (DBDPE) and hexabromocyclododecane (HBCD) at increasing loading levels in PP blends. In case of DBDPE, which is one of the most efficient halogenated FR, there is up to a 50% reduction in HRC with respect to virgin PP at 15% loading. HBCD, which is a moderate FR for PP is often used in combination with metal hydroxide additives for synergistic action.[3] In our study, HBCD showed a 24% reduction in HRC at 15% loading. Chemically synthesized polycardanol was efficient in the reduction of HRC, with up to 23% reduction with 15% additive incorporation. It is interesting to note that polycardanol shows promise in terms of improving the thermal stability of PP (increasing the thermal-decomposition temperature).

(iii)  Synthesis of synergistic combinations of inorganic nanoparticles and polyphenols

Figure 4. Proposed synthesis route for polyphenol-nanoclay synergists

Nanoclay and metal oxides have been previously shown to decrease rate of heat release and also HRC of polymer materials. However, most halogenated FR or metal oxides are not affective when used alone. Synergistic combinations are often more effective. In the case of nanoclays synergy can be obtained by physical adsorption of phenol on organically modified nanoclays followed by subsequent polymerization to create polyphenol nanocomposites (Figure 4). Another approach could be the decoration of surface of inorganic nanoparticles such as Titania with phenols followed by polymerization.

In the case of polyphenol-cloisite Na⁺, the composites were synthesized using in-situ polymerization of un-substituted phenol in the presence of cloisite Na⁺ in a mixed solvent system (alcohol and buffer). Horseradish peroxidase (HRP) enzyme is used as a catalyst and H₂O₂ is used as the oxidant for the synthesis of polyphenol modified clay. FTIR results (not shown) indicate mild intercalation of polyphenols into the both inorganic substrates.

For polyphenol-titania system, carboxylic acid functionalized phenol was adsorbed on the surface of the nanoparticle and subsequently polymerized. Alternatively, soluble polyphenol containing carboxylate functional groups was electrostatically anchored on the titania surface by dispersing (along with titania) at the appropriate pH.

Pyrolysis combustion flow calorimetry was used to characterize these materials. The results indicated that, polyphenol itself is a thermally stable material with a char yield of 33% at 750^oC and a low HRC (56 J/g-K) while acid functional polyphenol exhibits a char yield of 45% at 750^oC and HRC of 14 J/g-K. Cloisite Na⁺ and titania do not release heat upon combustion. Upon incorporation of polyphenol with cloisite Na⁺, there is heat released from the modified clay due to the contributions of the polyphenol (approximately 15% by weight of polyphenol in the modified clay). The final HRC of modified clays is roughly 6.5 J/g-K. However, no HRC peak is observed for polyphenol-titania composite although 10% by weight of polyphenol is incorporated onto titania surface.  

Characterization of blends of polyphenol-modified clay with Polypropylene: By blending PP, with polyphenol modified inorganic nanoparticles, HRC of PP can be decreased as shown in Figure 5 and 6. For, polyphenol-nanoclay composite system, the use of unmodified hydrophilic cloisite Na⁺ alone with PP did not reduce the HRC. This is due to the incompatibility of unmodified nanoclays with non-polar polymers like PP, resulting in a totally phase-separated blend. However, the use of polyphenol-cloisite Na⁺ composite helped increase the hydrophobicity of the clay and resulted in up to a 20% decrease in HRC with respect to virgin blends. For polyphenol-titania composite, the HRC is increased with increasing additive loading compared to PP blended with titania alone. Titania may also cause degradation of PP through random chain scission mechanism as reported for metal oxide-PP system.[4] However, in case of titania modified by polyphenol, the HRC decreases with increasing additive loading possibly due to gas-phase action of phenolic moieties in the polyphenol-titania complex.

Figure 5. Comparison of HRC of PP-cloisite Na+ versus PP-modified clay

Figure 6. Comparison of HRC of PP-titania versus PP-modified titania

(iv)  Study of degradation kinetics of polyphenols using Thermogravimetric Analysis - Fourier Transform Infrared Spectroscopy (TGA/FTIR)

Combination of thermogravimetric analysis and Fourier-transform infrared spectroscopy (TGA-FTIR) serves as a very useful tool for the possible determination of the degradation pathway of polymers, and the influence of FR additives on the degradation pathway. This tool has been previously used to monitor the degradation of several polymeric systems like polystyrene and polymethylmethacrylate.[5]

In this experiment, TGA equipped with a TGA-FTIR interface and an infrared spectrometer is used. The chamber inside the interface allows volatiles evolved from TGA furnace to flow through and allow the infrared beam to pass through to obtain the FTIR spectrum. Both the chamber and transfer line are heated to prevent the condensation of volatiles. The data obtained from infrared spectrometer are composed of IR spectra measured at constant time intervals. Gram-Schmidt chromatogram is used to represent the total infrared absorbance of volatiles versus time. It is calculated directly from the interferograms which are the original signals detected by the spectrometer before performing the Fourier transform. TGA-FTIR tests were conducted on Polycardanol, Polypropylene and Polypropylene/10% Polycardanol. The results are shown below.

Figure 7. TGA curves (up) and DTG curves (down) of polypropylene, polycardanol and polypropylene/10%polycardanol

Materials	Peak degradation temperature (oC)	Weight percent of Char remaining at 750 oC (%)
Polypropylene	470	0
Polycardanol	467	28.8
Polypropylene/10% Polycardanol	484	1.4

Table 2. Comparison of peak degradation temperature and char remaining among these three materials

In the TGA results (as shown in Figure 7 and Table 2), the addition of 10% polycardanol increased the peak degradation temperature of polypropylene about 15 ^oC. The addition of 10% by weight of polycardanol is the optimum. However the amount of char formed in samples with varying amounts of polycardanol blended into PP does not scale linearly with the amount of polycardanol added.

The Gram-Schmidt curves for evolution of gaseous products during the thermal decomposition is plotted in Figure 8.

Figure 8. The evolution of gaseous products for PP, polycardanol and PP/10%polycardanol

The peak height at specific wavenumber represents the absorption of characteristic compounds or functional groups. In the decomposition of polypropylene, both alkene and alkane were found. The volatiles are as shown in Figure 8. With addition of polycardanol, the release of gaseous products in polypropylene shifted to higher temperature. However the addition of polycardanol did not change the major gaseous products with the exception of the appearance of carbon monoxide due to the decomposition of aromatic entities.

(v)   Evaluation of toxicity of polycardanol

One of the goals of the project was to explore the possibility of developing non-toxic alternatives to toxic halogenated FR. While polycardanol has been shown to exhibit good char forming characteristics when blended into polyolefins it is also important to ascertain whether the newly developed is non-toxic. Although the monomer (cardanol) has been reported to be a skin irritant but biodegradable (96% of original weight degraded within 28 days) the toxicity of the polymer of cardanol needs to be evaluated.[6]  The molecular structure of polymer is different from that of the monomer and therefore the toxicity profile can be quite different.[7] To evaluate polycardanol as possible non-halogenated FR for plastic, the toxicity of this FR was evaluated in accordance to the Organization for Economic Co-Operation and Development (OECD) 425: guidelines for the testing of chemicals, Acute Oral Toxicity (up and down procedure) by Toxicon Corporation. The result from this test provided the LD₅₀ value.

These tests were not carried out at UML but were carried out by a third-party lab namely Toxicon Corporation. Five female Sprague Dawley rats were selected by Toxicon as test animals. Polycardanol at three different weights (0.1, 0.3 and 1 g) were mixed with cotton seed oil (CSO) at three different doses (175, 550 and 2000 mg/kg, the unit 'mg' is for the weight of polycardanol and the unit 'kg' is for the body weight of test animal). Then the mixture was fed to the test animals (175 and 550 mg/kg doses were used on animal #1 and #2 respectively and the dose at 2000 mg/kg was used on animal #3 - #5). After feeding, the test animals were observed for their response in first 4 hours and subsequently daily for the following 14 days. For the purpose of evaluation of acute toxicity, all abnormalities (including behavioral and clinical), gross lesion, bodyweight changes, effects on mortality and any other toxic effects were observed, measured and recorded. The LD₅₀ value and the confidence intervals were calculated and determined by using the statistical computer program (AOT425StatPgm) developed by the EPA.

The toxicity test results are shown in Table 3. It was found that the weight of all test animals increased and no unusual behaviors were observed during the test period. None of the animals dosed at three different levels died during the tests. The LD₅₀ of polycardanol was estimated to be greater than 2000 mg/kg.

Animal #	Polycardanol Dose (mg/kg)	Body weight (g)				Clinical Observations	Necropsy Observations
		Day 0	Day 7	Day 14	Weight Change
1	175	201.3	213.2	224.4	23.1	Normal	Normal
2	550	210.5	239.8	247.7	37.2	Normal	Normal
3	2000	230.4	258.7	264.6	34.2	Normal	Normal
4	2000	233.2	251.4	255.4	22.2	Normal	Normal
5	2000	240.1	253.6	267.1	27.0	Normal	Normal

Table 3. Dosing, animal weights, clinical observations and necropsy data

Future Activities:

In the phase-II of the EPA funded P3 project, the team has scaled up (to several hundred gram scale) the synthesis of polycardanol. The low cost, bio-derived phenol -cardanol could be polymerized to yield polycarandol with improved thermal stability and reasonable compatibility with polypropylene. Polycardanol could be easily dispersed into polyolefins such as PP and the blends, exhibited higher degradation temperatures, lower HRC and higher char yield. The performance of this polyphenol compares favorably with some commercially available halogenated FR (such as DBDPE and HBCD) purely from an heat release capacity (HRC) perspective. TGA-FTIR evolved gas analysis studies indicate that the release of inert carbon dioxide and formation of char contributes to the lower HRC of polycardanol and PP-polycardanol blends. Polycardanol also seems to retard/delay the release of hydrocarbon based degradation compounds from polypropylene upon heating. Polycardanol is also non-toxic toxicity evaluation done by OECD 425 method, polycardanol has estimated LD₅₀ greater than 2000 mg/kg.

References:

1. Ren, Q., Zhang, Y., Li, J., Li, J.C. 2011. Synergistic effect of vermiculite on the intumescent flame retardance of polypropylene. J. Appl. Polym. Sci. 120:1225-1233.
2. De Chirico, A., Armanini, M., Chini, P., Cioccolo, G., Provasoli, F., Audisio, G. 2003. Flame retardants for polypropylene based on lignin. Polym. Degrad. Stab. 79:139-145.
3. Tolinski, M. 2009. Additives for polyolefins: getting most out of polypropylene, polyethylene and TPO. Elsevier Inc., Burlington.
4. Shen, L., Chen, Y., Li, P. 2012. Synergistic catalysis effects of lanthanum oxide in polypropylene/magnesium hydroxide flame retarded system, Composit, Part A, 43:1177-1186.
5. Wilkie, C.A. 1999. TGA/FTIR: an extremely useful technique for studying polymer degradation. Polymer Degradation and Stability, 66, 301-306
6. Ravichandran, S., Bouldin, R.B., Kumar, J., Nagarajan, R. 2011. A renewable waste material for the synthesis of a novel non-halogenated flame retardant polymer. J. Clean. Product. 19: 454-458.
7. Weideli, H.J., 2008. Aspects of plastic additives related to health, safety and environment. In Plastics additives handbook, 6^th Ed., Zweifel, H., Maier, R.D., Schiller, M. Hanser, Munich.

Journal Articles:

No journal articles submitted with this report: View all 6 publications for this project

Supplemental Keywords:

Enzymatic polymerization, biocatalytic, non-halogenated flame-retardants, renewable feedstock, green chemistry, biomimetic catalyst, sustainable environment

Progress and Final Reports:

Original Abstract

2012 Progress Report

Final Report

P3 Phase I:

Novel ‘Greener’ Routes to Halogen-free Flame Retardant Materials  | Final Report