Grantee Research Project Results

2012 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, 2011 through August 14,2012
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 synthesis of polyphenols (specifically polycardanol developed in phase-I).
• Incorporation/blending of polycardanol with commercial polymers such as polyolefins and evaluation of thermal stability and heat release capacity using TGA and PCFC respectively.
• Development of new synergistic FR, combining polyphenols with organically modified nano-clays for efficient gas phase and condensed phase FR action.

Progress Summary:

(i)    Reaction scale up:

While scaling-up the synthesis of polycardanol based flame retardants, the ratio of the catalyst to reactant does not remain the same for producing similar yields at higher reaction volumes. It was found that higher amount of catalyst was required for larger reaction volumes. Figure 1(a) shows the plot used for optimization of catalyst/monomer ratios in a 5 L reactor for the synthesis of polycardanol. Figure 1(b) shows the reactor used for the scaled-up. A catalyst to monomer ratio of 2 was employed for all 5 L reactions to obtain maximum yields. After drying the product, it was ground using a blender prior to blending.

(ii)   Blending of polycardanol with polyolefins

Polycardanol contain repeat units with long aliphatic m-substituted side chains that can impart hydrophobicity to the additive. Hence it was envisioned that polycardanol would blend well with polyolefins like PP and LDPE. Theoretically calculated solubility parameter using Small & Van Krevelen�s method[1] for a C-O-C-O-C coupled polycardanol trimer is 9.45 (cal cm^-3)^1/2 and is very similar to that of PP (9.4) and LDPE (8.94). There is an increase in δ for perfectly C-C coupled products due to the presence of OH groups that increase the polarity of the polymer. However, chemically synthesized polycardanol has a mixture of C-C and C-O-C coupled linkages with a calculated solubility parameter δ ~ 9.6 fairly close to polyolefins.

Figure 1a. (Left) Scale-up reaction optimization; plot of ratio of catalyst to monomer with reaction yield

Figure 1b. Photograph of the reactor

Polycardanol was compounded with polyolefins using a CW Brabender type 6 mixer. After optimization, the barrel was maintained at a constant temperature of 185 and 195°C for LDPE and PP resins respectively. Blends containing several weight % of FR were prepared and thermal characterization was done on these compounded samples.

Results of Thermogravimetric Analysis (TGA): TGA of polycardanol/polyolefin blends are shown 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 at which Td 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 Td 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 LDPE blends. 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

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

a.     Char yield: With increase in concentration of polycardanol, char formation increases in both LDPE and PP blends as shown in Table 1. However, the total char formed in each of the blend ratios is higher than the char yield generated by polycardanol alone, possibly due to condensed phase interaction between polycardanol and the polyolefin.

b.     Total heat release: Polycardanol/Polyolefin blends show an 8% and 12% 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.[2] 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.

c.     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 492^oC and 501^oC respectively. However, with increase in polycardanol incorporation, there is a decrease in pHRR as well as HRC. Additionally, in both cases, there is formation of a new peak at around 510 - 520^oC. This is very close to the polycardanol peak centered around 510^oC. 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. This behavior is analogous to those previously observed for lignin/PP blends.[3] 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 simultaneous TGA combined with Fourier Transform Infrared Spectroscopy (TGA-FTIR) study in year 2 of Phase II. 

Polymer	THR (KJ/g)	Char Yield (%)	HRC (J/g-K)	% Decrease in HRC
LDPE	40.3   0.3	0	1375   28	-
LDPE + 1PC	40.2   0.2	2.2	1310   62	4.7
LDPE + 5 PC	38.6   0.2	4.5	1254   25	8.8
LDPE + 10 PC	37.2   0.2	6.2	999   20	27.4
LDPE + 15 PC	38.6   0.1	4.8	1070   43	22.2
PP	40.1   0.2	0	1250   35	-
PP + 1PC	39.8   0.3	2.2	1081   30	13.52
PP + 5 PC	37.8   0.2	3.9	990   22	20.8
PP + 10 PC	35.8   0.1	9.4	910   14	27.2
PP + 15 PC	35.5   0.1	9.2	840   13	32.8

Table 1. PCFC results summary of polycardanol/polyolefin blends

(iii)  Synthesis of synergistic FR combinations

Figure 3. Proposed synthesis route for polyphenol-nanoclay synergists

The use of nanoclays has been previously shown to decrease heat release rate of polymeric materials. Nanoclays can be intrinsically hydrophilic and can be modified using long aliphatic alkyl chains. However from the perspective of flame retardancy, these alkyl chains are not beneficial and contribute to decomposition and reduction in time to ignition when incorporated in polymer-clay nano-composites. The use of a thermally stable hydrophobic modification to the nanoclays can be beneficial for their use in FR applications. This synergy can be obtained by physical adsorption of phenol on organic nanoclays followed by subsequent polymerization (Figure 3). It is also worth mentioning that nanoclays are environment friendly mineral additives widely used in a lot of food and cosmetic applications to ensure good flow properties.[4]

Polymer	HRC (J/gK)	Total HR (kJ/g)	Char (%)
Clay	N.A.	N.A.	90.0 ± 1.9
Polyphenol	56.0 ±  0.7	5.1 ± 0.1	33.2 ± 1.2
Clay + Polyphenol	6.5 ±  0.8	0.5 ± 0.1	74.7 ± 1.9

 

 

 

 

 

 

 

Table 2. PCFC results summary for cloisite Na+ and polyphenol-cloisite Na+ composite

Figure 4. Comparison of pHRR of PP-cloisite Na+ versus PP-polyphenol modified cloisite Na+ blend

Polyphenol-cloisite Na+ composites are synthesized using in-situ polymerization of un-substituted phenol in the presence of cloisite Na+ in a mixed solvent system (alcohol and buffer). HRP enzyme is used as a catalyst and H₂O₂ is used as the oxidant for the synthesis of polyphenol modified clay. FTIR and XRD results (not shown) indicate mild intercalation of polyphenols into the nanoclay platelets.

Table 2 summarizes the results from PCFC of cloisite Na+ and modified clay. Polyphenol by itself is a thermally stable material with a char yield of 13% at 750^oC and a low HRC (56 J/g-K). Cloisite Na+ does 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. The final HRC of modified clays is roughly 6.5 J/g-K and is due to the polyphenol. This increase is proportional to approximately 15% by weight of polyphenol in the modified clay and is in agreement with TGA results. 

It is also observed that polyphenol modification decreases the pHRR as shown in Figure 4. The use of unmodified hydrophilic cloisite Na+ alone with PP did not reduce the pHRR. 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 pHRR with respect to virgin blends.

(iv)  Work in progress

• The mechanism of action of polycardanol based FR is investigated using (TGA-FTIR studies)
• Toxicity analysis of polycardanol based FR are underway
• The scope of the project has been expanded to include other tests (such as limiting oxygen index). To this effect we will obtain the appropriate equipment for carrying out these tests.

Future Activities:

• The reaction for synthesis of polycardanol-based FR was optimized and scaled-up to 100 gram scale
• Polycardanol has been blended with polyolefins. Polycardanol is reasonably compatible with PP and LDPE.
• Preliminary investigations (using TGA and PCFC) have revealed that polycardanol when blended with PP helps in lowering the heat release capacity and increasing the char yield.
• Polyphenol-nanoclay composites demonstrates the possibility of synergistic action enhancing the thermal stability

References:

1. Krevelen, V. 1972. Properties of polymers. Correlations with chemical structure. Elsevier Publishing, Amsterdam.
2. 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.
3. 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.
4. Wagener, R., Reisinger, T., 2003. A rheological method to compare the degree of exfoliation of nanocomposites. Polymer, 44, 7513

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

2013 Progress Report

Final Report

P3 Phase I:

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