Effect of Ammonium Polyphosphate in Combination with Zinc Phosphate and Zinc Borate on Thermal Degradation and Flame Retardation of Polyamide 6/Clay Nanocomposites

 

Sweety Monga  and J. B. Dahiya*

Department of Chemistry, Guru Jambheshwar University of Science & Technology, Hisar-125001, Haryana, India

*Corresponding Author E-mail: jbdic@yahoo.com

In this paper, PA6/clay nanocomposites containing flame retardant additives such as ammonium polyphosphate (AP760), zinc phosphate and zinc borate were prepared by melt blending method. The resulting nanocomposites were characterized by X-ray diffraction and Fourier Transform Infrared spectroscopy (FTIR). X-ray diffraction patterns indicated the formation of intercalated structure in PA6 nanocomposites. FTIR analysis of residues obtained at 400^o C revealed the formation of aromatic carbonaceous char and the presence of nitrile chain ends in the degradation products of PA6 composites. Thermogravimetric analysis and UL-94 test were performed to determine thermal stability and flammability of nanocomposites. Synergistic effects were observed on addition of zinc borate with AP760 in PA6/clay nanocomposites for increasing thermal stability by 38^o C. The PA6/AP760 composite containing 20 wt% of AP760 without clay achieved flame retardancy with V-2 rating in UL-94 test.

 

 

 

INTRODUCTION:

Polyamide 6 is one of the prominent members of the aliphatic nylons and commercially important due to its excellent physical properties. It meets great demand in electronics, electrical, telecommunication, transportation and textiles industries where thermal stability and fire resistance of the polymer are priorities. But, pure polyamide 6 is highly flammable and shows intensive dripping which increases fire hazard¹. Therefore, there is a great demand to improve the flame retardancy of polyamide 6 to prevent the fire hazard. The addition of conventional fire retardants (FRs) is the most common approach to improve the flame retardancy of a polymer. Despite the advantages of these conventional FRs, there are many drawbacks as they often require high levels of loading (40-50%) for adequate flame retardancy leading to processing difficulties as well as deterioration of physical and mechanical properties of the polymer. Also, halogen based FRs generate toxic and corrosive combustion products like dioxin which draw the concerns over their environmental impact².

 

Therefore, in the recent past, much attention has been diverted to use environmental friendly nanoscale additives as flame retarding agents in polymers because a very small amount of nanoclay can enhance barrier, mechanical, thermal as well as fire properties of the polymer significantly^3-5. Polyamide 6/clay nanocomposites were the first successful nanocomposites, as pioneered by the Toyota group⁶. Bourbigot et al.⁷ have observed the synergistic effect of clay in improving the FR performance of intumescent fire retardant PA6 system. Dogan M et al.⁸ have studied the combination of ammonium polyphosphate and zinc borate in polypropylene, which leads to a synergistic effect in fire resistance.

 

So, in this paper an attempt has been made to study the combined effect of inorganic additives such as zinc phosphate and zinc borate at low loading with halogen free intumescent FR (Exolit AP760) and nanoclay on thermal behavior and flammability of PA6. PA6/clay nanocomposites were prepared by melt blending method. Thermal behavior of nanocomposites was studied by differential scanning calorimetry and thermogravimetry techniques. The structure and interlayer spacing (d-spacing) of clay in polymer matrix was characterized by x-ray diffraction technique. The char residues of nanocomposites were also analyzed by FTIR to evaluate the structural changes of nanocomposites on heating. UL-94 flammability test was carried out to study the flammability behavior of PA6 nanocomposites.

 

EXPERIMENTAL:

Materials used:

Polyamide 6 (PA6) was purchased from Sigma Aldrich Co., India. Sodium montmorillonite (clay) was supplied by Southern Clay Products Inc., Germany. Exolit AP760 (a blend of ammonium polyphosphate and synergist tris(2-hydroxyethyl) isocyanurate) (AP760) was obtained from Clariant Inc., US. Zinc phosphate (ZP) and zinc borate (ZB) were purchased from Himedia Chemicals Co., India. All these chemicals were used as received without further purification.

 

Preparation of nanocomposites:

PA6 pellets were oven dried at 85^o C for 10 h and then grinded to a course material. The materials were mixed in different proportions as per Table 1. Then, the processing of samples was carried out by melt blending method in a single screw extruder (Maxwell mixing extruder, ¾ inch diameter) at a throughput of 200 g per hour. Extrusion was performed at 240^o C temperature at a screw speed of 30 rpm. The extrudate in the form of strands was air cooled and granules were formed. These granules pellets were again oven dried at 85^o C for 10 h and grinded for further study. The strands of samples were also collected for flammability test.      

 

Table 1: Materials and composition of PA6 composites

Sample  No.	Sample name	Materials	Percentage composition
1	PA6	Polyamide 6	100
2	PA6/clay	PA6+Cloisite Na	95+5
3	PA6/AP760	PA6+Exolit AP760	80+20
4	PA6/clay/AP760	PA6+Cloisite Na+ Exolit AP760	80+5+15
5	PA6/clay/AP760/ZP	PA6+Cloisite Na+Exolit AP760+zinc phosphate	80+5+10+5
6	PA6/clay/AP760/ZB	PA6+Cloisite Na+Exolit AP760+zinc borate	80+5+10+5

 

CHARACTERIZATION TECHNIQUES:

X-Ray Diffraction:

XRD spectra of composite samples were obtained with a Bruker D8 Advance instrument using Cu Kα radiation (λ =1.54 Å). The voltage and the current of the x-ray tube were 40 kV and 40 mA, respectively.

 

Thermal analysis:

Thermogravimetric analysis (TG) of samples was carried out using Perkin Elmer Diamond TG/DTA thermogravimetric analyzer instrument at a heating rate 10^o C/minute from ambient temperature to 700^o C under air at a flow rate of 50 mL per minute. Differential scanning calorimetry (DSC) was also performed with TA Instruments DSCQ10 (Differential Calorimeter Thermal Analyzer). Each sample was accurately weighted in mg before being placed in DSC pan. Then each sample was heated from ambient temperature to 500^o C with a heating rate of 10^oC per minute in nitrogen atmosphere.

 

FTIR analysis:

Char residues of PA6 and its composites were obtained by heating at 400^o C for 10 minutes in the muffle furnace. FTIR spectra of PA6 composites and the char residues were recorded at a resolution of 4 cm^-1 using a Shimadzu IR Affinity-I 8000 FTIR spectrometer. The spectra were averaged over 15 scans.

 

UL-94 flammability test:

UL-94 burn test in vertical and horizontal configuration were performed to test the flammability behavior of PA6 composites. The specimens used for the test were in the strand form with length 125 mm and average thickness of 1.2 mm.

 

RESULTS AND DISCUSSION:

X-ray Diffraction:

From XRD spectrum shown in Fig 1, the d-spacing of clay was determined equal to 1.13 nm corresponding to an intense diffraction peak at 2θ =7.75^o using Bragg eq., which is a characteristic peak of clay. The PA6 showed no diffraction peak in the 2θ range from 3 to 10^o, indicating that the polymer had no ordered structure in this dimension range. In case of PA6/clay and PA6/clay/AP760 composites, the diffraction peak of clay was shifted from 7.75 to 5.56 and 6.19^o with d-spacing 1.59 and 1.43 nm, respectively. These results indicated that the polymer molecular chains entered into the galleries of clay and thus intercalated structure is formed in the PA6/clay nanocomposites. The presence of microsized flame retardant (AP760) in composites has affected the intercalation to some extent as indicated by the small reduction in d-space from 1.59 to 1.43 nm.

 

 

Fig. 1:  XRD spectra of PA6, clay and PA6 composites

Thermal analysis:

TG thermograms of pure PA6 and its composites are shown in Figs 2-4. The TG data such as T_10wt% (temperature at 10% weight loss, i.e. onset temperature of degradation), T_50wt% (temperature at 50% weight loss), stages of thermal degradation and char yield at 700^o C are given in the Table 2. DSC curves of pure PA6 and its nanocomposites are shown in Fig 5 and the data is given in Table 3.

 

From the TG curve of pure PA6 (Fig 2), two stages of thermal degradation were observed, and most of the weight loss (87.0%) occurred in first stage of degradation in temperature range of 100–475^o C with corresponding first DTG peak at 445^o C. The initial weight loss in this stage was due to loss of residual moisture and volatilization of low molecular weight oligomers⁹. Later in this stage, the weight loss took place with the evolution of cyclic monomer, i.e. caprolactum and volatile gases like CO₂ and NH₃. The presence of oligomeric products with nitrile and vinyl chain ends have also been reported in earlier studies^10,11. The second stage of thermal degradation took place in the temperature range (475–700^o C) with a small DTG peak at 525^o C with 12.8% weight loss leading to no char yield. Herrera M et al.¹² has also reported the similar results in TG analysis of PA6 in air atmosphere. TG curve of PA6/clay nanocomposite showed two stages of thermal degradation similar to pure PA6 (Fig 2) but the degradation temperatures (T_10wt% , T_50wt%  and DTG peak)  were increased by about 5^o C in comparison to pure PA6 during the major degradation stage. Thus the formation of polymer nanocomposite with clay improved the thermal stability of PA6.

 

DSC study of pure PA6 and PA6/clay nanocomposite samples also supported the TG results by the fact that the endothermic peak of PA6 due to the decomposition was shifted slightly to higher temperature by 5^o C on formation of PA6/clay nanocomposite. No significant change in the melting point of PA6 (222^o C) was observed on the formation of PA6 nanocomposites (Fig 5).

 

Fig. 2: TG and DTG curves of (i) PA6 and (ii) PA6/clay

 

TG curve of PA6/AP760 (Fig 3) showed three stages of degradation indicating the change in decomposition path on addition of 20 wt% of AP760. Incorporation of flame retardant (AP760) resulted into the destabilization of the polymer matrix as seen by the significant reduction of onset temperature of degradation (304^o C) in comparison to pure PA6 (385^o C). T_50wt% was also lowered by 53^o C on addition of AP760. The flame retardant (AP760) decomposes on heating and releases polyphosphoric acid, which acts as a degradation catalyst¹³. The polyphosphoric acid attacks the alkyl-amide bond of PA6 and forms polyphosphate ester. At higher temperature, polyphosphate chains break resulting into the abstraction of hydrogen from PA6. The double bonds formed act as precursors¹⁴ for polymer charring giving rise to higher char yield of 8.4 wt% at 700^o C. The increase in char at the expense of combustible gases leads to improvement in the flame retardant property of the material

 

 

Table 2: TG data of PA6 and its various composites

Sample	Stage	Temp. Range (o C)	Wt. loss (%)	T10wt% (o C)	T50 wt% (o C)	DTG peak (o C)	Char  at 700o C (%)
PA6	1st 2nd	100-475 475-700	87.0 12.8	385	435	445 525	0.0
PA6/clay	1st 2nd	100-485 485-700	89.9 7.1	391	440	451 500	2.1
PA6/AP760	1st 2nd 3rd	100-360 360-490 490-700	30.6 48.8 10.7	304	382	308 386 511	8.4
PA6/clay/AP760	1st 2nd 3rd	100-350 350-465 465-700	20.2 50.4 17.3	308	397	303 388 520	10.9
PA6/clay/AP760/ZP	1st 2nd	100-470 470-700	73.0 18.2	335	416	426 525	7.9
PA6/clay/AP760/ZB	1st 2nd	100-475 475-700	81.4 7.5	346	404	407 525	9.5

Table 3: DSC data of PA6 and its various composites.  

Sample	DSC temp (o C)		Heat flow (J/g)	Nature of peaks
	Initiation	Maxima
PA6	210 403	222 451	63 699	Endo, sharp Endo, sharp
PA6/clay	211 401	221 456	48 703	Endo, sharp Endo, sharp
PA6/ AP760	208 286 360	220 300 380	73 64 295	Endo, sharp Exo, sharp Endo, broad
PA6/clay/AP760	208 295 374	220 304 411	58 38 228	Endo, sharp Exo, sharp Endo, broad
PA6/clay/AP760/ZP	209 283 363	220 315 404	39 52 480	Endo, sharp Endo, broad Endo, sharp
PA6/clay/AP760/ ZB	210 376 395	221 377 415	59 17 145	Endo, sharp Endo, sharp Endo, sharp

 

In TG analysis of PA6/clay/AP760 sample containing 5 wt% clay and 15 wt% AP760, three stages of thermal degradation (Fig 3) were observed similar to PA6/AP760. On substitution of 5% wt of AP760 by 5% wt of clay; both the onset temperature of thermal degradation and T_50wt% were increased by about 5^o C and char yield was also increased further to 10.9% in comparison to PA6/AP760 (8.4%). From Table 2, 20 wt% of AP760 in PA6/AP760 gave char yield equal to 8.4 wt%, which implies 15 wt% of AP760 in PA6/clay/AP760 equates to 6.3% char yield. Clay at 5 wt% in PA6/clay sample contributed to 2.1% of char yield. Thus, 10.9-6.3-2.1 % = 2.5 % is the additional char residue obtained due to interaction and combined effect of clay and AP760. Hence, a synergistic effect with regard to increase in char yield was observed with the presence of clay and AP760 together in PA6/clay/AP760 sample and also resulted into more thermal stability as compared to PA6/AP760.

 

The DSC curves of PA6/AP760 and PA6/clay/AP760 (Fig 5) showed one additional exothermic peak each in the temperature range 300-305^o C followed by endothermic peaks. This additional exothermic peak is attributed to the formation of crosslinked polyphosphate structure on heating of AP760 with the elimination of ammonia and water¹³. The released phosphoric acid attacks the alkyl-amide bonds of PA6 with the formation of phosphoric ester and primary amide chain ends. Therefore, the interaction of AP760 with PA6 changed the decomposition pathway of PA6 as observed by the appearance of DSC major endothermic peak at lower temperature 380^o C as compared to 451^o C for pure PA6 and also the heat involved was decreased to the large extent¹⁵.The shifting of endothermic peak due to decomposition from 380 to 411^o C on adding clay to PA6/AP760 composite is in agreement with the results obtained in TG analysis.     

 

TG curve of PA6/clay/AP760/ZP containing 10wt% AP760 (Fig 4) showed two steps of degradation similar to pure PA6, which suggested that at least 15 wt% of AP760 is required to change the decomposition path of PA6. However, thermal stability was improved as the onset temperature and T_50wt% were increased by 27 and 19^o C, respectively in comparison to PA6/clay/AP760. On comparing the effect of ZB and ZP, the TG thermogram of PA6/clay/AP760/ZB (Fig 4) showed further increase in the onset temperature of degradation by 11^o C in comparison to PA6/clay/AP760/ZP and the char yield was also increased to 9.5%, indicating that the ZB provided better stability and flame retardancy as compared to ZP. This was attributed to the synergistic effect between ZB and AP760 which was reported earlier also by Samyn et al.¹³. DSC curve of PA6/clay/AP760/ZP (Fig. 5) showed the absence of exothermic peak at 304^o C on substitution of 5 wt% of AP760 by ZP in PA6/clay/AP760. The heat change during endothermic decomposition was significantly increased from 228 to 480 J/g on addition of ZP to PA6/clay/AP760. Several small endotherms were also observed in case of PA6/clay/AP760/ZB and PA6/clay/AP760 in temperature range from 300 to 400^o C due to continuous evolution of ammonia before the major endothermic decomposition phenomenon occurs at 410-415^o C. DSC study supported the TG results by the fact that the endothermic peak due to decomposition for PA6/clay/AP760/ZP was shifted to higher temperature by 11^o C for the composite PA6/clay/AP760/ZB, suggesting better thermal stability provided by ZB in comparison to ZP.

 

Fig. 3: TG and DTG curves of (iii) PA6/AP760 and (iv) PA6/clay/AP760

 

Fig. 4: TG and DTG curves of (v) PA6/clay/AP760/ZP and (vi) PA6/clay/AP760/ZB

 

Fig. 5: DSC curves of

(i) PA6, (ii) PA6/clay, (iii) PA6/AP760, (iv) PA6/clay/AP760,

(v) PA6/clay/AP760/ZP and (vi) PA6/clay/AP760/ZB samples

 

 

FTIR analysis:

FTIR spectra of pure PA6, PA6/AP760 and PA6/clay/AP760/ZB samples and their residues obtained at 400^o C are shown in Fig 6. The characteristic bands of PA6 were observed at 3302 cm^-1 (N–H stretching hydrogen bonded (H-bonded)), 3082 cm^-1 (amide-II, overtone of N-H bending), 2941, 2870 cm^-1 (asymmetric and symmetric CH₂ stretching), 1670 cm^-1 (amide-I, C=O stretching), 1530 cm^-1 (amide-II, coupling of C–N stretching and N-H in plane bending), 1463 cm^-1 (CH₂ bending), 1270 cm^-1 (amide-III, C–N stretching), 1041 cm^-1 (CO-NH skeletal vibration), 681 cm^-1 (amide-V, N–H out of plane bending) in FTIR spectra of pure PA6¹⁶. Characteristic bands corresponding to different additives were also observed in spectra of PA6 nanocomposites.       

 

In FTIR spectrum of residue of PA6, apart from the typical absorptions of PA6, a new low intense band was found at 1616 cm^-1 due to C=C stretching suggesting the formation of carbonaceous char residue at 400^o C. In case of PA6/AP760 residue, presence of P-OH stretching band at 1105 cm^-1 and P-O-P symmetric stretching band at 987 cm^-1 indicated the presence of crosslinked polyphosphate at 400^o C. For residue of composites containing AP760, broad and intense bands were found in the range 1430-1610 cm^-1 corresponding to C=C stretching, which suggested the formation of significant amount of aromatic char on heating. A new band was appeared in spectra of residues of all composites in the range 2241-2245 cm^-1 (C≡N str.), which revealed the presence of nitrile chain ends in the degradation products¹⁷ of PA6.

 

UL-94 flammability test:

UL-94 flammability test measures the ignitability, flame spread and the self extinguishing behavior of plastic materials in vertical and horizontal positions when exposed to a small flame. Five tests were conducted for each sample. The results of UL-94 horizontal (HB) and vertical burn tests are given in Table 4.          

 

Pure PA6 dripped heavily with the flame spread rate 1.38 mm/sec and dripping rate 0.48 sec^-1; thus failed to pass HB test. On addition of 5 wt% of clay to PA6, the dripping rate was reduced to 0.21 sec^-1 but the flame spread rate was increased to 1.97 mm/sec. The composite PA6/AP760 containing 20 wt% AP760 self-extinguished immediately after removal of the flame and produced only one drip for each replicate test and gives HB rating. This is due to the formation of intumescent char layer on the surface of polymer composite¹⁸. PA6/clay/AP760, PA6/clay/ AP760/ZP and PA6/clay/AP760/ZB composites did not pass the UL-94 HB test.

 

 

Fig. 6: FTIR spectra of PA6, PA6/AP760, PA6/clay/AP760/ZB samples and their residues obtained at 400^o C.

 

Table 4: UL-94 flammability test for PA6 and its various composites.

Sample	Horizontal burn test			Vertical burn test
	FSR* (mm/sec)	kh (s-1)	Rating	t1, t2 (s)	k1, k2 (s-1)	Rating
PA6	1.38	0.48	BC, NR	BC	0.47, -	NR
PA6/clay	1.97	0.21	NR	BC	0.48, -	NR
PA6/AP760	0.98	0.19	HB	4.6, 9.2	0.43, 0.05	V-2
PA6/clay/AP760	3.34	0.26	NR	BC	0.53, -	NR
PA6/clay/AP760/ZP	2.54	0.18	BC, NR	BC	0.47, -	NR
PA6/clay/AP760/ZB	1.74	0.12	BC, NR	BC	0.31, -	NR

^*FSR = Average flame spread rate between two benchmarks; NR= No rating; BC= Burns to clamp;

k_h= Average dripping rate during horizontal burn test;

t₁= Average burning time after first application of flame during vertical burn test;

t₂= Average burning time after second application of flame during vertical burn test;

k₁= Average dripping rate after first application of flame during vertical burn test;

k₂ = Average dripping rate after second application of flame during vertical burn test

 

 

In vertical burn test, pure PA6 burnt quickly once ignited and constantly produced burning drips. The sample PA6/clay burnt up to the clamp with dripping rate k₁=0.48 sec^-1.The PA6/AP760 sample achieved V-2 rating with lower burning times (t₁=4.6 & t₂=9.2 sec) and lower dripping rates (k₁=0.43 & k₂= 0.05 sec^-1). All other samples did not pass vertical burn test as they burnt up to the clamp after first application of the flame. Thus, in UL-94 test, the PA6/AP760 composite showed best flammability properties with V-2 rating.

 

CONCLUSION:

PA6/clay nanocomposites were prepared via melt blending method. X-ray diffraction patterns indicated the formation of intercalated structure in the PA6 nanocomposites. Thermogravimetry studies showed that the addition of 20 wt% flame retardant for PA6/AP760 sample altered the path of degradation of PA6 by destabilizing the polymer matrix in initial stage but mass loss rate was slowed down at higher temperature with significant increase in the char yield. Synergistic effect for increasing the char yield and thermal stability was observed between clay, AP760 and ZB in PA6 composites. The FTIR spectra of residue of PA6 nanocomposites containing AP760,  showed broad and intense bands in the range 1430-1610 cm^-1 attributed to C=C stretching and suggested the formation of  significant amount of carbonaceous char residue at 400^o C. The PA6/AP760 composite containing 20 wt% AP760 achieved sufficient flame retardancy with V-2 rating in UL-94 test.

 

ACKNOWLEDGEMENT:

One of the authors (Sweety Monga) is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi, India, for the award of Senior Research Fellowship (SRF).

 

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Received on 22.11.2014         Modified on 08.12.2014

Accepted on 11.12.2014         © AJRC All right reserved