Chapter 13
Effect of Layered Silicate Nanocomposites on Burning Behavior of Conventionally FlameRetarded Unsaturated Polyesters Β. K. Kandola, S.Nazaré,A.R.Horrocks,and P. Myler Centre for Materials Research and Innovation, Bolton Institute, Deane Road, Bolton B L 3 5 A B , United Kingdom
Montmorillonite clay has been modified with a series of organic modifiers. Modified clays have been characterised by X-ray diffraction and thermal analytical techniques. Unsaturated polyester nanocomposites have been prepared by in-situ polymerisation. Some clays were fully exfoliated, some indicated combined nano-structures with ordered intercalation and partial exfoliation, and some indicated only microcomposite structures, depending upon type of organic modifier used. Thermal stability and flammability of these samples have been studied by thermal analysis and cone calorimetry. A l l clays reduce the onset of decomposition temperature of the resin, slightly increase ignition time and total burning time but decrease peak heat release rate (PHRR) values (9-36%) and total heat release rate (THR) (2-16%) compared to resin only sample. In the presence of conventional flame retardant, ammonium polyphosphate (APP), there is a synergistic effect with the nanoclay in terms of increased char formation, reduction in P H R R (60-67%), T H R (35-39%) and smoke production (1-13%) compared to resin. Mechanical testing in terms of flexural mode has indicated that inclusion of functionalised nanoclays enhances mechanical performance of the resin.
© 2006 American Chemical Society
156
Introduction There is a constant demand for efficient and environmentally friendly flame retardants. Organic-inorganic nanocomposites are of significant interest since they frequently exhibit unexpected hybrid properties synergistically derived from two components [1-3]. Montmorillonite, one of the layered silicate clay minerals, is composed of silicate layers with a thickness of about 1 nm. Organic ammonium ions and neutral organic molecules may be intercalated in the interlayer space between the silicate layers [4], which when present in a polymer matrix may lead to increased mechanical and reduced flammability properties at very low loading levels (2 - 5%) [2,3]. The efficiency of clay in modifying the properties of the polymer is primarily determined by its degree of dispersion in the polymer, which in turn depends on the clay particle size and chemistry of the clay [5]. Exfoliation depends upon processing shear conditions, melt rheology and the structure of aliphatic or organic compound to modify the clay [6]. The structure of a layered silicate and its charge density may also influence nanocomposite morphology and degree of exfoliation [6,7]. During the last five years we have been studying the flammability behaviour of unsaturated polyester resin, as cast laminates and glass - reinforced composite structures [8-10]. In our previous publications [10,11], we have used commercially available organically modified clays in preparing unsaturated polyester nanocomposites with and without conventional flame retardants. In this work, the effects of different organic modifiers on nanocomposite formation and thermal, flammability and mechanical properties of the resultant polymer are presented. Due to the limited sample size and numbers available, mechanical performances are only studied in the flexural mode, however, full mechnical characterisation of glass fibre reinforced composite laminates prepared from selected nanoclays and flame retardant unsaturated polyester resins will be discussed in a subsequent publication.
Experimental Materials Polyester resin : Orthophthalic, Crystic 471 PALV (Scott Bader) ; Catalyst M (methyl ethyl ketone peroxide, Scott-Bader) Clays: Cloisite Na montmorillonite, Na-MMT (Southern Clay Products, USA) modified with different organic modifiers are given in Table I. +
157
Table I. Treatment / properties of organically modified clays Clay
Organic modifier
Chemical structure of organic modifier
_ MMT CI (i)
Vinyl triphenyl phosphonium bromide
—,
Vinyl benzyl trimethyl ammonium chloride
—
1.17
Br > — Ph
1.77,0.89,0.58
Vh
Pff
Cl(ii)
XRD results d spacing (nm)*
Q \
cr *-* I^
1.46,-, 0.48 Me
M e
Me
Me
CI (iii)
H
Hexa decyl trimethyl ammonium chloride
M e
°™ ™ | CI
1.82, 0.94, 0.56
Me
Cl(iv)
CI (v)
Dodecyl ethyl dimethyl ammonium bromide N,N-dimethyl-N,Ndioctadecyl quaternary ammonium bromide
n
mb-^ +
1.12,0.98,0.61
B r
I M e
Oct^ ^ ^ I
0 c t
2.63, 1.29
N
B r
Me
Me= ~ C H
3
Et=
—C H 2
5
Oct = —CH (-CH -) 3
2
17
* Values in bold are characteristic silicate d-spacings, in italics are very small peaks Flame-retardant (FR) : Ammonium polyphosphate, APP (Antiblaze MCM, Rhodia Specialities)
158 Modification of sodium montmorillonite Sodium montmorillonite (Na-MMT) clay has been functionalised with a range of quaternary ammonium and phosphonium salts, as given in Table I. An appropriate salt was dissolved in distilled water and gently agitated to obtain a homogeneous solution of 0.1 M, to which 50g of Na-MMT was added and stirred for 6 hours at room temperature. The resulting mixture was filtered and washed repeatedly with hot water (60 °C) until free of excess organic modifier (tested with AgN0 solution). The exchange process was repeated for another 48 hrs and the resulting clay was collected by filtration, washed, finally dried in a vacuum oven (40 °C, 24 hrs) and then ground into a fine powder. Crude organo-modified clays contain impurities in form of (a) unexchanged / excess organic modifier and (b) an ion exchanged product (sodium bromide/chloride). The residual anions decrease thermal stability of organo clays, whereas the nature of counter cation dictates the onset of degradation of organic modifier on the organically modified clays [12]. Hence, the clays were extracted with ethanol first and then with tetrahydrofuran using routine soxhlet extraction procedures for 4 hours. The clays were dried under high vacuum for 18 hours at 120 °C. The extracted and dried organo-clays were analysed using XRD and TGA. 3
Preparation of polyester-clay nanocomposites The polyester-clay nanocomposites incorporating flame retardants have been prepared by in-situ intercalative polymerisation. 5%(w/w) clay was gradually added to the polyester resin, while stirring with a mechanical mixer under high shear (900 rpm). The mixing was carried out for 60 min at room temperature. For samples incorporating flame retardant, 20% (w/w with respect to resin-clay mixture) of the flame retardant was added to the mixture of resin and clay after 20 min of mixing. The percentages of various components in the formulations are given in Table II. Small amounts of samples were taken from the mixture for simultaneous DTA-TGA analysis. For cone calorimetric studies, 1 % (w/w with respect to resin) catalyst was added, laminates were cast and cured at room temperature for 24 hours and post cured at 80°C for 8 hours. Their nanocomposite structures were characterised by X-ray diffraction, XRD.
159 Table II. Mass percentages of various components in the formulations Sample
Sample description
Resin (%>
Res Res/Cl Res/FR Res/Cl/FR
Resin Resin + Clay Resin + FR Resin + Clay + FR
100 95 83 79
FR (%)
Clay (%)
-
5
17 17
4
-
Equipment X-ray diffraction (XRD) studies were carried out using a Siemens D500 powder diffractometer with a step size of 0.02°, a step time of 1 s and a range of 0-25° on the 2-theta scale. Simultaneous DTA-TGA analysis was performed using SDT 2960 TA instruments under flowing air (100 ml/min) and at a heating rate of 10K min" on 25 mg sample masses. A cone calorimeter (Fire Testing Technology Ltd., UK) was used at an incident heat flux of 50kW/m according to ISO 5660. The flexural modulii of the laminates (coupon sizes 120 x 12 x 3 mm) were measured in three point bending mode according to BS 2782-10: Method 1005:1997, EN 63:1977, with load applied via a computer controlled Instron 4303 tensometer. 1
2
Results and Discussion
X-ray diffraction studies The periodic structures of inorganic clays can be distinguished through Xray diffraction. Representative XRD patterns for the CI (i) and (v), their hybrids with resin and with/without APP are given in Fig.l (a) and (b), respectively. Characteristic d-spacing values for clays and their hybrids with resin and Res/APP for 2-theta in the range 2 - 1 0 ° only, are given in Tables I and III, respectively. Na-MMT shows a peak at 29 = 7.3° (d-spacing 1.17 nm, see Fig 1(a)), which has shifted to lower 20 angles : in CI (i) to 4.9° (d-spacing 1.77nm, Fig.l(a)), in CI (ii) to 6.0°(1.46 nm (Table I)), in CI (iii) to 4.8°(1.82nm (Table I)), in CI (iv) to 5.1° (1.72nm (Table I)), and CI (v) to 3.3° (2.63nm, Fig.l(b)).
160 This shows intercalation of the silicate layers by respective organic molecules. Some clays show up to three Bragg reflections, primary (d oi), secondary (d 02) and sometimes tertiary (d 03). These secondary or tertiary reflections are more prominent in certain clays compared to others, which may be explained by the height of the gallery spacing of the organoclay in relation to thickness of a silicate platelet [6]. This hypothesis is supported by the fact that these reflections are more prominent for organoclays with primary d-spacing 1.77 nm and higher (see Table I). CI (i) shows prominent secondary peaks at 20 = 9.8 and 15°, CI (iii) at 9.3 and CI (v) at 6.7°. From Fig 1(a), Tables I and III, it can be seen that the Res/Cl (i) and Res/Cl (ii) samples have characteristic clay peaks similar to those of the respective clays, indicating that there is no intercalation of the polymer between the clay layers. These samples correspond to conventionally filled polymers where, at least, each primary particle is dispersed in the polymer matrix. Additional presence of APP does not affect this pattern as seen for respective Res/Cl/APP curves. For Res/Cl (iii) sample, the main peak of CI (iii) at 20 = 4.8° has moved to lower 20 values (20 = 2.5°) and the d-spacing has increased from 1.76 nm to 3.40 nm, suggesting ordered intercalation of the polymer chains into the organoclay layers. Also, an extremely broad peak (at 20 = 5.0°) indicates exfoliation. According to Bragg equation, nA,=2d sinO and area under each reflection is proportional to sin (rotcp/2), where cp is the volume fraction of the structure [6], so broadening of peak can be interpreted as partial exfoliation [13]. The XRD pattern of Res/Cl (iii) system hence, illustrates combined structure with partly exfoliated clay layers and the remaining having ordered intercalation. Presence of APP in Res/Cl (iii)/APP sample has in fact assisted exfoliation of layers because as seen in Table III, the d-spacing is no longer observed for Res/Cl (iii)/APP sample. Res/Cl hybrids containing CI (iv) and CI (v) have no characteristic peaks in their respective XRD patterns suggesting that clay layers are completely exfoliated and dispersed at the molecular level into resin. For Res/Cl(iv),(v)/APP samples (see Table III and Fig. 1(b)) also the characteristic peaks are missing indicating exfoliation of clay layers. Characteristic XRD peaks of APP and broad resin peaks are unaffected by presence of clays in microcomposite or nanocomposite form as seen from Fig. 1(a) and (b). In conclusion, XRD studies on the modified clays suggest that the clays modified with longer chain molecules show better exfoliation in polymer-clay nanocomposites than the others. 0
0
0
2
Mechanical performance studies The mechanical performances in terms of flexural modulii of the samples containing nanoclays with and without flame retardants have been studied in
(b)
Fig. 1 XRD curves for unsaturated polyester with (a) CI (i) and (b) CI (v), with and without APP.
(a)
162
Table III. XRD and mechanical performance results for polyester-clay nanocomposites with and without FRs Sample
XRD results* d-spacing, (nm)
Res
-
Res/Cl (i) Res/Cl (ii) Res/Cl (iii) Res/Cl (iv) Res/Cl (v) Res/APP Res/Cl Res/Cl Res/Cl Res/Cl Res/Cl
(i)/APP (ii)/APP (iii)/APP (iv)/APP (v)/APP
1.76 1.44 3.04, 1.76 -
Mechanical properties "~ Flexural modulus, Stress at failure E (GPa) (MPa) 3.3 ±0.38
50.3 ± 2.3
4.5 3.3 3.5 3.3
63.7 ±3.1 45.5 ±4.7 42.1 ±1.9 38.2 ± 1.3
±0.41 ±0.07 ±0.01 ±0.12
*
*
-
2.7 ±0.01
24.7 ± 1.0
1.74 1.47 -
4.8 ± 0.34 3.8 ±0.18 3.8 ± 0.04 3.9 ±0.07
68.2 ± 6.9 56.9 ±4.1 49.3 ± 2.4 45.1 ±3.1
*
Diffraction peaks for 2-theta in the range 2 - 1 0 ° only are presented ; * Not tested
163 three point bending mode and the results are given in Table III. Inclusion of functionalised nanoclays maintains and in some cases enhances flexural modulus and hence, stiffness of the resin from 3.3 to 3.3 - 4.5 GPa, depending upon type of organic modifier of the clay. Stress at failure varies between 38.2 63.7 MPa for different Res/Cl samples. Inclusion of APP in resin (Res/APP sample) decreases both modulus (2.7 GPa) and stress at failure (24.7 MPa) of the resin. However additional presence of clay (Res/Cl/APP samples) enhances both modulus (3.8 - 4.8 GPa) and stress at failure (45.1 - 68.2 MPa). Res/Cl/APP samples show improved flexural modulii compared to resin only, Res/Cl and Res/APP samples, and stress at failure compared to Res/Cl and Res/APP samples (see Table III). In terms of reinforcing element, CI (i) shows better results compared to other clays (see Table III) in enhancing mechanical performance (flexural modulus and stress to failure) of resin, with and without APP.
Thermal analysis DTA and TGA analysis for all modified clays are given in Table IV. As seen from Table IV and also discussed in our earler communication [10], the DTA response of Na-MMT is featureless, showing inertness of the inorganic clay. All organically modified clays show exothermic peaks (Table IV) and two or three stages of weight loss represented by respective DTG peaks (given in Table IV). For CI (i), the main DTA decomposition peak is at 568°C, whereas for all other clays the decomposition peak is in the temperature range 300 352°C. This is also corroborated from DTG peaks, where the peak representing major weight loss for CI (i) is at 574°C and for others in the range 250 - 352°C. This suggests a higher thermal stability of CI (i). As discussed in detail earlier, on heating all of these clays, the organic component of the clay decomposes first, followed by dehydroxylation of clay layers [10,14]. Mass residues at 600 and 800°C are given in Table IV, which represent the residual silica content after decomposition of the organic component. Polyester resin starts to decompose above 200°C and the main decomposition occurs between 300 and 400°C [10]. Above 400°C, solid phase oxidation of the char occurs, leaving very little char residue at higher temperatures (1% at 800°C, see Fig 2(a)).
164 Table IV. Thermal analytical properties of organically modified clays Clays
DTA results Peak maxima*
CO Na-MMT Cl(i) Cl(ii) CI (iii) Cl(iv) Cl(v)
278 Ex(s,b); 568 Ex 335 Ex;612Ex(s,b) 305,342Ex(d); 618Ex(s,b) 308 Ex; 433 Ex(s); 621 Ex(s,b) 301,352 Ex(d); 455 Ex ; 609 Ex
TGA results % Mass residue at DTG peak * maxima 600 (°C) 800(°C) (°C) 88 91 76; 665 144 ; 351 ; 574 80 75 252; 592 73 66 256, 593 74 67 260,310(d); 445; 607 300; 462 ; 634
82
75
76
67
Key : Ex - Exotherm ; s = small; b = broad ; *Va!ues in bold are main decomposition peaks
All clays reduce thermal stability of the resin below 400°C and above 600°C increase slightly as can be seen from Fig 2(a) for CI (i). This is more clear in Fig.2(b), where the difference between TGA experimental and calculated (from weighted average component responses) masses versus temperature (for details see our previous publications [8,15] ) for all Res/Cl samples are plotted. Above 600°C, char formations are similar to those expected from respectively calculated values and the type of clay has no effect on residue formation at high temperatures. This gives indication that nanoclays on their own are not effective in increasing residue formation by additional char and hence, reducing flammability of the resins. Ammonium polyphosphate increases residue formation to 3% (see Fig 2(a)) and nanoclays increase this value further to 1214% as seen for CI (i) samples in Fig 2(a), presumably as a consequence of carbonaceous char. In Fig 2(c) the difference between experimental and calculated TGA responses for Res/APP and all Res/Cl/APP samples are plotted, where it can be seen that that the thermal stabilities of these formulations are less than expected below 500°C, but after that they are more stable, suggesting synergistic effect of clay and FR combination. Nanoclays are known [2,3,10] to increase thermal stability of the polymer due to formation of a protective surface insulative barrier layer consisting of accumulated silica platelets with a small amount of carbonaceous char. These insulative silicate platelets are claimed to protect the fast volatilization and degradation of the resin and giving more time to react with acid released from APP, leading to more char formation than expected. All clays promote additional char formation above 700°C. This same effect noted for commercially modified clays has been discussed in detail in our previous communication [10].
165 Effect of clays on residual char formation of resin with and without APP is shown in Fig.2(d), where residual chars for all Res/Cl and Res/Cl/APP samples at 600 and 800°C are reported, after substracting the residual silica content taken from Table IV. Clays enhance char formation of resin by 2%, whereas with additional presence of APP, more than 10% residual char is observed at 800°C. Thermal analytical results indicate that all clays have a similar effect on the thermal stability of resin with and without APP. None of the clays shows any distinct behaviour, although there is slight shift in decomposition temperature range, depending upon the type of organic modifier used.
Cone calorimetry 2
The various parameters recorded by the cone calorimetric test at 50kW/m heat flux are given in Table V and selected results are shown in Fig. 3. These are derived from respectively averaged curves obtained from three replicate runs for each sample. Thus heat release rate values have an error of ± 1 - 9% (See Table V) Presence of clays in general does not affect time to ignition (TTI) of the resin (TTI=34s), although CI (i) and (ii) slightly increase it to 45 and 40s, respectively. All clays increase flame out (FO) or total bum time of the resin from 136 to 139 - 170s (see Table V). Peak heat release rate (PHRR) of pure resin is reduced with all types of clays from 1153 kW/m to 743 - 1045 kW/m , depending upon the organic modifier used. These clays also help in reducing total heat release, whereas, smoke production is not affected and even increased in some samples. Effective heat of combustion, H is the quantity of heat produced by combustion of a unit quantity of material and hence, may be used to measure the possible flame retarding effects of components present. Except for Res/Cl (i) sample, effective heat of combustion for all other samples is unaffected by presence of clay. In Table V, the fire growth index (FIGRA) [16] values are also given, which is PHRR/TTP (kW/s) and is helpful in ranking the materials in terms of potential fire safety because it combines peak fire size (PHRR) and time to achieve this (time to peak, TTP). FIGRA index of resin (11.5 kW/s) is reduced to (6.5 kW/s) with CI (i) and 9.8 with CI (ii). CI (ii) - (iv) have little effect on this value, however. Mass loss versus time curve in Fig. 3(b) show that clay helps in increasing thermal stability and char formation. In Fig.2(d) char residue after 4 minute period is plotted, where the silica content is accounted for by substracting the silica values taken from TGA results at 800°C in Table IV and indicate that clays enhance 3 . 5 - 5 % char formation of resin. 2
c
2
k
«
Mass, % £
8
8
8
5
Mass difference, % U 3 o
2
Fig.2. (a) TGA responses of Res and Res/Cl(i) with/without APP in air; percentage residual mass differences (actual - calculated) as a function of temperature for (b) Res/Cl, (c) Res/Cl/APP ; (d) resisual masses of Res/Cl/APP samples from TGA curves at 600, 800°C and cone results after 4 min exposure at 50kW/m heat flux
2
Fig. 3. (a) HRR and (b) mass loss vs time curves for Res and Res/Cl(i) with/without APPat50kW/m
ON 00
169 The effect of different flame retardants, including ammonium polyphosphate, on the flammability of this particular resin is discussed in depth in a separate publication [11]. As seen from Table V, APP does not help in increasing time to ignition of the resin, but increases flame out time, which is further increased with additional presence of clays. APP in Res/APP sample is effective in reducing PHRR (478 kW/m ) and THR (52.2 MJ/m ) compared to pure resin. Clay presence further reduces PHRR (384 - 457 kW/m ) and THR (48.5 - 51.6 MJ/m ). There are no significant changes in smoke values except for Clay (v) containing sample. APP presence significantly reduces H of resin from 33.1 to 19.2 MJ/m , but that of clays increases it again to 23.7 - 24.7 MJ/m . FIGRA index is lowered by APP to 5.3 kW/s, which is further lowered by clays to within the range 3.1 - 4.6 kW/s. APP helps in increasing char formation as seen from Figs. 2(d) and 3(b), whereas additional presence of clay has little effect, once the amount of silica residue is accounted for. 2
2
2
2
c
2
2
2
Table V. Cone calorimetric results at 50kW/m heat flux for polyester-clay nanocomposites with and without FRs Sample
TTI (s)
FO (s)
PHRR (kW/m ) 2
FIGRA (kW/s)
THR* (MJ/m )
(MJ/kg)
Smoke* (1)
79.0
33.1
761
2
H;
34
136
1153
45 34 32 40 33
170 139 140 143 159
743 1045 1002 1034 958
6.5 11.6 11.1 10.8 9.1
66.5 68.8 70.0 71.7 77.9
30.1 32.1 37.8 35.3 33.5
814 683 712 732 835
Res/APP
31
190
478
5.3
52.2
19.2
754
Res/Cl Res/Cl Res/Cl Res/Cl Res/Cl
38 36 38 36 34
204 199 202 193 211
419 426 434 484 384
3.6 4.0 3.9 4.8 3.1
48.5 49.9 49.1 51.6 50.6
23.7 24.7 24.0 24.4 24.5
715 706 724 760 660
Res Res/Cl Res/Cl Res/Cl Res/Cl Res/Cl
(i) (ii) (iii) (iv) (v)
(i)/APP (ii)/APP (iii)/APP (iv)/APP (v)/APP
11.5
* Values for 4 minute period ; Coefficient of variation ranges are : TTI = 2 - 17 %, FO = 1 - 1 0 % , PHRR = 1 - 9 %, THR = 2 - 8 %, H = 6 - 19 % and Smoke = 1 - 4 %. c
In terms of effect of individual clays, CI (i) increases TTI, reduces PHRR and THR and shows minimum values of H and FIGRA (see Table V). Although c
170 this clay does not show intercalation or exfoliation in XRD studies, it surprisingly shows best results for cone studies of all clay/resin combinations tested. This may be due to flame retardant effect of phosphorus group present in the functionalised clay and its higher decomposition temperature (see Table IV). CI (ii) and (iii) do not show any significant improvement in any of the cone parameters except as reduced FO and smoke values in the Res/Cl (ii)/APP sample, but they do not indicate nanocomposite formation as well. CI (iv) shows complete exfoliation but does not show any significant improvement in cone parameters. CI (v) on its own is not very effective in reducing flammability of resin as seen for Res/Cl (v) sample in Table V, but shows best results in presence of APP indicating synergistic effect of organic modifier and APP. In Res/Cl (v)/APP sample, PHRR is reduced by 67%, THR by 36% and smoke by 13% compared to pure resin. This discussion shows that although nanoclays are effective in reducing flammability of unsaturated polyester resin but they do not do so to the same extent as seen for other polymer-nanocomposites systems. Bharadwaj et al [17] have proposed that exfoliation of clay reduces the cross-linking density of the resin, hence enhancement in certain thermal and mechanical properties due to exfoliated clay is counterbalanced by reduced crosslinking of the resin. This also explain why nanoclays are not very effective in reducing flammability of polymers showing low inherent tendencies to crosslink during thermal degradation, such as the polyester resin used in this work.
Conclusions In the unsaturated polyester resin used, organically modified nanoclays affect its thermal stability very slightly. Nanoclays reduce onset of decomposition temperature, peak heat release rate and total heat release values. In the presence of conventional flame retardants, typified by APP, flammability of resin-clay-nano/micro composites is considerably reduced compared to unmodified resin. Choice of organic modifier is an important factor affecting degree of intercalation/exfoliation, thermal stability, flammability and mechanical performance of the resultant polymer.
Acknowledgements The authors wish to acknowledge the financial support from the Engineering and Physical Science Research Council and National Institute of Standards and Technology (NIST), USA, in particular Dr Jeffrey W Gilman for technical and
171 financial support. They also want to thank Scott-Bader for providing samples and technical support.
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