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Synthesis and properties of a novel hyperbranched polyphosphate acrylate applied to UV curable flame retardant coatings
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 1302–1312
www.elsevier.com/locate/europolj
Synthesis and properties of a novel hyperbranched polyphosphate acrylate applied to UV curable flame retardant coatings Zhanguang Huang, Wenfang Shi
*
State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 9 November 2006; received in revised form 14 January 2007; accepted 15 January 2007 Available online 28 January 2007
Abstract A series of hyperbranched polyphosphate acrylates (HPPAs) being used for UV curable flame retardant coatings were prepared by the reaction of tri(acryloyloxyethyl) phosphate (TAEP) with piperazine at given ratios, and characterized using FTIR, 1H NMR and GPC measurements. HPPA was blended with TAEP in different ratios to obtain a series of f UV curable resins. Their maximum photopolymerization rates ðRmax P Þ and final unsaturation conversion (P ) in the cured f films at the presence of a photofragmenting initiator were investigated. The results showed that the P increased along with HPPA content and the pure HPPA has the maximum value of 82.1% in the photo-DSC analysis. The data from dynamic mechanical thermal analysis showed that HPPA has good miscibility with TAEP. The crosslinking density and Tg of the cured film decrease along with the content of HPPA in the blend. The mechanical properties of the cured films were also investigated. Less than 20% HPPA addition improved both the tensile strength and elongation at break without damaging the modulus. The HPPA20TAEP80 film with 20% HPPA addition has the highest tensile strength of 31.7 MPa and an elongation at break two times that of cured TAEP. The flame retardancy of the UV cured films was investigated by the limiting oxygen index (LOI). The cured TAEP/HPPA samples greatly expanded when burning, and the degree of expansion increased along with HPPA content. However, the LOI values decreased from 47.0 to 34.0 along with HPPA content, which can be ascribed to that the flame retardancy of TAEP is mainly acting in the gas phase, whereas HPPA mainly acting in condensed phase, and the gas phase mechanism holds the dominant effect while their blends are burning. 2007 Elsevier Ltd. All rights reserved. Keywords: Flame retardant; Hyperbranched; Phosphate; Photopolymerization kinetics; UV coating
1. Introduction
* Corresponding author. Tel.: +86 551 3603624; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi).
UV curing technology has been widely used in various areas because of it offers several benefits such as superior product quality, safer handling of coating materials, reduced cycle time, increased production capacity, reduced volatile solvents, selective
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.01.035
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312
area irradiation and also facilitates the use of heat sensitive substrates [1–3]. In addition to these important features, this technology can satisfy new requirements for traditional or advanced applications, as it can offer a broad range of the changes in formulation and curing conditions, and thus final properties. The network density depending on the chemical structure of a polymer chain determines the properties of the cured coating. UV curable resins, in general, provide hard, less flexible, even brittle films, which usually have poor adhesion with most substrates, due to the high crosslink density and internal stress derived from the short curing period. Therefore, the demand for polymeric materials of coatings with unique or improved properties relative to existing materials has been focused on the synthesis of novel polymers that possess unique structural features [4,5]. Hyperbranched polymers share some of the structural characters and physical properties of dendrimers, but hyperbranched polymers are much more readily accessible synthetically. They are characterized by a large number of end-groups, which greatly affect their properties. Hult and coworkers [6] have demonstrated that the hyperbranched acrylated polyesters have lower viscosities and higher curing rates as compared to conventional UV curable resins of similar molecular weight. It is also shown that these reactive polymers exhibited excellent hardness, good chemical resistance and low shrinkage upon curing. However, most conventional UV curable resins are flammable, which demands development of flame retardant systems to reduce the fire hazards for some applications, such as the matrix for optical fibers [7] and wood coatings [8]. Despite having a number of technical advantages in applications, the use of halogenated additives in polymers also has drawbacks, e.g. they tend to produce environmental and health problems due to the evolution of corrosive and obscuring smoke during combustion. Recent patents and technical reports indicate a growing interest in halogen-free systems with the predominance of literature focusing on phosphorus-based flame retardants. So non-halogenated systems are of interest, and much effort has been done to use phosphorus-based compounds as the substitutes for halogenated additives [9]. Among the phosphorus-based fire retardants, phosphorus–nitrogen synergistic combination is a more promising system and may be proceeding in the direction.
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In this work, we report the synthesis of a novel phosphorus–nitrogen containing hyperbranched polyphosphate acrylate (HPPA) used as a UV curable flame retardant oligomer in coatings by employing an A2 + B3 type polycondensation. Piperazine used as an A2 component reacted with tri(acryloyloxyethyl) phosphate (TAEP) synthesized in our previous work [10] as a B3 component in mole ratios of 1:0.8, 1:1 and 1:1.2. The obtained resins are denoted HPPA-0.8, HPPA and HPPA-1.2, respectively. HPPA was characterized with FTIR and 1H NMR. The molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC). The flame retardancy of the UV cured film was characterized by the limiting oxygen index (LOI). The viscosity and photopolymerization kinetics of the flame retardant systems, and the dynamic mechanical thermal properties and mechanical behaviors of the cured films were also investigated. 2. Experimental 2.1. Materials All reagents, purchased from the First Reagent Co. of Shanghai, China, were used as received. TAEP was synthesized using phosphorus oxychloride and hydroxylethyl acrylate in our laboratory [10]. 2-Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), used as a photoinitiator, was supplied by Runtec Co., Changzhou, China. 2.2. Synthesis 2.2.1. Synthesis of HPPA Into a 250 ml round-bottom flask equipped with a magnetic stirrer is charged with piperazine (4.00 g, 0.046 mol) and chloroform (100 ml) were added and cooled with an ice-water bath. TAEP (18.22 g, 0.046 mol) dissolved in 80 ml chloroform was added into the above solution dropwise through an addition funnel for 2 h, and left stirring for another 10 h at 25 C, followed by removing chloroform under vacuum, obtaining a yellowish liquid. The schematic outline of synthesis route for HPPA is given in Fig. 1. 2.2.2. Synthesis of HPPA-0.8 and HPPA-1.2 Synthesis of HPPA-0.8 and HPPA-1.2 are performed as above, except for 14.44 g TAEP
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Fig. 1. Synthetic route of HPPA.
(0.037 mol) and 21.86 g TAEP (0.056 mol), respectively used.
Table 1 Viscosity of TAEP, HPPA and their blends Sample
2.3. Sample preparation The mixtures of HPPA with TAEP in different ratios (Table 1) were stirred until the homogenous blends formed. HPPA, TAEP and their blends in the presence of 3 wt% Darocur 1173 exposed to a medium pressing mercury lamp (1 KW, Fusion UV systems, USA) to obtain the cured films. 2.4. Measurements The FTIR spectra were recorded with MAGNAIR 750 spectrometer (Nicolet Instrument Co., USA). The 1H NMR spectrum was obtained with a Bruker Avance 300 spectrometer operating at 300 MHz using chloroform-D as a solvent. The LOI values of cured films were measured using a
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50 HPPA
Formulation (wt%) HPPA
TAEP
0 10 20 30 40 50 100
100 90 80 70 60 50 0
Viscosity (cps, 25 C)
80 120 260 900 2500 5250 90,000
ZRY-type instrument (made in China) with the sheets of 120 · 6.5 · 3 mm3 according to the standard ASTM D635-77. The viscosity at room temperature was measured with a QNX Model spinning viscometer (Tianjin Insstrument Co., Tianjin, China). The photopolymerization rate was monitored by a CDR-1 differential scanning calorimeter (DSC) (Shanghai Balance Instrument Co., Shanghai, China) equipped with a UV spot cure system
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312
BHG-250 (Mejiro Precision Co., Japan). Photopolymerization was carried out in air with sample in an uncovered aluminum pan. The incident light intensity at the sample pan was measured to be 2.4 mW cm2 with a UV power meter. The unsaturation conversion (Pt) is calculated by the formula, Pt = Ht/H1, where Ht is the heat effect within t seconds, H1 is the heat effect of 100% unsaturation conversion. The DSC curves were unified by the weight of samples (g). The polymerization rate is defined by mmolC@C g1 s1, namely, the variation of unsaturation concentration (mmolC@C g1) per second. For calculating the polymerization rate and H1, the value for the heat of polymerization DH0 = 86 J mmol1 (per acrylic unsaturation) was taken [11]. The tensile storage modulus (E 0 ) and tensile loss factors (Tan d) of the UV cured films were measured by a dynamic mechanical thermal analyzer (Diamond DMA, PE Co., USA) at a frequency of 2 Hz and a heating rate of 5 C min1 in the range of 50 to 250 C on the sheets of 25 · 5 · 1 mm. The crosslinking density (Ve), that is, the molar number of elastically effective network chains per cube centimeter of the coatings, was calculated based on the storage modulus on the rubE0 bery plateau region according to [12,13]: V e ¼ 3RT , where E 0 is the elastic storage modulus on the rubbery plateau region, R is the ideal gas constant, and T is the temperature in Kelvin. The mechanical
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properties were measured with an Instron Universal tester (model 1185) at 25 C with a crosshead speed of 25 mm min1. 3. Results and discussion 3.1. Characterization Fig. 1 presents the synthesis route of HPPA. The FTIR spectrum of HPPA-0.8, HPPA and HPPA1.2 are given in Fig. 2. The formation of phosphate structures is revealed by the peaks observed at 1264 cm1 for P@O and 1034, 987 cm1 for P–O– C. Moreover, the FTIR spectrum shows the strong absorption bands at 1733, 1637, 1411 and 810 cm1, indicating the existence of acrylate groups. As shown in Fig. 3, the number average molecular weight (Mn) of HPPA-0.8, HPPA and HPPA-1.2 are experimentally to be 30,800, 54,300 and 39,800 g mol1, respectively, measured by GPC using DMF as an eluent. Their polydispersity are 1.28, 1.17 and 1.26, respectively, as listed in Table 2. The 1H NMR spectrum of HPPA with the assignment is shown in Fig. 4. The three groups of characteristic peaks at 5.80–6.56 ppm are obviously observed in the spectrum, which prove the existence of acrylate group in HPPA molecular structure. In addition to these acrylate main peaks, some additional peaks at 4.00–4.46 and 2.20–3.00 ppm
Fig. 2. FTIR spectra of HPPA-0.8, HPPA and HPPA-1.2.
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Fig. 3. GPC traces of HPPA-0.8, HPPA and HPPA-1.2.
Table 2 GPC data of HPPA-0.8, HPPA and HPPA-1.2 Sample
Mn
Polydispersity
HPPA-0.8 HPPA HPPA-1.2
30,800 54,300 39,800
1.28 1.17 1.26
can also be observed, which pointed to Hb and Hc groups, respectively. As can be seen, the substituent group attached to the same carbon with Hb is oxygen group, whereas that the substituent group attached to the same carbon with Hc is nitrogen or carbonyl group. Since the electronegativity of oxygen atom is more intensive, Hb appears low field (4.00–4.46 ppm) compared with Hc (2.20–3.00 ppm). Moreover, the peaks pointed to –CH2–N< group overlap with that of –CH2–C(@O)– group, which is corresponding to the similar shielding effect between nitrogen or carbonyl group.
to the low inherent viscosity of most hyperbranched polymers. Another reason may be due to its large numbers of low-polar end-groups of HPPA, which inhibited the formation of inter/intramolecular hydrogen bonds. The viscosity of a resin formulation is considered as one of the most important parameters as it affects its flow ability, air release rate and photopolymerization rate, and the final properties of the cured film. Reactive monomers, and sometime, organic solvents, are generally added into to adjust the formulation’s viscosity. As HPPA is a viscous polymer, the addition of modifiers as diluents is necessary to reduce the viscosity. As shown in Table 1, the viscosity of samples goes down sharply when adding co-monomer TAEP into HPPA due to the destruction of inter/intramolecular chain entanglements resulting from the lower viscosity of TAEP. Moreover, the rapid viscosity reduction further indicates that HPPA has good miscibility with TAEP.
3.2. Viscosity 3.3. Photopolymerization kinetics As general knowledge, the viscosity of an oligomer is usually related to the segment density within the volume of a molecule, intermolecular chain entanglement and inter/intramolecular hydrogen bond formed. As listed in Table 1, it can be seen that HPPA presents not so very high viscosity at room temperature with its high molecular weight of 54,300 g mol1, which is commonly corresponding
As known, the properties of a UV cured film, which are very important to its applications, depend not only on the resin composition but also on its photopolymerization kinetics. The most important parameters characterizing the photopolymerization behavior of a multifunctional oligomer are the rate at the peak maximum ðRmax P Þ and the final
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312
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Fig. 4. 1H NMR spectrum of HPPA.
unsaturation conversion (Pf) after a given irradiation time [14]. The UV curing kinetics of TAEP, HPPA and their blends at room temperature are shown in Figs. 5 and 6. Their Rmax and Pf are listed in Table 3. P From Fig. 5, it can be seen that the photopolymerization rate of each sample shows a steep increase at the start of reaction, reaching to a maximum rate Rmax P , and then drops rapidly. And it can also be observed that the formulations containing both HPPA and TAEP polymerized much faster (higher Rmax and shorter time needed to reach Rmax P P ) than that containing individual component. The lowest reactivity of HPPA results from its high viscosity (as high as 1000 times of TAEP), which restricts the diffusional mobility of HPPA molecules. This result is in good agreement with the results reported in the literature by Andrzejewska and Andrzejewski [14]. Moreover, the Rmax decreases with increasing P HPPA content in TAEP/HPPA blends. It is due to the fact that HPPA has much lower concentration of double bond but much higher viscosity than TAEP. The double bond concentration of HPPA is 2.09 mmol g1 compared with 7.65 mmol g1 for TAEP. However, an opposite trend of HPPA is found for Pf compared with Rmax from Table 3. P
The Pf increases from 66.9% to 82.1% with increasing HPPA content. This is mainly ascribed to two reasons. One reason can be due to its lower concentration of double bond, which could reduce the curing rate of the blends and then prevent the gel-point of system appearing prematurely. As a result, fewer double bonds trapped in the early-formed gel took part in further polymerization when more HPPA was added [15]. The other is the longer spacer chain between two double bonds in the molecular structure of HPPA. As has been proved that [16–18], the longer of the spacer chain is, the higher of the final unsaturation conversion. Increasing HPPA content results in the decrease of crosslinking density and increase of the mobility of double bonds, which restricted the vitrification. 3.4. Dynamic mechanical thermal properties The dynamic mechanical behavior of the UV cured TAEP films with HPPA addition is obtained as a function of the temperature beginning in the glassy state of each composition to the rubbery plateau of each material (Fig. 7). The crosslinking density of a polymer can be estimated from the plateau of elastic modulus in its rubbery state. As shown in
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Fig. 5. Photopolymerzation rates versus irradiation time of TAEP, HPPA and their blends (25 C).
Fig. 6. Unsaturation conversion versus irradiation time of TAEP, HPPA and their blends (25 C).
Table 4, the crosslinking density decreases from 17.26 to 3.26 as the HPPA content increases to 50 wt%. As far as the chemical structures of these networks are concerned, this behavior is to be expected since the lower concentration of double bond and a longer flexible molecular chain of HPPA
usually lead to more loose networks, as mentioned above. The incorporated hyperbranched polyphosphate, which greatly reduce the rigidity of the polymer chains, makes chain motion possible also at lower temperature. Fig. 7 also shows the plots of loss factor (Tan d) versus temperature. The glass
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312 Table 3 Photopolymerization rates at peak maximum and final unsaturation conversion for TAEP, HPPA and their blends Sample
Rmax ðJ g1 s1 Þ P
Pf (%)
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50 HPPA
21.97 47.36 38.08 33.86 30.89 27.40 10.53
66.9 69.5 72.4 74.3 76.8 79.0 82.1
transition temperature (Tg) of crosslinked materials can be detected as the relaxation peak of the loss factor. As can be seen from Table 4, there is a decrease in the glass transition temperature from 105 to 50 C with increasing HPPA content. Such a decrease can be ascribed to the lower crosslinking density and a greater level of flexibility and mobility of the molecular chain of HPPA. From the Tg of TAEP/HPPA blends, it is suggest that HPPA has potential to be used for high performance applications in particular in mixtures with TAEP. Moreover, the analysis of the height and width of the relaxation peak shows the trends in the crosslinking density and network homogeneity, as the composition of the material changes. The height of the Tan d peak, which is associated with the crosslinking density, increases as HPPA content increases. Because Tan d is the ratio of viscous com-
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ponents to elastic components, it can be assumed that the increased height is associated with the higher segmental mobility and more relaxing species, and is therefore indicative that the networks for the HPPA-rich samples are more looser. The Ts/Tg ratio expresses the width of Tan d peak, where Ts is the softening point defined as the extrapolated onset of the drop of storage modulus. It is a rule that a higher Ts/Tg ratio leads to a narrower Tan d peak. The peak width broadens as the number of branching modes increases, which produces a wider distribution of structures. The range of temperatures at which the different network segments gain mobility therefore increases. As can be seen from Table 4, the Ts/Tg values of all samples are 0.88 approximately. There are no significant differences among the samples, thus showing similar branching distribution for all samples. This uniformity of the Tan d peak indicates the absence of any network
Table 4 DMTA results of the UV cured TAEP and its blend films with HPPA Sample
Ts (C)
Tg (C)
Ts/Tg
Ve (mmol cm3)
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50
61 59 47 39 21 15
105 100 95 83 58 50
0.88 0.89 0.87 0.88 0.89 0.89
17.26 11.41 8.27 6.51 4.89 3.97
Fig. 7. DMTA curves of UV cured TAEP and its blend films with HPPA.
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heterogeneity [19], which implies the good miscibility between HPPA and TAEP.
3.6. Flame retardance of HPPA and its blends with TAEP
3.5. Mechanical properties
The LOI, which is as a qualitative method to rank the flammability of a material, is the minimum oxygen concentration in an oxygen/nitrogen mixture that will just support the combustion. As shown in Table 1, the LOI values of cured samples having different ratios of HPPA to TAEP decrease from 47.0 to 34.0 almost linearly by increasing the contents of HPPA. From Fig. 9, the expanding charred crusts were formed after the TAEP/HPPA blends burned, and the degree of expansion increases with the HPPA content increasing. As reported in our previous work [21–25], the flame retardants containing phosphorus and nitrogen generally possess a synergistic effect between phosphorus and nitrogen as they are burning. However, it was found that the LOI values decrease along with decreasing the phosphorus content from 7.90% to 6.48% and increasing the nitrogen content to 5.85%. In other word, there is not a distinct synergistic effect exists between nitrogen and phosphorus in this system. It may be ascribed to the fact that different flame retardant mechanisms are followed in different systems. Based on this fact, it can be supposed that the flame retardancy of TAEP is mainly acting in the gas phase, whereas HPPA mainly acting in condensed phase, and the gas phase mechanism holds the dominant effect while their blends are burning. As well known, the condensed phase mechanisms facilitates char
The stress–strain curves of UV cured blend films are shown in Fig. 8. The tensile strength and elongation at break are listed in Table 5. With an increase of HPPA content, the rubbery properties of the cured films are significantly improved. The elongation at break increases progressively, whereas the tensile strength decreases. In particular, the UV cured HPPA10TAEP90 and HPPA20TAEP80 films have higher tensile strength and elongation at break than pure TAEP. As has been discussed above in Section 3.3, the final unsaturation conversion increases with increasing HPPA content, which is responsible for the increase in the tensile strength of the films [20]. However, the addition of HPPA leads to improving the rubbery properties of cured blend films, which results in the increase of elongation-at-break value while decrease of the tensile strength. Therefore, the tensile strength and elongation at break of cured TAEP/HPPA films are the results of the competition of both factors. When less than 20% HPPA is added, an improvement of both the tensile strength and elongation at break of cured TAEP/HPPA films can be obtained. However, the tensile strength decreases with the excess addition of HPPA, although the elongation at break increases continually.
Fig. 8. Stress–strain curves of UV cured TAEP and its blend films with HPPA.
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Table 5 Flame retardancy, tensile strength and elongation at break of the UV cured TAEP films with HPPA addition Sample
Tensile strength (MPa)
Elongation at break (%)
Phosphorus (wt%)
Nitrogen (wt%)
LOI
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50 HPPA
13.1 21.9 31.7 26.1 19.4 16.3 –
2.5 3.3 5.0 5.8 7.1 8.3 –
7.90 7.76 7.62 7.47 7.33 7.19 6.48
0 0.59 1.17 1.76 2.34 2.93 5.85
47.0 43.0 40.5 38.5 37.0 35.0 34.0
Fig. 9. Photographs of the samples after and before combustion: (a) and (A) TAEP; (b) and (B) HPPA10TAEP90; (c) and (C) HPPA20TAEP80; (d) and (D) HPPA30TAEP70; (e) and (E) HPPA40TAEP60; (f) and (F) HPPA50TAEP50; (g) and (G) HPPA.
formation, and reduces the combustible volatiles and therefore, the total fuel support of the flame. Furthermore, the char can act as a barrier, decreasing the mass loss rate. While gas phase mechanism interrupts the exothermic process and thus suppress combustion by capturing free radicals through the phosphorus volatiles, such as P2, PO, PO2, HPO2, etc. The larger the degree of expansion, the thicker the insulating layer will be formed to protect the underlying material from burning. Simultaneously, in the presence of intumescent effect, some of phosphorus volatiles are also prevented emitting from
the expanding charred crust, thereby reducing the effect of gas phase mechanism with increasing HPPA content. Therefore, the LOI value decreases, whereas the degree of expansion increases with increasing HPPA content. 4. Conclusions A series of hyperbranched polyphospophate acrylates have been synthesized and characterized successfully. HPPA blended with TAEP can greatly reduce its viscosity. The photopolymerization
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kinetics study proved that the formulations containing both HPPA and TAEP can polymerize much faster than individual component. The Rmax P decreases with increasing HPPA content in the TAEP/HPPA blend, due to the lower concentration of double bond and higher viscosity of HPPA. The Pf increases linearly from 66.9% to 82.1% with increasing HPPA content, which due to the fact that HPPA has much longer spacer chain than TAEP. Moreover, the good miscibility between HPPA and TAEP is also observed from the dynamic mechanical thermal analysis. The crosslinking density and Tg of the cured film decrease along with the content of HPPA in the blend. The mechanical data show that the toughness of TAEP is improved by the addition of HPPA. Less than 20% HPPA addition improves both the tensile strength and elongation at break without damaging the modulus. The HPPA20TAEP80 film has the highest tensile strength of 31.7 MPa and an elongation at break two times of that for cured TAEP. Moreover, it has been found that the flame retardancy of TAEP is mainly acting in the gas phase, whereas HPPA mainly acting in the condensed phase, and the gas phase mechanism holds the dominant effect as their blends are burning. Acknowledgements The authors gratefully acknowledge the financial supports of the NKBRSF Project (No.
2001CB409600, China) and NSFC Project (No. 50633010, China). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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European Polymer Journal 43 (2007) 1302–1312
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Synthesis and properties of a novel hyperbranched polyphosphate acrylate applied to UV curable flame retardant coatings Zhanguang Huang, Wenfang Shi
*
State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 9 November 2006; received in revised form 14 January 2007; accepted 15 January 2007 Available online 28 January 2007
Abstract A series of hyperbranched polyphosphate acrylates (HPPAs) being used for UV curable flame retardant coatings were prepared by the reaction of tri(acryloyloxyethyl) phosphate (TAEP) with piperazine at given ratios, and characterized using FTIR, 1H NMR and GPC measurements. HPPA was blended with TAEP in different ratios to obtain a series of f UV curable resins. Their maximum photopolymerization rates ðRmax P Þ and final unsaturation conversion (P ) in the cured f films at the presence of a photofragmenting initiator were investigated. The results showed that the P increased along with HPPA content and the pure HPPA has the maximum value of 82.1% in the photo-DSC analysis. The data from dynamic mechanical thermal analysis showed that HPPA has good miscibility with TAEP. The crosslinking density and Tg of the cured film decrease along with the content of HPPA in the blend. The mechanical properties of the cured films were also investigated. Less than 20% HPPA addition improved both the tensile strength and elongation at break without damaging the modulus. The HPPA20TAEP80 film with 20% HPPA addition has the highest tensile strength of 31.7 MPa and an elongation at break two times that of cured TAEP. The flame retardancy of the UV cured films was investigated by the limiting oxygen index (LOI). The cured TAEP/HPPA samples greatly expanded when burning, and the degree of expansion increased along with HPPA content. However, the LOI values decreased from 47.0 to 34.0 along with HPPA content, which can be ascribed to that the flame retardancy of TAEP is mainly acting in the gas phase, whereas HPPA mainly acting in condensed phase, and the gas phase mechanism holds the dominant effect while their blends are burning. 2007 Elsevier Ltd. All rights reserved. Keywords: Flame retardant; Hyperbranched; Phosphate; Photopolymerization kinetics; UV coating
1. Introduction
* Corresponding author. Tel.: +86 551 3603624; fax: +86 551 3606630. E-mail address: [email protected] (W. Shi).
UV curing technology has been widely used in various areas because of it offers several benefits such as superior product quality, safer handling of coating materials, reduced cycle time, increased production capacity, reduced volatile solvents, selective
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.01.035
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312
area irradiation and also facilitates the use of heat sensitive substrates [1–3]. In addition to these important features, this technology can satisfy new requirements for traditional or advanced applications, as it can offer a broad range of the changes in formulation and curing conditions, and thus final properties. The network density depending on the chemical structure of a polymer chain determines the properties of the cured coating. UV curable resins, in general, provide hard, less flexible, even brittle films, which usually have poor adhesion with most substrates, due to the high crosslink density and internal stress derived from the short curing period. Therefore, the demand for polymeric materials of coatings with unique or improved properties relative to existing materials has been focused on the synthesis of novel polymers that possess unique structural features [4,5]. Hyperbranched polymers share some of the structural characters and physical properties of dendrimers, but hyperbranched polymers are much more readily accessible synthetically. They are characterized by a large number of end-groups, which greatly affect their properties. Hult and coworkers [6] have demonstrated that the hyperbranched acrylated polyesters have lower viscosities and higher curing rates as compared to conventional UV curable resins of similar molecular weight. It is also shown that these reactive polymers exhibited excellent hardness, good chemical resistance and low shrinkage upon curing. However, most conventional UV curable resins are flammable, which demands development of flame retardant systems to reduce the fire hazards for some applications, such as the matrix for optical fibers [7] and wood coatings [8]. Despite having a number of technical advantages in applications, the use of halogenated additives in polymers also has drawbacks, e.g. they tend to produce environmental and health problems due to the evolution of corrosive and obscuring smoke during combustion. Recent patents and technical reports indicate a growing interest in halogen-free systems with the predominance of literature focusing on phosphorus-based flame retardants. So non-halogenated systems are of interest, and much effort has been done to use phosphorus-based compounds as the substitutes for halogenated additives [9]. Among the phosphorus-based fire retardants, phosphorus–nitrogen synergistic combination is a more promising system and may be proceeding in the direction.
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In this work, we report the synthesis of a novel phosphorus–nitrogen containing hyperbranched polyphosphate acrylate (HPPA) used as a UV curable flame retardant oligomer in coatings by employing an A2 + B3 type polycondensation. Piperazine used as an A2 component reacted with tri(acryloyloxyethyl) phosphate (TAEP) synthesized in our previous work [10] as a B3 component in mole ratios of 1:0.8, 1:1 and 1:1.2. The obtained resins are denoted HPPA-0.8, HPPA and HPPA-1.2, respectively. HPPA was characterized with FTIR and 1H NMR. The molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC). The flame retardancy of the UV cured film was characterized by the limiting oxygen index (LOI). The viscosity and photopolymerization kinetics of the flame retardant systems, and the dynamic mechanical thermal properties and mechanical behaviors of the cured films were also investigated. 2. Experimental 2.1. Materials All reagents, purchased from the First Reagent Co. of Shanghai, China, were used as received. TAEP was synthesized using phosphorus oxychloride and hydroxylethyl acrylate in our laboratory [10]. 2-Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), used as a photoinitiator, was supplied by Runtec Co., Changzhou, China. 2.2. Synthesis 2.2.1. Synthesis of HPPA Into a 250 ml round-bottom flask equipped with a magnetic stirrer is charged with piperazine (4.00 g, 0.046 mol) and chloroform (100 ml) were added and cooled with an ice-water bath. TAEP (18.22 g, 0.046 mol) dissolved in 80 ml chloroform was added into the above solution dropwise through an addition funnel for 2 h, and left stirring for another 10 h at 25 C, followed by removing chloroform under vacuum, obtaining a yellowish liquid. The schematic outline of synthesis route for HPPA is given in Fig. 1. 2.2.2. Synthesis of HPPA-0.8 and HPPA-1.2 Synthesis of HPPA-0.8 and HPPA-1.2 are performed as above, except for 14.44 g TAEP
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Fig. 1. Synthetic route of HPPA.
(0.037 mol) and 21.86 g TAEP (0.056 mol), respectively used.
Table 1 Viscosity of TAEP, HPPA and their blends Sample
2.3. Sample preparation The mixtures of HPPA with TAEP in different ratios (Table 1) were stirred until the homogenous blends formed. HPPA, TAEP and their blends in the presence of 3 wt% Darocur 1173 exposed to a medium pressing mercury lamp (1 KW, Fusion UV systems, USA) to obtain the cured films. 2.4. Measurements The FTIR spectra were recorded with MAGNAIR 750 spectrometer (Nicolet Instrument Co., USA). The 1H NMR spectrum was obtained with a Bruker Avance 300 spectrometer operating at 300 MHz using chloroform-D as a solvent. The LOI values of cured films were measured using a
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50 HPPA
Formulation (wt%) HPPA
TAEP
0 10 20 30 40 50 100
100 90 80 70 60 50 0
Viscosity (cps, 25 C)
80 120 260 900 2500 5250 90,000
ZRY-type instrument (made in China) with the sheets of 120 · 6.5 · 3 mm3 according to the standard ASTM D635-77. The viscosity at room temperature was measured with a QNX Model spinning viscometer (Tianjin Insstrument Co., Tianjin, China). The photopolymerization rate was monitored by a CDR-1 differential scanning calorimeter (DSC) (Shanghai Balance Instrument Co., Shanghai, China) equipped with a UV spot cure system
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312
BHG-250 (Mejiro Precision Co., Japan). Photopolymerization was carried out in air with sample in an uncovered aluminum pan. The incident light intensity at the sample pan was measured to be 2.4 mW cm2 with a UV power meter. The unsaturation conversion (Pt) is calculated by the formula, Pt = Ht/H1, where Ht is the heat effect within t seconds, H1 is the heat effect of 100% unsaturation conversion. The DSC curves were unified by the weight of samples (g). The polymerization rate is defined by mmolC@C g1 s1, namely, the variation of unsaturation concentration (mmolC@C g1) per second. For calculating the polymerization rate and H1, the value for the heat of polymerization DH0 = 86 J mmol1 (per acrylic unsaturation) was taken [11]. The tensile storage modulus (E 0 ) and tensile loss factors (Tan d) of the UV cured films were measured by a dynamic mechanical thermal analyzer (Diamond DMA, PE Co., USA) at a frequency of 2 Hz and a heating rate of 5 C min1 in the range of 50 to 250 C on the sheets of 25 · 5 · 1 mm. The crosslinking density (Ve), that is, the molar number of elastically effective network chains per cube centimeter of the coatings, was calculated based on the storage modulus on the rubE0 bery plateau region according to [12,13]: V e ¼ 3RT , where E 0 is the elastic storage modulus on the rubbery plateau region, R is the ideal gas constant, and T is the temperature in Kelvin. The mechanical
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properties were measured with an Instron Universal tester (model 1185) at 25 C with a crosshead speed of 25 mm min1. 3. Results and discussion 3.1. Characterization Fig. 1 presents the synthesis route of HPPA. The FTIR spectrum of HPPA-0.8, HPPA and HPPA1.2 are given in Fig. 2. The formation of phosphate structures is revealed by the peaks observed at 1264 cm1 for P@O and 1034, 987 cm1 for P–O– C. Moreover, the FTIR spectrum shows the strong absorption bands at 1733, 1637, 1411 and 810 cm1, indicating the existence of acrylate groups. As shown in Fig. 3, the number average molecular weight (Mn) of HPPA-0.8, HPPA and HPPA-1.2 are experimentally to be 30,800, 54,300 and 39,800 g mol1, respectively, measured by GPC using DMF as an eluent. Their polydispersity are 1.28, 1.17 and 1.26, respectively, as listed in Table 2. The 1H NMR spectrum of HPPA with the assignment is shown in Fig. 4. The three groups of characteristic peaks at 5.80–6.56 ppm are obviously observed in the spectrum, which prove the existence of acrylate group in HPPA molecular structure. In addition to these acrylate main peaks, some additional peaks at 4.00–4.46 and 2.20–3.00 ppm
Fig. 2. FTIR spectra of HPPA-0.8, HPPA and HPPA-1.2.
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Fig. 3. GPC traces of HPPA-0.8, HPPA and HPPA-1.2.
Table 2 GPC data of HPPA-0.8, HPPA and HPPA-1.2 Sample
Mn
Polydispersity
HPPA-0.8 HPPA HPPA-1.2
30,800 54,300 39,800
1.28 1.17 1.26
can also be observed, which pointed to Hb and Hc groups, respectively. As can be seen, the substituent group attached to the same carbon with Hb is oxygen group, whereas that the substituent group attached to the same carbon with Hc is nitrogen or carbonyl group. Since the electronegativity of oxygen atom is more intensive, Hb appears low field (4.00–4.46 ppm) compared with Hc (2.20–3.00 ppm). Moreover, the peaks pointed to –CH2–N< group overlap with that of –CH2–C(@O)– group, which is corresponding to the similar shielding effect between nitrogen or carbonyl group.
to the low inherent viscosity of most hyperbranched polymers. Another reason may be due to its large numbers of low-polar end-groups of HPPA, which inhibited the formation of inter/intramolecular hydrogen bonds. The viscosity of a resin formulation is considered as one of the most important parameters as it affects its flow ability, air release rate and photopolymerization rate, and the final properties of the cured film. Reactive monomers, and sometime, organic solvents, are generally added into to adjust the formulation’s viscosity. As HPPA is a viscous polymer, the addition of modifiers as diluents is necessary to reduce the viscosity. As shown in Table 1, the viscosity of samples goes down sharply when adding co-monomer TAEP into HPPA due to the destruction of inter/intramolecular chain entanglements resulting from the lower viscosity of TAEP. Moreover, the rapid viscosity reduction further indicates that HPPA has good miscibility with TAEP.
3.2. Viscosity 3.3. Photopolymerization kinetics As general knowledge, the viscosity of an oligomer is usually related to the segment density within the volume of a molecule, intermolecular chain entanglement and inter/intramolecular hydrogen bond formed. As listed in Table 1, it can be seen that HPPA presents not so very high viscosity at room temperature with its high molecular weight of 54,300 g mol1, which is commonly corresponding
As known, the properties of a UV cured film, which are very important to its applications, depend not only on the resin composition but also on its photopolymerization kinetics. The most important parameters characterizing the photopolymerization behavior of a multifunctional oligomer are the rate at the peak maximum ðRmax P Þ and the final
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312
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Fig. 4. 1H NMR spectrum of HPPA.
unsaturation conversion (Pf) after a given irradiation time [14]. The UV curing kinetics of TAEP, HPPA and their blends at room temperature are shown in Figs. 5 and 6. Their Rmax and Pf are listed in Table 3. P From Fig. 5, it can be seen that the photopolymerization rate of each sample shows a steep increase at the start of reaction, reaching to a maximum rate Rmax P , and then drops rapidly. And it can also be observed that the formulations containing both HPPA and TAEP polymerized much faster (higher Rmax and shorter time needed to reach Rmax P P ) than that containing individual component. The lowest reactivity of HPPA results from its high viscosity (as high as 1000 times of TAEP), which restricts the diffusional mobility of HPPA molecules. This result is in good agreement with the results reported in the literature by Andrzejewska and Andrzejewski [14]. Moreover, the Rmax decreases with increasing P HPPA content in TAEP/HPPA blends. It is due to the fact that HPPA has much lower concentration of double bond but much higher viscosity than TAEP. The double bond concentration of HPPA is 2.09 mmol g1 compared with 7.65 mmol g1 for TAEP. However, an opposite trend of HPPA is found for Pf compared with Rmax from Table 3. P
The Pf increases from 66.9% to 82.1% with increasing HPPA content. This is mainly ascribed to two reasons. One reason can be due to its lower concentration of double bond, which could reduce the curing rate of the blends and then prevent the gel-point of system appearing prematurely. As a result, fewer double bonds trapped in the early-formed gel took part in further polymerization when more HPPA was added [15]. The other is the longer spacer chain between two double bonds in the molecular structure of HPPA. As has been proved that [16–18], the longer of the spacer chain is, the higher of the final unsaturation conversion. Increasing HPPA content results in the decrease of crosslinking density and increase of the mobility of double bonds, which restricted the vitrification. 3.4. Dynamic mechanical thermal properties The dynamic mechanical behavior of the UV cured TAEP films with HPPA addition is obtained as a function of the temperature beginning in the glassy state of each composition to the rubbery plateau of each material (Fig. 7). The crosslinking density of a polymer can be estimated from the plateau of elastic modulus in its rubbery state. As shown in
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Fig. 5. Photopolymerzation rates versus irradiation time of TAEP, HPPA and their blends (25 C).
Fig. 6. Unsaturation conversion versus irradiation time of TAEP, HPPA and their blends (25 C).
Table 4, the crosslinking density decreases from 17.26 to 3.26 as the HPPA content increases to 50 wt%. As far as the chemical structures of these networks are concerned, this behavior is to be expected since the lower concentration of double bond and a longer flexible molecular chain of HPPA
usually lead to more loose networks, as mentioned above. The incorporated hyperbranched polyphosphate, which greatly reduce the rigidity of the polymer chains, makes chain motion possible also at lower temperature. Fig. 7 also shows the plots of loss factor (Tan d) versus temperature. The glass
Z. Huang, W. Shi / European Polymer Journal 43 (2007) 1302–1312 Table 3 Photopolymerization rates at peak maximum and final unsaturation conversion for TAEP, HPPA and their blends Sample
Rmax ðJ g1 s1 Þ P
Pf (%)
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50 HPPA
21.97 47.36 38.08 33.86 30.89 27.40 10.53
66.9 69.5 72.4 74.3 76.8 79.0 82.1
transition temperature (Tg) of crosslinked materials can be detected as the relaxation peak of the loss factor. As can be seen from Table 4, there is a decrease in the glass transition temperature from 105 to 50 C with increasing HPPA content. Such a decrease can be ascribed to the lower crosslinking density and a greater level of flexibility and mobility of the molecular chain of HPPA. From the Tg of TAEP/HPPA blends, it is suggest that HPPA has potential to be used for high performance applications in particular in mixtures with TAEP. Moreover, the analysis of the height and width of the relaxation peak shows the trends in the crosslinking density and network homogeneity, as the composition of the material changes. The height of the Tan d peak, which is associated with the crosslinking density, increases as HPPA content increases. Because Tan d is the ratio of viscous com-
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ponents to elastic components, it can be assumed that the increased height is associated with the higher segmental mobility and more relaxing species, and is therefore indicative that the networks for the HPPA-rich samples are more looser. The Ts/Tg ratio expresses the width of Tan d peak, where Ts is the softening point defined as the extrapolated onset of the drop of storage modulus. It is a rule that a higher Ts/Tg ratio leads to a narrower Tan d peak. The peak width broadens as the number of branching modes increases, which produces a wider distribution of structures. The range of temperatures at which the different network segments gain mobility therefore increases. As can be seen from Table 4, the Ts/Tg values of all samples are 0.88 approximately. There are no significant differences among the samples, thus showing similar branching distribution for all samples. This uniformity of the Tan d peak indicates the absence of any network
Table 4 DMTA results of the UV cured TAEP and its blend films with HPPA Sample
Ts (C)
Tg (C)
Ts/Tg
Ve (mmol cm3)
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50
61 59 47 39 21 15
105 100 95 83 58 50
0.88 0.89 0.87 0.88 0.89 0.89
17.26 11.41 8.27 6.51 4.89 3.97
Fig. 7. DMTA curves of UV cured TAEP and its blend films with HPPA.
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heterogeneity [19], which implies the good miscibility between HPPA and TAEP.
3.6. Flame retardance of HPPA and its blends with TAEP
3.5. Mechanical properties
The LOI, which is as a qualitative method to rank the flammability of a material, is the minimum oxygen concentration in an oxygen/nitrogen mixture that will just support the combustion. As shown in Table 1, the LOI values of cured samples having different ratios of HPPA to TAEP decrease from 47.0 to 34.0 almost linearly by increasing the contents of HPPA. From Fig. 9, the expanding charred crusts were formed after the TAEP/HPPA blends burned, and the degree of expansion increases with the HPPA content increasing. As reported in our previous work [21–25], the flame retardants containing phosphorus and nitrogen generally possess a synergistic effect between phosphorus and nitrogen as they are burning. However, it was found that the LOI values decrease along with decreasing the phosphorus content from 7.90% to 6.48% and increasing the nitrogen content to 5.85%. In other word, there is not a distinct synergistic effect exists between nitrogen and phosphorus in this system. It may be ascribed to the fact that different flame retardant mechanisms are followed in different systems. Based on this fact, it can be supposed that the flame retardancy of TAEP is mainly acting in the gas phase, whereas HPPA mainly acting in condensed phase, and the gas phase mechanism holds the dominant effect while their blends are burning. As well known, the condensed phase mechanisms facilitates char
The stress–strain curves of UV cured blend films are shown in Fig. 8. The tensile strength and elongation at break are listed in Table 5. With an increase of HPPA content, the rubbery properties of the cured films are significantly improved. The elongation at break increases progressively, whereas the tensile strength decreases. In particular, the UV cured HPPA10TAEP90 and HPPA20TAEP80 films have higher tensile strength and elongation at break than pure TAEP. As has been discussed above in Section 3.3, the final unsaturation conversion increases with increasing HPPA content, which is responsible for the increase in the tensile strength of the films [20]. However, the addition of HPPA leads to improving the rubbery properties of cured blend films, which results in the increase of elongation-at-break value while decrease of the tensile strength. Therefore, the tensile strength and elongation at break of cured TAEP/HPPA films are the results of the competition of both factors. When less than 20% HPPA is added, an improvement of both the tensile strength and elongation at break of cured TAEP/HPPA films can be obtained. However, the tensile strength decreases with the excess addition of HPPA, although the elongation at break increases continually.
Fig. 8. Stress–strain curves of UV cured TAEP and its blend films with HPPA.
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Table 5 Flame retardancy, tensile strength and elongation at break of the UV cured TAEP films with HPPA addition Sample
Tensile strength (MPa)
Elongation at break (%)
Phosphorus (wt%)
Nitrogen (wt%)
LOI
TAEP HPPA10TAEP90 HPPA20TAEP80 HPPA30TAEP70 HPPA40TAEP60 HPPA50TAEP50 HPPA
13.1 21.9 31.7 26.1 19.4 16.3 –
2.5 3.3 5.0 5.8 7.1 8.3 –
7.90 7.76 7.62 7.47 7.33 7.19 6.48
0 0.59 1.17 1.76 2.34 2.93 5.85
47.0 43.0 40.5 38.5 37.0 35.0 34.0
Fig. 9. Photographs of the samples after and before combustion: (a) and (A) TAEP; (b) and (B) HPPA10TAEP90; (c) and (C) HPPA20TAEP80; (d) and (D) HPPA30TAEP70; (e) and (E) HPPA40TAEP60; (f) and (F) HPPA50TAEP50; (g) and (G) HPPA.
formation, and reduces the combustible volatiles and therefore, the total fuel support of the flame. Furthermore, the char can act as a barrier, decreasing the mass loss rate. While gas phase mechanism interrupts the exothermic process and thus suppress combustion by capturing free radicals through the phosphorus volatiles, such as P2, PO, PO2, HPO2, etc. The larger the degree of expansion, the thicker the insulating layer will be formed to protect the underlying material from burning. Simultaneously, in the presence of intumescent effect, some of phosphorus volatiles are also prevented emitting from
the expanding charred crust, thereby reducing the effect of gas phase mechanism with increasing HPPA content. Therefore, the LOI value decreases, whereas the degree of expansion increases with increasing HPPA content. 4. Conclusions A series of hyperbranched polyphospophate acrylates have been synthesized and characterized successfully. HPPA blended with TAEP can greatly reduce its viscosity. The photopolymerization
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kinetics study proved that the formulations containing both HPPA and TAEP can polymerize much faster than individual component. The Rmax P decreases with increasing HPPA content in the TAEP/HPPA blend, due to the lower concentration of double bond and higher viscosity of HPPA. The Pf increases linearly from 66.9% to 82.1% with increasing HPPA content, which due to the fact that HPPA has much longer spacer chain than TAEP. Moreover, the good miscibility between HPPA and TAEP is also observed from the dynamic mechanical thermal analysis. The crosslinking density and Tg of the cured film decrease along with the content of HPPA in the blend. The mechanical data show that the toughness of TAEP is improved by the addition of HPPA. Less than 20% HPPA addition improves both the tensile strength and elongation at break without damaging the modulus. The HPPA20TAEP80 film has the highest tensile strength of 31.7 MPa and an elongation at break two times of that for cured TAEP. Moreover, it has been found that the flame retardancy of TAEP is mainly acting in the gas phase, whereas HPPA mainly acting in the condensed phase, and the gas phase mechanism holds the dominant effect as their blends are burning. Acknowledgements The authors gratefully acknowledge the financial supports of the NKBRSF Project (No.
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