Flame-Retardant Pressure-Sensitive Adhesives Derived from

Mar 15, 2017 - The PSAs were based on renewable materials without any volatile organic compound, thus being environmentally friendly together with hav...
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Research Article pubs.acs.org/journal/ascecg

Flame-Retardant Pressure-Sensitive Adhesives Derived from Epoxidized Soybean Oil and Phosphorus-Containing Dicarboxylic Acids Xiao-Lin Wang, Li Chen,* Jia-Ning Wu, Teng Fu, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China ABSTRACT: In this study, novel biobased pressure-sensitive adhesives (PSAs) derived from epoxidized soybean oils and carboxylic acidterminated polyesters were developed with flame retardance, thermal stability, and peel strength comparable to those of current PSAs. The dynamic mechanical analysis indicated that the PSAs exhibited a dynamic mechanical response consistent with related high-performance PSAs. The thermal properties of the PSAs were investigated by thermogravimetric analysis, and the results suggested that the onset decomposition temperatures in both nitrogen and air atmospheres were improved by incorporating both 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10phosphaphenanthrene-10-oxide (DDP) and 2-(6-oxido-6H-dibenzoxaphosphorin-6-yl)-1,4-hydroxyethoxyphenylene (DOPOHQ-HE) as the flame-retardant monomer. Microscale combustion calorimetry, the limiting oxygen index test, UL-94, and the test method for flame resistance of PSA tapes were used to evaluate the flame retardance of the PSAs. With an increase in the content of the flame-retardant monomers, the flame retardance of two phosphorus-containing PSAs improved. The PSAs were based on renewable materials without any volatile organic compound, thus being environmentally friendly together with having the expected thermal stability and flame retardance. If we take advantage of these features, the PSAs can provide more opportunities for versatile applications. KEYWORDS: Epoxidized soybean oil, Pressure-sensitive adhesive, Peel strength, Frame retardance, Viscoelasticity

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co-workers21,22 obtained PSAs by reacting ESO with at least one dibasic acid (e.g., sebacic acid) or anhydride at molar ratios ranging from 3:1 to 1:3. The subsequent curing led to a tacky coating that had good adhesive properties for various substrates. Sun et al.23−25 proposed a renewable PSA derived from ESO, dihydroxyl soybean oil, fatty acid methyl ester, and phosphoric acid as a cross-linker through a simple curing process, showing attractive thermal stability, transparency, and tolerance to organic solvents as well as clear removal and peel strength comparable to those of commercial PSA tapes such as Postit notes and Scotch Magic Tape. Vendamme et al.26,27obtained renewable self-adhesive coatings showing tunable viscoelastic properties; hydroxyl-telechelic polyester as the network precursor with well-defined amounts of carboxylic acid groups was prepared with architectures by melt polycondensation of dimerized fatty acids with fatty diols or isosorbide, which was subsequently cured with maleic anhydride-modified triglycerides or epoxidized plant oils.

INTRODUCTION Pressure-sensitive adhesives (PSAs) are special types of adhesives that form a bond when external pressure is applied to make the adhesive cling to the adherend. Because of their soft nature and liquid viscoelasticity, they can form a contact with rough and nonwetting surfaces.1−5 According to the required technical specifications, such as permanent or removable applications, PSAs must bond materials efficiently in a fast and safe way.5−7 Except for natural rubbers, PSA categories are still based on petroleum.8−11 Polymers derived from renewable resources have recently experienced a remarkable resurgence because of long-term environmental and availability concerns associated with petroleum derivatives.10,11 Among the different kinds of renewable raw materials, plant oils are currently one of the most abundant, most biodegradable, least toxic, and cheapest renewable raw materials, which have been widely used as promising monomers of thermosetting composites in various applications such as foams, elastomers, coatings, and paints.12−17 In addition, the long aliphatic chains of the fatty acids impart unique properties to the resulting polymeric materials such as elasticity, flexibility, hydrolytic stability, hydrophobicity, and low glass transition temperatures (Tg), making them suitable for PSAs.18−20 Li and © XXXX American Chemical Society

Received: December 29, 2016 Revised: February 12, 2017

A

DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Chemical Compositions of the Carboxyl-Terminated Polyesters sample

SEAa

PDOa

DDPa

DOPO-HQ-HEa

Ptb (%)

Pec (%)

Mw (g/mol)

PDI

AVd (mg of KOH/g)

Tge (°C)

P1 P2 P3 P4 P5 P6

100 90 80 100 100 100

100 100 100 90 80 70

− 10 20 − − −

− − − 10 20 30

− 0.96 1.84 0.90 1.64 2.26

− 1.22 2.33 1.00 1.77 2.42

4890 5908 7450 7456 7000 6895

1.93 1.97 1.90 2.06 1.99 1.97

20.2 23.2 24.3 25.6 28.3 30.3

−45.5 −43.0 −33.1 −41.4 −31.0 −20.2

a

All the amounts are reported as molar ratios. bTheoretical phosphorus content of the carboxyl-terminated polyesters. cExperimental phosphorus content of the carboxyl-terminated polyesters. dThe acid values of the carboxyl-terminated polyesters, monitored via standard titration methods. e Glass transition temperature determined by differential scanning calorimetry in the second heating ramp at 10 °C/min.

Unfortunately, most PSAs are flammable materials, which limit their range of applications, such as automotive and aerospace industries.28,29 Many efforts have been made to improve the flame retardance of such materials. The addition of flame-retardant monomers into the polymer backbone or side groups through chemical bonds is an effective method.30−32 Among them, phosphorus-based compounds have been widely studied with regard to synthesis and application as flame retardants.33−36 In this study, biobased epoxy resins derived from ESO and the carboxylic acid-terminated polyesters as the curing agents were designed and synthesized. Via incorporation of 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10-phosphaphenanthrene-10-oxide (DDP) or 2-(6-oxido-6H-dibenzcoxaphosphorin-6-yl)-1,4-hydroxyethoxyphenylene (DOPO-HQ-HE) as flame-retardant monomers, a variety of formulations with good flame retardancy were obtained. These adhesives proved to be suitable materials for pressure-sensitive adhesive applications with respect to adhesion strength, viscoelasticity, and functionality.

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-CH2-CH2-), 3.8−4.2 [2H, -CH2-CH(CH3)-], 5.1 [1H, -CH(CH3)-]. P3: 1H NMR (DMSO-d6) δ 1.1 [3H, -CH(CH3)-], 1.5−2.3 (16H, -CH2-CH2-), 3.8−4.2 [2H, -CH2-CH(CH3)-], 5.1 [1H, -CH(CH3)-], 7.1−8.2 (8H, Ar-H). P6: 1H NMR (DMSO-d6) δ 1.1 [3H, -CH(CH3)], 1.5−2.3 (16H, -CH2-CH2-), 3.5−4.0 (8H, -CH2-CH2-), 3.8−4.2 [2H, -CH2-CH(CH3)-], 5.1 [1H, -CH(CH3)-], 6.7−8.5 (8H, Ar-H). Preparation of PSAs. Predetermined amounts of the carboxylterminated polyesters, ESO, and catalyst (Nacure XC-258) were weighed with a microbalance and homogenized at 90 °C in a roundbottom flask equipped with a magnetic stirrer. The resulting mixture was stirred to give a clear solution. The resin was then coated onto a 50 μm thick polyethylene terephthalate (PET) film at 5 cm/s. This bilayer film was then cured in oven at 160 °C for 1−3 h. The thickness of the dried adhesive layers was equal to 20 ± 1 μm. After being cured, the adhesive surfaces were covered with an antiadhesive siliconized paper. The adhesive formulations are always designated by the codes EβPα, where Pα refers to the base polymer (see Table 1) and β corresponds to the weight percent of ESO (relative to the weight of base polymer Pα). Compositions and properties of the PSAs are listd in Table 2.

Table 2. Compositions and Properties of the PSAs Synthesized by ESO Cured with the Carboxyl-Terminated Polymers

EXPERIMENTAL SECTION

Materials. Sebacic acid (SEA), 1,2-propanediol (PDO), and tetrabutyltitanate [Ti(OC4H9)4, AR grade] were purchased from Kelong Chemical Reagent Factory (Chengdu, China). The flameretardant diacid 10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10phosphaphenanthrene-10-oxide (DDP) was produced by Weili Flame Retardant Chemicals Industry Co. Ltd. (Chengdu, China). The flame-retardant diol (DOPO-HQ-HE), 2-(6-oxido-6H-dibenzoxaphosphorin-6-yl)-1,4-hydroxyethoxyphenylene, was prepared in the laboratory by following our previous publication.37 Epoxidized soybean oil (ESO) was purchased from Aladdin Industrial Corp. (Shanghai, China). The catalyst for the epoxy-carboxy curing reaction, Nacure XC-258, a zinc chelate (ZnCH) compound,38 was supplied by King Industries, Inc. (Norwalk, CT). Except for tetrabutyltitanate, which was dissolved in toluene, all the chemicals were used as received. Polycondensation of the Carboxyl-Terminated Polyesters. SEA, PDO, and a flame-retardant monomer (DDP or DOPO-HQHE) were weighed into a round-bottom glass reactor, fitted with a Dean-Stark apparatus, an Allihn condenser, and a magnetic stirrer. The mixture was continuously flushed with nitrogen to prevent oxidation and remove water. While being stirred, the mixture was heated to 180 °C for 4 h. The temperature was then increased to 200 °C, and vacuum processing (2−4 mbar) was applied for 30 min. Tetrabutyltitanate (0.02 mol % relative to the carboxylic acid functions) dissolved in xylene was added to the melt, and the reaction was continued under vacuum for 3 h at 220 °C. Finally, the resulting carboxyl-terminated polymer was cooled to 140 °C and discharged from the reactor. Compositions and properties of the synthesized polymers are listed in Table 1. The chemical structures of the resulting copolyesters were characterized by 1H nuclear magnetic resonance (NMR) spectroscopy. P1: 1H NMR (DMSO-d6) δ 1.1 [3H, -CH(CH3)-], 1.5−2.3 (16H,

sample

ESO content (wt %)

gel content (%)

Tga (°C)

E20-P1 E30-P1 E40-P1 E50-P1 E60-P1 E30-P2 E30-P3 E30-P4 E30-P5 E30-P6

20 30 40 50 60 30 30 30 30 30

33 69 75 88 73 − 65 − − 64

−47.4 −46.6 −46.1 −45.6 −46.3 −44.0 −34.3 −41.3 −32.0 −21.6

a

Glass transition temperature determined by differential scanning calorimetry in the second heating ramp at 10 °C/min. Characterization. The chemical structure of the polymers was investigated by 1H NMR spectroscopy at 400 MHz (Bruker AVANCE AV II-400). The samples were dissolved in dimethyl sulfoxide, while tetramethylsilane was used as an internal standard. Molecular weight determinations of the carboxyl-terminated polyesters were performed by GPC, using a Waters apparatus equipped with a model 1515 pump, a Waters model 717 auto sampler, and a 2414 refractive index detector. Chloroform and monodisperse polystyrene were used as the eluent and standard, respectively. The concentration of sample and the flow rate of the eluent were 2.5 mg/mL and 1.0 mL/min, respectively. The acid functionalities of the polyesters were monitored via a standard titration method with a normalized 0.5 N ethanolic KOH solution.27 The acid value (AV) is defined as the number of milligrams B

DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Scheme 1. Structures of (a) Carboxyl-Terminated Polyesters and (b) ESO and (c) Scheme of the Three Potential Curing Pathways, Ring-Opening Esterification of Epoxy with Carboxyl-Terminated Polyesters (path A), Etherification between Oxirane and Hydroxyl Groups (path B), and Condensation Esterification between Carboxyl and Resultant Hydroxyl Groups (path C)

of potassium hydroxide (KOH) required for neutralizing 1 g of polymer resin and can be calculated with the equation

AV = VsN × 56.1/Ws

The non-isothermal curing reaction was monitored by using a TA differential scanning calorimeter (DSC-Q200), calibrated with pure indium standards. The heat scan, in the range of 50−250 °C, was performed at heating rates equal to 5, 10, 15, and 20 °C/min. The curves of heat flow as a function of temperature were used to calculate the activation energy for all the samples. Around 0.15 g of a cured adhesive was packed into a sealed pocket made of a folded, porous PTFE membrane. After 2 weeks of immersion in a large excess of toluene, the dialysis bag was removed and dried. The gel content (gel percent) was determined gravimetrically from the average of two measurements. The sample weight after dialysis refers to the insoluble gel fraction, regardless of whether the difference between the initial and dialyzed weights corresponds to the sol fraction of the networks. The 180° peel test allows measurement of the force necessary to tear a strip from a solid support at a constant speed. The specimens were prepared with the following procedure: a 2 cm wide and 30 cm

(1)

where AV is the acid value (milligrams of KOH per gram), Vs is the volume (in milliliters) of the ethanolic KOH solution needed to titrate the sample, N is the normality of the KOH solution (moles per liter), 56.1 is the molar mass of KOH (grams per mole), and Ws is the sample weight (grams). All titrations were performed in duplicate. The thermal behavior of polyesters and PSAs was examined by a TA differential scanning calorimeter (DSC-Q200), calibrated with pure indium standards. The samples of ∼5 mg in aluminum pans were first heated to 140 °C at a heating rate of 10 °C/min (the first heating scan) and then held at 140 °C for 3 min to eliminate the thermal history. After that, it was cooled to −70 °C at a cooling rate of 10 °C/ min (cooling scan) and finally reheated to 140 °C at the same heating rate (the second heating scan). C

DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Activation energy plots based on (A) the Kissinger method and (B) the Ozawa method of 30 wt % ESO cured with the carboxylterminated polyesters.

where β is the heating rate, Tp is the exothermic peak temperature, Ea is the activation energy, R is the gas constant, and A is the pre-exponential factor. The activation energy is calculated by plotting −ln(β/Tp2) as a function of Ea/RTp. The Ozawa method is based on a different equation

long strip was cut and placed on a table (adhesive face on the top). Half of the tape length was covered with a 50 μm thick PET film, and then this assembly was turned upside down. The adhesive area was put on the reference surface, and a 2 kg cylinder was rolled twice on the tape. Peel tests were conducted by using a Zwick Z005 testing machine at a peeling speed of 300 mm/min. The initial adhesion was defined as the peel force measured after a dwell time of 15 min. The rheological profile was monitored at 25 °C with an Advanced Dynamic Rheometric Expansion System (TA, Discovery HR-2) in parallel plate geometry (25 mm diameter and 0.6 mm gap) in dynamic mode with a strain of 0.01. Frequency sweep measurements were performed from 0.01 to 100 Hz. Thermogravimetric analysis was performed using a Netzsch TG 209 F1 apparatus. Measurements were taken in a nitrogen or air flow in the temperature range of 40−700 °C at a rate of 10 °C/min. Sample weights of ∼5 mg were used in the experiment. Microcombustion behaviors of the investigated materials were measured using a MCC-1 microscale calorimeter (Fire Testing Technology). Samples of ∼7 mg were heated to 700 °C at a heating rate of 1 °C/s under a 80 cm3/min nitrogen flow. The volatiles produced by thermal degradation were mixed with a 20 cm3/min synthetic air flux before being placed in the combustion furnace set at 900 °C. The main parameters were the heat release rate (HRR), total heat evolved (THE), and heat release capacity (HRC). The flame retardance values of all the PSAs were characterized with the UL-94 and LOI tests according to ASTM 3801 and ASTM 2863, respectively. The small flame behavior of the flame-retardant PSA was also measured according to GB/T15903-1995 (the test method for flame resistance of pressure-sensitive adhesive tape standards).

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−ln β = −1.052 × Ea /RTp + ln AR /Ea − ln F(x) − 5.331 (3)

where F(x) is a conversion-dependent term. Figure 1 shows the curing activation energy plots of 30 wt % ESO cured with the carboxyl-terminated polyesters according to the Kissinger and Ozawa methods; the calculated Ea values are listed in Table 3. As reported, the Ea values of E30-P2 (63.8 Table 3. Peak Temperatures and Activation Energies of 30 wt % ESO Cured with the Carboxyl-Terminated Polyesters Kissinger method

Ozawa method 2

sample

Tp (°C)

Ea (kJ mol )

R

E30-P1 E30-P2 E30-P3 E30-P4 E30-P5 E30-P6

171.6 174.3 189.5 168.3 173.6 181.2

62.1 63.8 65.9 61.9 62.4 63.1

0.9986 0.9991 0.9998 0.9992 0.9999 0.9981

Ea (kJ mol−1)

R2

76.3 79.1 80.1 76.2 77.5 78.0

0.9997 0.9999 0.9999 0.9995 0.9999 0.9991

and 79.1 kJ mol−1) and E30-P3 (65.7 and 80.1 kJ mol−1) were both higher than those of E30-P1 (62.1 and 76.3 kJ mol−1). In other words, DDP-containing polyesters possessed a comparatively lower reactivity after the phosphorus-containing moiety was incorporated into the polyester chain. As shown in Table 1, the Mw values of the DDP-based carboxyl-terminated polyesters were different. The activity of the polyester with a longer carbon chain was weaker, resulting in the Ea of E30-P2 and E30-P3 being higher than that of E30-P1. E30-P4 was the only formulation that showed an Ea lower than that of E30-P1. For E30-P5 and E30-P6, the Ea values were also higher than those of E30-P1. Furthermore, because of random copolymerization, the phosphorus-containing monomers are located in different parts of the polymer chain. When they were terminals, the steric hindrance decreased the activity of the polymers, resulting in a higher Ea value. Dynamic Mechanical Analysis. The correlation between the bulk viscoelastic properties of PSAs and their adhesion performance was well-established.2,5,26,27 Adhesion is a lowfrequency process and occurs when the adhesive can flow and wet the substrate under gentle pressure. On the other hand, the debonding step is regarded as a high-speed behavior involving the deformation of the adhesive layer under stress. Chang and

RESULTS AND DISCUSSION

Curing Behavior. Three reactions had to be envisaged during the curing process of epoxides with carboxyl-terminated polyesters, as illustrated in Scheme 1: the ring-opening esterification between carboxyl and oxirane giving rise to a βhydroxyester (path A), the etherification of epoxide with alcohols (path B), and the condensation esterification between the residual carboxyl and the secondary hydroxyl groups (path C). To improve our understanding of the curing process of the samples, the non-isothermal curing behavior of the all the formulations was studied by differential scanning calorimetry (DSC).39−41 The heating curves showed that 30 wt % ESO cured with the carboxyl-terminated polymers had only one exothermic peak (180−190 °C, at a heating rate of 10 °C/min). The curing activation energy was determined by both Kissinger42 and Ozawa43 methods to ensure the high accuracy of the results. The Kissinger method is based on the following equation: −ln(β /Tp2) = Ea /RTp − ln AR /Ea

−1

(2) D

DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Yang demonstrated the ability to directly correlate the behavior of an adhesive during tack and peel testing to the response observed during DMA at bonding and debonding frequencies, respectively.44,45 Specifically, the response of the material during a bonding event correlated to a frequency of ∼1 rad s−1, while debonding under standard 180° peel testing conditions roughly corresponded to higher frequencies of ∼435 rad s−1. In this work, advanced dynamic rheometric analysis was performed for the PSA samples. The elastic and viscous response of P1-cured samples with different contents of ESO (G′ and G″, respectively) are shown on a log−log scale as a function of oscillation frequency ω in Figure 2A. The

Figure 3. Viscoelastic windows of PSAs as they relate to different regions on their rheological master curves: (●) bonding and (○) debonding.

frequencies on a plot of G′ versus G″. The region in which the viscoelastic window lay could then be used to qualitatively assess the behavior of the PSA and its area of application.44,45 The corresponding viscoelastic windows for E20-P1, E30-P1, E30-P3, and E30-P6 occupied the central region (overlapping part of the four quadrants), suggesting the general-purpose nature of these type of PSAs.46 This central area corresponded to medium modulus and dissipation. For E40-P1 with a higher cross-linking density, the window lay within the high-shear region (quadrant 2, top right quadrant). This quadrant corresponded to high modulus and dissipation. The high bonding modulus compensated by the high dissipation or flow made the bonding marginal. Adhesive Properties. In practice, the optimal balance between cohesion and adhesion, which depends on the composition and cross-linking density, is extremely hard to calculate precisely. Finding the right degree of cross-linking is vital: if it is too high, the film is too stiff to stick to the surface; otherwise, if it is too low, cohesion fails. Figure 4 displays the

Figure 2. Elastic and viscous response of the testing adhesives (G′ and G″, respectively): (A) Eβ-P1 and (B) E30-P3 and E30-P6 adhesives.

rheological profile was certainly influenced by the ESO amount. The network with the higher gel content (E30-P1 and E40-P1) exhibited an elastic response (G′ > G″) over the whole frequency range. The loosely cross-linked networks E20-P1 displayed an elastic behavior (G′ > G″) in the high-frequency region (typically when ω > 10 Hz); a crossover between G′ and G″ was then found at 9 Hz and was characterized by viscous behaviors (G″ > G′) at low frequencies. The loss modulus (G″) of E30-P1 was slightly lower than G′ at low frequencies, and the slope of G′ ∼ f(ω) of E30-P1 was higher than that of E40-P1, indicating that E30-P1 networks displayed a stronger liquid-like characteristic at low frequencies.46 Experimentally, it was found that E30-P1 samples were stickier to the touch (tacky). Figure 2B shows the viscoelastic properties of E30-P3 and E30-P6, which had gel contents similar to that of E30-P1. The elastic moduli of E30-P3 and E30-P6 in the low-frequency region were higher than that of E30-P1, indicating the higher Tg and the increased stiffness of the polymer chains.26 As a consequence, despite those two adhesives possessing similar cross-linking densities, the aromatic-containing network was more cohesive than its purely fatty acid counterpart. Figure 3 shows the viscoelastic window of the PSAs for an improved understanding of how the viscoelastic behavior at bonding and debonding frequencies was used to define the difference in adhesive qualities. The viscoelastic windows were constructed by plotting G′ and G″ at bonding and debonding

Figure 4. Peel test results of ESO cured with the carboxyl-terminated polyesters.

initial adhesion forces of adhesives (the carboxyl-terminated polyesters cured with ESO) as a function of the network formulation. As expected, adhesion levels and debonding modes were influenced by the compounding ratio of the adhesives. At a relatively lower cross-linking level (the carboxylterminated polyesters cured with 150 122 26

a

The self-extinguishing times and combustion lengths refer to the GB/ T 15903−1995 standard. bSamples self-extinguishing but cotton indicator ignited by flaming drops.

Table 5. Detailed Data from Microcombustion Calorimetry of the PSAs ΔPHRRa

sample

LOI (vol %)

UL-94 test and exhibited a very low LOI value. Via incorporation of a DDP molar ratio of 10, the LOI value was increased. E30-P3 showed a clear increase in its LOI value, and a VTM-2 ranking was obtained, meaning after ignition, the tape was extinguished but the dropping ignited the cotton. The LOI values of E30-P4 and E30-P5 were lower than those of the corresponding DDP-based formulations. E30-P6 achieved a better result in terms of the LOI and UL-94 test because of the large amount of flame-retardant monomer. This demonstrated that DOPO-HQ-HE was less efficient than DDP for the small flame tests. Both phosphorus-containing flame-retardant monomers (DDP and DOPO-HQ-HE) transferred into Pcontaining free radicals upon being heated; the phosphorus and the aryl structure of DOPO-HQ-HE would be expected to have a thermal stability higher than those of DDP-containing ones, exhibiting more activity in the condensed phase, therefore increasing the char residue, as shown in the TGA results.37,48 GB/T 15903−1995 is another test standard for the flame resistance of PSA tapes. The test results depend on the flameresistant level, average self-extinguishing time (AEB), and average combustion length. There are four levels. (1) For level 0, AEB = 0, denoting the tape is noncombustible. (2) For level 1, 0 < AEB ≤ 50 mm, meaning the tape self-extinguishes. (3) For level 2, 50 mm < AEB < 150 mm, suggesting the tape is combustible. (4) For level 3, AEB ≥ 150 mm, also suggesting the tape is combustible. With regard to this standard, as a reference, E30-P1 was combustible and burned completely. Via incorporation of the

peak temp (°C) 407.6 402.6 399.5 408.8 409.7 415.1

ΔPHRR = (PHRR − PHRRE30‑P1)/PHRRE30‑P1. bΔTHR = (THR − THRE30‑P1)/THRE30‑P1. a

The incorporation of DPP into PSA induced a small decrease in both PHRR and THR, and such a drop was proportional to the content of the flame-retardant monomer. Considering the limited amount of residue detected via TGA (Figure 5), this reduction was mostly due to the fuel dilution and reduction of the effective heat of combustion of volatiles. Because of the weak DOPO−aliphatic bond, DDP transferred into Pcontaining free radicals when heated, acting as trapping radicals in the gaseous phase.48 With the incorporation of DOPO-HQHE, the PHRRs of the PSA samples were also decreased compared with that of E30-P1, again indicating a reduction in fire danger with this modification. The phosphorus-containing aryl structure of DOPO-HQ-HE would be expected to increase char residue, in accord with the TGA results; therefore, it G

DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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flame-retardant monomers, the average self-extinguishing time was increased considerably, particularly for E30-P3 and E30-P6, which self-extinguished in a short time, suggesting a level 1 was therefore obtained.

REFERENCES

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CONCLUSIONS In this work, novel biobased PSA tapes derived from epoxidized soybean oils and carboxyl-terminated polyesters were produced. The preparation of the PSAs did not use any organic solvent or toxic chemicals, thus being environmentally friendly. The amount of ESO influenced the peel strength by adjusting the cross-linking density of the resulting materials. At a low crosslinking level, the adhesives behaved like a viscous liquid and were not sufficiently cohesive to sustain the mechanical stress during peeling. The adhesives cross-linked at or near the optimal stoichiometric conditions displayed an adhesive (interfacial) failure between the substrate and the adhesive layer, which was associated with the lowest adhesion levels. At a suitable cross-linking density, the PSAs exhibited a superior peel strength of >3.0 N/cm. Two different phosphoruscontaining moieties were utilized as the flame-retardant groups for PSA: for DDP-containing PSAs, their DOPO pendant groups were linked on the aliphatic diacid; as for the DOPOHQ-HE-containing ones, the DOPO pendants were in an aromatic environment. The incorporation of rigid DOPO pendants increased the Tg values and the cohesive strength of the flame-retardant PSAs. The thermal stability of the flameretardant PSAs increased with the incorporation of the phosphorus-containing moieties: both DOPO-containing segments increased the onset and the maximal decomposition temperature of the flame-retardant PSAs. DOPD-HQ-HEcontaining PSAs exhibited a higher level of decomposition residue under both nitrogen and air atmospheres; however, because of the unstable DOPO−aliphatic bond, DDPcontaining PSAs mostly generated P-containing volatiles during composition, and therefore, the char residue of the PSAs decreased. Different decomposition modes of action of two DOPO-derived PSAs resulted in comparable flame-retardant performance. Both HRR and THR values from MCC decreased. LOI, UL-94, and the test standard for flame resistance of PSA tapes suggested the same results. These biobased PSAs exhibited great potential to replace petro-based PSAs for a broad range of applications, including their use in flexible electronic devices, cars, and aircraft.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Li Chen: 0000-0002-9650-497X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 51273115 and 51421061) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT 1026) is sincerely acknowledged. The authors thank the Analysis and Testing Center of Sichuan University for the NMR measurements. H

DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.6b03201 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX