Preparation and Properties of Flame-Retardant Polyurethane Pressure Sensitive Adhesive and Its Application

Abstract

Using 10-(2,5-dihydroxyphenyl)-10-hydrogen-9-oxo-10-phosphine-10-oxide (DOPO-H Q), N,N-diethyl-bis(hydroxyethyl) aminomethylene phosphate diethyl (FRC-6), and (6-oxo- 6H-dibenzo[c,e][1,2]oxphosphine-6-yl) hydroxylmethyl-thiophene (DOPO-SF) as reactive flame retardants, the flame-retardant polyurethane pressure sensitive adhesive (FRPU-PSA) were prepared. The fourier transform infrared (FTIR), thermogravimetric analysis (TG), limiting oxygen index (LOI), vertical combustion (UL 94), 180° peeling, and inclined ball rolling were used to characterize and investigate the properties of FRPU-PSA. It was found that the LOI of PU/50mol%DOPO-HQ, PU/50mol%FRC-6, and PU/20wt%DOPO-SF were 30.7%, 29.3%, and 25.0%, respectively, the peel strength of PU/50mol%DOPO-HQ and PU/50mol%FRC-6 were 3.88N/25 mm and 3.42N/25 mm, respectively. FRPU-PSA not only had good bond strength, but also had good flame retardant performance.

1. Introduction

Pressure-sensitive adhesive (PSA) is a material that has adhesive properties after a short period of contact with small pressure. This adhesive does not require solvent evaporation or chemical reaction to achieve adhesion, Therefore the PSA is convenient and safe to use, and it is widely used in various fields of life in masking and protective tapes, packaging tapes, double-sided tapes, and labels [1]. The PSA is mainly divided into acrylic series, silicone series and rubber series, etc. Polyacrylic PSA has the advantages of excellent low temperature resistance, high temperature resistance, and no harmful gas volatilization, and is currently the most widely used PSA [2] Kajtna et al. synthesized a solvent-free acrylic acid UV-crosslinked PSA using acrylic acid monomers (2-ethylhexyl acrylate, acrylic acid and t-butyl acrylate), azobisisobutyronitrile, n-dodecylmercaptan and the unsaturated UV photoinitiator 4-acryloyloxybezophenone as raw materials. Their study showed that when acrylic acid was used as the copolymer monomer, the peel strength of the PSA decreased when the reduction of polymer molecular weight decreased, and also its peel increased. In contrast, the addition of tert-butyl acrylate monomer causes cohesive damage to the adhesive coating [3]. Jamaluddin et al. synthesized a UV-curable PSA using methyl methacrylate (MMA), trimethylolpropane triacrylate (TMPTA) and photoinitiator 1-hydroxycyclohexyl phenyl ketone. Their studying results showed that the strength of the PSA decreases with increasing MMA content, while the retention force shows an opposite trend. In contrast, the retention force of the PSA showed a significant increase with increasing TMPTA content and gel grade fraction under the effect of TMPTA cross-linking. The results showed that the peel strength of PSA decreases to zero when the oligomer content is greater than 40 wt%, while the retention force has a significant increase [4]. The unique properties of silicone such as high flexibility, low surface tension, and low intermolecular interactions give silicone PSA excellent electrical properties, chemical resistance and weather resistance [5]. However, at the same time, it is also difficult to wetting and adhesion on low surface energy substrates, such as polyimide, polyester and other materials [6]. Antosik et al. incorporated nanofiller montmorillonite into organosilicon PSA and evaluated their effect on performance. Their results showed that the viscosity of the binder modified with 1 wt% montmorillonite was twice as high as before the modification and increased with time. As the filler content increased, the maximum working temperature of the PSA then increased and shrinkage decreased. They further investigated a new method of making a double-sided adhesive tape, which is a double-sided silicone self-adhesive tape that can be used at high temperatures [7].

Polyurethane (PU) contains polar groups. It has a segmented structure. High molecular weight polyols make up its flexible chain segments and provide flexibility to the polymer backbone. Diisocyanates and low molecular weight diols or diamines make up the rigid chain segments and impart mechanical properties to the polymer. This structure PSA with good viscoelasticity. The flexible chain segment gives the elasticity and the soft phase gives the viscosity. The PSA prepared from PU has strong adhesion properties [6]. Therefore, PU-PSA with different viscoelasticities can be designed by adjusting the polymerization method and formula [8,9]. Fuensanta et al. synthesized different PU-PSA from 4,4′-diphenylmethane diisocyanate (MDI) blended with 1,4-butanediol and polypropylene glycol (PPG) of different molecular weights (1000 and 2000 Da). They exhibited low glass transition temperature, high viscosity and low 180° peel strength. However, the PU-PSAs prepared using polypropylene glycols with molecular weights of 1000 Da exhibited weak pressure sensitive properties [10]. Subsequently, Fuensanta et al. modified the hard chain segment content by mixing high and low molecular weight polyether polyols during synthesis. Among them, PU-PSA with higher hard chain segments exhibited low probe viscosity and low ring viscosity [11].

PU-PSA is widely used in the fields of medical treatment, electronic devices, and transportation et al., [12,13]. PU-PSA has good adhesion to substrates such as skin and is therefore often used as a surgical dressing in medical applications [14]. By studying the effect of different cross-linking agents on the performance of PU-PSA, Singh et al. showed that PU-PSA with Triallyl cyanurate (TAC) had the highest gel content and crystallinity, but the permeability decreased with increasing strength [15].

With the rapid development of society, people increasingly pay more attention to fire hazards. Nevertheless, most PSAs are flammable materials, a disadvantage that limits their use in the aerospace, automotive industries and medical treatment [16]. With the popularity of electric vehicles in life, the safety of electric vehicle work is also of great concern to the public. The flammable liquid electrolyte of lithium-ion battery, which is the energy supply of electric vehicles, may lead to fire or even explosion under overheating [17]. Therefore, the PSA used in power batteries often need to have high operating temperatures and flame retardant properties. Antosik et al. applied Attapulgite (ABSO-PRO L-16) modified by coupling agents 3-aminopropyltriethoxysilane (APTES) and 3-mercaptopropyltrimethoxysilane (MPTMS) to heat-resistant silicone PSA. This modified Attapulgitehas significantly improved chemical compatibility with specific resins. the PSA with 1 wt% modified Attapulgite added again showed the best performance. It can maintain cohesion >72 h at high temperatures, has an adhesion force of 10 N/25 mm, a stable shrinkage of 0.4%, and a thermal resistance higher than 255 °C [18]. In our previous study, organic boron IFRs [2,4,6-tris(4-boronic-2-thiophene)- 1,3,5-triazine(3TT-3BA); 2,4,6-tris(4-boron-phen- oxy)-(1,3,5)-triazine (TNB); and Hexakis (4-boronic acid-phenoxy)-cyclophosphaz ene (CP-6B)] were synthesized [19,20,21,22]. 3TT-3BA, TNB, and CP-6B havegood flame retardancy properties. Organic boron flame retardants can form a covering carbon layer containing boron to isolate volatiles and air [19,20,21,22,23,24]. It is possible for synergistic flame retardancy of intumescent flame retardant (IFR) and magnesium hydroxide (MH) to enhance theflame retardancy and decrease the amount ofMH and IFR added; thereby, the mechanical properties of the material are not afected. Many literature studies have reported that the combination of MH and otherIFRs containing phosphorus and nitrogen has successfully improved the flflame retardancy [25,26,27]. However, the total amount of flame retardants still needs to be reduced, and the report on the combination of MH and organic boron IFRs was not many. Ai et al. synthesized an organic/inorganic synergistic flame retardant using CP-6B and MH and applied it to epoxy resin (EP). The CP-6B/MH synergistic flame retardant showed better flame and heat resistance compared with CP-6B andMH working separately. The ultimate oxygen index of EP with 3 wt% CP-6B and 0.5 wt% MH reached 31.9% and the vertical combustion rating reached V-0. It indicates that C-6B/MH has a good synergistic effect [28]. Ai et al. synthesized two more organic IFRs, one of which is the DOPO derivative: 6,6-((sulfonylbis(4,1-phenylene)) bis(azanediyl))bis(thiophen-2-ylmethylene))bis(6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide(DOPO-N) and its synergistic flame retardant with inorganic boron flame retardant zinc borate (ZB) in PE. PE exhibited high thermal stability and flame retardant properties when 20 wt% ZB and 10 wt% DOPO-N were added. Its ultimate oxygen index reached 24.6% and its vertical combustion rating reached V-0 [29]. The other is an organic flame retardant containing phosphorus and nitrogen: melamine phenyl hypophosphite (MPHP). And it was applied to PP with silicon dioxide (SiO₂) in synergistic flame retardant. The PP added with 27 wt% MPHP and 3 wt% SiO₂ showed better flame retardant performance, and its ultimate oxygen index increased from 18.9% to 28.7% of pure PE, and the vertical combustion rating reached V-0. They further demonstrated that the addition of a small amount of silica to PP/MPHP not only reduced the heat dissipation rate, but also improved the mechanical properties of PP. Liu et al., [30]. prepared microencapsulated Mic-MBP by coating red phosphorus (RP) with melamine borate and applied it to PE in synergistic flame retardant with ZB. The flame retardant performance of the encapsulated Mic-MBP was significantly better than that of RP, and the ultimate oxygen index of PE reached 25.2% and the vertical combustion rating reached V-0. after the addition of 20 wt% Mic-MBP/10 wt% ZB [31]. On this basis, Lu et al. prepared a bilayer shell microcapsule (Mic-DP) using MBPand cross-linked β-cyclodextrin as the shell material to cover red phosphorus (RP) and applied it to polyamide 6 (PA6). The flame retardant properties of PA6 were greatly improved after the addition of 13 wt% Mic-DP. Its ultimate oxygen index reached 27.8% and its vertical combustion rating reached V-0. Their results further showed that Mic-DP has reduced hygroscopicity and better oxidation resistance compared to RP [32]. Recently, the adhesive and coat prepared by PU put forward the requirements of flame-retardant performance, especially PU pressure-sensitive adhesive (PU-PSA). However, PU is flammable, therefore it is necessary to prepare flame retardant polyurethane pressure-sensitive adhesive (FRPU-PSA) [33,34,35,36].

At present, flame-retardant PSA (FRPSA) can be divided into two types: one is the additive flame-retardant PSA (A-FRPSA) prepared by physical mixing of flame- retardant and PSA [15,37] and second is the intrinsic flame-retardant PSA (I-FRPSA) prepared by introducing flame-retardant reaction monomer into the preparation process. Compared with A-FRPSA, I-FRPSA can overcome the disadvantages of poor compatibility of flame retardants with PSA and the need for larger additions, while having less impact on the mechanical properties of PSA. The Preparation of PSA with excellent flame-retardant and pressure-sensitive properties and With comercial use has always been the focus of attention [38,39]. Wang et al. added9,10-dihydro-10-[2,3-bis(hydroxycarbonyl)propyl]-10-phosphene-10-oxide (DDP) and 2-(6-oxo-6H-dibenzo<c,e><1,2>oxaphosphorin-6-yl)-1,4-hydroxyethoxyphenyl (DOPO-HQ-HE) as flame retardant monomers to synthesize a flame retardant PSA. Theself-extinguishing time was greatly reduced. However, its vertical combustion rating (UL 94) can only reach V-2 [16].

In this study, using 10-(2,5-dihydroxyphenyl)-10-hydrogen-9-oxa-10-phosphen- anthrene-10-oxide (DOPO-HQ or DH), N,N-bis(hydroxyethyl)aminomethylenephos- phate diethyl (FRC-6 or F), and (6-oxo-6H-dibenzo[c,e][1,2]oxophoxahexacyclo-6-yl) hydroxymethylthiophene (DOPO-SF or DS) as reactive flame retardants (Scheme 1), I-FRPU-PSA was prepared and its flame-retardant and adhesive properties were investigated.

2. Materials and Methods

2.1. Materials

DOPO-HQ (DH) was purchased from Xijia Chemical Co., Ltd., FRC-6 (F) was purchased from Wuhan jinnuoxin Co., Ltd. (Wuhan, China), and DOPO-SF (DS) was purchased from Beijing HWRK Chem Co., Ltd. (Beijing, China), Polyetherdiol and toluene diisocyanate (TDI) were obtained from Wanhua Chemical Group Co., Ltd. (Shandong, China), Trimethylolpropane (TMP) was supplied by Tianjin kemio Chemical Reagent Co., Ltd. (Tianjin, China).

2.2. Characterization

The Fourier transform infrared (FTIR) spectra was recorded using a Nicolet 6700 FTIR spectrometer (Madison, WI, USA). Thermogravimetric analysis (TG) was performed using a Netzsch 209 F3 thermal analyzer (Selb, Germany) at a heating rate of 10 °C/min, and the nitrogen flow rate was 40 mL/min. The limiting oxygen index (LOI) was measured using a Fire Testing Technology instrument (FTT, East Grinstead, UK), according to ASTM D2863. The dimensions of each sample were 80 × 10 × 4 mm. According to the ASTM 3801 guidelines, vertical burning (UL 94) tests were performed with a FTT UL 94 instrument by using samples with dimensions of 125 × 12.7 × 3.2 mm. Depending on the self-extinguishing time and dripping, burning grades were classified as either V-0, V-1, V-2, or no rating (NR). The initial adhesion test was according to GB/T4852-2002. The 180° peel test was conducted with a BT1-FR010TH.A50 universal material testing machine (zwick, Germany) by using samples with dimensions of 125 × 50 × 1.5 mm and peeled at a descent rate of 300 mm/min, which according to GB/T2792-1998.

2.3. Preparation of I-FRPU-PSA

Polyetherdiol was weighed and added into a reactor to remove water by vacuum distillation. Afterwards, TDI was added into the reactor when the temperature was raised to 60 °C and then mixed and stirred under nitrogen atmosphere. PU prepolymer (PU-P) was obtained after 3h, and then TMP and flame-retardant monomer (DOPO-HQ or FRC-6 or DOPO-SF) was added into PU-P. I-FRPU-PSA was obtained after 30 min. And then 30 min of vacuum defoaming and was poured into a preheated standard dumbbell polytetrafluoroethylene mold and step temperature rise at 60 °C, 70 °C and 80 °C for 1 h each. After cooling to room temperature, I-FRPU-PSA samples were obtained.

Table 1. Feeding ratio of IFRPU-PSA samples.

Scheme 6	Mole Ratio a	PU-P b (g)	TMP (g)	DOPO-HQ (g)	FRC-6 (g)
PU	-	43	5.36	-	-
PU/10mol%DOPO-HQ	9:1:0	43	4.80	1.93	-
PU/30mol%DOPO-HQ	7:3:0	43	3.73	5.80	-
PU/50mol%DOPO-HQ	5:5:0	43	2.68	9.68	-
PU/10mol%FRC-6	9:0:1	43	4.80	-	1.52
PU/30mol%FRC-6	7:0:3	43	3.73	-	4.57
PU/50mol%FRC-6	5:0:5	43	2.68	-	7.65
PU/40mol%DOPO-HQ/10mol%FRC-6	5:4:1	43	2.68	7.73	1.52
PU/25mol%DOPO-HQ/25mol%FRC-6	2:1:1	43	2.68	4.83	3.80
PU/10mol%DOPO-HQ/40mol%FRC-6	5:1:4	43	2.68	1.93	6.08

^a: molar ratio of TMP/DOPO-HQ/FRC-6; ^b: the ratio OH/NCO for PU-P is 2.5.

shows the feeding ratio of I-FRPU-PSA samples. The molecular structure of DOPO-HQ and FRC-6 contains dihydroxyl groups, which are involved in the reaction of preparing flame retardant PU (FRPU), The molecular structure of DOPO-SF contains only one hydroxyl group, which is partially involved in the reaction of preparing FRPU (

Table 2. Feeding ratio of IFRPU-PSA samples for PU/DOPO-SF.

Samples	PU-P (g)	TMP (g)	DOPO-SF (g)
PU/5wt%DOPO-SF	43	5.36	2.42
PU/10wt%DOPO-SF	43	5.36	4.84
PU/15wt%DOPO-SF	43	5.36	7.25
PU/20wt%DOPO-SF	43	5.36	9.67
PU/25wt%DOPO-SF	43	5.36	12.09
PU/30wt%DOPO-SF	43	5.36	14.51

).

2.4. Preparation of I-FRPU-PSA Tapes

A 50 μm PET film was stretched and tiled in a vacuum coater, and then I-FRPU-PSA was evenly coated with a 50 μm diameter wire rod coater. The thickness of I-FRPU-PSA was 50 μm. After coating, it was heated at 100–120 °C for 5–10 min. After 24 h of curing in a closed and clean space, the PET sheet containing I-FRPU-PSA film was cut into 25 mm wide strips.

3. Results and Discussion

3.1. FTIR Spectra

The FTIR spectra of PU-P after chain extension was showed in Figure 1. The absorption peak of -NCO was very weak at about 2280 cm⁻¹, which indicated that -NCO reaction was almost complete after chain extension. The peak at 3425 cm⁻¹ was caused by the overlap of stretching vibration absorption peak of -OH (remaining in excess chain extender) and C-N (in carbamate group). Absorption peaks at 2920 cm⁻¹ and 2880 cm⁻¹ were -CH₃ and -CH₂- stretching vibrations. The stretching vibration at 1689, 1536, and 1250 cm⁻¹ belonged to the absorption peaks of -C-O in carbamate, secondary amine NH, and C-O, respectively. The peak at 1101 cm⁻¹ was caused by the overlap of stretching vibration of C-O-C in soft segment polyether and C-O in molecular chain.

3.2. Thermal Performance Analysis

Figure 2 showed the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of PU, PU/DH, PU/F, PU/DH/F and PU/DS, and their thermal decomposition data are summarized in

Table 3. Thermal decomposition characteristics of PU, PU/DH, PU/F, PU/DH/F and PU/DS.

Samples	Tonset a (°C)	Tmax1 b (°C)	Tmax2 (°C)	Tmax3 (°C)	W800 c (%)
PU	284.5	340.5	382.4	-	1.49
PU/50mol%DOPO-HQ	274.6	320.0	386.3	-	4.54
PU/50mol%FRC-6	239.8	277.3	373.4	-	10.57
PU/25mol%DOPO-HQ/25mol%FRC-6	252.7	288.3	363.2	-	7.52
PU/40mol%DOPO-HQ/10mol%FRC-6	262.3	300.1	385.9	-	4.56
PU/20wt%DOPO-SF	187.6	190.6	314.0	379.0	2.14

^a temperature corresponding to 5% of thermal weight loss; ^b Temperature corresponding to the maximum rate of thermal weight loss; ^c Percentage of residual carbon in the sample at 800 °C.

. The temperature at 5% thermal mass loss (T_onset) of PU/DH, PU/F, PU/DH/F, and PU/DS was lower than that of PU. This was because DOPO-HQ (DH), FRC-6 (F) and DOPO-SF (DS) contain phosphorus. When PU/DH, PU/F, PU/DH/F and PU/DS were burned, the C-P bond broke first and then pyrolyzed to produce acid substances such as polyphosphate and metaphosphoric acid, which catalyzed the dehydration of PU to carbon.

Based on the DTG curve, PU/DH, PU/F, PU/DH/F and PU/DS showed two weight loss stages corresponding to two different DTG peaks. The first stage of thermal weight loss of PU/FRC-6 was at 200–320 °C, corresponding to a DTG peak (Tmax1). In this stage, FRC-6 was heated to produce ammonia and water, which generated acidic substances, and catalyzed the early decomposition of PU. The second stage was at 320–410 °C, corresponding to a strong DTG peak (Tmax2), which was the further degradation of PU. The first stage of thermal weight loss of PU/DOPO-HQ was at 220–350 °C, corresponding to a DTG peak (Tmax1). In this stage, DOPO-HQ was pyrolyzed to produce acidic substances, which catalyzed PU decomposition in advance. The second stage was at 330–450 °C, corresponding to a strong DTG peak (Tmax2), which was the further degradation of benzene ring and PU. This indicated that the higher the DOPO-HQ content was, the higher was the Tmax1 of PU/DH. This was because the benzene and phenanthrene rings in DOPO-HQ made the sample to have higher thermal and chemical stabilities than those of FRC-6. At 320–450 °C, the thermal weight loss rate of PU was the largest.

In comparison, the thermal weight loss rates of PU/DOPO-HQ and PU/FRC-6 were reduced, and the higher the FRC-6 content was, the smaller was the thermal weight loss rate. Compared with DOPO-HQ, FRC-6 contained elements of P and N, which could not only generate acidic substances to promote the dehydration of the substrate into carbon but also generate noncombustible gases to dilute the oxygen concentration in the combustion environment, thus inhibiting the combustion.

Compared with PU/DOPO-HQ and PU/FRC-6, PU/DOPO-SF had three weight loss stages. The results showed that the thermal weight loss at 150–210 °C may be due to the dehydration and pyrolysis of the DOPO-SF. When DOPO-SF was added, the maximum thermal weight loss rate of PU/DOPO-SF decreased obviously, that is to say, the DOPO-SF containing samples are thermally unstable, which indicated that DOPO-SF had a certain flame-retardant effect on PU, but it was not as good as DOPO-HQ and FRC-6.

Compared with PU, the char yield of PU/50mol%DOPO-HQ, PU/50mol% FRC-6, PU/25mol%DOPO-HQ/25mol%FRC-6, PU/40mol%DOPO-HQ/10 mol%FRC-6, and PU/20wt%DOPO-SF at 800 °C (W800 °C) increased by 204.6%, 609.4%, 404.7%, 206.0%, and 43.6%, respectively, which indicated that the residual carbon of FRPU increased.

3.3. LOI and Vertical Combustion

When R (n (NCO):n (OH)) was 2.5, Polyetherdiol and TDI were used as raw materials to prepare PU-P, and DOPO-HQ and FRC-6 were introduced to extend the chain of PU-P. The influence of mole fraction (TMP/DOPO-HQ/FRC-6) on the flame retardancy of PU was studied under the premise of unchanged hydroxyl content of the chain extender, and the results are shown in

Table 4. LOI and UL 94 ratings of PU, PU/DH, PU/F, and PU/DH/F.

Samples	UL 94	LOI (%)
PU	NR	18.4
PU/10mol%DOPO-HQ	V-2	23.0
PU/30mol%DOPO-HQ	V-1	26.2
PU/50mol%DOPO-HQ	V-1	30.7
PU/10mol%FRC-6	V-2	23.0
PU/30mol%FRC-6	V-1	26.6
PU/50mol%FRC-6	V-1	29.3
PU/40mol%DOPO-HQ/10mol%FRC-6	V-1	28.8
PU/25mol%DOPO-HQ/25mol%FRC-6	V-1	28.3
PU/10mol%DOPO-HQ/40mol%FRC-6	V-1	28.0

.

Table 5. LOI values and UL 94 ratings of flame-retardant PU by DOPO-SF.

Samples	UL 94	LOI (%)
PU	NR	18.4
PU/5 wt%DOPO-SF	V-2	22.4
PU/10 wt%DOPO-SF	V-2	23.8
PU/15 wt%DOPO-SF	V-1	24.2
PU/20wt%DOPO-SF	V-1	25.0
PU/25 wt%DOPO-SF	V-1	24.7
PU/30 wt%DOPO-SF	V-1	24.3

shows the influence of DOPO-SF content on flame retardancy of PU.

When 10 mol%, 30 mol%, and 50 mol% of DOPO-HQ were added, the LOI of PU/DOPO-HQ were 23.0%, 26.2%, and 30.7%, respectively, which were 25%, 42%, and 67% higher than those of PU, and the vertical combustion test reached V-2, V-1, and V-1 grades, which indicated that with the increase of DOPO-HQ in the chain extender, the flame retardancy of PU was enhanced. When 10 mol%, 30 mol%, and 50 mol% of FRC-6 were added, the LOI of PU/FRC-6 were 23.0%, 26.6%, and 29.3%, respectively, which were 25%, 45%, and 59% higher than those of PU. The vertical combustion test was V-2, V-1, and V-1, which indicated that the flame retardancy of PU was enhanced with the increase of FRC-6 in the chain extender. When the molar ratios of DOPO-HQ/FRC-6 were 4:1, 1:1, and 1:4, the LOI of PU/DOPO-HQ/FRC-6 were 28.8%, 28.7%, and 28.5%, respectively, and when DOPO-HQ was further added, the LOI of PU/DOPO-HQ/FRC-6 were at around 28.6%. Even so, the LOI of PU/DOPO-HQ/FRC-6 was lower than that of PU/DOPO-HQ and PU/FRC-6, which indicated that DOPO-HQ and FRC-6 had no synergistic flame-retardant effect. This was because DOPO-HQ contained P element, which would release free radicals such as PO· and PO2· during combustion, and PO· and PO2· could quench H·and HO·from the pyrolysis of PU. Thus, the combustion chain reaction was interrupted.

In addition, due to its low bond energy, the C–P bond in DOPO-HQ broke first during combustion and then pyrolyzed to generate acid substances such as polyphosphoric acid and metaphosphoric acid, which catalyzed the dehydration of PU into carbon and formed a flame-retardant carbon layer on the surface of the substrate. The carbon layer further prevented the combustion heat and combustible gas from diffusing into the interior of the substrate, acting as a condensed-phase flame-retardant. Compared with small molecular phosphonates or acyclic organic phosphates, DOPO-HQ had higher thermal and chemical stabilities due to the high rigidity of the benzene and phenanthrene rings in its molecular structure, and the P-containing group was formed by the side joining the ring O=P–O bond. This also delayed the catalytic decomposition of PU by the acidic substances produced by DOPO-HQ pyrolysis to some extent. In the combustion process, FRC-6 contained P and N elements generated acid substances, promoted the dehydration of the substrate into carbon, formed carbon layer for heat insulation and oxygen isolation, and played the role of condensed-phase flame-retardant. Moreover, phosphorus-free radicals could be generated to inhibit the combustion chain transfer, and the noncombustible gases were formed to dilute the oxygen concentration in the combustion environment, which played as a P–N gas-phase synergistic flame-retardant.

It could be seen from Table 5 that the flame retardancy of PU/DOPO-SF was higher than that of PU. With the increase of DOPO-SF content, the LOI of PU/DOPO-SF increased at first and then decreased and reached the maximum when 20 wt% DOPO-SF was added. When DOPO-SF content increased from 5% to 20%, LOI values were 22.4% and 25.0%, which were 22% and 36% higher than those of PU.

3.4. Infrared Analysis of Residual Carbon

Figure 3 shows the FTIR spectra of the carbon residues of PU, PU/DOPO-HQ, PU/FRC-6, and PU/DOPO-SF in a muffle furnace at 650 °C for 15 min. The results showed that there was no obvious characteristic absorption peak of the PU residual carbon.The bending vibration and tensile vibration absorption peaks of P=O was at 1238 cm⁻¹ and 1142 cm⁻¹ in the FTIR of the PU/50mol%DOPO-HQ carbon residue, likewise, the PU/50mol%FRC-6 carbon residue was at 1238 cm⁻¹ and 1138 cm⁻¹ and the PU/20wt%DOPO-SF carbon residue was at 1250 cm⁻¹ and 1140 cm⁻¹. The bending and tensile vibration absorption peaks of P-O was at 1028 cm⁻¹ and 490 cm⁻¹ in the FTIR of the PU/50mol%DOPO-HQ, 1011 cm⁻¹ and 493 cm⁻¹ in the PU/50mol%FRC-6 carbon residue and 950 cm⁻¹ and 493 cm⁻¹ in the PU/20wt%DOPO-SF carbon residue.

3.5. Adhesive Properties

Table 6. Adhesive properties of FRPU-PSA and commercial PSA tapes.

Samples	Initial Viscosity (Steel Ball Number)	180° Peel Strength (N/25 mm)
PU tape	7	3.65 ± 0.13
PU/50mol%DOPO-HQ tape	8	3.88 ± 0.10
PU/50mol%FRC-6 tape	10	3.42 ± 0.10
PU/20wt%DOPO-SF tape	–	0.15 ± 0.10
Scotch tape (commercial)	7	1.90 ± 0.10
Electrical tape (commercial)	13	1.30 ± 0.10
Duct tape (commercial)	11	4.05 ± 0.10
Post-it tape (commercial)	4	0.20 ± 0.10

and Figure 4 show the adhesive properties of I-FRPU-PSA tapes and commercial PSA tapes. From Table 6, it can be seen that PU/50mol%DOPO-HQ and PU/50mol%FRC-6 tapes had similar adhesive properties with those of PU tape. It was indicated that the reactive flame-retardant did not reduce the adhesion of PU-PSA. Compared with that of PU tape, the peel strength of PU/50mol%DOPO-HQ tape (3.88 N/25 mm) was slightly higher, and the initial viscosity was also slightly higher. This was because DOPO-HQ contained aromatic rings, which enhanced the cohesion of PSA to a certain extent. In addition, when TMP was replaced by 50% DOPO-HQ, the chain flexibility were improved, which could enhance the wettability with the substrate. Compared with that of PU tape, the initial viscosity of PU/50mol%FRC-6 tape increased from 7 steel ball to 10 steel ball, and the peeling strength was slightly lower, which was 3.42 N/25 mm. Compared with that of PU/50mol%DOPO-HQ, the initial viscosity of PU/50mol%FRC-6 was also higher, but the peeling strength was lower. This was because FRC-6 had better flexibility than that of DOPO-HQ. In addition, when FRC-6 replaced TMP, the crosslinking point were fewer and the initial viscosity was higher.

Compared with those of PU tape, the initial adhesion and peel strength of I-FRPU-PSA tape prepared by PU/20wt%DOPO-SF decreased. It can be seen from Table 6 that the initial adhesion range of I-FRPU-PSA tape prepared in this study was 0–10 steel ball, and the peel strength range was 0.05 N/25 mm–3.98 N/25 mm. Under the same test conditions, four kinds of commonly used commercial tapes, such as Scotch tape, electrical tape, duct tapeand post-it note, were tested. The values of initial adhesion (4–11 steel balls) and peel strength (0.10 N/25 mm–4.15 N/25 mm) (yellow part in Figure 4) were obtained. There was no residual adhesive in the peeling process. By comparison, the I-FRPU-PSA could meet the requirements of initial adhesion and peel strength of commercial PSA products.

4. Conclusions

DOPO-HQ, FRC-6 and DOPO-SF were used as reactive flame retardants to prepare I-FRPU-PSA. The LOI values of PU/50mol%DOPO-HQ, PU/50mol%FRC-6, and PU/20wt% OPO-SF were 30.7%, 29.3%, and 25.0%, respectively, which passed UL 94 V-1 grade. The peel strength of PU/50mol%DOPO-HQ and PU/50mol% FRC-6 were 3.88 N/25 mm and 3.42 N/25 mm, respectively, and the initial adhesion was also good. Compared with PSA products such as commercial transparent tape, the prepared FRPU-PSA tape could meet the adhesive strength of different tapes and achieve practical application.