無(wú)鹵阻燃含磷環(huán)氧樹(shù)脂的研究進(jìn)展

葉龍健,錢立軍＊,佟芍朋,許國(guó)志

(1.北京工商大學(xué)材料科學(xué)與工程系,北京100048;2.北京理工大學(xué)材料科學(xué)與工程學(xué)院,北京100081)

0 前言

阻燃環(huán)氧樹(shù)脂除了擁有高效的阻燃性外,還具有優(yōu)異的黏結(jié)性、絕緣性、熱穩(wěn)定性、耐化學(xué)品性、低收縮性,因而被廣泛應(yīng)用于表層涂料、膠黏劑、粉末涂料、層壓板成型、復(fù)合材料以及半導(dǎo)體密封材料和電絕緣材料等領(lǐng)域[1-6]。目前對(duì)環(huán)氧樹(shù)脂的阻燃改性主要以引入鹵系元素為主[7-9],如溴化環(huán)氧樹(shù)脂[10-12]。但是,有鹵環(huán)氧樹(shù)脂燃燒會(huì)產(chǎn)生大量酸性氣體和濃煙,在環(huán)境中降解容易產(chǎn)生可持續(xù)污染物[13-14]。因此對(duì)于阻燃環(huán)氧樹(shù)脂的研究和應(yīng)用方向已轉(zhuǎn)向環(huán)境友好的無(wú)鹵阻燃環(huán)氧樹(shù)脂,特別是含磷阻燃環(huán)氧樹(shù)脂。與有鹵阻燃環(huán)氧樹(shù)脂相比,具有同樣優(yōu)異的熱穩(wěn)定性和阻燃性能的含磷環(huán)氧樹(shù)脂,在燃燒和降解過(guò)程中煙氣釋放量小,其降解產(chǎn)物環(huán)境友好[15-19]。

含磷環(huán)氧樹(shù)脂具有優(yōu)異的阻燃性能,這主要是由于在燃燒過(guò)程中,磷在凝聚相發(fā)揮催化成炭的作用使材料表面形成炭保護(hù)層;而在氣相中磷化合物產(chǎn)生PO·,以此來(lái)淬滅 H·和 HO·等自由基,使燃燒的連鎖反應(yīng)中斷[20-23]。而且磷與固化劑中氮或樹(shù)脂中的硅等元素具有協(xié)同作用,可以通過(guò)復(fù)合使用進(jìn)一步提高環(huán)氧樹(shù)脂的阻燃性能。顯然,含磷阻燃環(huán)氧樹(shù)脂以其優(yōu)異的阻燃性能以及降解產(chǎn)物環(huán)境友好,而成為無(wú)鹵阻燃環(huán)氧樹(shù)脂中最重要的研究方向[24]。

目前研究的含磷環(huán)氧樹(shù)脂主要包括DOPO型環(huán)氧樹(shù)脂、磷酸酯型環(huán)氧樹(shù)脂、磷腈型環(huán)氧樹(shù)脂、磷/硅復(fù)合環(huán)氧樹(shù)脂和環(huán)氧樹(shù)脂的含磷固化劑等類型。

1 DOPO環(huán)氧樹(shù)脂體系

1.1 DOPO環(huán)氧樹(shù)脂

DOPO結(jié)構(gòu)中含有 P—H鍵,可與萘醌、苯醌、衣康酸、馬來(lái)酸、環(huán)氧乙烷和甲醛等反生成具有新功能的衍生物,這些衍生物進(jìn)一步與環(huán)氧樹(shù)脂反應(yīng)可以獲得DOPO含磷環(huán)氧樹(shù)脂。

人們首先研究了DOPO與萘醌或苯醌反應(yīng)合成DOPO-NQ和DOPO-BQ與雙酚A二縮水甘油醚(DGEBA)反應(yīng)獲得的含磷環(huán)氧樹(shù)脂[27-29],如圖1所示。含磷環(huán)氧樹(shù)脂與4,4′-二氨基二苯砜(DDS)、PN線形酚醛樹(shù)脂、二異氰酸酯固化體系具有較高的玻璃化轉(zhuǎn)變溫度(Tg＞160℃),且固化后兩種含磷環(huán)氧樹(shù)脂的磷質(zhì)量分?jǐn)?shù)＞2.1%時(shí),UL 94測(cè)試達(dá)到了V-0級(jí),700℃時(shí)的殘?zhí)苛?0%左右,點(diǎn)燃時(shí)間大大減少,燃燒過(guò)程中低煙、無(wú)滴落。研究還發(fā)現(xiàn),在熱分解的初始階段,隨著磷含量的增加體系穩(wěn)定性會(huì)稍有下降,這主要是由于鍵比C—C鍵穩(wěn)定性弱造成的。另外,含磷環(huán)氧樹(shù)脂的殘?zhí)苛烤绕胀ōh(huán)氧樹(shù)脂高,含磷體系在降解初始階段發(fā)揮催化作用,并形成富含磷的殘?zhí)扛艚^層以防止可燃性氣體轉(zhuǎn)移至材料表面[30,31-33]。

圖1 DOPO與萘醌或苯醌反應(yīng)產(chǎn)物及其含磷環(huán)氧樹(shù)脂的結(jié)構(gòu)Fig.1 The structures of DOPO-NQ,DOPO-BQ and their phosphorus-containing epoxy resins

C S Wang等[34]用DOPO分別與馬來(lái)酸(MA)和衣康酸(ITA)反應(yīng)合成DOPO-MA及DOPO-ITA(圖2),然后進(jìn)一步與DGEBA進(jìn)行反應(yīng),可以得到DOPOMA/DGEBA和DOPO-ITA/DGEBA含磷環(huán)氧半固化物,再用DDS固化。結(jié)果表明,引入柔性脂肪羧酸鏈段的兩種含磷環(huán)氧樹(shù)脂Tg＞150℃,比DOPO-NQ、DOPO-BQ有所下降,但阻燃性能仍然優(yōu)異,在磷質(zhì)量分?jǐn)?shù)為1.7%時(shí),UL94測(cè)試達(dá)到V-0級(jí)。

圖2 DOPO與馬來(lái)酸、衣康酸反應(yīng)產(chǎn)物的結(jié)構(gòu)Fig.2 The structures of DOPO-MA and DOPO-ITA

1.2 DOPO基固化劑

通常情況下獲得DOPO阻燃環(huán)氧樹(shù)脂的方法有3種:第一,合成含DOPO的環(huán)氧化合物,然后再用不同的固化劑固化;第二,通過(guò)反應(yīng)型的DOPO衍生物與環(huán)氧樹(shù)脂反應(yīng)將DOPO基團(tuán)引入;第三,通過(guò)對(duì)普通環(huán)氧樹(shù)脂使用DOPO改性固化劑獲得含DOPO的環(huán)氧樹(shù)脂。因此,將DOPO引入環(huán)氧樹(shù)脂的固化劑也是制備含磷環(huán)氧樹(shù)脂的重要途徑。

C S Wang等[35]通過(guò)DOPO與甲醛反應(yīng),再分別與酚醛樹(shù)脂和三聚氰胺、苯酚反應(yīng)制備了兩種酚醛環(huán)氧固化劑DOPO-FP和DOPO-FMP,如圖3所示。研究表明,這兩種固化劑固化的樹(shù)脂在700℃時(shí),殘?zhí)苛浚?0%。DOPO-FP固化的樹(shù)脂在磷質(zhì)量分?jǐn)?shù)為1.45%時(shí)達(dá)到 UL 94 V-0級(jí),DOPO-FMP固化的樹(shù)脂達(dá)到UL94 V-0級(jí)的磷和氮質(zhì)量分?jǐn)?shù)分別為0.81%和2.36%。

圖3 含DOPO酚醛環(huán)氧固化劑的結(jié)構(gòu)Fig.3 The structures of the phenolic epoxy curing agents containing DOPO

Y L Liu等[36]通過(guò)DOPO和4-羥基苯甲醛反應(yīng)制得新型的含磷酚醛樹(shù)脂固化劑DOPO-PN,如圖4所示。由該固化劑固化的酚醛環(huán)氧樹(shù)脂的Tg高達(dá)160℃,在300℃以下無(wú)熱分解。在磷質(zhì)量分?jǐn)?shù)為2%時(shí),UL 94測(cè)試達(dá)到V-0級(jí)。

此外,Wu等[37-39]還研究了將阻燃固化劑DOPOSN用于固化含硅環(huán)氧樹(shù)脂,如圖4所示。結(jié)果表明,由此固化的含硅環(huán)氧樹(shù)脂(P/Si=4.71/2.22)初始熱分解溫度為 319℃,在700℃時(shí)殘?zhí)苛靠梢赃_(dá)到40.4%,極限氧指數(shù)達(dá)到29.5%,熱穩(wěn)定性阻燃性能良好。

圖4 DOPO-PN與DOPO-SN固化劑的結(jié)構(gòu)Fig.4 The structures of the curing agents DOPO-PN and DOPO-SN

1.3 添加型DOPO改性聚合物

相對(duì)于小分子阻燃劑,添加型DOPO改性聚合物阻燃劑與環(huán)氧樹(shù)脂的相容性好,加工和使用過(guò)程中不遷移,熱性能和力學(xué)性能良好,阻燃效率高。這是將DOPO引入環(huán)氧樹(shù)脂的又一個(gè)途徑。

Y Z Wang等[40]合成了1種添加型聚合物阻燃劑DOPO-PO,如圖5所示。因?yàn)槠渲麈満蛡?cè)基上均有含磷阻燃基團(tuán),磷質(zhì)量分?jǐn)?shù)高達(dá)13.8%,而且結(jié)構(gòu)中富含芳香結(jié)構(gòu),材料中酸源、炭源豐富。將其應(yīng)用于環(huán)氧體系中磷質(zhì)量分?jǐn)?shù)只為0.70%時(shí),即達(dá)到UL 94 V-0級(jí),阻燃效率非常高。與其他小分子添加型阻燃劑會(huì)明顯降低體系的力學(xué)性能不同,添加該阻燃劑對(duì)體系的拉伸強(qiáng)度、彈性模量等幾乎沒(méi)有影響。

Altst?dt等[41]制備1種新型的含磷改性聚砜DOPO-PUS,如圖5所示。并研究了其對(duì)DDS/DGEBA環(huán)氧體系阻燃性能的影響。研究表明,添加質(zhì)量分?jǐn)?shù)為5%DOPO-PUS的環(huán)氧樹(shù)脂體系的極限氧指數(shù)為27.4%,與純樹(shù)脂相比,提高了6.6%。當(dāng)DOPO-PUS的質(zhì)量分?jǐn)?shù)提高至20%,殘?zhí)苛刻岣咧?1%,熱釋放速率峰值降至472 kW/m2,表明該添加型含磷改性聚砜對(duì)環(huán)氧樹(shù)脂的阻燃性有明顯提高。

圖5 DOPO-PO與DOPO-PUS改性聚合物的結(jié)構(gòu)Fig.5 The structures of the modified polymers DOPO-PO and DOPO-PUS

2 磷酸酯環(huán)氧樹(shù)脂體系

磷酸酯類環(huán)氧樹(shù)脂是有機(jī)磷系無(wú)鹵阻燃環(huán)氧樹(shù)脂的重要系列,主鏈上連接有含磷酸酯類的環(huán)氧化合物與純樹(shù)脂相比表現(xiàn)出優(yōu)異的阻燃性能。

2.1 磷酸酯環(huán)氧化合物

W F Shi等[42]合成了環(huán)氧端超支化聚磷酸酯(E-HBPP),用于DGEBA/間苯二胺(m-PDA)體系的阻燃研究。結(jié)果表明,E-HBPP作為酸源,m-PDA作為發(fā)泡劑,高溫下 E-HBPP降解形成聚磷酸,與DGEBA的羥基反應(yīng)生成酯基,該酯基很容易脫水并促進(jìn)樹(shù)脂交聯(lián)生成膨脹性焦炭層。

Thompson等[43]制備了不同的主鏈上鍵接含磷環(huán)氧化合物,如圖6所示。結(jié)果表明,環(huán)氧樹(shù)脂的磷質(zhì)量分?jǐn)?shù)達(dá)到1.5%,在800℃時(shí)的殘?zhí)苛看笥?7%。研究發(fā)現(xiàn),磷鍵接的每個(gè)單元的質(zhì)量均提高,而熱分解溫度和活化能降低,綜合表現(xiàn)出磷的優(yōu)異的催化成炭作用。薛敬和等[44]對(duì)二縮水甘油基苯基磷酸/DDS體系的研究表明,在燃燒過(guò)程中,含磷環(huán)氧樹(shù)脂降解形成磷-氧自由基,然后與酯基反應(yīng)生成聚磷酸酯,進(jìn)一步降解后形成聚炭。因此,含磷組分對(duì)于樹(shù)脂在降解過(guò)程中聚集成炭和減少可燃性氣體具有重要作用。

圖6 不同磷酸酯環(huán)氧化合物的結(jié)構(gòu)Fig.6 The structures of the phosphate epoxy compounds

2.2 環(huán)狀磷酸酯環(huán)氧樹(shù)脂

多羥基化合物在阻燃體系中通常作為炭源使用,通過(guò)多羥基化合物與三氯氧磷反應(yīng)可以獲得環(huán)狀結(jié)構(gòu)磷酸酯環(huán)氧樹(shù)脂。這類環(huán)氧樹(shù)脂在燃燒時(shí)本身會(huì)形成一層焦炭保護(hù)膜,抑制進(jìn)一步燃燒。且環(huán)狀分子中P、O、C 3種元素組成了穩(wěn)定的雜環(huán)結(jié)構(gòu),兼具氣液兩相阻燃作用,熱穩(wěn)定性好,比脂肪類磷酸酯擁有更優(yōu)良的阻燃效率[45]。

Y Z Wang等[46]合成了具有環(huán)狀結(jié)構(gòu)的反應(yīng)型含磷單體,與DGEBA反應(yīng)獲得了含磷環(huán)氧樹(shù)脂,如圖7所示。低相對(duì)分子質(zhì)量聚酰胺(LWPA)與環(huán)氧樹(shù)脂混合時(shí),磷質(zhì)量分?jǐn)?shù)達(dá)到2.5%,UL 94測(cè)試為V-0級(jí),極限氧指數(shù)為30.2%。研究發(fā)現(xiàn),含磷環(huán)氧樹(shù)脂與LWPA體系生成的焦炭為致密多孔型,阻礙了燃燒過(guò)程中的熱釋放,提高了樹(shù)脂的阻燃性能[47,48]。

圖7 環(huán)狀磷酸酯環(huán)氧樹(shù)脂的結(jié)構(gòu)Fig.7 The structure of the cyclic phosphate epoxy resin

X Li等[49]在環(huán)氧樹(shù)脂中引進(jìn)雙環(huán)籠狀磷酸酯(PEPA)(圖 8)。研究表明,PEPA質(zhì)量分?jǐn)?shù)為19.2%,在600℃時(shí)的殘?zhí)苛繛?2%,極限氧指數(shù)為36%,UL 94測(cè)試V-0級(jí)。研究還發(fā)現(xiàn),這種籠狀結(jié)構(gòu)對(duì)于生成膨脹性殘?zhí)?發(fā)揮著至關(guān)重要的作用。

圖8 雙環(huán)籠狀磷酸酯(PEPA)的結(jié)構(gòu)Fig.8 The structure of the caged bicyclic phosphate

2.3 磷酸酯固化劑

C S Wang等[50]制備了含磷反應(yīng)型固化劑二(3-羥基苯基)苯氧基膦酸酯(BHPP)(圖9)。研究表明,由BHPP固化環(huán)氧樹(shù)脂的磷質(zhì)量分?jǐn)?shù)達(dá)到1.5%時(shí),UL 94測(cè)試達(dá)到 V-0級(jí),極限氧指數(shù)為31%,在700℃時(shí)殘?zhí)苛繛?2%,而達(dá)到相同阻燃級(jí)別的含溴環(huán)氧樹(shù)脂的殘?zhí)苛繛?4%。這是由于含磷基團(tuán)在380℃左右首先分解,然后生成富含磷的殘留物延緩了樹(shù)脂進(jìn)一步的降解,并導(dǎo)致高殘?zhí)苛俊?/p>

圖9 BHPP的結(jié)構(gòu)Fig.9 The structure of BHPP

3 其他含磷固化劑體系

通過(guò)含磷固化劑與環(huán)氧樹(shù)脂反應(yīng)是制備含磷環(huán)氧樹(shù)脂的一個(gè)重要途徑,而且已有的研究表明,通過(guò)該途徑制備的含磷環(huán)氧樹(shù)脂具有同樣優(yōu)異的阻燃性能。以下介紹的是除DOPO型和磷酸酯型以外的環(huán)氧樹(shù)脂的含磷固化劑體系。

Galià等[51]將二 -(間 -氨基苯基)-甲基氧化膦苯并噁嗪(Bz-BAMPO)與雙酚A縮水甘油醚共聚獲得一種熱固性酚醛環(huán)氧樹(shù)脂[52],如圖10所示。研究表明,含磷質(zhì)量分?jǐn)?shù)為2%的熱固性樹(shù)脂的極限氧指數(shù)為34.9%。這是由于燃燒過(guò)程中含磷基團(tuán)催化生成了炭絕緣保護(hù)層,其能抑制可燃性氣體轉(zhuǎn)移至材料表面,增加了材料高溫?zé)岱€(wěn)定性并提高了阻燃性。

圖10 Bz-BAMPO的結(jié)構(gòu)Fig.10 The structure of the Bz-BAMPO

Varma等[53]研究了含磷固化劑對(duì)DGEBA環(huán)氧樹(shù)脂的熱性能的影響,如圖11所示。在800℃時(shí)樹(shù)脂的殘?zhí)苛糠謩e為34.8%、38.5%、39.3%,并且隨著磷含量的增加成線性遞增,殘?zhí)苛吭礁咭脖砻髟摌?shù)脂具有更好的阻燃性。Levchik等[54]將二 -(間 -氨基苯)甲基氧化膦(DNMP)固化DGEBA環(huán)氧樹(shù)脂,如圖12所示。研究發(fā)現(xiàn),在600℃的殘?zhí)苛繌?2%提高至30%,阻燃機(jī)理可能是由于延緩了環(huán)氧樹(shù)脂的斷裂鏈的揮發(fā)。

圖11 含磷固化劑的結(jié)構(gòu)Fig.11 The structures of the phosphorus-containing curing agents

Wu等[37-39]研究了阻燃固化劑二 -(間 -氨基苯)苯基氧化膦(DNBP)對(duì)含硅環(huán)氧樹(shù)脂阻燃性的影響,如圖12所示。結(jié)果表明,DNBP固化的含硅環(huán)氧樹(shù)脂(P/Si為1.33/5.20)在800 ℃時(shí),殘?zhí)苛繛?8.6%,極限氧指數(shù)值為49%,這是由于磷/硅復(fù)合作用形成耐高溫殘?zhí)康慕Y(jié)果。

圖12 固化劑DNMP和DNBP的結(jié)構(gòu)Fig.12 The structures of the curing agents DNMP and DNBP

4 磷腈環(huán)氧樹(shù)脂體系

磷腈化合物磷氮骨架結(jié)構(gòu)具有較高的熱穩(wěn)定性、優(yōu)異的磷氮協(xié)效阻燃作用。將該結(jié)構(gòu)引入環(huán)氧樹(shù)脂體系中環(huán)氧樹(shù)脂也能夠獲得優(yōu)異的阻燃性能。

X D Wang等[55]制備出了磷腈環(huán)氧樹(shù)脂 PN-EP,如圖13所示。其由線形酚醛樹(shù)脂固化的含磷環(huán)氧樹(shù)脂具有比純樹(shù)脂高的Tg(144.6℃)和初始分解溫度(378.6℃,3%),在600℃時(shí)該樹(shù)脂的殘?zhí)苛扛哌_(dá)56.2%。如此高的殘?zhí)苛靠赡苁怯捎诃h(huán)三磷腈部分降解產(chǎn)生的磷酸和偏磷酸在固相表面促進(jìn)成炭的緣故。該樹(shù)脂的(P/N為2.18/0.98)極限氧指數(shù)為33.8%,UL 94測(cè)試達(dá)到V-0級(jí)。其阻燃性能優(yōu)異除去其促進(jìn)成炭的原因外,其環(huán)三磷腈部分產(chǎn)生PO·,能淬滅 H·和OH·自由基終止燃燒反應(yīng)[56],也是重要原因之一。

圖13 磷腈基環(huán)氧樹(shù)脂(PN-EP)的結(jié)構(gòu)Fig.13 The structures of the phosphazene-based epoxy resin

5 磷硅復(fù)合環(huán)氧樹(shù)脂

磷-硅體系在燃燒過(guò)程中,硅的存在使富含磷的殘?zhí)康臒岱€(wěn)定性有所提高[57,58],這是由于穩(wěn)定的硅遷移至聚合物表面形成保護(hù)層抑制了焦炭在高溫下的進(jìn)一步降解,但是硅本身對(duì)促進(jìn)成炭沒(méi)有明顯作用[59-64]?？偠灾?磷 -硅的協(xié)同阻燃作用要視不同的體系而論[65-66]。

圖14 磷 -硅環(huán)氧樹(shù)脂的結(jié)構(gòu)Fig.14 The structure of the phosphorus-silicon epoxy resin

Wu等[37-39]研究了DOPO-PhOH與二苯基二羥基硅烷(DPSD)雙酚A型環(huán)氧樹(shù)脂的熱穩(wěn)定性和阻燃性,如圖14所示。結(jié)果表明,P/Si為3.21/0.73的磷 -硅環(huán)氧樹(shù)脂的阻燃效果最好。初始分解溫度為365℃,在700℃時(shí)殘?zhí)苛繛?5.2%。這是由于含磷基團(tuán)在相對(duì)較低的溫度分解形成耐熱炭層,而硅由于其低表面能和高抗氧化性在高溫下遷移至材料表面形成保護(hù)層,進(jìn)一步增強(qiáng)了材料的耐熱性。Y L Liu等[65]還研究了DGEBA、亞磷酸二乙酯及不同含量的硅添加劑固化后得到的磷-硅復(fù)合環(huán)氧樹(shù)脂。其中添加質(zhì)量分?jǐn)?shù)為10%的四乙氧基硅烷的磷 -硅環(huán)氧樹(shù)脂的殘?zhí)苛吭?50℃時(shí)為45.3%,而未添加的含磷的環(huán)氧樹(shù)脂的殘?zhí)苛繛?4.1%。研究表明,四乙氧基硅烷能很好地從材料中遷移至表面,形成保護(hù)層,從而使材料獲得了更高的殘?zhí)苛?。用這種方法獲得的環(huán)氧-硅雜原子體系證實(shí)了磷和硅的阻燃性具有協(xié)同作用。

而 Galià等[66]研究的一類 P/Si復(fù)合環(huán)氧樹(shù)脂有很好的阻燃效果,卻未發(fā)現(xiàn)磷硅協(xié)效作用,該體系為(2,5-二羥基苯)二苯膦分別與二縮水甘油基氧甲基苯基硅烷和1,4-二(縮水甘油基氧基二甲基硅烷)-苯環(huán)氧單體反應(yīng)獲得的磷-硅復(fù)合環(huán)氧樹(shù)脂。

6 結(jié)語(yǔ)

含磷阻燃環(huán)氧樹(shù)脂作為高效、低毒、低煙的環(huán)境友好型材料,具有廣闊的發(fā)展前景。目前的研究雖然取得較大進(jìn)展,但由于含磷環(huán)氧樹(shù)脂或含磷改性單體的合成難度普遍較高,導(dǎo)致其實(shí)際應(yīng)用成本高、價(jià)格高,這個(gè)因素限制了含磷環(huán)氧樹(shù)脂大規(guī)模的使用,因此有必要通過(guò)研究新型含磷阻燃環(huán)氧樹(shù)脂的體系或探討不同組分與磷組分的協(xié)同作用來(lái)進(jìn)一步提高含磷環(huán)氧樹(shù)脂的阻燃效率并降低應(yīng)用成本。所以在今后的研究中需要重點(diǎn)研究?jī)?yōu)化現(xiàn)有含磷環(huán)氧樹(shù)脂的制備工藝、研制復(fù)合型環(huán)氧樹(shù)脂以及新型的含磷固化劑等。

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