葉龍健,錢立軍＊,佟芍朋,許國志

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

0 前言

阻燃環(huán)氧樹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、耐化學品性、低收縮性,因而被廣泛應用于表層涂料、膠黏劑、粉末涂料、層壓板成型、復合材料以及半導體密封材料和電絕緣材料等領(lǐng)域[1-6]。目前對環(huán)氧樹脂的阻燃改性主要以引入鹵系元素為主[7-9],如溴化環(huán)氧樹脂[10-12]。但是,有鹵環(huán)氧樹脂燃燒會產(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]。因此對于阻燃環(huán)氧樹脂的研究和應用方向已轉(zhuǎn)向環(huán)境友好的無鹵阻燃環(huán)氧樹脂,特別是含磷阻燃環(huán)氧樹脂。與有鹵阻燃環(huán)氧樹脂相比,具有同樣優(yōu)異的熱穩(wěn)定性和阻燃性能的含磷環(huán)氧樹脂,在燃燒和降解過程中煙氣釋放量小,其降解產(chǎn)物環(huán)境友好[15-19]。

含磷環(huán)氧樹脂具有優(yōu)異的阻燃性能,這主要是由于在燃燒過程中,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層;而在氣相中磷化合物產(chǎn)生PO·,以此來淬滅 H·和 HO·等自由基,使燃燒的連鎖反應中斷[20-23]。而且磷與固化劑中氮或樹脂中的硅等元素具有協(xié)同作用,可以通過復合使用進一步提高環(huán)氧樹脂的阻燃性能。顯然,含磷阻燃環(huán)氧樹脂以其優(yōu)異的阻燃性能以及降解產(chǎn)物環(huán)境友好,而成為無鹵阻燃環(huán)氧樹脂中最重要的研究方向[24]。

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

1 DOPO環(huán)氧樹脂體系

1.1 DOPO環(huán)氧樹脂

DOPO結(jié)構(gòu)中含有 P—H鍵,可與萘醌、苯醌、衣康酸、馬來酸、環(huán)氧乙烷和甲醛等反生成具有新功能的衍生物,這些衍生物進一步與環(huán)氧樹脂反應可以獲得DOPO含磷環(huán)氧樹脂。

人們首先研究了DOPO與萘醌或苯醌反應合成DOPO-NQ和DOPO-BQ與雙酚A二縮水甘油醚(DGEBA)反應獲得的含磷環(huán)氧樹脂[27-29],如圖1所示。含磷環(huán)氧樹脂與4,4′-二氨基二苯砜(DDS)、PN線形酚醛樹脂、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)氧樹脂的磷質(zhì)量分數(shù)＞2.1%時,UL 94測試達到了V-0級,700℃時的殘?zhí)苛?0%左右,點燃時間大大減少,燃燒過程中低煙、無滴落。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)氧樹脂的殘?zhí)苛烤绕胀ōh(huán)氧樹脂高,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與萘醌或苯醌反應產(chǎn)物及其含磷環(huán)氧樹脂的結(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分別與馬來酸(MA)和衣康酸(ITA)反應合成DOPO-MA及DOPO-ITA(圖2),然后進一步與DGEBA進行反應,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)量分數(shù)為1.7%時,UL94測試達到V-0級。

圖2 DOPO與馬來酸、衣康酸反應產(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,通過反應型的DOPO衍生物與環(huán)氧樹脂反應將DOPO基團引入;第三,通過對普通環(huán)氧樹脂使用DOPO改性固化劑獲得含DOPO的環(huán)氧樹脂。因此,將DOPO引入環(huán)氧樹脂的固化劑也是制備含磷環(huán)氧樹脂的重要途徑。

C S Wang等[35]通過DOPO與甲醛反應,再分別與酚醛樹脂和三聚氰胺、苯酚反應制備了兩種酚醛環(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脂在700℃時,殘?zhí)苛浚?0%。DOPO-FP固化的樹脂在磷質(zhì)量分數(shù)為1.45%時達到 UL 94 V-0級,DOPO-FMP固化的樹脂達到UL94 V-0級的磷和氮質(zhì)量分數(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DOPO和4-羥基苯甲醛反應制得新型的含磷酚醛樹脂固化劑DOPO-PN,如圖4所示。由該固化劑固化的酚醛環(huán)氧樹脂的Tg高達160℃,在300℃以下無熱分解。在磷質(zhì)量分數(shù)為2%時,UL 94測試達到V-0級。

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)氧樹脂,如圖4所示。結(jié)果表明,由此固化的含硅環(huán)氧樹脂(P/Si=4.71/2.22)初始熱分解溫度為 319℃,在700℃時殘?zhí)苛靠梢赃_到40.4%,極限氧指數(shù)達到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

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

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。因為其主鏈和側(cè)基上均有含磷阻燃基團,磷質(zhì)量分數(shù)高達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。將其應用于環(huán)氧體系中磷質(zhì)量分數(shù)只為0.70%時,即達到UL 94 V-0級,阻燃效率非常高。與其他小分子添加型阻燃劑會明顯降低體系的力學性能不同,添加該阻燃劑對體系的拉伸強度、彈性模量等幾乎沒有影響。

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DDS/DGEBA環(huán)氧體系阻燃性能的影響。研究表明,添加質(zhì)量分數(shù)為5%DOPO-PUS的環(huán)氧樹脂體系的極限氧指數(shù)為27.4%,與純樹脂相比,提高了6.6%。當DOPO-PUS的質(zhì)量分數(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環(huán)氧樹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)氧樹脂體系

磷酸酯類環(huán)氧樹脂是有機磷系無鹵阻燃環(huán)氧樹脂的重要系列,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的羥基反應生成酯基,該酯基很容易脫水并促進樹脂交聯(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)氧樹脂的磷質(zhì)量分數(shù)達到1.5%,在800℃時的殘?zhí)苛看笥?7%。研究發(fā)現(xiàn),磷鍵接的每個單元的質(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]對二縮水甘油基苯基磷酸/DDS體系的研究表明,在燃燒過程中,含磷環(huán)氧樹脂降解形成磷-氧自由基,然后與酯基反應生成聚磷酸酯,進一步降解后形成聚炭。因此,含磷組分對于樹脂在降解過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)氧樹脂

多羥基化合物在阻燃體系中通常作為炭源使用,通過多羥基化合物與三氯氧磷反應可以獲得環(huán)狀結(jié)構(gòu)磷酸酯環(huán)氧樹脂。這類環(huán)氧樹脂在燃燒時本身會形成一層焦炭保護膜,抑制進一步燃燒。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)的反應型含磷單體,與DGEBA反應獲得了含磷環(huán)氧樹脂,如圖7所示。低相對分子質(zhì)量聚酰胺(LWPA)與環(huán)氧樹脂混合時,磷質(zhì)量分數(shù)達到2.5%,UL 94測試為V-0級,極限氧指數(shù)為30.2%。研究發(fā)現(xiàn),含磷環(huán)氧樹脂與LWPA體系生成的焦炭為致密多孔型,阻礙了燃燒過程中的熱釋放,提高了樹脂的阻燃性能[47,48]。

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

X Li等[49]在環(huán)氧樹脂中引進雙環(huán)籠狀磷酸酯(PEPA)(圖 8)。研究表明,PEPA質(zhì)量分數(shù)為19.2%,在600℃時的殘?zhí)苛繛?2%,極限氧指數(shù)為36%,UL 94測試V-0級。研究還發(fā)現(xiàn),這種籠狀結(jié)構(gòu)對于生成膨脹性殘?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)氧樹脂的磷質(zhì)量分數(shù)達到1.5%時,UL 94測試達到 V-0級,極限氧指數(shù)為31%,在700℃時殘?zhí)苛繛?2%,而達到相同阻燃級別的含溴環(huán)氧樹脂的殘?zhí)苛繛?4%。這是由于含磷基團在380℃左右首先分解,然后生成富含磷的殘留物延緩了樹脂進一步的降解,并導致高殘?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

通過含磷固化劑與環(huán)氧樹脂反應是制備含磷環(huán)氧樹脂的一個重要途徑,而且已有的研究表明,通過該途徑制備的含磷環(huán)氧樹脂具有同樣優(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)氧樹脂[52],如圖10所示。研究表明,含磷質(zhì)量分數(shù)為2%的熱固性樹脂的極限氧指數(shù)為34.9%。這是由于燃燒過程中含磷基團催化生成了炭絕緣保護層,其能抑制可燃性氣體轉(zhuǎn)移至材料表面,增加了材料高溫熱穩(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DGEBA環(huán)氧樹脂的熱性能的影響,如圖11所示。在800℃時樹脂的殘?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)苛吭礁咭脖砻髟摌渲哂懈玫淖枞夹?。Levchik等[54]將二 -(間 -氨基苯)甲基氧化膦(DNMP)固化DGEBA環(huán)氧樹脂,如圖12所示。研究發(fā)現(xiàn),在600℃的殘?zhí)苛繌?2%提高至30%,阻燃機理可能是由于延緩了環(huán)氧樹脂的斷裂鏈的揮發(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)對含硅環(huán)氧樹脂阻燃性的影響,如圖12所示。結(jié)果表明,DNBP固化的含硅環(huán)氧樹脂(P/Si為1.33/5.20)在800 ℃時,殘?zhí)苛繛?8.6%,極限氧指數(shù)值為49%,這是由于磷/硅復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)氧樹脂體系中環(huán)氧樹脂也能夠獲得優(yōu)異的阻燃性能。

X D Wang等[55]制備出了磷腈環(huán)氧樹脂 PN-EP,如圖13所示。其由線形酚醛樹脂固化的含磷環(huán)氧樹脂具有比純樹脂高的Tg(144.6℃)和初始分解溫度(378.6℃,3%),在600℃時該樹脂的殘?zhí)苛扛哌_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成炭的緣故。該樹脂的(P/N為2.18/0.98)極限氧指數(shù)為33.8%,UL 94測試達到V-0級。其阻燃性能優(yōu)異除去其促進成炭的原因外,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[56],也是重要原因之一。

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

5 磷硅復合環(huán)氧樹脂

磷-硅體系在燃燒過程中,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層抑制了焦炭在高溫下的進一步降解,但是硅本身對促進成炭沒有明顯作用[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)氧樹脂的阻燃效果最好。初始分解溫度為365℃,在700℃時殘?zhí)苛繛?5.2%。這是由于含磷基團在相對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層,進一步增強了材料的耐熱性。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-硅復合環(huán)氧樹脂。其中添加質(zhì)量分數(shù)為10%的四乙氧基硅烷的磷 -硅環(huán)氧樹脂的殘?zhí)苛吭?50℃時為45.3%,而未添加的含磷的環(huán)氧樹脂的殘?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復合環(huán)氧樹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)氧單體反應獲得的磷-硅復合環(huán)氧樹脂。

6 結(jié)語

含磷阻燃環(huán)氧樹脂作為高效、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展,但由于含磷環(huán)氧樹脂或含磷改性單體的合成難度普遍較高,導致其實際應用成本高、價格高,這個因素限制了含磷環(huán)氧樹脂大規(guī)模的使用,因此有必要通過研究新型含磷阻燃環(huán)氧樹脂的體系或探討不同組分與磷組分的協(xié)同作用來進一步提高含磷環(huán)氧樹脂的阻燃效率并降低應用成本。所以在今后的研究中需要重點研究優(yōu)化現(xiàn)有含磷環(huán)氧樹脂的制備工藝、研制復合型環(huán)氧樹脂以及新型的含磷固化劑等。

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