PBT/PET共混物濕熱老化行為之研究

Abstract

聚對苯二甲酸乙二酯（PET）具高剛性、優異的機械性質與出色的阻氣性；聚對苯二甲酸丁二酯（PBT）則結晶速度快、韌性高，且加工效率高。將兩者熔融混摻可同時兼具PET的強度與PBT的成形優勢，因此廣泛用於汽車、電子與包裝等工程應用。然而兩種聚酯在高溫高濕條件下易發生水解，分子量迅速衰減，造成材料脆化並縮短使用壽命。 為探討其濕熱老化行為，本研究以熔融混煉製備不同比例的PBT/PET共混物，並加入不同含量的碳二亞胺（BDICDI）抗水解劑，於85°C/85% RH進行長達800小時老化，並使用拉伸試驗、SEC、DSC及SAXS分析機械性質、分子量、結晶行為與微觀結構的變化。 結果顯示，在85°C/85% RH濕熱老化下，PBT/PET共混物的抗拉強度隨老化時間顯著衰減。主要原因是鏈段水解使分子量急劇下降，隨後的化學結晶又縮短層狀週期與無定形區厚度，並伴隨tie-chains與分子糾纏的流失，最終導致材料脆化。然而僅添加0.5 phr BDICDI即能在500小時老化後維持抗拉強度；當添加量高於1.5 phr以上，經800小時老化後抗拉強度仍可保留75%以上。SEC、DSC及SAXS分析證實BDICDI可與聚酯末端羧酸以及水分反應，從而減緩分子量下降及隨後的化學結晶，並穩定層狀週期與無定形區厚度，保留充足的tie-chains與分子糾纏。此外，本研究進一步建立了抗拉強度與重量平均分子量（Mw）的對應曲線，並量化各配方的臨界分子量Mc，為聚酯材料的配方優化與壽命評估提供量化依據。 綜上所述，本研究闡明了PBT/PET共混物在濕熱環境下的降解與結晶機制，也證實BDICDI的抗水解能力。此成果可用於含回收料之聚酯配方，減輕二次加工帶來的分子量衰減，並為極端氣候下聚酯產品的設計與耐久預測提供具體參考。

Polyethylene terephthalate (PET) has high rigidity, excellent mechanical properties, and outstanding gas barrier performance. Polybutylene terephthalate (PBT) features fast crystallization, high toughness, and efficient processability. Blending the two through melt mixing allows the material to retain the strength of PET and the molding advantages of PBT, making it widely used in automotive, electronics, and packaging applications. However, both polyesters are prone to hydrolysis under high temperature and humidity, leading to rapid molecular weight degradation, material brittleness, and reduced service life. To evaluate their hygrothermal ageing behavior, PBT/PET blends with various compositions were prepared by melt compounding, and bis(2,6-diisopropylphenyl) carbodiimide (BDICDI) was incorporated at different loadings as a hydrolysis stabiliser. The blend samples were aged for up to 800 h at 85 °C and 85 % RH, and changes in mechanical properties, molar mass, crystallization behavior and microstructure were examined by tensile testing, SEC, DSC and SAXS. The results show that, at 85 °C and 85 % RH, the tensile strength of PBT/PET blends decreases markedly with aging time. The main reason is chain-scission hydrolysis, which sharply lowers the molecular weight. The resulting chemi-crystallization then shortens the long period and amorphous region thickness. At the same time, tie-chains and molecular entanglements are lost, and the material eventually becomes brittle. Adding only 0.5 phr BDICDI can keep the tensile strength after 500 h of ageing, and the doses above 1.5 phr still preserve more than 75% of the tensile strength after 800 h. SEC, DSC, and SAXS analyses confirm that BDICDI reacts with terminal carboxyl groups and moisture, thereby slowing the molecular-weight drop and subsequent chemi-crystallization, stabilizing the long period and amorphous region thickness, and retaining ample tie-chains and entanglements. In addition, the correlation curve between tensile strength and weight-average molecular weight (Mw) was established, and the critical molecular weight (Mc) for each formulation was quantified, providing a basis for optimizing polyester formulations and evaluating their lifespan. In conclusion, this study clarifies how PBT/PET blends degrade and crystallize under hygrothermal conditions and confirms the hydrolysis-blocking effect of BDICDI. The results can guide polyester formulations that include recycled resin by reducing molecular-weight loss during reprocessing and offer concrete reference points for the design and durability prediction of polyester products intended for extreme climates.