| 摘要: | 本研究以天然廢棄物牡蠣殼為原料製備碳酸鈣(CaCO₃)與鍛燒牡蠣殼後獲得的氧化鈣(CaO)，作為聚乳酸(PLA)之成核劑，探討其於非等溫與等溫結晶條件下對PLA結晶行為與熱性質之影響。透過差示掃描量熱分析(DSC)，系統分析各樣品之冷結晶溫度(Tcc)、結晶度(Xcc、Xm)、熔融行為(Tm)、玻璃化轉變溫度(Tg)及Avrami與Ozawa動力學參數。
等溫結晶結果顯示，105 °C為PLA結晶最有利溫度，不僅結晶度較高，k值較大、t₁/₂較短，亦呈現較穩定之Avrami指數(n ≈ 2.5)，而110與115 °C條件下結晶度下降且n值普遍偏低，反映結晶機制可能轉變。非等溫分析在Ozawa分析中，Blank樣品之m值約4.5-4.9，顯示三維球晶成長特性；添加成核劑後，m值下降顯著，尤以CaO樣品最低可達2.13，反映出成核密度顯著提高，晶體結構更為緻密。儘管CaO的log K(T)值相對較低、結晶速率稍慢，但具更高的結晶完整性與穩定性。結果顯示，CaCO₃與CaO均具促進成核效果，能降低結晶活化能並提升結晶度，尤其在升溫速率為4與8 °C/min條件下，CaO展現出更高的成核效率。然而，當升溫速率提升至12與16 °C/min時，PLA/CaO樣品之冷結晶度明顯下降，反映出快速加熱限制晶體成長與相變重組，導致結晶不完全。在Tcc、Xcc與Tm之分析中亦可觀察到升溫速率提升會使熔融峰由雙峰趨向單峰，顯示α′→α相轉換與重結晶機制受干擾。在Mo分析中PLA/CaCO₃複合材料隨CaCO₃含量由1 wt%提升至3–5 wt%，結晶速率明顯減慢(logF(T)增加)，但超過5 wt%後結晶速率又加快(logF(T)下降)。PLA/CaO複合材料的α與logF(T)普遍低於相同濃度的PLA/CaCO₃，表明其結晶行為更接近理想模型且速率更快。隨相對結晶度提升，α與F(T)皆呈上升趨勢，可能因次級結晶成為主導機制及晶體成長受限所致。純PLA(Blank-screw)樣品參數較穩定，結晶動力學表現較單一，且受雙螺桿擠出製程影響，剪切力可能誘發晶相轉變並影響成核行為。此外，PLA型號與D-乳酸含量對結晶速率亦有明顯影響，3251D因低D含量結晶速度較快。綜合分析可得，成核劑種類、添加濃度與熱處理條件皆為影響PLA結晶機構與性能的關鍵因素。彎曲強度測試中PLA中添加1至5 wt.%的碳酸鈣能顯著提升彎曲模量，最高提升12.9%；但隨碳酸鈣含量超過5 wt.%，彎曲模量增加趨緩。相較之下，PLA添加氧化鈣時，彎曲模量在3 wt.%添加量即顯著下降13.6%，且高含量時試片易碎裂無法成型。整體結果表明，碳酸鈣有效提升PLA剛性，而氧化鈣添加導致PLA分子量下降，對彎曲模量產生負面影響。
;This study utilized natural waste oyster shells as a raw material to prepare calcium carbonate (CaCO₃) and calcium oxide (CaO, obtained via calcination of oyster shells) as nucleating agents for polylactic acid (PLA). The effects of these agents on PLA crystallization behavior and thermal properties under non-isothermal and isothermal conditions were investigated. Differential scanning calorimetry (DSC) was used to systematically analyze the cold crystallization temperature (Tcc), crystallinity (Xcc, Xm), melting behavior (Tm), glass transition temperature (Tg), and the kinetic parameters from Avrami and Ozawa models for each sample.
Isothermal crystallization results indicated that 105 °C is the most favorable temperature for PLA crystallization, with higher crystallinity, larger k values, shorter t₁/₂, and a relatively stable Avrami index (n ≈ 2.5). At 110 °C and 115 °C, crystallinity decreased, and n values were generally lower, suggesting possible changes in the crystallization mechanism. In non-isothermal analyses, Ozawa plots showed m values of 4.5–4.9 for the blank sample, indicative of three-dimensional spherulitic growth. Upon addition of nucleating agents, m values significantly decreased, with the CaO sample reaching as low as 2.13, reflecting a marked increase in nucleation density and denser crystal structures. Although CaO exhibited relatively lower log K(T) values and slightly slower crystallization rates, it yielded crystals with higher completeness and stability.
Both CaCO₃ and CaO promoted nucleation, reduced crystallization activation energy, and increased crystallinity. At heating rates of 4 and 8 °C/min, CaO displayed superior nucleation efficiency. However, when the heating rate increased to 12 and 16 °C/min, the cold crystallinity of PLA/CaO samples dropped markedly, indicating that rapid heating restricted crystal growth and phase reorganization, leading to incomplete crystallization. Analyses of Tcc, Xcc, and Tm also showed that increasing heating rates caused the melting peaks to shift from double to single peaks, suggesting that the α′→α phase transition and recrystallization mechanisms were hindered.
Mo analysis revealed that for PLA/CaCO₃ composites, increasing CaCO₃ content from 1 wt% to 3–5 wt% slowed crystallization rates (increased log F(T)), but rates accelerated again when content exceeded 5 wt% (decreased log F(T)). PLA/CaO composites exhibited generally lower α and log F(T) values than PLA/CaCO₃ at the same concentrations, indicating crystallization behavior closer to ideal models and faster rates. With increasing relative crystallinity, both α and F(T) increased, likely due to secondary crystallization becoming dominant and restrictions on crystal growth. Pure PLA (Blank-screw) samples showed more stable parameters and simpler crystallization kinetics; the twin-screw extrusion process may have induced phase transitions via shear forces, affecting nucleation behavior. Additionally, PLA grade and D-lactic acid content had a notable impact on crystallization rates, with grade 3251D crystallizing faster due to its low D content.
Overall, nucleating agent type, additive concentration, and thermal treatment conditions were found to be key factors influencing PLA crystallization mechanisms and properties. In flexural strength tests, adding 1–5 wt% CaCO₃ significantly increased the flexural modulus of PLA, with a maximum improvement of 12.9%. However, further increases beyond 5 wt% led to diminishing returns. In contrast, adding CaO to PLA resulted in a sharp 13.6% decrease in flexural modulus at only 3 wt% content, and at higher contents, specimens became brittle and failed to mold properly. These results demonstrate that CaCO₃ effectively enhances PLA rigidity, whereas CaO addition reduces PLA molecular weight, negatively impacting flexural modulus. |
