Effects of Drawing Conditions and PEC Incorporation on the Structure and Properties of Uniaxially Oriented PLA Films

Abstract

ABSTRACT Polylactide (PLA) films have been considered to be a good alternative for packing applications due to its biodegradability. However, PLA films have the disadvantage of low thermal stability and mechanical properties, indicating that the use of PLA film is still limited. During drawing process, properties of PLA films are significantly affected by drawing conditions. It is obvious that understanding of the effects of drawing conditions can provide a means of controlling properties. To study the effects of drawing ratio on the structure and properties of PLA films induced by uniaxial drawing, PLA films were prepared by melt extrusion and then carried out using a biaxial stretcher. Uniaxially drawn PLA films were prepared using various draw ratios of 2, 3, 4, and 4.5 at a constant draw rate and temperature. It was confirmed that the conformational structure of the PLA films was composed of an α′ phase. With increasing draw ratio, the strain-induced crystallization and orientation of the α′ phase PLA films along the drawing direction increased, resulting in improved mechanical and thermal properties, as shown in the mechanical testing and dynamic mechanical analyses results. It is noteworthy that the crystallinity and orientation of the α′ -phase PLA increased significantly at draw ratio from 2 to 3 and were almost saturated at draw ratios above 3. The mechanical and thermal properties of the PLA films were significantly improved by using draw ratios above 3. To study the effects of drawing temperature on the structure and properties of PLA films induced by uniaxial drawing, as-cast PLA films were produced by melt processing and uniaxially drawn at drawing temperatures of 65, 85, and 120 °C at a constant draw rate. It was confirmed that at drawing temperatures of 65 and 90 °C, the PLA films were composed of the α′ phase. The crystallinity and molecular orientation of the α′ -phase PLA films increased with increasing draw ratio. Therefore, the thermal and mechanical properties of the PLA film at a drawing temperature of 90 °C were more enhanced than those at 65 °C due to more strain-induced crystallization and molecular chain orientation during uniaxial drawing. However, strain-induced crystallization and molecular orientation were not detected at 120 °C, which may have been because the rate of relaxation of the oriented segments was faster than the strain crystallization. It was confirmed that the mechanical and thermal properties of the PLA films were dependent on the processing conditions such as drawing temperature and draw ratio. It is expected that this paper will help determine the optimal process conditions for the commercial fabrication of PLA films. To study the preparation and characterization of PLA/poly(ethylene carbonate) (PEC) blend films, PLA/PEC blend films were prepared by melt extrusion and then uniaxially drawn using various draw ratios. The structural conformation was confirmed and the mechanical, thermal, and barrier properties were investigated. It was confirmed that the PLA/PEC blend films were composed of a strain-induced α´ phase. The degree of strain-induced crystallization and molecular orientation of α´-phase PLA/PEC films increased with increasing draw ratio, which enhanced the physical properties of the films. The elongration at bleak of the PLA/PEC blend film was higher than the neat PLA film, indicating that the PLA/PEC blend film has flexibility. As compared to the neat PLA films, the oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) of the PLA/PEC blend films improved, particularly OTR. In accordance with the above results, the PLA/PEC blend films are expected to be commercialized as ecofriendly flexible packing films. During drawing of the PLA films, strain-induced crystallization and orientation along the drawing direction were used to effectively improve the properties of PLA, and thus the optimum uniaxial drawing conditions for commercial fabrication were confirmed. Through blending with PEC, the disadvantages of PLA can be overcome. These blend films are expected to be implemented in various fields such as automobile interiors, clothing, biomedical devices, credit cards, and food packaging, replacing petroleum-based polymers.|Summary in Korean 본 연구에서는 일축 연신된 친환경 고분자 필름인 폴리락타이드 (polylactide, PLA)필름 및 PLA/폴리에틸렌 카르보네이트 (polyethylene carbonate, PEC) 블렌드 필름을 제조하고, 연신 공정에 따른 구조적인 특성 확인 및 물성에의 영향에 관해 고찰하였으며, 최적화된 연신 공정 조건에서 PLA 필름의 물성 향상을 통해, 친환경 필름으로 용도 확대 가능성을 확인하였다. PLA 필름은 우수한 친환경성으로 많은 지속적으로 연구되어 오고 있지만, PLA 필름의 부족한 내열성, 석유계 고분자 대비 낮은 기계적 물성 및 유연성으로 아직까지 상업적으로 널리 사용되지 못하고 있다. 연신 공정의 경우 연신에 의해 고분자 사슬의 배향 및 배향 결정화에 의해 고분자 물질의 물성을 향상시킬 수 있는 방법 중 하나로, PLA 도 연신 공정을 통해 물성을 향상 시킬수 있다. 일축 연신된 PLA 필름의 구조적인 발달을 연구하기 위해, 용융 압출 공정 및연신 공정을 진행하여 PLA 필름이 제조되었다. 연신비에 따른 영향을 확인하기 위해서 PLA 필름은 동일한 연신 온도 및 연신 속도에서 파단되기 전까지 2배, 3배, 4배, 4.5배의 다양한 연신비로 준비되었다. 일축 연신 공정에서 제조된 PLA 필름은 연신 결정화에 의한 α′ 상을 가진다는 것을 확인하였다. PLA 필름의 구조적인 변화는 필름의 특성과 직접적으로 관련이 있기 때문에, α′ 결정상을 가지는 PLA 필름의 연신비에 따른 연신 결정화 및 분자 배향에 관해 조사하였다. PLA 필름은 연신비의 증가에 따라 연신 결정화 및 배향도가 증가하며, 이에 따라 필름의 기계적 및 열적 물성도 향상된다. 특히, 연신비가 2배에서 3배로 증가할 때 연신 결정화 및 배향도의 증가 속도가 가장 빠르며, 이에 따라 기계적 열적 물성의 향상도 두드러지는 것을 확인하였다. 연신 온도에 따른 영향을 확인하기 위해서 PLA 필름은 동일한 연신 속도에서 60, 90 및 120 °C의 각 온도에서 파단 전까지 다양한 연신비로 연신되었다. 각 60 와 90 °C의 연신 온도에서 연신에 의해 α′ 상의 결겅상이 생성된 것을 확인할 수 있으며, 연신 결정화도와 배향도는 연신비가 증가함에 따라 증가하였다. 동일 연신비에서 비교하면, 90 °C에서 연신한 필름의 연신 결정화도와 배향도가 60 °C에서 연신한 필름보다 높은 경향이 있었다. 따라서 동일 연신비에서 필름의 기계적 및 열적 물성도 90 °C 에서 연신한 필름이 60 °C에서 연신한 필름 대비 우수한 경향이 있음을 확인하였다. 그러나, 연신온도가 120 °C 인 경우 연신 결정화와 배향이 거의 일어나지 않아, 결정상을 확인할 수 없었다. 이는 120 °C 의 연신 온도에서는 분자이완이 연신 결정화 속도보다 더 빠르게 일어나기 때문으로 판단된다. 상대적으로 낮은 연신 온도에서는 PLA 필름의 구조 및 물성은 연신비에 의한 영향이 우세하고, 높은 연신 온도에서는 연신비에 의한 영향은 크지 않은 것을 확인할 수 있었다. PLA 필름은 90 °C 의 연신 온도에서 가장 효과적으로 연신에 의해 구조적으로 발달하였으며, 기계적 및 열적 물성도 향상되었다. 본 연구의 결과는 PLA 필름의 제조 공정시에 공정 조건을 최적화하여 물성 향상을 통해 용도를 확장하는데 기여할 수 있다. PLA 필름의 경우 생분해성으로 포장용도에서 석유계 대체 고분자로 주목받고 있지만 낮은 유연성과 베리어성으로 용도가 한정되어 진다. PLA의 단점을 보완하기 위해 우수한 유연성과 베리어성을 가지는 CO2 를 기반으로 생산되어지는 친환경 PEC 고분자와 블렌드 필름을 제조하였다. 용융 압출 공정을 통해 블렌드 필름을 제조하고 다양한 연신비로 일축 연신하여 구조적인 특성 및 물성변화를 연구하였다. PLA/PEC 블렌드 필름은 연신 결정화에 의해 α′ 결정상을 가지는 것을 확인하였다. 이는 PEC 의 블렌딩이 PLA 필름의 결정 구조에 큰 영향을 주지 않은 것으로 판단된다. 연신비가 증가할수록 연신 결정화도와 배향도는 증가하였고, 기계적 및 열적 물성도 증가하는 경향이 있다. 또한, 유연한 사슬을 가지는 PEC 가 블렌딩된 함량만큼 순수 PLA 필름 대비 최대 연신비는 1.5 배 증가하였고, 최대 연신비에서의 연신 강도 및 modulus 는 다소 낮았다. 이는 필름의 유연화도가 증가함에 의한 영향으로 PLA/PEC 블렌드 필름의 유연성과 베리어성은 순수 PLA 필름 대비 우수하였다. 순수 PLA 필름 대비 PEC 10% 의 블렌딩을 통해 산소투과도는 약 70 % 정도 감소하였으며, 투습도의 경우 약 15 % 감소하였다. PEC 블렌딩에 의해 고분자 사슬간의 결합 강도가 높아졌거나 사슬간의 프리 볼륨 크기가 작아져서 산소투과도가 낮아진 것으로 판단된다. 투습도의 경우 PLA 의 에스터 그룹이 수분에 취약하기 때문에 PEC 가 블렌딩 된 함량에 절대적으로 의존하는 것으로 판단된다. 또한 연신비가 증가할수록 연신 결정화도 및 배향도가 증가하므로, 베리어성도 전반적으로 향상된다. PEC 블렌드를 통해 PLA 의 단점인 낮은 유연성 및 베리어성이 개선될 수 있으므로, PLA/PEC 블렌드 필름은 친환경적인 유연한 포장 필름으로 적용될 가능성이 있음이 확인되었다.; ABSTRACT Polylactide (PLA) films have been considered to be a good alternative for packing applications due to its biodegradability. However, PLA films have the disadvantage of low thermal stability and mechanical properties, indicating that the use of PLA film is still limited. During drawing process, properties of PLA films are significantly affected by drawing conditions. It is obvious that understanding of the effects of drawing conditions can provide a means of controlling properties. To study the effects of drawing ratio on the structure and properties of PLA films induced by uniaxial drawing, PLA films were prepared by melt extrusion and then carried out using a biaxial stretcher. Uniaxially drawn PLA films were prepared using various draw ratios of 2, 3, 4, and 4.5 at a constant draw rate and temperature. It was confirmed that the conformational structure of the PLA films was composed of an α′ phase. With increasing draw ratio, the strain-induced crystallization and orientation of the α′ phase PLA films along the drawing direction increased, resulting in improved mechanical and thermal properties, as shown in the mechanical testing and dynamic mechanical analyses results. It is noteworthy that the crystallinity and orientation of the α′ -phase PLA increased significantly at draw ratio from 2 to 3 and were almost saturated at draw ratios above 3. The mechanical and thermal properties of the PLA films were significantly improved by using draw ratios above 3. To study the effects of drawing temperature on the structure and properties of PLA films induced by uniaxial drawing, as-cast PLA films were produced by melt processing and uniaxially drawn at drawing temperatures of 65, 85, and 120 °C at a constant draw rate. It was confirmed that at drawing temperatures of 65 and 90 °C, the PLA films were composed of the α′ phase. The crystallinity and molecular orientation of the α′ -phase PLA films increased with increasing draw ratio. Therefore, the thermal and mechanical properties of the PLA film at a drawing temperature of 90 °C were more enhanced than those at 65 °C due to more strain-induced crystallization and molecular chain orientation during uniaxial drawing. However, strain-induced crystallization and molecular orientation were not detected at 120 °C, which may have been because the rate of relaxation of the oriented segments was faster than the strain crystallization. It was confirmed that the mechanical and thermal properties of the PLA films were dependent on the processing conditions such as drawing temperature and draw ratio. It is expected that this paper will help determine the optimal process conditions for the commercial fabrication of PLA films. To study the preparation and characterization of PLA/poly(ethylene carbonate) (PEC) blend films, PLA/PEC blend films were prepared by melt extrusion and then uniaxially drawn using various draw ratios. The structural conformation was confirmed and the mechanical, thermal, and barrier properties were investigated. It was confirmed that the PLA/PEC blend films were composed of a strain-induced α´ phase. The degree of strain-induced crystallization and molecular orientation of α´-phase PLA/PEC films increased with increasing draw ratio, which enhanced the physical properties of the films. The elongration at bleak of the PLA/PEC blend film was higher than the neat PLA film, indicating that the PLA/PEC blend film has flexibility. As compared to the neat PLA films, the oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) of the PLA/PEC blend films improved, particularly OTR. In accordance with the above results, the PLA/PEC blend films are expected to be commercialized as ecofriendly flexible packing films. During drawing of the PLA films, strain-induced crystallization and orientation along the drawing direction were used to effectively improve the properties of PLA, and thus the optimum uniaxial drawing conditions for commercial fabrication were confirmed. Through blending with PEC, the disadvantages of PLA can be overcome. These blend films are expected to be implemented in various fields such as automobile interiors, clothing, biomedical devices, credit cards, and food packaging, replacing petroleum-based polymers.