微波加熱及清洗設計對PET塑膠性質之影響

Abstract

一般而言，PET(Polyethylene Terephthalate)具有可熱塑及疏水之性質。然而粒子在貯儲時表面會產生物理性的水氣吸附，而在粒子射出成形為透明瓶前須進行水分之移除。此論文目的為利用微波設計加熱腔體以快速乾燥PET粒子及另一電漿清洗腔體處理可撓性PET基板，藉以檢測PET粒子及基板兩者的表面性質。 PET粒子加熱系統包含一圓柱筒狀腔體、活動門及其上之視窗、而由腔體另一端則裝設轉動葉片。當乾燥進行時，轉動葉片會迴轉以帶動PET粒子滾動。而腔體的上方裝置有四組2.45GHz 的微波源，為降低設計成本並沒有使用阻抗調節器。腔體設計在圓半徑/腔體長度時的Q值為0.5及振盪波長在2.45 GHz頻率時為2λ+λ/4的27 公分。另針對電漿清洗可撓性PET基板的電漿清洗系統設計則包括一方形真空腔體、活動門及其上之視窗、可固定PET基板的鋁製旋轉基座，微波源及泵抽氣單元。在此系統中，有一圓柱筒狀的共振腔藉由活塞式之圓形鼓狀板進行調整，並利用網路分析儀進行量測分析，以使微波能量做最大的輸入。 兩腔體之設計為減少微波洩漏量及增加人員操作的方便性，在活 動門上設計有一到三層的金屬包覆隔離網。隔離網層的多寡取決於門閉鎖時的緊密程度，較鬆的門鎖設計方式須較多的金屬隔離網。另為除去微波加熱時，腔內鋁製轉動葉片產生的不當火花，在材料表面先進行硬陽處理。也曾嘗試鐵弗龍及不□鋼鍍鐵弗龍膜層的設計方式，但前者為機械強度不夠，另一則為攪拌加熱中鍍層脫落而不適用。 PET粒子乾燥後的性質分析，以紅外線分析儀進行粒子含水率量測及SEM電子顯微鏡進行粒子表面分析。PET可撓性基板電漿清洗後的性質分析，則利用表面達因試劑及水滴角量測儀進行量測。實驗後結論如下所述。 PET粒子含水率由乾燥前的3700 ppm降至乾燥後的200 ppm，與真空熱風乾燥及除濕乾燥達到相同含水率的時間相比較，分別為0.5、 3、 4小時。除此之外，此設備微波乾燥能力可讓粒子在1.6小時內達到含水率37 ppm。另由SEM電子顯微鏡對粒子表面分析得知，乾燥後粒子表面會產生粗糙面，藉此評估PET水份之去除。在氧電漿對PET可撓性基板清洗效果上，經此清洗系統處理後基板表面張力由28~35達因提高到48~56達因，而基板水滴角則在150秒處理時間可由原90度降到10度以下。

Pet (Polyethylene Terephthalate), in general, is a thermoplastic material with hydrophobic property. Although, the surfaces of the PET grains may physically adsorb some moisture during storage; it is generally required to remove water right before using for container fabrication. The purposes of this work were to design microwave applicators to act as a heating chamber for quickly drying PET grains and another chamber for plasma cleaning the flexible PET plates to examine their effects on properties of grains and plates, respectively. The applicator for PET grains heating consists of a horizontally oriented cylindrical chamber, a door with a view port on one side and a rotating frame with blades in the chamber installed from another side of the chamber. The rotating frame is for agitating the PET grains during drying process. There are 4 units of 2.45 GHz microwave sources installed from the top of the chamber. The chamber is designed without using the microwave tuner to minimize the cost. The length and radius of the cylindrical chamber are based on the radius/length radio = 0.5 to obtain the maximum Q value and its resonant wavelength of the cavity for 2.45 GHz close to 27 cm which is 2λ+λ/4.Applicator for plasma cleaning of flexible PET plates consists of a square-shape vacuum chamber, a square door with a view port, a rotating Al-frame for fixing PET plates, one microwave power source and a pumping system. In this system, a cylindrical resonance cavity with an adjustable piston-like drum with the fence-wrapped sealing ring was used as a microwave tuner to tune the maximum power output to the cleaning chamber, where the power output was analyzed by a network analyzer. For both chambers, to minimize the wave leakage from the sealing ring of the door and to consider the convenient operation of door opening, the sealing rings of the door and drum were wrapped with one to three layers of grid metal fence. The number of fence layers is depend on the tightness of the door lock, a looser door lock requires a greater number of fences. To reduce the arcing at the rotating Al-frame during microwave heating, it was pretreated to become an electrically insulating film on the frame by aluminum surface positive electrode treatment. Teflon and Teflon-coated stainless steel rotating flame are not suitable for these applications due to weak mechanical support and weak Teflon-steel bonding, respectively. The PET drying process was characterized by precision IR moisture analyzer and SEM. The plasma cleaning process for flexible PET plates was characterized by surface dyne tests and water contact angle measurements. From the experimental results, the following conclusions can be drawn. The times to dry the PET grains from 3700 to 200 ppm H2O are 0.5, 3, and 4 hrs by using microwave heating, vacuum hot air drying and dehumidifier machine, respectively. In other words, this novel microwave drying machine can dry the PET grains to 37 ppm H2O around 1.6 hrs. SEM examination indicates that drying can make a rougher surface of the PET grains. In other words, SEM may be able to evaluate water content in the PET grains. Regarding the efficiency of oxygen-plasma cleaning for flexible PET plates, this novel system can change the surface tension of the plate from 28~35 to 48~56 mN/m, and water contact angle from 90o to 10o for 150 sec treatment time.