The study of thermal curing carbon nanotube/epoxy composite adhesives

熱固化奈米碳管/環氧樹脂複合膠料膠合研究

宋秉橙(Sung, Ping Cheng)

指導教授 ： 張士欽

國立清華大學/工學院/材料科學工程學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00197

摘要

環氧樹脂廣泛用於膠合結構與修補損傷。本研究使用奈米碳管加入環氧樹脂製成奈米碳管/環氧樹脂之複合膠料，使環氧樹脂具有可吸收微波以及可導電之性質，發展出以微波加熱以及直接電流加熱快速固化奈米碳管/環氧樹脂複合膠料的技術。
利用微波可以直接加熱複合膠料中的奈米碳管，使接合時間大幅降低至傳統加熱方式的1/3 ~ 1/4。使用傳統加熱奈米碳管/環氧樹脂複合膠料時，環氧樹脂的交連密度隨著碳管含量增加而降低；相對的，本研究發現，使用微波加熱對交連密度幾乎沒有影響。微波加熱還能使固化後殘留在膠料層中的氣泡縮小，使接合強度較傳統加熱者來得更高。
但是微波加熱無法簡單應用在接合會反射微波或是對微波具有強吸收性的材料，如金屬、碳纖維複合材料等。此外，微波加熱需要一定的微波系統設備，而試片之尺寸也會受到微波設備大小的限制。因此我們發展出了以電流直接加熱奈米碳管/環氧樹脂的技術。
當奈米碳管加入環氧樹脂的含量超過一門檻量(percolation threshold)時，奈米碳管可在環氧樹脂中形成導通網路，使得奈米碳管-環氧樹脂複合膠料的導電性大幅提高。然而奈米碳管具有溫度越高、電阻越低之特性，若使用一般施加電流的方式加熱此複合膠料，會使一開始已導通之區域有電流集中通過的溫度-電流之正回饋現象，導致膠料部分區域集中過熱燒毀。為了改善此問題，我們使用奈米碳管薄膜( buckypaper )當作使電流通過之加熱體加熱奈米碳管/環氧樹脂複合膠料，使用此種加熱方式可將加熱固化的時間從原本的60分鐘減少到25分鐘。由於使用buckypaper具有溫度-功率的負回饋效應而使加熱更均勻，再加上此加熱方式有自發排除氣泡的特性，能使膠料固化後殘留之氣泡大幅減少，而使接合強度相較於傳統加熱且未加入奈米碳管之接合提高了56%。
在接合導體材料如不銹鋼、黃銅、鋁板時，我們可以不使用奈米碳管薄膜，而直接以欲接合的導電基材作為兩電極，使電流自一基板經由導電膠料後從另一基板導出。由於此種加熱方式，電流流過的是膠料的厚度，其流過路徑極短而電流通過的面積極大，且金屬基板可快速導熱，因此可大幅緩解溫度-電流之正回饋現象而達成穩定且快速的加熱固化效果。然而，由於奈米碳管與金屬間之作用力極低，添加奈米碳管會使得金屬與膠料間接合強度下降。因此，碳管加入的含量僅需使複合膠料產生適當導電性質即可。
使用電流加熱固化奈米碳管/環氧樹脂複合膠料是理論上最節省能源的熱固化環氧樹脂方式，且不需要特殊的設備。不僅能大幅縮短固化時間，且能適用於各種環氧樹脂可接合之材料，無論是非導體材料、導體材料或是會吸收微波之材料皆可。因此可廣泛應用在各式材料之接合或修補。

關鍵字

奈米碳管 ； 微波加熱 ； 環氧樹脂複合材料 ； 直接電流加熱 ； 膠料接合

並列摘要

Carbon nanotubes (CNTs) could add in traditional thermal curing epoxy adhesive as effective strengtheners. The addition of CNTs not only strengthens the epoxy but also make it able to absorb microwave energy and conduct electrical current. In this work, processes of adhesive bonding by curing of carbon nanotube (CNT)/epoxy composite adhesives with microwave and direct application of electrical current were developed for fast and energy save bonding of materials.
Using microwave to cure epoxy adhesive containing multi-wall carbon nanotubes not only leads to large time and energy savings, but also results in much higher strength. The curing time by microwave heating was shortened to just 1/3 - 1/4 of that required by conventional heating means. The well-dispersed CNTs in the epoxy resin absorb microwave energy, acting as heat sources to cure the epoxy resin uniformly and effectively. Microwave curing can diminish the deterioration effect of lowering cross-link reaction by CNTs as observed in conventional heating. Thus, as much as three times addition of CNTs in the epoxy is allowed for optimum strength. As a result, 56% increase of the shear strength was observed when the epoxy adhesive doped with 3wt% CNTs was cured by microwave heating.
However, microwave curing of adhesive bonding cannot easily apply to materials that absorb or reflect microwave radiations strongly. The high equipment cost, size of microwave chamber and complexity in uniform power control put additional limitations to the application of microwave curing process. Therefore, there is definitely a need to develop a simple, time saving and cost effective process for curing CNT/epoxy adhesives. So, we developed the processes of thermal curing CNT/epoxy composite adhesives with direct application of electrical current.
Since CNTs are largely semi-conductive in nature, when apply electrical current directly through a CNT/epoxy composite adhesive film, the adhesive will burn out along a narrow path while leave other parts of the adhesive totally uncured due to the positive feedback effect. Therefore, two methods were developed to eliminate the detrimental positive feedback effect during Joule heating.
For joining conductive materials, the base materials to be bonded are used as current leads that make the current pass from one plate through the conducting CNTs/epoxy composites film to the other plate. For joining non-conductive materials, buckypaper was used as current carrier in a buckypaper-epoxy adhesive plaster patch. The electrical current passes through the buckypaper to raise the temperature of the epoxy resin for curing. With this process, not only the positive feedback effect can be avoided but also more uniform curing of the CNT/epoxy adhesive can be obtained with a beneficial negative feedback effect.
This process requires only simple equipment and is theoretically the most time and energy saving process for curing CNT/epoxy adhesive. The addition of CNTs in the epoxy can provide better heat transfer and strengthening of the epoxy adhesive. With the use of buckypaper/0.5% CNT/epoxy adhesive cured by Joule heating, the bonding strength increased significantly to be 67% higher compared to conventionally cured specimen with pure epoxy.

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