Epoxy bests polyester in boat repair trial

wumingsheng

Recent tests conducted at West System Inc. (Bay City, Mich.), a supplier of resins, hardeners, fillers, additives, reinforcements, application tools and instruction for boat repair, supported the company’s contention that today’s glass/polyester composite boat bulls can be repaired most effectively with epoxy-based repair systems.

Because the chopped strand glass mat used in early boats (30+ years ago) isn’t as strong as woven and stitched fabrics, their hulls were thick and stiff, so a repair technician could grind away the damage, sand a taper around the perimeter and apply an effective glass/polyester patch. But West System technical staffer Tom Pawlak points out that improved polyester resin chemistries make it possible to fabricate today’s hulls thinner, lighter and more flexible (impact-resistant) but also makes them more difficult to repair. Modern polyesters crosslink more completely during cure, he contends, resulting in fewer available sites for primary bonds with a polyester patch. Also, polyester is typically “air-inhibited” (it remains uncured or tacky when a thin film is applied on a surface exposed to air), making it less than desirable as an adhesive. In contrast, Pawlak says epoxies are able to form superior secondary bonds and readily adhere to properly prepared, cured polyester, without volatile emissions. Epoxy resists microcracking better than polyester and has excellent thin film characteristics, is more flexible with higher tensile elongation. Further, polyesters must be cured at temperatures above 60°F/16°C, but hardeners are available that make it possible to cure epoxy repairs at temperatures as low as 40°F/4°C. Pawlak also notes that polyester resin shrinkage, while fairly insignificant in a small repair, can become a factor in large repair areas, because the shrinkage subjects the bond line to stress before the repair has a chance to experience working loads.


Pawlak and his group backed these contentions with the results of testing that pitted West System epoxy against dicyclopentadiene (DCPD) polyester resin. Trials were conducted on large, well-cured fiberglass/DCPD laminate coupons made with triaxial fiberglass and glass mat, with G-10 fiberglass laminate tabs added to each end to provide a grip area for the test machine. Coupons were ground to a 12:1 bevel along one edge, sanded with 80-grit sandpaper, then repaired with multiple plies of fiberglass and various repair resins. The tensile breaking strength of the repaired laminates using the same DCPD polyester resin and fiberglass material averaged 18,460 psi or 70.5 percent of original strength (determined by testing unrepaired coupons). The breaking strength of the repaired laminates using West System epoxy and the same fiberglass material averaged 21,404 psi or 81.7 percent of the original laminate strength. Pawlak points out that all repairs were done using the minimum 12:1 bevel, but longer bevel angles at 15:1 or 20:1 would yield higher repair strengths.

yunwater2042

老兄能否對"secondary bonding"在多講講呢？
具體用Epoxy system 跟free-radical initiator system 的性能差別？
謝謝！

wumingsheng

secondary bonding. 就是指二次粘結，比如常見的修補就是二次粘結。

主要是對于環氧來說：第一，收縮較少，收縮導致的界面內應力，影響粘結強度，尤其對于大面積修補尤其如此。

第二, 環氧極性基團較多，容易與原有基體成健，具有較高的化學粘結能力。這是環氧的特性決定的。

對于不飽和體系：

第一與環氧相比，收縮較多，

第二， 由于原有的基體內，交聯程度隨著樹脂制造技術的提高，越來越高了。在二次粘結的時候，形成化學鍵的概率地多了。

不像環氧，你就是在實驗室用超聲波技術混合，他還是固化不完全。即使你能在分子水平上混合，由于固化條件的限制，還是不能固化完整，還就是本身的極性鍵就較多，容易與基體結合。

wumingsheng

表面處理也是很關鍵的。

一般的步驟是：

Degreasing----Grinding -sanding----Degreasing。

最好是不能暴露粘結面超過兩個小時。如果超過，可以涂供應商推薦的primer。

yunwater2042

多謝！

認識深入很多了。

實際操作就是狠命打磨然后再用環氧膠粘接上的，再加作了2層氈。