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

老兄能否对"secondary bonding"在多讲讲呢？
具体用Epoxy system 跟free-radical initiator system 的性能差别？
谢谢！

wumingsheng

secondary bonding. 就是指二次粘结，比如常见的修补就是二次粘结。

主要是对于环氧来说：第一，收缩较少，收缩导致的界面内应力，影响粘结强度，尤其对于大面积修补尤其如此。

第二, 环氧极性基团较多，容易与原有基体成健，具有较高的化学粘结能力。这是环氧的特性决定的。

对于不饱和体系：

第一与环氧相比，收缩较多，

第二， 由于原有的基体内，交联程度随着树脂制造技术的提高，越来越高了。在二次粘结的时候，形成化学键的概率地多了。

不像环氧，你就是在实验室用超声波技术混合，他还是固化不完全。即使你能在分子水平上混合，由于固化条件的限制，还是不能固化完整，还就是本身的极性键就较多，容易与基体结合。

wumingsheng

表面处理也是很关键的。

一般的步骤是：

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

最好是不能暴露粘结面超过两个小时。如果超过，可以涂供应商推荐的primer。

yunwater2042

多谢！

认识深入很多了。

实际操作就是狠命打磨然后再用环氧胶粘接上的，再加作了2层毡。