COMMONALITY OF PHENOMENA IN COMPOSITE MATERIALS Effect of UV exposure on the microstructure and mechanical properties of long fiber thermoplastic (LFT) composites A. Goel Æ K. K. Chawla Æ U. K. Vaidya Æ M. Koopman Æ D. R. Dean Received: 24 October 2007 / Accepted: 28 December 2007 / Published online: 18 April 2008 Ó Springer Science+Business Media, LLC 2008 Abstract In this work, the effect of ultraviolet (UV) exposure on the microstructure and dynamic Young’s modulus of polypropylene (PP)/21 vol.% E-glass LFT and neat PP was investigated. Microscopic observations revealed that the damage due to UV was confined to the surface region only in the form of surface cracking and exposure of fibers to the surface in the case of long fiber thermoplastic (LFT) and surface cracking in the case of neat PP. Fourier transform infrared spectroscopy showed that the crystallinity of PP in the damaged layer increased, both in neat PP as well as in LFT, with exposure time. This is due to chemicrystallization, which involves rearrange- ment of amorphous broken polymer chains into crystalline form. Crystallinity of PP in the damaged layer in LFT increased at a higher rate as compared to that in neat PP. Results of nanoindentation showed that the Young’s modulus of the PP in the damaged layer increased, with UV exposure time; the rate of modulus increase being higher in the case of LFT than in neat PP. Although the local Young’s modulus of the degraded layer increased, the dynamic Young’s modulus of the overall composite showed a decrease with UV exposure time. Introduction Thermoplastic matrix composites are increasingly used in a variety of applications such as mass-transit, automotive, and military structures. They hold a distinctive edge over traditional materials, such as steel and aluminum, in these applications due to their high specific strength, good damping capacity, and corrosion resistance [1]. The matrix in thermoplastic composites is generally comprised of polypropylene (PP), polyethylene (PE), nylon, or other inexpensive polymers. E-glass fiber is a commonly used reinforcing material. Long fiber thermoplastic (LFT) composites have seen fairly high growth rates in the plas- tics industry. The fibers in LFTs have large fiber aspect ratio [ratio of length (‘) to diameter (d)], often ‘/d [ 1,000 and thus take full advantage of the reinforcing fiber strength [2]. The final fiber lengths in the composite are typically [ 13 mm. Long fiber thermoplastics are frequently used for out- door structural applications and are exposed to UV radiation. Hence, it is important to investigate the effect of UV exposure on mechanical properties of these materials. LFTs provide enhanced stiffness to the struc- tural part due to the reinforcing fiber. An obvious disadvantage of using these composites in outdoor appli- cations, however, is their susceptibility to environmental degradation especially due to solar radiation. The wave- length of solar radiation ranges from 290 to 1,400 nm. The infrared region (780–1,400 nm) comprises *53% of A. Goel  K. K. Chawla (&)  U. K. Vaidya  M. Koopman  D. R. Dean Department of Materials Science and Engineering, University of Alabama at Birmingham, BEC 254, 1530 3rd Ave. S, Birmingham, AL 35294-4461, USA e-mail: kchawla@uab.edu A. Goel e-mail: ashutosh@uab.edu U. K. Vaidya e-mail: uvaidya@uab.edu M. Koopman e-mail: mkoopman@uab.edu D. R. Dean e-mail: deand@uab.edu 123 J Mater Sci (2008) 43:4423–4432 DOI 10.1007/s10853-007-2444-6