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This article is a part of a study of model and bulk composites, based on isotactic polypropylene (i-PP) and glass (or carbon) fibers, produced from knitted textile preforms of hybrid yarns. First, we report the results on crystallization and fusion of textile-grade i-PP, used for the processing of hybrid yarns and the corresponding knitted fabrics. The kinetics of the crystallization process, in the dynamic and isothermal regime, was followed by DSC, and the results were analyzed by Avrami, Ozawa, and Harnisch-Muschik methods. Isothermal crystallization of i-PP was carried out at 388-400 K, and values for the Avrami exponent ranging from 1.93 to 4.39 were determined. The equilibrium melting temperature was determined by the Hoffman-Weeks method, and g Å 2.54 was found. Double melting peaks were observed both when the crystallization was performed at lower temperatures (isothermal regime) and at higher cooling rates (nonisothermal regime). A single melting peak appeared upon melting following isothermal crystallization at 400 K. The nonisothermal kinetics data showed that the peak crystallization temperature changes from 377 to 386 K as the cooling rate decreases from 20 to 3 K/min. Applying the Ozawa method, a value of the exponent n Å 2.33 was determined, which is in agreement with the results for isothermal crystallization at 391-400 K. The Harnisch-Muschik approach was also applied to compare the results for n, and a similar trend in the results of isothermal and nonisothermal crystallization was found, due to the predominant homogeneous mechanism of nucleation at lower cooling rates (lower isothermal T c ) in spite of being heterogeneous at higher cooling rates (higher isothermal T c ).

Isothermal and nonisothermal crystallization of maleic anhydride grafted polypropylene (PP), which is used for the production of split warp knit composite preforms, 1 are analyzed in model composites to determine the influence of reinforcement glass fibers (GF) and poly(ethylene terephthalate) (PET) binding yarns on the crystallization kinetics. Basic energetic parameters of crystallization are determined, and the melting behavior of PP in model composites is analyzed. The crystallization of PP carried out in nonisothermal and isothermal regimes is facilitated in the presence of GF, and the additional effects of PET fibers are also shown. Better conditions for nucleation, resulting in lower energy for formation of a stable nucleus and lower critical dimensions, are proposed as a reason for this. The crystal structure of PP in model composites exhibits lower lamellae thickness and is less disposed to recrystallization.

Isothermal crystallization of iPP in model glass-fiber composites is studied by DSC, and the basic energetic parameters of crystallization are determined. Unsized untreated and thermally treated glass fibers are used in model composites to determine the role of the surface on nucleation and crystallization processes. Thermally treated glass fibers are found to exhibit a predominant nucleating effect as compared to unsized untreated ones, and the crystallization proceeds faster, resulting in lower values for the halftime of crystallization (10-120 s). The energy of formation of a nuclei of critical dimensions at a given T c is also lower, and it decreases as the content of the fibers in the composite increases. The surface free energy of folding, s e Å 140 1 10 03 J/m 2 , was determined for iPP in the composite containing 50% glass fibers, while for pure iPP, s e Å 170 1 10 03 J/m 2 was found.

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