a Highly Active and Selective β-Nucleating Agent for Isotactic Polypropylene and Crystallization Behavior of β-Nucleated Isotactic Polypropylene Under Rapid Cooling

Over the past few decades, much attention has been focused on the preparation of the β-crystal form of isotactic polypropylene (β-iPP) under laboratory and industrial conditions. The structure, morphology, processing, properties and application of β-iPP have been extensively studied. In comparison to the traditional α-form of iPP (α-iPP), β-iPP exhibits improved elongation at break and better impact resistance. These unique performance characteristics of β-iPP, which are advantageous in many applications, justify the extensive basic and applied research efforts that have been focused on this material.

β-iPP can be produced in the presence of efficient, selective β-nucleating agents (β-NAs) under practical conditions. Many available β-NAs have been previously reported. The first efficient β-NA was γ-quinacridone (Dye Permanent Red E3B), which was reported by Leugering. A two-component β-NA (i.e., compounds of pimelic acid and calcium stearate) was introduced by Shi et al. Additional compounds with more defined structure that were similarly efficient included calcium salts of pimelic and suberic acid, as reported by Varga et al.. These two β-NAs have been categorized as a class containing certain group IIA metal salts or their mixtures with some specific dicarboxylic acids. N,N′-Dicyclohexyl-2,6-naphthalenedicarboxamide (NU 100) is also an efficient β-NA that was developed in the past decade. Although these substances can act as β-NAs for iPP, only a few aromatic amide compounds, such as NU 100, have been marketed. Unfortunately, NU 100 is not a highly selective β-NA, and α-iPP is always formed in its presence. Therefore, a highly active and selective β-NA for isotactic polypropylene is still required. In recent years, a novel group IIB salt of alicyclic dicarboxylic acid that acts as a β-NA was investigated in our group. In this study, we reported a novel IIB salt of aliphatic dicarboxylic acid (i.e., zinc adipate), which acts as a highly active and selective β-NA for iPP. Our results further enrich the family of IIB salts of dicarboxylic acid for use as β-NAs of iPP.

β-iPP is thermodynamically metastable, which has been reported by Varga in detail. The formation of β-iPP strongly depends on the crystallization process, especially the cooling rate.  Both Varga and Wittmann attributed this “sensitivity” to the upper and lower critical temperature limit of the formation of β-iPP. The upper critical temperature limit is around 140 °C, and the lower limit is at 100-105 °C. Therefore, the growth rate of the β-phase is higher than that of α-phase between 100 and 140 °C. When the temperature is higher than 140 °C or lower than 100 °C, βα-growth transition will take place on the surface of a growing β-spherulite. Apparently, the content of β-iPP depends on the cooling rate. Therefore, the exploration of the dependence of the formation of β-iPP on the cooling rate under real processing conditions is technologically important and scientifically fascinating. Previous investigations have been extensively performed under laboratory conditions using standard differential scanning calorimetry (DSC).  However, operational processes occur much faster compared to typical laboratory rates. Therefore, achieving controlled, constant heating and cooling rates that are much higher than the typical rates of standard DSC instruments, which range from 10 to 100 °C/min, is experimentally challenging. Recent advances in chip fast scanning calorimetry (FSC) by Mettler-Toledo (e.g., the Flash DSC 1 calorimeter, introduced October 2010) may provide the ability to investigate crystallization and melting behavior under rapid cooling rates. In this approach, the cooling rate and heating rate can reach 240,000 °C/min. Therefore, FSC is a powerful tool for investigating the crystallization and melting behaviors of polymers by mimicking realistic conditions. However, to the best of our knowledge, the literature contains few studies that address the dependence of β-crystals on the cooling rates under rapid cooling and supercooling.

In the current study, Zinc adipate (Adi-Zn) was observed to be a highly active and selective β-nucleating agent for isotactic polypropylene (iPP). The effects of Adi-Zn on the mechanical properties and the β-crystals content of nucleated iPP were investigated. The impact strength of iPP nucleated with 0.2 wt% Adi-Zn was 1.8 times higher than that of neat iPP. In addition, wide-angle X-ray diffraction (WAXD) analysis indicated that the content of β-crystals in nucleated iPP (k_β value) reached 0.973 with 0.1 wt% Adi-Zn, indicating that Adi-Zn is a highly active and selective β-nucleating agent for iPP. Furthermore, fast scanning chip calorimetry (FSC) studies using cooling rates from 60 to 13,800 °C/min revealed that the formation of β-crystals significantly depended on the cooling rates. At cooling rates below 3000 °C/min, only β-crystals existed. However, at cooling rates above 6000 °C/min, β-crystals failed to form. Moreover, a lower critical crystallization temperature that corresponded to the generation of β-crystals was investigated using cooling-induced crystallization, and the results are in good agreement with those of a previous study. Finally, an exponential decay of T_cp as a function of the cooling rate is proposed.