Novel nucleating agents for polypropylene and modifier of its physical-mechanical properties

Highlights

• Based on dicalcium HEDP salt novel nucleators show high efficiency in the nucleation of iPP crystals.

• Polypropylene nucleation experiments were performed using differential scanning calorimetry.

• At of 0.25 wt% the nucleators demonstrate an increase in crystallinity + 1.5 % with Ca stearate and + 4% - with Zn stearate.

• When the dicalcium HEDP salt based nucleator is added to the iPP, the flexural modulus increases by 8–12 %.

• The main component of the nucleator is synthesized by the one-step reaction in a non-toxic aqueous.

Abstract

In this paper, we demonstrate that dicalcium salt of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) can be used as a component of nucleating agents for isotactic polypropylene (iPP) - one of the most widely used commercial polymers. Based on dicalcium HEDP salt novel nucleators show high efficiency in the nucleation of iPP α-crystals. At a concentration of 0.25 wt% the nucleators demonstrate an increase in crystallinity (+ 1.5 % in composition with calcium stearate and + 4% - with zinc stearate). When the dicalcium HEDP salt based nucleator is added to the iPP, the flexural modulus increases by 8–12 %. Assumptions are suggested about the mechanism of action of the new nucleator, based on its core-shell structure. The main component of the nucleating agent, the dicalcium HEDP salt, is synthesized by the one-step reaction in a non-toxic aqueous medium with commercially available reagents. The synthetic protocol gives a finely friable product that does not require further grinding. The synthesis is cheap and environmentally friendly since only deionized water is used as a solvent.

Introduction

Isotactic polypropylene (iPP) is one of the most common commercially available polymers due to its relatively high strength and low cost. The method of modifying base polymers to increase strength and give special properties to products is the introduction of various types of organic or inorganic fillers [[1], [2], [3], [4]]. The combination of moderately slow crystal growth at large undercooling together with the practical absence of sporadic nucleation makes iPP an ideal material for controlled nucleation [5].

Historically, polymers have a long time been divided into semi-crystalline and amorphous [6]. After crystallinity was determined as an essential factor in the thermomechanical behavior of such important bulk polymers as polyethylene, polypropylene, and most polyamides, interest arose in crystallization control methods both in industry and research. The crystallinity of the iPP is crucial for achieving the final physical and mechanical properties [[7], [8], [9], [10]]. The degree of crystallinity of iPP can be increased as a result of: (a) inducing shear (melt orientation in the stream during casting) [11,12], (b) applying cooling temperature gradient [1], (c) recrystallization of the melt of iPP chains (self-nucleation of polymer chains) [13], or administration of specific nucleating agents [[14], [15], [16]].

Three crystallographic modifications are known in which iPP can crystallize: α, β, and γ [[17], [18], [19]]. The α- and β-phases are probably the most common and technologically interesting crystalline forms of iPP [20]. For iPP homopolymers, the α -phase is more thermodynamically stable and has a monoclinic crystal structure [2]. In contrast, the β-phase is usually obtained almost only by specific nucleation [10,[21], [22], [23]]. For this reason, a seed addition is required to obtain any significant amount of the β-form, while many β-nucleators also contribute to the formation of the α –phase [2,17,24].

Both organic compounds (polycyclic aromatic hydrocarbons, organic acids/salts, and amides [25], benzoic acid salts [26], hydrazide compounds [27], various organic pigments) and inorganic compounds (salts, oxides, e.g., CaCO₃ [28,29], talc [30], and Al₂O₃) were used as nucleators for iPP [31]. The disadvantage of organic nucleators is that the technology of their producing is not environmentally friendly green, which usually include several stages of organic synthesis using specialty compounds, as well as flammable and toxic solvents, mineral acids. In this case, one stage of the synthesis can take up to three days, and the resulting product requires cleaning, vacuum drying, following grinding, and permanent hygroscopicity control to prevent slumping.

Abundant and cheap inorganic fillers (talc, wollastonite, silica, mica, kaolin, diatomite, and calcium carbonate) are still the most industrially used inorganic fillers for polypropylene [32]. In addition to a general improvement in mechanical properties and fire resistance, reducing warpage and shrinkage, they induce the formation of a crystalline structure in PP, which leads to the desired improvement in mechanical properties. However, inorganic nucleators usually exert an effect at high concentrations (up to 30 wt%) [28], which makes a detrimental contribution to the plasticity of the polymer (the transition from plastic to brittle fracture) and assumes color to polypropylene, as a result of which it is impossible to obtain a transparent colorless plastic.

As noted above, among β-nucleators for PP, a number of salts of dicarboxylic acids (pimelic, suberic etc.) were previously used [33]. However, there is controversial information about the ability of the calcium salts of these acids to act as β-nucleators [[34], [35], [36]]. Meanwhile, β-crystallization of PP is also promoted by calcium salts of malonic acid [37], phthalic, terephthalic, pyromellitic acids [38,39] and pimelic acid [40,41].

Among the metal dicarboxylic acid salts the most industrially used and effective iPP α nucleators are the Milliken Chemical products: Hyperform^(™) HPN-20E [[42], [43], [44]] (cis-1,2-cyclohexanedicarboxylic acid calcium salt), and Hyperform^(™) HPN-68L [45,46] (bicyclo[2.2.1]heptane-2,3-dicarboxylic acid disodium salt). In iPP, these nucleating agents exhibit a very strong and specific crystalline orientation, which leads to useful physical properties [46], low and balanced shrinkage, and excellent impact resistance. The Hyperform ^(™) HPN-20E nucleating additive was chosen as the reference in our work.

The use of metal salts with noncarboxylic acids as nucleators for PP is practically unknown except sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (Adekastab NA-11 ^(™) by Asahi Denka Kogyo KK, Japan) and hydroxy aluminium bis(2,4,8,10-tetra-trans-butyl-6-hydroxy-12H-dibenzo[d,g][1.3.2]dioxaphosphocin-6-oxide; main component (Adekastab NA-21 ^(™) by Asahi Denka Kogyo KK, Japan. The α-phase nucleating agent mainly affects crystallization at high rates of cooling and minimize crystallization halftime. Only minor increase of the crystallization temperature by few degrees is observed in presence of α-nucleating agent [47].

Earlier we investigated compounds of a number of metals with 1-hydroxyethane-1,1-diphosphonic acid (HEDP, C₂H₈O₇P₂) [48,49]. The low solubility of many metal/HEDP compounds is well-known, as well as the ability of calcium ions to form nanoscale polynuclear complexes [[50], [51], [52], [53], [54], [55], [56]]. With this in mind, we carried out a series of syntheses of calcium HEDP compound using various initial reagents and reaction conditions. In this work, we describe the synthesis of dicalcium HEDP salt by the reaction of HEDP with calcium carbonate and its use as an additive, affecting nucleation of iPP and its properties.

It is noteworthy that in almost all nucleation studies the samples of commercial PP that contain certain additives (antioxidants, nucleators, clarifying agents, etc.) are used. In the work, we also used polypropylene powder from the reactor as a polymer matrix to evaluate the “pure” effect of all additives, including nucleators and to calculate the efficiency of nucleating agents, and these results are given in the supplementary information.