1,3,5-Benzenetrisamide based nucleating agents for poly(vinylidene fluoride)

Abstract

This paper presents 1,3,5-benzenetrisamides as colorless α–nucleating agents for poly(vinylidene fluoride). In order to screen a large variety of 1,3,5-benzenetrisamide derivatives with respect to their nucleating potential an efficient and reliable test based on polarized light microscopy was established. For selected promising compounds the concentration dependence of the PVDF crystallization temperature, the dissolution behavior of the additive in the polymer melt, and the crystallization of the additive from the polymer melt was investigated in a concentration range between 1 wt% (10,000 ppm) and 70 ppm. It was found, that only two of the investigated compounds were able to raise the crystallization temperature about 8 °C at a concentration of 140 ppm and 580 ppm, respectively. These trisamides have the advantage being soluble in the polymer melt, not featuring absorption of visible light and therefore allowing the preparation of uniform and colorless PVDF products.

Introduction

Nucleating agents are wildly used in commercial semi-crystalline polymers to improve the materials properties and to reduce the cycle times in injection molding [1]. Polyvinylidene fluoride (PVDF; –(CH₂–CF₂)_n–) is a valuable thermoplastic resin due to its good mechanical strength, rigidity and toughness as well as its wide range of service temperatures and high chemical and temperature resistance [2], [3]. This property profile opens applications for PVDF for example in pipes, fittings, pumps, and plenum wires [2], [4].

The most common three crystalline polymorphs of PVDF are the α-phase (pseudo orthorhombic with the polymer chain conformation TGTG′), the β-phase (orthorhombic; TTTT) and the γ-Phase (monoclinic; T₃G T₃G) [5], [6]. The α-phase is the most common polymorph and normally obtained by cooling of the melt or from solution for example with a mixture of monochlorobenzene/N,N-dimethyl-formamide [6], [7], [8]. Organic pigments based on indanthrone, perlyene, quinophthalone and phthalocyanine as well as mineral fillers such as talc, mica and carbon black act as nucleating agents and influence the crystallization kinetics of PVDF [9]. Lotz and Wittmann determined an efficiency scale by self-seeding experiments and described two nucleating agents for the α-phase of PVDF, notably the polymer PTFE and the yellow pigment flavanthrone [10]. In the β-modification the alternating two hydrogen atoms and two fluorine atoms in the polymer chain –(CH₂–CF₂)_n– are leading to a dipole orientation in the polymer solid state, which therefore shows remarkable piezoelectric [11] and pyroelectric [12] properties. The β-phase is generally obtained by drawing films of the α-phase at relatively low temperatures (50 °C). Mechanical deformation at higher temperatures (150 °C) yields an increased percentage of the α-phase again [13]. Epitaxial growth between β-PVDF and KBr was found by Lovinger [14]. Organic sulfates and sulfites, in particular tetrabutylammonium hydrogen sulfate (TBAHS) [15], induce the β-phase from PVDF melt [15]. Solution derived PVDF films comprising magnesium nitrate hexahydrate on silicon substrates were reported to form the β-phase [16]. Also PVDF solutions in dimethyl sulfoxide with high concentrations of crystal violet form the β-polymorph while low concentrations yield the α- and γ-phases [17], [18]. Lovinger also investigated the unit cell of γ-phase crystals epitaxially grown on Mica and NaCl [19]. Routinely the γ-polymorph is obtained by high temperature annealing (∼165 °C) [20] or additivation with KBr powder [21].

Recently 1,3,5-benzenetrisamide based derivatives were introduced as a new class of nucleating agents and clarifiers for isotactic polypropylene (i-PP) [22], [23], [23](a), [23](b), [23](c). Certain derivatives are efficient nucleating agents to nucleate the α-phase of i-PP at very low concentrations smaller than 200 ppm. A few derivatives are additionally capable to substantially improve the optical properties (clarity and haze) of i-PP resulting in a highly transparent i-PP for packaging applications. Other derivatives promote the mechanically tough β-phase and hybrids of both crystallographic forms with new combinations of optical and mechanical properties. A major advantage of this class of additives are the organoleptic excellence, chemical and temperature stability preserving also performance in regrinds. This is particularly important in view of the properties of the currently applied sugar based (i.e. sorbitol or nonitol) clarifiers and nucleating agents, which have drawbacks such as blooming of the additive, thermal decomposition yielding bubble formation, increasing the yellowness index and forming odorous byproducts including migration of the additive itself [24], [25], [26].

This paper reports on the potential of 1,3,5-benzenetrisamides to nucleate PVDF in order to find colorless and soluble and therefore easy dispersible nucleating agents. In view of a large number of possible 1,3,5-benzetrisamide structures we established a reliable screening method using polarized light microscopy. Since the nucleation efficiency is strongly dependent on the additive concentration, promising derivatives were selected and investigated in concentrations ranging from 1 wt% (10,000 ppm) to 70 ppm. A comparison of the additive dissolution temperature in the polymer melt, the crystallization of the additive from the polymer melt and the crystallization temperature of PVDF for eight different 1,3,5-benzenetrisamides will be presented. These data are also compared to the commonly used reference compound flavanthrone.