Two thermotropic liquid crystalline polymers were evaluated for use in rotational molding: Vectra A 950 and Vectra B 950. Material is typically prepared by grinding. When ground, even under cryogenic conditions, TLCPs tend to yield particles of high aspect ratio translating to low bulk density, poor granular flow, and incomplete densification. A process was developed to generate spherical particles of controlled size and distribution. Particle coalescence and densification characteristics were determined and compared against model predictions. Introduction and Background Rotational molding is composed of four distinct steps. First the material is ground then mixed to ensure a homogenous powder exists. Next the powder is loaded into a mold, which is rotated biaxially. In this phase, a solid granular flow problem exists. As the material tumbles, the mold is heated eventually causing the particles to coalesce and adhere to the mold wall. Finally, once densification is complete, the mold is cooled and the solidified part is removed. Coalescence can be divided into two steps: initial interface bridge formation between particles, where there is little or no change in density, and densification in which the inter-particle cavities are eliminated [1]. Densification is a bulk effect and is modeled to account for pore closing. Neck formation is not dependent on the bulk powder because individual necks do not influence each other until pores form. Hence, it is typically modeled as the coalescence of two particles. This will be referred to as sintering in this paper, although it is technically the sintering of two particles. Frenkel [2] first explored viscous sintering by deriving an expression for the coalescence rate of two identical spherical particles, which was corrected for continuity by Eshelby [3]. Their model, Eq. 1, is limited to Newtonian materials at the beginning of sintering where the particle radius has not changed due to neck growth. It is the result of a mechanical energy balance between the work done by viscous forces to that presented by the reduction of surface area due to surface tension (see equations 2 and 3).

The coalescence in air of two polymeric drops into a single drop (also referred to as sintering) was investigated for two thermotropic liquid crystalline polymers. Initial coalescence via elastic contact was ruled out based on the magnitude of the equilibrium compliance values and the process was, therefore, believed to be driven by surface tension and resisted by means of viscous)low. Remarkably the viscous coalescence model developed for Newtonian fluids (an extension of the Frenkel and Eshelby approach) agreed well under some conditions of temperature with coalescence data (i.e., observation of neck growth under a microscope). On the other hand the extension of the Newtonian model to the viscoelastic case by incorporating the upper convected Maxwell model (UCM) assuming steady state stresses always underpredicted the rat( of coalescence. The viscous neck growth model using the UCM constitutive equation was extendcd to the transient stress case in order to incorporate the slow growth of viscosity at the startup of flow. The unsteady state UCM approach represented a qualitative improvement over the Newtonian and steady state UCM formulations because it predicted accelerated coalescence, relative to the Newtonian model, by increasing the relaxation time. However, the model was unable to quantitatively predict the experimental coalescence rates, as it overpredicted the acceleration of coalescence. (c) 2005 The Society of Rheology.

The initial theory of Frenkel and Eshelby for the coalescence of drops in air (or sintering) of Newtonian fluids, which equated the work of surface tension to the work done by viscous stresses while assuming biaxial extensional flow kinematics, was extended to the case of time-dependent material functions using the Upper Convected Maxwell (UCM) model. A numerical scheme was developed to solve the ordinary differential equations (ODE) for the stresses, which are embedded in the ODE based on the mechanical energy balance. Initial conditions required to solve the set of non-linear ODEs were obtained from visualization experiments of the coalescing drops as the theory for elastic contact gave unrealistically high values of the initial neck radius. The transient model predicted that coalescence was accelerated by increasing the relaxation time, the opposite relationship of what was predicted by the steady-state UCM formulation, and was capable of quantitatively predicting the experimental coalescence rates at times when viscoelasticity was important.

Rotational molding is a unique process for producing hollow plastic parts. Rotational molding offers advantages of low cost tooling and can produce very large parts with complicated shapes. Products made by rotational molding include water tanks with capacities up to 20,000 gallons, truck bed liners, playground equipment, air ducts, Nylon fuel tanks, pipes, toys, stretchers, kayaks, pallets, and many others. Thermotropic liquid crystalline polymers are an important class of engineering resins employed in a wide variety of applications. Thermotropic liquid crystalline polymers resins are composed of semi-rigid, nearly linear polymeric chains resulting in an ordered mesomorphic phase between the crystalline solid and the isotropic liquid. Ordering of the rigid rod-like polymers in the melt phase yields microfibrous, self-reinforcing polymer structures with outstanding mechanical and thermal properties. Rotational molding of liquid crystalline polymer resins results in high strength and high temperature hollow structures useful in a variety of applications. Various fillers and reinforcements can potentially be added to improve properties of the hollow structures. This paper focuses on the process and properties of rotationally molded liquid crystalline polymers.