Nucleating agents for thermoplastics

Nucleation can occur on the surface of a filler or reinforcement. Indeed the action of fillers is, in part, due to the changed crystal structure in the environment of the Mer. Nucleating agents must form very small particles within the polymer melt. They tend to have rigid planar structures which are expected to provide the site for the first few polymer segments to adsorb and provide stable embryo nuclei [1]. 

Crystallization occurs in two stages - nucleation and growth. The free energy of crystallization involves the sum of (1) the free energy for formation on a stable nucleus embryo (AG ) and (2) the free energy for 

When a folded layer on the surface of a crystal has finished growing, a new nucleus needs to form on the surface for continued growth of the crystal. Secondary nucleation requires a high undercooling because it has a low temperature dependence. The secondary crystallization layers are completed by an attachment-detachment mechanism [1]. 

When a polymer can exist in more than one crystalline form, the time-temperature conditions and type of nucleant can determine the form 

Nucleants may (1) be melt sensitive, i.e. they melt below or near the processing temperature, or (2) melt insensitive, i.e. they do not melt below normal processing temperatiu es. The melt sensitive nucleants form a physical gelation network within the pol)oner ([4] and Millad technical datasheet) while the melt insensitive nucleants provide single nucleation sites within the polymer. 

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Shanks R A and Tiganis B E (1998) Nucleating agents for thermoplastics, In Plastics additives, an A-Z reference, Pritchard G (Ed), Chapman Hall London, pp 464-472. 

Safoam. [Reedy Int l.] Chemical endothermic nucleating and blowing agent for thermoplastics. 

For particle volume fractions of 0.15 and more at low shear rates and shear stresses, the shear viscosity becomes increasingly non-Newtonian and yield values develop [8,9,16,22,25]. Calcium carbonate acts as a nucleating agent for various thermoplastic melts such as polypropylene. 

Typical additive packages for engineering thermoplastics have been described by Titzschkau [9], such as processing aids for PA, PP, or PET/PBT, three-component additive packages for polyamides and polyesters (nucleating agent, lubricant and process heat stabiliser) and coated copper stabilisers for polyamides. Additive packages or combinations of up to five or more additives are quite common. A typical white window PVC profile formulation comprises an acrylic impact modifier, TiC>2, CaCC>3, calcium stearate, a... 

The proper balance between viscosity and gas evolution can be controlled by a number of factors such as a suitable type and concentration of catalyst and surfactant, the presence of a nucleating agent (not always necessary) (17,18) and control of reaction temperature (or exotherm). Additional factors that must be considered are the use of a suitable chemical blowing agent, which is especially important for the production of thermoplastic foams, and the formation of oligomers (prepolymers) which exhibit higher viscosities than monomers in the preparation of thermoset foams (e.g. polyurethane foams). 

Polybond . [BP Performance Polymos] Polypropylene or polyethylene compounds thermoplastic for use as chemical coupling agent, compatibilizing agent, metal adhesive and nucleating agent impact modifier additive in polymer aUoys, for adhesion to metals and polar polymers. 

Considerable quantities of PET can be processed to injection-molded forms after addition of nucleation agents, but for this, very exact process conditions must be adhered to. The polymer is rigid, hard, exhibits very little wear and tear, shows very little creep, and can tolerate high mechanical loads. In industry, it is known, together with poly(butylene terephthalate), as a thermoplastic polyester. ... 

Polyester is mainly melt-spun into fibers. A smaller quantity is used for very thin films for the electrical industry. Higher molecular weight poly(ethylene terephthalate) with added nucleating agents (salts of carboxylic acids) can also be used as thermoplastic synthetics. A definite disadvantage of polyester fiber is that it yellows in light. The mechanism for this process... 

Improved impact strength with lower melt viscosity for better processability can be obtained by blending a thermoplastic polyester such as PTT with an ethylene copolymer. Including a nucleating agent increases crystallinity and impact strength (Talkowski et al. 2012). 

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