Crystallinity of semi-crystalline polymers is responsible for many of the characteristics such as dimensional stability, clarity, and toughness. For a defined part and process, the polymer structure, the formulation and the processing conditions that result in a specific balance of heat build-up and cooling, control the crystallinity. Consequently, crystallinity is often heterogeneous, the heat history being different for the skin and the core of the parts or goods.
Nucleating agents and clarifiers speed up and tune the crystallization, allowing adjusting the end properties of semi-crystalline polymers to the functional requirements. The shapes and sizes of the crystallites, the crystallinity ratios and, eventually the orientation of crystallites, characterize crystallinity.
Isotactic PP, depending on the conditions, can crystallize into four different phases denoted a, �, ? and mesomorphic smectic. The a and � phases are the most important. Monoclinic a is the predominant crystalline phase and as such it always occurs in articles prepared from PP. Trigonal � phase usually supplements a phase only at small amount in commonly manufactured samples. It can be prepared in higher quantity using a temperature gradient or a sheared or strained melt. For PE, studies of Ziegler-Natta based blown film morphology, show oriented lamellae in the form of cylindrites perpendicular to the machine direction. In contrast, the metallocene based films show no such preferential lamellar orientation. Apparently, the unbranched higher molecular weight chains in Ziegler-Natta based film orient during the film blowing process, allowing the more highly branched chains to crystallize onto these oriented chains.
Nucleators or nucleating agents are employed that provide sites for the initiation of crystals. Clarifiers are a subfamily of nucleators that provide smaller crystallites that scatter less light and, as a result, enhance the clarity for a same wall thickness of a part. The choice of the nucleator or clarifier determines the crystallization morphology and hence, processing, physical and mechanical properties. It must be noticed that nucleators can be involuntarily added, some additives such as colorants or fillers having a nucleating action.
•   The dimensional stability notably the warpage reduction and the low CLTE.
•   The aesthetic enhancement notably the contact and/or see-through clarity.
•   The thermal properties notably the HDT increase.
•   The mechanical property improvement.
•   The enhanced barrier effect.
Nucleators and clarifiers can be classified according to their general structure, organic or inorganic, their effect on crystallization, and the end properties of the nucleated polymer:
•   Inorganic agents are, for example talc or barium sulphate, nanoclays such as montmorillonite have a nucleating effect on PP thermoplastic polyesters, polyamide leading to faster nucleation rates and increased overall degrees of crystallization. Metal oxides such as titanium dioxide or magnesium oxide, phosphates, carbonates or sulphates, of preferably alkaline earth metals, are also used.
•   Organic additives are very diversified from sorbitol derivatives up to mono- or polycarboxylic acids and the salts thereof such as 4-tert-butylbenzoic acid, sodium or lithium benzoates, organophosphates or phosphate esters, adipic acid, diphenylacetic acid, sodium succinate or sodium benzoate; norbornane-carboxylic salt, polymeric compounds such as ionic copolymers (ionomers), nitrogen and more or less complex molecules, for example, triphenodithiazine, dicyclohexyl-2,6-naphtalenedicarboxamide, pimelic acid with calcium stearate or quinacridone dye permanent red.
The most effective a-nucleating agents are the sorbitol-based derivatives and the organic phosphates when the �-nucleating agents are among others triphenodithiazine, pimelic acid with calcium stearate or quinacridone dye permanent red. Latest developments concern the fourth generation but the oldest grades can be still interesting for economical features, the costs and the needed amounts of nucleators varying on a broad scale. The other ways of progress concern technical and environmental issues:
   •   Higher processing temperatures.
   •   Better mechanical properties.
   •   Better organoleptic properties : taste and odour, FDA compliance.
   •   Faster crystallization leading to improvements in cycle times.
   •   Involvement in multidirectional competitions:
      –  Homo- versus co-polymers of polypropylene.
     –  Ziegler-Natta versus metallocene polyolefines.
     –  Polypropylene versus PET and others.
   •   Suitability for bioplastics such as polylactic acids.

For HyperForm HPN68, the latest nucleator by Milliken Chemical, the following figure 'Nucleated-PP-Relative-Properties' displays the tensile strength, Gardner and Izod impacts @ -30�C of nucleated polypropylene versus the same properties of the virgin polymer. Tensile strength is 10% better for the nucleated PP but impact strengths @ -30�C are 5 or 10% inferior. For closure manufacture, the combination of high crystallization temperatures and isotropic shrinkage properties imparted by Hyperform to both impact copolymer and homopolymer PP result in high quality products produced at speeds 8-17% higher than before. In other applications including thermoforming, cycle times can be reduced in the order of 5 up to 40%.
New Japan Chemical markets NJSTAR TF-1, a crystal nucleating agent for eco-friendly plant derived polymers or Bioplastics (polylactic acids). New Japan Chemical claims that NJSTAR TF-1 reduces the polylactic acid moulding cycle by 17% with improved heat and shock resistance, and clarity. Efforts are made to ensure safety compliance with relevant laws for future applications in the food industry.
Viba Group has developed new masterbatches to meet the growing market for PLA (polylactic acid) in packaging. These include Vibatan PLA Nucleating Agent 03413, which has been specifically designed for the transformation of biopolymers. Nucleating agents increase several physical and mechanical properties of PP such as stiffness, impact properties, hardness, heat distortion temperature, etc. That leads to:
   •   Downgauging, thinwalling and weight reduction of the finished parts.
   •   Reduction of cycle-times and increase of productivity.
   •   Optimisation of the dimensional stability solving shrinkage and warpage issues. So, in certain cases post-machining         can be eliminated.
Some of the applications are:
•   Consumer products such as furniture, toys, sporting goods, lawn and garden furniture, flower pots, planters, clothe hangers, toolboxes, tackle boxes, home/office/craft supplies, luggage, stationery...
•   Household applications such as refrigerator and dishwasher linings and parts, washing machine drums, small appliances such as blenders, toaster ovens, coffee pots, food processors, hair dryers, and electric can openers, waste containers, food storage containers, clothe, basket shelving, cooler linings, tableware, clothe storage containers etc.
•   Automotive and transportation applications such as door panels, instrument handles, body panels, bumper fascias, battery cases, fan and radiator shields/shrouds, fluid containers, and others.
•   Medical applications such as syringes, labware, sharps containers, and other medical equipment and devices.