Article of the Week - Latest Innovation in Nucleating and Clarifying agents

Crystallinity of semi-crystalline polymers is responsible for many of the characteristics such as dimensional stability, clarity, toughness.

For a defined part and process, the crystallinity is controlled by the polymer structure, the formulation, and the processing conditions that result in a specific balance of heat build-up and cooling. 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 to adjust the end properties of semi-crystalline polymers to the functional requirements.

Only semi-crystalline polymers are involved but, for those, all the processing methods are concerned, including secondary processing methods: Injection Moulding, Blow Moulding, Film Extrusion, Rotomoulding, Thermoforming...

Both aesthetic and mechanical properties being involved, all the applications are concerned from packaging up to automobile.

Cursory glance at crystallinity

Crystallinity is characterized by the shapes and sizes of the crystallites, the crystallinity ratios and, eventually the orientation of crystallites.

For example, isotactic polypropylene (iPP), depending on the conditions, can crystallize into four different phases denoted α, β, γ and mesomorphic smectic. The α and β phases are the most important. Monoclinic α is the predominant crystalline phase and as such it always occurs in articles prepared from iPP. Trigonal β phase usually supplements α 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 polyethylene, 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.

The following figure 'Examples-of-Crystallites' shows three examples of crystallites obtained with various cooling conditions for a same polymer

The following figure 'Crystallisation-Heterogeneity-in-a-Moulded-Part' shows an example of heterogeneity of the crystallization according to the location in a same moulded part.

Nucleators and clarifiers: a good likeness but not quite the same thing

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, see figure 'Nucleators-&-Clarifiers', determines the crystallisation 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.

All the industrial sectors are concerned: Packaging, durables, automotive, appliance, sheets, films, agriculture, consumer goods, personal care market...

Nucleators and clarifiers: A rich panel of additives

Nucleators and clarifiers can be classified according to their general structure, organic or inorganic, their effect on crystallization, the end properties of the nucleated polymer...

Classification according to the chemical family: inorganic, organic

Inorganic agents are, for example talc or barium sulphate, nanoclays such as montmorillonite have a nucleating effect on polypropylene, 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 quoted.
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

Classification according to the favoured crystal types: a, b, g...

For example, the most effective α-nucleating agents are the sorbitol-based derivatives and the organic phosphates when the β-nucleating agents are among others triphenodithiazine, pimelic acid with calcium stearate or quinacridone dye permanent red.

Classification according to the end properties

The main difference concerns the clarifiers having mainly optical properties but also some thermal and mechanical effects. Clarifiers are often sorbitol derivatives and organophosphates.

Some marketed nucleators and clarifiers among others

Nucleators and clarifiers: latest and prospective development ways

Latest developments concern the third 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.

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.

Better mechanical properties

For HyperForm HPN68, the latest nucleator by Milliken Chemical, the following figure 'Nucleated-PP-Relative-Properties' displays the percentages of 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.

Faster crystallization: Improvements in cycle times

For closure manufacture, the combination of high crystallization temperatures and isotropic shrinkage properties imparted by Hyperform to both impact copolymer and homopolymer polypropylene 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%.

Suitability for bioplastics such as polylactic acid.

New Japan Chemical markets NJSTAR TF-1, a crystal nucleating agent for eco-friendly plant derived resins, so called 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 bio-polymers.

Some specific application overview

Nucleators and clarifiers enhance the aesthetic and mechanical properties of semi-crystalline plastics, notably the polypropylene. Numerous applications, see Figure 'Nucleated-polypropylene-Applications' take advantage of the improved properties such as clarity, toughness, better productivity, dimensional stability favouring the competition with other plastics or conventional materials such as glass.

Packaging

Versus glass and other commodity plastics, thanks to nucleators and clarifiers polypropylene can compete PET, polystyrene, PVC, HDPE, polycarbonate or glass according to the targeted application. Table displays some general indications of the advantages and drawbacks of nucleated polypropylene versus some other materials. For example, polypropylene is lighter than glass but is not so good as gas barrier.

There are a myriad of applications for beverage, cosmetics, food, household cleaners, media packaging, storage containers, water bottles, transparent bottles for consumer products, baby bottles, DVD cases, consumer electronics packaging, drug packaging, large storage totes, storage containers of all sizes and shapes, software packaging, dairy products packaging, drug packaging, juice packaging, deli containers, video cassette storage, containers, crates, caps & closures, housewares. Disposable packaging for food, consumer, media, electronics, etc. can be thermoformed or thin-wall and high speed injection moulded.

Danone's success story

Danone, for cream cheese, has replaced an existing glass application by a clarified polypropylene (PP) packaging using Milliken's clarifying agent, which creates a clear, cost-effective and environment-friendly plastic packaging.

Clarified polypropylene is an excellent choice for this application in several aspects:

lower production costs. Using clarified polypropylene, Danone was able to take their packaging weight from 150 grams down to 29 grams per cup that is to say a 81% weight saving
fuel and carbon dioxide saving for transport
the cup is unbreakable
the cap, cup and label are all made from polypropylene, which offers an environmental-friendly aspect in terms of recyclability.
the cup can be hot-filled at 90°C.

A myriad of other applications from automotive up to consumer goods

Nucleating agents increase several physical and mechanical properties of polypropylene 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.

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.

Suggar/Milliken/Nova success story

For its most popular washer models, Suggar, a leading appliance maker, switched from opaque plastic top doors to transparent doors. Consumers were enthusiastic and sales went up. Transparent SAN could not deliver the needed impact resistance for this part, which receives constant handling. Polypropylene nucleated with Millad NX8000 clarifier (Milliken) provides the beautiful appearance customers seek, combined with practical performance in everyday use.

Compared to SAN in the Suggar application, PP clarified with Millad NX8000 provides notched Izod impact that is up to five times higher, making it much more resistant to dropping, banging, and other handling impacts.

The following Figure 'Suggar Top Max 5.0 Washing Machine' shows the new model with clarified polypropylene.

Conclusion

Nucleating agents and clarifiers speed up and tune crystallization allowing to adjust the end properties of semi-crystalline polymers to the functional requirements. Clarity, dimensional stability, warpage, shrinkage, CLTE, HDT, mechanical properties, barrier effect can be improved by the careful choice of nucleators or clarifiers. Last generation of these are most performing bus previous grades are still of interest for economical reasons.

Apart from the universal ratio quality/cost, nucleators and clarifiers progress toward a better efficiency allowing cost savings, a better productivity, the improvement of organoleptic properties, the adaptation to bioplastics processing, the optimisation of homo- and co-polymers of polypropylene to compete other thermoplastics such as PET, PVC and others.

References

Technical guides and websites: Adeka Palmarole, Amfine Chemical, Chemtura, Chrostiki, Ciba, EC Chemical Industry, Milliken chemical, Mitsui Chemicals, New Japan Chemical, Polyvel, Rionlon Chemical, Roquette America, Viba Group...

Introduction