Role of Nucleating Clarifying Agents in PP
Role of Nucleating Clarifying Agents in PP
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 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.
In polypropylene formulations, adding nucleating agents (also called nucleators) result in improved performance & processing properties, such as:
- Improved clarity and reduced haze
- Improved strength and stiffness
- Improved Heat Deflection Temperature (HDT)
- Reduced cycle time
- Reduced warpage and more uniform shrinkage
- Reduced pigment sensitivity regarding property changes with different colors
- Improved processability in certain applications
Thus, nucleation is a powerful way to improve the physical, mechanical, and optical properties of polypropylene. Clarity, dimensional stability, warpage, shrinkage, CLTE, HDT, mechanical properties and barrier effect can be improved by the careful choice of nucleators or clarifiers.
Let's discover the crystallinity of polypropylene (PP) in detail, along with its nucleation process, and types of nucleators & clarifiers available to make the right selection.
Polypropylene and Its Crystallinity
Polypropylene and Its Crystallinity
Polypropylene is a widely used crystalline, commodity polymer made from the
polymerization of propene monomer. Upon polymerization, PP can form three basic chain structures (atactic,
isotactic, syndiotactic) depending on the position of the methyl groups. The crystallinity of the polymer is characterized by:
- The shapes and sizes of the crystallites
- The crystallinity ratios, and eventually
- The orientation of crystallites
Isotactic polypropylene (iPP) is a semi-crystalline polymer. It is characterized by an
excellent cost to performance ratio, making it very attractive in a wide range of applications like automotive, appliances, piping, packaging, etc.
Isotacticity index of iPP is directly linked to the degree of crystallinity
which has a major impact on polymer performance. Isotacticity increases
crystallization kinetics, flexural modulus, hardness & transparency, and decreases impact resistance
& permeability.
Table below compares properties of
two polypropylene homopolymers having a different isotacticity index.
Properties |
Standard |
PP1 |
PP2 |
Unit |
Density |
ISO R 1183 |
0.904 |
0.915 |
g/cm3 |
Isotacticity Index |
NMR C
13 |
95 |
98 |
% |
Flexural Modulus |
ISO 178 |
1700 |
2300 |
MPa |
Deformation Temperature |
ISO 75 |
102 |
131 |
°C |
Permeability |
ASTM D 1434 |
40000 |
30000 |
cm3-µm/m2-d-atm |
Effect of Isotacticity Index on iPP Properties
DID YOU KNOW?
Isotactic Polypropylene was discovered by
G. Natta in the '50s. Spheripol technology developed in 1988 allows today's production of polypropylene with isotacticity index higher than 97%. |
Crystallization of Polypropylene
Depending on the conditions, Isotactic Polypropylene can crystallize into four different phases denoted α, β, γ and mesomorphic smectic.
The α and β phases are the most important.
α Phase
- This phase is the most stable and the most known.
- The crystals are monoclinic.
|
β Phase
- This phase is metastable, and the crystals are pseudo-hexagonal.
- β-phase is mainly found in block PP copolymers and can be generated by addition of specific nucleating agents.
- This form was discovered by Padden and Keith in 1953 and can be improved by crystallization between 130 and 132°C or by orientation with high shear or through addition of specific nucleating agents.
- Presence of β-phase in PP homopolymer generally increase ductility in the finished parts. Maximum effect is observed at 65% of β-phase.
|
γ Phase
- This phase is also metastable with triclinic crystals.
- This form is not very familiar but appears mainly in low molecular weight polypropylene by crystallization at very high pressure and very low cooling rate.
|
Take the course by industry expert Philip Jacoby where he explains how to meet the urgent demands for greener polypropylene products (lighter, recyclable, high-performance PCR grades...) with beta nucleation to gain an edge over your competition.
Nucleation Process in Polypropylene
Nucleation Process in Polypropylene
It is well-recognized that the start-up point of crystallization of polymers is small germs (little particles) naturally included in the melt-like catalyst residues, impurities, dust, etc. It is then possible to modify and control crystalline morphology by the addition of
"artificial" germs introduced in the polymer melt. This operation is called Nucleation.
- 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 the same wall thickness of a part.
The role of these nucleating agents is to improve the physical and mechanical properties of finished parts.
How Do PP Nucleating Agents Work?
A nucleating agent is typically characterized as an insoluble particulate (discussed next) that increases the rate of crystallization.
When semi-crystalline polymers crystallize from the melt (typically during the cooling phase of a process), the lamellae organize from a primary nucleus to form complex macro-structures called
spherulites. These spherulites continue to grow until they
impinge on an adjacent spherulite at which point the growth ceases.
Properties of the polymers, including optical and physical characteristics depend on:
- The end size of the spherulite structures
- The crystalline orientation in the matrix
In nucleated polypropylene, crystallization occurs earlier in the cooling process and happens at a faster rate. This allows decreased cooling time of the polymer. Also, nucleation density is much higher and crystal spherulite size is much smaller.
Figure below shows an illustration of the heterogeneous nucleation process versus a non-nucleated resin for comparison:
Illustration of the Crystallization Process in Non-Nucleated or Nucleated Matrix
Polypropylene is recognized as a relatively easy material to nucleate. This is because the rate of crystallization is low enough to allow the nucleating agent to have a direct impact on the nucleation density. Further, the
effect of a nucleating agent depends on numerous parameters like:
- The nature of the polypropylene (homopolymer, random copolymer, block copolymer)
- The melt index
- The polydispersity index
- The processing conditions, and even
- The polymerization process
Nucleated PP molding formulations are often used for the production of thin-walled injection molded parts (< 0.4 mm) where stiffness is required. In some cases, cycle-time can be shortened by 30%. Nucleating agents are also used as a clarifier for films, sheets and molded parts, particularly for random PP copolymers.
Nucleators and Clarifiers: A Rich Panel of Additives
Nucleators and Clarifiers: A Rich Panel of Additives
Particulate Nucleating Agents
Particulate nucleating agents/nucleants are typically high melting compounds which are dispersed in the polymer melt via compounding. These particles act as distinct “point nuclei” on which polymer crystal growth can commence.
- The
high concentration of nuclei leads to more rapid crystallization (shorter cycle times), and higher levels of crystallinity, which improves the strength, stiffness, and HDT of the PP.
- The small size of the crystal aggregates (spherulites) leads to reduced light scattering and improved clarity.
The commonly used particulate nucleating agents include salts and minerals, such as talc, sodium benzoate, phosphate esters and other organic salts.
- Talc and sodium benzoate are considered to be low performance, low-cost nucleants, and provide a modest improvement in strength, stiffness, HDT, and cycle time.
- The high performance, high-cost nucleants, such as the phosphate esters and the bicycloheptane salts give better physical properties and some improvement in clarity.
Commonly Used Particulate Nucleating Agents
Soluble Nucleating Agents
Soluble nucleating agents, which are also referred to as “melt-sensitive”, typically have low melting points and dissolve in the molten PP.
- As the polymer melt cools in the mold, these nucleants crystallize out first forming a finely distributed network with extremely high surface area.
- As the temperature continues to drop the fibrils comprising this network function as nuclei to initiate the polymer crystallization.
- The extremely high concentration of nuclei leads to very small PP crystal aggregates, which give the lowest level of light scattering and the best clarity.
All clarifiers are nucleants, but not all nucleants are good clarifiers.
Some common nucleants, such as sodium benzoate and talc, do not reduce spherulite size by a sufficient amount to give a low haze and high clarity molded part. The
best clarity is generally achieved when soluble nucleants are used.
Soluble organic compounds which act as clarifiers include sorbitols, nonotols, trisamides.
Although these nucleants are mainly used to achieve high clarity and low haze, they also improve physical properties and reduce cycle time.
Nucleating Agent & Chemical Structure
|
Description |
Sorbitols
|
Strengths:
- Excellent clarity
- Minimal plate-out
- Very good organoleptics
Weakness:
- High molding temperatures required
- High loading levels
Cost & Loading:
- 1800 – 2500 ppm
- High cost
|
Nonitols
|
Strengths:
- Highest possible clarity
- Very good organoleptics
- Minimal plate-out
- Works with lower molding temperatures
Weakness:
- Very high loadings required
Cost & Loading:
-
2000 – 4000 ppm
-
High Cost
|
Trisamides
|
Strengths:
- Excellent clarity at low loading levels
- Excellent thermal stability & ultra-low discoloration
- No plate-out
- Excellent organoleptics (low taste & odor)
Weakness:
- Not as good clarity as the sorbitols and nonotols
Cost & Loading:
- 200 – 400 ppm
- High cost of additive is tempered by the low addition levels needed
|
Commonly Used Soluble Clarifying Agents
Other Classification Aspects
Nucleators and clarifiers can also be classified according to their chemical family as well as crystal types.
Inorganic and Organic Nucleating Agents
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 Favored Crystal Types: α, β, γ...
For example, the most effective α-nucleating agents are
the sorbitol-based derivatives and the organic phosphates while the β-nucleating agents are among others triphenodithiazine, pimelic acid with calcium stearate or quinacridone dye permanent red.
Benefit from a full range of nucleating and clarifying agents used to manufacture polypropylene products. Check their tech profile, ask for samples or discuss your case with producer’s tech staff.
Nucleating Agents for Polypropylene (PP) Clarifying Agents for Polypropylene (PP)
Now, let's move our attention towards the factors to consider while selecting the suitable nucleating or clarifying agent in order to improve the performance of your polypropylene (PP) applications.
Factors Impacting Performance of Nucleants and Clarifiers
Select the Right Nucleants and Clarifiers for PP
Select the Right Nucleants and Clarifiers for PP
Before selecting the suitable nucleating or clarifying agent for your PP application, determine which
property improvement you are most interested in:
-
If low haze and high clarity is important, then choose one of the soluble clarifiers.
-
For lower clarity requirements, the phosphate esters can be used.
-
If high modulus is of greatest importance, then choose one of the phosphate esters.
-
If low cost is of most importance, then choose sodium benzoate.
-
If low warpage and low pigment sensitivity is of most importance, then choose the bicycloheptane salt.
It is also imperative to decide how the nucleant will be incorporated into the PP resin. Always run appropriate tests to ensure that good dispersion and nucleation have been achieved.
-
Twin screw compounding is generally required unless well-dispersed masterbatches of the nucleants are prepared first.
-
Run DSC on the nucleated PP resin. Improvements in cycle time generally correlate with increases in the crystallization temperature (Tc). Test properties of molded specimens.
Find Suitable Nucleating Agent for Your PP Product
View a wide range of nucleating agents available today used for polypropylene, analyze technical data of each product, get technical assistance or request samples.
About Dr. Philip Jacoby
Dr. Phil Jacoby is the President of Jacoby polymer consulting. He received a Ph.D. in Physical Chemistry from the University of Wisconsin in Madison, Wisconsin. From 1975 – 2002 he worked for the Amoco Chemical company and BP Amoco as a Senior Research Associate in Polypropylene Product Development and R&D.
At BP Amoco, Dr. Jacoby helped to develop new polypropylene resin formulations for rigid packaging, film, and automotive applications. Dr. Jacoby holds 13 US patents and several international patents covering various polypropylene products, with emphasis on modifying the crystal structure of polypropylene. After leaving BP Amoco, he joined Mayzo Corporation as Vice President of Technology.
At Mayzo, Dr. Jacoby created a new business based on the use of Beta Nucleating agents in polypropylene. The beta nucleant masterbatches that Dr. Jacoby developed are used today in the production of microporous oriented PP films, thermoformed PP parts, and various other extruded and injection molded applications.
Dr. Jacoby is a past president of the Southern Section of the Society of Plastics Engineers (SPE), and a former board member of the Thermoforming Division of the SPE.
After retiring from Mayzo in March 2014, Dr. Jacoby started a consulting business (www.jacobypolymer.com) concentrating on Polypropylene, with particular emphasis on New Product Development, Problem Solving, and training & development courses.