Selection of Antistatic Agents for Polymers

Need for Anti-static Agents in Polymers

Plastics are insulating materials subjected to electrostatic build-up and discharge depending on the surface resistivity of the part. Plastics such as PP and PVC, tend to collect electrons and become negatively charged. Antistats are materials controlling the accumulation of static electrical charge especially on polymer surfaces.

This accumulation of charge at the surface makes the material prone to electric discharges, dust adhesion and static clings.

The dissipation of the static charge relies on creating conditions for unwanted electrons to move away from the surface. Most antistats make use of charge structures to dissipate the material accumulated charge. Other antistats rely solely on electronic lone pairs of electrons and/or hygroscopic properties.

In general, dissipative or ESD polymers have:

A surface resistivity in a range from 10⁵ or 10⁶ up to 10¹² ohms.
A static discharge half-life generally inferior to 60 seconds.

According to the targeted application, beware of too low resistivity leading to conductive polymers and inherent risks. Problems are of very various seriousness, from minor to very serious and even dreadful ones:

Dust and other pollutants attraction with marketing, use and processing problems
Electrostatic build-up or discharges when touching the plastic parts: synthetic carpets, knobs, car handles
Painting and printing defects
Fire or explosion of inflammable or explosive environment, organic powders
TV, radio, electronic interferences

Antistatic Strategies for Temporary or Long-Term Protection

Electrostatic build-up and discharges are widespread in:

Continuous processing of plastics such as films
Electronic manufacturing, handling and repairing
Electronic applications
Packaging of dusty organic materials
Aeronautics: lightning and interferences
Automotive: Electrostatic discharge of fuel lines leading to fires
Inflammable and explosive environments: healthcare, operating theatre, painting shops
Use in cleaning of rooms

Hence, the dissipation of the static charge relies on creating conditions for unwanted electrons to move away from the surface. Most anti-stats make use of charge structures to dissipate the material accumulated charge. Other anti-stats rely solely on electronic lone pairs of electrons and/or hygroscopic properties.

Antistatic agents may be liquids, semi-solids or solids. These materials are normally either:

Applied to the surface of a substrate, or
Can be incorporated into the materials itself

Applied anti-stats are normally for managing static charges during various stages of processing. They are considered transient use species.

Incorporation into a material matrix is required in cases where ‘lifetime’ static protection is a criterion of end-use. Examples are antistatic carpet fibers and certain composite materials prone to generating static charge.

Further, water (humidity) plays a key role in assisting anti-stats in the mechanism to dissipate charge, i.e. via conductivity.

After understanding the importance of anti-stats in polymer, let’s explore what are the main chemistries used to effective charge dissipation…

Antistatic Agents Chemistries

Anti-stats are categorized into two subsets: inorganic and organic. There is not any universal strategy to minimize the static build-up, but multiple ways are used, sometimes in combination. Hence, selection of the preferred antistats is based on need and use.

Anti-static Ways in Polymers

Inorganic Anti-static Agents

Inorganic salts and some basic organic elements can be incorporated into the polymer matrix to suppress long-term use static accumulation. Examples include:

Carbon that is used in carpet fibers to produce antistatic flooring
Carbon used in many non-woven wipe applications for clean rooms and aerospace applications

Various salts incorporated into a polymer matrix can offer some anti-static properties. Though, lock in the matrix, the ionic nature is not readily available (ion separation) to contribute to the dissipation of charge.

Organic Anti-static Agents

Organic antistats comprise the majority of materials used for assisting in the conductivity of excess charge away from the polymer surface. While some can be incorporated into the solid matrix, most are used externally to control static during processing and end-use applications.

The general subset classes of the organic systems are:

Phosphate, normally potassium or sodium salt of the corresponding free acid;
Quaternary amines;
Non-ionic hygroscopic materials i.e. surfactants of ethylene oxide and/or propylene oxide.

For the principal antistats, phosphates and quaternary amines are organic molecules with both positive and negative charged ions. The smaller the size of the species, the greater is the observed electron density around the molecule and thus greater enhance dissipation ability.

Sulfate or sulfonated chemicals can be used though they are not especially effective. An example, di-octylsulfosuccinate potassium salt, used as a surfactant, displays weak antistatic characteristics.

Non-ionic surfactants function due to their hygroscopic character and the lone pair of electrons on oxygen. The hydrophobic side interacts with the surface of the material, while the hydrophilic side interacts with the air moisture and binds the water molecules. Again, the antistatic effect is small compared to either the phosphates or the quaternary amines.

Let’s learn about some of the organic antistats in detail…

Phosphate Acid Ester Salts

These materials are normally produced reacting an organic alcohol (ROH) with either P₂O₅ or POCl₃. In both cases, both the mono and di-acid esters are formed (see Figure 1). In normal use, these free acid esters are transformed into the corresponding salt, preferably potassium (K⁺).

The P₂O₅ route normally leads to a 55:45 mono/di ratio with small amounts of trimester. The POCl₃ route tends to produce ~ 50% tri-ester, a component having little or no antistatic properties. Other aspects are outlined in the table below.

Phosphates ((RO)₂P(O)O-)
Pros	Cons
Extremely Effective Wide range of phosphoric acid ester is available Potassium salts are preferred In-situ acid salt preparation is common Low MW phosphate salts are more effective than higher MW analogs Phosphate ester from P₂O₅ preferred over POCl₃	Effectiveness decrease as MW increases Solid phosphates are difficult to formulate Un-neutralized acid esters are less effective antistats Many salts are not EPA and Reach compliant Low MW antistats are more likely to be absorbed in polymers esp. nylon and spandex POCl₃ prepared ester leave corrosive halide ion residues

Quaternary Amines

This antistat class is formed by the reaction of an appropriate amine with an alkyl halide or di-alkyl sulfate. This produces a pentavalent positively charged nitrogen coupled with the corresponding anion.

Use and constraints for quaternary amines are outlined in the table below.

Quaternary Amines
Pros	Cons
Range of systems available Readily formulated into lubricant systems Commonly used in the cosmetics industry Lower deposition issues versus phosphates Moderate antistat capabilities at even low RH conditions	Limited compliance with EPA and Reach on anion selection Effectiveness is more % dependent than phosphates (1.5-2 x more) Less irritating than phosphates in some cases^a Several anions, esp. methyl sulfate (CH₃SO^-₃) have H&E issues Log Rp more variable than Phosphate systems

Non-Ionic Surfactants

Non-ionic surfactant class covers a very wide variety of chemicals. These can include simple alcohols to complex bio-based polyhydric structures. For this section, the focus will be on those commonly associated with applications to polymers, alcohol or acid ethoxylated or ethoxylated / propoxylated systems. These systems tend to be hygroscopic in nature, with lone pairs of electrons available on the oxygen atoms, assisting in conducting static charge away from a polymer’s surface.

The following table gives an overview of capabilities for static control.

Non-ionics
Pros	Cons
Large number of products available Non-disruptive to antistat effect Excellent for providing hydroscopic properties to enhance antistat effect Compatibility assist in formulations with phosphate or quaternary antistats	Low antistat potential vs phosphates or quaternary amines Large %'s are used in formulations

Apart from these two main classes of anti-static agents, there are some conductive fillers & additives which are widely used for ESD, EMI or RFI shielding. Explore them in detail…

Conductive Fillers and Additives

These solutions lead to volume conductive plastics that can act as conductors receiving electrons from other electrostatic materials with the known risks of electrostatic discharges.

All suitably filled plastics, as listed below, can be used for ESD, EMI or RFI shielding.

Commodity plastics such as PE, PS, PP
Engineering plastics such as ABS, PA 6/6, PA 6, PC, POM, PBT, PPO, PPS
Specialty plastics such as PEI, PEEK
Alloys such as PC/PMMA, PC/ABS

For the ESD polymers, it is difficult to control the resistivity over the percolation threshold of fillers. The resistivity can be so low that the polymer becomes conductive. Let's learn about some of the key conductive additives used in plastics.

Carbon Blacks

There are various carbon blacks in the market that can provide anti-static or conductive properties. The main properties which will influence the conductive properties of the carbon black are specific surface area, structure and surface chemistry.

Most of the conductive carbon blacks available in the market have higher surface areas and structures and can contain a significant volume of micropores.

The resistivity of the final material depends on:

The surface area of the carbon black and the ion level on its surface
The level of carbon black
The grade of the polymer or, eventually, the alloy of polymers
The mixing method

Carbon blacks modify the other properties of the polymer especially its color.

Conductive Fibers

The carbon and steel fibers as well as conductive cellulose fiber highly filled with conductive carbon blacks are industrially used to make the plastics and composites conductive. The resistivity of the final material depends on:

The size, the aspect ratio, the chemical nature of the fibers
The level of fibers
The mixing method

There are specific conductive filler or fiber grades especially marketed as additives for conductive plastics and rubbers. The other properties of the final material, color, modulus, impact strength, etc. are modified.

Graphites

The resistivity of the final material depends on:

The type of graphite: some grades are specially developed for their electric conductivity
The aspect ratio
The level of graphite
The grade of the polymer
The mixing method

Furthermore, graphite has lubricating properties. It is claimed by certain producers that the resistivities can be in the order of those obtained with conductive carbon blacks, lower or higher according to the used grades.

Download Brochure: Enhance Electrical and Thermal Conductivity with NeoGraf’s Graf+® Graphite Powders

Metal Powders or Flakes

Aluminum, copper, nickel, silver powders or flakes are used to increase the electrical conductivity.

The resistivity of the final material depends on:

The particle size and form of the metal
The level of metal
The mixing method

There are specific grades especially marketed as additives for conductive plastics and rubbers. The polymer influences the metal choice. The Sulphur vulcanization can particularly cause some troubles with metals such as copper and silver attacked by Sulphur. The other properties like, color, modulus, impact strength etc. are modified.

The table below shows some examples concerning aluminium powder for EMI shielding.

Property Name	Unit	EMI Shielding ABS	Neat ABS	EMI Shielding PA66	Neat PA66	EMI Shielding PPO	PPO
Aluminium level	%	40	0	40	0	40	0
Specific gravity	g/cm3	1.57	1.1	1.48	1.1	1.45	1.1
Elastic modulus	GPa	2.5	1 to 3	5	1 to 1.35	5.2	2.5
Tensile strength	Mpa	23 to 29	30 to 65	41	40 to 85	45	45 to 65
Elongation break	%	2 to 5	3 to 60	4	-	3	2 to 60
CTE	10-5/ °C	4	6 to 10	2.2	5 to 14	1.1	3 to 8
HDT at 1.82 MPa	°C	95	100	190	85	110	110

Properties of EMI grades compared to neat polymers
Compared to general-purpose grades, the EMI grades have high modulus, HDT and density. The elongation, the coefficient of thermal expansion, and the strength are reduced.

Some grades made of Titanium and Zirconium are specially developed for applications in polymers to obtain ESD and other antistatic materials are used into numerous plastics such as ABS, EVA, polyethylene, polypropylene, PVC, PETG, polyamide, polyethersulfone, acrylics, polyurethane.

Carbon Nanotubes (CNT)

CNTs are fast-growing for mass-produced or specific devices. CNTs, relatively well known, are expensive despite a continuous cost drop. The very low resistivity of carbon nanotubes (CNT) allows obtaining EMI polymers with levels of CNT inferior to 1 %, far lower than used levels of conventional and conductive carbon blacks.

Polymer Resistivity Vs Carbon Loading

Inherently Conductive Polymers (ICP)

ICPs are the most exciting opportunities for mass produced or specific devices. They are used notably for transparent electronics, TCF (transparent Conducting Films) and photovoltaic.

For example, PEDOT, polyaniline, IonomerPolyElectrolyte (IPE®) and so on are proposed by several companies.

ICPs can be alloyed with various conventional plastics including, for example, ABS, acrylics, composites, polyamides, polycarbonate, polyesters, rubbers, and TPEs.

Watch Video: Permanent Antistatic Agents by Sanyo Chemical

Assessing Antistats Performance

A common test to assess the performance of an antistatic agent is the electrical resistivity test. The result is expressed as log of resistivity. Often, antistatic performance is specified as maximum permissible log R under specific humidity conditions. The following chart gives a good idea of the range of values that reflect a good antistat effect.

Static protection vs fiber surface resistivity

Log R_p can be obtained using a variety of instruments, Hayek-Chromey Wheel, Static Honestometer or more commonly with polymer, Rothschild’s Static Voltmeter.

Antistatic Agents Selection Criteria

The selection of anti-static agents would depend on the processing conditions and the nature of the polymer.

The following can have an effect on the static dissipation performance:

Humidity
Polymer Prep Temperatures
End-use Process Temperatures

Impact of Humidity (RH %) on Anti-static Behavior

While the non-ionics are less impacted by normal plant RH factors, phosphates and quaternary amines tend to exhibit marked behavior based on humidity.

Phosphate effectiveness significantly decreases with decreases in RH %. Decreasing RH during polymer processing from normally a 60-70% RH range to less than 45% can result in a 10 fold drop in ability to control static.

Quaternary amines are generally not impacted severely by humidity changes though effectiveness tends to be non-linear with MW increase.

The following chart illustrates the RH effect on static dissipation (measured by the log of resistivity) to the molecular weight of species.

Resistivity vs Molecular Weight

Resistivity (Log R) vs Molecular Weight

The following selection chart provides insight into dissipation performance, both within the individual type and between classes.

Type of Antistat	Physical State	Humidity % (RH)		Polymer Prep Temperatures		End-use Process Temperatures*		Storage Stability**
		High (55-70)	Low (< 45)	Internal use	External use	Internal use	External use
Inorganic Salts		3+	3+	5	n/a	5	n/a	5
Carbon		5	5	5	n/a	4	n/a	5
Phosphates	Liquid	5	4	1	5	1	5	3+
	Semi-solid	5	4	2	5	2	5	5
	Solid	4	3	3	4	3	3+	5
Quaternary amine	Liquid	4	4	0	4	0	3+	5
	Semi-solid	4	4	1	4	1	4	5
	Solid	3+	3	2	3	2	2+	5
Nonionics	Liquid	2	2	-2	2	-1	2	3
	Semi-solid	2	2	-1	2	-1	2	4
	Solid	1+	1+	1	2	-1	1+	5
*Normal process temperatures for most all classes of polymers **Normal warehouse; For staple, POY and PU polymer products, low MW phosphates are especially prone to absorb into the polymer structure, thus losing the capability to function optimally.

Anti-stat Selection Based on Type of Polymer

The general classes of polymers function well in combination with the antistats. The inorganic salts and carbon are considered fully compatible in use with all polymer systems if incorporation is feasible.

The following guide shows the ability to dissipate static relative to physical form and polymer types. (5 being excellent, -5 v. poor).

Polymer Type	Phosphates**			Quaternary Amine			Non-ionics
Polymer Type	Liquid	Semi-solid	Solid	Liquid	Semi-solid	Solid	Liquid	Semi-solid	Solid
Polyolefin	5	3+	2	4	3	0	-1	-1	-1
Polyester	5	5	5	4	4	3	1	1	-2
Polyamide*	4+	5	3+	4	3+	2+	1	1	-2
Aramid	5	5	5	4	4	4	1	1	1
Polyurethane	-1	1	3	-1	1	3	1	1	1
Polyketone	5	5	5	4	4	3	1	1	0
Fluoropolymers	5	5	5	4	4	4	2	1	0
Carbon Fiber	5	5	5	4	3	3	2	1	0
POY and staple nylon (6 & 6,6) are more sensitive to low MW absorption issues than FDY. MW esters of less than C12 will result in only short-term static protection and long storage times will result in poor processing. Physical form by themselves - normally K or Na neutralized salts

Find Suitable Anti-static Agents for Polymers

View a wide range of anti-static agent grades available in the market today, analyze technical data of each product, get technical assistance or request samples.

About Paul Seemuth

Paul Seemuth is Chief Executive Officer at Tribology Consulting International. Dr. Seemuth, who holds a PhD in Organic Chemistry, has more than 30 years of experience in tribology and lubrication of polymers. His fields encompass but are not limited to, organic chemistry, fiber lubrication technology and formulations, catalysis, polymer processing, specialty chemicals, fuel and oil additive formulations.

Work experience includes global technology leader at DuPont Fibers Finish Technology Group, responsible for global Fiber Finish technologies and strategies, related plant designing and plant start-ups, VP - Global Technology at SSC Industries and an Associate Professor at Chattanooga State in the Departments of Chemistry and Chemical Engineering.

A Fellow of the Royal Society of Chemistry (FRSC), he is a recognized world expert in the field of Tribology, the study of friction and wear. Paul has over thirty publications and over 15 patents covering scientific endeavors on automotive additives, lubricant technologies, fiber finish formulations, polymer production processes, heterogeneous catalysts and supercritical fluid applications. Recently, he completed a major chapter on Textile Fibers / Fabrics” in the Handbook of Lubrication and Tribology, Volume I Application and Maintenance, Second Edition, then served as Section Editor for the Encyclopedia of Tribology along with a contribution on Fiber Boundary Tribology.

Dr. Seemuth consults both domestically and internationally. He also presents regular presentations on scientific topics related to lubrication and surface science.

Suppliers

Brands