Additives for Polymers

 Additives for Polymers

During the processing of polymers, several selected substances are added to optimize specific properties, because only after the incorporation of an additive can a polymer fulfill those requirements. Supplementation of polymers with additives is quite common.

Common Additives

Additives can include:

• Antimicrobiological substances

• Antistatic agents

• Flame retardants

• Fillers

• Lubricants / dispersing agents / release agents

• Nucleating agents

• Stabilizers

• Blowing agents

• Plasticizers

Methods of Incorporation

There are principally two methods of adding additives:

1. As a component of a color batch

2. As a separate additive batch

Some additives must be incorporated by the polymer manufacturer due to the high concentrations required. For example, certain grades of polymers, such as glass fiber-reinforced types, contain up to 55–60% filler.

It is also common for customers to request the incorporation of an additive into the color preparation, for instance, a stabilizer to improve the light fastness of the plastic article.

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Considerations for Combining Color and Additives

A combination of color plus additive in one batch can sometimes be problematic:

• Space in the formulation: Additives take up space in the recipe, reducing the concentration of colorants, especially for very intensive colors. This may require a higher addition of the color batch for the final product.

• Color of additives: Some additives are not colorless, so they must also be colored, increasing the total amount of colorant needed.

Some additives are marketed as pre-made batches, so it is worth evaluating whether two separate batches (color batch + additive batch) or a combined batch is more economical.

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Nature and Function of Additives

• Additives can be inorganic or organic.

• Interactions between additives and colorants may occur and can be either positive or negative.

• The primary function of an additive is to optimize a specific property of the polymer.

• This requires tailor-made substances for each polymer type, depending on its chemical structure, which results in a large variety of additives.

Antimicrobiological Substances

Tests have shown that bacteria and fungi can settle on plastics and even penetrate the polymer surface. Consequently, mechanical cleaning alone is insufficient to remove them.

To prevent harmful contamination, an antimicrobiological substance can be added to the polymer. This supplementation serves as an additional precautionary measure and does not replace standard hygiene practices.

Fields of Application

Examples include:

• Hospitals: Floors, walls, appliances/equipment, protective gloves, bed linens, mattresses, packaging material for medical equipment, etc.
• Households: Kitchen, bathroom, bedroom
• Textiles: Work clothes for certain occupational groups

Characteristics of Effective Antimicrobiological Substances

Antimicrobiological substances used in polymers are well-known compounds previously applied successfully in disinfectants for many years.

A substance is suitable for polymer application if it meets the following requirements:

• Good activity in polymers
• Heat stability to withstand polymer processing temperatures
• Insolubility in water to prevent removal during cleaning or washing

Example:

• 2,4,4’-Trichloro-2’-hydroxydiphenolether (generic name: Triclosan), a phenol derivative widely used in polymer applications.

Antistatic Agents

Many polymers have good electrical insulating properties, which makes them susceptible to static electricity accumulation.

Static electricity is generated when two materials with different electron affinities are rubbed together. This can involve two polymers or different types of materials. Friction shifts the natural balance of electrons:

• One material loses electrons and becomes positively charged.
• The other gains electrons and becomes negatively charged.

When such materials come into contact with a conductive material, a discharge (spark) occurs. Dry conditions, particularly in winter, increase the risk of static buildup.

Mechanism of Antistatic Agents

Antistatic agents reduce static electricity by forming a very thin layer of humidity on the polymer surface, making it partially conductive.

Fields of Application

• Packaging: Reduces dust accumulation on consumer goods, improving customer appeal.
• Electronics: Ensures safety by preventing faulty circuits or short circuits.
• Industry: Provides a general safety precaution against dust explosions or hazards when handling flammable liquids.
• Plastics Processing: Helps avoid bridging of resins during conveying and feeding operations.

Practical Considerations

• During the feeding of color batches, volumetric metering units are often used. Each cavity is filled and emptied rhythmically, usually with different polymers in the cavity and the color batch.
• Rotation during feeding can build up static electricity, causing incomplete emptying and resulting in inhomogeneous or weak coloring of the polymer.
• Hoppers often have a small window to monitor material flow. If rotation causes static electricity, the color batch can stick to the window, leading to defective parts.
• Severe drying of polymers (e.g., PC and PET) can further increase static issues.

Problems related to static electricity during polymer coloring can be solved by adding 0.1–0.3% antistatic agent to the color preparation.

Antistatic Agents (Continued)

It should be noted that several dispersing agents also exhibit a side effect of antistatic activity.

Primary Compounds Used

The main compounds used as antistatic agents include:

• Fatty acid esters

• Ethoxylated fatty acid esters

• Alkylamine derivatives

• Ethoxylated alkylamine derivatives

• Alkylsulfonates

The antistatic effect works by forming a very thin layer of humidity on the polymer surface. This requires the migration of the antistatic agent to the surface.

Concentration and Polymer Type

• In partially crystalline polymers, the required concentration is 0.1–0.5%.

• In amorphous polymers, a higher concentration of 1.0–1.5% is needed.

Migration is not limited to the outer surface; it can also occur internally, which may result in contamination of the filled product.

Regulatory Considerations

Use of antistatic agents in consumer goods, especially those in contact with food, cosmetics, or other sensitive products, is subject to legal regulations to ensure consumer safety.

• Germany: All plastic items in contact with food, packaging for food, smoking materials, household products, cosmetics, and items in prolonged skin contact are regulated. Only approved antistatic agents within defined concentrations are allowed (e.g., ethoxylated alkylamine derivatives up to 0.15% in the final product).

• United States: Only FDA-approved antistatic agents may be used in consumer goods.

Practical Considerations for Incorporation

Many antistatic agents are liquid at room temperature or have melting points below polymer processing temperatures, such as ethoxylated alkylamine derivatives and ethoxylated fatty acids.

• Incorporating a liquid antistatic agent into a solid color preparation requires special precautions, especially at higher concentrations.

• Frequently used liquid antistatic agents are commercially available as solid concentrates, either:

  • Absorbed on a highly absorptive carrier like silica, or

  • As a highly concentrated batch

These solid concentrates are preferred because they do not cause problems during production of solid color preparations.

Flame Retardants

Most polymers are more or less combustible, and high standards in fire protection necessitate the use of flame retardants. To comply with regulations, polymers must often be equipped with a flame retardant.

Fields of Application

Flame retardants are commonly used in:

• Electrical/electronic appliances: TV compounds, fuse boxes, switches, plugs, etc.

• Interior decor of public transport: Buses, railways, airplanes

• Building protection: Interiors and exteriors of airports, railway stations, and similar structures

Evolution of Flame Retardants

• In the past, chlorinated and brominated substances were preferred due to their effectiveness at relatively low concentrations.

• However, they released gaseous halogen hydrides during fire, which, combined with water, were highly corrosive and could destroy sensitive electronic components, often causing more damage than the fire itself.

• As a result, halogenated flame retardants have lost importance in recent decades.

Modern Flame Retardants

Modern flame retardants are halogen-free, but:

• They require high concentrations to meet industrial standards such as UL 94V-0.

• Additions may reach 25–30%, and in extreme cases even higher.

Incorporation into Polymers

Flame retardants are typically added by the polymer manufacturer, due to:

1. The high concentration required, which makes incorporation into a color preparation impractical.

2. Regulatory compliance:

   • Legal requirements govern the type and concentration of flame retardant,

   • Test methods and approval procedures for the final part,

   • Proper documentation to customers in the industry.

These responsibilities are beyond the scope of color preparation manufacturers.

Challenges in Coloring Flame-Resistant Polymers

Flame-resistant polymers are usually available only in standard colors. However, there is increasing demand for custom colors (e.g., for branding purposes). Designing color preparations for flame-resistant polymers is challenging due to:

• Preserving flame retardancy: No loss of flame resistance is allowed.

• Coloring the flame retardant: Many flame retardants are not colorless and must be colored, increasing the required color preparation.

• High color concentration issues: Can cause problems in highly filled polymers.

• Potential interactions between flame retardant and colorant.

Coloring of Flame-Resistant Polymers

Due to the challenges involved, it is recommended to develop any color preparation for flame-resistant polymers only in cooperation with the polymer manufacturer.

In the electrical/electronic industry, the UL 94 standard is used for classifying flame-resistant plastic parts.

Practical Example

A case illustrates the difficulty:

• Titanium dioxide (Pigment White 6), a non-flammable powder, was added during injection molding of a flame-resistant polymer.

• Unexpectedly, the flame resistance was lost.

• Investigation revealed the cause:

  • UL 94 evaluates the behavior of ignited specimens.

  • Droplets of hot melt that fall and ignite cotton wool lead to failure.

  • Titanium dioxide, being a heavy substance, increased the weight of the polymer melt, causing droplet formation.

This demonstrates that even non-flammable colorants can affect flame resistance.

Common Flame Retardants

Substances used as flame retardants include:

• Aluminum hydroxide

• Zinc borates

• Red phosphorus

• Melamine derivatives (borates, cyanurates, phosphates)

• Phosphate derivatives (ammonium polyphosphates, aryl phosphates)

• Phosphorous acid esters (chlorinated and chlorine-free grades)

• Antimony pentoxide

• Chlorinated aliphatic hydrocarbons

The required concentration depends on:

• The chemistry of the flame retardant

• The type of polymer

• The UL 94 classification group (V-0, V-1, etc.)

Fillers

Many technical plastic parts require high mechanical strength and sometimes good durability at elevated temperatures. This is achieved by adding fillers.

• Fillers are usually incorporated by the polymer manufacturer.

• Concentrations typically range from 5% to 60%, depending on:

  • Type of filler

  • Type of polymer

  • Specific performance requirements of the final product

Special grades of polymers are available for many applications, developed specifically for reinforcement purposes.

Common Fillers

The following fillers are typically used to reinforce polymers:

• Glass fibers and glass spheres

• Carbon fibers

• Calcium carbonate (chalk)

• Talc (silicates)

• Barium sulfate

Considerations in Color Matching

When coloring reinforced polymers, several characteristic properties of fillers must be considered:

1. Possible Damage to Glass Fibers:

   • Some inorganic pigments are very hard and can break glass fibers, reducing mechanical strength.

   • Example: Titanium dioxide (Pigment White 6, rutile) can cause fiber breakage in glass-fiber-reinforced polyamide.

   • Solution: Replace with softer pigments, such as Pigment White 7 (Zinc sulfide) or Pigment White 5 (Lithopone).

   • This consideration applies to pure white colors as well as white reduction in formulations.

   • Example of another hard pigment: Chromium green hematite (Pigment Green 17), which is rarely used and can be replaced by organic pigments.

2. Heat Stability:

   • High filler content increases processing temperature and melt viscosity, leading to longer cycle times.

   • Heat-sensitive organic colorants may no longer be suitable for reinforced polymers.

3. Pigment Concentration in Color Preparation:

   • Most fillers are not colorless and require coloring.

   • This increases the amount of colorant needed, especially for intensive or brilliant colors.

   • Higher colorant concentration can lead to dispersion challenges and color specks in the final product.

4. Melt Viscosity of the Color Preparation:

   • Reinforced polymers have higher melt viscosity compared to the natural polymer.

   • The color preparation must be adjusted to match this viscosity; otherwise, color streaks or other defects may occur.

5. Intensive, Brilliant Colors:

   • Not all brilliant colors are achievable in reinforced polymers.

   • Fillers tend to lighten colors, and excessive addition of color preparation may exceed the polymer’s capacity, making certain colors impractical.

Use of Dyes in Reinforced Polymers

Dyes can be used to color reinforced polymers, but some considerations apply:

1. Polymer Proportion:

   • Reinforced polymers contain a lower proportion of polymer compared to naturally colored grades.

   • This may lead to dye migration, even at very low but otherwise safe concentrations.

2. Interactions:

   • Interactions between fillers and colorants cannot be excluded.

   • Such interactions have been reported, for example, with glass fibers.

Designing a color preparation for reinforced polymers is generally not difficult, provided the specific properties and behavior of the reinforced polymer are properly considered.

Dispersing Agents, Lubricants, and Release Agents

During polymer processing, various difficulties can arise, such as:

• Problems with demolding during injection molding

• Sticking of films during film blowing

• Difficulty in reproducing structured mold surfaces on plastic parts

A quick remedy in many cases is the addition of a lubricant and/or release agent.

Lubricant vs. Release Agent

A strict differentiation between lubricants and release agents is difficult because the phenomena of adhesion and glide merge.

• Most substances exhibit both effects, with the predominant effect depending on the polymer type and concentration.

• Release agent applications usually require higher concentrations than lubricant applications.

• Typical concentration ranges: 0.1–2.0%, depending on the efficiency of the substance, the intended effect, and the type of polymer.

Some polymers are already pre-equipped with these additives, but in some cases, additional addition is beneficial.

Recommended Substances

• Fatty acids: stearic acid, palmitic acid

• Fatty acid esters: sorbitan tristearate, pentaerythritol fatty acid ester

• Fatty acid amides: oleic acid amide, stearic acid amide, erucic acid amide

• Fatty alcohols: linear and branched grades

• Paraffin oil

• Waxes and derivatives

Comparison with dispersing agents shows that several of these substances are chemically identical.

• Every color preparation contains dispersing agents at variable concentrations.

• Often, the concentration is sufficient to provide a positive side effect as a lubricant or release agent.

• If the effect is insufficient, increasing the concentration is possible.

Mechanism of Action

A substance is effective as a lubricant or release agent when it migrates at least partially to the polymer surface.

• When plastic parts are in contact with food, this migration must be considered.

• Many countries regulate global migration, which includes lubricants, release agents, and dispersing agents.

• If migration exceeds allowable limits, it is recommended to analyze the chemical structure of each migrated substance to identify the source (polymer or color preparation).

Nucleating Agents

During injection molding, plastic parts must be cooled before demolding.

• In partially crystalline polymers (e.g., polyethylene (PE) and polypropylene (PP)), crystallization begins during cooling to form crystallites.

• Crystallization is a two-stage process:

  1. Nucleation: Formation of a stable nucleus through local ordering of polymer chains.

  2. Growth: Addition of polymer chains to the nucleus.

Dust, impurities, or catalyst residues can act as nuclei, but their presence is inconsistent and varies from batch to batch. This can lead to variable shrinkage, making it difficult to reproduce exact measurements in injection-molded parts.

Role of Nucleating Agents

Adding a nucleating agent provides several benefits:

• Crystallization starts at higher temperatures.

• More nuclei are available, producing smaller, more uniform spherulites.

• Faster overall crystallization, reducing shrinkage and warpage issues.

• Can help offset nucleating effects of certain organic pigments in PE-HD and PP.

Even when polymers already contain nucleating agents, adding them to the color preparation is common and effective.

Common Nucleating Agents

• Typical concentrations: 0.1–0.2% in the final product.

• Incorporation into color preparations is straightforward and does not present any processing issues.

Stabilizers

Like all organic substances, polymers age over time.

• The addition of a stabilizer can slow down the aging process, but it cannot completely prevent aging.

• Aging results in a loss of mechanical and physical properties, including changes in color, brittleness, and surface dullness.

Causes of Aging

Aging can result from internal factors (thermodynamically unstable conditions) or external factors (environmental effects). Common external influences include:

• Ionizing radiation: e.g., sterilization of medical devices by γ-rays

• Weather: e.g., outdoor use of polymers

• Biological impacts: e.g., bacteria and fungi; for biodegradable polymers, this is a positive effect

• Chemical impacts: e.g., interaction with contents or chemical plant environments

• Mechanical impacts: e.g., rotation, movement, pressure (conveyor belts)

• Thermal impacts: e.g., high temperatures during processing or use (chemical plants, automotive under-hood conditions)

Often, aging results from a combination of these factors.

Stabilization and Fastness

• Fastness properties, such as light fastness and weather resistance, depend on the whole system: both colorants and polymer.

• It is impossible to separate the fastness of the colorant from that of the polymer.

• Stabilizers are necessary to ensure strict fastness requirements.

Example: Polyethylene (PE)

• Ideal PE (perfect chain [-CH₂-CH₂-CH₂-]) is stable and does not require stabilization.

• Technical grades of PE contain defects (e.g., double bonds), which require stabilization using light stabilizers.

• Weathering of PE causes photooxidative degradation:

  1. Absorption of energy (e.g., UV light) forms a polymer radical.

  2. The radical reacts with atmospheric oxygen, forming a peroxide radical.

  3. This initiates a chain reaction, leading to polymer degradation.

Mechanism of Polymer Degradation

• The peroxide radical reacts with another polymer molecule, forming a peroxide hydroxide and a new polymer radical.

• The new polymer radical can either:

  • React with itself, terminating the chain reaction, or

  • React with atmospheric oxygen, restarting the chain reaction.

• The peroxide hydroxide, under the impact of energy, decomposes into a hydroxide radical and a polymer oxide radical, which react with further polymer molecules, continuing the chain reaction.

Simplified reaction steps:

• These are the primary steps, but additional reactions can occur, especially at double bonds or defective spots in the polymer chain.

• The key takeaway: energy initiates radicals, radicals start chain reactions, so avoiding energy would prevent degradation—but in practice, heat and light are unavoidable.

Stabilization Strategy

To protect polymers against heat, light, and oxygen, effective stabilizers are necessary:

1. Processing stabilizers – Protect the polymer during processing at temperatures >130 °C (266 °F).

2. Long-term thermal stabilizers – Protect the polymer during service life at lower temperatures.

3. Light stabilizers – Reduce damage from UV light and photooxidation.

Important points:

• UV absorbers primarily protect deeper layers, not the polymer surface.

• The main goal is to deactivate radicals before a chain reaction starts.

Processing Stabilizers

• Added immediately after polymer synthesis.

• Effective in the usual processing temperature range.

• Protect against heat-initiated decomposition.

Common chemical groups of processing stabilizers:

• Phosphites

• Hindered phenols

• Hydroxylamines

• Lactones

• Below 130 °C (266 °F), most processing stabilizers are ineffective, except for hindered phenols.

• For long-term applications, a separate long-term thermal stabilizer is required to ensure polymer durability.

Long-Term Thermal Stabilizers

• Long-term thermal stabilizers are added by the polymer manufacturer to protect the polymer during its service life.

• They are most effective at temperatures below 130 °C (266 °F) and are generally ineffective at processing temperatures, except for hindered phenols.

Chemical groups of long-term thermal stabilizers:

• Hindered phenols

• Hindered amine stabilizers (HAS)

• Thiosynergists (used in combination with hindered phenols)

• Together with processing stabilizers, they protect the polymer against thermal degradation.

Light Stabilizers

• Light stabilizers are added to improve light fastness and weather resistance of both colored and natural polymers.

• Even when polymers already contain light stabilizers, it is common to include them in color preparations.

• There are two main types:

  1. UV absorbers – Absorb harmful wavelengths of light that initiate radical formation in polymers.

  2. HALS (Hindered Amine Light Stabilizers) – Deactivate radicals before they start chain reactions.

• Synergy effect: Combining UV absorbers and HALS creates a synergistic effect, where the combined protective effect is greater than the sum of individual effects.

Chemical groups of UV absorbers:

• Benzotriazoles

• Benzophenones

• Cyanoacrylates

• Oxanilides

• Phenylsalicylic acid esters

• UV absorbers differ in their maximal absorption wavelength, which determines their area of activity.

UV light wavelength ranges:

• UV-A: 315 – ≤ 400 nm

• UV-B: 280 – ≤ 315 nm

• UV-C: 100 – ≤ 280 nm

• UV absorbers are assigned to specific ranges based on their maximal absorption wavelength, ensuring protection against the corresponding radiation.

Hindered Amine Light Stabilizers (HALS)

• HALS are light stabilizers whose chemical principle is indicated by the acronym.

• They consist of a heterocyclic, nitrogen-containing ring system with bulky substituents adjacent to the nitrogen.

• The diversity of commercial HALS products reflects the variability of this chemical principle.

Key points for HALS use:

1. Essential for light stabilization: A polymer always requires HALS to resist the damaging effects of light.

2. Type and concentration: Depends on the polymer, intensity of irradiation, and required light fastness. Typical concentrations for polyolefins are 0.2–0.4% in the final product.

3. UV absorber co-stabilizer: Recommended due to the synergistic effect, enhancing overall stabilization.

4. Physical form:

   • Most HALS are solid, making incorporation into solid color preparations straightforward.

   • For liquid color preparations, liquid stabilizers are preferred, though not always feasible depending on the polymer.

Interactions and precautions:

• HALS are not chemically inert, so interactions with other recipe components can occur:

  • Perylene pigments may deactivate HALS.

  • HALS are alkaline and can react with alkali-sensitive or acidic components, e.g., colorants, flame retardants, thiosynergists.

  • HALS may be absorbed by highly absorptive components, e.g., carbon black, special silica grades.

  • HALS can be deactivated by environmental factors, e.g., certain insecticides in agricultural films.

Blowing Agents

Blowing agents are added to the polymer melt to produce foamed polymers. Historically, fluorinated hydrocarbons were used, but they were banned due to their ozone-depleting effects.

Requirements for blowing agents:

• Must be lightly volatile, or

• Must release a large volume of gas upon thermal decomposition at processing temperatures.

Examples of thermal-decomposition blowing agents:

• Azodicarbonamide

• Toluene sulfohydrazide

Key notes on usage in color preparations:

• Rarely added to color preparations; only feasible in powdery or liquid forms.

• Excluded from masterbatches due to the thermal instability of blowing agents.

Considerations for coloring foams:

1. Homogeneous coloring requires a homogeneous cellular structure – uniform cell size and wall thickness.

2. Higher colorant concentration is often needed, comparable to film coloring.

3. Colorants may influence the foam’s cellular structure.

4. Color differences may appear compared to solid polymers, due to hollow cells and variable light absorption/reflection.

Plasticizers

Plasticizers are primarily used to increase the flexibility of polymers, with PVC being the most common application, although other polymers such as PUR and various rubbers also benefit from their addition.

Chemical groups of plasticizers commonly used:

• Phthalic acid esters

• Trimellitic acid esters

• Adipic acid esters

• Sebacic acid esters

• Fatty acid polyglycolesters

• Chlorinated paraffins

Safety considerations:

• Some plasticizers, like diethylhexylphthalic acid ester (DEHP), showed potential carcinogenic effects in high doses in animal studies. However, these effects are not considered applicable to humans.

• Certain countries, especially in Europe, ban DEHP in plastics intended for food contact, and some customers request products “free of any phthalic acid ester,” which is often unnecessarily restrictive.

Designing color preparations for plasticized polymers:

1. Migration:

   • Organic colorants may migrate within the polymer.

   • Migration tests under the intended use conditions are recommended to ensure the desired shade is achieved.

2. Influence on Shore hardness:

   • Colorants or carriers can affect the hardness of the final product.

   • Adjusting plasticizer content may not always correct this, so careful selection of colorant and carrier is crucial.

3. Plasticizers as carriers:

   • A simple approach is to use the polymer’s plasticizer as a carrier in liquid color preparations, commonly done with plasticized PVC.

   • The concentration of the color preparation depends on the plasticizer content in the final product.

4. Interactions:

   • Stabilizers in PVC can interact with colorants, so potential interactions must be considered.

Practical insight:
Experience indicates that all encountered issues with coloring plasticized polymers can be solved through close cooperation with the customer and careful adjustment of colorant, carrier, and formulation.

This section highlights the interplay between plasticizer properties, colorant selection, and final product performance, emphasizing the need for tailored color preparation design.

Additive masterbatches are used to enhance the functional performance of plastics. These include UV stabilizers, anti-block agents, antistatic additives, slip agents, optical brighteners and thermal stabilizers. By incorporating these additives into the polymer matrix, manufacturers can achieve improved durability, surface properties, safety, and processing performance in the final product.

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