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The material selection platform
Adhesives Ingredients
The material selection platform
Adhesives Ingredients

Selecting Antioxidants for Adhesives & Sealants

Antioxidants are used in a variety of adhesive formulations to protect against degradation caused by reaction with atmospheric oxygen. Excessive oxidation generally results in undesirable changes in the adhesive's mechanical, aesthetic, or bonding properties that may in turn cause damage (like discoloration, viscosity, skin formation, etc.) to your formulation.

Thus, selecting the right antioxidant for your adhesive or sealant applications is extremely important, but the selection process can be challenging.

Explore, in detail, the use of antioxidants in the adhesives & sealant industry and learn how to reduce the damage done by oxygen to adhesives.

Overview

Need for Antioxidants in Adhesives

Need for Antioxidants in Adhesives

Antioxidants are used in a variety of adhesive formulations to protect against degradation caused by reaction with atmospheric oxygen. The increasingly demanding requirements which modern adhesives are expected to meet, can only be achieved and retained by using appropriate stabilizer systems.

Adhesives and their raw materials react like other organic materials with oxygen in a process called "autoxidation". Autoxidation is initiated by various factors, such as:

  • Heat
  • Light
  • Mechanical stress
  • Catalyst residues, or
  • Reaction with impurities

Free radicals are formed and react in the presence of oxygen to form peroxy radicals, which further react with organic material leading to hydroperoxides (ROOH). These reactions ultimately lead to a change in chemical composition and polymer properties, such as molecular weight, resulting in immediate impact on adhesive's mechanical, aesthetic, or bonding properties that determine the service life of an adhesive. They key properties which can be impacted include:

  • Discoloration
  • Viscosity changes
  • Char formation
  • Cracking and
  • Loss of adhesion

When used in adhesives formulations, antioxidants work by interrupting the degradation process in different ways. They help to preserve quality and performance of adhesive formulations during all its life cycle.


Oxidation of Adhesives - When & Why Does it Occur?

Oxidation of Adhesives - When & Why Does it Occur?

Oxidation can occur at all stages of an adhesives life from synthesis to final end-use. It is usually recognized at high processing temperatures such as during mixing, compounding, or extrusion (in the case of hot-melt adhesives). However, oxidation can also occur at relatively low temperatures including ambient storage and also on exposure to UV light.

Oxidation of Adhesives
Oxidation of Adhesives


Life Cycle Stage Common Reason for Degradation
Raw material manufacturing
  • High temperature treatment
  • Catalyst residues
  • Shipping processes
Raw materials storage and shipment
  • Prolonged exposure to oxygen at ambient temperatures
Adhesive manufacturing
  • High temperature processing
  • Shear
  • Oxygen exposure
Application of adhesive to substrate
  • High temperature processing
  • Oxygen exposure
Service life
  • Prolonged exposure to oxygen at ambient or elevated temperatures
  • Exposure to UV light
Characteristics of Oxidation During an Adhesive's Typical Life Cycle

Components Susceptible to Oxidation


Polymers that are commonly used in adhesive formulations differ considerably in their sensitivity to oxidation. For example:

  • Acrylates are highly resistant to oxidation and do not generally require antioxidants at ambient temperatures
  • Unsaturated elastomers are highly susceptible to oxidative decomposition and require relatively high concentration of antioxidants for protection.

Adhesive components especially susceptible to oxidation include base synthetic polymers like ethylene vinyl acetate & styrene block copolymers, polyolefins, polyamides, natural rubber, polychloroprene, polyurethane, and butyl rubber.

Hydrocarbon additives, such as tackifiers and waxes, are also vulnerable to oxidation and can actually contribute to the oxidation of the base polymer. Metallic and other impurities in the adhesive can accelerate the oxidation process. Depending on the aging environment, most adhesives can benefit from antioxidants.

Role of an Antioxidant


The function of an antioxidant is to prevent the propagation of oxidation. Antioxidants interrupt the oxidation process in different ways according to their chemical structure. The antioxidant is usually optimized for a specific polymer or combination of polymers and for a specific environment or market segment.

As a result, there are many different families of commercial antioxidants that are specifically designed to meet the particular needs of the adhesives industry.

The role of antioxidants in providing longer life has taken on added importance with the growth of hot-melt adhesives and structural adhesives that must perform in elevated temperature environments.

Mechanism of Uninhibited Autoxidation

Mechanism of Uninhibited Autoxidation

To understand the function of an antioxidant, one must first understand the mechanism of degradation.

  • Oxidative degradation starts with the initiation of free radicals on exposure to heat, UV radiation, and mechanical shear or in the presence of reactive impurities such as catalyst residues.
  • These radicals attract neighboring labile hydrogen atoms to produce unstable hydroperoxides plus additional free radicals that keep the process going.

This mechanism is generally characterized by three distinct stages: initiation, propagation, and termination. These steps are shown and described below.

Major reactions occurring during polymer oxidation
Major reactions occurring during polymer oxidation

This cycle leads to degradation of the polymer. Only when a non-radical inert product is formed will the cycle be terminated. In preventing this type of degradation, antioxidants function by intercepting the radicals or by preventing radical initiation during the polymer's life cycle.


Initiation


Free radical initiation can be produced by several processes related to the input of some form of energy. Polymeric free radicals (R") can be formed by exposure to heat, light, and shear. Impurities and catalyst residue can also lead to the generation of free radicals, which then rapidly react with oxygen to form peroxy radicals.

It is virtually impossible to manufacture commercial polymers that do not contain traces of hydroperoxides. The peroxide bond is relatively weak and cleaves easily to yield alkoxy or hydroxy radicals. Once oxidation has started, the concentration of hydroperoxides becomes appreciable, and their decomposition is the main source of radical initiators.

The absorption of UV light also produces radicals by cleavage of hydroperoxides and carbonyl compounds. Most polymer degradation caused by the absorption of UV light results from radical initiated autoxidation. Thus, UV degradation and degradation from oxidation are tightly linked and often dealt with together.


Propagation


The propagation reactions involve the very rapid reaction of the free radicals with oxygen forming peroxy radicals (RCOO"). The peroxy radicals can then react with the polymer to produce hydroperoxides (ROOH), which can further decompose to react with the polymer to produce free radical species. Thus, the mechanism (often termed autoxidation) is cyclic and can be repeated many times before being terminated. It leads to an exponential growth of free radicals.

The rate of oxidation is usually very low at the start of aging and increases with time. Trace quantities of transition metal ions greatly accelerate the rate of oxidation by catalyzing the decomposition of the hydroperoxides into radical species. The most operative metal ions are copper, iron, cobalt, and manganese.


Termination


Conversion of radicals to non-radical species terminates the propagation reactions. The termination mechanisms generally result in crosslinking or chain scission. Thus, the resulting ramifications of oxidation include those physical effects summarized in the Table below. A change in color (yellowing) is also often evident from oxidation.

Property change to crosslinking Property change due to chain scission
Hardening Softening
Skinning Decrease in viscosity
Gel formation Increase in tack
Decrease in tack Loss of cohesive strength
Increase in viscosity
Property Change Due to Crosslinking and Chain Scission Resulting from Oxidation

Fortunately, there are a number of stabilization chemistries that disrupt and inhibit the autoxidation cycle by preferentially and sacrificially reacting with the radical species and hydroperoxides. Protection of the adhesive components from degradation is achieved by obstructing one or more of the oxidation mechanisms described above.

The specific antioxidants and concentrations employed have an important impact on the physical properties of the adhesive.

Antioxidants are generally added to each raw material component of an adhesive formulation that is prone to oxidation, and in most cases, they are also added to the final adhesive formulation. The use of antioxidants only in the final formulation process will not undo any oxidative damage that might be caused to the raw materials via storage, processing, etc.

Let's understand the different classes of antioxidants in detail.

Classification of Antioxidants

Classification of Antioxidants

Antioxidants interrupt the degradation process in different ways, according to their structure. The major classifications of antioxidants are listed below:

  • Primary Antioxidants – They are chain-breaking antioxidants which react rapidly with peroxy radicals (ROO*).
  • Secondary Antioxidants – They react with hydroperoxides (ROOH) to yield non-radical, non-reactive products and are also called as hydroperoxide decomposers.
  • Multi-functional Antioxidants – They combine both primary and secondary antioxidant functions in one compound, have only recently become available
  • Hydroxylamines – They are capable of scavenging C-radicals and act as both primary and secondary antioxidants.
  • Carbon-centered Radical Scavengers – They are effective in trapping alkyl radicals and provides powerful processing stability.


Primary Antioxidants


Primary antioxidants function by donating their reactive hydrogen to the peroxy free radicals so that the propagation of subsequent free radicals does not occur.

Mechanism of Antioxidation


  • Primary or free radical scavenging antioxidants inhibit oxidation via chain terminating reactions.
  • They have reactive OH or NH groups.
  • Inhibition occurs via a transfer of a proton to the free radical species.
  • The resulting radical is stable and does not abstract a proton from the polymer chain.

Mechanism of Antioxidation of Primary Antioxidants
Mechanism of Antioxidation of Primary Antioxidants

The two major groups among the primary antioxidants are hindered phenolics and aromatic amines.


Hindered Phenols


Phenolic stabilizers are primary antioxidants that act as hydrogen donors. They react with peroxy radicals to form hydroperoxides and prevent the abstraction of hydrogen from the polymer backbone. Often used in combination with secondary antioxidants, phenolic stabilizers are offered in an extensive range of molecular weights, product forms, and functionalities.

Sterically hindered phenols, are the most widely used stabilizers of this type. They are effective during both processing and long term thermal aging and many have FDA approvals.

These products generally resist staining or discoloration. However, they may form quinoid structures upon oxidation, which leads to yellowing. Phenolic antioxidants include simple phenolics, bisphenolics, polyphenolics, and thiobisphenolics.

Hindered phenolics, such as butylated hydroxy toluene (BHT), high molecular weight phenolics, and thiobisphenolics, are the most popular of the primary antioxidants. They are suitable as long-term stabilizers in almost all cases.

  • BHT has long been the workhorse of the antioxidant industry, and it possesses FDA approval for food contact. However, BHT is a relatively volatile material that is gradually being replaced by higher molecular weight antioxidants that resist migration.
  • Thiobisphenolics can also act as peroxide decomposers (secondary antioxidants) at temperatures above 100°C. Therefore, they are often found in higher temperature applications.

ROO* radicals are deactivated by hindered phenol via the following reaction.

Deactivation of RCOO* Radicals
Deactivation of RCOO* Radicals

The phenoxy radical generated are very stable due to their ability to build numerous mesomeric forms.

Mesomeric Forms of Phenoxy Radicals
Mesomeric Forms of Phenoxy Radicals


Secondary Aromatic Amines


Secondary aromatic amines act as primary antioxidants and are excellent hydrogen donors.

Mechanism of Secondary Aromatic Amines
Mechanism of Secondary Aromatic Amines


Also available in an extensive range of molecular weights and product forms, aromatic amines are often more active than hindered phenols, because of less steric hinderance. Aromatic amines, however, are more staining & discoloring than hindered phenols, especially on exposure to light or combustion gases (gas fade) and have limited FDA approvals for use in contact with food.

Amines are commonly used in the rubber industry, but they also find uses in applications such as wire and cable formulations and in polyurethane polyols.


Secondary Antioxidants


Secondary antioxidants are used in conjunction with primary antioxidants to provide added stability to the polymer. They destroy the unstable hydroperoxides that function as sources of free radicals during oxidative degradation. They react with hydroperoxides to yield non-reactive products. Thus, secondary antioxidants are also known as hydroperoxide decomposers.

Hydroperoxide decomposers prevent the split of hydroperoxides into extremely reactive alkoxy and hydroxy radicals. Organophosphorus compounds and Thiosynergist antioxidants are widely used hydroperoxide decomposers.

Mechanism of Antioxidation


  • Secondary antioxidants decompose hydroperoxides into non-radical, non-reactive, and thermally stable products.
  • They are often used in combination with primary antioxidants to yield synergistic stabilization effects.

Mechanism of Antioxidation of Secondary Antioxidants
Mechanism of Antioxidation of Secondary Antioxidants


Organophosphorus Compounds


Organophosphorus compounds are secondary antioxidants that decompose peroxides and hydroperoxides into stable, non-radical products. They are extremely effective stabilizers during processing and are normally used in combination with a primary antioxidant.

Trivalent phosphorus compounds are excellent hydroperoxide decomposers. Generally, phosphites (or phosphonites) are used and react according to the following general reaction, generating phosphates.

Phosphites (or Phosphonites) React to Generating Phosphates
Phosphites (or Phosphonites) React to Generating Phosphates


Some of these compounds are sensitive to water and can hydrolyze, leading to formation of acidic species. While the addition of an acid scavenger can minimize the effect, the industry has generally converted to hydrolysis-resistant compounds.

Commercially available phosphites differ by the nature of the aryl groups. Some typical structures are shown below.

Commercially Available Phosphites
Commercially Available Phosphites


Thiosynergists


Among sulfur-based hydroperoxide decomposers, esters of 3,3-thiodipropionic acid play an important role. Thiosynergists react according to the following general reaction, generating sulfoxides and sulfones.

Thiosynergists Reaction Generating Sulfoxides and Sulfones
Thiosynergists Reaction Generating Sulfoxides and Sulfones


Although thiosynergists do not improve the melt stability of polymers during polymer processing, they are very efficient for long-term thermal aging applications. Sulfur-based hydroperoxide decomposers are mainly used in combination with hindered phenol antioxidants.

The most common commercially available thiosynergists are based on either lauric or stearic acid.

The table below lists the chemical types of common primary and secondary antioxidants and their major resin applications.

Suitable Polymer Applications Comments
Primary Antioxidants
Amine Rubber, some pigmented polymers, and polyurethane polyols Arylamines tend to discolor and cause staining
Phenolic Polyolefins, styrenics, and most engineering resins Phenolics are generally stain resistant and include simple phenolics (BHT), various polyphenolics, and bisphenolics
Secondary Antioxidants
Organo-phosphite Polyolefins, styrenics, and most engineering resins Phosphites can improve color stability and property retention, but can be corrosive if hydrolyzed
Thioester Polyolefins and styrenics The major disadvantage with thioesters is their odor which is transferred to the host polymer
Common Antioxidants by Chemical Type with Major Resin Applications


Multifunctional Antioxidants


Due to their special molecular design, multifunctional antioxidants optimally combine primary and secondary antioxidant functions in one compound.

Structure of Multifunctional Antioxidants
Structure of Multifunctional Antioxidants


Having several stabilizing functions combined in the same molecule, multi-functional antioxidants eliminate the need for co-stabilizers, such as phosphites and thioethers. This not only simplifies the formulation, but it also simplifies the storage, handling, and use of the stabilizer.


Hydroxylamines


Hydroxylamines may act as both primary and secondary antioxidants, providing processing stability, comparable to phenol/phosphite systems. In addition, they provide excellent light stability when used in combination with hindered amines and are resistant to gas-fade discoloration. They are capable of scavenging C-radicals.

The mechanism of radical scavenging by hydroxylamine is shown below.

Mechanism of Radical Scavenging by Hydroxylamines
Mechanism of Radical Scavenging by Hydroxylamines


Carbon-centered Radical Scavengers


Carbon-centered radical scavengers are effective in trapping alkyl radicals, provides powerful processing stability.

Mechanism of Antioxidation


Radical scavengers are antioxidants capable of trapping radicals. Scavenging of alkyl radicals would immediately inhibit the autoxidation cycle. Under oxygen deficient conditions alkyl radical scavengers contribute significantly to the stabilization of the polymer. Scavenging the extremely reactive alkoxy and hydroxy radicals is practically not possible.

Antioxidation Mechanism of Radical Scavengers
Antioxidation Mechanism of Radical Scavengers

Carbon centered radical scavengers, such as lactones and acrylated bis-phenols, are extremely effective in oxygen deficient environments.

Lactones


Lactones (Benzofuranone derivatives) are powerful radical scavengers. Even when added in small amounts, they help control melt stability during polymer processing.

Antioxidation Mechanism of Lactones
Antioxidation Mechanism of Lactones


Substituted benzofuranone are excellent radical scavengers. They are mainly used in combination with phenolic compounds and phosphite to provide materials excellent performance even at low concentrations.

Acrylated Bis-Phenols


Acting as traditional hindered phenols, acrylate functionalized hindered phenols are efficient C-radical scavengers.

Antioxidation Mechanism of Acrylated Bis-Phenols
Antioxidation Mechanism of Acrylated Bis-Phenols 


They are very effective in preventing styrene copolymers from crosslinking or degrading during processing, particularly under oxygen deficient conditions. They are usually used in conjunction with other stabilization chemistries.

Benefits of Antioxidants

Benefits of Antioxidants

As adhesives or adhesive components can undergo degradation during its life cycle, the main benefits of antioxidants like color stabilization, viscosity stabilization, or retention of adhesive properties will the preserve quality and performance of adhesive formulations.

Any factor in the life cycle leads to an undesirable change in physical properties of the unstabilized adhesives, creating manufacturing problems, poor product appearance and reduction of adhesive strength. The role of antioxidants is to prevent or retard all these undesirable changes.





Color Stability


A common undesirable effect of degradation is discoloration. Both natural polymer materials and synthetic polymers in adhesives &sealants undergo induced discoloration as an effect of degradation. Typically observed by an increase in yellowness or darkening, discoloration often appears prior to any measurable loss in physical properties.

It is usually the first critical indicator of damage that is observed, which determines the useful service life of the product.

Although color development occurs at widely differing degrees in various adhesives & sealants and may not directly correspond to a decrease in the mechanical properties, a change in appearance is often unacceptable in many adhesive applications.

Adhesive without AO Adhesive with AO-1 Adhesive with AO-2
Adhesive without AO Adhesive with AO-1 Adhesive with AO-2
Positive Effect of Antioxidant Systems on Color Stability of an Adhesive
after Heat-aging at 170°C

Although not illustrated, additional benefits of antioxidants may include improved initial color of the adhesive prior to thermal aging.

Viscosity Stability


An important requirement for adhesives and their raw materials is viscosity stability. It is essential through all stages of the product's life cycle:

  • Processing
  • Storage
  • Shipment
  • Use
  • Aging

In formulated adhesives, the resins, plasticizers, and other polymers present must be protected.

Depending on the predominant degradation mechanism, the product may either decrease or increase in viscosity if not stabilized. For polymers where cross-linking predominates, an increase in viscosity is often accompanied by gel formation or skin formation. Chain scission, leading to a molecular weight decrease on aging manifests itself by a decrease of viscosity.

The most appropriate stabilizer combination will depend on the selection of other formulation ingredients, as well as on the aging conditions. Suitable phenolic antioxidants in combination with thiosynergists or multifunctional antioxidants exhibit the best performance.

Skin Formation


For certain hot-melt adhesives, high temperature aging may lead to skin formation. This occurs with the cross-linking reaction. Skinning is based on irreversible oxidative damage to the polymer and cannot be corrected by subsequent mixing or addition of stabilizers.

During processing, skin formation in the adhesive can lead to blocking of pipes and irregular coating. The addition of the proper additive package during the manufacturing of the hot melt significantly retards skin formation.

High Temperature Aging in HMAs May Lead to Skin Formation
High Temperature Aging in HMAs May Lead to Skin Formation

Note: Originally atmospheric conditions become anaerobic under a cross linked skin, due to reduced diffusion of oxygen.

Retention of Adhesive Properties


From a product use standpoint, the most important characteristic affected by degradation is adhesive performance. It is not only necessary to maintain the adhesive's ability to adhere, but also to ensure that the adhesive bond exhibit minimal change in adhesion/cohesion properties over the service life of the adhesive. These essential properties can be adversely affected if the adhesive system is not adequately protected from oxidative degradation.

The changes in bulk mechanical properties often observed in adhesives and their components reflect either oxidative cross-linking and/or chain scission. For adhesives and sealants, the consequences of degradation via cross-linking include:

  • Loss of tack and peel strength
  • Embrittlement
  • Decrease of solubility
  • Loss of elongation
  • Hardening of the polymer

Some polymers that predominantly degrade via cross-linking are – polyethylene, EVA, polymers that contain butadiene (i.e. SBR, SBS), and tackifier resins..

Degradation via chain scission often results in:

  • Increase in tack
  • Loss of cohesive strength
  • Embrittlement
  • Loss of physical properties
  • Softening of the polymer

Polymers that predominantly degrade via chain scission are polypropylene, polymers that contain isoprene (i.e. NR, SIS), and butyl rubber.

Through the proper choice of antioxidant or antioxidant combinations, the loss of adhesion or adhesive properties due to degradation can be avoided or at least minimized.

Stabilization of HMA polyken probe tack after film aging at 70°C
Stabilization of HMA Polyken Probe Tack After Film Aging at 70°C

Stabilization of HMA peel strength after film aging at 70°C
Stabilization of HMA Peel Strength After Film Aging at 70°C


The figures above illustrate how the use of antioxidants in a SBS hot-melt adhesive can help maintain adhesive performance throughout the aging cycle.

Having learnt about different types of antioxidants available and their benefits, now explore the use of antioxidants in different types of adhesive and sealant formulations.

Antioxidants for Hot-melt Adhesives

Antioxidants for Hot-melt Adhesives

Hot-melt adhesives and the components used in their manufacture are generally solid or highly viscous at room temperature. As a result, hot-melt adhesives are exposed to elevated temperatures, which are required to melt the components, during both formulation and end use. When exposed to elevated temperatures the hot-melt adhesives become susceptible to thermo-oxidative degradation.

Signs of adhesives degradation during formulation and processing Signs of degradation during end use
  • Skinning
  • Viscosity changes
  • Discoloration
  • Char Formation
  • Loss of adhesion
  • Loss of cohesive strength
  • Loss of peel strength, etc.
  • Discoloration

Antioxidants play a key role in the hot-melt systems utilizing polymers are discussed below.


Recent developments in the bulk delivery and storage of hot-melt adhesives have allowed manufacturers and their customers to increase productivity, reduce costs and decrease waste. To make this possible, the adhesive is stored and kept at elevated temperatures of greater than 100°C.

Chemical market offers a wide range of stabilizers and stabilizer systems to ensure that the quality and performance of the adhesive is protected during shipment, through storage and use.

Ethylene copolymers (EVA, EBA, EMA, etc.)


Hot-melts based on ethylene copolymers (EVA, EBA, EMA, etc.) are generally used in the packaging industry as quick-sealing adhesives, in disposables, for book and journal binding, and in the wood working industry as assembling adhesives.

There is a strong trend towards high heat resistant and low color hot-melt adhesives. Low color is perceived as a good indicator of adhesive quality. It is influenced by the choice of raw materials, as well as by the stabilizer package.

Effectively stabilized raw materials are critical for the production of hot-melts with constant and reproducible properties. However, this does not mean that the basic stabilization is sufficient to overcome degradation occurring during compounding and end use.

Additional Stabilization Package for Better Performance


An additional stabilization package is required to retard discoloration, viscosity changes, and skinning of EVA hot-melts during compounding at high temperatures. Irganox® 1010, a hindered phenol antioxidant, provides sound performance with improvements attainable with the use of phosphite co-additives.

For example, stabilized EVA hot-melts based on hydrocarbon resin show good color stability after oven aging at 170°C. (See figure below)

Gardner Color of EVA Hot-Melt Adhesives Based on Hydrocarbon Resin After Aging at 170°C
Gardner Color of EVA Hot-Melt Adhesives Based on Hydrocarbon Resin After Aging at 170°C

During high temperature aging of hot-melts, skin formation and increased melt viscosity can occur. Skinning is based on irreversible oxidative damage to the polymers and cannot be corrected by subsequent mixing or addition of stabilizers. During processing, skin formation in the adhesive can lead to blocking of pipes and irregular coating.

As shown in the figure below, the addition of additives significantly retards skin formation, with no skin formation after 72 hours at 170°C, and provides excellent viscosity stability.

A significant decrease in melt viscosity of ethylene copolymer based hot-melt adhesives can also occur on aging. This problem can also be significantly reduced with the addition of antioxidants.

In EVA hot-melt adhesives, the addition of 25% hindered phenol (Irganox® 1010) and 0.25% phosphite (Irgafos® 168) was found to significantly reduce the decrease in melt viscosity during aging at 170°C.1

Melt Viscosity of EVA based Hot-Melt After Aging at 170°C
Melt Viscosity of EVA based Hot-Melt After Aging at 170°C


Various tackifiers such as hydrocarbons, rosin esters, terpene resins, or dicyclopentadienes, or other additives used in the EVA hot-melt adhesive formulation generally do not affect the choice of antioxidant.


Styrene Block Copolymers


SIS-, SBS-, and SEBS- / SEPS- based adhesives have enjoyed strong growth in the pressure sensitive adhesive market, including tape and packaging applications, as well as other markets.

Adhesive formulations based on unsaturated elastomers such as SIS and SBS are more sensitive to oxygen, ozone, light and heat than those based on EVA. Although the use of a nitrogen blanket offers some protection during mixing, additional stabilization is essential to protect the adhesives against oxidative degradation during preparation, storage and end use.

The following sections will focus on the potential degradation and necessary stabilization of SIS, SBS, SEBS hot-melt adhesives during some of the main steps of their product life cycle:

  • Hot-melt manufacture, simulated by dynamic aging
  • Hot-melt coating, simulated by static aging at high temperatures
  • Hot-melt end use, simulated by static aging at low temperatures


Stabilization of SIS Hot-Melt Adhesives


A hindered phenol antioxidant such as Irganox® 1010 is considered to be the workhorse of the antioxidant in the adhesives industry and especially in SIS hot-melt adhesives for all-around performance. The additions of a phosphite co-antioxidant will generally provide further improvement in color stability.

Dynamic Aging

During compounding, SIS hot-melt adhesives are subjected to high heat and high mechanical shearing, which induce chain scission of the SIS polymer. This results in a lower melt viscosity of the hot melt. To limit the melt viscosity decrease and the discoloration of the hot melt, it is necessary to use a high concentration of stabilizers, over 1%, possibly up to 3%.

Initial Melt Viscosity After Mixing at 170°C
Initial Melt Viscosity After Mixing at 170°C


Static Aging at High Temperatures

During static aging at high temperatures, you can see excellent color retention (See figure below).

Discoloration of SIS Hot-Melt Adhesives During Oven Aging at 170°C
Discoloration of SIS Hot-Melt Adhesives During Oven Aging at 170°C

Stabilization of SBS Hot-Melt Adhesives


SBS polymers typically experience increased viscosities as a consequence of oxidation. The phenol-phosphite blend antioxidants provide consistent, stable viscosity over the duration of high processing temperatures, whereas systems with no antioxidant and those with phenol-only undergo significant viscosity change.

A new antioxidant, Irganox® 1726, has been found to provide significant improvements in HMA type applications. This antioxidant combines both phenol and thio-type moieties in the molecule. The interaction of these groups within the same molecule yields a very potent antioxidant.

Stabilization of SEBS Hot-Melt Adhesives


Although SEBS is inherently more stable than either SBS or SIS, it too requires an appropriate stabilization system. A stabilization package consisting of 0.5% hindered phenol (Irganox® 1010) and 0.5% phosphite (Irgafos® 168) will provide improvements in color, reduced changes in viscosity, and postponing skin formation.

Polyolefins


APAO polymers are used as a base for the manufacture of a wide variety of hot-melt adhesive and sealant systems. Because APAO is compatible with many solvents, tackifiers, waxes, and other polymers, it is well suited for many adhesive applications in the packaging, construction, medical, and personal care industry.

As an example, APAO hot-melt adhesives are widely used for hygienic applications in the adhesive industry such as disposable diapers.

Similar to styrene-isoprene systems, APAO polymers degrade via chain scission during high temperature aging. The appropriate choice of antioxidant or antioxidant system can prevent the effects of degradation such as color formation and most importantly, viscosity decrease.

Phenolic antioxidants provide very good viscosity control, but use of co-stabilizers such as phosphites can provide some additional improvements with regard to color, while still maintaining good viscosity control.

Dynamic Aging at High Temperatures

Gardner Color of APAO-HMA after Aging at 200°C in Mixer
Gardner Color of APAO-HMA after Aging at 200°C in Mixer

Viscosity of APAO-HMA after Aging at 200°C in Mixer
Viscosity of APAO-HMA after Aging at 200°C in Mixer


Static Aging at High Temperatures

The use of a phosphite in conjunction with the primary antioxidant improves the long-term color stability of the APAO hot-melt adhesive.

APAO-HMA Color After Aging at 170°C
APAO-HMA Color After Aging at 170°C

APAO-HMA Viscosity After Aging at 170°C
APAO-HMA Viscosity After Aging at 170°C


Polyamides


Polyamide hot-melt adhesives are used in a variety of industrial applications. Possessing a broad range of adhesive properties, most polyamide adhesives are characterized for exhibiting:

  • Excellent adhesion to a wide variety of substrates
  • Bonding well to porous substrates
  • Good chemical and oil resistance, and
  • Good adhesion over a wide range of service temperatures.

Most polyamides, particularly the dimer acid polyamides, are prone to oxidation and must be stabilized by antioxidants. Although the stabilizers will vary with the polyamide, the most common antioxidants that are used are hindered phenols, phosphites, phosphates, and hindered aromatic amines. Generally a blend of antioxidants is used.

Phenolic antioxidant, is able to stabilize polyamide hot-melt adhesives against degradation occurring at high temperatures. These antioxidants will contribute to improved initial color and long term thermal stability, particularly at lower end-use temperatures.

Alternative stabilizers are aromatic amine, which may be used either alone or in combination with other antioxidants. Check out the advantages and disadvantages of these antioxidant systems in the table below.

Antioxidant System

Advantages

Disadvantages

 Phenolics

  • Best performance at lower temperatures
  • Good initial and long-term color performance
  • Less effective at high temperature conditions such as under-the-hood auto applications

 Aromatic amines

  • Good long-term thermal performance
  • Discoloration
  • Requires relatively high concentrations
Antioxidant Systems for Polyamide Hot-Melt Adhesives


Antioxidants for Solvent-based Adhesives

Antioxidants for Solvent-based Adhesives

Solvent-based adhesives principally consist of polymers such as natural rubber, polychloroprene, etc., dissolved in aliphatic or aromatic solvents.

Natural and chloroprene rubbers are unsaturated elastomers and are therefore susceptible to attack by oxygen and light. Although they are not exposed to very high temperatures during processing, as is the case in the preparation of hot-melt adhesives, they do need to be stabilized against degradation in end use, especially when used in thin films.

Therefore, stabilizers should be added in the formulation to protect the adhesive during both processing (drying) and service life. There are products in market which are effective antioxidants for such applications:

  • Natural rubber-based adhesives
  • Polychloroprene-based adhesives


Natural Rubber-based Adhesives


Loss of adhesive properties and color formation are important parameters to be measured after film aging.

Films obtained from the following formulation, 100 parts NR, 80 parts rosin ester and 1000 parts toluene, are aged at 70°C for several days. In figures below you can see, better tack retention and improved color retention.

Loop Tack (PSTC-5) of NR solvent-based Adhesive After Film Aging at 70°C
Loop Tack (PSTC-5) of NR solvent-based Adhesive After Film Aging at 70°C


Adhesive films (-35 Nm) coated on polyester foils are aged covered with silicon paper. All values are averages of 3 replicates (± 10% relative).

Yellowness Index of NR Solvent-based Adhesive After Film Aging at 70°C
Yellowness Index of NR Solvent-based Adhesive After Film Aging at 70°C


Polychloroprene-based Adhesives


Polychloroprene is one of the most versatile materials used as a base for elastomeric adhesives. Contact adhesives made of chloroprene films develop high bond strength, high tack level, and auto-adhesion.

To determine the appropriate antioxidant and antioxidant level to use, it is necessary to conduct testing based on the application requirements. Non-staining, non-discoloring antioxidants are recommended. They offer overall good stabilizer performance.


Yellowness Index of CR SBA Films After Aging at 80°C


» Discover Antioxidants for Solvent-based Systems Here!


Masking Tape Adhesive Formulation


The table below shows a typical formulation for a masking tape employing an antioxidant additive.

Component
Part per hundred of resin (phr)
Natural rubber
100
Polyterpene
41
Calcium carbonate
58
Resole phenolic resin
51
Antioxidant (butylated hydroxytoluene, BHT)
2
Solvent (hexane:toluene, 70:30)
450
Masking Tape Adhesive Formulation 


Antioxidants for Water-based Adhesives

Antioxidants for Water-based Adhesives

Water-based adhesives consist mainly of polyvinylacetate (PVA), polyacrylate, natural rubber, carboxylated-SBR, chloroprene rubber, polyurethane, etc.

Most water-based adhesives need to be stabilized to achieve and maintain good properties. Liquid additives allow easy preparation of emulsions and are therefore particularly well-suited for these applications.

Depending on the type of water-based adhesive applications, solid additives may outperform after heat aging:

  • Natural rubber water-based adhesives
  • Carboxylated-SBR water-based adhesives


Natural Rubber Water-based Adhesives


The time to penetration into paper is an important criterion for envelope manufacturers, and this time can be significantly increased (See figure below).

Time to adhesive Failure After Film Aging at 105°C of NR Water-based Adhesive Coated on Paper, Aged Uncovered
Time to adhesive Failure After Film Aging at 105°C of NR Water-based Adhesive
Coated on Paper, Aged Uncovered.


Carboxylated-SBR Water-based Adhesives


For adhesives based on carboxylated-SBR (X-SBR) latex, a universal stabilization system is a mixture 2 antioxidants in the ratio 1:1. This combination shows good synergism, preventing discoloration and retention of tack properties during aging. (See figures below)

Yellowness Index of X-SBR Films During Oven Aging at 135°C
Yellowness Index of X-SBR Films During Oven Aging at 135°C


Polyken Probe Tack of X-SBR Films After Oven Aging at 70°C
Polyken Probe Tack of X-SBR Films After Oven Aging at 70°C


» Explore the Antioxidants for Water-based Adhesives Here!

Antioxidants for Reactive Adhesives

Antioxidants for Reactive Adhesives

Reactive adhesives are typically characterized by the formation of permanent bonds between substrates to provide resistance to chemicals, moisture and heat. These adhesives exhibit high bond strength and long-term durability under severe environmental conditions.

Categories of reactive adhesives include:
  • Hot-melt Polyurethanes
  • Solvent based Polyurethanes
  • Epoxies
  • Modified Acrylics
  • Anaerobics
  • Cyanoacrylates
  • Radiation Curable


Hot-Melt Polyurethanes


Reactive hot-melts, typically combine the traditional good properties of conventional hot-melts with the well-known characteristics of polyurethane adhesives. This combination results in products with enhanced performance.

Unlike conventional thermoplastic adhesives, which remelt when heated, PUR hot-melts cure to a thermoset material, retaining their structural integrity once cured.

In addition, the cured adhesive has excellent temperature and environmental resistance, generally withstanding exposure to temperatures from -30°C to +150°C, while maintaining strong bonds between similar and dissimilar substrates.

Yellowness Index (Aging at 130°C)
Yellowness Index (Aging at 130°C)


As with other hot-melts, PUR hot-melt adhesives are also exposed to thermal oxidative influences and in part, also to mechanical stress caused by shear. The rapid discoloration of an unstabilized PUR hot-melt adhesive is evident in the figure above.

Solvent-based Polyurethanes


Polyurethane adhesives are produced in many grades such as one-component, two-component, dispersion and solvent-based, for use in different application areas. Solvent-based polyurethane adhesives are suitable for a number of different industrial and construction applications, having good adhesion to rubber, leather, textiles, metal, paper, wood, and plastics. In addition, solvent-based polyurethane adhesives are used for a wide variety of laminating applications.

As with other adhesives, stabilizers should be added in the formulation to protect the adhesive from degradation during both processing and service life. Some antioxidants are effective for solvent-based polyurethanes.

Antioxidants for Sealants

Antioxidants for Sealants

Sealants and caulks are used to fill joints, gaps, and cavities between two or more similar or dissimilar substrates. Today, the number of applications for sealants in construction, industrial and consumer markets are growing. These materials are required to seal and adhere to the appropriate mating surfaces over a wide range of temperatures, environmental stress and joint movement conditions.

To ensure that the physical properties of sealants are maintained, it is necessary to prevent or retard the degradation of the sealant.

  • Polyurethane-based Sealants
  • Silyl-terminated Polyether-based Sealants
  • Butyl Rubber-based Sealants

Polyurethanes-based Sealants


Polyurethanes (PUR) are the most versatile polymer systems used to formulate elastomeric sealants. Urethane sealants are sold as both one and two-components and generally have good elongation and recovery, excellent abrasion resistance and tear strength.

To prevent the oxidative induced degradation of a polyurethane sealant, which results in a loss of physical and mechanical properties, as well as an increase in the discoloration, antioxidants are recommended.

Gardner Color after Oven Aging at 130°C
Gardner Color after Oven Aging at 130°C

Silyl-terminated Polyethers


Silyl-terminated polyethers are designed for non-isocyanate and moisture curable polymer systems. Sealants based on Silyl-terminated polyethers have found regular use in both construction and industrial applications.

Standard one component formulation

 Component    Parts (wt)  
  Silyl-terminated polyethers   100
Plasticizer 55
 Calcium Carbonate 120
Titanium Dioxide 20
Thixotropic agent 2
Antioxidant 1
 UV absorber / Hindered Amine   2
Dehydrating agent 2
Adhesion Promoter 3
Hardening catalyst 2

For improved stability and resistance to thermal and UV degradation, formulations based on these products generally contain both an antioxidant and UV absorber, as seen in the following standard one component formulation.

Butyl Rubbers


Butyl rubber is a copolymer of isobutylene and isoprene. It has excellent air retention, energy absorption and water resistance. These unique properties make it very suitable as a base for sealants and sealing tapes. Most butyl compositions are one component and many are used in industrial and construction applications.

Typical butyl sealants, as well as sealing tapes compositions, incorporate a phenolic type antioxidant to prevent the thermal oxidation of the butyl rubber by chain scission. Stabilizer blends are also recommended.


Raw Material Stabilization

Raw Material Stabilization

Market demand for improved adhesive systems has resulted in a concurrent demand for the improved quality of the main components of many of these systems. Stabilization of key comnponents of adhseives promote retention of adhesion properties, viscosity and color are critical during long term storage and adhesive manufacturing. Let's discuss different polymer stabilization, tackifier stabilization and role of waxes.

Stabilizing Polymers (EVA, SIS, SBS, SEBS)

EVA Stabilization

After exposure to temperatures up to 170-200°C, typical adhesive grade ethylene vinyl acetate (EVA) polymers (vinyl acetate content: 18-40%) undergo discoloration together with gel formation and a viscosity increase. During static aging at 170°C, the viscosity increase can be reduced by the use of relatively low levels of antioxidants. The same antioxidants also reduce gel formation after dynamic aging (Figure 1) and hinder color development.

Gel Content of EVA After Compounding in a Sigma Blade Kneader at 170°C
Gel Content of EVA After Compounding in a Sigma Blade Kneader at 170°C

The stabilization of neat EVA resin generally with by 2,6,-di-t-butyl-p-cresol (BHT) antioxidant provides an unsatisfactory level of performance for the hot-melt adhesive formulator. The presence of skinning and discoloration requires additional antioxidant to meet the processing requirements of hot-melt adhesives. Hindered phenols, such as Irganox® 1010, will provide the stability required, eliminating skin and gel formation and discoloration.

SIS Stabilization


Unsaturated elastomers such as styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) block copolymers contain double bonds in their structure. Therefore, they are more sensitive to oxidation than saturated polymers. SIS and SBS, however, behave quite differently during degradation as illustrated in the figure below. SBS is predominantly cross-linked by oxidation, leading to gel formation (torque increase), whereas SIS displays mainly chain scission, leading to molecular weight decrease (torque decrease).

Torque Development of SIS & SBS at 180°C in a Brabender Plasticorder
Torque Development of SIS & SBS at 180°C in a Brabender Plasticorder


The difference in the degradation mechanisms must be considered when preparing hot-melt adhesives. Generally a combination of hindered phenols and phosphites provide the optimum thermal stability in these applications.

Controlling the degradation of neat SIS during mixing at high temperatures and under high shear forces is more difficult than SBS. As a result, it is necessary to use an effective processing stabilizer, stabilizer system, or a multifunctional antioxidant to efficiently stabilize neat SIS under these conditions.

Melt Flow Index of SIS After Oven Aging at 70°C
Melt Flow Index of SIS After Oven Aging at 70°C

*Sample containing BHT is completely destroyed after seven days aging: MFI cannot be measured (sample is almost liquid).

SBS/ SEBS/ SEPS Stabilization


SBS copolymers, as with EVA, have the tendency to crosslink under thermo-oxidative conditions. Simulating compounding conditions in a Brabender Plasticorder, cross-linking is observed as an increase in torque (viscosity). Induction time, which is the time to an increase in torque of 1Nm after the minimum, is a useful parameter for evaluating the processability of SBS.

Induction Time After Brabender Plasticorder Aging of SBS at 160°C
Induction Time After Brabender Plasticorder Aging of SBS at 160°C


As with SIS polymers, a higher molecular weight, more effective stabilizers, such as 2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-tertbutylanilino)- 1,3,5-triazine (Irganox® 565), used alone or in combination with a phosphite antioxidant provides superior color stability and resistance to gel formation.2

SEBS and SEPS based hot-melt adhesives are inherently more stable than SBS or SIS. Although antioxidants may not be necessary for the raw material, hot-melt formulations generally require an appropriate stabilization system for protection during melt processing.


Stabilizing Tackifiers

Natural Tackifiers Stabilization

Rosin-based tackifiers find continued use in hot-melt adhesives because of their unique property of providing compatibility to a wide variety of adhesive components. Unmodified rosins, however, are easily oxidized on account of their main constituent, abietic acid (See figure below), containing a conjugated double bond (a), tertiary carbon atoms (b), and allylic hydrogen atoms (c).

Sites of potential oxidation (abietic acid)
Sites of Potential Oxidation (Abietic Acid)


Because of their tendency to degrade, natural rosins must be modified to meet the stability requirements of the hot adhesives. Improved stability can be developed by several processes such as hydrogenation, disproportionation, and dimerization, followed by esterification of the rosin with glycerol or pentaerythritol.

Esterification Catalyst and Bleaching Agent

Various catalyst systems such as TNPP, calcium acetate, zinc acetate, and zinc oxide are used to accelerate the esterification reaction times. Catalyst offers the shortest reaction time and produces a resin with reduced color and lower acid number. In addition, they impart a "carry through" antioxidant effect by offering improved heat stability of the final resin.

Rosin Esterification : influence of catalyst type
Rosin Esterification : Influence of Catalyst Type


As shown in the autoxidation mechanisms of adhesives, hydroperoxides are formed during degradation. These hydroperoxides are stable at ambient temperatures, and may accumulate in the raw material during storage. Upon heating, they may decompose and induce further degradation. Therefore, the determination of hydroperoxide content is a good method for quality control for these materials.

Hydroperoxide Formation of Rosin Ester During Aging
Hydroperoxide Formation of Rosin Ester During Aging


Hydroperoxide content correlates well with melt viscosity and color changes after heat treatment – ROOH formation (See figure above) and color development (See figure below) present similar profiles during oven aging. The color increase of tackifiers after heat treatment is caused by hydroperoxide decomposition and subsequent chromophore formation.

Gardner Color of Rosin Ester During Aging at 40°C
Gardner Color of Rosin Ester During Aging at 40°C


A suitable stabilizer system should therefore be used to provide long-term oxidation resistance to the tackifier and to ensure consistency of its key properties. In static oven agent at 40°C of a rosin ester tackifier, the rate of hydroperoxide formation can be reduced significantly using a hindered phenol (Irganox® 1010), with even better results using Irganox® 5652.

Synthetic Tackifiers Stabilization


Synthetic hydrocarbon resins make up another major group of tackifier resins. Synthetic tackifier resins consist of hydrocarbon resins (C5-, C9-, aliphatic or aromatic), a collection of modified or special resins, primarily phenolics, and polyterpenes.

As with rosin esters, synthetic tackifiers tend to oxidize during shipment and storage unless they contain an antioxidant. This oxidation is usually observed as an accumulation of hydroperoxides, which greatly influences tackifier properties that relate to end-use performance. The addition of a hindered phenol antioxidant such as Irganox® 1010 can greatly reduce the hydroperoxide formation during static oven aging at 40°C.

The figure below exhibits hydroperoxide formation in a C5-aliphatic hydrocarbon resin after low temperature aging at 40°C.

Hydroperoxide Formation in a C5-aliphatic Hydrocarbon Resin after 15 Days Oven Aging at 40°C
Hydroperoxide Formation in a C5-aliphatic Hydrocarbon Resin after 15 Days Oven Aging at 40°C


Because hydroperoxides lead to discoloration at high temperatures, the use of antioxidants can also prevent discoloration during long-term storage.

Yellowness of a Hydrogenated Hydrocarbon Resin After Oven Aging at 180°C.
Yellowness of a Hydrogenated Hydrocarbon Resin After Oven Aging at 180°C.


Other benefits that are obtained with the use of antioxidants are excellent viscosity stability and skin prevention. These results are applicable for other types of synthetic tackifier resins as well. It is strongly recommended that all tackifier resins (natural or synthetic) should be protected by the addition of stabilizers as early as possible in their life cycle, i.e. in the resin manufacturing process, to prevent hydroperoxide formation during storage.

Waxes


The two main reasons to include waxes in hot-melt adhesive formulations are to lower cost and reduce viscosity.

Generally included at levels of 20-30% in hot-melt adhesive formulations, properties affected by the wax content are blocking characteristics, softening point, and open time. The high melting microcrystalline waxes (m.p. 90°C) and synthetic waxes (m.p. 100-120°C) are used because they contribute to high temperature properties and greater cohesive strength. The high melting paraffin waxes (m.p. 65-70°C) are used extensively in hot-melt coatings for their barrier, anti-blocking and heat seal properties, as well as their lower cost.

Influence of wax preaging on EVA hot-melt initial color
Influence of wax preaging on EVA hot-melt initial color


As with other organic materials, waxes are susceptible to autoxidation and will lose their original properties if overlooked. Please refer the attached photo illustrating, there is not only a significant improvement in the initial color of the wax, but also in the overall color retention of the wax after aging. Other benefits include enhanced viscosity stability and adhesive properties, all contributing to the improved performance of the final hot-melt adhesive.

Various antioxidants can be used to improve the thermal stability of waxes. However, the use of a high molecular weight, less volatile antioxidant such as Irganox® 1010 significantly outperforms the more volatile antioxidants during high temperature processing.


Selection of Antioxidants

Selection of Antioxidants

The selection of an antioxidant system is mainly dependent upon the polymer to be stabilized. For example, polypropylene is less stable than the polyethylene and will require a more effective stabilization package. In addition, the anticipated melt processing temperature and the expected service temperature will determine the type of antioxidant that should be used.

Below mentioned are crucial parameters that need to be considered when selecting an antioxidant for any particular adhesive formulation:


Other parameters that must be considered during antioxidant selection include:


Let's visit each factor separately.


Compatibility and Solubility


Nearly all polymeric materials require the addition of antioxidants to retain physical properties and to ensure adequate service life. The nature of the polymeric resin and the solubility of the antioxidant in the polymer will influence the kind and amount of antioxidants used. The table below links common antioxidant families that are generally used with base polymers in the adhesive formulation.

Base Polymer Antioxidant Type
Amines Hydro-quinones Phenolics Phosphites Thioesters
Acrylics and acrylic copolymers X

X

Epoxies (EP) X
Ethylene co- and ter-polymers: Solids (EVA, EMA) X X X X
Natural rubber (NR) X X X X
Polyamides (PA)

X

X X X
Polychlorovinyls (PVC, PVDC) X X
Polyesters (PE) X X
Polyolefins (PO) X X X
Polyethylene derivatives X X
Polypropylene derivatives X X
Polyurethane (PU) X X X X
Isocyanates X X
PU prepolymers X X X X
PU dispersions X
Polyols X X X
Thermoplastic PU X X
Polyvinyl copolymer emulsions (PVAc) X X
Styrene block copolymers (SBCs) X X X
Styrene copolymers (SBR) X X X X
Sulfone based X X
Synthetic rubber X X X X X
Chloroprene (CR) and chlorinated rubbers X X X X
Antioxidant Families Used with Base Polymers in Adhesive Formulations

Antioxidants should be relatively soluble in the polymeric matrix. If the solubility limit of the antioxidant in the polymer is exceeded, exudation or migration of the antioxidant out of the formulation will occur.

Migration of the antioxidant can leads to performance issues, such as:

  • Diminish adhesion by providing a weak boundary layer and
  • Create surface tackiness to prevent unwinding of rolled tapes and labels.


Efficiency and Concentration


Antioxidants are used in small concentrations. The concentration will depend on the type polymer and antioxidant used and the short- and long-term exposure temperatures expected of the polymer. Antioxidants are generally used in concentrations of about 0.05 to 0.10% as processing stabilizers and 0.10 to 0.50% as long term stabilizers.

Higher concentrations of antioxidants are used to stabilize polymers such as natural rubber. Saturated polymers have greater oxidative stability and require relatively low concentrations of stabilizers. The specific concentration is generally determined by trial and error, but the technical literature and antioxidant supplier can be of great assistance.

Antioxidants are generally added to each raw material component of an adhesive formulation that is prone to oxidation, and in most cases they are also added to the final adhesive formulation.Low concentrations of antioxidants (< 0.01%) are added to the raw material after synthesis to protect the polymer during shipment and storage. Antioxidants applied by the formulator will not undo any oxidative damage that might be caused to the raw materials via storage, processing, etc.

The overall efficiency of antioxidants in polymers is determined by its:

  • Chemical reactivity
  • Physical loss during processing & service life
  • Minimal effective concentration

In most cases, loss of the stabilizer by physical processes (evaporation, leaching) is more significant than the loss by chemical consumption.

The minimal effective concentration of an antioxidant is dependent on its ability to reach the polymer’s attacked sites by diffusing through the polymer.

  • For this to effectively happen, the antioxidant must be compatible and soluble in the polymer.
  • It must also have a low degree of volatility so that minimal antioxidant is lost during processing.
  • The efficiency of an antioxidant during its service life is dependent on the above factors along with its resistance to extraction into the environment (migration).

The amine antioxidants are generally more powerful than the hindered phenols. This is due to a cyclic process which the amine antioxidant undergoes in which a nitroxyl radical is regenerated and consumes more radicals. The concentration level of phenols added to plastics is generally between 250 and 3000 ppm, depending on the character and expected lifetime of the polymer substrate. The concentration level of amines and phosphites is slightly less.

A mixture of antioxidants that function by different mechanisms might be synergistic and provide a higher degree of protection than the sum of the stabilizing activities of each component. The most frequently used synergistic mixtures are combinations of radical scavengers (primary antioxidants) and hydroperoxide decomposers (secondary antioxidants).

The table below illustrates the melt stabilizing efficiency of single antioxidants and blends for mixing polypropylene at 190°C. The melt stability is measured as reduction in melt flow index measured at 230°C.

Antioxidant Trade Name
(Supplier)
Concentration by Weight,
%
Melt Flow Index,
g/10 min
Control (no antioxidant) 0 11.7
Tetrakis (methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate) methane 0.05 5.9
0.10 4.6
0.20 3.9
Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate Irganox® 1076
(BASF)
0.20 3.7
2,6-di-t-butyl-p-cresol (butylated hydroxytoluene) BHT (Various) 0.20 3.5
Tris(2,4-di-tert-butylphenyl) phosphite Irgafos® 168
(BASF)
0.05 7.7
0.10 7.2
Thioester heat stabilizer blend Irganox® PS 800
(BASF)
0.10 8.4
Alpha-tocopherol (Various) 0.05 3.8
0.10 3.6
Blend of primary and secondary antioxidants Irganox® 1010 and Irgafos® 168 0.05 + 0.05 4.3
Irganox® 1010 and PS 800 0.05 + 0.10 4.9
Melt Stabilizing Efficiency of Antioxidants in Polypropylene (processed in an internal mixer at 190°C). Melt flow index (MFI) measured at 230°C for 2.16 kg charge.


Temperature During Compounding and Service


Antioxidants are not equally effective at all temperatures. The figure below illustrates the effective temperature ranges for selected types of antioxidants.

Effective temperature ranges for antioxidants
General Representation of Effective Temperature Ranges for Selected Types of Antioxidants.3


Phenolic antioxidants are useful during both processing and as long term stabilizers. Irganox® 1010 is the most well-known and generally used product in this family, and many others have been developed for specific applications.

Hindered amines (more commonly thought of as being UV stabilizers) are also useful for long-term thermal stability below the melting point of the polymer. This is due to the fact that hindered amines work by a free radical scavenging mechanism. However, hindered amines are ineffective at temperatures above 150°C. For overall thermal stability, hindered amines should always be used in combinations with an effective melt processing stabilizer.

Phosphorus, hydroxylamine, and lactone-based compounds are most effective at elevated temperatures, and they are commonly used as melt process stabilizers during processing and compounding. They are not effective long-term stabilizers. Often these antioxidants are used in stabilizer systems with phenolics to provide protection during melt processing and lessen the workload of the phenolic component.


Physical Property Deterioration


All polymers are subject to degradation on exposure to oxidative environments. These degradative factors will cause substantial changes in properties such as shown in the table below.

Type of Degradation Initiating / Accelerating Factors Degradation Causes Resulting Change in Properties
Storage aging Surrounding environmental conditions Oxygen, light, heat, humidity Loss of elasticity and tensile strength
Aging due to heat Heat Oxygen Loss of elasticity and tensile strength
Aging due to light and weathering Light, UV, heat, humidity, surrounding environmental conditions Oxygen Formation of crazed surfaces, loss of elasticity and tensile strength
Contribution of soluble metal ions to catalyze oxidation Cu, Fe, Mn, Co, and Ni ions due to catalysts and other components Oxygen Rapid loss in elasticity and tensile strength
Flex-fatigue cracking Intermittent mechanical strains Oxygen, ozone, flaws Appearance of surface cracking patterns
Ozone cracking Continuous / intermittent strains Ozone Extensive cracking at right angles to force causing strain
Type of Oxidative Degradation, Initiating Factors, Causes, and Resulting Changes in Properties


Therefore, the antioxidant should be selected to minimize changes to bulk properties such as tensile strength, elongation at break, and impact strength.

  1. Changes in these properties will also affect the following adhesive properties, such as:
    • Shear & peel strength
    • Tack
    • Toughness
    • Flexibility, and
    • Resistance to low temperature environments

  2. Processing property changes due to oxidation include:
    • Skin formation,
    • Gel formation, and
    • Changes in viscosity

  3. Aesthetic changes include:
    • Discoloration and
    • Loss of clarity

Information on selection of antioxidants relative to these changes is difficult to find due to the many different base polymers, antioxidants, and processing conditions that are possible. The formulator usually must rely on trial and error in specific formulations.


Volatility


Most additives are melt compounded into the polymer after the polymerization stage. The melt compounding and downstream processing steps represent significant heat histories. Storage of the product can also be quite warm in certain climates. It is important that the antioxidant, or the products resulting from its reaction that may also provide stability, not volatilize from the polymer.

Volatility of antioxidant is related to both the molecular weight and the type of molecule. Antioxidants with higher molecular weights generally have a lower degree of volatility and are more resistant to extraction or migration during service life. For example, Irganox® 1010 is exceptional among hindered phenolic antioxidants. It has one of the highest molecular weights in its class and is non-staining and non-discoloring. The type of molecule, however, has a greater effect than the molecular weight. For example, the hindered phenols have relatively high volatility in comparison with main antioxidants of the same approximate molecular weight.

The table below shows the significant effect that different antioxidants can have on volatility in natural rubber when aged at 100°C.

Antioxidant

Trade Name (Supplier)

Loss on Heating at 100°C, weight %

4 Days

8 Days

12 Days

N,N’-Bis (1,4-dimethylpentyl)-p-phenylenediamine

Santoflex 77PD (Eastman)

7.50

13.20

17.00

N-phenyl-N’ isopropyl-p-phenylenediamine

Antioxidant IPPD (Hainan Huarong Chemical)

4.25

6.40

9.00

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine

Santoflex 6PPD (Eastman)

2.50

4.50

6.20

Polymerized 2,2,4-Trimethyl-1,2-dihydroquinoline

Sovchem TMQ (Sovereign Chemical)

0.15

0.20

0.20

Volatility of Selected Antioxidants at 100°C.4



Migration


Migration of antioxidants out of the polymer during service life is especially important in certain applications such as food packaging where the antioxidants can affect taste and odor and adhesives where the antioxidants can form a weak boundary layer to reduce adhesive strength.

Similar to volatility, migration will depend strongly on the type of antioxidant and its molecular weight. However, it will also be affected by the type of base polymer, the environment to which the stabilized product is exposed, and the temperature.

In a study of high temperature migration of Irganox® 1010 and 1076 from polyolefins in water, aqueous ethanol, and corn oil at high temperatures. Goydan, et. al.5 found that the migrations varied with the square root of time, and the diffusion coefficients could be correlated with temperature in an Arrhenius fashion.

  • Under comparable test conditions, antioxidant migrations were largest from PP for aqueous simulants, but for non-aqueous simulants the highest losses were from LDPE.
  • In both instances lowest losses were from HDPE.
  • In most instances there was little difference between the migration behaviors of the two antioxidants tested.


Form


Antioxidants are commercially available in several forms – solids of various forms, liquids, and emulsions or dispersions in water. The following table provides examples of the forms available for the most common families of antioxidants. 

Base Polymer

Antioxidant Type

Amines

  Hydro-quinones 

Phenolics

Phosphites

Thioesters

Crystals

X

X

Dispersions

X

X

Emulsions

X

X

X

X

Flakes

X

X

X

X

Granules

X

X

X

X

Liquid

X

X

X

X

Pellets

X

X

X

X

Powder

X

X

X

X

X

Several Available forms of Antioxidants


Antioxidants in the form of liquids and pellets are challenging the powder form. Advantages include low dusting, improved safety, and lower cost. However, some polymer raw material manufacturers need the additives as fine powders in order to achieve good premixing with their reactor product before melt compounding into the traditional pellet form.


Color


Upon oxidation phenolic antioxidants impart much less color than aromatic amine antioxidants and are non-discoloring and non-staining. AO-1010 especially is non-staining, non-discoloring, and effective in non-black, colorable compounds.

Phosphorous compounds are used most frequently in combination with hindered phenols for a broad range of applications in rubber and polymers. They are also able to suppress color development caused by oxidation of the substrate and the phenolic antioxidant. Unlike phenols and secondary aromatic amines, phosphorus-based stabilizers generally do not develop colored oxidation products.

Hindered amines are extremely effective in protecting polyolefins and other polymeric materials against photodegradation. They usually are classified as light stabilizers rather than antioxidants.

It is important that antioxidants do not provide unwanted color due to the transformation chemistries associated with preventing oxidation. Some antioxidants are prone to forming color by their very nature, while other antioxidants discolor only when they have been over oxidized. In these cases, pigments can be used to mask the subtle changes in the base color of the polymer.


Toxicity and Regulatory Approvals


The required approvals for the specific application (e.g., for food packaging or medical applications) needs to be considered when selecting an antioxidant. Safety is generally assessed by subjecting the antioxidant to a series of animal toxicity tests similar to those used to test food packaging or medical polymer additives.

Granulated and liquid forms of antioxidants are receiving greater acceptance to minimize the inhalation of dust. 

A number of antioxidants have been regulated by approval of government authorities as indirect additives for polymers used in food contact applications. Acceptance is determined by:

  • Subchronic or chronic toxicity and
  • The concentration expected in the diet, based on the amount of the additive extracted from the polymer by solvents that simulate food in their extractive effects.

Broad food contact regulation is increasingly a requirement for the successful introduction of a new antioxidant.


Cost


It may seem apparent that increasingly higher levels of antioxidant will provide greater protection. However, this is often not true. Unfortunately, these are relatively expensive materials and excessive concentrations could cause migration and degrading effects on other properties. Their use, therefore, requires a balance between the desired prevention level and the cost of the raw material.

A number of the world's leading producers of polymer antioxidants have introduced price increases in recent years. This is generally attributed to increasing costs of raw materials, energy, packaging and freight.

Without the price increases the companies say they will be unable to ensure product availability or invest in innovation and increased capacity to meet growing demand for antioxidants. However, price increases are somewhat limited by the shift of antioxidant production to Asia.

Tests to Determine Efficiency of Selected Antioxidants

Tests to Determine Efficiency of Selected Antioxidants

There are a variety of test methods available for selecting and testing the right antioxidant for an adhesive formulation and specific application. These test methods generally monitor the effectiveness of the antioxidant package by change in physical property on aging.

Regular determinations of color, mechanical properties, or other parameters are used to follow the changes of the specimen. Analytical tests such as oxygen index time by differential scanning calorimetry (DSC) can also be used to determine the effectiveness of various antioxidants.

Suitable antioxidants for plastics have to be selected according to two important phases of their life cycle:

  • Stabilization of the resin during high temperature processing, and
  • Stabilization during its service life

Analytical Test Methods


Oxygen induction time is a sensitive measure of the efficiency of anti-oxidative additives within the polymer. Oxidative induction experiments can be made with differential scanning calorimetry (DSC) equipment and they are used to quickly assess the relative efficiency of various antioxidant systems6.

The experiments are typically conducted under severe testing conditions, typically at temperatures above the melting point of the polymer. This method is not useful for predicting long term thermal stability.

For the oxygen induction test, a sample is heated under a nitrogen atmosphere, typically to 200°C. Oxygen is then introduced to the sample cell. After a certain time, the exothermal oxidation of the sample starts, and this is noted as oxidation induction time.

The figure below illustrates the results of an oxygen induction time measurement using differential scanning calorimetry. The method is standardized in ASTM D 3895 and DIN EN 728 for polyolefins. The better the performance of the antioxidant, the longer is the oxygen induction time.

An oxygen induction time measurement using differential scanning calorimetry
Differential scanning calorimetry (DSC) diagram to determine oxygen induction time 7


High Temperature Processing Test Methods


During any high temperature processing step, the polymer is exposed to shear and high thermal stress within a short period. This can typically be simulated by aging in the melt form (as a hot-melt tank). Or if the adhesive is extruded into a film or onto a carrier, the aging process can be accelerated with multiple pass extrusions in the laboratory where the extrusion step is repeated at least five to ten times.

After hours of aging in the melt tank or after each extrusion step properties such as melt viscosity and color can be measured. GPC, IR, and other analytical methods can be used to follow the polymer degradation in more detail.


Long-term Aging Test Methods


In oven aging tests to monitor service life at temperatures below the melt temperature, one can conduct tests at three or more temperatures and then extrapolate the oxidative degradation on physical properties to other temperatures using an Arrhenius plot.


References
  1. Fay, J.J. and Fagouri, C., “Antioxidants, UV Stabilizers, and Other Functional Polymer Additives for Hot Melt Adhesives”, 2003 TAPPI Polymers, Laminations, and Coatings Hot Melt Symposium, Orlando, FL.
  2. Earhart, N.J., et. al., “Thermal Stabilization of Adhesives”, Chapter 19 in Handbook of Adhesives Technology, 2nd ed., Pizzi, A. and Mittal, K. eds., Springer Publishing, New York, 2003.
  3. Encyclopedia of Polymer Science and Technology, John Wiley and Sons, Inc., New York, 2014, Vol. 5, p. 179.
  4. Antioxidants and Antidegradants, Arvind Mafatlal Group, Nocil Limited, http://www.nocil.com/Downloadfile/ETechnicalNote-Antioxidants-Dec2010.pdf, accessed May 9, 2016.
  5. Goydan, R., et. al.,” High-Temperature Migration of Antioxidants from Polyolefins”, Food Additive Contamination, Vol. 7, No. 3, pp. 323-337, 1990.
  6. Determination of the Oxidation Induction Time or Temperature, Netzsch, https://www.netzsch-thermal-analysis.com/us/materials-applications/polymers/determination-of-the-oxidation-induction-time-or-temperature-oit-and-oot/.
  7. Ullmann’s Encyclopedia of Industrial Chemistry, “Antioxidants”, John Wiley and Sons, New York, 2014, p. 30.

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2 Comments on "Selecting Antioxidants for Adhesives & Sealants"
Syed Danish A Aug 20, 2020
I am developing solvent based adhesive using Neoprene NPR 2008. what could be the best: 1) antioxidant 2) solvent combo Market is local adhesive.
iqbal A Dec 19, 2018
I am looking for Antioxidant to be used in EVA hot melt adhesive

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