How to Enhance Stain Resistance in Adhesives and Sealants?

One of the primary causes of discoloration of adhesives and sealants is degradation due to UV exposure. UV light can initiate chemical reactions in polymers, which result in their breakdown and loss of properties.

UV degradation is not usually a problem if both adherends are opaque. However, it can be critically important when bonding glass, UV transparent plastics, or in designing joints where free edges of adhesive may be exposed to sunlight. UV resistance is a major issue with sealants, potting and encapsulating compounds, packaging compounds, labels and tapes, etc. when these materials must be used outdoors.

Ultraviolet radiation (UV) from sunlight is an important component in the outdoor durability of many adhesive systems. Although ultraviolet makes up less than 5% of sunlight, the photodegradation of polymeric materials is caused mainly by UV light. UV in the range of 315-290 nm causes the most damage to polymeric materials. The severity of UV damage depends largely on factors such as:
 

• Nature of the environment
• Geographic location
• Type of polymer formulation, and
• Duration of exposure
   

The effects of indoor and outdoor UV radiation exposure are similar. However, the radiation frequencies differ, and the intensity of radiation is less indoors. The main sources of indoor radiation are from florescent lamps and through window glass although window glass filters out some of the harmful radiation. Xenon arc lamps, sun lamps, and other artificial sources can emit harmful radiation when used indoors.

 

Mechanism of photodegradation

The energy in UV radiation is strong enough to break molecular bonds in many polymeric materials. This activity brings about embrittlement, discoloration and overall reduction in the physical properties of the material.

The amount of UV absorbed by an adhesive or sealant, the degree of degradation, and the nature of the chemical reactions that occur during degradation all depend on: 
 

• Molecular structure of the base polymer
• Substances present in the formulation
   

Efforts in improving UV resistance have concentrated on the relatively unstable thermoplastics. Examples include polyolefins, polvinyl acetate copolymers, styrene block copolymers, polyamides, and so forth. However, UV also affects thermoset polymers, such as polyurethanes and epoxies.

The process of UV degradation is perhaps better described as a photo-oxidation process. The photo-oxidation of most polymers proceeds by a radical chain mechanism. This involves initiation, propagation, and branching or termination as shown in Figure 1.
 

If the UV energy is not dissipated, it will begin to break the chemical bonds in the polymer molecule. The resulting lower molecular weight chain fragments will no longer exhibit the properties of the original polymer. The process also generates free radicals, initiating and propagating a chain degradation reaction. The end result can be embrittlement, discoloration, chalking, and loss of physical properties.

Linear or thermoplastic polymers may deteriorate by radical induced chain scission. UV light can easily break a C-C bond of a vinyl type polymer, producing smaller molecules. As a result of chain scission, cohesive strength is reduced, discoloration and/or staining can occur. 

Radiation can also produce chain branching. A photon or neutron can supply the activation energy necessary to provide crosslinking of polymeric molecules and the resulting increase in modulus. Under certain conditions this can lead to embrittlement, loss of tack and peel strength, and reduction in toughness. The resulting effects of chain scission and branching are summarized in Table 1.
 

The chemical consequences of photodegradation also include oxidation which introduces carbonyl, carboxyl, or peroxide groups into the polymer. This may also break the polymer chain and introduce unsaturation, hydrolysis of ester or amide groups, and other reactions. Thus, antioxidants greatly enhance the effectiveness of UV absorbers in stabilizing many polymers.

Two well-known examples of UV deterioration are the aging of polyethylene and natural rubber. Both show a loss of flexibility due to crosslinking by oxygen under the catalytic action of sunlight. Vulcanized rubber has only 5 to 20% of its possible positions anchored by sulfur crosslinks. Over time there are further crosslinks of oxygen by the reaction of UV and air. Further, the polymer gradually loses its elasticity and original performance properties.

For non-curing and solvent-based sealants, UV degradation is manifest as hardening of the surface by oxidation or in shrinkage through the loss of volatiles. Elastomer sealants are prone to surface degradation in these conditions. This could result in discoloration, staining, chalking, crazing, and surface cracking. This eventually leads to cohesive failure of the sealant.

In an ethylene vinyl acetate hot melt adhesive, the UV degradation results in loss of adhesive properties. These include tack, cohesive properties such as impact and tensile strength, and discoloration. Loss of adhesive properties for a PSA can result in a label or tape being readily peeled from a container or package.

 

Formulation alternatives
 

Adhesives and sealants used outdoors frequently require UV protection. There are four basic approaches to protecting an adhesive or sealant from UV light:
 

1. Inhibit the UV light from attacking the adhesive / sealant. This can be done possibly by using the substrate, itself, or the geographic location of the joint as a barrier to UV.
2. Incorporation of additives into the formulation that will stabilize it against the UV deterioration processes.
3. Alloying or blending two different polymers (one with better UV stability).
4. A combination of all approaches above.

Certain polymeric substrates can allow the passage of UV all the way through to the adhesive. However, the substrate can be surface coated, laminated, or coextruded with a more stable UV material. The substrate, if polymeric, can possibly also be compounded with materials that do not transmit UV light, as a result of their opacity, chemistry, or thickness. Such materials may be carbon black, TiO₂, pigments and fillers, or stabilizers as described below. However, this approach may not be technically feasible due to performance requirements of the substrate.

By proper selection or formulation of the substrate, it is possible to minimize the amount of UV that gets through to the adhesive. However, the edges of the joint remain exposed and may discolor, stain, and begin to peel from the substrates. Sealant applications, of course, generally have a significant amount of surface area exposed to the sunlight. Thus, discoloration and physical changes caused by UV deterioration are of special concern. A better approach at minimizing the effects of UV is through formulation of the adhesive or sealant, itself. In especially serious cases, both the substrate and the adhesive/sealant can be formulated for UV protection.

Light stabilizers, when added to the adhesive or sealant formulation, interrupt the sequence or reactions involved in UV degradation. Mechanisms include inhibition of the reactions by:
 

1. Screening or preferentially absorbing UV energy (and/or)
2. Quenching the excited state by reacting chemically with the free radicals and hydroperoxides as soon as they are formed

Several commercial sources of UV stabilizers available in our database. You should contact the supplier directly for recommendations with regard to specific applications, dosage, processing information, etc. The characteristics of these materials are summarized in Table 2.
 

The choice of UV stabilizer for a particular polymer and use is governed largely by economic factors and by technical considerations not related to ultraviolet absorbance. Some of the more important considerations are:
 

• High solubility of the stabilizer in the polymer
• Low rate of stabilizer loss from the polymer through volatilization, leaching, etc.
• Absence of chemical reactivity of the stabilizer with the polymer or other compounds in the formulation
• Low initial color and good color stability
• Low toxicity
• Ease of compounding
• Lowest possible cost consistent with the desired performance properties
   

In formulated adhesives, the base polymer is not the only component that may require UV protection. Other resins, plasticizers, tackifiers, extenders, etc. that are present may require protection as well. Oils containing small amounts of aromatic compounds are particularly unstable with regard to UV. The use of aromatic free "white oils" in thermoplastic rubber formulations, for example, will improve UV resistance markedly.

 

Many adhesives and sealants are prone to degradation by thermal oxidation. Depending on the base polymer, thermal oxidation can occur at relatively low temperature, including ambient storage.It can also occur at elevated service temperature or high temperature processing i.e., hot melt adhesives.

Polymers that are used in adhesive formulations differ considerably in their sensitivity to oxidation. 
 

• Acrylates, for example, are highly resistant to oxidation. They do not generally require antioxidants at ambient temperatures.
• Unsaturated elastomers are highly susceptible to oxidative decomposition. They require relatively high concentration of antioxidants for protection.
• Adhesive components especially susceptible to oxidation include base synthetic polymers such as ethylene vinyl acetate and styrene block copolymers, polyolefins, polyamides, natural rubber, polychloroprene, polyurethane, and butyl rubber.

Hydrocarbon additives, such as tackifiers and waxes, are also vulnerable to oxidation. They 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.

 

Mechanism of thermal oxidation

Degradation is initiated by the input of thermal energy. Several materials will lead to the formation of free radical species including:
 

• Impurities
• Catalyst residues, or
• Other inherently oxidation prone components

These rapidly react with oxygen to form peroxy radicals. They, in turn, react with hydrogen atoms in the adhesive matrix to form hydroperoxides and other free radicals.

Hydroperoxides are quite unstable. Left unchecked this reaction can lead to rapid discoloration and mechanical degradation of the adhesive or sealant. This mechanism of thermal oxidation is similar to UV degradation. 

 

Formulation alternatives
 

There are a number of stabilization chemistries that disrupt and inhibit the auto-oxidation 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 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
• A specific environment or market segment
   

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. 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.

Antioxidants can be divided into two basic classifications: primary and secondary antioxidants. 
 

• Primary antioxidants interrupt oxidation degradation by tying-up the free radicals. They react rapidly with peroxy radicals to break growing chains.
• Secondary antioxidants 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.
   

Table 3 lists the chemical types of common primary and secondary antioxidants and their major resin applications. 
 

Every stabilizer has a specific temperature range in which it develops its optimum properties. For this reason, antioxidant systems (stabilizer packages) combining two or more materials are often used. Such blends are commercially available as a single additive. The most effective mixture will combine a free radical inhibitor (primary antioxidant) with a peroxide decomposer (secondary antioxidant). The free radical inhibitor retards the initiation of reaction chains, but some hydroperoxide is nevertheless formed. A peroxide decomposer available to react with the hydroperoxide prevents it from decomposing with free radicals.

 

UV and oxidation are the most common sources of discoloration in adhesives and sealants. However, there are several other sources that can exist. These generally involve chemical reactions either within the adhesive, at the adhesive-substrate interface, or with the service environment.

Antioxidants, if not carefully chosen, can actually contribute to discoloration. These reactions generally involve hindered phenols, which are commonly added to polymers as primary antioxidants or processing stabilizers. During the process of stabilizing the polymer, the antioxidant can generate molecules that impart color to the polymer. Pigments can also interact with the antioxidants to cause yellowing. Among other common antioxidants, the arylamines tend to discolor and cause staining. However, certain phenolics are generally stain resistant and include simple phenolics (BHT), various polyphenolics, and bisphenolics.

Another source of discoloration is from pollutants in the atmosphere. Antioxidants, such as hindered amine light stabilizers, exposed to the oxides of nitrogen, for example, can cause discoloration ranging from yellow to red. This type of discoloration normally occurs in the absence of light. The color is usually concentrated at the most exposed surface of the adhesive or sealant.

A common source of discoloration is the migration or absorption of colored substances from nearby substrates. This type of effect is commonly observed with sealants on substrates having low molecular weight components that can leach-out during aging. It should also be noted that the ingredients within the sealant (e.g., plasticizers, colorizers) could migrate from the cured sealant during aging and stain the substrate.

Sealant systems can discolor by a variety of other mechanisms. For example, a sealant's color could change due to dirt accumulated on a tacky surface via electrostatic attraction. This is relatively common with certain silicone sealants. Urethanes and butyl sealants can also darken and change color from UV exposure and dirt pick-up.

 

Discoloration or staining in adhesives and sealants is a complex issue primarily caused by UV exposure and thermal oxidation. While not always critical, it can be detrimental in certain applications, affecting both aesthetics and performance. Formulators can mitigate discoloration or staining through careful selection of base polymers and incorporation of stabilizers such as UV absorbers and antioxidants. Understanding the various mechanisms of discoloration and available solutions is crucial for developing high-performance, long-lasting adhesive and sealant products.