Storage of Adhesives
A chapter on adhesive storage should be closely aligned with the current market practice. As such, a major part of this chapter focuses on the shelf life and safety aspect of adhesives. The adhesive shelf life is dependent on the adhesive system and the storage conditions, in particular, the temperature. For a stronger emphasis on the effect of temperature on adhesive shelf life, this chapter is subdivided into Storage at Room Temperature and Storage at Low Temperature.
Due to the flammability and possible health hazards of solvent-based adhesives and majority of the chemically reactive adhesives, their storage usually requires special safety measures.
Adhesive manufacturers are required to provide information on the shelf life and storage conditions in the product technical data sheet, as well as a description of the possible hazards in the safety data sheet (Material Safety Data Sheet, MSDS), e.g., in the event of a fire.
Due to the wide range of adhesive systems available, it is not possible to devise a standard storage guideline applicable to all adhesives. In this chapter, adhesives are divided into four categories: solvent-based, water-based, hot-melt, and reactive adhesives. For some adhesives, their physical characteristics should also be considered (e.g., liquid or solid in the form of powder, granules, or film). Considering the variety of adhesive systems, it is necessary to provide a short introduction on the adhesive to indicate the type of packaging and storage conditions needed and its potential hazards.
Similarly, in cases where the adhesive shelf life may be extended through storage at low temperatures, it should also be indicated on the packaging, although this applies to limited adhesive systems and applications. In order to explain the quantitative effect of low temperatures on adhesive storage, a brief subtopic on activation energy and temperature-dependent chemical reaction (Arrhenius equation) is provided.
KeywordsAdhesive storage Adhesives classification Shelf life Storage conditions packaging Quality assurance Safety data sheet Hazard potential Solvent based adhesives Reactive adhesives Low temperature storage Arrhenius equation
1.1 Basics of Adhesive Storage
The proper storage of an adhesive is a prerequisite to maintain the adhesive properties for the adhesion process and, beyond that, for the final bond performance (bonding strength, heat resistance, and other specific properties). The significance of the appropriate storage conditions is usually presented in the quality assurance documentation of the adhesive. Essentially, the adhesive is guaranteed by the manufacturer throughout the shelf life under the storage conditions specified in the product technical data sheet. The optimization of the adhesive shelf life normally commences with the adhesive development, with storage stability tests integral to the design specifications.
As a rule, manufacturers are required to inform customers of the adhesive shelf life under the recommended conditions in the product information. Within this period, the specified adhesive properties should be guaranteed for delivery as well as for the final bond performance after appropriate processing steps. The manufacturing date, shelf life, and suitable storage temperature of the adhesive should also be indicated on the packaging . While many products can be stored at room temperature, some may require low temperatures for a prolonged shelf life. The aspect of storage at low temperatures will be addressed separately.
Besides complying with international and national laws and regulations, institutional measures to ensure environmental protection and occupational health and safety must also be adhered to during adhesive storage. They are applicable throughout the supply chain, from production to delivery by the adhesive manufacturer, to distribution by the agent if necessary, and, finally, to the end user after processing. In the European standard, EN 12701 (European Standard 2001) titled “Determination of words and phrases relating to the product life of structural adhesives and related materials,” definitions and requirements applicable to structural adhesives with storage life limited by possible change of properties are clearly specified.
The compliance with safety procedures constitutes another notable aspect of adhesive storage. A series of adhesives are classified as hazardous materials due to the presence of highly flammable solvents or components presenting health hazards such as cyanoacrylates or acrylates containing high monomer contents. In addition, hardeners of reactive adhesives such as isocyanates, amines, or peroxides should be considered. An important reference is the REACH (Registration, Evaluation, Authorization and Restriction of Chemicals -EC 1907/2006) regulation, implemented by the European Parliament in 2007 to ensure that manufacturers take the responsibility to manage risks from chemicals and to provide safety information on the chemicals.
It is also an international practice to include exposure controls and other measures in the safety data sheet , so as to ensure the safety of personnel handling of the adhesives. The manufacturers are responsible for compiling the necessary documents and are required to provide them to customers at all times.
Special provisions and measures regarding the storage facilities of adhesives are established according to building and environmental regulations. Among them, fire control, which includes firefighting equipment, evacuation plan, and fire engine access route, is an important point for consideration. At the same time, attention should be accorded to the prevention of water pollution which may be caused by the leaching of hazardous materials into groundwater.
This chapter mainly covers the aspect of adhesive storage, according to the industrial standard. The storage of adhesives in households or offices, usually in small quantities (e.g., in tubes), is mostly subjected to other conditions and regulations.
1.2 Classification of Adhesives Based on Storage Criterion
Due to the existence of different adhesive systems and the potential hazards associated with each system, there are different types of packaging as well as storage conditions and shelf life. Adhesives can be classified according to specific criterion and properties. In the literature, one can find various classifications based on the assembly process, delivery form, adhesion mechanism, or application. Within the same adhesive group, the adhesives can be further categorized based on their physical states or characteristics: liquid, paste, or solid (through viscosity measurements and rheological characterization) or solvent based, water based, or those without volatile content (through solid content measurements). Another classification can be made based on the adhesion mechanism: physically dried adhesives from solution, solidified hot-melt adhesive, or chemically cross-linked single- and two-component reactive adhesives .
This chapter attempts to classify the different adhesives based on a few key points concerning storage. Based on storage criterion, adhesives can be classified into four main groups: solvent-based, water-based, hot-melt, and reactive adhesives. These groups mainly differ in their shelf life and potential hazards. Nonetheless, within such classifications, overlaps are possible. For instance, there are reactive hot-melt adhesives and reactive solvent-based adhesives.
Other considerations include the delivery form and packaging, which differentiate solid adhesives from liquid adhesives. Liquid adhesives are usually stored in sealed packaging, containers, barrels, or tanks at room temperature between 18 °C and 25 °C. Homogeneous products can be stored for a year or more due to the limited physical or chemical changes within the system. For nonhomogeneous products, due to the possible sedimentation of fillers or segregation of different phases during storage, stirring is usually necessary to return the system to its original state before use. Any premature chemical reactions caused by inappropriate storage conditions (e.g., higher temperature or longer duration) are irreversible, and they usually render the adhesives ineffective or unusable.
Solid adhesives such as hot melts are relatively easier to handle during packaging and storage, whether they are in the form of blocks, in crushed form as granules, in powdered form in cartons or sacks, or in the form of rolled sheets.
During the storage of reactive adhesives, the influence of humidity is another important consideration besides temperature, especially for moisture-sensitive bonding agents such as cyanoacrylates or single-component polyurethanes, which harden upon contact with moisture, like hardeners of reactive adhesives. Among special reactive adhesives are those which react under anaerobic conditions or in UV light and require storage under oxygen-rich or dark conditions, respectively. The packaging of such adhesives should also be adequate to prevent any undesired chemical reactions before use.
Reactive adhesives which are particularly sensitive require storage in a conditioned environment , with regulated temperature and humidity. To achieve fast adhesion during mass production, highly reactive adhesives containing catalysts are often required. However, they are not suitable for room temperature storage. Instead, low temperatures are required. Special measures of low-temperature storage and the affected adhesive groups will be addressed in Sect. 3.
2 Storage at Room Temperature
The term “room temperature” is sometimes misused. It was meant to define the temperature in a conditioned room, usually enclosed, such as an office or a work area, e.g., laboratory. The temperature range may vary according to the location, e.g., as narrow as 20–23 °C in a measurement laboratory or as wide as 18–25 °C in rooms where temperature sensitivity is not as crucial, such as storage hall for general goods. Unless specified, room temperature mentioned in this chapter refers to a temperature range of 18–25 °C.
Additionally, it is a basic condition in adhesive storage to keep the adhesive in its original sealed packaging before use, as far as possible, even if humidity does not always appear to influence the adhesive shelf life.
2.1 Solvent-Based Adhesives
Solvent-Based Adhesive Group
This class of adhesives consists of solutions of polymers, resins, and, where applicable, other additives in organic solvents. They have low to moderate viscosities and solid contents ranging from 15% to 40% in weight, which correspond to solvent contents ranging from 60% to 85% in weight. They are applied as PSA or contact adhesives.
The base polymer should be soluble in common solvents. The rate of dissolution is dependent on the polarities of both the base polymer and the solvent and the molecular weight of the polymer. Predominantly, acrylate, polyvinyl acetate, natural or synthetic rubber, polychloroprene, polyurethane, polyvinylpyrrolidone, or polyvinylchloride (PVC) constitute the base polymer for such adhesives. Nonetheless, solutions combining phenolic resins and rubber are also used.
Common solvents used are those which are easily available and cost effective. They should have a good solubility but low toxicity. Therefore, the use of chlorinated solvents such as dichloromethane or trichloroethylene has been effectively banned in the EU since December 2010 despite their good solubility, as they may be carcinogenic or pose health risks (e.g., optic neuropathy).
Properties of solvents used in solvent-based adhesives
Boiling point (°C) at 1,010 mBar
Flash point (°C) according to DIN EN 22719
Rate of evaporation ethyl ether = 1
The rate of evaporation is a ratio of the time required to evaporate a measured amount of liquid to that of a reference liquid (ethyl ether)
Storage and Shelf Life
Storage vessels of solvent-based adhesives which include jerry cans, drums, and containers with capacities of up to 30, 200, and 1,000 L, respectively, are made of metals such as aluminum or steel. Drums either have a sealable lid or are sealed but equipped with a faucet outlet. Sealed drums should only be stored upright. Otherwise, the drum lid may be easily damaged on contact with adhesives, especially those containing aggressive solvents, causing the contents to leak. Plastic vessels are not suitable as storage containers for solvent-based adhesives, as solvents may diffuse through the container walls – unless they are protected by a barrier coating.
Solvent-based adhesives are generally stable during storage, as they hardly undergo alterations as homogeneous solutions in sealed vessels. Hence, they usually have a recommended shelf life of up to 2 years. However, for some, the viscosity may change gradually with time. Though uncommon, some solvent-based adhesives may contain fillers or reactive components, which affect the storage stability through phase separation or cross-linking.
Solvent-based adhesives are classified as hazardous goods, as they contain a significant portion of highly flammable solvents. Thus, it is essential to comply with laws and regulations concerning fire prevention and firefighting.
Adhesive containers should be labeled with the appropriate hazard symbols . In storage facilities, no ignition sources should be present or allowed to come into contact with solvents or their fumes, and the generation of sparks should be avoided. Additionally, it is necessary for all electrical equipment to be explosion proof and all containers should be earthed. It is also mandatory for personnel to be equipped with safety shoes having conductive soles to minimize the accumulation of static electricity. Similarly, production equipment or vehicles should not generate any sparks.
Further information on this aspect may be obtained from national or international regulations such as REACH (the new European law concerning chemicals) or specialized contributions from corporate entities such as Fundamentals of Hazardous Materials by Denios, Germany Fundamentals of Hazardous Materials (2009/2010).
2.2 Water-Based Dispersion Adhesive
Water-Based Adhesive Group
Water-based dispersion adhesives consist of oligomers dispersed in water with surfactants as dispersing agents and stabilizers. Before dispersions, natural rubber latex and casein were used to manufacture such adhesives. Currently, polyvinyl acetate (PVAc) and polyacrylate serve as synthetic base polymers for water-based PSA, while polychloroprene and thermoplastic polyurethane serve the same purpose for water-based contact adhesives.
Compared to solvent-based adhesives, water-based dispersion adhesives have several advantages: environmentally friendly and nonflammable. In addition, a larger quantity of the adhesive can be dispersed in water without a significant increase in the viscosity. Thus, a solid content of 40–70% in weight is common, and this is almost twice of that compared to solvent-based adhesives.
Storage and Shelf Life
Water-based dispersion adhesives are mostly stored in plastic containers. Smaller containers with capacities of up to 30 L are made of polyethylene (PE), while larger containers which come in 100-L vessels or up to 800-L containers are made of polypropylene (PP). To ensure stability, the container thickness should be proportional to its size. In fact, such containers are often made of glass fiber-reinforced polypropylene. Larger containers can also be stabilized using metal casings or cages.
However, one drawback of water-based dispersion adhesives is the limited storage stability due to various influencing factors. In particular, they must be protected from frost, as freezing usually causes irreversible damage to the dispersions.
Furthermore, the stability of such dispersions is pH dependent. In fact, many of them (except PVAc dispersions) coagulate in neutral or acidic conditions, leading to the failure of the adhesive system. This can be avoided by the addition of neutralizing agents such as ammonium hydroxide or alkali hydroxides with pH values in the range of 9–10.
Water-based dispersion adhesives also face the problem of possible contamination through microbial attacks, especially when industrial water or mains water is used during their production. During microbial attacks, dispersions undergo partial coagulation, resulting in brownish discoloration and a strong odor. With that, the adhesive becomes unusable. Thus, a microbiological analysis is necessary to determine the bacteria count before storage.
With that in mind, most manufacturers of water-based dispersion adhesives recommend storage durations of 6–12 months at 5–25 °C.
As water-based dispersion adhesives do not pose any health hazards and are nonflammable, there is no critical concern associated with their storage. Despite so, precautions relating to the concerned chemicals and information provided in the safety data sheets should be taken note of, in the event of a spill. Contact with the skin and splashes into the eye should also be avoided, like most chemicals. Upon contact with chemicals such as ammonia, some dispersion adhesives emit an odor. For environmental concerns, contaminants such as alkaline or acidic materials should not be discharged into the groundwater.
2.3 Hot-Melt Adhesive
Hot-Melt Adhesive Group
Hot-melt adhesives range from viscous to solid forms in storage at room temperature. To allow proper application during the adhesion process, they have to be melted at elevated temperatures. Following that, they solidify on cooling to form strong bonds between the substrates. Polymers such as ethylene-vinyl acetate, polyamide, and polyester constitute the base polymers of harder hot-melt adhesives with higher melting points, while the softer ones consist of polyacrylates, styrene-butadiene copolymers, or thermoplastic polyurethane. However, as such polymers exhibit lower adhesion and tack properties, they are usually used in combination with tackifiers. While the soft and viscous hot-melt adhesives exhibit better tack than solid ones, the solid hot-melt adhesives provide a stronger bonding.
There are different delivery forms available: soft hot-melt adhesives are preferably packed into compact blocks or cartons, while highly viscous hot-melt granules are usually powdered with fine mineral powder such as talcum or silica to inhibit the clumping of the granules. This allows them to remain fit for storage. In contrast, as solid hot-melt adhesives do not form clumps easily, they are available in the form of powder, granules, or to some extent, sticks. Hot-melt adhesives also exist as films on release paper, winded into rolls. Such films can be precut into sheets, which are used immediately for mass productions, instead of being stored for later use.
Storage and Shelf Life
During manufacture, hot-melt adhesives are mostly filled in their molten state at high temperatures, after which they solidify upon cooling. They are available in compact forms in 200-L vessels or in cartons, which are coated with release agents, and are finally delivered as blocks. Pressure vessels are often used as special storage containers as they are suitable for adhesive storage under pressure, after the heating and melting of the adhesives. Many of these adhesives undergo crystallization during cooling after manufacture. However, this does not impact the adhesion properties, as crystallization is reversed during the subsequent heating process before bonding. Such reversible behavior is characteristic for thermoplastic materials.
Powdered and granulated hot-melt adhesives are usually prepared by crushing the solid form and then preferably packed and stored in 25-kg paper sacks. As hot-melt adhesives are sensitive to moisture, the paper sacks should be coated with polyethylene (PE) to prevent deterioration by the diffusion of moisture through the paper sacks. Polyamide is not used as a moisture barrier because it can absorb up to 3% weight of water during extended storage. Polyester is also not an option due to similar reason, though it has a lower moisture absorption capacity. The larger the surface area of the particle or smaller the particle size, the faster is the rate of absorption. During subsequent heating between 150 °C and 180 °C, the presence of moisture in the adhesive leads to the formation of bubbles or foam in the adhesive joint, which reduces the adhesive stability and leads to quality problems.
For adhesive tapes with liner, regardless of the size of the rolls, humidity should be taken into consideration. At a high humidity, the paper within the liner absorbs moisture from the environment despite the silicon interface, especially after drying. When this occurs, the paper tends to swell. This affects the structure of the paper, which may then be transferred to the adhesive film . However, this concerns only the outermost layers of the roll. This effect can be avoided either by air-tight packing of individual rolls or the conditioning of the entire storage room to a temperature of 20–23 °C and a humidity which is lower than 50%.
In general, by adhering to the moisture-related measures, hot-melt adhesives can be stored unchanged for a long period of time. The recommended shelf life of hot-melt adhesives ranges from 18 to 24 months, provided they are stored at 10–25 °C, under dry conditions.
Since most hot-melt adhesives do not contain volatile components, they are not highly flammable and do not pose any health hazard during normal handling. As such, they are also used in packaging for grocery. Unlike the rest, no special measures are required for an incident-free storage.
2.4 Reactive Adhesives
Reactive Adhesive Group
Reactive adhesives comprise all adhesives which undergo a chemical reaction during the adhesion process and in doing so, solidify. Single-component systems can react chemically in the presence of external influences such as heat, UV light, moisture, or anaerobic conditions. On the other hand, two-component systems usually react through the mixing of reactive components, e.g., resins and catalysts, at room temperature without external influences. During storage and before processing, reactive adhesives often exist in forms ranging from liquid to paste.
The most important types of two-component adhesives are moisture-sensitive polyurethane, comprising polyol resin and isocyanate catalyst, and heat-sensitive epoxy, comprising bisphenol-A or bisphenol-F resins and polyamide catalyst. Both single- and two-component adhesive systems are available in the market. Single-component reactive adhesives, which include cyanoacrylate and silicone, are also widely known, with the latter often being used as a sealant.
A particular adhesive group used for heat-resistant bonding is the single-component reactive adhesive, formulated as a combination of phenolic resin and nitrile rubber, in the form of heat-activated films (HAF) . These contain latent hardeners for the phenolic resins, which are activated at temperatures above 120 °C. The advantages include clean and precise handling.
UV-sensitive and anaerobic-sensitive adhesives also belong to the reactive adhesive systems, which should be stored under chilled conditions.
Storage and Shelf Life of Two-Component Reactive Adhesives
Resins and hardeners of two-component adhesives should be stored in separate metal drums, jerry cans, vessels, or larger containers. The quantity of each component to be stored is determined according to their mixing ratio. Polyurethanes and epoxy resins differ in the type of chemical reactions they undergo during the hardening process. On top of that, both systems are used in combination with different hardeners and have different storage requirements.
Polyurethanes are traditionally produced from the reaction of isocyanates and polyols.
Isocyanates (e.g., methylene diphenyl diisocyanate [MDI]), the most commonly used hardeners for polyurethanes have low viscosities and undergo crystallization in chilled conditions below 15 °C. At temperatures above 40 °C, isocyanates undergo a chemical reaction (i.e., dimerization), which increases their viscosities. In addition, upon exposure to moisture, they react with water molecules to produce polyurea and carbon dioxide gas, which results in outgassing and the formation of a layer of film. To prevent such occurrence, isocyanates must be stored at temperatures in the range of 20–35 °C and in the absence of moisture. Under these conditions, the shelf life is approximately 6 months.
Polyols, unlike isocyanates, are generally chemically stable. However, in many instances, they contain additives such as catalysts, fillers, and pigments for special applications. During storage, mineral fillers may sediment at different rates depending on their densities and the viscosity of the bulk solution. In such situations, the material should be stirred immediately before use to ensure homogeneity. However, this should be done without generating entrapped air in the mixture. Hence, it is essential to notify the user with a “Stir before use” label on the container.
Epoxy resins comprise bisphenol diglycidyl ethers. Having moderate to high viscosities, they are mostly delivered in jerry cans or vessels. While pure epoxy resins tend to crystallize upon storage for longer durations, this phenomenon can be reduced by mixing bisphenol-A with bisphenol-F. Often, such adhesives are also loaded with fillers. Hence, they should be monitored for any sedimentation during storage.
Hardeners are mostly polyether amines or similar amine composites. They have moderate to high viscosities and a typical amine smell.
The storage of resins and hardeners in sealed containers is less critical compared to that of polyurethanes, as every component is chemically consistent. Thus, they can be stored for 12 months at temperatures between 20 °C and 25 °C.
Storage and Shelf Life of Single-Component Reactive Adhesives
Single-component polyurethanes consist of prepolymers, a fraction of which is pre-reacted polyurethanes with terminal isocyanate groups. Due to their moisture sensitivity, they have to be stored away from moisture or possible contact with water, preferably in metal containers. They require similar storage conditions as MDI hardeners, although in the reduced forms, they are less reactive. In the absence of moisture, the viscosity increases during storage. As such, manufacturers usually recommend a shelf life of 3–6 months at 20–25 °C.
Single-component epoxy resins contain dicyandiamide as latent hardeners, which are activated at high temperatures – usually above 150 °C. As such, they belong to the class of heat-activated adhesives. They are available in viscous or paste forms, and those of even higher viscosities are available as films on release paper.
Apart from hardeners, some adhesives in this category also contain an additional activator, with which a faster rate of reaction can be achieved. This effectively shortens the duration needed for the adhesion process or lowers the activation temperature. However, this also leads to reduced storage stability of the adhesive.
Heat-activated films (HAF) can be stored for 12–18 months under dry conditions at room temperature, depending on the type of phenolic resins present in the system. For special applications, such as in the electronic industry, clean room environment and subsequent dust-proof packaging and storage are necessary.
Cyanoacrylate belongs to the single-component reactive adhesives. Due to its fast-acting characteristic, it is commonly known as “superglue.” In the presence of water, it undergoes rapid polymerization, causing the adhesive to set. Thus, it must be stored in leak-proof containers, completely protected from moisture or water during its storage. Otherwise, it may react prematurely and become unusable. This is guaranteed by the manufacturers through the use of suitable packaging , usually in small quantities from 5 g to 5 kg. When stored in their original packaging in dry conditions at 20–23 °C, the shelf life ranges from 6 to 12 months.
Anaerobic adhesives are derived from dimethacrylate esters and are often low in viscosities. They react only in the absence of oxygen and when in contact with a metal surface at the same time. In order to prevent premature polymerization, these adhesives should be packed in a way which allows them to be in constant contact with oxygen. Having a diffusion-resistant packaging can also ensure that oxygen does not diffuse out of the packaging during storage. They are mainly used as bolt adhesives or for the sealing and securing of mechanical devices. These adhesives should be stored in a cool environment. For instance, they can be stored at 5 °C in their original sealed packaging for 1 year.
UV-sensitive adhesives are based on acrylate-modified epoxy resins, polyurethanes, and, among others, polymers. They are single-component reactive adhesives, which undergo rapid polymerization upon irradiation with UV light with wavelengths ranging from 400 to 500 nm. These adhesives should be stored in small opaque containers of up to 50 mL in volume. They can be stored in their original containers for 6 months at 10–25 °C.
Due to their reactivity, reactive adhesives and hardeners have to be stored according to special regulatory requirements. The containers of such materials should be labeled with the appropriate warning symbols and possible hazards associated with the content, as indicated in the safety data sheets . Nonetheless, under normal storage in their original packaging, they are not expected to pose any hazards.
Nonetheless, mandatory precautions to contain a spill due to leaking or damaged packaging are necessary. Such precautions include secondary containment (e.g., trays) and spill kits or absorbents at accessible locations to soak up hazardous liquids for subsequent disposal. Personnel handling spilled adhesives should also be equipped with personal protective equipment (PPE) such as coats or gowns, gloves, safety goggles, and respiratory apparatus. In general, any contact with reactive adhesives should be avoided. While cyanoacrylates bond to the skin in seconds, epoxy resins and polyurethanes can trigger allergies. Other hazardous adhesive raw materials include hardeners used in the manufacture of polyurethanes. One example is methylene diphenyl diisocyanate (MDI), which poses health risks due to occupational exposure such as respiratory tract irritation and sensitization.
A more critical situation would be that of a fire. Suitable extinguishing agents or fire extinguishers should be available to combat fires which are small enough. Beyond that, larger fires may lead to uncontrollable chemical reactions with the accumulation of toxic by-products. For instance, single-component epoxy resins may trigger strong exothermic reactions with promoters at temperatures above 120 °C. In the presence of more than 1 kg of the materials, such exothermic reactions can hardly be extinguished. As a result, the surrounding may be heated up to temperatures of 500 °C or higher. Thus, in the event of a fire, the fire response team must be immediately summoned. The team should also be informed of the materials involved and provided with their safety data sheets .
3 Storage at Low Temperatures
3.1 Basics of Adhesive Storage at Low Temperatures
The main reason for adhesive storage under cool or cold conditions is to extend the adhesive shelf life. For some adhesives, storage at low temperature is absolutely necessary, while for some, it is recommended where longer storage durations are necessary. However, there are also some adhesives which can be damaged when stored under cold conditions.
For adhesives requiring storage at low temperatures, the same conditions should also apply during transportation from manufacturers to the customers. In the event that the adhesives are exported out of cold or temperate climates to warmer ones, insulated containers are essential.
To achieve the desired cooling, filled adhesive containers are stored in cooling chambers or freezers. In doing so, the cooling capacity of the adhesive should be balanced with the cooling efficiency and the temperature of the cooling system. As liquid adhesives are cooled, physical effects such as an increase in viscosity may occur through a reduction in the rate of filler sedimentation. Further cooling may even halt the sedimentation of the fillers. At the same time, solute components may crystallize out of the adhesive. Most importantly, cooling retards the chemical cross-linking reactions, thereby enhancing the adhesive shelf life.
The cooling of adhesives for storage followed by reconditioning to room temperature before use usually incurs additional costs, and for larger adhesive quantities, this may be a very time-consuming process. The tempering of full containers or vessels may take up to a few days depending on the temperature difference within the contents, especially when the bulk material is not stirred. Large rolls of reactive adhesive films , requiring storage at low temperatures, have similar behaviors. In practice, the conditioning of cooled adhesives before handling may take longer than expected, as the application of heat has to be conducted with caution. Overheating may lead to unwanted outcomes such as premature cross-linking in reactive adhesives or elevated vapor pressures, which present a flash fire hazard in the presence of solvent-based adhesives. Another effect is the condensation of water vapor on the cooled packaging , as the adhesives are brought into warm conditions. If the adhesives are applied too fast before conditioning, the condensed water droplets may lead to defective bonding between the adhesives and the substrates. These points should be taken into consideration during a good logistic practice.
The need for storage at low temperatures only concerns a small group of the commonly available adhesives in the market. In Sect. 1.2, adhesives are categorized into four groups: solvent-based, water-based, hot-melt, and reactive adhesives. In this section, key points of storage at low temperatures will be addressed based on these categories.
Most adhesives requiring cool or cold storage conditions are reactive adhesives, many of which are single-component adhesives. For some, cross-linking reactions begin even at room temperature, thus compromising the adhesive shelf life. Apart from fluid adhesives, this problem also concerns reactive adhesive films and prepregs (pre-impregnated sheets).
Unlike reactive adhesives, solvent-based adhesives seldom require such storage conditions as the reaction rates of reactive components, such as polyurethanes, epoxies, and phenolic resins, can be distinctly lowered by dilution with solvents. Thus, storage at low temperatures is only occasionally recommended for solvent-based adhesives. For normal storage in storage halls with higher ambient conditions, the temperature should be conditioned to a level not beyond the upper limit of room temperature, i.e., 25 °C (with the exception of up to 30 °C, depending on the solvent) through cooling.
Hot-melt adhesives do not require storage at low temperatures, because being pure thermoplastic materials, they are neither chemically reactive nor do they undergo physical changes at room temperature. Furthermore, they do not emit solvent vapors or other gases during their storage. Thus, at room temperature, these adhesives remain unchanged and can be stored for extended periods of time.
3.2 Temperature and Reactivity: The Arrhenius Equation
The shelf life of reactive adhesives can be extended by reducing their reactivity (i.e., rate of collision of reacting molecules per unit time). This can be achieved by increasing the free path of the molecules, through the dilution of the adhesive. However, for the dilution to be effective for this purpose, a reduction of the solid content by approximately 30–40% is necessary. Hence, in practice, dilution is only used as a means to reduce the adhesive viscosity. A more effective way to reduce the reactivity would be to decrease the mobility of the reacting molecules through cooling.
The Arrhenius equation describes the dependence of the chemical rate constant on the absolute temperature as follows:
k is the rate constant of the reaction.
A is the pre-exponent to indicate the number of molecular collisions per unit time.
EA is the activation energy of the reaction (J mol−1).
R is the gas constant, with a value of 8.314 (J mol−1 K−1).
T is the absolute temperature (K−1).
Effect of storage temperature of reactive adhesive on shelf life
In general, the trend observed fulfills the law of Arrhenius.
3.3 Storage Between Freezing Point and Room Temperature
Storage of different types of adhesives
Water-based dispersion adhesives
5–25 °C (Cooling chamber/room temperature)
5–15 °C (Cooling chamber/fridge)
Single-component adhesives polyurethane
15–25 °C (Cooling chamber/room temperature)
Methyl methacrylate adhesives
0–5 °C (Cooling chamber/fridge)
Phenolbutyral adhesive films
5–10 °C (Cooling chamber)
Phenolic resin/rubber adhesive films
5–15 °C (Cooling chamber)
UV-sensitive epoxy adhesives
5–15 °C (Fridge, dark)
3.4 Storage Below Freezing Point
Adhesives which require storage below freezing point are usually stored at temperatures between −20 °C and −25 °C. Such conditions are generally necessary for reactive adhesives, due to two main reasons as follows.
First, for normal reactive single-component adhesives, they are usually stored in large quantities after manufacture for a significant period of time, before they are filled into smaller quantities and delivered to the market. Hence, to eliminate any undesired chemical reactions during storage, low storage temperatures are essential. This group of adhesives includes cyanoacrylates and UV-sensitive epoxy resins. When stored at −20 °C, in dark conditions, they have a minimum shelf life of approximately 2 years.
Second, highly reactive single-component adhesives, such as catalyzed epoxy resins in the form of films, are viable at room temperature for only a few days. However, at −20 °C, they can be stored for approximately 6 months.
This category of adhesives also includes prepregs, which are used in the aviation industry for the construction of aircraft, e.g., to bond the molded parts together. Prepregs are made of woven glass fibers, or more recently carbon fibers, which have been soaked in and pre-reacted with a mixture of epoxy resins and hardeners. They usually undergo temporary storage at −20 °C and are conditioned to room temperature before use, where they are assembled with the substrates and cured. Apart from epoxy resins, phenolic resins can also be used as raw materials for prepregs.
In summary, this chapter has shown that, while the compliance to recommended measures during adhesive storage is absolutely necessary, it is usually manageable. The different types of storage and the corresponding shelf lives for different groups of adhesives and composites have also been addressed. In addition, the need to consider the potential hazards of the adhesives or their components during storage and transport is highlighted.
Generally, external influences on the adhesive characteristics during storage, such as temperature, humidity , and other physical effects, are controllable. Where necessary, measures such as suitable packaging and containers, as well as the conditioning of the storage halls, should be implemented.
On top of that, adhesive storage should also be taken into consideration in the development of new adhesives. Therefore, new adhesives should not only be tested against their applications but also their storability and shelf lives, before they are launched in the market. While such tests usually constitute the most time-consuming step during the design process, shortening the time taken for this step during the development phase could potentially result in customer claims later on.
It is a common practice to subject new products or bound parts to accelerated aging in order to simulate storage over long periods of time. This can be achieved through oven storage or environmental cycling tests for a defined period such as 6 weeks. Such tests are often helpful, except when there are other influencing factors apart from temperature and humidity. In such circumstances, it is difficult to establish the exact storage stability.
During storage, any unintended chemical reactions within the adhesives can also lead to problems during or after processing. Hence, new raw materials should be evaluated for potentially undesired chemical reactions to avoid disappointment when such problems arise during the storage of the end products.
Such measures also apply when changes to the adhesive contents are required, for instance, when replacements of hazardous raw materials with environmental-friendly substitutes are called for. In particular, there is an increasing drive toward the use of halogen-free adhesives in the electronic and automotive industries. In general, any change in adhesive formulations would require a series of testing for the storage stability, besides the standard property test profiles.
Currently, another challenge is the desire of many industries to have an adhesive with a shorter wetting-out time for the adhesion process and, yet, better adhesion properties. This would require a highly reactive adhesive, which normally requires storage at low temperatures to guarantee a reasonably long shelf life.