Pot life is referred to as the time until the paint is in workable condition after mixing the other components of a paint system. Shelf life is the time period during which the paint remains in workable condition on the shelf.
Both these terms are mainly based on the reaction between the polymer and the binder system. This helps formulators achieve the desired properties for the desired applications. However, pot life is specially used for two-part or multiple-part paint systems.
There are several factors that affect the pot life and shelf life of the paint system. These include the nature of polymer and binder, mixing ratio & volume, effect of temperature & pH, inclusion of additives, etc. But how do we differentiate between these terms.
Find solutions to all your queries related to pot life and shelf life of paints:
- What is the pot life of paints?
- What factors affect the pot life of paints?
- What test methods determine the pot life of paints?
- What is the shelf life of paints?
- What factors affect the shelf life of paints?
- What test methods determine the shelf life of paints?
What is the pot life of paints?
Pot life is the time until which the
viscosity of a paint doubles from its initial viscosity. This makes it inappropriate for painting or coating. It is a critical property of paint systems. As the pot lifetime lapses, the paint is no longer usable even after dilution or other corrections. This is because the chemical reactions between multiple components would already be started. Thus, the paint will not provide the desired results like bonding strength, adhesion, etc.
As the reactive or crosslinking multi-component systems are mixed with each other, an exothermic reaction occurs. Here the heat released cures the paint system. In other words, the viscosity increases as the crosslinking starts. This happens till a point, it cures (permanently hardens) the paint in the can. In fact, pot life can vary from seconds up to 24 hours or more depending on chemistry. Generally for epoxies the pot life is approximately 30 to 45 minutes.
Pot Life Reaction with Temperature and Viscosity of Two-component System
Understanding the two-part systems
The chemical reaction dynamics in a two-part system must be understood. This enables a better understanding of the pot life. It also helps to understand the implications of pot life on painting or coating. These systems include epoxy coatings, polyurethane, polyester, etc. The dependence of pot life is majorly on the
catalytic nature of the hardener and reaction propagation after mixing the two components.
Let’s take the example of epoxy coatings:
- The basic epoxy-amine reaction is an addition polymerization. Epoxy is a polymer that is made by the reaction of Bisphenol A with epichlorohydrin. There are different grades of epoxy resins that come as commercial grades. For e.g., Bisphenol A (BPA), Bisphenol F (BPF), Epoxy-Novolac (EN), and Epoxy-Phenolic (EP). All these grades show different properties and hence different applications.
- The second and critical component is the hardener, catalyst, or accelerator. These are also of different chemistries of hardeners like amine-based, amide-based, amidoamines, anhydrides, mercapto-based, etc.
Reaction mechanism of epoxy & amine-based coatings
- A primary amine has two active hydrogens for reacting with an epoxy group. Most primary amine curing agents that are used have more than one primary amine per molecule. This enables crosslinking and thus network development.
- A secondary amine will react with only one epoxy group. The reaction rate of the secondary amine with an epoxy resin is much slower than that of a primary amine.
- Tertiary amines which have no active hydrogens, will not react with epoxy resins. But they can cure epoxy resins catalytically, resulting in homopolymerization. Tertiary amines also accelerate the cure of primary and secondary amines with epoxy resins.
Reaction Propagation of Epoxy and Amine-based Coating
What factors affect the pot life of paints?
I. Effect of temperature on pot life & cure time
Generally, lower temperatures favor higher pot life and cure time and vice-versa.
| Temperature |
Pot Life |
Cure Time |
| ↑ |
↓ |
↓ |
| ↓ |
↑ |
↑ |
Effect of Temperature on Pot Life and Curing Time
II. The volume of mixing in two-pack systems
Small mixing volumes are recommended in the two-pack systems. This is due to better thermal dissipation and temperature control.
| Mass |
Pot Life |
Cure Time |
| ↑ |
↓ |
↓ |
| ↓ |
↑ |
↑ |
Effect of Mass (Volume) on Pot Life and Curing Time
III. Nature of hardener and polymer
The nature and chemistry of the hardener or cross-linked catalyst and polymer can have a great effect on pot life. For instance, the
amide type used as a hardener tends to offer longer pot life as compared to amine or anhydride types. Similarly, the use of pot life extenders or enhancers can increase pot life.
IV. Mixing ratio of catalyst or hardener to polymer
The right dosage of hardener to polymer amount is also critical. It can affect pot life, curing, and performance of the two-pack systems. For polyesters, the use of a hardener or catalyst is directly proportional to the reaction and inversely proportional to pot life. However, the hardener ratio is fixed for epoxies and urethanes. For instance, the amine dosage for epoxy is calculated by the PHR (parts per hundred grams of resin).
Curing Agent (Hardener) Dosage Calculation wrt Equivalent Weight of Epoxy and Amine
Where:
- PHR = Curing agent parts per 100 parts of resin (by weight)
- AHEW = Amine hydrogen equivalent weight of curing agent
- EEW = Epoxy equivalent weight of the resin
V. Nature of mixing of two components
Mixing processes and techniques can impact the pot life. Rigorous mixing can lead to excess heat generation. This can also lead to lower pot life as well as the curing of epoxy coating. A slow mixing process would help in achieving a longer pot life of two-part systems.
What test methods determine the pot life of paints?
- ISO 9514 — It determines the pot life of multi-component coating systems.
Testing Pot-life of 2K WB Epoxy Paint by ISO 9514
Source: Spektrochem Paint Technical Center
- ASTM D3056— It determines the gel time of solventless varnishes. It is also an indicative method for pot life and shelf life.
What is the shelf life of paints?
The shelf life of paints refer to the time period during which it will remain intact on the shelf. Generally paint life is more relatable with the time span of the sealed can of paint on the shelf. But, it can also be relatable with the leftover paint in cans, which are generally used for rework and touch-ups. The shelf life can be different for different types of paints. This is mainly based on the polymer or binder system of paints.
Shelf life of different types of paints
Based on the storage and conditions, paint can last from a few years to a decade and even longer on the shelf. Below are some of the paint’s shelf lives:
- Acrylic or latex paints – 2 to 10 years
- Oil, alkyd, or solvent paints – 2 to 15 years
- Epoxy resin – 6 months to 3 years
- Epoxy hardener – Up to 1 year.
What factors affect the shelf life of paints?
I. Rate of sedimentation and separation
Settling or sedimentation are key reasons for inferior and short shelf life. Shelf life can be improved by the:
Additionally, the
rheology of paint and the incorporation of anti-settling agents keeps the system intact.
Learn to Check & Avoid Sedimentation
II. Role of pH in paint systems
The alkaline nature of paints keeps the system in good condition. Especially in aqueous paints, the pH is more critical. It keeps the system protected from microbes and contaminants. It aids electrostatic stabilization of the paint system. A dropping pH may also signal a destabilizing system. It brings higher synergy with
dispersants,
biocides or preservatives,
rheology modifiers,
polymeric emulsions or even binders.
III. Presence of preservatives and biocides
In aqueous paint systems, biocides or preservatives (
in-can and
dry film) are added to give microbiocidal properties. This is against bacteria, fungi, yeasts, algae, and mold. Paint systems without biocides have a great
probability of gassing and spoilage. This means that the shelf life is impacted if there are no in-can or wet-state preservatives present in the paint system.
IV. Optimization of storage conditions
Storage condition is related to all the paints and coating systems. It can be aqueous, oil-based or two components. Temperature followed by moisture has a higher impact on paint shelf life. The
optimal storage conditions lead to longer shelf life. This enables better performance delivery of paint properties.
Factors Affecting Shelf Life of Paint (Aqueous Paints)
What test methods determine the shelf life of paints?
- ASTM D1849 — It determines the paint's stability and shelf life in controlled conditions.
Testing Package Stability of Paints by ASTM D1849
Source: Spektrochem Paint Technical Center
- ASTM D869 — It determines the degree of settling of paint. It estimates active shelf life by considering the settling rate and remixing.
References
- https://www.coatingsworld.com/
- https://www.ptm-w.com/
- https://ethox.com/product/ethoflex-er/
- https://concretecoloursystems.com.au/
- https://en.m.wikipedia.org/wiki/Epoxy