Walk into any cold chain procurement meeting and ask what is inside a gel pack. The answer is almost always “gel” — as if the word itself were an explanation. In reality, gel ice packs are engineered chemical systems whose performance characteristics are directly determined by their formulation. Two packs of identical size, at identical freeze temperature, from different suppliers may deliver meaningfully different cold chain performance because their gel chemistry differs.
This article breaks down the chemistry and physics of the most common gel pack formulations, explains how formulation choices affect thermal performance and safety, and gives buyers the technical vocabulary to evaluate gel packs as engineered products rather than commodity consumables.
The Two Fundamental Functions of Gel Pack Chemistry
A gel ice pack must do two things simultaneously: hold water (or a water-based solution) in a stable, non-spilling physical form, and deliver that water’s thermal capacity — specifically its latent heat of fusion — to absorb heat from a payload and surrounding environment.
These two functions create competing chemical design requirements. The gelling agent that holds water in a stable form should, ideally, impose minimal penalty on the thermal properties of the water it contains. Water’s latent heat of fusion — 334 kJ/kg — is the primary performance metric. Any gelling chemistry that depresses the freezing point of water, reduces the water content of the formulation, or introduces significant thermal mass without contributing latent heat is a performance trade-off.
Sodium Polyacrylate: The Dominant Gelling Agent
The most widely used gelling agent in commercial gel packs is sodium polyacrylate (NaPAA) — a superabsorbent polymer also used in nappy cores and agricultural water retention products. Sodium polyacrylate can absorb 200–300 times its own weight in deionised water, forming a viscous hydrogel at very low polymer concentrations (typically 0.5–2% by weight).
The mechanism is electrostatic and osmotic. Sodium polyacrylate polymer chains carry negative charges along their backbone (carboxylate groups — COO⁻). These charges repel each other, causing the polymer network to expand, and attract water molecules via hydrogen bonding and ion-dipole interactions. The result is a highly swollen polymer network that immobilises water against flow without chemically bonding it.
Thermal implications of NaPAA formulations: At typical use concentrations (≤2% polymer), sodium polyacrylate gel is thermally almost identical to water. The polymer mass is negligible relative to the water mass it holds, so the specific heat capacity and latent heat of fusion of the gel closely approximates that of pure water. Freezing point depression from the sodium and carboxylate ions is minimal — typically less than 0.5°C — so a NaPAA gel pack at 0°C freeze temperature genuinely delivers a freeze temperature very close to 0°C.
This makes sodium polyacrylate the ideal chemistry for packs targeting 0°C or near-0°C performance: maximum water content, minimum freeze point depression, maximum latent heat capacity per kilogram of pack.
Limitations: Sodium polyacrylate gels are sensitive to ionic contamination. Hard water (high calcium and magnesium ion content) can cause syneresis — the gel releases water rather than absorbing it, because divalent ions crosslink the polymer network and reduce its swelling capacity. Industrial-grade NaPAA packs should be manufactured with deionised water to avoid this. Packs manufactured with tap water may exhibit gel degradation or liquid release over multiple freeze cycles as dissolved minerals accumulate in the polymer network.
Carboxymethylcellulose: The Food-Grade Alternative
Carboxymethylcellulose (CMC) is a cellulose derivative created by reacting natural cellulose with chloroacetic acid and sodium hydroxide. The resulting polymer is water-soluble at low concentrations and forms viscous solutions rather than the superabsorbent gels of NaPAA — but at higher concentrations (2–5% by weight), CMC produces a firm, stable gel with acceptable thermal properties.
CMC’s primary advantage is its regulatory status: it is approved as a food additive (E466) under FSANZ and internationally, and is considered non-toxic by ingestion. In a food cold chain setting where gel pack leakage could result in contact with edible product, a CMC gel formulation presents substantially lower food safety risk than a NaPAA formulation. While sodium polyacrylate is not acutely toxic, it is not a food-grade material and its presence on food surfaces is technically a food contamination event requiring product disposition assessment.
Thermal implications of CMC formulations: The higher polymer concentration in CMC gels (2–5% vs ≤2% for NaPAA) means a slightly higher proportion of gel mass is polymer rather than water. This marginally reduces the effective latent heat capacity per kilogram relative to NaPAA at equivalent freeze temperature. The difference is small in absolute terms — a 4% CMC gel has approximately 4% less water per kilogram than pure water — but is theoretically calculable and should be accounted for in precision thermal engineering.
CMC gels also have freeze-thaw characteristics that differ slightly from NaPAA. The cellulose backbone confers some structural memory to the gel, which can result in slightly firmer gel texture after multiple freeze cycles — a performance stability advantage for reusable packs.
Propylene Glycol Mixtures: Engineering Sub-Zero Performance
Standard water-based gels with either NaPAA or CMC freeze at approximately 0°C. For applications requiring sustained sub-zero payload temperatures — frozen food distribution, certain pharmaceutical products, seafood in transit — a different chemistry is required.
The most common approach is partial replacement of water with propylene glycol (PG) — a diol (C₃H₈O₂) with a freezing point of -59°C. By adjusting the ratio of water to propylene glycol, the freeze point of the mixture can be precisely tuned:
- 10% PG in water: freeze point approximately -4°C
- 20% PG in water: freeze point approximately -9°C
- 30% PG in water: freeze point approximately -14°C
- 40% PG in water: freeze point approximately -22°C
Propylene glycol is GRAS (Generally Recognised as Safe) in the United States and is approved as a food additive in Australia and New Zealand, making PG-water gel formulations acceptable for use in food cold chain applications where leakage risk exists.
Thermal trade-off: The substitution of water with propylene glycol reduces the total latent heat capacity of the gel. Pure propylene glycol has a latent heat of fusion of approximately 99 kJ/kg — less than one-third the value of water (334 kJ/kg). A 30% PG, 70% water gel has an effective latent heat of approximately:
(0.70 × 334) + (0.30 × 99) ≈ 234 + 30 = 264 kJ/kg
This is approximately 79% of the thermal capacity of a water-only gel. To deliver equivalent cold duration at a given heat load, a PG-water gel pack must be 26% heavier than a water-only pack. This mass penalty is acceptable for most cold chain applications but should be factored into shipper weight calculations for air freight, where payload weight directly determines freight cost.
Ethylene Glycol: The Chemistry to Avoid
Ethylene glycol (EG — the compound in automotive antifreeze) provides even more aggressive freeze point depression than propylene glycol and is chemically straightforward to use in gel formulations. It is also significantly toxic: the LD₅₀ in humans is estimated at approximately 1.4 mL/kg, and ethylene glycol poisoning causes severe kidney damage and can be fatal.
Ethylene glycol should not be present in any gel pack used in food or pharmaceutical cold chain applications. Under FSANZ regulations, ethylene glycol is not permitted as a food additive. Under TGA GDP guidelines, pharmaceutical cold chain packaging materials must not present a contamination risk to medicines.
The practical risk is that some lower-cost gel packs — particularly imports from markets with less stringent chemical regulation — may use ethylene glycol formulations without adequate labelling. The pack colour and external appearance of an ethylene glycol pack are indistinguishable from a propylene glycol or water-only pack. If you are purchasing gel packs for food or pharmaceutical applications and you have not received chemical formulation disclosure from your supplier, request it. Ask specifically: does this product contain ethylene glycol? A reputable supplier will answer this question directly.
Thermal Conductivity and Freeze Behaviour
Beyond latent heat capacity, the thermal conductivity of the gel material affects how quickly cold is conducted from the frozen gel core to the pack surface — and from the pack surface to the payload and surrounding air.
Frozen water-based gels have thermal conductivity of approximately 2.0–2.2 W/m·K, close to that of ice (2.2 W/m·K). Thawed gels approximate liquid water at 0.6 W/m·K. The transition between these values occurs across the phase change — a frozen pack is a much more efficient conductor of cold than a thawed pack, which matters for understanding the initial performance of a pack placed in a shipper.
At the start of a transit, a fully frozen gel pack against a product surface will conduct cold efficiently through the solid ice phase. As the pack thaws from the surface inward, the outer thawed layer begins acting as an insulating barrier between the still-frozen core and the product surface. This is the “mushy layer” phenomenon — well-known in heat transfer engineering — where a solid-liquid mixture around the phase boundary develops lower effective conductivity than either the fully solid or fully liquid state.
Pack geometry affects how significant this mushy layer becomes. A thin pack (high surface-area-to-volume ratio) will develop a proportionally thin mushy layer relative to its total cross-section. A thick, single-compartment pack develops a proportionally thicker mushy layer. This is another reason why cell design — dividing the pack into smaller compartments — improves sustained thermal transfer: it limits the depth of the mushy layer in any individual cell.
What to Look for in a Quality Gel Pack
The following specifications should be confirmed when evaluating gel packs for Australian cold chain applications:
Formulation disclosure: Gelling agent type (NaPAA, CMC, PG mixture), propylene glycol content if applicable, confirmation of absence of ethylene glycol.
Water quality: Deionised or purified water (not tap water) for maximum gel stability over freeze-thaw cycling.
Freeze point (measured): Not nominal. Request DSC (differential scanning calorimetry) data or equivalent validated test showing actual onset of phase change at the specified temperature.
Latent heat capacity (measured): DSC data quantifying kJ/kg of latent heat. This is the single most important thermal performance metric and is rarely provided on standard data sheets without request.
Cycle testing: freeze-thaw cycling count at which gel integrity was maintained (no syneresis, leakage, or polymer degradation). For reusable applications, this should be >50 cycles minimum.
SDS availability: A Safety Data Sheet for the gel formulation, confirming chemical composition and toxicity classification. Mandatory for workplace chemical management under WHS Regulations.
Conclusion
Gel ice packs are engineered chemical systems whose performance is determined by the chemistry inside them. Sodium polyacrylate gels deliver maximum water content and thermal capacity at near-0°C freeze temperatures. CMC gels offer food-safe alternatives with good structural stability. Propylene glycol mixtures enable precisely tuned sub-zero performance at the cost of reduced latent heat per kilogram. Ethylene glycol has no place in food or pharmaceutical cold chain applications.
Buying gel packs on price per unit without understanding the formulation inside them is the cold chain equivalent of specifying insulation without knowing its R-value. The chemistry determines the performance — and in Australian conditions, where heat loads are extreme and cold chain failures are consequential, understanding what you are actually buying is a non-negotiable engineering discipline.