There are actually several thermal properties that should be considered when deciding on the best coolant for the given application.
Eutectic Point: This is the point at which the material changes phase. This point can be used to approximate the temperature at which the coolant will keep the payload. In the case of gel packs, this is the point where solid becomes liquid, the melt point (0°C / 32°F). In the case of dry ice, this is the sublimation point (-78°C /-109°F) where solid becomes gas.
Dry Ice sublimates at a much colder temperature than the gel pack melt point, meaning Dry Ice will keep payloads colder. The eutectic point is important when considering the application.
Category |
Application Examples |
Coolant Choice |
Fresh (2° – 8°C) |
Fruits, Vegetables, Fresh Meats, Dairy, Certain Biologics |
Gel Packs Only |
Frozen (~0°C) |
Frozen Meals, Frozen Meat, |
Either or Both |
Sub Frozen (-10°C or below) |
Certain vaccines, Ice Cream, Certain Seafood |
Dry Ice Only[1] |
Latent Heat of Fusion or Latent Heat of Sublimation: This is the amount of energy or heat that the material can “absorb” per unit mass. You might think of this as a measure of efficiency or thermal capacity. If coolant were analogized as a battery, this would be how much charge it has.
Water based Gel Packs have a Latent Heat of Fusion of ~334 J/g, meaning every frozen gram of gel pack can absorb 334 joules (a measure of heat or energy) by thawing. Dry Ice has a Latent Heat of Sublimation of ~590 J/g.
So, every gram of dry ice sublimating absorbs 77% more heat than a gram of gel pack melting.
However, to do an appropriate comparison of the two coolants you would also have to consider the specific heat of the gel packs. If the gel pack was conditioned in typical commercial freezer (-20C) and the cut off temperature for the shipment was 8C, the gel pack would be absorbing 390 J/g[2]. Therefore, a more correct comparison would be Dry Ice absorbing 51% more heat per gram. Specific heat of the Dry Ice would not need to be considered as the CO2 gas is vented out and does not provide thermal protection after sublimation.
Said another way, to get equivalent coolant levels purely from a net heat absorption perspective, you would match ~15 lbs. of gel packs for every 10 lbs. of Dry Ice.
Rate of Sublimation / Melt Rate: This is a measure of how long the coolant is active. Melt and Sublimation rates are a function of the delta between the eutectic point and the ambient or environment temperature.
The larger the difference between the eutectic point and the ambient temperature, the more rapid the sublimation or melt rate will be. For example, the melt rate of a gel pack in a 2°C ambient environment will be very slow. The sublimation rate of dry ice in that same 2°C environment will be faster as a function of % of starting material.
At room temperature (no protective insulation) a one lb. block of dry ice may completely sublimate in 20 minutes, room temperature being very far from its eutectic point of -109°F. A frozen gel pack may last for hours at room temperature without protective insulation.
This speaks to not only the importance of insulation in packaging but also the storage and handling of the coolants during packing and fulfillment stages.
Summary
Different tools for different use cases. There are many variables and the best approach to selecting a coolant and the amount of coolant is to do testing.
Minus Works is happy to help with your cold shipping application. We have a thermal environmental chamber to facilitate scenario testing and Tag Temperature Loggers to help with field testing.
[1] Certain Phase Change Materials (PCMs) are also appropriate. Though they are not in scope of this write up, Minus Works also produces low temperature PCMs.
[2] Even after the gel pack is fully thawed, it is still offering thermal protection in its liquid state by adding to thermal mass. This is driven by the specific heat of water: it still takes a fair amount of energy (4.18 J) to heat a gram of water one degree. Dry Ice offers no additional protection after sublimation as it converts to gas and escapes the package.
]]>We’ll divide up our investigation as follows:
Before we discuss sustainability, some cursory definitions:
Dry Ice is solid Carbon Dioxide (CO2) or frozen CO2. We mostly know CO2 to be a gas but at very low temperatures (below -78°C / -109°F) it behaves as a solid. At temperatures above -78°C Dry Ice will convert directly to CO2 gas, skipping the liquid phase, in a process called sublimation. This sublimation process absorbs heat from its environment making Dry Ice it a good coolant material.
Gel Packs are typically water-based refrigerants that contain gelling components to increase the viscosity of the liquid refrigerant. Gel Packs take on the thermal properties of water, freezing and melting around 0°C/32°F. The gel characteristic aids in processing and can promote uniformity in melt rates, and, depending on viscosity, prevent leakage. The gel material used will vary based on the gel pack manufacturer. Some use eco-friendly biodegradable hydrocolloids and some using non-biodegradable chemical components like sodium polyacrylate. The gel is often contained in a plastic film, often consisting of LLPDE or other flexible plastics. The gel pack is frozen and the thawing action absorbs heat from the environment, making it a good coolant material.
Dry Ice vs. Gel Packs: Sustainability
There are two major aspects of sustainability to consider when comparing Dry Ice to Gel Packs. The first is the product and material waste and the second is the carbon intensity of the production processes.
Product and Material Waste:
Gel Packs are typically encapsulated in 3 to 4 mil plastic, usually LLDPE, which is difficult to recycle and typically adds to landfill. Dry Ice is often also encapsulated in a vented plastic bag (HDPE or LLDPE) when it goes into direct-to-consumer shipments but the gauge of the plastic is typically lower than that of gel packs, meaning less material and less waste.
Gel Packs use water that can be reclaimed by the eco-system or that goes into landfill depending on the materials used. Minus Works uses readily biodegradable, plant-based components to enable the fresh water used in its gel packs to work its way back to the eco-system. Some gel pack manufacturers use Sodium Polyacrylate[1] which binds to water molecules and can bring them all the way to landfill. Minus Works also uses pre-recycled LLDPE in its gel pack containment film, thereby lowering the demand for virgin single use plastic.
Besides the vented plastic bag, dry ice does not leave behind any material that needs to be recycled or disposed of. However, sublimating dry ice is of course releasing CO2 into the atmosphere. One could make the argument that this carbon would have been released into the atmosphere anyway but that depends on whether the Dry Ice was produced as a by-product or if CO2 was purposely generated for the Dry Ice. This is a key point and discussed in more detail below.
Production Process:
The first step in the Dry Ice production process is to obtain raw material: CO2 gas. Many Dry Ice manufacturers will acquire this CO2 as a by-product of other processes like petroleum refining or the Ammonia production (for Agriculture or Defense). In the Ammonia example, natural gas is burned to separate carbon and hydrogen atoms, hydrogen is then combined with nitrogen to produce ammonia. The leftover carbon is then combined with oxygen to produce CO2.
Unfortunately there are some Dry Ice processes that do not start with CO2 being acquired as a by-product but rather fossil fuels being burned for the purpose of generating CO2 for Dry Ice. These purpose-built CO2 generators allow the dry ice plant to produce CO2 gas from the direct combustion of fossil fuels such as diesel, kerosene, or natural gas. This is a key point in the sustainability calculus of Dry Ice and shows that all Dry Ice is not created equal from an environmental stand point.
The rest of the Dry Ice production process involves cooling and high pressurization and is energy intensive. Typical processes have a yield of 33% of end product dry ice to weight of starting CO2. If the producer does not have an adequate capture mechanism to reuse that unconverted CO2 gas, that wasted carbon is bled into the atmosphere and contributes to carbon emission. So, given the yield, one lb. of dry ice can generate 3 lbs. of CO2 emissions.
The gel pack manufacturing process is a two-part process. The mixing phase is where water and additives come together to create the viscous gel. Typically the additives make up no more than 2% of the total gel by weight. The second part is a filling operating where the gel liquid is encapsulated in containment film and sealed, creating the gel pack. The gel pack manufacturing process is not energy intensive however the gel packs then need to be frozen. Generally modern commercial freezers are very efficient but by its nature, freezing water is an energy intensive exercise.
Summary
Using gel packs generates more single use plastic waste, however, using Dry Ice can generate a significantly greater carbon footprint, especially if the CO2 feedstock for the Dry Ice is not a by-product but purposely generated for the Dry Ice manufacture.
[1] Sodium Polyacrylate is the super absorbent material used in diapers which is contributing to a major landfill issue.
]]>Climate Change, Ozone Depletion, Freshwater ecotoxicity, freshwater eutrophication, marine eutrophication, terrestrial eutrophication, acidification, human toxicity (carcinogen), animal toxicity, particulate matter.
Yeah, it’s all of them
]]>Phenolic compounds have been listed by the United States Environmental Protection Agency (EPA) and the European Union (EU) as pollutants of priority concern. This enlistment is due to the fact that these chemicals are noted to be toxic and have severe short‐ and long‐term effects on humans and animals.
Here’s a quick quiz, which of the following impact categories have phenolics (via processing and disposal) been linked to:
Climate Change, Ozone Depletion, Freshwater ecotoxicity, freshwater eutrophication, marine eutrophication, terrestrial eutrophication, acidification, human toxicity (carcinogen), animal toxicity, particulate matter.
Yeah, it’s all of them[1].
Steady on, don't despair. Minus Works has developed a plant-based, biodegradable, non-toxic Cold Pack that can put the Freezer Brick on ice for good. Our new Structured Gel (called M-45STR) is a shape-constant material that will freeze and thaw in its original form. Our Structured Gel has more in common with a solid than a liquid and will not flow.
One of the additional benefits to our Structured Gel is that we are able to produce a fully LEAK-PROOF gel pack. Should the containment film burst or be punctured, the gel will remain fixed in place with no spillage. Request samples and experience it for yourself: cs@minusworks.com.
M-45STR can be supplied as an option for all our existing gel pack products.
For an over-the-top demonstration of “LEAK-PROOF” please give the below video a watch.
[1] Source: “The environmental impact of phenolic foam insulation boards,” Danielle Densley Tingley MEng, PhD, Abigail Hathway MEng, PhD, CEng, MCIBSE, Buick Davison BEng, PhD, CEng, MICE, Dan Allwood BSc, PhD Institute of Civil Engineers Publsihing. Peer reviewed and accepted 12/11/2014
]]>
Let’s pause this sad thought experiment there. I’m sure many have picked up on the obvious parallel between the playroom and our society’s use of single-use products. One of the more subtle analogs however is that toys made from legos are not a bad comparison to biodegradable products in terms of their potential for reuse. It is obvious that a toy made from legos can be taken apart into individual blocks, which in turn can be used to build something new. The same is true with biodegradable products; the constituent components are the basic building blocks for not only new products but also life. Organic material is commonly broken down into CO2, salts and minerals, biomass, water.
Not only is the overflowing wicker basket (*cough* Landfill) unsightly but it is taking valuable resources and components away from the playroom and forcing us to buy more legos (or drill/mine/manufacture more resources). In our industry, the product happens to be gel packs and one of the biggest problems is that water, fresh water, is being taken out of our ecosystem and into landfill or is being chemically bound to materials that do not biodegrade. Unfortunately, one of the most common materials used in gel packs is the non-biodegradable Sodium Polyacrylate, which grabs onto water and holds on tightly all the way to landfill. Although there are some companies who claim biodegradability, upon closer inspection they are using cellulose derivatives that have been so chemically altered for enzymatic resistance that they are no better than the non-biodegradable materials.
To solve this issue we have developed a refrigerant gel that is Readily Biodegradable. “Readily” or “Ready” is an important distinction in the definitional use of the term biodegradability. Being Readily Biodegradable assigns the biodegradation process to a timeline, 28 days to be precise, where most of the organic matter must biodegrade after its been exposed to oxygen. Our Readily Biodegradable certification can be viewed here.
Assigning a timeline to biodegradability is important for two reasons.
As of today, Minus Works is the only gel pack manufacturer to have achieved the Readily Biodegradability standard. We hope others join us. The overflowing wicker basket is a poor example to set for our little playmates.
]]>To understand these phenomena let’s first go over radiant heat. Heat can transferred be from one object to another in a few difference forms: conductive heat, convective heat, or radiant heat. In very basic terms, conductive is when heat is passed from one object to another by contact. Think about a hot skillet passing heat to frying bacon. Convective heat is heat that is passed from on object to another through a fluid (fluid can be liquid or gas). Think about hard boiling an egg; the hot pot transfers heat through the water to the egg. Finally, radiant heat is when heat is transferred via electromagnetic waves. That just means heat transference in the form of visible light, infrared, etc. The sun emits radiant heat, microwaves emit radiant heat and any object that is hot will emit radiant heat.
When we try and insulate our perishables from heat we care about protecting against all three methods of heat transfer but the reflective properties of insulation do a very good job in mitigating the effects of radiant heat.
When we try and gauge a material’s ability to insulate against radiant heat, the most important property is called emissivity.
When radiant heat hits an object, three things can happen. The radiant heat can be reflected, absorbed, or transmitted. From an insulation perspective, if the radiant heat is absorbed it makes the insulation hot, therefore increasing the conductive heat transfer to the payload it is protecting. If the radiant heat is transmitted, well then it’s just passing that heat along to the payload and that’s not great either. The best outcome would be reflecting that radiant heat back out.
The property of emissivity measures the amount of radiant heat that is reflected vs absorbed[1] or transmitted (emitted) and is measured on a scale of 0 to 1. Emissivity of 1 represents all radiant heat being absorbed or transmitted and 0 represents all radiant heat being reflected. Smooth, shiny materials like aluminum foil or polished silver will have very low emissivity values, close to 0[2]. Rough, unpolished materials like asphalt or brick will have emissivity values close to 1.
Cold chain packaging companies use aluminum foil box liners because the aluminum foil has a low emissivity, meaning it reflects back thermal radiation and reduces heat transfer from outside the package to the perishables you’re trying to keep cold. Marathon runners are trying to stay warm at the end of their race but they can still leverage the low emissivity properties of a metallic surface because their space blanket is reflecting their body heat back at them. Those black leather seats in your car that scorch and scald sunburnt skin have a high emissivity value, meaning they are absorbing that radiant heat that is coming in from the hot summer sun. In the vacuum of outer space, since there is no conductive or convective heat transfer, the space craft’s low-emissivity heat shields are protecting against radiant heat, e.g. solar flares from the sun.
OK, so how can we use this to protect perishables? Minus Works has produced a product to capitalize on the insulating qualities of low emissivity material. The CORDILLERO Series is an innovative product that consists of a gel pack contained in a low emissivity metallic coating. We offer it in a standalone pack or in a saddle-bag /linked chain format. The chain is designed to be wrapped around the perishable payload, providing refrigerant benefits as well as thermally reflective insulation.
Like other Minus Works products, the CORDILLERO is engineered for the environment and uses recycled material in the film containment and 100% renewable, plant-based inputs in the refrigerant gel.
Bonus Points: Emissivity, also changes with surface temperature. According to the Stefan-Boltzman Law and blackbody radiation theory, the colder the surface temperature, the lower the emissivity for metals. Because the CORDILLERO Series uses a radiant barrier outside of the gel refrigerant, the cold gel actually further lowers the emissivity of our material, allowing the product to reflect even more heat and be an even better insulator.
[1] According to Kirchhoff’s Law of Thermal Radiation, heat emissivity and heat absorption are equal for opaque materials.
[2] Shininess is not always a good indicator of emissivity because when we perceive something as shiny, we are only seeing it in the visible light spectrum whereas with thermal radiation we should also consider the infrared spectrum. Emissivity is wavelength dependent. An example of this dynamic would be white paint – it reflects a lot of visible light but it emits a whole lot of infrared radiation, giving it a high emissivity and making it a poor surface for a radiant barrier.
]]>
If you have purchased or used a “sustainable” gel pack in the past few years you’ve probably seen the above decal – the famous #4 recycling symbol which corresponds to Low Density Polyethylene (LDPE). LDPE is used for gel packs because it's flexible, it has good sealing properties. it doesn't get brittle when cold (with proper additives) and it's relatively cheap.
According the the EPA’s “Advancing Sustainable Materials Management: Facts and Figures 2018”, only 4.3% of LDPE generated in 2018 was recycled.
That means over 95% of the “sustainable” gel packs out there either went to landfill or were or diverted to Waste-to-Energy facilities, which is where material is essentially incinerated to generate electricity in a very dirty way. Think burning petroleum. Bad news.
Why is the recycle rate for LDPE so low? It’s not because consumers don’t want to recycle – we see significantly higher rates of recycling for other types of plastic resin like PET, the plastic used for water bottles. Its because recycling LDPE is hard. Unfortunately most gel pack companies use virgin LDPE in their products but claim they are recyclable.
Why is LDPE hard to recycle? For one, it’s very inconvenient for consumers. In most municipalities in the United States LDPE is not curbside recyclable. If you are asking a consumer to empty their gel pack, clean the empty wrapper of contaminants, get into their car and make a trip to the nearest MRF (Materials Recovery Facility), which are referred to verbally as “Murphs”, then your recycling strategy is doomed. Even when said consumer gets to the MRF, chances are the MRF won’t take it.
You may be asking, LDPE is a widely used plastic - why isn’t LDPE curbside recyclable? It’s because it’s not profitable right now.
To understand that last sentence let’s level set on the recycling industry. The reality is recycling is a business and the laws of supply and demand apply. Plastic recyclers are manufacturers of commodity products and their success depends on securing a steady supply of uncontaminated raw materials and having a demand for their end product.
Right now, there is not enough demand for recycled LDPE to warrant a curbside recycling program. It is not sought after enough, it is not valuable enough.
There are also some supply and processing issues with LDPE. MRFs are primarily set up to handle current curbside recyclable plastics: #1 PET and #2 HDPE, which can be thought of as those screw top bottles, gallon milk jugs, etc. Their sorting and grinding infrastructure is set up for these hard plastics. LDPE Plastic bags and films get wrapped around conveyors and sorting screens and are generally difficult products to recycle if the MRF does not have the relevant infrastructure. There are certainly facilities that can handle LDPE but they are not ubiquitous and they do not draw from curbside recycling.
But don’t lose hope. This is all a circular reference, and we can use the same laws of supply and demand to fix this. If there were more demand and the recycled LDPE were more sought after, MRFs would invest in infrastructure needed to recycle it efficiently. A supply chain would be formed to get inputs to the MRFs, vis-a-vis curbside recycling programs.
We believe the answer to including LDPE in the circular economy is to create demand for recycled material. This will drive up the value of the recycled material and create the consistency of demand needed for recyclers and communities to invest in the infrastructure, education, and pathways needed to recycle #4 plastic.
Instead of just slapping a #4 symbol on our gel packs and washing our hands of its inevitable trip to the landfill, we are using material that’s already out there and proving a business model that thrives off of recycled plastic. There are some geographies and strategies that have been successful recycling #4 plastic such as in store drop off for grocery bags. We know it's possible. Minus Works has been sourcing high quality recycled content from these strategies to use in our gel packs. Why generate new plastic when we can reuse what’s already been generated?
Why don’t other gel pack companies use recycled material? It took some innovation and development to get where we are. We have constructed a laminate that uses recycled polyethylene on the interior of the film, with a thin layer of virgin material on the exterior so as to keep our FDA approval for direct food contact. We have learned a lot when it comes to using recycled LDPE and achieving the right oxygen and moisture barrier properties and the right sealing performance.
Want to incorporate recycled material into your packaging? Send us a note, we are excited to hear from you.
Email us: cs@minusworks.com
]]>Sodium Polyacrylate is also used by most of the traditional cold packs as the chief component in the refrigerant gel. It’s cheap, it mixes readily with water at a variety of temperatures with minimal mixing energy, and yields a very high viscosity at low concentrations. Only one problem: its terrible for the environment.
Sodium Polyacrylate is a salt of Acrylic Acid, which is derived from petrochemicals in a highly carbon-intense process. Sodium Polyacrylate is also not biodegradable and the vast majority of products containing this chemical will go to landfill and exist there for centuries. According the EPA, at least 4.1 million tons of Sodium Polyacrylate-linked products went into landfill in 2018, which is 1.4% of US total Municipal Solid Waste generated[1].
Unfortunately, the vast majority of single use cold packs use this chemical. While there are some cold pack manufacturers that have more eco-friendly products lines, Sodium Polyacrylate based products are still represent the lion's share of the cold pack market.
At Minus Works, we decided to take a different approach. All of our hydrogel formulations use renewable components. Our formulations are plant-based and are 100% biodegradable and compostable. We believe this sets up apart – we have an eco-responsible product line that does not sacrifice performance or force the customer to pay a premium.
Check out our MONTT Series gel pack and see how our plant-based formulation can keep your perishables well protected.
[1] These figures do not include the urine or fecal matter in baby diapers, only the materials used in the products. The waste generated would be even higher if bio-matter were to be included.
]]>OK, fine, so we know what we want to maximize but how do we do that? What properties affect this mumbo jumbo called “Latent Heat of Fusion”?
We at Minus Works believe that one of the major, controllable properties that influences latent heat of fusion is the quality of crystallization, a fancy way of saying the quality of the freeze.
By way of analogy, let’s say you are a medieval monarch constructing a castle. Would you prefer a castle wall that looked like this:
Or a castle wall that looked like this:
We believe B would be the better choice. The tight, densely packed structure of Wall B will take more energy to break down when defending against those barbarian hordes. In this case the structure of the castle wall is analogous to the molecular structure of a frozen cold pack. A more orderly, more densely packed freeze will require more energy to melt, thus more heat can be absorbed and we can protect those perishables longer against higher ambient temperatures.
One of the key ways to get orderly crystallization is to engineer the hydrogel material for good thermal conductivity. Good thermal conductivity allows for different parts of the cold pack to experience similar temperatures at the same time, which allows the entire cold pack to freeze at a uniform rate – producing similar sized crystals and an orderly lattice structure. We have also set up our manufacturing process to optimize for homogeneity or consistency in our hydrogel, which improves crystal size and distribution uniformity. Additionally, the mechanism through which water is bound within our hydrogel formulation works as a natural governor on crystal ice, promoting uniformity.
Another very important factor in crystallization is the rate of freeze. This is something that you the user of cold packs can influence. A more rapid freeze will create smaller, more uniform crystal sizes and a more densely packed “Wall”. The working theory here comes from the way ice crystals are formed. Nucleation (or seeding) of crystals occurs at higher probabilities at lower temperatures so when you get colder faster, you will have more crystal seeds in your cold pack that’s on its way to be frozen. On the other hand, with a slow freeze, you will have a lower nucleation rate or fewer seeds. With the slow freeze, the seeds that do occur will act as the foundation for the overall freeze so you will have fewer, larger crystals in your cold pack, which will end up looking more like Wall A. How do you speed up the rate of freeze? The easiest way is to turn the temperature down on your freezer – a colder freezer creates a higher temperature differential, which increases the rate of freeze. There are also specialty freezers (blast freezers, shock freezers) that are purpose built to achieve this.
Unfortunately, the inconsistencies and voids we see in Wall A happen frequently in lower quality cold packs, which produce a weaker product that melts rapidly and just adds weight and cost to your package. Often times it’s hard to tell high quality from low quality refrigerants. There is no substitute for field testing – we are very happy to provide sample material for you to trial in your cold chain application or run comparison tests for you with our in-house test chamber.
Send us a note, we're excited to hear from you.
]]>Let’s start with a painfully obvious relationship between heat and temperature.
As shown above, as the amount of energy or heat you put into a substance increases, the temperature of the substance increases. Painfully obvious, we know, but bear with us. The relationship will hold true until you hit a phase change or a change in the substance’s state of matter (think solids transitioning into liquid as an example), then something odd starts to occur and the relationship starts to look like this:
Even though we have continued to add energy to the substance, temperature is not increasing. Interesting! We add heat, but no temperature increase! What’s going on here?
Said in a very basic way, energy is being absorbed during this phase change. The energy is being spent on overwhelming the bonds that hold together the molecules of the solid, thus converting the substance into a liquid. Not to get too Thermodynamic-y but when we say the energy is “spent” we do not mean lost, it is getting converted into potential energy that is contained in the liquid. Liquids, as a state of matter, have higher potential energies than their solid counterparts.
The amount of heat required to do this phase change is called the Latent Heat of Fusion, or the enthalpy difference between solid and liquid states. In physics articles you might see Latent Heat of Fusion abbreviated at LF, chemistry it’s usually ∆HF. Again, the energy needed to transition a substance from solid to liquid is the Latent Heat of Fusion. All else equal, the greater the Latent Heat of Fusion, the more heat that will be absorbed, the better the cold pack will perform. The cold pack is acting as the heat sink, the buffer that absorbs the ambient heat so that your food does not.
Once the phase change is complete we see our typical heat/temperature relationship resume:
But let’s focus on this dotted line representing the Latent Heat of Fusion and use an example to cement our understanding. Let’s use water.
Here we see our substance starting off as ice but when it gets to 0C /32F, it starts to melt, absorbing energy and maintaining a constant temperature. When all the ice is melted and we have completed the phase change to liquid, we once again see temperature rise as heat is added. Water has a latent heat of fusion equivalent to 334 J/g. That’s joules (a measurement of heat) per gram (a measurement of mass). It’s important to keep in mind that mass plays an important roll in the total heat of fusion: the more ice (more mass) we have the more heat we are able to absorb.
By the way, this same concept can be related to Dry Ice except that instead of the Latent Heat of Fusion, we would be talking about the Latent Heat of Sublimation as the solid form of carbon dioxide transitions directly to a gas form and absorbs a lot of energy along the way. Dry ice has a Latent Heat of Sublimation equivalent to 591 J/g, almost twice that of water’s Latent Heat of Fusion. Dry ice is not always the better refrigerant however as it is classified as a hazardous substance during shipping, it releases CO2 into the atmosphere, its expensive, and it may keep your perishables a lot colder than you want.
But back to Latent Heat of Fusion. Latent Heat of Fusion varies depending on the material and the nature of the bonds and structure of that material’s solid phase. Each material has its own Heat of Fusion. This emphasizes the importance of material selection and formulation when creating a gel for cold packs. We need to stretch our dotted line as long as possible so as to absorb the most heat as possible. At Minus Works we produce cold packs using a hydrogel material that optimizes for the Latent Heat of Fusion, allowing us to keep perishables colder, longer.
Want more science? Check out Part 2 of this Series: Designing a High Performance Cold Pack.
Interested in Cold Packs for your application? Reach out to our sales team.]]>