Chemical Resistance: How to Make Soap Safely Using Polypropylene and Stainless Steel

Updated: Apr 28

It is important to use the proper materials during soap making in order to prevent injury and damage. Let's explore what materials can be used to make soap safely by reading the following post!

Sodium hydroxide or NaOH is a caustic and highly alkaline chemical. When added to water, the Na+ and OH- ions dissociate. This part of the reaction is actually endothermic or energy-requiring. After they dissociate, the Na+ and OH- ions are immediately surrounded by water molecules, which are the exothermic or energy-releasing part of the reaction. The exothermic reaction can reach temperatures above 100C or more, and depending on other chemical additives and concentrations, it can be even higher. These considerations are also true for potassium hydroxide or KOH, although KOH is slightly less exothermic.

Because of the highly alkaline and caustic chemical composition of hydrated sodium and potassium hydroxide, in addition to the exothermic reaction produced when combined with water, special consideration is required when it comes to chemical reactivity and soaping materials. This leads us to ask some new questions- What exactly are these special considerations? How do we know what type of mixing bowls and containers can be used? How can you find out if a mixing spoon or spatula is safe for use? Can metal be used?

To find the answers to these questions and more information about chemical reactivity, we can use researched and reported information compiled into what is known as "chemical resistance data" or CRDs. By reviewing CRDs we can make an informed decision about the materials we use during soap making.

Materials that have an "A" rating are considered "resistant" and have very little to no chemical reaction. Those with a "D" rating are highly reactive and are not recommended for use. An example of a material that has a D rating with sodium and potassium hydroxide is aluminum. Aluminum should never be used during the process of soap making, as it can be very dangerous. When sodium hydroxide and aluminum are combined, sodium aluminate is formed. This chemical reaction is very exothermic and is accompanied by the rapid evolution of hydrogen gas. This is a very dangerous reaction because it is highly corrosive and produces an extreme amount of force due to the gas, especially if combined in an enclosed container. The corrosive nature and production of gas are what help clear drainpipes and are the functioning and chemical mechanism of drain cleaners commonly made from sodium hydroxide.

Glass is another chemical that should not be used with lye because it slowly reacts with the sodium hydroxide over time to form sodium silicate and becomes damaged with use. In addition, glass can have tiny imperfections, impurities, or poor formulations, and it can experience thermal shock and thermal stress. When there is a thermal expansion on one side of the glass, it induces stress. When this stress reaches a level above the yield strength of the material, it will cause the glass to crack. When glass cracks, it shatters due to glass's amorphous and brittle nature. Some soap makers call the tiny little stresses "etching." Eventually, the weakened areas or thermal stress will cause the container to break. Glass can also be knocked over or dropped, which can also cause it to break and glass pieces can end up everywhere with the potential for serious harm and product contamination. Shattered glass and hot caustic lye are a very dangerous combination.

But what about Pyrex containers? Everyone used to use Pyrex mixing containers, even me, and they are still recommended and used by some of the most popular soap makers and bloggers. Pyrex mixing containers were once made with a thick, high-quality tempered borosilicate glass. This made them much more resistant to chemical reactions and thermal stress. Modern Pyrex mixing bowls are made using cheaper materials that are prone to shattering and contain soda-lime glass, which reacts with the lye. Even though glass and Pyrex containers are an option, they should be a last resort. I personally will not use glass during soap making, and I no longer recommend or support the use of glass for my students, especially now that we have easy and affordable access to other safer materials.

Shattered new Pyrex container made from cheaper materials. Imagine if this had your lye solution in it?

So, now that we have a few examples of what not to use, what materials are safe for use in soap making? In hot process soap making, we not only need soaping materials that have a low chemical interaction risk with sodium hydroxide, but that also have high melting points. We need to look for materials that have an, "A" rating and a melting point above 250F/121C.

Two of the most commonly used materials in soap making are stainless steel and polypropylene. Stainless steel (304) is A-rated and has a melting point above 2552F/1400C. I think it is safe to say that our stainless-steel mixing bowls won't melt during hot process soap making! Polypropylene also has an A rating and has a melting point of 320F/160C. Polypropylene has advantages over stainless steel, including the fact that it is much more affordable, and it is microwave-safe, which allows for fast and easy heating. Other materials that are safe for use include titanium, PVC (plastic #3), and ceramic oxide. Crockpots that are coated in ceramic oxides, like most commercial crockpots today, have a high safety rating and are created specifically to tolerate high temperatures during cooking.

Although other plastic materials may not react with hydrated lye, many have a lower melting point. Many plastics will also react with other common soap additives, most commonly fragrance, which has a tendency to melt through thin plastics. I have personally had both a cracked Pyrex and a melted plastic bowl during my soap making career. If I was not taking other safety precautions, it could have been a disastrous occasion. I am sincerely appreciative that no serious injuries occurred, as a large pool of caustic lye can be very dangerous. After my newest Pyrex broke, I vowed not to use any mixing or additive container that is not made from an A-rated material with a high melting point.

Someone recently posted a comment on one of my boards that stated something along the lines of “You shouldn’t mix your lye with a metal spoon. Just based on other tutorials?!" From what we just learned, certain materials are safe for use in soap making and others should be avoided. The spoons that we use are made from 304 stainless steel and are A-rated with a melting point above 2500F. We also use other stainless-steel utensils during the process, including our 304 stainless steel whisks. Spatulas and mixing spoons used are made from rubber, polypropylene, and other safe materials with a high melting point, and all bowls and additive containers are also made from lye and heat-safe materials. At UG2HP, we don't use usually use stainless steel bowls and dishes because they are not microwave-safe and are more expensive; however, the use of stainless steel is certainly acceptable.

Safety is the absolute most important thing to consider in soap making.

Safe mixing bowls do not need to cost an arm and a leg. Both polypropylene and stainless-steel mixing bowls and tools can be found at online retailers, including Amazon, and also in-store at places like Walmart, Target, Bed Bath and Beyond, and more. Below are multiple suggestions for soap-safe mixing materials and are the same items that we use both personally and in our classes. You can easily purchase the same listings on items that have the "shop now" option by clicking on the image for the link to Amazon.

For more information about lye and heat-safe materials, be sure to use the resources available to you. CRDs are an excellent tool if you have questions about compatibility.

Questions or comments?