TDI vs MDI Urethane Rubber for Concrete Casting: Which to Use?

Urethane rubbers are versatile 2-part materials used in the concrete manufacturing industry to produce small and large precast pieces. The material you choose for your project will depend on your application, processing requirements, and manufacturing environment.

Because the chemical composition of polyurethane rubbers can be different, their properties are controlled by the types of prepolymers used to make them. As one of the core components of urethane, the type of isocyanate (A side material) is one material that changes the way it performs. The most common isocyanates used for urethane rubbers are TDI and MDI.

What Is TDI Rubber?

TDI rubber is a type of urethane that uses a prepolymer called toluene diisocyanate. TDI-based rubbers are standard in the industry and usually come with premium properties compared to MDIs.

The prepolymer is actually less hazardous than an MDI prepolymer and has a lower risk of causing respiratory irritation, skin sensitization, and other health issues if in direct contact. However, you should always utilize proper personal protective equipment (PPE) when using urethane rubbers. Regardless of potential hazards, they are an effective material for making durable concrete molds.

Benefits of using TDI:

• Lower exothermic reaction. The amount of heat generated while the rubber cures is lower than MDI. When less heat is used to cure the rubber, it will have better dimensional stability (lower shrinkage). The size of the form you are pouring will be consistent, and the rubber will retain its original shape.
• Lower sensitivity to moisture. Urethanes are sensitive to moisture and will bubble if exposed to a high amount in the environment or during application. What’s beneficial about TDI rubbers is that they can handle moisture better than MDI rubbers. This means you don’t have to work in as stringent conditions for the rubber to cure without bubbles.
• Extended pot life. If you’re pouring large and intricate forms, you will need ample time to work with the material. Most VFI TDI rubbers come with a 20-30-minute pot life, with some exceptions. This makes processing easier and reduces the risk of mistakes during mixing and pouring.
• More variability. TDI rubbers offer a wider range of properties and Shore A hardnesses, going from 25 A to 90 A. Due to the increase in options, there are also more application possibilities. Softer rubbers can be used to make more intricate molds, while harder rubbers can be used for less detailed but durable forms. The softness of lower durometers provides more give for easy demolding on complex parts. VFI also has a line of lower durometer TDI rubbers with improved release abilities. This means that concrete pieces will have lower breakage rates when demolded.

Where to Use?

• Manufactured stone: One of the most common applications for TDI rubbers is manufactured stone molds. At lower durometers (30-60 A), TDI rubbers capture intricate details without degrading after first use due to high abrasion resistance. A few materials VFI recommends include VFI-2143 45 A TDI Molding Rubber and VFI-2160 60 A TDI Molding Rubber. Our line of Max Release molding rubbers are also beneficial for this application because they demold from intricate details easier using less release.
• Cast stone: Like manufactured stone, cast stone applications benefit from using lower durometer TDI rubbers. They come in durometers as low as 25 A, which is good for making detailed features, such as wall trim, columns, ornaments, and more. Standard materials for this application include VFI-2123 25 A TDI Molding Rubber or VFI-2143 45 A TDI Molding Rubber.
• Formliners: Whether you need advanced detail or simple formliners, the range of TDI rubbers available can accommodate any project. 70-90 A rubbers withstand the weight of concrete for these projects due to their higher tensile and tear strengths. 50-70 A rubbers are recommended if more detail is needed. However, most concrete producers will use MDI rubbers instead if they are creating simple formliners due to cost.
• Stamps: Because stamps are made thin to comply with weight restrictions, a harder durometer material is used. Rubber between 70-90 A will offer the rigidity and durability needed, so the stamps can be stood on and moved. VFI’s recommended materials include VFI-2180 80 A TDI Molding Rubber and VFI-2190 90 A TDI Molding Rubber.

What Is MDI Rubber?

MDI rubber is a type of urethane that uses a prepolymer called methylene diphenyl diisocyanate. When using MDI rubbers, you need to work in a controlled, high-production environment because the material is highly sensitive to moisture.

Benefits of using MDI:

• Cost-effective. MDI rubbers are preferred due to their lower cost and ease of use. It is because you have to work in a high-production facility that manufacturers turn to TDI instead. There are also fewer options to choose from when it comes to MDIs because they work best at higher hardnesses.
• Better adhesion. When it comes to molding and casting, the last thing you want to see is your rubber sticking to other materials. However, there are some applications where some adhesion is desired. Some formliner or stamp manufacturers desire the ability to adhere their rubber to a rigid backing material like wood. This allows them to insert attachment points which helps with the movement of the formliner and prevents the liner from shrinking or expanding in extreme temperatures. Once bonded with the desired substrate, MDI becomes strong and is difficult to tear.
• Better processing. While the moisture sensitivity of MDI rubbers makes them less desirable, they do offer other benefits in terms of processing. MDI processes better at lower viscosities, so you don’t have to worry about working at room temperature (77°F) when making your molds. You also can choose between fast and slow options for quicker demold and turnaround on production. You just have to be wary of the pot life on faster materials.

Where to Use?

• Formliners: Since most MDI rubbers are harder in durometer, they are preferable for large projects like reusable concrete formliners. Their strength makes them able to maintain their shape under the weight and pressure of concrete. Since they have strong adhesion to other materials without tearing, they work well if they need to be adhered to backing panels for ease of movement with cranes. VFI’s recommended materials include VFI-3171 70 A MDI Molding Rubber and VFI-3181 80 A MDI Molding Rubber.
• Stamps: The rigidity of high-hardness MDI rubbers is also desirable for stamping projects. Harder rubbers have better strength, especially when poured thinner, and are better at printing higher detail. Their strength also allows them to maintain their shape when people are stepping on them during the stamping process. VFI’s recommended materials include VFI-3170 70 A MDI Fast Molding Rubber and VFI-3180 80 A MDI Fast Molding Rubber. These are faster MDIs since stamps are smaller and don’t tend to require longer pot lives like formliners.
• Rollers: The rigidity and durability of MDI rubbers are also desirable for creating concrete rollers. They withstand the abrasive qualities of concrete while also being sturdy enough to leave a desired pattern. They offer a quicker way to imprint concrete compared to rigid or flexible stamps. VFI would recommend the same materials as mentioned above.

Contact VFI if you’re still unsure whether you should use an MDI or TDI molding rubber.