Design Concepts And Analysis of A 245fa Polyurethane Spray Formulation for Cold Storage Facilities

With the ongoing implementation of the Montreal Protocol, HCFC-141b is being phased out. Among the numerous alternatives, HFC-245fa (pentafluoropropane) has emerged as the ‘preferred choice’ in the cold storage spray application sector, thanks to its zero ODP (Ozone Depletion Potential), excellent thermal conductivity and relatively mature, patented technology.

However, many engineers have reported that when used in spray applications, 245fa tends to become ‘brittle’, is prone to ‘blistering’ and is costly.

Today, drawing on my experience in rigid foam development over the years and approaching the subject from a molecular design perspective, I will provide an in-depth analysis of the core principles behind 245fa spray formulations for cold stores, and share a proven, practical system.

I. Why 245fa? — Not just for environmental reasons, but because of essential performance requirements

In cold store environments (particularly low-temperature cold stores, ranging from –18°C to –35°C), what are the greatest concerns regarding the insulation layer? Firstly, a rise in thermal conductivity (due to ice formation), and secondly, dimensional shrinkage (leading to cracking).

Compared to all-water foaming systems (where CO₂ has a high thermal conductivity of approximately 0.016 W/m·K), the vapour-phase thermal conductivity of HFC-245fa is only around 0.012 W/m·K. This means that, at the same density, the initial thermal insulation performance of the 245fa system is improved by approximately 15–20 per cent.

Furthermore, although the fourth-generation blowing agent LBA (1233zd) has a lower GWP, under the current cost structure, 245fa remains the optimal solution that balances ‘high cost-effectiveness’ with ‘high application tolerance’. It is non-flammable (compared to pentane) and has just the right solubility for polyethers and polyesters, without causing uncontrolled flow due to violent reactions as water does.

II. The Core Logic Behind the Design of 245fa Spray Formulations (Analysis)

Many novices simply apply 141b formulations directly to 245fa, resulting in foam powdering (poor strength) or shrinkage. This is incorrect.

The boiling point of 245fa (15.3 °C) is higher than that of 141b (32 °C), and it has a higher molecular weight. This means that between spray layers, it requires more energy to vaporise and emulsify. Consequently, our formulation design must adhere to the three-pronged principle of ‘high-functionality backbone + catalytic balance + solubility matching’.

1. Polyol System: Construction of a Rigid Backbone

Cold storage foam requires excellent compressive strength and dimensional stability.

· Polyether-based: It is recommended to use high-functionality (4.5–6.0) sucrose/sorbitol starting polyethers (such as types 403 and 8238). The proportion should be controlled at 60–70 per cent. This forms the backbone that ensures the foam does not shrink at low temperatures.

· Polyester as a supplement: Introduce 20–30 per cent aromatic polyester polyols XF-3152. Polyesters can significantly improve the core hardness and flame-retardant properties of the foam (this is also key to achieving B1/B2 class ratings with the 245fa + water system).

· Compatibility pitfalls: Whilst 245FA exhibits reasonable compatibility with standard polyethers, it is prone to separation when mixed with high-viscosity polyesters. It is necessary to use a highly compatible polyether ester or increase the intensity of agitation; otherwise, this can easily lead to coarse foam cells or even cell collapse.

2. Blowing agent gradient design: The ‘golden ratio’ for ‘245FA + water’

Although 245fa produces a fine-celled foam when used alone, it is extremely costly and tends to result in excessive density. The current industry standard is a ‘245fa + water’ mixed foaming system.

· Recommended proportions: In a standard formulation, based on 100 parts of polyol, it is recommended to add 10–18 parts of 245fa and 1.5–3.0 parts of water.

· Mechanism: 245fa is responsible for forming a fine, dense cell structure with high closed-cell content (low thermal conductivity); water (CO₂) serves to reduce brittleness and enhance cross-linking density. The combination of the two preserves the energy-saving properties (low thermal conductivity) whilst addressing the issues of high brittleness and tendency to crumble commonly associated with 245fa systems.

3. Catalysts and Silicone Oil: Microscopic Control of Flow Properties

This is the most challenging aspect of the 245fa formulation to master.

· Catalyst: The 245fa system is temperature-sensitive. The use of a delayed-gel catalyst (such as a blend of PC-5 and PC-41) is recommended. As the gasification of 245fa is an endothermic process, the initial reaction must not proceed too rapidly; the material must be allowed to mix and spread thoroughly. Once heat has accumulated, rapid post-curing should then take place. If foaming proceeds too rapidly in the early stages, the cell walls will be ‘frozen’, leading to cell rupture.

· Silicone oil cell-uniformity agent: It is essential to switch to a silicone oil specifically designed for the 245fa system. General-purpose silicone oils often lack sufficient emulsifying capacity in the 245fa system; surfactants with stronger nucleation capabilities must be selected to ensure that cells do not coalesce in the environment of a low-boiling-point blowing agent.