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Polyurethane spray foaming quality problem
Time:2020/4/16 11:47:39    

In the polyurethane foaming process, there are a series of problems such as foam shrinkage, crispness, too soft, heartburn, cracking, smoking, shedding, collapse, thick foam pores, pinch into fine powders, slow foaming in the initial reaction and slow enlightenment, etc.

Polyurethane foaming is affected by conditions, equipment, processes and ambient temperature during the construction process. Various abnormal phenomena will occur during the polyurethane reaction process which ultimately affects the quality of the foam.

  All possible situations are as follows:

  The following material A represents a polyurethane white material which is a combined polyether; material B represents a polyurethane black material which is a polymeric MDI (polyisocyanate).

1. The materials A and B do not foam after mixing: The material temperature is low; The distribution ratio between the two groups is inaccurate The material leakage is added with a catalyst; The material B is of poor quality;

2. Polyurethane hard foam shrinkage: There are many components in the A material, which causes the polyurethane rigid foam strength to decrease and cause shrinkage; The mixing of the liquid in the spray gun is uneven, the spray air is too small, or the viscosity of the material is too large; The curing is too fast and more closed cells are formed; The gas expands and shrinks due to thermal expansion and contraction.

3. The rigid polyurethane foam is crisp: too much components for material B;Excessive moisture; Working surface temperature is too low; Material B has a large acid value and contains many impurities; Material A contains too much flame retardant.

4. Polyurethane hard foam is too soft, and the curing is too slow: The component amount of material B is small; There is too little tin catalyst in material A; The temperature, material temperature and temperature of blanking working surface are low.

5. Polyurethane rigid foam collapse: The foaming gas generates too fast, and the amount of amine catalyst in material A should be reduced; The foam stabilizer in material A is invalid or alkaline;  If the catalyst fails or leaks, it should be added Tin catalyst in material A; Big acid value in raw materials.

6. The coarse pores of polyurethane hard foam: The foam stabilizer in material A is invalid or leaking;Excessive moisture (water in the foaming agent or polyether); The mixing of materials A and B is not uniform; The purity of material B is low and the total chlorine or acid value is high; The rate of gas generation is faster than that of gel.

7. Polyurethane hard foam cracking, or heartburn: The material temperature is high; The catalyst of material A is excessive; The amount of one-time pouring is too large and the foam is too thick; The amount of addition is too much when using water as a blowing agent;Metal salt impurities in the material.

8. The hard polyurethane foam falls off: The humidity of the spraying working surface is high which makes the reaction incomplete and the bottom foam is crispy, brittle and powdery; The sprayed working surface is unclean, oily, and dusty.

9. Polyurethane rigid foam easily emits smoke: The amount of catalyst in material A is too high; The hydroxyl value of polyether in material A is too high;The material temperature is too high.

From the analysis of the above problems, we know that in addition to objective factors, the material A component involves too many factors.In actual production, adjusting the viscosity, milking time, foaming time, curing time, etc. is mainly to adjust the material A.

Polyurethane rigid foam is not just a mixture of A and B materials. It is necessary to understand the principle of generating polyurethane rigid foam and the interaction between the components,comprehensive analysis of various factors, targeted analysis from which to determine the existence of problems, it is not difficult to produce high-quality polyurethane rigid foam products.

Influencing factors:

1. Hydroxyl value

Generally, the polyol compound has a large functionality and a high hydroxyl value, so the prepared foam has high hardness, good mechanical and physical properties and good temperature resistance. But the miscibility with other components such as isocyanate is poor.

  The reaction of hydroxyl compounds and isocyanates is one of the most common and basic reactions in polyurethane synthesis. When the hydroxyl content is the same, the reaction rate with big functionality will be high and the viscosity of the reactant increases rapidly. When the functionality is the same, Polyethers with a high hydroxyl value have a high reactivity with TDI and polyethers with a low hydroxyl value have a low reactivity with TDI. As the two main reactions in the foaming process: the reactivity of TDI with water is equivalent to its reaction with secondary alcohols, so the fluctuation of the polyether hydroxyl value will destroy the reaction balance of TDI with polyether and water in the foaming process. If the hydroxyl value is high, the reactivity of TDI with polyether is enhanced and the main reaction is enhanced. If the hydroxyl value is low, the reaction between TDI and water is enhanced, causing the main reaction speed not able to keep up with the foaming reaction speed which is likely to cause collapse.

  The fluctuation of polyether hydroxyl value has a greater impact on foam products.The hydroxyl value of the polyether directly affects the amount of TDI added.If the user does not adjust the amount of TDI, it is equivalent to adding too little TDI, resulting in foam cracks, poor resilience, poor strength and permanent compression and deformation increases. On the contrary, it is equivalent to adding too much TDI which will cause the foam to form large and closed cells, hardness increasing and heartburn caused by long aging time.

2. Unsaturation

The reaction temperature of the polyether and the amount of catalyst directly affect the unsaturation of the polyether product. If the reaction temperature is high or the catalyst content is high (the number of polyether chain activation centers is large), the side reaction increases and the unsaturation increases. The increase in the degree of unsaturation means that the aldehydes and other unsaturated substances generated during the reaction is increased. After forming an unsaturated double bond at the end of the polyether, the reaction activity is lost and the chain growth is terminated which affects the average molecular weight of the polyether, reduces the average molecular weight, increases the hydroxyl value, and reduces the functionality of the polyether. The level of functionality directly affects the performance of foam products. Therefore, in production control, the lower the degree of unsaturation, the less side reactions and the smaller the impact on the foam products.

2. Moisture

As a blowing agent, water is one of the most important reactions in foam plastics. A small amount of water can consume a large amount of TDI and generate a large amount of gas and emit heat. Increased water concentration in the reaction mixture can accelerate the reaction rate. If the moisture in the polyether (which is ignored when calculating the TDI index) increases,it will not only consumes a large amount of TDI to reduce the activity of polyether and causes foaming and collapse, but also reduces the density of foam products due to the release of a large amount of CO2 and heat which reduce the density of foam products. If the generated heat cannot be removed quickly, it will cause yellow heart or even on fire. If the moisture of polyether exceeds the standard, the amount of urea bond and CO2 increase, the foam density becomes smaller, the foam product becomes harder, and a large amount of heat is released, which is easy to cause heartburn. If the customer uses it without knowing it, it is equivalent to increasing the amount of water, but the amount of TDI and catalyst is not adjusted which is likely to cause cracking and foam collapse. Therefore, the lower the moisture in the process of polyether control, the better the stability of foam product formulation and performance.

4. Acid value

AV can be expressed in the form of hydroxyl value, so high acid value consumes more TDI, but can be negligible for small amount. High reaction temperature and increased catalyst concentration will increase side reactions and increase the acid value; high acid value consumes more amine catalysts in the foam formation process which reduces the reaction activity. In severe cases, it will be caused by too much consumption of amine catalysts Shrinkage and closed cell of foam products.

5. Molecular weight

  The properties of most polymers increase with increasing molecular weight, such as tensile strength, elongation, melting point, hardness, and glass transition temperature, etc., increase with increasing molecular weight, while the solubility is the opposite. This relationship does not change significantly after the molecular weight increases to a certain value. The molecular weight of polyether can be calculated from hydroxyl value and functionality. For the same brand, the size of the molecular weight is affected by the unsaturated bond, so the factors that cause the molecular weight change are similar to the factors that affect the degree of unsaturation, the two are a complementary relationship, that is, the same brand polyether, the degree of unsaturation increases molecular weight Decrease; unsaturation decreases molecular weight increases.

6. Potassium ion

  The presence of residual catalyst in polyether will directly affect the prepolymer preparation and the physical properties of polyurethane products, because potassium ions promote the formation of rigid and cross-linked allophanate and biuret groups in the chemical reaction of polyurethane, making foam products Hardens, elasticity decreases, and gels when severe. At the same time, the potassium ion content is high. When foaming with TDI as a raw material, the foaming heat cannot be radiated in time, causing foam heartburn.

7. Antioxidant

  In order to improve the oxidation resistance of foam products, we add antioxidants during the production and foaming of polyethers (the combustion temperature of foam products can be increased to above 180 ° C, and the amount added is related to the amount of water added). Increasing the burning temperature of foam products to avoid the occurrence of heartburn and  the color of foam products change accordingly.

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