What are the causes of poor wettability of chopped strand mat (CSM) and how can it be improved?

I. Three Core Signals of Poor Wetting

To determine if the wetting of chopped strand mat meets standards, observe the following three points:

1. Slow Penetration: Within approximately 5 minutes of resin application, a large area of ​​fiber remains unwetted;

2. Numerous Dry Spots: White fiber clumps appear on the cut surface after compaction, and air bubbles are difficult to expel;

3. Poor Adhesion: Delamination is easy after curing, the sound is hollow when tapped, and mechanical properties are substandard.

The root causes of these problems mainly lie in three aspects: the material itself, the construction process, and environmental conditions.

 

II. Four Core Reasons for Poor Wetting

(I) The Material Itself: The Source of the Problem

1. Incompatible or Ineffective Chemical Sizing Agents: The sizing agent on the surface of chopped strand mat is the "bridge" connecting the glass fiber and the resin. If the selected sizing agent film-forming agent (such as PVAc, polyester emulsion) is incompatible with the resin system, or if it has been stored for too long or exposed to moisture, causing the silane coupling agent to become ineffective, it will directly reduce compatibility. In addition, improper drying processes during production, resulting in insufficient film formation or migration and loss of the sizing agent, will also affect the impregnation effect.

2. Defects in the quality of chopped strand mat itself: Uneven distribution of the binder during production, or chopped strand length and weight not meeting process requirements, will lead to fiber agglomeration, making it difficult for the resin to penetrate. Simultaneously, excessively high alkali content in the glass fiber easily hydrolyzes to form silica colloids, which adsorb water films on the surface, hindering resin impregnation.

3. Inappropriate resin selection: Resin viscosity that is too high results in poor flowability, preventing rapid penetration into the fiber gaps; insufficient gel time causes the resin to cure before sufficient impregnation, leading to premature hardening.

(II) Construction Process: Operational details determine the final effect

1. Incorrect layering and coating methods: Applying resin all at once after dry-laying multiple layers, or applying too thick a layer, will prevent the lower fibers from contacting the resin. Failure to compact each layer during hand lay-up will trap air bubbles between the fibers, also affecting impregnation.

2. Inappropriate Curing Parameters: Improper accelerator/initiator ratios, or excessively low curing temperatures, will slow down the curing reaction and result in insufficient resin impregnation time; excessively high temperatures will accelerate curing, also leading to inadequate impregnation.

(III) Environmental Conditions: The Hidden Influence of Temperature and Humidity

When the ambient temperature is below 10℃, the resin molecular chain mobility decreases, the sizing agent activity decreases, the glass fiber hardens, and construction becomes more difficult; when the humidity is above 60%, the glass fiber surface easily absorbs a water film, the resin easily whitens, and the impregnation effect is significantly reduced.

(IV) Storage and Handling: Secondary Damage Cannot Be Underestimated

During transportation and storage, chopped strand mat may be exposed to moisture or pressure, or stored for extended periods, which can lead to sizing agent inactivation and fiber agglomeration, inevitably resulting in poor impregnation during use.

 

III. Five Key Improvement Strategies

(I) Source Control: Choosing the Right Materials is Key

1. Precise Matching of Sizing Agents and Resins: For unsaturated polyester resins, prioritize fast-dissolving PVAc or PVAc + polyester emulsion sizing agents for glass fiber chopped strand mat; for epoxy resins, choose products containing epoxy-based silane coupling agents (such as KH-560); for high-end lightweight products such as wind turbine blades and automotive parts, glass fiber chopped strand mat with polyester + polyurethane composite sizing agents can be used, balancing wettability and mechanical properties.

2. Strict Acceptance of Chopped Strand Mat Quality: Check for a smooth appearance and absence of agglomeration; verify that the product data and basis weight on the certificate of conformity meet requirements. Require suppliers to provide product testing reports to ensure that the silane coupling agent content is between 0.5% and 3%.

3. Optimizing Resin Selection and Proportioning: Prioritize low-viscosity resins, or add an appropriate amount of reactive diluent to reduce viscosity. Adjust the gel time to 15-45 minutes to ensure sufficient impregnation time. For low-temperature construction, add 1%-2% low-temperature accelerator to replace traditional cobalt salts.

(II) Standardized Construction: Pay Attention to Details

1. Layered Thin Coating, Layer-by-Layer Compaction: During hand lay-up, first apply a thin layer of resin to the mold surface, then lay a layer of chopped strand mat, compacting it in one direction with a roller or scraper to remove air bubbles. Ensure the resin is fully impregnated before applying the next layer. Each resin layer should be less than 0.5mm thick.

2. Adjusting the Curing Process: Adjust the accelerator ratio according to the ambient temperature. Increase the accelerator dosage appropriately when the temperature is low, and decrease it when the temperature is high. Control the curing temperature between 15-25℃. An insulated shed can be built, and electric heaters can be used to maintain the temperature to avoid low-temperature construction.

(III) Environmental Management: Creating Suitable Construction Conditions

Monitor temperature and humidity before construction. Maintain the temperature above 15℃ and the humidity below 60%. In low-temperature, high-humidity weather, use dehumidifiers and hot air blowers; suspend construction if necessary.

(IV) Scientific Storage: Avoiding Secondary Damage to Materials

Chopted strand mat should be stored in a dry, ventilated, and dark warehouse, raised off the ground using dedicated pallets to prevent moisture absorption. Once opened, the product should be used quickly and within its shelf life. Handle with care during transportation to avoid deformation under pressure.

(V) Emergency Handling: How to Remedy Existing Problems

If insufficient impregnation is found during construction, immediately stop curing, apply a low-viscosity resin, and re-compact. If dry spots or delamination appear after curing, remove the defective parts, grind them, and re-lay the layer to repair and ensure product quality.

 

IV. Conclusion

Poor impregnation of chopped strand mat is not solely due to raw material factors, but rather the result of multiple factors including materials, processes, and environment. This problem can be completely solved by selecting the right materials from the source, standardizing construction operations, and implementing effective environmental control. For those in the fiberglass industry, excellent impregnation not only improves product quality but also reduces rework costs and enhances customer satisfaction.