Application of Coupling Agents in Plastic Products

Coupling agent is a "bridge" connecting inorganic fillers (such as calcium carbonate and glass fiber) and organic resins in plastic products. By reacting different functional groups at both ends of the molecule with hydroxyl groups on the surface of the filler and resin molecular chains, the compatibility between the two is improved, and the problem of decreased mechanical properties of plastics caused by uneven dispersion of fillers and weak interfacial bonding is solved. It is suitable for almost all types of plastics such as PE, PP, PVC, PET, PA, etc., and is widely used for filling, modifying, and enhancing the production of modified plastics. It can increase the tensile strength of plastics by 10% -30% and the impact strength by 20% -50%. Currently, it is developing towards multifunctionality and low toxicity, and is the core additive for plastics to achieve high performance and low cost.

1、 The core mechanism of coupling agents: constructing inorganic organic interface bridges

In plastic modification, inorganic fillers (such as calcium carbonate and talc powder) have a large difference in polarity with organic resins, are prone to agglomeration, and have weak interfacial bonding. Coupling agents solve this problem through "bidirectional interaction", and the specific mechanism can be divided into three categories:

1. Chemical bonding type (silane coupling agent): forms stable chemical connections

Silane coupling agent molecules containing siloxane groups (reacting with inorganic fillers) and organic functional groups (reacting with resins) are the most commonly used types:

Action pathway: Siloxanes hydrolyze to form silanol groups, which then dehydrate and condense with hydroxyl groups on the surface of fillers to form Si-O-filler covalent bonds; The other end of the organic functional group (such as amino and epoxy groups) undergoes a chemical reaction with the resin molecular chain, forming a stable interface of "filler coupling agent resin";

Representative products: KH-550 (amino silane), KH-560 (epoxy silane);

Adaptation scenario: Glass fiber reinforced PA and talc powder filled PP significantly improve interfacial bonding strength.

2. Coordination bonding type (titanium ester coupling agent): enhances the dispersibility of fillers

Titanate coupling agents coordinate with hydroxyl groups on the surface of fillers through titanium oxygen bonds, while long-chain alkyl groups are compatible with resins, focusing on improving filler dispersion:

Action pathway: The inorganic end of the titanate molecule forms a coordination bond with the hydroxyl group on the surface of the filler, and the long-chain alkyl group at the organic end inserts into the resin molecule chain, reducing filler aggregation and improving dispersion uniformity;

Representative products: TMC-101 (monoalkoxy type), TMC-201 (chelating type);

Adaptation scenario: Calcium carbonate filled PE and PVC to reduce melt viscosity and improve plastic processing flowability.

3. Physical adsorption type (aluminate coupling agent): low-cost surface modification

Aluminum ester coupling agents adhere to the surface of fillers through physical adsorption, with a lower cost than silane and titanate esters, and are suitable for scenarios with low performance requirements:

Action pathway: The aluminum oxygen groups in the molecule physically adsorb with the hydroxyl groups on the surface of the filler, and the organic groups improve the compatibility between the filler and the resin;

Representative product: DL-411-A (monoalkoxy type);

Adaptation scenario: Lightweight calcium carbonate filled PP woven bags and PE pipes, reducing production costs while slightly improving mechanical properties.

2、 Mainstream coupling agent types and compatible plastics: characteristics and scene matching

Different coupling agents have significant differences in reaction activity, compatibility, and cost, and should be selected according to the type of filler, plastic variety, and modification target:

1. Silane coupling agent: preferred for enhancing modification, suitable for polar resins

Silane coupling agents have strong chemical bonding strength, especially suitable for hydroxyl containing fillers such as glass fibers and silica, and are compatible with polar resins

Core advantages: Improve interface bonding strength, significantly enhance the tensile and bending strength of plastics;

Compatible plastics: PA, PET, PC (polar resin);

Typical applications: Glass fiber reinforced PA automotive components (such as engine mounts), silica filled PET beverage bottles (to enhance rigidity).

2. Titanate coupling agent: filling modification main force, suitable for non-polar resins

Titanium ester coupling agents have good dispersibility and are suitable for non silicone fillers such as calcium carbonate and talc powder, as well as non-polar resins

Core advantages: reduce filler agglomeration, improve plastic processing flowability, and enhance impact strength;

Compatible plastics: PE, PP, PVC (non-polar/weakly polar resin);

Typical applications: Calcium carbonate filled PE film (improves toughness), talc filled PP household appliance shell (reduces shrinkage rate).

3. Aluminate coupling agent: low-cost choice, suitable for mid to low end products

Aluminum ester coupling agents have low prices and are easy to use, suitable for mid to low end filled plastics with low performance requirements:

Core advantages: Simplified modification process, no need for high-temperature hydrolysis, directly mixed with fillers;

Compatible plastics: PP, PE, PVC;

Typical applications: Calcium carbonate filled PP woven bags (reducing costs), talc filled PVC pipes (improving dimensional stability).

4. Other types of coupling agents: adaptation to special scenarios

Maleic anhydride grafting type: By reacting grafting groups with resin, it is suitable for PP and PE reinforcement modification, such as using maleic anhydride grafted PP for glass fiber reinforced PP;

Phosphorus based coupling agent: It has both coupling and flame retardant functions, and is suitable for flame retardant modification of PA and PC. For example, phosphorus based coupling agent is used to fill the outer shell of PA electronic components with red phosphorus.

3、 Application Practice of Coupling Agents in Key Plastic Products: Scenario based Formula Design

The application of coupling agents requires customized formulas based on "plastic type filler type modification target". The following are typical cases:

1. Enhance modified plastics: improve mechanical properties

Enhanced modification with glass fiber and carbon fiber as fillers, with the core goal of improving plastic strength, commonly using silane coupling agents:

Fiberglass reinforced PA automotive bracket:

Formula: PA resin+30% glass fiber+1.2% KH-550 silane coupling agent+0.3% antioxidant 1010;

Effect: The tensile strength has increased from 60MPa to 120MPa, and the bending strength has increased from 80MPa to 180MPa, meeting the stress requirements of automotive components.

Carbon fiber reinforced PC notebook shell:

Formula: PC resin+15% carbon fiber+0.8% KH-560 silane coupling agent+0.2% light stabilizer UV-327;

Effect: The impact strength has been increased from 60kJ/m ² to 90kJ/m ², and the weight has been reduced by 20%, balancing strength and lightweight.

2. Filling with modified plastics: reducing costs and improving performance

Filling modification uses calcium carbonate and talc powder as fillers, with the core goal of reducing costs and avoiding significant performance degradation. Commonly used coupling agents include titanium and aluminum esters

Calcium carbonate filled PE agricultural film:

Formula: PE resin+20% calcium carbonate+1.0% TMC-101 titanate coupling agent+0.2% antioxidant 1076;

Effect: The retention rate of film tensile strength is greater than 90%, the transmittance is greater than 85%, the cost is reduced by 15%, and it does not affect the light demand of crops.

Talc powder filled PP household appliance shell:

Formula: PP resin+25% talc powder+0.8% DL-411-A aluminate coupling agent+0.3% antioxidant 168;

Effect: The shrinkage rate of the shell has been reduced from 1.5% to 0.8%, the dimensional stability has been improved, and the temperature resistance has been increased from 100 ℃ to 120 ℃, meeting the requirements of home appliance use environment.

3. Special modified plastics: multifunctional synergy

Special modification requires a balance between coupling and other functions (such as flame retardancy and weather resistance), and commonly used special types of coupling agents include:

Flame retardant filled PA electronic component casing:

Formula: PA resin+20% red phosphorus+1.5% phosphorus coupling agent+0.5% flame retardant synergist;

Effect: The oxygen index has been increased from 24% to 32%, achieving UL94 V-0 flame retardancy, with a tensile strength retention rate of over 85%, suitable for the fire protection requirements of electronic components.

Weather resistant filled PVC door and window profiles:

Formula: PVC resin+30% calcium carbonate+1.2% titanate coupling agent+0.3% light stabilizer UV-531;

Effect: The profile can be used outdoors for 5 years without significant aging, with an impact strength retention rate of over 70% and a cost reduction of 20%.

4、 Challenges and Development Trends of Coupling Agents in Plastic Applications

Although coupling agents provide key support for plastic modification, current applications still face challenges such as reaction conditions, compatibility, and environmental protection. In the future, they will develop towards "high efficiency, multifunctionality, and green":

1. Current challenge: Balancing performance and application costs

Strict reaction conditions: Silane coupling agents require hydrolysis reaction, high humidity and temperature control requirements, which can easily lead to complex modification processes;

Compatibility limitations: Single coupling agents have a narrow adaptability range, such as silane coupling agents having weak effects on non-polar resins (PE, PP);

Environmental compliance pressure: Some titanium ester coupling agents contain heavy metals (such as lead and tin), which do not comply with EU RoHS and other environmental standards.

2. Development trend: Technological innovation drives upgrading

Multi functional coupling agent: Develop integrated products of "coupling+antioxidant+weather resistance", such as silane coupling agents containing antioxidant groups, simplify formulas, and adapt to high-end plastic modification;

Green coupling agent: Developing heavy metal free titanium acid esters and bio based silane coupling agents (such as plant-based silane) that meet environmental requirements and are suitable for food and pharmaceutical packaging plastics;

Nano coupling agent: Utilizing nano coupling agents to increase specific surface area, enhance interaction efficiency with fillers and resins, reduce addition amount (from 1% to 0.5%), and lower costs;

Reactive coupling agent: Develop coupling agents that can be polymerized in situ with resins, such as silane coupling agents containing double bonds, to form stronger interfacial bonds with PE and PP copolymerization, improving long-term stability.

5、 Summary: Coupling agent - the "performance bond" of plastic modification

From automotive components reinforced with glass fiber reinforced PA, to agricultural films filled with calcium carbonate PE, and to flame-retardant modified electronic component casings, coupling agents achieve a balance between "low-cost" and "high-performance" plastics by constructing inorganic organic interface bridges. It is not only a "basic additive" to solve the problems of filler dispersion and interface bonding, but also directly determines the application boundary of modified plastics (such as automobiles, electronics, building materials). In the future, with breakthroughs in the research and development of multifunctional and green coupling agents, the plastic modification industry will be further promoted towards high-end and environmentally friendly development, providing support for the development of more high-performance plastic products.