A new amine-functional dimer technology creates formulation freedom for high-end polyamide adhesives.

Figure 1. Extended Range of Dimer Functionalities

Dimerized fatty acids have been used for many years in polyamides, polyesters, epoxy resins and amine curatives. They are obtained through the conversion of unsaturated fatty acids (from natural oil and fat) by a combination of pressure, temperature and catalysis, followed by purification processes.

Starting from the C18fatty acids that nature typically provides, the dimer acid is a molecule with 36 carbon atoms, which makes it the longest dioic acid available. The benefits that the hydrocarbon character and non-crystallinity of these C36diacids add to coatings and adhesives are mainly related to the low glass-transition temperature (Tg) and their hydrophobic nature. These properties induce flexibility, hydrolytic resistance, thermo-oxidative resistance, moisture repellency and adhesion to a wide range of substrates.

For several years, dimer fatty diols and a range of polyester polyol derivatives of dimerized fatty acids have been available for use in various polyurethane applications to enhance durability, flexibility, and hydrophobicity.* New bio-based, amine-functional building blocks have been developed that extend the range of acid- and hydroxyl-functional fatty dimers and derived polyesters (see Figure 1).** This new technology provides benefits in many applications.

A Technological Development

In polyamide hot melts, the use of the new dimer diamine extends formulation freedom by allowing wider melting point adjustment. It increases hydrophobicity, which results in improved moisture repellency and stronger adhesion to plastics. The new amine-functional materials broaden application possibilities and bring additional environmental benefits through their durability, low volatility and renewable nature (100% renewable carbon).

Polyamide adhesives are used in a wide range of applications, such as footwear, electronic encapsulation and sealing, textiles, furniture assembly, and packaging. The amine component supplies intermolecular bonding and cohesive strength, a high melting point, green strength, and adhesion to polar substrates. Dimer fatty acids are used to create flexibility and stress absorption, enhanced flow and wetting, and adhesion to low-energy substrates. Other dioic acids in the carboxylic acid component are used to increase the melting point.

The availability of dimer diamine permits the use of dimer in the amine component, thereby allowing more freedom in the diacids used and giving wider control of the melting point. In addition, using both dimer acid and dimer diamine in one formulation leads to even better moisture resistance, improved adhesion to low-energy substrates and higher flexibility, which is of interest in applications such as electronics, packaging, and textiles.

Figure 2. Improved Moisture Repellency

Thermal Properties and Hardness

It is useful to compare applications using dimer acid or dimer diamine in a formulation. The polyamide hot-melt adhesives shown in Table 1 have been synthesized by heating the acid component and subsequently adding the amine component for the reaction. When the possible foaming stage passed, the temperature was gradually increased to 230°C (this temperature this can be higher, depending on viscosity). Reactions were continued until an acid value of 9 mg KOH/g had been reached, representing a molecular weight of 12,500 g/mol. The hot-melt adhesives have been analyzed with differential scanning calorimetry for the Tgand melting temperatures; Shore D hardness was also measured.

As expected, the presence of the C36dimer structure drastically reduces both the melting point and the Tg, making the polyamide more flexible than PA 6.6. Replacing dimer acid with dimer diamine increases the melting point and hardness while maintaining a low Tg.

A variation in total dimer content (83 vs. 78%) is caused by formulating with a carboxylic acid access. (When correcting the total dimer acid content by incorporating some adipic acid, dimer diamine has a very similar effect as dimer acid to the hardness and Tg). However, the dimer acid-based polyamide no longer has a sharp melting point, but a melting trajectory. This leads to undesired softening and threading of the adhesive. Dimer diamine thus extends the freedom in formulating, allowing wider melting point adjustment without compromising the performance. By altering the acid component in the formulation, the melting point can be further adjusted.

Figure 3. Lap Shear Adhesion to ABS

It is useful to compare applications using dimer acid or dimer diamine in a formulation. The polyamide hot-melt adhesives shown in Table 1 have been synthesized by heating the acid component and subsequently adding the amine component for the reaction. When the possible foaming stage passed, the temperature was gradually increased to 230°C (this temperature this can be higher, depending on viscosity). Reactions were continued until an acid value of 9 mg KOH/g had been reached, representing a molecular weight of 12,500 g/mol. The hot-melt adhesives have been analyzed with differential scanning calorimetry for the Tg and melting temperatures; Shore D hardness was also measured.

As expected, the presence of the C36dimer structure drastically reduces both the melting point and the Tg, making the polyamide more flexible than PA 6.6. Replacing dimer acid with dimer diamine increases the melting point and hardness while maintaining a low Tg.

A variation in total dimer content (83 vs. 78%) is caused by formulating with a carboxylic acid access. (When correcting the total dimer acid content by incorporating some adipic acid, dimer diamine has a very similar effect as dimer acid to the hardness and Tg). However, the dimer acid-based polyamide no longer has a sharp melting point, but a melting trajectory. This leads to undesired softening and threading of the adhesive. Dimer diamine thus extends the freedom in formulating, allowing wider melting point adjustment without compromising the performance. By altering the acid component in the formulation, the melting point can be further adjusted.

A second comparison was made of dimer diamine with several amines. Hexamethylene diamine (HMDA) was used as standard diamine, polyetheramine MW400 as flexibilizing diamine, and aminoethyl piperazine as a secondary amine for improved adhesion to plastics. Polyamide hot melts were prepared as shown in Table 2 (reactions were continued until an amine value of 7 mg KOH/g, representing a molecular weight of 16,000 g/mol). Similar to the polyetheramine soft block, dimer diamine reduces the hardness and Tgwhile maintaining a high melting point.

Figure 4. Lap Shear Adhesion to Nylon

Improved Moisture Resistance and Adhesion

Moisture uptake was tested by allowing adhesive samples to absorb moisture in 25°C water for a week and then analyzing the weight increase. The combination of dimer acid and dimer diamine clearly absorbs less water (see Figure 2). As expected, polyetheramine has a larger affinity for moisture.

Adhesion properties were analyzed by measuring lap-shear adhesion strength on a range of substrates at 500 µm adhesive thickness. Adhesion to beach wood resulted in no break (at 3.2 MPa) for all four samples. The results in Figures 3-5 show that, compared to HMDA, the three other amines give similar improved adhesion to ABS. On nylon, dimer diamine clearly brings a larger improvement, resulting in substrate failure. Interestingly, the hydrophobicity that dimer diamine provides clearly improves adhesion to the lowest-polarity substrate: untreated polypropylene.

Figure 5. Lap Shear Adhesion to PP

Expanding Opportunities

Amine-functional dimerized fatty acid technology can be used to great benefit in polyamide adhesives, broadening the application possibilities. These new bio-based building blocks provide several performance and environmental benefits through their renewable nature, reduction in volatility, and good thermal-oxidative stability for more durable applications.

In polyamide hot melts, the use of dimer diamine extends the freedom in formulating, enabling wider melting point adjustments. Moisture repellency and adhesion to plastics are improved. Combined with flexibility, this makes dimer diamine suitable for improving high-end applications, such as electronic encapsulation or sealing and demanding packaging, footwear, or textile applications.

For additional information, contact Croda Inc. at 300-A Columbus Circle, Edison, NJ 08837; phone (732) 417-0800; or visit www.croda.com.

*Offered by Croda under the Priplast product name.
**Priamine 1074, developed by Croda.

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