Tackifying Resins
In many applications, acrylic adhesives do not require the addition of tackifying resins to improve pressure sensitivity, but in most cases, they are necessary:
To increase initial tack and peel strength
To improve adhesion to low surface energy materials
Tackifying resins are commonly used in acrylic emulsion systems, such as paper labels and packaging tapes. They improve peel strength on difficult-to-bond surfaces, such as plastic films and biaxially oriented polypropylene (BOPP) films.
The table below shows the typical characteristics of adding 40 parts rosin ester. It can be noted that initial tack and peel strength are significantly increased, but shear strength is reduced. This is due to a decrease in modulus and softening. Tackifying resins can sometimes also reduce the cost of the final adhesive product. A typical addition amount is 30-40% by weight.
Tackifying resins with melting points significantly higher than the polymer's glass transition temperature can improve adhesive strength but reduce tackiness. Resins with lower melting points can increase adhesive tack and flexibility but sacrifice creep and shear strength.
Tackifying resins are responsible for regulating the initial tack, peel strength, and shear strength of adhesives, and a trade-off must be made among various aspects of the system.
The Effect of Tackifying Resins on Tg
Although tackifying resins lower the modulus and make the system more flexible, they can generally increase the glass transition temperature by reducing the rubber plateau. Since viscosity is measured by the energy required for failure, the adhesive needs a high modulus at both strain rate and strain amplitude during failure.
Tackifying resins increase the glass transition temperature of the elastomer, giving the adhesive mixture a high modulus at high strain rates and room temperature. Therefore, tackifying resins increase the modulus at low temperatures, short times, and high frequencies, but decrease the modulus at high temperatures, long times, and low frequencies.
Tackifying resins used in acrylic emulsions need to be compatible with the base polymer resin and the surfactant system. Pre-emulsified tackifying resins can be used in aqueous systems. Rosin and C5/C9 petroleum resins are common products.
Sinograce Chemical produces water-based tackifying resins for use in acrylic emulsion systems and water-based rosin for use in various adhesives.
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