If you’ve ever peeled a sticky note off a desk or removed a label from a jar, you’ve witnessed the magic of Pressure Sensitive Adhesive (PSA). But what makes a material "tacky" enough to stick instantly with just light finger pressure? For chemists and engineers working with Hot Melt Pressure Sensitive Adhesives (HMPSA), the answer lies in a famous rule of thumb known as the Dahlquist Criterion.
What is the Dahlquist Criterion?
Proposed by Dr. Carl A. Dahlquist in the 1960s, this criterion defines the fundamental mechanical property required for a material to behave as a pressure-sensitive adhesive at room temperature.
The adhesive must have a storage modulus (G') of less than 3 × 10⁵ Pa (300,000 Pascals, or 3× 106 dyn/cm2) at a frequency of 1 Hz. Furthermore, the Dahlquist criterion is best met by employing a tackifier on an adhesive at 40 to 70°C above Tg.
Figure 1. The Dahlquist criterion-PSA criterion
Why Does This Matter for HMPSA?
Hot melt adhesives are 100% solid thermoplastics. They are applied hot, cool down, and form a bond. To be a pressure-sensitive hot melt, the final cooled film must remain soft and flowable enough to "wet out" (flow into the microscopic valleys of a surface) under gentle pressure.
If G' is too high (> 3×10⁵ Pa): The adhesive is too stiff. It acts like a rigid plastic sheet. When pressed against a surface, it only touches the peaks of the surface texture, resulting in poor contact and no tack. It won't stick.
If G' is too low (< 3×10⁵ Pa): The adhesive is very soft and tacky. It flows easily into the surface, creating a large area of contact and strong initial adhesion (quick stick).
This balance is critical in HMPSA formulations. By adjusting the ratio of base polymer (e.g., SIS, SBS, APAO), tackifying resin, naphthalic oil, and wax, formulators tune the modulus to fall below that magical 300,000 Pa threshold.
Figure 2. Adjust the proportion of different ingredients to meet the Dahlquist Criterion
Real-World Application in HMPSA
Imagine formulating a label adhesive. If you add too much wax to reduce cost, the modulus rises above the Dahlquist limit. The label will feel dry and might not stick to a cardboard box. Conversely, if you add too much liquid oil, the modulus drops too low; the adhesive becomes overly soft, may ooze out from the edges of the label, and could fail under shear stress.
The Dahlquist Criterion is not a perfect law, but it is the first checkpoint every HMPSA developer uses when designing a new product.
Q&A Session: Dahlquist Criterion & HMPSA
Q1: Is the Dahlquist Criterion a fixed, universal law for all adhesives?
A: No. It is an empirical observation based on room temperature testing. It works best for conventional PSAs tested at 1 Hz. Some modern micro-structured adhesives can show tack even with a higher modulus, but for standard HMPSA, it remains the gold standard.
Q2: Does temperature affect the Dahlquist Criterion?
A: Absolutely. The modulus of a viscoelastic material changes with temperature. An HMPSA might meet the criterion perfectly at 23°C (room temp) but become brittle and fail the criterion at 5°C (cold application). This is why cold-temperature HMPSA formulations use different polymers and resins to keep the modulus low even in the cold.
Q3: How do we measure G' to check the criterion?
A: Using a Dynamic Mechanical Analyzer (DMA) or a Rheometer. You place a small sample of the cooled HMPSA between parallel plates, apply a small oscillating force (typically at 1 Hz), and measure the material's resistance to deformation (storage modulus G') and its ability to dissipate energy (loss modulus G").
Q4: What happens if my HMPSA has a G' of exactly 300,000 Pa?
A: It will likely be borderline. The criterion suggests the material needs to be below this value for good instant tack. At exactly 300,000 Pa, you might get acceptable adhesion on smooth surfaces (like glass) but poor performance on rough surfaces (like recycled cardboard). Formulators usually aim for a safety margin well below this number.
Q5: Can I use the Dahlquist Criterion for non-HMPSA adhesives like UV-cured or water-based PSAs?
A: Yes! While the article focuses on hot melts, the Dahlquist Criterion applies to any type of pressure-sensitive adhesive (solvent-based, water-based, or radiation-cured). The principle of needing a low modulus for wet-out is universal across all PSA technologies.
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