Process development for low-colour SEBS/PP tackified compounds requires simultaneous control of screw torque and surface tack because both respond to the same molecular weight and solubility parameters of the tackifier but in opposite directions. In a typical co-rotating twin-screw extrusion line with segmented screws, 40:1 L/D, 25 mm diameter, torque is recorded from the drive motor as a percentage of rated torque and calculated as specific mechanical energy in kWh/kg. The SEBS phase contributes ethylene-butylene midblock elasticity and styrenic endblock strength; the PP phase provides hardness and heat resistance. Hydrogenated hydrocarbon resins with softening points between 100 °C and 140 °C, Gardner colour below 1, and APHA values below 50 are commonly used when low yellowness is non-negotiable. Melt viscosity of the unfilled compound at 190 °C and 100 s⁻¹ typically lies in the 60 Pa·s to 120 Pa·s range. The addition of 10 wt% hydrogenated C9 resin can reduce shear viscosity by 15% to 30% and lower extrusion torque by 10% to 25% depending on the base PP molecular weight and SEBS/oil ratio. This same resin partitions into the ethylene-butylene midblock, raising the midblock T_g from about −55 °C to −35 °C or higher, which increases room-temperature surface tack measured by ASTM D2979 probe tack from roughly 0.6 N to 1.8 N. The conflict is that lower torque improves throughput and reduces thermal degradation but can also decrease specific energy below 0.18 kWh/kg, which is often insufficient for complete dispersion of antioxidant packages when the screw design relies on high shear kneading blocks. The compounder must therefore choose between a higher-torque, better-mixed compound with lower tack and a lower-torque, tackier compound with potential dispersion defects. The use of low-colour resins does not eliminate this tradeoff; it only shifts the yellowing boundary because hydrogenated C9 resins have residual aliphatic unsaturation that can still oxidise above 210 °C.

Molten tackifier lowers torque but only until phase saturation of the ethylene-butylene midblock is reached

The selective migration of hydrogenated C9 resin into the ethylene-butylene midblock is governed by solubility parameter matching and is not a simple linear dilution effect. When the tackifier concentration remains below the phase saturation limit, the Fox equation can be used to estimate the resin-midblock glass transition temperature: 1/T_g = w₁/T_g,1 + w₂/T_g,2. For an SEBS midblock with T_g,1 = 218 K and a hydrogenated C9 resin with T_g,2 = 333 K, the mixture at 15 wt% resin has a predicted T_g of approximately 230 K (−43 °C), and at 30 wt% resin the predicted T_g is approximately 243 K (−30 °C). The corresponding increase in midblock T_g increases the rate of bond formation in pressure-sensitive adhesion, which is why probe tack per ASTM D2979 rises steeply at first. Screw torque, however, reflects the bulk melt viscosity, which is more strongly influenced by the low molecular weight of the resin than by the localised rise in midblock T_g. The first 5 wt% addition of a 110 °C softening point hydrogenated C9 resin typically reduces extrusion torque by 8% to 12%; the next 5 wt% reduces torque by an additional 5% to 8%; and the third 5 wt% reduces torque by only 2% to 4%. Surface tack rises in the opposite direction with diminishing returns: the first 5 wt% may raise probe tack from 0.5 N to 1.2 N, the second 5 wt% to 1.8 N, and the third 5 wt% to 2.1 N. Published data for specific torque-tack curves in a fixed SEBS/PP matrix are limited because the response depends on PP melt flow rate, oil type, screw speed, and residence time distribution. Nevertheless, production-scale observations on a 40:1 L/D segmented co-rotating twin-screw extruder show that the torque plateau coincides with visual migration of low-molecular-weight resin to pellet surfaces, an effect detected by blocking tests per ISO 11502. Representative data for a constant barrel temperature of 190 °C, screw speed 350 rpm, and throughput 12 kg/h are shown in the following table.

Low-colour specifications for medical and optical SEBS/PP compounds routinely require yellowness index below 2.0 per ASTM E313 and haze below 10% on 2 mm injection-moulded plaques per ASTM D1003. Hydrogenated hydrocarbon resins achieve this because residual aromatic unsaturation is below 1 mol% and the Gardner colour after 1 h at 180 °C remains below 1. The antioxidant system is not optional in this formulation. A blend of 0.1 wt% primary hindered phenol, 0.2 wt% secondary phosphite, and 0.05 wt% acid scavenger protects both the resin and the PP phase during extrusion. Without the phosphite, residence times greater than 90 s at 200 °C generate conjugated carbonyl chromophores that increase yellowness index by 0.8 to 2.5 units. Screw torque should not be allowed to exceed 85% of the motor rating for more than 10 min because wall slip and viscous dissipation can create local melt temperatures 20 °C to 40 °C above barrel set points. These localised temperature excursions can trigger oxidative degradation even when the barrel setting is below 200 °C. Low-colour tackification therefore requires monitoring not only the average barrel temperature but also the melt temperature after the last kneading block and the die pressure fluctuation. If die pressure fluctuates more than 0.5 MPa over a 30 s interval, the melt is often experiencing intermittent wall slip, which can produce torque readings that understate the actual viscous response and allow temperature-sensitive resin to degrade.

What happens to surface tack when a paraffinic oil is replaced by a hydrogenated C9 resin at constant Shore hardness?

Replacing 10 wt% paraffinic oil with 10 wt% hydrogenated C9 resin at constant Shore A hardness requires a simultaneous reduction in PP content because oil and tackifier differ in molecular weight and in their effect on the ethylene-butylene midblock. Paraffinic oil has a number-average molecular weight around 250 g/mol to 500 g/mol, while a hydrogenated C9 resin has a number-average molecular weight of 500 g/mol to 1500 g/mol and a branched, cyclic structure. The oil reduces melt viscosity more efficiently per unit mass because it is a low-viscosity linear diluent; shear viscosity at 190 °C and 100 s⁻¹ may fall by 25% to 40% with 10 wt% oil, while the same loading of hydrocarbon resin reduces viscosity by 15% to 25%. The oil also reduces screw torque more aggressively but it can migrate to the surface within days and cause unpredictable changes in tack. In contrast, the resin raises the midblock T_g and provides a more stable surface-tack response after aging for 168 h at 60 °C. When hardness is fixed at Shore A 65 per ASTM D2240, the oil-replaced formulation usually shows an increase in 180° peel adhesion from 0.8 N/25 mm to 2.2 N/25 mm per ASTM D3330 and an increase in loop tack from 1.2 N/25 mm to 3.5 N/25 mm per ASTM D6190. The melt flow rate per ASTM D1238 at 230 °C and 2.16 kg increases from 9 g/10 min to 14 g/10 min because the resin remains partially miscible in the PP phase and reduces its viscosity. The processing consequence is that screw torque drops by only 8% to 14% instead of the 20% to 30% seen with oil, but the final surface tack remains stable and the colour remains lower because the hydrogenated resin does not exude and oxidise at the surface as easily as an oil-rich surface layer.

Injection moulding of tackified SEBS/PP compounds on a 100-tonne hydraulic press with a 35 mm screw and 20:1 L/D typically requires clamp force to remain below 60% of machine capacity. The reduced melt viscosity from 10 wt% to 15 wt% tackifier drops injection pressure from approximately 90 MPa to 65 MPa at a melt temperature of 200 °C. However, the same tackifier addition raises the coefficient of friction of the solidified part, increasing demoulding force and extending ejection time. A low-colour part with 15 wt% hydrogenated C9 resin may require mould release spray after 500 cycles unless the tool steel is coated with diamond-like carbon or a PTFE-impregnated nickel release layer. Surface tack measured one day after moulding per ASTM D6190 loop tack can rise by 20% to 40% compared with pellets because orientation and surface enrichment occur during injection. Therefore, torque reduction in the extruder does not translate directly to mould release behaviour, and processing trials must include a specified ejection force measurement, such as the force required to eject a standard cup or plaque mould, to avoid unscheduled downtime from parts sticking in the B-side cavity. In multi-cavity tools with long flow paths, the lower melt viscosity from tackifier addition can improve filling of thin-wall sections down to 0.5 mm, but the same low viscosity can cause flashing at parting lines if clamp force is not adjusted.

When the tackifier level exceeds the midblock solubility limit, surface tack becomes blocking after thermal aging

The phase saturation limit in an SEBS/PP/oil blend is not a single number because it depends on the aromaticity of the SEBS, the ethylene-butylene block length, the polypropylene crystallinity, and the presence of extender oil. For a typical 30 wt% SEBS, 50 wt% PP, 10 wt% paraffinic oil system, hydrogenated C9 resin remains miscible at room temperature up to about 18 wt% to 22 wt%. Above this level, a separate tackifier-rich phase begins to migrate to the surface over time. Surface tack then increases sharply in the first 24 h to 72 h after moulding, but the surface also becomes hazy and blocking force increases. Blocking force measured per ISO 11502 on 2 mm plaques can rise from 0.2 N/cm² to 1.5 N/cm² after thermal aging for 7 days at 60 °C and 90% relative humidity. This is not desirable for stacked components or roll-fed film because surfaces require separation forces that exceed the strength of thin sections. The torque advantage also disappears above the saturation limit: melt viscosity is already low enough that the resin is acting as an external lubricant at the metal surface rather than a homogeneous viscosity modifier. Capillary rheometry per ASTM D3835 at 190 °C and 100 s⁻¹ often shows a further apparent viscosity drop of 10% to 15% because of slip, but this is not representative of the bulk flow behaviour. A pressure transducer placed immediately before the die can detect slip when the measured pressure falls more quickly than the mass flow rate would predict. Consequently, torque readings alone are inadequate for process control above 18 wt% to 20 wt% tackifier loading; the process must be controlled by melt pressure, melt temperature, and an in-line rheometer if available.

Compounding screw design determines whether torque reduction from tackifier addition is beneficial or detrimental to dispersion quality. A segmented co-rotating twin-screw profile with 30°, 60°, and 90° kneading blocks in zones 4, 6, and 8 generates high elongational stress and disperses gel particles but also increases torque. When tackifier lowers viscosity too early in the machine, upstream melting can become uneven because the resin plasticises the PP before the SEBS has fully broken down. The result is a melt with undispersed SEBS domains, which can appear as surface defects even though screw torque is within an acceptable range. A compromise screw profile for tackified low-colour SEBS/PP places the first intensive kneading section after the resin injection point, not before it. Temperature zones should be profiled from 160 °C in the feed throat to 190 °C at the die, with the highest temperature in the zone immediately after the first kneading block. Venting must be at atmospheric pressure between zones 6 and 7, and a vacuum of −0.08 MPa to −0.09 MPa in zone 8 is required to remove low-molecular-weight volatiles from the hydrogenated resin. If the vacuum port becomes fouled with condensed resin droplets, the colour of the final compound degrades, and surface tack becomes unpredictable. In production, fouling of the vacuum port is an early indicator that the tackifier loading is too high for the specific screw temperature profile or that the resin softening point is too low for the barrel temperature. The power-law index of the melt is commonly reduced from approximately 0.45 to 0.35 by high tackifier loading, which increases shear sensitivity and narrows the metering-zone processing window to approximately ±5 °C. Operational boundaries are not limited to the compounding line. Compounds containing more than 15 wt% hydrogenated C9 resin should not be stored in stacks at temperatures above 40 °C because blocking force per ISO 11502 rises by approximately 0.5 N/cm² for every 10 °C increase above ambient. Pre-drying is generally unnecessary when the packaging remains sealed and relative humidity is below 60%; if condensation is visible on pellet surfaces, drying for 2 h at 80 °C is required before injection moulding. Amine-based antistatic additives and primary amide slip agents should be avoided in low-colour SEBS/PP compounds because they can promote yellowing in the presence of hydrogenated resins at elevated temperatures and create surface haze after ethylene oxide sterilisation at 55 °C.

The following test matrix is used to evaluate the tradeoff between screw torque and surface tack in a production environment. Each method is selected to eliminate anchorless comparisons: torque is recorded as percent of motor rating, surface tack as force, and colour as an optical index. The matrix is not a specification; it is a screening protocol for comparing formulations across extrusion campaigns and moulding lots.

Regulatory compliance for low-colour SEBS/PP tackified compounds used in food-contact or medical applications requires verification that the tackifier is a hydrogenated hydrocarbon resin meeting FDA 21 CFR 177.1520(c) for olefin polymers and FDA 21 CFR 177.1810 for styrene block copolymers, or equivalent EU 10/2011/EU migration limits. The antioxidant package must be within the positive lists of EU 10/2011/EU and, for medical use, meet ISO 10993-5 cytotoxicity requirements after ethylene oxide sterilisation at 55 °C. Hydrogenated C9 resins often have lower extractables than rosin esters and unhydrogenated C5 resins, but the final migration value depends on the dispersion quality achieved during compounding. If screw torque drops below 0.15 kWh/kg and specific energy is insufficient to disperse the resin uniformly, the product may fail extraction tests even though the incoming raw materials are compliant. This is the central operational link between screw torque, surface tack, and low-colour performance: the same rheological changes that reduce torque also alter the distribution of the tackifier, and that distribution controls both surface properties and regulatory extractables.