Polypropylene, BOPP film, and PET are not difficult substrates because they are chemically inert in every sense-they are difficult because their surfaces offer little for a water-based coating or ink to hold onto. Low surface energy and a shortage of polar functional groups mean waterborne acrylics, polyurethanes, and styrene-acrylic systems often wet the surface but fail to anchor to it. BOPP makes the problem especially visible: its inherent hydrophobicity directly challenges the adhesion of water-based inks, and solvent-based primers have traditionally been the workaround. The trade-off is VOC load and the handling measures that come with it.
WP-325 is a waterborne chlorinated polypropylene dispersion positioned squarely in that gap. It is not a universal adhesion solution, but it gives formulators and applicators a route to improve adhesion on PP, BOPP, PET, and nylon without returning to a solvent-borne CPP primer.
01 What WP-325 does at the interface
WP-325 is a chlorinated polypropylene dispersion with 30% effective content, a Tg of approximately 25°C, and a viscosity of 4,500 mPa·s at 25°C.
Its polymer structure carries two useful tendencies at once. The non-polar polypropylene backbone has an affinity for polyolefin surfaces such as PP and BOPP. Chlorination introduces polar character that supports interaction with waterborne acrylic and polyurethane resins. The adhesion between chlorinated polypropylene and a polyolefin substrate is generally understood to be dominated by dispersion forces rather than chemical bonding-waterborne chlorinated polyolefin primers have been shown to form adhesive structures through physical interactions at the interface. WP-325 is not "welding" a chemical bridge across the interface; it improves wetting and interfacial compatibility so that a water-based coating has a more workable surface to adhere to.
That mechanism also defines the boundary. WP-325 addresses insufficient adhesion; it does not replace substrate cleaning. Mold release agents, oil, and low-molecular-weight surface bloom will undermine any adhesion promoter, waterborne or solventborne.
02 Two use routes: formulation additive or stand-alone primer
WP-325 can be used in two ways, and they solve slightly different problems.
Route one: as an additive in an existing waterborne formulation
If a waterborne coating, ink, or adhesive already performs well except for adhesion to PP, BOPP, PET, or nylon, adding WP-325 is the least disruptive change. The recommended dosage is 5%–15% by total formulation weight, with the exact level best confirmed by a small-scale adhesion test on the target substrate.
At recommended levels, WP-325 is typically compatible with waterborne styrene-acrylic emulsions and polyurethane dispersions, without introducing stratification, sedimentation, or demulsification under normal storage conditions. It is also formulated not to reduce the hardness of the finished system and can contribute to water resistance. If the host formulation has marginal stability, a short storage comparison is still worth running before committing to a dosage.
Route two: as a water-based primer
When adhesion demands are higher, or the substrate is especially difficult to wet, diluting WP-325 and applying it as a stand-alone water-based primer usually gives a more direct adhesion response.
Dilute WP-325 with water at 1:6–8 by weight, preferably using deionized or purified water. Adding a small amount of a water-based wetting agent-around 0.1%–0.3% of the diluted primer weight-helps reduce surface tension, limit defects such as cratering on low-energy plastics, and improve substrate wetting. Spraying, roll coating, and brushing are all workable application methods. The primer layer is thin and typically touch-dry at room temperature within minutes, after which the topcoat can be applied.
Typical properties
| Property | Reference value |
|---|---|
| Effective content | 30% |
| Tg | 25°C |
| Viscosity at 25°C | 4,500 mPa·s |
| Specific gravity | 0.92 |
| Additive dosage | 5%–15% of total formulation |
| Primer dilution | WP-325 : water = 1:6–8 (by weight) |
| Wetting agent | 0.1%–0.3% of diluted primer |
| Target substrates | PP, BOPP, PET, nylon |
03 Where it fits-and where it does not
WP-325 is relevant to waterborne coating of plastic consumer goods and small appliance housings, water-based ink or functional coating on BOPP and PET film, waterborne primer for automotive interior plastic parts, and water-based adhesive formulations that need improved bonding to plastics.
Two selection boundaries are worth stating plainly. First, WP-325 is a chlorinated polypropylene dispersion. Its mechanism and substrate fit differ from silane coupling agents, titanate adhesion promoters, and oxazoline-functional crosslinkers; these should not be treated as interchangeable options without adhesion testing. Second, non-chlorinated modified polyolefin adhesion promoters exist as an alternative route, and they are sometimes preferred where chlorine content is a concern or where higher chemical and temperature resistance is required. Chlorinated and non-chlorinated routes are not identical in performance profile, and the choice should follow the end-use constraints rather than a general preference.
For any system where adhesion is critical, a cross-hatch or peel test remains the reference point. Dosage tables and dilution ratios are starting conditions, not guarantees.
In short: WP-325 offers a water-based path to improved PP, BOPP, and PET adhesion for formulators who want to stay out of solvent-borne CPP systems. It works either as a formulation component or as a thin waterborne primer, with defined starting dosages and dilution ranges. Final performance depends on substrate condition, resin compatibility, and application parameters, as it does with any adhesion promoter.