New Latex Paint Additives Improve Washability of Hydrophobic and Hydrophilic Stains

TAGS:  Architectural Coatings    Industrial Coatings 

This is a sponsored article by Stepan Company.

For architectural interior coatings, homeowners often need to wash off stains caused by accidental spills, children’s artwork, or heavily used areas to maintain clean walls or before re-painting. Unfortunately, newer low-VOC latex paints that have environmental and regulatory approvals have poor stain resistance and are not easily washable.

However, there are various ways to improve the washability of modern paint systems. One simple method is to use additives in the paint formulation that not only provides better washability but also prevent the strong attachment of common stains.

This article introduces two such additives available from Stepan Company (STEPCOTE® W-700 and STEPCOTE® W-714) that:

• Improve washability for various types of stains and
• Improve properties such as foam resistance and corrosion protection.

The recommended use level in latex formulations is only about 0.4 percent by weight solids, and the additives can be incorporated during the letdown phase of paint production, or they can be post-added to the final formulation. In this article, performance comparisons are made to other additives and established methods of improving washability in latex paint formulation are discussed.

Let’s explore how STEPCOTE® additives show improved washability from various types of stains in latex paint formulation.

Drivers for Improved Washability and Stain Resistance

Increased regulatory pressure and public awareness of environmental and health issues have led to the development of water-based, low-VOC paints and coatings, which make up many of the household paints on the market today. These stringent regulations and policies are expected to drive the continued growth of the global, low-VOC paint market.

Unfortunately, the environmental benefits of the low-VOC paint market come with some disadvantages. Typical low VOC latex paint forms soft, tacky coatings that show:

• Poor stain resistance and washability due to the types of polymers used,
• The elimination of volatile coalescents, and
• The need to lower minimum film formation temperature (MFFT).

The situation is further complicated in deeply colored paint formulations that require high loading of low or non-VOC colorants. These colorants generally have soft components, making hard film formation even more difficult. Another complication is the consumer’s desire for semi-gloss or matte (flat) paint finishes. These paints have rougher surfaces which tend to hold onto dirt making them difficult to clean. Aggressive cleaning can also remove some of the paint finish.

The main feature of stain-resistant or washable paints is the ease with which individuals can wipe marks and substances from the paint’s surface. This is the reason why homeowners today select stain-resistant paints. Stain-resistant paint is ideal for high-traffic areas of a home or business, such as kitchens, entryways, hallways, bathrooms, and storefronts. They allow walls to look clean and stain-free significantly longer than conventional paint formulations and reduce maintenance costs by preventing frequent re-painting.

The global stain-resistant coatings market is expected to expand to 4.5 Billion USD by 2027, growing at a CAGR of 5.4% from 2020-2027. The water-based segment of this market is projected to grow at 5.8% CAGR for the same period. This segment currently accounts for a 36% share of the global overall stain-resistant coatings market.^[1]

Types of Stains and Potential Solutions to their Problem

Common substances that can stain painted walls are generally divided into two groups: hydrophilic and hydrophobic. Solutions to staining problems must counteract both types of stains.

• Hydrophilic stains are generally water-soluble stains. They consist of common household stains: sauces, tea, coffee, red wine, colored markers, etc. The washability of hydrophilic stains depends on the water resistance of the film.


• Hydrophobic stains are generally oily, non-water-soluble stains. They mainly come from oily hands or fingerprints typically found around door frames and light switches. Other hydrophobic stains are oily foods, crayons and pencils, or cosmetic makeup.

To make a paint that is easily washable, the most important factor is to reduce the stain’s penetration deeper into the paint film. There are a few ways paint formulators can design a paint to achieve this — generally by variations in binder type, pigment volume content (PVC ratio), and the additives package. Stain washability depends largely on additives if the binder type and PVC level are set.

Formulation additives, such as surfactants or wetting agents, can also play an important role in providing stain resistance. Optimized additive packages can deliver cost-effective solutions for the paint formulator. There are a number of stain-resistant additives on the market. These include fluorosurfactants, EO/PO copolymeric wetting agents, and others.

It is difficult to find a stain-resistant additive that is effective with both types of stains. For example, fluorosurfactants, while effective against oily stains, have no positive impact on water-soluble stains.

The use of hydrophobic additives, such as silicone oils or waxes, can boost hydrophilic stain washability. However, this approach can also lead to poor oil resistance as well as surface defects, such as pinholes, craters, and fisheyes. Also, the use of certain surfactants, such as fluorosurfactants and alkylphenol ethoxylates (APEO) containing surfactants, may be constrained by existing and potential environmental regulations.

STEPCOTE® Additives and How They Work?

Stepan’s STEPCOTE® W-700 and STEPCOTE® W-714 are effective nonionic wetting agents for use in waterborne paint formulations. In addition to being a wetting agent, these additives can improve washability as well as reduce foam formation and flash rusting. Other basic paint properties such as gloss, scrub resistance, and dry adhesion remains unaffected.

The benefit of these STEPCOTE® additives for improving washability is primarily due to their ability to develop a tight monolayer film at the paint surface. Once the paint is applied to a substrate, surface-active ingredients such as emulsifiers, defoamers, wetting agents, and other additives quickly migrate to the interface. The composition of these additives will influence the monolayer formation at the surface.

For a base paint that contains only an emulsifier and no additive, the emulsifier will be the only surface-active ingredient that can be at the interface as shown in Figure 1 (top). However, when the paint contains one of these STEPCOTE® additives, both the emulsifier and the additive can interact with each other. The blend has a much lower critical micelle concentration (CMC) and smaller surface area compared to the pure emulsifier.


Assuming the emulsifier and additive have similar speeds moving toward the interface, then both molecules will be at the interface and result in much closer packing when compared to the pure emulsifier. This close packing is due to the much smaller surface area as shown in Table 1. The tight molecular surface packing is illustrated in Figure 1 (bottom).

Agent	CMC, mg/L	Minimum Surface Tension, mN/m	Area/Molecule, (Å2)
Anionic Emulsifier	353	36.1	108.6
STEPCOTE® W-714	3.8	32	25.9
Blend of STEPCOTE® W-714/Emulsifier at 1.6/1	3.2	28	49.5

Table 1: Physical Properties of Additive and Emulsifier
For a stain to be washed off easily from a painted surface, the stain must be prevented from penetrating deeply into the paint film. A tighter surface film can play a key role in doing this. In addition to the tight surface film, STEPCOTE® W-700 and STEPCOTE® W-714 can also interact with acidic stains^[2], such as coffee and grape juice, to keep the stain molecules on the surface.

Resulting Performance Properties of STEPCOTE® Additives

Stepan has found that its two specialty additives, STEPCOTE® W-700 and STEPCOTE® W-714, can improve the paint system’s washability for various stains, including both hydrophilic and hydrophobic types. The following describes the research that validates this conclusion.

The paint formulation used to compare washability in this study is given in Table 2. It is a semi-gloss paint with 29.8% PVC. The selected additives in Table 3 were post-added to the paint at a 4 lb/100 gal dosing level for the washability study.

Component	Function	Wt. %
Titanium Dioxide TR 93	TiO2 pigment	23.4
TAMOL™ 165A	Dispersant	1.2
KATHON™ LX 1.5%	Biocide	0.1
BYK-024	Defoamer	0.1
Acrylic Latex (~45% act.)	Binder	47.7
ROPAQUE™ ULTRA	Opaque Polymer	4.2
Texanol	Coalescing	0.7
Ammonia hydroxide (28%)	Neutralizing	0.3
RM 8W	Rheology modifier	0.2
RM 2020NPR		3.4
Water	Carrier	18.6

Table 2: Paint Formulation Used in This Study

Additive	Function
No Additive	Negative Control
Fluorosurfactant	Positive Control
EO/PO Copolymer	Common wetting agent
STEPCOTE® W-700	Specialty Additive
STEPCOTE® W-714	Specialty Additive

Table 3: Selected Additives for Paint

#1 - Study to Compare Surface Energy of Paints Containing Different Additives

Before conducting the wash test, the dried paint films were tested for their surface energy properties. The surface energy of paints with different additives is shown in Figure 2.

• The base paint without any additive showed predominant dispersive surface energy which indicates that it is more hydrophobic.
• The paint containing fluorosurfactant showed predominant polar surface energy suggesting that its more hydrophilic, which could have less affinity to oily stains^[3].
• The commonly used nonionic wetting agent (EO/PO copolymer) didn’t change the surface energy of the base paint, while STEPCOTE® W-700 and STEPCOTE® W-714 made a noticeable change to the paint’s surface energy and resulted in a balanced surface energy between hydrophilicity and hydrophobicity.

#2 - Study to Compare Washability of Different Types of Stains

Seven different types of stains were selected to study the washability of the formulated paints:

• Standard Black Oily Stain (Leneta ST-1),
• No. 2 Pencil (Staple),
• Blue Pen (Paper Mate),
• Purple Crayon (Crayola),
• Grape Juice (Welch’s, 100% natural),
• Coffee (Keurig Classic brewed in 4oz cup),
• Yellow Mustard (Heinz, 100% natural).

A paint sample was applied by drawdown perpendicular to the length of a Leneta P121-10N scrub chart using a 7 mil Dow blade. The film was then allowed to dry in a horizontal position for 7 days before stain was applied to the paint. Paint containing the additive and paint without additive (control) were applied on the same panel side-by-side to ensure the same testing condition for the wash study.

For Standard Black Oily Stain (Leneta ST-1), the washability test was done according to ASTM D3450. Three mils of soilant were applied to the dried paint films, and the stain was left for two hours before washing. Reflectance was measured, and the percentage of initial reflectance retained was calculated (100% reflectance = no stain).

For other stains, the wash test was done according to ASTM D4828. Various stains were applied to the dried paint films and left for two hours before the wash test. Stain removal was rated between 1-10 where 10 represents complete stain removal.

When the above paint films were tested for stain removal in wash tests using the ASTM D3450 test method, the removal of Leneta ST-1 oily stain was measured by reflectance change. The comparison results are graphed in Figure 3 using the paint without an additive as a control. The positive number versus the control in reflectance change indicated better cleaning. The differences can also be visually observed. The STEPCOTE® W-700 and STEPCOTE® W-714 additives demonstrated noticeable benefits for stain removal.


In addition to the ST-1 black stain, six other types of stains were tested using the ASTM D4828 method. The results are given in Figure 4 for four of the stains. When compared to the control (no additive) or to the fluorosurfactant, it was found that paints containing these STEPCOTE® additives showed better washability for pencil marks, crayon marks, grape juice, and coffee stains. However, no differences were apparent for the yellow mustard stain which was very easy to clean off and for a blue pen mark which was difficult to remove. The fluoro additive helped only slightly for hydrophobic stains such as pencil and crayon marks but did not show any effect on the hydrophilic stains. STEPCOTE® W-700 and STEPCOTE® W-714 demonstrated performance advantages over the fluorosurfactant control.

#3 - Improved Foam Control of STEPCOTE® Additives vs Emulsifier

STEPCOTE® W-700 and STEPCOTE® W-714 are nonionic. They have a low foam tendency and can even work as defoamers in some cases. The foaming tendency of the additives and a pure emulsion were measured using a mechanic cylinder shaker test. In this test, a 100 ml solution of 0.4% additives in tap water was added into a 500 ml cylinder. The cylinder was mounted to the shaker and inverted 10 times. The foam volume was then measured immediately. Two tests were completed for each product, and the resulting averages are given in Figure 5.

#4 - Improve Corrosion Resistance of Paints Containing Different Additives

STEPCOTE® W-700 and STEPCOTE® W-714 also provide anti-corrosion benefits due to their chemical structure^[4]. Paint containing these additives showed reduced flash rust when applied to a metal substrate. The results are shown in Figure 6.

#5 - STEPCOTE® Additives Maintain Other Performance Properties

In addition to the benefits shown above, the STEPCOTE® additives maintained other basic paint properties such as gloss, scrub resistance, and dry adhesion. Figure 7 displays the resulting properties when the paint with the STEPCOTE® additives is compared to the base paint without additives and to the paint containing a fluorosurfactant.

Typical Properties of STEPCOTE® Additives

Properties	STEPCOTE® W-700	STEPCOTE® W-714
Chemical Description	Ethoxylated tallow amine	Alkylamine Ethoxylate
CAS Registry Number	61791-26-2	61791-14-8
Appearance at 25°C	Clear liquid	Clear liquid
Actives, %	100	>99.5
Density at 38°C, g/ml (lbs/gal)	0.89 (7.4)	0.90 (7.5)
Viscosity at 25°C, cps	-	117
pH (5% in 1:1 IPA/H2O)	9.8	10.8
Flash Point, SETA, °C (°F)	>94 (>201)	>94 (>201)
Pour Point, °C (°F)	21 (70)	2 (36)
HLB Calculated	5	6
RVOC, US EPA, %	0	0
Water Solubility	-	Dispersible

These additives can be incorporated during the letdown phase of production or post-added to the final formation. The typical use level, based on active content is 2-6 lbs per 100 gallons; however, optimal dosing should be determined by performing a ladder study. Both additives are viscous liquids below their pour points and may thicken or solidify. These wetting agents may be fluidized by warming with a drum heater or in a steam room at a temperature less than 54°C.

Conclusion

STEPCOTE® W-700 and STEPCOTE® W-714 additives in latex paint formulations showed improved washability for various types of stains. The additives were effective on both hydrophilic and hydrophobic types of stains, thereby showing superior washability characteristics to fluorosurfactant and EO/PO copolymer additives. These STEPCOTE® additives also reduced foaming and substrate corrosion, without impairing good overall paint properties such as gloss, wet scrub resistance, and dry adhesion.


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