Non-Phthalate Benzoate Plasticizers Do More than Just Plasticize

Edward M. Petrie

EMP Solutions Cary, NC

Keywords: Plasticizers, benzoates, adhesives, sealants, waterborne, hot melt, volatility, migration,

adhesion, glass transition temperature, compatibility. Summary: This article reviews the various value propositions provided by benzoate plasticizers in

adhesive and sealant formulations. It primarily focuses on waterborne systems, but opportunities in other product forms are also noted. The main applications for plasticized adhesives and sealants are identified as well as processes for selecting and evaluating a suitable plasticizer.

Introduction

Plasticizers are used in all types of adhesive and sealant formulations: waterborne, hot melt, solvent based, and 100% solids reactive. They are used primarily to flexibilize the final product and provide improved low temperature resistance. However, plasticizers can also provide additional benefits that are

important to the formulator and end-user. Table 1 summarizes the main functions performed by plasticizers in adhesive and sealant formulations.

Table 1 Function of Plasticizers in Adhesives and Sealants

Function Solvent Based

Water-borne

Hot Melt 100% Solids

PVC Plastisol

Soften, decrease modulus, decrease Tg, improve low temperature properties

Increase solids content Replace co-solvents Improve adhesion, tack, and heat sealing temperature

Modify viscosity (+ and -), liquid adhesives Reduce melt viscosity, hot melt adhesives Improve coatability Extend open time and reduce set time Improve processing characteristics Lower gelation / fusion temperature Improve paintability

Over the years, benzoate esters have proven to be excellent plasticizers for adhesives and sealants of all

types and have especially claimed fame as non-phthalate plasticizers for waterborne latex systems. Benzoates were introduced as polyvinyl chloride (PVC) plasticizers in the 1990s. Although today the largest application area for these plasticizers is in adhesives and sealants (Figure 1), demand for

benzoates in PVC applications is growing very rapidly.

Eastman Chemical Company is one of the leading global suppliers of non-phthalate plasticizers. In 2010 Eastman acquired Genovique Specialties Corporation, an important supplier of benzoate plasticizers.

Table 2 provides a description of Eastmans product offerings and related applications. The Appendix summarizes the technical features for these benzoate plasticizers.

This article reviews the various value propositions provided by benzoate plasticizers in adhesive and sealant formulations. The focus is on waterborne systems, but opportunities in other product forms are also noted. The main applications for plasticized adhesives and sealants are identified, as well as

processes for selecting and evaluating a suitable plasticizer.

Figure 1 US consumption of benzoate plasticizers.

1

1 Plasticizers, Chemical Economics Handbook, IHS Chemical, November 2009.

Table 2 Non-Phthalate Eastman Plasticizers for the Adhesives and Sealants Market2

Types of Plasticizers and End-Use Applications

The following applications prominently use plasticized adhesives and sealant formulations, but this list is not exhaustive.

Packaging carton sealing (waterborne emulsions) Automotive adhesives and sealants Construction adhesives and sealants decorative wall panels, flooring, window glazing, caulks

Bookbinding (hot melt adhesives) Tapes and labels (pressure sensitive adhesives) Textiles nonwoven fabrics and fibers (hot melt and waterborne emulsions)

The use of plasticizers is very common in adhesives and sealants , but they are used in lower concentrations than in plastics or elastomers. Plastisol and cohesively removable pressure sensitive

adhesive products are exceptions as they require high concentrations of plasticizers for the conditions of their performance.

A large number of different resins and, thus, a large number of plasticizers are used in adhesives and sealants. Different plasticizers affect different physical and chemical properties of materials. There are over 38 general chemical families of plasticizers that are used for polymer modification.

3 The major

2 Eastman Flexible Solutions for Strong Brands, Publication L-249, Eastman Chemical Company, April 2012.

3 Wypych, G., Plasticizer Types, Chapter 2 in Handbook of Plasticizers, Wypych, G., ed., William Andrew

Publishers, New York, 2004.

chemical classes are listed in Table 3. Products may also use blends of plasticizers to achieve the desired end properties.

Table 3 Chemical Classes of Selected Plasticizers

Phthalates

Benzoates

Terephthalates

Adipates

Azelates

Trimellitates

Sulfonamides

Polyesters

Sebacates

Citrates

Epoxy oils

Soy and other oil derivatives

Glycol ethers and

esters

Hydrocarbons

Oils (paraffinic,

naphthenic, and aromatic)

Phosphates

Ethers

Glutarates

Terpenes

Pentaerythritol esters

Ricinoleates

Isobutyrates

Historically, phthalates have been the most popular choice of plasticizer because they usually provide the

best all-around performance (especially in flexibilizing PVC) and they are low in cost. However, phthalates have come under increasing regulatory and consumer scrutiny due to their migration potential and concern regarding their toxicity, safety, and environmental burden.

Non-phthalate plasticizers, such as benzoates, terephthalates, citrates, sulfonamides, and polymeric plasticizers, have begun replacing many of the phthalates due to regulatory and toxicological concerns.

Regulatory changes and shifting consumer preferences are driving rising demand for non-phthalate alternatives in the USD $12+ billion global plasticizer market. Demand for non-phthalate alternatives is expected to grow at a compounded annual rate of 7% over the next five years in North America and

Europe. Some adhesives and sealants demand plasticizers with special characteristics. For example, the use of

solid or high boiling point liquid plasticizers is generally required for hot melt adhesives. The plasticizer should melt just below the temperature of application but revert to solid form after the bond is formed. Reactive adhesive and sealants may also require high boiling point plasticizers because these products

are often cured at elevated temperatures. On the other hand, waterborne adhesives require the use of liquid plasticizers that can be easily

dispersed in water. Even though the plasticizer may be relatively insoluble in water, it is easily absorbed by the particles of the polymer emulsion, allowing for easy compounding.

While there are many types of plasticizers, there are relatively few for waterborne adhesives. This is an application where benzoates have a commercial and technical edge. Although other plasticizers are often used in waterborne adhesive and sealant formulations, benzoates appear to offer an optimal

balance of performance and value. Benzoate plasticizers are relatively polar which allows compatibility with many polymers that are used in

formulating emulsions, hot melts, solvent, and reactive adhesives and sealants. Common base polymers that are compatible with benzoate plasticizers and their end-use applications are indicated in Table 4.

Table 4 Common Polymers in Adhesive and Sealant Formulations Compatible with Benzoate Plasticizers

Polymer Abbreviation Primary Adhesive Form End-Use Polyvinyl acetate PVAc Waterborne Woodworking, packaging

Vinyl acetate ethylene VAE Waterborne Woodworking, laminating films Ethylene vinyl acetate EVA Hot melt Packaging Polyvinyl chloride PVC Plastisol Sealant (auto)

Styrene butadiene rubber SBR Waterborne and solvent Sealant (construction) Ethyl cellulose EC Solvent Solvent cement for plastics

Nitrocellulose NC Solvent Solvent cement for plastics Cellulose acetate butyrate CAB Solvent Solvent cement for plastics

Polyurethane PU Reactive, waterborne, solvent Sealant (construction), structural adhesive

Polysulfide PS Reactive Sealant (aircraft, auto)

Acrylic PMMA Waterborne and solvent Packaging, tapes and labels , sealants (construction)

Benzoate plasticizers include benzoates, dibenzoates, or any mixture thereof. Typical benzoate and

dibenzoate chemical structures are illustrated in Figure 2. Property ranges for the benzoate group of plasticizers compared to the phthalate group is provided in Table 5. Generally, the properties of these plasticizer families are comparable except for freezing point and solubility in water (favorable to

benzoates). There is some difference on weight loss during heating that also favors the benzoates.

Figure 2 Chemical structures of benzoate and dibenzoate.

Table 5 Property Range (low to high unless otherwise given) for

Benzoate and Phthalate Plasticizer Families4

Property Benzoates Phthalates

Molecular weight, daltons 178 - 1175 194 - 530 Freezing point, C -38 (low) -67 (low) Boiling point, C 249 - 323 283 - 523

Flash point, C 148 - 258 146 - 254 Refractive index at 20C 1.49 - 1.569 1.481 - 1.515

Vapor pressure, kPa (minimum) 2.5X10-8

at 25C 3.0x10-8

at 100C Solubility in water at 25C, weight % 3.04x10

-5 - 2.08x10

-3 3.0x10

-7 - 1.2x10

-3

Viscosity, MPa-s 15 - 406 at 25C 11- 320 at 20C Surface tension at 20C, mN/m (median) 40.4 33

Weight loss at 87C, 24 hrs, weight % 1.2 - 11.5 0.8 - 31

4 Wypych, G., Handbook of Plasticizers, Wypych, G., ed., William Andrew Publishers, New York, 2004.

Benzoates have replaced much of the C4 phthalate previously used in emulsion coatings and adhesives

(e.g., PVAc and acrylic). Certain performance characteristics, other than being a non-phthalate alternative, have produced a high level of interest in benzoate plasticizers. These added value propositions are shown in Table 6.

Table 6 Potential Value Propositions Offered by Benzoate Plasticizers

Property Improvements Processing Improvements Other

Low temperature properties

UV / light resistance Tensile and tear strength

improvement

Low volatility, migration resistant

Resistance to extraction by solvents, oils, etc.

Increased open times and reduced set times (waterborne and hot melt adhesives)

Improved adhesion

High degree of compatibility

Significant viscosity response High solvating power, fast fusion,

and good gelation properties

High boiling point Both solid and liquid types

available

Decreased processing time

Good coalescing agent

Possible tackifier or wax replacement

Elimination of risk labeling

Direct food contact approval Low VOCs

Regulatory compliance (REACH, ECHA, etc.)

Possible cost reduction (relative to base polymer)

Environmental and regulatory factors have been the driving force for the introduction and growth of benzoate and other non-phthalate plasticizers. Beyond their health and environmental benefits, the benzoate plasticizers have proven to offer equivalent performance to their phthalate counterparts in many

applications. To be considered as alternatives to general purpose phthalates, plasticizers must provide at least one of the following value propositions beyond being non-phthalate:

Reduced price Improved performance / processing Improved regulatory compliance (safety, health, environmental).

Price and Formulation Cost

Formulators are often reluctant to begin the costly and time consuming process of reoptimizing their formulations for a new plasticizer for the following reasons.

Higher price of non-phthalate plasticizers (although future prices are claimed to come down with volume and technology improvements) -- Prices are highly dependent upon raw material costs. Benzoate plasticizers utilizing toluene as a raw material are currently more costly than phthalate

plasticizers which are based on phthalic acid.

Availability (global availability is necessary in many high volume industries such as packaging) -- However, lack of availability is often more of a perception than reality. Recent years have seen

increases in production capacities of nearly all non-phthalate plasticizers around the globe.

Not being a drop-in replacement (reformulation and revalidation is likely in order to maintain current properties) -- Non-phthalate plasticizers can have properties that differ from phthalates,

with some properties being superior to their phthalate counterparts. The amount of reformulation necessary will depend on how tight the specificat ions for performance are and how closely the non-phthalate replacement mimics the properties of the plasticizer it is replacing.

Plasticizers that are produced in high volume, such as general purpose phthalates, have a manufacturing advantage over more specialty non-phthalate plasticizers simply due to the economies of scale generated

by producing larger volumes of product. Most non-phthalate plasticizers, including benzoates, are more expensive than phthalate plasticizers, and this is certainly a detriment to their usage especially in

applications that are not heavily regulated. Plasticizer price factors may be mitigated by formulation and processing advantages. For example, when

added to emulsion systems, plasticizer addition generally increases viscosity. This can allow improved coating and build and workability in certain applications without having to increase the concentration of the base polymer. Without using plasticizers, a converter would have to revert to using an emulsion

having higher solids content (and higher cost) to achieve high coating builds. Most plasticizers are manufactured from petrochemical feedstock, resulting in significant increases in

price and price volatility over the last several years. Biobased plasticizers (e.g., soy based) may prove to be less expensive in the long run due to the fact that they are manufactured from renewable feedstock , but this has not yet been the case. Biobased plasticizers have only recently been commercialized and

have not yet shown that they have comparable performance to other non-phthalate plasticizers based on different chemistries.

Performance / Processing

Compatibility Plasticizer compatibility is the key to plasticizer performance in adhesives and sealants. In latex systems,

compatibility can be assessed by observing the viscosity increase after the addition of the plasticizer and by observing the clarity of a cast film of the plasticized adhesives. A compatible plasticizer will give a greater viscosity response and will provide a clear dry film.

One of the main attributes of benzoate plasticizers is their good compatibility with many polymeric resins as was indicated previously in Table 4. This is due to their higher solubility and polar nature. With

respect to compatibility, benzoate plasticizers are considered to be at least equivalent to the BBP, DEHP , and other regulated phthalate plasticizers.

Benzoate plasticizers are popular in latex formulations which are common in the adhesive and sealant sectors. They have excellent compatibility with common polymeric emulsions such as polyvinyl acetate (PVAc), vinyl acetate ethylene (VAE), and acrylic.

Viscosity Response

Viscosity response is a key evaluation parameter in choosing a plasticizer. The addition of certain plasticizers to a polymeric emulsion will result in increased viscosity. This effect can be used to formulate

a waterborne adhesive having greater coating thickness build-up. When higher viscosity is not required the thickened emulsions are cut back to a specified constant viscosity, resulting in cost savings.

The degree of viscosity response is also a predictor of compatibility (i.e., the greater the viscosity response the greater the compatibility). Figure 3 illustrates the viscosity response of several benzoate plasticizers compared to conventional phthalate plasticizers such as dibutyl phthalate (DBP), diisobutyl

phthalate (DIBP), and butyl benzoate phthalate (BBP).

Figure 3 Polyvinyl acetate homopolymer viscosity response after 24 hrs.

Glass Transition Temperature Suppression

Suppression of the glass transition temperature, Tg, is an important plasticizer function as the formulator can use this to control many of the final properties of the adhesive or sealant (flexibility, low temperature

resistance, toughness, adhesion, etc.). Plasticizers that have a greater effect on the glass transition temperature are considered to be more efficient and more compatible with the base polymer.

Tg suppression is used mainly to improve the flexibility, adhesion, and low temperature properties of polyvinyl acetate homopolymers and vinyl acetate ethylene copolymers. Figure 4 illustrates the Tg suppression of benzoate plasticizers compared to conventional phthalates in reducing the Tg of a

polyvinyl acetate homopolymer.

Figure 4 Tg Suppression of a polyvinyl acetate homopolymer.

Adhesion Peel strength is often used as a measure of adhesion to a substrate. Flexible adhesives generally have

higher peel strength due to better distribution of stress and are preferred for flexible substrates such as packaging films and foils. Figure 5 illustrates the 180 peel strength of benzoate- and phthalate- plasticized polyvinyl acetate emulsions on a cloth substrate.

Figure 5 Peel strength (180 peel on dry cloth) of plasticized polyvinyl acetate homopolymer.5

Tack is also an important property in pressure sensitive adhesives. Tack can simply be defined as the

resistance of a liquid or semi-solid to separation or the stickiness of the adhesive as noticed by finger-touch. An important characteristic of plasticizers is the chemical compatibility with conventional tackifiers.

In pressure sensitive adhesive applications, benzoate plasticizers may provide additional value such as:

1. A cost reducer for tackified formulations without decreasing tack properties. For example,

dibenzyl or dicyclohexyl phthalate can act as both a plasticizer and a tackify ing agent.6

2. An additive to enhance the tack properties of tackifiers already present.

Set and Open Times

Plasticizers can significantly shorten the set time and extend the open time for waterborne or hot melt adhesives. Both effects are looked upon as improvements by the formulator as they are valued in many end-use applications including packaging. Figure 6 shows the effect of benzoate and phthalate

plasticizers on the set and open times of a polyvinyl acetate homopolymer.

5 Arendt, W.D., Benzoflex LA-705: A New Global Plasticizer for Waterborne Adhesive Market, Adhesive and

Sealants Council Meeting, 2006. 6 Wake, W.C., Adhesion and the Formulation of Adhesives, Applied Science Publishers, London, 1976, p. 261.

Set time is the time required for the adhesive to provide a measurable degree of bond strength after substrates are mated. Open time is the time that the adhesive can stand after being applied and before the substrates are mated. Longer open times may be required for substrate adjustment or production

timing. Long open times are also indicative of good wet tack.

Figure 6 Effect of plasticizers on the set and open times of a polyvinyl acetate homopolymer.

Volatility and Migration Volatility is typically a limiting factor for the use of many plasticizers. The fugitive nature of plasticizers is

important because of: (1) volatile loss during processing and the resulting necessity to increase plasticizer concentration, (2) degradation of properties with time that the adhesive bond is in service, and (3) volatility is an indication of VOCs. Benzoate plasticizers have exhibited lower volatility than DBP or other

phthalates (Figure 7.)

Figure 7 Plasticizer volatility per ASTM D2369 (weight loss after 1 hour at 110C).7

Thermal gravimetric analysis (TGA) measures the volatiles lost in a plasticizer or adhesive formulation as

a function of time at temperature. This is important in estimating migration potential as well as safety / toxicity issues. Many adhesives and coatings are processed at high temperatures (either during the compounding process or during application and cure). A high degree of volatility would signal significant

loss of plasticizer during these processes, resulting in degraded final product. Volatility can be measured in a standard thermal oven by ASTM D-2369 and this has become part of EPA-24.

Other Physical Property Improvements

Benzoate plasticizers also aid in the development of certain other physical properties. For example, improvements have been noticed in tensile properties of benzoate-plasticized films compared to conventional phthalate plasticizers. Diethylene glycol dibenzoate and triethylene glycol dibenzoate have

also been reported to be especially good in providing improved tear resistance. Benzoate plasticizers are recognized to have good resistance to extraction by aliphatic solvents and oils.

This leads to their use in applications such as flooring and wall coverings, food packaging, and medical devices. Benzoates also exhibit good environmental aging characteristics that are important for adhesives and sealants used in graphic arts, construction, transportation, and other exposed applications.

The benzoates are claimed to have better UV light stability when compared to conventional phthalate plasticizers.

7 Arendt, W.D., Benzoflex LA-705: A New Global Plasticizer for Waterborne Adhesive Market, Adhesive and

Sealants Council Meeting, 2006.

Regulatory Compliance The European Chemicals Agency (ECHA) manages the pan-European chemical oversight program

REACH. The following phthalate plasticizers have been cited as substances of very high concern (SVHC):

Benzyl butyl phthalate (BBP)

Di-(2-ethyl hexyl) phthalate (DEHP)

Dibutyl phthalate (DBP)

Diisobutyl phthalate (DIBP)

Pentyl iso-pentylphthalate (PIPP)

Diisopentylphthalate (DIPP)

Dimethoxyethylphthalate (DmoEP)

These chemistries have come under increased scrutiny under the REACH regulations because of their toxicity data in some animal species coupled with wide use and high volumes. Of particular concern are BBP, DBP, and DEHP, which have been targeted by ECHA for the authorization process in 2013.

Benzoate plasticizer chemistry has been identified by ECHA as a safer, viable alternative to phthalate plasticizers.

8 For example, ECHA concluded that dipropylene glycol dibenzoate (Benzoflex 9-88) is

viewed more favorably than the other alternatives to BBP examined. And benzoate plasticizers are also identified as potential alternatives for DBP and DEHP, showing the diversity of the benzoate chemistry in these applications.

In the US, the Consumer Product Safety Commission Act of 2008, restricted the sale of toys and other childrens products containing six specified phthalates.

Diisononyl phthalate (DINP) Diisodecyl phthalate (DIDP)

Bis(2-ethylhexyl) phthalate (DEHP) Dibutyl phthalate (DBP) Benzyl butyl phthalate (BBP)

Di-n-octyl phthalate (DnOP) This action mirrored regulation that had been introduced in Europe in 2005, when the European

Parliament restricted the use of those same phthalates in childrens toys. Subsequently, authorities from several other countries have enacted similar regulations restricting these phthalates in childrens goods.

Due primarily to their high biodegradation rates and low toxicity, benzoate plasticizers have been approved for various food packaging applications. For example, Eastmans Benzoflex LA-705 is approved in the US for the following FDA food contact clearances:

21CRF175.105 (indirect food contact, adhesives),

21CRF176.170 (indirect food contact, components of paper and paperboard in contact with aqueous foods), and

21CRF176.180 (indirect food contact, components of paper and paperboard in contact with dry food).

8 Deligio, T., Phthalate Alternative Recognized by ECHA, http://mpw.plasticstoday.com/articles/phthalate-

alternative-recognized-echa.

Studies submitted to and reviewed by the EPA have not revealed risks to human health with benzoate plasticizers. Benzoate plasticizers have been fully REACH registered and do not have any CLP labeling requirements for human health. No benzoate plasticizers are on the Substances of Very High Concern

(SVHC) list, nor have any been nominated for Community Rolling Action Plan (CoRAP) evaluations, which is the first step for identification of SVHCs.

Appendix: Description and Technical Features of Eastman Benzoate Plasticizers

Plasticizer Base Polymer Application Characteristics

Benzoflex 50 Polyvinyl acetate homopolymer and copolymer adhesives

Acrylic latex caulk Acrylic pressure sensitive adhesives

PVC plastisol sealants

Improved wet tack

Reduced set time, longer open times Excellent solvator for PVC

Benzoflex LC-531 Latex caulks Passes ASTM C-920 and ASTM C-834 performance standards

Benzoflex 2088 Polyvinyl acetate homopolymer and copolymer adhesives

Acrylic latex caulk Acrylic pressure sensitive adhesives

PVC plastisol sealants

Wet tack

Reduced set time, longer open times Excellent solvator for PVC

Improved adhesion in sealants

Fast fuser in PVC plastisols Low volatility

Used with Eastman 168 non-phthalate plasticizers for formulating pressure sensitive adhesives

Benzoflex LA-705 Polyvinyl acetate homopolymer and copolymer adhesives

Acrylic latex caulk

Acrylic pressure sensitive adhesives

Similar to Benzoflex 50 but reduces formulation cost

Benzoflex 9-88 Vinyl adhesives Cyanoacrylates

Polyurethane dispersions

Polyurethane sealants

High solvating plasticizer for a wide variety of applications

Benzoflex 9-88 SG 2K polyurethane adhesives and sealants

Highly compatible with polyurethane

Minimum cure interference Allows for hardness readings as low as 20-

25 Shore A Benzoflex 352 Hot melt adhesive (EVA, block

copolymers, polyurethane, polyamide, and polyester)

White, flake solid with a high melting point (118C)

Can replace wax in EVA hot melt formulations

Benzoflex PS-507 1K and 2K polysulfide sealants Decreases viscosity Improves flow, leveling, and workability