Applied Polymer Science - ACS Publications - American Chemical

40

Chemistry and Technology of Polyamide Resins from Dimerized Fatty Acids

Downloaded by UNIV OF ROCHESTER on June 7, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch040

MICHAEL A. LAUDISE Henkel Corporation, Minneapolis, ΜΝ 55413 Chemistry of Dimer-Based Polyamide Resins Solid Polyamide Resins Properties Viscosity Characteristics Solubility Surface Activity Applications Fluid Polyamide Resin Chemistry Thermoset Coatings Thermoset Adhesives Thermosetting Systems for Casting and Laminating Prior to 1900 the methodology of bodying oils for paint manufacture was well-known. However, the fatty acid chemistry involved was obscure until the early war years of 1940. Then an understanding started to emerge relative to the complex intermolecular condensation reactions occurring during oil bodying. These coupling reactions led to interesting high molecular weight polybasic fatty acids capable of entering into polymer formation. Continued study showed that these acids formed when unsaturated fatty acids containing one or more double bonds are polymerized. These studies also showed that the major products were complex dibasic acids containing 36 carbon atoms and smaller amounts of monocarboxylic, tricarboxylic, and higher carboxylic acids. It is these acid mixtures, either as such or refined to increase dibasic content, that are used to prepare polyamide resins via their reaction with polyamines. The first polyamides from these polybasic acids or their corresponding esters were described by Bradley and Johnston (1) and subsequently by Falkenburg et a l . (2). These chemists recognized that polyamide resins have unusual solubility in lower alcohols and that films, deposited from alcohol solution, have good water resistance, strong adhesion to various surfaces, and other 0097-6156/ 85/0285-0963506.50/ 0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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mechanical properties l i k e good tensile strength and f l e x i b i l i t y that contribute to toughness. This chapter w i l l describe the chemistry of these resins and their properties and applications. Polybasic acids are now available commercially in various purity grades. The chemistry of polybasic acids has been explained by Wheeler and coworkers (3-7). A common product of commerce contains 70-80% dibasic acid, 15-20% of higher functionality acids, and small amounts of monomeric acids. Much of the literature associated with commercial materials refer to i t as "dimer" or mixed dimerized fatty acids. This terminology, accordingly, w i l l be used here. The acid may be represented graphically as H00CDC00H where D i s a 34-carbon radical. Chemistry of Dimer-Based Polyamide Resins The simplest polyamide resin results from the condensation of dimer acid with ethylenediamine. The structure of the resulting polymer is represented as H0(-0C-D-C0NH-RNH-)nH where R i s C2H4- and η may be 5-15. Number-average molecular weights may be determined by use of a Mechrolab vapor pressure osmometer, u t i l i z i n g a solution of polyamide resin in chloroform at 37 °C. The procedure i s described in detail by Pasternak et a l . (8). Typical molecular weight values for ethylenediamine-based polyamides with varying viscosities are l i s t e d i n Table I. V i s c o s i t i e s of the melts were determined by utilizing a Brookfield viscometer, Model RVF, at 160 °C with a No. 3 spindle at 20 rpm. The polyamide resins melting above 100 °C, which result from the condensation of ethylenediamine or related higher molecular weight diamines with dimer acid, find application i n industry because of their film-forming and adhesive properties. These w i l l be discussed in detail later. If dimer acid i s condensed with diethylenetriamine or similar polyalkylene polyamines, a liquid resin usually results that melts below 100 ° C . The structure of such resins can be indicated diagrammatical 1 y as follows, although there i s evidence that both primary and secondary amine groups are involved in resin formation. •C00H

H00C—D-

+ NH2-C2H4-NH-C2H4-NH2

NH2-C2H4-NH-C2H4--N-- C—D—C-N-C2H4-NH-C2H4J-NH2

H

II 0

II I OH η

Some molecular weights of this type of polyamide resin are listed in Table II. Viscosity was determined at 150 °C by utilizing a Brookfield viscometer, Model RVF, No. 3 spindle at 20 rpms. If


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Table I. Downloaded by UNIV OF ROCHESTER on June 7, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch040

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Polyamide Resins from Dimerized Fatty Acids

Typical Molecular Weight Values for Ethylenediamine-Based Polyamides

160 °C Melt Viscosity of Polyamide (Ρ)

n

η (Average)

15.1

3070

5- 6

17.2

4000

6- 7

22.0

4230

7-8

Table II.

M

Molecular Weights of Polyamide Resins

150 °C Melt Viscosity of Polyamide (Ρ)

n

η (Average)

9.4

2780

4-5

11.7

2960

4-5

M


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the monomeric f r a c t i o n of dimer acid i s condensed with a polyalkylene polyamine l i k e diethylenetriamine, a new class of polyamides, sometimes referred to as fatty amido amines, results. These low-melting materials usually have viscosities below 10 Ρ at 25 ° C . The structure of such resins can be indicated diagram matical ly as follows. CH3 (CH2)n C00H +


Η

I

H2N-C2H4-N

C2H4

NH2

|-H 2 0 V CH3(CH2)nC—C2H4

Ν

C2H4

NH2

Η η = C16-C19 As may be seen from the above structures, the dimer-based liquid polyamides and monomeric fatty amido amines contain free amine groups that are available for further reaction. One such reaction i s imidazoline formation, which occurs intermolecularly (9). The structure of imidazoline can be indicated diagrammatically as follows. 0H

111!

CH3(CH2)nC-N-C2H4-NH2

I--H 0 2

CH3(CH2)nC=^ HN

CH2

\ /

CH2 η = C16-C19 S t i l l another reaction that accounts for the major application of l i q u i d polyamides and fatty amido amines is a curing agent for epoxy resins. These reactions and related applications w i l l be discussed in detail later. The molecular weights of fatty polyamides generally encompass the range of 3000-15,000. Anderson and Wheeler (10) evolved a relationship for number-average molecular weight determined by endgroup analysis and viscosity so that molecular weight can be


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estimated from solution viscosity measurements. From a practical point of view, solution and melt viscosities are more useful than molecular weight since viscosity values have important relationships to major application areas of coatings, inks, and adhesives. The s o l i d fatty dimer polyamides are related to nylons and, indeed, are considered nylons when based on highly purified dimer acid (11). When based on a less pure version of dimer acid, they are, of course, of considerably lower molecular weight than nylons and have a much lower order of crystallinity.


Solid Polyamide Resins Properties. The solid polyamide resins are alcohol soluble and i f properly formulated can supply some degree of hydrocarbon compatibility. Thus, they can be applied from solvent solution. However, they also can be applied as hot melts or from water dispersions. Water-based forms known as suspensoids have been described (12). Solid polyamides also may be finely divided for use as powders. The films are characterized by resistance to moisture, moisture vapor transmission, grease, and o i l s . They are resistant to many solvents and chemicals including aliphatic hydrocarbons and mineral and vegetable o i l s . They do not resist lacquer-type solvents and alcohols. They have a high degree of f l e x i b i l i t y and maintain their f l e x i b i l i t y upon aging. Also, they are heat sealable at r e l a t i v e l y low temperatures and adhere to an unusually wide variety of substrates. When fatty polyamides of different melting points are mixed, the resulting melting point is nonlinear with respect to composition as demonstrated in Figure 1. Thus, a formulator has the a b i l i t y to make many resin alloys. Viscosity Characteristics. As is t y p i c a l of certain polyamides, viscosity decreases rapidly above the melting point. Figure 2 shows the relationship of temperature to viscosity in poises for three polyamide resins melting in the 105 °C range. These resins differ in molecular weight only. The important point to observe is that a l l three resins have a very low viscosity range of 10-15 Ρ by the time temperature reaches 180 °C. Because of this steep viscosity drop, solid polyamide resins are useful as hot melt adhesives. The resins may tend to "skin" or oxidize when exposed to air and high temperatures for long periods. However, this negative property can be circumvented by the use of antioxidants and by proper application equipment design. Solubility. Solid polyamides are soluble in some alcohols and i f properly formulated tolerate significant amounts of hydrocarbon-type solvents. Actually these formulated polyamides are more soluble in a combination of alcohol and hydrocarbon solvent than alcohol alone (13). Thus, ethyl alcohol, isopropyl alcohol, propyl alcohol, and butanol can be used in combination with hexane, mineral s p i r i t s , xylene, and toluene. Isopropyl acetate may also be used when nitrocellulose compatibility is required. Surface Activity. As already indicated, the solid dimer polyamides adhere to numerous surfaces, have a high degree of f l e x i b i l i t y , and



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100

110

120

130

140

150

160

MELTING TEMPERATURE

Figure 1.

170

100

190

(C)

B a l l and ring softening points of mixtures of resins melting at 150 and 185 °C.

TEMPERATURE (F)

0

i

110

1 120

I

I

I

I

I

I

I

130

140

150

160

170

180

190

TEMPERATURE Figure 2.

1 200

(C)

(MACMICHAEL VISCOMETER)

R e l a t i o n s h i p of temperature to v i s c o s i t y polyamide resins melting at about 105 °C.


of

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have an excellent combination of mechanical properties. Because polyamide resins have a high fatty content coupled with highly polar amide, amine, and imidazoline groups, they have the structural characteristics of surface active agents. This surface activity is a particularly important property of liquid resins to be discussed later. The fatty characteristics also affect physical and chemical resistance properties such as compatibility with other resin types and melting point and water resistance. Indeed, fatty polyamides have considerably lower water absorption than c l a s s i c a l nylons, presumably because the fatty molecular portions shield the polar amide and amine groups. The fatty polyamides also demonstrate a high level of grease and o i l resistance combined with a low order of irritation and toxicity Applications. Adhesives. In order for a s o l i d polyamide to function e f f e c t i v e l y as a hot melt adhesive, i t must possess high t e n s i l e strength in the glassy or s o l i d state, melt in a range suitable for application by automated equipment, and have low enough viscosity when molten to wet surfaces to which i t must adhere. High t e n s i l e strength requires moderate to high molecular weight polyamides while low melt viscosity c a l l s for low molecular weights. Thus, an obvious compromise i s necessary in molecular weight to achieve an appropriate balance of properties (15). With the dimer polyamides, certain structural considerations interrelate with desirable adhesive properties. For example, even at moderate molecular weights around 10,000, the linear polyamides demonstrate a high degree of interchain attraction, l a r g e l y attributed to hydrogen-bond formation (14). Such polyamides tend to offer high t e n s i l e strengths even though molecular weights are moderate. Yet, when molten, the polyamides flow readily because hydrogen bonding breakage permits the molecules to s l i d e or flow past each other. Some examples of this type of polyamide are listed in Table III along with t e n s i l e data. Also, Figure 3 i l l u s t r a t e s viscosity p r o f i l e s over a r e l a t i v e l y wide temperature range for several polyamides. These polymers flow readily when molten, set quickly when cooled, and demonstrate high t e n s i l e strengths when glassy or solid. Thus, in general, dimer polyamides useful as the main component of hot melt adhesives are linear with the linearity contributing to tensile strength properties. Another distinguishing characteristic of dimer polyamide hot melt adhesives, which almost puts them in a class by themselves as engineering hot melts, is their unusually strong adhesion to many surfaces including most metals, leather, paper, many fabrics, p l a s t i c films including polyolefins, wood, and glass (15). It i s this characteristic that dictates polyamide hot melt use in such demanding automated applications as side seam sealants for metal cans (16-19); long-lasting construction adhesives for leather, ABS, and v i n y l (20); and lap seam adhesives for beer cans and sealants and insulators for electronic parts (21). Coatings. The solid dimer polyamide resins are useful as protective coatings and inks on many substrates, especially flexibile ones like polyolefin f i l m , paper, fiberboard, and metal f o i l s . Thus, paper converters use these resins by applying them from solvent by r o l l e r -


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Table III.

Some Properties of Macromelt Polyamide Resin Hot Melts

Softening Point


Product

Hot Melt a Application Temperature

Polymer Tensile Strength (psi)

Polymer (% Elongation)

Macromelt 6200

100 °C 212 °F

205 °C 401 °F

1700

550

Macromelt 6212

110 °C 230 °C

195 °C 383 °C

1800

450

Macromelt 6240

140 °C 284 °F

240 °C 464 °F

800

900

Macromelt 6264

160 °C 320 °F

210 °C 410 °F

1300

450

Macromelt 6300

200 °C 392 °F

250 °C 482 °F

3000

500

Note: a

Weight per Gallon: 8 lb/gal.

Macromelt resins weigh very close to

Melt viscosity of 35 poises was chosen as typical. Temperatures shown achieve this viscosity for each particular resin.

MACROMELT*POLYAMIDE RESINS TYPICAL VISCOSITY/TEMPERATURE CURVES

Figure 3.

Macromelt polyamide resins: perature curves.

typical viscosity/tera-


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coating, rotogravure, or flexographic printing techniques. They demonstrate excellent weatherability and adhesion properties when properly pigmented. Beside solvent application techniques, these resins can be applied as hot melts and from water dispersions. Water dispersions (12, 14) may be readily prepared by converting amine end groups to s a l t s such as substituted ammonium acetates. These i n t r i n s i c emulsifiers readily cause water dispersions c a l l e d suspensoids. Films from suspensoids must be heat fused in order to achieve barrier properties. Thixotropic Coatings. The hydrocarbon solubility characteristics of solid dimer polyamide resins impart thixotropic properties. This property was extended into oil-based alkyd coatings by Winkler (22) and has been discussed in a number of publications (25, 26). Best results are obtained when the polyamide i s incorporated into the alkyd vehicle by heating, either during or after formation, so a homogeneous system results. The nature of polyamide interaction with an o i l vehicle like an alkyd is not fully understood. However, 2-5% of s o l i d polyamide is usually required to achieve desired thixotropy. Above 5% there is a danger of coating gloss drop. As one would expect, the thixotropy induced by solid polyamides markedly reduces coating pigment s e t t l i n g and makes possible the application of thicker coats. Additional benefits include less absorption on porous substrates, better hiding from improvements in pigment dispersion quality, improved flow, improved leveling, and longer wet edge times without effecting sag resistance. Inks. Solid dimer polyamides occupy a dominant position as vehicles for flexographic inks. They are a d d i t i o n a l l y used as clear overprint varnishes and in rotogravure inks. Polyamide-epoxy combinations, to be discussed l a t e r , are also useful as s i l k screening inks, p a r t i c u l a r l y on difficult-to-wet surfaces such as molded polyethylene. Solid dimer polyamide overprint varnishes are prepared by dissolving the polyamide in propanol and/or lower boiling related solvents. Solvents are selected on the basis of requirements for viscosity, solvent release, speed of dry, gelation resistance, and other factors. S o l u b i l i t y can often be enhanced by adding small amounts of water. Resin s o l i d s are usually in the 25-40 v o l % range. However, newer polyamide technology has allowed for 50-60 volume solids (23). Most formulations include minor amounts of plasticizers, waxes, s l i p agents, and antiblocking agents. Solid dimer polyamides used as vehicles for flexographic inks (14, 24) possess the required alcohol solubility to make i t possible to use them on presses with rubber r o l l s . Their adhesion to films such as polyethylene, Saran, and Mylar is excellent, and their f l e x i b i l i t y ensures that they w i l l function properly on these f l e x i b l e substrates. In addition, they have excellent gloss and good blocking resistance. Thus, they are commonly used on polyolefin, Saran, cellophane, Mylar, cellulose acetate, and PVC films.


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Rotogravure polyamide ink formulations are particularly useful for the printing of gold colors on paper and cellophane. These formulations can contain n i t r o c e l l u l o s e r e s i n s . Solvent combinations include alcohols, hydrocarbon, ketones, and esters.


Fluid Polyamide Resin Chemistry The reactive dimer polyamides and fatty amido amines are l i q u i d resins with many of the properties described above for s o l i d polyamides. In addition, they are amine-like in that they are basic and form salts. They are more soluble than solid resins, although alcohols are s t i l l primary solvents. The l i q u i d dimer polyamides were f i r s t described by Bradley (25), but their reactivity with epoxy resins was first recognized by Renfrew and Wittcoff (26). In reaction with epoxy resins the aminecontaining dimer polyamides contribute to water r e s i s t a n c e , increased f l e x i b i l i t y and adhesion, and most importantly corrosion inhibition properties. Their pendent amine groups react readily with terminal epoxy groups to form cross-linking β-hydroxy amino and polyether reaction products. These reactions are i l l u s t r a t e d in the following equations: OH

I

-CH-CH2 + RNH2 -> -CH-CH2-NHR 0

1° Amine OH

-CH-CH2 + R2RNH -> -CH-CH2-NRR1 \ / 0 2° Amine -CH-CH2 + R1'R'RN -> -[0-CH 2 -CH 2 ] n 0

3° Amine

Polyether

The reaction mechanism of l i q u i d dimer polyamides and fatty amido amines with epoxy resins has been studied by Peerman et a l . (27), who employed infrared spectroscopic analysis to determine reaction rates. They showed that the terminal epoxy content of a blend of amino-containing polyamide and epoxy resin disappeared more rapidly at 150 °C than does the epoxy content of blends of epoxy r e s i n with t r i e t h y l e n e t e t r a m i n e or t r i s [ ( d i m e t h y l a m i n o ) raethy1]phenol. Both of these compounds are well-known for their fast cure at ambient temperatures. Correspondingly, the l i q u i d polyamide or fatty amido amines-epoxy combinations cure slower than the other two systems at ambient conditions. The reason for these surprising r e s u l t s i s postulated by utilizing a concept of immobilization in the cross-linking mechanism of epoxy resins. The aliphatic d i - and polyamines react rapidly at ambient conditions. At the higher temperature, these systems are



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postulated to have reacted so rapidly that the cross-linked network becomes r i g i d and immobile long before complete epoxy reaction occurs (Figure 4). Thus, although increased curing temperature reduces viscosity, i t also accelerates a state of immobility, which can have a pronounced effect on certain physical properties such as hardness, heat distortion, and flexural strength. Hardness effects caused by changing epoxy to polyamide mixtures are indicated by the data in Table IV. The f l u i d dimer polyamides and fatty amido amines also react with phenolic resins (23). These reactions are s i g n i f i c a n t l y different from those of epoxy resins. With the heat-reactive phenolic resins, the aminopolyamide portions react with methylol groups. A carbon-nitrogen bond or cross-link i s formed and a v o l a t i l e byproduct, water, i s produced. This reaction requires external heat to remove water. At temperatures near 150 °C the reaction proceeds smoothly. Since curing at elevated temperatures i s required, the pot l i f e or shelf l i f e at room temperature i s relatively long. The liquid dimer polyamide and fatty amido amines also react with alpha, beta unsaturated acids and esters (29) and with polyesters (30). The unsaturated esters reduce v i s c o s i t y , lengthen useful pot l i f e , and reduce heat of reaction. Thus, they are useful diluents when low viscosity is desired. Thermoset Coatings By far the most important coatings that make use of l i q u i d aminocontaining dimer polyamides and fatty amido amines are what i s referred to as epoxy-polyamide coatings. These coatings are used widely as maintenance paints. Coating formulations and properties have been detailed in technical b u l l e t i n s (31). The epoxy-polyamide system i s popular because i t provides an unusual degree of inherent corrosion resistance. This w i l l be discussed in detail later. The system is unusually tolerant as compared to related systems, such as amine cured, since the ratio of components is not particularly c r i t i c a l . Tolerance i s also demonstrated because i t may be applied to wet surfaces and to surfaces with t i g h t l y bound r u s t . Indeed, formulations are a v a i l a b l e that may be applied under water to structures such as submarines and off-shore o i l well riggings. Both the corrosion resistance and the tolerance relative to application on poorly prepared wet surfaces are believed to be functions of the surface a c t i v i t y of the polyamide resin. Related also to the surface activity are the unusually strong adhesive properties that the system demonstrates with a broad range of substrates. In addition, the f i l m has a long l i f e and i s resistant to the water, solvents, chemicals, acids, and a l k a l i normally found in chemical and industrial environments. The epoxy-polyamide coatings have less resistance to strong solvents than do amine-cured systems, and their heat resistance tends to be somewhat lower. The properties of these coatings have been described in d e t a i l in several publications (32, 33). The dramatic corrosion resistance of epoxy-polyamide coatings is demonstrated by the data in a Government m i l i t a r y specification, Mil-P-24441 (ships). This specification coating, at a relatively high solids of 61% by volume, i s formulated e n t i r e l y without


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TRIETHYLENE TETRAMINE

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