Wypych Handbook of Solvents

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IR spectra of PAAA-1 show absorptions at 1530 and 1660 cm-1, which are characteristic of amide bond, 1510 and 1590 cm-1 which are characteristic of benzene nuclei, and 1230 cm-1, which is characteristic of ether bond, but the absorption at 1380 and 1780 cm-l, which are characteristic of imide bond, are hardly observed. On the other hand, IR spectra of PAI-1 and PAI-3, obtained by heating of PAAA-1 and PAAA-3, respectively, show new absorptions at 1380 and 1780 cm-1.

Among the adsorptions of imide bond, the absorption at 1380 cm-1 is assigned to C-N stretching vibrations of all imide bond (cyclic and acyclic imide bond as shown in Eq. [ 13.4.3 ] and 1780 cm-1 is assigned to C=O stretching vibrations of five-member imide rings (cyclic imide bond). The absorption at 1530 cm-1 is assigned to N-H stretching vibrations of amide bond. Furthermore, the absorption at 1510 cm-1 is as-

signed to benzene nuclei that is stable to heat treatment.

Therefore, the content of cyclic imide, amide and all imide bond of PAI was estimated

by the absorbance ratio, D1380/D1510 (the absorbance ratio of absorbance at 1380 cm-1 to that of benzene nuclei), D1780/D1510, and D1530/D1510, respectively.

Table 13.4.2 shows the absorbance ratio of cyclic imide, amide, and all imide bond before and after heat treatment of PAAAs. The considerable increase of cyclic and all imide bond content of PAAAs and the decrease of amide bond were observed by heating. But the effect of solvent on the imidation of PAAAs was hardly observed.

13.4.2.2 Intrinsic viscosity and carboxyl group content

Table 13.4.3 shows the effect of solvent on the intrinsic viscosity, [η], and carboxyl group content of PAIs. The carboxyl content was determined by potentiometric titration. It is interesting to note that intrinsic viscosity of PAI from PAAA prepared in DMAc is larger than that in MEK/water mixed solvent. This suggests that the polymerization degree decreases with decreasing activity of TMAH-Cl in MEK/water mixed solvent because of the hydrolysis of TMAH-Cl by water.

Table 13.4.3. Properties of PAI samples [Data from reference 7]

aSolvent, NMP ; 30.0°C. bDetermined by potentiometric titration

In addition, the content of carboxyl groups in PAIs prepared in MEK/water mixed solvent was considerably larger than that prepared in DMAc. This also suggests the hydrolysis of TMAH-Cl in MEK/water mixed solvent.

13.4.3EFFECT OF SOLVENT ON THE CURING OF PAI BY HEAT TREATMENT

13.4.3.1 Chemical structure of PAI after heat treatment

Table 13.4.4 shows the effect of solvent on the change of chemical structure and formation of insoluble part of PAI-1 and PAI-2 after heat treatment. It became apparent that the con-

tent of cyclic imide bond and all imide bonds of PAI-1 and PAI-2 further increased by heating of the corresponding PAAAs at 180°C for 2 h followed by heat treatment at 260°C for 2 h. The content of amide bond decreased by heating of PAAA at 180°C for 2 h, but increased by post-heating at 260°C for 2 h.

Table 13.4.4. IR absorbance ratio and insoluble part of samples after heat treatment in air [Data from reference 7]

13.4.3.2 Curing PAI by post-heating

The formation of the insoluble part in NMP was observed and the amount of insoluble part formed by heating of PAI-1 (obtained in MEK/water mixed solvent) was larger than that by heating of PAI-2 (obtained in DMAc).

The increase of imide and amide bond content by post-heating at 260°C is considered as follows: the imidation of amic acid structure may be proceeded by both intermolecular and intramolecular imidation. The latter produces cyclic imide bond, but the former produces acyclic imide bond to give crosslinking material as shown in Eq. [13.4.3].

Since PAIs obtained by heating of PAAAs at 180°C are completely soluble in NMP, the intramolecular imidation preferentially proceeds at 180°C, but intermolecular imidation scarcely proceeds.

On the other hand, by post-heating at 260°C, the crosslinking reaction proceeds by the intermolecular imidation of terminal carboxyl groups of PAI with remaining amic-acid structure (Eq. [13.4.4]) and the amide bond in main chain of PAI (Eq. [13.4.5]) to give the insoluble part in NMP. The reaction induced the increase of imide bond content of PAI after post-heating. The increase of amide bond after post-heating may be due to the formation of crosslinking structure by the amidation of terminal amino groups of PAI with carboxyl groups of PAI in main chain (Eq. [13.4.5]-[13.4.8]).

Therefore, PAI obtained from the heating of PAAA obtained in MEK/water mixed solvent, which has many carboxyl groups, produces more insoluble part by post-heating.

Figure 13.4.2 shows the effect of heating temperature on the curing of PAI obtained by heating of PAAA at 180°C in MEK/water mixed solvent and DMAc. As shown in Figure 13.4.2, by post-heating at 280°C for 2 h in air, insoluble part in NMP reached 100%, indicating the almost complete curing of PAIs. It is interesting to note that PAIs formed by heating of PAAA obtained in MEK/water mixed solvent (PAI-1 and PAI-3) were found to be cured

Norio Tsubokawa

more easily than in DMAc (PAI-2 and PAI-4). This is due to the fact that PAI-1 and PAI-3 contains more carboxyl groups than PAI-2 and PAI-4.

13.4.4 CONCLUSIONS

The effects of polymerization solvent on the curing of PAIs were investigated and the following results were obtained:

(1) The carboxyl group content of PAIs from PAAAs prepared in MEK/H2O mixed solvent was larger than that in DMAc.

(2) The curing of PAIs prepared in MEK/H2O proceeded easier than those in DMAc.

(3) The curing of PAIs by heating was due to the intermolecular reaction between functional groups of PAIs, such as carboxyl and amino groups.

REFERENCES

1M.Fujita and T.Fujita, Rubber Digest, 18 (1), 21 (1966).

2 S.Terney, J.Keating, J.Zielinski, J.Hakala, and H.Sheffer, J. Polym. Sci., A-1, 8, 683 (1970).

3Y.Imai and T.Takahasi, Kobunshi Kagaku, 29, 182 (1972).

4 N.Tsubokawa, I.Yamamoto, and Y.Sone, Kobunshi Ronbunshu, 44, 389 (1987). 5 N.Tsubokawa, I.Yamamoto, and Y.Sone, Kobunshi Ronbunshu, 44, 831 (1987).

6H.Uchiyama and Y.Imai, Kobunshi Kagaku, 28, 73 (1971).

7N.Tsubokawa, M.Murata, and Y.Sone, Netsu Kokasei Jushi, 12, 12 (1991).

14

Solvent Use in Various Industries

Attempts to reduce solvent use in the production of various materials require background information on the current inventory, the reasons for selecting certain solvents, the effect of various solvents on the properties of final products, future trends and the possibilities for solvent replacement.

Information on solvent use investigates these areas. This chapter is possible only because of a thorough evaluation by large groups of scientists and engineers assembled by US Environmental Protection Agency. This has produced Compliance Sector Notebooks which contain invaluable information on solvent use. Full documents can be found on the EPA website at http://es.epa.gov/oeca/sector/index.html. These are useful in the investigation of a particular industry. Similar data from other countries were not available but open literature and patents have been included to help the reader to understand changes occurring in other countries.

14.1 ADHESIVES AND SEALANTS

George Wypych

ChemTec Laboratories, Inc., Toronto, Canada

Adhesives and sealants are manufactured from a variety of polymers. Their selection and their combinations used impact solvent selection. Most solvent systems are designed to optimize the solubility of the primary polymer. Adhesives can be divided into ones which bond by chemical reaction and ones which bond due to physical processes.1 Chemically reactive adhesives are further divided into three more categories for those that bond through polymerization, polyaddition, or polycondensation. Physically bonding adhesives include pressure sensitive and contact adhesives, melt, or solution adhesives, and plastisols. Polymerization adhesives are composed of cyanoacrylates (no solvents), anaerobic adhesives (do not contain solvents but require primers for plastics and some metals which are solutions of copper naphthenate),2 UV-curable adhesives (solvent-free compositions of polyurethanes and epoxy), rubber modified adhesives (variety solvents discussed below).

Polyaddition adhesives include epoxy and polyurethane polymers which can either be 100% solids, water-based, reactive or non-reactive hot melts or contain solvents mostly to regulate viscosity. Typical solvents include methyl ethyl ketone, acetone, mineral spirits, toluene, and xylene.3 Polycondensation adhesives include phenol-formaldehyde resin, polyamides, polyesters, silicones and polyimides. With the exception of polyesters (which require ethanol and N-methylpyrrolidone as solvents) and polyimides (which require

methyl amyl ketone, butyl acetate, methyl ethyl ketone, 2-ethoxyhexyl acetate as solvents), these adhesives can be made without solvents.

Pressure sensitive and contact adhesives are made from a variety of polymers including acrylic acid esters, polyisobutylene, polyesters, polychloroprene, polyurethane, silicone, styrene-butadiene copolymer and natural rubber. With the exception of acrylic acid ester adhesives which can be processed as solutions, emulsions, UV curable 100% solids and silicones (which may contain only traces of solvents), all remaining rubbers are primarily formulated with substantial amounts of solvents such as hydrocarbon solvents (mainly heptane, hexane, naphtha), ketones (mainly acetone and methyl ethyl ketone), and aromatic solvents (mainly toluene and xylene).

Melt adhesives and plastisols do not contain solvents. The solution adhesives group includes products made from the following polymer-solvent systems: nitrocellulose (typical solvents include solvent combinations usually of a ketone or an ester, an alcohol and a hydrocarbon selected from isopropanol, 2-butylhexanol, amyl acetate, acetone, methyl ethyl ketone), nitrile rubber (main solvent - methyl ethyl ketone), polychloroprene (which is usually dissolved in a mixture of solvents including a ketone or an ester, an aromatic and aliphatic hydrocarbon selected from naphtha, hexane, acetone, methyl ethyl ketone, benzene, toluene), and polyvinyl acetate (water).

In addition to the solvents used in adhesives, solvents are needed for surface preparation4 and primers. Their composition may vary and is usually designed for a particular substrate, often using fast evaporating solvents and environmentally unfriendly materials with significant adverse health effects.

Detailed data on the total amount of solvents used by adhesive industry could not be found. The adhesive manufacturing industry continues to grow at a very fast pace. Total adhesive production, according to Frost & Sullivan, was $18.25 billion in 1996 and this is expected to grow to $26.2 billion in 2003.5 Solvent-based materials in 1995 constituted 13% of total production in North America, 14% in Europe, 15% in Japan, and 25% in the Far East.6 Many industries which use solvent-based adhesives have moved to South America and Asia where regulations restricting emissions are less severe.5 The shoe industry is now concentrated in South America. There are many initiatives to decrease solvent emissions. For example, World Bank’s assistance program for developing countries focuses on this issue.5 But in spite of the fact that solvent-based adhesives lost some of their markets (3.3% during the period of 1994-1996),5 they still hold 14-15% of the European market.6 It is estimated that the use of solvents contributes 24% of all VOC emissions. According to one source adhesives were responsible for a 6% share in these emissions in 1993.7 Another source8 blames adhesives for 7% of total VOC emissions in Germany in 1995.

Sealants are divided into groups according to the generic names of polymer base. The main groups include: polyurethanes, silicones, acrylics, polysulfide and others (PVC, polybutylene, styrene-butadiene-styrene copolymers, polychloroprene, and several others). The amount of solvent used in sealants is controlled by the standards which previously divided sealants into two groups: these below 10% VOC and those above. Recently, a provision was made to include water-based acrylics and the limit of VOC for class A sealants was increased to 20%. Polyurethane sealants and structural adhesives can be made without solvent (the first solvent-free polyurethane sealant was made in 1994).9 Solvents are added to reduce sealant viscosity and to aid in the manufacture of polymer. Typical solvents used are mineral spirits, toluene, and xylene. A small amount of solvent is emitted from curatives

which contain methyl ethyl ketone. Most formulations of silicone sealants do not contain solvents. In some sealants, traces of benzene and toluene can be found.

Acrylic sealants are water-based but they may also contain ethylene and propylene glycols, mineral spirits and mineral oil. There are also solvent-based acrylic sealants which contain substantial amounts of solvents such as mineral spirits, toluene and xylene. Polysulfide sealants usually contain toluene but methyl ethyl ketone is also used. The group of class B sealants contains substantially more solvents (up to 40% by volume) but there are some exceptions. PVC sealants are based on plastisols and they can be made without solvents. Butyl rubber based sealants usually contain hydrocarbons (C6-C12). Styrene-buta- diene-styrene based sealants usually have a large amount of solvents selected from a group including toluene, heptane, hexane, methyl ethyl ketone, isobutyl isobutyrate, n-amyl acetate, n-amyl ketone. They are usually processed in solvent mixtures. Polychloroprene is usually dissolved in a mixture of solvents including ketones or esters, and aromatic and aliphatic hydrocarbons. The list includes naphtha, hexane, acetone, methyl ethyl ketone, benzene, and toluene.

The world market of sealants was estimated in 1996 at $2 billion and was expected to grow in 2003 to $2.75 billion with an annual growth rate of 4.5% which is slightly lower than that expected for adhesives (5.3%).5

The changing trends are clearly visible when developments in technology are studied but many barriers to reductions in solvent use exist such as the high investment required, longer processing time, frequently higher material cost of adhesives, and the psychological barrier of changing established adhesive practices. In many instances, adhesive performance is predicted by its superficial characteristics such as strong smell which might suggests that the material has superior properties, its initial green strength which for many indicates good bonding properties, and high viscosity often related to good processing characteristic.10 Since the alternative materials may not have much odor, or require of longer time to reach strength and have a low viscosity, users are suspicious that their potential performance may be inferior. The following information reviews some recent findings which may contribute to future changes.

In the shoe industry, a major breakthrough occurred in 1928 when polychloroprene was first introduced.1,10 The first, simple formulation is still manufactured and is used worldwide because the glue can be easily prepared by simply making a solution of the polymer. This gives a product with good adhesion to various substrates. Many new products are available today as potential replacements. Hot melt adhesives can be used in some applications but they still require solvents for cleaning, degreasing, and swelling. Also, their bond strength is frequently inadequate. Reactive systems are not yet used in the shoe industry but reactive hot melts are finding applications. Their broader use is hampered by their sensitivity to moisture which requires special equipment and special care.10 Water-based adhesives are the most likely replacement product. They also need special equipment for processing because of the high heat of evaporation of water (although water based adhesives contain 50% polymer compared with 15-20% in solvent based adhesives).10 Two sport shoe manufacturers, Nike and Reebok, already use this technology.

Traditional polychloroprene adhesives can be modified in several ways to be useful in water-based systems. Figure 14.1.1 shows peel strength of several adhesives. The solvent based adhesive (A) has excellent properties both in terms of green strength and bond strength. A simple emulsion of polychloroprene (B) has relatively good ultimate strength