Engineering Thermoplastics

Vinny R. Sastri , in Plastics in Medical Devices, 2010

7.2.3 Acrylics Sterilization

Acrylic polymers can be sterilized with ethylene oxide, gamma, and e-beam radiation. Steam sterilization is unsuitable for acrylic resins as they would warp and deform because of their low glass transition temperatures ( Table 7.4).

Table 7.4. Sterilization Resistance of Acrylic Polymers

Acrylic copolymers and blends can be tailored to have excellent gamma sterilization with the use of styrenic comonomers or blends. Figure 7.6 shows that such acrylic polymers retain over 80% of their properties when exposed to gamma radiation with doses ranging from 25 kGy to 100 kGy [1].

Figure 7.6. Property retention of acrylic resins after gamma radiation.

Acrylic polymers can be stabilized or tinted to prevent the materials from yellowing after exposure to gamma radiation (Figure 7.7a) [4]. Most polymers will yellow after gamma radiation. The yellow color will decrease after a few days. This decrease is not sufficient if the initial yellowness index is large, as with a standard acrylic resin shown in Figure 7.7b [5].

Figure 7.7. Yellowness index of acrylic resins after gamma radiation. (a) Yellowness index and radiation dose. (b) Yellowness index change over time.

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Plastics in Outdoor Applications

Jia Xin Chan , ... Khaliq Majeed , in Reference Module in Materials Science and Materials Engineering, 2020

Acrylic Polymers

Acrylic polymer is a strong, stiff, and transparent plastic material. Acrylic polymers are available in various colors and finishes. It is frequently used for lighting, electronics screen, automotive components and outdoor glazing in architecture and construction. Some of its trade names found in markets include Perspex®, Acrylite®, Europlex®, Optix®, Duraplex®, and Plexiglass®. The major monomers of acrylic polymers belong to two families of ester-acrylate (R=H) and methacrylate (R=CH 3) as shown in Fig. 2 (Campo, 2008; Jalal Uddin, 2010). The nature of the R and R′ groups determines the properties of monomers and their polymers.

Fig. 2

Fig. 2. Structure of acrylic ester

 Reprint with the permission from Jalal Uddin, A., 2010. Coatings for technical textile yarns. In: Alagirusamy, R., Das, A. (Eds.), Technical Textile Yarns. Cambridge: Woodhead Publishing Limited, pp. 140–184. Copyright (2020), Elsevier.

Poly(methyl methacrylate) (PMMA) is by far the most common acrylic polymer. PMMA is hard, optically clear and has excellent weather resistance as well as high thermal stability and heat resistance (Ali et al., 2015). Most commercial acrylic polymers have excellent UV stability. Despite of its hydrogen atoms that are susceptible to oxidation, PMMA oxidizes under the exposure of shorter-wavelength UV but not under solar UV. Considering both thermal and thermally oxidative decomposition temperatures of PMMA, it can hardly decompose at temperature below 200°C. This contributes to its weather resistance properties as well (Ali et al., 2015). PMMA commonly used as a shatterproof replacement for glass due to its lightweight, better impact resistance and glass-like appearance. It has a light transmission of 92% and can be easily thermoformed without losing in optical clarity (McKeen, 2012). PMMA has superior scratch resistance when compared to other transparent plastics such as polycarbonate. It exhibits low moisture absorption capacity, good chemical resistance and superior dimensional stability.

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Coatings for technical textile yarns

A. Jalal Uddin , in Technical Textile Yarns, 2010

5.4.5 Acrylic polymers

Acrylic polymers are commonly known as acrylics. The monomers are esters of acrylic and methacrylic acid. Their formula is given in Fig. 5.2. This is the general formula of acrylates (R   =   H for acrylates, R   =   CH3 for methacrylates). Some common esters are methyl, ethyl, n-butyl, isobutyl, 2-ethyl hexyl, and octyl. The esters can contain functional groups such as hydroxyl, amino and amido. The monomers can be multifunctional as well, such as trimethylol propane triacrylate or butylene glycol diacrylate. The nature of the R and R′ groups determines the properties of monomers and their polymers. Polymers of this class are noted for their outstanding clarity and stability of their properties upon ageing under severe service conditions.

5.2. Acrylic ester. R   =   H for acrylates, R   =   CH3 for methacrylates.

Polymerization of the monomers occurs by free radical polymerization using free radical initiators, such as azo compounds or peroxides. Acrylic polymers tend to be soft and tacky, while methacrylate polymers are hard and brittle. A proper adjustment of the amount of each type of monomer yields polymers of desirable hardness or flexibility. The vast majority of commercially available acrylic polymers are copolymers of acrylic and methacrylic esters. The polymerization can occur by bulk, solution, emulsion and suspension methods. The suspension-grade polymer is used for moulding powders. The emulsion and solution grades are used for coatings and adhesives.

Acrylate emulsions are extensively used as thickeners and for coatings. Acrylics have exceptional resistance to UV light, heat, ozone, chemicals, water, stiffening on ageing, and dry-cleaning solvents. As such, acrylics are used as backcoating materials in automotive upholstery fabric and carpets, window drapes, and pile fabrics used for outerwear.

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Paint Formulation

G.P. Bierwagen , ... M.S.H. Bhuiyan , in Reference Module in Materials Science and Materials Engineering, 2017

3.1.3 Acrylics

Acrylic polymers have been used in the coating industry for some time 58 and have great usefulness because of their UV stability and wide range of use. There are very many acrylic monomers so that copolymers of many types can be prepared using basic acrylic free radical polymerization. Similarly, very many side-groups of acrylic monomers are available, which allow crosslinking chemistries with many other polymer types. After 800   h of UVA-light ageing, the degradation process becomes minor for acrylic and styrene-acrylic paints, slightly more pronounced for vinyl products, suggesting that H.B. – Liquitex and Brera-Maimeri formulations suit the creation of outdoor murals, while Flashe-L&B paints do not. Notwithstanding this fact, it would be interesting to extend the ageing to verify if styrene-acrylic paints will remain stable or, as expected, they will undergo harsher photo-oxidative processes. 59

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Specialty Polymers & Polymer Processing

Hubert J. Fabris , Wolfgang G. Knauss , in Comprehensive Polymer Science and Supplements, 1989

5.2.1.3(i) Acrylic polymers

Acrylic polymers have recently achieved widespread use; they are white and exhibit much better resistance to aging than the old pressure-sensitive adhesives based on natural rubber, and are widely used where these two characteristics are desired.

Polyacrylates have been synthesized 37 which are inherently pressure-sensitive, i.e. without the need for the addition of a tackifier. Among the functional groups introduced by copolymerization are carboxylic, hydroxyl, epoxy, allylic amide and tertiary amine groups. Reversible cross-linking can be obtained by salt formation (reaction of carboxy groups with zinc, zirconium or titanium compounds), for example. 38 Permanent irreversible cross-links are established by the reaction of hydroxy groups with epoxide or formaldehyde resins, 39 or by reaction of epoxy groups with carboxy groups, amides or amines. Amide groups containing polymers are also cross-linked with formaldehyde resins. 40

Hot-melt pressure-sensitive adhesives have been introduced which have thermally reversible cross-links of a nondïsclosed nature. The material is applied to tape at 175   °C. Bonds with high levels of peel strength and permanency have been reported. 41

Acrylates are used for packaging, bandages, paper and film labels, decals and a variety of specialty items.

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Pyrolysis-gas chromatography

Karen D. Sam , in Gas Chromatography (Second Edition), 2021

11.4.2 Acrylics

Acrylic polymers include the acrylates such as ethyl acrylate and butyl acrylate and the methacrylates, such as methyl methacrylate and butyl methacrylate. Looking at line 1 in Fig. 11.1, for acrylates, R1, R3, and R4 are all hydrogen, and R2 is the ester group. This means that there is a hydrogen atom on the fifth carbon from the bond breaking that can be transferred; hence, the trimer can be formed by the mechanism shown in line 4, Fig. 11.1. For methacrylates, R1 is a methyl group; therefore, the bond breaking makes a tertiary free radical. There is also a methyl group on the fifth carbon; hence, there is no hydrogen to transfer, and the polymer simply unzips to monomer.

The acrylic material shown in Fig. 11.4 contains both acrylic and methacrylic monomers, specifically methyl methacrylate, butyl methacrylate, and butyl acrylate. Consequently, there is a large peak for methyl methacrylate monomer (peak 1) and a peak for butyl methacrylate (peak 3). There is also a considerable amount of butyl acrylate, which appears as the monomer, dimer, and trimer (peaks 2, 4, and 5). The trimer peak is the largest, which is typical, and there are two dimer peaks, also typical for acrylics. The relative sizes of the monomers (and other oligomers) are reflective of their abundance in the original copolymer [11]; hence, Py-GC-MS may be used to quantitate the monomer ratios in copolymers like these.

Figure 11.4. Pyrolysis of an acrylic copolymer at 750°C. Peak #1, Methyl methacrylate; 2, Butyl acrylate; 3, Butyl methacrylate; 4, Butyl acrylate dimers; 5, Butyl acrylate trimer.

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Chain Polymerization of Vinyl Monomers

S. Slavin , ... D.M. Haddleton , in Polymer Science: A Comprehensive Reference, 2012

3.09.5.10.2 Automotive base coats/color coats

Acrylic polymers containing hydroxyl groups are utilized in base coats as binders, which bind the pigments together, usually by a catalyzed cross-linking mechanism with a polyisocyanate hardener, providing a chip-resistant coating. These coatings are usually two-component systems, meaning the binder and cross-linker are stored separately and mixed to form a pot mix prior to application.

The acrylic polymers that comprise the binder can be synthesized by emulsion polymerization. It was found that emulsion polymerizations, which incorporated random and AB block copolymers synthesized using CCTP macromonomers, required no conventional surfactants to stabilize particles, as the block copolymers could be used as polymerizable surfactants. 191

Block copolymers can be synthesized by alternative techniques to CCTP, but these have disadvantages. Cationic polymerization is limited by the amount of monomers applicable; anionic polymerization requires low temperature, which is not feasible for application to plant reactors; and GTP requires protection of any acidic monomers. Certain living free radical techniques have also been investigated but drawbacks still remain with respect to conversion, color, and catalyst removal. 138 CCTP provides a new way to prepare hydroxyl-functional graft copolymers that show excellent drying.

Work by Huybrechts et al. on waterborne pigment dispersants for automotive paints utilized CCTP for synthesis of macromonomer copolymers of BA and methacrylic acid by standard solution polymerization in the formulation of automotive base coats. These amphiphilic block copolymer macromonomers form stable anionic dispersions in water and could be further used in the surfactant-free emulsion polymerization of MMA and BA. MMA and BA copolymerize with the macromonomer to form comb structures that are water dispersible and stabilize the final emulsion. The molecular weight of these comb copolymers can be reduced by incorporation of MMA macromonomers, also synthesized by CCTP as the MMA macromonomers act as CTAs. Tuning the molecular weight of comb structures has a profound effect on drying with both physical and chemical drying processes taking place. Physical drying is the increase in film T g caused by solvent evaporation, which is influenced by the T g of the comb copolymers. Chemical drying is the increase in T g due to cross-linking. On drying, hydroxyl groups will react with isocyanate cross-linkers leading to a network structure, with an overall drying performance comparable to solvent-borne systems when formulated correctly. 191

Macromonomers are utilized for a range of other applications for both solvent-borne 192 and aqueous systems used in the automotive industry, 193–195 and are usually utilized in the synthesis of comb polymers. In solvent-borne systems, the macromonomer facilitates less selective anchoring of the pigment to dispersants, hence creating a better color quality in resulting coatings. 192 In aqueous pigment dispersion, conventional coatings have an insufficient intensity of color with regard to jet black coatings; incorporation of macromonomers into the comb copolymer has been shown to increase the color intensity of jet black pigments. 193

Macromonomers have also been used in the production of fast drying coatings for the refinishing of base coats and clear coats. CCTP AB block macromonomers are first prepared; copolymerizing these in a sequential polymerization yields a comb polymer with random segmented side chains. Introduction of functional monomers is also possible creating more segments on the arm. These comb polymers can be added as binder resins to solvent- or water-based coatings for a range of paint compositions, such as primers, base coats, and clear coats, and many architectural compositions, such as house paints. 192 It was found that this formulation was particularly useful in fast drying coatings such as air-dry acrylic lacquer color coat compositions to be coated in a clear coat finish, these fast drying coatings are of particular use to the paint repair industry, as much repair work is carried out in auto repair shops, where ovens are not used to cure the paint, hence the paint must dry quickly under ambient conditions.

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Recent developments in flame-retarding thermoplastics and thermosets

Paul Joseph , John R. Ebdon , in Fire Retardant Materials, 2001

7.5.3 Acrylics: poly(methyl methacrylate) and polyacrylonitrile

Acrylic polymers are obtained from derivatives of acrylic and methacrylic acids; the group includes also their copolymers with various vinylic and allylic monomers. Monomers commonly used in the production of these polymers are acrylonitrile, acrylic and methacrylic acids, and their amide and alkyl ester derivatives. The largest applications of acrylic polymers in terms of tonnages used are in moulded and fabricated plastic articles of many kinds made from poly(methyl methacrylate) (PMMA). The crystal clarity, light weight, outstanding weather resistance, formability and strength of PMMA have resulted in numerous applications in different technical fields and in many domestic products. Since PMMA is odourless, tasteless and non-toxic, it may be used in food-handling equipment.

Polyacrylonitrile (PAN), and copolymers with acrylonitrile in a predominant amount, are white powders having relatively high glass transition temperatures, Tg . However, they have a low thermal plasticity and cannot therefore be used as a moulding material. Their high crystalline melting points, Tm (~   300   °C), limited solubility in certain solvents, and superior mechanical properties when used as fibres are due to the intermolecular forces between polymer chains. Staple acrylic fibres, being soft and resilient, are used as a substitute or diluent for wool, and fabrics made from them show good crease resistance and crease retention. PAN is also the most important raw material for the production of carbon fibres.

On heating, PMMA undergoes extensive chain unzipping or depolymerisation to produce a quantitative yield (>   90%) of monomer and is, as a consequence, highly flammable (LOI =   18). The oxygen of the ester group assists complete combustion of the pyrolysis products and is the reason for the low smoke production in the burning polymer. The material melts and volatilises so that no residue remains. Acrylic fibres also burn readily (LOI =   18) with melting and sputtering. The rate of burning and the amount of smoke produced depend on the acrylonitrile content of the fibre.

Generally, post-polymerisation chemical modification of PMMA with flame-retardant groups is far more difficult than in the case of PS, owing partly to the relatively less reactive ester-carbonyl groups, and partly to the fact that substitution reactions of backbone hydrogens with conventional modifying reagents invariably results in substantial chain degradation. However, chemical modification of PMMA by copolymerisation with a wide variety of comonomers bearing flame-retardant groups is relatively easy. Recent examples of such an approach include copolymerisation of methyl methacrylate with polymerisable cyclotriphosphazenes 78 , 99–101 and with a variety of phosphorus-containing unsaturated compounds. 74 , 75 The modified polymers were found to be significantly more flame retardant than PMMA, and predominantly a condensed-phase mechanism of flame retardation was found to be operative. In another study, phosphorus-containing groups, mainly phosphonate moieties, were incorporated in PMMA at specific positions on the polymer backbone, namely at chain ends and as pendent groups. 102 The degree of flame retardancy was found to depend on the topological dispositions of the modifying groups in addition to the extent of loading. Recently, enhanced char-forming tendency and increased flame retardancy were found in poly(methyl methacrylate-co-4-vinyl pyridine) polymers modified with transition metal complexes such as vanadium acetylacetonate, vanadyl chloride and ferric chloride. 80

Probably, a much more widely used strategy to flame retard acrylics, in general, is the incorporation of flame-retardant compounds as additives. Such additives for PMMA include red phosphorus or its compounds, especially in combination with other inorganic nitrogen and halogen-containing flame retardants. 103 Other additive-type flame retardants include inorganic and organic tin halides and sulphur compounds. Wilkinson's salt of the type RhCl(PPh3)3 has also been used as a flame retardant for PMMA. RhCl(PPh3)3 reacts with PMMA at the carbonyl groups promoting crosslinking and eventually leading to char formation. 104 More recently, novel ecologically safe flame-retardant systems based on silica gels have been used to flame retard PMMA. 105

There are several reports in the literature regarding the burning behaviour and the influence of various flame-retardant species on the flammability of fibre-forming homopolymer and copolymers of acrylonitrile. 106 , 107 A pressed, powdered, polymer sheet technique has been developed which allows a range of polymer compositions, in the presence and absence of flame retardants, to be assessed for LOI, burning rate and char residue. 106 It has been shown that the mechanism of thermal degradation of polymers of acrylonitrile is also dependent on the rate of heating. At low heating rates, cyclisation is the main reaction pathway whereas at high heating rates, commensurate with those encountered in fires, volatile-forming chain scission predominates. The common flame-retarding additives used for PAN and related polymers are red phosphorus, 108 brominated compounds, 109 and various aromatic phosphorus compounds in combination with metal oxides or hydroxides. 110

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Polymeric Materials and Properties

E. Alfredo Campo , in Selection of Polymeric Materials, 2008

1.14.3 Polymethyl Methacrylate (Acrylic, PMMA)

PMMA (acrylic) polymers have outstanding optical properties, weatherability, and a full range of transparent, translucent, and opaque colors. Acrylics are composed of polymers and copolymers in which the major monomerics belong to two families of ester-acrylates and methacrylates. Hard, clear acrylic sheets are made from methyl methacrylate; molding and extrusion resins are made in a continuous solution from methyl methacrylate copolymerized with small percentages of other acrylates or methacrylates.

The low strain optic coefficient of acrylics, coupled with their ability to be molded with low stress, makes them an ideal material for video disks. Sheets extruded from an acrylic base impact-modified grade have excellent thermoforming characteristics and can be stiffened by applying glass reinforced polyester to the inside surface with a spray gun to produce bathroom whirlpool tubs. The high flow grade has the best transparency, as it does not contain acrylonitrile, making it suitable for medical applications in which transparency is of prime importance.

Acrylic plastics can be cleaned with solutions of inorganic acids, alkalis, and aliphatic hydrocarbons. However, chlorinated and aromatic hydrocarbons, esters, and ketones will attack the acrylic plastics.

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Coating materials to increase pavement surface reflectance

N. Xie , ... D. Feng , in Eco-Efficient Materials for Mitigating Building Cooling Needs, 2015

2.2.3 Advantages and disadvantages of various polymers

Both epoxy and acrylic polymers provide advantages and disadvantages in their use as coating materials for pavements. Epoxy polymers typically bear the benefits of high fatigue strengths, high temperature properties, and chemical stability; however, during implementation, the surfaces that need to be joined have to be carefully cleaned. In addition, the curing time is relatively long and the curing temperature is sometimes high. Apart from the curing problems, the rigidity of the cured epoxy is another problem when used as a coating material for asphalt pavement. Acrylic ester polymers typically have good plasticity, which is a merit for asphalt pavement. They are also chemically stable in aggressive environments. However, both epoxy and acrylic polymers are not UV resistant. UV ageing is still the bottleneck problem that needs to be solved.

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