Applied Polymer Science - American Chemical Society

20 F o a m e d Plastics RUDOLPH D. DEANIN

Downloaded by UNIV OF IOWA on August 11, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch020

Plastics Engineering Department, University of Lowell, Lowell, MA 01854

Processes General Blowing Agents Special Processes Classes Properties Unique Properties of Foamed Plastics Structural Features of Foamed Plastics Leading Commercial Polymers Polyurethane Polystyrene Poly(vinyl chloride) Structural Foams Applications and Markets

When bubbles of air or other gas are dispersed in a solid polymer matrix, the product is classified as a foamed plastic. Although such foams were often observed accidentally in the early development of commercial polymers, commercialization of foams began more slowly and has accelerated more recently. Paradoxically, phenolic resins were commercialized in 1908 only when Baekeland learned to prevent foaming by molding them under high pressure. Rubber latex was first converted to foam rubber in 1914, and foamed ebonite has been in service as insulation for over half a century. The 1930s saw the invention of foamed polystyrene in Sweden and the commercial production of foamed urea-formaldehyde insulation and azo blowing agents in Germany. During the 1940s, foamed polyethylene was invented in England, and foamed epoxies were invented in the United States; commercial production of foamed polystyrene and poly(vinyl chloride) began in the United States, and that of vinyls and polyurethanes began in Germany. Polyurethane foams came to the United States in the 1950s and grew rapidly to major commodity 0097 6156/85/0285-0469S07.50/0 © 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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status. S y n t a c t i c and s t r u c t u r a l foams appeared i n the 1960s, followed by the rapid growth of reaction i n j e c t i o n molding (RIM) of polyurethanes since the l a t e 1970s. Current p r o d u c t i o n of foamed p l a s t i c s i s thus about 3 b i l l i o n l b / y e a r and i s growing much faster than t o t a l p l a s t i c s p r o d u c t i o n as k n o w l e d g e o f p r o d u c t i o n techniques, properties, and optimum a p p l i c a t i o n s increases. Much of t h i s growth has been recorded p e r i o d i c a l l y i n Modern P l a s t i c s ( p a r t i c u l a r l y every J a n u a r y ) , the J o u r n a l of C e l l u l a r P l a s t i c s , and the annual "Modern P l a s t i c s Encyclopedia." C o l l e c t i o n of information i n textbook form began with Ferrigno i n 1967, Benning i n 1969, and F r i s c h and Saunders i n 1972-73. More recently Meinecke and C l a r k , and most r e c e n t l y H i l y a r d , have made n o t a b l e contributions to the understanding of foam properties. This chapter w i l l summarize the processes by which foamed p l a s t i c s are produced, the major c l a s s e s , the r e l a t i o n s h i p s between foam s t r u c t u r e and properties, the leading polymers used i n foam production, and t h e i r major a p p l i c a t i o n s and markets. Processes G e n e r a l . The p r o d u c t i o n of foamed p l a s t i c s r e q u i r e s t h r e e s u c c e s s i v e s t e p s : l i q u i d s t a t e , bubbles of gas, and s o l i d i f i c a t i o n . The way these three s t e p s are c a r r i e d out depends upon the type of polymer and the type of foam being produced. Liquid State. The l i q u i d state i n most thermoplastics i s produced by h e a t i n g u n t i l m o l t e n ; i n v i n y l s i t i s g e n e r a l l y obtained by dispersing r e s i n p a r t i c l e s i n p l a s t i c i z e r to produce a p l a s t i s o l ; and i n rubber i t i s obtained by use of l a t e x . In t h e r m o s e t t i n g p l a s t i c s , the monomers are reacted only up to low molecular weight r e a c t i v e p r e p o l y m e r s , which are s t i l l l i q u i d or at l e a s t r e a d i l y f u s i b l e at low temperature. Bubbles of Gas. Bubbles of gas may be produced p h y s i c a l l y or chemically. P h y s i c a l blowing agents include permanent gases, such as a i r or n i t r o g e n , which can be whipped i n t o the l i q u i d ( f r o t h i n g of urethanes, rubber l a t e x , and v i n y l p l a s t i s o l s ) or compressed i n t o the molten polymer ( t h e r m o p l a s t i c s t r u c t u r a l foams). Physical blowing agents a l s o include v o l a t i l e l i q u i d s , which are b o i l e d by h e a t i n g the t h e r m o p l a s t i c s (pentane i n p o l y s t y r e n e ) or by the exothermic r e a c t i o n of t h e r m o s e t t i n g p l a s t i c s ( f l u o r o c a r b o n s i n polyurethanes). Chemical blowing agents are p r i m a r i l y organic azo compounds, such as azobisformamide, which are decomposed by heating thermoplastics, and thereby produce nitrogen and other s m a l l gaseous molecules. Other chemical reactions used to produce foaming include the r e a c t i o n of i s o c y a n a t e w i t h water to produce CO2 i n f l e x i b l e urethane foams, and the reaction of sodium bicarbonate with c i t r i c a c i d i n p o l y s t y r e n e to produce CO2 and thus n u c l e a t e the primary foaming action of pentane. In general, during the "blowing step," surface tension and melt strength may become extremely c r i t i c a l . Solidification. S o l i d i f i c a t i o n of the foamed l i q u i d may be a physical or a chemical process. In thermoplastics s o l i d i f i c a t i o n i s g e n e r a l l y accomplished by c o o l i n g the melt back to the s o l i d state; i n v i n y l p l a s t i s o l s and rubber l a t e x s o l i d i f i c a t i o n i s accomplished


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by more complex processes of p h y s i c a l g e l a t i o n ; i t i s sometimes aided by evaporation of s o l v e n t s such as pentane from polystyrene. In thermosetting p l a s t i c s , s o l i d i f i c a t i o n i s g e n e r a l l y accomplished by p o l y m e r i z a t i o n and c r o s s - l i n k i n g up to i n f i n i t e m o l e c u l a r weights. Blowing Agents. Aside from a i r and nitrogen, the choice of blowing agents for each polymer and each process depends upon t h e i r s p e c i f i c c h a r a c t e r i s t i c s , p a r t i c u l a r l y the b o i l i n g points of v o l a t i l e l i q u i d s ( T a b l e I) and the decomposition temperatures of o r g a n i c azo compounds ( T a b l e I I ) . P e n t a n e s a r e most commonly used f o r polystyrene foamable beads, and fluorotrichloromethane i s t y p i c a l of the f l u o r o c a r b o n s used f o r r i g i d urethane foams. Azobisformamide f o r m u l a t i o n s are the most w i d e l y used b l o w i n g agents f o r v i n y l p l a s t i s o l s and most thermoplastic s t r u c t u r a l foams, but the newer high-temperature engineering t h e r m o p l a s t i c s sometimes demand more s p e c i a l i z e d b l o w i n g a g e n t s w i t h much h i g h e r d e c o m p o s i t i o n temperatures. Concentration ranges for chemical blowing agents may t y p i c a l l y range from 0.05 to 15% depending on the a p p l i c a t i o n (Table III). Special Processes. Several more recent developments have opened the p o s s i b i l i t y of s p e c i a l foam processes that may grow to tremendous importance and perhaps even change the e n t i r e concept of foamed p l a s t i c s . These are s t r u c t u r a l foam, r e a c t i o n i n j e c t i o n m o l d i n g (RIM), and syntactic foam. S t r u c t u r a l foam i s produced by s l i g h t to moderate expansion of a normally s o l i d p l a s t i c . T y p i c a l l y , enough blowing agent i s used to reduce the density 10-50%, and the process i s carried out to produce a foamed core surrounded by a s o l i d s k i n . The polymers are u s u a l l y thermoplastics, and the blowing agent i s either compressed nitrogen or an o r g a n i c azo compound. There are a number of patented processes and types of equipment (see Box 1). The major advantages are m a t e r i a l s a v i n g , l o w e r - p r e s s u r e equipment, higher r i g i d i t y / w e i g h t r a t i o , and e l i m i n a t i o n of sink marks. This f i e l d i s growing so f a s t t h a t i t i s hard to say what percent of p l a s t i c s i s s t r u c t u r a l foam and what percent i s completely s o l i d . Reaction i n j e c t i o n molding (RIM) begins by metering two r e a c t i v e l i q u i d components ( p r i m a r i l y polyurethane) to a mixing head and then i n j e c t i n g the mix i n t o a large low-pressure mold. By using low-cost t o o l i n g and a high-speed p r o c e s s , l a r g e r i g i d or s e m i r i g i d p a r t s such as auto front ends are produced very r a p i d l y and economically. Most of these are somewhat foamed. There i s much current i n t e r e s t i n extending the RIM process to other polymers, so i t i s hard to say at present i n what d i r e c t i o n t h i s technique may continue to grow. Syntactic foam i s made by dispersing hollow microballoons i n t o a l i q u i d polymer and then s o l i d i f y i n g i t . Microbal loons are t y p i c a l l y hollow g l a s s or hollow phenolic microspheres, and the most common l i q u i d polymer i s an epoxy prepolymer, which i s then cured. Although some products are notably woodlike i n t h e i r properties and m a c h i n a b i l i t y , primary a p p l i c a t i o n s are high-performance products such as deep-sea instrumentation. These are t y p i c a l of some of the novel foam processes that may be unknowingly changing the e n t i r e concept of foamed p l a s t i c s .


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Table I V o l a t i l e L i q u i d s f o r Blowing


Name

Agents

B o i l i n g Temperature °C

Pentane Neopentane Hexane Isohexanes Heptane Isoheptanes Benzene Toluene Methyl C h l o r i d e Methylene C h l o r i d e Trichloroethylene Dichloroethane (sym.) Dichlorotetrafluoroethane Trichlorofluoromethane Trichlorotrifluoroethane Dichlorodifluoromethane

30-38 10 65-70 55-62 96-100 88-92 80-82 110-112 -24 40 87 84 4 24 48 -30

(Ref. 9, 1975-6, Pg. 127)

Table I I Organic Azo Compounds f o r Blowing Agents Decomposition Temperature ° F

Name

0xy-Bis-3enzene S u l f o n y l Hydrazide Azo-3is-Formamide + A d d i t i v e s Toluene S u l f o n y l Semicarbazide Trihydrazine Triazine 5-Phenyl T e t r a z o l e 5-Phenyl T e t r a z o l e Analogs

Gas Evolution cc/gm

315-320 329-419 442-456 527 464-482 680-72 5

(Ref. 9, 1981-2, Pg. 194)


12 5 140-230 140 175 200 200

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Table I I I Concentration Ranges f o r Chemical Blowing

Agents Concentration

PHR

Application


Compression Molding of Low-Density Foam C a s t i n g of S o f t P l a s t i s o l Foam E x t r u s i o n of S t r u c t u r a l Foam I n j e c t i o n Molding of S t r u c t u r a l Foam I n j e c t i o n Molding: E l i m i n a t i o n of Sink-Marks

5-15 2-4 0,2-1.0 0.3-0.5 0.05-0.1

(Ref. 9 , 1976-7, Pg. 194)

Box 1

S t r u c t u r a l Foam Processes Impco Trac: S e r i e s of molds t r a v e l a c l o s e d path through s t a t i o n s f o r clamping, i n j e c t i o n , s o l i d i f i c a t i o n , and unloading. Union Carbide: Extruder d e l i v e r s melt + blowing gas i n t o a p r e s s u r i z e d accumulator, from which a short shot i s i n j e c t e d i n t o a low-pressure mold. ICI Sandwich: 3 - s t a g e i n j e c t i o n molding: short shot of s o l i d polymer i s f o l l o w e d by a second shot of foamable polymer and a t h i r d shot o f s o l i d polymer, forming a s o l i d s k i n around a foamed i n t e r i o r . B e l o i t Two-Component: Two extruders feed an i n j e c t i o n mold simult'aneousTy. One feeds s o l i d polymer to form the s k i n , the other feeds foamable polymer t o form the core of the f i n i s h e d molding. USM: F u l l shot of foamable p l a s t i c i s i n j e c t e d i n t o a reduced-size mold c a v i t y . C o l l a p s i b l e core or movable w a l l s then expand the c a v i t y t o permit foam t o expand. TAF: P l a s t i c a t i n g screw f i l l s a high-pressure accumulator, which then f i l l s a reduced-size mold c a v i t y , which i s then expanded by use of movable w a l l s . LIM: Reactive l i q u i d s are mixed batchwise i n a s m a l l chamber and i n j e c t e d i n t o a mold where they foam and cure r a p i d l y . RIM: Reactive l i q u i d s are mixed c o n t i n u o u s l y and used t o feed a s e r i e s of molds i n which the mixture foams and cures. (Ref. 9 , 1975-6, Pg. 322)


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Classes A l t h o u g h foam s t r u c t u r e s can c o v e r a continuous spectrum, i t i s convenient to note s e v e r a l major classes of commercial importance. The most important d i s t i n c t i o n i s between c l o s e d - and o p e n - c e l l foams. In c l o s e d - c e l l ( u n i c e l l u l a r ) foams, each gas bubble i s separated from the others by t h i n w a l l s of polymer; these foams are optimal for f l o t a t i o n a p p l i c a t i o n s , s t r u c t u r a l r i g i d i t y , and thermal i n s u l a t i o n . In o p e n - c e l l foams, the c e l l s are a l l interconnecting, and f l u i d s and e s p e c i a l l y a i r can f l o w f r e e l y through the foam structure; these are o p t i m a l f o r sponge products and f o r s o f t f l e x i b l e materials. In the extreme case, when the l a s t few remaining c e l l w a l l s (windows) have been chemically d i s s o l v e d out of an o p e n - c e l l foam, i t i s sometimes c a l l e d " r e t i c u l a t e d . " Another important d i s t i n c t i o n i s based on d e n s i t y . For f l o t a t i o n , sandwich construction, thermal i n s u l a t i o n , and economy, very low-density foams are preferred, often as low as 1 l b / f t or l e s s . V i n y l , p o l y o l e f i n , and s y n t a c t i c foams are g e n e r a l l y of medium density. S t r u c t u r a l foams are of medium to f a i r l y high density and have a graded structure from s o l i d skin to f a i r l y low-density core. RIM foams are s i m i l a r , often e x h i b i t i n g a f a i r l y high density. In commercial practice, these d i s t i n c t i o n s separate most foams i n t o d i s c r e t e c l a s s i f i c a t i o n s . In t h e o r e t i c a l a n a l y s i s , i t i s d i f f i c u l t to use a s i n g l e process to produce a complete spectrum of structures, and i t i s d i f f i c u l t to r e l a t e properties to the complete spectrum of structure i n a s i n g l e unambiguous homologous series. I t i s hoped that such continuous a n a l y s i s w i l l be more f e a s i b l e i n the future. Properties Unique Properties of Foamed P l a s t i c s . Foamed p l a s t i c s have c e r t a i n unique properties that d i s t i n g u i s h them from s o l i d polymers and are p a r t i c u l a r l y u s e f u l i n p r a c t i c a l a p p l i c a t i o n s . B a s i c a l l y , these properties r e s u l t from t h e i r composite structure—a continuous phase of polymer t h a t i s of r e l a t i v e l y h i g h modulus, and a gas phase of n e g l i g i b l e modulus, which may be either dispersed as s i n g l e c e l l s i n a c l o s e d - c e l l foam, or continuous and interpenetrating i n an openc e l l foam. Buoyancy of c l o s e d - c e l l foams r e s u l t s from the low density of the gas c e l l s . High r a t i o of r i g i d i t y / w e i g h t and strength/weight i s p a r t i c u l a r l y n o t a b l e i n c l o s e d - c e l l foams and i n sandwich construction and s t r u c t u r a l foams; on the other hand, softness and f l e x i b i l i t y i n o p e n - c e l l foams r e s u l t from the low v i s c o s i t y of the continuous gas phase. Impact cushioning i s achieved p a r a d o x i c a l l y i n both f l e x i b l e and r i g i d foams. In s o f t f l e x i b l e foams, the mechanism of cushioning i s s e l f - e v i d e n t ; however, i n r i g i d foams, i t may be e x p l a i n e d by energy a b s o r p t i o n i n c r u s h i n g the t h i n c e l l w a l l s , or i n f l e x u r a l hysteresis of the s t i f f l y f l e x i b l e c e l l u l a r structure. A c o u s t i c a b s o r p t i o n i n o p e n - c e l l foams i s very u s e f u l i n quieting r e f l e c t e d noise. High thermal and high e l e c t r i c a l i n s u l a t i o n are due to the very low c o n d u c t i v i t y of the u b i q u i t o u s gas phase, and are d e f i n i t e l y best at lowest density; even choice of the



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blowing gas becomes important, with fluorocarbons g e n e r a l l y g i v i n g the highest thermal i n s u l a t i o n . Water a b s o r p t i o n of sponges r e s u l t s from the ease w i t h which water can r e p l a c e gas i n f l e x i b l e o p e n - c e l l foams, because both f l u i d s can flow f r e e l y on f l e x i n g . Economics are often c i t e d as an important f a c t o r i n the use of foam, e i t h e r through improved p r o p e r t y / w e i g h t r a t i o s as c i t e d e a r l i e r , or s i m p l y through the a b i l i t y of the processor to expand the s o l i d polymer and make i t go c o n s i d e r a b l y f u r t h e r at l i t t l e e x t r a cost. For any or a l l of these reasons, foamed p l a s t i c s are now growing at a much faster rate than the p l a s t i c s industry as a whole. At the same time, current notions of the basic r e l a t i o n s h i p s between foam structure and properties must be refined considerably to f a c i l i t a t e the proper production and use of foamed p l a s t i c s i n an even greater range of a p p l i c a t i o n s . S t r u c t u r a l Features of Foamed P l a s t i c s . The c r i t i c a l s t r u c t u r a l features of foamed p l a s t i c s may by itemized as f o l l o w s : polymer, gas, density, o p e n / c l o s e d - c e l l r a t i o , c e l l s i z e , and anisotropy. I t i s important to c o n s i d e r how each of these s t r u c t u r a l f e a t u r e s affects important properties of the foamed p l a s t i c . Polymer. There i s a n a t u r a l tendency f o r polymer s c i e n t i s t s to assume t h a t the r e l a t i o n s h i p s between polymer s t r u c t u r e and p r o perties carry over from the s o l i d polymer to i t s foams as w e l l . To some extent t h i s i s true. For example, the effects of p l a s t i c i z e r s i n v i n y l s and the e f f e c t s of c r o s s - l i n k i n g i n urethanes are w e l l understood i n the s o l i d polymers, and they have p a r a l l e l effects i n the foamed p l a s t i c s . In general, the properties of polymers i n the s o l i d phase can be r e f l e c t e d i n t h e i r foams i n the f o l l o w i n g ways. M e c h a n i c a l p r o p e r t i e s of s o l i d polymers g e n e r a l l y become c o n s i d e r a b l y " s o f t e r " i n the c o r r e s p o n d i n g foams. T h i s i s p a r t i c u l a r l y true i n o p e n - c e l l foams. In c l o s e d - c e l l foams, properties per unit area or unit volume are a l s o g e n e r a l l y somewhat softer; but p r o p e r t i e s per u n i t w e i g h t i n t h e expanded p o l y m e r may be considerably harder and stronger than i n the s o l i d polymer because of the p r i n c i p l e s of sandwich structures. Thermal mechanical behavior of foamed p l a s t i c s p a r a l l e l s that of t h e i r s o l i d polymers, but may be s h i f t e d to lower temperatures because of mechanical s o f t e n i n g ( d e s c r i b e d e a r l i e r ) . Thermal s t a b i l i t y a l s o p a r a l l e l s the s o l i d polymers, but i s somewhat lower because foams expose a h i g h s u r f a c e / v o l u m e r a t i o to a g g r e s s i v e environments such as hot oxygen. Flammability s i m i l a r l y p a r a l l e l s the s o l i d polymers but i s somewhat h i g h e r because of the h i g h surface/volume r a t i o of foamed p l a s t i c s exposed to oxygen. On the other hand, a p p l i c a t i o n of f lame-retardant a d d i t i v e s i s sometimes e a s i e r i n the foam, and i n c o r p o r a t i o n of f l a m e - r e t a r d a n t b l o w i n g agent gases such as fluorocarbons i n c l o s e d - c e l l foams can a c t u a l l y increase the flame retardance of the polymer. Chemical resistance of foams i s g e n e r a l l y s i m i l a r to t h e i r s o l i d polymers, but somewhat lower because of the h i g h s u r f a c e / v o l u m e r a t i o exposed to c h e m i c a l a t t a c k , and a l s o because of o r i e n t a t i o n s t r a i n s frozen i n during expansion. Aging i s s i m i l a r but somewhat faster, as discussed e a r l i e r , for thermal s t a b i l i t y . Permeability


476



i s s i m i l a r but h i g h e r because of the s m a l l amount of polymer t h a t must be permeated per u n i t t h i c k n e s s ; i n o p e n - c e l l foams i t i s p o s s i b l e to a c h i e v e h i g h f l o w - t h r o u g h r a t e s f o r f i l t r a t i o n a p p l i c a t i o n s . T o x i c i t y i s the same for s o l i d and foamed polymers, except t h a t l e a c h i n g of monomeric a d d i t i v e s from foams w i l l be faster because of t h e i r high surface-to-volume r a t i o . Thus, i n a l l these ways, the structure-property r e l a t i o n s h i p s i n s o l i d polymers do carry over q u a l i t a t i v e l y i n t o t h e i r foams as w e l l , but often with considerable q u a n t i t a t i v e modification. In terms of p r a c t i c a l p r o p e r t i e s of maximum importance, other s t r u c t u r a l features of the foam, p a r t i c u l a r l y gas and density, may be much more important than the p a r t i c u l a r polymer i t s e l f . Gas. The r o l e of the gas i n foam properties i s u s u a l l y obvious but too often o v e r l o o k e d i n the s c i e n t i f i c and e n g i n e e r i n g d e s i g n of materials and end products. In low-to-medium-density foams, the gas phase occupies most of the volume of the foam and, t h e r e f o r e , p l a y s a dominating r o l e i n c o n t r o l l i n g many properties. Most of the effects of the gas phase are due to i t s low d e n s i t y , low v i s c o s i t y , and low c o n d u c t i v i t y . Some of the e f f e c t s are e v i d e n t i n any type of foam, and other effects depend p r i m a r i l y on whether the foam i s an open- or c l o s e d c e l l structure. The low d e n s i t y of the gas d i r e c t l y l o w e r s the d e n s i t y of the foam, which i s important i n lightweight products. In c l o s e d - c e l l foams, the low d e n s i t y of the gas produces the buoyancy of the product, which i s important i n many marine products. The e f f e c t of the gas on mechanical p r o p e r t i e s depends on the paradox of c l o s e d - versus o p e n - c e l l construction. In a c l o s e d - c e l l foam, mechanical deformation compresses the gas, so the gas contributes to r i g i d i t y and strength of the p l a s t i c product. In an o p e n - c e l l foam, on the other hand, the f l u i d i t y of a i r permits i t to rush out when the foam i s deformed and to rush back when the deformation i s r e l e a s e d ; t h e r e f o r e , the foam c o n t r i b u t e s to the softness, f l e x i b i l i t y , and r e s i l i e n c e of the p l a s t i c product. Thermal conduction occurs p r i m a r i l y through the s o l i d polymer; i t i s the gas t h a t p r o v i d e s thermal i n s u l a t i o n . In c l o s e d - c e l l foams, r e s t r i c t i o n of convection i n s u l a t e s further; and choice of the gas, p a r t i c u l a r l y fluorocarbons, produces the maximum i n s u l a t i n g capacity. On the other hand, the gas i n a c l o s e d - c e l l foam aggravates the c o e f f i c i e n t of thermal expansion and contraction most s e r i o u s l y when i t i s used i n r e f r i g e r a t i o n and condenses to a lowvolume l i q u i d . I f the gas i n a c l o s e d c e l l i s a f l a m e - r e t a r d a n t f l u o r o c a r b o n r a t h e r than a i r , i t may c o n t r i b u t e to o v e r a l l flame retardance of the polymeric m a t e r i a l . E l e c t r i c a l conduction a l s o occurs p r i m a r i l y through the s o l i d polymer. R e l a t i v e l y s p e a k i n g , the gas i s the primary e l e c t r i c a l i n s u l a t o r , so foaming of p l a s t i c i n s u l a t i o n can g r e a t l y increase i t s i n s u l a t i n g c a p a c i t y . T h i s a p p l i e s not o n l y to r e s i s t a n c e and c o n d u c t i o n , but a l s o to d i e l e c t r i c constant and l o s s as w e l l . On the other hand, e l e c t r i c a l breakdown may be aggravated by the gas c e l l s i n a foam, perhaps by a c t i n g as i m p e r f e c t i o n s at which discharge can occur. Diffusion of blowing gas out of a c l o s e d - c e l l foam during normal aging g r a d u a l l y replaces the o r i g i n a l gas by normal a i r ; thus, h i g h -


20.

DEANIN

Foamed Plastics

477


performance p r o p e r t i e s of f l u o r o c a r b o n gases, such as t h e r m a l i n s u l a t i o n and flame retardance, may be l o s t unless the t o t a l foam i s completely sealed from the atmosphere. On the other hand, easy replacement of gas by water i n an o p e n - c e l l foam i s responsible for the function of p l a s t i c sponges. D e n s i t y . Because polymer and gas o f t e n have o p p o s i t e e f f e c t s on foam properties, density represents not only the r e l a t i v e amounts of the two substances i n the foam, but often a l s o t h e i r r e l a t i v e contributions to properties as w e l l . Thus, density i s u s u a l l y the most important s t r u c t u r a l f e a t u r e i n foam s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p s , much more s i g n i f i c a n t than the p a r t i c u l a r polymer t h a t happens to be used. T h i s i s an extremely important concept that foam s c i e n t i s t s and engineers must learn to use. In most p r a c t i c a l studies, a foam i s manufactured over a narrow range of densities (D), properties (P) are measured and p l o t t e d , and a f a i r l y s t r a i g h t - l i n e r e l a t i o n s h i p appears. This i s adequate for the p a r t i c u l a r p r o d u c t - l i n e i n v o l v e d , but i t i s very misleading i n terras of the e n t i r e density range from very low to completely s o l i d , and i n terms of the basic r e l a t i o n s h i p s and mechanisms i n v o l v e d . In more precise studies, p a r t i c u l a r l y over a wide or complete density range, the form of the r e l a t i o n s h i p i s g e n e r a l l y found to be P = KD , where the values of K and n depend on the s p e c i f i c polymer and the s p e c i f i c property being measured (Figure 1, Table IV). In more recent work, these e x p e r i m e n t a l o b s e r v a t i o n s have gained f u r t h e r strength from t h e o r e t i c a l mechanistic a n a l y s i s as w e l l . A number of important p r o p e r t i e s c o r r e l a t e d i r e c t l y w i t h increasing density. Those mentioned most often include n

1. 2. 3.

4. 5. 6. 7.

Modulus r e s u l t i n g d i r e c t l y from the s o l i d polymer. Strength, again r e s u l t i n g from the s o l i d polymer. U l t i m a t e e l o n g a t i o n , a p p a r e n t l y depending on the amount of m a t e r i a l present to permit d u c t i l e f l o w , or perhaps being i n v e r s e to the number of gas-bubble f l a w s , which can i n i t i a t e failure. Minimum load at which creep w i l l become serious. Hysteresis l o s s , a function of the s o l i d polymer. Thermal d i m e n s i o n a l s t a b i l i t y , being b e t t e r f o r the s o l i d polymer than for the gas. Thermal conduction, p r i m a r i l y through the s o l i d polymer rather than through the gas.

C o n v e r s e l y , a number of important p r o p e r t i e s are i n v e r s e to density. Those mentioned most often include 1. 2. 3.

Buoyancy, which i s the difference between the density of water and the density of the foam. Softness, f l e x i b i l i t y , and "rubberiness," p a r t i c u l a r l y i n openc e l l foams. These properties are due to the n e g l i g i b l e modulus and high f l u i d i t y of the gas phase. R e s i l i e n c e and recovery from both slow and impact deformation, p r i m a r i l y i n o p e n - c e l l foams, again because of the f l u i d i t y of a i r as i t rushes back i n t o the foam after the deforming force i s released.




478

Log Density Figure 1. Strength vs. density i n r i g i d urethane foams

(5).

Table V Experimental Values f o r K and n f o r R i g i d Urethane Foams K

Property i n PSI v s . PCF Compressive Strength F l e x u r a l Strength T e n s i l e Strength Shear Strength Compressive Modulus F l e x u r a l Modulus T e n s i l e Modulus Shear Modulus

12.8 19.0 23.0 14.9 293.8 186.3 573.5 169.9

(Ref. 4, V o l . I I , Pg. 487)


n 1.54 1.36 1.20 1.16 1.62 1.75 1.15 1.39

20. DEANIN 4.

5.


6.

7. 8. 9.

Foamed Plastics

479

Shock absorbance, both i n f l e x i b l e o p e n - c e l l foams because of the rush of escaping a i r , and i n r i g i d foams because of the c o n v e r s i o n of k i n e t i c energy i n t o p o t e n t i a l energy d u r i n g crushing of c e l l w a l l s to create new surface. Thermal d i m e n s i o n a l i n s t a b i l i t y of c l o s e d - c e l l foams due to expansion and contraction of gas and e s p e c i a l l y to condensation at very low temperatures i n r e f r i g e r a t i o n . A l s o s h r i n k a g e during oven aging, due to permeation and l o s s of gas through the thin c e l l walls. Thermal i n s u l a t i o n due to the low c o n d u c t i v i t y of the gas phase, p a r t i c u l a r l y i n c l o s e d - c e l l foams, which r e s t r i c t c o n v e c t i o n , and p a r t i c u l a r l y when they are f i l l e d with an e s p e c i a l l y lowconductivity gas such as fluorocarbon. E l e c t r i c a l i n s u l a t i o n due to the low c o n d u c t i v i t y and low p o l a r i z a b i l i t y of the gas phase. P e r m e a b i l i t y ( F i g u r e 2) due to the t h i n n e s s of the remaining polymer w a l l s acting as membranes, and of course p a r t i c u l a r l y i n o p e n - c e l l foams when t h i s property i s desired. Water absorption, p a r t i c u l a r l y i n f l e x i b l e o p e n - c e l l foams, for sponge-type a p p l i c a t i o n s .

A l l of these p r o p e r t i e s , and many more, depend more upon the d e n s i t y of the foam than upon any other s t r u c t u r a l feature; therefore, density should always be considered as the primary factor i n the design of both materials and end products. Open/Closed-Cell Ratio. The r a t i o of open to closed c e l l s i n a foam has important effects on many important properties. Although poor measurement techniques have reduced many studies from q u a n t i t a t i v e to simply q u a l i t a t i v e , and although many foam processes produce only a s e r a i c o n t r o l l a b l e mixture of open and c l o s e d c e l l s , the b a s i c r e l a t i o n s h i p s are of major t h e o r e t i c a l and p r a c t i c a l s i g n i f i c a n c e . In general, the percent of open c e l l s c o r r e l a t e s d i r e c t l y with many important properties 1.

Softness, f l e x i b i l i t y , and cushioning r e s u l t from the free flow of a i r out of the foam when i t i s deformed by mechanical forces (Figure 3). These properties are a major factor i n the comfort of c l o t h i n g and furniture. 2. Rebound and recovery depend on the ease with which a i r can rush back i n t o the foam when the mechanical deformation i s removed. 3. Mechanical properties i n general c o r r e l a t e better with those of the s o l i d polymer i n an o p e n - c e l l foam; however, they depend more on gas pressure i n a c l o s e d - c e l l foam. [ R e t i c u l a t i o n by chemically d i s s o l v i n g away the l a s t traces of any " c e l l windows" at a l l removes weak f l a w s t h a t c o u l d i n i t i a t e premature f a i l u r e , and thus u l t i m a t e s t r e n g t h ( F i g u r e 4) and elongation (Figure 5) are increased. However, t h i s i s the opposite of the normal e f f e c t s of c l o s e d - c e l l w a l l s , which a l s o i n c r e a s e strength and elongation. Thus, the two concepts are paradoxical and must not be confused.] 4. A c o u s t i c absorbance and i n s u l a t i o n are accomplished by d i s persion of sound waves i n the open c e l l s . 5. Thermal c o n d u c t i v i t y i n c r e a s e s d i r e c t l y w i t h i n c r e a s e d c o n vection through an o p e n - c e l l foam.



480

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ex.

Log Density-

Figure 2. Permeability v s . density ( 2 ) .

Percent Open C e l l s

Percent Compression Figure 3. Compressibility v s . open c e l l s i n polyethylene foams (7_).



DEANIN

Foamed Plastics

H O

50 Pores/Inch

100

Figure 4. Tensile strength of f l e x i b l e polyester urethane ( 2 ) .

0

50 Pores/Inch

100

Figure 5. Elongation of f l e x i b l e polyester urethane ( 2 ) .


482 6. 7.


Permeability i s easier and faster through an o p e n - c e l l foam than through the t h i n w a l l s of a c l o s e d - c e l l foam (Figure 6). Water absorption i n sponge-type a p p l i c a t i o n s and use as a f i l t e r medium depends on a h i g h percent of open c e l l s to permit easy passage of l i q u i d water through the foam.

The gas i n an o p e n - c e l l foam i s normally a i r , unless the e n t i r e foam i s enclosed i n an impermeable w a l l and then f i l l e d with a p a r t i c u l a r gas for a p a r t i c u l a r purpose. C o n v e r s e l y , the percent of c l o s e d c e l l s i n a foam a l s o c o r r e l a t e s with many important properties


1. 2. 3.

4. 5.

Buoyancy of the low-density gas i s only e f f e c t i v e i f c l o s e d - c e l l w a l l s protect i t against replacement by water. R i g i d i t y and strength of the polymer are enhanced by mechanical deformation, which increases gas pressure and thus increases the t o t a l load-bearing capacity of the structure. Compressive set, creep, mechanical damping, and hysteresis a l l occur because, under mechanical s t r e s s , gas permeates out of the c l o s e d - c e l l w a l l s to reduce the s t r e s s . When the mechanical stress i s released, the gas permeates back only s l o w l y and often incompletely. Thermal i n s u l a t i o n improves ( F i g u r e 7) because c l o s e d c e l l s reduce conduction by convection; thus, only r a d i a t i o n and s o l i d conduction remain as conductive mechanisms. Shrinkage during oven aging i s much worse i n c l o s e d - c e l l foams because the gas t h a t permeates out of the c e l l s cannot e a s i l y return when aging i s over.

Thus, the mechanistic and p r a c t i c a l s i g n i f i c a n c e of open versus c l o s e d c e l l s i s very i m p o r t a n t , and d e s e r v e s more p r e c i s e development of measurements and c o r r e l a t i o n s with properties. C e l l S i z e . M i c r o s c o p i c examination shows t h a t the c e l l s i z e and s i z e d i s t r i b u t i o n vary g r e a t l y between different foamed p l a s t i c s . I t i s natural to assume that the properties w i l l vary accordingly. Unfortunately, the exact r e l a t i o n s h i p s have proved e l u s i v e for two reasons: any attempt to vary c e l l s i z e experimentally i n v a r i a b l y changes other s t r u c t u r a l features simultaneously; thus, i t i s hard to separate these independent v a r i a b l e s ; and t h e r e i s a n a t u r a l p r e d i l e c t i o n to p r e f e r s m a l l e r c e l l s i z e as e v i d e n c e of b e t t e r experimental technique, and t h i s sometimes b l i n d s researchers to the hard evidence before them. D e s p i t e these d i f f i c u l t i e s , c e r t a i n r e l a t i o n s h i p s between c e l l s i z e and p r a c t i c a l p r o p e r t i e s are frequently reported. Larger c e l l s i z e i s o f t e n r e p o r t e d to c o r r e l a t e w i t h the f o l l o w i n g properties 1. 2. 3. 4. 5.

Modulus and s t i f f n e s s , presumably due to the thicker c e l l w a l l s . Shock absorption, presumably by these t h i c k e r c e l l w a l l s . Buckling f a i l u r e during compression. Higher thermal conductivity by both convection and r a d i a t i o n . Permeability, presumably because the permeating gas only has to go through a few c e l l w a l l s and can t r a v e l f r e e l y through large open spaces within the c e l l s .



20. DEANIN

483

Foamed Plastics

Figure 6. Effect of open/closed c e l l r a t i o on permeability

(.2).

Closed C e l l s % Figure 7. Thermal conductivity of r i g i d polyurethane foams ( 2 ) .


484


C o n v e r s e l y , s m a l l e r c e l l s i z e i s often r e p o r t e d to c o r r e l a t e with the f o l l o w i n g properties 1.


2. 3. 4.

Higher strength, because of the l a r g e r number of c e l l w a l l s that must be ruptured before f a i l u r e w i l l occur (Figure 8). Higher ultimate elongation for the same reason. Gradual f a i l u r e during compression. High thermal i n s u l a t i o n because s m a l l c e l l s reduce conduction by convection.

On the n i h i l i s t i c side, however, some researchers have f a i l e d to f i n d any s i g n i f i c a n t c o r r e l a t i o n between c e l l s i z e and p r a c t i c a l properties. The s p e c i f i c questions must thus remain open u n t i l more precise studies, i n which c e l l s i z e i s the only s t r u c t u r a l v a r i a b l e , are undertaken. Beyond t h i s point, c e l l - s i z e d i s t r i b u t i o n may w e l l prove to be another s i g n i f i c a n t subvariable. Anisotropy. When foam formation and expansion occur f r e e l y i n a l l d i r e c t i o n s , the c e l l structure should be symmetrical and i s o t r o p i c , and properties should s i m i l a r l y be i s o t r o p i c i n a l l d i r e c t i o n s . In many processes, however, the l i q u i d i s poured, and the gas bubbles t h a t form are a l l o w e d to r i s e f r e e l y i n the v e r t i c a l d i r e c t i o n . This process produces foam c e l l s that are elongated v e r t i c a l l y i n the d i r e c t i o n of foam r i s e . N a t u r a l l y , t h i s w i l l have a severely a n i s o t r o p i c e f f e c t on many p r o p e r t i e s . Although t h i s effect i s sometimes predictable and i s quite e a s i l y measurable, an unfortunate confusion i n nomenclature has prevented s c i e n t i s t s and e n g i n e e r s from making b e t t e r use of i t i n product d e s i g n . For c l a r i t y , any property t e s t s h o u l d i n d i c a t e whether the t e s t was taken i n the d i r e c t i o n of foam r i s e or a c r o s s the d i r e c t i o n of foam r i s e . A s i m p l e diagram c o u l d be used i n r e p o r t i n g a l l r e s u l t s and s h o u l d help c l a r i f y the meaning and f a c i l i t a t e use of the knowledge. In g e n e r a l , compressive l o a d i n g i n the d i r e c t i o n of foam r i s e must bear d i r e c t l y on the c e l l w a l l s , which thus act as columns to support the load and give greater compressive modulus and strength in this direction. Loading perpendicular to the d i r e c t i o n of foam r i s e w i l l tend to f o l d the foam e a s i l y l i k e an accordion (Figure 9). L i k e w i s e , permeation through a foam i n the d i r e c t i o n of foam r i s e r e q u i r e s the permeating gas m o l e c u l e s to t r a v e l through a r e l a t i v e l y s m a l l number of c e l l w a l l s , i n between which they can t r a v e l f r e e l y for r e l a t i v e l y long distances down the length of each elongated c e l l . Therefore, permeability i n t h i s d i r e c t i o n w i l l be h i g h . Permeation t r a n s v e r s e to the d i r e c t i o n of foam r i s e w i l l f o r c e the gas m o l e c u l e s to pass through very many c e l l w a l l s and only a l l o w them short t r a v e l distances from w a l l to w a l l ; therefore, permeability i n t h i s d i r e c t i o n w i l l be low (Figure 10). C a r e f u l measurement and r e p o r t i n g s h o u l d r e a d i l y expand the current understanding of such anisotropic effects. Leading Commercial Polymers Standard t e x t s have g i v e n good thorough d e s c r i p t i o n s of the i n d i v i d u a l commercial foamed p l a s t i c s , so o n l y b r i e f r e v i e w i s needed here to put them i n t o proper perspective.


20.

DEANIN

Foamed Plastics


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485

80

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0.1 0.2 C e l l S i z e Inches

Figure 8. Flexural strength v s . c e l l size of polystyrene foam (10).

5

10

15

20

Defe l cto i n, %

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Figure 9. Effect of anisotropy on compressive behavior ( 2 ) .



486


Polyurethane. Polyurethanes are formed by mixing l i q u i d a l i p h a t i c p o l y o l s with l i q u i d aromatic polyisocyanates, which react r e a d i l y . These mixes g e l i n s e v e r a l minutes at room temperature, but o f t e n b e n e f i t from f i n a l heat c u r e . Most p o l y u r e t h a n e s are foamed by introducing and expanding gas bubbles when the reacting l i q u i d s have reached the optimum v i s c o s i t y , then the c r o s s - l i n k i n g reaction i s completed to s t a b i l i z e the foam structure. For f l e x i b l e foams, the p o l y o l s are p r i m a r i l y l o n g - c h a i n p o l y o x y p r o p y l e n e d i o l s , w i t h a s m a l l amount of t r i o l to produce c r o s s - l i n k i n g and to ensure rubbery r e s i l i e n c e ; the polyisocyanate i s p r i m a r i l y t o l u e n e d i i s o c y a n a t e (see Box 2 ) . Gas bubbles are produced by the reaction of a measured amount of water i n the p o l y o l w i t h a measured excess of i s o c y a n a t e ; i n some f o r m u l a t i o n s , l o w b o i l i n g f l u o r o c a r b o n s may be used as a u x i l i a r y p h y s i c a l b l o w i n g agents. O p e n - c e l l s t r u c t u r e i s produced e i t h e r by mechanical crushing or by chemical formulation to burst the c e l l w a l l s before they cure. Such f l e x i b l e urethane foams have l a r g e l y replaced foam rubber and have taken a commanding p o s i t i o n i n automotive and furniture markets. For r i g i d foams, the p o l y o l s have h i g h e r f u n c t i o n a l i t y , g e n e r a l l y from three to s i x hydroxyls on a s m a l l compact backbone, and the polyisocyanate i s g e n e r a l l y two or three aromatic isocyanate groups j o i n e d by methylene b r i d g e s , to produce h i g h - c r o s s - l i n k density (see Box 3 ) . Blowing agents are p r i m a r i l y fluorotrichloromethane and mixed fluorocarbons, which are b o i l e d by the exotherm of the cure r e a c t i o n . The r e s u l t i n g c l o s e d - c e l l structure provides maximum thermal i n s u l a t i o n , e s p e c i a l l y i f the f l u o r o c a r b o n gases can be permanently s e a l e d i n t o the t o t a l structure; and the c l o s e d - c e l l r i g i d foam a l s o contributes to the o v e r a l l mechanical r i g i d i t y and s t r e n g t h of the t o t a l s t r u c t u r e . Such r i g i d urethane foams have thus become a major factor i n thermal i n s u l a t i o n for b u i l d i n g construction and r e f r i g e r a t i o n . More r e c e n t l y , s t r u c t u r a l foam and RIM have been produced by low to moderate foaming of semirigid to r i g i d urethane systems and have made a major contribution to furniture and automotive construction. Only the future w i l l t e l l how widely such developments may continue to grow. Polystyrene. Low-density polystyrene foam products are made by two different methods. Molded products such as cups and packaging are made by suspension polymerizing to t i n y beads, s w e l l i n g these with 5% pentane, pre-expanding w i t h steam, and then m o l d i n g the p r e expanded beads i n a steam chest to expand them further and fuse them into the f i n a l shape. Extruded food-packaging sheet and i n s u l a t i n g board i s made by melting polystyrene p e l l e t s i n a vented extruder, then i n j e c t i n g pentane or compressed n i t r o g e n gas through the "vent," and then extruding through a die and s i z i n g units to produce continuous sheet, board, or other p r o f i l e . The combination of low density, r i g i d i t y , thermal i n s u l a t i o n , nontoxicity, good c o l o r , and chemical resistance (Table V) has long been popular i n mass markets such as packaging and b u i l d i n g . P o l y ( v i n y l c h l o r i d e ) . When p l a s t i c i z e d p o l y ( v i n y l c h l o r i d e ) (PVC) i s l a m i n a t e d onto c l o t h , f o r c l o t h i n g and u p h o l s t e r y , the s o l i d v i n y l has a harsh f e e l often judged i n f e r i o r by the consumer. When


20.

DEANIN

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Plastics

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Applied Polymer Science - American Chemical Society

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