Food Packaging Materials (Chemical Structure & Properties) PDF

Chemical structure & properties of packaging materials Properties of packaging materials depend on the atomic & molecular structures at 4 levels: 1. Chemical constituents (what atoms are composed in the materials). 2. Chemical bonding (what forces hold the ato...

Chemical structure & properties of packaging materials Properties of packaging materials depend on the atomic & molecular structures at 4 levels: 1. Chemical constituents (what atoms are composed in the materials). 2. Chemical bonding (what forces hold the atom together to form a molecule). 3. Intermolecular forces (what forces attract molecules together to form a material). 4. Spatial arrangement (how molecules are arranged in 3 dimensional space).  Chemical structure & properties of packaging materials Level 1 & 2 are responsible for the chemical properties of materials. Determine the sensitivity of a material to chemical changes, which involve breaking of chemical bonds to transform part or all of the original materials into new substances. Chemical changes, e.g. oxidation, corrosion of metal containers, and incineration of packaging wastes.. Level 3 & 4 are responsible for the physical properties of materials. Determine the physical behavior of a material under conditions e.g. heat, pressure and concentration gradient, that do not involve breaking of chemical bonds and formation of new substances. Example : - gas permeation through package walls - migration of volatile compounds from package to food - shock and vibration during distribution.  Level 1 – Chemical constituents 4 basic types of materials of food package: plastics, paper, glass, metals. Plastic and paper: carbon based organic materials  lighter, weaker, more susceptible to chemical reactions, more likely to interact with food. Glass and metal: inorganic materials. Complexity of material: Single materials. Combination of 2 or more different materials to provide necessary functions. E.g.: aseptic carton for milk and juices is made from 70% paper, 6% aluminum and 24% polyethylene (PE) (layers: PE/paper/PE/Al/PE)  paper provides strength and stiffness, the thin aluminum foil layer provides gas and light barrier PE layers provide inner and outer protection, heat sealing, and binding between paper and aluminum.  Level 1 – Chemical constituents Material Commonly found atoms Chemical bond Intermolecular forces Plastics Major: C, H C-C backbone, van der Waals Minor: O, N, Cl, etc. ionic forces Paper C, H, O Covalent van der Waals forces Glass Major: Si, O Covalent, ionic Ionic solid Minor: Na, K, Ca, Al, etc. Metals Major: Fe, Al Metallic Coulombic forces Minor: Cr, Sn, etc.  Level 2 – Chemical bonding Chemical bonding determines how atoms are bound together to form a molecule. Chemical bond: formed when the outmost electrons from 2 atoms interact with each other to achieve a lower and more stable energy state. Types:  depends on the electronegativities of the atoms (i.e. the ability to attract electrons). Transferring of electrons: ionic bond. Sharing of electrons : covalent or metallic bonds. Ionic, covalent and metallic bonds = primary bonds  strong bond strengths A molecule may have more than one type of chemical bond. E.g. ionomer (packaging polymer, ion-containing polymer) consists of mostly covalent bonds and a small but significant number of ionic bonds (Na+ or Zn+ ions) to its side chains  unique properties (different from other polymers with only covalent bonds): heat seal strength, abrasion resistance, melt elasticity. Atom  Level 2 – Chemical bonding IONIC BONDING Formed when two atoms come together to transfer one or more electrons between each other. The bond occurs between two atoms of greatly different electronegativities  The oppositely charged ions are bound by electrostatic attraction together Typically, one atom is a metal (e.g. Li, Na, Al, Fe) because of its tendency to donate electrons, and the other one is a nonmetal (e.g. O, F, S, Cl) due to its tendency to accept electrons.  Ionic materials are: Strong. Hard. Brittle. Resistant to aggressive materials. Have a high melting point. Ionic bonds are less commonly found in packaging materials, compared to covalent and metallic bonds. Found in protective layers for packaging applications, e.g. alumina (Al2O3) on foil and tin oxide (SnOx) on tin plate. The ionic groups in an ionomers (copolymers containing both ionic and nonionic repeat units) may interact with each other to form a cluster  e.g. used as sealant between a plastic layer and a foil, in packaging applications where excellent heat sealability and hot tack are required.  Level 2 – Chemical bonding METALLIC BONDING Found in aluminum foil, metal containers, and other metal containing packaging materials. Atoms in metal: low electronegativities willing to share electrons freely, not only with neighbor atoms, but also with all other atoms in the lattice.  A metal atom loses its outer electrons  become positively charged  Those electrons form a ‘sea of electrons’ to surround the regularly spaced ions  The electrons form ‘delocalized’ or ‘free’ electrons (meaning that they are nondirectional and free to move in all directions in the arrangement; unlike the electrons in covalent bonds which are bound to specific atoms and locations).  Metallic bonds are responsible for:  the strength  malleability (the ability to be hammered into various shapes)  ductility (the ability to be drawn out into thin wires) of metals  change material shape without cracking or breaking; e.g. aluminum sheet rolled into a very thin aluminum foil; aluminum disk hammered into soda cans. Although strong, metallic bonds are not bound to specific metal ions  forces from hammering and stretching can cause layers of metal ions to slide over each another  the movement attracts the free electrons to their new locations, while the sea of electrons is not broken and still holds the metal ions together.  Metallic bonds are responsible for high electrical and thermal conductivities of metals. High electrical conductivity: due to the ability of the delocalized electrons to flow freely in the lattice of positive metal ions when a voltage is applied. High thermal conductivity: due to the ability of metal ions to absorb heat energy by vibrating faster  collision with neighboring ions  transfer of heat energy.  Level 2 – Chemical bonding COVALENT BONDING Most abundant chemical bond. Found in plastics, paper, and glass. Formed when two atoms come together to share their outermost electrons (these atoms have high and similar electronegativities  strong affinity to electrons and are willing to share electrons with their partner atoms). From nonmetal elements, e.g. C, O, N, Cl.  Level 2 – Chemical bonding COVALENT BONDING Can form polymer, e.g. polyethylene (thousands of C-C and C-H covalent bonds). For polymer: degree of polymerization determines the number of covalent bonds and MW  affect the property and performance of the material. Other bonds in plastics and paper: C=C, C=O, O-H. In glass: Si-O.  Level 2 – Chemical bonding COVALENT BONDING Bond polarity: determined by the difference in electronegativities between the atoms joined by the bond. Zero or negligible difference: electrons are shared equally between atoms  form a nonpolar covalent bond, e.g. C- C Small but significant difference: electrons are not shared equally between atoms  form a polar covalent bond, e.g. C=O Large difference: electrons are not shared, but transferred from one atom to another  form an ionic bond.  Level 2 – Chemical bonding COVALENT BONDING Bond polarity affect the polarity of the molecule and thus, the properties of the material. If all bonds are non polar  the molecule is non polar. If some of bonds are polar  the molecule could be polar or nonpolar, depending on the symmetry of the molecule. C-H and C-Cl are polar bonds  PVC is polar. C-H is polar, but since the bonds are arranged symmetrically  the dipoles cancel out resulting in no net dipole  PE is non polar. The more polar the molecule, the stronger the intermolecular forces  PVC has higher mechanical and barrier properties than PE.  Level 2 – Chemical bonding COVALENT BONDING Covalent bonds may be broken when packaging materials are exposed to severe conditions:  aggressive chemicals  high temperature (microwave oven or conventional oven)  high temperature + shear (e.g. during processing of polymer such as extruding polymer resins to form plastic films or containers)  possible formation of undesirable oligomer compounds which can migrate from package to food. A molecule may have more than one type of chemical bond e.g. ionomer which consists of mostly covalent bonds and a small but significant number of ionic bonds (Na+ or Zn+ ions) to its side chain  unique properties not found in polymers with only covalent bonds: heat seal strength, abrasion resistance, melt elasticity.  Level 2 – Chemical bonding COVALENT BONDING The strength of some selected covalent chemical bonds at 25 0C Bond Bond energy (kJ/mol) Bond Bond energy (kJ/mol) C-Cl 38 O-H 463 C-F 565 C-C 348 O-O 146 C=C 612 CI-O 466 C-N 305 C-H 412 C-O 360 N-H 388 C=O 743  Level 2 – Chemical bonding MAIN PROPERTIES Property Ionic bond materials Covalent bond materials Metallic bond materials Melting and boiling points High, due to strong attractions Relatively low thermal High; due to attractions of between atoms (ions). transition temperature due to metals ions with the weak attractive forces delocalized electrons. between molecules. Electrical and thermal No electric conductivity & The electron sharing makes High conductivity because of conductivity poor thermal conductivity due the electrons tightly bound to easy transmission of vibration to absence of mobile charged atoms  no charge available energy through the ‘electron particles. for conductivity. cloud’. Mechanical properties Generally hard because the Generally soft because the Dense, hard, and strong with (hardness and brittleness) ions are strongly bound to the intermolecular bonds are tightly packed atoms in the lattice; often brittle because of weak and easily displaced. lattice; malleable and ductile strong repulsion between (distortion is possible without equal charges at the close disrupting the inter-atomic proximity at the distortion. bond)  Level 2 – Chemical bonding BOND STRENGTHS Interaction type Bond energy (kJ/mol) Chemical Covalent bonds 200-800 bonding Metallic bonds 100-400 Ionic bonds 40-500 Covalent bonds are stronger than metallic and ionic bonds. The amount of energy required to break a covalent bond between two linked atoms is known as bond dissociation energy  Level 3 – Intermolecular forces Intramolecular forces/ primary bonds:  Forces which keep a molecule together  Forces that hold atoms together making molecules  Ionic, covalent, Metallic binding Intermolecular forces/ secondary bonds: Forces of attraction or repulsion that hold molecules together A group of molecules held together loosely by weak intermolecular forces. Covalent bonds are 10-100 times stronger than intermolecular forces.  Interaction type Bond energy (kJ/mol) Chemical Covalent bonds 200-800 bonding Metallic bonds 100-400 Ionic bonds 40-500 Intermolecular H-bonds 4-40 forces Dipole-dipole forces 0.15-15 Ion-dipole 5-60 Dispersion forces 0.4-4  Level 3 – Intermolecular forces ? Weak intermolecular forces  Plastic/ paper material: composed of many molecules hold together by intermolecular forces, or secondary bonds, which are much weaker than primary bonds  low strength and low melting points Eg: Punctured/ melted PE film: break only the intermolecular forces holding the PE molecules, whereas covalent bonds holding the C and H atoms remain strong and unaffected.  Level 3 – Intermolecular forces ? Weak intermolecular forces (continue) Metals and glass: not composed of a collection of molecules, instead, they are composed of a single giant molecule, in which all the atoms are bound together by primary bonds to form a network  stronger and stiffer than paper and plastics, since metallic bonds (metal) and covalent bonds (glass) are much stronger than intermolecular forces. Cutting a metal container : breaking metallic bonds. Needs higher Shattering a glass bottle: breaking covalent bonds. energy  Level 3 – Intermolecular forces Intermolecular forces are often called van der Waals forces. Different ways of attraction among molecules according to their dipole moment. 4 types: Dispersion forces (the weakest) Dipole-dipole interactions Ion-dipole interactions Hydrogen bonding (the strongest, about 1/10 the strength of covalent bonds) Weak bind among molecules in the solid state of organic materials that have covalent bonds  low melting point and low boiling point of the materials.  Level 3 – Intermolecular forces DISPERSION FORCES Exist between nonpolar molecules (between temporary dipole molecules as a result of positive nuclei of one molecule attracting the electrons of another molecule). Weakest intermolecular force. Also called induced dipole-induced dipole attraction. Important role in plastic and biopolymer structure.  Level 3 – Intermolecular forces DIPOLE-DIPOLE FORCES Between polar molecules (between the positive end of one polar molecule and the negative end of another polar molecule). Have a significant effect only when the molecules are close together (touching or almost touching). Much weaker than ionic or covalent bonds.  Level 3 – Intermolecular forces ION-DIPOLE FORCES An attractive force that results from the electrostatic attraction between an ion and a neutral molecule that has a dipole. Exist between charged ion and a polar molecule. Most commonly found in solutions. Responsible for the dissolution of ionic substances in polar solvents, e.g. aqueous salt solution.  Level 3 – Intermolecular forces HYDROGEN BONDING Occurs when a hydrogen atom is covalently bonded to a small highly electronegative atom, e.g. nitrogen or oxygen  dipolar molecule  stabilizing interaction that binds the two molecule together. Responsible for water properties, important in many organic solid materials, responsible in the behavior of many packaging materials like plastics and cellulosic. Much weaker than typical covalent bonds, but are stronger than dipole- dipole or dispersion forces.  Level 3 – Intermolecular forces Intermolecular forces can be cohesive between alike/similar molecules (e.g. in surface tension) or adhesive between different molecules. Cohesive forces in plastics and paper materials explain their strengths under tensile, tearing, or puncturing stress. Adhesion plays important role in lamination, coating and printing. Interaction type Bond energy (kJ/mol) Dispersion forces 0.4 – 4 Dipole-dipole forces 0.15 – 15 Ion-dipole forces 5 – 60 Hydrogen bonds 4 – 40  Level 3 – Intermolecular interaction COHESION, ADHESION AND SURFACE TENSION Surface tension results from cohesive forces between molecules inside the material which has an exposed surface. Whereas the molecules down in the liquid are shared with all the surrounding molecules in cohesive attraction, those on the surface have no neighboring molecules above  unbalanced forces  stronger attractive forces toward their nearest molecules at surface  expansion of this surface area requires energy to counteract the cohesive force (= surface tension). Measured in dyne/cm or N/m for liquid phases.  Level 3 – Intermolecular interaction COHESION, ADHESION AND SURFACE TENSION For two distinct phases such as one solid and one liquid (e.g. in coating or printing the surface of packaging material), or two solid surfaces in close contact with each other (e.g. during the sliding of one surface over another), the energy to increase the interface area = interfacial tension. It explains the behaviors of liquid on solid surface, e.g. wettability, capillarity, drop formation. If intermolecular adhesive forces between liquid and solid surface are stronger than the cohesive forces  wetting of the surface and upward meniscus in the capillary occur. If the cohesive forces are stronger than the adhesive forces  the liquid beads-up and does not wet the solid surface.  Level 3 – Intermolecular interaction COHESION, ADHESION AND SURFACE TENSION The generated interfacial tension between two different phases is important in several physical and chemical properties of packaging materials, e.g. friction, adhesion, cleaning ability, fogging, printing. A liquid phase can wet completely a solid surface (leading to a large surface area covered by a liquid layer and NOT to several separated small drops) only if its (liquid) surface tension is lower than the surface tension of the solid. Plastic materials generally have low surface tension compared to water solutions and several organic solvents  difficult to run important operations like printing, laminating, coating  therefore, it requires: Surface treatments to enhance the surface tension of plastic surfaces. Surface active agents (to lower the surface tension of the liquid). Antifogging and surfactant additives. Surface tension decreases with temperature. Hot solvent molecules more readily wet, reach and clean the solid surfaces than cold ones.  Level 3 – Intermolecular interaction COHESION, ADHESION AND SURFACE TENSION Surface tension of some packaging and reference materials Material Surface tension at 250C (dyne/cm) Polytetrafluoroethylene 20 Polyethylene 31 Polyethylene terephtalate 43 Cellophane 44 Polyamide (nylon 6.6) 46 Water 73 Glass (Pyrex) 170 Iron 1100  Level 4 – Spatial arrangements How molecules are arranged in the material. Important in determining the chemical and physical properties, esp. for polymeric materials e.g. plastics and paper. Morphology: the fine structure of material resulting from chemical composition, molecular arrangement and crystalline degree.  Level 4 – Spatial arrangements TACTICITY Tacticity: the atomic arrangement in three dimensional molecular configuration. Also called stereochemistry. The way the molecular or ionic units are geometrically organized  crystalline (symmetric, periodical, and well ordered structural organization) OR amorphous solids (asymmetric, not periodical and unsettled). Important in determining the ability of the solid material to crystallize  regular atomic arrangement in the molecular structure makes the molecules easily pack together into rigid crystals.  Level 4 – Spatial arrangements TACTICITY E.g.: methyl groups in PP Isotactic: all the methyl groups are on the same side of the chain  crystalline. Syndiotactic: the substitute group is alternately above and below the chain plane  crystalline. Atactic: random sequence  amorphous.  Level 4 – Spatial arrangements TACTICITY Crystalline structure of solids: structures composed of repeating identical units, orderly disposed with a fixed geometric relation between each other. Symmetric organization  low energy  more favored. Factors determining degree of crystallinity: stereochemistry, kinetics of solidification from a liquid state, presence of crystallization promoters.  Level 4 – Spatial arrangements CRYSTALLINE VS. AMORPHOUS Performances of packaging materials are greatly affected by the crystalline or amorphous structure. Glass is typically amorphous material. Ceramic material may be in different degrees of crystallinity depending on the chemical composition and manufacturing conditions. Most plastic materials are composed of crystalline and amorphous regions. Modulation of degree of crystallinity is possible by processing. The crystalline and amorphous state for general categories of materials used for packaging Material Spatial organization Glassy Amorphous Ceramic Crystalline/amorphous Cellulosic Crystalline (pure cellulose) Metallic Crystalline Plastic Amorphous/crystalline  Level 4 – Spatial arrangements CRYSTALLINE VS. AMORPHOUS Crystalline: high density. good mechanical properties. reduced free volume. low gas transmission. Some solids can crystallize in different forms (= polymorphism)  each form has its definite set of chemical and physical properties. Example: different resistance to corrosion and mechanical strength of different types of stainless steel or aluminum alloys. Amorphous solids (formed when the molecular chains have little orientation and regularity throughout the bulk structure), e.g. glass: no definite melting point. behave like solidified liquids. have properties of flowing, ductility and transparency. Most plastic materials are semi-crystalline  controlled combination of crystalline and amorphous structures can produce material with advantageous properties of strength and stiffness.  Level 4 – Spatial arrangements CRYSTALLINE VS. AMORPHOUS Generally, increase in crystallinity causes: Increased density. Increased tensile and compression strength. Increased gas and moisture barrier. Increased sealing temperature. Decreased transparency. Decreased tearing resistance. Decreased impact strength. Decreased toughness. Material Spatial organization Decreased elongation. Glassy Amorphous Ceramic Crystalline/amorphous Cellulosic Crystalline (pure cellulose) Metallic Crystalline Plastic Amorphous/crystalline  Chemical reactivity & susceptibility of packaging The chemical properties of food packaging materials are very important for the protective role of packaging. Chemical and other properties depend on what happen at the atomic and molecular levels  understanding it will help to select the best choice of food packaging materials. E.g.: type of polyethylene with linear chains and few side branches has higher density and lower gas permeability than the branched type. Chemical properties important to food packaging: Stability to oxidation, combustion and degradation, esp. for plastic and paper. Resistance to corrosion, e.g. for aluminum, stainless steel and tin plate. Resistance to aggressive chemicals.  Chemical reactivity & susceptibility of packaging Chemical properties important for food packaging materials Behavior Main materials involved When it might be important Oxidation Plastics, some cellulosic Weathering, aging, storage, combustion. Burning/flame Cellulosic, plastics Waste treatment, material identification. Corrosion Metals Weathering, storage, food contact. Biodegradation Cellulosic, plastics Waste treatment, food contact. Biodeterioration Cellulosic, plastics, metals Waste treatment, food contact, storage. Food contact, handling. Chemical resistance All Food contact. Etching/leaching All  Chemical reactivity & susceptibility of packaging - Oxidation Packaging materials are oxidized when exposed to atmospheric oxygen. Too fast oxidation rate  endanger its function as food packaging. Always happens in a reaction coupled with reduction (oxidation: loss of electrons from an atom or a molecules, which collected by other chemicals (reduced)) Redox reactions notable in packaging: Combustion or burning Bleaching Corrosion  Chemical reactivity & susceptibility of packaging - Oxidation Combustion or burning. Exothermic. Converts the potential energy of material into heat and light. A way to destroy organic packaging materials after their use. E.g. combustion of PP: 2 (C3H6)n + 9n O2  6n H2O + 6n CO2 + Heat Might be useful in identifying and recognizing the type and nature of material, esp. plastics, from: The flame’s color. The smoke produced. The burnt edge appearance. The combustion products.  Chemical reactivity & susceptibility of packaging - Oxidation The burning behavior of some polymers Polymer Flame Smoke Polyethylene/polypropylene Mostly yellow with blue inside Weak smoke, paraffin odor Polyvinyl chloride Bright yellow Soot (black), HCL odor Polyamide Bluish with yellow edge White smokes, horn odor Polyester Bright Sweet, provoking Polystyrene Yellow, softly bright Weak odor of plastic Polymethyl methacrylate Bright Fruit odor Cellophane Pale, orange, charred edges Paper, wood odor  Chemical reactivity & susceptibility of packaging - Oxidation Bleaching. Bleaching agents are used to chemically remove color from the packaging materials, e.g. paper and plastics. Similar phenomenon to the fading of color when package is exposed to sun and air. Most commercial chemical bleachers are oxidizing agents, e.g. sodium hypochlorite (NaOCl), other chlorine-based mixtures and hydrogen peroxide (H2O2) Their decolorizing action is mainly due to the removal of the electrons activated by visible light. Often become environmental concerns. Nowadays, fully bleached pulp can be obtained with elemental chlorine free (ECF) or totally chlorine free (TCF) bleaching processes.  Chemical reactivity & susceptibility of packaging - Oxidation Corrosion All metals used in food packaging (aluminum, stainless steel, tinplate) have the risk of corrosion from the interaction with the external environment (e.g. moist, salty air) and the food contact (e.g. acidic fruit or vegetable juices). Ways to reduce the possible damage of corrosion phenomena: to choose the best alloy for food contact that has the highest resistance to corrosion. to use coating technology (mostly organic coating  good barrier to oxygen and water) The danger of corrosion: the oxide formed does not firmly adhere to the metal surface  flakes off easily  structural weakness, possible container perforation, potential food contamination.  Chemical reactivity & susceptibility of packaging – Biodegradation & Biodeterioration Biodegradation: complete biochemical decomposition of organic molecules by microorganisms or the biologically- catalyzed decrease in the complexity of chemicals. Often involves mineralization, i.e. the conversion of C, N, S, and P content of organic compounds to inorganic products. Used to be undesirable. Now, due to the need of waste management, this property is more desired. Measurement: production of inorganic CO2 from the test material inoculated with mixed population of microorganisms. Percentage to the theoretically producible maximum CO2 amount is used as degree of biodegradation. A material is considered compostable if at least 90% of it biodegrades in a specified test (EN 14046 or ISO 14855) within 6 months (the European harmonized norm EN 13432).  Chemical reactivity & susceptibility of packaging – Biodegradation & Biodeterioration Theoretically, all naturally occurred materials (e.g. cellulose and derivatives) may undergo biodegradation, while synthetic materials cannot. Synthetic materials are chemically inertia (‘recalcitrance’ or ‘recalcitrancy’) due to: high average molecular weight. the presence of end groups incompatible with enzymatic attack. their hydrophobic nature. the presence of chemicals (additive or residuals) unfavorable for bacterial growth. the steric hindrance from branched side chains.  These characteristics mostly apply to plastic polymers, BUT may be modified by production technologies or additives. Synthetic or inorganic materials may also be deteriorated or affected by microbial action, e.g. corrosion of metal surface can be induced by microorganisms.  Chemical reactivity & susceptibility of packaging – Biodegradation & Biodeterioration Biodeterioration= Some degree of microbial damage or impairment of material = Applies mostly to relatively stable and durable materials. Example of biodeterioration: Formation of microbial biofilms (composed of immobilised cells embedded in an organic polymer matrix) on material surfaces that is in contact with aqueous environment  the films are absorptive and porous  they may contain solutes, heavy metals, inorganic particles as well as cellular constituents  they usually show a resistance to antimicrobial substances which enhances the possibility of bacteria proliferation  can cause deep corrosion (microbiologically-triggered corrosion) and other degradations. Surface characteristics of a material (e.g. roughness, electrostatic charges, surface tension) determine the adhesion of microbial cells on it. Those characteristics: Can influence the first adhesion of microorganisms. May affect the hygienic conditions of glass, stainless steel, aluminum, plastic surfaces. Can cause possible deterioration phenomena for paper, board, plastics, and even metal.  Chemical reactivity & susceptibility of packaging – Chemical resistance, etching & weathering Chemical resistance: the capacity of a material to maintain its fundamental characteristics when exposed to some chemically aggressive substances. Etching and leaching: the removal of matter from the surface of packaging materials by dissolution or scraping. Weathering: chemical (also physical) decomposition of a material on exposure to atmospheric conditions. Measured by controlled exposure of standardized sample material to standardized contact conditions (= abuse test). Changes of weight (absorption), dimension (absorption, interaction), mechanical properties, appearance (possible chemical changes) are measured objectively or evaluated subjectively after the time of exposure. Some frequently measured chemical resistance properties: behavior to oil, solvent, and some chemical substances.  Chemical reactivity & susceptibility of packaging – Chemical resistance, etching & weathering A selection of standards for chemical resistance Test Topic ASTM D 543 Chemical resistance (effects on weight, dimension and appearance during time of exposure to a standard list of chemicals). ASTM D 570 Water absorption (weight gain on water immersion). ASTM D 1693 Stress cracking (polyethylene samples with flaws are exposed to oils or other chemicals, and number of failures is counted). ASTM D 2651 Stress cracking. ISO 5634 Grease penetration (time to oil penetration under pressure). TAPPI 454 Grease penetration (time to oil penetration under pressure). ASTM F 119 Grease penetration (time to oil penetration under pressure).