Blowing agent(發(fā)泡劑)

1、Blowing Agentsv Introductionv Polymer Foamingv Blowing agent Physical blowing agent Chemical blowing agentvSelection criteria for blowing agent Polymeric foam Definition The substance that produces a cellular structure in a polymer mass is defined as a blowing agent. Function The blowing agents play

2、 a very important role in both the manufactoring and performance of polymer foam Effect of blowing agent on performance of polymer foamDensityCellular microstructureMorphology of the foam Insulation During manufactoring, the choice of blowing agent and the choice of processing condition/steps are in

3、ter-linked, i.e. each drives the other. Physical foaming process The blowing agent modifies melt viscosity and thermal history, and thus rheology during foaming. Chemical foaming process Foaming and shape all happens in one step, sometime, the release of blowing agent is also involved.Physical blowi

4、ng agents (PBA) provide gas for the expansion of polymers by undergoing a change in physical state.The change may involve volatilisition of a liquid, or the release of compressed gas to atmospheric pressure.Common liquid physical blowing agents are low boiling liquid and include short chain aliphati

5、c hydrocarbons and halogenated short chain hydrocarbons. Common gaseous physical blowing agent Include carbon dioxide, nitrogen, short chain aliphatic hydrocarbons and halogenated short chain hydrocarbons. The physical blowing agents are the only types of blowing agent when the foam density is low(5

6、0kg/m3)The choice of blowing agents is dependent upon the performance, cost-effectivenese and competetiviness of the finished product in a particular application. The ideal blowing agents for making insulation foam have some additional, specific requirements: low thermal conductivity and low diffusi

7、on coefficient through the foam. Halogenate hydrocarbons are one of the earlest blowing agent used to make foam. Large scale use of polymer foam was greatly accelerated by discovery and use of chlorofluorocarbons(CFC)Among Halogenate hydrocarbons, trichlorofluoromethane( CFC-11) to make PU foam, chl

8、orofluorocarbon-12(CFC-12) to make extrusion PS foam and 1,1-dichloro, 1,1,2,2-tetrafluoroethane (CFC-114) to make LDPE foam CFC has near ideal characteristics to be the blowing agent: low molecular weight, boiling point around room temperature, low toxicity, nonflamability and low thermal conductiv

9、ity, The most serious enviromental problem: ozone depletionvHCFCFeaturing at least one carbon hydrogen bond in the molecule, HCFC, is chemically less stable than CFC, and tend to break down at lower atmosphere into simple inorganic species, such as hydrogen halides. HCFC-141b was chosen as an excell

10、ent replacementFor CFC-11 in many rigid polyurethane foam applications. HCFC-141b blown rigid foams offer excellent thermal insulation, fire resistance and dimensional stability.Lower ODP HCFCs such as HCFC-142b and HCFC-22 are gaining more attention lately because their ODPs are only 50-59% of HCFC

11、-141b. However, HCFC-141b will eventually be phased out due to its residual ozone depletion potential (ODP) of 0.11. HCFC-142b (ODP = 0.065), HCFC-22 (ODP = 0.055) Blend of HCFC-22/HCFC-142b (40wt%/60wt%), are gaining more attention because they offer excellent thermal insulation, low cost and they

12、are commercially available.vHydrocarbons (HC)Many low boiling point aliphatic hydrocarbons have disired characteristics for blowing agent: low cost, specific volume, zero ODP, halogen-free, compatibility with common polymers, Disadvantage: flamability v Inert gasInert gas especially carbon dioxide a

13、nd nitrogen are the most widely used blowing agent. One reason CO2 and N2 are widely used as blowing agents is their relatively moderate critical temperature and pressure, especially CO2, Nitrogen is also used to make microcellular foam and conventional foam using many different polymers. Compressed

14、 N2 is used in injection molding to prepare microcellular foam. Advantages： low pressure and low temperature for supercritical state, non-toxic, low priceDrawbacks：High diffusivity、Low solubility in polymer(compared to CFC, HCFC, hydrocarbon)、High thermal conductivityGas is provide by chemical reaction. In general, CBA results in decomposition of molecule, one or more gas for polymer expansion, and one or more solid residues that remains in the foame