Effect Of Metal Hydroxide Encapsulated Expandable Graphite On Combustion And Physical And Mechanical Properties Of Polyurethane Foam

Rigid polyurethane foam（RPUF）has been applied as an insulation material in various industries owing to its low thermal conductivity,sound insulation and shock resistance.However,the limiting oxygen index（LOI）of RPUF is about 18%,and toxic gases are released during combustion.Therefore,it is necessary to add flame retardants to reduce its fire hazard.Expandable graphite（EG）,an environmentally friendly intumescent flame retardant,can effectively improve the flame retardant properties of RPUF.Nevertheless,the addition of EG to RPUF will cause the following problems:（1）The poor compatibility between the matrix and the additive causes the deterioration of the mechanical properties;（2）The good thermal conductivity of EG leads to the decrease of the thermal insulation performance;（3）EG is easy to produce fly ash in combustion,which resultes in the decrease in flame retardant performance.Metal hydroxide has excellent smoke suppression ability,but the flame retardant effect can only be achieved with its high amount.In this paper,the capsule coating technology was used to prepare magnesium hydroxide（MH）encapsulated EG（EG@MH）,iron hydroxide（FH）encapsulated EG（EG@FH）,and zinc hydroxide（ZH）encapsulated EG（EG@ZH）with EG as the core material,and MH,FH,ZH as wall materials,respectively.The influence of these additives together with APP on flame retardant and mechanical properties of RPUF composites was investageted.The specific research methods and results are as follows.1.EG@MH was prepared through the condensed phase separation method in the condition as follows:the amount of polyethylene glycol dispersant was 2 wt%of the magnesium chloride mass,the pH value was adjusted to 10 with ammonia,and the mass ratio of Mg Cl₂to EG was 2:1.Fourier transform infrared spectroscopy（FTIR）,scanning electron microscope（SEM）,X-ray diffraction spectroscopy（XRD）proved that EG was successfully coated by MH.The thermogravimetric data showed that the relative content of MH in EG@MH was31.06%.The physical and mechanical properties and combustion properties of the RPUF was investigated.Compared with 87RPUF/13EG,the cell integrity,mechanical properties and thermal insulation properties of 87RPUF/13EG@MH had been improved.Addition of EG@MH increased the LOI of 87RPUF/13EG@MH from 18.9%to 29.2%,and UL-94reached the V-0 level.The cone calorimetric test indicated that the peak of heat release rate（PHRR）,total heat release（THR）,smoke generation rate（SPR）,total smoke release（TSP）,CO and CO₂yield decreased significantly.Compared with 87RPUF/13EG,the THR,PHRR and TSP of 87RPUF/13EG@MH reduced by 20%,11%and 18%,respectively.The physical properties and flame retardancy of 87RPUF/6.5EG@MH/6.5APP were further improved.2.In the preparation of EG@FH,the amount of polyethylene glycol was 2 wt%of the ferric chloride mass,the pH was controlled as 10,and the mass ratio of FeCl₃to EG was 2:1.The relative content of FH in EG@FH was detected as 29.38%.Compared with87RPUF/13EG,the mechanical properties and thermal insulation properties of87RPUF/13EG@FH was improved,and the THR and TSP reduced by 25%and 35%respectively.The LOI of 87RPUF/13EG@FH increased to 28.5%,and UL-94 reached the V-0 level.Compared with one-component system,the physical properties and flame retardancy of 87RPUF/6.5EG@FH/6.5APP were further improved.3.In the preparation of EG@ZH,the dosage of polyethylene glycol was 4 wt%of the zinc sulfate mass,the pH was controlled as 10,and the mass ratio of ZnSO₄to EG was 1:1.In EG@ZH,the relative content of ZH was 35.58%.Compared with 87RPUF/13EG,the cell integrity,mechanical properties,thermal insulation properties and flame retardant of87RPUF/13EG@ZH had been improved,and its THR and TSP were reduced by 25%and18%respectively.The physical properties and flame retardancy properties of87RPUF/6.5EG@ZH/6.5APP were further improved.4.The effects of EG@MH,EG@FH,EG@ZH and their synergy with APP on the physical and mechanical properties and flame retardancy of RPUF were compared.The results showed that 87RPUF/13EG@MH had the highest compressive strength;the synergistic effect between EG@ZH and APP was the most significant;EG@MH persented the highest ability to reduce PHRR、THR、CO and CO₂yield;EG@FH indicated the highest smoke suppression capacity.The carbon-forming ability of EG@MH was the most significant.5.The flame retardant mechanism of the RPUF,encapsulated EG and APP system may be:the encapsulation treatment improved the flame retardancy of EG.The metal oxides produced in the decomposition of MH,FH and ZH can not only show packing action for residue and then suppress expanded graphite fly ash,but also exhibite the catalytic effect on the decomposition reaction of APP.The metal oxides can further reacte with ployphosphoric acid,produced by APP decomposition,and then generate stable phosphate,which further increase the residue yield at high temperature.As a result,each modified RPUF exhibited good flame retardant properties.