X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 12 Apr 2009 19:27:58 -0400 Message-ID: X-Original-Return-Path: Received: from web36903.mail.mud.yahoo.com ([209.191.85.71] verified) by logan.com (CommuniGate Pro SMTP 5.2.13) with SMTP id 3577877 for lml@lancaironline.net; Sat, 11 Apr 2009 21:24:10 -0400 Received-SPF: none receiver=logan.com; client-ip=209.191.85.71; envelope-from=chris_zavatson@yahoo.com Received: (qmail 60647 invoked by uid 60001); 12 Apr 2009 01:23:35 -0000 DomainKey-Signature:a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com; h=Message-ID:X-YMail-OSG:Received:X-Mailer:References:Date:From:Subject:To:In-Reply-To:MIME-Version:Content-Type; b=yisD5Q7qH39PGVw/s7arLVE4aVWIWF96b9CvqpYUB0sW9hVv9UnnStDBPX3AXr+/6jXsvjBxUGrmcDumpoq1tFvfzTps/g/d5Q0Ay+PcYjdreom+HPxbnkaJ+rR16Y59UMjy4kqdh1VnmI9Gl3sosWutwxFWbMDTwVJnlb1yfcs=; X-Original-Message-ID: <68517.60040.qm@web36903.mail.mud.yahoo.com> X-YMail-OSG: SkTNslsVM1miSRwZxjydFgYarfUev_QPwjcId0zg0Dnmaz7FguXqggZ3Ldi99ijkJMxY8.hrhQv15srm2O1DQiV7cWCgh_kV_RcxpFOLFwKBEWMp7EcyK_3m_aORftSN5AuNCETniw5yWKK_AIHvjZHMRNtipfx_xcHyCZWXoJZUuEewocUMKqrDL7xfEtx9j7X0xAmhXnbW7KXOe_oFx3vLPGoK4PTIi5BAq7nWrv1N1VP.nYhCQi5A9ETk1qF9goAFCbaoQsFCLh_RAqe5YZ1YKIPA4Vq.1uxIOs9vFCdC9Vkvetg- Received: from [76.246.40.52] by web36903.mail.mud.yahoo.com via HTTP; Sat, 11 Apr 2009 18:23:34 PDT X-Mailer: YahooMailRC/1277.35 YahooMailWebService/0.7.289.1 References: X-Original-Date: Sat, 11 Apr 2009 18:23:34 -0700 (PDT) From: Chris Zavatson Subject: Re: [LML] Re: Jefco X-Original-To: Lancair Mailing List In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="0-1100763376-1239499414=:60040" --0-1100763376-1239499414=:60040 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Researching what manufacturers have to say yields a=A0pretty consistent=A0s= tory on the topic:=0ATechnical Primer =0AEpoxy Crystallization =0AAll epoxy= resins are susceptible to a natural occurrence called crystallization. Thi= s phenomenon is similar to what happens to honey left undisturbed for long = periods. Crystallization starts out slowly, with single tiny seed crystals = forming at places called nucleation sites within the material. From these s= eed crystals, other crystals start forming and moving outward. =0AIn epoxie= s, crystallization shows up as cloudiness, free floating crystals, clumps o= f crystal masses or in extreme cases the liquid can turn to a hard, brittle= solid. Crystallization can occur in virtually all resins and hardeners. = =0AThe liquid polymeric materials that are common to all epoxy formulations= are classified as super cooled liquids at room temperature. Because of the= ir composition, they remain liquid at room temperature because the rate of = crystallization, or solidification, takes place at a relatively slow rate. = If certain conditions occur that allow the formation of small seed crystals= in the material, then the rate of crystallization increases. =0ABecause of= the complexity of crystal formation in nature, crystallization can be very= difficult to predict and control when working with epoxy adhesives. =0A...= ...................................=0AExtreme Cold =0AWhile cold storing an= epoxy material can slow down the rate of crystal formation by slowing down= the movement of the molecules, extreme cold storage even for short times c= an cause rapid crystal formations if small seed crystals are present. =0A..= .................................=0ATemperature Cycling =0ATemperature cycl= ing is perhaps the most frequent cause of most crystallization. Warming a m= aterial slightly allows the molecules to move around and contact seed cryst= als. The amount of temperature cycling required to induce crystallization i= s relatively small. A temperature swing of only 20oC can begin the crystall= ization process. As a result, temperature during shipping and warehouse sto= rage must be taken into consideration. When epoxy materials are shipped by = air, the combination of extreme cold in the cargo portion of a plane and ex= posure to multiple warm to cold temperature cycles can accelerate crystalli= zation greatly. =0A...........................=0AAccommodating Crystallizat= ion in Epoxies =0ACrystallization can be very frustrating to deal with in e= poxy adhesives, because it cannot be attributed to one single factor, and i= ts occurrence can be arbitrary. It is possible to have a shipment with hund= reds of small containers of epoxy, and discover only a few of them randomly= placed throughout the box will have varying degrees of crystallization. = =0AFortunately, two part epoxy systems can be treated easily to revert them= back to a liquid state so they are suitable for use. =0ASimply heating the= epoxy to 50oC for a short time is enough to revert it back to a liquid sta= te. It is important to heat the epoxy long enough to make sure that all of = the seed crystals have been liquefied. Once heating is complete, the materi= al should be allowed to return to room temperature before mixing, or the ex= cess heat will begin the crosslinking reaction more quickly when the two pa= rts are mixed. =0A...................................=0A=0A=0A --0-1100763376-1239499414=:60040 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable
Researching what manufacturers have to say yields a pretty c= onsistent story on the topic:
= =0A

Technical Primer <= /SPAN>

=0A

Epoxy Crystallizatio= n

=0A

All epoxy resins are susceptible to a natural occu= rrence called crystallization. This phenomenon is similar to what happens t= o honey left undisturbed for long periods. Crystallization starts out slowl= y, with single tiny seed crystals forming at places called nucleation sites= within the material. From these seed crystals, other crystals start formin= g and moving outward.

=0A

In epoxies, crystallization shows = up as cloudiness, free floating crystals, clumps of crystal masses or in ex= treme cases the liquid can turn to a hard, brittle solid. Crystallization c= an occur in virtually all resins and hardeners.

=0AThe liqu= id polymeric materials that are common to all epoxy formulations are classi= fied as super cooled liquids at room temperature. Because of their composit= ion, they remain liquid at room temperature because the rate of crystalliza= tion, or solidification, takes place at a relatively slow rate. If certain = conditions occur that allow the formation of small seed crystals in the mat= erial, then the rate of crystallization increases.

= =0A

Bec= ause of the complexity of crystal formation in nature, crystallization can = be very difficult to predict and control when working with epoxy adhesives.=

=0A

......................................

=0A

Extreme Cold

=0A

While cold storing an epoxy= material can slow down the rate of crystal formation by slowing down the m= ovement of the molecules, extreme cold storage even for short times can cau= se rapid crystal formations if small seed crystals are present. =

=0A

...................................

=0A

<= U>Tempera= ture Cycling

=0A

Temperature cycling is perhaps the = most frequent cause of most crystallization. Warming a material slightly al= lows the molecules to move around and contact seed crystals. The amount of = temperature cycling required to induce crystallization is relatively small.= A temperature swing of only 20oC can begin the crystallization = process. As a result, temperature during shipping and warehouse storage mus= t be taken into consideration. When epoxy materials are shipped by air, the= combination of extreme cold in the cargo portion of a plane and exposure t= o multiple warm to cold temperature cycles can accelerate crystallization g= reatly.

=0A

...........................

=0A

Accommodatin= g Crystallization in Epoxies

=0A

Crystallization can= be very frustrating to deal with in epoxy adhesives, because it cannot be = attributed to one single factor, and its occurrence can be arbitrary. It is= possible to have a shipment with hundreds of small containers of epoxy, an= d discover only a few of them randomly placed throughout the box will have = varying degrees of crystallization.

=0A

Fortunately, two par= t epoxy systems can be treated easily to revert them back to a liquid state= so they are suitable for use.

=0A

Simply heating the epoxy = to 50oC for a short time is enough to revert it back to a liquid= state. It is important to heat the epoxy long enough to make sure that all= of the seed crystals have been liquefied. Once heating is complete, the ma= terial should be allowed to return to room temperature before mixing, or th= e excess heat will begin the crosslinking reaction more quickly when the tw= o parts are mixed.

=0A

...................................

=0A


 


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